A perspective on gaseous biofuel production from micro-algae generated from CO2 from a coal-fired power plant. (15th June 2015) - Jacob, Amita
Highlights: 1 GWe coal-fired power plant can produce 2.7 Mt of micro-algal biomass (VS). A land area of 19, 200 ha would be required for a tubular PBR. 35% of the primary energy in coal can be retrieved as renewable gaseous fuel. Micro-algal gaseous biofuel from coal is sufficient to power 31 cars per hectare. The energy input to the cultivation system can be greater than the energy output. Abstract: There are significant resources of coal on the planet. It is likely that a lot of this coal will be combusted. A 1 GWe coal power plant operating at 35% electrical efficiency and a capacity factor of 75% produces 6.77 million tonnes of CO2 per annum. A closed cultivation system with a carbon capture efficiency of 80% allows production of 2.69 Mt of micro-algal (volatile solids), in a foot print of 19, 200 ha for a tubular photo-bioreactor (PBR) and 34, 000 ha for a Flat Plate PBR. An open system (raceway pond) at a carbon capture efficiency of 50% produces 1.68 Mt of micro-algal (volatile solids) and requires a footprint of 52, 303 ha. Employing a three stage sequential process (combining dark fermentation, photo fermentation and anaerobic digestion) to produce bio-hydrogen and bio-methane from the micro-algae could potentially generate 35% of the primary energy in the coal in the form of renewable gaseous fuel if a closed system of cultivation is used. This is sufficient to fuel 600, 000 cars per annum. In the cultivation of micro-algae, pumping and circulation is a considerable parasitic energy demand. The ratio of energy output (gaseous biofuel) to energy input (pumping and circulation) is less than 1 for all the three cultivation systems assessed, ranging from 0.71 for raceway ponds to 0.05 for a tubular PBR. If coal powered electricity is the source of this parasitic energy then a tubular PBR system produces more CO2 than the CO2 captured by the micro-algae.