Improving energy density and grindability of wood pellets by dry

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Biofuels and Biomass

Improving energy density and grindability of wood pellets by dry torrefaction Seunghan Yu, Jinje Park, Minsu Kim, Heeyoon Kim, Changkook Ryu, YongWoon Lee, Won Yang, and Yeong-gap Jeong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01086 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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Improving energy density and grindability of wood pellets by dry torrefaction Seunghan Yu †,#, Jinje Park †,#, Minsu Kim†, Heeyoon Kim†, Changkook Ryu†,*, Yongwoon Lee‡, Won Yang‡, Yeong-gap Jeong§ †School

of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, South Korea

‡Thermochemical

Energy Systems R&D Group, Korea Institute of Industrial Technology,

Cheonan, 330-825, South Korea §Human

Resource T/D Institute, Korea South-East Power Co., Jinju 52818, Republic of Korea

KEYWORDS: biomass; wood pellet; torrefaction; grindability; energy density

ABSTRACT: In dedicated wood pellet combustion and co-firing with coal in large pulverized fuel furnaces, poor grindability and low bulk density of biomass are important issues for lowering the unburned carbon in ash and achieving high co-firing ratios with coal for pulverized fuel combustion furnaces. In this study, the torrefaction of wood pellets was investigated for improvement of energy density and grindability. The torrefaction tests were performed using a fixed bed reactor for a temperature range of 210-310°C and holding time of 15-60 min. The mass yield varied from 86.18% to 39.46 % accompanied by an increase in the carbon content and heating value. The properties of torrefied wood pellets (TWP) were correlated with the mass

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yield for use with different time-temperature histories. The bulk density decreased by the mass yield raised to the power of 0.538. The energy density of TWP was higher in the initial torrefaction stage with a peak of 10.41 GJ/m3, but was below that for the original pellets when the mass yield was approximately ≤ 60 %. The grindability of TWP increased almost linearly with the degree of torrefaction and the mass yield of 80% attained the lower range of the grindability of coal.

1. INTRODUCTION Torrefaction of biomass is a well-established preprocessing technology to improve fuel quality [1]. It is performed typically over a temperature range of 250-300 °C under an inert atmosphere (dry torrefaction) or at 260 °C or lower using a hydrothermal process (wet torrefaction) [2]. Torrefaction involves the active decomposition of hemicellulose and the degradation of cellulose and lignin to some extent depending on the temperature [3, 4]. Hemicellulose is the most reactive polymer due to the small chain sizes and an amorphous structure. Above 200 °C, its degradation leads to the formation of acetic acid by desacetilization and methanol by the rupture of CH2OH groups and methyls. In parallel, depolymerization reactions release pentoses and hexoses, accompanied by the release of light gases such as CO2, CO, and formic acid. Above 250 °C, extensive decomposition of hemicellulose takes place while lignin and cellulose begin to degrade. Cellulose has a long chain of glucose with high crystallinity. It initially loses the degree of polymerization by the cleavage of glycosidic bonds, leading to char formation with the release of volatiles such as furfural, levoglucosan, and their derivatives, as well as light gases. The degradation of lignin involves demethoxylation with the release of acetic acid, methanol, and furfural, and modifications in the hydrogen bonds and in the connections of the aromatic rings.


