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Show 2 increasing with increasing radial distance near the burner damps turbulence and hence reduces air-fuel mixing, but further downstream due to vortex breakdown the mixing is vigorous, sufficiently to ensure complete burnout of the fuel. Standard grind Illinois No. 6 coal was fired at about 400 lblhr for a variety of operating conditions and various data collected. Burner parameters, staging, excess air and turndown were investigated. As will be seen, two modes of operation for practical application of this burner were found. In the first instance, the burner was operated with an air distribution of increasing fraction of air flow with increasing radial distance. We shall call this mode the Increasing Velocity Mode(lVM) for convenience. This mode was found to give the lowest NOx e~oDS for an unstaged furnace. This mode could not meet the emissions goal of below 120 ppm NOx- In the second instance a different burner operational mode was used. In this mode the transport air and pulverized coal and the air in the first burner annulus were premixed in a highly swirling flow upstream of the burner exit(the end of the fuel gun was recessed upstream of the burner exit). We shall call this mode the Premixed Primary Mode (pPM) for convenience. A central recirculation region was formed just downstream of the burner exit and within the quarl. This provided early, stable ignition, slowed or stopped the coal particles to increase residence time in this near burner, high temperature flame core. In this mode, staged combustion was found to give unusually low NOx emissions, well below the project goal. Theoretical Considerations. The effort of achieving low NOx emission (100-150 ppm) from coal flames involves the study of many chemical and physical processes taking place during coal combustion, such as flame ignition, volatile evolution and combustion, char particle burnout, chemical reactions of N-bearing species and the physics of turbulent, reacting, particle laden flows. Among these processes, only a few, can be considered in this report. Our attention will be focused on the most important mechanisms, which are governed by the following four principles: the rate of devolatilization and the volatile yield are strong functions of the flame temperature the rate of fuel nitrogen release from coal particles is proportional to the rate of volatile evolution and char combustion, the conversion of fuel nitrogen to molecular nitrogen is kinetically controlled in fuel rich regions of flames, and its rate is a strong function of local stoichiometry and temperature, sufficient residence time in fuel rich flame zones can be obtained by means of controlled radial mixing and/or external staging. As it is reflected in the above mentioned principles, the fate of fuel nitrogen is the primary focus of study since fuel nitrogen is known to be the major source of NOx emission from coal flames. When the |