Sulfur Capture and Oxidation of Calcium Sulfide in PFBC

Doctoral thesis, 2000

Sorbents such as limestone are successfully used for removal of SO2 during pressurized fluidized bed combustion (PFBC) processes. However, there still remains a strong incentive to reduce the consumption of such sorbents. In this study, sulfur capture by limestone and also the kinetics for the direct sulfation of limestone were examined under PFBC conditions. The findings of a detailed investigation of the separate effects of total pressure and the partial pressure of SO2 on direct sulfation are presented. The degree of sulfation was shown to decrease with rising total pressure when the partial pressure of SO2 was kept constant. A mathematical model has been suggested in order to describe the kinetic behavior of direct sulfation. The effective diffusivity of SO2 through the product layer of CaSO4 was determined using this model. It varied noticeably as the reaction proceeded and also in conjunction with changes in total pressure, SO2 partial pressure and temperature. Predictions based on the model agree well with experimental data. Further, the present work investigates the influence of periodically changing partial pressure of CO2 on the sulfation of limestone and the free lime content in residual products in PFBC. The degree of sulfation was found to be considerably higher with periodically changing partial pressure of CO2 than with a steady high partial pressure of CO2. The free lime content in the products proved remarkably different for various alternating conditions. The removal of sulfur using limestone during coal gasification generates hazardous calcium sulfide. The calcium sulfide must be quantitatively eliminated via oxidation or regeneration before disposal. The effects of total pressure, O2 partial pressure and temperature were examined under PFBC conditions to determine their influence on the oxidation behavior of CaS. The total pressure was shown to have only a relatively weak influence on the degree of conversion to sulfate, in spite of the fact that the conversion of CaS to CaSO4 increased to a certain extent with rising pressure at a constant oxygen volume fraction. However, over the 0.1 - 2.0 MPa range, temperature had a strong influence on the CaS oxidation, its effect being more pronounced at lower pressures. Also, the reaction of CaS with CO2 was found to occur above 550°C. For the oxidation of CaS at atmospheric pressure, the level of conversion of CaS to CaSO4 reached a maximum at approximately 920°C. Moreover, an entirely new process of regeneration of sulfided limestone particles has been proposed, by which CaS is regenerated effectively to CaO under alternating oxidizing and inert conditions. When the overall regenerative process is complete, more than 90% of CaS in the sample is converted to CaO.