Inside the chemistry race keeping CCGT cooling towers clean — and saving millions of gallons

Modern combined‑cycle gas turbine (CCGT) plants reject very high heat — often tens to hundreds of MWth (thermal megawatts) in the steam condenser — and circulate enormous water volumes. Typical withdrawals run on the order of 7,500–20,000 gallons per MWh of electricity generated (www.ucs.org). As water evaporates in the cooling tower, dissolved minerals concentrate (higher total dissolved solids, TDS), so simple makeup and blowdown cannot prevent scale.

It gets worse as temperature climbs: even saturated calcium carbonate (CaCO₃) readily precipitates as condenser‑water temperature rises (www.power‑eng.com). And the tiniest fouling matters — ~0.25 mm of biofilm can cut heat transfer by ~50% (irispublishers.com). Meanwhile, warm, oxygenated water attacks metals: mild steel corrosion can exceed 0.2 mm/yr if untreated, and copper‑nickel condenser alloys are also vulnerable.

Cooling towers are fertile grounds for biological growth — algae, bacteria, and Legionella — at 20–50 °C, mandating rigorous microbiological control. The upshot: very high heat load, high cycles of concentration (COC, the ratio of circulating water concentration to makeup water concentration), and large flow rates demand an aggressive, coordinated chemical regimen fine‑tuned by precise feed systems such as dosing pumps.

When plants get this right, the payoffs are visible on the water meter. One case showed that simply improving treatment (and polishing makeup) allowed operation at ~9.8× cycles instead of 2×, saving ~1.94×10^6 gallons of annual makeup (www.prochemtech.com); another raised cycles to 9.85× and saved ~5.0×10^6 gallons per year (~US$70.6k) (www.prochemtech.com).

Scale control chemistry and pH management

The primary scales in tower–condenser loops are calcium carbonate, calcium sulfate, calcium phosphate, silica, and metal hydroxides. Industry practice pairs chemical inhibitors with controlled blowdown. A straightforward tactic is acidity control — for example, H₂SO₄ dosing to convert bicarbonate alkalinity to CO₂ and H₂O (www.power‑eng.com) — delivered via accurate feed like a dosing pump.

Modern programs lean on phosphonate–polymer blends that sequester Ca²⁺ and “threshold‑inhibit” CaCO₃. Typical additions include phosphonates (e.g., HEDP, ATMP) at ~5–20 mg/L plus polyacrylate/related polymers at 1–5 mg/L to allow higher cycles (often 2–4× on hard water) (www.prochemtech.com). This is the sweet spot for scale inhibitor formulations, while dispersive polymers (e.g., polyacrylates/silicates) act as dispersants to stabilize precipitates.

Makeup water quality is leverage. In very hard waters, some plants pre‑soften makeup to <20 mg/L Ca²⁺, permitting ~10× cycles (see metrics below). Pretreatment options range from conventional softeners to hardness‑targeted pressure membranes such as nano‑filtration (qualitatively, NF removes multivalent hardness at lower pressure than RO), and in some cases polishing with brackish‑water RO if TDS is the constraint. Where RO is deployed, upstream protection with ultrafiltration helps when makeup is drawn from surface waters.

Historically, phosphate feeding (ortho‑ or polyphosphate) was used to precipitate calcium as calcium phosphate (www.power‑eng.com). But phosphates are also nutrients for algae and are now often restricted by environmental regulations (www.power‑eng.com). In one U.S. plant, eliminating phosphate ended algal blooms, while switching to a film‑forming organic starch inhibitor (FlexPro® RPSI) cut carbon‑steel corrosion from ~0.2–0.25 mm/yr to ~0.0025–0.0075 mm/yr (www.power‑eng.com). That trend — phosphate‑free programs using biodegradable polymers or polycarboxylates (e.g., polyaspartate, polysilicate blends) — is increasingly employed to control scale with negligible nutrient loading (www.power‑eng.com) (www.researchgate.net).

