Environmental
Chemistry
Acid rain
APCH211
Dr PG Ndungu
Acid Rain
“
Acidic Deposition
“Acid rain" – General term applied to any form of wet
precipitation, usually in the troposphere, with acidic species
stronger than CO2
±
Includes rain, sleet, snow, fog, or dew
“
±
±
Wet deposition
“
Acid rain is particularly damaging to lakes, streams, and forests
and the plants and animals that live in these ecosystems.
Nitrogen species:
“
NOx
Sources include burning of
fossil fuels, biomass, etc
Sulfur Species
“
SO2
“
H2S, & CS2:
±
Natural Sources; Anorexic
waters,
t
soils,
il etc
t
NH3
±
If the acidic chemicals in the air are
blown into areas where the weather
is wet, the acids can fall to the
ground in the form of rain, snow,
fog,
g or mist.
“
In areas where the weather is dry, the
acid chemicals may become
incorporated into dust or smoke and
fall to the ground through dry
deposition sticking to the ground,
buildings, homes, cars, and trees.
Later, when moisture content
i
increases,
acid
id solution
l ti iis produced.
d
d
Aqueous and dry deposition are collectively termed acid deposition
The precursors or chemical forerunners of acid rain formation
result from both natural sources, such as volcanoes and decaying
vegetation and man-made sources, primarily emissions of sulfur
dioxide (SO2) and nitrogen oxides (NOx) resulting from fossil fuel
combustion.
±
“
Can have deposition of dry gases and compounds – so called dry
deposition
“
±
refers to acidic rain, fog and snow.
Natural pH of rain water ~ 5.6 (from dissolved CO2)
Main Culprits in Acidic Precipitation
“
“
Dry deposition
Animal excreta, fertilizers and
microbiological release
±
“
Fossil fuels & sulfur ore smelting
The
principle
reaction
sequence
contributing to production of nitric acid
starts with nitric oxide, NO from
combustion processes.
Nitric oxide Chemistry: Daytime
‰
NO is oxidized by O2, O3 or ROO – e.g.
‰
This NO2 radical can then contribute to
ozone and
OH radical production i.e.
plays a role in smog formation
NO + O3 Æ NO2 + O2
Wetlands and submerged soils
Dimethylsulfide, (CH3)2S, carbonyl
sulfide, COS, methyl mercaptan,
CH3SH and Dimethyl disulfide,
CH3SSCH3.
±
Atmospheric Production Of Nitric Acid
Ocean and soils.
‰
Short lifetime, thus smog events don’t last,
& not that frequent
‰
Main removal sequence for NO is via
catalyst (M), OH rxns
NO2 + OH + M Æ HNO3 + M
1
Nitric acid Production at Night
™
Key species is the nitrate radical
(NO3).
™
Formed via O3 & NO2
™
Atmospheric
production
of
dinitrogen pentoxide (N2O5) occurs
when NO3 reacts with NO2 is the
only way to form in the atmosphere.
atmosphere
™
N2O5 is a store of NO3.
™
™
Reactions of NO3 with hydrocarbons
Easily removes an H from alkanes
o
NO3 + RH Æ R + HNO3
o
The R radical can then react with O2 to form peroxyl radicals
With alkenes the NO3 radical reacts via an addition mechanism producing nitro-oxy
substituted organic radicals which can regenerate NO2, or relatively stable organic nitrate
compounds (see - Paul S. Monks. Gas-phase radical chemistry in the troposphere. Chem.
Soc. Rev., 2005, 34, 376-395, for e.g. with propene)
o
NO3 + CnH2n Æ CnH2nNO3
o
With aldehydes, typically form nitric acid and the corresponding radical
o
Overall nighttime chemistry of NO3 can recycle NOx, or form HNO3, depending on the mix
of hydrocarbons
Can decompose back to NO3
and NO2
NO3 + RCHO Æ RCO + HNO3
Can react with water to form
nitric acid
Can be a nighttime source of OH radicals
o
Easily zapped by the
rising sun!
