TECHNOLOGIES FOR PARTICULATE
EMISSION CONTROL
TECHNOLOGIES FOR PARTICULATE
EMISSION CONTROL
Particle size
Defining particle size for spherical
particles is easy; it is simply the
diameter of the particle. For nonspherical particles, the term
"diameter" does not appear to be
strictly applicable. For example, what
is the diameter of a flake of material
or a fiber? Also, particles of identical
shape can be composed of quite
different chemical compounds and,
therefore, have different densities.
The differences in shape and density
could introduce considerable
confusion in defining particle size.
Particulate: Shape and Density
500 mm
asbestos
10 mm
carbon
10 mm
Iron rich
The aerodynamic diameter is the diameter of a spherical
particle having a density of 1 g/cm3 that has the same
inertial properties in the gas as the particle of interest.
Aerodynamic diameter
The aerodynamic diameter for all particles greater than
0.5 mm can be approximated using the following equation.
dpa = dps (rp)1/2
dpa = Aerodynamic particle diameter, mm
dps = Stokes diameter, mm
rp = Particle density, g/cm3
Particle density affects the motion of a particle through a fluid and is
taken into account in the Equation. The Stokes diameter for a particle is
the diameter of the sphere that has the same density and settling velocity
as the particle. It is based on the aerodynamic drag force caused by the
difference in velocity of the particle and the surrounding fluid. For smooth,
spherical particles, the Stokes diameter is identical to the physical or
actual diameter.
Particles that appear to have different physical sizes and
shapes can have the same aerodynamic diameter
Conversely some particles that appear to be visually similar
can have somewhat different aerodynamic diameters
Frequency particles / % by mass
Particle Size Terminology
8
Ultrafine
Fine
Coarse Supercoarse
6
4
2
0
0.002
0.01
0.1
Environmental Protection
Agency
1.0
10
100
1000
Particle diameter / mm
Total Suspended
Particulate Matter (TSP)
Frequency particles / % by mass
Particle Size Terminology
8
Ultrafine
Fine
Coarse Supercoarse
6
4
2
0
0.002
0.01
0.1
1.0
10
100
1000
Particle diameter / mm
PM10 (respirable )
Particles separated by a PM10 filter, characteristic of which is such
that 50% of particles diameter 10 mm are passing
Frequency particles / % by mass
Particle Size Terminology
8
Ultrafine
Fine
Coarse Supercoarse
6
4
2
0
0.002
0.01
0.1
Contain sulfate, nitrate,
organic compounds,
metals, acids….
1.0
10
100
1000
Particle diameter / mm
PM2.5 (high penetration into
human respiratory system)
Frequency particles / % by mass
Particle Size Terminology
8
Ultrafine
Fine
Coarse Supercoarse
6
4
2
0
0.002
0.01
0.1
1.0
10
100
1000
Particle diameter / mm
• Particle less than 0.1 mm (clusters of 20-50 molecules)
• Condensable Particulate Matter
Deposition fraction %
Airborne particle penetration capability
100
throat
80
60
adult’s
alveolus
40
20
chest
0
0
2
3
4 5
7
10
20
30
40
Aerodynamic diameter / mm
50
70
100
Particulate formation: Physical attrition
F 10-10.000 mm
Grinding
Wheel
Mining, construction
activities, in general
any part in movement
Particulate formation: Combustion particles
burnout
Fossil-FuelFired Boiler
• Injection of fuel
particles
• Volatile organic
compounds are
vaporized and
oxidized
• ash and char
formation
F 1-100 mm
Particulate formation hypothesis of radicals
Formation of
aromatic rings
CH4
Methane fuel
PAH
(Polycycle Aromatic
Hydrocarbons)
Desorption
Chemical
condensation
Pyrolise
Pyrens
Reactive small
molecules and radicals
• CH3
Methylradical
H
C
C
Graphitisation
H
Acetylene
Desorption
• C3H3
• C6H5
Propargylradical
Phenylradical
Corones
Ovalenes
Particulate formation: Homogeneous and
heterogeneous nucleation
Homogeneous nucleation is the formation of new particles
composed almost entirely of the vapor phase material (one
compound).
Heterogeneous nucleation is the accumulation of material
on the surfaces of existing particles (more than one
compound).
F 0.1-1 mm
Particulate formation: Homogeneous and
heterogeneous nucleation
Municipal and
medical waste
Incinerator
Condensation of
1. metal/metal
oxides like Hg,
Pb, Cd; As…
2. Organic
compounds
Particulate formation: Droplet
evaporation from wet scrubbers.
