316L stack after a wet FGD for a Orimulsion fired boiler 1

[fvincent]

Orimulsion is a bitumen in water emulsion (28% water) with very low Cl content and no F content. S content is 2.8%

Flue gases pass through an ESP and then throuh a FGD which utilizes lime as sorbent. Raw water used for lime milk preparation is low Cl (<5 ppm). Sulphite cake is discharged at 70% humidity.

Downstream the FGD system there is a stack made out of carbon steel and lined with a 1/8&quot; width stainless steel (316L).

Gases are saturated at 62oC. There is the possibility of re-heating gases by means of injecting a hot gases stream from an HFO (1%S) at 400oC, so that flue gases reach 80oC.

The stack is non-insulated

My questions are:
- what is the corrosion rate of the wall paper with or without the re-heating?
- Am I correct when I consider that no re-heating would be safer than re-heating? I assumed that the acid film on the wall paper would be more diluted and self cleaning of inner surfaces would be more effetive without the re-heating.
- is there any special care to be taken concerning corrosion monitoring
- what would be the effect on visible plume at the top of the stack?

Regards


fvincent
Figener S/A

 

[fvincent]

No hints??

fvincent
Figener S/A

 

[mcguire]

Fvincent
You're very familiar with your process, but it's a little hard for a stranger to follow clearly. What I surmise the environment that must be withstood is moist air at 62C, low Cl, some sulfur, and basic pH. If this is correct, then the 316L should remain passive and corrosion should be essentially nil.
Correct me on the environment and I'll reassess that answer.

 

[jvs3]

An analysis of condensed liquid off the flue gas...moisture content to help estimate dewpoint...ambient temperature and whether the stack is insulated would all help.

It's condensate pools that cause the corrosion...is this a wet or dry stack? I'm assuming it's wet.

 

[fvincent]

Thanks for your answer, mcguire... and sorry for the missing information

The problem corresponds to the flue gases from a boiler after being scrubbed by the associated flue gas de-sulfurization system. The boiler fires Orimulsion (70% bitumen S and 30% water; the S content of the bitumen is 4%) The scrubber operates with lime slurry.

Flue gases leave the scrubber saturated at 62oC and follow to the stack. Removal of SO2 is 90%.

Flue gases volume composition after the FGD system can be taken as:

N2 - 66.298% v
O2 - 0.832% v
CO2 - 12.120% v
H2O - 20.730% v
SO2 - 0.02% v

Additionally we have:
SO3 - 160 ppm (mass) as aerosol (formed by the reaction of SO3 + H2O)
Cl - 5.21 ppm as vapour or salts
F - 0.5 ppm as vapour or salts

Since the stack is non-insulated a certain water condensation occurs on the liner of the stack.

Part of H2SO4 aerosol reaches the liner too, so that an acid film forms.

According to some calculations based on the condensation rate of water and on a deposition rate of 50% of aerosols on the stack liner, I expect a 3-10% sulfuric acid concentration in the film.

This concentration is a capital value, I think. I have read that H2SO4 concentration can reach 30% , but I think such a high value can be obtained only when the stack is insulated and water condensation is very low.

Chloride and fluoride concentration in the film is hard to forsee. Chloride-containing droplets carried from the FGD by the exit gases reach the stack too. I expect that Cl and F concentration in the film will be low. But again I am worried with some numbers I got for coal fired boiler which show a concentration of up to 100,000 ppm for Cl on the condensate of flue gas after the FGD.

My supposition is that these figures are quite different from the present case, because Cl content of coal can reach 100 times the Cl content of Orimulsion or HFO and normally the stacks are somehow insulated.

So, the question is: Are crevice corrosion / pitting or distributed corrosion a real possibility for the 316L stack liner at 62oC? Or, 316L is a good solution for my case??

Thank you again




fvincent
Figener S/A

 

[fvincent]

Jvs3,

Hope the description above helps...
Flue gases are saturated at 62oC
Stack is not insulated
Ambient temperature is 30oC and it rains a lot

I think that the worst case is to consider no rain and warm clima, because the less the water condensates the higher is the acid concentration of the acid film, isn't it?

Thanks a lot

fvincent
Figener S/A

 

[mcguire]

To successfully use 316, and by that I mean to keep corrosion under 25 g/square meter/day, you should have H2SO4 concentration not over 7-8% at 62C. Based on the information you have given 316 is marginal for this application.

 

[martinwhelan]

why not consider something like Titan (316 Ti) instead of 316L.
It should be an improvement when any chlorides concentrate around welds.
Otherwise you may consider going higher up the nickel chain.

 

[Easel]

This is the application that led to the development of the 904 series of stainless steels. The 300 series were marginal. The added chromium and nickel have allowed 904 to survive in this applicaiton.

A second concern is 160 ppm of SO3. This will covert to sulfuric acid which will NOT be collected in the scrubber. It will leave as a 1 micron particle/droplet. This will lead to a serious opacity problem referred to as the Blue Plume.

 

[rolledalloy]

We have seen ductwork where as long as it is kept dry no corrosion occurred in mild steel, however the lower half of this horizontal duct did see condensate and hence corrosion. As a result, the utility had to go in a wall paper with AL-6XN.

The FGD market has been pretty active recently and one of the alloys that has been gaining in popularity is 2205 duplex stainless steel. When the first FGD systems went in the 317l, 317LMN type alloys were popular. 2205 offers better corrosion resistance at a lower price to the combined sulfuric acid/chloride conditions, hence it has grown in popularity and is specified on some current new FGD installations. It is about 20% more than 316L for raw material.

We have seen many units get specified this way, you may want to give it a look since it would get you an improved performance without jumping to the 6% moly alloys or hastelloy C grades. A data sheet listed below compares this alloy with the 300 series stainless steels.

http://www.rolledalloys.com/techcenter/corrosion/RA2205.pdf

A recent NACE paper published in the year 2000 by some Nickel Development Institute consultants is an excellent resource to compare alloys from 300 series to Hastelloy C in FGD absorber units. This is paper number 578. Available at

http://www.nace.org.