Bamboo Torrefaction in a High Gravity (Higee) - American Chemical

Research Article pubs.acs.org/journal/ascecg

Bamboo Torrefaction in a High Gravity (Higee) Environment Using a Rotating Packed Bed Ma. Katreena V. Pillejera,†,‡ Wei-Hsin Chen,*,‡ and Mark Daniel G. de Luna§ †

Energy Engineering Program, National Graduate School of Engineering, University of the Philippines − Diliman, Quezon City 1101, Philippines ‡ Department of Aeronautics and Astronautics, National Cheng Kung University, No. 1, University Rd., Tainan 701, Taiwan § Department of Chemical Engineering, University of the Philippines − Diliman, Quezon City 1101, Philippines ABSTRACT: Biomass torrefaction in various reactors has been extensively studied lately. Different from past studies, torrefaction of raw bamboo (Phyllostachys mankinoi) in a high gravity (Higee) environment is investigated where a rotating packed bed (RPB) for intensifying heat and mass transfer between gas and solid is used for the process. Three rotating speeds of 0, 900, and 1800 rpm, corresponding to the mean centrifugal forces of 0, 58, and 234 g, are taken into account. The results suggest that the Higee environments intensify the torrefaction performance drastically when the operations of light (206 °C) and mild (255 °C) torrefaction are practiced, stemming from the enhancement of heat and mass transfer in the rotating bed. In contrast, the torrefaction performance is affected slightly by the rotating speed when severe torrefaction (300 °C) is carried out. With the torrefaction conditions of 300 °C and 1800 rpm, the highest HHV (28.389 MJ/kg) with an HHV enhancement factor (EF) of 1.61 is obtained, yielding a coal-like fuel, and the energy yield is 63.51%. The torrefaction operation at 255 °C and 1800 rpm for 30 min upgrades the EF (1.53), HHV (26.988 MJ/kg), and energy yield (65.21%) values of the bamboo, which are close to those of the torrefied biomass under the most severe torrefaction conditions, and is thus recommended. The results suggested that torrefaction in a Higee environment is a promising process for upgrading biomass to produce carbon-neutral fuel utilized in industry. KEYWORDS: Torrefaction, Bamboo, Rotating packed bed (RPB), FTIR, High gravity (Higee) environment, Rotation

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INTRODUCTION The development of renewable and sustainable energy resources is a necessity to positively contribute to the economics of energy sector and energy security, and to reduce the environmental and ecological impact from burning fossil fuels, which is a prime route to get heat and power in industry.1,2 Extending to the energy resource and consumption status of Taiwan and the Philippines, both have large percentages of energy sources in conventional energy, particularly coal. The energy and power dependence of the Philippines and Taiwan in coal is about 29% (2014) and 38.4% (2015), respectively. In a global scale, the emissions declared by the Intergovernmental Panel on Climate Change (IPCC) in the 2014 Climate Change Synthesis Report indicated that 25% of greenhouse gases (GHGs) came from the electricity and heat production in the energy sector, where over 40% of this sector came from coal industries. Moreover, burning coal leads to the highest CO2 emissions in comparison to the other conventional energy sources. Among all the energy sources, one of the most promising alternatives to conventional energy is biomass.3 This alternative energy source is an environmentally friendly and sustainable renewable energy source. Its usage helps reduce dependence on © 2017 American Chemical Society

fossil fuels and promotes the conversion of wastes into biofuels. Moreover, in comparison to other energy resources, GHG emissions from burning biomass are low inasmuch as biomass pertains to a carbon-neutral fuel. Most common biomass feedstocks are lignocelluloses which comprise cellulose, hemicellulose, lignin, organic extractives, and inorganic minerals such as ash.4 Bamboo is a rapidly growing lignocellulose, which is abundant in Asian countries,5,6 and has a number of major fuel characteristics such as low ash and alkali contents.7 It is thus used as a model feedstock for this study. Nevertheless, from the viewpoint of biofuel development, it is crucial to use effective and efficient biomass conversion technologies for improving the material. Compared to coal, bamboo and other raw biomass species have the drawbacks of high moisture content, low calorific value, large volume, low bulk density, and hygroscopic and nonhomogeneous natures.8 One of the promising technologies to date is torrefaction, where biomass is thermally pretreated at temperatures of 200−300 °C in an Received: April 24, 2017 Revised: June 13, 2017 Published: June 24, 2017 7052

