Page 258 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

234 Biomass Gasification, Pyrolysis and Torrefaction

Neural network analysis is a relatively new simulation tool for modeling
a gasifier. It works somewhat like an experienced operator, who uses his or
her years of experience to predict how the gasifier will behave under a cer-
tain condition. This approach requires little prior knowledge about the pro-
cess. Instead, the neural network learns by itself from sample experimental
data (Guo et al., 1997).

7.5.2.1 Thermodynamic Equilibrium Models
Thermodynamic equilibrium calculation is independent of gasifier design
and so is convenient for studying the influence of fuel and process para-
meters. Though chemical or thermodynamic equilibrium may not be reached
within the gasifier, this model provides the designer with a reasonable pre-
diction of the maximum achievable yield of a desired product. However, it
cannot predict the influence of hydrodynamic or geometric parameters, like
fluidizing velocity, or design variables, like gasifier height.
Chemical equilibrium is determined by either of the following:
Equilibrium constant (stoichiometric model)
Minimization of the Gibbs free energy (non-stoichiometric model)
Prior to 1958, all equilibrium computations were carried out using the
equilibrium constant formulation of the governing equations (Zeleznik and
Gordon, 1968). Later, computation of equilibrium compositions by Gibbs
free energy minimization became an accepted alternative.
This section presents a simplified approach to equilibrium modeling of a
gasifier based on the following overall gasification reactions:
R1:CO 2 1 C-2CO (7.61)
R2:C 1 H 2 O-H 2 1 CO (7.62)

(7.63)
R3:C 1 2H 2 -CH 4
R9:CO 1 H 2 O-CO 2 1H 2 (7.64)
From a thermodynamic point of view, the equilibrium state gives the
maximum conversion for a given reaction condition. The reaction is consid-
ered to be zero dimensional and there are no changes with time (Li et al.,
2001). An equilibrium model is effective at higher temperatures (.1500 K),
where it can show useful trends in operating parameter variations (Altafini
et al., 2003). For equilibrium modeling, one may use stoichiometric or non-
stoichiometric methods (Basu, 2006).

7.5.2.2 Stoichiometric Equilibrium Models
In the stoichiometric method, the model incorporates the chemical reactions
and species involved. It usually starts by selecting all species containing