Effect of air humidity and natural gas composition on swirl burner combustion under unstable conditions

Abstract

Combustion instabilities are an undesired phenomenon in premix burners, leading to vibrations and potential flame problems such as lift-off or flashback. The combustion stability in a premix burner is highly sensitive to fuel composition and local environmental conditions. Consequently, this work presents the effect of air humidity and fuel composition on the degree of combustion instability in a swirl stabilized premix burner at atmospheric pressure, initiating in both unstable and stable states. The experimental setup consists of a swirl stabilized 20 kW burner with a square combustion section of 100 mm × 100 mm, with optical access to the reaction zone on three sides. Three types of fuel were evaluated: methane (CH4), CUSIANA (85% CH4; 10% C2H6; 5% C3H8), and propane (C3H8). Atmospheric humidity was emulated over a range from 0% to 5%, molar basis, the high point corresponding to the greatest atmospheric humidity. The degree of burner instability was characterized by the standard deviation of the pressure signal. It was found that atmospheric humidity can reduce combustion instability, affecting methane more significantly than propane at low air preheat temperature (315 K); at high preheat temperature, the water content was found to have no effect on combustion instability. The flame structure, i.e. shape and size, was similar for all fuel types at the same adiabatic flame and air preheat temperatures. Under unstable operating conditions, a similar flame shape and area of oscillation were found with the same resonant frequencies. This indicates that the variables characterizing thermoacoustic instability maintain similar values or proportions, regardless of the fuel type.