FIELD OF INVENTION
The present invention generally relates to a device capable of fluidizing bed
materials in fluidized Bed system effectively. More particularly, the invention
relates to a variable pressure drop distributor grid nozzle system for fluidization
of materials in the bed of a Fluidised Bed Combustion (FBC) Boiler.
BACKGROUND OF THE INVENTION
Fluidised Bed Combustion boiler is a system consisting of a fluidized bed
enclosure, where the solid particles are in suspended state. The state is achieved
by controlling the amount of air passing through the bed. At low velocities,
pressure drop across the bed is negligible and the buoyant force drivable from
low velocities is not sufficient to suspend the particles in the bed. The bed
remains static and undisturbed.
As the flow (velocity) is increased gradually, the buoyant force overcomes the
gravitational force exerted on the particle in suspended state in the fluidized bed.
The pressure drop increases till a point, where it matches with 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 increase allowing the high velocity

gas streams promoting a rapid mixing of the bed. At this stage the bed of solids
attain pseudo fluid properties.
In the FBC boiler, the mechanism as described hereinabove is employed to
fluidize the solid fuel particles and other entering inventory like inert bed
material, sorbent and recyclable ash particles. The entire bed is preheated to an
ignition temperature of the fuel. On reaching the ignition temperature, the fuel
particles are fed with a mixture of sorbent. The fuel particles including the
sorbent mixture undergo combustion and release heat which is imparted to the
whole volume of bed uniformly due to high turbulence and rapid mixing
characteristics of the fluidizing process.
Introduction of the fluidizing media requires an arrangement to ensures a
uniform distribution as any non-uniformity would result in a defluidizing condition
not conducive for the combustion process. This requires a sufficient pressure
drop across the distributing arrangement to ensure that the bed conditions do
not alter the distribution characteristics by virtue of a suitable pressure gradient.
The prior art of fluidizing the inventory in the bed is done by a part of the
combustion air with perforated plate, porous media like ceramics and several
nozzle arrangements. A plurality of air distributor arrangements provided for
particular operating conditions becomes inflexible due to other chanted operating
conditions. As the pressure drop across the air distributor arrangement is
proportional to the flow quantity, any change would affect the pressure gradient
and therefore the quality of fluidization in the bed. It may also be noted that in

case of specific operating regimes, the lower flow conditions develop which
causes maldistribution of the solids resulting in stoppage of the system. Further,
the non uniformity in the bed caused by dense reverse flow along the walls,
increases momentum of the solid at entry locations which severely affects the
radial concentration of the solids (and consequentially the pressure fluctuations
experienced in the zones). This situation causes mal distribution of gas solid
concentration leading to deteoriation in efficiency of the fluidization process and
affecting performance of the system.
The non-uniformity in the fiuidized bed system is further caused by the particles
entering the system at various locations and elevations. The process conditions
inside the fiuidized bed system especially with Geldart's group B and D type
particles, cause a severe fluctuation in the pressure conditions. The fluctuations
tend to significantly alter the pressure gradient across the distributor and cause a
backsifting of the particles into the wind box.
Prior art for example, US Patent No 4,402,605 describes a process which
envisages an intersection of the grid arrangement to improve air distribution
across the entire area of interest. Another known method (US Patent No
5,101,576) utilizes unidirectional nozzles to avoid backsifting of the solid back
into the air plenum/wind box. US Patent No 5,105,559 discloses a method to
prevent backsifting by employing bend tube nozzle configuration. Another known
alternative method (US patent no 5,183,641) employs two different nozzle
configurations to provide two different pressure drop characteristics for varying
the load conditions. Yet another known system (US patent no 5,286,188) utilizes

