Underground in situ coal thermal treatment for synthetic fuels production

Progress in Energy and Combustion Science 62 (2017) 1
32

Contents lists available at ScienceDirect

Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs

Underground in situ coal thermal treatment for synthetic fuels production 2XTagedPD1 XHongzhi XD R. Zhanga, 4XD3 XSuhui XD Lib,*, D5X XKerry E. KellyDa6X X , D7X XEric G. EddingsD8XaX a

TagedP Department of Chemical Engineering, University of Utah, Salt Lake City, UT 84112, USA b Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China

TAGEDPA R T I C L E

I N F O

Article History: Received 21 October 2016 Accepted 11 May 2017 Available online xxx TagedPKeywords: Underground coal pyrolysis Synthetic fuel Retorting Oil shale Carbon capture

TAGEDPA B S T R A C T

Underground coal thermal treatment (UCTT) is a promising concept that was recently proposed for extracting high-value hydrocarbon fuels from deep coal seams, which are economically unattractive for mining. UCTT is essentially an in situ pyrolysis process that converts underground coals into synthetic liquid and gaseous fuels, while leaving most of the carbon underground as a char matrix. The produced synthetic fuels have higher H/C ratios than coals. The remaining char matrix is an ideal reservoir for CO2 sequestration because pyrolysis significantly increases the surface area of the char. The UCTT concept is relatively new, and there is little research in this area. However, underground oil shale retorting, which is also an in-situ hydrocarbon fuels conversion process, shares key features with UCTT and has gained momentum in demonstration and commercial development. As such, there is a large body of literature available in this area. A review of the studies on underground oil shale retorting that are closely related to UCTT will shed light on the UCTT process. This paper presents a review of the recent literature on underground oil shale retorting that are most relevant to UCTT process. The review provides a background to the reader by comparing the properties of coal with oil shale, with an emphasis on the feasibility of applying oil shale retorting techniques to UCTT process. The review further discusses the coal and oil shale conversion issues and uses the knowledge of the latter as guidance for the development of UCTT. Published data on pyrolysis of large coal blocks at conditions relevant to UCTT process is scarce. Therefore, literature on conventional coal pyrolysis is reviewed for optimization of the UCTT process. Despite the abundant studies on pulverized coal pyrolysis, there are still many open questions on whether they can be directly applied to UCTT. A comparison of the unique environment of UCTT with conditions of conventional pulverized coal pyrolysis clearly shows there are knowledge gaps. Future research needs are then proposed to close these gaps. © 2017 Elsevier Ltd. All rights reserved.

Contents 1. 2.

*

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical, chemical and geological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Rank and elemental composition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Volatile matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Elemental composition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. Implications for UCTT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Formation depth and thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Coal formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Oil shale formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Implications for UCTT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Heat capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Specific heat capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corresponding author. E-mail address: [email protected] (S. Li).

http://dx.doi.org/10.1016/j.pecs.2017.05.003 0360-1285/© 2017 Elsevier Ltd. All rights reserved.

2 4 4 4 5 5 5 5 5 6 6 6 7 7 7

2

3.

4.

5.

6.

7.

8.

9.

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

2.5. Implications for UCTT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Potential technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Underground retorting technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.1. Internal combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.2. Wall conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.3. Geothermic Fuel Cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1.4. Externally generated hot gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1.5. Volumetric heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1.6. ExxonMobil electrofrac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Enabling technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.1. Directional drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.2. Hydraulic fracturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.3. Rubblization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3. Implications for UCTT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Coal pyrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1. Pyrolysis yield and product composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1.1. Coal size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1.2. Pyrolysis atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1.3. Implications for UCTT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2. Porous structure and permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2.1. External stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2.2. Coal size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2.3. New techniques for characterizing porous structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2.4. Permeability development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2.5. Permeability modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.2.6. Implications for mass transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.2.7. Implications for heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Process parameters, products and optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.1. Temperature and heating rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.2. Pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.3. Heating sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.4. Product yield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.5. Product composition and quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Economics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.1. Energy return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.2. Process optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Technical challenges and environmental issues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 7.1. Site selection and exploration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 7.2. Heat management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 7.3. Environmental issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 7.3.1. Water consumption and disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 7.3.2. Water contamination and treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 7.3.3. Greenhouse gas and pollutant emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 7.3.4. Land use and restoration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Future research perspectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 8.1. Heat transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.2. Mass transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.3. Permeability modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 8.4. Pyrolysis kinetics and modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 8.5. Heating technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

1. Introduction TagedPCoal is the most abundant fossil fuel on the earth, comprising over 50% of the world's total resources of fossil fuels [1]. It is the second largest part (about 30%) of the world's energy supply and maintains the largest share (about 43% in 2014) of the electricity generation in the world [1]. Coal will continue to be a major energy resource in the foreseeable future, particularly in some developing countries such as China and India, where natural gas is not abundant. Most of the coal resources, however, are economically unminable because these coals are buried too deep [2]. For example, China has 60% of coal located more than 1000 meters below the surface [3]. Estimates also suggest that tapping deeply buried coals could help

iTagedP ncrease the U.S. recoverable coal reserves by as much as 300%
400% [4,5]. On the other hand, coal has a bad image because of the high pollutant and greenhouse gas emissions associated with its combustion. According to International Energy Agency [6], coal accounted for 46% of the global CO2 emissions in 2014, the largest contributor among fossil fuels due to its heavy carbon content per unit of heating value, although it only represented 29% of the world's total primary energy share. Consequently, new methods are needed to extract and utilize these deeply buried coals more economically and cleanly. TagedPUnderground coal thermal treatment (UCTT) is a recently proposed concept that offers the prospect of converting deeply buried coals into high-value hydrocarbon fuels that have less CO2 emissions

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPper energy content. In this process, the coal seam is heated by external energy and decomposition products are recovered in the form of gaseous and liquid fuels. Much of the carbon remains underground and forms a carbon matrix. As such, UCTT is an in situ method that can extract hydrocarbon fuels from coal seams that are otherwise economically unattractive through traditional mining methods. The concept of UCTT is schematically shown in Fig. 1. TagedPUCTT is essentially an in situ pyrolysis process in a non-oxidizing environment, which distinguishes itself from underground coal gasification (UCG). The non-oxidizing environment offers UCTT significant environmental advantages over UCG [7-9]. UCTT does not have the risk of uncontrolled fire spread underground, a major concern in UCG [10,11]. With pyrolysis taking place at much lower temperatures (usually below 600 °C) than gasification, UCTT is expected to have much lower sulfur emissions. The inorganic sulfur, which is the dominant source of coal sulfur emissions, is unlikely to release from the coal at temperatures below 900 °C [12,13]. UCTT significantly reduces the risk of underground water contamination compared with UCG. The mineral matter in coal is unlikely to decompose at the low pyrolysis temperatures and will remain in the carbon matrix formed after pyrolysis. The carbon matrix can be envisioned as a huge molecular sieve that traps almost all contaminants. In contrast, UCG leaves little carbon in oxidized zones and releases some aromatic species to surrounding aquifers [11,14]. The carbon matrix left by UCTT also is less likely to collapse than the ashes left by UCG. The collapse of ashes may result in surface subsidence, which is a major risk in UCG [10,11]. In addition, the remaining carbon matrix has a significantly increased internal surface area and offers a great potential for CO2 storage. The carbon emissions from UCTT process itself plus that from burning the produced fuels could be balanced by utilizing the carbon matrix as a CO2 reservoir, i.e., the process could be “carbon neutral”. TagedPUCTT also produces fuels with higher H/C ratio than UCG. Coal pyrolysis tends to favor hydrogen removal than carbon removal, compared with gasification [15]. Pollutant and CO2 emissions from burning these higher H/C ratio fuels will be much less than burning

3

cTagedP oals. In summary, UCTT is an attractive concept for underground in situ fuel extraction in a more environmentally benign way. It deserves further investigation to evaluate its economics and technological issues, and to identify future research needs. This concept, however, is relatively new, and little literature is available for review and assessment. Current reviews relevant to UCTT are summarized in Table 1. Despite the large body of reviews on UCG and conventional coal pyrolysis, to date no reviews address the specific features of UCTT, i.e., in situ pyrolysis in a non-oxidizing underground (spatially confined) environment. Fortunately, UCTT largely resembles underground oil shale retorting in that the underground resources are thermally treated. Only the target resource differs - coal versus oil shale. In the last decade, underground oil shale retorting has been pilot-tested and is gaining momentum towards commercialization. The breakthrough in underground oil shale retorting technologies will likely provide guidance to a UCTT process, although it must be tailored to the specific properties of coal. In addition, coal pyrolysis has been well studied for conventional applications (pulverized coal combustion, gasification, coking, et al.). Leveraging the readily available literature on underground oil shale retorting and conventional pulverized coal pyrolysis, one can preliminarily evaluate the feasibility, economics, technological and environmental issues of UCTT. TagedPIn light of the great potential to address UCTT issues using oil shale retorting technologies, we present this review. In Section 2, a comparison on the most relevant properties of coal and oil shale is made to show the feasibility of UCTT. In Section 3, recent development of underground oil shale retorting technologies is reviewed in the context of tailoring them to UCTT. The review is focused on the conversion issues of coal and oil shale, leveraging the knowledge of the latter as guidance for the development of UCTT. In Section 4, studies on coal pyrolysis that are closely related to UCTT are reviewed for identifying the key operating parameters. In Section 5, recommendations are given to optimize the product yield and composition of UCTT by adjusting the operating parameters. In Section 6, economics of UCTT are discussed at system level. In Section 7,

Fig. 1. Conceptual illustration of UCTT process, not drawn to scale. Reprinted from [7] with permission of Elsevier.

4

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPtechnical challenges and environmental issues in operation of UCTT are discussed. In Section 8, the review reveals that there is a big gap when applying the knowledge of conventional coal pyrolysis to UCTT, by comparing the unique environment of UCTT with the conditions of conventional coal pyrolysis. Future research perspectives to address the gap are then proposed. The ultimate objective of the review is to spur more discussions and research activities on UCTT by initializing a framework, rather than giving definitive answers to specific problems.

Table 1 Summary of previous reviews relevant to UCTT. Author

Year

Relevance to UCTT

Difference from UCTT

Gregg & Edgar [16]

1978

Sury et al. [17]

2004

Khadse et al. [18]

2007

Friedmann et al. [5]

2009

Klimenko [19]

2009

Shafirovich & Varma [14] Roddy & Younger [11] Bhutto et al. [2]

2009

Imran et al. [20]

2014

Akbarzadeh & Chalaturnyk [21] Hobbs et al. [22]

2014

Anthony & Howard [23]

1976

Underground, in situ environment Underground, in situ environment Underground, in situ environment Underground, in situ environment Underground, in situ environment Underground, in situ environment Underground, in situ environment Underground, in situ environment Underground, in situ environment Underground, in situ environment Large coal size (mm-cm) Pyrolysis environment

Howard [24]

1981

Pyrolysis environment

Gavalas [25]

1982

Pyrolysis environment

€ ntgen [26] Ju

1984

Pyrolysis environment

Smoot & Smith [27]

1985

Pyrolysis environment

Solomon et al. [15]

1992

Pyrolysis environment

Solomon & Fletcher [28]

1994

Pyrolysis environment

Benfell et al. [29]

2000

Pyrolysis environment

Wall et al. [30]

2002

Pyrolysis environment

Yu et al. [31]

2007

Pyrolysis environment

Gasification conditions Gasification conditions Gasification conditions Gasification conditions Gasification conditions Gasification conditions Gasification conditions Gasification conditions Gasification conditions Gasification conditions Above ground, oxidizing conditions, Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles Above ground, unconfined particles

2010 2013

1993

2. Physical, chemical and geological properties TagedPBy comparing the properties of coal and oil shale, one can roughly assess the feasibility and economics of applying underground oil shale retorting technology to UCTT. The most relevant properties are rank, formation depth and thickness, permeability, and heat capacity. Coal rank impacts product yield and composition. Formation depth and thickness impacts the economics. Permeability largely determines mass transfer, i.e., collection of synthetic fuels from underground. Heat capacity impacts heating cost and temperature history in the pyrolysis process, which in turn affects product yield and composition. 2.1. Rank and elemental composition TagedPCoal rank and elemental composition are determined by proximate and ultimate analyses, respectively. For comparison, proximate and ultimate analyses of representative coal and oil shales are listed in Table 2 [32] and Table 3, by the rank from high to low. Here we focus on the most relevant properties concerning UCTT: volatile matter (VM), moisture, oxygen, and hydrogen contents. For a detailed description of coal ranks and classification, the readers are referred to Speight [33] and ASTM standard D388 [34]. TagedP2.1.1. Volatile matter TagedPVolatile matter content is the most important factor determining the yield of fuels that can be potentially extracted from coal, because the gas and liquid fuels are converted from the volatile matter during coal pyrolysis. Indeed, pyrolysis is usually not distinguished from devolatilization when performed in an inert environment [40]. Table 2 shows that high-volatile bituminous coals, as indicated by the name, contain the highest fraction of volatile matter (>33%). Medium and low-volatile bituminous coals, subbituminous coals and lignite, also contain a good amount of volatile matter (26
34%). Anthracite coals contain only 5
10%. Table 3 shows that most oil shales have 20
40% volatile matter by weight, which is comparable to coals (except anthracite). The rich oil shales contain more than 30% volatile matter, similar to the high-volatile bituminous coals. This comparison, in first principle, indicates that the potential yield

Table 2 Proximate and elemental analyses of representative coal samplesa, wt%, HHV in kJ/kg [32]. Classification

Moisture

Ash

VM

FC

C

H

N

O

S

HHV

Anthracite

Meta-anthracite Anthracite Semianthracite Low-volatile Medium-volatile High-volatile A High-volatile B High-volatile C Sub A Sub B Sub C Lignite

13.2 4.3 2.6 2.9 2.1 2.3 8.5 14.4 16.9 22.2 25.1 36.8

18.9 9.6 7.5 5.4 6.1 5.2 10.8 9.6 3.6 4.3 6.8 5.9

2.6 5.1 10.6 17.7 24.4 36.5 36.4 35.4 34.8 33.2 30.4 27.8

65.3 81 79.3 74 67.4 56 44.3 40.6 44.7 40.3 37.7 29.5

64.2 79.7 81.4 83.2 81.6 78.4 65.1 59.7 60.4 53.9 50.5 40.6

1.9 2.9 3.8 4.6 5 5.5 5.4 5.8 6 6.9 6.2 6.9

0.2 0.9 1.6 1.3 1.4 1.6 1.3 1 1.2 1 0.7 0.6

14.5 6.1 4 4.7 4.9 8.5 14.6 20.1 27.4 33.4 35.5 45.1

0.3 0.8 1.7 0.8 1 0.8 2.8 3.8 1.4 0.5 0.3 0.9

Bituminous

Subbituminous

Lignite a

Proximate analysis is as received, while ultimate analysis is moisture free. VM: volatile matter; FC: fixed carbon.

2164 2993 3226 3347 3326 3263 2714 2512 2475 2233 1989 1627

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

5

Table 3 Proximate and elemental analyses of oil shalesa, wt %, HHV in kJ/kg. Shale sample

Moist

Ash

VM

Himmetoglu Huadian Maoming Fushund Seyitomer Himmetoglu Mengen Beypazari

8.4 2.9 3.7 2.7 10.3 12.9 9.5 5.0

25.1 51.6 71.9 77.4 60.8 60.5 68.4 67.3

38.4 28.1 41.9 3.6 24.2 0.2 20.0 0 27.4 1.5 26.6b b 22.1 22.0 5.7

FC

C

H

N

55.43 33.64 22.31 17.88 23.60 20.22 14.69 14.36

6.29 4.65 2.70 2.24 2.82 2.21 2.78 1.18

1.80 7.34 0.77 8.28 0.45 2.30 0.47 1.58 0.94 8.78 15.56c c 12.86 0.52 14.65

O

S

HHV

Ref

4.04 1.06 0.31 0.38 3.06 15.56 12.86 1.99

11297 8374 7294 5701 5481 4540 3556 2971

[35] [36] [37] [37] [35] [38] [38] [35]

a

Proximate analysis is as received, while ultimate analysis is moisture free. VM: volatile matter; FC: fixed carbon. VM + FC. c N + O. d Also see He [39]. The range of ash content in Fushun rich samples is between 70.6 and 78.1%; that of VM is between 13.4 and 20.2%; that of heat value is between 4500 and 7600 kJ/kg. b

TagedPof UCTT is comparable to that of underground oil shale retorting. In turn, the UCTT concept is viable in terms of product yield. Bituminous coals, especially the high-volatile bituminous coals are most suited for UCTT considering maximum product yield. In addition, bituminous coals will consume less heat during UCTT compared to other lower rank coals, because bituminous coals contain less moisture than the low-rank coals. A significant amount of heat will be wasted in water vaporization and desorption. TagedP2.1.2. Water TagedPMoisture in coal is a significant energy barrier to UCTT, because it takes a significant amount of heat to vaporize the water. The heat capacity of liquid water is 3-4 times of dry coal [41] and 4 times of dry oil shale [42,43]. Moisture can also participate in pyrolysis reactions and affect the final pyrolysis products [44,45]. Therefore, moisture content is a critical property to be considered in the process design and economic analysis. However, if UCTT is performed on a coal-bed methane (CBM) production site, the moisture would have already been removed, which is a substantial energy saving. TagedPMoisture content in coal decreases with coal rank. Lignite usually contains highest water content, while anthracite has the lowest water content. Moisture content in lignite is in the range of 20%
40% of the total weight, and is often over 30% [46]. In pyrolysis, about 50% of energy input is consumed to heat the water in a 20% moisture content coal, a significant energy penalty. Moisture in subbituminous coals ranges from 15
30% [46]. The moisture content is usually below 10% for bituminous coals except for high-volatile C [46,47]. In comparison, moisture content of oil shales except for the Fushun, Maoming and Huadian oil shales, is mostly in the range of 5
10%. This value is generally comparable to that of high-volatile B and C bituminous coals, and higher than that of other bituminous coals. From energy cost perspective, coals that have ranks higher than subbituminous-A are recommended for UCTT. TagedP2.1.3. Elemental composition TagedPCoal elemental composition is also a concern for the feasibility of UCTT. The oxygen and hydrogen content significantly affects the quality of the fuels produced from underground thermal treatment. Oxygen content correlates closely with CO and CO2 emissions [48], which are subject to environmental regulations. Low oxygen content is preferred and is usually found in bituminous and anthracite coals. Lignite and sub-bituminous coals contain a higher percentage of oxygen at about 15
34% (Table 2), compared to 2
13% for bituminous coals and about 2
3% for anthracite coals. High hydrogen content, however, is desirable because it is essential for producing premium, high H/C ratio fuels. Liquids obtained from coal conversion process are often characterized by API gravity [49]. Premium light crude oils have greater API gravity by definition. A coal liquid with high hydrogen content has a larger API value and requires less

TagedP pgrading work. High hydrogen content also results in less CO2 u emissions in the utilization of the fuel. The coal hydrogen content is inversely related to its rank, i.e., a high rank coal has low hydrogen content. This is because the hydrogen element leaves coal during the coalification process. The hydrogen content in anthracite coals is between 2 and 4%, compared to 5%
6% for bituminous coals, and 4
6% for sub-bituminous and lignite coals. TagedPSimilar to coal, high oxygen content exists in lean shale samples that also feature low carbon content (Table 3). In general, rich oil shales have oxygen content comparable to high-volatile A and higher-rank bituminous coals. Hydrogen content in shale samples increases with heating value from 1
2% for lean shales to about 5
6% for rich shales. Hydrogen content in most coals excluding some anthracites is about 5
7%, comparable to that of rich shales. Rich oil shales, however, have a slightly higher H/C ratio than most of the coals. H/C ratio is a key factor in determining the API gravity. Therefore, it can be inferred that the product quality of UCTT will at the best approach that of underground oil shale retorting. TagedP2.1.4. Implications for UCTT TagedPAnalyses of volatile matter and moisture content suggest that comparable yield can be obtained from underground thermal treatment of high-volatile bituminous coals and rich oil shales. Analyses of oxygen and hydrogen content suggest that high-quality pyrolysis products (e.g., high API gravity oil) can also be obtained from bituminous coals. More unwanted byproducts, such as CO and CO2 are emitted from lower rank coals such as subbituminous coals and lignites. The energy cost for bituminous coal underground thermal treatment is also expected to be less than that for underground oil shale conversion considering the lower water in coal, although the exact number depends on the specific heat of coal and oil shale. Therefore, bituminous coals, especially high-volatile bituminous coals, are the targets for early-stage UCTT demonstration. The fuel quality of UCTT, however, will tend to be lower than that of oil shale because of the lower H/C ratio in coal. 2.2. Formation depth and thickness TagedP2.2.1. Coal formation TagedPThe depth of coal seam is an important factor in determining if mining or in situ thermal treatment should be used [50]. Currently 900
1200 m is the maximum depth for mining for coal to be considered as a reserve, whereas between 1200
1800 m it is subeconomic for mining and the coal is considered as resource only [50]. At such a subeconomic formation depth, underground thermal treatment is a promising alternative to extract the energy from coal in the form of pyrolysis gases and liquids. Using UCTT, the deeply buried coals will become economically available, and this greatly expands the recoverable coal reserves. For example, 60% of the Chinese coal is buried

6

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPmore than 1000 m below the surface [3]. Estimates also suggest that tapping deeply buried coals could increase the U.S. recoverable coal reserves by as much as 300%
400% [4]. TagedPThe thickness of the coal seam also significantly impacts the economics and technology of UCTT. A thin coal layer tends to waste energy by losing heat to the surrounding rock layers. Moreover, a thin coal layer increases infrastructure cost because more heat pipes are needed per unit volume. This economic consideration is similar as in coal mining when the formation is too thin or the formation thickness/depth ratio is too small, it becomes economically unattractive. The minimum thickness for a coal layer to be counted in as resources is 0.37 m for anthracite and bituminous coals, and this number increases for the coal to be classified as reserves [51]. Therefore, thick coal layer is preferred for UCTT to avoid wasting heat and save infrastructure investment. The thickness of a majority of coal seams is usually within 30 m, while the thickest can reach hundred meters. A record coal seam thickness was reported in Shaer Lake (Schallsee) field in Hami Basin, China [52] to be 217 m. In the U.S., the thickest single coal layer is estimated to be 75 m. It is located in the Wasatch Formation on the western edge of the Powder River Basin. However, most of the coal formation is buried deep and not economically minable. A UCTT approach might be able to extract these resources in an economical and environmental-friendly way. TagedP2.2.2. Oil shale formation TagedPOil shale deposits in the western United States are located at shallower depths than those deeply buried coals. The overburden layer in the Green River Formation can be up to 300 m, and the ratio between the overburden and oil shale formation thickness is around 1:2 [53], indicating that in some locations, one may find shale deposits of 600 m thick. In Daqing, China, the overburden of oil shale deposit is 30
90 m, and the average thickness of the deposit is only 6 m [54]. The cost for drilling wells and laying the heat pipes into deeply buried coals is expected to be higher than that for oil shales. TagedP2.2.3. Implications for UCTT TagedPCandidates for UCTT are the deeply buried coals (e.g., more than 1000 m below the surface), with a minimum thickness that is economically feasible. The greater depth of coal seams will increase infrastructure investment, compared with oil shales. The greater depth, however, also favors methane production and CO2 sequestration because of the increased hydrostatic pressure [55]. It might not be economically feasible to apply UCTT to thin coal seams separated by rock layers. In comparison, shale is more uniformly distributed in the rock without separating into different layers. Thick layers parallel to the surface such as the Big George Deposit in the Powder River Basin [56] are candidates for UCTT. A benefit of such a deposit is that it has been tapped for CBM and has been dewatered. 2.3. Permeability TagedPPermeability is critical for mass transfer. Sufficient permeability is required for the produced liquids and gases to transport out of the coal or shale. There is a huge difference in permeability between coal and oil shale formations. The permeability of raw oil shale is very low because oil shale is largely an inorganic rock formation, and the pores of the rock are filled with nondisplaceable organic material [57,58]. Rubblization is usually needed for efficient operation of underground oil shale retorting due to the low permeability and porosity in oil shale. Thanks to the low content of inorganic matter and massive internal pore structures, coal usually has much higher permeability than oil shale. TagedPCoal can be considered as a dual porosity/permeability system that consists of porous matrix and cleats, as shown in Fig. 2 [59,60].

