Biogas production of vitrinite from Shengli lignite pretreated with H2O2

Abstract:

[Objective] To study the effect of hydrogen peroxide (H₂O₂) treatment on biogas production of vitrinite from coal. [Methods] Shengli lignite collected from Inner Mongolia was used in this study. The methanogenic microbial consortium previously enriched and preserved in our laboratory was used as the inoculum. Coal macerals were separated by floatation, which yielded three samples containing high vitrinite (GJ), medium vitrinite (ZJ), and low vitrinite (DJ). After being characterized, the samples were treated with 10% H₂O₂ for 30 days at a solid-to-liquid ratio of 1:15. Biogas production experiments were conducted with the coal samples before and after treatment. Gas chromatography was employed to analyze the gas composition, and X-ray diffractometry and Fourier transform infrared spectroscopy were employed to study the physical and chemical properties of the coal before and after treatment as well as after gas production.[Results] The coal samples after H₂O₂ treatment showcased reduced vitrinite content and carbon fixation and increased volatile matter. The reaction was more intense in the coal sample with high content of vitrinite, accompanied by increased oxygen content and reduced carbon. Methane yields from untreated coal samples on day 100 followed the order of GJ (174.24 μmol/g coal)＞ZJ (164.31 μmol/g coal)＞DJ (135.52 μmol/g coal). However, the coal samples pretreated with H₂O₂ ceased gas production after day 20, with the gas yields of 39.63, 39.61, and 41.55 μmol/g coal, respectively, representing reductions of 77.26%, 75.89%, and 69.34%, respectively, compared with those from the coal samples without treatment. Furthermore, as vitrinite content increased, the coal samples demonstrated decreased layer spacing (d₀₀₂) of the aromatic ring lamellae, ductility (L_a) of the single-layer lamellae, and stacking degree (L_c) of the lamellae and increased number of aromatic layers (N) after H₂O₂ treatment, which indicated that the crystal nuclei appeared smaller. In addition, H₂O₂ treatment led to increased proportions of aromatic carbons, aromatic moieties, C=O groups, and C=C groups, enhanced aromatic ring condensation, and increased number of oxygen-containing functional groups. [Conclusion] Long-term H₂O₂ treatment reduces organic matter that is readily bioavailable in coal matrix, thereby decreasing gas production.