Coal Flame in Tangential Firing Boilers - How to Evaluate

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Understanding the quality of flame in any boiler furnace is very important in tuning the boiler to the optimal level of performance. The aspects of combustion tuning involve looking at the boiler furnace and making sure the quality of flame is acceptable and good.The gas and oil fired boilers do not pose much problem in establishing a good flame in the furnace. The available instruments like flame scanners, CO monitors and oxygen indicators, along with the exit gas temperature, give a good indication to perceive if the quality of the flame is good. In coal fired boilers and mainly in tangential fired boilers, the furnace acts as a single burner, so it is required to look at the flame and understand the quality of the flame.

It is necessary to start from the control room of boiler then go to the mill area, to the furnace, and then to the bottom ash and fly ash area to fully make sure of combustion quality in furnace.

The control room of the boiler

  • Look at the load at which the boiler is operating, availability of support fuel, SH & RH parameter
  • Look at the number of mills operating
  • Note the load, air flow, and outlet temperature on each mill
  • Check the oxygen level at Eco / APH outlet
  • Check the furnace pressure, scanner performance- watch for a few minutes for any fluctuations
  • Look at the coal proximate analysis within 8 hours- if not available then at the max 24 hrs
  • Check the PC fineness reading of each running mill if available
  • Keep a note of those mills which have plus 50 more than 2% and minus 200 below 65%

Check each running mill in the mills area

  • Bowl mills
    • Check each spring loading by feeling the bumping of the pressure spring shaft
    • Regular bump indicate the springs are loaded - how much cannot be estimated by feel - low minus 200 & or mill reject can be an indicator
    • Watch for any abnormal sound
    • Check the level of mill reject - look for coal in rejects - if nil or very low then ok
    • Look at classifier vane position – check if they are equal in each mill. Close further to improve fineness - if needed
  • Ball mills
    • Check for ball noise in the mill area - high noise indicates low coal levels or low speed
    • Check the mill speed and by-pass air
    • Check classifier settings
    • Look for any gear box noise

Checking the furnace and flame condition

  • Look at the flame front in each corner and elevation- stand in front of the peephole at the corner and see if you can find a tip of flame moving to and fro (visible and out of sight)
  • If it is not visible note the fuel air damper position in the windbox and close it to less than 5% open condition
  • Look at the flame in the furnace from just above wind box and count the number of flickers. Go to the furnace outlet plane elevation - normally at this elevation more peep holes are provided. Check the flicker at this elevation- if less than 10 per minute and bright orange red in colour, the flame is good
  • Open all peep holes one by one in the bottom hopper level - normally around 10 Mt level
  • Watch the particles falling each peep hole on both side walls
  • If large number of shooting star-like particles of pebble sizes or above is seen with frequent intervals or continuous this indicate deposition on the waterwalls
  • If this is experienced, then look at the bottom ash collection for any glazed lumps - this confirms deposition tending to slag
  • Open the other peep holes and see if they are loaded with ash particles or bridged by fused particles or glowing ash in with a narrow opening or black colour in center. If loose ash of some amount is found, this indicates friable ash and is normally seen in good operation / furnace condition. Higher fused ash and bridging indicates that furnace deposits are high and/or high temperature combustion
  • In the high heat flux region - just about 1 Mt above the wind box / top burner level - one will see fused lumps, but not bridging or flowing slag - if seen then higher furnace deposition is indicated
  • Look through the top most peep holes in the front wall - generally given at furnace outlet plane, apart from seeing the flame flicker level as said above, look at the Platen / Panel / Final SH which is provided
    • Watch for deposit on these. Large volume deposit seen indicate lower frequency of soot blower operation or high deposition levels
    • You may also see a swing in super heater panel / platen / final in this region. This is not uncommon and not to worry.
  • Look through the peep hole in the side walls near the reheater region.
    • If flame seen licking this area indicate the burning is getting completed at much higher elevation than envisaged.
    • This can be due to low reactive coal or improper air distribution or very high primary air
    • Watching carefully the flame / bright spots coming into this region with respect to width and intensity can give an indication on the elevation of combustion completion

Checking the bottom ash and fly ash

  • Bottom ash
    • Look at bottom ash, if you find glazed lumps then suspect clinkers
    • The higher the size and quantity, the higher the intensity of clinkering
    • A large amount of loading black particles indicates higher unburned in bottom ash
    • A higher quantity of friable ash along with coal particles indicates a higher plus 50 size % in PC or wall deposits shedding at frequent intervals
  • Fly ash
    • While collecting flyash from ESP field hopper, allow it to flow freely for some time so that locked up ash falls off
    • Look at the fly ash from the first working field of ESP - not to worry about the dummy field
    • If fly ash has higher blackish colour, this indicate higher unburneds
    • If higher black particles are seen, this indicate very low reactive constituents in coal