The Interaction Characteristics Between Biogas Generation And Coal Structure Evolution

The aim of this study was to investigate the interaction mechanism between the biogenic gas production and coal structure.Coal samples were collected from Baiyinhua mine,Qianqiu mine,Malan mine and Shaqu mine.The anaerobic fermentation experiments with coal as substrate were conducted in laboratory.The influence of biogenic gas production on the coal structure was analyzed by characterizing the changes of coal structure before and after biogenic gas production.Meanwhile,the extraction experiments were carried out with different solvents,the impact of structural differences of Baiyinhua,Qianqiu,and Malan raw coals on biogenic gas production was further analyzed by the aspects of solvent extraction rate,element content,surface functional groups,microcrystalline structure,pore structure and bioactive organic matter.Then the biogenic gas production effects of the coal residues after extraction were observed.The structure difference between raw coals and residues were investigated to analyze the influence of coal structure changes on the biogenic gas production.The results show that:a.After the primary biogenic gas production of raw coal,the residual coal still has a large potential for gas generation.Compared with the primary gas production process,the total gas production during the secondary gas production process is reduced,and the second gas production peak is relatively early.b.The surface element content and occurrence state of the residual coal after biogas generation have changed to varying degrees.The O/C atomic ratio of Baiyinhua residual coal sharply decreased,and the O/C atomic ratio of Qianqiu and Malan residual coals slightly increased.The aromatic units and the alkane substitutes（C-C,C-H）relative content increased in Baiyinhua,Qianqiu and Malan residual coals,and the relative content of aromatic carbon or ether carbon（C-O）decreased,the relative content of carbonyl carbon（C=O）increased,the relative content of carboxyl carbon（COO^—）increased.c.After anaerobic fermentation,the porosity of the coal samples increased,the connectivity of the pores was enhanced,and the lower the coal rank was,the more obvious the effect was.d.Baiyinhua raw coal has higher oxygen content and O/C atomic ratio,lower aromatization degree,more side chain functional groups and bioactive organic matter（e.g.,humic）,its three-dimensional space macromolecular structure cross-linking is even weaker and has a higher porosity,so that it has a higher biogas production capacity than Qianqiu and Malan raw coals.e.After extraction by NMP,acetone and 0.6 mol/L NaOH alkali solution,some small molecular organic matters such as aromatics,aliphatic hydrocarbons,and heteroatoms in the coal dissolved out.The aromatic lamellae spacing(d₀₀₂)and the stacking height of the aromatic layers（L_c）in Baiyinhua,Qianqiu,and Malan coal residues have a increasing trend,and the degree of cross-linking of the coal macromolecular network structure has decreased.f.After the extraction of N-methyl-2-pyrrolidone（NMP）and acetone,the biogas production effect of Baiyinhua,Qianqiu and Malan coal samples have been promoted.g.After the extraction of 0.6 mol/L NaOH alkali solution,the biogas production effect of Qianqiu and Malan coal samples have been promoted,but the biogas production effect of Baiyinhua coal samples were seriously suppressed.This study further demonstrated the methanogenic mechanism and process pattern in the coal seams,revealed the influence characteristics between biogenic gas and coal structure,and has important theoretical significance for the research of bioengineering of coalbed methane.