Causes of Poor Performance of ESP's (Electrostatic Precipitators)

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Causes of poor performance of ESP & discussions

There are many causes of poor performance of ESP (Electrostatic Precipitators). Some of the reasons are:

·       Excess gas volume

·       Poor gas distribution

·       Tracking and air inleakage

·       Electrode breakage

·       Ash resistivity

·       Particle size

·       Electrical conditions

·       Over-full  hoppers

·       Defective collector plates

Excess gas volume

If the gas volume is high, the gas velocity through the precipitator will be high so that the time available for the charged particles to migrate to the collector plates is reduced, possibly to the extent that the particles pass out of the working zone before they can be captured. The normal gas velocity is about 2 m/s, but this may be increased by:

·       Tramp air in leakage at the boiler.

·       High air heater gas outlet temperature causing increased volume flow.

·       Poor condition of rotary air heater seals.

·       Operating the boiler with too much excess air.

Poor gas distribution

It could be that the total gas flow is satisfactory but the spatial distribution is poor, causing good performance where the gas velocity is low and for it to be poor where the velocity is high. To be completely acceptable, all the traverse point velocities should be within 25% of the average for the duct.

Tracking and air in leakage

This is a common cause of trouble because precipitator structures are physically large and have numerous access doors, etc, at which leakage can occur. If the in leakage is sufficiently high, a draft of cool and possibly moist air will pass over the insulators, steady bars, etc. As the voltage inside a precipitator is very high (40-50 k V), tracking paths may result. Therefore, it is important that access -door seals, rapping rod covers, expansion joints and other ingress points are kept in good condition and are airtight.

Apart from above hazards, there are the additional disadvantages that in leakage increases the mass flow of the gas and lowers its temperature.

Poor rapping

In the usual precipitator arrangement the wires are negatively charged and the collector plates are earthed. In the immediate vicinity of the wires the electrostatic field is so strong that corona discharge occurs. The corona ionizes the flue gases and the entrained solid particles acquire negative ions, causing them to migrate towards the relatively –positive collecting plates. A few particles acquire positive ions and theses migrate towards the discharge electrodes.

The dust which adheres to the wires and collecting plates is periodically removed by rapping.

The timing of rapping operation is important as damage can be done to the plant if it is too frequent. Also, every time rapping takes place some of the dislodged dust is re-entrained, so reducing the collection performance. On the other hand, if the interval between raps is too great, the dust build-up on the wires and the plates will be sufficient to interfere with the operation of the plant and this also will reduce its performance. Efforts should be made to determine the optimum timing.

Upon inspection, the collector plate dust –layer should be less than 1 mm for high resistance dust and 3 mm for dust from coal with high sulphur. The discharge electrodes should have a very light deposit.

Electrode  breakage

The discharge electrodes consist of relatively thin wires which may be of flat, circular, star or square cross-section. In addition, they may be barbed or plain, twisted or straight and rigidly or loosely fastened, besides being made from a range of materials. Whichever arrangement is adopted, the essential requirement is that the wires should have excellent reliability. Out of the huge number in a zone, it only needs one to break to cause short-circuiting and electrical instability generally, which may seriously affect the performance of the plant.

Spark erosion is a common cause of wire failure.

The ‘unbreakable’ type of wire is becoming popular because of its inherent reliability. One method of limiting the effect on performance by breakage is to divide a complete zone into four sub-zones, so that a broken wire only affects a quarter of the zone.

Ash resistivity

An important factor in precipitator performance is the electrical conductivity of the ash itself. Poor combustion can result in a higher than normal carbon content in the ash. This causes the particles to have a lower resistivity which may cause them to be difficult to collect. This means that they can be readily re-entrained by the gas, particularly during rapping. On the other hand, if the resistivity is too high, the dust will accumulate on the collector plates and form an insulating layer, so preventing subsequent particles from surrendering their charge. Consequently, the charge on the surface layer will repel incoming dust, causing re-entrainment. In addition, a very high voltage- gradient will be built up across the dust layer which could result in flashover. Dust with high resistivity will suppress the corona discharge, seriously lowering the performance of the plant.

The resistivity of the ash is a function of the surface layers of sulfuric acid, salts and moisture, particularly the acid. If the coal contains about 2% sulfur, the resulting sulfuric acid deposition creates a desirable level of dust resistivity. If the sulfur content of the coal is very low, a highly resistive ash may be produced which is very difficult to precipitate. One remedy for this is to inject small quantities of suitable additives, such as ammonium sulfate, into the flue gas.

The desirable range of resistivity is between 10⁶ and 10¹² ohm-cm. Should the normal fuel supplies be low in sulfur and a change to alternate supplies be uneconomic, another possible remedy besides injection is to install ‘pulsed  energization’. In this, high frequency pulses are added to the rectified wave form. The resulting high ion-density corona acts along the length of the discharge electrode instead of just at discrete locations, so the dust particles are more easily precipitated. The improved performance is called the Enhancement factor given by (pulsed migration velocity)/ (unpulsed migration velocity).

Particle size

Particle sizing has an important bearing upon the efficiency of precipitations. Up to approximately 20 µm, the electrical and the drag forces combine to give a deposition velocity which increases in roughly efficient for sizes over 20 µm, the only problem being that of slight re-entrainment. Dust less than 20 µm is more difficult to capture, so the stack emission will contain a preponderance of fine particles.

Electrical conditions

Figure below shows the relationship between electrical power input and   precipitator efficiency. As one would expect, increased power results in improved performance, provided the power is useful.

For example, flashover and tracking will give high power consumption, but much of it is wasted. To find the demarcation between useful and wasted power, a simple graph is determined from tests on the plant. Starting at a low value the current is increased by suitable increments, noting the corresponding voltage. A typical diagram is shown below:

Notice that the current and voltage both increase until the voltage is about 45 k V. Thereafter increasing current is accompanied by reducing voltage. The knee of the curve determines the demarcation between stable and unstable operation, the highest stable current here being a little over 200 m A. The automatic voltage control always tries to operate the plant with the highest stable power input which can be maintained.

Causes of electrical instability include:

·       Tracking , possibly due to air ingress

·       Arcing and poor connections, possibly at sub-zone isolation selectors

·       Ineffective rapping

·       Overfull dust hoppers, causing bridging of electrodes

·       Misaligned electrodes

·       Broken discharge electrode wires

Defective collector plates

The plates are mechanically strong, but can become defective if the gas temperature falls below the dew point, allowing deposition of dilute acid. This can occur in discrete areas if cold air inleakage takes place. Alternatively, general corrosion can occur if the flue gas itself has a low temperature, possibly due to massive air ingress at defective seals on the ducts between the air heater and the precipitator, or if the ducting is corroded and holed. Frequent cold-starting of the boiler can also lead to low- temperature gas at the precipitator.

We undertake all types of works for Dry ESP  under Overhauling, Performance Enhancement, Technical troubleshooting and Erection Works. Visit our site for details.