Development of a Hybrid Catalytic Combustor

The pollution problems associated with unburned hydrocarbons and carbon monoxide in the idle mode, and NO_x and smoke production in the power mode of aircraft gas turbine operation can be minimized using hybrid catalytic combustion. A hybrid catalytic combustor (HCC) consists of a fuel-rich precombustor, secondary air quenching zone, and monolithic catalyst stage which rapidly oxidizes CO and UHC produced in the precombustor. The concentration of thermally produced NO_x in the precombustor is very low because of the lack of oxygen. However, the formation of NO_x precursors such as HCN and NH₃ produced under fuel rich conditions must be considered. Data showed that nitrogenous species produced in the rich precombustion zone were efficiently converted to NO_x by catalysts under very lean mixture conditions. The equivalence ratio in the precombustor was varied from 0.5 to 1.5, while the overall mixture, after secondary air injection, was in the range of 01–0.3. The noble metal catalysts on various monolithic support geometries and compositions were found to be the most active materials for CO and UHC oxidation in the temperature range of 700–1200 K. The HCC combustion efficiency of JP-4 which contained 535-ppm sulfur was determined to be 99.8 percent under realistic conditions. The combustor pressure drop was less than 6 percent. The average emission indices of CO, UHC, and NO_x leaving the HCC were on the order of 0.95, 0.43, and 1.8 g/kg of fuel, respectively, for metal supported Pt catalyst. This catalyst was effective in reducing CO by 86 percent and UHC by 94 percent, and increasing NO_x by 68 percent. Using approximate methods for calculating EPA emission parameters, it was estimated that the HCC can meet the 1979 new aircraft emission standards but fails to meet the 1981 new aircraft emission standards because UHC are slightly too high.