Corrossion in dry stacks 1

[Sandeep1970]

Can any one suggest what kind of corrosion can be expected in the tall dry stack. The gas contains 12 vol% CO2, H2O = 20 vol %, SO2 =0.165 vol %, NH3 = 0.014 Vol%, HCl = 0.00005 vol%. The temp range between 266 C to 170 C. Is there any software available to estimate the corrosion. Thanks

 

[rustbuster1]

A stack is like a condensing column: where the temperature is below the dew point of your stack gases, acidic condensate will form on the wall of the stack and produce internal corrosion.
A practical way to deal with this is to use the stack as it's own corrosion "coupon" and conduct periodic inspections and thickness profiles up the stack. I call these "line-scans": UT thickness measurements taken every foot or two lets you create a profile graph of the stack corrosion. This shows where it is worst and let's you determine rates too.
Corrosion prevention typically consists of internally lining the stack in the corroding length - metallic, inorganic and organic material options are available, depending on how durable you want it to be.
Alternatively, wait until the affected part of the stack is corroded to replacement condition and replace it with a more suitable material - duplex SS (e.g. S32304)is a good option in many cases.

I hope this assistance stacks up for you!!

 

[dickwilber]

After over 40 years in this field, I have to say, "no, there is no software available to estimate the corrosion rate in a stack". But, if the conditions you describe are constant, and the exterior surface is well insulated, then corrosion should not be a problem. The secret is to maintain the surface in contact with the gases well above the acid dew point. Such stacks have lasted decades with minimal attention.

(The acid dew point of sulfurous acid - H2SO3 - and nitric acid are in the 50 - 60 degree C range. But where there is sulfurous acid there is always a little bit of sulfuric acid - H2SO4 - which has a much higher acid dew point. Your temperature appears to be well above this, providing the stack is properly insulated. A 2" - 50 mm - layer, minimum, of insulation on the exterior of the gas tube or flue, including over all stiffeners and protrusions, maintains the inner surface above the acid dew point. However, if this insulation layer is compromised, corrosionwill occur at the resulting "cold spot", and can penetrate the steel flue within months.)

It is normal practice to construct the top course or two of a stainless (Type 316L is almost universal) to protect against the mild acids formed as the exiting gas mixes with the ambient air. This holds up quite well, but is completely inadequate without the external insulation. The insulation, in turn, must be fully protected from the wheather. If a dual wall design is used, with the insulation placed against the outer surface of the inner wall, then the exterior surface of the outer must be painted to protect against atmospheric corrosion.

An alternate solution is to protect the interior of a single wall stack with a 2" (50 mm) cementious layer (gunite). (Again painting the external surface of the stack.) This works well, but requires more maintenance, and periodic replacement. My experience does not favor internal paints or coatings for this service.

Dick