(λ ~ 600 -700nm)
Removal of Nitric Acid
Atmospheric Production of Sulfuric Acid
Oxidation of reduced sulfur species
•
¾
Removal is accomplished by either wet or dry
deposition
¾
One of the main contributors to acid precipitation.
¾
Nitric acid can react with ammonia:
¾
The ammonium nitrate, NH4NO3 can act as a
condensation nucleus for the formation of a water
droplet or it can be deposited as part of the solid
aerosol
•
NH3 + HNO3 Æ NH4NO3
Production of sulfuric acid is
more complex than that of nitric
acid as the starting materials
cover a wide range of reduced
sulfur and partially oxidized
sulfur compounds.
These include hydrogen sulfide,
carbon
disulfide,
carbonyl
sulfide,
methyl
mercaptan,
di th l disulfide,
dimethyl
di lfid and
d dimethyl
di th l
sulfide. All these compounds
contain sulfur in its oxidation
state (-2).
•
Mostly from natural sources
S
H
Sequence of Reactions of Sulfur Cmpds
•
Once sulfur compounds are
in the air, a sequence of
reaction begin as follows:
SH + O3 Æ SHO + O2
CS2 + OH Æ COS + SH
SHO + O2 Æ SO + HOO
COS + OH Æ CO2 + SH
The SO radical can then react
with either O2, O3 or NO2 to give
SO2 and other products.
™
™
The above reactions release
thionyl radical, SH as the initial
product.
Importance of OH!
Hydrogen sulfide and carbon
disulfide unlike carbonyl sulfide,
are very reactive and therefore
are quickly consumed
2SO + O2 Æ 2SO2
SO + O3 Æ SO2 + O2
SO + NO2 Æ SO2 + NO
NB.
SO2 is ultimately converted to sulfuric
acid, H2SO4.
S
C
S
O
C
S
More About SOx…
•
Dimethyl
sulfide
is
produced
by
phytoplankton living in surface waters of the
ocean. It is oxidized by hydroxyl radical (OH)
with a final product being sulfuric acid.
•
Sulfur dioxide SO2 is also released in large
quantities directly into the atmosphere from
sulfide ore smelting and fossil-fuel
combustion.
SH + O2 Æ SO + OH
H2S + OH Æ H2O + SH
NB:
™
Further oxidation of thionyl
radical
eventually produces
sulfur dioxide:
H
Where is the OH
coming from?
2
Peer to Peer Assignment: Mount
Pinatubo
Reducing
Acid Rain
There are several ways to reduce acid rain
(i.e. acid deposition). These range from
government policy to societal changes and
individual action.
e.g. given by epa
(http://www.epa.gov/acidrain/reducing/):
Understand acid deposition
deposition’ss causes and
effects
•
•
Clean up smokestacks and exhaust pipes
•
Use alternative energy sources
•
Restore a damaged environment
•
Look to the future
•
Take action as individuals
Recycle, Reuse, Reduce or the 3 R’s of
waste management has evolved from the
initial concepts championed in the 1970’s
& now includes, amongst other things,
prevention & minimization
The steps involved in reduction of acid
deposition are:
Steps to solve acid deposition
problem
Reducing Acid rain
Understand acid deposition’s causes and effects
‰
understand acid deposition’s causes and effects, and to track changes in the environment.
‰
Scientist Æ collect air, water & soil samples and measure them for various characteristics such as pH
and chemical composition, and investigate the effects of acid deposition on human-made materials.
‰
Scientists Æ understand the effects of sulfur dioxide (SO2) and nitrogen oxides (NOx), and any other
acid causing species
‰
People to understand the process of how acid rain damages the environment (Need to educate Policy
Makers!).
‰
People to find out what changes could be made to the air pollution sources that cause the problem
(Need to educate Policy Makers!).
WHY?
