|Application of Biomass Energy Technologies (HABITAT, 1993, 168 p.)|
|VI. CONVERSION OF BIOMASS INTO ELECTRICITY|
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. 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 (Larson and Svenningson, 1991). Gas turbines have lower unit-capital costs, can be considerably more efficient and have good prospects for improvements of both parameters.
The basic principles of gasification have been under study and development since the early nineteenth century, and during the Second World War nearly a million biomass gasifier-powered vehicles were used in Europe. Interest in biomass gasification was revived during the energy crisis of the 1970s and slumped again with the subsequent decline of oil prices in the 1980s. The World Bank (1989) estimated that only 1000 - 3000 gasifiers have been installed globally, mostly small charcoal gasifiers in South America.
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 worldwide have had a historical record of sensitivity to changes in fuel characteristics, technical hitches, manpower capabilities and environmental conditions (Sanday and Lloyd, 1991, p. 14).
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 (BIG/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.
It appears that the BIG/GT technology could be available for commercial power generating applications before the turn of the century. According to Williams and Larson (1992), efficiencies of 40 per cent or more will be demonstrated in the mid-1990s, and by 2025 these could reach 57 per cent using fuel-cell technologies being developed for coal. 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 (Foley and Barnard, 1983). 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 (Mendis, undated). For developing countries, the sugarcane industries that produce sugar and fuel ethanol are promising targets for near-term applications of BIG/GT technologies (Ogden et al, 1990).
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 macro-analysis 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.