In a limestone-gypsum wet flue gas desulfurization (FGD) system, the FGD gypsum cyclone is a core dehydration unit, performing the crucial function of "primary pre-concentration" of the gypsum slurry. This directly impacts the quality of the desulfurized gypsum, system operating efficiency, and energy consumption.

The core process of a limestone-gypsum wet FGD system is: flue gas desulfurization reaction → gypsum slurry formation → gypsum dehydration and recovery → wastewater treatment. After exiting the absorption tower, the gypsum slurry contains only 10%-20% solids (the remainder is water, unreacted limestone, and minor impurities). This slurry cannot directly enter subsequent dehydration equipment (such as a vacuum belt conveyor). Instead, it must first pass through the FGD gypsum cyclone for "pre-concentration + coarse separation." Therefore, it is considered a primary dehydration unit, serving as the critical bridge between the "slurry reaction" and "gypsum recovery." In the entire limestone-gypsum wet process, the FGD gypsum cyclone's workflow is closely linked to upstream and downstream equipment. The specific logic is as follows:

1. Upstream Feed: Absorption Tower and Gypsum Discharge Pump

In the absorption tower, limestone slurry (CaCO₃) reacts with SO₂ in the flue gas to produce calcium sulfite (CaSO₃). This is then oxidized by the oxidation blower to gypsum (CaSO₄・2H₂O), resulting in a gypsum slurry with a solids content of 10%-20%. When the slurry level reaches the set point, the gypsum discharge pump (typically a wear-resistant centrifugal pump) delivers the slurry tangentially to the FGD gypsum cyclone at a pressure of 0.2-0.4 MPa. (A single system typically features multiple cyclones in parallel to increase throughput.)

2. Inside the Cyclone: Centrifugal Separation and Dual-Path Output

After entering the cyclone, the slurry undergoes "solid-liquid classification + concentration separation" under the action of strong centrifugal force (speeds can reach 1000-3000 r/min):

· Bottom flow (concentrated gypsum): Larger gypsum crystals (primarily CaSO₄・2H₂O, typically > 40μm) and a small amount of unreacted limestone particles (small amounts) are centrifuged toward the vessel walls. They spiral downward along the conical wall and are discharged from the bottom "sand trap." The solids content is increased to 40%-60% and then directly conveyed to the downstream vacuum belt conveyor (secondary dehydration equipment) for deep dehydration. Overflow (slurry return): Small impurities (such as fly ash and fine gypsum particles (<20μm)) and excess water are discharged from the top "overflow pipe" along with the internal vortex flow. The solids content is only 5%-8%. After being collected in the overflow tank, the overflow returns to the absorber through the return pipe, achieving the recycling of "water, a small amount of limestone, and fine gypsum particles," reducing resource waste and wastewater discharge.

Downstream linkage: Secondary dehydration and wastewater diversion

The cyclone underflow (40%-60% solids content) enters a vacuum conveyor, where it is vacuum-extracted and dehydrated, ultimately producing a finished desulfurized gypsum product with a solids content greater than 90% (which can be used as a building material raw material, such as gypsum board and cement retarder).

If the system is equipped with a "wastewater cyclone," a portion of the cyclone overflow will be diverted to the wastewater treatment unit (to remove Cl⁻ and heavy metals), preventing impurity accumulation in the absorber and ensuring desulfurization reaction efficiency. Abnormal operation of the FGD gypsum cyclone can directly lead to system failure (such as vacuum conveyor blockage, excessive gypsum moisture content, and slurry imbalance in the absorber). Common problems and optimization solutions are as follows:

Problem 1: Low underflow solids (<35%)

· Cause: Oversized grit nozzle, insufficient feed pressure (<0.2 MPa), small gypsum crystal size in the slurry (<30 μm);

· Optimization: Replace the grit nozzle with a smaller diameter, increase the gypsum discharge pump pressure, and optimize the oxidation air volume in the absorber (to promote gypsum crystal growth).

Problem 2: High overflow solids (>10%)

· Cause: Excessive feed volume (exceeding the cyclone's processing capacity), overflow pipe blockage or misalignment;

· Optimization: Reduce the feed volume per cyclone (increase the number of cyclones in parallel), regularly clean the overflow pipe, and calibrate the overflow pipe center position.

Problem 3: Clogged Grit Nozzles

· Cause: Presence of large impurities in the slurry (e.g., fly ash agglomerates and undissolved limestone lumps);

· Optimization: Install a basket filter (5-10mm filtration accuracy) at the gypsum discharge pump inlet and regularly flush the grit nozzles (using high-pressure water backwash).

System-Level Optimization: Multiple Cyclones in Parallel + Intelligent Control

Large FGD systems (e.g., units over 300MW) typically utilize a "6-12 cyclones in parallel" design, evenly distributing the feed through distribution valves. Furthermore, an "online solids content monitor" (underflow and overflow) can be installed to adjust the feed pressure and grit nozzle diameter in real time, achieving "unmanned operation + dynamic optimization" and reducing O&M costs.