ARTICLE pubs.acs.org/est
Assessment of Fuel-Cycle Energy Use and Greenhouse Gas Emissions for Fischer-Tropsch Diesel from Coal and Cellulosic Biomass Xiaomin Xie,*,† Michael Wang,‡ and Jeongwoo Han‡ †
Key Laboratory for Power Machinery and Engineering of State Education Ministry, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, China, 200240 ‡ Center for Transportation Research, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
bS Supporting Information ABSTRACT: This study expands and uses the GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation) model to assess the effects of carbon capture and storage (CCS) technology and cellulosic biomass and coal cofeeding in Fischer-Tropsch (FT) plants on energy use and greenhouse gas (GHG) emissions of FT diesel (FTD). To demonstrate the influence of the coproduct credit methods on FTD life-cycle analysis (LCA) results, two allocation methods based on the energy value and the market revenue of different products and a hybrid method are employed. With the energy-based allocation method, fossil energy use of FTD is less than that of petroleum diesel, and GHG emissions of FTD could be close to zero or even less than zero with CCS when forest residue accounts for 55% or more of the total dry mass input to FTD plants. Without CCS, GHG emissions are reduced to a level equivalent to that from petroleum diesel plants when forest residue accounts for 61% of the total dry mass input. Moreover, we show that coproduct method selection is crucial for LCA results of FTD when a large amount of coproducts is produced.
1. INTRODUCTION The United States has competing strategic objectives related to energy: achieving energy supply security, attaining economic sustainability, and addressing concerns about global climate change.1 The U.S. transportation sector is the single largest consumer of petroleum oil, accounting for more than 70% of total U.S. petroleum use; moreover, more than 60% of this petroleum is imported.2 The combustion of petroleum fuel from the transportation sector is responsible for nearly 30% of U.S. greenhouse gas (GHG) emissions.3 Thus, the search for alternative transportation fuels that would reduce U.S. dependence on foreign sources of oil and help reduce GHG emissions is ongoing. Coal — the most abundant fossil fuel resource in the United States — could be used to produce transportation fuels, alleviate U.S. dependence on imported petroleum, and improve the nation’s energy security. The Fischer-Tropsch (FT) synthesis process can be employed to produce diesel fuel (i.e., FTD) from coal. The FT process, invented in the 1920s by Franz Fischer and Hans Tropsch,4 is a set of chemical reactions that convert a mixture of carbon monoxide (CO) and hydrogen into liquid hydrocarbons, especially diesel fuel. The synthetic gas (syngas) used in the FT synthesis process can come from coal, natural gas, or biomass. Generally, the properties of FT fuel can be made almost identical to those of conventional petroleum fuels. However, the high CO2 discharge of coal-to-liquids (CTL) processes has become a major diffusion barrier for coal-derived FTD use.5 Life-cycle analyses (LCAs) of FTD show that FT fuels made from coal result in a large amount of GHG emissions.1,6-8 One approach to reducing CTL GHG emissions is to use carbon capture and storage (CCS) technology.9 Another approach is to use biomass as a cofeedstock to offset CTL GHG emissions.7 For r 2011 American Chemical Society
biomass as the feedstock to produce FTD, the type of biomass input and the conversion rate to the final fuel are quite important with respect to the environmental evaluation of biomass FTD.10 The NETL/DOE (National Energy Technology Laboratory/U. S. Department of Energy) conducted a series of studies on FTD economic and LCA GHG emissions. Their latest results showed BTL achieves a 322% GHG reduction relative to petroleum diesel, assuming the electricity and naphtha coproducts displaces the U.S. average mix and petroleum-derived naphtha, respectively.1,6 Kreutz et al.7 estimated the fuel cycle GHG emissions for different FTD plant configurations, assuming that electricity produced from coal integrated gasification combined cycle (IGCC) by using CCS is displaced by electricity coproduct and showed that the fuel-cycle GHG emissions of FTL production and use are zero with 50% biomass feedstock input and CCS design. Vliet8 investigated the LCA of FTD in The Netherlands and confirmed that combining fuels produced from fossil resources with around 50% BTL and CCS technology can achieve net climate neutral. The choice of coproduct credit methods can significantly influence life-cycle results of transportation fuels. Wang et al.11 summarized five methods to address coproduct issues in biofuel LCA, and Huo et al.12 applied several coproduct methods in LCA of biodiesel and renewable diesel. Similarly, since the FT process produces multiple products (including electricity), the selection of coproduct method is critical for LCA, which has not been Received: May 25, 2010 Accepted: February 17, 2011 Revised: January 5, 2011 Published: March 03, 2011 3047
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Figure 1. Well-to-wheels analysis of FTD from coal and forest residue.
addressed in details in previous studies. This study combines data from previous studies, provides an LCA of the effects of CCS technology and cellulosic biomass and coal cofeeding in FTD plants on the energy use and GHG emissions of the full FTD fuel cycle compared with those of petroleum diesel, and employs three coproduct methods to explore their influence on FTD LCA results: two were allocation methods based on the energy value and the market revenue, and one was a hybrid method of combining the energy-based allocation method and the displacement method.
