Sulphur content of fuel oil and superheater fouling

[athomas236]

I am working on a project that includes a 500tonne/hour utility boiler. The fuel oils to be burned in the boiler have sulphur contents that range from 0.1% to 3.5%.

In discussions with the contractor he has stated that there is a relationship between the sulphur content of the fuel oil and the fouling of the superheater surfaces with the result that when burning 0.1% sulphur oil the heat absorption of the superheaters will be higher than when burning 3.5% sulphur oil.

I have searched the internet and have not been able to find evidence to support the contractor's statement. I would be pleased to receive any advice on this matter.


Best regards,

athomas236

 

[rmw]

Unless the fuel oil has some other constituent in it that the sulphur has some effect on changing the friability of the ash, I would worry more about the sulphur's effect on the back end say around the economizer.

I don't recall seeing sulphur effects as far forward in the boiler as the SH banks.

But I am still thinking. It has been some time since I burnt up a bunch of fire with high sulphur in it.

rmw

 

[quark]

I didn't come across this thing, so far. The main problem will be condensation of gases at lower temperatures. With high sulphur content fuel oil, you should maintain high flue gas temperture and thus you will lose some heat there.

During my early days, I worked in a plant that was also manufacturing sulphuric acid. Mainly, the deposits were of unburnt sulpur in the combustion chamber.

I faintly remember that I have a graph which shows the minimum flue gas temperature required with respect to various fuel oils of differing sulphur contents. Let me check it.

 

[JEB66]

The sulfur alone is not a problem for fouling. Has already written, it will be a problem with low temp corrosion, mainly with condensation of H2SO4.

Maybe what your contract is thinking about is, in coal fired furnaces, people use sulfur has an indicator of slagging characteristics. As with oil, the sulfur is not the problem but in coal, the iron is. In coal, the sulfur and iron is in the form of pyrite. So higher sulfur means higher iron and more slag.

 

[blacksmith37]

As JEB indicates; the high S oil is more likely to contain
"other stuff" - V, Ni, Fe, which can leave high temperature deposits. Also , I expect a modern boiler runs with very low excess air; reducing condditions are more likely to leave elemental S deposits with high S fuel. Are you talking about "bunker C" ? ; = flow problems also.

 

[rmw]

Athomas236,

I thought some more about this. I don't know if it is enough to make a difference or not, but I think I remember that burning sulphur is a reducing reaction which absorbs heat rather than giving off heat. I am too lazy to go check that out, but I do remember something about that with respect to burning sulphur in chemical recovery boilers in the paper industry.

That said, if I remember that right, reducing the sulphur will remove the reducing portion of the fuel even though it is a small percentage and replace it with BTU producing fuel oil.

That little difference may affect the heat absorption of the SH banks irrespective of the fouling.

Just a thought.

rmw

 

[quark]

Hi rmw,

I am not sure what that means by absorbing heat. Sulphuric acid is produced by burning molten sulphur in a boiler, converting it to SO3, then Oleum and etc. Steam is a byproduct there.

 

[athomas236]

Gentlemen,

Thank you for your information.

The boilers will burn three fuel oils with the following sulphur contents:

Main fuel 0.1%
1500 Redwood 3.0%
3500 Redwood 3.5%

The contractor is quoting from a report from Brookhaven National Laboratory (USA) on the burning of low sulphur home heating oil with sulphur contents upto 0.8%.

In my view this data cannot be extrapolated to 3.5% sulphur and to a power boiler.

Best regards,

athomas236

 

[rmw]

athomas, I agree that there is not much extrapolatable from a home heating furnace to a utility sized boiler other than they both burn fuel oil.

Quark,

I may have mis-stated it. I am trying to remember what the reason was (and I supplied oleum pumps and steam equipment to sulphuric acid plants and am grateful for the reminder that they produced steam as a by-product of the sulphur combustion process).

What I remember was that burning elemental sulphur was found to be unsatisfactory in paper mill chemical recovery boilers and an emulsified form of sulphur had to be used. I remember it being said that it had to do with the reducing nature of the sulphur combustion from elemental sulphur. It may have had to do with the reducing tendency of the SO2 and/or SO3 produced by the combustion. At the time, that wasn't the focus of my attention to the project, so I didn't dig any further to verify that comment. It also may have had to do with trying to combust the sulphur in a low O2 atmosphere in a chemical recovery furnace.

Since they had to add sulphur to the process to make up the sulphur lost, I thought that burning elemental sulphur sounded like the way to go to me.

rmw

 

[boilerone]

Athomas236,

You contractor is right. High-sulfur fuel oil DOES have big problem of generating sticky ash to the heating surfaces, especially on the superheater elements and thus reduces heat absorption and the steam production of boiler.

The other and bigger concern is, like most others mentioned, corrosion but it includes both high-temp and low-temp corrosions both related to sulfur content. At high temp S reacts with Fe of superheater and forms FeS then with O2 it becomes Fe3O4 (rust). At low temp below dew point the SOx in flue gas becomes H2SOx and corrodes economizer or air preheater whichever to be at the last stage before flue gas leaves the boiler.

Also confirming RMW that limiting sulfur on chemical recovery boilers must be related to the already (and required) reducing environment in the lower furnace and the 'char bed' area which is short on oxygen thus allows sulfur not to be able to be consumed by oxygen first but allows it to touch furnace walls and corrodes metal. This is more severe than regular boilers because for them they can intentionally add excessive air to avoid forming reducing zones in the furnace.

Boilerone

 

[rmw]

quark's post sent me back to the books and yesterday I read up thoroughly on recovery boiler furnace chemical reactions. One takeaway is that the sufphur we are talking about in fuel oil is probably not elemental sulphur but rather sulphur in the form of H2S. Elemental sulphur combustion is exothermic and quark is right that they use the heat released to generate steam for the process. H2S combuston on the other hand and especially in the presence of S is a reducing reaction.

Boilerone, I guess I never had the luxury of being around any boilers that burned low sulphur fuel oil. It was always high sulphur and it was accepted what it did all up and down the line. In general, it wasn't good for anything. The superheater tubes were never really a problem, but the economizer, air heater baskets, and back passes of the flue duct to the stack was always rotting out.

rmw