Insight into evolution characteristics of pyrolysis products of HLH and HL coal with Py-VUVPI-MS and DFT

To achieve a comprehensive understanding of the influence of chemical structure on the evolution characteristics of coal pyrolysis products, pyrolysis reaction of two kinds of low rank coal (Huolinhe and Huangling coal) were investigated with pyrolysis-vacuum ultraviolet photoionization mass spectrometry (Py-VUVPI-MS). The soft ionized mass spectral detection can provide evolved information of original pyrolysis product. The chemical structure of coal samples was characterized by solid-state ¹³C NMR, indicating HLH coal with more branched chain and longer aliphatic chain structure. The bond dissociation enthalpies (BDE) of β-C_al-C_al and β-C_al-O within model compounds were obtained with density functional theory. The difference of peak temperature with maximum evolution was collected to investigate the effect of the chemical environment on evolution behavior of pyrolysis products. The results reveal that branched and long aliphatic side-chains can reduces BDE of β-C_al-C_al and β-C_al-O, resulting in the lower peak temperature of pyrolysis products derived from HLH coal. Substituent groups (such as alkyl and hydroxyl groups) attached on the aromatic rings can reduce peak temperatures. Moreover, the effects of the different substituted position on the aromatic ring of the methyl group presented on differences of the BDE. An increase in aromatic ring size correlates with a certain degree of reduction in BDE for β-C_al-C_al; consequently, peak temperature of pyrolysis products with larger aromatic rings is lower. The pyrolysis behavior of coal were discussed based on the experimental observations and theoretical calculation, which are beneficial to understand the reaction route and mechanism of coal pyrolysis.