Oxygen transfer and evolution in microalgal culture in open raceways

•Oxygen mass transfer in raceway reactors mainly occurs place in paddlewheel and sump.•Mass transfer capacity of raceway reactors is a limiting factor for productivity.•Productivity increases at high mass transfer outweigh the higher power consumption.

The mass transfer characteristics of all sections of a 100 m2 raceway were evaluated. The efficiency of different diffusers was determined dynamically and the most effective was used for steady state system characterisation at water depth 0.2 m and velocity 0.22 m s−1. Mass transfer coefficients at a gas flow rate of 6 m3 h−1 were 164.50, 63.66, 0.87 and 0.94 h−1 for the paddlewheel, sump, straight and curved channel sections, with associated oxygen transfer rates of 106, 172, 27 and 39 g h−1. Oxygen supersaturation during algal cultivation led to a reduction in biomass productivity, which was more severe with pure CO2 than flue gas. Simulations showed the energy required to increase mass transfer and reduce oxygen concentrations was more than compensated for by increased biomass and potential energy yields. Oxygen removal is likely to be a critical criterion, and maintenance of mass transfer by sparging may be necessary even when CO2 is not required.