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The pyrolysis of lipid-extracted residue of Tribonema minus in a fixed-bed reactor
Accepted Manuscript Title: The pyrolysis of lipid-extracted residue of Tribonema minus in a fixed-bed reactor Author: Xiang Ji Bin Liu Guanyi Chen Wenchao Ma PII: DOI: Reference:
S0165-2370(15)30184-4 http://dx.doi.org/doi:10.1016/j.jaap.2015.09.006 JAAP 3568
To appear in:
J. Anal. Appl. Pyrolysis
Received date: Revised date: Accepted date:
15-3-2015 18-6-2015 7-9-2015
Please cite this article as: Xiang Ji, Bin Liu, Guanyi Chen, Wenchao Ma, The pyrolysis of lipid-extracted residue of Tribonema minus in a fixed-bed reactor, Journal of Analytical and Applied Pyrolysis http://dx.doi.org/10.1016/j.jaap.2015.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The pyrolysis of lipid-extracted residue of Tribonema minus in a fixed-bed reactor Xiang Jia,b, Bin Liub, Guanyi Chena,c,d* [email protected], Wenchao Maa,e a
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
b
School of Mathematics, Physics, and Biological Engineering, Inner Mongolia University of
Science and Technology, Baotou 014010, China c
School of Science, Tibet University, Lhasa 850012, China
d
Key Laboratory of Biomass-based Oil and Gas (Tianjin University), China Petroleum and
Chemical Industry Federation, Tianjin 300072, China e
Tianjin Engineering Center of Biomass-derived Gas and Oil, Tianjin 300072, China
*
Corresponding author at： School of Environmental Science and Engineering, Tianjin University,
Tianjin 300072, China. Tel/Fax: +86 2287401929. Highlights
The lipid-extracted residue of T. minus was firstly used as the pyrolysis feedstock.
The lipid-extracted residue of T. minus has lower pyrolysis temperature than lignocellulosic material.
The maximum liquid product yield (29.82 wt.%) was achieved at 450 ºC with 50 ml·min-1 nitrogen flow rate.
The liquid product obtained from residue of T. minus contained more alkane/alkene than that from lignocellulose biomass.
1
ABSTRACT The pyrolysis of lipid-extracted residue of Tribonema minus has been performed in a fixed-bed reactor. In this paper the influence of nitrogen flow rate and pyrolysis temperature on products yields and composition were investigated. The maximum liquid yield of 29.82 wt. % was obtained at 450 ºC with 50 ml·min-1 nitrogen flow rate. The major compounds of liquid product from microalgae were carbonyls, hydrocarbons and nitrogenous compounds with a high proportion of oxygen. CO2 and CO were the main compositions of gas and their contents changed with pyrolysis temperature. The comparison of product yields and properties was made among lipid-extracted residue of T. minus, reeds and woody chips pyrolyzed under similar conditions. The liquid product from microalgal residue contained more alkane/alkene and less aromatics than that from lignocellulose biomass. Keywords: Tribonema minus; Lipid-extracted residue; Pyrolysis; Fixed bed.
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1. Introduction Biomass, the only renewable source of organic carbon, which can be converted into liquid fuels, is considered as a promising alternative to deplete resources of petrodiesel [1]. Biomass energy can not only alleviate the dependence from fossil fuels but also reduce the emission of greenhouse gas [2]. Biomass can be converted into fuel usually through biochemical or thermochemical process. Pyrolysis is the most promising thermochemical process, which is typically conducted in the absence of oxygen at a moderate temperature [3]. The proportion and complex chemical composition of pyrolysis products depend on many factors, such as feedstock properties (biomass type, feedstock pretreatment, particle size, etc.) and operation parameters (temperature, heating rate, residence time, pressure, gaseous environment, catalyst, etc.) [4,5]. Biomass resources include wood, agricultural crops, aquatic plants and municipal wastes, etc, all of which can be converted into a liquid biofuel via pyrolysis. Liquid product contain about hundreds of different chemical compounds, which can be classed into aldehydes, ketones, alcohols, saccharides, hydrocarbons, organic acids, phenols and nitrogenous compounds [6]. Liquid product exhibit characteristics such as high oxygen concentration and moisture content making the high heating value lower than that of fossil fuels [7,8]. Note, the use of wood and agricultural crops have competed with food production, agricultural land and freshwater resources. Hence, microalgae have been suggested as a very good candidate [2]. Through photosynthesis, microalgae use the sunlight to convert water and carbon dioxide into organics [9]. Compared with other biomass resources, microalgae have (i) higher biological CO2 fixation, producing per ton of microalgae can fix 1.83 tons of CO2 roughly [10]; (ii) higher growth rate, they can
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increase twofold within 24 h, during exponential phase can be shortened to 3.5 h [10]; (iii) higher photosynthesis efficiency, the whole body can be carried out photosynthesis; (iv) higher lipid production, the productivity of microalgal lipid is 2-30 times of conventional crops per unit area and time [11]; and (v) greater adaptability, the microalgae can be cultivated on freshwater, waste water [12], aquatic medium, even the desert [13]. As we known, algae is an unicellular or simple multicellular structure species. So, it is difficult to get large microalgal biomass. As reported, harvesting microalgal biomass from the culture medium was estimated to account for 20-30% of the total biomass production cost [14]. The algal biomass as the material for pyrolysis focuses on
single-cell
microalgae,
such
as
Chlorella
[15],
Nannochloropsis
[5].
Comparatively speaking, the filamentous algal species with bigger size is easier to harvest [16]. Microalgae lipid, with a content of 60 % of dry biomass [11], can be used in the production of biodiesel by transesterification, which extracted mostly by enzyme reactions, ultrasound and microwave-assisted methods, supercritical fluid extraction, etc [14]. After biodiesel production, large amount microalgae residue still left as a new kind of waste. Nevertheless, there are few researches on the microalgal lipid-extracted residue utilization. In this paper, the work focuses on slow pyrolysis of lipid-extracted residue of microalgae. The lipid-extracted residue of Tribonema minus, a filamentous microalgae species, was used as the pyrolysis feedstock. The effect of pyrolysis parameters, such as nitrogen flow rate and pyrolysis temperature, on the product distributions was investigated, and the product characteristics were analyzed by gas chromatography-mass spectrometer (GC-MS), gas chromatography (GC), elemental
4
analyzer, etc. Furthermore, the product yields and properties from lipid-extracted residue of T. minus, reeds and woody chips pyrolyzed under similar conditions were compared. 2. Material and methods 2.1. Feedstock and characterizations Lipid-extracted residue of Tribonema minus, air-dried, used in this study was obtained from the Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, China, extracted the lipid by literature [16]. The reed and platanus woody chips were collected from Tianjin. The samples were oven-dried at 90 ºC for 24 h, then ground and sieved to a particle size in the range of 0.2-0.45 mm. The main characteristics of the samples are summarized in Table 1. The elemental composition was performed with Euro EA3000 elemental analyzer. The proximate analysis were determined according to the National Testing Standard of Proximate Analysis of Coal (GB/T 212-2008) [17]. The high heat value (HHV, MJ·kg-1) was calculated by the following equation [18]. HHV =33.5*C+142.3*H-15.4*O-14.5*N 2.2. Thermogravimetric analysis The pyrolysis characteristics of samples were studied by thermogravimetric analysis (TGA) using Exstar 6000, TG/DTA 6200 (SEIKO). The samples heated from room temperature to 600 ºC with a heating rate of 10 ºC·min-1 under a flow of 50 ml·min-1 of nitrogen gas (99.999%). 2.3. Pyrolysis reactor and experimental procedure Pyrolysis experiments were performed in a fixed bed reactor, as shown in Fig. 1. The fixed bed reactor was constituted by stainless-steel tube with an internal diameter
5
of 12 mm and a length of 500 mm and heated by an electrical furnace. The thermocouple was used to monitor the real temperature of the reactor. All experiments were carried out under atmospheric pressure and nitrogen conditions (99.9%), and the nitrogen flow rate was controlled by a mass flow controller (MFC). The volatile products leave the reactor together with the inert gas. Inside the reactor, the seperator sieve retained the pyrolysis solid particles in the gas stream. The condensable part was converted to liquid when the volatile products passed through the ice-bath condensers. The non-condensable part was collected by a gas collector and analyzed by GC. To study the effect of the pyrolysis temperature on product characteristics, 5 g of dried sample was fed into the fixed-bed reactor and electrically heated at 10 ºC·min-1 from room temperature to desired temperatures (300, 350, 400, 450 and 500 ºC) and held for 1 h. The gas collecting occurred for the temperature reached the preset temperature, each sampling interval 15 min and held 1 min. The char was collected after completing the pyrolysis reaction. The mass of char and liquid product was determined by an analytical balance. The gas yield was calculated by difference from mass balance. In this study, all the experiments were repeated three times and the product yields were calculated by an average value of three equivalent tests.
2.4. Product analysis Chemical compounds of the liquid product obtained were analyzed by Shimadzu GCMS-QP 2010 SE. The column used was a Rtx-5MS at 30.0 m capillary column, 0.25 mm i.d., 0.25 μm film thickness. The liquid samples (0.1 g) were diluted in methyl alcohol (2 ml); with 1.0 μl injected to the GC. The GC oven temperature was programmed to hold at 50 ºC for 3 min, then followed by a ramp at 6 ºC·min-1 to 270
6
ºC where it was hold for 5 min. The injector temperature was 280 ºC, and the injector split ratio was set to 20:1. The flow rate of helium carrier gas was 1 ml·min-1. The MS was set at an ionizing voltage of 70 eV with mass range (m·z-1) of 35-550 u; the interface temperature was maintained at 270 ºC. The GC-MS chromatogram could give the area percentage of various compounds, which was regarded as a good approach for semi-quantitative analyses of pyrolytic liquid product [19]. The elemental composition of liquid product was performed with Euro EA3000 elemental analyzer. The water content of liquid product was determined by Met-rohm-870 KF Titrino plus Karle-Fischer titrator. The gas product was analyzed by Agilent 7890A GC/TCD, equipped with Porapak Q column (3 m) and 5A molecular sieve (2 m). The oven was programmed to hold at 60 ºC for 2 min, ramp at 15 ºC·min-1 to 180 ºC and then at a temperature rate of 30 ºC·min-1 to 100 ºC where it was hold for 8 min. The TCD temperature was set at 250 ºC and reference gas was helium with a flow rate at 35 ml·min-1. The qualitative and quantitative of pyrolysis gas were determined by external standard method with a standard gas. 3. Results and discussion 3.1. Thermogravimetry behavior Thermogravimetric analysis (TGA) of lipid-extracted residue of T. minus was shown in Fig. 2. The TG process was divided into three stages. The first stage, less than the temperature of 150 ºC, was release of moisture content and volatile compounds with low molecular weight. The second stage, the thermal degradation of volatile component exhibiting a wide peak with a shoulder between 150 and 500 ºC with a maximum weight loss at 335 ºC, attributed to protein and polysaccharide
7
degradation. The third stage was the decomposition of inorganic materials (mainly carbonate decomposition) with the temperature over 500 ºC.
Compared with lingocellulosic biomass, the derivative weight loss peak of microalgal biomass presented at lower temperature, commonly. Yuan et al [20] obtained the temperature of the maximum mass loss rate of wood dust was 354, 375, 391, 396 ºC at the heating rates of 5, 15, 30 and 40 ºC·min-1 under argon atmosphere, respectively. López et al [21] compared the thermogravimetry behavior of four biomass, found the temperature tendency of the maximum mass loss rate was microalgae < rape < corn < sunflower. These results may be attributed to low molecular mass in microalgae can easily be degraded compared with the cellulose and hemicellulose content in lingocellulosic material at the same temperature [5]. 3.2. Product yields Pyrolysis products of biomass in the fixed-bed reactor have been divided into three parts: gas, liquid and char. The residence time of pyrolysis vapor in the reaction area, which affects the distribution of pyrolysis product yields, is closely correlation with carrier gas flow rate. The effect of nitrogen flow rate on the product yields of lipid-extracted residue of T. minus at a pyrolysis temperature of 450 ºC was shown in Fig. 3. The trend of liquid and gas yield with the nitrogen flow rate is similar with Pütün [22], who studied the effect of sweeping gas flow rate on cotton seed pyrolytic product yields. The low flow rate of nitrogen enhancing the secondary cracking of pyrolysis vapors, produced more gas; but the gas yield also increased with increasing nitrogen flow rate when its rate higher than 50 ml·min-1 since the uncondensed volatiles were removed from the reaction area by the nitrogen stream [22]. The 8
maximum liquid yield was obtained at a nitrogen flow rate of 50 ml·min-1. Below this rate, enhancement of the secondary cracking of condensed volatiles decreased the formation of liquid product in the condensate system. Increasing the nitrogen flow rate from 50 to 150 ml·min-1 also caused decrease of liquid yield since the deficient quenching of pyrolysis vapors during the condensate process. To some degree, the product yields can be controlled by changing the flow rate of the carrier gas while pyrolysis temperature keeps constant. Fig. 4 shows the product yields of lipid-extracted residue of T. minus pyrolysis at different temperatures. The char was the main production in the whole range of temperatures. When the pyrolysis temperature increased, the yield of char decreased from 66.07 wt.% (300 ºC) to 49.19 wt.% (500 ºC). The high char yield of microalgae have been reported in many literatures. Grierson et al [23] obtained the char yield of 34-63 wt.% from slow pyrolysis of six different microalgae at 500 ºC. Maddi et al [24] also indicated the algal pyrolysis product contained higher char content. The high amounts of ash in the microalgae and the inorganic elements in the ash can promote the formation of char during pyrolysis [24]. The highest yield of the liquid product obtained at 450 ºC, was 29.82 wt.%. The pyrolysis process was incomplete below 450 ºC, but over 450 ºC the yield of liquid product was slightly declined to 28.99 wt.% (500 ºC) since the secondary cracking reaction of volatiles was enhanced with the pyrolysis temperature increased at a constant carrier gas flow rate. The liquid yield was lower than literature [25] which obtained the liquid yield of 52.7 wt.% from slow pyrolysis of C. vulgaris in fixed-bed reactor at 500 ºC. Chaiwong et al [26] also got similar results that the liquid yield was 46 wt.% at pyrolysis temperature of 550 ºC from algae. This was because of the T.
9
minus subject to lipid extraction process before used for our study, which lipid compounds account for 50.23% of dry weight percentage of raw T. minus after 21-day-cultivation [16]. The enhancement of secondary cracking reaction of volatile was also the reason of gas yield increases from 14.17 wt.% at 300 ºC to 21.83 wt.% at 500 ºC. It was consistent with Maguyon [5], who studied the products yields and characteristics of N. oculata via pressurized pyrolysis. 3.3. Gas composition The gas composition of lipid-extracted residue of T. minus pyrolysis at different temperatures was shown in Table 2. The main compositions of gas were carbon dioxide and carbon monoxide. As the pyrolysis temperature rose from 300 ºC to 500 ºC, CO2 concentration decreased from 85.02 vol. % to 61.55 vol. %, while the CO increased from 9.10 vol. % to 15.17 vol. %. The trend was consistent with the work of Maddi et al [24], due to the enhancement of decarboxylation and decarbonylation reactions with the pyrolysis temperature increase. Furthermore, the gas was also composed of small amounts of H2, CH4 and C2-C3 hydrocarbons. The concentration of these in the gaseous product tended to increase with further heating of the biomass, since the severity of cracking reactions of the vapors forming incondensable gaseous compounds [5].
