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Pyrolysis of oleaginous yeast biomass from wastewater treatment: Kinetics analysis and biocrude characterization
Journal Pre-proof Pyrolysis of oleaginous yeast biomass from wastewater treatment: Kinetics analysis and biocrude characterization Dayu Yu, Shuang Hu, Weishan Liu, Xiaoning Wang, Haifeng Jiang, Nanhang Dong PII:
S0960-1481(20)30032-X
DOI:
https://doi.org/10.1016/j.renene.2020.01.028
Reference:
RENE 12890
To appear in:
Renewable Energy
Received Date: 4 June 2019 Revised Date:
29 November 2019
Accepted Date: 7 January 2020
Please cite this article as: Yu D, Hu S, Liu W, Wang X, Jiang H, Dong N, Pyrolysis of oleaginous yeast biomass from wastewater treatment: Kinetics analysis and biocrude characterization, Renewable Energy (2020), doi: https://doi.org/10.1016/j.renene.2020.01.028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Author Contribution Statement
DY and ND conceived and designed the experiments. DY, SH, WL, XW performed the experiments. DY, SH, HJ and ND analyzed data. DY, SH and ND wrote the paper. All authors read and approved the final manuscript.
Pyrolysis
Liquid Bio-oils obtained at different final temperatures (300-600°C) C H O N R
Recycling & Drying
TG-FTIR
Trichosporon fermentans biomass
Pyrolysis of oleaginous yeast biomass from wastewater treatment: kinetics analysis and biocrude characterization Dayu Yua, b, Shuang Hua, b, Weishan Liua, c, Xiaoning Wanga, b, Haifeng Jiangc, Nanhang Dong a, c, * a
Sci-Tech Center for Clean Conversion and High-valued Utilization of Biomass, Northeast Electric
Power University, Jilin 132012, China b
School of Chemical Engineering, Northeast Electric Power University, Jilin 132012, China
c
School of Energy and Power Engineering, Northeast Electric Power University, Jilin 132012, China
* Corresponding author. E-mail address: [email protected]
1
1
ABSTRACT
2
Pyrolysis of Trichosporon fermentans biomass, a type of oleaginous yeast from the
3
fermentation of refined soybean oil wastewater, was studied in the present work. Based
4
on the TG/DTG curves, the activation energy values for the thermal decomposition of
5
Trichosporon fermentans biomass were evaluated by the KAS method (111.69 kJ/mol)
6
and FWO method (116.93 kJ/mol), respectively. The pyrolysis behavior was represented
7
by considering the parallel degradation of four components, namely carbohydrates,
8
proteins, lipids, and others, and the fitting degree of the simulated curve was over 0.99.
9
According to the FTIR spectra of the end-products, the increasing hydrocarbons at a
10
temperature around 400
indicated the lipid degradation. Biocrude oil was collected
11
from a fixed bed reactor and the chemical composition was characterized by
12
GC-TOF/MS. The yield of hydrocarbons in the bio-oil was identified as being over 28%
13
when the pyrolysis temperature was higher than 500
14
compounds was less than 18%. The fractional yield distributions of the end-products at
15
different pyrolysis temperatures were compared and the maximum yield of bio-oil was
16
achieved (around 42%) at 500
17
environmentally friendly method for the preparation of biofuel from oleaginous yeast
18
pyrolysis.
19
Keywords: Pyrolysis; Oleaginous yeast; Trichosporon fermentans biomass; Bio-oil
20
characterization; Kinetics; Bio-oil yield
, whilst the content of nitrogen
. This study proposed a cost-effective and
2
21
1. Introduction
22
Oleaginous microorganisms such as algae, yeast, fungus, and bacterium with a lipid
23
accumulation of 20~25 wt% are considered to be promising feedstock to produce
24
biodiesel and other liquid hydrocarbon fuels [1-3]. The transesterification reaction is
25
always mentioned in order to produce biodiesel by oleaginous microorganisms where
26
microbial oil is extracted with an organic solvent ahead. Therefore, the large
27
consumption of organic reagents and the secondary wastewater pollution of oil
28
production should be of concern [4].
29
The thermochemical conversion methods including hydrothermal liquefaction and
30
pyrolysis are the optional techniques in which to transform oleaginous microorganisms
31
into liquid fuels. Studies on the hydrothermal liquefaction of microalgae have been
32
widely carried out with the main focus on the effect of temperature, pressure, and
33
dissolvent on the quality of the bio-oil [5, 6]. Meanwhile, the utilization of pyrolysis has
34
also attracted more and more interest due to its advantages of simple operation, low
35
equipment dependence and the direct utilization of end-products as biofuels or chemical
36
raw materials. Thermogravimetric Analysis (TGA) is generally used to analyze the
37
pyrolysis behavior of the microalgae biomass and to understand the degradation progress
38
during the heating process [7, 8]. While studying the thermal behavior, one or more
39
kinetic methods can be employed to calculate the kinetic parameters, such as the
40
activation energy value and pre-exponential factor [9-11]. Considering of its complex
41
components, the Derivative Thermogravimetric (DTG) curve of biomass pyrolysis can be
3
42
technically divided into several independent curves that simulate the degrading of
43
different components. Consequently the pyrolysis temperature range of each component
44
such as cellulose, hemicellulose, lignin, carbohydrates, proteins and lipids can be
45
predicted to obtain the better understanding of biomass pyrolysis behavior [12-14]. An
46
additional area of attention is given to the yield of pyrolysis products under different
47
experimental conditions. For instance, the pyrolysis of the Spirulina Sp. algae is studied
48
in a fixed-bed reactor, and the maximum yield of biochar and bio-oil is obtained at 500
49
and 550
50
distribution of the end-products is also studied and the experimental data show that the
51
liquid yield of algae bloom pyrolysis is the highest at 500
52
bio-oil is higher than that of pine sawdust derived bio-oil [16]. Analysis methods, such as
53
Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass
54
spectrometry (GC-MS) have also been widely introduced into the biomass pyrolysis
55
experiments. The spectrum analysis is considered to understand the volatilization process
56
during the thermal decomposition of the biomass, meanwhile the identification of the
57
bio-oil composition by GC-MS can be applied to evaluate the potential of products for
58
direct utilization or upgrading [17]. The pyrolysis behavior of different algae biomass is
59
studied by the pyrolysis-GC/MS experiment and high value chemicals such as xylene,
60
styrene, phenol, and toluene as well as a large number of nitrogen-containing compounds
61
are identified in the end-products [18, 19].
, individually [15]. The effect of the pyrolysis temperature on the yield
4
, and the calorific value of
62
Oleaginous yeasts cultivated by accumulating lipids can grow on non-traditional
63
substrates such as food wastewater [20], waste oil [21] and crude glycerin [22]; moreover,
64
a variety of wastewater can be used as a carbon source in the accumulation of microbial
65
oil [23]. Compared with algae biomass, the cultivation of oleaginous yeast is not affected
66
by light intensity and photoperiod, and the growth cycle is around 40 hours. Currently, a
67
series of studies on the hydrothermal liquefaction of oleaginous yeast is being carried out
68
[24, 25], but work on pyrolysis has been rarely performed yet. The limited reports on
69
oleaginous yeast pyrolysis indicated that the exothermic activity of the microorganisms
70
based on TGA can be considered to identify the presence of the lipid content [26].
