Experimental Study on the Effects of Different Heating Rates on Coalbed Methane Desorption and an Analysis of Desorption Kinetics.

Heat injection is an effective way to enhance coalbed methane (CBM) extraction. However, at present, the best way to inject that heat is not clear. To determine how the heating rate affects methane desorption, desorption tests at constant high (95 °C) and low (20 °C) temperatures and at three different heating rates (0.3, 0.6, and 0.9 °C/min to 95 °C) were conducted. The desorption content (the volume of gas desorbed per mass of coal) and the desorption rate under the constant 95 °C temperature were greater than those under the constant 20 °C temperature. For the heating rate tests, the total desorption contents under heating rates of 0.3, 0.6, and 0.9 °C/min were 1.322 times, 1.115 times, and 1.095 times that from the constant 95 °C temperature tests, respectively. The final desorption contents from the entire desorption process under heating rates of 0.3, 0.6, and 0.9 °C/min were 1.42 times, 1.30 times, and 1.20 times that from the constant 95 °C temperature tests, respectively. In the early parts of the heating stages, the desorption rates under the three heating rate tests were lower than those under the constant 95 °C temperature tests. When the heating stages ended, the desorption rates under the three heating rates were greater than those under the constant 95 °C temperature tests. After the heating ended, the desorption rates decreased rapidly. A higher heating rate was correlated with a faster decrease in the desorption rate. Kinetic analysis showed that heating coal to a high temperature before methane is desorbed did not suppress the diffusion coefficient decrease. Heating during desorption prevented the diffusion coefficient decrease. A lower heating rate is correlated with a slower diffusion coefficient decrease. Low heating rates were more effective for desorbing methane. The heat injection in the later stage of desorption had a more significant effect on promoting methane desorption than did the early desorption stage heat injection. An equation for calculating the optimal heat injection rate was proposed. These findings will offer significant references for the selection of a suitable way to inject heat to enhance CBM extraction.