DOI: 10.1021/acssuschemeng.7b01264 ACS Sustainable Chem. Eng. 2017, 5, 7052−7062

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ACS Sustainable Chemistry & Engineering oxygen-free environment. Torrefaction aims to produce coallike fuels9 and can efficiently overcome the obstacles mentioned above for biomass applications in industry. Examination of the reported literature suggests that the important parameters for torrefaction include temperature, duration (or holding time), particle size, atmosphere nature, pressure, heating rate, and carrier gas flow rate.9 In addition, a variety of reactors such as rotating drum, multiple hearth furnace, microwave reactor, belt dryer, screw conveyor, torbed reactor, compact moving bed, and fixed bed have been applied for biomass torrefaction.10,11 The rotating packed bed (RPB) has been extensively employed to enhance the mass transfer in absorption,12 distillation,13 stripping,14 etc. The reaction environment in an RPB is normally controlled at room temperature. In the study of Chen et al.,15 an RPB with elevated temperatures was developed to perform a water−gas shift reaction (WGSR) for hydrogen production. In their study, a dimensionless G number, giving the ratio of average centrifugal force and gravitational force acting on the subjected fluid element in the RPB, was determined. They reported that the CO conversion from the WGSR in a Higee environment could be increased up to 70% as compared to that without rotation. To date, no biomass torrefaction study has used the RPB. For this reason, the RPB is used as the torrefaction reactor in the present study where a new parameter of rotating speed is introduced.15 This study is intended to evaluate the feasibility of the new technology in the process of biomass torrefaction and to investigate the effect of rotating speed (0, 900, and 1800 rpm) on the solid yield, HHV, and energy yield of the product under various temperatures (206, 255, and 300 °C) at a fixed holding time (30 min). Also, the study extends the research to the effect of duration (15, 30, 45 min) at a fixed temperature (255 °C) on the performance of bamboo torrefaction. The structural characteristics of bamboo under various torrefaction conditions were analyzed by FTIR.

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Table 1. Basic Properties of Raw Bamboo Proximate analysis (wt %) volatile matter fixed carbon moisture ash Elemental analysis (wt %, dry-ash-free) C H N O (by difference) Fiber analysis (wt %) hemicellulose cellulose lignin others Higher heating value (MJ/kg, dry-basis)

72.09 10.93 13.54 3.44 43.82 6.20 1.95 48.03 24.88 44.55 23.34 7.23 17.66

controller, (F) rotameter, (G) carrier gas tank, (H) rotor speed controller, (I) cooling unit, and (J) exhaust gas treatment unit. Furthermore, Figure 1b displays the details of the RPB which was made up of (a) placement of feedstock, (b) rotating plate, (c) RPB wall, (d) heating unit, (e) rotating plate cover, (f) carrier gas inlet, (g) rubber, (h) high temperature insulator, (i) plate cover screws, (j) thermocouple, (k) cooling unit inlet, and (l) RPB cover screws. Experimental Procedure. The experiments were divided into two parts: (1) varying rotating speed and temperature at a fixed duration (30 min) and (2) altering duration at fixed temperature (255 °C) and rotating speed (900 rpm). For the rotating speed, a calibration curve between the controller and the real rotating speed was established which was used to precisely control the rotating speed. All the torrefaction experimentations were carried out in a nitrogen environment at a flow rate of 500 mL/min (25 °C and 1 atm). The number of bamboo chips used in each run was around 12−13 chips with a mass of approximately 8−9 g. The heating rate was 5 °C/min, and the cooling unit outside the RPB was set at 4 °C. In the first part of experiments, three different rotating speeds of 0, 900, and 1,800 rpm and three different torrefaction temperatures of 206, 255, and 300 °C were taken into consideration. Physically, the three temperatures correspond to light, mild, and severe torrefaction, respectively.17 In the second part, the experiments were carried out at a fixed rotating speed of 900 rpm along with three different durations of 15, 30, and 45 min. Performance Evaluation. The solid yield, enhancement factor (EF) of HHV, and energy yield are three crucial indexes to identify the performance of torrefaction.4 The solid yield is measured from the weight ratio of torrefied biomass to its parent one, while the EF of HHV is determined from the HHV ratio of torrefied biomass to the raw one. As for the energy yield, it stands for the ratio of energy amount of torrefied biomass to its raw counterpart. The three indexes are expressed as the following:

MATERIALS AND METHODS

Material (Raw Bamboo). A common bamboo species Phyllostachys makinoi in Taiwan was adopted as a feedstock for biochar production from an RPB. The bamboo was manually cut into chips with dimensions of 2 cm × 2 cm × 5 mm (length × width × thickness) using a saw. The chips were preheated at 65 °C for 24 h to establish a basis for sample tests. To figure out the basic properties of the biomass, fiber, proximate, elemental, and calorific analyses were performed. The fiber analysis followed the procedure of the Laboratory Analytical Procedure (LAP) for biomass provided by the National Renewable Energy Laboratory (NREL).16 The proximate analysis was performed in accordance with the standard procedures of American Society for Testing and Materials (ASTM), following ASTM D4442 (moisture), ASTM 872-82 (volatile matter), ASTM 1755-01 (ash), and ASTM E1534 (fixed carbon). The calorific values of the raw and torrefied bamboo were determined using a bomb calorimeter (IKA C5000). For the elemental analysis, a sample with a weight of 15−30 mg was placed into an elemental analyzer (PerkinElmer 2400 series II CHNS/O analyzer). The weight percentages of C, H, and N were measured from the analyzer, while the weight percentage of O was obtained by difference. The basic properties of raw bamboo are shown in Table 1. Overall, the biomass has high volatile matter (72.09%) and cellulose contents (44.55%). The HHV of the raw bamboo is 17.66 MJ/kg, which is typical in that the HHV of raw biomass is normally between 15 and 20 MJ/kg.4 Experimental Setup. The experimental setup for the process of torrefaction in a Higee environment is shown in Figure 1a. The entire system was composed of (A) rotating packed bed (RPB), (B) carrier gas inlet, (C) thermocouple, (D) heating unit, (E) temperature

