Environ. Sci. Technol. 2008, 42, 7992–7999
Life Cycle Assessment of an Advanced Bioethanol Technology in the Perspective of Constrained Biomass Availability K A R S T E N H E D E G A A R D , * ,† KATHRINE A. THYØ,‡ AND H E N R I K W E N Z E L * ,§ COWI A/S, 2800 Lyngby, Denmark, Copenhagen Energy Ltd., District heating, 2300 Copenhagen S, Denmark, and Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, University of Southern Denmark, 5230 Odense M, Denmark
Received February 05, 2008. Revised manuscript received June 18, 2008. Accepted August 14, 2008.
Among the existing environmental assessments of bioethanol, the studies suggesting an environmental benefit of bioethanol all ignore the constraints on the availability of biomass resources and the implications competition for biomass has on the assessment. We show that toward 2030, regardless of whether a global or European perspective is applied, the amount of biomass, which can become available for bioethanol or other energy uses, will be physically and economically constrained. This implies that use of biomass or land for bioethanol production will most likely happen at the expense of alternative uses. In this perspective, we show that for the case of a new advanced bioethanol technology, in terms of reducing greenhouse emissions and fossil fuel dependency, more is lost than gained when prioritizing biomass or land for bioethanol. Technology pathways involving heat and power production and/or biogas, natural gas or electricity for transport are advantageous.
Introduction The conversion of biomass to ethanol for use in the transport sector has received increasing attention motivated by objectives of reducing CO2 emissions and oil dependency. Along with the focus on the environmental and resource dependency benefits, the potential down-side of bioethanol is receiving increasing attention as well. The implications of using food for fuel especially are being discussed. In this context, much hope and research is directed toward advanced bioethanol technologies capable of fermenting nonfood lignocellulosic biomass feedstocks. The decisive issue for the environmental aspects of bioethanol is, however, not the issue of competition between food and fuel, but the issue of competition for the limited biomass in general, including competition between alternative energy uses of the biomass. Given the constraints on biomass availability and the magnitude of the new potential customers for biomass resources for transport, heat, elec* Address correspondence to either author. Phone: (0045) 2681 8298(K.H.); (0045) 6550 7374 (H.W.); e-mail:
[email protected] (K.H.);
[email protected] (H.W.). † COWI A/S. ‡ Copenhagen Energy Ltd. § University of Southern Denmark. 7992
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tricity, polymers, and bulk chemicals, comparisons between the alternative energy uses become decisive: we cannot have all, and for decades ahead use of biomass for ethanol will most likely take place at the expense of alternative energy uses. In this perspective, advanced bioethanol technologies capable of fermenting lignocellulosic biomass do not distinguish themselves from conventional bioethanol technologies. In the present paper, we show that a new advanced bioethanol technology case of fermenting whole-crops comes out environmentally disadvantageous due to this. To our knowledge, this article is the first to present a Life Cycle Assessment (LCA) of bioethanol seen in this light of the constraints on biomass availability. Several of the existing studies on bioethanol (e.g., refs 1-5) comprise a system to and including production of bioethanol only and are mainly based on an energy balance expressing how much fossil energy is used to produce the bioethanol compared to the energy content of the ethanol output, and in some cases including the energy content of byproduct. Another group of studies (e.g., refs 6-11) applies an expanded system delimitation, taking into account all displacement mechanisms including the displacement of petrol in cars with the corresponding avoided oil extraction and refining and the displacement of animal feed products by the coproducts from the ethanol production. The system delimitation in all of the above studies is, however, inadequate for supporting decisions on bioethanol, because the implications of the constraints on the biomass and land resources are ignored. Only three studies are known to adequately consider the implications of biomass constraints (12-14), and the results of these studies do not suggest an environmental benefit of bioethanol. This article is based on these studies. In the present article, we justify that toward 2030, use of biomass or land for bioethanol or other energy purposes will most probably take place at the expense of alternative uses. Since natural gas and coal will be used as fuels for heat and power production at least within this time frame, the lost alternatives include substitution of natural gas or coal in the heat and power sector. In the case study, we investigate the environmental feasibility of using advanced fermentation based bioethanol for transport, when held up against the consequence of losing alternative biomass utilizations. The biomass feedstock considered is an energy whole-crop in the form of whole-crop maize and the bioethanol technology considered includes fermentation of lignocellulosic biomass.
