A biogas plant consists of the following main components:
Receiving Unit: All feedstocks entering the biogas plant will be delivered to a receiving unit. The purpose of the receiving unit is to store feedstock and feed it into the digesters. The receiving unit can be an agitated storage tank for liquid biomass or a container with a feeding system for solid biomass. The feedstock is fed into the digesters according to a scheduled feeding plan.
Digester: The digestion of the biomass takes place in an oxygen free environment in a completely stirred tank digester (CSTR). In Renew Energy’s design, the digesters are equipped with top mounted agitators that assure complete mixing of the digesters so that the formation of floating layer and temperature pockets are avoided. Top mounted agitators have very low power consumption compared to other mixing options. The hydraulic retention time in the digesters is normally between 15 -35 days depending on the biomass. The digestion is most efficiently operated at either mesophilic temperatures (35-37 °C) or thermophilic temperatures (52-55 °C). The combined characteristic of the mixed feedstocks determines which temperature level is chosen. Stability is important for a good economy. Therefore, we prefer external heat exchangers in our design. Cleaning of external heat exchangers is a matter of only hours rather than days or weeks, and the digester will be left undisturbed to continue its gas production.
Biogas Desulphurization: In addition to methane and carbon dioxide, the biogas also contains hydrogen sulfide originating mainly from the degradation of sulfur containing proteins. To avoid corrosion, hydrogen sulfide needs to be removed from the biogas before the biogas can be utilized. There are several methods for removing hydrogen sulfide. Renew Energy offers a biological gas cleaning process that is performed in a separate scrubber tank. In this process, hydrogen sulfide is oxidized to elementary sulfur or sulfuric acid.
Gas Storage: The gas storage will balance fluctuations of the gas production. The size of the gas storage is normally equal to a few hours of gas production.
Flare: As a safety device, a gas flare for biogas burning is installed.
Gas Engine: The produced biogas is flammable, and it is converted into electricity and heat in an engine/generator. The electricity will be distributed through the power grid, and heat can be used for heating purposes. An alternative is to remove the CO2 through a biogas upgrading process. The upgraded biogas, primarily consisting of methane (CH4), can be distributed and sold to the natural gas network.
Storage Tank: After the anaerobic digestion in the biogas digester, the degassed biomass is pumped to a storage tank. The tank stores the degassed biomass, until it is delivered to farmers. Alternatively, the fertilizer components can be concentrated in Renew Energy’s nutrient recovery process.