Underground coal gasification: From fundamentals to applications

Progress in Energy and Combustion Science 39 (2013) 189e214

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

Review

Underground coal gasification: From fundamentals to applications Abdul Waheed Bhutto a, Aqeel Ahmed Bazmi b, c, Gholamreza Zahedi b, * a

Department of Chemical Engineering, Dawood College of Engineering & Technology, Karachi, Pakistan Process Systems Engineering Centre (PROSPECT), Chemical Engineering Department, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, Skudai 81310, Johor Bahru (JB), Malaysia c Biomass Conversion Research Centre (BCRC), Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 April 2012 Accepted 10 September 2012 Available online 22 October 2012

Underground coal gasification (UCG) is a promising option for the future use of un-worked coal. UCG permits coal to be gasified in situ within the coal seam, via a matrix of wells. The coal is ignited and air is injected underground to sustain a fire, which is essentially used to “mine” the coal and produce a combustible synthetic gas which can be used for industrial heating, power generation or the manufacture of hydrogen, synthetic natural gas or diesel fuel. As compared with conventional mining and surface gasification, UCG promises lower capital/operating costs and also has other advantages, such as no human labor underground. In addition, UCG has the potential to be linked with carbon capture and sequestration. The increasing demand for energy, depletion of oil, and gas resources, and threat of global climate change have lead to growing interest in UCG throughout the world. The potential for UCG to access low grade, inaccessible coal resources and convert them commercially and competitively into syngas is enormous, with potential applications in power, fuel, and chemical production. This article reviews the literature on UCG and research contributions are reported UCG with main emphasis given to the chemical and physical characteristic of feedstock, process chemistry, gasifier designs, and operating conditions. This is done to provide a general background and allow the reader to understand the influence of operating variables on UCG. Thermodynamic studies of UCG with emphasis on gasifier operation optimization based on thermodynamics, biomass gasification reaction engineering and particularly recently developed kinetic models, advantages and the technical challenges for UCG, and finally, the future prospects for UCG technology are also reviewed. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Underground coal gasification UCG kinetics Gasifier operation Post-burn coal processing Coal drilling

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .190 Underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193 2.1. UCG for synthetic fuel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 2.2. Process overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 2.2.1. Chemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 2.2.2. Physical process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 2.2.3. Effect of coal reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 2.2.4. Gasifying agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 2.2.5. Effect of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 2.2.6. Effect of heat loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 2.2.7. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 2.2.8. Cavity growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 2.2.9. Gas diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 2.2.10. Velocity of combustion front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 2.2.11. Compositions of syngas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 2.2.12. Optimization of UGC operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

* Corresponding author. Tel.: þ60 7 553583; fax: þ60 7 5566177. E-mail addresses: [email protected], [email protected] (G. Zahedi). 0360-1285/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pecs.2012.09.004

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Thermodynamics of UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201 3.1. Thermodynamic equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 3.2. Carbon-oxygen steam equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 3.3. Cold gas efficiency (hcg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Kinetic studies of UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 4.1. Single First Order Reaction model (SFORM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 4.2. Distributed activation energy model (DAEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 4.3. Reactions of formation of selected gas products in coal pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 4.4. Order of reaction and activation energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 4.5. Rate controlling step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 4.6. Chemical reaction rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Challenges for UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 5.1. Suitable site selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 5.2. Technical challenges for UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 5.2.1. The major issues in the use of UCG technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 5.2.2. Exploration of the UCG site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 5.2.3. Choice of a suitable drilling technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 5.2.4. Environment and safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 Advantages of underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 6.1. Advantages of underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 6.2. UGC challenge and promises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 6.3. UCG-CCS concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 6.4. Post-burn processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 6.5. Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .212 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

1. Introduction Coal, a fossil fuel created from the remains of plants that lived and died about 100e400 million years ago when parts of the Earth were covered with huge swampy forests is classified as a nonrenewable energy source because it takes millions of years to form. Coal has been used as a source of energy for nearly 3000 years. Although mined in Europe as early as the 13th century, it was not a highly desirable fuel because of its toxic combustion products. Coal did not become an important source of fuel until the beginning of the Industrial Revolution about 300 years later. The coal industry’s largest environmental challenge is removing organic sulfur, a substance that is chemically bound to coal. Traditional methods of burning coal produce emissions that can reduce air and water quality. Clean coal technologies remove sulfur and nitrogen oxides before, during, and after coal is burned, or convert coal to a gas or liquid fuel. Fluidized Bed Combustion is a clean coal technology which keeps both sulfur and nitrogen oxides in check. Coal Gasification is another clean coal technology bypasses the conventional coal burning process altogether by converting coal into a gas. This method removes sulfur, nitrogen compounds and particulates, before the fuel is burned, making it as clean as natural gas. Coal was first used in gas production during the late 18th century. Early production was used primarily for lighting, but as gasification techniques improved, applications grew wider. By the 19th century the conversion of coal to gas was a wellestablished commercial process. Globally, coal will still remain an indispensable source of chemical feedstock and energy for a long period of time. New and improved processes for its efficient and environmentally acceptable use will be a steady challenge for coming generations of coal scientists and for society to support the research required [1]. World energy policy is gripped by a fallacy d the idea that coal is destined to stay cheap for decades to come. This assumption supports investment in ‘clean-coal’ technology and trumps serious efforts to increase energy conservation and develop alternative energy sources. Underground coal gasification (UCG) is a promising

option for the future use of un-worked coal. UCG d may eventually make marginal coal reserves accessible, but it will take time and substantial investment to be commercialized on a large-scale [2]. Most current technologies of coal gasification such as entrained flow, fluidized bed, and moving bed use a surface reactor for gasification. The main differences between these technologies relate to the gas flow configuration, coal particle size, ash handling, and process conditions [3]. An alternative for surface gasifier is an underground coal gasifier. UCG is a is a combination of mining, exploitation and gasification that eliminates the need for mining and can be used in deep or steeply dipping, unmineable coal seam .UCG is an in situ technique to recover the fuel or feedstock value of coal that is not economically available through conventional recovery technologies. It has been regarded to be an important way to utilize low-rank and unmineable coals. The international experiences in the modeling and the experimental tests of underground coal gasification (UCG) show that UCG process offers an attractive option of utilizing unmineable coal [4e21]. Probably the strongest appeal of underground coal gasification at present is its potential value in exploiting marginal coal reserves that otherwise would remain unrecoverable [22]. Coal reserves significantly exceed those of oil and gas. World’s coal distribution on land is shown in Fig. 1. When coal resource totals is considered (including coal which it is uneconomic to mine), it dominates the fossil fuel picture. Estimates of total world coal resource (including unmineable coal) are usually stated in trillions of tons rather than billions. Recent estimates of the total remaining coal resource in the world quote a figure of 18 trillion tons [23]. Today, less than one sixth of the world’s coal is economically accessible. The chances of countries around the world choosing not to use this coal resource are very low indeed but unless cleaner and cheaper ways can be found to convert coal to gas or liquid fuels, coal is unlikely to become an acceptable replacement for dwindling and uncertain supplies of oil and natural gas. Underground coal gasification (UCG), taken on its own, offers the prospect of increasing the world’s usable coal reserves by a factor of at least three. Fortunately,

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Fig. 1. World’s coal distribution black areas on land: Map excludes Antarctica, which contains large coal deposits but is not usable by international convention [24].

potential sites for UCG operations correspond to locations where sites are plentiful for sequestering CO2 in geologic formations underground. UCG also enhances the storage capacity of the coal seam itself to store injected CO2. The generated gas, called syngas, would be taken from the ground and the by-products separated out. The CO2 would then be returned downhole nearby. The purpose of underground gasification of coal, regardless of method used, is to obtain the energy contained in the fuel for use on the surface, without mining in the usual sense of the term.

Underground gasification can be described as (1) a process where coal, in place, is consumed by partial combustion with air, oxygen, steam, or any combination of these to produce a low calorific value gas (80e300 Btu per cu ft) or (2) a complete combustion process in which air is used to produce a gas containing carbon dioxide, nitrogen, and considerable thermal energy [25]. UCG also lowers the capital investment by eliminating the need for specialized coal processing (transporting and stocking) and gasification reactors. UCG has other advantages such as increased

Fig. 2. Current world-wide status of UCG technology: Map shows underground coal gasification (UCG) sites worldwide, including planned sites and prior pilot test sites, current international UCG activities overlaying CO2 storage potential areas. Gray areas show potential areas for geological carbon storage [23,31,32].

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Fig. 3. Potential development of UCG: Step 1: well drilling and link establishment. Step 2: coal seam ignition and commencement of gasification and step 3: site clean-up by flushing cavity with steam and water to remove potential contaminants [19].

work safety, no surface disposal of ash, low dust, and noise pollution. It can be operated at high pressure to increase the reaction intensity and improve the efficiency of the process. UCG is particularly advantageous for deep coal deposits and steeply dipping coal seams since at these conditions less gas leakages to the surroundings and high pressures favor methane formation The successful application of such a process would provide a low to medium BTU gas (100e300BTU/SCF), depending on whether air or an oxygene steam mixture is used [26]. Composition and heating value of the product gas depends on the thermodynamic conditions of the operation as well as on the composition and temperature of the gasifying agent employed. In order to avoid potential environmental concerns, the reactor cavity is operated at less than hydrostatic pressure, which brings water into the gasification reactor in situ. As such, successful UCG operation relies on the natural permeability of the coal seam to transmit gases to and from the combustion zone, or on enhanced permeability created through reversed combustion, an in-seam channel, or hydro-fracturing [27]. The first recorded proposal for UCG was by Siemens. Sir William Siemens, a German scientist, was credited with first suggesting underground coal gasification in 1868 [28], followed by

Mendeleyev 20 years later. No further work was done until the 1930s, when an experimental station was started in the Donetsk coalfield in the then Soviet Union, to be followed by commercial installations in 1940 [29]. John et al. [25] has given excellent bibliography of the literature on UCG between 1945 and 60. Underground gasification continued at a number of locations in the Soviet Union until the late 1970s, with production of some 25,000 million Nm3 of gas from around 6.6 million tons of coal. Some of the well-documented UCG operations are those at AngrenUzbekistan, Queensland-Australia, Alberta-Canada, Walanchabi City-China, Majuba-South Aferica. A commercial-scale UCG plant is still being operated in Angren, Uzbekistan, where gas of an average heating value of 3.1e3.5 kJ/m3 is produced in an air-blown gasification process. The UCG gas produced is fed into a power station which is situated adjacent to the Yerostigaz operation in Angren. Yerostigaz has produced this gas to generate power at the 400 MW power station at Angren. Operators drill wells to inject air or oxygen that drives combustion and gasification in situ, and to produce the coal gas to surface for further processing, transport, or utilization. The most advanced UCG operation is at Chinchilla in Queensland, Australia, where the operator claims to be generating electricity from UCG product gas at a highly competitive cost (1.5 US cents per kWh). In October 2008, Carbon Energy successfully produced syngas from its unique UCG module based on the parallel controlled retractable injection point (CRIP) method. The trial, which ran for 100 days, reached coal gasification rates of around 150 tons per day and produced a high-quality syngas. Since then, Carbon Energy has installed two more modules and constructed a 5-MW electric power plant to be fed with syngas from Module 2. Module 1 is being carefully decommissioned. Plans for scaling up to 25 MW of electricity generation are under way, and a second project in Queensland, known as the Blue Gum Energy Park, is also in the early stages of planning. Swan Hills Synfuels recently produced syngas from its pilot project in Alberta, Canada. This project is the deepest UCG pilot ever undertaken, at a depth of 1400 m, and is using the linear controlled retractable injection point method. The ENN Group Co. Ltd. (a subsidiary of the Xinao company) produced syngas from a pilot project in Walanchabi City, Inner Mongolia, China, for 26 months, gasifying more than 100,000 tons of coal. Although not much information has been made available about this project, it is known that there were initially seven injection and production wells, which were first fired in October 2007 using air. ENN is now in its fourth year of operation at the plant. The Majuba UCG project has been producing syngas since January 2007 and began delivering UCG syngas to cofire with coal at the Majuba Power Station in late 2010. The project contributes about 3 MW to the overall output of 650 MW from the electric power station using the linked vertical well method. This project is now the longest running UCG trial in the western world. Plans are in place to expand the facilities to 1200 MWe output, with 30% of the plant’s fuel provided by syngas. There have been over 50 UCG tests or pilot operations worldwide. Trials were carried out at depths in excess of 500 m by a European consortium (UK, Spain and Belgium) between 1992 and 1998 at Teruel in Spain. Table 1 summarizes the history of the UCG and Fig. 2 illustrates the current world-wide status of the technology. The potential for UCG to access low grade, inaccessible coal resources and convert them commercially and, competitively into syngas is enormous, with potential applications in power, fuel, and chemical production. UCG research and development have been conducted in several countries, including long-term commercial operation of several UCG plants in the former Soviet Union.

