Oxygenated treatment

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

Traditionally, corrosion was prevented by keeping the oxygen in the system below 10 ppb by using a deaerator and injecting an oxygen scavenger. This process is called All-volatile treatment (AVT) in reference to the volatile alkalizing agents added to the feedwater. Ammonia (NH3) is added to keep the feedwater pH high, thus reducing the risk of FAC. In AVT(R), a reducing agent is added. AVT(R) causes the formation of a protective layer of magnetite (Fe3O4), which is a combination of FeO and Fe2O3. In AVT(O), no reducing agent is added and residual oxygen is present. AVT(O) allows a layer of hematite to form on top of the magnetite, similar to oxygenated treatment.

With oxygenated treatment, oxygen is injected into the feedwater to keep the oxygen level between 30-50 ppb. "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 condensate polisher.[5]

Comparison of AVT to OT

Characteristic All-Volatile Treatment (Reducing) All-Volatile Treatment (Oxidizing) Oxygenated Treatment (Neutral Water Treatment) Oxygenated Treatment (Combined Water Treatment)
Feedwater system piping ferrus or mixed metallurgy (e.g. copper feedwater train) all-ferrous metallurgy all-ferrous metallurgy all-ferrous metallurgy
Dissolved oxygen level < 10 ppb 1 to 10 ppb 30-50 ppb (drum), 30-150 (supercritical) 30-50 ppb (drum), 30-150 (supercritical)
Chemicals added a reducing agent (such as hydrazine), ammonia to raise pH ammonia to raise pH an oxidizing agent (such as hydrogen peroxide or oxygen) an oxidizing agent, ammonia to raise pH
pH[6] 9.0-9.3 9.2-9.6 9.2-9.6 8.0-8.5 (once-through), 9.0-9.4 (drum)
Top layer composition magnetite (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
Advantages Can be used with mixed metallurgy piping More protection against FAC than AVT(R), minimizes orifice founling [8] Less flow resistance, lower dissolved feedwater iron concentrations, FeOOH film is more stable, reduced boiler cleaning frequency -
Disadvantages Increased 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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