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Limestone Wet Scrubber Technology for SO₂ Removal: Principles and Performance

A limestone wet scrubber is a flue gas desulfurization (FGD) system designed to remove sulfur dioxide (SO₂) from exhaust gases generated by fossil fuel combustion, particularly in coal-fired power plants, steel mills, cement kilns and various chemical industries.

It operates using a limestone (CaCO₃) slurry as the alkaline sorbent, which reacts with acidic SO₂ to form stable, easily handled byproducts.

TORNADO-FB Fluidized Bed Scrubber

Process Overview

Flue Gas Desulfurization (FGD) Scrubber

The scrubber system comprises a lime scrubber tower, where hot flue gases flow upward through a countercurrent spray of finely atomized limestone slurry. The key chemical reactions are as follows:
1. Absorption and reaction:
SO₂(g) + H₂O(l) → H₂SO₃(aq)
H₂SO₃ + CaCO₃ → CaSO₃(s) + CO₂↑ + H₂O
2. Optional oxidation (for gypsum production):
2CaSO₃ + O₂ + 4H₂O → 2CaSO₄·2H₂O (gypsum)

The core chemical reaction can be summarized as:
SO₂ + CaCO₃ + H₂O → CaSO₃ + CO₂ + H₂O

Efficient operation depends on maintaining the correct slurry pH (~5.5–6.5), stoichiometric Ca/S ratio, and gas-liquid contact time. Removal efficiencies typically reach 90–98% for SO₂, depending on design and operating conditions.

Scrubber Tower Design Considerations

The wet lime scrubber includes multiple zones:

Absorber section with dual or triple spray levels.
Optional packed bed section to increase gas-liquid contact efficiency.
Demister to remove entrained droplets.
Recirculation pumps for slurry reuse.
Oxidation basin (optional) for gypsum crystallization.

Key operational parameters:

Liquid-to-gas ratio (L/G): ~8–12 L/m³
Slurry solids content: ~10–15 wt%
Inlet SO₂ concentration: 500–3,000 ppm
Pressure drop: 100–200 mmH₂O

Preferred Scrubber Types

Review of Packed Bed Wet Scrubber

In limestone wet scrubbers, the most commonly used designs are the spray tower (open spray chamber) and the packed bed scrubber.

The spray tower is the most widespread due to its simple design and resistance to scaling and clogging. It allows flue gas to flow upward while limestone slurry is sprayed downward, ensuring adequate contact without internal packing. It's ideal for handling high gas volumes with dust and particulate matter.

We recommend considering the TORNADO ST (Spray Tower or Spray Chamber) — a scrubber with a modular design that allows for quick scalability of system capacity. Each module handles between 600 and 30,000 CFM. The well-thought-out design of the scrubber facilitates the efficient passage of large air volumes for treatment.

The BOREAS-P1 Spray Tower features a horizontal tower design for more compact installation.

TORNADO ST Spray Tower Wet Scrubber

Performance: 600 — 30 000 cfm

BOREAS-P1 Horizontal Wet Scrubber

Performance: 100 — 175 000 cfm

The packed bed scrubber is chosen when higher mass transfer efficiency is required in a compact footprint. It uses structured or random packing to maximize gas-liquid interaction but is more susceptible to fouling due to calcium-based scale buildup.

TORNADO FB Fluidized Packed Bed Scrubber

Performance:
100 — 175 000 cfm

TORNADO RP Vertical Wet Scrubber

Performance: 600 — 24 000 cfm

TORNADO SP Packed Bed Scrubber

Performance: 600 — 30 000 cfm

BOREAS-P2 Horizontal Packed Bed Scrubber

Performance:
100 — 175 000 cfm

Venturi and foam scrubbers are rarely used in this context. Venturi scrubbers may serve as pre-treatment units for dust removal, while foam scrubbers are limited to small-scale or specialized applications.

Advantages of Limestone-Based Systems

Cost-effective sorbent: Limestone is inexpensive (typically ~$10–20/ton), widely available, and easy to handle.
High efficiency: Modern systems achieve >95% SO₂ removal with proper process control.
Byproduct utilization: The oxidation step yields commercial-grade gypsum suitable for wallboard manufacturing.
Robust scalability: Systems can handle large flue gas volumes (up to several million Nm³/h).

Comparison: Limestone vs Sodium Hydroxide (NaOH)

NaOH (sodium hydroxide) is also used to remove SO₂ from gases — it effectively neutralizes acidic pollutants. However, limestone (CaCO₃) offers significant advantages, especially at an industrial scale.

Parameter	Limestone (CaCO₃)	Sodium Hydroxide (NaOH)
SO₂ Removal Efficiency	90–98%	95–99%
Sorbent Cost	~$10–20/ton	~$300–500/ton
Reagent Availability	Globally abundant	Requires chemical-grade sourcing
Byproduct	Gypsum (useful)	Na₂SO₃/Na₂SO₄ (less valuable)
System Type	Slurry-based, bulk scale	Often used in small/precise systems
Corrosivity	Moderate	High (requires resistant materials)

Although NaOH offers faster kinetics and higher solubility, its high cost, corrosiveness, and less favorable byproducts make it impractical for large-scale continuous FGD. Conversely, limestone wet scrubbers provide a balanced solution between performance and economic feasibility.

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We always perform precise calculations and offer expert assistance in selecting the optimal dust collection or gas cleaning systems, typically completing this process within 1 to 2 days

Head of Engineering,
Vladimir Nikulin

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