FIELD OF INVENTION
The invention relates to an improved method of providing ceramic coating to
protect the anchor rods including retainer plates in a cyclone of CFBC boiler
against corrosion under SOx present in the flue gas. The invention further
relates to an improved ceramic coating to protect CFBC boiler components from
corrosive damage by Sox present in the flue gas.
BACKGROUND OF INVENTION
A continuous fluidization of solid substances (ash, coal, limestone etc.) in
fluidized-bed systems results in various deteriorating effects for example,
abrasion, erosion and corrosion of the system components. These effect system-
components are pronounced at varying degrees on the components, depending
on the geometry of the component, the flow conditions and the solids particles
concerned. In Circulating Fluidised Bed Combustion (CFBC) Boilers, high sulphur
fuel (4-6% S) like lignite, north eastern Assam coal, petcoke, etc is fired which
generates a rich flue gas including sulphur dioxide (SO2), which is absorbed by
adding limestone. The cyclones provided in CFBC boilers are normally lined with
refractory layers for protecting the cyclones from the high temperature of the
hot flue gas ranging between 700 and 900 DegC. Anchor rods and retainer
plates are used for holding the refractory lining
including the insulating bricks. However, during transient conditions the highly
rich SO2 gases leave the boiler and causes acidic corrosion, the acidic corrosion
in the cyclone area in the CFB boiler has a high negative impact. Fig 1 shows a
schematic diagram of a cyclone with internally refractory laid arrangement. The
cyclone shells in the CFBC boilers are subjected to high temperature thermal
shocks, chemical attacks because of high sulphur dioxide content in the flue gas.
The flue gas at high temperatures intrudes into the refractories at high
pressure, and causes a devastating effect on the refractories, thereby corroding
the cyclone shell.
During operation, high temperature ash particles with SOx enter in the cracks
(developed) on the refractory wall, and the entrapped flue gases condense in
between the refractory and cyclone shell. This entrapped flue gases start
corroding the anchor rods and the retainer plates as it touches those. Such a
phenomenon leads to falling of refractory bricks and the insulating bricks. Once
the hot flue gas touches the cyclone shell through the cracks or small linear
opening of the refractory, the adjacent refractory also starts to fall. This
phenomenon is very critical in the area of side/top portion of cyclone, because
the damage caused by this corrosive attack leads to falling of entire roof of the
cyclone refractory. Further, it leads to corroding of the cyclone shell area.
Maintenance costs for replacing the refractory and fixing up the corroded wall
opening are very high and the downtime associated with unscheduled
breakdowns due to failure of the refractory wall further adds to the cost factor.
US patent US4658761 teaches the means and method for protection of boiler
tubes from corrosion and erosion. According to the invention, a metal boiler tube
is proposed where in at least a selected part of the tube surface is initially
porous, and wherein the initially porous surface is coated with at least one stable
metal oxide by subjecting the porous surface to a solution containing salts, or by
treatment of such oxides of metals such that the stable metal oxides get
attached to the porous surface.
US3197291 relates generally to high temperature ceramic protective coatings,
and particularly concerns a product having material which readily oxidizes or
corrodes when exposed to oxidizing or corroding environments at elevated
temperatures and having a ceramic coating provided there on which develops an
improved resistance against oxidation or corrosion, in particular at temperatures
from approximately 1700 deg F to approximately 2150 deg F.
Accordingly, there remains a need in the art for an improved coating method,
and a coating composition that provides effective protection against corrosion,
such as is required for CFBC boiler components
Admittedly an improved coating method and/or a coating composition for the
regions of boiler components essentially need coatings with a combination of
high corrosion resistance, low residual stresses, and higher thickness including
long life, particularly for CFBC boiler operating conditions having coal with high
ash content or lignite .
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method of
providing ceramic coating to protect the anchor rods including retainer plates in
a cyclone of CFBC boiler against corrosion under SOx present in the flue gas.
