electrostatic precipitator design
DESCRIPTION
electrostatic precipitator design by use of simulation modelsTRANSCRIPT
The design of electrostatic precipitators
by use of physical models
G. Bacchiega - IRS S.r.l. – www.irsweb.itR. Sala -I. Gallimberti -P. Tronville - F. Zatti
2
Objective of the paper
• To show how modeling tools can be employed to design modern Electrostatic Precipitators (ESP) for industrial applications
• 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
• Considerations:– It is necessary a multidisciplinary approach (five authors!)– The presenter's area of expertise in this case is limited to
Computational Fluid Dynamics (CFD)
Design procedure
• Draft design based on: general specification analysis, expected exhaust particle concentration, site specific physical constraints, previous similar projects
• Simulation and optimisation of the gas flow in the ESP body (adoption of smoothing profiles at the inlet and exit)
• Simulation of the particle capture process and verification of the draft design efficiency (to make the necessary modifications and comply with specifications)
• Detailed engineering design and construction drawings, should include as a final result the mechanical specifications, cost evaluation and construction schedule
Modeling approach
• Time-dependent simulation of the main physical phenomena that take place inside the electrostatic precipitator
• Self-consistent physical and mathematical models for each phenomenon
• Modular structure of the models
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
Fluid-Dynamic simulation
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
3-D Fluid-DynamicFluid-dynamics
conditions of gas flow: stationary conditions of
3-D gas flow in the precipitator
2-D Fluid-DynamicFluid-dynamics
conditions of gas flow: 2-D gas flow in a single
cell
7
Calculation of flow field• Mass, momentum and energy conservation• Isotropic turbulence by k- model of the first order (kinetic
energy k and rate of turbulent dissipation )
ru
xi
i
0
u
t
u u
xpx x
Si j i
j i
ij
j
ui
k
t
u k
x xkx
Gi
i i
t
k i
t
t
u
x x xckG c
ki
i i
t
i
t
1 2
2
8
FLUE GAS OPERATING CONDITIONS Gas flow (on wet) Nm3/h 124000Operating temperature °C 402Operating Pressure kPa 98.7O2 Concentration % vol 11.25
Relative humidity % vol 8.7INLET PARTICLE CHARACTERISTICS Particle concentration (dry at 8% O2) mg/Nm3 4707
Furnace particles (average diameter 0.25 micron) % in mass 4.2Reaction particles (average diameter 6.0 micron) % in mass 95.8DRAFT ESP CHARACTERISTICSN° of fields 3N° of gas passages (d = 400 mm) 19N° of plates per field (h = 13.35 m, l = 0.5 m) 8N° of emitting electrodes per plate (RDE type) 1
Operating parameters - characteristics
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
Electric field section
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
Laplacian Fieldelectrostatic conditions defined by geometry
Electric Field
Time dependent electrostatic conditions defined by charge in the
space
14
Modeling the electrostatic fieldDefines characteristics of electric
dischargesDefines forces over the particles
Electrostatic field
Poisson equations
2
0
V
E V
Orthogonal embedded grid
Calculation domain
Calculation methodPotential and field: iterative FDM algorithm (Finite Differences Method) with convergence verification
15
Laplace potential and electric field
Characteristics:
• Refined mesh near high voltage-high divergence electrodes
• Electric field distribution proportional to a reference one
• The solution over the domain is calculated only once
0
0.5
1
1.5
2
2.5
3
0 5 10 15l [cm]
V(l
)/V
0
V(l)/V0
V(l)
70
35
0
V(l
) [k
V]
16
Potential and electric field
• Lower intensity comparing with laplacian field (~20%)
• Mesh with low refinement
0.0 kV
7.5 kV
6.0 kV
4.5 kV
9.0 kV
4.5 kV7.0 kV7.5 kV
3.0 kV
1.5 kV
0.0 kV
0
10
20
30
40
50
60
70
0 5 10 15l [cm]
V [
kV]
V=Vlapl
V=Vcar
17
Electric field contour in a collection cell
18
Voltage-Current characteristic
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 20 40 60 80Voltage (kV)
Curr
ent density
(m
A/m
2)
T= 20°C (Meas.)
T= 20°C (Sim.)
T= 400°C (Sim.)
Discharging characterization
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
Glow coronaStationary corona
discharge
Back coronaMicro-discharge in the dust
layer at the plates
20
Model of Glow Corona
Ionisationregion
Transportregion
V= 0
Vdc= V
Electrons emission by positive ions collisions
ne R ve R in R v R
Molecular ionization and attachment
neve neve
Ions drift by electric field force
ion ionE
iont ion
vion
Ddiff ion
0
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
• Global electric conditions (electric field in the dust)• Characteristics of the dust (particle size distribution, resistivity,
dielectric constant)
equiflux equiflux
0 1 plate
j
equipotentials
Charging section
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
Particle charging Mechanism of particle
charging
24
Model of particle chargingBy means of the field:
The particle modifies locally the electric field
E ERr
qrtot
r
r
0
3
30
2112 4
cos
Ions drift and attach to the particles
dqdt
eq
ionion s
s
4
10
2
The process ends when the electric field created by the particle is greater then the ambient field
0
1220
20q R Es
r
r
25
By diffusion:
dqdt
a e v TqeRk T
qqion
b s
2
041exp
Thermal agitation of ions produce collisions with the particles
Model of particle charging
Particle migration section
Ionic migrationIonic migration process
Particle migrationParticle 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 vp depends not only on forces, but also on inertia
Electric transport: charged particles are drifted by electric field to the plates
F qE
F R v vv f p 6
v vqmE v v
qmE ef f
t
0
Global instantaneous velocity
28
Efficiency %
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
0 5 10 15 20 25 30
Plates
%
Cl.1
Cl.2
Cl.3
Cl.4
Cl.5
Cl.6
Cl.7
Cl.8
Cl.9
Tot
Rend.tot. 98.95%
Particles collection section
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
Rapping-Reentrainment
Conditions of particles collection:
stationary simulation of dust over the plates
Collection and re-entrainment
Time-dependent evaluation of particles layer at plates
Objectives:
• Evaluation of particles re-entering in the main stream (re-entrainment)
• Evaluation of electric behavior (back-corona)• Definition of mechanical behavior (rapping,
collecting efficiency)
Collection and re-entrainment
Min
Mout-ree
Mout-
hop
Mass balance of re-entrained, collected and
fallen particles
32
Inlet particle size distribution
Percentage by mass
33
Particle size distribution (inlet and exit)
Mechanical layout
Conclusions
• It is possible to optimize the ESP characteristics by using engineering calculations and thus avoiding empirical correlations
• The balance between cost and performance can be carefully evaluated during the design phase
More info
www.irsweb.it