MECHANISM OF SO2 REMOVAL BY CARBON

The reaction of SO2 with carbon (C) in the presence of O-2 and H2O inv olves a series of reactions that leads to the formation of sulfuric ac id as the final product. The rate-determining step in the overall proc ess is the oxidation of SO2 to SO3. Three SO2 oxidation reactions are possible. Adsorbed SO2 (C-SO2) can react either with gas phase O-2 or with adsorbed oxygen (C-O complex) to form sulfur trioxide (SO3), or g as phase SO2 can react directly with the C-O complex. In optimizing th e SO2 removal capabilities of carbon, most studies only assume a given mechanism for SO2 adsorption and conversion to H2SO4 to be operable. The appropriate SO2 oxidation step and role of the C-O complex in this mechanism remain to be determined. The ultimate goal of this study wa s to prepare activated char from Illinois coal with optimal properties for low-temperature (80-150 degrees C) removal of sulfur dioxide from coal combustion flue gas. The SO2 adsorption capacity of activated ch ar was found to be inversely proportional to the amount of oxygen adso rbed on its surface. A temperature-programmed desorption technique was developed to titrate those sites responsible for adsorption of SO2 an d conversion to H2SO4. On the basis of these results, a mechanism for SO2 removal by carbon was proposed. The derived rate expression showed SO2 adsorption to be dependent only on the fundamental rate constant and concentration of carbon atoms designated as free sites. Recent stu dies indicate a similar relationship exists between the rate of carbon gasification (in CO2 or H2O) and the number of reactive sites as dete rmined by transient kinetics experiments. Utilizing the concept of act ive or free sites, it was possible to produce a char from Illinois coa l having an SO2 adsorption capacity surpassing that of a commercial ca talytic activated carbon.