Solid yield (%) =

Weight torrefied Weight raw

× 100 (1)

Enhancement factor of HHV =

HHVtorrefied HHVraw

Energy yield (%) = solid yield × enhancement factor

(2) (3)

An additional evaluation parameter is the improvement of HHV. This parameter is used to identify the improved extent of the biomass HHV influenced by torrefaction and is calculated as follows:

Improvement of HHV (%) =

HHVtorrefied − HHVraw × 100 HHVraw (4)

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Figure 1. (a) Experimental setup for bamboo torrefaction with Higee environment and (b) details of RPB.

Figure 2. Physical appearances of (a) raw bamboo and torrefied bamboo at (b) two durations with 900 rpm and 255 °C, (c) two rotating speeds with 206 °C for 30 min, and (d) two temperatures with 900 rpm for 30 min.

bamboo torrefied at 900 rpm and 255 °C (mild torrefaction) for 15 and 45 min are dark brown and heavy brown (Figure 2b). With light torrefaction at 206 °C and rotating speeds of 0 and 1800 rpm, the bamboo appearances are characterized by light brown and dark brown, respectively (Figure 2c), revealing that an increase in rotating speed intensifies biomass torrefaction and thereby carbonization extent. When the bamboo is torrefied at 900 rpm for 30 min, the surfaces of the bamboo at the temperatures of 206 and 300 °C are brown and dark brown, respectively (Figure 2d). The darkness of the torrefied bamboo may be attributed to the structural

The calorific and elemental analyses of torrefied bamboo were also measured by a bomb calorimeter and elemental analyzer, respectively. For the structural evaluation, Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum 100) was performed to analyze the structural modification of torrefied bamboo samples from its raw form using the model. The spectra were recorded in the wavenumber ranging from 4000 to 650 cm−1, and the data were collected in a computer.

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RESULTS AND DISCUSSION Bamboo Appearances. The physical appearances of raw bamboo and torrefied bamboo at various torrefaction conditions are shown in Figure 2. The appearances of the 7054


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Figure 3. Profiles of (a) solid yield, (b) HHV, and (c) energy yield with respect to rotating speed (solid lines) and G number (dashed lines). (30 min duration).

cannot keep up with the decrease in the solid yield, resulting in a decrease in the energy yield from 90.68% to 65.21% (Figure 3c). Past studies7,19,20 on bamboo torrefaction have reported that the decline in the solid yield was attributed to the moisture removal, weight loss of hemicellulose, and further degradation of cellulose components.21 When rotation is introduced, the solid yield at 206 and 255 °C obviously decreases with increasing rotating speed. This can be explained by two factors: (1) increased centrifugal force and (2) the elongated path of the carrier gas (N2) in the RPB.15 In other words, the two factors both have an impact on heat and mass transfer. It has been pointed out that the mass transfer in an RPB could be intensified when the rotating speed increased.13 Meanwhile, when the carrier gas is in a centrifugal field, the path of the carrier gas becomes bent.15 This leads to a longer path and forces the heat to stay on the bamboo surfaces longer than usual, hence giving a chance for the feedstock to liberate more moisture and volatiles and to break more molecular chains at the same temperature. During severe torrefaction (300 °C), however, variations in the rotating speed almost play no part on the three physical quantities. In the present study, the low sensitivity of torrefaction performance to

modification of lignin and the formation of more CO, which is a chromophoric group.18 After grinding the bamboo samples, it was found that the colors of the interior portions were no different from those of the outer surfaces. Raw and torrefied bamboo samples were ranked on a scale of 1 to 10 based on color darkness (1 being the lightest and 10 being the darkest). At light torrefaction (Figure 2c), with no rotation and at 1800 rpm, scales are 2 and 5, respectively. At mild torrefaction (Figure 2b) rotated at 900 rpm, subjected to 15 and 45 min, the scales are 6 and 8, respectively. For Figure 2d, where samples were torrefied at 206 and 300 °C at 900 rpm for 30 min, scales are 3 and 10, respectively. Effects of Rotating Speed and Temperature. At a fixed torrefaction duration of 30 min, the effects of rotating speed (i.e., 0, 900, and 1800 rpm) and torrefaction temperature (i.e., 206, 255, and 300 °C) on the solid yield, HHV, and energy yield are examined in Figure 3 (solid lines). It is shown that the solid yield at the condition without rotation decreases from 86.40% to 40.67% when the temperature increases from 206 to 300 °C (Figure 3a), whereas HHV increases from 18.53 to 28.31 MJ/kg (Figure 3b). However, the increase in HHV 7055


