Successful Real Time On-line Monitoring of High Temperature g g p Corrosion in Oxyfuel and Other Combustion Systems Combustion Systems William M. Cox, Corrosion Management Ltd., UK Kevin Davis, Andrew Fry, David Swensen, Reaction Engineering International, USA Martin de Jong DNV-KEMA, Netherlands
ABSTRACT Real time high temperature corrosion monitoring is useful to investigate troublesome degradation mechanisms, to reduce the time required to undertake comparative materials testing, and to evaluate the effects of novel combustion environments. However, some do not believe monitoring of high temperature corrosion processes is possible using electrochemical instrumentation. Others have felt it should be feasible but have found the results were unreliable. the results were unreliable This paper examines the development route of practical real-time high temperature corrosion rate determination, considers how the technology is consistent with traditional electrochemical theory and technology is consistent with traditional electrochemical theory and reviews results obtained in high temperature applications that demonstrate its use to characterize aggressive environments and fouling behavior in advanced combustion systems waste-to-energy fouling behavior in advanced combustion systems, waste-to-energy plants and oxy-fuel service in the UK, Europe, Asia and USA.
Relevant Applications pp High temperature corrosion is an issue for – fossil fuelled electric power generation systems – waste-to-energy boilers t t b il – gas turbines – reformers – reformers – fired heaters tube metal temperatures typically ~350 to ~650ºC (660 to 1200ºF)
Drivers e s There has been pressure in recent years for: • improved energy efficiency • CO 2 reduction This has resulted in developments in: p – Low NO x combustion systems – Larger waste incineration plants – Oxy-fuel combustion Oxy fuel combustion These initiatives have considerable implications for tube metal corrosion behavior
Coupons p Coupons provide useful data but: Coupons provide useful data, but: – often short-term or lab tests – compromise test environment p – give only cumulative results – give only retrospective information – do not provide any control option – results vary from full-scale performance – plant excursions can be catastrophic l t i b t t hi Results give sub optimal materials selection Results give sub-optimal materials selection
Relevant Electrochemical Theory Anodic reaction: Anodic reaction: Metal Me 2+ + 2e - Cathodic reaction: Cathodic reaction: O 2 + 4e - 2O 2- Combined: 2Me 2+ + 2O 2- 2MeO (dep) Similarly, sulfur sulfides chlorine chlorides
Other considerations • Electrochemical processes are completely consistent with established mechanical theories of high temperature corrosion protection p p i.e. chrome oxide formers or nickel oxide formers, etc. • “Electrolyte” can be molten but may be solid and semi-conductive i d ti • Corrosion is effectively just the reverse of electrochemical materials extraction e ec oc e ca a e a s e ac o
Development Route p • Early work on electrochemical potential (1930s) • High temperature dc polarization studies (1960’s) • Electrochemical impedance (1970s) El t h i l i d (1970 ) • Electrochemical noise (1985) • Full-scale Plant installations • Full-scale Plant installations
Advantages of EN approach g pp • Avoids need for a reference electrode Avoids need for a reference electrode • Does not require external polarization • Evaluates only spontaneous transients Evaluates only spontaneous transients generated by corrosion process itself • Allows temperature control of sample • Immediate response
EN is a DC Technique q From Ohm’s Law: V = IR V/I = R so: Δ V/ Δ I = R (p) and hence: V (n) / I (n) = R (n) Similarly: where: R(n) is equivalent to/analogous to R(p) is equivalent to/analogous to R ( ) q g (p) q g because in each case it is the ratio between the potential and current responses that gives the ‘R’ component
Specifically: Spec ca y: R n = V n / I n and from Stern Geary: y i corr = K . 1/R n use Faraday’s Law to convert i corr to mpy The value of K may/should be 1 but it can vary The value of K may/should be 1 but it can vary
