Boiler Process 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 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.
Case Study – Komati Power Station Unit 3 Background – Where this started Unit 3 Firing Floor showing Common Windbox and 12 Original PF Burners
Case Study – Komati Power Station Unit 3 Background – Where this started Photo of original Komati PF Burners – Top firing floor view, burner in situ with oil burner
Case Study – Komati Power Station Unit 3 Background – Where this started Photo of original Komati PF Burners – View from inside windbox Note wear on Swirl vanes
Case Study – Komati Power Station Unit 3 Implementation Photo of New Komati PF Burners – View from firing floor during installation
Case Study – Komati Power Station Unit 3 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.
Case Study – Komati Power Station Unit 3 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.
Case Study – Komati Power Station Unit 3 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
Case Study – Komati Power Station Unit 3 Burner Flow Measurement Philosophy ▪ Flow through the swirl burners are governed by adjusting the offset of cylindrical dampers from the burner back plate.
Case Study – Komati Power Station Unit 3 Burner Flow Measurement Philosophy ▪ 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.
Case Study – Komati Power Station Unit 3 Burner Flow Measurement Philosophy ▪ 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
Case Study – Komati Power Station Unit 3 Burner Flow Measurement Philosophy ▪ 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.
Case Study – Komati Power Station Unit 3 Burner Flow Measurement Philosophy ▪ 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
Case Study – Komati Power Station Unit 3 Burner Flow Measurement Philosophy ▪ 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
Case Study – Komati Power Station Unit 3 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.
Case Study – Komati Power Station Unit 3 Flow Evaluation To Improve Flame Stability Feedback on implementation successes Komati Coal – Supply Limits Unit Limit Jan Apr Jul Mar Value 2013 2013 2013 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 m m) % 98 min 96.8 95.6 - - PF fineness (passing 75 m m) % 65 min 62.2 66.7 - - PF mass flow variance from % 15 max 32.4 -23.2 - - mean o C Mill Outlet Temperature 75 min 69.8 82.9 87.4 84.8 Oil Burners in operation n 0 4 2 2 0
Case Study – Komati Power Station Unit 3 Flow Evaluation To Improve Flame Stability ▪ 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.
Case Study – Komati Power Station Unit 3 Flow Evaluation To Improve 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
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