Pulp and paper boilers are being run like power plants: makeup water at sub‑microsiemens, oxygen in the low‑ppb, and phosphate programs tuned to the decimal. The payoff is fewer tube failures, higher cycles, and measurable water and energy savings.
High‑pressure pulp/paper boilers live or die by their water chemistry. The modern spec starts with high‑purity demineralized makeup and ends with a tightly coordinated drum program—oxygen scavenger, phosphate/caustic pH control, and a polymer sludge conditioner—executed with power‑plant discipline. Industry guidance and mill data point to RO (reverse osmosis) followed by polishing to “near zero” conductivity before the feedwater ever touches a tube (Filtox), with target silica under 0.1–0.2 mg/L, hardness below detection, and chloride ± sulfate on the order of 0.01 mg/L, at conductivity ≈0.1 µS/cm or less (µS/cm = microsiemens per centimeter; mg/L ≈ ppm) (Filtox) (BQUA).
Those numbers matter. Even a few ppm of silica can polymerize into stubborn deposits, and every stray ion raises drum solids, lowers allowable cycles, and inflates blowdown heat loss. In an industry that averages roughly 54 m³ of raw water per tonne of product (Water Tech Online), “conductivity trending offers insight into blowdown requirements,” and tighter control has delivered “measurable reductions in specific water consumption” (Filtox).
High‑purity makeup water train
Raw water—often surface or groundwater—gets cleaned up via coagulation/flocculation, multimedia filtration, softening, and high‑efficiency purification before it qualifies as boiler makeup. Many mills standardize on integrated membrane systems—RO, NF (nanofiltration), and UF (ultrafiltration)—for the front end, with UF frequently used as RO pretreatment via ultrafiltration from surface waters and wells.
For the main salt removal step, mills typically deploy brackish‑water RO trains. Modern RO in paper mills routinely produces permeate below 10 µS/cm prior to polishing (Filtox), with RO recovery around 75–80% delivering roughly <50 µS/cm permeate before final treatment. Downstream, mixed‑bed polishing commonly finishes the job to <0.1 µS/cm; many mills choose mixed‑bed ion exchange or electrodeionization (EDI) for continuous ultra‑pure water production without chemical regeneration.
Ion exchange remains central: strong/weak cation/anion beds in a demineralizer configuration strip the last traces, and a softener upstream can protect the RO by removing residual hardness that drives scale formation. In practice, RO/IX systems routinely produce permeate below 0.01 mg/L TDS (total dissolved solids) (BQUA).
Makeup quality should meet or exceed high‑pressure guidelines (e.g., ABMA/APAVE), which for >60–70 bar boilers call for feed silica under about 0.1–1 ppm and negligible sodium or chlorides (Filtox) (BQUA). Two‑pass RO or RO + EDI are common routes to those specs. By contrast, boilers at 20–30 bar may tolerate tens of microSiemens and roughly 5–20 ppm silica—orders of magnitude looser than the regime described here.
Cleaner makeup enables higher cycles of concentration. Targeting >10–15× concentration (i.e., about 7–10% blowdown) reduces fresh water and heat loss. For a 10‑cycle boiler, approximately 10% of feed leaves as blowdown (Water Technology Report), and moving from 5× to 15× concentration cuts blowdown by about 67% (from 20% to ~6.7% of feed).
Oxygen control: deaeration and scavenging
Dissolved oxygen (DO) is corrosive at ppb levels. A mechanical deaerator strips roughly 95–99% of inlet O₂, leaving a residual around 10–30 ppb depending on pressure; the remaining trace must be chemically removed before the boiler. Common oxygen scavengers include sodium sulfite (Na₂SO₃) or bisulfite, hydrazine (N₂H₄), and organic scavengers such as hydroquinone and ascorbate (Veolia Water Handbook).
Dosing is stoichiometric plus a protective residual. The theoretical requirement is 7.9 mg sodium sulfite per 1.0 mg O₂, and a widely used practical rule is about 10 ppm sulfite per 1 ppm DO; for example, with 0.7 ppm O₂ in feed, roughly ~7 ppm sulfite is needed to neutralize it (Water Technology Report) (Water Technology Report). Typical boiler‑feed sulfite residuals are held near 20–40 ppm (Water Technology Report).
Hydrazine is more potent by weight: the stoichiometry is approximately 1 mg/L hydrazine per 1 mg/L O₂, with a small “passivation” residual of ~1.0–3.0 ppm in the boiler to keep DO ~0 (Water Technology Report) (Water Technology Report). Organic scavengers (carbohydrazide, etc.) are often effective at 0.2–2 ppm. Catalyzed sulfites can significantly speed reaction—heavy‑metal catalysts are among the more effective, with reaction rates roughly doubling in practice (Veolia Water Handbook).
Injection is continuous, typically into the deaerator storage or drop‑leg to ensure residence time. Hydrazine or amines form volatile films that protect copper alloys (gauges, turbines), which sulfite cannot. Hydrazine usage is restricted in some regions; organic alternatives are available. In all cases, plants dose with accurate dosing pumps and select from oxygen scavenger formulations to hold feed DO below roughly 0.02 ppm (20 ppb), with corrosion confirmed via low dissolved iron (e.g., <0.1 mg/L) (BQUA).