Fig. 2. Illustration of the coal porous/fracture system: matrix pores and cleats. Reprinted from [59] with permission of Elsevier.

TagedP ass transport in coal occurs at two scales: diffusion through the M pores of the coal matrix, and flow through the macroscopic cleats, the natural fracture system of coal [61
63]. Both pore diffusion and fracture convection contribute to the permeability of coal. Once diffusing out of the matrix pores, gases and liquids form streams in the cleats. The naturally occurring pores and cleats can change significantly in the pyrolysis process. Correspondingly, permeability of a coal seam can be classified as two categories: naturally occurring permeability and pyrolysis-induced permeability. TagedPResearch on the natural permeability of coal started in 1980s for the production of CBM [64]. The topic is being revisited due to the recent interest in enhanced CBM production and using coal seams as CO2 sequestration sites [65
67]. Extensive studies have been performed to identify key factors that influence coal permeability. For a complete summary of these studies, the reader is referred to McKee [64] and Pan and Connell [68]. A general consensus is that coal permeability strongly depends on the stress and strain from the overburden and matrix swelling/ shrinking, especially when gas adsorption/desorption occurs [69
76]. For deeply buried coal seams, the stress from the overburden tends to close the cleats, thus reducing its permeability. Studies have shown that coal permeability decreases by 1-2 orders of magnitude when effective stress is increased by 14 MPa [64]. On the other hand, gas desorption increases permeability by opening the pores, and causing coal matrix to shrink and cleats to expand. For example, well tests conducted at three wells in the Valencia Canyon area of the San Juan Basin (subbituminous to bituminous coals) found that permeability, during CBM production process, increased 2.7
7.1 times [71]. TagedPPyrolysis-induced permeability of coal increases drastically upon heating, due to the release of moisture, volatiles and increase of porosity [77
85]. When the devolatilization completes, the permeability of the coal can increase by more than 1000 times for subbituminous and bituminous coals [77
81] and more than 100 times for lignite coals [82]. In fact, rubblization was deemed unnecessary in Shell's UCTT patent [80,81], because high permeability and porosity in coal seams will develop during devolatilization. Studies on the effects of pyrolysis on coal permeability mostly target UCG applications. For a detailed review of the studies, the reader is referred to Akbarzadeh and Chalaturnyk [21]. In general, permeability is less of a concern in UCTT than that in underground oil shale retorting, primarily because of the low ash content of coal and the remarkable porosity growth in coal pyrolysis.

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

2.4. Heat capacity TagedPFor an axisymmetric system, the heat transfer into a unit volume is governed by Eq. (1) [86]

 @T 1 @ @T @ @T kr þ k ð1Þ rCp ¼ @t r @r @r @z @z TagedPIn Eq. (1), r, Cp, and k are density, specific heat and thermal conductivity, r and z are coordinates, and T and t are temperature and time, respectively. Eq. (1) shows that for heating the coal to a certain temperature, Heat transfer into a unit volume scales with the product of specific heat capacity and density. Consequently, specific heat capacity and density are critical properties to be considered when heating oil shales and coals for hydrocarbon fuels.

TagedP2.4.1. Specific heat capacity TagedPGilliam and Morgan [87] reported that the specific heat of Devonian shale, Pierre shale, and oil shale from the Green River formation is in the range of 1000
1500 J/(kg  K). Berkovich et al. [88] reported that for the kerogen and clay minerals in some Australian oil shales, the values are in the range of 1820
2500 and 690
1020 J/(kg  K) , respectively, at temperatures below 400 °C. Because kerogen is the minor component of the oil shale, it can be estimated that the specific heat of these Australian oil shales barely exceeds 1500 J/(kg  K) Model developed by Palmer et al. [89] predicts the specific heat of oil shale is generally in the range of 1000
1500 J/(kg  K) depending on the fraction of kerogen. TagedPIn comparison, the specific heat of coal is in the range of 1000
3000 J/(kg  K) , depending on the composition and temperature [90
92]. Merrick [93] proposed a model for calculating coal specific heat for coke-making applications, which predicts coal specific heat increases with temperature. The specific heat of coal is generally above 2000 J/(kg  K) at typical pyrolysis temperatures, as shown in Fig. 3 [90]. The two peaks occur at about 100 and 400 °C, corresponding to water vaporization and devolatilization chemical reactions, respectively. Higher specific heat of coal indicates more heat input than oil shale for heating the same amount of mass to achieve the same temperature increase.

Fig. 3. Specific heat capacity of a Hanna Basin Coal as a function of temperature. Reprinted from [90] with permission of ASME.

7

TagedP2.4.2. Density TagedPFor both oil shale and coal, density generally increases monotonically with depth because of the dense packing at the increased lithostatic and hydrostatic pressures from overburden and surrounding reservoir. The density of oil shale varies depending on the formation, and is generally in the range of 1.5
2.5 g/cm3, as shown in Fig. 4 [94]. The lower the shale's kerogen content, the higher the shale's density. The Maoming oil shale deposit in China has a bulk density of about 1.85 g/cm3 [95]. In comparison, coal density is usually lower than its surrounding sediments, and is in the range of 1.1
1.8 g/cm3, depending on the rank [96]. A general rule is that the density of coal increases with its rank because the lighter element content decreases as coal matures. TagedPCombining the specific heat and density, the specific heat of coal per volume basis is in the range of 2
5.4 J/(cm3  K), while for oil shale it is 1.5
2.5 J/(cm3  K). Therefore, coal will likely need more heat per unit volume compared to oil shale, to reach the same temperature increase. For conductive heating, this translates into more heat piping per unit volume or higher temperature of heating source, which will increase the infrastructure and energy cost of the heating system. 2.5. Implications for UCTT TagedPCoal and oil shale properties that are most relevant to underground thermal treatment were compared, and the results suggest that UCTT is economically and technically feasible. The best suited candidate for UCTT will be a high-volatile bituminous coal that is deeply buried and has a reasonable coal seam thickness. Ideally the

Fig. 4. Shale density as a function of depth from several sedimentary basins, reprinted from [94] with permission of Elsevier. 1 = methane-saturated clastic sedimentary rock (probable minimum density); 2 = mudstone—Po valley basin, Italy; 3 = average Mexican Gulf Coast shale densities derived from geophysical data; 4 = average Mexican Gulf Coast shale densities derived from density logs and formation samples; 5 = Motatan-1—Maracaibo basin, Venezuela; 6 = Pennsylvanian and Permian dry shales; 7 = Las Ollas-1—Eastern Venezuela.

8

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPcoal seam has already undergone CBM production. The pyrolysis product yield is comparable or even higher than the rich oil shale due to the higher volatile content of coal. The quality of liquid fuel is likely lower than that of oil shale because of the lower H/C ratio in coal. The infrastructure investment and energy cost of UCTT, however, can be higher than that of oil shale for the same operation temperature, due to the higher heat capacity of coal. To improve product quality and minimize production cost, the oil shale retorting technology needs to be tailored to the unique characteristics of UCTT. Therefore, potential technologies and coal pyrolysis are reviewed and discussed to provide a background for optimization of the UCTT process. 3. Potential technologies TagedPUnlike underground coal gasification, there is very little literature on UCTT. However, a large number of underground retorting technologies relevant to UCTT have been explored for in situ fuels production from oil shale [42,53,97
103]. In addition, progress in supporting technologies such as directional drilling and hydraulic fracturing has enabled commercial success of underground shale oil and shale gas exploration. These existing shale conversion technologies provide a starting point for developing an underground coal thermal treatment approach. In the following, we will provide an overview and discussion on the technologies relevant to UCTT. 3.1. Underground retorting technologies TagedPThe shale conversion concepts can be divided into four major categories according to the heating techniques: internal combustion, wall conduction, externally generated hot gas, and volumetric heating. Combined technologies of these four categories were also explored by industrial companies. An overview of these approaches is presented in Table 4. Comparison of the product quality and energy balance, advantages and drawbacks are summarized in Tables 5 and 6, respectively. TagedP3.1.1. Internal combustion TagedPThe internal combustion approaches feature the combustion of oil shale to generate a fire front moving through the oil shale deposit. The heat of the fire front breaks down macromolecular hydrocarbon clusters of the oil shale into oils and gases. In 1972, Occidental Petroleum became the first company in the U.S. to conduct a viability

sTagedP tudy of a modified in situ process using the internal combustion approach at Logan Wash, Colorado [101,104
107]. Since then, several companies have tested the internal combustion approaches, including the Lawrence Livermore National Laboratory (LLNL) [108
110], and Geokinetics Horizontal [111]. The internal combustion approach requires rubblization of an oil shale formation to create porosity that facilitates the flow of gas and oil toward the extraction wells before the combustion starts. A fraction of the shale deposit needs to be removed to allow oil shale expansion during the rubblization. In the LLNL RISE demonstration, 20% of an oil shale deposit was mined for this purpose. Explosives were often used for rubblization. TagedPInternal combustion method is not recommended for UCTT as the CO2 produced during combustion can cause gasification reactions. Indeed, internal combustion has been tested in UCG. There are significant efforts ongoing on UCG which, are beyond the scope of the present review. For the current status of UCG development, the reader is referred to Bhutto et al. [2], and Shafirovich and Varma [14].

TagedP3.1.2. Wall conduction TagedPWall conduction in situ technologies use heating elements or heating pipes inside the oil shale to thermally decompose the kerogen. Wall conduction technologies have been explored by several companies for oil shale conversion, including the Shell in situ conversion process (Shell ICP) in 1997 at Piceance Creek Basin, Colorado [112
114], and the Conduction, Convection, Reflux (CCR) Process by American Shale Oil (formerly E. G. L. Resources) at Western Colorado [115]. The Shell ICP uses electrical heating elements to heat the shale to 340-370°C for a very long time [107,116]. The production volume is isolated by a freeze wall (Fig. 5) filled with super-chilled fluid to protect the surrounding water [53,116,117]. Drawbacks of this process are the high consumption of electricity and water, and potential contamination of ground water [107]. The American Shale Oil CCR process heats the shale using heating pipes in which superheated steam or other heat carrier keeps circulating [101,115]. In the CCR process, both vertical and horizontal wells are used to promote heat transfer and facilitate product collection. A coal seam usually consists of many single coal layers with rock layers in between. Therefore, when a heating and extraction piping system is designed, the pipe orientation should consider the structure of the coal seam. To avoid wasting energy in the rock layers, pipes parallel to the coal layer make the most sense.

Table 4 Overview of various underground oil shale conversion approaches. Company/Technology

Site

Heating

Fracturing

Occidental Petroleum Geokinetics Shell ICP b Electrofrac Geothermic Fuel Cell Taiyuan University of Science and Technology Chevron CRUSH E. G. L. CCR Petroprobe, Omnishale Mountain West Energy in situ Vapor Extraction Radio frequency Microwave

Colorado Utah Colorado Jordan

Internal combustion Internal combustion Conduction; electrical; 2
4 years Planar conduction, 7-8 years Fuel cell Hot alkane convection CO2 convection Hot alkane convection Hot Air => Shale gas convection Natural gas convection; 2
4 years Radiation, 1-2 months Radiation

Rubblized Rubblized

a

China

Hydraulic Yes, Raised T Yes Yes Yes Yes Yes

Ta (°C)

P c (bar)

Efficiency

343
400 400

2
34 163

50
80%

400
705 400

136

400 low

88

Up to 90%

Kerogen decomposition rate depends on temperature: 90% decomposition occurs within 5000 min at 371 °C and within 2 min at 500 °C. Freeze wall is 3.1 m thick; freeze-wall wells are 2.5 m apart from each other; refrigerant at ¡40°C; stabilized within 1.5
2 years; maintained for 6.5
8 years. Heater wells are 7.8 m apart; the heating rate is 0.5°C per day; Heat loss to overburden is relatively small; At atmospheric pressure and ICP heating rate, 80% of FA oil yield can be reached; at higher pressure, 60% of FA oil yield was reported; Shell reported a 66% of the FA oil yield from test plots. Hydrocarbons travel to production well in vapor form, then was pumped to the surface as liquid at »200 °C. After production ceased, water is flushed to the production wells for 20 times of pore volumes to recover mobile HCs. c Pore pressure cannot exceed the lithostatic pressure that applied to pore space from the overlying formation. b

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

9

Table 5 Comparison of the products of various underground oil shale conversion technologies. Company/Technology Occidental petroleum Geokinetics Shell ICP Electrofrac Geothermic Fuel Cell Taiyuan University of Science and Technology Chevron CRUSH E. G. L. CCR Petroprobe, Omnishale Mountain West Energy in situ Vapor Extraction Radio Frequency Microwave

Product a

Quality

Water/oil production b

Depth, m

Energy Balance Outlet: Inlet

2/3 liquid + 1/3 syngas Gas + Liquid Electricity, gas, oil Oil, Gas and Water

> 30 °API

3: 1

300
600

1.2-3.5:1

Hydrogen; methane; condensate; and water Gas only Oil and gas

45 °API

18:1

900 Twice as other processes

a For a 101 liter/ton shale, the Fischer Assay yield is 84% oil, 6% gas, and 10% char. The FA involves heating the shale to 500 °C at 12 °C per minute and holding at that temperature for 10 min. Higher pressure, lower temperature, and slower heating leads to lower oil yield and higher gas yield. Synthetic crude has a high H:C ratio of 1.9:1. b Underground water that fills the porosity and fractures needs to be removed as much as possible because the heat capacity of water is 4 times that of shale. After all drainable water has been removed, water will occupy »7% of shale bulk volume.

Table 6 Comparison of the advantages and drawbacks of various underground oil shale conversion technologies. Company/Technology

Advantage

Drawback

Occidental petroleum

Utilizing more heat and chemical values; captures SO2; IRR 20% ⬄ $23-35/barrel Lower up-front cost; flexible heating temperature; low pollution; lower reclamation cost; available tested technology Fracturing to increase permeability; byproduct of Na2CO3; higher thermal efficiency of planar heating; reduced surface footprint Even heating; self-sustainable; producing electricity; $14/ barrel; lower pollutant and GHG emissions; minimal water usage; minimal surface footprint Fracturing to increase permeability; low demand for water; even heating Saves underground piping system; flexibility in adjusting CO2 temperature High thermal efficiency; low pollution; flexible operation temperature; uniform heating Fracturing to increase permeability; Low Pollution; self-sustaining; minimal surface footprint Natural gas is soluble in shale; even heating; some control in product distribution; fewer wells Short time heating; neutral carbon footprint; tunable process; targeted products Short time heating; volumetric heating; selective heating; low pollution; very high extraction rate

Gasification reactions; pollution and contamination of surrounding environment and ground watert Complex configuration; low thermal efficiency; excessive electricity consumption; low extraction rate No mention of ground water protection; excessive electricity consumption;

Shell ICP Electrofrac

Geothermic fuel Cell EPICC

Taiyuan University of science and technology Chevron CRUSH E. G. L. CCR Petroprobe, Omnishale MWE's in situ vapor extraction Schlumberger and LLNL's radio frequency Global resource Corp's microwave

TagedPThe wall conduction technique has later been adapted to coal. Underground coal thermal treatment using wall conduction heating was proposed by Shell Oil Company [80,81]. In their process, the coal was heated to 525 °C and gas and oil of very high quality were produced. The major products include a coal liquid with API gravity over 30. According to API manual [49], light crude oil has an API gravity of higher than 31.1 (less than 870 kg/m3), and medium crude oil has an API gravity between 22.3 and 31.1 (870 to 920 kg/m3). Therefore, the Shell ICP produces medium oil towards the lighter end, which can be considered high quality oil. In this process, steam is supplied by combusting fuels. Since direct heat exchange has a higher efficiency than electricity generation, the CCR process saves energy compared to the Shell ICP. TagedPBoth the electrical heating and steam heating methods can easily change the operation temperature. The two processes can be easily tuned to the optimized heating temperature for coal pyrolysis. In addition, wall conduction using hot pipes leads to very uniform thermal heating, which leads to a uniform distribution of porosity and a higher permeability. Therefore, wall conduction using heating elements or heating pipes are promising candidates for UCTT.

Fuel cell technology less mature; lacks flexibility in adjusting operation temperature; Less tolerant to fuel compositional variations

High water usage; low specific of CO2, low thermal efficiency; CO2 leakage; nonuniform flush Extensive piping investment; high water consumption low specific of hot gas, low thermal efficiency; nonuniform heating Heating efficiency is unknown Excessive electricity consumption to generate RF Excessive electricity consumption to generate microwave

TagedP3.1.3. Geothermic Fuel Cell TagedPAn interesting heating approach was proposed by Independent Energy Partners [118] by placing a “Geothermic Fuel Cell” stack within a formation to heat the oil shale. As the formation is heated, oils and gases are released and collected, which are then partially transferred to the fuel cell to generate the heat for shale pyrolysis as well as electricity. External oxidants are supplied to the fuel cell in the meantime. The advantage of this technology is that the fuel cell is self-sustained by the pyrolysis gases, once it is started by natural gas. This technology also has high efficiency and low carbon emissions because the waste heat of fuel cell (when converting chemical energy of the fuel into electricity) is used to heat the shale formation. Fuel cell technology, however, is less mature compared to electrical heating or steam heating [119]. Control of the oxidation rate and reaction temperature is still an issue. Fuel cell operation is also very sensitive to the fuel gas composition. Tiny amounts of sulfur gas or other impurities may cause serious problems to the fuel cell. The complex composition of pyrolysis gas poses a challenge to the fuel cell. The operation temperature is relatively fixed for a given type of fuel cell technology, which may not be flexibly adjusted to achieve

10

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

Fig. 5. Shell's freeze wall for in situ shale oil production separates the process from its surroundings. Reprinted from [117] with permission of Elsevier.

TagedPthe optimal heating temperature for coal pyrolysis. Therefore, the Geothermic Fuel Cell concept still needs development before it can be applied to UCTT. TagedPThe “Geothermic fuel cell” concept can be further combined with in situ carbon capture. This forms the concept of electricity production with in situ carbon capture (EPICC). Mulchandani and Brandt [102] estimate that the life cycle GHG emissions from EPICC amount to only 50% from those conventional fuel cycles and 33% from other in situ oil shale conversion processes. A key for low-carbon operation of the EPICC process is to maintain the shale temperature below the value that significant decomposition of carbonate minerals occurs. The optimum operation temperature of fuel cell for electricity generation may not overlap with this value, which will impair the process efficiency. TagedPGiven the fast pace of fuel cell technology development, Geothermic Fuel Cell and EPICC type concepts may be attractive for UCTT, considering their great potential in reducing GHG emissions. TagedP3.1.4. Externally generated hot gas TagedPExternally generated hot gas technologies, by the name, use hot gases that are generated on the ground and then injected underground to heat the oil shale. The gases can be CO2, air, or combustion exhaust. In 2006, Chevron developed an underground retorting technique, the Chevron Recovery and Upgrading oil from Shale (CRUSH) process that circulates heated CO2 through the shale formation [117,120]. Externally generated hot gases used by other companies for in situ shale oil production include superheated pressurized air and shale gas by General Synfuels International's Omnishale technology [117], and the combustion exhaust of natural gas by Mountain West Energy's in situ Vapor Extraction [121] and alkane gases by Taiyuan University of Science and Technology (TUST) [78]. The hot gases in general have a disadvantage of low specific heat compared to steam. It can be postulated that more piping is needed per unit volume of coal seam than that for steam heating. Heating the CO2 or the gases needs expensive gas heat exchanger. In addition, the hot CO2 and O2 can react with coal via gasification reactions. The hot gases also have a relatively low density and low viscosity. Therefore,

TagedP s the hot gases flush the coal formation in the horizontal direction, a they tend to go upward and bypass the bottom part of the coal formation, leaving the large part of coal untreated. To overcome this problem, horizontal wells need to be drilled to facilitate the hot gases flushing the entire coal formation. Vertical wells are also needed to inject hot gases at different horizontal locations of the coal formation. TagedPCalderon and Laubis [122] proposed an approach of underground coal pyrolysis using a heated hydrogen-rich recycle gas to extract syncrude and syngas, and subsequently to convert in situ CO2 to CO, SO2 to S, NOx to N2 using the residual coal char with a gas stream consisting of CO2 and air. The hydrogen-rich gas has an advantage of high specific heat. H2 leakage, however, can be a potential issue especially under pressurized, heated conditions. The H2, due to its low density and high diffusivity, has a high tendency to go through the upper layer of the coal seam and leave the rest of the coal seam untreated. TagedPIn general, technologies using externally heat gas save the heating pipe system compared to wall conduction heating. However, they have a drawback of incomplete and nonuniform treatment of the coal formation, due to the propensity of hot gases to go upward. To overcome this problem, extensive horizontal and vertical wells are needed to facilitate uniform flush of the coal formation, which adds the infrastructure cost of the process. In addition, gas heating is not as good as wall conduction heating in terms of permeability development of the coal. When a coal formation is heated by the combustion gases, channeling is usually observed which leads to a non-uniform porosity distribution [80,81]. TagedP3.1.5. Volumetric heating TagedPResearchers have also explored the possibilities of using radio frequency wave and microwave to heat oil shales [123]. In late 1970s, the Illinois Institute of Technology developed an in situ oil shale radio wave heating technique, which was further developed by Lawrence Livermore National Laboratory [124,125]. In the radio frequency wave method, oil shale is heated by radio waves emitted from electrodes that are installed underground. Raytheon and CF Technologies co-developed another radio frequency heating approach in conjunction with critical fluid flushing, which was acquired and patented by Schlumberger [126]. Global Resource Corporation tested a similar concept using microwave heating [125,117]. Radio wave and microwave deliver heat quickly and uniformly, but have the drawbacks of excessive electricity consumption and wave energy being adsorbed by water and residual chars. Realization of radio wave and microwave at high pressure conditions, which are typical of UCTT, demands high electric power. TagedP3.1.6. ExxonMobil electrofrac TagedPExxonMobil developed a combined approach of wall conduction and volumetric heating by injecting an electrically conductive material such as calcined petroleum coke into the fractures of an oil shale formation [127]. This conductive material then forms the heating element of the electrically heated underground oil shale retort. This process increases permeability by fracturing and has high thermal efficiency of planar heating, whereas has a drawback of high electricity consumption.

3.2. Enabling technologies TagedPTwo important technologies, directional drilling and hydraulic fracturing, have enabled the recent commercial success of unconventional oil and gas production, particularly in shale gas proliferation. The two technologies, having been employed in underground shale oil production, can be applied to UCTT process.

