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Integrating spent coffee grounds and silver skin as biofuels using torrefaction
Journal Pre-proof Integrating spent coffee grounds and silver skin as biofuels using torrefaction Chao-Heng Tseng, Sih-Yu Jhou, Ying-Chu Chen PII:
S0960-1481(19)31875-0
DOI:
https://doi.org/10.1016/j.renene.2019.12.005
Reference:
RENE 12714
To appear in:
Renewable Energy
Received Date: 11 July 2019 Revised Date:
19 November 2019
Accepted Date: 2 December 2019
Please cite this article as: Tseng C-H, Jhou S-Y, Chen Y-C, Integrating spent coffee grounds and silver skin as biofuels using torrefaction, Renewable Energy (2020), doi: https://doi.org/10.1016/ j.renene.2019.12.005. 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. © 2019 Published by Elsevier Ltd.
Author Contribution Statement Chao-Heng Tseng: Resources Sih-Yu Jhou: Formal Analysis, Data Curation. Ying-Chu Chen: Methodology, Writing- Original Draft, WritingReview & Editing, Funding Qcquisition.
1
Integrating Spent Coffee Grounds and Silver Skin as Biofuels Using
2
Torrefaction
3 Chao-Heng Tsenga, Sih-Yu Jhoub, Ying-Chu Chenc,*
4 5
a
6
University of Technology, Taipei City, 106, Taiwan (R.O.C.)
7
b
8
National Taipei University of Technology, Taipei City, 106, Taiwan (R.O.C.)
9
c
10
Professor, Institute of Environmental Engineering and Management, National Taipei
Master of Science, Institute of Environmental Engineering and Management,
Assistant Professor, Department of Civil Engineering, National Taipei University of
Technology, Taipei City, 106, Taiwan (R.O.C.)
11 12
*Corresponding author: Ying-Chu Chen
13 14
E-mail: [email protected]
15
Phone: +886-2-2771-2171#2634
16
Postal address: Department of Civil Engineering, National Taipei University of
17
Technology, 1, Sec. 3, Zhongxiao E. Rd., Taipei 10608 Taiwan, R.O.C.
18 19
Abstract
20
This study used the torrefaction method to innovatively integrate spent coffee
21
grounds (SCG) and silver skin into biofuels. The biofuels were dried, pelletized, and
22
torrefied at 300°C for 3 h. The mass yields and energy yields of the biofuels ranged 1
23
from 41% to 43% and from 52% to 58%, respectively. The high heat value (HHV)
24
range of the biofuels (24.23–27.28 MJ/kg) was higher than that reported in previous
25
studies. The results revealed that an increase in the percentage of silver skin increased
26
the hygroscopicity of the biofuels, which was unfavorable for storage. On average, the
27
weight increased by 0.24 to 0.57 wt% with a 10 wt% increase of silver skin in the
28
biofuels. The biofuels had zero sulfur and chlorine content and thus would be cleaner
29
energy sources than coal. The elemental compositions of the biofuels were similar to
30
that of lignite with 0.063–0.070 H/C and 0.34–0.44 O/C ratios. The sample most
31
similar to coal, based on heating value, element content, proximate analysis results,
32
and combustion characteristics, exhibited 62% similarity. Integrating silver skin with
33
other materials may be unsuitable for biofuels, but it is helpful for reducing the
34
environmental burden of landfilling or incineration.
35 36
Keywords: biofuel; coffee; pelletization; silver skin; spent coffee grounds;
37
torrefaction
38 39 40 41 2
42
1. Introduction
43
Coffee is one of the most popular beverages in the world [1] and the second most
44
traded product after petroleum [2]. More than 9 million tons of coffee beans were
45
produced worldwide in 2016, mostly in the EU, U.S., Brazil, and Japan [3]. In Taiwan,
46
29,248 tons of coffee beans were imported in 2018, an annual increase of 3.7% [4].
47
The coffee brewing process produces a dark brown solid residue with high moisture
48
content known as spent coffee grounds (SCG) [5,6]. In general, processing one ton of
49
unroasted coffee produces 650 kg of SCG [7]. The U.S. Department of Agriculture
50
(USDA) estimated that approximately 4 million tons of SCG were produced globally
51
in 2011 [8]. SCG is a highly potent pollutant, as the caffeine, tannins, and polyphenols
52
are toxic and require large quantities of oxygen to degrade [2,9]. Coffee and the
53
by-products of its production are mostly discharged into the environment without
54
proper management. Specifically, coffee companies produce more than 2 billion tons
55
of by-products annually, including SCG and silver skin, and most of it is sent to
56
landfill [10]. Growing environmental awareness has triggered an interest in using
57
waste products, such as SCG or silver skin, in the fabrication of environmentally
58
friendly materials.
59
Incineration and composting are the two traditional treatments for SCG [11],
60
although studies have also investigated using recycled SCG for animal feed [12], for 3
61
organic compost [13], as an adsorbent [14], as fuel pellets [15], as biodiesel and
62
ethanol [16], and in the production of active carbon [17]. Among these treatments,
63
recycling into fuel – known as second generation fuel – has been recognized as a
64
feasible technology given its compatibility with current incineration technologies. The
65
use of biofuels has been proposed for increasing the sustainability of fuels derived
66
from renewable sources [9], and SCG has the potential to become a raw material for
67
biofuels [16]. The highest heating value (HHV) of SCG is approximately 25 kJ/kg,
68
which is similar to that of coal [18]; the oil content of SCG ranges in mass fraction
69
from 11% to 20% depending on the type of coffee [16]. Moreover, the high calorific
70
value of SCG makes it a feasible renewable energy resource for incineration purposes
71
[14]. Silver skin is part of the structure of the fruit of the coffee tree, and is produced
72
when the coffee bean is roasted [19]. A few studies have examined whether silver
73
skin has the potential to be used as a sustainable material in the building construction
74
industry [19]. Ronix et al. (2017) successfully used hydrothermally carbonized coffee
75
husks to adsorb methylene blue dye [20]. Considerable research is still needed to
76
make energy and material recovery from coffee residues a technically viable option
77
[21]. To the best of our knowledge, no study has tried to integrate silver skin and SCG
78
into biofuels.
4
79
The energy efficiency of biomass can be improved via physical processes such as
80
compression and pelletization [22]. The pelletized biomass is characterized by lower
81
moisture content, higher bulk density, and higher volumetric energy density (GJ/m3)
82
[23] than the original biomass. Kondamudi et al. (2008) proposed reusing SCG after
83
oil extraction and pelletization [15]. Allesina et al. (2017) pelletized SCG with a
84
thermal efficiency of 41.2%, which is higher than the 37.7% obtained from wood
85
pellets [11]. Pelletized biomass residues are easier to transport and store than
86
unpelletized biomass residues.
87
Heating and drying technologies have been developed to increase the low
88
heating value of wet SCG (ca. 8.4 MJ/kg [24]). Torrefaction is viewed as a
89
technology with a low environmental impact that is useful for the sustainable
90
management of both energy and biomass [25, 26]. During the mild pyrolysis process
91
of torrefaction, which takes place at low operating temperatures of 200–300°C,
92
biomass undergoes dehydration, devolatilization, depolymerization, and carbonization
93
[27]. The process can efficiently destroy stubborn fibers and enhance energy density
94
[28]. Around 70% of the initial mass and a maximum of 90% of the initial energy
95
content is retained in the torrefied biomass [29]. Torrefied products have
96
thermo-technical characteristics that are comparable to coal [30]. Numerous studies
97
have been conducted on the torrefaction of different biomasses, such as bamboo, 5
98
willow, coconut shell [31], eucalyptus [32], reed canary grass [33], sawdust, rice husk
99
[34], and SCG [27]. The torrefaction of solid residues is limited due to the high
100
heterogeneity of biomass residues [26].
101
Given the benefits of biomass torrefaction and pelletization, this pilot study
102
combined both treatments to produce samples of integrated SCG and silver skin
103
biofuels. The composite morphology and mechanical and thermal properties of the
104
biofuels were analyzed using scanning electron microscopy, Fourier transform
105
infrared spectroscopy, element analysis, and thermogravimetric analysis. The effects
106
on the biofuel mass and energy yields of changing the blending ratio of the raw
107
materials (SCG, silver skin, and pine sawdust) were also investigated. Previous
108
studies have shown that the operating parameters of torrefaction should be carefully
109
considered and the interactions between these parameters should be taken into
110
account [35]. The results of this study provide new methods of fabricating
111
environmentally friendly biofuels by integrating SCG and silver skin, which is
112
important with regards to the need to reduce environmental burdens and the
113
dependence on fossil fuels.