Key Issue – Quantitative Rate y Q 1 1. Obtain a ‘representative’ sample of the Obt i ‘ t ti ’ l f th electrochemical activity 2. In fully immersed systems the cell factor may be 100% 3. In thin-film or semi-conductive conditions the cell factor may be less than 100% (say 10% for example) factor may be less than 100% (say 10% for example) 4. In a boiler application the area factor may be (say) 25% (because the area under attack is only at 10.00 and 02 00 on the tube circumference) and 02.00 on the tube circumference) 5. Multiply the electrochemical response by the cell factor and area factor to obtain the corrosion l loss/penetration rate on a boiler tube / t ti t b il t b
Typical Faults when using EN monitoring instrumentation EN monitoring instrumentation • Dissimilar electrodes are used • Only one electrode is used y • The ZRA is not properly balanced • The influence of the cell factor is not The influence of the cell factor is not understood • The influence of the area factor is not The influence of the area factor is not understood • The circuitry is not adequately grounded The circuitry is not adequately grounded
Instantaneous or Continuous? • EIM provides a ‘snap-shot’ EIM provides a ‘snap shot’ • EIM works best under activation control • EN provides continuous real-time output – Allows on-line correlation with change in furnace environment/composition/metal temperature – works well under both activation and diffusion control – enables real-time verification of effectiveness of remedial action
Independent Verification p • Waste to Energy (UK) • Circulating Fluidized Bed (Taiwan) Circulating Fluidized Bed (Taiwan) • Waste to Energy (Netherlands) • Petrochemical processing, (Japan) P t h i l i (J ) • Oxy-Fuel (USA)
#1: Waste-to-Energy (UK) gy ( ) • Work done at Tyseley Waste-to-Energy • Project supervised by CREED/Vivendi j p y • Two boilers handle 450,000 tpa MSW • Superheater tubes lasted <18 months p • Probes installed in radiant and superheater
TWD Boiler Detail Radiant Sensor Superheater Sensor Sensor
Concerns • Damage had significant effect on maintenance cost Damage had significant effect on maintenance cost and reduced the efficiency and availability of the plant. • The mechanisms and causes of attack were not well understood. • Rate of damage varied widely on similar types of plant and can be specific to local conditions d b ifi t l l diti • Conventional measurement of the corrosion behaviour is inherently difficult as it is normally retrospective and s e e t y d cu t as t s o a y et ospect e a d only possible over long operational periods.
Instrumentation
Superheater Corrosion Rate p CTD. ENCR Superheater Trend Plot 20 per. Mov. Avg. (CTD. ENCR) p g ( ) 4.00E+01 3.50E+01 Average rate 3 97mm/y Average rate 3.97mm/y 3.00E+01 (tube wall thickness 4.8mm) 2.50E+01 2.00E+01 1.50E+01 1.00E+01 1 00E+01 5.00E+00 0.00E+00 0 00 00 07/11/00 00:00 14/11/00 00:00 21/11/00 00:00 28/11/00 00:00 05/12/00 00:00 month 1 month 2
Stepped Temperature Traces pp p Period: ~3 days y
#2: Circulating Fluidized Bed g • Installation at a YFY Hsinwu Paper Mill • Project supervision by ITRI, Taiwan • 230 tph steam • Designed to run on pulverized coal • Fired instead on a high proportion of RDF, tire chips, paper waste, sewage sludge • Sensors in superheater and economizer • High interest in fouling behavior
Circulating Fluid Bed Boiler g
Coal Replacement p Fuel Fuel Coal Coal Waste Waste RDF - RDF Sludge Sludge Tires Diapers Type Inputs 40% 24% 13% 23% • • Coal and sludge contained Ca and Na Coal and sludge contained Ca and Na – ash composition unknown ash composition unknown • Tires typically contain Zn • RDF ash composition unknown: Ca? Na? • Overall sulfur-content ~1.5 wt% • Some questions on fuel analysis
Superheater Sensor Installation p
Economizer Corrosion
Superheater Corrosion p
Fouling/Sootblowing g/ g
#3: Waste-to-Energy, Netherlands gy • Installation at AEB Amsterdam • Installation at AEB, Amsterdam • Largest refuse-fired boiler in Europe • Installation supervised by KEMA Installation supervised by KEMA • Purpose – comparative materials testing • 4-probe installation, radiant and superheater 4 b i t ll ti di t d h t • 2 materials on sensor – standard and candidate candidate • Evaluation of real-time transients in the service environment service environment
AEB Waste to Energy Plant gy
Measurement Campaign at AEB p g • Corrosion measurement campaign was conducted at the high efficiency waste fired power plant in Amsterdam fired power plant in Amsterdam • Part of the EU-project NextGenBioWaste • Ultimate situation during campaign: Ultimate situation during campaign: simultaneous surveillance by 4 corrosion probes • No pre-selection of process parameters N l i f
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