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Microwave-assisted co-pyrolysis of Chlorella vulgaris and wood sawdust using different additives
Accepted Manuscript Microwave-assisted co-pyrolysis of Chlorella vulgaris and wood sawdust using different additives Lin Chen, Zhaosheng Yu, Hao Xu, Kuangyu Wan, Yanfen Liao, Xiaoqian Ma PII: DOI: Reference:
S0960-8524(18)31518-9 https://doi.org/10.1016/j.biortech.2018.10.086 BITE 20650
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
12 September 2018 29 October 2018 30 October 2018
Please cite this article as: Chen, L., Yu, Z., Xu, H., Wan, K., Liao, Y., Ma, X., Microwave-assisted co-pyrolysis of Chlorella vulgaris and wood sawdust using different additives, Bioresource Technology (2018), doi: https://doi.org/ 10.1016/j.biortech.2018.10.086
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Microwave-assisted co-pyrolysis of Chlorella vulgaris and wood sawdust using different additives Lin Chena,b, Zhaosheng Yua,b *, Hao Xua,b, Kuangyu Wan a,b, Yanfen Liaoa,b, Xiaoqian Maa,b a School
of Electric Power, South China University of Technology, 510640,
Guangzhou, China b
Guangdong Province Key Laboratory of Efficient and Clean Energy
Utilization, 510640, Guangzhou, China Abstract The microwave-assisted co-pyrolysis of Chlorella vulgaris (CV), wood sawdust (WS) and their blends with additives were investigated. There was a higher liquid and solid yield with silicon carbide (SiC) than activated carbon (AC) in most of samples. Microwave-assisted pyrolysis with additives behaved a positive effect on deoxygenation and aromatization, but not apparent denitrification. With the increase of CV proportion, aromatic hydrocarbons decreased, but aliphatic hydrocarbons increased using AC. High selectivity of phenols was reached at the sample of WS (relative content as 43.6%) using SiC; High selectivity of alkenes was reached at the sample of CV (relative content as 31.2%) and alkanes at the blend sample of 70% CV and 30% WS (relative content as 9.45%). Bio-oil and biochar from microwaveassisted pyrolysis of WS had higher calorific value than that of CV both with AC and
*
Corresponding author at: School of Electric Power, South China University of Technology, 510640
Guangzhou, China. Email: [email protected] (Z. Yu).
SiC. Calorific value of bio-oil decreased by 33.3% after mixing CV with WS. Keywords: Co-pyrolysis; Chlorella vulgaris; Sawdust; Microwave-assisted; Additives
1
2
1
Introduction
In pace with fossil fuel depletion aggravation, the utilization of renewable energy
3
has been a hot spot for research, and urgently needs to evolve. Compared to many
4
alternative energy resources such as wind power, solar power, tidal energy and hydro-
5
energy, biomass energy is the only one that can be conveyed and stored, and also the
6
oldest energy used by human beings. Moreover, biomass is the only sustainable
7
resource of organic carbon capable of producing petroleum-like oils and chemicals
8
(Yang et al., 2017). There are numerous methods for the conversion of biomass to
9
surrogate fuel and bio-chemical products, and they can be generally divided as
10
chemical methods, thermochemical methods and biochemical methods. Microwave-
11
assisted pyrolysis (MAP) is one of the most high-profile thermochemical process
12
under development in current years, because microwave heating owns the features of
13
volumetric heating, a fast heating rate, specified calefaction targets and automatic
14
control. MAP of biomass requires lower pyrolysis energy input and less average
15
activation energy than conventional pyrolysis, where heat conveys inefficiently from
16
exterior to interior. Luo et al. (Luo et al., 2017) found MAP of wood sawdust,
17
obtained more than 50% of liquid products, in which the yield of 78.7% was phenolic
18
chemicals at a 50-100°C lower temperature and 50-100 kJ/mol lower average
19
activation energy than that of conventional pyrolysis. Moreover, the MAP device is
20
less expensive due to its ability of rapid heating compared with conventional heating
21
devices (Wang et al., 2016).
22
Ordinary biomass usually has poor dielectric properties resulting in limited
23
valuable products during MAP (Namazi et al., 2015). While MAP with additives, the
24
yields and properties of the final products change significantly. For example, MAP of
25
lignin and low-density polyethylene with additives HZSM-5 and magnesium oxide
26
represented a higher bio-oil quantity and quality, the addition of HZSM-5 improved
27
the aromatization of chain hydrocarbons, and magnesium oxide improved the
28
selectivity of alkylated phenols (Fan et al., 2017). MAP of wood sawdust with
29
different additives resulted in significant effects on final products, by using silica
30
carbon decreased the gas and liquid yields, using potassium carbonate and sodium
31
hydrate increased gas yield strongly (Shang et al., 2015b). Generally, additives for
32
MAP participate in reaction not only as absorbents to enhance heating but also as
33
catalysts to improve the reaction rate, differentiate the reaction path and thus decrease
34
the energy consumption. Therefore, in order to improve the target valuable products
35
in biomass microwave-assisted heating process, additives are essential. Additives for
36
microwave can be categorized as carbon-based materials, metal oxides and zeolites. It
37
has been proved that the carbon-based materials are very suitable absorbents of
38
microwaves, which had a remarkable effect (Fang et al., 2018; Wang et al., 2016).
39
Two of the most commonly used carbon-based additives, activated carbon (AC) and
40
silicon carbide (SiC) were applied in this work. More studies about AC or SiC used in
41
MAP could be found in other literature and experiments (Borges et al., 2014a; Li et
42
al., 2016).
43
Chlorella vulgaris (CV) is a typical kind of algae biomass with widespread
44
growth globally. The main compositions of microalgae are lipids, proteins,
45
carbohydrates, inorganic elements (Fe, Cu, Mn, Zn, Se, etc.), vitamins as well as folic
46
acid. Some of the lipids proportion can even reach up to 50-55 wt.% on dry basis
47
(Zhang et al., 2017). On the other hand, algal biomass as the third-generation
48
sustainable biomass has its own predominant growth behavior such as fast growth
49
rate, high CO2 capture capacity, no need for arable land and fairly simple cultivation
50
conditions (Chen et al., 2018a; Maliutina et al., 2018). Therefore, it is a promising
51
feedstock for value-added chemical byproducts and/or biofuel. Wood sawdust (WS) is
52
a typical kind of lignocellulosic biomass, cellulose is its primary component (23.7-
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61.7 wt. %), followed by hemicellulose and lignin (Zhang et al., 2017). Previous
54
researches found cellulose and hemicellulose could produce more bio-oil during
55
pyrolysis than lignin (Zhao et al., 2016). Moreover, mass-produce of sawdust from
56
agroforestry make it easier for industrial production (Kumar et al., 2017). Hence, CV
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and WS were chose as typical biomass feedstock for MAP in this work.
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Considerable researches have been focused on co-pyrolysis of biomass, such as
59
co-pyrolysis of rice straw and polyethylene (Xiang et al., 2018), lignocellulosic
60
biomass and coal (An et al., 2017; Tchapda et al., 2017) and microalgae and scum
61
(Xie et al., 2015). While few aims at co-pyrolysis of microalgae and wood sawdust,
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which probably has a synergistic effect during the heating process, result in
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improvement of useful products. This paper studied MAP of CV, WS and their blends
64
using AC or SiC as the additive, trying to figure out an optimal MAP conditions to
65
obtain value-added chemicals or high caloric value products.
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2.1 Materials
Materials and methods
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Chlorella vulgaris (CV) in powder form was bought from SCIPHAR Natural
69
Products Co. Ltd. (Shaanxi Province, China) with particle size less than 178 μm. Wood
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sawdust (WS) was acquired from local wood processing plant in state of powder (＜
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200 μm). After air forced dried at 105°C until the weight was steady, all sample
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powders were stored in desiccator on standby.
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The additives, activated carbon (AC) and silicon carbide (SiC), were purchased
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from Guangzhou Cong Yuan Instrument Co. Ltd. (Guangdong Province, China). AC
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and SiC were all ground into 80 mesh, and correspond to standard with analytical
76
reagent (purity≥98.0%).
77
All samples and additives were measured according to ASTM D3176 for carbon
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(C), hydrogen (H), nitrogen (N), and sulfur (S) by Vario El Cube Elemental Analyzer
79
(Elementar Co. Ltd., German). Oxygen (O) content was calculated by difference.
