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c ISSN 0010-5082, Combustion, Explosion, and Shock Waves, 2014, Vol. 50, No. 4, pp. 454–462. Pleiades Publishing, Ltd., 2014. c H.Beidaghy Dizaji, F.Faraji Dizaji, and M. Bidabadi. Original Russian Text

Determining Thermo-Kinetic Constants in Order to Classify Explosivity of Foodstuffs H. Beidaghy Dizajia , F. Faraji Dizajib , and M. Bidabadic

UDC 662.612

Published in Fizika Goreniya i Vzryva, Vol. 50, No. 4, pp. 92–101, July–August, 2014. Original article submitted April 17, 2013; revision submitted October 21, 2013.

Abstract: The kinetics of devolatilization of some foodstuff materials like white wheat flour, sugar, and cocoa powders are studied by using thermogravimetric analysis, in order to measure their pyrolysis rate. The mean pyrolysis rate of these materials is used as a criterion to predict their explosivity. A comparison of the mean pyrolysis rates shows that the sugar powder is the most explosive material among others. Wheat flour explosivity is very close to sugar, and cocoa powder has the least tendency to explode. Our results are completely compatible with National Fire Protection Association reports. Keywords: dust explosion, thermogravimetric analysis, kinetics of devolatilization, pyrolysis rate. DOI: 10.1134/S0010508214040145

INTRODUCTION The most common dust explosions occur in underground coal mines. However, this general phenomenon is not restricted to coal mines. In fact, a dust explosion is likely to occur when a finely divided combustible solid (in practice, the mean diameter of the particles should not exceed 1 mm) happens to be dispersed as a cloud in air (with a typical mass loading between 10 and 1000 g/m3 ), and when an appropriate ignition source (hot body, flame, electrical or mechanical spark, etc.) is activated inside this mixture. The heat evolved from the ignition source initiates the combustion of the particles located in the vicinity of the ignition point. These particles act themselves as an ignition source for the adjacent slabs of the mixture so that the combustion zone a

Young Researchers and Elite Club, Tabriz Branch, Islamic Azad University, Tabriz, Iran; [email protected]; b School of Engineeing, The University of Vermont, 33, Colchecter Avenue, Burlington, Vermont 05405, USA; 33, ff[email protected]; c School of Mechanical Engineering Department of Energy Conversion, Combustion Research Laboratory, Iran University of Science and Technology, Narmak, Tehran, 16887 Iran; [email protected].

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is capable of propagating throughout the cloud without additional input of energy. This combustion zone has generally a finite thickness, and it is called the flame [1]. The vast majority of natural and synthetic organic materials, as well as some metals, can form combustible dust. Any industrial process that reduces a combustible material and some normally noncombustible materials to a finely divided state present a potential for a serious fire or explosion [2, 3]. A combustible dust explosion hazard may exist in a variety of industries, including food (e.g., candy, starch, flour, feed), plastics, wood, rubber, furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals (e.g., aluminum, chromium, iron, magnesium, and zinc), and fossil fuel power generation. Actually, when we want to displace these materials from one place to another place using pneumatic pipes (pneumatic powder transport), or storage process take longer than usual standards and without any appropriate ventilation, a dust explosion may happen. The two apparatus most often used for dust explosibility testing are the Hartmann vertical tube and the 20 liter sphere. The Hartmann tube was the first to be commonly used and a great deal of data exists, which have been generated in the pre-1980 era by this apparatus. A constant-volume (20 liters) sphere is used to calculate the maximum pressure rate [4] further used

c 2014 by Pleiades Publishing, Ltd. 0010-5082/14/5004-0454

Determining Thermo-Kinetic Constants in Order to Classify Explosivity of Foodstuffs to determine the absolute criterion to measure the dust explosivity Kst . However, other criteria are also used, such as the flame velocity, minimum ignition temperature, minimum ignition energy, particle size, etc. In the past, we studied organic dust combustion theoretically [5–8]; however, in this research we use an experimental method to study this phenomenon. In this article, we identify our material by using CNHSO and SEM experiments. To measure the explosive behavior of particles, we use thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG), which are methods of thermal analysis where changes in physical and chemical properties of materials are measured as a function of increasing temperature (with a constant heating rate), or as a function of time (with a constant temperature and/or constant mass loss). Then we investigate the activation energy and preexponential factors of sugar, cocoa, and wheat flour materials by using TGA, which allows us to compare the explosive behavior of these materials.

