Study On Catalyst And Process For Syngas Methanation In Transport Bed Reactor

Methanation of syngas from coal for the production of substitute natural gas (SNG) is considered to be an efficient way for the use of low-rank coal resource in remote areas. This is also a national strategic clean coal technology for securing the country’s supply of natural gas for civil utilization. As the only commercialized methanation process, the multi-stage adiabatic fixed bed process is complicated and relatively difficult to control, while its used catalyst is easy to be deactivated at high reaction temperatures. Taking advantage of the fast surface reaction characteristics of syngas methanation and the high heat and mass transfer efficiency as well as high superficial gas velocity in transport bed, an advanced methanation process was proposed in this study by combining a transport bed reactor and a tail-end clean-up fixed bed reactor. The use of catalyst particles which have high thermal diffusivity and high heat carrying capacity as the main heat carrier in the new process can realize the efficient removal of exothermic heat and the effective control of reaction temperature. Meanwhile, the new process can also achieve a sufficient conversion of syngas and require much less catalyst. Two big challenges for the transport bed methanation process are the development of highly attrition-resistant methanation catalyst and the operation of this new technology because they have not been studied before. This study covers the following five major research contents.1. Process simulation of two-stage syngas methanation processes for SNG. Process simulation using Aspen Plus was conducted to identify the technical feasibility and optimal reactor combination for a simple two-stage methanation process producing SNG. With two adiabatic fixed bed reactors in series the reaction temperature control requires high product gas recycle ratio which is much energy-consuming. An isothermal fluidized bed combining a fixed bed was shown to be an efficient one-pass two-stage methanation technology in which the CO conversion and the product gas quality was higher than that from the two-stage fixed bed process. Laboratory tests showed also the technical superiority of syngas methanation in fluidized bed than in fixed bed over the same catalyst in terms of the realized activity and carbon-deposition resistance. Nonetheless, the low operating velocity of the bubbling fluidized bed makes it difficult to be scaled up for the high-capacity system producing SNG. Taking advantage of the fast surface reaction characteristics of syngas methanation and high heat carrying capacity of solid catalyst particles, the methanation process based on a transport bed combing a tail-end clean-up fixed bed can not only simplifies the methanation process but also reduces the reactor size and catalyst amount required.2. Attrition-resistant Ni-Mg/Al2O3 catalysts with different binders for fluidized bed syngas methanation. Spray granulation of catalyst precursor prepared by co-precipitation using different binders was employed to make attrition-resistant Ni-Mg/Al2O3 catalysts with suitable particle size distribution for fluidized bed methanation. In this study, the tested binders included alumina sol (AS), acidic silica sol (SS), alumina-modified silica sol (AM) and alkaline silica sol (CC). The obtained catalysts are denoted respectively as C-33AS, C-33SS, C-33AM and C-33CC, where 33 means the weight percentage of binder. By air-jet attrition test it was found that the attrition strength of the resulting catalysts followed an order of C-33SS> C-33AM> C-33AS> C-33CC. The silica binders obviously improved the attrition strength of the prepared catalysts and the attrition index of C-33SS was 2.98%/h. Characterization shows that the higher volume of pores above 20 nm, the less attrition resistance of the catalyst was. Syngas methanation over the catalysts in a fluidized bed clarified an activity order of C-33AS> C-33SS> C-33AM≈C-33CC at 623-923 K. The AS binder enabled highly dispersed metallic Ni and more surface active sites for methanation reactions, thus C-33AS catalyst showed the better catalytic performance but with a large attrition index of 7.64%/h. Continuous methanation tests for 20 h at 900 K and 2.5 MPa verified the stability of the catalysts using binders AS, SS and CC. Analyzing the spent catalysts via TPO demonstrated a high amount of inactive carbon on C-33AM to cause its deactivation in the 20-h test.3. Ni-Mg/Al2O3 catalysts with different silica sources for fluidized bed syngas methanation. In this study, the tested silica sources included acidic silica sol (C-33SS, C-10SS), sodium silicate (C-10NS) and tetraethyl orthosilicate (C-10TEOS). Air-jet attrition tests showed that the attrition strength of the resulting catalysts followed an order of C-10TEOS> C-33SS> C-10NS ＞＞ C-10SS. Characterizations showed that the porosity and skeletal structure have strong correlation with the catalyst attrition strength. Simultaneous hydrolysis of TEOS and co-precipitation made the C-10TEOS have dense and continuous skeletal structure to cause the high strength of its precursor, which thus improved the attrition resistance of the sprayed catalyst to have an attrition index of only 2.18%/h. Atmospheric syngas methanation over the catalysts at an SV of 600 NL·g-1·h-1 in a fixed bed reactor clarified an activity order of C-10TEOS>C-10NS≈C-33SS. The C-10TEOS catalyst exhibited high activity and stability under the tested harsh conditions and also good resistance to carbon formation and Ni sintering. Therefore, C-10TEOS catalyst showed the better performance than C-33SS catalyst.4. Influence of other preparation parameters on performance of Ni-Mg/Al2O3 catalyst. Attrition-resistant Ni-Mg/Al2O3 catalysts for fluidized bed syngas methanation were prepared with different amounts of TEOS (C-5TEOS, C-10TEOS, C-15TEOS and C-20TEOS) or NiO (C-10Ni, C-15Ni, C-20Ni and C-25Ni), and then calcined at different temperatures (CC-773, CC-873, CC-973 and CC-1073). This study is expected to optimize the parameters for preparing attrition-resistant methanation catalyst without much sacrifice of catalytic activity. For catalysts prepared with different amounts of TEOS, air-jet attrition tests showed that attrition strength of these catalysts followed an order of C-10TEOS> C-15TEOS> C-20TEOS> C-5TEOS, but their catalytic performance decreased with increasing the SiO2 content in the catalyst. There was a non-linear relationship between the NiO content and particle strength resisting attrition. The catalyst with 20 wt.% NiO had the highest attrition resistance and there is little change on catalyst attrition strength before and after reduction. The CO conversion increased greatly with increasing the NiO content until it was over 20 wt.%. Raising the calcination temperature increased the interaction between primary particles and thus raised the attrition resistance of the resulting catalyst. However, only those catalysts with moderate metal-support interaction, such as calcined at 873 K, showed high and stable activity of methanation. In conclusion, the catalyst granulated with 10 wt.% SiO2,20 wt.% NiO and calcined at 873 K showed good activity and stability and high attrition resistance (2.18%/h) for fast fluidized bed methanation.5. Reaction characteristics of attrition-resistant catalyst in a laboratory transport bed reactor. The particle circulation rate increased with increasing the aeration gas velocity and superficial gas velocity. Varying superficial gas velocity had the larger effect on particle circulation rate than changing aeration gas velocity. Raising aeration gas velocity increased CO conversion, but the gas residence time reduced with increasing the superficial gas velocity to lower the CO conversion. The catalyst inventory in the reactor had little influence either on particle circulation rate or on catalytic performance, provided the inventory is above 100 g. The realized catalytic performance greatly varied with temperature of recycled particles, and CO conversion obviously increased with increasing the temperature of recycled particles. Due to the high heat transfer and reaction efficiency, the transport bed can be operated at high superficial gas velocity, such as 4.6 m/s (653 K), and the realized CO conversion can achieve 86%, with a bed temperature gradient less than 10 K. The pressure drop in the reactor increased with increasing the superficial gas velocity and the temperature of recycled particles. A heat balance calculation showed that the exothermic heat of the reactions in the transport bed under atmospheric pressure was mainly carried by the catalyst particles.