As one industry summary puts it: “sodium sulfite, sodium bisulfite, hydrazine, catalyzed variants, and organics (hydroquinone, ascorbate)” are the main classes (Veolia Water Handbook).
Phosphate/caustic alkalinity control
High‑pressure units typically run a coordinated phosphate program to control pH without free caustic. For roughly 1000–1500 psig (~70–105 bar), boiler phosphate residuals are commonly maintained at 5–15 ppm as PO₄, using a mix of trisodium phosphate (Na₃PO₄·12H₂O), disodium phosphate (Na₂HPO₄), and caustic (NaOH) to keep the sodium:phosphate molar ratio near 2.85–3.0:1.0 (Water Technology Report). The core equilibrium—PO₄³⁻ + H₂O ⇌ HPO₄²⁻ + OH⁻—lets phosphate “borrow” OH to buffer the drum, and coordinated programs “limit the amount of free hydroxide alkalinity” by design (Water Technology Report).
In practice, dose roughly 10–30 ppm PO₄ into the feed (accounting for carrydown) to hold 5–15 ppm in the drum, and trim with NaOH and Na₂HPO₄/Na₃PO₄ to maintain boiler water pH around 10.0–10.5 at 25 °C (about 11–11.5 at temperature), while keeping free OH near zero. Plants often integrate alkalinity control chemicals within a broader boiler treatment package. Field guidance places this coordinated approach as effective up to about 1500 psig; above ~10 MPa (1500 psig) many operators adopt congruent phosphate or all‑volatile treatment, but for pulp‑mill boilers at ~70–100 bar, the coordinated program is the standard (Water Technology Report).
Numbers to watch: total alkalinity around 50–200 ppm (CaCO₃), mostly as “P‑alk”; free OH below ~1 ppm; drum pH approximately 10.5–12 for 75–100 bar operation; phosphate about 5–20 ppm; and TDS near 100 mg/L (BQUA). Maintaining the Na:PO₄ ratio near 3:1 on the phosphate/caustic equilibrium curve is the practical way to stay on the safe side of caustic attack (Water Technology Report).
Sludge conditioning and dispersion
Trace iron, silica, or hardness still find their way into the boiler. To keep precipitates non‑adherent and mobile, mills feed polymeric dispersants such as polyacrylic acid or sulfonated styrene/maleic copolymers. Typical dosages are 5–20 ppm as active polymer, with some high‑performance variants effective at 1–10 ppm (Water Technology Report). These polymers remain effective up to roughly 1500 psig, dispersing iron oxides and calcium‑silicate fines so blowdown can remove them (Water Technology Report).
The operational goal is low boiler turbidity (under ~5 NTU), modest in‑drum polymer residuals (often ~5–10 ppm), and steady removal via continuous blowdown. Some mills address silica risk with a silica “conditioner”—for example, a soluble aluminate feed to convert silica to aluminosilicate gel—dosed near the economizer. Practical programs source dispersant chemicals that are non‑volatile so they stay in the water phase and avoid carryover.
Instrumentation, targets, and outcomes
Daily discipline is non‑negotiable. Typical targets for a ~100+ bar boiler are: feed DO <0.02 ppm; feed conductivity ≈0.1–1.0 µS/cm; silica <0.1 mg/L; drum pH about 10.5–11.5 (25 °C basis); phosphate residual ~5–15 ppm; free OH near zero; and boiler TDS on the order of 50–100 mg/L (BQUA). Chloride and sulfate should be under ~50 ppb in the steam (effectively zero in the drum) to avoid stress‑corrosion, and condensate conductivity should be “virtually zero.”
Plants log feedwater and condensate chemistry continuously or via high‑frequency grab sampling. Monitored parameters typically include pH, conductivity, O₂ (or redox), phosphate (PO₄), sodium (as a proxy for alkali), and silica. “Conductivity trending offers insight into blowdown requirements” (Filtox). In‑boiler probes (level, pressure, hydrostatic resistivity) flag carryover. Corrosion coupons or electrochemical probes at the economizer should confirm low attack—target corrosion rates on the order of 0.01–0.02 mm/yr or less. Routine lab panels (alkalinity profile, phosphate, silica, chloride, hydroxide) verify that the program is on spec (Filtox).
When the chemistry clicks, operations do, too. Plants report fewer tube failures and outages, a steadier drum, and even downstream benefits like “fewer sheet breaks” on paper machines—an indirect readout of cleaner steam (Filtox). Environmental compliance adds another reason: in Indonesia, boiler blowdown and effluent must meet permit limits (PermenLH 8/2009 for thermal power plants), so achieving high cycles and/or recycling blowdown is advantageous. Any RO reject should also meet local discharge standards for pH, TDS, phosphate, and metals.
Designing and running this program—high‑purity makeup, zero‑oxygen feed, coordinated phosphate, polymer dispersant—comes down to equipment and chemistry working in concert, from dual media filtration and activated carbon polishing upstream of the RO to amine selection in condensate returns. Volatile amines that protect copper alloys align with the role of a neutralizing amine in controlling pH and corrosion in return lines, while all non‑volatile treatment chemicals are kept low in the drum to protect steam purity (Veolia Water Handbook).
