Environmental Auditing-Day 2
AIR POLLUTION CONTROL DEVICES,
A factor that makes indoor air pollution a more significant problem, now than it was in
the past is the recent trend toward complete, insulation of buildings and homes for energy
conservation. Recently, the ventilation of the average home due to leakage has been
reduced from about one air change per hour to about 0.25 changes per hour. This
decreases in ventilation means that people are being exposed to staler and possibly more
polluted air.
Two of the major ambient pollutants, carbon monoxide and nitrogen dioxide, are also of
concern indoors. The highest levels of these pollutants seem to accumulate in the
kitchens of well – insulated houses, often exceeding the outdoor concentrations. Two
different gaseous substances that particularly affect indoor air quality are radon and
formaldehyde. Radon gas is produced continuously from the radioactive decay of radium,
which is a natural trace element in most rock and soil. Building materials, therefore
including concrete and brick, as well as the earth foundation itself, are a source of radon
gas. Indoor levels of this gas, a suspected carcinogen, are usually higher than outdoor
levels.
Formaldehyde is found in foam insulation, carpets, drapes, and other household items. It
is also used in plywood and particle board bonding agents, and is a by-product of natural
gas combustion. In addition to causing nausea, respiratory irritation, and other effects, it
is suspected of causing more serious long-term problems, such as cancer. In some cases,
household formaldehyde level have been measured in excess of 3 PPM, which is the
OSHA standard for the industrial workplace.
Suspended particulates are also an indoor air pollution problem of special concern. Even
in enclosed environments where there is no cigarette smoke, the level of respirable
suspended particulates (Less than 25 um in size) may be as much as 40 ug/m In
environments with cigarette smoke, the levels may be as high as 700 ug/m; this exceeds
the allowable NAAQS values for particulates by a large margin. It is of special concern to
RTI MUMBAI
1
Environmental Auditing-Day 2
researchers doing epidemiological studies that link respiratory disease to air pollution,
and particularly to cigarette smoke.
POLLUTION CONTROL STRATEGIES
There are several approaches or strategies for air pollution control. The most effective
control, of course, would be to prevent the pollution from occurring in the first place.
Complete source reduction would accomplish this, but source reduction is only practical
when cleaner technology is available.
Dilution seems to offer another alternative for air pollution control. Tall smoke stacks can
be used to reduce the ambient ground-level concentration of pollutants near the source.
Taller stacks or chimneys take the gases higher into the atmosphere for greater dilution
effects of dispersion and mixing.
Dilution may be effective to some degree, but the use of tall stacks and dispersion for air
pollution control has a serious limitation; generally, what goes up most come down. The
problem of pollutant transport and acid rain, demonstrates this weakness. In addition, tall
stack which at some power plants are as high as 300 m (1000 ft ), are very unattractive,
and may be hazardous to aircraft; Because of these limitations, stacks trend are supposed
to be a last resort for air pollution control.
Another option for air pollution control is to relocate the source in order to minimize
adverse environmental impacts in a particular locality. Community “air zoning” may be
included in the local planning process to require power plants or industrial facilities to be
located where fewer people will be affected by the pollutants. The location of these zones
can be established on the basis of prevailing wind and weather conditions. But this also
has its limitations; although local air quality may be protected, the pollutants can still be
“air-mailed” to a neighboring community.
A more positive approach to air pollution control is to make source reduction fuel
substitutions and or process change. For example, making more use of solar,
hydroelectric, and geothermal energy would eliminate some of the pollution caused by
RTI MUMBAI
2
Environmental Auditing-Day 2
fossil fuel combustion. (Nuclear power would do the same, but other problems related to
radioactive waste disposal and safety remain to be solved). Using natural low-sulphur
coal and oil would decreases SO2 emissions from fossil fuel power generating stations.
