Study on Density, Hardness, and Moisture Uptake of Torrefied Wood

(1-6) Biomass densification has been practiced commercially in large-scale ... oil and natural gas prices drive the rapid growth of wood pellet indust...
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Study on Density, Hardness, and Moisture Uptake of Torrefied Wood Pellets J. H. Peng,† H. T. Bi,†,* C. J. Lim,† and S. Sokhansanj†,‡ †

Clean Energy Research Centre and Chemical and Biological Engineering Department, University of British Columbia, Vancouver, British Columbia, Canada ‡ Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States ABSTRACT: Torrefied pellets, a transportable renewable energy source, have a higher energy density than the regular wood pellets (control pellets). The quality of torrefied pellets is determined mainly by the density, hardness, and the hygroscopicity or moisture uptake. In this study, the density and the hardness of torrefied pellets were systematically examined by using torrefied samples prepared at different conditions in a press machine. The hygroscopicity of prepared torrefied pellets was evaluated in a humidity chamber by measuring the moisture uptake rate of control and torrefied pellets. The results showed that the density and the hardness of torrefied pellets mainly depended on the densification die temperature and the weight loss of torrefied samples. To make strong torrefied pellets of high density and low moisture uptake from 30 wt % weight loss torrefied samples, a die temperature of 230 °C or above was needed. Preconditioning torrefied samples to a moisture content of ∼10% can improve the quality of torrefied pellets. The moisture uptake of torrefied pellets was more sensitive to the weight loss of torrefaction and the relative humidity of the storage environment. The saturated moisture uptake of torrefied pellets made from 30 wt % weight loss torrefied samples was at least 40% lower than the control pellets. and temperature.9 Rumpf (1962) classified the particle densification mechanisms into five major categories: attraction forces between solid particles, interfacial forces, and capillary pressure to move liquid such as water into surfaces, adhesion and cohesion forces, solid bridges, and mechanical interlocking or form-closed bonds.11 Li et al. (2000) and Liu et al. (2000) reported that the compaction pressure for making strong logs from sawdust at room temperature needed to be at least 100 MPa.12,13 Lignin is a natural binder for densification. Preheating of the sawdust to a certain temperature will help to reach the glass transition temperature of lignin during compaction. Normally, the glass transition temperature of lignin is 100−140 °C. At a moisture content of 8−10%, the transition temperature can be reduced to 60−100 °C.14 Torrefaction is a thermal treatment without air or oxygen at 200−300 °C. Torrefaction increases the wood energy density on the mass basis, improves the wood hygroscopicity, and reduces the microbial degradation. A number of lab and pilot scale torrefaction units has been in operation, under construction, or planned.15−17 So far, most torrefaction studies have been focused on the development of torrefaction kinetics and reactors, but few studies have been conducted on densification of torrefied particles into pellets. To build a commercial plant for the production of torrefied pellets, both the torrefaction and the densification technologies need to be developed and demonstrated in order to make torrefied pellets. Although wood densification has been practiced commercially in large-scale pellet plants, considering that the properties of torrefied wood are significantly different from the raw wood, the operating

1. INTRODUCTION Densification can enhance the bulk density of biomass particles from the initial 40−200 kg m−3 to the final 600−1400 kg m−3.1−6 Biomass densification has been practiced commercially in largescale pellet plants. Biomass densification is a process applying a mechanical force to compact biomass residues or wastes (sawdust, shaving, chip, or slab) into a uniform size shaped solid particles such as pellets, briquettes, and logs. The objectives of wood densification are to increase the volumetric energy density, to facilitate easy storage and handling, to reduce the transportation cost, and to be lower the moisture content. Wood pellets are usually 6.35 mm in diameter with the length ranging from 5 to 25 mm.7 Typically, the bulk density of wood pellets is between 500 and 650 kg m−3 with a moisture content of 7−10 wt %.8 The effort to reduce greenhouse gas emissions in Europe and the rising oil and natural gas prices drive the rapid growth of wood pellet industry in the world. Currently, wood pellets are considered as a transportable renewable energy source. Most wood pellets are used in combustion for the heating of singlefamily houses, district heating systems, and electricity generation. Briquettes are generally disk shaped, 50−100 mm in diameter and 20−50 mm in length. The bulk density of briquettes ranges from 320 to 560 kg m−3, and the moisture content is 10−12 wt %.9 The final use of briquettes is for both residential and industrial heat applications. Logs are cylindrical in shape, 50−100 mm in diameter and 300−400 mm in length. Similar to briquettes, the logs are mainly for residential uses such as fireplace and wood stove.10 The market of briquettes and logs is very limited. The quality of wood pellets strongly depends on the quality of the feedstock as well as processing conditions, such as fiber sources, particle size, particle moisture content, particle temperature, biomass feed rate, die size and shape, compacting speed, © 2013 American Chemical Society

