Effects of hydrogen peroxide pretreatment on physicochemical properties and biogas production of lignite

Abstract:

[Objective] To investigate the effects of hydrogen peroxide treatment on the physicochemical properties and biogas production of lignite. [Methods] We carried out orthogonal experiments to optimize the conditions of hydrogen peroxide pretreatment for Shenli No.5 lignite. Lignite was treated under the optimal conditions to obtain coal residues and treatment solutions. The physicochemical properties, including elemental and maceral composition, mineral components, microcrystalline structure, porosity, permeability, surface morphology, organic functional groups, and organic composition in the treatment solution were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), brunauer-emmett-teller (BET), gas chromatography-mass spectrometry (GC-MS), and high-performance liquid chromatography (HPLC). The physicochemical properties were then compared among the raw coal, treated residue, and treatment solution. [Results] The optimal pretreatment conditions of lignite were treatment with 5.0% hydrogen peroxide at a liquid-to-solid ratio of 30:1 for 20 days, under which the total organic carbon yield in the treatment solution was 105 mg/L. After treatment under these optimal conditions, the treated residue exhibited increased cracks and dents on the surface and loosened surface structures. In addition, the interlayer spacing of the aromatic plains of the coal increased while the aromatic ring structure became more open with smaller crystal nucleus structures. Both porosity and specific surface area increased after the treatment. Compared with that before treatment, the treated residue showcased decreased fixed carbon, carbon, and vitrinite and increased ash, volatile matter, oxygen and hydrogen, and inertinite. The content of functional groups such as O=C-O, C=C, and C=O increased in the treated residue, while that of N-H and C-H reduced. The biogas production of the treatment solution and the treated residue was 39.13% and 94.46%, respectively, lower than that of raw coal. Hydrogen peroxide pretreatment primarily acted on vitrinite, dissolving organic carbon and altering the functional groups of large molecular structures in coal. This altered the aromatic ring structure of coal, causing small molecules to dissolve into the treatment solution under oxidative conditions. The organic compounds in the treatment solution mainly consisted of short-chain fatty acids. After biogas production, the number of low-molecule-weight acids and organic compounds decreased in the treatment solution. The relative abundance of dominant microbial phyla and genera varied significantly among different microcosms. Regarding the archaea for biogas production, the dominant phylum and genus were Halobacteriota and Methanosarcina in the raw coal and Thermoproteota and Bathyarchaeia in the treatment solution, respectively. In terms of the bacteria for biogas production, the dominant phylum and genus were Actinomycetota and Gaiellales in the raw coal and Pseudomonadota and Delftia in the treatment solution, respectively. [Conclusion] The organic carbon dissolved from coal can be utilized by microorganisms for biogas production. However, the removal of organic components by over-oxidation may decrease the biogas production.