- Email: [email protected]
Upgrading fuel quality of moso bamboo via low temperature thermochemical treatments: Dry torrefaction and hydrothermal carbonization
Fuel 196 (2017) 473–480
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Upgrading fuel quality of moso bamboo via low temperature thermochemical treatments: Dry torrefaction and hydrothermal carbonization Wei Yan a, Sandy Perez b, Kuichuan Sheng a,⇑ a b
Zhejiang University, College of Biosystems Engineering and Food Science, 310058 Hangzhou, China University of Illinois at Urbana-Champaign, School of Earth, Society and Environment, IL 61801-4713, USA
h i g h l i g h t s Fuel potential was enhanced via hydrothermal carbonization and torrefaction. Hydrochar performed better than torrefied bamboo produced at the same temperature. Hydrophobicity was obtained at higher treating temperature. Removal of hemicellulose was the dominant reaction in hydrothermal carbonization. 260 °C was determined to be suitable for hydrochar production.
a r t i c l e
i n f o
Article history: Received 18 September 2016 Received in revised form 26 January 2017 Accepted 6 February 2017
Keywords: Moso bamboo Calorific value Dry torrefaction Hydrochar Hydrothermal carbonization
a b s t r a c t The application of raw bamboo as biomass energy is restricted due to its large particle size, high oxygen content, low energy density and weak water resistance. In order to upgrade the fuel quality of moso bamboo, dry torrefaction (DT) and hydrothermal carbonization (HTC) have been investigated in this study. The physicochemical properties, thermal stability and microstructures of solid products were examined by varying the reaction temperature among 220, 260 and 300 °C. The results showed that increasing temperature reduced the mass yield and energy yield, however, it significantly improved the calorific value of solid products. Through the HTC process, the bamboo hydrochar obtained the calorific value of 28.29 MJ/ kg, the energy yield of 59.77% and the fixed carbon content of 63.08% at the temperature of 260 °C, indicating enhanced potential as a solid fuel. In comparison, the grindability (including particle size and bulk density), hydrophobicity, and thermal stability of torrefied bamboo were considerably lower than that of bamboo hydrochar produced at the same temperature. Furthermore, the transformation of chemical bonds demonstrated that hydrolysis, dehydration and decarboxylation took place during the HTC process. The porous structure was only slightly enhanced with increasing reaction temperature, exhibiting variation consistent with that of hydrophobicity. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the past few decades, renewable biomass energy has received great attention due to the increasing environmental concerns and energy crisis associated with the exhaustion of conventional fossil fuels. Biomass is advantageous both alone in combustion and cofired with coal because it is widely available and demonstrates potential for greenhouse gas neutrality [1]. However, low calorific value and poor resistance to moisture absorption make the direct
⇑ Corresponding author. E-mail address: [email protected] (K. Sheng). http://dx.doi.org/10.1016/j.fuel.2017.02.015 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
use of biomass less attractive. To overcome these shortages, various types of thermochemical conversions have been attempted [2–5]. Among all the thermochemical treatments, dry torrefaction and hydrothermal carbonization are the two most practical methods with relatively low temperature requirements [6], costing less energy input when compared with slow pyrolysis for biochar, fast pyrolysis for bio-oil, or gasification. Dry torrefaction (DT), during which processed biomass is heated in an inert atmosphere at temperatures of around 200– 300 °C for a residence time of 30 min to a couple hours, is often regarded as a conventional thermochemical pre-treatment and proposed as an alternative to improve the physicochemical
474
W. Yan et al. / Fuel 196 (2017) 473–480
properties of biomass. Acharya et al. [7] reviewed that torrefaction undergoes devolatilization, depolymerization, and limited carbonization of lignocellulose components and generates a brown to black solid product with 70% mass and 90% energy reserved. Bach and Skreiberg [8] claimed that the improved properties of torrefied biomass, such as relatively superior handling and milling, highlight a beneficial opportunity to co-fire the clean renewable energy source with coal in an effort to relieve the energy crisis and mitigate environmental pollution. Nevertheless, the torrefaction process is unsuitable when disposing feedstocks with high moisture content, and the calorific value of torrefied biomass is still lowly graded. Moreover, the high O/C ratio and volatile content may affect the thermal stability and combustion profile. Hydrothermal carbonization (HTC), also referred to as wet torrefaction, is often used to prepare a solid product called hydrochar which exhibits reduced O/C ratio, increased calorific value, better grindability and improved hydrophobicity compared with untreated and torrefied biomass [9]. During the HTC process, feedstock is mixed with subcritical water at temperatures of 180– 350 °C under an inert atmosphere so that the products are not affected by the high moisture content. Hydrothermal carbonization has the ability to deal with not only extremely wet feedstock such as sewage sludge [10] or sludge from the pulp and paper mill [11], but also forestry processing residues and agricultural wastes [12]. Lynam et al. [13] prepared loblolly pine hydrochar at the temperature range from 200 to 260 °C. A 30% increase in calorific value of hydrochar was found with increasing temperature whereas the mass yield was slightly reduced. In addition, adding both 0.4 g acetic acid and 1 g lithium chloride per g pine resulted in an energy densification ratio of 1.34. Other research included the use of nut husks as feedstock [14]. The fuel properties of hydrochar were found to be most affected by the component weight ratios of the biomass. For example, lignin was the main contribution to the solid fuel yield and the reactivity of cellulose and hemicellulose in different biomass was affected by the biomass species. Moreover, the calorific value of solid fuels prepared at 260 °C, around 25 MJ/kg, were comparable to those of commercial coals. Therefore, the properties of hydrochar may greatly differ depending on the feedstock resource and reaction conditions such as temperature. Moso bamboo, an abundant natural cellulosic resource, has gained much attention in the comprehensive utilization of biomass over the past years due to its easy propagation, fast growth and regeneration, and high productivity as well as its rapid maturity [15]. It is widely used to produce furniture, veneers and flooring, but a significant amount of bamboo processing residues was treated as waste. Thus, the application of bamboo processing residues for bioenergy production is worth exploring. In order to dispose these solid waste residues, some researchers have evaluated moso bamboo residues directly as biofuel, but the energy density and the fuel quality is generally low for direct burning of moso bamboo. Some other researchers have prepared bamboo biochar by slow pyrolysis, but the mass yield is always lower than 30% and energy cost is high [16–18]. It is necessary to find an environmentally friendly and efficient method to upgrade fuel quality of moso bamboo. However, only a few researchers have studied hydrothermal carbonization on bamboo biomass [19]. Even fewer have examined the comparative assessment of moso bamboo for producing energy dense fuel via DT and HTC treatment. The main objectives of this study are to (1) upgrade fuel quality of the torrefied product and hydrochar obtained from moso bamboo by investigating the effect of reaction temperature on energy characteristics, such as calorific value, proximate analysis, hydrophobicity, and thermal degradation stability, (2) clarify the relationship between fuel quality and chemical structure as well as pore structure, (3) and compare the performance of these two
kinds of solid biofuels to identify the better option for energy application. 2. Materials and methods 2.1. Materials Moso bamboo (Phyllostachys pubescens) particles were supplied by China National Bamboo Research Center in Hangzhou. The raw moso bamboo was harvested in Anji, Zhejiang Province of China. The samples were ground into particles of 200–400 lm using a turbine grinder (XWDJ-130, Zhejiang Xinshiji grinder machine Co. Ltd, China) and the moisture content was measured to be 8%. 2.2. Samples preparation The torrefaction of bamboo particles was carried out in a benchscale fixed bed reactor, which was designed and fabricated by Bioenergy and Biomaterials Lab at Zhejiang University. The reactor consists of a stainless steel container with an inner diameter of 120 mm and a height of 330 mm. For each run, 100 g of moso bamboo particles was placed inside the reactor. The reactor was indirectly heated by electric heater at set temperatures of 220, 260 and 300 °C for 1 h, respectively. After the desired isothermal period, the sample was cooled down naturally to room temperature. The solid torrefied bamboo was collected in a sealed transparent plastic bag and stored in an airtight container at room temperature until analysis. Hydrothermal carbonization of bamboo particles was carried out in a cylindrical stainless steel reactor (4848, Parr Instrument Company, USA) with a working volume of 2 L. A weight of 100 g bamboo particles combined with 1 L purified water was fed into the reactor, which was then closely tightened. The reactor was held at the desired temperature of 220, 260 and 300 °C for 1 h, respectively. Temperature and pressure inside the reactor were measured and adjusted by the controller connected with a thermocouple and a pressure sensor. After treatment, the mixture inside the reactor was filtered by a vacuum pump (SHB-IA, Shanghai Yukang Science Teaching Instrument Equipment Co. Ltd), and then the hydrochar was dried at 105 °C until its weight reached a constant value. The samples were marked as BP for bamboo particle, DT220, DT260, DT300 for dry torrefied bamboo, and HTC220, HTC260, HTC300 for bamboo hydrochar at their respective temperatures. 2.3. Properties characterization Proximate analysis of the samples was conducted using ASTM E1756-08 to determine the moisture content, ASTM E872-82 for volatile content, and ASTM D1102-84 for ash content. The fixed carbon content was calculated by mass balance. The calorific value of the samples was measured using an Oxygen Bomb Calorimeter (5E-AC, Changsha Kaiyuan Instruments Co., Ltd, China). The mass yield (MY) and energy yield (EY) of solid products were calculated as follows:
MY ð%Þ ¼ m:SP =m:BP 100
ð1Þ
EY ð%Þ ¼ M Y cv :SP =cv :BP
ð2Þ
where m.SP and cv.SP are the mass and calorific value of solid products, m.BP and cv.BP are the mass and calorific value of bamboo particles, and cv.SP/cv.BP is energy density of the solid products. The bulk density was determined using a Bulk Density Tester (Buld-005, Hangzhou Tonfus Corporation, China). The particle size distribution was determined using a Laser Particle Size Analyzer
