Is it better to produce biomethane via thermochemical or biological routes? An energy balance perspective

Biomethane is a gaseous energy vector that can be produced via thermochemical (gasification) or biological (anaerobic digestion) routes. Biomethane may be used as a renewable fuel for transport, for power generation, or for heating. In this paper, the net energy of biomethane derived from a hectare of willow using a gasification process is detailed and shown to be in the range of 83 to 117GJ/ha/a; the precise value is dependent on the process efficiency assumptions. This was compared to existing studies on grass biomethane produced via biological routes. The net energy of both biological and gasification processes is very similar. The net energy per hectare of the biomethane systems is of the same order of Brazilian sugarcane ethanol. It is suggested that the rationale for choosing between these two methods of generating biomethane may be based on economic considerations and it is more likely that the biological route will win out in Ireland due to the investment required to set up such an industry.