# Decomposition Characteristics and Kinetics of Microalgae in N2 and CO2 Atmospheres by a Thermogravimetry

Authors: Xu Qing; Ma Xiaoqian; Yu Zhaosheng; Cai Zilin; Ling Changming
Journal: Journal of Combustion
 (2017)

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## Abstract

The thermal degradation characteristics of microalgae were investigated in highly purified N2 and CO2 atmospheres by a thermogravimetric analysis (TGA) under different heating rates (10, 20, and 40°C/min). The results indicated that the total residual mass in CO2 atmosphere (16.86%) was less than in N2 atmosphere (23.12%); in addition, the kinetics of microalgae in N2 and CO2 atmospheres could be described by the pseudo bicomponent separated state model (PBSM) and pseudo-multi-component overall model (PMOM), respectively. The kinetic parameters calculated by Coats-Redfern method showed that, in CO2 atmosphere, the apparent activation energy (E) of microalgae was between 9.863 and 309.381 kJ mol−1 and the reaction order (n) was varied from 1.1 to 7. The kinetic parameters (E,n) of the second stage in CO2 atmosphere were quite similar to those in N2 atmosphere.


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## Body

## 1. Introduction

Renewable and clean energy has become more and more important globally, especially with the current fuel crisis, economic crisis, and environmental pollution [1]. The bioenergy, as one form of renewable energy, is widely used in the third world. Its application can not only relax the energy crises but also restrain the environmental pollution [2, 3]. However, in recent years, the competition over biomass supply for fuel or for food has been intensified, which has resulted in growing interests in alternative, nonedible biomass resources such as perennial rhizomatous grasses, miscanthus (Miscanthus), and switch grass (Panicum virgatum) [4].Microalgae have many advantages over existing energy crops, such as faster growth rate, shorter growth time, higher biomass production, biochemicals and higher volume carbon abatement and no demand on arable land [5–8]. Furthermore, there is a possibility of direct generation of desired end products like bio-oil and hydrogen to be processed afterwards (like starch and biomass) [9]. Microalgae, as a source of biofuels and technological solution for CO2 fixation, are subject to intense academic and industrial research in its potential [10].Pyrolysis is the degradation of macromolecular materials with heat alone in absence of oxygen [11]. In recent years, many researches have studied the pyrolysis characteristics of microalgae in N2 atmosphere and established significant information on the pyrolysis behavior and kinetics [7, 11–13]. Both N2 and CO2 are inert gases. However, there have been no reports on the decomposition characteristics of microalgae in CO2 atmosphere. Whether the decomposition characteristics of microalgae in CO2 atmosphere are the same as in N2 atmosphere is still unknown.Chlorella, a genus of unicellular green microalgae, with a spherical shape of 2.0~10.0 μm in diameter, living in both fresh and marine water, can generally be found in fresh water of ponds and ditches, moist soil, or other damp conditions, for example, the surface of tree trunks, water pots, and damp walls [14].Chlorella includes eight species, one of which isChlorella vulgaris (C. vulgaris), growing in fresh water. Some scholars studied the pyrolysis kinetic of microalgae in N2 atmosphere by TGA [15, 16]. Besides, the conventional combustion, oxy-fuel combustion and hybrid combustion characteristics on microalgae were studied under different conditions by TGA [17–20]. However, the pyrolysis characteristics and kinetic analysis of microalgae pyrolysis under N2 and CO2 atmosphere by TGA were not reported yet.So far the pyrolysis technology is still in development. The less reports on the pyrolysis ofC. vulgaris in different atmospheres (N2 and CO2) were found. In this study, the decomposition characteristics ofC. vulgaris were studied by a thermogravimetric analysis (TGA), and the effects of atmospheres (N2 or CO2 atmosphere) and heating rates (10, 20 and 40°C/min) onC. vulgaris decomposition were investigated. Finally, kinetic triplets ofC. vulgaris were obtained with Coats-Redfern approximation method, includingE, reaction order (n), and preexponential factor (A). Thermogravimetric date (TG) and differential thermogravimetry (DTG) profiles, pyrolysis characteristics, the effects of heating rate, and kinetics analysis were analyzed to determine the optimal conditions forC. vulgaris treatment process.


## 2. Material and Method

### 2.1. Sample Preparation

The powder of microalgae,C. vulgaris, was used in this study provided by Jiangmen YueJian Biotechnologies Co, Ltd. (Guangdong Province, China). The ultimate analysis and proximate analysis were tested according to GB/T212-2008 [21], GB211-84 [22], and ASTM D5373-08 [23], respectively. Heat values were done based on ASTM D5468-02 [24] and ASTM E870-82 [25]. The proximate analysis, ultimate analysis, and low calorific value were listed in Table 1. TheC. vulgaris sample was dried in an oven at 105°C for 20 h and then milled and sieved with a screen of less than 200 μm in diameter. After the above treatment, the sample was stored in a desiccator for test.Table 1
Ultimate and proximate analysis and lower heating values ofChlorella vulgaris (on dry basis).
Ultimate analysis (wt%)
Proximate analysis (wt%)
Q
n
e
t
,
d (MJ kg−1)
C
H
O
N
S
Volatile
Ash
Fixed carbon
47.84
6.41
25
9.01
1.46
55.37
10.28
34.35
21.88
Note:Qnet,d, lower heating value on dry basis.


