Biomedical Engineering Reference
In-Depth Information
C, H, and O, or any other dominant elements. If other elements form a minor
part of the product gas, they are often neglected.
Let us take the example of 1 mol of biomass being gasified in d moles of
steam and e moles of air. The reaction of the biomass with air (3.76 moles
of nitrogen, 1 mol of oxygen) and steam may then be represented by:
CH
a
O
b
N
c
1
dH
2
O
1
e
ð
O
2
1
3
:
76N
2
Þ-
n
1
C
1
n
2
H
2
1
n
3
CO
(7.65)
1
n
4
H
2
O
1
n
5
CO
2
1
n
6
CH
4
1
n
7
N
2
where n
1
,
, n
7
are stoichiometric coefficients. Here, CH
a
O
b
N
c
is the chem-
ical representation of the biomass and a, b, and c are the mole ratios (H/C,
O/C, and N/C) determined from the ultimate analysis of the biomass. With d
and e as input parameters, the total number of unknowns is seven.
An atomic balance of carbon, hydrogen, oxygen, and nitrogen gives:
...
C:
n
1
1
n
3
1
n
5
1
n
6
5
1
(7.66)
H:
2n
2
1
2n
4
1
4n
6
1
a
1
2d
(7.67)
O:
n
3
1
n
4
1
2n
5
5
b
1
d
1
2e
(7.68)
N:
n
7
5
c
7
:
52e
(7.69)
1
During the gasification process, reactions R1, R2, R3, and R9 (see
Table 7.2
) take place. The water
gas shift reaction, R9, can be considered a
result of the subtraction of the steam gasification and Boudouard reactions,
so we consider the equilibrium of reactions R1, R2, and R3 alone. For a gas-
ifier pressure, P, the equilibrium constants for reactions R
1
,R
2
, and R
3
are
given by:
y
CO
P
y
CO
2
K
e
1
5
R1
(7.70)
y
CO
y
H
2
P
y
H
2
O
K
e
2
5
R2
(7.71)
y
CH
4
y
H
2
P
K
e
3
5
R3
(7.72)
where y
i
is the mole fraction for species i of CO, H
2
,H
2
O, and CO
2
.
The two sets of equations (stoichiometric and equilibrium) may be solved
simultaneously to find the coefficients, (n
1
,
, n
7
), and hence the product
gas composition in an equilibrium state. Thus, by solving seven equations
(
Eqs. (7.66)
...
,n
7
), which give
both the yield and the product of the gasification for a given air/steam-to-
biomass ratio. The approach is based on the simplified reaction path and the
chemical formula of the biomass.
(7.72)
) we can find seven unknowns (n
1
,
...
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