Air Pollution Control Devices

  Ambient Air sampling and control devices


Ambient Air Sampling and control devices

Air is considered to be polluted when it contains certain substances in concentrations high enough and for durations long enough to cause harm or undesirable effects. These include adverse effects on human health, property, and atmospheric visibility. The atmosphere is susceptible to pollution from natural sources as well as from human activities. 

Most air contaminants originate from combustion processes. The advent of mobile sources of air pollution--i.e., gasoline-powered highway vehicles--had a tremendous impact on air quality problems in cities.

The focus of air-pollution regulation in industrialized countries was initially on protecting ambient or outdoor air quality. This involved the control of a small number of specific criteria pollutants known to contribute to urban smog and chronic public health problems.

 

Techniques for controlling air pollution

Based on the state of matter the air pollutants are divided into two main categories. The air pollution control devices are designed according to the nature of air pollutants

  •  Particulate Matter
  • Gaseous Emissions

 Before starting the discussion on control devices it must be clear in your mind that; the pollutants can only be controlled at source from where they are released. Once they are released in atmosphere their fate is dicided by meteorology and can not be captured. Therefore all the techniques will talk about the control devices installed before chimney exit in industries.

  Air Sampling


Air Sampling

Survey for preliminary information

During ambient air pollutants sampling, it is also necessary to collect information on qualitative and quantitative data on the local sources of air pollution, topography, population distribution, land use pattern, climatology, etc, depending upon the objectives of the survey or measurement campaign.

For example, an area map to locate pollution sources and monitoring locations, sources of pollution situated at far distances, etc. and other relevant data that describe the behaviour of atmosphere for a specific pollutant to be sampled may also be required.

  1. Selection of sampling procedures including procedures of analysis of samples
  2. Sampling locations
  3. Period of sampling, frequency of sampling and duration
  4. Auxiliary measurements (including meteorological parameters)
  5. Processing of data

 

Selection of sampling procedure

There are two types of sampling –

  • Continuous Sampling - Continuous sampling is carried out by automatic sensors, optical or electrochemical, and spectroscopic methods which produce continuous records of concentration values 

 

  • Time averaged in–situ samplings -The specific time-averaged concentration data can then be obtained from continuous records. Time-averaged data can also be obtained by sampling for a short time – i.e. by sampling a known volume of air for the required averaging time. Samples are then analyzed by established physical, chemical, and biological methods for the concentration values which are the effective average over the period of sampling.

 

Sampling locations

Sampling locations are in general governed by factors like objectives, method of sampling and resources available. If the objective is to study health hazards and material damages, then locations should be kept close to the objects where the effects are being studied and should be kept at breathing level in the population centres, hospitals, schools, etc. For vegetation, it should be at foliage level. For background concentration, sampling location should be away from the sources of pollution. It can also be done by gridding the entire area to get statistically recommended values. The number of locations however depends upon the variability of concentration over the area under survey. A spot checking may be done to decide the location besides considering practical factors.

 

Period of sampling, frequency and duration:

Period, frequency and duration of sampling should be appropriate to the objectives of the study. It should be such that the measurable quantities are trapped in the sample at the end of the sampling. It is preferable to observe sampling period consistent with the averaging times for which air quality standards of the given pollutants are specified.

 

 

Stack sampling is the sampling from the chimney of industries however aimbient air sampling is done anywhere in environment.

 

  Dust Fall Collector


Dust Fall Collector

Dust fall collectors are the pre-cleaner technology used to capture heavy particle that can settle down because of gravity. The speed of the polluted air is reduced to an extent which cause gravitational settling of particles having size more than 50 microns. It is installed before any other device to prevent choking due to bigger particulate matter.

  • Particle Size > 50 microns
  • Pre-cleaner technology
  • Gravitational settling
  • Residence time, flow rate and width of chamber is designed to allow maximum settling

 

 

  Cyclone Seperator


Cyclone Seperator

A cyclone removes particulates by causing the dirty airstream to flow in a spiral path inside a cylindrical chamber. Dirty air enters the chamber from a tangential direction at the outer wall of the device, forming a vortex as it swirls within the chamber. The larger particulates, because of their greater inertia, move outward and are forced against the chamber wall. Slowed by friction with the wall surface, they then slide down the wall into a conical dust hopper at the bottom of the cyclone. The cleaned air swirls upward in a narrower spiral through an inner cylinder and emerges from an outlet at the top. Accumulated particulate dust is periodically removed from the hopper for disposal.

