Article pubs.acs.org/EF
Investigation on Pyrolysis of Low Lipid Microalgae Chlorella vulgaris and Dunaliella salina Xun Gong, Biao Zhang, Yang Zhang, Yongfu Huang, and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China S Supporting Information *
ABSTRACT: Chlorella vulgaris and Dunaliella salina are two kinds of microalgae, which are widely distributed in China. Thermal decomposition of low-lipid C. vulgaris and D. salina were performed using thermogravimetric analysis. The effect of heating rates on pyrolytic characteristics was investigated, and thermal decomposition kinetics was determined as well. Furthermore, pyrolysis experiments were carried out on a fixed-bed reactor. The gas, char, and tar yields were analyzed, and the mass balance was from 88.4 to 96.8%. C. vulgaris had higher H2 yields and lower CH4 yields than D. salina during pyrolysis. The theoretical calorific value of the pyrolytic gas of D. salina was higher than that of C. vulgaris because D. salina had a higher amount of high heating value components, such as C2H6, C2H4, and C2H2. The biochar from microalgae had a smaller Brunauer−Emmett−Teller surface area than the char from pyrolysis of lignocellulosic biomass. Highest yields of pyrolytic oil were 49.2 and 55.4% (water-free basis) for C. vulgaris and D. salina at 500 °C, respectively. The characteristics of bio-oil from microalgae pyrolysis, including water content, density, acidity, and heating value, were investigated as well as the chemical composition at different pyrolysis temperatures. The microalgae pyrolytic oil was found to have significant levels of alkanes, alkenes, alkines, and esters and be particularly high in nitrogenous compounds. In comparison to the bio-oils from common lignocellulosic biomass, the microalgae oil had lower oxygen and water contents, a lower total acid number, and a higher heating value.
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INTRODUCTION Microalgae are a diverse group of aquatic, photosynthetic organisms, which are typically unicellular. Microalgae have the potential to revolutionize biotechnology in a number of areas, including nutrition, aquaculture, pharmaceuticals, and biofuels.1 As a new type of energy crop, microalgae have many advantages over other feedstocks, such as impressive productivity,2 noncompetition with agriculture,3 flexibility on water quality,4 mitigation of CO2,5 and broad product portfolio.6 Microalgal biodiesel is known as the third-generation biofuel technology.7 Direct extraction,8 solvent extraction,9 and supercritical fluid extraction10 are three main developed technical routes for extraction of biodiesel from microalgae in recent years. However, there are still some problems with those processes, such as low-lipid content, incomplete extraction, and pollution from the extraction reagent recovery, resulting in poor economy. The annual production of algal biomass is estimated to be around 5 million kg/year, with a high market value of about 330 USD/kg.1 On the basis of conservative estimates, algal biofuels produced in large-scale production with current technologies would cost more than $8/gallon (in contrast to $4/gallon for soybean oil nowadays).11 However, the life-cycle assessment of microalgal biodiesel indicates that the cultivation of microalgae has the potential to produce an environmentally sustainable feedstock for the production of biodiesel.12 A high lipid content is always the limiting factor for traditional microalgae biodiesel technology. However, pyrolysis of low-lipid microalgae can produce high-quality bio-oils.13 Therefore, fast pyrolysis gradually becomes an alternative and promising technical route. Relatively few studies have been © 2013 American Chemical Society
performed about pyrolysis of microalgae. Pyrolysis of the residues after lipid extraction is an option. Pan et al.14 used the residue of Nannochloropsis sp. after lipid extraction as raw material. The liquid yield of 47.6% had been obtained in a fixedbed reactor at 400 °C. The direct pyrolysis bio-oil had an oxygen content of 30.1% and a calorific value of 24.6 MJ/kg. In the work by Wang et al.,15 Chlorella vulgaris was first extracted with solvent for lipid recovery and then the remnants were used as the feedstock for fast pyrolysis experiments using a fluidizedbed reactor at 500 °C. Yields of bio-oil, biochar, and gas were 53, 31, and 10 wt %, respectively. Using low-lipid microalgae as feedstocks of pyrolysis is a reasonable alternative because of the low cost for these kinds of microalgae. The maximum oil yield of 52.0% was achieved at 500 °C for 5 min using Chlorella protothecoides with 14.3% lipid content in the study by Peng et al.16 Miao et al.17 chose two kinds of low-lipid microalgae Chlorella protothecoides and Microcystis aeruginosa. Pyrolysis experiments were carried out on a fluidized-bed reactor. Bio-oil yields of 17.5 and 23.7% were obtained with a higher calorific value and a lower oxygen content compared to wood and straw. C. vulgaris is a kind of freshwater, fast-growing microalga, and D. salina is a type of green microalga especially found in sea salt fields. Both of them are widely distributed in China. The lowlipid content strains of microalgae provide lower cost of Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 31, 2013 Revised: October 12, 2013 Published: October 25, 2013 95
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column (30 m × 0.25 mm inner diameter, with 0.25 μm film thickness, Agilent Technologies, Inc., Wilmington, DE). Analysis was made using splitless mode. The GC oven temperature program began with 45 °C and held for a 3 min, then raised at 10 °C/min to 300 °C, and held for 3 min. The pyrolytic gas collected in the gas bag was analyzed by Agilent 3000A micro-GC (Agilent Technologies, Inc., Wilmington, DE) for gas constituents, immediately after the pyrolysis experiment. The Brunauer−Emmett−Teller (BET) surface area of the biochar was determined using an ASAP2020 physisorption analyzer (Micromeritics Instrument Corporation, Norcross, GA). Prior to analysis, samples were vacuum-degassed at 300 °C for 12 h. The biochar particle structure and surface topography were analyzed using a Quanta 200 scanning electron microscope (FEI, Hillsboro, OR).
