| Hydrogen had attracted broad attention as a basic chemical material and environmental-friendly energy. Reactive sorption enhanced reforming (ReSER) is a promising hydrogen production technology which enhances steam methane reforming by in-suit CO2removal with the advantages of lower energy consumption, low cost and controllable CO2emission. However, the activity of the sorption complex catalyst used in ReSER process was declined after multiple carbonation-calcination cycles. Thus the studies of instability reason and stability improvement of sorption complex catalyst were very important.First, we selected three different carbonation-calcination cyclic conditions under carbonation atmosphere of100%N2,20%CO2-80%N2and20%CO2-80%steam to indentify the influence of carbonation-calcination cycles on the microstructure and adsorption performance of sorption complex catalyst.30carbonation-calcination cycles test between600℃and800℃were performed in a fixed bed, and the samples under different multiple cycles were obtained for further test. The test results showed that the temperature variation between600℃and800℃was not the casuse of micro-structure and adsorption performance decline, while the carbonation-calcination cycles between CaO and CO2is the main reason, moreover the presence of steam accelerated the decline rate. XRD results showed the reaction between CaO and Al2O3was taken place during the carbonation-calcination cycles, and the formation of Ca12Al14O33made the microstructure varied and adsorption performance declined. However, the formation of Ca12Al14O33could enhance the stability during the later cycles. A possible mechanism was proposed to explain it.According to the above researches, in order to improve the stability of the sorption complex catalyst, we calcined nanoCaO/Al2O3support at900℃to form stable nanoCaO/Cai2Al14O33in advance and then impregnated Ni to prepare Ni-nanoCaO/Ca12Al14O33, and further evaluate it in a fixed bed reactor. The stability evaluation results showed that Ni-nanoCaO/Ca12Al14O33could endure10stable hydrogen production cycles with maximum96.5%hydrogen concentration and maximum86.2%methane conversion. The characterization results of BET、XRD、H2-TPD、H2-TPR showed that the specific surface area and pore structure of Ni-nanoCaO/Ca12Al14O33were stable and no growth of Ni grain was found during the hydrogen production process. The stronger interaction between Ni and CaO/Ca12Al14O33made the Ni grain disperse in the support surface uniformly which could inhibit Ni grain sintering. These factors increased the stability of catalyst.Further study, we pretreated nano CaCO3by coating TiO2to increase the stability of CaO during the carbonation-calcination cycles. Nano CaCO3coated by TiO2, and mixed with aluminum sol then impregnated Ni solution to prepare Ni-TiO2@nanoCaO/Al2O3, and then evaluate it in a fixed bed reactor. The stability evaluation results showed that Ni-TiO2@nanoCaO/Al2O3could endure15stable hydrogen production cycles with maximum96%hydrogen concentration and maximum82%methane conversion. The characterization results showed that Ni-TiO2@nanoCaO/Al2O3had a higher specific surface area and a larger pore volume, after seven ReSER cycles the specific surface area remained between13and15m2·g-1.The coated TiO2could prevent the micro-structure variation and adsorption properties declined by inhibiting the reaction of CaO with Al2O3. The results also showed that there is no obvious Ni grain growth during the cycles. These factors made Ni-TiO2@nanoCaO/Al2O3have the higher stability.Compared unmodified catalyst which was only four stable hydrogen production cycles, both modified methods of sorption complex catalyst preparation could improve the stability of catalyst, in which, the TiO2-coating method was much better. The results showed that the stability of the complex sorption catalyst could be improved by enhancing the stability of sorption support. The result of the study was of great significance to the preparation of the industry-application sorption complex catalyst. |
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