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

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 40C/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 kJmol 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.


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][6][7][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 CO 2 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 N 2 atmosphere and established significant information on the pyrolysis behavior and kinetics [7,[11][12][13].Both N 2 and CO 2 are inert gases.However, there have been no reports on the decomposition characteristics of microalgae in CO 2 atmosphere.Whether the decomposition characteristics of microalgae in CO 2 atmosphere are the same as in N 2 atmosphere is still unknown.
Chlorella, a genus of unicellular green microalgae, with a spherical shape of 2.0∼10.0m 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 is Chlorella vulgaris (C.vulgaris), growing in fresh water.Some scholars studied the pyrolysis kinetic of microalgae in N 2 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][18][19][20].However, the pyrolysis characteristics and kinetic analysis of microalgae pyrolysis under N 2 and CO 2 atmosphere by TGA were not reported yet.
So far the pyrolysis technology is still in development.The less reports on the pyrolysis of C. vulgaris in different atmospheres (N 2 and CO 2 ) were found.In this study, the decomposition characteristics of C. vulgaris were studied by a thermogravimetric analysis (TGA), and the effects of atmospheres (N 2 or CO 2 atmosphere) and heating rates (10, 20 and 40 ∘ C/min) on C. vulgaris decomposition were investigated.Finally, kinetic triplets of C. vulgaris were obtained with Coats-Redfern approximation method, including E, 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 for C. vulgaris treatment process.

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.The C. 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 m in diameter.After the above treatment, the sample was stored in a desiccator for test.

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 N 2 or CO 2 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.

Characteristic Parameters of Temperatures.
The initial and final thermal degradation temperatures represent how hard the reaction is.  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.  is the final degradation temperature which is obtained when the mass lose accounts for 98% of the total quality loss.DTG peak is differential thermal gravity value at a peak temperature and  peak is degradation peak temperature which is defined as the temperature at which the mass loss rate in DTG curve reached the local maximum.  is the average weight loss rate of the temperature ranged from 100 to 1000 ∘ C.

Kinetic Modeling.
The kinetic equation of common type can be generally expressed as follows [26]: where  is the conversion degree,  (min) is time,  (K) is the absolute temperature, () is a function, the type of which depends on the reaction mechanism, and () is the temperature dependent rate constant.() is usually described with the Arrhenius equation: where  (min −1 ) is preexponential or frequency factor,  (kJ mol −1 ) is the activation energy, and  (kJ/mol⋅K) is the universal gas constant.
The function () is expressed as follows: where  is the reaction order.The degree of conversion of the reduction process is expressed as follows: where   is the initial mass of the sample,   is the mass of the sample at time , and  ∞ 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 where In general,  is the heating rate; the term of 2/ can be neglected since it is much less than 1.A plot of ln[()/ 2 ] against 1/ should result in a straight line of slope −/ for the correct reaction mechanism, as using the method to find the suitable model function () (or ()) of global decomposition kinetics.Once the form of () is obtained, the apparent activation energy  and the frequency factor  can be calculated from the straight line in light of (5).With the PMOM, the C. vulgaris is considered as three pseudo components and individually decomposed over a temperature range, which can be expressed as follows: where  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][32][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).

Results and Discussion
3.1.Decomposition Process.The thermogravimetry (TG) and differential thermogravimetry (DTG) curves of C. 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 N 2 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.
As mentioned in the previous paragraph, in N 2 atmosphere, only one main decomposition stage occurs at the temperature range of 140-550 ∘ C, which is close to Chlorella protothecoides (150-540 ∘ C) reported by Peng et al. [11].
In CO 2 atmosphere, four stages could be distinguished during thermal degradation process of C. 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 CO 2 may react at a relatively high rate.In CO 2 atmosphere, there are two main decomposition stages (the second and fourth ones).Compared with the second stage, the fourth one is very narrow.

Comparison and Analysis of C. vulgaris Decomposition in
N 2 and CO 2 Atmospheres.Figures 1 and 2 show that the TG and DTG curves of C. vulgaris decomposition in N 2 atmosphere are different from CO 2 atmosphere.As mentioned in Section 3.1, the decomposition process of C. vulgaris in N 2 atmosphere can be divided into three stages, and one main decomposition process is obtained.However, in CO 2 atmosphere, the thermal decomposition process is divided into four stages and two main decomposition processes are obtained.
In addition, as shown in Figure 1, when the temperature is below 887 ∘ C, the weight loss in CO 2 atmosphere is less than in N 2 atmosphere.The decomposition process of C. vulgaris in CO 2 atmosphere is delayed due to the difference of the CO 2 from N 2 molecule.When the temperature reaches 887 ∘ C, the weight loss of C. vulgaris decomposition in N 2 atmosphere is equivalent in CO 2 atmosphere.However, when the temperature is above 887 ∘ C, a quick increase in the weight loss is observed in CO 2 atmosphere, which may be caused by the gasification of carbonaceous matters.
The characteristic parameters for C. 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 of C. vulgaris in N 2 atmosphere at 322 ∘ C, and at this temperature the rate of weight loss attains the maximum value.However, in CO 2 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 N 2 atmosphere is 23.12%, more than in CO 2 atmosphere (16.86%).
Figure 2 shows that in the low temperature range (210-470 ∘ C), the mass loss rate of C. vulgaris decomposition in CO 2 atmosphere is lower than in N 2 atmosphere, but between 760 ∘ C and 926 ∘ C, it is opposite.It may be that CO 2 molecule containing oxygen atom is different from N 2 .And char + CO 2 may react in higher temperature zone.Therefore, in CO 2 atmosphere, when the temperature rises from 760 to 926 ∘ C, the mass loss rate of C. vulgaris is remarkably increased for the char gasification by CO 2 [34].