475
W. Yan et al. / Fuel 196 (2017) 473–480
2.4. Energy consumption Energy efficiency (Ee) is calculated by EBP, Eheating, Eholding and ESP as follow.
Ee ¼
ESP 100% EBP þ Eheating þ Eholding
where EBP is the energy of raw bamboo particle, which equals to m∙cv.BP; Eheating is the energy input during the heating stage, which equals to CmΔT; Eholdting is the energy input during the holding stage, which equals to Powert; ESP is the energy of solid product, which equals to mcv.SP. Cwater (specific heat capacity) is 4.2 kJ/(kg°C); Cmoso bamboo is 33.30 kJ/(kg°C) based on the research of Lou et al. [20]. The electric power of dry torrefaction was recorded to be 1 kW while that of hydrothermal carbonization was 0.5 kW.
3. Results and discussion 3.1. Temperature and pressure profiles of HTC process The temperature and pressure profiles during the reaction process are shown in Fig. 1. As temperature increased, pressure provided by saturated water vapor inside the reactor also followed a similar increasing trend. The variance in trend between temperature and pressure was due to the unsaturated state of water vapor below 100 °C as this delayed increase in pressure. However, after cooling down to room temperature, the pressure inside the reactor was higher than the initial pressure. This was due to released micromolecular gases, such as CO2, in the decomposition reaction of bamboo fibers [21]. Non-isothermal conditions were also
established in the DT process, and an approximate constant heating rate of 3 °C/min made the reaction system stable. 3.2. Potential of energy application The mass yield, calorific value and energy yield of solid product at different thermochemical treatment conditions are shown in Table 1. With an increase in reaction temperature from 220 to 300 °C, the mass yield of torrefied bamboo and hydrochar decreased from 92.8% to 61.6% and 51.8% to 35.6%, respectively. Due to the presence of subcritical water, the degradation of hemicellulose in the HTC process was rapid and took place at a lower temperature when compared with dry torrefaction [22], so that bamboo particle treated by HTC showed higher mass loss than that treated by DT at the same reaction temperature. On the other hand, calorific value of both torrefied bamboo and hydrochar exhibited an increasing trend as the reaction temperature increased. HTC260 and HTC300 even contained high calorific value of 28.29 and 29.30 MJ/kg, which is comparable to the energy content of coal and biochar. Besides, hydrochar is obtained from biomass while coal contains sulfur, which therefore made hydrochar a kind of more clean energy with wider application. In addition, the calorific value of bamboo hydrochar produced at 220 °C was remarkably lower compared with those of HTC260 and HTC300. Considering both energy density and mass yield of all solid products, HTC260 had the highest energy yield of 59.77%. While in the lower temperature zones of the HTC process, the increase in temperature accelerated the decomposition of hemicellulose molecules [23] which then released a large amount of incombustible gas, such as CO2 and H2O. Volatile content decreased and fixed carbon content increased, leading to much improvement in calorific value. A higher temperature above 260 °C increased the treatment severity and therefore enhance the decomposition of biomass polymers to intermediate components with high calorific value into an aqueous phase which lessened the increase of calorific value. However, the decomposition rate of hemicellulose without water participation by DT was slower than that by HTC, so that torrefied bamboo obtained lower calorific value than bamboo hydrochar at the same reaction temperature. The energy consumptions of these two technologies are shown in Table 2. As can be seen, DT is considered to be a higher-yield technology, because it needs extra energy to heat up the water for the HTC process, many times of the bamboo feedstock weight. In a laboratory investigation, Yan et al. [24] calculated that it requires 15–20% extra energy to produce hydrochar, and the efficiency can be higher by an industrial company with optimal heat and mass integration. However, the energy per unit weight
280
(a)
HTC220 HTC260 HTC300
(b)
Temperature Pressure
210
Temperature (°C)
Temperature (°C)
300
200
4 3
140
100
2 70
0
0
100
200
Reaction time (min)
300
400
5
1
0
50
100
150
200
250
Reaction time (min)
Fig. 1. (a) temperature profile during HTC process; (b) Temperature and pressure profile of HTC260.
300
350
0 400
Pressure (MPa)
(MS3000, Malvern Instruments Ltd., UK). Water resistance was evaluated on water absorption for 24 h according to ASTM D5229-14. Thermogravimetric analysis was performed using a Thermogravimetric Analyzer (TA-60WS, Shimadzu Corporation, Japan). FTIR spectra were obtained using a FTIR Spectrometer (Nicolet 6700, Thermo Fisher Nicolet Corporation, USA) and the data were obtained in the wavenumber range from 4000 to 750 cm1. The specific surface areas and porosity characteristics were conducted with the method of isothermal adsorption of nitrogen using Automatic Nitrogen Adsorption Equipment (JWBK, Automatic Nitrogen Adsorption Equipment Co. Ltd., China). All of the data provided for the samples were the average value of three trials.
476
W. Yan et al. / Fuel 196 (2017) 473–480
Table 1 Physicochemical properties of torrefied bamboo and hydrochar. Sample
Mass yield/%
Calorific value/MJkg1
Energy density
Energy yield/%
Bulk density/102 gcm3
BP HTC220 HTC260 HTC300 DT220 DT260 DT300
– 51.8 ± 3.5 40.5 ± 1.7 35.6 ± 2.1 92.8 ± 1.5 86.6 ± 0.8 61.6 ± 2.4
19.17 ± 0.21 19.80 ± 0.64 28.29 ± 0.22 29.30 ± 0.39 19.38 ± 0.46 20.43 ± 0.05 23.32 ± 0.32
– 1.03 1.48 1.53 1.01 1.07 1.22
– 53.50 59.77 54.41 93.82 92.29 74.94
15 ± 0.05 27 ± 0.10 25 ± 0.45 29 ± 0.06 15 ± 0.22 15 ± 0.27 14 ± 0.10
a ab c d a b e
a b c d a a e
Particle size distribution Dx(10)/lm
Dx(50)/lm
Dx(90)/lm
164 30.7 4.58 6.84 164 146 144
298 179 55.7 84.5 298 284 282
498 422 347 354 499 488 487
Note: 1. D (10)/lm, D (50)/lm and D (90)/lm refer to that 10%, 50% and 90% of the particles are smaller than the recorded data, respectively. 2. The energy density is a non-dimensional parameter which equals to equals to cv.SP/cv.BP. 3. The different normal letters (a,b,c,d,e after the values) in the same column indicate significant difference among all treatments at 0.05 level.
Table 2 Energy consumption of dry torrefaction and hydrothermal carbonization.
HTC220 HTC260 HTC300 DT220 DT260 DT300
EBP/kJ
Eheating/kJ
Eholding/kJ
ESP/kJ
Ee/%
1917 1917 1917 1917 1917 1917
924.0 1092.0 1260.0 732.6 865.8 999.0
900 900 900 1800 1800 1800
1025.6 1145.7 1043.1 1798.5 1769.2 1436.5
27.42 29.31 25.58 40.42 38.61 30.46
(calorific value) of HTC samples are significantly higher than DT samples. The hydrochar might be more efficient in some occasion when high calorific value is needed, for the combustion point of view. The proximate analysis contents are shown in Fig. 2. The variation of fixed carbon, ash and volatile contents well explained the change in energy yield and calorific value. Ash content of torrefied bamboo and hydrochar kept in a relatively low range while that of coal was reported to be more than 10%. This indicated a capability of dissolving and washing out part of the inorganic components from biomass fuels [6], and therefore upgraded the fuel quality. Volatile and fixed carbon content showed the opposite trend in both torrefied bamboo and hydrochar. The fixed carbon difference between HTC260 and HTC300 was smaller than that between HTC220 and HTC260. By contrast, the fixed carbon difference between DT260 and DT300 was larger than that between DT220 and DT260. This result may also be explained by the initiation of the rapid hemicellulose degradation at lower temperatures in the HTC process. Hence, temperatures above 260 °C for HTC process seemed to have considerably higher fixed carbon content.
