For a long time, the reasoning of boiler water treaters was that oxygen causes corrosion, so it must be removed. To do so, procedures such as mechanical deaeration and oxygen scavengers were used for this task.
This approach worked very well for the corrosion problems known at the time, including systems with copper metallurgy. With the evolution of boilers, the first FAC events began to appear. As evolution never stops, it was discovered in supercritical plants that in high-purity feedwater systems (cation conductivity ≤0.15 µS/cm) with no copper alloys, the continuous injection of a small dose of pure oxygen produces an iron oxide layer that is more protective and adherent than traditional programs of mechanical deaeration and oxygen scavenger. This program came to be called Oxygenated Treatment (OT).
Under very high purity water conditions, the introduction of a small amount of dissolved oxygen induces the formation of a hematite layer (Fe₂O₃) over the existing magnetite. This double layer, denser and less soluble than magnetite alone, is able to drastically reduce iron transport through the system.
The practice consists of dosing oxygen into the condensate, typically right after the condensate polisher and again at the deaerator outlet, to maintain a residual dissolved oxygen range. Current IAPWS guidelines work with 30 to 150 ppb throughout the water-steam cycle. This is a much larger amount than any AVT(O) program, covered in previous articles, would tolerate. That is why OT depends on extreme feedwater purity. The usual market reference for conductivity after cation exchange in OT is up to 0.15 µS/cm.
Without this requirement, the dosed oxygen no longer forms protective hematite and starts to behave like any dissolved oxygen in water with contaminants, favoring localized corrosion instead of a uniform film. If well executed, it is able to keep total iron in the feedwater at around 1 ppb.
Thus, the decision to migrate to OT is a commitment to maintaining a system that guarantees condensate purity with consistently high performance, and that continuously monitors both dissolved oxygen and cation conductivity.
There are important observations about the risks of OT. First, any trace of copper in the cycle completely rules out the program, the same restriction already seen in AVT(O), only here with no margin of tolerance.
Second, there are documented cases of large units that suffered failures due to exfoliation of the duplex oxide layer in superheater and reheater tubes after conversion to OT. This problem is associated with the behavior of the oxide layer at higher temperatures, and the exact mechanism is still a matter of technical controversy.
These points do not invalidate the program, but they reinforce that converting a unit to OT requires specific metallurgical follow-up in the high-temperature sections, not just water chemistry control.
With this, we close the three main regimes of volatile conditioning, AVT(R), AVT(O) and OT, each one solving a different arrangement of metallurgy, pressure and boiler configuration. In the next article we will discuss the importance of condensate.
Does your plant operate under a supercritical once-through regime? Have you assessed whether the current condensate purity would support a migration to OT? Talk to us for a technical assessment.
References
- Power Engineering. Combined Cycle HRSGs: Ditch the oxygen scavenger. Available at: power-eng.com.
- International Association for the Properties of Water and Steam (IAPWS). Technical Guidance Document: Volatile treatments for the steam-water circuits of fossil and combined cycle/HRSG power plants. 2015 revision (original from 2010). Stockholm, July 2015. TGD3-10(2015).
- ChemTreat. The Challenges of Industrial Boiler Water Treatment.
- Power Engineering. Maintaining High Combined Cycle HRSG Efficiency and Reliability.
- ScienceDirect. Effect of oxygenated treatment on corrosion of the whole steam–water system in supercritical power plant.
- ScienceDirect Topics. Oxygenated Treatment - an overview.