Views: 0 Author: Wordfik Vacuum Publish Time: 2026-01-26 Origin: Wordfik Vacuum
Flue Gas Desulfurization (FGD) is one of the most critical emissions control technologies for coal-fired power plants, removing up to 95–99% of sulfur dioxide (SO₂) from flue gases. The most widely adopted FGD process—limestone-gypsum wet scrubbing—relies on vacuum systems to dewater the byproduct gypsum into a commercially usable form and to treat process wastewater.
Without effective vacuum technology, the FGD process would produce a wet, unmanageable sludge, increasing disposal costs and reducing the environmental benefits of SO₂ capture. This guide explains how vacuum systems function in FGD processes, the equipment involved, and how to optimize performance for maximum efficiency and regulatory compliance.
In a typical limestone-gypsum wet FGD system, flue gas containing SO₂ is sprayed with a limestone slurry in an absorber tower. The SO₂ reacts with calcium carbonate (limestone) to form calcium sulfite, which is then oxidized to calcium sulfate dihydrate—gypsum.
The chemical reaction sequence:
| Step | Reaction | Product |
| Absorption | SO₂ + H₂O → H₂SO₃ | Sulfurous acid |
| Neutralization | H₂SO₃ + CaCO₃ → CaSO₃ + CO₂ + H₂O | Calcium sulfite |
| Oxidation | CaSO₃ + ½O₂ + 2H₂O → CaSO₄·2H₂O | Gypsum |
The gypsum crystals are suspended in the absorber slurry. To recover a marketable byproduct, the gypsum must be separated from the liquid and dewatered—typically using vacuum belt filters.
The gypsum slurry from the absorber (typically 10–20% solids by weight) is fed onto a moving horizontal vacuum belt filter. As the belt passes over a vacuum box, the vacuum draws liquid through the filter cloth, leaving a solid gypsum cake. The cake is then washed (to remove chlorides), dried under vacuum, and discharged.
Why vacuum is essential:
Achieves 85–92% solids content in the gypsum cake
Removes chlorides and other soluble impurities
Enables gypsum to be sold to wallboard manufacturers or cement plants
Reduces landfill volume if gypsum cannot be marketed
A horizontal vacuum belt filter (also called a rubber belt filter) is the standard dewatering device for FGD gypsum. Key components include:
| Component | Function |
| Rubber drainage belt | Continuous moving belt with transverse grooves |
| Filter cloth | Porous fabric that retains solids while passing liquid |
| Vacuum box | Stationary chamber beneath the belt connected to the vacuum pump |
| Slurry feed system | Distributes slurry evenly across the belt width |
| Wash system | Spray bars for cake washing (chloride removal) |
| Cake discharge roll | Scrapes dewatered gypsum from the filter cloth |
| Pump Type | Suitability for FGD | Advantages | Limitations |
| Liquid ring vacuum pump | Excellent | Handles moisture, scale, and solids; robust; proven in FGD | Lower efficiency than dry pumps |
| Dry screw vacuum pump | Good | High efficiency; oil-free; VFD-capable | Higher first cost; less tolerant of liquid slugs |
| Dry claw vacuum pump | Limited | Oil-free; compact | Not suitable for high moisture or scale |
| Water-sealed piston pump | Poor (obsolete) | High maintenance, low reliability | No longer specified |
Industry standard: Liquid ring vacuum pumps are the dominant choice for FGD vacuum belt filters due to their ability to handle carryover slurry, scale, and saturated air without internal damage.
| Parameter | Typical Value | Notes |
| Vacuum level | 300–500 mbar abs (15–26 inHg vacuum) | Deeper vacuum increases cake solids |
| Air flow rate | 10–30 m³/h per m² of filter area | Depends on filter design and gypsum crystal size |
| Seal water (liquid ring pumps) | 1–2 m³/h per pump | Closed-loop cooling recommended |
Sizing rule of thumb: For a typical FGD vacuum belt filter producing 10–20 tons/hr of gypsum, a liquid ring vacuum pump with 500–1,500 m³/h capacity is required.
FGD slurries and gypsum process water contain chlorides, sulfates, and often low pH. Standard cast iron pumps may fail rapidly. Material selection is critical:
| Component | Standard Material | Recommended for Aggressive FGD |
| Casing | Cast iron | Duplex stainless steel (2205) or rubber-lined |
| Impeller | Bronze or cast iron | Duplex stainless steel or CD4MCu |
| Port plate | Cast iron | Duplex stainless steel |
| Seal water system | Carbon steel | 316 stainless steel |
The quality of dewatered gypsum—and therefore its marketability—directly depends on vacuum level.
| Vacuum Level (mbar abs) | Gypsum Cake Solids | Marketability |
| < 300 mbar | 90–94% | Premium (wallboard grade) |
| 300–400 mbar | 85–90% | Acceptable (cement grade) |
| > 400 mbar | < 85% | Poor (landfill likely) |
Factors affecting vacuum stability:
Air in-leakage in the filter sealing system
Scale buildup on filter cloth or vacuum box
Inadequate seal water flow or temperature
Worn pump internals (impeller, port plate)
FGD wastewater containing chlorides is increasingly regulated. In Europe and the US, zero liquid discharge (ZLD) requirements are driving more aggressive vacuum dewatering to minimize wastewater volume.
