1

Environmental Environmental ChemistryChemistryAPCH211APCH211

Dr PG NdunguDr PG Ndungu

Acid rainAcid rain

Acid RainAcid Rain

“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

Natural pH of rain water ~ 5.6 (from dissolved CO2)

Can have deposition of dry gases and compounds – so called dry deposition

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.

Acid rain is particularly damaging to lakes, streams, and forests and the plants and animals that live in these ecosystems.

Acidic DepositionAcidic DepositionWet deposition

refers to acidic rain, fog and snow.

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, or mist.

Dry deposition

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 id l ti i d d

gincreases, acid solution is produced.

Main Culprits in Acidic PrecipitationMain Culprits in Acidic Precipitation

Nitrogen species:

NOx

Sources include burning of fossil fuels, biomass, etc

Natural Sources; Anorexic t il t

Sulfur Species

SO2

Fossil fuels & sulfur ore smelting

H2S, & CS2:

Wetlands and submerged soilswaters, soils, etc

NH3

Animal excreta, fertilizers and microbiological release

Wetlands and submerged soils

Dimethylsulfide, (CH3)2S, carbonyl sulfide, COS, methyl mercaptan, CH3SH and Dimethyl disulfide, CH3SSCH3.

Ocean and soils.

Atmospheric Production Of Nitric AcidAtmospheric Production Of Nitric Acid

The principle reaction sequencecontributing to production of nitric acidstarts with nitric oxide, NO fromcombustion processes.

Nitric oxide Chemistry: Daytime

NO is oxidized by O2, O3 or ROO – e.g.

NO + O3 NO2 + O23

This NO2 radical can then contribute toozone and OH radical production i.e.plays a role in smog formation

Short lifetime, thus smog events don’t last,& not that frequent

Main removal sequence for NO is viacatalyst (M), OH rxns

NO2 + OH + M HNO3 + M

2

Nitric acid Production at NightNitric acid Production at Night

Key species is the nitrate radical(NO3).

Formed via O3 & NO2

Atmospheric production ofdinitrogen pentoxide (N2O5) occurswhen NO3 reacts with NO2 is theonly way to form in the atmosphereonly way to form in the atmosphere.

N2O5 is a store of NO3.

Can decompose back to NO3and NO2

Can react with water to formnitric acid

Easily zapped by the rising sun!

(λ ~ 600 -700nm)

Reactions of NOReactions of NO33 with hydrocarbonswith hydrocarbonso Easily removes an H from alkanes

NO3 + RH R + HNO3

o The R radical can then react with O2 to form peroxyl radicals

o With alkenes the NO3 radical reacts via an addition mechanism producing nitro-oxysubstituted organic radicals which can regenerate NO2, or relatively stable organic nitratecompounds (see - Paul S. Monks. Gas-phase radical chemistry in the troposphere. Chem.Soc. Rev., 2005, 34, 376-395, for e.g. with propene)

NO3 + CnH2n CnH2nNO3

o With aldehydes, typically form nitric acid and the corresponding radical

NO3 + RCHO RCO + HNO3

o Overall nighttime chemistry of NO3 can recycle NOx, or form HNO3, depending on the mixof hydrocarbons

o Can be a nighttime source of OH radicals

Removal of Nitric AcidRemoval of Nitric Acid

Removal is accomplished by either wet or dry deposition

One of the main contributors to acid precipitation.

Nitric acid can react with ammonia:

NH3 + HNO3 NH4NO3

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

Atmospheric Production of Sulfuric AcidAtmospheric Production of Sulfuric AcidOxidation of reduced sulfur species

• Production of sulfuric acid ismore complex than that of nitricacid as the starting materialscover a wide range of reducedsulfur and partially oxidizedsulfur compounds.

• These include hydrogen sulfide,carbon disulfide, carbonylsulfide, methyl mercaptan,di th l di lfid d di th ldimethyl disulfide, and dimethylsulfide. All these compoundscontain sulfur in its oxidationstate (-2).

• Mostly from natural sources

SSS C CO

S

HH

Sequence of Reactions of Sulfur Sequence of Reactions of Sulfur CmpdsCmpds

• Once sulfur compounds arein the air, a sequence ofreaction begin as follows:

H2S + OH H2O + SH

CS2 + OH COS + SH

COS + OH CO2 + SH

Further oxidation of thionylradical eventually producessulfur dioxide:SH + O2 SO + OH

SH + O3 SHO + O2

SHO + O2 SO + HOO

The SO radical can then reactNB:

The above reactions releasethionyl radical, SH as the initialproduct.

Importance of OH!

Hydrogen sulfide and carbondisulfide unlike carbonyl sulfide,are very reactive and thereforeare quickly consumed

with either O2, O3 or NO2 to giveSO2 and other products.2SO + O2 2SO2

SO + O3 SO2 + O2

SO + NO2 SO2 + NONB.SO2 is ultimately converted to sulfuricacid, H2SO4.

