During the site visit, part of the secondary superheater in the final loop was cut off and inspected. Continuous operation was not possible due to high production costs in the first year. In the superheater, where the temperature of the metal is high, FeCl2 is formed.
There may exist a possibility to reduce the contamination of ash in the last SH section.
Efficiency of the boiler is low. The unburnt coal in fly ash is around 79%
Vented seal device, a loop seal that allows collected solids to be returned to the furnace. Collected solids are returned to the combustion chamber via a loop seal that provides a pressure seal between the positive pressure in the bottom furnace and the negative draft in the solids separator. This prevents the flue gas from the furnace from shorting the dip leg of the separator and reducing the collection efficiency of the separator.
By injecting small amounts of high-pressure fluidizing air into the loop seal, the solids movement back to the lower furnace is maintained.
Even with a furnace temperature of 950 deg C & flue gas O2 at 8%, the steam generation is 62 TPH
In the input coal to coal handling plant the moisture can be at least 2-3% higher. Coal can dry up in the coal after coal handling plant the moisture can be at least 2-3% higher. Therefore, there is a need to ascertain the variation in Total moisture in raw coal and fired coal.
The moisture content and GCV of crushed coal are reliable because the coal is homogenized during the crushing and screening process.
The coal feeding is normally varied depending on the load. This variable can affect the
It was calculated that the total airflow requirement for loop sealing should be 1000 m3/hour while the current capacity of the roots blower is 3210 m3/hour and all its capacity was used up for loop sealing. The operation of the loop seal can be identified by the heavier airbox pressure seen in the dip leg and the riser leg which has less airbox pressure. See Figure 4, which shows that the airbox pressure in the dip leg is higher than on the riser side.
In a nut shell, the loop closure function is very vital in fines recirculation and this can contribute to the growth of the thin bed over the dense bed.
Vent control valve is required since a twin lobe blower is used here. Or else VFD will be required
The dip leg seal must be filled with material to a height greater than the riser side. After correcting the loop seal it was seen to contribute to dust load in free table. In fact, the dip leg airflow is kept at 0.3 m/s or less simply to promote aeration of material in the dip leg.
In case the fines increase above the limit and the furnace is loaded with dust, the same can be drained from dip bone.
Loop seal needs drain in order to control the lean bed density. Too much of dust will result in combustion temperature
Presence of adequate ash in dip leg is known by this
Comparison of the pressures tells the loop seal is functioning
Height of dense bed is known by this (includes drop across distributor)
Height of lean bed is known by this
Air flow meter after APH ensures the leakage air flow in APH is ignored. Moreover tube leakage is identifiable in case the damper opening increases for the same air flow
As APH develops tube leakage the pressure drop across APH can be used for inference
Pressure to ensure penetration is possible for CO ppm reduction is possible by choosing to close dampers optionally
As APH develops leakage the pressure drop can be used for diagnosis
The pipe is made to fill the space between the pipe and the pipe hole. After the pipe touches the pipe hole, the pipe is further thinned to ensure a tight joint is established. As the mandrel is moved inwards, the rollers develop the internal force which expands the tube.
In the case of drawn or roller-expanded pipes, the pipe holes in the pipe plates of drums, pockets or heads must be designed so that the pipes can be effectively pulled into them.
Expanded connections
Tube expansion or tube rolling is a process of cold working the end of the tube in contact with the metal of the drum or header containing the tube holes or seats. The residual radial pressure between the tube and tube seat resulting from a properly stretched tube will provide a pressure-tight joint of great strength and stability, as shown in figures 2 & 3. The strength of the joint is developed by the expanded joint, pipe threads or by the configuration as in the case of handhole fittings.
Where the pipes are normal to the slab, there shall be a neck or parallel seating strip at least ½ inch deep in a plane parallel to the axis of the pipe in the holes.
Tubes and integral pipes
Scope and restrictions
Expanding of tubes and pipes
- D12, Tube inner dia after pre-thinning = Sqrt [4 *A2 /142]
- The high contact stresses imposed by rolling make it more likely that stress corrosion will cause tube-end failure. The transition from the reduced wall is a possible trouble source in rolled tubes
- The extreme ease of operation of the HydroPro requires almost no training
- Hydro-expanding is successful in out-of-round holes and in holes distorted by tube plugging
Where the outside of the pipe is not accessible, the following formula is used for pipe expansion. A pipe expander is used to increase the circumference of the pipe ends until a proper joint is made. Expansion beyond the point that develops the greatest force in the weaker link of the pipe joint is overshoot.
