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The gasification reactivity of high-heating-rate chars in single and mixed atmospheres of H2O and CO2

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  • IMT Mines Albi , France

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Gasification reactivity of high-heating-rate chars (HHR-chars) in steam, carbon dioxide and their mixtures was investigated in a new macro-TG experimental device. The higher reactivity of the HHR-chars was highlighted by a comparison with reference chars prepared at a low heating rate (LHR-chars). It was found that the char reactivity in a mixed atmosphere of steam and carbon dioxide can be expressed as the sum of the individual reactivities obtained in single-atmosphere gasification experiments. This result was not dependent on the pyrolysis heating rate. In addition, gas-alternation gasification experiments – for both HHR-chars and LHR-chars – showed that gasifying the char with CO2 up to 30% of conversion does not affect its reactivity to H2O. Altogether, the results tend to indicate that the two reactant gases H2O and CO2 react on separate active sites when mixed atmospheres are used, and that CO2 does not affect the char structure to favor or inhibit the char–H2O gasification reaction.
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The gasification reactivity of high-heating-rate chars in single and mixed
atmospheres of H
2
O and CO
2
C. Guizani
, F.J. Escudero Sanz, S. Salvador
RAPSODEE, Mines Albi, Route de Teillet, 81013 ALBI CT Cedex 09, France
highlights
"
Reliable kinetic data for HHR-char gasification reactions with H
2
O and CO
2
.
"
Introducing CO
2
alongside H
2
O results in a higher char gasification reaction rate.
"
Gasifying a char under CO
2
does not affect its reactivity toward H
2
O.
"
Char reactivity in a H
2
O+CO
2
atmosphere is the sum of the individual reactivities.
article info
Article history:
Received 29 December 2012
Received in revised form 11 February 2013
Accepted 12 February 2013
Available online 6 March 2013
Keywords:
Biomass char
Gasification
H
2
O and CO
2
mixed atmospheres
Kinetics
abstract
Gasification reactivity of high-heating-rate chars (HHR-chars) in steam, carbon dioxide and their mix-
tures was investigated in a new macro-TG experimental device. The higher reactivity of the HHR-chars
was highlighted by a comparison with reference chars prepared at a low heating rate (LHR-chars). It
was found that the char reactivity in a mixed atmosphere of steam and carbon dioxide can be expressed
as the sum of the individual reactivities obtained in single-atmosphere gasification experiments. This
result was not dependent on the pyrolysis heating rate. In addition, gas-alternation gasification experi-
ments – for both HHR-chars and LHR-chars – showed that gasifying the char with CO
2
up to 30% of con-
version does not affect its reactivity to H
2
O. Altogether, the results tend to indicate that the two reactant
gases H
2
O and CO
2
react on separate active sites when mixed atmospheres are used, and that CO
2
does
not affect the char structure to favor or inhibit the char–H
2
O gasification reaction.
Ó2013 Elsevier Ltd. All rights reserved.
1. Introduction
Both industrialized and developing countries are today intensi-
fying their work on the development of renewable energies. This
increasing interest is a response to the unavoidable depletion of
fossil fuels and to continuous and alarming environmental prob-
lems, especially global warming. Global warming is a direct conse-
quence of the increasing concentration of greenhouse gases in the
atmosphere, particularly CO
2
, whose concentration has risen dras-
tically since the industrial revolution [1].
The conversion of biomass to energy is considered to be a path-
way towards clean and renewable energy production, because of
the availability of the resource and the carbon-neutral feature of
the thermochemical processes [2].
Among these thermochemical processes, biomass gasification is
gaining further interest as it allows the production of clean
fuel gases (e.g. H
2
, CO, CH
4
) that can be used either to produce
electricity and heat or as an input stream to produce chemicals
or transportation biofuels [3].
Biomass gasification can be processed with various gasifying re-
agents like air, steam or carbon dioxide [4]. Using carbon dioxide in
such a process would provide a long term solution to mitigate its
increasing concentration in the atmosphere. The CO
2
will be then
incorporated in a valorisation cycle for the production of market-
able fuels, rather than simply being captured and stored.
The biomass-gasification reaction includes three main steps:
pyrolysis, volatile-matter reforming and char gasification. The
char–gasification reaction is considered to be the limiting step of
the process because it is kinetically slow compared to the two other
steps.
A huge amount of studies can be found in the literature, as well
as very good reviews on char gasification in steam- or carbon diox-
ide-containing atmospheres [4–6]. Still, the majority of these stud-
ies do not tackle the issue of char gasification in mixed
atmospheres; only a few do so, and these are performed mainly
on coal–char gasification.
Their conclusions differ from one study to another; some
authors concluded that adding the carbon dioxide alongside the
0016-2361/$ - see front matter Ó2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.fuel.2013.02.027
Corresponding author. Tel.: +33 619572786.
E-mail address: cguizani@mines-albi.fr (C. Guizani).
Fuel 108 (2013) 812–823
Contents lists available at SciVerse ScienceDirect
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journal homepage: www.elsevier.com/locate/fuel
steam slows down the gasification reaction: (i) by inhibition and
competition for the same carbon active sites [7–9]. For instance,
in their work on coal–char gasification, Robert and Harris mea-
sured the gasification-reaction rate in single and mixed atmo-
sphere of H
2
O and CO
2
in a thermogravimetric apparatus and
concluded that the presence of CO
2
reduces the rate of the C–
H
2
O reaction [7].
Others think that the two gases operate on separate active sites:
(ii) by passive cooperation [10–12]. The work of Everson et al. de-
scribes this assertion well [11]. The authors studied the gasification
reactions of coal chars in a TG apparatus with different atmosphere
compositions and concluded that the char–gasification reaction
with mixtures of CO
2
and H
2
O is best described by the sum of
the single reaction rates. The same observation was made in the
work of Chen et al. on sewage–sludge–char gasification in a fluid-
ized bed [13]. The authors again found that the reaction rate in a
mixture of CO
2
+H
2
O is well represented by the sum of the individ-
ual reaction rates.
Other researchers think that there is a kind of (iii) synergy or
active cooperation between the two gases that leads to an en-
hanced char reactivity [14,15]. For instance, Tagutchou [14] found
that adding CO
2
alongside steam leads to enhanced char reactivity
which is superior to the sum of the individual reactivities obtained
respectively with steam and carbon dioxide. A more detailed liter-
ature review and discussion is presented later in this paper as an
approach for an extended experimental plan.
The different findings and conclusions in the literature make it
difficult to draw a clear conclusion on the unfolding of the gasifica-
tion reaction in a mixed atmosphere of H
2
O and CO
2
. The present
work was thus performed with the aim to further understand the
reaction mechanisms of biomass gasification in mixed atmo-
spheres of steam and carbon dioxide.
2. Materials and methods
2.1. Macro-TG experimental device
The new Macro-TG experimental device (Fig. 1) consists of three
major parts:
The heating system, including a liquid H
2
O evaporator, a gas
preheater and an electrically-heated alumina reactor.
The gas flow control system consisting of 3 mass-flow meters/
controllers.
The weighing system that comprises an electronic scale, a stand
and a platinum basket.
The 2-m long, 75-mm i.d. alumina reactor is electrically heated.
The temperatures of the three reactor zones (high, medium and
low) are independently controlled to ensure good temperature
homogeneity throughout the furnace.
