COAL ASH SLAGGING

PARAMETERS FOR COAL ASH EVALUATION

Parameters used for evaluating coal ash behaviour as they affect furnace-slagging

Ash-fusibility Temperature
Iron/Dolomite Ratio
Basic/Acid Ratio
Dolomite Percentage
Iron/Calcium Ratio
Ferric Percentage
Silica/Alumina Ratio

Molten or semi-molten coal-ash slag deposits will not form on clean water-wall tubes since, upon approaching relatively cooler tube surface, the slag particle becomes 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

MECHANISM OF COAL-ASH SLAG DEPOSITION

The following two types of primary deposits are most commonly observed:
Primary deposits that results from the setting of the finer fractions( smaller than 30 microns) of fly ash. This type of primary deposit is loose in structure and does not provide strong cohesive or adhesive bonds.
Primary deposits that result from the selective deposition of certain reactive components of ash components of ash ( iron, calcium or alkalies ). These components can be presents in the deposit in high concentrations as oxide and / or sulphur compounds, leading to the formation of lowͲmelting eutectic mixtures. This type of deposit is more dense in structure and has strong cohesive and adhesive bonds. During the deposition , there is a transitional stage where the plastic secondary deposits begin to stick to the primary layer. These secondary deposits/ 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 deposits is a function of the immediate gas temperature outside the deposit.

DEPOSIT STRUCTURE AND MAXIMUM THICKNESS

The figure 1 showing the crossͲsection of a typical water wall slag deposit has a slag surface that has reached a maximum steadyͲstate 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 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 build up , reducing the heat transfer efficiency of the boiler surfaces
This section discusses the operational techniques that the operator can use to control slagging. Although the predominant factor affecting ash deposition 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 :

(a) Furnace temperature
(b) Excess air
(c) Fuel fineness , and
(d) Secondary air distribution and
(e) Improper distribution of coal due to eroded coal burner nozzles/ nozzle tips/ impellers.
(f) Air-Fuel ratio ( 1.8 optimal)

Changing any one , however, will not eliminate operating problems completely. Although the individual variables affect 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 ruleͲofͲthumb, the higher the fusion temperature, the drier the slag in the furnace, and the easier it is to remove.

III )FUEL FINENESS:

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 optimizing 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.
Using programmed soot blower systems, ash deposits on the furnace walls can be minimized.
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 coalͲfired 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.

INTERMITTENT REMOVAL OF BOTTOM ASH HOPPER

Operating experience with waterͲfilled hoppers has shown that most of the ash will flow towards bottom ash hopper and through the feed gates by gravity; however, large slabs of fused ash do occur that will not move by gravity alone.
Their removal requires the assistance of high pressure ( 7 to 8 Kg/cm2) water jetting nozzles located on the sloping floor of the hopper.
In addition, jetting nozzles in the wall opposite each feeder gates breaksͲup the arching and packing of the ash that may occur at the gate.
The principal reasons for water-impounding include the following :

To break-up large pieces of slag by thermal shock as they fall into the hopper and to quench them.
To keep the ash and slag submerged so that they do not fuse into large unmanageable masses that would result if they were exposed to furnace heat.
To provide a resilient medium to decelerate the large pieces of slag. It is important to realise that the ash falling from the furnace walls does not pass through the furnace bottom opening in equal increments along the length of the opening.
About 25 percent of the total ash to the furnace bottom will go through a 3 to 4 feet portion near the side wall and less than 2 percent of the bottom ash will go through each remaining foot length of the bottom slot.

REASONS FOR ASH BUILD-UP

Bottom Ash hoppers are designed to carry the ash for about six hours (with intermittent deashing system), and four hours (for 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.
While firing the lowestͲheating value and highestͲashͲ content coal, the normal practice of emptying ash hoppers is once every 3 to 4 hours.
The usual time required to evacuate the ash is 1 to 1 ½ hours.
If deashing is delayed due to any reason, it must be attended immediately and bottom deashing started to avoid further ash accumulation.
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.
Both incomplete and delayed deashing will cause the ash level to come out of water, which in turn converts to hard clinkers on exposure to heat from the furnace.
This will in turn form a platform for further ash build up and if it crosses the S panel area it will be very difficult to dislodge it.
The ash buildͲup can be due to two primary reasons from ash removing strategy perspective:

Improper / Incomplete Bottom Ashing
Delayed Bottom Ashing

Typical Causes of Slagging and Fouling Problems in Boilers

Slag is molten ash and incombustible byproducts that remain following coal combustion. When the material cools to a certain temperature, it can stick to furnace components, such as waterwalls, which is called slagging.
A pulverized coal–fueled boiler is designed with a large furnace cavity that can tolerate liquid phase slag on the waterwalls. The furnace exit, however, should be at a low enough temperature that the slag will be cooled below its softening temperature.
The temperature of the first point—when the ash cone point becomes blunted—is called the “initial deformation temperature.” As the furnace is heated more, the temperature when the ash becomes soft and the height (H) of the cone equals the width (W), is recorded. This value is referred to as the “softening temperature.” Heating continues, resulting in the ash cone sagging further until H = 1/2 W. This temperature is called the “hemispherical temperature.” Finally, when the ash cone becomes a liquid, the temperature is noted and referred to as the ash “fluid temperature.”
For slagging and fouling, the most important issue is to have the furnace gases or “products of combustion” leaving the furnace at a temperature so that the ash is not too sticky. A good approximation is to have the furnace exit gases about 100F to 150F cooler than the ash softening temperature.
the furnace exit gases are above the fluid temperature, and it is possible to operate a boiler with liquid-phase ash flowing through the superheater and reheater, but it is not advisable for reasons of coal ash corrosion and the need for near-continuous long retractable sootblowing to mitigate the ash deposits.
“Fouling” typically refers to deposits that occur in the convection pass after the gases exit the furnace. Fouling is generally attributed to ash cinders and accumulations that form on the leading edges of the superheater and reheater tubes (Figure 1), especially the outlet legs, which are above 1,000F metal surface temperature. The deposits are dislodged by sootblowing.
When the long retractable sootblowers are used to blow the ash deposits free, the ash particles entrain into the flue gas stream and create cinders, which can block selective catalytic reduction (SCR) catalyst flow paths, plug air heater baskets, and bridge across boiler tubing in the convection pass. Usually, the areas of the boiler that are designated as being susceptible to slagging are from the burner belt to the furnace exit.
Heat flows from hottest to coolest and, therefore, to produce 1,000F to 1,100F superheated steam and reheated steam outlets, the furnace exit gas temperature (FEGT) must be above about 1,500F at the reheater gas inlet to drive the heat flow into the reheater and superheater to create the desired steam temperatures. Consequently, the sweet spot for the FEGT of a pulverized coal–fueled boiler should be about 2,150F to 2,250F to achieve desired steam temperatures without slagging. Lower than 2,150F it becomes difficult to achieve design steam temperatures. Above 2,250F on a bulk gas basis, it becomes close to the melting temperature of the ash in some fuels.
Fuels with extremely high ash fusion temperatures are thought of as being “boiler friendly” and forgiving. Fuels with lower ash fusion temperatures require more precise combustion tuning and increased sootblowing to mitigate slag deposits.
Some fuels are more unforgiving than others. The coal ash iron content is a huge factor. Ash that has as much as 15% to 20% iron content will have an ash fluid temperature in a reducing atmosphere as much as 500F lower than the same ash in an oxidizing atmosphere. Current plant operations with a strong regulatory limit on NOx tend to drive operators to operate at low excess oxygen levels. This practice, combined with fuel and air imbalances, can result in conditions in which flue gas lanes can have zero free oxygen and, therefore, are technically operating in a reducing atmosphere.
So, how can a reducing atmosphere or secondary combustion be created at the furnace exit? Here are six of the most common causes of boiler slagging and fouling in our experience:

■ Low furnace excess oxygen

■ Extreme stratifications of the FEGT flue gas lanes

■ High primary airflows

■ Burner damage and deficient mechanical condition/tolerances

■ Poor coal pulverizer performance

■ Inconsistent fuel properties and chemistry
Low Furnace Excess Oxygen

The No. 1 cause of furnace slagging is low furnace oxygen content. Most boilers are designed for 115% to 120% theoretical combustion air. This is generally expressed as 15% to 20% excess air. For coal furnaces, the oxygen levels would be 3% to 3.8%. Note the location of the oxygen analyzers at the economizer exit in Figure 2. This location is often subject to reading higher oxygen levels than the actual furnace oxygen content, due to air in-leakage between the furnace and the air heater flue gas inlet.

