Electros ta tic precip i ta tors

Working pr incip le o f ESP

Electrostatic Precipitators (ESP) Analysis Using CFDlearncax.com /blog/2013/11/26/electrostatic-precipitators-esp-analysis-using-cf d/

What are ESP ?The particulate emissions from process industries has received great attention due to the upcoming strictenvironmental protection agency (EPA) regulations and conservation in recent years. The Electrostatic precipitators(ESP) since its development in by Frederick G. Cottrell (Professor of chemistry at the University of California,Berkeley) have been the most common, effective and reliable technologies for removal of hazardous emissions likeflue gases, acid droplets and fine particles. The industrial ESP are capable of handling large gas volumes with awide range of inlet temperatures, pressures, dust volumes and gas conditions and exhibit complex interactionmechanism between electric field, fluid flow and particulate flows.

Electrostatic precipitators (ESP) since its development in by Frederick G. Cottrell (Professor of chemistry at theUniversity of California, Berkeley) have been the most common, effective and reliable technologies for removal ofhazardous emissions like flue gases, acid droplets and fine particles.

Working principle ofESP:Electrostatic precipitatorsare used for reducingpollutants from thedischarge (outlet) line ofprocess industries andare generally installed inthe ducts that dischargethe flue gases into theatmosphere. Theparticulate matter in theemissions contain electriccharges as they pass outof processes withinreaction chambers,combustion chambers,etc and are negativelycharged.

Taking advantage ofthese charged particles,a set of positivelycharged sheets is inserted inside thedevice in order to attract theparticulates. As the flow passes overthe parallel set of sheets, the particlesget attracted towards the sheet and getstuck over there. After some time ofoperation, the sheets get covered by alayer of such particles, at that momentthe sheets are vibrated which causesthe attracted particles to fall down andget collected in the bottom, i.e. in thetapered shape collection zone. Oncethis zone is sufficiently filled by suchparticulates, the bottom door is openedand the whole slug is removed out ofthe ESP. Hence, by the application ofelectrostatic force these pollutants arepulled out of the exhaust gas. This whole process is carried out inside an ESP and hence by reducing pollutants(carbon content majorly) a more eco- friendly gas is rejected into the atmosphere.

Geom etry o f a typ ica l ESP

CFD as a design tool for ESP:ESP though helps us to prevent pollutants from entering the atmosphere also actually adds to the operational costsin the industry & hence its very important to have an efficient ESP. An efficient ESP can be one that requires minimumelectrical energy to separate out the charged particles of the bulk gaseous waste. Further understanding the physicsof the process, one just needs to minimize the force required to attract & separate out the particulates by reducingtheir kinetic energy resulting by decreasing the flow velocity.The flow velocity can be reduced by increasing the cross sectional duct area that may further result in change of flowdistribution pattern. This then, becomes an important point to study as to how uniformly the flow pattern is ? This canbe a very good CFD study & let’s discuss here a case study to better understand the CFD work.

Object ive of study: To separate out maximum particulate pollutants with the minimum amount of power input.

From the velocity of the particles, we can find out the kinetic energy they possess. Based on this force, we need togenerate an electrostatic energy (from the electrodes inside the device) of a higher magnitude than the kineticenergy of the particles in order to successfully remove them out of the flow. As the target is to minimize the energyrequired, it is obvious that we reduce the velocity of the flow as much as possible. It is equally important to haveuniform & optimum velocity throughout the device. This means, velocity should not exceed the threshold valueanywhere inside the device. If velocity exceeds the max permissible value then, particles will not get trapped andhence escape into atmosphere. At the same time if at some place, the velocity is too low then the particles will getcollected at the beginning of the device itself and the energy supplied to the electrode sheets which are towards theend of the device will just go wasted. Hence it is very essential to maintain uniform velocity throughout the device. Inorder to maintain uniform velocity one needs to have a good design of the device as well as entry and exit evase.Here CFD plays a very crucial role in testing and validating the designs.

Objective of test case: To separate out maximum particulate pollutants with the minimum amount of power input.