71
In summary, extensive work has been carried out on the pyrolysis of algae biomass
72
including research on the analysis of pyrolysis behavior, pyrolysis kinetics, biocrude oil
73
quality, but the pyrolysis of oleaginous yeast has not been mentioned in depth up until
74
now. In the present work, the pyrolysis process of T. fermentans biomass, a kind of
75
oleaginous yeast, was studied systematically as follows: The pyrolysis process of T.
76
fermentans biomass was analyzed by TGA and the Gaussian fitting, and the thermal
77
behavior was simulated based on the pseudo components; the pyrolysis kinetic
78
parameters were calculated to evaluate the thermal resistance; the pyrolysis gas release
79
mechanism was explained by TG-FTIR and the optimum liquid yield of T. fermentans
80
biomass at different final temperatures was determined; in addition, the contents of the
81
hydrocarbons and nitrogen compounds, etc. were analyzed by GC/MS in order to fully
5
82
understand the distribution of the bio-oil composition produced by T. fermentans biomass
83
pyrolysis. All of the above-mentioned work will provide a theoretical basis for the
84
preparation of biofuels by oleaginous yeast pyrolysis.
85
2. Material and methods
86
2.1. Sample preparation and analysis
87
T. fermentans biomass was collected from the purification treatment of refined
88
soybean oil wastewater [27] and sieved into the size range of 150~180 µm. The samples
89
were dried at 105
90
analysis for the samples was carried out by an automatic proximate analyzer (Sundy
91
SDLA718, China) and the ultimate analysis was performed with a fully automatic
92
element analyzer (EuroVector EA 3000, Italy). The elements C, H, N, and S were
93
determined directly and the content of the element O was calculated by difference. The
94
main components of the sample such as carbohydrates, proteins, and lipids were
95
determined through the phenol-sulfuric acid method, the Kjeldahl method, and
96
acid-heating extraction.
97
2.2. TG-FTIR analysis
for 12 hours, and then sealed in sample bags. The proximate
98
TGA was conducted by using a thermogravimetric analyzer (PerkinElmer
99
TGA8000, US) to investigate the sample thermal degradation. The samples were heated
100
under a non-isothermal condition from room temperature to 800
101
atmosphere. The different heating rates of 10, 20, 30, 40, and 50
6
in a nitrogen
/min were applied to
102 103
calculate the kinetic parameters and the TG/DTG curves were plotted. At the heating rate of 20
/min, the decomposition of the sample progressed and
104
the gaseous end-product was analyzed online by TG-FTIR. The infrared synchronous
105
acquisition data were triggered in the temperature range of 105 to 600
106
residual rate was almost constant, and the infrared scanning wave number ranged from
107
4000 to 400 cm-1.
108
2.3. Kinetic analysis
109 110
Based on the TG experiment, the mass loss of sample was recorded and the conversion rate,
⁄ , can be expressed as a function of temperature [28]: =
1
111
where
112
function. The conversion, α, is written as [29]:
is the temperature-dependent rate constant and
= where
114
the final residue of sample in the reaction.
is the initial sample mass;
2
is the sample mass at time t; and
is
By coupling the Arrhenius equation, Eq. (1) can be rearranged as: =
116
where
117
gas constant, and
118
is the conversion
− −
113
115
when the solid
is the pre-exponential factor,
−
3 is the activation energy,
is equal to the heating rate,
/
The integrated form of Eq. (3) is introduced as:
7
.
is the universal
#
=$
%
= $
&
4
−
&'
119
The iso-conversional method which can be applied to multistep reactions is
120
supposed to obtain more reliable activation energy as no kinetic model function is
121
involved. The Kissinger-Akahira-Sunose (KAS) and the Flynn-Wall-Ozawa (FWO)
122
methods are widely used in the calculation of activation energy. Based on the two
123
methods, Eq. (4) can be introduced as follows, respectively: ln log
124
+
= log ,
-−
5
- − 2.315 − 0.4567
6
= ln , #
#
According to Eq. (5) or (6), the relation of
vs. T can be plotted as
125
With different heating rates, a straight line is derived, and then
126
2.4. Experimental procedure
is fixed.
can be calculated.
127
The pyrolysis experiments were carried out in a system consisting of a fixed bed
128
reactor, as shown in Fig. 1. For each run, the furnace was preheated to the set-point
129
temperature after a 20-min sweep of nitrogen. Samples weighing 0.5 g were loaded and
130
held for 30 minutes, which was sufficient to achieve complete decomposition. The
131
vapors were collected in a three-stage condensation facility including a cold bath, an ice
132
water bath, and an ice salt bath. Meanwhile, the syngas was washed by the tail gas
133
absorption bottle. The liquid yield was estimated by the total weight of bio-oil in the
134
condensing facility. The solid residue was collected and weighed until the furnace had
135
cooled down to room temperature by sweeping with cold nitrogen. The yield of the
8
136
syngas was determined by difference. The experiments were conducted at least three
137
times for each run to confirm the reproducibility, and the presented data are the mean
138
values ± standard deviations (SD).
139 140
The yields of the end-products were calculated according to the following equations: 56 =
6
5 = 5: = 141
where
142
liquid bio-oil; and
−
is the mass of feedstock; :
6
6
7 100%
7
7 100%
8
−
7 100%
is the mass of solid residue;
9 is the mass of
is the mass of the syngas.
Fig. 1. Schematic diagram of the pyrolysis system with a fixed bed reactor.
143
2.5. Bio-oil analysis
144
Bio-oil from the decomposition of T. fermentans biomass was analyzed by a GC
145
unit (Agilent 7890) and a LECO Pegasus 4D time of flight mass spectrometer
146
(TOF/MS). The GC oven was preheated at 40
9
, held for 3 min, and followed by
147
ramping to 300
at 5
/min. A total of 1.0 µL of bio-oil with an acetone solvent was
148
injected by the helium carrier gas. Full scanning mode was applied by the mass
149
spectrometer with the scanning range of 35~500 amu and the ionizing voltage was 70
150
eV. The ion source temperature was 280
151
the chromatograms were confirmed by the NIST 11 standard library of mass
152
spectrometry.
153
3. Results and Discussion
154
3.1. Feedstock characterization
. The compounds identified by the peaks in
155
The proximate and ultimate analyses and the chemical components of T.
156
fermentans biomass were listed and compared with two typical algae biomasses
157
reported in Refs. [12, 30]. As shown in Table 1, the T. fermentans biomass had the
158
highest content of volatile matter and the lowest content of fixed carbon. The ultimate
159
analysis exhibited the highest contents of carbon and hydrogen and the lowest content
160
of oxygen, nitrogen, and sulfur. The main components of the T. fermentans biomass
161
were comprised of carbohydrates, proteins, and lipids. Due to the different liquefaction
162
abilities of carbohydrates, proteins, and lipids [12], the T. fermentans biomass should
163
produce more liquid fuel than those from each of the listed algae biomass.
10
Table 1 Proximate and ultimate analyses and components of the different biomasses. T. fermentans biomass
Spirulina [12]
Chrysophyceae [12, 30]
Moisture
0
0
1.69
Volatile matter
90.48
75.55
79.79
Fixed carbon
6.94
16.39
11.63
Ash
2.57
8.06
6.89
C
62.10
49.14
49.26
H
9.72
6.68
7.5
O
22.8*
28.58
31.74
N
2.41
11.19
6.24
S
0.40
0.83
0.96
Carbohydrates
19.9
68.4*
15.2*
Proteins
15.1
23.44
35.9
Lipids
53.1
0.1
42.9
Others
9.33*
0
0
Proximate analysis (wt%)
Ultimate analysis (wt%)
Chemical composition analysis (wt%)
* by difference
164
3.2. Thermal degradation characteristics
165
The pyrolysis of the T. fermentans biomass was studied by TGA under
166
non-isothermal conditions. Fig. 2 shows the TG/DTG curves of T. fermentans biomass
167
pyrolysis at the different heating rates of 10, 20, 30, 40, and 50
168
lateral shifts observed in Fig. 2 (a) could be attributed to the thermal lag while the
169
increased maximum mass loss rate referred to the improved reaction rate under a high
170
heating rate. According to the pyrolysis process, three main stages were identified
171
including the water and light volatile release stage, the main pyrolysis stage, and the
11
/min. The distinct
172
carbonation stage.
Fig. 2. TG (a) and DTG (b) curves for T. fermentans biomass pyrolysis at different heating rates.
173
Fig. 3 shows the DTG curve with respect to the heating rate of 20
/min. In the
174
initial stage (~160
), the release of light volatiles led to the slight mass loss.
175
Subsequently, two overlapping peaks could be observed in the following stage
176
(160~500
177
parallel reactions. Two peaks at about 323 and 434
178
and 374
179
shoulder, respectively. A low mass loss rate in the final stage (500
180
slow decomposition of carbonaceous substances. The DTG curve pattern mainly
), where a large amount of organic matter was rapidly decomposed through , and two shoulders at about 282
were described as the first peak, second peak, first shoulder, and second
12
~) revealed the
181
indicated the different thermal resistance of the carbohydrates, proteins, lipids, and
182
others [31].
Fig. 3. Peak fitting of DTG curve for T. fermentans biomass pyrolysis at a heating rate of 20
/min.
183
Gaussian fitting was employed to perform the peak separation of the DTG curve in
184
Fig. 3. As reported in Ref. [13], the reaction stage of algae pyrolysis can be divided into
185
two, three, four, and seven sections, corresponding to the presumption of the
186
pseudo-component number. It was found that the four-reaction models were fitted and
187
close to the results of the component analysis. Hence, four fitting peaks were introduced
188
to represent the degradation of carbohydrates, proteins, lipids, and others here. The
189
fitting coefficient (
190
around 0.9952, which indicates that the four-reaction regime was acceptable. The fitting
191
peak at a temperature around 320 ℃ describes the maximum mass loss of carbohydrates
192
in the temperature range of 235 and 404 ℃. The curve with the fitting peak at a
193
temperature around 390 ℃ demonstrates the decomposition of proteins. Lipids are
194
supposed to decompose from 386 to 485 ℃ and the mass loss peak was located over
+
between the simulated curve and experimental DTG curve was
13
195
435
196
components [32], the variation can be attributed to the different proportions of
197
components and that the synergy effect caused variations in the pyrolysis characteristics.
198
In other words, the thermal degradation of T. fermentans biomass can be properly
199
interpreted based on the three main components.
200
3.3. Pyrolysis kinetic parameters
201
. Compared with the reported degradation temperature range of the three
The kinetic analysis of the pyrolysis process of T. fermentans biomass (105~600
)
202
was carried out by the KAS and FWO methods, respectively. The fitted lines at different
203
conversions in the range of 0.1 to 0.9 were derived by the plots of ln ( /T2) vs. 1/T in
204
Fig. 4 (a) and log ( ) vs. 1/T in Fig. 4 (b). The activation energy values calculated by
205
the KAS and FWO methods were listed in Table 2, together with the corresponding
206
fitting coefficients ranging from 0.9652 to 0.9992 for the different conversions. The
207
average activation energy values were 111.69 and 116.93 kJ/mol by the KAS and FWO
208
methods, respectively.
209
In comparison with the terrestrial biomasses of peanut shell and pine needle [33],
210
the relatively low value for the activation energy of the T. fermentans biomass
211
determined is close to that of algae biomass [10], which should be attributed to the
212
combination of high volatile matter and similar chemical composition to algae.
14
Fig. 4. Fitting curves for kinetic analysis of T. fermentans biomass pyrolysis by KAS method (a) and FWO method (b). Table 2 Activation energy values by KAS and FWO methods. KAS method
FWO method
α
E (kJ/mol)
R2
E (kJ/mol)
R2
0.1
78.67
0.9976
83.96
0.9984
0.2
92.43
0.9903
97.63
0.9942
0.3
101.67
0.9986
106.95
0.9992
0.4
110.48
0.9904
115.75
0.9933
0.5
110.24
0.9904
115.75
0.9933
0.6
124.98
0.9866
130.01
0.9914
0.7
124.81
0.9866
130.01
0.9914
0.8
124.64
0.9866
130.01
0.9914
0.9
137.25
0.9652
142.28
0.9744
Average
111.69
116.93
15
213
3.4. TG-FTIR analysis of pyrolysis products
214
The evolution of volatiles during the pyrolysis process can be examined online by
215
TG-FTIR. Based on the Beer-Lambert Law, the absorption peak intensity at the given
216
wavenumber is related to concentration of the substance, proportionally. Hence, the
217
varying absorbance is supposed to mark the evolved gas yields. From the 3D-IR
218
spectrum exhibited in Fig. 5 (a), the release of CO2 and H2O could be easily identified
219
as they corresponded to the visual peaks, and the steep shifts in the absorption intensity
220
were observed from 200 to 300
221
reaction and the dehydration reaction due to the reconstruction of hydroxyl, and the
222
hydrogen bonding in carboxyl during the thermal decomposition of carbohydrates. As
223
the pyrolysis temperature reached higher than 400
224
sharply with the increment of temperature, accompanied by the stable yield of H2O. The
225
appearance of the peak for CO, which was related to the decarbonylation reactions,
226
together with the strong absorption peak for C2+ aliphatics observed from the
227
temperature of 400
228
value for C2+ aliphatics at 450
229
optimum bio-oil yield. When the temperature was over 500
230
aliphatics and O-containing compounds such as aldehydes, alkanes, and alcohols
231
decreased considerably with regard to the weakened absorption intensity, whereas the
232
CO2 yield reached a higher level. The relevant results were consistent with the peak
. This could be explained by the decarboxylation
, the absorbance of CO2 increased
indicated the progress of lipid degradation. The maximum peak could be considered to estimate the temperature for the
16
, the yields of C2+
233
fitting of the TG curve and strengthened the understanding of the volatile release
234
scheme from T. fermentans biomass pyrolysis.
Fig. 5. IR spectra of volatiles at the pyrolysis temperature ranged in 150~600 temperature at 430
(a) and the pyrolysis
(b).
235
Fig.5 (b) shows the IR spectrum of the gaseous products at the temperature of
236
430 °C. The typical volatile species were identified including water (3950~3500 cm-1
237
and 1900~1300 cm-1), C2+ aliphatics (3000~2800 cm-1), carbon dioxide (2400~2270
238
cm-1 and 720~590 cm-1), carbon monoxide (2230~2030 cm-1), and O-containing organic
239
compounds (1900~1000 cm-1). Typically, O-containing compounds comprise the
240
aldehydes and acids (1900~1650 cm-1), ketones (1800~1650 cm-1 and 1400~1107 cm-1),
17
241
alkanes and ethers (1300~1200 cm-1), and alcohols (1150~1050 cm-1) [34].
