Pelletization of mixed Acacia dealbata and Salix atrocinerea with hazelnut and walnut shells: some experimental results
D. Almeida (1), T. Ferreira (1), J.M. Paiva (1) and C. Pinho (2)
(1) Escola Superior de Tecnologia, Instituto Politécnico de Viseu, Portugal
(2) Faculdade de Engenharia, Universidade do Porto, Portugal
1. Introduction - Wood species that have been currently used in pelletisation have become more and more a scarce resource, due to its common use in paper industry and, in a lesser scale, in furniture.
The present work consisted in harvesting and drying samples of Acacia dealbata, an invasive species, and in some additional processing phases, intended for further pelletisation. Salix atrocinerea samples were also used, as this is an autochthonous species with an interesting adaptation potential to very drought summers. Pellets were made using mixtures of the mentioned species and of nutshells (hazelnuts and walnuts). After densifying the materials, durability/quality and water resistance tests were carried out.
2. Experimental – Predetermined size samples were harvested, followed by drying in a solar kiln. Energy involved in this process was evaluated: Apogee SP215 pyranometers were used to measure solar radiation inside the solar dryer; T type thermocouples were used to measure temperatures and Honeywell HIH
4000 sensors to measure air relative humidity. National Instruments NI USB-6008 DAQ, Pico
Technology TC-08 USB and ADC-20 dataloggers were utilized to connect sensors. Pico Technology
Picolog Recorder and National Instruments Labview 8.6 software were chosen to read and collect data from drying process. Constant feed rate was ensured during the shattering/milling process, using a hammer mill, and product particle size characterization was done; J and K type thermocouples, as well as MTS strain gauges, were used to instrument the pelletizer machine, both internally and externally.
3. Results - The drying process took four days to come to a desired completion, so that a material with
16% (wb) moisture content was obtained. It was then shattered in a hammer mill. The resulting material had a 536 µm average diameter. Quality/ durability tests lay in an 84 to 98% index range.
4. Conclusions - Pellets fabricated exhibit acceptable quality indexes, though below international standards; water resistance was insufficient. New pelletisation conditions shall be tested to seek solutions to these problems.
1. Introduction - The importance in converting energy from biomass has grown in recent years due to the insufficient fossil energy providers and climate changes caused by carbon emissions. Pelletization of sawdust biomass materials increase handling characteristics, reduces transportation costs, improves volumetric calorific value and enhances control over combustion processes [1, 2].
Wood pellets are, basically, compacted sawdust, where lignin takes the role of thermoplastic glue binding loose particles; at temperatures between 80 and 200°C, lignin becomes softer and displays thermoplastic properties. Pellet appearance and quality depend on wood species lignin content [3-5].
Wood pellets are widely used for heating as well as for power production and have become more and more a scarce and valuable resource due to its common use in paper industry and, in a lesser scale, in furniture [3].
This work aimed the production of pellets made of mixtures of Acacia dealbata and Salix atrocinerea with nutshells (hazelnut and walnut shells) as a proportion 3:1 with good indexes to be commercialized.
2. Experimental setup - Predetermined size samples were harvested, followed by drying in a solar kiln. Energy involved in this process was evaluated: Apogee SP215 pyranometers were used to measure solar radiation inside the solar dryer; T type thermocouples were used to measure temperatures and Honeywell HIH 4000 sensors to measure air relative humidity. National Instruments NI USB-6008 DAQ, Pico
Technology TC-08 USB and ADC-20 dataloggers were utilized to connect sensors. Pico Technology
Picolog Recorder and National Instruments Labview 8.6 software were chosen to read and collect data from drying process.
Milling process occurred in an Agico wood crusher with constant feed rate, followed by pelletizing that happened with constant feed rate in an Agico pelletizing machine. J and K type thermocouples, and
MTS strain gauges were used to instrument the pelletizer machine, both internally and externally.
After pellets were made, a cooling period was provided before they were characterized by means of a set of standard procedures [6, 7]. Durability and water resistance tests were carried out to [6]. Sets of pellets were immersed in 25 mm of water at 27ºC for 30 seconds to estimate pellets resistance to water [8].
3. Results
3.1 Drying process - Drying process occurred during four days to come to a desired completion, so that a material with 16% (w.b.) moisture content was obtained.
Data were collected every one minute: irradiation values are shown in Fig. 1, temperatures in Fig. 2 and relative humidity in Fig. 3. Acacia dealbata solar drying data was similar to Salix atrocinerea. 0,28 kg of water per kg sample of Acacia dealbata was lost in solar drying process, so according to water vaporization latent enthalpy at an atmospheric pressure of 96 Pa (2261,129 kJ/kg), we know that to evaporate 7,84 kg of water from samples of raw material we needed 633 kJ/kg.