But overexpansion cannot be easily detected and causes excessive stresses in the tube and tube sheet material.
STRESS INDUCED CORROSION IN EVAPORATOR COILS
TUBE FAILURE IN WATERWALL
DUST POLLUTION FROM BOILER
DEAERATER STEAM HAMMERING EXPERIENCED AT TIMES
DUCTING SYSTEM DESIGN DEFECT
HIGH UNBURNT CARBON IN ASH
REPEATED SHELL TUBE WELDMENT IN 2 PASS INLET
LOW STEAM TEMPERATURE FROM BOILER
WATER LEVEL WAS NOT STEADY IN A 10 TPH BI DRUM BOILER
FREQUENT GAUGE GLASS FAILURE
REPEATED BANK TUBE FAILURE
CHIMNEY VIBRATION
FD FAN NOISE
NON-PERFORMANCE OF PA FANS
NON-PERFORMANCE OF ID FANS
NON-LOADING OF FD FANS
CAUSTIC GOUGING IN BED TUBES IN AFBC BOILER
MUD INSIDE BOILER AND INEFFECTIVE SOFTENING
BED COIL FAILURE
WET PRESERVATION UNDER HYDRAULIC PRESSURE
The pH should be maintained at 10 and at least 150 ppm of sodium sulfite should be maintained.
DRY PRESERVATION
In this case, the boiler is always kept under a hydraulic pressure of 5 kg/cm2 pressure. If the boiler has non-drainable superheater sections, it will be difficult to get rid of water.
WET PRESERVATION
It was incredible that all the bench tubes were leaking at the connections within a month of firing the boiler. The owner of the sugar mill requested me to stop by and inspect the boiler to find out if there was anything wrong. I checked with the operator if there is a low water level operation, even for a short time.
I checked for any possibility of general circulation failure due to incorrect internal drum arrangement. The next day I returned to see how the boiler was working, as the owner asked me to come again. The boiler was equipped with a pulsating grate for burning sugar mill cane.
SAW NEXT DAY!
Eroded / corroded APH tubes 16. Economiser casing joints
Roof panel to side walls side joints 19. Water drum to bank casing plates
Gas pressure tappings 23. Leaky view holes
Boiler bank casing plate joints HOW TO LOCATE THE AIR INGRESS?
When the ash is not completely burned in the furnace, the flowability of the ash is reduced. Excess air cannot be supplied to the furnace when the ID draws the exhaust air downstream. When excess air is not supplied to the furnace, the furnace temperature exceeds the design gas temperatures.
Photo 4: Presence of white marks along the edge of the primary dryer in the steam space. Dark colored water indicates that there has been high pH in the boiler water corroding the boiler. Fossil fuels such as coal, lignite are prepared, stored in bunkers and burned in boilers.
In the case of multi-fuels it is possible to have metering of all fuels. The fuel handling system will have to operate for all three shifts in the case of low bulk density/fibrous fuel. FBC furnaces are designed for bed temperatures of 800 to 900 degrees C in the case of the underbed feed system.
In the case of the overbed feeding arrangement, the overbed temperature will probably exceed 1000 degrees C. In the case of high sulfur fuels, limestone feed must be fed to the fluidized bed.
COCaO
In the case of the convection superheater, the counter-flow superheater will have more tendency to deposit slag compared to the parallel-flow superheater. Depending on the moisture content of the fuel, a minimum proportion of refractory material will be required to dry the fuel. When this is not done, ash deposition and suffocation are encountered in the pipe sheet I pass.
When using oil, an air preheater is not recommended due to a high combustion temperature and NOx formation. According to regulations, the chimney height must be kept at a minimum, as indicated in the formula below. Concrete chimney with refractory interior is usually the choice when the fuel is Bagasse.
The total ash produced is calculated and based on the percentage of collection expected in the various collection zones, the ash disposal equipment is selected. Bedas removal is dependent on the size of the fuel particles and the fuel itself. The bed ash is negligible in the case of Rice Husk and other agricultural waste.
