FABRIC FILTER

In a fabric filter, flue gas is passed through a tightly woven or felted fabric, causing PM in the
flue gas to be collected on the fabric by sieving and other mechanisms. Fabric filters may be in
the form of sheets, cartridges, or bags, with a number of the individual fabric filter units housed
together in a group. Bags are the most common type of fabric filter. The dust cake that forms
on the filter from the collected PM can significantly increase collection efficiency. Fabric filters
are frequently referred to as baghouses because the fabric is usually configured in cylindrical
bags. Bags may be 6 to 9m (20 to 30 ft) long and 12.7 to 30.5 centimeters (cm) (5 to 12 inches)
in diameter. Groups of bags are placed in isolable compartments to allow cleaning of the bags
or replacement of some of the bags without shutting down the entire fabric filter). Operating
conditions are important determinants of the choice of fabric. Some fabrics (i.e., polyolefins,
nylons, acrylics, polyesters) are useful only at relatively low temperatures of 95° to 150ºC
(200° to 300ºF). For high temperature flue gas streams, more thermally stable fabrics such as
fiberglass, Teflon, or Nomex must be used. Chemical nature of gas is also a consideration as
different fabrics have different resistance to acids or alkalies.

The practical application of fabric filters requires the use of a large fabric area in order to avoid
an unacceptable pressure drop across the fabric. Baghouse size for a particular unit is
determined by the choice of air-to-cloth ratio, or the ratio of volumetric airflow to cloth area.
The selection of air-to-cloth ratio depends on the particulate loading and characteristics, and
the cleaning method used. A high particulate loading will require the use of a larger baghouse
in order to avoid forming too heavy a dust cake.

Determinants of baghouse performance include the fabric chosen, the cleaning frequency and
methods, and the particulate characteristics. Fabrics can be chosen which will intercept a
greater fraction of particulate. In order to accomplish this, some fabrics are coated with a
membrane of very fine openings for enhanced removal of submicron particulate. However,
such fabrics tend to be more expensive. Cleaning intensity and frequency are important
variables in determining removal efficiency because the dust cake can affect the fine particulate
removal capability of a fabric. Cleaning procedures, which may be too frequent or too intense,
will also lower the removal efficiency of the fabric filter. On the other hand, if removal is too
infrequent or too ineffective, then the baghouse pressure drop will become too high.

Mechanical shaking of the bags has been a popular cleaning method for many years because
of its simplicity as well as its effectiveness. In a typical operation, dusty gas enters an inlet pipe
to the shaker. Very large particles are removed from the stream when they strike the baffle
plate in the inlet duct and fall into the hopper. The particulate-laden gas is drawn from beneath
a cell plate in the floor and into the filter bags. The gas proceeds from the inside of the bags to
the outside and through the outlet pipe. The particles are collected on the inside surface of the
bags and a filter cake accumulates. In mechanical shaking units, the tops of bags are attached
to a shaker bar, which is moved briskly (usually in a horizontal direction) to clean the bags.
The shaker bars are operated by mechanical motors or by hand, in applications where cleaning
is not required frequently. Reverse-air cleaning is another popular fabric filter cleaning method
that has been used extensively and improved over the years. It is a gentler but sometimes less
effective cleaning mechanism than mechanical shaking. Most reverse-air fabric filters operate
in a manner similar to shaker-cleaned fabric filters. Typically, the bags are open on bottom,
closed on top, and the gas flows from the inside to the outside of the bags with dust being
captured on the inside. However, some reverse-air designs collect dust on the outside of the
bags. In either design, forcing clean air through the filters in the opposite direction of the dusty
gas flow performs reverse-air cleaning. The change in direction of the gas flow causes the bag
to flex and crack the filter cake. In internal cake collection, the bags are allowed to collapse to
some extent during reverse-air cleaning. The bags are usually prevented from collapsing
entirely by some kind of support, such as rings that are sewn into the bags. The support enables
the dust cake to fall off the bags and into the hopper. Cake release is also aided by the reverse
flow of the gas because felted fabrics retain dust more than woven fabrics. Therefore, they are
more difficult to clean. For this reason, felts are usually not used in reverse-air systems.

Fabric filters in general provide high collection efficiencies on both coarse and fine
(submicron) particulates. Typical new equipment design efficiencies are between 99% and
99.9%.

