Coal combustion controlling

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. 

Today s major emissions control methods are sorbent injection and flue gas desulfurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulfur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulfur oxide emissions. 

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. 

Anthropogenic sources of fluoride include fossil fuel combustion and industrial waste. Hydrogen fluoride is water soluble and emissions are readily controlled by acid gas scrubbers. HF emission from coal combustion, that is considered to be the main anthropogenic source of HF, was estimated to be 0.18 Tg annually emission of HF from the combustion of petroleum and natural gas is almost certainly negligible [24]. Apparently only limited data are available concerning total annual emissions of HF from industrial operations however, there is evidence that emissions of fluorides have been declining [24,25]. 

The description of the workings of the control technologies is beyond the scope of this article. However, it is worth noting that many of these technologies create substantial amounts of solid or liquid waste that needs to be disposed of properly. For example, in the USA the total amount of fly ash produced from coal combustion alone is about 57 Mt/y (Kalyoncu 2000). About one-third is utilized as secondary raw material (e.g., for aggregate and asphalt), but the rest is usually disposed of in landfills. The wet and dry scrubbers for S02 control produce a sludge or dry waste that finds little secondary use, and a large amount is disposed in landfills. 

Based on the above description of the coal combustion process several conclusions become apparent. First, the type and amount of ash accumulated during coal combustion greatly depends on the mineralogy of the coal being used, the combustion process, and the presence of emission control devices. Secondly, the chemical forms in which elements are found in ash are affected by coal combustion process variables such as combustion temperature and the mode of combustion (e.g., pulverized-coal fired, fluidized bed, cyclone, stoker). Lastly, the amount of CCPs accumulated by power plants is predominantly a consequence of the presence of emission control devices. The latter is supported by the fact that the total amount of CCPs produced in the US has increased significantly since the use of electrostatic precipitators became prevalent in the early 1970s (Simsiman et al. 1987). 

Ash from pulverized coal combustion is a strategic material that has many critical applications from a source of aggregate to the most important source of pozzolan for addition to Portland cement concrete. Environmental control measures on the emissions of coal combustion have resulted in a loss of quality for these materials. In response we have seen the advent of beneficiation processes applying both proven and new technologies to produce high-quality consistent products from these materials. Currently we estimate that about one-fifth of all ash products marketed are processed through some form of beneficiation method. We expect that the demand for quality and consistency will continue and the relative amount of process ash products will increase in the future. 

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... 

CA 33, 2339(1939) (Expls contg K chlorate, antimony, sulfur Na bicarbonate with suitable binding and combustion controlling agents, such as flour, coal, wood, grease petroleum)... 

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). 

Gupta, H., Thomas, T.J., Park, A.-H.A., Iyer, M.V., Gupta, P Agnihotri, R Jadhav, R.A., Walker, H.W., Weavers, L.K., Butalia, T., Fan, L.-S. et al. (2007) Pilot-scale demonstration of the OSCAR process for high-temperature multipollutant control of coal combustion flue gas, using carbonated fly ash and mesoporous calcium carbonate. Industrial and Engineering Chemistry Research, 46(14), 5051-60. 

The addition of lime to control acid drainage from mining wastes typically produces calcium arsenates (Pichler, Hendry and Hall, 2001). Bothe and Brown (1999) further concluded that lime precipitates As(V) as a number of hydroxyl and hydrated calcium arsenates (Ca4(OH)2(AsC>4)2 4H2O, CaslAsCLLOH (arsenate apatite), and/or Ca3(AsC>4)2 3H2O) rather than anhydrous tricalcium orthoarsenate (Ca3(As04)2). Calcium arsenates also occur in coal combustion byproducts (Chapter 7). In the flue gas treatment systems of coal combustion facilities, volatile arsenic can readily react with calcium to form the arsenates on the surfaces of flyash and injected lime (Seames and Wendt, 2000 Yudovich and Ketris, 2005, 175). 

The determination of mercury in coal, and in coal combustion residues, can also be accomplished by controlled heating of the sample in oxygen (ASTM... 

The seasonal distribution of particle-associated PAHs is controlled by a combination of emission factors (EFs), dispersion conditions and chemical mechanisms (Caricchia et al., 1999 Menichini et al., 1999). This balance depends on the relative importance of degradation processes and emission sources (Guo et al., 2003b). The highest PAH concentrations of a sampling site were usually obtained from winter samples, and the differences were far higher in northern cities than southern ones, suggesting that coal combustion for space heating contributes the most PAHs in winter in Northern China. 

Precombustion control involves removal of sulfur compounds from fuel prior to combustion. Control during combustion employs techniques to minimize the formation and/or release of SO2 and N0X during the combustion process. Finally, SO2 and N0X can be removed from the combustion flue gas using various postcombustion control methods. This chapter discusses the potential of mitigating acid deposition through precombustion cleaning of coal to remove sulfur compounds. 

Coal contains most of the elements of the periodic table, the majority of which are present in concentrations of 100 ppm or less. Many of these trace elements are toxic to plant and animal life, even at low levels. Because U.S. power plants consune on the order of 600 million tons of coal annually for the production of electricity (1), coal combustion can mobilize thousands of tons of potentially hazardous trace elements into the environment each year. Due to the large quantities of coal combusted, even trace amounts of toxic elements present in the coals can accumulate to hazardous levels. Also, potentially deleterious effects of particulate emissions from coal combustion may be enhanced since many trace elements are surface-enriched (2) and concentrate preferentially in the smaller, more respirable particle sizes (3). Substantial amounts of some elements, such as As, Hg, and Se, are in the vapor phase in flue gases from coal combustion and are essentially unaffected by most particle control devices. Aside from the potential detrimental environmental aspects, trace elements in coal can pose adverse technological... 

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