Fundamental study on N2O formation/decomposition characteristics by means of low-temperature pulverized coal combustion

Abstract

N 2 O formation/decomposition characteristics and its mechanisms in pulverized coal combustion, especially at low temperature, are discussed by using a one-dimensional electrically heated laminar drop furnace for various coal types The behavior of nitrogen compounds along the furnace axis is studied by analyzing both the sampled burning particles and combustion gas and by calculating the mass balance of nitrogen. Additionally, the effects of the combustion efficiency and the ratio of fixed carbon to volatile matter content (fuel ratio) on N 2 O formation/decomposition characteristics are elucidated experimentally. As a result, coals that evolve more HCN than NH 3 produce higher N 2 O concentration. In the down-stream region beyond the point where volatile matter combustion is complete, N 2 O concentration increases but NO concentration decreases gradually. Increase of N 2 O in this region may be caused not only by the reactions between NO and carbon in char (NO+char-C→NCO and NCO+NO→N 2 O+CO) but also by the direct heterogeneous reaction between nitrogen in char and NO (char-N+NO→N 2 O). For the coals with low fuel ratio, N 2 O conversion decreases because of high flame temperature surrounding the coal particles caused by strong volatile matter combustion. The decomposition reaction of N 2 O by H radicals produced by the oxidation reaction of CO by OH radicals also contributes near the particle surface. Thereafter, the combustion atmosphere and temperature surrounding the coal particles affects, N 2 O formation/decomposition characteristics. The exit N 2 O concentration increases with decreasing combustion efficiency and decreasing fuel ratio. Data from testing nine different types of coal show that the exit N 2 O concentration has a good correlation with the combustion efficiency and the fuel ratio.