Lecture 6: Large Scale Combustion Testing Burners / Test Methods

Transcription

1 Lecture 6: Large Scale Combustion Testing Burners / Test Methods G Hesselmann APP OFWG Capacity Building Course 5 th & 6 th February 2009, Daejeon, Korea

2 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 N 2 (instead of NOx) where there is low O 2 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

3 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

4 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

5 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 O 2 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 O 2 via (for example) lances

6 Pilot Scale Oxyfuel Burner Pulverised Coal Primary FGR Primary O 2 Secondary O 2 Secondary FGR

7 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

8 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

9 Measurements Temperature Velocity Absolute Values Regions of Forward / Reverse Flow Gaseous Species NOx CO O 2 Etc Solid Samples Carbon in Ash Nitrogen release Heat Flux

10 Gas Temperature Suction Pyrometer / High Velocity Thermocouple

11 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 Actual Temperature (C) HVT Measured Temperature (C)

12 Gas Velocity Prandtl Probe

13 Gas Velocity 5-Hole Pitot Tube

14 Gas Velocity Hubbard Probe / S-Type Pitot

15 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)

16 Gaseous Species and Solid Sampling Doosan Babcock Water Quench Probe

17 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 NO 2 and measure NO 2 to give NOx, but can give NO if the converter is turned off; they do not measure N 2 O

18 Gaseous Species and Solid Sampling Wet chemistry sampling for Ammonia

19 Heat Flux Total Heat Flux Probe

20 Heat Flux Radiant Heat Flux Probe Ellipsoidal Radiometer

21 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

22 Emerging Technologies Laser Tomography T O2 CO

23 Emerging Technologies Multiple laser paths cross the measurement domain, arrange for paths to intersect Laser measurement of average property along path Temperature O 2,CO, CO 2, H 2 O 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

24 Emerging Technologies Wall Corrosion Aggressive atmospheres (CO, H 2 S, 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 H 2 S Higher peak temperature

25 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 C L 3 C L 1 ¼ 1 ¼ C L Probe Tip Probe Tip Threaded Tapping C L 6 mm Membrane C L Weld Inlay C L C L Thermocouple Guide Sniffer Port Corrosion Probe C L COLD SIDE

26 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

27 Practical Issues

28 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.

29 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

30 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

31 Useful References Chedaille & Braud Industrial Flames. Volume 1. Measurements in Flames. International Flame Research Foundation (IFRF) ISBN ICT Quick reference Guide Innovative Combustion Technologies Inc Risk Assessment Guide Industrial Emission Monitoring Source Testing Association