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As the results of these reactions, the torrefied biomass has a decrease in the O/C and H/C ratios [5, 6] and increase in the carbon content and the heating value [7]. The loss of hydroxyl groups to form hydrogen bonds with water reduces the adsorption of moisture after torrefaction [8]. Also, the fibrous structure becomes brittle during torrefaction, which can significantly improve the grindability [9]. Pelletization is another preprocessing technology for biomass to improve the fuel quality, increasing bulk density and creating a fairly uniform particle size, which are beneficial for transport, storage, and feeding. Wood pellets are widely used for heat and/or power production in residential, commercial, and industrial combustion plants as the main or co-firing fuel. Co-firing wood pellets, which are mostly imported from Vietnam, Malaysia, Russia, and other countries [10], in coal-fired power plants has become a common practice in Korea. In 2016, Korea imported 1.7 million tons of wood pellets. In contrast, domestic wood pellets are used mostly for commercial and residential purposes at smaller scales. With the need to increase renewable electricity production, more wood pellets are expected to be imported in Korea. In particular, the Yeongdong Unit 1 has recently been retrofitted to a dedicated wood pellet power plant with a capacity of 120 MWe. It has a wall-firing boiler for pulverized wood pellets, which was modified from arch firing for anthracite [11]. A similar retrofit for Unit 2 of the plant, with a double capacity, is also ongoing. In pulverized fuel combustion, the poor grindability of wood pellets causes many operational problems including unburned carbon and limitations in co-firing with coal. For example, the Yeongdong Unit 1 suffers from high unburned carbon contents (>60%) in the bottom ash which consisted of large particles (>1 mm). This lowers the boiler efficiency and causes an operation complexity with increased cost for the bottom ash recycling. Minimizing the proportion of large


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particles by improved grindability can accelerate the char conversion rate and increase the particles entrained to the hot gas flow and released as fly ash having a higher char conversion with longer residence time. Also, the co-firing ratio of wood pellets in coal-fired power plants is limited to about 8 % on a mass basis [12] because of the deterioration in the grindability when co-milled. Because the bulk density and heating value of wood pellets are lower than coal, increasing the co-firing ratio also requires a larger energy density for wood pellets to reduce the volume for feeding to a coal mill. Applying torrefaction before pelletization would be an ideal solution to fully benefit from its advantages [13, 14], but the torrefaction of imported wood pellets on-site has become one of the measures to alleviate the problem in Korea. Several studies are reported in literature investigated the torrefaction of wood pellets. Shang et al. [14] analyzed the properties of torrefied Scots pine pellets over a temperature range of 230 °C − 270 °C. The higher heating value (HHV) increased from 18.37 MJ/kg to 24.34 MJ/kg with a solid yield of 58.1 %. The compression energy leading to irreversible deformation significantly decreased by torrefaction and was correlated well with the grinding energy consumption. Peng et al. [15] torrefied three wood pellets using a fixed bed reactor over a temperature range of 270 °C - 450 °C. The energy density as well as the pellet density decreased by torrefaction, while pellets produced from torrefied sawdust increased in energy density from 12.9 GJ/m3 to 15.9 GJ/m3. At a mass yield of 70 %, the Meyer hardness of torrefied wood pellets (TWP) decreased by more than 40 %. Rollinson and Williams [16] conducted microwave-induced torrefaction for coniferous timber waste pellets, and the fuel properties and reaction characteristics were analyzed. The grindability was tested using the Bond work index (BWI), which was close to that of traditional torrefied pellets and coal. Microwave torrefaction was also used in the Ren et al. study [17] for Douglas fir pellets, and the products yield and HHV were correlated with the


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reaction temperature and time. The energy density decreased from 13.2 GJ/m3 to 11.6-13 GJ/m3 for mass yields of 52.61 % - 83.15 %. Brachi et al. [18] torrefied wood pellets in a fluidized bed and found that a torrefaction temperature of 200 °C with a reaction time of 7−15 min for 84−85 % mass yield was ideal in terms of moisture uptake, durability, and energy density compared to those of untreated wood pellets. Lee et al. [19] compared the influence of steam and nitrogen as purge gas during torrefaction of wood pellets in which the steam condition exhibits a slightly lower mass yield. This study investigates the torrefaction of wood pellets to improve fuel quality in terms of energy density and grindability required for high co-firing ratios and efficient carbon conversion with pulverized coal- or biomass-fired plants. The wood pellets were torrefied in a bench-scale fixed bed reactor for a temperature range of 210 °C − 310 °C with a holding time of 30 min and 250 °C − 290 °C with holding times of 15 min and 60 min. Using the mass yield and key fuel properties, the energy yield and energy density of the torrefied products were determined. The grindability of TWP was also tested using the thermally treated biomass grindability index (TTBGI) [20] modified from the Hardgrove grindability index (HGI) and compared with the values of commercial coals. The fuel properties were further analyzed to derive respective correlations in terms of mass yield so that these properties can be quickly estimated regardless of heating method.