Received: November 26, 2012 Revised: January 24, 2013 Published: January 25, 2013 967

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2. EXPERIMENTAL SECTION

conditions for compressing torrefied wood into pellets still need to be investigated. Density, hardness, and moisture uptake rate are the most important properties of pellets. A dense pellet reduces the transportation and handling costs significantly for pellets transported over long distances. A high die temperature, high compression pressure, and sample preconditioning can improve the density of torrefied pellets. Phanphanich (2010) reported that the single torrefied pellet density could be up to 1032 kg m−3 with the sample preconditioned to 3.5−10.5 wt % moisture content and compressed at a die temperature of 90 °C with 157− 353 MPa compression pressure for sawdust torrefied at 275 °C for 30 min.18 Verhoeff et al. (2011) reported that the single torrefied pellet density was higher than control pellets when a die temperature of 260 °C was used to compress torrefied sawdust prepared at 280 °C for 60 min.19 Peng et al. (2012) suggested that torrefied pellets of properties comparable to regular pellets could be made by preconditioning torrefied samples (weight loss 33−36% at 300 °C for 15 min) with 10% moisture and compressing with 110 °C die temperature and 156 MPa pressure.20 Both Phanphanich (2010) and Peng et al. (2012) reported that the single torrefied pellet density made from preconditioned torrefied samples was lower than control pellets.18,20 Strong pellets, as characterized by a high mechanical strength, can reduce the breakage during handling, transportation, and storage. The mechanical durability for large quantity of briquettes and pellets can be determined in a tumbling device following the international round robin test procedure.21 For single pellet or small sample of pellets, the pellet strength, as represented by its hardness, has been commonly determined by applying a force to deform or break the pellet located between a mobile probe and a fixed plate. Li et al. (2012) reported that the Meyer hardness of torrefied pellets made at a die temperature of 170 °C was lower than control pellets.22 With moisture preconditioning, Peng et al. (2012) reported that the hardness of torrefied pellets was lower than control pellets.20 Verhoeff et al. (2011) reported that torrefied pellets made at 225 °C die temperature were 29% stronger than control pellets and torrefied pellets made at 260 °C die temperature were 103% stronger than control pellets.19 A low hygroscopicity or moisture uptake pellet reduces the cost associated with pellets handling and storage and prolongs the pellet shelf life. The saturated moisture uptake of torrefied pellets as measured by placing the pellets into a humidify chamber was much lower than control pellets.20,22,23 Verhoeff et al. (2011) reported that, for torrefied pellets prepared at 225 °C die temperature, the final moisture uptake was only 40% of the control pellets and 10% for pellets made at 260 °C die temperature.19 In this study, a systematic investigation was carried out with an objective to identify a suitable range of torrefaction and densification conditions for making durable torrefied pellets. A broad range of compaction temperature (from 70 to 280 °C) and pressure (from 125 to 249 MPa) was studied on the density and the hardness of torrefied pellets from torrefied sawdust prepared at a wide range of weight losses (from 13.3 to 60.6 wt %) and preconditioned with different moisture contents (from 0.8 to 16.1 wt %). The hygroscopicity of torrefied pellets obtained from various torrefaction and densification conditions was evaluated using a humidity chamber operated at different temperatures and relative humidity.

