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Biogas powerplant

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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.

Overview

ArrowBio anaerobic digesters powering electricity generators, Hiriya, Tel Aviv, Israel

Biogas 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):

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.. (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.

Principal procedure

Biogas production

  • 1 Preparing the biomass
  • 2 Mixing
  • 3 Digester/fermenter (Heating 40-90 °C)

Gas input

  • 4 Raw biogas input 40 °C
  • 5 Liquid gas Separator → condensates
  • 6 Gas dryer (refrigerate) 4 °C → condensates
  • 7 Gas compressor about 400kPa
  • 8 Gas filter (cleaning of dust particles, less than 5 parts per billion by mass of siloxanes)
  • 9 Gas heating (minimum about 10 °C)

Gas combustion → thermal energy

  • 10 Gas turbine exhaust output 300 to 400 °C
  • 11 Generator → electric energy

Exhaust output → heat exchange

  • 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 engines

Plant types

Plant type depends on the type of biogas and usage of energy.

  • CHP combined heat & power or HRSG
  • CHCP combined heat, cooling & power production

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.

See also

External links

References

  1. ArrowBio 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

Education

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