Experimental Study About The Effect Of Coal Moisture Content On Carbon Dioxide Displacing Coalbed Methane

Energy consumption structure in the world has undergone several great revolutions. Coal began to replace firewood form the 18 th and gradually became a dominant energy in the mid-19 th century. In the early 1960 s, an abundant supply of cheap petroleum and natural gas undermined the coal economy and then coal industry gradually faded. The research effort on coal also diminished, leaving coal research at a standstill. Then subsequent petroleum crises revived the interest in coal and reshaped the fuel structure; thanks to the soaring petroleum price, coal regained its position in the energy structure. In 1980 s, environmental protection delivered a shock to the coal industry in its development and research process, spurring coal experts around the world to be mindful of various hazardous substances generated in coal production, burning, and use, particularly the hazards caused by sulfur dioxide and carbon dioxide to the ecological environment; as a result, the research was accelerated on methane, a type of associated energy in coal seams recovered in the process of coal production. It is of great significance to use carbon dioxide to displace coal seam methane because it can not only confine the greenhouse gas geologically but can also improves the recovery concentration of methane which is a kind of clean energy.In this study, raw coal specimens of large sizes（100×100×200mm） with a moisture content of 0%, 2%, 4%, 6%, and 8% were used in the tests, which investigated CH₄ and CO₂ permeability behavior in coal and CO₂ displacing CH₄. The tests were carried out using self-designed MCQ-II test equipment to acquire data for analysis. The main contents in this paper are listed as follows:（1） CH₄ and CO₂ permeability tests were performed using 1# and 2# specimens with a moisture content of 0%, 2%, 4%, 6%, and 8% respectively under constant temperature,pressure and,osmolarity（31MPa, 25oC,6MPa）. Results show that specimens with a moisture content of 8% and 6% has similar permeability, so is specimens with a moisture content of 4% and 2% and specimens with a moisture content of 8% and 6%. This suggests the existence of a critical moisture content, above which the permeability changes by order of magnitude.（2） Three set of conditions were selected for experiment at 25 ℃, osmolality setting 4MPa,4.5MPa,5MPa at volume stress 22 MPa, 25 MPa, 28 MPa. Gas permeability measured bagins to increase and then decrease with the raise of volume press, the relationship is expeonential. At constant temperature 25 ℃ and volume stress 25 MPa conditions, the gas permeability were measured at osmolarity 4MPa, 4.5MPa, 5MPa and 6MPa corresponding. Results show gas permeability increases with the raise of osmolarity. At osmotic pressure 4MPa, the permeability of methane and carbon dioxide is minimum at respectively 0.020 md and 0.070463 md. When osmotic pressure is 6MPa, the permeability of methane and carbon dioxide is biggest at respectively 0.202548 md and 0.1md.（3）Under the same temperature and pressure conditions, replacement ratio at coal moisture content of 8% is slightly higher then that of moisture content of 6%, which is respectively 31 and 30.2. Coal without water could obtain a lowest replacement ratio at only 2.478, suggesting that replacement ratio increases with the raise of moisture content（4） In the process of CO₂ displacing CH₄, the volune percent and cumulative production of exporting CH₄ in the outlet is maximum when the coal sample is without water. At the end of displacement, the percent of CH₄ reduces to 20%, and the cumulative output of CH₄ is 6.34 L. With the increase of water content, the volume percentage of CH₄ and the cumulative production decreases gradually. The lowest points are obtained at water content of 8%. At the end of the displacement, the volume percentage of CH₄ and the cumulative production are 0.3% and 0.018 L respectively.（5） In the process of CO₂ displacing CH₄, with the increase of the moisture content, the amount of CO₂ storage in coal reduces. When the coal is dry without water, volume of CO₂ storage is 15.707 L. When the coal is with 2% water content, the volume is 1.441 L, with 4% water content, the volume is 0.936 L, and with 6% water content, the volume decreases to 0.634 L. When the moisture content is as high as 8%, the volume of CO₂ storage is only 0.558 L, and this is 3.5% of the volume in the conditiong of coal sample without water.