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Cs
ðÞ
+2H 2 g
ðÞ
CH 4 g
ðÞ
ð
RX
:
10
:
5a
Þ
with 1 <
in reaction (RX. 10.1a) is subject to some controversy in
the literature. Some researchers suggest that this value cannot be found for the complex
interacting phenomena taking place during gasification (Denn et al., 1979). According
to Smith (1982), CO is the primary product (
α
< 2. The value of
α
= 2) for coal. Laurendeau (1978) and
Martens (1984) concluded that both CO and CO 2 are primary reaction products of this
oxidation reaction. High temperature and low pressure favor CO formation. Jensen
et al. (1995) showed that CO is the main primary product under pressurized fluidized
bed combustion (PFBC) process conditions, but relatively low levels of 10
α
30%of pri-
mary CO 2 formation are possible. For graphite and coal char, Arthur (1951) obtained,
under atmospheric conditions and suppressing further oxidation of gaseous CO using
POCl 3 , the following relation for themole ratioCO/CO 2 (Take note that R u is expressed
in cal
mol −1 in this equation):
CO
CO 2 =10 3 : 4 exp
12,400
R u T
ð
Eq
:
10
:
6
Þ
1000 C, the CO/CO 2
ratio is between 4.1 and 18.8, so CO is the major component of initial carbon oxi-
dation. Monson et al. (1995) give a relation for the molar ratio CO/CO 2 under pres-
surized bituminous coal char combustion conditions (verified in a drop tube reactor
with a temperature range of 1000
At temperatures typical for fluidized bed operation, 700
1500 K and O 2 concentrations in the range
of 5
-
21%):
CO
CO 2 =3×10 8 exp
30,178
T s
ð
Eq
:
10
:
7
Þ
1000 C and 1.02 × 10 −5
< CO/CO 2 < 1.52 × 10 −2 , the coefficients in equation (RX. 10.1a) for CO and CO 2
are practically 0 and 1, respectively.
In Figure 10.4 and in Figure 10.5, a graphical overview of experimental correla-
tions for the rate of char gasification reactions with CO 2 and H 2 O is given, obtained
from an excellent review of Di Blasi (2009). Only biochar data are shown here, and the
given rates are based on the intrinsic kinetics only (no effect of surface development;
see later in this section Equation (10.14)). The gasification reaction of char with H 2 is
observed to be much slower than the char
At temperatures typical for fluidized bed operation, 700
H 2 O/CO 2 reaction (Kosky and Floess,
1980) and is not further dealt with in this chapter. It can be observed that the reactivity
values for both reactions show a wide range of values. Biomass heterogeneity plays an
important role; certain ash elements can catalyze the reactions.
Char gasification reaction with CO 2 can be represented by the following mechan-
ism, as pointed out by Barrio et al. (2001):
-
k 1f
+CO 2 !
:
:
C f s
ðÞ
CO
ðÞ
+CO
ð
RX
10
4b
Þ
k 1b
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