EFFECTS OF FLUE-GAS RECIRCULATION ON NOX PRODUCTION AND PERFORMANCE OF PULSE COMBUSTION HOT-WATER BOILERS

Two hot-water boilers with rated inputs of 147 and 220 kW had NOx emissions near 40 and 50 ppm (3% O-2), respectively, without flue-gas recirculation. With recirculation, NOx was reduced to 20-25 ppm for the larger boiler and to less than 20 ppm for the smaller one. Emissions of CO were not correlated with NOx; they depended an overall equivalence ratio and on design of the combustion chamber, but remained within the acceptable range of < 400 ppm (air-free) when the equivalence ratio was below 0.89. Besides its effects on NOx, recirculation strengthened the pressure wave and slightly improved the overall thermal efficiency. These effects were due to the slower burning rate with recirculation and a resulting closer match between the heat release and the pressure wave. Similar effects on wave amplitude and efficiency were produced by delaying the mixing of the incoming streams of air and natural gas, but NOx was scarcely affected.