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedP3.2.1. Directional drilling TagedPDirectional drilling is a technique that the well bore can be deviated from the initial perpendicular angle to the surface [128]. It allows drilling horizontal wells through or under the shale formation. It also allows multiple horizontal wells from the same vertical well bore. The horizontal wells enable heating pipes to be installed horizontally through or under the formation, greatly expanding the heat transfer surface. The horizontal wells also provide channels for oil and gas moving into vertical wells, significantly improving the collection efficiency. Modern directional drilling technology can even drill along the inclination angle of coal seams. Directional drilling in UCTT process will be broadly used for heating element or heating pipe installation, heating fluid injection and product collection. TagedPIndeed, directional drilling has already been widely used in CBM production. Horizontal wells are drilled connecting vertical ones to facilitate methane collection. Substantial studies have been devoted to the mechanical stability and quality control of both vertical and horizontal wellbores [129
135]. These studies advanced the understanding of instability encountered in directional drilling in coal seams and proposed methods to improve quality control, which can be readily used by UCTT applications. TagedP3.2.2. Hydraulic fracturing TagedPHydraulic fracturing is a recently matured technique that has been applied in the “unconventional fuel” production, such as shale gas, tight gas, tight fuel and CBM [136]. As of 2012, 2.5 million "fracturing jobs" had been performed worldwide on oil and gas wells, over one million of which are within the United States [137]. To create fractures, a fracturing fluid (often water with additives) is pumped down to the wellbore to generate pressure to break the formation rock. The fracturing increases permeability of the formation, thus allows more flow of hydrocarbon to achieve a higher production. Water, often mixed with sand, gels, foams and other chemicals, is commonly used as a fracturing fluid. Conventional approaches used small volumes of fracturing fluid until the invention of highvolume horizontal slick-water fracturing in 1990s. The fracturing fluid in the new technology contains a lower amount of gelling agents, but adds friction reducers. To maintain the opening after fracturing, a proppant is usually introduced after the injection. For understanding the detailed physical mechanisms that control the efficiency and environmental impacts of hydraulic fracturing, the reader is referred to Hyman et al. [138]. TagedPHydraulic fracturing has been widely used in underground shale processing such as in ExxonMobil's Electrofrac [127] and TUST's Hot Alkane Convection [78] technologies. The temperature and pressure at which a shale rock formation fractures depend on the rock properties. For example, the Liushuhe oil shale sample fractures under pressures of 4.5
13 atm, in comparison with 80 atm for the Fushun samples [54]. TagedPObviously hydraulic fracturing adds water into the coal formation. The added water will likely cause energy penalty when heating the coal formation. Therefore, care should be exercised in the application of hydraulic fraction in UCTT. Unless necessary to increase the coal permeability, hydraulic fracturing is not recommended. TagedP3.2.3. Rubblization TagedPRubblization breaks a rock formation into small pieces or particles and creates voids, thereby increasing the permeability to promote mass and heat transfer. In underground oil shale retorting, rubblization is often required because of the low permeability of oil shale. Rubblization can be made by explosives or shock waves. For example, explosives were used to create a rubblized shale formation in Occidental Petroleum's modified in situ oil shale processing [106]. OP's underground retorts, with a dimension of 50 m wide £ 50 m deep £ 85 m high, were constructed by mining 3 retort void volumes

11

(TagedP upper, intermediate, and lower) to provide the expansion space for the shale formation after the detonation of explosives that were placed underneath the retort. Shockwaves can also be generated for oil shale rubblization using a high-pressure gas explosion and expansion [139]. For example, a combustible gas mixture can be injected into an oil field, oil shale or sand formation to create a shock wave after detonation. The particle size after rubblization is usually on the order of inches. Land distortion and surface subsidence are often associated with rubblization, although it significantly enhances convective mass and heat transfer. Therefore, use of rubblization should be limited in UCTT process unless it is imperative to increase the permeability by this approach. 3.3. Implications for UCTT TagedPReview of the potential technologies suggests that wall conduction is currently the most applicable method for UCTT. Conduction heating not only offers better process control which produces higher quality pyrolysis products, but also leads to a uniformly permeability. When steam or CO2 injection is used for enhancing oil extraction, the high, uniform permeability reduces the pressure drop and provides more contact surface for gas-surface reactions. It also reduces the number of extraction wells needed, which can be located at the end of the gas passage through the seam. The Shell ICP, in particular, is a proven technology that has undergone field test and produced high quality oil. To improve thermal efficiency, the CCR process can be considered, since direct heat exchange has a higher efficiency than electricity generation. 4. Coal pyrolysis TagedPUnderstanding pyrolysis is instrumental for identifying the most important parameters influencing product yield and composition, which determine the economics and product quality of UCTT. Coal pyrolysis has been well studied and extensively reviewed [15,23
31]. These reviews have greatly advanced the knowledge synthesis of coal pyrolysis, mainly for pulverized coal. A new review of coal pyrolysis is impractical and unnecessary here. Instead, we review the knowledge and data closely related to UCTT: 1) the pyrolysis yield and product composition, and 2) the porous structure and permeability development that impact mass and heat transfer. The results from conventional pulverized coal pyrolysis may not be directly applied to UCTT, due to the differences in coal size and pyrolysis conditions. The large coal size in UCTT makes a slow heating rate more reasonable, considering the heating cost. The large coal also features a long residence time for volatiles diffusing out, and has strong influence on the mass and heat transfer. The pyrolysis atmosphere may also be different if externally heated gas is used as heat carrier. The review is focused on these respects. 4.1. Pyrolysis yield and product composition TagedPMost of the pyrolysis reactions are endothermic. Therefore, heating conditions have strong effects on the pyrolysis yield and product composition, which are briefly summarized in Table 7. For a detailed review on this topic, the reader is referred to Yu et al. [31]. In the current work, these effects are reviewed in the context of UCTT applications, i.e., large coal size and different pyrolysis atmosphere. TagedP4.1.1. Coal size TagedPA distinctive feature of UCTT is the large coal size. The effect of coal size (especially large coal size) on pyrolysis yield is not well studied compared with the other effects, such as temperature and heating rate. Literature in this area is scarce, and sometimes, contradictory [31]. For coal particles smaller than 1 mm, coal size has a negligible impact on volatile yield. This is probably because the

12

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

Table 7 Effects of heating conditions on pyrolysis yield and product composition. Parameter

Effects on pyrolysis yield

Effects on product composition

References

Temperature

Strong effects; Tar favored at moderate temperature (500
800°C) and gases favored at high temperature (800
1000°C); More tars and carbon components due to more extensive thermal fragmentation and suppression of secondary reactions;

1

Pressure

Positive effect. Increases strongly with temperature but becomes less significant at high temperatures (>700
800°C) Positive effect. Increases by 10
50% with heating rate for low-swelling coal and 40
80% for high-swelling coal; Sometimes contradictory, no impact on lignites; Negative effect, decreases total volatile yield;

1, 3, 4

Residence time Coal size

Positive effect, increases total volatile yield; Not well studied; Minor decrease for coal particles less than 1 mm;

Decrease of tar but increase of gaseous products; Not significant for low-rank, low swelling coals; Increases mostly gaseous products; Tar decreases but gaseous volatiles increase;

Heating rate

1, 2

1 1, 5

€ ntgen [26]; 3 Shan [140]; 4 Wall et al. [30]; 5 Howard [24]. 1 Yu et al. [31]; 2 Ju

TagedPparticle is not big enough to distinguish the heating-rate controlled (kinetics) and diffusion-controlled (mass transfer) regime. The effect of coal size on pyrolysis yield can be interpreted from mass transfer and heating rate perspectives. Increasing coal size essentially increases the distance for volatiles diffusing out of the char and the distance for heat conducting into the char. The former increases the residence time of volatiles in the char and allows for more secondary reactions and condensation reactions, which inhibits tar production and promotes gas production. Overall, increasing coal size can be considered as having the same effects of increasing pressure or decreasing heating rate, both of which suppress tar production and promote light hydrocarbon gases, as shown in Table 7. Studies on conventional pulverized coal pyrolysis were mostly focused on submillimeter coal particles, far smaller than the coal size encountered in UCTT. TagedPGneshin et al. [9] used a fixed-bed reactor to study the global kinetics of a bituminous coal at sizes relevant to UCTT. For coal cores of up to 2 cm in diameter and 15 cm in length, they found that coal size has a strong impact on the tar yield, although its impact is negligible on total volatiles yield. The tar yield of the 2 cm diameter coal is substantially lower than that of the coal particles of 75 mm, over a wide range of heating temperatures (400
600 °C). Gneshin et al. [9] also found that the impact of coal size on tar yield is more prominent at slow heating rates (0.1 K/min) than that at higher heating rates (10 K/min). Tars were trapped in the pores of the large coals at slow heating rate, whereas it is not the case at high heating rates. Little difference, however, was found on the micro and mesoporous structure of the chars formed at the two different heating rates. Bulk transport of tars out of the pores is a pressure-driven flow. A Knudsen flow analysis in the mesopore system suggests that the pressure gradient needed to drive the tars out of the pores increases sharply with coal size. The pressure gradient is built up by the volatiles in the pores. Therefore, for large coals under slow heating conditions as encountered in UCTT, the pyrolysis reaction rate is insufficient to provide the pressure gradient to drive all the tars out of the pores. Retention of volatile liquids for large coals at slow heating conditions indicates a decrease of pyrolysis yield, a big difference as compared to the pyrolysis of pulverized coal particles. TagedPFor large coal, low heating rates are more realistic considering € ntgen [26] can be used for interthe heating cost. The review by Ju preting low-heating rate pyrolysis. At low heating rates, devolatilization tends to occur at lower temperatures. This will result in a greater retention of fixed carbon and an enhancement of secondary pyrolysis reactions, which in turn alters the final product yields [9]. A slow heating rate to about 390 °C was found to increase the ratio of the yield of gas to condensable hydrocarbons [80,81]. As such, the production of methane and light hydrocarbon gases can be significantly enhanced while the yield of condensables can be decreased, which indicates that the premium-value, high H/C ratio products can be produced in UCTT process. The effect of heating rate on pyrolysis yield is also dependent on the pyrolysis atmosphere. Khan and

Table 8 Effects of heating rate and atmosphere on the pyrolysis weight loss of a Pittsburgh No. 8 coal up to 900°C (Khan and Hshieh [142]). Heating rate (°C/min)

5 10 20 50

Weight loss He

Steam

38.0 37.0 37.0 37.0

50.0 44.5 40.5 37.0

TagedP shieh [142] compared the pyrolysis weight loss of a Pittsburgh No. H 8 coal under steam and helium atmosphere. For pyrolysis under steam atmosphere, the weight loss (volatiles yield) increased at low heating rate (Table 8). The low heating rate, which favors more secondary reactions, In contrast, heating rate has negligible effect on weight loss under helium atmosphere, indicating that hydrogen element in the gas phase is critical to secondary reactions such as steam reforming and water-gas reaction [44]. TagedPThe large coal size also means a longer residence time for the volatiles to diffuse out of the coal. Khan and Hshieh [142] studied the weight loss of devolatilization for Pittsburgh No. 8 and Wyodak coals, and they observed that larger weight loss correlates to a longer residence time. They concluded that more secondary reactions occur at longer residence time, which further decompose the coal and coal liquids in favor of gaseous products. Unless pyrolysis temperature is sufficiently high (above 1000 °C), the pyrolysis reactions are not fast enough to reach thermodynamic equilibrium in such a short time. Therefore, effect of residence time also needs to be taken into account when analyzing pyrolysis yield. The pyrolysis product yield (with the same heating temperature) increases with residence time. As such, the same yield can be reached at a low heating temperature and long residence, or at a high heating rate and short residence time. This gives choice for UCTT operation to save energy cost while reaching the desired yield. TagedP4.1.2. Pyrolysis atmosphere TagedPFor a non-oxidizing atmosphere, Tyler [143] reported that substituting an atmosphere of hydrogen for helium slightly increases the tar and methane yield. More researchers found significant influence of hydrogen and steam on pyrolysis yield and composition. These studies are listed in Table 9. TagedPThe influence of steam and H2 on volatiles can be explained by the role of hydrogen in pyrolysis reactions. The molecular structure of coal can be conceptualized as a network of various polyaromatic and hydroaromatic rings with aliphatic and functional groups. Pyrolysis is essentially the process of destructing the network by bondbreaking reactions and formation of new molecules. The most important pyrolysis reactions determining the volatiles yield are the ones involving hydrogen evolution, because: 1) hydrogen is the

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

13

Table 9 Effects of pyrolysis atmosphere on volatiles yield and product composition compared to N2 as baseline. Pyrolysis atmosphere Effects on pyrolysis yield

Effects on product composition

References

Steam

Improves product quality, lower molecular weight, higher H/C ratio; Increases tar yield; Decreases alkane/alkene ratio, increases benzene and other ring compounds; increases tar and oil yields; More CO Inert, no impact

[142,144
147]

H2 CO2 CH4

Increases yield for subbituminous coals, more significant at high temperatures Significantly increases total yield; more pronounced than steam; Increases at high temperatures (>550 °C); Inert, no impact

TagedPessential element in the hydrocarbon fuel, and 2) hydrogen in the coal structure is more prone to be depleted than carbon during the pyrolysis process. Steam and H2 can participate in various pyrolysis reactions by providing hydrogen, and the overall effect is to increase pyrolysis yield [31]. For example, steam can participate in the water-gas reaction to produce CO and H2 [44]. H2 in the pyrolysis atmosphere can react with radicals (OH, O, and CH2) to form water and aliphatics, as well as saturating the larger radicals to produce tar molecules. CO2 can directly react with carbon (the Boudouard reaction) to form CO at high temperatures, increasing volatiles yield. The presence of hydrogen can result in the largest gain in volatiles yield since hydrogen stabilizes the primary products from thermal degradation. TagedPResearchers [144
146] also found that the steam treatment provides a significant desulfurization effect on solid fuels and their liquid products. Sulfur content in the coal is transformed into H2S. Therefore, pyrolysis of coals and oil shales with steam yields higher concentrations of H2, CO2, and H2S and lower contents of CO and CH4. Yardim et al. [147] applied steam pyrolysis to four Turkish subbituminous coals and also reached the same conclusion on the occurrence of considerable desulfurization effect after steam treatment. They also found that the enhanced production of volatiles from the coals leads to a significantly higher porosity, specific surface area and adsorption capacity towards iodine of the resultant semi-cokes (Fig. 6). TagedPThese studies suggest that, if the retorting technology of externally generated hot gas is used in UCTT, type of the hot gas needs to be carefully selected because it can change the yield and composition of pyrolysis products. For example, if a hydrogen-rich hot gas (as proposed by Calderon and Laubis [122]) is used, the liquid fuel yield will be significantly increased. If combustion exhaust gas (which contains CO2 and steam) is used, more CO and higher H/C ratio fuels will be produced. TagedP4.1.3. Implications for UCTT TagedPParameters that have strong effects on pyrolysis yield and product composition are identified by reviewing the literature on conventional pulverized coal pyrolysis. These parameters can be adjusted in order to selectively promote the desired products. For example, low heating rate can increase H/C ratio in the pyrolysis product, and an externally generated hot gas with non-inert composition such as H2/CO2/steam can significantly increase the pyrolysis yield and change the composition. Some of the parameters, however, are essentially not adjustable in the UCTT process. For example, high pressure operation is almost a natural feature for the deeply-buried coal seam. Low heating rate is more realistic considering the large size of coal. These natural restrictions leave only heating temperature as a tunable parameter. Since the high pressure and slow heating rate all suppress tar production, the target product of UCTT should be light aliphatic gases such as methane. The light aliphatic gases are high-value products with large H/C ratios. These fuels have a high energy content per mass basis and low CO2 emissions per energy content, compared with large molecular-weight hydrocarbons. A medium heating temperature of 400
500 °C favors CH4

[12, 80,81,142,148
152] [152
154] [151,154]

TagedPproduction over tar production, while reducing heating cost. Although the tar yield can be decreased by a heating temperature above 600 °C, it comes at a high heating cost. In brief, careful control of the heating parameters and heating medium not only increases the premium-value products, but also saves operation cost. 4.2. Porous structure and permeability TagedPDuring pyrolysis, coal undergoes swelling and pore development, which subsequently determine the mass and heat transfer characteristics of the coal char. This in turn impacts the pyrolysis behavior of the coal. The change in porous structure also impacts the permeability of coal seam, a key property for collecting the volatiles. Xie et al. [155] conducted a comprehensive study on the physical, chemical and thermal properties of a packed coal bed during pyrolysis. They found that the permeability and thermal conductivity are strongly related to the swelling and porous structure development.

Fig. 6. The specific surface area (BET) and adsorption capacity towards iodine with steam treatment are compared with the data under inert atmosphere. Reprinted from [147] with permission of Elsevier.

14

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

Table 10 Effects of heating conditions on porous structure development during pyrolysis. Heating parameter

Effects on porosity

Temperature

Increases for subbituminous and bituminous coals, but peaks at intermediate temperature (500
650 ° C); more fracturing for large coal blocks; may increase or decrease upon presence of external pressure; Increases at low to intermediate heating rate; may decrease at very high heating rate (1000 K/s); For bulk coals, only increases macroporosity at low heating rate (10 K/ min) but changes little at very slow heating rate (0.1 K/min); Generally increases; may shrink upon high external pressure, depending on temperature; Increases with volatiles content; decreases with inertinites contents;

Heating rate

Pressure

Coal rank

References [9,31,77,79,82,157
159]

[9,31]

[9,82,157
159]

[9,77,79,159]

TagedPBecause of its critical role in coal pyrolysis and oxidation, the porous structure of coal has been extensively studied and reviewed. Yu et al. [31] reviewed the formation of porous structure during coal devolatilization and its thermoproperties. Bar
Ziv and Kantorovich [156] reviewed the mutual effects of porosity and reactivity in char oxidation. In general, porous structure development during coal pyrolysis is highly dependent on the heating conditions: temperature, heating rate, pressure, and coal size and type. Effects of these parameters on porous structure development are briefly summarized in Table 10. In the following subsections, the review is focused on conditions relevant to UCTT, i.e., external stress, large coal size, and permeability development that impact mass and heat transfer. TagedP4.2.1. External stress TagedPA unique feature of UCTT is the spatial confinement of the deeply buried coal by the overburden layer, which exerts stress on the coal. Recently, with triaxial high pressure experimental apparatus, pyrolysis-induced porosity was studied for coal blocks under strain and stresses [82,157
159]. The triaxial high pressure device is able to apply mechanical or hydraulic stresses on 3 axes, which simulates the spatial confinement of the underground environment. Studies have shown that porosity is strongly dependent on the heating temperature. The porosity decreases slightly with temperature until 200
300 °C, and increases sharply with temperature and peaks at 400
700 °C. Then the porosity decreases slightly again with temperature. The initial decrease at low temperature can be attributed to the closing of pores by softening of the coal and filling of the pores by heavy tars. The sharp increase at intermediate temperature is associated with the strong bubbling and volatiles release during pyrolysis. The slight decrease at higher temperature is due to the shrinking when all volatiles are released, especially under compression of overburden layer. TagedPAkbarzadeh and Chalaturnyk [21] attributed the shrinkage at the final pyrolysis stage to the weakened internal structure of coal when part of the carbon structure is consumed. Following the breakup of bubbles and release of volatiles, the swelling loses its driving force. Meanwhile, the internal structure of coal weakens due to the consumption of organic material. As a result, shrinking occurs when the

cTagedP oal is compressed by the overburden layer. The shrinking behavior influences the final porosity of the coal. The evolution of porosity during pyrolysis is indeed a competition between bubbling and shrinking. At an intermediate temperature (400
650 °C), the devolatilization is strong and bubbling dominates the pore development, thus the porosity increases significantly. At higher temperatures when most of the volatiles are released, shrinking dominates pore development, and the porosity decreases slightly. TagedP4.2.2. Coal size TagedPCoal size has a big impact on heating rate. The large coal size in UCTT makes it hard to achieve high heating rates. Increasing heating rate, however, leads to greater porosity. Gneshin et al. [9] found that heating rate mainly influences the macroporous structure, provided that the heating temperature is above 400 °C. For a bituminous coal core of 2 cm size, a substantial increase in macroporosity was observed for chars produced at 10 K/min, whereas little change occurred to the chars produced at 0.1 K/min. The macroporosity is responsible for the mass transfer in the coal. This indicates that a threshold heating rate exists to enhance the mass transfer by increasing macroporosity. Realization of higher heating rates, however, requires higher heating temperature or more heating elements per unit volume of coal. Therefore, it is important to balance this threshold value against the costs of additional heating. TagedP4.2.3. New techniques for characterizing porous structure TagedPRecently, new techniques have successfully characterized the coal structure. X-ray computed tomography (CT) is capable of in situ characterization of the structural deformation of the coal sample [160
164]. The CT imaging allows a quantitative mapping of the specific features inside the coal and a 3-dimensional reconstruction of the coal structure. Following the structure reconstruction, the swelling/shrinking strain can be calculated. CT imaging of coal core generated by Pone et al. [163] under stress conditions is shown in Fig. 7 as an example. Of course, the resolution of CT imaging is not as high as scanning electron microscopy (SEM), which can reach a few nanometers. But CT imaging is capable of non-destructive probing of the internal structure, whereas SEM imaging requires the sample to be sliced to be able to measure its internal structure. TagedPLow-field nuclear magnetic resonance (NMR) provides a quick means of measuring the pore size distribution and porosity of shale and coal samples [165
170]. NMR does not suffer from many of the inherent disadvantages of mercury porosimetry or nitrogen condensation, such as the inaccuracy caused by a pore shape assumption, long measurement time required by gas adsorption/desorption, limited pore size range caused by gas molecule size, and sample compression. At present, the echo time (TE) and the waiting time (TW) are considered as two of the key parameters when the rock porosity is measured with low-field NMR instruments, and different lithologies require different NMR experimental parameters [171
173]. A systematic comparison of coal pore size distribution characterization using low-field NMR and mercury intrusion porosimetry was provided by Yao and Liu [174]. TagedP4.2.4. Permeability development TagedPPermeability is the most critical measure of the mass transport capability of a coal seam. Akbarzadeh and Chalaturnyk [21] provided a summary of porosity and permeability development during the pyrolysis process. In general, coal permeability can increase by 2-3 orders of magnitude due to the pyrolysis, depending on the coal rank and heating temperature. For example, Thorsness et al. [77] reported that the permeability of a Wyodak subbituminous coal increases from 10 mD at 30 °C to 1200 mD at 400 °C, and further increases to 16,000 mD at 650 °C. Comparable trends were reported for four U.S. bituminous coals [79] and two U.K. bituminous coals [84,85]. Zhao et al. [158] observed that the permeability of a China

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

15

Fig. 7. CT imaging of a coal core. Reprinted from [163] with permission of Elsevier.

TagedPsteam coal (a coal between bituminous and anthracite) increased moderately in 200
300 °C and then increased exponentially at 400 ° C. The increase of permeability is less significant for the China steam coal, whereas it is still in the range of 1000 times. In contrast, the permeability of a China lignite coal only increases about 100 times after pyrolysis [82]. This clearly shows that coal rank and heating temperature are major factors influencing permeability development, similar to the porosity development. TagedPSimilar trend of coal permeability and coal porosity during pyrolysis suggests that the two properties are correlated. Xie et al. [155] concluded that the rapid increase of permeability after tars have fully evolved indicated that char structure became porous. Exponential correlations were developed to predict the coal permeability with porosity [21,77,79]. For example, Akbarzadeh and Chalaturnyk [21] combined the data of Zhao et al. [158] and Thorsness et al. [77] and plotted the logarithm of permeability against the change in porosity (Fig. 8). It must be kept in mind that initial softening of the coal and entrapping of heavy tars can partially close or fill the pores, leading to decreased porosity and permeability [155]. TagedP4.2.5. Permeability modeling TagedPCoal permeability modeling is an active research area because of the rising interest in using coal seams for CO2 sequestration and

TagedP nhanced coal bed methane (ECBM) production. Various permeabile ity models have been developed for reservoir simulation of gas and liquid flow in a coal bed. The models have been extensively reviewed [60,68,175
178]. Palmer [175] conducted a detailed review on four of the most widely used models. Ma et al. [176] introduced their own model based on matchstick strain and constant volume theory, in addition to the review of other models. Liu et al. [60] evaluated the models against laboratory data at free swelling/shrinking conditions and field data at constant volume conditions. They concluded that previous models have limited success in explaining the field data, because of the lack of understanding of the coal matrix-cleat interactions. Pan and Connell [68] recently provided a complete review of a broad variety of permeability models, and tested the models against laboratory and field data. They concluded that the coupling of gas adsorption/desorption induced swelling and shrinking is important for the accuracy of the model. Peng et al. [178] performed a benchmark assessment of the permeability models against the lab and in situ data. They also found that the permeability models failed to explain experimental results at conditions of the lab-controlled stresses, and only partially succeeded in explaining in situ data. In light of the existing reviews, we focus on the validity of the permeability models in UCTT simulation, by examining their basic approaches and major assumptions, and by comparing the CBM production conditions with the UCTT environment. TagedPA consensus among the reviews is that the permeability is linked to coal porosity, stress, and the strain of swelling/shrinking. Coal permeability modeling is generally based on the dual porosity/ permeability concept, as shown in Fig. 2. Gas and liquid are stored in the pores of the coal matrix, and diffuse into the cleats in CBM production. The flow in the cleats is modeled as Darcy flow, similar to the shale gas reservoir simulation approach. The most widely used model to describe the coal cleat system is the bundled matchstick concept, as shown in Fig. 9 [179]. For this model, the flow in the cleat can be calculated by the cubic law [180] as b3 @p qj ¼
i ¢ 12mai @xj

Fig. 8. Exponential correlation of coal permeability with porosity. Reprinted from [21] with permission of Elsevier.

