A wet stack is a chimney stack or flue that exhausts saturated, completely scrubbed flue gas downstream from a wet flue gas desulfurization (FGD) system. All recently designed and constructed wet FGD systems have installed wet stacks, allowing very significant cost savings compared to reheating. However, there are a number of technical issues that utilities must address to achieve a successful installation, whether it is a new-build or a retrofit.
FGD WET STACK FOR FLUE GAS DESULPHURIZATION WITHOUT SPITTING
Most – or all – of today's coal fired power stations are fitted with flue gas desulphurization plants ("FGD plants") to minimize their emissions of SO2. In the majority of cases, government regulators demand that the entire gas stream is desulfurized. Following its treatment in an FGD absorber, the flue gas stream is usually cool and saturated with water vapor.
Flue gas reheating
In many fossil-fired power stations built in the 1980's-2000's, especially in Europe and Southeast Asia, the flue gas stream exiting the FGD absorber was subsequently reheated to improve buoyancy of the exit plume and minimize the amount of flue gas condensate for-med in the chimney.
It is important to understand though that "reheating" is a very expensive technology, which makes a power station less efficient to run and which increases emissions, especially CO2 emissions as these are a direct function of power station efficiency.
Stack liquid discharge (SLD)
Operating FGD chimneys as wet stacks, without reheat, is economically attractive. However, it carries the risk of stack liquid discharge (SLD), also known as “spitting”. This phenomenon can be highly problematic, with acidic droplets raining down on to the environment surrounding the power station. In serious cases, such SLD can damage private property in the surrounding area such as cars and crops, as well as sensitive equipment within the power station itself.
According to the “EPRI Revised Wet Stack Design Guide”, chimneys lined with a PennguardTM borosilicate lining system can be operated safely, without spitting at velocities up to 18.3 m/s. If the Pennguard™ lining is applied in the 45 Degree High Velocity Pattern, velocities can increase to even 22.9 m/s without any spitting occurring.
FGD WET STACK AS AN ALTERNATIVE SOLUTION FOR FLUE GAS REHEATING
Based on experience gathered in the United States over a period of over 20 years, it is now possible to operate power stations without flue gas reheat responsibly, avoiding emissions of acid droplets (so called "spitting"). The guide lines for designing an environmentally safe FGD Wet Stack have been summarized in the EPRI Revised Wet Stack Design Guide, which was sponsored jointly by EPRI and CICIND.
Using an FGD Wet Stack, instead of reheating flue gas prior to entry into the power plant chimney, offers significant advantages. The cost of initial power plant construction is reduced by millions due to the avoidance of reheat equipment (usually Gas-to-Gas Heaters) and the simplification of the ductwork surrounding the absorber. The cost of power plant operation is significantly reduced as well, thanks to reduced pressure losses and reduced maintenance cost.
One important caution for using FGD Wet Stacks is, that these must be built using well-proven materials of construction and using a highly efficient system of liquid collectors and drains. The additional cost of these materials and systems are far smaller than the savings achieved through the elimination of reheat.
For additional cost saving and emissions reduction, power plant owners should consider Hadek’s innovative ideas: