tpe me326 environmental aspects

7/30/2019 TPE ME326 Environmental Aspects

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Thermal Power Engineering(ME326)

Environmental Aspects of Power Generation

Dr. K.SRINIVAS REDDYHeat Transfer &Thermal Power Lab.

Department of Mechanical Engineering

INDIAN INSTITUTE OF TECHNOLOGY MADRAS


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Environmental Aspects of Power Generation

Various kinds of fuel may be burned in thermal power

plants, all of which release emissions into the air.

Some fuels that are well known include fossil fuels:

Coal Natural gas, Oil

Methane (produced by biomass) and

wood waste.

Non-fossil : Nuclear


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The emissions can vary dramatically, depending upon

the fuel being burned and the plant technology.

Some of the environmental impacts which may be

associated with thermal power plants include:

Air pollution/airborne emissions, including

particulates, toxics, greenhouse gases and heat

Risk of spills of fuel on land, or contamination of

water or groundwater

Noise pollution


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The Thermal Plants produce emissions :

1. Fossil Power Plants

Sulfur Oxides(SOx) Nitrogen Oxides(NOx)

Carbon Oxides(CO2,CO)

Particulate Matter Thermal Pollution

2. Nuclear Power Plants Radioactivity release

Radioactive waste

Thermal pollution


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Air Pollution

Definition:

Any atmospheric condition in which certain substancesare present in such concentrations that they can

produce undesirable effects on man and his

environment.

The substances include

- Gases ( SOx , NOx, CO, CO2, HCs)

- Particulate matter(smoke, dust, fumes, aerosols)

- Radioactive materials etc.


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TYPES OF AIR POLLUTANTS

Primary pollutants are emitted directly from the

sources into the atmosphere, which include:

Particulate matter: Ash, Smoke, Dust, Fumes, Mist andSpray

Inorganic gases: SOx, H2S, NOx, NH3, CO,CO2, & H2F

Olefinic and aromatic hydrocarbons; and

Radioactive compounds

The primary pollutants in sufficient concentrations

to be of immediate concern are:

Particulate matter, SOx, NOx, CO, and HCs.


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Secondary Pollutants

Generated over time in the atmosphere from

chemical reactions involving primary pollutants.

The secondary pollutants include:

SO3, NO2, Peroxyacetylnitrate(PAN),

O3, Aldehydes, Ketones, and

various sulphate & nitrate salts.

Secondary pollutants are formed from chemical

and photochemical reactions in the atmosphere.


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The reaction mechanisms are influenced by such

factor as:

Concentration of reactants

Degree of photo-activation

Local topography

Meteorological forces and

Moisture content in atmosphere


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Sources of air pollutants

Industry

Fossil fuel combustion, smelting

Transport

Fossil fuel combustion

Agriculture

Animal effluent, fertilizers,

biomass burning

Domestic

Fossil fuel combustion

Pointand

Diffuse

sources


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MAJOR AIR POLLUTANTS

SO2 Gas Fossil fuel combustion, natural

NOx Gases Fossil fuel combustion, natural

CO Gas Fossil fuel combustion

VOCs Gases Cars, organic solvents, natural

NH3 Gas Agriculture, natural

TSP Particulate Oxidation, fossil fuel burning, dust

Heavy

metals

Particulate Metal processing, fossil fuel burning

Acidicaerosols

Particulate Secondary - reactions of pollutants from fossilfuel burning

Ozone Gas Secondaryfrom reaction of NOx and

VOCs(volatile organic carbon) under sunlight


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Air pollutant pathways


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AIR POLLUTION SOURCES, PATHWAYS

AND RECEPTORS


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Sulphur dioxide (SO2)

is a gas produced from burning coal, mainly in

thermal power plants.

Some industrial processes, such as production of

paper, power plants and smelting of metals,produce sulphur dioxide.

It is a major contributor to smog and acid rain.

Sulfur dioxide can lead to lung diseases.


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SO2 is a colourless gas with a characteristic, sharp,pungent odour.

It is moderately soluble in water(11.3g/100ml)forming weakly acidic H2SO3.

It is oxidized slowly in clean air to sulphurtrioxide.