ð2Þ

where qj is the flow in the jth direction due to pressure differential @p @xj , m is the fluid viscosity, and ai and bi are the cleat spacing and aperture, respectively. Eq. (2) assumes uniform cleat spacing and aperture.

16

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

Fig. 9. Bundled matchstick model to describe the flow in coal cleats. Reprinted from [179] with permission of Elsevier.

TagedPApplying Darcy's law, the permeability can be derived as ki ¼

b3i 12ai

ð3Þ

TagedPThe porosity of cleat is calculated as ’¼

X bi i

ai

ð4Þ

TagedPUnder isotropic conditions, and assuming two sets of cleats for coal, Eq. (4) simplifies to ’ ¼ 2b a . Substitute f into Eq. (3) and the per1 2 3 meability becomes k ¼ 48 a ; [181]. Although in more precise models the anisotropic nature and complex structure of cleats would be considered, the above process shows the basic approach how to relate the permeability to the porosity. The permeability is correlated to the porosity with a cubic factor. TagedPBy examining the derivation, one can see that in the calculation only the cleat's porosity is used, and the model assumes no contribution from the matrix porosity. This is valid in CBM or ECBM production because the matrix porosity is relatively small compared with the cleat porosity when there is no pyrolysis. In the UCTT process, however, the matrix porosity increases significantly due to the pyrolysis-induced porous structure development. In fact, an exponential correlation to porosity was found for permeability development during pyrolysis, as reviewed in the previous section. Therefore, the current models cannot predict the significant change in permeability during pyrolysis. Pyrolysis-induced porosity of the coal matrix needs to be incorporated into simulations of the UCTT process. This can be done by combining the non-pyrolysis based permeability models with pore models. In addition, the pyrolysisinduced matrix swelling is stronger than the gas adsorption-induced swelling. Correspondingly, the cleats will be compressed to an extent larger than non-pyrolysis conditions. The parameters for modeling the swelling effect on permeability thus need to be reevaluated against experimental data. The triaxial pyrolysis test cell will be able to provide such data, by applying mechanical loading and strain to various axes of the coal sample during pyrolysis process. TagedP4.2.6. Implications for mass transfer TagedPIn general, mass transport in coal occurs at two scales: convective flow through the macroscopic cleats (the natural fracture system of coal) and diffusion through the pores of the coal matrix [61
63]. The fractures and cleats are responsible for the collection of devolatilization products. The macropores and mesopores are mostly responsible for the mass transport into the cleats from the coal matrix [31]. In principle, the diffusion resistance decreases with the porosity of the coal and the pore size [182]. Therefore, the evolution of porous structure plays a key role in determining the transport

rTagedP ate of volatiles in the coal. Gneshin et al. [9] summarized the major categories of mass transfer in coal pyrolysis as internal flow and Knudsen flow. Internal flow mainly occurs through macropore transport and bubbling, and it is the major mechanism for volatiles transfer out of the coal matrix [28]. The time scale for gases moving out of coal particles less than 100 mm is in the order of 1 ms [183]. This suggests that as long as heating rate is below 1000 K/s, there is sufficient time for gaseous volatiles transfer out of the coal particle. For liquid volatiles, however, the situation is different. As heavy molecules move through the pores, secondary reactions and coking reactions may take place if the residence time is long enough, thereby reducing the tar yield and increasing gas yield [31]. In the UCTT process, the coal is in a large block, and the residence time for volatiles moving into the collection wells is significantly longer than that in particles. The extended residence time certainly is expected to affect secondary reactions and tar coking reactions, and subsequently the yield of tars and gases. Therefore, research on the mass transfer in large coal blocks is necessary to understand the impact of coal size on volatiles yield. TagedPThe phenomenon of Knudsen flow was first applied to petroleum engineering problems by Klinkenberg [184], known as Klinkenberg effect or gas slippage. Knudsen flow occurs when the mean flow path of gas molecules is longer than the diameter of the pores. Therefore, it mainly takes place in micropores and mesopores of coal [68], which are generally below 50 nm. When the mean flow path of gas molecules is longer than the pore diameter, collision between gas molecules and pore walls becomes frequent. This significantly increases the mass transfer resistance. Therefore, it plays an important role when pore transport is dominant, such as the case in UCTT in which slow heating and overburden confinement all tend to suppress bubbling and swelling. Studies [185
188] on Knudsen flow of CBM have been published. There is also abundant literature [189
194] on the Knudsen flow occurring in nano-scale pores of shale gas formations. These studies provide insight into the contribution of Knudsen flow to total gas flow and lead to the development of improved permeability models [195] including Knudsen flow. They found that the gas flow in nano-scale pores of the shale can be modeled with a diffusive transport regime with a constant diffusion coefficient and negligible viscous effects. The obtained diffusion coefficient is consistent with the Knudsen diffusivity which supports the slip boundary condition at the nano-scale pore surfaces. These studies show that Knudsen flow plays a critical role in determining the total flow, especially when the nano-scale pores are in abundance compared to large pores. Therefore, the Knudsen flow needs to be considered for UCTT process, where the slow heating conditions do not favor the growth of large pores, especially during the drying stage.

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

17

TagedPIn general, coal permeability increases significantly over the course of pyrolysis. Hydraulic fracturing or rubblization is thus not likely to be required. Studies on coal permeability show that there is an optimal temperature range that maximizes the permeability, depending on the coal rank. Comprehensive consideration should be given to the heating temperature in terms of balancing the permeability, volatile yields and heating cost. For example, a bituminous coal exposed to a heating temperature of 600
650 °C generates a permeability that is on the order of 10,000 mD, while this temperature also favors tar production. If a permeability of 1000 mD can meet the production requirement, lowering the temperature to 400
500 °C can promote gas production and save heating costs.

TagedP welling and fragmentation can increase the voids in the porous S char, or narrow or even break the connections between the microcrystals. Therefore, for a coal particle that is free to swell, the thermal conductivity can decrease significantly. For coal in UCTT, which is constrained by the surrounding formation, swelling will close the cleats and other fractures and lead to an increase in the thermal conductivity. Furthermore, the narrowing or breaking of the connections of the microcrystals may not be as significant as in a free swelling coal because shrinking may occur at the final stage of pyrolysis. Therefore, research is needed to understand the effect of a constrained environment on the evolution of thermal conductivity of the coal during pyrolysis.

TagedP4.2.7. Implications for heat transfer TagedPThe porous structure development influences the heat transfer properties of the char in two ways: heat convection through the pores and heat conduction via the pore connections. Heat convection increases with porosity, because increased porosity allows more gases and liquids carrying heat into pores. Pore development is accompanied by pore growth, carbon skeleton reorganization, and char fragmentation. Heat conduction of porous char is proportional to the thermal conductivity of the basal plane of microcrystals, and is also dependent on the porosity, pore size distribution, orientation of the microcrystals and nature of the inter-connections between microcrystals. These structural changes certainly have great impact on the thermal conductivity of coal during pyrolysis. TagedPThermal conductivity of coal particles has been extensively studied. For a review on these studies, the readers are referred to Kantorovich and Bar
Ziv [196]. In general, thermal conductivity of coal particles decreases with organic content and porosity, whereas it increases with temperature. Thermal conductivity of coal particles increases with temperature in the initial pyrolysis process, due to the structural changes [92,197]. Void space, or a pore filled with air, essentially has zero thermal conductivity. Therefore, high porosity results in low thermal conductivity. Zhang et al. [198] reported that the thermal conductivity of a highly porous char significantly decreases at the initial oxidation stage, due to narrowing or breaking of interconnections of pores. Therefore, it is expected that the thermal conductivity of coal particles will decrease during pyrolysis. TagedPThermal conductivity in a coal block, however, can be strikingly different if the coal is confined to a constant volume. For a bituminous coal, the thermal conductivity at around 350 °C will increase from 0.3
0.4 W/(m  K) if unconstrained [90] to 0.6 W/(m  K) if constrained [80,81]. For a packed coal bed that is partially confined in a tube, the thermal conductivity can rapidly increase at the onset of swelling because the void space between bed particles is reduced [155]. The dewatering and devolatilization of coal lead to significant increase of void volumes, which tend to decrease thermal conductivity. Heating rate also strongly influences thermal conductivity because porous structure is strongly dependent on heating rate. Slow heating allows full development of pores and rearrangement of microcrystals that interconnect the pores. Fast heating tends to cause thermal fragmentation [31] of the particle and break the microcrystals that link the pores, which are the conductive media of pores. Therefore, slow heating tends to increase the thermal conductivity of coal. The thermal conductivity of a coal block under slow heating conditions can reach 0.9 W/(m  K) , which is 3 times of that in an unstrained coal block with fast heating. In general, thermal conductivity of coal is lower than that of oil shales, which is in the range of 1-2 W/(m  K) [199
201]. The low thermal conductivity of coal will lead to low heating rate in UCTT, compared to underground oil shale retorting. TagedPBar-Ziv and Kantorovich [156] summarized that the main factors influencing thermal conductivity of porous char are: 1) changes in the dimensions of the microcrystals forming the microskeleton of char; 2) changes in the connections between the microcrystals.

5. Process parameters, products and optimization TagedPA literature review on coal pyrolysis reveals the key parameters influencing volatile yield and product composition. Dependence of product composition on heating conditions creates opportunities for selectively producing products. Heating conditions can be adjusted in favor of high-value, high H/C ratio products. For example, injection of steam and H2 increases both coal conversion and tar yield. From a mass transfer perspective, high temperature, fast heating and medium pressure are preferred because higher permeability is created at such conditions. From a heat transfer perspective, low temperature and slow heating are preferred because decrease in thermal conductivity is minimized. In addition, operation cost is strongly associated with heating requirements. For example, high temperature and high heating rate needs high energy input. Therefore, heating parameters must be carefully optimized for the UCTT process, considering the yield, quality, and heating cost. 5.1. Temperature and heating rate TagedPFor oil shale, kinetic data [202
204] show that the time needed to decompose the kerogen depends strongly on temperature. For example, Berchenko et al. [203] found that for one sample at 370 °C, 90% decomposition occurs within 5000 minutes, while at 500 °C, it was within 2 min. To maximize the decomposition yield, a high heating temperature is desired. However, slowly heating to a relatively low temperature can reduce heating and facility costs. In Shell's oil shale ICP [113], the heating period is 2
4 years, corresponding to less than 0.5 °C/day. The heating period for Shell's ICP process is typical among shale conversion approaches, i.e., 7-8 years for ExxonMobil's Electrofrac [127] and 1-2 years for Mountain West Energy's IVE [205]. The operation temperatures for underground oil shale processing in most approaches were set to a range of 340
400 °C [113,114,127,205]. It is noted that in TUST's Hot Alkane Convection and Western Research Institute's TREE approaches, the convection gases were heated to 700 °C. This gas temperature will probably lead to similar shale temperatures as other approaches, considering the temperature drop between hot gas and shale formation. TagedPSimilar principles apply to coal. A tradeoff must be made between volatile yield and energy cost by adjusting temperature and heating rate. In addition, the proportion of liquid and gaseous yields is strongly dependent on temperature and heating rate. Maximum yield is reached at high temperatures, whereas tar production is promoted at mild heating temperatures of 450
650 °C [206]. A slow heating rate to 390 °C increases the ratio of gas to condensable hydrocarbons [80,81]. The production of methane also can be significantly enhanced by minimizing the yield of condensables. This is a huge advantage because methane is a high hydrogen fuel that minimizes CO2 emissions. It also needs to be kept in mind that UCTT will require a higher temperature than oil shale retorting, because coal is a less thermal-decomposing material than oil shale [122]. Considering all these factors, a heating temperature of 400
500 °C with a slow to medium heating rate is recommended for UCTT process. This

18

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32 Table 11 Temperature breakdown favoring different products in UCTT. Target

Temperature (°C)

Maximum volatile yield Moisture removal Coal bed methane Liquid hydrocarbons SNG (Methane) Tar H2 Syngas (H2+CO)

600
700 200 < 300 < 400 400
500 450
650 > 500 450
700

TagedPtemperature range will yield most of the aliphatic gases, tar and other liquid hydrocarbons at a balanced energy cost. Detailed breakdown of temperature ranges for different products are listed in Table 11 [141].

TagedP hen electrical heating is used. The recycled gas can diffuse into the w formation and be exhausted to the surface after heat exchange with coal. Pyrolysis gases can be used to fire a gas turbine to generate electricity in addition to being used as a carrier for recycling energy. TagedPRecently, solar and wind technologies have undergone significant development for meeting future energy needs. The levelized costs of electricity by solar photovoltaic and land-based wind turbine are projected to becoming comparable (with tax credits) to natural gasfired combined cycle power plants in 2022 [207]. In addition, solar thermal technologies can be directly utilized for providing heat for the energy conversion process. For example, concentrated solar system with a fluidized bed core [208] can provide high-temperature heating gases at relatively low cost. Therefore, wind and solar have great potential for UCTT. In addition, the intermittent supply of energy from solar and wind is less of a concern for UCTT than in generating electricity for the grid. 5.4. Product yield

5.2. Pressure agedPIT n general, increasing pressure reduces total yield, particularly for high-volatile coals. As pressure increases, the tar yield and total yield drops sharply, while the gaseous volatiles increase significantly. In UCTT process, the coal is deeply buried and the pressure can reach 30
100 atm. At such high pressures, the tar yield can be reduced by 50% while the light hydrocarbon gas yield can be increased by 50% [141]. TagedPFor UCTT, the lithostatic pressure can reach 30
300 atm in deeply buried coal seams. As such, a high-pressure pyrolysis process is often chosen by default. Shell reported its UCTT process is maintained under pressures of up to 30 atm [80,81]. When a stream of gaseous fuels is produced from UCTT, it can be used to fire a gas turbine for electricity. The produced gas stream needs to be pressurized for transport and power generation. Therefore, a selection of a high operating pressure (of steam, CO2, air or O2) in underground thermal treatment does not increase the overall cost much. For underground oil shale retorting, due to different overburden thickness and operational considerations, the operation pressure was 2
30 atm in the Shell ICP process [42,203], 160 atm in the ExxonMobil's Electrofrac process [127], and 88 atm in the Mountain West Energy's IVE process [205]. TagedPSelection of operating pressure is also influenced by other design considerations, such as safety of personnel and equipment. During the underground heating stage, the porosity inside the coal formation increases dramatically as volatiles leave the coal. Subsidence and compaction of the formation becomes a possible issue, which could endanger the surface workers and underground equipment. A high operating pressure in the formation will prevent the subsidence, thus protecting the workers and equipment. 5.3. Heating sources TagedPHeating sources can be obtained off site or on site, and the heating cost can be quite high depending on the sources. In Shell's ICP, electrical heating was used and the electricity cost for oil shale retorting is estimated to be $12
15 per barrel [53]. The electricity can be purchased from grid, i.e., off site. In this manner, the loss during long-distance transmission will increase the cost. The electricity can also be generated onsite using coal, gas, nuclear power and renewable energy sources. The produced gas generated from underground oil shale thermal treatment is a natural source of fuel for a gas-fired power-plant. Similar conclusions can be reached for UCTT. TagedPWaste gases generated during the coal conversion can be heated and injected into the coal seam as a heat carrier. For approaches using externally generated hot gases, gaseous hydrocarbon fuels are often used as the heat source. This stream of gases with hydrogen rich content is also recommended to augment coal conversion even

TagedPCoal has much lower hydrogen content than petroleum. The much lower hydrogen content, in combination with the preferential reactions of hydrogen to form water, light aliphatic gases, and even H2 [28,31] makes it impossible to completely convert the coal into gaseous and liquid fuels, with much of its carbon remaining as char. Conversion ratios of other coal-to-liquid technologies (direct liquefaction, syngas-to-liquid) are generally in the range of 1-2 barrel of oil per ton of coal. The conversion ratio of pyrolysis is obviously lower than direct liquefaction because much of the carbon remains as char. Even for a high-volatile bituminous coal that has a volatile fraction of 40%, the maximum pyrolysis yield in theory is about 2 barrels of oil equivalent per ton of coal. TagedPCoal pyrolysis yield is strongly dependent on heating conditions. Although increasing heating temperature and heating rate can increase product yield, a low-to-medium temperature is recommended in the UCTT process considering the heating cost. Product yield also is influenced by other operating parameters. For example, the composition of an externally generated hot gas may determine the decomposition rate and product distribution. Injection of steam, H2 or CO2 will alter both the yield and composition of the pyrolysis product. Graff and Brandes [154] and Sharma et al. [148,149] reported the increase of tar yields with steam in coal devolatilization. 5.5. Product composition and quality TagedPHigh valuable oils are the most desired product of underground thermal treatment of oil shale and coal. In particular, light oils that need little upgrading have the highest value as transportation fuels. TagedPOil and gas are the main products in oil shale retorting. The synthetic crude obtained from underground oil shale retorting can have a H/C ratio of 1.9 [42], which has very high quality and high volatility compared to same molecular weight species. The quality of the synthetic crude obtained from kerogen is often characterized by the API gravity. According to API manual [49], light crude oil has an API gravity of higher than 31.1 (less than 870 kg/m3), and medium crude oil has an API gravity between 22.3 and 31.1 (870 to 920 kg/m3). The Shell ICP syncrude has an API gravity larger than 30, and General Synfuels International’ Omnishale technology claimed their API gravity to be about 45 [98]. These are in high quality, light crude oil range. Water is also a byproduct no matter how good the de-watering operation is before the extraction. Not much information is available for the synthesis gas composition, although it should be very similar to coal derived syngas that consists of CH4, H2, CO and other light gases [209,210]. TagedPFor UCTT, the selection of final products would be based on the economics and demand for products. Kelly et al. [7] reported the pyrolysis yield of a Utah Sufco coal (a high-volatile, low-moisture

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPbituminous coal) using a high pressure rubblized-bed reactor to simulate the effect of overburden pressure at 333 m depth. The gaseous and liquid volatiles are listed in Table 12 and Table 13, respectively. The gaseous product is a low-to-medium heating value syngas, while the liquid product has an elemental composition similar to conventional crude oil. Uncertainty analysis [7] shows that the LHV of the gaseous and liquid product combined ranges from 1.22 £ 104 to 1.93 £ 104 kJ/kg. The moderate content of CO2 in the gaseous product reduces the flame speed (a risk of flashback) of the syngas and suppresses NOx formation when burning in gas turbines, although it lowers the heating value of the product. TagedPKhan and Hshieh [142] reported that rapidly heating coal leads to poorer quality tars in terms of H/C ratio. Primary products obtained at higher heating rates contain loosely bounded fragments of coal that inherit many functional characteristics of the parent structures. Rapid heating rate also leads to less volatile pyrolysis liquids, with relative higher fractions of poly-phenolic compounds and a lower content of alkane/neutral aromatic compounds. In comparison, slow heating improves the quality of liquid products with less rings, although slow heating yields a higher amount of gaseous volatiles.

Table 12 Gaseous volatiles composition of a Utah Sufco coal pyrolyzed using a high pressure rubblized-bed reactor. Composition

H2

H2 O

CO

CH4

C2H4

C2H6

CO2

wt% vol%

3 25

7 7

10 6

35 40

7 4

10 6

28 12

Table 13 Liquid volatiles composition of a Utah Sufco coal pyrolyzed using a high pressure rubblized-bed reactor. Composition

C

H

N

S

wt% molar%

85 38.96

11 60.51

0.5 0.18

2 0.34

Table 14 Product composition of coal conversion with a slow heating to a low temperature (Coal cube, [80,81]) and rapid heating to an elevated temperature (Fluidized bedUtah and Fluidized bed-Illinois #6 HVB, [211]).

Char (wt%) CO2 (wt%) Gas (wt%) Water (wt%) Naphtha (wt%) Jet (wt%) Diesel (wt%) Tar/Bottoms (wt%) Total oil API

Coal cube

Fluidized bed-Utah

Fluidized bed-Ill.#6

74.7 2.6 9.7 6.8 1.6 2.0 2.2 0.4 37

56.7 4.6 10.5 4.6 0.0 1.0 6.2 16.4 ¡3.5

57.1 2.5 10.7 6.1 0.2 2.4 5.0 15.0 ¡13.1

19

TagedPVinegar et al. [80,81] in their in situ coal pyrolysis obtained coal liquids with an API gravity of 23
36 by slow heating, and found that the API gravity increases with decreasing heating rate. Furthermore, no tar was formed because of the slow heating, as shown in Table 14. Therefore, many disadvantages of tar formation are eliminated, including the clogging of pores, the low quality of coal liquids, and higher levels of air pollution. Tar formation in the two fluidized bed experiments [211] resulted in an API gravity of 2
17 (specific gravity of 1.061
1.240). These products contain significant fraction of devolatilization bottoms (fractional distillation temperature above 370 °C), which need to be upgraded. TagedPTable 14 shows that UCTT results in a comparable yield of gaseous products as a fluidized bed gasification process. The total gas yield subtracting CO2 is about 10% for all three processes. Despite the similar yield, the gas composition and quality have dramatic differences, as shown in Table 15. Coal gasification with oxygen presence (air or pure oxygen) results in a high content of oxygenated gases [80,81]. CO from gasification is about 2
5 times that from UCTT, and CO2 is 1-2 times. If air is used, then almost 50% of the product is nitrogen. UCTT results in negligible N2, with a small percentage of CO and CO2. The major components from UCTT in an inert atmosphere are CH4 (43%), H2 (26.9%), and C2+ (10.8%) [7]. The synthesis gas from UCTT has a high heating value of 24702.3 MJ/Nm3, which is 2
4 times of that from gasification processes. Pyrolysis in a reducing atmosphere [80,81] can make the CH4 content and heating value even higher, approaching substitute natural gas (SNG). TagedPTable 14 also suggests that the liquids proportion is a not a major contributor to the total yield. This is particularly true for large coal blocks under slow heating conditions, due to the retention of liquids in the pores [9,206]. In addition, the liquids often need further upgrading to be able to meet refinery requirements. The gaseous products are easier to collect compared to liquids, due to the lower resistance in mass transport. Therefore, the UCTT process should target maximizing gaseous fuels. As shown in Table 15, synthesis gas produced in UCTT has a much higher CH4 content compared to the synthesis gas produced by coal gasification. Gasification syngas consists of mainly CO and H2 with a lower heat content. The heating value of syngas from UCTT is 2
5 times that of syngas from gasification. Therefore, UCTT leads to a high quality gas stream, which offsets its lower liquid yield in. It is well documented in the literature that higher operating pressures correlate to higher yield of gaseous products and improve the quality of liquids. Fortunately, the deeply buried coal seam provides a natural environment for high pressure operation.

