Page 257 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 233

Simulation can never be a substitute for good experimental data, espe-
cially in the case of gas solid systems such as gasifiers. A mathematical
model, however sophisticated, is useless unless it can reproduce real opera-
tion with an acceptable degree of deviation (Souza-Santos, 2004). Still, a
good mathematical model can:
Find optimum operating conditions or a design for the gasifier.
Identify areas of concern or danger in operation.
Provide information on extreme operating conditions (high temperature,
high pressure) where experiments are difficult to perform.
Provide information over a much wider range of conditions than one can
obtain experimentally.
Better interpret experimental results and analyze abnormal behavior of a
gasifier, if that occurs.
Assist scale-up of the gasifier from one successfully operating size to
another, and from one feedstock to another.

7.5.2 Gasifier Simulation Models
Gasifier simulation models may be classified into the following groups:

Thermodynamic equilibrium
Kinetic
Computational fluid dynamics (CFD)
Artificial neural network (ANN)
The thermodynamic equilibrium model predicts the maximum achievable
yield of a desired product from a reacting system (Li et al., 2001). In other
words, if the reactants are left to react for an infinite time, they will reach
equilibrium yield. The yield and composition of the product at this condition
are given by the equilibrium model, which concerns the reaction alone with-
out taking into account the geometry of the gasifier.
In practice, only a finite time is available for the reactant to react in the
gasifier. So, the equilibrium model may give an ideal yield. For practical
applications, we need to use the kinetic model to predict the product from a
gasifier that provides a certain time for reaction. A kinetic model studies the
progress of reactions in the reactor, giving the product compositions at dif-
ferent positions along the gasifier. It takes into account the reactor’s geome-
try as well as its hydrodynamics.
The CFD models (Euler type) solve a set of simultaneous equations for
conservation of mass, momentum, energy, and species over a discrete region
of the gasifier. Thus, they give distribution of temperature, concentration,
and other parameters within the reactor. If the reactor hydrodynamics is well
known, a CFD model provides a very accurate prediction of temperature and
gas yield around the reactor.