Coal combustion particle changes

There is a general understanding that the size of ash particles produced during coal combustion decreases with decreasing coal particle size and with decreasing mineral content of the parent coal particles. There are, however, no fundamental models that allow the researchers to predict the change in the size of ash particles when coal is finely ground or beneficiated or how ash size is affected by combustion conditions. 

When sources are studied, several things should be done to provide data needed for receptor-model applications. First, particles should be collected In at least two different size fractions corresponding to the division at about 2.5-ym dlam now used In many studies of ambient aerosols. In some cases. It may be desirable to have more size cuts. As noted above, compositions of particles from coal combustion change dramatically below about 0.5-pm dlam (44, 46). Above we Identified a minimum of about twenty elements that should be measured. Also, In order to develop adequate markers for sources that emit carbonaceous particles, measurements of organic compounds and other properties related to carbonaceous particles should be made. 

The surface chemical composition of brown coal ash particles formed during combustion has been determined by Ersez and Liesegang (1991) to detect any sputter-induced composition changes or to observe any difference between the surface and bulk compositions of the ash. They have observed that certain steels used for heat exchanger piping may well be predisposed to aluminosilicate fouling due to their intrinsic A1 content. 

Unlike the catalytic reaction discussed above, gas-solid reactions involve the solid particle as well as the gas in the reaction. Typical examples of industrial applications include spent FCC catalyst regeneration, calcination, coal combustion, gasification, and silicon chlorination. Owing to the solid particle involvement in the reaction, significant changes in the chemical compositions and physical properties of the particles occur during the reaction. Particles reduce in size and/ or increase in porosity in some reactions like coal combustion, whereas particles increase in size and/or decrease in porosity in other reactions such as limestone sulfation. As a result, the particle properties vary unlike those particle properties in catalytic reactions. However, as with catalytic reactions, gas-solid reactions take place on the particle surface as gas reactant adsorbs to the surface. 

Applications include coal and trash combustion, coal and biomass gasification, and chemical, including polymer, synthesis. Both particles and the fluidizing gas stream change properties during the process. Coal is converted to ash, carbon dioxide, and carbon monoxide. The coal particle can also become fragmented. Coal and steam can yield hydrogen and carbon monoxide. 

The physical differences between inherent and extraneous ash are important not only to those interested in cleaning coal but also to those concerned with the fireside behavior of coal ash. Inherent material is so intimately mixed with coal that its thermal history is linked to the combustion of the coal particle in which it is contained. Therefore, it will most likely reach a temperature in excess of the gas in the immediate surroundings. The close proximity of each species with every other species permits chemical reaction and physical changes to occur so rapidly that the subsequent ash particles formed will behave as a single material whose composition is defined by the mixture of minerals contained within the coal particle. The atmosphere under which the individual transformations take place will, no doubt, approach a reducing environment. Figure 2 illustrates a model of the coal and mineral matter as fed to the combustor and the fate of the minerals after combustion [13]. 

Schneider (9 ) with brown coals of different origin in a drop tube furnace similar to that used by Field clearly showed the influence of different mineral substances on ash deposition. Sand particles (Figure 2) maintained their former shape in combustion or showed changes only at their edges while other ash constituents fused together into dark brown or glassy-clear spheres. 

Coal beneficiation involves a series of steps to separate the mineral matter from the combustible portion of the coal. Current coal characterization for beneficiation is usually limited to measurements of the particle specific gravity distribution (washability). It is further assumed that the properties of the coal feed stream and related mineral matter remain constant during the separation or cleaning process, but the compositions of the streams do change. These changes are important in understanding the lack of expected separations. The effects of specific mineral constituents on different unit operations are described. Better measurement and analytical systems will permit improved control of the processes and better separations. 

On cooling the gas phase some gas components condense and thus lead to the formation of deposits in the boiler. Table 7.9 shows the composition of the condensed phases and the residual waste gas. The change in concentration of the main gas components is low on rapid cooling from 1600 to 700 °C. The condensed phases mainly consist of compounds of K and Na with Cl, P, and S. The low-melting sulfates, phosphates, and chlorides form first a liquid layer where solid dust particles can be deposited by sticking. The composition of the input fuel (coal, and waste material) besides the temperature and O2 partial pressure in the combustion chamber, determine... 

The characteristics of several QCM instruments for aerosol measurement have been reviewed (ll). Particles are collected by impaction, electrostatic precipitation or both. The mass sensitivity is reported to be affected by the location of deposited particles on the crystal, the size of the particles, and the type of coating. In addition, the sensitivity changes as the crystal becomes loaded. Despite some limitations, most of the studies Indicated that QCMs can be successfully used for aerosol measurement with good correlation coefficient with the reference filtration method. Applications included measurement of aerosol in ambient air, particulate emission from automobiles and diesel engines, smoke plume from a coal-fired power plant, solid fueled rocket plvune, and particulate matter in the effluents in combustion sources. 

For fluid solid reactions, such as the combustion of coal or the dissolution of limestone particles in acid solution, the reaction rate is based on the mass of solid or, for some fundamental studies, on the estimated external surface area of the solid. The mass and the area change as the reaction proceeds, and the rates are sometimes based on the initial amount of solid. 

This increase in combustion efficiency and the introduction of particle arrestors at source, in addition to the implementation of successively more rigorous control legislation, has meant that despite the continued increase in fuel consumption, in many industrialised areas, particulate emissions have started to decline. In addition, in some countries, such as the UK, changes in policy have led to a decline in heavy industry, reducing emissions both directly and indirectly via a reduced demand for electricity, as well as a move away from the use of traditional fuels such as coal and fuel-oil to natural gas. 

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