Effect of CO2 on high temperature chlorination behavior of calcined limestone in an innovated fixed-bed reactor

Abstract

This work aimed to elaborate on the influence of CO 2 on the reactivity of CaO with HCl under conditions similar to combustion environment. This study was conducted in an innovated fixed-bed reactor to investigate the influences of simulated CO 2 concentrations on the chemistry of CaO with HCl at varied temperatures. Tests of sole CaO carbonation and chlorination were performed in a thermogravimetric analyzer and fixed-bed reactor, respectively, to provide fundamental information for the CaO chlorination experiments in the presence of CO 2 . The HCl concentrations were continuously monitored in a Fourier Transform Infrared (FTIR) cell and the repeatability tests yielded good results. These results indicated that the presence of CO 2 was beneficial for the apparent reaction rates of CaO chlorination performed at 650 ˚C and 700 ˚C, compared with the case of only chlorination; nevertheless, this benefit accompanied a slight reduction of HCl capture capacity. The results were related to the CaO carbonation behavior. On the © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 other hand, further increased CO 2 concentrations had no effect on the extent of the chlorination reaction.

Removal of HCl in Flue Gases by Calcined Limestone at High Temperatures

“…Second, the time needed to achieve complete chlorination has also been greatly shortened, suggesting a faster reaction rate. Such a finding may not be surprising, because, in our previous study, the presence of CO 2 was found to promote the reaction rate of the chlorination process, particularly at a temperature of ∼650 °C . At this temperature, the CO 2 partial pressure used in this work is sufficient to achieve the carbonation of the calcined limestone, thus affecting the absorption behavior.…”

“…The calculation results of the HCl uptake ability of the calcined limestone for different gas atmospheres and temperatures, as a function of time, are shown in Figure , in which the ordinate expresses the cumulative uptake of HCl. As has previously been suggested to generally be the case for the temperature dependence, the HCl uptake ability of the calcined limestone decreases dramatically as the reaction temperature increases from 650 °C to 850 °C. − ,, The capture of HCl by calcined limestone is more difficult in the flue gas atmosphere at temperatures of 750 °C or above, except for the test at 650 °C.…”

“…In a word, this can be attributed to the greater diffusion resistance of the CaCl 2 layer. More specifically, the direct chlorination process is mainly determined by chemical reaction at temperatures above 750 °C; nevertheless, the chlorination process is almost governed by combination of the chemical reaction and the product layer diffusion at temperatures of 650–700 °C . The difference of diffusion limitation leads to the variation in the apparent reaction rate observed.…”

“…At the combustion temperatures, the main calcium compound that reacts with HCl is calcined limestone. Previous studies have led to the conclusion that the reaction temperature is a key parameter that determines the removal efficiency of HCl by calcined limestone at high temperature. − However, there is also a need to explore how other species in the flue gases affect the chlorination reactivity of the calcined limestone, because the information available in the literature is very limited. The flue gas composition is one of the crucial parameters that determine the high-temperature chlorination behavior of calcium-based compounds, apart from the temperature sensitivity.…”

“…Generally speaking, it would appear that the capture of HCl by CaO is more difficult in the flue gas atmosphere. − At elevated temperatures, calcium-based solids can simultaneously react with acidic gases, and hence there probably exists an interaction or competing mechanism between the co-capture of acidic gases. Carbonation is one such competing reaction that is a result of the presence of CO 2 in the gas atmosphere. ,, …”