STAR-CCM+ an invaluable tool for coal fired power plant design ...mdx2.plm.automation.siemens.com/sites/default/files/Presentation/SGC... · Case Study – Komati Power ... burner

STAR-CCM+ an invaluable tool for coal fired

power plant design optimization Ignus le Roux, Aerotherm Computational Dynamics & Warwick Ham, Steinmüller Africa Bilfinger | 2014 STAR Global Conference

Boiler Process

18 – 19 March 2014

Presentation Structure

▪ Aerotherm Computational Dynamics & Bilfinger Steinmüller Africa History

▪ Problem Statement

▪ Secondary Air Flow Measurement Error

▪ Combustion Instability

▪ Case study – Komati Power Station Unit 3

▪ Model creation

▪ CFD Revision of the burner flow measurement philosophy

▪ Flow only evaluation to improve burner stability

▪ Drop Tube Furnace Modelling - char burnout rate validation

▪ Full Furnace Combustion Model

▪ Conclusion

▪ Further development

▪ Questions

Aerotherm CD & Bilfinger Steinmüller Africa History

▪ Aerotherm CD have been supporting Steinmüller Africa with Computational Fluid Dynamic Simulation

Services for the past 8 years (since 2006).

▪ Projects varying from burner optimization studies, detail coal combution modelling to boiler erosion analyses.

▪ Projects performed initially using STAR-CD and since 2008 solely in STAR-CCM+

Case Study – Komati Power Station

▪ Komati Power Station, is a coal-fired power plant operated by Eskom.

▪ The station is situated between the city of Middelburg and town of Bethal in South Africa’s Mpumalanga

province.

▪ Technical details:

▪ Five 100MW units

▪ Four 125MW units

▪ Installed capacity: 1,000MW

▪ The first unit was commissioned in 1961 and the last in 1966.

▪ In 1988, three units at Komati were mothballed, one was kept in reserve and the other five were only

operated during peak hours. In 1990 the complete station was mothballed.

▪ In 2008 unit 9 was the first to be recommissioned under Eskom's return to service project.

Case Study –Komati Power Station Unit 3

Implementation - Background

▪ There are 12 Pulverised Coal Burners in a common

windbox firing through the rear furnace wall of the

boiler.

▪ The original Pulverised Fuel burners were of 1950’s

design

▪ During RTS (Return To Service) the old PF burners

were found to be severely worn.

▪ The old burners used Light Fuel Oil for ignition and

Komati was to be converted to Heavy Fuel Oil to save

operating costs.

▪ It was noted that the original furnace burner openings

were smaller on units 1-3 than on other units, but could

not be replaced due to financial constraints.

▪ The old burners were to be removed and replaced with

new PF burners.

▪ The new burners were designed, manufactured and

installed in units 1-3 during 2011.


Background – Where this started

Unit 3 Firing Floor showing Common Windbox and 12 Original PF Burners



Photo of original Komati PF Burners – Top firing floor view, burner in situ with oil burner



Photo of original Komati PF Burners – View from inside windbox Note wear on Swirl vanes


Implementation

Photo of New Komati PF Burners – View from firing floor during installation


Implementation

Photo of New Komati PF Burners – View from firing floor during operation

Case Study – Komati Power Station Unit 3

Problem Statement

Secondary Air Flow Measurement Error.

▪ At low flow settings featuring small cylindrical damper openings, a lower pressure is

measured at the back plate than in the burner throat producing complications with the

flow rate calibration.


Model Creation

▪ The Komati PS Unit 3 features 12 swirl burners fired from the rear wall.

▪ Sufficient 2D drawing were available to create the model of the furnace, the detailed burners and the

common windbox (incl. internal structures).

▪ However no geometric information was available of the ducting from the outlet of the regenerative air

preheater up to the entrance to the common windbox.

▪ This section needed to be included in the analysis as it has a prominent influence on temperature and flow

distribution in the windbox.


Model Creation

▪ The world class spatial laser scanning capabilities of Venter Consulting Engineers (Pty) Ltd. were used to

capture the missing detail.

Raw point cloud scan data


Burner Flow Measurement Philosophy

▪ Flow through the swirl burners are governed by adjusting the offset of cylindrical dampers from the burner

back plate.



▪ The burner flow rate is based on the pressure differential measured across a specific burner.

▪ The upstream pressure is measured at a single position against the burner back plate.

▪ The downstream pressure is based on the average of six pressure measurements measured in the burner

throat.