Almost all of the electricity that powers
modern life comes from burning fossil
fuels such as coal (Over 80% in RSA),
RSA)
natural gas, and oil. Sulfur dioxide (SO2)
and nitrogen oxides (NOx) are the main
acid chemicals.
•
•
Use alternative energy sources
•
Other sources of electricity besides
fossil fuels. They include nuclear
power, hydropower, wind energy,
tidal, geothermal energy, and solar
energy.
•
Alternatives to internal combustion
engines
•
batteries, solar cells, and fuel
cells
Options for reducing SO2 emissions,
include: using coal containing
less
sulfur, washing the coal, and using
devices
called
“scrubbers”
to
chemically remove the SO2 from the
gases leaving the smokestack.
Power plants to change type of fuels
e.g, burning natural gas creates much
less SO2 than burning coal.
Steps to solve acid deposition…
Steps to solve acid deposition…
b)
Clean up smokestacks and exhaust
pipes
a)
Restore a damaged environment
c)
NB!
It takes many years for ecosystems to recover from acid deposition, even
after emissions are reduced and the rain pH is restored to normal.
There are some things that people can do to bring back lakes and streams
more quickly.
i kl
•
Limestone or lime (a naturally occurring basic compound) can be added
to acidic lakes to “cancel out” the acidity. Liming, has been used
extensively in Norway and Sweden.
NB: All sources of energy have
environmental costs as well as
benefits.
3
Steps to solve acid deposition…
•
e)
Take action as individuals
Yes! You too can make a difference!
9
Turn off lights, computers, and other appliances when you're not using them
(Unplug chargers when not in use!).
9
Use energy-efficient appliances for lighting, air conditioners, heaters, refrigerators,
washing machines, etc.
If the depositions are reduced,
reduced environmental
protection agency (EPA) scientists must assess the
reductions to make sure they are achieving the
anticipated results.
9
Use public transportation,
transportation or better yet,
yet walk or bicycle whenever possible
9
Buy vehicles with low NOx emissions, and properly maintain your vehicle (In the
News, Mercedes, VW, etc, complaining RSA fuel quality has too much sulfur for
latest technologies).
If no changes, to consider additional ways to reduce
emissions that cause acid deposition. Example:
focus on energy efficiency and alternative energy.
9
6. Be well informed.
Evaluation of the progress made on acid rain
reduction process
d)
Monitoring Very Important!
•
•
Steps to solve acid deposition…
WHAT ABOUT TALKING TO GOVERNMENT
What Policies, Programs, or commitment in general has RSA made (PEER TO PEER
ASSIGNMENT)?
The Chemistry of Urban & Indoor Atmospheres
THE CHEMISTRY OF
URBAN AND
INDOOR
ATMOSPHERES
Urban & Indoor Atmospheres…
“
Use of petroleum products especially
in motor vehicles result in groundlevel emissions of carbon monoxides,
volatile hydrocarbons, nitrogen oxides
and sometimes, lead compounds.
These emissions are accompanied by
aldehydes and other secondary
pollutants.
“
The combustion of biomass and coal
produces substantial concentrations
p
of solid particulate matter along with
nitrogen oxides, polyaromatic
hydrocarbon (PAHs) compounds as
well as sulphur dioxide.
“
Open burning refuse or garbage
cause air pollution is a source of
volatile organic carbon compounds
and solid particulate matter (SPM).
“
Hurricanes and wind are the source
of particulate matter such as dust
especially in dry areas.
™
The chemical composition of air in places where people
live & work (urban areas, homes, offices, etc) vary with
modernization or industrialization of the locality.
™
Urban areas are likely to be affected by atmospheric
pollution due to the following major factors:
™
Combustion of fossil fuels (mostly cars)
™
In-space heating and cooling
™
Power generation and industrialization
™
Incineration of waste materials
Pollutants In The Urban Atmosphere
World Health Organization (WHO) Standards for Air Quality
“
The WHO guidelines for air quality take into account time period over
which measurements is done. This is known as human exposure.
