Interactions of Sulfur Dioxide with Coals: Implications for Oxy-coal Combustion Flue Gas Sequestration in Deep Coal Seams

The flue gas emitted from the oxy-coal combustion process, mainly including the concentrated CO2 with volume fraction above 95% and a minor quantity of SO2 and NOx, directly sequestrated in the coal reservoirs can not only mitigate CO2 emissions but also eliminate the cost of denitration and desulfurization for the flue gas. The fluid storage in the coal reservoirs is realized by the adsorption capability of the coal matrix. Therefore, the adsorption process of SO2 on the four coal samples was discussed in this work with the aim to provide basic knowledge about the oxy-coal combustion flue gas storage in the unmineable coal reservoirs. The potential mechanism existing during the interactions of SO2 with various rank coal samples was further elucidated. Research results show that the Freundlich isotherm model perfectly fits the adsorption equilibrium behaviors for SO2 on various rank coals. The kinetics process for SO2 adsorption on the coals can be described by the pseudo-second-order kinetics model. The dependence of SO2 adsorption capacity upon the moisture of coals relies on the coal rank. Furthermore, characterizations including the elemental analysis, the Fourier transform infrared spectroscopy analysis, and the X-ray photoelectron spectroscopy analysis reveal that the chemisorption occurs between SO2 molecules and all of the test coal samples. Specifically, the formations of sulfate and sulfone are the main mechanisms during the interactions of SO2 with various rank coal samples. The aforementioned chemisorption effect is beneficial to stably store SO2 in the unmineable coal reservoirs. In addition, the chemisorption effect between SO2 and the coals can also generate higher recovery of coalbed methane than the conventional CO2-enhanced coalbed methane process.