Thermal and Catalytic kinetics of Charcoal Oxidation
Author: F.Lemos; M.A.N.D.A Lemos; A. Saqib
H I G H L I G H T S
Charcoal impregnated with 1%V, 1%Cu and their corresponding mix.
Thermogravimetric analysis under air between 400 and 800 oC.
Char conversion with and without catalyst.
Evaluation of char conversion to CO through heat flow data.
Char oxidation modelling
A R T I C L E I N F O
G R A P H I C A L A B S T R A C T
A B S T R A C T
History:2016
Key Words: Charcoal Oxidation Effect of catalyst CO evaluation
This work reports a thermal and catalytic kinetic study of the activated
charcoal air oxidation. The study was performed by atmospheric
pressure thermogravimetry over a temperature range of 400 – 800 oC.
TG, DTG and DSC data was used to observe the expected catalytic
activity. Also, char conversion to fuel gas i.e. carbon monoxide was
evaluated through heat flow data curves in exothermic boundaries.
Charcoal impregnated with Vanadium showed the maximum CO
formation at around 400 oC in comparison to simple charcoal which
was found to give maximum conversion to CO at 500 oC. Finally,
Langmuir-Hinshelwood type kinetic model was developed for
estimation of kinetic parameters.
1. Introduction
Char oxidation is a complex heterogeneous process
which often governs the overall rate of combustion and
gasification [1-2]. Oxidation rates are partially governed
by surface properties of the char and reactions catalyzed
by minerals within the char matrix. Biomass chars have
inherently some alkali and alkaline-earth minerals and
their catalytic effects have also been observed during
pyrolysis [3-5]. However, the effect of transition metals
on char reactivity during combustion had not gathered
much attention. Hence, the present effort has been made
to look for the expected catalytic effects of transition
metal i.e. Vanadium, Copper and their mix on the char
reactivity during air gasification at various temperatures.
2. Materials and Methods
2.1. Sample:
Air dried charcoal along with charcoal impregnated with
1% Vanadium, 1% Copper and their corresponding mix
were used. Raw charcoal proximate analysis observed
through TGA is as follows:
Parameter Percentage
Moisture 10
Volatile Matter 3.2
Fixed Carbon 83
Ash 4.3
Table 2.1 Proximate Analysis
2.2. Thermogravimetric Analysis
STA 6000 instrument by PerkinElmer was used to
perform TGA experiments. The study was focused to
determine the temperature conditions appropriate for
the maximum conversion of char to fuel gas carbon
monoxide by using air as gasifying agent. Both samples
i.e. charcoal and charcoal impregnated with 1%
Charcoal STA TG DTG DSC Model
vanadium, copper and their corresponding mix were
subjected to series of isothermal experiments between
400 – 800 oC using heat rates of 20 and 100 oC/min. High
heat rate was also used because the experiments were
programmed to switch the gasifying agent immediately
as the run starts. At high temperatures, low heat rate
consumes large portion of carbon during temperature
scan before reaching the desired isothermal condition so
high heating rate was utilized there to observe maximum
mass loss in isothermal regime. Air flow rate was
maintained at 20 ml/min. The experiments were
performed using sample mass less than 10 mg in an
alumina crucible.
A typical experiment proceeded as follows: In step 1;
after introducing the sample crucible in the equipment;
the unit is first purged with nitrogen gas before starting
the run. The sample was then heated to 40 oC and hold
there for 10 minutes to equilibrate the system. In step 2,
the heat rates of 20 or 100 oC/min were used to reach
the desired temperature. The system was made
isothermal till complete carbon loss. Finally, the system is
cooled to the initial temperature.
Table 2.2 Experimental Procedure
Step Description Stage
1 Flow of Pure Air (20ml/min) Initial
2 Ramp to Tisothermal with specified
heat rate
Heating
3 Hold at Tisothermal for specified
Time
Isothermal
4 Cool to room temperature Cooling
3. Results and discussion
3.1. TG/DTG
TGA results have been used to graphically demonstrate the
analytical process. Thermogravimetric analysis for char
gasification takes the general form of mass loss over time for a
specified temperature profile. The isothermal segments were
exported and normalized from 0% to 100% char conversion
according to equation 3.1.
𝑿 = 𝒎𝒐−𝒎𝒕
𝒎𝒐−𝒎𝒂 Eq: 3.1
Where, mo denotes the sample mass at the start of
experiment, mt the sample mass at time t and ma the
mass of ash remained after almost complete carbon loss.
Fig 3.1 Charcoal conversions at various temperatures
Fig 3.2 Time Derivative of char mass fractions at
low temperatures
Fig 3.3 Time Derivative of char mass fractions at
high temperatures
It was observed from Fig 3.1 that the carbon conversion
already occurs at 500 °C, albeit with a very slow pace. It
increases with a faster pace at 550 °C till 600 °C. After that the
mass loss occurred in a more uniform way till 850 °C. These
trends showed that the combustion of charcoal follows
complete combustion pattern from 600 °C. At low
temperatures till 500 °C the combustion is partial and very
slow.
From DTG curves i.e. Fig 3.2 and 3.3. It was observed that the
reactivity of char increases with increasing temperature. The
char oxidation reaction begins at around 500 °C and increases
rapidly with time till 650 °C. After reaching a peak value, the
reactivity starts to decrease due to the combustion of the less
reactive portion of the char. Char reactivity was more or less
same for temperatures 700 to 850 °C.
[A]: 1% Vanadium
[B]: 1% Copper
[C]: 1%Vanadium + 1%Copper
Fig: 3.4 [A], [B] and [C]. Conversion plots for charcoal
impregnated with catalysts.
It is evident from the Fig 3.4 that the charcoal
impregnated with catalyst showed reactivity at lower
temperatures in comparison to non-impregnated
charcoal. Particularly, charcoal impregnated with 1% V
started to produce gas at temperature as low as 400 oC.
However, the observed rate was very low and the
conversion was partly endothermic as shown in Fig 3.5. In
case of sample impregnated with 1%V+1%Cu, no
synergistic effect observed. The overall reactivity for the
impregnated samples was remarkably high compared to
simple charcoal and shown in Fig 3.6. Also among the
samples, 1% Cu showed high temperature sensitivity.
Fig 3.5 Conversion Regimes (1%V)
It was observed from Fig. 3.5 that time required for
complete carbon conversion decreases with the increase
of temperature, as expected. The sample tested at 400 °C
took the longest time and it was far away from
comparison with samples tested at 450 and 500 °C.
Furthermore at lower temperature, mass loss occurred in
an exothermic manner till 80 percent of conversion after
that remaining mass loss occurred in an endothermic
way. A decrease in mass loss at fractional conversion
greater than 80 percent may be attributed due to
decrease in available surface area. With the progress in
mass loss; the pores may collapse and coalesce; thus
limiting the reaction of oxygen with carbon [6]. The
phenomenon of decrease in carbon conversion tendency
was very limited at temperature 450 °C and absent at
temperature 500 °C.
[A] 1% V
[B] 1% Cu
[C] 1%V+1%Cu
Fig 3.6 Time derivative of impregnated chars
Decrease in rate at higher conversions may be considered due
to the known factors having a detrimental effect on char
gasification. Over time, carbonaceous material remaining in
the char is gradually annealed. Annealing reduces char
reactivity by increasing the ordering of the char structure,
destroying carbon edges, and reducing structural defects. As
carbon is simultaneously depleted from the char, micropores
coalesce into meso and macropores, reducing char reactivity
by effectively reducing the available surface area for
gasification [6-7]. Finally, deactivation of the inherent catalytic
inorganic species may occur over time. As conversion
increases, each of these interrelated processes has an
increasing effect on the gradually decelerating gasification
rate
3.2. DSC/CO Evaluation
Thermodynamics data have been used in order to
calculate the percentage of carbon monoxide in the
product gas mix during gasification. The standard
enthalpy of formation for char gasification products in
the presence of air are as follows:
∆H for CO2 = - 393.2 KJ/mol at 298 oK
∆H for CO = - 110.2 KJ/mol at 298 oK
These values are corrected for the working temperature
ranges i.e. 400 – 800 oC by using Shomate equation which
is explained below [8]:
Ho = A*t + B*t2/2 + C*t3/3 +D*t4/4 – E/t + F Eq: 3.2
Where,
H° = standard enthalpy (kJ/mol)
t = temperature (K) / 1000.
Table 3.4 Shomate Equation Constants
Temperature (K) 298. – 1200. 1200. – 6000.
A 24.99735 58.16639
B 55.18696 2.720074
C -33.69137 -0.492289
D 7.948387 0.038844
E -0.136638 -6.447293
F -403.6075 -425.9186
G 228.2431 263.6125
H -393.5224 -393.5224
Reference [9-10] Chase, 1998 Chase, 1998
Table 3.5 Heat of formations
Temperature
(°C) Heat of Formation (Theoretical) KJ/g
CO CO2
400 -8.278 -31.151
450 -8.278 -31.151
500 -8.149 -30.883
550 -8.017 -30.603
600 -7.750 -30.007
650 -7.615 -29.692
700 -7.478 29.365
750 -7.340 -29.026
800 -7.200 -28.675
850 -7.06 -28.314
For a particular isothermal experiment; the total energy
released with a given mass was calculated by integrating the
heat flow data in the exothermic region. Energy released per
unit mass was then calculated in KJ/g.
