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electrostatic precipitator design by use of simulation models

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- 1. The design of electrostatic precipitators by use of physical models G. Bacchiega - IRS S.r.l. – www.irsweb.it R. Sala -I. Gallimberti - P. Tronville - F. Zatti
- 2. Objective of the paper <ul><li>To show how modeling tools can be employed to design modern Electrostatic Precipitators (ESP) for industrial applications </li></ul><ul><li>To demonstrate the power of modern CFD tools to tackle turbulent gas flow, electrical field and discharge phenomena, particle charging and transport, particle collection and re-entrainment </li></ul><ul><li>Considerations: </li></ul><ul><ul><li>It is necessary a multidisciplinary approach (five authors!) </li></ul></ul><ul><ul><li>The presenter's area of expertise in this case is limited to Computational Fluid Dynamics (CFD) </li></ul></ul>
- 3. Design procedure <ul><li>Draft design based on: general specification analysis, expected exhaust particle concentration, site specific physical constraints, previous similar projects </li></ul><ul><li>Simulation and optimisation of the gas flow in the ESP body (adoption of smoothing profiles at the inlet and exit) </li></ul><ul><li>Simulation of the particle capture process and verification of the draft design efficiency (to make the necessary modifications and comply with specifications) </li></ul><ul><li>Detailed engineering design and construction drawings, should include as a final result the mechanical specifications, cost evaluation and construction schedule </li></ul>
- 4. Modeling approach <ul><li>Time-dependent simulation of the main physical phenomena that take place inside the electrostatic precipitator </li></ul><ul><li>Self-consistent physical and mathematical models for each phenomenon </li></ul><ul><li>Modular structure of the models </li></ul>
- 5. Data structure Gas Flow: 3D Fluid-Dynamic Gas Flow: 2D Fluid-Dynamic Sect. 1 Laplacian Field Electric Field time loop Back Corona Glow Corona Streamer Corona Breakdown Electric Field Sect. 2 Ion Migration Particle Charging Particle Migration Space Charge Distribution Sect. 3 Particle Collection Rapping Reentrainment Process Efficiency Sect. 4
- 6. Fluid-Dynamic simulation 3-D Fluid-Dynamic Fluid-dynamics conditions of gas flow: stationary conditions of 3-D gas flow in the precipitator 2-D Fluid-Dynamic Fluid-dynamics conditions of gas flow: 2-D gas flow in a single cell Data structure Gas Flow: 3D Fluid-Dynamic Gas Flow: 2D Fluid-Dynamic Laplacian Field Electric Field time loop Back Corona Glow Corona Streamer Corona Breakdown Electric Field Ion Migration Particle Charging Particle Migration Space Charge Distribution Particle Collection Rapping Reentrainment Process Efficiency Sect. 1 Sect. 2 Sect. 3 Sect. 4
- 7. Calculation of flow field <ul><li>Mass, momentum and energy conservation </li></ul><ul><li>Isotropic turbulence by k- model of the first order (kinetic energy k and rate of turbulent dissipation ) </li></ul>
- 8. Operating parameters - characteristics FLUE GAS OPERATING CONDITIONS Gas flow (on wet) Nm 3 /h 124000 Operating temperature °C 402 Operating Pressure kPa 98.7 O 2 Concentration % vol 11.25 Relative humidity % vol 8.7 INLET PARTICLE CHARACTERISTICS Particle concentration (dry at 8% O 2 ) mg/Nm 3 4707 Furnace particles ( average diameter 0.25 micron) % in mass 4.2 Reaction particles ( average diameter 6.0 micron) % in mass 95.8 DRAFT ESP CHARACTERISTICS N° of fields 3 N° of gas passages (d = 400 mm) 19 N° of plates per field (h = 13.35 m, l = 0.5 m) 8 N° of emitting electrodes per plate (RDE type) 1
- 9. 3-D mesh of the ESP
- 10. Calculated velocity contours (central section)
- 11. Smoothing velocity profile See white lines and dashes (perforated plates)
- 12. Fluid dynamic optimization Perforated plates with variable permeability
- 13. Electric field section Laplacian Field electrostatic conditions defined by geometry Electric Field Time dependent electrostatic conditions defined by charge in the space Data structure Gas Flow: 3D Fluid-Dynamic Gas Flow: 2D Fluid-Dynamic Laplacian Field Electric Field time loop Back Corona Glow Corona Streamer Corona Breakdown Electric Field Ion Migration Particle Charging Particle Migration Space Charge Distribution Particle Collection Rapping Reentrainment Process Efficiency Sect. 1 Sect. 2 Sect. 3 Sect. 4
- 14. Modeling the electrostatic field Defines characteristics of electric discharges Defines forces over the particles Poisson equations Calculation method Potential and field: iterative FDM algorithm (Finite Differences Method) with convergence verification Electrostatic field Orthogonal embedded grid Calculation domain
