Hydrated Lime Ca(OH)2 in Boiler Flue Gas Desulfurization (FGD): Scrubbing SO2, HCl & HF Under QCVN 19:2024/BTNMT

Table of contents

To control stack emissions from coal-fired, fuel-oil, and biomass boilers as well as industrial waste incinerators under QCVN 19:2024/BTNMT (SO₂ ≤ 300 mg/Nm³, HCl ≤ 50 mg/Nm³, HF ≤ 5 mg/Nm³), Industrial Hydrated Lime Ca(OH)₂ serves as the primary Flue Gas Desulfurization (FGD) sorbent. Because Ca(OH)₂ neutralizes acid gases rapidly (1.00 kg SO₂ requires 1.157 kg theoretical Ca(OH)₂) and precipitates sulfur out of the liquid loop as solid Gypsum (CaSO₄·2H₂O), ≥ 325 mesh Ca(OH)₂ is deployed across Wet Forced-Oxidation FGD Spray Towers, Semi-Dry Spray Dryer Absorbers (SDA), and Dry Sorbent Injection (DSI) baghouse systems.

Moisture-proof 25kg bags of Hydrated Lime Ca(OH)2 stored on racks at Loc Thien Chemical warehouse

1. Reaction Chemistry of SO₂, HCl, HF Capture & Forced Oxidation to FGD Gypsum

Stage 1: Acid Gas Absorption (`SO₂`, `SO₃`, `HCl`, `HF`)

Sulfur Dioxide Capture: SO₂ (gas) + Ca(OH)₂ (slurry/solid) → CaSO₃·½H₂O↓ (Calcium Sulfite Hemihydrate) + ½H₂O
Sulfur Trioxide / Acid Mist Capture: SO₃ (gas) + Ca(OH)₂ + H₂O → CaSO₄·2H₂O↓ (Gypsum)
Hydrogen Chloride & Fluoride Capture: 2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O and 2HF + Ca(OH)₂ → CaF₂↓ + 2H₂O

Stage 2: Forced Air Oxidation Converting CaSO₃ into Commercial Gypsum (`CaSO₄·2H₂O`)

At sump pH = 5.2 – 5.8, compressed air blowers oxidize thin, hard-to-dewater CaSO₃·½H₂O platelets into large prismatic FGD Gypsum (CaSO₄·2H₂O, 50–100 µm) crystals:

Forced Oxidation Reaction: 2CaSO₃·½H₂O + O₂ (air) + 3H₂O → 2CaSO₄·2H₂O↓ (FGD Gypsum)

Table 1: Specific Consumption of Commercial Hydrated Lime Ca(OH)₂ 92% per 1 kg Acid Gas

Flue Gas Pollutant	Molar Mass (`M`, g/mol)	Stoichiometric Molar Ratio (`Ca / Gas`)	Theoretical `100% Ca(OH)₂` per `1 kg` Gas (`kg/kg`)	Wet FGD Dosage of `Ca(OH)₂ 92%` (`Ca/S = 1.08`)	Dry DSI Dosage of `Ca(OH)₂ 92%` (`Ca/S = 1.65`)
Sulfur Dioxide (`SO₂`)	`64.06`	`1 : 1`	`1.157 kg`	`1.36 kg Ca(OH)₂ 92%` / kg SO₂	`2.07 kg Ca(OH)₂ 92%` / kg SO₂
Sulfur Trioxide (`SO₃`)	`80.06`	`1 : 1`	`0.925 kg`	`1.09 kg Ca(OH)₂ 92%` / kg SO₃	`1.66 kg Ca(OH)₂ 92%` / kg SO₃
Hydrogen Chloride (`HCl`)	`36.46`	`1 : 2`	`1.016 kg`	`1.19 kg Ca(OH)₂ 92%` / kg HCl	`1.82 kg Ca(OH)₂ 92%` / kg HCl
Hydrogen Fluoride (`HF`)	`20.01`	`1 : 2`	`1.851 kg`	`2.17 kg Ca(OH)₂ 92%` / kg HF	`3.32 kg Ca(OH)₂ 92%` / kg HF

2. Comparing Wet FGD, Semi-Dry SDA, and Dry Sorbent Injection (DSI)

Table 2: Engineering Comparison of Three Ca(OH)₂ Flue Gas Desulfurization Technologies

Technical Criterion	1. Wet Lime FGD (Open Spray Tower)	2. Semi-Dry Spray Dryer Absorber (SDA)	3. Dry Sorbent Injection (DSI + Baghouse)
`Ca(OH)₂` Feed State	`10% – 15% w/w` Milk of Lime slurry sprayed via SiC hollow-cone nozzles in an unpacked tower.	`15% – 25% w/w` Lime slurry atomized by rotary disk (`10,000 rpm`), evaporating dry in `140–180 °C` flue gas.	Dry `≥ 325 mesh` `Ca(OH)₂` powder pneumatically injected into duct upstream of baghouse filter.
`SO₂` & `HCl` Removal	Superior: `92% – 98%` (`SO₂`), `99%` (`HCl/HF`).	High: `85% – 92%` (`SO₂`), `95%–98%` (`HCl/HF`).	Moderate: `60% – 80%` (`SO₂`), `90%–95%` (`HCl/HF`).
Operating `Ca / S` Ratio	Economical: `1.03 – 1.12` (`> 90%` utilization).	`1.25 – 1.50`	`1.50 – 2.20` (reacts across filter cake).
Wastewater Discharge	Requires `FGD Blowdown` bleed for Gypsum dewatering and Chloride purge.	Zero Liquid Discharge (`ZLD`) — dry ash collected in hoppers.	`100% Dry Process` — optimal equipment `CAPEX`.

3. Preventing Gypsum Scaling & 25-Ton/h Coal Boiler Calculation Example

Three Anti-Scaling Rules for Wet Lime FGD: (1) Use an Open Spray Tower with large-bore SiC nozzles (never use random plastic packing with lime slurry); (2) Lock recirculation sump pH at 5.4 – 6.2 so flue-gas CO₂ (10%–14%) is not absorbed as CaCO₃ scale; and (3) Maintain 10%–15% TSS Gypsum seed crystals in the sump so new CaSO₄·2H₂O grows on suspended seeds rather than vessel walls.

Technical Illustration (Reference Engineering Model): A 25-ton/h coal-fired boiler exhausts Q = 45,000 Nm³/h with inlet SO₂ = 1,450 mg/Nm³, targeting outlet SO₂ ≤ 120 mg/Nm³ (QCVN 19:2024/BTNMT Column B limit ≤ 300 mg/Nm³).

Hourly SO₂ Mass Removed: 45,000 × (1,450 - 120) × 10⁻⁶ = 59.85 kg SO₂/h (1,436.4 kg SO₂/day).

Hourly Ca(OH)₂ 92% Consumption (Ca/S = 1.08, 1.36 kg/kg SO₂): 59.85 × 1.36 = 81.40 kg Ca(OH)₂ 92%/h (1.954 tons/day).

Feed Rate of 10% w/w Milk of Lime Slurry (d = 1.072 kg/L): 81.40 ÷ 0.1072 = 759.3 L/h.

5–30-ton truck and tanker delivery network of Loc Thien Chemical

4. Supply of 325-Mesh FGD / DSI Grade Hydrated Lime Ca(OH)₂ by Loc Thien Chemical

Loc Thien Investment Development Co., Ltd. (LOC THIEN INVESTMENT DEVELOPMENT COMPANY LIMITED — Tax ID: 0313650856) is a specialized commercial supplier and distributor of industrial chemicals across Vietnam:

FGD / DSI Grade Ca(OH)₂ Specifications: Active Ca(OH)₂ ≥ 90%–96%, ultra-fine ≥ 325 mesh (≤ 45 µm), moisture ≤ 0.8%–1.0%, silica grit ≤ 1.0%, packed in 25 kg PE-lined bags or 500–1,000 kg Jumbo bags.
6 Strategic Warehouses (3,000+ tons/month, 5–30-ton fleet) & Delivery SLA: Ba Ria - Vung Tau (Phu My 1 IP), Dong Nai (Long Thanh IP), Binh Duong (Di An City), Da Nang (Hoa Khanh IP), Can Tho (Tra Noc IP), and Bac Ninh (Tien Son IP). Headquarters: 452/6B Tinh Lo 10, Binh Tri Dong Ward, Binh Tan District, Ho Chi Minh City. Technical quotation within 15 minutes; delivery within 4 hours in inner HCMC, 24–48 hours across Southern & Mekong Delta industrial zones, and 2–3 days in Northern & Central Vietnam. Contact hotline 0979 891 929 for detailed delivery procedures.

Product Pages: Industrial Hydrated Lime Ca(OH)₂ 96% | Liquid Caustic Soda NaOH 32%–50% | Soda Ash Light Na₂CO₃ 99.2%
GHS Safety Data Sheets (MSDS): MSDS Calcium Hydroxide Ca(OH)₂ | MSDS Liquid Caustic Soda NaOH 32%
Related Engineering Guide: Designing Packed Bed Wet Scrubbers with Liquid Caustic Soda NaOH 32%
Technical Inquiry & 15-Minute Quotation: Hotline 0979 891 929 (tel:0979891929) — Email: [email protected].