Air Pollution Control

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Air Pollution Control Part CElectrostatic
Precipitation Basic Principles

• Yaacov Mamane
• Visiting Scientist, CNR
• Rome, Italy

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• Electrostatic precipitation has been a reliable
  technology since the early 1900's. Originally
  developed to abate serious smoke nuisances. Zinc,
  copper, and lead industries found ESP a cost
  efficient way to recover valuable product. Today
  ESP are found mainly on large power plants,
  incinerators, cement plants,  
                              
•   In wood products industry, ESP preceded by
  multi clones is considered the best available
  control technology for wood fired boiler
  emissions. Wet ESP have found renewed interest
  from particle board, and plywood veneer
  manufactures for controlling dryer exhaust.
• An ESP can consistently provide 99 removal
  reducing emissions levels to 0.002 - 0.015 grains
  per dry standard cubic foot of exhaust gas.   
• Precipitators are designed to handle flow form
  10,000 cfm to 300,000 cfm and can operate at
  temperatures as high as 750 degrees F. Normal gas
  flow through a precipitator is 2-5 feet per
  second, consequently, the pressure drop is only
  0.5" wc.    

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• Many countries around the world depend on coal
  and other fossil fuels to produce electricity. A
  natural result from the burning of fossil fuels,
  particularly coal, is the emission of flyash. Ash
  is mineral matter present in the fuel. For a
  pulverized coal unit, 60-80 of ash leaves with
  the flue gas. Flyash emissions have received the
  greatest attention since they are easily seen
  leaving smokestacks.
• Two emission control devices for flyash are
• The traditional fabric filters - The fabric
  filters are large baghouse filters having a high
  maintenance cost (the cloth bags have a life of
  18 to 36 months, but can be temporarily cleaned
  by shaking or back flushing with air). These
  fabric filters are inherently large structures
  resulting in a large pressure drop, which reduces
  the plant efficiency
• The electrostatic precipitators- ESP have
  collection efficiency of 99, but do not work
  well for flyash with a high electrical
  resistivity (as commonly results from combustion
  of low-sulfur coal). In addition, the designer
  must avoid allowing unburned gas to enter the
  electrostatic precipitator since the gas could be
  ignited.

  
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• DESIGN AND OPERATION
• A precipitator is a relatively simple device.
• The main components are as follows
• An insulated and lagged shell
• Collection plates or tubes
• Discharge electrodes
• Collection Plate Rappers/Electrode Vibrators
• Hoppers

Dust laden gases are pushed or pulled through the
box with the assistance of a fan. The air flow is
channeled into lanes formed by the collection
plates or tubes. Discharge electrodes are
centered between each collection plate/tube to
provide a negative charge to the surrounding dust
particles. The collection plates/tubes are
positively grounded and act as a magnet for the
negatively charged dust particles. The collected
dust is transported down the collection plates
and electrode with the assistance of a rapper or
vibrator system into the collection hopper. For
a more detailed overview of an electrostatic
precipitator, see www.powerspancorp.com/news/preci
pitator.shtml and http//www4.ncsu.edu/frey/apces
pph.html.
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• Insulated Steel Housing The development of
  modular, factory built units has significantly
  lowered the installed cost of precipitators. Dry
  precipitators are normally fabricated from 3/16"
  thick steel plate, insulated and lagged with
  aluminum. The electrodes are made of steel tubing
  and the collection plates are made of rolled
  steel. The housing can last 15-20 years.
• Wet precipitators are traditionally fabricated of
  stainless steel for corrosion resistance.
• Discharge Electrodes In the past dust particles
  were charged by a series of small diameter wires
  which were suspended from a ceiling rack and
  weights at the bottom. This maze of electrodes
  was subject to electric erosion. Today, discharge
  electrodes are rigid and constructed of 2" steel
  tubing. Ten years of continuous service is the
  expected norm.
• Rappers and Vibrators Heavy duty rappers are
  used in the wood industry. They consist of 30
  pound piston hammers designed to rap small
  sections of collection plates. A timer
  periodically releases the rapper to transfer the
  dust on the collection plates to the hopper.
  Electric vibrators are placed on the electrode
  rack to transfer any collected dust to the hopper
  and are operated by a timer.
• Power ESP will take 480 volt AC and transform /
  convert the power to 55-70,000 Volt DC.
  Electrostatic precipitator use the lowest power
  to accomplish the job. Electrostatic forces are
  applied directly to the particles and not the
  entire gas stream. Combining this feature with
  the low pressure drop (0.5" wc) across the system
  results in power requirements approximately 50
  of comparable wet systems and 25 of equivalent
  bag filter systems.

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• When are Electrostatic Precipitators not a
  suitable solution?
• As the size of the required precipitator
  increases, other technologies become more cost
  effective. For low sulfur utility applications,
  fabric filters are an attractive alternative. As
  part of the overall precipitator/fabric filter
  cost evaluation, operating costs need to be
  included. Typically, the pressure drop across a
  flange to flange fabric filter will be in the 6
  to 8" w.c. range whereas an electrostatic
  precipitator will have approximately a 0.5 to 1"
  w.c. pressure drop. This pressure drop penalty
  for a fabric filter will be somewhat offset by
  its lower power consumption.
• Another benefit of a fabric filter is high acid
  gas, SO2, chlorides, fluorides and Hg removal
  capability.
• When operating downstream of a spray dryer
  absorber, removal efficiencies of 90 or greater
  can be attained for some species when operating
  in conjunction with a fabric filter. The fabric
  filter dust layer acts as a fixed bed where high
  acid gas removal efficiency can take place. Since
  most of the particulate is removed from the
  collecting electrodes of a precipitator during
  normal operation, acid gas removal capability is
  much reduced.

