Effects of ammonia on combustion of coal in stoichiometric premixed methane–air flames

NH3 has long been considered a promising alternative fuel·NH3 from renewables can be used for energy storage·NH3 is a zero-carbon molecule and can be burned directly and used as a hydrogen carrier. Compared with hydrocarbon fuels, NH3 has several drawbacks, such as an inferior flammability limit, flame temperature, and flame burning velocity, resulting in a lower reactivity rate and the production of pollutants. Therefore, cofiring coal with NH3 is an approach that has been growing in popularity. This study investigated the combustion of coal cofired with NH3 and conducted two-color pyrometry to measure the temperature profiles of coal particles in hybrid flames, that is, methane–coal–air and methane–NH3–coal–air premixed flames. The axial temperature distribution at the center of the flame was measured using two-color pyrometry and a thermocouple. The temperature values obtained were not identical but exhibited similar trends. The results indicated that the temperature at the origin significantly decreased upon ammonia addition, but as the measuring height increased, the final temperatures approached similarity. In addition, the emissions of pollutants such as CO and NOx were also measured. The results indicate that the addition of coal particles without ammonia gas leads to an increase in CO concentration due to the pyrolysis of the pulverized coal. However, there was no significant impact on NOx emissions. In contrast, the addition of coal particles under CH4 − NH3 gas combustion conditions results in a significant decrease in both CO and NOx concentrations. Specifically, under the 80 %CH4 + 20 %NH3 condition and with a coal particle feeding rate increasing from 0 to 0.24 g/min, there was a reduction of approximately 34% in CO concentration and 35% in NOx concentration. In order to confirm the coal consumption rate, coal particles were captured before and after flame sheet using a metal grid, and the particle projected area was determined based on scanning electron microscopy (SEM) images. These results provided evidence that the addition of NH3 to the CH4 − air flame enhances coal combustion, as indicated by a decrease in the particle shrinkage rate and sampling density.