GASIFICATION
Usually, electricity from biomass is produced via the condensing steam turbine, in which the biomass is burned in a boiler to produce steam’ which is expanded through a turbine driving a generator. The technology is well-established, robust and can accept a wide variety of feedstocks. However, it has a relatively high unit-capital cost and low operating efficiency with little prospect of improving either significantly in the future. There is also the inherent danger in steam. Steam occupies about 1200 times the volume of water at atmospheric pressure (known as “gage” pressure). Producing steam requires heating water to above boiling temperature under pressure. Water boils at 100° C at sea level. By pressurizing the boiler it is possible to raise the boiling temperature of water much higher. Elevating steam temperature has to be done to use the generated steam for any useful work otherwise the steam would condense in the supply lines or inside the cylinder of the steam engine itself.
Gasification is the newest method to generate electricity from biomass.  Instead of simply burning the fuel, gasification captures about 65-70% of the energy in solid fuel by converting it first into combustible gases.  This gas is then burned as natural gas is, to create electricity, fuel a vehicle, in industrial applications, or converted to synfuels-synthetic fuels.  Since this is the latest technology, it is still under development.
A promising alternative is the gas turbine fuelled by gas produced from biomass by means of thermochemical decomposition in an atmosphere that has a restricted supply of air. Gas turbines have lower unit-capital costs, can be considerably more efficient and have good prospects for improvements of both parameters.

Biomass gasification systems generally have four principal components:
(a) Fuel preparation, handling and feed system;
(b) Gasification reactor vessel;
(c) Gas cleaning, cooling and mixing system;
(d) Energy conversion system (e.g., internal-combustion engine with generator or pump set, or gas burner coupled to a boiler and kiln).

When gas is used in an internal-combustion engine for electricity production (power gasifiers), it usually requires elaborate gas cleaning, cooling and mixing systems with strict quality and reactor design criteria making the technology quite complicated. Therefore, “Power gasifiers world-wide have had a historical record of sensitivity to changes in fuel characteristics, technical hitches, manpower capabilities and environmental conditions”.
Gasifiers used simply for heat generation do not have such complex requirements and are, therefore, easier to design and operate, less costly and more energy- efficient.. All types of gasifiers require feedstocks with low moisture and volatile contents. Therefore, good quality charcoal is generally best, although it requires a separate production facility and gives a lower overall efficiency.

In the simplest, open-cycle gas turbine the hot exhaust of the turbine, is discharged directly to the atmosphere. Alternatively, it can be used to produce steam in a heat recovery steam generator. The steam can then be used for heating in a cogeneration system; for injecting back into the gas turbine, thus improving power output and generating efficiency known as a steam-injected gas turbine (STIG) cycle; or for expanding through a steam turbine to boost power output and efficiency - a gas turbine/steam turbine combined cycle (GTCC) (Williams & Larson, 1992). While natural gas is the preferred fuel, limited future supplies have stimulated the expenditure of millions of dollars in research and development efforts on the thermo-chemical gasification of coal as a gas-turbine feedstock. Much of the work on coal-gasifier/gas-turbine systems is directly relevant to biomass integrated gasifier/gas turbines (BlG/GTs). Biomass is easier to gasify than coal and has a very low sulphur content. Also, BIG/GT technologies for cogeneration or stand-alone power applications have the promise of being able to produce electricity at a lower cost in many instances than most alternatives, including large centralized, coal-fired, steam-electric power plants with flue gas desulphurization, nuclear power plants, and hydroelectric power plants.
Gasifiers using wood and charcoal (the only fuel adequately proved so far) are again becoming commercially available, and research is being carried out on ways of gasifying other biomass fuels (such as residues) in some parts of the world. Problems to overcome include the sensitivity of power gasifiers to changes in fuel characteristics, technical problems and environmental conditions. Capital costs can still sometimes be limiting, but can be reduced considerably if systems are manufactured locally or use local materials. For example, a ferrocement gasifier developed at the Asian institute of Technology in Bangkok had a capital cost reduced by a factor of ten. For developing countries, the sugarcane industries that produce sugar and fuel ethanol are promising targets for near-term applications of BIG/GT technologies.

Gasification has been the focus of attention in India because of its potential for large scale commercialization. Biomass gasification technology could meet a variety of energy needs, particularly in the agricultural and rural sectors. A detailed micro- and macroanalysis by Jain (1989) showed that the overall potential in terms of installed capacity could be as large as 10,000 to 20,000 MW by the year 2000, consisting of small-scale decentralized installations for irrigation pumping and village electrification, as well as captive industrial power generation and grid fed power from energy plantations. This results from a combination of favourable parameters in India which includes political commitment, prevailing power shortages and high costs, potential for specific applications such as irrigation pumping and rural electrification, and the existence of an infrastructure and technological base. Nonetheless, considerable efforts are still needed for large- scale commercialization.

CO-FIRING
Co-firing of biofuels (e.g. gasified wood) and coal seems to be the way how to reduce emissions from coal firing power plants in many countries. In 1999 a new co-firing system - biomass and coal - started its operation in Zeltweg (Austria). A 10 MW biomass gasification unit was installed in combination with an existing coal fired power station. The gasifier needs 16 m3 woody biomass (chips and bark) per hour. The calorific value of the gas ranges between 2,5 - 5 MJ/Nm3. The project named “Biococomb” is an EU demonstration project. It was realised by the “Verbund” company together with several other companies from Italy, Belgium, Germany and Austria and co-financed by the European Commission.

COGENERATION
Biomass-Fired Gas Turbine
A current trend in industrialized countries is the use of increasing number of smaller and more flexible biomass based plants for cogeneration of heat and electricity. A newly developed biomass cogeneration plant in Knoxville, Tennessee, USA, is at the cutting edge of one of the promising technologies behind this development. The plant combines a wood furnace with a gas turbine. A hot, pressurized flue-gas filter cleans the exhaust gas from the furnace before it drives the power turbine. The plant can run on fresh cut sawdust (40% humidity), and produces 5.8 MW of electricity, while consuming 10 tons sawdust/hour, and delivering heat as hot exhaust gas at 370°C. This gives an electric efficiency of about 19% and overall efficiency of up to about 75%. The exhaust gas can be used in a steam turbine, increasing electric output to 9.6 MW, and electricity efficiency to over 30%. The plant in Knoxville has been operating since spring 1999.


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