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Table 1 History of the UCG [30]. Test site

Country

Year

Coal type

Seam thickness (m)

Seam depth (m)

Dipa (degrees)

Coal gasified (t)

Syngas cv (mj/m3)

Lisichansk Lisichansk Gorlovka Podmoskova Bois-la-Dame Newman Spinney Yuzhno-Abinsk Angren Hanna 1 Hanna 2 Hoe Creek 1 Hanna 3 Hoe Creek 2A Hoe Creek 2B Hanna 4 Hoe Creek 3A Hoe Creek 3B Pricetown Rawlins 1A Rawlins 1B Rawlins 2 Brauy-en-Artois Thulin Centralia Tono A Centralia Tono B Haute-Duele Thulin Rocky Mountain 1A Rocky Mountain 1B El Tremedal

Russia Ukraine Russia Russia Belgium UK Russia Uzbekistan USA USA USA USA USA USA USA USA USA USA USA USA USA France Belgium USA USA France Belgium USA USA Spain

1934e36 1943e63 1935e41 1940e62 1948 1949e59 1955e89 1965enow 73e74 75e76 1976 1977 1977 1977 77e79 1979 1979 1979 1979 1979 1979 1981 1982e84 84e85 84e85 1985e86 1986e87 87e88 87e88 1997

Bit Bit N/A SBB A SBB Bit SBB HVC HVC HVC HVC HVC HVC HVC HVC HVC Bit SBB SBB SBB A SA SBB SBB A SA SBB SBB SBB

0.75 0.4 1.9 2 1 1 2-Sep 4 9.1 9.1 7.5 9.1 7.5 7.5 9.1 7.5 7.5 1.8 18 18 18 1200 860 6 6 2 6 7 7 2

24 400 40 40 N/A 75 138 110 120 84 100 84 100 100 100 100 100 270 105 105 130e180 N/A N/A 75 75 880 860 110 110 600

N/A 0 N/A 0 N/A N/A 60 N/A 0 0 0 0 0 0 0 0 0 0 63 63 63

N/A N/A N/A N/A N/A 180 2 mt Over 10 mt 3130 7580 112 2370 1820 60 4700 290 3190 350 1330 169 7760

3e4 3.2 6e10 6 with O2 N/A 2.6 9e12.1 3.6 5.3 3.6 4.1 3.4 9.0 4.1 3.9 6.9 6.1 5.6 8.1 11.8

14 14

190 390

9.7 8.4

0 0

157 11200 4440

9.5 8.8

HVC ¼ High Vol Bit, Bit ¼ Bituminous, SBB ¼ Sub Bituminous, SA ¼ Semi-anthracite, A ¼ Anthracite. a Dip is the maximum angle between the inclined plane and the horizontal plane. Dip is always perpendicular to strike, and has both a compass direction and an angle. Inclinometer is used to measure the amount of dip in degrees (a plane lying flat along the horizontal as zero dip).

Information on UCG technology, however, is limited and there is a lack of compact review articles in this area [33]. Most of the current available literature on UCG emphasizes on its geological implications, environmental concerns and numerical analysis, modeling and simulation based on laboratory or pilot scale studies. However, and in spite of the significance of all this, there is no comprehensive review on UCG process description with emphasis on its thermodynamic and kinetic studies. We believe that, in all these respects, this is a timely contribution. We anticipate that this review will promote research and development efforts, scale-up of the gasification process, and large-scale implementation of UCG in future. In this article, research contributions are reported according to the following sections:  In Section 2, we reviewed the UCG with main emphasis given to the chemical and physical characteristic of feedstock, process chemistry, gasifier designs, and operating conditions. This is done to provide a general background and allow the reader to understand the influence of operating variables on UCG.  In Section 3, we discussed thermodynamic studies of UCG with emphasis on gasifier operation optimization based on thermodynamics.  In Section 4, we reviewed coal gasification reaction engineering and particularly we reviewed the recently developed kinetic models.  Section 5 has discussed challenges for UCG and the proposed approach which has been implemented to overcome the existing challenges.  Section 6 summarized the advantages and limitations of UCG and in Section 7, we provide concluding remarks and future prospects for UCG technology.

2. Underground coal gasification 2.1. UCG for synthetic fuel production UCG permits coal to be gasified in situ within the coal seam, via a matrix of wells. The coal is ignited and air is injected underground to sustain a fire, which is essentially used to produce and transport combustible synthetic gas to surface. This synthetic gas can be used for industrial heating, power generation or the manufacture of hydrogen, synthetic natural gas or other fuels. As compared with conventional mining and surface gasification, UCG promises lower capital/operating costs and also has other advantages, such as no human labor underground for coal mining. In addition, UCG has the potential to be linked with carbon capture and sequestration [34]. The increasing demand for energy, depletion of oil and gas resources, and threat of global climate change have led to growing interest in UCG throughout the world. The primary components of UCG syngas are H2, CO, CO2, CH4, and H2S. The pressures and temperatures of produced gas are similar, at 30e50 bars for a 300e500 m deep seam, and 500e 800  C outlet temperatures for sub-bituminous coals and up to 1000  C for bituminous coals. The product gas requires cleaning once it has reached the surface, either to meet the specification for input into a gas turbine (for electricity generation), or to be of sufficient purity for use as a chemical feedstock for conversion to synthetic fuels. 2.2. Process overview UCG has been approached in many different ways. The old technique to gasify the coal in situ uses two-vertically drilled wells as the Injection and Production wells. The procedure consists of three steps as shown in Fig. 3. In the first step an injection and

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production well are drilled from the surface to the coal seam and highly permeable path within the coal seams are established between these two well. Prior to the gasification step a linkage path is created between injector and producer. Several techniques can be used for linking the wells, including the Reverse Combustion Linking (RCL), Forward Combustion Linking (FCL), hydro-fracking, electro-linking, explosive and in-seam linking. Other techniques for the in situ gasification include CRIPs, long and large tunnel gasification, and two-stage UCG [35e37]. The RCL is a method of linking which includes injection of an oxidant into one well and ignition of coal in the other so that combustion propagates toward the source of oxidant as shown in Fig. 4(a). In the course of the FCL coal is ignited in the injection well, and the fire propagates toward the production well as shown in Fig. 4(b). During forward gasification, the flame working face gradually moves to the outlet, making the dry distillation zone shorter and shorter. At the time when forward gasification is nearly complete, the reduction zone also becomes shorter [38]. Flow of oxidant into the injection well is maintained until the fire reaches the bottom of the injection well in the RCL or that of the production well in the FCL. This outcome is accompanied by a significant drop in the injection pressure indicating creation of a low hydraulic resistance link between the wells, which establishes a low hydraulic resistance path between the two wells. CRIP technique is suitable for thin, deep coal seams, replaces the vertical injector by a horizontal injector [39]. During the gasification process, the burning zone grows in the upstream direction, in contrast to the gas flow in the horizontal direction. This occurs by cutting off or perforating the injection linear at successive new upstream locations. The CRIP technique produces higher quality gas, results in lower heat loss than the two-vertical well configuration, and improves the overall efficiency of the UCG process [40]. Once a successful link has been established the second step is ignited. The gasification step starts with ignition of the coal and the injection of air or air enriched with oxygen. Both permeable bed gasification and natural convection driven surface gasification will

Fig. 4. Schematic views of the reverse and forward combustion linking in UCG. (a) Reverse combustion linking. (b) Forward combustion linking [36].

occur. When the gas quality deteriorates the injection well is burnt to allow injection further upstream. Gasification occurs when a mixture of air or oxygen and steam is forced into the coal seam through injection well and react chemically with the coal, generating a synthesis gas, which is recovered through product well. At the surface the raw product gas is cleaned for industrial uses [20]. As gasification proceeds, an underground cavity is formed. Water from the surrounding strata will enter the cavity and participate in the gasification process leading to a drop in the local water table. At some point, the coal in the vicinity of the injection well will be exhausted and steps one and two will be repeated to access fresh coal to sustain gas production. In the commercial operations several underground gasifiers will be operated simultaneously. Once the gasification operations in a section of coal seam have finished, the third step is to return environment back to its original state. This is achieved by flushing the cavities with steam and/or water to remove pollutants from cal seams to prevent them from diffusing into surrounding water aquifers. Over the time, the water table will return to a level close to that existing prior to the start of gasification [20]. The composition of the product gas from UCG can very substantially depending on the injected oxidant used, operating pressure and mass and energy balance of the underground reactor. CRIP technique, is suitable for thin, deep coal seams, replaces the vertical injector by a horizontal injector [39]. The CRIP method requires two horizontal wells drilled along a coal seam. One is near the top of the seam and the other near the bottom. The bottom (injection) well is lined with metal pipe. The upper well is the production well. As pyrolysis proceeds, the burn cavity moves toward the base of the wells, progressively exposing more and more of the injection pipe. At an appropriate time, the pipe is melted or burned off and a new period of pyrolysis begins. In effect, the old problems of well plugging are circumvented by simply starting a new burn periodically along the horizontal wells [41]. The CRIP method was first tried successfully in early 1982 with a threeday trial, gasifying a 40-ton cavity. The injection pipe was then burned off and a second 10-ton cavity started. The original cavity cooled to 500  C, and the second achieved the typical operating temperature of 1000  C. The average heating values of the product gases were between 265 and 277 Btu per standard cubic foot. Burning is started by pyrophoric silane and propane gases. The silane ignites upon encountering the oxygen in the burn cavity and burns long enough to subsequently ignite the propane, which is injected into the well. The propane actually ignites the coal in the cavity. At a suitable time, the propane is shut off and the pyrolysis sustains itself. This method has proved reliable since its adoption. Burning can also be started by passing LPG through the injection well for a short period of time (3e5 min) to initiate the combustion. An electric spark is generated for ignition of the liquefied petroleum gas (LPG) in the channel of the coal block near the mouth of the injection well. Once coal is ignited, the LPG supply is stopped and oxygen is continuously passed through the channel created in the coal block until the completion of the experiment [42]. CRIP technique uses a combination of conventional and directional drilling to drill the process wells. First, the vertically-drilled Production Well is drilled until it intersects the coal seam. Then the vertical section of the Injection Well is drilled to a predetermined depth, after which directional drilling is used to deviate the hole and drill along the coal seam until it intersects the Production Well. This technique enables the injection point (i.e. the end of the coiled tubing) to be retracted back along the coal seam, which is of benefit because it allows for fresh coal to be accessed each time the syngas quality drops as a result of cavity maturation. Retraction of the injection point along the coal seam is known as

A.W. Bhutto et al. / Progress in Energy and Combustion Science 39 (2013) 189e214

a CRIP maneuver, and between 10 and 20 such maneuvers are expected during the course of a module’s lifetime. Directional drilling is a proven technology in the oil and gas industry. The in-seam drilling of coal seams has been part of coal exploitation since at least the 1950s. Underground steering of boreholes made its commercial entrance in the oil and gas industry around 1990, when operators established the benefits of lateral drilling for extending the life of wells and fixed drilling platforms and for reaching inaccessible locations. Nowadays directional drilling has become common for coal bed methane (CBM) and enhanced CBM applications; there are specialist drilling companies around who supply services to CBM operators. The focus to-date has been on reducing costs. UCG has a tighter requirement on accuracy. The ability of directional drilling to meet these requirements at an affordable cost is still under review [37]. The CRIP technique produces higher quality gas, results in lower heat loss than the two-vertical well configuration, and improves the overall efficiency of the UCG process [40]. Two-stage UCG is a technique of supplying air and steam cyclically [10,43]. In the first stage, air is supplied to make the coal burn and store heat to produce air gas; in the second stage, steam is supplied to produce water gas. Only if sufficient heat is stored in the first stage can the decomposition reactions in the second stage run smoothly and the water gas with high heating value be ensured. Meanwhile, the degree of the coal layer decomposition and the production volume of the gas are totally determined by the temperature distribution in the coal layers [44]. During in situ coal gasification remote sensing technique may be used for mapping underground fracture systems, locating tunnels or water-bearing strata and mapping burn fronts [45]. 2.2.1. Chemical processes The study considers the quasi-steady burning of a carbon particle which undergoes gasification at its surface by chemical reactions, followed by a homogeneous reaction in the gas phase. The main chemical processes occurring during coal gasification are drying, pyrolysis, combustion and gasification of the solid hydrocarbon. These processes occur in all methods of coal gasification, whether conducted in surface gasifiers or in situ. From the chemical and thermodynamic point of view, the UCG process runs analogically to gasification in the surface reactors [46]. The most important chemical reactions taking place during underground coal gasification are listed in Table 2. Chemical reactions (1)e(4) take place on the wall plane of the coal seams (heterogeneous reactions), while (6) and (7) reactions occur at the gaseous stage (homogeneous reactions). In addition to these listed, reactions involving nitrogen and sulfur are also important. The final product gas consists of hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. Composition and heating value of the product gas depends on the thermodynamic conditions of the operation as well as on the composition and temperature of the gasifying agent employed [46]. During in situ combustion of coal different processes of vaporization (drying), pyrolysis, and combustion and gasification of char take place collectively. The UCG process has a zonal character and the main gasification reactions occur both in the solid and the gaseous phases as well as on their boundaries. Qualitative description of phenomena at the UCG cavity wall is explained in Fig. 5. In the solid phase mainly the pyrolysis and the drying processes take place. Along with the migration of the gaseous product of the thermal decomposition through the pores and slots of the solid phase, various homo- and heterogenic reactions occur. The rates of

