Detailed Investigation into Torrefaction of Wood in a Two-Stage

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A Detailed Investigation into Torrefaction of Wood in a Two-Stage Inclined Rotary Torrefier David Alejandro Granados, Prabir Basu, Farid Chejne, and Daya Ram Nhuchhen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02524 • Publication Date (Web): 25 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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A Detailed Investigation into Torrefaction of Wood in a Two-Stage Inclined Rotary Torrefier D.A. Granadosa, P. Basub*, F. Chejnea and D. R. Nhuchhenb, a

TAYEA Group, Faculty of Mines, National University of Colombia, Colombia

b

Mechanical Engineering Department, Dalhousie University Halifax, Canada *[email protected]

Abstract: A two-stage, inclined continuous rotary torrefier with novel flights has been developed in the Biomass Conversion Laboratory at Dalhousie University for improving biomass torrefaction process. Experimental work on torrefaction of small Poplar wood particles (0.5-1.0 mm) in the torrefier was undertaken for a deeper understanding of the working of such torrefiers where the volatile gas released was used as the torrefaction medium instead of nitrogen. The rotary torrefier is operated under different operating conditions by varying its rotational speed, tilt angle and temperature. Measured chemical and physical properties of the torrefied products included ultimate and proximate analysis, structural analysis, energy density, mass yield, energy yield, and bulk density. A novel probe was developed to collect samples of biomass and measure temperature at different interior points along the length of the rotary torrefaction reactor while the biomass was being progressively torrefied in it. Axial temperature distribution of the rotary torrefier showed a parabolic profile but the fixed carbon content, volatile and energy density of biomass undergoing torrefaction varied linearly along the length of the torrefier. For torrefaction at 300°C and 5 RPM and 1° of tilt angle the change in heating value was 40%, while the mass yield and energy yield of torrefied biomass were 34% and 48% respectively. Results showed that temperature is the most important parameter in this torrefaction process. 1

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Keywords: Biomass torrefaction, Rotary torrefier, Volatiles atmosphere, small biomass particles 1. Introduction Owing to its negligible greenhouse gas emissions, abundance, low costs, and acceptable performance in thermal processes, biomass emerged as a promising energy alternative to fossil fuels. Biomass, however, suffers from a number of limitations such as its high moisture content, low mass density, low energy density and rapid dry mass losses due to bacterial decompositions, which may discourage its use as an energy source. This, in turn, brings additional disadvantages in a production and supply chain management including those in transportation and handling 1. Torrefaction is a thermal pre-treatment in the temperature range of 200-300°C, at low heating rates (600 °C). This formation increases with temperature, while carbon dioxide formation remains relatively constant for lignin. Condensable volatiles were not characterized, but from previous works

37–39,42

we can assume it to contain

acidic compounds and aldehydes (C=O), alkenes (C-C), and ethers (C-O-C) and water when torrefied in the temperature range 200-400 °C for hemicellulose 38, phenolic compounds (monomers or oligomers) to the case of lignin, and 5-HMF, HAA, HA and furfural in the case of cellulose 37. Torrefaction primarily involves decomposition of hemicellulose, and cellulose38,39 and mainly carbon monoxide and carbon dioxide are generated through decarboxylation of acid groups attached to this polymers. Condensable volatiles contain water, acid acetic, methanol, furfural, lactic acid, which are formed from acetyl group and methyl monomers bound to polymers, which, during heating are detached by deacetylation and demethylation reactions9,40. Water is formed from the dehydration process of the polymers through the expulsion of hydroxyl groups 9,40. 3.5.

Structural analysis 19


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Structural analysis for selected torrefied biomasses were also performed to determine the final main polymeric compositions such as hemicellulose, cellulose and lignin. From Van Soest tests, the amount of Ash and Extractives can also be estimated. The selected torrefied biomass used for these test was produced at temperatures of 280 °C and 300 °C, with residence times of 7, 9.6 y 16.4 minutes. Results are presented in Figure 8 where one can note the effects of temperature and residence time on the final amount of hemicellulose, cellulose, and lignin of the biomass. Figure 8. Structural analysis for some torrefied biomass. (TXXX-YY, Torrefaction at XXX °C with YY minutes of residence time).