Effect of Heating Rates on Decomposition of C. vulgaris in N 2
Atmosphere. Figure 3 shows the DTG curve of C. vulgaris decomposition at the heating rates of 10, 20, and 40 ∘ C min −1 in N 2 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.
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).

Effect of Heating Rates on the Decomposition of C. vulgaris in CO 2
Atmosphere.The DTG curve of C. vulgaris decomposition at different heating rates in CO 2 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.Note:  1 is the temperatures for the first weight losses reaction peak; DTG 1 is the reaction rate for the first weight losses reaction peaks;  2 is the temperatures for the second weight losses reaction peak; DTG 2 is the reaction rate for the second weight losses reaction peak; DTG max is the maximum reaction rate.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).

Kinetics Analysis.
As shown in Figure 2, the C. vulgaris decomposition reaction in N 2 atmosphere is considered as a single pseudo component.But in CO 2 atmosphere, the decomposition process can be represented by the pseudo multicomponent overall model (PMOM) [31,32,37].As mentioned in Section 3.1, in N 2 atmosphere, C. vulgaris mainly devolatilizes at 140-550 ∘ C, while in CO 2 atmosphere, the decomposition mainly occurs at 140-560 ∘ C and 790-1000 ∘ C (fourth stage), respectively.
The results on variance and correlation coefficient ( 2 ) 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.
Table 5 shows the kinetic parameters of C. vulgaris in N 2 and CO 2 atmosphere at a heating rate of 10 ∘ C min −1 .The correlation coefficient ( 2 ) between predicted and experimental values is 0.93684-0.99506in two atmospheres, indicating that the predicted values can match the actual experimental values well (Table 5).The kinetic parameters of the second stage in N 2 atmosphere are  = 11.255kJ/mol,  = 1.2, and  = 4.827, quite similar to those in CO 2 atmosphere ( = 9.863 kJ/mol,  = 1.1 and  = 2.361).This phenomenon is the same as that of the sewage sludge reported by Jindarom et al. [38]: in CO 2 atmosphere, the apparent activation energy and the reaction order of sewage sludge for low temperature are quite similar to those in N 2 atmosphere.However, the kinetic parameters of C. vulgaris in N 2 atmosphere are different from the reported Chlorella 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].
As shown in Table 5, in CO 2 atmosphere,  = 9.863, 57.619, and 309.381 kJ/mol,  = 1.1, 1.6, and 7, and  = 2.361, 3757.358, and 6.520 × 1017 min −1 corresponding to the second, third and fourth stages of C. vulgaris.The maximum kinetic parameters (, , ) occur in the fourth stage, while the minimum values occur in the second one.

Conclusion
Thermogravimetric analysis is carried out to investigate decomposition characteristics of C. vulgaris in different atmospheres and different heating rates.It can be concluded as follows.
(2) At the heating rate of 10 ∘ C/min, there is only one peak for the DTG curve of C. vulgaris decomposition in N 2 atmosphere at 322 ∘ C and maximum rate of weight loss occurs at this peak.However, in CO 2 atmosphere, there are two peaks for the DTG curve of C. 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 CO 2 atmosphere (16.86%) is less than in N 2 atmosphere (23.12%).
(3) At the heating rate of 10 ∘ C/min, before 887 ∘ C, the weight loss in CO 2 atmosphere is less than that in N 2 atmosphere, while above 887 ∘ C, a new increase in the weight loss is observed in CO 2 atmosphere.In addition, when the temperature ranges 210-470 ∘ C, the mass loss rate of C. vulgaris decomposition in CO 2 atmosphere is lower than that in N 2 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 CO 2 atmosphere, the maximum kinetic parameters (, , ) occur in the fourth stage, while the minimum kinetic parameters occur in the second one.The kinetic parameters of second stage in CO 2 atmosphere are quite similar to those in N 2 atmosphere.

Figure 1 :
Figure 1: TG curves of Chlorella vulgaris decomposition at a heating rate of 10 ∘ C min −1 in N 2 and CO 2 atmospheres.

Figure 2 :
Figure 2: DTG curves of Chlorella vulgaris decomposition at a heating rate of 10 ∘ C min −1 in N 2 and CO 2 atmosphere.

Table 1 :
Ultimate and proximate analysis and lower heating values of Chlorella vulgaris (on dry basis).
Note:  net, , lower heating value on dry basis.

Table 3 :
Results from thermogravimetric analysis in N 2 atmosphere with different heating rates., the initial degradation temperature;   , the final degradation temperature,  peak , degradation peak temperature; DTG peak , differential thermal gravity value at a peak temperature.is the average weight loss rate of the temperature ranging from 100 to 1000 ∘ C.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. Note:

Table 5 :
Kinetic parameters of Chlorella vulgaris at heating rate of 10 ∘ C min −1 .
Note:  2 is related coefficient.