3.2 Milling and sawdust characterization - Constant feed rate was ensured during the shattering/milling process. 150 kg/h were processed using a hammer mill, and the product particle size characterization was done. The results are shown in Figure 4, a characteristic graphic of a particle size distribution. The calculated average diameter was calculated according to the equation 1 and 536 µm average particle diameter was obtained.
Equation 1:
dp=1∑xdp
Were:
dp represents the average particle diameter (µm);
dp is the sieve mean diameter (µm);
x is the retained mass fraction (g).
3.3 Pelletizing and pellets characterization – Mixes of Acacia dealbata and nutshells (hazelnuts and walnuts) were done with a proportion of 3:1 followed by pelletizing process. The same process was done to Salix atrocinerea that was mixed with nutshells in the same proportion.
Two pelletizing essays for each species Acacia dealbata and Salix atrocinerea and mixes with hazelnuts and walnuts were done. Figure 5 represents the temperature evolution with time during the Acacia
dealbata and Salix atrocinerea pelletizing process for the first essay.
Figure 5: Temperature data from pelletizing process (blue- axis; red- cover upper part; green- cover lower part);
A: Pelletizing mixed Acacia dealbata with nutshells (I- Acacia dealbata; II- Mixed Acacia dealbata with hazelnut; III- Mixed Acacia dealbata with walnut); B: Pelletizing mixed Salix atrocinerea with nutshells (I- Salix atrocinerea; II- Mixed Salix atrocinerea with hazelnut; III- Mixed Salix atrocinerea with walnut).
As verified in Figure 5 A, Acacia delabata pellets production happened at a constant average temperature rate of 104 ºC, than adding hazelnut shells temperature decreased and pellet production of mixed hazelnut shells with Acacia delabata occurred at a constant average temperature rate of 95 ºC, adding walnut shells temperature suffered a soft decrease and pelletizing mixed Acacia dealbata with walnut shells happened at a constant average temperature rate of 88,3 ºC. Adding mixed Acacia dealbata with nutshells decreased temperature because of his higher moisture content, as showed in table 1.
As observed in Figure 5 B, 82ºC was the constant average temperature rate of Salix atrocinerea pellet production, later, addition of hazelnut shells increased temperature about 16ºC, being constant during mixed Salix atrocinerea with hazelnut and walnut shells pellets production. Adding nutshells to produce pellets decrease raw material moisture content, leading to an increase of temperature.
Table 1 contains the main principal characteristic values of mixed Acacia dealbata and nutshells (hazelnuts and walnuts) pellets (with a proportion of 3:1) for the two essays at an approximate constant mass flow rate of 120 kg/h. The difference between the first and the second essay is an increase of dp and decrease of moistures content sawdust.
Table 1: Acacia dealbata pellets characterization.
Acacia dealbata / Acacia dealbata + hazelnut shells / Acacia dealbata + walnut shells1st Essay / 2nd Essay / 1st Essay / 2nd Essay / 1st Essay / 2nd Essay
Sawdust / dp (µm) / 448 / 594,0 / 541,0 / 594,0 / 541,0 / 594,0
Moisture (%, w.b.) / 12,6 / 7,0 / 13,3 / 7,0 / 13,3 / 7,0
Pellet / Diameter (mm) / 6,2 / 6,1 / 6,4 / 6,1 / 6,3 / 6,1
Length (mm) / 16,8 / 28,7 / 19,6 / 25,7 / 18,3 / 23,3
Weight (g) / 0,5 / 0,9 / 0,6 / 0,8 / 0,5 / 0,7
Fine content (%) / 1,8 / 0,8 / 4,0 / 3,7 / 3,6 / 3,7
Moisture (%, w.b) / 9,6 / 5,0 / 10,6 / 5,8 / 9,5 / 6,8
The main physical characteristics of pellets made of Salix atrocinerea and mixtures of Salix atrocinerea with nutshells are available in table 2. Pellets production was made at a constant mass flow rate of 70 kg/h, 50 kg/h lower than Acacia dealbata pelletizing feed.
The main difference between the first and the second essay lies on the increase of dp and particle moisture content.
Table 2: Salix atrocinerea pellets characterization.
