- Email: [email protected]
Comparative study of MSW heat treatment processes and electricity generation
Accepted Manuscript Comparative Study of MSW Heat Treatment Processes and Electricity Generation Hossam A. Gabbar, Mohamed Aboughaly, Nasser Ayoub PII:
S1743-9671(17)30012-0
DOI:
10.1016/j.joei.2017.04.009
Reference:
JOEI 327
To appear in:
Journal of the Energy Institute
Received Date: 3 February 2017 Revised Date:
24 April 2017
Accepted Date: 26 April 2017
Please cite this article as: H. A. Gabbar, M. Aboughaly, N. Ayoub, Comparative Study of MSW Heat Treatment Processes and Electricity Generation, Journal of the Energy Institute (2017), doi: 10.1016/ j.joei.2017.04.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Comparative Study of MSW Heat Treatment Processes and Electricity Generation
RI PT
Hossam A. Gabbara, b, *, Mohamed Aboughaly b, Nasser Ayoubc a Faculty of Energy Systems and Nuclear Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, L1H 7K4 ON, Canada b Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, L1H 7K4 ON, Canada c Faculty of Industrial Education, Helwan University, Sawah Street, Amiriah, Cairo, Egypt
M AN U
SC
Abstract: This paper discusses the latest analysis in solid waste thermal treatment methods including life cycle assessment (LCA), process systems, economic and energy analysis. The MSW collected by municipalities such as paper, plastics, organic materials, glass, metals, food, leather and rubber can be utilized, via primary treatment methods which are mainly pyrolysis, gasification and combined pyrolysis gasification (P-G) cycles to generate thermal energy or electricity. The importance of solid waste treatment comes from its potential to convert waste into several sources of energy or fuels such as gasoline, syngas or diesel, eliminate waste, and reduce CO2 emissions. The process systems are explained in terms of process stages, carrier gases, operating pressures and temperatures, end products and reaction residence time. According to MSW management statistics, landfilling and incineration are considered a major activity and is incompatible with the exponential increase of global production of MSW per year. Environmental analysis shows that combined pyrolysis-gasification has lowest environmental impact with acceptable results for pyrolysis and gasification. Economic analysis shows highest capital cost for combined pyrolysis gasification (P-G) followed by pyrolysis and gasification process systems respectively. Operating and maintenance cost shows highest for pyrolysis, while gasification systems shows highest revenue per ton.
1. Introduction
TE D
Keywords: Sustainable Engineering, pyrolysis, gasification, chemical recycling, energy production, economic analysis, mechanical recycling, incineration.
AC C
EP
Municipal waste management includes solid waste control, storage, processing, disposal, and recycling of waste generated from residential and non-residential areas. The MSW, management methods are considered a public service managed by governmental organizations that, in most cases, have limited resources. However, this can’t continue the same way, in the near future, due to the exponential increase of the generated solid waste. Furthermore, various assessments showed that 20-40% of carbon in MSW is derived from fossil fuels during manufacturing [1]. To solve this issue, recently, a great attention was given to MSW recycling [2]. Currently, mass incineration is still the most applied global waste treatment method followed by gasification [3]. However, the MSW chemical recycling, investigated in this paper, is considered a breakthrough that can solve environmental challenges faced by modern societies owing to its ability to eliminate most of treated waste and convert it to energy as discussed by [4]. The proportion of combustible components in MSW is about 81.26% on average that makes thermochemical treatment methods (pyrolysis and gasification) the most efficient and effective for MSW recycling [5]. Hence, the chemical recycling of MSW has become a prominent waste management method due to its promising sustainable energy production and waste elimination potential [6]. The thermal cracking, in these methods, releases syngas that can be used in form of district heating, power generation fuel, or other combustion purposes. The amount of energy recovered from thermal cracking of MSW varies significantly depending on its chemical composition that affects the thermodynamics process (i.e. impurities reduce energy recovery). The important thermodynamic parameters include; mass composition, mass flow rate, and HHV as well as characteristics of thermodynamic cycle [7].
1
ACCEPTED MANUSCRIPT
2. Literature Review 2.1 The MSW Management
Prevention Re-use or mechanical recycling Chemical recycling Thermal energy recovery Disposal
SC
a) b) c) d) e)
RI PT
The MSW composition can be divided into two main components (e.g. renewable and non-renewable) as illustrated in Table 1. Per the European Union classification (2008), the hierarchy of MSW management is arranged in the following order (Chen, 2013):
M AN U
As seen figure1, expected MSW generation per capita in most of European countries are in range of 500-700 kg / person per year. Therefore, a need for MSW treatment and conversion to waste is needed for a sustainable future. MSW per capita per year ( kg/person/year)
800 700 600 500
300 200 100 0
Switzerland
EP
United Kingdom
TE D
400
Germany
2001 2010
Finlad
AC C
Fig. 1 - MSW generated per capital in 2001 and 2010 [7]
In terms of municipal waste management, Figure 2 shows a statistic of implemented waste to energy recycling methods in 32 European countries from 2001 to 2010. The statistics show that landfilling is still the most common waste treatment method with expected landfilling of 115 - 155 million tons/ year, while Incineration is the most common thermal recycling method followed by gasification and pyrolysis.
2
ACCEPTED MANUSCRIPT
MSW management in 32 European countries 300 250 150 100 50 0
2001
2004
2008
Recycling
60
70
90
Incineration
50
80
50
Landfilling
155
140
no treatment
20
20
RI PT
200
2010 90
SC
MSW million tons
350
50
133
115
17
15
M AN U
Fig. 2 - MSW chemical recycling in 32 European countries from 2001 to 2010 [8]
Vast research has been done on MSW collection, treatment, disposal, and recycling methods including the following:
TE D
• • • • •
MSW yearly production in industrial countries such as Canada, Japan and USA. MSW composition and amount of landfilling per year. Thermal plasma systems in terms of operating temperature, energy duties, MSW product yield, and thermal plasma control. MSW gasification incineration and pyrolysis operating conditions. Life cycle assessment of chemical and mechanical recycling methods. Metal catalysts and its utilization in pyrolysis and gasification. Thermal and electrical efficiency of different process systems Mass and energy balance of MSW chemical recycling
EP
• • •
AC C
The scale of MSW production around the globe has a direct impact on its importance and development on a sustainable solution that can eliminate the traditional method of landfilling. The most common four waste management strategies are landfilling with methane utilization, landfilling without methane utilization Incineration (i.e. mass burning), and waste sorting. The expected GHG emissions from different thermal processes which affect global warming are CO2, CH4, N2O, SO2, NOx, HCl, H2S, and HF [9]. Below is a schematic diagram of waste management strategies.
3
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3 – MSW sorting and chemical recycling block diagram
AC C
EP
TE D
MSW sorting and separation is essential to ensure that only treatable MSW is fed to the gasifier. MSW contamination is divided as physical or chemical. Metal contamination is considered as physical contamination and has a huge impact on the gasification process. Among common metal contaminants is stainless steel, carbon steel, screws, and aluminum alloys. Metal contaminants are divided to ions and solid contaminants which are later separated and used in the manufacturing industry [10]. Different metal contamination processing pathways is shown in figure 4.
Fig. 4 – Pathways to MSW Contaminant removal
4
ACCEPTED MANUSCRIPT
Most metal contaminants in MSW are from car batteries, consumer electronics, ceramics, housing, and construction sites. Nearly 20% of lead batteries which are not recycled contribute to around 66% of lead in MSW in the US [10].
