What is Bioenergy ?

Monday, August 30, 2010

What is Bioenergy ?

Bioenergy is energy derived from biomass, which is organic material such as wood, plants, or animal wastes. Bioenergy can be used to generate electricity, produce heat, and also for the production of biofuels. The three main areas of bioenergy are:-  
  • Biomass
  • Biogas
  • Biofuels
  • Co-firing
Biomass can be burned to produce electricity, hot water or hot air. Direct combustion is the simplest and most common method of generating energy from biomass. For small-scale applications, the fuel usually takes the form of wood pellets, chips and logs. Wood pellets are a compact form of wood, with low moisture content & high energy density. Larger applications use fuels such a miscanthus and willow, which are short rotation crops.
The energy content of the fuel used depends on the moisture content. A higher moisture level will slow down the combustion process, as the water has to boil off before the fuel burns.
Agricultural Waste
Wood is not the only form of biomass that can be burnt to produce energy. Other forms of biomass produced by farmers are by-products of conventional agricultural activity. They include 'dry' agricultural wastes such as straw, damaged grain or seeds, even animal bedding has been tried as a fuel. These wastes vary in their efficiency, but for farmers who burn these wastes for heat, they can represent a useful and cheap fuel source.
'Wet' agricultural wastes such as manure can also be used to generate heat and electricity by a process called Anaerobic Digestion.
In biomass electricity generation, biomass is combusted and heat is produced, which is then used to generate electricity.
 Biomass Heating
Biomass heating systems produce either hot water or hot air. The biomass is combusted in a furnace, where it heats either air or water. Hot air production is used primarily for space heating, and hot water can be used either as part of a domestic hot water system or for heating systems such as central heating and underfloor heating. In hot water systems, a heat exchange system can be used to heat water tanks etc.
 Combined Heat and Power (CHP)
Combined heat and power systems can also be used – the biomass is burned to produce both electricity and heat. CHP is an efficient way to use the biomass, as it uses the waste heat from generation as useful energy.
Biogas is a mixture comprising mainly methane and carbon dioxide. It is produced when organic matter decomposes in the absence of oxygen. This can take place in a landfill site to give landfill gas or in an anaerobic digester to give biogas. Sewage gas is biogas produced by the digestion of sewage sludge.
An anaerobic digester produces conditions that encourage the natural breakdown of organic matter by bacteria in the absence of air, so that the biomass breaks down much faster than usual.
Anaerobic digestion is a method for turning residues from livestock farming and food processing industries into biogas, fibre and a liquid called 'digestate'. The outputs from the digestion process are:
Biogas – a mixture of 60% methane, 40% carbon dioxide and traces of other gases. Biogas can be burnt to produce either heat or energy. The biogas is burnt in the same was as natural gas, and the resultant heat can either be used for space heating, hot water systems, or to generate electricity or as a road fuel.
Digestate – an inert and sterile wet product containing valuable plant nutrients and organic humus which can be applied to land as a soil conditioner and biofertilizer.
Biofuels are fuels made from biomass (organic matter) which can be used instead of traditional fossil fuels. Biofuels are most commonly used for transport, but are also used for small heating applications
The three most common biofuels are:
  • Biodiesel
  • Bioethanol
  • Biomethanol
Biodiesel is derived from vegetable oils, and can either replace diesel completely or be mixed in different proportions. It is most commonly blended, as it gives better performance, and in the UK car warranties are made invalid if biodiesel is used to replace diesel completely. This type of biofuel can be used in diesel engines with no modifications. Typical feedstocks for biodiesel are mainstream agricultural crops such as oilseed rape (Northern Europe) and sunflower oil (Southern Europe).
Bioethanol is produced from a variety of agricultural feedstock, including starch crops, sugar crops and woody crops. By-products from the sugar industry, such as molasses, can also be used. The most typical feedstocks are wheat and sugar beet (Northern Europe) and sweet sorghum (Southern Europe). Bioethanol can be used in existing petrol engines, although some petrol is needed in addition to the fuel when cold starting.
Biomethanol is produced from wood, and is used in existing petrol engines in the same way as bioethanol. It is not as common as either biodiesel or bioethanol.
Co-firing refers to the simultaneous combustion of a supplementary fuel (i.e. biomass) with a base fuel (i.e. coal). Co-firing biomass with coal is seen as the cheapest way of generating green power in utility plants. In addition it also reduces the emissions of fossil based carbon dioxide and is accredited under the Renewables Obligation. From a community services perspective, biomass co-firing represents an opportunity to add value to raw materials.
After 31st March 2006 co-firing coal with biomass will only be eligible under the Renewables Obligation if 75 % or more of the energy content of the biomass derives from energy crops. The co-firing of coal and biomass will only be eligible under the Renewables Obligation until 31st March 2011.
Co-firing can be subdivided into: Direct Co-firing: biomass and coal are fired in the same compartment. Indirect Co-firing: combustion/ gasification of biomass occurs in a separate facility.


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