air pollution
DESCRIPTION
Air PollutionTRANSCRIPT
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Content
• Definition, • Unit of measurement, • Sources of air pollution, • Classification of air pollutants, • Type of air pollutants,• Air quality index (AQI),• Effects of pollutant to environment and human, • ozone depletion, • acid rain, • green house effect, • climate change, • global warming, • Meteorology, • air pollution monitoring device, • control and preventive action.
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Definition
Air pollution may be defined as the presence in the air (outdoor
atmosphere) of one or more contaminants or combinations thereof in such
quantities and of such durations as may be or tend to be injurious to
human, animal or plant life, or property, or which unreasonably
interferes with the comfortable enjoyment of life or property or conduct
of business.
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Unit of Measurement
• Air quality measurement are commonly reported in terms of:
– micrograms per cubic meter (µg/m3)
– parts per million (ppm) or parts per billion (ppb)
• For particulate matter, sizes are expressed in micron or micrometer.
– ppm is a volume-to-volume ratio, which makes it independent of local temperature and pressure.
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T
PV
T
PV
gastheofweightmoleculargramMW
molLV
ggxmLxMWxppmxmg
Standard conditions: V=22.4 L/mol @ 273K & 760 mmHg
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Source of Air Pollution
• Mobile sources -- automobiles, gas-powered lawn tools and mowers, boats, planes etc
• Agriculture sources -- Enclosed farm animals – manure release gases eg ammonia; aerial drift of excess fertilizers and pesticides;
• Natural -- forest fires, volcanoes, dust stormVolcanoes – ash, acid mist, hydrogen sulfide & other toxic gas.Sea spray & decaying vegetation – major source of reactive sulfur
compound.Trees & bushes – volatile organic compoundsStorms in arid regions –dust clouds.Fermentation in swamp & break-down of cellulose in guts of
termites & ruminant animals – ⅔ of world methane (natural gas).
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• Sources and emission rates of pollution
• Topography: Mountains as barriers for air movement, forming temperature inversion layer and promoting pollution over certain areas
• Atmospheric conditions: Temperature, cloud cover, and wind affecting the transportation or dispersion of pollutants
Geographic Factors
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Classification of Air PollutantsCategories of pollutants
● Primary – emitted directly from a source● Secondary – formed in the atmosphere from a reaction of primary pollutants
It is a substance or effect dwelling temporarily or permanently in the air , which adversely alters the environment by interfering with the health, the comfort, or the food chain, or by interfering with the property values of people.
Basic Pollutants
PrimarySO2Sulfur Dioxide
Primary & SecondaryPMParticulate Matter
Primary & SecondaryHCHydrocarbon Compounds (also called VOCs – volatile organic
compounds )
SecondaryNO2Nitrogen Dioxide
SecondaryO3Ozone
PrimaryCOCarbon MonoxideTypeAbbreviationPollutant
Primary & SecondaryPbLead
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Type of Pollutants • Suspended particulate matter – complex mixture of solid particles & aerosols
(liquid particles) suspended in the air. We see these particles as dust, smoke, spores, algal cell & haze. PM smaller than 2.5 micrometers (PM2.5)–most dangerous.
• Volatile organic compounds (VOCs) – gasoline, paint solvents & organic cleaning solutions, which evaporate & enter the air in a vapor state, as well as fragments of molecules resulted from the incomplete oxidation of fuels & wastes. VOCs are prime agents of ozone formation. Plants = largest sources of VOCs, ~350 million tons of isoprene (C5H8) & 450 million tons of terpenes (C10H15) each year. About 400 million tons of methane are produced from wetlands & bacteria.
• Carbon Monoxide (CO) – Invisible, odorless gas. From incomplete combustion of fuel (coal, oil, charcoal or gas), incineration of biomass or material. ~ 1 billion metric tons release each year, half from human activities.~90% of CO is consumed in photochemical reaction that produce ozone.
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• Hazardous Air Pollutants (HAPs) -Air toxic include carcinogenic chemicals, radioactive materials & other chemicals (asbestos, vinyl chloride & Benzene). EPA has identified 166 categories of major sources and 8 area sources for HAPs.
