MINIMIZATION OF NO EMISSIONS FROM MULTI-BURNER COAL-FIRED BOILERS

The focus of this program is to provide insight into the formation and minimization of NO{sub x} in multi-burner arrays, such as those that would be found in a typical utility boiler. Most detailed studies are performed in single-burner test facilities, and may not capture significant burner-to-burner interactions that could influence NO{sub x} emissions. Thus, investigations of such interactions were made by performing a combination of single and multiple burner experiments in a pilot-scale coal-fired test facility at the University of Utah, and by the use of computational combustion simulations to evaluate full-scale utility boilers. In addition, fundamental studies on nitrogen release from coal were performed to develop greater understanding of the physical processes that control NO formation in pulverized coal flames--particularly under low NO{sub x} conditions. A CO/H{sub 2}/O{sub 2}/N{sub 2} flame was operated under fuel-rich conditions in a flat flame reactor to provide a high temperature, oxygen-free post-flame environment to study secondary reactions of coal volatiles. Effects of temperature, residence time and coal rank on nitrogen evolution and soot formation were examined. Elemental compositions of the char, tar and soot were determined by elemental analysis, gas species distributions were determined using FTIR, and the chemical structure of the tar and soot was analyzed by solid-state {sup 13}C NMR spectroscopy. A laminar flow drop tube furnace was used to study char nitrogen conversion to NO. The experimental evidence and simulation results indicated that some of the nitrogen present in the char is converted to nitric oxide after direct attack of oxygen on the particle, while another portion of the nitrogen, present in more labile functionalities, is released as HCN and further reacts in the bulk gas. The reaction of HCN with NO in the bulk gas has a strong influence on the overall conversion of char-nitrogen to nitric oxide; therefore, any model that aims to predict the conversion of char-nitrogen to nitric oxide should allow for the conversion of char-nitrogen to HCN. The extent of the HCN conversion to NO or N{sub 2} will depend on the composition of the atmosphere surrounding the particle. A pilot-scale testing campaign was carried out to evaluate the impact of multiburner firing on NO{sub x} emissions using a three-burner vertical array. In general, the results indicated that multiburner firing yielded higher NO{sub x} emissions than single burner firing at the same fuel rate and excess air. Mismatched burner operation, due to increases in the firing rate of the middle burner, generally demonstrated an increase in NO{sub x} over uniform firing. Biased firing, operating the middle burner fuel rich with the upper and lower burners fuel lean, demonstrated an overall reduction in NO{sub x} emissions; particularly when the middle burner was operated highly fuel rich. Computational modeling indicated that operating the three burner array with the center burner swirl in a direction opposite to the other two resulted in a slight reduction in NO{sub x}