242
3.5. Product distribution and bio-oil characterization
243
The effect of the pyrolysis temperature on the product distribution and the quality
244
of bio-oil from T. fermentans biomass pyrolysis was investigated in a fixed bed reactor.
245
Fig. 6 shows the product yield distributions at different pyrolysis temperatures.
Fig. 6. Yield distributions (mean ± SD) of end-products at different pyrolysis temperatures.
246
With an increase in temperature from 300 to 600
, a sharp reduction in the solid
247
yield was visible from 74.20 to 12.87% and there was an opposite trend for syngas from
248
4.88 to 53.06%. The monotonic trend could not be employed to represent the varied
249
liquid yield. With the rising temperature, the yield of liquid increased from 20.92 to
250
42.32%, which was followed by a decline to 34.07% at 600
251
achieved at 500
252
located somewhere between 450 and 550
253
pyrolysis in a fixed-bed reactor where the maximum liquid yield of the protein-rich
. The maximum yield was
experimentally, which indicates that the liquid yield peak would be . Referring to the reported data for algae
18
254
algae was about 40% while that of the fat-rich algae was close to 50% at 500
255
it was found that the lipid content may dominantly affect the liquid collection.
256
It should be noted that the yield of solid and syngas varied significantly from 400
257
where a large amount of solid was consumed, corresponding to the considerable growth
258
in the syngas yield and the gradual increase of liquid.
Fig. 7. The GC-TOF/MS total ion chromatogram of the bio-oil obtained at 500 and 600
259
[12, 35],
,
.
The chemical compounds of the bio-oil collected from the T. fermentans biomass
260
pyrolysis at 500 and 600
was determined by GC-TOF/MS, and the total ion
261
chromatogram of the bio-oil can be seen in Fig. 7. The carbon distribution range of the
262
pyrolysis oil was mainly between C2~C18, which was similar to other pyrolysis bio-oils
263
[36]. In the previous study, it was found that the main compounds of microbial oil
264
extracted from T. fermentans biomass were 22.9% palmitic acid, 35.3% linoleic acid,
265
34.1% oleic acid and 7.7% stearic acid [27]. Hence, aliphatic and alicyclic hydrocarbons,
266
aromatic hydrocarbons, acids, esters, phenols, furans and nitrogen compounds were
267
identified in the pyrolysis end-product, bio-oil, which makes it acidic and complex. A
19
268
quantitative analysis was carried out based on the area ratio defined by the percentage of
269
the compound’s chromatographic area over the total area (area percentage method) [32].
270
Compounds with a total area of less than 0.1% were not identified and the possible
271
compounds are listed in Table 3. Table 3 Main compounds identified in bio-oil by GC-TOF/MS. Area Percentage (%) Compound Types
Chrysophyceae [12] (500
)
T. fermentans biomass (500
T. fermentans )
biomass (600
)
Aliphatic and Alicyclic 11.98
11.45
4.31
Aromatic hydrocarbons
2.09
17.18
37.08
Nitrogen compounds (1-3)
40.11
13.56
18.01
1. Nitriles
1.98
1.86
6.49
2. Amines and Amides
0.17
4.27
3.54
3. N-heterocyclic compounds
37.96
7.43
7.98
Carboxylic acids
14.89
34.45
15.60
Ketones and Aldehyde
9.47
13.91
17.31
Alcohols
6.60
1.19
1.54
Furans
1.56
0.86
0.74
Phenols
8.55
3.62
4.57
Esters
0.33
2.87
0.32
Others
2.23
0.91
0.52
hydrocarbons
272
Hydrocarbons are considered to be valuable compounds in bio-oils, and lipids have
273
been proven to be a main source of hydrocarbons. Free fatty acids are typical
274
intermediate products of lipid pyrolysis. At moderate temperature, lipids are first
275
converted to free fatty acids and then converted to hydrocarbons by the decarboxylation
276
reaction [37]. As shown in Table 3, the yield of aliphatic and alicyclic hydrocarbons of
20
277
the T. fermentans biomass pyrolysis at 500
was nearly equal to that of Chrysophyceae
278
[12]. In theory, the T. fermentans biomass, which possesses a higher lipid content,
279
produces more hydrocarbons than that from Chrysophyceae, however, there was no
280
significant difference, which can be attributed to the aromatization of aliphatic
281
hydrocarbons [38]. The decrease in aliphatic hydrocarbons compensated for the increase
282
in aromatic hydrocarbons and the trend was further enhanced at high temperatures.
283
Hence, a higher content of aromatic hydrocarbons had been identified in the T.
284
fermentans biomass derived bio-oil. In the analysis and detection of bio-oil, there was a
285
certain content of free fatty acids, which can be due to incomplete secondary pyrolysis.
286
Additionally, with the increase in pyrolysis temperature from 500 to 600
287
depletion of carboxylic acids progressed while maintaining a high CO2 yield as shown
288
in Fig. 5(a) and the higher fractional yield of hydrocarbons at 600
289
reaction of acids and esters leads to the growth of light oxygenates and hydrocarbons.
, the further
. The decarboxylic
290
At a higher pyrolysis temperature, the fractional yield of nitrogen compounds is
291
enlarged. However, when compared with the algae pyrolysis oil, the content of nitrogen
292
compounds was relatively small, almost half [39]. The experimental results showed that
293
the bio-oil of the T. fermentans biomass contained high amides, which indicates that
294
high lipid content is more conducive to the protein fatty-acylation of proteins with lipids.
295
The content of nitriles increased with pyrolysis temperature, which can be attributed to
296
further reactions of amino acids and amides, and was consistent with findings in other
21
297
studies [40]. N-heterocycle compounds mainly include indole, piperidine, and pyridine
298
compounds, which were formed by the condensation of protein fragments, and the
299
content of N-heterocyclic compounds was relatively low due to the low protein content.
300
In consideration of the end-product yields shown in Fig. 6, it seems that the
301
depletion of the solid creates more gas and contributes less to the bio-oil yield. However,
302
there were differences in the detailed composition of the bio-oil as above-mentioned.
303
Therefore, the given pyrolysis temperature has to be carefully determined based on the
304
specific target product.
305
4. Conclusions
306
The kinetics and thermal behavior of T. fermentans biomass were studied by using
307
TGA and the calculated results indicated the lower thermal resistance of the T.
308
fermentans biomass than that of the traditional terrestrial biomass. The pyrolysis
309
behavior can be explained by degradation of four pseudo-components and the simulated
310
DTG curve was obtained with fitting degree over 0.99. The gas products evolved from
311
400
312
hydrocarbons meanwhile the maximum liquid yield of 42.32% was achieved at 500
313
The contents of hydrocarbons in the collected bio-oil were over 28% (500
314
(600
315
Amides, was beneficial to produce hydrocarbon based fuels.
316
Acknowledgements
included CO2, CO, H2O, O-containing compounds and an amount of .
) and 41%
) whilst the low content of the N-compounds such as Nitriles, Amines and
22
317
This work was supported by grants from the National Natural Science Foundation
318
of China (31470787), Science and Technology Research Project of Jilin Province, China
319
(20190902014TC, 20170519015JH) and National Key R&D Program of China
320
(NO.2018YFB1501405).
