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SUN ET AL .
time in multiple cycles. It can be seen that the LM shows much slower carbonation rate during the initial fast reaction stage, while it presents higher carbonation rate during the fol- lowing diffusion controlled stage compared with the lime- stone in the same cycle. The point of maximum carbonation rate in each carbon- ation process can be obtained from Figure 4B, according to which the X u can be confirmed from Figure 4A. Then, the carbonation curves of the LM and limestone during the chemical reaction controlled stage were fitted by Equation (3), as shown in Figure 5. The correlation coefficients of all the curves are in the range of 0.980-0.998, which means it is feasible to use this surface reaction-controlled kinetic model to describe the carbonation of the LM and limestone during the chemical reaction controlled stage. The kinetic parame- ters obtained by this model are shown in Table 2. The param- eter t crcs in Table 2 denotes the time duration of the chemical reaction controlled stage of the sorbents. The values of k and t crcs of the LM are much smaller compared with those of the limestone in the same cycle, as can be seen in Table 2. Taking the 1st cycle as an ex- ample, the values of k and t crcs of the LM are only 62.6% and 31.3% those of the limestone, which means that the LM shows much slower carbonation rate and much shorter time duration in the chemical reaction controlled stage. Thus, the X u that can be achieved in this stage for the LM, which is mainly determined by k and t crcs , is much smaller than that for the limestone. We also can see from Table 2 that the values of X u of the LM in different cycles are very small, with a value that no more than 0.03 after 15 cycles. Such a low CO 2 capture capacity of the LM in the chemical reac- tion controlled stage makes it not suitable to be used as CO 2 sorbent in calcium looping process. The main components of the LM and limestone are all CaCO 3 , and the corrected CaO contents that in the LM and limestone are almost the
same, as shown in Table 1. Maybe, the complex impurities in the LM or the specific microstructure characteristics of the calcined LM were the reasons why the LM showed such low carbonation capacity during the chemical reaction con- trolled stage.
3.2 | Effect of Cl on the carbonation kinetics of calcium-based sorbent
As shown in Table 1, the chemical components of the LM are much more complex compared with the limestone. Especially, the content of Cl in the LM is relatively high. The other impurities except Cl have been proved to be beneficial or not attributable to CO 2 capture capacity of the calcium- based sorbents. 21,22 The effect of Cl on CO 2 capture capacity of the calcium-based sorbent was detected by doping CaCl 2 into CaCO 3 through wet impregnation method. The carbona- tion conversion and carbonation rate of Cl-doped CaCO 3 with carbonation time during the first cycle are shown in Figure 6, and the fitting results of the carbonation process of Cl-doped CaCO 3 with different Cl/Ca molar ratio during the chemical reaction controlled stage are shown in Figure 7. The obtained kinetic parameters were presented in Table 3. We can see from Figures 6 and 7 and Table 3 that addition of Cl has adverse effect on the CO 2 capture performance of the calcium-based sorbent, especially the carbonation pro- cess during the chemical reaction controlled stage. The val- ues of k and t crcs of Cl-doped CaCO 3 decrease dramatically with increasing the Cl/Ca molar ratio. It indicates that the Cl-doped CaCO 3 shows much slower carbonation rate and shorter duration time during the chemical reaction controlled stage with increasing the additive amount of Cl in CaCO 3 . Therefore, smaller carbonation conversion in chemical reac- tion controlled stage is achieved. When the Cl/Ca molar ratio is larger than 2:100, the X u that can be achieved is very low. It has been proven that doping the calcium-based sorbent with small amount of chloride can improve the long-term CO 2 capture capacity, while larger doping leads to decrease in pore volume and gives marked reduction in capacity. 23,24 The similar result was obtained here. As is shown in Table 3, the pore volume and BET surface area decreased dramatically with the molar ratio of Cl/Ca when the value was higher than 0.25:100. The pore volume and surface area of the calcined sorbent decreased by 71.3% and 44.5% respectively when the molar ratio of Cl/Ca in the sorbent was increased from 0 to 2:100. After Cl doping, the CO 2 diffusion and carbon- ation reaction were limited during the carbonation process. The corresponding lower X u , k, and t crcs were achieved for Cl doped sorbent. The Cl/Ca molar ratio in the LM which can be calculated according to the XRF results shown in Table 1 is 2.6:100. Therefore, the relatively high content of Cl in the LM can be judged as one of the major reasons that lead to
FIGURE 5 Fitting results of the carbonation conversions of LM and limestone during the chemical reaction controlled stage in multiple cycles
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