Coal combustion pollution controls

Ash particles produced in coal combustion are controlled by passing the flue gases through electrostatic precipitators. Since most of the mass of particulate matter is removed by these devices, ash received relatively little attention as an air pollutant until it was shown that the concentrations of many toxic species in the ash particles increase as particle size decreases. Particle removal techniques become less efiective as particle size decreases to the 0.1-0.5 pm range, so that particles in this size range that escape contain disproportionately high concentrations of toxic substances. 

The production of electric power from coal is a mature and well-established technology in the industrialized countries of the world. However, with the advent of stricter environmental controls on effluents from power plants, especially with respect to snlfnr oxides, new types of combustion/ pollution control technology are emerging. In fact, the increased utilization of coal in place of oil and gas for combustion applications in the United States is motivating near-term development and implementation of alternative technologies for electricity generation from coal. 

L. B. Heia, A. B. Phillips, and R. D. Young, "Recovery of Sulfur Dioxide from Coal Combustion Stack Gases," ia F. S. MaHette, ed. Problems and Control of A.ir Pollution, Reioliold Publishing Corporation, New York, 1955, pp. 155—169. 

A growing use of lime is in pollution control, where lime scrubbers placed in combustion stacks remove sulfur dioxide present in combustion gases from the burning of high sulfur coal. 

Several studies investigating the environmental effects of controlled tyre combustion have been conducted. It is evident that atmospheric emissions can be greatly reduced if proper air pollution control systems are installed. Laboratory and field data provide evidence indicating that concentrations of some environmental pollutants, especially NOx, may decrease due to tyre combustion, whereas others increase compared to pure coal combustion. Zinc is an example of an element that increases in both solid combustion products and atmospheric emissions. The geochemical impact of higher Zn contents in fly and bottom ash on leaching processes in disposal sites remains to be tested. 

The pollutants which increased dramatically in converting from oil/gas to coal combustion are particulate matter and sulfur dioxide. State laws, especially those governing emissions in urban areas, specify limits for both species. They could be met when low sulfur fuel oil or gas was burned, but coal combustion required new or upgraded pollution control devices. To understand the problem of coping with both of these emissions, it is simpler to treat the response to particulate control separate from that of... 

Beryllium and mercury were two of the three air pollutants promulgated as hazardous by the EPA in 1973 and subject to stringent controls (30). While coal combustion releases these elements to the atmosphere, EPA, on the basis of available information, held that this source did not generate sufficient concentrations to be considered hazardous even under restrictive dispersion conditions (29, 31). Coal combustion is thought to be a significant industrial source of atmospheric As (13), Bi (14), Cd (32), FI (33), Hg (3), and Ni (34). 

There are several approaches available to a utility to construct a boiler that will meet New Source Performance Standards. These approaches can be classified according to the position in the combustion system at which pollutant control technology is applied. Precombustion control involves removal of sulfur, nitrogen, and ash compounds from the fuel before it is burned. For coal combustion this approach involves the application of coal-cleaning technology. Combustion control relies on modifications to the combustion process itself or the addition of material to the combustion process to reduce pollutant formation or capture the pollutants formed in the combustion chamber. Examples of combustion control include staged combustion, boiler limestone injection, and fluidized-bed combustion with limestone addition. Post-combustion control involves removal of pollutants after they have been formed but before they are released into the atmosphere. Traditionally, flue gas desulfurization has meant the application of postcombustion control either alone or in conjunction with another... 

Modern pollution control technology can remove —99% of the particulates in the combustion gases. Improved coal cleaning procedures and pollution control technology promise to be even more efficient in reducing the amount of particulates produced by coal combustion. 

Existing pollution control devices can remove as much as 99% of the trace elements (except mercury and selenium) from the combustion gases. Substantial amounts of mercury and, to a lesser extent, selenium are emitted with the combustion gases. There are a number of ways to reduce trace-element emissions due to coal combustion. These pollution control options include switching to coals with a lower trace-element content, selectively mining those parts of coal beds with lower trace-element contents, cleaning the mined coal, and using pollution control devices such as fabric filters, electrostatic precipitators, and combustion gas scrubbers. 

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