- 15. Laplace potential and electric field <ul><li>Refined mesh near high voltage-high divergence electrodes </li></ul><ul><li>Electric field distribution proportional to a reference one </li></ul><ul><li>The solution over the domain is calculated only once </li></ul>Characteristics: 70 35 0 V(l) [kV]
- 16. Potential and electric field <ul><li>Lower intensity comparing with laplacian field (~20%) </li></ul><ul><li>Mesh with low refinement </li></ul>0.0 kV 7.5 kV 6.0 kV 4.5 kV 9.0 kV 4.5 kV 7.0 kV 7.5 kV 3.0 kV 1.5 kV 0.0 kV
- 17. Electric field contour in a collection cell
- 18. Voltage-Current characteristic
- 19. Discharging characterization Glow corona Stationary corona discharge Back corona Micro-discharge in the dust layer at the plates Data structure Gas Flow: 3D Fluid-Dynamic Gas Flow: 2D Fluid-Dynamic Laplacian Field Electric Field time loop Back Corona Glow Corona Streamer Corona Breakdown Electric Field Ion Migration Particle Charging Particle Migration Space Charge Distribution Particle Collection Rapping Reentrainment Process Efficiency Sect. 1 Sect. 2 Sect. 3 Sect. 4
- 20. Model of Glow Corona Electrons emission by positive ions collisions Molecular ionization and attachment Ions drift by electric field force Ionisation region Transport region V= 0 V dc = V
- 21. Glow Corona: calculation procedure Electric field distribution Ions current injection Space charge distribution Time dependent solution of transport equation Ions transport
- 22. Back Corona: physical description Micro-discharge in the dust layer at the plates <ul><li>Global electric conditions (electric field in the dust) </li></ul><ul><li>Characteristics of the dust (particle size distribution, resistivity, dielectric constant) </li></ul>equiflux equiflux 0 1 plate j equipotentials
- 23. Charging section Particle charging Mechanism of particle charging Data structure Gas Flow: 3D Fluid-Dynamic Gas Flow: 2D Fluid-Dynamic Laplacian Field Electric Field time loop Back Corona Glow Corona Streamer Corona Breakdown Electric Field Ion Migration Particle Charging Particle Migration Space Charge Distribution Particle Collection Rapping Reentrainment Process Efficiency Sect. 1 Sect. 2 Sect. 3 Sect. 4
- 24. Model of particle charging By means of the field: The particle modifies locally the electric field Ions drift and attach to the particles The process ends when the electric field created by the particle is greater then the ambient field
- 25. By diffusion: Thermal agitation of ions produce collisions with the particles Model of particle charging
- 26. Particle migration section Ionic migration Ionic migration process Particle migration Particle migration process Space charge distribution Time dependent variation of ionic and particles distribution Data structure Gas Flow: 3D Fluid-Dynamic Gas Flow: 2D Fluid-Dynamic Laplacian Field Electric Field time loop Back Corona Glow Corona Streamer Corona Breakdown Electric Field Ion Migration Particle Charging Particle Migration Space Charge Distribution Particle Collection Rapping Reentrainment Process Efficiency Sect. 1 Sect. 2 Sect. 3 Sect. 4
- 27. Particles migration section Fluid transport: particles are dragged by the gas in the duct Velocity v p depends not only on forces, but also on inertia Electric transport: charged particles are drifted by electric field to the plates Global instantaneous velocity
- 28. Rend.tot. 98.95%
- 29. Particles collection section Rapping-Reentrainment Conditions of particles collection: stationary simulation of dust over the plates Data structure Gas Flow: 3D Fluid-Dynamic Gas Flow: 2D Fluid-Dynamic Laplacian Field Electric Field time loop Back Corona Glow Corona Streamer Corona Breakdown Electric Field Ion Migration Particle Charging Particle Migration Space Charge Distribution Particle Collection Rapping Reentrainment Process Efficiency Sect. 1 Sect. 2 Sect. 3 Sect. 4
- 30. Collection and re-entrainment Time-dependent evaluation of particles layer at plates Objectives: <ul><li>Evaluation of particles re-entering in the main stream (re-entrainment) </li></ul><ul><li>Evaluation of electric behavior (back-corona) </li></ul><ul><li>Definition of mechanical behavior (rapping, collecting efficiency) </li></ul>
- 31. Collection and re-entrainment Mass balance of re-entrained, collected and fallen particles M in M out-ree M out-hop
- 32. Inlet particle size distribution Percentage by mass
- 33. Particle size distribution (inlet and exit)
- 34. Mechanical layout
- 35. Conclusions <ul><li>It is possible to optimize the ESP characteristics by using engineering calculations and thus avoiding empirical correlations </li></ul><ul><li>The balance between cost and performance can be carefully evaluated during the design phase </li></ul>More info www.irsweb.it

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