Condensing economizer materials with biogas

[spud22]

We are considering installing a condensing economizer on a NG fired boiler which will also be fired with 10% biogas. Concern is H2S, with approximately 400 ppm H2S in condensate. Materials proposed are 304 SS tubes and 1100 Al fins. Is this a proper application, or do we need to go to 304 or 316 SS fins and/or tubes?

 

[EdStainless]

Do you have any idea as to the chloride content and pH of the condensate?

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Plymouth Tube

 

[metengr]

I would not recommend any austenitic stainless steel in a boiler economizer because of concerns related to stress corrosion cracking. This has been an issue with ASME B&PV Code, Section I, and this warning regarding the use of austenitic stainless steels in water-wetted service remains today for boiler circuits.

 

[metengr]

and... intergranular corrosion. Make the economizer out of carbon steel tubing with ferritic stainless steel or carbon steel fins for heat absorption.

 

[spud22]

My concern is not so much with the tubing, as we are circulating a glycol mix at relatively low pressures. The questionable component is the fins with possible corrosion attack. pH of condensate is anticipated to be above 4.0, but will not know for sure until 10% biogas is burned in boiler, which is intended to be concurrent with commissioning of economizer.

 

[metengr]

spud22;
What exactly are you doing with this economizer? Typically, a economizer is used to raise feedwater inlet temperature to the boiler to improve cycle efficiency. In this case are you simply cooling boiler exit gas temperature using a stand alone economizer? Also, what is your definition of low pressure? If it is greater than 15 psig, this must be designed to ASME B&PV Code, Section I or Section VIII requirements, if installed in the US.

 

[spud22]

We are heating process water, with glycol/water exit temp of 140 F and gas exit temps of 100 F. Pressure on water side is estimated at 50 psig. Chloride attack should be minimal, but sulfide concentrations and corrosion on the gas side is a major concern.

 

[EdStainless]

At these temps you might consider 439 for the tubing and 409 for the fins. Not great corrosion resistance, but prob enough and good heat transfer.

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Plymouth Tube

 

[davefitz]

may be able to use the same alloys now used in wet FGD service, such as 6% moly alloys etc. As edstainless says, get the condensate pH and Cl content and then access the corrosion curves typically used for selection of metals in corrosive service, such as seawater service and wet FGD service.AL6XN for example, but very rich.

 

[rmw]

Is this a clean environment, as in pristine? Any ambient dust, etc that the boiler fans ingest will end up being caught by the moisture in your wet economizer. Are you making any provisions to clean it either on line or off line? The air around you may look clean, but look at your face cloth the next time you shower.

rmw

 

[spud22]

Good point, RMW. We were looking at staggered tubes, but may need to consider in line arrangement with soot blowers. Also, what is anyone's experience with Heresite coatings for corrosion protection?