89
CHAPTER EIGHT
8 THE COMBUSTION AIR - FLUE GAS SYSTEM
8.1 Function of the combustion air-flue gas system
The equipment considered as the gas loop has the function of
1. Creating the proper conditions for combustion
2. Supplying the ingredient of combustion in the proper proportions
3. Moving the product of combustion to the atmosphere.
The gas loop consist of auxiliary heat transfer surfaces and draft fans, the gas passages, the
chimney, the arrangement for supplying fuel and air to the combustion equipment and
removal of ash from it.
8.2 Fuel storage
Fuel storage is necessary as insurance against complete shut down of plant due to failure
of fuel supply. For coal, a storage of 10% of the annual consumption is required. Proper
care should be taken to avoid spontaneous burning especially coal with high sulphur. Gas
may be supplied directed from the pipeline and thus does not give any storage problem.
Oil storage creates less problem than coal storage due to its high heating value per unit
area.
8.3 Flue gas cleaning
The products of combustion of coal-fed fires contain particles of solid matter floating in
suspension, which must be collected so that the gas released to atmosphere is clean. This
is affected by using commercial dust collector. Commercial dust collector can be classified
as mechanical and electrical. Mechanical dust collectors include settling chamber,
cyclones, bughouse filters as well as wet type (scrubbers), which operates with water
sprays to wash dust from the gas. It requires large quantity of water if used in a power
station thus not popularly used. Dry scrubbers are the most common commercially. Dust
MPE 571E: POWER PLANT ENGINEERING
, 90
collectors can be classified according to operating principle as gravitational separators and
inertial separators
Gravitational separators act by slowing down gas flow so that particles remains in the
chamber long enough to settle to the bottom. They are not very suitable because of large
chamber volume needed. Inertial Separators act by rapid change of direction of gas.
Common focus are the baffle and the cyclone separator.
8.3.1 Settling Chambers
A settling chamber is a large box through which the effluent gas stream flows and in
which particles in the stream settle to the floor by gravity. Gas velocity must be kept low
enough so that settling particle are all maintained. The gas velocity is usually reduced by
expanding the ducting into a chamber large enough so that sufficiently low velocities
results. Although the principle of settling chambers could be used to remove even the
smallest particles, practical limitation in the length of such chambers restrict their
application to p 750 mm.
The settling chambers are normally used as pre-cleaner to remove large and possibly
abrasive particles, prior to passing the gas stream through other collector devices.
The performance of a settling chamber depends on the flow characteristics (laminar flow,
plug flow with no vertical mixing or plug flow with no vertical mixing and plug flow with
vertical mixing).
y
Unclean Clean Flue
u Flue Gas Gas out
Height, H
Vt
y critical
Length, L
x l l
Fig 8.1 Settling Chamber
Just like in the case of all particle control devices, the collection efficiency depends on the
particle diameter and the axial position of the particle at inlet to the chamber. There will
be a critical height ycrit such that a particle of a given diameter initiatly at x = 0, y = ycrit
will be at y = 0 at x = L. This particle will be the ‘last’ particle of that diameter to be
collected. Any particle which entered the chamber above y = y* will not be collected
The vertical position of the particle at any time after entering the chamber depends on the
terminal velocity of the particle, Vt
MPE 571E: POWER PLANT ENGINEERING
CHAPTER EIGHT
8 THE COMBUSTION AIR - FLUE GAS SYSTEM
8.1 Function of the combustion air-flue gas system
The equipment considered as the gas loop has the function of
1. Creating the proper conditions for combustion
2. Supplying the ingredient of combustion in the proper proportions
3. Moving the product of combustion to the atmosphere.
The gas loop consist of auxiliary heat transfer surfaces and draft fans, the gas passages, the
chimney, the arrangement for supplying fuel and air to the combustion equipment and
removal of ash from it.
8.2 Fuel storage
Fuel storage is necessary as insurance against complete shut down of plant due to failure
of fuel supply. For coal, a storage of 10% of the annual consumption is required. Proper
care should be taken to avoid spontaneous burning especially coal with high sulphur. Gas
may be supplied directed from the pipeline and thus does not give any storage problem.
Oil storage creates less problem than coal storage due to its high heating value per unit
area.
8.3 Flue gas cleaning
The products of combustion of coal-fed fires contain particles of solid matter floating in
suspension, which must be collected so that the gas released to atmosphere is clean. This
is affected by using commercial dust collector. Commercial dust collector can be classified
as mechanical and electrical. Mechanical dust collectors include settling chamber,
cyclones, bughouse filters as well as wet type (scrubbers), which operates with water
sprays to wash dust from the gas. It requires large quantity of water if used in a power
station thus not popularly used. Dry scrubbers are the most common commercially. Dust
MPE 571E: POWER PLANT ENGINEERING
, 90
collectors can be classified according to operating principle as gravitational separators and
inertial separators
Gravitational separators act by slowing down gas flow so that particles remains in the
chamber long enough to settle to the bottom. They are not very suitable because of large
chamber volume needed. Inertial Separators act by rapid change of direction of gas.
Common focus are the baffle and the cyclone separator.
8.3.1 Settling Chambers
A settling chamber is a large box through which the effluent gas stream flows and in
which particles in the stream settle to the floor by gravity. Gas velocity must be kept low
enough so that settling particle are all maintained. The gas velocity is usually reduced by
expanding the ducting into a chamber large enough so that sufficiently low velocities
results. Although the principle of settling chambers could be used to remove even the
smallest particles, practical limitation in the length of such chambers restrict their
application to p 750 mm.
The settling chambers are normally used as pre-cleaner to remove large and possibly
abrasive particles, prior to passing the gas stream through other collector devices.
The performance of a settling chamber depends on the flow characteristics (laminar flow,
plug flow with no vertical mixing or plug flow with no vertical mixing and plug flow with
vertical mixing).
y
Unclean Clean Flue
u Flue Gas Gas out
Height, H
Vt
y critical
Length, L
x l l
Fig 8.1 Settling Chamber
Just like in the case of all particle control devices, the collection efficiency depends on the
particle diameter and the axial position of the particle at inlet to the chamber. There will
be a critical height ycrit such that a particle of a given diameter initiatly at x = 0, y = ycrit
will be at y = 0 at x = L. This particle will be the ‘last’ particle of that diameter to be
collected. Any particle which entered the chamber above y = y* will not be collected
The vertical position of the particle at any time after entering the chamber depends on the
terminal velocity of the particle, Vt
MPE 571E: POWER PLANT ENGINEERING
