Formation of Oxides of Nitrogen in Pulverized Coal Combustion

Nitrogen oxides are a potential atmospheric pollutant. Their formation and decomposition were studied in an experimental pulverized-coal-fired furnace. The concentration of nitrogen oxides (NO_x) was a maximum in the combustion zone and decreased as the combustion gas cooled. At a coal burning rate of 2 Ib/hr and 22% excess air, reduction of nitrogen oxides was obtained by selective secondary-air distribution. With 105% cf the stoichiometric air fed to the coal-combustion zone and 17% additional air fed just beyond the flame front, 62% reduction of NO_x occurred with good combustion efficiency. Lowering the quantity of excess air lowered the NO_x concentration, but at the expense of combustion efficiency. When 22% excess air was fed to the primary combustion zone, NO_x concentration in the effluent was 550 ppm and carbon in the fly ash 2.0%. With 5% excess air, the NO_x concentration fell to 210 ppm and carbon in the fly ash rose to 13.8%. With stoichiometric combustion the NO_x was 105 ppm a reduction of 81 %, and the carbon was 42.3%. Recirculation of combustion gas was not an effective means of lowering NO_x formation.