In combined‑cycle heat recovery steam generators (HRSGs), a few parts per billion of dissolved oxygen or a pH slip can spike corrosion. The fix is unapologetically chemical: remove the last traces of O₂ and keep the cycle mildly alkaline.
High‑pressure feedwater for combined‑cycle HRSGs (heat recovery steam generators) must be very pure to minimize corrosion. Uncontrolled dissolved oxygen (DO) or acidic condensate (from CO₂, air ingress, etc.) can trigger flow‑accelerated corrosion (FAC, thinning of carbon‑steel surfaces by rapid dissolution) in carbon‑steel piping and economizers (Power Engineering) (Chemical Engineering). As a precaution, feedwater is typically deaerated mechanically (thermal degasification) and then chemically treated. Chemical oxygen scavengers are dosed to consume the last traces of DO (often achieving residuals <5–10 ppb, per Chemical Engineering), and alkaline agents (ammonia/neutralizing amines) are added to maintain a mildly basic pH (~9–10) that protects steel surfaces (Power Engineering) (POWER).
In practice, steam‑cycle chemistry guidelines for HRSGs call for final feedwater DO in the single‑digit ppb range and pH (at 25 °C) typically in the 9.6–10.0 range (Power Engineering) (Chemical Engineering). Maintaining these targets has been shown to reduce iron corrosion product carryover to only a few ppb (Corrosion journal (Allen Press)) (Chemical Engineering), whereas out‑of‑spec water can raise Fe levels an order of magnitude higher.
DO scavengers and steel passivation
After deaeration, chemical “oxygen scavengers” remove the remaining DO. Hydrazine (N₂H₄) has long been a standard scavenger because it reacts stoichiometrically with O₂: N₂H₄ + O₂ → 2H₂O + N₂ (the reaction consumes 1 mg/L O₂ per 1 mg/L hydrazine, per Scribd) and adds no solids to the water. It also passivates steel by reforming a protective Fe₃O₄ film (Chemical Engineering).
However, hydrazine is highly toxic and suspected carcinogen, and it decomposes into ammonia at boiler temperatures. For these reasons many plants replace or limit hydrazine. Carbohydrazide (N₂H₄CO) is a safer alternative that likewise scavenges oxygen: a boiler study found that ~0.7 mg/L carbohydrazide was effective at eliminating DO and behaved similarly to hydrazine (ResearchGate). In high‑pressure feedwater at ≥250 °C, carbohydrazide decomposes to hydrazine + CO₂ (thus slowly releasing NH₄⁺), and controlled dosing can maintain a low residual hydrazine (on the order of 30–40 ppb) (ResearchGate). Experiments show that carbohydrazide and its byproducts do not accelerate carbon‑steel corrosion (Corrosion journal (Allen Press)).
Another common scavenger is DEHA (diethylhydroxylamine). DEHA is volatile and distributes through the steam/condensate loop. In practice DEHA must be fed at higher temperatures or concentrations than hydrazine to be effective: its O₂‑scavenging efficiency is significantly lower than hydrazine’s (Corrosion journal (Allen Press)). In hot water DEHA decomposes into diethylamine, ethylamine, acetaldehyde and acetate, which raises conductivity (Corrosion journal (Allen Press)). In tests DEHA required much higher loads or heat input to reduce DO, and it modestly raised ORP (oxidation‑reduction potential) unlike hydrazine (Corrosion journal (Allen Press)). Nevertheless, DEHA plus its breakdown products have been shown not to increase carbon‑steel corrosion rates (Corrosion journal (Allen Press)). The choice of scavenger thus involves trade‑offs: hydrazine is efficient but toxic, inactive to blowing down, whereas DEHA (and other organics like erythorbate or hydroquinone) are safer but contribute some volatile conductivity and carbonic‑ or organic‑acid load.
Targets and tuning for DO removal
In all cases, dosing is adjusted to achieve near‑zero DO. Industry practice aims for residual DO well below 10 ppb (often <5 ppb) to prevent corrosion (Chemical Engineering). Properly applied initial Fe₃O₄ layers will become maintained by a FeOOH overlayer if DO is controlled; if DO rises, copper and steel can rapidly corrode (Chemical Engineering). One feedwater monitoring program recommends DO = 5–10 ppb and cation conductivity ≤0.2 µS/cm as target purity levels (Chemical Engineering).
Oxygen scavenger dosages are accordingly calculated: roughly 1 mg/L hydrazine neutralizes 1 mg/L O₂ (Scribd), or about 0.7 mg/L carbohydrazide per 1 mg/L O₂ (ResearchGate). In practice, scavenger feed rates are tuned using online DO or ORP monitors. When DO in‑leakage occurs, feed is increased; conversely, if conductivity spikes from organic byproducts, feed is reduced. Modern HRSG control may rely on cation conductivity after cation exchange (CACE) to discount ammonia/amines and reveal true contamination (Power Engineering) (Chemical Engineering).
Chemical feed in these programs is delivered as controlled chemical dosing integrated with DO and conductivity monitoring.
Ammonia and neutralizing amines for pH
To combat acidic corrosion from residual CO₂, oxygen and other contaminants, feedwater is kept mildly alkaline. The basic reaction is NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, which buffers carbonic acid (H₂CO₃) and keeps pH elevated (Power Engineering). Practically, feedwater pH at 25 °C is held around 9.0–9.6 (often cited as 9.6–10.0 for all‑ferrous systems) (Power Engineering) (POWER). Maintaining this range minimizes single‑phase FAC in carbon steel; experiments show that iron dissolution on carbon steel sharply rises below pH ~8–9, especially at high temperatures (Power Engineering). Thus virtually all HRSG programs add a neutralizing amine or ammonia to target pH ≈9.6–10.0.
