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The Effect of Adding Torrefied Fraction to Biomass Pellet from Corn Stover: Physical Properties and Fuel Qualities
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 17 (2019) 1872–1879
www.materialstoday.com/proceedings
MRS-Thailand 2017
The Effect of Adding Torrefied Fraction to Biomass Pellet from Corn Stover: Physical Properties and Fuel Qualities T. Phonlayuta, N. Soykeabkeawa,b, U. Intathaa,b, N. Tawichaia,b,* a
b
School of Science, Mae Fah Luang University,333 M 1, Muang, Chiang Rai 57100, Thailand Materials for Energy and Environment Research Group, Mae Fah Luang University, 333 M 1, Muang, Chiang Rai 57100, Thailand
Abstract Biomass pellet is one of a superior renewable energy. This work aims to improve efficiency of biomass pellet from corn residues, which are abundantly available after harvesting in Thailand by adding a torrefied fraction. Torrefaction treatment (225 – 300 C in nitrogen gas atmosphere for 30 min) was applied to corn stover (only leaves and stalks) and then mixed with the biomass in various ratios (i.e.,100:0, 90:10, 20:80, 0:100). The pellets were formed using uniaxial pressing with an 8 mm diameter die at pressure of 80 MPa. The physical properties of pellets such as bulk density, expansion ratio, moisture content, and durability are measured. The ash content and heat value are determined using standard laboratory methods. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Biomass; Torrefied; Corn Stover; Heat value; Durability
1. Introduction Biomass pellet is one of the renewable energy which is now the fastest growing energy sector [1] and it is considered as carbonaceous fuels. Forestry and agriculture sector are two main resources as the primary source of biomass, especially, wood pellet is the most prominent biomass energy sources [2]. Because of the limited supply of wood for pellet production, attention is turning to using a variety of agricultural products and wastes as raw material.
* Corresponding author. Tel.: +66 5391 6774; fax: +66 5391 6776. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.
T. Phonlayut et al. / Materials Today: Proceedings 17 (2019) 1872–1879
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Many researchers have worked on pelleting biomass from corn feedstock [3-5]. However, biopellet form corn still has high moisture content and low heat value. Then, some techniques [6, 7] were used to improve the properties of the biopellet. Torrefaction is an advanced technique, in which biomass is roasted under controlled conditions (heating rate, temperature, time) in an inert atmosphere, retaining most of its energy [8]. Normally, the torrefaction process performed at 200 - 300C [9] and torrefaction time was controlled within 1 h [10]. The benefits accomplished by torrefaction include higher heating value or energy density; lower moisture content; higher hydrophobicity or water-resistivity; improved grindability and reactivity; and more uniform properties of biomass [11]. Unfortunately, the densification of torrefied biomass samples was more difficult under the same operating conditions as used for the densification of untreated samples [12]. The high compression pressure and high die temperature are required to produce torrefied pellets. Previous reports showed only single torrefied pellet properties, there is no report about mixed torrefied pellet with untreated biomass. Then, the objectives of this research were to study the effect of torrefied fraction on the physical properties (size, unit density, expansion ratio, and durability) of pellets made from corn stover. Moreover, the moisture content, heating value and ash content of the obtained pellets were also studied. 2. Materials and Methods 2.1. Sample preparation In this study, corn stover samples (stalk and leaves) were collected from Chiang Rai Province, northern Thailand. The samples were dried in the oven at 105°C for 24 h, and then ground to less than 1 mm particle size. materials were stored at room conditions. A laboratory oven was used to torrefy biomass. The ground samples were initially weighted and then placed in ceramic crucibles and put into the oven at room temperature. The nitrogen gas was then purged through the chamber at the flow rate of 5 ml/min by an external flow meter. The samples were heated from ambient temperatures to the desired temperature (225, 250, 275, and 300C) at a rate of 5°C/min and held at the torrefaction temperature for 30 min. After cooling down to room temperature, the torrefied powders were removed from the oven, and weighed to measure the weight loss of torrefied sample before placing in a desiccator. The fuel properties of the torrefied samples were measured by a IKA C6000 Calorimeter (Germany). The torrefied powder with high heating value was mixed with the untreated biomass with various ratio (0, 10, 20 and 100% by weight). The tapioca starch was used as a binder. The pellets were then manufactured using a uniaxial pressing with an 8 mm diameter die at pressure of 80 MPa holding for 1 min. The obtained pellets were dried in an oven at 70°C for about 3 h to reduce moisture content before further characterization. 2.2. Expansion ratio The expansion ratio was calculated by using the pellet diameter data collected for the unit density measurement. The diametrical expansion ratio was calculated using the below equation:
Expansion ratio
D2 d2
(1)
where D is the diameter of the pellet compressed (mm) and d is the diameter of the die (mm). The reported values are an average of 10 measurements. 2.3. Density The density of individual pellets was calculated by dividing the mass of an individual pellet by its volume. A Vernier caliper was used to measure length and diameter to find volume of the pellets and a balance was used to weigh the sample. The reported values are an average of 10 measurements.