2. MATERIALS AND METHODS 2.1 Properties of wood pellets


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The wood pellet sample was acquired from Yeongdong plant (Korea South-East Power Co.), which was originally imported from Vietnam. The pellets had a diameter of 8 mm and typical length between 10 and 30 mm and contained 8.90 % moisture, 73.77 % volatile matter (VM), 14.85 % fixed carbon (FC), and 2.48 % ash. The organic fraction consisted of 49.65 % C, 5.62 % H, 44.32 % O, and 0.41 % N on a dry ash-free basis. The higher heating value (HHV) was 17.11 MJ/kg on a wet basis. The bulk density was approximately 547 kg/m3, which corresponded to a bulk energy density of 9.35 GJ/m3. The lignocellulosic composition was 41.0 % cellulose, 28.2% hemicellulose, and 30.8 % lignin, based on the wood chemistry method [21−23]. 2.2 Torrefaction tests in a fixed bed Torrefaction tests were carried out using the bench-scale fixed bed reactor shown in Fig. 1. The wood pellets were placed over a perforated plate inside the reactor (diameter 7.0 cm and height 15.0 cm) and were heated by an electric heater and by preheated purge gas simultaneously. The purge gas (N2 at 10 l/min) was injected at the atmospheric pressure from the top of the reactor after preheating to the target torrefaction temperature to help uniform heating of the particles and to sweep the vapor products out of the reactor. The temperature distribution inside the reactor were monitored by four K-type thermocouples, located 3 cm apart along the centerline from 3 cm above the perforated plate. The bed of particles naturally developed a temperature distribution in which the upper part was heated first by direct contact with the hot purge gas. Attaining the target torrefaction temperature was judged by T2, measured at 6 cm above the perforated plate. The temperature was maintained for a particular duration (holding time). With the help of the purge gas, the vapors released from the moisture evaporation and torrefaction reactions were immediately passed through a condensation system for tar and water below the reactor. The condensation system consisted of a series of bottles submerged in warm


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and cold baths maintained at 20 °C and -20 °C, respectively. The composition of the noncondensable gasses was monitored by an on-line gas analyzer (A&D System, model A&D 9000) composed of nondispersive infrared sensor and thermal conductivity detector for O2, CO, CO2, H2, and CH4. The TWP and condensed liquids were weighed after the test to determine the product yields. In particular, the TWP was weighed immediately after it cooled inside the reactor under N2 purge to prevent moisture adsorption from the atmosphere. The yield of noncondensable gases was determined by difference. N2 (10 LPM)

Preheater Torrefaction reactor (ID 7 x H 15 cm)

T gas T4 T3

Heater

T2 T1

Gas analyzer Vapor / N2

20°C

-20°C Condensors for tar and water

Fig. 1. Schematic of batch type fixed bed reactor for torrefaction tests. The influence of torrefaction temperature was tested for a wide temperature range between 210 °C and 310 °C with a holding time of 30 min. The effect of holding time was investigated for 15 min and 60 min in a temperature range of 250 °C and 290 °C, which covered typical torrefaction temperatures. The weight of the wood pellets in each test was approximately 270 g. 2.3 Analytical methods for fuel properties


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The proximate analysis was carried out for the wood pellets before and after torrefaction based on the standard methods (ASTM E871-82 for moisture content, ASTM E872-82 for VM content, ASTM D1102-84 for ash content, and FC content by difference). The ultimate analysis of the raw and torrefied samples, including the liquids, were performed using an elemental analyzer (CE Instruments, EA 1108). The higher heating value (HHV) of each material was determined using the correlation of Channiwala and Parikh [24]. The bulk density of wood pellets and TWP was measured based on ASTM E873-82 but using a 60 mm cube container. The apparent density was measured directly from the weight, diameter, and length of the individual pellets. In order to compare the mass loss profile in a powder form with that for pellets, thermogravimetric analysis (TGA) was performed using a TG analyzer (Scinco, TGA N-1000) for the raw pellet powder with a sample weight of 15 mg. It was heated to a final temperature of 250, 270, or 290 °C at a heating rate of 10 °C/min under N2 and then maintained for up to 1 h. This can identify the effect of holding time, minimizing the intraparticle heating effect. Another set of TGA tests to a final temperature of 700 °C were performed to compare the pyrolysis characteristics of the raw wood pellet and TWP samples produced from different temperatures with a holding time of 30 min. 2.4 Grindability tests The grindability of biomass has been tested using various methods including the modified HGI [25], the hybrid work index [26], BWI [27], and the grindability criterion based on the grinding energy [28]. In this study, the grindability of the TWP was evaluated using the TTBGI test [20]. It is based on the HGI test for coal which is the standard method (ASTM D409) routinely applied in industry. TTBGI modifies the HGI test to consider the low bulk density of biomass and is


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calibrated to the same scale with HGI. Therefore, the two indices are directly comparable and the grindability of TWP to attain that of coal can be determined easily. In the HGI test, the sample with a weight of 50 g is ground in a specific mill developed for the test, and the material passing 75 µm is determined and compared to standard reference samples. However, it is not suitable for biomass which has a low bulk density and does not require such small particle sizes due to its high reactivity. In the TTBGI test, the powder of raw or torrefied pellets sieved between 0.6-1.18 mm is ground in a ball mill having a volume of 75 cm3 for 60 revolutions at 41 RPM. The mass fraction below 500 µm is determined using a sieve for the sizing. The TTBGI is calibrated by a predetermined correlation derived from two reference samples (wood-based charcoal and wood pellets) and four standard reference samples of coal with HGI ranging between 38 and 96.

3. RESULTS AND DISCUSSION 3.1 Mass yields of torrefaction products from wood pellets Fig. 2 shows the product yields from the torrefaction tests. The error bars are not shown because the deviations of the yields in repeated tests were within 0.58 %. The mass yield of TWP decreased gradually from 86.2 % at 210 °C to 68.8 % at 270 °C and then to 39.5 % at 310 °C when the holding time was fixed at 30 min. The liquid yield (water, tar, etc.) at 210 °C was attributable to the release of moisture. The non-condensable gas yields, such as those for CO2 and CO, were only about 1.1 % at 210 °C, which conforms to previous studies in the literature [17, 29, 30] The liquid and gas yields increased to 46.1 % and 14.4 %, respectively, at 310 °C. Increasing the holding time from 15 min to 60 min decreased the TWP yield by 5.6 % − 10.7 % at temperatures of 250 °C − 290 °C, because longer reaction times were allowed at the elevated


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temperatures [2, 3]. The effect of holding time became greater at higher temperatures, but was smaller than that of temperature.

100 15 min 30 min 60 min

80

Product yield (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Torrefied biomass 60

40

20

Tar + water Gas

0 210

230

250

270

290

310

Temperature (oC)

Fig. 2. Product yields from torrefaction of wood pellets. Fig. 3 shows the temperature history measured inside the reactor during the torrefaction test at 270 °C with a holding time of 30 min which represents the midpoint of all test conditions. The dense pellet particles required a long time to reach the target temperature. The pellets in the upper parts were heated earlier by the hot purge gas compared to those in the lower parts. The holding time began when T2 at 6 cm above the perforated plate reached 99 % of the target value, which is t=132 min in the figure. Therefore, there was a significantly long time allowed for the pellets to decompose even before they reached the target value, especially for those in the upper part. However, the bottom part (T1) had slightly higher temperature in the later stage. In terms of the volatile matter content, the difference in the TWP from the top and bottom parts of the bed was 0.24 % and 0.14 % at 210 °C and 250 °C with a holding time of 30 min, respectively. Therefore, the TWP had a satisfactory uniformity.


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The above results suggest that the heat transfer condition in particular reactor would significantly influence the actual time-temperature history of particles and, therefore, the results for combinations of temperature and holding time in the specific reactor cannot be applied to other reactors.