At temperature of 280 °C, we note that the proportion of extractives and hemicellulose in the torrefied samples is reduced by as much as 36% and 87% respectively with the longer residence time, but at a temperature of 300°C these components were nearly absent in the final product for longer residence time. This was expected, because from previous works reported in the literature it was found that this polymer begins its further degradation around 260 °C and accelerates as the temperature increases. A raw biomass with 56.2% cellulose is turned in a torrefied biomass with 68.5% cellulose after torrefaction at 280ºC and with a short residence time. This increase is due to rapid decomposition of hemicellulose, which increased the cellulose percentage initially. For longer residence times and temperature of 300°C the proportion of cellulose reduced to 76% of its initial amount. Meanwhile lignin is increased in proportion, and a lignitic torrefied material is obtained from the torrefaction process, which was expected. This increase of lignin content in the torrefied material can be due to some condensed products of torrefaction process could not be degraded by 24N acid what underlies lignin overestimation. 3.6.

BET analysis

Tests on measurements of the pore area using N2 and CO2 methods were performed for samples torrefied at temperatures of 280 °C and 300 °C. Nitrogen adsorption gives surface areas of overall pores while carbon 20


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dioxide adsorption gives surface areas of narrow micro-pores (< 1 nm). According to measured values shown in Table 2, it not so clear that torrefaction process improves the porosity and superficial area of the product when the temperature and residence time increases. Torrefied samples at low temperature seem to have a decrease in superficial area in comparison with raw sample, maybe due at pore obstruction by some melted phases inside sample during devolatilization process. An increase in the superficial area of the samples was observed when the residence time increases for each temperature. But, from the residence time of 9.6 minutes, the area decreases, which means a unexplained behavior of the analysis equipment or an initiation of some additional plastic behavior, due to the generation of new liquid inside the solid that causes a blockage or plugging of the porosity. This could be a study of future works. In a previous work, Raut

46

noted that the

surface area for the same raw poplar wood was close to zero (~0.0098 m2/gram), which is different to our data shown in Table 2. The maximum surface area found was 2.0 m2/gram with N2 analysis when poplar wood is torrefied at severe conditions. This low value indicates that torrefied poplar wood is not a promising material for adsorptions applications in meso-porous scale. Table 2. Superficial area with meso and micro-porosity for torrefied biomass.

Adsorption with CO2, shows the surface area to be as high as 142 m2 / gram. This indicates that the devolatilization of the material generates largely micro-pores. In the meso-porosity, a pore formation around 3nm was observed for all samples, and in the micro-porosity, the formation of pores occurs in approximately 0.52nm and 0.8nm of pore sizing, being predominant pore formation in first size. This information is important for identification end-use of the torrefied product, because from pore sizes decides which species it can capture for commercial use. 3.7.

Characterization of intermediate products sampled inside kiln

The biomass was sampled inside the rotary kiln while it was still being torrefied, and was analyzed for its structural and proximate composition. This helped study changes in polymeric decomposition, and variations 21


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in volatiles and fixed carbon content during torrefaction. The sampling of intermediate products was conducted for 7 RPM, 1° tilt angle at torrefaction temperature of 300°C. Five samples were obtained inside the kiln for different axial positions (Figure 9). The first sample (collected at 11.5 cm) was withdrawn from a point near the entrance of the rotary reactor while the last sample (61.5 cm) was captured near the reactor exit. Figure 9. Structural analysis (a), proximate analysis (b), and predicted Mass yield and HHV with correlations shown in Figure 6 (c) for torrefied biomass taken inside kiln sample T300-11. (TXXX-YY, torrefaction at XXX °C with YY minutes of residence time).

As seen in this Figure 9a, hemicellulose contents of biomass decreased from 23.6% to 2.3% as biomass moves from front to the rear end of the reactor. This means that the residence time in these operating conditions, were not sufficient to decompose this polymer entirely. It thus produced a less energy dense product because the hemicellulose is the most oxygenated component in the biomass. The lignin, increases its percentage share (not absolute amount) in the biomass as the torrefaction process progresses. This is due to primary reactions of hemicellulose and cellulose, which generate char with aromatic structures, which are very similar to those of lignin. Cellulose decomposes and decreases its amorphous structure, as mentioned above, but its slight increase in the middle of the reactor could be a result of higher temperature. In addition to obtaining information on the structural changes of the material through knowledge of its polymeric compositions, a characterization was also performed by the proximate analysis in order to know the variations of the fixed carbon and volatile material along the reactor length as the process proceeds. This helps to estimate degree of torrefaction or degree of devolatilization of the torrefaction process. As seen in Figure 9b, the volatile matter decreases as it moves down the reactor or as the torrefaction process proceeds. Opposite behavior is noted for the fixed carbon as the biomass is torrefied. From this analysis, it is possible to obtain information about changes in FR during the process, which also increases. The Figure 9c, also shows the values of HHV and MY predicted from correlation (Equations 9 and 10). A good agreement

22


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with experimental values is apparent in Figure 9. These correlations thus, allow one to obtain predictive product qualities for a given set of operating condition. This could help control and manipulation of the process. 3.8.

