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1. Background :
A few CFBC Boilers have suffered furnace explosion in the past. Apart from causing severe losses to the businesses concerned, the occurrences have shaken the confidence of CPP professionals. Due to inherent characteristics, Pulverized Fired (PF) boilers are more prone to such type of explosion than a CFBC version. However, with an operation history of centuries, the general principles necessary to avoid these dangers of PF boilers are clearly established and our power plant personnel working in PF boiler based units are well versed in the subject.
As regards CFBC boilers, these are comparatively newer generation of technology and explosion avoidance measures are not clearly understood by the operating engineers.
In view of above and on the requests received from a number of CPP units, this Instruction Write Up has been prepared for the benefit of the concerned.
2. Type of Explosion in CFBC Boilers
Most of the explosions faced in CFBC boilers are dust explosions caused by small particles of coal in the bed and in the free board kept under suspension by fluidizing air fans. However, explosion due to FO/ HSD/LDO used in duct burners and/ or load carrying burners has also been reported.
3. Definition of Dust Explosion
A dust explosion is the rapid combustion of a dust cloud. In a confined or nearly confined space, the explosion is characterized by relatively rapid development of pressure with flame propagation and the evolution of large quantities of heat and reaction products. The required oxygen for this combustion is mostly supplied by the combustion air.
The condition necessary for a dust explosion is a simultaneous presence of a dust cloud of proper concentration in air that will support combustion and a suitable ignition source.
Explosions are either deflagrations or detonations. The difference depends on the speed of the shock wave emanating from the explosion. If the pressure wave moves at a speed less than or equal to the speed of sound in the unreacted medium, it is a deflagration; if it moves faster than the speed of sound, the explosion is a detonation. The term dust is used if the maximum particle size of the solids mixture is below 840 μm.
Minor flue gas explosions are called puffs or blow backs.
4. Fire Triangle and Explosion Pentagon
Air or Oxygen
There are three necessary elements which must occur
simultaneously to cause a fire: fuel, heat, and oxygen. These
elements form the three legs of the fire triangle. By removing
Fuelany one of these elements, a fire becomes impossible. For
Fire Triangleexample, if there were very little or no oxygen present, a fire
could not occur regardless of the quantities of fuel and heat
that were present. Likewise, if insufficient heat were
available, no concentrations of fuel and oxygen could result in a
On the other hand, for an explosion to occur, there are five
Air or Oxygen
Suspensionnecessary elements which must occur simultaneously: fuel, heat,
oxygen, suspension, and confinement. These form the five sides
of the explosion pentagon. Like the fire triangle, removing any
Ignition Sourceone of these requirements would prevent an explosion from
Confinementpropagating. For example, if fuel, heat, oxygen, and confinement
occurred together in proper quantities, an explosion would still
not be possible without the suspension of the fuel. However, in
Fuelthis case, a fire could occur. If the burning fuel were then
placed in suspension by a sudden blast of air, all five sides of
Explosion Pentagonthe explosion pentagon would be satisfied and an explosion would
Remembering the three sides of the fire triangle (fuel, heat, oxygen) and the five sides of the explosion pentagon (fuel, heat, oxygen, suspension, confinement) is important in preventing fires and explosions at any facility. By eliminating the possibility of either suspension or confinement, an explosion cannot occur, but a fire may occur. By eliminating the fuel, the heat, or the oxygen requirements, neither a fire nor an explosion can occur.
5. Explosiveness of Coal
Coal, as a primary fuel, must meet several requirements in order to be explosive. These requirements are:
· Volatile ratio
The volatile ratio is defined as the volatile matter divided by the summation of volatile matter and fixed carbon of the coal. It has been determined that coals with a volatile ratio exceeding 0.12 present a dust explosion hazard. All bituminous coals fall into this category.
· Particle size
The particle sizes that can contribute to an explosion are 840 microns (0.84 mm) and below. Lesser the particle size more severe would be the explosion. Hence PF boilers are more susceptible to explosions.
