THE EFFECTS OF PRESSURE, OXYGEN PARTIAL-PRESSURE, AND TEMPERATURE ON THE FORMATION OF N2O, NO, AND NO2 FROM PULVERIZED COAL

The main features of a new, pressurized, entrained-flow reactor are de scribed and results presented of experiments investigating the formati on of nitrogen oxides (N2O, NO, and NO2) from pulverized Polish coal, burned in the reactor at temperatures (T) 800-1300 degrees C, pressure s (p) 1-20, bar and oxygen partial pressures (pO(2)) 0.05-2.4 bar. The experimental results are compared with the results of detailed gas-ph ase kinetic calculations at 850 degrees C, where HCN was used as the s ource of coal-nitrogen, and H-2, H2O, CO and C2H4 were used to describ e the gaseous products of pyrolysis and char combustion. The new react or made it possible to control the experimental conditions with high p recision. Regression equations were obtained between the dependent, y- variables (conversions of fuel-N to N2O, NO, and NxOy) and independent , x-variables (p, pO(2) and T). NO formation decreased sharply with pr essure, and increased, but not as strongly, with oxygen partial pressu re and temperature. Total pressure and oxygen partial pressure did not affect N2O formation in the pO(2) range 0.15-0.6 bar. At higher pO(2) the conversion of fuel-N to N2O decreased with both total pressure an d oxygen partial pressure. An increase in temperature strongly reduced N2O formation, independently of pressure and pO(2). No N2O was found at or above 950 degrees C. NO2 was formed in sufficient concentrations to find a regression model at high partial pressures (> 0.5 bar) of o xygen. Like N2O formation, the yield of NO2 decreased with temperature . But like NO, and in contrast to N2O, the formation of NO2 increased with pO(2). NO was the only nitrogen oxide produced above 1000 degrees C at 4-16 bar pressure. Under these conditions its formation obeyed a simple regression equation. Concentrations of NO, NO2 and N2O obtaine d in kinetic computations showed similar trends to the measured values . Calculations also showed the concentrations of O, OH and H radicals to decrease with pressure, and also that HO2 becomes the dominating ra dical at high pressures. These changes probably originate mostly from the three-body reaction H + O-2 + M --> HO2 + M, which at 850 degrees C begins to compete with and finally dominates over the reaction H + O -2 --> OH + O as the pressure increases. The decrease in NO formation with increasing pressure follows as a consequence, because O and OH ar e key radicals in the production of NO.