Cyclones are best at removing relatively coarse particulates.

They can routinely achieve efficiencies of 90 percent for particles larger than about 20 micron. By themselves, however, cyclones are not sufficient to meet stringent air quality standards. They are typically used as precleaners and are followed by more efficient air-cleaning equipment such as electrostatic precipitators and baghouses.

Point to remember:

In all devices the dirty air will enter from base as it is hot and will automatically rise up. The particle size and efficiency are given in relative range; for example for in cyclone seperator 5µ particles removing efficiency will be 50% however 40µ particle removing efficiency will be 99%.

 

 

  Baghouse/Fabric Filters


Baghouse/Fabric Filters

baghouse (BH, B/H) or fabric filter (FF) is an air pollution control device that removes particulates out of air or gas released from commercial processes or combustion for electricity generation. Example Power plants, steel mills, pharmaceutical producers, food manufacturers, chemical producers and other industrial companies often use baghouses to control emission of air pollutants.

One of the most efficient devices for removing suspended particulates. Its simply like covering your face with a cloth when lot of dust is there; and dust particle stick to cloth rather than coming on your face.

A typical baghouse comprises an array of long, narrow bags-each about 25 cm (10 inches) in diameter-that are suspended upside down in a large enclosure.

Dust-laden air is blown upward through the bottom of the enclosure by fans. Particulates are trapped inside the filter bags, while the clean air passes through the fabric and exits at the top of the baghouse.

A fabric-filter dust collector can remove very nearly 100 percent of particles as small as 1 m (0.00004 inch) and a significant fraction of particles as small as 0.01 m (0.0000004 inch). Fabric filters, however, offer relatively high resistance to airflow, and they are expensive to operate and maintain. Additionally, to prolong the useful life of the filter fabric, the air to be cleaned must be cooled (usually below 300 °C [570 °F]) before it is passed through the unit; cooling coils needed for this purpose add to the expense. (Certain filter fabrics--e.g., those made of ceramic or mineral materials--can operate at higher temperatures.)

Several compartments of filter bags are often used at a single baghouse installation. This arrangement allows individual compartments to be cleaned while others remain in service. The bags are cleaned by mechanical shakers or by reversing the flow of air, and the loosened particulates are collected and removed for disposal.

 

  Wet Scrubber


Wet Scrubber/ Wet Collectors

As the name indicates; liquid medium is used to capture the particulates. Scrubbers or wet collectors remove particles or gases from the exhaust stream by using water sprays. Gases can be absorbed if they are water-soluble or by adding various chemicals to the spray. Particles of dust or soot can also be captured in microscopic liquid mists. Before the exhaust leaves the scrubber, the liquid mists must be collected before the exhaust enters the public air.

As described earlier the hot gas flow up and water is sprayed down, spray captures the particulate and is collected as waste water.

Scrubbers are generally better at removing particles than cyclones, but not as good as electrostatic precipitators or baghouses unless operated at high power. However, it give rise to water pollution by controlling air pollution. Therfore the waste liquid need to be treated.

 

Their are mainly three types of wet scrubbers:

 

  Spray Tower


Spray Tower

It consists of empty cylindrical vessel made of steel or plastic and nozzles that spray liquid into the vessel. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed downward from one or more levels. 

  Cyclone Scrubber


Cyclone Scrubber

The inlet gas enters the chamber tangentially, swirls through the chamber in a corkscrew motion, and exits. At the same time, liquid is sprayed inside the chamber. As the gas swirls around the chamber, pollutants are removed when they impact on liquid droplets, are thrown to the walls, and washed back down and out.

The principle and working are exactly similar to cyclone seperator, only difference is the medium of capture. Here the particulates are trapped in liquid stream rather than falling in dust hopper.

  Venturi Scrubber


Venturi Scrubber

A venturi scrubber consists of three sections:

  • a converging section,
  • a throat section, and
  • a diverging section

The inlet gas stream enters the converging section and, as the area decreases, gas velocity increases. Liquid is introduced either at the throat or at the entrance to the converging section.

The inlet gas, forced to move at extremely high velocities in the small throat section, shears the liquid from its walls, producing an enormous number of very tiny droplets.

Particle and gas removal occur in the throat section as the inlet gas stream mixes with the fog of tiny liquid droplets. The inlet stream then exits through the diverging section, where it is forced to slow down.

Venturis can be used to collect both particulate and gaseous pollutants, but they are more effective in removing particles than gaseous pollutants.

