1 A CASE STUDY OF FURNACE EXPLOSION IN A CIRCULATING FLUIDISED BED BOILER By K.K.Parthiban, B.Tech ( IIT), M.E, boiler specialist, Venus energy audit system Introduction PF boilers have furnace safeguard supervisory system in which flame sensing devices are used to check the presence of flame continually. Other combustion technologies such as grate combustion, Fluidised bed combustion, do not have flame combustion and hence operators are dependent on temperature sensors and to some extent on oxygen sensors. Temperature sensors and oxygen sensors are practically slow sensors as compared to flame sensors which are positive in saying flame is there or not and instantaneously. Explosion in CFBC Boilers 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. The explosion results in distortion of waterwall panel. The photos at the end show the damage caused by an explosion in the case study presented. Definition of an 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 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. In a PF boiler, the particle size is around 85 μm. In a BFBC boiler only 30% of the particles are below 30%. Fire Triangle and Explosion Pentagon 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 Fuel any one of these elements, a fire becomes impossible. For Fire Triangle example, 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.
2 Likewise, if insufficient heat were available, no concentrations of fuel and oxygen could result in a fire. On the other hand, for an explosion to occur, there are five Suspension necessary 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 Source one of these requirements would prevent an explosion from Confinement propagating. 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 Fuel this 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 Pentagon the explosion pentagon would be satisfied and an explosion would be imminent. 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. 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. Quantity 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.
3 Heat Furnace explosions in CFBC boilers are rare when both bed and free board temperatures are above 760 deg C. Chances of explosions are very high when these temperatures are below 540 deg C. Though not fully established, yet chances of explosions cannot be ruled out when bed temperature remains between 540 deg C and 760 deg C. Causes of Furnace Explosion 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. Sudden firing after a boiler is banked or stopped for a short period without proper purging. Insufficient purging of the furnace. Unbalanced fuel air ratio to allow the fuel concentration to fall within explosive range. Leaking fuel supply system Basic Philosophy of Explosion Prevention The basic principles of avoidance of explosion are: Fuel should never be fed into the furnace continuously for more than 12 seconds when there is no fire. Furnace is completely purged of the explosive mixture and then fired. Fuel supply is stopped immediately if fire / flame is not established and re-purging is surely done before restart. Correct air fuel ratio is maintained so that dust concentration within explosive limits is never achieved. Comparison between a PF Boiler and a CFBC Boiler Following aspects need to be considered before sequencing the operation of a CFBC Boiler: 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 smoulder. 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 cannot 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. 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
4 fuel supply, coal on the bed continues to remain present in the furnace unless boiler is cooled and those materials are taken out. The only parameter that would indicate whether combustion is being established in a CFBC boiler is the reducing oxygen / increasing CO 2 % in flue gas at furnace outlet. Operation to Prevent Explosion in CFBC Boilers The explosions mostly occur when during start up or during a hot restart. During this period air / fuel controls are not proper. Even draft control is kept on manual mode. Hence operators who do not have a feel for the air / fuel ratio control land the boiler for an explosion. 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. Case study This is a case study of a boiler explosion in a 85 TPH CFBC boiler fired with South African coal. The boiler had a furnace explosion after 12 hrs after a fresh start up. There were extensive damages to buckstay, boiler waterwall and economiser casing. The incident took place a week before our visit was made. The boiler was offered for inspection after cleaning. The boiler insulation was removed almost fully so that the waterwall rectification wok can be commenced. The CFBC boiler log parameters were made available for review at the time of visit. Subsequent to this, few important trends were also taken for review. The findings are summarised below. Chronology of events Boiler was started on 1.30 AM on first day. Boiler was connected to main steam header at 6.30 AM on the first day. Boiler was running at a load of 43 TPH till PM on the first day. Coal was not feeding to furnace at hrs on second day. There was an incident of furnace high pressure trip prior to incident of explosion. PA fan and SA fan got tripped. Fans were restarted at hrs. Coal feed was initiated at 0.38 hrs. Just after hrs furnace explosion took place. Review of log sheet and trend The log data was made available for duration from pm on the first day to am of next day. Following were the observations of undersigned. Duration between of first day to of second day 1. Bed temperature was at 830 deg C average.
5 2. The furnace pressure was hunting between -20 mmwc and -50 mmwc. 3. PA air flow had been TPH, which is almost the full load air flow. 4. SA flow was in between 131 to 137 TPH though the load was 73 TPH. 5. Air box pressure had been mmwc to mmwc. 6. Main steam flow had been TPH. 7. Oxygen went up from % to 13.18% at hrs. Duration between of second day to of second day 1. Bed temperature came down to 636 deg C. 2. As the oxygen went up to 18%, it was clear that there was no coal flow to the furnace. 3. Coal feeder rpm was raised from PA air flow had been TPH, which is almost the full load air flow. However PA fan VFD was brought down from 85 to ID VFD remained at Air box pressure had come down from 1200 mmwc to 1000 mmwc. 7. Main steam flow came down to 21 TPH. 8. SA flow remained at 134 TPH and SA pressure was at 640 mmwc. 9. Furnace draft increased from -40 mmwc to -90 mmwc. 10. PA fan, SA fan and fuel feeder, DCF tripped at hrs. 11. In the one minute interval data, there is no record of furnace pressure going high. However on furnace pressure high activation only the feeders, PA and SA got tripped. Duration between of second day to of second day 1. The PA fan and SA fan were brought in service. 2. PA pressure went up to 1096 mmwc. 3. The coal feeder rpm was at 15 for three minutes. Then it was reduced to SA discharge pressure was at 433 mmwc. 5. The PA fan VFD was at increased up to 78%. 6. Suddenly the ID fan VFD input was changed from 75% to 20% by operator. Just at this point the furnace pressure had gone high and resulted in explosion. The reduction of ID fan VFD made the furnace pressure to + ve. Yet there must be fuel rich situation for the incident to occur. There were two possibilities of fuel accumulation. Theory 1: Actually coal flow was there from feeder and it might have accumulated in the fuel feed pipe without entering in to furnace. At one point the fuel could have come in to furnace. For a steam load of 45 TPH the feeder speed was 6.5. Approximately the coal feed rate had been 10 TPH. In a minute the feeder can feed 167 kg for 1% rpm. Cumulatively there must have been coal feed to an extent of 3384 kg of coal for the period of coal flow problem. Out of this the coal must have burnt to an extent of 50% since the steaming was at a rate of 20 TPH. There can be a balance of coal to about 1700 kg inside the coal pipe which got fed suddenly. The bed inventory can be about 7740 kg for a bed height of 600 mm. This will leave a coal percent of 18% in the bed. But the VM was about 40%, which got used up in explosion. The remaining coal in bed should have been 10.8%. Under normal condition the carbon percent in a fluidised bed does not
6 exceed 1.5%. This proves there was coal accumulation. Theory 2: Coal burning needs adequate air flow for combustion. In case of excess air flow, coal will accumulate without combustion. When there is PA flow / SA flow reduction, the accumulated coal could burn rapidly. The first trip which occurred was at hrs. This time the furnace was dilute. There was no coal flow for a period. As the furnace was with excess air, the coal stopped burning. As fuel flow was raised and air flow was brought down, the furnace pressure went high due to sudden burning of high VM fresh coal. The same situation was occurred during the explosion. The fuel flow was increased and ID fan rpm & damper was reduced drastically. On pressurisation of furnace, the accumulated coal had burnt rapidly. This had resulted in explosion. In this explanation, there would be a quite a high fuel accumulation. It was informed that when the LOI of bed material was analysed to be 12%. It meant that there was sufficient accumulation of fuel without burning. RECOMMENDATIONS The loading of the boiler has to be done with regulated air flow, fuel flow and bed material flow. Without bed material addition, the load rise will take time in a CFBC boiler. With less bed material, the rise and fall of bed temperature will be abrupt during feeder rpm change. The combustion is in the form of suspension firing rather than a fluidised bed firing. In this plant, the fuel spreader air was taken from SA. Initially the SA flow would be less and thus the SA pressure would be less. With less SA pressure fuel spreading would not be proper. The fuel spreading air shall be tapped from windbox itself, where the PA pressure is always in the range of 1000 mmwc to 1200 mmwc. An explosion door can be added to upper furnace. Rupture disc can also be another choice. In the present design, the fuel port is made of refractory. Refractory surface not being smooth can accumulate fuel. Air cooled plate formed chutes are recommended. EXTENT OF DAMAGE On inspection of the furnace, it was clear that the waterwall got opened due to improper closure of the buckstay at corners. If the buckstays were closed properly, the waterwall would not have got distorted. Instead some other weak point such as APH hopper could open up. The economiser casing got opened up since casing stiffeners were not closed at all. Stiffening is incomplete, if the corners were not closed. The photos explain the designer s and erector s mistakes that led to more damage to the combustor panels. CONCLUSION Knowledge of minimum fluidisation air flow is very essential during start up. With less air flow and with less fuel, the continuous combustion has to be established. High SA flow during start up, results in quenching of the fire. This explosion is a result of confused state of the operation team during start up. The boiler manufacturer did not consider explosion doors which are essential these days wherein high VM coals are used. Once we know that there is excess fuel inside the furnace, the ID draft alone has to be increased to avoid a mishap.
7 Photo 1 & 2: The buckstays got distorted at several levels. The corner plate weldment broke away since the weld was incomplete. Otherwise waterwall might have been spared. Other vulnerable areas would have given way under pressure.
8 Photo 4: The scalloped plate welding was not sufficient. The weld length was not sufficient for the pressure, the furnace underwent. Photo 3: The economiser casing stiffeners are not closed at corners. Photo 5: A corner without a weld at bottom of corner plate. The incomplete job during construction is also a cause for extensive damage to waterwall.
9 Photo 6: A seismic guide for second pass. Second pass should be guided at casing stiffener. This is a designer s mistake. Photo 7: Boiler guide shall be at buckstay and not at waterwall plain area. This is again a designer s fault to have improperly located the buckstay guide.
10 Photo 8: The economiser casing hangs from Steam cooled wall panel bottom header. The weld is incomplete. During the explosion, the improperly fused weld joint had given way. Photo 9: A view of the economiser casing which is not welded.
11 Photo 10: Just before boiler explosion, the PA rpm was being raised till the explosion occurrence.
12 Photo 11: Prior to explosion incident, it is seen that the ID fan current came down and then the furnace explosion took place. The damper % and VFD percent both were reduced with no reduction in PA damper & SA damper setting. The furnace got exploded due to fuel accumulation present & simultaneously as the ID draft was reduced.
13 Photo 12: As the damper closure and VFD rpm reduction took place simultaneously, the furnace pressure went up. As there was fuel accumulation earlier, the furnace got exploded.
14 Photo 13: Typical explosion doors in a CFBC boiler by another manufacturer. Figure 1: The above sketch shows a typical explosion door arrangement suggested to suit the site condition. Hinged door / rupture discs can be the alternate arrangement.