Advantages of Fabric Filters

• Very high collection efficiency

• They can operate over a wide range of volumetric flow rates
• The pressure drops are reasonably low.
• Fabric Filter houses are modular in design, and can be pre-assembled at the factory

Disadvantages of Fabric Filters

• Fabric Filters require a large floor area.
• The fabric is damaged at high temperature.
• Ordinary fabrics cannot handle corrosive gases.
• Fabric Filters cannot handle moist gas streams
• A fabric filtration unit is a potential fire hazard as some fabrics are flammable; some
dust are explosive

ELECTROSTATIC PRECIPITATORS
An ESP is a PM control device that uses electrical forces to move particles entrained within an
exhaust stream onto collection surfaces. The entrained particles are given an electrical charge
when they pass through a corona, a region where gaseous ions flow. Electrodes in the center
of the flow lane are maintained at high voltage and generate the electrical field that forces the
particles to the collector plates. The high voltage electrodes are long wires or rigid “masts”
suspended from a frame in the upper part of the ESP that run through the centre of each channel.
Rigid electrodes are generally supported by both an upper and lower frame. The power supplies
for the ESP convert the industrial AC voltage to pulsating DC voltage in the range of 20,000
to 100,000 volts as needed. The voltage applied to the electrodes causes the gas between the
electrodes to break down electrically, an action known as a “corona.” The electrodes are usually
given a negative polarity because a negative corona supports a higher voltage than does a
positive corona before sparking occurs. The ions generated in the corona follow electric field
lines from the electrode to the collecting plates. Therefore, each electrode-plate combination
establishes a charging zone through which the particles must pass. As larger particles (>10μ
m diameter) absorb many times more ions than small particles (>1μm diameter), the electrical
forces are much stronger on the large particles. When the collection plates are filled to capacity,
the particulate is removed from the plates by “rapping,” which is a mechanical means to
dislodge the particulate. The collected particulate material slides downward into a hopper
located below the unit. The collection efficiency of an ESP is quite reliably about 99 percent
for particles less than 10 micrometers.
Control of gaseous pollutants from stationary sources (stack) – Adsorption

When a gas or vapor is brought into contact with a solid, part of it is taken up by the solid. The
molecules that disappear from the gas either enter the inside of the solid, or remain on the
outside attached to the surface. The former phenomenon is termed absorption (or dissolution)
and the latter adsorption. Adsorption is the binding of molecules or particles to a surface. In
this phenomenon molecules from a gas or liquid will be attached in a physical way 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. Activated carbon is the universal standard for purification and removal
of trace organic contaminants from liquid and vapor streams. Carbon adsorption uses activated
carbon to control and/or recover gaseous pollutant emissions. In carbon adsorption, the gas is
attracted and adheres to the porous surface of the activated carbon. Removal efficiencies of 95
percent to 99 percent can be achieved by using this process. Carbon adsorption is used in cases
where the recovered organics are valuable. For example, carbon adsorption is often used to
recover perchloroethylene, a compound used in the dry cleaning process. Carbon adsorption
systems are either regenerative or non-regenerative. A regenerative system usually contains
more than one carbon bed. As one bed actively removes pollutants, another bed is being
regenerated for future use. Steam is used to purge captured pollutants from the bed to a
pollutant recovery device. By "regenerating" the carbon bed, the same activated carbon
particles can be used again and again. Regenerative systems are used when concentration of
the pollutant in the gas stream is relatively high. Non-regenerative systems have thinner beds
of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it
becomes saturated with the pollutant. Because of the solid waste problem generated by this
type of system, nonregenerative
carbon adsorbers are usually used when the pollutant concentration is extremely low.

Regenerative Carbon Adsorption System Non-Regenerative Carbon Adsorption System

Control of gaseous pollutants from stationary sources (stack)– Absorption

The removal of one or more selected components from a gas mixture by absorption is probably
the most important operation in the control of gaseous pollutant emissions. Absorption is a
process in which a gaseous pollutant is dissolved in a liquid. Water is the most commonly used
absorbent liquid. As the gas stream passes through the liquid, the liquid absorbs the gas, in
much the same way that sugar is absorbed in a glass of water when stirred. Absorption is
commonly used to recover products or to purify gas streams that have high concentrations of
organic compounds. Absorption equipment is designed to get as much mixing between the gas
and liquid as possible. Absorbers are often referred to as scrubbers, and there are various types
of absorption equipment. The principal types of gas absorption equipment include spray
towers, packed columns, spray chambers, and venture scrubbers. The packed column is by far
the most commonly used for the absorption of gaseous pollutants. The packed column absorber
has a column filled with an inert (non-reactive) substance, such as plastic or ceramic, which
increases the liquid surface area for the liquid/gas interface. The inert material helps to
maximize the absorption capability of the column. In addition, the introduction of the gas and
liquid at opposite ends of the column causes mixing to be more efficient because of the counter-
current flow through the column. In general, absorbers can achieve removal efficiencies greater
than 95 percent. One potential problem with absorption is the generation of waste-water, which
converts an air pollution problem to a water pollution problem.

Typical packed column diagram