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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.

Refined bio-crude production from hydrothermal liquefaction of algae holds the potential to replace fossil-based conventional liquid fuels. The microalgae act as natural carbon sequestrators by consuming CO2. However, this absorbed CO2 is released to the atmosphere during the combustion of the bio-crude. Thus, the life-cycle greenhouse gas (GHG) emissions of refined bio-crude are linked to the production and supply of the materials involved and the process energy demands. One prominent raw material is CO2, which is the main source of carbon for algae and the subsequent products. The emissions associated with the supply of CO2 can have a considerable impact on the sustainability of the algae-based refined bio-crude production process. Furthermore, the diurnal algae growth cycle complicates the CO2 supply scenarios. Traditionally, studies have relied on CO2 supplied from existing power plants. However, there is potential for building natural gas or biomass-based power plants with the primary aim of supplying CO2 to the biorefinery. Alternately, a direct air capture (DAC) process can extract CO2 directly from the air. The life-cycle GHG emissions associated with the production of refined bio-crude through hydrothermal liquefaction of algae are presented in this study. Different CO2 supply scenarios, including existing fossil fuel power plants and purpose-built CO2 sources, are compared. The integration of the CO2 sources with the algal biorefinery is also presented. The CO2 supply from biomass-based power plants has the highest potential for GHG reduction, with a GHG footprint of −57 g CO2 eq./MJ refined bio-crude. The CO2 supply from the DAC process has a GHG footprint of 49 CO2 eq./MJ refined bio-crude, which is very similar to the scenario that considers the supply of CO2 from an existing conventional natural gas-based plant and takes credit for the carbon utilization.

This study describes the response of Arthrospira platensis to a variety of temperature conditions as reflected in variations of photosynthetic parameters, pigmentation, and biomass productivity in indoor photobioreactor (PBR) cultivations. These experiments are designed to better understand the impact of temperature, seasonal variations, and acclimation effects on outdoor biomass production. The irradiance level and temperature range (20–39°C) are chosen to enable modeling of semi‐continuous operation of large‐scale outdoor PBR deployments. Overall, the cultivations are quite stable with some pigment‐related instabilities after prolonged high‐temperature exposure. Changes in productivity with temperature, as reflected in measured photosynthetic parameters, are immediate and mainly attributable to the temperature dependence of the photosaturation parameter, a secondary factor being variation in pigment content on a longer time scale corresponding to turnover of the culture population. Though pigment changes are not accompanied by significant changes in productivity, prolonged exposure at 35°C and above yields a clear degradation in performance. Productivities in a semi‐continuous operation are quantitatively reproduced with a productivity model incorporating photosynthetic parameters measured herein. This study confirms the importance of temperature for biomass and pigment production in Arthrospira cultivations and provides a basis for risk assessments related to temperature mitigation for large‐scale outdoor cultivations.