“
Potential toxicity depend on both atmospheric concentration and
duration of contact with the atmosphere. That is;
Exposure = Concentration x time
4
Quality guidelines
“
“
“
“
The quality guidelines must specify the acceptable
concentration to be exposed to humans over a specified
period.
Example: carbon monoxide at 20mg.m-3 (20ppbv) may
be acceptable if exposure time is 1 hour but not
acceptable
t bl ffor llonger period
i d times.
ti
For longer exposure such as 8hrs, the allowed
concentration of carbon dioxide should not exceed
10mg.m-3, that is, 0.01ppmv or 10ppbv.
WHO works closely with United Nations Environmental
Program (UNEP) to carry out air quality monitoring.
Table 1 is a summary of WHO guideline values for air
quality-values in µg.m-3 or parts per trillion in volume (pptv).
Pollutant
Max.
time Average
weighted
time
(µg/. m-3)
SO2
500
10 min
CO
30,000
1 hr
NO2
400
1 hr
O3
150-200
1 hr
SPM (black smoke) 100-150
24hr
SPM=Suspended
particulate matter
TSP=Total suspended
particulate
RSP; PM10
150-230
24hr
RSP = Respirable
suspended particulate,
PM10 with particle size
Pb
70
24hr
< 10µm
0.5-1
1yr
TSP
Updated values (2005) can be found at:
http://www.who.int/mediacentre/factsheets/fs313/en/index.html
You
Need it!
Air quality parameters
Suspended Particles Matter (SPM)
•Concentration of atmospheric particulates is
severe in some megacities (cities > 10million
population) and average levels may range from
200 to 600 µg.m-3 or pptv.
“
Carbon Monoxide (CO): depend on high traffic density – vary from city
to city.
“
Sulphur dioxide (SO2): Usually produced by coal. Sulfur dioxide conc. Is
low in cities that use low coal fuels
“
Nitrogen dioxide (NO2): Higher levels expected indoors with poor
ventilation where kerosene or natural gas used for heating and cooking.
“
Ozone (O3): from reaction of gases in the troposphere; trace amounts may
result from mass transfer from stratosphere.
“
Lead (Pb): Airborne lead depends on the population of cars, the
concentration of lead additives in the fuel and availability of unleaded fuel.
Concentrations in leaded gasoline vary between 0.1 and 2.0 g.L-1.
•Human
health associated with high values depend on the nature of
particulates.
particulates
Examples: those derived from coal and those in the PM10 or PM2.5 categories,
have been shown to be hazardous.
NB: PM10 is particulate matter size < 10µm ;
PM2.5 < 2.5µm.
NB. Use of tetraethyl lead to augment the octane number (?) is becoming less.
More info from:
http://www.bbc.co.uk/dna/h2g2/A16407173
You
Need it!
Major factors that determine the quality of indoor air
INDOOR AIR QUALITY
•
Many people spend most of their
time indoors (home, office, etc.).
The atmospheres encountered
indoor vary a great deal.
•
The materials of house construction
may vary from clay-rich soils or
other fresh or baked earth materials.
•
In some cases
cases, the homes are open
and air exchange is rapid while in
others heating may be done over an
open fire in a room without a
chimney and also a variety of fuels
may be used.
•
The building materials may range
from bricks, stones, wood, various
plastics and metals.
•
Activities in the homes include:
cleaning, cooking, heating over
open or closed fires with varying
smoke conditions.
•
•
•
•
•
•
The nature of the ambient air, outdoor around the building plays a
role. In this case, the outdoor atmosphere is influenced by air
outside.
Design and site of the building is important. This will dictate the
quality of exchange of indoor atmosphere.
Nature of materials present in the building such as polymers. The
latter could be a source of formaldehydes or other partially
oxidized organic compounds.
Building materials from clays,
clays concrete,
concrete etc.,
etc may contain traces of
radioactive elements such as uranium.
Activities that take place inside the house. These may include
combustion of wood for heating, cooking gas, electric cookers, etc.
Cleaning of the house may involve mechanical devices such as
vacuum cleaner, that create dust.
Use of cleaning solvents and detergents, insecticide sprays, toilet
sprays and air fresheners.
Another reference you will need:
Indoor Air Quality Guidelines: selected pollutants(PDF)
http://www.who.int/indoorair/publications/en/index.html
5
COMMON INDOOR AIR CONTAMINANTS
Radioactivity
Air contaminants refer to levels above the outdoor background level.
1. Radioactive compounds:
•
Radioactivity is usually associated with Radon noble gas, Rn released by
Uranium isotope 238 and also by Thorium isotope 232 with half-lifes of 4.5
and 14 billion years, respectively. These elements are found in geological
materials such as rocks and fossils
Radioactive elements
“
Radioactive elements, such as uranium (239U) thorium (234Th) and
potassium (40K) break down (decay) fairly readily to form lighter atoms e.g
Be, B.
ƒ
ƒ
ƒ
The energy that is released in the process is made up of small, fast-moving
particles and high-energy waves.
These particles and waves are, of course, invisible. (The level of radioactivity of an
element varies according to how stable its atoms are).
Other elements with naturally occurring radioactive forms, (isotopes) are carbon
(C13), bismuth (210Bi), radon (223R) and strontium (88Sr).
Example: Calculation of HalfHalf- Life of
Radioactive Elelemnts
“
•
Consider strontium-90 which has a half-life of
approximately 28 years.
Initially, at time t=0, the sample is 100% strontium-90
•
After 28 years, only half the original amount of
strontium will remain: ½ x 100% = 50%
•
After another 28 years, only half of this amount of
strontium-90 will remain: ½ x 50% = 25%
•
After another 28 years, only half of this amount of
strontium will remain: ½ x 25% = 12.5%
•
and so on.
•
At any given time, the amount of strontium-90 that has
undergone decay can be calculated:
amount of strontium-90 decayed = the original amount the amount remaining.
“
The spontaneous emission of particles and/or
energy from atomic nuclei.
“
The spontaneous emission of radiation from the
nucleus of an atom. Radionuclides lose particles
and energy through this process of radioactive
decay.
Conti… Radioactivity process
ƒ
Radioactivity is a random process that happens
naturally as the isotopes in particular elements decay.
The isotopes continue to break down over time.
ƒ
The length of time that is taken for half of the nuclei in
an element to decay is called its 'half-life'.
ƒ
A half
half-life
life can be very short (milliseconds to hours) or
very long (hundreds of thousands of years).
ƒ
Radiation also arises from nuclear fission.
ƒ
Fission can be spontaneous but is usually initiated in a
nuclear reactor. Fission is a radioactive process; it
releases energy as the heavy nucleus is split into two.
Calculations
“
“
The amount of radioactive isotope remaining
can be calculated:
Nt = No x (0.5)number of half-lives
Nt = amount of radioisotope remaining
No = original amount of radioisotope
number of half-lives = time ÷ half-life
Example
p
Calculate the percentage of strontium-90
remaining after 280 years.
Nt = No x (0.5)number of half-lives
Nt = ? %
No = 100%
number of half-lives = time ÷ half-life = 280
÷ 28 =10
Nt = 100 x (0.5)10 = 0.098%
6
Strontium-90 half-lifes
Strontium-90 half-lifes
%
%
Number
StrontiumTime Strontiumof Half90 that
(years)
90
lives
has
remaining
decayed
0
0
100
0
1
28
50
50
2
56
25
75
3
84
12.5
87.5
4
112
6.25
93.75
5
140
3.125
96.875
6
168
1.5625
98.4375
2. Volatile Organic Compounds (VOCs)
3. Polybrominated diphenyl ether
Sources are:
Polybrominated diphenyl ether (PBDE) is toxic. General
structure is shown below. PBDE is used in commercial
household products such as plastics casings for appliances,
in fabrics used for clothing, carpets, etc.
•
Paints: toluene, ethylbenzene, 2-isopropanol and butanone.