321
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Highlights Oleaginous yeast from wastewater treatment was a potential feedstock of pyrolysis. Activation energy of oleaginous yeast pyrolysis was estimated by KAS and FWO methods. The simulated DTG was obtained by Gaussian fitting method and R2 > 0.99 at 20 °C/min. The hydrocarbon content in the bio-oil of oleaginous yeast was greater than 28%.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0960-1481(20)30032-X
DOI:
https://doi.org/10.1016/j.renene.2020.01.028
Reference:
RENE 12890
To appear in:
Renewable Energy
Received Date: 4 June 2019 Revised Date:
29 November 2019
Accepted Date: 7 January 2020
Please cite this article as: Yu D, Hu S, Liu W, Wang X, Jiang H, Dong N, Pyrolysis of oleaginous yeast biomass from wastewater treatment: Kinetics analysis and biocrude characterization, Renewable Energy (2020), doi: https://doi.org/10.1016/j.renene.2020.01.028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Author Contribution Statement
DY and ND conceived and designed the experiments. DY, SH, WL, XW performed the experiments. DY, SH, HJ and ND analyzed data. DY, SH and ND wrote the paper. All authors read and approved the final manuscript.
Pyrolysis
Liquid Bio-oils obtained at different final temperatures (300-600°C) C H O N R
Recycling & Drying
TG-FTIR
Trichosporon fermentans biomass
Pyrolysis of oleaginous yeast biomass from wastewater treatment: kinetics analysis and biocrude characterization Dayu Yua, b, Shuang Hua, b, Weishan Liua, c, Xiaoning Wanga, b, Haifeng Jiangc, Nanhang Dong a, c, * a
Sci-Tech Center for Clean Conversion and High-valued Utilization of Biomass, Northeast Electric
Power University, Jilin 132012, China b
School of Chemical Engineering, Northeast Electric Power University, Jilin 132012, China
c
School of Energy and Power Engineering, Northeast Electric Power University, Jilin 132012, China
* Corresponding author. E-mail address: [email protected]
1
1
ABSTRACT
2
Pyrolysis of Trichosporon fermentans biomass, a type of oleaginous yeast from the
3
fermentation of refined soybean oil wastewater, was studied in the present work. Based
4
on the TG/DTG curves, the activation energy values for the thermal decomposition of
5
Trichosporon fermentans biomass were evaluated by the KAS method (111.69 kJ/mol)
6
and FWO method (116.93 kJ/mol), respectively. The pyrolysis behavior was represented
7
by considering the parallel degradation of four components, namely carbohydrates,
8
proteins, lipids, and others, and the fitting degree of the simulated curve was over 0.99.
9
According to the FTIR spectra of the end-products, the increasing hydrocarbons at a
10
temperature around 400
indicated the lipid degradation. Biocrude oil was collected
11
from a fixed bed reactor and the chemical composition was characterized by
12
GC-TOF/MS. The yield of hydrocarbons in the bio-oil was identified as being over 28%
13
when the pyrolysis temperature was higher than 500
14
compounds was less than 18%. The fractional yield distributions of the end-products at
15
different pyrolysis temperatures were compared and the maximum yield of bio-oil was
16
achieved (around 42%) at 500
17
environmentally friendly method for the preparation of biofuel from oleaginous yeast
18
pyrolysis.
19
Keywords: Pyrolysis; Oleaginous yeast; Trichosporon fermentans biomass; Bio-oil
20
characterization; Kinetics; Bio-oil yield
, whilst the content of nitrogen
. This study proposed a cost-effective and
2
21
1. Introduction
22
Oleaginous microorganisms such as algae, yeast, fungus, and bacterium with a lipid
23
accumulation of 20~25 wt% are considered to be promising feedstock to produce
24
biodiesel and other liquid hydrocarbon fuels [1-3]. The transesterification reaction is
25
always mentioned in order to produce biodiesel by oleaginous microorganisms where
26
microbial oil is extracted with an organic solvent ahead. Therefore, the large
27
consumption of organic reagents and the secondary wastewater pollution of oil
28
production should be of concern [4].
29
The thermochemical conversion methods including hydrothermal liquefaction and
30
pyrolysis are the optional techniques in which to transform oleaginous microorganisms
31
into liquid fuels. Studies on the hydrothermal liquefaction of microalgae have been
32
widely carried out with the main focus on the effect of temperature, pressure, and
33
dissolvent on the quality of the bio-oil [5, 6]. Meanwhile, the utilization of pyrolysis has
34
also attracted more and more interest due to its advantages of simple operation, low
35
equipment dependence and the direct utilization of end-products as biofuels or chemical
36
raw materials. Thermogravimetric Analysis (TGA) is generally used to analyze the
37
pyrolysis behavior of the microalgae biomass and to understand the degradation progress
38
during the heating process [7, 8]. While studying the thermal behavior, one or more
39
kinetic methods can be employed to calculate the kinetic parameters, such as the
40
activation energy value and pre-exponential factor [9-11]. Considering of its complex
41
components, the Derivative Thermogravimetric (DTG) curve of biomass pyrolysis can be
3
42
technically divided into several independent curves that simulate the degrading of
43
different components. Consequently the pyrolysis temperature range of each component
44
such as cellulose, hemicellulose, lignin, carbohydrates, proteins and lipids can be
45
predicted to obtain the better understanding of biomass pyrolysis behavior [12-14]. An
46
additional area of attention is given to the yield of pyrolysis products under different
47
experimental conditions. For instance, the pyrolysis of the Spirulina Sp. algae is studied
48
in a fixed-bed reactor, and the maximum yield of biochar and bio-oil is obtained at 500
49
and 550
50
distribution of the end-products is also studied and the experimental data show that the
51
liquid yield of algae bloom pyrolysis is the highest at 500
52
bio-oil is higher than that of pine sawdust derived bio-oil [16]. Analysis methods, such as
53
Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass
54
spectrometry (GC-MS) have also been widely introduced into the biomass pyrolysis
55
experiments. The spectrum analysis is considered to understand the volatilization process
56
during the thermal decomposition of the biomass, meanwhile the identification of the
57
bio-oil composition by GC-MS can be applied to evaluate the potential of products for
58
direct utilization or upgrading [17]. The pyrolysis behavior of different algae biomass is
59
studied by the pyrolysis-GC/MS experiment and high value chemicals such as xylene,
60
styrene, phenol, and toluene as well as a large number of nitrogen-containing compounds
61
are identified in the end-products [18, 19].
, individually [15]. The effect of the pyrolysis temperature on the yield
4
, and the calorific value of
62
Oleaginous yeasts cultivated by accumulating lipids can grow on non-traditional
63
substrates such as food wastewater [20], waste oil [21] and crude glycerin [22]; moreover,
64
a variety of wastewater can be used as a carbon source in the accumulation of microbial
65
oil [23]. Compared with algae biomass, the cultivation of oleaginous yeast is not affected
66
by light intensity and photoperiod, and the growth cycle is around 40 hours. Currently, a
67
series of studies on the hydrothermal liquefaction of oleaginous yeast is being carried out
68
[24, 25], but work on pyrolysis has been rarely performed yet. The limited reports on
69
oleaginous yeast pyrolysis indicated that the exothermic activity of the microorganisms
70
based on TGA can be considered to identify the presence of the lipid content [26].
71
In summary, extensive work has been carried out on the pyrolysis of algae biomass
72
including research on the analysis of pyrolysis behavior, pyrolysis kinetics, biocrude oil
73
quality, but the pyrolysis of oleaginous yeast has not been mentioned in depth up until
74
now. In the present work, the pyrolysis process of T. fermentans biomass, a kind of
75
oleaginous yeast, was studied systematically as follows: The pyrolysis process of T.
