FIELD OF THE INVENTION
The present invention generally relates to effective removal upon enhanced
cooling of bed ash from a combustor of fluidized bed system for example,
circulating fluidized bed ( CFB) boilers. In particular, the invention relates to a
new mechanism to enchance heat transfer from ash (along with fluidizing air
cooling) to cooling water. More particularly the invention relates to a serpentine
coil-panel device for systematic plug flow cooling of ash with air in an ash
evacuation chamber of circulating fluidized bed (CFB) boiler.
BACKGROUND OF THE INVENTION
Fluidized Bed Combustion boiler functions in a system comprising a fluidized bed
enclosure, where the solid particles are in a suspended state. The suspended
state is achieved by controlling the amount of air passing through the bed.
However, at low velocities, the pressure drop across the bed is negligible and
buoyant force derived from low velocities is not sufficient to maintain the
particles suspended in the bed. Thus, at low-velocities, the bed remains static
and undisturbed.

As the air flow is increased gradually, the buoyant force overcomes the
gravitational force exerted on the particles and causes the particles to suspend in
a state of fluidized bed. A pressure drop is allowed to increase till a point where
where it equals the weight of the particles fluidized bed. A pressure drop is
allowed to increase till a point where it equals the weight of the particles unit
area. At this point, the bed, particles are in a suspended state and it is termed as
"minimally fluidized bed".
When the flow is increased further, the bed becomes highly turbulent as the
interstitial spaces between the bed particles increases allowing generation of
high velocity gas streams which promote a rapid mixing of the bed. At this state,
the bed of solids attain pseudo fluid properties. With further increase in flow, the
particles are also carried along with the gas and then separated in a cyclone and
returned back to the dense bottom bed. This stage of fluidization for combustion
and subsequent heat transfer to generate steam is termed as "circulating
fluidized bed boiler".
In the circulating fluidized bed boiler, the above mechanism is used to circulate
solid coal particles after the entire bed is preheated to ignition temperature of
the fuel (coal). The solids undergo combustion and release heat which is
uniformly imparted to the whole volume of the bed due to high turbulence and

rapid mixing characteristics of the fluidizing process. Large portions of the fuel
and fine limestone are suspended in the gas stream. In case of a CFB, a velocity
in the combustor is sufficient enough to form particle clusters. The size of these
clusters increases making the velocity of the gas below entrainment velocity, and
thus causing the material to fall back down in the combustor and is called
primary circulation loop. On their way down, these clusters collide with the rising
material, get broken-down, and the smaller particles rise up again.
The phenomenon promotes the mixing and leads to a gas to solid contact in the
bed. Some of the material remaining in the bed is too large to be entrained until
it is reduced in size by combustion and attrition. Fresh feed of coal and limestone
maintains the bed level. The majority of the particles/clusters contact the walls of
the CFB furnace and thus transfer heat to the water-walls of the furnace. The
uniform distribution of the air is ensured by utilizing various configurations of the
air nozzles with objective of achieving uniformity while minimizing the pressure
drop extended across the nozzles.
The enormous quantity of ash thus generated and residing in the system needs
to be controlled as the differential pressure across the bed needs to be
maintained. Also the bed inventory requires quick evacuation in case of tube
leakage. Thus, the effective and regular removal of ash inventory from the
chamber is of utmost importance to ensure effective and continued operability of
the system.

US Patent No. 2,75,808 describes a fluid heater method utilizing an ash collection
chamber as part of a furnace, and allowing water wall tubes to cool the ash slags
in a typical pulverized furnace. This arrangement only addresses the bottom ash
evacuation methods and not for the complete ash evacuation for a fluidized bed
system.
US Patent 5,95,4000 describes apparatus for cooling ash exiting from a
circulating fluidized bed boiler, which comprises an enclosure having a floor, a
plurality of walls disposed around the floor and a ceiling. This apparatus enables
removal of bottom ash by fluidization and doesn't address the requirement of
systematic ash cool down.
US Patent 7,46,4669 teaches an integrated fluidized bed ash cooler for a
fluidized bed boiler, particularly a circulating fluidized bed (FFB) boiler, which
employs at least two fluidized bed sections positioned in series along a solids-
flow path. Each section contains fluidizing means, the first section along the
solids path being separated from a following section with a threshold. This
arrangement does not mention about any mechanism for efficient heat transfer
from the ash to enhance rate of ash removal.