195

Table 2 Chemical reactions taking place during underground coal gasification. Reaction equation

Reaction rate (Ri)

DHo298 (MJ/kmol)

Equation number

C þ O2 /CO2

R1

þ393.8

(1)

C þ CO2 /2CO2

R2

162.4

(2)

C þ H2 O/H2 þ CO

R3

131.4

(3)

C þ 2H2 /CH4

R4

þ74.9

(4)

1 O /CO2 2 2 1 H2 þ O2 /H2 O 2 CO þ H2 O/CO þ H2

R5

þ285.1

(5)

CO þ

R6

0.242

(6)

R7

0.041

(7)

these processes depend mostly on the temperature. On the phase boundary in the gasification channel heterogenic reactions take place. Their rates are determined by the diffusion and the accessible reaction area. The major products of the reaction of oxygen with carbon in the gasification area (oxidation zone) are carbon dioxide and carbon monoxide [46]. Based on the differences in major chemical reactions, the temperature, and the gas compositions, the gasification channel can be divided into three zones: oxidization zone, reduction zone and dry distillation zone as shown in Fig. 6 [21]. In the oxidization zone, the multi-phase chemical reactions between oxygen contained in the gasification agent and the carbon in the coal seam occur, producing heat and making the coal seam very-hot. The coal seams become incandescent with temperature ranging from 900  C to 1450  C [47]. Inherent water plays a role in coal oxidation, affecting oxygen transport within coal pores and participating in the chemical reactions during the oxidation process. Details of chemical reactions involving water have not yet been elucidated [48]. With the O2 burning up gradually, the air stream gets into the reduction zone. In the reduction zone H2O(g) and CO2 are reduced to H2 and CO under the effect of high temperature, when they meet with the incandescent coal seams. The temperature ranges from 600  C to 1000  C, and the length is 1.5e2 times that of the oxidation zone with its pressure being 0.01e0.2 MPa [49]. Additionally, under the catalytic action of coal ash and metallic oxides, a certain methanation reaction occurs [Eq. (4)]. The above endothermic reactions cause the temperature at the reduction zone to drop until it is low enough to terminate the reduction reactions. After the endothermic reactions in the reduction zone, the gas current temperature drops, and then it begins to flow into the destructive distillation and dry zone (200 Ce600  C). The main physical changes for coal with high water content are dewatering and cracking, as well as absorption and contraction of the coal, when the temperature is below 100  C. When the temperature is not higher than 300  C, only small amounts of paraffin hydrocarbon, water, and CO2 are separated out. Over 300  C, the slow chemical changes take place, accompanied with a light polymerization and depolymerization. In the meantime, appropriate amounts of volatile and oil-like liquid are separated out, which take on a gelatinous state afterward. When the temperature of the coal seam rises to 350 Ce550  C, a large proportion of tar oil is separated out (500  C at its peak) and a certain amount of combustible gas is yielded. The hydrocarbon gas is given out when the temperature stands at 450 Ce500  C. As the temperature of the coal seam continues to rise until it is over 550  C, semi-coke remains begin to solidify and contract, accompanied with the yield of H2, CO2, and CH4 [47,50].

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Fig. 5. Qualitative description of phenomena at the UCG cavity wall [16,19].

The overall UCG process is strongly exothermic, and temperatures in the burn zone are likely to occasionally exceed 900  C. Even after cooling (through conductive heat loss to surrounding strata and convective heat loss to native groundwater), syngas typically flows through production wells at temperatures between 200  C and 400  C. Around the burn zone, the high buoyancy of hot syngas relative to groundwater will tend to lead to large pores getting invaded with bubbles of syngas, which will heat the groundwater and turn it into steam. A dynamic interface between steam and hot groundwater will develop around the UCG burn zone, in which steam will mix with the syngas [23]. Passing through these three reaction zones, the gas with the main combustible compositions of CO, H2 and CH4 is formed, whose proportion of contents varies from one gasification agent and air injection method to another. These three zones move toward the outlet along the direction of the air flow, which, in turn, ensures the continuous run of the gasification reactions [21]. Figs. 6 and 7 illustrate different chemical regions of gasification of coal in situ. In the drying zone, surface water in the wet coal is vaporized at temperatures above the saturation temperature of seam water at a specified pressure, which makes the coal more porous. The dried coal undergoes the pyrolysis process upon more heating in the next phase. During pyrolysis, coal loses about 40e 50% of its dry weight as low molecular weight gases, chemical

Fig. 6. Division of gasification channel into three zones: oxidization zone, reduction zone and dry distillation zone [21].

water, light hydrocarbons and heavy tars, and after evolving the volatile matters, a more permeable solid substance called char will be combusted and gasified by the injected oxidant agents and exhausted gases from the previous steps [51,52]. The rates of the gaseous phase reactions are determined mostly by the temperature and concentration of the particular gaseous compounds. Development of these reactions is frequently supported by the catalytic influence of some chemical compounds, e.g. iron oxides. 2.2.2. Physical process In the process of underground coal gasification (UCG), the gas movement not only influences the concentration distribution and movement of fluid in the burning zone directly, but also restricts the diffusion of the gasification agent in the whole gasifier.

Fig. 7. Thermal wave propagation through coal seam during in situ gasification which demonstrates the different regions [3].

A.W. Bhutto et al. / Progress in Energy and Combustion Science 39 (2013) 189e214

Therefore, it eventually determines the rate of chemical reaction between gas and solid, and the process of burning and gasification. Evidently, Lanhe 2003 [15] suggested the study of moving patterns of fluid in the gasifier should precede the study of the process of chemical reaction, the moving patterns of agents, and the distribution regularity of temperature fields near the flame working face. In the process of underground coal gasification, under the effect of high temperature, that a temperature field forms in the coal layer to be gasified within the coal and rock mass, which makes the coal and rock layersdoriginally full of stratification, joints, and fracturesdsoften, melt, cement, and solidify. Accordingly, the internal molecular structure is rearranged and reorganized, which leads to qualitative changes of organizational structure and morphological appearance. Hence, obvious changes take place in the physicomechanical properties of the coal and rock mass. In the process of underground coal gasification, a high temperature field comes into being in the coal body under the high temperature, which makes the coal seam, full of layers and joints and interstices, soften, melt, glue, and solidify. Under the high temperature, the internal molecular structure reorganizes, which completely changes the coal seam’s surface morphology. Hence, dramatic changes take place in the physical and mechanical properties of the coal body. As a result, its corresponding physical and mechanical properties are no longer constants, but functions of temperature. The differences in the heat expansion coefficient among coal grains and anisotropy generate new cracks, whose extension leads to the connected net structure. Thus, all these improve the connectivity of the pore passageway and increase the seepage pressure of the dry distillation gas [53]. Research indicates that, under the non-isothermal condition, the densities of the solid media and pore water are greatly affected by the temperature and pressure [49]. However, the small deformation of the solid skeleton still produces a certain effect on the distribution of the temperature field and seepage of underground water in the gasification panel. Therefore, the deformation of the solid particle is not negligible and can be regarded as compressible [9]. The coal rock is extended and deformed by the pore fluid pressure. The fluid inside the pores affects the cracks inside the skeleton of the coal rock and the pores’ opening and closing; second, the relation between the stress and strain of the coal rock is changed by the fluid in the pores, which in turn changes the elastic modulus and compressive strength of the coal rock [54e56]. The changes in the temperature field of the coal seam are due mainly to the flame working face. When the temperature in the coal seam rises, the desorption rate of the dry distillation gas in the coal seam accelerates. The free dry distillation gas content in the coal increases. The mass of the dry distillation gas which participates in the seepage increases too. On the other hand, with the rise of the temperature, the amount of absorbed dry distillation gas in the coal seam drops. 2.2.2.1. Operating conditions. The investigation by Perkins and Sahajwalla [18] has found that the operating conditions that have the greatest impact on cavity growth rate are temperature, water influx, pressure, and gas composition in underground coal gasification. In this section, the effect of operating conditions and coal properties, namely, coal reactivity, operating pressure, heat loss, and the type of oxidant used are investigated [16]. Lanhe [13] while establishing the mathematical models on the underground coal gasification in steep coal seams according to their storage conditions and features of gas production process concludes that numerical simulation on the temperature field, concentration field and pressure field is reasonable in the

197

underground gasification of steep coal seams on the experimental condition. 2.2.2.2. The thickness of coal layers. UCG is influenced by several natural factors as described in Table 3. Most UCG operations were carried out in more gas permeable conditions of brown coal beds and younger formations of hard coals. Generally, these deposits occurred at shallower depths, down to 300 m, and ignited relatively easily. Strongly swelling and coking coals have the tendency to block gas flow through the coal bed, thus hindering the course of the reaction. The gasification of beds 1 m thick or more improves economics [57]. Beds that are thinner than 0.5 m are not considered suitable for UCG. In the process of UCG, the burning area and gas are not only cooled down through heat exchange but a part of the heat is also lost into the coal seam and surrounding rocks (floor, roof), thus having an adverse effect on the stability of the underground gasification process. Eliot [58] suggested that when the thickness of coal seam is smaller than 2 m, the cooling action with a dramatic change for surrounding rocks affects the heat value of coal gas considerably. As for comparatively thin coal seam, enhancing the blowing velocity or oxygen-enriched blowing can improve the heating value of gas. In the former Soviet Union, Lischansk underground gasification station adopted oxygen-enriched blowing in the coal seam, for which the thickness is less than 2 m [58]. When the thickness of coal layers is decreased or the intake rate of water is increased, the CO2 content in the gas will rise [58,59]. 2.2.3. Effect of coal reactivity The chemical reactivity of the coal is potentially very important for UCG. The reported intrinsic reactivities of low rank coals differ by up to 4 orders of magnitude when extrapolated to typical gasifier operating temperatures [18]. The coal intrinsic reactivity has a big impact on the distributions in the gasifier and on the final product gas. In particular, high reactivity favors the production of methane via the char-H2 reaction. Because this reaction is exothermic, the increased reactivity for this reaction can lead to big changes in the final product gas calorific value. 2.2.4. Gasifying agents Gasification under different atmospheres such as air, steam, steam-oxygen, and carbon dioxide has been reported in the literature. In general, the gasifier atmosphere determines the calorific

Table 3 Classification criteria for UCG. Criterion

Characteristics/remarks

Coal type Any Physicochemical properties of coal Recommended: high content of volatile matter, low agglomerating capacity or its lack, ash content < 50% by weight Occurrence depth Profitability criterion Bed thickness More than 1 m Angle of inclination of coal bed Any Type and tightness of rock mass Recommended: firmness and tightness of rock mass, thickness and lithology of rock massdoverburden in slightly permeable layers (clays, silts, shale clays) Hydrogeological conditions Recommended: lack of fissures, faults, aquiferous layers, water reservoirs causing water inflow Deposit tectonics Recommended homogeneity of deposit (lack of fissure, faults) Quantity of resources Profitability criterion Methane presence in the bed Causes gas hazard Conditions of infrastructure Recommended lack of building development

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value of the syngas produced. When one uses air as the gasifying agent, a syngas with low heating value is obtained. This is mainly due to the syngas dilution by the nitrogen contained in air. However, if one uses steam or a combination of steam and oxygen, a syngas with a medium calorific value is produced. Adding steam changes carbon-oxygen system balance to carbon-oxygen-steam system balance in the combustion process. Oxygen-steam gasification not only utilizes the surplus heat to improve the energy efficiency of the process, but also increases the gas production volume per ton of coal and lowers the oxygen consumption volume per ton of coal. The changing relationships between gas compositions and steam/oxygen ratios are shown in Fig. 8 [60]. The experiment results show that pure-oxygen underground coal gasification, the water in the coal seams, or the leaching water on the roof can be used to produce water gas. However, because water evaporation consumes heat, and it is impossible to control steam volume, gas compositions often present the wide fluctuations. Therefore, it is required to adjust the oxygen supplying volume so as to keep the stable proceeding of gasification process. From Fig. 8, it can be seen that with the rise in the steam/oxygen ratio, the volume of steam increases, the H2 content in the coal gas improves, the CO content drops, and the CH4 content is heightening a little [60]. The syngas produced has a by UCG process has low calorific value approximately one-eighth of natural gas if air injection is used, and double this figure if oxygen injection is used. Oxygenenriched steam forward gasification has remarkable effects on gas compositions. Under the testing environment, in pure oxygen gasification, the average rising rate for the temperature of the gasified coal seams is about 2.10  C/h; in the oxygen-enriched steam forward gasification phase, the high temperature field mainly concentrates around gasification gallery, and the highest temperature in oxidation zone reaches over 1200  C [61]. The air injected into a gasification channel is at a low speed, the flame tends to propagate toward the injection point but, if the air flow rate increases, the cavity tends to grow in the downstream direction. It is also known that flame propagation is faster when oxygen is used instead of air. This behavior is also expected since oxygen-fed flames are hotter and have higher reaction rates [62]. Saulov et al. [62] considered the limit of high temperatures, high activation energy and a strong air flow. Under these conditions the surface of the channel has two zones, cold and hot. The temperature is insufficiently high in the cold zone to initiate reactions, while in the hot zone any oxygen on the surface reacts instantly. Since the activation energy is high, these zones are separated only

