01.prevention and safe clearing of ash build-up in boiler bottom ash hoppers

NTPC LTD KORBA

Ref.: OPERATION GUIDANCE NOTE :

COS-ISO-00-OGN/OPS/SYST/001

LOCATION MANAGEMENT INSTRUCTION

LMI/OGN/OPS/SYST/001

Rev. No.:2 Date: January 07

Prevention and Safe Clearing of Ash Build-up in Boiler Bottom Ash Hoppers

Approved for Implementation by ……………………………………………………..

GM(K) Date: ……22/01/2007


INDEX

Sl. No. Contents Page No. 1.0 Introduction 1

2.0 Superseded Documents 1

3.0 Coal-Ash Slagging 1

4.0 Control of Slagging 3

5.0 Continuous Removal - Bottom Ash Hopper 6

6.0 Reasons for Ash build-up 6

7.0 The means of Preventing and Clearing Ash build-up 7

8.0 Review 12

Page - 1 LMI/OGN/OPS/SYST/001 Rev. No.: 2


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1.0 INTRODUCTION :

This LMI based on OPERATION GUIDANCE NOTE : COS-ISO-00-OGN/SYST/001 , Rev. No.:2 , Dated: December 2005 , identifies the need to exercise great care to avoid slag formation in the boiler furnace which results in poor heat transfer of furnace walls, large slabs of fused ash, blocking of clinker grinders & ash slurry pumps and choking of ash disposal lines. The LMI also covers safe means to clear ash build up in case it so happens. Operating experience has shown that excessive slag formation or improper bottom deashing leads to build-up of bottom ash to a height of 16-18 meters in the furnace bottom, forcing unit shutdown which could last several days. Build-up of this magnitude can also result in catastrophic failure of boiler support system.

2.0 SUPERSEDED DOCUMENTS -

LMI/OPN/008 based on Operation Guidance Note “Ash Build-Up in Boiler Bottom Ash Hoppers : OGN/OPS/SYST/001 Issue: 1 dated August 1994”

3.0 COAL-ASH SLAGGING

Slagging occurs when fused or solidified molten material forms on surfaces exposed to high radiant heat fluxes and gas temperatures.

3.1 Coal parameters used for evaluating coal-ash behaviour as they affect furnace slagging are:

* ash-fusibility temperatures

* base/ acid ratio (More basic nature implies more tendency for slagging)

* iron/calcium ratio

* silica/alumina ratio

* iron/dolomite ratio

* dolomite percentage

* ferric percentage



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The figure below shows a typical cross section of a water wall slag deposit that has reached a maximum steady-state thickness.

Figure 1. Cross section of a typical furnace-wall slag deposit

Molten to semi-molten coal-ash slag deposits will not form on clean water-wall tubes since, upon approaching relatively cooler tube surface, the slag particles become less adhesive because of rapid cooling in the wall-adjacent area. Accordingly, coal-ash deposition is a two-stage process. A primary layer of deposit first forms on the water wall tube surface. The resulting rise in the surrounding surface temperatures subsequently allows the adherence of rigid plastic secondary deposits.



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3.2 Mechanism of Coal-Ash Slag Deposition

The following two types or primary deposits are most commonly observed:

i) Primary deposits that result from the settling of the finer fractions (smaller than 30 microns) of fly ash. This type of primary deposit is loose in structure and does not provide cohesive or adhesive bonds.

ii) Primary deposits that result from the selective deposition of certain reactive components of the ash (iron, calcium, or alkalies). These components can be present in the deposit in high concentrations as oxides and/or sulphur compounds, leading to the formation of low-melting eutectic mixtures. This type of deposit is denser in structure and has stronger cohesive and adhesive bonds.

During the deposition, there is a transitional stage where the plastic secondary deposits begin to stick on the primary layer. These secondary deposit/droplets of low viscosity molten glass are strengthened by time and increasing temperatures. The temperature at which the secondary deposits begin to form corresponds to approximately 950 degree C. The transition from primary to secondary deposit is a function of the immediate gas temperature outside the deposit.

3.3 Deposit Structure and Maximum Thickness

The figure 1 showing the cross-section of a typical water wall slag deposit has a slag surface that has reached a maximum steady-state thickness.

The rate at which glass droplets fuse together is directly dependent upon the time available for bonding to develop, and is inversely proportional to the viscosity of the melt, i.e., the more fluid the droplets are the more readily they will run together thus strengthening the mechanical bonding in the deposits, and ultimately thick slabs may built up, reducing the heat transfer efficiency of the boiler surfaces.

4.0 CONTROL OF SLAGGING

This section discusses the operational techniques that the operator can use to control slagging. Although the predominant factors affecting ash deposition



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are the amount and composition of coal ash; boiler operating conditions have also been demonstrated to affect deposition.

The operating variables that influence slagging are:

(i) furnace temperature

(ii) excess air

(iii) fuel fineness

(iv) secondary air distribution

(v) distribution of coal

(vi) air fuel ratio

Changing any one, however, will not eliminate operating problems completely. Although the individual variable affects boiler performance, they must be studied collectively to determine the effectiveness of a given programme.

Effect of each operating variable is considered as follows :-

i) Furnace Temperature

Higher the furnace temperature on account of improper combustion, poor heat absorption in the walls or sometimes due to higher load, greater the potential for slagging. Load reduction can reduce the potential for slagging but this is not always possible, because the rated output may be necessary to meet power generation. Changes in excess air, fuel fineness, and secondary air distribution are less drastic methods for minimising slagging.

ii) Excess Air

In a reducing atmosphere, the ash fusion temperature reduces. Here the Fe2+ ions will increase at the expense of more highly oxidized Fe3+ ions. The ferrous ions act as a flux and this reduces the initial deformation temperature of ash. If slagging is a problem with high iron coals, furnace deposits can be reduced drastically by increasing the amount of excess air. As a rule-of-thumb, the higher the fusion temperature, the drier the slag in the furnace, and the easier it is to remove.

iii) Fuel fineness



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Slagging conditions often can be improved by control of the pulveriser fineness. Since coarse particles take longer to burn, hence these burn away from the intended zone of combustion. They are more prone to producing slag. High retention on +50 mesh often increases slagging tendencies. This is better accomplished by maintaining air/fuel ratio at designed level.

iv) Secondary air distribution

Basically, the objective is to provide a good mixing of fuel and air so that combustion is efficient, and local zones with reducing atmosphere are avoided. In units with tangential firing, for example, slagging can be reduced by optimising secondary air flow to fuel compartments. It is accomplished by regulating fuel air and combustion air distribution. The extent of Fuel air damper opening can be decided depending upon the amount of volatile matter in coal (Higher the Volatile Matter, lesser can be the opening). In changing coal profile, such dampers have to be readjusted to get efficient combustion. Fuel air dampers of a non running mill (especially bottom mills) should be kept closed as they cause an increase in furnace temperature. Generally any major temperature mismatch of steam and flue gas (Left/right) should warrant a check up of SADC system.

(v) Fuel distribution

Improper distribution of coal, due to eroded coal burner nozzles/ nozzle tips/ impellers will affect the combustion and hence increase the furnace temperature.

vi) Other Operational measures

Failure to remove deposits at the proper time may result in chain reaction of deteriorating events. For example, excessive furnace slagging can result from not using the wall blowers at proper intervals. This condition imposes higher gas temperatures in the convective section of the furnace because of the reduced rate of heat absorption. The attemperation levels indicate the level of deposits in a boiler for a constant steam temperature. High attemperation flow denotes a high deposit on the boiler tubes. Thus it is best to do soot blowing whenever such a situation arises.



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Using programmed soot blower systems, ash deposits on the furnace walls can be minimised. Waiting too long between operations can seriously hamper the effectiveness of soot blowers. Observation of furnace conditions at least twice per shift helps eliminate unexpected problems. Because the soot blower system is so important for reliable, full capacity operation of a coal-fired unit, this equipment must be maintained in good operating condition.

After overhauling of boiler / maintenance of furnace, the boiler should be inspected and loose material if any, is to be removed from furnace / bottom ash hopper. Falling of loose materials in the bottom ash hopper will obstruct ash flow inside the hopper / full closing of feed gates / jamming of clinker grinder / increase the time of deashing, etc.

6.0 REASONS FOR ASH BUILD-UP

Bottom Ash hoppers are designed to carry the ash for about two hours (with scrapper conveyor system). However the carrying capacity in terms of time depends on various factors like coal firing rate, ash content in coal, bottom ash/fly ash ratio etc. During BAH equipment maintenance , the BAH gates are normally closed and if the work is likely to get prolonged ( more than four/two hours ) , bottom ash hopper deasing must be started after every four/two hours with proper care alerting all working personnel in the vicinity , to avoid further ash build up inside the BAH. The operation staff will be in continuous touch with maintenance staff during BAH work..

Falling slag / hard clinkers also can cause accumulation inside the bottom ash hopper as they will block the hopper gates and prevent free flow of ash. These may lead to incomplete deashing also as the clinkers remain undetected inside the hopper.

6.1 The ash build-up can be of two types from ash removing strategy perspective:

A. Build-up during normal unit operation with bottom ash gates wide open.

B. Build-up during / after scrapper conveyor maintenance activities with bottom ash gates in closed condition.



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7.0 THE MEANS OF PREVENTING AND CLEARING ASH BUILD-UP

7.1 Routine observations:

i) On getting indications of slagging or high furnace exit temperature, the combustion related parameters (mentioned earlier) are to be checked and necessary corrective actions taken accordingly.

ii) To prevent build up in bottom ash hopper and to start deashing in time, peep holes above bottom ash hopper (at 10.5 / 7m m in case of 500/200 MW units) on both side view glasses are to be regularly inspected for any sign of build up by local supervisor/Unit Controller. This is to be recorded in the log book. This observation is to made preferably three hours after completion of deslagging.

7.2 During Operation:

i) Observation is to be made from the BAH view glasses for deposition of bottom ash, if any, in the hopper. A typical empty hopper demonstrates intermittent flame. View glass cleaning water supply should be ensured regularly. If view glasses are not clean , the same may be got cleaned by shift mech.mtce.

ii) Utmost care is to be taken to ensure that the entire bottom ash is evacuated from the bottom ash hopper and there is no build up of ash over Z-panel of furnace. Any abnormality must be reported to SCE and Head (Operation). Both the furnace and bottom ash hopper must be inspected at least twice a shift for any ash build up in these locations.

iii) It is to be ensured that the inspection view glasses/peepholes are easily accessible and a clear view maintained.

7.3 In case of ash build up

i) In case slag deposits / ash build-up are not getting cleared by normal deashing, it is to be cleared immediately by poking / flushing with a high pressure water jet/ lances through peep holes (availability of fire hoses, nozzles etc. should be ensured always).



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ii) If massive slag deposits are observed, unit load should be restricted to the minimum, with oil support and all efforts must be made to remove slag in a running condition.

iii) Shift Charge Engineer is fully empowered to reduce load under such conditions.

iv) Manhole in the furnace mounted above Z-panel should be opened only when unmanageable ash build up is there. Efforts should first be made to dislodge ash through peep holes with the help of lancers and fire water. Whenever such operation is carried out, persons not connected with the job should not be present around and below that area. While opening the manhole door near Z-panel for removing the ash build up inside the boiler, care should be taken by providing the wire mesh cover near the manhole door of Z-panel. Due to this the furnace will not be completely exposed to the poking people and minimizing the unsafe conditions.

v) Activities like manual poking of bottom ash hopper and area above Z-panel of furnace should be carried out in consultation with operation, boiler mechanical maintenance, ash handling maintenance and Safety Departments, ensuring all safety measures like Personal Protective Equipments, operating parameters have been taken care of. Operating staff should take extra care to ensure that the furnace does not get pressurized during such activity.

vi) Whenever manual poking of bottom ash hopper and area above Z-panel of furnace is being done through manhole etc. no mill is to be stopped or cut in, and soot blowing is not to be carried out, as it causes sudden detachment and fall of clinkers.

vii) Concerned persons involved in ash / slag / clinker removing activities should stand at safe distance from bottom ash hoppers so that risk of splashing ash / hot water causing injuries is minimised. Water lances should have sufficient long metallic pipes so that cleaning is done from a safe distance.

viii) A practice of carrying out one job only at a time, i.e., either clearing of bottom ash hoppers or clearing of slag formation above Z-panel is to be adhered to.

ix) During clearing of ash build-up, special care and lot of patience is required to be taken for avoiding over loading leading to failure of



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scrapper conveyors since any such failure increases total time for ash removal considerably. So, specific precautions to be taken during slag / clinker removing activities.

x) No direct control on rate of ash falling on scrapper conveyor is available to concerned O&M staff. If suddenly heavy amount of ash / slag / clinkers fall on scrapper conveyor (S/C), it may lead to over loading / failure of S/C. Apart from this, sudden falling of ash is also dangerous for people working in near by areas since splashed ash/hot water can reach up to considerable distances.

xi) Due precaution is to be taken by restricting no. of water lances in use at a time & their point of application so that risk mentioned above is minimised. All concerned people must move to safe distances when there is chance of sudden ash falling in large amount. The surrounding area shall be kept free of debris for this.

xii) Personal Protective Equipments (PPE) are to be used by personnel, supervising and executing the job of deashing. The PPE includes safety helmets with face shield, gum boots, fire retardant hand gloves and fire resistant overall suit. All Personal Protective Equipments (PPE) are to be kept in common place like shift maintenance room in a separate box for easy availability during the emergency of chockage removal for saving time.

xiii) All efforts should be made to avoid the practice of by-passing the clinker grinders and dumping the slurry on boiler floor to be manually removed later. If required bypassing may be resorted to with additional water jetting arrangements.

xiv) If all actions fail to remove/dislodge the massive slag deposits, the unit should be taken under shut down for removal of clinkers/ slag accumulated in the furnace. Immediately after unit shut down bottom ash hopper is to be flooded with water to avoid hard clinkers formation from burning slags. In no case unit is to be run with uncontrolled and unmanageable slag deposits or without bottom ash evacuation.

xv) While shutting down Unit, sufficient care needs to be taken to avoid overheating of boiler tubes. Boiler is not to be drained and drum level is to be maintained.

xvi) If build up has taken place during or after maintenance on scraper conveyor, the rate of bottom ash gates opening is VERY CRITICAL



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for controlling loading on S/C. The gates are to be opened only in 5%-10% increment in order to avoid overloading of S/C and splashing of ash in near by areas.

xvii) After each incremental opening of gates, S/C is to be seen physically for amount of ash being handled by it. Once, there is reasonable reduction from peak amount of ash being carried away by S/C then only further opening of gates is to be carried out.

7.4 Other Precautionary Measures

i) Since BAH gates and Hyd, punp (Stg-I) & VC feed gates and Oil pump units (Stg-II) are very critical items their reliability must be ensured .

ii) In Stg.II , Winches meant for shifting Scraper conveyor are to be regularly checked and maintained for their availability.

iii) In Stg.II , Routine trial of standby scrapper conveyors / clinker grinders shall be carried out regularly as per the schedule.

iv) In case any of the above critical equipments is under break down it must be attended at the earliest.

8.0 REVIEW

This LMI shall be reviewed by the GM(K) , on a 3-yearly basis or as necessary.

Document for Distribution to:

1. GM(K) 2. AGM(O&M) 3. DGM(O/EEMG/MM-ash) 4. DGM(TS) 5. Sr.Mgr.(Safety) 6. SCE 7. All UCBs 8. O&M Library 9. Local intranet – soft copy