As the energy paradigm has been changed, research on renewable energy production have been actively conducted. In particular, carbon dioxide (a greenhouse gas) is a rising issue causing global warming. Therefore, the development of CO2 capture and utilization technologies to reduce carbon dioxide emissions is required. Methane production by hydrogenation of carbon dioxide is a technology that can convert carbon dioxide into methane, liquefy it, and store it as high-density energy. This can be expected to be an approach utilizing greenhouse gases.
The methanation reaction of carbon dioxide causes a temperature increase in the catalytic reaction process due to a high exothermic reaction. As consequence, the yield of methane production can be reduced due to a decrease in the equilibrium conversion rate. Therefore, research has been conducted to apply a fluidized bed reactor as an approach to maintain the temperature in the methanation reactor at isothermal conditions. In order to utilize the fluidized bed reaction process, the high performances of catalyst''s reaction activity, durability, and attrition resistance must be needed at the same time. Therefore, a high-efficiency catalyst design, manufacturing and molding technology are required for the application to a fluidized bed.
In this study, the basic design conditions of a catalyst for methane synthesis by hydrogenation of carbon dioxide for application to a fluidized bed reaction process were investigated. The generation of reaction heat during the methanation reaction causes a decrease in process efficiency due to an increase in the catalyst bed temperature. In order to control this problem, a fluidized bed reaction process was employed.
To find the optimal conditions for catalyst activity and sttrition resistance, the experiments changing the content of nickel as an active material and the calcination temperature were conducted. It was confirmed that the higher the nickel content, the higher the activity of the catalyst, and the lower the calcination temperature, the higher the activity. On the other hand, as a result of raising the sintering temperature to increase the wear resistance, it was confirmed that the catalytic activity of the nickel component rapidly decreased due to the formation of a composite metal oxide of nickel and alumina used as a support.
A spinel structure such as NiAl2O4 has the effect of increasing the durability of the catalyst, but lowers the catalytic activity. Therefore, the design of the catalyst was attempted as a way to prevent the formation of spinel structures. In particular, it was confirmed to have high catalytic activity in the catalyst composition in which the nickel content is 60 wt% or more. According to this result, since the content of nickel is higher than the content of alumina used as a support, even after all the alumina used forms a composite oxide, the remaining Ni0 component is sufficiently present. As a result, the high activity of the catalyst was observed. However, the composition of such a catalyst is inappropriate because of its high price. Therefore, in this study, an additive to inhibit the formation of NiAl2O4 was employed. In addition, it was confirmed that since MgO combines with alumina to form MgAl2O4, it has an influence so that Ni, a catalytically active material, can exist independently.
As the content of MgO increased, the structure of NiO of the Ni-Mg-Al-based catalyst exhibited high peak intensity in XRD analysis. However, in the case of a catalyst containing a large amount of MgO, the CO2 conversion rate was low and the yield of methane since the production of CO was reduced. In order to reveal the origin this problem, the catalytic activity and reaction behavior of NiAl2O4 and MgAl2O4 spinel, which are composite metal oxides, for the CO2 hydrogenation reaction were investigated. Nickel aluminate was confirmed to have a relatively high activity in the hydrogenation reaction for methane production, and magnesium aluminate showed very low catalytic activity and low methane yield. Magnesium aluminate resulted in an increase in the selectivity of carbon monoxide. The addition of MgO affects the formation of Ni0 active sites with high catalytic activity. However, there was no positive effect on the CO2 hydrogenation reaction because the selectivity of CO increased as the amount of MgO increased.
In order to effectively remove the heat of reaction in the methanation reaction to maintain a pseudo-isothermal state, a catalyst applicable to the fluidized bed reaction process was prepared. As a result of using inorganic binders such as calcium silicate and potassium carbonate to improve the attrition resistance of the fluidized bed catalyst, high attrition resistance was obtained by the addition of potassium carbonate. However, the reaction activity of the catalyst was low and the selectivity of carbon monoxide was increased. Therefore, it is considered that the application of calcium silicate provides more advantageous in order to improve the attrition resistance of the catalyst for the fluidized bed for producing methane.