Where is the OH coming from?

More About More About SOSOxx……

• Dimethyl sulfide is produced byphytoplankton living in surface waters of theocean. 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.

3

Peer to Peer Assignment: Mount Peer to Peer Assignment: Mount PinatuboPinatubo

Reducing Reducing Acid RainAcid RainThere 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’s causes and • Understand acid deposition s 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

The steps involved in reduction of acid deposition are:

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

Reducing Acid rainReducing Acid rainUnderstand acid deposition’s causes and effects

understand acid deposition’s causes and effects, and to track changes in the environment.

Scientist 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 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 People to find out what changes could be made to the air pollution sources that cause the problem (Need to educate Policy Makers!).

Steps to solve acid deposition Steps to solve acid deposition problemproblem

a) Clean up smokestacks and exhaust pipes

WHY?Almost all of the electricity that powers modern life comes from burning fossil fuels such as coal (Over 80% in RSA) fuels such as coal (Over 80% in RSA), natural gas, and oil. Sulfur dioxide (SO2) and nitrogen oxides (NOx) are the main acid chemicals.

• Options for reducing SO2 emissions,include: using coal containing lesssulfur, washing the coal, and usingdevices called “scrubbers” tochemically remove the SO2 from thegases leaving the smokestack.

• Power plants to change type of fuelse.g, burning natural gas creates muchless SO2 than burning coal.

Steps to solve acid deposition…Steps to solve acid deposition…b) Use alternative energy sources

• Other sources of electricity besidesfossil fuels. They include nuclearpower, hydropower, wind energy,tidal, geothermal energy, and solarenergy.

• Alternatives to internal combustion engines

• batteries, solar cells, and fuel cells

NB: All sources of energy haveenvironmental costs as well asbenefits.

Steps to solve acid deposition…Steps to solve acid deposition…

c) Restore a damaged environment

NB!

It takes many years for ecosystems to recover from acid deposition, evenafter emissions are reduced and the rain pH is restored to normal.

There are some things that people can do to bring back lakes and streamsi klmore quickly.

• Limestone or lime (a naturally occurring basic compound) can be addedto acidic lakes to “cancel out” the acidity. Liming, has been usedextensively in Norway and Sweden.

4

Steps to solve acid deposition…Steps to solve acid deposition…

d) Evaluation of the progress made on acid rain reduction process

• Monitoring Very Important!

If the depositions are reduced environmental • If the depositions are reduced, environmental protection agency (EPA) scientists must assess the reductions to make sure they are achieving the anticipated results.

• If no changes, to consider additional ways to reduce emissions that cause acid deposition. Example: focus on energy efficiency and alternative energy.

Steps to solve acid deposition…Steps to solve acid deposition…

e) Take action as individualsYes! You too can make a difference!

Turn off lights, computers, and other appliances when you're not using them(Unplug chargers when not in use!).

Use energy-efficient appliances for lighting, air conditioners, heaters, refrigerators,washing machines, etc.

Use public transportation or better yet walk or bicycle whenever possible Use public transportation, or better yet, walk or bicycle whenever possible

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).

6. Be well informed.

WHAT ABOUT TALKING TO GOVERNMENT

What Policies, Programs, or commitment in general has RSA made (PEER TO PEER ASSIGNMENT)?

THE CHEMISTRY OF THE CHEMISTRY OF URBAN AND URBAN AND INDOOR INDOOR ATMOSPHERESATMOSPHERES

The Chemistry of Urban & Indoor AtmospheresThe Chemistry of Urban & Indoor Atmospheres

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

Urban & Indoor Atmospheres…Urban & Indoor Atmospheres…Use of petroleum products especially in motor vehicles result in ground-level 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 pof solid particulate matter along with nitrogen oxides, polyaromatichydrocarbon (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.

Pollutants In The Urban AtmospherePollutants In The Urban AtmosphereWorld Health Organization (WHO) Standards for Air Quality

The WHO guidelines for air quality take into account time period overwhich measurements is done. This is known as human exposure.

Potential toxicity depend on both atmospheric concentration andduration of contact with the atmosphere. That is;

Exposure = Concentration x time

5

Quality guidelinesQuality 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

t bl f l i d tiacceptable for longer period times.

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).

SPM=Suspendedparticulate matter

TSP=Total suspendedparticulate

Pollutant Max. time

weighted

(µg/. m-3)

Average

time

SO2

CO

NO2

500

30,000

400

10 min

1 hr

1 hr particulate

RSP = Respirablesuspended particulate, PM10 with particle size < 10µm

O3

SPM (black smoke)

TSP

RSP; PM10

Pb

150-200

100-150

150-230

70

0.5-1

1 hr

24hr

24hr

24hr

1yr

Updated values (2005) can be found at: http://www.who.int/mediacentre/factsheets/fs313/en/index.html

You Need it!

