The influence of fuel composition on emissions of CO, NO, and NO 2 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 NO 2 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 NO 2 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.