300 250

o

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

200 150 100 T1 (Bottom) T2

50

T3 T4 (Top)

0 0

20

40

60

80

100 120 140 160

Time (min)

Fig. 3. Temperature history inside the reactor during torrefaction test at 270 °C. One example to exhibit the different influence of holding time would be to compare the TWP yield of the bench-scale tests with those from the TGA for powder at the same final temperature. Fig. 4 shows the TGA results for the final temperatures of 250, 270, and 290 °C in which the temperature was maintained after it reached the target value. As indicated by the symbols in Fig. 4, the mass yields of the bench-scale tests for a holding time of 30 min were significantly lower than the TGA results at the same holding time at each respective temperature. The difference was the largest (9.63 %) at 290 °C. The TGA tests were performed for 15 mg sample with particles ground to less than 0.3 mm. Therefore, the large surface area and small particle size allowed faster heating and shorter time to reach the thermal equilibrium with the gas supplied during the


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test compared to the large wood pellets in the fixed bed. The large differences in the bench-scale tests and TGA results show that the exact yield at different temperatures and holding times vary considerably by the reactor type and heating method. Therefore, the properties of TWP in this study were interpreted as a function of mass yield so that the results can be generalized for torrefaction with different time-temperature histories or heat transfer conditions. Similarly, it has been reported in the literature that the solid yield can be used as a synthetic indicator for torrefaction [31−33]. 100

Target temp. reached

↓↓↓

90

TWP yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 70 250oC (TGA) 270oC (TGA) 290oC (TGA) 250oC (30 min) 270oC (30 min) 290oC (30 min)

60 50 40 0

20

40

60

80

100

Time (min)

Fig. 4. Comparison of solid yields between TGA for powder and bench-scale tests for pellets.

3.2 Fuel properties and energy density of TWP Table 1 lists the proximate and ultimate analysis results of TWP. The maximum deviation was 0.57% in the volatile matter content, 0.22% in the ash content, and 0.25% in the elemental composition when repeated three times. With the release of volatiles, the fixed carbon and ash contents increased gradually to 50.3 % and 5.9 %, respectively, at 310 °C. The elemental


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composition suggests that the release of O was prominent during the torrefaction mainly in the form of CO2, CO, and H2O, decreasing the O content from 44.3 % in the raw pellets to 22.8 % at 310 °C. In contrast, the C content increased from 49.65 % in the raw pellets to 71.41 % at 310 °C. Table 1. Fuel properties of torrefied wood pellets.

Raw

Holding Yield time (wt.%) (min) 100

Proximate analysis (wt.%)

Ultimate analysis (%daf)

M

VM

FC

A

C

8.90

73.77

14.85

2.48

49.65 5.62

44.32 0.41

Energy HHV yield (MJ/kg) (%) 17.11 -

210

30

86.18

80.32

16.66

3.02

50.22 6.12

43.22 0.44

19.60

98.7

230

82.64

78.78

18.50

2.72

52.12 6.04

41.43 0.41

20.40

98.5

250

76.11

76.08

20.86

3.05

53.95 5.95

39.71 0.39

21.01

93.4

270

68.81

71.10

25.47

3.44

56.23 5.82

37.47 0.48

21.76

87.5

290

55.05

63.23

32.94

3.83

60.72 5.59

33.18 0.50

23.34

75.1

310

39.46

43.78

50.33

5.89

71.41 4.95

22.82 0.82

26.59

61.3

80.11

77.58

19.53

2.90

52.83 5.93

40.89 0.35

20.52

96.1

270

72.15

73.41

23.71

2.89

54.68 5.78

39.06 0.48

21.16

89.2

290

58.51

65.05

31.63

3.32

59.68 5.63

34.22 0.48

23.06

78.8

74.48

74.79

22.17

3.04

54.35 5.85

39.37 0.43

21.06

91.7

270

65.67

70.31

26.73

2.96

56.99 5.81

36.80 0.40

22.19

85.2

290

49.61

57.47

37.86

4.67

64.83 5.50

29.09 0.59

24.77

71.8

Temp (°C)