CO contribution in the total energy released was calculated
using the enthalpy of formation data as follows:
For y KJ/g of energy released:
Y = X . ∆ H CO + (1 − X). ∆ H CO2 Eq: 3.3
Applying the algebraic manipulations, fraction of carbon
converted to CO as:
X = ∆H CO2−Y
∆H CO2− ∆H CO Eq: 3.4
Typical DSC data used for carbon monoxide evaluation is
presented in Fig 3.7 [A] and [B]. Same approach was used for
other samples and the results are presented in Fig 3.8.
[A] 1% Vanadium at 400 oC
[B] 1% Vanadium at 450 oC
Fig: 3.7 [A], [B]. Specific Heat Flow (Endo up) and Fractional
mass loss as a function of time.
Fig 3.8 CO formation comparison
Fig 3.8 presents the overall trend of CO formation during air
oxidation of both types of charcoal samples i.e. simple and
impregnated. Impregnated samples not only showed fuel gas
formation at low temperatures in comparison to simple
charcoal but also made it possible to achieve near complete
combustion at particular high temperature.
3.3 Kinetic Modelling
Central to surface catalysis are reaction steps involving
one, or more than one, surface bound (adsorbed)
intermediate species. In case of unimolecular surface
reaction, we have:
A ● s → B ● s 3.5
Where, A ● s is a surface bound species involving A and
site s. The rate of this reaction is given by [11]:
(-rA) = k ᶿA 3.6
Where, ᶿA is the fraction of the surface covered by
adsorbed species A.
By combining surface-reaction rate laws with the Langmuir
expressions for surface coverages, Langmuir-Hinshelwood
(LH) rate laws for surface-catalyzed reactions are obtained
as:
Following Langmuir isotherm for competing species [12-
13]:
θA = KA
𝐶𝐴
1+ KACA+ KBCB 3.7
For the overall reaction A → B, if the rate determining step
is the unimolecular surface reaction by eq(3.5), then the
rate of reaction is obtained by using eq(3.7) for θA in
eq(3.6) to result in:
(−𝑟A) = k KA
𝐶A
1+ KACA+ KBCB 3.8
Above explained L-H type kinetics has been made the basis
in order to develop an appropriate kinetic model for the
explanation of the experimental mass loss data.
For reactions at low temperatures, the rate of reaction is
controlled by the chemical reactivity of the char and hence
chemical reaction rate is relatively slow compared to the
diffusion rate of the reactant gases to the internal surface
of the particles [14-6]. Under these conditions the rate of
chemical reaction can be expressed as:
𝑟𝑐 = 𝑘 𝑃𝑂2(𝑆) 3.9
Where, 𝑃𝑂2(𝑆) is the partial pressure of oxygen at the
reactant surface when there are no diffusion limitations.
At low temperatures this partial pressure of oxygen will be
the same as in the bulk gas phase 𝑃𝑂2(𝑆)=𝑃𝑂2(𝑔).
It is noteworthy that all the experiments have been
carried-out at a constant air flow, and so the bulk oxygen
concentration can be considered as being the same at all
times and the influence of oxygen in the kinetic data will
only be relevant in the context of diffusion limitations of
the oxidant.
Apart from the order of the reaction in relation to the
oxygen, one also has to consider the apparent order in
relation to the carbonaceous material itself. In this
respect, it is clear from the experimental data that there is
significant segment in the beginning of the reaction where
there is a linear trend in the mass loss, indicating that the
rate of reaction is not directly proportional to the amount
of carbon material. This can be explained according to
several mechanisms but there are two main
interpretations that can be put forward. On one hand the
reaction can occur mostly catalyzed, either by the added
catalyst or by the inorganic contaminants present
beforehand and the reaction proceeds by a CASA ‘contact
active surface area’ mechanism [17]. In this case the
reaction rate will depend only on the amount of catalyst
present until the carbon amount is relatively low. On the
other hand, if the combustion occurs in the micro porous
surface, as discussed above, the reaction will be mostly
dependent on the outer surface of the particle and not
directly related to the inner surface and this will also
reduce the dependence on the amount of the carbon
present in the sample.