Wet collectors are considered better than dry collectors but also have some disadvantages.

Scrubber benefits:

  1. Can collect both particles and gases
  2. Can handle high temperature gases
  3. Fire and explosion hazards found in some dry-collection systems are eliminated with wet collection
  4. Once the pollutants are collected, they cannot escape easily, unlike dry collection systems where dust can be released from hoppers
  5. The water slurry can sometimes be easier to handle than dry dust.

Scrubbers also have disadvantages:

  1. Water and absorbed gases can become very corrosive, so the scrubber must be properly designed to meet each specific industrial process
  2. Because scrubbers use water, high-humidity air leaving the scrubber can cause large water vapor plumes
  3. Fog and precipitation can cause local meteorological problems or driving hazards near the industry
  4. And because water is used to clean the air, the dirty water also needs to be cleaned
  5. Settling ponds and sludge handlers are often needed to clarify the water slurry
  6. High-powered scrubbers are costly to operate when using high fan speeds

  Wet collectors versus Dry Collectors


Wet collectors versus Dry Collectors

 

  Wet Collectors for Gases


Wet Collectors for Gases 

Wet scrubbers that remove gaseous pollutants are referred to as absorbers. Good gas-to-liquid contact is essential to obtain high removal efficiencies in absorbers. A number of wet scrubber designs are used to remove gaseous pollutants, with the packed tower and the plate tower being the most common.

 

  Electrostatic Precipitator


Electrostatic Precipitator

An electrostatic precipitator (ESP) is a filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.

A negative voltage of several thousand volts is applied between wire and plate. If the applied voltage is high enough, an electric corona discharge ionizes the gas around the electrodes. Negative ions flow to the plates and charge the gas-flow particles

The effectiveness of electrostatic precipitators in removing fly ash from the combustion gases of fossil-fuel furnaces accounts for their high frequency of use at power stations. In a typical unit the collection electrodes comprise a group of large rectangular metal plates suspended vertically and parallel to each other inside a boxlike structure. There are often hundreds of plates having a combined surface area of tens of thousands of square metres. Rows of discharge electrode wires hang between the collection plates. The wires are given a negative electric charge, whereas the plates are grounded and thus become positively charged.

Wet Electrostatic Precipitators (WESP) are also introduced for collection of pollutant gases but not commercially used.

  Control of Gaseous Emission


Gaseous contaminants can be divided into two main categories:

  • Primary pollutants
  • Secondary pollutants

Primary pollutants are compounds that are emitted directly from the stack and/or process equipment of the source. Typical examples of primary pollutants include sulfur dioxide emissions from combustion sources and organic compound emissions from  surface coating facilities.

Secondary pollutants are gaseous and vapor phase compounds that form due to reactions between primary pollutants in the atmosphere or between a primary pollutant and naturally occurring compounds in the atmosphere. Important categories of secondary pollutants include ozone and  other photochemical oxidants formed because of sunlight-initiated reactions  of nitrogen oxides, organic compounds, and carbon monoxide.

Primary Gaseous Contaminants

  • Sulfur dioxide and sulfuric acid vapor
  • Nitrogen oxide and nitrogen dioxide
  • Carbon monoxide and partially oxidized organic compounds
  • Volatile organic compounds and other organic compounds
  • Hydrogen chloride and hydrogen fluoride
  • Hydrogen sulfide and other reduced sulfur compounds (mercaptans sulfides)
  • Ammonia

Secondary Gaseous Contaminants

  • Nitrogen dioxide
  • Ozone and other photochemical oxidants
  • Sulfuric acid

  Important Gas Stream Properties


 

The selection and design of a gaseous contaminant control system must be based on specific information concerning the gas stream to be treated. The following is a partial list of the gas stream parameters that are often useful:

  • Flow rate
  • Temperature
  • Pressure
  • Contaminant concentration
  • Contaminant ignition characteristics
  • Oxygen concentration

 

  Gaseous Contaminant Control Techniques


Six major control technologies are used commercially for the capture and/or destruction of gaseous contaminants

 

  • Adsorption onto solid surfaces
  • Absorption into liquids
  • Biological oxidation
  • Chemical oxidation
  • Chemical reduction
  • Condensation of vapors

 

  Adsorption


Adsorption 

Adsorption involves the interaction between gaseous contaminants and the surface of a solid adsorbent. The adsorbent can be in a wide variety of physical forms such as pellets in a thick bed, small beads in a fluidized bed, or fibers pressed onto a flat surface.