Elect rostat ic Precipitator Device:A simple demo case presented here was designed on the basis of space availability and hence it is quite visible inthe geometry image (seen below) that this design is not an optimized one. There is an expected flow-separation atthe inlet evase straight away visible, but in order to predict this design’s ineffectiveness a complete CFD simulationneeds to be done and based on the results the design will be optimized and another CFD study is conducted on themodified design. The CFD study will contain a very basic flow prediction inside the device. The discrete pollutantsbeing so light in weight, that they won’t be influencing the flow behavior at all. Hence, the discrete pollutant phase isnot modeled in this study and instead just air is modeled. The velocity prediction would give the understanding ofthe effectiveness of the device.

A steady-state singlephasesimulationwas carriedout withReynolds-averagedNavier–Stokesequationscoupled withtheturbulencemodelequations.Defaultconvergencecriteria of forall the equations were considered.

CFD Results:

Let us have a look at the analysis data. The images below clearly show the non-uniformity in the velocity distributioninside the ESP Device.

Non-un i form flow d is tr ibu tion in ESP

In this setup, the available power was just sufficient enough to separate the particles (from the bulk flow) havingvelocities of or below. The following images will explain it in detail.

In the above images, can be clearly seen that in the colored zone the velocity is within range and the centre part(i.e. unfilled area) indicates that the velocity is not in operational range of the device. The percentage of area fallingwithin the operational range is about and in horizontal and vertical cross-section respectively. This means that onlyaround pollutants would be removed from the exhaust gases whereas the energy supplied to it was sufficientenough to remove them completely. This gives an idea about how important role a proper design plays in each andevery engineered product.Non-uniformity in velocity distribution was observed within the device due to the sudden expansion of the duct whichcaused a flow-separation at the divergence section of the inlet. It would have been much more uniform if theincrease of cross sectional area was done gradually, however though at times the decision is based on purely non-technical factors like space availability & others.

Opt imizat ion study:In the existing design, a steel wire- frame (filter) was introduced at the entrance of the ESP to assist in the uniformdistribution of flue gases.

The CFD approach was kept same as that of baseline study performed earlier, except the filter section that wasmodeled as a “porous zone”. Let us now analyze the results from the simulation study of this modified geometry.

The above images have made it clear that the flow has become much more uniform than earlier. Now let us analyzeit more quantitatively.

The above images display the area which falls in the optimum working range of the ESP. The percentage areacomes out to be . This clearly states that the efficiency of the device has increased from to around and above.

The insertion of filter at the inlet helped delay the flow separation at the inlet evase and improved the distribution ofvelocity in a more uniform pattern around all the electrode plates thus improving the efficiency of the ESP from to .

Also below is shown the video animation of the flow distribution in a ESP with and without filter:

Summary:

The initial design of the ESP was studied with the help of commercial CFD tool “ANSYS Fluent” and afterunderstanding its ineffectiveness, the design was modified by the addition of a filter. The insertion of filter at the inlethelped delay the flow separation at the inlet evase and improved the distribution of velocity in a more uniform patternaround all the electrode plates thus improving the efficiency of the ESP from to . Hence, CFD has proven as a verycost effective and useful tool in designing the Electrostatic precipitator.References:

Zhengwei Long, Qiang Yao Ã; Evaluation of various particle charging models for simulating particle dynamics inelectrostatic precipitators Journal of Aerosol Science 41 (2010), 702–718.

F.J. Gutiérrez Ortiz , B. Navarrete, L. Cañadas; Dimensional analysis for assessing the performance ofelectrostatic precipitators Fuel Processing Technology 91 (2010), 1783–1793.

G. Skodras, S.P. Kaldis, D. Sofialidis, O. Faltsi, P. Grammelis, G.P. Sakellaropoulos; Particulate removal viaelectrostatic precipitators—CFD simulation Fuel Processing Technology 87 (2006), 623 – 631.

Shah M.E. Haque, M.G. Rasul, A.V. Deev, M.M.K. Khan, N. Subaschandar; Flow simulation in an electrostaticprecipitator of a thermal power plant Applied Thermal Engineering 29 (2009), 2037–2042.

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