Article pubs.acs.org/IECR
Investigation on Pyrolysis of Microalgae Botryococcus braunii and Hapalosiphon sp. Yong-Qiang Liu,*,† Lingo R. X. Lim,† Jing Wang,† Rong Yan,† and Aparat Mahakhant‡ †
Institute of Environmental Science and Engineering, Nanyang Technological University, 18 Nanyang Drive, Singapore, 637723 Bioscience Department, Thailand Institute of Scientific and Technological Research (TISTR), 35 Mu 3, Khlong 5, Khlong Luang, Pathum Thani 12120, Thailand
‡
ABSTRACT: In this study, the thermochemical characteristics of two types of microalgae, namely Hapalosiphon sp. and Botryococcus braunii, were investigated by use of a thermogravimetric analyzer. The low calorific value of Hapalosiphon sp. was 14.75 MJ kg−1, which is close to that of sewage sludge or lignocellulosic biomass. However, the low calorific value of B. braunii was as high as 35.58 MJ kg−1 due to 89% of total C and H contents in biomass. Pyrolytic results indicated that the two microalgae had similar pyrolytic temperature ranges. In addition, with increased heating rates, the pyrolytic curves of the microalgae shifted to a higher temperature. However, derivative thermogravimetric (DTG) profiles of the two microalgae are distinct, which showed that different reactions were involved in the pyrolysis process. Hapalosiphon sp. has a lower mean activation energy than B. braunii in the pyrolysis process.
1. INTRODUCTION With increasing concern over global warming and the depletion of limited fossil fuels, algae are getting more and more attention for their functions to fix CO2 and provide renewable biofuel.1−4 Compared with terrestrial plants, microalgae have higher growth rates, more flexible cultivation ways, and less land requirements.1 In addition, there is no competition between algae and crops; thus, algae cultivation will not negatively impact food prices or other human activities. Microalgae contain carbohydrates, proteins, lipids, and valuable nutrients. In some cases, the lipid content in microalgae can reach as high as 70% of dry biomass weight.1 The lipids in microalgae thus could be used for biodiesel production. Sometimes, carbohydrates such as starch, glycogen, or cellulose accumulate a lot in certain microalgae species as much as above 50% dry biomass weight, which could be applied as a carbon source for bioethanol or biomethane production.5−7 Some algae species even exhibit the capability to produce hydrogen,8 or algae could be used as a carbon source for hydrogen production. Except for biochemical conversion for biofuel production, algae could also be processed thermochemically to produce energy or biofuel. Cofiring with other feedstock to generate power is one of the thermochemical ways for algae to produce energy. Hydrogen or syngas could be produced by gasification of algae.9,10 Hydrothermal liquefaction or pyrolysis of algae is able to produce liquid biofuel for easy storage and transportation.11 Among the different ways to convert algae to biofuel, biodiesel production from algae is investigated the most extensively. However, the price of biodiesel from algae is still too high for biodiesel to replace fossil fuels. Since lipids are an intracellular product of algae, oil extraction has to be carried out for its followed transesterification. If the residual biomass after oil extraction could be further utilized effectively to recover energy, the economic benefit of algae-based biofuel could be improved for its potential application in practice.1 Boateng et al.12 reported promising production of pyrolysis liquids from the oil© 2012 American Chemical Society
seed press cakes of mustard family plants to further recover energy from oil seeds. Similarly, anaerobic digestion is used to process algal waste after oil extraction for methane production to get an energetic balance of the microalgae to biofuel process.13 However, algal biomass is not particularly easy to digest and is characterized by low specific gas yields and solids destruction, which means a lower specific energy recovery per kilogram of biomass.14 Pyrolysis is an alternative promising way to further recover energy from microalgae.20 Furthermore, it was noted that it is hard to extract all intracellular oil from algae in an economical way. The oil extraction yield and efficiency greatly depend on both the specific extraction method and the specific algae species. Different algae oil extraction strategies have been reported and compared. It was found that oil yield might vary greatly with different extraction ways.15−18 In practice, for large scale oil extraction from algae, incomplete oil extraction is possible. In this