The Complete Guide to Modern Flue Gas Desulfurization: Technologies, Trends, and Industrial Applications

Air-quality regulations around the world have become increasingly strict over the past decade, pushing power plants, steelworks, cement producers, and chemical enterprises to upgrade their flue-gas cleaning systems. At the center of these environmental requirements is flue gas desulfurization (FGD)—the essential process for removing sulfur dioxide (SO₂) from industrial exhaust streams.

As industries transition toward greener and more efficient operations, FGD technologies continue to evolve. From the well-established limestone-gypsum method to newer ammonia-based approaches, each solution offers different advantages in efficiency, cost, operating stability, and by-product recovery.

This article provides a comprehensive overview of desulfurization technologies, core mechanisms, application scenarios, and global industry trends—designed for engineers, procurement managers, EPC contractors, and environmental professionals seeking reliable, up-to-date insights.

1. Why Desulfurization Matters

Sulfur dioxide is a major pollutant generated by the combustion of fossil fuels, metallurgical reactions, and heavy industrial processes. Without proper treatment, SO₂ emissions contribute to:

• Acid rain

• Smog formation

• Severe respiratory health problems

• Soil acidification

• Damage to equipment, buildings, and crops

Regulations in Europe, the Middle East, Southeast Asia, and China now commonly require SO₂ emissions to reach as low as 35 mg/Nm³, making FGD systems mandatory for many plants.

Industrial clients are also facing growing pressure from international buyers, ESG investors, and carbon-neutral commitments, all of which make emission control a strategic priority—not just a compliance obligation.

2. Core Technologies Used in Flue Gas Desulfurization

FGD methods can be broadly categorized into wet, semi-dry, and dry processes. Each has its own chemical principles, operating conditions, and suitable industries.

2.1 Limestone–Gypsum Wet Desulfurization (WFGD)

This is the most widely applied desulfurization method in coal-fired power plants and large industrial boilers.

Process principle:

SO₂ in the flue gas reacts with limestone slurry (CaCO₃) to form calcium sulfite, which is further oxidized to gypsum (CaSO₄·2H₂O).

Key advantages:

• High and stable SO₂ removal efficiency (95–99%)

• Mature, reliable technology

• Applicable to large-scale plants

• Gypsum by-product can be sold for building materials

Limitations:

• High water consumption

• Larger footprint

• High initial investment

• Scaling and slurry pipeline maintenance requirements

Despite the drawbacks, limestone-gypsum remains the global mainstream for power plants and large combustion systems due to its stability and proven track record.

2.2 Ammonia-Based Desulfurization (NH₃-FGD)

In recent years, ammonia desulfurization has gained strong momentum, especially in chemical plants, steelworks, ferrosilicon smelting, coking plants, and industrial boilers.

Process principle:

SO₂ reacts with ammonia to form ammonium sulfite/bisulfite, which is then oxidized to produce ammonium sulfate fertilizer.

Advantages:

• SO₂ removal efficiency 97%

• NO₂ absorption ability—simultaneous desulfurization & partial denitrification

• Zero wastewater discharge

• Valuable by-product ammonium sulfate

• No scaling, simpler operation than limestone gypsum

Challenges:

• Requires stable ammonia supply

• Ammonia slip control

• Higher safety and ventilation requirements

For industries seeking both emission reduction and resource efficiency, ammonia-based desulfurization is increasingly becoming a preferred choice.

2.3 Semi-Dry Desulfurization (SDA) / Spray Dryer Absorber

Semi-dry systems are common in cement plants, waste-to-energy facilities, small power units, and biomass boilers.

Features:

• Uses hydrated lime

• Requires minimal water

• Medium SO₂ removal efficiency (70–90%)

• Low investment cost

• Simple operation and low maintenance

Although semi-dry systems cannot reach the ultra-low emission levels required in some countries, they remain a cost-effective solution for smaller or older facilities.