The minimum concentration of dust in suspension that will propagate a coal dust explosion is called Minimum Explosive Concentration (MEC). The MEC for bituminous coal is approximately 100 grams per cubic meter.
The upper explosive limit of coal dust concentration is 380 grams per cubic meter that would propagate a low-velocity explosion.
Furnace explosions in CFBC boilers are rare when both bed and free board temperatures are above 7600C. Chances of explosions are very high when these temperatures are below 5400C. Though not fully established, yet chances of explosions can not be ruled out when bed temperature remains between 5400C and 7600C.
6. Causes of Furnace Explosion
a. Pushing the fuel (coal or oil) into the boiler when there is loss of ignition known as ‘Delayed operation of Fuel Trip Relay’. Loss of ignition in a boiler causes explosive mixture to form. Such mixture must be purged out before initiating firing.
b. Sudden firing after a boiler is banked or stopped for a short period without proper purging
c. Insufficient purging of the furnace
d. Unbalanced fuel air ratio to allow the fuel concentration to fall within explosive range.
e. Leaking fuel supply system
7. Basic Philosophy of Explosion Prevention
The basic principles of avoidance of explosion are:
v Fuel should never be fed into the furnace continuously for more than 12 seconds when there is no fire.
v Furnace is completely purged of the explosive mixture and then fired.
v Fuel supply is stopped immediately if fire/flame is not established and re-purging is surely done before restart.
v Correct air fuel ratio is maintained so that dust concentration within explosive limits is never achieved.
8. Difference between a PF Boiler and a CFBC Boiler
Following aspects need to be considered before sequencing the operation of a CFBC Boiler:
ü Tripping of PF boiler with fuel supply shut off would result in complete flame failure (barring slag burning). This may not be true in case of a CFBC boiler where coal on bed may continue to smolder.
ü Failure of flame or complete loss of ignition in PF boiler immediately gives the indication that any fuel supply under such condition would form explosive mixture. Since loss of complete/ partial ignition can not be ascertained in CFBC boiler (due to non- availability of flame sensing device), it becomes extremely difficult to judge whether such mixture is being formed or not.
ü Incorporation of furnace safe guard systems in PF boiler which completely shuts off the fuel supply when actuated by flame sensing device greatly helps operation staff in preventing explosions. CFBC boilers are not fortunate in this regard. In this boiler, even after stopping the fuel supply, coal on the bed continues to remain present in the furnace unless boiler is cooled and those materials are taken out.
ü The only parameter that would indicate whether combustion is being established in a CFBC boiler is the reducing oxygen / increasing CO2 % in flue gas at furnace outlet.
9. Operation to Prevent Explosion in CFBC Boilers
The explosions mostly occur when
Ø Boiler is restarted after a trip out.
Ø Boiler is restarted after a short period of stoppage
Ø Fast cooling of boiler is resorted to following a tube leakage especially when the leaking water falls on the bed
Major operation steps recommended to be followed to avoid any risk of explosion in such cases are
A. Restart after a trip out
Precondition: Bed temperature in this case will remain above 7600 C.
I. Stopping & Purging
ü Ensure that coal feeders and oil burners stop immediately.
ü Continue to run the ID fan with adequate furnace draft for 15 minutes.
ü Start Secondary Air Fan (fan meant to supply combustion air over the bed) and purge the furnace at least for 5 (five) minutes with a minimum of 25% of rated air flow.
ü Open the dampers in air/flue gas circuits during purging so that any accumulation of explosives is driven out through ID Fan.
ü Ensure that air flow to siphons (from PA fan or Roots blowers) is established.
ü The vent line and dampers in air inlet and outlet paths of bed ash coolers (those cooling the ash with primary air) must remain open during purging and then can be closed.