Biomass Constraints Physical Biomass Constraints. We have assessed the physical biomass availability on a European and global scale toward 2030. A European perspective is relevant since biofuel trade is to some extent regionally oriented, and a global perspective has relevance since the market for liquid biofuels and for some solid biofuels (wood pellets, wood chips, tree trunks, and straw pellets) is expected to become very internationally oriented in the future (12). The assessment is based on comparing studies of technical biomass potentials with the biomass required to meet conflicting uses of biomass. The technical biomass potential is defined as the amount of biomass that, based on the available technology, can become available for energy purposesswhile still securing the demand for food, animal feed, and biomaterials. The technical biomass potential is based on inventories of potential bioenergy sources, including organic residues and waste and use of land for biomass production, with an evaluation of possibilities to utilize the sources for energy purposes (15). The potential is limited by 10.1021/es800358d CCC: $40.75
2008 American Chemical Society
Published on Web 10/04/2008
TABLE 1. Biomass Potentials Compared to Biomass Required for Full Fossil Substitution biomass potential (EJ/y)
study 21 22
17 23 24 25 26 30 27 28 29
geogr. scope
temporal scope
resource focused
EU25 EU27 EU27 EU27 EU27 EU27 global global global global global global global global global global
2030 2015-2025 2025-2045 2025-2045 >2040 >2040 2030
X X X X X X X X X X X
2030 2025-2050 2020 2025 2025 2025 2030 2025
n.d Xb Xb
demand driven
scenario
low high low high low high
X n.d Xb X Xb
BI FFES RIGES
residues
energy crops
6.7 2.8 2.9 3.5 2.5 3.1 96 96 87 31 15
5.2 1.8 5.6 7.2 15.4 19.9 219 315 151 267 112
56
17
65
80
total
biomass req. for full fossil fuel subst.a (EJ/y)
fossil fuel subst. (%)
11.9 4.6 8.5 10.7 17.9 23 315 411 238 298 127 74 85 74 91 145
79-90 89-102 89-102 89-102 89-102 89-102 631-716 631-716 631-716 631-716 631-716 631-716 631-716 631-716 631-716 631-716
16-18 4-5 8-9 10-12 17-19 21-25 42-48 55-62 32-36 40-45 17-19 10-11 11-13 10-11 12-14 19-22
a
Based on energy demand scenarios in refs 19 and 20, assumed fossil fuel substitution efficiencies, and supplementary energy statistics in ref 31. For conversion of biomass into transport biofuels, an average energy conversion efficiency of 60% is assumed (liquid biofuel output versus biomass feedstock input and net fuel use after crediting for an assumed use of byproduct as fuels). In the heat and power sector, an overall fossil fuel substitution efficiency of 90% is assumed, considering the conversion losses in obtaining high-quality biomass fuels, providing electric efficiencies on level with fuel-based power generation. b The study has an upper limit of biomass energy availability for the demand driven scenarios. n.d.: not documented. BI: Biomass Intensive variant. FFES: Fossil Free Energy Scenario. RIGES: Renewables-Intensive Global Energy Scenario.