2.1. Preparation of Samples. SPF (a mixture of spruce, pine, and fir) shavings from the Wood Pellet Association of Canada and pine woodchip from FPInnovation were used in this study. Pine samples were prepared by drying in a THELCO laboratory PRECISION oven (Thermo Electron Corporation) at 105 °C for 24 h and crushing in a hammer mill (Glenmills Inc., U.S.A.; Model 10HMBL) installed with different size screens (0.79 mm and 3.18 mm). Three wood samples, one SPF shaving and two pine sawdust, were used for making control pellets and torrefied pellets. The properties of all raw materials are given in Table 1. From Table 1, the true density (also called the particle density

Table 1. Properties of SPF Shaving and Pine Sawdust Samples SPF shaving screen size, mm 4.00 moisture content, wt % 9.94 bulk density, kg m−3 156 true density, kg m−3 1346 high heating value, MJ kg−1 18.13 Sauter mean particle size (diam.), mm 1.10 Proximate Analysis volatile, wt % 93.06 fixed carbon, wt % 6.76 ash, wt % 0.18 Elemental Analysis, wt % C 49.54 H 6.17 O (by difference) 43.93 N 0.18

pine sawdust 0.79 7.44 225 1412 18.60 0.23

3.18 9.33 215 1361 18.79 0.67

82.79 16.81 0.40 51.22 6.02 42.24 0.12

50.67 5.94 42.95 0.04

that makes up a powder or particulate solid) of three wood samples was much higher than the bulk density of three wood samples. A bench-scale fixed-bed tubular reactor was used for preparing torrefied sawdust samples. The torrefaction conditions of SPF shaving samples were 240, 270, 300, and 340 °C for 60 min. The 0.67 mm size pine sawdust sample was torrefied at 300 °C for 15 min, and the 0.23 mm size pine sawdust sample was torrefied at 280 °C for 52 min. 2.2. Densification Procedure. A MTI 50K press machine (Measurement Technology Inc.) was used for compressing samples into pellets. A cylinder of 6.35 mm inside diameter and 70 mm length, with a piston 6.30 mm in diameter and 90 mm in length, was installed on the machine for making a single pellet. The cylinder unit was wrapped by a heating tape to preheat the inside cylinder to a certain temperature (called die temperature) as monitored by a thermocouple connected to a temperature controller (see Figure 1). In this study, the top hole of the cylinder was filled with approximately 0.5 g sawdust samples to make a single pellet of 6.5 mm in diameter and ∼12 mm in length. The sample was pressed by applying a pressure of 125 to 249 MPa and held for 1 min. The machine then continued to press the sample until a maximum pressure of 156 to 280 MPa was reached. The maximum pressure is typically about 30 MPa higher than the normal compression pressure. The die temperature was controlled in the range 70−280 °C. For making control pellets, the die temperature was typically maintained at 70 °C with 125 MPa compression pressure and 1 min holding time, and the maximum pressure was 156 MPa. Torrefied SPF shaving samples with different torrefaction weight loss were used for the densification test and the hygroscopicity test. The 0.67 mm size torrefied pine sawdust samples were preconditioned over 24−72 h to 0, 5, 10, and 15% moisture contents before being used for densification to study the effect of moisture content and die temperature. Torrefied pine sawdust samples (0.23 mm size) were used to investigate the effect of die temperature and pressure. Table 2 summarizes the pelletization test conditions. The energy consumption for densification was derived by integrating the pelletization force (N) and the rod displacement (mm) curves determined from the MTI machine. 968

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Table 3. Properties of Torrefied SPF Shaving and Sawdust Samples SPF shaving particle mean size, mm torrefaction temp., °C residence time, min weight loss, wt % bulk density, kg m−3 true material density, kg m−3 moisture, wt % high heating value, MJ kg−1 energy yield, % volatile fixed carbon ash C H O (by difference) N

