2015 BF2RA Technical Session

On Biomass Milling for Power Generation

Presenter: Orla Williams, University of Nottingham

Co-Authors: Dr Carol Eastwick, Prof Ed Lester, Dr Donald Giddings, Prof

Ed Lester, Mr Stephen Lormor (EDF Energy plc)

Tuesday 6th October 2015

BF2RA

BF2RA

4 year EngD with BF2RA completed in September

2015

Part of the Efficient Fossil Energy Technologies

Centre at the University of Nottingham

Research theme: biomass milling in the power

generation sector

Academic supervision: Dr Carol Eastwick & Dr

Donald Giddings (University of Nottingham)

Industrial supervision: Stephen Lormor EDF Energy

Project Outline

BF2RA

How different woody, herbaceous, fruit and thermally

treated densified biomasses comminute in different mills

How mills impact densified biomass particle shape

factors

Suitability of standard coal grindability test for densified

biomass

Differences between grindability tests and classification

on mill product

Investigation of industrial operational issues associated

with biomass, such as mill choking and olive caking

Key Research Areas

BF2RA

Samples UsedMixed Wood Eucalyptus Sunflower Miscanthus

Olive Cake Steam Exploded Microwave La Loma Coal

BF2RA

Planetary Ball Mill

Laboratory scale Retsch PM100

planetary ball mill

High impact compression forces

100ml of sample milled at 300 RPM

for 3 minutes

BF2RA

Cutting Mill

Retsch SM300 cutting mill

Operated at speed of 1500

RPM, feed rate of

0.0025kg/s, screen size of 4mm

Continuous throughout mill fed

by vibrating bed feeder

Cutting mills use primarily shear

forces to reduce the particle

size of the material

BF2RA

Hardgrove Grindability Index

Hardgrove Grindability

Index (HGI) most widely

used grindability test for

coal for vertical spindle

mills

Test conducted according

to BS 1016-112:1995 [1]

Limited data for

grindability of densified

biomass

BF2RA

Bond Work Index

Test conducted on a Bond Work

Index testing mill

The Bond Work Index Wi expressesthe resistance of the material to

grinding to a specified product size

(kWh/ton) [2]

Study recently published in journal

Fuel “Investigation into the

applicability of Bond Work Index

(BWI) and Hardgrove Grindability

Index (HGI) tests for several

biomasses compared toColombian La Loma coal” [3]

BF2RA

Ring-Roller Mill

The Lopulco LM 1.6

laboratory scale ring-

roller mill

Continuous through put

mill with dynamic

classification

(separator)

No biomass

comminution literature

published for this type

of laboratory scale mill

BF2RA

Standard Coal Grindability Tests

BF2RA

Energy Consumption Comparison

BF2RA

Laboratory scale milling tests assess the grindability of a

material to a given size

Full scale mills use classifiers to select particles

Stokes number of a biomass particle should match that of an

equivalent coal particle in order to pass through the classifier

[4].