feedstocks with faster growth rates. The objective of this research is to investigate the yields and characteristics of bio-oil, biochar, and noncondensable gas from the pyrolysis of C. vulgaris and D. salina.
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SAMPLING AND TEST PROCESS
Characterization of the Microalgae Sample. Microalgae powder of C. vulgaris and D. salina was obtained from Shandong Firstspirulina Biotech Co., Ltd. (Binzhou, Shandong, China). The feedstock was dried at 45 °C in a nitrogen atmosphere until reaching a constant weight and then mechanically sieved into a size fraction of 45−150 μm as well as the compared biomass sample, which were pretreated by crushing and grinding, including sawdust, straw, and rice husk. Ultimate analysis was performed using a Vario EL-II elemental analyzer (Elementar Analysensysteme GmbH, Hanau, Germany), and proximate analysis was performed on a TGA-2000 thermogravimetric analyzer (Navas Instruments, Conway, SC). The calorific value of the feedstocks and microalgae char was obtained using a Parr 6300 oxygen bomb calorimeter (Parr Instrument Company, Moline, IL). Main mineral elements in the sample were measured by ELAN DRC-e inductively coupled plasma−mass spectrometry (PerkinElmer, Waltham, MA) after all of the solid had been digested in an ETHOS E microwave-assisted system (Milestone S.r.l., Via Fatebenefratelli, Sorisole, Italy). Crude fat and protein contents in the microalgae were analyzed according to GB5009.5-201018 and GB/T 5009.62003.19 Simple sugars, oligosaccharides, polysaccharides, and their derivatives were determined by the phenol−sulfuric method.20 Thermogravimetric analysis (TGA, STA409C, NETZSCH-Gerätebau GmbH, Wittelsbacherstrasse, Selb, Germany) was carried out for both feedstocks to study their decomposition behavior. Purified N2 (purity > 99.99 vol %) at a flow rate of 100 mL/min was used as the carrier gas to provide an inert atmosphere. The samples were heated from room temperature to 800 °C at different heating rates from 5 to 40 K/min. Pyrolysis Experiments. Pyrolysis experiments were carried out on a quartz fixed-bed reactor using an atmospheric flow of N2 (purity > 99.99 vol %). The inner diameter of the reactor was 40 mm. The outlet of the reactor was only 6 mm, which can minimize the secondary reaction of volatile. The temperature error range within the center of 200 mm was smaller than 5 °C through the temperature calibration from 300 to 700 °C. A total of 1 g of each microalgae sample was used in this study. The gas flow rate of 0.4 L/min was controlled by a mass flow meter. In the experiment, a relatively fast heating rate could be achieved by pushing the sample crucible into the center of the reaction zone after the temperature had reached the set value. The holding time for pyrolysis is 20 min, which had been proven to be enough for completing the pyrolysis reaction. Liquid nitrogen was used to quench the volatile through a U-shaped tar trap in the quantitative analysis of the tar yield and components. After the pyrolysis, the trap was washed with mixed solvents of chloroform and methanol [high-performance liquid chromatography (HPLC) grade, 80:20, vol/vol], and then the tar was dissolved into the solvent. A weighed portion of the tar solution was transferred onto a preweighed aluminum dish. The solvents were then evaporated from the aluminum dish in a nitrogen-swept oven controlled at around 40 °C for 8 h. The tar yield was determined by the weight loss of the dish during this process. It should be noted that some light hydrocarbons with higher vaporization partial pressure might vaporize during the drying process at 40 °C. Ice water was only used in the tar trap when the pyrolytic gas was collected by a gas bag for further GC analysis. Characterization of Pyrolysis Products. The water content in the collected liquid product was measured by the Karl Fischer method.21 The acid value of the oil was analyzed using the American Oil Chemists’ Society (AOCS) method,22 while the density of pyrolytic oil was determined according to ASTM D5002-99.23 Tar compounds were determined by Agilent 7890A/5975C gas chromatography−mass spectrometry (GC−MS) (Agilent Technologies, Inc., Wilmington, DE) equipped with an Aminex HP-5MS
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RESULTS AND DISCUSSION Feedstock Characterization. The results of proximate and ultimate analyses and higher heating value (HHV) of two kinds of microalgae are shown in Tables 1 and 2. As a comparison, Table 1. Proximate Analysis and HHV of C. vulgaris and D. salina Compared to a Typical Biomass Sample proximate analysis (wt %) moisturea
asha
volatile mattera
fixed carbona
HHV (MJ/kg)
4.4 4.0 4.7 8.5 7.7
11.4 7.2 1.4 10.5 15.0
75.2 76.3 84.2 65.8 60.7
9.0 12.5 9.7 15.2 16.6
19.3 21.2 17.7 16.3 16.0
C. vulgaris D. salina sawdust straw rice husk a
Air-dried basis.