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2.4. Heating value Analytical methods according to ASTM D3286 by brought sample to completely burned in bombs with enough oxygen. The heat of combustion results in a higher temperature of the jacket and the calculated heat value can be calculated and the value obtained which show on display screen of Bomb calorimeter. For method start from brought sample about 0.5 g and without moisture content to cup and weigh, then rope 10 cm long was tie wire to touch with sample for burning, press the oxygen in the bomb to have a pressure of 28-30 atmosphere and put the Jacket water in a 2,000 millimeter and insert the Bomb into the Jacket then close the lid and open the stirrer. Which value obtained which show on display screen of Bomb calorimeter when finish of system. 2.5. Ash contents A crucible is preheated in a muffle furnace at 575 ± 25°C for at least 60 min. After being taken out of the furnace, the crucible is cooled for 5–10 min and then placed in a desiccator, when the mass of the crucible reaches a constant level of 0.1 mg, the mass is recorded. And brought sample to drying at 105 ± 3°C for 24 h then to weighed about 1020 g move to a crucible then take to furnace and heated according to the following heating schedule: The furnace is heated to 575 ± 25°C at a rate of 5–6°C/min maintained at this temperature for at least 120 min. The crucible is taken out of the furnace and cooled in a desiccator.
Ash content
m3 m1 100 m2 m1
(2)
where ash content of oven-dried pellets on a dry mass basis, m1: mass of the crucible, m2: mass of the crucible + sample, m3: mass of the crucible + ash. 2.6. Durability of pellets The pellet durability was determined according to EN 15210-1. Firstly, fines were removed from the sample by gentle hand sieving using a sieve with aperture size of 3.35 mm. A 500 ±10 g sample of sieve pellets was tumbles at 50 ±2 rpm. After 500 rotations the drum has to be emptied and fine material will be sieved again. The biomass pellets will be weighed and the mechanical durability will be determined using following formula:
Durability (%)
M2 100 M1
(3)
where M1 is mass of pre-sieved biomass pellets before tumbling, and M2 is mass of the sieved biomass pellets after tumbling. 2.7. Moisture contents The measurement bottle with lid is dried at 105 ± 3°C till the mass is constant and then cooled down to room temperature. The mass of the empty bottle, including the lid, is measured and recorded to 0.01 g precision. The measurement bottle is evenly filled with at least 20 g of pellets, and the loaded mass, including the lid, is measured. After removing the lid, drying is continued at 105 ± 3°C for 24 h or until no fluctuations are observed in the loaded mass of the bottle and sample. The lid is dried in the same oven. The lid is again placed in the oven and the bottle is moved to a desiccator, where it is cooled to room temperature. The loaded measurement bottle, including the lid, is measured to 0.01 g precision. Measurements should be made at least in duplicate. Moisture content is calculated to 0.01% precision using the following formula, and the mean values are rounded to the nearest 0.1% for record.