114 115
2. Materials and Methods
116
2.1. Preparation of material 6
117
SCG was the main feedstock for the production of biofuels in this study. The
118
SCG samples (SCG-A and SCG-B) were acquired from two major convenience stores,
119
where they had originally been treated as waste. The wet feedstock samples had a
120
55% moisture content, which was eliminated by drying at 105 ± 2°C for 24 h. The
121
dehydrated SCG was then sealed in plastic bags and stored in a desiccator at room
122
temperature until required for analysis.
123
However, a preliminary test showed that it was difficult to compress dehydrated
124
SCG into stable pellets. A mixture of ≥50% pine sawdust with a 10% moisture
125
content was used to increase the stability of the pelletized products. Table 1 shows the
126
ratios, by weight percentages, of the SCG, silver skin, and pine sawdust in the
127
different samples.
128 129
Table 1 here. 2.2. Experimental apparatus
130
A schematic of the experimental setup and the mass and energy flows is shown
131
in Fig. 1. The torrefaction reactor (TF55030A, Thermo) consisted of a quartz tube, a
132
tube furnace, a mass flow meter, and a product gas treatment unit. The quartz tube had
133
a 23 mm outer diameter and was 900 mm long (Thermo, USA). In each test, 10 ± 1 g
134
of pelletized material was loaded into the tube in a sample container, to measure the
135
torrefaction temperature. The temperature was increased at a constant rate of 7
136
10°C/min until it reached the desired temperature. The sample was torrefied for 3 h at
137
300°C to maximize the efficiency of torrefaction. After this time, the temperature was
138
decreased at a rate of 4.5°C/min; when the temperature fell below 120°C, the samples
139
were weighed and characterized. On average, 57.7% of the mass of the pellets was
140
lost during torrefaction (Fig. 1). Nitrogen (99.99 vol%) stored in a steel cylinder was
141
used as a carrier gas to purge the tube so that the torrefaction occurred in an
142
oxygen-free environment. The volumetric flow rate of the carrier gas was fixed at 100
143
mL/min by a mass flow meter, which was connected to a readout device. A
144
customized heated single-pellet die system was used in this study. A load force of 4
145
kgf/cm2 was applied by a piston for 3 min to compress the SCG into pellets that were
146
8.5 ± 1 mm in diameter and 20 ± 3 mm in height. A die temperature of 150°C was
147
sufficient to make control pellets. The energy of the pellets before torrefaction ranged
148
from 20.22 to 20.38 MJ/kg, depending on the ratios of the SCG, silver skin, and pine
149
sawdust. The measured and controlled variables are shown in Table 2.
150
Post-torrefaction gas and oil analyses will be performed in future studies.
151
Figure 1 here.
152
Table 2 here.
153
2.3. Properties of the SCG
8
154
For each experiment, the moisture content, HHV, volatile fraction, ash content,
155
and hygroscopicity of the sample were measured. Moisture content was measure
156
using the variating weights of the samples dried at 105°C for 24 h in a circular heating
157
oven (Model OVP-30, Hong-Siang Co., Taiwan). The calorific value obtained from
158
the bomb calorimeter (Model 1341, Parr Instrument Co., USA) was the HHV, which
159
included the latent heat of the vapor emitted from the specimen. Proximate analysis
160
was performed in accordance with the standard procedure of the American Society for
161
Testing and Materials (ASTM) for obtaining volatile fraction and ash content. The
162
average temperature and relative humidity of Taiwan in 2018 were 29.55 °C and
163
80.79%; these values were used as the basis of the hygroscopicity test [36]. About 1 g
164
of each sample was placed in the humidity simulator at 29.08±2°C and 79.83±3%
165
humidity. A fog generator with the supersonic frequency of 68 W and a fog producing
166
rate of 20 mL/min was installed. Both 1-h and 5-h hygroscopicity samples were
167
weighed to evaluate the preservation of the torrefied products. The mass and energy
168
yields are defined by Eqs. (1) and (2), as used by Bridgeman et al. (2008) [33].
169
170
!"
= #
$% &
=
× 100%
×
''(
)
''(
9
* *
(1) × 100% .
(2)
171
The experiment was repeated at least three times under each given condition. The data
172
shown in the tables are average values and the relative error between repetitions was
173
less than 5%.
174
2.4.Characterization of SCG
175
The SCG samples were characterized using thermogravimetric analysis (TGA),
176
Fourier transform infrared spectroscopy (FTIR), an element analyzer (EA), and a
177
scanning electron microscope equipped with an EDX spectrometers (SEM/EDX).
178
2.4.1. Thermogravimetric analyses
179
The pyrolysis characteristics of the samples were examined using a thermogravimetric
180
analyzer (Model Q500, TA Instruments, Inc., New Castle, USA). The samples were
181
loaded into a crucible that was placed inside the analyzer and the weight of the sample
182
was constantly measured. The analyzer measured and recorded the sample weight loss
183
as the temperature increased. For each test, a sample of around 5 mg with a particle
184
size of 100–200 mesh was used. The temperature increased from 25°C to 900°C, and
185
the heating rate was controlled at 10°C/min. Nitrogen gas was used as the carrier gas
186
in the analyzer and the flow rate was fixed at 40 mL/min. The relative error between
187
the TGA measurements was controlled to less than 5%. Two comprehensive indexes
188
(+ and , ) representing the combustion characteristics [37] were evaluated and
189
calculated as follows: 10
-./012 ×-./0314
190
+=
191
, = 8* ln -./
192
@8A
193
respectively. 8 , 8C , 8* , and 8
194
corresponding temperature, and peak temperature, which reflected the samples’
195
thermal behavior during the combustion process. These parameters can be derived
196
from TG and DTG curves. ∆8D/E is the temperature range of
197
2.4.2. Fourier transform infrared spectroscopy (FTIR) analyses
198
The chemical groups and constituents bonding arrangements found in the SCG were
199
determined by Fourier transform infrared spectroscopy (FTIR) using an FTIR
200
spectrophotometer (NICOLETiS5, Thermo, USA) equipped with a single reflection
201
diamond crystal ATR module. The spectra were recorded from the 400 to 4000 cm-1
202
spectral range, at a rate of 32 scans and a spectral resolution of 44 cm-1. The pelletized
203
samples were not favorable for FTIR analysis due to low transmission and
204
reflectance.
205
2.4.3. Element analyses
206
The elemental analysis was performed using an elemental analyzer (FlashEA 1112
207
HT, Thermo, USA) to measure the weight percentages of C, H, N, S, and Cl in the
208
SCG, whereas the weight percentage of O was obtained by considering difference.
(3)
.5 6 ×.7
∆.
B
10>? .
and @8A
(4)
are the maximum and average combustion rates (mg/min), B
are the ignition temperature, burnout temperature,
11
-./ -./012
= 0.5.
209
2.4.4. Scanning electron microscopy analyses
210
The particle shapes and surface textures were evaluated using scanning electron
211
microscopy (SEM) equipped with an EDX spectrometers using a model JSM-7610F
212
(JEOL, Japan), operating at 0.1 kV. The samples were mounted on aluminum stubs
213
using carbon tape. All of the specimens were sputter-coated with gold.
214
2.5. Similarity measurement
215
For fabricated biofuels that are designed to replace coal, the similarity of the
216
fabricated material’s characteristics to coal’s characteristics can be evaluated using
217
Eqs. (5) and (6) as follows:
218
H=
219
H = 1−
220
where ℎ, N, O, and P represent heating value, element content, proximate analysis
221
results, and combustion characteristics. T and U are the characteristics of the
222
biofuel and coal, respectively.