Gas flow rates are controlled by means of mass-flow meters/
controllers. Before entering the reaction zone, the reactant gases
(N
2
,CO
2
and H
2
O) are preheated up to the reactor temperature.
When H
2
O is added in the gasification medium, the H
2
O + gas mix-
ture passes first through an electrically-heated evaporator main-
tained at 180 °C to vaporize the water. The reacting gas flow
inside the reactor is laminar and flowing at an average velocity
of 0.2 m/s.
The weighing system comprises a set of electronic scales with
an accuracy of ±0.1 mg, a metallic stand placed over the balance,
on which are fixed three ceramic hollow tubes with a length of
1 m and a 2.4 mm external diameter. These ceramic tubes hold
up a platinum basket with a 50 mm diameter, a solid bottom and
a side wall made from a 500
l
m grid to allow the gas to pass
through it. The biomass samples are placed into it and are then
pyrolysed and gasified in the hot furnace.
The weighing system can be moved in the vertical direction
using a crank handle. The platinum basket can hence be introduced
into the hot furnace within less than 15 s.
Altogether, the macro-TG experimental device has the advan-
tage of being of a far greater experimental scale than conventional
TG-devices. This makes sample representativity better because an
average result for several woodchips is obtained for each run. The
wood chips or char particles are not ground as they use to be when
the gasification was performed in a classic TG device. This is impor-
tant because the size-reduction process leads to modifications in
the structural and chemical composition, loss of fibrous texture
and heterogeneous dispersion of catalytic minerals, which vary
according to the biomass particle-size range [16]. This would affect
the biomass reactivity data, which may not be representative of the
raw biomass. Moreover, the biomass particles are submitted to a
thermal shock similar to that endured when they are introduced
in a fluidized bed.
Finally, to our best knowledge, it is not possible to perform iso-
thermal gasification experiments in a classic TG device without
preheating the char sample to the desired temperature over a con-
siderable time. This thermal treatment has an impact on the char
reactivity. In fact, It has been widely demonstrated for several bio-
masses such as maize stalk, rice straw, cotton straw, rice husk, Bra-
zil Nut shells and eucalyptus that the char reactivity decreases
with thermal treatment as a result of morphological modifications
encompassing the evolution of the level and type of porosity and
the average pore size [17–21]. The char structure becomes increas-
ingly condensed and ordered when increasing the heat-treatment
temperature and duration. This thermal annealing phenomenon
would surely distort the real char-reactivity data. On the contrary,
in our case, the biomass sample is introduced into the reactor with-
in 15 s. The pyrolysis takes less than 1 min to be fulfilled.
2.2. Biomass feedstock and char preparation
2.2.1. Biomass feedstock
The biomass feedstock (beech wood chips) was provided by a
company called SPPS (France). The woodchips were firs sieved to
select particles with a size ranging between 4 and 5 mm and a
thickness of 1–2 mm.
The size and thickness of biomass particles may greatly influ-
ence the rate of the gasification reaction if they impact on the heat
and mass transfer inside the particle [6,22]. In a recent study
[23,14], the authors demonstrated that the gasification rate was
not influenced when varying the char particle size in the range of
10.5–15 mm. The influencing characteristic dimension was rather
the particle thickness, as it slowed down the reaction rate by
1.6 times when it was increased from 1.5 to 6 mm for a constant
size of 10.5 mm. No differences were observed between thick-
nesses of 2.5 and 1.5 mm, which suggests that the reaction is
chemically controlled below a 2.5 mm particle thickness. On the
basis of these observations and of the similarities between the
raw biomasses used (beech wood chips) and between the experi-
mental devices (Macro-TG), we performed the gasification reac-
tions with biomass and char particles having a size in the range
of 4–5 mm and a thickness of about 1–2 mm.
2.2.2. Experimental procedure for char preparation and gasification
A mass of wood chips of 0.8–1 g is introduced in the platinum
basket; the biomass particles are spaced widely enough to avoid
chemical and thermal interactions. After heating the reactor to
the desired temperature, the weighing system is lifted up using
the crank handle; the platinum basket – containing the wood chips
– is introduced into the hot furnace in less than 15 s. The biomass
C. Guizani et al. / Fuel 108 (2013) 812–823 813
is firstly pyrolysed in a flow of nitrogen until reaching a constant
mass, corresponding to the char. This procedure produces special
chars prepared at a high heating rate which was estimated to be
around 100 K/s [24]. When the mass of char is stabilized, the dis-
played weight on the electronic scales is reinitialized to zero and
the reactant gas flow is established, marking the beginning of the
char gasification stage. When introducing the reacting gas (CO
2
or H
2
O) alongside the nitrogen, we observe a flow-deceleration ef-
fect (gas pressure on the platinum basket) on the force registered
by the scales. This effect is corrected afterwards in the data pro-
cessing phase. The mass of char begins to decrease progressively
until it reaches a plateau corresponding to the end of the experi-
ment. The weighing system is then moved downwards using the
crank handle and the residual ashes are weighed after cooling. Fol-
lowing this procedure, we can obtain an accurate value for the
mass of char that was converted during the gasification reaction.
The study focuses mainly on the gasification of biomass chars
prepared at a high heating rate in the Macro-TG device, as de-
scribed in the previous paragraph. However, in order to highlight
the higher reactivity of these ‘‘HHR-chars’’ (in terms of reactivity
towards H
2
O and CO
2
), we also conducted gasification experiments
with chars prepared at a low heating rate ‘‘LHR-chars’’.
These LHR-chars were obtained after a slow pyrolysis in a retort
furnace with a heating rate of 5 °C/min up to 550 °C and a
residence time at the final temperature of 30 min. The slow pyro-
lysis was performed under a nitrogen flow of 2 Nl/min. The LHR-
char samples were then cooled to room temperature and stocked
in a sealed recipient for gasification tests in the M-TG device. With
a low heating rate, the char yield was about 24.8%, whereas it was
much lower with a high heating rate. It decreased slightly when
increasing the reaction temperature, from 9.89% at 850 °Cto
7.87% at 950 °C. These results were expected in view of previous
studies on biomass pyrolysis [25,19,4].
Table 1 lists the results of the proximate and ultimate analysis
of the raw biomass wood chips, the LHR-chars and the HHR-chars
obtained at three temperatures. The results are reported on a dry
basis.
The ash content rose with an increase in the pyrolysis heating
rate, due to lower char yields and little devolatilisation of the min-
eral species. One can note that the fuel-nitrogen remained in the
char. The concentrations of hydrogen and oxygen decreased with
an increase in the temperature.
2.2.3. Experimental conditions and data analysis method
The gasification experiments were performed at atmospheric
pressure with operating conditions similar to those encountered
in fluidized-bed gasifiers. The reactor temperature was in the range
of 800–950 °C and the gasifying-medium partial pressure in the
Fig. 1. Macro-TG experimental device scheme.
814 C. Guizani et al. / Fuel 108 (2013) 812–823
range of 0.1–0.3 atm. Table 2 gives the operating conditions for the
different gasification experiments.
2.2.4. Method of data analysis
The normalized mass or conversion ratio ‘X’ during the gasifica-
tion reaction is calculated according to:
X
ðtÞ
¼m
ð0Þ
m
ðtÞ
m
ð0Þ
m
ðashÞ
ð1Þ
where m
0
,m
(t)
and m
ash
are respectively the initial mass of char, the
mass at any time ‘‘t’’ and the mass of the residual ash.