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Moisture, ash, volatile matter and fixed carbon were obtained by drier (DHG-9070A,
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Shanghai Qixin Science Instruments Co. Ltd., China) and Ash Tester (MF-2000,
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Hegang Haotian Electrical Co. Ltd., China) according to GB/T212-2008 and GB211-
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84. Higher calorific value (HHV) was tested by Oxygen Bomb Calorimeter (TE-C610,
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Changsha Titen Electronic Co. Ltd., China) according to ASTM D5865. Dielectric
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properties of parent samples and additives were interpreted by dielectric loss tangent,
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tanδ, using Eq. 1 (Sun et al., 2016):
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tanδ =
𝜀'' 𝜀'
(1)
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Where ε'' is the dielectric loss factor, which indicates microwave dissipation
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efficiency of materials. ε' is dielectric coefficient, which represents microwave
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absorption intensity of materials. There is no explicit relationship between ε' and ε'',
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while tanδ is a rough estimate for the ability of converting electromagnetic energy into
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heat. The higher tanδ is, the better capacity of microwave absorption for materials
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becomes. Basic characteristics of biomass feedstock and additives were listed in Table.
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1. Blending ratios of CV and WS were set up 5 levels by mass percentage, i.e. 100%CV,
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70%CV with 30%WS, 50%CV with 50%WS, 30%CV with 70%WS, and 100%WS,
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abbreviated as CV, 70CV30WS, 50CV50WS, 30CV70WS and WS, hereafter.
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Blending ratio of parent samples and microwave additive was 10:1 by mass percentage.
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2.2 Experimental apparatus and procedure
99
The co-pyrolysis experiments were carried out on a self-built microwave-assisted
100
pyrolysis (MAP) platform, as shown in Fig. 1. This platform consists of four parts:
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Microwave occurrence system, material reaction system, product collection system
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and data acquisition system. The microwave rated power was 1000 W, 2.45 GHz
103
uniformly in this work. The temperature sensor used in this experiment was 4 mm
104
diameter K-type thermocouple with the thermometric range between 0-1600°C,
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temperature precision of ±1.5°C. Magnetrons were uniformly distributed as shown in
106
Fig. 1 in order to ensure the homogeneous microwave distribution during the reaction.
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30.00±0.01 g of blending samples with 3.00±0.01 g AC or SiC were fully mixed and
108
placed in the clean and dry quartz flask. After checking out the sealing property of gas
109
flow unit, high purity N2 as an inert carrier gas was introduced to the flask
110
consistently for 10 min with a flow rate of 300 mL/min, in order to vent detained air.
111
Afterwards, switching on microwave heating system for 25 min. Then, microwave
112
heating system was switched off and the reaction chamber was naturally air-cooled to
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room temperature with uninterrupted N2 atmosphere. Finally, solid residue was
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remained in the flask. Condensable gas was cooled by chill water (5°C) and collected
115
as bio-oil in conical bottles. Incondensable gas was obtained by difference on the
116
basis of mass equilibrium. Three phase products under different blending ratios and
117
additives were weighed up and calculated. Micro positive pressure in gas flow unit
118
was used during the whole MAP process. Each MAP experiment was carried out
119
twice. Standard deviation of two batches were presented in the form of the error bar.
120
2.3 Analytical methods
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The acquired liquid content was immediately diluted by dichloromethane solvent
122
to a proper concentration. Processed liquid products (designated as bio-oils hereafter)
123
were examined by the Gas Chromatography-Mass Spectrometry (GC-MS) (Agilent
124
7890B & 5977A, Agilent Technologies Inc. U.S.A). The metal capillary column type
125
was HP-5ms, 30 m×0.25 mm ID×0.25 μm. The oven temperature was set from 50°C
126
(held for 1 min) to 260°C (held for 5 min), at a heating rate of 5 °C/min. The injector
127
temperature was maintained at 290°C and the injection volume was 1 μL using a split
128
ratio of 1:50. The carrier gas (high pure helium) rate was 1 mL/min. The scan range of
129
the quadrupole mass spectrometer was between 50 and 500 m/z. The chemical
130
compounds in bio-oils were identified by NIST data library and relevant literature. The Higher calorific value (HHV) of bio-oils (i.e. liquid products after
131 132
dehydrated) and residues (i.e. solid products after fully ground) were tested by
133
Oxygen Bomb Calorimeter (TE-C610, Changsha Titen Electronic Co., Ltd, China)
134
according to ASTM D5865.
135
3
136
3.1 Temperature profiles of additives
137
Results and discussion
2.45 GHz frequency of microwave power, corresponding to 12.5 cm wavelength
138
is the conventional set value for microwave-assisted pyrolysis (MAP) of biomass
139
(Antunes et al., 2018; Motasemi & Afzal, 2013b). Thus, it has been used as a constant
140
value in this work. However setting up an accurate microwave temperature during
141
MAP was tricky (Du et al., 2011), even for the same sample, the ability of microwave
142
heating was still affected by numerous factors such as initial temperature, sample
143
stacking density, particle size, sample moisture, magnetron distribution, reactor shape
144
and especially additive type. Additives are the major accelerator of temperature,
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sketch of temperature to time for sole activated carbon (AC) and sole silicon carbide
146
(SiC) can help explain its microwave absorption capacity. Fig. 2 illustrates
147
temperature rise curves of AC and SiC. Though both AC and SiC are excellent
148
microwave absorbers, they obeyed different absorption regularity. AC at the
149
beginning had a rapid temperature rise before 200 s, after that, the rise speed slowed
150
down during 200-360 s owing to the great vaporization of volatiles and inner water,
151
who were strong absorbers of microwave. Temperature later ascended markedly to
152
nearly 800°C at around 450 s. finally, temperature with AC additives were settled at
153
around 760°C. As for SiC, temperature increased quickly before 220 s, slowed down
154
the heating pace during 220-560°C, promptly raised at 470-490°C and declined at
155
490-520°C, this phenomenon might come from the instability of MAP system in the
156
early stage. This stage happened in the first 10 min of heating stage, and the whole
157
heating stage lasted for 25 min to guarantee adequate heating. Finally, temperature
158
with SiC rebounded a little and waved at around 660°C. By calculating the first-order
159
derivative of Fig. 2, the temperature profile of both AC and SiC fluctuated at first
160
owing to heat conduction inhomogeneity of additives and instability of MAP system
161
at the very start. In the main pyrolysis stage, the rapid temperature rise period of AC
162
was earlier and more violent than SiC.
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3.2 Yields of three-phase products
164
Fig. 3 charted the yield distribution of solid, liquid and gas products of MAP
165
under different blending ratios and additives. It was noted that both blending ratios
166
and additives affected yield distribution greatly. With regard to the additives, samples
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of MAP with AC had a higher gas yield and lower solid yield under all Chlorella
168
vulgaris (CV) and wood sawdust (WS) blending ratios. As for liquid yield, AC as the
169
additive had a slight lower value except that of 100% CV proportion. The result
170
revealed that AC could absorb microwave irradiation more strongly, which may due
171
to the existence of local high temperature at hot spots in AC case. Active dipolar
172
friction intensity and interfacial polarization effects could generate sparks where
173
pyrolysis vapors were further pyrolyzed into small gas molecules (Dai et al., 2017).
174
Some research also found AC can give rise to hot spots in the form of small sparks
175
and electric arcs termed as micro plasmas (Mushtaq et al., 2014). AC hereby
176
presented more active dipolar friction intensity and interfacial polarization effects
177
than SiC, consequently inducing more gas yield and liquid yield at all CV proportion
178
levels. Another factor that resulted in the three-phase distribution difference between
179
AC and SiC probably was the density difference. Heterogeneity by big density
180
difference between parent samples (CV and WS) and additives would affect gas and
181
liquid yields(Shang et al., 2015a). The density of AC and SiC was 0.38-0.65 g/cm3
182
and 3.2 g/cm3, respectively. Apparently the former one was more similar to the
183
density of CV and WS, which urged CV to be more active in these experiments.
184
As for the effect of different blending ratios, higher CV proportion palpably
185
resulted in less gas yield, more liquid and solid content both in AC and SiC as the
186
additive cases. This was due to the component difference between CV and WS. Lipid
187
and protein in CV could be pyrolyzed into liquid. The main composition of WS was
188
cellulose, followed by hemicellulose and lignin, whose main pyrolysis output tended
189
to be small light molecules gases. MAP of KW only obtained 20.2-21.2% of liquid
190
products, which was lower than conventional pyrolysis method. In contrast, MAP of
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CV obtained 45.3-49.3% of liquid products, which was higher than conventional
192
pyrolysis. For instance 34% of liquid product from CV could be received in the batch
193
pyrolysis reactor (Rizzo et al., 2013). But gas products from KW improved greatly by
194
MAP. Further investigations could pay attention to evaluation of gas products. In
195
brief, MAP of sole CV with SiC as the additive had a high yield of liquid products
196
and MAP of sole KW with AC as the additive had a high yield of gas product.
