FIVE in Visual C#

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plants, especially air emissions, and the potential for lower-cost control of greenhouse gases than other coal-based systems Fluctuations in the costs associated with natural-gas-based power, which is viewed as a major competitor to coal based power, can also play a role An IGCC plant includes three main processes (Fig 512) The gasifier turns coal into fuel-gas, which has about half the energy of natural gas; standard cleaning processes remove any sulfur in the gas The fuel-gas then burns in the combustion chamber of a gas turbine, which turns an alternator to generate electricity In the third process, heat recovered from the gasifier and from the turbine exhaust converts water into steam which drives a steam turbine, which turns another alternator to generate yet more electricity

Steam turbine generator Electricity Steam Hot Clean exhaust Flue fuel Heat gas Combustion gas recovery gas turbine steam generator generators Steam Off-gas Hydrogen separation H2 product

The combined cycle links the gas turbine cycle with its high inlet temperature to the steam turbine cycle with its low outlet temperature; the wider the temperature range, the higher is the overall efficiency of converting fuel into electricity An IGCC plant may achieve efficiency well above 40 percent, compared with around 36 percent from a conventional power station IGCC design is suitable for staged construction and is inherently modular: electrical utilities can therefore increase the size of a plant gradually, in line with demand They can begin by installing a gas turbine burning oil or natural gas As the load increases, the utility can add a boiler to recover heat from the turbine exhaust to raise steam, and it can add a steam turbine to generate more electricity When the premium fuel becomes too expensive, the supplier can add a coal gasifier 551 Gasifiers Four types of gasifier are currently available for commercial use: countercurrent fixed bed, cocurrent fixed bed, fluid bed, and entrained flow In all cases, the oxygen supplied is

insufficient to produce complete combustion and the presence of water as steam favors the reactions that yield fuel-gas However, to raise the efficiency of a power plant, commercial gasifiers must recover the heat released during gasification Most designs do this by cooling the gasifier with water, to produce steam and fuel-gas The fuel-gas itself may have to be cooled and reheating reduces the overall efficiency of the plant The countercurrent fixed bed (up draft) gasifier consists of a fixed bed of carbonaceous fuel (eg, coal or biomass) through which the gasification agent (steam, oxygen, and/or air) flows in countercurrent configuration The ash is either removed dry or as a slag The slagging gasifiers require a higher ratio of steam and oxygen to carbon in order to reach temperatures higher than the ash fusion temperature The nature of the gasifier means that the fuel must have high mechanical strength and must be noncaking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent The throughput for this type of gasifier is relatively low Thermal efficiency is high as the gas exit temperatures are relatively low However, this means that tar and methane production is significant at typical operation temperatures, so product gas must be extensively cleaned before use or recycled to the reactor The major advantages of this type of gasifier are its simplicity, high charcoal burnout, and internal heat exchange leading to low gas exit temperatures and high gasification efficiency In this way, also fuels with high moisture content (up to 50 percent by weight) can be used Major drawbacks are the high amounts of tar and pyrolysis products, because the pyrolysis gas is not led through the oxidation zone This is of minor importance if the gas is used for direct heat applications, in which the tars are simply burnt In case the gas is used for engines, gas cleaning is required, resulting in problems of tar-containing condensates The cocurrent fixed bed (down draft) gasifier is similar to the countercurrent type, but the gasification agent gas flows in cocurrent configuration with the fuel (downward, hence the name down draft gasifier) Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources The produced gas leaves the gasifier at a high temperature, and most of this heat is often transferred to the gasification agent added in the top of the bed, resulting in energy efficiency on level with the countercurrent type Since all tars must pass through a hot bed of char in this configuration, tar levels are much lower than the countercurrent type Drawbacks of the downdraft gasifier are: (a) the high amounts of ash and dust particles in the gas, (b) the inability to operate on a number of unprocessed fuels, often pelletization or briquetting of the biomass is necessary (c) the outlet gas has a high temperature leading to a lower gasification efficiency, (d) the moisture content of the biomass must be less than 25 percent to maintain the high temperature and the mineral content (ash yield) should also be low and nonslagging, and (e) the feed must have uniform particle size In the fluid bed gasifier, the fuel is fluidized in oxygen (or air) and steam The ash is removed dry or as heavy agglomerates that defluidize The temperatures are relatively low in dry ash gasifiers, so the fuel must be highly reactive; low-grade coals are particularly suitable Fluidized bed reactors feature extremely good mixing with good heat and mass transfer Gasification is efficient and typically exceeds 90 percent of the feedstock, often falling in 95 to 99 percent range of carbon being converted Ash is carried with gas and separated from the gas in cyclones The agglomerating gasifiers have slightly higher temperatures, and are suitable for higher rank coals Fuel throughput is higher than for the fixed bed, but not as high as for the entrained flow gasifier The conversion efficiency is rather low, so recycle or subsequent combustion of solids is necessary to increase conversion Fluidized bed gasifiers are most useful for fuels that form highly corrosive ash that would damage the walls of slagging gasifiers Biomass generally contains high levels of ash-forming constituents