Advanced approach: Two-stage vacuum filtration:
Primary vacuum belt filter – produces main gypsum cake
Secondary vacuum filter (or centrifuge) – treats side stream to reduce chlorides below 100 ppm
| Energy-Saving Measure | Typical Saving | Implementation |
| VFD control on vacuum pump | 20–35% | Retrofit existing fixed-speed pumps |
| Closed-loop seal water cooling | 15–25% (water pumping) | Reduce seal water temperature |
| Vacuum box optimization | 10–15% | Reduce unnecessary filter area under vacuum |
| Dry screw vs. liquid ring (new plants) | 30–40% | Higher efficiency, higher first cost |
In addition to gypsum dewatering, vacuum systems are used in advanced FGD wastewater treatment:
Reverse osmosis (RO) reject water from FGD wastewater treatment can be further concentrated using vacuum evaporators. Operating under vacuum (100–200 mbar abs) lowers the boiling point, reducing energy consumption by 30–50% compared to atmospheric evaporation.
In the lime softening step of FGD wastewater treatment, vacuum filters dewater the calcium carbonate/magnesium hydroxide sludge, reducing haul-off volume by 70–80%.
Dissolved gases (CO₂, O₂) are removed from FGD wastewater under vacuum to prevent scaling in downstream reverse osmosis membranes.
| Issue | Cause | Prevention / Corrective Action |
| Scale formation on port plate | Hard water, high pH | Closed-loop seal water with softened or demineralized water |
| Impeller erosion | Sand/silica in slurry | Install primary cyclone; upgrade to duplex stainless steel |
| Loss of vacuum | Seal water too warm | Reduce temperature; increase flow rate |
| Cavitation | Inadequate NPSH | Lower seal water temperature; reduce pump speed (VFD) |
| Frequent seal failure | Abrasive particles | Upgrade to mechanical seal with tungsten carbide faces |
| Motor overload | Gas load too high | Investigate air in-leakage on filter; clean filter cloth |
New EPA regulations in the US and upcoming revisions to China's GB standards will require even lower SO₂ emissions (down to <10 mg/Nm³) and near-zero liquid discharge. This will drive demand for more efficient vacuum dewatering and vacuum evaporation.
Liquid ring pumps have dominated FGD due to their robustness, but dry screw pumps are gaining ground for new plants because of:
30–40% lower energy consumption
No seal water consumption
Lower overall maintenance (no oil or seal water systems)
Limitation: Dry screw pumps are less tolerant of liquid carryover; they require effective knockout pots ahead of the pump.
IoT-enabled vacuum pumps with real-time monitoring of vacuum level, motor power, and bearing vibration are being deployed in FGD systems to predict maintenance needs and optimize energy use.
As coal plants explore post-combustion carbon capture, amine-based systems require vacuum regeneration of the solvent. FGD vacuum expertise positions power plant operators well for this transition.
Vacuum systems are indispensable to modern limestone-gypsum Flue Gas Desulfurization. They enable:
Efficient gypsum dewatering (85–92% solids)
Removal of chlorides for regulatory compliance
Marketable byproduct that reduces waste and generates revenue
For FGD service, liquid ring vacuum pumps remain the industry standard due to their tolerance for moisture, scale, and challenging process conditions. Proper material selection (duplex stainless steel for corrosive service), VFD control for energy efficiency, and regular maintenance are key to long-term reliability.
As coal plants face tighter emissions limits and water discharge regulations, optimizing FGD vacuum systems is not only an environmental requirement—it is a competitive necessity.
Q: What is the typical vacuum level for gypsum dewatering in FGD?
A: Operating vacuum is typically 300–500 mbar absolute (approximately 15–26 inHg vacuum). Deeper vacuum produces drier gypsum cake but increases energy consumption and may blind the filter cloth.
Q: Why do FGD vacuum pumps require special materials?
A: FGD process water and gypsum slurry contain chlorides, fluorides, and low pH. Standard cast iron pumps corrode rapidly, leading to frequent failure. Duplex stainless steel (2205) or rubber-lined construction is recommended for aggressive service.
Q: Can dry vacuum pumps replace liquid ring pumps in FGD service?
A: Yes, in some applications. Dry screw pumps offer higher efficiency and no seal water consumption. However, they are less tolerant of liquid carryover and scale. For new FGD plants with tight water budgets and stable operation, dry screw pumps are a viable option.
Q: How does FGD vacuum dewatering affect gypsum marketability?
A: Gypsum containing more than 200–300 ppm chlorides is generally rejected by wallboard manufacturers. Achieving >90% cake solids and low chlorides requires well-maintained vacuum systems with effective cake washing.
Q: What causes loss of vacuum on an FGD belt filter?
A: Common causes: air in-leakage through belt seals, clogged filter cloth, worn vacuum pump, seal water too warm, or scale buildup in the vacuum box.
