Power, Steel & Flue-Gas Desulfurization
High-chloride (10,000–30,000 mg/L), high-suspension (TSS 1–10%), trace-heavy-metal (Hg, Se, As) wastewater from coal-fired power, steel sintering and FGD bleed streams — engineered for ZLD compliance with pre-treatment that actually handles Hg <1 µg/L and Se <50 µg/L discharge limits.
What Makes FGD & Power-Plant Wastewater Hard
These streams sit at the intersection of chloride attack, suspended-solids loading, trace heavy-metal compliance and the zero-liquid-discharge mandate. The challenge is rarely the evaporator — it is the pre-treatment that decides whether the evaporator survives.
High Chloride Attack & Sulfite Chemistry
FGD purge water chlorides are set by the coal, and the scrubber loop adds unoxidized sulfite carryover — a reducing-acid combination that eats standard alloys.
- Cl⁻ 10,000–30,000 mg/L in FGD purge water — driven by coal chlorine content of 0.01–0.3 wt%
- 316L fails by pitting in months at these levels; its safe envelope is <200 mg/L Cl⁻ at evaporation temperature
- Unoxidized SO₃²⁻ carryover from the absorber loop complicates pre-treatment chemistry ahead of evaporation
High Suspended Solids
Gypsum carryover, fly ash and unreacted limestone turn the evaporator feed into an abrasive slurry unless solids are removed first.
- TSS of 1–10% (10,000–100,000 mg/L) — gypsum (CaSO₄·2H₂O), fly ash, unreacted limestone
- Clarification, hydrocycloning and filter press are the system’s survival mechanism — not optional extras
Mercury, Selenium & Coal-Trace Metals
Coal-trace Hg and Se report to the purge stream together with associated species (As, B) — and are regulated at the µg/L level, orders of magnitude below their inlet concentrations.
- Coal-trace Hg 0.05–10 mg/L and Se 0.1–5 mg/L report to FGD purge water
- US ELG: Hg <1.0 µg/L, Se <10–18 µg/L at discharge
- Hg speciation: particulate HgS 30–60%, ionic Hg²⁺ 10–40%, organomercury 5–20%
- Removal must sit upstream — the crystallizer does not remove Hg or Se
ZLD Mandate & the Salt Route
Thermal power and steel sintering plants in many regions operate under ZLD or near-ZLD wastewater mandates — the system must produce reuse-grade condensate and a managed salt / solids stream, with no liquid discharge to surface water.
- Mixed NaCl / CaSO₄ / heavy-metal-bearing salt is in the majority of cases stabilized and landfilled as solid waste
- Saleable-grade salt only when upstream separation yields a clean NaCl stream free of sulfate and heavy metals
- Do not assume salt sales offset operating cost without verified pilot data
Typical Streams & Application Map
Common feed streams in power, steel and FGD wastewater, with the technology route typically considered for first-pass screening.
| Chemical / Stream | Source & Context | Key Behavior | Typical Route | Material Focus |
|---|---|---|---|---|
| FGD purge water | Wet limestone FGD bleed, coal-fired power | Cl⁻ 10,000–30,000 mg/L · TSS 1–5% · Hg 0.05–5 µg/L · Se 0.1–2 mg/L | Pretreat + MVR + FC | Titanium / Hastelloy |
| Power plant circulating blowdown | Cooling tower bleed, water-softening residue | TDS 1–3% · Ca²⁺ 500–1,500 mg/L · Mg²⁺ 100–500 mg/L — scale-prone | Softening + MVR | Duplex 2205 |
| Steel sintering FGD wastewater | Sinter machine de-SOx bleed (limestone / seawater) | SO₄²⁻ 2,000–15,000 mg/L · Fe / Zn / Pb 10–500 mg/L · pH 2–5 | Pretreat + Crystallization | Duplex / Titanium |
| Coke-oven wastewater concentrate | Coking plant ammonia stripper bottom — see Coal Chemical & Mining | COD 2,000–10,000 mg/L · CN⁻ 5–50 mg/L · phenol 100–500 mg/L | Pre-treat + MVR | Titanium / Duplex |
| Desulfurization brine | Seawater FGD, soda-ash FGD bleed | NaCl 15–30 g/L · Na₂SO₄ 3–10 g/L · Mg²⁺ trace | MVR + FC | Titanium |
| Boiler blowdown concentrate | High-pressure boiler continuous blowdown | TDS <1% · SiO₂ 50–200 mg/L — scale-prone | Multi-Effect | 316L |
Related steel-plant streams — cold-rolling pickling rinsewater and direct-reduced-iron (DRI) scrubber water — are covered under Coal Chemical & Mining.
