Biogas typically refers to a gas mixture produced by the anaerobic degradation of organic matter. It is mainly composed of methane and carbon dioxide and other trace gases (H2S, H2O,..). Biogas can be produced from different natural or man-made substrates that disintegrate by bacterial processes. Being a renewable carbon neutral energy source biogas is used to generate electricity, heat or transportation fuel.
Composition
Biogas consists mainly of methane and carbon dioxide, and traces of hydrogen sulphide, nitrogen and in some cases of siloxanes.
| Typical biogas composition | ||
| Compound | Formular | Concentration |
| Methane | CH4 | 50 - 75 vol.-% |
| Carbon dioxide | CO2 | 25 - 45 vol.-% |
| Water vapour | H2O | 2 - 7 vol.-% |
| Oxygen | O2 | < 2 vol.-% |
| Nitrogen | N2 | < 2 vol.-% |
| Ammonia | N2 | < 1 vol.-% |
| Hydrogen | H2 | < 1 vol.-% |
| Hydrogen sulphide | H2S | 20 - 20.000 ppm |
| Source: Fachagentur Nachwachsende Rohstoffe e.V. (FNR) | ||
The energy content of biogas is determined by the proportion of methane. The biogas composition depends primarily on the design and operational parameters of the anaerobic digestion process and on the sources.
Feedstock
Biogas can be produced from a large range of feedstocks suitable for anaerobic digestion. Potential feedstocks are biomass or waste substrates in the form of solids, slurries and liquids arising from the agricultural, industrial or public sector.
- Agricultural waste
- Liquid and solid manure
- Sludge from waste water treatment plants
- Municipal waste (Biowaste, food leftovers)
- Energy crops
The biogas yields and the methane content from different organic matters are reported in Figure 1.
Figure 1: Biogas yields from different organic matters
Source: Fachagentur Nachwachsende Rohstoffe e.V. (FNR)
Production
Biogas is produced by anaerobic digestion. In this biological process the organic matter is decomposed by bacteria in the absence of oxygen.
The main steps of the anaerobic digestion process are:
- Hydrolysis: Break-down of the organic matter by micro-organisms into soluble organic compounds
- Acido-&Acetogenesis: Conversion of the decomposed matter into organic acids
- Methanogenesis: Conversion of the organic acids into a methane-rich gas (i.e. biogas)
The left over indigestible material and dead micro-organisms containing valuable plant nutrients like nitrogen and potassium is referred to as digestate. The digestate can be used as a substitute to chemical based fertilisers and soil conditioner.
The anaerobic process producing biogas takes place in a digester. The digester is covered or encapsulated to capture the biogas for flaring, heat and power generation or for feeding a natural gas network.
Advantages
Biogas has many advantages, especially with regard to sustainable development and environmental protection:
- Renewable resource and mostly local resource:
- Readily available and sustainable
- Independent supply
- Carbon neutral when combusted (Not contributing to the greenhouse effect)
- Decreased waste product
- Digestion products (organic fertilizers in solid and liquid form) replace industrial mineral fertilizers
Application
Biogas is combusted or oxidized in gas engines, combined heat and power (CHP) engines or fuel cells to generate renewable energy.
The main applications are:
- Electricity generation
- Heat generation
- Space heating
- Hot water
- Cold generation (Trigeneration: cold, heat and power)
- Transportation fuel (compressed natural gas)
The typical global efficiency of a CHP is 85-90%, with an electrical efficiency of 28-47% and a thermal efficiency of 34-55%.
Facts & Potentials
An average biogas plant with an installed capacity of 190kWel can supply 450 homes with electricity and about 30 homes with heat. This avoids, on average, 650 tonnes of CO2 and substitutes 20,000 kilograms of mineral fertilizer.
Some typical values for biogas plants are:
| 1 m3 biogas | = 5.0 to 7.5 kWh total energy = 1.5 to 3 kWhel |
| 1 ha silage maize | = 7800-10000 m3 biogas = 16 -18 MWhel. |
| 1 head of cattle | = 20m3/y liquid manure = 650 m3/y biogas = 0.15 kWel |
| Data source: Fachverband Biogas E.V. | |
Regional development
The development of biogas as energy source varies greatly in Europe (Figure 2). Germany represents with around 50% of the total amount of biogas produced in Europe, the biggest producer. In Luxembourg, the biogas production increased by a factor 8 between 2002 and 2013, reaching 15,2 ktoil eq. of biogas in 2013.
Figure 2: Evolution of biogas production (expressed in oil equivalents) in different European countries
Source: Adapted from Eurostat database
