Pulverizers and Fuel Oil Handling System

Kumar Rupesh

Contents



Evolution of Boilers



Why coal Pulverization



Type of Pulverizers – Brief Overview



Mill Operating Window



Emergencies related to Pulverizers



Mill Operational Aspects



Mill Healthiness Record Keeping



Fuel Oil Characteristics



Fuel Oil Handling General Schemes



Fuel Oil Firing in Boiler

April 27, 2020 Footer text 2

Introduction – Evolution of Boilers and Comparison of different technologies

Boiler - Evolution

 Water tube Boilers range from - Capacity from 5TPH to 4500TPH - Steam Pressure from 3.5bar to 320bar - Steam Temperature from saturation to 620 Deg C Broad Classification of Boilers: April 27, 2020 Footer text 4

Industrial Boilers – a brief overview



Burners or Oil & Gas Fired Boilers

- Quick and relatively clean burning - High cost of production - Mostly used now as package boilers for support steam (up to 200TPH), but may be field erected also.

- 1960s, large Utility boilers based on Oil and gas were made but trend died out due to oil crisis & rising oil prices and increased reliability of Gas Turbines.

- Still prevailing in Oil rich countries 

Stoker fired boilers – Grate firing and suspension firing

- Dumping grate, chain grate, spreader stokers and reciprocating grates - Can not burn high ash coal due to deashing limitations, high sufur coal due Sox limitations and produce high NOx emissions - More suitable for wastes, biofuels and and bagasse 5 April 27, 2020 Footer text

Utility Boilers – a brief overview



Fluidized bed combustion

- Air is passed through chamber with bed of solids resting on bottom plate with provision of air distribution.

Static bed (Stokers): no apparent disturbance, air pressure drop increases proportionately to air flow as no voids are created - Bubbling bed (BFBC) : At minimum fluidization velocity onset of fluidization takes place lifting the bed from the plate, at higher velocity bubbling of air starts with bed expansion - Turbulent bed (CFBC): Even at higher velocity, bubbles get larger and coalesce to larger voids. Heavier particles settle and lighter particles swirl at top. DP is constant - Entrained Flow (PF Boilers): As velocity further increases, dense cloud also rise and start leaving the boiler called transport regime. DP drops down.

- Viscosity, density & particle size of bed material decides transport velocity April 27, 2020 Footer text 6

Boilers – Operating range of flue gas velocities

April 27, 2020 Footer text 7

CFBC and PF Boilers comparison

April 27, 2020 Footer text 8

Industrial Boilers – a brief overview

April 27, 2020 Footer text 9

Industrial Boilers – a brief overview

CFBC boilers

 comparable to PF boilers  have better emission levels

PF Boilers

 the higher Flue Gas velocities is related with higher heat release and larger output.

 Availability of low NOx burners and low sulfur coal

PF type boilers favored over CFBC for larger size units.

CFBC is a choice for difficult to burn fuel, high sulfur fuel and tighter NOx emission control Other boilers limited to miniscule or limited use

April 27, 2020 Footer text 10

Pulverizers – Types, Operation and Control

Why Coal Pulverization and Coal Pulverizers

       Solid or liquid combustible burn on surface – oxidant availability on surface only Increased surface area of the combustible - same volume will have quicker burning and high heat release.

Increasing surface area – breaking into number of smaller particles.

Done in Pulverizers - The breaking of particles with smaller pre defined size is called coal pulverization.

Higher flue gas velocities - high heat release and hence slender boiler with smaller footprints Higher flue gas velocities – require smaller coal particles - provided by Coal Pulverizers.

Coal contains moisture - Hot air to remove moisture and avoid coal pipe choking.

It also improves pulverization action.

Coal pulverizers do coal drying, grinding, classification and transportation.

Why Coal Pulverization and Coal Pulverizers

Typical Milling Circuit

Type of Pulverizers

Type of Coal Mills

 Slow speed mills – Ball and tube mills - Used for coals ranging from sub bituminous to anthracite  Medium speed mills – Bowl, Ball and race, Roller mills - Used for coals ranging from sub-bituminous to anthracite  High speed impact mill - Used mainly for lignite coal We will take-up briefly the low speed Ball and tube mills and medium speed mills for better understanding.

April 27, 2020 Footer text 14

Ball and Tube Mills

Ball and Tube Mills

Ball and Tube Mills

 This mill consists of a horizontal heavy cylinder lined up with wear resistant liners and filled up to 25-30% with balls of different sizes (generally 30mm, 40mm and 50mm).

 Rotational speed is 80% of that at which centrifugal forces would overcome gravity and cause the balls to cling to the shell wall. Hence the balls fall back and collide with each other.

 The grinding happens with coal particles trapped between the tumbling balls.

 They may be either single or double ended i.e. fed from one or both ends.

 The coal residence time in the mill is higher – preferable for harder coals.

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Ball and Tube Mills – General features

Ball and Tube Mills

 Classifiers are external to the mills and oversize material is fed back to the mill with raw feed.

 Mill reject can not be taken out.

 They do not develop fluidized bed in the classifier section and hence not very effective in particle distribution.

 Coals with very high moisture > 20% can not be used as poor intrinsic coal air mixing capability.

 They are more difficult to control.

 They are most suitable with highly abrasive and difficult to grind coals.

April 27, 2020 Footer text 17

Ball and Tube Mills – Controls and Operation

Ball and Tube Mills

Operational Aspects  Very heavy weight of the mill - slowing motor to start with at very slow RPM (2-3 RPM).

 Jacking oil at these low RPMs - hydrodynamic films do not form.

 Kept on hot stand-by by keeping under rotation with slowing motor.

 They are more prone to mill fire especially at low loading due to presence of very high volume of coal under suspension, the ignition source and high % of oxygen in the mill. (The fire rectangle)  Gnerally provided with steam inerting device which come on auto on start and stoppage of mill.

April 27, 2020 Footer text 18

Ball and Tube Mills – Controls and Operation

Tube Mill Controls Permissives

 Mill Jacking Oil Pressure adequate – Oil film and babbit damage  Mill Lube Oil Pressure adequate - Oil film and babbit damage  Reducing Gear Oil Pump ON – Gear cooling and lubrication  Mill motor cooling air fan ON – Motor cooling and insulation protection  Primary Air to Furnace DP not Low – Proper coal transportation  Seal Air to PA DP is normal – Bearing coal ingress protection  Mill Bearings temp normal - oil viscous properties and babbit damage  Mill Motor Winding Temp Normal – Insulation Protection  Mill Outlet temperature is normal and discharge MOVs closed – Furnace backfire and mill fire protection.

19 April 27, 2020 Footer text

Ball and Tube Mills – Controls and Operation

Tube Mill Controls Protections

 Boiler MFT – The mill stops and its discharge gates close.

 Motor/ Mill Bearing Temperature Hi- Hi - oil viscous properties and babbit damage  Mill Outlet Temperature Hi – Depends upon type of coal  Elevation Flame Failure (1/4 or no scanners sense flame) – Unhealthy burning  Coal Feeder Stopped (TD – 15 minutes) – Mill outlet temperature Hi and seal damage  Mill running and mill discharge gate closed (TD – 30 Secs) – Mill choking  Mill running and mill primary air damper closed - Mill choking  PA to seal DP Lo-Lo – Bearing coal ingress  All PA fans stopped or DP between PA header press and WB Lo – Coal/ Air velocity  Lube Oil Pressure Lo-Lo – Babbit damage April 27, 2020 Footer text 20

Ball and Tube Mills – Controls and Operation

Mill Controls

 Loop-1: Boiler Master – Fuel Master – Air Dampers.

 Loop-2: Mill Level – Mill DP – Coal Feeder Speed  Loop-3: Mill outlet temperature – Mill Cold Air Damper  The basic difference between tube mill control and roller/ bowl mill control is that for tube mill, the coal output increases with air input flow but for Bowl mill it happens with feeder speed. The difference is related with residence time of coal in mills.

Mill Operation Sequence (if on auto)

 Steam inerting for 5 minutes to contain low load condition fires  Purge Air damper opens for 2 minutes for Coal Pipe cleaning  Mill outlet gates open  Mill mix air gate opens  Mill main motor starts 21 April 27, 2020 Footer text

Ball and Tube Mills – Controls and Operation

Manual Operation

 Open cold air gate, hot air gate and capacity air dampers for mill warming.

 Start the coal feeder when mill outlet temperature reaches 80 Deg C (depends upon VM of coal fed).

 Observe that coal scanners sense flame. When 3/4 scanners sense flame, elevation flame failure protection is armed.

 Load the mill slowly by increasing coal feeder speed and air dampers. As the mill loading increases, the coal level in mill increases. This is reflected by Mill DP.

 Put Mill DP control, mill capacity air control, mill outlet temperature controls on auto.

22 April 27, 2020 Footer text

Bowl Mills – General features

Medium Speed Mills (Ball/ Roller) – Speed is in the range of 30 RPM

 Generally vertically mounted - use rolling elements (Ball/ Roller) on rotating table to grind coal.

   

How Pulverization happens:

Roller passes over a layer of granular material – coal compression against a moving table.

Movement of the table hence layer - motion between particles Motion under applied pressure within the particle layers causes particle break-up by friction.

Due to the rotation of ball/ roller and motion of table, combined effect of centrifugal force and displacement of the coal layer is pushed to the outside edge of the table.

April 27, 2020 Footer text 23

Bowl Mills – General features

April 27, 2020 Footer text 24

Bowl Mills – General features

      

Coal Fineness control 1 st Separation:

Air introduction through throat/ air port ring or nozzle ring - fluidized bed particle just above throat.

Air velocities are low enough to remove the smaller particles only.

2 nd Separation:

As the air-fuel mixture flows upward, the flow area increases and velocity reduces gravity separation - Larger particles are returned back to grinding zone.

3 rd Separation

Classifier at top of mill - centrifugal separator with angled openings to impart spin movement - Larger particles impact perimeter and fall back.

What will happen if air flow is very high or low with respect of coal flow

High – Improper separation and hence particle size control Low – Mill choking or excessive grinding/ power consumption April 27, 2020 Footer text 25

Bowl Mills – General features

General Understanding:

1.

Fluidization – Assists drying, high moisture coal can be dealt well 2. Grinding through impact – Even abrasive and hard to grind coal can be ground 3. Residence Time - Makes it suitable for fine grinding at the expense of high specific power consumption and slow mill response 4. Mill Speed – Fast response and less inventory 

What is the gap between moving surfaces

The compressed granular layers have cushioning effect which reduce grinding effectiveness but also prevent excessive roller wear. When rollers are very close to the table in order of particle size, wear due to three body contact (roller, particle and table) is very high. Example: tramp iron or quartz trapping in the mill April 27, 2020 Footer text 26

Bowl Mills – Improved performance over Ball and Tube mills

 Mill reject can be taken out.

 Coals with very high moisture > 20% can be used due to fluidization and good coal air mixing capability.

 They are easy to control.

 Loading and withdrawal is faster  Less prone to mill fires  More maintenance prone April 27, 2020 Footer text 27

Bowl Mills – Controls and Operation

Bowl Mill Controls

April 27, 2020 Footer text 28

Bowl Mills – Controls and Operation

Bowl Mill Controls Protections

 Boiler MFT – The mill stops and its discharge gates close.

 Motor/ Mill Bearing Temperature Hi- Hi - oil viscous properties and babbit damage  Mill Outlet Temperature Hi – Depends upon type of coal  Elevation Flame Failure (1/4 or no scanners sense flame) – Unhealthy burning  Coal Feeder Stopped (TD – 15 minutes) – Mill outlet temperature Hi and seal damage  Mill running and mill discharge gate closed (TD – 30 Secs) – Mill choking  Mill running and mill primary air damper closed - Mill choking  PA to seal DP Lo-Lo – Bearing coal ingress  All PA fans stopped or DP between PA header press and WB Lo – Coal/ Air velocity  Lube Oil Pressure Lo-Lo – Babbit damage April 27, 2020 Footer text 29

Mill Operating Window

Mill Operating Window

25 20 15 10 Milling Capacity Limit Operating Point Flame Stability Limit 5 0 0 10

Individual Mill Total Air Flow in kg/sec

20 30 40 50 60 April 27, 2020 Footer text Coal Transportation Limit Erosion Limit Drying Limit Tempering Air Limit Milling Limit Flame Stability Limit Operating Point 30

Mill Operating Window

 Flame Stability Limit: - Minimum coal flow at which flame stability is ensured - Generally taken as 50% of nominal mill coal flow rating - If number of mills are in service or support fuel oil in adjacent elevation in service this can be lower - Guided by mill manufacturer (turndown ratio) and operation experience  Milling Capacity Limit: - Maximum coal flow provided by manufacturer to avoid mill choking and skids - Correction should be applied for age of grinding pieces. E.g. linear correlation from o Hrs equivalent to 100% to 75% for 15000 hours - Coal blend also affects the limit  Coal Transportation Limit: - Coal & Air flow required to avoid coal stagnation in ducts - Chances of fire explosion if operated outside - Generally, minimum velocity is taken as 20m/sec April 27, 2020 Footer text 31

Mill Operating Window

 Erosion Limit: - Maximum velocity in ducts to avoid excessive wearing in ducts, valves and bends - As rough estimate maximum velocity is taken as 1.5 times the nominal velocity - Very high velocity may lead to unstable flame as well as high unburnt in flue gas  Drying Limit: - Maximum coal flow that can be dried with air at maximum available temperature - Improper drying leads to mills skids, overloading, duct choking, second pass coal burning & reduced boiler efficiency  Tempering Air Limit: - Low coal flows also to leave the mill at safe operating temperature to avoid spontaneous combustion.

April 27, 2020 Footer text 32

Emergencies related to Mill Operation – Mill Fires

 Mill Fires: High mill outlet/ PA temperature Lean Mixture Ignition Source Sufficient coal in suspension - Low load operation during start-up and shutdown - To avoid this Steam Inerting is recommended during shutdown and start-up - Very High VM coal with high mill outlet temperature - Low mill outlet temperature is recommended for high VM coal - Tube mills more prone than Bowl mills April 27, 2020 Footer text 33

Emergencies related to Mill Operation – Mill Fires

 Mill Fires:

Typical recommended Mill outlet temperature vis-à-vis type of coal

April 27, 2020 Footer text 34

Emergencies related to Mill Operation – One PA Fan Tripping

 

One PA Fan Tripping (2 X 50% configuration)

The discharge damper of tripped PA Fan takes time to close full. Due to fan tripping and running fan air short circuiting the PA Header pressure falls below expected.

 Take the running PA on manual and restrict its BP from overloading.

 Stop upper mills such that half of the running mills from bottom are in service.

 Immediately close the BP of fan tripped, Interconnecting damper of PA fans and CADs & HADs of mills stopped to reduce running fan demand.

 Take the oil support only after PA header pressure has recovered. An early action may result into furnace explosion due to high coal input and oil input simultaneously when PA header pressure recovers.

 April 27, 2020 Due to sudden pressure drop, drum level also surges violently. Maintain drum level properly.

Footer text 35

Emergencies related to Mill Operation – Lowest Mill Tripping

 

Lowest Mill tripping

As lower mills cover the most of the furnace water wall circuit, heat addition to water wall is impacted strongly.

 This results into lower heat evaporation and SH and RH steam temperatures rise rapidly.

 Drum Pressure also drops rapidly due to lower evaporation. This results into lower BFP discharge Pressure.

 Increase SH and RH spray to maximum. Also, increase ΔP across FRS for higher BFP pressure and hence spray.

 Bring burner tilt to lowest position. Take-out the highest mill out of service.

April 27, 2020 Footer text 36

General Instruction

 

N-1 mills to N mills operation

Assume that a lower number of mills are operating near full load and additional mill is taken in service.

 Generally, bowl mills operating at full load have choking tendency. Before cutting in the new feeder, it is advisable to clear the choking of one mill at a time.

 Reducing coal flow in all mills and simultaneous command to extra feeder pumps excess coal in Boiler and all the pressures rise rapidly. This may result into furnace pressurization and Unit tripping, Safety valve operation, HP/ LP bypass operation, vacuum deterioration etc.

April 27, 2020 Footer text 37

Emergencies related to Mill Operation – HPHs group bypass

        

HPHs group bypass

The HPHs extraction steam may amount to 8-10% of full load steam flows.

Expansion of this amount of steam in turbine leads to sudden load shoot up.

As sensible heat is no more added in HPHs, this shall be added in Economiser and Water Wall. Thus lower heat is available for steam evaporation.

This results into lower heat evaporation. Also, there is higher FG heat consumption in water wall. Out of these competing features, former dominates and SH and RH steam temperatures rise rapidly.

Also, due to very high heat consumption in Economiser, PA temperature pick-up in APH is low and its difficult to maintain Mill outlet temperature.

Drum Pressure also drops rapidly due to lower evaporation. This results into lower BFP discharge Pressure.

Increase SH and RH spray to maximum. Also, increase ΔP across FRS for higher BFP pressure and hence spray.

Bring burner tilt to lowest position. Take-out the highest mill out of service.

When temperature stabilises, higher coal than condition before disturbance shall be required to restore load.

April 27, 2020 Footer text 38

General Mill Operational Aspects Mill shutdown/ stand-by precautionary steps –

 Ensure upper/ lower elevation mill/ fuel oil is in service to maintain flame stability  Bring Feeder speed to minimum slowly  Do steam inerting  Close Hot air gate and increase cold PA flow to maintain at least minimum flow in coal pipes  Close Feeder outlet gate followed by stopping feeder  Empty the mill – this is indicated by sudden drop in mill current/ very low mill DP/ sudden rise in PA flow  Stop the Mill  Close Cold Air Damper

If on stand-by

 Mill discharge gates should remain closed  Observe mill outlet temperature regularly April 27, 2020 Footer text 39

General Mill Operational Aspects Mill under maintenance precautionary steps

 During normal coal firing due disturbance in coal feeding or draft system, furnace pressurization may happen.

 This pressurization may feed hot FG/ fire to mill under maintenance.  To avoid this, keep the mill discharge gates at mill outlet and coal feed gate at furnace inlet closed.

 Also recommended is to keep the coal pipes pressurized with cold PA.

Important observation during unit start-up

 Once Boiler firing is on without PA fan in service, Mill outlet temperature may rise: An indication of high APH seals passing (SA to PA), APH dampers passing and mills gates and dampers passing  When PA fan is in service, this tendency will be more. This may cause damage of soft seals and mill fires, if coal is present. Also feeder fires and bunker fires are probable though with remote possibility.

April 27, 2020 Footer text 40

Mill Observation and Record Sheets

Mill observation and record sheets

Typical Mill log sheet should essentially include:  Mill Differential pressure – indicates mill levels/ levels of choking/ Air coal ratio. If DP maintains high with lower current, reduce coal flow, increase air flow, open reject, check if fuel is wet.  Seal Air to Primary Air DP – lower value may damage the bearings, seal air filter DP monitoring and regular filter cleaning  Mill Outlet temperature – coal drying helps in pulverization, transportation and effective burning – value depends upon coal VM and moisture value – High mill outlet temperatures should be in line with internal lubricant and soft seal temperature – increases chances of mill fires 41 April 27, 2020 Footer text

Mill Observation and Record Sheets

Mill observation and record sheets

Typical Mill log sheet should essentially include: 

Mill current and coal flow –

- Higher power consumption for same loading - fineness is more - spring loading should be reduced - Lower power consumption for same mill loading - fineness is less - spring tightening is required.

- An optimization on Mill fineness and APC needs careful & prolonged study  PA Flow – Coal transportation and erosion limits  CAD and HAD opening – Comparison between mills 42 April 27, 2020 Footer text

Mill Observation and Record Sheets

Mill Healthiness Record Sheets



Dirty and clean air pitot tube test

- The clean air pitot tube test is done offline after every overhaul before coal firing. The clean air balance exclusively establishes the pulverizer outlet line conditions. This also establishes the healthiness of PA flow transmitters.

- The dirty air test can be done online. This is used to monitor the balance across coal pipes in running condition. Also as a crosscheck the readings of Mill instrumentation viz. PA flow, mill outlet temperature, coal flow etc.

- The temperature, static pressure and DP across the pipe cross section are measured to calculate the coal & air mixture velocity and flow in each pipe.

- The dirty air balance is expressed as a deviation from the mean velocity of all the pulverizers’ individual coal transport lines. A maximum deviation of +/-5% indicates a satisfactory results.

- At least two test ports 90 degrees apart are used on pipe. Sampling at a number of points (usually 12) at each port along different depths of section - Sampling port is chosen at least 5D downstream and 10D upstream of the nearest bend.

April 27, 2020 Footer text 43

Mill Observation and Record Sheets

Mill Healthiness Record Sheets



Dirty and clean air pitot tube test

Indicators of imbalance: • Temperature imbalance between LHS and RHS of boiler for Furnace exit FG temperature, MST, RHT • High content of combustible in fly ash • Poor coal fineness • Slag formation around certain burners of same pulverizer • Poor flame appearance • Excessive mill reject for a particular mill • Burner panel tube leakage due to short term or long term overheating 44 April 27, 2020 Footer text

Mill Observation and Record Sheets

Mill Healthiness Record Sheets

 Critical dimensions  Wear Data  When lubricants are drained – quantity and oil sample analysis  Mill fineness data – Requirements for spring tension adjustment/ classifier adjustment or internal inspection  Mill reject data – Per shift operation and recording the observations  Gearbox vibration data – early warning for bearing failure  Coal analysis data  Roller replacement history April 27, 2020 Footer text 45

General Guideline

Mill Performance Results – Competing parameters of NOx, Auxiliary Power Consumption and Unburnt losses

 The full range of particle distribution of coal at mill outlet should be done.

 General acceptability, >70% from 200 mesh  Very high fineness contributes to unburnt carbon loss in FG and higher auxiliary power consumption  Low NOx requires delayed combustion and hence not so fine coal  Low fineness contributes to unburnt carbon loss in BA April 27, 2020 Footer text 46

Fuel Oil Handling and Firing System

Fuel Oil characteristics comparison chart

2 3

4 5

6 7 1 Sl. No.

Characteristics Flash Point a. Abel, min, deg C b. Pensky Martin closed cup, min

8

Kinematic viscosity, cSt @ 40 DegC Sediments, percent by mass, max

Total Contamination, mg/ kg Density at 15 Deg C, kg/ m3

Total Sulphur, percent by mass, Max Water content, percent (v/v)

Total sediments, mg per 100 ml, Max

HSD 35 66** 2.0 - 5.0

0.05

24 820 - 860

1 0.05

1.5

Value LDO 1.8

0.25

NA

HFO (HV Grade) Method of Test (as per ISO:1448) 66 2.5 - 15.7

66 180 - 370 @ 50 Deg C P:20 P:21 P:25/ ISO 3014 0.1

NA

0.25

NA 840 – 890 940-990

P:30

EN 12662 P:16 / P:32

1 4.5

NA

**: High flash point HSD is available for specific industries only

April 27, 2020 Footer text

IP 336 or 4294 P:40

Annex A/ ISO 11205 48

Fuel Oil characteristics comparison chart

Sl. No.

9

10 11 12 13 14 15

Characteristics Ash, Percent by mass, Max

Carbon residue (Ramsbottom) on 10 percent residue, percent by mass, max Cetane number, min

HSD 0.01

0.3

51

LDO 0.02

1.5

45

Value HFO (HV Grade) 0.1

NA NA Cetane index, Min 46 Pour Point, Deg C, max a. Winter b. Summer 3 15 Copper strip corrosion for 3 hr @ 100 Deg C Not worse than no. 1 Distillation, percent (v/v) recovered @ 360 Deg C 95 NA NA NA 12 21 24 Not worse than no. 2 NA NA

Method of Test (as per ISO:1448) P:4/ ISO 6245

P:8/ ISO 10370 P:9/ ISO 5165 D4737/ ISO 4264 P:10/ D5949 P:15/ ISO 2160 P:18/ ISO 3405 April 27, 2020 Footer text 49

Property Comparison

 Flash Point - minimum temperature at which petroleum yields the vapour which will give a momentary flash when ignited.

  Petroleum Act 1934 and Petroleum Rules 2002 guide the import, transport and storage of the Petroleum Products.

Definition of Class is based on Flash Point as Per Petroleum Act 1934 Class A < 23 Deg C Class B > = 23 Deg C and < 65 Deg C Class C > = 65 Deg C and < 93 Deg C   The Flash Point of LDO and HFO both are minimum 66 Deg C - Class – C The Flash Point of HSD >23 Deg C and <66 Deg C - Class - B petroleum.



CCOE clearance is not required for storage of class C petroleum up to 45 KL and Class B Petroleum up to 2.5 KL.

April 27, 2020 Footer text 50

Property Comparison

 Viscosity – Flowability of the fuel - HFO requires external heating through Tank floor coils heaters, suction heater, inline heater and steam tracing line.

 LDO and HSD do not require any external heating.

 Recommended temperature value for HFO: Tank Oil temperature < 66 deg C, Oil temperature at burner inlet 110-120 degc.

 Sediments and ash content in Fuel oil - strainers choking, burner choking and frequent draining requirements.

    Density: low density of LDO and HSD - separation of sludge and water easier.

High density of HFO - settling time is required for sludge/water separation.

For HFO with prolonged settlement the density across the levels in the tank vary All level sampling is recommended.

 Moisture: High quantity leads to flame instability.

April 27, 2020 Footer text 51

Property Comparison



Generally, the density and moisture content of the fuel is recommended to be considered as quality parameters as acceptance criteria.

 The GCV of all the three fuels in the order of 10,500 kcal/kg.

  Sulfur Content: Higher sulfur content – lower acid dew point - higher FG outlet temperature to avoid cold end corrosion.

SCAPH (steam cooled APH) is recommended to be used to avoid this.

52 April 27, 2020 Footer text

Typical Fuel Oil Schematic

P&ID for HFO Unloading System P&ID for HFO Firing system P&ID for LDO Firing system April 27, 2020 Footer text 53

Property Comparison - HFO handling difficulties

• The high viscosity, ash/ sludge content and water content of HFO makes it the “difficult to handle” fuel oil.

• Due to high viscosity - Continuous circulation and external heating - temperature of 110 Deg C at Burner inlet and 65 Deg C in the Tank - Fuel, DM water and Generation

loss.

• Any leakage in the steam system may spoil the complete product.

•    High Ash/ Sludge and water content : Firing properties are inferior and less stable.

Choking & replacement of strainers/ filters and burners are frequent High maintenance cost 54 April 27, 2020 Footer text

Property Comparison - HFO handling difficulties

•   Time Delays: The furnace temperature > 200 Deg C, before HFO firing for stable flame.

For a cold start-up of system, HFO heating (6-7 Hrs).

•    The Unburnt loss and oil carry over high especially during very cold start-up APH fouling & fire ESP fouling & loss of efficacy Bag Filters (DFA) fouling April 27, 2020 Footer text 55

Property Comparison – HSD/ LDO improved performance

• As the characteristics suggest, the firing of HSD in the boiler is the simplest, cleanest and least maintenance prone followed by LDO and then HFO.

• The Firing reliability - HSD followed by LDO. This is quite warranted during unit emergency and very cold start-up.

• No steam heating - Additional MW generation, lesser coal consumption, savings on DM make up consumption • The Unburnt loss and oil carry over – Least for HSD followed by LDO.

•    Consequential impacts APH fouling & fire - Least for HSD followed by LDO.

ESP fouling & loss of efficacy - Least for HSD followed by LDO.

Bag Filters (DFA) fouling - Least for HSD followed by LDO.

April 27, 2020 Footer text 56

Line-up for Boiler light-up

Boiler Line-up for Light-up



Furnace purging is completed

 All the wall blowers and LRSBs are available and in retracted condition.

 Furnace temperature probes are available.

 Boiler spring loaded safety valves are in gagged condition if light-up is done before safety valve setting. If safety valve setting has already been done, the gagging should be removed.

 Bottom ash equipment such as clinker grinder, hydraulic pumps, jet pumps etc.

are available.

 Again ensure that Boiler water circuit drains are closed. All Boiler vents are open.

All Boiler steam circuit drains are opened.

 Ensure that Boiler stop valves along with IBVs are closed.

 Start-up vents are open full.

 The SCAPH is charged. If the aux steam is not available, this activity to be taken after Boiler light-up. (Purpose of SCAPH and cold end corrosion).  APH steam blowers/ air blower is in service  APH fire sensing and oil carryover probes are in service.

April 27, 2020 Footer text 57

Boiler Light-up – Oil Firing

Boiler Light-up operation – Taking Oil Guns in service

 Increase the LDO and HFO pressure to set value. Wait till HFO temperature reaches 110 Deg C (if aux steam is available).

 Insert the furnace probe till it advances 80-100%.

Note: The HFO should be taken in service only if the furnace temperature reaches >

200 Deg C. The light-up should start with LDO guns only..

April 27, 2020 Footer text 58

Boiler Light-up – Oil Firing

• • • • •  • • • •

Boiler Light-up operation – Taking Oil Guns in service

 Important parameters to observe: LDO/HFO header pressure, atomizing air/ Steam pressure, LDO/ HFO pressure after LONV. Typical value for oil pressure is 10 12 ksc for LDO burner and 8-10 ksc for HFO burner. Typical value for atomising air is 5-7 ksc and atomising steam is 6-8 ksc.

Press pair 1-3 start button for LDO/ HFO and observe the following sequence: AB-1 oil gun advances The spark rod advances.

The ignitor is ignited for 15 secs.

The atomising steam/ air valve opens.

LONV/ HONV valve opens.

The header pressure drops, open the LDO/ HFO pressure control valve to maintain pressure.

Discriminating scanner senses flame.

HEA ignitor is retracted after 15 secs.

AB-3 oil gun advances after a time delay of 30 secs.

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Boiler Light-up – Oil Firing

Boiler Light-up operation – Taking Oil Guns in service

If gun fails to prove  Check that gun has retracted to its recommended position and ensure that LONV/ HONV is closed. A check on Bottom Ash system oil accumulation may also be done.

 Do scavenzing and try the burner again  Check for spark rod actual spark intensity and time for which spark persists, the same can be done after removing this outside also.

 Check if the gun advances to its recommended position, if this does not spark shall not be able to fire the gun  Check the fuel oil and atomizing medium pressure at as well as after LONV.

 Check for healthiness of scanners  If the problem persists, remove the gun assembly for cleaning and inspection April 27, 2020 Footer text 60

Boiler Light-up – Oil Firing contd.

Boiler Light-up operation – Taking Oil Guns in service

• • • • • • • • • •

Note: it is advisable to start the gun after scavenging first. The scavenging is done to ensure gun nozzle cleaning and dislodge any oil accumulation in the gun and hence choking. The scavenging sequence has following steps:

Press corner gun scavenge button.

Oil gun advances The spark rod advances.

The igniter is ignited for 15 secs.

LONV/ HONV valve remains closed.

Atomizing steam/ air valve opens.

Scavenging valve opens.

Scavenging valve closes after 15 secs.

Atomizing air/ steam valve closes.

HEA igniters is retracted after 15 secs.

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Boiler Pressure Raising

Boiler Pressure Raising

 With the corner gun taken in service the corresponding corner SAD shall also be opened.

 After establishing the AB elevation LDO/ HFO guns as indicated by discriminating scanner, the next elevation HFO guns may be taken in service.



The rate of rise of Boiler metal temperature should be maintained as per OEM guidelines. For, very cold start-up it is advisable to maintain rate of rise of drum metal temperature at 1.5-2 Deg C / minute and rate of rise of steam temperature at 5.0 Deg C/ minute.

April 27, 2020 Footer text 62

Thank You