Biogas-based electric power rather than electricity from fossil fuels is an option for reducing green house gas emissions. The positive effects are even greater if the biogas energy content is used in cogeneration systems and consumption of fossil fuel for heat is avoided.
Using biogas from organic waste flows may also make a very significant contribution to climate change mitigation. The natural decomposition of organic matter produces methane, which has a high global warming potential (25 times that of carbon dioxide). The global warming potential of these emissions is significantly reduced if the methane is used to generate electricity and heat.
Electric power from biogas represents a competitive option for ensuring decentralised access to electricity. The technology is available for small off-grid solutions to supply electricity to one or several users. This allows people and organisations to participate directly in their energy supply. Biogas electric power represents a powerful tool in addressing energy poverty.
Greenhouse gas emissions from biogas combustion are climate neutral. Other types of emissions are comparable with those from energy generation using natural gas (possibly the cleanest fossil fuel). Emissions standards can therefore be met under proper combustion conditions and with the use of conventional exhaust treatment systems.
Greenhouse gas emissions from the biomass supply chain need to be taken into account in addition to operational emissions (which are climate neutral). These mainly include the mechanical preparation of soil, fertilisation, harvest and transport (see Environmental issues of Biomass - Electric Power).
Emission levels are generally lowest when waste flows are used as substrates and highest when crop production involves extensive inputs and energy. The environmental effects can also be enhanced when heat losses are used productively and consumption of fossil fuel is avoided. The overall effects on the environment are therefore case specific.
Absolute reductions in greenhouse gas emissions can be achieved in systems operating solely on waste flows. An analysis of German biogas cogeneration systems shows that a system using only manure is set to achieve a net reduction of greenhouse gases of around 850 g of CO2 (equivalent) emissions per kWh of electricity1. The net reduction results mainly from the lack of methane emissions that would be released in a conventional animal breeding system, without the treatment of manure, via anaerobic digestion.
The same system using only corn to produce biogas is estimated to generate between 200 g and 260 g of CO2 (equivalent) emissions per kWh of electricity. This value still represents a sizeable reduction potential when compared with the overall emissions from efficient power plants driven by fossil fuels: i.e. over 900 g of CO2 (equivalent) per kWh of electricity in large coal facilities and over 400 g of CO2 (equivalent) per kWh of electricity in large combined cycle power plants running on natural gas2.
Generating electric power from biogas is already a commercial standard. The main components required (the biodigester and the engine-generator set) are well known technologies and widely available.
Using cogeneration units to utilise biogas from sewage plants is a common practice in many countries. Generating electric power from landfill gases has received special interest in developing countries, particularly because of the possibility of obtaining carbon credits by participating in the clean development mechanism (CDM).
Producing biogas from energy crops is becoming increasingly widespread in Europe. Around one-third of European biogas production in 2007 came from facilities using agricultural products. Cogeneration units are the preferred option3.
Despite the maturity of these technologies, improvements are possible and even necessary in various areas: cleaning and upgrading the biogas, improving different types of substrates, integrated plant management (heat management) and improving conversion technologies (e.g. gas engines, micro turbines or fuel cells) to suit the properties of the biogas.
Further moves towards commercialising small and compact trigeneration units will allow for more effective use of biogas energy. Trigeneration units are a single source of electricity, heat and cooling. Technical improvements and cost-reductions must be made if small units that can cover building and household needs are to be commercialised. They must also be user-friendly.
The biodigester and the generation set constitute the main investment outlay in terms of generating electric power using biogas. The resulting power generation costs depend heavily on the source of substrates. Biomass from waste flows often has no market price. The costs of substrates are negligible in such cases and only the costs of collection and transport activities need to be taken into account. The situation with energy crops is entirely different, however. The variability of market prices has a strong impact on the financial feasibility of a project.
The Energy Sector Management Assistance Program has assessed the costs of a small 60 kW electric power system4. The capital costs for such a system are between USD 2,260 and USD 2,720 per kilowatt (cost in 2004). These figures are not expected to vary significantly in the future, as the main cost factors (biodigester and engine-generator set) are mature technologies with low cost reduction potentials.