natural circumstances as well as the given technology level (16, 17). By definition, the technical potential is assessed without considering how much of the biomass would be economically feasible to utilize. As such, the technical bioenergy potential defines the upper physical limit of the amount of biomass that can become available for energy use (16, 18). The potential utilizations of biomass for energy include electricity, heat, and transport fuel production, substituting fossil fuels both in the energy sector and in the transport sector. The amount of biomass required for fossil fuel substitution therefore represents the maximum potential demand for biomass for energy. We estimate the biomass required for fossil fuel substitution based on energy demand scenarios set up by the International Energy Agency, concerning future use of fossil and renewable energy sources (19, 20). In Table 1, various studies of biomass potentials toward 2030 in a European and global scope are presented. The biomass potentials are held up against the amount of biomass required for full fossil fuel substitution for the given geographical and temporal scope. Studies applying a purely resource focused approach (17, 21-25) represent technical biomass potentials. It can be seen that in a European perspective, toward 2030, technical biomass potentials (21, 22) only provide a small contribution, 4-25%, compared to the biomass required for substitution of all fossil fuels. In a global perspective, the technical biomass potentials (17, 23-25) correspond to a fossil fuel substitution of 17-62% in 2030. Thus, the technical biomass potentials are far from reaching the level required to meet all potential utilizations of biomass for energy. This indicates that toward 2030, in a European perspective as well as a global perspective, the amount of biomass which can become available for energy purposes cannot satisfy all potential new biomass customers. As can be seen, the constraint is particularly strong in a European perspective. Economic Biomass Constraints. While the technical potentials express the upper physical limit of the biomass
that can become available, economic limitations also exist. We have investigated the economic biomass constraints based on studies which, rather than assessing biomass potentials, estimate the biomass supply in the energy system, based on a demand-driven approach. This biomass supply depends on factors such as projected population growth, economic development, climate policies, energy intensity of economic activity, expected development in energy technologies, competition with other energy technologies, and considerations regarding the amount of biomass that can become available at given costs (15). According to demand-driven studies (26-29), the global biomass supply in 2020-2030 corresponds to 10-22% of the biomass required for full fossil fuel substitution, i.e. a significantly lower level than the global technical biomass potentials (Table 1). This indicates that when considering economic constraints, the limitation of biomass is even more significant. The explanation is that in reality, bioenergy technologies have to compete with other energy technologies in satisfying the energy demand. As a result, only the fraction of the biomass potentials realizable at cost levels providing economic competitiveness, or only the fraction available within the limits of available economic incentives, will be utilized. Systems Description. The identified constraints on biomass availability to meet the new conflicting demands for it strongly indicate that whenever biomass or land is used for bioethanol production, it will happen at the expense of alternative utilizations, e.g., for heat and power generation. When assessing the environmental aspects of bioethanol, this use of biomass or land should, therefore, be held up against alternative utilizations. Based on this recognition, we have performed an environmental assessment of a new Danish advanced bioethanol technology. The bioethanol technology analyzed is the Integrated Biomass Utilization System (IBUS) (developed by the Danish energy group DONG Energy A/S) where the ethanol plant is integrated with a coal-fired power plant. Via the IBUS process, bioethanol is produced together with byproduct in the form VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Energy Crop Utilization Pathways Compared in the Environmental Assessment transport sector
heat and power sector
reference system
petrol
coal or natural gas CHP production
bioethanol scenarios
bioethanol from whole-crop maize, IBUS concept. Use of fodder byproduct as animal feed (baseline)
coal or natural gas CHP production
bioethanol from whole-crop maize, IBUS concept
coal or natural gas CHP production, use of fodder byproduct as fuels displacing coal
bioethanol from whole-crop maize, stand-alone plant use of fodder byproduct as animal feed (baseline)
coal or natural gas CHP production
bioethanol from whole-crop maize, stand-alone plant
coal or natural gas CHP production, use of fodder byproduct as fuels displacing coal
alternative scenarios
biogas from maize silage petrol compressed natural gas (CNG) (displaced in the heat and power sector) petrol compressed natural gas (CNG) (displaced in the heat and power sector) petrol electricity (use of electric cars) (produced from willow in the heat and power sector)
