Design and Performance of Precipitators | Environmental Engineering

The design and performance of precipitators are affected by a plurality of factors.

These are divided into three main categories, namely – efficiency, local, high turbulence eddies scour:

(i) Properties of the gas stream,

(ii) Properties of the dust, and 

(iii) Process conditions.

(i) Properties of the Gas Stream:

The precipitator is designed for a particular throughput rate. Increase in the throughput rate that for which the precipitator has been designed reduces the efficiency as can be seen from the Deustsch-Anderson equation. This in turn increases the outlet concentration of dust in the gas.

Temperature and Pressure:

Higher gas pressure enhances the possibility of operating the ESP at higher electric field intensity. This facilitates corona discharge at a higher discharge electrode voltage. The increase in temperature decreases the voltage on the discharge electrodes without flash over (Sparks) and greatly hampers the efficiency of the collector.

If the temperature is around dew point of the moisture, any moisture condensation will absorb the toxic and corrosive gases present in the stream and will have detrimental effect in the precipitator material. It is therefore essential that the precipitator be properly insulated.

Compositions of the gas:

The presence of gaseous components like moisture, SO₂ and/or SO₃ is important. Trace amounts of SO_x increase the dew point of the gas, and decreases the resistivity of the dust cake. Moisture content is also beneficial in that it also increases the breakdown voltage and decreases the resistivity of the cake.

The dust concentration increases the load on the precipitator and the efficiency of collection will be impaired. It is always useful to analyse the composition of the gas as regards the gaseous like moisture, SO_x, HC CO₂,2 etc.

(ii) Properties of the Dust:

Size distribution is a very important parameter in assessing the performance characteristics of ESP’s. The dust particle size determines the magnitude of the charge held by the particle under a given electric field as also the migration velocity of the charged particle toward the collection electrodes.

The saturation charge of particle larger 1µ m, size is proportional to the field intensity and the square of the particle diameter. Hence under such condition, at any field intensity, particle diameter has profound effect on the saturation charge. For particle less than 1µ m size, the charging takes place by diffusion and the charge magnitude is proportional to the size. Hence the bigger the size, the more will be the charge magnitude.

The migration velocity of particles greater than 1 µ m size varies proportionately to the particle size and the square of the field intensity. The particles having sizes less than 1 µ m will have migration velocity independent of their sizes and dependent on the field intensity only.

The dust particles in the gas stream have a wide size distribution and therefore will have differing migration velocities. The migration velocity is also affected by the charge, the particle holds.

Also, the turbulence intensity in the collection zone, dust concentration at various points in the zone, the precipitator dimension, the dimension and layout of precipitator internal etc. also play important role in affective migration velocity. The back corona has strong influence on the migration velocity.

Dust Resistivity:

Dust resistivity is a critical parameter in the design and performance of an ESP.

Specific dust resistivity is defined as:

Where R is the mean electrical resistance of the dust A is the collection area and S_d is the thickness of the dust layer on the collection electrode.

Based on the resistivity, the dust particles are classified in three groups namely:

(i) Group (I p_α ≤ 10⁴ ohm cm)

(ii) Group II 10⁴ < p_α ≤ 10¹⁰ ohm cm.

(iii) Group III p_d> 10¹⁰ ohm.

Group I particles, having excellent conductivity transfer their negative charge to the collection electrode very easily and are propelled back in the gas stream either to escape from the precipitator or become recharged by the corona field. In order to avoid this, liquid spraying resorted to, which arrests the dust particles. Dry collection of such dusts like coal particles is impossible.

Group II particles do not easily give off their charge to the collecting electrode and hence will form a porous layer of dust progressively increasing with time. This increases progressively the voltage drop across the dust layer ∆V_d, as given by equation (ii)

The cake on a collecting electrode consists of only 10-50%, dust depending on the size of the particles, the remaining part being pores and channels filled with gas. Because of difference in the values of dust and gas dielectric permeabilities, the lines of force concentrate inside the channels. When the voltage is high, and electric breakdown takes place across the dust cake, and the gas inside the pores is ionized.

