The effects of pressure, oxygen partial pressure, and temperature on the formation of N 2O, NO, and NO 2 from pulverized coal

Abstract

The main features of a new, pressurized, entrained-flow reactor are described and results presented of experiments investigating the formation of nitrogen oxides (N 2O, NO, and NO 2) from pulverized Polish coal, burned in the reactor at temperatures ( T) 800–1300°C, pressures ( 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-phase kinetic calculations at 850°C, where HCN was used as the source of coal-nitrogen, and H 2, H 2O, CO and C 2H 4 were used to describe the gaseous products of pyrolysis and char combustion. The new reactor made it possible to control the experimental conditions with high precision. Regression equations were obtained between the dependent, y-variables (conversions of fuel-N to N 2O, NO, and N gO y) and independent, x-variables ( p, pO 2 and T). NO formation decreased sharply with pressure, and increased, but not as strongly, with oxygen partial pressure and temperature. Total pressure and oxygen partial pressure did not affect N 2O formation in the pO 2 range 0.15-0.6 bar. At higher pO 2 the conversion of fuel-N to N 2O decreased with both total pressure and oxygen partial pressure. An increase in temperature strongly reduced N 2O formation, independently of pressure and pO 2. No N 2O was found at or above 950°C. NO 2 was formed in sufficient concentrations to find a regression model at high partial pressures (> 0.5 bar) of oxygen. Like N 2O formation, the yield of NO 2 decreased with temperature. But like NO, and in contrast to N 2O, the formation of NO 2 increased with pO 2. NO was the only nitrogen oxide produced above 1000°C at 4–16 bar pressure. Under these conditions its formation obeyed a simple regression equation. Concentrations of NO, NO 2 and N 2O obtained 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 HO 2 becomes the dominating radical at high pressures. These changes probably originate mostly from the three-body reaction H + O 2 + M → HO 2 + M, which at 850°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 are key radicals in the production of NO.