Kinetic and Thermodynamic Evaluation of Pyrolysis of Plant Biomass using TGA

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ScienceDirect Materials Today: Proceedings 21 (2020) 2087–2095

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ISFM-2018

Kinetic and Thermodynamic Evaluation of Pyrolysis of Plant Biomass using TGA Tanveer Rasool*, Shashikant Kumar Department of Chemical Engineering, National Institute of Technology. Srinagar, India, 190006

Abstract The kinetic and thermodynamic analysis of the plant biomass waste of Banyan tree (Ficus Benghalensis) was carried out using thermo gravimetric analysis (TGA) at different heating rates of 10, 25 and 50°C min-1 under an inert atmosphere of nitrogen. Using iso-conversional models of Kissenger-Akahira-Sunrose (KAS) and Ozawa-Flynn-Wall (OFW), the average activation energy values were found to be of the range of 73.03 kJmol-1 and 79.74 kJmol-1, respectively. The application of Coats-Redfern method predicted that most appropriate mechanisms followed by thermal degradation of Banyan tree biomass are D5 and F3, based on diffusion and chemical reaction respectively, involving both exothermic and endothermic reactions for the temperature range under study. A kinetic compensation effect was found to follow the equation: lnA0=-4.1575+0.2042E. The Gibbs free energy value of ~176.24 kJmol-1 and a difference of about ~5 kJmol-1 between activation energy and enthalpy of the reaction indicate favorable product formation with considerable bio energy potential of Banyan tree biomass waste. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications. Keywords: Plant biomass; Banyan tree; pyrolysis; Gibbs free energy

* Corresponding author. Tel.: +91-194-2422032; fax: +91-194-2420475. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications.

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1. Introduction There are limited resources of fossil fuels (coal, petroleum and gas) are available for production of heat energy and electricity generation. Due to this shortage problem of fossil fuels attention is being paid world-over by many researchers for the use of alternative source of energy in power generation and other energy oriented industries. There are various types of renewable energy sources available such as solar energy, wind energy, hydropower, and biomass energy etc. and out of these renewable sources of energy, biomass is more economically and feasible compared to other source of energy [1]. Biomass is a carbon rich material and produce thermal energy with fewer oxides, where as other renewable energy sources produce more oxides. Ash materials content in biomass is very less compared to other solid fuel like coal etc. so biomass is the purest solid fuel. It has been estimated that renewable sources of energy supply around 19% of the world energy demand and 9% of this demand met by biomass, with an increasing rate of 2.5% per annum [2]. The power generation through renewable energy sources in India clearly indicates that the biomass has potential to generate approximate more than 20000 MW of electricity per year in India. However, the country is lacking in use of biomass in power generation. Till date, India has been generating approximately 2000 MW electricity so it needs to be enhanced by

utilization of biomass. In this context, the

thermal exchange of biomass to get biofuels with high energy has proved as a promising technology [3]. However, pyrolysis has been more popular thermo chemical process compared to gasification and combustion as it efficiently converts the biomass directly into fuels and chemicals in an inert atmosphere [4]. As such, much work has been carried out to study pyrolytic behaviour of numerous types of biomasses and their waste using thermo gravimetric analysis under nitrogen atmosphere to evaluate kinetic parameters of biomasses such as Rice husk and Elephant grass [5], Pine [6] and Willow [7]. El-Sayed et al. [8] investigated two Egyptian biomasses (sugarcane bagasse and cotton stalks). Thermo gravimetric analysis data of biomass is used to determine kinetic parameter like activation energy, pre exponential factor, and order of the reaction using Arrhenius equation. Ceylon and Tapcu [9] found the physical and chemical properties along with kinetics parameters of Hazelnut husk, an in abundance found agricultural biomass waste in Turkey. The TGA analysis of biomass was carried out under inert environment with varying heating rates of 5, 10 and 20°Cmin-1. There are three well known kinetic models Kissinger-Akahira-Sunrose (KAS), Ozawa-Flynn-Wall (OFW) and Coats-Redfern methods were used on TGA data of Hazelnut husk to calculate kinetic triplet. Ningbo et al. [10] carried out a study on the continuous pyrolysis of pine saw dust using a screw reactor and find the effect of pyrolysis temperature and heat residence time on products and energy circulation. TG analysis was carried out to find kinetic parameters while products were analyzed using GC/MS. The objective of this study was to investigate the thermal degradation behavior of Banyan tree (Ficus Benghalensis) plant waste (BTW) for energy conversion and to predict whether or not the biomass can be used as a potential fuel on an industrial level. The characterizations of Banyan tree were carried out to determine the chemical composition and other properties and compare them with several other biomass sources that have been under study for a long time in the same direction. It was to put on record that very little or no work had been done on the usage of this biomass as a source of energy; however, extensive work has been done in synthesizing gold and silver nano-

