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Part-financed by the European Union (European Regional Development Fund PlasTEP training course and Summer school 2011 Warsaw / Szczecin Saulius Vasarevicius, VGTU AIR CLEANING TECHNOLOGIES
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AIR CLEANING TECHNOLOGIES

Jan 02, 2017

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Page 1: AIR CLEANING TECHNOLOGIES

Part-financed by the European Union (European Regional Development Fund

PlasTEP training course and Summer school 2011 Warsaw / Szczecin

Saulius Vasarevicius, VGTU

AIR CLEANING TECHNOLOGIES

Page 2: AIR CLEANING TECHNOLOGIES

2PlasTEP

including

our club

site.

Every place on Earth is an

ecosystem,

Page 3: AIR CLEANING TECHNOLOGIES

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Page 4: AIR CLEANING TECHNOLOGIES

Two Types of Air Pollutants

Particulate (Visible)

Gaseous

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Stationary Source Control

Philosophy of pollution prevention (3P’s)

• Modify the process: use different raw materials

• Modify the process: increase efficiency

• Recover and reuse: less waste = less pollution

Philosophy of end-of-pipe treatment

• Collection of waste streams

• Add-on equipment at emission points

Control of stationary sources

• Particulates

• Gases

Page 6: AIR CLEANING TECHNOLOGIES

Three Types Of Control

Mechanical

Chemical

Biological

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Particulates

Regulated Particles

●10 microns or less diameter

Human hair averages 25 microns

25 microns is 1/1000 inch

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Example Sources OfParticulate Pollution

• Wood Processing

• Rock Quarries

• Coal Power Plants

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Particulate Control(Mechanical)

• Electrostatic precipitator

• Bag house fabric filter

• Wet scrubber

• High efficiency cyclones

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Particulate Control TechnologiesRemember this order:

●Settling chambers

●Cyclones

●ESPs (electrostatic precipitators)

●Spray towers

●Venturi scrubbers

●Baghouses (fabric filtration)

All physical processes

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Settling Chambers

• “Knock-out pots”

• Simplest, cheapest, no moving parts

• Least efficient

●large particles only

• Creates solid-waste stream

●Can be reused

• Picture on next slide

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Disadvantages

Large space requirement

Relatively low overall collection efficiencies (typical 20 - 60 %)

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Flue Gas Cleaning – The state of the art

Selection criteria

ESP Bag house Scrubber Cyclones

(normal)

Spraycone

Cyclones

Emission

mg/Nm3100 30 200 250 < 100

Reliability ++ + ++ ++++ ++++

Cost ++++ ++++ +++ + +

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Page 16: AIR CLEANING TECHNOLOGIES

Gas Cleaning – The state of the art

Evaluation of ESP for industrial boilers:• High cost (investment, maintenance & operation)• Complex large size plant with sub-systems• Requires constant gas conditions (sulphur, temp, moisture)

Evaluation of bag filters for industrial boilers:• High cost (investment, maintenance & filter bags)• Difficult to handle sulphur and sparks• Not robust (one faulty bag destroys efficiency

Evaluation of wet scrubbers for industrial boilers:• High cost (large water treatment plant)• Difficult to separate fine particulate• Sulphur control costly & difficult

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Flue Gas Cleaning – The state of the art

Evaluation of cyclone “grid arrestor” :

• Low collection efficiency due to:• Wrong design (see velocity analysis)• Air ingress• Bad manufacturing quality• Lack of maintenance (blockage of cyclone cells)

But cyclone system advantages are low cost and robust installation

Can a cyclone reach efficiencies of ESP / Bag filter / Wet scrubber?

This question triggered our cyclone development Program in 1994 to improve cyclone efficiency and to invent the“dry spray agglomeration principle”

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Cyclone

• Most Common

• Cheapest

• Most Adaptable

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CYCLONES (CONTD.)

Construction and Operation

The gas enters through the inlet, and is forced into a spiral.

• At the bottom, the gas reverses direction and flows upwards.

• To prevent particles in the incoming stream from contaminating the clean gas, a vortex finder is provided to separate them. the cleaned gas flows out through the vortex finder.

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CYCLONES (CONTD.)

Advantages of Cyclones

• Cyclones have a lost capital cost

• Reasonable high efficiency for specially designed

cyclones.

