FIELD OF INVENTION :
The present invention generally relates to enhancement of the
combustion efficiency of pulverized coal based tangentially fired
Boilers. More particularly, the invention relates to an under-fire air
system for pulverized coal-based tangentially fired boiler.
BACKGROUND OF THE INVENTION :
Fossil fuel is generally burnt directly in the boiler furnace to provide
heat energy, although a waste heat energy from another source may
be used for the furnace. In the pulverized coal fired boilers the loss
due to un-burnt carbon, which is wasted with the bottom ash account
for nearly 2-4%.
US Patent 4241672 discloses a method of metering under fire air for
increasing efficiency and reducing particulate emissions from wood-
fire, spreader-stoker boilers. According to this invention, a portion of
the combustion air, approximately one pound of air per pound of
wood, is fed through the grate into the fuel bed, while the remainder
of the combustion air is distributed above the fuel in the furnace, and
the fuel bed is maintained at a depth sufficient to consume all oxygen
admitted under fire and to insure a continuous layer of fresh fuel
there over to entrap charred particle inside the fuel bed.

US patent 4278034 describes a furnace adapted to burn a low grade,
granulated fuel. The furnace has a combustion vessel to receive a
fuel and spaced inner and outer walls. A lid is positionable on the top
of the combustion vessel and there is an exhaust vent in the lid. A
bottom to the combustion vessel. A false bottom to the combustion
vessel spaced above the bottom to define a passageway. There is at
least one perforate column extending from the bottom through the
combustion vessel to adjacent the lid. Air may be fed to the fuel and
the burned residue of any fuel can be supported.
However, none of the prior art provides an under fire air system for a
pulverized coal-based tangentially fired boiler to achieve a complete
combustion of the coal particles in the furnace so as to increase
efficiency and lessen the cost. Further, Indian coal used in such
boilers contains a high degree of impurities and called high ash-
content coal.
It is therefore important to achieve a complete combustion in the
furnace by minimizing losses due to partial combustion of the solid
fuel or providing excess air.

The inventors have observed that an improved mixing including
turbulence in the furnace environment increases residence time of
the air present and consequently provides sufficient time and spatial
distribution of the fuel and its constituents to completely combust.
Accordingly, a complete combustion including higher combustion
efficiency can be achieved.
OBJECTS OF THE INVENTION :
It is therefore an object of the invention to propose an under fire air
system for pulverized coal based tangentially fired boilers, which
eliminates the disadvantages of prior art.
Another object of the invention is to propose an under fire air system
for pulverized coal based tangentially fired boilers, which enhances
the combustion efficiency of the boilers.
A still another object of the invention is to propose an under fire air
system for pulverized coal based tangentially fired boilers, which is
enabled to fluidize the coal with the under fired air on bottom of the
furnace.

Yet another object of the invention is to propose an under fire air
system for pulverized coal based tangentially fired boilers, in which
the fluidization acts as a screen.
A further object of the invention is to propose an under fire air
system for pulverized coal based tangentially fired boilers, in which
the under fired air forms a sheet of air in the bottom of the furnace
to generate a turbulence for the coal particles to combust completely.
A still further object of the invention is to propose an under fire air
system for pulverized coal based tangentially fired boilers, which
enhances the residence time for the fuel particles to achieve
complete combustion at the bottom of the furnace.
SUMMARY OF THE INVENTION :
According, there is provided An under fire air system for pulverized
coal based tangentially fired boilers; comprising an under fire air flow
nozzle provided at the bottom area of the furnace, air from a
secondary source of the boiler being injected to generate a
fluidization of the fluid-mixture; and a combustion zone configured at
the lower part of the furnace such that the under fired air generates
a fluidization adjacent the area of bottom ash accumulation which

acts as a screen to enable separation of the unburnt carbon particles
from the bottom ash including provisioning of oxygen to achieve a
complete combustion of the solid fuel.
According to the invention, the under fire air system provides a
fluidization environment in the bottom of the furnace. This
fluidization environment which is present in the bottom of the
furnace allows combustion of the un-burnt carbon particles from
bottom ash which is falling downward due to gravity. The under fired
air supply the necessary oxygen for the un-burnt carbon to burn and
hence achieve a complete combustion in the boiler. This under fired
air allows higher residence time for the coal to burn. The under fired
air form a sheet of air in the bottom of the furnace which acts as a
screen and provides a turbulence including higher residence time for
the coal particles to combust completely. The Under fired air is
supplied from the secondary air source of the boiler.
The under fire air system of the invention is placed on bottom of the
furnace and is used for pulverized coal based tangentially fired
boilers. This system of fluidization acts as a screen and separates the
fuel and gives the oxygen for allowing a complete combustion. This
system provides the necessary turbulence and residence time for the
coal particles to combust completely. The under fire air system is

used to enhance the combustion processes and separate the unburnt
carbon particles from bulk bottom ash. The wind box is providing
sufficient quantity of oxygen to the combustion chamber but in prior
art, the bottom of the furnace is devoid of oxygen necessary for
complete combustion in absence of air flow because products of
combustion are flowing upwards.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows an embodiment of an under fire air system for
pulverized coal based tangentially fired boilers according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION :
Figure 1 shows an embodiment of the under fire air system for
pulverized coal based tangentially fired boilers. According to the
invention, an under fire air flow nozzle (1) is provided at the bottom
of the furnace which creates a combustion zone (8) at the lower part
of the furnace. The system enhances the combustion efficiency.

The wind box (5) of the boiler provides the fuel and air mixture for
the combustion process (6). But in pulverized coal fired boilers, the
loss due to un-burnt carbon (3), which goes out of the boiler along
with bottom ash (4) is in the tune of 2-4%.
The fly ash and flue gases (7) normally flow towards top of the boiler
due to the negative - suction pressure created at the other end of
the boiler. The suction pressure is provided by a plurality of fans
which are provided in the power plant.
The fluidization created by the under fired air acts as a screen and
separates the unburnt carbon particle (3) from the bulk bottom ash
(4) so as to provide the oxygen to achieve a complete combustion
process.

WE CLAIM :
1. An under fire air system for pulverized coal based
tangentially fired boilers; comprising:
an under fire air flow nozzle (1) provided at the bottom area
of the furnace, air from the secondary source of the boiler
being injected to generate a fluidization of the fluid-mixture;
and
a combustion zone (8) configured at the lower part of the
furnace such that the under fired air generates a fluidization
(2) adjacent the area of bottom ash accumulation which acts
as a screen to enable separation of the unburnt carbon
particles (3) from the bottom ash (4) including provisioning
of oxygen to achieve a complete combustion of the solid
fuel.

2. An under fire air system for pulverized coal based
tangentially fired boilers as substantially described and
illustrated herein with reference to the accompanying
drawing.

The invention relates to an under fire air system for pulverized coal
based tangentially fired boilers; comprising: an under fire air flow
nozzle (1) provided at the bottom area of the furnace, air from the
secondary source of the boiler being injected to generate a
fluidization of the fluid-mixture; and a combustion zone (8)
configured at the lower part of the furnace such that the under fired
air generates a fluidization (2) adjacent the area of bottom ash
accumulation which acts as a screen to enable separation of the
unburnt carbon particles (3) from the bottom ash (4) including
provisioning of oxygen to achieve a complete combustion of the solid
fuel.