pellet feedstock characteristics and pellet...

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Bioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short Course

Pellet feedstock characteristics and pellet quality

Dr. David DeVallance

Assistant Professor & Program Coordinator

Wood Science and Technology Program

Division of Forestry and Natural Resources

West Virginia University


Quality Pellets

What makes a quality pellet?

• Hardwood vs. Softwood?

• Single or Multiple Species?

• Moisture Content Level = 6-8% or 8-12% or 12-15%?

• Particle Size = 1-2mm, 3-5mm, 5-7mm?

• Absence of Bark or Contaminants?

• Consistent Feedstock?

• Luck?


Feedstock and Process Parameters: Overview

Feedstock characteristics, additive, and processing parameters influence pellet strength and durability

Moisture Content

Temperature or Steam

Lubricant

Chemical Composition

Particle Size/Shape

Binders

Feed Rate

Gap Between Roll and Die

Die Dimensions

Die Speed

Feedstock Characteristics Additives

Processing Parameters

Material and Species

Others?


Moisture content (M.C.) in pellet feedstock acts to promote binding and provide lubrication during the pelleting process

Feedstock and Process Parameters: Moisture Content

Journal of Applied Polymer ScienceVolume 102, Issue 2, pages 1445-1451, 28 JUL 2006 DOI: 10.1002/app.24299

http://onlinelibrary.wiley.com/doi/10.1002/app.24299/full#fig3

Wood

Glass transition temperature (Tg)

for wood ≈ 170 oC

Research varies on optimal M.C.

for wood, but is generally anywhere from 6-15%, with some

saying max 12% (Li and Lui 2000, Ohmberger and Thek 2004)

Corn Stover and Switch Grass

Mean glass transition temperature (Tg) ≈ 75 oC for

range of M.C. from 10-20%(Kaliyan and Morey 2009)

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Particle size is important from a flow and mechanical interlocking aspect during the pelleting process

Feedstock and Process Parameters: Particle Size/Shape

More Flow Less Flow

Fibers are difficult to

compress, but

can produce tough pellets

Mixture of particle sizes more optimum?

More Energy Needed?

More Durable Pellets?

Bioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short Coursehttp://pelletheat.org/pfi-standards/pfi-standards-program/

The Pellet Fuel Institute sets some fuel standards

Pellet Quality: Why Do We Care?


Chemical composition can impact resulting properties and quality of pellets

Feedstock and Process Parameters: Chemical Composition

• High combinations of N, S, and Cl are problems during combustion in terms of pollution potential

• Chlorine can also lead to metal corrosion

Chlorine Spec ≤ 300


Biomass feedstock possess a variety of ash contents

Feedstock and Process Parameters: Chemical Composition

http://forages.org/bioenergy/downloa

ds/Bioenergy_Info_Sheet_5.pdf

Ash %

Ash melting temperature

[some ash sintering observed]

(C)

Corn stovera 9.8 - 13 5 b

Sweet sorghum 5.5 b

Sugarcane bagassea 2.8 - 9.4 b

Sugarcane leaves 7.7 b

Hardwood 0.45 [900]

Softwood 0.3 b

Hybrid poplara 0.4 - 2.4 1,350

Bamboo 0.8 - 2.5 b

Switchgrassa 2.8 - 7.5 1,016

Miscanthus 1.5 - 4.5 1,090 [600]

Giant reed 5 - 6 b

Bioethanol b N/A

Biodiesel <0.02 N/A

Coal (low rank;

lignite/sub-bituminous) 5 - 20 ~1,300

Coal (high rank

bituminous/anthracite) 1 - 10 ~1,300

Oil (typical distillate) 0.5 - 1.5 N/A

Source:

Notes:

N/A = Not Applicable.aUpdated using http://www1.eere.energy.gov/biomass/feedstock_databases.htmlb Data not available.

Oak Ridge National Laboratory, Bioenergy Feedstock Development Program.

Bioenergy Feedstocks

Liquid Biofuels

Fossil Fuels

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Binders can be used to increase pellet durability and reduce dust and fines produced during handling

Feedstock and Process Parameters: Binders and Lubricants

• Typical binders include: calcium lignosulfonate, starches (i.e., corn starch), proteins, molasses, gluten,

distiller grain, vegetable oils, etc.

• Binders add addition cost and also can impact fuel

quality (i.e., higher heat value) and by-product content

• The most common lubricant is vegetable oil

• In general, less likely to need lubricants when dealing with softwoods than hardwoods

• Lubricants reduce friction between particles and die walls Bioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short Course

So We Care About Durability, Ash Content, Moisture Resistance, Chlorine….

What about heat values?

Can we also improve durability at the same time?


Pretreatment: Means to Improve Pellet Quality

While previously mentioned characteristics can impact pellet quality, what about instead using some pretreatment

methods?

Hot water extractionHemicellulose extraction

from hardwood (Xylans)

Different conditions of Temperature and time.


