Electrostatic Precipitator for Bagasse Fired Steam Generators (Boilers)By K.Suresh Kumar Head Technical UNICON ENGINEERS COIMBATOREThe electrostatic precipitator has been used for decades to collect particles from flue gases in many industrial processes. Its popularity is based on three important features: 1. Low pressure drop in the gas flow 2. Low sensitivity to high temperatures and aggressive gases 3. It can be sized for collecting efficiencies well in excess of 99 percent. PRINCIPLE The electrostatic precipitator utilizes electrostatic forces to separate dust particles from the gas to be cleaned. The ESP basically consists of a plates placed in rows to form a set of parallel metal curtain. In between a pair of such curtain are placed an array of discharge elctrodes. A high-negative voltage, is connected between the framework of discharge electrodes and the ground, thereby creating a strong electrical field between the wires in the frame- work and the steel curtains. The electrical field becomes strongest near the surface of the wires, so strong that an electrical discharge - 'the corona discharge" - develops along the wires. The gas is ionized in the corona discharge and large quantities of positive and negative ions are formed. The positive ions are immediately attracted towards the negative wires by the strength of the field. The negative ions, however, have to traverse the entire space between the electrodes to reach the nearest positive curtains. Thus, one obtains a flow of negative ions from the discharge electrodes. En route towards the steel curtains. the ions collide with and adhere to the particles in the gas. The particles thereby become electrically charged and also begin to migrate in the same direction as the ions, towards the nearest steel curtains. The dust is collected in large quantities on the curtains or collecting electrodes. As a result of periodic rapping, the caked dust is loosened and slides, due to its weight, into a dust hopper. CHOICE OF ELECTROSTATIC PRECIPITATOR It has been determined through laboratory tests that the amount of dust deposited on the collecting plates decreases exponentially from the electrostatic precipitator's inlet to its outlet. Deutsch developed the following well-known formula:

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 2 of 14Dustoutlet = Dustinlet * e -W . A/Q) W = velocity of the dust towards the collecting surface A = total collecting area Q = total gas flow A represent the geometric dimensions of the precipitator and Q represent the amount of gas handled. W is a characteristic of the dust particles. Assuming that Si and Q are given and that W is known, A - and thereby the precipitator size - is calculated from the formula to give the accepted dust emission. How to go about selecting an ESP First we should understand that we are looking for an economic particulate collection solution. So we need to be very clear regarding the parameters that are to be considered for arriving at the ESP design. That is the design parameters. These are : a) Gas Volume or gas flow rate that is to be treated by the ESP (cubic meter per sec) b) Inlet dust loading to the ESP ( Grams per Normal Cubic meter of Gas) c) Out let dust emission desired (milli gram / normal cubic meter of gas) d) Gas temperature (Deg.C)

e) Type of dust to be handled or fuel being used (Bagasse in this case)Gas Volume has a direct impact on the size of the precipitator. Higher the gas volume bigger the precipitator and more the cost. Figure 1. shows the effect of gas volume on ESP size. The ratio of ESP size for a gas flow 80% to that of gas flow 120% would be about 1.6 This would mean a very undersized ESP if a low gas volume is indicated or an oversized ESP if a high gas volume is indicated. Similarly Inlet dust concentration will again effect the size of ESP. For bagasse firing an inlet dust load of 6gms/Nm3 could have an ESP size which is 120% the size of an ESP if the inlet dust load is only 3 gms/Nm3.

This requires that the buyer should do some good homework to arrive at the correct figures of the above parameters (Gas Volume, Gas temperature, Dust loading before ESP and desired outlet emission).Some of the other ESP details which effect the performance of ESP are as follows: i ) Gas velocity inside the ESP chamber: gas velocity inside ESP is a very important parameter. For Bagasse it has been found that if the velocity exceeds 1.0m/s then re-entrainment of collected particles take place. It has been found that if the gas velocity is greater than 1.0 m/s then the outlet emission cannot be limited below 100mg/Nm3 .