Fig. 8. Gas composition variation with steam/oxygen (v/v) [60].

by a very small distance. The overall reaction rate is determined by the rate of diffusion of oxygen to the hot zone, while the oxygen concentration on hotwalls is essentially zero. Under such conditions the turbulent flame is fully controlled by diffusion and the injection rate has no control over the flame position. Combustion of coal begins with devolitalization reactions at low temperatures and can be cooled by the air stream. If these reactions play a noticeable role in initiating the rest of the oxidation process or in the overall energy balance, the flame position is affected by the air speed and becomes controllable. When other factors are the same, increases in flow rate and operation time result inmonotonic increases in all the dimensions of the cavity, and its volume. However, when the distance between the injection and production wells is increased, the overall cavity volume decreases, due to significant reduction in the rate of growth of the cavity in the forward direction [42]. 2.2.5. Effect of pressure Pressure is known to positively impact the performance of coal gasification [63]. At close to atmospheric pressure, the gas calorific value is very low because of the kinetic limitations of the gasification reactions. The changes in operating pressure can perfect the underground gasification process to a great extent. Under the cyclically changing pressure condition, heat loss was obviously reduced, and heat efficiency and gasification efficiency and the heat value of the product gas are increased greatly. The underground gasifier with a long channel and big cross-section could improve the combustion and gasification conditions to a large extent, markedly bettering the quality of the product gas and the stability of gas production. Therefore, the large-scale underground gasifier is a condition necessarily met by the industrial production [50]. 2.2.6. Effect of heat loss Heat losses from underground coal gasification are not easy to estimate. If the cavity remains completely in the coal seam, then heat losses to the surrounding strata will probably be small and can be ignored. However, as the overburden is progressively exposed, irreversible heat loss to the surrounding will increase. It is not easy to estimate this heat loss, because if the overburden undergoes spalling, some of the energy used to heat it to cavity temperatures may be recovered through preheating of the injected gas. The heat loss mechanisms can probably be more easily investigated using a dynamic model, in which cavity growth and heat loss are estimated as functions of time, simultaneously. 2.2.7. Effect of temperature The process of UCG is virtually one of a self-heat balance. The heat produced by coal combustion contributes to the establishment for ideal temperature field in the underground gasifier and also leads to the occurrence of gasification reactions and, eventually, the generation of gas. Temperature is a key factor in determining the continuous and stable production in the process of underground coal gasification. The patterns of variation for temperature field in the gasifier are closely related to the nature of the gasification agent, gasification modes, and the changes of cavity [8,49,61,64,65]. Under the pure oxygen gasification condition, the average rising rate for the temperature of the gasified coal seams is about 4.15  C/h; in the oxygen-steam forward gasification phase the high temperature field mainly concentrates around loosening zones arising from the thermal explosions, and the highest temperature in the oxidation zone approaches 1300  C [6]. Compared with forward gasification, the average temperature in the gasifier for backward gasification is lower [61]. The drop of temperature results in a decrease in CO content while H2, CH4 and CO2 contents increases [50].

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In thermal-explosion gasification method, under the pure oxygen gasification condition, the average rising rate for the temperature of the gasified coal seams is about 4.15  C/h; in the oxygen-steam forward gasification phase, temperature field mainly concentrates around loosening zones arising from the thermal explosions, and the highest temperature in the oxidation zone approaches 1300  C. Test data showed that the forward oxygensteam gasification with moving points can obviously improve the temperature conditions in the gasifier. During the backward oxygen-steam gasification, with the passage of time, the temperature of the gasification coal seams continuously increases, approaches stable little by little, and was basically the same with that of the forward gasification. Therefore, backward gasification can form new temperature conditions and improve the gasification efficiency of the coal seams. In the process of coal gasification, the changes of the temperature in the coal seam are due mainly to the heat transfer medium of the flame working face, which corresponds to a source of heat [53]. In the process of underground coal gasification, the temperature of coal seams around the gasification channel rises along with the conducted heat. When the coal surface is heated by the hot gas or the neighboring incandescent coal, its temperature distribution expands toward the coal grains or the interior of the coal seam, which inevitably results in the thermal effects of absorption, desorption, and seepage movement of dry distillation gas stored in the coal seam [49,53,66,67]. King and Ertekin [68] study shows that under non-isothermal conditions, either the absorption-desorption process or the permeation-expansion process is linked to the temperature. According to the gasification theory, the temperature above 1000  C indicates a high-speed diffusion of the water decomposition reaction constituting the fundamental process for the production of a hydrogen rich gas in the course of the UCG steam stage. On the other hand, the temperature drop below 700  C slowed down the reaction speed considerably. For these reasons, special attention was paid to keeping parameters preferable for the production of gas with a high content of the combustible components, mainly hydrogen. The oxygen stage was therefore continued to achieve temperatures in the range between 1100 and 1200  C. According to the simulated calculation results [13], with the increase of the length for the gasification channel, the heating value of the gas improves. However, behind the reduction zone, it increases with a smaller margin. The influence of the temperature field on the heating value for the gas is noticeable. Due to the effect of temperature, in high temperature zone, the change of the measured value of the concentration field for the gas compositions is larger than that of calculated value. The underground gasification of a large quantity of coal at temperatures higher than 1000  C results in the typically argillaceous overburden rocks overlying the coal becoming thermally

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affected. Most of thermal reactions in argillaceous rocks are endothermic. 2.2.8. Cavity growth As the coal gasification reaction precedes a cavity consisting of coal, char, ash, rubble, and void space, is created underground. The size of the cavity formed during UCG impacts directly the economic and environmental factors crucial to its success. Lateral dimensions influence resource recovery by determining the spacing between modules, and ultimate overall dimensions dictate the hydrological and subsidence response of the overburden. The exact shape and size of the gasification channel during UGC are of vital importance for the safety and stability of the upper parts of the geological formation [69]. Due to upward growth the cavity eventually reaches the interface between the coal seam and the overburden. From that point onwards the development of the cavity can be strongly influenced by the interaction of the gas mixture with the overburden. At the start of the UCG process, typically, the exothermic coal combustion reaction is required in order to create a sufficiently large underground cavity. In this early stage, cavity growth is unconstrained by roof interactions. Once a stable temperature field is attained, steam is introduced in the cavity for gasification of the coal in order to obtain the combustible product gases [38]. The shape and rate of growth of this cavity will strongly impact other important phenomena, such as reactant gas flow patterns, kinetics, temperature profiles, and so on [42]. The cavity size at any given time depends on the rate of coal consumption and its shape depends on the non-ideal flow patterns inside the cavity. The cavity shape is almost symmetric around the injection well. The cavity evolution behind the injection well (i.e. backward length) is less than the height (in the vertical direction) and the width at the injection point (in the transverse direction). The forward length of the cavity (i.e. distance from injection well to the end point of the cavity dome in the forward direction) is larger than the height and the backward length. The convective flux of the reactant gases in the forward direction (i.e. toward the production well) contributes to the additional growth of the cavity in this direction. The observed final cavity dome that is associated with a long outflow channel is nevertheless nearly hemispherical in shape. Fig. 9 is a schematic of the final cavity shape, indicating the vertical, forward, backward and transverse directions as defined here. The temperature at the cavity roof is in the range of 950e1000  C whereas the floor temperature varies between 650 and 700  C. The volume of the cavity increases progressively with coal consumption and thermomechanical spalling, if any, from the roof. As the cavity growth is irregular in three dimensions, the flow pattern inside the UCG cavity is highly non-ideal. The complexity increases further because of several other processes occurring simultaneously, such as heat transfer due to convection and radiation, spalling, water intrusion from surrounding aquifers, several

Fig. 9. Schematic diagram defining forward length, backward length, height and width of the final cavity [42].

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chemical reactions, and other geological aspects [57]. Several mathematical models have been developed considering the UCG cavity as either a packed bed or a free channel Most of the existing models consider the UCG cavity as a rectangular or cylindrical channel [13,16,18,19,35,70e75]. Perkins and Sahajwalla [18] predicted cavity growth rate between 1.6 and 5 cm/h using their mathematical model which links linear cavity growth rates to reactivity and mass transport properties. Daggupati et al. [38] measured the linear, vertical growth rate of 1.1 cm/h (obtained using the measured cavity heights at different times, with the other operating conditions being the same). The cavity volume is directly proportional to the coal consumption whereas the shape depends on the relative rates of growth taking place in each of the four identified representative directions. While the coal consumption is governed by the extent or rate of reaction that takes place in the cavity reactor, the growth in each individual direction is a function of the complex reactant gas flow field inside the cavity, and other effects such as thermo mechanical spalling of the coal. Chen et al. [69] has developed model to calculate the temperature distribution in the vertical direction, and the combustion volume. According to the physical and chemical properties of coal and the mining geology conditions of the burial for the coal seams, two kinds of gasification channels can be formed in the gasification panel; namely, free channel without solid phase and the percolation patterned porous loose channel. In the longitudinal (or radial) direction, the free channel can be divided into three zones (Fig. 10), i.e., free flowing zone, reaction zone and the coal seams zone. The gas phases flow under the condition of wall plane of the channel continuously exchanging heat, consuming or producing certain compositions. At the same time, the homogeneous reactions also occur to the gas phases. In the reaction zone, the oxidation, reduced reactions and the pyrolysis reactions of the coal occur. The heat transfer to the gas phases, the consumption and production of the compositions can be regarded as the boundary conditions for the flowing of the gas phases. In the coal seams zone, part of the heat in the reaction zone loses in the coal seams mainly in the form of the heat conduction, making the dry and distillation of the coal seams. Therefore, we can observe the characteristics of the gas phase moving and establish the control equation set of the free channel gasification process. The cavity growth directly impact on the coal resource recovery and energy efficiency and therefore the economic feasibility. Cavity growth is also related to other potential design considerations including avoiding surface subsidence and groundwater contamination.

Installation of well pairs (injection and production wells) is costly and therefore it is desirable to gasify the maximum volume of coal between a well pair. As gasification proceeds, a cavity is formed which will extend until the roof collapses. This roof collapse is important as it aids the lateral growth of the gasifier. Where the roof is strong and fails to break, or where the broken ground is blocky and poorly consolidated, some fluid reactants will by-pass the coal and the reactor efficiency could decline rapidly. In general, as depth increases, conditions should become increasingly favorable to gasifier development with a lower risk of bypass problems occurring, except possibly in strong roof conditions [76]. 2.2.9. Gas diffusion In the process of combustion and gasification for the coal seams in the gasifier, the major reactions are multi-phase reactions. At each stage of multi-phase reactions, the gas state reactant spreads to the surface of the solid state reaction by the diffusion method. Gas diffusion mainly has two kinds: molecular diffusion and convection (eddy) diffusion. The process of the combustion for coal seams depends on the gas diffusion features and the dynamic characteristics for the chemical reactions. According to the diffusion-dynamic theory for combustion [49], under the low temperature condition, the overall velocity of the combustion and gasification process is mainly determined by the dynamics conditions of the chemical reactions; under the high temperature condition, the overall velocity of combustion and gasification process mostly depends on the speed for oxygen to diffuse from the main current to the carbon surface and the velocity of its product diffusing from the carbon surface to the main current. Seeing from the circumstances of the field test of underground gasification and model experiment, the temperature within the gasifier (the vicinity of the flame working face, in particular) is very high. Moreover, considering the movement conditions for the fluid, we can conclude that the convection diffusion for gas is the significant factor influencing the process of the underground gasification. Under the condition of high temperature, molecular diffusion results from the existence of concentration gradient, temperature gradient and pressure gradient [14]. While studying the basic features of convection diffusion for the gas produced in underground coal gasification, on the basis of the model experiment, through the analysis of the distribution and patterns of variation for the fluid concentration field in the process of the combustion and gasification of the coal seams within the gasifier, Lanhe [14] established the 3-D non-linear unstable mathematical models on the convection diffusion for oxygen. Same study concludes that oxygen concentration is in direct proportion to its distance from the flame working face, i.e. the longer its

Fig. 10. Gasification channels in coal seems [11].

