Kinetics of the Reduction of CuO/Bentonite by Methane (CH4) during Chemical Looping Combustion

Chemical looping combustion (CLC) is a process that uses an oxygen-carrier metal, instead of air or pure oxygen, to provide oxygen for combustion. The products of CLC of methane are CO2 and H2O. After condensation of H2O, a concentrated CO2 gas stream is produced and ready for sequestration. An important issue for the CLC process is the selection of metal oxide as an oxygen carrier, because it must retain its reactivity through many cycles. In this study, isothermal thermogravimetric analysis is used to evaluate the rates of reduction of CuO impregnated in bentonite with methane (CH4) over the range 1023-1173 K for 20%, 50%, and 100% CH4 over 10 reduction cycles. The mechanism and reactivity of the CuO oxygen carrier were evaluated by 10 different rate models. The results indicate that the transformation kinetics described by the Johnson-Mehl-Avrami (JMA) model was the best fit. The Avrami exponent n ranges from 1.55 to 2.16. The average value of 1.77 indicates that the crystallization mechanism is mainly two-dimensional diffusion-controlled. The activation energy was estimated to be 37.3 +/- 1.3 kJ/mol. No deactivation was observed over 10 cycles at any CH4 concentration. In the first 10 reaction cycles, the reaction rates increased slightly with the increasing number of cycles. Moreover, the rate-time and rate-conversion curves for all the temperatures show that the maximum rate occurred at t > 0. This was confirmed by the outlet gas measurements. The experimental results suggested that the CuO/bentonite oxygen carrier is a promising candidate for the CLC system burning methane.