A new FGD limestone grinding project should be planned from the absorber’s maximum reagent demand, the actual limestone source, and the required operating availability. The mill selection comes after these process inputs are defined—not before—because capacity, fineness, storage, slurry preparation, and redundancy must all match the maximum sulfur dioxide removal duty.

Modern wet limestone FGD systems are commonly designed for high SO₂ removal, with advanced systems capable of more than 95% removal. Their limestone preparation systems must deliver consistent, reactive powder, commonly near 95% passing 325 mesh, while maintaining enough inventory to protect the absorber during grinding interruptions.

1. Define the Design Basis

The design basis is the technical foundation of the entire project. It should use the worst credible operating condition rather than annual average values. If the plant may burn several coal types, use the highest expected sulfur case or develop separate design cases for each fuel range.

The FGD system should be designed around maximum and minimum gas flow, SO₂ concentration, flue-gas temperature, dust loading, limestone quality, emission requirement, and critical-equipment redundancy.

2. Calculate Limestone Demand

The required grinding capacity starts with the amount of SO₂ to be removed at maximum duty. The basic wet limestone reaction is:

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

One tonne of sulfur creates approximately two tonnes of SO₂. One tonne of SO₂ removed theoretically requires 1.5625 tonnes of pure CaCO₃. Actual consumption must then be corrected for limestone purity and the operating Ca/S molar ratio.

SO₂ generation (t/h) = Coal consumption (t/h) × sulfur content × 2

Removed SO₂ load (t/h) = SO₂ generation × required removal efficiency

Required limestone powder (t/h) = Removed SO₂ load × 1.5625 × Ca/S ratio ÷ CaCO₃ purity

For preliminary planning, a Ca/S molar ratio around 1.03 and limestone containing 90% CaCO₃ can be used as initial assumptions. Final values should be confirmed through FGD process design and limestone reactivity testing.

For example, consider a project with 250 t/h coal consumption, 1.0% sulfur, 98% SO₂ removal, a Ca/S ratio of 1.03, and 90% CaCO₃ limestone:

Sulfur load = 250 × 0.01 = 2.50 t/h sulfur

SO₂ generated = 2.50 × 2 = 5.00 t/h SO₂

SO₂ removed = 5.00 × 0.98 = 4.90 t/h SO₂

Required limestone powder = 4.90 × 1.5625 × 1.03 ÷ 0.90 = 8.76 t/h

The grinding plant should not be selected at exactly 8.76 t/h. It needs margin for changing limestone quality, equipment wear, maintenance periods, and short-term high sulfur conditions.

3. Select the Process Route

The main process route should be selected after reviewing limestone moisture, feed size, required fineness, water availability, site layout, and the plant’s maintenance strategy.

For a dry grinding route, the process should normally include raw limestone crushing, controlled feed storage, fine grinding, classification, dust collection, finished-powder storage, accurate powder dosing, slurry preparation, slurry storage, and duty-and-standby pumps.

Dry grinding provides a useful operating buffer because powder can be stored in a silo before slurry preparation. This allows the grinding mill to operate independently from short-term changes in absorber demand and gives the plant more flexibility during maintenance.

4. Select the Grinding Mill

The selected mill must meet the guaranteed output at the specified FGD fineness using the actual limestone source. The mill should not be selected from a nominal capacity range alone. Feed moisture, Bond Work Index, hardness, silica content, feed size, target particle-size distribution, and operating hours can significantly change the achievable capacity.

The capacity calculation should include an availability correction:

Required mill capacity = Maximum limestone consumption × production margin ÷ expected mill availability

For example, if maximum consumption is 8.76 t/h, the project requires a 10% production margin, and expected mill availability is 90%:

Required mill capacity = 8.76 × 1.10 ÷ 0.90 = 10.71 t/h

The selected mill should therefore guarantee at least about 11 t/h of qualified limestone powder at the required FGD fineness under actual feed conditions.

5. Plan Storage and Redundancy

Grinding capacity alone cannot protect FGD availability. The project needs a deliberate reserve strategy covering raw limestone, finished powder, slurry inventory, pumps, critical feeders, and major maintenance periods.

For reference, a wet FGD system upgrade study notes that increasing SO₂ removal can require higher limestone consumption, finer grinding, and a review of reagent preparation and dewatering capacity. The absorber, reagent preparation, and solids-handling systems should therefore be planned as one connected process rather than as separate packages.

Finished-powder silo capacity can be estimated using:

Usable powder storage (t) = Maximum limestone consumption (t/h) × required backup time (h)

For a plant consuming 11 t/h of limestone powder, a 24-hour operating reserve requires at least 264 t of usable powder inventory. The physical silo must be larger because it also needs allowance for dead stock, freeboard, and bulk-density variation.

6. Complete the Engineering Package

Once the design basis, material balance, process route, mill selection, and reserve strategy are defined, prepare a complete engineering package before procurement. This avoids mismatches such as a correctly sized mill feeding an undersized silo, powder conveyor, slurry tank, or pump.

The package should include:

• Design basis document covering boiler load, fuel range, SO₂ duty, limestone quality, required fineness, and availability target.

• Representative limestone test report covering CaCO₃, MgO, SiO₂, moisture, hardness, abrasiveness, bulk density, and reactivity.

• FGD limestone consumption calculation for normal, maximum, and future operating cases.

• Process flow diagram showing crushing, grinding, classification, collection, storage, slurry preparation, and absorber feed.

• Material balance and slurry balance for each operating case.

• Equipment list with capacity, duty, standby requirement, materials of construction, and power demand.

• General arrangement drawing with access routes, maintenance space, lifting points, and truck or rail unloading arrangements.

• Electrical load list, motor-control philosophy, variable-frequency-drive scope, and emergency power requirements.

• Control narrative covering mill feed, classifier control, powder-silo level, slurry density, tank level, and absorber-demand response.

• Dust collection, ventilation, drainage, fire protection, noise control, and safety-access design.

• Commissioning plan with performance-test requirements for capacity, fineness, energy consumption, and slurry quality.

• Critical-spare-parts list and preventive-maintenance plan for grinding parts, classifier components, filters, fans, feeders, and pumps.

A new FGD limestone grinding project succeeds when the whole reagent-preparation chain is sized from real plant conditions: limestone quality, peak SO₂ load, required fineness, operating reserve, and maintenance philosophy. For large, centralized supply systems, the LM Vertical Mill provides the appropriate integrated grinding route. For medium-capacity projects, the MTW European Mill offers a practical fine-powder solution. In both cases, the mill, storage system, slurry preparation tanks, conveying equipment, and pumps should be engineered as one continuous system.