76
fermentans biomass was analyzed by TGA and the Gaussian fitting, and the thermal
77
behavior was simulated based on the pseudo components; the pyrolysis kinetic
78
parameters were calculated to evaluate the thermal resistance; the pyrolysis gas release
79
mechanism was explained by TG-FTIR and the optimum liquid yield of T. fermentans
80
biomass at different final temperatures was determined; in addition, the contents of the
81
hydrocarbons and nitrogen compounds, etc. were analyzed by GC/MS in order to fully
5
82
understand the distribution of the bio-oil composition produced by T. fermentans biomass
83
pyrolysis. All of the above-mentioned work will provide a theoretical basis for the
84
preparation of biofuels by oleaginous yeast pyrolysis.
85
2. Material and methods
86
2.1. Sample preparation and analysis
87
T. fermentans biomass was collected from the purification treatment of refined
88
soybean oil wastewater [27] and sieved into the size range of 150~180 µm. The samples
89
were dried at 105
90
analysis for the samples was carried out by an automatic proximate analyzer (Sundy
91
SDLA718, China) and the ultimate analysis was performed with a fully automatic
92
element analyzer (EuroVector EA 3000, Italy). The elements C, H, N, and S were
93
determined directly and the content of the element O was calculated by difference. The
94
main components of the sample such as carbohydrates, proteins, and lipids were
95
determined through the phenol-sulfuric acid method, the Kjeldahl method, and
96
acid-heating extraction.
97
2.2. TG-FTIR analysis
for 12 hours, and then sealed in sample bags. The proximate
98
TGA was conducted by using a thermogravimetric analyzer (PerkinElmer
99
TGA8000, US) to investigate the sample thermal degradation. The samples were heated
100
under a non-isothermal condition from room temperature to 800
101
atmosphere. The different heating rates of 10, 20, 30, 40, and 50
6
in a nitrogen
/min were applied to
102 103
calculate the kinetic parameters and the TG/DTG curves were plotted. At the heating rate of 20
/min, the decomposition of the sample progressed and
104
the gaseous end-product was analyzed online by TG-FTIR. The infrared synchronous
105
acquisition data were triggered in the temperature range of 105 to 600
106
residual rate was almost constant, and the infrared scanning wave number ranged from
107
4000 to 400 cm-1.
108
2.3. Kinetic analysis
109 110
Based on the TG experiment, the mass loss of sample was recorded and the conversion rate,
⁄ , can be expressed as a function of temperature [28]: =
1
111
where
112
function. The conversion, α, is written as [29]:
is the temperature-dependent rate constant and
= where
114
the final residue of sample in the reaction.
is the initial sample mass;
2
is the sample mass at time t; and
is
By coupling the Arrhenius equation, Eq. (1) can be rearranged as: =
116
where
117
gas constant, and
118
is the conversion
− −
113
115
when the solid
is the pre-exponential factor,
−
3 is the activation energy,
is equal to the heating rate,
/
The integrated form of Eq. (3) is introduced as:
7
.
is the universal
#
=$
%
= $
&
4
−
&'
119
The iso-conversional method which can be applied to multistep reactions is
120
supposed to obtain more reliable activation energy as no kinetic model function is
121
involved. The Kissinger-Akahira-Sunose (KAS) and the Flynn-Wall-Ozawa (FWO)
122
methods are widely used in the calculation of activation energy. Based on the two
123
methods, Eq. (4) can be introduced as follows, respectively: ln log
124
+
= log ,
-−
5
- − 2.315 − 0.4567
6
= ln , #
#
According to Eq. (5) or (6), the relation of
vs. T can be plotted as
125
With different heating rates, a straight line is derived, and then
126
2.4. Experimental procedure
is fixed.
can be calculated.
127
The pyrolysis experiments were carried out in a system consisting of a fixed bed
128
reactor, as shown in Fig. 1. For each run, the furnace was preheated to the set-point
129
temperature after a 20-min sweep of nitrogen. Samples weighing 0.5 g were loaded and
130
held for 30 minutes, which was sufficient to achieve complete decomposition. The
131
vapors were collected in a three-stage condensation facility including a cold bath, an ice
132
water bath, and an ice salt bath. Meanwhile, the syngas was washed by the tail gas
133
absorption bottle. The liquid yield was estimated by the total weight of bio-oil in the
134
condensing facility. The solid residue was collected and weighed until the furnace had
135
cooled down to room temperature by sweeping with cold nitrogen. The yield of the
8
136
syngas was determined by difference. The experiments were conducted at least three
137
times for each run to confirm the reproducibility, and the presented data are the mean
138
values ± standard deviations (SD).
139 140
The yields of the end-products were calculated according to the following equations: 56 =
6
5 = 5: = 141
where
142
liquid bio-oil; and
−
is the mass of feedstock; :
6
6
7 100%
7
7 100%
8
−
7 100%
is the mass of solid residue;
9 is the mass of
is the mass of the syngas.
Fig. 1. Schematic diagram of the pyrolysis system with a fixed bed reactor.
143
2.5. Bio-oil analysis
144
Bio-oil from the decomposition of T. fermentans biomass was analyzed by a GC
145
unit (Agilent 7890) and a LECO Pegasus 4D time of flight mass spectrometer
146
(TOF/MS). The GC oven was preheated at 40
9
, held for 3 min, and followed by
147
ramping to 300
at 5
/min. A total of 1.0 µL of bio-oil with an acetone solvent was
148
injected by the helium carrier gas. Full scanning mode was applied by the mass
149
spectrometer with the scanning range of 35~500 amu and the ionizing voltage was 70
150
eV. The ion source temperature was 280
151
the chromatograms were confirmed by the NIST 11 standard library of mass
152
spectrometry.
153
3. Results and Discussion
154
3.1. Feedstock characterization
. The compounds identified by the peaks in
155
The proximate and ultimate analyses and the chemical components of T.
156
fermentans biomass were listed and compared with two typical algae biomasses
157
reported in Refs. [12, 30]. As shown in Table 1, the T. fermentans biomass had the
158
highest content of volatile matter and the lowest content of fixed carbon. The ultimate
159
analysis exhibited the highest contents of carbon and hydrogen and the lowest content
160
of oxygen, nitrogen, and sulfur. The main components of the T. fermentans biomass
161
were comprised of carbohydrates, proteins, and lipids. Due to the different liquefaction
162
abilities of carbohydrates, proteins, and lipids [12], the T. fermentans biomass should
163
produce more liquid fuel than those from each of the listed algae biomass.
10
Table 1 Proximate and ultimate analyses and components of the different biomasses. T. fermentans biomass
Spirulina [12]
Chrysophyceae [12, 30]
Moisture
0
0
1.69
Volatile matter
90.48
75.55
79.79
Fixed carbon
6.94
16.39
11.63
Ash
2.57
8.06
6.89
C
62.10
49.14
49.26
H
9.72
6.68
7.5
O
22.8*
28.58
31.74
N
2.41
11.19
6.24
S
0.40
0.83
0.96
Carbohydrates
19.9
68.4*
15.2*
Proteins
15.1
23.44
35.9
Lipids
53.1
0.1
42.9
Others
9.33*
0
0
Proximate analysis (wt%)
Ultimate analysis (wt%)
Chemical composition analysis (wt%)
* by difference
164
3.2. Thermal degradation characteristics
165
The pyrolysis of the T. fermentans biomass was studied by TGA under
166
non-isothermal conditions. Fig. 2 shows the TG/DTG curves of T. fermentans biomass
167
pyrolysis at the different heating rates of 10, 20, 30, 40, and 50
168
lateral shifts observed in Fig. 2 (a) could be attributed to the thermal lag while the
169
increased maximum mass loss rate referred to the improved reaction rate under a high
170
heating rate. According to the pyrolysis process, three main stages were identified
171
including the water and light volatile release stage, the main pyrolysis stage, and the
11
/min. The distinct
172
carbonation stage.