Fixed carbon
Proximate analysis (%)
100
ash
Volatile
80
60 40
20
0
3.3. Appearance feature and grindability The appearance features of torrefaction and hydrothermal carbonization treated samples are significantly different as shown in Fig. 3. As temperatures increased, both torrefied bamboo and hydrochar grew darker which meant the degradation degrees increased, while the homogeneity could only be macroscopic in bamboo hydrochar samples. This was because subcritical water under the temperature over 100 °C obtains increased vapor pressure, reduced surface tension and larger ionization constant. The water were not only involved in the reaction to accelerate the decomposition of hemicellulose but also reacted as medium to increase heat transfer and made the solid product distributed homogenously. However, DT process occurred in an inert environment, and the weakening of cell wall due to the decomposition of hemicellulose along with partially depolymerization of cellulose and thermal softening of lignin resulted in the appearance change of torrefied bamboo [25]. The bulk density and particle size distribution of torrefied bamboo and hydrochar samples are exhibited in Table 1 and the surface structure observed under SEM is shown in Fig. 4. The particle size was obviously reduced after HTC treatment with 50% of HTC260 and HTC300 particles smaller than 85 lm. And the bulk density of bamboo hydrochar was approximate double times of that of torrefied bamboo and raw bamboo particle, which indicated it could have more potential to be converted to densified form of fuel. For raw bamboo particle, a thick and unbroken fiber structure could be observed. However, in the image of hydrochar sample, the rupture of fiber could be noticed at low temperature, and an almost complete destruction of fiber could be observed for bamboo hydrochar produced at 260 °C. Bach et al. reported similar results using forest residues as feedstocks [26]. The gradual change in surface structure supported that the degradation degree increased with increasing temperature. 3.4. Hydrophobicity
BP
DT220 DT260 DT300 HTC220 HTC260 HTC300
Thermal treatment Fig. 2. Proximate analysis content of torrefied bamboo and hydrochar.
The tendency to absorb moisture from the ambient, even after drying, make the material rot easily with time, and reduce the energy content of solid fuel, which is one of the major limitations
477
W. Yan et al. / Fuel 196 (2017) 473–480
HTC220
BP
DT220
HTC260
DT260
HTC300
DT300
Fig. 3. Appearance feature of thermal treated bamboo.
Fig. 4. SEM images of the samples (a) BP, (b) HTC220, and (c) HTC260.
to application of solid fuel [27]. In this case, improvement of thermochemical treatment on the hydrophobicity of solid fuel could reduce the energy spent in evaporating water prior to combustion. The results for hydrophobicity behavior of raw bamboo particle,
torrefied bamboo and hydrochar samples are shown in Table 3. Both HTC and DT process at high temperature (HTC260, HTC300 and DT300) improved the hydrophobicity of solid fuel, and bamboo hydrochar samples were found to be more hydrophobic in nature
478
W. Yan et al. / Fuel 196 (2017) 473–480
Table 3 Hydrophobicity of bamboo particles, torrefied bamboo and hydrochar. Sample Moisture content/%
1h 24 h
BP
HTC220
HTC260
HTC300
DT220
DT260
DT300
7.15 ± 0.17a 7.17 ± 0.16a
2.53 ± 0.19bc 13.66 ± 0.21b
2.49 ± 0.20bc 3.09 ± 0.21c
1.60 ± 0.44b 2.41 ± 0.44c
4.25 ± 0.16d 11.48 ± 0.30d
6.31 ± 0.08a 9.91 ± 0.09d
3.52 ± 0.07 cd 5.27 ± 0.06e
Note: The different normal letters (a,b,c,d,e after the values) in the same row indicate significant difference among all treatments at 0.05 level.
than the torrefied bamboo samples at the same temperature. As mentioned previously in 3.2 potential of energy application that the degradation of hemicellulose was more rapid for HTC, the HTC process underwent a series of hydrolysis, condensation, decarboxylation, and dehydration reactions while the DT process underwent depolymerization, recondensation, devolatilization and limited carbonization [28,29]. Therefore, the removal of hydrophilic hemicellulose and the fractional increase of hydrophobic lignin content improved hydrophobicity of torrefied bamboo and hydrochar. BP sample obtained the highest percentage moisture content after 1 h due to the composition of hydrophilic hemicellulose and cellulose, and almost reached the equilibrium moisture content around 7.17%. However, DT220, DT260 and HTC220 showed worse water resistance after 24 h than BP. It might be explained by the observation of agglomeration because of sticky intermediate components, some of which may be hydrophilic, generated in the low intensity HTC and DT process. Porosity might also play a role [30]. All of these findings supported the reason behind the high hydrophobicity of hydrochar samples compared with raw bamboo particle.
5% to the final weight loss) increased and the pyrolysis interval (offset temperature minus onset temperature) significantly narrowed with reaction temperature. The maximum DTG peak for pyrolysis was moved to higher temperature as revealed in Table 4. During DT process, the amorphous hemicellulose and small cellulose crystallites were partially removed, while large cellulose crystallites with higher thermal stability were preserved in solid residues [31], resulting in the enhancement of thermal stability of torrefied bamboo samples. Particularly, HTC220 behaved with a similar variation in trend as was the case with DT samples. However, it exhibited better thermal stability with higher onset temperature of 322 °C and narrower pyrolysis interval of 23.58 °C than all of the torrefied bamboo samples. Moreover, the maximum pyrolysis rate (weight loss rate) of HTC220 was higher than other samples. This also might be due to the active intermediate components produced in relatively low temperature. In a word, the thermal stability increased with increasing temperature and bamboo hydrochar were more stable than torrefied bamboo in thermal environment.
3.6. FTIR 3.5. Thermogravimetric analysis As to the thermal stability, bamboo hydrochar and torrefied bamboo performed quite different in thermogravimetric analysis. TGA (weight loss) and DTG (weight loss rate) curves are shown in Fig. 5. The characteristic parameters of thermal degradation were presented in Table 3. For raw bamboo particle sample, a two-step pyrolysis process took place as shown by two prominent peaks in DTG curves. Apparently, the first DTG peak was due to the thermal decomposition of active hemicellulose, while the cellulose decomposition led to the second DTG peak. HTC260 and HTC300 were most stable in thermal degradation process with weight loss of only 45.42% and 45.89% respectively, and no weight loss peak were found in DTG curves of these two samples. It was because most of the hemicellulose and cellulose had already been removed from bamboo particle during HTC process. For DT samples, the onset temperature of pyrolysis (corresponding to a weight loss of
The degradation of hemicellulose is also supported by the FTIR spectra obtained from torrefied bamboo and hydrochar, which is shown in Fig. 6. The absorption bands in the spectra were assigned as follows: (1) the peaks around 3300 cm1 are attributed to O–H stretching vibration in hydroxyl group; (2) the peaks around 2900 cm1 are attributed to C–H stretching vibration in alkane; (3) the peak at 1730 cm1 is attributed to C = O stretching vibration, mainly from carbonyl and ester groups of hemicellulose; (4) the peak at 1376 cm1 is attributed to C-OH symmetric stretching vibration in carboxyl group; (5) the peak at 1235 cm1 is attributed to C-OH stretching vibration in phenolic hydroxyl group. As can be seen, the peaks attributed to hydroxyl and carboxyl groups decreased with increasing temperature, which therefore suggested that dehydration and decarboxylation did occur in both DT and HTC process. The signal of peaks between 1400 and 1600 cm1 attributed to complicated lignin existed but became weak as
0.3
(b)
(a)
100
0.0 -0.3
40 20
100
200
0.0
-0.6 -0.9 -1.2
300
400
Temperature (°C)
500
600
-1.5 200
BP DT220 DT260 DT300 HTC220 HTC260 HTC300 300
-0.4
DTG (%/°C)
BP DT220 DT260 DT300 HTC220 HTC260 HTC300
60
0
DTG (%/°C)
TGA (%)
80
-0.8
-1.2
250
300
350
400
Temperature (°C)
400
Temperature (°C)
Fig. 5. (a) TGA and (b) DTG curve of bamboo particle, torrefied bamboo and hydrochar.
500
600
479
W. Yan et al. / Fuel 196 (2017) 473–480 Table 4 Thermal degradation characteristics of bamboo particle, torrefied bamboo and hydrochar. Sample
BP
DT220
DT260
DT300
HTC220
HTC260
HTC300
Onset temperature/°C Pyrolysis interval/°C Max pyrolysis rate temperature/°C Weight loss/%
274.24 139.62 305.97 95.35
280.03 84.43 341.70 82.97
299.79 66.89 345.70 71.97
313.88 59.93 350.03 60.00
322.08 23.58 338.51 87.01
/ / 471.18 45.42
/ / 561.19 45.89
Note: There are no significant onset temperature and pyrolysis interval of HTC260 and HTC300.
HTC300
C-OH
HTC260
Transmittance (%)
HTC220
DT300 DT260 DT220 BP C=O C-H
O-H
4000
3200
O=C-OH
2400
1600
800
wavelength (cm-1) Fig. 6. FTIR spectra of bamboo particle and treated bamboo.
Fig. 7. Pore volume with increased pore size.
