A Solution for Preventing Fouling in CFBC Boiler by K.K.parthiban

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 onefourth 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

Table 1: Fly ash particle distribution at silo ash +850 µm

% Nil

+500 µm

% 0.34

+180 µm

% 1.68

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

+ 90 µm

% 3.58

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

+75 µm

% 2.54

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

+45 µm

% 12.52

fineness of ash.

- 45 µm

% 79.34

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

Table 2: Fly ash particle distribution in CFBC boilers Boiler no

CFBC boiler no 1

CFBC boiler 2

ESP field no

ESP

ESP

field 1

field 2 field 3 field 1 field 2 field 3 field 1 field 2 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

ESP

ESP

ESP

CFBC boiler 3 ESP

ESP

ESP

ESP

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.

Combustor pressures

Gas pr at eco outlet Gas pr at eco inlet

Gas pr after APH

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.

Eco inlet flue gas temperature

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.

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.

Ash classification & fouling index – as per B&W steam generation & use When Fe2O3 > CaO + MgO, the ash is termed as bituminous. Otherwise it is called lignitic Fouling index for bituminous ash, Fouling index, Rf is calculated from an expression B/A * Na2O, where, B = CaO+MgO+Fe2O3+Na2O+K2O A=SiO2+Al2O3+TiO2 Na2O = weight % from ash analysis 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, Fouling index for lignitic ash When CaO+MgO+Fe2O3 < 20% by wt, When CaO+MgO+Fe2O3 > 20% by wt, For Na2O < 3, fouling potential is low to For Na2O < 1.2, fouling potential is low to medium, medium, For 3.0 < Na2O < 6, fouling potential is high, For 1.2 < Na2O < 3, fouling potential is high, For Na2O > 6, fouling potential is severe, For Na2O > 3, fouling potential is severe, CASE STUDY 2 Fouling Index calculation - based on B&W steam generation & use Coal ash chemical constituents SA coal Fouled SiO2 Silica 54.46 42.25 TiO2 Titanium oxide 1.3 1.52 Al2O3 Alumina 34.13 28.04 Fe2O3 Iron oxide 2.23 3.57 Calcium oxide CaO 3.36 0.72 Magnesium oxide MgO 0.46 4.14 Na2O Sodium oxide 3.56 17.33 K2O Pottasium oxide 0.5 2.43 Total 100 100 Type of ash- Lignitic / Bituminous Fe2O3 / CaO+MgO 0.58 0.73 Lignitic / Bituminous Lignitic Lignitic Fouling Index for bituminous ash B, Base CaO+MgO+Fe2O3+Na2O+K2O A, Acid SiO2+Al2O3+TiO2 B/A ratio B/A ratio x Na2O Fouling potential is Fouling index for lignitic ash CaO+MgO+Fe2O3 6.05 8.43 Na2O 3.56 17.33 Fouling potential is Severe Severe

Lime 15.86 1.62 9.53 3.23 67.6 1.79 0.24 0.13 100

Petcoke 33.87 1.8 1.08 48.86 8.91 2.9 1.69 0.89 100

0.05 Lignitic

4.14 Bitumin 63.25 36.75 1.72 0.03 Low

72.62 0.24 low

CASE STUDY 2 Alkali loading in CFBC boiler during petcoke firing & South African coal firing Alkali loading in CFBC boiler during petcoke firing Petcoke firing in CFBC Petcoke fired TPD Ash percent in petcoke % Ash from petcoke TPD Alkali percent in ash % Total alkali input to boilerTPD

Fly ash alkali in AFBC boilers Petcoke fired TPD Ash percent in petcoke % Ash from petcoke TPD Alkali percent in ash % Total alkali input to boiler TPD

166.630 1.000 1.666 2.580 0.043

Limestone addition Ratio of LS to petcoke no Limestone added TPD Alkali percent in LS % Total alkali input to boilerTPD

0.250 41.658 0.230 0.096

Ash fed from AFBC Alkali load from AFBC Combined effect Total alkali input to CFBC Total fly ash produced % Alkali in fly ash

1.000 1.000 0.010 2.580 0.000

Limestone addition in AFBC boiler Ratio of LS to petcoke no 0.250 Limestone added TPD 0.250 Alkali percent in LS % 0.230 Total alkali input to boiler TPD 0.001 Alklai percent after firing % 0.320 TPD 51.01 TPD 0.163428 TPD TPD %

0.302 38.38 0.8

SA coal fired Ash percent in SA coal Ash from coal Alkali percent in ash Total alkali input to boiler

TPD % TPD % TPD

204.880 14.000 28.683 4.060 1.165

Fly ash from AFBC boilers Ash added Alkali percent Alkali input from AFBC Total alkali input Total ash produced Alkali percent in fly ash

TPD % TPD TPD TPD %

93.570 4.060 3.799 5.0 122.3 4.1

South African coal firing in CFBC

 

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.