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 3 of 14This has to be kept in mind while designing the ESP. With emission requirements becoming more stringent with time, a gas velocity >1.0 m/s would mean that any future requirements of improving emission level would not be practical. Hence it would be a good design to have the gas velocity below 0.9m/s. This velocity will differ for different dusts. For example in the case of Coal firing, it would be a good design for this value to be 0.8m/s. ii ) Aspect ratio: this is the ratio of effective field length to the height of the field. This value should be greater than 0.8 and a good design value would be 1.0 for bagasse and >1.5 for coal etc. iii ) Treatment time. This the the total period of time in seconds that the gas is under the effect of active fields. For bagasse fired boilers the treatment time greater than 10 seconds would suffice. A good design value would be 11 secs. For bagasse and > 16 seconds for coal etc. iv ) Number of fields in series: More number of fields would mean that the effect of disturbances inside the field would be lower and hence better performance. During sizing of ESP a larger number of fields in series could mean a lower SCA requirement. It has been found that for Bagasse fired boiler a two field ESP is sufficient to ensure emission levels below pollution norms. It would be three or more when coal is used. A practical Approach Once again it is stressed here that we are looking for an economic & effective particulate collection solution. This is possible if we obtain offers from various ESP vendors which cater to a specific requirements of parameters (Gas volume, gas temperature, inlet dust loading, outlet emission etc.). If this is not ensured we would receive technically different solutions and naturally commercially different and they naturally cannot be compared to determine which is most suitable both technically and commercially. As given in the Deutche equation the factor which plays a major role in the performance of the ESP is the Specific Collecting Area (m2 / (m3 / sec). That is to state that an electrostatic precipitator which has a larger SCA will necessarily perform better than one with a smaller SCA. This of course assumes that all other factors remain the same. Next we should ensure that the ESP designed has the gas velocity below allowed value Having ensured that the ESPs being offered by all the vendors meet the criteria of SCA, Gas velocity, gas temperature and the guarantee parameters of inlet dust loading, outlet emission etc we may evaluate them commercially to obtain the most economical solution.

the buyer should do some good homework to arrive at the correct figures of the design parameters (Gas Volume, Gas Temperature, ESP InletHence

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 4 of 14 dust loading and required emission level. It is also recommended that the buyer should also specify the reasonable SCA that needs to be provided and also the maximum velocity allowed in the ESP chamber, for bagasse this should preferably be 0.95 m/s.Gas Distribution: It is also recommended that gas distribution studies are carried out inside the ESP before taking the ESP into operation. The importance of gas distribution cannot be undermined when the emission levels required are very low in the order of 50 to 150 milligrams per Normal meter cube of gas flow. provided. The gas to the ESP is taken through small cross section duct where in the gas velocity is in the order of 12 to 15 m/s in the case of bagasse fired boiler etc and could be as high as 25m/s in some other applications. This gas is suddenly expanded before entering the ESP so that the velocity inside the ESP is less than 1 m/s. This sudden expansion makes it very difficult to obtain a good distribution inside the ESP. Special care has to be taken to ensure the acceptable distribution. Poor distribution would result that the ESP collecting system is not utilized effectively with some areas being over loaded and others under utilized. The result a poor performance. Further certain areas would be subject to very high velocities resulting in re-entrainment of collected dust and resultant poor performance of the ESP. Many vendors including MNCs declare that model studies have been conducted and no test is required at site. But site studies some of these equipments have proved otherwise. This leads to the conclusion that model studies are not correctly simulated. A confirmative Distribution study is a must in all cases. Maintanance requirements The requirement of maintenance should also be kept in mind. Many ESP vendors do not provide proper maintenance approach inside the ESP. Approach platforms should be provided at the inlet and outlet of each field. If any structural members are provided between fields clear space for movement of an average built person should be provided between the structural members and the collecting / discharge systems. A good distributions effectively means equal loading of the collecting system

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 5 of 14Proper size man hole should be provided for easy approach into the ESP chamber by an average built person.