Release of small particles during the evaporation
of solids containing droplets in hot gas streams.
F 0.1-20 mm
Technology for particle emission’s
control from stationary sources
• Gravity settling chamber;
• Mechanical collectors;
• Particulate wet scrubbers;
• Electrostatic precipitator;
• Fabric filters.
Gravity settling chamber
Gas Flow
OUT
W
Outlet Baffles
H
L
Gas Flow
IN
Inlet Baffles
Mechanical collectors:
Top-Inlet Large-Diameter Cyclone
Cyclone F 30-180 cm
90% efficiency for particles
of F > 20 mm
Low capital cost
Ability to operate at high T
No moving part = low
maintenance cost
Low efficiency for small
particles
Pressure drop high
operating cost
Mechanical collectors:
Small-Diameter Multi-Cyclone Collector
Cyclone F 8-30 cm
Small-Diameter Multi-Cyclone Collector
Small,
WoodFired
Boiler
Particulate wet scrubbers
Liquid in
Gas in
Gas out
Liquid in
Gas in
Liquid out
Gas out
Liquid out
Scheme of a cross-flow scrubber
Scheme of a counter-flow scrubber
Gas in
Gas out
Liquid out
Liquid in
Scheme of a co-flow scrubber.
Particulate wet scrubbers:
Adjustable-Throat Venturi Scrubber
Efficiency depends on :
• size and velocity of
particles
• droplets velocity
Simultaneous collection of
particulate and gas
(including explosives)
Compact and high
efficiency
Make-up water
Treatment of waste water
Wet Scrubbers:
Spray Tower and Impingement Plate Scrubber
Electrostatic Attraction
In air pollution control, electrostatic precipitators (ESPs) use
electrostatic attraction for particulate collection. Electrostatic
attraction of particles is accomplished by establishing a strong
electrical field and creating unipolar ions. The particles
passing through the electrical field are charged by the ions
being driven along the electrical field lines. Several
parameters dictate the effectiveness of electrostatic attraction
including the particle size, gas flow rate, and resistivity.
The particles will eventually reach a maximum or saturation
charge, which is a function of the particle area. The saturation
charge occurs when the localized field created by the already
captured ions is sufficiently strong to deflect the electrical field
lines. Particles can also be charged by diffusion of ions in the
gas stream. The strength of the electrical charges imposed on
the particles by both mechanisms is particle size dependent.
Electrostatic Precipitator
High-voltage Wires for
Corona Discharge
L
2H
Upper
Wire
Support
Dust-collection
Plates
Corona Discharge
along the Length
of a Wire
Clean Gas
h
Dirty Gas
Lower
Wire
Support
Collected Dust
on Plates
Ground
Duct Removed from
Plates to Hoppers
Electrostatic Precipitator
High
efficiency
for small
particles
Arrangements of fields and Chambers
in an ESP
Reverse Air Fabric Filter
Pulse Jet Fabric Filter
Efficiency of particle’s control systems
Degree of separation / %
50
venturi
70
cyclotron
90
Wash-out cyclones
electrostatic
96
fibre
98
99.5
99.9
sinter-ceramics
0.05
0.1
0.5 1.0
5
10
50
100
Average particle size / mm
Particulate Matter Sources
Is PM sticky or
wet?
Yes
PM Wet Scrubber Wet
ESPs
No
Are PM gases or vapor
explosive?
Yes
PM Wet Scrubber
Mechanical Collectors
No
What are size and collection
efficiency criteria?
0 - 0.5 mm
High Efficiency
0.5 - 5 mm
Medium Efficiency
>5 mm
Low Efficiency
Filtration Systems
Filtration Systems
ESPs
PM Wet Scrubbers
Filtration Systems
ESPs
PM Wet Scrubbers
Mehanical Collectors
Hot vapor
Low volatility
vapor
condensation
Homogeneous nucleation
Primary particles
coagulation
Windblown dust
Emissions
Sea spray
Volcano
Plant particles
Condensation
Chain aggregates
growth of nuclei
coagulation
droplets
coagulation
coagulation
Rainout
sedimentation
and washout
0.002
0.01
0.1
1
2
10
Particle diameter / mm
100
Particulate Wet Scrubbing System
Particulate emission from vehicles
• Soot from fuel combustion
• Attrition of tires (cars 70 mg/Km, trucks 1 g/Km
f 15 mm)
• Attrition of clutches (3 mg/Km, f 5 mm)
• Attrition of brakes (20 mg/Km, f 10-15 mm)
• Engine wear
• Catalyst (0.0005 mg/km of metal oxides, f 5 mm )
• Fiber-particles from silencers
Factors that influence soot formation
• The flame temperature:
High activation energy of pyrolysis.