•
Cleaning agents: households solvents, detergents.
•
Wood-building materials such as plywood produce formaldehyde.
Chemical structure of PBDEs
Conti…air contaminants
4. Emissions from indoor combustion.
5. Indoor particulates
These include: solid aerosols from dust;
combustion of coal & biomass material.
This is combustion of fuel that contains VOCs;
burning of coal, wood and biomass.
•
Tobacco smoking is a source of many VOCs
iincluding
l di aldehydes,
ld h d ketones,
k t
organic
i bases
b
such
h
as nicotine, organic acids.
Particle size is usually in the range PM10
(<10µm). Particle size <2µm, can easily enter
p
y track.
respiratory
•
Smoking contributes to respirable particulate
matter inside a building.
•
Polyaromatic hydrocarbons
emitted from coal & biomass.
ƒ
ƒ
(PAHs)
are
7
Particles in the atmosphere
•
Particulate is a term that has come to stand for particles in the
atmosphere.
•
Particulate matter makes up the most visible and obvious form of
air pollution.
•
Particles in the atmosphere range from 0.5 mm (size of sand)
down to molecular size level (nanometer).
•
Particles may consist of either solids or liquid droplets.
•
Atmospheric aerosols are solid or liquid particles smaller than 100
µm in diameter.
•
Pollutant particles in the 1 nm to 10 µm range are commonly
suspended in the air near sources of pollution such as the urban
atmosphere, industrial plants, highways and power plants.
Nature of particles
•
•
D ESC R IPT IO N
PA R T IC L ES
Very small solid particles include (1 nm-10 µm ): carbon
black, silver iodide, combustion nuclei, sea-salt nuclei- tend
to be acidic.
Larger particles include (100 µm -500 µm ) : cement dust,
wind blown soil dust, foundry dust and pulverized coal- tend
to be basic.
•
Li id particles-mist,
Liquid
i l
i include
i l d raindrops,
i d
fog
f andd sulfuric
lf i acid
id
mixture
•
Particles of biological origin: viruses, bacteria, bacterial
spores, fungal spores and pollen.
•
Important atmospheric contaminants- mainly inorganic and
organic particles.
Effects of atmospheric particles…
OF
A TM O SPH E R IC
T erm s
1. A erosol
M eaning
Colloidal-sized
atm ospheric
particle
2. C ondensation Form ed by condensation of
aerosol
vapors or reactions of gases.
3. D ispersion
Form ed by grinding of solids,
aerosoll
atom
t
ization
i ti
off liquids
li id
or
dispersion of dusts.
4. Fog
D enotes high level of w ater
droplets
5. H aze
D enotes decreased visibility
due to presence of particles
6. M ists
Liquid particles
7. Sm oke
Particles
form ed
by
incom plete com bustion of fuel
Effects of atmospheric particles
•
Effects on climate
•
Damage buildings
•
Impact on human health (people with asthma)
•
Reduced visibility & causes undesirable aesthetic effects
NB: Aerosols, natural and anthropogenic, can affect the climate by changing
the way
a radiation is transmitted through
thro gh the atmosphere.
atmosphere
Physical behaviour of particles
“
All aerosols both absorb and scatter solar and terrestrial radiation.
•
“
If a substance absorbs a significant amount of radiation, as well as
scattering, it is called absorbing.
Small colloidal particles undergo diffusion processes and coagulate
together to form larger particles.
•
“
This is quantified as the ratio of scattering alone to scattering plus
absorption (extinction) of radiation by a particle.
Mechanism for removal of particles from the atmosphere is mainly through
sedimentation & scavenging by rain drops and then precipitation.
•
Particle size refers to diameter of the particle but in some cases radius may
b used.
be
d
8
Composition of Inorganic Particles
Process for particle formation
•
Physical Process: particle formation is mainly through disintegration of
larger particles > 1 µm
•
Many dispersion aerosols originate from natural sources: sea-spray,
windblown dust, volcanic dust.
