Comments on mass transfer around carbon particles burning in fluidised beds

Dear Sirs,

The paper by Guedes de Carvalho, Pinto and Pinho, Trans IChemE 1991 69, 63-70 provides experimental data which should clearly support the mechanism of direct oxidation to CO followed by combustion of monoxide in the gas phase, when carbon particles are burned in a fluidized bed.

The first point I would like to make concerns the way in which the authors try to prove that combustion of CO to CO2 takes place in the gas phase only. The authors assume that using a platina catalyst complete oxidation to CO2 must take place. This is very doubtful, because the platina catalyst also has to deal with mass-transfer limitations.
What frightens me is that the results found are not consistent with the work of Prins [ref. 2]. Prins recently made an extensive study to the CO/CO2 ratio at the carbon surface and found 70 % formation of CO2. Guedes de Carvalho et al. however find 0 % formation of CO2. The authors failed at this point, by not comparing their results with the work of Prins.

The second comment I would like to make concerns the way in which the experimental data of La Nauze et al. [ref. 4] are interpreted. Figure 7 to 11 of the paper show the mass transfer data from La Nauze et al. assuming negligible CO oxidation close to the burning particle. This data is compared to the model of La Nauze et al. [ref. 4] (equation 2) (a comparison with the corrected model [ref. 3] (equation 2, corrected) should have been more appropriate, but I am not arguing that) and the model of Coelho and Guedes de Carvalho [ref. 5] (equation 6).

Agarwal and La Nauze [ref. 1] recently also reinterpreted the data of La Nauze et al., but Agarwal and La Nauze assumed that 70 % of the CO was oxidated to CO2 at the carbon surface. The value of 70 % is based on an the already mentioned work by Prins [ref. 2]. Agarwal and La Nauze compared the data with about 10 mass transfer models including the model of Coelho and Guedes de Carvalho [ref. 5] (equation 6) and they concluded that this last model results in Sherwood numbers which are far too low.

In the figure the original data of condition 11 of La Nauze et al. [ref. 4] are shown. The data as interpreted by Agarwal and La Nauze [ref. 1] as well as the data as interpreted by Guedes de Carvalho et al. are also shown. It is clear from this picture that equation (2, corrected) as well as the models of Prins [ref. 2] and Agarwal et al. [ref. 1] fit well with the data assuming 70 % oxidation to CO2. It is also clear that the model of Coelho and Guedes de Carvalho [ref. 5] (equation 6) predicts much lower Sherwood numbers.

I hope this letter made clear that the question which mass transfer model is the best cannot be answered before CO/CO2 ratios have been cleared up. The experimental data of Guedes the Carvalho et al. is not consistent with the much more refined data of Prins [ref. 2]. Therefore the mechanism of direct oxidation to CO, followed by combustion of the monoxide in the gas phase, is very doubtful. The conclusion that mass transfer coefficients for oxygen diffusion are shown to be well predicted by equation 6 is premature.

E.P. van Elk

References:

1. Agarwal, P.K.; La Nauze, R.D. "Transfer Processes Local to the Coal Particle: A Review of Drying, Devolatilization and Mass Transfer in Fluidized Bed Combustion" Chem Eng Res Des 1989, 67, 457-480.
2. Prins, W. "Fluidized Bed Combustion of a Single Coal Particle" Thesis University of Twente 1987.
3. Guedes de Carvalho, J.R.F.; Coelho, M.A.N. "Comments on Mass Transfer to Large Particles in Fluidized Beds of Smaller Particles" Chem. Eng. Sci 1986 41, 209-210.
4. La Nauze, R.D.; Jung, K.; Kastl, J. "Mass Transfer to Large Particles in Fluidized Beds of Smaller Particles" Chem Eng Sci 1984, 39, 1623-1633.
5. Coelho, M.A.N.; Guedes de Carvalho, J.R.F. "Transverse Dispersion in Granular Beds Part I. Mass Transfer from a Wall and the Dispersion Coefficient in Packed Beds" Chem Eng Res Des 1988, 66, 165-177.