∑J K
, H = ℎ, N, O, P R5 >S5 S5
(5)
,
(6)
223 224
3. Results and Discussion
225
3.1. Properties of the biofuels
226
The properties of the biofuels are shown in Table 3. Samples 1 to 3 were
227
fabricated from SCG-A and Samples 4 to 6 were fabricated from SCG-B. Their mass 12
228
yields ranged from 41% to 43%; however, their energy yields were higher, at 52% to
229
58%. To increase the stability of the pellets, pine sawdust had to make up at least 50
230
wt% of the material. As the proportion of silver skin increased, both the mass and
231
energy yields decreased due to the HHV and fix carbon content of silver skin were
232
lower than SCGs. Poudel et al. (2015) found that a lower mass yield results in a lower
233
energy yield [38]. Also, the mass yield was decreased with an increase in temperature
234
or oxygen concentration [39]. The biofuels fabricated from SCG-A had higher mass
235
and energy yields than those fabricated from SCG-B. Previous studies found the mass
236
yield and energy of pine chips, logging residues, rice husk, and peanut husk fell in the
237
ranges of 52–89%, 71–94%, 41–79%, and 55–98% [28,40]. The yields are dependent
238
on the raw biomass, torrefaction temperature, residence time and reactor type [41].
239
The HHV of Sample 1 reached the same level as the HHV of coal (27.28 MJ/kg),
240
as shown in Table 3. The experimental set up and blended materials achieved higher
241
HHV values (ranging from 24.23 to 27.28 MJ/kg, shown in Fig. 1) than reported in
242
previous studies. Limousy et al. (2013) and Jeguirim et al. (2014) blended SCG and
243
pine sawdust into pellets [42,43]. They used the pyrolysis method to fabricate biofuels
244
and their HHVs were 18–20 MJ/kg at 11.78% moisture content. This study confirmed
245
that torrefaction has higher energy efficiency of training the initial energy content of
246
the biomass. Lower moisture content of the samples (1.77–3.32%) was also favorable 13
247
for increasing their energy properties. Previous study discovered that when SCG was
248
incorporated into wood chip logs, the HHV decreased to 17.39 MJ/kg at 10%
249
moisture content in a residential stove [44]. The comparison of the combustion
250
characteristics shows that the blended material had a higher reactivity than pure SCG
251
or sawdust.
252
Table 3 here.
253
The SCG had better heating values after drying. The moisture content of the
254
samples in this study ranged from 1.77% to 3.32%, as shown in Table 3. The original
255
moisture content of SCG-A and SCG-B was about 50%. The oven drying process
256
reduced the moisture content and made the pellets more suitable for longer storage.
257
When the proportion of fixed carbon increased, the HHV of the sample increased. In
258
this study, the volatile fraction of the samples was higher than that of coal. Some
259
harmful emissions, particularly nitrogen oxides in flue gas, were discovered by Kang
260
et al. (2017) [24]. There were about 2.79 g of nitrogen in 100 g of dry SCG [31]. The
261
ash content of the samples was about 1 to 3%, which is lower than coal. Thus, the
262
biofuels fabricated in this study have the potential to reduce the environmental burden
263
caused by landfilling.
264
Figure 2 shows the hygroscopicity of the samples. The increase in weight (wt%)
265
over a 5-h period was limited. Exposure of the samples to atmospheric conditions for 14
266
1 h allowed the samples to absorb water molecules and reach saturation. When the
267
proportion of silver skin increased, the hygroscopicity of the sample increased. The
268
composition of the biofuels with silver skin was unstable in the atmospheric
269
environment. On average, the weight increased by 0.24 to 0.57 wt% with a 10 wt%
270
increase of silver skin in the biofuels. Sample 1, which had 50% SCG-A and 50%
271
pine sawdust, was favorable for storage and had high hydrophobic ability.
272 273
Figure 2 here. 3.2.Characterization of the biofuels
274
As noted above, the biofuels were characterized using element analysis, FTIR,
275
SEM/EDX, and TGA. The carbon, hydrogen, oxygen, nitrogen, sulfur, and chlorine
276
contents of the samples were measured by elemental and EDX analysis, as shown in
277
Table S1. Consistent with previous studies, carbon sources were the primary
278
substances in the biofuels, including glucose and lignin, which is a single unit of
279
cellulose and hemicellulose [45]. The results of elemental and EDX were consisted
280
and the carbon content in the biofuels was 65–71 wt% in this study. SCG also
281
contains protein, which was measured using the nitrogen and sulfur sources in the
282
samples [46]. Neither sulfur nor chlorine were detected in the analysis by the
283
elemental analysis. None of the samples emitted dioxin or sulfur, which are air
15
284
pollutants, when heated. The fabricated biofuels were cleaner energy resources than
285
coal.
286
The element composition of the samples was similar to that of lignite, as shown
287
in the Van Krevelen diagram in Fig. 3. The benefits of torrefaction included lower
288
atomic oxygen-to-carbon (O/C) and hydrogen-to-carbon (H/C) ratios. The decrease in
289
the O/C ratio during torrefaction is attributed to the generation of volatiles rich in
290
oxygen, such as CO2 and H2O [47,48]. A reduced O/C ratio increases the HHV of the
291
torrefied biomass [38]. These changes in O/C and H/C ratios are mainly due to water
292
release and to the partial elimination of oxygen by decarboxylation, decarbonylation,
293
and dehydration reactions as well as in the form of volatile oxygenated compounds
294
such as acetic acid or methanol [49]. Lignite provides environmentally friendly power
295
generation with an energy content of 10 to 20 MJ/kg [50]. The biofuels made in this
296
study also had an energy content in a range of 18–20 MJ/kg, as shown in Table 3.
297
Figure 3 here.
298
Figure 4 shows the FTIR results for the SCG, silver skin, and pine sawdust. A
299
typical lignocellulosic material is composed of polysaccharides and aromatic
300
polymers. The broad peak between 3600 and 3200 cm-1 is related to the hydroxyl
301
group O-H stretching vibration [45]. The peaks at both 2920 cm-1 and 2850 cm-1 were
302
attributed to asymmetric and symmetric stretching of the C-H bonds of the methyl 16
303
group in the caffeine molecule along with sugars [51]. The C-H bonds decreased after
304
torrefaction, as shown in Figs. 4(a) and (b). A band at 1742 cm-1, which was attributed
305
to carbonyl vibration (C=O) in the aliphatic ester or triglycerides that originated in the
306
lipids [52], remained after torrefaction. Several peaks, 1245 cm-1 and 1032 cm-1 (C-O
307
stretching) verified that SCG contains a great variety of monosaccharides together
308
with diverse acid molecules, such as chlorogenic acid, caffeic acid, and coumaric acid
309
[53]. The band between 1700 and 1600 cm-1 was highly associated with chlorogenic
310
acids and caffeine [54]. The broad band between 1135 and 952 cm-1 was the result of
311
the stretching vibration of C-O in C-O-H bonds such as glycosidic bonds in
312
galactomannan polysaccharide sugars [55].
313
Figure 4 here.
314
Consistent with previous studies, the TGA curves for biofuels showed three
315
defined weight loss stages (Fig. 5). The first one occurred in the 105–360°C
316
temperature
317
low-molecule-mass volatile compounds [56]. During this state, there was about 5%
318
weight loss. The higher weight loss (35%) in the second range (360–490 °C)
319
corresponds to polysaccharide depolymerization and decomposition. Hemicellulose
320
and cellulose mainly decompose at 220–315°C and 315–400°C [57]. In the third
321
phase (490–800 °C), the formation of carbonaceous materials resulted in a further
range,
reflecting
the
water
17
evaporation
and
the
release
of
322
weight loss of around 10%. The residue at 800°C for biofuels was around 30%. Thus,
323
weight loss increased with temperature until burnout was reached.
324
Figure 5 here.
325
The combustion characteristics of the biofuels are listed in Table S2. The peak
326
temperature (8* ) of the biofuels was in the 409–419°C range, which is lower than that
327
of coal (about 441°C). Therefore, the whole combustion process of biofuels occurs
328
earlier than that of coal. The ignition temperatures (8 ) of Samples 1 and 4 were lower
329
than those of the other samples due to their higher volatile content, as shown in Table
330
3. The volatile content was increased when percentage of the silver skin in the
331
samples increased. The faster devolatilization and oxidation rates of the volatile
332
matter caused lower particle ignition temperatures [58]. The comprehensive +
333
indexes of Samples 1 and 4 were greater than those of the other samples, indicating
334
that their combustion performance was superior. The results of the comprehensive ,
335
index were the inverse of the results of the + index. A high ash content may have
336
inhibited the combustion reaction of the samples, leading to a decrease in combustion
337
performance [37].