In a polluted atmosphere, SO2 reacts photo-

chemically or catalytically with other pollutantsor normal atmospheric constituents to form SO3,H2S, H2SO4, and salts of H2SO4.


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Nitrogen oxides (NOx NO and NO2)causes smog and acid rain.

It is produced from burning fuels including petrol,

diesel, and coal.

Nitrogen oxides can make children susceptible to

respiratory diseases in winters.

Nitrous oxide(N2O), Nitric oxide(NO) and

Nitrogen dioxide(NO2) are formed in appreciable

quantities in the atmosphere.


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NO is a colorless, odorless gas produced largely by fuel

combustion.

It is oxidized to NO2 in a pollutant atmosphere through

photochemical secondary reaction.

NO2 is a brown pungent gas with an irritating odor.

NO2 is emitted by fuel combustion and nitric acid plants

Small concentrations of NO2 are detected in the lower

stratosphere( oxidation of NO by Ozone).


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Oxides of Carbon: CO and CO2(largest)

Carbon monoxide (CO): colorless, odorless gas is

produced by the incomplete burning of carbon-basedfuels including petrol, diesel, wood, natural and

synthetic products.

It has affinity towards the hemoglobin of the bloodstream

and is a dangerous asphyxiant.

It can slow our reflexes and make us confused and sleepy.

The rate of oxidation of CO to CO2 in the atmosphere

seems to be very slow.


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Carbon dioxide (CO2):is more abundant and is largely

contributed by power plant.

CO2 is the principle greenhouse gas emitted as a result of

human activities such as the burning of coal, oil, and

natural gases.

Hydrocarbons(HCs)

The gaseous and volatile liquid hydrocarbons are of

particular interest as air pollutants.

HCs can be saturated or unsaturated, branched or

straight-chain or ring structure.

I t t d l CH i b f th t b d t


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In saturated class, CH4 is by far the most abundanthydrocarbon constituting about 40-80% of total HCs

present in an urban atmosphere.

The unsaturated class includes Alkenes(Olefins) andAcetylenes.

Among the alkenes the prominent pollutants are Ethyleneand propane.

The HCs in air by themselves alone cause no harmfuleffects.

They are of concern because they undergo chemical

reactions in the presence of sunlight.


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Suspended Particulate Matter (SPM)

consists of solids in the air in the form of smoke, dust, and

vapour that can remain suspended for extended periods.

SPM can be suspended droplets or solid particles

or mixtures or the two.

Particulates can be composed of inert or extremely

reactive material ranging in size 0.1-100m.

These reactive materials could be oxidized or may reactchemically with the environment.

SPM is the main source of haze which reduces

visibility.


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The finer of these particles, when breathed in can lodge in

our lungs and cause lung damage and respiratory

problems.

The classification of various particulates include:

DustIt contains particles of 1-200m size.

These are formed by natural disintegration of rock and

soil or by mechanical processes of grinding and spraying.

They have large settling velocities and are removed from

air by gravity and other inertial processes.


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SmokeIt consists of fine particles of 0.01-1m size, which can be liquid or

solid and are formed by combustion or chemical processes.

FumesThese are solid particles of the size ranging from0.1 to1m.

Fumes are normally released from chemical and metallurgical

processes.MistIt made of liquid droplets smaller than 10m which are formed by

condensation in the atmosphere or are released from industrial

operations.

FogIt is the mist in which the liquid is water and is sufficiently dense to

obscure vision.


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Pollution Control Methods

1. Source Correction/ Pollution Prevention(P2)

2. Effluent Gas Cleaning


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Source Correction /Pollution Prevention(P2)

Reducing or eliminating pollution at the source so that itnever enters the environment in the first place.

P2 is a proactive approach to environmental management.

P2s health and environmental benefits include

cleaner air and water

fewer greenhouse gas emissions

less toxic waste to manage

less solid waste going to landfills

greater workplace safety

better stewardship of natural resources.


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Effluent Gas Cleaning

When Source Correction methods cannot achieve thedesired goal of air pollution control, use is made of

Effluent Gas Cleaning

Pollution control at the "end of the pipe" after it has beencreated.