Case in point: a plant raised cooling cycles from 2× to ~10× (via softened makeup and advanced inhibitors) so that bulk Ca hardness fell from ~186 to ~19 mg/L as CaCO₃ (www.prochemtech.com) (www.prochemtech.com). Total alkalinity climbed to ~1,660 mg/L (because most calcium was removed, as expected at high cycling). No visible CaCO₃ deposits occurred even on “blowdown‑limited” heat exchangers. In practice, water analysis sets targets — for example, keeping hardness <50 mg/L (as CaCO₃) and silicon <10–30 mg/L means carbonate or silica scale stays well below critical levels.

Corrosion inhibitor packages by metallurgy

Cooling loops mix carbon steel piping, copper‑nickel condenser tubes, stainless steels, and sometimes galvanized (zinc‑coated) components. A multi‑component inhibitor package is typical, delivered as part of a broader cooling tower chemical program.

For carbon steel, operating bulk water at mildly alkaline pH (typically 8.0–9.0) helps minimize corrosion. Orthophosphate or silicate (30–60 mg/L as Na₂SiO₃) forms protective films, and traditional polyphosphate “passivators” (10–30 mg/L PO₄³⁻) deposit an amorphous Fe/PO₄ layer on steel (watertechnologyreport.wordpress.com). Newer branched polymers (e.g., polyacrylates or polysilicates) can form thin barriers that yield very low rates (~0.1 mm/yr or less) (www.power‑eng.com), the domain of modern corrosion inhibitors. In practice, a well‑treated tower can see mild‑steel corrosion 0.25–0.5 mils/yr (0.006–0.012 mm/yr) at 10× cycles (www.prochemtech.com).

Copper–nickel tubes require azoles — tolyltriazole or benzotriazole at ~1–5 mg/L — which adsorb onto copper surfaces to block attack. A cited program includes tolyltriazole for copper corrosion control (www.prochemtech.com).

Galvanized steel brings a different risk: “white rust” on zinc surfaces can occur above pH ~8.2 (www.prochemtech.com). Modern formulations add a specific “white rust inhibitor” (often borate‑ or silicate‑based); one program maintained 4–12 mg/L of such inhibitor and fully eliminated white rust at pH ~9+ (www.prochemtech.com). Stainless alloys are generally self‑passivating, but maintaining low chloride (<200 mg/L) avoids pitting.

Performance data from a COC‑10 program showed corrosion coupon rates of mild steel <0.2 mil/yr (0.005 mm/yr) and copper <0.05 mil/yr (www.prochemtech.com), judged “excellent” by AWT guidance. Another example yielded copper corrosion ~0.01 mil/yr in a glass‑plant system (www.prochemtech.com).

Biocide schedules and microbial targets

Warm, oxygenated tower water is ideal for [microbial growth, including Legionella](https://beta.co.id/en/blog/inside-the-highstakes-war-on-legionella-in-powerplant-cooling-towers). Programs combine oxidizing biocides (chlorine, bromine, chlorine dioxide) with periodic non‑oxidizing agents, implemented through controlled feed equipment and, where appropriate, on‑site oxidant generation such as electrochlorination. A curated portfolio of biocides underpins the regimen.

Oxidizing biocides typically run a combined continuous/slug strategy: a low‑level continuous dose (e.g., ~0.2–0.5 mg/L Cl₂ or Br₂) to maintain background control, plus periodic “shock” treatments (e.g., 2–5 mg/L for 10–60 minutes) to disrupt established biofilms (irispublishers.com). Chlorine dioxide (ClO₂) remains effective at high pH, yields no THMs, and does not interact with ammonia or inhibitors; it is typically applied at ~0.5–1.0 mg/L as a continuous dosing or short feed, with intermittent boosts to 1–3 mg/L (irispublishers.com).

Non‑oxidizing biocides tackle chlorine‑resistant strains and entrenched slimes: aldehydes (e.g., glutaraldehyde) or organic bromides (DBNPA, THPS) are dosed 5–25 mg/L for short contacts (1–4 h) once per week or month. Glutaraldehyde (50% active) is often fed at 15–25 ppm (as active) for 2–4 h weekly. Algaecide combinations (e.g., magnesium or zinc bromides/nitrates) and periodic polyquat shocks can help; many programs alternate halogen oxidizers and non‑oxidizers to prevent resistance.

