Chemistry of nitrogen oxides (NOx) formation in flameless combustion

One of the major drivers beyond the development of flameless technology is its potential to cut the emissions of nitrogen oxides (NOx) far below the harmful levels for both human health and the environment. Yet, although decades of research have consolidated the knowledge on the processes of NOx formation at high temperature, a complete understanding of their chemistry, especially in the operating conditions typical of flameless, is still far from being reached. As a matter of fact, at lower temperatures and with higher premixing levels, the significant reduction of the thermal and prompt pathways increases the relative importance of the chemistry of fuel-nitrogen, especially in the next-generation fuels like biomass-derived ones. Moreover, the presence of NOx in the fuel mixture, even in trace amounts, may alter the reactivity of the major hydrocarbons at lower temperatures. In this chapter, each of the NOx formation mechanisms was reviewed in detail, and their role in the flameless combustion was discussed by using several recent experimental campaigns as case studies, and a comprehensive kinetic mechanism was leveraged to identify the elementary steps governing nitrogen conversion and selectivity to N2, NO, and N2O. The use of the kinetic mechanism also allowed to frame the chemical role of NOx in enhancing the hydrocarbons reactivity at lower temperatures and the key role of the intermediate methoxy radical. A deeper focus was then put on the fate of fuel nitrogen in the flameless regime, considering cyanic acid (HCN), ammonia (NH3), and pyrrole (C4H5N) as surrogate species to represent fuel-N. It was found that at lower temperatures, nitrogen selectivity favors the formation of N2 and N2O over NO and NO2, with NH3 being the “cleanest” precursor, i.e., mostly providing N2.