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distance, the higher the oxygen concentration; otherwise, the lower. In the vicinity of the combustion zone, due to the very high temperature, the oxygen is almost exhausted in the reaction with carbon; in loosening zone, the oxygen concentration drops to a very low point where it almost approaches zero; in dropped out zone, owing to the comparatively low temperature, the drop of the oxygen concentration is slow [14]. During UCG processes, the surrounding rock acting as the furnace walls will be affected by high temperature, and its mechanical properties will change with the increased temperatures. At the same time, stress and displacement will happen among rocks due to the high temperature. Gasifier instability would result in steam interruption, and incomplete contact between gasification agents and coal. Two mechanisms can play a role in a gas transport through the porous stratum above the gas source, viz. diffusion and permeation. The diffusion driving force is the composition gradient (expressed through gas component mole fractions); the driving force for permeation is the total pressure gradient. It was found that the pressure increase influences the speed of the gas front movement more significantly than the temperature increase that is almost negligible. Nevertheless, for all tested conditions CO2 appears at the distance of the few hundred meters after some years only. The direct proportionality of the effective permeability coefficient to the effective squared mean pore radius was confirmed [77]. 2.2.10. Velocity of combustion front In packed bed gasification, the combustion front moves slowly down the bed parallel to the flow of gases. Hot combustion gases always have intimate contact with the unburned coal ahead of the combustion zone until the fire breaks through to the production well. In channel gasification, the combustion zone moves outward at nearly right-angles to the flow of air and combustion gases. During UCG a thermal wave is formed which gradually travels through the coal bed toward the gas production well. The shape of the thermal wave tends to change very little. Since the shape of the wave remains unchanged, the processes occurring at each temperature level in the moving wave remain unchanged in time, and an apparent steady-state or psuedo-steady-state condition prevails. Under these conditions in a one-dimensional system, it is possible to transform the mathematical model to a moving coordinate system which converts partial differential to ordinary differential equations, a major simplification of the problem. This transformation is [78]: n ¼ x e vt Where x ¼ fixed spatial coordinate t ¼ time v ¼ velocity of thermal wave or combustion front n ¼ coordinate system moving with frontal velocity v When the physical properties of coal tend to vary widely over short distances even in a single coal seam making the task of modeling such as UCG process very complex. Gasification of typical 9 m seam of sub-bituminous coal proceeds at a rate of 0.3e0.6m/ day consuming all the coal in a swath 12 to 15 m wide for a well spacing of approximately 18 m. 2.2.11. Compositions of syngas The precise proportions of the various component gases in any particular syngas mixture are a function of quality and rank of

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coal, seam depth, steam: oxygen ration and oxygen injection rate and other parameter discussed in Section 2. Compositions of syngas from a variety of coals as reported in literature reveals component fractions in the following ranges [8,18,26,79e81]. At constant steam/oxygen ratio gas compositions remained stable [8]. H2:11e35%; CO: 2e16%; CH4: 1e8%; CO2: 12e28%; H2S:0.03e 3.5%. 2.2.12. Optimization of UGC operation Underground gasification cannot be controlled to the same extent as a surface process as the coal feed cannot be processed. The UCG process can be operated with stability and flexibility, as input flow has been shown to have a direct relationship to production flow, with little effect on product gas quality. The power output from the gasifier could be rapidly increased or reduced by increasing or decreasing the O2 flow rate. Although elevated depth and pressure are not pre-requisites for a high quality gas, the benefit is in higher mass flows and hence greater efficiency of energy transmission to the surface. The energy output of a UCG system depends on the flow rate of gaseous products and the heat value of the gas mixture. The volume flow of the product gas is typically four times the injection flow so the limiting factor is the dynamic resistance of the production well. The mass flow capability of a well is proportional to input pressure. Increasing well depth increases the product gas density and pressure. The mass flow gain due to pressure increase exceeds the frictional loss due to increased bore hole length. Increasing the diameter of production tubing also raises the limiting flow rate. Increasing the diameter of production tubing, or the number of production wells, also raises the limiting flow rate [76]. Information on the process conditions must be constantly monitored and updated as the gasification process moves forward. The ideal temperatures of above ground coal gasification are about 1000  C, however, it may or may not be possible to achieve these temperatures in UCG, primarily because of the lack of control on water influx and reactant gas flow patterns [57]. Blinderman et al. [36,82] has used intrinsic disturbed flame equations to determine the key parameters of the RCL process. Wang et al. [83] performed field trial with various operational maneuvers, such as implementing controlled moving injection points, O2-enriched operation and variation of operational pressure to ensure the gas flow comparatively controllable and hence improve efficiency of heat and quality of the production syngas. Lawrence Livermore National Laboratory (LLNL) is evaluating commercial computational fluid dynamics (CFD) code to model cavity gas flow and combustion in two and three dimensions. Fig. 11 [84] show a typical cavity configuration at a mid-to-late stage of a linked vertical well module. Nitao et al. [84] has provided the details of models and simulators. It will be more useful to couple the UCG process models with full scale process simulator so that the entire process can be modeled at once, rather than sequentially.

3. Thermodynamics of UCG The gasification performance is controlled by both of kinetic and thermodynamic factors. The thermodynamic properties are, by definition, point functions of the gasification process, indicating the conditions of a system at equilibrium, regardless of the reaction path followed in attaining equilibrium or the time required. On the other hand, the kinetics of a reacting system defines a particular sequence of reaction paths, as well as the rates at which the chemical changes take place.

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Fig. 11. A typical cavity configuration is shown at a mid-to-late stage of a linked vertical well module. UCG involves several distinct multiphysical/chemical-process domains, including the cavity, the wall zone containing coal, the wall zone containing rock, and the rubble zone.

3.1. Thermodynamic equilibrium In gasification, both homogeneous and heterogeneous reactions occur simultaneously in a complex reacting system [85]. As underground coal gasification processes are thermally autobalanced, at given constant pressure and initial enthalpy, the equilibrium state is reached when;

dS  0 at constantðp; HÞ

(8)

Therefore, at equilibrium, when conditions of constant pressure and enthalpy are applied, the total entropy is at maximum. Some of the processes are at specific pressure and temperature, exothermic or endothermic. Constraining the unit to constant T and p, we find that;

dG ¼ dSg

(9)

and at equilibrium under these conditions, the following equation must be satisfied;

dG  0 at constantðp; HÞ

(10)

Therefore, at equilibrium, the Gibbs free energy must reach a minimum when the state is defined by the pressure and temperature. As gasification reactions are highly endothermic, heat is supplied through the combustion reactions. Since heat is supplied by direct carbon combustion with oxygen, the carbon-oxygen-

steam ratio is optimized such that the global reaction is thermal self-balanced. Oxygen-carbon reactions are highly exothermic and can supply heat to other gasification reactions or increase the product gas temperature, while the steam-carbon gasification will absorb the heat into chemical energy in syngas. The adiabatic gasification temperature increases with increasing O2/C ratio and decreasing H2O/C ratio. However, the greater oxygen consumption in during gasification will require more energy consumption for air separation, and thus lead to higher efficiency penalty. 3.2. Carbon-oxygen steam equilibrium Fig. 12 presents equilibrium compositions for a carbon-steam system. It can be seen that one of the primary additions to the list of products is methane, which increases in concentration as the temperature decreases. Fig. 13 shows the equilibrium compositions for the carbon-oxygen-steam system under zero enthalpy change and the same pressure-temperature conditions as in Fig. 12. This is more closely represents a possible combusting system than does Fig. 12, since no heat is added or subtracted externally. The principle difference between the systems lies in the product gas hydrogen/ carbon monoxide ratio at high temperatures. However, the behavior of the carbon monoxide/carbon dioxide ratio vs. temperature as well as the occurrence of methane remains essentially the same. The carbon monoxide/carbon dioxide ratio is also not significantly altered from the case where steam is not present (Fig. 14).

Fig. 12. Equilibrium gas compositions of the carbon-steam system [86,87].

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Fig. 14 presents the fractional composition of carbon monoxide and carbon dioxide in equilibrium with b-graphite at 1 atm as a function of temperature. It can be seen that if one wants to produce carbon monoxide preferentially, it is essential to maintain a combustion temperature above 1000 K. As the void space underground grows with the consumption of coal, there is more potential for gas phase Oxidation of carbon monoxide to carbon dioxide and the resulting extreme temperatures in the gas phase; this phenomenon is generally referred to as oxygen bypassing [87]. One of the most interesting features of carbon-oxygen steam equilibrium calculations (Fig. 13) is that throughout the temperature range considered, the heating value of the product gas does not decrease significantly as the temperature decreases, in contrast to the case where one is considering the carbon monoxide carbon dioxideecarbon equilibrium in Fig. 14. This result from the fact that as the carbon monoxide and hydrogen concentrations fall off with decreasing temperature, the loss in their contribution to the heating value of the product gas is partly made up for by the increase in the concentration of methane. Figs. 12e14 also show that increased pressure will enhance the formation of methane. However, owing to subsidence of the overburden, which is unavoidable for a system that removes essentially all of the coal, the gasification zone is usually sufficiently porous to require that the process be carried out at the lowest possible pressure to minimize gas losses. This is a major obstacle to producing pipeline quality gas (synthetic natural gas) by UCG [87]. 3.3. Cold gas efficiency (hcg) The cold gas efficiency (hcg) is a key factor assessing the energy conversion efficiency in gasification process, which is defined as

hcg ¼


   nH2 LHVH2 þ nCO LHVCO þ nCH4 LHVCH4 ðLHVcoal Þ  100 (11)

Where LHV ¼ lower heating value n ¼ Number of moles.

hcg is mainly a function of the O2/C ratio, while the H2/CO ratio is a more determined by the H2O/C ratio. 0.5 is the stoichiometric

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ratio of oxygenecarbon reaction to produce carbon monoxide, smaller O2/C ratio will not convert carbon sufficiently, and CO will burn at greater O2/C ratios. So hcg reaches the maximum value at this ratio. On the other hand, the water gas shift reaction is nearly thermal-neutral compared with other reactions, so it will convert CO into H2 without changing the hcg much. If the optimized O2/C ratio (about 0.5) is chosen, although lower H2O/C ratio will slightly increase the hcg meanwhile lower O2/C ratio and higher H2O/C ratio is favorable to produce H2. Were CH4 the final product, then low temperature of gasification is favorable in terms of its higher H2/CO ratio (about 3e40) in syngas. 4. Kinetic studies of UCG The main factors that influence the reaction rate include the physical state of the reactants, the concentrations of the reactants, the temperature at which the reaction occurs, and whether or not any catalysts are present in the reaction. Chemical kinetics investigate how different experimental conditions influence the speed of a chemical reaction and yield information about the reaction’s mechanism and transition states, as well as the construction of mathematical models that describe the characteristics of a chemical reaction. This section reviewed coal gasification reaction engineering. A simplified reaction sequence for coal gasification is described in Fig. 15 [88]. Coal pyrolysis is the initial step in most coal conversion processes followed by gas phase and gasification reactions. The speed at which it takes place has an influence on the subsequent steps, so that it is of great importance in any accurate model [89]. The pyrolysis step is most dependent on the properties of the coal. Heating of coal causes its complex structure to decompose. The weaker bonds rupture at lower temperatures and the stronger ones at higher temperatures. There are many models of coal devolatilization such as Single First Order Reaction model (SFORM), multiple parallel reaction model (MPRM), distributed activation energy model (DAEM), multiple competing reaction model (MCRM), and consecutive competing char-forming reactions model (CCCRM). SFORM and DAEM suggest that for a single block of coal, the time-evolution of its constituent parts should be considered averaged over the whole block. This approach ignores spatial variation of temperature and is

Fig. 13. Equilibrium gas compositions of the carbon-oxygen-steam system at adiabatic conditions [86,87].

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Ei RTðtÞ

ki ¼ koi exp

(13)

Where koi is the pre-exponential or frequency factor in sec1, Ei is the apparent activation energy for constituent i in J/mol, R is the ideal gas constant in (J/mol kelvins) and T(t) is the absolute temperature of the coal particle in Kelvins. Values of koi, Ei, and Vi*are estimated from matching with experimental data.

