8 |
SUN ET AL .
3.4 | Effect of prolonged carbonation treatment on the carbonation kinetics of the PLM We can see from the above discussion that prewash treat- ment process dramatically enhances the CO 2 capture capac- ity of the LM during the chemical reaction controlled stage. However, the CO 2 capture capacity of the PLM is still lower than that of the limestone during the chemical reaction con- trolled stage. One of the major problems for the LM is that the pores of the calcined LM distributed in 10-100 nm were very less. Here, we proposed a prolonged carbonation pro- cess to improve the microstructure of the calcined PLM, in the hope of further enhancing the CO 2 capture capacity of the PLM. The CO 2 capture performance of the PLM after prolonged carbonation treatment is shown in Figure 11. As shown in Figure 11A, prolonged carbonation treatment suc- cessfully improves the carbonation conversion of the PLM in multiple cycles. The CO 2 capture capacity of the PLM increases with increasing the prolonged carbonation time when the prolonged carbonation time is shorter than 9 hours. Further extending the prolonged carbonation time has lit- tle effect on the CO 2 capture of the PLM when longer than 9 hours. It seems that 9 hours is the optimum prolonged car- bonation time for the PLM. The carbonation conversion and carbonation rate of the PLM after 9 hours prolonged carbona- tion in different cycles is shown in Figure 11B. The fitting results of the carbonation process of the PLM after 9 hours prolonged carbonation treatment during the chemical reac- tion controlled stage are shown in Figure 12 and the obtained kinetic parameters were presented in Table 2. As is expected, prolonged carbonation treatment further enhances the carbonation performance of the PLM during the chemical reaction controlled stage. The values of k and t crcs of the PLM are increased by 51.5% and 6.3% after prolonged
carbonation treatment in the 1st cycle, and the value of X u is finally increased by 51.7%. The value of X u of the PLM after prolonged carbonation treatment is still higher than that of the PLM in the 15th cycle. It means that prolonged carbon- ation treatment further enhances the carbonation performance of the PLM during the chemical reaction controlled stage. Though still lower compared with that of the limestone, the CO 2 capture capacity of the LM during the chemical reaction controlled stage is effectively enhanced by prewash treatment and subsequent prolonged carbonation treatment. The PLM after prolonged carbonation treatment shows higher car- bonation rate but shorter duration time during the chemical reaction controlled stage compared with the limestone. The final values of X u for the PLM after prolonged carbonation treatment are approximate to those of the limestone during multiple cycles, which means that the LM can be effectively utilized as an alternative of the limestone to capture CO 2 in calcium looping process after prewash and following pro- longed carbonation treatment process. Table 4 showed the pore distribution parameters of the limestone, LM, PLM before and after prolonged carbon- ation treatment in the 1st calcination. Figure 13A,B showed the pore volume distributions and cumulative pore volumes of the calcined limestone, LM, PLM before and after pro- longed carbonation treatment in the 1st cycle. The BET sur- face area of the calcined LM is even larger than that of the calcined limestone, which means that the calcined LM can afford more places for the carbonation reaction. It seems that the LM should present higher carbonation conversion com- pared with limestone. However, the pore volume distributes in 1-100 nm ( V 1-100nm ) of the calcined LM is less than that of the calcined limestone. Furthermore, the pore volume of calcined LM is mainly distributed in the range of 1-10 nm, as shown in Figure 13B.The value of V 10-100nm of the cal- cined LM, which has been proved to be significant for the
(a)
(b)
FIGURE 11
The CO 2 capture performance of the PLM after prolonged carbonation treatment during multiple cycles. A, X N with cycle
number and B, X N and r N with carbonation time
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