FORMATION AND CONTROL OF NITROGEN AND SULFUR OXIDES IN SUSPENSION COAL COMBUSTION (INERT PYROLYSIS, CALCIUM OXIDE)

The increased usage of coal as a combustion material in the United States has intensified existing health hazards due to the formation of nitrogen oxides and sulfur oxides. The first objective of this research was to identify and characterize the parameters that influence NO(,x) and SO(,x) formation and define possible control techniques that could be used in the suspension-phase of a spreader stoker. The lack of in situ techniques for control of sulfur oxides, prompted the inclusion of the second objective: investigation of a potential SO(,2) control technology, dry sorbent injection. This portion of the program was specifically designed to establish the relationship between physical characteristics of the calcium-based sorbents and the gas-phase sulfur removal.; An eight foot, drop-tube furnace with vertical hot-gas flow was used to achieve both objectives. In general the results indicate nitrogen conversion to NO remains relatively constant above 4 percent O(,2) while NO formation is insensitive to particle size and gas bulk temperature. Fuel composition does influence the rate of evolution of fuel nitrogen, but fuel chemistry has little effect on the conversion of evolved nitrogen to NO.; The initial sulfur distribution in the raw coal varies with particle size; naturally occurring fines are usually high in sulfate sulfur. The evolution and oxidation of organic sulfur appears to depend primarily on the rate of carbon evolution; however, the rate of sulfate sulfur decomposition is strongly influenced by local oxygen concentration and temperature. The initial sulfur distribution does not strongly influence the overall SO(,2) formation.; The results obtained during the sorbent injection studies indicate the strong influence of sorbent physical characteristics (i.e., particle size, pore volume, and surface area) on calcium utilization. Particle size and pore size distribution dictate the ease of sulfur accessibility to the calcium sites; conversely surface area indicates the potential of the crystallites for reaction with sulfur (or a competing reaction such as sintering). Therefore, the ideal sorbent is one that has a small mean particle size (minimizing internal diffusion resistances), a moderate mean size of the microporosity (if the pores are too small they will block rapidly), and a high surface area (indicating high availability of calcium sites).