It is extremely important to apply the attention necessary to optimize the furnace burner belt “inputs,” because combustion must be completed within the furnace cavity. Of absolute importance is providing sufficient combustion airflow to the fuel before the products of combustion exit the furnace. One of the most common causes of slagging and fouling is secondary combustion at the upper furnace. The most common cause of secondary combustion is insufficient excess oxygen within the burner belt.

the oxygen analyzers are usually located at the economizer outlet, the excess oxygen that is measured at the economizer exit includes any ambient air that has leaked into the boiler setting after combustion should have been completed. This lack of excess free oxygen at the furnace causes the active combustion to stretch out and actively continue into the superheater section. The flue gas temperature, due to such secondary combustion, can and has been measured to be well over 1,000F above optimum.

The second factor is that when the coal ash has an iron content of over about 10%, the melting temperature of the ash is lower in a reducing atmosphere. In other words, not only does the secondary combustion elevate the FEGT, but also, if the coal ash contains significant amounts of iron, the fusion temperature can be drastically lower as a result of the ash chemistry. That is, the ash will melt at a much lower temperature in a reducing atmosphere from what the fusion temperature would be in an oxidizing atmosphere. As noted previously, the ash fusion temperature can be reduced by as much as 500F.

Extreme Stratifications of the FEGT Flue Gas Lanes

The limited residence time of large utility boilers demands that the furnace inputs of fuel and air be optimized (Figure 3). If not properly controlled, fuel/air inconsistencies can contribute to slagging and fouling issues due to secondary combustion and elevated FEGTs. Optimizing the fuel and air inputs to the furnace and making certain that the furnace exit is an oxidizing atmosphere are the first steps in reducing furnace slagging.

Fuel input optimization includes ensuring that:

■ Coal fineness meets the following guidelines: At least 75% passes a 200-mesh screen and less than 0.2% remains on a 50-mesh screen with representative and isokinetically removed coal fineness samples.

■ Coal distribution to each burner must be balanced plus or minus 10%.

Combustion air optimization includes ensuring that:

■ Primary airflow quantities are optimized and air/fuel ratios are repeatable.

■ Measured and controlled secondary airflow is uniformly distributed to the individual burners.

■ Measured and controlled overfire airflow is optimized.

FEGT and excess oxygen can be measured with a water-cooled high-velocity thermocouple (HVT) probe. The measurements by HVT probe should be a minimum of 3% excess oxygen with maximum temperatures about 100F to 150F below the ash fusion temperature. It is when the FEGT approaches the ash fusion temperature that slagging occurs.

Often, the most useful data obtained by using a water-cooled HVT probe is the furnace exit, excess oxygen levels, and profiles. All points at the upper furnace should be oxidizing and preferably above 3% excess oxygen.

The word “slagging” is usually used to describe slagging in the furnace, whereas fouling is generally used to describe cinders or ash that have carried over into the convection pass and created flow obstructions due to the deposition. As discussed previously, fouling of the convection pass, SCR, and air heater are the result of ash accumulations on the leading edges of superheater and reheater tubing that is removed by long retractable sootblower operation.

Minimizing slagging and fouling begins by optimizing the burner belt combustion performance. This is necessary because there is only about 1 or 2 seconds of residence time between the top of the burner belt and the superheater flue gas inlet. At the furnace exit, the superheater and reheater tube spacing becomes closer and closer, resulting in narrowing gas flow lanes.

The typical FEGT is about 2,150F to 2,250F, assuming that the prerequisites for optimum burner belt combustion are present for the inputs. In the superheater shown in Figure 4, the peak furnace flue gas temperatures were well above the melting temperature of Alloy 310 stainless steel (about 2,780F). The active secondary combustion peak temperatures were truly about 1,000F above the FEGT with optimized burner belt inputs. When optimized, the FEGT was a uniform 1,950F to 2,100F across the boiler width. Prior to optimization, temperatures of 2,850F to 3,100F were present. These extremes have been documented in numerous cases.

The cause of the high temperature in this case was threefold. First, primary air velocities were high, which drove the fuel deep into the furnace, away from the secondary air provided at the burners. Second, the burner belt suffered from combustion air starvation due to the overfire airflow being too high and exceeding 20% of total airflow, with only about 115% of total theoretical airflow to the boiler. In other words, the burner belt was deeply staged at sub-stoichiometric excess oxygen levels. Third, fuel fineness and distribution was not optimized. The flue gas temperatures were over 3,000F at the superheater gas side inlet. At this temperature, the ash condition was fluid, and it only took a couple of shifts to completely slag the furnace exit.

It is common to find between 0.5% and 1% oxygen rise from the furnace to the air heater inlet flue gas. Why? Well, for one reason, the average 500-MW pulverized coal–fueled boiler is more than 30 years old. Therefore, the potential for air infiltration is increased due to age alone, even when diligent and thorough maintenance repairs are practiced. The only excess oxygen that matters from a slagging and fouling viewpoint is the excess oxygen present at the furnace exit. Keep in mind that the available residence time from the top to the burner belt may be less than 1.5 seconds.

High Primary Airflows

High primary airflow, especially on wall-fired boilers, contributes to poor fuel balance, poor fuel fineness, and longer flame lengths. Primary air is basically transport air and provides 15% to 25% of the total air for combustion. Therefore, when the primary airflow is very high, the fuel particles “outrun” the secondary air and result in longer-than-optimum flames (Figure 5)

High primary airflow on nearly any modern low-NOx burner will drive the fuel deep into the furnace, thus outrunning the secondary airflow. As a consequence, fuel-rich zones can form in the upper furnace, resulting in secondary combustion, elevated temperatures, and zones of localized reducing atmosphere—all of which contribute to slagging and fouling.

Burner Damage and Deficient Mechanical Condition/Tolerances

One of the 13 essentials of optimum combustion is burner tolerance at plus or minus one-quarter inch. The photographs shown in Figure 6 offer some examples of typical as-found burners.

Poor Coal Pulverizer Performance

The most frequent cause of extreme fuel imbalances at the furnace exit is coal pulverizer performance. Poor fuel fineness nearly always contributes to poor fuel balance. At best, pulverized fuel balance will be in the range of plus or minus 5% to 15% deviation.

When classifiers are not set for best fineness (usually to substitute more coal pulverizer throughput for reduced fineness), the fineness may deteriorate to less than 70% passing 200 mesh. Along with the reduced fineness, there will be less-uniform fuel balance. Poor fuel fineness nearly always results in poor fuel distribution. It is not unusual to find fuel deviations of plus or minus 25% when the pulverizers are not optimized.

The coal pulverizers are the heart of a pulverized coal–fueled boiler. About 75% of the opportunities for improvements in tuning are with the coal mills, primary airflow, and fuel line balancing. Figure 7 shows the important points for achieving optimum combustion with minimum slagging and fouling.

THE MEANS OF PREVENTING ASH BUILD-UP

Before Deashing

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

Furnace exit temperature
Excess air 20 % ( O2 3.0-3.5%)
Fuel fineness , ( 70% In 200 Mesh ,< 10 % in 100 mesh ,<0.5 % in +50 Mesh ,)
Secondary air distribution ( SADC)
Improper distribution of coal due to eroded coal burner nozzles/ nozzle tips/ impellers.

( CA flow test result ± 2.0 % from mean ,
Fuel line balance through Dirty Air Velocity Test result ± 5.0 % from mean )

Air-Fuel ratio ( 1.8 FOR Stage II OPTIMAL)

Responsibility- (SCE/Unit Controller/Boiler Desk Engineer)

To prevent build up in bottom ash hopper and to start deashing in time, peep holes above bottom ash hopper (at 10.5 m in case of 500MW BHEL units) on both sides are to be regularly inspected for any sign of build up by Unit Controller. This is to be recorded in his log book. This observation is to made preferably 2.5-3.0 hours after completion of deashing or whenever slagging /ash build up tendency exists.

Responsibility (Unit Controller/Boiler Desk Engineer)

On detecting any sign of ash build up, bottom deashing is to be commenced immediately preferably by both grinders on that side . Due to any reason if it cannot be started immediately, load on the unit should be reduced and ash level shall be monitored continuously.

Responsibility (Unit Controller /ADPH Engr)

In the normal course, frequency of bottom ashing is to be decided based on the coal firing rate and hopper filling/ash build up trend

Responsibility (Unit Controller/ADPH Engr)

Monitoring through 10.5 mtr Boiler peep holes at beginning & end of each shift to be done and to be recorded in Unit Control Engineer’s log book..

Unit Controller (UCE).

During Deashing

During bottom deashing it is to be ensured that the feed Sluice gates are opened fully for free flow of bottom ash.
Ash Disposal Pump House should confirm for proper discharge in both lines (BC/AD) after the BA is started .
The ejector water pressure is important as a lower pressure will make evacuation slower. Should there be any problem in the discharge, back flushing of the ejector can be done to normalize the discharge.
Ash build up on the side of the hopper (Left or right) is to be tackled by opening the side flushing valves.
Immediately after the BA deashing is started , after a predefined time period say 20-30 minutes the flame should be visible in all 4 front view glasses simultaneously .
If not then it may be due to possibly poor discharge / ash accumulation inside the hopper. Slope jetting nozzles are to be used , grinder changeover may be carried out . hopper may be refilled up to overflow level to avoid further aggravating the situation .
Problems to be highlighted to SCE immediately .

Responsibility (Unit Controller/ADPH Engr)

After the flame is visible in all 4 view glasses in the front in stͲ II or in front & side view glasses stͲI either UCE / Desk engr should witness the emptiness of hopper thoroughly through view glasses after closing cooling water & to be opened after inspection .

Responsibility (Unit Controller/ASPH Engr)

11.2.3. Deashing of bottom ash hopper is to be carried out in 'maintained level mode'. This would

result in additional suction head for jet pump and also eliminate problem of dry ash as ash will always be quenched. In order to check for any ash build up in the bottom ash hopper, deashing may be done occasionally (say, once in a day) in ' draw down mode' for a short duration in all the units on rotational basis .

Responsibility (SCE/Unit Controller/ASPH Engr)

11.2.4. Immediately after starting bottom deashing the side view glasses of the hopper need to be

monitored for level drop. If the view glasses are getting cleared on one side only, it indicates a build up/improper deashing on the other side (This is assuming that and both view glasses are at the same level). The reasons for the same shall be identified and attended.

During bottom deashing, loading on clinker grinders is to be observed from the ammeters provided in AHP Control Room/ local panels/UCB as the case may be . Observations are also to be made at ash slurry falling in the ash slurry pit for sufficient quantity of ash in slurry reaching there. Bottom ash discharge line pressure also indicates the quality of discharge (More for proper discharge and vice versa)

Responsibility (Unit Controller/ASPH Engr)

11.2.5. 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 SͲpanel of furnace. Any abnormality

must be reported to SCE and Head (Operation).

Responsibility (Unit Controller/ASPH Engr)

11.2.6. When the hopper becomes empty observation is to be made from the view glasses for

deposition of bottom ash, if any, in the hopper. In case there is any left over bottom ash, the other set of diagonally opposite clinker grinders are to be taken in service after stopping the two clinker grinders which have been in service. Operation of hopper flushing nozzles will also help in removing the remaining ash. After ensuring that bottom ash hoppers are empty, observation is made through peep holes provided above SͲpanel of furnace to look for any ash build up there. Wherever peepholes are available above the grinders, visual inspection of the hopper is to be done to ensure an empty hopper. A typical empty hopper demonstrates fast moving ash pieces.

Responsibility (Unit Controller/ASPH Engr)

11.2.7. It is to be ensured that the inspection view glasses/peepholes are easily accessible and a

clear view maintained.

Responsibility (Unit Controller/ASPH Engr)

11.2.8. The bottom ash hoppers are not to be left dry for long durations to avoid ash build up on

the slopes of the hopper. If required for some reason then sufficient care should be taken to keep filled the other half & slop jetting nozzles can be kept charged as per requirement .

Responsibility (Unit Controller/ASPH Engr)

11.2.9. Inspection of bottom ash hopper through view glasses (all 4 nos. at 5.5 mtr.) just after deͲ

ashing, to ensure emptying of hopper.. The same to be recorded in UCE’s log book.

Responsibility (Unit Controller/Boiler Desk Engineer)

11.2.10. If ash flow is not proper through both the CG of any side , then the clinker grinders may be byͲpassed and clinkers to be dumped on boiler floor by opening the CG circular manhole and after wards opening the BAHG . This is to be carried out along with the AHD group. Under no circumstances ash will be allowed to buildͲup inside the hopper & bridge the “S” panel area . Effort shall be put by operation group to lift the ash as early as possible by proper mobilisation of manpower & machineries ( Tractor/Tripper/JCB etc) .

Responsibility (HOO/Operation Support Group/SCE/UCE/ AGM(AHD)

12.0 Steps to be Taken In case of Ash BuildͲup:

If ash build up is suspected then the following techniques should be considered .

12.1. In case slag deposits / ash buildͲup 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, flushing nozzle lances s etc. should ensured always).

In this case immediate information should be given to Shift BMD /Pressure parts group

to seek their support .

Responsibility (SCE/Unit Controller)

12.2. 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.

Responsibility (HOO /Oprn Support Group /SCE/Unit Controller/ASPH Engr)

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

Responsibility (HOO / SCE)

12.4.

Manhole in the furnace mounted above SͲpanel 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 lances 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 SͲpanel for removing the ash build up inside the boiler, care should be taken by providing the wire mesh cover near the manhole door of SͲ panel. Due to this the furnace will not be completely exposed to the poking people and minimizing the unsafe conditions.

Responsibility (HOO /Oprn Support Group /SCE/Unit Controller/BMD )

12.5. Activities like manual poking of bottom ash hopper and area above SͲpanel of furnace

should be carried out only after certification on protocol (as per annexureͲ1) by a committee consisting of operation, mechanical maintenance, ash handling maintenance and Safety Departments, stating that all safety measures like Personal Protective Equipments, operating parameters have been taken care of. SCE shall inform safety department immediately when such situation arises to monitor the safety aspects .

This protocol format shall be kept with SCE in a specific file .

Operating staff should take extra care to ensure that the furnace does not get pressurized

during such activity.

Responsibility (HOO /Oprn Support Group / BMD/Safety )

12.6. Whenever manual poking of bottom ash hopper and area above SͲpanel 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.

Responsibility (SCE/Unit Controller/Blr Desk Engr )

12.7. 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.

Responsibility (Unit Controller/ASPH Engr )

12.8. 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 SͲpanel is to be adhered to.

Responsibility (SCE/Unit Controller )

12.9.

During clearing of ash buildͲup, special care and lot of patience is required to be taken for

avoiding over loading / failure of clinker grinders , since any such failure increases total

time for ash removal considerably. So, specific precautions to be taken during slag / clinker

removing activities.

Responsibility (SCE/Unit Controller )

12.10. 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.

Responsibility (Operation Support Group /SCE/Unit Controller/BMD )

12.11. 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 chokage removal for saving time.

Responsibility (Operation Support Group /SCE/Unit Controller/BMD/Safety )

12.12. All efforts should be made to avoid the practice of byͲpassing the clinker grinders and

dumping the slurry on boiler floor to be manually removed later.

Responsibility (Operation Support Group /SCE/Unit Controller/ASPH Engr )

12.13. 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.

Responsibility (HOO /SCE)

12.14. 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.

Responsibility (Operation Support Group /SCE/Unit Controller )

Other Precautionary Measures :Ͳ

12.15. The damaged BA Hopper refractory causes the nucleolus for ash build up inside the BA hopper & causes hindrance to smooth ash flow during evacuation .This aspect is to be taken care during annual OH & proper cooling of refractory to be ensured always .

Responsibility (Operation Support Group /UCE/AGM(AHD )

12.16. Since feed gates are very critical items their reliability must be ensured for at least one year

from overhaul, by carrying out thorough maintenance during overhauls.

Responsibility (Operation Support Group /AGM(AHD )

12.17. Routine changeover of clinker grinders shall be carried out regularly in successive CG

operation .

Responsibility (Operation Support Group /SCE/Unit Controller/ASPH Engr )

12.18. In case any of the critical equipments like feed gates, clinker grinders, hydro ejector is

under break down it must be attended at the earliest, if required by taking unit shut down

because effect of keeping it in such condition can be far reaching.

Responsibility (Operation Support Group /SCE/Unit Controller/ASPH Engr/AGM(AHD) )

12.19. Facility of quick filling of bottom ash hopper (within 10 – 15 minutes), wherever not

provided, should be provided.

Responsibility (Operation Support Group /AGM(AHD) )

13.0 Record :

13.1 ASPH Engr /Unit Controller/SCE should record the details of deͲashing & hopper evacuation

during the shift in their respective log books as per format TS/OPN/R/Log/008/003/001.

Responsibility : SCE/UCE/ Boiler Desk Engineer

Energy Tab

Saturday, November 11, 2017

COAL ASH SLAGGING

Typical Causes of Slagging and Fouling Problems in Boilers

Low Furnace Excess Oxygen

Extreme Stratifications of the FEGT Flue Gas Lanes

High Primary Airflows

Burner Damage and Deficient Mechanical Condition/Tolerances

Poor Coal Pulverizer Performance

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