Large-Scale Preparation Of The High-Temperature Methanation Catalyst And Its Trial Research Of Industrial Applications

There have long been the problems of resource shortage, rise in price and energy security for the natural gas supply in our country. The abundant coal resources in the western areas can be utilized to produce substitute natural gas with the heating value of 31.4 MJ/m3 by coal gasification, followed with gas clean up and methanation, and thus the produced gas can make up the gap of natural gas shortage in our country. Among the coal-to-natural gas technologies,methanation catalysts with high temperature resistance is the key that has been monopolized by foreign countries, herefore developing catalysts with proprietary intellectural property rights is imperative. This dissertation firstly obtained the suitable reaction conditions for methanation via thermodynamic calculation. The methanation catalysts with high Ni dispersion and high temperature resistance ability were prepared with microwave radiation and homogeneous precipitation methods. The as-prepared catalysts were compared with those made with conventional methods. The optimized catalysts were subjected to high-temperature lifetime test, and applied to the low-temperature heat exchanger coke oven gas methanation industrial test. The main conclusions are as follows:1. The thermodynamic equilibrium constants of the sixteen reactions involved in the methanation process were calculated using the Van't Hoff equation. The results indicate that the reaction temperature below 600 ? is optimum for CO methanation, which is relatively easier than CO2 methanation.The carbon deposition occurs and reduces in the order of CO>CO2>CH4, and the coke elimination follows the order of H2 > H2O> CO2. A detailed thermodynamic analysis empolying minimazation of the Gibbs free energy method demonstrates that low temperature, high pressure, and high H2/CO are favorable for methanation. Methanation is an exothermic reaction, and thus it can not be carried out at very low temperature in the industrial scale. Keeping the temperature between 200-600 ? is suitable. Insignificant effects on the feed gas conversion and product yields was observed when the pressure is above 3.0 MPa, and therefore 3.0 MPa pressure is suitable. To meet the standards of natural gas made from coal, H2/CO of 3 is appropriate. Adding steam to the feed gas improves the C02 yield, but steam addition consumes a large amount of energy, and therefore adjustments should be made in industrial applications according to the real conditions. Adding CH4 to the feed gas results in the reduction of H2 conversion and an obvious increase of C yield, so the CH4 amount should be well controlled. Adding CO2 to the feed gas results in the reduction of CH4 yield and a significant increase of C yield, so the C02 amount should be well controlled. When the industrial feed gas has the composition of H2:CO:CH4 equivalent to 3:1:1, the steam to CH4 ratio is 1:2 and the reaction temperature is 600?, the H2 and CO conversion and CH4 yield are not affected,and simultaneously the coke yield decreases to the minimum. Thus, the steam addition amount of 1/2 CH4 is appropriate.2. A series of catalysts defined as xNi-5Ce/?-Al2O3(x = 5, 10, 15, 20) were prepared using industrial Al2O3 microspheres as surpports through traditional impregnation and calcination treated by traditional and microwave heating,separately. The as-prepared catalysts are tested for the methanation of simulated coke oven gas in a fixed bed reactor to investigate the influence of temperature,pressure and space velocity. The results indicate that the conversion of 100% CO,C02, and 02, with part conversion of C2H6, was obtained on the 15Ni-Ce5/y-Al2O3 catalyst prepared by microwave heating following reaction conditions: 260 ?, 3.0 MPa and 20,000 mL·g-1·h-1 weight hourly space velocity. Analysis in light of BET, XRD, H2-TPR, H2 pulse, TPSR, XPS and TEM cooperatively manifest that the free state NiO poses a significant effect on the catalytic performace, and microwave energy absorption promotes the formation of a large amount of highly dispersed amorphous NiO, which has a weak interaction with the support. The discussion focuses on the relationships between catalyst structures and properties, and preparation mechanisms of microwave heating and traditional calcination.3. A series of Ni-Al2O3 catalysts were prepared by a single-step homogeneous co-precipitation method to study the effects of different Ni content (10-50 wt.%) and calcination temperature (350-750 ?) on the stability and selectivity of CO methanation. The 40NiAl-450-HC catalyst shows the optimal catalytic activity with 99 % CO conversion in the following operating conditions: T=220 ?, P=1.0 MPa, and GHSV=20,000 mL·g-1·h-1. Various characterization (BET, XRD, H2-TPR, H2 pulse, XPS, Raman and SEM) of the fresh catalysts indicate that the well- dispersed Ni species can be isolated by Al2O3, efficiently protecting Ni from aggregation and agglomeration. The high activity at low reaction temperature can be attributed to the larger specific surface area and active surface area. However, a higher calcination temperature gives rise to enhancing gradually the interactions between Ni and Al2O3 supports,finally leading to the catalyst deactivation. Compared with the catalyst prepared by the conventional co-precipitation and homogeneous co-precipitation methods,the catalytic performances have little difference at low temperature, whereas obvious distinctions can be observed when undertaking tests of high temperature resistance. At 300 ?, CO methanation higher than 99 % of 40NiAl-800-HC can be achieved. For 40NiAl-800-CC, CO conversion is only 60.1 % until 380 ?.In summary, different preparations form different structures, resulting in the differences of catalytic activity, stability and anti-coking ability. As illustrated in TPR results, the lower content of nickel-aluminate spinel, the better anti-coking performance.4. Based on the analysis above, TGJ-L and TGJ-H catalysts were prepared.After a long run evaluation test at high temperature, both CO conversion and CH4 yield keep stable. By comparing TGJ-H to GYC catalysts, CO conversion remain stable with a slightly decline of CH4 yield at 800 ?, 1.0 MPa and a space velocity of 10,000 mL·g-11·h-1 for 100 h. Characterizations via BET, XRD and HOT show that although undergone a decrease, especially for GYC catalyst,the BET surface area as well as active surface area of GYC catalyst are consistently larger than that of TGJ-H catalyst.5. Several kilograms of TGJ-L methanation catalysts made by microwave radiation method were applied to the low-temperature heat exchanger coke oven gas methanation industrial test under the conditions of recycling, non-recycling and carbon supplement respectively for 700 h, but the experiments ceased due to sulfur poisoning to the catalyst. The results show that the optimum concentration of COx in the feed gas is CO+C02<12 %. In addition, short technological process can reduce the equipment and pipeline investment, energy cost and has low recycling percentage. Most importantly, it can solve the problems of methanation catalyst deactivation due to coking and sintering under high temperature. Therefore, superior technologies can not only avoid temperature sudden increase, but also take into account the comprehensive utilization of energy. Large amount of coke formed for all the three technologies, but the coke mainly deposited on the outer surface of catalyst, thus not deactivating the catalyst. The main reason of catalyst deactivation is thiophene-S poisoning,because thiophene is adsorbed on the Ni surface, which are the adsorption sites for H2 and CO.