Observation and classification of the fouling mechanism in rotary air

Observation and
classification of the fouling
mechanism in rotary
air-heaters
Presenter: Vanessa Mathebula
Academic Mentor: Prof Walter Schmitz
Industrial Mentor: Dr. Chris van Alphen and Mike Lander
Date: 05 May 2014
Acknowledgement
This research was conducted
through the Eskom Power Plant
Engineering Institute (EPPEI)
Eskom Specialisation Centre for
Combustion Engineering at
The University of the Witwatersrand
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Overview
• Introduction
• Problem Statement
• Objectives
• Investigational Method
• Analyses Procedures and Results
• Conclusion
(Shah and Sekulić, 2003)
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Introduction
• Air-heaters are heat
exchangers used in
fossil-fuelled power
stations to heat up boiler
air.
• Improves the boiler
efficiency and the plant
efficiency.
• Recuperative and
Regenerative
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Introduction
• Ljungström and Rothemühle airheaters
• Rotors of the Ljungström and
Rothemühle twin-flow air-heaters
are similar
• Corrugated closed channel steel
plates (elements) packed closely
together in each basket
• On load cleaning of passages
between the elements is conducted
using steam sootblowing equipment
• Steam supply nozzles on top and
bottom of rotor, on gas side
• Off load cleaning conducted using
high pressure water washing
EPRI, 1998
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Introduction
• Fouling occurs as a result of a
build-up of ash particles and other
deposits in the passages of the airheater elements
• Hot end fouling/plugging:
Caused by boiler conditions such as
incombustibles and coarse debris.
This is sometimes called plugging.
• Cold end fouling
Closely related to the condensation of
sulphuric acid at temperatures below
the dew point (Raask, 1985).
• AD: acid deposition
• TAD: acid dewpoint temperature
Redrawn from Raask, 1985
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Problem Statement
• Fouling and blockage of air-heaters is
experienced
• This results in an increase in the pressure
drop across the air heater.
• More power is required from the induced
draft fan.
• Severely fouled air-heater element packs
have a negative impact on the boiler
efficiency, due to reduced heat transfer
• Difficult to clean elements with hardened
deposits
• In extreme cases complete replacement
of air-heater elements is required, which
costs millions for each unit.
Matimba
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Objectives
• Identify the mechanism and extend of cold end fouling in the airheaters.
• A qualitative observation of the fouling distribution across the airheater rotor.
• An inlet design verification of the velocity profile of the flue gas and air
in the air-heater inlet duct in order to determine its effect on the
temperature distribution in the air-heater rotor and that of the gases
flowing in it.
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Investigational Method
1
Access to the air-heater
matrix
Qualitative
observation of fouling
distribution at cold end
Take ash deposit
samples
Determine the mineralogical
composition of the samples
QEMSCAN analyser
(2µm lateral resolution)
RAMAN Spectrometer
(1µm lateral resolution)
Compare results and location
where samples were taken to
the Qualitative observation
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Investigational Method
2
Collect coal analysis
results of Power Station
coal
Determine the flue gas
composition
Determine the dew point
temperatures of H2SO4 and H2SO3
Conduct a sensitivity study of
the dew point temperatures to
changes in coal component
ratios
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Investigational Method
3
RAH simulation inputs for Eskom’s
Rothemühle twin-flow air-heater
Velocity profile of the air and flue gas
flow into the air-heater, from
Aerotherm’s (2012) CFD results
Station operating data
Air-heater matrix
dimensions
Run simulation for mal-distribution and
uniform velocity distribution case
Compare primary and
secondary section plate
temperature distribution
results
Compare mal-distribution
results to the uniform
distribution results
Use dew point temperature
results to determine where
condensation of H2SO4 and
H2SO3 would occur
Compare with the
Qualitative observation
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Analysis procedures,
results and discussion
Qualitative observation and air-heater deposits
1
2
4
3
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QEMSCAN
(Van Alphen, 2013)
• AlSi-sulphate, kaolinite, cenospheres and Al(Si)-sulphate were the
four most dominating minerals/phases
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Raman
Filters
Mirrors
Monochromator
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Formation of H2SO4 and H2SO3
• 1% to 5% of the SO2 in the flue gas will be converted to SO3 (Ganapathy, 1989).
