Comparison of Kinetic Models for CO₂ Gasification of Coconut-Shell Chars: Carbonization Temperature Effects on Char Reactivity and Porous Properties of Produced Activated Carbons

Abstract

Solid chars were prepared from coconut shell at different carbonization temperatures in the range from 250 - 750°C and gasified in a thermogravimetric analyzer under the atmosphere of carbon dioxide at 850°C. The kinetic analysis showed an accelerating increasing of char conversion with reaction time, indicating an increase in the instantaneous gasification rate as the reaction proceeded. Four kinetic models for gas-solid reactions including, the volume- reaction model (VRM), the shrinking-core model (SCM), the random-pore model (RPM) and the modified volume-reaction model (MVRM) were tested against the measured kinetic data and the MVRM was found to predict the gasification kinetics most accurately. The char reactivity index was computed from the apparent rate constant of the MVRM and used to assess the reactivity of char towards carbon dioxide gasification. It was found that the char reactivity index decreased with increasing carbonization temperature, with the char produced at the lowest temperature of 250°C giving the highest reactivity. Surface area of activated carbon, produced from the gasification of various chars at 850°C for 60 and 120 min, correlated well with the char reactivity index, showing a proportional increasing of surface area with increasing reactivity index and passing through a maximum near the reactivity index of 0.02 min^-1.