Environmental Engineering Reference
In-Depth Information
Example 10.1 Calculation of CC and CGE for a gasifier
h
−1
of
Miscanthus
into product gas. The composition of this fuel on a dry mass basis is 48% C, 6% H,
42%O, 0.5%N, 3.5%ash; its moisture content on an as received (ar) basis is 10 wt%.
The LHV of the fuel measured is 16 MJ
An air-blown, pilot-scale fluidized bed gasifier converts 277.8 kg
kg
−1
(ar basis). The used air stoichiometry
value is
λ
= 0.33. The molecular weights of the elements are as follows: MW
C
=12
kmol
−1
.
Assume that the fuel-bound N is converted to NO in case of complete stoichiometric
combustion.
Fly ash is tapped off from cyclone(s) and filters, and it contains 35 wt% carbon
(pure) and 65 wt%mineral matter. Assume that all ash in the fuel goes to the fly ash
fraction and that no carbon accumulation takes place in the bed. The gas composition
is as follows: 4.5 vol.% methane, 15.5 vol.% carbon monoxide, 13.1 vol.% carbon
dioxide, 11.6 vol.%hydrogen, 10.1 vol.%water, and 0.3 vol.%ammonia, while the
remainder of the gas consists of nitrogen. The values of LHV of the gas constituents
CH
4
, CO, NH
3
, andH
2
are, respectively, 50.016, 10.104, 22.5, and 119.97 MJ
kmol
−1
,MW
H
=1kg
kmol
−1
,MW
N
=14kg
kmol
−1
,andMW
O
=16kg
kg
kg
−1
.
a. Calculate the carbon conversion, CC.
b. Calculate the cold gas efficiency, CGE.
Solution
a. Start with an inert balance (ash):
φ
m
,
flyash
φ
m
,
fuel
,
dry
m
ash
,
db
=1
−
m
C
,
fly ash
)
φ
m
,
fuel
,
ar
1
−
m
moisture
,
ar
m
ash
,
db
=
φ
m
,
fly ash
1
−
m
C
,
fly ash
Substituting the known data gives
h
−1
φ
m
,
fly ash
=13
:
46 kg
Thus, the carbon flow as part of this fly ash mass flow is
h
−1
:
φ
m
,
C
,
flyash
=4
71 kg
This results in
CC = 96
:
1
%
b. To calculate the CGE, use Equation (10.5). Here, we know all the parameters
in the denominator of the equation on an ar basis. Now, we only need to
derive the mass flows of each species. First, we need to calculate the total
mass flow of the gas produced. This can be derived from the fuel mass flow,
Search WWH ::
Custom Search