338
Fig. 6 shows the SEM images of the raw materials’ morphologies. In addition to
339
its caloric value, SCG has a unique microporous structure with a high surface area of
340
about 300–1,000 m2/g [59]. SCG has been proven to be an effective adsorbent for a 18
341
wide range of contaminants [60]. The SCG particles mainly have a diameter range of
342
20–75 μm. Previous studies have suggested that particles of random size showed a
343
good response in electrorheology performance [46]. The torrefaction process may
344
have increased porosity, and similar results were noted by Mendes et al. (2019) [56].
345 346
Figure 6 here. 3.3. Similarity results
347
The study explored the creation of biofuels that were similar to coal with the aim
348
of reducing dependence on fossil fuels. The new biofuels’ similarity to coal was
349
evaluated by comparing their heating value, element content, proximate analysis
350
results, and combustion characteristics. Figure 7 shows the results of the comparison:
351
there was a similarity of at least 55%. The most similar sample was Sample 4, which
352
was 62% the same as coal. As the percentage of silver skin increased, the similarity to
353
coal decreased. The characteristics of the fabricated biofuels that were most similar to
354
coal were heating value (83–94%), followed by combustion characteristics (80–84%),
355
proximate analysis results (24–38%), and element content (27–42%). Previous studies
356
have shown that the heating value of SCG is similar to coal and higher than wood and
357
other biomass residues [61]. Future studies should consider other indicators.
358
Figure 7 here.
359 19
360
4. Conclusions
361
This study used the torrefaction method to fabricate biofuels that integrated SCG
362
and silver skin. The biofuels were dried, pelletized, and combusted at 300°C for 3 h.
363
To test whether the fabricated biofuels could replace coal, the similarity of their
364
characteristics was evaluated based on the parameters of heating value, element
365
content, proximate analysis results, and combustion characteristics. The mass yields
366
ranged from 41% to 43%; however, their energy yields were higher at 52% to 58%.
367
As the percentage of silver skin in the biofuels increased, both the mass and energy
368
yields decreased. The HHV of the biofuels ranged from 24.23 to 27.28 MJ/kg, which
369
are higher values than reported in previous studies. The results showed that increasing
370
the percentage of silver skin increased the hygroscopicity of the biofuels. On average,
371
there was a 0.24 to 0.57 wt% increase with an increase of 10 wt% of silver skin. The
372
carbon content in the samples ranged from 65 to 71 wt% and the sulfur and chlorine
373
contents were 0. These results demonstrate that these biofuels are cleaner energy
374
resources than coal. The element contents in the biofuels were similar to lignite, as
375
shown in the Van Krevelen diagram. The sample most similar to coal, Sample 4, was
376
62% similar. Integrating silver skin may reduce the quality of the biofuels, but it helps
377
reduce the environmental burden of landfilling or incineration.
378 20
379 380
Acknowledgments The authors would like to thank the Ministry of Science and Technology,
381
Republic
of
China,
for
financial
support
under
Contract
No.
MOST
382
106-2627-M-002-028. They also acknowledge the Taiwan EPA and Ministry of
383
Economic Affairs, and other governmental agencies for assisting with data collection.
384
The authors further thank the anonymous reviewers for their invaluable comments
385
and suggestions.
386 387
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576
Table 1
Sample composition Pine
Content
SCG-A
SCG-B
Silver skin
(%)
(%)
(%)
sawdust Sample
Weight (g)
(%) 1
50.00
0.00
0.00
50.00
3.0026
2
40.00
0.00
10.00
50.00
3.0273
3
30.00
0.00
20.00
50.00
3.0110
4
0.00
50.00
0.00
50.00
3.3830
5
0.00
40.00
10.00
50.00
3.0596
6
0.00
30.00
20.00
50.00
2.9985
577 578 579 580 581 582 583 584 585 586 587
31
588
Table 2
Variables used in this study
Variable
Unit
Value
Controlled variable pelletized load force
kgf/cm3
4
pellet size
mm
8.5±1 (diameter) and 20±3 (height)
heating rate
o
10
torrefied temperature
o
300
torrefied time
h
3
C/min C
Measured variable moisture content
wt%
HHV
MJ/kg
volatile fraction
wt%
ash content
wt%
fix carbon
wt%
hygroscopicity
wt%
mass yield
%
energy yield
%
similarity
%
589 32
590
Table 3 Sample properties moisture Content
mass yield
energy yield HHV (MJ/kg)
Sample
(%)
volatile content
fix carbon
(wt%)
(wt%)
ash content content
(%)
(wt%) (wt%)
591
a
SCG-A
－
－
22.12
9.49
1.36
81.58
17.06
SCG-B
－
－
21.81
9.46
1.40
82.37
16.23
Silver skin
－
－
17.82
7.48
3.24
80.69
16.07
1
42.72
58.33
27.28
3.32
3.0026
60.59
34.54
2
41.94
54.51
25.52
2.29
3.0273
63.78
32.30
3
41.68
53.45
24.73
2.84
3.0110
68.66
25.91
4
42.51
56.94
26.55
2.41
3.3830
64.15
32.21
5
41.73
52.91
24.73
1.77
3.0596
66.80
28.42
6
41.57
52.50
24.23
3.18
2.9985
68.26
27.14
Coal
－
－
27.28b
1.28-21.62 a
2.93-16.73 a
22.38-46.42 a
24.26-70.05 a
[62]
33
592
b
[63]
34
593 594
Fig. 1. The experimental setup and mass and energy flow.
595 596 597 598 599
35
6 1-h 5-h
5
weight (wt%)
4
3
2
1
0 1
2
4
5
sample
600 601
3
Fig. 2
Hygroscopicity of the biofuels.
602 603
36
6
0.18 0.16 0.14
H/C ratio
0.12
anthracite bituminous coal lignite peat biomass sample
0.10 0.08 0.06 0.04 0.02 0.00 0.0
0.4
0.6
0.8
1.0
1.2
O/C ratio
604 605
0.2
Fig. 3
Van Krevelen diagram of the biofuels and coal.
606 607 608 609 610 611
37
1.4
102
(a)
103
(b)
100 102
98
transmittance (%)
101
2882 cm C-H 2920 cm C-H
94
transmittance (%)
-1
3600~3200 cm C-O
96
-1
-1
92
-1
1245 cm C-O 1032 cm-1 C-O
90 SCG-A torrefied SCG-A
88
3600~3200 cm-1 C-O
99 98
SCG-B torrefied SCG-B 2882 cm-1 C-H
97 96
-1
1742 cm C=O
86
100
2920 cm-1 C-H
95
4000
3000
612
2000
1000
4000
3000
-1
1245 cm-1 C-O 1742 cm-1 C=O 1032 cm-1 C-O
2000
1000 -1
wavenumber (cm )
wavenumber (cm )
700
(d)
500
(c)
500
300
3600~3200 cm-1 C-O
-1
2829 cm C-H
2882 cm-1 C-H
200
1245 cm-1 C-O 1742 cm C=O
100 2920 cm C-H
transmittance (%)
transmittance (%)
400
3600~3200 cm-1 C-O 2882 cm-1 C-H
300
1245 cm-1 C-O
-1
100
-1
2920 cm-1 C-H
silver skin torrefied silver skin
1742 cm-1 C=O
0 4000
613
615
400
200
0
614
pine sawdust torrefied pine sawdust
600
3000
2000
4000
1000
Fig. 4
3000
2000
1000
wavenumber (cm-1)
wavenumber (cm-1)
FTIR spectra for of (a) SCG-A, (b) SCG-B, (c) silver skin, and (d) pine sawdust.
38
100
weight (%)
90
80
70 sample 1 sample 2 sample 3 sample 4 sample 5 sample 6
60
50 200
400
temperature (oC)
616 617
600
Fig. 5
TGA curves of the biofuels.
39
800
618 619
Fig. 6
SEM images of (a) SCG-A, (b) torrefied SCG-A, (c) SCG, (d) torrefied
620
SCG-B, (e) pine sawdust, (f) torrefied pine sawdust, (g) silver skin, and (h)
621
torrefied silver skin. 40
100
0.63 0.62
80
0.60
60
0.59 40
20
0.58 0.57
SCG (%) silver skin (%) pine sawdust (%) similarity result
0.56
0
0.55 1
2
4
5
6
sample
622 623
3
Fig. 7
Similarity analysis of the biofuels and coal.
624
41
similarity results
composition (%)
0.61
Research Highlights Spent coffee grounds (SCG) and silver skin were integrated into biofuels. The high heat value (HHV) was in range of the biofuels (24.23–27.28 MJ/kg). An increase in silver skin increased the hygroscopicity of the biofuels. The sample most similar to coal exhibited 62% similarity.