Table 2. Ultimate Analysis of C. vulgaris and D. salina Compared to a Typical Biomass Sample (wt %) C. vulgaris D. salina sawdust straw rice husk a
Ca
Ha
Oa,b
Na
Sa
molecular formula
44.7 48.1 47.4 39.3 38.5
6.5 7.1 6.2 6.2 5.7
24.0 23.3 40.0 33.4 32.2
7.6 9.4 0.2 1.4 0.6
1.4 0.9 0.1 0.7 0.3
CH0.15O0.54 CH0.15O0.49 CH0.13O0.85 CH0.16O0.85 CH0.15O0.84
Air-dried basis. bBy difference.
the results of typical lignocellulosic biomass samples, including sawdust, straw, and rice husk, are also listed. Relatively high contents of carbon and hydrogen led to the high calorific value for microalgae. Likewise, the HHV of D. salina was higher than that of C. vulgaris. The N content of the microagal biomass was much higher because the peptide bonds were widely distributed in the microalgae proteins. The data showed that the S content of microalgae is also higher than that of ordinary biomass because it is a possible element existing in the proteins. Other significant features for microalgae were a low O content, O/C ratio, and O/H ratio. Results for main mineral elements are shown in Table 3. The microalgae cell should absorb inorganic salts from the culture media during growth, because the accumulations of Na, Ca, and Fe were found in the microalgae, especially for C. vulgaris. Microalgae contain carbohydrates, proteins, lipids, and some other nutrients. The analysis results of biochemical composition are given in Table 4. The lipid contents of both microalgae were smaller than 16%. As a result, chemical extraction technology is not suitable for microalgae with such low lipid 96
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Table 3. Main Mineral Elements in C. vulgaris and D. salina Compared to a Typical Biomass Sample (wt %) C. vulgaris D. salina sawdust straw rice husk a
Na
Mg
K
Ca
Fe
1.35 0.82 0.01 0.01 nd
0.40 0.56 0.01 0.05 0.02
0.67 0.51 0.07 1.23 0.32
1.66 0.48 0.05 0.39 0.02
0.36 0.10 nda 0.01 nd
degradation of C. vulgaris was more marked than that of D. salina in the initial phase when the temperature was below 200 °C. However, the degradation of D. salina was more significant between 200 and 300 °C, which resulted in a larger weight loss. The residual yields were 24.8 and 21.1%. C. vulgaris had more solid residual after thermal degradation mainly because of a relatively high ash content as well as alkali and alkali earth metal contents. The maximum weight loss occurred at 310 and 270 °C for C. vulgaris and D. salina, respectively. The results above indicate that different microalgae species have different decomposition characteristics, although they are both mainly composed of carbohydrates, proteins, and lipids. Different subspecies of these main components may lead to different decomposition profiles. In the DTG plot, the vertical coordinate is also equivalent to the reaction rate. Thus, several important parameters about the pyrolysis can be valued, including (1) initial temperature of decomposition (Ts), (2) temperature of the maximum reaction rate (Tmax), (3) maximum reaction rate (Dmax), and (4) average reaction rate (Dave), which are listed in Table 5. It was found that the initial
nd = not detected.
Table 4. Biochemical Composition of C. vulgaris and D. salina (wt %) C. vulgaris D. salina a
protein
lipid
carbohydrate
mfcarbohydratea
47.4 58.8
15.6 10.5
13.2 11.9
21.2 19.5
By difference.
contents. In general, lower oil strains grow faster than higher oil strains.24 For instance, microalgae containing 30% oil grow 30 times faster than those containing 80% oil.25 The microalgae in this study grow rapidly, and thus, they are more efficient for pyrolysis use. The carbohydrates mass fraction (mf, %) can be determined by difference.15
Table 5. Thermal Decomposition Parameters at 5 K/mina C. vulgaris D. salina
mfcarbohydrate = 100 − mf protein − mflipid − mf moisture
Ts (°C)
Tmax (°C)
Dmax (%/min)
Dave (%/min)
160 150
300 270
2.46 3.74
0.50 0.53
a
Ts, initial decomposition temperature; Tmax, temperature of the higher DTG peak; Dmax, maximum reaction rate; and Dave, average reaction rate.
