Flue Gas Desulfurization Limestone Grinding Mill: Raymond Mill, Vertical Roller Mill & More

Flue Gas Desulfurization Limestone Grinding Mill: Raymond Mill, Vertical Roller Mill & More

Introduction: The Critical Role of Limestone Grinding in FGD

Flue Gas Desulfurization (FGD) is a cornerstone technology for reducing sulfur dioxide (SO₂) emissions from power plants and industrial facilities, a key contributor to acid rain and air pollution. Among various FGD methods, the wet limestone-gypsum process is the most widely adopted globally due to its high efficiency, reliability, and byproduct usability. At the heart of this process lies a critical preparatory step: the grinding of raw limestone into a fine, reactive slurry. The choice of grinding equipment directly impacts the desulfurization efficiency, operational cost, and overall plant economics. This article explores the primary grinding technologies—Raymond Mill, Vertical Roller Mill, and others—and their suitability for FGD limestone preparation.

1. The FGD Process and Limestone Slurry Specifications

In a typical wet limestone FGD system, a slurry of finely ground limestone (CaCO₃) is sprayed into an absorber tower. The SO₂ in the flue gas reacts with the limestone to form calcium sulfite, which is then oxidized to produce marketable gypsum (CaSO₄·2H₂O). The reactivity of the limestone is highly dependent on its particle size distribution (PSD). Generally, a fineness of 90% passing 325 mesh (44 μm) or finer is required to ensure rapid dissolution and complete reaction with SO₂. This stringent requirement places significant demands on the grinding mill, which must deliver consistent, high-volume production of ultra-fine powder with optimal energy consumption.

2. Key Grinding Technologies for FGD Limestone

Several milling technologies are employed for limestone grinding, each with distinct operational principles and performance characteristics.

2.1 Raymond Mill (Pendulum Roller Mill)

The traditional Raymond Mill, or its modern evolution like the MTW Series European Trapezium Mill, operates on a pendulum roller principle. Centrifugal force causes rollers to swing outward and press against a stationary grinding ring, crushing the material fed by a central shovel.

• Advantages for FGD: Well-established technology, relatively low initial investment, capable of producing powder in the 30-325 mesh range suitable for some FGD applications. Modern versions like the MTW series feature improved wear resistance and transmission efficiency.
• Considerations: For high-capacity plants or requirements for ultra-fine powder (beyond 325 mesh), its energy efficiency may be lower compared to more modern vertical mills. It is often an excellent choice for mid-scale or retrofit projects.

2.2 Vertical Roller Mill (VRM)

The Vertical Roller Mill, such as the LM Series, has become the industry standard for large-scale FGD limestone grinding. Material is fed onto a rotating table and crushed under hydraulic pressure applied by grinding rollers. A hot gas stream dries and transports the ground powder to an integrated classifier.

• Advantages for FGD: Superior energy efficiency (30-50% less power than ball mills), excellent drying capability (can handle limestone with some moisture), compact footprint, and integrated grinding-drying-classification. It excels at producing the consistent, fine powder required for high-efficiency FGD systems.
• Considerations: Higher initial investment but lower lifetime operating costs. Requires more sophisticated maintenance expertise.

For FGD applications, our LM Series Vertical Roller Mill is a top-tier solution. Its integrated design reduces plant footprint by 50%, while its low operating cost—achieved through a non-contact grinding design that triples wear part life and reduces energy consumption by 30-40% compared to traditional systems—makes it highly economical. The mill’s intelligent control system ensures stable operation and produces the precise fineness (30-325 mesh, up to 600 mesh for special models) critical for effective SO₂ absorption.

2.3 Ball Mill

The traditional ball mill grinds material via impact and attrition as steel balls tumble inside a rotating cylinder.

• Advantages for FGD: Extreme reliability, ability to produce very fine powder, and can handle wet grinding directly to produce slurry. Suitable for situations where limestone feed moisture is very high.
• Considerations: Highest energy consumption among the options, significant noise, large footprint, and higher wear rates on liners and media. Often used in older installations or specific wet-grinding circuit configurations.

2.4 Ultrafine Grinding Mills

For advanced FGD systems or applications requiring exceptionally high reactivity, ultrafine mills like the SCM Series are employed to produce powder in the 325-2500 mesh range.

• Advantages for FGD: Produces ultra-fine powder that can significantly enhance reaction kinetics, potentially allowing for lower limestone consumption or a smaller absorber design.
• Considerations: Higher specific energy consumption for ultrafine grinding. Typically used for specialized applications or as a secondary polishing mill.

When ultra-fine limestone powder is specified, our SCM Series Ultrafine Mill delivers unparalleled performance. With an output fineness adjustable between 325 and 2500 mesh (45-5μm), it ensures maximum limestone surface area for SO₂ capture. Its high-efficiency classification system guarantees no coarse powder mixing, resulting in a uniform slurry that optimizes the desulfurization chemical process. Despite its fine output, it maintains high efficiency, offering capacity 2x that of jet mills while consuming 30% less energy.

3. Selection Criteria for FGD Limestone Mills

Choosing the right mill involves a holistic analysis of several factors:

• Plant Capacity & Limestone Throughput: VRMs are ideal for large, base-load power plants (>200 MW). Raymond/MTW mills suit medium-scale facilities, while ball mills may be considered for specific wet grinding scenarios.
• Required Product Fineness & PSD: VRMs and modern Raymond mills reliably achieve 90% < 325 mesh. Ultrafine mills are for special < 10 μm requirements.
• Moisture Content of Feed: VRMs have superior drying capacity (can handle up to 15-20% moisture with hot gas). Ball mills can handle slurry feed directly.
• Total Cost of Ownership (TCO): While VRMs have a higher CAPEX, their lower OPEX (energy and wear parts) often results in a lower TCO over a 10-15 year lifespan.
• Space Constraints: VRMs have a significantly smaller footprint than ball mills of equivalent capacity.
• Environmental Compliance: Modern mills like the LM Series and SCM Series feature fully sealed negative pressure operation and integrated high-efficiency bag filters, ensuring dust emissions and noise levels meet stringent standards.

4. Conclusion

The efficiency of a Flue Gas Desulfurization system is fundamentally linked to the performance of its limestone grinding circuit. While several technologies are available, the Vertical Roller Mill, particularly advanced series like the LM, stands out as the most efficient and economical solution for large-scale, modern FGD plants due to its integrated design, low energy consumption, and precise control. For applications demanding standard fineness with cost-effectiveness, the modernized Raymond Mill (MTW Series) remains a robust choice. Ultimately, the selection must be based on a detailed technical and economic analysis tailored to the specific project parameters, ensuring reliable, clean, and cost-effective compliance with environmental regulations for years to come.