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ACS Sustainable Chemistry & Engineering rotating speed should be due to the obvious disruption of the lignocellulosic structure under the severe torrefaction,17 where the solid yield has been down to a level of around 40% (Figure 3a). Overall, the highest HHV (28.389 MJ/kg) is achieved at 300 °C and 1800 rpm, which is slightly higher than the values obtained in the absence of rotation (28.309 MJ/kg) and at 900 rpm (28.335 MJ/kg) at the same temperature. The energy yield ranges from 65.21% to 63.51%. Among all the varying torrefaction conditions under a fixed torrefaction duration of 30 min, the lines at 255 °C have the steepest progression on the three responses showing the most impact of rotation upon the torrefaction performance. For the temperature of 255 °C, the highest HHV (26.988 MJ/kg) is obtained at the maximum rotating speed (1800 rpm) with a corresponding energy yield of 63.36%. The HHVs of the upgraded feedstocks at the aforementioned conditions are in the range of coal’s HHV (25−35 MJ/kg).4 It has been known that there exists a centrifugal force in a rotational field. Chen et al.15 conducted a dimensionless G number, in terms of the inner and outer rings of the RPB, to quantify the average centrifugal force at various rotating speeds. The RPB used in the study of Chen et al.,15 with dimensions 5.6, 7.3, 2.1, 10, and 10 cm for the inner ring, outer ring, bed height, chamber radius, and chamber height, respectively, was also adopted in this study. On the basis of their theory, the G number, which is a measure of the ratio of the average centrifugal force to the gravitational force acting on the material in the bed of the RPB, is expressed as G=

Fc̅ 2mπ 2f 2 (ro + ri) 2π 2f 2 (ro + ri) = = mg mg g

Figure 4. Profiles of the EF of HHV and its improvement versus rotating speed. (30 min duration).

(52.84%) relative to its counterpart without rotation is at 255 °C and 1800 rpm. Although the highest improvement in HHV (60.78%) is at 300 °C, it can be observed that there is no significant difference on its improvement when the speed varies from 0 to 1800 rpm, showing that the bars are leveled. The EF of HHV versus rotating speed at 300 °C is characterized by a horizontal line where the values are around 160%. The contour maps of the solid yield, EF of HHV, and energy yield are shown in Figure 5. The contour map of the solid yield depicts that the parameter declines from the bottom left corner to the top right one (Figure 5a). It is worth noting that the contour gradient at around 40% of the solid yield is relatively small as compared to the other regions. This implies, in turn, that the variations of temperature and rotating speed may not have a pronounced effect on the solid yield at this region. The contour map of the EF exhibits a reverse trend (Figure 5b). At a fixed duration (30 min), when the temperature is at 300 °C and the rotating speed is higher than 900 rpm, the EF of HHV is higher than 1.5, yielding a high torrefaction quality. The energy yield is an important indicator to evaluate the effect of torrefaction in that it is a simultaneous representation of the solid yield and the EF of HHV.22 Overall, the contour map of the energy yield (Figure 5c) is similar to that of the solid yield, declining from the bottom left corner to the top right one. It follows that the densification of the calorific value of the bamboo from torrefaction cannot keep up with the mass loss when the torrefaction severity is lifted. As observed, the contour gradients along the rotating speed of 1800 rpm are bigger than those along the temperature of 300 °C. This conveys that the solid yield, HHV, and energy yield are more sensitive to the variation of the temperature at 1800 rpm than the variation of the rotating speed at 300 °C. Effect of Duration. The effect of duration on the solid yield, HHV and energy yield are examined in Figure 6. In the adopted torrefaction temperatures and rotating speeds, 255 °C and 900 rpm are the intermediate values. Consequently, the temperature and speed are chosen as the baseline of experiments to investigate the effect of duration on bamboo torrefaction. It is clear that the duration has an effect on the solid yield, enhancement factor of HHV, and energy yield of the torrefied bamboo. The prolongation of the torrefaction duration results to the lowering of the solid yield (from 66.03%

(5)

where F̅c is the centrifugal force upon the biomass (N), g is the acceleration of gravity (= 9.81 m/s2), f is the rotating speed (rps), ro is the radius of the outer ring (5.6 cm), and ri is the radius of the inner ring (7.3 cm). Accordingly, the G numbers of the RPB at the rotating speeds of 0, 900, and 1800 rpm are 0, 58, and 234, respectively, implying that the average centrifugal forces in the RPB are 0, 58, and 234 g, respectively. The profiles of the solid yield, HHV, and energy yield with respect to the G number at the three temperatures are also shown in Figure 3 (dashed lines). Equation 5 suggests that the values of the G number are proportional to the square of rotating speed. Thus, there is a shift in values (from 900 rpm to 58 g), implying that the average rotating speed (900 rpm) does not necessarily correspond to the average centrifugal (117 g) force exhibited by the reactor. For the temperatures of 206 and 255 °C, altering the G number has an evident change in the three quantities, especially at 255 °C. In contrast, the variation of the three quantities with respect to the G number at 300 °C is slight. It can be said that the influence coming from the high temperature is greater than the force induced by the rotor, hence merely posing a tiny change from the latter. The observations also imply that there must be a threshold value for the increase or decrease in torrefaction parameters at varying temperature and rotating speed such that the values increase or decrease gradually, then drastically, and last reach a state of plateau. Figure 4 shows the profiles of the enhancement factor (EF) and the improvement of HHV, namely, eq 4. Generally, the EF and the improvement increase when the temperature and the rotating speed increase. The most drastic improvement 7056