case, the residual algae biomass after oil extraction might still have a relatively high heating value, which could be preferably further converted into biofuel by a thermochemical method. Furthermore, if oil content in microalgae is too low, it might not be worthwhile to extract oil from microalgae in terms of the energy balance between consumption and recovery. Thus, the thermochemical method is also preferable for energy recovery from microalgae biomass with low oil content. Pyrolysis is considered to be one of the promising thermal approaches in converting biomass to energy, which have been investigated extensively for recovering energy from biomass such as lignocellulose and sewage sludge. Unlike lignocellulosic materials, which are mainly composed of cellulose, hemicellulose, and lignin, sewage sludge and algae contain mainly Received: Revised: Accepted: Published: 10320
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30, and 40 °C min−1 from ambient temperature to 900 °C. Nitrogen purging is used to displace air in the pyrolysis site and avoid any possible oxidation of the samples in TGA. The sample size was controlled around 15 mg, and the nitrogen purging gas rate was set at 40 mL min−1 based on previous experience. The weight loss of samples in response to temperature was recorded and used to plot the thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). 2.4. Kinetic Parameters of Pyrolysis. The kinetic study of pyrolysis is very important to the reactor design for large scale pyrolysis operation. A number of decomposition reactions are involved during the pyrolysis, but the kinetic study of each individual reaction is impossible and unnecessary. The macro kinetics is typically used in chemical reactor design. Therefore, the kinetic study could be simplified into one macro reaction as assumed in eq 1.
carbohydrates, proteins, and lipids. These compounds are reported to be probably pyrolyzed more easily than those contained in lignocellulosic materials especially with high lignin content.19 So far, although several studies on algae pyrolysis have been reported,20,21 knowledge on algae pyrolysis is still quite limited and the fundamentals relative to algae pyrolysis have not been fully understood. Pyrolysis is a highly complex process that is influenced by many factors such as the properties of the feedstock, components of the feedstock, and operational conditions. Considering the different components with different levels in algae, it is necessary to investigate the pyrolysis characteristics of typical algae with distinct components. Two kinds of typical oils, i.e., lipid and hydrocarbon, exist in microalgae. The former could be usually converted into biodiesel by transesterification, while the latter could be often processed to produce gasoline. In this study, two microalgae species, namely, Botryococcus braunii producing hydrocarbons and Hapalosiphon sp. producing lipids were chosen as representative microalgae biomass for the pyrolysis study. In addition, the pyrolysis of residual biomass after oil extraction was also investigated to compare with the microalgae biomass without oil loss, which could be used to guide the further application of residual microalgae biomass from the microalgaebased biofuel industry.
A(s) → B(s) + C(v)
(1)
Here, A(s) represents the reactant solid biomass, B(s) represents the solid residue from the biomass pyrolysis at high temperature, and C(v) represents the volatiles at high temperature. The rate of the general reaction of eq 1 could be described by eq 2.
2. MATERIALS AND METHODS 2.1. Algae Species and Sample Preparation. Two kinds of microalgae biomass, Hapalosiphon sp. TISTR 8236 and B. braunii, respectively, were applied in this study and were provided kindly by the Thailand Institute of Scientific and Technological Research (TISTR). Hapalosiphon sp. TISTR 8236 was cultivated in an outdoor raceway pond, while B. braunii was collected from natural bloom with B. braunii dominance around 95%. After filtering by plankton net, two kinds of biomass were lyophilized and then ground in a Rocklab bench top ring mill for oil extraction. Based on an oil extraction method reported by Bligh and Dyer,22 the oil contents in Hapalosiphon sp. TISTR 8236 and B. braunii were 13.6 and 35.5%, respectively. For thermochemical conversion study, a Soxhlet extractor was used to extract oil from lyophilized and ground biomass with hexane (99% grade AR) for a large amount of extracted oil and for convenience. The oil yields by Soxhlet extractor for Hapalosiphon sp. TISTR 8236 and B. braunii were 2.5 and 23.9%, respectively. The original biomass, residual biomass after extraction, and extracted oil were stored in a desiccator until thermogravimetric analysis. 