Controlling pollutant emissions in a high-pressure combustor with fuel-diluent blending

Abstract

This paper investigates the formation of nitric oxide (NO) and carbon monoxide (CO) emissions as a function of nitrogen (N2) or carbon dioxide (CO2) diluent content, added to a premixed reacting jet in crossflow. Reaction characteristics of a rich methane-air-diluent jet injected into a lean vitiated crossflow were analyzed at an elevated pressure level 5 atm, which is critical to obtain data scalable to industry conditions. The jet was pre-heated and enriched with 0%, 15%, 30%, and 50% dilution by mass percentage to quantify the effect on pollutant emissions. Simulated results of the full chemistry Star-CCM + CFD model were verified with data taken in an experimental high-pressure combustion facility, which provides pressure, temperature, and velocity profiles, as well as line-of-sight chemiluminescence and exit emission measurements. The analysis revealed the significant influence of the diluent to increase the axial flame lift-off and delay axial combustion. Increasing the diluent content increased the timescale for the flame to stabilize, which allowed for greater entrainment of crossflow oxidizer into the axial jet stream, and led to decreased pollutant emissions. Hence, crossflow entrainment is a critical driving force at high diluent content. Local nitric oxide (NO) emission formation in the axial stage was predicted numerically, showing a correlation between diluent addition, axial heat release, and the formation of nitric oxide pollution. The high diluent levels delayed axial combustion, while reducing prompt and thermal NOx by mitigating flame hotspots and minimizing the timescale that the products remain at high temperatures.