Mechanism of SO2 Removal by Carbon

The reaction of SO 2 with carbon (C) in the presence of O 2 and H 2 O involves a series of reactions that leads to the formation of sulfuric acid as the final product. The rate-determining step in the overall process is the oxidation of SO 2 to SO 3 . Three SO 2 oxidation reactions are possible. Adsorbed SO 2 (C−SO 2 ) can react either with gas phase O 2 or with adsorbed oxygen (C−O complex) to form sulfur trioxide (SO 3 ), or gas phase SO 2 can react directly with the C−O complex. In optimizing the SO 2 removal capabilities of carbon, most studies only assume a given mechanism for SO 2 adsorption and conversion to H 2 SO 4 to be operable. The appropriate SO 2 oxidation step and role of the C−O complex in this mechanism remain to be determined. The ultimate goal of this study was to prepare activated char from Illinois coal with optimal properties for low-temperature (80−150 °C) removal of sulfur dioxide from coal combustion flue gas. The SO 2 adsorption capacity of activated char was found to be inversely proportional to the amount of oxygen adsorbed on its surface. A temperature-programmed desorption technique was developed to titrate those sites responsible for adsorption of SO 2 and conversion to H 2 SO 4 . On the basis of these results, a mechanism for SO 2 removal by carbon was proposed. The derived rate expression showed SO 2 adsorption to be dependent only on the fundamental rate constant and concentration of carbon atoms designated as free sites. Recent studies indicate a similar relationship exists between the rate of carbon gasification (in CO 2 or H 2 O) and the number of reactive sites as determined by transient kinetics experiments. Utilizing the concept of active or free sites, it was possible to produce a char from Illinois coal having an SO 2 adsorption capacity surpassing that of a commercial catalytic activated carbon.