197
Microwave-assisted co-pyrolysis of CV and WS with AC leaded to acquire more
198
liquid product, and with SiC leaded to more gas product in general. Only from yields
199
of three-phase products, it is difficult to tell whether there was synergism between CV
200
and WS. Therefore, pyrolysis products of bio-oils were discussed in the next section.
201
3.3 GC-MS characterization of bio-oils
202
Semi quantitative analysis results of bio-oils by GC-MS method were
203
summarized in Fig. 4. Specific pyrolysis products of bio-oils could be found in E-
204
supplementary data. E-supplementary data of this work can be found in online version
205
of the paper. Apparently, the content of chemical constituents varied with additive
206
types and blending ratios of CV and WS. On the one hand, comparing Fig. 4(a) with
207
Fig. 4(b), samples with AC as the additive inclined to produce more aromatic
208
hydrocarbons, particularly polycyclic aromatic hydrocarbons (PAHs) and BTEXs (a
209
generic name for benzene, toluene, ethylbenzene and o-, m- and p-xylenes). PAHs
210
were the most concentrated chemical composition for WS-AC (notated as parent
211
sample name-additive name, the same below), 30CV70WS-AC and 50CV50WS-AC
212
with peak area of 48.65%, 27.28% and 29.92%, respectively. Phenols were the second
213
concentrated chemical composition for WS-AC (a peak area of 22.65%) and
214
30CV70WS-AC (a peak area of 21.27%). With the increase of CV proportion,
215
relative content of PAHs and phenols declined to its bottom at CV-AC with a peak
216
area of 14.07% and 3.30%, respectively. Higher content of WS in samples preferred
217
to produce more PAHs, as well as phenols when use AC as the additive. This was
218
attributed to MAP of cellulose, as well as lignin in WS, which had highly polymerized
219
phenyl structure inside (Fan et al., 2018). Furthermore, the phenolic compounds
220
generally came from the thermal degradation of the phenylpropanoid lignin structure
221
(Zheng et al., 2018). CV barely have no lignin, while lignin accounts for 13-30%
222
among the ingredients in raw WS (Chen et al., 2018b; Gu et al., 2013). Besides, With
223
the increase of CV proportion, BTEXs content improved as shown in Fig.4(a). High
224
BTEXs yield occurred at CV-AC, 70CV30WS-AC and 50CV50WS-AC with peak
225
area of 18.80%, 15.90% and 12.31%, respectively. Samples with higher CV
226
proportion tended to obtain more BTEXs chemicals, as well as nitrogenated
227
compounds probably owing to abundant protein and lipids in CV. BTEXs in CV-AC
228
and CV-SiC bio-oils mainly consisted of benzene, 1,3-(Fang et al., 2016)dimethyl-,
229
benzene and benzene, propyl-. However, BTEXs in 50CV50WS-AC and
230
50CV50WS-SiC bio-oils mainly consisted of ethylbenzene and benzene, 1,3-
231
dimethyl-. Different types of BTEXs between CV pyrolysis and 50CV50WS
232
pyrolysis indicated that interaction possibly happened between CV and WS.
233
Furthermore, compared to Fig. 4(b), Fig. 4(a) demonstrated that the presence of AC
234
which performed a strong absorptivity promoted further cracking of furans, especially
235
for WS sample. Furans were mainly derived from cellulose, hemicellulose and lignin
236
(Zheng et al., 2018), in accord with the main components of WS. In summary, it could
237
be safe to conclude that MAP with AC behaved a positive effect on aromatization.
238
WS-AC was an excellent collocation to obtain concentrated PAHs.
239
On the other hand, from Fig. 4(b), BTEXs yield when using SiC was generally
240
low in the range of 0.14% at WS-SiC to 10.09% at CV-SiC. And PAHs yield first
241
increased and then decreased with the increase ratio of CV; it achieved a maximum
242
value at 70CV30WS-SiC with peak area of 26.62%. Prominent constituents when
243
using SiC were aliphatic hydrocarbons and phenols. High alkanes and alkenes
244
emerged at CV-SiC and 30CV70WS-SiC with peak area of 9.83% for alkanes,
245
31.23% for alkenes and 9.45% for alkanes, 21.85% for alkenes, respectively. This was
246
1.5-3 times higher than that at AC conditions. As for phenols, peak value happened at
247
WS-SiC and 50CV50WS-SiC with peak area of 43.62% and 22.14%, respectively.
248
Analogously, previous work showed alkanes and aromatics formation during MAP of
249
soap stock were increased when using SiC particles as the microwave absorbent bed
250
(Wang et al., 2016). The specific phenolic components varied over blending ratios.
251
phenols in WS-SiC bio-oil were mainly comprised of phenol,2-methoxy-, creosol and
252
phenol, 4-ethyl-2-methoxy-, while phenols in 50CV50WS-SiC bio-oil mainly
253
included creosol, mequinol and phenol, 2-methoxy-4-(1-propenyl)-, (Z)-. This was
254
inferred that the alkylation reaction mechanism of phenol had changed after adding
255
CV into WS. From the perspective of phenols, aliphatic hydrocarbons and PAHs,
256
WS-SiC,CV-SiC and 70CV30SiC was the optimal condition. Respectively.
257
High proportion of value-added constituents with little or without useless
258
constituents were considered as high-quality bio-oils. Hydrocarbons are valuable
259
components in bio-oils from the perspective of fuel application (Bundhoo, 2018). For
260
example, adding aromatics can improve octane number for transportation fuel (Borges
261
et al., 2014b), adding alkanes and alkenes will enhance caloric value of combustion
262
fuel for heat generation. BTEXs could be applied as important industrial chemicals to
263
produce synthetic fibers, synthetic rubber, adhesives, dyestuff, flavors, etc. Phenols
264
could be served in agriculture industry, pharmaceutical industry and preservative
265
field. From Fig. 4 and previous discussion in this section, bio-oils from MAP with
266
carbon-based additives of CV, WS and their blends contained many useful
267
components; negligible acids and oxygen-containing substance such as esters,
268
ketones, aldehydes and alcohols were mixed in. Compared to traditional direct
269
pyrolysis of microalgae (Zainan et al., 2018) and wood dust (Chen et al., 2018b),
270
MAP apparently lowered acids and oxygenated compounds of products. But with
271
regard to nitrogenated compounds, refining effect by MAP was not obvious. High
272
selectivity of PAHs in bio-oils could be received at WS-AC 30CV70WS-AC,
273
50CV50WS-AC and 70CV30WS-SiC. High selectivity of phenols could be received
274
at WS-SiC. And high selectivity of aliphatic hydrocarbon could be received at CV-
275
SiC and 50CV50WS-SiC. The next investigation direction points to the enhancement
276
and purification of targeted products during or after pyrolysis under these optimal
277
blend ratios and additives.
278
3.4 Calorific value distribution of products
279
Higher calorific value (HHV) was an important evaluation index to assess the
280
quality of products. From HHV of bio-oil and bio-char listed in Table 2., fluctuation
281
trend could be found. With regard to bio-oil, WS-AC and WS-SiC had the highest
282
HHV, the value of 28.66-29.30 MJ/kg for WS pyrolysis bio-oil was higher than that
283
from conventional pyrolysis such as fluidized bed reactor (Wang et al., 2013) or fixed
284
bed reactor (Kabir & Hameed, 2017), compared to WS-AC and WS-SiC, CV-AC and
285
CV-SiC had a relative low HHV. But it was similar to those conventional pyrolysis
286
(Chen et al., 2015). This was in contrast to parent sample (HHV of WS was 18.42
287
MJ/kg, HHV of CV was 22.06 MJ/kg). Similar trends were happened in bio-char,
288
HHV of bio-char from WS had a higher value, especially when using SiC as the
289
additive, it reached up to 31.84 MJ/kg. While the bio-char of CV obtained a low
290
HHV. The reason was that microalgae bio-char had less carbon content than
291
lignocellulose bio-char (Azizi et al., 2018). In other words, MAP of WS with carbon-
292
based additive to produce high calorific value products is more practical than that of
293
CV. The mixing degree of CV and WS also affected HHV greatly. HHV of bio-oil
294
decreased by one third to half after mixing, 50CV50WS attained the worst. Moreover,
295
HHV of bio-char decreased with the increase of CV proportion in samples and using
296
AC as the additive did not gain much privilege for HHV of bio-char. In general, bio-
297
char of WS by MAP improved most apparently compared to WS feedstock, which
298
may be applied as solid fuels for energy plant, feedstock for gasification process,
299
feedstock for catalyst or restoration of land, purifier for water pollution, etc
300
(Motasemi & Afzal, 2013a).