Red Oak & Yellow Poplar hot water extracted145°C - 45 min

160°C - 45 min160°C - 90 min

170°C - 90 min

Study 1: Investigation of surface energy of hot water extracted red oak and yellow-poplar (Oporto et al. 2012)

Looked at Surface Energy: The energy required to form a unit area

of new surface at the interface

(i.e. ��

��)

Hot-Water Extraction: Methods

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Hot water extraction increased the surface energy of both red oak and yellow-poplar feedstock (to a point)

Hot-Water Extraction: Findings – Surface Energy


Hot water extraction increased the compression strength (i.e., hardness) of both red oak and yellow-poplar feedstock

Hot-Water Extraction: Findings – Hardness

Red Oak

Yellow-poplar


Hot water extraction process resulted in pellets that were more resistant to moisture

Red Oak – No Treatment

Original Soaked for 20s Original Soaked for 10 min.

Red Oak – 160 oC for 90 min.

Original Soaked for 20 min.

Red Oak – 170 oC for 90 min.

Hot-Water Extraction: Findings – Water Resistance


Findings:

• Hot-water extraction increased the surface energy of the

red oak and yellow-poplar feedstock

• Hot-water extraction increased the compressive strength

of the red oak and yellow-poplar feedstock

• Hot-water extraction improved the water resistance of the

red oak and yellow-poplar pellets

Hot-Water Extraction: Summary

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Study 2: Investigation of increasing HHV of feedstock through torrefaction and pelleting of red oak

Research Objectives included determining:

• The effects of torrefied feedstock particle size and moisture

content on pellet hardness

• The effects of moisture content, particle size, and torrefaction

level (250 oC vs 300 oC) on compaction behavior (i.e., work or

energy)

• The effect of addition of binder to HHV and durability of

torrefied pellets


Torrefaction: Why Pelletize

Torrefied biomass has poor handling properties: porous brittle structure, low bulk density, dust, etc.

Figure 3. SEM images (x500) of (a) raw wood and torrefied wood at the

torrefaction temperatures of (b) 220°c, (c) 250°c and (d) 280 °c

(torrefaction time =1 h) (Chen et al. 2011).

a b c d

Current Solution: Pelletizing and Briquetting


Torrefied Biomass Pellets: Challenges

Pelleting of torrefied biomass is energy intensive (high friction), can results in weaker pellets, and may require

binders

Figure 4. Pellets made from torrefied wheat straw (Stelte W. et al. 2013).

Grind & Pelletize


Methods: Pellet Fabrication

Red Oak Chips were torrefied at 250 oC and 300 oC for 30 minutes using a pilot-scale torrefaction unit at WVU

1mm screen

Particle Size

0.5-0.7 mm

0.7-1.0 mm

Co

nd

ition

Sieve

MC

1.5 %

5.0%

385 oF at 1,000 lbf

for 3

min.

+ Binder

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Methods: Pellet Testing

Final pellets were tested for diametric compression properties and higher heat value (HHV)


Results: Pellet Hardness

Red oak untreated pellets performed extremely better in diametric compression (i.e., hardness) than any of the

torrefied pellets

0.0

266.7

533.3

800.0

RO1 RO2 RO3 RO4 TOR1 TOR2 TOR3 TOR4

Box Plot

Pellet Group

Pe

llet H

ard

ne

ss (

lbf/in

ch

le

ng

th)

R01: Non-Torrefied, 0.7-1.0mm, 5% MC

R02: Non-Torrefied, 0.5-0.7 mm, 5% MC

R03: Non-Torrefied, 0.7-1.0mm, 1.5% MC

R04: Non-Torrefied, 0.5-0.7 mm, 1.5% MC

TOR1: Torrefied, 0.7-1.0mm, 5% MC

TOR2: Torrefied, 0.5-0.7 mm, 5% MC

TOR3: Torrefied, 0.7-1.0mm, 1.5% MC

TOR4: Torrefied, 0.5-0.7 mm, 1.5% MC


Results: Torrefied Pellet Hardness

Torrefied red oak with 0.7-1.0mm particle size at 1.5% moisture content resulted in a statistically significant higher

average hardness

TOR1: 0.7-1.0mm, 5% MC

TOR2: 0.5-0.7 mm, 5% MC

TOR3: 0.7-1.0mm, 1.5% MC

TOR4: 0.5-0.7 mm, 1.5% MC

40.0

66.7

93.3

120.0

TOR1 TOR2 TOR3 TOR4

Box Plot

Pellet Group

Pe

llet H

ard

ne

ss (

lbf/in

ch

le

ng

th)

Without outlier, only difference

was between

TOR3 and TOR4 Bioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short Course

Results: Pellet Compaction Energy

Torrefied red oak required a significantly higher energy to pelletize to 1,000 compaction force vs. non-torrefied red oak

1.5% MC

5.0% MC

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Results: Pellet Compaction Energy

