Transcript Slide 1

Environmental Physics
Chapter 8:
Air Pollution
Copyright © 2012 by DBS
Cartoon
Introduction
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Power production and energy use produces considerable adverse environmental effects
Air pollution knows no boundaries, state or national
Effects can be seen far from the source
Introduction
Mega Cities
Source: UNEP/WHO, 1992
Eastern Europe
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Eastern Europe after the fall of Communism
Environmental concerns passed over in the name of progress
Power plants burning high sulfur coal wcontaiing arsenic and cadmium
Lack of air pollution controls on power plants
Carbon black (soo
metal smelting
Air pollution in Copsa Mica, Romania, dubbed the most
polluted city in the world.
Properties and Motion of the Atmosphere
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A
Permanent
Variable
Properties and Motion of the Atmosphere
Properties and Motion of the Atmosphere
Air Pollutant:
Substances added by humans that are toxic or irritant to animals, vegetation, or property
Examples?
Properties and Motion of the Atmosphere
Anthropogenic
Natural
Anthropogenic vs. Natural
Sources
Mt. Pinatubo,
Phillipines
June, 1991
A coal barge on the Monongahela
River moves past a U. S. Steel
Corporation coke plant at Clairton,
Pennsylvania, 1973
Wildfires across
Oklahoma
and Texas
Jan 2, 2006
Properties and Motion of the
Atmosphere
Pressure
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Thin gaseous envelope – how thin?
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99 % of the mass of the atmosphere is below 33 km (20 miles)
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We experience pressure,
P=f/A
Where P = pressure (psi, N/m2 = Pa), f = force (N)
and A = area (m2)
30 km
6400 km
Question
Which exerts more pressure, a 4000 lb elephant standing on one leg with a foot size of
8" x 8" or a 120 lb woman standing on one leg in high-healed shoes with a heal size of
1"x1"?
P = f/A
4000/64 = 62.5 psi
120/1 = 120 psi
so the woman exerts more pressure
Question
What is the pressure exerted by your finger if you hold back water leaking through a
hole in a dike, as shown below?
Weight density of water Is 62 lb/ft2
P = 62 lb/ft3 x 1 ft = 62 lb
ft2 x(144 in2/ft2 )
= 0.43 psi
Properties and Motion of the Atmosphere
Pressure
Mean sea level pressure of air
= 15 lb/sq in
= 30 in. Hg (inches of mercury)
= 1000 mb (millibars)
= 100,000 Pa (SI system)
Properties and Motion of the Atmosphere
Figure 8.1: Examples of atmospheric pressure. (a) The maximum weight you can lift with a 4-inch
diameter suction cup is area × pressure = π (2)2 in2 × 14.7 lb/in2 = 184 lb. (b) Turning a glass of water
(covered with a piece of paper) upside down forces some of the air out of the glass. The pressure of the
remaining air plus the weight of the water is less than atmospheric pressure, so the water remains in the
glass. (c) Sipping soda: atmospheric pressure forces the drink up the straw to the region of lower
pressure in your mouth.
Demonstrations
Explain the following results
A: Hard-boiled egg sucked into a
flask
B: Can Crushing
Properties and Motion of the Atmosphere
Mercury barometer
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("bar" o "meter" - an instrument that measures "bars")
– (invented by Toricelli, 1643)
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Height of mercury in column is proportional to air
pressure
Corrected for changes in temperature
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1 bar = 1000 mb = 760 Torr = 76 cm Hg = 30” Hg
Properties and Motion of the Atmosphere
Buoyant Force and Air Temperature Profiles
A solid object will float if its density is equal or less than the density of the medium it is in
Archimedes principle states that the
buoyant force on an object is equal to the
weight of the fluid displaced by the object
Figure 8.3: The buoyant force on a submerged object is equal to the weight of the displaced fluid. This
is a result of the difference in pressure between the top and the bottom of the object. An object will float
if the buoyant force is equal to or greater than the object’s weight.
