The US Air Force is currently assessing the use of Fischer-Tropsch (FT) fuels, blended with conventional jet fuel, as aircraft fuel. However, a potential barrier to the use of FT fuels is the greenhouse gas emissions associated with their manufacture. Emerging guidelines for procurement of fuels place limits on the life cycle greenhouse gas emissions for fuels purchased by the US federal government. Since life cycle greenhouse gas emission estimates depend on inputs and emissions along the entire manufacturing chain, and depend on the fates of co-products of manufacturing processes, they are not directly measurable and must be estimated using models. This work examines the sensitivity of greenhouse gas emission estimates for Fischer-Tropsch (FT) fuels to FT processing conditions and to the assumptions made in the life cycle emission modeling. Overall, the greenhouse gas footprints for FT fuels were most sensitive to the choice of catalyst in the FT reactor, the internal recycling of synthesis gas and heavy ends, the use of biomass as a feedstock rather than coal, and the carbon number distribution of liquid hydrocarbons from the FT reactor. Life cycle assessment methodological choices (i.e., allocation methods) also had a significant impact on the estimated greenhouse gas footprint. These sensitivities were often coupled, with the sensitivity of process conditions depending on the allocation choices. Collectively, these results suggest that potential fuel suppliers will need to document both the details of process configurations and their emission estimation methodologies to obtain precise greenhouse gas emission estimates.
The US Air Force is assessing the use of Fischer-Tropsch (FT) fuels, blended with conventional jet fuel, in a variety of aircraft. However, a potential barrier to the use of FT fuels is the greenhouse gas emissions associated with their manufacture. The Energy Independence and Security Act of 2007 prohibits the federal government from purchasing alternative fuels for transportation that have greater greenhouse gas footprints than fuels produced from conventional petroleum sources. Specifically, Section 526 of the Energy Independence and Security Act of 2007 provides that: No Federal agency shall enter into a contract for procurement of an alternative or synthetic fuel, including a fuel produced from nonconventional petroleum sources, for any mobility-related use, other than for research or testing, unless the contract specifies that the lifecycle greenhouse gas emissions associated with the production and combustion of the fuel supplied under the contract must, on an ongoing basis, be less than or equal to such emissions from the equivalent conventional fuel produced from conventional petroleum sources.
Previous work on estimating greenhouse gas emissions from FT fuels (for example, reference 1) have been based on a small number of processing conditions, however, a wide variety of process configurations are possible for FT fuel synthesis, and these different processing configurations could lead to significantly different greenhouse gas emission estimates.
The goal of this work is to expand on previous work by developing parametric models of greenhouse gas emissions of FT fuel processing. The parameters to be considered include a variety of process variables (e.g., fraction of the unreacted syngas recycled to the FT reactor; molecular weight distribution of the waxes produced in the FT reactor; fraction of heavy fuels recycled to the fuel upgrading process) that would be expected to vary among facilities producing FT fuels.