Effect of Benzoic Acid on Biogenic Gas Production with Different Rank Coals and the Fluorescence Spectra Characteristic of Produced Organic Products

Biogas production was conducted using samples from different coal beds and laboratory-domesticated microbes to investigate the effect of the addition of benzoic acid on biogas production. Furthermore, the response properties of produced organic substances at different gas production stages were analyzed with ultraviolet-visible (UV-Vis) spectroscopy and three-dimensional fluorescence spectroscopy. The results showed that adding benzoic acid significantly enhanced the microbial gas production with different rank coals. Obvious spectroscopic differences were observed in the gas production effects and liquid-phase composition across varying rank coals. The UV-vis spectroscopy findings indicated that soluble organic matter gradually increased in molecular weight during gas production, leading to increased aromatization and an increase in aromatic ring substituents with hydrogen and oxygen functional groups. Fluorescence spectroscopy revealed changes in protein-like substances during gas production, indicating the involvement of humic acid-like substances from coal in microbial gas production. The results of the fluorescence index supported the biological origin of humic acid during the gas production process. Benzoic acid augmentation promoted biogas production in different coal grades, and distinct differences were observed in the organic spectral properties during gas production, suggesting that the metabolic pathways of the same microbes acting on different coal grades vary.