RI PT
2.2 Waste to energy technologies
M AN U
SC
The U.S-EPA considers MSW a renewable energy resource due the possibility of its conversion to useful products or energy sources [11]. The chemical recycling, considered in this work, is used as a Waste-to-Energy, MSW management option. The thermal and chemical recycling methods of MSW are; direct combustion (i.e. Incineration), pyrolysis, or gasification. Incineration has advantage of sustainable reduction of MSW by 90% as well as minimal treatment of waste preprocessing but has high CO2 emissions in comparison with other thermal treatment methods [12]. Disadvantages of incineration include high capital and operational cost, high pollution, and mandatory treatment of flue gas [13]. The high CO2 emissions also limit large scale recycling for landfilling and incineration. Added to that, its broader widespread is restricted, owing to harmful emissions, especially with dioxins and acidic gases (SO2, NOx, HCl, etc.). Besides, excessive corrosion by HCl has limited the maximum steam super-heater temperature and thus led to lower energy efficiency [14]. Table 1 - MSW renewable and non-renewable components [15] Renewable MSW Cellulose paper (i.e. newsprint, newspapers) Containers, Packaging
Rubber
Thermosetting plastics (i.e. Polyurethanes, polyesters etc.)
TE D
Food and organic compounds Leather and clothes
Non-Renewable MSW Thermoplastics plastics (i.e. Polyethylene, Polystyrene, Polyethylene tetraphalate)
Metals (i.e. copper, iron steel, etc.)
2.2.1
AC C
EP
Waste to energy technologies have been implemented as large scale chemical plants successfully throughout the world. It is estimated that about 130 million tonnes of MSW are chemically recycled annually in over 600 waste to energy chemical plants worldwide. The syngas or liquid fuels end-products can either produce electricity through a high-pressure steam generation cycle for district heating or used as combustion fuels [16]. Syngas requires purification and clean-up and can also be converted to ethanol, methanol, gasoline, or diesel using catalysts. Gaseous products of these methods are in form of CH4, H2, and hydrocarbon liquids, which are used in the thermal cycle to produce high pressure steam (HPS) followed by electrical production using a steam turbine [17]. Non-renewable portions such as metals and non-decomposable components lower heat efficiency and syngas yield in thermal processes [16] and it should be separated using density mass separation equipment or as unaccepted slag/tar after gasification. Thermo-Chemical MSW Treatment
The high fossil fuel consumption and the environmental impacts arising from GHG emissions, unsustainable sources, and high cost of refining and fossil fuel extraction have attracted great attention for MSW recycling, and alternative fuels. Thermo-chemical transformation via pyrolysis/gasification of MSW is highly efficient due to lower greenhouse-gas emissions in comparison with other recycling methods more sustainable solution for MSW deposits [18]. Added to that, continuous process with high conversion can be achieved. The major two thermo-chemical MSW treatment methods are thermal pyrolysis and gasification which are viewed as the economically viable approaches for residential waste, commercial waste, or industrial waste methods [17]. Gasification is defined as thermal conversion of MSW to a gaseous product known as syngas. The synthesized
5
ACCEPTED MANUSCRIPT
RI PT
gas is a mixture of two main components CO and H2, with minor traces of CH4, CO2 and gaseous hydrocarbons [18]. On the other hand, MSW pyrolysis is a thermal composition in limited oxygen conditions to synthetic fuels in range of 400 °C - 650 °C in long residence time [19]. As seen in table 2, expected end products from incineration are flue gas mainly CO2 and H2O. While gasification yields CO, H2 and CH4. Combustion is carried in excess oxygen supply, while gasification is carried in limited oxygen supply [20]. Pyrolysis should be carried in inert conditions and the most common medium is nitrogen gas due to its abundance and low cost in comparison with inert gases. Table 2 - Main characteristics MSW thermo-chemical treatment methods [21]
Incineration
Gasification
Pyrolysis
Convert MSW in a combustion process to high temperature flue gas CO2 and H2O in excess oxygen
Convert MSW to HHV syngas which consists mainly of CO, H2 and CH4
MSW thermal cracking in absence of oxygen to hydrocarbon gases, liquids and wax
Reaction environment
Oxidizing reaction
Air, pure oxygen, steam
Nitrogen gas or any inert gas
Reactant gas
Excess Air
Air, O2, Steam
No reactant gas
Temperature
850℃ to 1200℃
Pressure range
Atmospheric
End-products
CO2, H2O
Undesired effluents
SO2, nitric oxides, HCl
Essential installations
Air pollution control
M AN U
SC
Process Description
500℃ to 800℃
Atmospheric
Over atmospheric
CO, H2, CO2, H2O, CH4
CO, H2, CH4 hydrocarbon liquids
H2S, HCl, Sulphur oxides, NH3, HCN, tar Syngas cleaning is required
H2S, HCl, NH3, HCN, tar. Particulates No treatment is needed
TE D
550 ℃ to 900 ℃
EP
For catalytic gasification, fixed bed reactors are used. The catalytic gasification reduces the operating temperature from 1200℃ to 700℃ . As the operating temperature increases, the gas content increases and the tar content is reduced to less than 10 wt % with expected carbon conversion of 60 - 75 wt % [22]. Below are the possible reactors for pyrolysis and gasification: Table 3 - Pyrolysis and gasification reactors [23]
AC C
MSW Treatment Method
Recommended Reactor Type
Possible Reactors for pyrolysis
Rotating kiln, heated tube, Surface contact
Possible reactors for gasification
Fluidized bed, Fixed bed
In MSW thermal treatment processes the products are utilized to produce steam in a steam cycle using a boiler to produce steam followed by a steam turbine to generate electricity in an electrical cycle. For steam cycle incineration shows a higher conversion rate than gasification and pyrolysis. For steam cycle, incineration is in range of 19-27 % while in pyrolysis and gasification 9-20% [24].
6
ACCEPTED MANUSCRIPT
2.2.2. Disadvantages of Incineration and Landfilling
RI PT
Incineration is combustion of solid waste in excess oxygen at very high temperatures (i.e. above 1000℃ ) with a complex process system that requires huge capital investment and high operating costs. Most of the operating cost is due to complying with gaseous emissions and pollution control. The incinerator chamber requires complete burnout of flue gases which causes erosion, corrosion and boiler fouling causing high maintenance cost and more expensive steel grades. As the environmental precautions are being stricted day by day, incineration process systems face lower profits due to higher operating and quality control costs [25]. Existence of dioxins and heavy metals in incineration gaseous emissions makes it a substantial source of environmental emissions and prevents further expansion of incineration plants.
SC
Solid waste landfilling is still the most common system for waste disposal worldwide which involves direct burying of solid waste under the ground in environmental friendly land site. The disadvantage of this process is high methane emissions due to anaerobic digestion of solid waste and inability of waste elimination but rather decomposition. Materials such as plastics and inorganic materials are inapplicable for such method.
TE D
3. Pyrolysis and Gasification Methods
M AN U
The process involves separation of hazardous materials and compacting the solid waste into dense self-contained packages before burying into safe underground systems. A major disadvantage of this method is acquiring of large land area sites and inability to implement on large quantities of MSW. An ordinary MSW landfilling plant consists of collection and transportation of MSW trays to a landfill underground system. The landfilling area has a separate leachate collection at the bottom of the while methane gases are collected at the top of the landfilling system and can be utilized for energy production [26].
EP
Pyrolysis and gasification in recent years have increased over incineration with the goal of reducing incineration environmental impact and improve electricity generation dependency from waste in Italy [24]. MSW chemical recycling has advantage of feedstock flexibility, hydrocarbon products, chemical synthesis, and electricity production. Large scale implementation of waste to energy chemical plants could even replace fossil fuel dependency. Every ton of chemical recycled MSW replaces 0.4 ton of coal used for electricity production in the US. It is expected that MSW chemical recycling large scale implementation in the US replaces coal mining by 100 million tons/ year [27].