301
4
302
Conclusions
AC could improve the aromatization of chain like hydrocarbons in bio-oils from
303
MAP of WS, 30CV70WS and 50CV50WS, SiC could improve the selectivity of
304
aliphatic hydrocarbons in bio-oils from MAP of CV and 30CV70WS. Bio-oil with
305
high selective can be applied as blending stock for conventional fuel or industrial
306
chemical feedstock after further refining. The synergies resulted from co-pyrolysis of
307
CV and WS were obvious in terms of calorific value fluctuation. In conclusion, using
308
carbon-based additives and combination CV with WS could be alternative methods to
309
improve products quality of MAP of biomass.
310
Acknowledgments
311
This work was supported by the National Natural Science Foundation of China
312
(51406058, 51476060, 51606071); the Guangdong Natural Science Foundation
313
(2015A030313227); the China Scholarship Council (201706155065); the Guangdong
314
Province Engineering Research Center of Highly Efficient and Low Pollution Energy
315
Conversion; the Key Laboratory of Efficient and Clean Energy Utilization of
316
Guangdong Higher Education Institutes, South China University of Technology
317
(KLB10004).
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Antunes, E., Jacob, M.V., Brodie, G., Schneider, P.A. 2018. Microwave pyrolysis of sewage biosolids: Dielectric properties, microwave susceptor role and its impact on biochar properties. Journal of Analytical and Applied Pyrolysis, 129, 93-100.
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Azizi, K., Keshavarz Moraveji, M., Abedini Najafabadi, H. 2018. A review on bio-fuel production from microalgal biomass by using pyrolysis method. Renewable and Sustainable Energy Reviews, 82, 3046-3059.
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Borges, F.C., Du, Z., Xie, Q., Trierweiler, J.O., Cheng, Y., Wan, Y., Liu, Y., Zhu, R., Lin, X., Chen, P., Ruan, R. 2014a. Fast microwave assisted pyrolysis of biomass using microwave absorbent. Bioresource Technology, 156, 267-274.
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Borges, F.C., Xie, Q., Min, M., Muniz, L.A.R., Farenzena, M., Trierweiler, J.O., Chen, P., Ruan, R. 2014b. Fast microwave-assisted pyrolysis of microalgae using microwave absorbent and HZSM-5 catalyst. Bioresource Technology, 166, 518526.
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Bundhoo, Z.M.A. 2018. Microwave-assisted conversion of biomass and waste materials to biofuels. Renewable and Sustainable Energy Reviews, 82, 11491177.
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Chen, L., Yu, Z., Liang, J., Liao, Y., Ma, X. 2018a. Co-pyrolysis of chlorella vulgaris and kitchen waste with different additives using TG-FTIR and PyGC/MS. Energy Conversion and Management, 177, 582-591.
8.
Chen, W.-H., Lin, B.-J., Huang, M.-Y., Chang, J.-S. 2015. Thermochemical conversion of microalgal biomass into biofuels: A review. Bioresource Technology, 184, 314-327.
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Chen, W.-H., Wang, C.-W., Kumar, G., Rousset, P., Hsieh, T.-H. 2018b. Effect of torrefaction pretreatment on the pyrolysis of rubber wood sawdust analyzed by Py-GC/MS. Bioresource Technology, 259, 469-473.
10. Dai, L., Fan, L., Duan, D., Ruan, R., Wang, Y., Liu, Y., Zhou, Y., Yu, Z., Liu, Y., Jiang, L. 2017. Production of hydrocarbon-rich bio-oil from soapstock via fast microwave-assisted catalytic pyrolysis. Journal of Analytical and Applied Pyrolysis, 125, 356-362. 11. Du, Z., Li, Y., Wang, X., Wan, Y., Chen, Q., Wang, C., Lin, X., Liu, Y., Chen, P., Ruan, R. 2011. Microwave-assisted pyrolysis of microalgae for biofuel production. Bioresource Technology, 102(7), 4890-4896. 12. Fan, L., Chen, P., Zhang, Y., Liu, S., Liu, Y., Wang, Y., Dai, L., Ruan, R. 2017. Fast microwave-assisted catalytic co-pyrolysis of lignin and low-density polyethylene with HZSM-5 and MgO for improved bio-oil yield and quality. Bioresource Technology, 225, 199-205. 13. Fan, L., Chen, P., Zhou, N., Liu, S., Zhang, Y., Liu, Y., Wang, Y., Omar, M.M., Peng, P., Addy, M., Cheng, Y., Ruan, R. 2018. In-situ and ex-situ catalytic upgrading of vapors from microwave-assisted pyrolysis of lignin. Bioresource Technology, 247, 851-858. 14. Fang, S., Gu, W., Dai, M., Xu, J., Yu, Z., Lin, Y., Chen, J., Ma, X. 2018. A study on microwave-assisted fast co-pyrolysis of chlorella and tire in the N2 and CO2 atmospheres. Bioresource Technology, 250, 821-827. 15. Fang, S., Yu, Z., Lin, Y., Lin, Y., Fan, Y., Liao, Y., Ma, X. 2016. Effects of additives on the co-pyrolysis of municipal solid waste and paper sludge by using thermogravimetric analysis. Bioresource Technology, 209, 265-272. 16. Gu, X., Ma, X., Li, L., Liu, C., Cheng, K., Li, Z. 2013. Pyrolysis of poplar wood sawdust by TG-FTIR and Py–GC/MS. Journal of Analytical and Applied Pyrolysis, 102, 16-23. 17. Kabir, G., Hameed, B.H. 2017. Recent progress on catalytic pyrolysis of
lignocellulosic biomass to high-grade bio-oil and bio-chemicals. Renewable and Sustainable Energy Reviews, 70, 945-967. 18. Kumar, P., Kumar, P., Rao, P.V.C., Choudary, N.V., Sriganesh, G. 2017. Saw dust pyrolysis: Effect of temperature and catalysts. Fuel, 199, 339-345. 19. Li, H., Li, X., Liu, L., Li, K., Wang, X., Li, H. 2016. Experimental study of microwave-assisted pyrolysis of rice straw for hydrogen production. International Journal of Hydrogen Energy, 41(4), 2263-2267. 20. Luo, H., Bao, L., Kong, L., Sun, Y. 2017. Low temperature microwave-assisted pyrolysis of wood sawdust for phenolic rich compounds: Kinetics and dielectric properties analysis. Bioresource Technology, 238, 109. 21. Maliutina, K., Tahmasebi, A., Yu, J. 2018. The transformation of nitrogen during pressurized entrained-flow pyrolysis of Chlorella vulgaris. Bioresource Technology, 262, 90-97. 22. Motasemi, F., Afzal, M.T. 2013a. A review on the microwave-assisted pyrolysis technique. Renewable & Sustainable Energy Reviews, 28, 317-330. 23. Motasemi, F., Afzal, M.T. 2013b. A review on the microwave-assisted pyrolysis technique. Renewable and Sustainable Energy Reviews, 28, 317-330. 24. Mushtaq, F., Mat, R., Ani, F.N. 2014. A review on microwave assisted pyrolysis of coal and biomass for fuel production. Renewable and Sustainable Energy Reviews, 39, 555-574. 25. Namazi, A.B., Allen, D.G., Jia, C.Q. 2015. Microwave-assisted pyrolysis and activation of pulp mill sludge. Biomass & Bioenergy, 73, 217-224. 26. Rizzo, A.M., Prussi, M., Bettucci, L., Libelli, I.M., Chiaramonti, D. 2013. Characterization of microalga Chlorella as a fuel and its thermogravimetric behavior. Applied Energy, 102, 24-31. 27. Shang, H., Lu, R.-R., Shang, L., Zhang, W.-H. 2015a. Effect of additives on the microwave-assisted pyrolysis of sawdust. Fuel Processing Technology, 131, 167174.