• They can be used under almost any operating condition.

• Cyclones can be constructed of a wide variety of

materials.

• There are no moving parts, so there are no maintenance

requirements.

• Disadvantages of Cyclones

Page 22: AIR CLEANING TECHNOLOGIES

Mechanical Collectors –Cyclones

Advantages: Good for larger PM

Disadvantages: Poor efficiency for finer PM

Difficult removing sticky or wet PM

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Cyclone Operating Principle

“Dirty” Air Enters The Side.

The Air Swirls Around The

Cylinder And Velocity Is Reduced.

Particulate Falls Out Of The Air To The Bottom Cone And Out.

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Cyclone

PlasTEP 26

d0.5 = cut diameter at 50% removalm = dynamic viscosity of gas, Pa-sB = width, mH = height, mrp = particle density, kg/m3Qg = gas flow rate, m3/sq = effective number of turns

21

2

5.0

9

gpQ

HBd

De

L3D2

L1

L2

Dd

H

B

212 LLH

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Ex. 6-9Given:D2 = 0.5 mQg = 4 m3/sT = 25 oCrp = 800 kg/m3

For standard Cyclone:B = 0.25 D2 = 0.13 mH = 0.5 D2 = 0.25 mL1 = L2 = 2 D2 = 1 m

Q = What is the removal efficiency for particles with avediameter of 10 mm?

7.371)1(225.0

)(41.2

1041.2)7.37)(4)(800(

)25.0()13.0)(105.18(9 6

5.026

5.0

m

xx

d

@ d =10 mm 15.441.2

10

5.0

d

d

h = 0.95

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Flue Gas Cleaning – The state of the art

History:

1975 - 1980 work with Prof. Stairmond (UK)Cyclones for coal fired gas turbine 80 MW PFBC

1993 – 1998 basic research in cyclone technology

CyDesign - Cape Town

development center

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Flue Gas Cleaning – The state of the art

On site technology studies & testing:

JTA boiler training center

Glass melting furnace

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Flue Gas Cleaning – The state of the art

Commercial applications of high efficiency cyclones:

BurnerMaxFluidized bed furnaceHigh efficiency cyclonesoperating at 400 C

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Flue Gas Cleaning – The state of the art

Project 1996 / 9730 year old Büttner solid fired wood chip dryer equipped with low efficiency cyclone. Emission > 650 mg / Nm3

Solution:

Step 1 = retrofitting of high efficiency cyclones

Reduction of emission to < 100 mg / Nm3

Step 2 = installation of THERMAX agglomeration spraysupstream of cyclones

Reduction of emission to < 20 mg / Nm3

First idea of agglomeration spray in 1997

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Flue Gas Cleaning – The state of the art

High efficiency cyclone plantwith fully evaporativefine agglomeration sprays(Installation 1997)

Efficiency > 99 %

Emission < 20 mg / Nm3

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Flue Gas Cleaning – The state of the art

What is the agglomeration spray?

•Water spray with very fine droplets•Fully evaporative (dry system)•Droplets capture small particles •and agglomerate them•Larger particles are easily collected

Typical water consumption30 liter / t steam generation

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Flue Gas Cleaning – The state of the art

Wet scrubber installed at a130 t/h JTA water-tube boiler

Inlet conditions:

• 5000 mg / Nm3

• 5 % (250 mg / Nm3) < 5 micron• Emission > 300 mg / Nm3

• Required 120 mg / Nm3

Case study:

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Flue Gas Cleaning – The state of the art

Solution: Installation of 3 horizontal “spraytrees”

Water consumptionminimal.

Fully evaporative

Final emission< 100 mg / Nm3

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Flue Gas Cleaning – The state of the art

Current CyDesign specifications foradvanced cyclone technology:

Integrated cyclone system withfully evaporative agglomerationwater spray (20 l / h per cyclone)

To provide a robust, economic, low maintenance, very high performance,flue gas cleaning system

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Flue Gas Cleaning – The state of the art

Cyclone parts are manufactured to high precision aserosion & corrosion resistant castings

Cyclone vortex blade ringwith pressure recovery

Wax dies

Castings

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Cyclone Separator - Cheap

Dirty gas

Dust discharge

Cleaned gas

Does NOT produce hazardous materials like other

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Multiple Cyclones(Multi clone)

Smaller Particles Need Lower

Air Flow Rate To Separate.

