100m³/d desulfurization wastewater triple-effect evaporator

I. Project Overview 

A coal-fired power plant's limestone-gypsum wet desulfurization unit generates 100 m³/d of end-of-life wastewater, containing 12-18% chloride ions, 4 g/L Ca²⁺, 2 g/L Mg²⁺, 30 g/L SO₄²⁻, 3500 mg/L COD, 1200 mg/L ammonia nitrogen, and mercury and arsenic total heavy metals ≤2 mg/L. In 2024, the power plant decided to construct a zero-discharge end-of-life wastewater project. Our company provided a 100 m³/d triple-effect evaporator system under an EPC model. To date, it has been operating continuously for >7200 hours, with a water production rate ≥85%, a miscellaneous salt content ≤0.5%, and a comprehensive energy cost per ton of water <35 yuan.

II. Process Scheme

1. Influent Characteristics

• Boiling point elevation Δb = 14℃ (TDS 160g/L, 70kPa·A)

• High hardness, high chlorine, high COD, scaling factor RF = 0.92, highly corrosive

• Requires coupling with the existing triplet softening and DTRO concentrate system

2. Process Overview

• Desulfurization wastewater → Softening and clarification (NaOH + Na₂CO₃, Ca ≤ 30mg/L, Mg ≤ 20mg/L) → Tubular microfiltration (SDI ≤ 3) → DTRO (60% recovery rate, concentrate TDS ≥ 120g/L) → Concentrate tank → 100m³/d triple-effect co-current evaporator → Crystallizer → Centrifuge → Drying → Ton bag packaging

• Live steam parameters: 0.5MPa(g), 158℃. The first-effect heating chamber uses 2205 duplex steel, and the second and third effects use 2507 + titanium composite tube sheets to ensure chloride ion corrosion allowance.

3. Process Innovation

• Coupling of Co-current and Forced Circulation: The first-effect falling film separator handles 65% of the evaporation load, while the second and third effects utilize forced circulation to prevent scaling, extending the cleaning cycle from 3 days to 10 days.

• Segmented Preheating: Secondary steam from the second and third effects is used to progressively heat the inlet water to 95℃, reducing live steam consumption by 12%.

• Separate Crystallization: The DTB crystallizer controls supersaturation ΔC < 3g/L, slurry density 25%, d50 = 0.6mm, C.V. ≤ 8%, and centrifugal moisture content ≤ 5%.

• Online Defoaming: The first-effect separator is equipped with double-layer inclined plates and wire mesh, achieving a foam carryover rate of < 0.1%, ensuring water quality for the subsequent steam compressor (with a reserved MVR interface).

III. Equipment Configuration

Evaporators: First-effect 110m², Second-effect 95m², Third-effect 80m², total heat exchange area 285m², design margin 20%. Crystallizer: Φ1600×3500mm DTB, built-in guide tube, 2507 stainless steel + rubber lining, design pressure -0.1MPa.

Centrifuge: LW450×1800-N, separation factor 1400, frequency conversion speed regulation, screen gap 0.08mm, condensate used in the washing section, impurities Cl⁻≤0.3%.

Drying: Paddle dryer, steam 0.4MPa, drying capacity 500kg/h, discharge temperature ≤60℃, to prevent agglomeration.

IV. Automation and Energy Saving

DCS+APC: Multi-variable predictive control of evaporation rate, liquid level, density, and vacuum degree, fluctuation ±3%.

Dual closed-loop system with steam flow feedforward + temperature feedback, live steam consumption 0.28t/t water, better than the design value of 0.30t.

Secondary steam condensation uses a titanium plate heat exchanger, cooling water consumption 2.5t/t water, saving 15% of electricity compared to traditional surface condensers. Dual Redundant SIS: 12 interlocks including low liquid level, high vacuum, compressor surge (reserved), and mixed salt silo level, with a response time <500ms.

V. Operational Data (Performance Assessment, April 2025)

Processing Capacity: 100 m³/d (Design 100)

Water Production Rate: 85% (DTRO 60% + Evaporation 85% × 40%)

Live Steam: 0.28 t/t water (Design 0.30)

Power Consumption: 18 kWh/t water (Circulating Pump + Vacuum Pump + Centrifuge)

Mixed Salt Production: 0.9 t/d, Moisture Content 0.4%, TDS ≥ 98%, Heavy Metal Leaching Concentration Meets Owner's Requirements, Delivered to Hazardous Waste Landfill

Condensate: TDS ≤ 150 mg/L, Cl⁻ ≤ 50 mg/L, Reused for Desulfurization Process Water, Overall Plant Water Consumption Reduced by 10%