α 0

1 A dα = (1 − α)n H

The kinetics of pyrolysis has been usually described by the first-order Arrhenius law [9, 10]. In this paper, it is assumed that the process of pyrolysis and combustion is a multi-step procedure (according to TGA and DTG), and each stage is governed by the first-order Arrhenius law. The kinetics of pyrolysis can be described as dm = −k(m − mf )n , (1) dt where E , k = A exp − RT n is the order of the reaction, m is the current mass, mf is the final particle mass at the end of the heating experiment, A is the pre-exponential factor, E is the activation energy, and R is the global gas constant. When the particle mass reaches mf , the reaction stops (dm/dt = 0). Defining the percent change of the particle mass as α = (m0 − m)/(m0 − mf ) (m0 is the initial mass of the particle), which varies from zero at the beginning of the reaction to unity at the end, we have dα = k(1 − α)n , (2) dt dα dα dT dα = = H, (3) dt dT dt dT where H is a prescribed heating rate. Considering the Arrhenius kinetic form for the reactions that take place in this experiment, we have

T

E dT . exp − RT

(5)

0

If E/RT is replaced by x and the integration limits are changed, Eq. (5) becomes α 0

1 AE dα = n (1 − α) HR

∞ x

exp(−x) dx. x2

(6)

Introducing the notations α g(α) = 0

1 dα, (1 − α)n

∞ p(x) = x

exp(−x) dx, x2

we rewrite Eq. (6) as g(α) =

KINETIC ANALYSIS

455

n dα A E E → = exp − (1 − α . k = A exp − RT dT H RT (4) By integrating Eq. (4), we obtain

AE p(x). HR

(7)

The exponential integral p(x) has no analytical solutions, but it has many approximations [11, 12]. Among model-fitting methods, it is possible to find a suitable model to describe the dependence of the relative change in the particle mass α on temperature and simultaneously to determine the activation energy E and the preexponential factor A. There are several non-isothermal model-fitting methods. One of the most popular methods is the Coats and Redfern method [13]. This method utilizes the asymptotic series expansion for approximating the exponential integral in Eq. (7), which yields g(α) 2RT E AR ln 2 = ln 1− − . (8) T HE E RT As 2RT /E 1, we have ln

g(α) E AR − . = ln 2 T HE RT

(9)

The antilogarithm is used for both sides: g(α) E AR − = exp ln T2 HE RT AR E AR E = exp ln exp = exp , (10) HE RT HE RT whence it follows that AR E g(α) = T 2 exp . HE RT

(11)

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Beidaghy Dizaji et al.

Fig. 1. ln(− ln(1 − α)) as a function of 1/T for white wheat flour (a), sugar powder (b), and cocoa powder (c).

The logarithm is used for both sides of Eq. (11): E ART 2 − ln exp , (12) ln(g(α)) = ln HE RT ART 2 E ln(g(α)) = ln − , (13) HE RT where α g(α) = 0

t1

1 dα (1 − α)n

⎧ ⎪ ⎨ − ln(1 − α) ← n = 1, =

1−n ⎪ ⎩ 1 − (1 − α) ← n = 1. 1−n

order to compare the rate constants k of these material at identical steps, we use the general first-order reaction model (n = 1). The mean pyrolysis rate is calculated by using the pyrolysis kinetic constants: t2

dm = − [k(m − mf )]dt Δt, (15) dt t2 dm E = − A exp − (m − mf ) dt Δt. (16) dt RT t1

