Desulfurization Limestone Raw Material Vertical Roller Mill Process and Configuration

Introduction to Flue Gas Desulfurization (FGD) and Limestone Preparation

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. The wet limestone-gypsum process is the most prevalent FGD method due to its high efficiency, reliability, and cost-effectiveness. The heart of this process is the chemical reaction between sulfur dioxide and a limestone (CaCO₃) slurry. The quality of the limestone powder, specifically its fineness and reactivity, is paramount to the efficiency and economics of the entire desulfurization system. This is where the selection of the optimal grinding equipment becomes crucial.

Vertical Roller Mills (VRMs) have emerged as the industry standard for preparing limestone powder for FGD applications. Their superior energy efficiency, ability to handle moist feed, and precise control over product fineness make them vastly superior to traditional ball mill systems for this specific purpose.

The Vertical Roller Mill: Ideal Technology for Limestone Grinding

Core Working Principle

A Vertical Roller Mill for limestone grinding operates on the principle of bed comminution. The process can be broken down into several key stages:

1. Feed Introduction: Crushed limestone feedstock (typically ≤50mm) is fed onto the center of a rotating grinding table via a screw feeder.
2. Grinding: The material moves outwardly under centrifugal force and passes under the grinding rollers. Hydropneumatic springs or hydraulic cylinders exert significant pressure, forcing the rollers against the bed of material on the table, crushing it through compression and shear forces.
3. Drying: Simultaneously, hot gas (often waste heat from the plant) is introduced into the mill housing through a nozzle ring. This hot gas fluidizes the ground particles, drying the material and transporting the finer particles upwards to the classifier.
4. Classification: The gas-powder mixture enters an integrated, high-efficiency dynamic classifier located at the top of the mill. The classifier rotor, with its adjustable speed, acts as a precise size selector. Particles that are too coarse are rejected by the classifier blades and fall back onto the grinding table for further size reduction.
5. Product Collection: The fine powder that meets the target fineness (typically 90% passing 45μm or 325 mesh for FGD) exits with the gas stream to a downstream baghouse or electrostatic precipitator for collection. The cleaned gas is then vented or recirculated.

Why VRMs are Superior for FGD Limestone

• High Energy Efficiency: VRMs consume 30-50% less energy than ball mills. This is because grinding occurs by pressure rather than impact, and the integrated classifier eliminates the need for external conveying and classifying equipment.
• Excellent Drying Capability: The ability to use hot gases allows VRMs to handle feed moisture levels of up to 15-20% without the need for a separate, energy-intensive dryer.
• Precise Particle Size Control: The dynamic classifier provides instant and precise control over product fineness, allowing operators to easily adjust the fineness to optimize SO₂ removal efficiency and limestone utilization.
• Compact Design and Low Noise: VRMs have a much smaller footprint than traditional ball mill circuits and operate at significantly lower noise levels.

Key Process Considerations and Mill Configuration

Target Product Specification

For effective FGD, the ground limestone must have a very high surface area to facilitate rapid dissolution and reaction. The standard industry specification is a minimum of 90% of the particles passing 44 microns (325 mesh). Some advanced systems may require even finer grinds. The VRM’s classifier is perfectly suited to achieve and maintain this tight specification consistently.

System Configuration

A complete FGD limestone grinding system based on a VRM typically includes:

1. Feed Hopper and Belt Feeder: For controlled feed to the mill.
2. Vertical Roller Mill: The core grinding unit with its motor, grinding elements, and classifier.
3. Hot Gas Generator: To provide the necessary heat for drying if sufficient waste heat is not available.
4. Main Fan: Creates the gas flow through the mill system for material transport and drying.
5. Product Collection System: A high-efficiency pulse-jet baghouse is most common, ensuring dust emissions are well below regulatory limits (<20 mg/Nm³).
6. PLC Control System: Automates the entire process, monitoring key parameters like mill power, pressure drop, temperature, and fan speed to ensure optimal and stable operation.

Recommended Product: LM Series Vertical Roller Mill

For large-scale FGD limestone preparation projects, our LM Series Vertical Roller Mill represents the pinnacle of efficiency and reliability. Engineered specifically for mineral grinding applications, the LM mill integrates crushing, grinding, drying, classifying, and conveying into a single, compact unit.

The LM series is perfectly suited for producing the high volumes of fine limestone powder required by modern power stations. Its key advantages include:

• Low Operating Costs: The non-contact design of the grinding elements (rollers and table) and the use of high-wear-resistant materials extend service life dramatically, reducing maintenance downtime and cost. Its energy consumption is 30-40% lower than traditional ball mill systems.
• Intelligent Control: An expert automatic control system allows for remote operation and real-time monitoring and adjustment of all process parameters, ensuring the product consistently meets the required fineness with minimal manual intervention.
• Proven Environmental Compliance: The mill operates under full negative pressure, ensuring no dust leakage. Combined with a high-efficiency baghouse, particulate emissions are guaranteed to be below the strictest international standards.

For example, the LM190K model, with a grinding table diameter of 1900mm and a main motor power of 500kW, is capable of processing 23-68 tons of limestone per hour to the desired FGD fineness, making it an ideal choice for mid-to-large-scale power plants.

Conclusion

The selection of the right grinding technology is a fundamental decision in designing an efficient and economical FGD system. The Vertical Roller Mill has proven itself to be the superior solution for preparing limestone reagent, offering unmatched energy efficiency, precise product quality control, and operational reliability. For any new project or retrofit aiming to reduce emissions and operating costs, investing in a modern VRM system, such as our robust LM Series, is the most strategic and effective choice. Its ability to deliver a consistently high-quality product while minimizing energy use and environmental impact ensures compliance and maximizes the return on investment over the entire lifecycle of the FGD plant.