Suspended Particles Matter (SPM)Suspended Particles Matter (SPM)•Concentration of atmospheric particulates issevere in some megacities (cities > 10millionpopulation) and average levels may range from200 to 600 µg.m-3 or pptv.•Human health associated with high values depend on the nature of particulatesparticulates.

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.

Air quality parametersAir quality parameters

Carbon Monoxide (CO): depend on high traffic density – vary from cityto city.

Sulphur dioxide (SO2): Usually produced by coal. Sulfur dioxide conc. Islow in cities that use low coal fuels

Nitrogen dioxide (NO2): Higher levels expected indoors with poorventilation where kerosene or natural gas used for heating and cooking.

Ozone (O3): from reaction of gases in the troposphere; trace amounts mayresult from mass transfer from stratosphere.

Lead (Pb): Airborne lead depends on the population of cars, theconcentration 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.

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!

INDOOR AIR QUALITYINDOOR 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 constructionmay vary from clay-rich soils or other fresh or baked earth materials.

• In some cases the homes are open• In some 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 fuelsmay 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.

Major factors that determine the quality of indoor airMajor factors that determine the quality of indoor air

• The nature of the ambient air, outdoor around the building plays arole. In this case, the outdoor atmosphere is influenced by airoutside.

• 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 concrete etc may contain traces of• Building materials from clays, concrete, 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

6

COMMON INDOOR AIR CONTAMINANTSCOMMON INDOOR AIR CONTAMINANTS

Air contaminants refer to levels above the outdoor background level.

1. Radioactive compounds:

• Radioactivity is usually associated with Radon noble gas, Rn released byUranium isotope 238 and also by Thorium isotope 232 with half-lifes of 4.5and 14 billion years, respectively. These elements are found in geologicalmaterials such as rocks and fossils

RadioactivityRadioactivity

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.

Radioactive elementsRadioactive elementsRadioactive elements, such as uranium (239U) thorium (234Th) and potassium (40K) break down (decay) fairly readily to form lighter atoms e.g Be, B.

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).

Conti… Radioactivity processConti… Radioactivity processRadioactivity is a random process that happens naturally as the isotopes in particular elements decay.The isotopes continue to break down over time.

The length of time that is taken for half of the nuclei in an element to decay is called its 'half-life'.

A half-life can be very short (milliseconds to hours) or A half life can be very short (milliseconds to hours) or very long (hundreds of thousands of years).

Radiation also arises from nuclear fission.

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.

Example: Calculation of HalfExample: Calculation of Half--Life of Life of Radioactive ElelemntsRadioactive 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% 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.

CalculationsCalculations

The amount of radioactive isotope remaining can be calculated:Nt = No x (0.5)number of half-lives

Nt = amount of radioisotope remainingNo = original amount of radioisotopenumber of half-lives = time ÷ half-life ExamplepCalculate 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 =10Nt = 100 x (0.5)10 = 0.098%

7

Number of Half-

lives

Time (years)

% Strontium-

90 remaining

% Strontium-

90 that has

decayed 0 0 100 0

Strontium-90 half-lifes

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

Strontium-90 half-lifes

2. Volatile Organic Compounds2. Volatile Organic Compounds (VOCs)(VOCs)Sources are:

• Paints: toluene, ethylbenzene, 2-isopropanol and butanone.

• Cleaning agents: households solvents, detergents.

• Wood-building materials such as plywood produce formaldehyde.

3. Polybrominated diphenyl ether3. Polybrominated diphenyl ether

Polybrominated diphenyl ether (PBDE) is toxic. Generalstructure is shown below. PBDE is used in commercialhousehold products such as plastics casings for appliances,in fabrics used for clothing, carpets, etc.

Chemical structure of PBDEs

Conti…air contaminantsConti…air contaminants

4. Emissions from indoor combustion.

This is combustion of fuel that contains VOCs;burning of coal, wood and biomass.

Tobacco smoking is a source of many VOCsi l di ld h d k t i b hincluding aldehydes, ketones, organic bases suchas nicotine, organic acids.

5. Indoor particulates5. Indoor particulates

These include: solid aerosols from dust;combustion of coal & biomass material.

• Particle size is usually in the range PM10(<10µm). Particle size <2µm, can easily enterrespiratory track.p y

• Smoking contributes to respirable particulatematter inside a building.

• Polyaromatic hydrocarbons (PAHs) areemitted from coal & biomass.

8

Particles in the atmosphereParticles in the atmosphere

• Particulate is a term that has come to stand for particles in theatmosphere.

• 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.

DESCRIPTION OF ATM OSPHERIC PARTICLES Terms M eaning 1. Aerosol Colloidal-sized atmospheric

particle 2. Condensation

aerosol Formed by condensation of vapors or reactions of gases.