Another object of the invention is to propose, an improved method of providing
ceramic coating to protect the anchor rods including retainer plates in a cyclone
of CFBC boiler against corrosion under SOx present in the flue gas, which
increases the life of refractory including insulating bricks of the cyclone shell
of CFBC boiler.
Another object of the invention is to propose an improved ceramic coating to
protect CFBC boiler components from corrosive damage by Sox present in the
flue gas.
A still another object of the invention is to propose an improved method of
providing ceramic coating to protect the anchor rods including retainer plates in
a cyclone of CFBC boiler against corrosion under SOx present in the flue gas,
which is implementable in-situ without work handling devices.
A further object of the invention is to propose an improved method of providing
ceramic coating to protect the anchor rods including retainer plates in a cyclone
of CFBC boiler against corrosion under SOx present in the flue gas, which is
enabled to provide coatings with a combination of high corrosion resistance, low
residual stresses, and higher thickness preventing acidic corrosion in cyclone
shells and cyclone refractory damages.
These and other objects and advantages of the invention will be apparent from
the ensuing description.
At the outset of the description, which follows, it is to be understood that the
ensuing description only illustrates a particular form of this invention.
However, such a particular form is only an exemplary embodiment and the
teachings of the invention are not intended to be taken restrictively.
SUMMARY OF THE INVENTION
The present invention provides a method and a coating to prevent corrosion of
anchor rods, retainer plates, cyclone shell areas, in CFBC boiler, and improves
the life of the refractory against damages caused by the Sox present in the flue
gas. According to this method, the cyclone shell, anchor rods and retainer
plates exposed to acid corrosion, are coated with a high corrosion resistant
coatings of Zirconium of a particular composition and coated with a
predetermined thickness.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1- Schematic view of a cyclone with internally refractory laid arrangement.
Figure 2 - Sectional view of a cyclone shell of the cyclone exhibiting cracks due
to multiphase flow.
Figure 3- Sectional view of a refractory lay in a cyclone shell according to prior
art.
Figure 4 - Sectional view of a refractory lay in a cyclone shell with ceramic
coating according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
According to the invention, a Zirconium rich ceramic coating compound is
produced which is capable to withstand high temperatures and control erosion /
corrosion of the steel shells, refractory linings, chimneys, boilers etc.
The produced Zirconium rich ceramic coating compound has been vigorously
tested at 1000-1500 DegC in high temperature kilns and its performance has
been found to be extremely satisfactory.
The Zirconium rich ceramic coating compound ensures that maximum heat
energy is preserved in the heating chambers and damage to the surface of the
components is minimized. The outstanding feature of this product is the long
lasting protection that it is enabled to provide to the cyclone shells and refractory
in a corrosive environment.
The Zirconium rich ceramic coating compound can be applied by using simple
hand tools like brushing and spraying. The coating compound is non hazardous
and environmental friendly.
Unlike epoxy based coatings used in prior art, and which can vaporize around
500 °C, the Zirconium rich ceramic coating compound of the invention is a
refractory compound capable to withstand a temperature up to 1500 °C.
The novel compound possesses an excellent dry strength and resistance to
powdering, and having higher adhesion and chemical resistance properties. The
compound is odourless and can be easily mixed.
The Zirconium rich ceramic coating compound of the invention yields the most
dense corrosion resistant coatings and as such, is generally preferred for forming
corrosion resistant coatings
Referring to the drawings, wherein like reference numerals designate identical or
corresponding parts throughout the several views, the schematic view of the
cyclone with internally refractory laid arrangement is shown in Fig.1.
In a prior art cyclone, a refractory material (2) is laid over a cyclone shell (1) for
insulating at the boiler operating conditions. A plurality of anchor rods (5) and
retainer plates (6) are provided within the refractory for holding the refractory
against falling. Due to presence of high solid flux and entry velocity of flue gas, a
damage in the refractory layer (2) takes place which create cracks (3) in the
refractory laid inside the cyclone shell (1) as shown in sectional view of Fig 2.
The cracks (3) expose the cyclone shell (1) directly to the high sulphur flue gas.