Figure 1. Photo of a single pellet press with a heated die unit. 2.3. Hardness Test Procedure. The Meyer hardness (HM) is measured to represent the durability of pellets.24 To measure HM, the pellet was placed between two anvils under the MTI cross-head. The force was diametrical. The maximum force (F) to break a pellet was recorded. The equation of HM is then obtained as follows:

HM =

F π(Dh − h2)

pine sawdust

1.1

1.1

1.1

1.1

0.67

0.23

240

270

300

340

300

280

60

60

60

60

15

52

13.3 150

33.8 133

50.7 120

60.6 107

33.9 174

28.5 175

1365

1355

1447

1447

1526

1528

1.12 20.89

1.22 23.11

1.13 25.32

1.98 29.49

1.10 23.27

0.80 22.11

90.89 76.81 62.72 58.33 Proximate Analysis, wt % 84.5 73.72 63.83 48.95 15.3 26 35.79 50.56 0.19 0.28 0.38 0.48 Elemental Analysis, wt % 52.36 58.14 63.51 71.91 6.03 5.68 5.38 4.77 41.26 35.73 30.54 22.66 0.16 0.17 0.19 0.18

75.13

77.34

76.16 23.23 0.61

79.00 20.44 0.56

56.09 5.73 37.46 0.11

55.58 5.85 37.89 0.12

increased, but the bulk density, energy yield, hydrogen content, and oxygen content decreased. The moisture content of torrefied samples, as shown in Table 3, was very low compared to 10 wt % moisture content for the raw material, and the volatile component decreased with increasing the weight loss. The bulk density of torrefied samples was lower than raw materials because the weight loss of torrefaction took place inside the particles with little shrinkage in particle size. The true material density of torrefied samples, however, was higher than raw samples because of the removal of water and volatiles. Because of the removal of water and oxygen-containing volatiles during torrefaction, the carbon content of the torrefied sawdust increased, giving rise to a higher HHV of torrefied sawdust. At the same time, the removal of volatiles decreased the energy yield. During densification, the water and volatiles as natural binders between particles play a very important role.25 Water also can reduce the glass transition temperature of lignin, help the feedstock to soften, and improve the feedstock lubrication.14 3.2. Density of Torrefied Pellet. The single torrefied pellet density depends on the torrefaction conditions and the densification conditions. Because of the removal of moisture and volatiles from raw materials, torrefied samples are more difficult to be compressed into strong pellets under the same densification conditions as those used for making the control pellets.26

(1)

where D is the probe diameter and h is the indentation depth. 2.4. Moisture Uptake Measurement Procedure. A humidity chamber (ESPEC CORP, Japan; Model LHU-113) was used for the measurement of moisture uptake of pellets. Before the moisture uptake tests, pellets were dried in a convection oven at 105 °C for 24 h. Then, pellets were placed in a Petri glass dish and put into the humidity chamber. During the weight measurement, the Petri dish was covered with a glass cap to reduce the moisture loss. For the test of the moisture uptake rate, the chamber was set at 30 °C at 90% relative humidity. The weight of pellets was measured every 20 min for the first 2 h followed by every 30 min for the next 4 h. After 6 h, the weigh was measured by a few hours until the weight was constant. For the measurement of the saturated moisture uptake, the pellets were placed in the chamber for 48 h, with the temperature of the chamber varied from 20 to 35 °C and the relative humidity from 40% to 95%.

3. RESULTS AND DISCUSSION 3.1. Physical and Chemical Properties of Torrefied Samples. Table 3 summarizes the properties of torrefied samples at different torrefaction conditions. As can be seen in Table 3, as the torrefaction temperature increased, weight loss, true material density, higher heating value, and carbon content all Table 2. Summary of Pelletization Test Conditions samples raw pine sawdust and SPF shaving 1.10 mm torrefied SPF shavings 0.67 mm torrefied pine sawdust (300 °C for 15 min) 0.23 mm torrefied pine sawdust (280 °C for 52 min)

moisture content, wt % 7−10% 1.1 1.1, 6.1, 11.1, 16.1 0.8

die temp., °C 70 170, 230 70, 90, 100, 110,120,130,140,150 170, 200, 230, 260, 280