3. Dispersion l

Formed by grinding of solids, t i ti f li idaerosol atomization of liquids or

dispersion of dusts. 4. Fog Denotes high level of water

droplets 5. Haze Denotes decreased visibility

due to presence of particles 6. M ists Liquid particles 7. Smoke Particles formed by

incomplete combustion of fuel

Nature of particlesNature of particles

• Very small solid particles include (1 nm-10 µm ): carbonblack, silver iodide, combustion nuclei, sea-salt nuclei- tendto 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 i l i i l d i d f d lf i id• Liquid particles-mist, include raindrops, fog and sulfuric acid mixture

• Particles of biological origin: viruses, bacteria, bacterial spores, fungal spores and pollen.

• Important atmospheric contaminants- mainly inorganic and organic particles.

Effects of atmospheric particlesEffects 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 a radiation is transmitted thro gh the atmospherethe way radiation is transmitted through the atmosphere.

Effects of atmospheric particles…Effects of atmospheric 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.

This is quantified as the ratio of scattering alone to scattering plus absorption (extinction) of radiation by a particle.

Physical behaviour of particlesPhysical behaviour of particles

• Small colloidal particles undergo diffusion processes and coagulatetogether to form larger particles.

• 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 dbe used.

9

Process for particle formationProcess for particle formation

• Physical Process: particle formation is mainly through disintegration oflarger 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.

• Organic particles are produced mainly through internal combustion engines.

N.B.

Recall: PHOTOCHEMICAL SMOG

Composition of Inorganic ParticlesComposition of Inorganic Particles

• Aluminium oxide, iron oxide, calcium oxide and silicon dioxide are due to soilerosion, 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

Composition of Organic ParticlesComposition 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.

Radioactive particles• Main source of radionuclides in atmosphere is randon: it is• 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, 32Pand 33P

Control of Particulate EmissionsControl 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.

Methods of particle removalMethods of particle removalThese 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 dropletsfor scavenging particles from the gas stream.g g p g

Air Pollution Control for Particulate EmissionsAir Pollution Control for Particulate Emissions

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 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.

10

Settling ChambersSettling ChambersAre the simplest and commonly used.

Advantages

Simple to build, low cost, low maintenance, low pressure drop, simple to dispose of collected materials

Disadvantages

Limited to removal of particles larger than40-60 μm diameter

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 adsorptionand absorption.

Types of Settling ChambersTypes of Settling Chambers

Simple expansion chamber

Multiple-tray settling chamber.

Supp Slide 1

Momentum SeparatorsMomentum 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

CombustionCombustion

Fabric Filters: FiltrationFabric Filters: FiltrationFabric 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

• Large Footprint• Temperature Limitations• Requires Dry Environment• Fire or Explosion Potential• High Maintenance Cost

FiltrationFiltration

11

Electrostatic PrecipitatorElectrostatic PrecipitatorElectrostatic 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

Electrostatic PrecipitatorElectrostatic Precipitator

ScrubbersScrubbersScrubbers 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

Absorption MethodAbsorption Method

AdsorptionAdsorption Liquid ScrubberLiquid Scrubber

12

CyclonesCyclones

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.

Once in contact with the wall, the particles slide down the inner surface of the cone to a collection containerthe inner surface of the cone to a collection containerbelow it.

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.

Stokes LawStokes Law

Where

vt = terminal velocity of particles in m.s-1;

d it f ti l i 3

ηρρ

ν18

d g C ) -(

2pp a

t =

pp = density of particle in g.cm-3

Pa = density of air = 1.2 x 103 g.m-3 at Po and 25° C;

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.

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

CycloneCyclone CombustionCombustion

Minimize Emission from point Minimize Emission from point Source: example SOSource: example SO22

Minimize emissions of aerosol particles from point of source such as thermal electrical generating stations or industrial smelting units.

EXAMPLE: EXAMPLE: Sulfur Dioxide ControlSulfur Dioxide Control

http://www.apt.lanl.gov/projects/cctc/factsheets/puair/adflugasdemo.html

13

Clean Coal TechnologiesClean 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?Why Clean Coal?

Coal Usage

In RSA Coal used to generate over 85% of th l t i it

Impact of Coal

Mining

Water scarcitythe electricity

Globally Coal accounts for more than 40% of Electricity produced

Only realistic technology for next 20 – 50 years

Water scarcity

Water Pollution

Burning

SOx, NOx, particulates, CO2, fly ash

COCO22 SequestrationSequestration

• 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

Underground Coal Gasification (UCG)Underground Coal Gasification (UCG)

Historically a lot of the work was done in the former USSRResurgence in interest (China, Australia, Europe, Americas & RSA

SASOLMajubahttp://www.eskom.co.za/live/content.php?Item_ID=14077