And the technology is available for treating and desulfurizing “dirty” fuels prior to
combustion, but it is expensive. Completely changing certain industrial manufacturing
processes can serve to reduce air pollution, one example of this is to use oxygen or
electric furnaces instead of open-hearth furnaces in the steel industry.
Even if the fuel or process is not changed, the use of correct operation and maintenance
practices can be very important for minimizing air pollution, and should not be
overlooked as an effective control strategy. For example, if a power plant operator allows
too much excess air into the boiler furnace, flash emission will increase. And adding too
much sulphur at a sulphuric acid manufacturing plant, without providing enough air can
cause excessive sulphur dioxide emission. Even the failure to properly lubricate a fan
motor at an incinerator could lead to unnecessary pollution.
POLLUTION CONTROL EQUIPMENT
The most realistic option for air quality protection is to reduce the most of pollution at its
source. When fuel substitutions or process changes are not possible, or they are simply
insufficient to accomplish the goal, then some type of air cleaning equipment must be
installed.
There are several types of air pollution control devices that can collect or trap the
pollutants before they are emitted into the atmosphere. Some of these devices serve to
control only suspended particulates, and other control only gaseous pollutants. The
design or selection of a particular type of air cleaning equipment depends on the physical
properties of the pollutant to be removed as well as on the temperatures, corrosivity, and
other characteristic of the carrier or bulk gas stream.
Control of Particulates
RTI MUMBAI
3
Environmental Auditing-Day 2
Most ambient particulate air pollution comes from stationary sources, particularly from
power plants, industrial processes, and incinerators. The primary factors that determine
which type of equipment is best for a specific application are the average particle size and
particle density.
SETTLING CHAMBER
The simplest device is the setting chamber, which is drawn schematically in the Figure. It
is basically an enlarged section or compartment in the flue in which the velocity of the
carrier gas is reduced. When the air stream velocity is slowed down sufficiently, the
coarse particulates (those more than about 40um in size) can settle out under the force of
gravity. A settling chamber of this type usually serves as a “precleaner” to prevent
clogging of the more efficient small-particle collectors that must follow it. Sometimes
baffles are added inside the chamber, to increase its efficiency.
Figure: A setting chamber or enlarged flue section provides a simple way to remove
settleable particulates at the source.
CYCLONE
A device called a cyclone is also used for reducing TSP emissions. Instead of relying
only on velocity reduction and gravity settling, the particles are subjected to centrifugal
force and friction, which separates them from the carrier gas. In a cyclone, which is
illustrated in Figure the carrier gas enters from a tangential direction at the outer wall of
the device and forms a vortex as it swirls around inside a cylindrical and conical shell.
The particulates are forced against the wall by centrifugal force, where they are allowed
down by friction, and then they slide down into a dust hopper. The cleaned gas steam
them swirls upwards in a narrower spiral, through an inner cylinder and toward the outlet.
A cyclone is most efficient when serving to remove coarse particulates, but it is also
somewhat effective in removing smaller particles. A typical “cut diameter” for a cyclone
is about 15 um; the cut diameter is that of which 50 percent of the particles are not
RTI MUMBAI
4
Environmental Auditing-Day 2
collected, and 50 percent of the particles are collected. Unlike the settling chamber, the
removal efficiency for small particles in a cyclone increases as the velocity increases.
Scrubber:
A device called a wet scrubber serves to trap suspended particles in a liquid aerosol. In
effect, a wet scrubber” washed the particulates out of the gas stream, as they collide and
are intercepted by the countless number of tiny droplets in the aerosol. There are several
types of wet scrubbers, the simplest of which is the spray tower. In a spray tower, the
upward flowing carrier gas stream is washed by water sprayed downward from a series of
nozzles. Spray towers are most efficient for removing particles that are 10 um of greater
in size. A schematic diagram of a spray tower scrubber is shown in Figure
Figure: In a cyclone collector, particulates are spun out toward the outer wall by
centrifugal force. They are slowed down by friction and settle to the bottom, clean air
flows upward and out the top.