2.2. Analytical Methods. The proximate analysis of the samples was conducted using a thermogravimetric analyzer (TA Model 2050, USA). The moisture content was evaluated around 105 °C,33 and the volatiles content was determined at 900 °C under N2 purging. The residue at 900 °C after switching N2 purging to O2 purging for 10 min is considered as ash. Fixed carbon is a calculated value of the difference between 100% and the sum of the percentages of moisture, volatile matter, and ash. For easy comparison, volatile matter, fixed carbon, and ash were further calculated on a dry basis without moisture in this study. The ultimate analysis was performed in a Leco TruSpec CHNS analyzer (USA). The low heating value (LHV) of the studied samples was measured in a bomb calorimeter (Parr 1260, Parr Instrument Co., Moline, IL). 2.3. Pyrolysis of the Samples. The pyrolysis of samples was conducted in the thermogravimetric analyzer (TA Model 2050, USA) in nitrogen atmosphere at heating rates of 5,10, 20,
da = kf (a) dt
(2)
where a is the thermal conversion and is given as a = (w0 − w)/ (w0 − wf). w, w0, and wf represent the mass of solid sample in response to reaction temperature, the initial sample amount, and the final residual amount after pyrolysis, respectively. The term f(a) is the reaction rate. If supposing the pyrolysis is a simple reaction, then the reaction rate could be symbolized as f(a) = (1 − a)n, and n is the reaction order. The reaction rate constant k could be expressed as k = A exp(−E/(RT)) according to the Arrhenius equation, where the kinetic parameters E and A represent the activation energy and the pre-exponential factor, respectively. Considering that the heating rate is β = dT/dt, eq 2 could be rewritten as
⎛1⎞ da = ⎜ ⎟kf (a) dT ⎝β⎠
(3)
If we replace k and f(a) by A exp(−E/(RT)) and (1 − a)n, respectively, eq 3 could be rewritten as ⎛A⎞ ⎛ E ⎞ da n ⎟(1 − a) = ⎜ ⎟ exp⎜ − ⎝ RT ⎠ dT ⎝β⎠
(4)
To calculate the kinetics parameters E and A, the nonisothermal method was used. Equation 4 could be transformed into24 G (a) =
∫T
T
0
⎛A⎞ ⎛ E ⎞ ⎟ dT ⎜ ⎟ exp⎜ − ⎝ RT ⎠ ⎝β⎠
(5)
Usually, for most temperatures and activation energies,1 − 2RT/E ≈ 1, so the kinetic mechanism equation can be simplified as ⎛ AR ⎞ ⎛ E ⎞ ⎟ + ln⎜ ln(G(a)) = −⎜ ⎟ ⎝ RT ⎠ ⎝ βE ⎠ 10321
(6)
dx.doi.org/10.1021/ie202799e | Ind. Eng. Chem. Res. 2012, 51, 10320−10326
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ln(1 − a) T2
(for n = 1)
1 − (1 − a)1 − n (1 − n)T 2
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Hapalosiphon sp. The higher nitrogen content in Hapalosiphon sp. probably came from the higher protein content. The 37.58% oxygen content in Hapalosiphon sp. decided the low heating value of Hapalosiphon sp. biomass. Based on molecular formulas of B. braunii and Hapalosiphon sp., both the higher H/C ratio and the lower O/C ratio in B. braunii led to the higher heating value of biomass. The oil extracted from Hapalosiphon sp. had a much higher heating value than that of the biomass, which is in agreement with the higher heating values of lipids than those of proteins and carbohydrates. From Tables 1 and 2, it could be known that the heating value and elemental contents of Hapalosiphon sp. are close to those of sewage sludge and lignocellulosic biomass24,25 while the properties of B. braunii with total 89% C and H contents and 35.58 MJ/kg low heating value are close to those of crude oil.26 This indicated that more energy could be recovered from B. braunii than from Hapalosiphon sp. 3.2. Pyrolysis in Thermogravimetric Analyzer. The pyrolysis characteristics―both the thermogravimetric curves (TG, in units of wt %) and differential thermogravimetric curves (DTG, in units of wt %/°C)―of the original Hapalosiphon sp. biomass, residual Hapalosiphon sp. biomass, and extracted oil are shown in Figure 1. Moisture was generally removed at 105 °C. After that, they started to degrade at 110 °C and the weight loss was observed. There were four distinct degradation temperature zones from 110 to 540 °C. The maximum weight loss occurred at 310 °C for both original biomass and residual. Since oil content was low in Hapalosiphon sp. biomass, the residual biomass and original biomass had similar components resulting in similar pyrolytic profiles and zones. In addition, the oil content in Hapalosiphon sp. was 13.6% by the Bligh and Dyer method and only 2.5% oil was removed from Hapalosiphon sp. biomass by the Soxhlet method; the degradation profiles of original Hapalosiphon sp. and residual Hapalosiphon