28. Shang, H., Lu, R.R., Shang, L., Zhang, W.H. 2015b. Effect of additives on the microwave-assisted pyrolysis of sawdust. Fuel Processing Technology, 131, 167174. 29. Sun, J., Wang, W., Yue, Q., Ma, C., Zhang, J., Zhao, X., Song, Z. 2016. Review on microwave–metal discharges and their applications in energy and industrial processes. Applied Energy, 175, 141-157. 30. Tchapda, A.H., Krishnamoorthy, V., Yeboah, Y.D., Pisupati, S.V. 2017. Analysis of tars formed during co-pyrolysis of coal and biomass at high temperature in carbon dioxide atmosphere. Journal of Analytical and Applied Pyrolysis, 128, 379-396. 31. Wang, K., Brown, R.C., Homsy, S., Martinez, L., Sidhu, S.S. 2013. Fast pyrolysis of microalgae remnants in a fluidized bed reactor for bio-oil and biochar production. Bioresource Technology, 127, 494-499. 32. Wang, Y., Dai, L., Wang, R., Fan, L., Liu, Y., Xie, Q., Ruan, R. 2016. Hydrocarbon fuel production from soapstock through fast microwave-assisted pyrolysis using microwave absorbent. Journal of Analytical and Applied Pyrolysis, 119, 251-258. 33. Xiang, Z., Liang, J., Morgan, H.M., Liu, Y., Mao, H., Bu, Q. 2018. Thermal behavior and kinetic study for co-pyrolysis of lignocellulosic biomass with polyethylene over Cobalt modified ZSM-5 catalyst by thermogravimetric analysis. Bioresource Technology, 247, 804-811. 34. Xie, Q., Addy, M., Liu, S., Zhang, B., Cheng, Y., Wan, Y., Li, Y., Liu, Y., Lin, X., Chen, P., Ruan, R. 2015. Fast microwave-assisted catalytic co-pyrolysis of microalgae and scum for bio-oil production. Fuel, 160, 577-582. 35. Yang, Z., Kumar, A., Apblett, A.W., Moneeb, A.M. 2017. Co-Pyrolysis of torrefied biomass and methane over molybdenum modified bimetallic HZSM-5 catalyst for hydrocarbons production. Green Chemistry, 19(3), 757-768. 36. Zainan, N.H., Srivatsa, S.C., Li, F., Bhattacharya, S. 2018. Quality of bio-oil
from catalytic pyrolysis of microalgae Chlorella vulgaris. Fuel, 223, 12-19. 37. Zhang, Y., Chen, P., Liu, S., Peng, P., Min, M., Cheng, Y., Anderson, E., Zhou, N., Fan, L., Liu, C., Chen, G., Liu, Y., Lei, H., Li, B., Ruan, R. 2017. Effects of feedstock characteristics on microwave-assisted pyrolysis - A review. Bioresource Technology, 230, 143-151. 38. Zhao, C., Jiang, E., Chen, A. 2016. Volatile production from pyrolysis of cellulose, hemicellulose and lignin. Journal of the Energy Institute. 39. Zheng, Y., Tao, L., Yang, X., Huang, Y., Liu, C., Zheng, Z. 2018. Study of the thermal behavior, kinetics, and product characterization of biomass and lowdensity polyethylene co-pyrolysis by thermogravimetric analysis and pyrolysisGC/MS. Journal of Analytical and Applied Pyrolysis, 133, 185-197.
Fig. 1. A schematic diagram of reaction system. Fig. 2. Temperature profiles of AC and SiC. Fig. 3. The yields of solid, liquid and gas on mass ratio. Fig. 4. Chemical compounds of co-pyrolysis bio-oil with different additives: (a) AC; (b) SiC.
Table 1
Characteristics of CV, WS, AC and SiC. Ultimate analyses (wt.%, dry
Proximate analyses (wt.%,
Ta HHV
basis)
dry basis)
H/
O/
nδ
Cc
Cc
×1
(MJ/ Moist
Volati
As
Fixed C
ure
les
0.66
71.63
C
h
47.
79.76
S
S
33.6
kg)
0.8
47.
6 6.6
1
3
1.7
0.5
7
3
1.6
0.7
<0.
9
3
1d
22.06 9
2
0.15
0
46.1
14.21 2
11.1
7 33
0.7 5.32
N
0-4
19.19 2
W
Ob
Carbona
8.5
V
H
—
18.42
3
0.5 A
80. 10.35
7.9
4.9
1.7
13.7
76.85
C
0.0 4.36
09
3
3
725.19
—
—
9
0.8 d
0.1 Si
—
—
—
—
—
—
—
—
—
C
—
—
— 0.4 7d
a
calculated by difference (fixed carbon=100-moisture-ash-volatiles).
b
calculated by difference (O (%)=100-C-H-N-S-Ash).
c
Molar ratio.
d
Refs. (Salema et al., 2017), (Atwater & Wheeler, 2003), (Duan et al., 2016; Polaert et al., 2017),
respectively
Table 2 HHV of bio-oil and bio-char. Additive
AC
SiC
CV proportion (%)
0
30
50
70
100
0
30
50
70
HHV of bio-oil
29.3
24.8
15.0
24.7
19.9
28.6
25.9
17.9
24.3
（MJ/kg）
0
6
2
8
3
6
8
7
3
HHV of bio-char
28.0
27.4
27.1
25.3
24.8
31.8
23.0
22.7
23.0
（MJ/kg）
9
6
0
1
9
4
9
7
3
100
23.61
19.27
Highlights
Co-pyrolysis of chlorella and sawdust was investigated.
Activated carbon had better temperature response to microwave than silicon carbide.
Aromatics in bio-oil improved by co-pyrolysis of sawdust and chlorella.
Microwave pyrolysis with activated carbon had higher gas and liquid yield.
High calorific bio-char was made by microwave-assisted pyrolysis of sawdust.
S0960-8524(18)31518-9 https://doi.org/10.1016/j.biortech.2018.10.086 BITE 20650
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
12 September 2018 29 October 2018 30 October 2018
Please cite this article as: Chen, L., Yu, Z., Xu, H., Wan, K., Liao, Y., Ma, X., Microwave-assisted co-pyrolysis of Chlorella vulgaris and wood sawdust using different additives, Bioresource Technology (2018), doi: https://doi.org/ 10.1016/j.biortech.2018.10.086
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Microwave-assisted co-pyrolysis of Chlorella vulgaris and wood sawdust using different additives Lin Chena,b, Zhaosheng Yua,b *, Hao Xua,b, Kuangyu Wan a,b, Yanfen Liaoa,b, Xiaoqian Maa,b a School
of Electric Power, South China University of Technology, 510640,
Guangzhou, China b
Guangdong Province Key Laboratory of Efficient and Clean Energy
Utilization, 510640, Guangzhou, China Abstract The microwave-assisted co-pyrolysis of Chlorella vulgaris (CV), wood sawdust (WS) and their blends with additives were investigated. There was a higher liquid and solid yield with silicon carbide (SiC) than activated carbon (AC) in most of samples. Microwave-assisted pyrolysis with additives behaved a positive effect on deoxygenation and aromatization, but not apparent denitrification. With the increase of CV proportion, aromatic hydrocarbons decreased, but aliphatic hydrocarbons increased using AC. High selectivity of phenols was reached at the sample of WS (relative content as 43.6%) using SiC; High selectivity of alkenes was reached at the sample of CV (relative content as 31.2%) and alkanes at the blend sample of 70% CV and 30% WS (relative content as 9.45%). Bio-oil and biochar from microwaveassisted pyrolysis of WS had higher calorific value than that of CV both with AC and
*
Corresponding author at: School of Electric Power, South China University of Technology, 510640
Guangzhou, China. Email: [email protected] (Z. Yu).
SiC. Calorific value of bio-oil decreased by 33.3% after mixing CV with WS. Keywords: Co-pyrolysis; Chlorella vulgaris; Sawdust; Microwave-assisted; Additives
1
2
1
Introduction
In pace with fossil fuel depletion aggravation, the utilization of renewable energy
3
has been a hot spot for research, and urgently needs to evolve. Compared to many
4
alternative energy resources such as wind power, solar power, tidal energy and hydro-
5
energy, biomass energy is the only one that can be conveyed and stored, and also the
6
oldest energy used by human beings. Moreover, biomass is the only sustainable
7
resource of organic carbon capable of producing petroleum-like oils and chemicals
8
(Yang et al., 2017). There are numerous methods for the conversion of biomass to
9
surrogate fuel and bio-chemical products, and they can be generally divided as
10
chemical methods, thermochemical methods and biochemical methods. Microwave-
11
assisted pyrolysis (MAP) is one of the most high-profile thermochemical process
12
under development in current years, because microwave heating owns the features of
13
volumetric heating, a fast heating rate, specified calefaction targets and automatic
14
control. MAP of biomass requires lower pyrolysis energy input and less average
15
activation energy than conventional pyrolysis, where heat conveys inefficiently from
16
exterior to interior. Luo et al. (Luo et al., 2017) found MAP of wood sawdust,
17
obtained more than 50% of liquid products, in which the yield of 78.7% was phenolic
18
chemicals at a 50-100°C lower temperature and 50-100 kJ/mol lower average
19
activation energy than that of conventional pyrolysis. Moreover, the MAP device is
20
less expensive due to its ability of rapid heating compared with conventional heating
21
devices (Wang et al., 2016).