Figure:- The spray tower is a type of wet scrubber that removes suspended particulates
from the carrier gas.
Another type of wet collector is the wet centrifugal scrubber. This device services to
increase the relative velocity between the water droplets and the dirty gas stream, thereby
increasing particulate removal efficiency; particles as small as 2 um can be removed. It
should be recognized that in any type of scrubber, the spent wash water that is generated
must be collected and treated before discharge into a river or lake. The wet scrubber is a
good example of the close interrelationship between air and water pollution control
problems.
Electrostatic Precipitator:A relatively large piece of equipment used to remove very tiny particulates from the
carrier gas, especially at power generating stations, is the electrostatic precipitator. This
RTI MUMBAI
5
Environmental Auditing-Day 2
device applies a high voltage in order to charge the particulates electrically; they can then
be attracted in an electric field toward collecting surfaces, on which they become trapped.
The collecting surfaces usually comprise a series of large rectangular metal plates, which
are suspended vertically in a paralleled arrangement within a box like structure. The
charged particles that adhere to the plates are removed when the plates are mechanically
vibrated or rapped. The falling particles are collected at the bottom of the unit, and
disposed of, usually in a landfill. A cutaway view of a typical electrostatic precipitator.
Figure:- A cutaway view of an electrostatic precipitator, (Western Precipitation Division,
Jay Manufacturing Company)
An electrostatic precipitator can remove particles as small as 1um. With an efficiency
exceeding 99 percent. It the TSP level in the carrier gas is very high, a setting chamber of
cyclone is generally installed in front of the precipitator, for precleaning of the gas. The
removal efficiency of a precipitator is very sensitive to the velocity and distribution of the
gas stream flowing across the collecting plates. The flow must be slow and uniform, that
is the same across all of the plates, form top to bottom.
The installation of precipitator units at a power generating facility or at an industrial plant
represents a very large investment for the owner. Removal efficiencies must be
guaranteed by the equipment manufacturer, so as to ensure the owner that air quality or
source performance standards will not be violated Because of this, the manufacturers
usually build and test exact scale models of each precipitator unit in a laboratory before
they are actually constructed in the field. In this way, the required shape or configuration
of ductwork and baffles can be predetermined, so as to provide uniform flow velocities
across the plates.
Baghouse Filters:
One of the most efficient devices for removing suspended particulates from a carrier gas
stream is the baghouse filter. It can remove particles as small as 0.01 um. A typical
RTI MUMBAI
6
Environmental Auditing-Day 2
installation comprises a series of long and narrow filter bags, which are suspended upside
down in a large enclosure. It is illustrated schematically.
Dirty air is blown through the bottom of the enclosure by fans. The particulates are
trapped inside the bags, whereas the clean air passes through the filter fabric and exits at
the top of the “baghouse”. This equipment is generally preceded by settling chambers or
cyclones to reduce the particulate load and the required cleaning frequency for the filter
bags. Several compartments can be cleaned while the others remain in service.
Figure:- A section view of a baghouse filter. The filters may be cleaned by mechanical
vibrations or by blowing clean air back through the unit.
In cleaning, bag filters are mechanically shaken or vibrated to collect the particulates so
that they can be removed for disposal. The air cleaning efficiency of the baghouse filter
approaches 100 percent for particles 1 um or larger in size, and particles as small as 0.01
um can also be removed to a significant extent. But baghouse filters cause relatively high
pressure losses, and they are expensive to maintain. Also, the flue gas or carrier air
stream generally must be cooled before passing through the unit; cooling coils needed for
this purpose add to the expense.
Control of Gases
Gaseous air pollutants can be controlled using the techniques of absorption or adsorption,
similar to the methods used for air sampling, but not a much larger scale. A third method
for controlling gaseous pollutants involves combustion. The specific control method
selected depends on the nature and properties of the pollutant to be controlled.