6. Economics 6.1. Energy return TagedPA critical measure for evaluating the economics of oil shale retorting is the ratio of the energy produced by a resource to the energy consumed in extraction and processing of that resource, "Energy Returned on Energy Invested" (EROEI). Significant variation

Table 15 Gaseous product composition (vol%) and quality of UCTT compared with gasification processes. Gasification with air[80,81] Gasification with oxygen [80,81] UCTT-pyrolysis in an UCTT-pyrolysis in a reducing inert atmosphere [7] atmosphere [80,81] H2 (vol%) 18.6 CH4 (vol%) 3.6 N2 and Ar (vol%) 47.5 CO (vol%) 16.5 CO2 (vol%) 13.1 C2+ (vol%) 0.6 Lower heating value (kJ/Nm3) 5998.7

33.5 6.9 0.0 31.5 25.0 1.1 11438.5

26.9 43.0 0.0 6.4 12.9 10.8 24702.3

16.7 61.9 0.0 0.9 5.3 15.2 39121.9

20

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPexists in the energy returns for different technologies. In addition, care must be taken when comparing EROEIs to ensure that the system boundaries, internal energy, and end products are well defined. For example, EREOIs are significantly higher for wellhead products than for refined fuels because upgrading and refining require additional energy. Internal energy is generated during shale and UCTT resource extraction and processing, and this can be used to power the process itself. TagedPCleveland and O'Connor [212] summarized EROEI estimates for oil shale processing technologies in the range of 2
16 for products at the wellhead, considering only the external energy. Depending on the specifics of the process, EROEIs for oil shale products decrease to less than 1-2 if one considers refined products and internal energy is included in the calculation. According to the Brandt et al. [42,213,214], the EROEI of ex-situ oil shale processing is 1
3, and the EROEI for in situ shale processing is 1.2
2.5. TagedPThe same evaluation approach can be applied to coal conversion. Coal liquefaction has an estimated EROEI of 0.5
8.2 [215], based on the General Accounting Office report [216]. Wellington et al. [217] estimated the EROEI of a UCTT process in Shell's patents. They propose to apply conduction to heat the coal to 390 °C for gas and oil production and then gasify the remaining char underground for syngas production (i.e., UCTT + UCG). They estimated the total energy output from the synthesis natural gas and synthetic crude to be more than 20 times the energy input needed for heating elements. This EROEI is surprisingly high although it is for the wellhead product, uses the product from gasifying the char to provide process energy, and excludes this internal energy from EROEI calculation. In contrast, Kelly et al. [7] estimated a net energy return (NER) for gasoline produced by UCTT for a low-moisture, high-volatile coal ranged from 0.4
4.5. Their NER accounts for the energy needed to drill wells, heat the formation, and recover, refine, and transport the products. It does not account for electricity transmission loss, gas separation, and environmental protection. They found that the NER decreases after 1 year because large volumes of coal are heated to low temperatures, resulting in little product. Consequently, the operation time would need to be balanced with capital costs. In summary, they concluded that the NER and GHG emissions for UCTT are less favorable than those for oil sands and are in the general range of that for oil shale. TagedPIt can be difficult to compare energy return for different processes, such as coal and oil shale in situ approaches. Even for the same process, researchers provided drastically different estimations. For example, the Shell ICP originally claimed a very favorable energy return of 3
3.5. The energy return was revised down by Brandt et al. [42,213,214] to 1.2
1.6 for the refined product, including internal energy and up to 2.5 excluding internal energy. The large difference resulted from the inclusion of additional, necessary energy inputs, from using refined gasoline as the product, and from including the internal energy in the calculation. Shell only considered the wellhead product and the electricity cost in estimating their energy balance, leaving out energy needs in preliminary operations, drilling, pumping, freezing wall, and remediation. Such differences are common because the cost estimation is usually overly optimistic when a new technology is trying to gain acceptance. Merrow et al. [218] estimated that the actual capital investments of new energy processes have been routinely understated by 100%. 6.2. Process optimization TagedPThe UCTT process can be optimized to reduce the operation costs and enhance product values products. Heating conditions can be adjusted in favor of high-value, high H/C ratio products. A demonstrative control parameter is the selection of operating pressure. It is well documented in the coal literature that higher operating pressures increase the yield of gaseous products (SNG) and improve the

TagedP uality of liquids. High pressure operation also reduces the size of q collection conduits and the cost of product gas compression. TagedPOther operating parameters, such as temperature and heating rate, can be adjusted to achieve a better return on the investment. However, market demands change over time and UCTT takes a long operation time. Therefore, the process design should incorporate flexibility so that key parameters can be adjusted when the market changes. TagedPCost can also be reduced if some of the costly steps in coal conversion can be eliminated or reduced. This is the basic incentive behind the UCTT process by greatly reducing the costly reclamation of the mining pit and treatment of spent char. Although environmental damages cannot be completely eliminated by in situ production, the burdens are lighter. Another cost-reduction strategy is to minimize the need for upgrading products. Retorting with air and/or oxygen will lead to very heavy fuel liquids (specific gravity of 1.061
1.240) with an API gravity of 2
17 that need to be upgraded, which certainly increases the cost [211,219]. Shell claimed a cost reduction of $6
10 per barrel from their in situ coal conversion process [80,81] by eliminating the upgrading step. However, their in situ method has lower yields of liquid fuels compared to direct combustion conversion. It is also possible to convert the remaining char into syngas as suggested by Shell, which may reduce cost but increase GHG emissions. TagedPUsing the remaining char matrix as a CO2 reservoir can also provide incentives for cost reduction. The porous structure of the char matrix needs to be characterized for accurate prediction of its capacity for CO2 adsorption. 7. Technical challenges and environmental issues TagedPChallenges and issues in the development of UCTT process can be anticipated to be largely similar to those encountered in underground oil shale retorting and UCG processes. These challenges and issues, however, are inevitably influenced by the unique characteristics of coal seam compared to oil shale and the differences between pyrolysis and gasification. The technical challenges and environmental issues are described in this section. 7.1. Site selection and exploration TagedPSite selection for a UCTT process is primarily determined by two factors: economics and environmental impact. Because UCTT targets deeply buried coal, the thickness of coal must meet a minimum requirement for the process to be economically feasible. The depth also poses challenges for gathering geology and hydrogeology data for the coal surrounding formations, and for directional drilling that is in parallel to the coal seam. Regarding site selection criteria, those of underground coal gasification can be used as a reference [2]: (1) minimum 4.2 m thick; (2) depth of 600
1200 m; (3) 4500 m away from abandoned mine working license areas and (4) greater than 100 m vertical separation from major aquifers. The minimum coal thickness may need to increase because UCTT has much lower yield and thermal efficiency than UCG. The restrictions on separation distances, however, may not be as strict as UCG because UCTT has lower subsidence and water-contamination risks. The thermal heating of coal seams may cause surrounding formations to crack or fracture, or change the mechanical properties of rocks. Such changes are associated with risks of gas leakage, land distortion and overburden layer collapse. All these potential risks need to be thoroughly evaluated in selecting the appropriate site. A geological survey is a necessary step in this evaluation. TagedPThe heating value and physical properties of the coal need to be considered. High-volatile bituminous coal is the ideal candidate for a UCTT process. Selection criteria of the site also need to include the potential for CO2 sequestration. The massive pore network after coal

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPpyrolysis is an attractive candidate for CO2 storage. The geology data of the surrounding formations need to be evaluated for potential CO2 leakage or reactions with CO2. 7.2. Heat management TagedPConsidering the lower yield of valuable oils and the higher heating temperature of UCTT compared to underground oil shale retorting, the energy return of UCTT is likely lower than underground oil shale retorting. Therefore, heat management is critical to ensure thermal efficiency and economics of the process. Heat transfer is probably the biggest challenge in UCTT because: (1) coal has a greater heat capacity than oil shale while it requires higher pyrolysis temperature than oil shale; (2) the evolution of thermal conductivity in large coal blocks during the pyrolysis process is not well understood; (3) the increase in porosity of coal during pyrolysis creates void space for heat convection. The large heat capacity and high heating temperature requires vast heat input. The evolution of heat transfer properties requires dynamic adjustment of heat input and intensity along with the progress of the process. This requirement poses great challenges to the transient response of the heating system and real-time assessment of the coal properties, which are inherently slow given the large scale of the heating system and the huge depth of the coal seam. The wall conduction and volumetric heating methods (electric, radio frequency or microwave) have the advantage in fast response to the requirement of adjusting heating temperature and intensity. In comparison, externally heated gas method has much slower response because it takes a long time for the gas temperature to be changed and to be transported to the deeply buried coal. The fuel cell method inherently lacks the flexibility in adjusting the operating temperature. TagedPThe increase of void space during pyrolysis process creates the opportunity for convective heating, which is more efficient than conductive heating. Good use of convective heating can significantly reduce the heating cost and turn-around time of the UCTT process. Externally heated gas method has significant advantage in convective heating, especially when the coal is heated by direct contact with the heated gas. For example, heated CO2 can easily diffuse into the micropores of the coal [220], making it an ideal candidate for convective heating. In contrast, N2 takes a much longer time to diffuse into the micropores of the coal [220]. Use of externally heated gas also improves volatiles collection by promoting convective mass transfer. While the diffusion of heating gases into pores brings in heat, it also helps drive the volatiles out of the pores. TagedPSelection of the heating scheme is driven by comprehensive considerations of the unique heat transfer properties of coal and its dynamic evolution with pyrolysis process, as well as the characteristics of the heating methods. 7.3. Environmental issues TagedPThe primary environmental concerns for UCTT are water consumption, water contamination, GHG and air pollutant emissions, and land disturbance. The UCTT process has lower environmental burdens compared with UCG, and it offers benefits over traditional coal mining and above-ground combustions. It eliminates the mining process, coal transportation, ash disposal, particulate matter emissions and air pollutant emissions of coal combustion. It also avoids the mining hazards that coal miners face, such as coal dust explosions, firedamp explosions, roof collapsing, and lung diseases. TagedPUCTT has similar types of environmental concerns compared to UCG and underground oil shale retorting, as these processes share some common features. The impact of UCTT, however, can be much smaller than UCG because of UCTT's non-oxidizing environment. Table 16 provides a brief summary of the environmental issues of UCTT compared to UCG and underground oil shale retorting. The

21

TagedP otential solutions are also suggested by leveraging the solutions for p UCG and underground oil shale retorting. Detailed discussions are presented in subsections. TagedPWhen comparing fuels derived from UCTT to other transportation fuel sources, it is important to consider the fuel's entire life cycle, from raw material extraction to transportation, processing (including upgrading), refining, and delivery to the pump. Comparisons of life-cycle metrics, such as CO2 footprint, can be difficult between UCTT, oil shale, and conventional crude sources. Even for the same process, researchers sometimes provide drastically different estimations because of different assumptions and system boundaries. Table 17 provides a summary of life-cycle assessment (LCA) metrics for benchmarking UCTT to more widely-studied processes. These metrics are discussed in more detail in the following subsections. TagedP7.3.1. Water consumption and disposal TagedPLife-cycle water consumption for UCTT could not be identified. However, because of process similarities, UCTT's water consumption is anticipated to be in the same ballpark as underground oil shale processing. Extensive water consumption is not uncommon during underground oil shale processing. The Shell ICP requires 3 barrels of water per barrel of oil equivalent produced [98]. Water is usually consumed in underground thermal treatment of coal and oil shale in two ways: heat carrier (steam) and flush agent. If steam is used for heating the coal or oil shale, large quantities of clean water are needed for producing steam. Water is also used to recover the remaining hydrocarbons at the end of the thermal treatment process. In the Shell ICP, water equivalent to 20 times of pore volumes was injected into the production wells for oil recovery [112]. Once the process finishes, clean water is pumped underground to drive the contaminated water out for cleanup. This step repeats until the water meets environmental regulations. Large consumption of water is a serious problem in water-scarce region. If CO2 sequestration follows the UCTT process, the remaining hydrocarbon products can be driven out of coal by CO2 instead of water. This will reduce the consumption of clean water and the cost associated with water cleanup. Separation of CO2 with hydrocarbon fuels and reusing is a common practice in CO2-enhanced oil recovery. Water is also required for the generation of electricity, needed to heat the formation, and for upgrading and refining the UCTT product. Over its life cycle, the Shell ICP process is estimated to require 10 to 20% more water than conventional crude [212], as shown in Table 17. TagedPIn addition, water can be produced during underground thermal treatment. Underground water often fills pores and fractures, and water in large void volumes may even form subterranean aquifers. Liquid water's heat capacity is 4 times of that of oil shale rock, and water absorbs a large amount of heat that could be used to heat the shale [42,43]. Therefore, water needs to be removed before heating and extraction operation starts. Dewatering wells are drilled to remove formation water although some of the water will remain. For example, the remaining undrainable water accounts for about 7% of the shale bulk volume after dewatering [112]. Similar issues are present in underground coal thermal treatment. Water needs to be pumped out before heat treatment. If hydraulic fracturing is used to increase the permeability of the coal seam, water needs to be processed before it can be disposed. This is because chemical additives are often used in the fracturing water to prevent pipe corrosion or as a proppant. TagedP7.3.2. Water contamination and treatment TagedPContamination of underground or ground water is probably the most prominent environmental concern in underground thermal treatment of oil shale and coal. For example, in the Shell ICP the remaining water occupies about 7% of shale bulk volume after the dewatering stage [112]. Remaining water and flushing water will be contaminated by the hydrocarbons during the heating and

22

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32 Table 16 Comparison of major environmental issues of UCTT with UCG and underground oil shale retorting. Issues

Major causes

Severity vs. UCG

Severity vs. underground oil shale retorting

Potential solutions

Water contamination

Migration of pyrolysis gases into aquifers; Water flushing for enhanced recovery; Hydraulic fracturing

Much less

Less

Combustion of fuels for heating; Fugitive gases

Much less

Air pollutants emissions Combustion of fuels for heating; Fugitive gases

Much less

Land distortion and subsidence

Replenish of underground water; Collapse of remaining char matrix

Much less

Uncontrollable reactions underground

Loss of control of subsurface heating source, such as fuel cell

Much less, very unlikely Comparable

Careful site selection; Process control; in situ monitoring; Use CO2 as flushing agent; Post-cleanup before disposal Combine with CO2 capture by using UCTT site as CO2 reservoir; Use CO2 as flushing agent; Maintain operating pressure below lithostatic pressure of overburden layer; Precise control of heating temperature; in situ monitoring Careful site selection to avoid geological faults; Water and CO2 reinjection; Replanting; In situ monitoring; Shut down power/fuel supply in time

GHG emissions

TagedPextraction processes. Leakage of the hydrocarbons can also cause contamination to surrounding aquifers. As in UCG, typical hydrocarbon pollutants include phenols, benzene, PAHs and heterocyclics [2,222]. Among the inorganic species, heavy metals, ammonia and cyanides were the most serious group of contaminants [222]. Formation and transport of these contaminants is strongly dependent upon heating conditions and the site. For example, formation of PAHs is accelerated at high heating rates. In a two-week UCG field test at the experimental mine Barbara in Poland, Kapusta et al. [222] observed these major hydrocarbon and inorganic contaminants in the process water and nearby ground water. They also observed an interesting phenomenon that the ground water self-restored to original conditions 200 days after the test ended. TagedPGround water contamination by UCTT, however, should be a much lower concern than for UCG because of the slow heating rate and the remaining char behaving as a huge molecular sieve to hold the contaminants. As long as the remaining water does not leak into the surroundings, the contamination of ground water by UCTT can be minimized. In UCG, the char is converted to syngas and only

Comparable to more

More

Comparable

References

[2,5,10,11,17, 42, 221
225]

[2,5,7,10, 11,17,42, 102,221,222,226,]

[2,5,10, 11,17,42, 102,221,222]

[2,5,10, 11,17,21, 42,102, 221,222] [10,42,102]

tTagedP he ash remains. The remaining ash has little capability to hold contaminants, and may even collapse causing ground subsidence. The most well-known case is the UCG site in Hoe Creek, Wyoming, where poor siting and operation led to cavity roof collapse and product gas loss into the local groundwater system [232,233]. TagedPAnother source of water contamination is hydraulic fracturing. If hydraulic fracturing is used for enhancing permeability, additives are often used for various purposes such as anti-corrosion and friction reduction. When the fracturing fluids return to the surface, they can bring trace contaminants to the surface from the underground. These fracturing-fluids can contaminate surface and ground water, and they require treatment before disposal or reuse. For a detailed summary of chemical additives used in hydraulic fracturing and water management issues arising from hydraulic fracturing, the reader is referred to the reviews by Gregory et al. [223], Osborn et al. [224], Vengosh et al. [225], and Elliott et al. [234]. Ward et al. [235] provided an overview of the public health risks due to hydraulic fracturing.

Table 17 Summary of LCA metrics for relevant synthetic fuels. Fuel

Source

g CO2e /MJ

EROEI

Water (l/MJ)

Gasolinea Gasoline Refined crudec Refined crudec Gasolined

Conventional crude UCTT In situ oil shale Ex situ oil shale Coal to liquide

5.1
18.7 [7,227] 50
550 [229] 38
63 [42] 40
180 [213,214] 102 [231]

4.5
4.7 [228] 0.4
4.5 [229] 1.2
2.5 [6] 1.1
3 [228,230] 0.5
8.2 [215]

0.116
0.228 [212],b NA 0.129
0.274 [212],b 0.132
0.279 [212],b NA-

NA: not available; CO2e: CO2 equivalent, which describes the global warming potential of all greenhouse gases emitted in terms of an equivalent amount of CO2; EROEI: energy return on energy invested. a Conventional US crude. b Assumed energy content of crude or crude equivalent is 5,729 MJ/barrel. c Product not specified. d Assumed energy content of 34.2 MJ/l. e Fischer
Tropsch reaction.

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPThe major water protection strategies include prevention and post-processing cleanup. Shell proposed an innovative technique to prevent water contamination [113]. A freeze wall was established in their demonstration case by drilling wells 2.5 m apart from each other and circulating a refrigerant at ¡40 °C through the wells [42]. The freeze wall stopped the flow of water between the established retorting cell and the surrounding volume, and contained the water contamination within the boundary of the heating zone. Shell ICP also employed another water treatment technique involving a cleanup step after retorting. At the end of the process, underground water was pumped to the surface for hydrocarbon and water separation. The cleaned water was injected back into the underground cell, and the process repeated until the water met environmental standards. Monitoring wells should also be drilled to examine the changes in underground ecosystems, including water, temperature, pressure and bacterial populations. TagedPCareful evaluation of the UCTT site is the first step in minimizing water contamination. A drinking water-quality aquifer should be avoided. In UK, a minimum of 100 m separation distance was proposed for UCG [236]. Although the UCTT process has significant advantages over UCG in terms of potential water contamination, the 100 m separation distance should be used a first approximate. TagedP7.3.3. Greenhouse gas and pollutant emissions TagedPProducing fuels from underground oil shale processing generates more GHG emissions than from regular oil extraction on a per barrel oil equivalent basis, because hydrocarbon fuels are burned to heat the oil shale to recover products, as shown in Table 17. Brandt et al. [42,213,214] concluded that fuel produced from the Shell ICP process generates 21
47% more GHG emissions than conventional petroleum production. Kelly et al. [7] reported that gasoline produced from UCTT generated at least 1.4 times more GHG emissions than a refined fuel from the Shell ICP process, although UCTT's GHG emissions are highly dependent on the properties of the coal formation. The UCTT GHG emissions are generally within the wider range of GHG emissions reported for fuels produced from oil shale or from coal-to-liquid fuel production via the Fischer Tropsch process (Table 17). Using the waste heat of a fuel cell as heating source, however, can significantly improve the process efficiency and reduce CO2 emissions [102]. However, oil shales tend to have larger quantities of carbonate minerals than coal, and when oil shale is heated to 565 °C or higher these carbonate minerals begin to decompose and release CO2 [108,230]. TagedPThe porous char network left by the UCTT process is an ideal CO2 sequestration site. Injection of CO2 into UCTT site can actually help drive the remaining hydrocarbons out of the char network, thus improving production yield. Indeed, studies [186,237,238] have already started on enhancing CBM recovery via CO2 sequestration. White et al. [74] provided a review on the previous work in this area. CO2 is also a common flushing agent for enhanced oil recovery [226]. However, public concerns exist over the long-term safety of CO2 storage. TagedPA UCTT process would result in air pollution emissions from site preparation, well drilling, power generation, product separation and transport. These burdens would be similar to those from an in situ oil shale development process, which are discussed in detail in Kelly et al. [8]. Fugitive gas leakage is a concern for a UCTT process similar to an oil shale process. The high-pressure gaseous products may leak through the cracks or fractures in the overburden layer. Coal contains more oxygen than oil shale. It can be expected that more oxygenated species will be formed during UCTT. Liu et al. [239] reported that in addition to CO and CO2, nitrogen-containing species such as HCN, NH3, and NOx can also be produced during coal pyrolysis. Coal also contains more sulfur content than oil shale, and this sulfur will be converted to H2S or other species during pyrolysis, which are significant pollutants.

