FIELD OF THE INVENTION
The present invention, generally relates to fuel feeding arrangement for a
Circulating Fluidized Bed (CFB) combustors which is operating at pressures
slightly above atmospheric pressure. More particularly, the invention relates to a
device for creating negative pressure for feeding crushed solid fuel into a Cii
Circulating fluidised bed combustor.
BACKGROUND OF THE INVENTION
Circulating Fluidised Bed Combustor is a system consisting of fluidized bed
enclosure, where the solid particles are in a suspended state. The state is
achieved by controlling the amount of air passing through the bed. At low
velocities, the pressure drop across the bed is negligible and buoyant force
derived from low velocities is not sufficient to suspend the particle in the bed.
The bed remains static and undisturbed.
As the air flow (velocity) is increased gradually, the buoyant force overcomes the
gravitational force exerted on the particle which remain suspended in the state of
fluidized bed. The pressure drop increases till a point, where it matches the
weight of particles per unit area. At this stage, the bed particles are in
suspended state and it is called "minimally fluidized bed".
When the flow is increased further, the bed becomes highly turbulent as the
interstitial spaces between the bed particles increased allowing the high velocity
gas streams promoting rapid mixing of the bed. At this stage, the bed of solids
attain pseudo fluid properties.
In a CFB combustor, velocities much higher than the minimum fluidised condition
are maintained wherein most of the bed solids are carried out of the combustor
along with the air. These solids are collected in a cyclone and returned back to
the combustor by means of a non mechanical seal pot. A substantial portion of
the particles are also internally circulated which is evidenced from the downpour
of the solids along the walls of the combustor. The entire bed is preheated to an
ignition temperature of the fuel. On reaching the ignition temperature, the fuel
particles are fed. They undergo combustion and release heat which is imparted
to the whole volume of the combustor uniformly due to high turbulence and
rapid mixing characteristics of the fluidizing process.
In some of the systems, the pressure at which the fuel fed is maintained
constitutes a near atmospheric, and in some other cases, the pressure is
maintained higher than atmospheric (around 40 -60 mbar above atmospheric
pressure). The fuel feed points are maintained at higher pressure which further
falls down into a pressurized combustor. However, in a pressurized combustor,
feeding solid fuel involves several factors. Enormous volume of high temperature
gases which contain carbon monoxide gets leaked out and poses a threat to the
equipment and human in the surrounding area.
US patent no. 5113771 describes a coal injector which contains an acceleration
section to improve the uniformity of coal-air mixture to be burned. An integral
splitter is provided for splitting the coal-air mixture into a number of separate
streams or jets, and a center body directs the streams at a controlled angle into
the primary zone of a burner.
US patent no. 5558473 describes a labyrinth seal coal injector for continuously
injecting dry, sized, coal into a pressurized hydraulic pipeline for transport. The
injector consists of a rotor or screw rotated by a motor. Coal is compacted in the
injector and forms a hydraulic seal against the pressure in the pipe line.
US Patent No. 4,446,799 discloses a Fluidised Bed Fuel Feed System which uses
movable nozzle on the furnace wall for feeding fuel on the bed of the fluidised
bed boiler.
US Patent No. 4,259,911 describes a Fluidised Bed Boiler Feed System which
utilises fuel pump arrangement for feeding the fuel / sorbent mixture in dense
phase transport with higher air pressures.
US Patent No. 4,203,391 discloses a Fluidised Bed Fuel Feeder which utilises T-
Shaped nozzle for feeding the coal into the fluidised bed boiler.
US Patent No. 5,868,083 describes a method and a device for feeding fuel into a
fluidised bed which uses a fuel pipe with storage vessel for feeding over bed.
US Patent No. 4,275,668 describes a coal feed system for a fluidized bed
combustor which uses a separator with agglomerator for feeding onto the
surface of the bed of the Fluidised bed boiler.
The prior art of fuel feeding devices to the pressurized system is shown in Figure
1. Here, the solid fuel from the bunker is metered in a drag link chain feeder or a
belt type weigh feeders. The discharge from the metering feeder falls onto a
rotary air lock feeder which prevents the leakage of hot gases from the
pressurized combustor. However the rotary feeders are difficult to be maintained
in leak tight condition during operation due to wear and tear of the moving
parts. A sophisticated high temperature tight shut off valve is employed to isolate
the pressurized combustor in case of any maintenance work in rotary feeder or
the metering feeder. In spite of providing such sophisticated system due to wear
and tear hot gases are found leaking around the feeding area which is very much
hazardous from the equipment and human safety point.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a device for creating a
negative pressure for feeding solid fuel into a pressurised combustor to prevent
high temperature gas leakage.
Another object of the invention is to propose a device for creating a negative
pressure for feeding solid fuel into a pressurised combustor to prevent high
temperature gas leakage, which eliminates the use of costly components for
example, high temperature leak tight gate valves and rotary air lock feeders.
