Preparation Of Zn-Mn-Cu Based Sorbents Loaded On Semi-Coke By High-Pressure Impregnation And The Performance Of Removing H2S From Coal Gas At Mid-temperature

The dual gas coal-based poly-generation technology is now considered as one of the most promising clean coal techniques. By this technology, the syngas with optimum composition can be acquired by reforming the pyrolysis gas rich in CH4combining with gasification gas rich in CO2, and the step utilization of raw material and rational proportion of energy can be achieved by producing coke, alcohol ether fuel and power. The limitation for the content of H2S in syngas used to synthetize chemicals is very strict (below0.1ppmv) in this technology. To avoid the heat loss caused by heating and cooling gas, the removal of H2S from hot coal gas at mid-temperature has been selected as the preferred procedure of this technology for cleaning syngas. The preparation of sorbent and its desulfurization performance are the main concerns.On the basis of above analyses, the high-pressure impregnation method is proposed for the hydrothermal synthesis of the sorbent and the performance of prepared sorbent removing H2S from hot coal gas at mid-temperature is researched in this paper. Firstly, the support and active component precursors were screened out. Then, the optimal concentration of precursor solution and operation parameters of impregnation pressure, impregnation time, calcination temperature and calcination time were determined. The experiment results show that the high-pressure impregnation is an effective sorbent preparation method, and semi-coke (SC) and zinc nitrate are the suitable support and precursor, respectively. The high-pressure impregnation method presents an outstanding advantage of perfectly improving the pore structure of SC support at the same time of effectively impregnating the active component precursor on semi-coke support. The optimal Zn-based sorbent preparation conditions are shown as follows:35ml semi-coke support and20%Zn(NO3)2precursors solution were placed in an autoclave through high-pressure impregnation at20atm for5h, then the sample was filtered and dried at50℃for5h, dried at100℃for5h. Finally, the sample was calcined at500℃in pure N2for5h. The BET results show that by high-pressure impregnation, the specific surface area and pore volume of the raw semi-coke are expanded from16.65m2/g and0.01cm3/g to265.49m2/g and0.07cm3/g, respectively.Zinc oxide is recognized as high accuracy sorbent, but it is unstable at high temperature in the strong reducing atmosphere. While manganese oxide shows good desulfurization activity and excellent mechanical stability, and copper oxide exhibits excellent mechanical stability and good dispersion in the high temperature desulfurization process. So, an idea is designed that the desulfurization performance of Zn-based sorbent is modified by the join of manganese oxide and copper oxide, and the Zn-Mn-Cu-based sorbent with good desulfurization activity and mechanical stability is expected to get. Zinc nitrate, manganese nitrate, and copper nitrate aqueous solutions were used as precursors and the Z20M4C6SC sorbent was prepared at the optimal operation conditions by high-pressure impregnation. This sorbent possesses a good desulfurization performance and it can effectively remove H2S from500to below0.1ppmv (H2S removal efficiency is above99.98%) in a temperature range of300-550℃. It can maintain this high desulfurization precision of0.1ppmv for56h, with the maximal sulfur capacity of13.84%at500℃.The desulfurization performance of three metal Zn-Mn-Cu-based sorbent is obviously better than that of single metal sorbent, and its sulfur capacity is54.12%greater than the simple summation (8.98%) of three single metal oxide sorbents Z20SC, M4SC, and C6SC. The sulfur capacity (7.98%) of bi-metal Z20M4SC sorbent is nearly similar to the simple summation (8.02%) of Z20SC and M4SC. These results show that copper has a significant promoting function in the process of desulfurization of three metal Zn-Mn-Cu sorbent.XRD results show that CuO, ZnO, MnO2and ZnMnO3are the active components of Z20M4C6SC sorbent. The equilibrium constant of the reaction between ZnMnO3and H2S is the maximum (5.499×1018) and ZnMnO3in Z20M4C6SC sorbent plays an important role in the process of removing H2S from hot coal gas. The addition of copper in the Zn-Mn-sorbent promotes the formation of new phase ZnMnO3, and improves the dispersion of active components on support, thus enhances the reactivity of active components with H2S. Meanwhile, the addition of copper also enhances the mechanical strength of the sorbent and it is helpful for improving the sulfidation/regeneration performance.By the equivalent grain model, dynamic parameters of Z20SC, Z20M4SC and Z20M4C6SC were calculated respectively. It is suggested that the desulfurization reaction of the Zn-based sorbents is mainly controlled by the chemical reaction in the initial stage and later by the gas diffusion through the reacted layers. The chemical reaction activation energy Ea and diffusion activation energy Ep of three metal Z20M4C6SC sorbent (6.23and9.39kJ/mol) are lower than bi-metal Z20M4SC sorbent (22.89and33.87kJ/mol) and single metal Z20SC sorbent (18.10and51.21kJ/mol). These results show that the combined action of manganese and copper effectively reduces the desulfurization reaction active energy and improves the desulfurization activity of Zn-based sorbent.CO, H2and H2O are the main components of coal-based gas. The experiment results that the adaptability of Z20M4C6SC sorbent on ambient gas indicates that CO or H2independently existing in feeding gas clearly inhibites its desulfurization reaction. And this phenomenon is mainly caused by the reduction of active component ZnO and CuO, and decomposition of ZnMnO3in the reducing environment. But CO mixed in the gas containing H2, or H2mixed in the gas containing CO obviously weakens their inhibition degree for desulfurization performance. As one of the products of the desulfurization reaction, the effect of H2O on desulfurization reaction is negative, according to chemical reaction equilibrium principle. Obviously, the sulfur capacity of Z20M4C6SC (13.84%>9.46%>7.64%>6.57%>6.15%) in different ambient gas of H2/CO/N2/H2S, H2/CO/H2O/N2/H2S, H2O/N2/H2S, H2/N2/H2S and CO/N2/H2S is related to the diffusion activation energy Ep (9.39<13.44<13.96<14.31<18.10kJ/mol). And the diffusion activation energy Ep is higher than chemical reaction activation energy Ea. So, it is suggested that the diffusion through the reacted layers is very important for the desulfurization reaction of Zn-based sorbents and H2S.