Lecture 6: Large Scale Combustion Testing
Burners / Test Methods
G Hesselmann
APP OFWG Capacity Building Course
5th & 6th February 2009, Daejeon, Korea
Burner Technology – Air Firing
• Modern low NOx coal burners
operate on the principle of air staging
– Coal is rapidly heated in an oxygen
deficient region
– Fuel nitrogen reactions favour the
formation of N2 (instead of NOx) where
there is low O2 availability
– Main combustion air is added
sequentially to complete burnout
• Stabilisation is predominantly by
aerodynamic measures
– Swirled air streams create a strong
internal recirculation zone that draws
hot gases into the burner quarl
Burner Design Considerations
• The burner should perform in such a way that the overall combustion efficiency is
high (typically less than 5% CIA, 99.8% burnout)
• Flame length and shape should be appropriate for the furnace
• Flame stability limits should be “robust” – e.g. the flame should typically be
stabilised in the burner quarl at all normal operating conditions
• Turndown performance should not impact plant operability – typically burners
should be able to operate at 40% of design capacity without oil support
• The flame itself should have an overall oxidising envelope to minimise the potential
for high temperature corrosion at the furnace walls
• Performance of the burner should be acceptable for a wide range of coals covering
the typical range of supply to pulverised coal fired utility plant
Doosan Babcock Mk3 Low NOx Axial Swirl Burner
Over 3000 (45GWe) Doosan Babcock Mk3 low NOx
burners installed worldwide
Primary Air and
Pulverised Coal
Core Air
Secondary Air
Tertiary Air
Burners for Oxyfuel Firing
• “Simulated Air” Burners
– Basic design is the same as for air firing, but the comburrant (air) is replaced by a mixture
of nearly pure oxygen and recycled flue gas
– Primary air conditions are constrained by the milling plant
• Gas velocities in the pulverising mill must be high enough to convey the coal
• O2 content has an upper limit (21%) to minimise risk of mill fires
– Simulated air burners are likely to be the first to be installed in large utility plant
• Oxyfuel Burners
– Design allows for direct injection of O2 via (for example) lances
Pilot Scale Oxyfuel Burner
Pulverised Coal
Primary FGR
Primary O2
Secondary O2
Secondary FGR
Oxyfuel Burner Experience
Schwarze Pumpe
• Simulated Air
– Some experience of large (30MWt, ¾ scale)
burners at Schwarze Pumpe and B&W
– Bright stable flame
• Oxyfuel Burners
– Coal firing experience limited to pilot scale
(typically 1MWt)
– Considerable experience for gas fired oxyfuel
flames in non-utility applications, mostly lower
thermal input than required for utility plant
B&W
Oxyfuel Burner Experience
• Experimental studies at
Chalmers University (Sweden)
shows the differences
between air and oxyfuel firing
in a simulated air burner
(100kWt)
• It is important to understand
flame structure in order to
establish viable burner
designs
– There is a need to measure
key parameters in the
aggressive combustion
environment
CO
Temperature
Measurements
• Temperature
• Velocity
– Absolute Values
– Regions of Forward / Reverse Flow
• Gaseous Species
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NOx
CO
O2
Etc
• Solid Samples
– Carbon in Ash
– Nitrogen release
• Heat Flux
Gas Temperature
Suction Pyrometer / High Velocity Thermocouple
Gas Temperature
• Suction pyrometry is the “correct” way
to measure gas temperature in
furnaces
– Radiation shields are fragile
– Prone to blockage
– More robust !
– Need to correct measured temperature
for radiation losses – measured value is
lower than actual
• Simple Unshielded Thermocouples
have large radiation losses – indicative
temperatures only
– Robust
– Fine wire thermocouples minimise
radiation error, but are fragile
1800
1600
Actual Temperature (C)
• High Velocity Thermocouples (HVT)
are similar in construction, but have a
simple cylindrical radiation shield
2000
1400
1200
1000
800
800
1000
1200
1400
HVT Measured Temperature (C)
1600
1800
2000
Gas Velocity
Prandtl Probe
Gas Velocity
5-Hole Pitot Tube
Gas Velocity
Hubbard Probe / S-Type Pitot
Gas Velocity
• Based on standard “clean air” techniques
– Ruggedised for combustion environment
– Generally incorporated into a water cooled probe
• Simple in principle, very difficult in practice !
– Small holes in pitot type devices are prone to blockage
– Absolute velocities are low (low ∆P to be measured)
– Velocities tend to fluctuate widely and rapidly
• Usually the best that can be achieved is an indication of the region of reverse flow,
using the S-type pitot (the most robust of the various probes available)
Gaseous Species and Solid Sampling
Doosan Babcock Water Quench Probe
Gaseous Species and Solid Sampling
• Water quench probe
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Rapid cooling of gas/solids at probe tip freezes combustion reactions
Water flushes solids to collection vessel
Quench not needed for just gases
Can also use HVT probe for gas sampling
• Robust and reliable method
• Gases analysed using conventional CEM-type analysers
– Specify appropriate measurement range
– Consider interactions between species – e.g. NOx measurement error at high CO levels
– Be aware of what is measured – e.g. NOx analysers convert NO to NO2 and measure NO2
to give “NOx”, but can give “NO” if the converter is turned off; they do not measure N2O
Gaseous Species and Solid Sampling
Wet chemistry sampling for Ammonia
Heat Flux
Total Heat Flux Probe
Heat Flux
Radiant Heat Flux Probe – Ellipsoidal Radiometer
Heat Flux
• Difficult to measure absolute values of total heat flux
– Deposition of ash on probe acts as an insulator; indicated heat flux reduces with time
– Calibration of probes is expensive – requires a black-body furnace
– Useful technique to get relative measure of radiation from a flame
• Ellipsoidal radiometer
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Not sufficiently robust for large coal firing applications
Blockage of “window” by particulate material
Fouling of inner surface causes calibration drift
Calibration of probes is expensive – requires a black-body furnace
Emerging Technologies
Laser Tomography
T
O2
CO
Emerging Technologies
• Multiple laser paths cross the measurement domain, arrange for paths to intersect
• Laser measurement of average property along path
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Temperature
O2,CO, CO2, H2O
Self-calibrating
Semi-continuous
• Mathematical analysis generates profile
• Many practical issues
– Laser alignment
– Attenuation of laser signal due to particulate material
– Not demonstrated for in-flame measurements
Emerging Technologies
Wall Corrosion
• Aggressive atmospheres (CO, H2S,
molten ash) can lead to high rates of
metal loss
• Oxyfuel combustion has the potential to
create an environment that is more
corrosive than for air firing
– Higher levels of CO
– Higher levels of H2S
– Higher peak temperature
Emerging Technologies
Wall Corrosion / Sniffer Port
Long term corrosion tests on large boiler plant
Measure gaseous species at wall, and metal loss from test coupon
HOT GAS SIDE
CL
CL
3”
1 ¼”
CL
1 ¼”
Probe Tip
Threaded
Tapping
Probe Tip
6 mm
Membrane
CL
CL
Weld
Inlay
CL
CL
Thermocouple
Guide
Sniffer Port
Corrosion Probe
CL
COLD SIDE
Practical Issues
Furnace Measurements – Practical Considerations
• Restricted Access
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limited access points,
restricted space
limited galleries
movement of equipment
• Environmental conditions
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Dust / Fume
Temperature
Noise
Light
• Services
– Power
– Cooling water
– Compressed Air
Practical Issues
Practical Issues
Probe Handling
Photo’s show probing gallery on Doosan Babcock’s test facility – probes are typically 6m
to 10m long, 75mm diameter, stainless steel, water filled – i.e. heavy. Issues for manual
handling.
Safety
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Access/Egress
Multi - level Working
Fall of Person/Objects
Slips and Trips
Confined Space
Restricted Means of escape/rescue
Contact with Process
Electricity
Fire + Explosion
Vibration
Lifting Operations
Vehicles
Manual Handling
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Mobile or Fixed Plant
Adjacent Live Plant or Equipment
Environmental
Weather
Temperature
Lighting
Noise
Hazardous Substances
Asbestos
Dust / Fume
Lone Working
Interfacing with Third Parties
Safety
• Pre-Test Site Visit
– Know what to expect when you arrive on site
• Risk Assessment – review on arrival
– Assess hazards, and identify mitigation
– Review – things are likely to have changed between your pre-test visit and testing
• Method Statement
– Define clearly what procedures you are going to follow
• ‘Toolbox Talk’
– Ensure that all members of the test team know what the hazards are, what to do in an
emergency, etc
Useful References
• Chedaille & Braud
Industrial Flames. Volume 1. Measurements in Flames.
International Flame Research Foundation (IFRF)
ISBN 0 7131 3265 5
• ICT Quick reference Guide
Innovative Combustion Technologies Inc
www.innovativecombustion.com
• Risk Assessment Guide – Industrial Emission Monitoring
Source Testing Association
www.S-T-A.org