T. Phonlayut et al. / Materials Today: Proceedings 17 (2019) 1872–1879
Moisture content
m2 m3 100 m2 m1
1875
(4)
where moisture content of pellets on a wet mass basis, m1: mass of the empty measurement bottle + lid, m2: mass of bottle + lid + sample before drying, and m3: mass of bottle + lid + sample after oven-drying. 3. Results and Discussion Fig.1 shows the effect of the torrefaction temperature on the product color. It is clear from the figure that the darkness of the torrefied biomass increases with temperature. Tumuluru et al. [13] assume that color changing can be mainly attributed to the chemical compositional changes that occur in the biomass components, such as hemicellulose, lignin, and cellulose. A change in the color of biomass is a good indicator to show the degree of torrefaction [14]. Moreover, the colors of the solid particles are uniform, indicating a same level of thermal degradation to all particles in the torrefier.
Fig. 1. Ground biomass before (a) and after torrefaction (b) 225C; (c) 250C; (d) 275C; and (e) 300C.
The effect of torrefaction temperature on the heating value (HV) and weight loss of the biomass is shown in Fig. 2. The HV significantly increased from 16.5 MJ/kg for untreated biomass to 22.9 MJ/kg for torrefied biomass at 300C. This result is in agreement with the report of Ru et al.[15]. The torrefaction temperature has an impact on biomass weight loss. This might be due to the removal of water and volatile compounds present in the biomass as an initial reaction of the thermo-degradation of biomass. In the temperature ranged from 225 to 300 C, most of the hemicelluloses, and some of the cellulose and lignin content in the biomass could be degraded during torrefaction. From this result, the torrefied powder at 300C was selected to mix with the untreated biomass.
Fig. 2. Effect of torrefaction temperature in high heating value and weight loss during torrefaction.
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T. Phonlayut et al. / Materials Today: Proceedings 17 (2019) 1872–1879
Photographs of the pellets at different torrefied fraction contents were shown in Fig.3. The torrefied fraction content resulted in changes in pellet color from light brown to dark brown. The diameter of the pellets was in the range of 10.1 – 10.5 mm and the length of the pellets was in the range of 7.04 – 7.79 mm after cooling. These pellets were further dried in a laboratory oven at 70 °C for about 3 h. Drying resulted in a decrease in pellet diameter of about 0.3 – 0.4 mm. The major reason for a decrease in diameter after drying was due to contraction of the pellets.
Fig. 3. Photographs of the pellets at different torrefied fraction contents (a) untreated pellets; (b) 10%; (c) 20%; and (d) 100%.
Fig. 4 shows the dependency of the torrefied fraction on the density and expansion ratio of the pellets. The unit density of the pellets varied between 743.3 and 1051.0 kg/m3. It clearly seen that the density of the pellets decreases with increasing torrefied fraction. Nhuchhen et al. [14] mentioned that the torrefaction process, which releases light volatile gases leaving the solid product more porous, decreases the density of torrefied pellet. The other possible explanation for lower density at higher torrefied fraction content can be due to volumetric expansion of the pellet as it exits the die. The measurement of the expansion ratio helps to observe the diametrical expansion of the pressed biomass pellets. The results indicated that the expansion ratio values did change dramatically by increasing the torrefied fraction from 20% to 100%. Due to an increasing hydrophobic nature of torrefied biomass, the bonding between particles is likely to decrease.[16]
Fig. 4. Influence of the torrefied fraction on the density and expansion ratio of the pellets.
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The initial HV observed for the biopellet was 17.23 MJ/kg as shown in Fig.5. Increasing the torrefied fraction to 10% and 20% slightly increased the heating value to about 17.78 and 18.11 MJ/kg, respectively. The maximum HV was about 22.94 MJ/kg for the sample with a 100% of torrefied fraction. The ash content in the pellets increased with the increase of torrefied fraction. The changes in the ash content are mainly due to breakdown of carbon-hydrogen bonds, resulting volatile loss and further concentrating the ash content in the biomass. The ash content was 9.60%, 10.86%, and 11.21% for the sample with 0, 10 and 20% of torrefied fraction, respectively. The highest ash content of about 20.95% was observed for samples with a 100% of torrefied fraction. The increase was about 118% with respect to the initial value.
