Fuel 84 (2005) 885–892
www.fuelfirst.com
On the intrinsic reaction rate of biomass char gasification
with carbon dioxide and steam
Wolfgang Klose*, Michael Wölki
Institute of Thermal Engineering, FB 15, Thermodynamics, University of Kassel, Kurt-Wolters-Str. 3, D-34109 Kassel, Germany
Received 21 June 2004; received in revised form 18 November 2004; accepted 23 November 2004
Available online 15 December 2004
Abstract
The gasification of beech wood char and oil palm shell char with carbon dioxide and steam was studied. To avoid heat and mass transport
limitations during gasification, the amount of char, particle size and flow rate were varied in isothermal experiments. A rate expression of the
Langmuir–Hinshelwood-type was applied to match the experimental data at different partial pressures and reaction temperatures in the
intrinsic regime. Furthermore, the reactive surface area (RSA) of the biomass chars was determined as a function of the degree of conversion
by the temperature-programmed desorption technique (TPD). The results show that the reaction rate is in general proportional to the RSA.
The surface related reaction rates for the studied biomass chars are comparable to surface related reaction rates for coal chars at similar
reaction temperatures.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Biomass; Intrinsic reaction rate; Temperature-programmed desorption; Reactive surface area
1. Introduction
As a renewable carbon source, biomass can be used as
an alternative to fossil fuels for the production of
hydrogen and carbon monoxide enriched gases by
gasification with carbon dioxide and steam. As a
heterogeneous reaction involving a porous solid and a
gas can be influenced generally by mass transfer
limitations under isothermal conditions, the investigation
of reaction kinetics requires conditions under which the
reaction is solely determined by the chemical surface
reaction. For the modelling of gasification processes, the
intrinsic reaction rate of the gasification is required.
The Fig. 1 from Rossberg and Wicke [1] shows ideally
the three zones representing the change of reaction rate
of a porous carbon with temperature.
Since the surface of the char is generally accepted as the
location of the chemical reaction during gasification, the
reaction rate should be related to a relevant surface area of
the char.
* Corresponding author. Tel.: C49 561 804 3268; fax: C49 561 804
3993.
E-mail address: [email protected] (W. Klose).
0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2004.11.016
An appropriate surface area should be proportional to the
reaction rate. The following surface areas are known from
literature:
† total surface area (TSA) determined by physical adsorption of nitrogen and/or carbon dioxide in conjunction with
the BET-equation, the Dubinin–Polanyi (DP) equation or
the Dubinin–Radushkevich (DR) equation [2],
† active surface area (ASA) [3,4] and
† reactive surface area (RSA) [3,5,6].
It is well known that chars with the same total surface area
can have widely differing gasification rates [4,7,8]. Since the
reactive surface area (RSA) was used successfully to explain
the different reactivities of coal chars during gasification,
the RSA should be applied in this study to calculate a
surface related intrinsic reaction rate for the gasification of
biomass chars with carbon dioxide and steam [5,6].
For the determination of a surface related reaction rate,
the following expression has to be calculated:
r ðaÞ
ra Z m
(1)
aðaÞ
The use of an appropriate surface area should result in a
surface related reaction rate, which is independent from
886
W. Klose, M. Wölki / Fuel 84 (2005) 885–892
Nomenclature
a
EA
DSHm
k0
k
K
n
P
pi
r
t
specific surface area, m2/g
activation energy, kJ/mol
adsorption enthalpy, kJ/mol
pre-exponential factor, sK1
reaction rate constant, sK1
sorption equilibrium coefficient, PaK1
amount of substance, mol
pressure, Pa
partial pressure, Pa
reaction rate, sK1
time, s
T
ai
FV
Subscripts
a
related to the surface area
i
substance
m
related to the mass
ASA
active surface area
RSA
reactive surface area
TSA
total surface area
the degree of conversion. Besides, the reaction rate should
be proportional to the relevant surface area [5,9].
In the literature, the reaction rate is usually related to the
instantaneous mass of the char. A survey of different
specific reaction rates from different authors during the
gasification of various biomass chars with carbon dioxide
and steam is given in Figs. 2 and 3. Since the reaction rate
depends on the degree of conversion, it is mentioned in
these figures as well. Ergun and Reif [10,11] suggested a
reasonable reaction mechanism for the gasification of
carbon with carbon dioxide
k1
Cf C CO2 %
CðOÞ C CO
0
(2)
k1
k2
CðOÞ /
CO C Cf
(3)
and steam
k3
CðOÞ C H2
Cf C H2 O%
0
(4)
k3
k4
CðOÞ /
CO C Cf
temperature, K
degree of conversion
flow rate, m3 sK1
(5)
Fig. 1. The three zones representing the change of reaction rate of a porous
carbon with temperature.
