Ammonia is an ideal renewable, carbon-free fuel and hydrogen carrier, which produces nitrogen and water after complete combustion in the presence of oxygen. However, ammonia has low reactivity, slow flame-propagation speed, and carries risks of high nitrogen oxide (NO_x) emissions. Co-firing ammonia with an industrial by-product gas (with CH₄ and CO being the main combustible materials) is a cost-effective and convenient method of improving the combustion characteristics of ammonia, but attention still needs to be paid to the NO_x generation. Currently, the research on NO_x formation during co-firing of ammonia with other fuel gases is still insufficient. In this study, a high-temperature furnace reaction system was used to investigate the NO_x formation and inhibition mechanisms during the combustion of NH₃/CH₄ and NH₃/CO mixtures. By varying the ammonia blending ratio, excess air coefficient (α), temperature, residence time, and fuel concentration, the key factors influencing NO_x generation and inhibition were further analyzed. The results showed that when α was no less than 1, the production of NO_x initially increased and then decreased with an increasing proportion of ammonia in the fuel gas. Within the temperature range of 900 °C to 1500 °C, the amount of NO_x generated during the combustion of the mixed gas gradually decreased with the increase in temperature. Under the conditions of NH₃/CH₄ and NH₃/CO, the emissions of NO_x were higher than those during pure ammonia combustion.

Gas-fired boilers, gas stoves, and wall-mounted gas boilers are the main consumers of gas fuel, but they generally encounter problems when operating at high altitudes, such as reduced thermal efficiency and increased pollutant emissions. Previous studies on gas combustion characteristics under sub-atmospheric pressure were mostly carried out in a large space, which is quite different from chamber combustion equipment. Therefore, it is insufficient to guide the design and operation optimization of plateau gas equipment. In this paper, experimentations were carried out to explore the characteristics of a methane diffusion flame under sub-atmospheric pressures. The mass flow rates of methane and air remain consistent under different pressure conditions. The centerline temperature ($T_c$) distribution, flame appearance, smoke point, CO emission, and NO_x emission under different pressures (ranging from 61.66 to 97.75 kPa) were examined under both fuel rich and lean conditions. The results show that $T_c$ at the rear and front of furnace variation with pressure is opposite under fuel-lean and -rich combustion. The $T_c$ at the front of furnace decreases with decreasing pressure, whereas $T_c$ at the rear of furnace increases with decreasing pressure. With decreasing pressure, flame length decreases under lean combustion, but increases under rich combustion. The smoke point fuel flow rate, flame length, and residence time increases with decreasing pressure, following the law of negative exponent. The CO emission decreases with decreasing pressure, which indicates that the reduced pressure makes methane combustion more complete. For NO emission, the reduced pressure results in an opposite tendency under fuel-lean and -rich combustion. With decreasing pressure, the NO emission decreases under fuel-lean combustion but increases under fuel-rich combustion.