− mfash
The mfcarbohydrate calculated for C. vulgaris was 21.2%, which is very similar to the result of 20.99% by Wang et al.15 However, mfcarbohydrate was 8% larger than the result by the phenol− sulfuric method. The results showed that D. salina had a higher protein content and lower lipid and carbohydrate contents. Thermal Decomposition Behavior. Thermogravimetry (TG) and differential thermogravimetry (DTG) curves of C. vulgaris and D. salina in a thermogravimetric analyzer at 5 K/ min are shown in Figure 1. For both microalgae, the weight loss
temperature of thermal decomposition (160 °C) of C. vulgaris was higher than that of D. salina (150 °C). Dmax of D. salina (3.7%/min) at 270 °C was much higher than that of C. vulgaris (2.5%/min) at 300 °C. DTG curves of C. vulgaris and D. salina under different heating rates range from 5 to 40 K/min are presented in Figures 2 and 3, respectively. The curves of both microalgae
Figure 2. DTG curves of C. vulgaris under different heating rates. Figure 1. TG and DTG curves of C. vulgaris and D. salina in a thermogravimetric analyzer at 5 K/min.
shifted to a higher temperature when the heating rate increased. This is a common phenomenon for non-isothermal biomass pyrolysis. The temperature for the maximum mass loss rate at heating rates from 5 to 40 K/min had a difference of 30 °C for C. vulgaris and 20 °C for D. salina. At a lower heating rate, the samples could start to decompose rapidly because of lower heat-transfer resistance. The maximum mass loss rate decreased with the increase of the heating rate, which was more obvious for C. vulgaris. The kinetic parameters were calculated according to the TG results by the Freeman−Carroll method.26 The results are given
had undergone three stages. In stage I, the free water in the microalgae was totally evaporated below 150 °C. In stage II, the weight loss was the most significant because the protein, lipid, and carbohydrate are significantly broken down between 150 and 550 °C. Stage III was from 550 to 800 °C, in which the C− C and C−H bond cleavages might happen in the primary char. Further discussion is as follows. Moisture is generally removed at 105 °C. Both microalgae started to degrade at 110 °C, and the weight loss was observed. The thermal 97
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the outer surface of the plasma membrane layer in the cell of D. salina is wrapped only by a thin elastic membrane, the chemical compositions of which are a small amount of neuraminidase glycoprotein.35 Pyrolysis Product Distribution. Pyrolysis product distribution of C. vulgaris and D. salina is given in Figures 4 and 5,
Figure 3. DTG curves of D. salina under different heating rates.
in Table 6. The correlation coefficients for all samples and conditions were larger than 0.94. The activation energy Table 6. Pyrolysis Kinetic Parameters of C. vulgaris and D. salina at Different Heating Ratesa heating rate (K/min) C. vulgaris
D. salina
5 10 20 40 5 10 20 40
E (kJ/mol)
A (min−1)
n
R2
48.4 45.2 43.6 42.9 43.8 41.1 39.3 38.8
× × × × × × × ×
2.05 1.92 1.83 1.86 1.95 1.86 1.85 1.88
0.94 0.97 0.95 0.96 0.98 0.94 0.97 0.95
1.8 1.6 1.3 1.0 9.2 6.6 7.6 6.2
4
10 104 104 104 103 103 103 103
Figure 4. Pyrolysis product distribution of C. vulgaris.
a
E, activation energy; A, pre-exponential factor; n, reaction order; and R2, correlation coefficients.
Figure 5. Pyrolysis product distribution of D. salina.
