Mechanism of SO₂ removal by carbon

The reaction of SO₂ with carbon (C) in the presence of O₂ and H₂O 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 SO₂ to SO₃. Three SO₂ oxidation reactions are possible. Adsorbed SO₂ (C−SO₂) can react either with gas phase O₂ or with adsorbed oxygen (C−O complex) to form sulfur trioxide (SO₃), or gas phase SO₂ can react directly with the C−O complex. In optimizing the 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 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 SO₂ 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 SO₂ and 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 studies indicate a similar relationship exists between the rate of carbon gasification (in CO₂ or H₂O) 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 SO₂ adsorption capacity surpassing that of a commercial catalytic activated carbon.