The prior art of extracting hot ashes do not mention about systematic cooling for
CFB system. Known methodology of handling hot ashes involve simple drainage
onto ground or onto bins and handles after waiting for a natural cooling process.
Thus, the prior art envisages only natural cooling process of the hot ash, outside
the boiler system, without contemplating a systematic cooling and removal of the
cooled ash, and therefore does not teach an effective way of heat transfer. The
present invention uses novel design of serpentine coil-panel configuration in
which ash evacuated from combustor is cooled in streams of cold air by Plug flow
of the ash. This results in effectively extracting the cooled ash from the cooler
before fresh hot ash enters.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a serpentine coil-panel
device for systematic plug flow cooling of ash with air in an ash evacuation
chamber of CFB boiler which ensures evacuating of ash, before entry of fresh
ash.
Another object of the invention is to propose a serpentine coil-panel device for
systematic plug flow cooling of ash with air in an ash evacuation chamber of CFB
boiler in which fluidized ash flow is achieved by plug flow dense phase conveying
of the ash at ash drain point to provide superior cooling effect.

A still another object of the invention is to propose a serpentine coil-panel device
for systematic plug flow cooling of ash with air in an ash evacuation chamber of
CFB boiler in which heat transfer in which heat transfer from ash to cooling
water is enhanced.
A further object of the invention is to propose a serpentine coil-panel device for
systematic plug flow cooling of ash with air in an ash evacuation chamber of
CFB boiler in which coarser particles expusion including reduction in erosion of
coil-panels is achieved.
SUMMARY OF THE INVENTION
Accordingly, there is provided an ash evacuation system comprising a
combination of air cooling means for cooling of ash along with a serpentine
arrangement of coil-panels that would allow complete bch discharge of the
entering ash to the exterior, thus providing for effective heat transfer to occur.
The ash from the combustion chamber of the boiler is allowed to pass through
the plug flow ash cooler, which by an air diversion mechanism and coil-panel
arrangement caters to the cases of emergency ash evacuation or in very low
load transients to improve flow distribution. The ash, guided by a horizontal air
flow regime, flows out by plug flow mechanism and in the process gets cooled
down and flows out completely. The ash then falls on to the intermediate ash
storage bin for transport and disposal.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a typical CFB combustor, cyclone, Sealpot and ash cooler with
Evacuation points.
Figure 2 - shows side elevation of the Plug ash cooler.
Figure 3 - shows plan view of the plug ash cooler of the invention.
Figure 4 - shows front elevation view of the plug ash cooler with air nozzle.
DETAILED DESCRIPTION OF THE INVENTION
A typical CFB boiler comprises at least one windbox (1), a combustor (2), a
plurality of cyclone (3), a seal pot (4) and an External Ash cooler (5) as shown in
figure 1.
As seen from the figure 1, the ash cooler (5) is an essential component in
ensuring periodic removal of ash from the combustion chamber (2), failing which
an improper fluidization and consequential loss of process results. Thus the
inventory of solids which builds up in chambers of the combustor, and the seal

pot, needs to be evacuated in order to control differential pressure including
water ingress (from water wall tube failure). As this ingressed water reacts with
calcined lime in the ash to form cement-hard mixture, it becomes difficult to
operate the boiler system in such cases. Prior art includes use of ash gate valves
located at the ash discharge piping disposed at bottom of the grate and windbox
to evacuate ash from the system.
The inventive device operating as a plug flow ash evacuation system, utilizes the
principles of a dense phase vertical conveying of ash and a systematic removal of
the ash through sequential horizontal solid flow. This configuration employs
innovative ash cooler coil-panels to allow once through flow in a serpertine
manner (10). The ash enters the cooler at the top (6), flows into the first coil-
panel shelf (9) created by rows 1 and 2). Then the flow is preserved by the plug
flow of the ash entering subsequently behind this batch and also the air injected
by the horizontal nozzles. The ash flow takes a serpentine turn and enters the
next shelf of the subsequent coil-panels (created by rows 2 and 3). The flow
sequence cpntinues till the ash reaches the last shelf of coil-panels (created by
rows n and (n+1)). This is shown in figure 2. By providing a sufficient pressure
drop, the critical pneumatic moving bed transport is achieved.