(14)

The plot of ln(g(α)) versus 1/T should correspond to a straight line with a slope of (−E/R), giving the activation energy. The pre-exponential factor A can be calculated by Eq. (9), extrapolating the straight line up to its intersection with the ordinate axis and knowing the activation energy. Figure 1 shows these results for white wheat flour, sugar powder, and cocoa powder. In

EXPERIMENTAL PROCEDURE Materials By using the elemental combustion system, we determined the chemical compositions of white wheat flour, sugar powder, and cocoa powder; the ultimate results are listed in Table 1. The proximate results extracted from TGA are presented in Table 2.

Determining Thermo-Kinetic Constants in Order to Classify Explosivity of Foodstuffs

457

Table 1. Chemical composition of materials (ultimate analysis of CNHSO tester equipment) White wheat flour

Sugar powder

Cocoa powder

Element name

Weight, Weight fraction, mg %

Element name


Element name


Nitrogen

0.0835

1.654

Nitrogen

0.0004

0.009

Nitrogen

0.1072

2.493

Carbon

1.9668

38.97

Carbon

2.0560

41.51

Carbon

1.7854

41.53

Sulphur

0

0

Sulphur

0

0

Sulphur

0.0792

1.843

Hydrogen

0.3694

7.319

Hydrogen

0.3302

6.667

Hydrogen

0.2598

6.045

52.05

Oxygen∗

51.81

Oxygen∗

2.0669

48.08

Oxygen∗

2.6268

Molecular formula C3.24 H7.26 O3.25 N0.12 Note:

∗ The

2.5661

Molecular formula C3.46 H6.61 O3.24 N0.0006

values are calculated by the difference.

Table 2. Proximate analysis from TGA Material

Molecular formula C3.46 H6 O3.01 N0.18 S0.06

Moisture Volatile Ash content, content, % content, % %

White wheat flour

3.87

83.13

13

Sugar powder

0.23

85.54

14.23

Cocoa powder

6.27

72.58

21.15

The SEM pictures of white wheat flour, sugar powder, and cocoa powder (Fig. 2) show that the smallest particles are found in cocoa powder (approximately mono-sized particles smaller than 10 μm). The maximum particle size of white wheat flour is about 30 μm; however, the sizes of flour particles randomly differ, and there is no reference size for it. The last material is sugar powder, which has the greatest particles among the three materials (50 μm), but there is a huge difference between the sugar particle diameters, as is seen from Fig. 2). It should be noted that sugar has the least moisture content among other investigated materials. However, its particles agglomerate intensively due to its nature; in fact, sugar particles tend to be crystallized. TGA and DTG To obtain information on the thermal behavior of the discussed materials, thermogravimetric analysis (TGA) was conducted, and the weight loss percentage due to heating is shown in Fig. 3. It should be noted that the temperature itself in TGA is a function of time according to Eq. (3). Consequently, exothermic or endothermic reactions in the pyrolysis procedure are ne-

glected. The particle weigh loss procedure is generally divided into two processes; drying and pyrolysis. The pyrolysis procedure itself can be divided into several steps, which are identified from DTG. Each minimum in the DTG curve nominates a different step (component) in the pyrolysis procedure. From the TGA and DTG curves, some characteristic parameters of pyrolysis are calculated for each step: the temperature of the initial weight loss (Ti ), the temperature at the end of the step (Tf ), the activation energy E, and the preexponential factor A. There parameters are summarized in Tables 3–5, where the errors of calculations are determined by using regression (see the values of R2 in Tables 3–5) [14, 15]. The TGA results are compared in Fig. 4. The drying procedure approximately starts at 50 and finishes at 190◦ C for all studied materials. Then, the pyrolysis procedure starts (Fig. 4a). At the end of pyrolysis, the maximum remained ash content is observed for cocoa powder, which means that cocoa powder has the least tendency to pyrolyze. As all of our studied materials are organic, three-component devolatilization (without considering the drying process) is observed according to the DTG curves. As is shown in Fig. 4b, several events can be distinguished during the heating process of the studied samples. (a) White Wheat Flour. In the first event, which lasts up to around 163◦ C, the moisture is removed. In the second event, between 163–396◦C, a sharp peak is observed. This peak nominates cellulose devolatization, which is a dominant phenomenon in comparison to hemicellulose and lignin pyrolysis. Finally, in the third event, heavier volatile compounds are evolved. This latter process, which is usually identified as lignin decomposition, occurs slowly and covers a broad temperature range [16, 17].