The gasification experiments were reproduced 2–5 times and
showed a good repeatability with deviation less than 12%, which
is considered acceptable regarding the heterogeneity of the wood
material. Mass-loss data curves were firstly smoothed using a poly-
nomial least-square function covering a fixed time period before
and after each point. Precautions were taken for restoring
smoothed data, with high fidelity to the experimental results.
These data were then used to calculate the instantaneous reactivity
of the char throughout the gasification.
Reactivity data were obtained following the next equation:
R
ðXÞ
¼ 1
m
ðtÞ
dm
ðtÞ
dt ¼1
1X
ðtÞ
dX
ðtÞ
dt ð2Þ
The char undergoes structural modifications throughout the
gasification reaction due to phenomena such as pore enlargement,
coalescence or blocking. This leads to variations in the number of
carbon-active sites C
t
ðXÞavailable for the gasifying agents. The
reactivity, which is a function of temperature, gas partial pressure
and available reactive surface, is therefore continuously changing
during the gasification. It is consequently expressed by means of
a chemical kinetic term accounting for temperature and partial
pressure effects RðXÞ
ð
T;P
i
Þ, and a reactivity profile FðXÞthat aims
to describe the effects of available reactive surface.
The reactivity must therefore refer to a specific conversion level.
Reactivities at 10% or 50% of char conversion are often used for the
determination of the kinetic parameters; the latter is actually the
most commonly selected parameter in several similar investiga-
tions [26,27,7]. In our study, reactivity at 50% conversion level is
taken as a reference. Assuming that the structural function does
not depend on the temperature and pressure ranges of the gasifica-
tion experiments, the reactivity can be expressed as follows:
R
ðT;P
i
;XÞ
¼Rð50Þ
ðT;P
i
Þ
FðXÞð3Þ
An nth order kinetic model following the Arrhenius law for the
reactivity-temperature dependence and a power law for the
reactivity-gas partial pressure dependence is considered for
the determination of the kinetic parameters of the steam and car-
bon dioxide gasification reactions. The reactivity at 50% conversion
level is given by:
Rð50Þ
ðT;P
i
Þ
¼k
ðTÞ
P
n
i
ð4Þ
k
ðTÞ
¼Aexp E
RT
 ð5Þ
The reactivity profile expression can be developed from general
structural models such as uniform conversion models, shrinking-
core models, grain models or random-pore models that may con-
tain one or more parameters to adjust so as to match the parame-
ters of the experimental data [6]. It is however worth noting that
the structural modifications are not solely responsible for the reac-
tivity change throughout the gasification. Other factors intervene,
such as the char inner mineral species concentration and types
[28–30,25,31], the thermal annealing phenomena occurring in par-
allel with the gasification reaction [32], and also the type of gasify-
ing media, as it has been demonstrated that the char contact with
the steam drastically changes its structure into a more ordered one
[33–35]. Owing to these observations and to the difficulties in
determining the individual contribution of each of these parame-
ters on the gasification reactivity, we opted for a determination
of an empirical formulation for the structural term FðXÞwhich is
assumed to encompass all the influencing parameters that cause
the reactivity change along the gasification reaction.
The structural function FðXÞ, which is a normalized reactivity,
can be calculated at any conversion level as follows:
FðXÞ¼ RðXÞ
Rð50Þð6Þ
In the literature, the ratio
RðXÞ
RðrefÞ
is calculated within a conversion
level range where experimental errors are, in the author’s appreci-
ation, acceptable. For instance, some authors referred to a (0.2–0.8)
conversion range with a reference at X= 0.2 [36], others chose
ranges between (0.2–0.8) and (0.15–0.9) with a reference reactivity
at X= 0.5 [26,23]. In our study, FðXÞis determined in the conver-
sion level range of 0.2–0.9. This range was selected to minimize
weight measurement uncertainties at the small mass loss in the
early stages of the reaction (X= 0–0.2), and to avoid high reactivity
values as the mass goes to zero in the final stages of the gasification
reaction (X= 0.9–1). A 5th order polynomial regression is applied
to the experimental Xand F(X) data to determine the reactivity
profile.
3. Results and discussion
In this section we shall first present results of HHR-char gasifi-
cation in single atmospheres containing H
2
OorCO
2
and then
determine the intrinsic kinetic parameters and the reactivity pro-
files for each case. The high reactivity of the HHR-chars will be also
highlighted through a comparison with LHR-chars. Finally we shall
take a comprehensive, experiment-based approach to the under-
standing of the mechanisms involved in char gasification in mixed
atmospheres of steam and carbon dioxide.
Table 1
Proximate and ultimate analysis of the biomass samples (dry basis).
Proximate analysis (%) Ultimate analysis (%)
VM Ash FC C H O N
Wood chips 88.1 0.4 11.6 46.1 5.5 48.1 0.10
LHR-Char 20.03 1.88 78.09 82.06 2.85 12.88 0.30
HHR-char 850 °C – 3.75 83.51 0.86 11.60 0.28
HHR-char 900 °C – 4.14 85.56 0.80 8.42 1.04
HHR-char 950 °C – 4.15 85.83 0.91 8.07 1.05
Table 2
Operating conditions of the gasification experiments.
Reacting medium Reacting gas partial pressure (atm) Temperature (°C)
H
2
O 0.1, 0.2, 0.3 800, 850, 900, 950
CO
2
0.1, 0.2, 0.3 850, 900, 950
H
2
O/CO
2
0.1/0.1;0.1/0.2; 0.2/0.1 900
C. Guizani et al. / Fuel 108 (2013) 812–823 815
3.1. HHR-char gasification in single atmospheres of steam and carbon
dioxide
To determine the kinetic parameters and the reactivity profiles
for the H
2
O and CO
2
char–gasification reactions, we performed
experiments in which we varied the gas partial pressure at con-
stant temperatures and vice versa. Although all the possible com-
binations of temperature and gas partial pressure were tested, in
the following sections we present only some results of reference
experiments.
3.1.1. Char–H
2
O gasification experiments
3.1.1.1. Influence of the temperature and H
2
O partial pressure on the
char–H
2
O gasification reaction. Fig. 2a and b illustrate respectively
the effect of the temperature and steam partial pressure on the
H
2
O–char gasification rate.
The influence of temperature was evaluated in the range of
800–950 °C. Fig. 2a shows the char conversion versus time in gas-
ification experiments with 20% H2O in the gasifying medium at
800, 850, 900, and 950 °C. Increasing the temperature from 800
to 950 °C reduced the time required for 90% conversion by a ratio
of more than fivefold.
A temperature of 900 °C was taken as a reference to evaluate
the role of H
2
O partial pressure in the char–H
2
O gasification reac-
tion. A conversion level of 90% was reached respectively after 220,
330, and 580 s with H
2
O concentrations of 30%, 20%, and 10% in the
gasifying medium. That is to say, increasing the H
2
O concentration
from 10% to 30% results in 2.5 times higher char reactivity.
3.1.1.2. Determination of kinetic parameters for the H
2
O–char gasi-
fication reaction. Adopting an nth-order model and a reference
reactivity at 50% of conversion provides a set of linear equations
– when taking the logarithm of both sides of Eq. (4) – for the differ-
ent temperatures and H
2
O partial pressures.
LnðR
50
Þ¼lnðAÞE
R
1
TþnlnðP
H
2
O
Þð7Þ
The set of equations was put in a matrix form to determine A,E
and nwith a minimization of the error according to the least-
squares method. The logarithm of R
50
is plotted versus the inverse
of the temperature in Fig. 3 to illustrate the temperature depen-
dence of the reactivity following an Arrhenius law for the different
steam partial pressures.