various valves at the bottom of the nozzles to prevent backsifting at shutdown
stage.
In general, the distributor plate of the fluidized bed combustion boiler has a fixed
flow bubble cap nozzle arranged in a standard pattern to perform the basic
purpose of fluidization. The distributor plate is connected to a wind box which
receives the fludizing medium (air) from the air heater after getting heated by
the flue gas or steam with a head produced by a fan or blower.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a variable pressure drop
distributor grid nozzle system in a fluidized bed combustion boiler, which
optimizes the non uniform fluidization conditions and prevents backsifting of
solids into wind box of the boiler.
Another object of the invention is to propose a variable pressure drop distributor
grid nozzle system in a fluidized bed combustion boiler, which provides a method
of controlling the fluidizing pattern in the combustion chamber in a fluidization
bed combustion boiler.
A still another object of the invention is to propose a variable pressure drop
distributor grid nozzle system in a fluidized bed combustion boiler, which is
enabled to vary the nozzle pressure drop individually or by a group of nozzle

arrangement by suitably adjusting the flow resistance across the nozzles even
during turn down operation.
SUMMARY OF THE INVENTION
The inventive system adapts a double sheathed nozzle configuration for varying
the pressure drop in the distributor grid.
The variable pressure drop distributor grid nozzle system is enabled to provide a
high pressure fludizing air to the required areas. From the feedback observed
during the operation, individual nozzles / group of nozzles in a particular zone
can be suitably adjusted to provide a high/low resistance to the nozzle air flow to
have a uniform fluidization across the bed in the combustor.
The invention adopts a combination of interconnected inner sheath of orifice
holes/blanked holes which align with /obstruct outer sheath of the orifice holes
to provide the required variation in the flow resistances. This arrangement is
utilized for regulating the flow for fluidization, utilizing the pressure drop due to
counteract vagaries of pressure fluctuations experienced in the fluidizing
chamber. The improved system employs fludizing nozzles of variable orifice
across the cross section of the distributor plate for optimizing the pressure drop
for each of the operating conditions and also preventing the backsifting of the
particles.

In addition, the inventory coming from the recyclable part like cyclone, external
fluidised bed heat exchanger and seal pot return, the solids which flow back to
the grate grid along the walls, are also taken care of by the variable orifice
configuration.
The innovated configuration and selection of the fludizing nozzle is integrated
with a feedback system, considering the differential pressure across the grate,
the position of ash valve opening, the amount of fuel and limestone mixture fed
into the combustor, the coarser materials falling along the combustor walls, the
recyclable material from the cyclone through loop seal or seal pot at various
locations in the distributor plate. The variable pressure drop distributor plate
provides the varying pressure across the grid plate to counteract the varying
inventory at various locations.
The fluidizing nozzle in the grate is connected so as to create the variable
pressure drop across the grid. It is held at a position to have a uniform
fluidization and to have a uniform differential pressure across the distributor
plate.
According to the invention the pressure usage is optimized by altering the nozzle
configuration and by preventing backsifting of particles. For blocking the flow at
fluidizing nozzle air line, a connecting rod mechanism is provided.
The inventory quantity (material fall back along the walls, recyclable from
primary loop, return leg of seapot, fuel sorbent mixture) is considered to decide

the quantity of delivered fludizing medium.
The inventive system is adaptable to all fluidized bed combustion boil irrespective
of applications. It enhances the flexibility of operation at various inventory levels.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a typical arrangement of fluidizing nozzle system in a FBC
boiler
Figure 2 - shows a variable pressure drop distributor grid nozzle system in a FBC
. boiler according of the invention.
DETAILED DESCRIPTION OF THE INVENTION
A conventional arrangement of typical air nozzle for fludizing the inventory in
fluidized bed boilers is shown in Figure 1, which comprises at least one fluidizing
nozzle (1) with a bubble cap (5), airflow hole arranged in a distributor plate (2)
with windbox (6).
The invented system for fludizing the inventory in fluidized bed boilers as shown
in Figure 2, comprises at least one fluidizing nozzle (1) with a bubble cap (5),
and a plurality of airflow holes (4), the nozzle (1), is arranged on a distributor