• Some of the HAPs (EPA)
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• Nitrogen oxides (NOx).- highly reactive gas formed when nitrogen in fuel or combustion air is heated above 650ºC in the presence of oxygen. Converted to nitric acid=major source of acid deposition. NOx is a lung irritant.
• Sulfur oxides (SOx), mainly sulfur dioxide (SO2)-poisonous gas to plant & animal. Converted to sulfuric acid in air. Major source of acid deposition.
– Natural sources =evaporation from sea spray, erosion of sulfate-containing dust from arid soils, fumes from volcanoes, biogenic emission of hydrogen sulfide (H2S) & organic sulfur-containing compounds (i.e. dimethylsulfide, methyl mercaptan, carbon disulfide ).
– Yearly input to air: 114 million metric ton. Anthropogenic sources = ⅔ of worldwide sulfur flux. Mainly from combustion of sulfur-containing fuel (coal & oil), purification of sour (sulfur-containing) natural gas or oil, smelting of sulfide ores. China & US = largest sources of anthropogenic sulfur (from coal burning).
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• Lead & heavy metal- World wide lead ~ 2 million metric tons/year, or ⅔ of all metallic air pollution. Most from leaded-gasoline. Lead=metabolic poison & neurotoxin. ~20% of all inner-city children suffer some degree of mental retardation from high Pb level. Mercury (Hg) =neurotoxin. 2 largest sources = coal burning power plants & WASTE INCINERATION. Arsenic (from metal smelter, coal combustion & pesticides.
• Ozone & other photochemical oxidant -ozone=ground level pollutant, strong oxidizing reagent & damages vegetation, building material (such as paint, rubber & plastic) & sensitive tissue (eyes & lungs). Hydrocarbons in the air contribute to accumulation of ozone by removing NO in the formation of compounds (such as peroxyacetyl nitrate (PAN), which is another damaging photochemical oxidant.
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• Haze - Opaque condition of atmosphere caused by tiny suspended solid or liquid particles; Normally from open burning
• Industrial & Photochemical Smog -
14Source: EPA website
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Smog Production
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Temperature Inversion
• Several weather conditions intensify levels of industrial & photochemical smogs.
• Most significant – temperature inversion.
• Temperature inversions are defined as the increase of air temperature with altitude. Such an increase represents a reversal of the normal temperature condition of the troposphere (the region of the atmosphere in contact with the Earth's surface), where temperature usually decreases with height. (Britannica Online, 1998)
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Temperature Inversion
• In (a), daytime temperature highest near ground – earth absorbs heat & radiates to the air near the ground. The warm air near ground rises, carrying pollutants.
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Temperature Inversion
• In (b), 2 types of mechanisms create inversion:1. Unstable type – cold front slides under an adjacent warmer air mass
OR cool air subsides down a mountain slope to displace warmer air in the valley below.
2. Stable type – Rapid night-time cooling in a valley/basin where air movement is restricted. Best example (Los Angeles)-a city surrounded by 3 side mountains & climate is dry & sunny. Lots of aerosol & gaseous chemical from motor vehicles. Skies are generally clear at night, allowing rapid radiant heat loss, and the ground cools quickly. Surface areas are cooled by conduction, while upper layers remain relatively warm. During the night, cool & humid onshore breezes slide in under the contaminated air. Density differences retard vertical mixing.
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• Environmental health hazards at homes and workplaces
• A variety of substances: Smoke, chemicals, microbes, and radon
• Different sources: Insulation materials, wood products, poisonous gases due to poor ventilation, cleaning chemicals, etc.
Indoor Air Pollution
http://gsc.nrcan.gc.ca/gamma/radon_e.php
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Adverse Effects of Air Pollution on Humans
• Air pollution is not a single entity.• Synergistic effects = 2 or more factors combine to produce an effect
greater than their simple sum.• I.E. plants & animals may be so stressed by pollution – become >
vulnerable to other environmental factors (drought/attack by parasites & disease).