2. LCA OF FTD WITH THE GREET MODEL The system boundary for this study is from wells to wheels (WTW) (Figure 1), which includes a well-to-pump (WTP) stage covering the production and transportation of feedstock and the production, transportation, and distribution of fuel and a pump-to-wheel (PTW) stage covering vehicle operational activities. To this end, this study uses the GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation) model that has been developed by Argonne National Laboratory.13 It has been updated frequently to reflect new feedstocks, processing technologies, and fuels. The latest version, GREET 1.8d, can analyze more than 100 transportation fuel pathways, including renewable natural gas from landfill gas.14 Since the pathways for petroleum diesel and FTD are already in the GREET model, this study updates FTD fuel production simulations with new data on energy efficiencies and the production of coproducts. Moreover, we expand GREET to include explicit simulations of the coproduction of electricity in FTD plants. Because many key input parameters involve uncertainties, stochastic simulations built in GREET are conducted in this study to address uncertainties.15 To use this tool, probability distributions need to be developed for key input parameters, such as energy efficiencies and emission factors of key activities, including feedstock recovery, fuel production, and transportation.16-18 GREET takes into account the probability distributions and provides results in the form of statistical distributions rather than point estimates.19 3. DATA AND KEY ASSUMPTIONS 3.1. Feedstocks. U.S. coal deposits come in four major types, also known as ranks: bituminous, subbituminous, lignite, and anthracite. They represent approximately 47, 45, 6.5, and 1.5% of U.S. production, respectively.20 Bituminous coal has properties that make it good for producing gas.21 Because a considerable amount of information is available on Illinois No. 6 bituminous coal, we used its properties to represent typical U.S. bituminous
coal for CTL plants, as shown in Table S1 (where “S” refers to Supporting Information). During the coal mining process, CH4 contained in coal beds is released. The total CH4 emissions of coal production in the United States were estimated to be 3221 thousand metric tons in 2007, while coal production in the United States was 1.172 billion short tons on an as-received basis,20,22 resulting in an average CH4 emissions of 122.1 g/ mmBtu coal (LHV). We assumed that 50% coal is transported by pipelines 65 miles and 50% is transported by trains 65 miles one way from the mine site to an FTD plant. A significant amount of forest residue is potentially available in the United States for fuel production.23,24 Forest residue includes logging residue; other “removals” from the forest inventory as a result of precommercial thinning, land clearing, and changes in land use (from forest use to nonforest or developed use); and mill residue.25,26 Table S2 lists the amount of forest residue in the United States in 2006. In this analysis, we consider logging residue and other removals from forests. The properties of forest residue biomass are shown in Table S1. The harvesting of forest residue includes stumpage and harvesting. Forest residue is assumed to be transported 75 miles one way from the collection site to an FTD plant by using heavy-duty trucks with a payload of 17 tons. Table S3 lists the energy use associated with forest residue collection and transportation. 3.2. FTD Production. In FTD plants, solid feedstocks, such as coal and biomass, are fed into a gasifier to produce syngas, which mainly consists of carbon monoxide, hydrogen, and CO2. Before syngas is sent to the FT reactor, CO2 and sulfur compounds are removed from it. In FT reactors, a catalyst is used27-29 to convert CO and hydrogen into the desired hydrocarbon products. The CO2 may be vented or captured and sequestered (with the CCS technology). During FTD production, electricity could be produced from unconverted syngas, some of which could be exported to the electric grid as a coproduct. There are two general designs for FTD production:7,30 recycling (RC) design and once-through (OT) design, as illustrated in Figure 2. In the RC design, the unconverted syngas from FT reactors is recycled back into them for additional conversion, and the final tail gas is used to generate electric power. The OT design passes the syngas only once through a synthesis reactor; it maximizes the power generation from the plant. We evaluate a few recently completed studies on FTD production. On the basis of these studies, key input parameters for 15 FTD production pathways are developed for our WTW analysis and summarized in Table 1. As mentioned above, a stochastic simulation is conducted in this study. Since the amount of data available from those completed studies is limited, we decided to use the triangular distribution function by defining the minimum, maximum, and most likely values for each key parameter. In particular, the lowest, 3048
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Figure 2. Generic process block diagram for FTD plants.