AC C
3.1 MSW Gasification
MSW gasification is the transformation of solid waste to volatized gases in limited oxygen flow to gaseous products including mainly CO, CO2, H2, CH4 and hydrocarbon liquids. Alternatively, MSW pyrolysis is thermal decomposition and thermal cracking in absence of oxygen to hydrocarbon fuels [18]. Below is a process block diagram of MSW gasification:
7
ACCEPTED MANUSCRIPT
Syngas cleanup Power Generation + Steam Generation + Electrical Energy
Feedstock Municipal Solid Waste (MSW)
RI PT
Synthesis Gas (SYNGAS)
Gasifier Reactor
SC
Slag, and/or Ash
M AN U
Fig. 5 - MSW gasification process schematic diagram
MSW gasification have several 100 waste treatment sites worldwide. The characteristic of these process systems has the following operating condition (1,100℃ , 1 - 20 bar), short residence time (i.e. few seconds) and high conversion in comparison with pyrolysis and incineration. The MSW needs preparation by means of crushing and sieving the feed with controlled moisture content to increase heat transfer area during gasification [24]. The mass composition of the MSW affects the gasification process. Thus, an ultimate analysis of MSW is required which involves determination of main components such as carbon, oxygen, nitrogen, Sulphur and ash [28].
TE D
3.2 MSW Pyrolysis
AC C
EP
Power Generation + Steam Generation
Feedstock Municipal Solid Waste (MSW)
Hydrocarbon Gas
Pyrolysis Reactor
Ash, Tar Fig. 6 - MSW pyrolysis process schematic diagram
8
ACCEPTED MANUSCRIPT
Table Main
Parameters
Conventional pyrolysis
Fast pyrolysis
Operating temperature(℃) 850 - 1250
0.1-1
10-200
(seconds)
300-3600
0.5-10
4-
Flash pyrolysis
1050 - 1300 >1000
SC
550 - 900 Heat flux (℃/s)
RI PT
For MSW pyrolysis, slow pyrolysis has longer residence time, slower heating rates up to 20 ℃ /min and lower operating temperatures in comparison with alternative pyrolysis methods. Fast pyrolysis is a process in which very high heat flux are applied to MSW leading fast conversion to syngas in absence of oxygen achieving maximum liquid yields at high heating rates. The reaction temperature is higher than slow pyrolysis (i.e. 900°C). Alternatively, very fast pyrolysis is also referred as flash pyrolysis, its operating temperature is slightly higher than fast pyrolysis. The main product distributions are similar to fast pyrolysis [29].
<0.5
M AN U
operating conditions for MSW pyrolysis process [29]
TE D
As seen above, conventional pyrolysis is suitable for producing liquid hydrocarbons. Fast and flash pyrolysis process mainly CH4 and light gaseous hydrocarbons for electricity generation. 4. Environmental and Economic Comparison
EP
4.1 Environmental LCA of pyrolysis and gasification
AC C
Life cycle assessment of MSW thermal treatment methods are evaluated in terms of GHGs emissions including the following factors: • Acidification potential - Acid gases such as SO2 react with water in the atmosphere forming acid rain • Ozone photochemical formation - Tropospheric ozone formation occurs when nitrogen oxides (e.g.: NO2) or carbon monoxide and volatile organic compounds react in the atmosphere in presence of sunlight. • Global warming - Release of GHGs such as carbon dioxide which traps heat inside the atmosphere causing elevated temperatures. Below are gas emissions analysis of MSW different chemical recycling methods: Table 5 - LCA of pyrolysis and gasification of MSW in kg/Ton of product gas [30]
Recycling Method
Nitrogen dioxide
Sulphur dioxides
Carbon monoxide
HCl
9
ACCEPTED MANUSCRIPT
Co-pyrolysis – gasification
2.8010
2.8110
5.6210
Pyrolysis
7.81 10 1.9510
1.9510
3.9110
Gasification
4.52 10 1.3610
1.3610
1.7210
RI PT
1.9610
SC
As seen above NO2 emissions are highest for pyrolysis and least for co-pyrolysis-gasification systems. In terms of SO2 and carbon monoxide, gasification process shows lowest emissions. MSW pyrolysis is also the second highest contributor to acidification [30]. Overall, pyrolysis showed acceptable results for SO2, carbon monoxide and HCl but highest for NO2 emissions.
M AN U
A major advantage of pyrolysis/gasification over incineration systems is less NO2 emissions in comparison with incineration due to limited or no air supply during operations. Also, air emissions of pyrolysis/gasification plants easily meet European, and Japanese standards. In gasification, HCl can be removed prior to combustion and inhibition to avoid the formation of dioxins which is a major advantage over incineration [14]. 4.2 Economic Analysis of pyrolysis and gasification of MSW
TE D
A detailed economic analysis is completed on several MSW thermal processes with MSW feedstock capacity of 500 TPD as well as net revenue before taxes. The capital cost is calculated for large scale thermal plants and represented in terms of $/ton of municipal feedstock. As seen below are the different parameters including capital, operating & maintenance cost, and expected revenue.
Table 6 - Economic analysis of MSW treatment methods in $/ton of MSW feedstock [17]
Economic Parameter
Pyrolysis /Gasification (P-G) ($/ton)
Gasification ($/ton)
173,873.8
205,186.8
160,675.6
143,874
15,422.2
13,743.6
400
300
6,400
EP
Capital Investment
Pyrolysis ($/ton)
AC C
Operating & maintenance Cost Revenue
As seen above, in terms of capital cost, the gasification capital costs, it is significantly lower than other treatment methods. For O&M cost, pyrolysis has higher cost than other treatment methods due to generation of tar, production, and condensation of hydrocarbon liquids unlike other treatment methods. In terms of revenue, gasification exceeds 16 times higher the revenue per in comparison with alternative thermal treatment methods [17]. 4.3 Energy Analysis of pyrolysis and gasification
10
ACCEPTED MANUSCRIPT
RI PT
The energy consumption of both gasification and pyrolysis are shown below. The functional unit is per ton of treated MSW. As seen below, gasification shows the lowest energy required for startup due to mass-burn. Combined pyrolysis and gasification (P-G) shows highest energy consumption due to integrated reactors, thus causing higher start-up energy consumption. In terms of MSW residue, pyrolysis shows higher residue than other thermal treatment methods.
Table 7 - Energy consumption of MSW Pyrolysis and gasification methods [17]
Energy required for start-up (KWh) 302.95
Pyrolysis
190.32
19.55 90.13
M AN U
Pyrolysis-gasification (P-G)
Residual/solid waste for disposal (g/ kg)
SC
Technology
Thermal cracking gasification
12.035
18.02
TE D
The net electrical and thermal efficiency varies from a waste to energy process system to another. In case of only waste to electricity production chemical plants, net electrical efficiency may reach up to 31% and the achievable values are strongly dependent on plant size [31]. The reasons for relatively low performance for waste to electricity only plants has many factors including conservation of steam parameters, high condensing pressure and large rate of in- plant energy consumption [31].
EP
The thermal efficiency is the conversion efficiency achieved inside the reactor and can be maximized by combining the thermal cycle with natural gas turbine generator [32]. The reactor thermal efficiency can exceed 80% in heat only waste to energy plants. In order to maximize thermal efficiency, non-corrosive precautions should be taken since corrosion limits steam temperatures [32]. As seen below, incineration and pyrolysis shows highest thermal efficiency inside the reactor. Pyrolysis show lower electrical efficiency due to hydrocarbon liquid production, unlike gasification and incineration which produce hydrocarbon gas ready for combustion.