### 2.2. Decomposition Experiment

Decomposition was carried out on NETZSCH STA 409 PC simultaneous analyzer with the heating rate of 10, 20, and 40°C/min from room temperature to 1000°C in either N2 or CO2 atmosphere. About 6±0.2 mg dried sample was used for each run in nonisothermal conditions. The thermogravimetric (TG) and differential thermogravimetric (DTG) data were used to differentiate the decomposition as well as estimate the kinetic parameters. Three repeated experiments were accomplished for data confirmation, in order to verify reproducibility and decrease the experiments error.


### 2.3. Characteristic Parameters of Temperatures

The initial and final thermal degradation temperatures represent how hard the reaction is.TI is the initial degradation temperature which is defined as the intersection of the tangent and the horizontal curve, and the temperature at which the DTG has its peak value is the location of the TG curve tangent. TF is the final degradation temperature which is obtained when the mass lose accounts for 98% of the total quality loss. DTGpeak is differential thermal gravity value at a peak temperature and Tpeak is degradation peak temperature which is defined as the temperature at which the mass loss rate in DTG curve reached the local maximum. DV is the average weight loss rate of the temperature ranged from 100 to 1000°C.


### 2.4. Kinetic Modeling

The kinetic equation of common type can be generally expressed as follows [26]:(1)dαdt=kTfα,where α is the conversion degree, t (min) is time, T(K) is the absolute temperature, f(α) is a function, the type of which depends on the reaction mechanism, and k(T) is the temperature dependent rate constant. k(T) is usually described with the Arrhenius equation:(2)k=Aexp⁡-ERT,where A (min−1) is preexponential or frequency factor, E (kJ mol−1) is the activation energy, and R (kJ/mol·K) is the universal gas constant.The functionf(α) is expressed as follows:(3)fα=1-αn,where n is the reaction order.The degree of conversion of the reduction process is expressed as follows:(4)α=mi-mtmi-m∞,where mi is the initial mass of the sample, mt is the mass of the sample at time t, and m∞ is the final mass of sample in the reaction [27, 28].By derivation of (2) and (3) in (1) and is integrated by using Coats-Redfern approximation method [29, 30], it becomes(5)ln⁡gαT2=ln⁡ARβE1-2RTE-ERT,where(6)gα=-ln⁡1-α,n=1,1-1-α1-n1-n,n≠1.In general,β is the heating rate; the term of 2RT/E can be neglected since it is much less than 1. A plot of ln⁡[g(α)/T2] against 1/T should result in a straight line of slope -E/R for the correct reaction mechanism, as using the method to find the suitable model function g(α) (or f(α)) of global decomposition kinetics. Once the form of g(α) is obtained, the apparent activation energy E and the frequency factor A can be calculated from the straight line in light of (5).With the PMOM, theC. vulgaris is considered as three pseudo components and individually decomposed over a temperature range, which can be expressed as follows:(7)dαdt=w10-w1∞w10-w3∞dα1dT,w10<w<w1∞,w20-w2∞w10-w3∞dα2dT,w1∞=w20<w<w2∞,w30-w3∞w10-w3∞dα2dT,w2∞=w30<w<w3∞,dα1dT=A1βexp⁡-E1RTf1α1,w10<w<w1∞,dα2dT=A2βexp⁡-E2RTf2α2,w20<w<w2∞,dα2dT=A3βexp⁡-E3RTf3α3,w30<w<w3∞,where w is the mass percentage of solid and the subscripts 0 and ∞ refer to the initial and residual amounts, respectively. The subscripts 1, 2, and 3, respectively, correspond to the pseudo components 1, 2, and 3 [31–33].The analogy to (7) is considered when single and three pseudo components are involved. To obtain the kinetic parameters, the TG-DTG information and a nonlinear regression scheme are used in fitting equations (5) and (6).