Multiple Cyclones Allow Lower Air Flow Rate, Capture Particles to 2 microns

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Air Filtration

PlasTEP 42

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Filtration Mechanisms

Diffusion

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Q: How does efficiency change with respect to dp?a. Efficiency goes up as dp decreasesb. Efficiency goes down as dp decreases

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Filtration Mechanisms

Impaction

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Q: How does efficiency change with respect to dp?a. Efficiency goes up as dp decreasesb. Efficiency goes down as dp decreases

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Filtration Mechanisms

Interception

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Fat Man’s Misery, Mammoth Cave NP

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Filter efficiency for individual mechanism and combined mechanisms

dp (m)

0.01 0.1 1 10

Eff

icie

ncy

0.0

0.2

0.4

0.6

0.8

1.0

Interception

Impaction

Diffusion

Gravitation

Total

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FILTRATION

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Fiber filter

Q: Do filters function just as a strainer, collecting particles larger than the strainerspacing?

a: yesb: no

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Filter Drag Model

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spf PPPP

VLVtKVK 21

Areal Dust Density LVtW

Filter drag

V

PS

WKKS 21

Ks

Ke

Ke & Ks to be determined empiricallyDPf: fabric pressure dropDPf: particle layer pressure dropDPs: structure pressure drop

Time (min)

P, Pa

0 150

5 380

10 505

20 610

30 690

60 990

Q: What is the pressure drop after 100 minutes of operation? L = 5 g/m3 and V = 0.9 m/min.

se

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Case A: Pore blocking

Case B: Pore plugging

Case C1: Pore narrowing

Case C2: Pore narrowing w/lost pore

Case D: Pore bridging

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Air Filtration

• Impaction

• Diffusion

• Straining (Interception)

• Electrostatics

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Air Filter• High removal

efficiency for < 5 m particles

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Cleaned gas

Dirty gas

Baghouse Filter – only one to remove hazardous fine particles

Dust discharge

Bags

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Advantages/Disadvantages

• Very high collection efficiencies

• Pressure drop reasonably low (at beginning of operation, must be cleaned periodically to reduce)

• Can’t handle high T flows or moist environments

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Fabric Filters / BaghousesAdvantages: Good efficiency for various sizes of particles

Disadvantages: Not to be used around corrosive substances, explosive gases, or sticky and wet particles

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DESIGN OF FABRIC FILTERS

• The equation for fabric filters is based on Darcy’s law for flow through porous media.

• Fabric filtration can be represented by the following equation:

S = Ke + KswWhere,

S = filter drag, N-min/m3

Ke = extrapolated clean filter drag, N-min/m3

Ks = slope constant. Varies with the dust, gas and fabric, N-min/kg-m

W= Areal dust density = LVt, where

L = dust loading (g/m3), V = velocity (m/s)

• Both Ke and Ks are determined empirically from pilot tests.

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Fabric Filters

Δ P Total pressure drop

Δ Pf Pressure drop due to the fabric

Δ Pp Pressure drop due to the particulate layer

Δ Ps Pressure drop due to the bag house structure

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ADVANTAGES OF FABRIC FILTERS

Very high collection efficiency

They can operate over a wide range of volumetric flow rates

The pressure drops are reasonably low.

Fabric Filter houses are modular in design, and can be pre-assembled at the factory

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FABRIC FILTERS (CONTD.)

• Disadvantages of Fabric Filters

Fabric Filters require a large floor area.

The fabric is damaged at high temperature.

Ordinary fabrics cannot handle corrosive gases.

Fabric Filters cannot handle moist gas streams

A fabric filtration unit is a potential fire hazard

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Wet Particle Scrubbers

• Particulate control by impaction, interception with water droplets

• Can clean both gas and particlephases

• High operating costs, highcorrosion potential

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Baghouse Filters

• Particulate control by impaction, interception, diffusion on fabric & dust layer

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Baghouses

• Fabric filtration – vacuum cleaner

• High removal efficiency for small particles

• Not good for wet or high temperature streams

• Uses fabric bags to filter out PM

• Inexpensive to operate (process based)

• Bags cleaned by periodic shaking or air pulse

• Creates solid-waste stream

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Pulse-Air-Jet Type Baghouse

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Fabric Filter(Baghouse)

• Same Principle As Home Vacuum Cleaner

• Air Can Be Blown Through Or Pulled Through

• Bag Material Varies According To Exhaust Character

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Baghouse

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About Baghouses

Efficiency Up To 97+%

(Cyclone Efficiency 70-90%)

Can Capture Smaller Particles Than A Cyclone

More Complex, Cost More To Maintain Than Cyclones

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Types of Baghouses

• The three common types of baghouses based on cleaning methods

a. Reverse-air

b. Shaker

c. Pulse-jet

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ELECTROSTATIC PRECIPITATOR (CONTD.)

