Study On Characteristics Of Adsorption/desorption And Diffusion For CO₂, CH₄, N₂ And Their Mixtures In Coal

CO2 or flue gas enhancing coalbed methane recovery (CO2/N2-ECBM) technology can improve the recovery lof coalbed methane and sequestrate CO2 at the same time when injection gas into the deep coal seam, and it is of great to both environment and economy. The process involved in the CO2/N2-ECBM includes gas adsorption, desorption and diffusion in coal seam, which has an important effect on CH4 displacement efficiency and CO2 safe storage. Consequently, the measurement and model of adsorbed amount and diffusion in gas-coal systems are crucial for CO2/N2-ECBM.In this paper, some measurements have been done on the adsorption, desorption and diffusion characteristics of CO2, CH4, N2 and their mixtures using the high pressure volumetric analyzer (HPVA) and the magnetic suspension balance (MSB). Also the prediction model of adsorbed amount and diffusion model were investigated. These studies provide data support and theoretic basis for CBM resource, CO2 sequestration and reservoir simulation.Crushed and block coal from the same coal rank were chosen to perform the adsorption/desorption experiments at 294-353 K and pressures up to 18 MPa. The static and dynamic adsorption/desorption data of CO2, CH4, N2 and their mixtures were established at wide temperature and pressure. Uncertainties of excess adsorption capacity were estimated based on the theory of multivariate error propagation. The adsorbed/desorbed amount versus temperature, pressure and feed gas composition were analyzed systematically. It’s found that the changed trend of CO2 and CH4 adsorption isotherms in crushed coal were similar to the results of block coal, and the adsorbed amount of gas on crushed coal was larger than that on block coal in the experimental temperature and pressure range. The results showed that it is more favorable to CH4 recovery and CO2 storage when the temperature is higher 311 K and the pressure is less than 10 MPa. The desorption hysteresis for multi-componet showed the residual gas was stored by micropore close due to the gas adsorption on coal.On the basis of the static adsorption data, the adsorbed amount of pure gases were predicted using Langmuir and DRp equation, and results within the scope of the uncertainty in the experiment basically. Fluid-solid interaction parameter εis/k in the Ono-Kondo lattice model was improved as functions of temperature and pressure, which improved the accuracy significantly compared with Langmuir, DRp and 3OK model. The improved 3OK model could extend from pure to multi-component gases successfully, and predict the adsorbed amount of each component with greater accuracy than EL, DY and E3OK model, which showed strong extension.The dynamic adsorption data of pure and multi-component gases were obtained over a wide temperature, pressure and feed gas compositions. On the basis of these data, the diffusion model was extended to multi-componet gases to calculate the effective diffusion coefficient. It’s found that CO2 effective diffusion coefficient in crushed coal showed decrease with the increasing pressure due to the swelling of coal matrix, but CH4 and N2 effective diffusion coefficient showed an increasing trend. The effective diffusion coefficient of CH4/CO2, CH4/N2 and CH4/CO2/N2 at high pressure increased with the increasing temperature and pressure. The effect of feed gas compositions were not remarkable.