28 May, 2011 REPORT ON 24 TPH FBC BOILER PERFORMANCE AT APOLLO TYRES, CHENNAI By K.K.Parthiban, Venus energy audit system Agenda The following problems were reported at this installation. 1. 2. 3. 4.
Erratic bed temperature readings Small amount of clinkers in APH and Bank hopper chutes Unable to operate 3rd/4th compartment independently. Steam loss from De-aerator is high.
About the boiler
The boiler is a FBC boiler design to give an output of 24 TPH at 21 kg/cm2 pressure at saturated condition with the feed water temperature at 105 deg C. The boiler is designed for Indonesian coal. Four compartments with two 125 nb PA lines are provided. A Mechanical dust collector and an ESP in parallel are provided. This is a Bi drum boiler with an economiser. The economiser is provided with a gas side & water side by pass arrangement. Multilouver dampers are provided for by passing the flue gas.
The visit was made on 25th May 201. The boiler was in operating condition. The following are observations. Boiler was being operated with two compartments only. The steam demand in a day itself was hardly 200 Tons. Minimum steam demand comes down to 8 TPH. Maximum demand is around 9.5 TPH only. The boiler had been in operation for more than a year. There had been a bed tube failure due to erosion. The boiler was being operated with Indonesian coal since commissioning. Review of the boiler design / operation with respect to agenda points 1. Erratic bed temperatures The bottom temperature thermocouples are reading less at times due to settling. Incidentally these are located on bunker side. Bed drains are located on opposite side. Hence when the bed settles down at lower load, the temperatures are falling below. At the time of visit, the PA header pressure was reading 1000 mmWC. Lesser PA will contribute to more fluidising air and can thus avoid localised settling. At lower load, the bed is shallow. Hence the bed would not fluidise thoroughly. If we have to continue with lower loads, high DP nozzle drop is advised. Or else the bed area has to reduced by blinding the end row of nozzles and by building a refractory wall over the plugged nozzles to a height of one meter / up to bed coils top. 2. Small clinkers present in bed ash The bed ash was found to be reddish. This is typical of Indonesian coal where the iron content is
above 5%. Iron content makes the bed heavier. Operating at lower steam generation rate leads to formation of clinkers due to static burning. 3. Operation of third & fourth compartments independently Currently the bed height was found to be less as the steam generation is less. Bed temperature was maximum 850 deg C even with lesser bed height. There is no rear inspection door provided for 3rd & 4th compartment as the bank as hopper prevents the access. Hence it is not possible to start the bed from 4th compartment. There are boilers with bed SH compartment in the rear side. Such boilers are always started with 1st compartment. We cannot consider this as handicap for boiler operation. In the sense not all boilers would have the same kind of flexibilities for boiler operation. 4. Deaerator vent loss is high The condensate rate returns virtually at the same pressure from the bladders. Since the condensate pressure is killed only at deaerator, the flash steam production is high. This leads to excess steam venting at deaerator. We close one of the vent valves (there are two vent valves at deaerator), then the deaerator would pressurise and the condensate return line will be pressurised. If the condensate is flashed at plant, then the steam loss at deaerator will come down. Clinkers from first field ash hopper of ESP At shallow bed operation, it is possible that the coal fines keep escaping to ESP hoppers. It is advised to check loss on ignition (LOI) of ESP ash. It should be less than 18%. If there was ash plugging up to field, then the ash could clinker due to heat energy from sparks. Only internal inspection can throw more light. Ash plugging is possible since there are cold spots in ESP hoppers and as the ESP outlet gas temperature is less than 60 deg C. The moisture (25% of coal weight) in flue gas would condense and make the ash lumpy even at the field itself. 1. There is cold air ingress seen in ESP roof. See photo in annexure. This must be arrested. There is no penthouse in this ESP. There is no space between the top chequered plate and the ESP roof. Hence it is not possible to inspect / repair from outside. The ESP has to be inspected from inside roof. There will be any ash at air ingress point. This is to be observed from inside and corrective action is to be done. 2. The oxygen percent is seen to go up by 3% across ESP as measured by portable O2 analyser. 3. There are no hopper heaters provided at ESP hoppers. This makes the ash flowability difficult. 4. The inspection doors are simple plate type doors instead of mineral wool packed doors. The cold surface of inspection doors plates act as sites for condensation of moisture. 5. Air ingress is seen in inspection doors due to absence of fasteners and due to improper seal rope fit up. It is recommended to use 40 mm flat rope at all doors. 6. The ash by pass chute should also be insulated to avoid cold spots. 7. The ESP columns are seen to be uninsulated. This can lead to condensation of flue gas moisture. These columns are to be insulated. 8. The O2 level in flue gas is seen to be 9.8%, 12.5% and 15.8% at ECO outlet, APH outlet and at ESP outlet respectively. There is air ingress along the flue path in the flanges where asbestoes rope is provided. It is advised to seal weld all remaining duct flanges.
