Air Pollution Notes

Air Pollution

Definition of Air Pollution

• WHO Definition:
  • Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere.
  • Common sources include household combustion devices, motor vehicles, industrial facilities, and forest fires.
• EPA Definition:
  • Air pollution is one or more chemicals or substances in high enough concentrations in the air to harm humans, other animals, vegetation, or materials.
  • These chemicals or physical conditions (like excess heat or noise) are called air pollutants.

Key Factors Influencing Air Quality

• Sources of air pollution
• Meteorology and topography
• Air quality effects on receptors

Air Quality Index (AQI)

• Purpose:
  • Transforms individual air pollutant levels (e.g., $SO<em>2$, CO, $PM</em>{10}$) into a single number for easy understanding of air quality.
  • Relates to health impacts, allowing citizens to avoid unnecessary exposure to air pollutants.
  • Indicates compliance with National Air Quality Standards.
  • Prompts local authorities to take quick actions to improve air quality.
  • Guides policymakers to make broad decisions.
  • Encourages citizen participation in air quality management.

Pollutants Considered for AQI and Air Quality Standards

• Pollutants: $SO<em>2$, $NO</em>2$, $PM<em>{2.5}$, $PM</em>{10}$, $O<em>3$, CO, Pb, $NH</em>3$
• Averaging Times (h): 24, 1, 8
• Indian Standards ($\mu$g/m3):
  • $SO_2$: 80 (24-hour)
  • $NO_2$: 80 (24-hour)
  • $PM_{2.5}$: 60 (24-hour)
  • $PM_{10}$: 100 (24-hour)
  • $O_3$: 180 (1-hour)
  • CO: 100 (8-hour)
  • Pb: 4 (1-hour), 2 (8-hour)
  • $NH_3$: 1 (24-hour), 400 (24-hour)

Particulate Matter Classification

• $PM_{10}$: Particulate matter less than 10 millionths of a meter (10 micrometers or 10 $mm$) in diameter; generally not visible to the naked eye.
• $PM<em>{2.5}$: Subset of $PM</em>{10}$; particulate matter less than 2.5 millionths of a meter (2.5 $mm$) in diameter (approximately 70% of total $PM_{10}$ in ambient air).

AQI Categories and Ranges

• AQI Category (Range), Breakpoint Concentrations:
  • Good (0-50)
  • Satisfactory (51-100)
  • Moderate (101-200)
  • Poor (201-300)
  • Very Poor (301-400)
  • Severe (401-500)

Classification of Air Pollutants

• Primary Air Pollutants: Materials released that pose health risks in their unmodified forms or are emitted directly from identifiable sources.
• Secondary Air Pollutants: Primary pollutants interact with one another in the presence of sunlight or natural gases to produce new, harmful compounds.

Sources of Air Pollution

• [Image of sources of air pollution, source not specified]

Primary Air Pollutant Components

Suspended Particulate Matter (SPM)

• Complex mixture of airborne particles that differ in size, origin, and chemical composition.
• One of the six EPA “Criteria Pollutants” (SO2, NOx, CO, Pb, VOCs, & PM).
• Consists of solids in the air in the form of smoke, dust, and vapor that can remain suspended for extended periods.
• Main source of haze which reduces visibility.
• Fine particles can lodge in lungs, causing lung damage and respiratory problems.
• Can also constitute pollen dust and microbial organisms.

Particle Sizes

• Coarse (Inhalable)
• Fine (Lungs)
• Ultrafine (Blood)
• $PM_{10}$ means particulate size less than 10 $µm$
• $PM_{2.5}$ means particulate size less than 2.5 $µm$
• $PM_{1.0}$ means particulate size less than 1.0 $µm$

Carbon Monoxide (CO)

• Colorless, odorless gas produced by the incomplete burning of carbon-based fuels (petrol, diesel, wood).
• Lowers the amount of oxygen that enters our blood.
• Can slow reflexes and cause confusion and sleepiness.

Chlorofluorocarbons (CFCs)

• Gases released mainly from air-conditioning systems and refrigeration.
• Rise to the stratosphere and reduce the ozone layer, which protects the earth from harmful ultraviolet rays.

Carbon Dioxide ($CO_2$)

• Principle greenhouse gas emitted as a result of human activities such as the burning of coal, oil, and natural gases.