458


Fig. 2. SEM pictures of white wheat flour, sugar powder, and cocoa powder with different magnifications. Table 3. Kinetic parameters from TGA for white wheat flour, H = 10◦ C/min−1 Step

Ti , ◦ C

Tf , ◦ C

mi , mg

mf , mg

Δα, %

E, J/mol

A, s−1

R2

1

50

162.87

5.2500

5.0470

3.85

73 583

5.73 · 108

0.921

1012

0.957

2

162.87

396.04

5.0470

1.8447

60.99

164 352

3

396.04

850

1.8447

0.6826

22.14

56 472

(b) Sugar Powder. The drying procedure happens between 50 and 186◦ C. Three-component devolatization is clearly distinguishable; between 186–337◦ C, the lightest volatile compounds (hemicellulose and cellulose) are evolved. The lower temperature peak represents decomposition of hemicellulose present in the biomass sample, and the higher temperature peak represents decompo-

2.87 ·

4.90

0.966

sition of cellulose. The degree of overlapping between these two peaks is due to the mineral matter present in the sample, which catalyses thermal decomposition of the material [18, 19]. At the end, lignin pyrolysis starts at 337◦ C and continues to the end of pyrolysis. (c) Cocoa Powder. Moisture evaporates between 50 and 132◦C. Unlike other materials, the DTG curve


459

Table 4. Kinetic parameters from TGA for sugar powder, H = 10◦ C/min−1 Step

Ti , ◦ C

Tf , ◦ C

mi , mg

mf , mg

Δα, %

E, J/mol

A, s−1

R2

1

50

186.08

5.2500

5.2380

0.22

—

—

—

2

186.08

261.13

5.2380

3.6671

30.08

278 394

3.79 · 1026

0.980

3

261.13

337.01

3.6671

2.0408

30.14

142 252

1.03 · 1011

0.910

4

337.01

529.17

2.0408

1.2012

16.05

78 962

3.40 · 103

0.892

5

529.17

850

1.2012

0.7470

9.04

88 499

168.96

0.930

Table 5. Kinetic parameters from TGA for cocoa powder, H = 10◦ C/min−1 Step

Ti , ◦ C

Tf , ◦ C

mi , mg

mf , mg

Δα, %

E, J/mol

A, s−1

R2

1

50

131.44

3.1100

2.9150

6.27

48 440

1.38 · 105

0.828

2

131.44

483.75

2.9150

1.1586

56.48

52 422

266.59

0.978

3

483.75

850

1.1586

0.6577

16.11

77 142

40.20

0.962

for cocoa powder has no sharp peaks. It happens due to a slow and steady pyrolysis procedure of cocoa. In fact, cellulose, hemicellulose, and lignin simultaneously pyrolyze in a broad range of temperatures (132–484◦ C). Finally, the remaining lignin pyrolyzes.

Experimental Conditions As the purpose of this study is to model the pyrolysis process (and not combustion), an inert gas (nitrogen) is used in TGA tests as an ambient gas. Consequently, no oxidation occurs during the pyrolysis process. The difference between the oxidative ambient medium (air) and non-oxidative ambient medium (nitrogen) is that an extra peak related to the combustion process is observable in the DTG curve in the oxidative ambient medium. Consequently, the non-oxidative ambient (nitrogen) test does not predict the ignition condition. We perform some experiments with lycopodium particles in order to determine the accuracy of our TGA tester equipment. The results are completely compatible with those of other researchers [20, 21] (Fig. 5a). The experiments for white wheat flour and cocoa powder are repeated three times for each material, but we repeat TGA tests for sugar powder five times because there are very little changes in the drying process of this material. TGA tests for each material are reasonably compatible with each other. The mean values of the conducted experiments (TGA tests) are represented in this paper.

