Kinetics of the Boudouard reaction for low-rank Western-Canadian coals

The kinetics of gasification of two Alberta sub-bituminous coal chars with CO₂ have been investigated in the temperature range of 800-950°C. The reactor utilized in the experimental work was a laboratory-size batch fluidized bed. The overall gasification kinetics were followed by measurements of gas composition and flow rates. Chars in the particle size -841 + 420 μm were gasified with gas mixtures involving CO, CO₂ and He. Initially it was determined that beds containing 20.0 g of char (L/D ≈ 0.25), and a total inlet flow rate of 10 l/min were able to provide an adequate mixing of the reactants and near isothermal conditions in most of the experiments. Moreover these operating conditions allowed the measurement of reaction rates with minimum influence of CO₂ starvation, elutriation of char particles and fluidization hydrodynamics. Chars prepared with longer soak time and lower heating rate were less reactive due to their lower surface area and more closed pore structure. For the conditions studied, Highvale chars were more reactive than similarly prepared Smoky Tower chars primarily due to differences in surface area. Increasing PCO₂ in the inlet gas caused a non-linear increase in the reaction rate, and CO strongly retarded the reaction, mainly at low temperatures, suggesting that Langmuir-Hinshelwood kinetics are followed. The reaction also was heavily affected by temperature, especially with increasing concentration of CO due to the poisoning effect of this gas. The external appearance of char particles and the changes in pore structure and ash characteristics with the extent of reaction were examined by SEM. The initial rate of reaction follows the LH equation. The apparent activation energies for the rate constants k₁ and k₂ of this equation are respectively 176 and 286 kJ/mole. However the LH equation was not the most suitable equation to represent the kinetics of the reaction under the conditions investigated. Therefore a power-law rate equation that accounted for the variation in surface area of the chars was employed. The orders of reaction obtained for Highvale and Smoky Tower chars are 0.4 and 0.7 respectively. The initial apparent activation energies for Highvale chars are 143,210 and 255 kJ/mole for CO/CO₂ ratios of 0, 0.25 and 0.50 respectively. For Smoky Tower chars the initial apparent activation energy is 202 kJ/mole. The equations of Bhatia and Perlmutter and Dutta et al. were used to correlate the reactivity of both chars with the extent of reaction. The equation of Bhatia and Perlmutter was able to represent the data better at any temperature for Smoky Tower chars and up to 900°C for Highvale chars. For Highvale chars, the equation of Dutta et al. fitted the data better at 950°C. These results and the values of apparent activation energy obtained suggest that, for the conditions investigated, the gasification reaction was mainly chemically controlled with pore diffusion effects increasing for Highvale chars at 950°C. In addition, the increase in apparent activation energy when the carbon conversion and the CO/CO₂ratio increased, supports the contention that the reaction was under chemical control.

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