• Humans breathe ~ 14 kg of air/day.• Some symptoms of air pollution involve moist surfaces of eyes, nose &
throat, the major site of impact is the lungs.• 3 categories of impact:
– Chronic: Pollutants cause the gradual deterioration of a variety of physiological functions over a period of years.
– Acute: Pollutants bring on life-threatening reactions within a period of hours/days
– Carcinogenic: Pollutants initiate changes within cells that lead to uncontrolled growth & division (cancer).
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Air Pollution – Chronic effects on Humans
• Chronic Effects-Almost everyone living in areas of urban air pollution suffers from chronic effects.
– Sulfur dioxide (SO2) – long term exposure → bronchitis (inflammation of the bronchi)
– Ozone & particulate – inflammation → fibrosis of the lung (scarring that permanently impairs lung function).
– CO – reduce the capacity of blood to carry O2 → heart disease.
– NO (Nitrogen Oxides) – impair immune system → leaving the lungs open to attack by bacteria & viruses.
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fig_12_02
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Air pollution effects on agriculture & forest• Plants are more sensitive than humans.• The pollutant responsible is usually sulfur dioxide.• Forest under stress from pollution = more susceptible to damage by insects
& pathogens.• 2 probable ways :
– Direct toxic. Within a few days of exposure, molting (discoloration) occurs in leaves due to chlorosis (Bleaching of chlorophyll) and then necrotic (dead) spots develop. If injury is severe, the whole plant may be killed.
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Air pollution effects on agriculture & forest
– Air pollutants (ethylene=component of automobile exhaust & release from petroleum refineries & chemical plant), that act as metabolic regulators or plant hormones & disrupt normal patterns of growth & development.
– The concentration of ethylene around highways & industrial areas is often high enough to cause injury to sensitive plant.
– Synergistic effect – when white pine seedlings are exposed to sub-threshold [Ozone] & [SO2] individually, no visible injury occurs. If the same [ ] of ozone & SO2 given together, visible damage occurs. In alfalfa, ozone & SO2 together cause less damage than either one alone.
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Aquatic Effects
• Reproduction is the most sensitive stage in fish life cycles.
• Eggs & fry of many spp. of fish are killed when pH drops to ~ 5.0.
• pH lower than 5 disrupt food chain, killing aquatic plants, insects & invertebrate & adult fish.
• Trout, salmon & other game fish are usually the most sensitive.
• Carpgar, suckers & other less desirable fish are more resistant.
• Acidity alters body chemistry, destroys gills & prevents oxygen uptake, causes bone decalcification, & disrupt muscle contraction.
• Acid water also leaches toxic metals such as mercury & aluminium out of soil & rocks.
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Air pollution effects on materials
• Mechanism of deterioration
– Abrasion
– Deposition
– Indirect chemical attack
– Direct chemical attack
– Electrochemical corrosion
• Factor influence the rate of deterioration
– Moisture
– Sunlight
– Temperature
– Position of exposed material
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Acid Rain
• Pure rainwater is slightly acidic, (dissolved carbon dioxide).• Air also contains naturally occurring organic acids and acidic
particles. • The pH of unpolluted rainwater ranges from about 6 to just below 5. • Air compounds containing oxides of sulfur and nitrogen :
– may then dissolve in cloud droplets, making rainwater more acidic (wet deposition), or
– may mix through the atmosphere, eventually coming into direct contact with the ground and vegetation (dry deposition).
• Both forms can harm soil, lakes, plants, buildings and people • More industrial activity in the northern hemisphere.• Industry also tends to be concentrated in particular regions. This is
why acid rain problems are worse in the northern hemisphere.
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Ozone Depletion
• Ozone forms a layer in the stratosphere (20-40km and up) that provides a barrier to ultraviolet (UV) radiation.
• Although oxygen serves as a barrier to UV radiation, it absorb only over a narrow band centered at wavelength of 0.2 μm.
http://ozone.unep.org
38http://ozone.unep.org
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Montreal Protocol 1988
• Following the discovery of the Antarctic ozone hole in late 1985, governments recognized the need for stronger measures to reduce the production and consumption of a number of CFCs (CFC 11, 12, 113, 114, and 115) and several Halons (1211, 1301, 2402).