AC C
Table 8 - Thermal and electrical efficiency for gasification and gasification methods [33]
Waste to energy method
Gross electricity efficiency (%)
Net electricity efficiency (%)
Net thermal efficiency (%)
33 - 34
34
26-40
Pyrolysis
18
15.25
70.3
Pyrolysis- gasification
35
30
40
Gasification
11
RI PT
ACCEPTED MANUSCRIPT
5. Conclusion
AC C
EP
TE D
M AN U
SC
In terms of life cycle assessment, pyrolysis shows highest NO2 emissions but showed acceptable results for SO2, CO and HCl. Pyrolysis-gasification (P-G) showed lowest NO2, SO2, CO emissions and acceptable HCl. Gasification shows acceptable NO2, SO2 but highest CO and HCl emissions. Overall, combined pyrolysis – gasification shows the best optimized process with low environmental emissions. In terms of operating conditions, pyrolysis used nitrogen gas in absence of oxygen as a gaseous medium to prevent side reactions and combustion. In gasification and incineration limited and excess oxygen supply are used respectively with higher elevated temperatures than pyrolysis. In terms of economic analysis, the capital cost for combined pyrolysis gasification shows highest cost per ton followed by pyrolysis and gasification respectively. For operating and maintenance cost, pyrolysis shows 7 times higher in comparison with alternative thermal treatment methods. In terms of revenue, gasification shows highest revenue per unit ton due to shorter residence time, high reaction conversion, syngas production and high HHV of syngas. Incineration has high GHG emissions and low conversion in comparison with gasification. Pyrolysis has a disadvantage of longer residence time but is the only thermal process that supplies liquid hydrocarbons such as gasoline and diesel with accepted environmental emissions.
12
RI PT
ACCEPTED MANUSCRIPT
REFERENCES
SC
[1] N. J.Andrews, Energy Science: Principles,technologies,and impacts, Second edition, Oxford, 2013.
[2] L. S. R.Pai, "Impact of urbanization on municipal solid waste management: A system dynamics approach," International Journal Renewable Energy and Environmental Engineering, vol. 2, 2014.
22, no. 5, 2002.
M AN U
[3] M. J. R.-E. B.K.Lee, "Analyses of the recycling potential of medical plastic wastes," Waste Management 461–470, vol.
[4] C. V. D.Xevgenos, "Success stories for recycling of MSW at municipal level: A Review," Waste and biomass valorization, vol. 6, no. 5, pp. 657 - 684, 2015.
[5] A. Y. S.Chen, "TGA pyrolysis and gasification of combustible municipal solid waste," Elsevier, Journal of the Energy Institute, vol. 88, pp. 332 - 343, 2015.
[6] C. N.J.Themelis, "Municipal solid waste management and waste to energy in the United States, China and Japan," in
TE D
2nd International Academic Symposium on Enhanced Landfill Mining, 2013. [7] L.Branchini, Waste-to-Energy: Advanced Cycles and New Design Concepts for Efficient Power Plants, Springer, 2015. [8] L. Branchin, "Chapter 2 : Municipal solid waste overview," in Waste-to-Energy: Advanced Cycles and New Design Concepts for Efficient Power Plants, Springer, 2015.
EP
[9] S. S. F.Cherubini, "Life cycle assessment (LCA) of waste management strategies: Landfilling, sorting plant and incineration," Elsevier, Energy, vol. 34, p. 2116 – 2123, 2009.
AC C
[10] D. W. S.Yu, "Trace metals in municipal solid waste compost: sources and research methodology," Compost Science & Utilization, vol. 19, pp. 79 - 86, 2011. [11] U. E. P. Agency, "Regulation of fuels and fuel additives: Changes to renewable fuel standard program," Environmental Protection Agency (EPA) , 75, 2010. [12] C. L. L. C. Jia.Hong Kuo, "Emission of carbon dioxide in municipal solid waste incineration in Taiwan: A comparison with thermal power plants," Elsevier, International Journal of Greenhouse Control, vol. 5, p. 889–898, 2011. [13] S. o. t. W. Bank, Municipal solid waste incineration, The International Bank for Reconstruction and Development, 1999. [14] Y. Y. J.Dong, "Effect of operating parameters and moisture content on municipal solid waste pyrolysis and gasification," Energy & Fuels, vol. 30, no. 5, 2016.
13
ACCEPTED MANUSCRIPT
[15] U. D. o. Energy, "Methodology for allocating municipal solid waste to biogenic and non-biogenic energy," EIA , U.S Department of Energy, 2007. [16] Y. H.Cheng, "Municipal solid waste (MSW) as a renewable source of energy: Current and future practices in China," Bioresource technology , vol. 101, p. 3816 –3824, 2010.
Processes, , Wiley, 2010, p. 2.
RI PT
[17] G. C.Young, "Economic, technical, and renewable comparisons," in Municipal Solid Waste to Energy Conversion
[18] K. ,. E.Kwon, "Transforming municipal solid waste (MSW) into fuel via the gasification/pyrolysis process," in 8th Annual North American Waste-to-Energy Conference, 2010.
[19] D. Himawanto, "Multisteps global kinetic analysis of MSW slow pyrolysis," The Journal for Technology and Science,
SC
vol. 24, pp. 19 - 22, 2013.
[20] C. V. F.Fabry, "Waste gasification by thermal plasma: A review," Waste biomass valorization, vol. 4, pp. 421 - 439, 2013.
M AN U
[21] U.Area, "Process and technological aspects of municipal solid waste gasification. A review," Elsevier, Waste Management , vol. 32, no. 4, pp. 625 - 639, 2012.
[22] Z. B. X. M.He, "Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of catalyst and temperature on yield and product composition," Elsevier, International Journal of Hydrogen Energy, vol. 34, no. 1, p. 195 – 203, 2009.
[23] E. C. Limited, "Advanced thermal treatment of municipal solid waste," Department of environment, Food & Rural
TE D
affairs, government of UK, 2005.
[24] V. E. G. D.Panepinto, "Environmental performances and energy efficiency for MSW gasification treatment," Waste Biomass Valorization, vol. 6, no. 1, p. 123–135, 2015.
[25] A.Buekens, "Evaluation of waste incineration," in Incineration technologies, Springer, 2012.
EP
[26] H. N.Yanga, "Greenhouse gas emissions during MSW landfilling in China: Influenceof waste characteristics and LFG treatment measures," Journal of Environmental Management, vol. 129, pp. 510-521, 2013. [27] C. N.J.Themelis, "Energy and economic value of municipal solid waste (MSW), including non-recycled plastics (NRP),
AC C
currently landfilled in the fifty states," Columbia University, Earth engineering centre , advancing the coals for sustainable waste management, 2014. [28] J. K.K.H.Choy, "Process design and feasibility study for small scale MSW gasification," Elsevier, Chemical engineering journal , vol. 105, no. 1, pp. 31 - 41, 2004. [29] M. Y.Chhiti, "Thermal conversion of biomass, pyrolysis and gasification: A review," The International Journal of Engineering And Science (IJES), vol. 2, no. 3, pp. 75-85, 2013. [30] H.H.Khoo, "Life cycle assessment of various waste conversion technologies," Waste Management 29, vol. 29, p. 1892– 1900, 2009. [31] E. A. L.Lombardia, "A review of technologies and performances of thermal treatment systems for energy recovery from
14
ACCEPTED MANUSCRIPT
waste," Elsevier, Waste Management, vol. 37, pp. 26 - 44, 2015. [32] D. C.Ryu, "Combined heat and power from municipal solid waste: current status and issues in south korea," Energies, vol. 6, pp. 45-57, 2013. [33] D. A. T.Astrup, "Life cycle assessment of thermal waste-to- energy technologies: Review and recommendations,"
RI PT
Elsevier, Waste Management, vol. 37, p. 104 –115, 2015. [34] Y. G. R. Z. Y.B. Yang, "Mathematical modelling of MSW incineration on a travelling bed," Pergamon, Waste management 22, vol. 22, p. 369–380, 2002.
[35] A.Tukker, Plastic waste - feedstock recycling, chemical recycling and incineration, RAPRA technology LTD., 2002. [36] T. R. S. W. M. L. Aalborg University, "Report on assessment of relevant recycling technologies," European union
AC C
EP
TE D
M AN U
SC
LIFE10 ENV/DK/000098, 2013.