S0960-1481(19)31875-0
DOI:
https://doi.org/10.1016/j.renene.2019.12.005
Reference:
RENE 12714
To appear in:
Renewable Energy
Received Date: 11 July 2019 Revised Date:
19 November 2019
Accepted Date: 2 December 2019
Please cite this article as: Tseng C-H, Jhou S-Y, Chen Y-C, Integrating spent coffee grounds and silver skin as biofuels using torrefaction, Renewable Energy (2020), doi: https://doi.org/10.1016/ j.renene.2019.12.005. 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. © 2019 Published by Elsevier Ltd.
Author Contribution Statement Chao-Heng Tseng: Resources Sih-Yu Jhou: Formal Analysis, Data Curation. Ying-Chu Chen: Methodology, Writing- Original Draft, WritingReview & Editing, Funding Qcquisition.
1
Integrating Spent Coffee Grounds and Silver Skin as Biofuels Using
2
Torrefaction
3 Chao-Heng Tsenga, Sih-Yu Jhoub, Ying-Chu Chenc,*
4 5
a
6
University of Technology, Taipei City, 106, Taiwan (R.O.C.)
7
b
8
National Taipei University of Technology, Taipei City, 106, Taiwan (R.O.C.)
9
c
10
Professor, Institute of Environmental Engineering and Management, National Taipei
Master of Science, Institute of Environmental Engineering and Management,
Assistant Professor, Department of Civil Engineering, National Taipei University of
Technology, Taipei City, 106, Taiwan (R.O.C.)
11 12
*Corresponding author: Ying-Chu Chen
13 14
E-mail: [email protected]
15
Phone: +886-2-2771-2171#2634
16
Postal address: Department of Civil Engineering, National Taipei University of
17
Technology, 1, Sec. 3, Zhongxiao E. Rd., Taipei 10608 Taiwan, R.O.C.
18 19
Abstract
20
This study used the torrefaction method to innovatively integrate spent coffee
21
grounds (SCG) and silver skin into biofuels. The biofuels were dried, pelletized, and
22
torrefied at 300°C for 3 h. The mass yields and energy yields of the biofuels ranged 1
23
from 41% to 43% and from 52% to 58%, respectively. The high heat value (HHV)
24
range of the biofuels (24.23–27.28 MJ/kg) was higher than that reported in previous
25
studies. The results revealed that an increase in the percentage of silver skin increased
26
the hygroscopicity of the biofuels, which was unfavorable for storage. On average, the
27
weight increased by 0.24 to 0.57 wt% with a 10 wt% increase of silver skin in the
28
biofuels. The biofuels had zero sulfur and chlorine content and thus would be cleaner
29
energy sources than coal. The elemental compositions of the biofuels were similar to
30
that of lignite with 0.063–0.070 H/C and 0.34–0.44 O/C ratios. The sample most
31
similar to coal, based on heating value, element content, proximate analysis results,
32
and combustion characteristics, exhibited 62% similarity. Integrating silver skin with
33
other materials may be unsuitable for biofuels, but it is helpful for reducing the
34
environmental burden of landfilling or incineration.
35 36
Keywords: biofuel; coffee; pelletization; silver skin; spent coffee grounds;
37
torrefaction
38 39 40 41 2
42
1. Introduction
43
Coffee is one of the most popular beverages in the world [1] and the second most
44
traded product after petroleum [2]. More than 9 million tons of coffee beans were
45
produced worldwide in 2016, mostly in the EU, U.S., Brazil, and Japan [3]. In Taiwan,
46
29,248 tons of coffee beans were imported in 2018, an annual increase of 3.7% [4].
47
The coffee brewing process produces a dark brown solid residue with high moisture
48
content known as spent coffee grounds (SCG) [5,6]. In general, processing one ton of
49
unroasted coffee produces 650 kg of SCG [7]. The U.S. Department of Agriculture
50
(USDA) estimated that approximately 4 million tons of SCG were produced globally
51
in 2011 [8]. SCG is a highly potent pollutant, as the caffeine, tannins, and polyphenols
52
are toxic and require large quantities of oxygen to degrade [2,9]. Coffee and the
53
by-products of its production are mostly discharged into the environment without
54
proper management. Specifically, coffee companies produce more than 2 billion tons
55
of by-products annually, including SCG and silver skin, and most of it is sent to
56
landfill [10]. Growing environmental awareness has triggered an interest in using
57
waste products, such as SCG or silver skin, in the fabrication of environmentally
58
friendly materials.
59
Incineration and composting are the two traditional treatments for SCG [11],
60
although studies have also investigated using recycled SCG for animal feed [12], for 3
61
organic compost [13], as an adsorbent [14], as fuel pellets [15], as biodiesel and
62
ethanol [16], and in the production of active carbon [17]. Among these treatments,
63
recycling into fuel – known as second generation fuel – has been recognized as a
64
feasible technology given its compatibility with current incineration technologies. The
65
use of biofuels has been proposed for increasing the sustainability of fuels derived
66
from renewable sources [9], and SCG has the potential to become a raw material for
67
biofuels [16]. The highest heating value (HHV) of SCG is approximately 25 kJ/kg,
68
which is similar to that of coal [18]; the oil content of SCG ranges in mass fraction
69
from 11% to 20% depending on the type of coffee [16]. Moreover, the high calorific
70
value of SCG makes it a feasible renewable energy resource for incineration purposes
71
[14]. Silver skin is part of the structure of the fruit of the coffee tree, and is produced
72
when the coffee bean is roasted [19]. A few studies have examined whether silver
73
skin has the potential to be used as a sustainable material in the building construction
74
industry [19]. Ronix et al. (2017) successfully used hydrothermally carbonized coffee
75
husks to adsorb methylene blue dye [20]. Considerable research is still needed to
76
make energy and material recovery from coffee residues a technically viable option
77
[21]. To the best of our knowledge, no study has tried to integrate silver skin and SCG
78
into biofuels.
4
79
The energy efficiency of biomass can be improved via physical processes such as
80
compression and pelletization [22]. The pelletized biomass is characterized by lower
81
moisture content, higher bulk density, and higher volumetric energy density (GJ/m3)
82
[23] than the original biomass. Kondamudi et al. (2008) proposed reusing SCG after
83
oil extraction and pelletization [15]. Allesina et al. (2017) pelletized SCG with a
84
thermal efficiency of 41.2%, which is higher than the 37.7% obtained from wood
85
pellets [11]. Pelletized biomass residues are easier to transport and store than
86
unpelletized biomass residues.
87
Heating and drying technologies have been developed to increase the low
88
heating value of wet SCG (ca. 8.4 MJ/kg [24]). Torrefaction is viewed as a
89
technology with a low environmental impact that is useful for the sustainable
90
management of both energy and biomass [25, 26]. During the mild pyrolysis process
91
of torrefaction, which takes place at low operating temperatures of 200–300°C,
92
biomass undergoes dehydration, devolatilization, depolymerization, and carbonization
93
[27]. The process can efficiently destroy stubborn fibers and enhance energy density
94
[28]. Around 70% of the initial mass and a maximum of 90% of the initial energy
95
content is retained in the torrefied biomass [29]. Torrefied products have
96
thermo-technical characteristics that are comparable to coal [30]. Numerous studies
97
have been conducted on the torrefaction of different biomasses, such as bamboo, 5
98
willow, coconut shell [31], eucalyptus [32], reed canary grass [33], sawdust, rice husk
99
[34], and SCG [27]. The torrefaction of solid residues is limited due to the high
100
heterogeneity of biomass residues [26].
101
Given the benefits of biomass torrefaction and pelletization, this pilot study
102
combined both treatments to produce samples of integrated SCG and silver skin
103
biofuels. The composite morphology and mechanical and thermal properties of the
104
biofuels were analyzed using scanning electron microscopy, Fourier transform
105
infrared spectroscopy, element analysis, and thermogravimetric analysis. The effects
106
on the biofuel mass and energy yields of changing the blending ratio of the raw
107
materials (SCG, silver skin, and pine sawdust) were also investigated. Previous
108
studies have shown that the operating parameters of torrefaction should be carefully
109
considered and the interactions between these parameters should be taken into
110
account [35]. The results of this study provide new methods of fabricating
111
environmentally friendly biofuels by integrating SCG and silver skin, which is
112
important with regards to the need to reduce environmental burdens and the
113
dependence on fossil fuels.