• The first general ozone agreement was on 1985. This agreement, known as the Vienna Convention for the Protection of the Ozone Layer.
• Later, the Montreal Protocol on Substances that Deplete the Ozone Layer was adopted on 16 September 1987 at the Headquarters of the International Civil Aviation Organization in Montreal. The Protocol came into force on 1st January 1989, when it was ratified by 29 countries and the EEC. Since then several other countries have ratified it.
• The Montreal Protocol and its Amendment constitute a mechanism for the phasing out of ozone depleting substances.
• The control measures and phase out schedules cover both the production and the consumption of the target substances. However, even after phase out both developed and developing countries are permitted to produce limited quantities in order to meet the essential uses for which no alternatives have yet been identified
http://ozone.unep.org
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Helsinki Declaration 1989
• Encourage all states that have not done so to join the Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol
• Agree to phase out the production and the consumption of CFCs controlled by the Montreal Protocol as soon as possible but not later than the year 2000
• Agree to phase out the production and the consumption of CFCs controlled by the Montreal Protocol as soon as possible but not later than the year 2000
• Agree to commit themselves, in proportion to their means and resources, to accelerate the development of environmentally acceptable substituting chemicals, products and technologies
• Agree to facilitate the access of developing countries to relevant scientific information, research results and training and to seek to develop appropriate funding mechanisms to facilitate the transfer of technology and replacement of equipment at minimum cost to developing countries.
http://ozone.unep.org
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Climate Change/ Global Warming
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Climate Change/ Global Warming
• Since 1850, the average global temperature is up by 0.76 °C
• Most of the warming happened in the last 50 years, due to human activities.
• Causes: burning fossil fuels for energy, agriculture, deforestation
• We are already seeing climate change impacts like heat waves and more extreme weather events
• Countries began discussing climate change in 1992, with the creation of the UN Framework Convention on Climate Change (UNFCC)
• The first international commitments to cut CO2 came with the Kyoto Protocol in 1997
• The EU played a leading role in past agreements and advocates strong future action
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Effects Of Climate Change On Mercury And Persistent Organic Pollutants
Precipitation
Winds
Exchange withSurface reservoirs
Ocean currents
biota
Chemistry
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Kyoto Protocol
• Only a first step• It only runs until the end of 2012 • Not all countries are on board – including the US• It doesn't contain commitments for developing countries, and their
emissions are fast catching up with those of developed countries
• A new agreement is needed
• It must be more ambitious, with long-term commitments to deeper cuts
• Developed and developing countries need to act
• We must keep the commitment to limit warming to 2ºC
• Global CO2 emissions need to peak by 2020 and halve by 2050
• Kyoto's successor must include support for developing countries and adaptation
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EU actions
In December 2008, European leaders agreed the climate and energy package, with ambitious targets for 2020.
The package means:
– 20% cut in greenhouse gas emissions by 2020 – 30% if other developed countries agree
– 20% of energy from renewable sources – 20% increase in energy efficiency
Concrete steps in package include: • An extension of emissions trading system• Support for carbon capture and storage• Country-specific targets for renewable energy
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Air Pollution Meteorology
• An air pollution problem involves three parts: the source, the movement of the pollutant and the recipient.
• All meteorological phenomena are a result of interaction of the elemental properties of the atmosphere, heat, pressure, wind and moisture.
• The rotation of the earth couple with heat conductivities of the ocean and land produced weather.
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Mechanical Turbulence• Random fluctuation of wind velocity (speed and direction)
• Wind is zero at ground surface and rise with elevation to near the speed imposed by the pressure gradient.
• The greater the mean wind speed, the greater the turbulence.
• The more the mechanical turbulence, the easier it is to disperse the spread the atmospheric pollutants.
Thermal Turbulence• Different of air circulation during day time and nights.