Red oak with lower moisture content required less energy to compact

Lower moisture content torrefied material required more energy to compact

inch-pounds

Joules

(newton-meters)

RO1: 0.7-1mm, 5% MC 157.0 17.7

RO2: 0.5-0.7 mm, 5% MC 153.7 17.4

RO3: 0.7-1mm, 1.5% MC 131.6 14.9

RO4: 0.5-0.7 mm, 1.5% MC 94.5 10.7

TOR1: 0.7-1mm, 5% MC 355.6 40.2

TOR2: 0.5-0.7 mm, 5% MC 343.0 38.8

TOR3: 0.7-1mm, 1.5% MC 392.7 44.4

TOR4: 0.5-0.7 mm, 1.5% MC 377.7 42.7

Work of Compaction (to 1,000 lbf)

Pellet Type


Results: Pellet Hardness

Results indicated a statistically significant difference in average hardness between pellets made from torrefied wood at 250 oC and

300 oC and with and without lignin for 250 oC material

10

100

1000

High T High TL Low T Low TL

Box Plot

Pellet Type

Hard

ness (

lb/in

ch

len

gth

)

High T: 300 oC no lignin

High TL: 300 oC, 8% lignin

Low T: 250 oC, no lignin

Low TL: 250 oC, 8% lignin


Results: Pellet Porosity

SEM images show obvious lower porosity compared to torrefiedred oak chips and some indications of lignin presence within

pellet

Without Binder

With 8% Binder Bioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short Course

Results: Pellet HHV

Results indicated there was not a statistically significant difference in average higher heat value between pellets with vs.

without lignin, but was between pellets made from torrefied wood

at 250 oC and 300 oC.

1000

10000

High T High TL Low T Low TL

Box Plot

Pellet Type

HH

V (

Btu

/lb

)

High T: 300 oC no lignin

High TL: 300 oC, 8% lignin

Low T: 250 oC, no lignin

Low TL: 250 oC, 8% lignin

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Results: Pellet HHV

Additionally, as weight loss is increased during torrefaction, HHV values are higher

Sample

Weigth Loss

(%) Binder

Higher Heat Value

(HHV), BTU/lb.

44.3 No 10,549

No 9,940

Yes 9,647

No 9,064

Yes 8,830

300 oC for 30 Minutes

250 oC for 30 Minutes

33.7

14.1


Summary

Findings:

• Pelletization of torrefied red oak resulted in pellets with

lower hardness, as compared to red oak

• Pelletization of torrefied red oak required more energy

than red oak

• Torrefied red oak with a lower MC required more energy to

pelletize

• Higher torrefaction levels resulted in pellets with lower

hardness, but higher BTU values

• The addition of 8% lignin as a binder did not improve

torrefied red oak pellet quality or BTU values


Future Work

The results of this research are being used to scale up pelletization research on full-size pelleting machine and

selection of lubricants and binders

• Investigate binders

• Look at L/D ratio effects

• Pellet RPM effects

• Energy use

• Moisture content

• Particle size

• Mixtures with coal

• Other biomass Bioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short CourseBioenergy Pellets and Power Short Course

Questions

Biomaterials and Wood Utilization Research Center(http://www.wdscapps.caf.wvu.edu/BioMatWURCtr)

http://forestry.wvu.edu/faculty_staff/david_devallance

Funding provided by: WVU Advanced Energy Initiative (AEI) Grant Program

Acknowledgements: Dr. Gloria Oporto, Juan Carlos Carrasco Moraga, and Tianmiao Wang (WVU)

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References

Chen W., Hsu H., Lu K., Lee W., Lin T. 2011. “Thermal pretreatment of wood (Lauan) block by torrefaction and its influence on the properties of biomass.” Energy, 2011(36): 3012-3021.

Kaliyan, N., and R.V. Morey. 2009. Densification Characteristics of Corn Stover and Switchgrass. Transactions of the ASABE. 52(3):907-920.

Li, Y., and H. Liu. 2000. High-pressure densification of wood residues to form an upgraded fuel. Biomass and Bioenergy 19: 177-186.

Obernberger, I., and G. Thek. 2004. Physical 61haracterization and chemical composition of densified biomass fuels with regard to their combustion behavior. Biomass and Bioenergy 27: 653-669.

Oporto, G.S., R.H. Jara, D. DeVallance, T. Wang, and J. Armstrong. 2012. Pre-treatment of Appalachian woody biomass for enhanced biofuel properties – Part I. Hot water extraction and pelletizing. Submitted to Biomass and Bioenergy. Manuscript Reference Number: JBB-D-12-01038.

Stelte, W., Holm, J.K., Sanadi, A.R., Barsberg, S., Ahrenfeldt, J., Henriksen, U.B. 2011. A study of bonding and failure mechanisms in fuel pellets from different biomass resources. Biomass Bioenergy, 35: 910–918.

Van Loo, S., and J. Koppejan. 2008. The handbook of biomass combustion and co-firing. 2008 ed. London, Earthscan. 442 pp.

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