Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 35 – 40
The 8th International Conference on Applied Energy – ICAE2016
Microwave torrefaction of leucaena to produce biochar with high fuel ratio and energy return on investment Yu-Fong Huang, Pei-Hsin Cheng, Pei-Te Chiueh, Shang-Lien Lo* Graduate institute of Environmental Engineering, National Taiwan University, 71, Chou-Shan Rd., Taipei 106, Taiwan, ROC
Abstract In this study, microwave torrefaction of leucaena was studied to find out the potential applications and energy usage benefit of this technique. Both maximum temperature and heating rate increased with increasing microwave power level. Processing time was also an important operational parameter, but its effect was weaker than that of microwave power level. The heating value of torrefied product was higher at higher power level and longer processing time, but the mass and energy yields were lower due to higher energy input and thus more severe reaction. Heating value was approximately 30 MJ/kg at a microwave power level of 250 W for 30 min processing time. The fuel ratio of torrefied leucaena was up to 3.7, which is much higher than that of bituminous coal and thus can be regarded as an alternative fuel to replace or co-fire with coal. The energy return on investment of microwave torrefaction of leucaena can be 1.4, 17, and 34 when handling capacities are 8, 100, and 200 g, respectively. Therefore, microwave torrefaction of leucaena is a promising technique, and it can be more competitive when it is scaled up. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. Keywords: Microwave torrefaction; Leucaena; Biochar; Fuel ratio; Energy return on investment; Reaction kinetics
1. Introduction The utilization of renewable energy has become an important and urgent issue due to the continuously increasing consumption of fossil fuels and the worsening global climate change situation. With the increasing population and industrialization, more carbon dioxide is emitted by the combustion of fossil fuels to worsen the climate change. The worse climate change would make the summer hotter and the winter colder, and thus more electricity and fossil fuels are required. This vicious circle could lead to an irreversible consequence, which may make it impossible to survive on the earth for all the lives. One of the promising renewable energy sources is biomass. Biomass is an abundant carbon-neutral renewable * Corresponding author. Tel.: +886-2-2362-5373; fax: +886-2-2392-8821. E-mail address:
[email protected].
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.276
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resource to produce bioenergy and biomaterials, and its enhanced utilization can address some societal demands [1]. In addition to being an alternative energy, the use of bioenergy can offset anthropogenic carbon dioxide emissions. In the next few decades, massive growth of the bioenergy industry will be required to address societal needs to reduce net carbon emissions [2]. The woody biomass feedstock used in this study was Leucaena leucocephala. Leucaena is a fast growing tree with very high biomass production and re-sprout capacity, and it is capable of growing with high yield on arid soils [3–7]. Therefore, leucaena has a high potential for the production of bioenergy [3], and it can be an alternative to the use of traditional biomass feedstocks [5]. In this study, the reactivity of leucaena torrefaction using microwave heating and the properties of torrefied products were studied to find out the potential applications and energy usage benefit of this technique. 2. Methods 2.1. Material Leucaena wood was provided by the Kenting National Park, Taiwan. Before applying to microwave torrefaction experiments, the leucaena wood was shredded and ground into powdered samples. 2.2. Experimental device and procedure This study used a single-mode microwave oven operated at 2.45 GHz frequency. Reaction tube and sample holder were both made of quartz. The shredded and sieved (50 mesh) biomass feedstock (approximately 8 g) was added to a quartz crucible and then placed inside a quartz tube that was located in the pathway of the microwaves. A thermocouple sensor was placed at the bottom of the quartz crucible to measure the temperature of the biomass sample. To maintain anoxic conditions, nitrogen gas was purged into the system at a flow rate of 35 mL/min. After sufficient purging was performed to maintain an inert atmosphere, the power supply was turned on and switched to a microwave power level of 100, 150, 200, or 250 W, and the processing time was 15, 20, 25, or 30 min. 2.3. Product analysis The proximate analyses of raw and torrefied leucaena were performed according to the standard test methods D7582-12 and D3172-07a of the American Society for Testing and Materials (ASTM). The ultimate analyses were carried out by using a Perkin–Elmer 2400 II CHNS/O elemental analyzer. The higher heating values (HHV) of biomass samples were measured in a Parr 1341 adiabatic oxygen bomb calorimeter. The surfaces of the biochar were observed by using a Hitachi S-4800 field emission scanning electron microscope (SEM). 