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Biogas

What is Biogas?

Biogas is the gas produced from the breakdown of organic matter in the absence of oxygen. The raw gas is typically composed of 60% methane (CH4) and 40% carbon dioxide, however, depending on the source, other components can exist which include oxygen (O2), hydrogen (H2), hydrogen sulphide (H2S), siloxanes, ammonia (NH3) and water vapour (moisture).


  

Compound

Chemical formula

%

Methane

CH4

50-85

Carbon dioxide

CO2

5-50

Hydrogen

H2

0-1

Hydrogen sulphide

H2S

0-3

Nitrogen

N2

0-5

Oxygen

O2

0-2

Typical composition of biogas


For most purposes, biogas can be divided into two categories: anaerobic digestion (AD) type and land-fill type. Land-fill type biogas is produced by allowing natural decay to occur within a land-fill producing a gas that is captured, while anaerobic digestion biogas is produced in purpose-designed above-ground plants to optimise the gas-producing decay process for greater efficiencies.

Production of biogas from anaerobic digesters

Anaerobic digestion is essentially a 3 stage biological process. The first stage is the breakdown of the complex organic molecules into simpler molecules, volatile fatty acids (VFAs), NH3, CO2 and H2S. The simpler molecules are then further digested to produce more CO2, hydrogen and acetic acid. The final stage involves further breakdown of the fatty acids into CH4, CO2 and water. Each of these 3 stages uses completely different bacteria that operate at different conditions.

Processing biogas from anaerobic digesters

Biogas from anaerobic digesters can be processed to a gas with higher methane content. When this has been done, it is then referred to as biomethane or renewable gas. The amount of unwanted contaminants removed from the produced biogas depends on the final end-use of the gas.

Biogas cleaning methods

Water vapour must be removed in order to meet pipeline quality standards or CNG vehicle fuel standards. The removal methods for water vapour are either based on the physical separation of condensed water or chemical drying.

H2S removal is required to avoid corrosion issues in piping, compressors, gas storage tanks and engines. H2S is extremely reactive with most metals, and this reactivity is enhanced by the presence of water, and elevated temperatures, pressures and concentrations.

CO2 removal is essential for enhancing the energy value of biogas. As the CO2 is removed, the relative density of the gas is decreased and the calorific value increased - increasing the Wobbe Index.

Biomethane utilisation

Purified biogas or biomethane can be utilised in a variety of ways. The main uses are listed below[5]:

  • It can be burnt in boilers to provide heat
  • It can be used to generate electricity in gas turbines or engines.
  • It can be compressed for use as a vehicle fuel
  • It also be injected to the gas grid for subsequent use

Injecting biomethane into the UK gas grid

The most promising method of utilising biomethane would be introducing it into the natural gas distribution network. There is some work going on in the UK looking into the feasibility of the concept. The final composition of the injected biomethane depends on the grid specifications.
 

UK compliance requirements

Biomethane injected into the UK gas network must meet the specifications outlined in the Gas Safety (Management) Regulations (GS(M)R). The GS(M)R approach was initially developed by the Health and Safety Executive (HSE), and the former British Gas. The approach uses Wobbe Number, which is a measure of the energy input through an appliance based predominantly on the discharge through a burner nozzle, as the main parameter to compare between different gas qualities.


Bio-SNG

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