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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)

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

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).

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

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.