Role of ammonia addition on the growth of polycyclic aromatic hydrocarbon and coke surface at deep cracking of endothermic hydrocarbon fuels

The active cooling technology based on endothermic hydrocarbon fuels can cause serious coking problems. Ammonia addition inhibits the coke formation at the mild fuel cracking. However, the mechanism of coke deposition under higher fuel conversion conditions remains unclear. Herein, the deep pyrolysis experiments of n-decane with ammonia addition was conducted using the electric heating method. The anti-coking performances of additives were obtained from the coking characteristics at different pyrolysis stages. The results indicate that under deep cracking conditions, the additive reacts with unsaturated free radicals produced by fuel cracking to produce aromatic heterocyclic compounds, inhibiting the formation and growth of polycyclic aromatic hydrocarbons (PAHs), thereby reducing the content of coking precursors. More importantly, the C-N compounds interact with the metals to form the spherical carbon with a predominant diamond structure, depositing as a thin film with passivation effect. As the reaction time proceeds, the film gradually smooths and maintains a thickness of about 4 mu m. In addition, passivation coating of TiN is used to further evaluate the effect of additives on the formation of surface coke. These results reveal that ammonia can ensure effective coking inhibition under long-term deep cracking conditions by affecting the growth of PAHs and coke surface.