22
Ordinary biomass usually has poor dielectric properties resulting in limited
23
valuable products during MAP (Namazi et al., 2015). While MAP with additives, the
24
yields and properties of the final products change significantly. For example, MAP of
25
lignin and low-density polyethylene with additives HZSM-5 and magnesium oxide
26
represented a higher bio-oil quantity and quality, the addition of HZSM-5 improved
27
the aromatization of chain hydrocarbons, and magnesium oxide improved the
28
selectivity of alkylated phenols (Fan et al., 2017). MAP of wood sawdust with
29
different additives resulted in significant effects on final products, by using silica
30
carbon decreased the gas and liquid yields, using potassium carbonate and sodium
31
hydrate increased gas yield strongly (Shang et al., 2015b). Generally, additives for
32
MAP participate in reaction not only as absorbents to enhance heating but also as
33
catalysts to improve the reaction rate, differentiate the reaction path and thus decrease
34
the energy consumption. Therefore, in order to improve the target valuable products
35
in biomass microwave-assisted heating process, additives are essential. Additives for
36
microwave can be categorized as carbon-based materials, metal oxides and zeolites. It
37
has been proved that the carbon-based materials are very suitable absorbents of
38
microwaves, which had a remarkable effect (Fang et al., 2018; Wang et al., 2016).
39
Two of the most commonly used carbon-based additives, activated carbon (AC) and
40
silicon carbide (SiC) were applied in this work. More studies about AC or SiC used in
41
MAP could be found in other literature and experiments (Borges et al., 2014a; Li et
42
al., 2016).
43
Chlorella vulgaris (CV) is a typical kind of algae biomass with widespread
44
growth globally. The main compositions of microalgae are lipids, proteins,
45
carbohydrates, inorganic elements (Fe, Cu, Mn, Zn, Se, etc.), vitamins as well as folic
46
acid. Some of the lipids proportion can even reach up to 50-55 wt.% on dry basis
47
(Zhang et al., 2017). On the other hand, algal biomass as the third-generation
48
sustainable biomass has its own predominant growth behavior such as fast growth
49
rate, high CO2 capture capacity, no need for arable land and fairly simple cultivation
50
conditions (Chen et al., 2018a; Maliutina et al., 2018). Therefore, it is a promising
51
feedstock for value-added chemical byproducts and/or biofuel. Wood sawdust (WS) is
52
a typical kind of lignocellulosic biomass, cellulose is its primary component (23.7-
53
61.7 wt. %), followed by hemicellulose and lignin (Zhang et al., 2017). Previous
54
researches found cellulose and hemicellulose could produce more bio-oil during
55
pyrolysis than lignin (Zhao et al., 2016). Moreover, mass-produce of sawdust from
56
agroforestry make it easier for industrial production (Kumar et al., 2017). Hence, CV
57
and WS were chose as typical biomass feedstock for MAP in this work.
58
Considerable researches have been focused on co-pyrolysis of biomass, such as
59
co-pyrolysis of rice straw and polyethylene (Xiang et al., 2018), lignocellulosic
60
biomass and coal (An et al., 2017; Tchapda et al., 2017) and microalgae and scum
61
(Xie et al., 2015). While few aims at co-pyrolysis of microalgae and wood sawdust,
62
which probably has a synergistic effect during the heating process, result in
63
improvement of useful products. This paper studied MAP of CV, WS and their blends
64
using AC or SiC as the additive, trying to figure out an optimal MAP conditions to
65
obtain value-added chemicals or high caloric value products.
66
2
67
2.1 Materials
Materials and methods
68
Chlorella vulgaris (CV) in powder form was bought from SCIPHAR Natural
69
Products Co. Ltd. (Shaanxi Province, China) with particle size less than 178 μm. Wood
70
sawdust (WS) was acquired from local wood processing plant in state of powder (＜
71
200 μm). After air forced dried at 105°C until the weight was steady, all sample
72
powders were stored in desiccator on standby.
73
The additives, activated carbon (AC) and silicon carbide (SiC), were purchased
74
from Guangzhou Cong Yuan Instrument Co. Ltd. (Guangdong Province, China). AC
75
and SiC were all ground into 80 mesh, and correspond to standard with analytical
76
reagent (purity≥98.0%).
77
All samples and additives were measured according to ASTM D3176 for carbon
78
(C), hydrogen (H), nitrogen (N), and sulfur (S) by Vario El Cube Elemental Analyzer
79
(Elementar Co. Ltd., German). Oxygen (O) content was calculated by difference.
80
Moisture, ash, volatile matter and fixed carbon were obtained by drier (DHG-9070A,
81
Shanghai Qixin Science Instruments Co. Ltd., China) and Ash Tester (MF-2000,
82
Hegang Haotian Electrical Co. Ltd., China) according to GB/T212-2008 and GB211-
83
84. Higher calorific value (HHV) was tested by Oxygen Bomb Calorimeter (TE-C610,
84
Changsha Titen Electronic Co. Ltd., China) according to ASTM D5865. Dielectric
85
properties of parent samples and additives were interpreted by dielectric loss tangent,
86
tanδ, using Eq. 1 (Sun et al., 2016):
87
tanδ =
𝜀'' 𝜀'
(1)
88
Where ε'' is the dielectric loss factor, which indicates microwave dissipation
89
efficiency of materials. ε' is dielectric coefficient, which represents microwave
90
absorption intensity of materials. There is no explicit relationship between ε' and ε'',
91
while tanδ is a rough estimate for the ability of converting electromagnetic energy into
92
heat. The higher tanδ is, the better capacity of microwave absorption for materials
93
becomes. Basic characteristics of biomass feedstock and additives were listed in Table.
94
1. Blending ratios of CV and WS were set up 5 levels by mass percentage, i.e. 100%CV,
95
70%CV with 30%WS, 50%CV with 50%WS, 30%CV with 70%WS, and 100%WS,
96
abbreviated as CV, 70CV30WS, 50CV50WS, 30CV70WS and WS, hereafter.
97
Blending ratio of parent samples and microwave additive was 10:1 by mass percentage.
98
2.2 Experimental apparatus and procedure
99
The co-pyrolysis experiments were carried out on a self-built microwave-assisted
100
pyrolysis (MAP) platform, as shown in Fig. 1. This platform consists of four parts:
101
Microwave occurrence system, material reaction system, product collection system
102
and data acquisition system. The microwave rated power was 1000 W, 2.45 GHz
103
uniformly in this work. The temperature sensor used in this experiment was 4 mm
104
diameter K-type thermocouple with the thermometric range between 0-1600°C,
105
temperature precision of ±1.5°C. Magnetrons were uniformly distributed as shown in
106
Fig. 1 in order to ensure the homogeneous microwave distribution during the reaction.
107
30.00±0.01 g of blending samples with 3.00±0.01 g AC or SiC were fully mixed and
108
placed in the clean and dry quartz flask. After checking out the sealing property of gas
109
flow unit, high purity N2 as an inert carrier gas was introduced to the flask
110
consistently for 10 min with a flow rate of 300 mL/min, in order to vent detained air.
111
Afterwards, switching on microwave heating system for 25 min. Then, microwave
112
heating system was switched off and the reaction chamber was naturally air-cooled to
113
room temperature with uninterrupted N2 atmosphere. Finally, solid residue was
114
remained in the flask. Condensable gas was cooled by chill water (5°C) and collected
115
as bio-oil in conical bottles. Incondensable gas was obtained by difference on the
116
basis of mass equilibrium. Three phase products under different blending ratios and
117
additives were weighed up and calculated. Micro positive pressure in gas flow unit
118
was used during the whole MAP process. Each MAP experiment was carried out
119
twice. Standard deviation of two batches were presented in the form of the error bar.
120
2.3 Analytical methods
121
The acquired liquid content was immediately diluted by dichloromethane solvent
122
to a proper concentration. Processed liquid products (designated as bio-oils hereafter)
123
were examined by the Gas Chromatography-Mass Spectrometry (GC-MS) (Agilent
124
7890B & 5977A, Agilent Technologies Inc. U.S.A). The metal capillary column type
125
was HP-5ms, 30 m×0.25 mm ID×0.25 μm. The oven temperature was set from 50°C
126
(held for 1 min) to 260°C (held for 5 min), at a heating rate of 5 °C/min. The injector
127
temperature was maintained at 290°C and the injection volume was 1 μL using a split
128
ratio of 1:50. The carrier gas (high pure helium) rate was 1 mL/min. The scan range of
129
the quadrupole mass spectrometer was between 50 and 500 m/z. The chemical
130
compounds in bio-oils were identified by NIST data library and relevant literature. The Higher calorific value (HHV) of bio-oils (i.e. liquid products after
131 132
dehydrated) and residues (i.e. solid products after fully ground) were tested by
133
Oxygen Bomb Calorimeter (TE-C610, Changsha Titen Electronic Co., Ltd, China)
134
according to ASTM D5865.
