September 3, 2012

REPORT ON DIAGNOSIS OF LESS STEAM GENERATION IN WHRB

By K.K.Parthiban, Venus Energy Audit System

At the time visit, WHRB1, 2 and 4 were in operation. WHRB 3 was just taken out of service for clearing flue path of ABC outlet duct and for kiln accretion removal. The AFBC boiler was running on dolochar and coal mix above its capacity.

PROBLEMS AND SOLUTIONS – LESS STEAM GENERATION IN WHRB

GAS FLOW MEASUREMENT

In waste heat recovery application the steam generation is based on the gas flow and the gas temperature. When there is a problem of steam generation, the gas flow needs to be measured. Earlier Mr.Muthukumar visited with pitot tube and measured the gas flow and submitted a report on gas flow. Part of the report is reproduced in annexure 1. The gas flow was measured at ID fan inlet duct in WHRB 3 & 4. WHRB 1 & 2 were under shut down at the time of visit. The results are as below.

At WHRB 3, the gas flow was measured to be 30688 Nm3/h.

At WHRB 4, the gas flow was measured to be 25198 Nm3/h.

The above includes false air as well. Design gas flow was supposed to be 26000 Nm3/h.

Air ingress – Observations by undersigned during this visit

All boilers except WHRB 1 are provided with gas cooler system in working condition. The gas cooler system at WHRB 1 was already dummied. There were so many places in the gas cooler system, where in the false air could come in to the system and cause additional gas flow. Wherever the flue gas leaks one can see the marks of corrosion.

False air ingress from the isolation gate in gas by pass duct

The false air can come in to the system in the gap around the plate gate provided in the bypass duct. This can drop the WHRB inlet gas temperature, depending on the draft maintained at ABC outlet. See photo 1 in annexure 2.

False air ingress from the gas cooler inspection doors

The false air can enter in to ESP due to leakages in the gas cooler inspection doors. See photo 2. Photo 3 shows the air ingress location in the bypass gas cooler at kiln 2. Flue gas condensation marks are seen in photo 3.

Photo 4 shows the kiln stack vent. There is air ingress here. Again care should be taken not to maintain the draft to much negative here. False air would be high at higher negative pressure.

False air ingress from the fabric joint at inlet to WHRB

There are two thermocouples in the WHRB inlet gas duct. One is above the fabric expansion joint. It is being called as ABC outlet duct temperature. The thermocouple below the fabric expansion joint is called ash WHRB inlet gas temperature. See mimic attached in photo 5. See the temperature difference in all WHRB in photo 6. Photo 7 shows the location of the thermocouples. Photo 8 shows the flue gas leakage in expansion joint. The corrosion sign is an indication of the flue gas. The whitish color can be due to condensation of alkalis from gas (Na2O / K2O / P2O5 in flue gas). The flame test indicated air ingress in fabric. See photo 9. Photo 10 shows the three locations of air ingress in the expansion joint. The WHRB 3 was shut down during the visit. Its slagging pattern at gas inlet duct confirms that there is air ingress at the inlet of WHRB. Just after the air ingress point no accretions are seen. See photo 11 & 12.

False air ingress in between the gas duct and the waterwall

Photo 13 & 14 show the air ingress between that gas duct and the waterwall of radiation chamber. It is possible that the duct sealing is not properly designed. It is advised to apply plaster of paris over the insulation in this area for better sealing. See typical application in a FBC boiler roof. The insulation is plastered with plaster of paris and then covered with hessian cloth and black bitumen paint is applied to make it leak proof.

False air ingress through the failed fabric joints at ESP inlet / outlet duct

The fabric joints have failed in some cases. The expansion joint needs replacement. Weather protection is required to prevent the failure. Rain hoods are being provided by fabric manufacturer.

False air ingress through the ESP penthouse and inspection doors

The ESP of WHRB 3 was checked for air ingress by physical inspection of ESP penthouse.. There is no sign of air ingress. However the bypass ash chutes and improperly sealed inspection doors are the source of air ingress.

IN SUMMARY THERE ARE MANY LOCATIONS IN WHICH AIR INGRESS IS SEEN. THESE AFFECT THERMAL PERFORMANCE OF RADIANT SECTION OF THE WHRB. GAS FLOW MEASURMENTS SHOW HIGHER GAS FLOW. IN ACTUAL CASE, THE GAS FLOW COULD BE LESS.

GAS COMPOSITION AT ECONOMISER OUTLET AND AT ID FAN INLET

CO in gas from WHRB

The CO level was measured in three WHRB which were in operation. The photographs 20 to 24 show the gas analysis as indicted by Kane portable gas analyser. CO gets reduced when there is air ingress. We can see even with oxygen of 9%, CO levels are high. When CO is present in gas, it implies the combustion is incomplete at ABC.

Lower ash fusion temperature and slagging

Presence of CO means starvation of air at ABC. The flue gas contains Fe2O3. In reducing

atmosphere, the fouling will be high. See a photo 25 from the Babcock steam handbook on the ash melting point due to reducing atmosphere.

Condition of ABC blowers and air leaks

The ABC blowers are to be kept in operating condition. Generally less importance is given by the kiln operating team. The air leak at inspection doors are to be arrested. The air leakage in non operating fans should be plugged by means of an inlet dummy plate. See photo 26.

STEAM FLOW MEASUREMENT

Flowmeter installation

The steam flowmeter can be seen in photo 27. This is a flow nozzle. The flowmeter pipe size is lower than the steam line. There is hardly any straight portion before and after the flowmeter. This will cause error. At least 20D is maintained for steam flowmeter.

Calibration range of flowmeter

At present the steam flow transmitter is calibrated for 8000 mmWC, which is for a steam flow of 15000 kg/h. The steam generation is not likely to go beyond 12 TPH. In that case for better resolution, the transmitter can be calibrated for a lesser range.

FOULING IN WHRB & HIGH EXIT TEMPERATURE

Fouling in economiser

The CO level was measured in three WHRB which were in operation. The photographs 20 to 24 show the gas analysis as indicted by Kane portable gas analyser. CO gets reduced when there is air ingress. We can see even with oxygen of 9%, CO levels are high. When CO is present in gas, it implies the combustion is incomplete at ABC.

Fouling components in ash

Not all the coals are same with respect to their ash chemistry. Some ash cause fouling and some slag. Kiln being a source of iron, slagging is possible at higher gas temperature and when there is starvation of air. Alkalis are source of fouling (at convective passes). Sodium and potassium in ash cause this effect. It is possible to clean this, by periodical injection of bed ash in the economiser gas path.

Steam operated soot blowers / sonic blowers can be used for this purpose.

It is advised to analyze the ash chemical composition of coal ash, deposits for better understanding of the coal being used for kiln as well.

TIPS FOR KILN OPERATION AND FOR INCREASED STEAM GENERATION

Generally kiln operators maintain only draft and ABC outlet temperature. The gas generation is however dependent on the volatile matter and the coal fines elutriated from the kiln. If the steam generation is less, it is because the volatile matter / fines combustion at ABC is inadequate.

It is possible to add some amount of coal fines along with raw coal at kiln inlet. This will see to it that WHRB is better loaded. It should be the interest of kiln operators to generate steam as per WHRB rating. Fines addition only would bring this about. As the fines are added, combustion air is required at ABC. Hence the blowers are to be made in working condition. As pointed out earlier the leakages in non operating fans are to be corrected.

WATER CHEMISTRY AND FOULING ON WATER SIDE

During the visit of our engineer, the WHRB steam drum was inspected. The boiler was found to be corroding inside. See photo 1 to 4 in annexure 3.

Photo 5 & 6 show the magnetite layer in a well maintained water chemistry regime. The aim of water chemistry is to achieve the magnetite layer formation and maintain the same as well. Due to removal of iron from parent tube material, the silica also goes high calling for high blow down rate. There is silica carryover to steam as indicated by the water chemistry reports. See photo 8. There is foaming inside the drum due to presence of corrosion products.

Actual blow down rate was measured by bucket method. Ignoring flash steam loss, the blow down rate is seen to be 4%.

High pressure boilers are generally susceptible to caustic attack. For this reason the phosphate level is maintained at < 3 ppm. The WHRB circulation would be less if heat input is less. The evaporator tubes & waterwall tubes at 4th pass would have less circulation due to low gas temperatures. Hence the caustic can concentrate and cause gross corrosion of the boiler. This was informed at design stage itself. See photo 7.

All volatile treatment with 1-2 ppm of phosphate is recommended for boiler water chemistry. The pH can be at 9.2 to 9.4.

Fouling in boiler tubes can bring down the heat transfer. Low load operation also affects the circulation and generates more corrosion products.

It is advised to take up with NALCO on the proper treatment for this plant. Phosphate at 15- 20 ppm is alarmingly high. At every shut down drum photographs are to be taken and NALCO representative should be involved in the achievement of magnetite layer at drum.

K.K.Parthiban.

ANNEXURE 1 – PITOT STUDY

Photo 01: This is the photograph taken at site. It shows the tapping taken for pitot study in WHRB-3. Like this the tapping was taken for the WHRB-4 also. At the time of inspection, these two boilers were in running conditions and pitot study was conducted to conclude the quantity of flue gas flow is as per design.

Photo 02: The photograph shows the ID fan inlet duct dimension where the tappings were made for pitot study. A1 to D4 are location points inside the duct where individual readings are noted and tabulated.

PITOT STUDY TEST READINGS AND CALCULATION FOR WHRB-3:

As per this calculation, the quantity of flue gas coming out of the boiler is 30,688 Nm3/hr. This includes false air ingress.

PITOT STUDY TEST READINGS AND CALCULATION FOR WHRB-4:

As per this calculation, the quantity of flue gas coming out of the boiler is 25,200 Nm3/hr. This includes false air ingress.

ANNEXURE 2: CAUSES FOR LESS STEAM GENEATION IN WHRB

Photo 1: The vertical gate at the gas cooler duct is not leak-proof. Air ingress here adds to flue gas at ID fan inlet.

Photo 2: The air cooler duct has large size inspection doors. Air ingress through these doors adds to gas volume at ID fan inlet.

Photo 3: Condensation marks in the doors are due to gas leakage / air ingress in to the system.

Photo 4: The kiln cap does not seat properly at the stack outlet. This can cause outside air to come in to WHRB depending on the draft maintained. Ingress of air can reduce the gas temperature. It can also help in reducing unburnt gases from ABC. It depends on the exit CO condition from ABC.

Photo 5: There is a temperature drop of 140 deg C between the ABC outlet gas temperature and the WHRB inlet gas temperature. In all WHRB the same trend is seen. This low temperature leads to less steam generation by the radiant chamber ( I pass of the boiler).

Photo 6: Log sheet of WHRBs. From the WHRB log sheets, we can see that the gas temperatures are less at WHRB inlet as compared to ABC outlet. There are some readings showing less difference. This can be due to the draft effect. Less negative draft can lead to higher temperature at WHRB inlet.

 

Photo 7: ABC outlet temperature location and WHRB inlet gas temperatures are shown here. Both are in refractory lined duct.

Photo 8: Flue gas condensation at the boiler inlet duct expansion joint is an indication of air ingress.

Photo 9: flame test at boiler inlet expansion joint shows air ingress. This air ingress drops the gas inlet temperature at boiler inlet.

Photo 10: Locations of air ingress are marked above. The outer fabric (pressure barrier) seems to have got burnt / lost due to weathering. SS Metallic expansion joints can also be tried here. Rain hood is a must.

Photo 11 & 12: There are fused accretions in the bend circular duct above the expansion joint. In fact the passage is plugged (top photo). There is no sign of accretion in the transition duct above WHRB (bottom photo). As the gas gets oxygen from outside, the fouling of ash gets reduced.

Photo 13 & 14: Air is being sucked around the gas inlet duct to boiler.

Photo 15: Photo shows the application of Plaster of paris / cement fluff over the insulation of hard casing finish and for leak proof arrangement.

Photo 16: Failed expansion joint at WHRB 3. The gas flow was measured to be 30688 NM3/h in WHRB 3.

Photo 17: Failed fabric joint at WHRB 2 ESP.

Photo 18: Failed fabric joint is covered with asbestoes cloth. The gas flow measurement have gone wrong with this. This is at WHRB 3.

Photo 19: Improperly closed manholes would add O2 in the flue gas. The hoppers must be insulated to avoid ash flow problems.

Photo 20: The CO reading and O2 reading at ID inlet of WHRB 4 are seen here. The CO level at WHRB 4 was seen to be 2566 ppm. The O2 level was 3.4%. This is inclusive of air ingress.

Photo 21 & 22: The O2 and CO in WHRB 1. The left side readings are at ID inlet. Right side readings are at economiser outlet. Across ESP there is about 2.7% O2 increase.

Photo 23 & 24: The CO level at WHRB 2 is very high. The left side reading is at ID fan inlet. The right side photo is at economiser outlet. There is 3.4% rise in O2 across the ESP.

Photo 25: The effect of iron in ash is to reduce the fusion temperature. In addition the effect of oxygen starvation is to lower the ash fusion temperature further.

Photo 26: The air leak from non operating blower at ABC shall be arrested by a dummy plate. All the blowers shall be kept fit for service. The leak in inspection door of air header must be arrested.

Photo 27: The steam flowmeter needs 20D straight portion before and after the flowmeter. Otherwise the DP measurement goes wrong.

Photo 28: Fouling of ash in economiser and higher exit temperature in WHRB.

Photo 29: Present exhaust gas temperature in WHRB is seen to be as high as 180 deg C even at less steam generation. Soot blowers are advised.

Photo 30: Coal ash – base to acid ratio

Photo 31: coal ash type – lignitic / bituminous

Photo 32: Ash fouling index and slagging index

Photo 33 & 34: The above photos show the sonic blowers used in CFBC boiler. Depending on gas temperature and loose ash, this can be used. One manufacturer is Nirafon India at Kolkata.

ANNEXURE 3: CORROSION OF WHRB & WATER CHEMISTRY

Photo 1 to 4: Condition of boiler drum at our WHRB. The total boiler is undergoing corrosion. This corrosion can contribute to silica rise in boiler water. The silica gets fed from the dissolved iron from tube material. This can happen due to low steam generation and the high PO4 in boiler water.

Photo 5 & 6: condition of steam drum at some other plants. Photo 6 shows the phosphate treatment with PO4 at 2-3 ppm. Photo 5 shows drum with AVT with 1 ppm of PO4.

Photo 7: The above is the report given at design clearance stage. Since the boiler has poor circulation design, water chemistry is important. The caustic gouging phenomenon will experienced in the boiler and hence volatile treatment was recommended. Phosphate dosing shall be maintained at 1 ppm, since cooling water can contaminate any time and bring down the boiler pH anytime.

Photo 8: Silica values are seen to be higher than 0.02 ppm in steam. Silica in boiler water is seen uncontrollable.

Photo 9: The blow down rate is 4%. This is too high when pure water is being supplied through UF-RO-MB stream.