Nitrogen Oxide (NOx)

• Produced from burning fuels, including petrol, diesel, and coal.
• At high temperatures, nitrogen in the air can combine with oxygen to produce NOx.
• Can make children susceptible to respiratory diseases in winters.
• Causes smog and acid rain.

Sulfur Dioxide ($SO_2$)

• Gas produced from burning fuel due to presence of sulfur in fuel.
• Some industrial processes, such as paper production and smelting of metals, produce sulfur dioxide.
• Major contributor to smog and acid rain.
• Can lead to lung diseases.

Effects of Air Pollutants

• Humans
• Materials
• Animals
• Plants

Meteorology

• Meteorology is concerned with the description of the transport of pollutants from source to receptor.
• Meteorological elements have the most direct and significant effects on the distribution/dispersion of air pollutants in the atmosphere.

Dispersion

• Pollutants released into the atmosphere are subjected to transportation, dilution, modification, and removal.
• Transport and dilution cause the dispersion of pollutants into the atmosphere, reducing the background concentration of pollutants.
• Modification and removal result in the atmosphere getting rid of pollutants after chemical alterations.
• Dispersion depends on climatic conditions like temperature, pressure, prevailing winds, humidity, etc.
• The atmosphere can be highly turbulent with violent eddies or calm with only molecular movement.
• It is important to discuss the turbulence level of atmosphere.

Meteorological Parameters (M.P.)

• Primary M.P.:
  • Meteorological elements that have the most direct and significant effects on the turbulence level of the atmosphere, which ultimately impacts the distribution/dispersion of air pollutants.
  • Examples: wind speed, wind direction, temperature, atmospheric stability, and mixing height.
• Secondary M.P.:
  • Effects on primary parameters.
  • Examples: Humidity, solar radiations, precipitation, atmospheric pressure, topography, etc.

Turbulence

• Irregular air motion is known as turbulence, which may be either mechanical or thermal in nature.
• At the surface, turbulence is commonly identified in terms of eddies, whirls, and gusts.

Mechanical Turbulence

• Wind blowing over the ground causes air to become gusty and helps spread pollutants by allowing them to mix more easily with the surrounding unpolluted air.

Thermal Turbulence

• Mixing also happens when the air is heated to a greater extent than the air above (temperature gradient).
• Causes bubbles of warm air (thermal turbulence) to rise.
• These cause eddies, which often take pollutants higher into the atmosphere.
• The type and magnitude of turbulence decide the size and velocity of eddies.

Wind Roses

• A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location.
• The directions of the rose with the longest spoke show the wind direction with the greatest frequency.
• Presented in a circular format, the modern wind rose shows the frequency of winds.
• The length of each "spoke" around the circle is related to the frequency that the wind blows from a particular direction per unit time.
• Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles.
• A wind rose plot may contain additional information, in that each spoke is broken down into color-coded bands that show wind speed ranges.
• Wind roses typically use 16 cardinal directions, such as north (N), NNE, NE, etc., although they may be subdivided into as many as 32 directions.
• Wind speed determines the travel time of a pollutant from its source to a receptor and accounts for the amount of pollutant dispersion
• Wind direction determines the direction of the pollutant movement and the receptor which is affected at what time
• Both parameters defined by Windrose. This is a graphical display of wind in a given location during a defined period. It statistically defines the frequency, direction, force and speed.
• Limitations include lack of precision and vertical representation, and is location specific

Thermal Turbulence

• Thermal turbulence is associated with instability and convective activity.
• It is similar to mechanical turbulence in its effects on surface winds, but extends higher in the atmosphere.
• Since it is the result of surface heating, thermal turbulence increases with the intensity of surface heating and the degree of instability indicated by the temperature lapse rate. It therefore shows diurnal changes and is most pronounced in the early afternoon when surface heating is at a maximum and the air is unstable in the lower layers.
• It is at a minimum during the night and early morning when the air is more stable. Mechanical and thermal turbulence frequently occur together, each magnifying the effects of the other.

