Innovative Solutions for Controlling Slagging and Fouling in Coal Fired BFBC and CFBC

Boilers

K.K.Parthiban Venus energy audit system

India

SYNOPSIS

Coal is widely used fuel in a thermal power plant / process steam boilers. Coal has combustibles

and non combustibles. Non combustible is moisture and mineral matter. The mineral matter after

combustion is termed as ash. This ash can slag and / or foul over the heat transfer surfaces affecting

the heat transfer to tubes and sometimes lead to corrosion as well. Slagging of ash means sticking

of ash in the molten form over the furnace wall tubes and in some cases, on the superheaters located

in the high temperature zone. Fouling occurs on low temperature zones, where some minerals

which get vaporized at furnace condense at temperatures below 750 deg C.

Since the mineral fractions of coal ash varies, not all the coals behave in the same manner in a

boiler. Again depending on the combustion technology adopted, the ash behaves in a different

manner from one boiler to another boiler.

CASE STUDY 1

This plant has 3 x 95 TPH BFBC boilers with under bed feeding arrangement. The boilers are used

for captive power requirement of the cement plant. The coal used was an E grade Indian coal with

40% ash. The loss on ignition of fly ash used to be 5.5% to 6%. In order to reduce the LOI to less

than 2%, one of the boilers was converted from BFBC to CFBC in the year 2009. The conversion

resulted in following problems.

The problems

1. The steam temperature was reduced to 450 deg C as against rated temperature of 490 deg C.

2. Less steam generation as limited by ID fan draft. The steam flow was about 80 TPH as against

the earlier design of 95 TPH.

3. The gas temperature at ID inlet touched 195 deg C as against 140 deg C at lesser load itself.

First visit

The first visit was made when the boiler was in operation. The second visit was made when the

boiler was in shut down.

DESIGN REVIEW

A new external CFB combustor with three hot refractory cyclones was provided. See figure 1.1.

The external combustor was with In-bed & free board coils. The dip legs of three hot cyclones were

connected back to combustor. There was no SH coil addition. The flue gas from the cyclone outlet

was connected to the old boiler bottom where earlier BFB combustor was there. Basically the CFB

combustor was found to be a BFB combustor with ash recycling facility. A complete review of

design was done. The following were the summary of findings.

1. The new furnace was with a bed area of 67.67 m2. With 20% excess air & with a bed

temperature of 900 deg C, the fluidization velocity worked out to be 1.85 m/s. The new furnace

was provided with over bed feeding.

2. The bed coil length required to restrict the combustion temperature to 900 deg C was 908 m.

The equivalent available length of bed coil was 940 m.

3. The bed coil was laid out at 550 mm above bed bottom and ended at 1515 mm. This meant that

the bed height inside had to be at 750 mmWC. The DP drop was expected to be of the order of

1040 mmWC. That meant for full load the air box pressure should be at 1790 mmWC for MCR

steam generation. The new FD fans were selected considering this high head pressure drop.

4. The over bed coils could pick up radiation heat transfer and compensate for the waterwall

pickup which was available prior to conversion.

5. Assuming 10 % over fire air port velocity worked out to be 60 m/s.

6. The new bed coils were with independent downcomers & independent risers. The outlet of bed

coils (both inbed & over bed) were not connected to old waterwall section. The heat pick up by

the old boiler would be less and hence its circulation would not be as good as before. Poor

circulation could give deposit induced corrosion occasionally.

7. The new combustor was provided 4.5 meter free board height above the bed coil top. Since the

new combustor was provided with over bed feed system, the finer particles would first travel to

cyclone and return via dip leg. Yet the cyclones do have a cut off particle size, which may be

135 microns as indicated by ash particle analysis. That meant the carbon particles of size less

than 135 microns could leave the furnace and reach the SH without much time for combustion.

In traditional CBFC boiler, the intense mixing available in the lower and upper furnace helps in

better carbon burn up. The turbulence was absent in this furnace.

8. The furnace volume above the lower bed coil was found to be 360 m3 including the gas pipes to

cyclones. The residence time was calculated to be to be 3 seconds.

9. No compartments were envisaged and hence the turndown had to be done using the entire bed.

The startup would consume huge amount of oil when the entire bed has to be heated for coal

feeding. The new hot air generator was feeding hot gas to the entire bed. In the earlier BFB

combustor, the hot gas generator was connected to start up compartment which was only one-

fourth of the total bed.

10. The refractory lining adopted for the combustor was 300 mm only. The skin temperature ranged

from 85 to 125 deg C when measured by an IR Camera.

11. The superheater now did not receive the direct radiation component from bed. Direct radiation

would add a steam temperature raise of 25 deg C. Hence the superheater surface should have

been increased. It was checked that the other boilers were capable of achieving steam

temperature with 20% excess air and with a bed temperature of even 850 deg C even at a load

of 70 TPH.

12. The ID fans were connected in series to make up for draft loss across the cyclone & connecting

gas ducts to old boiler. The interconnection gas duct between two ID fans was found to be with

abrupt bends instead of with smooth curved inlet & outlet. In addition the gas duct had two

successive 90 deg duct bends very close by. This could cause high draft loss. See photo 1.10.

OPERATIONAL INSPECTION

1. The boiler load was about 75 TPH at the time of visit. The O2 level in APH inlet was

maintained at 4.5%. CO ppm at APH inlet & economiser outlet was about 2400 ppm.

2. The ID fans were found to run at full damper open position. The exit gas temperature touched

198 deg C. The ID fans were designed for 140 deg C gas temperature. Prior to conversion, the

boiler draft loss was only 95 mmWC. Now the both ID fans are found to do a duty of 110

mmWC each. The ESP outlet draft is measured to be -185 mmWC. The ID fan design head was

180 mmWC at 140 deg C. The ID fan head would be less due to high gas temperature.

3. The SA air was full open. The FD fan dampers were open to 50% & 30%. The limitation was

by ID fan at this high gas temperature. Further air pumping was not possible and hence the

boiler got stuck at about 75 TPH load.

4. It was seen that the cross over duct gas temperature was more than the bed temperature &

cyclone outlet temperature. This implied the combustion was not complete in the furnace.

Combustion was extending beyond the combustor. This resulted in high LOI. In ideal condition,

the volatile / powder combustion should have been completed in the furnace itself. The finer

particle or volatile matter should not burn in the cross over duct.

5. The LOI of fly ash was around 2.5% to 3.0%.

6. SH & Economiser thermal performance of CFBC boiler was compared AFBC boiler. It proved

that there had been fouling of ash. See fig 1.3 & 1.4.

7. There was good response in steam generation to coal feed in last compartment. The bed

temperature was running higher in first compartment though one feeder was down. The air

distribution arrangement was checked. See figure .The air duct tapping from the HAG (hot air

generator) to windbox was from a manifold in which the air velocity was calculated to be 19

m/s. The air velocity in compartment opening is 17.3 m/s for an air temperature of 190 deg C.

The first air box should be getting a disturbed air supply because its tapping was at the bend

itself. The last compartment was getting more air.

From the study, conclusion could not be drawn as to whether the tubes were coated with soot or fly

ash. Hence a shut down inspection was requested. Four interim suggestions were given. They were,

ID duct layout modification was suggested to minimize draft loss. See figure 1.2.

Refractory lining over the waterwall portion below the final SH in old furnace.

SA pipe size of 100 mm with tips of 50 mm dia was recommended, in order to make more

SA available.

Extra air ducts between the main duct to the air plenum to reduce the flow unbalance

between windbox chambers.

THE SHUT DOWN INSPECTION

The boiler heating surfaces were inspected before any cleaning was done. Ash was scraped and no

soot was seen anywhere in SH coils and economiser coils. APH showed ash mounts over the tube

sheet. It became clear that the fly ash had fouling constituents. See photo 1.1 to 1.4. It was

important to note that the same coal was being fired for several years before conversion and in the

other two boilers and there was no such fouling problems reported in the BFBC boilers.

Reasoning of Fly ash depositions over the convection pass tubes

The coal was the same in all the three boilers. The fly ash is formed by fragmentation of ash

particles and also due to condensation of vaporized alkalis in coal. In BFBC all the ash constituents

leave the bed simultaneously without any enrichment of alkalis. In CFBC the particles are recycled

in to the furnace and the major ash removal is from the furnace, in the case of high ash coals (here

40% ash). The fineness of fly ash coupled with condensation of volatilized alkalis is the cause for

the increased fouling as compared to BFBC. In BFBC the relatively coarse ash particles leaving

with flue gas clean up the convective heating surfaces and there is no enrichment of fly ash by

volatile species. In fact, there were no soot blowers in BFBC boilers.

SOLUTIONS TO REMOVE THE DEPOSITS

The following four solutions are well known among the boiler designers and users.

Use of fireside additive

Use of sonic horns

Steam operated soot blower

Use of air blaster

Sonic horns / air blasters could be fitted in the inspection doors in I pass and II pass. Since the ash

was little softer, these were recommended. Incorporating steam operated soot blowers needed lot of

change in boiler pressure parts.

We recommended another solution which was little innovative.

We suggested that the bed ash cooler outlet ash could be injected in to the flue path of the old

boiler at the ash hopper below the old furnace. The idea was simply to simulate the condition as

before.

FURTHER FINDINGS ON SHUT DOWN INSPECTION- RELATED TO ID DRAFT

1. ID fan duct system

The ID duct system was modified as per the recommendations given in first visit.

2. APH leaks& blockage

There were tube failures in APH – upper block. The leakages could be seen by the shining wall

plates. See photo 1.5. Air leak test was done by closing the furnace side dampers and

pressurizing the APH with only FD fan. It revealed the extent of leakage.

The ash accumulated inside the APH increase the pressure drop. See photo 1.7. Subsequent to

modification to CFBC, the air side pressure had nearly doubled. This should have led to further

tube failures, since the tubes must have worn out already. The APH was never re-tubed in the

past. It was advised to replace the APH cold block tubes in full at the earliest.

3. gas side baffles in APH to ESP inlet duct

The gas side baffles were eroded badly. See photograph 1.8. The baffles were removed as the

eroded baffles create unnecessary pressure drop otherwise.

4. Roof air ingress

The seal box inside the boiler roof had developed leaks. See photo 1.6. This was arrested in the

shut down.

5. Leakage around the SH tubes of Sealbox of SH header

There was air ingress through the annular gap around the SH tubes that penetrate through the

waterwall. This could lead to leakage of SH tubes over a period. This was attended in the shut

down.

6. Gas side velocity check at cyclone

The gas inlet arrangement to cyclone was with an opening of 2056 x 700. The gas velocity

worked out to be 27.9 m/s. This could result in very finer fly ash.

The cyclone configuration is a standard Stairmand cyclone. It did not have a volute inlet.

The gas outlet velocity was worked out to be 20.4 m/s.

FEED BACK AFTER MODIFICATIONS & THE IMPLEMENTATION OF WILD SOLUTION

There was improvement in ID draft and steam generation after the duct modification & after

arresting air ingress. The high back end temperature and low main steam temperature problems

remained. The plant engineers wanted our concurrence to implement the innovative suggestion.

This was nothing but injecting the bed ash from bed ash coolers directly in to the flue path of old

boiler. The call came in a morning from the plant manager that all their problems were solved in a

matter of 2 hrs after the bed ash cooler ash was injected in to the ash hopper below final

superheater. The boiler is in operation till date with the above solution.

CASE STUDY 2

This is a cement manufacturing unit having captive power plants. The plant has 4 x 85 TPH AFBC

boiler and 3 x 85 TPH CFBC boilers- see figure 2.1. The plant had been in operation with petcoke.

When the petcoke price & availability scenario change, coal is selected. Recently the plant had to

change over to firing South African coal in AFBC and CFBC boilers. It coincided with the lean

period for cement production and power export. One AFBC boiler & one CFBC boiler were in

operation. The following problems were reported from plant.

1. The boiler exit temperature went up by 15 deg C at AFBC boiler. The AFBC boilers were not

provided with any Soot blower system.

2. The CFBC boiler no 3 was alone in operation due to lesser power demand. In this boiler also,

the ESP inlet gas temperature went up by 20 deg C. The fouling of pressure parts was sensed

and the soot blowers were put to operation at least two times in a shift.

3. In CFBC boiler, another strange phenomenon started occurring. The combustor pressure ( lower

) at times gone up from + 80 mmWC to + 125 mmWC in few seconds and returned normal after

some time without any reason. The boiler started tripping 5 to 7 times in a day due the high

furnace pressure. At times the tripping occurred during steam blowing cycle as well.

In a nut shell, two problems were experienced. One was the fouling and the other was sudden boiler

pressurization in gas side.

DURING THE VISIT

Furnace pressurization

The furnace pressurization was analyzed through the DCS trend profile. See figure 2.2. It could be

identified that some blockage was occurring downstream of economiser. It was also seen that the

economiser inlet gas temperature was alarmingly high. See figure 2.3. It was clear that there was

ash fouling in superheater area.

Fouling in general

Fouling of the boiler heat transfer surfaces in convective sections are due to chemistry of ash. The

Na2O and K2O (alkalis) present in fuel ash vaporize at furnace and condense at flue path at

temperatures ranging from 750 to 650 deg C. Some coals do have high alkalis and hence pose

problems by way of decreasing steam generation, decreasing main steam temperature and

increasing the boiler exit gas temperature. Ashes have been characterized and fouling potential is

predicted by empirical relations. A good input is covered in the book- B&W Steam generation &

use.

Coal ash analysis of South African coal

To confirm that the ash is fouling, coal ash analysis was carried out. See analysis attached, in which

the South African Coal ash, ash deposit collected from Final superheater, Limestone and Petcoke

ash were compared. When petcoke was used, it was always mixed with limestone and

predominantly the ash was created from limestone. Since SA coal was being used without any other

inert material, then the ash characteristics remained as that of Coal ash itself. By analysing the ash

nature by empirical relations, it was confirmed that the south african ash coal ash was fouling

nature. Interestingly the sodium content was more in the deposits confirming the deposits were

caused by sodium.

Severity of fouling / rate of fouling

The rate at which fouling occurred was dependent on how much Na2O+K2O passed through the

furnace and second pass. CFBC handled additional Na2O due to AFBC ash being fired additionally.

Hence the fouling rate was high. More the ash was refired in CFBC; faster would be the fouling

rate. The soot blowing cycles had to be more for refiring AFBC ash.

Yet fouling cannot be avoided as the coal ash can foul due to higher sodium content. Sodium +

Potassium less than 1% would pose less fouling.

A material balance was worked out on sodium loading in flue gas for the case of Petcoke firing

and South African coal firing. It was estimated that the Alkali content in fly ash during South

African coal firing could be 5 times than that of Petcoke firing case.

Particle size & cyclone cut off- difference between BFBC & CFBC

CFBC produces ash of very fine particles. This causes to form deposits

by particle adhesion. CFBC cyclone design will decide the particle

size. The same coal used in BFBC did lead to higher exhaust

temperature. In BFBC there were no soot blowers.

An earlier data on CFBC particle analysis report indicated that the

CFBC was filtering a very high percentage of finer particles. Almost

90% of the particles were less than 75 microns. Average particles

could be around 50 microns, which is very fine cut off. See table 1 &

2. The difference between the CFBC and the BFBC is the difference in

fineness of ash.

Table 1: Fly ash particle

distribution at silo ash

+850 µm % Nil

+500 µm % 0.34

+180 µm % 1.68

+ 90 µm % 3.58

+75 µm % 2.54

+45 µm % 12.52

- 45 µm % 79.34

Table 2: Fly ash particle distribution in CFBC boilers

Boiler no CFBC boiler no 1 CFBC boiler 2 CFBC boiler 3

ESP field no ESP

field 1

ESP

field 2

ESP

field 3

ESP

field 1

ESP

field 2

ESP

field 3

ESP

field 1

ESP

field 2

ESP

field 3

+500 µm Nil Nil Nil Nil Nil Nil Nil Nil Nil

+180 µm 2.8 Nil Nil 2.2 Nil 2.6 3.5 Nil 1.2

+90 µm 1.6 2.8 2.2 3.8 2.2 4.2 4.4 2.6 2.3

+75 µm 2.1 4.2 3.5 2.7 4.6 10.2 7.6 2.1 9.5

+45 µm 36.1 38.5 42.1 30.2 70.8 43.4 18.2 6.9 35.1

-45 µm 57.4 54.5 52.2 61.1 22.4 39.6 66.3 88.4 51.9

Sudden pressurization & boiler tripping

It was suggested to start the second boiler in order to find where exactly the choking was occurring.

After the boiler was cooled, the boiler was inspected physically. The ash deposits over superheater,

economiser and airpreheater blocks were to an alarming level. See photographs 2.1 to 2.4. The

fouled ash from SH & Eco could suddenly fall on the APH block and might act as block for the flue

path. There was no soot blower system provided for APH. In BFBC air preheater, the gas flow was

through the tubes of dia 63.5 mm. In BFBC the particles leaving the furnace are coarser as

compared to CFBC. In CFBC boiler, APH was designed for the flue gas flow over the tubes. The

APH tube OD was 60. 3 and the tubes were pitched at 80 mm. The tube to tube clearance was 20

mm only. This was the main reason for furnace tripping.

Actions to counter sudden pressurization & boiler tripping

We had given three suggestions for countering the superheater fouling and blocking of airpreheater

block.

Addition of air blasters for APH cleaning

It was advised to fit air blasters below & in between APH blocks. Since the ash was of severely

fouling characteristics this would help. The fineness of coal ash could cause plugging.

ECO / APH hopper modifications

It was advised to modify the gas side interconnecting duct between the APH hoppers as it was a 3

meter long horizontal duct. Horizontal ducts always trap ash. Bottom plates of the ducts were to be

provided with inclined plates directing the ash flow towards the hopper. This would avoid the

temporary plugging of interconnection duct. The ash released from the soot blowing operation /

sudden self dislodging would not block the gas path.

Refiring of ash

It was advised to stop re-firing at CFBC. Few days of operation in petcoke or any non fouling fuel

could clean up the boiler. This was the simplest solution for cleaning the boiler. But there was no

alternate fuel available at plant.

Injection of bed ash in to second pass

In CFBC boiler all the Na2O+K2O would leave the furnace by volatilization. Remaining ash at

furnace / or the bed ash would be containing less Na2O + K2O. The bed ash should be injected

through the manhole available above the superheater. This was advised based on experience in case

study 1. It was advised to add sand / low grade limestone for generation of more bed ash. The dense

phase line going to bed ash silo was to be provided with a ‘Y’ diverter.

Feed back after the incorporation of recommendations

The tripping continued even after addition of large size air blasters on both sides of APH blocks.

When the bed ash was injected in second pass, the problem vanished. The unit was inspected nearly

after a year to check whether there was any erosion or not. The boiler did not show any sign of

erosion. Since the average particle size in bed ash was about 600 microns, there was no problem

with respect to erosion.

CASE STUDY 3

This captive power plant had a 38 TPH BFBC boiler. The boiler was designed for lignite available

from nearby mines. The boiler was commissioned about 3 months back. The boiler could not run

for more than a week with the available lignite, called Barmer lignite. The boiler had been

clinkering heavily. The photographs 3.1 to 3.4 exhibit the extent of clinkering & slagging.

SOLVING THE PROBLEM Barmer lignite fuel & ash analysis As it could be seen that the bed ash was clinkering, the fuel & ash were analyzed immediately. See

table 3.1. One would say there is no problem with the coal as the ash fusion temperature is well

above 1200 deg C, whereas in FBC, the combustion temperature is only 900 deg C maximum.

Ratio of magnetic content versus non magnetic content The percent of iron oxides present in bed was found to exceed more that 50%. See photo 3.5. There

was a mechanism of iron concentration in the bed. For start up of the bed, the bed material chosen

was crushed refractory grog. The iron content in the startup bed material was less than 1%. The ash

content in barmer lignite being 23%, the bed ash inventory keeps increasing. As the bed height

increases, the ash is drained out maintaining a windbox pressure of 550 mmWC. As days go by, the

bed material becomes replaced with ash from lignite. The particles which contain iron being high in

density do not break down much, and do not leave the bed as well. Sooner the bed becomes

heavier. The bulk density goes up as high as 1300 kg/m3. The increase in amount iron oxide

increases the slagging potential. See photo 3.6, a case with high iron Indonesian coal in BFBC.

DILUTION WITH BED MATERIAL / SAND While we looked in to the possibility of dilution of iron with alternate bed material, we had the

choice of limestone, crushed refractory and fine sand. The limestone available at the industry was

very pure and was being used to make white cement. Crushed refractory pricing was higher than

the fuel cost. The only alternate material was sand, which was available at an affordable price. An

estimate on quantity of fresh bed material for dilution purpose was worked out. See table 3.2. The

plant went on to consume all the Barmer lignite that was purchased for next six months. Now the

plant found that petcoke is the better fuel as its ash with high unburnt (4000 kcal/kg) is usable as a

supplement fuel in the cement kiln.

FINAL WORD

Boilers with BFBC are quite large in population in India. There had not been a case of heavy

fouling of flue path. This must have been due to self cleaning effect by the coarse ash. Slagging of

bed was also not experienced and the case referred here was a typical one. Indonesian coals of

some mines had posed problem in bed ash chemistry control in few other boilers. Such boilers had

to use more of fresh bed material to prevent the bed from clinkering.

 

Figure 1.1: General arrangement of the new CFBC combustor and hook-up to existing

combustor.

Photo 1.1: On shut down inspection the bed coils were found to be clear of ash deposits.

The coal was good with respect to slagging characteristics.

Photo1.2: The ash deposition in FSH area. The ash was found to bridge between the SH coils.

Photo 1.3: Ash Deposits over the Primary SH tubes.

Photo 1.4: Ash fouling at economiser area.

Photo 1.5: The difference on color of APH tubes confirmed the extent of air leaks /

blocks. The polishing mark on the casing confirmed the APH tubes had failed.

Photo 1.6: Air ingress through seal box.

Photo 1.7: APH choking and ash build up.

Photo 1.8: Eroded gas baffles at APH inlet. This can cause higher pressure drop. It was

removed during the shutdown.

Photo 1.9: Inside view of the cyclone confirms that the ash was not slagging type but only

fouling type. Iron containing ash would slag in the furnace itself.

Photo 1.10: Air manifold modification done.

Photo 1.11: The 2 x 100% ID fans were put in series to handle the additional draft loss.

Improper ducting engineered during the conversion. 2 no 90 deg bends were seen very

closely placed. Abrupt 90 deg bend was seen. Multilouver damper was used for isolation

purpose, which would practically have higher leakage level.

Figure 1.2: The modification included straightening the duct as shown with smooth bends.

Figure 1.3: During the first visit, the thermal performance of secondary SH was checked. It

was clear that there was abnormal fouling in secondary SH.

Figure 1.4: In order to ascertain whether there was fouling of economiser, thermal

performance analysis was done. The fouling factor had to be increased from 0.002 to

0.013, which is about 6.5 times the normal value. Since the boiler no 2 was running around

the same load, its thermal performance was analysed. The performance was as per general

design practice. Based on the above, it was clear that the boiler had fouled by soot / ash.

 

Figure 2.1: General arrangement of the CFBC boiler with FSH, LTSH, Economiser and

APH hot blocks in the second pass. The fouling was extensive at FSH and LTSH. The

ash which got dislodged during soot blowing plugged APH block. This led to tripping of

the boiler. Steam operated soot blowers were available at second pass for Superheater

and economiser and not for APH blocks.

Figure 2.2: A screen shot on gas side draft profile at the time of draft upset. At times the

combustor pressures went highly positive and the boiler got tripped.

Figure 2.3: The trend on various flue gas temperatures. Gas temperature at economiser

inlet showed that three was fouling of superheater coils above. In fact the secondary SH

was in the furnace and only FSH and LTSH were in the second pass.

Gas pr after APH

Combustor pressures

Gas pr at eco inlet

Gas pr at eco outlet

Eco inlet flue gas temperature

Photo 2.1: Fouling of final superheater at second pass.

Photo 2.2: Fouling of LTSH superheater and second pass.

Photo 2.3: Fouling of economiser at second pass.

Photo 2.4: Fouling of airpreheater at second pass.

Photo 2.5: Since the APH block did not have a soot blowing arrangement, air blasters

were installed. It could not stop the tripping of the boiler.

Photo 2.6: Installation of bed ash injection at superheater. This resolved the problem of

gas temperature profile and plugging of air preheater.

For 3.0 < Na2O < 6, fouling potential is high,For Na2O > 6, fouling potential is severe,

SA coal Fouled Lime PetcokeSiO2 54.46 42.25 15.86 33.87TiO2 1.3 1.52 1.62 1.8Al2O3 34.13 28.04 9.53 1.08Fe2O3 2.23 3.57 3.23 48.86CaO 3.36 0.72 67.6 8.91MgO 0.46 4.14 1.79 2.9Na2O 3.56 17.33 0.24 1.69K2O 0.5 2.43 0.13 0.89

100 100 100 100

0.58 0.73 0.05 4.14Lignitic Lignitic Lignitic Bitumin

B, Base 63.25A, Acid 36.75

1.720.03Low

6.05 8.43 72.623.56 17.33 0.24Severe Severe low

For 1.2 < Na2O < 3, fouling potential is high,For Na2O > 3, fouling potential is severe,

CASE STUDY 2

When CaO+MgO+Fe2O3 > 20% by wt, When CaO+MgO+Fe2O3 < 20% by wt,

For Na2O < 3, fouling potential is low to medium,

For Na2O < 1.2, fouling potential is low to medium,

Ash classification & fouling index – as per B&W steam generation & useWhen Fe2O3 > CaO + MgO, the ash is termed as bituminous. Otherwise it is called lignitic

Fouling index for bituminous ash,

Fouling index for lignitic ash

For Rf < 0.2, fouling potential is low,For 0.2< Rf < 0.5, fouling potential is medium,For 0.5< Rf < 1.0, fouling potential is high,For 1.0 < Rf, fouling potential is severe,

B = CaO+MgO+Fe2O3+Na2O+K2OA=SiO2+Al2O3+TiO2

Na2O = weight % from ash analysis

Fouling index, Rf is calculated from an expression B/A * Na2O, where,

Na2OFouling potential is

CaO+MgO+Fe2O3+Na2O+K2OSiO2+Al2O3+TiO2

B/A ratioB/A ratio x Na2OFouling potential isFouling index for lignitic ashCaO+MgO+Fe2O3

Type of ash- Lignitic / BituminousFe2O3 / CaO+MgOLignitic / BituminousFouling Index for bituminous ash

Fouling Index calculation - based on B&W steam generation & useCoal ash chemical constituents

SilicaTitanium oxideAluminaIron oxideCalcium oxideMagnesium oxideSodium oxidePottasium oxideTotal

TPD 166.630 TPD 1.000% 1.000 % 1.000TPD 1.666 TPD 0.010% 2.580 % 2.580TPD 0.043 TPD 0.000

Ratio of LS to petcoke no 0.250 Ratio of LS to petcoke no 0.250Limestone added TPD 41.658 Limestone added TPD 0.250Alkali percent in LS % 0.230 Alkali percent in LS % 0.230

TPD 0.096 TPD 0.001Alklai percent after firing % 0.320

TPD 51.01TPD 0.163428

TPD 0.302TPD 38.38% 0.8

TPD 204.880% 14.000TPD 28.683% 4.060TPD 1.165

TPD 93.570% 4.060TPD 3.799TPD 5.0TPD 122.3% 4.1

Total alkali input

Total alkali input to CFBC

Ash percent in SA coalAsh from coalAlkali percent in ashTotal alkali input to boiler

Total fly ash produced% Alkali in fly ash

Total alkali input to boiler

South African coal firing in CFBC

Combined effect

Total ash producedAlkali percent in fly ash

Fly ash from AFBC boilersAsh addedAlkali percentAlkali input from AFBC

Ash fed from AFBCAlkali load from AFBC

SA coal fired

Total alkali input to boiler

Total alkali input to boiler

Petcoke firing in CFBC Fly ash alkali in AFBC boilersPetcoke firedAsh percent in petcokeAsh from petcoke

Petcoke firedAsh percent in petcokeAsh from petcokeAlkali percent in ashTotal alkali input to boiler

Limestone addition Limestone addition in AFBC boiler

Alkali percent in ash

Alkali loading in CFBC boiler during petcoke firing & South African coal firing

Alkali loading in CFBC boiler during petcoke firing

CASE STUDY 2

 

Photo 3.1: Fused ash coating over the bed Superheater coils.

Photo 3.2: Fused clinkers of the entire bed.

Photo 3.3: Fused ash capping the air nozzles. The blockage would the air flow and bed

temperature would shoot up.

Photo 3.4: The fused layers of fly ash over the radiant SH placed above the furnace.

Photo 3.5: Ratio of magnetic to non-magnetic material in bed ash. It was over 50%.

Table 3.1: Barmer fuel & ash chemical analysis & prediction of slagging, fouling nature.

Table 3.2: Calculation for dilution of iron in bed ash by fresh iron-free bed material.

Photo 3.6: Slagging nature of high iron Indonesian coal in another BFBC boiler.