of solid biofuel and animal fodder, i.e., Dried Distillers Grain Soluble (DDGS) and molasses. The solid biofuel residue after ethanol fermentation can be incinerated directly at the power plant, displacing coal. The power plant integration means that in periods with low or no district heating demand, steam for the bioethanol process can be extracted from the power plant with low or no additional fuel use. As a result of this synergy effect, the average fossil fuel consumption for producing steam for the ethanol process is low (approximately 0.60 GJ coal/GJ steam) compared to using a separate boiler (12). However, it is questionable whether the gain of this synergy effect can be attributed to the ethanol production, since alternative utilizations of the low fuel steam extraction exist, and since a longer term aim of the energy system infrastructure is to plan for reducing these excess heat losses from power production. Against this background, the performance of a stand-alone bioethanol plant is also investigated. For this case variant, the steam is assumed to be produced at a natural-gas-fired boiler (with a typical boiler efficiency of 0.95 GJ steam/GJ natural gas). As a consequence of an increasing bioethanol production, saturation with DDGS on the global fodder market is probable in a future perspective. Thus, a situation is likely to occur where an alternative use of DDGS such as incineration with energy recovery will be applied. Based on these considerations, scenario variants have been set up for use of DDGS and molasses, entirely as fuels (displacing coal) instead of fodder. The assessment concerns whole-crop based bioethanol production, representing a large-scale production stretching further than use of biomass residues or waste. In order for bioethanol to become a major global transport fuel, energy crop production in temperate regions would also be required (12). Against this background, energy crop production in a 7994
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coal or natural gas CHP production biogas from maize silage and subsequent CHP production displacing decentralized natural gas CHP plants biogas from maize silage and subsequent CHP production displacing decentralized natural gas CHP plants thermal gasification of willow and subsequent CHP production displacing natural gas on the fuel input side thermal gasification of willow and subsequent CHP production displacing natural gas on the fuel input side willow wood pellet manufacturing and subsequent CHP production (cofiring) displacing coal on the fuel input side willow wood pellet manufacturing and subsequent CHP production (cofiring) displacing coal on the fuel input side
temperate climate, represented by Danish conditions, is considered. When prioritizing land for biomass production targeted toward bioethanol, the lost alternatives comprise other energy crops and/or energy utilization pathways. In this context we consider other biomass conversionssbiogas production, thermal gasification, and wood pellet manufacturingscombined with subsequent combined heat and power (CHP) production, as well as other alternative transport fuels and vehicles technologiessbiogas, Compressed Natural Gas (CNG), and electricity (use of electric cars). Based on discussions with experts in agricultural production, we have considered whole-crop maize to represent a high yield and suitable whole-crop for advanced bioethanol technologies capable of fermenting both starch and lignocellulosic biomass. For biogas production, whole-crop maize is assumed used as well, while the perennial crop, willow, is applied for heat and power production. The biomass technologies compared are currently at pilot or demonstration scale and are expected to reach commercial level toward 2030 (12, 14). In the environmental assessment, the consequence of choosing each biomass utilization route is investigated by modeling changes compared to a reference system comprising the transport sector and the heat and power sector. We apply a temporal scope toward 2030, representing a scope where natural gas and coal will continue to be used in the heat and power sector and petrol in the transport sector. Table 2 gives an overview of the various scenarios considered in the assessment. Method. The assessment is performed as an LCA and is based on the EDIP method (32) and further updates of this method (33-35), which are in agreement with the standards of the International Organization for Standardisation, ISO. The study is conducted according to the principles of
FIGURE 1. Scenario examples of alternative utilizations of 1 ha year agricultural land for energy use: (a) reference system (for coal displacement scenarios), (b) IBUS bioethanol scenario (baseline), (c) willow CHP production displacing coal, and (d) willow CHP production displacing coal and using electric vehicles for transport. Induced (red lines), avoided (green scattered lines), and unchanged (gray lines) processes and flows compared to the reference system. The modeling of the scenarios expresses the induced and avoided processes and flows. The diagrams are simplified and therefore do not cover the full modeling performed. consequential LCA, which is today’s best scientific practice. It implies that the LCA is comparative and dedicated to identifying the environmental consequence of choosing one alternative over the other. The consequential and comparative approach ensures that all compared alternatives are equivalent and provide the same services to society, not just regarding the primary service, which in this case is a specified transportation, electricity, and heat service, but also on all secondary services. Secondary services are defined as products/services arising, e.g., as coproducts from processes in the studied systems. In the case of liquid biofuels, such