decreased as the heating rate increased from 5 to 40 K/min. At the same heating rate, the activation energy of C. vulgaris was higher than that of D. salina. This indicates that thermal decomposition of D. salina is more likely to occur. C. vulgaris had a reaction order from 1.86 to 2.05 with a higher preexponential factor compared to that of D. salina. The biochemical composition analysis showed that the content of crude protein in D. salina (58.8%) was much higher than that in C. vulgaris (47.4%), which resulted in a smaller activation energy for D. salina. The activation energy of some typical biomass at a similar heating rate27−34 is listed in Table 7. The
respectively. The gas, tar, and char yields were all obtained by direct experimental analysis. The proportion of undetected substance was less than 10%, except an 11.6% for C. vulgaris pyrolysis at 300 °C. The mass balance was from 88.4 to 96.8%. The loss in mass balance was mainly due to the deposit of highmolecular-weight tar compounds between the sample zone and the liquid collector, which was hard to be totally recovered by the solvent. On the other hand, some small molecular compounds with a low boiling point in the tar could be released in the process of solvent evaporation at 40 °C. With an increasing temperature range from 300 to 700 °C, the gas yields increased and the char yields decreased correspondingly. Pyrolytic gas from C. vulgaris pyrolysis was higher than D. salina in all experimental conditions. Oil yields increased as the temperature increased between 300 and 500 °C and reached more than 60% (wet basis) at 500 °C reactor temperature for both microalgae. Secondary reactions for the formation of pyrolytic gas and char increased with the further increase of the temperature from 500 to 700 °C. For different feedstocks, D. salina had higher oil yields within the experimental temperature range. Maximum bio-oil yields (wet basis) were 60.7 and 64.9% for C. vulgaris and D. salina at 500 °C, respectively. Pyrolytic Gas Characterization. The composition of gases from pyrolysis of both microalgae in different temperatures is shown in Tables 8 and 9. CH4 was the only constituent that has been detected in the pyrolytic gas at 300 °C. The fraction of CH4 dropped rapidly to 21−26% as the temperature increases to 700 °C. There was no H2 when the temperature was lower than 500 °C. The volume fraction of H2 increased to 24−26% at 700 °C. CO had a similar trend to H2; the pyrolytic gas was not detected with the presence of CO before 600 °C. The
Table 7. Activation Energies from Pyrolysis of Microalgae and Different Biomass C. vulgaris D. salina sawdust straw rice husk
heating rate (K/min)
E (kJ/mol)
5−40 5−40 5−40 10−15 10−30
42.9−48.4 38.8−43.8 65−12127−29 70−17030−33 92−20030,33,34
compared results showed that microalgae have a lower activation energy than traditional biomass. There is no rigid cell wall in the biology structure in microalgae compared to lignocellulosic biomass.35 The cell wall of lignocellulosic biomass is a heterogeneous solid composed of a carbohydrate fraction tightly interlinked with a complex alkyl−aromatic fraction,36 which is hard to break down at the initial stage of pyrolysis, especially at a low reaction temperature. In contrast, 98
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Table 8. Composition of Gases from Pyrolysis of C. vulgaris at Different Temperatures (vol %) H2 CH4 CO CO2 C2H4 C2H6 C2H2 Qvb (MJ/Nm3)
300 °C
400 °C
500 °C
600 °C
700 °C
nda 100.0 nd nd nd nd nd 35.9
nd 36.5 nd 61.6 nd 1.9 nd 14.3
22.2 34.9 nd 38.6 nd 4.3 nd 17.7
25.5 22.8 24.5 22.1 2.4 2.8 nd 17.2
25.9 21.5 29.9 14.5 5.8 2.3 0.2 19.3
had transformed to volatile from raw materials and, subsequently, to final pyrolytic oil and gas. These characteristics of microalgae char are similar to traditional coal and biomass char. Of particular note, char of microalgae had a higher N content than typical biomass because of the high protein content in the microalgae. These N elements might be in the form of ammonium salts or nitrates but most likely with highmolecular-weight N heterocyclic compounds on the biochar surface or inside the biochar pores.15 The biochar contained higher concentrations of mineral elements than biochar from lignocellulosic biomass, in particular plant nutrients, such as Na, Mg, Ca, and Fe (Table 12). The BET surface area of the char from C. vulgaris and D. salina pyrolysis is given in Figure 6. The structures of microalgae powder and microalgae char at 500 °C are shown in Figure 7. In comparison of the SEM images of microalgae to those of its char, the cell wall structure appeared to have been totally destroyed. There was evidence of melting and resolidification of cell structures, which might be due to the lower melting points of the pyrolysis products of the protein and lipid. This is in contrast to biochar from lignocellusosic biomass, which clearly retains the plant structure of the feedstock.37 The surface area of the microalgae char increased as the pyrolysis temperature rose. The C. vulgaris char had a slightly higher BET surface area than that from D. salina when the temperature was below 500 °C. The differences of the surface area became significant as the temperature was further raised to 700 °C. The largest BET surface area was less than 1.5 m2/g, which was relatively low compared to biochar obtained from lignocellulosic biomass.38 Bio-oil Properties and Composition. The ultimate analysis of bio-oils from C. vulgaris and D. salina is listed in Table 13. The oxygen content of bio-oils is usually 35−40 wt %.39 The high oxygen content is indicative of the presence of many highly polar groups, leading to high viscosities and boiling points as well as relatively poor chemical stability and energy density.40 The data showed that the oxygen content in bio-oils from microalgae was from 24.8 to 53.8%. At 500 °C, when the highest oil yield has been reached, the oxygen contents were 33.3 and 29.8% for C. vulgaris and D. salina, respectively. Therefore, a lower oxygen content could be obtained when the microalgae were taken as pyrolysis feedstock. As the temperature increased, cracking of the vapors and formation of gases would be enhanced, leaving the organic liquid with less oxygen. Another notable feature of bio-oils from microalgae was high N contents, which will be discussed later. Figure 8 presents the content of water in the oil obtained from the pyrolysis of microalgae. Water is hard to remove from bio-oils. The presence of water has many negative effects on the storage and use of bio-oils, although it helps to reduce viscosity and facilitate atomization and pollutant emissions during
a
nd = not detected. bQv = 12.64VCO + 10.79VH2 + 35.88VCH4 + 59.44VC2H4 + 56.49VC2H2 + 64.35VC2H6.