The outside of the cyclone shell (1) is subjected to an atmospheric temperature
which condenses the sulphur present in the flue gas into the inside surfaces of
the cyclone, which in turn causes corrosion in the cyclone shell (1).
The present invention teaches means and method to protect the components
against the corrosion caused by the flue gases in the cyclone shell (1) by
providing a ceramic coating (4) over the inner portion of the cyclone shell (1)
before applying the refractory (2) inside the boiler. A thin layer of ceramic
coating of composition ZrO2-60-70%; SiO2-25-31%; Al203-~1%; CaO-0.5-1.5%;
is applied over the entire inside surface of the cyclone shell (1), anchors (5) and
retainer plates (6). The ceramic coating (4) has the property to withstand high
temperatures of around 1,200°C.
The properties of ceramic coating (4) as disclosed hereinabove enables to
protect the cyclone shell (1) from corrosion even if the refractory(2) failure
occurs. The corrosion resistance coating on the cyclone shell area is shown in
Fig. 4. The application of the corrosion resistant coating according to the
invention can be made with least possible efforts and expenses. The inventive
method enables coating of the Zirconium rich ceramic compound over the
corrosive surfaces of the boiler components.
The advantages of the invention can be seen, inter alia, that the ceramic coating
can be simply and manually applied without adapting any laying equipment,
and can be carried-out any elevation in the CFBC boiler site for cyclone shells.
We Claim:
1. An improved method of providing ceramic coating to protect the anchor
rods including retainer plates in a cyclone of CFBC boiler against corrosion
under Sox present in the flue gas, the method comprising the steps of:
-preparing the surface of the refractory structure including the cyclone
area;
-producing a ceramic coating compound and preparing a ready-to-
apply coating;
- providing the ceramic coating composition on the prepared surfaces
normally adapting a paint brush or a spraying means;
-allowing the coating to be air-dried for a time period between 4 to 6
hours; and
- restricting the thickness of the coating approximately between 0.5 to
0.7 mm.
2. The method as claimed in claim 1, wherein the step of surface preparation
of the refractory structure including the cyclone area comprises sand-
blasting and cleaning of the surfaces.
3. The method as claimed in claim 1, wherein the step of preparing the
ceramic coating compound comprises mixing the compound with
approximately 10% (v/v) water to form a paint-consistent coating layer.
4. An improved ceramic coating to protect CFBC boiler components from
corrosive damage by Sox present in the flue gas , comprising: Zirconium
oxide, silicon oxide, Aluminium Oxide, and calcium oxide, wherein the
compound having a composition of ZrO2, SiO2, AI2O3, and Cao respectively
around 60-70%, 25-31%, 0.1-1%, and 0.5-1.5%, and wherein the
physical properties of the compound comprise a density between 3.0 to
3.28 gm/cc, a cold crushing strength between 300-350 kg/sq.cm, a
temperature-resistant strength above 1500°C, a thermal expansion less
than 1.0% in a temperature range of 0-1500°C, a bulk factor less than
2%, and a shrinkage property less than 0.25% at 1500°C.
5. An improved method of providing ceramic coating to protect the anchor
rods including retainer plates in a cyclone of CFBC boiler against corrosion
under Sox present in flue gas, the method as substantially described and
illustrated herein with reference to the accompanying drawings.
6. An improved ceramic coating to protect CFBC boiler components from
corrosive damage by Sox present in the flue gas as substantially described
and illustrated herein with reference to the accompanying drawings.

The invention relates to an improved method of providing ceramic coating to
protect the anchor rods including retainer plates in a cyclone of CFBC boiler
against corrosion under Sox present in the flue gas, the method comprising the
steps of preparing the surface of the refractory structure including the cyclone
area; producing a ceramic coating compound and preparing a ready-to- apply
coating; providing the ceramic coating composition on the prepared surfaces
normally adapting a paint brush or a spraying means;allowing the coating to be
air-dried for a time period between 4 to 6 hours; and restricting the thickness of
the coating approximately between 0.5 to 0.7 mm.