Effect of Coal Combustion on the Reactivity of a CaO-Based Sorbent for CO2 Capture

CaO is usually decomposed from limestone to capture CO2. CaO regeneration (limestone decomposition) is an endothermic reaction and needs a lot of heat, which could be from coal combustion through mixing the coal particles and limestone particles. However, the effect of coal combustion on the CaO sorbent reactivity for CO2 capture needs to be clarified. In this study, the reactivity of the sorbent decomposed from the limestone/coal mixture under various conditions was investigated using thermogravimetric analysis. The results show that the sorbent reactivity decreases with carbonation-calcination cycles because of sorbent sintering under a high temperature. Although coal combustion could supply heat for limestone decomposition, it is also found that sorbent reactivity decreases after adding the coal particles to the limestone particles. Evidently, coal ash remains in the CaO sorbent after coal combustion, but the results show that coal ash does not react with CaO or CaCO3 in a fixed-bed reactor. Coal ash has little influence on the sorbent reactivity. Also, most coal sulfur released out, and little reacted with the CaO sorbent. The main explanation for the reactivity decrease after adding the coal particles to limestone particles may be the combustion of fixed carbon and volatile matter in the coal particle, which results in a high local particle temperature. Therefore, the CaO sorbent sintering occurs during coal combustion. CO2 capture capacities decrease in both the chemical- and diffusion-controlled stages in the carbonation reaction after coal addition. Fortunately, steam activation could enhance the sorbent reactivity, even in the presence of coal combustion.