Oxidation of Fuel Bound Nitrogen in a Transitional Metal Oxide Catalytic Combustor

Abstract

Experimental results are reported concerning the oxidation of nitrogen containing fuels in a binary transition metal oxide catalytic combustor for both fuel-lean and fuel-rich operations

The catalyst consisted of chromium and cobalt oxides deposited on high surface area alumina pellets (5.63 percent Cr₂O₃, 187 percent C0₃O₄, balance alumina). The fuel used was propane doped with 1 percent by wt. ammonia and the oxidizer was a mixture of O₂/Ar to eliminate thermal NO_x formation. The experiments were conducted over a wide range of inlet conditions and for every set of these, the exit concentrations. of the dominant fixed nitrogen species (NO_x, NH₃ and HCN) as well as other major species (like O₂, CO₂, CO, H₂O and C₃H₈) were measured

Under fuel-lean operation the sole product of fuel-N conversion was NO_x, while in fuel-rich operation HCN and unconverted NH₃ were also detected. The conversion to HCN and the amount of unconverted NH₃ increase, and the conversion to NO_x decreases, as the operation becomes more fuel rich. The concentration of HCN increases, while those of NH₃ and NO_x decrease as the inlet temperature (T_in) increases. The sum of the fixed nitrogen species shows a minimum at an equivalence ratio (φ _in) of about 1.43

The most significant result was that at sufficiently high Tin (600-640°C) the total emission levels or fixed nitrogen compounds (NO_x, NH₃ and HCN combined) can be reduced to very low levels (5-20 percent of the fuel-N present at the combustor inlet) in both fuel-lean as well as fuel-rich legions. Most of the fuel-N is converted to molecular nitrogen. This was also found to be true for lean fuel combustion when air was the oxidant. By comparison, fuel-lean operation with noble metal catalysts, typically yields 50-90 percent fuel-N conversion to NO_x