23

TagedPThe operating pressure for underground thermal treatment needs to be maintained below the lithostatic pressure of the overburden formation to prevent out-leakage of gas. Precise control of the heating temperature can also reduce the formation of sulfur gases and oxygenated gases, as these gases are favored at a relatively high temperature range [12,13]. As in UCG, installation of monitoring wells in permeable strata above the pyrolysis zone will provide early detection of gas escape [17]. Regular water sampling and monitoring will also help detect leakage of gases into underground water. TagedP7.3.4. Land use and restoration TagedPRubblization, hydraulic fracturing, heating, and site preparation/ operation all lead to land disturbance. When UCTT operations complete, land-restoration will be necessary. Most underground oil shale thermal treatment techniques require a rubblization step that often uses explosions. Underground rubblization can cause surface distortion or even subsidence, and surface damage to the surface need to be restored production. In some modified in situ processes [106], oil shale was mined to create void space for rock expansion during rubblization. The mined rock was often processed ex situ for oil and gas. Due to the higher permeability of coal, rubblization is often not necessary in UCTT. Therefore, the damage of UCTT to landscape and underground formations is much smaller compared to underground oil shale retorting. TagedPHeating the coal formation may cause coal swelling, release of volatiles, and/or fracturing of the overburden and underburden rocks, potentially leading to land distortion. Real-time monitoring of the subsurface pressure is necessary to adjust heating rate and prevent over-expansion of the surrounding formations and overburden layer. Hydraulic fracturing can also cause surface distortion and land damage. The recent large-scale shale-gas operations have led to increased experience with hydraulic fracturing, which can be leveraged in UCTT to minimize environmental impact. TagedPThe stability of overburden is also dependent upon the mechanical properties of the coal layer, particularly the strength and stiffness. Akbarzadeh and Chalaturnyk [21] reviewed the influence of temperature on coal strength and stiffness. They concluded that the shear strength and stiffness generally decrease with temperature, as evidenced by the decrease of shear modulus and elastic modulus with temperature. As a result, the overall coal structure is expected to be weakened. Akbarzadeh and Chalaturnyk [21] suggest that the influence of temperature on mechanical properties be included in geomechanical simulation to better predict the effect of underground coal thermal conversion on the strata. As coal is being heated, it undergoes softening, thermoplastic transformation, swelling and porous structure development. When devolatilization completes, gas leaves the porous structure and the coal shrinks, exhibiting compressional deformation [90,157,195]. Heating the coal also generates cracks and fractures [240,241]. At higher temperature, coal specimens generally fail under smaller compressive (or shear) stress while exhibiting larger axial (or shear) strain [90,240,242]. To alleviate the impacts of UCTT on overburden layer, slow heating is recommended. Slow heating results in a mild change in coal structure by reducing the extent of swelling and formation of cracks, because of the lower devolatilization rate and smaller thermal gradient in coal. 8. Future research perspectives TagedPA literature review of coal pyrolysis shows that although there are numerous studies on coal pyrolysis, they were mostly conducted on micron-to-millimeter sized coal particles for conventional pulverized coal combustion and gasification. For these size ranges, the effects of temperature, heating rate, and pressure on pyrolysis yield and composition of coal particles are well

24

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

Table 18 Comparison of characteristics of UCTT with conventional pulverized coal pyrolysis. Characteristics

UCTT environment

Conventional pulverized pyrolysis

Coal size Spatial constraint Swelling and pore growth Heat transfer Mass transfer

Large block (centimeter to meter) Yes Limited by overburden layer Initially by conduction, then changes with conversion due to porous structure development Initially by diffusion through native pores and convection through cleats and fractures, then changes with conversion due to porous structure development and cleat closing

Particle (micrometer to millimeter) No Free Convection and conduction Convection and diffusion

TagedPunderstood. Mass transfer effects, however, did not receive much attention because of the small particle size. Applicability of the results of pulverized coal pyrolysis in underground coal pyrolysis has yet to be validated. Pyrolysis of large-size coals in an underground environment has distinctive features that are not typically present in conventional pulverized coal pyrolysis [141]. The coal size is several orders of magnitude larger than coal powders, which introduces non-negligible heat and mass transfer effects. Coal swelling and pore growth that are characteristic in pulverized coal pyrolysis are spatially constrained by the overburden layer and surrounding formations. At the onset of pyrolysis, mass transfer is limited to native pores and cleats, making product diffusion and collection far more difficult than in a highporosity particle. Swelling of coal under the spatial constraints may close the cleats and fractures, further reducing the permeability of coal seam. Heat transfer in coal blocks is also hindered due to the reliance on conduction over larger length scales. Thermal conductivity of coal, which strongly depends on the pore structure, is not well understood because pore growth is limited by the spatial constraint. The largely different pore growth and transport properties in turn impact the pyrolysis kinetics. Table 18 compares key characteristics of underground block coal pyrolysis with conventional pulverized coal pyrolysis. TagedPThese distinctive characteristics create a knowledge gap in the fundamental understanding of coal pyrolysis in UCTT environment. For devolatilization of coal blocks, there is scarcity in literature regarding the kinetics and transport phenomena under slow heating and mechanical loading conditions. Furthermore, literature is scarce regarding the life-cycle impacts of UCTT, with a particular lack of information on water consumption. In addition, because UCTT is a relatively new concept, challenges exist in correlating the lab-scale data to system level towards field operation. The knowledge gap and challenges are defined in this review. Research needs are proposed to bridge the gap and solve the challenges. 8.1. Heat transfer TagedPHeat transfer in underground coal pyrolysis heavily depends on coal properties. A signature of coal pyrolysis is the porous structure evolution. Pores open up and grow as volatiles form bubbles and leave the coal. The evolution of coal's pore structure during pyrolysis determines the thermal conductivity of the coal [196]. The narrowing and breaking of microcrystals that occurs during pore growth tends to reduce the thermal conductivity. The pore growth, however, is suppressed by the overburden layer and surrounding formation. Therefore, the spatial constraint of underground coal pyrolysis helps ease the reduction of thermal conductivity caused by pyrolysis. Meanwhile, the suppression of pore growth also undermines the potential benefit of convective heating. Therefore, the overall impact of spatial constraint on heat transfer of the coal during pyrolysis process depends on whether conduction or convection dominates the heat transfer. TagedPFor wall conduction heating and pipe heating methods, heat is mainly transferred to coal by conduction from the heating elements. Suppression of pore growth helps keep the microcrystal structure

TagedP nd pore connections, thus minimizing the reduction of thermal a conductivity. For gas heating methods, coal is mainly heated by convection of the hot gases. Suppression of pore growth limits the gas flow. Furthermore, swelling of coal under spatial constraints may close the cleats and other fractures, hindering convective flow. Understanding heat transfer properties is essential for the heat management in UCTT. Therefore, pore growth and coal swelling behavior during pyrolysis need to be characterized under physically constrained conditions, as a function of coal conversion, temperature and heating rate. Thermal conductivity needs to be measured in situ for providing reference data for heat transfer calculations in a UCTT environment. TagedPIn addition, because the thermal conductivity strongly correlates to the porous structure [196,198], the evolution of thermal conductivity can be used to infer the porous structure changes during coal conversion. This provides an in-situ method to monitor the porous structure changes of coal during the UCTT process because the electrical signal used to calculate thermal conductivity responds in a real-time manner. Therefore, research is needed to develop a method for measuring the thermal conductivity of coal blocks during the conversion process and establish the correlation between thermal conductivity and porous structure (or coal conversion). 8.2. Mass transfer TagedPAnalogous to heat transfer, mass transfer in UCTT is strongly dependent on the evolution of pore structures. In conventional pulverized coal pyrolysis, mass transfer time scale of gaseous volatiles (1 ms) is usually smaller than the heating time scale for a high heating rate (1000
10,000 K/s). Therefore, mass transfer is usually not the rate-limiting step in conventional pulverized coal pyrolysis, and the reactions can be described as kinetics controlled, unless the temperature and heating rate are extremely high. In underground coal pyrolysis that features large coal blocks, the mass transfer time may exceed the heating time scale, depending on the heating conditions and heat transfer properties of the coal. Under such conditions, the mass transfer effects on pyrolysis cannot be neglected. Pyrolysis reactions can shift from a kinetics-controlled regime to a diffusioncontrolled regime or a regime between the two. Gneshin et al. [9] used a coal block reactor to study the heat and mass transfer in large coal blocks. The block reactor (Fig. 10) was designed to accommodate coal blocks up to 15 cm square and physically confine the coal, mimicking the spatial confinement conditions of UCTT process. Preliminary results showed that retention of tars in pores and fractures occurred to a coal block of 15 cm3 (»5 kg) at 10 K/min heating rate, partly contributing to a reduction of tar yields. In contrast, such retention was not found in a coal core of 2 cm diameter pyrolyzed at the same heating rate in a fixed-bed reactor. This demonstrates the effect of large coal size in retaining the tars due to its extended length scale (and time scale) of mass transfer. In addition, mass transfer also affects product evolution. For example, secondary reactions and tar coking become more prominent at extended mass transfer time. TagedPSince UCTT will most likely be conducted at low temperature and slow heating conditions, the pyrolysis reaction rates are expected to be slow. Mass transfer will also be slow, considering the large size of

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

Fig. 10. The coal block reactor for studying pyrolysis of large coal blocs. Reprinted from [9] with permission of American chemical society.

TagedPcoal blocks. It is unclear which factor, heat transfer or mass transfer, will determine pyrolysis reaction rates. Studies on mass transfer of large-size coals under slow heating conditions will provide insight into the controlling mechanism. Studies on this topic will also be instrumental by providing reference data for the design and operation of heating and hydrocarbon collection systems in a UCTT process. If mass transfer needs to be improved, hydraulic fracturing can be employed to enhance the permeability of the coal seams. Alternatively, if heat transfer is the limiting factor, heating system needs to provide more heating elements or higher heating temperature, or to use convective heating methods. Due to the high complexity and cost, field test is premature at this time. Thus, lab-scale studies are recommended using a coal block reactor [9], a triaxial test cell [158], or a uniaxial loading cell [195] to obtain pyrolysis kinetics and porosity data. These apparatus are capable of applying thermal and mechanical loadings simultaneously, in a way that closely represents the spatially confined environment in UCTT process while providing well-controlled experimental conditions. The X-ray CT and low-field NMR are powerful tools for in situ characterization of the porous structure development. 8.3. Permeability modeling TagedPCoal permeability plays a crucial role in the production and collection of the pyrolysis yields. The permeability model is essential for the predictions of the convective mass and heat transfer during pyrolysis. Such predictions are critical for resolving the physics that impact the reactions rates of pyrolysis and the convection of pyrolysis products. Previous studies on coal permeability modeling were mostly focused on CBM production and CO2 sequestration applications, i.e., non-reacting conditions. For example, researchers [66,76,243,244] have attempted to describe the evolution of permeability caused by coal matrix shrinking when CBM leaves the pore. They found that coal permeability strongly correlates to strain and stress. The evolution of coal permeability during pyrolysis is also expected to be impacted by strain and stress. Wang et al. [195] also observed that both thermal swelling and desorption of gas flow are strongly affected by mechanical loading when heating coal blocks in a specially designed reactor. The coal undergoes thermal swelling, and the pores open up as being heated. Swelling of the coal may close the cleats and other fractures, reducing the permeability of the coal seam. On the other side, opening-up of the pores increases coal permeability. Evolution of coal permeability under physically constrained conditions is thus determined by the overall effects of coal swelling, pore growth and fracturing. Indeed, Xie et al. [155] observed that the permeability of a packed coal bed initially

25

TagedP ecreased due to swelling, which closes the voids between coal pard ticles, and then started increasing rapidly once tar release completed. The rapid increase in permeability is obviously due to the pyrolysis-induced porosity. TagedPIn a UCTT environment, pyrolysis takes place within large coal blocks. Fu et al. [245] compared the swelling and shrinking behavior of raw coal blocks and processed coal cakes (made by pressing pulverized coal particles) heated in a coke oven. These coal blocks and cakes are about 10
20 mm in diameter and 10
14 mm in height. They found that the swelling ratio of raw coal blocks is much larger than that of processed coal cakes, while the shrinkage ratio has the opposite behavior. The swelling ratio also increases with the size of the coal block. These differences were attributed to the laminated macrostructure of the raw coal, which was broken in pulverized coal particles. These findings suggest that the swelling and shrinkage behavior of coals blocks in a UCTT environment can be quite different from the conventional pulverized coal particles, although these results were obtained at unconfined conditions. TagedPA unique feature of UCTT is the spatial confinement and mechanical loading from the overburden and surrounding formation. The spatial confinement suppresses the coal swelling, while the mechanical loading tends to cause coal deformation. The confinement imposed on coal body will certainly have an impact on the bubbling mechanism, which subsequently determines the porous structure during pyrolysis. Gneshin et al. [9] found that confinement has a pronounced effect on both pore size distribution and pore morphology when coal swelling is suppressed, as shown in Fig. 11. They used an aluminum tube enclosing a coal core of 2 cm in diameter to mimic the physical confinement in UCTT environment, and compared the effects of confinement at two different heating rates. At a heating rate of 10 K/min, pore size distribution moves towards the larger end while strong metaplastic deformation and violent bubble ruptures occurs. This is because higher pore pressure builds up to resist the spatial confinement. At 0.1 K/min, however, pore size distribution and pore morphology remain essentially unchanged because coal swelling and thermoplastic deformation did not occur at such a slow heating rate. In summary, confinement of coal tends to result in more violent swelling at fast heating conditions while having little impact at slow heating. Alternatively, swelling of coal in a constrained space tends to close the cleats and other fractures. These factors suggest that the swelling and shrinkage behavior of coal blocks in a UCTT environment may be quite different from conventional pulverized coal pyrolysis. TagedPThe mechanical loading exerted by the overburden layer influences the coal structure development and gas transport property. He et al. [246] studied the gas transport and deformation behavior of coal blocks under stressed conditions. They used a uniaxial test apparatus to apply mechanical loading to the cylindrical coal block (5 cm in diameter and 10 cm in height) after slow heating the sample to 45 °C, and used X-ray CT to examine the coal structure before and after the test. Results show that micro and meso cracks (Fig. 12) formed due to mechanical loading, and gas transport were strongly associated with the formation of the cracks. The researchers suggest that coal under mechanical loading can be considered as a brittle material, which tends to crack. The cracks may promote the deformation of the coal, and lead to non-uniform pressure and gas transport in the deformed region. For example, some clustered cracks may act as a gas reservoir, while other cracks may interconnect the fractures and serve as flow channels. The cracks will in turn affect the gas transport in the coal. TagedPThe conditions relevant to a UCTT process have unique impacts on permeability development. Spatial confinement on coal swelling favors the formation of macropores of a bituminous coal heated at 10 K/min (Fig. 11, A), which tends to increase the permeability. In contrast, pores remain essentially unchanged because no thermoplastic swelling or deformation occurred at a slow heating rate of

26

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

Fig. 11. SEM micrographs of bituminous coal cores heated at 10°C/min to 600°C with (A) full volumetric confinement and (B) no volumetric confinement. All scale bars are 100 mm. Reprinted from [9] with permission of American chemical society.

TagedP0.1 K/min, which indicates that the native pores are capable of handling the mass transfer at slow heating rate. Furthermore, it indicates that the native pore distribution of raw coals can be used in modeling the mass transfer at slow heating conditions, thus significantly simplifying the modeling effort. Thermal-stress-induced fracturing, however, occurred to a large coal block (15 cm cubed) even if the coal was slowly heated at 0.1 K/min. The fractures obviously contribute to permeability development. These results, although

TagedP btained at atmospheric pressure, suggest that heating rate and o temperature can be adjusted to control the permeability development of the coal. Further, modeling of mass transfer at UCTT conditions need to be adjusted accordingly. For example, a steep temperature gradient can be applied to the coal block to induce large thermal stress and fractures. Meanwhile, these results suggest that previous coal permeability models need to be revised taking into account the interaction of coal swelling with spatial confinement

Fig. 12. Example of cracks formed in a coal block due to mechanical loading, reprinted from [246] with permission of Elsevier: (a) The 3-D reconstructed images of pre-test, (b) the 3D reconstructed images of post-test, (c) the 3-D reconstructed images of cracks illustrated by green color that are processed by MimicsTM , (d) the CT image of pre-test slice, (e) the CT image of post-test slice, (f) the image corresponding to (e) processed by the binary algorithm, for #2 coal sample. (unit: mm).

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedPand the effect of thermal stress, in addition to the mechanical stress imposed by the overburden. Fundamental understanding of the transient behavior of coal permeability as a function of coal conversion is critical for capturing the physics of convective heat and mass transfer in UCTT process. Permeability studies under conditions closely resembling UCTT process (large size, high pressure, slow heating, spatial confinement, mechanical loading) will provide more precise data for describing the mass transfer in UCTT process. 8.4. Pyrolysis kinetics and modeling TagedPPyrolysis kinetics has been well studied for coal particles. In addition to temperature, heating rate and pressure, pyrolysis kinetics is also dependent on the porous structure. The pyrolysis yields drive pore growth and coal swelling. Meanwhile, the porous structure determines heat and mass transfer. The relative time scales of heat transfer and mass transfer determine if the reaction is kinetically controlled or mass transfer controlled. The wealth of pyrolysis data on coal particles provides a fairly good understanding on the correlation between pyrolysis yields and heating conditions. The extrapolation of these data to coal blocks in underground in situ pyrolysis, however, is not well understood. Westmoreland and Dickerson [247] suggest that pyrolysis data for large coal blocks are essential to perform such an extrapolation. TagedPFor underground coal pyrolysis, the effects of coal size and spatial confinement on porous structure evolution cannot be neglected, as discussed in previous sections. The much larger coal size drastically increases heat and mass transfer time. The slow heating rate, which is characteristic of a UCTT process, may also shift the reaction pathways of tar formation. Decrease in liquids yield that occurred to slowly heated large coals was partly attributed to the change in tarformation pathways [9]. Secondary reactions, tar coking, cross-linking and reticulation reactions become more pronounced at extended residence times [248], which is typical for large size coal blocks. The measurement of secondary reactions has large uncertainty [249]. The confinement of overburden layer suppresses pore growth and coal swelling, resulting in complex effects on heat transfer and mass transfer. The discussion in Sections 8.1 and 8.2 shows that the relative time scales of heat transfer and mass transfer are not well understood for large coal that is pyrolyzed at slow heating conditions. This underscores the importance of studying the effects of spatial confinement and slow heating on pyrolysis kinetics of large coal blocks. TagedPFor coal pyrolysis models, the prediction accuracy is dependent on the heating conditions [250
253]. Previous coal devolatilization models were developed targeting pulverized coal combustion and gasification; i.e., high temperature and high heating rate conditions. Their accuracy and prediction capability have yet to be tested at conditions relevant to underground thermal treatment. For example, even for the Sommariva model [254], which was tested in a very wide range of heating conditions, the lowest heating rate was 30 K/ min. In UCTT, the heating rate is expected to be much lower and the coal size is much larger. Residence time of volatiles in the large coal block is much longer than that in coal particles. Retention of volatiles in the char at extended residence time was found to increase the proportion of CH4 and H2 in the gaseous products in a fixed-bed reactor experiment [255]. Secondary reactions, tar coking, crosslinking and reticulation reactions become more important at extended residence time [31,25,42,48]. These reactions affect the tar yield and gas species yield. Therefore, the pyrolysis models need to be validated against experimental data at conditions relevant to UCTT process; i.e., large blocks, low temperature, slow heating and high pressure with spatial confinement. Kinetic parameters need to be tuned to optimize the performance of the models at the specific conditions. To perform such a validation, experimental data using coal block reactors or fixed-bed reactors are recommended.

27

TagedPAnother important difference between in situ coal pyrolysis and conventional pulverized coal pyrolysis is the self-gasification by inherent moisture of the coal. Westmoreland and Dickerson [247,256] found that the yield of CO and CO2 could not be reconciled from the oxygen content of the parent coal, when they studied the in situ pyrolysis of large blocks of bituminous and lignite coals. The more-than-expected yield of CO and CO2 was attributed to the self-gasification of coal with moisture. In addition, the high pressure of the UCTT environment also impacts reaction kinetics, favoring gas yields. The work by Gneshin et al. [9] was performed at atmospheric pressure. Kinetic studies of large coal under pressurized conditions will provide more accurate data for UCTT process design and optimization. 8.5. Heating technique TagedPThe heating technique is the key for cost reduction in heat management. Wall conduction using electricity is associated with a high energy penalty because a large fraction of the energy is wasted in power generation. Although state-of-the-art combined cycle power plants have thermal efficiency over 60%, the thermal efficiency of power plants at the scale for UCTT is likely lower [257]. Therefore, heating using an externally generated hot gas circulating through pipes or directly injected into coal seams is recommended in underground coal thermal treatment in order to improve the thermal efficiency and the energy return. The hot gas directly injected into coal seams promotes convective heat and mass transfer. TagedPHowever, the wall conduction approach has advantages of generating uniform porosity and enhancing permeability of coal formation. It also leads to less swelling in the coal seam due to the more uniform and mild heating. Wall conduction provides better process control, which may lead to higher quality pyrolysis products. Volumetric heating provides a unique advantage for the short time delay between process initiation and production. Within 1-2 months, the extraction wells can be yielding coal liquids. Volumetric heating also provides an even temperature profile so that the coal conversion rate is uniform and the process is more controlled. TagedPWaste gases generated during the coal conversion can be heated and injected into the coal seam as a heat carrier. This stream of hydrogen-rich gases can also augment coal conversion even when electric heating is used. The recycled gases can diffuse into the formation and be extracted to surface after exchanging heat with coal. Product gases can be used to fire a gas turbine to generate electricity in addition to providing a carrier for recycling energy. TagedPUsing the heat from fuel cell operation is an energy-saving way to heat the coal formation. It utilizes the heat that otherwise would be wasted in conventional internal combustion engine when burning the fuels. The geothermic fuel cell and EPICC concepts are thus high efficiency and low-carbon in nature. Given the potential to reach 70% thermodynamic efficiency [119], they offer great promise for UCTT. Major barriers towards the application of fuel cell technology in UCTT are the subsurface operation and maintenance of fuel cell, degradation of cathodes, high operating temperature, and the significantly high cost [119]. System integration can also be a problem when electricity generation and heat generation of the fuel cell need to be balanced. TagedPRecently, solar and wind have gained more market share and show great future for energy demand. Cost of these renewable energies is dropping sharply as technology advances. Therefore, besides the onsite generation of energy sources, wind and solar have a great potential in coal and oil shale underground thermal treatment. The intermittent supply of energy from solar energy and wind is less of a problem for UCTT than in providing electricity to the grid.

28

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

9. Summary TagedPThis review provides a general description of the UCTT process and discusses its feasibility by comparing it with underground oil shale retorting and UCG. A comparison of the physical properties of coal with oil shale suggests that UCTT is a promising technology for extracting high-value hydrocarbon fuels from deeply buried coal seams while leaving behind an ideal site for CO2 sequestration. Leveraging the readily available literature on underground oil shale retorting, promising technologies that can be adapted to UCTT have been identified. Challenges and issues of the UCTT process are discussed from systems-level and operational perspectives. Studies on conventional pulverized coal pyrolysis are reviewed in the context of identifying the key process parameters and their impacts on product yield and composition. The review is then extended to whether these studies can be readily applied to a UCTT environment. It clearly shows that there are knowledge gaps between conventional pulverized coal pyrolysis and underground in situ thermal treatment. To bridge these gaps, future research needs are identified. TagedPIn brief, underground coal thermal treatment is a slow-heating, low-to-medium temperature, high-pressure pyrolysis process that takes place in large coal blocks. The ideal site for UCTT is a deeply-buried high-volatile bituminous coal that has undergone CBM production. Light hydrocarbon gases dominate the pyrolysis products with a composition similar to SNG. The underground environment imposes spatial constraints and lithostatic pressures on the coal, which suppress the swelling and pore growth that are typical in conventional pulverized coal pyrolysis. This distinctive feature of UCTT gives rise to the question whether the previous literature on pulverized coal pyrolysis can be directly applied to the UCTT process. In particular, the evolution of pore structure determines heat and mass transfer behavior, which will strongly impact the pyrolysis kinetics and product transport in large coal blocks. In addition, the heat and mass transfer properties are the prerequisite for design of the heating and product collection systems. Therefore, fundamental studies need to focus on pyrolysis of physically-constrained coal, with an objective to understand the impact of spatial constraints on the evolution of porous structure, and its effects on heat and mass transfer properties. Kinetic studies at slow-heating and high-pressure conditions also deserve to be revisited for large coal blocks.

