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Show researchers have shown this conversion to be small (typically between 5 and 300/0 depending on oxygen concentration and fuel properties). The formation of nitrogen from the volatile fuel species has a high dependency on the local stoichiometry and temperature. Since there is a difference between volatile and char nitrogen conversion effiCiencies, fuel NO conversion is significantly affected by operational and design parameters (such as burner velocities, and staging at the burner front to control the flame speed) that control the partitioning of coal nitrogen into the volatile and char fractions. In addition, since volatile nitrogen conversion is dependent on oxygen availability, fuel NO formation would be expected to be highest when the coal particles are hottest and well mixed in an atmosphere containing excess oxygen. As summarized recently by Williams, et al. (1994) and Costa, et al. (1994), a key combustion step and major input to NO formation is the devolatilization of coal in the near burner region. In this region, the flow patterns are complex (sometimes involving swirling flow), are influenced by turbulent effects, and involve high radiant heat fluxes. In addition, the coal particle may soften, become plastic, and change topological features, thus influencing particle drag and the resulting trajectories. During this process the coal particles undergo decomposition into char and volatile material, with the char burning slower in the later stages of the flame, and, in the most simplified theory, the volatile material is assumed to form CO rapidly, and then C02. In general, however, the process involves the breakdown of the coal particle into a complex mixture of light gases, tars, etc. that later react forming CO, C02, and soot. For NO formation, investigators (Pohl and Sarofim, 1977) conclude that under normal coal combustion conditions, the evolution of nitrogen begins to occur after approximately 10 to 200/0 of the coal weight is lost. It is a generally accepted fact that most nitrogen contained in the coal is associated with the organic coal ring structure. This is consistent with experimental findings, as initial heating is required before the ring begins to break and release nitrogen. Following the initial delay in nitrogen release, the kinetics of nitrogen release typically parallel that of the total volatile release or coal mass loss. Because devolatilization occurs on such a rapid time scale, most fundamental analytical models assume that the nitrogen release parallels the mass release from the onset of devolatilization. Under fuel-rich or staged combustion conditions, it has been found that NO is formed primarily (60-950/0) from the nitrogen remaining in the char, whereas under normal . combustion conditions up to 800/0 of the NO is formed from coal nitrogen evolved as volatile matter (Wall, 1987). Temperature effects on atmospheric nitrogen are well understood and can be described through an application of the Zeldovich equations. The formation of thermal NO is highly dependent on temperature to initiate the dissociation of oxygen molecules which initiates a simple sequence of chain reactions in which molecular nitrogen is converted to NO (Glassman, 1987). These reactions are also significantly dependent on the local oxygen concentration and can, therefore, be influenced by combustion staging. Initial investigations by Blair, et al. (1977), Pershing and Wendt (1979), and Pohl and Sarofim (1977), showed that the amount of nitrogen evolved with the volatile matter was largely a function of the particle heat rate and the local pyrolysis temperature. For example, Blair, et al. (19n) noted that the total nitrogen volatilized is a more sensitive function of the pyrolysis temperature than is the total mass evolved during pyrolysis. 5 |