3.4. Liquid product composition The properties of the liquid product were shown in Table 3. As observed the liquid product contains higher water content. Water was obtained from the moisture in the feedstock and most of it formed via the dehydration and/or decarboxylation reactions 10
throughout pyrolysis process [2,27]. Higher water content can lower the heating value of liquid product, while can also reduce the oil viscosity and enhance the fluidity which was beneficial to the atomization and combustion of liquid fuels [26]. It can also be seen that with the pyrolysis temperature increase, the C content in liquid was increased, the O content decreased, while the H and N content were hardly affected by pyrolysis temperature. The N content in the microalgae-derived liquid was obtained from protein and pigment in the algae, most of nitrogen compounds into the liquid phase via the pyrolysis process [28]. The liquid product contains a high proportion of oxygen, resulting in the poor stability and lower heating value. A deoxygenated process was necessary to upgrade the liquid product. The chemical composition of the liquid product obtained at different pyrolysis temperatures was analyzed by GC-MS. The liquid product contained more than 100 kinds of compounds, and grouped in Table 4 according to their functional groups. The major compounds of microalgal pyrolytic liquid product were carbonyls, hydrocarbons, phenols, and nitrogenous compounds. These oxygenate compounds will correspondingly increase the oxygen content of liquid product, making it unstable. The amount of heterocyclic compounds in liquid product was easy to its polymerization [27]. With the pyrolysis temperature raise, the cracking reaction of pyrolysis compounds was more severe leading to more light compounds and less heavy compounds. For instance, the phenols content was increased from 0.99 wt.% (300 ºC) to 4.83 wt.% (500 ºC). Furthermore, the higher temperature was beneficial to hydrocarbons generation, which was also caused by the severity of cracking reactions. The alkanes/alkenes content in liquid product increase from 2.78 wt.% to 10.96 wt.% with
11
pyrolysis temperature rising from 300 ºC to 500 ºC, whereas the aromatics were hardly affected by pyrolysis temperature. Ketones, the main compounds in the liquid product, was reduced from 21.07 wt.% (300 ºC) to 9.57 wt.% (500 ºC). Which were formed by condensation reactions of the carbohydrate-derived fraction and decomposition of the miscellaneous oxygenates and sugars [27]. The contents of alcohols and saccharides also reduced with temperature rising. There were also carboxylic acids, esters and nitrogenous compounds in liquid product, which concentrations were hardly changed with pyrolysis temperature. The presence of acids may significantly contribute to the strong acidity of liquid product. They can corrode the pyrolysis equipment and limit its application in the engine [27]. The nitrogenous compounds (nitriles, amines and nitrogen heterocycles such as pyridines, pyrazines, pyrroles, etc.) were produced by the re-polymerization of small organic materials, formed from decarboxylation and deamination of the protein, via Fischer-Tropsch type reactions [28]. There were a small amount of aldehydes in liquid product, which obtained from the cracking reactions of cellulose and hemicellulose, since the algae biomass contains only a small amount of cellulose and hemicellulose in the cell wall. 3.5. Comparison of pyrolysis of residue of T. minus with lignocellulosic biomass 3.5.1. Product yields The yield of pyrolysis product is related to the characteristics of biomass. Comparison of three kinds of biomass pyrolysis at temperature of 450 ºC was shown in Table 5. The char yields of reeds and woody chips were 32.14 wt.% and 28.90 wt.%, respectively, obviously lower than that of microalgae residue of T. minus. Maddi et al [24] obtained similar results through comparative study of pyrolysis of 12
algal biomass with lignocellulosic biomass. It is possible that inorganic elements in the ash of algae samples catalyzed the char-forming reactions during pyrolysis, and the proteins and carbohydrates could have degraded into char. However, almost of the cellulose in the lignocellulosic biomass can break up into volatile [24]. The yields of gas and liquid of reeds and woody chips were higher than that of residue of T. minus, since their volatile matter were higher than residue of T. minus as shown in Table 1. Moreover, the bio-oil formation from the algal samples during pyrolysis follows the trend lipids > proteins > carbohydrates [28]. The residue of T. minus has been extracted the lipid before used for the study, which is another significant reason for the lower yield of liquid product. 3.5.2. Liquid product composition The compounds of liquid product
were closely
bound up with the
chemical composition of biomass. The lignocellulosic biomass consists of different contents of cellulose, hemicellulose and lignin. However, algae biomass was composed of protein, lipid and polysaccharide, as well as a small amount of cellulose in cell walls. The chemical composition of biomass can disintegrate different compounds by pyrolysis process. Fig. 5 shows the main liquid product compositions of residue of T. minus , reeds and woody chips at a pyrolysis temperature of 450 ºC. Compared with the lignocellulosic biomass, the liquid product from residue of T. minus contained more alkane/alkene compounds and less aromatic compounds. The mostly aromatic compounds and their derivatives (phenols) were derived by thermal decomposition of lignin [24]. Since microalgae do not contain lignin and the side chain of protein contained only a small amount of benzene compounds, this caused 13
these compounds were rarely present in the liquid product from residue of T. minus pyrolysis. The ketones and aldehydes were obtained from pyrolysis of cellulose and hemicellulose (polysaccharide in algal feedstock) fraction [24]. Therefore, the ketone/aldehyde contents (especially the aldehyde compounds) in liquid product obtained from pyrolysis of residue of T. minus was less than that obtained from reeds and woody chips. The liquid product from T. minus residue pyrolysis contained a series of nitrogenous compounds, such as nitriles, amines, pyrazines, pyridines, indoles and other heterocyclic nitrogen compounds, most of which were not present in the liquid product obtained from lingocellulosic biomass. The result was consistent with the literature [24]. This is a disadvantage when using bio-oils as fuel due to NOx emissions during combustion [5]. These acid compounds were formed by the deacetylation of hemicelluloses (pigment and protein in microalgae) at higher temperature. The breaking of peptide bond among protein (abundant in the microalgae) formed the carboxylic compounds, which caused the acid content of microalgal pyrolytic liquid product was higher than that of the other two biomasses. 4. Conclusions Microalgae have great potential for fossil fuel substitution in energy generation and conversion, and have been paid great attention in recent years. In this study, pyrolysis of lipid-extracted residue of T. minus in a fixed-bed reactor was primarily carried out. Furthermore, the product yields and properties from pyrolysis of residue of T. minus, reeds and woody chips under similar conditions were compared. 14
For lipid-extracted residue of T. minus pyrolysis, the char was the main production in the whole range of temperatures with yield decreasing from 66.07 wt.% (300 ºC) to 49.19 wt.% (500 ºC). The maximum yield of liquid product was 29.82 wt. %, achieved at 450 ºC with 50 ml·min-1 nitrogen flow rate. The major compounds of liquid product were carbonyls, hydrocarbons, phenols, and nitrogenous compounds, and their content was affected by temperature. The main components of CO and CO2 have diverse trends with temperature rising. Moreover, a slight increase of concentration of H2 and C1-C3 hydrocarbons occurred as temperature increasing. Compared with lignocellulose feedstock, the residue of T. minus has lower pyrolysis temperature. The microalgal pyrolytic liquid product contained more alkane/alkene and nitrogenous compounds, less aromatic, phenol, ketone and aldehyde compounds. Acknowledgements This work is financially supported by National Natural Science Foundation of China through project (NO.51306131), Tianjin Research Program of Application and Advanced Technology (15JCQNJC06600). Greatly thanks to Prof. Tianzhong Liu, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, China, for supplying the lipid-extracted residue of T. minus .
