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Evaluation of two pathways to

produce torrefied pellets

Shahab Sokhansanj

Oak Ridge National Laboratory

2 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Question?

• Is it better to do torrefaction before or after pelletization?

3 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Wood Pellet Production –North America

1

2

3

5

6

7

8

9

Million tons

4 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

5 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

5

Collaborate with wood pellet industry in

developing durable pellets and quality

standards

6 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Advantage of wood pellets over other

forms of cellulosic biomass

• Higher bulk density- Pellets are 2-5 time denser

• Better flowability characteristics – feeding to reactors

• Predictable quality and thermal performance

• Access and upgrade low quality low cost residues and wastes

• A good form of feedstock for biochemical conversion

But pellets are

• Susceptible to moisture ingress and damage

• Easily become dusty during handling

• Low in energy density (MJ/kg)

7 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Torrefaction is a solution to a few of

problems with regular pellets

• The calorific value of torrefied pellets increases 10-25%

• T. pellets are hydrophobic – can be stored outside

• T. feedstock takes less energy to grind

• Bio-oil produced from T. feedstock is less acidic and more stable

But

• Torrefied biomass does not bind to form durable pellets

• Increased carbon fraction makes the torrefied biomass more abrasive to press mill.

7

8 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

One possible solution to making pelletize pellets

8

A. Pelletization Before Tarrefaction (PBT)

B. Pelletization After Torrefaction (PAT)

B

A

9 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Process equipment to test the question:

Torrefier BTGA Hammermill grinder

Pellet mill

10 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems 10

Grinding and Pelletizing

Comparison

11 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems 11

Torrefied material produced a

larger fraction of small

particles.

Grinding power input

and particle size

5 x difference in power

12 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems 12

Torrefaction

Comparison

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Torrefaction of woodchips and regular

pellets in BTGA

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 100 120 140 160

Mas

s ra

tio

Time (min)

Chips Pellets

290oC

100oC

The initial mc of pellets was 5.5%. The initial m.c. of wood chips 11.1%.

Not much difference in mass loss ratio

14 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems 14

A. Pelletized

and torrefied

A. Torrefied and

pelletized

Testing hydrophobicity

Immersing pellets in water for two hours

15 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Physical properties of pellets made from

untreated and torrefied wood chips Pellet type Diam.

(mm)

MC

(%)

Particle

density

(g/cm3)

Bulk

density

(kg/m3)

High

heat

value

(MJ/kg)

Durability

DURAL

(% )

Pellets made from untreated wood chips

6.43 6.7 1.16 674 18.82 80.7

Pellets made from torrefied woodchips mixed with 7% wheat flour binder , Temp 260oC

6.47 8.6 1.21 - - 85.0

Regular white pellets torrefied at 260oC

6.28 1.9 1.14 614 21.08 63.9

Regular white pellets torrefied at 280oC

6.12 1.7 1.04 579 21.97 62.0

Regular white pellets torrfied at 300oC

6.12 1.5 0.96 510 23.00 60.9

16 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

0

20

40

60

80

100

120

140

Drying Grinding Torrefaction Pelltization

Ene

rgy

inp

ut

(MJ)

Pelletize before torrefaction (Pathway I) Pelletize after torrefaction (Pathway II)

Pelletize before torrefaction

(Pathway A)

Pelletize after torrefaction

(Pathway BI)

m.c. in

(%)

m.c. out

(%)

Dry matter

loss

(%)

m.c. in

(%)

m.c. out

(%)

Dry matter

loss

(%)

Drying 45 15 0 45 2 2

Grinding 15 10 1 2 2 2

Pelletization 12 10 2 12 10 10

Torrefaction 10 2 27 5 2 27

Overall mass and energy balance

Calculations for 100 kg

Pathway I 3.55 MJ/kg

Pathway II 2.62 MJ/kg

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Conclusions

• Hydrophobic pelletized pellets can be made from regular pellets (good)

• Energy input to make torrefied pellets from regular pellets is higher than pellets made from torrefied wood chips (not so good).

• The rates of torrefaction (mass loss vs. time) for the two pathways are similar (good)

• Untreated ground wood particles are larger than ground torrefied wood chips (good)

• Torrefied pellets made from regular pellets have a lower density and durability (not so good)

18 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Acknowledgement

• The Thermochemical Platform of the Bioenergy Technology Office (BETO), EERE, DOE supported this project.

19 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

Carbon, Hydrogen, Cellulose, Hemicellulose and Lignin content of wood chips

and pellets. Numbers in the parenthesis are (+/-) ranges of at least triplicated

tests

Biomass and

treatments

Carbon

(%)

Hydrogen

(%)

Glucan

(Celulose)

(%)

Hemicellu-

loses

(%)

Acid insoluble

lignin/resi-due

Untreated wood

chips 45.7 6.4

43.2

(0.8)

21.5

(0.4)

31.7

(0.4)

Untreated wood

pellets 46.5 6.4

43.8

(0.6)

21.1

(0.3)

32.3

(0.7)

Pelletised and

then torrefied 51.2 6.0

49.1

(1.0)

6.2

(0.2)

43.3

(0.2)

Torrefied and

then pelletixed 49.5 5.4

54.7

(1.4)

5.2

(0.2)

38.3

(0.5)

20 Managed by UT-Battelle for the U.S. Department of Energy ORNL – Bioenergy Resource and Engineering Systems

20

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

0 50 100 150 200 250 300

Pe

lle

t d

en

sit

y

(g/cm

3)

Die temperature (oC)

Density vs. compacting die temperature

Tooyserkani et al. 2012. Torrefaction and pelletization od steam treatment of soft wood species. Applied Energy

(under review).