4.1. Single First Order Reaction model (SFORM) The simplest method for the description of the kinetics of the pyrolysis reactions is to use a first order reaction for overall weight loss of the volatile and for individual species evolution.

  dV=dt ¼ ki V *  V Fig. 14. Fractional composition of CO and CO2 in equilibrium with b graphite at 1 atm as a function of temperature.

appropriate for transient weight loss of pulverized coal and can be used as a component of a more complicated traveling wave model [90e93]. As the process of thermal decomposition of coal evolves, i denotes one particular reaction and coal’s constituents are numbered with i ¼ 1.n. The thermal decomposition of coal is assumed to comprise large numbers of independent chemical reactions. Large fragments of the coal molecule are present due to depolymerization and the rupture of various bonds within the coal molecule. The strength of chemical bonds depends on the coal type and rank, related to the occurrence of different reactions at various temperature intervals. Vi is the released mass fraction of volatiles corresponding to the ith constituent. Thus Vi* is the initial mass of constituent i in the coal. The contribution to evolution by a particular reaction is described by a first order equation, so that the rate of pyrolysis is

  dVi ¼ ki Vi*  Vi dt

(12)

Where ki is the rate coefficient that is typically associated with temperature by Arrhenius equation as under,

(14)

The rate constant (k) in above equation is typically correlated with temperature by an Arrhenius expression

 E k ¼ k0 exp  RT

(15)

According to Howard [92], the most serious problem of Equations (14) and (15) in the SFOR model is the apparently asymptotic yield of volatiles that is observed after some time at the final temperature. As a result, the apparent value of V* as a function of final temperature is mechanistically inconsistent with the equations and is mathematically unmineable [92]. 4.2. Distributed activation energy model (DAEM) The DAEM is one of the multi-reaction models used widely to clarify the thermal decomposition processes of coal pyrolysis. This model was originally developed by Pitt [91] and later adapted by Anthony [90]. Pitt [91] assumed that the evolution of a certain substance involves an infinite number of independent chemical reactions by considering a continuous distribution of reactants. That is, many irreversible first-order parallel reactions that have different rate parameters occur simultaneously. In the DAEM model, the dependence on i is replaced by a continuous dependence on activation energy E so the values of koi, Ei and Vi* cannot be predicted earlier and must be estimated from the experimental data. DAEM has been applied to represent the change in overall conversion and the change in the yield of a given component

Fig. 15. Simplified reaction sequence for coal gasification.

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during the coal pyrolysis. The increase in the number of reactions required can cause a problem. This problem is simplified by assuming that the ki’s differ only in activation energy so a common assumption is then to take all the pre-exponential factors, koi, to have the same value ko for all constituents i. Then the number of reactions is large enough to permit the distribution of energy to be expressed as a distribution function f(E), where f(E) is the distribution of activation energies, representing the differences in the activation energies of many first order irreversible reactions. Then f(E) dE represents the fraction of the potential volatile loss V* that has an activation energy between E and E þ dE. Thus, the total amount of volatile material available for release from the coal can be written as:

dV * ¼ V * f ðEÞdE

(16)

With the distribution function f(E) normalized to satisfy

ZN f ðEÞdE

(17)

0

The solution then becomes

V*  V ¼ V*



Zt k0 ðEÞexp

E du f ðEÞdE RTðuÞ

(18)

0

The model is represented as follows when it is applied to represent the change in total volatile

V 1 * ¼ V

Zt



E k0 ðEÞexp du f ðEÞdE RTðuÞ

(19)

0

Miura [94] has discussed two major weak points in DAEM model. The first is the assumption of a constant ko value for all reactions. The other is the assignment of the Gaussian distribution to f(E). It is possible to estimate f(E) from experimental data without assuming the Gaussian distribution as performed by Vand [95]. However, in order to use the Gaussian distribution, a constant value must be assigned to ko beforehand in order to estimate f(E). The DAEM is generally recognized to be the most appropriate approach to model coal pyrolysis. Hashimoto and coworkers [96] correlated ko with E based on experimental data by the equation

ko ¼ aebE

(20)

Where a and b are constants dependent on reaction system. Once a and i determined, f(E) can be determined by a procedure presented by Vand [95] to uniquely to fit experimental data. However, it is not easy to determine a and b experimentally [94]. Let coal is heated from a low temperature T0 (low temperature under which reaction does not occur) at a constant heating rate a, Then the temperature of coal at time t is given by

T ¼ T0 þ at

(21)

The Arrhenius equation can be described as follows:

 ln

a T2



 ¼ ln

ko R E þ 0:6:75  E RT

(22)

Above equation is used to estimate both E and ko from the Arrhenius plot of ln(a/T2) vs. 1/T at the selected V/V* values for different a values. The relationship between V/V* vs. E could be obtained by plotting the V/V* value against the corresponding E value. The distribution curve of the activation energy, f(E), can be

205

obtained by simply differentiating the V/V* vs. E as shown in following equation. No assumptions are required for the functional forms of f(E) and k0(E) [94,97].

V ¼ 1 V*

Zf

ZES f ðEÞdE ¼

ES

f ðEÞdE

(23)

0

Two methods has been proposed to estimate f(E) and ko(E), a differential method [94] and an integral method [98], from a set of three TG experiments at different heating rates without assuming ko value and functional form for f(E). It was found for three coals that the f(E) curve spreads over 150e400 kJ/mol and that the frequency factor ko increases with the increase of E. The assumption of a constant ko value could not be employed for these coals. Estimated ko vs. E relationship was little dependent on coal types, and ko increased from 1011 to 1026 sl as E increased from 150 to 400 kJ/ mol [94]. Rate constant for Multiple Parallel Reaction Model are shown in Table 4. Comparing the two models, three parameters, k, Eo, and s are required in addition to V* for the DAEM model. However for the SFOR model, only two parameters, frequency factor and activation energy are required for analysis. In other word DAEM requires only one additional parameter, s, from SFOR model but it is applicable to the description of thermal decomposition processes with different heating rates [92,100]. In DAEM reactions are assumed to consist of a set of irreversible first order reactions that have different activation energies and a constant frequency factor. The differences in the activation energies are represented by a Gaussian distribution [103]. Paea [103] evaluated the SFOR and DAEM model and found that SFOR model is problematic because it can only be applied in limited conditions and cannot be expected to represent data accurately over a wide range of conditions and concludes Evidence is found that the DAEM is more powerful than the SFOR model in evaluating the complex experimental conditions of coal pyrolysis [103]. 4.3. Reactions of formation of selected gas products in coal pyrolysis Assuming that the coal pyrolysis is a first order reaction and that the temperature increase is of linear type, the evolution rate of a given gaseous product of coal pyrolysis can be expressed by the equation.



 dV ko E ko RT 2 E Vo e  exp  ¼  m m E dT RT RT

(24)

Table 4 The rate constants of MPRM based on individual volatile constituents. Species

ka

Eo [kcal/mol]

Reference

CO2 CO H2O HCN C2H6 CH4 H2 NH3 H2S Tar

6.5  1016 2.2  1018 1.4  1018 1.7  1013 8.4  1014 7.5  1013 1.0  1014 1.2  1012 2.91  109 4.3  1014 8.6  1014

67.0 60.0 79.0 59.6 59.1 59.0 80.0 13.7 36.89 54.6 27.5

[99,100] [99,100] [99,100] [99,100] [99,100] [99,100] [99e101] [99e101] [90,102] [99] [100]

a These constants are taken from DAEM with the standard deviations, si, are set zeroes.

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Where

k3

CO ðCOÞ # 0

dV/dT Rate of evolution of a given gaseous product ko Frequency factor m Rate of heating E Activation energy T Temperature R Gas constant Vo Total volume of resulting gas during pyrolysis

(The parentheses indicate adsorbed species). This mechanism leads to the rate expression

g¼ 

Knowing the total volume of the material evolved, one can fit Equation (22) to the experimental results and obtain E and a. closeness of the fit is a good indication of the validity of the assumed reaction order. As noted in the introduction, coal is such a complicated heterogeneous mixture of different organic and inorganic compounds that the reported pyrolysis kinetics are undoubtedly an average for a vast number of different reactions that give the same product. For this reason, the activation energy and frequency factor for a particular gas evolution process areonly “effective” values for the whole process. Porada [104] has determined values of activation energy and frequency factor of particular constituent reactions of hydrogen and methane formation during coal pyrolysis under non-isothermal conditions. The highest rate of the reaction occurs at the temperature which depends on the activation energy, frequency factor, and the rate of heating [Equation (20)]. The analysis of total kinetic curves for methane and hydrogen rests on the determination of kinetic parameters, such as activation energy; frequency factor and total volume of resulting gas for each constituent reaction. Therefore it is necessary to resolve the function expressing the formation rate of a given product into a number of functions expressing the formation rates of particular constituent reactions. This can be done in a graphical or analytical way. The rate of total formation reaction is described by the equation

  dV dV1  dVn  ¼ E1 ; k0;1 ; V0;1 þ En ; k0;n ; V0;n dT dT dT

(25)

The rate of each constituent reaction can be calculated by means of Equation (24). As gasification progresses, a decline in rate are usually observed since carbon of progressively lower reactivity remains. It has been observed that, upon heating, coal first becomes metaplastic and gives off volatile matter leaving a rather stable coke. Thus, coal or char may be regarded to be composed of two distinguished portions differing greatly in reactivity. The highly reactive portion is related to the volatile portion of coal characterized by the aliphatic hydrocarbon side chain, and to oxygenated functional groups present. The low reactive portion is the residual carbonaceous coke. Thus, the gasification of coal at elevated temperatures can be divided into the first and second phase reactions; each reaction represents one of the two distinctly different reactivities of carbon present in coal. The following mechanism was presented by Von Fredersdorff [86] for the CeCO2 reaction where the equilibrium adsorption of CO2 on the carbon surface is followed by surface reaction and the adsorbed and gas phase products are in equilibrium [105,106] k1

CO2 # ðCO2 Þ

(26)

ðCO2 Þ þ C/ðCOÞ

(27)

k2

(28)

k3

kf sc pCO2

0

1 þ f0 pCO þ b pCO2



(29)

k0 k1 k2 0 k a ¼ 3 , b0 ¼ 10 and sc is the active surface area k01 k3 k1 available for reaction per unit weight of carbon. Von Fredersdorff [86] proposed a similar mechanism for the Ce H2O reaction

Where kf ¼

k1

H2 O # ðH2 OÞ

(30)

ðH2 OÞ þ CO/ðH2 Þ

(31)

k2

k3

H2 ðH2 Þ # 0

(32)

k3

This leads to the rate expression

g¼ 

kf sc pH2 O

0

1 þ f0 pH2 þ b pH2 O



(33)

k0 0 k1 k2 0 k ,a ¼ 3 ; b ¼ 10 and sc is the active surface area 0 k1 k3 k1 available for reaction per unit weight of carbon. In the literature the Von Fredersdorff rate equation, is generally known as a Langmuire Hinshelwood rate expression expressed as under. Where kf ¼

g¼ 

kbpCO2  1 þ fpCO þ bpCO2

(34)

Where k is a rate constant and N and b are equilibrium constants. Turkdogan and Vinters [107] postulated a reaction mechanism involving two rate-controlling reactions in series-namely, the dissociation of CO2 and the formation of CO on the surface of carbon, for which the reaction rate is proportional to the partial pressures of CO2 and to the square root of the partial pressure of CO2, respectively. They found that at low CO content the rate is proportional to the square root of the partial pressure of C02 and, in the presence of more than 10% CO, that the rate is proportional to the partial pressure of CO2. Rao and Jalan [108] presented a critical evaluation of several theories and rate equations for the CeCO2 reaction. They concluded that a two-stage mechanism involving oxygen exchange between the carbon surface and the gas phase, followed by the rate-limiting carbon gasification step and a langmuir hinshelwood rate expression, represented a large part of the published data. They obtained activation energy of 79.6 kcal/g mol for the CeCO2 reaction. Katta and Keairns [105] reviewed the recent studies on carbon gasification reactions to shows that rate of reaction of char with H2O and CO2 appears to depend on the following factors: (a) the nature of the carbon (i.e., the type of coal from which char is formed) and the temperature used in the preparation of the char; (b) the change in reactivity during gasaification (increase or decrease in the effective surface area available for reaction); (c) the temperature and pressure of the reactions; (d) the gas composition

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(concentration of steam, CO2, CO, and H2); (e) the mineral content of the char; and (f) the particle size if the reactions are diffusion controlled. It is apparent from a large number of studies that the carbon gasification reactions are chemically controlled below a temperature of about 1100  C for the particle sizes normally used for gasification. Above this temperature level the diffusion effects become important [105e108].

 DE5 PCO R5 ¼ Ac e RT

(41)

 DE6 PH2 R6 ¼ Ac e RT

(42)

Rate and order of the surface reaction are determined under such conditions that diffusion is unimportant. This may be achieved by maintaining a sufficiently high flow rate and a sufficiently low temperature or by the use of low pressure. Mayers [109] found the reaction to be first order between 930 and 1650  F. Above 1650  F, however, diffusion controlled the reaction. Riley [110] based on his experiments on the oxidation of coal, reported energies of activation between 13.7 and 16.4 kcal.