969

compression pressure, MPa

holding time, min

max. pressure, MPa

125 125 125

1 1 1

156 156 156

125, 156, 187, 218, 249

1

156, 187, 218, 249, 280

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To make torrefied pellets, Pyle (1976) suggested a die temperature of 93 °C for making durable torrefied pellets without any binder applied, although the single pellet density was only 800 kg m−3.27 Verhoeff et al. (2011) reported that it would be hard to make the strong torrefied pellets when the die temperature was below 225 °C.19 Figure 2 shows the single

Figure 3. Relative density of torrefied pellets as a function of the weight loss of torrefaction and the die temperature of densification (1.1 mm SPF shaving; control pellet density 1200 kg m−3; the straight line represents the control pellets).

Figure 2. Density of raw and torrefied pellets as a function of die temperature and compression pressure (0.23 mm torrefied pine sawdust at 280 °C for 52 min).

torrefied pellet density as a function of die temperature and compression pressure. The test sample was 0.23 mm pine sawdust, prepared at 280 °C for 52 min with a weight loss of 29 wt %. The single torrefied pellet density was seen in Figure 2 to increase with increasing the die temperature and the compression pressure and was more sensitive to the die temperature. At a die temperature of 230 °C or above, the torrefied pellet density was around 1250 kg m−3, which was close to the control pellet density. The higher die temperature and higher pressure are required to make torrefied pellets of density similar to that of the control pellet. From Figure 2, it is also seen that the density of single control pellet was less sensitive to the compression pressure when compared to the torrefied pellets. The density of torrefied pellets is also significantly affected by the torrefaction conditions. The relative density, defined as the ratio between the single torrefied pellet density and the single control pellet density, is plotted Figure 3 as a function of the weight loss of torrefaction and the die temperature of densification for 1.10 mm SPF shaving. The density of control pellet was 1200 kg m−3. From Figure 3, the relative torrefied pellet density is seen to decrease with the severity of torrefaction and increase with increasing the die temperature. At a die temperature of 170 °C, the relative torrefied pellet density was always lower than 1 and became close to 1 at a die temperature of 230 °C. Li et al. (2012) observed that the torrefied pellet density decreased with increasing the degree of torrefaction and was lower than the control pellets at a die temperature of 170 °C.22 Verhoeff et al. (2011) reported that higher torrefied pellet density was achieved at a die temperature of 225 °C and above.19 Due to the low moisture content of torrefied sawdust, preconditioning with water/steam can improve the densification properties of torrefied samples. Figure 4 shows the relative density of torrefied pellets as a function of the moisture content and the die temperature. The test sample was 0.67 mm pine

Figure 4. Relative density of torrefied pellets as a function of moisture content and die temperature (0.67 mm torrefied pine sawdust with 34% weight loss prepared at 300 °C for 15 min; control pellet density 1230 kg m−3; the straight line represents the control pellets).

sawdust torrefied at 300 °C for 15 min with a 34 wt % weight loss. The control pellet density was 1230 kg m−3. From Figure 4, it can be seen that the highest pellet density was obtained at 11% moisture content with a die temperature of 100−110 °C, although the relative torrefied pellet density was always lower than 1. It should be noted that a too high die temperature might have caused the moisture evaporation, which reduces the actual water content. Therefore, for compression of preconditioned torrefied samples, the die temperature should not be too high. Figure 5 shows the energy consumption for making torrefied pellets as a function of compression pressure and die temperature. 0.23 mm pine sawdust was used as the test sample, torrefied at 280 °C for 52 min with a weight loss of 29 wt % weight. It can be seen from Figure 5 that the energy consumption for making torrefied pellets was higher than the control pellets and decreased with increasing die temperature and decreasing 970