•
Chemical process: Inorganic particles are mainly metal oxides formed by
oxidation of the metal by oxygen.
•
•
Aluminium oxide, iron oxide, calcium oxide and silicon dioxide are due to soil
erosion, rock dust, coal combustion.
•
Carbon particles- due to incomplete combustion
•
Sodium and chlorine compounds- due to marine aerosols
•
Antimony and selenium- due to combustion of oil, coal or refuse.
•
Lead from combustion of leaded fuels & wastes
Organic particles are produced mainly through internal combustion
engines.
N.B.
Recall: PHOTOCHEMICAL SMOG
Composition of Organic Particles
A wide variety of organic compounds most of which are
toxic: polycyclic aromatic hydrocarbons (PAHs) such as
benzo(a)pyrene, chrysene, benzo-fluoranthene, acridine.
Control of Particulate Emissions
•
Removal of particulate matter from gas streams is the most practiced means
of air pollution control.
•
Techniques for removal depends on particle size, loading, nature of
particles and type of scrubbing system.
Radioactive particles
•
Main source of radionuclides in atmosphere is randon: it is
a noble gas produced from radium decay.
•
Cosmic rays in the atmosphere produce radionuclides
which are isotopes of: 7Be , 10Be, 14C, 39Cl, 3H, 22Na, 32P
and 33P
Methods of particle removal
Air Pollution Control for Particulate Emissions
These include:
•
Sedimentation and inertia, i.e gravitational settling as a continuous process.
•
Particle filtration using fabric filters that allow gas molecules to pass
through but retain the particulate matter.
•
Scrubbers- this involves use of scrubbing liquid which forms small droplets
g gp
particles from the g
gas stream.
for scavenging
“
It is possible to minimize emissions of aerosol particles
from point of source such as thermal electrical
generating stations or industrial smelting units.
“
Containment of particulate matter is achieved using
devises that remove the aerosols from fast moving
stack gas stream. Common collection methods include:
settling chambers, cyclones, fabric filter, scrubbers,
and electrostatic precipitators as shown in the slides
that follow.
9
Settling Chambers
“
Are the simplest and commonly used.
“
Advantages
±
Types of Settling Chambers
Simple to build, low cost, low maintenance, low pressure drop,
simple to dispose of collected materials
“
Disadvantages
“
Construction includes variety of baffles and open space designed
to allow the particles sufficient time to settle under the force of
gravity.
“
Settling rates are limited by gravity therefore method effective for
large particle size >10µm.
“
They come in different design. The mechanism include adsorption
and absorption.
±
“
Simple expansion chamber
“
Multiple-tray settling chamber.
Limited to removal of particles larger than40-60 μm diameter
Supp Slide 1
Combustion
Momentum Separators
“
These differ slightly from your typical settling chambers via the
addition of simple chamber features that allow for directional
changes in air flow that add a downward inertial force to
supplement the gravitational force.
Supp Slide 2
Fabric Filters: Filtration
“
Fabric filter or bags operate in a similar principle as
vacuum cleaner.
“
The air stream is made to pass thro a porous fabric
material and is effective for particulates size in the
range 0.01 - 10µm range.
“
Bags or fabric filters are sensitive to temperature and
humidity. The fine particles clog the filters and there4
must be periodically cleaned.
Advantages
Disadvantages
High Collection Efficiency (>99%)
Effective for a Wide Range of Dust Types
Modules Can be Factory Assembled
Operates Over Wide Range of Gas Flow
Rates
• Reasonably Low Pressure Drop
• Good Efficiency for Small Particles
• Dry Collection and Disposal
•
•
•
•
•
•
•
•
•
Filtration
Large Footprint
Temperature Limitations
Requires Dry Environment
Fire or Explosion Potential
High Maintenance Cost
10
Electrostatic Precipitator
“
Electrostatic precipitator causes the particles in a gas stream to become
charged by electrons produced thro an electrical discharge between two
electrodes.
