Reaction kinetics of limestone - SOx in a thermogravimetric analyzer and a fluidized bed combustor

Abstract

The effects of reaction temperature, $CO_2$ partial pressure, the properties of six different sorbents and sorbent size on the reactivities of calcination and sulfation of limestones have been determined in a thermogravimetric analyzer (TGA). The calcination reactivity of limestone under air atmosphere increases with increasing reaction temperature and its apparent activation energy is found to be 31,500kcal/kmol. The calcination rate decreases with an increase in $CO_2$ partial pressure in the ambient gas stream but increasee increases with increasing reaction temperature. The apparent thermal decomposition reaction order of limestone under $CO_2$ gas mixture atmosphere and the apparent sulfation reaction orders of precalcined limes-$SO_2$ are found to be close to unity. The apparent activation energies are found to be 17,000 kcal/kmol for sulfation of precalcined limes with $SO_2$ and 19,500kcal/kmol for direct sulfation. The initial sulfation reaction rate of precalcined limes increases with increasing temperature, whereas the sulfur capture capacity exhibits a maximum value at 900$^\circ$C. In the direct sulfation, the sulfation reactivity and sulfur capture capacity of limestone increase with increasing temperature and decreasing $CO_2$ partial pressure. From the XRD spectra, $CaCO_3$ in sulfated limestone under 10%-$CO_2$ gas stream at 850$^\circ$C decreases sharply with reaction time; CaO increases with time up to 20 min and thereafter decreases with time. Also, $CaSO_3$ slowly increases with increasing calcination and sulfation reaction time. The decomposition temperature of limestone has been correlated with $CO_2$ partial pressure as: $$T_{\mbox{doc}}=35\;\mbox{In}(P_g)+915$$. The effects of reaction temperature, $CO_2$ partial pressure, exposure time, and physical properties of three different sorbents on the pore structure of limestone in sulfation and calcination reactions have been determined. The surface area of CaO formed by rapid calcination in...