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Because the high temperature treatment during torrefaction removes the low melting point volatiles (see Table 3), the high melting point volatiles need a high die temperature during compaction. Verhoeff et al. (2011) suggested that the high torrefied pellet density could be obtained by using a die temperature 20−30 °C below the torrefaction temperature.19 For torrefied samples preconditioned with water, a die temperature of around 100 °C can be sufficient to soften the tracheid, but it cannot melt the high melting point lignin. For this reason, the density of torrefied pellets made from preconditioned samples was still lower than the control pellet. 3.3. Hardness of Torrefied Pellets. In this study, the Meyer hardness for both control pellet and torrefied pellet were measured, and a relative hardness is defined as the ratio of the torrefied pellet Meyer hardness over the control pellet Meyer hardness. Figure 6 shows the relative hardness of torrefied pellets

Figure 5. Energy consumption for making pellets as a function of compression pressure and die temperature (0.23 mm pine sawdust torrefied at 280 °C for 52 min with 29% weight loss).

compression pressure. At the die temperature range 170−230 °C, however, the energy consumption was not sensitive to the temperature. At 260 °C die temperature, the energy consumption for making torrefied pellets was close to that for the control pellet. Similarly, Li et al. (2012) and Peng et al. (2012) reported that the energy consumption for making torrefied pellets increased with increasing the compression pressure.22,20 The energy consumption of torrefied pellets at 260 °C die temperature was close to the control pellets because the torrefied samples became soft and easy to be compressed to pellets. Moisture content and volatile content of wood samples can affect the densification process. During densification under high temperature and high pressure, water and volatiles can increase interfacial forces, adhesion and cohesion forces, solid bridges, and the interlocking forces. The structures of wood are extremely important for wood densification. The density of pine board is typically around 440 kg m−3,28 and the true material density of pine is 1361−1412 kg m−3 (see Table 1). The main pores or empty space of wood are located in the tracheid, with a size in micrometers. At a moisture content of 8−10 wt %, the wood glass transition temperature is in the range of 60−100 °C.14 It means that the tracheid becomes softened at 60−100 °C. During the wood densification process, the tracheid is significantly reduced by the compression force. The highest density of control pellets was 1270 kg m−3, which was close to the true density of the wood (1412 kg m−3) for the 0.23 mm raw pine sawdust. Wood is composed of microfibrils, which are bundles of cellulose molecules surrounded by hemicelluloses. In between the microfibrils, lignin is deposited. During torrefaction, the weight loss was found to be mainly associated with the decomposition of hemicelluloses, although there was also certain degree of decomposition of cellulose and lignin.29 The new pores are created in and between microfibrils. For torrefaction in a temperature range from 200 to 300 °C with a low particle heating rate, the wood particle size did not change significantly.20 The experimental data shows that the bulk density decreased after torrefaction, while the true material density increased (see Tables 1 and 3). This suggests that the torrefied pellet density has potential to be higher than the control pellet.

Figure 6. Relative hardness of torrefied pellets as a function of die temperature and compression pressure (0.23 mm pine sawdust torrefied at 280 °C for 52 min with 29 wt % weight loss; control pellet Meyer hardness 7.48 N mm−2; the straight line represents the control pellets).

as a function of die temperature and compression pressure. The test sample was 0.23 mm pine sawdust torrefied at 280 °C for 52 min with a weight loss of 29 wt %. The control pellet Meyer hardness was 7.48 N mm−2. From Figure 6, it can be seen that the relative hardness of torrefied pellets increased with increasing die temperature and compression pressure and was more sensitive to the die temperature. At a die temperature of 240 °C or above, the relative hardness of torrefied pellets was higher than 1, indicating that the Meyer hardness of torrefied pellets is higher than the control pellets when the die temperature is higher than 240 °C. At 260 °C die temperature with 187 MPa compression pressure, the torrefied pellets are 1.24 times as strong as the control pellets. Verhoeff et al. (2011) also reported that the torrefied pellets made with a die temperature of 225 °C or above were stronger than the control pellets.19 Figure 7 shows the relative hardness of torrefied pellets as a function of the weight loss of torrefaction and the die temperature of densification. The test sample was 1.10 mm SPF shaving, with a control pellet Meyer hardness of 6.39 N mm−2. From Figure 7, it can be seen that the relative hardness of torrefied pellets decreased with the severity of torrefaction and increased with the die temperature. At a die temperature of 230 971

