Other Processes Anaerobic Digestion of Biomass
One possible way to make use of a unique property of biomass is to convert it by means of biochemical reactions, but this subject, however interesting, falls outside the scope of the present book. Suffice it to mention that anaerobic digestion is the most elegant and efficient gasification process in which (dirty) liquid water can be used as a gasifying agent and a cold-gas efficiency of about 95% is obtained. Unfortunately, only the (hemi-)cellulose part of the biomass is converted in the presently available processes. The reaction for cellulose can be written as:
(C6H10O5)n+nH2O = 3n CH4+ 3nC02
Anaerobic digestion is only applied in small-scale units of below 5 MW to convert agricultural and liquid domestic waste (Kruger 1995). The use of thermophilic microorganisms has made this conversion more attractive, but for a reasonable conversion of the biomass a period of two to three weeks is still required, which makes this process less suitable for large-scale plants. It could be that with hyper-thermophilic microorganisms this period could be reduced such that it could also be applied for larger-scale plants. Moreover, by using hyper-thermophiles, no sterilization stage is required, which is necessary where the digestion waste will be recycled to farms and/or forests. Finally, hyper-thermophiles may eventually be found that also convert lignin. The gas produced is essentially a mixture of methane and C02 and can only be used advantageously for heating purposes and in combined heat and power (CHP) schemes using a gas motor or small gas turbine for power generation.