Biogas production and variation of the chemical composition of the high temperature water extract of lignite

The interaction process between microorganism and coal is complicated. In order to deeply analyze the function and change rule of organic matter in coal in the process of biological gas production, taking Yima lignite and methanogens preserved in laboratory as research objects, the gas production characteristics of water-soluble organic matter of lignite and its chemical composition change before and after gas production were studied. The water-soluble organic components of Yima lignite obtained by deionized water extraction at 70℃ and the extraction residue were used as substrates for biogas production. High performance liquid chromatography-mass spectrometry (HPLC-MS), gas chromatography(GC) and gas chromatography-mass spectrometry(GC-MS) were used to analyze the change of gas production and the chemical composition of fermentation broth and coal methanol extract. The results showed that the gas production of raw coal, water-soluble organic matter and extraction residue were 0.46, 0.45 and 0.15 mmol/g coal respectively. At the beginning of gas production, the chemical composition of water-soluble organic compounds of which the molecular weight was mainly concentrated in 200-300 Da was complex. After biogas production, the species of compounds decreased, and the molecular weight decreased to 150-200 Da. Some oxygen-containing and nitrogen-containing compounds with phenyl rings were detected in the fermentation broth. GC-MS analysis showed that a small number of water-soluble compounds(e.g., formamide, acetamide and dimethyl sulfite) appeared in the methanol extract of extraction residue after biogas production. The conclusion was that the water-soluble organic matter of lignite extracted at 70℃ can be utilized by methanogens for biogasification and some water-insoluble organic compounds were converted into water-soluble organic compounds in the process of biodegradation. This study explored the potential role of water-soluble organic compounds in the biogasification process of lignite.