Table 5 BET surface area and characteristics of pore structure. Sample 2
1
BET surface area/m g Total pore volume/103cm3g1 Mean pore size/nm
BP
DT220
DT260
DT300
HTC220
HTC260
HTC300
2.17 ± 0.41a 8.01 ± 0.51a 11.21 ± 0.08a
7.03 ± 0.75bc 18.98 ± 1.10b 9.52 ± 0.10b
6.10 ± 0.18bd 15.21 ± 0.22bc 8.32 ± 0.06c
2.65 ± 0.19a 10.03 ± 1.15a 11.13 ± 0.06a
5.14 ± 0.26d 56.45 ± 2.74d 37.62 ± 0.15d
7.81 ± 0.11c 24.95 ± 1.45e 11.40 ± 0.09a
3.61 ± 0.62a 11.02 ± 0.89ac 10.71 ± 0.15e
Note: The different normal letters (a,b,c,d,e after the values) in the same row indicate significant difference among all treatments at 0.05 level.
temperature increased, which implied that lignin was not completely decomposed and some fragrant and intermediate structure from lignin remained in torrefied bamboo and hydrochar. The peak at 1730 cm1 appeared in the spectra of BP, DT220, DT260 samples but was not obviously observed in the spectra of the other samples, indicating the degradation of hemicellulose during the HTC and DT process.
3.7. Porosity Generally, a porous structure is often regarded as a good quality in activated carbon, biochar or hydrochar when applied in soil remediation and carbon sequestration rather than solid fuel. However, a porous structure would increase the tendency to absorb ambient moisture which may degrade the quality of the fuel source. The BET surface area and characteristics of pore structure of torrefied bamboo and hydrochar samples are shown in Table 5. The BET surface areas of torrefied bamboo and hydrochar kept to a low range of less than 10 m2 g1, which meant the DT and HTC processes had dismissible effect on the BET surface area of the solid product samples. An obvious decreasing trend was also found in total pore volume of both torrefied bamboo and hydrochar samples with increasing reaction temperature, which weakened moisture absorption ability. The positive correlation between single pore volume and pore size is shown in Fig. 7. As clearly demonstrated, most of the pores were mesoporous with pore size of 2–6 nm.
Single pore volume of torrefied bamboo samples remained fairly constant regardless of increase in pore size unlike those of the HTC samples which had a positive correlation with pore size. All three DT samples reached larger volumes than raw bamboo particles. It can be noted that HTC220 had the most drastic increasing tendency, which likely contributed to the sample’s poor hydrophobicity as mentioned in 3.4 Hydrophobicity. Overall, increasing temperature would reduce the total pore volume and help resist ambient moisture, which therefore upgraded the fuel quality.
4. Conclusion Both DT and HTC processes can upgrade the fuel quality of bamboo biomass. DT is advantageous in mass yield, energy yield and energy efficiency, while HTC is better in calorific value and fixed carbon content. Bamboo hydrochar has smaller particle size, higher bulk density and obtains excellent hydrophobicity at a temperature of 260 and 300 °C. Temperature played an important role in thermochemical treatment of both processes. With increasing temperature, mass yield, energy yield, and volatile content decrease, while calorific value, fixed carbon content, hydrophobicity, and thermal degradation stability increase. FTIR and pore structure analysis provide evidence of change in chemical structure and hydrophilic group as well as demonstrate altered fuel quality of torrefied bamboo and hydrochar. According to all the data in this study, we can draw a conclusion that high quality solid fuel could
480
W. Yan et al. / Fuel 196 (2017) 473–480
be obtained by HTC at a temperature of 260 °C, and high-yield solid fuel could be obtained by DT at a temperature of 300 °C. Acknowledgement This work is financial supported by China National Key R&D Program (No. 2016YFD0800804) and the Natural Science Foundation of Zhejiang Province, China (No. LY16E030003). References [1] Liu Z, Han G. Production of solid fuel biochar from waste biomass by low temperature pyrolysis. Fuel 2015;158:159–65. [2] Li M-F, Li X, Bian J, Chen C-Z, Yu Y-T, Sun R-C. Effect of temperature and holding time on bamboo torrefaction. Biomass Bioenergy 2015;83:366–72. [3] Ronsse F, Van Hecke S, Dickinson D, Prins W. Production and characterization of slow pyrolysis biochar: influence of feedstock type and pyrolysis conditions. GCB Bioenergy 2013;5:104–15. [4] Ross A, Biller P, Kubacki M, Li H, Lea-Langton A, Jones J. Hydrothermal processing of microalgae using alkali and organic acids. Fuel 2010;89:2234–43. [5] Sarvaramini A, Larachi F. Integrated biomass torrefaction–chemical looping combustion as a method to recover torrefaction volatiles energy. Fuel 2014;116:158–67. [6] Bach Q-V, Tran K-Q, Skreiberg Ø, Khalil RA, Phan AN. Effects of wet torrefaction on reactivity and kinetics of wood under air combustion conditions. Fuel 2014;137:375–83. [7] Acharya B, Sule I, Dutta A. A review on advances of torrefaction technologies for biomass processing. Biomass Convers Biorefin 2012;2:349–69. [8] Bach Q-V, Skreiberg Ø. Upgrading biomass fuels via wet torrefaction: a review and comparison with dry torrefaction. Renewable Sustainable Energy Rev 2016;54:665–77. [9] Gao Y, Wang X, Wang J, Li X, Cheng J, Yang H, et al. Effect of residence time on chemical and structural properties of hydrochar obtained by hydrothermal carbonization of water hyacinth. Energy 2013;58:376–83. [10] He C, Giannis A, Wang J-Y. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior. Appl Energy 2013;111:257–66. [11] Mäkelä M, Benavente V, Fullana A. Hydrothermal carbonization of industrial mixed sludge from a pulp and paper mill. Bioresour Technol 2016;200:444–50. [12] Benavente V, Calabuig E, Fullana A. Upgrading of moist agro-industrial wastes by hydrothermal carbonization. J Anal Appl Pyrolysis 2015;113:89–98. [13] Lynam JG, Coronella CJ, Yan W, Reza MT, Vasquez VR. Acetic acid and lithium chloride effects on hydrothermal carbonization of lignocellulosic biomass. Bioresour Technol 2011;102:6192–9.
[14] Yang W, Shimanouchi T, Kimura Y. Characterization of the residue and liquid products produced from husks of nuts from Carya cathayensis Sarg by hydrothermal carbonization. ACS Sustainable Chem Eng 2015;3:591–8. [15] Li X, Li M-F, Bian J, Wang B, Xu J-K, Sun R-C. Hydrothermal carbonization of bamboo in an oxalic acid solution: effects of acid concentration and retention time on the characteristics of products. RSC Adv 2015;5:77147–53. [16] Huang PH, Jhan JW, Cheng YM, Cheng HH. Effects of carbonization parameters of moso-bamboo-based porous charcoal on capturing carbon dioxide. Sci World J 2014. [17] Lin SH, Hsu LY, Chou CS, Jhang JW, Wu P. Carbonization process of moso bamboo (Phyllostachys pubescens) charcoal and its governing thermodynamics. J Anal Appl Pyrolysis 2014;107:9–16. [18] Yamashita N, Machida M. Carbonization of bamboo and consecutive low temperature air activation. Wood Sci Technol 2011;45:801–8. [19] Yang W, Wang H, Zhang M, Zhu J, Zhou J, Wu S. Fuel properties and combustion kinetics of hydrochar prepared by hydrothermal carbonization of bamboo. Bioresour Technol 2016;205:199–204. [20] Lou R, Wu SB, Lv GJ, Yang Q. Chemical structure and thermochemical properties of lignin from herbaceous plant. J South China Univ Technol (Nat Sci Ed) 2010;38:1–6. [21] Chen D, Zhou J, Zhang Q. Effects of heating rate on slow pyrolysis behavior, kinetic parameters and products properties of moso bamboo. Bioresour Technol 2014;169:313–9. [22] Kambo HS, Dutta A. Comparative evaluation of torrefaction and hydrothermal carbonization of lignocellulosic biomass for the production of solid biofuel. Energy Convers Manage 2015;105:746–55. [23] Bach Q-V, Tran K-Q, Khalil RA, Skreiberg Ø. Comparative assessment of wet torrefaction. Energy Fuels 2013;27:6743–53. [24] Yan W, Hastings J, Acharjee T, Coronella C, Vasquez V. Mass and Energy Balances of Wet Torrefaction of Lignocellulosic Biomass. Energy Fuels 2010;24:4738–42. [25] Funke A, Ziegler F. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuels Bioprod Biorefin 2010;4:160–77. [26] Bach Q-V, Tran K-Q, Skreiberg Ø. Hydrothermal pretreatment of fresh forest residues: effects of feedstock pre-drying. Biomass Bioenergy 2016;85:76–83. [27] Guo W. Self-heating and spontaneous combustion of wood pellets during storage. University of British Columbia; 2013. [28] Donar YO, Çag˘lar E, Sınag˘ A. Preparation and characterization of agricultural waste biomass based hydrochars. Fuel 2016;183:366–72. [29] Liu Z, Quek A, Hoekman SK, Balasubramanian R. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 2013;103:943–9. [30] Qian SP, Wang H, Zarei E, Sheng KC. Effect of hydrothermal pretreatment on the properties of moso bamboo particles reinforced polyvinyl chloride composites. Composites Part B 2015;82:23–9. [31] Zheng A, Zhao Z, Chang S, Huang Z, Zhao K, Wei G, et al. Comparison of the effect of wet and dry torrefaction on chemical structure and pyrolysis behavior of corncobs. Bioresour Technol 2015;176:15–22.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Upgrading fuel quality of moso bamboo via low temperature thermochemical treatments: Dry torrefaction and hydrothermal carbonization Wei Yan a, Sandy Perez b, Kuichuan Sheng a,⇑ a b
Zhejiang University, College of Biosystems Engineering and Food Science, 310058 Hangzhou, China University of Illinois at Urbana-Champaign, School of Earth, Society and Environment, IL 61801-4713, USA
h i g h l i g h t s Fuel potential was enhanced via hydrothermal carbonization and torrefaction. Hydrochar performed better than torrefied bamboo produced at the same temperature. Hydrophobicity was obtained at higher treating temperature. Removal of hemicellulose was the dominant reaction in hydrothermal carbonization. 260 °C was determined to be suitable for hydrochar production.