A Langmuir–Hinshelwood-type rate expression is often
applied in the literature to satisfy these mechanisms:
rm Z
n Z CO2 ;
k a pn
1 C kb pn C kc pm n Z H2 O;
m Z CO
m Z H2
(6)
2. Experimental
For the experiments, biomass chars from beech wood and
oil palm shell were used. The biomass pyrolysis was
performed in a vertical tube furnace. The biomass was
heated from room temperature to 1173 K at a heating rate of
3 K/min, and devolatilized in a flow of nitrogen for 30 min.
After pyrolysis, the chars were ground with an agate and
pestle and separated into different fractions. To remove
oxygen, carbon dioxide and argon were passed over hot pure
copper wires at about 800 K. For the removal of water,
magnesium chloride anhydrous was used. In Table 1 the
elemental composition and some initial properties of the
biomass chars are summarized.
For the investigation of the reaction kinetics, isothermal
experiments are done in a thermogravimetric apparatus.
Since beech wood char and oil palm shell char show explicit
Fig. 2. Literature survey over different studies about the reaction kinetics of
the gasification of biomass chars with carbon dioxide [13–22].
W. Klose, M. Wölki / Fuel 84 (2005) 885–892
887
A typical TPD experiment began with in situ partial
gasification of the char in the gasifying agent. After reaching
the desired degree of conversion, the sample (100–200 mg)
was quenched in reactant gas to a temperature at which no
significant gasification occurred (473 K). The reactant gas
was then removed from the system with argon. Once the
concentration of carbon monoxide and carbon dioxide
returned to baseline levels, the sample was heated in flowing
argon (FV (TZ323.15 K; PZ101 kPa)Z50 cm3/min) at a
constant heating rate of 3 K/min to a final temperature of
1173 K. This temperature was held to achieve complete
desorption of carbon monoxide and carbon dioxide from the
char surface. After each experiment, the mass spectrometer
was calibrated to permit a quantitative analysis of the partial
pressure data. The TPD spectra obtained in this way
represent the extent and energetic of total CO-complexes
formed on the char surface after partial gasification at a
certain degree of conversion.
For the direct determination of the amount of stable COcomplexes formed on the char surface after partial
gasification, the experimental procedure was slightly
modified. The char was gasified to a specified degree of
conversion. Instead of quenching it immediately in reactant
gas, a flow of argon (50 cm3/min) was permitted to flow
over the sample to desorb the reactive C(O) intermediate
formed during gasification. The decay of carbon monoxide
with time was monitored by the mass spectrometer. Once
the concentration of carbon monoxide reached its baseline
level, the sample was quenched in argon to a significantly
lower temperature. Beginning at this temperature, a TPD
experiment, as previously described, was performed. The
described TPD experiments were repeated for different
degrees of conversion.
From the TPD experiments, the number of reactive
surface complexes results from the difference of the total
amount of surface complexes and the amount of stable
surface complexes at a specified degree of conversion:
Fig. 3. Literature survey over different studies about the reaction kinetics of
the gasification of biomass chars with steam [23–31].
different reaction rates during gasifiction at the same
reaction conditions, different reaction temperatures were
chosen for the biomass chars. Beech wood char was gasified
in isothermal experiments at 993, 1003, 1013 and 1023 K,
whereas gasification of oil palm shell char took place at
1023, 1033, 1043 and 1053 K. The partial pressures of
steam and carbon dioxide were usually 70 and 100 kPa,
respectively. For the reactions of beech wood char and oil
palm shell char, a reaction temperature of 1023 and 1053 K,
respectively, is chosen as standard temperature.
In order to study the intrinsic chemically controlled
kinetics, preliminary experiments were carried out. In these
experiments, the amount of char, the particle size and the
flow rate of the gasifying agent were varied. According to
Fig. 1, in the intrinsic regime the reaction rate must not be a
function of these parameters. Gas flow rates from FVZ200–
1500 cm3/min (STP) were chosen to vary the flow rate of the
gasifying agent. Besides, the particle size was varied
between 63 mm and 2 mm and sample masses between 5
and 200 mg were used.