15
ACCEPTED MANUSCRIPT
review latest advancements in MSW waste to energy conversion Illustrates two main methods of MSW chemical recycling Discuss energy duties, conversion, economic and environmental analysis in MSW chemical recycling
AC C
EP
TE D
M AN U
SC
RI PT
Compares chemical vs. mechanical recycling for MSW
S1743-9671(17)30012-0
DOI:
10.1016/j.joei.2017.04.009
Reference:
JOEI 327
To appear in:
Journal of the Energy Institute
Received Date: 3 February 2017 Revised Date:
24 April 2017
Accepted Date: 26 April 2017
Please cite this article as: H. A. Gabbar, M. Aboughaly, N. Ayoub, Comparative Study of MSW Heat Treatment Processes and Electricity Generation, Journal of the Energy Institute (2017), doi: 10.1016/ j.joei.2017.04.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Comparative Study of MSW Heat Treatment Processes and Electricity Generation
RI PT
Hossam A. Gabbara, b, *, Mohamed Aboughaly b, Nasser Ayoubc a Faculty of Energy Systems and Nuclear Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, L1H 7K4 ON, Canada b Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, L1H 7K4 ON, Canada c Faculty of Industrial Education, Helwan University, Sawah Street, Amiriah, Cairo, Egypt
M AN U
SC
Abstract: This paper discusses the latest analysis in solid waste thermal treatment methods including life cycle assessment (LCA), process systems, economic and energy analysis. The MSW collected by municipalities such as paper, plastics, organic materials, glass, metals, food, leather and rubber can be utilized, via primary treatment methods which are mainly pyrolysis, gasification and combined pyrolysis gasification (P-G) cycles to generate thermal energy or electricity. The importance of solid waste treatment comes from its potential to convert waste into several sources of energy or fuels such as gasoline, syngas or diesel, eliminate waste, and reduce CO2 emissions. The process systems are explained in terms of process stages, carrier gases, operating pressures and temperatures, end products and reaction residence time. According to MSW management statistics, landfilling and incineration are considered a major activity and is incompatible with the exponential increase of global production of MSW per year. Environmental analysis shows that combined pyrolysis-gasification has lowest environmental impact with acceptable results for pyrolysis and gasification. Economic analysis shows highest capital cost for combined pyrolysis gasification (P-G) followed by pyrolysis and gasification process systems respectively. Operating and maintenance cost shows highest for pyrolysis, while gasification systems shows highest revenue per ton.
1. Introduction
TE D
Keywords: Sustainable Engineering, pyrolysis, gasification, chemical recycling, energy production, economic analysis, mechanical recycling, incineration.
AC C
EP
Municipal waste management includes solid waste control, storage, processing, disposal, and recycling of waste generated from residential and non-residential areas. The MSW, management methods are considered a public service managed by governmental organizations that, in most cases, have limited resources. However, this can’t continue the same way, in the near future, due to the exponential increase of the generated solid waste. Furthermore, various assessments showed that 20-40% of carbon in MSW is derived from fossil fuels during manufacturing [1]. To solve this issue, recently, a great attention was given to MSW recycling [2]. Currently, mass incineration is still the most applied global waste treatment method followed by gasification [3]. However, the MSW chemical recycling, investigated in this paper, is considered a breakthrough that can solve environmental challenges faced by modern societies owing to its ability to eliminate most of treated waste and convert it to energy as discussed by [4]. The proportion of combustible components in MSW is about 81.26% on average that makes thermochemical treatment methods (pyrolysis and gasification) the most efficient and effective for MSW recycling [5]. Hence, the chemical recycling of MSW has become a prominent waste management method due to its promising sustainable energy production and waste elimination potential [6]. The thermal cracking, in these methods, releases syngas that can be used in form of district heating, power generation fuel, or other combustion purposes. The amount of energy recovered from thermal cracking of MSW varies significantly depending on its chemical composition that affects the thermodynamics process (i.e. impurities reduce energy recovery). The important thermodynamic parameters include; mass composition, mass flow rate, and HHV as well as characteristics of thermodynamic cycle [7].
1
ACCEPTED MANUSCRIPT
2. Literature Review 2.1 The MSW Management
Prevention Re-use or mechanical recycling Chemical recycling Thermal energy recovery Disposal
SC
a) b) c) d) e)
RI PT
The MSW composition can be divided into two main components (e.g. renewable and non-renewable) as illustrated in Table 1. Per the European Union classification (2008), the hierarchy of MSW management is arranged in the following order (Chen, 2013):
M AN U
As seen figure1, expected MSW generation per capita in most of European countries are in range of 500-700 kg / person per year. Therefore, a need for MSW treatment and conversion to waste is needed for a sustainable future. MSW per capita per year ( kg/person/year)
800 700 600 500
300 200 100 0
Switzerland
EP
United Kingdom
TE D
400
Germany
2001 2010
Finlad
AC C
Fig. 1 - MSW generated per capital in 2001 and 2010 [7]
In terms of municipal waste management, Figure 2 shows a statistic of implemented waste to energy recycling methods in 32 European countries from 2001 to 2010. The statistics show that landfilling is still the most common waste treatment method with expected landfilling of 115 - 155 million tons/ year, while Incineration is the most common thermal recycling method followed by gasification and pyrolysis.
2
ACCEPTED MANUSCRIPT
MSW management in 32 European countries 300 250 150 100 50 0
2001
2004
2008
Recycling
60
70
90
Incineration
50
80
50
Landfilling
155
140
no treatment
20
20
RI PT
200
2010 90
SC
MSW million tons
350
50
133
115
17
15
M AN U
Fig. 2 - MSW chemical recycling in 32 European countries from 2001 to 2010 [8]
Vast research has been done on MSW collection, treatment, disposal, and recycling methods including the following:
TE D
• • • • •
MSW yearly production in industrial countries such as Canada, Japan and USA. MSW composition and amount of landfilling per year. Thermal plasma systems in terms of operating temperature, energy duties, MSW product yield, and thermal plasma control. MSW gasification incineration and pyrolysis operating conditions. Life cycle assessment of chemical and mechanical recycling methods. Metal catalysts and its utilization in pyrolysis and gasification. Thermal and electrical efficiency of different process systems Mass and energy balance of MSW chemical recycling
EP
• • •
AC C
The scale of MSW production around the globe has a direct impact on its importance and development on a sustainable solution that can eliminate the traditional method of landfilling. The most common four waste management strategies are landfilling with methane utilization, landfilling without methane utilization Incineration (i.e. mass burning), and waste sorting. The expected GHG emissions from different thermal processes which affect global warming are CO2, CH4, N2O, SO2, NOx, HCl, H2S, and HF [9]. Below is a schematic diagram of waste management strategies.
3
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3 – MSW sorting and chemical recycling block diagram
AC C
EP
TE D
MSW sorting and separation is essential to ensure that only treatable MSW is fed to the gasifier. MSW contamination is divided as physical or chemical. Metal contamination is considered as physical contamination and has a huge impact on the gasification process. Among common metal contaminants is stainless steel, carbon steel, screws, and aluminum alloys. Metal contaminants are divided to ions and solid contaminants which are later separated and used in the manufacturing industry [10]. Different metal contamination processing pathways is shown in figure 4.
Fig. 4 – Pathways to MSW Contaminant removal
4
ACCEPTED MANUSCRIPT
Most metal contaminants in MSW are from car batteries, consumer electronics, ceramics, housing, and construction sites. Nearly 20% of lead batteries which are not recycled contribute to around 66% of lead in MSW in the US [10].
