The influence of fuel composition on emissions of CO, NO, and NO2 from a gas-fired pulsed combustor

Abstract

A commercially available nonpremixed pulsed combustion room heater designed to burn natural gas has been used to study emissions of NO(x) and CO using a wide range of fuel flowrates and fuel compositions. In particular, the effect of addition of hydrogen and propane to the fuel has been investigated. When burning methane alone, NO(x) emissions increased as the fuel flowrate increased, and there was also a marked switch from emitting predominantly NO2 at high excess air (low temperature) to predominantly NO at low excess air (high temperature). NO(x) emissions appear to be determined by the residence time at high temperature in the near-stoichiometric burning zone; the proportion of NO(x) emitted as NO2 is determined by the temperature after mixing with excess air and the backflow of exhaust gases. Adding small amounts of hydrogen to the fuel increased NO(x) emissions by lowering the excess air, but if the proportion of hydrogen in the fuel rose to over 20%, too high a phase angle between the heat release and pressure oscillations led to a deterioration in combustor performance, a sudden increase in the emission of CO, and a decrease in the emission of NO(x) owing to a reduction in temperature. Addition of propane increased NO(x) emissions by lowering the excess air in the combustion chamber and hence increasing the chamber temperature. Results from running the combustor with a wide variety of fuel compositions and operating conditions suggest that, despite its being a nonpremixed combustor, calculated adiabatic flame temperature can be used as an indicator of the quantity and nature of NO(x) emissions, provided that combustion remains essentially complete.