Wet Stack Design and Operation was the Hot Topic November 20

In the U.S. and much of the world the wet stack concept has replaced reheat. The disadvantage of wet stacks is the danger of corrosion and liquid discharge if they are not properly designed. Factors that most affect wet stack operation are stack diameter, flue gas velocity, and the flow of condensed liquid on typical liner materials. The sources of liquid in a wet stack are mist-eliminator carry-over, condensation due to cooling by ambient air, and condensation due to adiabatic expansion along the height of the stack. Trouble-free operation of an FGD unit with wet ducts and stacks requires consideration of the collection, handling and discharge of condensed liquid during the design of the stack.

The following speakers discussed the issues to consider when designing and installing a wet stack:

David Anderson, P.E. Vice-President Alden Research Laboratory, Inc. (ARL) and Lewis Maroti, Senior Technical Consultant at ARL, told participants that liquid collectors and drains are needed in most coal-fired units when operating with wet FGD systems. Understanding and properly applying the physics and thermodynamics related to wet operation are essential for the development of effective liquid collection systems. Physical flow modeling is a powerful tool to minimize the liquid discharge from the stacks. Physical flow models ensure that site specific geometry and arrangement issues are properly addressed.

James L. Naylor, III, Vice-President Marketing and Sales Pullman Power LLC, discussed structural considerations when designing and installing liquid collection systems for wet stacks.

Paul Nederlof, Vice-President Sales & Operations Hadek (Penn Iron Works, Inc.), discussed the proper design of a wet stack and operational benefits that come with a good design. He cited the following Pennguard Block Lining System advantages:

• Completely resistant to sulfuric acid (diluted acids)
• Impermeable to acidic flue gas and condensate (positive pressure, diluted acids)
• Strongly insulating (condensation in chimney/droplet carry-over/low dispersion)
• Lightweight.

The Alert this week includes details on the first FGD system to discharge into a cooling tower in the U.S.  This will be at AEP Cardinal.

When we conducted the wet stack webinar we quoted Hans Hartenstein as saying that there was an additional benefit, which was SO₃ reduction. After the webinar we called Evonik and talked to Mark Ehrnschwender.  He explained that the SO₃ migrates to the sidewalls of the cooling tower stack and that droplets are formed. These droplets gravitate downward and therefore there is a big reduction in acid mist from the top of the cooling tower stack.

We also talked to Paul Lindahl of SPX. He cited the buoyancy of the plume as a major advantage.  Some environmentalists have worried about mercury hot spots.  Elemental mercury will travel long distances, but oxidized mercury will fall out much closer. So buoyancy should mitigate the “hot spot” concern.

The individual slides are located in our FGD Decision Tree as follows:

David Anderson and Lewis Maroti

FGD Continuing Decision Process For: Stacks

Wet Stack Liquid Collection System Development, presented by David Anderson and Lewis Maroti, Alden Research Laboratory, Inc. Hot Topic Hour November 20, 2009.

http://www.mcilvainecompany.com/FGD_Decision_Tree/subscriber/Tree/DescriptionTextLinks/Anderson and Maroti Alden - Nov. 20.pdf

Paul Nederlof - Hadek

FGD Continuing Decision Process For: Products

Proper Design of Wet Stack. Pennguard Block Lining System advantages, presented by Paul Nederlof, Hadek. Hot Topic Hour November 20, 2009.

http://www.mcilvainecompany.com/FGD_Decision_Tree/subscriber/Tree/DescriptionTextLinks/Paul Nederlof, Hadek - 11-20-09.pdf

James Naylor – Pullman Power LLC

FGD Continuing Decision Process For: Products

Structural considerations when designing and installing liquid collection systems for wet stacks. Presented by James Naylor, Pullman Power. Hot Topic Hour November 20, 2009.

http://www.mcilvainecompany.com/FGD_Decision_Tree/subscriber/Tree/DescriptionTextLinks/Jim Naylor - Pullman Power 11-20-09.pdf