Table 9. Composition of Gases from Pyrolysis of D. salina at Different Temperatures (vol %) H2 CH4 CO CO2 C2H4 C2H6 C2H2 Qvb (MJ/Nm3)
300 °C
400 °C
500 °C
600 °C
700 °C
nda 100.0 nd nd nd nd nd 35.9
nd 43.4 nd 54.2 nd 2.4 nd 17.1
17.4 43.1 nd 34.4 nd 5.1 nd 20.6
23.2 28.6 24.0 18.7 2.3 3.2 nd 19.2
24.3 25.8 29.6 11.0 6.8 2.4 0.2 21.3
a nd = not detected. bQv = 12.64 VCO+10.79 VH2+35.88 VCH4+59.44 VC2H4+56.49 VC2H2+64.35 VC2H6.
volume fraction of CO2 decreased rapidly with the increase of the CnHm content, while the temperature rose from 400 to 700 °C. The theoretical calorific value Qv (MJ/Nm3) can be determined by the following formula: Q v = 12.64VCO + 10.79VH2 + 35.88VCH4 + 59.44VC2H4 + 56.49VC2H2 + 64.35VC2H6
C. vulgaris had higher H2 yields and lower CH4 yields than D. salina. The Qv of the pyrolytic gas of D. salina was higher than that of C. vulgaris because of the higher heating value components, such as C2H6, C2H4, and C2H2. Biochar Analysis. Proximate, ultimate, and HHV analyses of C. vulgaris and D. salina pyrolysis char are shown in Tables 10 and 11. As the pyrolysis temperature rose, the contents of volatile matter in the char residue decreased sharply. The ash contents increased at the same time mainly because of the metal elements and mineral salts in the raw algae biomass, which were hard to evaporate in experimental temperatures. The concentrations of C, H, N, and O decreased as the pyrolysis temperature increased, indicating that these elements
Table 10. Proximate, Ultimate, and HHV Analyses of C. vulgaris Pyrolysis Char proximate analysis (wt %)
a
ultimate analysis (wt %)
temperature (°C)
moisturea
asha
volatile mattera
fixed carbona
Ca
Ha
Na
Sa
Oa,b
HHV (MJ/kg)
300 400 500 600 700
3.3 3.8 4.6 4.9 5.0
18.8 27.5 32.7 36.7 39.1
59.5 49.0 45.1 29.7 27.4
18.4 19.7 17.6 28.7 28.5
52.0 49.4 47.1 46.5 46.1
4.6 3.5 2.9 2.0 1.6
8.7 7.7 7.1 6.6 5.9
1.4 1.8 1.3 1.2 1.2
11.2 6.3 4.3 2.1 1.1
22.3 20.1 17.7 17.0 16.4
Air-dried basis. bBy difference. 99
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Table 11. Proximate, Ultimate, and HHV Analyses of D. salina Pyrolysis Char proximate analysis (wt %)
a
ultimate analysis (wt %)
temperature (°C)
moisturea
asha
volatile mattera
fixed carbona
Ca
Ha
Na
Sa
Oa,b
HHV (MJ/kg)
300 400 500 600 700
2.6 3.1 4.5 5.3 5.4
12.2 19.3 24.4 27.2 29.3
56.8 40.0 29.8 22.3 21.8
28.5 37.6 41.2 45.2 43.6
57.9 57.2 55.2 54.7 54.0
4.7 3.8 2.9 2.2 2.0
10.7 9.8 9.0 8.2 7.4
0.6 0.5 0.3 0.2 0.2
11.3 6.3 3.7 2.2 1.7
25.4 24.1 21.8 20.6 19.6
Air-dried basis. bBy difference.
