Lecture 6: Large Scale Combustion TestingBurners / Test Methods

G HesselmannAPP OFWG Capacity Building Course5th & 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

Core Air

Primary Air and Pulverised Coal

Secondary Air

Tertiary Air

Over 3000 (45GWe) Doosan Babcock Mk3 low NOx burners installed worldwide

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 CoalPrimary FGRPrimary O2

Secondary O2

Secondary FGR

Oxyfuel Burner Experience

• 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

Schwarze Pumpe

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– 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

• High Velocity Thermocouples (HVT) are similar in construction, but have a simple cylindrical radiation shield

– 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

800

1000

1200

1400

1600

1800

2000

800 1000 1200 1400 1600 1800 2000

HVT Measured Temperature (C)A

ctua

l Tem

pera

ture

(C)

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– 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– 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

T O2 CO

Laser Tomography

Emerging Technologies

• Multiple laser paths cross the measurement domain, arrange for paths to intersect• Laser measurement of average property along path

– 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

• 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

Wall Corrosion

Emerging Technologies

Corrosion ProbeSniffer Port

Probe Tip

CL

CL

CL

HOT GAS SIDE

1 ¼” 1 ¼”

3”

CL

CL

CL CL

CL

COLD SIDE

ThreadedTapping

6 mmMembrane

Thermocouple Guide

WeldInlay

Probe Tip

CL

CL

CL

HOT GAS SIDE

1 ¼” 1 ¼”

3”

CL

CL

CL CL

CL

COLD SIDE

ThreadedTapping

6 mmMembrane

Thermocouple Guide

WeldInlay

Probe Tip

Wall Corrosion / Sniffer Port

Long term corrosion tests on large boiler plantMeasure gaseous species at wall, and metal loss from test coupon

Practical Issues

Furnace Measurements – Practical Considerations

• Restricted Access– limited access points,– restricted space– limited galleries– movement of equipment

• Environmental conditions– 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

• 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

• 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 & BraudIndustrial Flames. Volume 1. Measurements in Flames.International Flame Research Foundation (IFRF)ISBN 0 7131 3265 5

• ICT Quick reference GuideInnovative Combustion Technologies Incwww.innovativecombustion.com

• Risk Assessment Guide – Industrial Emission MonitoringSource Testing Associationwww.S-T-A.org