Review of effects of zero-carbon fuel ammonia addition on soot formation in combustion

Carbon-free fuel ammonia is considered to contribute to the decarbonization of energy consumption structure. Blending ammonia with conventional carbon-based fuels can often reduce soot emission while improving the combustion performance of ammonia. This work provided an overview of soot formation characteristics and regulation mechanisms in the case of ammonia blending with gaseous/liquid/solid carbon-based fuels. Effects of adding ammonia were divided into three categories, namely dilution, thermal and chemical effects. These competed with each other and ultimately resulted in a reduction in primary particle size and soot volume fraction compared to the combustion of pure carbon-based fuels. Of these, the regulation mechanism of chemical effect was the most complex, mainly divided into two aspects. First, nitrogen-containing species formed by ammonia decomposition could inhibit soot formation by robbing available carbon atoms which should have formed soot. On the other hand, nitrogenous species might participate in carbon layers growth by combining with polycyclic aromatic hydrocarbons, and also consume hydroxyl radicals in flames to inhibit soot oxidation. However, there were some controversial parts of current research, which hindered the efficient utilization of ammonia and pollution control. Future research should focus on the interactions between nitrogen-containing species and hydrocarbons lager than C3 at broad pressures, to accurately understand detailed mechanisms by which adding ammonia regulates soot emission.