Environmental Engineering Reference
In-Depth Information
λ
, and the given dry fuel composition basis. For complete combustion of the
daf part of the fuel, one needs, according to the stoichiometry of the reaction
below, x moles of air:
C
a
H
b
N
c
O
d
+x O
2
+3
ð
:
76N
2
Þ−
>aCO
2
+
bH
2
O+cNO+3
:
76a N
2
½
Now, x = a + ¼ b +
c
-
½
d.
½
The stoichiometric air-to-fuel ratio (fuel on daf basis!) is
Air
Fuel
=
4
:
76x
1
MW
air
MW
fuel
stoichiometric
We have 48/12 mol = 4 mol C, 6/1 = 6 mol H, 0.5/14 = 0.036 mol N, and
42/16 = 2.63 mol O. The fuel then can be represented (daf ) and normalized to
1 C per mol fuel by CH
1.5
O
0.656
N
0.0089
. This leads to an air-to-fuel stoichiomet-
ric ratio of 4.76 × (28.85/24.13) = 5.99 kg air
kg
−1
fuel (daf ), which is on a
kg
−1
fuel.With the given 277.8 kg
h
−1
fuel (ar basis),
dry basis (db) of 5.78 kg air
h
−1
dry feed flow. Now, the actually fed airflow, considering
we have 250 kg
λ
=
h
−1
. The total gas flow follows from an overall mass
balance and is the sumof fuel and air minus the carbon-containing ash flow. This
is 277.8 + 476.5
0.33, is then 476.5 kg air
h
−1
product gas flow. Now, the only
unknown is the product gas LHV. This is the sum of mass fractions of combus-
tible gas compounds times their specific LHV. Given are volume fractions,
which, with ideal gas lawvalid under these conditions, are equal tomole fractions
(y
i
). Mass fractions (Y
i
) are calculated from these mole fractions (y
i
)by
h
−1
=740.8kg
−
13.46 kg
MW
i
MW
Y
i
=y
i
Here, MW is the average molecular weight of the gas. The following mass
fractions for the combustible species are obtained:
Y
CH
4
=0
:
028; Y
CO
=0
:
17; Y
H
2
=0
:
009; Y
NH
3
=0
:
002
:
The LHV of the mixture then directly follows, given the known LHV values
for each species. This gives LHV = 3.42 MJ
kg
−1
, so that
CGE =
740
:
8×3
:
42
8×16
100
%
=57
:
0
%
277
:
10.1.4 A Thermodynamic Equilibrium Approach to Biomass Gasification
As a first approach to modeling biomass gasification processes, usually, a calculation
of the chemical reaction equilibrium is performed so as to gain insight into which
Search WWH ::
Custom Search