In order to describe this type of relationship the
dependence on the weight of carbon material was
introduced in the kinetic rate expression in the following
form:
𝑟𝑐 = 𝑘𝑤
𝑤0+𝑤 𝑃𝑂2(𝑆) 3.10
Let us now consider what will happen as the temperature
increases. With the increase in temperature, the chemical
reaction rate and hence the consumption of the gaseous
reactant will be higher than the diffusion rate of the
reactant gas [18-19]. The reactant gas will not be able to
penetrate through the pores to the interior of the reacting
solid particle. This diffusion related phenomenon will start
limiting the rate of reaction. This can happen only due to
internal diffusion limitations, thus reducing the apparent
activation due to external diffusion limitations, where the
process will be fully controlled by the diffusion of the gas
from the bulk of the gas to the surface of the material. To
understand this effect another reaction path is considered
which is as:
𝑟𝑑 = 𝑘𝑔(𝑃𝑂2(𝑔) − 𝑃𝑂2(𝑆)) 3.11
At very high chemical reaction rate, the 𝑃𝑂2(𝑆) = 0 at
extreme diffusion limitation.
Furthermore, at quasi steady-state;
𝑟 = 𝑟𝐶 = 𝑟𝑑 3.12
By equating equations 3.9 and 3.11 and following algebraic
manipulations; 𝑃𝑂2(𝑆) was evaluated as:
𝑃𝑂2(𝑆) = 𝑘𝑔𝑃𝑂2(𝑔)
𝑘+ 𝑘𝑔 3.13
Utilizing eq 3.13; equation 3.9 becomes:
𝑟 =𝑘𝑘𝑔𝑃𝑂2(𝑔)
𝑘+ 𝑘𝑔 3.14
Equation 3.14 yielded [k/] as 𝑘 𝑘𝑔
𝑘+ 𝑘𝑔 .
It can be observed that overall rate constant is governed
by individual k and kg. To evaluate the k/, the concept of
resistances was used. By this, individual rates were added
by converting them to the reciprocal form as:
1
𝑘/ = 1
𝑘+
1
𝑘𝑔 3.15
Fig 3.9 Model fit at 550 oC (Charcoal)
Fig. 3.9 shows that the proposed model dictates higher
reactivity at low temperature. As the model was
developed utilizing assumption of uniform reaction and
the experimental mass loss data was comprised of partial
combustion and pyrolysis segments dictating the global
reactions. Perhaps, it may become the reason for model
deviation at low temperature.
Fig 3.10 Model fittings (Charcoal)
Figures 3.10 represent the model fitting at high
temperatures. It is observed that the model fits very well
for temperatures above 600 °C. However, at low
temperature the model predicts a higher rate of reaction
which was not in accordance with experimental data.
Table 3.6 Estimated Kinetic Parameters
Temperature oC 600-850
Frequency Factor ‘K’ 0.068
Activation Energy Kcal/mol 1540
Frequency Factor ‘Kg’ 0.0000005
Activation Energy Kcal/mol 21000
Order ‘n’ 1.1
4 Conclusions
4.1 TG
Raw charcoal conversion occurs around 500 o C
Impregnation of catalysts reduces the temperature for
carbon conversion and it was lowest for 1% V.
Charcoal impregnated with 1 % Cu and (1%V+1%Cu)
showed conversion at 450 °C.
For low temperatures; inhibiting effect was observed
after around 80 % conversion.
Least conversion time was shown by 1% Cu
impregnation.
Raw charcoal and 1%V didn’t give sharp change in
conversion at high temperatures.
4.2 DTG
Raw charcoal had the lowest reaction rate among others,
also the rate increases slowly with increase in
temperature.
For Vanadium the rate was more pronounced at high
temperatures.
Copper showed sharp increase in rate with temperature.
For catalyst mix; the rate first increases and then
decreases at around 700 °C, followed by an increase
again.
4.3 DSC
For all samples the product was a gas mix.
Maximum CO2 in the gas mix was attained with raw
charcoal.
Extreme low and high temperatures favored CO
formation.
Moderate temperatures favored CO2 formation.
At high temperatures CO formation was attributed to the
diffusion limitations.
Cu in the form of mix and alone found to promote
diffusion limitations more effectively in comparison to
Vanadium.
4.4 Kinetic Modelling
Langmuir-Hinshelwood type kinetics seems appropriate
to be considered as basis for the charcoal oxidation
reaction modeling.
For all samples the predicted kinetic model gave best fits
at moderate to high temperatures.
At low temperatures model suggested high rates which
was not in accordance with actual experimental data.
Model was found valid for complete oxidation reactions
only as at low temperatures the samples were following
partial combustion.
Acknowledgements:
This work was supported by Experts-Sustain scholarship
program of ERASMUS MUNDUS and conducted at Institute
Superior Tecnico, [Lisbon-Portugal] as an exchange masters
student. (experts_20127208)
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