3. Results and discussion 3.1. Characteristics of leucaena The main characteristics of the raw and torrefied leucaena samples are listed in Table 1. After microwave torrefaction, part of the volatile matter of leucaena was thermally decomposed, and the decomposition extent increased with increasing microwave power levels. The decomposition of volatile matter is mainly owing to the thermochemical reaction of hemicellulose that primarily happens in the temperature range of 200–300 ºC for biomass torrefaction [8,9]. When the microwave power level was
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200 or 250 W, the volatile matter content of leucaena was decreased to approximately 20 wt.%, whereas the fixed carbon content was substantially increased to 73–76 wt.%. Table 1. Characteristics of the raw and torrefied leucaena samples. Raw leucaena
Torrefied leucaena 100 W
150 W
200 W
250 W
9.42
7.62
8.56
10.28
14.91
Volatile matter
81.97
64.19
52.82
20.46
20.37
Fixed carbon
16.06
29.88
44.22
75.69
72.79
Ash
1.97
5.92
2.96
3.85
6.84
C
39.36
58.17
76.94
78.08
76.29
H
5.41
5.02
2.66
2.43
2.64
Moisture (wt.%) Proximate analysis (wt.%) a
Ultimate analysis (wt.%) b
N
5.92
0.82
0.99
1.09
1.02
O
31.43
35.15
14.95
14.96
15.07
18.34
23.50
29.18
29.19
29.72
HHV (MJ/kg) a a
Dry basis. b Dry and ash-free basis.
The SEM images of the raw and torrefied leucaena are illustrated in Fig. 1. It can be seen that the surface texture of the raw leucaena was flat and smooth with only a few cracks, whereas the torrefaction product was severely fractured into smaller pieces with various sizes. This should be owing to the removal of hemicellulose component done by torrefaction. Since the particle size of torrefied leucaena was smaller than that of the raw one, it could have more pores and bigger surface area.
Fig. 1. SEM images of (a) raw leucaena and (b) torrefied leucaena.
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3.2. Fuel ratio The proximate compositions and fuel ratios of microwave torrefaction products are listed in Table 2. In general, the volatile matter content decreased whereas the fixed carbon content increased with increasing microwave power level and processing time. This is because that, under the relatively lower heating temperatures of biomass torrefaction, part of the volatile matter was thermally decomposed but most of the fixed carbon remained. Fuel ratio is the ratio of fixed carbon content to volatile matter content and can be used to evaluate the combustibility of coal and biochar [10–12]. The fuel ratio of raw leucaena was approximately 0.2. After microwave torrefaction at the power levels of 200 and 250 W, the fuel ratio of the biochar produced was up to 3.7, which is much higher than that of bituminous coal (1.5–2.0) [10,12]. Therefore, the biochar produced by microwave torrefaction of leucaena can be an alternative solid fuel, and it should have a high potential to replace or co-fire with coal. Table 2. Proximate compositions and fuel ratios of torrefied leucaena. Microwave power (W)
Processing time (min)
100 100
Proximate composition (wt.%)
Fuel ratio
Volatile matter
Fixed carbon
Ash
15
73.83
23.89
2.28
0.32
20
66.70
29.80
3.50
0.45
100
25
72.14
23.72
4.14
0.33
100
30
64.19
29.88
5.92
0.47
150
15
68.10
29.14
2.75
0.43
150
20
66.10
31.77
2.14
0.48
150
25
66.13
31.57
2.30
0.48
150
30
52.82
44.22
2.96
0.84
200
15
29.09
67.90
3.01
2.33
200
20
34.33
60.71
4.96
1.77
200
25
28.78
66.99
4.23
2.33
200
30
20.46
75.69
3.85
3.70
250
15
47.56
46.90
5.53
0.99
250
20
21.35
72.47
6.18
3.39
250
25
22.55
71.43
6.02
3.17
250
30
20.37
72.79
6.84
3.57
3.3. Energy return on investment To evaluate the energy usage benefit of microwave torrefaction technique, the energy return on investment (EROI) was applied. EROI is the ratio between the energy delivered by a particular fuel to society and the energy invested in the capture and delivery of this energy [13]. The EROI for the coal production in China was approximately 27 in 2010 [14]. Here are three scenarios based on different sample weights (capacities). The weight of leucaena sample was approximately 8 g in this study, which is Scenario 1. Scenarios 2 and 3 assume that the sample weights are 100 and 200 g, respectively. The weight of 200 g was used because, theoretically, it is the allowable quantity for microwave torrefaction when the microwave power level is 100 W and the processing time is 15 min (i.e., an overall energy input of 90 kJ).