• Advantages of Electrostatic Precipitators

Electrostatic precipitators are capable very high efficiency, generally of the order of 99.5-99.9%.

Since the electrostatic precipitators act on the particles and not on the air, they can handle higher loads with lower pressure drops.

They can operate at higher temperatures.

The operating costs are generally low.

• Disadvantages of Electrostatic Precipitators

The initial capital costs are high.

Although they can be designed for a variety of operating conditions, they are not very flexible to changes in the operating conditions, once installed.

Particulate with high resistivity may go uncollected.

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Page 78: AIR CLEANING TECHNOLOGIES

Note about pulse-jets• blast of compressed clean air flows briefly into

the bags, while they are still filtering dusty air, knocking off some dust

• bottom of the bag is closed and filtration can occur on this surface - need to account for in area calculations

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Number of Compartments

Q: What is one of the parameters that affect our decision on the number of compartments to be used?a. fabric typeb. time required to clean a compartmentc. temperature and RH

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Parallel Flow Operation

PlasTEP 80

How do you determine when to clean?

Page 81: AIR CLEANING TECHNOLOGIES

Design procedure for baghouses• Identify filter velocity (V) for dust that your are trying

to treat

• Estimate net cloth area A = Q/V

●Remember to add area at bottom of filter if you are using pulse-jet

• Estimate number of compartments using table

●Not needed for pulse-jet

• May need to develop filter-drag model to understand relationship between pressure drop P, filtering velocity V

●For estimating how long you can run

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Electrostatic Precipitator(ESP)• High Efficiency

• Able to Handle Large Air Flow Rates

• Or Can Be Very Small (Smoke Eaters In Bars and Restaurants)

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Type 4: Electrostatic Precipitators

Types include:

• Dry, negatively charged

• Wet-walled, negatively charged

• Two-stage, positively charged

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Electrostatic Precipitators

Advantages: Good efficiency

Disadvantages: Dependent upon resistivity of PM, cannot be used around explosive gases

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http://www.ppcbio.com/ppcdespwhatis.htm

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Electrostatic Precipitator Drawing

PlasTEP 87

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How An ESP Operates

PlasTEP 88

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ESPs

• Electrostatic precipitator

• More expensive to install,

• Electricity is major operating cost

• Higher particulate efficiency than cyclones

• Can be dry or wet

• Plates cleaned by rapping

• Creates solid-waste stream

• Picture on next slide

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Electrostatic Precipitator Concept

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Electrostatic Precipitator

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Electrostatic Precipitator – static plates collect particlesDirty gas

Dust discharge

Electrodes

Cleaned gas

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Principle

High-Voltage Charges Wires

Gases Are Ionized

Particles Become Charged

Collection Plates (Opposite Charge) Attract Particles

Rapper Knocks Plates So That The Collected Dust Layer Falls Into Hoppers

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Wet Type

• Venturi

• Static packed

• Moving bed

• Tray tower

• Spray towers

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Scrubbers

• Gas Contacts A Liquid Stream

• Particles Are Entrained In The Liquid

• May Also Be A Chemical Reaction

●Example: Limestone Slurry With Coal Power Plant Flue Gas

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Scrubbers

Advantages: Good efficiency, can collect (potentially explosive) gaseous pollutants as well as PM, small size

Disadvantages: Requires a lot of water, generates waste stream

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WET SCRUBBERS (CONTD.)

Advantages of Wet ScrubbersWet Scrubbers can handle incoming streams at high temperature, thus

removing the need for temperature control equipment.

Wet scrubbers can handle high particle loading.

Loading fluctuations do not affect the removal efficiency.

They can handle explosive gases with little risk.

Gas adsorption and dust collection are handled in one unit.