Fig. 5. Effect of the torrefied fraction on the heat value and ash content of the pellets.
Durability values indicate the ability of the biomass pellets to withstand impact and shear force during transportation [17]. Durability value above 80% is considered high to retain the pellet integrity, and it is considered medium to retain the pellet integrity when the value is between 70% - 80% [18]. Fig. 6 shows the effect of torrefied fraction on durability values of the pellets. It is clear that adding torrefied fraction in small amount (10 and 20%) did not change much in the durability values (93 – 94%). However, the pure torrefied pellet shows low durability value (71.6%).
Fig. 6. Effect of the torrefied fraction on the durability of the pellets.
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Moisture content is another important property of biomass pellets. A high moisture content in a pellet may lead a high-energy loss in the course of burning [11]. The effect of the torrefied fraction on the moisture contents presents in Fig.7. The initial moisture content of the biomass pellet was about 9.52%. Normally, biomass pellet has a high moisture content about 20 – 30% because moisture can be absorbed into the cell walls and hydrogen-bonded to the hydroxyl groups of the cell wall components [19, 20]. After adding torrefied fraction, the moisture content was in the range of 9.50%. The lowest moisture content observed was about 9.49% in torrefied pellets. Felix et al. [16] suggested that the hygroscopic nature of biomass is changed by torrefaction process and turned it to hydrophobic, resulting in low moisture content of the torrefied pellet.
Fig. 7. Effect of the torrefied fraction on moisture content of the pellets.
4. Conclusion The research presented was carried out to understand the effect of torrefied fraction on the physical properties and fuel quality of the pellets. Increasing the torrefied fraction significantly decreases the density of the pellet, which might be due to the volumetric expansion of the pellet, whereas the heating value increased from its initial value of about 17.23 MJ/kg to about 22.94 MJ/kg. This indicated that good quality pellets in terms of heating value can be made using torrefied fraction. However, the ash content increases as well. Acknowledgements This work was supported by Materials for Energy and Environment Research Group (MEE) and Mae Fah Luang University, Thailand. References [1] I.E.A. (IEA), Renewable Energy: Medium-Term Market Report 2013, IEA, Paris, France, 2013. [2] P.B.a.B.A. Daya Ram Nhuchhen, International Journal of Renewable Energy & Biofuels (2014). [3] J.S. Tumuluru, C.C. Conner, A.N. Hoover, (2016) e54092. [4] K. Theerarattananoon, F. Xu, J. Wilson, R. Ballard, L. McKinney, S. Staggenborg, P. Vadlani, Z.J. Pei, D. Wang, Industrial Crops and Products 33 (2011) 325-332. [5] B. Zhou, K. E. Ileleji, G. Ejeta, Transactions of the ASABE 51 (2008) 581. [6] M. Sami, K. Annamalai, M. Wooldridge, Progress in Energy and Combustion Science 27 (2001) 171-214. [7] M.J.C. van der Stelt, H. Gerhauser, J.H.A. Kiel, K.J. Ptasinski, Biomass and Bioenergy 35 (2011) 3748-3762. [8] P.K. Prins MJ, Janssen FJJG., J Anal Appl Pyrolysis 77 (2006) 28-34. [9] W.-H. Chen, P.-C. Kuo, Energy 36 (2011) 803-811.
