study guide
EHST 3600: Air Pollution 1
PARTICULATE CONTROL
EHST 3600: Air Pollution
SESSION OBJECTIVES
• To review the sources of particulate matter
• To understand particulate collection efficiency, particle size distributions, and terminal or settling velocity
• To describe elements of hood and duct design
• To know different mechanisms of particulate collection
• To understand how various particulate control equipment work
SESSION OUTLINE
• Distribution and sources of particulate matter
• Particulate collection efficiency
• Particle size distributions
• Terminal or settling velocity
• Hood and duct design
• Particulate collection mechanisms
• Particulate control equipment
Definition of Terms that Describe Airborne Particulate Matter
Particulate matter
Any material, except uncombined water, that exists in the solid or liquid state in the atmosphere or gas stream at standard condition.
Aerosol A dispersion of microscopic solid or liquid particles in gaseous media.
Dust Solid particles larger than colloidal size capable of temporary suspension in air.
Fly ash Finely divided particles of ash entrained in flue gas. Particles may contain unburned fuel
Fog Visible aerosol
Fume Particles formed by condensation, sublimation, or chemical reaction, predominantly smaller than 1 um (tobacco smoke).
Mist Dispersion of small liquid droplets of sufficient size to fall from the air.
Particle Discrete mass of solid or liquid matter.
Smoke Small gas-borne particles resulting from combustion.
Soot An agglomeration of carbon particles.
PM10 Refer to particles with an aerodynamic diameter less than or equal to a nominal 10 micrometers.
PM2.5 Refer to particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers. Also referred to as the fine fraction of PM10.
PM10-2.5 Refers to those particles with an aerodynamic diameter less than or equal to a nominal 10 um but greater than 2.5 um. Also referred to as the course fraction of PM10.
ADVERSE EFFECTS OF PARTICULATE MATTER
• Health hazard to the lungs
• Enhances chemical reactions in the atmosphere
• Reduces visibility
• Increases the possibility of precipitation, fog and clouds
• Reduces solar radiation
• Range of particle sizes in the local atmosphere
• Particle concentration
• Chemical and physical composition of the particulate matter
SETTLING VELOCITIES FOR PARTICLES
Aerodynamic diameter, m*
Settling velocity, cm/s
0.1 4 x 10-5
1 4 x 10-3
10 0.3
100 30 *Particles with a density of 1 g/cm3
F ig
. 5
-1
EHST 3600: Air Pollution 2
CONCENTRATION OF PARTICULATE MATTER
• Expressed as mass of the particles in a given volume of gas
• g/m3
• 1.0 gr/ft3 = 2.29 g/m3 = 2.29 x 106 g/m3
• Examples of atmospheric dust loading
• Over the middle of the ocean: < 10 g/m3
• Severe dust storm: 109 g/m3
• Industrial gases: 104 to 108 g/m3
DISTRIBUTION AND SOURCES OF PARTICULATE MATTER
Table 5-2. Particle Distribution by Count and Volume Percent of a Typical Atmosphere Sample as a Function of Size
Size Range (m) Average size (m) Particle Count Volume %
10 – 30 20 1 27
5 – 10 7.5 112 53
3 – 5 4 167 12
1 – 3 2 555 5
0.5 – 1 0.75 4,215 2
0 – 0.5 0.25 56,900 1
PARTICULATE GENERATION
• 5 particle types produced during combustion by: • Heat vaporizing materials
condensation 0.1 – 1 m
• Chemical reactions of combustion process short-lived particles of unstable molecular clusters <0.1 m
• Mechanical processes ash or
fuel particles 1 m • Liquid fuel sprays very fine ash • Partial combustion of fossil fuels soot
MAJOR SOURCES OF PARTICULATE EMISSIONS
• Transportation
• Fuel combustion (e.g., electric utilities)
• Industrial processes
• Miscellaneous sources (e.g., household, commercial)
7%
6% 3%
84%
Both
Stationary fuel combustion
Industrial and other processes
Transportation
Miscellaneous
EPA, 2014
AIR POLLUTION CONTROL TECHNIQUES
• Gas cleaning
• Source relocation
• Fuel substitution
• Process changes
• Good operating practice
• Source shutdown
• Dispersion
1. Is the atmospheric contaminant in fact a necessary consequence of the operation?
2. Can the rate of generation of the contamination be reduced and can high bursts of release be avoided?
3. Does the process lend itself to
control by local exhaust ventilation equipment such as hoods?
