Thermodynamic Analysis And Catalytic Performance For Methanation Reaction Coupled Water-Gas-Shift Reaction Over Ni-Based Catalyst

Coal-to-SNG contains coal gasification, water-gas-shift, purification and methanation process. Among which, the CO methanation is the key technology. Theorily, the CO methanation(CO + 3H2 â†' CH4 + H2O) reaction prefers syngas with the H2/CO ratio of 3. However, the syngas from coal gasification has a low H2/CO ratio, ususlly not more than 1. Therefore, the water-gas-shift(CO + H2 O â†' CO2 + H2) is necessry to control the ratio. Combining CO methanation with water-gas-shift(WGS) reaction, 2CO + 2H2 â†' CH4 + CO2, is a another way to sythesis natural gas. This coupled reaction can significantly decrease the H2/CO ratio to 1 and diminish the damage of water steam on methanation catalyst. However, at present, there is little studies about the thermodynamics calculation of coupled reaction and catalytic performance. under the coupled conditions.In this paper, the thermodynamic properties of the copled reaction were calculated using HSC6.0 software and reaction conditions were optimized firstly. Secondly, the NiO-ZrO2 nano-particles were prepared by itric acid aided sol-gel method and the coated catalysts of Ni O@SiO2 and NiO-ZrO2@SiO2 were prepared by St?ber method. Moreover, the effect of thickness of SiO2 layer on catalytic coupling reaction was studied. With characterization technologies of N2-adsorption, XRD, H2-TPR, H2-chemsorption, XPS, TG and TEM, the relationship between structure and acticity of catalysts was discussed and the deactivation reasons of catalyst were also analyzed. The results obtained are as follows:(1) The Gibbs free energy minimization method was used for the thermodynamic calculation of the coupled reaction. It was shown that lower temperature(200–500 °C), higher pressure(1–5 MPa) and H2/CO ratio(at least 1) promote CO conversion and CH4 yield and decrease carbon yield. Water steam introduced into the feed gas accelerates CO2 selectivity and decreases carbon yield. Extra methane elevates CH4 selectivity, but leads to more solid carbon produced at higher temperatures. A trace amount of CO2 and O2 reduce CH4 selectivity and carbon yield. The added C2H4 are prone to cracking reaction leading to the generation of more carbon.The comparation between experimental results with the calculations indicates that the gibbs free energy minimization method is effective for thermodynamic analysis.(2) The characterize results of NiO-ZrO2 catalyst and NiO@SiO2 and NiO-ZrO2@SiO2 catalysts demonstrate that the support of ZrO2 promotes the dispersion of active Ni species.The coated SiO2 layer enhances the interaction between NiO and ZrO2 and inhibits the agglomeration of active Ni species. Coating SiO2 layer on NiO or NiO-ZrO2 nanoparticles could increase reduction temperature of NiO species. Under the coupling reaction conditions that the H2/CO is 1, pressure is 1 MPa, space vesolity is 30,000 mL g-1·h-1, reaction temperature is fistly 400 oC, then heat up to 550 oC, and then decreaesd to 400 oC, the coated catalyst NiO-ZrO2@SiO2 had the best catalytic activity and stability. The apparent sintering occuerd on NiO-ZrO2 catalyst after reaction. The metal Ni crystal increased from 13.0 nm to 32.7 nm, and the amount of deposited carbon is 63.4%; NiO-ZrO2@SiO2 catalyst does not appear obvious sintering. The metal Ni crystal increased from 7.6 nm to 12.0 nm, and the amount of deposited carbon is only 53.1%.(3)A series of coated catalysts of NiO-ZrO2@SiO2 catalysts were prepared by St?ber method with a uniform silica shell thickness of 9 nm, 12 nm and 26 nm. The results of catalystic couped reaction show that the under the conditions that the H2/CO is 1, pressure is 1 MPa, space vesolity is 30,000 mL g-1·h-1, reaction temperature is fistly 300 oC, then heat up to 600 oC, and then decreaesd to 300 oC, the coated catalyst Ni O-ZrO2@SiO2 with silica shell thickness of 12 nm had the best catalytic activity and stability. The Ni-baesd catalyst with silica shell thickness of 26 nm is better. The Ni-baesd catalyst with silica shell thickness of 9 nm is the lowest. Characterization results shown that the sintering of Ni crystal is most serious for the catalyst with silica shell thickness of 9 nm. The metal Ni crystal increased from 15.3 nm to 25.1 nm, and the amount of deposited carbon is only 68.2%. When the silica shell thickness of 12 nm. The metal Ni crystal increased from 12.3 nm to 18.4 nm, and the amount of deposited carbon is only 51.7%.