Absorption of HCl by limestone in hot flue gases. Part III: simultaneous absorption with SO2

In this work, the simultaneous absorption of HCl and SO2 with limestone was studied using thermogravimetry. The objective was to investigate the effects of the temperature and gas atmosphere on the absorption results. Special emphasis was placed on elucidating the formation of the molten phases while these gases were being absorbed. Partially reacted absorbent particles were examined using scanning electron microscopy and energy dispersive spectrometry to study the reaction patterns inside them. This research showed that the chlorination of both calcined and uncalcined limestone was considerably suppressed when simultaneous sulphation occurred. Furthermore, the experimental results indicated that limestone sulphation was significantly enhanced by the presence of HCl. The results for both the suppressed chlorination and enhanced sulphation were similar under all experimental conditions, i.e. for all the temperatures and gas atmospheres studied. Molten phases were observed to have been formed in the samples at both 850 and 650 °C, and our thermodynamic modelling results for the CaCO3–CaSO4–CaCl2 system indicated that the system has a eutectic point at approximately 580 °C. The formation of molten product phases could explain the enhancement seen in SO2 absorption, although it cannot explain the suppressed chlorination during the simultaneous absorption of HCl and SO2. The results of this study suggested that the calcium chloride initially formed is consumed in a subsequent reaction with SO2.