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

Abstract

Algal biofuel production requires CO 2 , electricity, and process heat. Previous studies assumed CO 2 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 CO 2 delivery. For ethanol produced by cyanobacteria in photobioreactors, we design onsite systems that provide heat, power and CO 2 (CHP-CO 2), fueled by natural gas or biomass. Meeting the CO 2 requirement produces excess electricity, which can be sold back to the grid. The scale of the CHP-CO 2 can be reduced by nighttime capture and refrigerated storage of CO 2. The lifecycle greenhouse gas (GHG) emissions for 1 MJ ethanol are about −19 g CO 2 e for biomass CHP-CO 2 , and +31-35 CO 2 e g for natural gas CHP-CO 2 options, compared with +19 g CO 2 e for the direct use of coal flue gas, and 91.3 g CO 2 e for 1 MJ of conventional gasoline. This work evaluates the energy and GHG implications of onsite CHP-CO 2 for algal ethanol production and other CO 2 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.

Greenhouse Gas Impact of Algal Bio-Crude Production for a Range of CO2 Supply Scenarios

“…The results of that study, with biomass sourcing providing the lowest emissions and purposebuilt natural gas sourcing providing the highest emissions, are reflected in the results here. In other previous work [21] we evaluated the life-cycle greenhouse gas emissions for ethanol produced in a similar process, for different CO 2 sources; that study had similar findings: biomass is the lowest carbon CO 2 source, existing fossil fuel power plants are an intermediate source, and purpose built natural gas power plants provide the highest fuel emissions. The process modeled here for direct air capture is a current-technology high emitting scenario powered by fossil fuel; more energy-efficient systems that use low-carbon energy sources have the potential to significantly lower the greenhouse gas emissions.…”

“…For the supply of CO 2, the following five different supply scenarios are considered: These CO 2 supply scenarios have been extensively discussed by the authors in previous publications [10,21] and are briefly described in the Supplementary Information. The process flowsheet for the base-case scenario is presented in Figure 1.…”

Environmental Impacts of Formalin and Hexamine Production Units Using Life Cycle Assessment: A Case Study in the Gameron Petro Industry Complex, Iran

“…[11]. The LCA deals with environmental aspects throughout the life cycle of a product from processed raw material to production, consumption, end of biological acts, recycling and final disposal [12].…”