Investigation On Ni-Mo-based Sulfur Resistant Catalyst For Methanation

The energy structure of rich coal, poor oil and deficient gas makes coal to synthetic natural gas(SNG) an effective way to exploit coal resource cleanly and also meets the demand for natural gas in China. Coal to SNG generally requires the following steps: Coal gasification/ pyrolysis, syngas conversion, desulfurization and decarbonization processing, syngas methanation, product separation. Ni-based catalyst is widely used in the methanation process due to its high catalytic activity and low price. However, it is very sensitive to the presence of H2 S in the feed gas and quite easy to be poisoned. Therefore, the composition of H2 S in the feed gas into the methanation reactor must be lower than 0.1- 0.02 ppm. In order to shorten the process of coal-to-SNG and reduce the operation cost, Ni-Mo-based catalysts were developed to improve the sulfur-resistance of methanation catalyst.Firstly, Ni/Al2O3 and Ni-Mo/Al2O3 were prepared by precipitation-impregnation method. The catalytic performance and sulfur-resistance of these two catalysts were investigated at 550 oC and SV of 120 L/(g·h) under atmospheric pressure, and the composition of feed gas was n(H2):n(CO):n(N2)=3:1:1. The results reveal that the addition of Mo decreased the initial catalytic activity(mainly the CO conversion), but the formation of the Ni-Mo alloy can obviously increased the sulfur-resistance of the Ni-based catalyst, and it should be noted that the catalytic stability of Ni-Mo/Al2O3 was much better than Ni/Al2O3. Characterizations of the samples before and after reaction indicate that the addition of Mo into the Ni-Al2O3 catalyst improved the dispersity of Ni on the surface and formed Ni-Mo alloy which would improve the sulfur sulfur-resistance and the stability of Ni-based catalyst.Considering the poor sulfur-resistance and CO conversion of Ni-Mo/Al2O3 catalyst, a series of 15Ni-5Mo catalysts with different supports(PB, NaY, USY, MCM-41, kaolin and Al2O3) were prepared. The methanation performance tests reveal the stability followed an order of: Ni-Mo/MCM-41> Ni-Mo/K aolin> Ni-Mo/USY> Ni-Mo/Al2O3> Ni-Mo/NaY> Ni-Mo/PB, and the activity of Ni-Mo/MCM-41 approached to that of a commercial catalyst(28Ni-5Mo/Al2O3). The characterizations indicate that the 15Ni-5Mo/MCM-41 catalyst showed the best dispersity and the smallest particle size of metallic Ni, so the interaction between Ni and Mo was stronger than other catalysts. Therfore, 15Ni-5Mo/MCM-41 exhibited a better sulfur-resistance and stability. Thus MCM-41 was applied as the support of Ni-Mo-based catalyst in the following experiments.Comparison of the catalytic performances of Ni-Mo/MCM-41 catalysts with different Ni/Mo ratios reveals that the increase of Mo in the catalyst improved the sulfur-resistance and stability. The CO conversion of the catalysts with 5% Mo decreased with time during methanation, and the stability of the catalyst with 6.5% Mo was better while it kept stable for the catalyst with 10% Mo. The characterizations indicate that more Mo could lead to better dispersity of Ni and form more Ni-Mo alloy, and then the catalysts showed better sulfur– resistance. The particle size of metallic Ni also decreased with the increase of Mo, and it is conducive to improve the stability of the catalyst. Therefore, it was found that 20Ni-10Mo/MCM-41 was the best catalyst.