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Electrostatic Precipitator Sizing Electrostatic
precipitators are being used in cement, refinery
and petrochemical, pulp and paper and power
generation. Although the physical operation of a
precipitator is simple and essentially the same
for each industry, involving particle charging,
collection, dislodging and disposal, the sizing
of a precipitator is more complex combining both
art and science. The typical equation used in
precipitator sizing is the modified Deutsch
equation
                                                             
   gas volume
   precipitator inlet loading
   precipitator outlet loading required
   outlet opacity required
   particulate resistivity
   particle size
Where A is the collecting electrode surface
area, V is the gas volume and w is the
precipitation rate. y is a variable based on
test data for each specific application.
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Factors that influence precipitator sizing are
   gas volume
   precipitator inlet loading
   precipitator outlet loading required
   outlet opacity required
   particulate resistivity
   particle size
Particle size of the incoming particulate has a
dramatic impact on the sizing of an electrostatic
precipitator. Fluid Catalytic Crackers and
Recovery Boilers, which have particle
resistivities in the medium range, exhibit very
fine particulate. The size of the precipitator
must be increased in these cases because the fine
particulate is easily re-entrained into the gas
stream. In the power industry, generally the
higher the fuel ash content, the larger the ash
particle size.
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Resistivity is the resistance of a medium to the
flow of an electrical current. By definition,
resistivity, which has units of ohm-cm, is the
electrical resistance of a dust sample 1 cm2 in
cross sectional area and 1 cm thick.
Resistivity levels are generally broken down into three categories Low lt10 5 ohm-cm Medium 10 5 to 10 11 ohm-cm High gt10 11 ohm-cm  
Particles with medium resistivity are the most
acceptable for electrostatic precipitators.
Particles in the low range are easily charged,
however upon contact with the collecting
electrodes, they rapidly loss their negative
charge and are repelled by the collecting
electrodes back into the gas stream. Particles in
the high resistivity category may cause back
corona which is a localized discharge at the
collecting electrode due to the surface being
coated by a layer of non-conductive material.
Resistivity is influenced by flue gas temperature
and conditioning agents, such as flue gas
moisture and ash chemistry. Conductive chemical
species, such as sulfur and sodium will tend to
reduce resistivity levels while insulating
species, such as SiO2, AL2O3 and Ca will tend to
increase resistivity (ash from low sulfur coal
is). Flue gas conditioning with SO3 can reduce
resistivity to a more optimum value thus reducing
the size of the precipitator needed.
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• ESP Advantages
•                                                   
      
• They have high efficiencies (exceeds 99.9 in
  some applications)
• Fine dust particles are collected efficiently
• Can function at high temperatures (as high as 700
  degree F 1300 degree F)
• Pressure and temperature changes are small
• Difficult material like acid and tars can be
  collected
• They withstand extremely corrosive material
• Low power requirement for cleaning
• Dry dust is collected making recovery of lost
  product easy
• Large flow rates are possible

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ESP Disadvantages High initial cost Materials
with very high or low resistivity are difficult
to collect Inefficiencies could arise in the
system due to variable condition of airflow
(though automatic voltage control improves
collector efficiency) They can be larger than
baghouses (fabric collectors) and cartridge
units, and can occupy greater space Material in
gaseous phase cannot be removed by electrostatic
method Dust loads may be needed to be reduced
before precipitation process (precleaner may be
needed)
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Wet Electrostatic Precipitators Wet
electrostatic precipitators (WESPs) operate in
the same three-step process as dry ESPs
charging, collecting and finally cleaning of the
particles. However, cleaning the collecting
electrode is performed by washing the collection
surface with liquid rather than mechanically
rapping the collection plates. While the
cleaning mechanism would not be thought to have
any impact upon performance, it significantly
affects the nature of the particles that can be
captured, the performance efficiencies that can
be achieved, and the design parameters and
operating maintenance of the equipment.
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Example 1

• Five thousand tons of coal are burned daily in an
  electric power plant.
• The coal has an ash content of 12 by mass. Forty
  percent of the ash falls out the bottom of the
  furnace.
• The rest of the ash is carried out of the furnace
  with the hot gases into an electrostatic
  precipitator (ESP).
• The ESP is 99.5 efficient in removing the ash
  that comes into it. Draw a diagram representing
  this process and calculate the mass emissions
  rate of ash into the atmosphere from this plant.

• Source Ni-Bin Chang

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Diagram for Example 1
The rest is emitted to the the stack
208300 kg/hr coal is burnt. Coal has 12 ash
Furnace burns coal, ash is being formed
ESP treats the rest of the ash, with efficiency
of gt99
40 of the ash is deposited in the boiler

• Source Ni-Bin Chang

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Solution for example 1
5000 tons coal/day 365 days 0.12 ash 219000
tons ash / year 219000 tons ash 0.4 87600
tons / year deposit in boiler. 21900 87600
131400 tons/year reach ESP 131400 0.995
130743 tons/year deposit in ESP 219000 87600
130743 657 tons/year emitted to the atmosphere
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Example 2

• A 600 MW coal burning power plant is burning
  Illinois bituminous coal with 8 ash content.
  The plant is 39 efficient. 35 of the ash drops
  out in the furnace. The electrostatic
  precipitator is 99.0 efficient.
• Draw a simplified sketch of the process.
• Draw an energy balance diagram for the plant and
  calculate the rate of heat emitted to the
  environment, in J/s.
• Calculate the rate of coal input to the furnace,
  in kg/day.
• Calculate the rate of ash emissions to the
  atmosphere, in kg/s.

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Example 2
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Example 2

• Energy Balance
• Waste heat 1579 - 600 979MW

Heat in
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Example 2

• Coal input
• Ash to atmosphere