2 |
SUN ET AL .
lime mud was widely investigated to remove pollutants in different phase and as catalysts in chemical industry. Li et al 6 used lime mud to prepare heterogeneous base transesterifi- cation catalyst by doping with potassium fluoride, and up to 99% oil conversion can be achieved under the optimum conditions. Lime mud was also investigated to absorb SO 2 at fluidized bed conditions. 7 The result shows that lime mud exhibits much better SO 2 capture capacity compared with limestone due to the beneficial microstructure of the cal- cined lime mud for SO 2 removal. CO 2 emission from fos- sil fuel combustion has been identified as one of the major greenhouse gases and leads to global warming. Since fossil fuel fired power plants are the major sources of CO 2 emis- sion, how to capture the CO 2 from flue gas in power plant has drawn public attentions. 8-10 The reuse of the lime mud in environmentally friendly way and CO 2 capture from flue gas can be simultaneously realized if lime mud can be used as CO 2 sorbent in carbon capture system. Calcium looping pro- cess is widely considered as one of the most potential meth- ods that can achieve deep CO 2 emission reduction in view of its advantages including low-cost CO 2 sorbent 11 and proven CFB technology. 12 The CO 2 capture capacity of the calci- um-based sorbent can be further enhanced by synthesizing inert supported sorbent 13,14 and template-assisted synthesis approach. 15,16 Therefore, the cost of this technology can be further reduced. Our previous research has proposed lime mud as CO 2 sorbent in calcium looping process. 17 After prewash and prolonged carbonation treatment, the lime mud presented relatively higher CO 2 capture capacity compared with lime- stone. Ma et al 18 used lime mud as CaO precursor to synthe- size highly reactive calcium-based sorbent. The carbonation conversion of the obtained calcium-based sorbent can be higher than 38% after 50 cycles. The carbonation time, that is the time duration of the carbonation stage, was chosen in the range of 10-40 minutes when testing the CO 2 capture capac- ity of the sorbents. However, the lasting time that the sorbent can stay in the carbonator was only 1-5 minutes in calcium looping process. 19 Therefore, the carbonation conversion that the sorbent can obtain during the initial fast chemical reac- tion controlled stage, usually lasts for 1-3 minutes, is rela- tively significant to judge the CO 2 capture capacity of the sorbents. A surface reaction-controlled kinetic model with a Boltzmann equation, developed by Lan and Wu, 20 was proven to be more appropriate than the core shrinking model and the random core model to describe the carbonation re- action between CO 2 and CaO during the chemical reaction controlled stage. In this manuscript, the carbonation kinetics of the lime mud in the chemical reaction controlled stage was analyzed by this surface reaction-controlled kinetic model. The effects of prewash and prolonged carbonation treatment on the carbonation kinetics of the lime mud were also dis- cussed in detail.
FIGURE 1
XRD spectrum of lime mud and calcined lime mud
2 | EXPERIMENTAL 2.1 | Samples
The lime mud (LM) in this research was sampled from a paper mill located in Shandong province, China. The com- ponents of the LM, detected by XRD analysis, were CaCO 3 and a small amount of Ca(OH) 2 , as shown in Figure 1. All the CaCO 3 and Ca(OH) 2 in LM decomposed to CaO after calcination. A kind of limestone was employed as contrast sample. The X-ray fluorescence (XRF) results of the LM and limestone are shown in Table 1. CaCO 3 was doped with CaCl 2 by wet impregnation method to check the effect of Cl on carbonation performance of the LM, with the Cl/Ca molar ratio differed from 0.25:100 to 2:100. The detailed modification process was presented elsewhere. 17 In order to mitigate the adverse effect of Cl on the CO 2 capture capac- ity, the LM was prewashed to decrease the content of Cl. The prewash process was presented as follows: 100 g LM and 400 mL distilled water were firstly mixed in a beaker at ordinary temperature. After stirring for 2 hours, the mixture was filtered to remove the liquid. Then, the solid residue and 200 mL distilled water were mixed and stirred for 2 hours. After filtration, the mixture was dried in the oven at 120°C. Then, the solid residue obtained was called prewashed lime mud (PLM). The particle sizes of all the sorbents were below 0.125 mm. A prolonged carbonation process was proposed to improve the microstructure and enhance the CO 2 capture capacity of the calcined LM and PLM. The prolonged carbonation of the calcined sample was only performed in the 1st carbonation. Before used as CO 2 sorbent, the calcined PLM was firstly carbonated in 100% CO 2 for 3-12 hours at 700°C. Then, the PLM after prolonged carbonation treatment was sent for car- bonation kinetics test in the dual-fixed bed reactor and the thermogravimetric analyzer.
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