This phenomenon is termed as brack corona of positive polarity and is accompanied by the release inside the ESP of positive ions which move counter current with respect to the dust particles and partly neutralize their negative charges, so that the dust collection is adversely affected and current intensity of the ESP greatly increases.

At the same time, the positive ions, given up by the collecting electrodes, transform the electric field between the electrodes of an ESP into a field similar to that between two points. These types of field can be readily disrupted, and to prevent breakdowns, it is necessary to reduce the voltage on the precipitator.

This reduction is often so great that it affects substantially the velocity of dust particles drifting toward the collecting electrodes, and the dust collection efficiency in the ESP is reduced still more.

Thus, a back corona has a very detrimental effect upon the operation of ESP. The dust resistivity of group III particles could be reduced by reducing the temperature of the gas and/or by adding moisture. At low temperature the moisture is adsorbed on dust surface and it enhances the conductivity of the dust. At high temperatures, the adsorption phenomena is adversely affected.

The maximum resistivity in most cases lies in the temperature range of 100-200°C. The fly-ash emanating from India boilers have a resistivity between 10¹³10¹⁴ ohm cm at the operating conditions. The resistivity of most of the industrial dusts could be reduced by injecting small (trace) amounts of SO₃ and Ammonia.

The Indra Prashtha Power Station in Delhi has successfully tested the performances improvement by injecting ammonia at a concentration of 20 ppm. This has greatly reduced the dust emission from I.P. power station.

Table 9.5 presents the maximum value of specific resistivity of various industrial dusts.

Fig. 9.11 presents a qualitative picture of the effect of moisture, SO₃ and NH₃, on the specific resistivity at different temperatures. Thus, it could be argued that sulphur bearing coals should give better collection properly because of reduced resistivity of flyash. Fig. 9.12 presents the influences of sulphur in coal on the dust resistivity.

(iii) Process Conditions:

The process conditions include, the dust composition, variation of the gas, the velocity and distribution of gas through different chambers, the collection of dust through hoppers having baffles, electric field intensity etc.

The just load variation will give fluctuating efficiency of the ESP. Normally up to 30-35 mg/l of dust concentration could be handled providing proper rapping arrangement is made, both at discharge and collecting electrodes. Higher concentration may suppress corona and for this, the velocity of the stream should be minimised and the voltage on the discharge electrode should be increased.

ESP’s are normally so designed as to have gas velocities in the range of 30 to 80 cm/s. High velocity tends on train the dusts, a tendency most frequency observed during rapping operation. The gas velocity must be uniformly distributed across the cross section ESP. otherwise the efficiency of collection will be adversely affected.

For this purpose, guide genes, baffles sieve plates, screens etc. are successfully used, at the entrance to the ESP. The precipitator in the duct has much more pronounced in most circumstances the flow changes, upstream the reduction of collection efficiency than the downstream duct flow changed Local high turbulence eddies scour particulates off the collection efficiency than the downstream duct flow changes Local high turbulence eddies scour particulates off the collection electrodes.

Once a flow mal-distribution occurs, the situation continuously deteriorates, because low velocity portions of the gas stream deposit solids in the upstream duct work, making the velocity profile, even more non-uniform. Gas-by passing may occur between the end plates and the shell, over the top of the electrical fields or in the hoppers, seriously dipping in the efficiency. Such kind of by-passing and its removal are shown in Fig. 9.13.

In the upstream duct to the precipitator, the velocity normally ranges between 12-20 m/s, in order to obviate any change of dust deposition in the duct and also to save money on duct installation. In each precipitator duct, the flow reduction in velocity in the precipitator will disturb the whole flow profile.

In order to offset any eventual loss in efficiency due to flow mal-distribution, and angular duct with guide vanes in its below, delivering the gas from above to the precipitator, as given in Fig. 9.14 or a kind of perforated plate distributors in the diverging conical duct attached to the precipitator inlet as gine in Fig. 9.15 are preferred.

Variable porosity distribution plates fitted at the precipitator outlet and proper buffing will inhabit those pressure gradients that would promote re-entrainment of dust from the hoppers.