Lapse Rate

• Lapse Rate: Rate of Decrease of Temperature with Altitude.
• This is an important concept when describing the vertical movement of air.
• Essentially it is the description of how quickly air cools as a response to an increase in elevation, or vice versa.
• There are three lapse rates to consider.
  • The environmental lapse rate (ELR) is the actual measured decrease in air temperature with an increase in altitude. It is usually around $6.5^\circ C / 1000 m$. The environmental lapse rate (ELR) is the actual measured decrease in air temperature with an increase in altitude. It is usually around $6.5^\circ C / 1000 m$.
  • The dry adiabatic lapse rate (DALR) is the theoretical rate of cooling of air with no moisture in it over an increase in altitude. This rate is $9.8^\circ C / 1000 m$.
  • The wet adiabatic lapse rate (WALR) is a theoretical reduction in temperature with elevation in saturated air. It is much less than the DALR, and usually around $5.6^\circ C / 1000 m$.
  • Process L.R. (PLR) is the rate at which temperature of pollutant mass (puff) is decreasing with altitude due to expansion of flue gases and its magnitude is somewhat between DALR and WALR since puff of pollutants are neither completely dry nor completely saturated.
• Reason why WALR is less than DALR: As pollutants rise, temperature decreases due to expansion and partial pressure becomes equal to atmospheric pressure which causes condensation due to saturation. Condensation leads to liberation of heat equivalent to latent heat of condensation, hence the rate of decrease of temperature will be less so WALR is less than DALR.

Stability of Atmosphere

• The stability of the atmosphere is dependant on the ELR compared to the PLR/ALR. Three conditions can occur
  • Stable (ELR < PLR)
  • Unstable (ELR>PLR)
  • Neutral conditions (ELR = PLR)
• Unstable atmospheric conditions occur when ELR>PLR. Absolute instability occurs when the ELR is of greater magnitude than the DALR. For example: ELR is $12^\circ C/1000 m$, DALR is $10^\circ C/1000 m$. A parcel of air with an initial temperature of $20^\circ C$ will cool to $10^\circ C$ at 1000 m, but the surrounding air is at $8^\circ C$. Therefore the parcel will continue to rise.
• Stable conditions: When ELR < PLR, pollutants will tend to sink.
  • Absolute stability occurs when ELR is of lesser magnitude than WALR.
  • E.g. ELR is $6.5^\circ C/Km$, PLR is $8^\circ C/Km$. If initial temperature of puff and ambient air is $20^\circ C$, then at 1000 m altitude, the parcel would be $12 ^\circ C$, but the surrounding air is $13.5^\circ C$. So pollutants will be suppressed and it is undesirable state as far as dispersion is concerned.
• Neutral conditions
  • Neutral stability occurs when the ELR is the same as PLR.
  • Conditional instability is a existance of both stable and unstable atmospheres within the stability zones
  • Atmosphere neither helps nor suppress the dispersion of pollutants

Air Pollution Control Devices (APCDs)

Control of Air Pollutants: Mechanisms Involved

• Particulates:
  • Gravity Settling
  • Centrifugal Forces
  • Electrostatic Forces
  • Filteration
  • Scrubbing
• Gaseous Pollutants:
  • Absorption
  • Adsorption
  • Combustion
  • Masking

Air Pollution Control System & Devices (Particulate Control)

• Considering factors:
  • Particulate concentration
  • Particulate Size
  • Volume of flue gases
  • Composition of flue gases
  • Moisture content in flue gases
  • Temperature of flue gases
• Designing considerations:
  • Efficiency
  • Pressure Drop
  • Cost

Gravitational Settling Chambers

• Operating temperature – 1000 deg.C
• Applications – precleaners for removing dry dust produced by grinding in cement and lime kilns, grain elevators, rock crushers, thermal coal dryers, furnaces
• Dust particle size > 50 mm
• One of the first devices to control particulate emission
• Types – Simple expansion Chamber; Multi-tray settling chamber
• Force – Gravitational
• Once the horizontal gas velocity is reduced, large particles can be overcome by gravity and fall into the hopper
• Typical horizontal velocity range of 0.3 to 3.0 m/s
• Expansion chamber
• Howard Settling Chamber (Multiple Tray)
• Baffle Chamber

Cyclones

• The suspended gas-particle mixture is fed tangentially to the inlet at a high velocity and pressure.
• This creates a spiral flow wherein the suspended particles are forced towards the wall due to centrifugal force.
• These particles which collide at the wall, spiral downwards in the gas boundary layer and are collected at an outlet at the bottom.
• Single-cyclone separators create a dual vortex to separate coarse from fine dust. The main vortex spirals downward and carries most of the coarser dust particles. The inner vortex, created near the bottom of the cyclone, spirals upward and carries finer dust particles.
• Types of design
• Reverse flow or straight through
• Flue Gas Entry points – tangential, Scroll and Swirl vane
• Gas Flow rates – 50 –50,000 m3/hr
• Pressure drop – 0.5 to 0.8 inches in water
• Operating temperatures - ~1000 deg.C
• Applications – Cement and lime kilns, furnaces, milling operations, power drying processes
• Dust particle sizes - > 25µm
• Dust concentration - > 2000g/m3
• Cyclones are efficient in removing large particles but are not as efficient with smaller particles. For this reason, they are used with other particulate control devices.