▪ At low flow settings featuring small cylindrical damper openings, a lower pressure is measured at the back

plate than in the burner throat producing complications with the flow rate calibration.

General damper position Small damper position



▪ The high amount of internal detail included in the burner windbox model allowed the cause of the negative

burner dP measurement to be identified and allowed Aerotherm CD and Steinmüller Africa to determine an

optimal upstream pressure measurement location.

▪ The recommendation was made to replace the single pressure measurement against the burner back plate

with a small diameter pipe located inside a local stagnation region with several openings distributed around

the burner effectively obtaining an average pressure measurement.



▪ It is clear that measurements in the proposed location will yield a positive pressure drop for all damper

positions evaluated.

General damper position Small damper position

Pressure at proposed pressure probe location



▪ Certain spatial restrictions on site prevented installation of the measurement probe in the proposed location.

▪ The model was used to develop a conical fence which allowed the implementation of the probe in the

proposed location as it increased the size of the stagnation regions sufficiently.

Conical flow restriction


Problem Statement

Combustion Instability.

▪ Unit 3 went into operation during 2012.

▪ During summer of 2012-2013 coal moisture content rose above design limits (10%

max) and burner stability became problematic.

▪ Several oil burners had to be in service during high and low load operation to support

PF combustion.

▪ ESKOM suggested cutting back PF and core air part to improve mixing between PF

and Secondary Air.

▪ Windbox and burner model used to assess ESKOM proposed burner modifications.

▪ Several burner cutback modifications investigated (9 in total) and presented to ESKOM

on a weekly basis.

▪ Operating requirement exceeds design specification.


Flow Evaluation To Improve Flame Stability

Feedback on implementation successes

Komati Coal – Supply Limits

Unit Limit Value

Jan 2013

Apr 2013

Jul 2013

Mar 2014

Coal CV MJ/kg 20 min 19.7 19.45 18.62 20.33

Ash in coal (as received) % 32 max 28 28.2 29.3 27.3

Volatile matter in coal % 18.6 min 18.5 16.2 19.9 20.7

Total Moisture in coal % 10 max 14.7 20.4 13.6 12.6

PF fineness (passing 300 mm) % 98 min 96.8 95.6 - -

PF fineness (passing 75 mm) % 65 min 62.2 66.7 - -

PF mass flow variance from mean

% 15 max 32.4 -23.2 - -

Mill Outlet Temperature oC 75 min 69.8 82.9 87.4 84.8

Oil Burners in operation n 0 4 2 2 0



▪ Under certain operational conditions, the flame ignition front is located very far from the burner face on some

of the burners in the Komati Power Station Unit 3 Boiler.

▪ To prevent losing the flame in such instances, the station will introduce oil burners to stabilize and maintain

the flame.

▪ As the time scheduled was very tight to evaluate possible burner modifications, the highly detailed windbox

model which extends up to the furnace exit was used to evaluate possible burner modifications which from a

purely flow behaviour perspective could improve the flame stability.



▪ 9 geometric burner modifications were considered to strengthen the reversed flow towards the burner

▪ This draws more hot flue gas closer to the burner

▪ The hot flue gas activates the newly introduced pulverised fuel at the point where maximum velocity and

turbulence forces mixing of the pulverised fuel with the secondary air.

▪ This ensures high heating up rates of the coal particles and stabilizes the flame.

1 Burner return flow

3 2



▪ In the existing burner design, the termination point of the secondary air, primary air and core air sections line

up at the entrance to the cowls section.



▪ A X-Y Plot visualizing the axial velocity component as a function of the radial position component (relative to

the burner axis) on a section at the entrance to the burner cowl provided an excellent tool for comparison of

the different burner modifications’ ability to increase the return flow behind the burner.



▪ The second proposed modification dubbed Mod2 considered changes which would reduce the energy level

in the primary air flow (increase flow area, add diffuser) and increase the energy in the swirled secondary air

flow (reduce flow area). The core air and primary air termination points were trimmed back by 90mm to

create earlier interaction between the fuel carrying primary air and the secondary air.



▪ The Mod2 changes managed to reduce the energy in the primary air annulus by 19.9% whilst increasing the

energy in the secondary air annulus by 11.5%. The increased swirl in the secondary air and the earlier

interaction with the lower energy primary air flow results in an stronger and larger return flow region behind

the burner. However practically it required major burner modification which was not ideal.