1. SO2 + O2
SO3 (in the flame)
2. SO2 + Catalyst (Vanadium pentoxide and iron oxide)
SO3 + catalyst product
(low temperature zone between 602 ºC to 752 ºC)
• SO3 + H2O
H2SO4 (below 350 °C)
• Condensation of the water vapour in the flue gas takes place at temperatures below
the water dew point.
• H2O + SO2
H2SO3
• Raask (1985) stated that the concentrated solution of H2SO4 would combine with
alkaline ash and a reaction would occur with the air-heater element surface.
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What does that mean?
• The fly ash may have adhered to the H2SO4 that condensed, out of the
flue gas, onto the air-heater plates.
• Then reacted with some fly ash components or fuel oil char to form the
fouling phases/minerals.
• The mixture may have dried up when exposed to higher temperatures.
• Quartz and cenospheres would have added to the hardness of the fly
ash deposits.
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Acid dew point calculations and sensitivity for
Duvha coal
• From Ganapathy (1989) and Niessen (2002)
• Sulphurous acid (H2SO3):
1000/஽௉ 3.9526 0.1863 lnுమ ை 0.000867 lnௌைమ 0.000913 lnுమ ை lnௌைమ • Sulphuric acid (H2SO4):
1000/஽௉ 2.276 0.0294 lnுమ ை 0.0858 lnௌைయ 0.0062 lnுమ ை lnௌைయ • Ganapathy (1989) stated that the calculated dew point temperature for
sulphurous acid was lower by 6°K and that of sulphuric acid was lower by
9°K than published data.
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Acid dew point calculations and sensitivity for
Duvha coal
• Sensitivity
Component
Change in mass %
Change in dew point
H2SO4 [°C]
Change in dew point
H2SO3 [°C]
Sulphur
0.2
±2
± 0.01
Hydrogen
0.41
± 0.5
± 1.33
Total Moisture
0.8
± 0.15
± 0.34
Water Vapour
0.8
± 1.5
± 2.00
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Even versus mal-distribution (plate temps from
RAH)
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Even versus mal-distribution (plate temps from
RAH)
Annular Division 1 Third Layer and Cold End Plate Temperatures
200
180
Temperature (°C)
160
Uniform third layer inlet
140
Non-uniform third layer inlet
120
Uniform third layer outlet (cold end inlet)
100
Non-uniform third layer outlet (cold end
inlet)
80
Uniform cold end outlet
60
Non-uniform cold end outlet
40
20
0
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360
Rotation Angle (Degrees)
300mm - 16.7%
600mm - 33.3%
600mm - 33.3%
300mm - 16.7%
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RAH simulation for Duvha
Average Primary and Secondary Plate Temperatures
260
240
143.15 °C
Temperature (°C)
220
200
Secondary third layer inlet
180
primary third layer inlet
160
Secondary third layer outlet (cold end
inlet)
Primary third layer outlet (cold end
inlet)
Secondary cold end outlet
140
120
100
Primary cold end outlet
80
60
300mm - 16.7%
40
20
Un-even
600mm - 33.3%
600mm - 33.3%
0
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360
Rotation angle (Degrees)
300mm - 16.7%
• It was determined that approximately 35.8% of the rotor volume percentage
would be exposed to temperatures below the dew point of H2SO4
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Conclusion
Conclusion
• The qualitative observation at the cold end of a twin-flow Rothemühle
air-heater showed that the primary section was more blocked than the
secondary section.
• The QEMSCAN and RAMAN results of the air-heater deposits showed
that AlSi-sulphate, kaolinite, cenospheres and Al(Si)-sulphate were
the four most dominating minerals/phases.
• Using RAH simulation model, the plate temperatures of the primary
section were observed to be lower than those of the secondary
section.
• It was determined that approximately 35.8% and 25.3% of the rotor
volume percentage would be exposed to temperatures below the dew
point of H2SO4
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Conclusion
• Therefore, soot blowing would not be effective for hardened deposition
able to dislodge the solid from the air-heater plates.
• RAH is a very useful tool
• And CFD as well
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Thank you

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