Experimental and DFT study on the relationships between the evolution mechanism of PAHs and NOx precursors during coal pyrolysis

Abstract

The evolution mechanism of coal carbon skeleton to polycyclic aromatic hydrocarbons (PAHs) with temperature has been researched by in-situ Raman and solid-state 13C NMR. A theoretical exploration with density functional theory at M06-2X/6-31G(d,p)//def-TZVP level was performed, in order to investigate the mutual conversion pathways among different functional nitrogen, and the relationships between their evolution to NOx precursors and PAHs. The aromatic nucleus radicals produced by homolysis reaction of aliphatic side chains can be connected by carbon-carbon single bonds to form PAHs remaining in char at medium pyrolysis temperature. The participation of olefin compounds cracked from aliphatic side chains is favorable to the conversion of pyrrolic and pyridinic (N-6) nitrogen to NOx precursors and PAHs, and the cyclopentadiene and butadiene obtained by the cleavage of different functional nitrogen in coal skeleton are important intermediates for the formation of PAHs. The conversion of pyridine oxide and quaternary nitrogen (N-Q) to N-6 around 440 °C can compensate for the own thermal decomposition of N-6, while the conversion of N-Q to N-6 is gradually dominated by the reverse reaction from around 690 °C.