BRIEF INTRODUCTION

K. SURESH KUMARBachelors Degree in Electrical & Electronics Engineering, Has been in the field of Electrostatic Precipitator from 1976 to date. Was in BHEL from 1976 to 2003. After leaving BHEL continues to be in the field or ESP to date. Has been involved in Sizing and selection of ESPs for wide range of Industries Utility Power Plants, Steel, Cement, Calcination Plant, Sugar, Captive Power Plants etc. Crores. Is also providing consultancy for Maintenance, Trouble shooting, Commissioning, Gas distribution studies etc. for Various Utilities and also to various other industries and captive power plants. Consultancy also included an overseas work for M/s. Worley Parsons USA for a Nickel Plant of PTINCO. He is an expert in selection and sizing of ESP, Flow model studies, and Gas Distribution studies, Optimization of flow and pressure losses in ESP and Flue systems

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 6 of 14 Factors effecting the performance of ESP: 1. Type of Application : Bio mass Coal Metal Chemical 2. Chemical composition of dust to be handled Sulphur Sodium Potassium Magnesium Iron / other metals Alumina Silica Moisture Lithium Etc Gas Volume Gas Temperature Specific Collecting Area Provided Gas velocity in Electrode Zone Number of fields in series. Collecting Area provided per field Aspect Ratio Electrode between Rows of Collecting Electrodes.

3. 4. 5. 6. 7. 8. 9. 10.

Brought out below are some typical curves to highlight some of the above.

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 7 of 14 SCA required for a given ESP Efficiency with Variation in No. Of FieldsESP SIZE 70 60 50 40 30 20 10 0 0 2 4 6 8 10 ESP SIZE

SCA required for a given ESP Efficiency with Variation in Aspect Ratio

ESP SIZE 60 50 40 30 20 10 0 0 0.5 1 1.5 2 2.5 3 3.5 4 ESP SIZE

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 8 of 14 SCA required for a given ESP Efficiency with Variation in Field sizeESP Size 50 45 40 35 30 25 20 15 10 5 0 0 500 1000 1500 2000 2500 3000 3500 4000 ESP Size

ESP Performance (for given SCA) with variation in Number of fields in Series

ESP EFF 100 99.5 99 98.5 98 97.5 97 96.5 96 0 2 4 6 8 10 ESP EFF

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 9 of 14

ESP Performance (for given SCA) with variation in Aspect Ratio

ESP EFF 99.8 99.6 99.4 99.2 99 98.8 98.6 98.4 0 0.5 1 1.5 2 2.5 3 3.5 4 ESP EFF

ESP Performance (for given SCA) with variation in Field SizeESP EFF 99.8 99.6 99.4 99.2 99 98.8 98.6 98.4 0 500 1000 1500 2000 2500 3000 3500 4000 ESP EFF

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 10 of 14 ESP Performance (for given Application) with variation in SCAEmission 2500 2000 1500 Emission 1000 500 0 0 50 100 150 200 250

ESP Performance (for given Application) with variation in SCAESP EFF 100.5 100 99.5 99 98.5 98 97.5 0 50 100 150 200 250 ESP EFF

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 11 of 14 Effect of gas velocityGiven below the performance of two ESPs where the effect of gas velocity is highlighted ESP1 Appolication SCA No. of fields Gas Velocity Treatment time Aspect Ratio Field size Emission BAGASSE 59 2 0.9 11.81 1.16 2362 100 COMPARATIVE WHICH IS BETTER ESP2 ESP2 ESP1 ESP2 ESP2 ESP2 ESP1

Inspite of ESP2 design being better in 5 out of 6 ESP1 performance is better. Clearly showing the importance of keeping Velocity below o.9m/s for bagasse or coal

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 12 of 14Gas distribution measured at site for an ESP where an MNC had insisted that Model study was carried out and hence no GD test at site is required.

115 to 140% Average Greater than 140 60 to 115% Average %

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 13 of 14 Gas distribution obtained in same ESP after GD test at site

Paper for Seminar on Bagasse fired ESP by K.Suresh Kumar Head Technical UNICON ENGINEERS Page 14 of 14

115 to 140% Average 60 to 115% Average Greater than 140 %