## 2.1. Sample Preparation

The powder of microalgae,C. vulgaris, was used in this study provided by Jiangmen YueJian Biotechnologies Co, Ltd. (Guangdong Province, China). The ultimate analysis and proximate analysis were tested according to GB/T212-2008 [21], GB211-84 [22], and ASTM D5373-08 [23], respectively. Heat values were done based on ASTM D5468-02 [24] and ASTM E870-82 [25]. The proximate analysis, ultimate analysis, and low calorific value were listed in Table 1. TheC. vulgaris sample was dried in an oven at 105°C for 20 h and then milled and sieved with a screen of less than 200 μm in diameter. After the above treatment, the sample was stored in a desiccator for test.Table 1
Ultimate and proximate analysis and lower heating values ofChlorella vulgaris (on dry basis).
Ultimate analysis (wt%)
Proximate analysis (wt%)
Q
n
e
t
,
d (MJ kg−1)
C
H
O
N
S
Volatile
Ash
Fixed carbon
47.84
6.41
25
9.01
1.46
55.37
10.28
34.35
21.88
Note:Qnet,d, lower heating value on dry basis.


## 2.2. Decomposition Experiment

Decomposition was carried out on NETZSCH STA 409 PC simultaneous analyzer with the heating rate of 10, 20, and 40°C/min from room temperature to 1000°C in either N2 or CO2 atmosphere. About 6±0.2 mg dried sample was used for each run in nonisothermal conditions. The thermogravimetric (TG) and differential thermogravimetric (DTG) data were used to differentiate the decomposition as well as estimate the kinetic parameters. Three repeated experiments were accomplished for data confirmation, in order to verify reproducibility and decrease the experiments error.


## 2.3. Characteristic Parameters of Temperatures

The initial and final thermal degradation temperatures represent how hard the reaction is.TI is the initial degradation temperature which is defined as the intersection of the tangent and the horizontal curve, and the temperature at which the DTG has its peak value is the location of the TG curve tangent. TF is the final degradation temperature which is obtained when the mass lose accounts for 98% of the total quality loss. DTGpeak is differential thermal gravity value at a peak temperature and Tpeak is degradation peak temperature which is defined as the temperature at which the mass loss rate in DTG curve reached the local maximum. DV is the average weight loss rate of the temperature ranged from 100 to 1000°C.


## 2.4. Kinetic Modeling

The kinetic equation of common type can be generally expressed as follows [26]:(1)dαdt=kTfα,where α is the conversion degree, t (min) is time, T(K) is the absolute temperature, f(α) is a function, the type of which depends on the reaction mechanism, and k(T) is the temperature dependent rate constant. k(T) is usually described with the Arrhenius equation:(2)k=Aexp⁡-ERT,where A (min−1) is preexponential or frequency factor, E (kJ mol−1) is the activation energy, and R (kJ/mol·K) is the universal gas constant.The functionf(α) is expressed as follows:(3)fα=1-αn,where n is the reaction order.The degree of conversion of the reduction process is expressed as follows:(4)α=mi-mtmi-m∞,where mi is the initial mass of the sample, mt is the mass of the sample at time t, and m∞ is the final mass of sample in the reaction [27, 28].By derivation of (2) and (3) in (1) and is integrated by using Coats-Redfern approximation method [29, 30], it becomes(5)ln⁡gαT2=ln⁡ARβE1-2RTE-ERT,where(6)gα=-ln⁡1-α,n=1,1-1-α1-n1-n,n≠1.In general,β is the heating rate; the term of 2RT/E can be neglected since it is much less than 1. A plot of ln⁡[g(α)/T2] against 1/T should result in a straight line of slope -E/R for the correct reaction mechanism, as using the method to find the suitable model function g(α) (or f(α)) of global decomposition kinetics. Once the form of g(α) is obtained, the apparent activation energy E and the frequency factor A can be calculated from the straight line in light of (5).With the PMOM, theC. vulgaris is considered as three pseudo components and individually decomposed over a temperature range, which can be expressed as follows:(7)dαdt=w10-w1∞w10-w3∞dα1dT,w10<w<w1∞,w20-w2∞w10-w3∞dα2dT,w1∞=w20<w<w2∞,w30-w3∞w10-w3∞dα2dT,w2∞=w30<w<w3∞,dα1dT=A1βexp⁡-E1RTf1α1,w10<w<w1∞,dα2dT=A2βexp⁡-E2RTf2α2,w20<w<w2∞,dα2dT=A3βexp⁡-E3RTf3α3,w30<w<w3∞,where w is the mass percentage of solid and the subscripts 0 and ∞ refer to the initial and residual amounts, respectively. The subscripts 1, 2, and 3, respectively, correspond to the pseudo components 1, 2, and 3 [31–33].The analogy to (7) is considered when single and three pseudo components are involved. To obtain the kinetic parameters, the TG-DTG information and a nonlinear regression scheme are used in fitting equations (5) and (6).