9. Insulation of APH to ESP inlet gas duct is pending. The duct shall be laid over the stiffeners as well. 10. There is a rack & pinion gate provided above the knife edge gate of ash transmitting vessels. The rack & pinion gate must be removed. Only knife edge gate is adequate for the maintenance. 11. The chute up to knife edge gate must be insulated. 12. ESP field inspection doors are seen with a single door system. This should be with a double door system as shown in annexure. There has to be insulation mattress between the outer and inner doors. 13. The space around rapping shaft penetration in ESP shall be provided with complete insulation to avoid cold spots. 14. Since the boiler is operating at lower load, the flue gas temperature would come down and cause wetness of ash. Hence the economiser by pass can be used to improve the gas inlet temperature to APH. Additional parameters for monitoring 1. Some of the Indonesian coals are known to have slagging / fouling ash constituents. Slagging / clinkering in bed occur in boiler due to Fe2O3 in ash. Na2O and K2O cause the ash to foul in boiler bank and economiser coils. The bed ash must be checked for iron content by a permanent magnet. The iron content should be restricted to below 15% by weight by adding fresh bed material. Recycled ash is not allowed in case it has iron. The log sheet should be designed to contain the reading of iron in ash. The limit should also be pre-printed in log sheet. A daily analysis is recommended. 2. Bulk density of bed ash has to be < 1200 kg/m3. This parameter should also be logged to note the deviation. Bulk density goes up due to presence of iron. A daily analysis is recommended. Observation of failed bed coil The bed coil seems to have thinned down on the left side of boiler where the coal nozzle comes closer to bed coil. In addition the coil seems to have sagged. 1. If the load is going to be lesser for more time period, the erosion rate will be high. It is advised to keep the PA header pressure as low as possible. The direction of coal transport air is upwards at lesser bed height. 2. At present settling of bed (lowering of bottom bed temperatures) is seen due to less air flow in 2 compartments. It is possible to reduce the bed area to cover some portion of bed coil & air nozzle. This results in improvement of fluidisation quality. 3. It is advised to inspect the bottom header of bed coil. If there is any sludge / foreign material / corrosion products are present, the same shall be removed. Clinker / tar formation in coal pipe 1. The decoking of coal lines should be done only in slumped bed condition. IF we do dechoking with the bed in fluidised condition, it leads to drawing of hot bed ash in to the coal pipe. Indonesian coals have high Volatile matter and they burn in coal lines. The ignited coal forms a clinker due to coal ash tendency.
2. It is advised to specify that the ash fusion temperature of coal should be above 1200 deg C. For every coal lot, the ash fusion temperature must be checked before use. Clinkers are known to occur due to ash constituents as mentioned earlier. Outside laboratory service shall be taken for this purpose. 3. The moisture running down from coal drain pipe is nothing but the moisture from coal that vaporised due to hot air. The condensation is due to cooling by air inside airbox. This can be seen in any high moisture fuel. Full load trials on 28 may 2011 The plant steam consumption was about 200 TPD. Hence for trial purpose the venting arrangement was required. Fresh bed material had to be arranged to take care of full load requirement and to make for the drains to remove clinkers from the bed. The boiler was operated at full load of 24 TPH for nearly 40 minutes duration. During this period the feed water temperature was only 75 deg C as the plant condensate return was not available. The coal and ash samples were taken for analysis. The only limitation that was experienced was the ID fan current & furnace pressurisation. This was diagnosed to be the partial closure of main damper in the Economiser to APH inlet gas duct. The boiler was again taken to full load and this time the Apollo executives witnessed the full load trial. The coal & ash samples were taken for analysis purpose. The DCS snap shots are taken by Dalkia engineers. However I have attached the photos with this report. Following improvement is required to avoid settling and clinker generation The present air nozzle is with 24 no of 3.5 mm holes diameter. The pressure drop at MCR is 94 mmWC only. There are two options available to improve the pressure drop. In the option 1, the hole diameter can be 3 mm and 24 holes. This will give a pressure drop of 174 mmWC at MCR. In option 2, the hole dia can be 3.5 mm with 18 holes and it will give a pressure drop of 167 mmWC. Less pressure drop leads to clinker formation on a regular basis.