Properties and Motion of the Atmosphere
Buoyant Force and Air Temperature Profiles
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Air warmed by the ground rises due to buoyancy
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Bouyant foce = upward force exerted on a parcel of air
As warm air rises it
expands and cools
Properties and Motion of the Atmosphere
Buoyant Force and Air Temperature Profiles
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Energy used for expansion of rising air is taken from the thermal energy of the air, air cools
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Air parcel will rise until its temperature equals that of the surrounding air
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If the warm-air parcel cools less rapidly than the surrounding air, its temperature will always be
above the temperature of the surrounding air
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Mixing occurs
Figure 8.4: The temperature of ambient air normally decreases with increasing altitude. A parcel of air
will rise until its temperature is equal to that of the surrounding air. An elevated inversion layer (a region
in which the temperature of the ambient air increases with altitude) will put a lid on the rising air parcels.
Properties and Motion of the Atmosphere
Natural Dispersion of Air Pollutants
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Vertical motion (convection) and the winds are
important dispersal mechanisms
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Mixing is restricted when there is an inversion
Troposphere usually
well mixed (warm air
rises, replaced by
cooler air)
Cool air trapped below
warm air prevents
mixing
Inversion increases
with calm winds
Properties and Motion of the Atmosphere
Natural Dispersion of Air Pollutants
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Vertical motion (convection) and the winds are important dispersal mechanisms
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Mixing is restricted when there is an inversion
Figure 8.5: View of Houston 15 miles from city center.
Photochemical Smog
Properties and Motion of the Atmosphere
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Temperature differences are also responsible
for global air circulation
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Pollutants can travel
long distances before settling out
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Emissions from one country affect another
e.g. US pollution from Midwest power plants
goes to Canada
Figure 8.6: Earth’s wind engine. The rotation of the earth produces a complex wind pattern, as the cold
polar air sinks toward the equator and the warm tropical air rises and moves toward the poles.
End
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Air Pollutants and their Sources
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Gases, particulates (solids) and aerosols (liquid suspend in a gas)
CO, SO2, PM, HC’s, NOx
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Levels of pollutants in a particular area depend on type, amount emitted, method of release,
meteorological conditions
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In general two sources: stationary or mobile
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We are affected by these primary pollutants and also by the products of chemical reactions that
these pollutants undergo in the atmosphere
Air Pollutants and their Sources
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US EPA reports air quality data for 5 principal pollutants
Every year more
than 180 million
tons emitted in US,
3.3 lb per person
per day
(~amount of trash
we generate)
1970-2003 US population increased by 39 %, vehicle
miles increased
by 155
%, total
emissions
of Particulates
the 5
Figure 8.7: Emissions of major air pollutants
in the United
States
by source:
2003.
principal
pollutants decreased by 50 %
refer to those of size less than 10 microns
(PM-10).
Air Pollutants and their Sources
Removal mechanisms:
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Sinks or scavenging mechanisms
– Absorption by vegetation, ground, and water
– Oxidation and conversion to precipitates
Measuring concentration
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No. molecules of a pollutant in one million molecules of air (N2 and O2)
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Expressed as a mixing ratio - Parts per million (v/v) (ppm)
e.g. 10 ppm = air containing one million molecules has 10 molecules of a pollutant
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Also mass per unit volume (μg / m3)
Conversion (at SATP 25 ºC and 1 atm):
concentration (ppm) = concentration (mg m-3) x 24.45
Molar mass
Mixing ratio is conserved if temperature or pressure changes
Air Pollutants and their Sources
Carbon Monoxide (CO)
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Colorless, odorless, poisonous gas produced by incomplete combustion of carbon in gasoline:
C + ½ O2 → CO
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Binds with hemoglobin, prevents oxygenation of blood
Average exposure 10-30 ppm, heavy traffic 50 – 100 ppm – causes dizziness, headaches and
visual aberrations
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Levels
– Air quality standard (AQS) for CO is 9 ppm (10,000 μg/m3) for 8 hours
– No physiological effects are known to appear at outdoor AQS levels
– OSHA limits long-term workplace exposure to 50 ppm
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Total amount of CO emitted is more than all other pollutants combined, only a health hazard in
heavy traffic
Air Pollutants and their Sources
Sulfur Oxides (SO2/SO3)
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Colorless gas with a suffocating odor at concentrations > 3 ppm
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Produced by burning fossil fuels:
S + O2 → SO2
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Coal contains as much as 6% sulfur by weight, contributes most of the 16 million tons per year
Also natural sources (sea spray) which add about 2x as much sulfur
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Damages respiratory tract, lung tissue
Effects are pronounced in young, old and those with existing respiratory ailments
Air Pollutants and their Sources
London Smog 1952 / Donora Smog 1948
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Over 4,000 deaths were attributable to
the Great London Smog of December
1952
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Donora Death Fog of October 1948
Pollutants from Steel mill, zinc smelter,
sulfuric acid plant
Results: 7,000 inhabitants fell ill, 20 died
Smog = smoke + fog
Air Pollutants and their Sources
Sulfur Oxides
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Further oxidizes to SO3 which combines
with water to form sulfuric acid
2SO2 + O2 → 2SO3
SO3 + H2O → H2SO4
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Acid rain!
