Environmental Engineering Reference
In-Depth Information
with Y
O2
being the O
2
mass fraction, k
1
=10
7
kmol
s
−1
, and T
a1
= 2.5 ×
10
4
K. The reactor is operated at steady state and at isothermal conditions with
T = 1000 K.
a. Calculate
m
−3
.
b. Suppose that just enough air is fed into the reactor for complete wood con-
version into CO and H
2
. Compute
λ
ϕ
m,
air
.
c. Write down the conservation equations for total mass and O
2
(mass
fraction), respectively.
d. Determine Y
O2
in the reactor by solving the equations. N. B. Y
O2
>0,
though just enough air is introduced in the reactor for complete conversion.
10.5
For rice husk char derived from slow pyrolysis, the following kinetic rate
expression for the char
-
CO
2
reaction is presented by Di Blasi (2009):
p
CO
2
1
87 × 10
12
T
:
d
X
dt
=
9
197
R
u
T
exp
−
½
−
X
Plot
X
versus t at 800
C and 1 atm for the case of conversion with pure CO
2
as gasifying agent. What changes when the surface factor [1
−
X
] changes to
−
X
)
2/3
? Which models for surface area development are represented by
these two factors?
3(1
10.6
Char gasification reactions are generally slow and the rate-limiting step during
biomass gasification. Therefore, a good understanding of char reactivity kinet-
ics is essential for the effective modeling and operation of gasification pro-
cesses. Reaction data concerning char gasification using 10 vol.% CO
2
(C +
CO
2
!
2CO) are presented in the following table, where the weight (%)
and the derived weight loss (%.min
−1
) at three different temperatures, 900,
1000, and 1100
C, are given. The residual weights at 900, 1000, and
1100
C are 2.52, 1.931, and 2.141%, respectively.
a. Plot the curve of weight versus time at different temperatures, and explain
the influence of temperature on char conversion. Assume that the overall
reaction rate of char gasification can be expressed as
f
X
d
X
dt
=k
0
exp
E
a
R
u
T
−
ðÞ
C
CO
2
=
2
b. Use two different models, f ðÞ
=1
−ð Þ
and f ðÞ
=3 1
−ð Þ
3
, to calculate
the activation energy (E
a
) and pre-exponential factor (k
0
).
c. Use the determined E
a
and k
0
to recalculate the weight change (%) and then
compare it with the experimental value. Which model is better and why?
Search WWH ::
Custom Search