(602a) Gasifier Performance Modeling Including Mass Transfer Effects

In the development of multiphase reacting CFD codes, a number of simplifications were incorporated into the codes and models. One of these simplifications was the use of a simplistic mass transfer correlation for the faster reactions and omission of mass transfer effects completely on the moderate speed and slow speed reactions such as those in a fluidized bed gasifier. Another problem that has propagated  is that the mass transfer correlation used in the codes is not universal and is being used far from its developed bubbling fluidized bed regime when applied to riser reactors. These problems are true for the major codes, which at this time does not consider mass transfer effects.

            To alleviate this problem, a mechanistic based mass transfer coefficient algorithm has been developed based upon earlier work by Breault et al. [1, 2, 3 and 4]. This fundamental approach uses the local hydrodynamics to predict a local, time varying mass transfer coefficient. The algorithm previously developed has been generalized to work on cells of any volume. As was the case for the non-generalized cell volume, the predicted mass transfer coefficients and the corresponding Sherwood numbers agree well with literature data and are typically about an order of magnitude lower than the correlation noted above. The incorporation of the new mass transfer model into the C3M code gives the expected behavior for all the gasification reactions evaluated in the paper. The new algorithm should be used such that the conversions are not over predicted. Additionally, its behavior with changes in solids flow rate are consistent with the changes in the hydrodynamics.

1. 1.         Breault, R.W, “A review of gas–solid dispersion and mass transfer coefficient correlations in circulating fluidized beds”, Powder Technology 163 (2006) 9–17
2. 2.         Breault, R. W. and Guenther, C. P., “Mass transfer in the core-annular and fast fluidization flow regimes of a CFB”, Powder Technology 190 (2009) 385–389
3. 3.         Breault, R. W. and Guenther, C. P., “Mass transfer coefficient prediction method for CFD modeling of riser reactors”, Powder Technology 203 (2010) 33–39
4. 4.         Breault, R. W., Li, Tingwen, and Nicoletti, P., “Mass Transfer Effects in a Gasification Riser”, submitted to Powder Technology