Fig. 2. TG (a) and DTG (b) curves for T. fermentans biomass pyrolysis at different heating rates.
173
Fig. 3 shows the DTG curve with respect to the heating rate of 20
/min. In the
174
initial stage (~160
), the release of light volatiles led to the slight mass loss.
175
Subsequently, two overlapping peaks could be observed in the following stage
176
(160~500
177
parallel reactions. Two peaks at about 323 and 434
178
and 374
179
shoulder, respectively. A low mass loss rate in the final stage (500
180
slow decomposition of carbonaceous substances. The DTG curve pattern mainly
), where a large amount of organic matter was rapidly decomposed through , and two shoulders at about 282
were described as the first peak, second peak, first shoulder, and second
12
~) revealed the
181
indicated the different thermal resistance of the carbohydrates, proteins, lipids, and
182
others [31].
Fig. 3. Peak fitting of DTG curve for T. fermentans biomass pyrolysis at a heating rate of 20
/min.
183
Gaussian fitting was employed to perform the peak separation of the DTG curve in
184
Fig. 3. As reported in Ref. [13], the reaction stage of algae pyrolysis can be divided into
185
two, three, four, and seven sections, corresponding to the presumption of the
186
pseudo-component number. It was found that the four-reaction models were fitted and
187
close to the results of the component analysis. Hence, four fitting peaks were introduced
188
to represent the degradation of carbohydrates, proteins, lipids, and others here. The
189
fitting coefficient (
190
around 0.9952, which indicates that the four-reaction regime was acceptable. The fitting
191
peak at a temperature around 320 ℃ describes the maximum mass loss of carbohydrates
192
in the temperature range of 235 and 404 ℃. The curve with the fitting peak at a
193
temperature around 390 ℃ demonstrates the decomposition of proteins. Lipids are
194
supposed to decompose from 386 to 485 ℃ and the mass loss peak was located over
+
between the simulated curve and experimental DTG curve was
13
195
435
196
components [32], the variation can be attributed to the different proportions of
197
components and that the synergy effect caused variations in the pyrolysis characteristics.
198
In other words, the thermal degradation of T. fermentans biomass can be properly
199
interpreted based on the three main components.
200
3.3. Pyrolysis kinetic parameters
201
. Compared with the reported degradation temperature range of the three
The kinetic analysis of the pyrolysis process of T. fermentans biomass (105~600
)
202
was carried out by the KAS and FWO methods, respectively. The fitted lines at different
203
conversions in the range of 0.1 to 0.9 were derived by the plots of ln ( /T2) vs. 1/T in
204
Fig. 4 (a) and log ( ) vs. 1/T in Fig. 4 (b). The activation energy values calculated by
205
the KAS and FWO methods were listed in Table 2, together with the corresponding
206
fitting coefficients ranging from 0.9652 to 0.9992 for the different conversions. The
207
average activation energy values were 111.69 and 116.93 kJ/mol by the KAS and FWO
208
methods, respectively.
209
In comparison with the terrestrial biomasses of peanut shell and pine needle [33],
210
the relatively low value for the activation energy of the T. fermentans biomass
211
determined is close to that of algae biomass [10], which should be attributed to the
212
combination of high volatile matter and similar chemical composition to algae.
14
Fig. 4. Fitting curves for kinetic analysis of T. fermentans biomass pyrolysis by KAS method (a) and FWO method (b). Table 2 Activation energy values by KAS and FWO methods. KAS method
FWO method
α
E (kJ/mol)
R2
E (kJ/mol)
R2
0.1
78.67
0.9976
83.96
0.9984
0.2
92.43
0.9903
97.63
0.9942
0.3
101.67
0.9986
106.95
0.9992
0.4
110.48
0.9904
115.75
0.9933
0.5
110.24
0.9904
115.75
0.9933
0.6
124.98
0.9866
130.01
0.9914
0.7
124.81
0.9866
130.01
0.9914
0.8
124.64
0.9866
130.01
0.9914
0.9
137.25
0.9652
142.28
0.9744
Average
111.69
116.93
15
213
3.4. TG-FTIR analysis of pyrolysis products
214
The evolution of volatiles during the pyrolysis process can be examined online by
215
TG-FTIR. Based on the Beer-Lambert Law, the absorption peak intensity at the given
216
wavenumber is related to concentration of the substance, proportionally. Hence, the
217
varying absorbance is supposed to mark the evolved gas yields. From the 3D-IR
218
spectrum exhibited in Fig. 5 (a), the release of CO2 and H2O could be easily identified
219
as they corresponded to the visual peaks, and the steep shifts in the absorption intensity
220
were observed from 200 to 300
221
reaction and the dehydration reaction due to the reconstruction of hydroxyl, and the
222
hydrogen bonding in carboxyl during the thermal decomposition of carbohydrates. As
223
the pyrolysis temperature reached higher than 400
224
sharply with the increment of temperature, accompanied by the stable yield of H2O. The
225
appearance of the peak for CO, which was related to the decarbonylation reactions,
226
together with the strong absorption peak for C2+ aliphatics observed from the
227
temperature of 400
228
value for C2+ aliphatics at 450
229
optimum bio-oil yield. When the temperature was over 500
230
aliphatics and O-containing compounds such as aldehydes, alkanes, and alcohols
231
decreased considerably with regard to the weakened absorption intensity, whereas the
232
CO2 yield reached a higher level. The relevant results were consistent with the peak
. This could be explained by the decarboxylation
, the absorbance of CO2 increased
indicated the progress of lipid degradation. The maximum peak could be considered to estimate the temperature for the
16
, the yields of C2+
233
fitting of the TG curve and strengthened the understanding of the volatile release
234
scheme from T. fermentans biomass pyrolysis.
Fig. 5. IR spectra of volatiles at the pyrolysis temperature ranged in 150~600 temperature at 430
(a) and the pyrolysis
(b).
235
Fig.5 (b) shows the IR spectrum of the gaseous products at the temperature of
236
430 °C. The typical volatile species were identified including water (3950~3500 cm-1
237
and 1900~1300 cm-1), C2+ aliphatics (3000~2800 cm-1), carbon dioxide (2400~2270
238
cm-1 and 720~590 cm-1), carbon monoxide (2230~2030 cm-1), and O-containing organic
239
compounds (1900~1000 cm-1). Typically, O-containing compounds comprise the
240
aldehydes and acids (1900~1650 cm-1), ketones (1800~1650 cm-1 and 1400~1107 cm-1),
17
241
alkanes and ethers (1300~1200 cm-1), and alcohols (1150~1050 cm-1) [34].
242
3.5. Product distribution and bio-oil characterization
243
The effect of the pyrolysis temperature on the product distribution and the quality
244
of bio-oil from T. fermentans biomass pyrolysis was investigated in a fixed bed reactor.