## 3. Results and Discussion

### 3.1. Decomposition Process

The thermogravimetry (TG) and differential thermogravimetry (DTG) curves ofC. vulgaris decomposition at a heating rate of 10°C min−1 in two atmospheres are shown in Figures 1 and 2, respectively. From Figure 2, two noteworthy peaks in pyrolysis are shown in curves; the decomposition process in N2 atmosphere can be divided into three stages for interpretation. The first stage is from room temperature to 140°C, corresponding to a loss of moisture and a slight volatile compound. The second one is from 140 to 550°C, where most of the organic materials are decomposed; this is the main decomposition process. The third one is from 550 to 1000°C; during this stage, the carbonaceous matters in the solid residuals are continuously decomposed at a very low rate.Figure 1
TG curves ofChlorella vulgaris decomposition at a heating rate of 10°C min−1 in N2 and CO2 atmospheres.Figure 2
DTG curves ofChlorella vulgaris decomposition at a heating rate of 10°C min−1 in N2 and CO2 atmosphere.As mentioned in the previous paragraph, in N2 atmosphere, only one main decomposition stage occurs at the temperature range of 140–550°C, which is close toChlorella protothecoides (150–540°C) reported by Peng et al. [11].In CO2 atmosphere, four stages could be distinguished during thermal degradation process ofC. vulgaris. The first stage is from room temperature to 140°C (dehydration). The second one is from 140 to 560°C (devolatilization), corresponding to the first main decomposition process. The third one is from 560 to 790°C (devolatilization); at this stage, the rate of reaction is very low. The fourth one (second main decomposition process) is from 790 to 1000°C; in this stage, the residual carbonaceous matters and CO2 may react at a relatively high rate. In CO2 atmosphere, there are two main decomposition stages (the second and fourth ones). Compared with the second stage, the fourth one is very narrow.


### 3.2. Comparison and Analysis ofC. vulgaris Decomposition in N2 and CO2 Atmospheres

Figures1 and 2 show that the TG and DTG curves ofC. vulgaris decomposition in N2 atmosphere are different from CO2 atmosphere. As mentioned in Section 3.1, the decomposition process ofC. vulgaris in N2 atmosphere can be divided into three stages, and one main decomposition process is obtained. However, in CO2 atmosphere, the thermal decomposition process is divided into four stages and two main decomposition processes are obtained.In addition, as shown in Figure1, when the temperature is below 887°C, the weight loss in CO2 atmosphere is less than in N2 atmosphere. The decomposition process ofC. vulgaris in CO2 atmosphere is delayed due to the difference of the CO2 from N2 molecule. When the temperature reaches 887°C, the weight loss ofC. vulgaris decomposition in N2 atmosphere is equivalent in CO2 atmosphere. However, when the temperature is above 887°C, a quick increase in the weight loss is observed in CO2 atmosphere, which may be caused by the gasification of carbonaceous matters.The characteristic parameters forC. vulgaris decomposition can be obtained from Figure 2, as shown in Table 2. It is clear that there is a peak for the thermal decomposition process ofC. vulgaris in N2 atmosphere at 322°C, and at this temperature the rate of weight loss attains the maximum value. However, in CO2 atmosphere, there are two peaks of the DTG curve occurring at 323.8°C and 901.1°C, respectively. The maximum rate of weight loss is attained at 323.8°C. The total residual mass in N2 atmosphere is 23.12%, more than in CO2 atmosphere (16.86%).Table 2
Results from thermogravimetric analysis forChlorella vulgaris in different atmospheres with the heating rate of 10°C/min.
Atmosphere
Heating rate (°C/min)
T
1 (°C)
DTG1 (% min−1)
T
2 (°C)
DTG2 (% min−1)
D
T
G
m
a
x (% min−1)
Residual mass (%)
CO2
10
323.8
−3.54
901.1
−3.26
−3.54
16.86
N2
10
322
−3.94
—
—
−3.94
23.12
Note:T1 is the temperatures for the first weight losses reaction peak; DTG1 is the reaction rate for the first weight losses reaction peaks; T2 is the temperatures for the second weight losses reaction peak; DTG2 is the reaction rate for the second weight losses reaction peak; DTGmax is the maximum reaction rate.Figure2 shows that in the low temperature range (210–470°C), the mass loss rate ofC. vulgaris decomposition in CO2 atmosphere is lower than in N2 atmosphere, but between 760°C and 926°C, it is opposite. It may be that CO2 molecule containing oxygen atom is different from N2. And char + CO2 may react in higher temperature zone. Therefore, in CO2 atmosphere, when the temperature rises from 760 to 926°C, the mass loss rate ofC. vulgaris is remarkably increased for the char gasification by CO2 [34].