In many modern combined‑cycle plants, ammonia + an organic amine blend is used. For example, one 830 MW plant (Siemens H‑class units) employed a mixture of ammonia and monoethanolamine (MEA) to target condensate pH ≈9.4–9.8 (POWER). In this case about 10 ppm of active NH₃/MEA yields pH 9.6 in pure water, implying only ~2.1 gal/day of makeup chemical at 99% condensate return – yet the plant was dosing ~22 gal/day (nearly 10× the theoretical need) (POWER). This discrepancy underscored losses (amine venting) and the importance of precise feed rate control. In general, to achieve pH ≈9.6 in high‑purity water, ammonia concentrations of the order 5–20 ppb (µmol/L) may be required, depending on temperature and CO₂ level (Power Engineering) (POWER). Conductivity measurements after cation exchange can confirm the actual ammonia/amine level (as ammonia forms NH₄⁺ and adds to CACE).
Neutralizing amines beside ammonia (NH₃) include morpholine, cyclohexylamine (CHA), ethanolamine (ETA), methoxypropylamine (MPA) (Water Technology). These organics have different volatility and basicity (“distribution ratios”). They generally yield higher liquid‑phase pH than ammonia alone at the same dose (Water Technology), but also decompose to some extent in hot water (forming CO₂, organics, NH₃) (Water Technology). Guidelines recommend restricting ammonia levels to ≤0.5 ppm (0.5 mg/L) in systems with any copper alloys to avoid copper corrosion or carryover (Scribd). HRSG circuits are usually copper‑free, but this advisory informs distribution. Moreover, any amines must be chosen for thermal stability: morpholine, CHA, ETA and MPA are common in high‑pressure plants due to their relatively low degradation (Water Technology). The overall goal is to ensure fluid pH stays mildly alkaline throughout the loop. For example, one study found an LP evaporator pH of ~9.1, while consensus guidelines targeted ≥9.6 (pH drop below ~8 triggers shutdown) (Scribd) (Water Technology).
When CO₂ ingress or oxygenation occurs, ammonia neutralizes the resulting acidity. In practical terms, controllers use either direct pH measurement (difficult in pure water) or conductivity‑based dosing. Because ammonia adds NH₄⁺, the specific conductivity rises, so CACE is preferred to monitor true impurities (Power Engineering) (Chemical Engineering). In all‑volatiles treatment (AVT, dosing only volatile alkalizing agents and scavengers), typical control logic ties ammonia feed to maintain a setpoint CACE corresponding to the desired pH. This ensures that, for example, 20–70 ppb NH₃ in the feedwater will keep pH around 8.0–8.5 if ammonia‑only (as in oxygenated treatment) (Scribd), or ~9.6 if fully all‑volatile reducing. The key outcome is protecting metal surfaces: corrosion tests confirm that water at pH ≈9.6 leaves minimal active corrosion, whereas lower pH leads to accelerated iron release (Power Engineering) (Chemical Engineering).
Neutralizing amines in these programs are delivered as part of a neutralizing amine feed strategy aligned to the plant’s AVT or OT regime.
Outcomes, oxygenated treatment, and field data
In summary, a robust feedwater chemical program yields measurable results: residual DO <5–10 ppb and high pH dramatically reduce iron transport. An oxygenated water treatment (OT) program reports dissolved‑iron concentrations dropping from ~10 ppb (with conventional scavengers) to ~1–2 ppb once a small O₂+ammonia overlay is established (Scribd). While inert conditions (near‑zero O₂) can sometimes encourage FAC, most HRSGs prefer negligible DO with all‑volatile treatment. The resulting iron levels (e.g. ≤2 ppb) and low corrosion rates are consistent with textbook predictions of steel stability at pH 9–10 (Chemical Engineering) (Corrosion journal (Allen Press)).
Quantitatively, implementing correct dosing achieves clear outcomes. A case study in an Indonesian boiler system found that adding ~0.084 kg/hr of ammonia to makeup water raised feed pH from ~8.0 to ~9.2–9.53, matching the plant’s design target (ResearchGate). This stoichiometric approach (balancing moles of NH₃ to CO₂) is standard practice. On the O₂ side, dosing rates are based on flow and measured DO; a rule of thumb is ~1 mg/L hydrazine per 1 mg/L O₂, or the equivalent in alternative scavengers (Scribd). In humidified CO₂‑laden feedwater, ammonia neutralizes ∼1 mg/L CO₂ per ~0.7 mg/L NH₃ (via formation of NH₄HCO₃), although exact ratios depend on temperature.
Overall, industry research and regulatory guidelines converge on these chemical goals. Major utilities expect feedwater pH ≈9.6–10.0 and ultra‑low DO (Power Engineering) (Chemical Engineering). In Indonesia, while no special local regulation mandates unique treatment chemicals, plants are required to meet boiler water quality equivalent to international standards (e.g., ABMA/ASME levels of conductivity and silica; “Silica: 10 ppb” is cited in Power Engineering) (Chemical Engineering). Local studies confirm that achieving the global norms (DO ≪10 ppb, pH ~9.5) is both technically feasible and essential: failure to do so measurably increases corrosion risk. Thus, data‑driven cycle chemistry programs — including periodic monitoring of DO, conductivity, and pH — form the backbone of reliable HRSG operation.
These pH programs are typically implemented with neutralizing amines and ammonia setpoints aligned to CACE, while the DO program uses oxygen scavengers sized to the measured in‑leakage and tuned by ORP.
Source links
Industry guidelines and research on HRSG chemistry control (Chemical Engineering) (Power Engineering) (POWER) (Corrosion journal (Allen Press)) (ResearchGate) (ResearchGate) support these practices.
