TPE ME326 Environmental Aspects1

7/30/2019 TPE ME326 Environmental Aspects1

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Environmental Aspects of Power

Generation

Various kinds of fuel may be burned inthermal power plants, all of which releaseemissions into the air.

Some fuels that are well known includefossil fuels:

Coal Natural gas, Oil

Methane (produced by biomass) andwood 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 certainsubstances are present in such concentrationsthat 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 largelyby fuel combustion.

It is oxidized to NO2 in a pollutant atmospherethrough photochemical secondary reaction.

NO2 is a brown pungent gas with an irritatingodor.

NO2 is emitted by fuel combustion and nitric acid

plants

Small concentrations of NO2 are detected in thelower 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 ofcarbon-based fuels including petrol, diesel,wood, natural and synthetic products.

It has affinity towards the hemoglobin of thebloodstream and is a dangerous asphyxiant.

It can slow our reflexes and make us confused andsleepy.

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 aresult of human activities such as the burning ofcoal, 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.


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In saturated class, CH4 is by far the most abundant

hydrocarbon constituting about 40-80% of total

HCs present in an urban atmosphere.

The unsaturated class includes Alkenes(Olefins)

and Acetylenes.

Among the alkenes the prominent pollutants are

Ethylene and propane.

The HCs in air by themselves alone cause no

harmful effects.

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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AIR POLLUTION CONTROL

Particulate Emission ControlParticle 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


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Particulate Control Mechanisms1. Gravitational Settling

2. Centrifugal Impaction3. Inertial Impaction

4. Direct interception

5. Diffusion

6. Electrostatic Precipitation

Particulate Control Equipment1. Gravitational Settling Chambers

2. Cyclone Separators3. Fabric Filters

4. Electrostatic Precipitator

5. Wet Collectors(Scrubbers)


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Gravitational Settling Chambers Used to remove large abrasive particles(> 50) from gas steams

Offer low pressure drop and require low maintenance

Collection efficiency can be improved by inserting several trays

Used as pre-cleaners

The minimum particle size that can be removed from the gas with

100% efficiecy is:

Where QVolumetric flow rate of gas steam

nNo. of trays of L X W X H

p and g densities of particle and gas respectively

min

8

( )p

p g

Qd

nWLg


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CYCLONES

Cyclones operate to collect relatively large sizeparticulate matter from a gaseous streamthrough the use of centrifugal forces.

Dust laden gas is made to rotate in a decreasingdiameter pathway forcing solids to the outeredge of the gas stream for deposition into the

bottom of the cyclone.

Efficiencies of 90% in particle sizes of 10

microns or greater are possible.


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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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Performance & Collection Efficiency

Linear increases with: particle density, gas

stream velocity, and rotational passes

Linear decrease with fluid viscosity

Exponential increase with particle diameter


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Limitations / Advantages / Problems

Reduces internal access needs

Optimal flow rate difficult to adjust

Prone to internal erosion / corrosion

Operation at elevated temperatures possible

Low efficiency for small diameter material

Hopper recirculation / flow distribution problems

High energy costs for volumetric flow requirements

Dew point agglomeration, bridging, and plugging Few moving parts, few mechanical / electrical ignition

sources


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BAGHOUSES(Fabric Filters)

Baghouses utilize the principles to remove solids

from a gaseous exhaust stream are:Sieving(Direct Interception)

Impaction,

Agglomeration(Diffusion) andElectrostatic Filtration

Baghouses maximize the filtration area byconfiguring the fabric filter media into a seriesof long small-diameter fabric tubes referred toas bags.


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Baghouse are tightly packed into a housing wherein the

dust laden air moves across the bag fabric thereby

removing 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 shakingmeans.


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Fabric-filter Baghouse


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Performance & Efficiency Parameters

The operating pressure drop across the bags is

described by:

Pressure drop

dP = SeV + KCV2t

where Se = drag coefficient

V = velocity

K = filter cake coefficient

C = inlet dust concentration

t = Collection running time


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High collection efficiencies

Internal condensation / corrosion

Over-temperature limitations

Need for internal inspection / access

Possible to have variable flow rates Plugging / short-circuiting / break-through/ collection

media fouling

Accumulation of flammable gases/ dusts and ignitionsources

Unexpected bag failure due to changes in operatingparameters


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ELECTROSTATIC PRECIPITATORS(ESP)

This utilizes gaseous ions to charge particles which are

then moved through an electric field to be depositedonto charged collection plates.

ESP consists of two set of electrodes:

1. Collection Electrodes

rows of electrically grounded vertical parallel plates

2. Discharge Electrodes

wire electrodes centrally located.

The wires carry a unidirectional, negatively charged,

high voltage(20-100kV) current from an external

source.


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The applied high voltage generates a unidirectional,

non-uniform electric field whose magnitude is

greatest near the discharge electrode.

When that voltage is high enough, a blue luminous

glow, called a corona, is produced around them.

The corona is an indication of the generation of

negatively charged gas ions.

The gas close to the negative electrode is ionized upon

passing through the corona.


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The charged gas ions travel from the wires to the

grounded collection electrodes as result of the

strong electric field between them.

The electrostatic field then draws the particles to the

collector surface where they are deposited.

Collected particulate material is then removed by

rapping or washing of the plates.

Usually, a large number of these collectors are placed

in parallel in a single housing.


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The internal gas passages within a precipitator are

relatively open with small pressure drops and lower

energy costs to move the gas stream.

Collection Efficiency

High collection efficiencies are possible, but efficiencymay drastically change with changes in operating

parameters.

The efficiency is a function of gas flow rate and

precipitator size is given by Deutsch equation.


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Collection Efficiency(Deutsch equation)

= 1 - eVm.A/Q

where A = collecting electrode area

Q = volumetric gas flow rateVm = particle migration velocity

andDrift/migration velocity =

Where E = Applied voltage(V)

s = Distance between electrodes(m)d = Particle diameter(m)

g = Gas viscosity(kg/m.s)

p = Particle dielectric constant(1.52.4)

12 22.95 10 ( / )m

g

X p E s dV


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Large installation space required High efficiencies for small particles possible

Low pressure drops and air moving costs

High potential for ignition sources Re-entrainment, spark-over, back corona

problems

High temperature operation possible Susceptible to changes in moisture and

resistivity


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GASEOUS POLLUTANT CONTROL

ABSORPTION & WET SCRUBBING


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ABSORPTION & WET SCRUBBING

EQUIPMENT

The removal of gases and particulate matter by causing the

gaseous contamination to become dissolved into the

liquid stream and the solids to be entrained in the liquid.

The rate of gas transfer into the liquid is dependent upon

the solubility,

mass transfer mechanism, and

equilibrium concentration of the gas in solution.


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Gas collection efficiencies in the range of 99% are

possible.

The rate of particulate matter collection at constant

pressure drops is inversely proportional to

the aerodynamic mean dia. of the particulate matter andscrubber droplets.


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For gas collection, the maximum equilibrium concentration insolution is described by Henry's law:

[Cgas

] = (Hk) [C

liquid]

Where (Hk) is Henry's constant

[Cgas] is the concentration in the gas stream

[Cliquid] is the concentration in the liquid stream


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High pressure drops required

Internal plugging, corrosion, erosion

Increased need for internal inspection

Formation / precipitation of solids

Few internal moving parts

Reduced opportunity for gas ignition

Gas and liquid chemistry control important Increased relative velocity between scrubbing the

fluid and gas stream, increases efficiency for solids


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TPE ME326 Environmental Aspects1