Catalyst Preparation And Process Simulation For Slurry Bed Methanation

Coal-to-synthetic nature gas (CTG) which mainly includes gasification, purification and methanation three steps, is an important way for the clean and efficient utilization of coal. The methanation is the key technology of the CTG. At present, the industrial methanation technology usually uses the fixed bed adiabatic reactor. However, the methanation is a strongly exothermic process. Some measures such as several reactors in series, a lot of heat exchanger and a large number of the recycle gas must be use to avoid the damage of the equipment and the sintering of catalysts caused by the high reaction temperature, which lead to the increasing of equipment investments and energy consumption cycle. The introduction of paraffins with high heat conductivity and high thermal capacity leads to the advantages of good heat transfer performance and bed isothermal in slurry bed reactor. So the introduction of slurry bed reactor in methanation technology not only can conquer the problems of more equipment, more cycle gas and catalyst easy sintering in fixed bed reactor, but also enable achieve the replacement of catalyst online. Therefore, the development of slurry bed methanation has important theoretical value and industrial significance.In this paper, the thermodynamic of methanation was firstly investigated, and got the law of thermodynamics. Secondly, the influence of dipping sequence, promoters and calcination temperature on the activity and selectivity of catalysts was studied, and the relationship between catalyst microstructure and performance was further investigated by the characterization of TG、XRD、 BET、H2-TPR、H2-TPD、CO-TPD、TEM and XPS. Thirdly, the macrokinetics of slurry bed methanation was studied, and the kinetic equation was obtained. Finally, the process of slurry bed methanation was simulated by Aspen Plus, and the results were compared with the industrial fixed bed methanation. The main results as follows:1. The results of thermodynamic for methanation showed that the conversion of CO, conversion of H2, selectivity of CH4 and yield of CH4 gradually reduced with the increasing of reaction temperature at the pressure of 0.1-4 MPa. While the selectivity of CO2 gradually increased, especially when the temperature exceeds 600 K, the tendency above was more obvious. At the same temperature conditions, the conversion of CO, conversion of H2, selectivity of CH4 and yield of CH4 gradually decreased with the decreasing of pressure, especially when the pressure was below 1.0 MPa, the tendency above became intensified. Considering of the effects of temperature and pressure on the thermodynamic equilibrium of each reaction in methanation, it can be known that the equilibrium conversion of CO, equilibrium conversion of H2, selectivity of CH4 and yield of CH4 are equal and higher than 99.7%,95.2%,96% and 90%, respectively. While the selectivity of CO2 is equal and lower than 2%.2. The influence of dipping sequence, promoters and calcinations temperature on the structure of catalyst and performance of slurry bed was studied. The results showed that the additive of Zr, Co, Ce and La excepting Mg could improve the NiO dispersion, decrease the Ni grain size and decrease the reduction temperature, in which the effect of La was most obvious and the performance of catalyst adding La was best. Comparing with the catalysts prepared by step impregnation, the catalysts prepared by co-impregnation possesses more active center, smaller Ni grain and optimum performance. With the increasing of calcination temperature, the surface area and the Ni dispersion of Ni-La/Al2O3 catalyst first increases and then decreases, while the Ni grain size first decreases and then decreases. The performance of catalysts calcined at 350 ℃ is best.3. The stability of 12Ni-4La/Al2O3 prepared by co-impregnation and calcined at 350 ℃ was studied using CO/H2=3:1 and lurgi as the feed gas, respectively. the results of which were further compared with the industrial PK-7R and GCC-low temperature catalyst. The results showed that CO conversion gradually decreased from initial 94.2% to 50% in stability evaluation of the period of 1563 h, and the average deactivation rate of which was only 0.028%/h. The further characterization of XRD and BET for fresh and used catalysts showed that the pore narrowing caused by coking and the growing of Ni grain is two main causes for catalysts deactivation. The stability evaluation of 12Ni-4La/Al2O3, industrial PK-7R and GCC-low temperature catalyst using lurgi gas showed that the CO conversion and selectivity of PK-7R catalyst was significantly lower than those of 12Ni-4La/Al2O3, especially the CH4 selectivity of PK-7R was only 55%, which much less than 80% of 12Ni-4La/Al2O3. Although the selectivity of CH4, CO2 and C2-4 of GCC-low temperature catalyst was slightly higher than those of 12Ni-4La/Al2O3, but the CO conversion of GCC-low temperature catalyst was less than 50% from first to last.4. According to Langmuir-Hinschelwood hyperbolic kinetics uniform adsorption model theory, the macroscopic kinetic model for hydrogenation of CO and CO2 methanation was obtained. Basing on the kinetic data, the model parameters were estimated using marquardt method. The results tested by mathematical statistical (F test) showed that the regression of model was obvious. The experimental data validation further showed that relative error of the reaction rate (rjc) from experiment and the reaction rate (rj) from model calculation is between -5.43%～8.56%, which means the experimental values agree well with the and the calculated values.5. The Aspen Plus simulation results of the methanation process with product gas size of 1.43 billion Nm3/a showed that the energy efficiency of slurry bed methanation using lurgi gas and H2/CO=3 gas were of up to 95.43% and 97.52%, respectively, which were better than 93.28% of fixed bed methanation. The total equipment number of the slurry bed methanation using lurgi gas and the slurry bed methanation using H2/CO=3 are 36 and 30 units, respectively, which is less of total equipment number for fixed bed methanation process. All of products from three process meet the first class of national nature gas standards. Because of the lower pressure drop of the two slurry bed methanation, the both products of slurry bed methanation using lurgi gas and H2/CO=3 gas meet the class A of high pressure urban nature gas. Because of the longer process and the higher pressure dorp in fixed bed methanation, the product gas only meet the class B of high pressure urban nature gas.