The linear dependence of the logarithm of R
50
on the inverse of
the temperature was verified with a good correlation. Similarly, the
dependence of the logarithm of R
50
on the steam partial pressure
for the different gasification temperatures was also verified. The
correlation coefficient is superior to 0.99, thus validating the pro-
posed model. The derived kinetic parameters are respectively:
E= 139 kJ/mol, A= 26.30 10
3
s
1
bar
n
and n= 0.64. A compari-
son of the char reactivity with literature data is shown in Fig. 4
for a steam partial pressure of 0.2 atm. The conversion level at
which the reactivity is calculated is indicated on the figure with
the references.
The derived kinetic parameters are in accordance with recent
studies on steam-beech char gasification. Moreover, the activation
energy and the reaction order are in the range of the values re-
ported in Di Blasi’s review for biomass char–steam gasification
reactions, i.e. 40–240 kJ mol
1
for ‘E’ and 0.4–1 for ‘n[4].
3.1.1.3. Determination of the reactivity profile F(X)-H
2
O. Reactivity
profiles for the different H
2
O–char gasification experiments are
plotted in Fig. 5. Except for a few irregularities, probably due to
the measurement uncertainties, all reactivity profiles are mono-
tonically increasing functions and almost superposed.
The temperature and H
2
O partial pressure would not therefore
seem to affect the reactivity evolution tendency in the studied
range of parameters. The average of the functions obtained for
the different gasification experiments can be expressed through:
F
H
2
O
ðXÞ
¼0:008230X
5
þ0:02038X
4
þ0:11367X
3
þ0:23074X
2
þ0:56013Xþ1:12488 ð8Þ
The weak influence of the temperature and H
2
O partial pressure
on the shape of the reactivity profile was also observed by [23]
whereas in Barrio’s study there was a clear influence of the temper-
ature on the reactivity profile, especially in the final stages of con-
version, as the function decreased with an increase in the
temperature. This can be imputable to mineral species loss or
accentuated thermal annealing of the char which exhibited a lower
reactivity [37].
0 250 500 750 1000 1250 1500
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (s)
Conversion level X
20 % H
2
O 800
°
C
20 % H
2
O 850
°
C
20 % H
2
O 900
°
C
20 % H
2
O 950
°
C
0 100 200 300 400 500 600 700 800
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (s)
Conversion level X
10 % H
2
O 900
°
C
20 % H
2
O 900
°
C
30 % H
2
O 900
°
C
(a)
(b)
Fig. 2. Influence of the temperature (a) and steam partial pressure (b) on the char
gasification rate.
8 8.2 8.4 8.6 8.8 9 9.2 9.4
x 10
-4
-7
-6.5
-6
-5.5
-5
-4.5
-4
1/T (K
-1
)
ln (R
50
)
P
H
2
O
=0.1 atm
P
H
2
O
=0.2 atm
P
H
2
O
=0.3 atm
Mode l : P
H
2
O
=0.1 atm
Mode l : P
H
2
O
=0.2 atm
Mode l : P
H
2
O
=0.3 atm
Fig. 3. Arrhenius plots for H
2
O gasification of HHR-chars.
816 C. Guizani et al. / Fuel 108 (2013) 812–823
3.1.2. Char–CO
2.
gasification experiments
3.1.2.1. Influence of the temperature and CO
2
partial pressure on the
char–CO
2
gasification reaction. Fig. 6a and b illustrate respectively
the effect of the temperature and CO
2
partial pressure on the
CO
2
–char gasification rate. Experiments with CO2 partial pressure
of 0.2 atm were taken as references to evaluate the role of the tem-
perature. As shown in Fig. 6a, increasing the temperature by 100 °C
reduced the time required for a 90% conversion by more than
3.5 times.
The results of the char gasification experiments at a reference
temperature of 900 °C with CO
2
partial pressure ranging from 0.1
to 0.3 atm illustrate the effect of the CO
2
partial pressure on the
gasification rate. Increasing the CO
2
concentration in the gasifying
medium from 10% to 30% allows 50% of char conversion to be
reached in almost half the time. 90% conversion times are respec-
tively 800, 590, and 460 s in gasifying atmospheres containing 10%,
20%, and 30% of CO
2
. Similar trends were found at 850 °C and
950 °C.
3.1.2.2. Determination of kinetic parameters for the CO
2
–char gasifi-
cation reaction. The same procedure as for steam gasification was
followed to determine the kinetic parameters for the char–CO
2
gas-
ification reaction. Fig. 7 shows the Arrhenius dependence of the
char reactivity on the temperature for the different CO
2
partial
pressures. The linear dependence between the logarithm of R
50
and the reciprocal temperature is verified with a good correlation.
The solid lines in the Arrhenius represent the calculated reactivity
in the temperature range of the study.
Likewise, we obtained a linear dependence of the logarithms of
R
50
and CO
2
partial pressure at the different gasification tempera-
tures. The results are not plotted here.
The model is also verified for CO
2
gasification experiments with
a very good determination coefficient R
2
= 0.996. The derived ki-
netic parameters are: E= 154 kJ/mol, A= 55.18 10
3
s
1
bar
n
and n= 0.55, which are in the respective value ranges reported in
Di Blasi’s review for biomass char–CO
2
gasification reactions [4].
3.1.2.3. Determination of the reactivity profile F(X)-CO
2
.Fig. 8 shows
the reactivity profiles obtained for the CO
2
–char gasification exper-
iments at different temperatures and CO
2
partial pressure. The char
reactivity increases with the gasification; this reactivity tendency
is typical of the majority of biomass chars in contrast with that
of coal chars, which decreases as the conversion level increases
[38].
88.2 8.4 8. 6 8.8 99.2 9.4
x 10
-4
-9
-8
-7
-6
-5
-4
-3
Fig. 4. Literature review on the reaction kinetics of the gasification of beech chars
with steam (P
H2O
= 0.2 atm).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Conversion level X
Rea c tivity p rofile F(X)-(H
2
O)
20 % H2O 800
°
C
30 % H2O 800
°
C
10 % H2O 850
°
C
20 % H2O 850
°
C
30 % H2O 850
°
C
10 % H2O 900
°
C
20 % H2O 900
°
C
30 % H2O 900
°
C
10 % H2O 950
°
C
30 % H2O 950
°
C
Average reactivity profile
Fig. 5. Reactivity profile F(X) in the H
2
Ochar gasification experiments.
0 100 200 300 400 500 600 700 800 900 1000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (s)
Conversion level X
Conversion level X
10 % CO
2
900
°
C
20 % CO
2
900
°
C
30 % CO
2
900
°
C
0 200 400 600 800 1000 1200 1400
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (s)
20 % CO
2
850
°
C
20 % CO
2
900
°
C
20 % CO
2
950
°
C
(b)
(a)
Fig. 6. Influence of the temperature (a) and CO
2
partial pressure (b) on the char
gasification rate.
8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9
x 10
-4
-7
-6.5
-6
-5.5
-5
-4.5
1/T (K
-1)
ln (R
50)
PCO2
=0.1 atm
PCO2
=0.2 atm
PCO2
=0.3 atm
Model : PCO 2
=0.1 atm
Model : PCO 2
=0.2 atm
Model : PCO 2
=0.3 atm
Fig. 7. Arrhenius plots for CO
2
gasification of HHR-chars.
