Biomass is a broad term used to describe material of recent biological origin that can be used as a source of energy.This includes wood, crops, algae and other plants as well as agricultural and forest residues. Biomass can be used for a variety of end uses: heating, electricity generation or as fuel for transportation.The term ‘bio energy’ is used for biomass energy systems that produce heat and/or electricity and ‘bio fuels’ for liquid fuels for transport. Biodiesel manufactured from various crops has become increasingly used as vehicle fuel, especially as the cost of oil has risen.
Biological power sources are renewable, easily stored, and, if sustainably harvested, CO2 neutral.This is because the gas emitted during their transfer into useful energy is balanced by the carbon dioxide absorbed when they were growing plants.
Electricity generating biomass power plants work just like natural gas or coal power stations, except that the fuel must be processed before it can be burned.These power plants are generally not as large as coal power stations because their fuel supply needs to grow as near as possible to the power plant. Heat generation from biomass power plants can result either from utilising the heat produced in a Combined Heat and Power plant (CHP), piping the heat to nearby homes or industry, or through dedicated heating systems. Small heating systems using specially produced pellets made from waste wood, for example, can be used to heat single family homes instead of natural gas or oil.
A number of processes can be used to convert energy from biomass. These divide into thermal systems, which involve direct combustion of either solids, liquids or a gas via pyrolysis or gasification, and biological systems, which involve decomposition of solid biomass to liquid or gaseous fuels by processes such as anaerobic digestion and fermentation.
direct combustion Direct combustion is the most common way of converting biomass to energy, for heat as well as electricity.Worldwide it accounts for over 90% of biomass generation.Technologies can be distinguished as either fixed bed, fluidised bed or entrained flow combustion. In fixed bed combustion, such as a grate furnace, primary air passes through a fixed bed, in which drying, gasification and charcoal combustion takes place.The combustible gases produced are burned after the addition of secondary air, usually in a zone separated from the fuel bed. In fluidised bed combustion, the primary combustion air is injected from the bottom of the furnace with such high velocity that the material inside the furnace becomes a seething mass of particles and bubbles. Entrained flow combustion is suitable for fuels available as small particles, such as sawdust or fine shavings, which are pneumatically injected into the furnace.
gasification Biomass fuels are increasingly being used with advanced conversion technologies, such as gasification systems, which offer superior efficiencies compared with conventional power generation. Gasification is a thermochemical process in which biomass is heated with little or no oxygen present to produce a low energy gas.The gas can then be used to fuel a gas turbine or a combustion engine to generate electricity. Gasification can also decrease emission levels compared to power production with direct combustion and a steam cycle.
pyrolysis Pyrolysis is a process whereby biomass is exposed to high temperatures in the absence of air, causing the biomass to decompose.The products of pyrolysis always include gas (‘biogas’), liquid (‘bio-oil’) and solid (‘char’), with the relative proportions of each depending on the fuel characteristics, the method of pyrolysis and the reaction parameters, such as temperature and pressure. Lower temperatures produce more solid and liquid products and higher temperatures more biogas.
These processes are suitable for very wet biomass materials such as food or agricultural wastes, including slurry.
anaerobic digestion Anaerobic digestion means the breakdown of organic waste by bacteria in an oxygen-free environment.This produces a biogas typically made up of 65% methane and 35% carbon dioxide. Purified biogas can then be used both for heating or electricity generation.
fermentation Fermentation is the process by which plants of high sugar and starch content are broken down with the help of microorganisms to produce ethanol and methanol.The end product is a combustible fuel that can be used in vehicles.
Biomass power station capacities typically range up to 15 MW, but larger plants are possible of up to 400 MW capacity, with part of the fuel input potentially being fossil fuel, for example pulverised coal.The world’s largest biomass fuelled power plant is located at Pietarsaari in Finland. Built in 2001, this is an industrial CHP plant producing steam (100 MWth) and electricity (240 MWe) for the local forest industry and district heat for the nearby town.The boiler is a circulating fluidised bed boiler designed to generate steam from bark, sawdust, wood residues, commercial bio fuel and peat.
A 2005 study commissioned by Greenpeace Netherlands concluded that it was technically possible to build and operate a 1,000 MWe biomass fired power plant using fluidised bed combustion technology and fed with wood residue pellets.16
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