114 115
2. Materials and Methods
116
2.1. Preparation of material 6
117
SCG was the main feedstock for the production of biofuels in this study. The
118
SCG samples (SCG-A and SCG-B) were acquired from two major convenience stores,
119
where they had originally been treated as waste. The wet feedstock samples had a
120
55% moisture content, which was eliminated by drying at 105 ± 2°C for 24 h. The
121
dehydrated SCG was then sealed in plastic bags and stored in a desiccator at room
122
temperature until required for analysis.
123
However, a preliminary test showed that it was difficult to compress dehydrated
124
SCG into stable pellets. A mixture of ≥50% pine sawdust with a 10% moisture
125
content was used to increase the stability of the pelletized products. Table 1 shows the
126
ratios, by weight percentages, of the SCG, silver skin, and pine sawdust in the
127
different samples.
128 129
Table 1 here. 2.2. Experimental apparatus
130
A schematic of the experimental setup and the mass and energy flows is shown
131
in Fig. 1. The torrefaction reactor (TF55030A, Thermo) consisted of a quartz tube, a
132
tube furnace, a mass flow meter, and a product gas treatment unit. The quartz tube had
133
a 23 mm outer diameter and was 900 mm long (Thermo, USA). In each test, 10 ± 1 g
134
of pelletized material was loaded into the tube in a sample container, to measure the
135
torrefaction temperature. The temperature was increased at a constant rate of 7
136
10°C/min until it reached the desired temperature. The sample was torrefied for 3 h at
137
300°C to maximize the efficiency of torrefaction. After this time, the temperature was
138
decreased at a rate of 4.5°C/min; when the temperature fell below 120°C, the samples
139
were weighed and characterized. On average, 57.7% of the mass of the pellets was
140
lost during torrefaction (Fig. 1). Nitrogen (99.99 vol%) stored in a steel cylinder was
141
used as a carrier gas to purge the tube so that the torrefaction occurred in an
142
oxygen-free environment. The volumetric flow rate of the carrier gas was fixed at 100
143
mL/min by a mass flow meter, which was connected to a readout device. A
144
customized heated single-pellet die system was used in this study. A load force of 4
145
kgf/cm2 was applied by a piston for 3 min to compress the SCG into pellets that were
146
8.5 ± 1 mm in diameter and 20 ± 3 mm in height. A die temperature of 150°C was
147
sufficient to make control pellets. The energy of the pellets before torrefaction ranged
148
from 20.22 to 20.38 MJ/kg, depending on the ratios of the SCG, silver skin, and pine
149
sawdust. The measured and controlled variables are shown in Table 2.
150
Post-torrefaction gas and oil analyses will be performed in future studies.
151
Figure 1 here.
152
Table 2 here.
153
2.3. Properties of the SCG
8
154
For each experiment, the moisture content, HHV, volatile fraction, ash content,
155
and hygroscopicity of the sample were measured. Moisture content was measure
156
using the variating weights of the samples dried at 105°C for 24 h in a circular heating
157
oven (Model OVP-30, Hong-Siang Co., Taiwan). The calorific value obtained from
158
the bomb calorimeter (Model 1341, Parr Instrument Co., USA) was the HHV, which
159
included the latent heat of the vapor emitted from the specimen. Proximate analysis
160
was performed in accordance with the standard procedure of the American Society for
161
Testing and Materials (ASTM) for obtaining volatile fraction and ash content. The
162
average temperature and relative humidity of Taiwan in 2018 were 29.55 °C and
163
80.79%; these values were used as the basis of the hygroscopicity test [36]. About 1 g
164
of each sample was placed in the humidity simulator at 29.08±2°C and 79.83±3%
165
humidity. A fog generator with the supersonic frequency of 68 W and a fog producing
166
rate of 20 mL/min was installed. Both 1-h and 5-h hygroscopicity samples were
167
weighed to evaluate the preservation of the torrefied products. The mass and energy
168
yields are defined by Eqs. (1) and (2), as used by Bridgeman et al. (2008) [33].
169
170
!"
= #
$% &
=
× 100%
×
''(
)
''(
9
* *
(1) × 100% .
(2)
171
The experiment was repeated at least three times under each given condition. The data
172
shown in the tables are average values and the relative error between repetitions was
173
less than 5%.
174
2.4.Characterization of SCG
175
The SCG samples were characterized using thermogravimetric analysis (TGA),
176
Fourier transform infrared spectroscopy (FTIR), an element analyzer (EA), and a
177
scanning electron microscope equipped with an EDX spectrometers (SEM/EDX).
178
2.4.1. Thermogravimetric analyses
179
The pyrolysis characteristics of the samples were examined using a thermogravimetric
180
analyzer (Model Q500, TA Instruments, Inc., New Castle, USA). The samples were
181
loaded into a crucible that was placed inside the analyzer and the weight of the sample
182
was constantly measured. The analyzer measured and recorded the sample weight loss
183
as the temperature increased. For each test, a sample of around 5 mg with a particle
184
size of 100–200 mesh was used. The temperature increased from 25°C to 900°C, and
185
the heating rate was controlled at 10°C/min. Nitrogen gas was used as the carrier gas
186
in the analyzer and the flow rate was fixed at 40 mL/min. The relative error between
187
the TGA measurements was controlled to less than 5%. Two comprehensive indexes
188
(+ and , ) representing the combustion characteristics [37] were evaluated and
189
calculated as follows: 10
-./012 ×-./0314
190
+=
191
, = 8* ln -./
192
@8A
193
respectively. 8 , 8C , 8* , and 8
194
corresponding temperature, and peak temperature, which reflected the samples’
195
thermal behavior during the combustion process. These parameters can be derived
196
from TG and DTG curves. ∆8D/E is the temperature range of
197
2.4.2. Fourier transform infrared spectroscopy (FTIR) analyses
198
The chemical groups and constituents bonding arrangements found in the SCG were
199
determined by Fourier transform infrared spectroscopy (FTIR) using an FTIR
200
spectrophotometer (NICOLETiS5, Thermo, USA) equipped with a single reflection
201
diamond crystal ATR module. The spectra were recorded from the 400 to 4000 cm-1
202
spectral range, at a rate of 32 scans and a spectral resolution of 44 cm-1. The pelletized
203
samples were not favorable for FTIR analysis due to low transmission and
204
reflectance.
205
2.4.3. Element analyses
206
The elemental analysis was performed using an elemental analyzer (FlashEA 1112
207
HT, Thermo, USA) to measure the weight percentages of C, H, N, S, and Cl in the
208
SCG, whereas the weight percentage of O was obtained by considering difference.
(3)
.5 6 ×.7
∆.
B
10>? .
and @8A
(4)
are the maximum and average combustion rates (mg/min), B
are the ignition temperature, burnout temperature,
11
-./ -./012
= 0.5.
209
2.4.4. Scanning electron microscopy analyses
210
The particle shapes and surface textures were evaluated using scanning electron
211
microscopy (SEM) equipped with an EDX spectrometers using a model JSM-7610F
212
(JEOL, Japan), operating at 0.1 kV. The samples were mounted on aluminum stubs
213
using carbon tape. All of the specimens were sputter-coated with gold.
214
2.5. Similarity measurement
215
For fabricated biofuels that are designed to replace coal, the similarity of the
216
fabricated material’s characteristics to coal’s characteristics can be evaluated using
217
Eqs. (5) and (6) as follows:
218
H=
219
H = 1−
220
where ℎ, N, O, and P represent heating value, element content, proximate analysis
221
results, and combustion characteristics. T and U are the characteristics of the
222
biofuel and coal, respectively.
∑J K
, H = ℎ, N, O, P R5 >S5 S5
(5)
,
(6)
223 224
3. Results and Discussion
225
3.1. Properties of the biofuels
226
The properties of the biofuels are shown in Table 3. Samples 1 to 3 were
227
fabricated from SCG-A and Samples 4 to 6 were fabricated from SCG-B. Their mass 12
228
yields ranged from 41% to 43%; however, their energy yields were higher, at 52% to
229
58%. To increase the stability of the pellets, pine sawdust had to make up at least 50
230
wt% of the material. As the proportion of silver skin increased, both the mass and
231
energy yields decreased due to the HHV and fix carbon content of silver skin were
232
lower than SCGs. Poudel et al. (2015) found that a lower mass yield results in a lower
233
energy yield [38]. Also, the mass yield was decreased with an increase in temperature
234
or oxygen concentration [39]. The biofuels fabricated from SCG-A had higher mass
235
and energy yields than those fabricated from SCG-B. Previous studies found the mass
236
yield and energy of pine chips, logging residues, rice husk, and peanut husk fell in the
237
ranges of 52–89%, 71–94%, 41–79%, and 55–98% [28,40]. The yields are dependent
238
on the raw biomass, torrefaction temperature, residence time and reactor type [41].