C. Guizani et al. / Fuel 108 (2013) 812–823 817
Apart from small discrepancies, all the reactivity profiles are
superposed. Neither the temperature nor the CO
2
concentration af-
fects the reactivity profile. The reactivity profile is clearly related to
other phenomena such as the structural modifications and the
increasing concentration of minerals in the biomass char, as sug-
gested in many similar studies. Weak effects of the temperature
and CO
2
partial pressure on the reactivity profile were observed
in the work of Vandesteene et al. [23], while the temperature
clearly affected the reactivity profile for birch char gasification
experiments, as presented elsewhere [36].
The average of the obtained functions in the different gasifica-
tion experiments has the following expression:
F
CO2
ðXÞ
¼0:014422X
5
þ0:081024X
4
þ0:1379X
3
þ0:2142X
2
þ0:5254Xþ1:1175 ð9Þ
3.1.3. HHR-chars high reactivity
In order to highlight the higher reactivity of the HHR-chars, we
performed gasification experiments under respectively 20% of
steam and 20% of CO
2
at 900 °C with the beech char particles pre-
pared with a low heating rate (5 °C/min). The results are plotted in
Fig. 9 in terms of average reactivity calculated in the conversion
range of 20–90%. The effect of the heating rate is clear, as the
HHR-char reactivity was more than 3.5 times higher in 20% of
steam than for the LHR-char in the same operating conditions. Sim-
ilarly, the LHR-char reactivity in an atmosphere containing 20%
CO
2
was estimated at 0.001 g/(g s) while it was 4.3 times higher
for the HHR-char in the same operating conditions.
These results are in accordance with the literature [25,19,39],
although the effect of the heating rate on the char reactivity is
much more pronounced in the present study than in that of
Guerrero et al. [19] for eucalyptus char gasification, and comes
close to the observations made by Mermoud et al. [25].
In the work of Guerrero et al. [19], the increase in the char reac-
tivity with the HR was not as marked as in the present work or in
that of Mermoud et al. [25]. Mermoud et al. found that the char
yields under low and high heating rates were respectively 24.3%
and 14.2% with an increase in the ash amount when increasing
the heating rate. The effect of the heating rate on the char reactiv-
ity with steam was as significant as it is in the present work, since
the HHR-char reactivity was 2.6 times higher than that of the LHR
char. The authors concluded that besides the more porous struc-
ture obtained in HHR-chars, the mineral species seems to play a
crucial role.
In the present work, the HHR-chars exhibited high gasification
rates which may be imputable to two main factors: the small par-
ticle thickness and the high pyrolysis heating rate. The former fac-
tor minimizes the mass-transfer limitations while the second leads
to a highly porous char having a mineral content more than twice
as high as that of LHR char. These combined parameters are known
to greatly enhance the char reactivity.
3.2. Char gasification in mixture of H
2
O+CO
2
In order to study the effect of introducing the CO
2
as a co-reac-
tant alongside steam, we performed gasification experiments at
900 °C with a steam concentration of 10% and a CO
2
concentration
increasing from 0% to 30%. Conversion levels versus time plots are
shown in Fig. 10.
The CO
2
introduction clearly enhances the reaction rate. 90%
conversion time was about 580 s with 10% H
2
O and no CO
2
in
the gasifying medium but decreased with the increase in the CO
2
concentration in the input gas to 215 s with a CO
2
molar fraction
of 30%. The evolution of the char average reactivity – calculated
in the conversion range of 20–90% – with the increasing CO
2
con-
centration is illustrated in Fig. 11. The average reaction rate with
20% of CO
2
introduced alongside steam was twice that of free-
CO
2
gasification experiments with only 10% steam in the gasifying
medium.
It is clear that the introduction of the CO
2
would not inhibit the
gasification reaction, as proposed elsewhere [7]. This assumption is
not valid for the present gasification experiments.
To gain greater understanding of the gasification reaction in the
CO
2
/H
2
O mixture, we compared the char reactivity obtained in
mixed atmospheres with the sum of the reactivities obtained in
single-atmosphere experiments for the same steam and carbon-
dioxide partial pressures.
Reactivity curves in the conversion range of 20–90% are plotted
in Fig. 12. Apart from small discrepancies probably due to experi-
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
1
2
3
4
5
6
Conversion level X
Reactivity profile F(X)-(CO
2
)
10 % CO2 850
°
C
20 % CO2 850
°
C
30 % CO2 850
°
C
10 % CO2 900
°
C
20 % CO2 900
°
C
30 % CO2 900
°
C
10 % CO2 950
°
C
20 % CO2 950
°
C
30 % CO2 950
°
C
Average reactivity profile
Fig. 8. Reactivity profile F(X) in the CO
2
char gasification experiments.
HHR-20% H
2
O LHR-20% H
2
O HHR-20% CO
2
LHR-20% CO
2
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
Average reactivity (g/(g.s)
LHR
HHR
HHR
LHR
CO2 gasificationH2O gasification
Fig. 9. HHR and LHR char average reactivity at 900 °C.
0 100 200 300 400 500 600 700 800
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (s)
Conversion level X
10 % H2O 900
°
C
10 % H2O + 10 % CO2 900
°
C
10 % H2O + 20 % CO2 900
°
C
10 % H2O + 30 % CO2 900
°
C
Fig. 10. Effect of the CO
2
co-feeding next to steam on the gasification reaction rate.
818 C. Guizani et al. / Fuel 108 (2013) 812–823
mental errors, an additive law is valid to describe char gasification
under mixed atmospheres.
A first a priori conclusion would be that H
2
O and CO
2
are oper-
ating on separate active sites (Passive cooperation). Nevertheless,
this observation may be due to two opposed actions resulting in
an ‘‘apparent additive law’’. To further interpret this result, we pro-
pose in the next section a detailed reflexion based on a literature
review and on additional gasification experiments.
3.2.1. On the understanding of the char gasification reaction in mixed
atmospheres of CO
2
+H
2
O
3.2.1.1. Literature review and discussion. Concerning the mecha-
nisms involved in the char gasification reaction in mixed atmo-
spheres, three main assumptions are held in the literature:
CO
2
and H
2
O gasification reactions occurring on common active
sites (Inhibition):In a previous study on coal–char gasification,
the authors observed that the char reactivity decreased when add-
ing CO
2
alongside steam and concluded that there was a competi-
tion between the two gases for the access to the carbon active sites
[7]. Although the chars are different (coal and biomass chars) –
which may lead to different results, the authors limit their obser-
vations to the first 10% of char conversion, which is not represen-
tative of the overall gasification reaction. The coal char may
exhibit a different behavior beyond 10% of conversion, mainly
when pores open more widely and gasification spreads homoge-
neously through the particle. In a more recent study, the authors
found the char–H
2
O gasification reaction was independent of the
char–CO
2
reaction, while the latter is inhibited by the former [9].
Others proposed a model based on a partial sharing of active sites.
The two gases are competing in part of the active sites, while react-
ing separately in their own ones [8].
This assumption is clearly not valid regarding the result of the
present study. Other studies on coal–char gasification showed that
there was no inhibition between the gasifying agents, but rather a
passive cooperation on separate active sites [11,10,34]. This will be
discussed in the next paragraph.
CO
2
and H
2
O gasification reactions on separate active sites (Passive
cooperation):The model of passive cooperation assumes that the two
gases react on separate active sites without influencing each other.