Comparison with data of large particles

Present results of small particles (0.5 – 1.0 mm ) were compared with those from previously work by Nhuchhen et al. 13 in the same two-stage rotary reactor but with large particles of about 5 mm of the same poplar wood. The torrefaction was conducted in the same atmosphere of released volatiles from biomass, but the evaluation was done for temperatures of 260 °C, 290 °C and 320 °C. Results of two works with different particles sizes but under same torrefaction process are compared in Table 3. Table 3. Comparison between a torrefaction process with poplar for big and small particles 13.

From the above table it can be seen that the torrefaction had a great impact on small particles. At 260 °C, the differences in results between two particle sizes is very prominent. Large particles were not torrefied to the same extent as small particles torrefied at this temperature. A small increase in HHV, fixed carbon and an almost imperceptible decrease in volatile material was noticed in large particles while small particles showed major changes in their final properties. Increases of up 59% in the fixed carbon and 7% in HHV was evidenced. The mass loss in small particles is considerable, approximately 29% compared with 5.7% in the large particles confirms the difference. This is because a thermal gradient between surface and core occurs in large particles, causing a delay in the heating of the material and in its devolatilization process. This means that, to obtain a similar conversion as in small particles large particles must be allowed a longer residence time under the same thermal conditions. This may also suggest that the thermal gradient in large particles may lead to non-uniform conversion at the outer and inner surfaces of large particles. This, however, requires further investigation at different particle sizes. 4. CONCLUSIONS

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Torrefaction of small particles of poplar wood in a novel continuous two-stage, indirectly heated rotary torrefier was investigated under different operating conditions. It investigated the effect of temperature and residence time on the properties of torrefied biomass. Results showed that the torrefaction is this type of reactor is similar to those in other types. Compared to residence time torrefaction temperature has a stronger effect on all properties of the torrefied biomass. Axial temperature distribution of the rotary torrefier showed a parabolic profile due heat losses from two ends. Samples taken from within the reactor shows continuous degradation of the biomass as it moves along the reactor. With increases in temperature, HHV, fixed carbon, mass loss increase also, but with increase in RPM and tilt angle of the rotary reactor an opposite trend is observed because these provided shorter residence time of torrefaction. Important parameters such as HHV and fuel ratio (FR) increased when temperature and residence time were increased. Increments up to 40% for HHV and 485% for FR when temperature and residence time taken the maximum values. Results of different characteristics of the torrefied biomass and process parameters indicate that the torrefaction process in a volatile gases medium could be developed. A comparison of torrefaction of particles (5 mm approximately) with that of small particles in the same rotary reactor under same torrefaction condition showed that the torrefaction is much more effective for small particles, because for large particles, owing to thermal gradient within it do not under go uniform degradation. For a similar conversion level large particles need longer residence time or more severe torrefaction. Acknowledgments The work was performed in the Biomass Conversion Laboratory at Dalhousie University under the guidance of Prof. Prabir Basu, who acknowledge the support of Natural Science & Engineering Council and from Greenfield Research Incorporated of Canada. The first authors thank the Colombian Administrative Department of Science, Technology and Innovation (COLCIENCIAS) (Departamento Administrativo de Ciencia, Tecnología e Innovacion) for providing him with financial support. 24


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FIGURES

(a)

(b) Figure 1. Kiln layout (a) and torrefied biomass collector (b) 29.

Thermocouple

(a)

(b)

Figure 2. Designed scoopers for biomass sampling (a); and temperature measurement (b).

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(a)

(b)

(c) Figure 3. Axial kiln and biomass temperatures inside the kiln. Kiln wall temperatures measured in static conditions and biomass temperature measured at 5 RPM and 2° of inclination (9.6 minutes of residence time). (a) 260°C; (b) 280°C; (c) 300°C. (BT – biomass temperature; WT – Temperature on the inner wall of the torrefier)

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Process severity

Figure 4. Van krevelen diagram for raw and torrefied biomass.

Figure 5. Influence of process temperature in Mass yield and energy yield for all operational conditions evaluated in experiments. (TXXX-YY, torrefaction at XXX °C with YY minutes of residence time).