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33−43 wt % weight loss was lower than the Meyer hardness of control pellets at a die temperature of 110 °C.20 3.4. Moisture Uptake Performance. The moisture uptake of torrefied pellets was examined in the humidity chamber. Figure 9 shows the measured moisture uptake rate of control and

Figure 7. Relative hardness of torrefied pellets as a function of torrefaction weight loss and the die temperature (1.1 mm SPF shaving; control pellet Meyer hardness 6.39 N mm−2; the straight line represents the control pellets).

°C, the relative hardness of torrefied pellets could be higher than 1 at sample torrefaction weight loss less than 50%. Li et al. (2012) also observed that the torrefied pellet Meyer hardness decreased with increasing the degree of torrefaction.22 Figure 8 shows the relative hardness of torrefied pellets as a function of the moisture content and the die temperature for 0.67

Figure 9. Moisture uptake rate of control and torrefied pellets made from torrefied SPF shaving (humidity chamber with air of 90% relative humidity at 30 °C).

torrefied pellets made from torrefied SPF shaving samples. It can be seen that it took less than 10 h for most tested pellets to reach saturation. The saturated moisture uptake of torrefied pellets mainly depended on the torrefaction condition. The saturated moisture uptake of torrefied pellets was around 10% for torrefied samples with 34% weight loss, which is about 40% lower than the control pellets of 19%. Further increase in torrefaction weight loss to 51 and 61% only slightly decreased the saturated moisture uptake. The decrease in saturated moisture uptake for torrefied wood pellets likely results from the removal of hydrophilic hydroxyl (−OH) groups from the biomass and the increase of hydrophobic carbon content.30,31 Table 4 shows the elemental Table 4. Elemental Ratios of 1.10 mm Shaving Raw and Torrefied Samples raw materials weight loss, % wt O/C H/C (O + H)/(C + H + O + N)

Figure 8. Relative hardness of torrefied pellets as a function of sample moisture content and the die temperature (0.67 mm pine sawdust torrefied at 300 °C for 15 min; control pellet Meyer hardness 7.63 N mm−2; the straight line represents the control pellets).

240 °C, 60 min

270 °C, 60 min

0.00 13.30 33.78 Elemental Ratios (mole) 0.67 0.59 0.46 1.49 1.38 1.17 0.68 0.66 0.62

300 °C, 60 min

340 °C, 60 min

50.65

60.59

0.36 1.02 0.58

0.24 0.80 0.51

ratios of 1.10 mm SPF shaving raw and torrefied samples. It can be seen that both the hydroxyl groups and volatiles decreased with increasing the severity of torrefaction, while the fixed carbon content increased. Figure 10 shows the measured moisture uptake rate of control and torrefied pellets made from 0.23 mm pine sawdust with different die temperatures. The torrefaction condition was 300 °C for 15 min with 34 wt % weight loss. Increasing the pelletization die temperature is also seen to slightly lower the

mm pine sawdust samples. The torrefaction condition was 300 °C for 15 min with 34 wt % weight loss, and the control pellet Meyer hardness was 7.63 N mm−2. It can be seen from Figure 8 that the relative hardness of torrefied pellets was always lower than 1 with a die temperature of less than 150 °C, although the density increased with increasing the die temperature. A moisture content of 5 to 10% appeared to be optimal for densification within this die temperature range. Peng et al. (2012) reported that the Meyer hardness of torrefied pellets with 972

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Figure 10. Moisture uptake rate of control and torrefied pellets made from 0.23 mm pine sawdust (humidity chamber with air at 90% relative humidity and 30 °C).

