Particulates coal-combustion

Environmental considerations also were reflected in coal production and consumption statistics, including regional production patterns and economic sector utilization characteristics. Average coal sulfur content, as produced, declined from 2.3% in 1973 to 1.6% in 1980 and 1.3% in 1990. Coal ash content declined similarly, from 13.1% in 1973 to 11.1% in 1980 and 9.9% in 1990. These numbers clearly reflect a trend toward utilization of coal that produces less SO2 and less flyash to capture. Emissions from coal in the 1990s were 14 x 10 t /yr of SO2 and 450 x 10 t /yr of particulates generated by coal combustion at electric utiUties. The total coal combustion emissions from all sources were only slightly higher than the emissions from electric utiUty coal utilization (6). 

A significant issue in combustors in the mid-1990s is the performance of the process in an environmentally acceptable manner through the use of either low sulfur coal or post-combustion clean-up of the flue gases. Thus there is a marked trend to more efficient methods of coal combustion and, in fact, a combustion system that is able to accept coal without the necessity of a post-combustion treatment or without emitting objectionable amounts of sulfur oxides, nitrogen oxides, and particulates is very desirable (51,52). 

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. 

Examples of the need for multimedia models are found in contemporary problem areas. Polynuclear aromatic hydrocarbons and metals are emitted into the atmosphere as trace impurities with the products of coal combustion. The organics have low vapor pressure and partially condense on emitted particulates in a stack plume. The particulates are transferred to the soil by dry deposition, rainout or washout. The metals manifest... 

The 1970 s also brought about increased use of three-phase systems in environmental applications. A three-phase fluidized bed system, known as the Turbulent Bed Contactor, was commercially used in the 1970 s to remove sulfur dioxide and particulates from flue gas generated by coal combustion processes. This wet scrubbing process experienced several... 

S02 and NOx in flue gas from coal combustion contribute to smog and acid rain. Methods to remove these pollutants include alkaline wet scrubber systems that fix S02 to solid CaS04, and selective catalytic reduction by metal/metal oxide systems of NO/NOz to N2 and steam in the presence of ammonia. Particulate active carbons have also been used in flue gas decontamination, especially as they avoid costly scrubber processes and can operate at lower temperatures. The potential of active carbon fibers in this application has been explored by a... 

A coal combustion pilot plant is used to obtain efficiency data on the collection of particulate matter by an electrostatics precipitator (ESP). The ESP performance is varied by changing the surface area of the collecting plates. Figure E2.2 shows the data collected to estimate the coefficients in a model to represent efficiency 17 as a function of the specific collection area A, measured as plate area/volumetric flow rate. 

Coal-fired power plants release very large amounts of particulate material. The question Is, however, what fractions of the various elements In ambient air can be accounted for by particles from coal-fired plants A major fraction of an element can be contributed by coal combustion only If (1) coal accounts for an appreciable fraction of the A1 In the local atmosphere and (2) the EF value of the element on particles from coal combustion Is as great as for ambient particles. Only for those elements In Figure 1 for which there Is considerable overlap between the ranges for cities and for coal-fired plants can coal possibly be a major contributor. Even If there Is overlap, coal Is not necessarily a major source, as condition 1 above may not be met. On this basis, coal combustion could be a major source of many llthophlles plus Cr, N1, As, Se and, In cities where little residual oil Is used (Charleston and St. Louis), V. The very high EF values for As and Se and low values for V and Nl In Charleston, where little oil and a great deal of coal are burned, lends credence to this Interpretation. 

A laboratory study has been undertaken to characterize the aerosol produced during pulverized coal combustion. The emphasis in this work has been on the particulate matter present in the flue gases at the inlet to the gas cleaning equipment rather than that leaving the stack. Coal is burned at conditions which simulate the combustion region of coal-fired utility boilers. 

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... 

Research indicates that a significant fraction (50-90% ) of mercury is volatilized and lost during coal combustion (10, 11, 12) and that many of the potentially hazardous trace elements appear concentrated upon finer particulate emissions (13, 14). Several investigators have observed enrichment of these hazardous elements upon particulates in urban areas... 

With respect to coal emissions, mercury, for example, was found to exist principally (as much as 96% ) in the elemental form (30). Previously, it was argued by some that ultraviolet radiation transformed it to the less toxic mercuric oxide (30). Sunlight tends to degrade mercurial compounds to the elemental form (47). Beryllium emissions from coal combustion may be in the nontoxic elemental form (46), but this is not known for certain. Fluoride, which is generally assumed to be 100% volatized (19), may be trapped with lime in particulates (33), but this also is questionable. Highly toxic nickel carbonyl (48) and arsine (49) emissions have not been reported to date, although the former is a distinct possibility (50). 

In most developed countries, coal combustion and smelting facilities are required to treat gaseous and particulate arsenic in flue gases before the gases are released into the atmosphere (Chapters 5 and 7). Potentially effective sorbents for flue gas arsenic include hydrated lime (Ca(OH)2, portlandite), lime (CaO), calcium carbonate, limestone, fly ash, and sometimes activated carbon ((Jadhav and Fan, 2001 Lopez-Anton et al., 2007 Helsen and Van den Bulck, 2004), 287, 289 (Taerakul et al., 2006 Gupta et al., 2007) Chapters 5 and 7). The injection of hydrated lime is especially effective and probably... 

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... 

Pressurized FBC may make possible the use of coal in turbocharged combustors and combined cycle power systems. Success is dependent on reducing the erosive and corrosive character of hot, pressurized flue gas generated from coal combustion before expanding it through the system s gas turbine. The problems to be overcome are manifold. The usual requirement for reducing particulate emissions to environmentally acceptable levels is supplemented with stringent... 

If one is performing CMBs of an area in which particulate emissions from coal combustion are dominated by those from one to several plants, it is obviously desirable to use a coal component based on measurements of compositions of the particles released by those specific plants, preferably particles collected in their plumes. However, as those data are rarely available, we have placed data from as many multielement studies as possible in the library so the user can construct a component from results of the most appropriate studies that have been done. 

Coal combustion is a major source of particulate emissions into the atmosphere, especially of particles 0.1 to 5 pm in diameter [9]. Certain toxic elements, e.g., As, Se, Sb, and Zn, are preferentially concentrated in the smaller sizes. The characteristics of coal combustion aerosols depend on a number of factors, such as the type, properties, and size of the parent coal and the furnace design and operating condition. 

Emissions of sulfur oxides, nitrogen oxides, and particulates from coal combustion are problems of increasing concern and regulation. Coal combustion contributes about 25 percent of the particulate matter, 25 percent of the sulfur oxides, and 5 percent of the nitrogen oxides emitted to the atmosphere. Much of the particulates are derived from the mineral content of the coal, but some particulates also result from sulfur and nitrogen oxides that react to form various sulfate and nitrate salts. A major concern about particulate matter is that the smallest particles are respirable and may pose a health hazard. Particulate matter is recovered in most power plants by the use of electrostatic precipitators, which have been developed to very high efficiencies (>99%). Other methods of particulate removal include baghouses and cyclone collectors. 

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