Wednesday, March 29, 2017

Control of SPM by equipment (gravitation, centrifugation, filtration, scrubbing, electrostatic precipitation)

Suspended Particulate Matter is a collective name given to fine liquid or solid particles added to the atmosphere by processes in the Earth's surface. A few examples of particulate matter are dust, soot, smoke, fumes, mist, pollen, soil particles, etc. SPM may be further classified into RSPM and TSPM based on the average size of particles.
RSPM refers to Respirable Suspended Particulate Matter and it is of size small enough to enter the respiratory tract of human beings and damage the lungs. The average size of RSPM is of the order of 0.1 microns or less.
Particulate matter generates from natural sources like volcanoes, salt sprays, dust storms, grassland fires and living vegetation. Man made sources of SPM is primarily burning of fossil fuels for vehicles, operation of power plants, industrial operations and coal combustion for heating homes and supplying energy.
The composition of SPM depends upon the source. Wind blown mineral dust is composed of mineral oxides and other material blown from Earth's crust. Sea salt is composed of NaCl originates from sea spray.Salt sprays from sea reflect  the composition of sea water and may contain chlorides and sulphates of Magnesium and Potassium. Secondary particles are derived from oxides of primary gases like oxides of Sulphur and Nitrogen. Organic matter may be primary (from anthropogenic or biogenic activities) or secondary (from oxidation of VOCs).
Secondary Organic Aerosols (SOAs) emitted due to internal combustion engines are a danger to human beings.
Control of SPM can be achieved by the equipment listed below.

  1. Control of SPM by gravitation 
Equipment used: Gravitational Settling Chamber A typical gravitational chamber is shown below.

  • The dust laden gas enters at the inlet and due to the sudden increase in cross-section the particulate matter settles at the bottom and can be removed from the dust hoppers as shown
  • The clean gas free from particulate matter exits  from the outlet
  • Simple to construct and maintain
  • Efficient to remove particles of diameter greater than 50 mm from gas streams
  • They are used as pre-cleaners before passing gases through high efficiency collection devices
  • They rely on gravitational settling and are the simplest and oldest mechanical collectors for removal of particulates from gas streams
  • Flow within the chamber must be uniform without macroscopic mixing
  • Dust removal system must be sealed to prevent production of turbulence due to air from leaking into chamber
  • Efficiency of the equipment increases with increased residence time of the waste gas. Hence, the equipment is operated at lowest possible gas velocity
  • The size of the unit depends on:
    • gas velocity which should preferably be less than 0.3 m/s
  • Low capital and energy cost
  • Low maintenance and operating costs
  • Low pressure drop
  • Reliable
  • Equipment is not subjected to abrasion due to low gas velocity
  • Equipment provides incidental cooling of gas stream
  • Temperature and pressure limitations depend on material of construction
  • Pollutants are collected in dry state
  • Low particulate matter collection efficiency
  • Unable to handle sticky materials
  • Large size
  • Trays in multiple tray settling chamber may warp under high temperatures.
2.  Control of SPM by centrifugation
Equipment used: Cyclonic separator
Centrifugation is a process that involves the use of centrifugal force for sedimentation of a heterogeneous mixture with a centrifuge. It involves removal of particulates from air, gas or a liquid stream without use of filters with a vortex separation. When removing particulates from a gaseous stream, a gas cyclone is used while a hydrocyclone is used to remove particulates from a liquid stream. This method can also be used to separate fine droplets of liquid from a gaseous stream. 
A high speed rotating air flow is formed in a cylindrical or conical container called a cyclone.
Air flows in a helical pattern from the top to a narrow bottom as show,

 Cyclones use the principle of inertia to remove particulate matter from a gas stream. Several cyclones operating in parallel is known as multicyclone. In a cyclone separator, dirty gas is fed into a chamber where a spiral vortex exists. The large particles hit the inside walls of the container and drop down into the collection hooper. The clean flue gas escapes from the top of the chamber. Cyclones can be used efficiently to remove particles of size 10 microns or more. High efficiency cyclones can remove particles of dimeter as small as 2.5 microns. They are the least expensive of all particulate collection devices. They are used as rough separators before the gas is passed through fine filtration systems. Their efficiency is between 50-99%. Cyclone separators work best on flue gases that contain large amount of big particulate matter. 
  • Cyclones are less expensive to install or maintain as they do not contain any moving parts
  • It is easy to dispose particulate matter as it is collected in the dry state
  • Space requirement is very less
  • They are not efficient in collecting particulate matter smaller than 10 microns
  • They cannnot handle sticky material
3.   Control of SPM by filtration

In a fabric filter system, a stream of the polluted gas is made to pass through a fabric that filters out the particulate pollutant and allows the clear gas to pass through. The particulate matter is left in the form of a thin dust mat on the insides of the bag. This dust mat acts as a filtering medium for further removal of particulates increasing the efficiency of the filter bag to sieve more sub mi­cron particles (0.5 ┬Ám).