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[10] J.H. Peng, H.T. Bi, S. Sokhansanj, J.C. Lim, Energy & Fuels 26 (2012) 3826-3839. [11] W.-H. Chen, J. Peng, X.T. Bi, Renewable and Sustainable Energy Reviews 44 (2015) 847-866. [12] J.H. Peng, H.T. Bi, C.J. Lim, S. Sokhansanj, Energy & Fuels 27 (2013) 967-974. [13] J. Shankar Tumuluru, S. Sokhansanj, J.R. Hess, C.T. Wright, R.D. Boardman, Industrial Biotechnology 7 (2011) 384-401. [14] D.R. Nhuchhen, P. Basu, B. Acharya, Energy & Fuels 30 (2016) 1027-1038. [15] B. Ru, S. Wang, G. Dai, L. Zhang, Energy & Fuels 29 (2015) 5865-5874. [16] F.F. Felfli, C.A. Luengo, J.A. Suárez, P.A. Beatón, Energy for Sustainable Development 9 (2005) 19-22. [17] L. Tabil, Sokhansanj, S.,, Appl. Eng. Agric. 12 (1996) 345-350. [18] Z. Colley, Fasina, O.O., Bransby, D., Lee, Y.Y.,, T ASAE 49 (2006) 1845-1851. [19] J.S. Tumuluru, Energy Science & Engineering 3 (2015) 327-341. [20] M. Andersson, A.M. Tillman, Journal of Applied Polymer Science 37 (1989) 3437-3447.
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ScienceDirect Materials Today: Proceedings 17 (2019) 1872–1879
www.materialstoday.com/proceedings
MRS-Thailand 2017
The Effect of Adding Torrefied Fraction to Biomass Pellet from Corn Stover: Physical Properties and Fuel Qualities T. Phonlayuta, N. Soykeabkeawa,b, U. Intathaa,b, N. Tawichaia,b,* a
b
School of Science, Mae Fah Luang University,333 M 1, Muang, Chiang Rai 57100, Thailand Materials for Energy and Environment Research Group, Mae Fah Luang University, 333 M 1, Muang, Chiang Rai 57100, Thailand
Abstract Biomass pellet is one of a superior renewable energy. This work aims to improve efficiency of biomass pellet from corn residues, which are abundantly available after harvesting in Thailand by adding a torrefied fraction. Torrefaction treatment (225 – 300 C in nitrogen gas atmosphere for 30 min) was applied to corn stover (only leaves and stalks) and then mixed with the biomass in various ratios (i.e.,100:0, 90:10, 20:80, 0:100). The pellets were formed using uniaxial pressing with an 8 mm diameter die at pressure of 80 MPa. The physical properties of pellets such as bulk density, expansion ratio, moisture content, and durability are measured. The ash content and heat value are determined using standard laboratory methods. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Biomass; Torrefied; Corn Stover; Heat value; Durability
1. Introduction Biomass pellet is one of the renewable energy which is now the fastest growing energy sector [1] and it is considered as carbonaceous fuels. Forestry and agriculture sector are two main resources as the primary source of biomass, especially, wood pellet is the most prominent biomass energy sources [2]. Because of the limited supply of wood for pellet production, attention is turning to using a variety of agricultural products and wastes as raw material.
* Corresponding author. Tel.: +66 5391 6774; fax: +66 5391 6776. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.
T. Phonlayut et al. / Materials Today: Proceedings 17 (2019) 1872–1879
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Many researchers have worked on pelleting biomass from corn feedstock [3-5]. However, biopellet form corn still has high moisture content and low heat value. Then, some techniques [6, 7] were used to improve the properties of the biopellet. Torrefaction is an advanced technique, in which biomass is roasted under controlled conditions (heating rate, temperature, time) in an inert atmosphere, retaining most of its energy [8]. Normally, the torrefaction process performed at 200 - 300C [9] and torrefaction time was controlled within 1 h [10]. The benefits accomplished by torrefaction include higher heating value or energy density; lower moisture content; higher hydrophobicity or water-resistivity; improved grindability and reactivity; and more uniform properties of biomass [11]. Unfortunately, the densification of torrefied biomass samples was more difficult under the same operating conditions as used for the densification of untreated samples [12]. The high compression pressure and high die temperature are required to produce torrefied pellets. Previous reports showed only single torrefied pellet properties, there is no report about mixed torrefied pellet with untreated biomass. Then, the objectives of this research were to study the effect of torrefied fraction on the physical properties (size, unit density, expansion ratio, and durability) of pellets made from corn stover. Moreover, the moisture content, heating value and ash content of the obtained pellets were also studied. 