Flue Gas Desulfurization (FGD) plays a critical role in controlling sulfur dioxide (SO2) emissions and managing sulfur balance in industrial plants. Based on moisture content during reaction and the physical state of the end-product, sulfur removal technologies are categorized into three main routes: Wet, Semi-Dry, and Dry FGD processes.
Wet Flue Gas Desulfurization (Wet FGD)
Wet FGD involves gas-liquid reactions, making it the most mature and widely adopted technology globally. It accounts for over 80% of total installed sulfur abatement capacity.
- Pros: Fast reaction rate, exceptionally high sulfur removal efficiency (>90%–95%), stable operation, and broad industrial applicability.
- Cons: Generates liquid or sludge byproduct, highly corrosive environment, high energy consumption (requires flue gas reheating), large footprint, and substantial water consumption. Best suited for large power plants.
Key Technological Routes
A. Limestone / Lime-Gypsum Process
- Mechanism: Uses limestone (CaCO3) or lime (Ca(OH)2) slurry to absorb flue gas SO2, forming calcium sulfite (CaSO3), which is oxidized to calcium sulfate (CaSO4• 2H2O) to recover high-purity gypsum.
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Challenge & Solution : Traditional calcium-based systems can cause scaling and clogging due to low salt solubility. Dual-Alkali processes solve this scaling issue by using soluble sodium-based solutions for primary sulfur absorption, followed by lime regeneration outside the tower.
B. Magnesium Oxide (MgO) Recovery Method
- Mechanism: Utilizes magnesium oxide slurry to react with sulfur compounds, forming magnesium sulfite, which is oxidized and crystallized into heptahydrate magnesium sulfate (MgSO4•7H2O) as a saleable byproduct.
C. Wet Ammonia Method
- Mechanism: Uses ammonia (NH3) as the absorbing agent to convert sulfur emissions into high-value chemical fertilizer, such as ammonium sulfate ((NH4)2SO4).
D. Industrial Waste Alkali Utilization
- Carbide Slag Method : Reuses calcium-rich waste slag from acetylene generation, offering 20% higher sulfur absorption performance at lower raw material costs.
- Paper White Liquor Method: Leverages paper mill waste rich in CaCO3 and MgO for cost-effective sulfur capture.
Dry Flue Gas Desulfurization (Dry FGD)
Dry FGD relies on gas-solid reactions, eliminating wastewater treatment requirements entirely. Sorbents (e.g., lime or active carbon) are injected directly into the furnace or reaction chamber.
- Pros: Simple layout, zero liquid discharge, low energy consumption, lower equipment corrosion, and high outlet flue gas temperatures (prevents stack “white plume”).
- Cons: Lower baseline efficiency (60%–80%), lower sorbent utilization rate, higher mechanical wear, and operational instability.
Prominent Dry Methods:
- Activated Carbon Adsorption : Catalytically converts SO2 into sulfuric acid or elemental sulfur, enabling full sulfur resource recovery with efficiency up to 95%.
- Electron Beam Irradiation: Irradiates flue gas with high-energy electrons to generate reactive radicals, oxidizing sulfur oxides into sulfates.
- Charged Dry Injection (CD/SI): Electrostatically charges sorbent particles to disperse them evenly across the gas stream, improving gas-solid sulfur contact.
Semi-Dry Flue Gas Desulfurization (Semi-Dry FGD)
Semi-Dry FGD strikes a balance between wet efficiency and dry process simplicity, making it ideal for medium-to-small industrial boilers.
- Spray Dryer Absorption (SDA): Fine droplets of lime slurry are atomized into the flue gas. The sulfur absorption reaction occurs rapidly alongside water evaporation, yielding a dry solid byproduct.
- Spouted Bed & Duct Injection: Injects dry sorbent and fine water mist directly into flue ducts, reducing initial capital investment while maintaining 70%–85% sulfur removal rates.
Emerging & Next-Gen Sulfur Control Technologies
As environmental regulations tighten, advanced methodologies focus on lower lifecycle cost and higher sulfur recovery rates:
- Sodium Sulfide Method: Uses industrial Na2S to absorb sulfur dioxide and yield high-value elemental sulfur (S) or sodium thiosulfate (Na2S2O3).
- Membrane Absorption: Combines polymeric hollow-fiber membranes with alkaline solutions to capture sulfur compounds at lower operational footprints.
- Biological Desulfurization: Uses specialized bacteria (e.g., Thiobacillus) to oxidize sulfur oxides under ambient conditions, reducing operating costs by up to 50%.
How to Choose the Right FGD System?
| FGD Category | Efficiency | Investment / OpEx | Water Usage | Best Application |
| Wet FGD | High (>90%–95%) | High | High | Large-scale thermal power plants, major chemical hubs |
| Semi-Dry FGD | Medium-High (70%–85%) | Moderate | Moderate | Medium/small industrial boilers, municipal plants |
| Dry FGD | Moderate (60%–80%) | Low | Zero | Steel plants, metallurgical furnaces, water-scarce regions |
Selecting the optimal sulfur management process requires evaluating flue gas volume, inlet SO2 concentration, regional water constraints, and downstream byproduct utilization options.
