Gas to Gas Heat Exchanger System (GGH) in FGD System for Coal based Power Plants

Introduction

Gas to Gas Heat Exchanger (GGH) is a heat exchanger device in which heat is transferred from the high temperature untreated flue gas to the treated low temperature flue gas.

Main Function of GGH is to transfer the heat from raw flue gas to outlet clean gas from absorber. GGH is used in the coal power plant mainly for the purpose of reducing the SO2 from exhaust Flue Gas.

The working principle of GGH heat exchanger is slowly rotating rotor filled with heating elements (Baskets) which transfer heat between two gas flows. The hot gas flow to be cooled down has to be in counter flow with the cold gas to be heated up.

• GGH is set with two electric drive units with one for main drive and the other for standby.
• Rotor is of counter flow bisector type. Rotor Fan bearings are oil grease lubricated. Motor Rotor bearings are sealed cooled by seal air fan.
• There are two bearing cooling fans to cool the fan bearings as bearing are located in top and bottom side. Motor has air cooled winding system.
• There are two LRSB in GGH top side and bottom side. Soot blowing using steam or water or compressor air.
• Two low leakage fans are in GGH one for main and another one is stand by. Purpose of low leakage fan is to seal the raw flue gas leakage from the Sector plate by clean flue gas.

GGH Design Description

On one side, GGH takes in the hot untreated flue gas and passes this flue gas into FGD system. Whereas on the other side there will be flow of treated flue gas and then passes into the Absorber. In the GGH, there will be heat transfer from the hot untreated flue gas to the treated flue gas. This will decrease the temperature of untreated hot flue gas below the dew point of sulfuric acid which results in condensation of sulfuric acid and consequently SO₂ can be removed at FGD by the treatment of injected compounds such as limestone.

Pre Start-Up Checks:

5.2.1 

• All manholes in raw flue gas, clean flue gas path, are in closed condition.
• LRSB top and bottom need to be checked.
• GGH Gear box oil level and bearings oil levels are Normal.
• Rotor shaft bearing cooling fan (A, B) power supply is ON.
• Low leakage fan (A,B) Power supply is ON

Starting permissive of GGH:

• GGH main motor breaker control supply is in healthy condition.
• No trip indication in breaker control panel.
• FGD raw and clean flue gas damper are closed.
• GGH rotor stalling alarm to be checked
• Lube oil for gear box and support bearing to be checked
• All man holes to be closed.

GGH Sequence start up procedure:

• Ensure FGD raw and clean flue gas damper are closed, the standby motor is stopped, start the GGH sealing fan
• Open the seal air valves /dampers to the rotor seal and sector plate seal portions from the Seal air fans and Low leakage fans
• Start the main seal air fan and put another fan as a standby mode
• Start the GGH main motor VFD until the normal RPM delaying for 60s, start the rotor parking alarm system and check the direction of rotation
• The rotor runs at the normal revolving speed of main drive motor open the FGD raw and clean flue gas dampers
• The low-leakage fan inlet and outlet dampers are closed, start the main low-leakage fan keep the standby low-leakage fan stopped, close the standby low-leakage fan inlet and outlet dampers, Open the main low-leakage fan inlet and outlet dampers.
• Open the sealing valves for soot blower
• Start the LRSB soot blowers sequence with air compressor.

GGH running Maintenance checks:

• Check the drive gear box lube oil level.
• Check the bearing shaft cooling fan running condition.
• Check the low leak fan running condition.
• Check the LRSB top and bottom soot blower running condition.
• Check the rotor main and support bearing temperature.
• Check the rotor cooling fan & LLF fan air pressure.
• Check the GGH rotor running condition and feedback.
• Check any abnormal noise and vibration in above equipment’s.

GGH Stop procedure:

• Ensure the booster fan is stopped. Then stop the low leakage fan.
• Start the soot blowing sequence controller keep GGH rotating for 1h turn off the on/off switch for alarm system
• Stop the main and auxiliary motors
• Close the sealing valves for soot blower
• Inlet and outlet damper of FGD closed condition, absorber vent in open condition

Corrosion Resistance:

In most GGH applications, the risk of condensation and corrosion is much greater. The GGH operates at lower temperature than Air Pre heater (APH). This means that the GGH is exposed to fly ash and acid condensing on the untreated side and to a combination of water droplets (often with high chlorine content), Unreacted limestone and the remaining SO₂ on the clean gas side. 

Heating Elements (Baskets):

Removal of GGH Baskets:

1. Maintenance opening is to be provided where removal and installation of GGH baskets is convenient (remove insulation and cut open a hole at a suitable location)
2. Install a channel Beam from inside to Outside of GGH and install Hoist
3. Before carrying out any welding activities on the rotor, it is necessary to make sure that the rotor is properly earthed to avoid any possible damage to the GGH bearing
4. Hook up the GGH baskets and remove the baskets from compartments in the sequence of A to G and move the baskets outside through the maintenance opening
5. Rotate rotor to reach next compartment and repeat the above same procedure
6. All old GGH elements shall be removed before installation of new ones to prevent clogging.All the accumulated dust and miscellaneous materials on the rotor and cold-end & hot-end sector plates shall be thoroughly cleaned off.
7. Welding of sector plates shall be checked, if loose welding point is found, must apply welding and buffing again.
8. Bearing verticality shall be within the required range (0.40 mm/meter)

Installation of GGH Baskets:

1. Move the GGH baskets to be installed through the maintenance opening to the inside of GGH.
2. Baskets installation should start with A, and to B, and to C, and to D, and to E1 & E2, and to F1 & F2, and to G1 & G2 installation is complete
3. Rotor balancing is very important criteria we have to follow while installation of baskets.
4. Levelness adjustment after completion of all GGH baskets installation to be carried out
5. Installation of All Apex Seal, Axial Seal and Bypass Seals to be carried out

Key Note:

1. If you insist that installation be carried out after partial removal of old GGH element, then the installed GGH modules should be properly covered so that objects or dust may not fall into the GGH element plates and form clogging.
2. Walking and stacking of any materials on the newly installed GGH elements is strictly prohibited.
3. Collision and hitting of GGH elements is also prohibited.

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.