RI PT
2.2 Waste to energy technologies
M AN U
SC
The U.S-EPA considers MSW a renewable energy resource due the possibility of its conversion to useful products or energy sources [11]. The chemical recycling, considered in this work, is used as a Waste-to-Energy, MSW management option. The thermal and chemical recycling methods of MSW are; direct combustion (i.e. Incineration), pyrolysis, or gasification. Incineration has advantage of sustainable reduction of MSW by 90% as well as minimal treatment of waste preprocessing but has high CO2 emissions in comparison with other thermal treatment methods [12]. Disadvantages of incineration include high capital and operational cost, high pollution, and mandatory treatment of flue gas [13]. The high CO2 emissions also limit large scale recycling for landfilling and incineration. Added to that, its broader widespread is restricted, owing to harmful emissions, especially with dioxins and acidic gases (SO2, NOx, HCl, etc.). Besides, excessive corrosion by HCl has limited the maximum steam super-heater temperature and thus led to lower energy efficiency [14]. Table 1 - MSW renewable and non-renewable components [15] Renewable MSW Cellulose paper (i.e. newsprint, newspapers) Containers, Packaging
Rubber
Thermosetting plastics (i.e. Polyurethanes, polyesters etc.)
TE D
Food and organic compounds Leather and clothes
Non-Renewable MSW Thermoplastics plastics (i.e. Polyethylene, Polystyrene, Polyethylene tetraphalate)
Metals (i.e. copper, iron steel, etc.)
2.2.1
AC C
EP
Waste to energy technologies have been implemented as large scale chemical plants successfully throughout the world. It is estimated that about 130 million tonnes of MSW are chemically recycled annually in over 600 waste to energy chemical plants worldwide. The syngas or liquid fuels end-products can either produce electricity through a high-pressure steam generation cycle for district heating or used as combustion fuels [16]. Syngas requires purification and clean-up and can also be converted to ethanol, methanol, gasoline, or diesel using catalysts. Gaseous products of these methods are in form of CH4, H2, and hydrocarbon liquids, which are used in the thermal cycle to produce high pressure steam (HPS) followed by electrical production using a steam turbine [17]. Non-renewable portions such as metals and non-decomposable components lower heat efficiency and syngas yield in thermal processes [16] and it should be separated using density mass separation equipment or as unaccepted slag/tar after gasification. Thermo-Chemical MSW Treatment
The high fossil fuel consumption and the environmental impacts arising from GHG emissions, unsustainable sources, and high cost of refining and fossil fuel extraction have attracted great attention for MSW recycling, and alternative fuels. Thermo-chemical transformation via pyrolysis/gasification of MSW is highly efficient due to lower greenhouse-gas emissions in comparison with other recycling methods more sustainable solution for MSW deposits [18]. Added to that, continuous process with high conversion can be achieved. The major two thermo-chemical MSW treatment methods are thermal pyrolysis and gasification which are viewed as the economically viable approaches for residential waste, commercial waste, or industrial waste methods [17]. Gasification is defined as thermal conversion of MSW to a gaseous product known as syngas. The synthesized
5
ACCEPTED MANUSCRIPT
RI PT
gas is a mixture of two main components CO and H2, with minor traces of CH4, CO2 and gaseous hydrocarbons [18]. On the other hand, MSW pyrolysis is a thermal composition in limited oxygen conditions to synthetic fuels in range of 400 °C - 650 °C in long residence time [19]. As seen in table 2, expected end products from incineration are flue gas mainly CO2 and H2O. While gasification yields CO, H2 and CH4. Combustion is carried in excess oxygen supply, while gasification is carried in limited oxygen supply [20]. Pyrolysis should be carried in inert conditions and the most common medium is nitrogen gas due to its abundance and low cost in comparison with inert gases. Table 2 - Main characteristics MSW thermo-chemical treatment methods [21]
Incineration
Gasification
Pyrolysis
Convert MSW in a combustion process to high temperature flue gas CO2 and H2O in excess oxygen
Convert MSW to HHV syngas which consists mainly of CO, H2 and CH4
MSW thermal cracking in absence of oxygen to hydrocarbon gases, liquids and wax
Reaction environment
Oxidizing reaction
Air, pure oxygen, steam
Nitrogen gas or any inert gas
Reactant gas
Excess Air
Air, O2, Steam
No reactant gas
Temperature
850℃ to 1200℃
Pressure range
Atmospheric
End-products
CO2, H2O
Undesired effluents
SO2, nitric oxides, HCl
Essential installations
Air pollution control
M AN U
SC
Process Description
500℃ to 800℃
Atmospheric
Over atmospheric
CO, H2, CO2, H2O, CH4
CO, H2, CH4 hydrocarbon liquids
H2S, HCl, Sulphur oxides, NH3, HCN, tar Syngas cleaning is required
H2S, HCl, NH3, HCN, tar. Particulates No treatment is needed
TE D
550 ℃ to 900 ℃
EP
For catalytic gasification, fixed bed reactors are used. The catalytic gasification reduces the operating temperature from 1200℃ to 700℃ . As the operating temperature increases, the gas content increases and the tar content is reduced to less than 10 wt % with expected carbon conversion of 60 - 75 wt % [22]. Below are the possible reactors for pyrolysis and gasification: Table 3 - Pyrolysis and gasification reactors [23]
AC C
MSW Treatment Method
Recommended Reactor Type
Possible Reactors for pyrolysis
Rotating kiln, heated tube, Surface contact
Possible reactors for gasification
Fluidized bed, Fixed bed
In MSW thermal treatment processes the products are utilized to produce steam in a steam cycle using a boiler to produce steam followed by a steam turbine to generate electricity in an electrical cycle. For steam cycle incineration shows a higher conversion rate than gasification and pyrolysis. For steam cycle, incineration is in range of 19-27 % while in pyrolysis and gasification 9-20% [24].
6
ACCEPTED MANUSCRIPT
2.2.2. Disadvantages of Incineration and Landfilling
RI PT
Incineration is combustion of solid waste in excess oxygen at very high temperatures (i.e. above 1000℃ ) with a complex process system that requires huge capital investment and high operating costs. Most of the operating cost is due to complying with gaseous emissions and pollution control. The incinerator chamber requires complete burnout of flue gases which causes erosion, corrosion and boiler fouling causing high maintenance cost and more expensive steel grades. As the environmental precautions are being stricted day by day, incineration process systems face lower profits due to higher operating and quality control costs [25]. Existence of dioxins and heavy metals in incineration gaseous emissions makes it a substantial source of environmental emissions and prevents further expansion of incineration plants.
SC
Solid waste landfilling is still the most common system for waste disposal worldwide which involves direct burying of solid waste under the ground in environmental friendly land site. The disadvantage of this process is high methane emissions due to anaerobic digestion of solid waste and inability of waste elimination but rather decomposition. Materials such as plastics and inorganic materials are inapplicable for such method.
TE D
3. Pyrolysis and Gasification Methods
M AN U
The process involves separation of hazardous materials and compacting the solid waste into dense self-contained packages before burying into safe underground systems. A major disadvantage of this method is acquiring of large land area sites and inability to implement on large quantities of MSW. An ordinary MSW landfilling plant consists of collection and transportation of MSW trays to a landfill underground system. The landfilling area has a separate leachate collection at the bottom of the while methane gases are collected at the top of the landfilling system and can be utilized for energy production [26].
EP
Pyrolysis and gasification in recent years have increased over incineration with the goal of reducing incineration environmental impact and improve electricity generation dependency from waste in Italy [24]. MSW chemical recycling has advantage of feedstock flexibility, hydrocarbon products, chemical synthesis, and electricity production. Large scale implementation of waste to energy chemical plants could even replace fossil fuel dependency. Every ton of chemical recycled MSW replaces 0.4 ton of coal used for electricity production in the US. It is expected that MSW chemical recycling large scale implementation in the US replaces coal mining by 100 million tons/ year [27].
