Silica in raw water can sit anywhere between 1–100 mg/L SiO₂, yet the steam leaving a palm oil mill boiler is expected to carry no more than 0.02 mg/L—20 parts per billion. Miss those limits and glassy deposits can sap heat-transfer efficiency by about 28%.
Industry: Palm_Oil | Process: Boiler_&_Power_Generation
Silica is ubiquitous in natural waters—typical surface and well sources run at 1–100 mg/L SiO₂ (freedrinkingwater.com). Inside a boiler, as feedwater evaporates, silica concentrates and even volatilizes with steam, forming tenacious glassy deposits across steam drums and turbines that insulate heat surfaces and degrade efficiency (ResearchGate; saka.co.id).
Clark (2014) put a number on the pain: silica scale can cut heat-transfer efficiency by ~28% (saka.co.id). That’s why boiler codes, including ASME and Indonesia’s SNI, set tight limits: steam silica must be ≤0.02 mg/L (20 ppb) (saka.co.id; SNI 7268:2009). In practice, that often means boiler‑water silica (normalized to drum pressure) must sit at ≲0.3–0.4 mg/L (ResearchGate; SNI 7268:2009).
Standards do flex with pressure. Indonesian SNI 7268:2009 allows feedwater silica up to 50 mg/L at low pressures, declining to 5 mg/L at very high pressures, but still mandates boiler (steam drum) silica ≤0.02 mg/L (SNI 7268:2009; SNI 7268:2009). In a study of 39 palm boilers, several mills fed by rivers or wells exceeded ASME silica limits, raising deposit risk (ResearchGate).
Demineralization system design and monitoring
Meeting these limits starts outside the boiler. In practice, most plants rely on softened or demineralized feedwater (Veolia Water Handbook). For many mills, that means pairing a softener with a demineralizer or deploying reverse osmosis within broader membrane systems.
Silica removal hinges on strong‑base anion exchange because dissolved silica is a weak ionic species; resins must be strongly charged to capture it (Lenntech). It’s also the first impurity to break through exhausted resin—and a spent demin bed can release slug concentrations ~4× higher than the raw water (Lenntech). That’s why high‑pressure plants add online silica analyzers to catch spikes that conductivity alone can miss (Lenntech; saka.co.id).
Well‑maintained DM trains can push feed silica to negligible levels: one high‑pressure design targeted <0.02 mg/L in the demineralizer effluent (ResearchGate). When a resin‑regeneration error nudged silica above 0.02 mg/L, installing an ultrafiltration polish unit brought silica back down, conductivity and fuel use fell, and steam generation increased (ResearchGate). In this role, ultrafiltration is a practical polish stage alongside ion exchange resins.
Technology choices matter. A fertilizer plant that switched to a high‑capacity anion resin (Purolite PFA870) extended its silica‑depletion time by ~290% (Purolite). On targets, one Indonesian boiler case held boiler‑feedwater silica at ≲3 mg/L using softeners/DM and saw very low boiler demands (iptek.its.ac.id). A softener often anchors the front of this train.
Verification is non‑negotiable: periodic spectrophotometry or online analyzers to track silica in both feed and boiler cycles (saka.co.id; Lenntech). When demineralization falters—or a high‑silica source is tapped—scale can rise quickly. The immediate moves are rapid blowdown and source fixes (Nautilus Shipping; ResearchGate).
Phosphate precipitation for hardness control
Residual hardness (calcium and magnesium) always sneaks in via makeup or condensate. Inside the boiler, phosphate‑based internal programs—often with polymer or phosphonate aids—convert calcium into calcium phosphate (hydroxyapatite) precipitated in the bulk water rather than as adherent CaCO₃ scale (Veolia Water Handbook). At high pH (~11–12), these particles stay as suspendable sludge that blowdown can remove (Veolia Water Handbook). Dosing consistency matters; many mills feed these chemistries with a dosing pump and rely on scale-control formulations aligned to boiler operating pressure.
Magnesium follows different pathways: with silica present, Mg tends to form magnesium silicate—a benign “soft” sludge—or Mg(OH)₂ if silica is absent (Veolia Water Handbook). If pH runs too low, magnesium phosphate can form, which is hard and adherent; high alkalinity is therefore essential in a phosphate program to avoid Mg‑phosphate build‑up (Veolia Water Handbook). Where pH setpoint management is the lever, alkalinity-control chemistries support the window indicated below.
Operating targets and field results
One sugar‑mill “Yoshimin” boiler at 21 bar ran with feedwater hardness ≲3 ppm as CaCO₃ and silica ≲3 ppm, holding boiler water at pH ~10.5–11 with residual phosphate ~4–10 ppm and alkalinity ~350–800 ppm (iptek.its.ac.id). The program, plus regular blowdown, “had a good impact on the condition of the boiler drum pipe” (iptek.its.ac.id).
