Page 267 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 243

entire zone. Therefore, each zone shows a homogeneous char concentration
in the bed material and a uniform temperature. Additional input parameters
to the model are geometric data, particle properties, and flow-rates.

7.6.2.1 Hydrodynamic Submodel (Bubbling Bed)
The dense zone (assumed to be the bubbling bed) is modeled according to
the modified two-phase theory. Bubble size is calculated as a function of bed
height (Darton and LaNauze, 1977), and it is assumed that all bubbles at any
cross-section are of uniform size:
ffiffiffiffiffiffiffi 0:8
0:4 r
ðU2U mf Þ A
d b 5 0:54 z14 (7.81)
g 0:2 N or
where (N or /A) is the number of orifices per unit of cross-section area of the
bed.
The interphase mass transfer between bubbles and emulsion, essential for
the gas solid reactions, is modeled semiempirically using the specific bubble
surface as the exchange area, the concentration gradient, and the mass-transfer
coefficient. The mass-transfer coefficient, K BE , based on the bubble emulsion
surface area (Sit and Grace, 1978), is:
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
U mf 4ε mf D r U B
K BE 5 1 (7.82)
4 πd B
where U mf and ε mf are, respectively, minimum fluidization velocity and voi-
dage at a minimum fluidizing condition, D r is the bed diameter, and U B is
the rise velocity of a bubble of size d B .
The axial mean voidage in the freeboard is calculated using an exponen-
tial decay function.

7.6.2.2 Reaction Submodel
Gasification reactions proceed at a finite speed; this process is divided into
three steps: drying, devolatilization, and gasification. The time taken for dry-
ing and devolatilization of the fuel is much shorter than the time taken for
gasification of the remaining char. Some models assume instantaneous dry-
ing and devolatilization because the rate of reaction of the char, which is the
slowest, largely governs the overall process.
The products of devolatilization are CO 2 , CO, H 2 O, H 2 , and CH 4 .The
gases released during drying and devolatilization are not added instantaneously
to the upflowing gas stream, but are added along the height of the gasifier in a
predefined pattern. The total mass devolatilized, m volatile , is therefore the sum
of the carbon, hydrogen, and oxygen volatilized from the solid biomass.

(7.83)
m volatile 5 m char 1 m hydrogen 1 m oxygen