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(2011) 215-225.
MFC Furnace N2
Gas collector Seperator sieve GC
Heating belts
Thermocouple
Condenser
0.0
90
-0.5
80
-1.0
TG (%) -1 DTG (%·ºC )
70
-1.5
60
-2.0
50
-2.5
40
-1
100
Derivative Weight Loss (%·ºC )
Residual Weight (%)
Fig. 1. Schematic diagram of the fixed-bed reactor
-3.0 0
100
200
300
400
500
600
Temperature (ºC)
Fig. 2. TG and DTG curves of lipid-extracted residue of T. minus
19
50 Char Liquid Gas
Yield (wt. %)
40
30
20
10 30
50
80
150 -1
Nitrogen flow rate (mlmin )
Fig. 3. Effect of nitrogen flow rate on the product yields (450 ºC) 70 Char Liquid Gas
60
Yield (wt. %)
50 40 30 20 10 300
350
400
450
500
Temperature (C)
Fig. 4. Effect of temperature on the product yields
20
25
Reeds Woody chips Residues of T. minus
Percentage (%)
20
15
10
5
0 s s ne s tic ka ene ma l k o A al Ar
＆
ols en Ph
es ton e K
ids Ac
Compositions
s s de ou ds hy en oun e g d tro p Al Ni com
Fig. 5. The main composition of liquid product from pyrolysis of different feedstock
21
Table 1. Properties of the three kinds of feedstock Elemental analysis(wt.%) a
Proximate analysis (wt.%)
Feedstock
M
Ash
VM
FC
Residues of T. minus
3.42
20.55
65.27
Reeds
1.37
4.16
Woody Chips
3.16
1.26
a
b
H
N
10.76
32.27
5.09
4.03
58.61
8.45
76.68
17.79
47.43
5.65
0.26
46.66
16.71
77.72
17.86
48.89
5.93
0.47
44.71
17.86
on dry and ash-free basis; by difference.
Table 2. Influence of temperature on gas composition (vol. %) 300 ºC
350 ºC
400 ºC
450 ºC
500 ºC
CO2
85.02
81.42
76.01
72.51
61.55
CO
9.10
11.20
12.74
13.23
15.17
CH4
0.67
1.51
2.82
5.79
7.30
H2
3.02
2.22
2.15
3.27
7.49
C2-C3
2.19
3.65
6.28
5.20
8.49
Table 3. Properties of the liquid product at different temperatures Temperature
300 ºC
350 ºC
400 ºC
450 ºC
500 ºC
Water (wt. %)
77.30
69.45
67.25
65.43
64.16
Elemental analysis (wt. %) C
13.86
15.68
18.86
19.81
21.08
H
9.49
10.10
9.47
8.55
8.43
N
7.22
7.63
8.38
8.28
8.62
69.43
66.59
63.29
63.36
61.87
6.41
8.26
8.83
7.84
8.28
O
a -1
HHV (MJ·kg ) a
by difference.
22
O
(MJ·kg-1)
C
b
Temperature
HHV b
Table 4. Influence of temperature on liquid product chemical composition Compounds (wt. %) Alkanes/Alkenes
300 ºC
350 ºC
400 ºC
450 ºC
500 ºC
2.78
4.51
7.11
7.87
10.96
Heptane
—
—
0.82
0.55
0.36
Octane
0.11
0.29
0.33
—
1.01
Nonane
—
0.86
—
1.82
2.36
Propene
0.70
0.41
0.49
0.16
—
Aromatics
2.86
1.49
3.99
2.31
2.51
Styrene
0.38
0.27
0.43
0.42
0.41
Indene
—
—
0.66
1.56
1.65
0.99
2.71
3.42
4.46
4.83
Phenol
0.40
0.45
0.67
1.44
1.42
Cresols
—
0.39
0.69
1.42
1.66
21.07
16.68
13.66
11.01
9.57
Ethanone, 1-(2-furanyl)-
4.13
2.70
2.45
1.97
1.80
2-Butanone, 1-(acetyloxy)
3.24
1.68
0.90
—
0.45
2(5H)-Furanone
1.93
1.96
1.24
1.14
—
—
—
1.26
1.62
2.53
Acids
3.62
5.82
9.26
9.08
7.22
Aldehydes
4.34
3.38
0.21
0.17
0.29
Esters
11.86
8.08
9.77
11.94
11.39
Alcohols
4.42
11.10
10.26
7.91
7.98
Saccharides
10.74
16.18
15.72
12.50
11.97
Nitrogenous compounds
20.90
16.68
18.14
18.09
17.25
Others
16.42
13.37
8.46
14.66
16.03
Phenols
Ketones
2-Cyclopenten-1-one
Table 5. Distribution of product yields of different feedstock (450 ºC) Feedstock
Product yields (wt. %) Char
Liquid
Gas
Residues of T. minus
50.35
29.82
19.83
Reeds
32.14
39.54
28.32
Woody chips
28.90
43.59
27.51
23
S0165-2370(15)30184-4 http://dx.doi.org/doi:10.1016/j.jaap.2015.09.006 JAAP 3568
To appear in:
J. Anal. Appl. Pyrolysis
Received date: Revised date: Accepted date:
15-3-2015 18-6-2015 7-9-2015
Please cite this article as: Xiang Ji, Bin Liu, Guanyi Chen, Wenchao Ma, The pyrolysis of lipid-extracted residue of Tribonema minus in a fixed-bed reactor, Journal of Analytical and Applied Pyrolysis http://dx.doi.org/10.1016/j.jaap.2015.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The pyrolysis of lipid-extracted residue of Tribonema minus in a fixed-bed reactor Xiang Jia,b, Bin Liub, Guanyi Chena,c,d* [email protected], Wenchao Maa,e a
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
b
School of Mathematics, Physics, and Biological Engineering, Inner Mongolia University of
Science and Technology, Baotou 014010, China c
School of Science, Tibet University, Lhasa 850012, China
d
Key Laboratory of Biomass-based Oil and Gas (Tianjin University), China Petroleum and
Chemical Industry Federation, Tianjin 300072, China e
Tianjin Engineering Center of Biomass-derived Gas and Oil, Tianjin 300072, China
*
Corresponding author at： School of Environmental Science and Engineering, Tianjin University,
Tianjin 300072, China. Tel/Fax: +86 2287401929. Highlights
The lipid-extracted residue of T. minus was firstly used as the pyrolysis feedstock.
The lipid-extracted residue of T. minus has lower pyrolysis temperature than lignocellulosic material.
The maximum liquid product yield (29.82 wt.%) was achieved at 450 ºC with 50 ml·min-1 nitrogen flow rate.
The liquid product obtained from residue of T. minus contained more alkane/alkene than that from lignocellulose biomass.