secondary services can typically be animal feed and nutrients. Consequential LCA ensures equivalence on all such services by identifying and including the displacements of alternative products that will occur when choosing one alternative over the other. The modeling has been facilitated in Gabi4 LCA software. The LCA covers all environmentally significant induced and avoided processes and flows compared to the reference system occurring as a consequence of choosing the given technology pathway. The approach is illustrated in Figure 1, presenting simplified diagrams of the reference system and VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Greenhouse gas emissions in a life cycle perspective for alternative energy utilizations of 1 ha year agricultural land (a) shown distributed on sources and (b) as net greenhouse gas emissions (positive values represent induced emissions and negative values represent avoided emissions). CO2-eq.: CO2-equivalents. CNG: Compressed Natural Gas. a few of the scenarios considered. The scenarios all provide the same primary service to society, namely 154,000 km transport in a typical European compact size 5-seater passenger car, 87 GJ power to the grid, and 113 GJ district heating to the grid, and are thus perfectly comparable in this respect. This forms the functional unit of the environmental assessment. These quantities reflect the largest biomass based transport service, power service, and heat service, respectively, delivered among the scenarios. For example, the transport service of 154,000 km is the transport distance delivered in the scenario where biomass is used for producing electricity for transportation by electric cars (Figure 1d). In the other scenarios, the reference fuel, petrol, will then be used up to the deliverance of this transport service. Functional outputs from the systems in terms of animal feed products and utilized nutrient value are kept equal by including the alternative feed and fertilizer products substituted by these functional outputs. Detailed descriptions of the modeling can be found in Jensen and Thyø (12). 7996
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Toward 2030, average yields in practice of 15 tons of dry matter per hectare per year have been assumed for both of the energy crops in question, i.e., for whole-crop maize and willow. Thus, differences between the scenarios reflect differences in the technology pathways alone. It is reasonable to assume similar yield levels for the two crops since their current average yields are similar: 10.6 and 11 tons DM per hectare per year for whole-crop maize and willow, respectively (the 75% percentile) (12). Possible differences in carbon dioxide sequestration during production of maize and willow, respectively, have not been included. Electricity consumption is modeled as coal CHP, which in many countries is likely to cover marginal electricity demand (34). However, for the most essential electricity flows, natural gas based electricity is also modeled, and the assumption regarding marginal electricity demand is found not to be crucial for the scenario comparison. With respect to the assessment parameters, we focus on the two key parameters that are most often used to justify
TABLE 3. Energy Input/Output Data for Different Biomass-to-Energy Technologies fuel inputa (primary energy, GJ) feedstock bioeth. bioeth. bioeth. bioeth.
technology
IBUS concept IBUS concept IBUS concept stand-alone plant anaerobic digestion, biogas whole-crop maize biogas upgrading
CHP CHP CHP
maize kernels maize stover whole-crop maize whole-crop maize
wood pellet manufacturing., cofiring at coal CHP plants thermal gasification, cofiring willow at natural gas CHP plants anaerobic digestion, whole-crop maize biogas CHP willow
product outputa (primary energy, GJ) liquid/gaseous solid fodder total biofuel biofuel
net fossil fuel subst. efficiencyb
power
heat
total
0.10 0.12 0.11 0.11
0.15 0.17 0.16 0.28
0.25 0.29 0.27 0.39
0.54 0.43 0.49 0.49
0.41 0.18 0.18
0.40 0.11 0.27 0.27
0.94 0.96 0.95 0.95
0.23
0.04 0.27
0.90
-
-
0.90 0.71
0.11
0.01 0.12
-
1.00
-
1.00 0.90
0.05
-
0.05
0.95
-
-
0.95 0.91
0.05
0.04 0.09
0.90
-
-
0.90 0.83
0.43 (0.75) 0.65 (0.74) 0.54 (0.75) 0.49 (0.68)
a Per GJ biomass feedstock input (lower heating values used). b Fossil fuel energy substituted at the power plant or in the car engine, per biomass feedstock energy and primary fuel use for biofuel processing. Indirect fossil fuel substitution as a consequence of fodder byproduct displacing fodder production is included in the environmental assessment but not in the substitution efficiencies given here. It is assumed that 1 GJ bioethanol substitutes 1 GJ petrol in the engine and that 1 GJ willow wood pellet or producer gas substitutes 1 GJ fossil fuel at the power plant. Values in parenthesis indicate substitution factors in case of incineration of possible fodder byproduct.
the promotion of bioethanol from an environmental perspective, i.e., GHG mitigation and reduction in fossil fuel dependency.