Figure 8.8: Effects of sulfur dioxide pollution on health. The figure shows ranges of concentrations and
exposure times in which (a) the number of deaths reported was above expectation (light gray), (b)
significant health effects have occurred (dark gray), and (c) health effects are suspected (middle gray).
Air Pollutants and their Sources
Particulates
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Natural sources: soil, volcanic ash, pollen,
sea salt spray
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Man-made sources: combustion products,
fly ash
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Source determination:
– PM diameter > 1 μm
associated with natural source
– PM diameter < 1 μm
usually from combustion
Air Pollutants and their Sources
Mineral dust from weathering
of rocks and soils
Coarse
Ultra-fine
e.g NH4Cl,
SO42- / NO3- salts
Fine
Particulates
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Affect breathing, aggravate existing cardiovascular disease, possibly damage immune system
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Fine particles remain suspended in air
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PM-10 (diameter < 10 microns) and PM-2.5 reach lower respiratory tract
Natural: forest fires, volcanoes etc.
Man-made: fossil-fuel combustion, industry
Air Pollutants and their
Sources
Particulates
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Absorb trace metals + toxic organics
Air Pollutants and their Sources
Air Pollution and the 3rd world
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For most developing countries, industrial development is the priority (not environment!)
Total particulates
(TSP) and SOx are
much higher than
industrialized nations
due to fewer emission
controls
NOx concentrations
are comparable
Nature of air pollutants in the developing world.
Air Pollutants and their
Sources
Hydrocarbons and VOC’s
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Also known as volatile organic compounds (VOC’s)
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Natural - methane from wetlands, ruminants, rice paddies, landfills, emissions of isoprene and
terpenes from tress account for 85 % of all HC’s in air
Man-made sources – gasoline, natural gas etc. are far more reactive
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For this reason often divided into methane and non-methane hydrocarbons
Air Pollutants and their Sources
Nitrogen Oxides, Photochemical Smog and Ozone
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Nitrogen oxides family: NO, NO2, N2O, N2O5
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Produced by oxidation of N2 in air
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Role in the formation of photochemical smog
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Health effects: eye irritation, reduced visibility and respiratory ailments
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Combination of many different gaseous and particulate pollutants, dominated by ozone
NO2 + Light energy → NO + O
O + O2 → O3
O3 + NO → NO2 + O2
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Equilibrium
HC’s react with O, NO and NO2 to form reactive organic radicals e.g. PAN (peroxyacetyl nitrates)
Produce more NO2 and prevent destruction of O3
Air Pollutants and their Sources
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Ozone production is stimulated by sunlight and warm temperatures (summer problem)
Many US urban areas do not meet 8-hour O3 standard of 0.080 ppm
Pollution varies with time of day
Figure 8.9:
Variation of NOx
and ozone
concentrations with
time of day in the
Los Angeles basin.
Air Pollutants and their Sources
Acid Rain
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Rain falling through atmosphere polluted by SOx / NOx produces sulfuric + nitric acids
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pH scale is a measure of hydrogen ion concentration (H+)
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Each pH unit represents a factor of 10 change in solutions acidity
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Normal rainwater is weakly acidic (5.7) due to presence of dissolved CO2 as carbonic acid
Acid Rain
Figure 8.10: Acidity is expressed by pH, which is a logarithmic number. A 1-unit change in pH represents a
change in acidity by a factor of 10.