Empirical and modeling work back up the chemistry: controlling feedwater hardness and silica reduces boiler dissolved solids, keeping cycles of concentration manageable when these species are held to sub‑ppm levels (ResearchGate). Hydroxyapatite is also far less insulating than CaCO₃ for moderately hard feeds (Bond Water).
Dose ranges scale with cycles. In boilers limited to ~15 cycles, a phosphate feed liquor around 200 ppm might suffice; in very high‑cycle smaller boilers (30–40×), 500–800 ppm may be needed (Bond Water; Bond Water). Online monitoring then verifies a few ppm residual phosphate and tracks hardness. In the field example above, the boiler ran trouble‑free through 140 days of testing with zero tube failures under these chemistry specs (iptek.its.ac.id).
Troubleshooting boiler water chemistry
Operators benefit from plotting pH, P‑alkalinity (phenolphthalein alkalinity), conductivity, hardness, silica, and iron over time—using control charts as in Rusdi et al. (ResearchGate). Typical issues and moves:
Rising silica in steam or drum: check demineralizer performance. Measure silica in feed/DM effluent; a spike (e.g., >0.02–0.05 mg/L) often signals resin exhaustion or bypass (ResearchGate; Lenntech). If DM is off‑spec, regenerate or replace resins—or add a polishing stage such as a mixed‑bed. Increase blowdown until boiler silica returns to safe levels (Nautilus Shipping). Probe the raw water source for seasonal shifts. Since silica carryover drops as boiler pH rises (ResearchGate), aim for the upper end (≈10.5–11) before other remedies.
Unexpected high hardness/alkalinity: boiler hardness creeping up often traces to softener exhaust or leakage. Verify regeneration cycles and bypass valves on the softener. In the drum, check residual phosphate—if too low, less Ca is precipitated. Adjust phosphate and caustic feed accordingly. Any new hardness in makeup must be caught by phosphate. Blowdown removes sludge; if deposits form, an offline acid clean (or EDTA) may be required to restore heat transfer.
Foam or carryover: foaming can entrain silica and carbonates into steam. Control organics/oils in condensate and filters. A small dose of neutralizing or filming amines can help, aligned to a neutralizing amine program, as can a targeted anti‑foam. Lowering the TDS setpoint (more frequent surface blowdown) also helps, since highly alkaline solutions foam more readily.
High conductivity/TDS: a surge usually means blowdown rate is too low for current cycles. Adjust the blowdown controller. Because silica and hardness add to TDS, keeping those low via strong DM or effective phosphate action also curbs conductivity rise (ResearchGate).
Low/high pH or alkalinity: boiler water should sit near pH 10–11. If pH drifts low, verify sufficient caustic soda or trisodium phosphate; a low‑alkalinity phosphate program risks Ca‑phosphate deposition on surfaces (Veolia Water Handbook). If pH runs too high (caustic embrittlement risk), reduce alkali or use carbonate/tribasic phosphate instead.
Sampling and speciation underpin all of the above. Spectrophotometric or online silica kits (saka.co.id), cartridge hardness titrations, alkalinity/pH meters, and iron tests should be daily or online. Comparing feed vs. drum analytics enables cycle calculations and mass balances for Ca, Mg, Cl⁻ (if relevant), and silica.
The payoff is hard to ignore. One study concluded that while “some palm mills maintain boiler quality, controlling silica, hardness and iron needs improvement,” the combined approach—tight feed specs and an aggressive phosphate regime—eliminated unplanned downtime (ResearchGate). With these improvements, mills report no hard scale on tubes, stable thermal efficiency, and compliance with even the strictest limits—<20 ppb silica in steam (saka.co.id; SNI 7268:2009).
Key data points
Targets referenced across studies and standards: feed hardness <3–5 ppm as CaCO₃, feed silica <3–5 ppm, boiler phosphates ~5–15 ppm, boiler pH ~10.5–11, boiler hardness <15–20 ppm, boiler silica <20–50 ppm (pressure‑dependent) (iptek.its.ac.id; SNI 7268:2009). Indonesian standards specify boiler (steam drum) silica ≤0.02 mg/L (SNI 7268:2009). Demineralizer effluent silica is ideally <0.02 mg/L (ResearchGate); if it creeps higher, expect scale. Unlimited blowdown throughput or more pretreatment costs money, but those outlays are cheaper than unplanned outages and tube replacement.
Sources
Authoritative boiler‑water sources and studies underpin these figures and practices: Veolia’s Water Handbook – Boiler Deposits (Veolia Water Handbook; Veolia Water Handbook), Indonesian boiler RT guidelines SNI 7268:2009 (SNI 7268:2009; SNI 7268:2009), research on palm‑mill boilers (ResearchGate; iptek.its.ac.id; ResearchGate), and industry/technical literature on silica monitoring and ion exchange (saka.co.id; Lenntech).
