Limestone Powder Specifications for Power Plant FGD - FGD Limestone Grinding Plant

In a limestone–gypsum flue gas desulfurization (FGD) system, limestone powder is not simply a bulk raw material. Its chemical composition, particle-size distribution, moisture level, and grinding consistency directly affect slurry preparation, limestone dissolution, reagent utilization, sulfur dioxide removal, gypsum quality, and long-term operating stability.

For most wet FGD applications in coal-fired power plants, the preferred feedstock is a high-calcium limestone powder with stable quality from batch to batch. The powder is mixed with water to prepare an absorbent slurry, which reacts with SO₂ in the absorber. A fine and reactive powder helps the system maintain sufficient alkalinity and reduces the amount of unreacted limestone leaving with the by-product gypsum.

Core Quality Requirements

The specification should be determined by the absorber design, coal sulfur content, target SO₂ removal rate, slurry concentration, oxidation mode, and local limestone source. Nevertheless, the following parameters are commonly used as a practical starting point for power-plant FGD projects.

Parameter	Typical FGD Requirement	Why It Matters
Calcium carbonate content	CaCO₃ ≥ 90%; commonly 92–95% or higher	Higher CaCO₃ content provides more active alkaline material for SO₂ absorption.
Calcium oxide equivalent	Usually above 45%	CaO equivalent is often used as an indicator of the available calcium content in limestone.
Particle size	Typically 90% passing 325 mesh, approximately 44 μm	Fine particles dissolve faster in the slurry and provide more surface area for reaction.
Particle-size distribution	Stable and reasonably narrow distribution	A balanced distribution improves slurry stability and avoids excessive coarse, unreacted particles.
Moisture	Preferably low and controlled	Low moisture supports storage, conveying, metering, and dry grinding efficiency.
Insoluble matter	As low as practical	Clay, silica, sand, and other insolubles can increase wear, reduce effective reagent content, and accumulate in the process.
Magnesium content	Controlled according to process design	Excessive magnesium-bearing minerals may change slurry chemistry and affect gypsum handling.
Chloride and trace impurities	Monitored according to plant requirements	Some impurities can contribute to corrosion, wastewater-treatment load, or operating complications.

Why Fineness Is Critical

Limestone reacts only after it dissolves into the circulating slurry. Reducing particle size increases the exposed mineral surface area, allowing calcium carbonate to dissolve more rapidly and participate more effectively in the absorption reaction.

In a wet limestone FGD absorber, the main reaction pathway can be simplified as follows:

CaCO₃ + SO₂ + ½O₂ + 2H₂O → CaSO₄·2H₂O + CO₂

The final product is generally gypsum, provided that the oxidation system operates properly. If limestone particles are too coarse, they may remain partially unreacted, increasing limestone consumption and reducing gypsum purity. Coarse particles can also settle in slurry tanks, pipelines, or recirculation systems, making operation less stable.

A common target is 90% passing 325 mesh. However, the exact requirement should always follow the power plant’s engineering specification. Some installations use a coarser feed where residence time and absorber conditions are favorable, while other high-efficiency systems require a finer and more tightly controlled powder.

Balancing Fineness and Operating Cost

Grinding limestone more finely is not always the best solution. Extremely fine powder can improve dissolution, but it also increases grinding energy consumption and may create handling challenges if the powder becomes too light or dusty. The objective is to achieve the fineness needed for reliable SO₂ removal without producing unnecessary ultrafine material.

A well-designed FGD limestone grinding system should therefore deliver:

Consistent powder fineness at the mill outlet.
Low coarse-particle content to improve reagent utilization.
Stable throughput matched to the power plant’s peak limestone demand.
Low moisture in the finished powder for smooth storage and pneumatic conveying.
Reliable classification to prevent oversized particles from entering the slurry-preparation system.
Flexible adjustment capability when coal quality, sulfur content, or boiler load changes.

Recommended Grinding Equipment

For power-plant FGD limestone powder production, the grinding system should be selected according to required capacity, feed size, site conditions, finished-powder target, and automation requirements.

For medium and large projects requiring continuous production, the LM Vertical Coal Mill from Liming Heavy Industry can be configured for limestone grinding. Its integrated grinding, drying, classification, and conveying functions support a compact process layout. The vertical design is particularly suitable where a large production capacity, stable operation, and centralized control are required.

For projects that need flexible fineness adjustment, efficient classification, and a mature dry-powder process, the MTW European Trapezium Grinding Mill from Liming Heavy Industry is a practical option. It is suitable for producing fine limestone powder for FGD slurry preparation, especially where the required product is around the common 325-mesh range.

Both mill types should be paired with suitable crushing, feeding, dust collection, finished-powder storage, and conveying equipment. The final configuration should ensure that the powder delivered to the slurry tank remains stable in fineness and chemical quality throughout the operating cycle.

Feed Limestone Before Grinding

Good FGD powder starts with suitable raw limestone. Before entering the mill, limestone should be sampled and evaluated for CaCO₃ content, SiO₂, MgO, moisture, loss on ignition, and other site-specific indicators. Variation in quarry quality can be more damaging to FGD stability than small variations in mill output.

The raw stone should also be crushed to a stable feed size. Uniform feed material supports steady mill loading, reduces fluctuation in grinding pressure, and helps maintain a consistent finished-powder distribution. Oversized stone, excessive clay, and high moisture can reduce capacity and increase the risk of blockage in the feeding system.

Inspection and Acceptance Checklist

Before accepting limestone powder for an FGD unit, plant operators commonly review the following items:

CaCO₃ content and CaO equivalent.
Residue on the specified control sieve, often 325 mesh.
Particle-size distribution measured by sieve analysis or laser particle-size analysis.
Moisture content at delivery and before slurry preparation.
Silica, clay, magnesium, chloride, and other impurity levels.
Slurry density and settling behavior after mixing with water.
Limestone reactivity under the plant’s operating conditions.
Consistency between different supply batches and production periods.

High-quality limestone powder is a foundation for efficient FGD operation. A stable high-calcium feed, suitable 325-mesh-level fineness, low impurity content, and properly selected grinding equipment allow the desulfurization system to operate with better reagent utilization, smoother slurry preparation, and more consistent gypsum production. Industry references commonly identify CaCO₃ above 90% and fine grinding near 325 mesh as important starting points for wet limestone FGD applications.

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