2. Materials and Methods 2.1. Sample preparation In this study, corn stover samples (stalk and leaves) were collected from Chiang Rai Province, northern Thailand. The samples were dried in the oven at 105°C for 24 h, and then ground to less than 1 mm particle size. materials were stored at room conditions. A laboratory oven was used to torrefy biomass. The ground samples were initially weighted and then placed in ceramic crucibles and put into the oven at room temperature. The nitrogen gas was then purged through the chamber at the flow rate of 5 ml/min by an external flow meter. The samples were heated from ambient temperatures to the desired temperature (225, 250, 275, and 300C) at a rate of 5°C/min and held at the torrefaction temperature for 30 min. After cooling down to room temperature, the torrefied powders were removed from the oven, and weighed to measure the weight loss of torrefied sample before placing in a desiccator. The fuel properties of the torrefied samples were measured by a IKA C6000 Calorimeter (Germany). The torrefied powder with high heating value was mixed with the untreated biomass with various ratio (0, 10, 20 and 100% by weight). The tapioca starch was used as a binder. The pellets were then manufactured using a uniaxial pressing with an 8 mm diameter die at pressure of 80 MPa holding for 1 min. The obtained pellets were dried in an oven at 70°C for about 3 h to reduce moisture content before further characterization. 2.2. Expansion ratio The expansion ratio was calculated by using the pellet diameter data collected for the unit density measurement. The diametrical expansion ratio was calculated using the below equation:
Expansion ratio
D2 d2
(1)
where D is the diameter of the pellet compressed (mm) and d is the diameter of the die (mm). The reported values are an average of 10 measurements. 2.3. Density The density of individual pellets was calculated by dividing the mass of an individual pellet by its volume. A Vernier caliper was used to measure length and diameter to find volume of the pellets and a balance was used to weigh the sample. The reported values are an average of 10 measurements.
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2.4. Heating value Analytical methods according to ASTM D3286 by brought sample to completely burned in bombs with enough oxygen. The heat of combustion results in a higher temperature of the jacket and the calculated heat value can be calculated and the value obtained which show on display screen of Bomb calorimeter. For method start from brought sample about 0.5 g and without moisture content to cup and weigh, then rope 10 cm long was tie wire to touch with sample for burning, press the oxygen in the bomb to have a pressure of 28-30 atmosphere and put the Jacket water in a 2,000 millimeter and insert the Bomb into the Jacket then close the lid and open the stirrer. Which value obtained which show on display screen of Bomb calorimeter when finish of system. 2.5. Ash contents A crucible is preheated in a muffle furnace at 575 ± 25°C for at least 60 min. After being taken out of the furnace, the crucible is cooled for 5–10 min and then placed in a desiccator, when the mass of the crucible reaches a constant level of 0.1 mg, the mass is recorded. And brought sample to drying at 105 ± 3°C for 24 h then to weighed about 1020 g move to a crucible then take to furnace and heated according to the following heating schedule: The furnace is heated to 575 ± 25°C at a rate of 5–6°C/min maintained at this temperature for at least 120 min. The crucible is taken out of the furnace and cooled in a desiccator.
Ash content
m3 m1 100 m2 m1
(2)
where ash content of oven-dried pellets on a dry mass basis, m1: mass of the crucible, m2: mass of the crucible + sample, m3: mass of the crucible + ash. 2.6. Durability of pellets The pellet durability was determined according to EN 15210-1. Firstly, fines were removed from the sample by gentle hand sieving using a sieve with aperture size of 3.35 mm. A 500 ±10 g sample of sieve pellets was tumbles at 50 ±2 rpm. After 500 rotations the drum has to be emptied and fine material will be sieved again. The biomass pellets will be weighed and the mechanical durability will be determined using following formula:
Durability (%)
M2 100 M1
(3)
where M1 is mass of pre-sieved biomass pellets before tumbling, and M2 is mass of the sieved biomass pellets after tumbling. 2.7. Moisture contents The measurement bottle with lid is dried at 105 ± 3°C till the mass is constant and then cooled down to room temperature. The mass of the empty bottle, including the lid, is measured and recorded to 0.01 g precision. The measurement bottle is evenly filled with at least 20 g of pellets, and the loaded mass, including the lid, is measured. After removing the lid, drying is continued at 105 ± 3°C for 24 h or until no fluctuations are observed in the loaded mass of the bottle and sample. The lid is dried in the same oven. The lid is again placed in the oven and the bottle is moved to a desiccator, where it is cooled to room temperature. The loaded measurement bottle, including the lid, is measured to 0.01 g precision. Measurements should be made at least in duplicate. Moisture content is calculated to 0.01% precision using the following formula, and the mean values are rounded to the nearest 0.1% for record.