Corrosive gases and dusts are neutralized.

Disadvantages of Wet Scrubbers

High potential for corrosive problems

Effluent scrubbing liquid poses a water pollution problem.

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Venturi Scrubber

• High intensity contact between water and gas => high

pressure drop

• Venturi action modified spray tower

• High removal efficiency for small particles

• Creates water pollution stream

• Can also absorb some gaseous pollutants (SO2)

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Venturi Scrubber

Detail illustrates cloud atomization from highvelocity gas stream shearing liquid at throat

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Vertical Venturi Scrubber

PlasTEP 102

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Packed Bed Scrubber

PlasTEP 103

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Dry Scrubber System

PlasTEP 104

http://www.fkinc.com/dirctspraydry.htm#topca

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Tower Scrubber

PlasTEP 105

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Spray Towers

• Water or other liquid “washes out” PM

• Less expensive than ESP but more than cyclone, still low

pressure drop

• Variety of configurations

• Higher efficiency than cyclones

• Creates water pollution stream

• Can also absorb some gaseous pollutants (SO2)

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Spray Tower

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Types Of Scrubber

Tray Tower Scrubbers

Impingement Tray

Sieve Tray

Packed Bed Scrubbers

Cylinder Filled with Media

Which Promotes Gas-Liquid Contact

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Types Of Scrubber

Fiber Bed Scrubber

Vertical Mesh Pads Of Interlaced Fibers Promote Gas-Liquid Contact

Spray Tower Scrubber

Nozzles Spray Liquid Across the Inlet Gas Flow Path

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Gaseous Pollutant Control

PlasTEP 110

•Absorption

•Adsorption

•Combustion

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Control of Air Pollutants

Gaseous pollutants - Combustion

• 3 types of combustion systems commonly utilised for pollution control

● direct flame,

● thermal, and

● catalytic incineration systems

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Control of Air Pollutants

Gaseous pollutants - Adsorption

• physical adsorption to solid surfaces

• Reversible - adsorbate removed from the adsorbent by increasing temp. or lowering pressure

• widely used for solvent recovery in dry cleaning, metal degreasing operations, surface coating, and rayon, plastic, and rubber processing

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Control of Air Pollutants

Gaseous pollutants - Adsorption

• limited use in solving ambient air pollution problems – with its main use involved in the reduction of odour

• Adsorbents with large surface area to volume ratio (activated carbon) preferred agents for gaseous pollutant control

• Efficiencies to 99%

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Carbon Adsorption

• Will do demonstration shortly

• Good for organics (VOCs)

• Both VOCs and carbon can be recovered when

carbon is regenerated (steam stripping)

• Physical capture

●Adsorption

●Absorption

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Adsorb

Absorb

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Control of Air Pollutants

Gaseous pollutants - Absorption

• Scrubbers remove gases by chemical absorption in a medium that may be a liquid or a liquid-solid slurry

• water is the most commonly used scrubbing medium

• Additives commonly employed to increase chemical reactivity and absorption capacity

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Pollutants Of Interest

• Volatile Organic Compounds (VOC)

• Nitrogen Oxides (NOx)

• Sulfur Oxides (SOx)

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Example Sources OfGaseous Pollutants

• Surface Coating Processes

• Printing

• Combustion (Boilers)

• Dry Cleaning

• Bakeries

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Mechanical Control

• For Burners, Air/Fuel Ratio Control, Called Low-NOx Burners.

• For Dry Cleaners And Similar Processes Using Solvent In Closed Vessels, Refrigerated Condensers.

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Chemical Control

• Flue Gas Control

• Solvent Destruction

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Flue Gas Control

To Reduce Emissions of NOx From Burners:

Break NOx into O2 And N2 with a catalyst.

Same Process as in automobiles.