135
3
136
3.1 Temperature profiles of additives
137
Results and discussion
2.45 GHz frequency of microwave power, corresponding to 12.5 cm wavelength
138
is the conventional set value for microwave-assisted pyrolysis (MAP) of biomass
139
(Antunes et al., 2018; Motasemi & Afzal, 2013b). Thus, it has been used as a constant
140
value in this work. However setting up an accurate microwave temperature during
141
MAP was tricky (Du et al., 2011), even for the same sample, the ability of microwave
142
heating was still affected by numerous factors such as initial temperature, sample
143
stacking density, particle size, sample moisture, magnetron distribution, reactor shape
144
and especially additive type. Additives are the major accelerator of temperature,
145
sketch of temperature to time for sole activated carbon (AC) and sole silicon carbide
146
(SiC) can help explain its microwave absorption capacity. Fig. 2 illustrates
147
temperature rise curves of AC and SiC. Though both AC and SiC are excellent
148
microwave absorbers, they obeyed different absorption regularity. AC at the
149
beginning had a rapid temperature rise before 200 s, after that, the rise speed slowed
150
down during 200-360 s owing to the great vaporization of volatiles and inner water,
151
who were strong absorbers of microwave. Temperature later ascended markedly to
152
nearly 800°C at around 450 s. finally, temperature with AC additives were settled at
153
around 760°C. As for SiC, temperature increased quickly before 220 s, slowed down
154
the heating pace during 220-560°C, promptly raised at 470-490°C and declined at
155
490-520°C, this phenomenon might come from the instability of MAP system in the
156
early stage. This stage happened in the first 10 min of heating stage, and the whole
157
heating stage lasted for 25 min to guarantee adequate heating. Finally, temperature
158
with SiC rebounded a little and waved at around 660°C. By calculating the first-order
159
derivative of Fig. 2, the temperature profile of both AC and SiC fluctuated at first
160
owing to heat conduction inhomogeneity of additives and instability of MAP system
161
at the very start. In the main pyrolysis stage, the rapid temperature rise period of AC
162
was earlier and more violent than SiC.
163
3.2 Yields of three-phase products
164
Fig. 3 charted the yield distribution of solid, liquid and gas products of MAP
165
under different blending ratios and additives. It was noted that both blending ratios
166
and additives affected yield distribution greatly. With regard to the additives, samples
167
of MAP with AC had a higher gas yield and lower solid yield under all Chlorella
168
vulgaris (CV) and wood sawdust (WS) blending ratios. As for liquid yield, AC as the
169
additive had a slight lower value except that of 100% CV proportion. The result
170
revealed that AC could absorb microwave irradiation more strongly, which may due
171
to the existence of local high temperature at hot spots in AC case. Active dipolar
172
friction intensity and interfacial polarization effects could generate sparks where
173
pyrolysis vapors were further pyrolyzed into small gas molecules (Dai et al., 2017).
174
Some research also found AC can give rise to hot spots in the form of small sparks
175
and electric arcs termed as micro plasmas (Mushtaq et al., 2014). AC hereby
176
presented more active dipolar friction intensity and interfacial polarization effects
177
than SiC, consequently inducing more gas yield and liquid yield at all CV proportion
178
levels. Another factor that resulted in the three-phase distribution difference between
179
AC and SiC probably was the density difference. Heterogeneity by big density
180
difference between parent samples (CV and WS) and additives would affect gas and
181
liquid yields(Shang et al., 2015a). The density of AC and SiC was 0.38-0.65 g/cm3
182
and 3.2 g/cm3, respectively. Apparently the former one was more similar to the
183
density of CV and WS, which urged CV to be more active in these experiments.
184
As for the effect of different blending ratios, higher CV proportion palpably
185
resulted in less gas yield, more liquid and solid content both in AC and SiC as the
186
additive cases. This was due to the component difference between CV and WS. Lipid
187
and protein in CV could be pyrolyzed into liquid. The main composition of WS was
188
cellulose, followed by hemicellulose and lignin, whose main pyrolysis output tended
189
to be small light molecules gases. MAP of KW only obtained 20.2-21.2% of liquid
190
products, which was lower than conventional pyrolysis method. In contrast, MAP of
191
CV obtained 45.3-49.3% of liquid products, which was higher than conventional
192
pyrolysis. For instance 34% of liquid product from CV could be received in the batch
193
pyrolysis reactor (Rizzo et al., 2013). But gas products from KW improved greatly by
194
MAP. Further investigations could pay attention to evaluation of gas products. In
195
brief, MAP of sole CV with SiC as the additive had a high yield of liquid products
196
and MAP of sole KW with AC as the additive had a high yield of gas product.
197
Microwave-assisted co-pyrolysis of CV and WS with AC leaded to acquire more
198
liquid product, and with SiC leaded to more gas product in general. Only from yields
199
of three-phase products, it is difficult to tell whether there was synergism between CV
200
and WS. Therefore, pyrolysis products of bio-oils were discussed in the next section.
201
3.3 GC-MS characterization of bio-oils
202
Semi quantitative analysis results of bio-oils by GC-MS method were
203
summarized in Fig. 4. Specific pyrolysis products of bio-oils could be found in E-
204
supplementary data. E-supplementary data of this work can be found in online version
205
of the paper. Apparently, the content of chemical constituents varied with additive
206
types and blending ratios of CV and WS. On the one hand, comparing Fig. 4(a) with
207
Fig. 4(b), samples with AC as the additive inclined to produce more aromatic
208
hydrocarbons, particularly polycyclic aromatic hydrocarbons (PAHs) and BTEXs (a
209
generic name for benzene, toluene, ethylbenzene and o-, m- and p-xylenes). PAHs
210
were the most concentrated chemical composition for WS-AC (notated as parent
211
sample name-additive name, the same below), 30CV70WS-AC and 50CV50WS-AC
212
with peak area of 48.65%, 27.28% and 29.92%, respectively. Phenols were the second
213
concentrated chemical composition for WS-AC (a peak area of 22.65%) and
214
30CV70WS-AC (a peak area of 21.27%). With the increase of CV proportion,
215
relative content of PAHs and phenols declined to its bottom at CV-AC with a peak
216
area of 14.07% and 3.30%, respectively. Higher content of WS in samples preferred
217
to produce more PAHs, as well as phenols when use AC as the additive. This was
218
attributed to MAP of cellulose, as well as lignin in WS, which had highly polymerized
219
phenyl structure inside (Fan et al., 2018). Furthermore, the phenolic compounds
220
generally came from the thermal degradation of the phenylpropanoid lignin structure
221
(Zheng et al., 2018). CV barely have no lignin, while lignin accounts for 13-30%
222
among the ingredients in raw WS (Chen et al., 2018b; Gu et al., 2013). Besides, With
223
the increase of CV proportion, BTEXs content improved as shown in Fig.4(a). High
224
BTEXs yield occurred at CV-AC, 70CV30WS-AC and 50CV50WS-AC with peak
225
area of 18.80%, 15.90% and 12.31%, respectively. Samples with higher CV
226
proportion tended to obtain more BTEXs chemicals, as well as nitrogenated
227
compounds probably owing to abundant protein and lipids in CV. BTEXs in CV-AC
228
and CV-SiC bio-oils mainly consisted of benzene, 1,3-(Fang et al., 2016)dimethyl-,
229
benzene and benzene, propyl-. However, BTEXs in 50CV50WS-AC and
230
50CV50WS-SiC bio-oils mainly consisted of ethylbenzene and benzene, 1,3-
231
dimethyl-. Different types of BTEXs between CV pyrolysis and 50CV50WS
232
pyrolysis indicated that interaction possibly happened between CV and WS.
233
Furthermore, compared to Fig. 4(b), Fig. 4(a) demonstrated that the presence of AC
234
which performed a strong absorptivity promoted further cracking of furans, especially
235
for WS sample. Furans were mainly derived from cellulose, hemicellulose and lignin
236
(Zheng et al., 2018), in accord with the main components of WS. In summary, it could
237
be safe to conclude that MAP with AC behaved a positive effect on aromatization.
238
WS-AC was an excellent collocation to obtain concentrated PAHs.