2.4 Dry Desulfurization

Dry processes involve injecting dry sorbents directly into the flue gas. They are typically used for:

• Small industrial furnaces

• Glass kilns

• Low-SO₂ exhaust streams

• Retrofit projects with limited space

Dry systems are compact and easy to maintain, but their efficiency and reaction completeness are lower than wet systems.

3. How to Choose the Right Desulfurization Technology

Selecting an appropriate FGD system involves evaluating several factors:

3.1 SO₂ Concentration and Flue Gas Flow Rate

• High SO₂ + large flow → prefer wet systems (limestone or ammonia)

• Medium SO₂ → semi-dry

• Low SO₂ → dry absorption

3.2 Water Resources and Local Regulations

• Water-scarce regions (Middle East) may prefer semi-dry

• For the strictest standards, ammonia or limestone-gypsum are required

3.3 By-Product Utilization

• If a plant has fertilizer buyers, ammonia desulfurization becomes more economical

• Gypsum markets vary internationally

3.4 CAPEX & OPEX Considerations

Total cost includes electricity, sorbents, maintenance, manpower, consumables, and gypsum or ammonium sulfate handling. Many clients now prioritize long-term operating cost over initial investment.

4. Key Components of an Efficient FGD System

Modern desulfurization units include:

• Absorber tower or scrubber

• Slurry preparation system

• Oxidation air equipment

• Mist eliminators

• Circulation pumps

• By-product handling systems (gypsum, ammonium sulfate)

• Drying and packaging systems (for ammonia-based solutions)

• Automation and online monitoring

High reliability of the absorber, pumps, and mist eliminators directly determines the SO₂ removal performance.

5. Global Trends in Desulfurization Technology

5.1 Shift to Resource-Recovery FGD

Governments and clients increasingly demand circular-economy solutions. Ammonia-based systems align well with this trend, producing fertilizer-grade ammonium sulfate rather than waste gypsum.

5.2 More Hybrid and Integrated Systems

FGD is now often combined with:

• SCR/SNCR denitrification

• Dust removal

• Broadband pollution control

• VOCs treatment

Modern systems are optimized to achieve ultra-low emissions in a single integrated process.

5.3 Digitalization and Smart Control

AI-driven monitoring, optimized pH/ammonia feed rate, and automated scaling prediction are becoming standard in advanced plants.

5.4 Expansion in Emerging Markets

Countries in the Middle East, Southeast Asia, Africa, and South America are rapidly upgrading environmental standards. Demand growth is especially strong in:

• Saudi Arabia

• UAE

• Indonesia

• Vietnam

• India

• Kazakhstan

For EPC contractors and equipment suppliers, these regions represent major market opportunities.

6. Case Applications: Where FGD Makes the Biggest Impact

6.1 Coal-Fired Power Plants

Still the largest installation base worldwide, typically using limestone-gypsum or ammonia systems to achieve ultra-low emission compliance.

6.2 Ferrosilicon & Metallurgical Plants

Flue gas often contains high SO₂ and particulates. Ammonia desulfurization paired with dust removal is highly effective.

6.3 Coking & Coal Chemical Industry

Ammonia-rich environments and variable SO₂ loads make ammonia-FGD particularly suitable.

6.4 Cement & Waste-to-Energy Plants

Semi-dry and dry systems dominate due to restricted space and lower water availability.

7. Future Outlook: Toward Zero-Emission Combustion

As the industrial world moves toward carbon neutrality, desulfurization technology will continue to evolve toward:

• Zero wastewater

• Lower energy consumption

• Higher by-product value

• Full-process digital control

• Integration with CO₂ capture

FGD remains one of the most essential environmental technologies for heavy industry, and its role will only grow as air-quality standards tighten globally.

Conclusion

Flue gas desulfurization is no longer just an environmental requirement—it’s a long-term investment in sustainable, competitive industrial operation. Whether a plant chooses limestone-gypsum, ammonia-based, semi-dry, or dry desulfurization depends on emissions requirements, local regulations, operating costs, and by-product value.

For companies pursuing ultra-low emissions and economic benefits, modern ammonia-based desulfurization and hybrid multi-pollutant control systems represent the new direction of the industry.