Stokes number is dependent on the sphericity and Reynolds

number of the particle

Possible for large biomass particles to have similar

aerodynamic properties to that of a much smaller coal

particle provided it has a sufficiently low sphericity

Particle Size and Shape

BF2RA

Camsizer® Digital Analysis

CAMSIZER P4

simultaneously

measures both

particle size and

shape

CAMSIZER P4 uses

Dynamic Image

Analysis principle

to detect each

particle and

record its size and

shape

BF2RA

Mixed Wood Pellets Particle Size

BF2RA

Mixed Wood Pellets Particle Shape

Characteristic Pre-milledPlanetary

Ball Mill

Bond

Work

Index

Cutting MillRing-Roller

Mill

Particle Size d80

1373 µm 975 µm 787 µm 1105 µm 1201 µm

Sphericity Q50

0.347 0.381 0.387 0.384 0.403

Symmetry

Q50

0.821 0.769 0.766 0.796 0.764

Aspect Ratio

Q50

0.471 0.519 0.527 0.522 0.548

Circularity

Q50

0.502 0.427 0.465 0.509 0.433

BF2RA

Eucalyptus Pellets Particle Size

BF2RA

Eucalyptus Pellets Particle Shape

Characteristic Pre-milledPlanetary

Ball Mill

Bond

Work

Index

Cutting MillRing-Roller

Mill

Particle Size d80

1215 µm 705 µm 751 µm 1171 µm 958 µm

Sphericity Q50

0.404 0.398 0.409 0.409 0.382

Symmetry

Q50

0.804 0.789 0.764 0.810 0.794

Aspect Ratio

Q50

0.552 0.540 0.554 0.555 0.520

Circularity

Q50

0.551 0.506 0.466 0.555 0.500

BF2RA

Miscanthus Pellets Particle Size

BF2RA

Miscanthus Pellets Particle Shape

Characteristic Pre-milledPlanetary

Ball Mill

Bond

Work

Index

Cutting MillRing-Roller

Mill

Particle Size d80

1251 µm 1181 µm 813 µm 1069 µm 1377 µm

Sphericity Q50

0.324 0.330 0.339 0.355 0.343

Symmetry

Q50

0.826 0.781 0.757 0.783 0.769

Aspect Ratio

Q50

0.437 0.446 0.460 0.482 0.464

Circularity

Q50

0.511 0.457 0.413 0.482 0.441

BF2RA

Sunflower Pellets Particle Size

BF2RA

Sunflower Pellets Particle Shape

Characteristic Pre-milledPlanetary

Ball Mill

Bond

Work

Index

Cutting MillRing-Roller

Mill

Particle Size d80

1744 µm 1220 µm 757 µm 1145 µm 1523 µm

Sphericity Q50

0.347 0.384 0.418 0.397 0.382

Symmetry

Q50

0.831 0.782 0.745 0.803 0.793

Aspect Ratio

Q50

0.470 0.521 0.567 0.540 0.517

Circularity

Q50

0.571 0.479 0.452 0.530 0.496

BF2RA

Olive Cake Particle Size

BF2RA

Olive Cake Particle Shape

Characteristic Pre-milledPlanetary Ball

Mill

Bond Work

IndexCutting Mill

Particle Size d80

3660 µm 1553 µm 589 µm 1461 µm

Sphericity Q50

0.562 0.577 0.490 0.535

Symmetry

Q50

0.886 0.871 0.833 0.870

Aspect Ratio

Q50

0.708 0.714 0.645 0.685

Circularity

Q50

0.806 0.778 0.682 0.781

BF2RA

Steam Exploded Pellets Particle Size

BF2RA

Steam Exploded Pellets Particle Shape

Characteristic Pre-milledPlanetary

Ball Mill

Bond

Work

Index

Cutting MillRing-Roller

Mill

Particle Size d80

1196 µm 466 µm 346 µm 1412 µm 521 µm

Sphericity Q50

0.428 0.454 0.455 0.464 0.445

Symmetry

Q50

0.840 0.785 0.770 0.846 0.823

Aspect Ratio

Q50

0.580 0.608 0.608 0.620 0.600

Circularity

Q50

0.645 0.522 0.506 0.690 0.630

BF2RA

Microwave Pellets Particle Size

BF2RA

Microwave Pellets Particle Shape

CharacteristicPlanetary Ball

Mill

Bond Work

IndexCutting Mill Ring-Roller Mill

Particle Size d80

285 µm 888 µm 1091 µm 538 µm

Sphericity Q50

0.464 0.385 0.414 0.403

Symmetry

Q50

0.763 0.827 0.836 0.819

Aspect Ratio

Q50

0.621 0.524 0.563 0.549

Circularity

Q50

0.520 0.605 0.651 0.597

BF2RA

La Loma Coal Particle Size

BF2RA

La Loma Coal Particle Shape

Characteristic Planetary Ball Mill Bond Work Index Ring-Roller Mill

Particle Size d80

2723 µm 78 µm 402 µm

Sphericity Q50

0.503 0.506 0.403

Symmetry

Q50

0.862 0.796 0.804

Aspect Ratio

Q50

0.656 0.657 0.660

Circularity

Q50

0.579 0.628 0.636

BF2RA

Particle Size & Shape Relationship

BF2RA

Project Findings

Biomass Characterisation

Pre-milled biomass pellets are composed of similar particle sizedistributions, but show a large variance in particle shape