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The EROI of microwave torrefaction under the three scenarios are listed in Table 3. In general, the EROI decreased with increasing microwave power level and processing time. In Scenario 1, the EROI was 0.6– 1.4 at a microwave power level of 100 W, and it was only 0.1–0.3 at 200 and 250 W. If the sample weight can be bigger, the EROI would be much better and thus could make microwave torrefaction more feasible for commercial applications. Hall et al. reported that the minimum EROI for a sustainable society is 3 [15], which can be generally satisfied in Scenarios 2 and 3. According to Scenario 3, the EROI can be 8.3 when the HHV of torrefied leucaena is 27.5 MJ/kg. Despite the magnitude of HHV, the EROI can be as high as 19–34 at low microwave power levels and short processing time. Table 3. EROI of microwave torrefaction. Microwave power (W)
Processing time (min)
100 100
EROI 8 g capacity
100 g capacity
200 g capacity
15
1.4
17.0
34.0
20
1.0
12.9
25.8
100
25
0.7
9.3
18.5
100
30
0.6
7.4
14.8
150
15
0.7
9.3
18.7
150
20
0.5
6.7
13.5
150
25
0.4
5.4
10.8
150
30
0.3
4.1
8.3
200
15
0.3
4.2
8.3
200
20
0.3
3.3
6.6
200
25
0.2
2.3
4.6
200
30
0.1
1.7
3.3
250
15
0.2
3.1
6.1
250
20
0.2
2.1
4.2
250
25
0.1
1.6
3.2
250
30
0.1
1.1
2.3
4. Conclusions Microwave torrefaction of leucaena can produce biochar with high HHV and fuel ratio at low microwave power levels and short processing time. Leucaena is a fast growing tree with very high biomass production and re-sprout capacity, so it can be a source of biofuels with high mass and energy yields. Both microwave power level and processing time were important parameters affecting heating performance, mass and energy yields, and biochar characteristics. In general, maximum temperature, heating rate, HHV, energy densification ratio, and fuel ratio increased but mass and energy yields decreased with increasing microwave power level and processing time. Besides, the effect of microwave power level was stronger than that of processing time. The HHV and fuel ratio of the biochar can be up to 30 MJ/kg and 3.7, respectively, which are much higher than those of bituminous coal. The EROI can be 19–34 at low microwave power levels and short processing time. Therefore, the biochar produced by microwave torrefaction of leucaena can be an alternative fuel to replace or co-fire with coal, and this technique can be more competitive as long as it is scaled up.
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Biography Shang-Lien Lo is a Distinguished Professor at National Taiwan University. He is also an IWA Distinguished Fellow. His specialization and research interests include water supply and sewage engineering, water quality control, environmental mathematics, soil and groundwater pollution, environmental economics analysis, sustainable development, and renewable energy.
