Applicability of

Mechanical Strength

Tests for Biomass

Pellet

Characterisation

Presented by: Dr Orla Williams

Co-Authors: Carol Eastwick, Donald Giddings, Sam Kingman,

Peter Sage, Edward Lester

Scope of Study

Aims: To explore how biomass pellet mechanical strength relates to milling

behaviour

Objective: Determine durability, quasi-static and dynamic strength of biomass

pellets and correlate results to milling energy in 3 different mills; Hardgrove

Grindability Index (HGI), Ring-Roller, and Knife Mill

Wood Pellets Sunflower

Pellets

Eucalyptus

Pellets

Miscanthus

Pellets

Steam Exploded

PelletsMicrowave

Pellets

Pellet Durability Test

• Pellet durability

tested in

accordance

with BS EN

15210-1:2009

• 𝐷𝑢 =𝑚𝐴

𝑚𝐵× 100

• Sunflower and

mixed wood

pellets failed to

meet standards

92 93 94 95 96 97 98 99 100

EUCALYPTUS PELLETS

MIXED WOOD PELLETS

STEAM EXPLODED PELLETS

SUNFLOWER PELLETS

MISCANTHUS PELLETS

Pellet Durability (%)

96.5% 97.5%

Correlation between Durability and Milling Energy

y = 1.4495x - 127.2

R² = 0.9204

y = 1.7657x - 144.95

R² = 0.9957

0

5

10

15

20

25

30

35

92 93 94 95 96 97 98 99 100

Mill

ing

En

ergy

(kW

h/t

)

Durability (%)

Ring-Roller Mill

Ring-Roller Mill Outlier

Knife Mill

Knife Mill Outlier

97.5% Durability Limit

Correlation between HGI and Durability

Eucalyptus Pellets

Miscanthus Pellets

Steam

Exploded Pellets

Olive Pellets

Mixed Wood Pellets

Sunflower Pellets

y = 7.9907x - 763.73

R² = 0.9937

0

5

10

15

20

25

30

35

92 93 94 95 96 97 98 99 100

HG

I

Durability (%)

97.5% Durability Limit

Quasi-Static Mechanical Tests

• Samples measured in 3 orientations axial, diametral and flexure using

Instrom Mechanical Testing Machine

• Stress and strain values obtained from Force-Extension curves

• Test gives low strain rate (1 mm/min) mechanical strength for biomass

pellets

Correlation between Diametral Strain and HGI

Eucalyptus Pellets

Mixed Wood Pellets

Miscanthus Pellets

Sunflower Pellets

Steam Exploded Pellets

y = -0.8005x + 28.63

R² = 0.9432

0

5

10

15

20

25

15 17 19 21 23 25 27 29 31

Stra

in (

%)

HGI

Correlation between Diametral Stress and Ring-Roller Mill Energy

Eucalyptus Pellets

Mixed Wood Pellets

Miscanthus Pellets

Sunflower Pellets

Microwave Pellets

y = 0.1607x + 4.2578

R² = 0.8628

0

2

4

6

8

10

12

5 10 15 20 25 30 35

Dia

met

ric

Stre

ss (

MPa

)

Milling Energy (kWh/t)

Dynamic Mechanical Tests – Split Hopkinson Pressure Bar

SHPB Test

SHPB Slow Motion

SHPB Outputs

• Impact creates strain single which is converted into

voltage output

• Engineering strain is obtained from:

𝑒𝑠 𝑡 = 2𝐶𝑏𝐿𝑠 0

𝑡

𝜀𝑅𝑑𝑡

• Engineering stress is obtained from:

𝑠𝑠 𝑡 =𝐴𝐵𝐴𝑠𝐸𝑏𝜀𝑇(𝑡)

Correlation between Axial Young’s Modulus and Knife Milling Energy

Mixed Wood PelletsMiscanthus Pellets

Sunflower Pellets Microwave Pellets

y = 1.3552x - 6.1374

R² = 0.9461

0

10

20

30

40

50

60

15 20 25 30 35

You

ng'

s M

od

ulu

s (M

Pa)

Milling Energy (kWh/t)

Correlation between Axial Young’s Modulus and Ring-Roller Milling Energy

Mixed Wood PelletsMiscanthus Pellets

Sunflower PelletsMicrowave Pellets

y = 1.1362x + 11.174

R² = 0.9248

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

You

ng'

s M

od

ulu

s (M

Pa)

Milling Energy (kWh/t)

Conclusions

• The pellet durability correlated with HGI (provided the pellet durability was

over 97.5%) and the milling energies from a knife mill and ring-roller mill

• Low durability will result in pellets that are unsuitable for transport and

comminution.

• The compressive strength of the biomass pellets at low and high strain rates

and in multiple orientations was obtained using quasi-static and dynamic

mechanical tests.

• At low strain rates, the biomass pellets showed the highest rigidity in axial

orientations under a uniformly distributed load (axial and diametral

orientations), and poor rigidity when subjected to point loads (flexure test).

• Dynamic mechanical strength and rigidity were highest in the diametral

orientation.

Conclusions

• Pellet strength was greater at high strain rates than at low strain rates.

• Orientation was an important factor in relating mechanical strength to milling

energy.

• Correlations were only observed in the diametral orientation at low strain

rates and in the axial orientation at high strain rates.

• Different torrefaction methods have varying impacts on pellet strength.

• Mechanical tests of biomass pellets can provide an indication of how a pellet

will break down in the mill, but further work is required to enhance their

accuracy for commercially produced biomass pellets.

Thank you for listening

For further information:

Orla.Williams@Nottingham.ac.uk