• During clear nights when the ground radiates its heat away to the cold night sky and the cold air above it causing a sinking density current.
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Cyclonic conditions• Areas of Low pressure are generally
– fast moving,
– associated with strong winds and bad weather (tornadoes and hurricanes)
– upward motion, clouds and precipitation
all result in low pollutant concentrations
Anticyclonic conditions• High pressure areas have the opposite conditions:
– Often slow moving and stagnant
– Associated with weak pressure gradients and light winds
– Downward motion – clear skies and good weather
– Formation of a subsidence inversion that stabilizes the atmosphere and limits vertical mixing
Conditions that lead to stagnation and high pollutant concentrations
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Atmospheric Stability• Tendency of atmosphere to resist or enhance vertical motion.• Affects dispersion of pollutants• It is related with wind speed and change of air temperature with height (lapse rate,
Г)• Lapse rate is the indicator for atmospheric stability. • 3 stability categories: neutral, unstable and stable. • Unstable atmosphere: Mechanical turbulence enhanced by thermal turbulence. The
temperature of atmosphere greater than Г : Superadiabatic. Eg. Hot air balloon.• Neutral stability: Г increased or decreased by the parcel of air that expand or
contracts adiabatically as it is raised through atmosphere. • Stable atmosphere: Temperature of the atmosphere less than Г it is call
subadiabatic.
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Determine Atmospheric Stability
• Stability is the tendency of the atmosphere to resist and enhance vertical motion.
• Lapse rate is the change of air temperature with height.
• There are 3 stability categories:
– Neutral Atmosphere
– Unstable Atmosphere
– Stable Atmosphere
• Mathematically, atmospheric stability can be determine as:
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12:ZZ
TT
Z
TRateLapse
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Vertical Temperature Profiles
Environmental lapse rate (ELR)Dry adiabatic lapse rate (DALR)
If, ELR > DALR =sub adiabatic condition, atmosphere is stable.ELR >> DALR= Inversion conditions. Very stable atmosphere.ELR= DALR= atmosphere is neutral.ELR< DALR = super adiabatic condition, atmosphere is unstable.
Shapes of plumes depends upon atmospheric stability conditions.
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Subsidence inversion due to adiabatic warming of downward moving air (usually in an area of High Pressure)
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Stability Classes• Developed for use in dispersion models• Stability classified into 6 classes (A – F)• A: strongly unstable• B: moderately unstable• C: slightly unstable• D: neutral• E: slightly stable• F: moderately stable
Surface wind speed at 10 m (m/s)
Day Night
Incoming Solar radiation Cloud Cover
Strong Moderate Slight Thinly Overcast Mostly Cloudy
< 2 A (s = 1) A-B B (s = 2)
2-3 A-B B C (s = 3) E (s = 5) F (s = 6)
3-5 B B-C C D E
5-6 C C-D D (s = 4) D D
>6 C D D D D
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Atmospheric Dispersion
• Emission point characteristic
• Nature of pollutant
• Meteorology
• Terrain
• Anthropogenic structures
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Effect of wind
• Wind diffuses pollutants by stretching them along the wind direction.
• Wind speed also enhances turbulence, and thus vertical and horizontal diffusion.
• Variations in wind direction are also important as they lead to sinuous plumes
• The greatest potential for pollution is in low wind situations because horizontal transport and turbulent diffusion are both curtailed.
• Local circulations (land/sea breezes etc.) are not good pollution ventilators because they are associated with low wind speeds, they are closed systems, and there usually is a diurnal reversal → pollution comes back.
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C: Inversions
• Temperature inversions represent a situation in which the atmosphere is very stable and the mixing depth is significantly restricted.
• When an inversion exists and winds are light, diffusion is inhibited and high pollution concentrations are to be expected in areas where pollution sources exist.
• Surface temperature inversions form because the ground is a more effective radiator than the air above. Inversions aloft are associated with sinking air that characterizes centers of high air pressure (anticyclones).
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Inversion
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This is an example of a generalized temperature profile for a surface inversion.
Temperature-profile changes in bottom diagram after the sun has heated the surface.