A typical filter is a tubular bag which is closed at the upper end and has a hopper attached at the lower end to collect the particles when they are dislodged from the fabric. Many such bags are hung in a baghouse. For efficient filtration and a longer life the filter bags must be cleaned occasionally by a mechanical shaker to prevent too many particulate layers from building up on the inside surfaces of the bag. A typical bag house filter is shown in the figure below.
  • Bag filter is a high quality performance instrument to effectively control particulate emissions and its efficiency is as high as 99%
  • Collection efficiency is not affected by sulphur content in fuel
  • It is not sensitive to particle size distribution
  • It does not require high voltage
  • It can be used to collect flammable dust
  • Special fiber or filter aids can be used to sub-micron level smoke and fumes
  • Fabric life is reduced due to presence of highly acidic or alkaline atmospheres, especially at high temperatures
  • Maximum operating temperature is 500 F
  • Collection of hygroscopic materials or condensation of moisture can lead to fabric plugging, loss of cleaning efficiency and large pressure losses.
  • Certain dusts may require special fabric treatments to aid in reducing leakage or to help in cake removal
  • Fabric bags are prone to burning or melting at extreme temperatures.
4.  Control of SPM by scrubbing
A scrubber is a system used to remove harmful materials from industrial exhaust gases before they are released into the environment. The two main ways to scrub pollutants out of exhaust are:
  1. Dry scrubbing and
  2. Wet scrubbing
In dry scrubbing, harmful components of exhausted flue gas are removed by introducing a solid substance (usually in the powdered form) in the gas stream.

Wet scrubbing involves removal of harmful components from exhaust by spraying a liquid substance through the gas.

Both methods work similarly and perform the same process of removing pollutants. The difference lies in the materials they use to remove the pollutant from the gas stream. By removing acidic gases from the exhaust before it is released into the atmosphere, scrubbers help in the prevent the formation of acid rain.
Scrubbing is sometimes referred to as flue gas desulfurization.

Scrubbing is the most effective technique for the removal of oxides of sulphur and is widely used. Scrubbers remove sulphur oxides from flue gases by passing the gases through a spray of water in a wet scrubber that contains many chemicals, mainly calcium carbonate.
If a dry scrubber is used, the flue gas comes in contact with pulverised limestone. The chemical reaction between suphur dioxide and calcium carbonate yields calcium sulphite. The calcium sulphite either falls out of the gas stream or is removed with other particulates.
Scrubbers are highly efficient and remove almost 98% of sulphur from flue gases. However, they are expensive to maintain and install. They are also energy intensive as the flue gas must be reheated after coming into contact with water vapour in the wet scrubber to make the gas buoyant to exit the smoke stacks.

5.  Control of SPM by Electrostatic precipitator
An Electrostatic precipitator is mainly used to control particulate matter. An Electrostatic precipitator uses 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 as shown in the figure below.

Air borne particles receive a negative charge as they pass through the ionized field between the electrodes. These charged particles are then attracted to the oppositely charged electrode and stick to it. The collected material is then removed by rapping or vibrating the electrodes. Cleaning the electrodes is done without interrupting the air flow.
The main components of all electrostatic precipitators are:

  • a power supply unit to supply high voltage DC power
  • ionizing section to impart a charge to the particulates in the gas stream
  • an attachment to remove the collected particulates
  • a housing to enclose the precipitator zone
The following factors influence the collection efficiency of electrostatic precipitators:
  • Larger collection surface areas and lower gas flow rates increase efficiency of electrostatic precipitators due to increased time for the electrical activity to collect the dust particles
  • The dust particle migration velocity to the collecting electrodes can be increased by:
    • Decreasing gas velocity
    • Increasing gas temperature and
    • Increasing the voltage field
There are two types of precipitators:
  • Single-stage precipitators that combine an ionization and collection step also known as cottrell precipitators. It is mainly used in mineral processing operations.
  • Low voltage, two stage precipitators that use a similar principle, but in this case, the ionization section is followed by collection plates. It is mainly used for filtration in air-conditioning systems. 
Electrostatic precipitators may be:
  1. Plate precipitators in which particles are collected on flat parallel surfaces about 20 to 30 cm apart with a series of discharge electrodes spaced along the centerline of two adjacent plates. The contaminated particles pass through the passage between the plates and the particles get charged and adhere to the collection plates. The particles are eventually removed by rapping the plates and the dust is collected in the hoppers or bins placed at the base of the precipitator.
  2. Tubular precipitators consist of cylindrical collection electrodes with discharge electrodes located on the axis of the cylinder. The contaminated gases flow around the discharge electrode and through the inside of the cylinders. The charged particles are collected on the grounded walls of the cylinder. The collected dust is removed from the bottom of the cylinder. They are generally used for collection of mist or fog or for adhesive, sticky, radioactive or extremely toxic materials.

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