Oxygenated treatment (OT) is a technique used to reduce corrosion in a boiler and its associated feedwater system in flow-through boilers.
Process 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."Brad Buecker, Flow-Accelerated Corrosion: A Critical Issue Revisited 2007, Power Engineering 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.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 Also, OT reduces the risk of flow-accelerated corrosion.Daniels, D., "HRSG Failure Mechanisms - Waterside," Proceedings of the 22nd Annual Electric Utility Chemistry Workshop, Champaign, Illinois, May 7–9, 2002.
When OT is used, conductivity after cation exchange (CACE) at the economiser inlet must be maintained below 0.15μS/cm 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" this can be achieved by the use of a full-flow condensate polisher.Frank Gabrielli and Horst Schwevers, "Design Factors and Water Chemistry Practices - Supercritical Power Cycles" PREPRINT-ICPWS XV Berlin, September 8–11, 2008
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 pHSharat 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 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)Frank Gabrielli and Horst Schwevers, "Design Factors and Water Chemistry Practices - Supercritical Power Cycles" PREPRINT-ICPWS XV Berlin, September 8–11, 2008, Page 10 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 fouling Frank Gabrielli and Horst Schwevers, "Design Factors and Water Chemistry Practices - Supercritical Power Cycles" PREPRINT-ICPWS XV Berlin, September 8–11, 2008, Page 10 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 Heat recovery steam generator Flow-accelerated corrosion Oxygen scavenger
References
External links Lamella Clarifier