Industrial Applications

• Used in many applications for precleaning before sending to precipitators of filters
• Recovery and recycling of solid particles such as food products or catalysts
• Used in metallurgical industries as a first stage for control of particulate matter emissions from roasters, kilns and furnaces
• Used after spray drying operations in food industry
• Recovery of process materials after crushing and grinding operations.

Advantages

• No moving parts, low maintenance costs
• Can operate over wide range of temperature (upto 540 C) and pressure (upto 500 atm)
• No pretreatment necessary
• Small space requirements
• Comparatively low capital and operating costs

Disadvantages

• Low collection efficiencies when particle size is small (<10 micrometers)
• Cannot handle sticky particles which have high inter-particulate attraction
• High efficiency units experience high pressure drops and are costlier to operate

Fabric Collectors/Bag Filters/Bag House

• Commonly known as baghouses, fabric collectors use filtration to separate dust particulates from dusty gases.
• They are one of the most efficient and cost effective types of dust collectors available and can achieve a collection efficiency of more than 99% for very fine particulates.
• Dust-laden gases enter the baghouse and pass through fabric bags that act as filters.
• The bags can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape
• The high efficiency of these collectors is due to the dust cake formed on the surfaces of the bags. The fabric primarily provides a surface on which dust particulates collect through the following four mechanisms:
  • Inertial Collection - Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
  • Interception - Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size.
  • Brownian Movement - Submicron particles are diffused, increasing the probability of contact between the particles and collecting surfaces.
  • Electrostatic Forces - The presence of an electrostatic charge on the particles and the filter can increase dust capture.
• As classified by cleaning method, three common types of baghouses are:
• Mechanical shaker
• Reverse air
• Reverse jet

Electrostatic Precipitators (ESP)

• Electrostatic Precipitators use electrostatic forces to separate dust particles from exhaust gases.
• A number of high-voltage, direct-current discharge electrodes are placed between grounded collecting electrodes.
• The contaminated gases flow through the passage formed by the discharge and collecting electrodes.
• The airborne particles receive a negative charge as they pass through the ionized field between the electrodes.
• These charged particles are then attracted to a grounded or positively charged electrode and adhere to it.

Working Principle

• Corona Generation
  • When high voltage is applied in the charging electrodes, a blue luminous glow called Corona is generated around the electrodes and it causes ionization of gas molecules, due to which +ve ions, -ve ions and free electrons are generated.
• Particle Charging
  • When –ve ions travel towards +ve electrodes (Collection electrode), the –ve charges get attached to the dust particles and thus, dust particles are electrically charged
• Particle Collection
  • Attracted towards oppositely charged electrodes and form a layer on surface of electrode
• Removal of particles
  • Rapping mechanism or wet removal method

ESP Particle Charging - Corona formation

• This state is transient, once the excitation has ceased, the molecule reverts to its ground state releasing energy
• Part of this energy gets converted light (bluish glow) – Corona
• The space between the wire and the plate can be divided into active zone and the passive zone
• In the active zone, defined by corona glow discharge, electrons leave the wire electrode
• In the passive zone, the electrons impact gas molecules thereby ionizing the molecules which continues as chain of ionization called Avalanche effect

Classification of Electrostatic Precipitators

• Tubular and parallel Plate
• Dry and wet bottom ESP
• Negative and positive corona ESP
• Single and double stage ESP

Particle Removal

• Dust that has accumulated to a certain thickness on the collection electrode is removed by one of two processes, depending on the type of collection electrode.
• Tubes are usually cleaned by water sprays, while plates can be cleaned either by water sprays or a process called rapping.
• Rapping is a process whereby deposited, dry particles are dislodged from the collection plates by sending mechanical impulses, or vibrations, to the plates.
• Precipitator plates are rapped periodically while maintaining the continuous flue-gas cleaning process.
• Plates are rapped when the accumulated dust layer is relatively thick (0.08 to 1.27 cm or 0.03 to 0.5 in.). This allows the dust layer to fall off the plates as large aggregate sheets and helps eliminate dust re-entrainment.
• Dislodged dust falls from the plates into the hopper.