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particles from its leaves, roots and straws for various medical applications. Ras-Sindoor is yet another beneficial product that has been derived from its leaves. 2. Experimental 2.1. Material preparation The waste biomass of Banyan tree, a national tree of India, was collected around the area densely populated with Banyan trees, from Kolkata province of India. The samples collected consist of leaves, straw and small branches. The samples were washed to remove any dirt or impurity followed by air drying in dry oven for 8 hours. The dried samples were grind in small pieces of particle size 0.5mm or less. The prepared biomass sample was stored in desiccators for further analysis. 2.2. Kinetic theory Pyrolisis is a solid state decomposition reaction of biomass under inert atmosphere leads to formation of char residue, gases and volatiles. In the present study of degradation reactions of biomass two well-known mathematical models of Kissinger-Akahira-Sunrose and Ozawa-Flynn-Wall were used to evaluate kinetic and thermodynamic parameters and in turn the reaction mechanism. The results of thermo gravimetric experiments are expressed as mass change in biomass sample, given as:

mi − m .......... .......... .......... .......... .......... .......... .......... .......... ......(1) mi − m f where mi, m and mf are the initial, at any time and final mass of the biomass sample. On the other hand, the rate

α=

of conversion in mass is given as:

dα dt = k (T ) f (α ) = A0 exp( − E RT ) f (α ) ....................................( 2) where k(T) is the rate constant and its value depend only on temperature described by Arrhenius Law as,

k(T) = A 0 exp(−E / RT) . f(α) is the rate constant model in which constant term are, A0 the pre-exponential Arrhenius factor (s-1), E the activation energy (kJmol-1) at a reaction temperature T (K) and R the gas constant (kJmol-1K-1). After using the value of k(T) in Eq. (2) with initial conditions of no mass loss α = 0 at T = To and taking E/RT as 'χ' gives: α

T

A0 − E dα = e f (α ) To β

F (α ) =  0

RT

dT =

A0 E m( χ ).......................................(3) βR

The above equation (3) does not give exact solution and this equation can be solved by using various approximations leading to generation of different is conversional methods including KAS and OFW mathematical models. These two models are based on the following two equations [8-11].

(

)

KAS method

: ln β T 2 = ln [ A0 R Ef (α ) ] − E RT .......... .......... .......... .......... .......( 4 )

OFW method

: ln (β ) = ln [A0 E Rf (α ) ] − 5.331 − E RT .......... .......... .......... .......... ...( 5)

For constant α, activation energy can be getting through the inclination of the straight line by plotting the L.H.S of Eq. (4) and Eq. (5) vs. inverse of temperature 1/T. Coats-Redfern method involves thermal degradation

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mechanism. Rearrangement of Eq. (3) using Doyle approximation [9, 12] yields ln

AR F (α ) E = ln 0 − .......... .......... .......... .......... .......... .......... .......... .......... ...( 6) β E RT T2

The equation (6) is used to obtained activation energy E, and pre-exponential factor A0, in terms of algebraic expressions replacing F(α) represent different kinetic mechanisms as given in Table 4. The thermodynamic parameter of activation Gibbs free energy ( ΔG ) is calculated using following equation, given elsewhere [12].

ΔG = E + RTh ln (K bTh hA0 )........................................ ........................(7) 3. Results and discussions 3.1. Physico-chemical analysis The proximate analysis and Energy Dispersive X-ray spectroscopy (EDS) of BTW were carried out and results are shown in Table 1. From table it can be seen that BTW contains around 70 wt. % volatile matters and 30 wt. % carbon element. The FTIR spectrum of BTW is shown in Fig. 1. The FTIR spectrum of the BTW shows absorption band at 3300-3600 cm-1, which is shows that H-bonded O-H groups of cellulose are present [13]. A sharp peak at wave number 2929 cm-1 is corresponded to C-H stretching which is of methylene of cellulose. The peaks in between 3000-3500 cm-1 wave number is attributed to presence of hydroxyl groups from carbohydrates and those of lignin and as well as to the symmetric and asymmetric stretching vibrations associated with H2O molecules. The sharp peak at 1241 cm-1 is for alkyl ester of acetyl group in hemicellulose structure and/or the linkage between hemicelluloses and lignin. The peak of C=O groups at 1623 cm-1 and peaks at various wave number of 2843, 1623, and 1418 cm-1 have been attributed to absorption due to C-H deformation within the methoxyl groups of lignin.