Air Pollutants and their Sources
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Primarily effects Eastern US, Canada and N. Europe
Rainfall with pH 4.0-4.5 is common in NE and SE states
NADP map
Figure 8.11: Change in annual average pH of
precipitation in the eastern United States between 1955
and end of twentieth century.
Air Pollutants and their Sources
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Effect on crops
Leaches nutrients e.g. Ca/Mg/Al from soil, bleaches chlorophyll from leaves
Effects of acid rain on a forest in Europe.
Air Pollutants and their Sources
Change in distribution of pH for Adriondack Lakes between 1930s and 1970s
All fish die at pH < 5.0
Lakes most at risk have hard insoluble bedrock, thin surface soils with low buffering capacity
Naturally alkaline soils are resistant to acidification
In acidified lakes the number of
fish and amphibians is
declining due to reactions of
aluminum ions with proteins in
the gills of fish and the
embryo's of frogs
No. lakes
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High aluminum concentrations
do not only cause effects upon
fish, but also upon birds and
other animals that consume
contaminated fish
Figure 8.12: Change in pH and fish population for 200 Adirondack lakes (in
New York) above 600 meters altitude between the 1930s and 1970s.
Acid Deposition
Results from technological fix of one problem
(local air pollution)
What was the fix?
Air Pollutants and their Sources
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Taller smokestacks emit pollutants higher into the atmosphere
Westerly winds transport pollutants to Canada and NE US from mid-west utilities
Solutions: burning lower sulfur fuels, emissions control technology
Air Pollutants and their Sources
• Toxic metals leaching into water supplies
• Damage to structures (limestone, marble)
• Damage to vehicles (rusting)
• Decreases visibility
• Decreases productivity of fisheries,
forests and farms
Cartoon
Air Pollutants and their Sources
Indoor air pollution usually is a greater threat to human health than outdoor air pollution.
Why?
Indoor environments often concentrate chemical and biological contaminants
e.g. weather proofing / reducing home heating and cooling costs
We spend 90% of our time indoors
Air Pollutants and their Sources
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Indoor Air Pollution
Air Pollutants and their Sources
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According to the American
Medical Association 50%
of all illness is caused or
aggravated by polluted
indoor air
Air Pollutants and their Sources
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According to the EPA, the most dangerous indoor air pollutants in developed countries are:
– Tobacco smoke and fine particulates
– Formaldehyde
– Radioactive radon-222 gas
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The only one of these that has a recommended indoor level is radon
Radon decay products build up
in confined space –are breathed
in, stick to surface of airways
and emit α-particles
Health Effects
Radon Gas
Highly energetic α, β particles rip
through tissue causing cellular
and genetic damage
Air Pollutants and their Sources
Upper
estimate
25,000
15,000
10,000
Lower estimate
Deaths per year
20,000
5,000
0
Drunk
Driving
Radon
Drownings
Fires/Burns
Air
Transportation
Source: U.S. EPA’s Home Buyer’s and Seller’s Guide (Radon: National Academy of
Sciences, Non-radon: National Safety Council)
End
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Air Quality Standards
Signs of progress?
Air Quality Standards
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London and Donora were turning points
Death is no longer the sole factor to consider
Changes in life expectancy and quality of life are now key factors
Air Quality Standards
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Clean Air Act (CAA) –
begun in 1955, followed by 1963 and
1967
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Amended in 1970
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National air quality standards (NAAQS)
for six pollutants linked to public health
effects
SOx, NOx, PM, O3, CO and lead
Nonattainment 2007
PPG 05-01-2008
Air Quality Standards
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1990 CAA amendments dealt with:
– Acid rain – proposals to cut sulfur by installing emission control tech (scrubbers)
– Smog – reduction using alternative fuels (ethanol, methanol, natural gas, electricity)
– air toxics
– Introduced evaporative emission controls
– Mandated new gas formulations for summer months
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California enacted its own plan for ultra-low emission vehicles (ULEV) and eventually zero
emission vehicles (ZEV’s)
Air Quality Standards
Lead recently
changed by
factor of 10
TSP revised in
1987 and 1997
Primary – protect human health
Secondary – protects welfare
Air Quality Standards
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Overall, emissions from all sources have been in decline
50 % decrease since 1970, despite 200 % increase in miles travelled
EPA: Latest Findings on National Air
Quality – Status of Trends through
2006
Air Quality Standards
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Megacities – total population > 10 million
Figure 8.13: Comparison of ambient levels of average annual total suspended particulate (TSP)
matter, sulfur dioxide, and nitrogen dioxide concentrations among selected cities, expressed in μg/m3.
