In order to investigate coke degradation behavior due to CO 2 gasification reaction in the blast furnace, mass transfer analyses with the reaction and stress analyses for coke considering its structure after the reaction were performed. Using the finite element method, CO 2 gas diffusion in a coke lump and consumption of coke matrices owing to the gasification reaction were considered for the coke model in which the actual coke structure was reproduced. The rate-controlling step was also evaluated calculating the Thiele modulus and the effectiveness factor of catalyst obtained from CO 2 concentration distribution in a coke lump. Further, stress analyses assuming a uniaxial tensile test were carried out for the coke model after CO 2 gasification reaction, and the effect of the gasification reaction on a stress state in a coke lump was investigated. As a result, the reaction progressed mainly in the vicinity of the external surface with reaction temperature of 1 673 K while it did uniformly in the whole coke lump with 1 273 and 1 473 K. Thus, the rate-controlling step shifted from the reaction-controlling step to the diffusion-controlling step with an increase in a reaction temperature, and the Thiele modulus and the effectiveness factor of catalyst also showed the same trend. From the stress analysis, coke strength decreased uniformly in the whole coke lump in case of the reaction-controlling step whereas it did mainly in the vicinity of the external surface in case of diffusion-controlling step.
A distributed activation
energy model (DAEM) was applied to the
kinetic analysis of CO
2
and H
2
O gasification
reactions for pulverized metallurgical coke. The results of the scanning
electron microscopy observations and CO
2
gas adsorption
suggested that the gasification reaction occurs at the particle surface.
Therefore, a grain model was employed as a gasification reaction model.
The reaction rates of CO
2
and H
2
O gasification
were evaluated based on the DAEM. The activation energy changed as
the reaction progressed and hardly depended on the particle size.
The activation energies were 200–260 kJ/mol in CO
2
gasification and 220–290 kJ/mol in H
2
O gasification.
The frequency factor of H
2
O gasification was approximately
10 times larger than that of CO
2
gasification, regardless
of the progress of the reaction. At the same activation energy level,
the frequency factor showed a higher value with a decrease in the
particle size. This result was consistent with the theory of the grain
model and indicated that the gasification reaction of the pulverized
coke with a micrometer scale occurs on the surface of the coke particle.
Furthermore, the value predicted by the DAEM was in good agreement
with the experimental one.
To quantitatively evaluate the temperature dependency of coke degradation by CO 2 gasification reaction in a blast furnace, kinetic analyses of gasification reaction with mass transfer for the coke model with approx. 200 million voxels developed from X-ray CT images at the reaction temperatures of 1 373, 1 573, 1 773 and 1 973 K were performed. At high reaction temperature, the gas concentration of CO 2 was high in the external area of the coke model, and the coke matrix voxels vanished mainly around the external surface. Distinguishing surface area of interface between a carbon matrix voxel and a pore voxel with the gas concentration of CO2 at a neighbor pore voxel, although the surface area with the high gas concentration of CO 2 accounted for the majority of the total surface area at 1 373 K, the ratio was lower at over 1 573 K than at 1 373 K. In addition to this, from the effectiveness factor of catalyst, the initial rate-controlling step was chemical reaction at 1 373 K but pore diffusion at over 1 573 K. Also, although the frequency distribution of local porosity showed unimodal regardless of the progress of reaction, the standard deviation calculated from the distribution was changed by reaction. The change rate of the standard deviation by reaction seemed to be larger at high reaction temperature than at low reaction temperature. The logarithm of the change rate hardly depended on reaction temperature under 1 573 K but was proportional to the inverse of the temperature over 1 573 K. This study quantitatively showed that the rate-controlling step affects the coke structure after reaction largely.
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