OCR Text |
Show be reduced to N2 by heterogeneous reaction with coal particles or char. During this devolatilization process, inorganic and organic sulfur species are released as sulfur intermediates (designated XS). In general, the XS species are essentially quantitatively converted to sulfur oxides (SO2 or SO3) at some point during the combustion process; however, these species may undergo different reactions or may influence other reactions in this devolatilization zone. Two examples may be cited. First a significant amount of the sulfur in some western coals can be retained in the ash, probably as a sulfate; however, this retention may be enhanced if the sulfur could be captured during devolatilization. Second, sulfur species can influence the conversion of nitrogen species (XN) to NO and other products; therefore, the history of sulfur intermediates relative to nitrogen intermediates (XN) is important in the devolatilization zone. Finally, a portion of the mineral matter from the coal is vaporized in the devolatilization zone and subsequently condenses and/or coalesces to form submicron particulate. The temperature and stoichiometry during devolatilization probably influences the potential for fine particulate formation. Following devolatilization, the residual coal matter (called char) is burned out. The composition of the char depends strongly on the conditions in the devolatilization zone; however, its major components are generally carbon and mineral matter with variable amounts of nitrogen and sulfur species. By the nature of coal combustion, char combustion occurs under predominantly fuel lean conditions. By design, carbon burnout is nearly complete, thereby maximizing energy efficiency and minimizing the carbonaceous particulate. The residual nitrogen species in the char form NO and N2 during burnout in a mode of combustion which appears to promote N2 formation. The sulfur species are either oxidized to SO2 or retained with the mineral matter. The residual mineral matter forms particulate (flyash) in the 0.1 to 50 um size range. Simultaneous with char burnout, residual gaseous species (CO, H2, HC) must also be burned out. This mechanistic understanding of pollutant formation processes forms the basis for optimization of combustion techniques for emission control which are based on tailoring the air and fuel mixing history to minimize all objectionable species. 3-19 19 |