What is FGD (Flue Gas Desulfurization), Types of FGD, Major Systems in FGD

Flue Gas Desulfurization (FGD) 

Flue Gas desulfurization (FGD) is a set of technologies used to remove Sulphur di-oxide (SO2) from exhaust flue gas.

Following types of Flue Gas Desulfurization systems are in practice:

1. Limestone FGD (wet)
2. Seawater FGD
3. Dry FGD

Lime Stone FGD 

Major systems in FGD

1. Lime stone handling and lime slurry preparation.
2. Wet Ball Mill
3. Water pump house
4. Absorber/Scrubber (Slurry R/C pumps, GGH, Oxidation blower)
5. GYPSUM Dewatering system
6. Waste water system.

Lime Stone Handling System:

Main Equipment's:

1. Bucket Elevator
2. Ground Hopper
3. Magnetic Seperater
4. Lime Stone Silo
5. Gravimetric Feeder
6. Vibro Grate Feeder
7. Vibro Feeder Motor
8. Dust Collector
9. Air Blaster

Lime Stone Size Requirement for slurry preparation is ≤ 20mm. Lime Stone is transported from storage yard to ground hopper through trucks. Vibro grate feeder used to maintain proper flow to the bucket elevator. Magnetic separator is used to remove iron particles from limestone. Unloaded lime stone in ground hopper stocked in silo (1748MT) using vibrating motor & bucket elevator(80TPH). Lime stone from silo to ball mill is fed by the gravimetric feeder (0-25Tph). Air blaster (360 l/min) placed in silo to ensure proper flow to gravimetric feeder.

Wet Ball Mill:

Main Equipment's:

1. Lube oil system
2. Bearing Gear Motor
3. Ball mill main motor
4. Ball mill
5. Mill re-circulation pump
6. Hydro cyclone 
7. Lime slurry feed pump

From gravimetric feeder lime stone is fed to the mill and filtrate water /process water in the ratio of 1:1 and 1:3 for R/c tank. Lime slurry is pumped by re-circulation pump to the hydro cyclone. Hydro cyclone separate the slurry according to the density. The more density slurry will be transferred to lime stone slurry tank through distribution box. Lower density slurry will be transferred back to ball mill and R/C tank through distribution box. Lime slurry is pumped to absorber through lime slurry feed pump

Water pump house:

Main Equipment's:

1. Cooling water pump A & B
2. Process water pump A & B 
3. Mist eliminator pump A, B & C
4. Emergency slurry pump A & B

Cooling water tank – Purpose of cooling water tank is to store the service water, for equipment (gearbox, pumps etc.) cooling for both units. Water is pumped through cooling water pump. 

Process water tank – Water in the tank is used for the purpose of all lines flushing and sealing of vacuum pump of both units. Water is pumped through process water pump

Mist eliminator pump – Purpose of the ME pump is to supply water for flushing/cleaning the absorber’s mist eliminator system. It eliminates the mist generated in the absorber during gas water absorption.

Emergency slurry tank – Stores the slurry in absorber during maintenance or for operational emergencies. Slurry can be transferred both sides i.e from absorber to EST and EST to absorber. 

ABSORBER:

Main Equipment's:

1. Booster fan
2. Booster fan lube oil system
3. Slurry re-circulation pumps
4. Oxidation blower
5. Air compressor and dryer
6. Gas to Gas heat Exchanger (GGH)

                                 

Absorber tank height 40.2 m. Slurry level ranges from 11.7 to  12.2 m. Slurry feed to absorber continuously for maintaining the PH. Raw flue gas from ID Fan is drawn by booster fan  and  gas flows to the absorber through the GGH by duct. The flue gas & slurry flow are in counter flow direction. While the gas is passing upward direction the slurry is sprayed through R/C pumps nozzles. SOx in the gas will be absorbed by the lime slurry and treated flue gas goes upwards in the absorber and then pass to stack through GGH. Oxidation blower oxidizes the slurry inside the absorber continuously. Now the reaction will take place for gypsum crystal formation at particular retention time 

Booster fan:

   

Purpose of Booster fan is to draw the flue gas to FGD system from ID fan outlet. According to blade pitch it will draw the gas to absorber. Is equipped with lube oil pump (1w+1s/b) and  tank circulation oil pump (1w+1S/b). Damper seal air fan (1w+1s/b) supply's the air through heaters for absorber INLET,OUTLET and BYPASS flue gas dampers, while unit is shutdown/ Maintenance.

 

GGH & Oxidation blower:

 

Gas to Gas Heater – Purpose of GGH is to transfer the heat from raw flue gas to clean flue gas. Of the two motors of GGH (main motor and auxiliary motor) one motor is stand by. Low leakage fan in GGH is used to seal the raw flue gas by clean gas. This Oxidation blower is used for oxidation purpose in absorber system. (1W+1s/b). Air from oxidation fan/ blower enters absorber after spray water humidification and then the calcium sulfite generated from desulfurization shall form calcium sulfate in the forced-oxidation process and then form dehydrate gypsum after crystallization.

Gypsum Dewatering Systems:

Main Equipment's:

1. Bleed pump
2. Hydro cyclone
3. Vacuum pump
4. Vacuum belt filter (VBF)
5. Cloth wash pump
6. Cake wash pump
7. Gas separator tank

Gypsum slurry is pumped by bleed pump to hydro cyclone from absorber, based on the density of the slurry of absorber. The gypsum slurry flows through hydro cyclone to vacuum belt filter by distribution chamber. Higher density slurry goes to VBF and less density goes to filtrate water tank/waste water tank (based on the Operation requirements). Gypsum slurry dewatering done by vacuum pump in the VBF. Cloth wash pump run continuously to clean VBF cloth and sealing of vacuum pump. Dry gypsum after dewatering falls to the gypsum storage yard.

WASTE WATER SYSTEM:

Main Equipment's:

1. Waste water pump
2. Waste water hydro cyclone
3. Waste water transfer pump
4. Outlet transfer pump
5. Dosing system 

Waste water generated in the dewatering system is treated and discharged through various stages in waste water system i.e neutralization, precipitation, flocculation, Clarifier. Fecl3 ,TMT (Organic sulfur) , Polyacromite ,HCL and Lime Milk chemical use for water treatment process. PH and heavy metal are treated in this system.