Flue gas desulphurization (FGD) systems typically use mist eliminators to remove liquid droplets from the gas stream. Fine droplets and those generated by thermal and adiabatic condensation within the stack can result in significant quantities of liquid collecting on the walls of the absorber outlet duct and stack breach, as well as on the inside of the stack liner itself. If uncontrolled, larger droplets can form, which are then re-entrained into the stack flow and ejected from the top of the stack in a phenomenon called “stack liquid discharge” or “rainout.”
Minimizing stack discharge problems requires the installation of liquid collectors and drains at key locations inside the absorber outlet duct and stack. Alden determines the optimum design and location of these collectors and drains through physical flow modeling.
Many factors go into the design and development of an effective liquid collection system including:
- Proper choice of absorber outlet geometry
- Breaching duct aspect ratio
- Stack inlet design
- Overall gas velocity and liquid loading
When properly configured, secondary flows and recirculation zones are developed which guide the liquid films on the walls of the unit to collection troughs and drains.
If a plant is considering the installation of a WFGD system or conversion to wet stack operation, it is highly recommended that Alden be brought into the process during the early design phase. Based on our many years of experience, we can often recommend simple design changes that will result in the system being more favorable for wet operation, thus minimizing the potential for stack liquid discharge.
