Production of “high quality torrefied wood pellets” with minimum energy consumption

Is it better to do torrefaction before or after pelletisation?

Bahman  Ghiasi,  Shahab  Sokhansanj,  Jim  Lim,  Linoj  Kumar    Email:  bghiasi@chbe.ubc.ca/  shahabs@chbe.ubc.ca    

Outline of the presentation •  Why torrefied biomass? •  Why to densify torrefied biomass? •  How to combine densification and torrefaction

to make high quality torrefied pellets? •  Comparison of two different process pathways

— Detailed energy and mass balance

— Comparison of the wood pellet properties

•  Take home messages

Torrefaction improve the biomass properties to be used as an energy/chemical commodity

• Can be co-fired efficiently with coal —  Torrefaction can enhance the calorific value closer to coal —  Torrefied particles are brittle – Easy to grind with less energy

consumption

• Improves biomass properties for other applications —  Torrefied biomass based bio-oil is less acidic and more stable

compared to the raw biomass — Higher carbon content can lead to a better quality syngas production

•  Easy to store ―  Torrefaction can enhance the hydrophobic properties of biomass, can be stored even in an

open environment

•  Produce homogenous fuel ―  Upgrade low quality residues and wastes

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Torrefied biomass currently has some limitations

• Density of torrefied biomass is lower and challenging to transport economically over long distances

• Need to be densified for the tradability as an energy commodity

• Hard  to  bind  during  densifica1on    

Why to densify torrefied biomass?

• Increase in bulk density • Better flowability characteristics – feeding to reactors • Predictable quality and thermal performance But torrefied biomass particles should be densified • With minimum energy consumption • The resulting pellets must be stable and durable • Must maintain the higher properties of torrefied biomass (high energy value, less susceptible to moisture ingress and mechanical damage)

How to make torrefied pellets?

• Can we densify torrefied biomass? • Torrefied biomass is not easy to bind as the raw biomass particles

• High energy or the use of binder seems to be required to densify the torrefied biomass

• Alternatively, can we torrefy densified biomass? • Can we maintain the physical compaction of the pellets if we torrefy them?

• Can pellets be torrefied as effective as wood chips?

One possible solution to making Torrefied pellets

A. Pelletization Before Tarrefaction (PBT)

B. Pelletization After Torrefaction (PAT)

B  

A  

Which is better?

Process instruments used for the work

Torrefaction unit (BTGA)

Hammer mill grinding unit

Pelletisation unit

Drying unit

Grinding of raw wood chips vs. torrefied wood chips

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Comparison

Grinding of torrefied wood chips was easier compared to grinding wood chips

10 5 – 6 times less energy for grinding torrefied wood chips

Grinding torrrefied material produce more fines!

Pelletizing of raw biomass vs. torrefied biomass

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Comparison

Pelletisation of torrefied material was energy intensive, but use of binder minimised the energy consumption

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Opera1ng  Time  [sec]  

Torrefied  biomass  with  the  use  of  binder  

Raw  biomass  

Torrefaction

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Torrefaction of raw biomass vs. pellets

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Torrefaction of woodchips and regular wood pellets are fairly similar

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Mass loss-wood chips Mass loss - pellets temperature

Overall energy/mass balance

Pathway  I  (PelleJzaJon  before  torrefacJon)  

   Unit  energy    required  KJ/Kg  

Ini1al    mass  (Kg)  

Consumed    energy  kJ  

Drying     1327.7   1   1327.7  Grinding     291.9   0.71   206.1  Pelle1za1on   756.9   0.65   492.0  Torrefac1on   522.9   0.62   322.1  Total           2347.9  

Pathway  II  (PelleJzaJon  aRer  torrefacJon)  

   Unit  energy    required  KJ/Kg  

Ini1al    mass  (Kg)  

Consumed    energy  kJ  

Drying   1327.7   1   1327.68  Torrefac1on     545.1   0.706   384.86  Grinding     39.1   0.525   20.53  Pelle1za1on   461.1   0.589   271.58  Total           2004.66  

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A.  PelleJzed  and  torrefied  

B.  Torrefied    and  pelleJzed  

Hydrophobicity of pellets: Immersing pellets in water

Physical properties of pellets made from untreated and torrefied wood chips

Pellet  type   MC  (%)  

ParJcle  density  (g/cm3)  

High  heat  value  (MJ/kg)  

Durability  DURAL  (%  )  

Pellets  made  from  untreated  wood  chips      

6.7   1.16   18.82   80.7  

Regular  white  pellets  torrefied  at  260oC    

1.9   1.14   23.2   63.9  

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

8.6   1.21   21.0   85.0  

Chemical composition of the pellets made from the two pathways

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)  

PelleJsed  and  then  torrefied    

51.2   6.0  49.1  (1.0)  

6.2  (0.2)  

43.3  (0.2)  

Torrefied    and  then  pelleJxed  

49.5   5.4  54.7  (1.4)  

5.2  (0.2)  

38.3  (0.5)  

Application of pathway 1 for agri-biomass torrefied pellet production

There are several concerns regarding densification of agricultural biomass

• Agri biomass has less bulk density and need more compression for pelletization

• Some agri biomass coated with waxy materials that prevent proper pelletization

• Conditioned ground agri-biomass loss moisture faster than the woody biomass

 

Torrefied Agri-Biomass pellet samples at 260oC for 15 minutes

Switch  grass   Corn  Stover   Miscanthus     Wheat  straw  

Densification of raw material production of denser pellet

 Effect of pretreatment 1. Steam explosion pretreatment 2. Effect of die thickness on pellet density 3. Effect of size distribution on pellet density

 

Steam explosion of raw materials

•  Denser pellet •  Durable pellet •  Reduce the energy require for grinding •  Lowers energy required for densification •  Faster drying

 

Douglas fir and Switch grass pellet quality in different stages

SAMPLE HHV MG/Kg

DENSITY g/cm3 Durability

Torrefied steam treated D F pellet 22.4 1.23 99.2

Torrefied steam treated SG pellet 21.3 1.2 99

Torrefied D F pellet 23.2 1.14 98.4

Torrefied SG pellet 21.8 1.05 98.2

steam treated D F pellet 20.3 1.29 99.7

steam treated SG pellet 19.2 1.3 99.3

SG pellet 17.3 1.15 97.8

DF pellet 18.8 1.17 98.1

Effect of die thickness on pellet density

• Effect of L/D of die on compression of the materials

1. Densification of woody biomass 2. Densification of agri-biomass

• Expanded surface are for more friction 1. Increase surface temperature 2. Increase moisture content drop 3. Help lignin replacement properly

Key conclusions

• It was very difficult to bind the torrefied material with reasonable energy consumption

• The use of binder minimized the energy required for pelletizing torrefied wood chips

• Alternatively, torrefied pellets can be made from torrefying regular wood pellets with reasonable energy consumption. The resulting torrefied pellets had many promising properties (Enhanced energy value, higher hydrophobicity, higher carbon content etc.)

• Pretreatment prior to densificacion can help to produce more denser pellet for torrefaction.

Questions? Thank you!