Effect of metal-containing additives on NOx reduction in combustion and reburning

This paper describes experimental and modeling studies on the effect of met al-containing compounds on NOx reduction. Sodium and potassium carbonates, calcium acetate, and Ay ash were injected into a 300 kW combustor in one of the following configurations: with the main fuel, with the reburning fuel, or into the reburning zone. Natural gas was used as the main fuel and the reburning fuel. Tests showed that co-injection of Na-, K-, and Ca-containin g compounds with the main fuel reduced NOx emissions with and without rebur ning. Co-injection of Na, K. and Ca-containing additives along with the reb urning fuel and into the reburning zone downstream of the reburning fuel ha d a smaller effect than co-injection with the main fuel. Fly ash showed a m inimal effect on NOx reduction. A potential benefit of using calcium-contai ning additives in reburning: is reduced SO2 emissions during coal combustio n. About 50% SO2 reduction was achieved with injection of 1000 ppm of Ca(OH )(2) with the main fuel. Modeling was used to determine the mechanism of the effect of Na-containing additives on NOx reduction. The model combined a detailed description of t he reburning chemistry with a simplified representation of mixing. Modeling suggested that reduction of NOx emissions in the presence of sodium-contai ning compounds was due to the inhibition of the combustion process by sodiu m. Because NOx formation via thermal and fuel-NO mechanisms strongly depend s on the local combustion environment, NO, formation in the main combustion zone was inhibited because H. OH, and O concentrations in the combustion z one decreased in the presence of Na. The increase in NOx reduction in the r eburning zone was due to slower oxidation of the reburning fuel in the pres ence of Na. (C) 2001 by The Combustion institute.