Experimental study of ammonia-methane diffusion flames

Ammonia could either be mixed with or replace conventional fuels in combustion processes to provide a low or zero-carbon solution for these systems. Several studies of ammonia-methane combustion have been reported in the literature. However, the literature still needs to include experimental data on the combined effects of ammonia and nitrogen doping on methane-air diffusion flames to delineate the chemical impact of fuel-bound nitrogen and the thermal/dilution effect of free nitrogen on ammonia-methane-air combustion. Therefore, this study aims to provide insight into the properties of ammonia/methane combustion in the presence of nitrogen using various flame configurations under laboratory conditions. This work investigates a range of ammonia/methane/nitrogen mixtures in three combustion rigs: laminar counterflow diffusion burner, co-flow diffusion burner, and swirl-stabilised co-flow diffusion burner. The experimental campaign included temperature measurement using fine wire thermocouple and planar laser-induced fluorescence studies to provide NO and OH species concentration profiles through the flames and a spectroscopic method of temperature measurement to compare to the thermocouple measurements. The recorded LIF signals were corrected for quenching and Boltzmann population fraction to quantify the OH and NO concentrations. Preliminary tests and analyses were conducted to select a suitable transition pair for the OH PLIF thermometry.
Furthermore, chemiluminescence visualisation studies were conducted to record the OH* and CH* species in the flames. A flame stability test was also performed to establish the flame stabilisation region and the blow-off transient behaviour. The corrected thermocouple temperature measurement reproduced the OH PLIF temperature results excellently. Increasing the strain rate of the counterflow diffusion flame extended the detectable temperature range. This study contributes to the experimental database of ammonia/methane combustion in the presence of nitrogen. The results revealed opposite effects of nitrogen addition on the temperature and OH LIF intensity in the mid-section and tip of the co-flow flame. The CH* Chemiluminescence profiles of the co-flow flame featured an inflexion point between the peak and vanishing points. The OH* chemiluminescence did not feature any inflexion points. Nitrogen addition to the ammonia-containing flames suppressed the NO emissions in the co-flow flames. Primary and secondary NO formation zones in the counterflow ammonia-methane-air flames are reported for the first time. The data presented in this work facilitates the chemical kinetic and CFD modelling of ammonia/methane diffusion flames. Further studies to determine the temperature of the swirling co-flow ammonia-methane-air diffusion flame using two lasers to record the OH PLIF intensity at a transition pair simultaneously and PIV imaging to capture the flow field are suggested.