Table 12. Main Mineral Elements in the Char of Microalgae Pyrolysis (wt %) C. vulgaris
D. salina
temperature (°C)
Na
Mg
K
Ca
Fe
Na
Mg
K
Ca
Fe
300 400 500 600 700
2.2 2.8 3.7 4.1 3.2
0.7 0.9 1.2 1.3 1.1
1.3 1.5 2.1 2.2 1.7
2.6 3.2 4.5 5.7 3.7
0.6 0.8 1.1 1.2 0.8
1.3 2.3 2.5 2.7 2.9
0.9 1.5 1.7 1.9 1.6
0.9 1.4 1.7 1.9 1.8
0.8 1.3 1.4 1.7 1.7
0.2 0.3 0.4 0.4 0.4
combustion. The water lowers heating values and may cause phase separation of bio-oils. Moreover, it increases ignition delay and reduces combustion rates and adiabatic flame temperatures during the combustion process.41 The water contents of bio-oils usually vary in the range of 15−30 wt %, depending upon the initial moisture in feedstock and pyrolysis conditions.39 Most of the water in bio-oils is probably hydrogen-bonded to polar organic compounds with a small portion in the form of aldehyde hydrates.42 The results from the Karl Fischer titration showed that the water contents in the pyrolytic oil of microalgae were above 25% at low temperatures but below 20% when the temperature was greater than 500 °C. Bio-oils from D. salina had lower water contents in all experimental conditions. A minimum moisture content of 12.4% had been achieved at 600 °C. At the same time, waterfree bio-oil yields can be calculated. The maximum water-free bio-oil yields were 49.2 and 55.4% for C. vulgaris and D. salina at 500 °C, respectively. The density of lignocellulose bio-oils was about 1.2 g/mL compared to that of petroleum fuels, which is 0.8−1.0 g/mL.42 The density of microalgae bio-oils is given in Figure 9. Both microalgae bio-oils had similar density at a pyrolysis temperature of 300 °C. However, bio-oils from D. salina had higher density values, especially when the temperature was above 500 °C. The density values of C. vulgaris and D. salina reached 1.22 and 1.28 g/mL at 500 °C, respectively. Higher density determines a greater volumetric energy density of bio-oils. Bio-oils usually contain about 7−12 wt % acids and have a pH of 2−4 and an acid number of 50−200 mg of KOH/g.43−45 It has been reported that bio-oils are very corrosive to aluminum, mild steel, and nickel-based materials.42 The acid number of the pyrolytic oil from C. vulgaris and D. salina is given in Figure 10. The range of the acid numbers was from 50 to 140 mg of KOH/g. Pyrolytic oil from D. salina had a lower acid number, which indicates that less functional acidic groups exist. The lowest acid numbers were 57.5 and 52.1 mg of KOH/g for C. vulgaris and D. salina at 600 °C, respectively. Previous studies46 have shown there is a certain function between the density and water content. In general, the density of the bio-oils will drop with the increase of the water content. A comparison and a linear fitting for the water content and density of bio-oils from microalgae are shown in Figure 11.
Figure 6. BET surface area of the char from C. vulgaris and D. salina pyrolysis. The error bars indicate a standard deviation for three results.
Figure 7. SEM structure of microalgae powder and microalgae char at 500 °C: (a) C. vulgaris; (b) D. salina; (c) C. vulgaris char; (d) D. salina char.
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Table 13. Ultimate Analysis of C. vulgaris and D. salina Pyrolytic Oil (wt %) C. vulgaris
a
D. salina
temperature (°C)
Ca
Ha
Na
Sa
Oa,b
Ca
Ha
Na
Sa
Oa,b
300 400 500 600 700
30.6 42.8 51.4 53.5 55.1
8.6 8.3 7.8 7.6 7.5
6.5 6.8 7.1 8.0 9.3
0.5 0.4 0.4 0.4 0.4
53.8 41.7 33.3 30.5 27.7
32.0 45.8 53.0 55.6 56.3
9.0 8.7 8.3 8.2 7.8
8.1 8.3 8.6 9.4 10.8
0.4 0.2 0.3 0.2 0.2
50.6 37.0 29.8 26.5 24.8
As-received basis. bBy difference.
Figure 8. Water content in pyrolytic oil of C. vulgaris and D. salina. The error bars indicate a standard deviation for three results.
Figure 10. Acid number of pyrolytic oil from C. vulgaris and D. salina. The error bars indicate a standard deviation for three results.
Figure 9. Density of pyrolytic oil from C. vulgaris and D. salina. The error bars indicate a standard deviation for three results.
Figure 11. Comparison and linear fitting for the water content and density of bio-oils from microalgae.