Fig. 1: FTIR analysis of Banyan tree waste

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Table 1: Composition of Banyan tree waste. Proximate analysis (wt. %) Ultimate analysis (wt. %) Moisture Volatile Ash Fixed C N Al Si matter content Carbon**

8.15

70.23

1.3

20.10

36.14

59.13

1.19

1.24

2091

Ca

2.3

The peaks of lignin can be seen at 1241 cm-1 (C-O ring). Another band around 1000 cm-1 corresponds to the functional groups of C-O-H or C-O-R (alcohols or ethers). The peak at 2929 cm-1 is related to the presence of C-H stretching vibration together with bending vibrations around 1400 cm-1 of aliphatic chains (-CH2 and –CH3) forming the basic structure of these lignin cellulose materials. The peak at 1600 cm-1 may be due to aliphatic or unsaturated aromatic compounds [13-14]. 3.2. Thermal Degradation The result of pyrolysis of BTW at different heating rate under nitrogen atmosphere is shown in Fig. 2. The curve of TG represents the mass loss with respect to temperature and the curve DTG represents the rate of mass loss with respect to temperature at different heating rate. The curves indicate that the decomposition of BTW included three major stages, an initial stage, the pyrolytic stage and stage of char decomposition. The first stage corresponds to moisture vaporization and lasts till 300°C. The second stage is sudden mass loss stage and occurs between 350-500°C and corresponds to loss of volatile matter (Mass loss of around 70%). It is found that the maximum mass loss happens in this stage only for all heating rates studied. This stage is followed by char pyrolysis stage at temperatures of above 500°C. The behaviour of TG and DTG curves are similar to other cellulose based biomass wastes were already reported [8-10]. With an increasing heating rate, the maximum rate of mass loss also increased from 10 to 65% min-1 at same temperature. The similar trends of results were also reported by some other researchers [11]. 10000

-1

- --- 5 0

0

C m in

8000

-1

80 6000

TG(wt%)

TG 60

DTG

4000

DTG wt%/min

0

1 0 C m in 0 -1 - -- - -- 2 5 C m in

100

40 2000 20 0 0 0

200

400

600

800

1000

0

T e m p e r a tu r e ( C )

Fig. 2: TG/DTG curves of pyrolysis of Banyan tree biomass at different heating rates 3.3. Kinetic and thermodynamic study The relative studies of activation energies have been carried out at different heating rates using KAS and OFW models. By using Eq. 4 and 5, activation energies have been calculated using KAS and OFW models respectively.

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The plot between ln β T 2 vs. 1/T (using KAS model) and ln β vs. 1/T were plotted to calculate the slope and in turn the values of activation energy E. Fig. 3 shows one of the plots using OFW model. The values of activation energies calculated at each conversion rates using KAS and OFW model are given in Table 2 and 3 respectively.

Fig. 3: Linear fit plots of ln(β) against 1/T for OFW model. Table 2: Kinetic and thermodynamic values using KAS method. R2 lnA0 Conversion E ΔG (α)

(kJmol-1)

0.2

70.27

0.957

9.36

175.84

0.3

68.35

0.936

9.29

175.92

0.4

63.51

0.914

8.66

176.22

0.5

84.77

0.956

13.46

174.95

0.6

121.21

0.990

21.00

173.33

0.7

55.59

0.842

7.67

176.72

0.8

45.85

0.961

5.81

177.43

Average

73.03

-

-

175.77

(kJmol-1)

However, the mean value of activation energies calculated using KAS and OFW method are 73.032 and 79.742 kJmol-1 respectively. The lower activation energy values correspond to higher reaction rate. The activation energies show a suddenly increased after α=0.4 (for KAS and OFW model), this result show the initiation of main pyrolysis process. Results reveal that activation energy is strongly dependent on percentage conversion and reiterates the fact that pyrolysis of biomass is a complex process. It can also be seen that best fitting lines at conversion (α) between 0.2 to 0.8 are nearly of same nature indicating that reaction mechanism is almost same for all conversions. The