a r t i c l e
i n f o
Article history: Received 18 September 2016 Received in revised form 26 January 2017 Accepted 6 February 2017
Keywords: Moso bamboo Calorific value Dry torrefaction Hydrochar Hydrothermal carbonization
a b s t r a c t The application of raw bamboo as biomass energy is restricted due to its large particle size, high oxygen content, low energy density and weak water resistance. In order to upgrade the fuel quality of moso bamboo, dry torrefaction (DT) and hydrothermal carbonization (HTC) have been investigated in this study. The physicochemical properties, thermal stability and microstructures of solid products were examined by varying the reaction temperature among 220, 260 and 300 °C. The results showed that increasing temperature reduced the mass yield and energy yield, however, it significantly improved the calorific value of solid products. Through the HTC process, the bamboo hydrochar obtained the calorific value of 28.29 MJ/ kg, the energy yield of 59.77% and the fixed carbon content of 63.08% at the temperature of 260 °C, indicating enhanced potential as a solid fuel. In comparison, the grindability (including particle size and bulk density), hydrophobicity, and thermal stability of torrefied bamboo were considerably lower than that of bamboo hydrochar produced at the same temperature. Furthermore, the transformation of chemical bonds demonstrated that hydrolysis, dehydration and decarboxylation took place during the HTC process. The porous structure was only slightly enhanced with increasing reaction temperature, exhibiting variation consistent with that of hydrophobicity. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the past few decades, renewable biomass energy has received great attention due to the increasing environmental concerns and energy crisis associated with the exhaustion of conventional fossil fuels. Biomass is advantageous both alone in combustion and cofired with coal because it is widely available and demonstrates potential for greenhouse gas neutrality [1]. However, low calorific value and poor resistance to moisture absorption make the direct
⇑ Corresponding author. E-mail address: [email protected] (K. Sheng). http://dx.doi.org/10.1016/j.fuel.2017.02.015 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
use of biomass less attractive. To overcome these shortages, various types of thermochemical conversions have been attempted [2–5]. Among all the thermochemical treatments, dry torrefaction and hydrothermal carbonization are the two most practical methods with relatively low temperature requirements [6], costing less energy input when compared with slow pyrolysis for biochar, fast pyrolysis for bio-oil, or gasification. Dry torrefaction (DT), during which processed biomass is heated in an inert atmosphere at temperatures of around 200– 300 °C for a residence time of 30 min to a couple hours, is often regarded as a conventional thermochemical pre-treatment and proposed as an alternative to improve the physicochemical
474
W. Yan et al. / Fuel 196 (2017) 473–480
properties of biomass. Acharya et al. [7] reviewed that torrefaction undergoes devolatilization, depolymerization, and limited carbonization of lignocellulose components and generates a brown to black solid product with 70% mass and 90% energy reserved. Bach and Skreiberg [8] claimed that the improved properties of torrefied biomass, such as relatively superior handling and milling, highlight a beneficial opportunity to co-fire the clean renewable energy source with coal in an effort to relieve the energy crisis and mitigate environmental pollution. Nevertheless, the torrefaction process is unsuitable when disposing feedstocks with high moisture content, and the calorific value of torrefied biomass is still lowly graded. Moreover, the high O/C ratio and volatile content may affect the thermal stability and combustion profile. Hydrothermal carbonization (HTC), also referred to as wet torrefaction, is often used to prepare a solid product called hydrochar which exhibits reduced O/C ratio, increased calorific value, better grindability and improved hydrophobicity compared with untreated and torrefied biomass [9]. During the HTC process, feedstock is mixed with subcritical water at temperatures of 180– 350 °C under an inert atmosphere so that the products are not affected by the high moisture content. Hydrothermal carbonization has the ability to deal with not only extremely wet feedstock such as sewage sludge [10] or sludge from the pulp and paper mill [11], but also forestry processing residues and agricultural wastes [12]. Lynam et al. [13] prepared loblolly pine hydrochar at the temperature range from 200 to 260 °C. A 30% increase in calorific value of hydrochar was found with increasing temperature whereas the mass yield was slightly reduced. In addition, adding both 0.4 g acetic acid and 1 g lithium chloride per g pine resulted in an energy densification ratio of 1.34. Other research included the use of nut husks as feedstock [14]. The fuel properties of hydrochar were found to be most affected by the component weight ratios of the biomass. For example, lignin was the main contribution to the solid fuel yield and the reactivity of cellulose and hemicellulose in different biomass was affected by the biomass species. Moreover, the calorific value of solid fuels prepared at 260 °C, around 25 MJ/kg, were comparable to those of commercial coals. Therefore, the properties of hydrochar may greatly differ depending on the feedstock resource and reaction conditions such as temperature. Moso bamboo, an abundant natural cellulosic resource, has gained much attention in the comprehensive utilization of biomass over the past years due to its easy propagation, fast growth and regeneration, and high productivity as well as its rapid maturity [15]. It is widely used to produce furniture, veneers and flooring, but a significant amount of bamboo processing residues was treated as waste. Thus, the application of bamboo processing residues for bioenergy production is worth exploring. In order to dispose these solid waste residues, some researchers have evaluated moso bamboo residues directly as biofuel, but the energy density and the fuel quality is generally low for direct burning of moso bamboo. Some other researchers have prepared bamboo biochar by slow pyrolysis, but the mass yield is always lower than 30% and energy cost is high [16–18]. It is necessary to find an environmentally friendly and efficient method to upgrade fuel quality of moso bamboo. However, only a few researchers have studied hydrothermal carbonization on bamboo biomass [19]. Even fewer have examined the comparative assessment of moso bamboo for producing energy dense fuel via DT and HTC treatment. The main objectives of this study are to (1) upgrade fuel quality of the torrefied product and hydrochar obtained from moso bamboo by investigating the effect of reaction temperature on energy characteristics, such as calorific value, proximate analysis, hydrophobicity, and thermal degradation stability, (2) clarify the relationship between fuel quality and chemical structure as well as pore structure, (3) and compare the performance of these two
kinds of solid biofuels to identify the better option for energy application. 2. Materials and methods 2.1. Materials Moso bamboo (Phyllostachys pubescens) particles were supplied by China National Bamboo Research Center in Hangzhou. The raw moso bamboo was harvested in Anji, Zhejiang Province of China. The samples were ground into particles of 200–400 lm using a turbine grinder (XWDJ-130, Zhejiang Xinshiji grinder machine Co. Ltd, China) and the moisture content was measured to be 8%. 2.2. Samples preparation The torrefaction of bamboo particles was carried out in a benchscale fixed bed reactor, which was designed and fabricated by Bioenergy and Biomaterials Lab at Zhejiang University. The reactor consists of a stainless steel container with an inner diameter of 120 mm and a height of 330 mm. For each run, 100 g of moso bamboo particles was placed inside the reactor. The reactor was indirectly heated by electric heater at set temperatures of 220, 260 and 300 °C for 1 h, respectively. After the desired isothermal period, the sample was cooled down naturally to room temperature. The solid torrefied bamboo was collected in a sealed transparent plastic bag and stored in an airtight container at room temperature until analysis. Hydrothermal carbonization of bamboo particles was carried out in a cylindrical stainless steel reactor (4848, Parr Instrument Company, USA) with a working volume of 2 L. A weight of 100 g bamboo particles combined with 1 L purified water was fed into the reactor, which was then closely tightened. The reactor was held at the desired temperature of 220, 260 and 300 °C for 1 h, respectively. Temperature and pressure inside the reactor were measured and adjusted by the controller connected with a thermocouple and a pressure sensor. After treatment, the mixture inside the reactor was filtered by a vacuum pump (SHB-IA, Shanghai Yukang Science Teaching Instrument Equipment Co. Ltd), and then the hydrochar was dried at 105 °C until its weight reached a constant value. The samples were marked as BP for bamboo particle, DT220, DT260, DT300 for dry torrefied bamboo, and HTC220, HTC260, HTC300 for bamboo hydrochar at their respective temperatures. 2.3. Properties characterization Proximate analysis of the samples was conducted using ASTM E1756-08 to determine the moisture content, ASTM E872-82 for volatile content, and ASTM D1102-84 for ash content. The fixed carbon content was calculated by mass balance. The calorific value of the samples was measured using an Oxygen Bomb Calorimeter (5E-AC, Changsha Kaiyuan Instruments Co., Ltd, China). The mass yield (MY) and energy yield (EY) of solid products were calculated as follows:
MY ð%Þ ¼ m:SP =m:BP 100
ð1Þ
EY ð%Þ ¼ M Y cv :SP =cv :BP
ð2Þ
where m.SP and cv.SP are the mass and calorific value of solid products, m.BP and cv.BP are the mass and calorific value of bamboo particles, and cv.SP/cv.BP is energy density of the solid products. The bulk density was determined using a Bulk Density Tester (Buld-005, Hangzhou Tonfus Corporation, China). The particle size distribution was determined using a Laser Particle Size Analyzer