### 3.3. Effect of Heating Rates on Decomposition ofC. vulgaris in N2 Atmosphere

Figure3 shows the DTG curve ofC. vulgaris decomposition at the heating rates of 10, 20, and 40°C min−1 in N2 atmosphere. The DTG curves for different heating rates (Figure 3) show that the rate of decomposition shifts to a higher magnitude as the heating rate increases, because the minimum heat required for the cracking of particles is reached later at higher temperatures, since the heat transfer is not as effective and efficient as slower heating rates [35]. As the heating rate increases from 10 to 20 and finally to 40°C/min, the maximum weight loss rate increases from 3.89 to 16.8%/min, and the corresponding temperature increases from 321.92 to 340.16°C, as shown in Table 3.Table 3
Results from thermogravimetric analysis in N2 atmosphere with different heating rates.
Heating rate (°C/min)
T
I (°C)
T
p
e
a
k (°C)
D
T
G
p
e
a
k (%/min)
T
F (°C)
D
V (%/min)
10
165.35
321.92
−3.89
568.38
−0.78
20
171.37
330.01
−8.47
560.57
−1.67
40
176.73
340.16
−16.8
556.54
−3.44
Note:TI, the initial degradation temperature; TF, the final degradation temperature, Tpeak, degradation peak temperature; DTGpeak, differential thermal gravity value at a peak temperature. DV is the average weight loss rate of the temperature ranging from 100 to 1000°C.Figure 3
DTG curves ofChlorella vulgaris decomposition at the heating rates of 10, 20, and 40°C min−1 in N2 atmosphere.In addition, the initial degradation temperatures at different heating rates are slightly increased, while the final degradation temperature is decreased. So the temperature range of the main pyrolysis process is narrowed as the heating rate increased. Moreover, the similar findings are also found in the case of decomposition of rapeseed [36]. The average weight loss rate of whole temperature range (100–1000°C) is changed from 0.78 to 3.44%/min when the heating rate varies from 10 to 40°C/min (Table 3).


### 3.4. Effect of Heating Rates on the Decomposition ofC. vulgaris in CO2 Atmosphere

The DTG curve ofC. vulgaris decomposition at different heating rates in CO2 atmosphere is shown in Figure 4. There are two peaks temperatures for each DTG curve: a strong peak (first peak) and one small one (second peak). As the heating rate increases, the two temperature peaks, especially the small one, shifts to high temperature. When the temperature reaches to 531.15, 608.35, and 651.98°C, the first main decomposition process at the heating rates of 10, 20, and 40°C/min is completed. When the heating rate changes from 10 to 40°C/min, the maximum weight loss rates corresponding to the first peak temperature increase from 3.52 to 15.26, while the weight loss rate corresponding to the second peak temperature increase from 3.22 to 8.27°C/min. The average weight loss rate of whole temperature range (100–1000°C) is changed from 0.87%/min to 3.77%/min when the heating rate varies from 10 to 40°C/min (Table 4).Table 4
Results from thermogravimetric analysis in CO2 atmosphere with different heating rates.
Heating rate (°C/min)
T
I (°C)
T
1 (°C)
DTG1 (°C/min)
T
2 (°C)
DTG2 (°C/min)
T
F (°C)
D
V (%/min)
10
202.35
326.43
−3.52
900.79
−3.22
531.15
−0.87
20
233.54
336.21
−7.35
931.26
−5.7
608.35
−1.81
40
235.36
348.06
−15.26
969.82
−8.27
651.98
−3.77Figure 4
DTG curves ofChlorella vulgaris decomposition at the heating rates of 10, 20, and 40°C min−1 in CO2 atmosphere.


### 3.5. Kinetics Analysis

As shown in Figure2, theC. vulgaris decomposition reaction in N2 atmosphere is considered as a single pseudo component. But in CO2 atmosphere, the decomposition process can be represented by the pseudo multicomponent overall model (PMOM) [31, 32, 37].As mentioned in Section3.1, in N2 atmosphere,C. vulgaris mainly devolatilizes at 140–550°C, while in CO2 atmosphere, the decomposition mainly occurs at 140–560°C and 790–1000°C (fourth stage), respectively.The results on variance and correlation coefficient (R2) are obtained from the experimental data analyzed with Excel 2003 software produced by Microsoft Corporation and Origin 7.0 software produced by Origin Lab Corporation.Table5 shows the kinetic parameters ofC. vulgaris in N2 and CO2 atmosphere at a heating rate of 10°C min−1. The correlation coefficient (R2) between predicted and experimental values is 0.93684–0.99506 in two atmospheres, indicating that the predicted values can match the actual experimental values well (Table 5). The kinetic parameters of the second stage in N2 atmosphere are E=11.255 kJ/mol, n=1.2, and A=4.827, quite similar to those in CO2 atmosphere (E=9.863 kJ/mol, n=1.1 and A=2.361). This phenomenon is the same as that of the sewage sludge reported by Jindarom et al. [38]: in CO2 atmosphere, the apparent activation energy and the reaction order of sewage sludge for low temperature are quite similar to those in N2 atmosphere. However, the kinetic parameters ofC. vulgaris in N2 atmosphere are different from the reportedChlorella protothecoides (42.2~52.5 kJ·mol−1) (Peng et al., 2001); it may be because the thermal behavior is greatly influenced by composition of biomass materials, and there may be obvious differences in decomposition kinetics among the similar species of biomass [13]. In addition, the values of kinetic parameters are varied depending on the assumptions decomposition kinetic models. Due to the different models used, the values may not be comparable [37].Table 5
Kinetic parameters ofChlorella vulgaris at heating rate of 10°C min−1.
stage
Gas
Model
E (kJ/mol)
n
A (min−1)
R
2
Second stage
N2
PSOM
11.255
1.2
4.827
0.99321
CO2
PMOM
9.863
1.1
2.361
0.99506
Third stage
N2
PSOM
—
—
—
—
CO2
PMOM
57.619
1.6
3757.358
0.93684
Fourth stage
N2
PSOM
—
—
—
—
CO2
PMOM
309.381
7
6.520 × 1017
0.97603
Note:R2 is related coefficient.As shown in Table5, in CO2 atmosphere, E=9.863, 57.619, and 309.381 kJ/mol, n=1.1, 1.6, and 7, and A=2.361, 3757.358, and 6.520 × 1017 min−1 corresponding to the second, third and fourth stages ofC. vulgaris. The maximum kinetic parameters (E,n,A) occur in the fourth stage, while the minimum values occur in the second one.