239
On the other hand, from Fig. 4(b), BTEXs yield when using SiC was generally
240
low in the range of 0.14% at WS-SiC to 10.09% at CV-SiC. And PAHs yield first
241
increased and then decreased with the increase ratio of CV; it achieved a maximum
242
value at 70CV30WS-SiC with peak area of 26.62%. Prominent constituents when
243
using SiC were aliphatic hydrocarbons and phenols. High alkanes and alkenes
244
emerged at CV-SiC and 30CV70WS-SiC with peak area of 9.83% for alkanes,
245
31.23% for alkenes and 9.45% for alkanes, 21.85% for alkenes, respectively. This was
246
1.5-3 times higher than that at AC conditions. As for phenols, peak value happened at
247
WS-SiC and 50CV50WS-SiC with peak area of 43.62% and 22.14%, respectively.
248
Analogously, previous work showed alkanes and aromatics formation during MAP of
249
soap stock were increased when using SiC particles as the microwave absorbent bed
250
(Wang et al., 2016). The specific phenolic components varied over blending ratios.
251
phenols in WS-SiC bio-oil were mainly comprised of phenol,2-methoxy-, creosol and
252
phenol, 4-ethyl-2-methoxy-, while phenols in 50CV50WS-SiC bio-oil mainly
253
included creosol, mequinol and phenol, 2-methoxy-4-(1-propenyl)-, (Z)-. This was
254
inferred that the alkylation reaction mechanism of phenol had changed after adding
255
CV into WS. From the perspective of phenols, aliphatic hydrocarbons and PAHs,
256
WS-SiC,CV-SiC and 70CV30SiC was the optimal condition. Respectively.
257
High proportion of value-added constituents with little or without useless
258
constituents were considered as high-quality bio-oils. Hydrocarbons are valuable
259
components in bio-oils from the perspective of fuel application (Bundhoo, 2018). For
260
example, adding aromatics can improve octane number for transportation fuel (Borges
261
et al., 2014b), adding alkanes and alkenes will enhance caloric value of combustion
262
fuel for heat generation. BTEXs could be applied as important industrial chemicals to
263
produce synthetic fibers, synthetic rubber, adhesives, dyestuff, flavors, etc. Phenols
264
could be served in agriculture industry, pharmaceutical industry and preservative
265
field. From Fig. 4 and previous discussion in this section, bio-oils from MAP with
266
carbon-based additives of CV, WS and their blends contained many useful
267
components; negligible acids and oxygen-containing substance such as esters,
268
ketones, aldehydes and alcohols were mixed in. Compared to traditional direct
269
pyrolysis of microalgae (Zainan et al., 2018) and wood dust (Chen et al., 2018b),
270
MAP apparently lowered acids and oxygenated compounds of products. But with
271
regard to nitrogenated compounds, refining effect by MAP was not obvious. High
272
selectivity of PAHs in bio-oils could be received at WS-AC 30CV70WS-AC,
273
50CV50WS-AC and 70CV30WS-SiC. High selectivity of phenols could be received
274
at WS-SiC. And high selectivity of aliphatic hydrocarbon could be received at CV-
275
SiC and 50CV50WS-SiC. The next investigation direction points to the enhancement
276
and purification of targeted products during or after pyrolysis under these optimal
277
blend ratios and additives.
278
3.4 Calorific value distribution of products
279
Higher calorific value (HHV) was an important evaluation index to assess the
280
quality of products. From HHV of bio-oil and bio-char listed in Table 2., fluctuation
281
trend could be found. With regard to bio-oil, WS-AC and WS-SiC had the highest
282
HHV, the value of 28.66-29.30 MJ/kg for WS pyrolysis bio-oil was higher than that
283
from conventional pyrolysis such as fluidized bed reactor (Wang et al., 2013) or fixed
284
bed reactor (Kabir & Hameed, 2017), compared to WS-AC and WS-SiC, CV-AC and
285
CV-SiC had a relative low HHV. But it was similar to those conventional pyrolysis
286
(Chen et al., 2015). This was in contrast to parent sample (HHV of WS was 18.42
287
MJ/kg, HHV of CV was 22.06 MJ/kg). Similar trends were happened in bio-char,
288
HHV of bio-char from WS had a higher value, especially when using SiC as the
289
additive, it reached up to 31.84 MJ/kg. While the bio-char of CV obtained a low
290
HHV. The reason was that microalgae bio-char had less carbon content than
291
lignocellulose bio-char (Azizi et al., 2018). In other words, MAP of WS with carbon-
292
based additive to produce high calorific value products is more practical than that of
293
CV. The mixing degree of CV and WS also affected HHV greatly. HHV of bio-oil
294
decreased by one third to half after mixing, 50CV50WS attained the worst. Moreover,
295
HHV of bio-char decreased with the increase of CV proportion in samples and using
296
AC as the additive did not gain much privilege for HHV of bio-char. In general, bio-
297
char of WS by MAP improved most apparently compared to WS feedstock, which
298
may be applied as solid fuels for energy plant, feedstock for gasification process,
299
feedstock for catalyst or restoration of land, purifier for water pollution, etc
300
(Motasemi & Afzal, 2013a).
301
4
302
Conclusions
AC could improve the aromatization of chain like hydrocarbons in bio-oils from
303
MAP of WS, 30CV70WS and 50CV50WS, SiC could improve the selectivity of
304
aliphatic hydrocarbons in bio-oils from MAP of CV and 30CV70WS. Bio-oil with
305
high selective can be applied as blending stock for conventional fuel or industrial
306
chemical feedstock after further refining. The synergies resulted from co-pyrolysis of
307
CV and WS were obvious in terms of calorific value fluctuation. In conclusion, using
308
carbon-based additives and combination CV with WS could be alternative methods to
309
improve products quality of MAP of biomass.
310
Acknowledgments
311
This work was supported by the National Natural Science Foundation of China
312
(51406058, 51476060, 51606071); the Guangdong Natural Science Foundation
313
(2015A030313227); the China Scholarship Council (201706155065); the Guangdong
314
Province Engineering Research Center of Highly Efficient and Low Pollution Energy
315
Conversion; the Key Laboratory of Efficient and Clean Energy Utilization of
316
Guangdong Higher Education Institutes, South China University of Technology
317
(KLB10004).
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Fig. 1. A schematic diagram of reaction system. Fig. 2. Temperature profiles of AC and SiC. Fig. 3. The yields of solid, liquid and gas on mass ratio. Fig. 4. Chemical compounds of co-pyrolysis bio-oil with different additives: (a) AC; (b) SiC.
Table 1
Characteristics of CV, WS, AC and SiC. Ultimate analyses (wt.%, dry
Proximate analyses (wt.%,
Ta HHV
basis)
dry basis)
H/
O/
nδ
Cc
Cc
×1
(MJ/ Moist
Volati
As
Fixed C
ure
les
0.66
71.63
C
h
47.
79.76
S
S
33.6
kg)
0.8
47.
6 6.6
1
3
1.7
0.5
7
3
1.6
0.7
<0.
9
3
1d
22.06 9
2
0.15
0
46.1
14.21 2
11.1
7 33
0.7 5.32
N
0-4
19.19 2
W
Ob
Carbona
8.5
V
H
—
18.42
3
0.5 A
80. 10.35
7.9
4.9
1.7
13.7
76.85
C
0.0 4.36
09
3
3
725.19
—
—
9
0.8 d
0.1 Si
—
—
—
—
—
—
—
—
—
C
—
—
— 0.4 7d
a
calculated by difference (fixed carbon=100-moisture-ash-volatiles).
b
calculated by difference (O (%)=100-C-H-N-S-Ash).
c
Molar ratio.
d
Refs. (Salema et al., 2017), (Atwater & Wheeler, 2003), (Duan et al., 2016; Polaert et al., 2017),
respectively
Table 2 HHV of bio-oil and bio-char. Additive
AC
SiC
CV proportion (%)
0
30
50
70
100
0
30
50
70
HHV of bio-oil
29.3
24.8
15.0
24.7
19.9
28.6
25.9
17.9
24.3
（MJ/kg）
0
6
2
8
3
6
8
7
3
HHV of bio-char
28.0
27.4
27.1
25.3
24.8
31.8
23.0
22.7
23.0
（MJ/kg）
9
6
0
1
9
4
9
7
3
100
23.61
19.27
Highlights
Co-pyrolysis of chlorella and sawdust was investigated.
Activated carbon had better temperature response to microwave than silicon carbide.
Aromatics in bio-oil improved by co-pyrolysis of sawdust and chlorella.
Microwave pyrolysis with activated carbon had higher gas and liquid yield.
High calorific bio-char was made by microwave-assisted pyrolysis of sawdust.