▪ The fourth modification dubbed Mod4 considered changes which could be affected without removing the

burner. The modification would however not address any energy imperfections in the different annuli but

would guarantee very early interaction between the fluid stream. The core air and primary air termination

points were trimmed back by 200mm



▪ The Mod4 changes managed through the earlier interaction with the primary air flow to create a much

stronger and larger return flow region behind the burner. The simple implementation of this modification

resulted in this modification being selected to replace the destroyed burner.



▪ Mod4 was implemented, i.e. cut back burner PF pipe and core air tube by 200 mm. Boilers have been

running with stable combustion since April 2013.

▪ When the coal total moisture exceeds the design range of 10% combustion stability is affected and 2 oil

burners are required to be in service to support pulverised fuel stability.

▪ Boiler Pyro positions were moved towards the back wall to allow both pyros to give a stable flame signal.

▪ Unburnt carbon in ash is between 5 and 9% for units 1-3 depending on coal quality, pulverised fuel fineness

and excess air.

▪ The decision was made to create a detail coal combustion model to evaluate the burner design

▪ PF and SA Cross Section

▪ Individual Burner Swirl Direction


Drop Tube Furnace Model

▪ A coal sample is passed through the furnace in pure Nitrogen more than 1000°C, devolatilizing the sample

leaving only the combustible solid matter.

▪ The sample is then dropped through the furnace, burning in a 3% Oxygen, 97% Nitrogen composition gas at

three different temperatures: 1000°C, 1200°C & 1400°C.

▪ The sample is introduced and collected with water cooled probes, which ensures the reaction only starts in

the furnace and stops when collected.

▪ Two size ranges are tested: 0-38µm and 38-75µm. The residence time is calculated based on the distance

from the particle release point in the furnace that the sample is collected. The composition is analyzed and

the percentage burnout established. The results are mass-weighted and presented in the form shown below.



▪ A CFD model of the drop tube furnace was used to evaluate the coal char burnout rates (shown below) were

used to calibrate the combustion model for application in the full furnace model.

▪ Three temperatures were considered: 1000°C, 1200°C & 1400°C.

▪ Two size samples were evaluated: 0-38µm and 38-75µm. Distribution described using a Rosin-Rammler

distribution with 19µm & 49µm mean sizes respectively. (Exponent 2)

▪ The inlet and collection probes were not modelled explicitly, but the effect of the cold carrier fluid trans-

porting the sample to the furnace have been accounted for in the analyses.



▪ First order char oxidation reactions. (Smoot, D.J., and Smith, P.J. 1985. “Coal Combustion and Gasification”,

The Plenum Chemical Engineering series, New York).

▪ First order reactions for char oxidation in O2 as well as Arrhenius coefficients Smoot and Smith calculated

describing the gasification.

C+0.5O2→CO

C+CO2→2CO (Gasification, Boudouard reaction)

C+H2O→CO+H2 (Gasification)

Char oxidizes in the 3% oxygen background gas as it falls through the Drop

Tube Furnace.



▪ Mass weighted burnout curves from CFD DTF overlaid onto the experimental DTF results.

0

10

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5 3 3.5

1000°C Mass Weighted Burnout 1200°C Mass Weighted Burnout 1400°C Mass Weighted Burnout


Furnace Combustion Model

▪ Half of the boiler was modelled as with the detailed windbox model.

▪ The velocity vector field, temperature distribution and turbulence levels were mapped from the highly detailed

windbox model.

Furnace cutting plane modelled as a zero

shear wall or symmetry plane.

Burner throats modelled as velocity boundaries with

velocity profiles, temperature and turbulence levels mapped

from the detailed windbox analysis.

Outlet from radiation section of the boiler

modelled as pressure boundary.

Heat extracted through furnace walls.





▪ As raw coal is introduced, the moisture is driven off using a quasi-steady evaporation model.

▪ A single rate devolatilization model defined in Arrhenius form (shown below) developed by Badzioch and

Kawsley, 1970 was built into the model.

k=Ae-(E/RT)

▪ The gaseous portion of the volatile matter produces the following products where the coal composition

determines the stoichiometry.

CHON (coal volatile) → CH4 + CO + H2 + N2

CH4 + O2 → CO + H2

H2 + O2 → H2O

CO + O2 → CO2



▪ Full furnace combustion model provided insight into the complex interaction between the different burners.

▪ Iso-surfaces proved to be a handy diagnostic tool to understand flow interaction (see example below).

Iso-contours of flow towards the rear wall.



▪ Animating the particle tracks also provides clear insight into the complex interaction in the furnace.







▪ The first burner modification considered is based on the knowledge gained during the first phase of flame

stability improvement simulations where purely flow were considered.