TagedP

Acknowledgments

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

T his material is based upon work supported by the United States agedPT Department of Energy under Award Numbers DE-FE0001243 and DE-NT0005015, and the China Young Thousand Talents Program under Award Number 20161710305. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or Chinese Government or any agency thereof. TagedPSpecial thanks to Professor Adel F. Sarofim, Presidential Professor, University of Utah (deceased) for inspiring and initiating this international partnership to evaluate UCTT and for sharing his immense knowledge on the subject of coal, coal pyrolysis and heat transfer. It was his vision into the future of UCTT that encouraged us to undertake this investigation into this uncharted area. TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

TagedP

References TagedP TagedP

TagedP

1 International Energy Agency. World energy outlook. Paris: IEA; 2016. p. 2016. 2 Bhutto AW, Bazmi AA, Zahedi G. Underground coal gasification: From fundamentals to applications. Progr Energy Combust Sci 2013;39(1):189–214. TagedP

3 Song HZ. The distribution characteristics and exploration prospect of coal resource of China PhD thesis (in Chinese). China: China University of Geoscience; 2013. p. 41–53. 4 Friedmann J. Accelerating development of underground coal gasification: priorities and challenges for US research and development. In: Pettus A, Tatsutani M, editors. Coal without carbon: an investment plan for federal action. Boston, MA: Clean Air Task Force; 2009. p. 1–16. 5 Friedmann SJ, Upadhye R, Kong FM. Prospects for underground coal gasification in carbon-constrained world. Energy Procedia 2009 Feb 28;1(1):4551–7. 6 International Energy Agency. CO2 emissions from fuel combustion highlights. Paris: IEA; 2016. p. 11. 7 Kelly KE, Wang D, Hradisky M, Silcox GD, Smith PJ, Eddings EG, Pershing DW. Underground coal thermal treatment as a potential low-carbon energy source. Fuel Process Technol 2016;144:8–19. 8 Kelly KE, Ruple JC, Wilkey J. Oil shale development, air quality, and carbon management. In: Spinti J, editor. Utah oil shale: science, technology, and policy perspectives. Boca Raton, FL: CRC Press; 2016. p. 307–28. 9 Gneshin KW, Krumm RL, Eddings EG. Porosity and structure evolution during coal pyrolysis in large particles at very slow heating rates. Energy Fuels 2015;29 (3):1574–89. 10 Shackley S, Mander S, Reiche A. Public perceptions of underground coal gasification in the United Kingdom. Energy Policy 2006;34(18):3423–33. 11 Roddy DJ, Younger PL. Underground coal gasification with CCS: a pathway to decarbonising industry. Energy Env Sci 2010;3(4):400–7. 12 Chen HK, Li BQ, Yang JI, Zhang BJ. Transformation of sulfur during pyrolysis and hydropyrolysis of coal. Fuel 1998;77(6):487–93. 13 Chen H, Li B, Zhang B. Effects of mineral matter on products and sulfur distributions in hydropyrolysis. Fuel 1999;78(6):713–9. 14 Shafirovich E, Varma A. Underground coal gasification: a brief review of current status. Ind Eng Chem Res 2009;48(17):7865–75. 15 Solomon PR, Serio MA, Suuberg EM. Coal pyrolysis: experiments, kinetic rates and mechanisms. Progr Energy Combust Sci 1992;18(2):133–220. 16 Gregg DW, Edgar TF. Underground coal gasification. AIChE J 1978;24(5):753–81. 17 Sury M, White M, Kirton J, Carr P, Woodbridge R, Mostade M, et al. Review of environmental issues of underground coal gasification-best practice guide. Review of environmental issues of underground coal gasificationbest practice guide, 126. United Kingdom Department of Trade and Industry. p. 1–26. 18 Khadse A, Qayyumi M, Mahajani S, Aghalayam P. Underground coal gasification: a new clean coal utilization technique for India. Energy 2007;32(11):2061–71. 19 Klimenko AY. Early ideas in underground coal gasification and their evolution. Energies 2009;2(2):456–76. 20 Imran M, Kumar D, Kumar N, Qayyum A, Saeed A, Bhatti MS. Environmental concerns of underground coal gasification. Renew Sustain Energy Rev 2014;31: 600–10. 21 Akbarzadeh H, Chalaturnyk RJ. Structural changes in coal at elevated temperature pertinent to underground coal gasification: a review. Int J Coal Geol 2014;131:126–46. 22 Hobbs ML, Radulovic PT, Smoot LD. Combustion and gasification of coals in fixedbeds. Progr Energy Combust Sci 1993;19(6):505–86. 23 Anthony DB, Howard JB. Coal devolatilization and hydrogastification. AIChE J 1976;22(4):625–56. 24 Howard JB. Fundamentals of coal pyrolysis and hydropyrolysis. Chem Coal Util 1981;2:665–784. 25 Gavalas GR. Coal pyrolysis. Amsterdam, Oxford, New York: Elsevier Scientific Publishing Company; 1982. € ntgen H. Review of the kinetics of pyrolysis and hydropyrolysis in relation to 26 Ju the chemical constitution of coal. Fuel 1984;63(6):731–7. 27 Smoot LD, Smith PJ. Coal combustion and gasification. New York: Plenum Press; 1985. 28 Solomon PR, Fletcher TH. Impact of coal pyrolysis on combustion. Symposium (International) on combustion, 25; 1994. p. 463–74. 29 Benfell KE, Liu GS, Roberts DG, Harris DJ, Lucas JA, Bailey JG, Wall TF. Modeling char combustion: the influence of parent coal petrography and pyrolysis pressure on the structure and intrinsic reactivity of its char. In: Proceedings of the combustion institute, 28; 2000. p. 2233–41. 30 Wall TF, Liu GS, Wu HW, Roberts DG, Benfell KE, Gupta S, et al. The effects of pressure on coal reactions during pulverised coal combustion and gasification. Progr Energy Combust Sci 2002;28(5):405–33. 31 Yu J, Lucas JA, Wall TF. Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: a review. Progr Energy Combust Sci 2007;33(2):135–70. 32 Bartok W, Sarofim A F. Fossil fuel combustion: a source book. New York, NY: John Wiley & Sons; 1991. p. 835. 33 Speight JG. The chemistry and technology of coal. second ed. New York: Marcel Dekker; 1994. p. 15. 34 ASTM Standard classification of coals by rank., D388
15 35 Dogan OM, Uysal BZ. Pyrolysis of three Turkish oil shales and analysis of shale oils using FT-IR and NMR spectroscopy. Oil Shale 2002;19(4):399–410. 36 Jiang X, Yang L, Yang C, Huang X, Yang H. Experimental investigation of SO2 and NOx emissions from Huadian oil shale during circulating fluidized-bed combustion. Oil Shale 2004;21(3):249–58. 37 Yue C, Li S. Combustion of oil shale particles under elevated pressures. Oil Shale 2002;19(4):411–8. € k MV, Pamir MR. Comparative pyrolysis and combustion kinetics of oil shales. J 38 Ko Anal Appl Pyrol 2000;55(2):185–94.

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32 TagedP 39 He YG. Mining and utilization of Chinese Fushun oil shale. Oil Shale 2004 Jan 1;21 (3):259–64. TagedP 40 Hayhurst AN, Lawrence AD. The devolatilization of coal and a comparison of chars produced in oxidizing and inert atmospheres in fluidized beds. Combust Flame 1995;100(4):591–604. TagedP 41 Speight JG. Handbook of coal analysis. Hoboken, NJ: John Wiley & Sons; 2005. TagedP 42 Brandt AR. Converting oil shale to liquid fuels: energy inputs and greenhouse gas emissions of the Shell in situ conversion process. Env Sci Technol 2008 Aug 23;42 (19):7489–95. TagedP 43 Hendrickson TA. Synthetic fuels data handbook. Denver, CO: Cameron Engineers. Inc; 1975. TagedP 44 Westmoreland PR, Dickerson LS. Pyrolysis of blocks of lignite. In Situ 1980;4(4). TagedP 45 Yip K, Wu H, Zhang DK. Mathematical modeling of Collie coal pyrolysis considering the effect of steam produced in situ from coal inherent moisture and pyrolytic water. In: Proceedings of the combustion institute, 32; 2009. p. 2675–83. TagedP 46 Palmer CA, Oman CL, Park AJ, Luppens, JA. The U.S. geological survey coal quality (COALQUAL) database version 3.0: U.S. Geol Surv Data Ser 975, 43 p. TagedP 47 Palmer TJ, Vahrman M. The smaller molecules obtainable from coal and their significance: Part 4. Composition of low-temperature tars. Fuel. 1972;51(1):22–6. TagedP 48 Serio MA, Hamblen DG, Markham JR, Solomon PR. Kinetics of volatile product evolution in coal pyrolysis: experiment and theory. Energy Fuels 1987;1(2):138– 52. TagedP 49 API. Manual of petroleum measurement standards. Chapter 11, Volume XI/XII. Adjunct to: ASTM D1250; 2004. TagedP 50 Luppens JA, Rohrbacher T, Osmonson LM, Carter MD. Coal resource availability, recoverability, and economic evaluations in the United States-A summary. In: Pierce BS, Dennen KO, editors. The national coal resource assessment overview: U.S. geological survey professional Paper 1625
F, Chapter D, 17 p. Reston, Virginia: U.S. Geological Survey; 2009. TagedP 51 Wood GH, Kehn TM, Carter MD, Culbertson WC. Coal resource classification system of the US geological survey. US Geol Surv Circ 1983;65:891. TagedP 52 Liu B, Wan M. The thickest coal bed was discovered in Xinjiang
a total thickness of 301 meters and the thickest of 217.14 meters (in Chinese), http://news. xinhuanet.com/fortune/2009-11/15/content_12461438.htm; 2009 [accessed 16.10.16]. TagedP 53 Bartis JB, LaTourrette T, Dixon L, Peterson D, Cecchine G. Oil shale development in the United States. Prospects and policy issues, prepared for the national energy technology laboratory of the United States department of energy, 50; 2005. p. 15–8. TagedP 54 Zheng D, Wang H, Liu D, Li J, Ge Z. Oil shale features and the retorting process for oil shale in the Liushuhe Basin in Daqing. Nat Gas Ind (China) 2008;28(12):1–3. TagedP 55 Green MB. Underground coal gasification: a joint European trial in Spain, London: Department of Trade and Industry ETSU/DTI Report COALR169, DTI/PUB, URN99/1093, Energy Technology Support Unit; 1999. TagedP 56 U.S. Department of Energy. Powder river basin coalbed methane development and produced water TagedP 57 Lee S. Oil Shale Technology, Boca Raton, FL: CRC Press; 1991, p. 85. TagedP 58 Zargari S, Canter KL, Prasad M. Porosity evolution in oil-prone source rocks. Fuel 2015;153:110–7. TagedP 59 Harpalani S, Schraufnagel RA. Shrinkage of coal matrix with release of gas and its impact on permeability of coal. Fuel 1990;69(5):551–6. TagedP 60 Liu J, Chen Z, Elsworth D, Qu H, Chen D. Interactions of multiple processes during CBM extraction: a critical review. Int J Coal Geol 2011;87(3):175–89. TagedP 61 King G. Numerical simulation of the simultaneous flow of methane and water through dual porosity coal seams during the degasification process. University Park (USA): Pennsylvania State Univ.; 1985 Jan 1. TagedP 62 Kolesar JE, Ertekin T, Obut ST. The unsteady-state nature of sorption and diffusion phenomena in the micropore structure of coal: Part 1-Theory and mathematical formulation. SPE Form Eval 1990;5(01):81–8. TagedP 63 Kolesar JE, Ertekin T, Obut ST. The unsteady-state nature of sorption and diffusion phenomena in the micropore structure of coal: Part 2-Solution. SPE Form Eval 1990;5(01):89–97. TagedP 64 McKee CR, Bumb AC, Koenig RA. Stress-dependent permeability and porosity of coal and other geologic formations. SPE Form Eval 1988;3(1):81–91. TagedP 65 Stevens SH, Kuuskraa VA, Gale J, Beecy D. CO2 injection and sequestration in depleted oil and gas fields and deep coal seams: worldwide potential and costs. Env Geosci 2001;8(3):200–9. TagedP 66 Cui X, Bustin RM, Chikatamarla L. Adsorption-induced coal swelling and stress: implications for methane production and acid gas sequestration into coal seams. J Geophys Res 2007;112(B10). TagedP 67 Liu HH, Rutqvist J. A new coal-permeability model: internal swelling stress and fracture
matrix interaction. Transp Porous Media 2010;82(1):157–71. TagedP 68 Pan Z, Connell LD. Modelling permeability for coal reservoirs: a review of analytical models and testing data. Int J Coal Geol 2012;92:1–44. TagedP 69 Harpalani S, Zhao X. An investigation of the effect of gas desorption on coal permeability formation. In: Proc. international coalbed methane symposium; 1989 Apr. p. 57–64. TagedP 70 Palmer I, Mansoori J. How permeability depends on stress and pore pressure in coalbeds: a new model. SPE annual technical conference and exhibition. Society of Petroleum Engineers; 1996 Jan 1. TagedP 71 Mavor MJ, Vaughn JE. Increasing absolute permeability in the San Juan basin fruitland formation. In: Proceedings of the coalbed methane symposium. University of Alabama, Tuscaloosa: Alabama; 1997. p. 33–45. TagedP 72 Mavor MJ, Russell B, Pratt TJ. Powder river basin Ft. Union coal reservoir properties and production decline analysis. SPE annual technical conference and exhibition. Society of Petroleum Engineers; 2003. Jan 1.

29

TagedP 73 Robertson EP. Measurement and modeling of sorption-induced strain and permeability changes in coal. United States. Department of Energy; 2005 Oct 1. doi: 10.2172/911830. TagedP 74 White CM, Smith DH, Jones KL, Goodman AL, Jikich SA, LaCount RB, et al. Sequestration of carbon dioxide in coal with enhanced coalbed methane recovery a review. Energy Fuels 2005;19(3):659–724. TagedP 75 Lin W, Tang GQ, Kovscek AR. Sorption-induced permeability change of coal during gas-injection processes. SPE Reserv Eval Eng 2008;11(04):792–802. TagedP 76 Pan Z, Connell LD, Camilleri M. Laboratory characterisation of coal reservoir permeability for primary and enhanced coalbed methane recovery. Int J Coal Geol 2010;82(3):252–61. TagedP 77 Thorsness CB, Grens EA, Sherwood A. One-dimensional model for in situ coal gasification. Livermore (USA): California Univ., Lawrence Livermore Lab Report number UCRL-52523; 1978. TagedP 78 Zhao Y, Yang D, Guan K, Liu S, Liang W, Feng Z, et al. A method for producing oil and gaseous fuels by convectively heating oil shale using high temperature alkane gases. Chinese. China Patent Number: CN101122226, inventors. Taiyuan University of Science and Technology; 2008 Feb 13. assignee. TagedP 79 Singer JM, Tye RP. Thermal, mechanical, and physical properties of selected bituminous coals and cokes. Bureau of Mines: Washington, DC; 1979 Jan 1. TagedP 80 Vinegar HJ, Wellington SL, De Rouffignac EP, Karanikas JM, Berchenko IE, Stegemeier GL, et al. In situ thermal processing of a coal formation to produce hydrocarbon fluids and synthesis gas. Inventors; shell oil company, assignee. United States patent US 6; 2004 Jul 13. p. 216. TagedP 81 Vinegar HJ, Wellington SL, De Rouffignac EP, Karanikas JM, Berchenko IE, Stegemeier GL, et al. Production of synthesis gas from a coal formation. Inventors; shell oil company, assignee. United States patent 7; 2006. p. 661. TagedP 82 Niu S, Zhao Y, Hu Y. Experimental investigation of the temperature and pore pressure effect on permeability of lignite under the in situ condition. Transp Porous Media 2014;101(1):137–48. TagedP 83 Cai Y, Liu D, Yao Y, Li Z, Pan Z. Partial coal pyrolysis and its implication to enhance coalbed methane recovery, Part I: an experimental investigation. Fuel 2014;132:12–9. TagedP 84 de Koranyi A, Balek V. Structural changes in coal during pyrolysis. Thermochimica Acta 1985;93:737–40. TagedP 85 Balek V, de Koranyi A. Diagnostics of structural alterations in coal: porosity changes with pyrolysis temperature. Fuel 1990;69(12):1502–6. TagedP 86 Ozisjk MN. Heat conduction. second ed. New York: John Wiley & Sons; 1993. TagedP 87 Gilliam TM, Morgan IL. Shale: measurement of thermal properties (No. ORNL/ TM-10499). Oak Ridge National Lab., TN (USA), 1987. DOI: 10.2172/6163318. TagedP 88 Berkovich AJ, Levy JH, Schmidt SJ, Young BR. Heat capacities and enthalpies for some Australian oil shales from non-isothermal modulated DSC. Thermochimica acta 2000;357:41–5. TagedP 89 Palmer CD, Mattson E, Huang H. Models for thermal transport properties of oil shale. Presented at the 30th Oil Shale Symposium. Colorado School of Mines; 2010. October 18-22. TagedP 90 Glass RE. The thermal and structural properties of a Hanna basin coal. J Energy Resour Technol 1984;106(2):266–71. TagedP 91 Tomeczek J, Palugniok H. Specific heat capacity and enthalpy of coal pyrolysis at elevated temperatures. Fuel 1996;75(9):1089–93. TagedP 92 Maloney DJ, Sampath R, Zondlo JW. Heat capacity and thermal conductivity considerations for coal particles during the early stages of rapid heating. Combust Flame 1999;116(1):94–104. TagedP 93 Merrick D. Mathematical models of the thermal decomposition of coal: 2. Specific heats and heats of reaction. Fuel. 1983;62(5):540–6. TagedP 94 Chilingarian GV, Robertson JO, Kumar S. Surface operations in petroleum production, II (Vol. 19) editors. Amsterdam: Elsevier; 1989. p. 110. TagedP 95 Dyni JR. Geology and resources of some world oil-shale deposits. Report 2005
5294, p. 15 U.S. Geological Survey Scientific Investigations. TagedP 96 Thomas L. Coal geology. West Sussex, England: John Wiley & Sons; 2002. p. 186. TagedP 97 Qian J, Wang J. World oil shale retorting technologies. Int. conf. on oil shale: recent trends in oil shale; 2006. p. 7–9. TagedP 98 Crawford P, Biglarbigi K, Dammer A, Knaus E. Advances in world oil-shale production technologies. SPE annual technical conference and exhibition, society of petroleum engineers. SPE, 116570; 2008. p. 1–11. TagedP 99 Liu DX, Wang HY, Zheng DW, Fang CH, Ge ZX. World progress of oil shale in-situ exploitation methods. Nat Gas Ind (China) 2009;29(5):128–32. TagedP100 Crawford PM, Killen JC. New challenges and directions in oil shale development technologies. ACS symposium series. Oxford University Press; 2010. (Vol. 1032, pp. 21-60). TagedP101 Crawford PM, Stone J. Secure fuels from domestic resources: profiles of companies engaged in domestic oil shale and tar sands resource and technology development. 5th ed. Prepared by INTEK, inc. for the U.S. department of energy, office of petroleum reserves; 2011. TagedP102 Mulchandani H, Brandt AR. Oil shale as an energy resource in a CO2 constrained world: the concept of electricity production with in situ carbon capture. Energy Fuels 2011;25(4):1633–41 Mar 11. TagedP103 Bolonkin A, Friedlander J, Neumann S, Group SS. Innovative unconventional oil extraction technologies. Fuel Process Technol 2014;124:228–42. TagedP104 U.S. Office of Technology Assessment. An assessment of oil shale technologies. Washington, DC: United States Government Printing Office; 1980. TagedP105 Agarwal AK. Assessment of solid-waste characteristics and control technology for oil-shale retorting. Final report for September 1983-February 1985 (No. PB86-198371/XAB). Monsanto Co: Dayton, OH (USA) May 1; 1986. TagedP106 Hulsebos J, Pohani BP, Moore RE. Zahradnik RL Modified-in-situ technology combined with aboveground retorting and circulating fluid bed combustors could

30

TagedP107 TagedP108 TagedP109 TagedP110 TagedP111 TagedP112 TagedP113 TagedP114 TagedP115

TagedP116 TagedP117 TagedP118 TagedP119 TagedP120 TagedP121 TagedP122 TagedP123 TagedP124

TagedP125 TagedP126

TagedP127 TagedP128 TagedP129 TagedP130 TagedP131 TagedP132 TagedP133 TagedP134 TagedP135 TagedP136 TagedP137 TagedP138

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32 TagedPoffer a viable method to unlock oil shale reserves in the near future. 21st Oil Shale Symposium. Golden: Colorado: Colorado School of Mines; 1988. p. 440–7. Lee S, Speight JG, Loyalka SK. Handbook of alternative fuel technologies. Boca Raton, FL: CRC Press; 2007. p. 286–90. Burnham AK. Chemical kinetics and oil shale process design. In: Snape C, editor. Composition, geochemistry and conversion of oil shales. Akcay, Turkey: Springer Science & Business Media; 1995. p. 263–76. Lewis AE, Rothman AJ. Rubble in situ extraction (RISE): a proposed program for recovery of oil from oil shale. Lawrence Livermore National Laboratory UCRL51768; 1975. Rothman AJ. Research and development on rubble in-situ extraction of oil shale (RISE) at Lawrence Livermore Laboratory. 8th Oil Shale Symposium. Golden, Colorado: Colorado School of Mines; 1975. Lekas MA. Progress report on the geokinetics horizontal retort process. 12th oil shale symposium; 1979. p. 229–36. Colorado Division of Reclamation and Mining Safety. Department of natural resources. Designated mining operation reclamation permit application for the Shell Frontier Oil and Gas, Inc. Oil shale test project, Denver, CO; 2007. Nair V, Ryan R, Roes G. Shell ICP-Shale oil refining. 26th oil shale symposium. Golden, Colorado: Colorado School of Mines; 2006. Fowler TD, Vinegar HJ. Oil shale ICP - Colorado field pilots. SPE western regional meeting, San Jose, California, 24-26 March. SPE: Society of Petroleum Engineers; 2009. p. 1–15. doi: 10.2118/121164-MS. Burnham AK, Day RL, Hardy MP, Wallman PH. AMSO's novel approach to in-situ oil shale recovery. In oil shale: a solution to the liquid fuel dilemma. In: Ogunsola OI, Hartstein AM, Ogunsola O, editors. ACS Symposium Series. Washington, DC: American Chemical Society; 2010. p. 149–60. Speight JG. Synthetic fuels handbook: properties, process, and performance. New York: McGraw-Hill; 2008, p. 13; 182; 186. Bolonkin A, Friedlander J, Neumann S. Innovative unconventional oil extraction technologies. Fuel Process Technol 2014;124:228–42. Savage MT. Geothermic fuel cells. 26th oil shale symposium. Golden, Colorado: Colorado School of Mines; 2006. Choudhury A, Chandra H, Arora A. Application of solid oxide fuel cell technology for power generation-A review. Renew Sustain Energy Rev 2013;20:430–42. Hazra KG, Lee KJ, Economides E, Moridis GJ. Comparison of heating methods for in-situ oil shale extraction. 17th European symposium on improved oil recovery. St Petersburg: Russia; 2013. p. 16–8. doi: 10.3997/2214-4609.20142631. Shurtleff K, Doyle D. Single well, single gas phase technique is key to unique method of extracting oil vapors from oil shale. World Oil 2008;229(3). Calderon A, Laubis TJ, inventors. Method for recovering energy in-situ from underground resources and upgrading such energy resources above ground. United States patent US 8,002,033. 2011 Aug 23. Kinzer D. Past, present, and pending intellectual property for electromagnetic heating of oil Shale. 28th oil shale symposium. Golden, Colorado: Colorado School of Mines; 2008. p. 13–5. Carlson RD, Blase EF, McLendon TR. Development of the IIT research institute RF heating process for in situ oil shale/tar sand fuel extraction
an overview. 14th oil shale symposium proceedings. Golden, Colorado: Colorado School of Mines; 1981. p. 138–45. Mutyala S, Fairbridge C, Pare JR, et al. Microwave applications to oil sands and petroleum: a review. Fuel Process Technol 2010;91(2):127–35. Considine BC, Cogliandro JA, Cogliandro MP, Moses JM, Hannon JR, Markiewicz JP, inventors; Schlumberger Technology Corporation, assignee. Method for extraction of hydrocarbon fuels or contaminants using electrical energy and critical fluids. United States patent US 7,461,693. 2008 Dec 9. Symington WA, Kaminsky RD, Meurer WP, Otten GA, Thomas MM, Yeakel JD. ExxonMobil's ElectrofracTM process for in situ oil shale conversion. ACS symposium series. Oxford University Press; 2010. p. 185–216. Bourgoyne AT, Millheim KK, Chenevert ME, Young FS. Applied drilling engineering. Richardson, TX: Society of Petroleum Engineers; 1986. Gentzis T. Stability analysis of a horizontal coalbed methane well in the rocky mountain front ranges of southeast British Columbia, Canada. Int J Coal Geol 2009;77(3):328–37. Gentzis T, Deisman N, Chalaturnyk RJ. Effect of drilling fluids on coal permeability: impact on horizontal wellbore stability. Int J Coal Geol 2009;78(3):177–91. Gentzis T, Deisman N, Chalaturnyk RJ. A method to predict geomechanical properties and model well stability in horizontal boreholes. Int J Coal Geol 2009;78 (2):149–60. Han JZ, Sang SX, Cheng ZZ, Huang HZ. Exploitation technology of pressure relief coalbed methane in vertical surface wells in the Huainan coal mining area. Min Sci Technol (China) 2009;19(1):25–30. Shen RC, Qu P, Yang HL. Advancement and development of coal bed wellbore stability technology. Petrol Dril Tech (China) 2010;20(3):1–7. Nie RS, Meng YF, Guo JC, Jia YL. Modeling transient flow behavior of a horizontal well in a coal seam. Int J Coal Geol 2012;92:54–68. Mao Z, Zhao B, Li S. Research on wellbore quality control technology for coalbed methane deviated wells. Procedia Eng 2014;73:237–42. Montgomery CT, Smith MB. Hydraulic fracturing: history of an enduring technology. J Petrol Technol 2010;62(12):26–40. King GE. Hydraulic fracturing 101. Hydraulic fracturing technology conference. Society of Petroleum Engineers, SPE; 2012. nez-Martínez J, Viswanathan HS, Carey JW, Porter ML, Rougier E, Hyman JD, Jime Karra S, Kang Q, Frash L, Chen L, Lei Z. Understanding hydraulic fracturing: a multi-scale problem. Phil Trans R Soc A 2016;374(2078):20150426.

