Effects of Functional Groups in Coal with Different Vitrinite/Inertinite Ratios on Pyrolysis Products

Maceral composition is one of the key factors affecting the liquefaction and gasification of coal, which has attracted extensive attention of researchers working on coal chemical industry. To elucidate the impact of vitrinite and inertinite in coal on pyrolysis products, vitrinite and inertinite were extracted from a single coal sample and mixed to create six samples with varying vitrinite/inertinite ratios. The samples were subjected to thermogravimetry coupled online with mass spectrometry (TG-MS) experiments, and the Fourier transform infrared spectrometry (FITR) experiment was used to determine the macromolecular structures before and after the TG-MS experiments. The results show that the maximum mass loss rate is proportional to the vitrinite content and inversely proportional to the inertinite content, and increased vitrinite content accelerates the pyrolysis process and shifts the pyrolysis peak to low temperatures. Based on FTIR experiments, the sample's CH₂/CH₃ content, representing the length of aliphatic side chains, decreases significantly after pyrolysis, and the greater the loss of CH₂/CH₃, the greater the intensity of organic molecule production, indicating that aliphatic side chains are likely to yield organic molecule products. The aromatic degree (I) of samples rises sharply and steadily with increasing inertinite content. After high-temperature pyrolysis, the polycondensation degree of aromatic rings (DOC) and relative abundance of aromatic and aliphatic hydrogen (H_ar/H_al) within the sample increased significantly, indicating the thermal degradation rate of aromatic hydrogen content is much lower than that of aliphatic hydrogen. When the pyrolysis temperature is lower than 400 °C, the higher the inertinite content, the easier it is to produce CO₂, whereas an increase in vitrinite leads to an increase in CO production. At this stage, the "-C-O-" functional group is pyrolyzed to produce CO and CO₂. When the temperature exceeds 400 °C, the CO₂ output intensity of vitrinite-rich samples is much higher than that of inertinite-rich samples, while the CO output intensity of vitrinite-rich samples is lower, and the higher the vitrinite content, the higher the peak temperature of CO gas production of samples, indicating that when the temperature exceeds 400 °C, the increase of vitrinite inhibits CO production and promotes CO₂ production. At this stage, the reduction of each sample's "-C-O-" functional group after pyrolysis is positively correlated with the maximum CO gas production intensity, and the reduction of each sample's "-C=O" functional group after pyrolysis is positively correlated with the maximum CO₂ gas production intensity. As a result, the "-C-O-" functional group is more likely to produce CO, whereas the "-C=O" functional group is more likely to be pyrolyzed to CO₂. Hydrogen is primarily produced during the polycondensation and aromatization processes, and its production is proportional to the dynamic DOC values after pyrolysis. The higher the I value after pyrolysis, the lower the maximum gas production peak intensity of CH₄ and C₂H₆, which indicates that increasing the aromatic proportion is detrimental to CH₄ and C₂H₆ production. This work is expected to provide theoretical support of the liquefaction and gasification of coal with different vitrinite/inertinite ratios.