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Controlling Gaseous Pollutants: SO2 & NOx

• Modify Process (recall 3P’s)

●Switch to low-sulfur coals

●Desulfurize coal (washing, gasification)

• Increase efficiency

●Low-NOx burners

• Recover and Reuse (heat)

●staged combustion

●flue-gas recirculation

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Controlling Gaseous Pollutants: CO & VOCs

• Wet/dry scrubbers

●Absorbers

●NOx and SOx included

• Proper operating conditions

• Thermal and catalytic oxidation

●Chemical

• Carbon adsorption

●Physical

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VOC / CO Process Control

• Keep combustion HOT

●Reuse & recycle heat

• Control cold start-ups, shut-downs, wet inputs

●wood-fired, chemical incinerators, boilers

• Increase residence time of gas in combustor

• Unfortunately, things that reduce NOx tend to increase VOC’s

●Atmosphere in air combustion 78% N2

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Scrubbers / Absorbers

• SO2 removal: “FGD” (flue gas desulfurization)

●Lime/soda ash/citrate absorbing solutions

●Can create useable by-product OR solid waste stream

• NOx removal—catalytic and non-catalytic

●Catalyst = facilitates chemical reaction

●Ammonia-absorbing solutions

●Process controls favored over this technology

• CO & CO2 removal

• Some VOC removal

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Flue Gas SOx Control

SOx Forms Sulfuric Acid With Moisture In Air Producing Acid Rain.

Remove From Flue Gas By Chemical Reaction With Limestone

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Control Technologies for Nitrogen Oxides

• Preventive●minimizing operating temperature

● fuel switching

● low excess air

● flue gas recirculation

● lean combustion

●staged combustion

● low Nox burners

●secondary combustion

●water/steam injection

• Post combustion●selective catalytic reduction

●selective non-catalytic reduction

●non-selective catalytic reduction

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Thermal Oxidizers

For VOC Control

Also Called Afterburners

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Thermal Oxidation

• Chemical change = burn

●CO2 and H2O ideal end products of all processes

• Flares (for emergency purposes)

• Incinerators

●Direct

●Catalytic = improve reaction efficiency

●Recuperative: heat transfer between inlet /exit gas

●Regenerative: switching ceramic beds that hold heat, release

in air stream later to re-use heat

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Two Types Of Oxidizer

• Catalytic

• Non-Catalytic

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Thermal Oxidizer(Non-Catalytic)

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Catalytic Thermal Oxidizer

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Biological Method

• Uses Naturally Occurring Bacteria (Bugs) To Break Down VOC

• “Bugs” Grow On Moist Media And Dirty Gas Is Passed Through. Bugs Digest The VOC.

• Result Is CO2 And H2O

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A Bio Filter For VOC Removal

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Design procedure for Scrubbers

• Identify type of scrubber

●Counter current

●Cross flow

●Venturi

• Calculate Kp, impaction parameter

• Calculate single droplet target efficiency, hd

• Calculate Penetration, Ptd

• Collection efficiency is 1 - Ptd

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Its Raining in Boulder

• Liken particle scrubbers to a good rain storm

• Some particles in air are removed because they are collected on or in raindrops and deposited on ground

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Dirty gas

Dirty water

Cleanwater

Wet Scrubber – Expensive

Wetgas

Cleaned gas

Remove 98% of SO2 and

PM from emissions

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Other Technologies

• High-temp ceramic filter

• Operates at T > 500 F (limit for baghouses)

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E-beam flue gas treatment process(Prof. A.Chmielewski)

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Control of Air Pollutants

Gaseous pollutants – Odour

The main approaches include

• wet scrubbing,

• charcoal filtration and

• incineration

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Control of Air Pollutants

Gas pollutants – Vehicle emissions

• generally involve simple procedures such as maintaining the correct tuning for the engine, or the use of catalytic converters

• catalytic converters use Pt and Pd attached to some form of ceramic material

• extremely high surface area (in hundreds of m2) allows catalytic materials to contact exhaust gases, oxidising them to CO2 and water vapour

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Automobile Emission Control System

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Control of Air Pollutants

Gas pollutants – Vehicle emissions

• all the measures which decrease CO and hydrocarbon emissions, increase NOx emissions

• measures such as changing engine spark plug timing and reduction of compression ratios allow NOx emissions to be lowered without greatly increasing other pollutant emissions

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Environnemental friendly technologies (Pollution Prevention)

• Preparation of fuel materials and by-product energy sources (clean coal, RDF, wood, biofuels, etc…)

• Low NOx burner

• Pollution abatement devices (gas) :

– DeNOx

– DeDiox

– Acid neutralization

– Desulfurization

• Filtration of particles :

– Electrostatic precipitator

– Baghouse

• Cogeneration (heat and power)

• Use of renewable energy sources

• Promotion of low carbon power generation technologies

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Thank You for Your attention!

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