(363g) Thermal Management of CO2 methanation Process Based on Experimental and Numerical Approach Using Ni-YSZ Tubular Catalysts

CO₂ methanation (eq. 1) has attracted much interests as a potential CO₂ utilization technology to reduce the CO₂ emission from power plants and other industrial plants for realizing the low carbon society[1–3].

CO₂+ 4H₂→CH₄+ 2H₂O, ΔH_298K= 165 kJ mol^-1 (1)

The reaction has long historical background and has been well known as Sabatier reaction[3]. Much efforts have been devoted to the development of highly-active catalysts and there has been many literatures reporting kinetic analysis and modeling of the methanation reactions[4–7].

Since the CO₂ methanation is highly exothermic reaction as shown in eq. 1, temperature inside the reactor easily increase when using high-concentration CO₂, and furthermore, localized hotspot forms at the inlet region of the reactor. Increase in temperature results in decrease in the CO₂ conversion (i.e., CH₄ yield) because low temperature is thermodynamically favorable for CO₂ methanation. In addition, the formation of hotspot may cause the thermal degradation of the catalysts. Therefore, investigating the temperature distribution in the reactor and preventing the formation of hotspot in the reactor are important.

In this study, we prepared Ni-YSZ tubular catalysts with different Ni contents as model catalysts for CO₂ methanation. Using tubular catalysts, fluid dynamic characteristics can be simplified in comparison with that in conventional packed bed reactor. We prepared Ni-YSZ tubular catalysts with different Ni contents in a range of 25-100wt% and discussed the effect of the Ni contents on the temperature distribution in the reactor. The CO₂ methanation was conducted under the furnace temperature between 200-400℃ and the pressure of 0.1-0.9 MPa. Temperature inside the reactor was monitored with several thermocouples and on-line reaction product analysis was also performed using gas chromatographywith thermal conductivity detector. In addition, numerical simulation of the CO₂ methanation was conducted and the effect of the catalytic activity and reaction conditions on the temperature profile in the reactor was discussed.

Using the catalyst with small Ni-content (25-75 Ni-YSZ), both increase in temperature in the reactor and CH₄ yield were small due to the low catalytic activity of the catalyst. On the other hand, using the Ni-YSZ tubular catalysts with high Ni contents (50 and 100wt% Ni-YSZ), we observed high CH₄ yield (more than 90%). However, rapid increase in temperature around the inlet region of the reactor and large temperature gap between inlet and outlet regions of the reactor were observed with the catalysts. At first, temperature at the outlet region of the reactor rapidly increased, and then, the temperature peak gradually shifted to the inlet region of the reactor.The result suggests that temperature increase due to heat generation derived from CO₂ methanation interacted with the acceleration of CO₂ methanation activity, resulting in further increase in temperature and CO₂ methanation rate at the inlet region in a chain reaction. The result suggests that too high catalytic activity for CO₂ methanation causes rapid increase in temperature in a chain reaction and forms localized hotspot at the inlet region of the reactor. The formation of hotspot was also demonstrated by the numerical simulation based on the coupled analysis of fluid dynamics, heat conduction, and reaction kinetics. The result suggests that there is a threshold of the catalytic activity and temperature causing the formation of hotspot in the reactor. To prevent the formation of hotspot in the reactor, appropriate balance between reaction rate, i.e., catalytic activity, and heat conduction is required for designing the reactor and catalysts with long lifetime.

Acknowledgment

This study is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

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