Understanding the effect of sodium on NOx precursors and PAHs formation during coal pyrolysis: A combined experimental and DFT study

The effect of Na on NOx precursors and polycyclic aromatic hydrocarbons (PAHs) formation during coal pyrolysis has been researched by thermogravimetry - mass spectrometry and solid-state 13C NMR. A theoretical exploration with density functional theory is performed to investigate the impact mechanism of Na on the conversion among different functional nitrogen, and on the evolution of coal skeleton to NOx precursors and PAHs. Na can promote pyrrolic nitrogen (N-5) cracking while inhibit pyridinic nitrogen (N-6) cracking and to N-5, resulting in the reduction rate of N-5 and the band area ratio of N-6 in NaCl-loaded coal char being higher than those in raw coal char with temperature. Na can also promote oxidized pyridinic nitrogen (N-X) cracking to form N-6, and the carbon condensation of N-6 to quaternary nitrogen (N-Q), leading to the decrease of N-X while increase of N-Q in NaCl-loaded coal char. The H free radicals in the environment are beneficial to the cleavage of N-5 and N-6 to NH3 and HCN, and Na can significantly promote H transfer. Na can also promote NH3 formation (33.41 kcal/mol), while inhibit the cleavage of N-5 and N-6 to HCN (11.40 kcal/mol). Moreover, Na has a promoting effect on the formation of PAHs.