THE USE OF AIR STAGING TO REDUCE THE NOX EMISSIONS FROM COAL BURNING RIJKE PULSE COMBUSTORS

This paper describes an investigation of the effect of combustion air staging upon NOx emissions from coal burning Rijke pulse combustors, w hich offer many advantages in domestic and industrial heating applicat ions. This NOx reduction technique was investigated because of its dem onstrated effectiveness in steady state coal combustors. The basic con figuration of the investigated Rijke pulse combustor consisted of a ve rtical tube in which unpulverized coal was burned on a rotating bed lo cated at a distance of L/4, where L is the combustor length, from the combustor entrance. Combustion air was supplied from below the bed and its reaction with the bed resulted in spontaneous excitation of the f undamental acoustic mode of the combustor. The resulting acoustic and pressure oscillations in the bed resulted in bed fluidization and inte nsification of the combustion process. A series of experiments was con ducted without air-staging in order to determine the baseline NOx emis sions. A bituminous coal with about 1.5 percent nitrogen and about 1.3 percent sulfur was burned in all of the experiments. Under pulsating combustion conditions at a sound pressure level of about 160 dB and a frequency of about 65 Hz, NOx emissions (3% oxygen basis) ranged from about 250 ppm for extremely fuel rich combustion to about 700 ppm for large excess air conditions. Air staging experiments were conducted fo r total air/fuel ratios ranging from 1.1 to 1.4 and primary air/fuel r atios ranging from 0.6 to 0.9. Secondary air was injected at a height of 52 cm above the coal bed. Air staging was effective in reducing NOx emissions over a range of the combustor operating conditions. The lar gest reductions in NOx emissions were obtained for primary air/fuel ra tios of about 0.7 with secondary air injection sufficient to yield fin al excess air values greater than about 20 percent. Under optimum cond itions, reductions in NOx emissions of up to 56 percent were obtained.