T. Phonlayut et al. / Materials Today: Proceedings 17 (2019) 1872–1879
Moisture content
m2 m3 100 m2 m1
1875
(4)
where moisture content of pellets on a wet mass basis, m1: mass of the empty measurement bottle + lid, m2: mass of bottle + lid + sample before drying, and m3: mass of bottle + lid + sample after oven-drying. 3. Results and Discussion Fig.1 shows the effect of the torrefaction temperature on the product color. It is clear from the figure that the darkness of the torrefied biomass increases with temperature. Tumuluru et al. [13] assume that color changing can be mainly attributed to the chemical compositional changes that occur in the biomass components, such as hemicellulose, lignin, and cellulose. A change in the color of biomass is a good indicator to show the degree of torrefaction [14]. Moreover, the colors of the solid particles are uniform, indicating a same level of thermal degradation to all particles in the torrefier.
Fig. 1. Ground biomass before (a) and after torrefaction (b) 225C; (c) 250C; (d) 275C; and (e) 300C.
The effect of torrefaction temperature on the heating value (HV) and weight loss of the biomass is shown in Fig. 2. The HV significantly increased from 16.5 MJ/kg for untreated biomass to 22.9 MJ/kg for torrefied biomass at 300C. This result is in agreement with the report of Ru et al.[15]. The torrefaction temperature has an impact on biomass weight loss. This might be due to the removal of water and volatile compounds present in the biomass as an initial reaction of the thermo-degradation of biomass. In the temperature ranged from 225 to 300 C, most of the hemicelluloses, and some of the cellulose and lignin content in the biomass could be degraded during torrefaction. From this result, the torrefied powder at 300C was selected to mix with the untreated biomass.
Fig. 2. Effect of torrefaction temperature in high heating value and weight loss during torrefaction.
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Photographs of the pellets at different torrefied fraction contents were shown in Fig.3. The torrefied fraction content resulted in changes in pellet color from light brown to dark brown. The diameter of the pellets was in the range of 10.1 – 10.5 mm and the length of the pellets was in the range of 7.04 – 7.79 mm after cooling. These pellets were further dried in a laboratory oven at 70 °C for about 3 h. Drying resulted in a decrease in pellet diameter of about 0.3 – 0.4 mm. The major reason for a decrease in diameter after drying was due to contraction of the pellets.
Fig. 3. Photographs of the pellets at different torrefied fraction contents (a) untreated pellets; (b) 10%; (c) 20%; and (d) 100%.
Fig. 4 shows the dependency of the torrefied fraction on the density and expansion ratio of the pellets. The unit density of the pellets varied between 743.3 and 1051.0 kg/m3. It clearly seen that the density of the pellets decreases with increasing torrefied fraction. Nhuchhen et al. [14] mentioned that the torrefaction process, which releases light volatile gases leaving the solid product more porous, decreases the density of torrefied pellet. The other possible explanation for lower density at higher torrefied fraction content can be due to volumetric expansion of the pellet as it exits the die. The measurement of the expansion ratio helps to observe the diametrical expansion of the pressed biomass pellets. The results indicated that the expansion ratio values did change dramatically by increasing the torrefied fraction from 20% to 100%. Due to an increasing hydrophobic nature of torrefied biomass, the bonding between particles is likely to decrease.[16]
Fig. 4. Influence of the torrefied fraction on the density and expansion ratio of the pellets.
