The mixing or agitation processes is typically solid suspension in applications like in thermal power plants for fly ash handling systems; effluent or pollutant particles in water treatment; lime slurry mixing in flue gas desulphurization (FGD). Agitators are used for the solid suspension of fly ash or bottom ash, effluent and pollutant particles.
Ash Handling Systems
Fly ash and bottom ash from the boiler are collected into the ash silos. From the silos, ash is conveyed to Ash Retention Tank for homogenization of ash with water for pumping to Ash Ponds located a few kilometres from the plant. The ash can be used for brick making, cement, road etc. This system is also called High concentration slurry disposal (HCSD) system. It is deployed initially where water is in shortage.
In these systems, agitator design is critical as the ash is fed into the tank from agglomerates which then floats on the liquid surface. When agglomerates get enough wet, they then drops suddenly inside the tank and can damage the agitator parts by creating unbalanced forces. Breaking such agglomerates, in the initial stage, is very important.
Top MP4 - Pitch Blade Turbines help in breaking the agglomerates and suck the floating solid into the flow generated by the bottom impeller. Bottom hydrofoil impeller helps to maintain uniform solid concentration at the bottom so that the pump doesn’t get choked. The material of construction of the impeller is used to C–45 with case hardening up to 400 BHN. Case hardening is done to increase the life of impeller as the ash are highly abrasive.
Wastewater is typically recycled for reuse in plant. Top entry agitators, side entry agitators and static mixers are widely used in these applications all of which can be designed and offered based on specific requirements.
Flue Gas Desulfurization (FGD)
FGD is a set of technologies used to remove sulfur oxide (SOx) from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes. FGD applications are both wet and dry. Agitators are installed in wet FGD plants.
Absorber is the heart of an FDG system. Untreated Gas, Oxidation Air, Wash water, Gypsum Cake and limestone are fed into the absorber column. After treatment of untreated gas with limestone, and oxidation of air, treated gas, gypsum cake and water are released from the absorber. As the tower heights are considerable, top entry agitators are not recommended for this case. Multiple side entry angled agitators are designed to get off- bottom suspension. Since the slurry is a combination of gypsum and limestone the agitator should generate enough flow so that solid particles do not get settled down.
Other applications include utilities for absorber (Limestone slurry, Mill slurry, Wastewater booster tank, Gypsum Dewatering Feed tank, Hydrocyclone overflow tanks). Top entry agitators with MH3/MF3 - Hydrofoil are preferred and suspension is the only criterion for designing the agitator.