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Figure 5. Contour maps of (a) solid yield, (b) EF of HHV, and (c) energy yield. (30 min duration).

color of the bamboo torrefied at 45 min tends to be darker than the bamboo torrefied at 15 min. Also, by inspection, the change with respect to the average duration (30 min) is more evident at the 45 min duration than the 15 min duration. With the 45 min torrefaction, the HHV reaches 24.303 MJ/kg. C, H, and O Components. Another crucial factor that should be considered is the C, H, and O components of the feedstock after torrefaction. The elemental analyses of the bamboo after undergoing torrefaction at various conditions is shown in Tables 2 and 3. The atomic H/C and the O/C ratios play prominent roles in the energy density of a fuel.23 On the basis of the values in the two tables, the van Krevelen diagrams under fixed duration (30 min), as well as fixed temperature (255 °C) and rotating speed, are demonstrated in Figure 7a and b, respectively. A strong linear distribution between the atomic H/C and O/C ratios is exhibited in the two figures inasmuch as the coefficients of determination (R2) are equal to 0.9820 and 0.9762, respectively. The slopes in the plots of the O/C ratio versus the H/C ratio in Figure 7a and b are equal to 0.1106 and 0.1041, respectively. These values imply that torrefaction has more influence on the O/C ratio than on the H/C ratio, by factors of approximately 9.04 and 9.61, respectively. For torrefied samples, the highest H/C and O/C ratios in Figure 7a are the bamboo samples torrefied at 206 °C

Figure 6. Profile of the torrefaction performance of bamboo at fixed temperature and rotation (255 °C and 900 rpm) at varying durations.

to 54.51%), to the increasing of the enhancement factor (from 1.22 to 1.37), and consequently, to the lowering of the energy yield (from 80.52% to 74.73%). As shown in Figure 4b, the 7057


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To show the general spread of H/C and O/C ratios, their contour maps under the combinations of various temperatures and rotating speeds are shown in Figure 8. Spread for the duration is not considered due to limited data. The H/C and O/C ratios are all less than 1 and decrease from the bottom left corner to the top right one. This means that the H and O values are decreasing as the torrefaction severity increases. In this regard, the carbon content of the torrefied biomass enriches as the torrefaction severity increases.5 Within the investigated environments for the torrefaction of bamboo, H/C and O/C ratios fall in the ranges of 0.078−0.126 and 0.482− 0.975, respectively. Seeing that the distances between the contour lines along the rotating speed at 1800 rpm are shorter when compared to the distances along the temperature at 300 °C, it suggests that atomic H/C and O/C ratios are more sensitive to the variation in temperature at 1800 rpm compared to the variation of rotating speed at 300 °C, where in fact there is no evident change of the ratios at this temperature. FTIR Spectroscopy. The modification in the structure of the bamboo from torrefaction is examined by FTIR spectroscopy, and the spectra within the wavenumber range of 650− 4000 cm−1 are shown in Figure 9. It is observed that the signal between 3200 and 3500 cm−1 decreases as the torrefaction severity increases, especially in Figure 9b where the signal is the first to level down at relatively lower torrefaction conditions. The range of signals from 3200 to 3600 cm−1 can be attributed to the O−H bond stretch24 where, even in the early stages of the treatment, the intensity of this bond around these regions showed a decrease. The flattening thus indicates that moisture is released from the breaking of the bonds; the withered hydroxyl groups also weaken the hydrogen bonding, thereby improving the hydrophobicity of the material.4,7 The thermal decomposition temperatures of the three prime lignocellulosic constituents are as follows: hemicellulose (220− 315 °C), cellulose (315−400 °C), and lignin (160−900 °C).4 At these temperatures, hemicellulose undergoes a two-step degradation: (1) deformation of monosaccharides and polysaccharides, and (2) degradation into CO and CO2. Also, cellulose is subjected to aromatization and cross-linking, producing insoluble products, and lignin is subjected to polymerization (mainly of β-aryl and ether linkages) and

Table 2. Elemental Analysis (wt % dry ash free) of Torrefied Bamboo under Varied Rotating Speeds and Temperatures at a Fixed Duration (30 min) Temperature, °C Rotating Speed, rpm

206

255

300

Carbon (C) 47.07 51.90 51.97 55.12 53.75 63.88 Hydrogen (H) 5.94 5.30 5.44 5.11 5.49 4.04 Nitrogen (N) 1.09 1.15 1.32 1.10 1.24 1.29 Oxygen (O, by difference) 45.91 41.66 41.28 38.68 39.53 30.79

0 900 1800 0 900 1800 0 900 1800 0 900 1800

68.45 68.41 69.19 4.19 4.18 4.16 1.04 1.13 1.28 26.33 26.29 26.38

Table 3. Elemental Analysis (wt% dry ash free) of Torrefied Bamboo under Varied Torrefaction Duration at a Fixed Temperature (255 °C) and Rotating Speed (900 rpm) Elements, wt % Duration, min

C

H

N

O

15 30 45

54.98 55.12 62.91

5.43 5.11 4.94

2.34 1.10 1.87

37.26 38.68 30.30

in the absence of rotation; the lowest ratios are the bamboo samples torrefied at 300 °C, regardless of the rotating speed. However, in Figure 7b, it can be observed that the H/C and O/ C ratios obtained at 15 and 30 min are close to each other. This result is consistent with the results presented in Figure 5, where the enhancement factors obtained from the durations of 15 and 30 min only differ by 0.04, as compared to the difference in the values obtained from 30 and 45 min which are equal to 0.11.

Figure 7. Van Krevelen diagrams of raw and torrefied bamboo samples at fixed (a) duration (30 min), (b) rotating speed (900 rpm), and temperature (255 °C). 7058


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Figure 8. Contour maps of atomic (a) H/C and (b) O/C ratios.

recondensation.21 The signals at 780 cm−1, which are attributed to carboxylic acid,25 start to decrease at 255 °C along with 900 rpm and at 15 min torrefaction. The signal at 1043 cm−1, which is a typical band for an O−H bond in hemicellulose,26 starts to decrease at the conditions of 255 °C without rotation which explains the improved hydrophobicity of the biomass.4,24 The signal at 1182 cm−1, which is due to the CH3 in acetate,19 gradually decreases from 206 °C with no rotation and disappears at 300 °C. The signal at 1110 cm−1, ascribed to cyclic ether,25 remains unchanged through the various duration conditions (Figure 9b) but starts to decrease from the torrefaction conditions of 255 °C along with 1800 rpm and then becomes undetectable at 300 °C in all levels of rotating speed. An overlap between hemicellulose and cellulose for the signal at 1375 cm−1, attributed to C−H deformation and aromatic skeletal vibration,27 is also found to decrease its peak from the torrefaction conditions of 255 °C without rotation and from a 15 min holding time. Another overlapped band is the signal at 1160 cm−1, which is attributed to the antisymmetric stretching of C−O−C glycosidic linkages due to polymerization,28 decreases from 255 °C rotated at 900 rpm and then becomes absent at 300 °C. Moreover, the signal at 1064 cm−1, which is a typical band for cellulose,29 starts to decrease at the conditions of 255 °C and 1800 rpm. The signal at 1427 cm−1, ascribed to aromatic C−H out-of-plane deformation,25 decreases from 206 °C at 900 rpm. The signal at 898 cm−1, which may be attributed to the C−H deformation of polysaccharides in cellulose,19,25 decreases from 255 °C without rotation. For lignin, a decrease in the signal at 1300 cm−1, which is due to the aromatic C−O stretch,30 is exhibited at the conditions of 255 °C with 900 rpm and at 15 min torrefaction and then disappears at 300 °C in all levels of rotating speed. The signal at 1451 cm−1, likely attributed to methoxy,7 starts to decrease at 300 °C without rotation. The signal at 1510 cm−1, which may be ascribed to the condensed syringyl ring and guaiacyl or more specifically, due to the CC aromatic ring vibrations,24 starts to decrease at 300 °C with 0 rpm. The signal at 1462 cm−1, which is a typical band of lignin component,25 starts to decrease at 255 °C with 900 rpm. The signal at 857 cm−1,25 begins to decrease at 255 °C rotated at 1800 rpm. The signal at 1254 cm−1 is attributed to a syringyl ring and C−O stretch as an overlap between the lignin and hemicelluloses.27