EHST 3600: Air Pollution 3
PARTICULATE COLLECTION EFFICIENCY
• Overall collection efficiency, 0 • Generalized parameter employed to indicate the performance
level of a gas cleaning device
• A function of: • Particle size distribution of the particles to be collected • Other particle and gas stream characteristics.
• Predicted by: • Mass or weight distribution among the particle sizes of interest
• Collection efficiency as a function of the particle diameter, dp
0 = 𝐶
𝐴 =
𝐶
𝐵+𝐶 =
𝐴−𝐵
𝐴
• A – entering loading or concentration
• B – leaving loading or concentration
• C – amount caught or retained by the
cleaning device
FRACTIONAL COLLECTION EFFICIENCY, d
• Collection efficiency as a function of the particle diameter
• A.k.a. grade efficiency curve
• d increases as particle size increases
• May be a function of the types of dust
Fig. 5-3
PARTICLE DIAMETER
• Based on physical characteristic
• Diameter based on surface
area, dSA
• Volumetric diameter, dv
• Based on particle behavior
• Stokes’ diameter, ds
• Aerodynamic equivalent diameter, dA
d
PARTICLE SIZE DISTRIBUTIONS
• Modal diameter
• Median diameter
• Number (count) median
diameter, dNM
• Mass (volume) median
diameter, dMM
• Mean diameter, dmean
• Summing all values of the variable and then dividing the sum by the total number of samples
Fig. 5-4
TERMINAL OR SETTLING VELOCITY, Vt
• Constant downward speed that a particle attains in a direction parallel to the earth’s gravity as it overcomes the buoyancy (Fb) and frictional drag forces (Fd).
Fg
Fb + Fd
Fg
Fb + Fd
Fg Object released from rest
(no air drag force yet) Air drag force increases as
the object speeds up Air drag force is equal to the force of
gravity. Object falls at terminal velocity.
• Fb – buoyancy force
• Fd – drag force
• Fg – force of gravity
Fb + Fd = Fg
TERMINAL VELOCITY VIDEO
http://study.com/academy/lesson/what-is-terminal-velocity-definition-formula-calculation-examples.html
EHST 3600: Air Pollution 4
TERMINAL OR SETTLING VELOCITY
dp Vt
10 m 0.3 cm/s
100 m 25.5 cm/s
1000 m 400 cm/s
Fig. 5-8
HOOD AND DUCT DESIGN
• Control velocity
• Air velocity that will just overcome the
dispersive motions of the contaminant,
plus a suitable safety factor
• Adjusted to obtain the least air flow rate that gives satisfactory control results for
minimum gas volume and maximum contaminant loading
• Optimum values depend on:
• Size and shape of the hood
• Hood position relative to points of emission
• Nature and quantity of air
contaminants
VELOCITY CONTOURS
• Flanges • Eliminate the entrainment
of air from areas where no
contaminant exists • Increase the effective
capture zone in front of the duct
Fig. 5-9
CENTERLINE AIR VELOCITY, Vx • Aspect ratio 0.2
• No flange
• Flanged
• Aspect ratio < 0.2 (slots)
• No flange
• Flanged
Fig. 5-10. Duct showing aspect ratio, W/L
𝑉𝑥 = 𝑄ℎ
10𝑥2 + 𝐴ℎ
𝑉𝑥 = 1.33𝑄ℎ
10𝑥2 + 𝐴ℎ
𝑉𝑥 = 0.27𝑄ℎ 𝐿 𝑥
𝑉𝑥 = 0.36𝑄ℎ 𝐿 𝑥
NULL POINT
• General position of the particle when its original velocity has been reduced to approximately zero
Fig. 5-11
PARTICULATE COLLECTION MECHANISM
1. Gravitational settling
2. Centrifugal impaction
3. Inertial impaction (a)
4. Direct interception (b)
5. Diffusion (c)
6. Electrostatic effects
Fig. 5-12
Agglomeration
EHST 3600: Air Pollution 5
PARTICULATE CONTROL EQUIPMENT• Physical and chemical properties
of the particles
• Range of the volumetric flow rate
of the gas stream
• Range of expected particulate
concentrations
• Temperature and pressure of the
flow stream
• Humidity
• Nature of the gas phase
• Required condition of the treated
effluent
Proper choice of
particulate control
equipment
PARTICULATE CONTROL EQUIPMENT
• Gravity settling chamber
• Cyclone separators
• Wet collectors
• Spray chamber scrubbers
• Cyclonic scrubbers (wet cyclones)
• Venturi scrubbers
• Air filtration
• Electrostatic precipitators