Salix atrocinerea / Salix atrocinerea + hazelnut shells / Salix atrocinerea + walnut shells1st Essay / 2nd Essay / 1st Essay / 2nd Essay / 1st Essay / 2nd Essay
Sawdust / dp (µm) / 304,0 / 402,4 / 304,0 / 402,4 / 304,0 / 402,4
Moisture (%, w.b.) / 8,5 / 9,9 / 8,5 / 9,9 / 8,5 / 9,9
Pellet / Diameter (mm) / 8,7 / 6,1 / 6,0 / 6,1 / 6,0 / 6,2
Length (mm) / 20,3 / 16,6 / 22,5 / 17,3 / 20,9 / 16,6
Weight (g) / 0,7 / 0,5 / 0,7 / 0,5 / 0,7 / 0,5
Fine content (%) / 1,9 / 0,4 / 2,5 / 0,4 / 3,3 / 0,5
Moisture (%, w.b) / 6,5 / 8,7 / 7,4 / 7,8 / 6,7 / 9,1
4. Discussion – Pelletizing process occurred maintaining temperatures between 80-120ºC, ensuring plastic deformation of lignin to accomplish saw particles unification [4].
In table 3 it’s possible to observe the quality tests results of Acacia dealbata and mixtures of Acacia dealbata with hazelnut and walnut pellets.
Table 3: Quality tests results of Acacia dealbata pellets.
Acacia dealbata / Acacia dealbata + hazelnut shells / Acacia dealbata + walnut shells1st Essay / 2nd Essay / 1st Essay / 2nd Essay / 1st Essay / 2nd Essay
Durability (%) / 86,1 / 95,5 / 84,4 / 86,8 / 83,7 / 86,4
Water resistance (%) / 51,6 / 15,7 / 31,4 / 25,0 / 33,0 / 20,9
The first essay realized revealed acceptable values of durability but lower than the international standards of 97,5%, a 448 µm dp for Acacia dealbata pellets and a 541 µm dp for mixtures of Acacia dealbata with hazelnut and walnut shells were used. For the second essay, 594 µm dp for the three types of pellets were used, corresponding to a 146µm for Acacia dealbata and 53µm for mixed Acacia dealbata with nutshells increase of dp. Moisture content of the sawdust used was lower (almost half moisture content reduced), obtaining better quality pellets, but not good enough and bellow international standards. The water resistance results reveal that pellets with higher durability absorb less percentage of water.
The quality tests results of pellets made of Salix atrocinerea and mixtures of Salix atrocinerea hazelnut and walnut shells are presented in table 4.
Table 4: Quality tests results of Salix atrocinerea pellets
Salix atrocinerea / Salix atrocinerea + hazelnut shells / Salix atrocinerea + walnut shells1st Essay / 2nd Essay / 1st Essay / 2nd Essay / 1st Essay / 2nd Essay
Durability (%) / 89,6 / 98,0 / 91,4 / 96,8 / 88,5 / 96,2
Water resistance (%) / 38,8 / 15,9 / 39,5 / 17,2 / 37,7 / 21,1
Using 304 µm particle size average diameter and 8,5 particle moisture (w.b.) Salix atrocinerea sawdust, a 89,6% , 91,4% and 88,5 % durability pellet was obtained, for Salix atrocinerea, mixed Salix atrocinerea with hazelnut shells and mixed Salix atrocinerea with walnut shells, respectively for the first essay.
In the second essay from a 402,4 µm particle size average diameter and 9,9 particle moisture (w.b.) sawdust of Salix atrocinerea a better quality pellet was produced, a 98 %, 96,8 and 96,2% durability pellet of Salix atrocinerea and mixed with hazelnut and walnut, respectively, was obtained and a reduced percentage of absorbance of water, of 15,9%, 17,2% and 21,1% respectively.
5. Conclusions - The drying process took four days to come to a desired completion, so that a material with 16% (w.b.) moisture content was obtained in a similar way for the two species.
Using a 594 µm particle size average diameter and 7% moisture (w.b.) sawdust of Acacia dealbata has produced better quality pellets of Acacia dealbata and mixed with nutshells (hazelnuts and walnuts) in this work.
Better quality pellets of Salix atrocinerea and mixed Salix atrocinerea with nutshells were obtained with sawdust with 402,4 µm particle size average diameter and with 9,9% moisture (w.b.). Salix atrocinerea pellets produced into these conditions achieved a pellet quality assessing the compliance to international standards of 97,5%, so this pellets could be commercialized.
Pellets produced using mixed hazelnut shells with Salix atrocinerea as a raw material presents better durability and resistance to water percentages than pellets made of mixed walnut shells with Salix atrocinerea.The same situation is verified to pellets produced from mixed Acacia dealbata with hazelnut and walnut shells.
Adding nutshells to Acacia dealbata and Salix atrocinerea produced pellets with lower durability index and lower water resistance pellets.