Experimental Study on the Influence of Ammonia and Hydrogen addition on Soot Formation in Laminar Coflow Ethylene Diffusion Flames

Abstract

Ammonia and hydrogen are two alternative fuels that can help decarbonization as they can be produced using renewable energy. Ammonia has transportation, handling, and storage advantages over hydrogen even though its combustion characteristics are worse. One intermediate strategy to use ammonia or hydrogen as a fuel is to co-fire it with hydrocarbons. However, co-firing with hydrocarbons may emit harmful pollutants such as NOx and soot. This study investigates the effects of ammonia and hydrogen addition on soot formation in laminar coflow nitrogen-diluted-ethylene normal diffusion flames using experimental techniques. Ammonia and hydrogen were added separately to the fuel flow. Flame conditions from 0 to 50 vol% of the added species (ammonia or hydrogen) were tested. Laser diagnostics for measuring the distributions of polycyclic aromatic hydrocarbons (PAHs) and soot volume fraction (SVF), and intrusive measurements (using a thermocouple and probe sampling) were performed. Based on the results, ammonia addition suppressed soot formation while hydrogen addition enhanced it. In conditions with ammonia addition, the temperature measurements with a Type S thermocouple and adiabatic flame temperature simulations using CHEMKIN PRO showed similar temperature profiles and negligible adiabatic flame temperature differences respectively. The qualitative PAH measurements using planar laser induced fluorescence (PLIF) showed that the concentration of PAHs of four or larger rings reduced with ammonia addition. Soot volume fraction (SVF) measurements using planar laser induced incandescence (PLII) showed that the peak SVF decreased exponentially with ammonia addition. Particle size distributions showed that the incipient particles were formed, however growth to mature primary particles was limited with 25% or higher ammonia addition in the flame. Based on similar temperature profiles and decreasing trends in the distribution of PAHs and SVF, soot suppression with ammonia addition was linked to chemical effects. PLIF measurements with hydrogen addition could be affected by the temperature difference between the flames, therefore further investigation is needed. PLII measurements, however, showed that the soot volume fraction increased linearly with hydrogen addition.