Page 265 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 241

The gas flows upward as a plug flow.
The interchange between two phases takes place by diffusion.

The mass balance of a gas species, j, can be written (Souza-Santos, 2004,
p. 134) as:
2
dρ g;j d ρ g;j
u g 5 D g;j 2 1 R m;j (7.79)
dz dz
where u g is the superficial gas velocity, z is the distance, ρ g,j is the density of
the jth gas, and D g,j is the diffusivity of the jth gas. R m,j , the production or
consumption of the jth gas element, is related to Q gasification heat generation
or absorption.
Similarly, an energy balance equation can be written for a dz element as:
2
dT d T
ρ C pg u g 5 λ g 1 Q gasification 1 Q conv 1 Q rad 1 Q mass (7.80)
g 2
dz dz
where, Q gasification , Q conv , Q rad , and Q mass are the net heat flow into the ele-
ment due to gasification, convection, radiation, and mass transfer, respec-
tively. These terms can be positive or negative. ρ g , C pg , and λ g are the den-
sity, specific heat, and thermal conductivity of the bulk gas, respectively.
Equations (7.79) and (7.80) can be solved simultaneously with appropri-
ate expression for the reaction rate R m,j .

7.6.2 Fluidized-Bed Gasifiers

The kinetic modeling of fluidized-bed gasifiers requires several assumptions
or submodels. It takes into account how the fluidized-bed hydrodynamics is
viewed in terms of heat and mass transfer, and gas flow through the fluidized
bed. The bed hydrodynamics defines the transport of the gasification medium
through the system, which in turn influences the chemical reaction on the
biomass surface. Each of these is subject to some assumptions or involves
submodels.
One can use several versions of the fluidization model:
Two-phase model of bubbling fluidized bed: bubbling and emulsion
phases.
Three-phase model of bubbling fluidized bed: bubbling, cloud, and emul-
sion phases.
Fluidized bed divided into horizontal sections or slices.
Core-annulus structure.
Gas flow through the bed can be modeled as:
Plug flow in the bubbling phase; ideally mixed gas in the emulsion phase.
Ideally mixed gases in both phases.
Plug flow in both phases (there is exchange between phases).