Life cycle assessment of a microalgae biomass cultivation, bio-oil extraction and pyrolysis processing regime

•PBR cultivation of T. chui biomass delivers a net result of − 222 kg of CO2 eq/t DW.•Fertiliser usage/drying energy is a major component of environmental impact.•Global warming typically represents only around half of all impacts.•The lowest impact comes from electricity production and biochar application.•Microalgae bio-CCS is not currently viable without considerable process innovation.

Life cycle assessment (LCA) of a microalgae biomass cultivation, bio-oil extraction and pyrolysis processing regime is a useful means to gauge the likely environmental impact of this prospective new development on an industrial scale. Coupled to thermal conversion via slow pyrolysis, the prospect of biologically ‘sequestering’ carbon derived from microalgae biomass as biochar, added to soil, is considered. However, an intensive closed culturing photobioreactor system coupled to a pyrolysis process incurs a net increase in global warming and overall life cycle impact, notwithstanding biochar application to soil. Results indicate that up to 50% of environmental impact in certain categories stems from the upstream influence of fertiliser production. Energy used in flue gas delivery and pumping during cultivation is also considerable, suggesting that current practice in closed cultivation systems does not yet adequately trade-off biomass productivity against operating intensity. Drying of the harvested microalgae biomass for pyrolysis processing is potentially a major hurdle in terms of process viability also. Overall, utilisation of nutrients derived from waste streams, integrating renewable energy and capture of process heat for more efficient drying are essential levers for reducing the environmental impact of this proposition.