239
The HHV of Sample 1 reached the same level as the HHV of coal (27.28 MJ/kg),
240
as shown in Table 3. The experimental set up and blended materials achieved higher
241
HHV values (ranging from 24.23 to 27.28 MJ/kg, shown in Fig. 1) than reported in
242
previous studies. Limousy et al. (2013) and Jeguirim et al. (2014) blended SCG and
243
pine sawdust into pellets [42,43]. They used the pyrolysis method to fabricate biofuels
244
and their HHVs were 18–20 MJ/kg at 11.78% moisture content. This study confirmed
245
that torrefaction has higher energy efficiency of training the initial energy content of
246
the biomass. Lower moisture content of the samples (1.77–3.32%) was also favorable 13
247
for increasing their energy properties. Previous study discovered that when SCG was
248
incorporated into wood chip logs, the HHV decreased to 17.39 MJ/kg at 10%
249
moisture content in a residential stove [44]. The comparison of the combustion
250
characteristics shows that the blended material had a higher reactivity than pure SCG
251
or sawdust.
252
Table 3 here.
253
The SCG had better heating values after drying. The moisture content of the
254
samples in this study ranged from 1.77% to 3.32%, as shown in Table 3. The original
255
moisture content of SCG-A and SCG-B was about 50%. The oven drying process
256
reduced the moisture content and made the pellets more suitable for longer storage.
257
When the proportion of fixed carbon increased, the HHV of the sample increased. In
258
this study, the volatile fraction of the samples was higher than that of coal. Some
259
harmful emissions, particularly nitrogen oxides in flue gas, were discovered by Kang
260
et al. (2017) [24]. There were about 2.79 g of nitrogen in 100 g of dry SCG [31]. The
261
ash content of the samples was about 1 to 3%, which is lower than coal. Thus, the
262
biofuels fabricated in this study have the potential to reduce the environmental burden
263
caused by landfilling.
264
Figure 2 shows the hygroscopicity of the samples. The increase in weight (wt%)
265
over a 5-h period was limited. Exposure of the samples to atmospheric conditions for 14
266
1 h allowed the samples to absorb water molecules and reach saturation. When the
267
proportion of silver skin increased, the hygroscopicity of the sample increased. The
268
composition of the biofuels with silver skin was unstable in the atmospheric
269
environment. On average, the weight increased by 0.24 to 0.57 wt% with a 10 wt%
270
increase of silver skin in the biofuels. Sample 1, which had 50% SCG-A and 50%
271
pine sawdust, was favorable for storage and had high hydrophobic ability.
272 273
Figure 2 here. 3.2.Characterization of the biofuels
274
As noted above, the biofuels were characterized using element analysis, FTIR,
275
SEM/EDX, and TGA. The carbon, hydrogen, oxygen, nitrogen, sulfur, and chlorine
276
contents of the samples were measured by elemental and EDX analysis, as shown in
277
Table S1. Consistent with previous studies, carbon sources were the primary
278
substances in the biofuels, including glucose and lignin, which is a single unit of
279
cellulose and hemicellulose [45]. The results of elemental and EDX were consisted
280
and the carbon content in the biofuels was 65–71 wt% in this study. SCG also
281
contains protein, which was measured using the nitrogen and sulfur sources in the
282
samples [46]. Neither sulfur nor chlorine were detected in the analysis by the
283
elemental analysis. None of the samples emitted dioxin or sulfur, which are air
15
284
pollutants, when heated. The fabricated biofuels were cleaner energy resources than
285
coal.
286
The element composition of the samples was similar to that of lignite, as shown
287
in the Van Krevelen diagram in Fig. 3. The benefits of torrefaction included lower
288
atomic oxygen-to-carbon (O/C) and hydrogen-to-carbon (H/C) ratios. The decrease in
289
the O/C ratio during torrefaction is attributed to the generation of volatiles rich in
290
oxygen, such as CO2 and H2O [47,48]. A reduced O/C ratio increases the HHV of the
291
torrefied biomass [38]. These changes in O/C and H/C ratios are mainly due to water
292
release and to the partial elimination of oxygen by decarboxylation, decarbonylation,
293
and dehydration reactions as well as in the form of volatile oxygenated compounds
294
such as acetic acid or methanol [49]. Lignite provides environmentally friendly power
295
generation with an energy content of 10 to 20 MJ/kg [50]. The biofuels made in this
296
study also had an energy content in a range of 18–20 MJ/kg, as shown in Table 3.
297
Figure 3 here.
298
Figure 4 shows the FTIR results for the SCG, silver skin, and pine sawdust. A
299
typical lignocellulosic material is composed of polysaccharides and aromatic
300
polymers. The broad peak between 3600 and 3200 cm-1 is related to the hydroxyl
301
group O-H stretching vibration [45]. The peaks at both 2920 cm-1 and 2850 cm-1 were
302
attributed to asymmetric and symmetric stretching of the C-H bonds of the methyl 16
303
group in the caffeine molecule along with sugars [51]. The C-H bonds decreased after
304
torrefaction, as shown in Figs. 4(a) and (b). A band at 1742 cm-1, which was attributed
305
to carbonyl vibration (C=O) in the aliphatic ester or triglycerides that originated in the
306
lipids [52], remained after torrefaction. Several peaks, 1245 cm-1 and 1032 cm-1 (C-O
307
stretching) verified that SCG contains a great variety of monosaccharides together
308
with diverse acid molecules, such as chlorogenic acid, caffeic acid, and coumaric acid
309
[53]. The band between 1700 and 1600 cm-1 was highly associated with chlorogenic
310
acids and caffeine [54]. The broad band between 1135 and 952 cm-1 was the result of
311
the stretching vibration of C-O in C-O-H bonds such as glycosidic bonds in
312
galactomannan polysaccharide sugars [55].
313
Figure 4 here.
314
Consistent with previous studies, the TGA curves for biofuels showed three
315
defined weight loss stages (Fig. 5). The first one occurred in the 105–360°C
316
temperature
317
low-molecule-mass volatile compounds [56]. During this state, there was about 5%
318
weight loss. The higher weight loss (35%) in the second range (360–490 °C)
319
corresponds to polysaccharide depolymerization and decomposition. Hemicellulose
320
and cellulose mainly decompose at 220–315°C and 315–400°C [57]. In the third
321
phase (490–800 °C), the formation of carbonaceous materials resulted in a further
range,
reflecting
the
water
17
evaporation
and
the
release
of
322
weight loss of around 10%. The residue at 800°C for biofuels was around 30%. Thus,
323
weight loss increased with temperature until burnout was reached.
324
Figure 5 here.
325
The combustion characteristics of the biofuels are listed in Table S2. The peak
326
temperature (8* ) of the biofuels was in the 409–419°C range, which is lower than that
327
of coal (about 441°C). Therefore, the whole combustion process of biofuels occurs
328
earlier than that of coal. The ignition temperatures (8 ) of Samples 1 and 4 were lower
329
than those of the other samples due to their higher volatile content, as shown in Table
330
3. The volatile content was increased when percentage of the silver skin in the
331
samples increased. The faster devolatilization and oxidation rates of the volatile
332
matter caused lower particle ignition temperatures [58]. The comprehensive +
333
indexes of Samples 1 and 4 were greater than those of the other samples, indicating
334
that their combustion performance was superior. The results of the comprehensive ,
335
index were the inverse of the results of the + index. A high ash content may have
336
inhibited the combustion reaction of the samples, leading to a decrease in combustion
337
performance [37].
338
Fig. 6 shows the SEM images of the raw materials’ morphologies. In addition to
339
its caloric value, SCG has a unique microporous structure with a high surface area of
340
about 300–1,000 m2/g [59]. SCG has been proven to be an effective adsorbent for a 18
341
wide range of contaminants [60]. The SCG particles mainly have a diameter range of
342
20–75 μm. Previous studies have suggested that particles of random size showed a
343
good response in electrorheology performance [46]. The torrefaction process may
344
have increased porosity, and similar results were noted by Mendes et al. (2019) [56].
