Gasification study of a single Indian sub-bituminous coal char particle is carried out in the temperature range of 880−910 °C in a pure CO 2 atmosphere. Two temperatures, 900 and 800 °C, are used for char preparation, and the char obtained at higher temperature is found to be more reactive. A fully transient nonisothermal model is developed incorporating the reaction kinetics along with transport limitations. Spatial and temporal variation of thermo-physical properties like thermal conductivity, diffusivity, and density of the gas mixture and variable specific pore surface area and accessible porosity are included in the model. The model computation shows that the combined kinetics and heat transfer model is more effective to predict the experimental findings of the present authors and that reported in the literature. The simulation study is carried out to assess the effect of reaction temperature, particle size, and char reactivity on the particle temperature, conversion, gasification rate, and CO and CO 2 mass fractions within the porous volume of the particle.

No abstract

A generalized unsteady-state kinetic model, coupled with all modes of heat transfer, was developed to describe the combined coal devolatilization and the subsequent combustion of the residual char under oxy−fuel condition in both O 2 −CO 2 and O 2 −N 2 environments. Experiments were conducted to validate the model, which was also found to predict the experimental data published in the literature well. The effect of coal particle diameter, temperature of the reactor, and oxygen concentration on devolatilization time was investigated. Peaks in devolatilization and char combustion rates and particle center temperature were studied, and the effect of different parameters assessed. Higher reaction time was observed in an O 2 −CO 2 environment compared to that in an O 2 −N 2 environment due to lower particle temperatures resulting from endothermic gasification reaction and the difference in thermo-physical properties. Simulation studies were carried out to generate temperature, carbon, O 2 , CO, and CO 2 contours to understand the char combustion reaction mechanism. The reaction started at the external surface of the particle, following unreacted shrinking core model with two zones; the solid product layer and the unreacted shrinking core, separated by a thin reaction front. Gradually, the reaction front thickness increases, leading to the shrinking reactive core model, consisting of three zones: completely reacted ash layer, partially burnt reacting char called reaction zone, and unreacted zone or slightly reacted char core. At the outset, O 2 cannot penetrate into the particle and CO produced near the surface diffuses out to the boundary layer, forming a thin flame. Subsequently, O 2 diffuses through porous ash layer into the char, and CO burns within the pores, with hardly any CO detected in the boundary layer as the particle temperature increases.

No abstract

No abstract