GRAVITY SETTLING CHAMBER
• Create a drop in the velocity as air enters the chamber
• Cause large particles to fall from the air stream
• Good for large, coarse particles (e.g. chips)
• Disadvantage: excessive space requirements
GRAVITY SETTLING CHAMBER
• Settling velocity: > 25 ft/min
• Particle size
• >50 m (if density is low)
• 10 m (if density is high)
• <10 m require excessive horizontal flow distances excessive chamber volumes
• Minimum particle size removed with 100% efficiency
Fig. 5-13
𝑡 = 𝐻
𝑉𝑡 = 𝐿
𝑉
BASIC CHARACTERISTICS OF GRAVITY SETTLING CHAMBERS
• Very low energy cost
• Low maintenance cost
• Low installation cost
• Excellent reliability
• Very large physical size
• Low to very low collection efficiency
CYCLONE SEPARATORS
• Gas cleaning devices that employ a
centrifugal force generated by a
spinning gas stream to separate the
particulate matter (solid or liquid) from
the carrier gas
• A.k.a. centrifugal collectors (cyclone)
• Moderate-sized particles (wood dust,
shavings, chips); 10-40 um diameter
• Advantage: low initial cost and maintenance
• Disadvantage: not effective for finer dusts (<5 um)
http://www.iowadnr.gov /Env ironment/AirQuality/HowAirPollutionIsControlled/Cyclones.aspx
EHST 3600: Air Pollution 6
CYCLONE SEPARATORS
• Construction types
• A single large chamber
• A number of small tubular chambers in parallel or series
• A dynamic unit similar to a blower
• 2 major classes
• Vane-axial cyclone separator
• Involute centrifugal separator
Fig. 5-15
CYCLONE SEPARATORS
• For removing particles with size 10 m
• Conventional cyclones seldom remove particles with an efficiency >90% unless particle size
25 m.
• High efficiency cyclones are effective with particle sizes down
to 5 m.
Fig. 5-16
WET COLLECTORS
• Used to capture particle dust or to increase the size of aerosols
• Effectively removes both liquid and solid particulates (0.1 to 20 m) from gas stream
• Primary aim: Adequate dispersion of the liquid phase to achieve good contact between particulate phase and liquid phase
• 3 major types
• Spray chamber scrubbers
• Cyclonic scrubbers (wet cyclones)
• Venturi scrubbers
• (Packed towers – primarily for gas absorption)
http://www.forbesgroup.co.uk/products/venturi-scrubber-range/
WET COLLECTORS: DISADVANTAGES
1. Handling and disposal of wet sludge
2. Freezing of sludge in cold weather
3. Increase in corrosiveness of materials due to presence of water
4. High power input required for good dispersal of the liquid phase
Fig. 5-20. Sketch of a spray tower scrubber ht tp
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SPRAY CHAMBER SCRUBBERS
Typical countercurrent- flow spray tower
SPRAY CHAMBER SCRUBBERS
Fig. 5-21. Sketch of a wetted impingement baffle scrubber
EHST 3600: Air Pollution 7
SPRAY CHAMBER SCRUBBERS
• Water rate through scrubber: 2 – 10 gal/min for every
1000 ft3/min of gas flow
• Makeup water must be replenished
• Water used must be recycled
• Pressure drop is quite small (1-2 inH2O)
• Effectiveness of conventional spray tower
• 94% for 5-m particles
• 99% for 25-m particles
• Effectiveness with use of baffles, packed bed
• 97% for 5-m particles
• ~100% for 10-m particles
• Use of high-pressure fog sprays
• High efficiencies for particle sizes down to 1 m
http://www.iowadnr.gov /Env ironment/AirQuality/HowAirPollutionIsControlled/ScrubbersOrWetCollectors.aspx
CYCLONIC SCRUBBERS (WET CYCLONES)
https://en.wikipedia.org/wiki/Cyclonic_spray_scrubber#/media/File:Irrigcyclone.gifFig. 5-22. Cyclonic spray tower
CYCLONIC SCRUBBERS (WET CYCLONES)
• Water circulation rate: 1 – 8 gal/1000 ft3
of treated gas
• Draft loss/ pressure drop: 1 – 4 inH2O
• Collection efficiency
• 100% for droplets of 100 m
• 99% for droplets of 50-100 m
• 90-98% for droplets of 5-50 m
VENTURI SCRUBBERS
• Velocity in throat section: 160-600 ft/s