345 346
Figure 6 here. 3.3. Similarity results
347
The study explored the creation of biofuels that were similar to coal with the aim
348
of reducing dependence on fossil fuels. The new biofuels’ similarity to coal was
349
evaluated by comparing their heating value, element content, proximate analysis
350
results, and combustion characteristics. Figure 7 shows the results of the comparison:
351
there was a similarity of at least 55%. The most similar sample was Sample 4, which
352
was 62% the same as coal. As the percentage of silver skin increased, the similarity to
353
coal decreased. The characteristics of the fabricated biofuels that were most similar to
354
coal were heating value (83–94%), followed by combustion characteristics (80–84%),
355
proximate analysis results (24–38%), and element content (27–42%). Previous studies
356
have shown that the heating value of SCG is similar to coal and higher than wood and
357
other biomass residues [61]. Future studies should consider other indicators.
358
Figure 7 here.
359 19
360
4. Conclusions
361
This study used the torrefaction method to fabricate biofuels that integrated SCG
362
and silver skin. The biofuels were dried, pelletized, and combusted at 300°C for 3 h.
363
To test whether the fabricated biofuels could replace coal, the similarity of their
364
characteristics was evaluated based on the parameters of heating value, element
365
content, proximate analysis results, and combustion characteristics. The mass yields
366
ranged from 41% to 43%; however, their energy yields were higher at 52% to 58%.
367
As the percentage of silver skin in the biofuels increased, both the mass and energy
368
yields decreased. The HHV of the biofuels ranged from 24.23 to 27.28 MJ/kg, which
369
are higher values than reported in previous studies. The results showed that increasing
370
the percentage of silver skin increased the hygroscopicity of the biofuels. On average,
371
there was a 0.24 to 0.57 wt% increase with an increase of 10 wt% of silver skin. The
372
carbon content in the samples ranged from 65 to 71 wt% and the sulfur and chlorine
373
contents were 0. These results demonstrate that these biofuels are cleaner energy
374
resources than coal. The element contents in the biofuels were similar to lignite, as
375
shown in the Van Krevelen diagram. The sample most similar to coal, Sample 4, was
376
62% similar. Integrating silver skin may reduce the quality of the biofuels, but it helps
377
reduce the environmental burden of landfilling or incineration.
378 20
379 380
Acknowledgments The authors would like to thank the Ministry of Science and Technology,
381
Republic
of
China,
for
financial
support
under
Contract
No.
MOST
382
106-2627-M-002-028. They also acknowledge the Taiwan EPA and Ministry of
383
Economic Affairs, and other governmental agencies for assisting with data collection.
384
The authors further thank the anonymous reviewers for their invaluable comments
385
and suggestions.
386 387
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576
Table 1
Sample composition Pine
Content
SCG-A
SCG-B
Silver skin
(%)
(%)
(%)
sawdust Sample
Weight (g)
(%) 1
50.00
0.00
0.00
50.00
3.0026
2
40.00
0.00
10.00
50.00
3.0273
3
30.00
0.00
20.00
50.00
3.0110
4
0.00
50.00
0.00
50.00
3.3830
5
0.00
40.00
10.00
50.00
3.0596
6
0.00
30.00
20.00
50.00
2.9985
577 578 579 580 581 582 583 584 585 586 587
31
588
Table 2
Variables used in this study
Variable
Unit
Value
Controlled variable pelletized load force
kgf/cm3
4
pellet size
mm
8.5±1 (diameter) and 20±3 (height)
heating rate
o
10
torrefied temperature
o
300
torrefied time
h
3
C/min C
Measured variable moisture content
wt%
HHV
MJ/kg
volatile fraction
wt%
ash content
wt%
fix carbon
wt%
hygroscopicity
wt%
mass yield
%
energy yield
%
similarity
%
589 32
590
Table 3 Sample properties moisture Content
mass yield
energy yield HHV (MJ/kg)
Sample
(%)
volatile content
fix carbon
(wt%)
(wt%)
ash content content
(%)
(wt%) (wt%)
591
a
SCG-A
－
－
22.12
9.49
1.36
81.58
17.06
SCG-B
－
－
21.81
9.46
1.40
82.37
16.23
Silver skin
－
－
17.82
7.48
3.24
80.69
16.07
1
42.72
58.33
27.28
3.32
3.0026
60.59
34.54
2
41.94
54.51
25.52
2.29
3.0273
63.78
32.30
3
41.68
53.45
24.73
2.84
3.0110
68.66
25.91
4
42.51
56.94
26.55
2.41
3.3830
64.15
32.21
5
41.73
52.91
24.73
1.77
3.0596
66.80
28.42
6
41.57
52.50
24.23
3.18
2.9985
68.26
27.14
Coal
－
－
27.28b
1.28-21.62 a
2.93-16.73 a
22.38-46.42 a
24.26-70.05 a
[62]
33
592
b
[63]
34
593 594
Fig. 1. The experimental setup and mass and energy flow.
595 596 597 598 599
35
6 1-h 5-h
5
weight (wt%)
4
3
2
1
0 1
2
4
5
sample
600 601
3
Fig. 2
Hygroscopicity of the biofuels.
602 603
36
6
0.18 0.16 0.14
H/C ratio
0.12
anthracite bituminous coal lignite peat biomass sample
0.10 0.08 0.06 0.04 0.02 0.00 0.0
0.4
0.6
0.8
1.0
1.2
O/C ratio
604 605
0.2
Fig. 3
Van Krevelen diagram of the biofuels and coal.
606 607 608 609 610 611
37
1.4
102
(a)
103
(b)
100 102
98
transmittance (%)
101
2882 cm C-H 2920 cm C-H
94
transmittance (%)
-1
3600~3200 cm C-O
96
-1
-1
92
-1
1245 cm C-O 1032 cm-1 C-O
90 SCG-A torrefied SCG-A
88
3600~3200 cm-1 C-O
99 98
SCG-B torrefied SCG-B 2882 cm-1 C-H
97 96
-1
1742 cm C=O
86
100
2920 cm-1 C-H
95
4000
3000
612
2000
1000
4000
3000
-1
1245 cm-1 C-O 1742 cm-1 C=O 1032 cm-1 C-O
2000
1000 -1
wavenumber (cm )
wavenumber (cm )
700
(d)
500
(c)
500
300
3600~3200 cm-1 C-O
-1
2829 cm C-H
2882 cm-1 C-H
200
1245 cm-1 C-O 1742 cm C=O
100 2920 cm C-H
transmittance (%)
transmittance (%)
400
3600~3200 cm-1 C-O 2882 cm-1 C-H
300
1245 cm-1 C-O
-1
100
-1
2920 cm-1 C-H
silver skin torrefied silver skin
1742 cm-1 C=O
0 4000
613
615
400
200
0
614
pine sawdust torrefied pine sawdust
600
3000
2000
4000
1000
Fig. 4
3000
2000
1000
wavenumber (cm-1)
wavenumber (cm-1)
FTIR spectra for of (a) SCG-A, (b) SCG-B, (c) silver skin, and (d) pine sawdust.
38
100
weight (%)
90
80
70 sample 1 sample 2 sample 3 sample 4 sample 5 sample 6
60
50 200
400
temperature (oC)
616 617
600
Fig. 5
TGA curves of the biofuels.
39
800
618 619
Fig. 6
SEM images of (a) SCG-A, (b) torrefied SCG-A, (c) SCG, (d) torrefied
620
SCG-B, (e) pine sawdust, (f) torrefied pine sawdust, (g) silver skin, and (h)
621
torrefied silver skin. 40
100
0.63 0.62
80
0.60
60
0.59 40
20
0.58 0.57
SCG (%) silver skin (%) pine sawdust (%) similarity result
0.56
0
0.55 1
2
4
5
6
sample
622 623
3
Fig. 7
Similarity analysis of the biofuels and coal.
624
41
similarity results
composition (%)
0.61
Research Highlights Spent coffee grounds (SCG) and silver skin were integrated into biofuels. The high heat value (HHV) was in range of the biofuels (24.23–27.28 MJ/kg). An increase in silver skin increased the hygroscopicity of the biofuels. The sample most similar to coal exhibited 62% similarity.