sp. biomass were thus almost the same. However, the oil extracted from Hapalosiphon sp. biomass exhibited a different degradation profile. The maximum weight loss rate of 0.72 wt %/°C took place at the temperature of 260 °C for the oil, while it was 0.50 wt %/°C at 310 °C for the biomass. The main pyrolysis reactions of biomass occurred over the temperature range 230−400 °C corresponding to 48% weight loss in this temperature range. The pyrolysis of oil extracted from biomass at 110−340 °C led to 71% weight loss in this temperature range. In addition, it was noted that there was still a slight weight loss after 540 °C for original and residual Hapalosiphon sp. biomass. This indicated that the carbonaceous matter in the solid residue continuously decomposed at a very slow rate above 540 °C. However, for oil extracted from Hapalosiphon sp. biomass, the weight loss rate was close to zero and almost no weight loss was observed above 540 °C. Figure 2 shows TG and DTG curves of B. braunii. Obviously, the pyrolysis characteristics of B. braunii biomass differed from that of Hapalosiphon sp. biomass distinctly. There were two identifiable peaks for pyrolysis of original B. braunii biomass, while there were three identifiable peaks for oil extracted from B. braunii biomass. The degradation of biomass started from 150 °C, and the weight loss rate gradually increased. After the maximum weight loss rate was reached at 480 °C for the original biomass, residual biomass, and oil, it was reduced steeply to almost zero at 520 °C. At the same heating rate of 20 °C min−1, the temperature range for the pyrolysis of
(7)
(for n ≠ 1)
(8)
If the reaction order is correct, the plot of ln(G(a)) versus −1/ T should be a straight line, the activation energy E can be obtained from the slope, and the pre-exponential factor A can be obtained from the intercept. When the entire pyrolysis process is divided into several stages, E is just the activation energy of each individual temperature range, which has no direct relation to the entire process. To obtain the overall pyrolysis characteristics, Cumming et al.23 determined the weight mean activation energy Em and used it to analyze the overall sample reactivity: Em = (FE 1 1) + (F2E 2) + ... + (FnEn)
(9)
where E1−En is the activation energy of every stage (1−n); F1− Fn is the corresponding relative weight loss amount. Both E for a specific temperature stage and the mean activation energy Em for an overall evaluation of biomass pyrolysis were calculated in this work.
3. RESULTS AND DISCUSSION 3.1. Proximate and Ultimate Analysis of Microalgae. The original microalgae biomass, residual microalgae biomass after oil extraction, and extracted oil were analyzed to characterize their main properties. The proximate and ultimate results of these samples are listed in Tables 1 and 2. It was Table 1. Proximate Analyses of Two Types of Microalgae proximate analysis (wt.%)
Hapalosiphon sp. original biomass residual biomass after partial oil extraction oil extracted from original biomass B. braunii original biomass residual biomass after partial oil extraction oil extracted from original biomass a
moisture
volatile mattera
fixed carbona
asha
LHVa (MJ kg−1)
3.97 6.47
74.29 74.29
11.73 11.37
13.98 14.34
14.75 14.60
0.78
93.60
5.26
1.14
34.24
1.64 1.92
99.15 97.82
0.15 0.04
0.70 2.14
35.58 35.18
0.30
100.00
0
0
39.04
Dry basis.
found that there was a distinct difference between two microalgae species. For Hapalosiphon sp., the fixed carbon was around 11% for both original biomass (without oil loss) and residual biomass (after oil extraction). The ash content in Hapalosiphon sp. reached as high as 14%. However, fixed carbon and ash content in B. braunii were lower than 0.15 and 2.14%, respectively, for both original biomass and residual biomass. The volatile matter was much higher in B. braunii than in Hapalosiphon sp. The ultimate analysis revealed around 77% carbon content in B. braunii, while it was only 48% in 10322
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Table 2. Ultimate Analyses of Two Types of Microalgae ultimate analysisa (wt %) Hapalosiphon sp. original biomass residual biomass after partial oil extraction oil extracted from original biomass B. braunii original biomass residual biomass after partial oil extraction oil extracted from original biomass a
C
H
N
S
Ob
mol formula
47.94 48.68 70.09
7.44 7.25 10.98
6.45 6.71 0.96
0.58 0.61 0.20
37.58 36.74 17.77
CH1.86O0.59 CH1.79O0.57 CH1.88O0.19
77.04 76.16 80.49
12.40 12.05 13.42
1.23 1.54 0
0.18 0.20 0.07
9.86 10.06 6.01
CH1.93O0.10 CH1.90O0.10 CH2.00O0.06
Dry ash free basis. bCalculated by difference.