K.K.Parthiban
ANNEXURE 1: PHOTOGRAPHS & COMMENTS
Photo 1- The clinker formation noted in coal drain line. This happens when the coal ash has slagging / fouling properties. This can be known from a chemical analysis and ash fusion temperature.
Photo 2- This photo shows granular clinkers formed inside the bed. This implies the bed is settling due to low load near the wall / along the compartment borders. Frequent mixing will help to avoid this. Ash characteristics need to be understood. Indonesian coals are known for slagging & fouling properties.
Photo 3: The ESP column shall be insulated to prevent flue gas condensation / corrosion inside the ESP. The ESP inlet duct should be insulated.
Photo 4: A view of the ESP hopper is seen here. Hopper heaters are a must for smooth flow of ash. The inspection door must be with insulation box. It is advised to have double door system so that inside condensation can be avoided. Another way is to provide removable insulation box outside this door.
Photo 5: The ash by pass chute in ESP needs to be insulated so that cold spot can be avoided in the hopper.
Photo 6: The insulation of seal box is very important to avoid flue gas corrosion and condensation inside the ESP. The ash flowability can be improved if we insulate the seal box.
Photo 7: The rack & pinion gate shall be removed. Only bottom knife gate is sufficient for maintenance. The pipe up to knife edge gate shall be insulated for smooth flow.
Photo 8- The ESP is provided with a single door system. This is prone for corrosion and condensation inside ESP.
Photo 9: we can see that there is air ingress in ESP roof area. Since this ESP is without penthouse, the leakage can be arrested only after a shut down by going in to the gas path of ESP.
Photo 10: ESP hopper doors are to be fitted with flat rope. The door has to be with a insulation box too.
Photo 11: There are too many flange joints which are packed with round asbestos rope. These are to be checked for leaks. Ideally these joints are to be seal welded and insulated too.
Photo 12: The inspection door at APH hopper was found to be admitting air inside the gas path.
Photo 13: The expansion joint at FD fan delivery is not leak proof. The rope is seen to have come out. Flat rope is advised here.
Photo 14: The flange joint of air flow meter before APH is found to be leaky. Flat rope is advised here.
Photo 15: The corrosion of ESP where a single door was fitted. This is a case from another plant. The photo shows the importance of a proper double door system for ESP.
Photo 16 & 17: ESPs are to be provided with a double door system. This ensures that there is no exposed uninsulated area around the door. In the absence of double door system, the corrosion of casing is more. These problems are addressed by double door system. The photo at right is the arrangement with double door system. In double door system placement of LRB mattress can help to keep the inner door warmer.
Photo 18: The corrosion of ESP casing inside where the rapping shaft penetrates the casing. This shows the need for providing insulation over the sealbox of rapping system.
Photo 19: When the insulation is not proper in ESP, the flue gas condenses. Actually this is a case where the flue gas has condensed inside penthouse in a different boiler.
Photo 19: The ESP casing seen damaged in a plant where the main column & tie beams were not insulated. Hence the ESP collapsed in two years time. This is from a case of high moisture in fuel.
AT HOPPER JUNCTIONS TYPICAL CASING
TYPICAL FUNNEL
DOOR FITTING
DETAIL OF INSPECTION
Figure 1: The insulation detailing is very important for ESP. The above is an extract from the insulation arrangement drawing by a good ESP manufacturer.
ANNEXURE 2: EFFECT OF INDONESIAN COAL / LOW ASH FUSION TEMPERATURE COAL IN SEVERAL INSTALLATIONS
Photo 1: The behaviour of low ash fusion temperature coal. This photo is from Birla white cements, where Barmer lignite was used. The bed used to defluidise and tubes used to fail due to erosion. Only after balancing the bed with sand, the bed could operate. In addition, the bed temperature had to be controlled.
Photo 2: The capping of ash over the nozzles at Birla white cements due to high iron in the lignite. Fusion of ash due to low ash fusion temperature coal leads to this.