AC C
3.1 MSW Gasification
MSW gasification is the transformation of solid waste to volatized gases in limited oxygen flow to gaseous products including mainly CO, CO2, H2, CH4 and hydrocarbon liquids. Alternatively, MSW pyrolysis is thermal decomposition and thermal cracking in absence of oxygen to hydrocarbon fuels [18]. Below is a process block diagram of MSW gasification:
7
ACCEPTED MANUSCRIPT
Syngas cleanup Power Generation + Steam Generation + Electrical Energy
Feedstock Municipal Solid Waste (MSW)
RI PT
Synthesis Gas (SYNGAS)
Gasifier Reactor
SC
Slag, and/or Ash
M AN U
Fig. 5 - MSW gasification process schematic diagram
MSW gasification have several 100 waste treatment sites worldwide. The characteristic of these process systems has the following operating condition (1,100℃ , 1 - 20 bar), short residence time (i.e. few seconds) and high conversion in comparison with pyrolysis and incineration. The MSW needs preparation by means of crushing and sieving the feed with controlled moisture content to increase heat transfer area during gasification [24]. The mass composition of the MSW affects the gasification process. Thus, an ultimate analysis of MSW is required which involves determination of main components such as carbon, oxygen, nitrogen, Sulphur and ash [28].
TE D
3.2 MSW Pyrolysis
AC C
EP
Power Generation + Steam Generation
Feedstock Municipal Solid Waste (MSW)
Hydrocarbon Gas
Pyrolysis Reactor
Ash, Tar Fig. 6 - MSW pyrolysis process schematic diagram
8
ACCEPTED MANUSCRIPT
Table Main
Parameters
Conventional pyrolysis
Fast pyrolysis
Operating temperature(℃) 850 - 1250
0.1-1
10-200
(seconds)
300-3600
0.5-10
4-
Flash pyrolysis
1050 - 1300 >1000
SC
550 - 900 Heat flux (℃/s)
RI PT
For MSW pyrolysis, slow pyrolysis has longer residence time, slower heating rates up to 20 ℃ /min and lower operating temperatures in comparison with alternative pyrolysis methods. Fast pyrolysis is a process in which very high heat flux are applied to MSW leading fast conversion to syngas in absence of oxygen achieving maximum liquid yields at high heating rates. The reaction temperature is higher than slow pyrolysis (i.e. 900°C). Alternatively, very fast pyrolysis is also referred as flash pyrolysis, its operating temperature is slightly higher than fast pyrolysis. The main product distributions are similar to fast pyrolysis [29].
<0.5
M AN U
operating conditions for MSW pyrolysis process [29]
TE D
As seen above, conventional pyrolysis is suitable for producing liquid hydrocarbons. Fast and flash pyrolysis process mainly CH4 and light gaseous hydrocarbons for electricity generation. 4. Environmental and Economic Comparison
EP
4.1 Environmental LCA of pyrolysis and gasification
AC C
Life cycle assessment of MSW thermal treatment methods are evaluated in terms of GHGs emissions including the following factors: • Acidification potential - Acid gases such as SO2 react with water in the atmosphere forming acid rain • Ozone photochemical formation - Tropospheric ozone formation occurs when nitrogen oxides (e.g.: NO2) or carbon monoxide and volatile organic compounds react in the atmosphere in presence of sunlight. • Global warming - Release of GHGs such as carbon dioxide which traps heat inside the atmosphere causing elevated temperatures. Below are gas emissions analysis of MSW different chemical recycling methods: Table 5 - LCA of pyrolysis and gasification of MSW in kg/Ton of product gas [30]
Recycling Method
Nitrogen dioxide
Sulphur dioxides
Carbon monoxide
HCl
9
ACCEPTED MANUSCRIPT
Co-pyrolysis – gasification
2.8010
2.8110
5.6210
Pyrolysis
7.81 10 1.9510
1.9510
3.9110
Gasification
4.52 10 1.3610
1.3610
1.7210
RI PT
1.9610
SC
As seen above NO2 emissions are highest for pyrolysis and least for co-pyrolysis-gasification systems. In terms of SO2 and carbon monoxide, gasification process shows lowest emissions. MSW pyrolysis is also the second highest contributor to acidification [30]. Overall, pyrolysis showed acceptable results for SO2, carbon monoxide and HCl but highest for NO2 emissions.
M AN U
A major advantage of pyrolysis/gasification over incineration systems is less NO2 emissions in comparison with incineration due to limited or no air supply during operations. Also, air emissions of pyrolysis/gasification plants easily meet European, and Japanese standards. In gasification, HCl can be removed prior to combustion and inhibition to avoid the formation of dioxins which is a major advantage over incineration [14]. 4.2 Economic Analysis of pyrolysis and gasification of MSW
TE D
A detailed economic analysis is completed on several MSW thermal processes with MSW feedstock capacity of 500 TPD as well as net revenue before taxes. The capital cost is calculated for large scale thermal plants and represented in terms of $/ton of municipal feedstock. As seen below are the different parameters including capital, operating & maintenance cost, and expected revenue.
Table 6 - Economic analysis of MSW treatment methods in $/ton of MSW feedstock [17]
Economic Parameter
Pyrolysis /Gasification (P-G) ($/ton)
Gasification ($/ton)
173,873.8
205,186.8
160,675.6
143,874
15,422.2
13,743.6
400
300
6,400
EP
Capital Investment
Pyrolysis ($/ton)
AC C
Operating & maintenance Cost Revenue
As seen above, in terms of capital cost, the gasification capital costs, it is significantly lower than other treatment methods. For O&M cost, pyrolysis has higher cost than other treatment methods due to generation of tar, production, and condensation of hydrocarbon liquids unlike other treatment methods. In terms of revenue, gasification exceeds 16 times higher the revenue per in comparison with alternative thermal treatment methods [17]. 4.3 Energy Analysis of pyrolysis and gasification
10
ACCEPTED MANUSCRIPT
RI PT
The energy consumption of both gasification and pyrolysis are shown below. The functional unit is per ton of treated MSW. As seen below, gasification shows the lowest energy required for startup due to mass-burn. Combined pyrolysis and gasification (P-G) shows highest energy consumption due to integrated reactors, thus causing higher start-up energy consumption. In terms of MSW residue, pyrolysis shows higher residue than other thermal treatment methods.
Table 7 - Energy consumption of MSW Pyrolysis and gasification methods [17]
Energy required for start-up (KWh) 302.95
Pyrolysis
190.32
19.55 90.13
M AN U
Pyrolysis-gasification (P-G)
Residual/solid waste for disposal (g/ kg)
SC
Technology
Thermal cracking gasification
12.035
18.02
TE D
The net electrical and thermal efficiency varies from a waste to energy process system to another. In case of only waste to electricity production chemical plants, net electrical efficiency may reach up to 31% and the achievable values are strongly dependent on plant size [31]. The reasons for relatively low performance for waste to electricity only plants has many factors including conservation of steam parameters, high condensing pressure and large rate of in- plant energy consumption [31].
EP
The thermal efficiency is the conversion efficiency achieved inside the reactor and can be maximized by combining the thermal cycle with natural gas turbine generator [32]. The reactor thermal efficiency can exceed 80% in heat only waste to energy plants. In order to maximize thermal efficiency, non-corrosive precautions should be taken since corrosion limits steam temperatures [32]. As seen below, incineration and pyrolysis shows highest thermal efficiency inside the reactor. Pyrolysis show lower electrical efficiency due to hydrocarbon liquid production, unlike gasification and incineration which produce hydrocarbon gas ready for combustion.