Table 1. Parameters of FTD Pathways for Life-Cycle Analysis of This Studya feedstock share (dry
feedstock share
mass %)
(LHV %)
forest pathway 1
2
3
coal
CTL w/o CCS RC
100
CTL w/CCS RC
100
CB10TL w/CCS RC
90
residue 0
0
10
coal 100
100
93.9
plant
total LHV
electricity
CO2
forest
design
efficiency
coproduct share
capture
residue
type
(%)
by LHV (%)
ratio (%)
0
RC
0
RC
6.1
RC
ref 1,7,31
min ave
49.2 50.6
1.1 1.2
max
52.2
1.4
min
46.6
3.8
85.2
ave
47.6
6.9
88.2
max
52.8
0
91
min
46.9
1.5
86.3
ave
48.4
2.8
88.6
max
4.0 4.1
90.9 91.2
0 1,7,32,33
31,33
4
CB20TL w/CCS RC
80
20
87.2
12.8
RC
49.8 46.5
5
CB55TL w/o CCS RC
45
55
63
37
RC
51.0
13.4
0
7
6
CB55TL w/CCS RC
45
55
63
37
RC
49.5
10.7
87.5
7
b
33
7
CB61TL w/o CCS RC
39
61
63
37
RC
51.0
13.4
0
7
8
BTL w/o CCS RC
0
100
0
100
RC
51.9
10.9
0
7
9
BTL w/CCS RC
0
100
0
100
RC
50.3
8.0
89.9
7
10
CTL w/o CCS OT
100
0
100
0
OT
49.3
35.7
0
7
11
CTL w/CCS OT
100
0
100
0
OT
min ave
44.8 46.3
20.9 26.4
73.3 75.7
7,33
max
47.6
31.8
79.5
12
CB34TL w/o CCS OT
66
34
76.7
23.3
OT
50.2
37.2
0
7
13
CB34TL w/CCS OT
66
34
76.7
23.3
OT
47.1
33.1
73.4
7
14
CB50TL w/o CCS OT
50
50
63
37
OT
50.8
38.2
0
7
15
CB50TL w/CCS OT
50
50
63
37
OT
48.4
35.1
73.7
7
a
LHV = lower heating value, RC = recycling, OT = once through, CBxxTL = coal and xx% dry basis mass of biomass cofeedstock; CO2 capture ratio (%) refers to captured CO2 as a percentage of produced CO2. b Case CB61TL w/o CCS RC uses the same energy efficiency and electricity coproduct data as Case CB55TL w/o CCS RC.
average, and highest energy efficiency values from the completed studies are used as the minimum, likeliest, and maximum values for the triangular distribution, respectively. Similarly, we develop triangular distribution functions for other parameters, such as the electricity credit of FTD plants and CO2 sequestration rates in FTL plants. The electricity used to compress CO2 is part of the electricity used within the plant; the former amounts to 326-348 kWh per ton of carbon compressed.7
3.3. Key Parameters for Coproduct Credits. In addition to FTD and electricity, FTD plants produce a mixture of hydrocarbons, the liquid portion of which is refined into finished FT diesel, naphtha (or gasoline). Total energy use and emissions of the FTD plants need to be allocated among these products in order to generate FTD-specific results. Table 2 shows that among the six OT FTD plant design cases, FTD accounts for 40-50% of total product output, on an energy output basis; 3049
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Table 2. Allocation Ratios between Primary Products and Coproducts energy value allocation pathway
diesel (%)
gasoline (%)
market value allocation electricity (%)
diesel (%)
gasoline (%)
electricity (%)
CTL w/o CCS OT
39.0
25.4
35.7
32.4
26.8
40.9
CTL w/CCS OT
49.6
24.1
26.3
42.6
26.3
31.1
CB34TL w/o CCS OT
38.0
24.8
37.2
31.5
26.1
42.5
CB34TL w/CCS OT
40.5
26.4
33.1
33.9
28.0
38.1
CB50TL w/o CCS OT
37.5
24.4
38.2
31.0
25.6
43.5
CB50TL w/CCS OT
39.3
25.6
35.1
32.7
27.0
40.3
gasoline (liquid coproduct in the selected cases7) accounts for about 25%; and electricity accounts for about 26-38%. Because of space limitation, we selected the six OT designs to explore the influence of three different coproduct credits methods on the results of FTD LCA: two were allocation methods based on the energy value and the market revenue and one hybrid method combining the energy allocation between diesel and gasoline and the displacement method for electricity (with two displacement cases - the U.S. average generation mix and the coal/biomass IGCC). The displacement method assumes that a new coproduct displaces a conventional product. The life-cycle energy emissions that would have been involved with the production of the displaced product are counted as credits for the main product. In the allocation method, the emissions and energy burdens of a process or plant are allocated among products according to their energy output shares or market revenue shares. The