475
W. Yan et al. / Fuel 196 (2017) 473–480
2.4. Energy consumption Energy efficiency (Ee) is calculated by EBP, Eheating, Eholding and ESP as follow.
Ee ¼
ESP 100% EBP þ Eheating þ Eholding
where EBP is the energy of raw bamboo particle, which equals to m∙cv.BP; Eheating is the energy input during the heating stage, which equals to CmΔT; Eholdting is the energy input during the holding stage, which equals to Powert; ESP is the energy of solid product, which equals to mcv.SP. Cwater (specific heat capacity) is 4.2 kJ/(kg°C); Cmoso bamboo is 33.30 kJ/(kg°C) based on the research of Lou et al. [20]. The electric power of dry torrefaction was recorded to be 1 kW while that of hydrothermal carbonization was 0.5 kW.
3. Results and discussion 3.1. Temperature and pressure profiles of HTC process The temperature and pressure profiles during the reaction process are shown in Fig. 1. As temperature increased, pressure provided by saturated water vapor inside the reactor also followed a similar increasing trend. The variance in trend between temperature and pressure was due to the unsaturated state of water vapor below 100 °C as this delayed increase in pressure. However, after cooling down to room temperature, the pressure inside the reactor was higher than the initial pressure. This was due to released micromolecular gases, such as CO2, in the decomposition reaction of bamboo fibers [21]. Non-isothermal conditions were also
established in the DT process, and an approximate constant heating rate of 3 °C/min made the reaction system stable. 3.2. Potential of energy application The mass yield, calorific value and energy yield of solid product at different thermochemical treatment conditions are shown in Table 1. With an increase in reaction temperature from 220 to 300 °C, the mass yield of torrefied bamboo and hydrochar decreased from 92.8% to 61.6% and 51.8% to 35.6%, respectively. Due to the presence of subcritical water, the degradation of hemicellulose in the HTC process was rapid and took place at a lower temperature when compared with dry torrefaction [22], so that bamboo particle treated by HTC showed higher mass loss than that treated by DT at the same reaction temperature. On the other hand, calorific value of both torrefied bamboo and hydrochar exhibited an increasing trend as the reaction temperature increased. HTC260 and HTC300 even contained high calorific value of 28.29 and 29.30 MJ/kg, which is comparable to the energy content of coal and biochar. Besides, hydrochar is obtained from biomass while coal contains sulfur, which therefore made hydrochar a kind of more clean energy with wider application. In addition, the calorific value of bamboo hydrochar produced at 220 °C was remarkably lower compared with those of HTC260 and HTC300. Considering both energy density and mass yield of all solid products, HTC260 had the highest energy yield of 59.77%. While in the lower temperature zones of the HTC process, the increase in temperature accelerated the decomposition of hemicellulose molecules [23] which then released a large amount of incombustible gas, such as CO2 and H2O. Volatile content decreased and fixed carbon content increased, leading to much improvement in calorific value. A higher temperature above 260 °C increased the treatment severity and therefore enhance the decomposition of biomass polymers to intermediate components with high calorific value into an aqueous phase which lessened the increase of calorific value. However, the decomposition rate of hemicellulose without water participation by DT was slower than that by HTC, so that torrefied bamboo obtained lower calorific value than bamboo hydrochar at the same reaction temperature. The energy consumptions of these two technologies are shown in Table 2. As can be seen, DT is considered to be a higher-yield technology, because it needs extra energy to heat up the water for the HTC process, many times of the bamboo feedstock weight. In a laboratory investigation, Yan et al. [24] calculated that it requires 15–20% extra energy to produce hydrochar, and the efficiency can be higher by an industrial company with optimal heat and mass integration. However, the energy per unit weight
280
(a)
HTC220 HTC260 HTC300
(b)
Temperature Pressure
210
Temperature (°C)
Temperature (°C)
300
200
4 3
140
100
2 70
0
0
100
200
Reaction time (min)
300
400
5
1
0
50
100
150
200
250
Reaction time (min)
Fig. 1. (a) temperature profile during HTC process; (b) Temperature and pressure profile of HTC260.
300
350
0 400
Pressure (MPa)
(MS3000, Malvern Instruments Ltd., UK). Water resistance was evaluated on water absorption for 24 h according to ASTM D5229-14. Thermogravimetric analysis was performed using a Thermogravimetric Analyzer (TA-60WS, Shimadzu Corporation, Japan). FTIR spectra were obtained using a FTIR Spectrometer (Nicolet 6700, Thermo Fisher Nicolet Corporation, USA) and the data were obtained in the wavenumber range from 4000 to 750 cm1. The specific surface areas and porosity characteristics were conducted with the method of isothermal adsorption of nitrogen using Automatic Nitrogen Adsorption Equipment (JWBK, Automatic Nitrogen Adsorption Equipment Co. Ltd., China). All of the data provided for the samples were the average value of three trials.
476
W. Yan et al. / Fuel 196 (2017) 473–480
Table 1 Physicochemical properties of torrefied bamboo and hydrochar. Sample
Mass yield/%
Calorific value/MJkg1
Energy density
Energy yield/%
Bulk density/102 gcm3
BP HTC220 HTC260 HTC300 DT220 DT260 DT300
– 51.8 ± 3.5 40.5 ± 1.7 35.6 ± 2.1 92.8 ± 1.5 86.6 ± 0.8 61.6 ± 2.4
19.17 ± 0.21 19.80 ± 0.64 28.29 ± 0.22 29.30 ± 0.39 19.38 ± 0.46 20.43 ± 0.05 23.32 ± 0.32
– 1.03 1.48 1.53 1.01 1.07 1.22
– 53.50 59.77 54.41 93.82 92.29 74.94
15 ± 0.05 27 ± 0.10 25 ± 0.45 29 ± 0.06 15 ± 0.22 15 ± 0.27 14 ± 0.10
a ab c d a b e
a b c d a a e
Particle size distribution Dx(10)/lm
Dx(50)/lm
Dx(90)/lm
164 30.7 4.58 6.84 164 146 144
298 179 55.7 84.5 298 284 282
498 422 347 354 499 488 487
Note: 1. D (10)/lm, D (50)/lm and D (90)/lm refer to that 10%, 50% and 90% of the particles are smaller than the recorded data, respectively. 2. The energy density is a non-dimensional parameter which equals to equals to cv.SP/cv.BP. 3. The different normal letters (a,b,c,d,e after the values) in the same column indicate significant difference among all treatments at 0.05 level.
Table 2 Energy consumption of dry torrefaction and hydrothermal carbonization.
HTC220 HTC260 HTC300 DT220 DT260 DT300
EBP/kJ
Eheating/kJ
Eholding/kJ
ESP/kJ
Ee/%
1917 1917 1917 1917 1917 1917
924.0 1092.0 1260.0 732.6 865.8 999.0
900 900 900 1800 1800 1800
1025.6 1145.7 1043.1 1798.5 1769.2 1436.5
27.42 29.31 25.58 40.42 38.61 30.46
(calorific value) of HTC samples are significantly higher than DT samples. The hydrochar might be more efficient in some occasion when high calorific value is needed, for the combustion point of view. The proximate analysis contents are shown in Fig. 2. The variation of fixed carbon, ash and volatile contents well explained the change in energy yield and calorific value. Ash content of torrefied bamboo and hydrochar kept in a relatively low range while that of coal was reported to be more than 10%. This indicated a capability of dissolving and washing out part of the inorganic components from biomass fuels [6], and therefore upgraded the fuel quality. Volatile and fixed carbon content showed the opposite trend in both torrefied bamboo and hydrochar. The fixed carbon difference between HTC260 and HTC300 was smaller than that between HTC220 and HTC260. By contrast, the fixed carbon difference between DT260 and DT300 was larger than that between DT220 and DT260. This result may also be explained by the initiation of the rapid hemicellulose degradation at lower temperatures in the HTC process. Hence, temperatures above 260 °C for HTC process seemed to have considerably higher fixed carbon content.