## 3.1. Decomposition Process

The thermogravimetry (TG) and differential thermogravimetry (DTG) curves ofC. vulgaris decomposition at a heating rate of 10°C min−1 in two atmospheres are shown in Figures 1 and 2, respectively. From Figure 2, two noteworthy peaks in pyrolysis are shown in curves; the decomposition process in N2 atmosphere can be divided into three stages for interpretation. The first stage is from room temperature to 140°C, corresponding to a loss of moisture and a slight volatile compound. The second one is from 140 to 550°C, where most of the organic materials are decomposed; this is the main decomposition process. The third one is from 550 to 1000°C; during this stage, the carbonaceous matters in the solid residuals are continuously decomposed at a very low rate.Figure 1
TG curves ofChlorella vulgaris decomposition at a heating rate of 10°C min−1 in N2 and CO2 atmospheres.Figure 2
DTG curves ofChlorella vulgaris decomposition at a heating rate of 10°C min−1 in N2 and CO2 atmosphere.As mentioned in the previous paragraph, in N2 atmosphere, only one main decomposition stage occurs at the temperature range of 140–550°C, which is close toChlorella protothecoides (150–540°C) reported by Peng et al. [11].In CO2 atmosphere, four stages could be distinguished during thermal degradation process ofC. vulgaris. The first stage is from room temperature to 140°C (dehydration). The second one is from 140 to 560°C (devolatilization), corresponding to the first main decomposition process. The third one is from 560 to 790°C (devolatilization); at this stage, the rate of reaction is very low. The fourth one (second main decomposition process) is from 790 to 1000°C; in this stage, the residual carbonaceous matters and CO2 may react at a relatively high rate. In CO2 atmosphere, there are two main decomposition stages (the second and fourth ones). Compared with the second stage, the fourth one is very narrow.


## 3.2. Comparison and Analysis ofC. vulgaris Decomposition in N2 and CO2 Atmospheres

Figures1 and 2 show that the TG and DTG curves ofC. vulgaris decomposition in N2 atmosphere are different from CO2 atmosphere. As mentioned in Section 3.1, the decomposition process ofC. vulgaris in N2 atmosphere can be divided into three stages, and one main decomposition process is obtained. However, in CO2 atmosphere, the thermal decomposition process is divided into four stages and two main decomposition processes are obtained.In addition, as shown in Figure1, when the temperature is below 887°C, the weight loss in CO2 atmosphere is less than in N2 atmosphere. The decomposition process ofC. vulgaris in CO2 atmosphere is delayed due to the difference of the CO2 from N2 molecule. When the temperature reaches 887°C, the weight loss ofC. vulgaris decomposition in N2 atmosphere is equivalent in CO2 atmosphere. However, when the temperature is above 887°C, a quick increase in the weight loss is observed in CO2 atmosphere, which may be caused by the gasification of carbonaceous matters.The characteristic parameters forC. vulgaris decomposition can be obtained from Figure 2, as shown in Table 2. It is clear that there is a peak for the thermal decomposition process ofC. vulgaris in N2 atmosphere at 322°C, and at this temperature the rate of weight loss attains the maximum value. However, in CO2 atmosphere, there are two peaks of the DTG curve occurring at 323.8°C and 901.1°C, respectively. The maximum rate of weight loss is attained at 323.8°C. The total residual mass in N2 atmosphere is 23.12%, more than in CO2 atmosphere (16.86%).Table 2
Results from thermogravimetric analysis forChlorella vulgaris in different atmospheres with the heating rate of 10°C/min.
Atmosphere
Heating rate (°C/min)
T
1 (°C)
DTG1 (% min−1)
T
2 (°C)
DTG2 (% min−1)
D
T
G
m
a
x (% min−1)
Residual mass (%)
CO2
10
323.8
−3.54
901.1
−3.26
−3.54
16.86
N2
10
322
−3.94
—
—
−3.94
23.12
Note:T1 is the temperatures for the first weight losses reaction peak; DTG1 is the reaction rate for the first weight losses reaction peaks; T2 is the temperatures for the second weight losses reaction peak; DTG2 is the reaction rate for the second weight losses reaction peak; DTGmax is the maximum reaction rate.Figure2 shows that in the low temperature range (210–470°C), the mass loss rate ofC. vulgaris decomposition in CO2 atmosphere is lower than in N2 atmosphere, but between 760°C and 926°C, it is opposite. It may be that CO2 molecule containing oxygen atom is different from N2. And char + CO2 may react in higher temperature zone. Therefore, in CO2 atmosphere, when the temperature rises from 760 to 926°C, the mass loss rate ofC. vulgaris is remarkably increased for the char gasification by CO2 [34].


