Micromixing Diffusion Combustion Characteristics of Ammonia Cracking Gas

Ammonia cracking combustion can significantly enhance combustion stability, reducing fuel-NOx and ammonia emissions. The combustion characteristics of a micromixing diffusion flame were studied by simulating ammonia cracking gas with ammonia, hydrogen, and nitrogen in this paper. The results indicate that the flame height increases as the cracking ratio decreases and the equivalence ratio increases. Additionally, the flame changes from a yellow color to light blue with insignificant flame height change during increasing cracking ratio. For a 10 kW flame, combustion stability can be maintained when X-cra is above 50%, but there is a risk of flame lifting and blow-off when it is below 40%. When the cracking ratio increases, the nozzle temperature rises. When the equivalence ratio increases, the nozzle temperature reaches a peak and moves with the cracking ratio. As the cracking ratio decreases, the NOx emission increases and then decreases, reaching a maximum value of 910 ppm. The NOx emission is lowest at a cracking ratio of X-cra = 100%, ranging from 2 to 4 ppm. In the case of the lowest cracking ratio (X-cra = 50%), the ammonia in the low-temperature ambient of the combustion chamber plays a significant role in the reduction, resulting in NOx emissions ranging from 44 to 80 ppm. Overall, NOx emission decreases with increasing equivalence ratio, but higher power is accompanied by a greater concentration of NOx emission. Hydrogen near-completely combusts at high cracking ratios. Conversely, decreasing the cracking ratio leads to an increase in ammonia, reduces combustion efficiency, and consequently, results in higher hydrogen emissions, with a peak emission of 28.1 ppm at a 50% cracking ratio and 0.9 equivalence ratio. The results demonstrate that combining ammonia cracking gas with micromixing diffusion combustion can achieve stable and efficient combustion. Low NOx emission can be achieved by controlling influencing factors such as combustion temperature, equivalence ratio, and cracking ratio.