1
ABSTRACT The pyrolysis of lipid-extracted residue of Tribonema minus has been performed in a fixed-bed reactor. In this paper the influence of nitrogen flow rate and pyrolysis temperature on products yields and composition were investigated. The maximum liquid yield of 29.82 wt. % was obtained at 450 ºC with 50 ml·min-1 nitrogen flow rate. The major compounds of liquid product from microalgae were carbonyls, hydrocarbons and nitrogenous compounds with a high proportion of oxygen. CO2 and CO were the main compositions of gas and their contents changed with pyrolysis temperature. The comparison of product yields and properties was made among lipid-extracted residue of T. minus, reeds and woody chips pyrolyzed under similar conditions. The liquid product from microalgal residue contained more alkane/alkene and less aromatics than that from lignocellulose biomass. Keywords: Tribonema minus; Lipid-extracted residue; Pyrolysis; Fixed bed.
2
1. Introduction Biomass, the only renewable source of organic carbon, which can be converted into liquid fuels, is considered as a promising alternative to deplete resources of petrodiesel [1]. Biomass energy can not only alleviate the dependence from fossil fuels but also reduce the emission of greenhouse gas [2]. Biomass can be converted into fuel usually through biochemical or thermochemical process. Pyrolysis is the most promising thermochemical process, which is typically conducted in the absence of oxygen at a moderate temperature [3]. The proportion and complex chemical composition of pyrolysis products depend on many factors, such as feedstock properties (biomass type, feedstock pretreatment, particle size, etc.) and operation parameters (temperature, heating rate, residence time, pressure, gaseous environment, catalyst, etc.) [4,5]. Biomass resources include wood, agricultural crops, aquatic plants and municipal wastes, etc, all of which can be converted into a liquid biofuel via pyrolysis. Liquid product contain about hundreds of different chemical compounds, which can be classed into aldehydes, ketones, alcohols, saccharides, hydrocarbons, organic acids, phenols and nitrogenous compounds [6]. Liquid product exhibit characteristics such as high oxygen concentration and moisture content making the high heating value lower than that of fossil fuels [7,8]. Note, the use of wood and agricultural crops have competed with food production, agricultural land and freshwater resources. Hence, microalgae have been suggested as a very good candidate [2]. Through photosynthesis, microalgae use the sunlight to convert water and carbon dioxide into organics [9]. Compared with other biomass resources, microalgae have (i) higher biological CO2 fixation, producing per ton of microalgae can fix 1.83 tons of CO2 roughly [10]; (ii) higher growth rate, they can
3
increase twofold within 24 h, during exponential phase can be shortened to 3.5 h [10]; (iii) higher photosynthesis efficiency, the whole body can be carried out photosynthesis; (iv) higher lipid production, the productivity of microalgal lipid is 2-30 times of conventional crops per unit area and time [11]; and (v) greater adaptability, the microalgae can be cultivated on freshwater, waste water [12], aquatic medium, even the desert [13]. As we known, algae is an unicellular or simple multicellular structure species. So, it is difficult to get large microalgal biomass. As reported, harvesting microalgal biomass from the culture medium was estimated to account for 20-30% of the total biomass production cost [14]. The algal biomass as the material for pyrolysis focuses on
single-cell
microalgae,
such
as
Chlorella
[15],
Nannochloropsis
[5].
Comparatively speaking, the filamentous algal species with bigger size is easier to harvest [16]. Microalgae lipid, with a content of 60 % of dry biomass [11], can be used in the production of biodiesel by transesterification, which extracted mostly by enzyme reactions, ultrasound and microwave-assisted methods, supercritical fluid extraction, etc [14]. After biodiesel production, large amount microalgae residue still left as a new kind of waste. Nevertheless, there are few researches on the microalgal lipid-extracted residue utilization. In this paper, the work focuses on slow pyrolysis of lipid-extracted residue of microalgae. The lipid-extracted residue of Tribonema minus, a filamentous microalgae species, was used as the pyrolysis feedstock. The effect of pyrolysis parameters, such as nitrogen flow rate and pyrolysis temperature, on the product distributions was investigated, and the product characteristics were analyzed by gas chromatography-mass spectrometer (GC-MS), gas chromatography (GC), elemental
4
analyzer, etc. Furthermore, the product yields and properties from lipid-extracted residue of T. minus, reeds and woody chips pyrolyzed under similar conditions were compared. 2. Material and methods 2.1. Feedstock and characterizations Lipid-extracted residue of Tribonema minus, air-dried, used in this study was obtained from the Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, China, extracted the lipid by literature [16]. The reed and platanus woody chips were collected from Tianjin. The samples were oven-dried at 90 ºC for 24 h, then ground and sieved to a particle size in the range of 0.2-0.45 mm. The main characteristics of the samples are summarized in Table 1. The elemental composition was performed with Euro EA3000 elemental analyzer. The proximate analysis were determined according to the National Testing Standard of Proximate Analysis of Coal (GB/T 212-2008) [17]. The high heat value (HHV, MJ·kg-1) was calculated by the following equation [18]. HHV =33.5*C+142.3*H-15.4*O-14.5*N 2.2. Thermogravimetric analysis The pyrolysis characteristics of samples were studied by thermogravimetric analysis (TGA) using Exstar 6000, TG/DTA 6200 (SEIKO). The samples heated from room temperature to 600 ºC with a heating rate of 10 ºC·min-1 under a flow of 50 ml·min-1 of nitrogen gas (99.999%). 2.3. Pyrolysis reactor and experimental procedure Pyrolysis experiments were performed in a fixed bed reactor, as shown in Fig. 1. The fixed bed reactor was constituted by stainless-steel tube with an internal diameter
5
of 12 mm and a length of 500 mm and heated by an electrical furnace. The thermocouple was used to monitor the real temperature of the reactor. All experiments were carried out under atmospheric pressure and nitrogen conditions (99.9%), and the nitrogen flow rate was controlled by a mass flow controller (MFC). The volatile products leave the reactor together with the inert gas. Inside the reactor, the seperator sieve retained the pyrolysis solid particles in the gas stream. The condensable part was converted to liquid when the volatile products passed through the ice-bath condensers. The non-condensable part was collected by a gas collector and analyzed by GC. To study the effect of the pyrolysis temperature on product characteristics, 5 g of dried sample was fed into the fixed-bed reactor and electrically heated at 10 ºC·min-1 from room temperature to desired temperatures (300, 350, 400, 450 and 500 ºC) and held for 1 h. The gas collecting occurred for the temperature reached the preset temperature, each sampling interval 15 min and held 1 min. The char was collected after completing the pyrolysis reaction. The mass of char and liquid product was determined by an analytical balance. The gas yield was calculated by difference from mass balance. In this study, all the experiments were repeated three times and the product yields were calculated by an average value of three equivalent tests.