This feature can be verified by the experimental results. An adjusted R2 of 0.811 can be obtained by a linear fitting. Beyond that, a better linear fitting result has been achieved with the water content and acid number in the bio-oils from microalgae. An adjusted R2 reaches 0.951, which is shown in Figure 12. The HHV of bio-oils is typically 15−20 MJ/kg, which is much lower than that of petroleum fuels.42 The results in Figure 13 indicate that the bio-oils obtained above 500 °C have a higher HHV than general biomass. A high heating value can be achieved by increasing the temperature for thermal decomposition of microalgae. In comparison of C. vulgaris to D. salina, the latter has a higher HHV. Compounds identified by GC−MS after the pyrolysis of C. vulgaris and D. salina at 500 °C are shown in the Supporting Information. More than 100 compounds are identified in the spectrum using the National Institute of Standards and Technology (NIST) database. Peak area percentage statistics
according to GC−MS results are given in Figure 14. The undetected percentages are 14.1 and 7.9% for C. vulgaris and D. salina, respectively. Carbohydrate-derived compound contents were less than 8% mainly because of the low content of carbohydrate in the raw material. The mineral elements and ash contents of C. vulgaris were relatively high, which could promote the catalysis of the decomposition of carbohydrate to small molecule gases and ester derivatives. For this reason, high-carbohydrate C. vulgaris produced less carbohydrate-derived compounds in pyrolytic oil compared to D. salina. Protein derivatives in pyrolytic oil include phenols, nitrogencontaining aromatic derivatives, nitriles, amides, pyrrole, nitro compounds, pyridine, amine compounds, etc. Many of these products are evolved from the original amino acid structure in microalgae protein. The main phenols and alkylphenols detected by GC−MS in the bio-oils from C. vulgaris and D. 101
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more protein-derived chemicals were found in pyrolytic oil from D. salina. Pyrolytic oil contains a large number of straight-chain alkanes, alkenes, alkynes, fatty acids, aldehydes, and esters with carbon atoms in the range from 12 to 20. These compounds are classified as lipid-derived substances. C. vulgaris has more lipidderived substances in its pyrolytic oil than D. salina. It is noteworthy that the main acids in both oils are straight-chain fatty acids, such as palmitic, linolenic, myristic, and linoleic acids, while the important acids in lignocellulosic biomass pyrolysis oil are mainly volatile organic acids with a low boiling point, such as acetic acid.
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CONCLUSION In this study, TGA and pyrolysis experiments were carried out for two kinds of low-lipid microalgae C. vulgaris and D. salina. The characteristics of main pyrolysis products, including pyrolytic oil, char, and gas, were investigated. The following conclusions can be drawn from this study: (1) The activation energy of D. salina during pyrolysis at 5−40 K/min in TGA ranged from 38.8 to 43.8 kJ/mol, which was lower than that of C. vulgaris and other lignocellulosic biomass. (2) C. vulgaris had higher H2 yields and lower CH4 yields than D. salina. The theoretical calorific value of the pyrolytic gas of D. salina was higher than that of C. vulgaris because of the higher heating value components, such as C2H6, C2H4, and C2H2. The pyrolytic char from microalgae had a BET surface area less than 1.5 m2/g. These char were rich in N, Na, Mg, Ca, and Fe. (3) Highest yields of pyrolytic oil were 49.2 and 55.4% (water-free basis) for C. vulgaris and D. salina at 500 °C, respectively. The pyrolytic oil from D. salina had a lower water content and acid number and a higher density and HHV than the oil from C. vulgaris. A good linear relationship in the pyrolytic oil was found between the water content and acid number in this study. GC−MS analysis shows there are more carbohydrateand protein-derived compounds but less lipid-derived substances in the oil from D. salina compared to that from C. vulgaris.
Figure 12. Comparison and linear fitting for the water content and acid number of pyrolytic oil from microalgae.
Figure 13. HHV of pyrolytic oil from C. vulgaris and D. salina. The error bars indicate a standard deviation for three results.
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ASSOCIATED CONTENT
S Supporting Information *
Compounds identified by GC−MS after the pyrolysis of C. vulgaris (Appendix A) and D. salina (Appendix B) at 500 °C. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 86-27-87546631. Fax: 86-27-87545526. E-mail:
[email protected].
Figure 14. Peak area percentage statistics according to GC−MS results.
Notes
The authors declare no competing financial interest.
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salina were phenol, 4-methylphenol, and 3-ethylphenol. Phenols and alkylphenols are generally considered mainly from the decomposition of tyrosine in microalgae15 and the decomposition of lignin in lignocellulosic biomass.47 Indole and alkylindoles may be contributed by tryptophan, which is a main amino acid in microalgae.48 At the same time, N-containing heterocyclic compounds were detected, which were mainly from the primary pyrolysis of amino acids and the interactive reaction between N-containing compounds and sugar compounds in microalgae.48 From the comparison in Figure 14,
ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Basic Research Program of China (2011CB201505), the National Natural Science Foundation of China (51306062 and U1261204), the Ph.D. Program Foundation of the Ministry of Education of China (20120142120036), the Natural Science Foundation of Hubei Province of China (2012FFB02218), and the Basic Research Foundation from Huazhong University of Science and 102
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