Standard coal abrasion index test does not work for biomass as itcaused the apparatus to catch fire

Standard Coal Grindability Test

HGI is a poor indicator of the grindability of biomass in a verticalspindle mill

Grindability tests which aim to analyse the grindability of biomassto 75µm are inappropriate – should be informed by classifiersettings

BWI can be used replicate mill choking

Biomass pellets should be composed of particles close to therequired size

BF2RA

Olive Caking

Olive caking phenomenon is associated with the pulp section of

the material

Sugar and moisture content of the pulp section are such that

when milled a glass transition step occurs which results in the

caking of the olive

Olive cake fines below 1mm should be sieved out and sent

directly to the burner and not comminuted

Ring-Roller Mill

Strong negative correlation was obtained between comminuted

particle size and shape for mill with separator

Biomass mill classifiers should be set to the Stokes number

requirements for coal rather than a blanket particle size for all

biomasses

Project Findings 2

BF2RA

The critical particle size for comminution through compression is key to

understanding milling behaviour in different mills.

By knowing a materials critical particle size and the target size of the specific

biomass based on the classifier settings for a mill, it will be possible to tell:

i. Whether or not a biomass pellet will break down easily within the mill

ii. If the biomass particles which compose the pellet will need extra

comminution to reach its required particle size for combustion

iii. If the biomass particles will struggle to reduce in size if comminution is

required due to their critical particle size being larger than the target

particle size for combustion

Milling had little impact on particle shape even when an order of magnitude

difference was seen in the particle size.

Particle shape is inherent to a pellet and is determined by the pre-densified

comminution processes.

Overall Project Findings

BF2RA

Follow on Engineering Doctorate currently under way with BF2RA which is

developing numerical models of biomass comminution based on the test results

Development of a new standard test to quantify the abrasiveness of biomass in

pneumatic conveying systems

New HGI test for biomass which incorporates larger volumes and targets s

Models to predict the required particle size for a classifier based on the physical

characteristics of the pre-milled particles to inform grindability test

Need to find the critical particle size for compressed fracture for biomass for all

types of biomasses, and compared to the particle size distributions of

disintegrated pellets

Ring-roller mill: energy consumption and particle size correlations should be

compared to those of the full scale Lopulco ring-roller mill at EDF West Burton

and the pilot scale ring-ball mill at Doosan [5] for standard materials

Future Work

BF2RA

Thank you for listening

The authors would like to thank the University of

Nottingham, the Biomass and Fossil Fuel

Research Alliance (BF2RA), and EDF Energy plc

for their support during this project.

Questions?

BF2RA

[1] The British Standards Institution, 1995. BS 1016-112: 1995 Methods for Analysis and testing of coal and coke — Part 112: Determination of Hardgrove grindability index of hard coal. Milton Keynes: BSI.

[2] Bond FC. The Third Theory of Comminution. Trans AIME 1952;193:484–94.

[3] Williams, O., Eastwick, C., Kingman, S., Giddings, D., Lormor, S., & Lester, E. (2015). Investigation into the applicability of Bond Work Index (BWI) and Hardgrove Grindability Index (HGI) tests for several biomasses compared to Colombian La Loma coal. Fuel.

[4] Mandø, M., Rosendahl, L., Yin, C., & Sørensen, H. (2010). Pulverized straw combustion in a low-NOx multifuel burner: Modeling the transition from coal to straw. Fuel, 89(10), 3051–3062

[5] Livingston, W. R., Horne, P. A., McGhee, B. F., Gibson, J. R., & Chakraborty, R. K. (1998). The Characteristics of Coal Blends -Milling Behaviour & Ash Fusion Temperatures

References