Photosynthetic biogas upgrading using microalgae: Effect of light/dark photoperiod

The use of biogas for grid injection or vehicle fuel requires purification steps to obtain biomethane, process normally called biogas upgrading. The use of microalgae cultures has been proposed as a new alternative for CO₂removal from biogas. Full-scale systems for biogas upgrading using microalgae should be able to deal with natural existing day/night photoperiods. This research evaluated the effect of a light/dark photoperiod on the operation of a photosynthetic biogas upgrading system, at lab-scale conditions. A system based on an open-photobioreactor connected to a mass transfer column was used for that purpose. Using a continuous biogas flow, an upgraded biogas with a CO₂concentration between 2 and 4.5% was obtained throughout light and dark periods. O₂concentrations below 1% in final biogas were observed. Mass balances showed that CO₂desorption was the main process behind CO₂removal. CO₂removal during the dark phase was possible, under the tested conditions, as a result of inorganic carbon desorption from the photobioreactor and accumulation in the liquid phase.