flue-gas-desulfurization

name	flue-gas-desulfurization
description	Flue gas desulfurization (FGD) — wet limestone scrubbing, SO2 removal efficiency, L/G ratio, spray tower sizing, gypsum byproduct, dry/semi-dry FGD, EPA MATS limits, forced oxidation.
metadata	{"priority":7,"promptSignals":{"phrases":["flue gas desulfurization","FGD","SO2 scrubbing","limestone scrubbing","SO2 removal","wet scrubber SO2","gypsum byproduct"],"minScore":3}}

Flue Gas Desulfurization — Complete Skill

SO₂ Chemistry — Wet Limestone Process

Absorption reaction: SO₂(g) + H₂O → H₂SO₃ → H⁺ + HSO₃⁻ HSO₃⁻ → H⁺ + SO₃²⁻

Limestone neutralization: CaCO₃ + SO₂ + ½H₂O → CaSO₃ · ½H₂O + CO₂

Forced oxidation (air injection — modern standard): CaSO₃ · ½H₂O + ½O₂ → CaSO₄ · 2H₂O (gypsum) Full reaction: CaCO₃ + SO₂ + ½O₂ + 2H₂O → CaSO₄ · 2H₂O + CO₂

pH control: Optimal pH: 5.5–6.0 (below → calcium sulfate precipitation risk; above → limestone dissolves slower) Limestone quality: > 95% CaCO₃; d50 < 30 μm (fine grinding improves reaction rate)

Overall stoichiometry: Limestone consumption rate: L_CaCO₃ = ṁ_SO₂ × M_CaCO₃ / (M_SO₂ × η × stoich_excess) Stoich excess: typically 1.03–1.10 (3–10% excess limestone)

Gypsum production: ṁ_gypsum = ṁ_SO₂ × M_CaSO₄·2H₂O / M_SO₂ = ṁ_SO₂ × 172/64 = 2.69 × ṁ_SO₂ [kg/kg SO₂]

Process Description — Wet Limestone FGD

Flue gas path: Boiler → ESP/baghouse (remove fly ash) → FGD absorber tower → mist eliminators → clean stack Optional: flue gas reheater (GGH) to prevent condensation in stack

Absorber types:

1. Open spray tower: most common; vertical; countercurrent spray
2. Jet bubbling reactor: submerged gas injection through liquid pool
3. Packed tower: structured packing; higher efficiency; fouling risk with high solids
4. Venturi scrubber: high-velocity contact; high ΔP; less common for FGD

Spray Tower Design

Gas-liquid contact: Countercurrent: gas up; slurry spray down; maximum driving force Cross-flow: gas through; spray perpendicular; lower pressure drop

L/G ratio (liquid to gas): L/G = (liquid flow [L/hr]) / (gas flow [m³/hr NTP]) = 10–25 L/m³ Higher L/G → more contact → higher removal; more pump energy

Number of transfer units (NTU): NTU = ln[(y_in - y_out)/(y_out - y_in)] / LMTD [for dilute systems] or NTU = ∫ dy/(y - y*) [general form; y = SO₂ mole fraction]

Height of transfer unit (HTU): HTU = G / (K_ya × A_tower) [m; K_ya = overall volumetric mass transfer coefficient] K_ya ≈ 0.05–0.5 s⁻¹ depending on spray density and droplet size

Tower height: Z = NTU × HTU [m]

Tower diameter: Gas velocity: V_gas = 2–5 m/s in absorber (low to prevent mist entrainment) A_tower = Q_gas / V_gas → D = √(4A/π)

SO₂ removal efficiency: η = (y_in - y_out) / y_in × 100% = 95–99% achievable

Forced Oxidation System

Air injection: compressed air to the absorber sump Agitators: mix slurry; prevent settling; distribute oxidation air Oxidation completion: 96–99% conversion of CaSO₃ → CaSO₄ required for dewatering Incomplete oxidation → CaSO₃ slurry (hard to dewater; not marketable)

Air flow requirement: Q_air = ṁ_CaSO₃ × (M_air/M_SO₂) × (1/0.21) × 1.2 [stoichiometric + 20% excess]

Dewatering — Gypsum Product

Primary dewatering: hydrocyclones; reduce solids to 40–60% moisture Secondary dewatering: vacuum belt filter or centrifuge; 90% dry solid (10% moisture) Gypsum quality: > 90% CaSO₄ · 2H₂O for wallboard market; < 1% chloride; < 50 ppm MgO

Gypsum market: can be sold for wallboard (calcium gypsum plasterboard) if specification met Or: landfill disposal (zero-value)

Reagent Preparation

Limestone grinding: Wet ball mill → limestone slurry 20–30% solids; d50 < 30 μm (fine = better reactivity) Ball mill motor: P = f(throughput, grindability index)

Fresh water: To makeup for gypsum moisture + water vapor in flue gas + leakage Water balance: Q_makeup = Q_gypsum_moisture + Q_vapor_exit - Q_vapor_inlet

Chloride buildup: Chlorides (from coal HCl) accumulate in slurry → corrosion; gypsum contamination Blowdown required: Cl⁻ ≤ 20,000 ppm in slurry (typical control point)

Dry / Semi-Dry FGD

Dry Lime Injection

CaO or Ca(OH)₂ powder injected into hot flue gas No liquid handling; simpler; lower removal efficiency (50–70%) Used for smaller boilers; distillate fuels; low sulfur applications

Spray Dry Absorber (SDA / SDS)

Slaked lime slurry atomized into flue gas; evaporation cools gas + reacts SO₂ 60–90% removal; produces dry powder waste (CaSO₃ + CaSO₄ + excess lime) Requires downstream baghouse for product capture

Operating temperature: 20–40°C above adiabatic saturation temperature to maintain dry product

Circulating Dry Scrubbing (CDAS)

Recirculate spent absorbent; high cake-to-fresh reagent ratio; efficiency up to 95%

EPA / Regulatory Limits (MATS — Mercury and Air Toxics Standards, 40 CFR Part 63)

New Source Performance Standards (NSPS 40 CFR Part 60 Subpart Da): New coal boilers: SO₂ ≤ 1.4 lbs/MMBtu OR 90% removal (whichever is less stringent) Best Achievable Control Technology (BACT): site-specific

MATS (2012, existing sources): SO₂: addressed separately as acid gas surrogate; 0.20 lb/MMBtu PM: 0.03 lb/MMBtu

EU IED (Industrial Emissions Directive 2010/75/EU): For coal plants > 300 MW: SO₂ ≤ 150 mg/Nm³ (new BAT conclusions 2017) Existing: 200 mg/Nm³ with derogations

Liquid Waste Treatment

FGD wastewater: Contains: heavy metals (Hg, Se, As, Pb), chlorides, suspended solids Treatment: neutralization → precipitation → clarification → chemical oxidation (selenium) → filtration Discharge to surface water: permit-based (EPA ELG effluent limits for steam electric)

Output

Provide: SO₂ removal efficiency [%], L/G ratio [L/m³], tower diameter D [m] and height Z [m], limestone consumption [kg/hr], gypsum production [tonne/day], oxidation air flow [Nm³/hr], absorber gas velocity [m/s], pressure drop through absorber [Pa], reagent slurry concentration [% solids], chloride control blowdown rate [m³/hr], applicable regulatory limit (MATS or EU IED), and annual reagent + disposal cost [$].