AC C
Table 8 - Thermal and electrical efficiency for gasification and gasification methods [33]
Waste to energy method
Gross electricity efficiency (%)
Net electricity efficiency (%)
Net thermal efficiency (%)
33 - 34
34
26-40
Pyrolysis
18
15.25
70.3
Pyrolysis- gasification
35
30
40
Gasification
11
RI PT
ACCEPTED MANUSCRIPT
5. Conclusion
AC C
EP
TE D
M AN U
SC
In terms of life cycle assessment, pyrolysis shows highest NO2 emissions but showed acceptable results for SO2, CO and HCl. Pyrolysis-gasification (P-G) showed lowest NO2, SO2, CO emissions and acceptable HCl. Gasification shows acceptable NO2, SO2 but highest CO and HCl emissions. Overall, combined pyrolysis – gasification shows the best optimized process with low environmental emissions. In terms of operating conditions, pyrolysis used nitrogen gas in absence of oxygen as a gaseous medium to prevent side reactions and combustion. In gasification and incineration limited and excess oxygen supply are used respectively with higher elevated temperatures than pyrolysis. In terms of economic analysis, the capital cost for combined pyrolysis gasification shows highest cost per ton followed by pyrolysis and gasification respectively. For operating and maintenance cost, pyrolysis shows 7 times higher in comparison with alternative thermal treatment methods. In terms of revenue, gasification shows highest revenue per unit ton due to shorter residence time, high reaction conversion, syngas production and high HHV of syngas. Incineration has high GHG emissions and low conversion in comparison with gasification. Pyrolysis has a disadvantage of longer residence time but is the only thermal process that supplies liquid hydrocarbons such as gasoline and diesel with accepted environmental emissions.
12
RI PT
ACCEPTED MANUSCRIPT
REFERENCES
SC
[1] N. J.Andrews, Energy Science: Principles,technologies,and impacts, Second edition, Oxford, 2013.
[2] L. S. R.Pai, "Impact of urbanization on municipal solid waste management: A system dynamics approach," International Journal Renewable Energy and Environmental Engineering, vol. 2, 2014.
22, no. 5, 2002.
M AN U
[3] M. J. R.-E. B.K.Lee, "Analyses of the recycling potential of medical plastic wastes," Waste Management 461–470, vol.
[4] C. V. D.Xevgenos, "Success stories for recycling of MSW at municipal level: A Review," Waste and biomass valorization, vol. 6, no. 5, pp. 657 - 684, 2015.
[5] A. Y. S.Chen, "TGA pyrolysis and gasification of combustible municipal solid waste," Elsevier, Journal of the Energy Institute, vol. 88, pp. 332 - 343, 2015.
[6] C. N.J.Themelis, "Municipal solid waste management and waste to energy in the United States, China and Japan," in
TE D
2nd International Academic Symposium on Enhanced Landfill Mining, 2013. [7] L.Branchini, Waste-to-Energy: Advanced Cycles and New Design Concepts for Efficient Power Plants, Springer, 2015. [8] L. Branchin, "Chapter 2 : Municipal solid waste overview," in Waste-to-Energy: Advanced Cycles and New Design Concepts for Efficient Power Plants, Springer, 2015.
EP
[9] S. S. F.Cherubini, "Life cycle assessment (LCA) of waste management strategies: Landfilling, sorting plant and incineration," Elsevier, Energy, vol. 34, p. 2116 – 2123, 2009.
AC C
[10] D. W. S.Yu, "Trace metals in municipal solid waste compost: sources and research methodology," Compost Science & Utilization, vol. 19, pp. 79 - 86, 2011. [11] U. E. P. Agency, "Regulation of fuels and fuel additives: Changes to renewable fuel standard program," Environmental Protection Agency (EPA) , 75, 2010. [12] C. L. L. C. Jia.Hong Kuo, "Emission of carbon dioxide in municipal solid waste incineration in Taiwan: A comparison with thermal power plants," Elsevier, International Journal of Greenhouse Control, vol. 5, p. 889–898, 2011. [13] S. o. t. W. Bank, Municipal solid waste incineration, The International Bank for Reconstruction and Development, 1999. [14] Y. Y. J.Dong, "Effect of operating parameters and moisture content on municipal solid waste pyrolysis and gasification," Energy & Fuels, vol. 30, no. 5, 2016.
13
ACCEPTED MANUSCRIPT
[15] U. D. o. Energy, "Methodology for allocating municipal solid waste to biogenic and non-biogenic energy," EIA , U.S Department of Energy, 2007. [16] Y. H.Cheng, "Municipal solid waste (MSW) as a renewable source of energy: Current and future practices in China," Bioresource technology , vol. 101, p. 3816 –3824, 2010.
Processes, , Wiley, 2010, p. 2.
RI PT
[17] G. C.Young, "Economic, technical, and renewable comparisons," in Municipal Solid Waste to Energy Conversion
[18] K. ,. E.Kwon, "Transforming municipal solid waste (MSW) into fuel via the gasification/pyrolysis process," in 8th Annual North American Waste-to-Energy Conference, 2010.
[19] D. Himawanto, "Multisteps global kinetic analysis of MSW slow pyrolysis," The Journal for Technology and Science,
SC
vol. 24, pp. 19 - 22, 2013.
[20] C. V. F.Fabry, "Waste gasification by thermal plasma: A review," Waste biomass valorization, vol. 4, pp. 421 - 439, 2013.
M AN U
[21] U.Area, "Process and technological aspects of municipal solid waste gasification. A review," Elsevier, Waste Management , vol. 32, no. 4, pp. 625 - 639, 2012.
[22] Z. B. X. M.He, "Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of catalyst and temperature on yield and product composition," Elsevier, International Journal of Hydrogen Energy, vol. 34, no. 1, p. 195 – 203, 2009.
[23] E. C. Limited, "Advanced thermal treatment of municipal solid waste," Department of environment, Food & Rural
TE D
affairs, government of UK, 2005.
[24] V. E. G. D.Panepinto, "Environmental performances and energy efficiency for MSW gasification treatment," Waste Biomass Valorization, vol. 6, no. 1, p. 123–135, 2015.
[25] A.Buekens, "Evaluation of waste incineration," in Incineration technologies, Springer, 2012.
EP
[26] H. N.Yanga, "Greenhouse gas emissions during MSW landfilling in China: Influenceof waste characteristics and LFG treatment measures," Journal of Environmental Management, vol. 129, pp. 510-521, 2013. [27] C. N.J.Themelis, "Energy and economic value of municipal solid waste (MSW), including non-recycled plastics (NRP),
AC C
currently landfilled in the fifty states," Columbia University, Earth engineering centre , advancing the coals for sustainable waste management, 2014. [28] J. K.K.H.Choy, "Process design and feasibility study for small scale MSW gasification," Elsevier, Chemical engineering journal , vol. 105, no. 1, pp. 31 - 41, 2004. [29] M. Y.Chhiti, "Thermal conversion of biomass, pyrolysis and gasification: A review," The International Journal of Engineering And Science (IJES), vol. 2, no. 3, pp. 75-85, 2013. [30] H.H.Khoo, "Life cycle assessment of various waste conversion technologies," Waste Management 29, vol. 29, p. 1892– 1900, 2009. [31] E. A. L.Lombardia, "A review of technologies and performances of thermal treatment systems for energy recovery from
14
ACCEPTED MANUSCRIPT
waste," Elsevier, Waste Management, vol. 37, pp. 26 - 44, 2015. [32] D. C.Ryu, "Combined heat and power from municipal solid waste: current status and issues in south korea," Energies, vol. 6, pp. 45-57, 2013. [33] D. A. T.Astrup, "Life cycle assessment of thermal waste-to- energy technologies: Review and recommendations,"
RI PT
Elsevier, Waste Management, vol. 37, p. 104 –115, 2015. [34] Y. G. R. Z. Y.B. Yang, "Mathematical modelling of MSW incineration on a travelling bed," Pergamon, Waste management 22, vol. 22, p. 369–380, 2002.
[35] A.Tukker, Plastic waste - feedstock recycling, chemical recycling and incineration, RAPRA technology LTD., 2002. [36] T. R. S. W. M. L. Aalborg University, "Report on assessment of relevant recycling technologies," European union
AC C
EP
TE D
M AN U
SC
LIFE10 ENV/DK/000098, 2013.
15
ACCEPTED MANUSCRIPT
review latest advancements in MSW waste to energy conversion Illustrates two main methods of MSW chemical recycling Discuss energy duties, conversion, economic and environmental analysis in MSW chemical recycling
AC C
EP
TE D
M AN U
SC
RI PT
Compares chemical vs. mechanical recycling for MSW