245
Fig. 6 shows the product yield distributions at different pyrolysis temperatures.
Fig. 6. Yield distributions (mean ± SD) of end-products at different pyrolysis temperatures.
246
With an increase in temperature from 300 to 600
, a sharp reduction in the solid
247
yield was visible from 74.20 to 12.87% and there was an opposite trend for syngas from
248
4.88 to 53.06%. The monotonic trend could not be employed to represent the varied
249
liquid yield. With the rising temperature, the yield of liquid increased from 20.92 to
250
42.32%, which was followed by a decline to 34.07% at 600
251
achieved at 500
252
located somewhere between 450 and 550
253
pyrolysis in a fixed-bed reactor where the maximum liquid yield of the protein-rich
. The maximum yield was
experimentally, which indicates that the liquid yield peak would be . Referring to the reported data for algae
18
254
algae was about 40% while that of the fat-rich algae was close to 50% at 500
255
it was found that the lipid content may dominantly affect the liquid collection.
256
It should be noted that the yield of solid and syngas varied significantly from 400
257
where a large amount of solid was consumed, corresponding to the considerable growth
258
in the syngas yield and the gradual increase of liquid.
Fig. 7. The GC-TOF/MS total ion chromatogram of the bio-oil obtained at 500 and 600
259
[12, 35],
,
.
The chemical compounds of the bio-oil collected from the T. fermentans biomass
260
pyrolysis at 500 and 600
was determined by GC-TOF/MS, and the total ion
261
chromatogram of the bio-oil can be seen in Fig. 7. The carbon distribution range of the
262
pyrolysis oil was mainly between C2~C18, which was similar to other pyrolysis bio-oils
263
[36]. In the previous study, it was found that the main compounds of microbial oil
264
extracted from T. fermentans biomass were 22.9% palmitic acid, 35.3% linoleic acid,
265
34.1% oleic acid and 7.7% stearic acid [27]. Hence, aliphatic and alicyclic hydrocarbons,
266
aromatic hydrocarbons, acids, esters, phenols, furans and nitrogen compounds were
267
identified in the pyrolysis end-product, bio-oil, which makes it acidic and complex. A
19
268
quantitative analysis was carried out based on the area ratio defined by the percentage of
269
the compound’s chromatographic area over the total area (area percentage method) [32].
270
Compounds with a total area of less than 0.1% were not identified and the possible
271
compounds are listed in Table 3. Table 3 Main compounds identified in bio-oil by GC-TOF/MS. Area Percentage (%) Compound Types
Chrysophyceae [12] (500
)
T. fermentans biomass (500
T. fermentans )
biomass (600
)
Aliphatic and Alicyclic 11.98
11.45
4.31
Aromatic hydrocarbons
2.09
17.18
37.08
Nitrogen compounds (1-3)
40.11
13.56
18.01
1. Nitriles
1.98
1.86
6.49
2. Amines and Amides
0.17
4.27
3.54
3. N-heterocyclic compounds
37.96
7.43
7.98
Carboxylic acids
14.89
34.45
15.60
Ketones and Aldehyde
9.47
13.91
17.31
Alcohols
6.60
1.19
1.54
Furans
1.56
0.86
0.74
Phenols
8.55
3.62
4.57
Esters
0.33
2.87
0.32
Others
2.23
0.91
0.52
hydrocarbons
272
Hydrocarbons are considered to be valuable compounds in bio-oils, and lipids have
273
been proven to be a main source of hydrocarbons. Free fatty acids are typical
274
intermediate products of lipid pyrolysis. At moderate temperature, lipids are first
275
converted to free fatty acids and then converted to hydrocarbons by the decarboxylation
276
reaction [37]. As shown in Table 3, the yield of aliphatic and alicyclic hydrocarbons of
20
277
the T. fermentans biomass pyrolysis at 500
was nearly equal to that of Chrysophyceae
278
[12]. In theory, the T. fermentans biomass, which possesses a higher lipid content,
279
produces more hydrocarbons than that from Chrysophyceae, however, there was no
280
significant difference, which can be attributed to the aromatization of aliphatic
281
hydrocarbons [38]. The decrease in aliphatic hydrocarbons compensated for the increase
282
in aromatic hydrocarbons and the trend was further enhanced at high temperatures.
283
Hence, a higher content of aromatic hydrocarbons had been identified in the T.
284
fermentans biomass derived bio-oil. In the analysis and detection of bio-oil, there was a
285
certain content of free fatty acids, which can be due to incomplete secondary pyrolysis.
286
Additionally, with the increase in pyrolysis temperature from 500 to 600
287
depletion of carboxylic acids progressed while maintaining a high CO2 yield as shown
288
in Fig. 5(a) and the higher fractional yield of hydrocarbons at 600
289
reaction of acids and esters leads to the growth of light oxygenates and hydrocarbons.
, the further
. The decarboxylic
290
At a higher pyrolysis temperature, the fractional yield of nitrogen compounds is
291
enlarged. However, when compared with the algae pyrolysis oil, the content of nitrogen
292
compounds was relatively small, almost half [39]. The experimental results showed that
293
the bio-oil of the T. fermentans biomass contained high amides, which indicates that
294
high lipid content is more conducive to the protein fatty-acylation of proteins with lipids.
295
The content of nitriles increased with pyrolysis temperature, which can be attributed to
296
further reactions of amino acids and amides, and was consistent with findings in other
21
297
studies [40]. N-heterocycle compounds mainly include indole, piperidine, and pyridine
298
compounds, which were formed by the condensation of protein fragments, and the
299
content of N-heterocyclic compounds was relatively low due to the low protein content.
300
In consideration of the end-product yields shown in Fig. 6, it seems that the
301
depletion of the solid creates more gas and contributes less to the bio-oil yield. However,
302
there were differences in the detailed composition of the bio-oil as above-mentioned.
303
Therefore, the given pyrolysis temperature has to be carefully determined based on the
304
specific target product.
305
4. Conclusions
306
The kinetics and thermal behavior of T. fermentans biomass were studied by using
307
TGA and the calculated results indicated the lower thermal resistance of the T.
308
fermentans biomass than that of the traditional terrestrial biomass. The pyrolysis
309
behavior can be explained by degradation of four pseudo-components and the simulated
310
DTG curve was obtained with fitting degree over 0.99. The gas products evolved from
311
400
312
hydrocarbons meanwhile the maximum liquid yield of 42.32% was achieved at 500
313
The contents of hydrocarbons in the collected bio-oil were over 28% (500
314
(600
315
Amides, was beneficial to produce hydrocarbon based fuels.
316
Acknowledgements
included CO2, CO, H2O, O-containing compounds and an amount of .
) and 41%
) whilst the low content of the N-compounds such as Nitriles, Amines and
22
317
This work was supported by grants from the National Natural Science Foundation
318
of China (31470787), Science and Technology Research Project of Jilin Province, China
319
(20190902014TC, 20170519015JH) and National Key R&D Program of China
320
(NO.2018YFB1501405).
321
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Highlights Oleaginous yeast from wastewater treatment was a potential feedstock of pyrolysis. Activation energy of oleaginous yeast pyrolysis was estimated by KAS and FWO methods. The simulated DTG was obtained by Gaussian fitting method and R2 > 0.99 at 20 °C/min. The hydrocarbon content in the bio-oil of oleaginous yeast was greater than 28%.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: