A biogas powerplant is a system where biogas is used to generate electricity. The gas which is produced via anaerobic digestion is used to drive an electricity generator. By-products of this process are steam and hot water. The hot water can be recycled in a combined heat and power cycle to increase the temperature of the digesters to optimal conditions.
ArrowBio anaerobic digesters powering electricity generators, Hiriya, Tel Aviv, IsraelBiogas power plants are a combination of anaerobic digestion systems with associated electricity generators such as gas turbines or gas engines. The electricity they produce is classified as renewable or green energy and if sold into the national grid may attract subsidies (such as Renewables Obligation Certificates in the UK).
Feedstock into the biogas power plants must be biodegradable in order to produce methane. Suitable feedstocks include (but are not limited to):
- Biodegradable waste
- Sewage treatment sludge (primary or raw sludge and/or secondary sludge)
- Slaughterhouse waste
- Food waste
- Farm waste
- Organic component of mixed municipal waste (in mechanical biological treatment)
- Biomass like maize
There are three stages of anaerobic digestion: hydrolysis, acidogenesis, and methanogenesis. These stages can occur in the same digestion tank or can be controlled independently and optimised according to the requirements of the different bacterial processes.
The more complex and efficient a biogas plant the more expensive it will be for the locality. Biogas plants can be simplified to produce gas for villages in countries where organic wastes are available and funds are limited. Alternatively, in more developed countries pressure in the form of legislation and high energy costs is increasing the amount of projects generating renewable energy from waste.
Biogas plants can be found in countries such as India, China, Philippines, Germany, Austria and Turkey.
Advanced processing systems can recover the organic fraction mixed waste streams. These systems are a subgroup of mechanical biological treatment plants. They sort the recyclable elements of the waste and process the organic fraction into a high surface area low solids soup which are then passed into a biogas power plant (anaerobic digester). Advanced systems like this can be found in Israel.[1]. (ArrowBio) and Australia and are being widely considered in Europe to meet the EU Landfill Directive.
Further energy can be produced by the combustion of the digestate which may be classified as a biofuel.
Principle procedure
Biogas production
1 Preparing the biomass
2 Mixing
3 Digester/fermenter (Heating 40-90 °C)
4 Raw biogas input 40
°C
5 Liquid gas Separator ? condensates
6 Gas dryer (refridge) 4 °C ? condensates
7 Gas compressor about 400kPa
8 Gas filter (cleaning of dust particles, less then 5 parts per billion
by mass of siloxanes)
9 Gas heating (minimum about 10 °C)
10 Gas turbine exhaust
output 300 to 400 °C
11 Generator electric energy
12 HRSG heat recovery
steam generation
13 Heat exchanger for hot water
Siloxane might be present in the biogas and must be removed prior to input
in the gas engines as it erodes moving parts. Hydrogen sulphide may also
be produced in the process if there are high levels of sulphur in the biogas.
The exhaust gas must be cleaned up as sulphur dioxide is toxic.
Plant
types
Plant type depends on the type of biogas and usage of energy.
CHP combined heat &
power or HRSG
CCHP combined cooling heat & power
Plant sizes
Size Power Plant Size
Small 500W to 5 kW 10 m²
Medium 5 kW to 75 kW 15 to 100 m²
Large 75 kW to 4MW 1 km²
The plant can also be segmented including gas motors and gas turbines.
References
1ArrowBio Process Finstein, M. S., Zadik, Y., Marshall, A. T. & Brody,
D. (2004) The ArrowBio Process for Mixed Municipal Solid Waste – Responses
to “Requests for Information”, Proceedings for Biodegradable
and Residual Waste Management, Proceedings. (Eds. E. K. Papadimitriou &
E. I. Stentiford), Technology and Service Providers Forum, p. 407-413
