CFBC, preferred Technology for lignite fuel - Experiences - Infraline

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CFBC boiler is operational friendly. This boiler is called dark boiler because of absence of any flame in the combustor. Hence, there is no boiler tripping due to flame failure/ furnace disturbance. As this boiler has huge refractory and very large amount of bed ash in circulation, there is no need to trip the turbine in case of all fuel cut off to boiler for about 20 minutes. Boiler continues generating steam within specified parameters for sustaining turbine at reduced loads. CFBC combustor is a reservoir of heat energy. This energy is used in case of all fuel failure for short duration before restoration of one of the fuel available (lignite or oil). CFBC boilers do not experience large sharp fluctuations in boiler pressure due to very stable firing system. Oil support is not required even at part loads of about 25 % and combustor temperatures, steam parameters are maintained. CFBC boilers have almost zero combustibles in bottom ash with share of bottom ash of about 30-40%. The intense heat storage capacity of this boiler makes it useful for quick hot re-starts. One of the disadvantages of CFBC boiler is longer cold start-up and shut down time. Cold start-up time is about 10-12 hours as boiler is to be heated up slowly at specified heating rates as per refractory heat up curves. Similarly during shutdown also, gradual cooling is required. A typical boiler start-up is given Fig: 1. As lime is added in CFBC boiler for capturing Sulfur, the units with CFBC boilers have more number of auxiliary equipments such as air blowers, limestone handling, milling and transporting system. Hence, auxiliary power consumption is on higher side for lower capacity units. Figure-1: Typical Boiler start-up curve from light-up to Synchronization of Unit 700 600 Temperature in deg C 500 400 300 200 100 0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600 Time in Minutes Lower bed Temp Middle bed Temp FG temp after Cyclone FG temp after Economiser FG temp after APH Using Lignite as fuel with CFBC boilers, following environmental parameters can be achieved. SOx (ppm) NOx (ppm) CO (mg/N.Cu.M) SPM(mg/N.Cu.M) < 50 < 20
Other General operation problems being experienced are: • Back sifting of bed ash in to primary air wind box trough combustor nozzle grate • Pulsation in cyclones at lower loads, bed ash/bed materials hang up in cyclones. • Agglomeration of ash in combustor to cyclone ducts. • Deposition of ash on back-pass coils. • Clinker formation in Start up burners ,Bed lances Maintenance Experiences: CFBC boilers experience higher erosion rates in water wall tubes at refractory transition zone in general. CFBC combustor has refractory lining up to about 9 meters height to protect water wall against erosion. But erosion problems experienced at refractory transition zone due to bed material sliding back to combustor along water wall from different heights. Erosion of water wall at this zone is due to scooping action of bed material. Some designs use kick-out tubes to overcome this problem. To protect the water wall tube from erosion, protective coating is being used in some units and is giving satisfactory results. Area requiring metal spray can be identified with the help of tube thickness survey of water wall and identifying erosion pattern. Life of water wall tubes in these areas extended to three times with metal spray. Due to large quantities of refractory and bed ash in circulation, CFBC boilers require more down time to repair as it takes longer periods to cool down boiler to working temperature. A typical cooling curve is shown Fig 2. This is results in increased down time and decreased availability. These boilers also demand major refractory repairs particularly in combustor water wall refractory, ash cooler refractory and refractory along fuel path. In case a tube leakage in combustor or Fluidized bed Heat Exchangers (FBHE) in some designs requires immediate evacuation of all bed material to avoid solidification of the material (due to presence of Calcium sulfate). Non Metallic expansion Joints (NMEJs) in ash cycle path also require very special attention. Any ingress of hot bed material in to it will damage the NMEJ. Combustor grate nozzle is another major maintenance area. Severe erosion is observed in the nozzles due to bed material. About 10% of nozzles require replacement every year. Lignite chain feeders, Bead ash handling equipment also require more maintenance due to corrosion and erosion.
Page 1 and 2: CFBC, preferred Technology for lign
Page 3: Simplified Fuel Preparation And Fee
Page 7: India has good reserves of lignite

CFBC, preferred Technology for lignite fuel - Experiences - Infraline

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