TagedP139 Adams TF, Schmidt SC, Carter WJ. Permeability enhancement using explosive techniques. J Energy Resour Technol 1981;103(2):110–8. TagedP140 Shan G. A comprehensive model for coal devolatilisation PhD thesis. Australia: Department of Chemical Engineering, University of Newcastle; 2000. p. 5–55. TagedP141 Smith PJ, Deo M, Eddings EG, Hradisky M, Kelly KE, Krumm R, Sarofim AF, Wang D. Underground coal thermal treatment. Utah Clean Coal Program, submitted to U.S. Department of Energy Task 6 Topical Report.http://www.osti.gov/scitech/ servlets/purl/1176872; 2015 [accessed 16.10.16; January 2015. TagedP142 Khan MR, Hshieh FY. Influence of steam on coal devolatilization and on the reactivity of the resulting Char, Preprints of Papers - American chemical society. Division of Fuel Chemistry 1989;34(4):1245–55. TagedP143 Tyler RJ. Flash pyrolysis of coals. Devolatilization of bituminous coals in a small fluidized-bed reactor. Fuel. 1980;59(4):218–26. TagedP144 Angelova GK, Minkova VN, Goranova MD. Thermal degradation of solid fuels at atmospheric pressure in the presence of steam. Izvestiya po Khimiya 1981;14:251–7. TagedP145 Angelova GK, Minkova VN. Pyrolysis of solid fuels in a stream of water vapor: Bulgarian coals and shales. Khimiya Tverdogo Topliva (Moscow, Russian Federation) 1986;3:94–9. TagedP146 Minkova V, Razvigorova M, Goranova M, Ljutzkanov L, Angelova G. Effect of water vapour on the pyrolysis of solid fuels: 1. Effect of water vapour during the pyrolysis of solid fuels on the yield and composition of the liquid products. Fuel. 1991;70(6):713–9. TagedP147 Yardim MF, Ekinci E, Minkova V, Razvigorova M, Budinova T, Petrov N, Goranova M. Formation of porous structure of semicokes from pyrolysis of Turkish coals in different atmospheres. Fuel 2003;82(4):459–63. TagedP148 Sharma DK, Sulimma A, van Heek KH. Hydropyrolysis of coal in the presence of steam. Fuel 1986;65(11):1571–4. TagedP149 Sharma DK, Sulimma A, van Heek KH. Comparative studies of pyrolysis of coal in inert gas, steam, and hydrogen under pressure, Erdoel Kohle, Erdgas, Petrochem, 1986; 39(4), 173. s R, Furfari S. Low-temperature hydropyrolysis of coal under pressure of TagedP150 Cypre H2-CH4 mixtures. Fuel 1982;61(8):721–4. TagedP151 Liao H, Li B, Zhang B. Co-pyrolysis of coal with hydrogen-rich gases. 1. Coal pyrolysis under coke-oven gas and synthesis gas. Fuel. 1998;77(8):847–51. TagedP152 Wu DM, Harrison DP. Volatile products from lignite pyrolysis and hydropyrolysis. Fuel 1986;65(6):747–51. TagedP153 Avid B, Purevsuren B, Born M, Dugarjav J, Davaajav Y, Tuvshinjargal A. Pyrolysis and TG analysis of Shivee Ovoo coal from Mongolia. J Therm Anal Calorim 2002;68(3):877–85. TagedP154 Graff RA, Brandes SD. Coal liquefaction by steam pyrolysis. Papers-American Chemical Society; 1984. p. 104–11. TagedP155 Xie W, Stanger R, Wall TF, Lucas JA, Mahoney MR. Associations of physical, chemical with thermal changes during coking as coal heats
Experiments on coal maceral concentrates. Fuel 2015;147:1–8. TagedP156 Bar-Ziv E, Kantorovich II. Mutual effects of porosity and reactivity in char oxidation. Progr Energy Combust Sci 2001;27(6):667–97. TagedP157 Feng ZJ, Zhao YS, Wan ZJ. Experiment study of the thermal deformation of in-situ gas coal editor. In: Cai M, editor. Rock mechanics: achievements and ambitions. Boca Raton, FL: CRC Press; 2012. p. 103–8. TagedP158 Zhao Y, Qu F, Wan Z, Zhang Y, Liang W, Meng Q. Experimental investigation on correlation between permeability variation and pore structure during coal pyrolysis. Transp Porous Media 2010;82(2):401–12. TagedP159 Yu Y, Liang W, Hu Y, Meng Q. Study of micro-pores development in lean coal with temperature. Int J Rock Mech Min Sci 2012;51:91–6. € Karacan, Mitchell GD. Behavior and effect of different coal microlithotypes TagedP160 CO during gas transport for carbon dioxide sequestration into coal seams. Int J Coal Geol 2003;53(4):201–17. TagedP161 Hile ML. CO2 sorption by Pittsburgh-seam coal subjected to confining pressure Master Thesis. University Park: The Pennsylvania State University; 2006. TagedP162 Pone JD, Hile M, Halleck PM, Mathews JP. Three-dimensional carbon dioxideinduced strain distribution within a confined bituminous coal. Int J Coal Geol 2009;77(1):103–8. TagedP163 Pone JD, Halleck PM, Mathews JP. 3D characterization of coal strains induced by compression, carbon dioxide sorption, and desorption at in-situ stress conditions. Int J Coal Geol 2010;82(3):262–8. TagedP164 Mathews JP, Pone JD, Mitchell GD, Halleck P. High-resolution X-ray computed tomography observations of the thermal drying of lump-sized subbituminous coal. Fuel Process Technol 2011 Jan 31;92(1):58–64. TagedP165 Martinez GA, Davis LA. Petrophysical measurements on shales using NMR. In: The 2000 SPE/AAPG western regional meeting held in long beach, California, June 19
23, 2000; SPE 62851. TagedP166 Yao Y, Liu D, Che Y, Tang D, Tang S, Huang W. Petrophysical characterization of coals by low-field nuclear magnetic resonance (NMR). Fuel 2010;89(7):1371– 80. TagedP167 Ding Y, Guo B, Yan X, Li J, Lu Q. Validity of identification shale reservoirs and physical parameters of quantitative evaluation method. International conference & exhibition on reservoir surveillance and management, Xi'an, China; September 2013. p. 16–8. ICRSM 00191. TagedP168 Lewis R, Singer P, Jiang T, Rylander E, Sinclair S, Mclin RH. NMR T2 distributions in the Eagle Ford shale: reflections on pore size. SPE unconventional resources conference-USA. Society of Petroleum Engineers; 2013 Apr 10. TagedP169 Li S, Tang D, Pan Z, Xu H, Huang W. Characterization of the stress sensitivity of pores for different rank coals by nuclear magnetic resonance. Fuel 2013;111: 746–54.

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32 TagedP170 Cai Y, Liu D, Pan Z, Yao Y, Li J, Qiu Y. Petrophysical characterization of Chinese coal cores with heat treatment by nuclear magnetic resonance. Fuel 2013;108:292–302. TagedP171 Dunn KJ, Bergman DJ, LaTorraca GA, editors. Nuclear magnetic resonance: petrophysical and logging applications. The Netherlands: Elsevier; 2002. TagedP172 Mai A, Kantzas A. An evaluation of the application of low field NMR in the characterization of carbonate reservoir. Presentation at SPE annual technical conference and exhibition held in San Antonio, Texas, 9 September-2; October 2002. SPE 77401. TagedP173 Xu H, Tang D, Zhao J, Li S. A precise measurement method for shale porosity with low-field nuclear magnetic resonance: a case study of the Carboniferous
Permian strata in the Linxing area, eastern Ordos Basin, China. Fuel 2015;143:47–54. TagedP174 Yao Y, Liu D. Comparison of low-field NMR and mercury intrusion porosimetry in characterizing pore size distributions of coals. Fuel 2012;95:152–8. TagedP175 Palmer I. Permeability changes in coal: analytical modeling. Int J Coal Geol 2009;77(1):119–26. TagedP176 Ma Q, Harpalani S, Liu S. A simplified permeability model for coalbed methane reservoirs based on matchstick strain and constant volume theory. Int J Coal Geol 2011;85(1):43–8. TagedP177 Peng Y, Liu J, Wei M, Pan Z, Connell LD. Why coal permeability changes under free swellings: new insights. Int J Coal Geol 2014;133:35–46. TagedP178 Peng Y, Liu J, Zhu W, Pan Z, Connell L. Benchmark assessment of coal permeability models on the accuracy of permeability prediction. Fuel 2014;132:194–203. TagedP179 van Golf-Racht TD. Fundamentals of fractured reservoir engineering. Dev Petrol Sci 1982;12 Amsterdam: Elsevier. TagedP180 Bai M, Elsworth D. Coupled processes in subsurface deformation, flow, and transport. Reston, VA: American Society of Civil Engineers Press; 2000. TagedP181 Reiss LH. The reservoir engineering aspects of fractured formations. Houston: Gulf Publishing Co.; 1980. TagedP182 Laurendeau NM. Heterogeneous kinetics of coal char gasification and combustion. Progr Energy Combust Sci 1978;4(4):221–70. TagedP183 Russel W, Saville P, Greene M. Truncated tar yields from rapid pyrolysis of wyodak particles. AIChE J 1979;58:231–43. TagedP184 Klinkenberg LJ. The permeability of porous media to liquid and gases. In: Paper presented at the API 11th midyear meeting, Tulsa, Oklahoma, API Drilling and Production Practice, May 1941, p. 200
13. TagedP185 Harpalani S, Chen G. Influence of gas production induced volumetric strain on permeability of coal. Geotech Geol Eng 1997;15(4):303–25. TagedP186 Harpalani S, Prusty BK, Dutta P. Methane/CO2 sorption modeling for coalbed methane production and CO2 sequestration. Energy Fuels 2006;20(4):1591–9. TagedP187 Gilman A, Beckie R. Flow of coal-bed methane to a gallery. Transp Porous Media 2000 Oct 1;41(1):1–6. TagedP188 Zhu WC, Liu J, Sheng JC, Elsworth D. Analysis of coupled gas flow and deformation process with desorption and Klinkenberg effects in coal seams. Int J Rock Mech Min Sci 2007;44(7):971–80. TagedP189 Javadpour F, Fisher D, Unsworth M. Nanoscale gas flow in shale gas sediments. J Canad Petrol Technol 2007;46(10). TagedP190 Javadpour F. Nanopores and apparent permeability of gas flow in mudrocks (shales and siltstone). J Canad Petrol Technol 2009;48(08):16–21. TagedP191 Wang FP, Reed RM. Pore networks and fluid flow in gas shales. SPE annual technical conference and exhibition. Society of Petroleum Engineers; 2009 Jan 1. TagedP192 Guo C, Bai B, Wei M, He X, Wu YS. Study on gas flow in nano pores of shale gas reservoir. Unconventional resources conference Canada. Society of Petroleum Engineers; 2013 Nov 5. TagedP193 Guo C, Xu J, Wu K, Wei M, Liu S. Study on gas flow through nano pores of shale gas reservoirs. Fuel 2015;143:107–17. TagedP194 Song H, Yu M, Zhu W, Wu P, Lou Y, Wang Y, Killough J. Numerical investigation of gas flow rate in shale gas reservoirs with nanoporous media. Int J Heat Mass Transfer 2015;80:626–35. TagedP195 Wang G, Ren T, Wang K, Zhou A. Improved apparent permeability models of gas flow in coal with Klinkenberg effect. Fuel 2014;128:53–61. TagedP196 Kantorovich II, Bar-Ziv E. Heat transfer within highly porous chars: a review. Fuel 1999;78(3):279–99. TagedP197 Zhang X, Bar-Ziv E. A novel approach to determine thermal conductivity of micron-sized fuel particles. Combust Sci Technol 1997;130(1-6):79–95. TagedP198 Zhang X, Dukhan A, Kantorovich II, Bar-Ziv E. The thermal conductivity and porous structure of char particles. Combust Flame 1998;113(4):519–31. ^ te  J, Konrad JM. Thermal conductivity of base-course materials. Canad Geotech TagedP199 Co J 2005;42(1):61–78. TagedP200 Tihen SS, Carpenter HC, Sohns HW. Thermal conductivity and thermal diffusivity of green river oil shale. Natl. Bur. Stand.(US), Spec. Publ. p. 302. (United States); 1968. TagedP201 Rajeshwar K, DuBow JB, Rosenvold RJ. Dependence of thermal conductivity on organic content for green river oil shales. Ind Eng Chem Prod Res Dev 1980;19 (4):629–32. TagedP202 Campbell JH. Pyrolysis of subbituminous coal in relation to in-situ coal gasification. Fuel 1978;57(4):217–24. TagedP203 Berchenko IE, De Rouffignac EP, Fowler TD, Karanikas JM, Ryan RC, Shahin Jr GT, Stegemeier GL, Vinegar HJ, Wellington SL, Zhang E. inventors; Shell Oil Company, assignee. In situ thermal processing of an oil shale formation using a pattern of heat sources, 991; 2006 Jan 31. p. 032. TagedP204 Shih SM, Sohn HY. Nonisothermal determination of the intrinsic kinetics of oil generation from oil shale. Ind Eng Chem Process Des Dev 1980;19(3):420–6. TagedP205 Shurtleff K, Doyle D. Single well, single gas phase technique is key to unique method of extracting oil vapors from oil shale. World Oil 2008;229(3)

TagedP206 TagedP207 TagedP208 TagedP209 TagedP210 TagedP211 TagedP212 TagedP213 TagedP214

TagedP215 TagedP216 TagedP217 TagedP218 TagedP219 TagedP220 TagedP221 TagedP222 TagedP223 TagedP224 TagedP225 TagedP226

TagedP227 TagedP228 TagedP229 TagedP230 TagedP231 TagedP232 TagedP233 TagedP234 TagedP235

31

TagedP ttp://www.worldoil.com/magazine/2008/march-2008/features/single-wellh single-gas-phase-technique-is-key-to-unique-method-of-extracting-oil-vaporsfrom-oil-shale; [accessed 16.10.16]. €o € k M, Aleklett K. A review on coal-to-liquid fuels and its coal consumption. Int Ho J Energy Res 2010;34(10):848–64. U.S. Energy Information Administration. Levelized cost and levelized avoided cost of new generation resources in the Annual Energy Outlook 2016. Report Number: DOE/EIA-0383ER(2016). Salatino P, Ammendola P, Bareschino P, Chirone R, Solimene R. Improving the thermal performance of fluidized beds for concentrated solar power and thermal energy storage. Powder Technol 2016;290:97–101. Whitty KJ, Zhang HR, Eddings EG. Emissions from syngas combustion. Combust Sci Technol 2008;180(6):1117–36. Zhang HR, Eddings EG, Sarofim AF. A review of coal devolatilization with very low heating rates. CD-ROM, proc. american institute of chemical engineers annual meeting, Salt Lake City, Utah; 2007. Jacobs HE, Jones JF, Eddinger RT. Hydrogenation of COED process coal-derived oils. Ind Eng Chem Process Des Dev 1971;10(4):558–62. Cleveland CJ, O'Connor PA. Energy return on investment (EROI) of oil shale. Sustainability 2011;3(11):2307–22. Brandt AR, Farrell AE. Scraping the bottom of the barrel: greenhouse gas emission consequences of a transition to low-quality and synthetic petroleum resources.. Clim Change 2007;84(3
4):241–63. Brandt AR, Boak J, Burnham AK. Carbon dioxide emissions from oil shale derived liquid fuels. In: Ogunsola OI, Hartstein AM, Ogunsola O, editors. Oil shale: a solution to the liquid fuel dilemma. Washington, DC: ACS Symposium Series; American Chemical Society; 2010. p. 219–48. 1032(11). Cleveland CJ, Costanza R, Hall CA, Kaufman R. Energy and the US economy: a biophysical perspective. Science 1984;225:890–8. Bowsher CA. DOE funds new energy technologies without estimating potential net-energy yields. Washington, DC (USA): General Accounting Office Office of the Comptroller General; 1982 Jul 26. Wellington SL, Vinegar HJ, Berchenko II, Maher KA, deRouffignac E, Karanikas JM, et al. Emissionless energy recovery from coal. Patent US Provisional Application (60/199,213); 2001. Merrow EW, Chapel SW, Worthing C. Review of cost estimation in new technologies: implications for energy process plants. Santa Monica, CA (USA): RAND Corp.; 1979 Jul 1. Cortez DH, Humphries JJ. In: Process for the gasification of coal, 5. UK Patent Application GB 2,068,014 A. Jagiello J, Thommes M. Comparison of DFT characterization methods based on N2, Ar, CO2, and H2 adsorption applied to carbons with various pore size distributions. Carbon. 2004;42(7):1227
32. Imran M, Kumar D, Kumar N, Qayyum A, Saeed A, Bhatti MS. Environmental concerns of underground coal gasification. Renew Sustain Energy Rev 2014;31:600– 10.  czyk K, Wiatowski M, Checko J. Environmental aspects of a fieldKapusta K, Stan scale underground coal gasification trial in a shallow coal seam at the Experimental Mine Barbara in Poland. Fuel 2013;113:196–208. Gregory KB, Vidic RD, Dzombak DA. Water management challenges associated with the production of shale gas by hydraulic fracturing. Elements 2011;7 (3):181–6. Osborn SG, Vengosh A, Warner NR, Jackson RB. Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing. Proc Natl Acad Sci 2011;108(20):8172–6. Vengosh A, Jackson RB, Warner N, Darrah TH, Kondash A. A critical review of the risks to water resources from unconventional shale gas development and hydraulic fracturing in the United States. Env Sci Technol 2014;48(15):8334–48. Enick RM, Olsen DK, Ammer JR, Schuller W. Mobility and conformance control for CO2 EOR via thickeners, foams, and gels
A literature review of 40 years of research and pilot tests. SPE improved oil recovery symposium. Jan 1. Society of Petroleum Engineers; 2012. Wang M. Greenhouse gases, regulated Emissions, and energy use in transportation model. Argonne National Lab; 2016. US Environmental Protection Agency. Renewable fuel lifecycle greenhouse gas (GHG) emissions results spreadsheets. Docket ID: EPA-HQ-OAR-2005-01610938; 2005. Mangmeechai A, Jaramillo P, Griffin WM, Matthews HS. Life cycle consumptive water use for oil shale development and implications for water supply in the Colorado River Basin. Int J Life Cycle Assess 2014;19(3):677–87. Brandt AR. Converting oil shale to liquid fuels with the Alberta Taciuk processor: energy inputs and greenhouse gas emissions. Energy Fuels 2009;23(12):6253–8. Jaramillo P, Samaras C, Wakeley H, Meisterling K. Greenhouse gas implications of using coal for transportation: life cycle assessment of coal-to-liquids, plug-in hybrids, and hydrogen pathways. Energy Pol 2009;37(7):2689–95. Stuermer DH, Ng DJ, Morris CJ. Organic contaminants in groundwater near an underground coal gasification site in northeastern Wyoming. Env Sci Technol 1982;16(9):582–7. Thorsness CB, Creighton JR. Review of underground coal gasification field experiments at Hoe Creek. ACS symp. ser. United StatesLivermore, CA: Lawrence Livermore National Laboratory; 1983. Elliott EG, Ettinger AS, Leaderer BP, Bracken MB, Deziel NC. A systematic evaluation of chemicals in hydraulic-fracturing fluids and wastewater for reproductive and developmental toxicity. J Expos Sci Env Epidemiol 2016 Jan 6. Ward H, Eykelbosh A, Nicol AM. Addressing uncertainty in public health risks due to hydraulic fracturing. Env Health Rev 2016;59(2):57–61.

32

H.R. Zhang et al. / Progress in Energy and Combustion Science 62 (2017) 1
32

TagedP236 DTI. Cleaner technology programme progress report. DTI/Pub Contract No.: URN 01/937; 2001. TagedP237 Stevens SH, Spector D, Riemer P. Enhanced coalbed methane recovery using CO2 injection: worldwide resource and CO2 sequestration potential. IOGCEC: international oil & gas conference and exhibition in China; 1998. TagedP238 Gale J, Freund P. Coal-bed methane enhancement with CO2 sequestration worldwide potential. Env Geosci 2001;8(3):210–7. TagedP239 Liu J, Jiang X, Shen J, Zhang H. Pyrolysis of superfine pulverized coal. Part 3. Mechanisms of nitrogen-containing species formation. Energy Convers Manage 2015;94:130–8. TagedP240 He MC, Wang CG, Feng JL, Li DJ, Zhang GY. Experimental investigations on gas desorption and transport in stressed coal under isothermal conditions. Int J Coal Geol 2010;83(4):377–86. TagedP241 Su F, Nakanowataru T, Itakura KI, Ohga K, Deguchi G. Evaluation of structural changes in the coal specimen heating process and UCG model experiments for developing efficient UCG systems. Energies 2013;6(5):2386–406. TagedP242 Shoemaker HD, Shuck LZ, Haynes RR, Advani SH. The mechanical properties of the Pittsburgh coal at elevated temperatures. J Press Vess Technol 1977;99(1):192–8. TagedP243 Cui X, Bustin RM. Volumetric strain associated with methane desorption and its impact on coalbed gas production from deep coal seams. Aapg Bull 2005;89 (9):1181–202. TagedP244 Connell LD, Lu M, Pan Z. An analytical coal permeability model for tri-axial strain and stress conditions. Int J Coal Geol 2010;84(2):103–14. TagedP245 Fu Z, Guo Z, Yuan Z, Wang Z. Swelling and shrinkage behavior of raw and processed coals during pyrolysis. Fuel 2007;86(3):418–25. TagedP246 He MC, Wang CG, Feng JL, Li DJ, Zhang GY. Experimental investigations on gas desorption and transport in stressed coal under isothermal conditions. Int J Coal Geol 2010;83(4):377–86.

TagedP247 Westmoreland PR, Dickerson LS. Pyrolysis of blocks of Texas lignite. Oak Ridge, TN (United States): Oak Ridge National Laboratory DOE Report No. CONF790405
17; 1979. TagedP248 Sommariva S, Maffei T, Migliavacca G, Faravelli T, Ranzi E. A predictive multi-step kinetic model of coal devolatilization. Fuel 2010;89(2):318–28. TagedP249 Zeng D, Fletcher TH. Effects of pressure on coal pyrolysis and char morphology. Energy fuels 2005;19(5):1828–38. TagedP250 Smith JD, Smith PJ, Hill SC. Parametric sensitivity study of a CFD-based coal combustion model. AIChE J 1993;39(10):1668–79. TagedP251 Brewster BS, Smoot LD, Barthelson SH, Thornock DE. Model comparisons with drop tube combustion data for various devolatilization submodels. Energy Fuels 1995 Sep;9(5):870–9. TagedP252 Bradley LC, Miller SF, Miller BG, Tillman DA. A study on the relationship between fuel composition and pyrolysis kinetics. Energy Fuels 2011 Apr 19;25(5):1989– 95. € nnenbeck C, Leyssens G, Brilhac JF, Porcheron L. TagedP253 Authier O, Thunin E, Plion P, Scho Kinetic study of pulverized coal devolatilization for boiler CFD modeling. Fuel 2014;15:254–60 122. TagedP254 Sommariva S, Maffei T, Migliavacca G, Faravelli T, Ranzi E. A predictive multi-step kinetic model of coal devolatilization. Fuel 2010;89(2):318–28. TagedP255 Cloke M, Lester E, Leney M. Effect of volatile retention on the products from low temperature pyrolysis in a fixed bed batch reactor. Fuel 1999 Nov 30;78(14): 1719–28. TagedP256 Westmoreland PR, Dickerson LS. Review of supporting research at Oak Ridge National Laboratory for underground coal conversion. TN (USA): Oak Ridge National Laboratory (DOE Report No. CONF-790630-9); 1979. TagedP257 Hada S, Yuri M, Masada J, Ito E, Tsukagoshi K. Evolution and future trend of large frame gas turbines: a new 1600 °C, J class gas turbine. In: Proceedings of ASME turbo expo 2012, Copenhagen, Denmark; 2012. p. 599–606.