How Does Limestone/Lime-Gypsum Flue Gas Desulfurization Work? A Guide by Vertical Mill Manufacturers

Introduction to Flue Gas Desulfurization (FGD)

Flue Gas Desulfurization (FGD) is a critical technology employed by power plants and industrial facilities worldwide to remove sulfur dioxide (SO₂) from exhaust flue gases. As environmental regulations become increasingly stringent, effective SO₂ control has become mandatory for coal-fired power stations and various industrial processes. Among the various FGD technologies available, wet scrubbing systems using limestone or lime-gypsum reagents have proven to be the most efficient and widely adopted method, achieving removal efficiencies exceeding 95%.

This comprehensive guide explores the fundamental principles, chemical processes, and system components of limestone/lime-gypsum FGD systems, with particular emphasis on the crucial role of grinding equipment in preparing the absorbent materials to optimal specifications.

The Chemistry Behind Limestone/Lime-Gypsum FGD

Understanding the chemical reactions involved is essential to appreciating how limestone/lime-gypsum FGD systems effectively remove SO₂ from flue gases.

Limestone-Based FGD Chemistry

In limestone-based systems, the primary reactions occur as follows:

1. SO₂ Absorption: SO₂ from the flue gas dissolves in the slurry droplets: SO₂(g) + H₂O → H₂SO₃ (aq)
2. Neutralization: The sulfurous acid reacts with limestone (CaCO₃): H₂SO₃ + CaCO₃ → CaSO₃ + CO₂ + H₂O
3. Oxidation: Air is introduced to oxidize calcium sulfite to calcium sulfate (gypsum): 2CaSO₃ + O₂ → 2CaSO₄
4. Crystallization: The calcium sulfate precipitates as gypsum crystals: CaSO₄ + 2H₂O → CaSO₄·2H₂O (s)

Lime-Based FGD Chemistry

Lime-based systems follow a similar pathway but begin with quicklime (CaO) or hydrated lime (Ca(OH)₂):

1. Slaking (if using quicklime): CaO + H₂O → Ca(OH)₂
2. SO₂ Absorption: SO₂(g) + H₂O → H₂SO₃ (aq)
3. Neutralization: H₂SO₃ + Ca(OH)₂ → CaSO₃ + 2H₂O
4. Oxidation and Crystallization: Similar to limestone systems, producing gypsum as the final product.

Key Components of a Wet FGD System

A complete wet FGD system comprises several integrated components that work together to efficiently remove SO₂ from flue gases.

1. Absorbent Preparation System

The heart of the FGD system begins with proper preparation of the absorbent material. Limestone or lime must be ground to a specific fineness to maximize surface area and reactivity. Typically, limestone for FGD applications requires grinding to 90-95% passing 325 mesh (44 microns) or finer. This optimal particle size ensures complete dissolution and reaction with SO₂ while minimizing reagent consumption.

The grinding system typically includes crushers for primary size reduction followed by fine grinding mills. The choice of grinding equipment significantly impacts the overall efficiency and operating costs of the FGD system.

2. Absorber Tower

The absorber tower is where the actual SO₂ removal occurs. Flue gas enters the tower and comes into contact with the limestone or lime slurry. Modern FGD systems utilize various absorber designs including spray towers, tray towers, and jet bubbling reactors. Each design optimizes gas-liquid contact to enhance SO₂ absorption efficiency.

3. Reagent Feed System

This system precisely controls the feed rate of the prepared absorbent slurry to maintain optimal stoichiometric ratios relative to the SO₂ concentration in the flue gas. Automated control systems adjust feed rates based on real-time SO₂ measurements to ensure consistent performance while minimizing reagent consumption.

4. Oxidation Air System

Forced oxidation is critical in producing marketable gypsum as a byproduct. Compressed air is injected into the reaction tank to convert calcium sulfite to calcium sulfate (gypsum). Proper oxidation ensures the formation of high-quality, easily dewaterable gypsum crystals.

5. Gypsum Dewatering System

This system removes water from the gypsum slurry produced in the absorber. Hydrocyclones and vacuum belt filters typically achieve final moisture content of 10% or less, producing gypsum suitable for commercial applications in wallboard manufacturing, cement production, or agricultural uses.

The Critical Role of Grinding in FGD Efficiency

The efficiency of an FGD system is profoundly influenced by the quality of the absorbent preparation. Proper grinding achieves several crucial objectives:

Particle Size Optimization

Finer limestone particles dissolve more rapidly in the scrubber slurry, providing more surface area for reaction with SO₂. This translates to higher SO₂ removal efficiency and lower limestone consumption. The target fineness for FGD-grade limestone is typically 90-95% passing 325 mesh, with some advanced systems requiring even finer grinds.

Reactivity Enhancement

Smaller particles react more completely with SO₂, reducing the amount of unreacted limestone in the gypsum byproduct. This is particularly important for producing high-purity gypsum suitable for commercial applications.

System Reliability

Properly ground limestone with consistent particle size distribution prevents operational issues such as nozzle clogging, pipeline scaling, and pump wear, ensuring continuous, trouble-free operation of the FGD system.

Recommended Grinding Solutions for FGD Applications

Selecting the appropriate grinding equipment is paramount for optimizing FGD system performance. Based on extensive experience in supplying equipment for power plants and industrial facilities, we recommend the following solutions for FGD absorbent preparation:

MTW Series Trapezium Mill for High-Capacity Applications

For large-scale power plants requiring high throughput of limestone slurry, the MTW Series Trapezium Mill offers an ideal solution. With a capacity range of 3-45 tons per hour and the ability to produce powder in the 30-325 mesh range (adjustable to 0.038mm), this mill meets the demands of even the largest FGD systems.

The MTW Series incorporates several advanced features specifically beneficial for FGD applications:

• Anti-wear shovel blade design with combined shoveling plates reduces maintenance costs
• Curved air duct optimization minimizes air resistance and improves transmission efficiency
• Integral transmission with bevel gear achieves 98% transmission efficiency
• Wear-resistant volute structure with non-blocking design enhances air classification efficiency

For FGD systems in medium to large power plants, the MTW215G model is particularly recommended, offering a processing capacity of 15-45 tons per hour with a main motor power of 280kW. This model efficiently handles feed materials up to 50mm in size, producing the precisely controlled fineness required for optimal FGD performance.

LM Series Vertical Roller Mill for Integrated Solutions

When space constraints or integrated operations are considerations, the LM Series Vertical Roller Mill provides a compact, efficient solution for FGD absorbent preparation. With capacities ranging from 3-250 tons per hour and the ability to produce powder from 30-325 mesh (with special models achieving 600 mesh), this versatile mill adapts to various FGD requirements.

Key advantages of the LM Series for FGD applications include:

• Compact integrated design combining crushing, grinding, and classification functions, reducing footprint by 50%
• Low operating costs with non-contact grinding roller and grinding disc design, increasing wear part life by 3 times
• Energy efficiency with 30-40% lower energy consumption compared to ball mill systems
• Intelligent control system with expert automatic control supporting remote/local switching
• Environmental compliance with fully sealed negative pressure operation, dust emission < 20mg/m³

The LM220K model is especially suitable for large-scale FGD systems, offering a processing capacity of 36-105 tons per hour with an 800kW main motor. Its ability to handle feed materials up to 50mm in size while producing powder in the 170-45μm (80-325 mesh) range makes it an excellent choice for power plants seeking reliable, high-volume limestone preparation.

Operational Considerations for FGD Grinding Systems

Successful FGD operation requires attention to several key factors in the grinding process:

Moisture Content Management

Limestone feedstock typically contains surface moisture that must be managed during grinding. Proper drying capabilities or the use of grinding systems that can handle certain moisture levels are essential for consistent operation.

Wear Protection

Limestone grinding presents significant abrasion challenges. Selecting mills with appropriate wear protection, such as hardfaced grinding elements and wear-resistant liners, extends maintenance intervals and reduces operating costs.

Particle Size Distribution Control

Consistent particle size distribution is critical for FGD performance. Modern grinding systems with integrated classifiers allow precise control over the final product fineness, ensuring optimal reactivity in the absorber.

System Integration

The grinding system must be seamlessly integrated with slurry preparation, storage, and feed systems. Proper design of this integrated system ensures continuous, reliable operation of the entire FGD process.

Economic Considerations: Cost vs. Performance

When evaluating FGD grinding solutions, several economic factors must be considered:

Capital Investment

Vertical roller mills typically represent a higher initial investment compared to traditional ball mills but offer significant advantages in operating costs and footprint requirements.

Operating Costs

Energy consumption represents the largest portion of operating costs for grinding systems. Modern vertical mills can reduce specific energy consumption by 30-40% compared to ball mills, delivering substantial savings over the system lifespan.

Maintenance Requirements

Equipment selection significantly impacts maintenance costs and availability. Systems with longer wear part life and easier maintenance access reduce downtime and associated costs.

Byproduct Quality

The quality of the ground absorbent directly affects gypsum byproduct quality and marketability. Higher quality gypsum can generate additional revenue, partially offsetting FGD operating costs.

Future Trends in FGD Technology

The evolution of FGD technology continues with several emerging trends:

Ultra-Fine Grinding for Enhanced Reactivity

Research indicates that ultra-fine limestone particles (D90 < 10μm) can significantly improve SO₂ removal efficiency while reducing reagent consumption. Advanced grinding technologies capable of producing these finer particles are becoming increasingly important.

Integration with Carbon Capture Systems

As carbon capture technologies develop, FGD systems may be integrated with these processes, requiring adaptations in absorbent preparation and system design.

Digitalization and Smart Optimization

The incorporation of IoT sensors, predictive analytics, and AI-based optimization algorithms is transforming FGD operation, enabling real-time adjustment of grinding parameters based on changing fuel quality and operating conditions.

Conclusion

Limestone/lime-gypsum flue gas desulfurization represents a proven, highly effective technology for controlling SO₂ emissions from industrial sources. The efficiency and reliability of these systems depend fundamentally on the proper preparation of the absorbent material, making the selection of appropriate grinding equipment a critical decision.

Modern vertical grinding technologies, such as the MTW Series Trapezium Mill and LM Series Vertical Roller Mill, offer significant advantages in energy efficiency, particle size control, and operational reliability compared to traditional grinding systems. By partnering with experienced grinding equipment manufacturers who understand the specific requirements of FGD applications, power plants and industrial facilities can optimize their emission control systems for maximum performance and economic viability.

As environmental regulations continue to tighten globally, the importance of efficient, reliable FGD systems will only increase. Investing in advanced grinding technology today positions operators for compliance tomorrow while optimizing operational costs and maintaining competitive advantage in an increasingly environmentally conscious marketplace.