A spray tower scrubber, similar to the type used for particulate control, can also be used
to control gaseous emissions by absorbent. Instead of water a reactive liquid absorbent
may be used to capture the pollutant molecules by chemical reaction. In addition to spray
towers, other types of absorbers include packed towers, plate towers, liquid jet scrubbers,
or agitated tanks. Each of these devices has a different physical design, but they all
operate on the same basic principle, the gaseous pollutant to be removed is put into
RTI MUMBAI
7
Environmental Auditing-Day 2
contact with a liquid, which absorbs it by solution or chemical reaction, and separates it
from the flue gas.
Sulphur dioxide, for example, can be removed from the flue gases at coal-fired power
plants by scrubbing with a solution of lime, CaO. The SO2 reacts with the lime, forming
an insoluble precipitate of calcium sulphite, CaSO3. The concentrated slurry of insoluble
solids that form in this reaction is called flue gas desulfurization (FGD) sludge.
Application of this processes would help to reduce ambient SO2 levels, and could
mitigate the acid rain problem. But in addition to the great expense (which would be
passed on directly to the consumer as higher rates for electricity), there remains a major
disposal problems for the millions of tons of PGD sludge that would be generated each
year.
Gas adsorption, as contrasted with absorption, is a surface phenomenon. The gas
molecules are sorbed (attracted and held) on the surface-area-to-volume, it is very porous
and has an extremely high surface-area-to volume ration. A schematic diagram of an
activated carbon adsorption unit is shown in Figure.
Figure:- Activated carbon can be used to adsorb certain gaseous air pollutants. The
chemical process of combustion (rapid oxidation) may be used to convert gaseous
hydrocarbon, pollutants to carbon dioxide and water. The hydrocarbons, usually the
product of incomplete combusion, are in effect reburned in a control device, to form the
products of complete combusion. The incinerator of afterburner must be designed to
provide sufficient oxygen, turbulence, burning time, and temperature, so that complete
combustion will occur. A schematic diagram of a combustion device is shown in Figure.
Figure:- Direct combustion can be used to control emissions of combustible gases and
odors Natural gas is used as an auxiliary fuel to initiate and maintain a stable flame.
Certain substances, such as platinum, may act in a manner that will assist the combustion
or oxidation reaction. Such substances are called catalysts. A device called a catalytic
converter is installed in the exhaust system of an automobile. It allows complete
RTI MUMBAI
8
Environmental Auditing-Day 2
oxidation of the combustible gases in the exhaust to occur at relatively low temperatures,
in effect this is a form of “flameless combustion”
The control of gaseous emissions from mobile sources, primarily the automobile, cannot
be accomplished by the catalytic converter alone. Certain modifications to the internal
combustion engine are needed to first minimize the emission of hydrocarbons, as well as
of carbon dioxide and nitrogen dioxide. The use of positive crankcase ventilation (PVC)
systems, for example, serves to recycle gases that slip by the piston rings back to the
engine intake manifold. A small PVC check valve prevents the build up of excess
pressure in the engine.
Engine operating parameters can also be controlled in order to reduce certain emissions.
For example, increasing the air-to-gas ratio of the fuel and air mixture helps combustion
and reduces carbon monoxide and hydrocarbon emissions. But the higher combustion
temperatures that then occur cause an increase in the emission rate of nitrogen dioxide.
Other operating conditions of the engine can be modified, but then there may be a loss of
power and fuel economy. Obviously, controlling emissions from the internal combustion
engine is a complex problem.
Changing the fuels used in mobile sources can be effective in controlling air
pollution. The use of lead-free gasoline, for instance, protects public health from
airborne lead-gasoline must be used to prevent damage to catalytic converters.
Ultimately, the use of electric vehicles, or Hydrogen Gas or CNG or other
unconventional engines, could be necessary to eliminate serious air pollution
problems from mobile sources.
RTI MUMBAI
9