displacement method may be appropriate as long as the main product accounts for a major part of the total output (see Wang et al.11). In the six cases shown in Table 2, FTD shares are relatively small. Thus, application of the displacement method to FTD LCA is dubious. However, if we treat energy in FTD, naphtha, and gasoline by the same method and combine the three together, the share of coproduced electricity is relatively small. It makes application of the displacement method to electricity somewhat reasonable. Therefore, we use this hybrid method in our analysis. Energy and market allocation ratios between primary products and coproducts are shown in Table 2. Table S4 lists the LHV and C ratio of FT diesel, naphtha, and gasoline. This study uses the average price over the last 10 years for the market allocation method. Also, the FT naphtha price is assumed to be 90% of the gasoline price because its lower octane rating depreciates its value as a transportation fuel, even if the trading prices are similar. Table S5 lists the prices of conventional diesel, gasoline (as surrogates for the FTD and FT gasoline price, respectively), and electricity in the United States for the last 10 years. Two types of electricity generation are assumed to be displaced by FT plant electricity for the hybrid method: (1) electricity generated from the U.S. average generation mix (Table S6) and (2) electricity generated from IGCC that uses the same feedstock combination as the FT plant (coal-based or coal-biomass-based IGCC but without CCS).
4. RESULTS AND ANALYSIS Results in this study are reported for a million Btu of fuel produced and used in diesel passenger cars. Total energy use includes all energy sources (fossil and renewable energy). Fossil
Figure 3. WTW total energy and fossil energy use.
energy includes petroleum, natural gas, and coal. The figures in Sections 4 and 5 provide error bars superimposed on bars to represent ranges of 10th and 90th percentiles and dots to represent average values. Because of the large coproduct shares and volatility of market prices, this section applies the energybased allocation method. The influence of different coproduct credits methods on FTD LCA is discussed in the next section. 4.1. Energy Use Results. As shown in Figure 3, total energy use for FTD production is more than 75% larger than that for petroleum diesel production, no matter how much forest residue is fed. Moreover, CCS incurs an energy penalty of 3-10%. On the other hand, fossil energy use decreases linearly as the share of biomass increases. For example, when forest residue feedstock is increased to 55% by mass (CB55TL), fossil energy use is 96% (without the CCS) and 99% (with the CCS) of that of petroleum diesel. Moreover, since process fuel use in FT plants and FTD combustion during vehicle operation for the BTL pathways come from biomass, fossil energy use is reduced by 86% with BTL relative to that of petroleum diesel. Because of the lower total efficiency, the OT design consumes slightly more total and fossil energies than the RC design if the same feedstock is used (for example, CTL w/o CCS RC case and CTL w/o CCS OT case). Results indicate that the energy use and the fossil energy results have moderate uncertainties of 5-10% and 7-13%, respectively. 4.2. GHG Emissions Results. Figure 4 compares WTW emissions of GHGs for the 15 FTD pathways with the petroleum diesel pathway. FTD from coal increases life-cycle GHG emissions by more 200% without CCS and 5-29% (5% for RC design and 29% for OT design) with CCS relative to petroleum diesel. Increasing forest residue share reduces GHG emission linearly because carbon in forest residue is from the atmosphere via photosynthesis during feedstock growth. For example, 3050
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Figure 4. WTW GHG emissions of FTD and petroleum diesel.
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Figure 6. Fossil energy use with different coproduct approaches.