▪ The modification features a reduction in the secondary air annular cross section and an increase in the

primary air annular cross section to respectively increase and reduce the amount of energy in these regions.

Increase in PF cross section



▪ Evaluation of the particle tracks reveals that the modification drastically improved flow uniformity. The

increase in energy in the secondary air swirl annuli and the reduction in energy in the primary air annuli also

managed to establish properly defined swirl coned in front of each burner.

First Proposed Modified Design Base Case Burner Design









▪ With the improvement in burner flow uniformity, the global interaction changed to such an extent that fewer of

the burners managed to pull back hot gas from the furnace.

▪ It became apparent that the interaction resulting from the base case burner flow distribution problems

assisted the central burner columns to pull back hot gas from the furnace.

First Proposed Modified Design



Base Case



▪ Three additional geometric burner modifications were considered, but none of these improved the overall

flame stability.

▪ The final investigation considered reversing the swirl direction of the middle row of burners as this result in a

situation where the flow from all burners sustain each others swirl action.

Modified Swirl Direction Original Swirl Direction



▪ Evaluation of the particle tracks reveals that the modification drastically improved flow uniformity. The

increase in energy in the secondary air swirl annuli and the reduction in energy in the primary air annuli also

managed to establish properly defined swirl coned in front of each burner.

First Proposed Modified Design First Proposed Modified Design Incl.

Swirl Mod.




Swirl Mod.




Swirl Mod.



First Proposed Modified Design Incl. Swirl Mod




Swirl Mod.

▪ Based on the summary of ash fusion temperatures for several samples taken from site, the likelihood for

fouling to occur could be investigated. The algorithm identifies all particles with a temperature exceeding the

ash fusion temperature located within 10mm from a wall with a velocity component towards that wall.



Feedback on implementation successes Operating data from 2013 - 2014 showing differences: * these coal CV values calculated by unit controller.

Unit Jul 2013 Mar 2014

Load MW el 95.6 97.8

SA flow kg/s 29.3 29.1

SA Temp oC 219 227.7

PA flow kg/s 8.33 9.1

Mill out T oC 90.2 88.1

Coal flow kg/s

4.7 5.9

Coal CV MJ/kg 20.5 17.7

No Oil Burners i/s n 2 0


Implementation success

Feedback on implementation successes Operating data from Jul 2013:


Implementation success

Feedback on implementation successes Operating data from Mar 2014:



Komati Furnace Pyrometers – During operation



Komati Furnace Pyrometers – During operation

Conclusion

Video of healthy PF flame

Komati PF combustion 001.mov



Video of oil flame



Video of pf flame

Conclusion

▪ Original burner design installed during 2011. Unit 3 went into operation during 2012.

▪ During Jan 2013 burner stability became problematic. Several oil burners had to be in service during low

load operation to support PF combustion.

▪ The highly detailed windbox model provided clear insight into the complex flow behaviour in the windbox and

the resultant individual burner flow distribution.

▪ The explicit modelling of all internal structures and leakage flow paths allowed for the modification of the

measurement device which will produce a positive pressure drop across the burner for the entire range of

cylindrical damper positions.

▪ The model allowed the evaluation of 9 burner modifications to from a flow perspective maximised the amount

of return flow behind the swirl burners.

▪ Mod4 Implemented during April 2013.

▪ Burners operated during the rest of 2013 with no stability problems.

▪ Mar 2014 PF combustion stability is maintained without oil burner firing in spite of excessive coal moisture

content.

▪ The full combustion model provided valuable insight into the complex interaction between the 12 swirl

burners firing from the rear wall. The model also revealed the strong influence of the individual burner flow

distribution emanating from the common windbox on the combustion behaviour.

▪ Model results used to predict furnace wall fouling locations

Further Development

▪ Steinmüller is purchasing a suction pyrometer and gas sampling probe to measure Furnace Exit Gas

Temperature (FEGT).

▪ This will allow traverses to be made measuring flue gas temperature as well as flue gas species such as CO,

and O2 plus sampling particulates for unburnt carbon content

▪ This will allow verification of the results of the CFD model by comparison with the control room operating

results.

▪ Suction pyrometer has multiple ceramic shielded tip with high induced flue gas flow rate for high convective

heat transfer with the measuring tip.

▪ Obtain data on coal combustion to verify devolatilisation rate model.

Questions?

Thank you for your support, do you have any questions?

Documents

STAR-CCM+ an invaluable tool for coal fired power plant design ...mdx2.plm.automation.siemens.com/sites/default/files/Presentation/SGC... · Case Study – Komati Power ... burner