2.4. Product analysis Chemical compounds of the liquid product obtained were analyzed by Shimadzu GCMS-QP 2010 SE. The column used was a Rtx-5MS at 30.0 m capillary column, 0.25 mm i.d., 0.25 μm film thickness. The liquid samples (0.1 g) were diluted in methyl alcohol (2 ml); with 1.0 μl injected to the GC. The GC oven temperature was programmed to hold at 50 ºC for 3 min, then followed by a ramp at 6 ºC·min-1 to 270
6
ºC where it was hold for 5 min. The injector temperature was 280 ºC, and the injector split ratio was set to 20:1. The flow rate of helium carrier gas was 1 ml·min-1. The MS was set at an ionizing voltage of 70 eV with mass range (m·z-1) of 35-550 u; the interface temperature was maintained at 270 ºC. The GC-MS chromatogram could give the area percentage of various compounds, which was regarded as a good approach for semi-quantitative analyses of pyrolytic liquid product [19]. The elemental composition of liquid product was performed with Euro EA3000 elemental analyzer. The water content of liquid product was determined by Met-rohm-870 KF Titrino plus Karle-Fischer titrator. The gas product was analyzed by Agilent 7890A GC/TCD, equipped with Porapak Q column (3 m) and 5A molecular sieve (2 m). The oven was programmed to hold at 60 ºC for 2 min, ramp at 15 ºC·min-1 to 180 ºC and then at a temperature rate of 30 ºC·min-1 to 100 ºC where it was hold for 8 min. The TCD temperature was set at 250 ºC and reference gas was helium with a flow rate at 35 ml·min-1. The qualitative and quantitative of pyrolysis gas were determined by external standard method with a standard gas. 3. Results and discussion 3.1. Thermogravimetry behavior Thermogravimetric analysis (TGA) of lipid-extracted residue of T. minus was shown in Fig. 2. The TG process was divided into three stages. The first stage, less than the temperature of 150 ºC, was release of moisture content and volatile compounds with low molecular weight. The second stage, the thermal degradation of volatile component exhibiting a wide peak with a shoulder between 150 and 500 ºC with a maximum weight loss at 335 ºC, attributed to protein and polysaccharide
7
degradation. The third stage was the decomposition of inorganic materials (mainly carbonate decomposition) with the temperature over 500 ºC.
Compared with lingocellulosic biomass, the derivative weight loss peak of microalgal biomass presented at lower temperature, commonly. Yuan et al [20] obtained the temperature of the maximum mass loss rate of wood dust was 354, 375, 391, 396 ºC at the heating rates of 5, 15, 30 and 40 ºC·min-1 under argon atmosphere, respectively. López et al [21] compared the thermogravimetry behavior of four biomass, found the temperature tendency of the maximum mass loss rate was microalgae < rape < corn < sunflower. These results may be attributed to low molecular mass in microalgae can easily be degraded compared with the cellulose and hemicellulose content in lingocellulosic material at the same temperature [5]. 3.2. Product yields Pyrolysis products of biomass in the fixed-bed reactor have been divided into three parts: gas, liquid and char. The residence time of pyrolysis vapor in the reaction area, which affects the distribution of pyrolysis product yields, is closely correlation with carrier gas flow rate. The effect of nitrogen flow rate on the product yields of lipid-extracted residue of T. minus at a pyrolysis temperature of 450 ºC was shown in Fig. 3. The trend of liquid and gas yield with the nitrogen flow rate is similar with Pütün [22], who studied the effect of sweeping gas flow rate on cotton seed pyrolytic product yields. The low flow rate of nitrogen enhancing the secondary cracking of pyrolysis vapors, produced more gas; but the gas yield also increased with increasing nitrogen flow rate when its rate higher than 50 ml·min-1 since the uncondensed volatiles were removed from the reaction area by the nitrogen stream [22]. The 8
maximum liquid yield was obtained at a nitrogen flow rate of 50 ml·min-1. Below this rate, enhancement of the secondary cracking of condensed volatiles decreased the formation of liquid product in the condensate system. Increasing the nitrogen flow rate from 50 to 150 ml·min-1 also caused decrease of liquid yield since the deficient quenching of pyrolysis vapors during the condensate process. To some degree, the product yields can be controlled by changing the flow rate of the carrier gas while pyrolysis temperature keeps constant. Fig. 4 shows the product yields of lipid-extracted residue of T. minus pyrolysis at different temperatures. The char was the main production in the whole range of temperatures. When the pyrolysis temperature increased, the yield of char decreased from 66.07 wt.% (300 ºC) to 49.19 wt.% (500 ºC). The high char yield of microalgae have been reported in many literatures. Grierson et al [23] obtained the char yield of 34-63 wt.% from slow pyrolysis of six different microalgae at 500 ºC. Maddi et al [24] also indicated the algal pyrolysis product contained higher char content. The high amounts of ash in the microalgae and the inorganic elements in the ash can promote the formation of char during pyrolysis [24]. The highest yield of the liquid product obtained at 450 ºC, was 29.82 wt.%. The pyrolysis process was incomplete below 450 ºC, but over 450 ºC the yield of liquid product was slightly declined to 28.99 wt.% (500 ºC) since the secondary cracking reaction of volatiles was enhanced with the pyrolysis temperature increased at a constant carrier gas flow rate. The liquid yield was lower than literature [25] which obtained the liquid yield of 52.7 wt.% from slow pyrolysis of C. vulgaris in fixed-bed reactor at 500 ºC. Chaiwong et al [26] also got similar results that the liquid yield was 46 wt.% at pyrolysis temperature of 550 ºC from algae. This was because of the T.
9
minus subject to lipid extraction process before used for our study, which lipid compounds account for 50.23% of dry weight percentage of raw T. minus after 21-day-cultivation [16]. The enhancement of secondary cracking reaction of volatile was also the reason of gas yield increases from 14.17 wt.% at 300 ºC to 21.83 wt.% at 500 ºC. It was consistent with Maguyon [5], who studied the products yields and characteristics of N. oculata via pressurized pyrolysis. 3.3. Gas composition The gas composition of lipid-extracted residue of T. minus pyrolysis at different temperatures was shown in Table 2. The main compositions of gas were carbon dioxide and carbon monoxide. As the pyrolysis temperature rose from 300 ºC to 500 ºC, CO2 concentration decreased from 85.02 vol. % to 61.55 vol. %, while the CO increased from 9.10 vol. % to 15.17 vol. %. The trend was consistent with the work of Maddi et al [24], due to the enhancement of decarboxylation and decarbonylation reactions with the pyrolysis temperature increase. Furthermore, the gas was also composed of small amounts of H2, CH4 and C2-C3 hydrocarbons. The concentration of these in the gaseous product tended to increase with further heating of the biomass, since the severity of cracking reactions of the vapors forming incondensable gaseous compounds [5].
