Abstract
CO2 sequestration in coal is mainly attributed to adsorption. The adsorption experiments of CO2 were conducted at injection pressures ranging from 1 to 3 MPa on coal samples with five kinds of particle sizes. The fitting degree of four classical adsorption models to experimental adsorption data was systematically compared. The adsorption properties of CO2 were comprehensively discussed. The temperature changes of coal samples at different positions during CO2 adsorption were measured by using the improved adsorption tank, and then the energy conversion law was obtained. The results showed increasing gas injection pressure can effectively increase the adsorption capacity of CO2 on coal samples. The BET equation had the best fitting accuracy for CO2 adsorption on various size coal samples. There was a significant exothermic effect during CO2 adsorption and storage. With the rise of injection pressure, the peak value of the rising temperature of coal samples increased, but the change rate decreased. The maximum temperature rise of coal samples was up to 13.6 °C at 3 MPa, which should be of great concern for the prevention of coal spontaneous combustion. During the sequestration process of CO2, the adsorption resulted in a decrease in coal surface free energy and then partial conversion to heat, leading to the rise of coal temperature. In addition, the CO2 adsorption on the pore surface caused the expansion and deformation of coal.
Highlights
With the increasing consumption of fossil fuel energy, CO2 emissions continue to rise, which poses a serious threat to the global climatic and ecological environment [1,2,3]
The adsorption capacity of the same size coal samples is positively related to the gas injection pressure
When the adsorption occurs in a coal sample with smaller particle size, along with enlarging the contact area between the unit mass coal sample and CO2 molecule, the adsorption probability increases, leading to the increase in adsorption capacity
Summary
With the increasing consumption of fossil fuel energy, CO2 emissions continue to rise, which poses a serious threat to the global climatic and ecological environment [1,2,3]. Based on the strong adsorption capacity of CO2 in coal, injecting CO2 into coal seam can realize the dual purposes of geological storage of CO2 and displacement mining of coalbed methane [7,8,9]. The adsorption capacity of CO2 in moisture coal is 11.1% lower than that in dry coal on average [14]. Likar and Tajnik, using the gravimetric method, concluded that the adsorption capacity of lignite increased with the increase in pressure and up to 14 m3/ton at 23 ◦C and 4 MPa [15]. The adsorptive capacity and storage rate of CO2 in coal decrease with the increase in temperature [18]. The temperature significantly impacts coal permeability and further affects the adsorption capacity of the different rank coal samples [19]. Hao et al indicated that the equilibrium adsorption capacity of CO2 in coal is logarithmic to pressure and proposed a prediction model of CO2 adsorption capacity, whose predictions are slightly different from the measured data [20]
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