250

250

15

60

H

O

N

From the elemental composition of TWP, the release of atoms from the combustible fraction (excluding moisture and ash) can be calculated as shown in Fig. 5. As shown in Fig. 5a, the release of C, H, and O were roughly linear to the mass loss on a dry ash free basis (100-Ydaf) between 20 - 50 % regardless of the holding time applied. However, the O release was larger in the initial stage due to the release of CO2, while there was little H release. For quick estimations using the TWP yield, the composition of volatiles released can be approximated as 31.6 % C, 5.3 % H, and 63.1 % O or, as a chemical formula, C1H2.02O1.50. Note that this is the total volatile composition accumulating the instantaneous release, which would vary by temperature. N was


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Energy & Fuels

not included in this analysis because its proportion is very small. Fig. 5b shows the atomic yield in the TWP. Using the results, respective correlations were derived for the C, H, and O yields of TWP as a function of (100−Ydaf) with a high accuracy as listed in Table 2. The correlations can be used to predict the atomic yields, composition, and HHV for the mass yield of TWP with different heating methods. Hasan et al. [34] proposed similar correlations for the C and H yields of woody biomass as a function of dry yield, which are also plotted in Fig. 5b. The predicted C yield using the correlation (0.7405 Ydry + 28.47) was reasonable, but the H yield using the correlation (1.067 Ydry − 11.45) was underestimated for the wood pellet. In a similar approach, Campbell and Evitts [35] used the C content as an indication of torrefaction severity. However, expanding the correlations for H and O as well as other properties would be more helpful for numerical modeling and process design [36, 37].

100

100 C release H release O release

80

y=1.546x-0.00352x2

80

Atomic yield (%)

Atomic release excluding moisture (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 y=-5.685+1.1498x

40

y=0.550x+0.00237x

20

C

60

40

H

2

20

O Correlations derived Hasan et al.'s correlations

0

0 0

10

20

30

40

50

60

70

0

10

100 - Ydaf (%)

20

30

40

50

60

70

100 - Ydaf (%)

(a)

(b)

Fig. 5. Changes in the elemental composition by torrefaction as a function of dry, ash-free yield: (a) atoms released as volatiles and (b) atomic yields of the torrefied wood pellet.


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Table 2. Correlations of C, H, and O yield in torrefied wood pellets as a function of mass yield on a dry, ash free basis. Atomic yield

Correlation

R2

C

100−YC = 0.5499(100−Ydaf)+0.002374 (100−Ydaf)2

99.86%

H

100−YH = min{100, −5.6847+1.1498 (100−Ydaf)}

99.86%

O

100−YO = 1.5458(100−Ydaf)−0.00352 (100−Ydaf)2

99.91%

Ydaf=(100-Ash)Y/(100-Asho-Mo)

The loss of volatiles with a high O content led to a significant increase in HHV up to a high of 26.59 MJ/kg, as presented in Table 1. Combining these results with the mass yields, the energy yield of TWP was calculated. When plotting the mass loss by torrefaction (Fig. 6), the moisture evaporation led to a rapid increase in HHV, while the energy yield did not decrease. After the initial torrefaction stage, both the HHV and energy yield exhibited a linear relationship with the mass loss by torrefaction.

28

100

HHV (MJ/kg)

26

90

24 80 22 70 20

Energy yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60

18

HHV Energy yield

16

50 0

20

40

60

Mass loss by torrefaction (wt.%)

Fig. 6. HHV and energy yield of torrefied wood pellets.


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Table 1 also presents the bulk density of TWP. Compared to the original wood pellets (546 kg/m3), the bulk density of TWP was lower because of the release of volatiles and the resultant formation of intraparticle pores. It decreased from 519 kg/m3 at 210 °C to 329 kg/m3 at 310 °C. The diameter of the pellets also decreased from 8 mm for the original pellets to approximately 7.2 mm at 310 °C. Using the measured density, a correlation could be derived to predict the apparent density (ρapp) of a pellet as a function of TWP yield (Y) with the introduction of a burning mode parameter, ɑ, as ρapp/ρapp,o=Yɑ For a characteristic length (Dp), the above equation could be expressed as Dp/Dp,o=Y(1-ɑ)/3 Using the void fraction (ε), the bulk density (ρbulk) became ρbulk=(1-ε)ρapp If ɑ=0, the density remains constant, while Dp reduces by Y1/3. If ɑ=1, Dp does not change, while the bulk density decreases by Y. As shown in Fig. 7a, the measured ρapp correlated well with Y when ɑ=0.538. This implied that the intraparticle pores develop simultaneously with the shrinkage of pellets at the particular ratio. ρbulk measured separately also correlated reasonably with ρapp for ε fixed at 0.495, as shown in Fig. 7b.


