Power Plant Water Purification: Boiler Purity and Zero Liquid Discharge

In thermal power and nuclear power plants, boiler feed water treatment is the most critical part of the entire water purification system. Ultra-supercritical units require the silica content in the feed water to be below 10 μg/L, and the hardness to be virtually zero. The auxiliary equipment typically employs a four-stage demineralization process of “disc filter—ultrafiltration—two-stage reverse osmosis—packed bed ion exchange or continuous electrodeionization .” Among these, EDI units have become the preferred choice for the green retrofitting of new power plants because they generate no acid or base waste during operation. The core difficulty lies in the fact that the resin in the condensate polishing mixed bed must withstand high temperatures and repeated regeneration. Any trace ion leakage will concentrate inside the steam drum and superheater tube walls, subsequently causing severe corrosion problems. Therefore, this demands that the system effluent quality must always be maintained at the ultimate purity level.

Circulating cooling water treatment is a key area for power plants to achieve water conservation and emission reduction. When the circulating cooling water operates under high cycles of concentration, the plant needs to dose enhanced corrosion and scale inhibitor formulations and deploy electrochemical descaling equipment for side-stream softening treatment, thereby significantly reducing the blowdown volume. The difficulty in implementing high cycle operation lies in the fact that the threshold effect of scale inhibitors can be easily broken under varying water quality conditions, which leads to a sudden increase in scaling risk. Therefore, dynamic adjustment must be made by relying on online water quality monitoring and automatic chemical dosing systems. Meanwhile, the sterilization and algae control of the cooling tower is equally important, its purpose being to prevent the proliferation of Legionella and the clogging of biological slime, thereby ensuring that the condenser maintains a high vacuum and the unit’s thermal efficiency.

The terminal zero liquid discharge of flue gas desulfurization (FGD) wastewater is a major intractable challenge in the current power environmental protection field. The desulfurization slurry collects nearly all the heavy metals and suspended solids from the coal combustion. During treatment, it must first undergo chemical precipitation and clarification through the “triple-tank” process to remove heavy metals and fluorides, and then be sent to low-temperature flash evaporation or bypass flue gas evaporation equipment for solidification treatment, thus completely eliminating any wastewater discharge. With the large-scale integration of new energy into the grid, coal-fired power units need to frequently participate in deep peak shaving and start-stop operations; therefore, the water purification equipment must possess extremely strong load-adaptive capacity. This means that the integrated membrane system and evaporation crystallization unit must maintain a stable operation state without scaling or fouling even under partial load conditions. This is the major integrated control challenge faced by the water purification system during the transition of coal-fired power to a peak-shaving power source.