II. Fluidizing the bed by PA Fan & Fuel Charging
This is the most vulnerable operation requiring a high level of competence and skills of the engineers and operators.
a. View through peep holes before fluidization to make sure that coal is burning
b. Ascertain that all personnel are at safe distance from the boiler furnace
c. Fluidize the bed and watch carefully that oxygen % in flue gas at furnace outlet starts dropping, an indication that combustion is getting established.
d. If within 12 seconds, the oxygen content in flue gas does not come down then trip the PA fan and restart purging as described above.
e. Time lag of 12 seconds above has been arrived at with the consideration that the time taken by the coal dust to reach lower explosive limit in the confined space would be more than 15 seconds.
f. Attempt to maintain excessive rich fuel air ratio is again a potential hazard indicated by high level of CO2 or low level of oxygen in flue gas. Never allow oxygen %age to fall below 2.5 %.
g. Purging and restarting in above manner to be continued till fire in boiler is established.
h. Feed coal and adjust air as per normal operation. It is assumed that the control room/ shift charge engineers are well experienced in normal operation of plant and equipment.
B. Restart after a short period of stoppage
¨ Purging with SA fan and ID fan should continue since bed contains smoldering coal
¨ PA fan to be stopped to avoid bed temperature falling sharply.
Precondition -1: Bed temperature> 7600C
Same as above
Precondition -2: 6500C
a. Load carrying oil burners, if provided, should be switched on to increase the temperature while following the procedure described under Restart after a trip out.
b. Boilers not equipped with load carrying burners to follow the same procedure described under Restart after a trip out. A very high degree of vigilance is needed for establishing the ignition.
Precondition – 3: Bed temperature < 6500C
a. Boilers not equipped with load carrying burners are normally provided with hot gas generators wherein oil is fired and resultant flue gases are admitted into the furnace. When bed temperature drops below 6500C, heating of bed must be done by firing hot gas generator.
i. As bed temperature rises, keep constant watch on oxygen % in flue gas because rapid drop in this parameter will indicate that coal has caught fire. Never allow oxygen %age to fall below 2.5 % .
b. Adjust promptly the recommended air fuel ratio when coal starts burning.
A well structured control logic incorporating above particulars with suitable interlocks can be provided in the process cycles so that human errors during such situation can be avoided altogether.
C. Cooling of Boiler following a Tube Leakage
ü The steps a) to g) will be the same as given above.
ü Instead of step h) cooling of furnace will be completed after burning the coal already present in the furnace without feeding any additional coal.
ü Additional steps/ precautionary measures need to be taken are:
o When the tube leakage is at a location wherefrom water/ steam is impinging on the bed, the bed will quench down. The fall in bed temperature will depend on the magnitude of tube leakage.
o Fluidization to be attempted very carefully if bed temperature and freeboard temperature are well above 7600C. Time lag allowed between PA fan starting and its tripping (when combustion fails to occur) in this case should also be 12 seconds.
o If proper combustion is not achieved in a couple of attempts, it is advisable not to try any more. Allow bed to cool down while continuing with purging operation. Cooling will be fast due to water falling on it from the leaking tube. It may please be noted that allowing the bed to cool down on its own will never invite explosion since fuel in suspension can only cause explosion (refer explosion pentagon above).
o Never attempt to fluidize the bed when tube leakage has caused bed temperature to fall below 7600C because the prevailing adverse condition in the furnace in such case may quickly take out the heat energy from the air and bed temperature may fall down to disastrous level of 5400C or below. Allow bed to cool with purging on and drain out the bed when cool.
10. Protection of Boiler against Explosion
¨ Explosion doors/ vents of adequate sizes and at suitable locations must be provided in any type of boiler to mitigate the impact of explosion. Boiler manufacturers may please be asked to provide the same.
¨ Opportune test must be carried out periodically to ensure that the explosion vent /door is perfectly operational.
We undertake all types of consultancy works for Boiler troubleshooting. We conduct study of your boiler (AFBC/CFBC) operations/maintenance to help you avoid such misshapen in future.
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