EFFECTIVE DIFFUSION COEFFICIENTS IN COAL CHARS

摘要

Knowledge of effective diffusion coefficients in char particles is important when interpreting experimental reactivity measurements and modeling char combustion or NO and N_2O reduction. In this work, NO and N_2O reaction with a bituminous coal char was studied in a fixed-bed quartz glass reactor. Particle sizes in the range 0.05-5 mm were tested, and the effective diffusion coefficients were estimated from measured effectiveness factors using the Thiele modulus. At 1079 K the effective diffusion coefficients were 5.5 X 10~(-6) m~2/s and 6.8 X 10~(-6) m~2/s for N_2O and NO, respectively. The experimental results were compared with theoretical values calculated from the mean pore radius and the cross-linked pore model. The method of mean pore radius underestimated the effective diffusion coefficient more than an order of magnitude. Using the cross-linked pore model, the bimodal pore size distribution, and a tortuosity factor of 5, a complete agreement between the experimental and the theoretical value was found. The conclusion was that for a char with a wide pore size distribution, the cross-linked pore model is a good choice for a theoretical calculation of the effective diffusion coefficient. In the case of strong pore diffusion limitations, the error in the interpretation of experimental results using the mean pore radius could be a factor of 5 on the intrinsic rate constant. For an average coal char reacting with oxygen at 1300 K, this would be the case for particle sizes larger than about 50μm.