The number of coils to be provided can be attained either by increasing the
number of starts from the headers (14) into the ash cooler or by increasing the
number of coil-panel shelves required, or both, depending on the total heating
area required for cooling the ash (as in figure 3).
If each batch of ash is considered as a separate entity, it may be seen that there
is complete removal of that particular batch out of the cooler, by the plug flow
mechanism. By this process, effective heat transfer takes place and thus leads to
a reduction in the number of coils required and in turn the size of the Ash cooler.
This flow property is maintained by a horizontal fluidization generated by jet air
nozzles located in the sides of the ash cooler (as opposed to air nozzles' location
in the grate at the bottom of a fluidized bed ash cooler).
The plug ash cooler houses the key feature of serpentine flow coil-panel device
as detailed in figure 2. It consists of a horizontal fluidizing means (11,12)
consisting of multiple air nozzles (as shown in figure 4). It has inlet and outlet
vertical headers (7), located near the outer edges of the ash cooler, which supply
cooling water to the coil-panels. Air supply is rerouted through the sides by
operating a damper gate arrangement and connecting duct.

This arrangement is activated from the control room which diverts air from a
primary path into the connecting duct arrangement. Further, solenoid valves may
also be used to regulate alternate air flow to the coil-panel shelves, to maintain
consistent flow operation. The ash flow in said arrangement causes lesser
erosion of coils in the panels due to lesser velocities, which inturn increases the
longevity of the panels and reduces the number of replacements per interval
resulting from erosion. In addition to this advantage, this ash cooler can be
expended for extraction of higher size particles than those extracted by
conventional ash coolers. The dense phase conveying mechanism adopted to this
ash cooler provides effective heat extraction and cooled material removal
periodically. The cooled ash doesn't deposit at the bottom as in conventional
coolers. This feature prevents cooling of the ash that is entering (by mixing with
already cooled ash) and more efficiently utilizes the heat of ash only to increase
the cooling water temperature. The continually cooled ash is transported to the
lowest shelf of coil-panels and removed (8) by simultaneous fluidization and
influx of external hot ash.
This is then discharged into an intermediate ash bin for further disposal.

WE CLAIM :
1. A serpentine coil-panel device for systematic plug flow cooling of ash with
air in an ash evacuation chamber of circulating fluidized bed (CFB) boiler,
the boiler essentially comprising:
- at least one windbox, a combustor having a plurality of chambers, a seal
pot, and a plurality of cyclones,
- the device comprises a plug flow ash cooler housing having at least one
serpentine flow coil-panel operating under the principle of dense phase
vertical conveying of ash; a horizontal fluidizing means consisting of
multiple air nozzles; inlet and outlet vertical headers disposed adjacent
the outer edges of the cooler; a damper gate means for diverting air from
the primary path to a connecting duct,
wherein the plug flow ash cooler comprises a plurality of coil panel shelf,
a first shelf (9) formed by coils of first and second rows, a second coil

panel shelf formed of coils of second and third rows, and subsequent coil-
panel shelves formed of coils of corresponding rows, wherein the ash is
allowed to enter the cooler from top portion and flow into the plurality of
coil panel shelves in a serpentine manner, and wherein the horizontal
fluidizing means are enabled to inject air on the ash flow stream.
2. The device as claimed in claim 1, wherein the coil panels are disposed
along the passage joining the inlet and outlet headers of the ash cooler.

ABSTRACT

The invention relates to a serpentine coil-panel device for systematic plug flow
cooling of ash with air in an ash evacuation chamber of CFB boiler, the boiler
essentially comprising : at least one windbox, a combustor having a plurality of
chambers, a seal pot, and a plurality of cyclones, the device comprises a plug
flow ash cooler housing having at least one serpentine flow coil-panel operating
under the principle of dense phase vertical conveying of ash; a horizontal
fluidizing means consisting of multiple air nozzles; inlet and outlet vertical
headers disposed adjacent the outer edges of the cooler; a damper gate means
for diverting air from the primary path to a connecting duct, wherein the plug
flow ash cooler comprises a plurality of coil panel shelf, a first shelf (9) formed
by coils of first and second rows, a second coil panel shelf formed of coils of
+second and third rows, and subsequent coil-panel shelves formed of coils of
corresponding rows, wherein the ash is allowed to enter the cooler from top
portion and flow into the plurality of coil panel shelves in a serpentine manner,
and wherein the horizontal fluidizing means are enabled to inject air on the ash
flow stream.