The effluent gas cleaning includes: Removal of Particulate Pollutants

Cleaning of Gaseous Effluents


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Pollution Control Equipment


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Air Pollution Control Equipment

Particulate Removal Gaseous Removal

Settling Chamber

Cyclone

Filtration

Electrostatic Precipitators(ESPs)

Scrubbers

Wet Scrubbers / Absorption

Catalytic

Adsorption


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Air Pollution Control


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Particulate Emission Control


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Particulate Emission Control

Particle sizes range from 0.1m to 100m.

The choice of collection devices depends upon:

Physical and chemical characteristics of particulates

Particulate size and concentration in the gas

Volume of particulates to be handled and

Temperature and Humidity of gaseous medium

Toxicity and inflammability (very important)


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The major controlling performance parameters are:

Particle size, weight, shape

Particle velocity Gas temperature/density

Solubility and pH

System pressure drop and mass transferconditions

Particle size distribution

Gas viscosity

Humidity level

Chemical stoichiometry

Residence time


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Particulate Control Mechanisms

1. Gravitational Settling

2. Centrifugal Impaction

3. Inertial Impaction

4. Direct interception

5. Diffusion

6. Electrostatic Precipitation


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Particulate Control Equipment

1. Gravitational Settling Chambers

2. Cyclone Separators

3. Fabric Filters

4. Electrostatic Precipitator

5. Wet Collectors(Scrubbers)


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Gravitational Settling Chamber

Used to remove large abrasive particles(> 50m) fromgas steams

Offer low pressure drop and require low maintenance

Efficiency is quite low for particles


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Gravitational Settling Chamber-Design Concepts


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Q

L

W

H

h

uv

Particles with velocities greater than v are completely removed

Particles with velocities less than v are partially removed

QVolumetric flow rate of gas steam

nNo. of trays of L X W X H

pand g densities of particle and gas respectively

uvs

Gravitational Settling Chamber-Design Concepts


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Driving force: gravity

Characteristics: good for particles with high settling velocities

low for dp< 10s ofm

typically laminar flow to allow for settling

Simple


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3D g p gF d v v

3

6g p g

dF g

Drag force

Buoyancy

At steady state, forces equal; particle falls at constant velocity

2 ( )

18

p g

p g c

g

d gv v C

Can neglect gas density and velocity; equation holds good for

0.01


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If Rep is in the range of 3-400, then

2 2( )

8

D g p g

D

C v v d

F

CD is a dimensionless drag coefficient (a particle friction factor)

Equating this force to the buoyant force Fg

4 ( )

3

p g

t p g

D g

d gv v v

C

2 21

2D D p g tF C d v


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Q

L

W

H

h

0

o

Lt

u

H Hu HuW Q

v t L LW A

u

v uvs

If a particle with settling velocity < v enters the settling chamber,

then, it will be removed only if it is at a height h, such that

0sh v t


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For laminar conditions i.e. Re


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The generally be turbulent rather than laminar

The collection efficiency is expressed as:

1 exp tnWLvQ


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CYCLONES

Operate to collect relatively large size particulate matter

from a gaseous stream through the use of centrifugal

forces.

Dust laden gas is made to rotate in a decreasing

diameter pathway forcing solids to the outer edge of thegas stream for deposition into the bottom of the cyclone

The particles lose K.E. in cyclone and are separated

from the gas stream.


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CYCLONES

Particles are then overcome by gravitational force and

collected.

Centrifugal and gravitational forces are both

responsible for particle collection in a cyclone.

Efficiencies of 90% in particle sizes of 10 microns or

greater are possible.


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Particulate Control - Cyclones

Driving force: gravity and centrifugal force

Principle: increase by imparting an additional

centrifugal force on the particles

Characteristics:

no moving parts generally for dpof 10 to 10s of microns

standard: 90 input to axis of cyclone

29

2)(WHg

eNQppd

pd

21 cLbe LH

N

D


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De

HW

S

D

Lb

Lc

Dd

Ne = number of turns of vortex


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Efficiency dependent on

particle diameter,

number of vortex turns(length of cyclone),and

inlet velocity

Inversely dependent on cyclone inlet width.


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1

29 gpc

e p g

W

d N V

Lapple correlated collection efficiency in terms of the cut size dpc.

