Coal to biomass (wood pellet) mill conversion

presented by

Corniel Zwaan

Coal Milling Projects

STAR Global Conference, Berlin 2017

Company background

• CMP is a South African company and was formed in December 1997. Celebrating 20 years

this year.

• The company consists of three departments namely:

• Design and CAD department.

• Manufacturing and refurbishing department.

• Maintenance department.

• License agreements:

• Southwestern Corporation – USA

• Merrick Feeders – USA

• Synergy – Australia

• Patented technologies:

• Version 1 to 6 rotating throats (vane wheels, port rings or nozzle rings).

• Ultra high performance static classifiers (UHPSC).

• Static modified air re-entrainment technology (SMART) for Low NOx purposes.

• High ash coal de-sanding technology to remove abrasive materials from coal.

• Coal to biomass mill conversion classifier.

Company background

• CMP’s design department started to use in 2008 as CAE software to do

computational fluid dynamic analysis (CFD) and discreet element modelling (DEM).

• Since May 2014, CMP have a full time design engineer that uses to do

CFD and DEM in house.

• 9 years using

Company background

Presentation content

• Project background

• Project outcomes

• CFD setup

▪ Geometry

▪ Computational mesh

▪ Physics

▪ Results

▪ Lagrangian Multiphase

▪ CFD compared to site tests

• How CFD helped the milling industry

• Questions

Project background

• Drax Power Station (UK) approached CMP to help convert their

Babcock 10 E vertical spindle mills from coal to biomass equivalents.

• 3960 MW Power station (6 Units)

• Replacing coal with biomass pellets as primary energy source.

• The mill is used to break up the pellet into its primary shape called a

“flake”.

• Ultimately replacing coal which is a carbon accumulator, with

biomass, a carbon neutral, to help reduce CO₂ emissions.

Project background

• The modifications imposed before our involvement had the following

issues:

• Could not get the mill throughput up (maximum load required)

• Settling of biomass material in one of the production pipes.

• The accumulative settling effect blocked the pipes causing uneven distribution to the

burners.

• Due to material settling, they continuously need to open the mill to clear the blockages.

• The mill primary air (PA) fan continuously trips on high amp-alarm.

• The mill was not able to run on auto operation.

Project background

Project outcomes

• All biomass flakes (broken pellets) must pass to the burners.

• No passing of biomass pellets to the burners.

• No reject of biomass material into the mill reject boxes

• Increased throughput to accommodate lower calorific value of biomass.

• Equal fuel distribution to the burners.

• Static classifier design.

• Use most of the existing mill components (i.e. the primary air fan)

• Easy to manufacture and maintain.

Biomass with a coal classifier?

CFD SetupGeometry

PA inlet

PA ducting

Biomass classifier

Biomass feed pipe

Loading cylinders

Loading ring

Grinding elements

Biomass build up (bed)

CMP v 4 port ring

Mill table

Plenum chamber

CFD SetupGeometry

CFD SetupComputational mesh

• 10.7 million polyhedral cells

• Volume refinement in areas with high

gradients.

• High Y+ (2 layer) prism layer to solve the

boundary layer.

• Extruded mesh on production pipe

outlet generating a separate region to

incorporate mill back pressure effects.

CFD SetupPhysics

• Steady state analysis

• Reynolds stress model (RSM) to solve turbulence

• Energy extraction field function, used in a cell set, to

simulate heat losses due to conduction and moisture

driven off.

• Extruded region on pipe outlets, set with desired

porous inertial resistances to simulate wind-box back

pressures.

Operating Data Value Units

PA flow rate 21.21 kg/s

PA inlet temp 170 ᵒC

Mill outlet temp 87 ᵒC

Barometric pressure (Ermelo)

101.325 kPa

CFD SetupResults – mill temperature

CFD SetupResults – mill velocities

CFD SetupResults – mill velocities

0 °

CFD SetupResults – mill pressure

With just replacing the existing port ring with a CMP version 4,

the system resistances came down with 30 %.