Fixed carbon
Proximate analysis (%)
100
ash
Volatile
80
60 40
20
0
3.3. Appearance feature and grindability The appearance features of torrefaction and hydrothermal carbonization treated samples are significantly different as shown in Fig. 3. As temperatures increased, both torrefied bamboo and hydrochar grew darker which meant the degradation degrees increased, while the homogeneity could only be macroscopic in bamboo hydrochar samples. This was because subcritical water under the temperature over 100 °C obtains increased vapor pressure, reduced surface tension and larger ionization constant. The water were not only involved in the reaction to accelerate the decomposition of hemicellulose but also reacted as medium to increase heat transfer and made the solid product distributed homogenously. However, DT process occurred in an inert environment, and the weakening of cell wall due to the decomposition of hemicellulose along with partially depolymerization of cellulose and thermal softening of lignin resulted in the appearance change of torrefied bamboo [25]. The bulk density and particle size distribution of torrefied bamboo and hydrochar samples are exhibited in Table 1 and the surface structure observed under SEM is shown in Fig. 4. The particle size was obviously reduced after HTC treatment with 50% of HTC260 and HTC300 particles smaller than 85 lm. And the bulk density of bamboo hydrochar was approximate double times of that of torrefied bamboo and raw bamboo particle, which indicated it could have more potential to be converted to densified form of fuel. For raw bamboo particle, a thick and unbroken fiber structure could be observed. However, in the image of hydrochar sample, the rupture of fiber could be noticed at low temperature, and an almost complete destruction of fiber could be observed for bamboo hydrochar produced at 260 °C. Bach et al. reported similar results using forest residues as feedstocks [26]. The gradual change in surface structure supported that the degradation degree increased with increasing temperature. 3.4. Hydrophobicity
BP
DT220 DT260 DT300 HTC220 HTC260 HTC300
Thermal treatment Fig. 2. Proximate analysis content of torrefied bamboo and hydrochar.
The tendency to absorb moisture from the ambient, even after drying, make the material rot easily with time, and reduce the energy content of solid fuel, which is one of the major limitations
477
W. Yan et al. / Fuel 196 (2017) 473–480
HTC220
BP
DT220
HTC260
DT260
HTC300
DT300
Fig. 3. Appearance feature of thermal treated bamboo.
Fig. 4. SEM images of the samples (a) BP, (b) HTC220, and (c) HTC260.
to application of solid fuel [27]. In this case, improvement of thermochemical treatment on the hydrophobicity of solid fuel could reduce the energy spent in evaporating water prior to combustion. The results for hydrophobicity behavior of raw bamboo particle,
torrefied bamboo and hydrochar samples are shown in Table 3. Both HTC and DT process at high temperature (HTC260, HTC300 and DT300) improved the hydrophobicity of solid fuel, and bamboo hydrochar samples were found to be more hydrophobic in nature
478
W. Yan et al. / Fuel 196 (2017) 473–480
Table 3 Hydrophobicity of bamboo particles, torrefied bamboo and hydrochar. Sample Moisture content/%
1h 24 h
BP
HTC220
HTC260
HTC300
DT220
DT260
DT300
7.15 ± 0.17a 7.17 ± 0.16a
2.53 ± 0.19bc 13.66 ± 0.21b
2.49 ± 0.20bc 3.09 ± 0.21c
1.60 ± 0.44b 2.41 ± 0.44c
4.25 ± 0.16d 11.48 ± 0.30d
6.31 ± 0.08a 9.91 ± 0.09d
3.52 ± 0.07 cd 5.27 ± 0.06e
Note: The different normal letters (a,b,c,d,e after the values) in the same row indicate significant difference among all treatments at 0.05 level.
than the torrefied bamboo samples at the same temperature. As mentioned previously in 3.2 potential of energy application that the degradation of hemicellulose was more rapid for HTC, the HTC process underwent a series of hydrolysis, condensation, decarboxylation, and dehydration reactions while the DT process underwent depolymerization, recondensation, devolatilization and limited carbonization [28,29]. Therefore, the removal of hydrophilic hemicellulose and the fractional increase of hydrophobic lignin content improved hydrophobicity of torrefied bamboo and hydrochar. BP sample obtained the highest percentage moisture content after 1 h due to the composition of hydrophilic hemicellulose and cellulose, and almost reached the equilibrium moisture content around 7.17%. However, DT220, DT260 and HTC220 showed worse water resistance after 24 h than BP. It might be explained by the observation of agglomeration because of sticky intermediate components, some of which may be hydrophilic, generated in the low intensity HTC and DT process. Porosity might also play a role [30]. All of these findings supported the reason behind the high hydrophobicity of hydrochar samples compared with raw bamboo particle.
5% to the final weight loss) increased and the pyrolysis interval (offset temperature minus onset temperature) significantly narrowed with reaction temperature. The maximum DTG peak for pyrolysis was moved to higher temperature as revealed in Table 4. During DT process, the amorphous hemicellulose and small cellulose crystallites were partially removed, while large cellulose crystallites with higher thermal stability were preserved in solid residues [31], resulting in the enhancement of thermal stability of torrefied bamboo samples. Particularly, HTC220 behaved with a similar variation in trend as was the case with DT samples. However, it exhibited better thermal stability with higher onset temperature of 322 °C and narrower pyrolysis interval of 23.58 °C than all of the torrefied bamboo samples. Moreover, the maximum pyrolysis rate (weight loss rate) of HTC220 was higher than other samples. This also might be due to the active intermediate components produced in relatively low temperature. In a word, the thermal stability increased with increasing temperature and bamboo hydrochar were more stable than torrefied bamboo in thermal environment.
3.6. FTIR 3.5. Thermogravimetric analysis As to the thermal stability, bamboo hydrochar and torrefied bamboo performed quite different in thermogravimetric analysis. TGA (weight loss) and DTG (weight loss rate) curves are shown in Fig. 5. The characteristic parameters of thermal degradation were presented in Table 3. For raw bamboo particle sample, a two-step pyrolysis process took place as shown by two prominent peaks in DTG curves. Apparently, the first DTG peak was due to the thermal decomposition of active hemicellulose, while the cellulose decomposition led to the second DTG peak. HTC260 and HTC300 were most stable in thermal degradation process with weight loss of only 45.42% and 45.89% respectively, and no weight loss peak were found in DTG curves of these two samples. It was because most of the hemicellulose and cellulose had already been removed from bamboo particle during HTC process. For DT samples, the onset temperature of pyrolysis (corresponding to a weight loss of
The degradation of hemicellulose is also supported by the FTIR spectra obtained from torrefied bamboo and hydrochar, which is shown in Fig. 6. The absorption bands in the spectra were assigned as follows: (1) the peaks around 3300 cm1 are attributed to O–H stretching vibration in hydroxyl group; (2) the peaks around 2900 cm1 are attributed to C–H stretching vibration in alkane; (3) the peak at 1730 cm1 is attributed to C = O stretching vibration, mainly from carbonyl and ester groups of hemicellulose; (4) the peak at 1376 cm1 is attributed to C-OH symmetric stretching vibration in carboxyl group; (5) the peak at 1235 cm1 is attributed to C-OH stretching vibration in phenolic hydroxyl group. As can be seen, the peaks attributed to hydroxyl and carboxyl groups decreased with increasing temperature, which therefore suggested that dehydration and decarboxylation did occur in both DT and HTC process. The signal of peaks between 1400 and 1600 cm1 attributed to complicated lignin existed but became weak as
0.3
(b)
(a)
100
0.0 -0.3
40 20
100
200
0.0
-0.6 -0.9 -1.2
300
400
Temperature (°C)
500
600
-1.5 200
BP DT220 DT260 DT300 HTC220 HTC260 HTC300 300
-0.4
DTG (%/°C)
BP DT220 DT260 DT300 HTC220 HTC260 HTC300
60
0
DTG (%/°C)
TGA (%)
80
-0.8
-1.2
250
300
350
400
Temperature (°C)
400
Temperature (°C)
Fig. 5. (a) TGA and (b) DTG curve of bamboo particle, torrefied bamboo and hydrochar.
500
600
479
W. Yan et al. / Fuel 196 (2017) 473–480 Table 4 Thermal degradation characteristics of bamboo particle, torrefied bamboo and hydrochar. Sample
BP
DT220
DT260
DT300
HTC220
HTC260
HTC300
Onset temperature/°C Pyrolysis interval/°C Max pyrolysis rate temperature/°C Weight loss/%
274.24 139.62 305.97 95.35
280.03 84.43 341.70 82.97
299.79 66.89 345.70 71.97
313.88 59.93 350.03 60.00
322.08 23.58 338.51 87.01
/ / 471.18 45.42
/ / 561.19 45.89
Note: There are no significant onset temperature and pyrolysis interval of HTC260 and HTC300.
HTC300
C-OH
HTC260
Transmittance (%)
HTC220
DT300 DT260 DT220 BP C=O C-H
O-H
4000
3200
O=C-OH
2400
1600
800
wavelength (cm-1) Fig. 6. FTIR spectra of bamboo particle and treated bamboo.
Fig. 7. Pore volume with increased pore size.
