Life Cycle Assessments of Ethanol Production via Gas Fermentation: Anticipated Greenhouse Gas Emissions for Cellulosic and Waste Gas Feedstocks

Abstract

LanzaTech has developed novel microbial bioreactor systems capable of direct gas fermentation to produce ethanol from carbon-containing gases. In this study, a life-cycle assessment method is used to quantify the global warming potential of several scenarios for producing renewable ethanol with the LanzaTech process. Scenarios considering ethanol produced from steel mill waste gases or biomass (corn stover, forest residue, or switchgrass, via gasification) have been considered, using input data from peer-reviewed literature, government reports, life cycle inventory databases, and LanzaTech process engineering estimates. Using standardized life-cycle assessment methods, ethanol produced via LanzaTech fermentation appears to result in greenhouse gas emissions that are at least 60% lower than that of conventional fossil gasoline, with biomass-based ethanol achieving close to 90% emission reductions. Results indicate that the LanzaTech gas fermentation technology can be a viable alternative for producing next-generation biofuels that satisfy United States Renewable Fuels Standard policies concerning fuels with a reduced greenhouse gas emissions footprint.

Efficiency Comparison of Large‐Scale Standalone, Centralized, and Distributed Thermochemical Biorefineries

“…The biochemical pathway (syngas fermentation) to ethanol was selected as it offers a high efficiency, tolerates sulfur impurities in the syngas (in contrast to metallic catalysts), and is less affected by inert gases, such as methane. Furthermore, the production technology has recently reached the stage of maturity necessary for industrial applications …”

“…Theb iochemicalp athway (syngas fermentation) to ethanol was selected as it offers ah igh efficiency,t olerates sulfur impurities in the syngas( in contrast to metallic catalysts), and is less affected by inert gases,s uch as methane.F urthermore,t he production technology has recently reached the stage of maturity necessary for industrial applications. [26,27] Theh igh-energy demand for distillation is ak ey issue to achieve ah igh efficiency for ethanolp roduction,a nd the direct integration of the distillation process into the rest of the plant would lead to an overall low efficiency as extensive streams with high exergy would be used to meet ad emand that could instead be achievedu sing al ow-exergy stream. However, the water/ethanol mixture can be stored in regular tanks,w hich provides an opportunity to use the distillation process to balance the load of other low-temperature heat sources,f or example,adistrict heating plant, or to balance the electricity demandwithin the grid.…”

“…In the LanzaTech process, the syngas is introduced into the bioreactor and mixed with the liquid medium that contains the biocatalyst (the bacterium Clostridium autoethanogenum ), which is consumed throughout the reactor. The product at the outlet of the reactor is directed to the steadfast separation system (which includes distillation), which recycles the liquid that contains the microorganisms to the bioreactor and separates the main product from the byproducts . As the information available from the process manufacturers is limited, the flow‐sheet simulation (Figure ) is modeled by introducing data from other studies [Eqs.…”

“…Ther ange of operation is calculated by investigating the zero El case and the maximum gasification case max El . syngasf ermentation [26,27,[44][45][46] inlet pressure [bar] 1.8 [26] makeup process water [t t eth À1 ]8.5 [26] overall CO conversion[%] 50 [44] -80 [45] overall H 2 conversion [%] 45 [44] -75 [45] char gasification char gasification range [%] 40-70 electrolyzer [41]…”

“…Thep roduct at the outlet of the reactor is directed to the steadfast separations ystem (which includes distillation), which recycles the liquid that containst he microorganisms to the bioreactor and separatest he main product from the byproducts. [26] As the informationa vailable from the process manufacturers is limited,t he flow-sheets imulation ( Figure 5) is modeledb yi ntroducingd ata from other studies [Eqs.…”