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1200

600 =0

=0 500

3

1000

Bulk density (kg/m )

3

Apparent density (kg/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

=0.538 800

=0.538

400

300

600 =1

=1

200 0

10

20

30

40

50


60

0

10

20

30

40

50

60


(a)

(b)

Fig. 7. Density of torrefied wood pellets: (a) apparent density, (b) bulk density. Based on the HHV and bulk density of TWP, the energy density was evaluated, as shown in Fig. 8. The original pellets had an average density of 9.35 GJ/m3. The torrefaction at lower temperatures increased the energy density to over 10 GJ/m3 at mass yields around 80 %. This implied that the influence of the increase in HHV was larger than that for the decrease in the bulk density in the initial torrefaction stage due to the moisture evaporation and release of CO2. However, the energy density dropped rapidly. When the TWP yield was lower than 60 %, the energy density was below that of the original pellets. If the TWP is fed directly to coal pulverizers for co-milling, this decrease in the energy density (i.e., increase in the volume to deliver the same thermal input) will be an issue at high co-firing ratios despite the improvement in the grindability.


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Energy & Fuels

11.0

Bulk energy density (GJ/m3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10.5 10.0 9.5 9.0 Holding time: 15 min Holding time: 30 min Holding time: 60 min Raw pellets

8.5 8.0 0

10

20

30

40

50

60

70


Fig. 8. Bulk energy density of torrefied wood pellets. Fig. 9 shows the DTG curves of the raw pellet and TWPs treated at different temperatures with a fixed holding time of 30 min. In Fig. 9a, the DTG curve of the raw pellet is compared to a curve synthesized by the mass-weighting of individual curves for cellulose (41.0 %), hemicellulose (28.2 %), and lignin (30.8 %) with rate constants taken from [4, 38, 39]. The DTG curve of the raw wood pellet comprised the thermal degradation of lignocellulosic components but was shifted toward the high-temperature side by approximately 20 °C when compared to the synthesized curve. In particular, the shoulder at approximately 300 °C by an overlap of hemicellulose and cellulose decomposition became not clear. Also, the peak of cellulose decomposition appeared at 367 °C was higher than that in the synthesized curve. This is attributable to the inhibition of heat and mass transfer in the biomass compared to the decomposition of the pure component. As shown in Fig. 9b, the shoulder around 300 °C gradually became small with increasing the torrefaction temperature and the cellulose peak became more dominant because the temperature range of the torrefaction test was mainly for


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hemicellulose decomposition. The TWP processed at 310 °C (with a mass yield of 39.46%), the cellulose peak almost disappeared, leaving the lignin decomposition curve with a peak at 420 °C. For the TWP processed at 250 °C which had a mass yield of 76.11%, the mass loss rate of -0.1 dW/dT was delayed by 32 °C compared to that of raw pellet. Due to the increase in the onset of pyrolysis and the decrease in the volatile matter, more severe torrefaction would increase the ignition temperature during combustion. Further investigations are required to evaluate the ignition behaviors of TWP.

1.2

1.2 1.0

Raw pellet Synthesized Hemicellulose Cellulose Lignin

1.0

0.8

DTG(-dW/dT)

DTG (-dW/dT)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.6 0.4 0.2 0.0 100

0.8

210 oC 230 oC 250 oC 270 oC 290 oC 310 oC Raw pellet

0.6 0.4 0.2

200

300

400

500

600

0.0 100

200

o

300

400

500

600

Temperature (oC)

Temperature ( C)

(a)

(b)

Fig. 9. DTG curves of (a) raw pellet and synthesized biomass based on the lignocellulosic composition, and (b) raw and torrefied pellets (holding time of 30 min).