For the determination of the RSA, a surface analyzer
from ThermoQuest was applied. The RSA was determined
by the temperature-programmed desorption technique
(TPD). Temperature-programmed desorption experiments
were done in a separate fixed bed reactor system. A
quadrupole mass spectrometer (Balzers, QMG 420) was
used to continuously monitor the evolution of gaseous
species. The relevant data were collected as ASCII-files by a
personal computer.
½reactive CðOÞ Z ½total CðOÞ K ½stable CðOÞ
(7)
Typical TPD desorption spectra used for the determination
of the total and stable complexes are shown in Fig. 4.
The possibility of inter-particle diffusion limitations was
investigated by comparing the reaction rates obtained from
partial gasification in the fixed bed reactor system with the
reaction rates determined from thermogravimetric experiments in the intrinsic regime.
Table 1
Elemental composition and some initial properties of the biomass chars
Biomass char
cdaf /%
hdaf /%
odaf
(by diff.)/%
ndaf /%
adf /%
rHg /
g cmK3
rHg /
g cmK3
3/1
aBET /
m2 gK1
Beech wood char
Oil palm shell char
89.0
95.0
1.7
1.0
9.0
3.9
0.3
0.1
2.0
2.7
1.16
1.34
2.17
2.06
0.47
0.35
11
7
888
W. Klose, M. Wölki / Fuel 84 (2005) 885–892
Table 2
Reaction conditions for experiments in the intrinsic regime
Biomass
char
Gasifying
agent
Sample
mass (mg)
Particle size
(mm)
Flow rate
(cm sK1)
Beech wood
char
CO2
z10
dK!125
17
H2 O
CO2
z10
z10
dK!125
dK!125
38
17
H2 O
z10
dK!125
38
Oil palm
shell char
a non-linear regression was done to describe the reaction
rate in the intrinsic regime as a function of partial
pressure and reaction temperature. The initial values of
the kinetic parameters used for the non-linear regression
and the final values after 10 iterations are presented in
Table 3.
According to the reaction mechanism suggested by
Ergun and Reif [10,11], the following Langmuir–Hinshelwood-type rate expression was used for the non-linear
regression to describe the reaction kinetics of
Fig. 4. (A) Typical procedure for the determination of the total amount of
surface complexes. (B) Typical procedure for the determination of the
amount of stable surface complexes.
3. Results and discussion
3.1. Reaction kinetics
Since the reaction kinetics should be investigated in
the intrinsic regime, the appropriate reaction conditions
were determined by varying the particle size, flow rate
and the amount of char. In Table 2 the reaction
conditions for the intrinsic regime are summarized. In
the kinetic studies, the reaction rate was determined as a
function of partial pressure of the gasifying agent and the
reaction temperature. The dependency of the reaction rate
on the partial pressure for the biomass chars is shown in
Fig. 5 and the influence of the reaction temperature is
shown in Figs. 6 and 7. In addition to these experiments,
Fig. 5. Specific reaction rate as a function of partial pressure for the
gasification with carbon dioxide and steam.
W. Klose, M. Wölki / Fuel 84 (2005) 885–892
Fig. 6. Specific reaction rate as a function of temperature for the gasification
with carbon dioxide.
the gasification of biomass chars with carbon dioxide and
steam:
EA;2
k20 exp K RT
rm;c Z
(8)
K0
1 C Ds H10 exp K
rm;c Z
k40
1C
m;1
RT
pCO2
EA;4
exp K RT
0
K30
Ds Hm;3
RT
exp K
0
Z
K03
pH 2
zconst:
K01
3.2. Surface related reaction rates
According to the typical desorption spectra in Fig. 4, the
amount of desorbed surface complexes was calculated by
numerical integration with the trapezoidal rule
tbegin
n_i dt yDt0
C DtnK2
pH2 O
whereas the following assumptions were made
p
0
K01
Z CO zconst:
K01
Fig. 7. Specific reaction rate as a function of temperature for the gasification
with steam.
ð tstart
(9)
(10)
(11)
The resulting rate expressions are shown in Figs. 5–7.
For the non-linear regression, the Levenberg–Marquardt
algorithm was used.
889
n_0 C n_ 1
n_ C n_2
C Dt1 1
C.
2
2
n_nK2 C n_ nK1
n_
C n_ n
C DtnK1 nK1
2
2
(12)
The amount of the reactive surface complexes was
obtained by subtracting the amount of stable surface
complexes from the total amount of surface complexes.
The mass of the surface complexes was related to the
according mass of the sample at a certain degree of
conversion. The specific reaction rates were determined by
gas analysis.