Some signals in the spectra exhibit an increase. The signal at 1599 cm−1 attributed to the aromatic skeletal stretching vibrations appears during the treatment25 where the peak increases as the intensity of the torrefaction increases. It can be observed that the peak starts to increase from 255 °C with 1800 rpm and at 45 min torrefaction. The increase is in accordance to the findings of Rousset et al.7 Moreover, starting at the 15 min torrefaction, another evident appearance of a peak is observed in the signal at 1620 cm−1, which can be ascribed to the formation of ketone species.31 For the signal at 1705 cm−1, which may be attributed to the pyranose rings of the cellulose component,31 it starts to increase its peak at 255 °C rotated at 1800 rpm and at 45 min torrefaction. The observed results above indicated that when the torrefaction severity is raised, more of the structures of the feedstock are degraded. It is important to note that, compared to the reported literature, some of the signals attributed to that of cellulose and lignin exhibit degradation at a lower temperature when combined with high rotating speed. This implies that the intensified heat and mass transfer caused by the rotation may have compensated for the conventional amount of temperature to degrade a certain lignocellulosic component. With the rotation introduced as a new parameter in the process of torrefaction, the results lean toward a competitive and promising operating condition. In line with this, a summary of the studies on the torrefaction of bamboo is shown in Table 4. Comparing the results in this study with the results in other studies, using 300 °C without rotation as the first basis, this study has the advantage in terms of the HHV (28.308 MJ/kg) and at par with the energy yield (65.21%). By inspection, the highest HHV obtained by other works was 28.51 MJ/kg which was attained at 340 °C for 30 min22 with an energy yield of 63.01%. In another study,19 at the torrefaction conditions of 340 °C for 60 min, the obtained HHV was 27.45 MJ/kg with an energy yield of 67.23%, while for the bamboo torrefied at 300 °C for 90 min the HHV was only 22.06 MJ/kg. Wen et al.20 obtained a sample with an HHV of 25.28 MJ/kg and an energy yield of 72.7% at 300 °C for 60 min. On the basis of the recommended operating conditions in the present study (at 255 °C and 1800 rpm for 30 min), the torrefied bamboo with an HHV of 26.988 MJ/kg and energy yield of 63.36% is still more compelling than that of the reviewed studies such that at 280 °C for 60 min the HHV obtained by Li et al.5 was 20.4 MJ/ 7059


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Figure 9. FTIR spectra of the torrefied bamboo samples as compared to raw samples depicting the changes from the (a) effect of rotation and temperature and (b) effect of duration at 255 °C and 900 rpm.

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kg and that by Rousset et al.7 was 23.1 MJ/kg with a corresponding energy yield of 78%. This suggests that at a lower temperature a better HHV can be attained with the aid of rotation.

CONCLUSIONS

A rotating packed bed (RPB) has been utilized as a new technology for a biomass torrefaction process, with bamboo as a feedstock. In addition to torrefaction temperature and 7060


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ACS Sustainable Chemistry & Engineering Table 4. Studies on Torrefaction of Bamboo Species Phyllostachys sulphurea19

Torrefaction Condition temperature: 280, 300, 320, 340 °C duration: 10−90 min environment: nitrogen equipment: horizontal tube reactor

Solid Yield

HHV

Energy Yield

from 280 to 340 °C, 84.08% to at 60 min, from 280 to 340 °C, 41.93% 19.13 to 27.45 MJ/kg at 300 °C, from 10 to 90 min: 84.83% to 44.20%

at 300 °C, from 10 to 90 min, 19.11 to 22.06 MJ/kg

ranged from 93.95% to 67.23%

Phyllostachys acuta5

temperature: 220, 240, 260, 280 °C duration: 10, 30, 60 min environment: nitrogen equipment: microwave heater

at 220 °C, 10 min: 98.31%; at 280 °C, 60 min: 74.60%

from 220 to 280 °C: 17.17 to 20.4 MJ/kg

no data provided

Phyllostachys acuta22

temperature: 240, 260, 280, 300, 320, 340 °C duration: 30 min environment: carbon dioxide equipment: horizontal tube reactor

from 240−340 °C: 96.89% to 40.58%

from 240 to 340 °C: 18.78 to 28.51 MJ/kg

from 240 to 340 °C: 99.09% to 63.01%

Bambusa vulgaris7

temperature: 200, 250, 280 °C duration: 60 min environment: nitrogen equipment: oven chamber

from 220 to 280 °C: 91% to 61%

from 220 to 280 °C: 19.3 to 23.1 MJ/kg

from 220 to 280 °C: 96.1% to 78%

Phyllostachys pubescens20

temperature: 200, 225, 250 275, 300 °C duration: 60 min environment: nitrogen equipment: tubular furnace

from 200 to 300 °C: 92.3% to 47%

from 200 to 300 °C: 16.22 to 25.28 MJ/kg

from 200 to 300 °C: 94% to 72.7%

Phyllostachys makinoi (this study)

temperature: 206, 255, 300 °C at three rotating speeds (0, 900, 1800 rpm)

range of solid yield over varying torrefaction:

range of HHV over the varying torrefaction:

range of energy yield over varying torrefaction:

at fixed temperature (255 °C) and rotor speed (900 rpm) duration: 15, 30, 45 min

(a) temperature and rotor speed: 86.40% to 39.50% (b) duration: 66.03% to 54.51%

(a) temperature and rotor speed: 18.532 to 28.389 MJ/kg (b) duration: 21.533 to 24.204 MJ/kg

(a) temperature and rotor speed: 90.68% to 63.51% (b) duration: 80.52% to 74.73%

environment: nitrogen equipment: rotating packed bed

For further research, the region from 255 to 300 °C enclosed by rotations 900 and 1800 rpm may be closely analyzed to come up with an optimum operating condition and for threshold analysis. Moreover, it is recommended to gather information on the energy cost and economic analysis to give a better perspective on its potential in the industry. This may bring light to the idea if the energy demand of the rotation coupled with low temperature will compensate the energy needed for the torrefaction process at high temperature without rotation. Moreover, before considering a scale-up model, it would be pertinent to conduct kinetic modeling to deepen the understanding of the effect of rotating speed to the torrefaction performance and structural variation.