“
The negatively charged particles then migrate to the positive electrode and
are collected and removed from the emission stream. Positively charged
particles move to negative electrode.
Advantages
Disadvantages
High Collection Efficiency
Dry Collection and Disposal
Small Pressure Drop
Capable of Handling Large Gas Flow
Rates
• Low Electrical Power Requirements
• Low Maintenance
• Disadvantages
• High Capital Cost
• Particle Resistivity Limitations
• May Require Injection of SO3 or
NH3 to Control Resistivity
• Relatively Large Footprint
• Special Precautions for Safe
Operating at High Voltage
•
•
•
•
Absorption Method
Scrubbers
“
Scrubbers allow gas stream to be in contact with a
fine mist or spray of water.
“
The water droplets capture many small particles and
these settle more rapidly into a collector container.
“
Scrubbers come in different designs as shown below.
Advantages
Disadvantages
• High Collection Efficiency
• Capable of Handling Flammable and
Explosive Dusts
• Can Handle Mists
• Low Maintenance
• Simple Design and Easy to Install
• Provides Cooling for Hot Gases
• Neutralizes Corrosive Gases and Dusts
• Waste Water Must be Treated
• Collected Particulates are in
Sludge Form
• High Corrosion Potential
• High Pressure Drop
• May Require Protection Against
Freezing
• Final Exhaust Must be
Reheated
• Sludge Disposal May be
Expensive
Adsorption
Electrostatic Precipitator
Liquid Scrubber
11
Cyclones
“
“
“
Cyclones are cone-shaped devices that cause the waste
gas stream to swirl rapidly in spiral fashion causing
larger particles to move towards the wall of the cone by
centrifugal force.
Stokes Law
νt =
( ρ p - ρ a ) C g d 2p
18η
Where
Once in contact with the wall, the particles slide down
the inner surface of the cone to a collection container
below it.
vt = terminal velocity of particles in m.s-1;
Stoke’s law determines the extent of removal of
particles but the settling rates can be greatly enhanced
by the increased force due to cyclone action. In this case
removal of particles <10µm can be achieved.
C = Stokes correction factor for assuming spherical shape and
discontinous of fluid interactions when the particle size is small
compared with the molecular mean path in air.
pp = density
d
it off particle
ti l in
i g.cm-33
Pa = density of air = 1.2 x 103 g.m-3 at Po and 25° C;
g = acceleration due to gravity = 9.8 m.s-2
dp = particle diameter in meters and
= viscosity of air = 1.9 x 10-2 g.m-1.s-1 at P° and 25° C
Cyclone
Combustion
Minimize Emission from point
Source: example SO2
EXAMPLE: Sulfur Dioxide Control
“
Minimize emissions of aerosol particles from point
of source such as thermal electrical generating
stations or industrial smelting units.
http://www.apt.lanl.gov/projects/cctc/factsheets/puair/adflugasdemo.html
12
Clean Coal Technologies
“
Advanced Flue Gas Desulfurization
Demonstration Project - e.g. of a series of
initiatives
“
Others Include
±
Carbon Capture and storage
±
Underground coal gasification
Why Clean Coal?
Coal Usage
“
“
“
CO2 Sequestration
In RSA Coal used to
generate over 85% of
th electricity
the
l t i it
Globally Coal accounts
for more than 40% of
Electricity produced
Impact of Coal
“
“
Mining
±
Water scarcity
±
Water Pollution
Burning
±
Only realistic
technology for next 20
– 50 years
SOx, NOx,
particulates,
CO2, fly ash
Underground Coal Gasification (UCG)
Historically a lot of the work was done in
the former USSR
Resurgence in interest (China, Australia,
Europe, Americas & RSA
•
•
•
•
Clean and treat the CO2 then store it
Currently used in enhanced oil recovery
Options to use saline (very salty) aquifers
Abandoned coal mines, other geologic caverns etc
SASOL
Majuba
http://www.eskom.co.za/live/content.php?Item_ID=14077
13