Results and Discussion As shown in Figure 2b, the ethanol baseline scenarios, representing use of DDGS and molasses as fodder, provide by far the lowest net GHG mitigation compared to the alternative utilizations of land for energy purposes. For instance, use of willow for CHP substituting coal provides GHG mitigation more than twice as high, and even higher if combined with electric cars. Assuming use of the fodder byproduct as fuels, significantly larger GHG mitigation is obtained; however, the ethanol scenarios are still in the low end compared to the alternatives. The low net GHG mitigation obtained in the ethanol scenarios is mainly caused by the considerable amounts of steam and electricity consumed in the process of converting biomass into bioethanol, particularly for pretreatment, hydrolysis, extract concentration, distillation, and drying processes (12). In comparison, less energy is required for catalyzing the anaerobic digestion in the biogas process, for the thermal gasification of willow, or for wood pellet manufacturing. This results in lower net fossil fuel substitution efficiencies for ethanol compared to the alternatives, as presented in Table 3. Other factors contributing to the difference in GHG mitigation across the scenarios comprise the high CO2 content of coal, providing high GHG emissions reduction when prioritizing biomass for coal displacement and the high energy efficiency of electric motors compared to combustion engines. As revealed in Figure 3b, the ethanol scenarios provide a low net fossil fuel displacement compared to several of the alternative technology pathways. Assessing how well the technology pathways perform in terms of reducing fossil fuel dependency, one should, however, not merely rely on energy balances. As reflected in the fuel prices, oil and natural gas are considered to be of higher value than coal, per energy content. This is due to factors such as higher energy densities, the fact that natural gas and oil can be used directly as transport fuels, and that they are more scarce, i.e., assuming continuation of current production rates, global oil and natural gas reserves offer supply horizons of 40-50 years
and 60-70 years, respectively, compared to 120-130 years for coal (36). Thus, when interpreting the results in Figure 3a, oil or natural gas displacements should be valued higher than coal displacements. Figure 3a reveals that in terms of obtaining the highest reduction in oil dependency, use of the land for ethanol production is far from being the optimal solution. Up to 2.5 times as high oil savings can be obtained with the alternative energy crop utilization pathways. Moreover, for a stand-alone bioethanol plant using steam from a natural gas boiler, considerable net consumption of natural gas occurs. Overall, for the case presented, the reductions in GHG emissions and fossil fuel dependency, obtained by producing whole-crop maize for bioethanol production happens at the expense of other land/biomass utilizations, which would provide considerably larger reductions. Thus, for this technology case and perspective, more is lost than gained when prioritizing land/biomass for bioethanol. This is mainly caused by the significant energy conversion losses in bioethanol production compared to use of biomass in the energy sector. The losses lie in the need for pretreatment (lignocellulosic based production), the relatively low fermentation yield of ethanol, the need to dry and further process the byproduct and residual unconverted matter in order to make use of them, and the need to separate ethanol and water, implying distillation in all known cases. Such losses are not present in alternative technologies, e.g., biomass conversion to electricity and/or heat by incineration or conversion to biogas. As long as fermentation-based conversion of biomass to ethanol implies these losses, bioethanol will come out disadvantageous to the alternatives studied heresand this is the case for presently known bioethanol technologies including both starch and lignocellulose based production. Thus, the results question the assumed justification for lignocellulosic fermentation based bioethanol: instead of reductions in GHG emissions and fossil fuel dependency, net increases will much more likely be the outcome, when considering the alternative biomass/land utilizations deprived on behalf of bioethanol. Among the scenarios investigated, willow CHP production combined with using electric cars for transport yields the highest GHG mitigation and reduction in oil dependency. VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Fossil fuel displacement in a life cycle perspective for alternative energy utilizations of 1 ha year agricultural land, shown as (a) net consumption/displacement of crude oil, hard coal, and natural gas, respectively, and as (b) net fossil fuel displacement (positive values represent fuel consumptions and negative values represent fuel displacements). CNG: Compressed Natural Gas. Another advantage of electric cars is that they can serve as a buffer in the electricity grid allowing for further implementation of fluctuating wind and solar power in the energy system. However, the electric car and plug-in hybrid cars (electric motor and combustion engine) are still in a development stage and commercialization is not expected to occur until 10 to 15 years from now (37). Technologies for use of CNG or biogas as transport fuel are commercially available and are used to an increasing degree in a number of countries, e.g., in Germany and Sweden. The natural gas supply horizon of approximately 60-70 years, based on current global reserves and production rate, makes it a relevant alternative transport fuel. Evidently, natural gas will be present in our energy sector worldwide for the period ahead until the electric car is expected available. Maximizing both GHG mitigation and fossil fuel displacement, the optimal biogas scenario is thus for biogas to substitute natural gas in heat and power production and to use the displaced natural gas to substitute oil in the transport sector (Figures 2b, 3b). This renders the scenario for upgrading biogas (removing CO2) directly for transport less interesting, as it will imply unnecessary losses. 7998
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