SUMMARY OF THE INVENTION
The inventive concept implements a combination of the principle of venturi and
vortex to create a negative pressure. The reduction in the flow area of the
annular air space accelerates the air (increases the velocity) and the static
pressure is reduced. In the reducing section, a swirler is provided which forms a
vortex at the exit. The combination of the static pressure reduction and the
vortex formation reduces the static pressure at that location below atmospheric
pressure and creates a zone which pulls the solids fuel which is already falling
under the influence of gravity. The crushed solid fuel comes in contact with air at
this point and air plus solid fuel mixture flows down. To recover the operating
pressure of the combustor, a diffuser section is provided.
According to the inventive configuration of the device solid fuel is allowed to fall
by gravity from the metering feeder into a chute which may be rectangular or
circular in shape, surrounding which pressurised air enters in the annular area.
Reduction of annular cross sectional flow area along with a swirler assembly
enables creation of negative pressure (below atmospheric) condition. The solids
enter the region of negative pressure and are carried to the combustor. The
coal-air mixture then passes through a recovery diffuser nozzle portion where
some of the lost air pressure is recovered. Since fuel is fed into negative
pressure no gas leak will be there and there is no need for sophisticated rotary
air lock feeder or high temperature leak tight gate valve.
The device eliminates the need for sophisticated rotary air lock feeder and high
temperature gas tight knife gate valve, and keeps the area around the feeding
system at low temperature thereby avoiding equipment damage due to high
temperature. The generated negative pressure prevents gas leakage during fuel
feeding and also when fuel is not fed, and further avoids pollution by harmful
gases like the CO, SOX etc which are present in high amounts at high pressure in
the combustor making the place in the area safe for working.
The device is capable of feeding any granular solid material into pressurized
chamber and prevents the leakage of the leak of contents of the pressurized
chamber to the surrounding area.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Shows a schematic view of a crushed solid fuel feeding system of
prior art employed in a pressurized CFB combustor.
Figure 2 Shows a schematic view of a crushed solid fuel feeding system in a
pressurized CFB combustor according to the invention
Figure 3 Shows the details of a negative pressure creating device in a
crushed solid fuel feeding system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION
A conventional arrangement of typical fuel feed system for feeding crushed solid
fuel into a pressurized circulating fluidised bed combustor is shown in Figure (I)
which comprises a crushed solid fuel bunker (I), a Drag link chain feeder (2), a
Rotary air lock feeder (3), and a High temperature Knife gate valve (4).
The present invention provides a device (5) to create negative pressure i.e.,
pressure below atmospheric pressure. Solid fuel is introduced at that point. Due
to this, there is no passing of hot gases into the upstream fuel feed system like
the drag link feeders. In this system a low temperature gate valve (6) is
sufficient to act as a fuel isolation valve.
The crushed solid fuel that is metered falls on to chute (7). Surrounding this
chute (7) is an air entry duct (8). Air passes through an annular space. Velocity
of the air is increased in an accelerating section (9). Inside this accelerating
section (9), a swirler (10) is placed to give swirl to the accelerating air flow. The
swirling air with high velocity comes in contact with the free downward flowing
crushed solid fuel in a mixing section (11). The air fuel mixture then passes
through a pressure recovery diffuser section (12).
The advantage of the invention lies in fact that no complex/costly components
like the rotary air lock feeder or the high temperature gas tight knife gate valve
is required to prevent the high temperature, high pressure gases from the
combustion chamber from leaking out. The entire fuel feeding system is kept at
low temperature and pollution around the feeding system is totally avoided.
WE CLAIM
1. A device (5) to create a negative system in a fuel feeding system for
feeding crushed solid fuel into a pressurized circulating fluidised bed
combustor, the negative pressure being a pressure below atmospheric
pressure when the crushed solid fuel is introduced into the system, the
device comprising:
- a chute (7) along which the solid fuel ingress the system at a
predetermined quantity;
- an air entry duct (8) surrounding the chute (7) and having an annular
space including an accelerating section (9) to flow high velocity air;
characterized by comprising :
- a swirler (10) disposed into said accelerating section (9) to generate a
swirl to the accelerated air-stream which is allowed at a mixing section
(11) to contact the crushed solid fuel free-flowing downwardly; and
- a pressure recovery diffuser section (12) disposed downbelow the mixing
section (11) and allowing the air-fuel mixture to pass through, the
negative pressure created enables to prevent any leakage of gas and
allow recovery of the lost pressure.
2. A device (5) to create a negative system in a fuel feeding system for
feeding crushed solid fuel into a pressurized circulating fluidised bed
combustor, as substantially described and illustrated herein with reference
to the accompanying drawings.

The present invention relates to a device (5) to create a negative system in a
fuel feeding system for feeding crushed solid fuel into a pressurized circulating
fluidised bed combustor, the negative pressure being a pressure below
atmospheric pressure when the crushed solid fuel is introduced into the system,
the device comprising a chute (7) along which the solid fuel ingress the system
at a predetermined quantity; an air entry duct (8) surrounding the chute (7) and
having an annular space including an accelerating section (9) to flow high
velocity air; a swirler (10) disposed into said accelerating section (9) to generate
a swirl to the accelerated air-stream which is allowed at a mixing section (11) to
contact the crushed solid fuel free-flowing downwardly; and a pressure recovery
diffuser section (12) disposed downbelow the mixing section (11) and allowing
the air-fuel mixture to pass through, the negative pressure created enables to
prevent any leakage of gas and allow recovery of the lost pressure.