Automobile Emission Control
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Carbon monoxide control
– reduced by using excess air (O2 sensor) to complete combustion
Nitrogen oxides control
– EGR – exhaust gas recirculation, lowers peak combustion temperature
Hydrocarbon control
– from fuel evaporation and unburnt fuel)
– captured by charcoal filter, engine re-design to eliminate ‘blow-by’ gases
Catalytic converter
– Further reduces pollutants
– Precious metals
– HC and CO oxidized, NOx reduced
Figure 8.14: Automobile emission controls.
Automobile Emission Control
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Emission standards required by CAA Amendments
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In additional to emission controls for gasoline, decreased emissions via:
– Switch to diesel
– Gas turbine
– Hybrid
– Electric
– Mass transit
Stationary Source Air Pollution Control Systems
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Fossil fuel plants produce majority SO2 and PM
General philosophies for meeting air quality:
1. use low sulfur fuel
2. remove sulfur prior to combustion
3. remove particulates and PM after combustion
4. shift fuels or power output in response to air quality
5. dilute and disperse gases through taller stacks
Stationary Source Air Pollution Control Systems
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Use of very tall chimney stacks
Wind is higher at altitude
Dilute and disperse pollutants
Figure 8.19: Chimneys are some of the tallest structures built by humans.
Stationary Source Air Pollution Control Systems
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After combustion control devices
Particulates are removed via settling chambers, cyclone
or inertial chambers, electrostatic precipitators, filters and scrubbers
Settling chambers - larger particles settle out
Cyclone collector – further settling of heavier particles,
removes particles > 50 microns
Figure 8.16: Inertial or cyclone collector. As the gas undergoes
circular motion, the heavier particles collide with the collector’s
walls and fall to the bottom, where they are collected for disposal.
Stationary Source Air Pollution Control Systems
Electrostatic precipitator – metal wires and plates under large negative charge. Electric field
ionizes the gas molecules which stick to the positive plate
Can remove ~99 % of particles, not good for < 1 micron
Bag filter – cloth or fiberglass, 99.9 % efficient
down to 0.1 microns
Figure 8.17: A before-and-after sequence showing the effect of an electrostatic precipitator on stack gas
emissions from a coal-fired power plant.
Stationary Source Air Pollution Control Systems
Since SO2 is a gas we need different technology for its removal…
Scrubbers – gases pass through a water spray
Flue-gas desulfurization – slurries of lime or limestone or dolomite react with SO2 to form
calcium and magnesium sulfate
Lime:
SO2 + CaO → CaSO3
2CaSO3 + O2 → 2CaSO4
Limestone:
2SO2 + 2CaCO3 + O2 → 2CaSO4 + 2CO2
Removes 98% of sulfur dioxide
Figure 8.20: Schematic of a typical fossil-fuel plant, showing equipment used to
remove pollutants from the flue gas or boiler exhaust.
Fig. 8-20, p. 278
Stationary Source Air Pollution Control Systems
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Reduce amount of SO2 formed before combustion
– Use low sulfur coal
– Remove sulfur from fuel prior to combustion
– Convert coal to synthetic oil or gas
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Fluidized bed combustion (FBC) removes SO2 by
burning crushed coal on a moving bed of air and sand
to which limestone is added
Figure 8.21: A fluidized bed combustion (FBC) unit. When air is forced up from below, the bed of ash
becomes fluidized (that is, the solids “flow” like a liquid). Crushed solid fuel burning in the bed heats the
ash. Limestone added to the fluidized mixture reduces the amount of SOx emitted in the flue gases.
Summary
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Major air pollutants – SO2, PM, NOx, HC’s, O3 and CO
Photochemical smog (primarily O3) is secondary in origin and formed by the photochemical
reaction of NOx and HC’s
Pollution control devices have reduced mobile and stationary pollution
About half of currently operating coal-fired plants were built pre-1975 and have no sulfur controls
Debate continues over the economic and human health benefits of air pollution control