3.3 Grindability of TWP Torrefaction is known to improve the grindability of biomass owing to the degradation of fibrous structure [40,41]. Fig. 10 shows the grindability determined by the TTBGI tests versus


19

Energy & Fuels

the weight loss for TWP from different processing conditions. For comparison purpose, the HGI values of 10 commercial coals from a domestic 500 MWe power plant were also plotted, which ranged between 39 and 62 [42]. The TTBGI of the raw wood pellets was only 16, which is known to increase the power consumption and also deteriorate the coal grindability when comilled. By the progress of torrefaction, the TTBGI increased almost linearly from 26 at a mass loss of 13.8 % by torrefaction (i.e., TWP yield of 86.2 %) to 71 at 60.54 %. In particular, the mass loss of 20% can attain the minimum HGI value (39) of coal, and the mass loss of 20% becomes comparable to the lower range of most coals. Therefore, the TWP yield of 80% can be targeted in terms of energy density and grindability for pulverized fuel combustion. This can be achieved by various time-temperature combinations depending on the reactor type and heat transfer condition. For co-milling with coal, however, a new test method is required to evaluate the grindability of TWP and coal blends.

80

290°C

Grindability (TTBGI)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 270°C 250°C

40

Holding time: 15 min Holding time: 30 min Holding time: 60 min Raw wood pellets Coals

20

0 0

20

40

60


Fig. 10. Grindability (TTBGI) of torrefied wood pellets vs weight loss.


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Energy & Fuels

4. CONCLUSIONS The poor grindability and low energy density of wood pellets increase the unburned carbon and limits the co-firing ratio with coal in pulverized fuel applications. In this study, improving the fuel quality of wood pellets by torrefaction was investigated using a batch type fixed bed reactor to increase the energy density and to achieve the grindability comparable to that of coal. The torrefied pellet yield decreased from 86.2 to 39.5 % at reaction temperatures ranging between 210 - 310 °C and holding times of 15 - 60 min. Comparing the mass yields to the TGA curves, the effects of peak temperature and holding time depended on a particular heating method and could not be generalized. Therefore, the properties of torrefied products were correlated with the mass yield so that they can be quickly estimated and compared for different heating methods. The bulk density of the torrefied pellets was correlated with the mass yield raised to the power of 0.538, indicating an increase in the intra-particle pores as well as a reduction in the particle size. Combining this with the trend of HHV, the energy density peaked to 10.41 GJ/m3 at a mass yield of 82.6 % but became lower than that of the original pellets (9.35 GJ/m3) when the mass yield was approximately below 60 %. The grindability measured using TTBGI increased almost linearly by torrefaction from 16 for the original pellets and attained the lower range of coal at the mass yield of 80 %. Therefore, the ideal mass yield for pulverized fuel combustion was 80 % that maximized the energy density while attaining satisfactory grindability. This can be achieved by various time-temperature combinations depending on the reactor type and heat transfer condition.

AUTHOR INFORMATION Corresponding Author


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*E-mail:

Page 22 of 27

[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. #These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) affiliated with the Ministry of Trade, Industry and Energy of the Korean Government (Grant No. 20173010092550). REFERENCES (1) Chew, J. J.; Doshi, V. Recent advances in biomass pretreatment–Torrefaction fundamentals and technology. Renew. Sust. Energ. Rev. 2011, 15(8), 4212−4222. (2) Yan, W.; Acharjee, T. C.; Coronella, C. J.; Vasquez, V.R. Thermal pretreatment of lignocellulosic biomass. Environ. Prog. Sustain. 2009, 28, 435−440. (3) Nunes, L. J. R.; Matias, J. C. D. O.; Catalao, J. P. D. S. Torrefaction of biomass for energy applications. From fundamentals to industrial scale. Massachusetts: Academic Press, 2018, 89−124. (4) Basu, P. Biomass gasification, pyrolysis and torrefaction: practical design and theory. 2nd ed. Massachusetts: Academic Press; 2013, 87−145.


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Improving energy density and grindability of wood pellets by dry

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