plate (2) in the windbox (6). An inner sheath (3) of holes/blanked pipe (3) which
is connected individually or in group in a mechanical rotation/elevation
arrangement and is operable to obtain a desired position of the inner sheath (3)
in relation to the outer orifice nozzles to obtain the desired variation.
The invention proposes a variable orifice area which enables the fludizing nozzles
(1) to utilize the pressure for uniform fluidization with the varying inventory level
according to the operating parameters. The modus operandi for guiding this
blocker assembly is to provide a plurality of guide rods connected to the inner
sheath (3). The locating of the blocking elements (3) is made possible by
translation/rotation of a connecting rod mechanism. This assembly can be driven
thorough a drive mechanism to vary the nozzle area for variable pressure across
the distributor plate (2).
This novel system ensures a fludizing condition of highly variable inventory levels
by altering the dimensional configuration of the fluidizing air flow with the
fludizing nozzle (1) and an optimal pressure generated through the blocker
assembly configuration. The area for air flow required to fluidized the material in
the plenum can be varied by adjusting the connecting rod mechanism.
The advantage of the invention lies in the pressure utilization process over entire
arrangement is not only affected by the improved nozzle configuration, but also
provides an affirmative measures to disturb the bridging thus ensures an ideal
operating condition for operating of the fluidizing system. This results in more

uniform bed temperatures improving fuel utilization and better control over
thermal NOx production as all the material in the boiler-combustor is actively in
service.
The other benefits associated with variable fluidizing nozzles is a better pressure
utilization in other fluidised bed applications, thus reducing the pressure to be
developed by the fan system and improves overall cycle efficiency.
A significant benefit stem from this unique control allows adjusting for wide
varying inventory levels by going in for variable nozzle for varying with a part
load to ensure a good mixing by uniform fluidizing without backsifting.

WE CLAIM
1. A variable pressure drop distributor grid-nozzle system in fluidised bed
combustion boilers, comprising a plurality of fludizing nozzles (1) with
bubble caps (5), and a plurality of airflow holes (4), the nozzles (1)
arranged on a distributor plate (2) in the windbox (6), characterized in
that the plurality of nozzles (1) is arranged in a grid configuration, and a
blocker assembly consisting of a plurality of guide rods including a
connecting rod mechanism to provide a translational and/or a rotational
movement of a device enabling variation of the air-flow resistance through
the plurality of nozzles (1), the blocker assembly being operated via an
electrical drive means, and in that the device comprising a plurality of
blocker pipes (3) connected to the inner sheaths of the air-flow holes (4)
individually or in groups in a rotational including elevational configuration
to allow the blocker pipes (3) to align and obstruct the outer sheaths of
the air flow holes (4) so as to regulate the flow for fluidization through
providing a variation in the flow-resistance in the airflow outcoming from
the plurality of nozzles (1).
2. A variable pressure drop distributor grid-nozzle system in fluidised bed
combustion boilers, as substantially described and illustrated herein with
reference to the accompanying drawings.

The invention relates to a variable pressure drop distributor grid-nozzle system in
fluidised bed combustion boilers, comprising a plurality of fluidizing nozzles (1)
with bubble caps (5), and a plurality of airflow holes (4), the nozzles (1)
arranged on a distributor plate (2) in the windbox (6), the plurality of nozzles (1)
is arranged in a grid configuration, and a blocker assembly consisting of a
plurality of guide rods including a connecting rod mechanism to provide a
translational and/or a rotational movement of a device enabling variation of the
air-flow resistance through the plurality of nozzles (1), the blocker assembly
being operated via an electrical drive means, and in that the device comprising a
plurality of blocker pipes (3) connected to the inner sheaths of the air-flow holes
(4) individually or in groups in a rotational including elevational configuration to
allow the blocker pipes (3) to align and obstruct the outer sheaths of the air flow
holes (4) so as to regulate the flow for fluidization through providing a variation
in the flow-resistance in the airflow outcoming from the plurality of nozzles (1).