So for fossil fuels such as coal/lignite, only the bed-ash cooling system and bed-ash transport systems are planned. The % ash content in the fuel and erosion of ash determine the flue gas velocities to be assumed.
NCVkg/h x
The ash settling below final SH was found to spill more towards front portion of LTSH
FINAL SH
LTSH
In one case the economiser was eroding along the rear wall. The gas baffle arrangement was altered based on the CFD analysis and the erosion pattern was removed
The spaces were so much smaller that the ash could easily be caught in the narrow space of the also swaying. The spaces were so small that ash could easily fit into the narrow space of the pipes.
The SH coil bends had come close to the casing, that the bends had got eroding. The
The protection shield at bends was not placed properly during maintenance. This had
This boiler suffered erosion of tubes all over in the first pass whereas in the second pass suffered no erosion though the design gas velocities were same. The small window opening had
- See photo The fuel feed point is just above the ash collection chamber wall. The fuel will go directly to ash chamber
- The ID fan draft required will be 250 mmWC. At present what is selected is 210 mmWC
- A leak proof casing shall be provided for the furnace to prevent the air ingress in to the furnace
- MDC outlet pipe to vane clearance is too high. This gap shall be filled with a 5 mm rod all around to prevent erosion of outlet tubes
- When a compartment is slumped its air bleed off valve must be kept open
- The bed coil area required for 950 deg C combustion temperature @ 30% Excess air is 113m2
- At present the water after pressurizing may be going in to the economiser due to the long piping. It is recommended to maintain a minimum leakage flow in economiser while the bed temperature
- The bed coil thickness was measured at 12 0 clock position at the lower part of the coils, where caustic gouging phenomenon was experienced earlier. The readings did not indicate any such
- It was confirmed that the feed water temperature could be maintained at 105 deg C after the additional vent was incorporated at the deaerator tower manhole. The steam drum did not seem to
- It was seen the many birds have come through the FD fan and plugged the I pass of the APH tubes
- The tube bends of economizer were found to be eroding in between the banks. The tubes can be provided with cassette baffles as shown in the figure 2
- It is seen that the bed ash is being screened manually. It is recommended to use mechanized system so that the bed material specification can be easily adhered to
- The waterwall outlet duct to economizer duct is found to be distorted as the fabric expansion joint filled with ash. The fabric expansion joint needs to be cleared of ash to permit the thermal
- At many places manholes are buried inside the insulation. See photograph 20
The percentage of transfer of each constituent is a function of the boiler operating pressure. The internal assembly of the drum must be inspected by an expert who is familiar with the design. In the companies, the old faithful have not installed systems for design revision & field revision of the boiler &.
Some of the calls I receive are due to a lack of design knowledge within the internal project team. I have chosen to include the reports of some of the audits I have conducted in the recent past. In addition, the boiler operating engineers gained insight into the equipment they were working with.
Pump manufacturers recommend that the top of the pipe should be flat with no chance of air lock / cavity. With staggered pitch, the second rows of tubes must also be protected with shields. The dust expansion joint at water wall to wind box was found to be tight, indicating the filling of the bed material.
The dust expansion joint must be cleaned of ash to allow the thermal expansion of the boiler. The heat is extracted from the bed by the leaving flue gases and the bed evaporator tubes. Similar changes in operating loads may also warrant fine-tuning of the boiler's operating parameters.
In the continuous improvement of the design / operation of the FBC boilers there is always room for additions to this list.
Measure and maintain adequate Distributor plate drop
Underbeds are best suited to fuels with a higher carbon content such as Australian and African coals. The design of a fluid bed combustion boiler has a lot to do with the fuel type and fuel conditions. In such a case, the bed area and bed coil area may need to be covered until steam demand increases.
The use of VFD/use of smaller capacity fans would benefit the user in terms of energy savings and operational efficiency. There are many possibilities for a revision of the original design to current operating conditions. TIPS FOR IMPROVING OPERATION / CHANGES FOR IMPROVEMENT The following pages explain the tips with illustrations where necessary.
The tips are based on the operating experience of several brands of FBC boilers in India.
Check bed coil pitch for studded bed coils
Consider reduction of bed size
Inadequate instrumentation
Care of idle bed
Provide additional drain points
Care for idle bed
Use Optimum primary air pressure
Care for shutting PA damper in idle bed
Replace the Worn-out venturi / mixing nozzles promptly