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Figure 6. Relationship between Mass yield and HHV, FC, and VM.

Figure 7. Products distribution in the process for different temperatures and 11 minutes of residence time.

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Figure 8. Structural analysis for some torrefied biomass. (TXXX-YY, Torrefaction at XXX °C with YY minutes of residence time).

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(a)

(b)

(c) Figure 9. Structural analysis (a), proximate analysis (b), and predicted Mass yield and HHV with correlations shown in Figure 6 (c) for torrefied biomass taken inside kiln sample T300-11. (TXXX-YY, torrefaction at XXX °C with YY minutes of residence time)

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TABLES

Table 1. Proximate (%wt), elemental analysis (%wt), higher heating value (MJ/kg), and structural analysis for all samples.

Raw

260

280

300

RT

VM*

FC*

FR

C**

H**

N**

-

82.40 ± 3.33

16.20 ± 0.39

0.19 ± 0.06

45.90 ± 0.12

6.10 ± 0.23

0.40 ± 0.00

16.40 ± 0.36

73.40 ± 1.63

25.80 ± 0.57

0.35 ± 0.04

49.50 ± 0.22

5.44 ± 0.08

9.60 ± 0.04

76.60 ± 0.27

22.70 ± 0.43

0.3 ± 0.02

49.10 ± 0.01

7.00 ± 0.20

76.70 ± 2.96

22.50 ± 1.10

0.29 ± 0.08

11.00 ± 0.50 75.60 ± 3.43

23.10 ± 0.53

6.60 ± 0.10

76.80 ± 2.35

5.20 ± 0.30

MY

E** H** C** L**

47.60 ± 0.35 18.40 ± 0.10

-

8.5 23.5 56.2 11.7

0.02 ± 0.01

45.10 ± 0.32 19.60 ± 0.20

0.71 ± 0.01

-

-

-

-

5.67 ± 0.11

0.00 ± 0.00

45.30 ± 0.13 18.70 ± 0.12

0.75 ± 0.00

-

-

-

-

49.00 ± 0.43

5.16 ± 0.00

0.01 ± 0.00

45.90 ± 0.44 18.50 ± 0.09

0.80 ± 0.00

-

-

-

-

0.31 ± 0.06

48.10 ± 0.21

5.53 ± 0.08

0.00 ± 0.00

46.40 ± 0.29 19.10 ± 0.21

0.74 ± 0.00

-

-

-

-

22.50 ± 0.88

0.29 ± 0.07

48.50 ± 0.03

5.39 ± 0.03

0.00 ± 0.00

46.10 ± 0.06 19.20 ± 0.12

0.81 ± 0.00

-

-

-

-

77.80 ± 1.79

21.70 ± 1.72

0.28 ± 0.10

49.10 ± 1.88

5.11 ± 0.61

0.00 ± 0.00

45.80 ± 2.49 19.20 ± 0.08

0.82 ± 0.00

-

-

-

-

16.40 ± 0.36

59.10 ± 0.12

39.60 ± 0.95

0.67 ± 0.02

53.60 ± 3.91

4.87 ± 0.19

0.03 ± 0.02

41.50 ± 4.13 22.50 ± 0.33

0.49 ± 0.00

5.5

3

9.60 ± 0.04

71.60 ± 1.47

27.40 ± 0.36

0.38 ± 0.03

49.00 ± 1.49

5.21 ± 0.25

0.05 ± 0.00

45.80 ± 1.75 20.40 ± 0.19

0.59 ± 0.00

6.9

4.2 67.3 21.6

7.00 ± 0.20

74.90 ± 3.33

24.10 ± 1.06

0.32 ± 0.08

50.00 ± 2.41

5.33 ± 0.19

0.04 ± 0.02

44.60 ± 2.63 19.80 ± 0.10

0.68 ± 0.00

7.3

4.6 68.5 19.5

11.00 ± 0.50 70.60 ± 0.03

28.60 ± 0.18

0.41 ± 0.00

48.50 ± 1.45

4.80 ± 0.03

0.11 ± 0.01

46.60 ± 1.49 21.50 ± 0.40

0.62 ± 0.00

-

-

-

-

6.60 ± 0.10

73.40 ± 3.50

25.50 ± 1.24

0.35 ± 0.09

50.00 ± 0.90

5.15 ± 0.07