Particles > dpc will have a collection efficiency greater than 50%

< dpc will be collected with lesser efficiency

The cut size is given by


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Primary operating costs are due to pressure drop.

The pressure drop ma be estimated as

Where K is a constant and ranges from 7.5-18.5

De, W and H are Cyclone dimensions

Vg is inlet gas velocity

2

22

g g

e

K v HWP

D

M lti l C l (M lti l )


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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 2m


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Particulate Control - Filtration

Filtration


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Filtration

Principle:

increase by retaining particles smaller than filter openings

Characteristics:

commonly used for small-particle collection

typical of 98 to 99.9%

filters cleaned of particulate cake when pressure drop throughfilter exceeds preset value

pressure drop can be predicted by knowing filter and cakecharacteristics

typical arrangements of multiple socks in a compartment --baghouse


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Air Filtration

BAGHOUSES(Fabric Filters)


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BAGHOUSES(Fabric Filters)

The particles are trapped in Fabric Filters due thefollowing mechanisms:

Inertial Impaction

Direct Interception(Sieving)

Diffusion(Agglomeration)

Electrostatic Filtration

I ti l I ti


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Inertial Impaction

It occurs when the particle with high inertial follows a fluid

streamline.

Theoretical investigations based on potential flow theory and

experimental results reveal that

the collection efficiency

( , Re )impact f f

Inertial Impaction

Wh i St k i ti l i ti t d fi d b


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Where is Stokes or inertial impaction parameter defined by

Where C is Cunningham correction factor and the magnitude is

given by Davies:

In which is the mean free path of the gas molecules.

For Std. Air = 0.066m.The Reynolds number Ref is given by

2

18

p g p po

g f

C d v

d

0.5521 1.257 0.4pd

p

C ed

Reo f g

f

g

v d

Di t I t ti (Si i )


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The particles have less inertia and almost follow the streamlines

around the obstruction. The particles clear the obstacle but their outer peripheries come

in contact with the fibre.

The particle will touch the fibre and will be intercepted


The collection efficiency by interception of a cylindrical target may


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The collection efficiency by interception of a cylindrical target maybe calculated: (if potential flow is assumed)

For a viscous flow situation, Strauss recommends

The combined mechanisms of impaction and interception usually

account more than 99.9% of the collection of the particles


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Particularly for the sub-micron range 0.001 to 0.05 m.

These particles usually do not follow the gas streamlines

surrounding the fibre because of individual motion.

This zigzag random Brownian motion causes the particles to

impinge and adhere to the surface fibre.


The collection efficiency by diffusion is by ( Torgeson):


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The collection efficiency by diffusion is by ( Torgeson):

Where CDfis drag coefficient of the fibre and

Peclet number is defined as

Where Sc is the Schmidt number

And Diff is particle diffusivity and is given by

Where k = Boltzmann constant(=1.38X10-23 J/K)

0.4

e 0.6R

0.775

2

Df f

D

CPe

Re . o g f g o f fg g iff iff

v d v d Pe ScD D

. .3

iff

g p

k T CDd


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In practice the collection mechanisms of impaction, interception

and diffusion are not independent and hence, all three

mechanisms must be combined.

The combined efficiency of collection is given by

int1 (1 )(1 )(1 )f impact er D



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The electrostatic forces between particles and fibres increase the

collection efficiency.

The generation of electrostatic charges in filter fabrics may be due

to friction between

gas and fabrics

particles and fabric at high gas velocities(1.5-2.0m/s).

The increase in on account of electrostatic forces becomes greater

with increase in the strength of the electric charge.



