Valorisation of biogas on-site

Anaerobic digestion of sewage sludge generates biogas, which is usually stored in gas storage tanks and valorised on-site at the WWTP. Burning the biogas in a combined heat and power (CHP) plant generates electricity and heat, which can then be used to cover the internal demand of the WWTP in self supply. Other potential options include a micro-gas turbine, or a gas boiler to maximise heat production in case of high heat demand on the plant. On-site valorisation of biogas represents an attractive option to cover a major fraction of the energy costs of the WWTP by own production. Economic viability of on-site valorisation of biogas has to be checked individually for each site based on local rules, taxes and pricing structure of the energy market.


Combined heat and power (CHP) plant

Biogas produced in anaerobic digestion can be valorised on-site in combined heat and power (CHP) plants. These units burn biogas of different quality and convert it into electricity and heat, reaching a high total utilisation efficiency of the energy content in the biogas. CHP plants at WWTPs are usually gas motors, which drive a generator for electricity generation with electrical efficiencies of 35-44% depending on size and load rate. Heat is supplied at two distinct temperature levels: the cooling cycle of the gas motor typically supplies heat at 90°C, while exhaust gas contains heat at >350°C.

Both types of heat can be utilised with heat exchangers, e.g. for heating of anaerobic digestor or buildings or to supply heat for additional processes at the WWTP (e.g. stripping, thermal hydrolysis). Thermal efficiency of usable heat is 41-50% and typically adds up to 85-90% with electrical efficiency. CHP plants should be operated with relatively constant gas quality and in an optimum load range to prevent loss in efficiency.

Gas boiler

This technology was not part of the POWERSTEP project.