Absorption of HCl by limestone in hot flue gases. Part I: the effects of temperature, gas atmosphere and absorbent quality

In the present work, the objective was to elaborate the chemistry of limestone and HCl in hot flue gases, especially under conditions that are typical of fluidised bed combustors. The effects of temperature, absorbent quality and particle size as well as gas atmosphere on the absorption reactions of HCl have been investigated, as have the products of chlorination. In addition, special emphasis was placed on studying the effects of molten phase formation during limestone chlorination. Experiments were performed in a laboratory using a special thermogravimetric apparatus. In the experiments, the absorption of HCl at 850 °C was found to be strongly dependent on the humidity of the flue gas. The presence of water vapour depressed the attainable conversion at a specific reaction time. Higher conversions to calcium chloride were seen at lower temperatures for the same gas atmosphere in the experiments. This was also true in the presence of CO2, which led to the recarbonation and chlorination taking place simultaneously at 650 °C. At 850 °C, the chlorination of CaO was not affected by the presence of CO2. At 650 °C, the concurrent recarbonation of CaO led to higher conversions for chlorination than was achieved when CO2 was not present in the reacting gas. From the tested absorbent materials, the Nordkalk Kurevere dolomite gave the highest conversions. Furthermore, the smaller the absorbent particles, the higher the reactivities and conversions that were achieved at 650 °C. At 850 °C, the effect of particle size on the conversion of CaO was most likely interfered with by the formation of a molten product phase. This work on the absorption of HCl has shown the phenomenon to be very complex and affected by several factors that are not fully understood. Molten product phases were observed to form at both temperatures that were studied, which significantly complicated the interpretation of the experimental results.