One field example: at the Kamojang power plant (Indonesia), injecting 2 ppm NaOCl into the cooling water dropped bacterial counts from ~19,000 to ~100 CFU/mL and raised cooling efficiency ~17.4% (www.researchgate.net). Targets after treatment typically include <10³ CFU/mL total heterotrophic bacteria (CFU, colony‑forming units; heterotrophic plate count) and undetectable Legionella (while no universal limit exists, many guidelines use <100 CFU/100 mL for potable systems). Regular ATP or HPC testing, plus on‑line ORP or UV254 monitors, confirm biocide effectiveness.

Cycles, setpoints, and verification data

COC is controlled primarily by blowdown to hold conductivity/TDS at a setpoint that balances water conservation against fouling risk. Plants commonly target ~5–10× cycles on moderate TDS; extreme cases (with softened makeup) can reach 10–15×. One program held COC ~9.8 at pH 9.4 (www.prochemtech.com), significantly reducing blowdown. Pretreatment that supports higher cycles includes selective polishing and makeup conditioning using systems like softeners or RO, with supporting equipment (valving, control, tanks) drawn from water treatment ancillaries.

Indicative water quality limits under treatment include Ca²⁺ <20–50 mg/L, silica <30–50 mg/L (as reactive silica), alkalinity <1,500 mg/L, and pH 8.5–9.5. Regular lab tests confirm scale indices near zero (e.g., “Pendans” or Langelier index ≤–2). Corrosion verification hinges on coupons or probes, with targets of <0.1 mm/yr (4 mils/yr) for mild steel and <0.01–0.02 mm/yr (0.4–0.8 mils/yr) for copper alloys; the successful cases above achieved <0.2 mil/yr for steel and <0.05 mil/yr for copper (www.prochemtech.com).

Scale and fouling are tracked by periodic tube inspections and heat‑transfer “approach” measurements; the goal is negligible scale on condenser tubes and drift eliminators. With good chemistry, [condensers stay clean enough](https://beta.co.id/en/blog/inside-the-chemistry-race-keeping-power-plant-condensers-clean-and-megawatts-on-line) to maintain design pressure‑drop and heat transfer. Over time, plants quantify gains in fewer unplanned cleanings, steadier temperature rise, and reduced blowdown. In cited examples, aggressive programs delivered a 54–80% reduction in makeup/blowdown and a 17–18% increase in tower heat rejection efficiency (www.prochemtech.com) (www.researchgate.net).

Compliance, discharge, and environmental choices

Modern programs aim to minimize environmentally harmful discharges. Phosphorus‑free chemistries protect downstream ecosystems (eutrophication concerns) (www.power‑eng.com). Biocide residuals are neutralized (e.g., bisulfite addition to blowdown to quench chlorine) to meet limits on halogen or biocide in effluent, and corrosion inhibitors are selected without chromium or heavy metals under stringent standards. In Indonesia, for example, power plants must comply with wastewater standards (e.g., Permen‑LH) that effectively constrain heavy metals and nutrients from cooling tower blowdown.

What a coordinated program looks like

An effective CCGT cooling‑water program melds precise chemical dosing with active monitoring. A robust inhibitor blend — typically phosphonates, polymers, silicates, and azoles — is fed continuously to deter scale and corrosion, supported by targeted products such as scale inhibitors and corrosion inhibitors. A mixed biocide schedule (oxidizing + non‑oxidizing) suppresses bio‑growth, guided by controlled feeds and instruments. Where needed, makeup conditioning with nano‑filtration or softening raises feasible cycles, and cleanliness is maintained with routine services alongside a cooling tower chemical program.

The outcomes are quantifiable: higher COC (often doubling or tripling effective cycling), minimal condenser fouling (maintaining >90% design heat transfer), and corrosion rates on the order of 0.01–0.02 mm/yr on steel/copper (www.power‑eng.com) (www.prochemtech.com). Those data guide blowdown setpoints and chemical feeds — the operational levers that ultimately protect plant efficiency and lifespan.

Sources: Authoritative industry literature and case studies were used to obtain data and guidelines (www.power‑eng.com) (irispublishers.com) (irispublishers.com) (www.power‑eng.com) (www.prochemtech.com) (www.prochemtech.com) (www.researchgate.net).