Page 264 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

240 Biomass Gasification, Pyrolysis and Torrefaction

(k-ε) model or large eddy simulation filters are two means of accounting for
turbulence in the flow.
For a fluidized bed, the flow is often modeled using the Eulerian
Lagrange concept. The discrete phase is applied to the particle flow; and the
continuous phase to the gas. Overmann et al. (2008) used the Euler Euler
and Euler Lagrange approaches to model wood gasification in a bubbling
fluidized bed. Their preliminary results found both to have comparable
agreement with experiments. If the flow is sufficiently dilute, the parti-
cle particle interaction and the particle volume in the gas are neglected.
A two-fluid model is another CFD approach. Finite difference, finite ele-
ment, and finite volume are three methods used for discretization.
Commercial software such as ANSYS, ASPEN, Fluent, Phoenics, and
CFD2000 are available for solution (Miao et al., 2008). A review and compar-
ison of these codes is given in Xia and Sun (2002) and Norton et al. (2007).
Recent progress in numerical solution and modeling of complex gas solid
interactions has brought CFD much closer to real-life simulation. If success-
ful, it will be a powerful tool for optimization and even design of thermo-
chemical reactors like gasifiers (Wang and Yan, 2008). CFD models are most
effective in modeling entrained-flow gasifiers, where the gas solid flows are
less complex than those in fluidized beds and the solid concentration is low.
Models developed by several investigators employ sophisticated reaction
kinetics and complex particle particle interaction. Most of them, however,
must use some submodels, fitting parameters or major assumptions into areas
where precise information is not available. Such weak links in the long array
make the final result susceptible to the accuracy of those “weak links.” If the
final results are known, we can use them to back-calculate the values of the
unknown parameters or to refine the assumptions used.
The CFD model can thus predict the behavior of a given gasifier over a
wider range of parameters using data for one situation, but this prediction
might not be accurate if the code is used for a different gasifier with input
parameters that are substantially different from the one for which experimen-
tal data are available.

7.6 KINETIC MODEL APPLICATIONS

This section briefly discusses how kinetic models can be applied to the three
major gasifier types.

7.6.1 Moving-Bed Gasifiers
A basic moving-bed or fixed-bed gasifier can use the following assumptions:
The reactor is uniform radially (i.e., no temperature or concentration gra-
dient exists in the radial direction).
The solids flow downward (in an updraft gasifier) as a plug flow.