Jolly, M., Knight, G., Polo, C., Black & Veatch(free)
The conversion of biosolids using anaerobic digestion generates biogas which is a mainly a mixture of methane and carbon dioxide with small quantities of other gasses. The methane can be burned to generate electricity in an internal combustion engine or a gas turbine or it can be cleaned up and injected into the natural gas grid. UK government incentives currently allow biogas generated from CHP to receive 0.5 times the renewable obligation certificates (ROC) value which will come to an end in March 2017. However biogas clean up and injection into the national gas network receives an incentive under the heading “biomethane”. The value and timescale for these payments is constantly under review. Assessment of the biogas usage options requires consideration of the following parameters:
1. Heat to the process
2. Gas conditioning
3. Emissions to atmosphere,
4. Incentive payments,
5. TOTEX (Whole life cost).
This paper gives a technical and economic appraisal of the technologies for biogas usage.
Keywords: Biogas, biomethane to grid, combined heat and power, gas turbine, methane.
Anaerobic digestion generates biogas which can be used in various ways. Traditionally mesophilic anaerobic digestion on its own has fitted well with the use of internal combustion engines as combined heat and power (CHP) plant. However for projects considering thermal hydrolysis (TH), the process heating requirements shift from the relatively low grade heat required for conventional mesophilic digestion (i.e. hot water) to the higher grade heat needed to heat and pressurize Thermal Hydrolysis reactors (i.e. steam 190oC).
The need for high grade heat in the form of steam means that where turbine technology may not have compared favourably to internal combustion engines in the past for conventional applications, its use requires more careful evaluation for TH facilities.
In addition the option to clean up the biogas to produce a gas with 98-99% methane has been incentivised by the UK government and this has led to a number of UK Water Companies installing biomethane to grid plants.
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