Page 256 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

232 Biomass Gasification, Pyrolysis and Torrefaction

rise in the surface temperature, the kinetic rate increases and therefore the
overall reaction moves from the kinetic to the diffusion-controlled regime,
resulting in less reaction within the pores.
The overall gasification rate of char particles, Q, when both mass-transfer
and kinetic rates are important, may be written as:
P g 2
Q 5 kg carbon=m s (7.60)
ð1=h m Þ 1 ð1=R c Þ
where P g is the concentration in partial pressure (bar) of the gasifying agent
2
outside the char particle, h m is the mass-transfer rate (kg carbon (m bar s))
2
to the surface, and R c is the kinetic rate of reaction: kg carbon (m bar s).

7.5 GASIFICATION MODELS
Optimal conversion of chemical energy of the biomass or other solid fuel
into the desired gas depends on proper configuration, sizing, and choice of
gasifier operating conditions. In commercial plants, optimum operating con-
ditions are often derived through trials on the unit or by experiments on pilot
plants. Even though expensive, experiments can give more reliable design
data than those can be obtained through modeling or simulation. There is,
however, one major limitation with experimental data. If one of the variables
of the original process changes, the optimum operating condition chosen
from the specific experimental condition is no longer valid. Furthermore, an
experimentally found optimum parameter can be size specific; that is, the
optimum operating condition for one size of gasifier is not necessarily valid
for any other size. The right choice between experiment and modeling, then,
is necessary for a reliable design.

7.5.1 Simulation Versus Experiment

Simulation, or mathematical modeling, of a gasifier may not give a very
accurate prediction of its performance, but it can at least provide qualitative
guidance on the effect of design and operating or feedstock parameters.
Simulation allows the designer or plant engineer to reasonably optimize the
operation or the design of the plant using available experimental data for a
pilot plant or the current plant.
Simulation can also identify operating limits and hazardous or undesir-
able operating zones, if they exist. Modern gasifiers, for example, often oper-
ate at a high temperature and pressure and are therefore exposed to extreme
operating conditions. To push the operation to further extreme conditions to
improve the gasifier performance may be hazardous, especially if it is done
with no prior idea of how the gasifier might behave at those conditions.
Modeling may provide a less expensive means of assessing the benefits and
the associated risk.