Figure 12. Saturated moisture uptake as a function of the humidity chamber temperature and the torrefaction weight loss for 1.10 mm SPF shaving pellets in a humidity chamber with 90% relative humidity.

saturated moisture uptake for torrefied wood pellets, likely due to the additional thermal treatment of torrefied pellet surface at higher die temperature. Figure 11 shows the saturated moisture uptake as a function of the relative humidity of humidity chamber and the torrefaction

was seen to be insensitive to the humidity chamber temperature at a 90% relative humidity. In conjunction with the data in Figure 10, it can be concluded that the saturated water uptake is much more sensitive to the relative humidity than the temperature of the storage environment.

4. CONCLUSIONS The density, hardness, and hygroscopicity are the three most important quality indicators for torrefied pellets, because torrefied samples are more difficult to be compressed into dense and strong pellets under the same densification conditions as used for making the control pellets. The density and the hardness of torrefied pellets were found to be a main function of the torrefaction weight loss and the compression die temperature. The torrefied pellet density increased with increasing the die temperature and the compression pressure, and was more sensitive to the die temperature. With the increase in the severity of torrefaction, the torrefied pellet density decreased. At 230 °C die temperature, the torrefied pellet density with 10−50% weight loss was close to the control pellet density. Preconditioning torrefied samples to 5 to 15% moisture content can increase the density of torrefied pellets, with the highest pellet density being obtained at ∼10% moisture content with a die temperature of 100 °C, but the density and the hardness of torrefied pellets were still lower than control pellets. The energy consumption for making torrefied pellets was higher than the control pellets. At 260 °C die temperature, the energy consumption for making torrefied pellets was close to that for the control pellets. The Meyer hardness of torrefied pellets decreased with increasing the torrefaction weight loss. The Meyer hardness of torrefied pellets increased with increasing the die temperature and the compression pressure and was more sensitive to the die temperature. At a die temperature of 240 °C or above, the Meyer hardness of torrefied pellets could be equal to or higher than the control pellets. At 260 °C die temperature with the 187 MPa compression pressure, the torrefied pellets were 1.24 times as strong as the control pellets. Torrefied samples preconditioned to 5 to 10% moisture content improved the hardness of torrefied pellets.

Figure 11. Saturated moisture uptake as a function of the relative humidity and the torrefaction weight loss for 1.10 mm SPF shaving pellets in a humidity chamber at 30 °C.

weight loss for 1.10 mm SPF shaving pellets. The 0% weight loss corresponded to the control pellets. The saturated moisture uptake is seen to significantly depend on the torrefaction weight loss and the relative humidity of the storage environment, increasing with increasing the environment relative humidify and decreasing the torrefaction severity. The saturated moisture uptake of torrefied wood pellets with 34 wt % mass loss was more than 40 wt % lower than that of control wood pellets, especially in the high relative humidify environment. Figure 12 shows the saturated moisture uptake as a function of the humidity chamber temperature and the torrefaction weight loss 1.10 mm SPF shaving pellets. The saturated moisture uptake 973

dx.doi.org/10.1021/ef301928q | Energy Fuels 2013, 27, 967−974

Energy & Fuels

Article

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The moisture uptake of torrefied pellets was very sensitive to the torrefaction weight loss and the relative humidity of the environment but not to the storage temperature. The saturated moisture uptake of torrefied pellets made from 34 wt % weight loss torrefied samples was at least 40% lower than that of the control pellets. Based on the three key quality properties of torrefied pellets investigated in this study, it appears that a suitable torrefaction condition is a temperature of 250 to 300 °C with a weight loss of about 30%. The torrefied samples are compressed with a die temperature of 170−230 °C. Compared to regular control pellets, the torrefied pellets made in such a way will have a similar strength, a density of about 5 to 10% lower and a hygroscopicity of 50% or better.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the financial support from Natural Science and Engineering Research Council (NSERC) of Canada, Wood Pellet Association of Canada, and BC Innovation Council.

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NOMENCLATURE D = probe diameter in mm F = maximum force to break a pellet in N h = indentation depth in mm HM = Meyer hardness in N mm−2 min = torrefaction time in minutes

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REFERENCES

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