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An Inversion Aloft
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D: Air Mixing Depth
• The direct effect of wind speed is to influence the concentration of pollutants.
• Atmospheric stability determines the extent to which vertical motions will mix the pollution with cleaner air above the surface layers.
• The vertical distance between Earth's surface and the height to which convectional movements extend is called the mixing depth.
• Generally, the greater the mixing depth, the better the air quality.
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Mixing Height of atmosphere
The height of the base of the inversion layer from ground surface.
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Stacks in Industry
Emissions from industrial stacks are regulated to protect human and environmental health
Industrial facilities are required to obtain permits to emit into the atmosphere and to demonstrate their compliance with regulations
In the process of applying for permits, dispersion models are generally used to assess the impact of point source emission
• A dispersion model is essentially a computational procedure for predicting concentrations downwind of a pollutant source
• Routinely used in: Environmental impact assessments Risk analysis Emergency planning
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Classes of Air Quality Models The air quality modeling procedures can be categorized into four generic classes:
Gaussian, numerical, statistical or empirical and physical
The emphasis is on Gaussian-plume type models for continuous releases, which are at the core of most U.S. Environmental Protection Agency (EPA) regulatory models
Gaussian models are the most widely used techniques for estimating the impact of nonreactive pollutants.
Model Parameters The model is based on our knowledge of the following parameters: The emissions characteristics (stack exit velocity, plume rise, temperature, stack
diameter) Terrain (surface roughness, local topography, nearby buildings) State of the atmosphere (wind speed, stability, mixing height, wind direction)
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Dispersion = Advection (Transport) + Dilution (Diffusion)
Fick’s law of diffusion J= - D * D C/Dx
Where, J= Mass Flux; D = Diffusivity coefficient,; D C/Dx = Concentration gradient
Diffusion of pollutants occur due to turbulence, which further depends upon many factors:a. Ambient temperatureb. Temperature of emissionsc. Roughness factorsd. Wind velocitye. Wind directionf. Humidityg. Stability
Source Receptor
Transport
Re-entrainment
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Air Pollutants Cycle
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General Characteristics of Stack Plumes• Dispersion of pollutants
• Wind – carries pollution downstream from source
• Atmospheric turbulence -- causes pollutants to
fluctuate from mainstream in vertical and crosswind directions
• Mechanical & atmospheric heating both present at same time but in varying ratios
• Affect plume dispersion differently
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Plume Types• Plume types are important because they help us understand under what
conditions there will be higher concentrations of contaminants at ground level.
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Looping Plume
• High degree of convective turbulence• Superadiabatic lapse rate -- strong
instabilities• Associated with clear daytime conditions
accompanied by strong solar heating & light winds
• High probability of high concentrations sporadically at ground level close to stack.
• Occurs in unstable atmospheric conditions.
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Coning Plume
• Stable with small-scale turbulence• Associated with overcast moderate to
strong winds• Roughly 10° cone• Pollutants travel fairly long distances
before reaching ground level in significant amounts
• Occurs in neutral atmospheric conditions
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Fanning Plume
• Occurs under large negative lapse rate
• Strong inversion at a considerable distance above the stack
• Extremely stable atmosphere
• Little turbulence
• If plume density is similar to air, travels downwind at approximately same elevation
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Lofting Plume
• Favorable in the sense that fewer impacts at ground level.
• Pollutants go up into environment. • They are created when atmospheric
conditions are unstable above the plume and stable below.
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Fumigation• Most dangerous plume:
contaminants are all coming down to ground level.
• They are created when atmospheric conditions are stable above the plume and unstable below.
• This happens most often after the daylight sun has warmed the atmosphere, which turns a night time fanning plume into fumigation for about a half an hour.
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Legislation & Standards
• Topographic, meteorological, and land-use characteristics of areas within an air region will vary.
• The social and economic development of an area will result in different degrees of air pollution and demands for air quality.