In their study on coal–char gasification, Bliek et al. [12] found that
the overall carbon-conversion rate in the presence of CO
2
and H
2
O
is the sum of the single char reactivities. This assumption was also
held by Tay et al. [34] for coal char gasification. The authors found
that the gasification rate in mixed atmospheres of O
2
+H
2
O+CO
2
was approximately equal to the sum of the gasification rates in the
respective single atmospheres. They suggest that the additivity in
10% H
2
O10% H
2
O + 10%
CO
2
10% H
2
O + 20%
CO
2
10% H
2
O + 30%
CO
2
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
Average reactivity (X=0.2-0.9) (g/(g.s)
10 % H2O 900
°
C
10 % H2O + 10 % CO2 900
°
C
10 % H2O + 20 % CO2 900
°
C
10 % H2O + 30 % CO2 900
°
C
0.005
0.0077
0.0097
0.0134
Fig. 11. Char average reactivity evolution with the increasing amount of CO
2
introduced next to 10% steam.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.005
0.01
0.015
0.02
0.025
0.03
Conversion level X
Reactivity (g/(g.s)
Rmix [ 10 % H2O + 10 % CO2]
Rsing [10 % H2O] + R sing[10 % CO 2]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.005
0.01
0.015
0.02
0.025
0.03
Conversion level X
Reactivity (g/(g.s)
Rmix [ 10 % H 2O + 20 % CO2]
Rsing [10 % H2O] + Rsing [20 % CO2]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.005
0.01
0.015
0.02
0.025
0.03
Conversion level X
Reactivity (g/(g.s)
Rmix [ 10 % H 2O + 30 % CO2]
Rsing [10 % H2O] + R sing[30 % CO2]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.005
0.01
0.015
0.02
0.025
0.03
Conversion level X
Reactivity (g/(g.s)
Rmix [ 20 % H 2O + 10 % CO2]
Rsing [20 % H2O] + R sing[10 % CO2]
Fig. 12. Comparison of the HHR-char reactivity in mixed atmosphere with the sum of the single reactivities for different gasifying medium composition at 900 °C.
C. Guizani et al. / Fuel 108 (2013) 812–823 819
char-conversion rates means that O
2
,H
2
O and CO
2
do not compete
for the same active sites on the coal char but are rather operating
on separate active sites. Similar conclusions can be found elsewhere
for the coal–char gasification reaction in mixed atmospheres con-
taining CO
2
and H
2
O[11,10]. In both of these studies, 2 models based
on the assumptions of char–CO
2
and char–H
2
O reactions occurring
on common and separate active sites were tested. The model assum-
ing reactions on separate active sites fitted the experimental results
well, whereas the model assuming competition for the same carbon
active sites under-predicted the experimental results.
Minkova et al. [40] also performed gasification experiments of
birch wood, straw and miscanthus pellets in non-isothermal condi-
tions (HR = 10 °C/min) up to a temperature of 750 °C with a resi-
dence time of 2 h, and found that the yield of solid product is
lower in a mixed atmosphere of CO
2
+H
2
O than with steam only,
which means that the CO
2
is also participating in the global gasifi-
cation reaction. They also observed that the CO
2
+H
2
O gasification
environment leads to a more developed pore structure and surface
area, which is merely the result of an advanced stage of the bio-
mass gasification reaction.
In a more recent study on sewage–sludge gasification in a fluid-
ized bed, the authors found that the reaction rate in a mixture of
CO
2
+H
2
O is well represented by the sum of the individual reaction
rates obtained with CO
2
and H
2
O individually [13].
Synergy between CO
2
and H
2
O(Active cooperation):This model
assumes that besides reaction on separate active sites, there is an
active cooperation between the gases for the accessibility to the
carbon active sites. At least one of the reactants is supposed to
act in a certain way as to enhance the char reactivity towards
the second gas. Such an action can be, for example, the creation
of additional porosity, as proposed by Butterman and Castaldi
[15], or the retention of catalytic mineral species inside the char,
as mentioned elsewhere [34].
Recently, in their study on pine–char gasification, Tagutchou
et al. [14] proposed a model where the CO
2
and H
2
O cooperate to-
gether for accessibility to the carbon active sites. The authors found
that the char reactivity in a mixed atmosphere was higher than the
sum of the reactivities obtained in single atmospheres of CO
2
and
H
2
O. They concluded that a cooperative effect of the two reactants
with char–gasification reactions occurred on separate active sites,
but they did not go further into the understanding of the mecha-
nisms involved.
Butterman and Castaldi [15] also think that CO
2
alongside
steam may lead to an enhanced reactivity, as it could further devel-
op the porosity inside the char particle and provide a greater reac-
tive surface. The authors performed gasification experiments on
several biomasses in a TG apparatus with a heating rate of 10 °C/
min and observed that the total number of pores during CO
2
ther-
mal treatment was an order of magnitude greater than that ob-
served during H
2
O/N
2
processing. Even the range in pore sizes
was much more extended with CO
2
(2–50
l
m) than with steam
(10–20
l
m). They also observed that the gasification was com-
pleted when introducing 30% of CO
2
with the steam, while a black
char residue remained when using only steam as a gasification
medium.
As regards the results obtained in the present study, the most
plausible assumption would be that the two reactants are operat-
ing in separate active sites without any kind of synergy. Still, we
cannot draw a definitive conclusion without taking into consider-
ation the observations of Tagutchou et al. [14] and Butterman
and Castaldi [15]. For this reason, we performed two other types
of char–gasification experiments: (i) with LHR-chars, so as to come
closer to Tagutchou’s experimental conditions in terms of HR, (ii)
char–gasification experiments with gas transition, wherein the
char is firstly operated with CO
2
up to a defined conversion level
and then gasified with steam to see if the CO
2
influences the char
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
1
2
3
4
5
6
7
8
9x 10-3
Conversion level X
Reactivity (g/(g.s)
Rmix [ 20 % H 2O + 10 % CO2]
Rsing [20 % H2O] + R sing[10 % CO2]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
1
2
3
4
5
6
7x 10-3
Conversion level X
Reactivity (g/(g.s)
Rmix [ 10 % H 2O + 20 % CO2]
Rsing [10 % H2O] + R sing [20 % CO2]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
1
2
3
4
5
6
7x 10-3
Conversion level X
Reactivity (g/(g.s)
Rmix [ 10 % H 2O + 30 % CO2]
Rsing [10 % H2O] + R sing [30 % CO2]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
Conversion level X
Reactivity (g/(g.s)
Rmix [ 20 % H 2O + 20 % CO2]
Rsing [20 % H2O] + R sing [20 % CO2]
Fig. 13. Comparison of the LHR-char reactivity in mixed atmosphere with the sum of the single reactivities for different gasifying medium composition at 900 °C.
820 C. Guizani et al. / Fuel 108 (2013) 812–823
physical properties and consequently impacts its reactivity toward
steam.
3.2.2. Reactivity of LHR-chars in mixed atmospheres of H
2
O+CO
2
Unlike in our experiments, Tagutchou et al. used char particles
with a greater thickness (5 mm) and prepared with a relatively low
heating rate (60 °C/min) in a screw pyrolysis reactor. Unsurpris-
ingly, the differences in operating conditions for the char prepara-
tion led to chars with different reactivities and morphological
features (available reactive surface and pore opening). Chars pre-
pared at a high heating rate already have their pores open and
present a high surface area, whereas those prepared at a low heat-
ing rate have a less-developed reactive surface and a narrower por-
ous network [39,19,25].
