(768e) Methane-Derived Biofuels: Choosing Among Options

It has been estimated that the proven natural gas reserves in the U.S. could provide enough energy to meet the country’s transportation energy demand for the next 50 years [1]. This, coupled with the increasing price gap between natural gas and gasoline over recent years [2], makes natural-gas derived fuels an attractive option for use in the transportation sector. However, the infrastructure to use natural gas directly (either as CNG or LNG) has not been developed, and so natural gas must first be converted to a liquid fuel. Chemical processes for accomplishing this (e.g. GTL,  based on the Fischer-Tropsch reaction) suffer from high capital costs and low yields, thus there is significant interest in developing alternative processes for the high-efficiency conversion of methane to a drop-in transportation fuel [3].

Various microorganisms have evolved enzyme systems capable of metabolizing methane, either consuming it (methanotrophs) or producing it (methanogens), thus it is conceivable that, with the application of metabolic engineering technologies, a strain capable of producing a liquid fuel from methane could be developed. Before embarking on such an undertaking, however, it is critical to establish the inherent limitations of the chosen pathway, both from a carbon yield and energy yield standpoint, to maximize the likelihood of selecting a commercially viable process, and not just a novel scientific investigation. To that end, we present here an analysis of the various biological routes for the conversion of methane to a liquid fuel. Each process is divided into two segments: the upstream conversion of methane to an activated intermediate (either aerobically, anaerobically, or catalytically), followed by the downstream synthesis of a fuel molecule using several alternative pathways (Serine cycle, RuMP pathway, Wood-Ljungdahl pathway). We develop stoichiometric models for each scenario that allow calculation of the maximum potential yields, and conclude with the implication of these results in selecting an organism and pathway for the conversion of methane to a liquid fuel.

[1] “America’s Energy Future: Technology and Transformation” (National Academy of Sciences, 2009).

[2] U.S. Energy Information Administration, Annual Energy Outlook 2012 (EIA Publication, DOE/EIA-0383, 2012;

http://www.eia.gov/forecasts/archive/aeo12/index.cfm).

[3] "Reducing Emissions using Methanotrophic Organisms for Transportation Energy (REMOTE)" 2013. ARPA-E. March 2013. <https://arpa-e-foa.energy.gov/FileContent.aspx?FileID=261ed0b9-6695-4c51-a8a5-60f58b81e4f4>