Figure 1. TG and DTG curves of Hapalosiphon sp. in thermogravimetric analyzer.
Hapalosiphon sp. biomass was 110−540 °C while it was 150− 520 °C for B. braunii biomass. The results above indicated that different microalgae species have different decomposition zones and different decomposition profiles although they are mainly composed of carbohydrates, proteins, and lipids. Since carbohydrates, proteins, and lipids include many types of subspecies, different subspecies of these main components may lead to different decomposition profiles. For example, there was only one DTG peak for the pyrolysis of the autotrophic microalga Chlorella protothecoides,27 the pyrolysis of the microalga Dunaliella tertiolecta, 28 or the pyrolysis of Enteromorpha prolifera.29 However, two DTG peaks were observed for the heterotrophic microalga C. protothecoides27 and more than two DTG peaks were observed for B. braunii and Hapalosiphon sp. in this study and for Synechococcus sp.30 Grierson et al.30 investigated the thermal characterization of six microalgae species under slow
pyrolysis conditions and found that DTG profiles of six microalgae species were distinct. Even for the same microalgae species C. protothecoides, the DTG profiles under pyrolysis conditions were different if microalgae were cultivated under different conditions such as autotrophic or heterotrophic conditions. This further demonstrated that thermal devolatilization characteristics of microalgae greatly depend on the compositions of the microalgae. It is well-known that various subcategories of compounds exist in each main category, i.e., carbohydrates, proteins, and lipids. For example, algae containing high carbohydrate fractions are dominated by alginates, laminarin, manntitol, and fucoidan depending on the species and culture conditions.31 Proteins in microalgae may include Ile, Leu, Val, Lys, Phe, Met, Cys, Thr, Ala, etc. Algae lipids typically have a carbon number range from 12 to 22. Therefore, it is hard to predict the devolatilization profiles of microalgae merely based on total amounts of carbohydrates, 10323
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Figure 2. TG and DTG curves of B. braunii in thermogravimetric analyzer.
proteins, and oil. Based on the literature and results listed here, the devolatilization of all microalgae species could be finished around 500−550 °C. 3.3. Effects of Heating Rate on Microalgae Pyrolysis. The curves of mass loss rates obtained from the pyrolysis of Hapalosiphon sp. and B. braunii after oil extraction at different heating rates are shown in parts a and b, respectively, of Figure 3. When the heating rate was increased, the DTG curves of biomass shifted to higher temperature. The temperature corresponding to the maximum mass loss rate at heating rates of 5 and 40 °C min−1 had a difference around 27 °C for Hapalosiphon sp. and 28 °C for B. braunii. The lateral shift of DTG curves to higher temperature with the increased heating rates a the common phenomenon for nonisothermal biomass pyrolysis. At a lower heating rate, samples may be heated uniformly from the outside to the inside without heat transfer resistance, so a sample could thus start to decompose at a lower temperature. Here, it needs to be pointed out that the DTG curves at different heating rates in this study are very close, namely a 27−28 °C shift at heating rates from 5 to 40 °C min−1, which indicates that heat and mass transfer inside a sample could be negligible. In addition, no evident difference of maximum mass loss rate in the main devolatilization zone was observed in parts a and b of Figure 3 at different heating rates. This is different from the result reported for the pyrolysis of cellulose25 that the maximum mass loss rate (wt %/°C) decreased greatly with the increase of the heating rate and its mass loss occurred over a wider temperature range. In this study, the total volatiles content at different heating rates was almost the same. The main reason is likely due to the loose
Figure 3. DTG curves of microalgae biomass after oil extraction under different heating rates: (a) residual biomass of Hapalosiphon sp.; (b) residual biomass of B. braunii.
