Environmental Engineering Reference
In-Depth Information
between bulk and inner concentration of Xmust be less than 5%. In this case,
it has been derived (see Westerterp et al., 1988) that
2
d
p
6
R
j j
2
−
Φ
X
1
δ
=
≤
0
:
05
−ð Þ
V
r
D
i
c
X
,
i
D
i
c
X
,
i
Now,
R
X
was calculated under subproblem (c). c
X,i
—
when the criterion is
is within 5% of c
X
(the bulk concentration), so we can substitute
c
X
. The only unknown left then is
D
i
, the effective internal diffusion coef-
ficient. This can be calculated via
fulfilled
-
8R
u
T
r
with
D
K
=
D
i
=
τ
p
1
1
D
+
1
D
K
d
pore
3
ε
π
MW
X
D
K
is the Knudsen diffusion coefficient. The binary diffusion coefficient
D
at 900
C is 1.30 × 10
−4
m
2
s
−1
. We take
ε
0
as a conservative value for the
voidage. Filling out the relation for the Knudsen diffusion coefficient, we
arrive at
D
K
= 2.50 × 10
−4
m
2
s
−1
. Thus, the internal effective diffusion coef-
ficient is 2.92 × 10
−5
m
2
s
−1
.c
X
is calculated based on the assumption that
ideal gas law holds:
c
X
=
y
X
p
04 × 10
−3
kmol
m
−3
:
R
u
T
=1
Now, the aforementioned criterion can be evaluated given the calculated
values, and it reaches a value of 0.014, which is OK.
10.2 A SHORT HISTORY OF GASIFICATION
Already around 1850, there was a considerable gasification industry in Europe. In
those early days, the gas was used for lighting and industrial heating and as fuel
for the internal combustion (IC) engine (power generation). So-called
gen-
eration was mostly based on coal. This was a two-step process of consisting of thermal
cracking of solid fuel and subsequent steam blowing of the remaining char in a fixed
bed reactor. A real breakthrough in gasification technology was the Siemens gasifier
(1861). The Siemens gasifier was the first continuous gasification reactor and had spa-
tially separated combustion and gasification sections. All early gasifiers were air-
blown fixed bed reactors with a maximum temperature in the gasification zone of
ca. 900
C. Around the WWII era, a large part of the cars and trucks was powered
by gas produced by built-in fixed bed wood and waste gasifiers. Winkler introduced
the first alternative to the fixed bed gasifier in 1926 by developing a (relatively)
low-temperature fluidized bed gasifier. The advantages of a fluid bed over a fixed
bed were claimed to be the ability to accept all types of coal, smaller-sized coal,
and more ash removal flexibility. The availability of oxygen supply on plant scale
“
town gas
”
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