Fundamental of Electrostatic Precipitator (ESP)

ESPElectrostatic precipitators (ESP) are particulate collection devices that remove particles from a flowing gas (e.g. air) by using the force of an induced electrostatic charge. ESPs have a highly efficient filtration performance with a minimal reduction of the flow of gases through the device

All electrostatic precipitators, regardless of their particular designs, contain the following essential components:

• Discharge electrodes

• Collection electrodes

• High voltage electrical systems

• Rappers

• Hoppers

• Shell

Precipitator Components

Discharge electrodes are either small-diameter metal wires that hang vertically (in the electrostatic precipitator), a number of wires attached together in rigid frames, or a rigid electrode made from a single piece of fabricated metal. Discharge electrodes create a strong electrical field that ionizes flue gas, and this ionization charges particles in the gasCollection electrodes collect charged particles. Collection electrodes are either flat plates or tubes with a charge opposite that of the discharge electrodes.

High voltage equipment provides the electric field between the discharge and collection electrodes used to charge particles in the ESP

Rappers impart a vibration, or shock, to the electrodes, removing the collected dust. Rappers remove dust that has accumulated on both collection electrodes and discharge electrodes. Occasionally, water sprays are used to remove dust from collection electrodes

Hoppers are located at the bottom of the precipitator. Hoppers are used to collect and temporarily store the dust removed during the rapping process.

The shell provides the base to support the ESP components and to enclose the unit

Typical view of rappers of collection plates

Hoppers

When the electrodes are rapped, the dust falls into hoppers and is stored temporarily before it is disposed in a landfill or reused in the process. Dust should be removed as soon as possible to avoid packing, which would make removal very difficult. Hoppers are usually designed with a 50 to 70° (60° is common) slope to allow dust to flow freely from the top of the hopper to the bottom discharge opening.

Some manufacturers add devices to the hopper to promote easy and quick discharge. These devices include strike plates, poke holes, vibrators, and rappers. Strike plates are simply pieces of flat steel that are bolted or welded to the center of the hopper wall. If dust becomes stuck in the hopper, rapping the strike plate several times with a mallet will free this material. Hopper designs also usually include access doors, or ports. Access ports allow easier access for cleaning, inspection, and maintenance of the hopper (Figure 2-15).

In Hopper vibrators are occasionally used to help remove dust from the hopper walls. Hopper vibrators are electrically operated devices that cause the side walls of the hopper to vibrate, thereby removing the dust from the hopper walls. These devices must be carefully designed and chosen so that they do not cause dust to be firmly packed against the hopper walls, and thereby plug the hopper. Before installing vibrators to reduce hopper plugging, make sure they have been successfully used in other, similar industrial applications

Rotary airlock valves are used on medium or large-sized ESPs. The valve is designed with a paddle wheel that is shaft mounted and driven by a motor The rotary valve is similar to a revolving door; the paddles or blades form an airtight seal with the housing, and the motor slowly moves the blades to allow the dust to discharge from the hopper.

After the dust leaves the discharge device it is transported to the final disposal destination by screw, drag, or pneumatic conveyers. Screw conveyors can be used as discharge devices when located in the bottom of the hopper as shown in Figure 2-19 or as a separate conveyor to move dust after it is discharged. Screw conveyers employ a revolving screw feeder to move the dust through the conveyor. Drag conveyors usepaddles, or flaps, that are connected to a drag chain to pull the dust through the conveyor trough (Figure 2-20). Drag conveyors are used frequently for conveying sticky or hygroscopic dusts such as calcium chloride dust generated from municipal waste combustors (collected fly ash/acid gas products). Pneumatic conveyers use blowers to blow or move the dust through the conveyor (Figure 2-21). Pneumatic conveyorscan be positive pressure (dust is moved by a blower) or vacuum type systems (dust is pulled by a vacuum).

Brief NOTES on technical issue of ESP at plant level

Factor depends on ESP performance

1- Type of dust ( Chemical composition , dew point, corrosive ness &

combustive)

2- Size

3- Concentration of gas stream.

4- Dust / Particle Resistivity.

Now if we analyze any issue Related ESP performance , we should go through the above points .

Lets say in our cooler ESP(L-1) , We are operating at high temperature (300-345 0C) and higher the operating temperature lower the performance ESP and that is because of resistivity.

Note: In case of clinker dust ESP ,the resistivity is low at high temperature and high at low operating temperature where as in case of pre-heater exit dust ESP / raw mill ESP , it is just Reverse of Cooler ESP.( Next slide plz see the graph for clear understanding )

What is resistivity ?• Particle resistivity is a measure of the resistance of the dust particle

to the passage of current. It is expressed as ohm-cm. For practical operation the resistivity should be 107 and 1011 ohm-cm. At higher resistivities, particles is too difficult to charge and lead to decrease in efficiency. At times, particles with higher resistivity may be conditioned with moisture to bring them to the desired range. If the resistivity is too low, particles accept a charge easily but dissipate it so quickly that the particles are not collected at the electrode and are re-entrained in the gas stream. Particle resistivity depends upon the Chemical composition ,continuity of dust, gas temperature, SO3 content in burned coal and voltage gradient that exists across the dust layer.

Resistivity curve

Resistivity Vs ESP efficiency

Resistivity Range

Normal resistivity:

ESPs work best under normal resistivity conditions. Particles withnormal resistivity do not rapidly lose their charge on arrival at the collection electrode. These particles slowly leak their charge to grounded plates and are retained on the collection plates by intermolecular adhesive and cohesive forces. This allows a particulate layer to be built up and then dislodged from the plates by rapping. Within the range of normal dust resistivity (between 107 and 1010 ohm-cm), fly ash is collected more easily than dust having either low or high resistivity.

