2. DESIGN OF RADIANT SUPER
HEATER AND IMPROVEMENT
OF SUPER HEATER TUBES IN
AFBC BOILER
3. ABSTRACT
The bed super heater of boiler is vulnerable to problems like
clinker formation, erosion, space constraint, frequent replacement etc., In
order to overcome such problems we came up with two solutions.
The first option will be the design of Radiant Super Heaters which
involves the replacement of bed super heaters completely and shifting it to
the radiant zone where we do not have a constraint of space and more
temperature can be achieved. This greatly reduces the problem of frequent
replacement of tubes.
The second option will be coating of the super heater tubes with
Infiltration Brazed Tungsten Carbide Cladding which reduces corrosion and
erosion problems and increases the life of the super haters to four times.
Both these options increase the availability of super heater tubes to
a great extent.
4. INDIAN PAPER INDUSTRY
The Indian paper industry took birth in 1812,
when the first mill was set up at Bengal which has
made considerable progress during the last fifty
years. At the time of independence there were less
than 20 mills in India with an annual production of
1 lakh tones. Today the industry output is over 40
lakhs tones from more than 400 mills.
6. BOILERS – GENERAL
DESCRIPTION
DEFINITION
Boiler also known as steam generator is
a closed vessel in which water is converted
into steam above atmospheric pressure by the
application of heat.
DESCRIPTION OF TNPL BOILER
NO 4
Make – M/s Bharath Heavy Electrical
Limited (BHEL)
Pressure at superheater outlet = 44 kg/cm2
Temperature at superheater outlet = 440° C
Flow at superheater outlet = 60 TPH
7. FLUIDIZED BED COMBUSTION
BOILER
A packaged fluidized bed boiler
concept has been introduced for the first
time in the country in the range 4 to 10
T/hr steam generation capacity. Fuels such
as coal, lignite, spent bagasse. Rice husk
have been tried retrofit on FBC to existing
boilers as a great potential.
8. MECHANISM OF FBC
If the sand in a fluidized state is heated to the ignition
temperature of the coal and the coal is injected continuously in the bed,
the coal burns rapidly and the bed attains a uniform temperature due to
effective mixing. This in short is fluidized bed combustion. While it is
essential that temperature of bed should be at least equal to combustion
temperature to avoid melting of ash. The combustion must be carried
out essentially at a temperature below ash fusion temperature. This is
achieved by abstracting heat from the bed through walls of the bed
through heat transfer tubes immersed in the bed as well as through walls
of the bed.
If the gas velocity becomes too high, the particles are entrained
in the gas stream and are lost. Hence, to sustain stable operation of the
bed, it must be ensured that gas velocity maintained between minimum
fluidization velocity and particle entrainment velocity.
9. TYPES OF FBC BOILER
1. Atmospheric “Classic” Fluidized
Bed Combustion System (AFBC)
2. Pressurized Fluidized Bed
Combustion (PFBC)
3. Atmospheric Circulating (fast)
Fluidized Bed Combustion
System(CFBC)
12. BED SUPERHEATER
The bed super heater is one of the important
accessories in the FBC boiler. It is located inside the
evaporator tubes, which are placed inside the furnace bed.
In the bed super heater even though the area is relatively
less the heat transfer is more because it is located inside
the furnace bed. The high velocity air fuel mixture is very
less, about 2 to 3 years. The space availability inside the
furnace is very less.
13. PROBLEM PREVAILING THE
EXISTING FBC BOILER
1.Clinker formation
2.Erosion
3.Space constraint
4.Frequent replacement of super heater
tubes.
14. CLINKER FORMATION
Clinker formation is generally because of the static
combustion caused either as or by local de- fluidization
due to the reasons like,
1.Fuel feed size exceeding the limits.
2.Increased average bed material size.
3.Blockage of air nozzles leading to non uniform
fluidization.
4.In adequate air flow leading to de- fluidization.
5.Bed temperature exceeding recommended limits.
6.Leakage of air through a hot slumped bed.
15.
16.
17. METHODOLOGY USED
Option one will be the design of radiant super heaters.
These radiant superheaters will receive radiant energy from the
furnace walls. These require a larger area than the superheaters
that are kept in the fluidization bed. The length and the number
of banks of the tubes can be evaluated by the design calculation.
Regarding the maintenance issues, the replacement period of
the radiant superheaters is once in 25 years which is pretty
much higher when compared to conventional tubes which are
replaced once in three years.
18. Option two will be the coating of infiltration
brazed tungsten carbide cladding. This material is
welded at an angle of 270° on the superheater
tubes that are kept in the bed. It is not necessary to
shift the entire set up to a different zone as the
super heated steam will be obtained by keeping the
superheaters in the fluidization bed itself. This
infiltration brazed tungsten carbide cladding has
appealing performance figures over the other
metals taken into consideration. They provide
better results in tests like erosion resistance,
impact angle, corrosion resistance etc. The
investment cost for this is also much lower when
compared to the other option.
19. INFILTRATION BRAZED
TUNGSTEN CARBIDE CLADDING
Infiltration brazed tungsten carbide wear protection, as an
alternative to weld overlay, has been shown to be superior in pulp and
paper mill applications, where severe erosion, corrosion and abrasion
are the primary mechanisms of failure. A material’s erosion resistance
is determined by several factors, including the size and spacing of the
material’s hard particles and the method of application. Other important
variables include the characteristics, impact angle, and velocity of the
eroding media. The erosion of steel equipment can create costly
equipment replacements, unscheduled downtime, and reductions in mill
productivity. The use of wear protection on components, such as
induced draft fans, chutes, screws, elbows, pumps, and other material
conveyance components, can enable mills to reliably extend scheduled
outage cycles while reducing maintenance budgets and diminishing the
risk of unexpected downtime.
20. RESULTS
It is evident from the design calculations of radiant super heater
that the area is much larger when compared to bed super heater which in
turn means that the heat convection will be much higher. MTBF values of
this option are also high.
The properties of infiltration brazed tungsten carbide cladding like
erosion resistance, hard particle distribution, corrosion distribution, impact
angle provides good results when compared to other metals taken into
consideration. The evaluation of this option revealed that super heated
steam temperature of 443º C is produced at the outlet. This shows that not
only the corrosion and erosion problems are reduced by this method but the
super heated steam temperature is produced too.
21. SCOPE FOR FUTURE WORK
In the case of radiant superheater the flow
analysis can be done for it. But the problem in doing
that is that the radiant temperature of the flue gas from
the walls are highly unpredictable as it varies greatly
depending upon the load of the boiler. This requires a
separate study and it is a time consuming process too.
Steps can also be taken to reduce the area of radiant
superheaters once the radiation is fully studied.
In the case of infiltration brazed tungsten carbide
cladding the simulation can be done for the tubes coated
with the material at an angle of 180° and 360° as we
have considered the case of 270° only. Higher results can
be achieved with slight changes over the material
composition. The thickness of the cladding can also be
varied as we have evaluated a thickness of 15 micron.
Thickness can be varied up to 50 micron.
