Abstract

As one of the most effective ways to produce energy from a solid carbon, gasification becomes more popular among the industries. The feedstock of gasification passes through multiple steps and reactions to produce syngas, which is a mixture of CO and H2. However, diffusion limitations play an important role in the kinetics of gasification reaction and should be determined. A thermogravimetric analyzer (TGA) is used to investigate the gasification kinetics by studying the mass loss of the sample over time. The influence of mass transfer limitations is typically evaluated using an effectiveness factor, which is a function of the Thiele modulus.
In this work, a mathematical model is developed in MATLAB to investigate the mass transfer limitations in the CO2 gasification of petroleum coke and activated carbon using different diffusion theories and various scenarios. Unlike previous studies, various multicomponent and binary diffusion flux theories are compared in this work. Starting with Fick’s law of diffusion, which is the simplest diffusion theory for binary mixtures to the most recent ones such as the dusty-gas model (DGM), and the cylindrical pore interpolation model (CPIM).
Simulation results show that using Fick’s law (binary case) may underestimate the diffusion limitations in the bed and the gas part. Equally important are the assumptions made about the fate of minor elements in the carbonaceous material, such as oxygen or hydrogen. Therefore, a multicomponent flux theory is required to model the diffusion. The dusty-gas model provides a better estimation of the experimental rates compared to the other theories (CPIM and Fick’s law). Moreover, the diffusion limitations are more significant for the more reactive component, activated carbon, and cannot be eliminated. For petcoke, external diffusion is the mass transfer limiting step in a closed-bottom crucible. For activated carbon, the effects of external and inter-particle diffusion on the overall rate are almost the same order of magnitude. Eventually, the model is verified with the experimental results of Malekshahian et al. (2014). The proposed model shows a good consistency with the experimental results in all initial masses. However, the intrinsic rate constant should be modified with a kinetic model accounting for CO inhibition to enhance the match with the experimental results.