High Resistivity: If the voltage drop across the dust layer becomes too high, several adverse

effects can occur. First, the high voltage drop reduces the voltage difference between the discharge electrode and collection electrode, and thereby reduces the electrostatic field strength used to drive the gas ion - charged particles over to the collected dust layer. As the dust layer builds up, and the electrical charges accumulate on the surface of the dust layer, the voltage difference between the discharge and collection electrodes decreases. The migration velocities of small particles are especially affected by the reduced electric field strength.

Another problem that occurs with high resistivity dust layers is called back corona. This occurs when the potential drop across the dust layer is so great that corona discharges begin to appear in the gas that is trapped within the dust layer. The dust layer breaks down electrically, producing small holes or craters from which back corona discharges occur. Positive gas ions are generated within the dust layer and are accelerated toward the "negatively charged" discharge electrode. The positive ions reduce some of the negative charges on the dust layer and neutralize some of the negative ions on the "charged particles" heading toward the collection electrode. Disruptions of the normal corona process greatly reduce the ESP's collection efficiency, which in severe cases, may fall below 50%

Low resistivity: In low resistivity dust layers, the corona current is readily passed to the grounded collection electrode. Therefore, a relatively weak electric field, of several thousand volts, is maintained across the dust layer. Collected dust particles with low resistivity do not adhere strongly enough to the collection plate. They are easily dislodged and become reentrained in the gas stream.

Most probable reason stated under performance of ESP.

Now if we compare with our Pre-Coal ESP(L-1) , it will give clear understanding that because of high resistivity , Its performance is low And another could be a common reason that we are firing Indian coal (ash% from 35-45) and ash dust has high resistivity( means the coating over the collecting plate are not allowing the current for charge the particles.

Solution for high resistivity

Two other methods that reduce particle resistivity include increasing the collection surface area and handling the flue gas at higher temperatures. Increasing the collection area of the precipitator will increase the overall cost of the ESP, which may not be desirable. Hot-side precipitators, which are usually located in front of the combustion air preheater section are also used to combat resistivity problems. However, the use of conditioning agents has been more successful and very few hot-side ESPs have been installed since the 1980s.

Sulfur trioxide andsulfuric acidAmmoniaAmmonium sulfate1TriethylamineSodium compounds

Few example of Conditioning agent

Note: Normally for high resistivity , the collection efficiency of ESP will be bellow 50%.

The basic design criteria for ESP is the determination of the principal

parameters for precipitator sizing, electrode arrangement and the electrical

energisation needed to provide specified levels of performance.

Specific collection area (SCA)( Typical value 11-45 m2/1000 m3/hr)The collection surface of an ESP required for a given gas flow and

efficiency is usually computed from the modified Deutsch-Andersion

Equation. The practical values of SCA usually range between 140 and

250 m2/m3/s, the higher values for higher collection efficiency.

ESP Design Criteria

Gas velocity

The importance of gas velocity is in relation to rapping and re-entrainment

losses of fly ash from the collecting electrode. Above some critical velocity,

these losses tend to increase rapidly. The critical velocity depends upon

the composition, temperature and pressure of gas flow, plate

configuration, ESP size. The gas velocity is calculated from the gas flow

and cross section of ESP. The maximum gas velocity is 1.1 m/s and the

optimum limit is 0.8 m/s for high efficiency ESP.

Aspect ratio (Typical value L/H=1.3 to 2.4 )

The importance of aspect ratio is due to its effect on rapping loss. Aspect

ratio is defined as the ratio of the total active length (L) of the fields to the

height of the field (H). Collected fly ash is released upon rapping and is

carried along the gas flow path. If the total field length is too short

compared to height, some of the carried particles will not reach the

hopper and goes out. The minimum aspect ratio should be between 1.3

to 2.4, the highest figure for highest efficiency.

High tension sectionalisation

The optimum number of high tension section per 1000 m3/m of gas flow

rate is between 0.73 to 0.78, the lower value is for higher ESP

performance. The performance of ESP improves with degree of high

tension sectionalisation due to the following reasons:

1- Small sections have less electrode area for sparks to occur.

2-Electrode alignment and spacing are more accurate for smaller sections.

3-Smaller rectifiers are needed that are more stable under sparking conditions

4- Outages of one or two sections have a lesser effect on ESP performance.

Migration velocity:

The ESP manufacturers based on individual experience determinemigration velocity. The important variables that are used to determinemigration velocity of fly ash are its resistivity, size distribution, gas velocitydistribution, re-entrainment and rapping.

Series fieldGood design practices calls for at least 5 or 6 separately energised seriesof high tension sections in an ESP. The number of fields in series neededfor ESP depends mainly on the efficiency required and on the redundancynecessary to ensure performance with section outages

Question ?

• The exhaust rate of the gas being processed is given as

1,000,000 ft3/min. The inlet dust concentration in the gas as it enters the ESP is 8 gr/ft3. If the emission regulations state that the outlet dust concentration must be less than 0.04 gr/ft3, how much collection area is required to meet the regulations? Use the Deutsch-Anderson equation for this calculation

and assume the migration velocity is 0.3 ft/sec.

Solution

• use this version of the Deutsch-Anderson equation to solve the problem:

Where: A = collection area, ft2 Q = gas flow rate, ft3/sec w = migration velocity, ft/sec η = collection efficiency ln = natural logarithm• In this example, Q = 1,000,000 ft3/min × 1 min/60 sec = 16,667 ft3/sec And w = 0.3 ft/sec

Calculate the collection efficiency, η.

Calculate the collection area, A, in ft2.