Case studies of CO2 utilization in the production of ethanol

This policy brief, produced within the framework of IEA Bioenergy Task 39 (Biofuels to decarbonize transport) presents an analysis of two case studies: (1) the use of biogenic CO2 from sugar-to-ethanol fermentation in Brazil, and (2) the utilization of industrial exhaust gas from the steel industry in China for bioethanol production. The findings include an assessment of mass and energy balances integrated into a life cycle assessment to evaluate the environmental and economic potential of bioethanol production from these sources as part of broader carbon neutrality strategies.

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The first case study evaluated the environmental and economic feasibility of bioenergy with carbon capture, utilization, and storage (BECCUS) in sugarcane biorefineries. The findings demonstrate that carbon storage technology (BECCS) can produce ethanol with negative CO2 emissions (-8.5 gCO2e /MJ) even without the capture of CO2 produced in biomass furnaces. Meanwhile, the gas-to-bioethanol process (BECCU), which repurposes CO2 from the sugar-to-ethanol fermentation process, faces economic challenges related to the energy requirements of producing hydrogen via water electrolysis. In terms of emissions, gas-to-bioethanol fermentation offers a reduction in overall ethanol emissions but will not achieve negative emissions. This approach contributes to a more circular carbon economy, enhancing the use of carbon captured by biomass. Additionally, for gas-to-bioethanol to become economically feasible, alternative hydrogen sources like biomass gasification should be explored in future research.

The second case study evaluated the costs and environmental benefits of using steelmaking exhaust gas to produce ethanol. This gas stream contains CO2, CO, and H2, and can be used as feedstock in the gas-to-bioethanol fermentation process. Two technology pathways were compared: an existing process, from Beijing Shougang LanzaTech, in China, and an optimized process. Besides marginal improvements, the key difference in the optimized process is that unreacted CO2 is electrochemically reduced to CO and recycled back into the gas-to-bioethanol fermentation process, thus increasing the carbon utilization rate. Life cycle assessment shows a reduction in carbon emissions from 3.95 tCO2eq/t in the existing route to 1.46 tCO2eq/t in the optimized route. Despite the environmental benefits, the optimized route faces significant challenges, particularly in terms of high energy consumption and the associated costs of the CO2 reduction process.

The challenges listed for the two case studies of this report underscore the need for continued research and technological advancements to enhance the energy efficiency and economic viability of ethanol production via industrial exhaust gas. To leverage these findings, we recommend developing policies to support and optimize the CCU process, contributing to the domestic security of energy supply and the circular economy while also setting a framework for the utilization of CO/CO2-containing industrial waste gases in other energy-intensive sectors.

Additionally, we recommend the creation of incentives for BECCS in ethanol production, particularly given the high purity of CO2 from sugar-to-ethanol fermentation, which lowers CCS costs associated with the capture process.

CO2 utilization in the production of ethanol.