In the fluidized incineration of municipal solids waste and other fuels containing significant amounts of chlorine, it is possible for limestone, present to capture sulphur, to react with HCl to produce CaCl2. This work considers the fate of CaCl2 as it circulates in a fluidized combustor encountering regions of high O2 and moisture content. Rather than attempting this in a complicated fluidized bed, a thermogravimetric analyzer (TGA) was used: the mass of samples of CaCl2 was measured as a function of time as the solid reacted with various gases. Calcium chloride was reacted with mixtures of gases containing combinations of 0, 3.2 or 11.6 vol% O2, 0, 5, 10 or 20 vol% H2O, 0, 1000 or 2000 vppm SO2 and 0, 1000, 3000 or 5000 vppm HCl. The balance was N2. The temperature range investigated was 680-900 °C. It was found that even in pure N2 there was a slow decrease in the mass of CaCl2, which could be attributed to evaporation. Using 3.2 or 11.6 vol% O2 in N2 resulted in a faster, yet still slow, mass change. In this case, the CaCl2 was being oxidized to CaO. The presence of water greatly increased this reaction such that with 10 vol% water at 900 °C, complete conversion to CaO (releasing HCl) could be achieved. With mixtures of water and HCl, however, the ratio of CaO to CaCl2 in the reacted particle varied with the concentration of water. At moisture levels typically found in combustion products, a fully reacted particle contained about 66 wt% CaO and 34 wt% CaCl2. This degree of conversion, however, took nearly 2 h. In short, limestone is not likely to act as a significant in situ capture method for HCl in a fluidized bed. CaCl2 that is formed will tend to release the HCl as the solid circulates in the fluidized bed.
2000. Vol. 55, no 24, 6129-6137 p.