FIELD OF INVENTION
The present invention, in general relates to systems involving protection of the
interface of the furnace wall and the refractory junction from erosion in
Circulating Fluidised Bed Combustion (CFBC) Boiler. In particular, the invention
adapts a new mechanism to eliminate or reduce erosion on top of the refractory
furnace wall interface at various locations in the furnace normally exposed to
higher solid reflux. More particularly, the invention relates to a nozzle-assisted
anti-erosion device to protect refractory water wall interface in a circulating
fluidized bed combustion boiler.
BACKGROUND OF THE INVENTION
FluidisedBed Combustion boiler is 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 suspend the particles in the bed.
Thus, at the low-velocities, the bed remains static and undisturbed.
As the flow (velocity) is increased gradually, the buoyant force overcomes the
gravitational force exerted on the particles and suspends in the state of fluidized
bed. The pressure drop is allowed to increase till a point, where it matches with
the weight of particles per unit area. At this stage, the bed particles are in
suspendedstate 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. With further increase in flow, the particles are also
carried along with the gas and then usually separated in a cyclone and returned
back to dense the bottom bed.
In the CFBCboiler, the above mechanism is employed to circulate the solid coal
particles. The entire bed is preheated to ignition temperature of the fuel (coal).
On reaching the ignition temperature, the coal particles are fed. They undergo
combustion and release heat which is imparted to the whole volume of the bed
uniformly due to high turbulence and rapid mixing characteristics of the fluidizing
process. Large portions of the fuel and limestone are suspended in the gas
stream and the gas-solid phase extends throughout the furnace. Even though
the velocity in the combustor is sufficient, particles tend to form clusters. The
size of these clusters increases making the velocity of the gas below entrainment
velocity, and thus causing the 'material to fall down the furnace. On their way
down, these clusters collide with the rising material, get broken-down, and the
smaller particles travel up again. This action 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 condition. The majority of
particles/clusters tend to fall along the walls of the furnace and thus transfer
heat to the water-walls of the furnace.
The configuration of the furnace is such that the total combustion air
requirement is split into parts and around 50% is admitted at the bottom as the
fluidizing air. The rest is admitted as a secondary air along the height of the
bottom portion. This split of the combustion air quantity results in the bottom
portion operating under reducing conditions which is unfavorable for the exposed
metal surfaces. Also the lower dense bed presents a significant erosion potential.
This is avoided by applying a layer of the refractory to protect the metal surface
(of water-wall) from a corrosive and erosive environment existing in the bottom
of the furnace. This refractory layer is extended upwards and creates an
interface at the location where the heat transfer requirement dictates the
exposed water-wall surface. This interface induces a change of direction of
refluxing the solids which creates the potential for erosion in spite of lower
velocities. This leads to erosion of the fins and tube wall causing forced
shutdown of the boiler. Also this erosion makes the refractory to fall down from
it's place and further causing damage to the operating boiler.
The prior art of reducing the erosion potential are discussed in the patents
referred hereinbelow. USpatent No.4, 554,967 uses a variety of studs members
of various shapes which are attached and they extend along the periphery of the
tube to protect the tube surface from a direct contact from the refluxing solids.
USPatent No 5, 893, 340 teaches that the furnace walls be bent bend and kept
out of the solid flow stream to reduce the incidence of solids on this refractory
discontinuity.
US Patent No.6, 491000 adopts swaged sections of tubes above the refractory
layer covered with abrasion resistant shaped refractory to protect the wastage of
metal surface at the interface.
Thus, the prior art envisages only measures to protect the tube surfaces
mechanicallyor utilizes a modified shape for the water wall panel or a specialized
refectory and complicated attachment procedures.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a nozzle-assisted antierosion
device, which protects the refractory water wall interface from erosion
inside a circulating fluidized bed combustion boiler furnace.
Another object of the invention is to propose a nozzle-assisted anti-erosion
device, which adapts an air feed arrangement at the interface with a varying
media feed rate at various locations along the width and corners of the interface
to obtain a boundary layer at the interface thereby protecting the metal surface
from erosion.
SUMMARY OF THE INVENTION
Accordingly, there is provided a nozzle-assistedanti-erosion device comprising a
combination of nozzles with openings at the strategic locations at the interface
with specific angles for diverting solids refluxing on the walls by injecting a fluid
media at the interface.
