Organic Geochemical Characteristics Of Low And Middle Rank Coals And Their Effects On Biogas Generation

It was an important theoretical basis for further understanding the formation process of coalbed biogas and innovating evaluation method of coalbed methane resources to clarify the effect and mechanism of coal on biogas generation. Therefore, by modern test technology and method, this paper selected lignite in Longkou coalfield and fat coal in Huainan coalfield as research objects, to study organic geochemistry of low and middle rank coals and then carry out the simulation experiment of biogas generation, on the basic of analyzing basic characteristics of coal quality. Through comparing and analyzing the characteristics of biogas production, rate and components, this paper studied the effect of organic geochemical characteristics of low and middle rank coals on the biogas generation.Longkou lignite had high water, high volatile, low ash and special low sulfur. Huainan fat coal had high volatile, low ash and sulfur. The micro components of coal were mainly vitrinite, and the content of structured vitrinite was the highest. Exinite mainly contained sporinite and bituminite. The main inorganic components were clay minerals and pyrite. The extraction rate of chloroform bitumen "A" from coal samples were above 2.0%, and they were good gas source rock. The organic matter abundance of Longkou lignite was higher than that of Huainan fat coal. Non-hydrocarbon content of chloroform asphalt "A" in Longkou lignite was the highest, and it showed that the maturity of organic matter was lower. The distribution characteristics of normal alkanes in saturated hydrocarbons were single peak type. The carbon number of main peak was C₃₁, and distribution range of carbon number was wide. Kerogen was rich in H and O, and had the lighter carbon isotope. It showed that organic matter type of coal samples of Longkou coalfield belonged to the sapropelic humic type. The asphalt content of chloroform bitumen "A" in Huainan fat coal was the highest, content of saturated hydrocarbon was the lowest, and it showed that the maturity of organic matter was higher. The distribution characteristics of normal alkanes in P1 and XJ were single peak type, and the carbon number of main peak were C₂₅. The distribution characteristics of normal alkanes in P3 and ZJ were double-peak type, and the front peak and post peak were C₂₅ and C₃₁. Kerogen was rich in C, had the heavier carbon isotope. It showed that the organic matter type of coal samples of Huainan coalfield belonged to the humic type.The results of simulation experiment of biogas generation within 90 days showed that process of biogas generation was divided into 3 stages: first stage was the rapidly rising period, and generated a lot of gas; the second stage was the steady decline period, and accumulation growthed slowly; third stage was gradually steady period, and less gas gengeration. However, there was a difference in the maximum gas generation rate of the low and middle rank coals. Gas generation rate of Longkou lignite reached the maximum at day 30, but Huainan fat coal reached the maximum at day 20. The gas components were mainly CH₄ and CO₂, and some of them contained a small amount of heavy hydrocarbons. The gas was dry gas, which was in accordance with the characteristics of biogas. CH₄ content showed an upward trend with time, while the content of CO₂ was the opposite, and it indicated that the metabolic type in the late stage of the experiment was mainly acetic acid fermentation, and CO₂ reduction was partly involved;Organic geochemical characteristics of high organic matter abundance, low organic matter maturity and mixed organic matter types were favorable to the generation of biogas, which was characterized by high gas generation rate and generating large amount of gas. The ash content in coal had an effect on the gas generation rate. The effect of pyrite and clay minerals was significant, and it can improve the biological activity and accelerate gas generation rate. The organic matter in coal had more small molecular side chain, and higher level of isomerization of organic molecules, which was conducive to the biological degradation.