14 - environmental impacts

8/19/2019 14 - Environmental Impacts

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Power Plants – ME 471

Faculty of Mechanical Engineering

GIK Institute Pakistan

Topic 14

Environmental Impacts – CH 17


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Power Plants, Faculty of Mechanical Engineering – GIK Institute Pakistan. 2/22

Sections

• 17 – 1 to 17 – 8 (excluding particulate matter distribution).

– [you are not expected to memorize various gas concentrations]

• 17 – 9

• 17 – 10 (only working principle)

•

17 – 11


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•Most of the worlds industry lies between 30 – 60

o

N latitude.

• The atmospheric mixing between hemispheres is slow, so the region remains

isolated containing a higher concentration of contaminants.


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What are the contaminants?

• UNNATURAL addition of materials, radiations and thermal

effects to the environment.

• Nature puts around 10 times the amount of contaminants in

to the environment as compared to man [1960].

SO WHAT ARE POLLUTANTS ???

• The contaminants which adversely affect something people

value (such as environment and health).

• Things which disturb the eco-system.


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Constituents of the atmosphere



NOTE: These emissions are produced by IC Engines but similar pollutants are

emitted by gas turbines and fossil-fueled powerplants.



Oxides of Sulfur

• Sulfur in atmosphere exists as:

– H2S

– Sulfates

– SO2

• H2S: comes from natural sources (natural gas called sour gas,

volcanoes). – It colorless, very toxic and has a foul (rotten egg) odor.

• Sulfates: Sea spray and oxidation of SO2.

• SO2 is an important concern, since it is artificially added to

the atmosphere.



SO2 (less than 25% of all the sulfur in atmosphere)

• Sources:

– Combustion of COAL and OIL mainly in powerplants but in steelmills, smelters etc.

– Coal is responsible for 70% of the total sulfurous emissions.

– SO2 has no ill effects below 0.6 ppm. – Adverse effects appear near 5 ppm.

– At 10 ppm sever irritation is caused.

– 1 hr exposure at 10 ppm can cause breathing difficulties and

removal mucus.



Oxides of Nitrogen

• Examples: NO, N2O, NO2, NO3 etc.

• Significant artificially added oxides of nitrogen are NO andNO2 termed as NOx.

• NO is formed in the combustion of all fossil fuels.

• Significant formation occurs at high combustion

temperatures.

• Primary contributor to NO are IC engines.

• NO rapidly oxides to NO2 which has more adverse effects.



NO2

• Concentrations higher than 0.4 ppm can be detected through

smell.• However a constant exposure to 0.06 to 0.1 ppm of NO2 can

cause respiratory illness.

• NO2 has an affinity for hemoglobin.

• Hemoglobin carries less oxygen if NO2 is present.

• It forms acid in the lungs.

• A few minutes exposure to 150 – 200 ppm can cause sever

damage to the lungs (bronchiols).• Average conc. in 30 – 60oN is 1 ppb.

• Local urban conc can be higher (0.02 – 0.9 ppm)



Oxides of Carbon

• Most widely produces of all contaminants are methane, CO

and CO2.

• Carbon monoxide (CO)

– Natural sources: mashes, mines, vegetation, lightening. But this

emission in very small.

– Artificial sources: 90% of CO emission is from motor vehicles.

– Less than 1% is produced by powerplants.



Carbon monoxide (CO)

• Concentrations and effects:

– Global average: 0.1 ppm. – Regional values may be high, e.g in LA, it is 7 – 11 ppm (1958-1963)

– Effects are like NO2. (deprivation from oxygen).

– 100 ppm (headaches)

– 500 ppm (collapse)

– 1000 ppm is fatal.

• Cigarette smoke may have as high as 42000 ppm CO.

• Exhaled smoke contains 200 – 300 ppm.



CO2

• Most of CO2 is emitted by powerplants.

• It is commonly considered a contaminant but not a pollutant.


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THE PROBLEM WITH CO2 IS GREEN HOUSE EFFECT


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Green house effect: (Electromagnetic spectrum)

Thermal radiations

Most energetic


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Greenhouse effect

• Sun at around 6000K emits radiation mostly in visible and

shortwaves (UV).• Some of the incoming radiations are reflected back.

• Around half of the radiation reach the surface of the earth

heating it up.

• Earth gives off this heat in all possible modes of heat transfer.

• CO2 and vapor only absorb and emit long waves (infrared).

• This increases the temperature of the earths surface and the

atmosphere in a phenomenon called “green house effect”.


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Contaminant removal

• SO2: precipitation and gravitation

• NOx: precipitation and vegitation

• CO: oxidation to CO2, precipitation, vegitation

• CO2: Photosynthesis, ocean absorption



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FLUE-GAS DESULFURIZATION (FGD)

• Wet Flue-Gas Desulfurization system:

• Wet FGD system is also called wet scrubber.

– SO2 is reacted with a slurry of CaO and CaCO3.

– The reaction mechanism is not well known.


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Wet - FGD

SLURRY

Reaction products

Flue gas from

particulate

absorbers

CaO and CaCO3

CaSO3.2H2O

CaSO4.2H2O

Additional Sulfites

and Sulfates are

formed

Some are recycled back


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Wet - FGD

2 2

2 2 3 2

2 2 3 2

3 2 2 3 2 2

3 2 2 4

3

2

2

+ H O Ca(OH)Ca(OH) + CO CaCO + H O

+ CO + H O Ca(HCO )

Ca(HCO ) + + H O CaSO 2H O + 2CO

1CaSO 2H O +

CaO

CaCO

O CaSO 2H O

O

2

S


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Wet FGD

• Advantages:

• High SO2 removal efficiency.• Remaining flue gas particulate matter is also removed.

• Disadvantage:

• Buildup of scale.


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Dry – FGD

• The lime water slurry is sprayed.

• As it interacts with hot flue gases, desulfurization occurs andthe slurry dries as particles.

• The reaction products are separated from the gas in fabric

filter.

• Advantages:

• Dry waste is easy to handle.

• Disadvantage:

•Low SO2 removal efficiency.

• The flue gases should be kept in safe temperature ranges to

avoid condensation.


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Single Alkali Scrubbing

• In order to avoid slurry plugging problems:

– An aqueous solution of Na2SO

3 can be used to absorb SO

2.

• Example of such a system:

– Welman-Lord SO2 recovery system.

2 2 3 2 3

3 2 3 2 2

Reaction in

SO + Na SO + H O 2NaHSO

Reaction

Absorber

Evaporator-Crystallizer in

2NaHSO Na SO + SO

+ H O


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Electrostatic PrecipitatorsUsed for the removal of particulate matter.


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Questions


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S


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An ESP

El i P i i (ESP)


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Electrostatic Precipitator (ESP)

14 - environmental impacts

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