The device further employs a combination of valves and correspondingly-sized
openings/nozzles in the fins of the water walls including the refractory to ensure
a cushioning layer of the fluid media. The solid refluxing on the water wall
changes the direction in this boundary layer of media instead at the metal
surface.
The new device utilizes air from primary/secondary fans disposed at the
admission location in the furnace.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a typical interface of the refractory and furnace wall.
Figure 2 - is a view showing the erosion at the interface of the refractory.
Figure 3 - Shows a nozzle-assisted anti-erosion device according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
A typical refractory and water wall interface is shown in Figure 1, which
comprisesa plurality of water wall tubes (1), a refractory lining (3), a plurality of
refluxing solid clusters (4), a refractory and water wall interface (5).
A typical erosion pattern occurring at the interface (5) of the refractory and the
water wall is illustrated in Figure 2. The inventive device as shown in figure - 3,
comprises an air line (6), at least one fin nozzle (7), a refractory nozzle feed line
(8), at least one refractory nozzle (9), and a refractory nozzle branch air line
(10).
The present invention thus provides a nozzle-assisted anti-erosion device to
protect the refractory interface discontinuity arising out of the refluxing solids on
the water wall. The nozzle assembly (7, 9) ensures generating a layer of air
stream in which the solids/clusters (4) changes their flow direction and prevents
erosion to the water wall interface (5). The modus operandi of operation of the
device is to control the quantum of air into the fin nozzles (7) and refractory
nozzles(9) to provide an air cushion without disturbing the reflux pattern of the
solids (4) above the interface (5). This is made possible by a configuration and
combination of the admission nozzles (7, 9) used in the fin and refractory area.
The nozzles (9) located in the refractory area are aligned along the tubes to
provide a boundary cushion layer. The location and quantity of air to the corner
is also adjusted using a plurality of valves to provide an air quantity sufficient to
overcomethe solid refluxes (4) faced along the corners and other areas.
The nozzle assisted anti erosion device provides a protection against the high
solid reflux (4) by altering the boundary layer at the interface (5) of the water
wall and refractory by utilizing the nozzle configuration (7, 9). The direction
change of the solids (4) induces the avoidance of erosion by providing this
dynamic layer of air cushion.
The advantage of the invention lies in fact that the proposed device can not only
be installed in the existing units without affecting the pressure part constructions
but further provides a significant advantage over methods like kick out design
which involves changes in the tube/panel configurations. The other
disadvantages of the prior art for example, annual renewal of cladding in
alternative protection mechanism are eliminated by the invention. The invention
adopts affirmative measures to provide aerodynamic layer for accommodating a
directional change of the solids, and thus ensures an ideal prevention of erosion
at the refractory water wall interface (5). This results in elimination of the
erosion observed in the water walls (5) with the prior art device, thereby
improving boiler availability with least effort on maintenance.
WE CLAIM
1. A nozzle-assisted anti-erosion device to protect refractory water wall
interface (5) in a circulating fluidized bed combustion boiler, the refractory
water wall interface (5) comprises a plurality of water wall tubes (1), a
refractory lining (3), and a plurality of refluxing solid clusters (4), the
device comprising:
- at least one fin nozzle (7), and at least one refractory nozzle (9)
constituting a nozzle assembly (7, 9) which generates a layer of air
stream in which the solids clusters (4) changes their flow direction;
- a first air line (6) feeding a controlled quantum of air to the fin
nozzle (7), and a second airline (8) providing an air cushion without
disturbing the reflux pattern of the solids (4) above the interface
(5), the refractory nozzle (9) being aligned along the water wall
tubes (1) to provide a boundary cushion layer;
- a refractory nozzle branch air line (10) connected between the first
air line (6) and the refractory nozzle (9); and
- a plurality of valves to control the quantity of air supply to be
sufficient to overcome the solid refluxes (4) faced along the
corners, the direction change of the solids/clusters (4) including the
alteration of boundary cushion layer due to configuration and
disposition of the nozzle assembly (7, 9) providing a protection at
the interface (5) of the refractory and water wall.
2. A nozzle-assisted anti-erosion device to protect refractory water wall
interface (5) in a circulating fluidized bed combustion boiler, as
substantially described and illustrated herein with reference to the
accompanying draWings.