3.4. Liquid product composition The properties of the liquid product were shown in Table 3. As observed the liquid product contains higher water content. Water was obtained from the moisture in the feedstock and most of it formed via the dehydration and/or decarboxylation reactions 10
throughout pyrolysis process [2,27]. Higher water content can lower the heating value of liquid product, while can also reduce the oil viscosity and enhance the fluidity which was beneficial to the atomization and combustion of liquid fuels [26]. It can also be seen that with the pyrolysis temperature increase, the C content in liquid was increased, the O content decreased, while the H and N content were hardly affected by pyrolysis temperature. The N content in the microalgae-derived liquid was obtained from protein and pigment in the algae, most of nitrogen compounds into the liquid phase via the pyrolysis process [28]. The liquid product contains a high proportion of oxygen, resulting in the poor stability and lower heating value. A deoxygenated process was necessary to upgrade the liquid product. The chemical composition of the liquid product obtained at different pyrolysis temperatures was analyzed by GC-MS. The liquid product contained more than 100 kinds of compounds, and grouped in Table 4 according to their functional groups. The major compounds of microalgal pyrolytic liquid product were carbonyls, hydrocarbons, phenols, and nitrogenous compounds. These oxygenate compounds will correspondingly increase the oxygen content of liquid product, making it unstable. The amount of heterocyclic compounds in liquid product was easy to its polymerization [27]. With the pyrolysis temperature raise, the cracking reaction of pyrolysis compounds was more severe leading to more light compounds and less heavy compounds. For instance, the phenols content was increased from 0.99 wt.% (300 ºC) to 4.83 wt.% (500 ºC). Furthermore, the higher temperature was beneficial to hydrocarbons generation, which was also caused by the severity of cracking reactions. The alkanes/alkenes content in liquid product increase from 2.78 wt.% to 10.96 wt.% with
11
pyrolysis temperature rising from 300 ºC to 500 ºC, whereas the aromatics were hardly affected by pyrolysis temperature. Ketones, the main compounds in the liquid product, was reduced from 21.07 wt.% (300 ºC) to 9.57 wt.% (500 ºC). Which were formed by condensation reactions of the carbohydrate-derived fraction and decomposition of the miscellaneous oxygenates and sugars [27]. The contents of alcohols and saccharides also reduced with temperature rising. There were also carboxylic acids, esters and nitrogenous compounds in liquid product, which concentrations were hardly changed with pyrolysis temperature. The presence of acids may significantly contribute to the strong acidity of liquid product. They can corrode the pyrolysis equipment and limit its application in the engine [27]. The nitrogenous compounds (nitriles, amines and nitrogen heterocycles such as pyridines, pyrazines, pyrroles, etc.) were produced by the re-polymerization of small organic materials, formed from decarboxylation and deamination of the protein, via Fischer-Tropsch type reactions [28]. There were a small amount of aldehydes in liquid product, which obtained from the cracking reactions of cellulose and hemicellulose, since the algae biomass contains only a small amount of cellulose and hemicellulose in the cell wall. 3.5. Comparison of pyrolysis of residue of T. minus with lignocellulosic biomass 3.5.1. Product yields The yield of pyrolysis product is related to the characteristics of biomass. Comparison of three kinds of biomass pyrolysis at temperature of 450 ºC was shown in Table 5. The char yields of reeds and woody chips were 32.14 wt.% and 28.90 wt.%, respectively, obviously lower than that of microalgae residue of T. minus. Maddi et al [24] obtained similar results through comparative study of pyrolysis of 12
algal biomass with lignocellulosic biomass. It is possible that inorganic elements in the ash of algae samples catalyzed the char-forming reactions during pyrolysis, and the proteins and carbohydrates could have degraded into char. However, almost of the cellulose in the lignocellulosic biomass can break up into volatile [24]. The yields of gas and liquid of reeds and woody chips were higher than that of residue of T. minus, since their volatile matter were higher than residue of T. minus as shown in Table 1. Moreover, the bio-oil formation from the algal samples during pyrolysis follows the trend lipids > proteins > carbohydrates [28]. The residue of T. minus has been extracted the lipid before used for the study, which is another significant reason for the lower yield of liquid product. 3.5.2. Liquid product composition The compounds of liquid product
were closely
bound up with the
chemical composition of biomass. The lignocellulosic biomass consists of different contents of cellulose, hemicellulose and lignin. However, algae biomass was composed of protein, lipid and polysaccharide, as well as a small amount of cellulose in cell walls. The chemical composition of biomass can disintegrate different compounds by pyrolysis process. Fig. 5 shows the main liquid product compositions of residue of T. minus , reeds and woody chips at a pyrolysis temperature of 450 ºC. Compared with the lignocellulosic biomass, the liquid product from residue of T. minus contained more alkane/alkene compounds and less aromatic compounds. The mostly aromatic compounds and their derivatives (phenols) were derived by thermal decomposition of lignin [24]. Since microalgae do not contain lignin and the side chain of protein contained only a small amount of benzene compounds, this caused 13
these compounds were rarely present in the liquid product from residue of T. minus pyrolysis. The ketones and aldehydes were obtained from pyrolysis of cellulose and hemicellulose (polysaccharide in algal feedstock) fraction [24]. Therefore, the ketone/aldehyde contents (especially the aldehyde compounds) in liquid product obtained from pyrolysis of residue of T. minus was less than that obtained from reeds and woody chips. The liquid product from T. minus residue pyrolysis contained a series of nitrogenous compounds, such as nitriles, amines, pyrazines, pyridines, indoles and other heterocyclic nitrogen compounds, most of which were not present in the liquid product obtained from lingocellulosic biomass. The result was consistent with the literature [24]. This is a disadvantage when using bio-oils as fuel due to NOx emissions during combustion [5]. These acid compounds were formed by the deacetylation of hemicelluloses (pigment and protein in microalgae) at higher temperature. The breaking of peptide bond among protein (abundant in the microalgae) formed the carboxylic compounds, which caused the acid content of microalgal pyrolytic liquid product was higher than that of the other two biomasses. 4. Conclusions Microalgae have great potential for fossil fuel substitution in energy generation and conversion, and have been paid great attention in recent years. In this study, pyrolysis of lipid-extracted residue of T. minus in a fixed-bed reactor was primarily carried out. Furthermore, the product yields and properties from pyrolysis of residue of T. minus, reeds and woody chips under similar conditions were compared. 14
For lipid-extracted residue of T. minus pyrolysis, the char was the main production in the whole range of temperatures with yield decreasing from 66.07 wt.% (300 ºC) to 49.19 wt.% (500 ºC). The maximum yield of liquid product was 29.82 wt. %, achieved at 450 ºC with 50 ml·min-1 nitrogen flow rate. The major compounds of liquid product were carbonyls, hydrocarbons, phenols, and nitrogenous compounds, and their content was affected by temperature. The main components of CO and CO2 have diverse trends with temperature rising. Moreover, a slight increase of concentration of H2 and C1-C3 hydrocarbons occurred as temperature increasing. Compared with lignocellulose feedstock, the residue of T. minus has lower pyrolysis temperature. The microalgal pyrolytic liquid product contained more alkane/alkene and nitrogenous compounds, less aromatic, phenol, ketone and aldehyde compounds. Acknowledgements This work is financially supported by National Natural Science Foundation of China through project (NO.51306131), Tianjin Research Program of Application and Advanced Technology (15JCQNJC06600). Greatly thanks to Prof. Tianzhong Liu, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, China, for supplying the lipid-extracted residue of T. minus .