The specific RSAs and reaction rates are summarized in
the Table 4. For the calculation of the RSA, it was assumed
890
W. Klose, M. Wölki / Fuel 84 (2005) 885–892
Table 3
Initial and final values of the kinetic parameters for the non-linear regression
K 0 01 resp. K 0 03/Pa
Biomass char; gasifying agent
Initial values
Beech wood char; CO2
Beech wood char; H2O
Oil palm shell char; CO2
Oil palm shell char; H2O
Final values
Beech wood char; CO2
Beech wood char; H2O
Oil palm shell char; CO2
Oil palm shell char; H2O
DSHm,1 bzw.
DSHm,3/kJ molK1
k20 bzw. k40/sK1
EA,2 bzw. EA,4/kJ molK1
3.1!106
1.1!107
1.0!107
2.2!106
K40
K40
K40
K40
1.8!106
1.4!107
5.9!1010
1.1!1011
200
200
300
300
3.2!106
2.0!107
1.1!107
2.5!106
K39
K35
K39
K37
1.8!106
2.1!107
6.5!1010
1.2!1011
200
196
299
299
that one surface complex consists of one oxygen atom per
edge carbon atom [3]. For each reaction, the specific
reaction rates are plotted against the specific surface area
in Fig. 8. The surface related reaction rate was then
determined by linear regression. For the linear regression,
the equation
rm Z rRSA aRSA
(13)
was applied. The surface related reaction rates are presented
in Table 5. In addition, in Table 5, the reaction rates are
compared to reaction rates obtained for the gasification of
coal chars with carbon dioxide [5,6]. To get reliable results,
all experiments were repeated three times. The average
values for the reaction rates and for the RSAs were used in
the Figs. 5–8. The repeatability was in a range of 3%.
4. Conclusions
The reaction mechanisms suggested by Ergun and Reif
[4,5] lead to Langmuir–Hinshelwood rate expressions with
which the dependency of the reaction rate on the partial
pressure of the gasifying agent and the reaction temperature
can be described quite well. Furthermore, the adsorption
enthalpies of carbon dioxide and steam for the adsorption
onto carbon [12] can be used as initial values for the nonlinear regression to get suitable rate expressions. Activation
energies of about 200 and 300 kJ molK1 were chosen to
describe the influence of temperature on the reaction rate of
beech wood char and oil palm shell char, respectively.
The obtained Langmuir–Hinshelwood-type rate
expressions can be used for the modelling of gasification
processes. From TPD studies, the RSA was determined
Table 4
Surface related reaction rate as a function of the degree of conversion for the gasification with carbon dioxide
Biomass char
Carbon dioxide
Beech wood char
Oil palm shell char
Steam
Beech wood char
Oil palm shell char
ac/1
rm,eff/10K5 sK1
aRSA/mgC gCK1
rRSA,eff/10K3 sK1
0.02
0.16
0.61
0.86
0.98
0.01
0.08
0.14
0.25
0.47
0.70
0.91
7.6
4.5
13
22
35
2.7
2.4
2.3
2.4
3.8
5.1
14.2
4
9.6
30.7
89.3
187
1.0
2.5
2.6
3.1
4.9
9.4
28.1
19
4.7
4.2
2.5
1.9
27
9.6
8.5
7.7
7.8
5.4
5.1
0.03
0.24
0.60
0.91
0.45
0.60
0.77
0.90
1.5
2.4
4.7
30
0.7
1.6
2.8
4.9
10.9
21.5
29.1
211
1.9
4.6
15
26
13.8
11.2
16.2
14.2
36.8
34.8
18.7
18.8
W. Klose, M. Wölki / Fuel 84 (2005) 885–892
891
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Fig. 8. Specific reaction rate as a function of the specific reactive surface
area.
and surface related reaction rates were calculated. The
surface related reaction rates are comparable to reaction
rates determined for the gasification of coal chars with
carbon dioxide by other authors at similar reaction
temperatures [5,6]. The reactive surface seems to be
appropriate to describe the relevant surface of different
carbon materials during gasification.
Table 5
Comparison of surface related reaction rates determined for the gasification
of biomass chars and coal chars with carbon dioxide and steam
Carbon material
Gasifying
agent
TR/K
rRSA /(10K3 sK1)
Saran char
PSOC-1098 char
Carbosieve
Beech wood char
CO2
CO2
CO2
CO2
H2O
CO2
H2O
1133
1093
1133
1023
1023
1053
1053
6
4
39
2
14
5
19
Oil palm shell char
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