Soot formation occurs above 1000 -1300 °C.
Soot emissions when burnout ceases at
temperatures below 1000 °C.
Factors that influence soot formation
• The flame temperature;
• Local oxygen concentration:
local depletion in oxygen concentration favors soot
formation.
Old diesel cars Large fuel drops (stacked at the
walls) - low excess of air.
New diesel cars fine equally distributed fuel drops
(reduced agglomeration) - large excess of air.
Factors that influence soot formation
• The flame temperature;
• Local oxygen concentration;
• Pressure:
High pressure
• increases cracking promoting soot formation.
• decrease fuel droplets penetration into the
combustion chamber. High fuel concentration
enhances soot formation.
Modern diesel: new fuel and air injectors
homogeneous gas mixture
Factors that influence soot formation
• The flame temperature;
• Local oxygen concentration;
• Pressure;
• Chemical impurities and fuel additives :
Sulfur from fuel is oxidized to SO2 and SO3
different soot formation/agglomeration mechanism
sulfuric acid + Ca from lubrication oil
gypsum
Halogens cause increase of hydrocarbon radicals by
scavenging H radicals.
Factors that influence soot formation
• The flame temperature;
• Local oxygen concentration;
• Pressure;
• Chemical impurities and fuel additives :
Emulsification with water may deplete soot
formation.
Metals such as Ni or Mn, charge the soot particles
electrostatically and reduce agglomeration. The
smaller particles thus formed burn more easily.
Emissions of fine ash containing metals.
Factors that influence soot formation
• The flame temperature;
• Local oxygen concentration;
• Pressure;
• Chemical impurities and fuel additives;
• Characteristics of the fuel:
Viscosity
atomization characteristics.
Volatility
evaporation.
Thermal stability
cracking
smoking tendency:
aromatics > alkanes > alkenes > alkynes
Diesel particulate control systems:
requirement
• High temperature and fast temperature
changes
• High space velocity
• High degree of separation (fine particles)
• Long lifetime
• Easy-fast regeneration step
• Low pressure drop
• Low cost
Wall-flow filter
Filtered exhaust
Soot is deposited on
the walls forming a
cake which improves
separation
Surface
filters
Depth filter
Filtered exhaust
Efficiency depends on
• Depth of filter
• Pore size (much bigger
than diesel particles!)
Ceramic monolith cell filters
Pressure
Regeneration Time - Pressure Drop
Time
Regeneration
Combustion of accumulated soot occurs
at temperature > 500°C
Exhaust temperature < 500 °C
active
or
passive
burner or electrical power
catalysis
off-site vs on site
Diesel particle filter equipped with total
flow burner
Diesel
tank
off
Engine
Exhaust
Compressor
Air
Soot
accumulation
Exhaust
Filter
monolith
Working period
Diesel particle filter equipped with total
flow burner
Diesel
tank
on
Engine
Exhaust
Compressor
Air
Soot
combustion
Exhaust
Filter
monolith
Regeneration period
Twin-filter system with flap control
Period of regeneration
burner
Dirty gas in
Period of trapping
burner
Clean gas
out
Twin-filter system with flap control
Period of trapping
burner
Clean gas
out
Dirty gas in
Period of regeneration
burner
Catalytic filters: passive regeneration
CO2 evolution
V2O5
Pt - CeO2
K4V2O7
Ag2V2O6
0
100
200
300
400
500
600
700
Temperature / °C
Continuously regenerating trap
PM filter
Pt/Al2O3 catalyst
Exhaust
gas in
• (1) NO + ½ O2 = NO2
• (2) NO2+PM= CO2 + H2O + N2
Sulphur tolerance is an issue
Exhaust
gas out
State of the art in particulate emission
control
Particulate is emitted by any moving part
Production can’t be eliminated but emissions may
be controlled.
Stationary sources
mature technology, widespread use in developed
countries
Transitional and Developing countries?
Mobile sources
not adequately developed
Perspectives and target for the future
Increase emission control of fine and
ultra fine particles;
Design of new control technology
Enhance growth of ultra fine particles;
Prevent ultra fine particles formation:
use of hydrogen as clean fuel for
mobile sources