OCR Text |
Show Fuel screening. Of all the fossil fuels currently used in the United States, coal is not only the most abundant, but also presents the most complex problem of combustion and emission control. In addition, there is no typical coal: properties of a given coal can vary within the same seam. In spite of the wide ranges of composition that affect both the way the fuel burns and the pollutant emissions, a general picture of the important pollutant formation mechanisms can be presented by discussing phenomena that occur in terms of a single coal particle. In fact, coal probably does not burn as single particles and the following discussion is phenomenological. For combustion in practical systems, pulverized coal is mixed with a fraction of the combustion air (called primary air) and introduced into the furnace through the fuel injector of the burner. The amount of primary air is determined by both the fuel properties and burner design; however, it is normally 10 to 30 percent of the theoretical air required for complete combustion. The actual stoichiometry under which any fuel particle reacts will depend on the fuel and air mixing history. The sequence of events occurring for a single coal particle is shown in Figure 5, which indicates two combustion modes (volatile evolution and char burnout) as discussed below. Although every coal particle undergoes similar types of processes, the environment under which pollutant formation reactions occur is governed by the aggregate coal particle cloud reaction history. As the coal particle is heated by radiation and convection, the volatile portion of the coal substance begins to evolve. The initial products contain carbon and hydrogen and probably represent side chains and cross linkages between the ring structures in the coal molecule. These initial volatiles react with the surrounding air and partially deplete the available oxygen. As the temperature increases and the ring structures begin to fragment, nitrogen containing intermediates (designated XN) are evolved and begin to react with oxygen to form N0X. Subsequent reactions of XN with N0X and other species produce molecular N2» The amount of nitrogen evolved in the volatile fraction depends on the ultimate particle temperature; the fate of the XN compounds depends on the local stoichiometry around the particle. For fuel lean conditions, a substantial fraction will be converted to NO. For fuel rich environments, the production of molecular nitrogen increases until an optimum stoichiometry is reached; then, for even richer stoichiometries, the residual nitrogen species (XN) are retained unreacted and burn in leaner secondary combustion zones. It also appears that some of the N0X formed can 3-17 17 |