FIELD OF INVENTION
The present invention relates to an improved erosion protection device for hanger
tubes in bubbling fluidizing bed combustion boilers (BFBC). More particularly, the
present invention relates to an improved erosion protection device for hanger tubes in
BFBC boilers which is having a cost effective method.
BACKGROUND OF THE INVENTION
The BFBC is an environment friendly and efficient system of burning various grades of
fuels. The fuel burns between 700-900° C. This temperature ensures that Nox is not
formed. Sorbent in the form of limestone is added to capture the sulphur in the fuel
thereby preventing the formation of Sox. The principle heat transfer surfaces like
evaporator and super heater are immersed in the fluidized bed. The primary air of
combustion is directed through the bed at a pre determined velocity, at which the bed
behaves like fluid and effective heat transfer takes place due to fluidization.
Hanger tubes constituting evaporator tubes are used for supporting super heater coils
inside the furnace of the boiler. Typical arrangement is shown in Figure 1.
During the course of the process of fluidization there is a continuous impingement of
bed material particles on the hanger tubes supporting the super heater coils in the bed.
Due to continuous impingement, the tubes are quickly eroded.

Conventionally to increase the tube life, welding of small (about 8 mm) diameter
pin studs are used to prevent direct impingement of the particles on the
tube.(Fig 2)
These pin studs are electrical resistance welded on the tubes. Both carbon steel
and stainless steel studs are adopted. The studs are welded in closed pitch on
the tubes. They prevent direct impingement of bed material on the tubes
The studs get eroded first and then only erosion starts in tubes. Thereby tube
life is enhanced. Once the studs are fully eroded the tubes have to be replaced
with a freshly studded tube
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved erosion
protection device which protects the hanger tubes from erosion due to
impingement of the bed material.
An another object of the invention is to propose an improved erosion protection
device which efficiently protects the hanger tubes from erosion due to
impingement of the bed material thereby enhances the life of the hanger tubes.

A further object of the present invention is to propose an improved erosion
protection device which efficiently protects the hanger tubes from erosion due to
impingement of the bed material which enables routing of the hanger tubes
close to coal nozzles and ash re injection nozzles, without subjecting the tubes to
significant erosion.
A still further object of the present invention is to propose an improved erosion
protection device which efficiently protects the hanger tubes from erosion due to
impingement of the bed material which makes the furnace further compact.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved erosion protection device for hanger
tubes in BFBC boilers comprising a hot air distributor plate disposed at the
bottom boundary of the combustor enclosed by a membrane, said plate having a
plurality of air nozzles, a hot air source for continuous supply of hot air through
said nozzles into the fluidized bed, a first super heater coil immersed in the bed
of the boiler; a second super heater coil above the bed supported by a plurality
of hanger tubes; and a tube assembly comprising a tube and a plurality of
ceramic sleeves fitted over the tube.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1- shows a typical arrangement of hanger tubes in a BFBC boiler.

Figure 2 shows a cross sectional view of a studded tube used in prior art.
Figure 3 shows a ceramic ring fitted on to a hanger tube.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION
With the above and other objectives in view, the present invention provides an
alternative method of erosion protection. The present invention relates to a BFBC boiler
to overcome the above said drawbacks. It has been the endeavor of the applicant to
develop alternate ways employing ceramics to avoid erosion.
The present invention provides an improved and cost effective device for erosion
protection. According to present invention, ceramic rings are designed to form a ring
over a tube (11) (both straight and bent portion) so that they form a protective layer.
(Fig. 3)
The type of ceramic used has high erosion resistant properties. The ceramic material
used is suitably selected with the right amount of alumina etc. to get the requisite
hardness properties. Further it is tested with suitable erosion tests to determine the
suitability for the intended working atmosphere. Having ensured so the ceramic
material is selected.

Now the exact ring shapes required for covering the tubes (11) are determined.
While doing so care has been taken to ensure that no exposed part of hanger
tubes will be visible once the coating is done. The rings are then die cast in
specially developed dies. They are then sintered at a predetermined temperature
for a pre-determined time.
The rings (6) are now assembled over the tubes to form a protective layer.
Arrangements are provided to lock the rings in place. Actual tests at site have
indicated that life of the hunger tubes with ceramic rings is much more than the
pin studded tubes.
A hot-air distributor plate (1) is arranged fixedly on the boundary wall of the
BFBC boiler by a membrane (2). The plate (1) is having a number of air-injecting
nozzles (7). Hot air form a source is continuously passed to the fluidized bed (8)
of the boiler via the nozzles (7). One each heater coil (3,4) is placed inside the
bed (8) and above the bed (8). The second heater coil (4) is supported by a
plurality of hanger tubes (5). Instead of pin studs (9) as of the prior art, a
number of ceramic sleeves (6) are arranged over a tube (11) to entirely cover
the tube (11) thereby forming a tube assembly (10). The tube assembly (10)
covers the hanger tubes (5) from getting impinged by the bed material thereby
protecting the hanger tubes (5) from quick erosion.

WE CLAIM
1. An improved erosion protection device for hanger tubes in BFBC boilers
comprising a hot air distributor plate (1) disposed at the bottom boundary of
the combustor enclosed by a membrane (2), said plate (1) having a plurality of
air nozzles (7); a hot air source for continuous supply of hot air through said
nozzles into the fiuidized bed (8); a first super heater coil (3) immersed in the
bed (8) of the boiler; a second super heater coil (4) above the bed (8)
supported by a plurality of hanger tubes (5);
Characterized in that,
a plurality of ceramic sleeves (6) are disposed over a tube (11) for covering
entirely the said tube and forming a tube assembly (10) for covering the hanger
tubes (5) for protecting the impingement by the bed material and for protecting
the hanger tubes from quick erosion.
2. The device as claimed in claim 1, wherein the ceramic sleeves (6) are die-cast
using ceramic material having high erosion resistance properties.
3. The device as claimed in claim 1 or 2, wherein the ceramic sleeves (6) are
subjected to sintering at a predetermined temperature for a predetermined time
after die-casting.

ABSTRACT

AN IMPROVED EROSION PROTECTION DEVICE FOR HANGER TUBES
IN BUBBLING FLUIDIZING BED COMBUSTION BOILERS (BFBC)
An improved erosion protection device for hanger tubes in BFBC boilers consists
of a hot air distributor plate 1 disposed at the bottom boundary of the combustor
enclosed by a membrane. The said plate has a plurality of air nozzles (7). A hot
air source is disposed for continuous supply of hot air through these nozzles (7)
into the fluidized bed (8). A first super heater coil (3) is immersed in the bed
(8). A second super heater coil (4) is placed above the bed and is supported by
a plurality of hanger tubes (5). A number of ceramic sleeves (6) are arranged
over a tube (11) to entirely cover the said tube (11) thereby forming a tube
assembly (10). The tube assembly cover the hanger tubes (5) from getting
impinged by the bed material thereby protecting the hanger tubes (5) from quick
erosion.