Biomedical Engineering Reference
In-Depth Information
gas, we can write the energy balance of the gasifier taking 0
C as the
reference:
ACp
a
T
0
1
FCp
f
T
0
1
WH
0
1
F
HHV
Q
ext
5
ð
C
CO
V
CO
1
C
CO
2
V
CO
2
3
1
1
C
CH
4
V
CH
4
1
C
H
2
V
H
2
1
C
O
2
V
O
2
1
C
N
2
V
N
2
Þ
T
g
1
ð
1
2
X
g
Þ
WH
g
1
P
c
q
c
1
Q
gasification
1
Q
loss
1
Q
product
(8.22)
where H
0
and H
g
are the enthalpies of steam at the reference temperature
and the gasifier exit temperature; C
i
and V
i
are the volumetric specific heat
and the volume of the gas species, i, at temperature T
g
leaving the gasifier;
(1
X
g
)W is the net amount of steam remaining in the product gas of the
gasification reaction; P
c
is the amount of char produced; and q
c
is the heating
value of the char. Here, Q
loss
is the total heat loss through the wall, radiation
from the bed surface, ash drain, and entrained solids, corresponding to
1Nm
3
of gas generation. This allows computation of external heat addition,
Q
ext
kJ/N m
3
of product gas to the system. Q
product
is the amount of energy
in the product gas and Q
gasification
is the net heat of reaction.
2
8.7 PRODUCT GAS PREDICTION
A typical gasifier design starts with a desired composition of the product gas.
Equilibrium and other calculations are carried out to check how closely that
targeted composition can be achieved through a choice of design parameters.
The product of combustion reactions is predominantly made up of carbon
dioxide and steam, the percentages of which can be estimated with a fair
degree of accuracy from simple stoichiometric calculations. For gasification
reactions, this calculation is not straightforward; as the fraction of the fuel
gasified and the compositions of the product gas need to be estimated care-
fully. Unlike combustion reactions, gasification reactions do not always
reach equilibrium, so only a rough estimate is possible through an equilib-
rium calculation. Still, this can be a reasonable start for the design until
detailed kinetic modeling is carried out in the design optimization stage.
8.7.1 Equilibrium Approach
An equilibrium calculation ideally predicts the product of gasification if the
reactants are allowed to react in a fully mixed condition for an infinite period
of time. There are two types of equilibrium model. The first one is based on
equilibrium constants (stoichiometric model). The specific chemical reac-
tions used for the calculations have to be defined, so this model is not
suitable for complex reactions where the chemical formulae of the com-
pounds,
the reaction path, or the reaction equations are unknown. This
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