• Scrubbing action occurs through introduction of water either:
• In the venturi throat region
(recommended)
• At the beginning of the
convergent section
• Water circulation rate: 2-12 gal/ft3
• Pressure drop: 3-100 inH2O
• Collection efficiency
• 99% in submicron range
• 99.5% for 5-m particles
Fig. 5-23 http://oouah.dev iantart.com/art/V enturi-Scrubber-307266786
VENTURI SCRUBBERS
http://www.mikropul.com/uploads/pdf/wet_scrubbers.pdf
Use of wet cyclone in series
with a venturi scrubber
PACKED-BED SCRUBBER
• Fixed-bed scrubbers
• Fluidized-bed scrubbers
• Flooded-bed scrubbers
http://www.mikropul.com/uploads/pdf/wet_scrubbers.pdf
EHST 3600: Air Pollution 8
AIR FILTRATION • Filter
• Any porous structure composed of granular or fibrous materials that tends to retain the particulate matter as the carrier gas passes through the voids of the filter
• Constructed of any material compatible with the carrier gas
and particulate matter
• May be arranged as a deep bed, mat, pleated filter, or as fabric filter
• May be non-cleanable (throwaway) or cleanable
AIR FILTRATION
• HEPA – High Efficiency Particulate
Air filters
• Made from fiberglass fibers
• 99.9% efficient at 0.1 m
• ULPA – Ultra High Efficiency
Particulate Air filters
• 99.9999% efficient at 0.1 m
• Cartridge filter (modified pleated
filter)
• Cleanable
• Historically made of paper
• Can now be manufactured from a wide variety of materials
Fig. 5-25
AIR FILTRATION: FABRIC FILTER
• Usually formed into cylindrical tubes (bags)
• Hung in multiple rows to provide large surface areas for gas passage
• Housing referred to as baghouse
• Overall efficiencies: 99 – 99.99%
• 0.5 m particles – 99%
• 0.01 m particles – removes substantial quantities
• Made from woven, felted, and knitted materials with varying filter weights
http://www.iowadnr.gov /Env ironment/AirQuality/HowAirPollutionIsControlled/BagHousesOrFabricFilters.aspx
AIR FILTRATION: FABRIC FILTER
http://encyclopedia.che.engin.umich.edu/Pages/SeparationsMechanical/Filters/Filters.html
Baghouse
Fabric type Maximum continuous
temperatures of operation
Cotton 180°F 82°C
Polypropylene 190°F 88°C
Fiberglass 500°F 260°C
Nylon 200°F 93°C
Polyester 275°F 135°C
Wool 200°F 93°C
Nomex 400°F 204°C
Teflon® 450°F 232°C
Ryton® 375°F 190°C
P84 polyamide 500°F 260°C
Ceramic 1800°F 980°C
AIR FILTRATION: FABRIC FILTER
• Choice of fabric based on:
• Type of fabric filter collector
• Cost of the media
• Operating temperature
• Physical and chemical characteristics of particulates and carrier gas
• Corrosiveness
• Abrasiveness
• Combustibility
• Resistance to alkalinity
• Moisture content
AIR FILTRATION: FABRIC FILTERS
• Disadvantages of fabric filters
• Necessity of relatively frequent cleaning to avoid unreasonable pressure drops
• Large overall size of equipment, expensive maintenance
• Cleaning methods
• Mechanical vibration or shaking
• Pulse jets
• Reverse airflow
Fig. 5-26. Typical baghouse with mechanical shaking Fig. 5-28
Fig. 5-27
EHST 3600: Air Pollution 9
PRESSURE DROP ACROSS FABRIC FILTERS
Increased filtration time
Increased removal efficiency
Increased pressure drop
Overall pressure drop (PD) =
PD due to new filter (Pf)
+ PD due to formed dust cake (Pp)
Filter cake
PRESSURE DROP ACROSS FABRIC FILTERS
Fig. 5-29. Pressure drop as a function of time at constant loading rate for 3 different filters
Smooth woven filter
Felted filter
FRACTIONAL EFFICIENCY OF FABRIC FILTER
• Cleaned state (after 10 shakes)
• <90% efficiency 0.1-0.5 m
• >98% efficiency >1 m
• Loaded state
• >95% efficiency all particle
sizes
• >99.6% efficiency >1 m
Fig. 5-32
AIR FILTRATION: FABRIC FILTERS
ADVANTAGES
1. High collection efficiency over a broad range of particle sizes
2. Extreme flexibility in design
3. Volumetric capacities in a
single installation (100 – 5M ft3/min)