We performed additional gasification experiments with LHR-
char in mixed atmospheres of steam and carbon dioxide to see if
the additivity of single reactivities is valid for LHR-chars. The re-
sults are plotted in Fig. 13.
The reactivity curves obtained in mixed atmospheres match
very well with those of the added single reactivities, except for a
small deviation observed for the gasifying atmosphere composed
of 10% CO
2
+ 20% H
2
O. Additivity of reactivities is again a valid
assumption for the LHR-char. The heating rate would therefore
only impact the reaction rate but not on the reaction mechanisms
in a mixed atmosphere. Other authors found that the contributions
of the char–H
2
O reaction and char–CO
2
reaction in the global
mixed-atmosphere reaction rate remained the same indepen-
dently of the pyrolysis heating rate [9]. This is in accordance with
our findings.
If we assume that there is no influence from the type of biomass
(pine and beech wood), we believe that in Tagutchou’s work, it is
more likely that it is the char particle thickness that influences
the global reaction rate and the mixed-atmosphere gasification
mechanisms. Because of internal diffusion limitations for the CO
2
molecules, due to a greater particle thickness (5 mm) in compari-
son with the present case, the enhancement of the gasification
reaction observed by Tagutchou in mixed-atmosphere conditions
may be due to the fact that the steam-gasification reaction further
developed the char’s internal porosity and ameliorated the access
of the CO
2
molecules to the heart of the char particle, which re-
sulted in an apparent reactivity that was greater than the sum of
the respective single reactivities.
The additive law would therefore be valid as long as the char
particle is thin enough to prevent internal diffusion limitations to-
ward the CO
2
molecules. This possible explanation must be further
investigated by comparing the effect of the particle thickness
respectively on the H
2
O and CO
2
gasification reaction rates.
Another fact worth noting is that the average reactivity profiles
for H
2
O, CO
2
and mixed-atmosphere gasification experiments for
HHR and LHR chars are practically the same, except for some devi-
ations for higher conversion levels that can be attributed to mea-
surement uncertainties, as depicted in Fig. 14.
This tendency was not observed, for example, by Tagutchou
et al. [14,23] who found different reactivity profiles for CO
2
and
H
2
O gasification experiments. The authors observed that the H
2
O
reactivity profile showed a continuous increase throughout the
conversion while the CO
2
reactivity profile did not go beyond the
value of 1 until a conversion level of 80%, from which it began to
increase. In other words the char reactivity did not increase in
the range of 50–80% of conversion. This may be due to the limited
access of the CO
2
molecules to the heart of the char particle, de-
spite the advanced gasification stage. In the present study, the sim-
ilarity of the reactivity profiles may be imputed to the absence of
internal diffusion limitations due to the particle’s lower thickness.
Once again, this assumption needs to be investigated further.
3.2.3. Gas-alternation gasification experiments
The aim of such experiments is to verify whether or not there is
a kind of synergy, as claimed by Butterman et al. [14,15], that leads
in the present case to an apparent additive law. The CO
2
is first
introduced as a gasifying reagent to establish whether or not it cre-
ates additional porosity and further develops the reactive surface
for H
2
O. The unfolding of this type of experiment comprises three
stages:
Char gasification with CO
2
up to a certain conversion level.
Stopping the CO
2
flow, stabilization of the mass and purge of
the reactor under N
2
.
Introduction of steam and pursuance of the gasification reaction
up to total conversion.
The first experiment was performed on a HHR-char that was
gasified with 20% CO
2
up to a conversion level of 28% and subse-
quently operated with 10% of steam. The second one, on a LHR-
char, was performed with 20% of CO
2
up to a conversion level of
35% followed by steam gasification with a steam concentration of
20% in the gas flow. Fig. 15a and b illustrate the unfolding of these
experiments. On these figures are plotted reference char reactivi-
ties obtained in single atmospheres of steam and carbon dioxide
(gray and black solid lines) and the char reactivity in the gas-alter-
nation experiment (black dashed line).
For the HHR and LHR-chars, in the first stage of the gas-alterna-
tion experiment, the char reactivity curve naturally follows the ref-
erence curve obtained under a CO
2
-containing atmosphere. No
results are reported during the gas-transition zone. We can clearly
see on the two figures that the char–reactivity curve in the H
2
O
gasification stage re-joins the reference curve obtained with the
reference steam-containing atmosphere. If the CO
2
had changed
the char’s properties, we would have seen an enhancement or a de-
crease in the char’s reactivity towards H
2
O; it is not the case here.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Conversion level X
Reactivity profile F(X)
F(X)-[CO
2
+ H
2
O]
F(X)-[CO
2
]
F(X)-[H
2
O]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.5
1
1.5
2
2.5
3
3.5
4
Conversion level X
Reactivity profile F(X)
F(X)-[CO
2
]
F(X)-[H
2
O]
F(X)-[CO
2
+ H
2
O]
LHR-chars
HHR-chars
Fig. 14. Comparison of the average reactivity profiles for steam and carbon dioxide
gasification experiments.
C. Guizani et al. / Fuel 108 (2013) 812–823 821
On the basis of these observations, the most likely explanation for
the observed additive law would be that the CO
2
and H
2
O are oper-
ating on separate active sites without any kind of synergy. This con-
clusion is to be taken with precaution, as there can be other
phenomena when the two gases are reacting simultaneously. None-
theless it constitutes a step forward in the understanding of the
mechanisms involved in char gasification in mixed atmospheres.
Finally, the observation of Butterman and Castaldi [15] concern-
ing the incomplete char burnout with only steam as reactant may
be a consequence of an ordering of the carbon structure due to the
thermal annealing that is promoted by the contact with steam, as
proposed elsewhere for coal and biomass char gasification [33,34].
Introducing 30% of CO
2
would have overcome the structural order-
ing of the carbon matrix as the rate of the gasification reaction
would then be higher than that of the carbon ordering and result
in complete char burnout.
For our study, in view of the results obtained and the literature
review and discussion, the char reactivity in a mixed atmosphere
of CO
2
+H
2
O can be written as the sum of the single reactivities:
R
H
2
OþCO
2
¼R
H
2
O
þR
CO
2
ð10Þ
4. Conclusion
The new macro-TG experimental device allowed us to perform
gasification experiments on beech chars prepared at high heating
rates. These experimental conditions are of interest as they come
close to those encountered in fluidized beds. The fast pyrolysis
was followed directly by gasification experiments without addi-
tional heating of the char particle to the gasification temperature,
as is usually done in conventional TG devices. Reliable kinetic data
were obtained for HHR-char gasification reactions with H
2
O and
CO
2
and can be used for the design and optimization of fluidized
bed gasifiers.
The heating rate greatly affects the char reactivity to H
2
Oas
well as to CO
2
. The HHR-char reactivity was 3.5 times higher in
H
2
O gasification and greater than fourfold with CO
2
in comparison
with LHR-chars.
Introducing CO
2
alongside steam resulted in a higher reactivity
of the beech char, regardless of the pyrolysis conditions (low or
high heating rate). For a HHR-char, increasing the CO
2
concentra-
tion from 0% to 30% in a 10% steam-containing atmosphere re-
sulted in a 2.7-times-higher char reactivity.
