The biogas produced by anaerobic digestion typically contains 55-65% methane (CH4), 34-44 % carbon dioxide (CO2) and smaller quantities of hydrogen sulphide (H2S), ammonia (NH3) and water vapour (H2O). Trace amounts of hydrogen, nitrogen, oxygen, and siloxanes may also be present.
In many smaller scale AD plants the biogas is burnt directly in boiler systems to produce heat for heating the digester and buildings. However, for many applications the quality of the biogas has to be improved before use.
Biogas production from different wastes
Biogas production from organic wastes can vary in different digestion systems with different feedstocks, loading regimes, total solid content percentages, mixing efficiencies, operating temperatures and retention times among other parameters. The organic fraction of municipal solid waste (OFMSW) and source segregated biodegradable municipal waste (BMW) are high energy wastes, with industrial processes producing 50-100 m3 and 70-170 m3 of biogas per tonne of waste, respectively.
Biogas production can be enhanced by mixing low-energy substrates such as agricultural slurries or sewage sludge with higher energy wastes e.g. grass & maize silage and rape seed cake. Co-digestion may be of value in order to manage inhibition conditions that can be related to high nitrogen content, lack of trace elements or overload of light metal ions. Co-digestion of a number of feedstocks can deliver a recipe for improved biogas yield and stability of feedstock.
The main upgrading required before the biogas is utilised in most gas engines is the removal or reduction of hydrogen sulphide (H2S) to levels below 250 ppm to prevent corrosion but should even be further reduced when considering Health & Safety. A biogas de-sulphurisation unit is a common feature of most anaerobic digestion plants. The processes used for hydrogen sulphide removal are fairly well developed and include; biological de-sulphurisation, iron oxide treatments or water scrubbing.
Further upgrades are required if the biogas is to be added to the gas networks or utilised as a transport fuel.
Biogas produced by anaerobic digestion has many uses. It can be used to generate heat and power, upgraded and used as a transport fuel or injected directly into the gas distribution networks. Further details of each method are discussed below.
On-site electricity and heat requirements
The most common biogas utilisation route involves the burning of the biogas in CHP engines, producing electricity and heat. In these cases, the electricity and heat produced usually cover all on-site requirements. For example, electricity is required for lighting, macerating, pumping and other necessary applications. Heating is required to keep the anaerobic digester at the required temperature and to heat and in some cases to pasteurise the incoming waste stream.
The on-site electricity requirements for systems treating centrally separated OFMSW are usually considerable higher than those for source separated systems. This is primarily because centrally separated OFMSW requires more upfront mechanical separation and pre-treatment than source segregated BMW, and this is more energy intensive.
Biogas as a transport fuel has been demonstrated in countries including Sweden, Germany, Spain, France, Switzerland, Austria and others. In the UK a number of trials are being carried out by councils and supermarket chains with vehicles powered by compressed biomethane (CBM).
However, before it can be used as a vehicle fuel, biogas must first be upgraded, to approximately 97% methane. This can be achieved by bubbling biogas through a counter-current flow of water at high pressure (known as water scrubbing). Carbon dioxide and hydrogen sulphide dissolve in the water due to their higher solubility in water compared to methane. Other upgrading technologies to remove CO2 include pressure-swing adsorption, temperature-swing adsorption, amine scrubbing, cryogenic separation and membrane separation. The bio-methane is then compressed to around 200-250 bar. The same compression and storage technology as for natural gas can be used.
Benefits of using upgraded biogas (biomethane) as a vehicle fuel include:
• Can be produced from waste and a wide range of energy crops
• Reduced reliance on foreign fossil fuels
• Lower emissions (air quality benefits)
• Vehicle noise reduction
• Low carbon fuel
• No blending with other fuel is required
Utilisation by Local Gas Networks
Alternatively, upgraded biogas can be injected into the existing gas distribution infrastructure (gas grids). In the UK there are 8 gas distribution networks covering separate geographical areas. Biogas must comply with gas quality requirements set out in the Gas Safety (Management) Regulations 1996. Key components to be removed from biogas are H2S, water, CO2 and siloxanes. The gas must also meet a minimum calorific value and Wobbe index. Enrichment with propane to match the calorific value and combustion stability of natural gas may also be necessary. For health & safety considerations, an odorant is usually added to give the biogas a characteristic smell.
Despite the technology already being deployed in many other countries in Europe, injection of bio-methane derived from AD into the gas grid is still in its infancy in the UK. The UK is ideally suited for this technology since it has a good distribution infrastructure with a dense coverage. Householders in Didcot, Oxfordshire were the first in the country to use gas in their homes derived from anaerobic digestion of their own sewage. Biogas produced from a Suffolk brewery has also entered the national grid to heat homes. See the following links to read more: Didcot householders and Suffolk brewery
Advantages of upgrading biogas for injection into the gas grid include:
• Renewable gas can be delivered using existing gas distribution infrastructure
• Low transportation costs by pipelines
• Distribution network has a very dense coverage
• Increase security of gas supply, reducing external dependencies
• Utilised for heating at efficiency rates in excess of 90%
• Low carbon dioxide emissions when compared with natural gas.