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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