Biomass and Bioenergy
What is biomass and bioenergy?
Bioenergy covers a diverse range of technologies and raw materials utilising biomass. The energy in biomass results from the conversion of solar energy to sugar by the process of photosynthesis. The source of carbon is carbon dioxide from air.
Since biomass results from the current fixation of carbon, it is renewable whereas fossil fuels are not. It is more or less carbon neutral: the carbon released on conversion to bioenergy is counter-balanced by the carbon fixed in the formation of new biomass.
Bioenergy may either be used directly as a fuel or processed into liquids and gases for the production of heat, electricity or transport fuels.
Why biomass?
Reasons for using biomass as a renewable energy source include
- mitigating against global warming by replacing fossil fuels which are a major source of emissions of carbon dioxide (and other greenhouse gases) which cause global warming. Wales, as part of the UK, must play its part in reaching lower greenhouse gas targets.
- long-term fuel security. This is more of an issue in N America than in the UK.
- supporting sustainable forestry and agriculture
- the creation of new business and job opportunities in Wales.
- offsetting the costs of getting rid of industrial and agricultural wastes.
Biomass resources
It is estimated that about 180,000t of woodfuel is available to the biomass sector from public forestry. The largest proportion of timber volume economically available to the biomass sector comes from the lower value chip/pulpwood proportion of any forest harvesting operation.
About 1.6 m t of biomass from municipal solid waste and industry that could be used for energy generation is landfilled each year. However, there are major barriers to the use of waste for energy. These include low calorific value, planning difficulties, public perception, alternative recycling strategies such as composting, risk of emissions and the need to meet the Waste Incineration Directive (WID) and Animal By-Products regulation (ABPR) controls.
Energy crops are a reasonably uniform source of biomass which can fill the gap between imminent demand for biomass and that which can be provided by forestry or waste wood. Energy crops are normally either woody species such as short-rotation coppice willow or grasses such as Miscanthus. Both species have high yields, can stand upright at low moisture contents, and are perennials so that they do not need re-establishing every year.
Biomass can also be converted to road fuels. Currently, biofuels (bioethanol and biodiesel) are derived from annual arable crops but apart from critisicms concerned with poor energy balances, direct competition between these crops for biomass and for food and high risk of environmental damage, these crops are not as suited to Welsh agriculture as willow or Miscanthus.
If Wales set itself a target for 100,000 ha of biomass using one-tenth of the grassland and arable areas in Wales from energy crops, this would produce 1 million tonnes of biomass per annum. Compared with most existing agricultural land use types in Wales, all energy crops and particularly willow, have far higher biodiversity values and could be usefully included in current agri-environment schemes.
Factors holding energy crops back include lack of confidence, high costs of establishment, uncertainty about renewable obligation certificates, and the lack as yet of 5-10 year contracts to give producers and end-users long term security.
The use of imports is acknowledged as healthy competition in the EU Action Plan 2003) but there is concern that they should be sustainable. It is likely that legislation will be put in place to ensure that both biofuels and biomass imports are ‘sustainable’.
In addition to these sources there is potential to use alternative or waste feedstock such as biofuel co-products (e.g. rape meal and distiller’s grains), agricultural products (e.g. straw and grain) and recovered wood waste from municipal waste. Feedstock viability depends on energy density, transport distance and moisture content.
Conversion technologies
Most current bioenergy generation is based on thermal processes. Combustion means burning but it implies a more efficient process.
At the small scale, heating boilers involving woodchips or pellets connect to pipework which can be used to heat radiators or provide hot water. Larger systems have automated feed of fuel directly from a hopper, feed and de-ashing systems.
There is less scope for district heating - networks of pipes carrying hot water serving a number of buildings - in Wales than elsewhere in Europe. However, there may be opportunities in small communities providing heat for homes, businesses and the village school.
Electricity can also be generated from combustion. Combined heat and power generates electricity plus heat that can be used locally. As yet, installations are expensive. When producing electrical power from combustion, the energy output is most efficient when heat from the process is also used, typically greater than 80%. CHP schemes can be fossil fuelled, but the biomass feedstocks regularly used for combustion (e.g. wood products and energy crops) are equally applicable.
Gasification is a more advanced technology in which biomass such as woody material is heated at temperatures between 500-1200oC with a controlled oxygen supply. The gases so produced (synthesis gas, or more commonly, syngas hydrogen and carbon monoxide) are used to fuel an internal combustion engine (small-scale) or a gas turbine (>5MWe). The engine or turbine is used to turn a generator. This technology becomes more cost effective at large scale, generating electricity from 100kW up to many MW with a cost up to many millions of pounds.
In pyrolysis, biomass is heated in the absence of air, and degrades to produce a liquid fuel (bio-oil), a solid char and syngas.
Co-firing involves the generation of a percentage (circa 5%) of power from renewables, in this case biomass fuels, in existing coal combustion power stations in response to the Renewable Obligation legislation. Co-firing reduces capital costs but does affect the performance of the power station and the amount of biomass that can be handled. RWE npower plans to produce 55MWe through co-firing at its Aberthaw power station.
A dedicated power station offers more control over firing and can handle larger quantities of biomass.Other biomass technologies include
- Anaerobic digestion which uses bacteria in the absence of oxygen to breakdown ‘wet and non-woody’ biomass such as grass cuttings, food waste, sewage and animal manures to yield a methane rich biogas and a digestate residue.
- Hydrogen for transport fuel produced from woody biomass, municipal waste and maybe plastic wastes using gasification.
- Next generation liquid transport fuels derived from structural plant material such as stems, leaves or crop residues by breaking cellulose down into the simple sugar building blocks. Cellulose can be digested, by chemicals or enzymes, to produce bioethanol, but it is more difficult to break down than cereal starch, requiring up to 100 times more enzyme. A recently commissioned project at IGER is concerned with the production of bioethanol from high sugar ryegrasses developed fro ruminants.
- Biomass for Biorefineries in which chemicals, fuels and energy in one one production facility. This increases production efficiency and the economies of scale.
Biomass supply chains
Good opportunities exist for bioenergy in Wales. However, it is important to match supply of biomass with demand. It is not economic or environmentally logical to transport biomass over 30-50 km. Energy generators need to secure supplies meeting their demands in their own catchments. This may necessitate contracting growers, through intermediaries, to grow specialised energy crops. Since planting grants and planting and harvesting machinery do not exist in Wales, some support will be needed to cover high establishment costs.
1252 wordsAn overview of different biomass resources and technologies has been prepared as part of WEST. This deals with
- Understanding biomass, the reasons for its use for energy and its origin.
- Biomass resources particularly from forests, energy crops, wastes and co-products.
- The key differences between perennial biomass and annual biofuel crops
- Energy balances for a range of bioenergy options
- The role of perennial biomass crops in Wales
- The economics of a perennial biomass crop
- Biofuels crops, their characteristics and issues of energy balances, land availability, effects on the environment and competition between food and fuel
- Conversion technologies
- Thermally based processes
- Anaerobic digestion
- Hydrogen from biomass
- Next generation liquid transport fuels from ligno-cellulosic raw materials, steps in production using existing biomass-to-liquid (BTL) and enzymatic technologies and progress towards economic production
- The biorefinery concept
- Pic of woodchips
- Pic of energy crops