Study On The Simulation Of Biogenic Gas And Thermogenic Gas Of Low-Rank Coal And Its Structure Evolution

For this paper, low-rank coal from Da nan hu area of Tuha basin and Fukang mining of Junggar basin is taken as the research object. Based on the analysis of coal quality, pore structure, element and mineral composition, we have carried out the simulation experiments of biogenic gas and thermogenic gas. Gas characteristics, such as composition, yield, isotopic composition of carbon and hydrogen, from these two kinds of simulation experiments were analyzed. It was found that there are biomarker evidences for the degradation in low-rank coal by microbes. Thus, a comprehensive criteria for origin of biogenic gas with secondary biological action was proposed. In addition, we explained coal evolution mechanism in the process of biological degradation, and revealed the reasons of changes in carbon isotopic compositions of pyrolysis gases, and analyzed the relationship between alkane gas and its carbon isotope composition and coal structure evolution. The main results are as follows:(1)Biogenic gases are mainly composed of CH4 and CO2 during 90 d simulation experiments of secondary biogenic gas, and almost does not contain heavy hydrocarbon gas. Changes in the content of CH4 and CO2 show a synchronization effect with continuing bio-degradation time. Namely, they all show a way of "rippling" slowly rising. Furthermore, the smaller the δ13C1 value is, the larger the δ13CCO2 value is. Carbon isotopic compositions of CH4 and CO2 present a "mirror image" symmetric relation, which is mainly related to inheritance effect of carbon isotopic composition originated from the parent material. The inheritance effect results in the light carbon isotope being fractionated into CH4, while heavy carbon isotope being fractionated into CO2.(2)Biodegradation ways are diversified, It was found that soluble and insoluble organic matter in low-rank coal could be degraded. In the bio-degradation process, the distribution of long-chain normal alkanes step by step is shifted towards lower carbon number wavelike. Furthermore, Biomarker compounds, such as C29-hopene, C30-hopene, C29~C32—ββ biological hopanes series, C24 tetracyclic diterpenoid, C31~C34 pentacyclic triterpane, were detected in the soluble organic matter, which strongly confirms the existence of biogenic gas.(3)During the bio-degradation process, aliphatic structure suffered greater degradation by microbes, and the aromatic structure also can be degraded by microbes. In addition, the dual characters of the biological degradation, promoting or hindering the development of aromatization, were found.(4)In the process of coal pyrolysis, carbon isotopic changes of CH4, C2H6 and C3H8 are mainly caused by tendentious fracture of aliphatic chain when Ro,max<2.0%, and carbon isotopic changes of these alkane gases are maybe mainly due to the matter in the loops, containing heavy carbon isotope, being removed into the products. In addition, Ro,max = 1.3% and 2.0% is also the inflection point between alkane gas and its carbon isotope composition and coal structure evolution. The research results have a guiding role in enrichng the formation mechanism of CBM in low-rank coals.