Table 5 BET surface area and characteristics of pore structure. Sample 2
1
BET surface area/m g Total pore volume/103cm3g1 Mean pore size/nm
BP
DT220
DT260
DT300
HTC220
HTC260
HTC300
2.17 ± 0.41a 8.01 ± 0.51a 11.21 ± 0.08a
7.03 ± 0.75bc 18.98 ± 1.10b 9.52 ± 0.10b
6.10 ± 0.18bd 15.21 ± 0.22bc 8.32 ± 0.06c
2.65 ± 0.19a 10.03 ± 1.15a 11.13 ± 0.06a
5.14 ± 0.26d 56.45 ± 2.74d 37.62 ± 0.15d
7.81 ± 0.11c 24.95 ± 1.45e 11.40 ± 0.09a
3.61 ± 0.62a 11.02 ± 0.89ac 10.71 ± 0.15e
Note: The different normal letters (a,b,c,d,e after the values) in the same row indicate significant difference among all treatments at 0.05 level.
temperature increased, which implied that lignin was not completely decomposed and some fragrant and intermediate structure from lignin remained in torrefied bamboo and hydrochar. The peak at 1730 cm1 appeared in the spectra of BP, DT220, DT260 samples but was not obviously observed in the spectra of the other samples, indicating the degradation of hemicellulose during the HTC and DT process.
3.7. Porosity Generally, a porous structure is often regarded as a good quality in activated carbon, biochar or hydrochar when applied in soil remediation and carbon sequestration rather than solid fuel. However, a porous structure would increase the tendency to absorb ambient moisture which may degrade the quality of the fuel source. The BET surface area and characteristics of pore structure of torrefied bamboo and hydrochar samples are shown in Table 5. The BET surface areas of torrefied bamboo and hydrochar kept to a low range of less than 10 m2 g1, which meant the DT and HTC processes had dismissible effect on the BET surface area of the solid product samples. An obvious decreasing trend was also found in total pore volume of both torrefied bamboo and hydrochar samples with increasing reaction temperature, which weakened moisture absorption ability. The positive correlation between single pore volume and pore size is shown in Fig. 7. As clearly demonstrated, most of the pores were mesoporous with pore size of 2–6 nm.
Single pore volume of torrefied bamboo samples remained fairly constant regardless of increase in pore size unlike those of the HTC samples which had a positive correlation with pore size. All three DT samples reached larger volumes than raw bamboo particles. It can be noted that HTC220 had the most drastic increasing tendency, which likely contributed to the sample’s poor hydrophobicity as mentioned in 3.4 Hydrophobicity. Overall, increasing temperature would reduce the total pore volume and help resist ambient moisture, which therefore upgraded the fuel quality.
4. Conclusion Both DT and HTC processes can upgrade the fuel quality of bamboo biomass. DT is advantageous in mass yield, energy yield and energy efficiency, while HTC is better in calorific value and fixed carbon content. Bamboo hydrochar has smaller particle size, higher bulk density and obtains excellent hydrophobicity at a temperature of 260 and 300 °C. Temperature played an important role in thermochemical treatment of both processes. With increasing temperature, mass yield, energy yield, and volatile content decrease, while calorific value, fixed carbon content, hydrophobicity, and thermal degradation stability increase. FTIR and pore structure analysis provide evidence of change in chemical structure and hydrophilic group as well as demonstrate altered fuel quality of torrefied bamboo and hydrochar. According to all the data in this study, we can draw a conclusion that high quality solid fuel could
480
W. Yan et al. / Fuel 196 (2017) 473–480
be obtained by HTC at a temperature of 260 °C, and high-yield solid fuel could be obtained by DT at a temperature of 300 °C. Acknowledgement This work is financial supported by China National Key R&D Program (No. 2016YFD0800804) and the Natural Science Foundation of Zhejiang Province, China (No. LY16E030003). References [1] Liu Z, Han G. Production of solid fuel biochar from waste biomass by low temperature pyrolysis. Fuel 2015;158:159–65. [2] Li M-F, Li X, Bian J, Chen C-Z, Yu Y-T, Sun R-C. Effect of temperature and holding time on bamboo torrefaction. Biomass Bioenergy 2015;83:366–72. [3] Ronsse F, Van Hecke S, Dickinson D, Prins W. Production and characterization of slow pyrolysis biochar: influence of feedstock type and pyrolysis conditions. GCB Bioenergy 2013;5:104–15. [4] Ross A, Biller P, Kubacki M, Li H, Lea-Langton A, Jones J. Hydrothermal processing of microalgae using alkali and organic acids. Fuel 2010;89:2234–43. [5] Sarvaramini A, Larachi F. Integrated biomass torrefaction–chemical looping combustion as a method to recover torrefaction volatiles energy. Fuel 2014;116:158–67. [6] Bach Q-V, Tran K-Q, Skreiberg Ø, Khalil RA, Phan AN. Effects of wet torrefaction on reactivity and kinetics of wood under air combustion conditions. Fuel 2014;137:375–83. [7] Acharya B, Sule I, Dutta A. A review on advances of torrefaction technologies for biomass processing. Biomass Convers Biorefin 2012;2:349–69. [8] Bach Q-V, Skreiberg Ø. Upgrading biomass fuels via wet torrefaction: a review and comparison with dry torrefaction. Renewable Sustainable Energy Rev 2016;54:665–77. [9] Gao Y, Wang X, Wang J, Li X, Cheng J, Yang H, et al. Effect of residence time on chemical and structural properties of hydrochar obtained by hydrothermal carbonization of water hyacinth. Energy 2013;58:376–83. [10] He C, Giannis A, Wang J-Y. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior. Appl Energy 2013;111:257–66. [11] Mäkelä M, Benavente V, Fullana A. Hydrothermal carbonization of industrial mixed sludge from a pulp and paper mill. Bioresour Technol 2016;200:444–50. [12] Benavente V, Calabuig E, Fullana A. Upgrading of moist agro-industrial wastes by hydrothermal carbonization. J Anal Appl Pyrolysis 2015;113:89–98. [13] Lynam JG, Coronella CJ, Yan W, Reza MT, Vasquez VR. Acetic acid and lithium chloride effects on hydrothermal carbonization of lignocellulosic biomass. Bioresour Technol 2011;102:6192–9.
[14] Yang W, Shimanouchi T, Kimura Y. Characterization of the residue and liquid products produced from husks of nuts from Carya cathayensis Sarg by hydrothermal carbonization. ACS Sustainable Chem Eng 2015;3:591–8. [15] Li X, Li M-F, Bian J, Wang B, Xu J-K, Sun R-C. Hydrothermal carbonization of bamboo in an oxalic acid solution: effects of acid concentration and retention time on the characteristics of products. RSC Adv 2015;5:77147–53. [16] Huang PH, Jhan JW, Cheng YM, Cheng HH. Effects of carbonization parameters of moso-bamboo-based porous charcoal on capturing carbon dioxide. Sci World J 2014. [17] Lin SH, Hsu LY, Chou CS, Jhang JW, Wu P. Carbonization process of moso bamboo (Phyllostachys pubescens) charcoal and its governing thermodynamics. J Anal Appl Pyrolysis 2014;107:9–16. [18] Yamashita N, Machida M. Carbonization of bamboo and consecutive low temperature air activation. Wood Sci Technol 2011;45:801–8. [19] Yang W, Wang H, Zhang M, Zhu J, Zhou J, Wu S. Fuel properties and combustion kinetics of hydrochar prepared by hydrothermal carbonization of bamboo. Bioresour Technol 2016;205:199–204. [20] Lou R, Wu SB, Lv GJ, Yang Q. Chemical structure and thermochemical properties of lignin from herbaceous plant. J South China Univ Technol (Nat Sci Ed) 2010;38:1–6. [21] Chen D, Zhou J, Zhang Q. Effects of heating rate on slow pyrolysis behavior, kinetic parameters and products properties of moso bamboo. Bioresour Technol 2014;169:313–9. [22] Kambo HS, Dutta A. Comparative evaluation of torrefaction and hydrothermal carbonization of lignocellulosic biomass for the production of solid biofuel. Energy Convers Manage 2015;105:746–55. [23] Bach Q-V, Tran K-Q, Khalil RA, Skreiberg Ø. Comparative assessment of wet torrefaction. Energy Fuels 2013;27:6743–53. [24] Yan W, Hastings J, Acharjee T, Coronella C, Vasquez V. Mass and Energy Balances of Wet Torrefaction of Lignocellulosic Biomass. Energy Fuels 2010;24:4738–42. [25] Funke A, Ziegler F. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuels Bioprod Biorefin 2010;4:160–77. [26] Bach Q-V, Tran K-Q, Skreiberg Ø. Hydrothermal pretreatment of fresh forest residues: effects of feedstock pre-drying. Biomass Bioenergy 2016;85:76–83. [27] Guo W. Self-heating and spontaneous combustion of wood pellets during storage. University of British Columbia; 2013. [28] Donar YO, Çag˘lar E, Sınag˘ A. Preparation and characterization of agricultural waste biomass based hydrochars. Fuel 2016;183:366–72. [29] Liu Z, Quek A, Hoekman SK, Balasubramanian R. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 2013;103:943–9. [30] Qian SP, Wang H, Zarei E, Sheng KC. Effect of hydrothermal pretreatment on the properties of moso bamboo particles reinforced polyvinyl chloride composites. Composites Part B 2015;82:23–9. [31] Zheng A, Zhao Z, Chang S, Huang Z, Zhao K, Wei G, et al. Comparison of the effect of wet and dry torrefaction on chemical structure and pyrolysis behavior of corncobs. Bioresour Technol 2015;176:15–22.