15
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[9] A. Demirbas.;1; Use of algae as biofuel sources. Energy Conversion and Management. 51 (2010) 2738-2749 . [10] Y. Chisti.;1; Biodiesel from microalgae beats bioethanol. Trends in Biotechnology. 26 (2008) 126-131. [11] S.A. Scott, M.P. Davey, J.S. Dennis, I. Horst, C.J. Howe, D.J. Lea-Smith, A.G. Smith.;1; Biodiesel from algae: challenges and prospects. Current Opinion in Biotechnology. 21 (2010) 277-286. [12] Y.J. Feng, C. Li, D.W. Zhang.;1; Lipid production of Chlorella vulgaris cultured in artificial wastewater medium. Bioresource Technology. 102 (2011) 101-105. [13] X.L. Yang, S.Q. Deng, R.D. Philippis, L.Z. Chen, C.Z. Hu, W.H. Zhang.;1; Chemical composition of volatile oil from Artemisia ordosica and its allelopathic effects on desert soil microalgae, Palmellococcus miniatus. Plant Physiology and Biochemistry. 51 (2012) 153-158. [14] T.M. Mata, A.A. Martins, N.S. Caetano.;1; Microalgae for biodiesel production and other applications: A review. Renewable and Sustainable Energy Reviews. 14 (2010) 217-232. [15] A.M. Rizzo, M. Prussi, L. Bettucci, I.M. Libelli, D. Chiaramonti.;1; Characterization of microalga Chlorella as a fuel and its thermogravimetric behavior. Applied Energy. 102 (2013) 24-31. [16] H. Wang, L.L. Gao, L. Chen, F.J. Guo, T.Z. Liu.;1; Integration process of biodiesel production from filamentous oleaginous microalgae Tribonema minus. Bioresource Technology. 142 (2013) 39-44. [17] GB/T 212–2008. The national testing standard of proximate analysis of coal of China, 2008. [18] A. Campanella, M.P. Harold.;1; Fast pyrolysis of microalgae in a falling solids reactor: Effects of process variables and zeolite catalysts. Biomass and Bioenergy. 46 (2012) 218-232. [19] G.Y. Chen, C. Liu, W.C. Ma, X.X. Zhang, Y.B. Li, B.B. Yan, W.H. Zhou.;1; 17
Co-pyrolysis of corn cob and waste cooking oil in a fixed bed. Bioresource Technology. 166 (2014) 500-507. [20] H.R. Yuan, S.Y. Xing, Huhetaoli, T. Lu, Y. Chen.;1; Influences of copper on the pyrolysis process of demineralized wood dust through thermogravimetric and Py-GC/MS analysis. Journal of Analytical and Applied Pyrolysis. 112 (2015) 325-332. [21] R. López, C. Fernández, J. Fierro, J. Cara, O. Martínez, M.E. Sánchez.;1; Oxy-combustion of corn, sunflower, rape and microalgae bioresidues and their blends from the perspective of thermogravimetric analysis. Energy. 74 (2014) 845-854. [22] E. Pütün.;1; Catalytic pyrolysis of biomass: Effects of pyrolysis temperature, sweeping gas flow rate and MgO catalyst. Energy. 35 (2010) 2761-2766. [23] S. Grierson, V. Strezov, G. Ellem, R. Mcgregor,;1; J. Herbertson. Thermal characterisation of microalgae under slow pyrolysis conditions. Journal of Analytical and Applied Pyrolysis. 85 (2009) 118-123. [24] B. Maddi, S. Viamajala, S. Varanasi.;1; Comparative study of pyrolysis of algal biomass from natural lake blooms with lignocellulosic biomass. Bioresource Technology. 102 (2011) 11018-11026. [25] S. Thangalazhy-Gopakumar, S. Adhikari, S.A. Chattanathan, R.B. Gupta.;1; Catalytic pyrolysis of green algae for hydrocarbon production using H+ZSM-5 catalyst. Bioresource Technology. 118 (2012) 150-157. [26] K. Chaiwong, T. Kiatsiriroat, N. Vorayos, C. Thararax.;1; Study of bio-oil and bio-char production from algae by slow pyrolysis. Biomass and Bioenergy. 56 (2013) 600-606. [27] J. Alvarez, G. Lopez, M. Amutio, J. Bilbao, M. Olazar.;1; Bio-oil production from rice husk fast pyrolysis in a conical spouted bed reactor. Fuel. 128 (2014) 162-169. [28] P. Biller, A.B. Ross.;1; Potential yields and properties of oil from the hydrothermal liquefaction of microalgae with different biochemical content. Bioresource Technology. 102
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(2011) 215-225.
MFC Furnace N2
Gas collector Seperator sieve GC
Heating belts
Thermocouple
Condenser
0.0
90
-0.5
80
-1.0
TG (%) -1 DTG (%·ºC )
70
-1.5
60
-2.0
50
-2.5
40
-1
100
Derivative Weight Loss (%·ºC )
Residual Weight (%)
Fig. 1. Schematic diagram of the fixed-bed reactor
-3.0 0
100
200
300
400
500
600
Temperature (ºC)
Fig. 2. TG and DTG curves of lipid-extracted residue of T. minus
19
50 Char Liquid Gas
Yield (wt. %)
40
30
20
10 30
50
80
150 -1
Nitrogen flow rate (mlmin )
Fig. 3. Effect of nitrogen flow rate on the product yields (450 ºC) 70 Char Liquid Gas
60
Yield (wt. %)
50 40 30 20 10 300
350
400
450
500
Temperature (C)
Fig. 4. Effect of temperature on the product yields
20
25
Reeds Woody chips Residues of T. minus
Percentage (%)
20
15
10
5
0 s s ne s tic ka ene ma l k o A al Ar
＆
ols en Ph
es ton e K
ids Ac
Compositions
s s de ou ds hy en oun e g d tro p Al Ni com
Fig. 5. The main composition of liquid product from pyrolysis of different feedstock
21
Table 1. Properties of the three kinds of feedstock Elemental analysis(wt.%) a
Proximate analysis (wt.%)
Feedstock
M
Ash
VM
FC
Residues of T. minus
3.42
20.55
65.27
Reeds
1.37
4.16
Woody Chips
3.16
1.26
a
b
H
N
10.76
32.27
5.09
4.03
58.61
8.45
76.68
17.79
47.43
5.65
0.26
46.66
16.71
77.72
17.86
48.89
5.93
0.47
44.71
17.86
on dry and ash-free basis; by difference.
Table 2. Influence of temperature on gas composition (vol. %) 300 ºC
350 ºC
400 ºC
450 ºC
500 ºC
CO2
85.02
81.42
76.01
72.51
61.55
CO
9.10
11.20
12.74
13.23
15.17
CH4
0.67
1.51
2.82
5.79
7.30
H2
3.02
2.22
2.15
3.27
7.49
C2-C3
2.19
3.65
6.28
5.20
8.49
Table 3. Properties of the liquid product at different temperatures Temperature
300 ºC
350 ºC
400 ºC
450 ºC
500 ºC
Water (wt. %)
77.30
69.45
67.25
65.43
64.16
Elemental analysis (wt. %) C
13.86
15.68
18.86
19.81
21.08
H
9.49
10.10
9.47
8.55
8.43
N
7.22
7.63
8.38
8.28
8.62
69.43
66.59
63.29
63.36
61.87
6.41
8.26
8.83
7.84
8.28
O
a -1
HHV (MJ·kg ) a
by difference.
22
O
(MJ·kg-1)
C
b
Temperature
HHV b
Table 4. Influence of temperature on liquid product chemical composition Compounds (wt. %) Alkanes/Alkenes
300 ºC
350 ºC
400 ºC
450 ºC
500 ºC
2.78
4.51
7.11
7.87
10.96
Heptane
—
—
0.82
0.55
0.36
Octane
0.11
0.29
0.33
—
1.01
Nonane
—
0.86
—
1.82
2.36
Propene
0.70
0.41
0.49
0.16
—
Aromatics
2.86
1.49
3.99
2.31
2.51
Styrene
0.38
0.27
0.43
0.42
0.41
Indene
—
—
0.66
1.56
1.65
0.99
2.71
3.42
4.46
4.83
Phenol
0.40
0.45
0.67
1.44
1.42
Cresols
—
0.39
0.69
1.42
1.66
21.07
16.68
13.66
11.01
9.57
Ethanone, 1-(2-furanyl)-
4.13
2.70
2.45
1.97
1.80
2-Butanone, 1-(acetyloxy)
3.24
1.68
0.90
—
0.45
2(5H)-Furanone
1.93
1.96
1.24
1.14
—
—
—
1.26
1.62
2.53
Acids
3.62
5.82
9.26
9.08
7.22
Aldehydes
4.34
3.38
0.21
0.17
0.29
Esters
11.86
8.08
9.77
11.94
11.39
Alcohols
4.42
11.10
10.26
7.91
7.98
Saccharides
10.74
16.18
15.72
12.50
11.97
Nitrogenous compounds
20.90
16.68
18.14
18.09
17.25
Others
16.42
13.37
8.46
14.66
16.03
Phenols
Ketones
2-Cyclopenten-1-one
Table 5. Distribution of product yields of different feedstock (450 ºC) Feedstock
Product yields (wt. %) Char
Liquid
Gas
Residues of T. minus
50.35
29.82
19.83
Reeds
32.14
39.54
28.32
Woody chips
28.90
43.59
27.51
23