ESP ( Electrostatic Precipitator)

                   Boiler ESP (Electrostatic Precipitator)

   Electrostatic Precipitator is a device that removes suspended dust particles from a gas or exhaust by applying a high voltage Electrostatic charge and collecting the particles on collecting plates .

                               

GDS – Gas Distribution screen
Inlet gas distribution .
Velocity after gas distribution screen reduce below 1 m/s .
Emitting Electrode collecting plate
Discharge electrode (-VE Charge)
Collecting Plate ( +VE Charge)
 One emitting  electrode collecting plate between two collecting plate . create a group transfer rectifier set .
    Transfer rectifier is an electronic device that can be used to convert alternating current (AC) , which reverse direction , in to direct current (DC) . AC to DC Rectifier .
                 
               

                                      Corona Effect

  If the applied voltage is high emitting an electric corona discharge ionizes the air around the electrodes which then ionizes the particles in the air steam . The ionized particles , due to the electrostatic force , are diverted towards the grounded plates .
                     
                              
                     

GD PRM Hammer  -  Gas distribution plate rapper mechanism
CRM Motor – Collecting rapper Motor
DRM Motor – Discharge rapper Motor
v  Ash hopper
v Pent house
v TR Set *    -VE  Charge from  TR Set
v Drum / conical insulator
v Heater temperature maintain
v Support insulator
v Purge air blower
v Hopper heater arrangements
 
                   

          Ash handling system
 1 field – 60 to 70%
2 field – 20%
3 field – 10%
Power – milli amp
Flow rate of gas i.e. gas velocity .
 When flue gas velocity decreases then efficiency of ESP increases .
Flue gas velocity is inversely proportional to efficiency of ESP .
  Two forces acts on a particle at ESP (1) Flue gas flow
                                                              (2) Electrostatic force
 Both forces are right angle to each other . So the particle flow in resultant direction .
          (1)  V_FG  Increase       
 ESP  Increase

(2)V_FG   Decrease       

 If flue gas velocity is higher than Electrostatic force then particles does not charge properly , escape from collecting plate which leads to erosion .
                          
                           

   

       If flue gas velocity is lower then Electrostatic force then sufficient time available for proper charging & collecting of dust particles .
 Gas velocity 0.75 m/s is best for ESP .
Resistivity of Ash particles .
Resistance to Electrical conduction measurement
Resistance unit - Ω ohm cm
Resistivity of coal fly ash = 10 ⁸ to 10 ¹² ohm cm
       R is too low
       R is too high
  Charge particles supposed to transfer their charge to collecting plate .
    If resistivity is too low particles discharge their charges too quickly & escape from collecting plate &  come back to gas steam . Re-net rain process .
   If resistivity is too high particles do not give up their charge completely to collecting plate , so the charge accumulate on collecting plate & leads to back corona . This reduce ionization  & particles escape with flue gas .
                       
                               
     

       Resistivity depends on temperature of flue gas temperature is directly proportional  to viscosity gas .
 Viscosity means resist  flow .
T     Viscosity    Resistance
   Temperature of flue gas increases viscosity increase . Resistance of ash particles also increase .
Resistivity is maximum between 149 ⁰ c to 240 ⁰ c
(2) Carbon Content in as particles .
      Carbon is good conductor of electricity . If carbon content is more due to bad combustion of or high FC fuel then resistivity of ash particles decreases .
 If ash contain 10% higher unburnt carbon , sparking rate increases in ESP ultimately reduce secondary voltage .               
(3) Sulphur content in ash particles .
      Sulphur % is inversely proportional to resistivity of ash particles .
  Sulphur trioxide (SO₃) injected in to flue gas when low Sulphur coal is used in boiler to reduce the resistivity of flue gas . 10 PPM to 20 PPM of SO₃  is sufficient to maintain resistivity in flue gas . If SO₃  level is high then Ammonia(NH₃) is injected into flue gas for maintain resistivity .
  Dust concentration in flue gas .
  If ash concentration is increased beyond design value , collection efficiency  is reduced .
                           
                 

 Size of ash particles
   If size of ash particles increase the efficiency of ESP decrease .
Operating voltage
  ESP works at higher efficiency when higher voltage is maintained .
What should care before charging ESP ?
Ø Purge air blower should be starting condition .
Ø ESP inlet flue gas temperature is more than 120⁰C .
Ø All manhole door should be locked condition .
Ø Earth switch (1) Normal mode
       (2) Earthing mode
RPM – Rotation per minute
Combustible elements in fuel :- Carbon , Hydrogen & Sulphur
Hydrogen + oxygen = Water
H₂O          +   O        =  H₂O
    The temperature at which water vapor in flue gas of boiler start condensing (First drop water) is known as DT . ( 50⁰C TO 80⁰C) .
Sulphur + oxygen =  Sulphur Dioxide  ( S+O₂ = SO₂)
Excess air is present , some of this SO₂ oxide to SO₃
Sulphur dioxide + ½ oxygen = Sulphur trioxide
Blow acid due point temperature
Sulphur trioxide + water = Sulfuric acid
SO₃ + H₂O = H₂SO₄
If 0.5% Sulphur in fuel then acid dew temperature -119⁰C
   The temperature of flue gas at which the Sulfuric acid condensed on the surface of boiler parts is called ADT (Acid dew temperature) acid serious corrosive action on boiler parts such as Economizer , Air pre heater ducts , fans & chimney .
                             
                                             

                  
 Due to condensation of sulfuric acid always ESP charged at above ADT temperature [119⁰C] .
If sulfur % increased in fuel acid due point temperature also increases .
0.5 %  sulfuric - ADT - 119⁰C
1 % sulfuric – ADT – 126.7⁰C
2 % sulfuric – ADT – 132.2⁰C
3 % sulfuric- ADT – 137.8⁰C
4 % sulfuric – ADT – 143.3⁰C
5 % sulfuric – ADT – 148.9⁰C
 Sulphur % is directly proportional to acid due point temperature (ADT)
SL  Increased  ADT Increased
SL Decreased  ADT Decreased
Excess Air

  The amount  of air which is more then the stoichiometric  /  Theoretical air quantity required for combustion of fuel is called Excess air .

                                 

                                           

 

 
    Stoichiometric air or theoretical air is the exact amount of air required to provide the right amount of oxygen for complete combustion .
   Excess air ensures that there is enough air for complete combustion  . Excess air is expressed as a percentage of theoretical air required .
 Calculation of EA from O₂ %
     O₂ %= 5
Excess air from O₂ = O₂ %  / 21 -O₂ % * 100
                              = 5/21-5 * 100 = 500 / 16 = 31.25 %
                           
Good effect of EA
·       Better  combustion , less unburnt is Ash .
·       Less co generation ( Co – Carbon monoxide)
·       Better control of furnace and bed temperature
Excess air for different type of fuel combustion –
1.       Stoker fire Boiler – 25 % to 35 %
2.     Wood fire Boiler – 25 % to 50 %
3.     Coal fire Boiler – 10 % to 40 %
4.    Gas fire Boiler – 5 % to 10 %
5.     Oil fire Boiler – 10 % to 20 %
Bed effect of Excess air
v  Boiler efficiency is affected badly .
v Fires carry over is more is flue gas .
v More erosion in flue gas path .
 Every 10 %  excess air high results in a 3 % increases in fuel combustion .
 
Thank you  For read
Best wishes from
                        

     Suvendu  Singha . (Asia , India , Odisha , Balasore)