The concept of variable activation energy in solid-state kinetics under nonisothermal conditions has been suffering from doubt and controversy. Rate equations of nonisothermal kinetics of solid decomposition, which involve the factors of thermodynamics conditions, pressure of gaseous product, structure parameters of solid, and/or extent of conversion, are derived from the models of the interface reaction, the diffusion of gaseous product, and the nuclei growth of the solid product, respectively. The definition of the validity function in the rate equations represents the influence of the factors on the reaction rate. A function of variable activation energy depending on the validity function is also developed. The changing trend and degree of activation energy are extrapolated from the function of variable activation energy and based on the data of nonisothermal thermal decomposition of calcium carbonate. It is shown that the concept of variable activation energy is meaningfully applicable to solid-state reactions under nonisothermal conditions.

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Stefan flow that is caused by non-equimolar diffusion in the boundary layer is often neglected in studies of the coke gasification reaction (Ccoke + CO2 = 2CO). An inaccurate and incomplete understanding of the metallurgical coke gasification reaction mechanism has had a major influence on the coke-making industry. Meanwhile, it is adverse to the reasonable use of the resources of coking coal as well as energy savings. Therefore, this paper has treated the external diffusion coefficient, mass transfer coefficient, concentration of carbon dioxide (CO2) on the outside and inside of coke, and kinetic parameters as objects to study the effect of Stefan flow. Isothermal experiments of metallurgical coke gasification with CO2 were carried out for 1100–1300 °C and at atmospheric pressure. After modification, the coefficients of external diffusion and mass transfer decreased and the deviation of corrections were 45 and 35%, respectively. Under the different temperatures, the concentration of CO2 on the outer surface of coke together with its distribution inside coke all decreased. Meanwhile, the region of the coke gasification reaction was reduced as well as close to the outer surface area of coke because of Stefan flow. Kinetic parameters were affected by Stefan flow, and they increased to a certain degree.

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