Burning coal

For central station power generation the open cycle system using electrically conducting coal combustion products as the working fluid is employed. The fuel typically is pulverized coal burned directly in the MHD combustor, although in some plant designs cleaner fuels made from coal by gasification or by beneficiation have been considered (8—10) (see Fuels, synthetic). 

Some of the advantages of fluidized beds include flexibiUty in fuel use, easy removal of SO2, reduced NO production due to relatively low combustion temperatures, simplified operation due to reduced slagging, and finally lower costs in meeting environmental regulations compared to the conventional coal burning technologies. Consequently, fluidized-bed combustors are currently under intensive development and industrial size units (up to 150 MW) are commercially available (Fig. 10). 

Fig. 19-13. Three-parameter averaging-time model fitted through the arithmetic mean and the second highest 3-hr and 24-hr SOj concentrations measured in 1972 a few miles from a coal-burning power plant. Source From Larsen (21).

Consider coal burning in a boiler house. The assessor may not be able to measure the mass of sulfur dioxide (SOj) leaving the boiler stack, because of access problems and the lack of suitable sampling ports on the stack. The only information available is that the coal is of soft quality, containing 3% sulfur by weight and, on average, 1,000 kg of coal is burned each day. 

The IGCC cycle was described in Section 7.4.2. Obviously, there is an attraction in burning cheap coal instead of expensive gas, but the IGCC plant will discharge as much carbon dioxide as a normal coal burning plant unless major modifications are made to remove the CO2 (Table 8.IE). 

Over the past decades, advances have been made that reduce environmental impacts of coal burning in large plants. Some arc standard and others experimental. Limestone (mainly calcium carbonate) scrubber smokestacks react with the emitted sulfates from the combustion and contain the chemical products, thereby reducing the release of SO., into the atmosphere by a large factor (of ten or more). Pulverization of coal can also allow for the mechanical separation of some sulfur impurities, notably those in the form of pyrites, prior to combustion. Currently deployed—with more advanced versions in the development stage—are various t yies of fluidized bed reactors, which use coal fuel in a pulverized form, mixed with pulverized limestone or dolomite in a high temperature furnace. This technique reduces sulfate release considerably. There are... 

Specialized practical configurations for combustion have a number of practical applications such as coal burning for energy production. The study of these specialized combustion setups is necessaiy for better application. 

NO, emissions are less dependent on the type of coal burned, and two oxidation mechanisms are associated with the release of NO, into the atmosphere during the combustion process. Thermal NO results from the reaction of nitrogen in the comhustion air with excess oxygen at elevated temperatures, and fuel NO., is a product of the oxidation of nitrogen chemically hound in the coal. 

Continuing dependence on fossil fuels raises several major ethical issues. Ethical questions concerning our responsibilities to future generations arc raised by the fact that fossil fuels are a nonrenewable energy source, so that eveiy barrel of oil or ton of coal burned today is forever lost to future generations. Further, the by-products of fossil fuel combustion pose hazards to both present and future generations. 

When the power is a large number, as in the case of an electric power plant, it is convenient to express the power in megawatts (MW) where one megawatt equals one million watts. An electric power of 1,000,000,000 watts would be expressed as 1,000 MW. A large coal-burning or nuclear power plant produces about 1,000 MW of electric power. The sum total of the electric power produced by all electric power plants is expressed m units of gigawatts (GW). One gigawatt equals one billion watts. An electric power of 1,000,000,000,000 watts would be expressed as 1,000 GW. 

Environmental considerations in recent years have dictated that sulphur bearing compounds are removed from the exhaust gases of coal burning power stations in order to reduce the incidence of acid rain . The flue gas... 

A different type of exact number arises in certain calculations. Suppose you are asked to determine the amount of heat evolved when one kilogram of coal burns. The implication is that because one is spelled out, exactly one kilogram of coal bums. The uncertainty in the answer should be independent of the amount of coaL... 

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