 K1 1=2 1=2  pH2 K2 K3 PH2 PCO2  K2 K3 PH2 O PCO K1
  K2 PCO þ K3 PH2

 R7 ¼

4.4. Order of reaction and activation energy

(43) Here;

 K1 ¼ a1 T b1 e

4.5. Rate controlling step

 K3 ¼ a3 e K1 ¼



(35)

The above equation can be expressed in term of the partial pressure, based on finding of the Fredersdorff and Elliott and further work by Guo [116] and Yang [112,113]

DE1 PO2 R1 ¼ Ac e RT : 

R2 ¼ Ac e T  R3 ¼ Ac e

 bT

DE1 RT

(36)



PH2 O 

RT

DE2

(44)

RT

K3 ¼ a3 e

(45)

(46) (47)





DE2



RT

DE3

(48)

RT

(49)

Where; Kj The equilibrium constant of chemical reaction j, Kf The equilibrium constant expressed in terms of the partial fugacity of every composition; Ej The activation energy of chemical reaction j, R universal gas constant; G The free enthalpy of standard formation T Temperature Pi partial pressure of composition i in the mixed gas. Key aspects of kinetic assessment of UGC are summarized in Table 5. 5. Challenges for UCG

PCO2





RT



DE

a

DE3



4.6. Chemical reaction rate

K ¼ Ac e RT :



DE1

a1 T

K2 ¼ a2 e

The diffusion rate of the mole number for gas compositions equals the dynamic reaction rate of coal surface. On the basis of the above, we can obtain every chemical reaction rate Rj. The influence of temperature on the reaction rate is mainly reflected on the constant of reaction rate (K). According to literature the Arrhenius formula is:



 K2 ¼ a2 T b2 e

Increase in the rate of reaction with increasing gas velocity then indicates mass-transport effects. From the work reported, it may be concluded, with reservation, that diffusion controls the rate of oxidation of carbon at temperatures greater than 1350  F, for velocities up to 50,000 feet per second. The rate varies as the 0.4 to 0.7 power of the mass velocity with an apparent energy of activation between 2.3 and 5.3 kcal per g mol [111].

207

(37) 1 PCO PH2 kf

! (38)

The UCG process characteristics are strongly related not only to the technological but also to the natural factors, such as geology and hydrogeology of coal seams, their thickness, quality of coal and the surrounding stratum. Virgin coal seams are reported to be the optimal in terms of the UCG process.

Where DG

kf ¼ e RT

(39) 

R4 ¼ Ac e

DE4 RT

2 PH  2O

1 P kf CH4

! (40)

5.1. Suitable site selection Geological, geotechnical and hydrogeological issues are paramount in the selection of a site suitable for application of UCG techniques. The geological evaluation of the site requires a comprehensive representation of the coal seam, as well as of the overlying and underlying strata. Geotechnical factors such as the

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Table 5 Key aspects of kinetic assessment of UGC. Parameters

Expression/Model

Key aspects

Kinetic Studies of UCG

Single First Order Reaction Model (SFORM) dV=dt ¼ ki ðV *  V Þ

Expression is use to evaluate overall weight loss of the volatile and for individual species evolution. Two parameters, frequency factor and activation energy are required for analysis. Value of V* as a function of final temperature is mathematically unamenable Expression widely used to evaluate overall weight loss during the thermal decomposition processes at different heating rates Three parameters, k, Eo, and s are required for analysis Expression is used to evaluate rate of formation of a given gaseous product during coal pyrolysis.

Reactions of formation of selected gas products in coal Pyrolysis

Order of reaction and activation energy-Arrhenius formula

Distributed Activation Energy Model (DAEM)

Z t V E du f ðEÞdE k0 ðEÞexp 1 * ¼ RTðuÞ V 0 

E ko RT 2 E  exp   dV ko RT RT m E ¼ Vo e dT m dV dV1 dVn ¼ ðE ; k ; V Þ þ ðEn ; k0;n ; V0;n Þ dT dT 1 0;1 0;1 dT DE K ¼ Ac e RT

strength, jointing and deformability of the overlying strata all have a role to play in the response of the profile to the process occurring in the coal seam. The groundwater regime must be defined, both regionally and in the vicinity of the operation. Groundwater pressures in the coal seam and its permeability to water flow are integral to the UCG process operation. They govern the oxidant injection pressure that can be used and the potential water flow into the gasification cavity, which will impact on the chemistry of the reaction in the chamber. The presence of significant aquifer systems in the profile, particularly above the coal seam, may be impacted by subsidence resulting from roof collapse into the process cavity with potentially disastrous consequences to the operation [114].

Expression is used to the evaluate rate of total formation reactions Equation can be expressed in term of the partial pressure If rate and order of the surface reaction are determined at high flow rate and a sufficiently low temperature or by the use of low pressure, the diffusion is unimportant Above 1650  F diffusion control the reaction

quantity and quality of coal available. In the UK for the UCG site the following selection criteria are used by DTI [115]: (i) Coal seam 4.2 m thick, (ii) depth between 600 and 1200 m, (iii) the availability of good density and bore hole data, (iv) standoff 4.500 m from abandoned mine working license areas and (v) greater than 100 m vertical separation from major aquifers. A good knowledge of the adjacent strata is required to ensure well bore and environmental integrity. The explorations present no exceptional technical problems for the UCG process though there is always a chance that the site may get rejected as the study proceeds, due to the presence of a surrounding good quality water aquifer, low strength overburden or discontinuous coal seam layers. The cost of exploratory drilling and 3D seismic survey is high but is necessary for successful UCG operation [73].

5.2. Technical challenges for UCG There are many practical difficulties still to be overcome, and it is already clear that the technology can only be applied to certain types of coal seam. The hydrogeology of the seam is important, since excessive ingress of water would render the process uneconomic, and leakage of gas into underground water supplies could represent an environmental hazard. Both air and oxygen gasification has been tried. With air a very low Btu gas is produced, whereas with oxygen the cost of the blast and the losses make the process very costly. For these and other reasons, no commercial development has yet emerged [114]. Successful application of the UCG process requires the integration of a wide range of technical disciplines, which may also explain its slow commercial acceptance. Such specialist skills as are used in the fields of chemistry, chemical engineering, geology, geotechnical engineering and geohydrology are all necessary to plan and execute a successful UCG project [114]. 5.2.1. The major issues in the use of UCG technology UCG requires an understanding of various aspects of the selected site. The geology, hydrology, mining, drilling, exploration, chemistry and thermodynamics of the gasification reactions in the cavity are important parameters for successful operation. Before starting UCG, many issues should be considered. Some of them are discussed in following section [26]. 5.2.2. Exploration of the UCG site The potential of the UCG site can be estimated by identifying the geological structure of the coal seam, its depth and thickness,

5.2.3. Choice of a suitable drilling technique A good drilling technique is necessary to connect the injection and production wells. The cavity between these two wells is considered as the gasification reactor. 5.2.4. Environment and safety The various environmental issues associated with UCG are discussed in this section. 5.2.4.1. CO2 emissions. In the UCG process CO2 separation from the product gas and storage are the major concerns. CO2 is produced in significant amounts during the gasification. CO2 must be captured before venting to the atmosphere and stored or utilized for various applications. The higher pressure of the gas is an advantage offered by UCG for CO2 storage. CO2 sequestration work is under development internationally via the Intergovernmental Panel on Climate Change (IPCC) and Carbon Sequestration Leadership Forum [116]. 5.2.4.2. Groundwater contamination. Groundwater pollution is caused by the diffusion and penetration of contaminants generated by underground gasification processes toward surrounding strata and the possible leaching of underground residue by natural groundwater flow after gasification. The source of inorganic pollutants is primarily ash leachate, while the source of organics and ammonia is primarily condensed vapors. Typical organic pollutants include phenols, benzene, minor components such as PAHs and heterocyclics. Inorganic pollutants involve cations and anions [117e119]. Polycyclic aromatic hydrocarbons (PAHs) are hydrocarbons containing two or more fused benzene

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rings. They are environmental pollutants and are considered a health concern due to their potential carcinogenic, mutagenic, and toxic characteristics, as well as for the possible synthesis of dioxins from them. Therefore, PAHs have been listed as priority controlled pollutants by most of countries. Generally, the formation of PAHs is accelerated by higher heating rates. This is especially true of the lower molecular weight PAHs. When the heating rate changes from 15 to 40 C/min the total PAH yield increases by nearly 2.23 times. The formation of contaminants is the function of coal rank, the elemental composition of coal, and the gasification temperature. In case of hard coal gasification, the total load of inorganic and organic pollutants in the process water is substantially higher in comparison to lignite. It has been identified that the reaction pH is the parameter affecting concentrations of heavy metals in process waters [117]. The process of underground coal gasification is accompanied by heating of the roof and floor rocks of the coal bed and subsurface waters. It is known that an increase of temperature of subsurface waters leads to a change in the chemical and physical properties of water: a) the solvent power of water increases; b) the density of water decreases (the volume of liquid increases); c) the viscosity of water decreases. The transport of aqueous phase contaminants depends on the permeability of in situ rocks, the geological setting of the gasification reactor and the hydrogeology of the area [118]. Study results of large-scale UCG projects conducted during the late 1950’s and early 1960’s have revealed that groundwater contaminants, resulting from gasification, to be widespread and persistent, even up to five years after production had ceased. There are other reports stating that phenols were found with another aquifer in the former Soviet Union which extended over an area of 10 km2 [118]. The UCG site should be carefully evaluated for groundwater contamination. The UCG site should be away from the water aquifers. Detailed analysis is needed and after UCG start up, regular checkup of the water near the UCG site should be done [116]. 5.2.4.3. Surface subsidence. The multiwell technology can be used to reduce the chances of surface subsidence. The bore diameter in UCG is smaller than in usual mining operations. So there are less chances of surface subsidence when compared to conventional coal mining [116]. 5.2.4.4. Volatilization of mercury, arsenic and selenium. It is necessary to determine the amount of the volatile elements in the raw coals and the modes of their occurrence, in order to understand their volatilization behavior during the UCG process. Coal type and its UCG behavior also have an effect on volatilization. Being the highly volatile elements, mercury, arsenic and selenium are released from coal during the chemical and physical changes involved and become distributed mainly in the gas phase. The fuel gas produced from UCG process will inevitably be mixed with trace elements, especially the highly volatile ones such as mercury, arsenic and selenium, as a result of the high temperatures involved in the gasification process. However, UCG fuel gas cools as it passes through the production well and a significant proportion of the contaminants will deposit or condense in this transit. Those trace elements that do exit from the production well will be discharged into the atmosphere through direct combustion and will additionally cause deposition and corrosion when the gas is used for gas turbine power generation [120]. Set against these, its outstanding environmental advantages are the elimination of coal stock piles and coal transport and much of the disturbance at surface, low dust and noise levels, the absence of health and safety concerns relating to underground workers, the

209

avoidance of ash handling at power stations, and the elimination of SO2 and NOx emissions [37]. Table 6 summarized the challenge in UGC and their prospective solutions. 6. Advantages of underground coal gasification This section discusses the advantages and limitations of UGC. 6.1. Advantages of underground coal gasification The advantages of UCG over conventional underground or strip mining are related to its resource recovery, environmental impact, health and safety benefits, process efficiency and economic potential. Table 7 summarized the advantages and limitations of UGC. 6.2. UGC challenge and promises Whilst the underlying science can be developed through modeling work backed up by laboratory-scale experimental work, most countries active in the field have found the need to move to pilot-scale trials in order to explore UCG performance at depth in coal seams. The cost of these trials tends to be dominated by drilling costs and usually amount to millions of pounds. Extended trials to explore the consistency of operation over a period of time are particularly expensive. The trials reported todate are at relatively shallow depths (100e200 m). There are clear conclusions that can be drawn from the work to date: 6.3. UCG-CCS concept All fossil fuels produce CO2 when burned conventionally, but some liberate more energy than others in the process. Although calorific values vary by source, typical figures would be 50 GJ t1 for natural gas, 45 GJ t1 for crude oil and 30 GJ t1 for coal e which means that coal has the highest CO2 emissions per unit of energy produced. If we are going to see a major swing toward the use of coal, technologies for capturing the downstream CO2 emissions become increasingly important [37]. The broad technology options available for capturing CO2 are physical absorption, chemical absorption, membrane separation and cryogenic separation [37,121]. CO2 storage capacities of deep deposits mainly depend on their accessibility to carbon dioxide with respect to pore space for free gas storage and surface area for storage by sorption. The UCG process creates voids deep underground following gasification of the coal. These voids will inevitably collapse, just as voids produced by longwall coal mining do, leaving high permeability zones of artificial breccias e known as ‘goaf’ (meaning a cave) e which are almost invariably isolated from surface by low permeability superincumbent strata. Where UCG has taken place at depths in excess of about 700e800 m, storage of CO2 in these artificial high permeability zones is a very attractive proposition. A combined UCGeCCS project can then offer integrated energy recovery from coal and storage of CO2 at the same site [37,121]. Physical sorption represents a feasible option for CO2 storage in underground gasification cavities [122]. If UCG can be successfully linked to CCS, then the combined UCGeCCS offering provides a way of harnessing the energy contained within huge untapped coal resource whilst remaining within the ever-tightening targets for reducing CO2 emissions. Regarding the safety of CCS, the failure of the underground CO2 storage system appears to have limited consequences, suggesting a low risk [123].