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results conclude that pyrolysis of BTW follows a multistep complex reaction mechanism depicted by the progressive change in activation energy with conversion. Table 3: Kinetic and thermodynamic values using OFW method. Conversion (α)

E (kJmol-1)

R2

0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average

75.66 74.45 70.01 90.61 125.76 63.33 54.88 79.24

0.976 0.883 0.992 0.965 0.934 0.950 0.967 -

ΔG

lnA0

(kJmol-1) 176.21 176.35 176.71 175.28 173.51 177.36 178.32 176.25

11.18 11.24 10.78 15.09 22.02 10.21 8.86 -

Varying the activation energy with enthalpies ( ΔH ) of activation, a mere difference of around 5 kJmol-1 was found for each conversion point. This result shows that activated carbon is formed with small potential energy barrier. The Gibbs free energy ( ΔG ) of activation of BTW was calculated and its value remain nearly same as ~176.24kJ mol-1 , little higher than camel grass [12], rice bran and rice straw [15]. The values are indicative of BTW as potential feedstock for bio energy. The activation energy (E) value, correlation coefficient (R2) and preexponential factor A0 (ln A0) are also present in Table-4 against the algebraic expression used for different mechanisms predicted for heating rate of 25°Cmin-1. Table 4: Algebraic expression for F(α) for different solid state reaction mechanism and corresponding kinetic variables at heating rate of 25°Cmin-1. Mechanism

Integral form

R2

E

A0 (s-1)

(kJmol-1)

F(α) Power Law (P3/2)

α1 3

0.79

11.933

596.36

Power Law (P1/2)

α1 2

0.74

-3.382

-0.013

Diffusion Law(D2)

(1 − α )ln(1 − α) + α

0.85

26.886

17161.44

Diffusion Law (D5)

[(1

0.85

60.388

10290845.11

− α

)1

3

− 1

]

2

Contracting sphere (R2)

1 − (1 − α )

0.72

10.469

-8645.56

Contracting cylinder (R3)

1 − (1 − α )

0.75

12.911

56.846

Chemical Reaction (F3/2)

(1 − α )1 2 − 1

0.80

28.113

1458.91

0.82

66.807

10473358.52

Chemical Reaction (F3)

12

13

[1 (1 − α )

2

]

−1

2

The comparison of apparent values of activation energies obtained against each method shows that E value corresponding to thermal degradation via D5 and F3 mechanism are almost in line with the values obtained by KAS

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and OFW models. The findings reveal that the thermal degradation of BTW is based on diffusion and chemical reaction involving both endothermic and exothermic reactions. 3.4. Kinetic compensation effects The Fig. 4 shows relation between activation energy and pre-exponential factors. The variation of preexponential factors was linear with activation energy as: ln A0 = aE + b . where a and b are constants and it is called compensation coefficients. The compensation effect can be observed for different types of reaction models. However, OFW model being more credited was used in this study. Plotting lnA0 against E from OFW model gives an excellent linear relationship with R2=0.9889, shown in Fig. 4.

LnAo (s-1 )

25 20

E vs lnA

15

Regression result

10 lnAo = 0.2042E-4.1575 R² = 0.9889

5 0 0

25

50

75

Activation Energy E

100

125

(kJmol-1

150

)

Fig. 4: Plot of lnA0 against E of BTW thermal degradation. The linear equation representing the data can be expressed as lnA0=-4.1575+0.2042E, signifying the intense interaction between various kinetic parameters. 4. Conclusions The kinetic and thermodynamic data were calculated using Kissenger-Akahira-Sunrose (KAS) and Ozawa-FlynnWall (OFW) models and the mean activation energies of pyrolysis of Banyan tree waste was calculated and the values were found to fall in the range of 73.032 kJ mol-1 and 79.742 kJ mol-1 respectively. With an increase in heating rate, the maximum rate of mass loss experienced a shift towards higher temperatures in line with the previous studies. The pyrolysis of Banyan tree most probably follows D5 and F3 mechanism based on diffusion and chemical reaction. The outcome from this study has proved BTW as a potential low cost renewable source for bio energy.

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