Routes shown are for preliminary screening only. Mercury and selenium removal requires dedicated precipitation pilot testing on a representative sample. Final material selection is confirmed by coupon testing at operating temperature.
Four Steps That Decide Everything Downstream
The evaporator only sees what pre-treatment lets through. This is the canonical conditioning sequence for high-chloride, high-TSS, Hg/Se-bearing FGD feed.
Coagulation, Flocculation & TSS Removal
Clarifier + hydrocyclone + filter press take out gypsum, fly ash and limestone solids. Target <1,000 mg/L TSS to the evaporator.
Lime–Soda Softening
Calcium and magnesium hardness down to <20 mg/L by lime-soda dosing — protecting heat-transfer surfaces from sulfate and carbonate scale.
Sulfide Precipitation for Hg & Se
TMT-15 (or NaHS) at 1.5–3× stoichiometric dose at pH 5–7; ferrihydrite adsorption polishes Se(IV) — always upstream of the evaporator.
Ion-Exchange Polishing
Final hardness and trace-metal trim on the softened feed — the last line of protection before MVR / multi-effect duty.
Softening and heavy-metal precipitation chemistry require site-specific jar testing on a representative sample. The crystallizer does not remove Hg or Se — both must be removed before the evaporator.
FGD & Power-Plant Wastewater ZLD Route
Indicative process flow for FGD purge water with high chloride, gypsum TSS and trace Hg / Se — from bleed stream to managed salt and reuse condensate.
FGD Purge Water
Cl⁻ 10,000–30,000 mg/L · TSS 1–10%
Clarification & Hydrocyclone
TSS down to <1,000 mg/L
Lime-Soda Softening
Ca²⁺ / Mg²⁺ <20 mg/L
Sulfide Precipitation (Hg · Se)
TMT-15 + ferrihydrite
MVR / Multi-Effect
15–25 kWh/t water — or 0.25–0.40 t steam/t
FC Crystallizer
Mixed-salt slurry
Salt + Reuse Condensate
Managed solids · reuse-grade water
Forced-Circulation Crystallization
The workhorse closing stage for scaling, high-TDS FGD brines — high liquid circulation velocity suppresses wall scaling while mixed salt is produced as a free-flowing solid.
MVR Concentration + FC Crystallizer
Electric-driven concentration ahead of crystallization — strongest where power is available and steam is dear. Watch the BPE penalty at the top of the chloride range.
Multi-Effect with Falling-Film Pre-Concentration
Falling-film (tube) effects on clarified, softened feed, finishing in FC — competitive above ~15,000 mg/L Cl⁻ when low-pressure steam ($15–25/t) is available from the power plant itself.
Salt Classification & Separation
NaCl vs Na₂SO₄ / CaSO₄ split when a sellable clean-salt route is pursued instead of landfill — additional process complexity, justified only with verified pilot data.
Simplified PFD for indicative routing only. Actual configuration depends on chloride load, TSS profile, regulatory Hg / Se limits and site utility availability. Softening and heavy-metal precipitation chemistry require site-specific jar testing.
High-Chloride Material Selection
Chloride exposure — not TDS — picks the alloy. Map your stream to the material envelope before any evaporator geometry is discussed.
| Chloride Exposure | Typical Service | Material | Selection Logic & Watch-Outs |
|---|---|---|---|
| <200 mg/L Cl⁻ at evaporation temperature | Mild streams — boiler blowdown concentrate (TDS <1%, SiO₂ 50–200 mg/L) | 316L | Safe envelope only — pitting begins beyond it; three orders of magnitude below FGD chloride levels |
| ~1,000 mg/L Cl⁻ | Moderate streams — power-plant circulating blowdown (TDS 1–3%, Ca²⁺ 500–1,500 mg/L) | Duplex 2205 | Handles moderate Cl⁻ but fails under high Cl⁻ combined with low pH |
| 10,000–30,000 mg/L Cl⁻ — coal Cl 0.01–0.3 wt% | FGD brine — FGD purge water, seawater / soda-ash FGD bleed | Titanium Grade 2 | The workhorse material for FGD evaporator duty |
| SO₄²⁻ 2,000–15,000 mg/L · pH 2–5 — sulfate-rich | Steel sintering FGD wastewater (Fe / Zn / Pb 10–500 mg/L) | Duplex / Titanium | Sulfate-rich service shifts the choice; heavy metals removed upstream |
| >30,000 mg/L Cl⁻ + low pH + oxidizing species (Fe³⁺, Cu²⁺) | Most aggressive FGD brines | Hastelloy C-276 | Reserved for the harshest combination — a cost driver, not a default |
Final material selection is confirmed by coupon testing at operating temperature — never from a table alone.
What Usually Owns the Project
In this industry the driver is almost always regulatory or energetic. Find yours — it decides the pre-treatment depth and the evaporation route.
Related Solutions & Projects
EvapCryst organizes its delivery capability around four solution lines. Power, steel and FGD projects most often lead with S1 and S4.
ZLD & Wastewater Minimization
FGD purge water ZLD, mixed-salt management and reuse-grade condensate recovery.
Concentration & Product Crystallization
Where FGD by-product sodium or ammonium sulfate can be recovered as a sellable salt.
Mother Liquor Recovery
Purge optimization for sintering and coke-oven wastewater crystallizer loops.
Energy Retrofit & Decarbonization
MVR retrofits for existing steam-heated FGD concentrators — energy and carbon reduction.
Frequently Asked Questions
FGD purge water chlorides typically sit at 10,000–30,000 mg/L — three orders of magnitude above 316L’s safe operating envelope (<200 mg/L at evaporation T). Duplex 2205 handles moderate Cl⁻ (~1,000 mg/L) but fails under high Cl⁻ plus low pH; titanium Grade 2 is the workhorse material for FGD evaporator duty. Hastelloy C-276 is reserved for the most aggressive combinations of Cl⁻ >30,000 mg/L, low pH and oxidizing species (Fe³⁺, Cu²⁺).
Mercury in FGD purge is typically present as particulate HgS (30–60%), ionic Hg²⁺ (10–40%) and a small organomercury fraction (5–20%). Standard treatment is dedicated sulfide precipitation (TMT-15 at 1.5–3× stoichiometric dose, pH 5–7) combined with ferric co-precipitation and polythiocarbonate polishing. Selenium is harder — Se(IV) is removed by ferrihydrite adsorption at pH 4–6 (Fe / Se mass ratio 10:1–20:1); Se(VI) requires prior reduction. Both must be removed before the evaporator; the crystallizer does not remove them.
In the majority of cases the mixed NaCl / CaSO₄ / heavy-metal-bearing salt from FGD ZLD must be managed as a solid waste — typically stabilized and landfilled in a compliant facility. Saleable-grade salt is achievable only when upstream separation produces a clean NaCl stream free of sulfate and heavy metals, which requires additional process complexity. Do not assume salt sales offset operating cost without verified pilot data.
High chloride raises boiling-point elevation (BPE 3–8°C at 15,000–30,000 mg/L Cl⁻) and narrows MVR’s effective ΔT. For FGD purge above ~15,000 mg/L Cl⁻ with significant concentration factor, multi-effect with motive steam (0.25–0.40 t steam per tonne water) often becomes competitive — particularly when low-pressure steam is available from the power plant itself at $15–25/t. The choice is project-specific and driven by a full energy balance including compressor electricity ($0.05–0.10/kWh) vs. steam cost.
Discuss Your Power, Steel or FGD Project
Send us your feed analysis (Cl⁻, TSS, Hg, Se, TDS) and regulatory limits. Within 2 business days you will receive a feasibility assessment, simplified PFD and indicative scope.