Super Heater Failure in Bio Mass Boilers

[cponnu]

I have noticed frequent failures of secondary super heater tubes (Material of Construction T22) in the body section in biomass fueled power boilers. Thinning due to erosion and corrosion on the flue gas side is the main reason for failures on these small capacity boilers, with capacities ranging from 35 Tonnes to 100 Tonnes of steam output per hour. Multiple fuels are normally blamed though no definite solution is offered for multi fuel firing ( various types of wood, sugar cane trash,shells of ground nut, coconut etc.) by boiler vendors. Helpful ideas from members will be appreciated.

Recently a vendor offering injection of aqueous Magnesium and Ammonium Nitrate solutions, is claiming that the problem will reduce by such failures. Any comments

C. Ponnu

 

[metengr]

cponnu;
First, I would strongly suggest you have one or more of the failed superheater tubes evaluated by a competent metallurgical laboratory to determine the type of corrosion damage or if something else is occurring in service. Normally, any of the large boiler OEM's can do this type of metallurgical work for you at nominal cost.

I would suspect that with the wide range and blend of fuels you have external corrosion occurring that means the T22 tube material may not be suitable for this type of operation. A metallurgical evaluation of the failed tubes would confirm this, and allow you to decide if you need to upgrade superheater tube material. I don't believe fuel additives are going to be the answer.

 

[davefitz]

You indicated "trash" - this is a source of lead and chlorides- both of which can generate an agressive ash flux at metal temperatures above 600 F. There are also high levels of other metals in wood ( mercury, potassium , etc) which can form an agressive flux, either due to eutectic melting with sulphur or simply cause high fouling ( which leads to frequent sootblower use- see below).

Increased frequency of sootblower use plus high silica in ash leads to sandblasing of the superheater tubes- this removes the protective oxide scale of the tube and allows agressive corrosion of the fresh unprotected metal . To avoid this sandbalst , conseravtive designrules do not permit use of sootblowers and instead use "rapper hammers" inthe penthouse- just like a piano hammer tapping the piano strings.

 

[JEB66]

Some of the scrub trees contain high amounts of silica because they are drug on the ground. That can lead to accelerated flyash erosion.

The bark and annual type plants contain high amounts of potassium and alkalides. This can be very corrosive.

To the eye, either case is just thinned walls. Follow the advice above to determine if it is corrosion or erosion.

 

[macmet]

The fuels you mention are high in chlorides and alkalis and therefore susceptible to corrosion. Wood is generally rather low in chlorides and it may help if you increase the wood portion of your fuel.

You do not say what temperature you are burning at, but if you could lower it you may reduce the amount of chlorides released as vapor. This may not be possible but might be something to look at it.

 

[dtn6770]

You've done well to not taking your vendor at face value. Additive can quickly become a significant operating expense that, at best, masks the real problem.

Short and long term economics should be evaluated tanking into account different levels of additive effectivness.

Ask for references and avoid being anyone's beta site.

 

[MJCronin]

Ponnu.....

It sounds to me like you have the same problem as the WTE (Waste-to-energy) plants had in the late 1980s with thier "mass burn" boilers.....rapid exterior loss of tube wall in the superheater section as well as the lower boiler sections. Suspected hydrocholric acid wastage

Unfortunately, after trying many solutions, the best fix was one of the most expensive......tube wall cladding with low Fe content Inconel 625. Babcock & Wilcox "Volund" division has many years of research in this area

See these websites:

http://www.dongenergy.com/NR/rdonlyres/0306854A-5A07-42A6-B2A9-630BBC1BC1D0/0/23.pdf

http://www.volund.dk/content/download/1040/6429/file/Symposium on waste treatement technologies.pdf

Please do the "common professional courtesy" and complete this thread by coming back to us with your fonal solution

Good luck


-MJC

 

[cponnu]

I thank everyone who had responded to my query;

could not respond earlier due to another breakdown with the boiler.

the metallurgical lab reports indicate that the failure is due to erosion on the outside;

the manufacturer is trying out few tubes with SS310 material;

during the recent shut down we have found large scale settling of ash also at the bottom of super heater banks.

I am continuing the observations and would share the findings again with the group.

Thanks again

C Ponnu

 

[rmw]

For that problem you can resort to cladding as MJCronin has suggested, or tube shields in high velocity zones, or metal spray applications with hard coatings. All have been used in similar applications.

rmw