Photo 3: Severe fouling seen in SPB by one lot of coal that caused immediate increase of the ESP inlet temperature. Boiler was stopped and coal was changed immediately.
Photo 4: Fouled ash seen in the superheater section. A deposit analysis revealed presence of iron and sodium.
Photo 5: The repeated bed coil failure seen in Sun paper mill boiler. Since the inner coil was not there and the pitch being closer, it led to frequent bed coil failures.
Photo 6: Ash fouling in SH area in sun paper mill, when low ash fusion temperature coal was used.
Photo 7: This picture is from ITC Mettupalayam, when the coal was with low ash fusion temperature. Incidentally, the sand loading was more as this boiler was running along with saw dust. It did not lead to failure. The bed temperatures used to be 850 deg C.
Photo 8: Slagging and erosion seen when all the coils were with studs in an installation at GFL, Gujarat.
Photo 9: Localized fouling was seen in a boiler when one day low ash fusion temperature coal was fired.
Photo 10: The failure of bed coils in MCL when the coal was having low ash fusion temperature.
Photo 11: At MCL, it was learnt that the coal ash was influencing the bed ash fusion temperature depending on ash content / Fe2O3 / Na2O. Lot to lot Indonesian coals are seen to vary in fusion temperature.
Photo 12: The ash clinkers forming if the bed is slagging. Seen at Madras cements.
Photo 13: The studs of the bottom coils are seen covered with slagged ash. Nearby inner coil is seen flattened due to erosion right from the bend. This is from the recent failure in bed coil at DCW due to coal ash chemistry.
Photo 14: The ash is seen accumulated above the bottom coil. If this was the pattern during boiler operating condition, then the failure pattern will be at seen at 4o – 5o clock position / 7o – 8o clock position. This picture is from DCW. Inner coil being plain, there is flatness of tube seen.
Photo 15: Air nozzles seen capped with molten ash. This kind of slagging is seen in boilers fired with low ash fusion temperature. This picture is from DCW. It is learnt this was same here as well.
ANNEXURE 3: FULL LOAD TRIAL
Photo 1- Small clinkers are seen coming from all compartments during activation of idle beds. For this reason bed material was frequently drained and replenished with fresh bed material.
Photos 2- Plate type clinker are seen in 3rd & 4th compartment before activation. This has happened due to powder coal burning as a layer on top of idle bed. Operating with deeper bed would generally reduce this risk. This type of clinkers can be avoided by frequency mixing of idle bed and by operating with a air box pressure of 450 mmWC.
Photo 3: The boiler load was raised to 24 TPH with feed water temperature of 75 deg C. The feed water temperature was low since flash steam return was stopped. The above was the O2 & CO readings in APH inlet & ID inlet. We can see the O2 increase from 4.2% to 11.7%.
Photo 4: A screen shot of boiler during full load operation. Even though the boiler was with small clinkers and with settled bed in some part of the bed due to low load operation, the full load could be obtained. The steam was vented for this purpose. Bed temperature indication was not available in 4th compartment due to hardware problem.
Photo 5: During the full load trial, the airbox pressure was maintained at 450 mmWC. Fresh bed material was arranged by M/S Apollo tyres for capacity trial.
Photo 6: The feed water temperature at deaerator outlet was 71 deg due to some trials being carried out at plan end. With this temperature the boiler could reach the load of 24 TPH.
Photo 7: The APH inspection door needs to be sealed properly. Approach for gas sampling is not available at APH outlet. Also the APH maintenance door needs proper fasteners. CI wheels are breaking down.
Photo 8: The ECO to APH main duct damper was in partly closed condition. During the capacity trial this was identified. Open // Close signs are not seen on the dampers.
Photo 9: These type clinkers are still in the bed. As the oil fired boiler back up is available, the bed must be cleaned off clinkers.
Photo 10: The FD fan is seen with a flap damper very close to the inlet. The inlet conditions should be improved for better performance of the FD fan.
Photo 11: It is seen that the PA fan inlet damper is of smaller size and placed at fan inlet. It should be of higher size and should be placed away from the fan inlet.
Photo 12: The ID dampers are also seen to be close to the inlet of fan. These result in poor performance of the fan.
Photo 13: The pant chute of feeder is not proper. It dumps more coal in one line. The design needs improvement.
Photo 14: There is no access provided for PA lines.