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Air Quality Monitoring : Air Pollutant Index (API) or Air Quality Index (AQI)• A general index used to assess air quality;• Values calculated based on the average concentration of each of the
monitored pollutants: CO, SO2, NO2, O3, Fine SP Matter (PM10)• The dominant pollutant with the highest concentration will determine the
API value;• Normally PM10 will be dominant;• During late afternoon and early evening O3 can be high too
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Air Quality Index (AQI) Values Levels of Health Concern
151 to 200
201 to 300
301 to 500
Unhealthy
Very Unhealthy
Hazardous
0 to 50
51 to 100
101 to 150
Good
Moderate
Unhealthy for Sensitive Groups
The Air Quality Index
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Threshold level
• Threshold level = the pollutant level below which no ill effects are observed.
• Above threshold level, the effect of pollutant depends on dose (dose = concentration X time of exposure).
• 3 factors determine level of pollution:– Amount of pollutants entering the air– Amount of space into which the pollutants are dispersed.– Mechanisms that remove pollutants from air
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Environmental Quality (Clean Air) Regulations 1978
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Air Quality Management
• Air quality management is fundamentally concerned with the achievement of economic, awareness and regulatory objectives.
– Identifying threats to natural ecosystems or population health
– Informing the public about air quality and raising awareness
– Determining compliance with national or international standards
– Providing objective inputs to AQM
– Policy development and prioritization of management actions
– Development/validation of management tools
– Assessing point or area source impacts
– Trend qualification, to identify future problems or progress against management/control targets
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Instrumentation Selection
• Consider monitoring objectives and data quality objectives
• Required time resolution of measurement
• Resource availability
• Talk to other users
• Independent type approval/designation
• Cost/Budget
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Air Pollution Measurement
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Air Pollution Control• Two different approaches:
- Pollution Prevention at the source – the better alternative
- Treatment of fumes as they are formed – the classical approach
• Emission control equipment is designed to remove or reduce particulates,
aerosols (solids and liquid forms), and gaseous byproducts from various sources and, in some instances, emissions resulting from inefficient design and operation.
• The operating principles of aerosol collection equipment include:
– 1. inertial entrapment by altering the direction and velocity of the effluent;
– 2. increasing the size of the particles through conglomeration or liquid
mist entrainment to subject the particles to inertial and gravitational
forces within the operational range of the control device;
– 3. impingement of particles on impact surfaces, baffles, or filters; and
– 4. precipitation of contaminants in electrical fields or by thermal convection.
• The collection of gases and vapors is based on the particular physical and chemical properties of the gases to be controlled.
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Particulate Collectors and Separators
• Settling chambers
– the settling of particles larger than 40 m in diameter in trays that can be removed for cleaning. Special designs can intercept particles as small as 10 m.
• Cyclones
– impose a downward spiraling movement on the tangentially directed incoming dust-laden gas, causing separation of particles by centrifugal force and collection at the bottom of the cone. Particle sizes collected range from 5 to 200 m at gas flows of 30 to 25,000 ft3 /min. Removal efficiency below 10 m particle size is low. Cyclones can be placed in series or combined with other devices to increase removal efficiency.
95cyclone
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Collection EfficiencyIn order to calculate the efficiency, first the particle size with 50% collection
efficiency (dp50%) as the baseline needs to be determined
μ is dynamic viscosity of air (based on gas temperature)
b is inlet width,
Ne is the number of effective turns (number of turns the flow makes from the
entrance to the midpoint of the core section),
Vi is inlet velocity and
ρp is particle density.
)2( ccc
e ZLH
N
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Empirical Cyclone Collection Efficiency
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Properties of Dry Air at 1 atm
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Air Filters: Baghouse & cloth screen
• The filter medium governs the temperature of the gas to be filtered, particle size removed, capacity and loading, and durability of the filter.
• Filter operating temperatures vary from about 200F (93C) for wool or cotton to 450 to 500F (232–260C) for glass fiber.