Figure 5. Total energy use with different coproduct approaches.
without CCS, GHG emissions are (1) reduced to a level equivalent to that from petroleum diesel when forest residue accounts for 61% of the total mass input to FT plants and (2) reduced by 77% compared to petroleum diesel with 100% biomass feed. With CCS, WTW GHG emissions of FTD can be almost zero when the forest residue share is 55%, while FTD from 100% biomass can achieve a 254% reduction in GHG emissions compared to petroleum diesel. The reduction in GHG emissions by CCS also increases as biomass share increases, even with consistent CCS ratios. For example, CCS reduces GHG emissions by 77 and 171 kg CO2e per mmBtu for CTL and BTL, respectively. The increased reduction with a larger biomass share is the result of forest residue containing more carbon per mmBtu than coal (39 kg C/mmBtu for forest residue and 28 kg C/ mmBtu for coal). GHG emission results have a moderate uncertainty of 7-16%. The CO2 capture ratio can reach 91% for the RC design but just about 76% for the OT design in this study, but the WTW GHG emission results of the two designs share the same trend line with the forest residue share. The trend lines are in agreement because when there is less CO2 capture, less energy is used, and thus more electricity is exported to the electric grid. Since the allocation method is used to handle the coproducts, the CCS ratio and the total efficiency of the FTL are the two major factors for the GHG emissions results, while the GHG emissions of displaced conventional products are an additional important factor with the displacement method.
Figure 7. GHG emissions of FTD with different coproduct approaches.
5. COPRODUCT CREDITS DISCUSSION Figures 5 and 6 present total and fossil energy uses with different coproduct credit methods, which differ by 5-10% and 3-34% among the three methods (with two displaced electricity cases) for the same pathway, respectively. The total energy results are relatively consistent with each other since the average energy efficiencies of the U.S. average mix and the IGCC are similar to the efficiency of FTD plants. In terms of fossil energy, as the forest residue share increases, the reduction resulting from the displacement method with the U.S. average electricity mix is higher than that resulting from the other methods because a higher forest residue share means less fossil energy is used for the electricity coproduction (Table S7). On the other hand, the displacement method with the coal/biomass IGCC shows that fossil energy use is similar to that associated with the allocation methods because the displaced electricity is assumed to have the same feedstock shares of coal and biomass as the FTD plants do. Figure 7 shows the results for GHG emissions with different coproduct methods in comparison with the corresponding results from Kreutz et al.7 The results of GHG emission differ by 24-325% (maximum value minus minimum value, then divided by maximum) among the three coproduct methods for 3051
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Environmental Science & Technology the same pathway. For pathways with coal as the feedstock, the displacement method involving coal IGCC yields the lowest GHG emissions, and the energy-value-based allocation method yields the highest. For pathways with coal and forest residue as the feedstock, the displacement method involving the U.S. average electricity mix yields the lowest GHG emissions because more than 50% of U.S. average electricity is generated with conventional coal-fired power plants. If the displacement method is used to evaluate the pathways with CCS, GHG emissions are much lower than those based on energy and market allocation method, especially for pathways with coal and forest residue as the feedstock. CB34TL with CCS and CB50TL with CCS result in a large negative GHG emission value if the displacement method is used, and the results are 148%-325% lower than the results calculated by using energy and market allocation methods. Contribution of the coproducts in relation to the primary product amplifies the effect of the displacement value on the final results for the pathways with small shares of FTD. Grid electricity generation for displacement by FTD plant electricity in this study is assumed without CCS. Thus, the amount of GHG emissions subtracted from the total GHG emissions of FT fuel pathway by the displaced electricity is considerably larger than that by the electricity with CCS. Therefore, the GHG emissions are reduced for FTD significantly, especially in the CBTL pathways with CCS achieving negativevalue GHG emissions. On the other hand, Kreutz et al. used electricity from coal IGCC with CCS as the displaced product.7 GHG emissions of electricity displaced in ref 7 is 138 kgCO2eq/ MWh, are much larger than those using the displacement method with coal/biomass IGCC in this study (Table S8) because the GHG emission credit is much smaller in ref 7 as a result of CCS in grid electricity generation. Displacement methods can yield widely different results based solely on the LCA choice of coproduct displacement value. We demonstrated the effect of the choice of coproduct methods on the LCA results of FTD. In addition to the arbitrary choice of grid electricity generation for displacement, the small shares of FTD make the results by the displacement methods dubious. The market-value-based allocation method is subject to the great variation in prices of different energy products. Energybased allocation method is appropriate to use for FTD LCA, especially for pathways with small shares of FTD.
’ ASSOCIATED CONTENT
bS
Supporting Information. Properties of Illinois No. 6 Bituminous Coal and Forest Residue, data on energy use in forest residue collection and transportation, U.S. historical prices of diesel, gasoline fuel and electricity, U.S. Electric Generation Mix and fossil energy use and GHG emissions of electricity with different coproduct approaches. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy,
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Vehicle Technology Program, under contract DE-AC0206CH11357. We thank Mr. Kevin Stork of that DOE office for his support of this study and the anonymous reviewer for the helpful comments on our draft manuscript.
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