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Table 3. Kinetics Parameters of the Samples under Nonisothermal Conditions in Nitrogen with Heating Rate of 20 °C min−1 sample Hapalosiphon sp. original biomass
residual biomass after partial oil extraction
oil extracted from original biomass B. braunii original biomass residual biomass after partial oil extraction oil extracted from original biomass
Tm range (°C)
peak Tm
activation energy, E (kJ mol−1)
106−168 168−234 234−396 106−168
148 216 306 148
11.31 12.45 45.63 5.50
5.6 7.7 4.2 7.5
× × × ×
168−234 234−396 110−340 340−406
216 306 261 350
9.29 44.76 53.34 11.78
3.0 3.7 5.6 1.6
× × × ×
110−440 440−540 110−456
407 480 425
43.29 84.40 51.87
456−540 110−300 300−440 440−540
480 270 400 480
96.21 28.48 47.79 109.79
correln coeff, R2
weight loss (wt %)
10−3 10−3 101 10−4
0.9958 0.9972 0.9988 0.9726
5.79 7.24 48.60 6.47
10−3 101 102 10−2
0.9992 0.9992 0.9984 0.9994
6.24 48.81 71.03 13.20
4.4 6.5 × 103 1.8 × 101
0.9988 0.9974 0.9998
55.15 36.83 56.22
× × × ×
0.9966 0.9984 0.9988 0.9954
36.23 13.54 47.90 38.24
A (s−1)
4.3 2.1 1.2 6.0
Em (kJ mol−1)
38.50
37.03
46.83
59.75 69.25
104 10−1 101 105
68.96
rate had a weak influence on the pyrolysis of two types of microalgae biomass. The mean activation energy of Hapalosiphon sp. was 38.50 kJ mol−1 while it was 59.75 kJ mol−1 for B. braunii, which indicated that Hapalosiphon sp. is more easily pyrolyzed than B. braunii.
structure of microalgae and negligible heat transfer resistance inside the sample. 3.4. Kinetics Study of Microalgae Pyrolysis. Based on eqs 6−9 described in foregoing statements, the kinetic parameters were calculated with the assumption of different reaction orders such as 0.5, 1, 2, and 3 in different pyrolysis zones. It was found that the reaction order of 1 is the best fit for the pyrolysis of two types of microalgae and oil extracted from microalgae. The calculated kinetic parameters with the reaction order of 1 are listed in Table 3. It can be found that the activation energy in three different decomposition zones for original and residual Hapalosiphon sp. biomass were close although the activation energy corresponding to each specific zone was different. A similar phenomenon was observed for original and residual B. braunii biomass. This indicated that the pyrolysis characteristics for original microalgae biomass are similar to those for residual microalgae biomass after oil extraction. The mean activity energy E m for residual Hapalosiphon sp. biomass is only 37.03 kJ/mol with a lower pre-exponential factor, while it is 69.25 kJ/mol for B. braunii biomass. Obviously, Hapalosiphon sp. biomass could be pyrolyzed more easily than B. braunii biomass at the same heating rate and the pyrolytic reactions involved in two types of microalgae biomass are also different. The mean activation energy for oil extracted from Hapalosiphon sp. is 46.83 kJ mol−1 while it is 68.96 kJ mol−1 for oil from B. braunii, which indicates the difference in oil compositions from Hapalosiphon sp. and B. braunii. It has been reported that hydrocarbons, mainly terpenoids, are the main component of oil from B. braunii32 while the main oil component from Hapalosiphon sp. is glyceryl lipids. Therefore, it showed the different pyrolytic parameters even for oil extracted from two types of microalgae.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to express our sincere acknowledgment to the National Environment Agency of Singapore (NEA) for its support.
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REFERENCES
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4. CONCLUSION The higher calorific value of B. braunii showed that more energy could be recovered from B. braunii than from Hapalosiphon sp. Distinct pyrolytic zones and DTG profiles were exhibited for two types of microalgae. However, for each microalga, the pyrolytic characteristics of original biomass and residual biomass after oil extraction were similar. The heating 10325
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Industrial & Engineering Chemistry Research
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dx.doi.org/10.1021/ie202799e | Ind. Eng. Chem. Res. 2012, 51, 10320−10326