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Fabric Filters

Several fabric types- primarily chosen bychemical

thermal and

mechanical resistance

Cleaning methods-

Mechanical shaking,

reverse air flow,

pulse jet cleaning

P f P t


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Performance Parameters

The operating pressure drop across the bags is

described by:Pressure drop = dP = SeV + KCV

2t

where Se

= drag coefficient

V = velocity

K = filter cake coefficient

C = inlet dust concentration

t = Collection running time


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Sizing of Bag Filters

Based on recommended air to cloth ratios

ratio

Q

A CA

ae

nc/

,

Multiply net cloth area by factor to get gross cloth area

A/C Ratios Recommended For Cleaning Method (ft min)


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Dust or

fume

Shaker Reverse

Air

Pulse-jet

Abrasives 2-3 * 9

Alumina 2.25-3

Paintpigments

2

Machining 3 16

Metalfumes 1.5 1.5-1.8 6-9

Fly Ash 2 2.1-2.3 9-10

Chrome 1.5-2.5 9-12

A/C Ratios Recommended For Cleaning Method (ft.min)


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Anc Factor to get GrossCloth Area

1-4,000 2

4,001-12,000 1.512,001-24,000 1.25

24,001-36,000 1.17

36,001-48,000 1.125

48,001-60,000 1.11


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Fabric Filters

Temperatures up to 285 oC.

Least efficient for 0.1-0.3 micron range.

High removal efficiency for < 5 m particles

Highly sensitive to moisture content.


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Baghouse maximize the filtration area by configuring the

fabric filter media into a series of long small-diameter

fabric tubes referred to as bags.

Baghouse are tightly packed into a housing wherein the

dust laden air moves across the bag fabric therebyremoving it from the gas stream and building up a

filter cake which further enhances air cleaning.

The filter cake is removed to hoppers by various shaking

means.

Fabric filter Baghouse


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Fabric-filter Baghouse


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Particulate Control

Electrostatic Precipitators (ESPs)


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Driving force: gravity and electrical force

Principle: increase by imparting an additional electrical

force on the particles

Characteristics:

complex electrical systems

generally for dpof < 1to 10s of microns

laminar or turbulent flow



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A strong electric field is established.

This creates a corona and ions

Ions attach themselves to particulate material chargingthem

Charge saturation is reached, function of particle area

If the localized field on the charged particle is strongenough, it is deflected towards a collector electrode,here it is captured


ELECTROSTATIC PRECIPITATORS


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This utilizes gaseous ions to charge particles which are then

moved through an electric field to be deposited onto chargedcollection plates.

Collected particulate material is then removed by rapping or

washing of the plates.

small pressure drops and lower energy costs to move the gas

stream.

High collection efficiencies are possible, but efficiency may

drastically change with changes in operating parameters.


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ESP Variables


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ESP Variables

Electric field strength; based on applied voltage and distance

between collecting plate and electrode.

Maximum voltage with minimum sparking

Cleaning frequency and intensity;

particles accumulating need to be removed; either wet or dry.For dry operation, control re-entrainment

ESP


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ESP

Particulate resistivity important variable; Very low resistive

particles are good conductors and will lose charge.Very highly resistant particles will accumulate and insulatecollector plate.Preferred resistivity 108 to 1010 ohm-cm

Least efficient for 0.2-0.5 micron

Sensitive to moisture content. Temp up to 1000 F.

Not for organic materials due to fire hazard.

To produce the free ions and electric field, high internalvoltages are required.


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Performance & Efficiency Parameters


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Performance & Efficiency ParametersThe Collection Efficiency of ESP is obtain by Deutsch

Also

For a cylindrical type collectors Ac/V = 4/Dc

a parallel-plate type collectors Ac/V = 2/S

Where

Ac = collecting electrode area; Q = volumetric gas flow rate

vpm = particle migration/drift velocity; V = volume of ESP

s = distance between two parallel plates

pm cv A

Qe

pm cv A L

v Ve



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Collection Efficiency

where Ac = collecting electrode areaQ = volumetric gas flow ratevpm = particle migration/drift velocity

and

Drift velocity (vpm) = Eo Ep rpC()

Where Eo = charging fieldEp = collecting fieldrp = particle radiusC = proportionality constant = gas viscosity

pm cv A

Qe

Drift velocity values


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Application Drift velocity (m/s)

Pulverized coal fly ash 0.10 - 0.13Paper mills 0.08

Open hearth furnaces 0.06

Portland cement -wet manufacturing

-dry manufacturing

0.1-0.11

0.06-0.07

Gypsum 016-0.20

Sulphuric Acid mist 0.06-0.08

Catalyst dust 0.08Hot Phosphorus 0.03

Secondary Blast Furnace 0.12

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