Energy balance of algal biogas production

Abstract

A mechanistic energy balance model was successfully developed for the production of biogas from the anaerobic digestion of micro-algal biomass from raceways. The energy balance model was used to consider the energetic viability of a number of production scenarios, and to identify the most critical parameters affecting net energy production. The output of the model demonstrated that no single method of harvesting studied (centrifugation, settlement or flocculation), produced a sufficiently greater energy output over operational energy inputs to make algal biogas production viable. Combinations of harvesting methods produced energy outputs 2.3 to 3.4 times greater than the operational energy inputs. Electrical energy to power pumps, mixers and harvesting systems is 5 to 8 times greater than the heating energy requirement. If the energy to power the plant is generated locally in a combined heat and power unit a considerable amount of “low grade heat” will be generated that is not required by the algal biogas process and for the plant to be efficient must be exploited. It is concluded that the production of micro-algal biogas may be energetically viable, but is dependent on the exploitation of the heat generated by the combustion of biogas in combined heat and power units to show a positive balance