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The initial HV observed for the biopellet was 17.23 MJ/kg as shown in Fig.5. Increasing the torrefied fraction to 10% and 20% slightly increased the heating value to about 17.78 and 18.11 MJ/kg, respectively. The maximum HV was about 22.94 MJ/kg for the sample with a 100% of torrefied fraction. The ash content in the pellets increased with the increase of torrefied fraction. The changes in the ash content are mainly due to breakdown of carbon-hydrogen bonds, resulting volatile loss and further concentrating the ash content in the biomass. The ash content was 9.60%, 10.86%, and 11.21% for the sample with 0, 10 and 20% of torrefied fraction, respectively. The highest ash content of about 20.95% was observed for samples with a 100% of torrefied fraction. The increase was about 118% with respect to the initial value.
Fig. 5. Effect of the torrefied fraction on the heat value and ash content of the pellets.
Durability values indicate the ability of the biomass pellets to withstand impact and shear force during transportation [17]. Durability value above 80% is considered high to retain the pellet integrity, and it is considered medium to retain the pellet integrity when the value is between 70% - 80% [18]. Fig. 6 shows the effect of torrefied fraction on durability values of the pellets. It is clear that adding torrefied fraction in small amount (10 and 20%) did not change much in the durability values (93 – 94%). However, the pure torrefied pellet shows low durability value (71.6%).
Fig. 6. Effect of the torrefied fraction on the durability of the pellets.
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Moisture content is another important property of biomass pellets. A high moisture content in a pellet may lead a high-energy loss in the course of burning [11]. The effect of the torrefied fraction on the moisture contents presents in Fig.7. The initial moisture content of the biomass pellet was about 9.52%. Normally, biomass pellet has a high moisture content about 20 – 30% because moisture can be absorbed into the cell walls and hydrogen-bonded to the hydroxyl groups of the cell wall components [19, 20]. After adding torrefied fraction, the moisture content was in the range of 9.50%. The lowest moisture content observed was about 9.49% in torrefied pellets. Felix et al. [16] suggested that the hygroscopic nature of biomass is changed by torrefaction process and turned it to hydrophobic, resulting in low moisture content of the torrefied pellet.
Fig. 7. Effect of the torrefied fraction on moisture content of the pellets.
4. Conclusion The research presented was carried out to understand the effect of torrefied fraction on the physical properties and fuel quality of the pellets. Increasing the torrefied fraction significantly decreases the density of the pellet, which might be due to the volumetric expansion of the pellet, whereas the heating value increased from its initial value of about 17.23 MJ/kg to about 22.94 MJ/kg. This indicated that good quality pellets in terms of heating value can be made using torrefied fraction. However, the ash content increases as well. Acknowledgements This work was supported by Materials for Energy and Environment Research Group (MEE) and Mae Fah Luang University, Thailand. References [1] I.E.A. (IEA), Renewable Energy: Medium-Term Market Report 2013, IEA, Paris, France, 2013. [2] P.B.a.B.A. Daya Ram Nhuchhen, International Journal of Renewable Energy & Biofuels (2014). [3] J.S. Tumuluru, C.C. Conner, A.N. Hoover, (2016) e54092. [4] K. Theerarattananoon, F. Xu, J. Wilson, R. Ballard, L. McKinney, S. Staggenborg, P. Vadlani, Z.J. Pei, D. Wang, Industrial Crops and Products 33 (2011) 325-332. [5] B. Zhou, K. E. Ileleji, G. Ejeta, Transactions of the ASABE 51 (2008) 581. [6] M. Sami, K. Annamalai, M. Wooldridge, Progress in Energy and Combustion Science 27 (2001) 171-214. [7] M.J.C. van der Stelt, H. Gerhauser, J.H.A. Kiel, K.J. Ptasinski, Biomass and Bioenergy 35 (2011) 3748-3762. [8] P.K. Prins MJ, Janssen FJJG., J Anal Appl Pyrolysis 77 (2006) 28-34. [9] W.-H. Chen, P.-C. Kuo, Energy 36 (2011) 803-811.
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