• Baghouse filter: The tubular bags are 5 to 18 in. in diameter and from 2 to 30 ft in length. The dust-laden gas stream to be filtered passes through the bags where the particles build up on the inside and, in so doing, increase the filtering efficiency. Periodic shaking of the bags (tubes) causes the collected dust to fall off and restore the filtering capacity. The baghouse filter has particular application in cement plants, heavy metallurgical operations, and other dusty operations. Efficiencies exceeding 99% and particle removal below 10 m in size are reported, depending on the major form and buildup. Baghouses are usually supplemented by scrubber systems.
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Cloth-screen filters used in the smaller grinding, tumbling, and abrasive
cleaning operations. Dust-laden air passes through one or more cloth screens in series. The screens are replaced as needed. Other types of filters use packed fibers, filter beds, granules, and oil baths.
Electrostatic Precipitation (ESP) charge the particles by using electrostatic force to attract them to wall
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Baghouse Cleaning Method1. Pulse Jet System:
Air to Cloth Ratio: 0.033 to 0.083 m/s. 2. Shaker Style System:
Air to Cloth Ratio : 0.01 to 0.017 m/s
3. Reverse Air System: Air to Cloth Ratio : 0.01 to 0.02 m/s
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Calculation for Baghouse • Estimate Fan Size
– air flow rate in cubic feet per minute (cfm), using the surface area of the bags and assume a typical air to cloth ratio. The air to cloth ratio is the air volume per square foot of bag (cfm).
Bag Circumference = п x Bag Diameter
Bag Area = Bag length x bag circumference
Area of baghouse = Gas flow rate/air to cloth ratio (m/s)
Number of bag = Area of baghouse / Bag Area
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Scrubbers are of different types, selected for specific applications. They include spray towers, ejector venturis, venturi scrubbers, and packed-bed, plate, moving-bed, centrifugal, impingement, and entrainment types.
• Wet collectors are generally used to remove gases such as hydrogen chloride, nitrous oxides, and sulfur dioxide and particles that form as a dust, fog, or mist. A high-pressure liquid spray is applied to the gas passing through the washer, filter, venturi, or other device.
• The three important operations of air washers or wet gas scrubbers:
– Humidification (of air): This process increases size of fine dust particles and makes collection easier
– Contact of liquid and dust filled air: This is the key operation that governs collection efficiency of the scrubber. Water droplets in the path of the air stream collect dust particles while the air stream flows around it
– Separation of dirty liquid and clean air: During this operation, the dust-water mixture accumulates, and as they grow larger, they collect into the hopper.
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109
table_12_14
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111
table_12_13
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Removing Sulfur Dioxide – “scrubbers”
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114
Example:
• An aggregate plant at Lime Ridge has been found to be in violation of particulate discharge standards. A mechanical shaker baghouse has been selected for particulate control. Estimate the number of bags required for a gas flow rate of 20m3/s if each back is 15cm in diameter and 12 m in length. The manufacturer’s recommended air to cloth ratio for aggregate plants is 0.01 m/s.
• Determine the efficiency of a cyclone having the following characteristic for 10μm in diameter with density of 800kg/m3.
– Cyclone barrel diameter=0.5m
– Gas flow rate=4m3/s
– Gas temperature=25°C
– Dynamic viscosity, μ=18.5 μPa.s
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Particulate Matter Chemistry (1 of 4)
Coagulation: Particles collide and stick together.
Condensation: Gases condense onto a small solid particle to form a liquid droplet.
Chemical Reaction: Gases react to form particles.
Cloud/Fog Processes: Gases dissolve in a water droplet and chemically react. A particle exists when the water evaporates.
Sulfate
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NOx
Ammonia
VOCs
Particulate Matter Composition (2 of 3)
PM contains many compounds
Primary Particles(directly emitted)
Secondary Particles(from precursor gases)
Other(sea salt)
Other(sea salt)
Crustal(soil,dust)
Crustal(soil,dust)
OrganicCarbon
OrganicCarbon
Carbon(Soot)
Carbon(Soot)
SO2
AmmoniumSulfate
AmmoniumSulfate
AmmoniumNitrate
AmmoniumNitrate
MetalsMetals
Composition of PM tells us about the sources and formation processes
Gas
ParticleParticle