Oxygenated treatment (OT) is a technique used to reduce corrosion in a boiler and its associated feedwater system in flow-through boilers.

With oxygenated treatment, oxygen is injected into the feedwater to keep the oxygen level between 30 and 50 ppb. OT programs are most commonly used in supercritical (i.e. >3250psi) power boilers. The ability to change an existing sub-critical boiler over to an OT program is very limited. "Common injection points are just after the condensate polisher and again at the deaerator outlet."[1] This forms a thicker protective layer of hematite (Fe2O3) on top of the magnetite. This is a denser, flatter film (vs. the undulation scale with OT) so that there is less resistance to water flow compared to AVT.[2] Also, OT reduces the risk of flow-accelerated corrosion.[3]

When OT is used, conductivity after cation exchange (CACE) at the economiser inlet must be maintained below 0.15μS/cm [4] this can be achieved by the use of a full-flow condensate polisher.[5]

Comparison of AVT to OT

CharacteristicAll-Volatile Treatment (Reducing)All-Volatile Treatment (Oxidizing)Oxygenated Treatment (Neutral Water Treatment)Oxygenated Treatment (Combined Water Treatment)
Feedwater system pipingferrus or mixed metallurgy (e.g. copper feedwater train)all-ferrous metallurgyall-ferrous metallurgyall-ferrous metallurgy
Dissolved oxygen level< 10 ppb1 to 10 ppb30-50 ppb (drum), 30-150 (supercritical)30-50 ppb (drum), 30-150 (supercritical)
Chemicals addeda reducing agent (such as hydrazine), ammonia to raise pHammonia to raise pHan oxidizing agent (such as hydrogen peroxide or oxygen)an oxidizing agent, ammonia to raise pH
pH[6]9.0-9.39.2-9.69.2-9.68.0-8.5 (once-through), 9.0-9.4 (drum)
Top layer compositionmagnetite (Fe3O4) on steel piping, cuprous oxide (Cu2O) on copper piping hematite (Fe2O3) forms on top of the porous magnetite (Fe3O4)[7] ferric oxide hydrate (FeOOH) or hematite (Fe2O3) forms over the porous magnetite ferric oxide hydrate (FeOOH) or hematite (Fe2O3) forms over the porous magnetite
AdvantagesCan be used with mixed metallurgy pipingMore protection against FAC than AVT(R), minimizes orifice fouling [8]Less flow resistance, lower dissolved feedwater iron concentrations, FeOOH film is more stable, reduced boiler cleaning frequency-
DisadvantagesIncreased risk of FAC, a deaerator is required, more frequent chemical cleaning is required, hazardous chemicals (hydrazine) are used.A deaerator is required.Air leakage is more serious. Two-phase FAC can be a concern.Condensate polishers are required.

See also

References

  1. ↑ Brad Buecker, "Flow-Accelerated Corrosion: A Critical Issue Revisited", 2007, Power Engineering, http://www.power-eng.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited.html
  2. ↑ Mitsuhiro Yamagishi, Masamichi Miyajima, "Evaluation of Oxygenated Water Treatment" 14th International Conference on the Properties of Water and Steam in Kyoto, August 29-September 3, 2004
  3. ↑ Daniels, D., "HRSG Failure Mechanisms - Waterside," Proceedings of the 22nd Annual Electric Utility Chemistry Workshop, Champaign, Illinois, May 7–9, 2002.
  4. ↑ IAPWS Technical Guidance Document: "Volatile treatments for the steam-water circuits of fossil and combined cycle/HRSG power plants (July 2010) http://www.iapws.org/techguide/Volatile.html"
  5. ↑ Frank Gabrielli and Horst Schwevers, "Design Factors and Water Chemistry Practices - Supercritical Power Cycles" PREPRINT-ICPWS XV Berlin, September 8–11, 2008
  6. ↑ Sharat Kumar and S.K. Gupta "Feed Water Treatment Optimization for Controlling Flow Accelerated Corrosion (FAC)" http://www.infraline.com/power/presentations/others/ntpc/n_50_fac_sharatkumar_chem.pdf
  7. ↑ Frank Gabrielli and Horst Schwevers, "Design Factors and Water Chemistry Practices - Supercritical Power Cycles" PREPRINT-ICPWS XV Berlin, September 8–11, 2008, Page 10
  8. ↑ Frank Gabrielli and Horst Schwevers, "Design Factors and Water Chemistry Practices - Supercritical Power Cycles" PREPRINT-ICPWS XV Berlin, September 8–11, 2008, Page 10
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