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Table 6 UGC challenge and solutions. Parameter

Challenges

Potential solutions

Site Selection

The presence of significant aquifer systems in the profile, particularly above the coal seam, may be impacted by subsidence resulting from roof collapse into the process cavity with potentially disastrous consequences to the operation. Deeper seams require guided drilling technology to initiate a well at the surface that is deviated to intercept and follow a coal seam for many hundreds of meters, and establish a link between injection and production wells. This results in higher drilling costs. Deep seams require higher injection and operating pressure, and increase the cost of any subsequent pump-and-treat option. The evaluation of potentially productive sites must include the determination of the amount of coal available in a gasification project in conjunction with a consideration of the potential applications of the produced gas.

Excellent treatise on UCG site selection has been published by the U.S. Department of Energy. During in situ coal gasification remote sensing technique may be used for mapping underground fracture systems, locating tunnels or water-bearing strata and mapping burn fronts. Shallower seams are more likely to produce surface subsidence. As extraction depth increases, surface subsidence decreases. Deeper seams are less likely to be linked with potable aquifers, thus avoiding drinkable water contamination problems. If the product gas is to be used in gas turbines, additional compression may not be necessary. UCG cavities at depths of more than 800 m could be used for CO2 sequestration. For each potential site, the productive lifetime of the site must be determined as a function of required gas yield. For 20-year continuous operation of a 300 MW UCG-based combined-cycle power plant (efficiency, 50%), it is necessary to produce 75.6  109 Nm3 of syngas with a heating value of 5 MJ/m3. Based on 95% recovery of the coal resource and 75% total energy recovery, 33  106 metric tons coal will be required to be gasified for this purpose [33]. A combined UCGeCCS project can offer integrated energy recovery and storage of CO2 at the same site. UCG below the water table avoid inducing uncontrolled ingress of oxygen from the surface.

Depth of the coal seam

Evaluation of potentially productive sites

CO2 Emissions Uncontrolled ingress of oxygen from the surface Groundwater Contamination

Surface Subsidence

The productive lifetime of the site Volatilization of Mercury, Arsenic and Selenium.

In the UCG process CO2 separation from the product gas and storage are the major concerns. There is a risk of uncontrolled oxygen ingress occurring, potentially supporting a wildfire in the coal seam, which is highly undesirable. Groundwater pollution is caused by the diffusion and penetration of contaminants generated by underground gasification processes toward surrounding strata and the possible leaching of underground residue by natural groundwater flow after gasification.

Under the high temperature, the internal molecular structure reorganizes, which completely changes the coal seam’s surface morphology. Stress and displacement will happen among rocks due to the high temperature.

Being the highly volatile elements, mercury, arsenic and selenium are released from coal during the chemical and physical changes involved and become distributed mainly in the gas phase.

Careful evaluation of UCG site help in avoiding for groundwater contamination. The UCG site should be away from the water aquifers. Detailed analysis UCG at start up and regular checkup of the water near the UCG site should be done for efficient monitoring. Younger [79] has provided checklist of key hydrogeological issues, and actions required for dealing with them, for all of the main phases of the life-cycle of UCG (-CCS) operations. Numerical simulation on the temperature field, concentration field and pressure field is reasonable in the underground gasification of steep coal seams on the experimental condition. Same can be used to predict the operating condition during UGC. Multiwell technology can be used to reduce the chances of surface subsidence. Coal type and its UCG behavior effect volatilization. It is necessary to determine the amount of the volatile elements in the raw coals and the modes of their occurrence, in order to understand their volatilization behavior during the UCG process. UCG fuel gas cools as it passes through the production well causing significant proportion of the contaminants to deposit or condense in transit conduit. Those trace elements that do exit from the production well can be handled using pollution presentation technology.

6.4. Post-burn processing

6.5. Economics

The product gas can serve different purposes such as being used as fuel gas or as feedstock for liquid fuels and chemicals [33]. The raw product gas from UCG is similar to that produced by surface gasifiers for which gas cleaning technologies have already been developed. Additionally, the product gas can be processed to remove its CO2 content before it is passed to the end users; the captured CO2 can then be stored in the underground cavity, thereby contributing to climate change mitigation [26]. The post-burn, rawgas processing is tailored to the impurities present and the requirements for the final product gas. When the product gas reaches the surface it contains energy not only in its chemical composition but also in its high temperature, pressure and velocity. The gas treatment system must accept product gas at elevated temperatures and high pressures (5.3 MPa) from the production wellhead clean and dry this stream and route it to the end use while maximizing the potential for energy conversion.

Most estimates for conventional oil and gas suggest a peak in production in the next 10e15 years, with reserves being substantially depleted by 2050 for oil and 2070 for gas. Compared with these, coal reserves are usually estimated at more than 200 years [37,121]. Against this background, if UCG technology can effectively increase usable coal resources three-fold, by releasing the energy from coals that are inaccessible using conventional mining techniques, the economic case for UCG as a bridging technology becomes compelling. In broad terms, the economic case for UCG is a balance between positive and negative factors. On the positive side, UCG offers a low-cost route to emissions reduction; the cost is lower than for surface gasification plants because there is no need to mine, store or transport coal, there are no solid residues to dispose of, and there is no need to purchase a gasifier; it converts an abundant natural resource into a secure, economic supply of gas; it enables stranded coal resources (e.g. deep or offshore) to be

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211

Table 7 Advantages and limitations of UGC. Advantages of UGC

Limitations

 It can be used to recover the energy content of low rank coals that are not economically or technically feasible to recover by conventional technologies because of their seam thickness, depth, high ash and/or excessive moisture content, large dip angle, or undesirable overburden properties.  Coals that are unmineable (too deep, low grade, thin seams) are exploitable by UCG, thereby greatly increasing resource availability.  Lignite and sub bituminous coal are ideal for gasification, as is bituminous coal provided no significant swelling characteristics exist.  CV’s in the range of 12e14 MJ/m3 are recorded when using oxygen feed and this may be slightly increased as the process develops at a greater depth.  Deeper coals offer the opportunity to have much higher pressures in the reactor resulting in higher methane content and resultant higher heat value gas.  The UCG process has a higher thermal efficiency than surface gasification processes.  The CRIP concept has led to the highest gasification efficiency in terms of oxygen usage and will allow subsidence to be minimized or possible eliminated, by using wider barrier pillars between panels.  UCG also lowers the capital investment by eliminating the need for specialized coal mining, processing (transporting and stocking) and gasification reactors, reducing operating costs, surface damage and eliminating mine safety issues such as mine collapse and asphyxiation.  UGC uses lesswater than surface gasification processes which must maintain a high steam-to-air ratio to avoid slagging. Furthermore. It does not use high quality surface water, but utilizes the water within the coal seam itself.  Most of the ash in the coal stays underground, thereby avoiding the need for excessive gas clean-up, and the environmental issues associated with fly ash waste stored at the surface.  Reclamation of the land is not a serious problem since the surface disturbance is minimal.  There is no production of some criteria pollutants (e.g., SOx, NOx) and many other pollutants (mercury, particulates, sulfur species) are greatly reduced in volume and easier to handle.  UCG eliminates much of the energy waste associated with moving waste as well as usable product from the ground to the surface;  UCG, compared to conventional mining combined with surface combustion, produces less greenhouse gas and has advantages for geologic carbon storage.  The well infrastructure for UCG can be used subsequently for geologic CO2 sequestration operations. It may be possible to store CO2 in the reactor zone underground as well in adjacent strata.  UCG research and development have been conducted in several countries, including long-term commercial operation of several UCG plants in the former Soviet Union.

 Shallow seams are not suitable for gasification because of high gas losses, potential breakthrough to surface and possible contamination of groundwater. The shallowness of the coal seams means the pressure in the reactor is low and the quality of the gas produced is reduced compared to deeper systems. Heat losses are considerable with such seams, leading to low thermal efficiency and lower product gas quality.  Thin seams, less than 3 m thick, will be difficult to exploit economically unless in a multi-seam environment.  Installation of well pairs (injection and production wells) is costly and therefore it is desirable to gasify the maximum volume of coal between a well pair.  During UCG processes, the surrounding rock acting as the furnace walls will be affected by high temperature, and its mechanical properties will change with the increased temperatures.  Stress and displacement will happen among rocks due to the high temperature.

converted into commercial reserves; there is a range of potential end uses and markets, e.g. power generation, heating, synthetic fuels, chemicals and hydrogen; it is largely immune to crude oil price swings (unlike conventional coal mining which relies on diesel-fueled equipment and transportation); and it is cheaper than natural gas for power generation [121]. The negatives are: technical and commercial uncertainty, since the technology has not yet been widely deployed; syngas production

rates and composition are variable compared with pipeline-delivered natural gas; open-cast coal mining (where acceptable) is cheap; there is a risk of ground subsidence and a risk of aquifer contamination (especially freshwater aquifers); trials and prospective site evaluation are expensive; there can be significant costs in transporting the syngas to the point of use; and planning approval processes are still under review in various countries. This is the background against which prospective investors have to make a decision [121].

Table 8 Economic and environmental benefits of UCG. Aspects relevant to the commercial viability of UCG

Economic and environmental benefits of UCG over coal mining for power generation

1. The productive lifetime of the site (must be determined as a function of required gas yield) 2. Geological variables and relationship to drilling difficulty and cost 3. Drilling difficulty and drilling cost 4. Power output and life of the gasifier 5. Subsidence effects 6. Safety of production operations 7. Commercial perspective of UCG industry 8. Strategic value of UCG 9. Environmental sensitivities 10. The potential market e chemical and power generation industries

1. Potentially lower overall capital and operating costs 2. Flexibility of access to mineral 3. Larger coal resource exploitable 4. Lower fugitive dust, noise and visual impact on the surface 5. Lower water consumption 6. Low risk of surface water pollution 7. Reduced methane emissions 8. No dirt handling and disposal at mine sites 9. No coal washing and fines disposal at mine sites 10. No ash handling and disposal at power station sites 11. No coal stocking and transport 12. Smaller surface footprints at power stations 13. No minewater recovery and significant surface hazard liabilities on abandonment

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The economic case for UCG syngas displacing natural gas or coal for power generation is relatively straightforward. Alternative uses, such as conversion of syngas into liquid fuels, chemical intermediates or hydrogen, are more difficult because, whilst the added value is well known (and much higher than for power generation), there is a tighter requirement for syngas clean-up. Technologies for cleaning up UCG syngas to chemical feedstock standard are still under development and so the costs are less well known [121]. The size of the coal resource is a major commercial factor for the development of the underground coal gasification process. In general, the requisite resource properties include availability of at least 50 million tons of coal in place in a coal seam at least 15 feet thick. The seam depth should be no more than 1200 feet for horizontal seams and no more than 1500 feet for steeply dipping beds [22]. The market for the product gas is the second major factor for commercial development of UCG. If the markets for utilizing the gases are located near the gasification site then gas can be economically transported. Key economic and environmental benefits of UCG are listed in Table 8. The investigation shows that underground coal gasification will inevitably cause a potential environmental problem, viz. degrading the groundwater quality in the vicinity of the gasification panel. As time progresses, the situation will become more serious. So, as clean energy is developed, it may result in secondary pollution or even destroy the ecological safety at the same time [7].

[5] [6]

[7]

[8]

[9]

[10]

[11]

[12]

[13] [14]

[15]

7. Conclusions and future prospects [16]

In spite of the significant advantages of UGC technology summarized in Table 7 there was no comprehensive review on UCG process description with emphasis on its thermodynamic and kinetic studies. There was also lack of compact review articles in this area. We anticipate that this review will promote research and development efforts, scale-up of the gasification process, and largescale implementation of UCG in future. Classification Criteria for UCG is summarized in Table 3 whilst the underlying science is being developed through modeling work backed up by laboratory-scale experimental work. In this study the key aspects of kinetic assessment of UGC are given in Table 4, however most countries active in the field have realized the need to move to pilot-scale trials in order to explore UCG performance at depth in coal seams. The cost of these trials tends to be dominated by drilling costs and usually amount to millions of pounds. Extended trials to explore the consistency of operation over a period of time are particularly expensive. The technology of UCG has been technically proven to work at numerous locations and different depths ranging from several hundred meters up to 1.4 km of depth. Nevertheless, field tests have been encouraging, and there is a growing body of positive economic projections as the technology becomes gradually established. An upcoming project now being planned by an industrial consortiumd what will be the first commercial burndmay be the most promising event for the future of underground coal gasification. References

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