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H-15 APPENDIX D Life-Cycle Greenhouse Gas Emissions To accurately assess the impact of fuel combustion on The life-cycle GHG emissions from a variety of potential global climate change, one must consider the full fuel life alternative jet fuels are plotted in Figure 6; these data are from cycle, from feedstock extraction through fuel combustion. If the analysis of Stratton et al. (2010). These results include an one only considers combustion, then for the fuels considered assessment on the anticipated impact of variations in feedstock here (conventional jet fuel, SPK, and ULSJ fuel) the emis- properties and process efficiencies on life-cycle GHG emissions sions of an alternative fuel will vary by less than 4%, and this as well as an analysis of the impacts of land-use changes. Five is true regardless of the feedstock used to create the fuel life-cycle steps were considered: feedstock recovery (e.g., min- (petroleum, natural gas, coal, or biomass) or how the fuel is ing, farming, pumping), feedstock transportation, feedstock processed. It is only from a life-cycle standpoint that one processing (e.g., gasification, F-T synthesis, refining), trans- can see that biofuels offer the potential to reduce aviation's portation (of finished fuel), and fuel combustion. Because of the impact on global climate change. Biofuels can lessen avia- increased energy intensity of feedstock extraction, unconven- tion's production of greenhouse gases because the biofuel tional petroleum fuels (oil sands and oil shale) have increased feedstock was created by photosynthetic reaction of water life-cycle carbon dioxide emissions relative to fuels created with carbon dioxide; thus, if atmospheric carbon dioxide from crude oil. A ULS fuel has a slight increase in life-cycle was used to grow the biomass, then the combustion of the carbon dioxide emissions because of the additional process- biofuel results in the carbon dioxide being returned to the ing (i.e., refining) that is necessary to desulfurize the fuel. To atmosphere from which it came and there is zero net emis- achieve emissions comparable to conventional fuels, F-T fuels sions of carbon dioxide into the atmosphere from fuel com- must either use carbon capture and sequestration (CCS) or bustion. This is not true for fossil fuel combustion, where the incorporate biomass. Without CCS, F-T fuels from coal will fuel feedstock contains carbon that has been sequestered have roughly twice the life-cycle carbon dioxide emissions. from the atmosphere for millions of years. Further back- Hydroprocessed renewable jet (HRJ) fuels have emissions that ground information and guidance on creating a life-cycle are highly dependent on the feedstock that is being used, with GHG inventory can be found within AFLCAWG (2009). emissions from either land-use change dominating (Table 1).
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H-16 Figure 6. Life-cycle greenhouse gas emissions from a variety of potential alternative fuel pathways that could result in SPK, ULS, or conventional fuels [from Stratton et al. (2010) with permission]. Table 1. Land-use change scenarios explored for HRJ pathways [from Stratton et al. (2010) with permission]. Land-Use Scenario 0 Scenario 1 Scenario 2 Scenario 3 Change Carbon depleted soils Switchgrass None converted to switchgrass n/a n/a cultivation Tropical rainforest Grassland conversion to Soy oil None conversion to soybean n/a soybean field field Logged-over forest Tropical rainforest Peatland rainforest Palm oil None conversion to palm plantation conversion to palm conversion to palm field plantation field plantation field Set-aside land converted to Rapeseed oil None n/a n/a rapeseed cultivation Desert land converted to Salicornia None n/a n/a Salicornia cultivation field