Optimizing heat transfer rate for efficient CO₂-to-chemical conversion in CO₂ methanation and CO₂ hydrogenation reactions

CO₂ utilization is an evolving technology that converts CO₂ into CH₄ or other chemicals upon its reaction with H₂, which can be supplied from water electrolysis using the surplus solar and wind power. CO₂ methanation and hydrogenation reactions require thermal management as they are exothermic. The average heat transfer coefficient (h_o) was measured and analyzed at various temperatures (200^°C–400^°C), gas velocities (0.7–9.1 U_o/U_mf), and pressures (1–25 bar) using two different fluidized-bed setups for CO₂ methanation and hydrogenation reactions. In addition, the effects of the heat exchanger location at different heights in a fluidized-bed reactor were studied along with the local heat transfer coefficients (h_L) of the tube. The glass bead resulted in higher h_o (200–340 W/m²∙°C) compared to the nickel-based material (133–183 W/m²∙°C) owing to its bubble formation characteristics. An increase in gas velocity, temperature, and pressure enhanced the heat transfer efficiency. The reason for the different h_o values at different heights was inferred from the local heat transfer coefficients (h_L) of the tube. The highest heat transfer rate was observed at the bottom of the tube, and the lowest heat transfer rate was found to be on either side or at the top of the tube depending on the location of the heat exchanger in the fluidized-bed reactor. Finally, an empirical equation was derived for h_o under CO₂ hydrogenation conditions, showing a p-value of <0.0001. Our study can pave the way for highly effective heat exchanger design for CO₂ utilization.