Oxygenated treatment
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 30-50 ppb. OT programs are most commonly used in supercritical (ie >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." 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. Also, OT reduces the risk of flow-accelerated corrosion.
When OT is used, conductivity after cation exchange (CACE) at the economiser inlet must be maintained below 0.15μS/cm this can be achieved by the use of a full-flow condensate polisher.
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 | 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) | 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 | 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. |