We use sulfanilic acid (C6 H7 NO3 S) in order to determine the accuracy of our CNHSO tester equipment. We repeat each experiment two times for each material, and both experiments are very close together. In order to determine the accuracy of our SEM picture, we use lycopodium particles as a test sample (Fig. 5b). The results are completely compatible with the previous studies [20–23].

RESULTS AND DISCUSSION Figure 6 shows the log(k) dependences on temperature for each step of pyrolysis for all materials. It is seen that the same substance in each material has an almost the same rate constant of the corresponding process, in particular, moisture content evaporation and lignin pyrolysis. In order to evaluate the accuracy of our model, the weight loss curves versus temperature obtained by using the calculated thermo-kinetic constants are plotted in Fig. 7. These curves are in reasonable agreement with experimental data (TGA tests). In this article, the mean pyrolysis rate is used as a criterion to compare explosivity of materials. In order to calculate this parameter, the right side of Eq. (16) is presented as a sum of several integrals according to the physical characteristics of each material. The mean pyrolysis rate for cocoa powder is written for both wet and dry bases as

460


Fig. 4. Comparison of TGA and DTG curves of the studied materials. Table 6. Mean pyrolysis rates and explosivity of materials

Material

Fig. 3. TGA and DTG curves for white wheat flour (a), sugar powder (b), and cocoa powder (c) extracted at a heating rate of 10◦ C/min.

dm dt

tdry

=− wet

tinit

t pyr1

+ tdry

tfin + tpyr1

E1 (m − mf ) dt A1 exp − RT

E2 (m − mf ) dt A2 exp − RT

E3 (m − mf ) dt A3 exp − Δt, RT

dm dt

Pyrolysis rate, μg/s

Kst , bar · m/s [4]

wet base

dry base

White wheat flour 0.933

0.890

139

Sugar powder

0.928

0.927

154

Cocoa powder

0.501

0.461

128

dry

tfin + tpyr1

tpyr1 E2 (m − mf ) dt A2 exp − =− RT tdry

E3 (m − mf ) dt A3 exp − Δt RT

[limits of integration are init (initial), dry (drying), pyr1 (pyrolysis 1), and fin (final)].


461

Fig. 6. The quantity log(k) as a function of temperature.

Fig. 5. Lycopodium TGA curve (a) and SEM pictures of lycopodium particles (b).

If only TGA curves are used (wet base) to compare explosivity of the discussed materials, it is possible to make a mistake and choose white wheat flour as the most explosive material among other materials. However, sugar powder is the most explosive material according to the dry base comparison [4]. Therefore, it is necessary to analyze the mean pyrolysis rate and use the dry base mean pyrolysis rate as a criterion to compare explosivity of materials. According to Table 6, it is understood that, if dry bases of examined particles are compared, sugar powder has the highest mean pyrolysis rate, white wheat flour is the next, and cocoa powder has the least mean pyrolysis rate, which are completely compatible with National Fire Protection Association reports [4].

Fig. 7. Weight loss as a function of temperature. The points and curves show the experimental results and the calculation based on the thermo-kinetic constants obtained.

CONCLUSIONS We use the mean pyrolysis rate as a criterion to compare explosivity of white wheat flour, sugar powder, and cocoa powder. The mean pyrolysis rate is a criterion to predict the volatility of different materials. A comparison of the mean pyrolysis rates shows that the explosivity of the examined materials gradually decreases in the following row: sugar powder, white wheat flour, and cocoa powder.

462

Beidaghy Dizaji et al. REFERENCES

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