## 3.3. Effect of Heating Rates on Decomposition ofC. vulgaris in N2 Atmosphere

Figure3 shows the DTG curve ofC. vulgaris decomposition at the heating rates of 10, 20, and 40°C min−1 in N2 atmosphere. The DTG curves for different heating rates (Figure 3) show that the rate of decomposition shifts to a higher magnitude as the heating rate increases, because the minimum heat required for the cracking of particles is reached later at higher temperatures, since the heat transfer is not as effective and efficient as slower heating rates [35]. As the heating rate increases from 10 to 20 and finally to 40°C/min, the maximum weight loss rate increases from 3.89 to 16.8%/min, and the corresponding temperature increases from 321.92 to 340.16°C, as shown in Table 3.Table 3
Results from thermogravimetric analysis in N2 atmosphere with different heating rates.
Heating rate (°C/min)
T
I (°C)
T
p
e
a
k (°C)
D
T
G
p
e
a
k (%/min)
T
F (°C)
D
V (%/min)
10
165.35
321.92
−3.89
568.38
−0.78
20
171.37
330.01
−8.47
560.57
−1.67
40
176.73
340.16
−16.8
556.54
−3.44
Note:TI, the initial degradation temperature; TF, the final degradation temperature, Tpeak, degradation peak temperature; DTGpeak, differential thermal gravity value at a peak temperature. DV is the average weight loss rate of the temperature ranging from 100 to 1000°C.Figure 3
DTG curves ofChlorella vulgaris decomposition at the heating rates of 10, 20, and 40°C min−1 in N2 atmosphere.In addition, the initial degradation temperatures at different heating rates are slightly increased, while the final degradation temperature is decreased. So the temperature range of the main pyrolysis process is narrowed as the heating rate increased. Moreover, the similar findings are also found in the case of decomposition of rapeseed [36]. The average weight loss rate of whole temperature range (100–1000°C) is changed from 0.78 to 3.44%/min when the heating rate varies from 10 to 40°C/min (Table 3).


## 3.4. Effect of Heating Rates on the Decomposition ofC. vulgaris in CO2 Atmosphere

The DTG curve ofC. vulgaris decomposition at different heating rates in CO2 atmosphere is shown in Figure 4. There are two peaks temperatures for each DTG curve: a strong peak (first peak) and one small one (second peak). As the heating rate increases, the two temperature peaks, especially the small one, shifts to high temperature. When the temperature reaches to 531.15, 608.35, and 651.98°C, the first main decomposition process at the heating rates of 10, 20, and 40°C/min is completed. When the heating rate changes from 10 to 40°C/min, the maximum weight loss rates corresponding to the first peak temperature increase from 3.52 to 15.26, while the weight loss rate corresponding to the second peak temperature increase from 3.22 to 8.27°C/min. The average weight loss rate of whole temperature range (100–1000°C) is changed from 0.87%/min to 3.77%/min when the heating rate varies from 10 to 40°C/min (Table 4).Table 4
Results from thermogravimetric analysis in CO2 atmosphere with different heating rates.
Heating rate (°C/min)
T
I (°C)
T
1 (°C)
DTG1 (°C/min)
T
2 (°C)
DTG2 (°C/min)
T
F (°C)
D
V (%/min)
10
202.35
326.43
−3.52
900.79
−3.22
531.15
−0.87
20
233.54
336.21
−7.35
931.26
−5.7
608.35
−1.81
40
235.36
348.06
−15.26
969.82
−8.27
651.98
−3.77Figure 4
DTG curves ofChlorella vulgaris decomposition at the heating rates of 10, 20, and 40°C min−1 in CO2 atmosphere.