CFD SetupLagrangian Multiphase – Calibration

0.01

0.1

1

10

100

1000

0.1 1 10 100 1000 10000Dra

g C

oef

fici

ent

Reynold's Number

Drag coefficient data for a sphere vs. a square prism

SN (Sphere)

Stoke's Law

Square Prism (Experimental Data)

Square Formula

P1

P3

P2

𝐷𝑟𝑎𝑔 𝐹𝑜𝑟𝑐𝑒 = 12𝜌𝑉2 × 𝐶𝐷 × 𝐴𝑟𝑒𝑎

𝐺𝑟𝑎𝑣𝑖𝑡𝑦 𝐹𝑜𝑟𝑐𝑒 = 𝑚𝑎𝑠𝑠 × 𝑔

CFD SetupLagrangian Multiphase – particle size distribution

Height/Thickn

ess (mm)

Length =

Width (mm)

Dh (mm)

0.6 5 0.00305983

0.6 4 0.00263688

0.6 3.8 0.00254824

0.6 3.4 0.00236612

0.6 3 0.00217670

0.6 2.6 0.00197864

0.6 2.2 0.00177011

0.6 1.8 0.00154846

0.6 1.4 0.00130959

0.6 1 0.00104645

0.6 0.6 0.00074442

0.6 0.2 0.00035788

0.6 0.08 0.00019429

0.6 0.04 0.00012239

Diameter (mm) Length (mm) Dh (mm)

12 42 0.031666

12 36 0.028573

12 28 0.024166

12 20 0.01931

12 12 0.013737

6 6 0.006868

Flake to sphere

Pellet to sphere

Flake specification

CFD SetupLagrangian Multiphase – setup

Value Units

Number of flake injectors 14 #

Number of pellet injectors 6 #

Particles per injector 300 #

Density of biomass 700 kg/m³

Track time 30 Seconds

Pellet injection plane Flake injection plane Biomass material passed plane

CFD SetupLagrangian Multiphase – flakes animation

CFD SetupLagrangian Multiphase – pellets animation

CFD SetupLagrangian Multiphase – pellets and flakes animation

CFD SetupLagrangian Multiphase – flakes in pipes animation

CFD SetupLagrangian Multiphase – biomass passing

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

100.00 1000.00 10000.00 100000.00

Per

cen

tage

of

inje

cted

par

ticl

es p

assi

ng

to b

urn

ers,

%

Particle Size, hudraulic diameter (µm)

Particle Size Passed

Flakes passing Pellets passing

CFD SetupCFD compared to site tests

Pipe 1Pipe 2

Mill load (Tons/h) Pipe split (%)

Pipe 1 Pipe 2

42 48 % 52 %

CFD maximum load run (incident mass flux

run)

49.05 % 50.95%

• Pipe 1 and 2 temp were within 0.5 °C indicating no settling (on site test).

• System resistance (mill differential pressure) reduced with 50 % (confirmed on site).

How CFD helped the milling industry

Accurately predict Eddy currents that might lead to particle suspensionAccurately predict wear patterns

How CFD helped the milling industry

Optimise patented technology

Conclusions

• STAR-CCM+ played an integrate role in the design process of the biomass classifier.

• The ability to incorporate field functions determined from calibration experiments etc.

• STAR-CCM+ identified areas of focus to help overcome problems that will not be able to be

solved with conventional design methodologies.

• The CFD model correlated well with on site results and this ultimately led to solving the

distribution problems at Drax.

• With the help of STAR-CCM+ we managed to solve all the outcomes of this project and the

mill was able to run on auto for the first time.

• The STAR-CCM+ software is a platform where a set CFD methodology for mills can be applied

and still correlate well with conventional design methodologies and on site test results.

• Ultimately, STAR-CCM+ helped to understand mills better

• Help solve real-world engineering problems.

• Complex Multiphysics solution

Any questions ?

Email: corniel@coalmilling.co.zaTel: +2772 851 0608Website: www.coalmilling.co.za

Thank you