A comprehensive approach was established in order to clarify
further the mechanisms involved in mixed atmosphere gasification
reactions. The present work demonstrates the validity of an addi-
tive law reflecting a passive cooperation of steam and carbon diox-
ide in the gasification reaction. Specific experiments carried out in
this work showed that converting a char under CO
2
to approxi-
mately X= 30% did not affect its reactivity during further conver-
sion under H
2
O.
The additive law would appear to be valid as long as the particle
is thin enough to prevent diffusional limitation phenomena toward
the CO
2
molecules. We believe that the steam-gasification reaction
would facilitate the access of CO
2
molecules to the heart of the char
particle in the case of thick particles, resulting in an apparent en-
hanced reactivity.
Acknowledgements
The authors acknowledge the national research agency ANR-
France for its financial support in the RECO2 project. They also
00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
Conversion level X
Reactivity (g/(g.s)
20 % CO
2
10 % H
2
O
20 % CO
2
followed by 10 % H
2
O
data4
00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
1
2
3
4
5
6
7
8x 10
-3
Conversion level X
Reactivity (g/(g.s)
20 % H
2
O
20 % CO
2
20 % CO
2
followed by 20 % H
2
O
CO
2
gasification
Gas transition zone
H
2
O gasification
(b)
CO
2
gasification
H
2
O gasification
(a)
LHR-char
Gas transition zone
HHR-char
Fig. 15. HHR-char (a) and LHR-char (b) gasification experiments with alternance of CO
2
and H
2
O at 900 °C.
822 C. Guizani et al. / Fuel 108 (2013) 812–823
wish to express their appreciation to Bernard Auduc for his techni-
cal support. Finally, the authors would also like to thank Laurent
Bedel and Sylvie Valin for their assistance and insightful
conversations.
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... The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/su14052596/s1. Figure S1: Detailed review on biomass gasification. References [162][163][164][165][166][167][168][169][170][171][172][173][174][175][176][177][178][179][180] ...
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Rising level of atmospheric CO2 and consequent global warming is evident. Global surface temperature have already increased by 0.8 °C over the 20th century and is projected to increase by 1.4–5.8 °C during the twenty-first century. The global warming will continue till atmospheric concentrations of the major greenhouse gases are stabilized. Among them, CO2 is mainly responsible and is expected to account for about 60% of the warming over the next century. This study reviews advances on causes and consequences of global climate change and its impact on nature and society. Renewable biomass has tremendous potential to mitigate the global warming. Renewable biomass is expected to play a multifunctional role including food production, source of energy and fodder, biodiversity conservation, yield of goods and services to the society as well as mitigation of the impact of climate change. The review highlights the different management and research strategies in forestry, agriculture, agroforestry and grasslands to mitigate the global warming.
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Char–CO2 gasification reactions in the presence of CO and char–steam gasification reactions in the presence of H2 were studied at the atmospheric condition using a thermogravimetric apparatus (TGA) at various reactant partial pressures and within a temperature range of 1123 K–1223 K. The char was prepared from a lignite coal. The partial pressure of H2 and CO varied from 0.05 to 0.3 atm. The experimental results showed that Langmuir–Hinshelwood (L–H) kinetic equation was applicable to describe the inhibition effects of CO and H2. The kinetic parameters in L–H equations were obtained. Interactions of char gasification by steam and CO2 in the presence of H2 and CO were discussed. It was found that the kinetic parameters determined from pure or binary gas mixtures can be used to predict multi-component gasification rates. The results confirmed that the char–steam and char–CO2 reactions proceed on separate active sites rather than common active sites.
Article
The rate of gasification of char from dried sewage sludge (DSS) with a gas containing CO2 and H2O was studied. The tests were conducted in an atmospheric laboratory fluidized bed (FB), using CO2–H2O–N2 mixtures as fluidizing gas and temperatures in the range of 800–900 °C. The gasification rates were compared with those previously obtained for DSS char using one single reactant (either CO2 or H2O) in the gas, i.e. in mixtures of CO2 and N2 or H2O and N2. It was found that the char gasification rates measured in a mixture containing both reactants, i.e. CO2 and H2O, were well approximated as the sum of the individual rates measured separately with CO2 and H2O. Besides this result, an alternative method to accurately measure the char gasification in an FB using CO2–H2O–N2 mixtures was employed, based on determining the char remaining in the reactor up to a certain time of gasification from the CO and CO2 concentrations in the gas during the combustion of this char. It was shown that this method compares well with the traditional method (based on tracking the CO and CO2 concentrations during gasification) when using a single reactant, but improves significantly the reliability of measurements for tests where both H2O and CO2 react simultaneously with char.
Article
In a coal gasifier, CO2 gasification and H2O gasification occur at the same time. Many researchers have constructed CO2 gasification model and H2O gasification model separately. Only a few models explain the competition between CO2 gasification and H2O gasification. These models are divided into two types according to the properties of the active sites. Some models assume that CO2 gasification and H2O gasification occur at separate active sites, whereas others assume that all of the active sites are shared and are common to both CO2 and H2O gasification. There are some coals whose gasification reaction rates, with CO2 and H2O in coexistence, cannot be described by the conventional models. In this study, a Langmuir–Hinshelwood type gasification model was modified to explain the char gasification by CO2 and H2O. In the proposed model, CO2 gasification and H2O gasification partially share active sites. Coal chars, which were prepared through pyrolysis using a drop tube furnace at ambient pressure and 1673 K, were gasified with CO2 and H2O using a pressurized drop tube furnace (PDTF) and a thermogravimetric analyzer (TGA) at various temperatures. The proposed model was found to describe the gasification reaction rates of the experiments more accurately than the conventional models.
Article
A series of carbon dioxide gasification tests of waste biomass chars were performed in a thermogravimetric analysis system, at non-isothermal heating conditions. The effects of the inorganic constituents of the fuels on thermal conversion characteristics were examined. Reaction rates were determined by developing a power law model.The bulk of char gasification process occurred between 800 and 950°C. Maximum reaction rate and conversion were exhibited by waste paper char, due to its higher surface area.Inherent alkaline and alkaline earth carbonates and sulphates acted as catalysts, by increasing the reactivity of the fuels in carbon dioxide and causing their degradation to start at lower temperatures (60–75°C).The kinetic model fitted the experimental results accurately. Activation energy values and reaction order ranged from 180 to 370kJ/mol and 0.4 to 0.6, respectively, among the chars, indicating a chemically controlled process.
Article
Steam plays a vital role in the gasification process. This study aims to investigate the changes in char structure and reactivity during the gasification of Victorian brown coal. A Loy Yang brown coal sample was gasified at 800°C in a novel fluidised-bed/fixed-bed reactor in three different gasification atmospheres: 15% H2O balanced with argon, 4000ppm O2 balanced with CO2 and 4000ppm O2+15% H2O balanced with CO2. The intrinsic reactivities of chars with air were measured with a thermogravimetric analyser (TGA) at low temperatures (380 or 400°C). The char structural features were characterised using FT-Raman spectroscopy followed by spectral deconvolution. Our results indicate that steam, when it is present in the gasifying atmosphere, has a drastic effect on char structure and the subsequent reactivity of char with air at low temperatures. The presence of steam during the gasification at 800°C also impacts on the volatilisation of Mg and Ca by altering the char structure. Our data provide evidence that the char–H2O gasification follows a different reaction pathway from the char–CO2 gasification, at least for the gasification of Victorian brown coal under the current experimental conditions.