An experimental and modeling study of the contribution of coal ash to SO₂ capture in fluidized bed combustion

The process of sulfur self-retention (SSR) occurs as a result of the reactions between the mineral matter in coal ash and the SO₂ evolved during coal combustion. Consequently, the emission of SO₂ may be significantly reduced. The results of experimental investigations and modeling of SSR is presented in this work. The transformations of sulfur forms during devolatilization are taken into account via a correlation for the amount of sulfur that remains in the char, after devolatilization. A novel approach has been applied for modeling SSR during char combustion, closely related to the grain model used for SO₂ retention by limestone as a sorbent. It is assumed that SSR is a result of the reaction between SO₂ and CaO in the form of uniformly distributed micro-grains in char. An unreacted shrinking core model is adopted for the reactions between the CaO micro-grains and SO₂. The comparison with the experimentally obtained values in a fluidized bed reactor and in a laboratory oven, using coals of different rank (fixed carbon over volatile matter ratio, C_fix/VM=0.75-7.40), content of sulfur forms (total 0.84-6.04%, organic 0.71-4.71%, pyritic 0-2.57%) and molar Ca/S ratio (0.34-3.17), has shown that the model can adequately predict the kinetics of the process, the levels of the obtained values of SSR efficiencies, as well as the influence of temperature, coal particle size and the surrounding conditions.