Lifecycle greenhouse gas emissions for an ethanol production process based on genetically modified cyanobacteria: CO2 sourcing options

Abstract

AbstractAlgal biofuel production requires CO2, electricity, and process heat. Previous studies assumed CO2 sourcing from nearby coal or natural gas power plants. This may not be viable at a large scale or for the long term. The diurnal algal growth cycle imposes additional system design challenges for CO2 delivery. For ethanol produced by cyanobacteria in photobioreactors, we design onsite systems that provide heat, power and CO2 (CHP‐CO2), fueled by natural gas or biomass. Meeting the CO2 requirement produces excess electricity, which can be sold back to the grid. The scale of the CHP‐CO2 can be reduced by night‐time capture and refrigerated storage of CO2. The lifecycle greenhouse gas (GHG) emissions for 1 MJ ethanol are about −19 g CO2e for biomass CHP‐CO2, and +31–35 CO2e g for natural gas CHP‐CO2 options, compared with +19 g CO2e for the direct use of coal flue gas, and 91.3 g CO2e for 1 MJ of conventional gasoline. This work evaluates the energy and GHG implications of onsite CHP‐CO2 for algal ethanol production and other CO2 sourcing options. Combined heat and power (CHP) facilities, fueled by natural gas or biomass, could be co‐located with algal ethanol production, capturing and utilizing carbon dioxide to make biofuel, and thus providing an essentially stand‐alone biofuel operation, free from the constraints of co‐location with anthropogenic sources. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd