Fly Ash Collection System The most important factor in determining the type of fly ash collection technology to be used is the permissible particulate outlet emission concentration in mg/Nm3.

If an invisible plume is required, then this would require an outlet emission of typically 50 mg/Nm3 or less.

For a high ash coal, the inlet burden to the flue gas cleaning plant could be as high as 3O gm/Nm3.

The collection efficiency of a plant to give an invisible outlet plume would therefore be (Ein - Eout)Efficiency = X 100 Ein

where Ein = inlet dust concentration = 30 gm/Nm3 Eout = outlet dust concentration = 0.05 gm/Nm3

Therefore: Required Efficiency =

(30 - 0.05) x 100 = 99.83% 30

Whole of life cost comprises

1. Capital Cost and

2. Operation and maintenance costs over the projected project life.

On a net present value basis

ESP designed for Ash Content =25% Max

Coal Quality Design Actual

Carbon 37.3% 23%

V.M. 27.60% 20.8%

Ash 25% 45%

CV 5000 kcal/kg 3000 kcal/kg

Rate of Emission with ESP (Average) >600 mg/NM3

Air & Evaporative

Pulsing Gas Volume 374 m3/sec

Cloth Area 24037 m2

Velocity 0.0157 m/sec

Length 7.2 m

No of Bags 8248 Nos.

Pressure Drop 135 mm wc

Gas Temp 1350C

APH out Temp 1600C

Cooling Pressure 3.60 kg/cm2

Evaporative cooling

Atomised Water size 50 microns

No of Nozzles 42 Nos

Water Pressure at Valve 7.00 kg/cm2

Water Pump out pressure 9.5 kg/cm2

No of water pumps 2 Nos

Water filters Course & Fine 2 Nos

Max water flow 384 LPM

Evaporating compressors 2 Nos + 2 Nos (Standby)

Air Pressure 8 Kg/cm2

Air requirement Max 24 M3/Min

Air pressure at Valve station 7.00 kg/cm2

Max DP Air/ water 1.00 kg/cm2

Flue gas Temperature Measurement

RTDs have been installed in all locations in the fabric filter inlet duct. RTDs are enclosed in erosion shields except for the tips.RTD tips have been left exposed to ensure quick response time. RTDs downstream of the spray system are shorter and inserted from sides of duct.Access platforms have been provided to RTDs.

RTDs Fitted to the Flue Gas Inlet to Fabric Filter

(After Air Heater & before Attemperator)

Temperature is measured downstream of the water sprays, with water flow controlled to maintain the set point.The PLC reads all RTD temperatures, and calculates the average of the 3 highest temperatures for each zone (Tmax), plus the average of the 3 lowest temperatures of each duct Tmin).These maximum and minimum averages are then used to control the various spray and attemperating air systems.

The temperature set point is adjustable from the PLC, nominally 148 oC.The set point for the attemperating air system is set 7 oC higher than the water spray system, so that it will only operate if the water spray system cannot maintain the set point temperature.If the attemperating air system operates, an alarm is given, “fabric filter inlet temperature high”.

If the temperature Tmax reaches 160 oC an alarm is given “fabric filter temperature very high”If the temperature Tmax reaches 175 oC automatic changeover from fabric filter to ESP is initiated.If the temperature Tmax reaches 180 oC an alarm is given “fabric filter temperature near trip set point ”.

If the temperature Tmax reaches 190 oC the emergency water spray system is activated and a trip signal is sent to the boiler control system.The emergency water spray system will automatically shut down when the inlet temperature drops below 190 oC.If the temperature Tmin drops to 120 oC, an alarm “Fabric Filter Inlet Temperature Low” will be given.If the temperature Tmin drops to 110 oC, an alarm “Water Spray System Shut Down, Spray System Fault” will be given, and the water supply to the spray system will shutdown.

To change back to Water Spray operation it is recommended that manual control be used to open the water control valves until the temperature set point is reached. The plant can then be changed over to automatic operation.In additional to the temperature trips and alarms provision has also been made to initiate an automaticchangeover to ESP mode if the airheater rotation slow or stops

The emergency water spray system has been modified by the addition of a non-return valve in the water supply to the sprays after the solenoid valve.This allows the negative pressure in the duct to draw air in from outside, through the nozzles, keeping them clean.

Emergency Water System

Bag Cleaning SystemThe plant contains 436 Optipow pulse control valves. Each gas path has 2 rows of valves, each row consisting of 109 valves. The 109 valves are mounted in 5 pulse tanks.The cleaning sequence is such that odd numbered valves are pulsed first, 1 in each tank in sequence, followed by even numbered valvesWhen the valve is fired, a pulse of compressed air is directed down each row of bags, dislodging accumulated ash from the outside of the bag.The differential pressure (DP) across the bag plate in each gas path is monitored.

Filter Bags

When the DP reaches the DP set-point, nominally 130 mm WG, adjustable in the PLC, one row of filter bags (2 pulse valves, 1 in each row of tanks) is pulsed. Pulsing is dependant on the pressure in the pulse tank being above 350 kPa. If the tank is not above the set point, the valves are fired after 7 seconds, and the system steps on to the next valve in the sequence.When the DP drops below the set-point pulsing stops.

If the DP does not rise above the setpoint within 120 seconds, the next valve in the sequence is fired. This means that regardless of low DP all bags will be cleaned once every 218 minutesIn addition to the normal automatic cleaning, a fast clean function is available in the PLC. When manually initiated, every bag in the gas path will be cleaned once, with the time only limited by the supply of compressed air.

An alarm system has been provided to monitor pulse valve operationThe system checks that the pressure in the pulse tank drops after a signal has been given to fire.If the pressure in the tank fails to rise above the setpoint by the time the next valve in the tank is due to be fired, a fail to rise “R” alarm is given in the PLC.To test that all pulse valves are operating, the following procedure is used:

Go to BF Pulsing Pass A or D screenChange “F to Fall Mode Sw” to “On”.Reset the “Pulse on Time” from 10 to 100 millisecondsWith these settings, if the pressure fails to drop when a signal is sent for a valve to fire, a fail to fall (f) alarm is given in the PLC in the location of the valve.It is recommended that this procedure be carried out for one complete cleaning cycle on each gas path in turn, on a once a month basis.After testing all valves in one gas path the “F to Fall Mode Sw” should be changed back to “Off” and the “Pulse on Time” back to 10 milliseconds.

The air supply pressure is also monitored in the PLC. Should the filter bag plate DP rise to 170 mm WG, an alarm ”Fabric Filter DP High” is givenShould the filter bag plate DP rise to 200 mm WG, an automatic changeover to ESP operation is initiated

The pulse valves have been upgraded to the latest design using an upgrade kit. This was done to overcome problems with the mounting of the solenoid valvesOrifice plates have been installed at the inlet to each pulse tank. This has the following benefits:

System pressure is not completely lost if a valve fails to close.Pressure in tanks is not significantly affected when another pulse tank fires, assisting the operation of the pulse alarm system.

Optipow Pulse Valve

Pulse Tank

Pulse Pipes Inside Casing

Clean compressed air supply filters as required (alarm of high DP in PLC)Monitor emissions using opacity monitor and/or visual observation of stack daily.Inspect clean gas chambers through viewing windows weeklyReplace or cap leaking bags when detected.Monitor bag plate DPCheck all pulse tanks for leaks monthly (max 50 kPa pressure loss in 60 s when isolated at 350 kPa)

Bag Filter Routine Maintenance

Check operation of sonic horns weekly

Routine MaintenanceMonitor cleaning frequency at full load weekly and reset DP set-point if necessaryMaintain record of all failed bags:

locationservice hoursfailure typecause

Maintain air compressors and dryers.Clean viewing windows (weekly)Repair or replace faulty RTD’s as faults occur.

Boiler Shutdown

Inspect clean gas plenums both gas paths internally. Clean and replace faulty bags as required.Inspect inlet ducts internally.

Wear on spray erosion protection,wear on gas flow guide vanestest correct operation of both control and emergency spray lancesinspect and replace work nozzle capscheck RTD’s for erosion damage

Leak test all pulse tanks

Pulse tank leaks can be caused by:tank drain valve not properly closedvalve plunger stuck open due to grit.valve plunger worn or damagedvalve diaphragm worn or damagedhose to solenoid leakingfaulty O ringsfaulty or loose solenoidworn or corroded valve seat

Recalibrate all DP transmitters and switches during major boiler outages.

TroubleshootingHigh emissions can be caused by excessive bag cleaning, by damaged bags, or by leakage through the ESP isolation dampers.In the case of high emissions the opacity monitors will indicate which gas path is at fault. Inspection through the clean gas compartment will then enable the area of the fault to be determined.Internal inspection will then be required to locate the faulty bag. This is normally found by a build-up of ash around the bag.Because of this the compartment should be cleaned when a faulty bag is capped or replaced.

Excessive cleaning is normally due to mal-operation of the DP transmitters, or the DP Cleaning Set-Point being set too low.The DP transmitter and associated tapping points should be checked.If the DP transmitter is OK, the DP Set-Point should be raised to give a cleaning cycle of no less than 12 minutes when operating at full load.

High DP is normally caused by mal-operation of the DP Transmitter or the bag cleaning system, but can also be caused by excessive boiler gas flow, boiler tube leaks, operation at very low temperatures, and gradual aging (blinding) of the filter bags.First check the pressure of the air supply to the pulse tanks in the PLC.Check pulse air compressors running.

Then check for faulty pulse valve operation in PLC.Inspect pulse tanks. One tank lower pressure than others indicates a leaking valve.Isolate faulty tank and check that supply pressure rises.Check tank pressure falls by 150 kPa minimum when pulsed. Done by closing isolation valve before pulsing.Check DP transmitter and boiler operation

THE END