duration, the rotating speed ranging from 0 to 1800 rpm is introduced as a third factor for torrefaction where the maximum mean centrifugal force in the RPB is 234 g located at 1800 rpm. When light (206 °C) and mild (255 °C) torrefaction is carried out, the rotating speed is able to enhance the torrefaction performance to a great extent, especially at the mild torrefaction. This arises from the factors of intensified heat and mass transfer and elongated pathway of the carrier gas in a Higee environment. In contrast, the rotating speed almost plays no part on the torrefaction performance at severe (300 °C) torrefaction. This may be explained by the obvious disruption in the bamboo structure at this temperature so that it is insensitive to the rotating speed. The highest HHV of 28.389 MJ/kg was obtained at the most severe conditions (300 °C and 1800 rpm) with an energy yield of 63.51%. However, the recommended operating condition is at 255 °C along with 1800 rpm inasmuch as the HHV (26.988 MJ/kg) and energy yield (65.21%) of the upgraded bamboo are close to those of the bamboo treated at 300 °C and 1800 rpm. The FTIR spectra suggest that the cellulose and lignin components may have been degraded at a lower temperature with the aid of a high rotating speed. Overall, from the results presented, it can be concluded that torrefaction in a Higee environment is a promising process to upgrade biomass for producing carbonneutral and coal-like fuels used in industry.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +886-6-2004456. Fax: +886-6-2389940. E-mails: [email protected]; [email protected]. ORCID

Wei-Hsin Chen: 0000-0001-5009-3960 Notes

The authors declare no competing financial interest. 7061


Research Article


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(20) Wen, J.-L.; Sun, S.-L.; Yuan, T.-Q.; Xu, F.; Sun, R.-C. Understanding the chemical and structural transformations of lignin macromolecule during torrefaction. Appl. Energy 2014, 121, 1−9. (21) Chen, W.-H.; Kuo, P.-C. Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass. Energy 2011, 36 (2), 803−811. (22) Li, M.-F.; Li, X.; Bian, J.; Xu, J.-K.; Yang, S.; Sun, R.-C. Influence of temperature on bamboo torrefaction under carbon dioxide atmosphere. Ind. Crops Prod. 2015, 76, 149−157. (23) Chen, W.-H.; Lu, K.-M.; Liu, S.-H.; Tsai, C.-M.; Lee, W.-J.; Lin, T.-C. Biomass torrefaction characteristics in inert and oxidative atmospheres at various superficial velocities. Bioresour. Technol. 2013, 146, 152−160. (24) Ibrahim, R. H. H.; Darvell, L. I.; Jones, J. M.; Williams, A. Physicochemical characterisation of torrefied biomass. J. Anal. Appl. Pyrolysis 2013, 103, 21−30. (25) Lv, P.; Almeida, G.; Perre, P. TGA-FTIR analysis of torrefaction of lignocellulosic components (cellulose, xylan, lignin) in isothermal conditions over a wide range of time durations. BioResources 2015, 10 (3), 4239−4251. (26) Park, J.; Meng, J.; Lim, K. H.; Rojas, O. J.; Park, S. Transformation of lignocellulosic biomass during torrefaction. J. Anal. Appl. Pyrolysis 2013, 100, 199−206. (27) Zheng, A.; Zhao, Z.; Chang, S.; Huang, Z.; Wang, X.; He, F.; Li, H. Effect of torrefaction on structure and fast pyrolysis behavior of corncobs. Bioresour. Technol. 2013, 128, 370−377. (28) Shang, L.; Ahrenfeldt, J.; Holm, J. K.; Sanadi, A. R.; Barsberg, S.; Thomsen, T.; Stelte, W.; Henriksen, U. B. Changes of chemical and mechanical behavior of torrefied wheat straw. Biomass Bioenergy 2012, 40, 63−70. (29) Bilgic, E.; Yaman, S.; Haykiri-Acma, H.; Kucukbayrak, S. Limits of variations on the structure and the fuel characteristics of sunflower seed shell through torrefaction. Fuel Process. Technol. 2016, 144, 197− 202. (30) Nam, H.; Capareda, S. Experimental investigation of torrefaction of two agricultural wastes of different composition using RSM (response surface methodology). Energy 2015, 91, 507−516. (31) Sarvaramini, A.; Assima, G. P.; Larachi, F. Dry torrefaction of biomass − Torrefied products and torrefaction kinetics using the distributed activation energy model. Chem. Eng. J. 2013, 229, 498− 507.

ACKNOWLEDGMENTS The authors acknowledge the financial supports of the Ministry of Science and Technology (MOST) in Taiwan under the contracts MOST 102-2221-E-006-288-MY3 and MOST 1062923-E-006-002-MY3 and of the Department of Science and Technology (DOST) in the Philippines for this research.

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Bamboo Torrefaction in a High Gravity (Higee) - American Chemical

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