Flue Gas Desulfurization (FGD) Scrubber: Design, Chemistry, Technology, and Available Models

Author: Michael Klepik, Chief Executive Officer

A typical FGD wet scrubber diagram consists of several key components. The housing is a cylindrical or rectangular tank made from corrosion-resistant materials. Spray towers, usually located at the top, disperse the solution. Airstream enters the unit through a pipeline at the bottom and rises upward. Inside the chamber, packing materials or cascading plates are employed to maximize contact between the phases. At the top, a gas outlet duct allows the cleaned airflow to exit. Solid waste is collected in a system located at the base of the setup. Additionally, pumps circulate the solvent, and filters are installed to purify the solution.

The wet scrubbing procedure works by bringing contaminated mixture into contact with a liquid that absorbs harmful components. Sulfur-containing airstream is introduced into the chamber, where it is sprayed with the solution — typically water or a mixture with reagents such as lime or soda. The spray scrubber flue gas desulfurization technique ensures an even distribution of the liquid, preventing the formation of dead zones. As the airflow interacts with the solution, harmful substances, particularly sulfur dioxide, react to form neutral compounds (most commonly calcium sulfate, CaSO₄), which are then removed as sludge.

In a wet purifiers with a packed bed, the stream flows through a layer of specially selected packing materials (such as Raschig rings, saddles, or balls) that increase the surface area for phase contact. The solution trickles down over these packings, creating a thin film through which the airstream passes. The packing materials evenly distribute the liquid flow, optimizing conditions for the interaction of chemicals.

In bubbling-type flue gas desulfurization scrubber systems, instead of packings, fluidized bed is used. The mass transfer section consists of perforated trays that hold the liquid. As the airflow passes through the holes in the trays, it creates an intense foam on the surface, increasing the contact area and enhancing the efficiency of the scrubbing method.

In configurations equipped with a venturi tube, additional turbulence is generated during the cleaning operation, enhancing the efficiency of pollutant absorption.

The hard yellow rock in a scrubber FGD, formed during reactions, collects at the bottom of the build and is removed via the drainage mechanism. The cleaned airflow rises and passes through mist eliminators that capture liquid droplets, preventing them from being released with the exhaust stream. Afterward, the purified mixture is vented to the atmosphere through the outlet duct. Since pumps continuously supply liquid to the nozzles, the scrubbing procedure remains stable and operates continuously.

Depending on the chosen FGD wet scrubber design, the equipment can be utilized to remove a range of pollutants, including sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, and nitrogen oxides.

When selecting a neutralizing agent, the key requirements include ensuring stable operation at the specified concentration of sulfur compounds, minimizing corrosive effects on the equipment, and maintaining economic viability. The most commonly utilized options in the industry is a flue gas desulfurization limestone wet scrubber and its equivalent based on Ca(OH)₂.

In some cases, such as at coal-fired power plants with high impurity content, sodium hydroxide (NaOH) solutions may be preferred due to their high reactivity; however, this option can be more costly.

The proper selection of equipment type, choice of absorbent reagent, and adjustment to the production process parameters — such as temperature, flow rate, and composition of the exhaust mixture — are crucial for achieving high efficiency in sulfur dioxide removal. Under optimal conditions, removal efficiencies can reach 95-99%, meeting stringent environmental standards.

The primary reaction in wet units is the interaction between SO₂ and the absorbent.

When using calcium hydroxide, the reaction is as follows:
SO₂ + Ca(OH)₂ → CaSO₃ + H₂O

If limestone is applied as the reagent, the method also produces carbon dioxide:
SO₂ + CaCO₃ → CaSO₃ + CO₂

The resulting calcium sulfite can further oxidize to calcium sulfate in the presence of oxygen, forming gypsum:
2CaSO₃ + O₂ → 2CaSO₄

When the collection and processing of by-products are prioritized, this oxidation is carried out more thoroughly.
For sodium-based agents in a wet flue gas desulfurization scrubber, such as sodium hydroxide, a similar reaction occurs:
SO₂ + 2NaOH → Na₂SO₃ + 2H₂O

The resulting sodium sulfite can also be oxidized to sulfate:
2Na₂SO₃ + O₂ → 2Na₂SO₄

During equipment operation, the wash solution is continuously monitored across several key parameters to maintain optimal cleaning efficiency. The primary indicators tracked include pH, dissolved oxygen concentration, and levels of sulfates and sulfites. These measurements enable timely adjustments to the reagent composition and operation conditions, minimizing the risk of precipitate formation.

To reduce waste and improve the overall efficiency of FGD scrubber systems, the collection and processing of by-products are often employed. For example, when lime is used as the neutralizing agent, gypsum (CaSO₄) is produced, which can be utilized in construction, such as in the manufacturing of drywall. As part of this procedure, periodic FGD scrubber blowdown is carried out to remove dissolved compounds. This approach not only lowers the environmental impact but also provides additional economic benefits for the facility.

Additionally, maintenance includes regular inspections of the equipment, cleaning and replacing filters, calibrating sensors and instruments, and checking pumps and nozzles. It also involves training personnel on the operation and maintenance procedures for the equipment.

If you have any further questions about sulfur removal methods for your facility's emissions, feel free to reach out to our engineers for a free consultation.