## 3.5. Kinetics Analysis

As shown in Figure2, theC. vulgaris decomposition reaction in N2 atmosphere is considered as a single pseudo component. But in CO2 atmosphere, the decomposition process can be represented by the pseudo multicomponent overall model (PMOM) [31, 32, 37].As mentioned in Section3.1, in N2 atmosphere,C. vulgaris mainly devolatilizes at 140–550°C, while in CO2 atmosphere, the decomposition mainly occurs at 140–560°C and 790–1000°C (fourth stage), respectively.The results on variance and correlation coefficient (R2) are obtained from the experimental data analyzed with Excel 2003 software produced by Microsoft Corporation and Origin 7.0 software produced by Origin Lab Corporation.Table5 shows the kinetic parameters ofC. vulgaris in N2 and CO2 atmosphere at a heating rate of 10°C min−1. The correlation coefficient (R2) between predicted and experimental values is 0.93684–0.99506 in two atmospheres, indicating that the predicted values can match the actual experimental values well (Table 5). The kinetic parameters of the second stage in N2 atmosphere are E=11.255 kJ/mol, n=1.2, and A=4.827, quite similar to those in CO2 atmosphere (E=9.863 kJ/mol, n=1.1 and A=2.361). This phenomenon is the same as that of the sewage sludge reported by Jindarom et al. [38]: in CO2 atmosphere, the apparent activation energy and the reaction order of sewage sludge for low temperature are quite similar to those in N2 atmosphere. However, the kinetic parameters ofC. vulgaris in N2 atmosphere are different from the reportedChlorella protothecoides (42.2~52.5 kJ·mol−1) (Peng et al., 2001); it may be because the thermal behavior is greatly influenced by composition of biomass materials, and there may be obvious differences in decomposition kinetics among the similar species of biomass [13]. In addition, the values of kinetic parameters are varied depending on the assumptions decomposition kinetic models. Due to the different models used, the values may not be comparable [37].Table 5
Kinetic parameters ofChlorella vulgaris at heating rate of 10°C min−1.
stage
Gas
Model
E (kJ/mol)
n
A (min−1)
R
2
Second stage
N2
PSOM
11.255
1.2
4.827
0.99321
CO2
PMOM
9.863
1.1
2.361
0.99506
Third stage
N2
PSOM
—
—
—
—
CO2
PMOM
57.619
1.6
3757.358
0.93684
Fourth stage
N2
PSOM
—
—
—
—
CO2
PMOM
309.381
7
6.520 × 1017
0.97603
Note:R2 is related coefficient.As shown in Table5, in CO2 atmosphere, E=9.863, 57.619, and 309.381 kJ/mol, n=1.1, 1.6, and 7, and A=2.361, 3757.358, and 6.520 × 1017 min−1 corresponding to the second, third and fourth stages ofC. vulgaris. The maximum kinetic parameters (E,n,A) occur in the fourth stage, while the minimum values occur in the second one.


## 4. Conclusion

Thermogravimetric analysis is carried out to investigate decomposition characteristics ofC. vulgaris in different atmospheres and different heating rates. It can be concluded as follows.(
1
) In N2 atmosphere, only one main decomposition stage occurs at 140–550°C (devolatilization). However, in CO2 atmosphere, two main decomposition stages are observed, occurring at 140–560°C and 790–1000°C.(
2
) At the heating rate of 10°C/min, there is only one peak for the DTG curve ofC. vulgaris decomposition in N2 atmosphere at 322°C and maximum rate of weight loss occurs at this peak. However, in CO2 atmosphere, there are two peaks for the DTG curve ofC. vulgaris decomposition at 323.8°C and 901.1°C, respectively, and the maximum rate of weight loss occurs at 323.8°C. Moreover, the total residual mass in CO2 atmosphere (16.86%) is less than in N2 atmosphere (23.12%).(
3
) At the heating rate of 10°C/min, before 887°C, the weight loss in CO2 atmosphere is less than that in N2 atmosphere, while above 887°C, a new increase in the weight loss is observed in CO2 atmosphere. In addition, when the temperature ranges 210–470°C, the mass loss rate ofC. vulgaris decomposition in CO2 atmosphere is lower than that in N2 atmosphere, but it is opposite when the temperature is between 760°C and 926°C.(
4
) As the heating rate increases, the maximum weight loss rate, temperature corresponding to the maximum weight loss rate, and the average weight loss rate in temperature range of 100–1000°C are increased.(
5
) In CO2 atmosphere, the maximum kinetic parameters (E,n,A) occur in the fourth stage, while the minimum kinetic parameters occur in the second one. The kinetic parameters of second stage in CO2 atmosphere are quite similar to those in N2 atmosphere.



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*Source: 6160234-2017-03-15.xml*