0.29 ± 0.06

44.60 ± 1.04 19.80 ± 0.20

0.68 ± 0.00

-

-

-

-

5.20 ± 0.30

74.90 ± 0.05

24.50 ± 1.69

0.33 ± 0.06

48.10 ± 0.34

5.01 ± 0.00

0.28 ± 0.02

46.60 ± 0.37 19.60 ± 0.21

0.71 ± 0.00

-

-

-

-

16.40 ± 0.36

45.90 ± 2.26

52.70 ± 1.24

1.15 ± 0.07

58.50 ± 4.25

3.57 ± 0.29

0.50 ± 0.01

37.50 ± 4.57 25.80 ± 0.30

0.34 ± 0.00

0

0

9.60 ± 0.04

57.70 ± 3.72

41.20 ± 0.46

0.71 ± 0.07

53.40 ± 2.88

4.62 ± 0.87

0.64 ± 0.07

41.40 ± 3.82 23.70 ± 0.11

0.44 ± 0.00

1.5

0.3 39.2

7.00 ± 0.20

64.90 ± 0.92

33.60 ± 1.17

0.52 ± 0.04

56.70 ± 0.07

4.59 ± 0.03

0.80 ± 0.03

38.70 ± 0.14 21.10 ± 0.15

0.54 ± 0.00

7.6

2.7 54.9 34.9

11.00 ± 0.50 52.60 ± 2.27

46.10 ± 0.46

0.87 ± 0.05

52.20 ± 0.42

3.74 ± 0.03

0.90 ± 0.00

42.40 ± 0.46 23.90 ± 0.23

0.38 ± 0.00

-

-

-

-

6.60 ± 0.10

37.90 ± 0.68

0.62 ± 0.03

53.10 ± 0.98

4.35 ± 0.16

0.64 ± 0.03

41.90 ± 1.17 21.80 ± 0.14

0.48 ± 0.00

-

-

-

-

5.20 ± 0.30 69.40 ± 1.92 29.40 ± 1.23 * Analysis in dry basis. ** Analysis in dry and ash free basis

0.42 ± 0.06

50.60 ± 0.08

4.77 ± 0.09

0.73 ± 0.02

43.90 ± 0.20 20.00 ± 0.32

0.59 ± 0.00

-

-

-

-

61.10 ± 1.29


O**

HHV*

44.9 46.6

13.3 86.7 59

34

Page 35 of 35

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Energy & Fuels

Table 2. BET pore surface area for raw and torrefied biomass as measured in nitrogen and carbon dioxide methods. Sample

N2 (m2/g)

CO2 (m2/g)

Sample

N2 (m2/g)

CO2 (m2/g)

Raw

1.33 ± 0.07

65.81 ± 1.22

T300-5.2*

0.66 ± 0.02

68.32 ± 0.54

T280-5.2*

0.48 ± 0.04

50.40 ± 0.50

T300-6.6*

0.48 ± 0.01

81.02 ± 0.76

T280-6.6*

0.68 ± 0.04

50.11 ± 0.23

T300-7.0*

1.46 ± 0.01

97.14 ± 1.04

T280-7.0*

0.76 ± 0.01

53.30 ± 0.74

T300-9.6*

2.11 ± 0.01

142.35 ± 2.50

T280-9.6*

0.92 ± 0.02

60.28 ± 0.55

T300-11*

1.7 ± 0.18

141.16 ± 1.01

T280-11*

0.51 ± 0.02

79.31 ± 0.98

T300-16.4*

1.14 ± 0.01

125.59 ± 1.50

T280-16.4*

0.67 ± 0.02

108.69 ± 1.50

*TXXX-YY, torrefaction at XXX °C with YY minutes of residence time

Table 3. Comparison between a torrefaction process with poplar for large and small particles. Small particles (0.5 – 1 mm)

Large particles (5 mm) 13 Properties T260-16.4*

T290-16.4*

T320-16.4*

T260-16.4*

T300-16.4*

HHV (% Increase)

4.1%

8.3%

13.6%

7.2%

40.50%

FC (% Increase)

6.1%

31.57%

73.7%

59.5%

225%

FR (% Increase)

6.6%

36.7%

90.8%

78.7%

485%

VM (% Decrease)

0.5%

3.7%

9.0%

10.9%

44%

MY

94.3

90.3

82.9

71

34

EY

98

97.3

96.1

76

48

*TXXX-YY, torrefaction at XXX °C with YY minutes of residence time


35

Detailed Investigation into Torrefaction of Wood in a Two-Stage

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