Advantages of ESPs:

• Very high efficiencies, even for very small particles
• Can handle very large gas volumes with low pressure drop
• Dry collection of valuable material, or wet collection of fumes and mists
• Can be designed for a wide range of gas temperatures
• Low operating costs, except at very high efficiencies

Disadvantages of ESPs:

• High capital costs
• Will not control gaseous emissions
• Not very flexible, once installed, to changes in operating conditions
• Take up a lot of space
• Might not work on particulates with very high electrical resistivity

Collection Efficiency of ESP:

The collection efficiency of an ESP as a function of gas flow rate and precipitator size is given by the Deutsch-Andersen Equation
Assumptions:

• Repulsion effect is neglected
• Uniform gas velocity throughout the cross section
• Particles are fully charged by field charging.
• No hindered settling effect
  Collection efficiency mathematically expressed as follows
  where ƞ=Fractional Collection Efficiency; Ac=Area of the collection electrode; Vpm=Particle migration velocity; Q=Av=Volumetric flow rate of gas; v=gas velocity.
  $\eta = 1- exp {(-Vpm \times Ac)/Q}$

Wet Scrubbers

• PM Control: PM having Size 2.5-10 µm & hazardous air pollutants (HAP) in particulate form
• Gaseous Control: Inorganic fumes, vapors, and gases (e.g. chromic acid, hydrogen sulfide, ammonia, chlorides, fluorides and SO2)
• Control of volatile organic compounds (VOC)

Scrubbers (Used both for removal of particulates and gaseous pollutants)

• There is a large variety of wet scrubbers; however, all have of three basic operations:
  • Gas-Humidification - The gas-humidification process conditions fine particles to increase their size so they can be collected more easily.
  • Gas-Liquid Contact - This is one of the most important factors affecting collection efficiency.
  • Gas-Liquid Separation - Regardless of the contact mechanism used, as much liquid and dust as possible must be removed. Once contact is made, dust particulates and water droplets combine to form agglomerates. As the agglomerates grow larger, they settle into a collector.
• Absorption
• Selection of the solvent
  • Solubility is the most important consideration in the selection of a solvent for absorption
  • Higher the solubility, lower the amount of solvent required
  • The solvent should also be non-volatile to prevent an excessive carry over in the gas effluent
  • Other properties of a good solvent are
  • Low flammability and viscosity
  • High chemical stability
  • Acceptable corrosivity
  • Low toxicity and pollution potential
• Plate type scrubber
  • It consists of a hollow vertical tower with one or more plates .
• Gas comes in from the bottom and must pass through perforations, valves, slots, or other openings in each plate and exit from top.
• Liquid from the top plate, flows successively across each plate and exit at the bottom.
• Gas passing through the openings in each plate mixes with the liquid flowing over the plate.
• The gas and liquid contact allows the mass transfer or particle removal.
• These have the ability to remove gaseous pollutants to any desired concentration provided a sufficient number of plates are used.
• Spray Tower
  • It utilizes spray nozzles for liquid droplet atomization.
• The sprays are directed into a chamber to conduct the gas through the atomized liquid droplets.
• These are designed for low pressure drop and high liquid consumption.
• Applicable to the removal of gases which have high liquid solubility.
• Many nozzles are placed at different heights.
• Theoretically, the smaller the droplets formed, the higher the collection efficiency achieved
• Gaseous Emission Control Mechanisms:
  • -Absorption
  • -Condensation
  • -Adsorption
• Condensation
• In cases where pollutants have low vapour pressures, condensation is effective for removing a significant part of the vapour
• The condenser works by cooling the feed gas to a temperature below the dew point of the feed gas
• Two types of condensers are normally used
• Surface condensers contact condensers
• Removal efficiencies of condensers typically range from 50 percent to more than 95 percent, depending on design and applications.
• Adsorption
• Adsorption is the binding of molecules or particles to a surface.
• In this phenomenon molecules from a gas will be attached physically to a surface.
• The binding to the surface is usually weak and reversible.
• The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they have enormous surface areas per unit weight.
• In selecting the adsorbent, the adsorption isotherm of the adsorbent should be favourable
• An adsorption isotherm for a single gaseous adsorptive on a solid is the function which relates at constant temperature the amount of substance adsorbed at equilibrium to the pressure (or concentration) of the adsorptive in the gas phase.