Application of Limestone Vertical Mill in Power Plant Flue Gas Desulfurization

Introduction

Flue gas desulfurization (FGD) is a critical process in coal-fired power plants to reduce sulfur dioxide (SO₂) emissions and comply with stringent environmental regulations. Among various FGD technologies, wet limestone-gypsum desulfurization is the most widely adopted due to its high efficiency, reliability, and cost-effectiveness. A key requirement for this process is the preparation of fine limestone powder, typically with a fineness of 250 mesh (61 μm) or finer, to ensure rapid dissolution and high reactivity in the absorber. Limestone vertical mills have emerged as the preferred grinding equipment for this application, offering superior grinding efficiency, drying capability, and operational flexibility. This article explores the technical advantages and practical applications of limestone vertical mills in power plant FGD systems, with a focus on two advanced products: the LM Series Vertical Roller Mill and the LUM Ultrafine Vertical Roller Mill.

Importance of Limestone Grinding in FGD

In wet FGD systems, limestone (calcium carbonate, CaCO₃) is ground and mixed with water to form a slurry. The slurry is sprayed into the absorber tower, where it reacts with SO₂ in the flue gas to produce calcium sulfite (CaSO₃), which is then oxidized to gypsum (CaSO₄·2H₂O). The particle size of the limestone powder directly affects the reaction rate and desulfurization efficiency. Finer particles provide a larger surface area for reaction, leading to higher SO₂ removal efficiency and better limestone utilization. Typically, a fineness of 90% passing 250 mesh (61 μm) or 95% passing 325 mesh (44 μm) is required. Therefore, selecting the right grinding mill is crucial for optimizing FGD performance and reducing operating costs.

Why Vertical Mills Are Preferred for FGD

Vertical roller mills (VRMs) have become the dominant technology for limestone grinding in FGD applications due to several inherent advantages over traditional ball mills:

• High Grinding Efficiency: VRMs use a grinding table and rollers to crush and grind material in a bed, consuming 30-40% less energy than ball mills for the same fineness.
• Integrated Drying: Hot air from the power plant can be used to dry the limestone while grinding, eliminating the need for separate drying equipment.
• Compact Layout: The vertical arrangement allows a smaller footprint and lower civil construction costs.
• Product Fineness Control: Dynamic classifiers provide precise particle size control, ensuring consistent product quality.
• Low Maintenance: Wear parts are designed for long life, and maintenance can be performed quickly with hydraulic systems.

Among the various vertical mill designs, the LM Series and LUM Series from our company have been specifically engineered to meet the demanding requirements of FGD systems.

Product Recommendation: LM Series Vertical Roller Mill

The LM Series Vertical Roller Mill is a flagship product designed for high-capacity grinding of limestone in power plant FGD. With a capacity range of 3 to 250 t/h and output fineness adjustable from 30 to 325 mesh (up to 600 mesh for special models), it is ideally suited for large-scale FGD systems. Its key technical advantages include:

• Integrated Design: Crushing, grinding, and classification are combined in one unit, reducing floor space by 50% and enabling outdoor installation, which cuts infrastructure costs by 40%.
• Low Operating Cost: The non-contact design between rollers and table minimizes wear, extending wear part life by three times. Energy consumption is 30-40% lower than ball mill systems.
• Intelligent Control: An expert-level auto-control system supports remote/local switching and real-time monitoring, reducing manual intervention.
• Environmental Compliance: Fully sealed negative pressure operation ensures dust emission is below 20 mg/m³, and noise levels are reduced.

For FGD applications, the LM Series can be equipped with a hot air generator or connected to the plant’s flue gas duct to utilize waste heat for drying. The mill’s robust construction and wear-resistant materials ensure reliable operation even with varying limestone quality.

Product Recommendation: LUM Ultrafine Vertical Roller Mill

For FGD systems requiring ultrafine limestone powder (325-2500 mesh), the LUM Ultrafine Vertical Roller Mill is an excellent choice. It is designed for high-efficiency grinding of materials with a feed size of 0-20mm and can produce products with fineness up to 2500 mesh. Its main technical features include:

• Efficient Grinding: Unique roller and liner curves improve grinding efficiency and reduce energy consumption.
• Precise Classifying: Multi-rotor technology ensures no coarse particles in the finished product, which is critical for maximizing desulfurization reactivity.
• Smart Control: PLC automation ensures stable operation and easy parameter adjustment.
• Environmental: Negative pressure operation prevents dust leakage, maintaining a clean working environment.

With models like LUM1525 (1.6-11.5 t/h) and LUM1836 (2.3-15 t/h), this mill is suitable for medium-scale FGD systems or as a fine grinding stage in larger plants. Its ability to produce ultrafine powder can enhance SO₂ removal efficiency and reduce limestone consumption.

Working Principle of LM Series in FGD

In the LM Series vertical mill, the main motor drives the grinding table via a reducer. Limestone is fed through the central feed port and spreads evenly on the table due to centrifugal force. Grinding rollers apply hydraulic pressure to crush the material bed. Hot air enters the mill from the air inlet and carries the ground particles to the dynamic classifier. Fine particles are classified as finished product and collected by a bag filter or electrostatic precipitator, while coarse particles fall back to the table for re-grinding. The dried and ground limestone powder is then transported to the slurry preparation system.

Case Study: Application in a 600 MW Power Plant

A 600 MW coal-fired power plant in China upgraded its FGD system by replacing two ball mills with a single LM220K vertical roller mill. The LM220K has a capacity of 36-105 t/h and produces limestone powder with 90% passing 325 mesh. The results were remarkable: energy consumption for grinding dropped by 35%, maintenance costs decreased by 40%, and the SO₂ removal efficiency improved from 95% to 98.5%. The compact layout also freed up space for other equipment. This case demonstrates the significant benefits of using advanced vertical mills in FGD retrofits.

Conclusion

Limestone vertical mills are indispensable in modern power plant FGD systems, offering high efficiency, reliability, and environmental compliance. Our company’s LM Series and LUM Series vertical roller mills are engineered to meet the specific demands of limestone grinding for desulfurization. Whether you need high-capacity grinding or ultrafine powder production, these mills provide superior performance and cost savings. By choosing the right vertical mill, power plants can enhance SO₂ removal efficiency, reduce operating costs, and contribute to a cleaner environment.