There are two types of adsorption mechanisms: (1) physical, and (2) chemical The basic difference between physical and chemical adsorption is the manner in which the gas or vapor molecule is held to the adsorbent surface.

In physical adsorption, the gas or vapor molecule is weakly held to the solid surface by intermolecular cohesion.  Physical adsorption is easily reversed by the application of heat or by reducing the pressure surrounding the adsorbing material. 

In chemical adsorption, a chemical reaction occurs between the adsorbent and the gaseous contaminant.  This reaction is not easily reversed. Physical adsorption is commonly used for the capture and concentration of organic compounds.  Chemical adsorption is frequently used for the control of acid gases such as hydrogen chloride, hydrogen fluoride, and hydrogen sulfide.  Chemical adsorption is also used for the control of mercury vapor. 

 

 

 

 

  Instruments used for adsorption


Thin bed absorber

Thin bed adsorbers are flat, cylindrical, or pleated.  The granules of activated carbon are retained by porous support material, usually perforated sheet metal.  An adsorber system usually consists of a number of retainers or panels placed in one frame.  Figure shows a nine-panel, thin-bed adsorber.  The panels are similar to home air filters except that they contain activated carbon as the filter instead of fiberglass.

 

 

Fluidized Bed Adsorbers

A fluidized bed uses the motion of the solvent-laden air stream to entrain adsorbent material and thereby facilitate good gas-adsorbent contact.  The adsorbent flows down through the vessel from tray to tray until it reaches the desorption section. 

 

  Types of adsorbents


Activated Carbon

Activated carbon can be produced from a variety of raw materials such as wood, coal, coconut, nutshells, and petroleum-based products and is activated. 

Zeolites (Molecular Sieves)

Unlike activated carbon adsorbents that are amorphous in nature, molecular sieves have a crystalline structure.  The pores are uniform in diameter. Molecular sieves can be used to capture or separate gases on the basis of molecular size and shape.

 

  Absorption in liquids


Absorption

Gaseous contaminants that are soluble in aqueous liquids can be removed in absorbers.  This is one of the main mechanisms used for the removal of acid gas compounds (e.g., sulfur dioxide, hydrogen chloride, and hydrogen fluoride) and water soluble organic compounds (e.g., alcohols, aldehydes, organic acids). 

The contaminant gas or vapor is absorbed from the gas stream as it comes into contact with the liquid.  The rate of pollutant capture increases as the contact between the liquid and the pollutant-laden gas increases.  Therefore, factors such as (1) turbulent mixing of the pollutant-containing gas stream and the liquid, and (2) increased surface area of the aqueous liquid promote absorption.

 

  Instruments used for absorption


Spray towersspray chambers

They consist of empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed downward from one or more levels. This flow of inlet gas and liquid in the opposite direction is called countercurrent flow.  They are used for both particulate and gases emission control.

 

Venturi Scrubber

the same we have studies in particulate emission control are used for gaseous emission control.

Cyclone Spray Scrubber

Cyclone Spray Scrubber uses the features of both the dry cyclone and the spray chamber to remove pollutants from gas streams. Generally, the inlet gas enters the chamber tangentially, swirls through the chamber in a corkscrew motion, and exits. At the same time, liquid is sprayed inside the chamber. As the gas swirls around the chamber, pollutants are removed when they impact on liquid droplets, are thrown to the walls, and washed back down and out.

 

 

  Oxidation for gaseous emission control


Oxidizers can be used for the destruction of a wide variety of organic compounds.  There are three main categories.

  • Thermal oxidizers
  • Catalytic oxidizers
  • Flares

Thermal oxidizers and catalytic oxidizers are used for sources such as surface coaters, gasoline storage and distribution terminals, and synthetic organic chemical plants. 

Flares are used primarily to treat emergency vent gases in synthetic organic chemical plants and petroleum refiners. 

Catalytic Oxidizer

A catalytic oxidizer operating in a 600°F to 850°F (320°C to 450°C) range can achieve the same efficiency as a thermal oxidizer operating between 1,000°F and 2,000°F (540°C and 1,100°C).  Because of the lower operating temperatures, it is often possible for catalytic oxidizers to operate without supplemental fuel except during start-up.

 

 

  Adsorption and Absorption medium


S.No.

Pollutants

Absorbent

Adsorbent

1

SO2

Water, Ca (OH)2

Al and Sodium oxides

2

H2S

NaOH

Iron Oxide

3

NOX

Water, aq HNO3, Urea

Silica Gel

4

Chlorine

NaOH

Activated carbon

5

Organic Vapours

-

Activated carbon

6

Nitrogen Gas

H2SO4

-

  High Volume Air Sampler


High Volume Air Sampler (HVS)

The high volume filtration method is popular for measurement of the mass concentration of suspended particulates smaller than 10 µm.In this method, a known volume of air is sucked by a high speed blower through a fine filter and the increase in weight due to the trapped particles is measured. The filter, usually made of fibrous or granular material, provides a dense porous medium through which an air stream must change direction in a random fashion, allowing the entrained particles to impact on the filter material.

 

  Advantages of HVS

  • High flow rate at low pressure drop
  • High particulate storage capacity
  • No moisture regain
  • high collection efficiency
  • Low cost
  • Not appreciable increase in air flow resistance
  • Filter is 99% efficient and can collect the particles as fine as 0.3 µm
  • Absorption principle is 99% efficient in collecting the gases

Errors in sampling by HVS

  • Particulates may be lost in sampling manifold – so not too long or too twisted manifold must be used.
  • If ’Isokinetic’ conditions are not maintained, biased results may be obtained for particulate matters.

  Respirable Dust Sampler (RDS)


Respirable Dust Sampler (RDS)

Respirable dust is dust that is smaller than 10-microns in size. Total Suspended Particles (TSP) in ambient air and also simultaneous sampling of pollutant gases like SO2, NOx, Cl2 H2S, CS2, etc. These gases are analyzed subsequently by simple chemistry method to determine concentration of specific pollutant.

Particles below 10 microns size are collected on filter paper and bigger than 10  Micron will be collected in a separate sampling bottle, through a Cyclone separator. 

 

  Stack Gas Sampling


Stack gas sampling is done at the source of origin of pollutants from where they enter into the atmosphere. Generally stacks are used to discharge the pollutants into the atmosphere hence the sampling is done in the stack at a specific height from the ground level.

The sample collected must be representative in terms of time and location. The sample volume should be large enough to permit accurate analysis. The sampling rate must be such as to provide maximum efficiency of collection. The contaminants must not be modified or altered in the process of collection.

The major problem in stack sampling is of obtaining a representative sample. If a representative sample is not obtained then the concentration and composition of the actual gas stream will be different, and serious errors would result in the analysis. The important factors in obtaining a representative sample are the selection of the sampling site and the number of sampling points required.

The sampling site should be located at least eight stack or duct diameters downstream and two diameters upstream from any source of flow disturbance such as bends, fittings, or constrictions. In some stacks, it is not always possible to ensure uniform flow, so concentration patterns and, hence, multiple samples are required to obtain a representative sample. That is, the actual sampling must be performed at a number of traverse points in the stack.

Objectives

  • To determine the quantity and the type of pollutants emitted from a specific source
  • Determine the efficiency of pollution control device
  • Monitor the emission standards to review the air quality management programme

Selection of sampling location in stack

The sampling point should be as far as possible from any disturbing influence, such as elbows, bends, transition pieces, baffles. The sampling point, wherever possible should be at a distance of 5-10 diameters down-stream from any obstruction and 3-5 diameters up-stream from similar disturbance.

To get average concentration, samples are collected at various points across the stack. The temperature and velocity of gas varies from point to point within the diameter of the stack.

In circular stacks, traverse points are located at the center of equal annular areas across two perpendicular diameters as shown in Figure

Traverse points for circular stack

 

In case of rectangular stacks, the area may be divided in to 12 to 25 equal areas and the centers for each area are fixed.

Traverse points for rectangular stack

Number of Traverse points on the basis of area of stack

Cross-section area of stack sq. m

No. of Points

0.2

4

0.2-2.5

12

2.5 and above

20

  Pollution Prevention Measures


Raw material Substitution

Example : High sulfur in coal can be substituted or can go through desulphurization process before using in industry. Coal gasification can also be used to recover sulfur from coal.

Process Modification

Example : the process can be modified to reduce the emission of any pollutants. Petroleum industry releases SO2 when H2S is passed through flare, the process modification can give elemental S rather than SO2.

Alteration of Equipments

Changes in engine etc.

Cleaning of effluent gases and pollutant

Construction of high stacks, pollution checking of vehicles, planting trees and installation of pollution control devices in industries.

  Summary of methods used in Air Pollutant Analysis


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