4. Reasonable operating pressure drops and power requirements
5. Ability to handle a diversity of solid materials
DISADVANTAGES
1. Space factors may prohibit consideration of bag houses.
2. Possibility of explosion or fires if sparks are present in vicinity of a bag house.
3. Hydroscopic materials usually cannot be handled, owing to cloth cleaning problems.
4. Limited usefulness regarding gas streams containing caustic materials.
ELECTROSTATIC PRECIPITATORS
• Based on the mutual attraction between particles of one electrical charge and a collecting electrode of opposite polarity
• Advantages
• Capacity to handle large gas volumes
• High collection efficiencies even for
submicron-size particles
• Low energy consumption and draft
loss
• Ability to operate with relatively high- temp gases (up to 1200ºF)
http://www.iowadnr.gov /Env ironment/AirQuality/HowAirPollutionIsControlled/ElectrostaticPrecipitators.aspx
ELECTROSTATIC PRECIPITATORS
• fs
EHST 3600: Air Pollution 10
ELECTROSTATIC PRECIPITATORS: DESIGNS
• Tube type
• Wires between rows of parallel plates
Fig. 5-32
ELECTROSTATIC PRECIPITATORS 1. Gas ionization
• Electrons attach to gas molecules form negative ions • Heavily ionized gas near the wire visible blue corona effect
2. Charging of dust particles in gas stream
• Collision of negatively charged gas ions with particles
3. Migration of charged particles to plate
electrodes • Migration or drift velocity – the speed at
which the migration takes place
4. Actual deposition of charged particles on electrode
• Dust electrical resistivity: 104 – 1010 ohm· cm 5. Removal of deposited particulate matter
• Rapping or vibration
SELECTION OF PARTICULATE CONTROL EQUIPMENT
1. Total volume flow rate
2. Maximum collection efficiency rating
3. Physical and chemical characteristics
of the particles
4. Dust loading of the gas stream
5. Temperature range and possibility of sudden temp increases
6. Maintenance requirements
COMPARISON OF PARTICULATE CONTROL EQUIPMENT
1. Cyclones are typically used when:
• The dust is course;
• Concentration are fairly high (>1 gr/ft3)
• Classification is desired;
• Very high efficiency is not required.
2. Wet scrubbers are typically used when:
• Fine particles need to be removed at a relatively high efficiency;
• Cooling may be desirable and moisture is not objectionable;
• Gases are combustible;
• Gaseous, as well as particulate pollutants, need to be removed.
3. Fabric filters are typically used when:
• Very high efficiencies are required;
• Valuable material is to be collected dry;
• The gas is always above its dew point;
• Volumes are reasonably low;
• Temperatures are relatively low.
4. Electrostatic precipitators are typically used when:
• Very high efficiencies are required for removing fine dust;
• Very large volumes of gas are to be handled;
• Valuable materials needs to be recovered.
COMPARISON OF PARTICULATE CONTROL EQUIPMENT
COMPARISON OF PARTICULATE CONTROL EQUIPMENT
Fig. 5-40
A – High-throughput cyclone
B – High-efficiency
cyclone
C – Electrostatic precipitator
D – Venturi scrubber
E – Baghouse
EHST 3600: Air Pollution 11
SUMMARY
• Particulate matter (PM) includes dust, fog, fume, mist, smoke, soot, PM2.5 and PM10.
• PM adversely affects human health and the environment.
• Finer particles make up most of the PM by count, but course particles make up most the PM by volume and mass.
• PM is generated by various mechanisms including vapor condensation,
chemical reaction, mechanical processes, liquid spray and partial combustion of fossil fuels.
• The major source of PM is transportation.
• Air pollution control techniques range from gas cleaning to process change to dispersion.
• Overall collection efficiency indicates indicate the performance level of a gas cleaning device.
SUMMARY
• Particle diameter can be characterized in terms of PM’s physical characteristic or behavior.
• Particle size distribution can be described in terms of mean, median and
modal diameter. • Terminal or settling velocity is constant downward speed that a particle
attains in a direction parallel to the earth’s gravity as it overcomes the buoyancy and frictional drag forces.
• Hood design and PM location relative to the hood are some of the
factors considered in particle collection. • PM is collected by various mechanisms from gravitational settling to
electrostatic effects. • Particulate control equipment include gravity settling chamber, cyclone
separators, wet collectors, air filtration, and electrostatic precipitators.