Unusual emissions on heavy fuel oil

[op9]

Hi All,
I have to investigate a fault next week on board a ship where they have an Aarlborg oil fired boiler (Danish). The burner is steam atomised operating at approx. 15MW (50 MM Btu/hr)max. The problem they are having is that they are emitting small carbon/ash balls out of the stack when the burner is on HFO. (see attached photo). They are soft, light and hollow. The stack appears clear from a distance so no visual smoke. When they operate on light oil with air atomising, no problem. I have not worked on this burner before. The ships engineers report that they have tried all sort of adjustments which sometimes appear to fix the problem, but it soon returns. Obviously atomising fuel temp/pressures, steam pressures etc need to be correct, but I am assuming that they have made sure that is as per the manual.

I am "guessing" from the shape of the carbon that there may be a problem that the atomising steam is wet and that the "balls" are due to water droplets in the flame attaching to burning fuel which cools and stops the combustion process producing carbon, the water evaporating, hence the hollow structure.

Would appreciate any comments, especially if this phenomenon has been experienced before.

Thanks in advance,
Rod

Rod Nissen.
Combustion & Engineering Diagnostics

 

[ccfowler]

Interesting photo! What is the size range for these balls?

I think that you are starting on the right approach.

Is firing HFO something new, or is this a new problem after much trouble-free operation?

I would want to check on the condition and suitability of both the steam and the fuel nozzles. Beyond the obvious issue of properly selected nozzles, erosion or corrosion damage seem possible to be involved.

In my experience in sorting out problems, it has so very often been the item(s) that "could not be bad" that were actually what was "bad."

If orifice meters are involved for flow metering, I would insist on having them removed, inspected, and measured. In addition to erosion damage, don't be surprised if they were installed backwards. Pressure sensing ports may be troublesome from erosion, corrosion, or partial clogging.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[op9]

Hi CC,
I am guessing the size is 1 - 3mm diameter.

Unfortunately I don't know the complete history of the problem as the ship's engineers do change. I will be asking the appropriate questions when I attend.

On another forum the suggestion was "cenospheres" which I have heard about years ago. Do a search...The explanation is that in the fuel droplets the more volatile components, incl hydrogen are consumed first leaving the unburnt porus carbon shell behind. So my thinking is make the atomisation better..smaller droplets, then maybe less problem??? The steam atomisation efficiency could also be aggravating the issue whether by worn or incorrectly assembled nozzles as you suggest and/or wet steam.
Regards,
Rod

Rod Nissen.
Combustion & Engineering Diagnostics

 

[Compositepro]

These do look like cenospheres. Cenospherse are not made of carbon but ash and minerals. I imagine that salt water in the fuel oil might be a factor.

 

[ccfowler]

I've had a touch of involvement with cenospheres in the flyash resulting from pulverized coal combustion. They are usually very tiny (think near microscopic). Obviously, there was no steam atomization involved, but there was plenty of turbulence in the flame zone.

I wan't much concerned with the formation of the cenospheres, there were plenty of them available whether wanted or not. The concern was the possible practicality of extracting them from the flyash stream for commercial uses.

In pulverized coal combustion, heavy slag formation is usually associated with regions with an inadequate supply of air (reducing rather than oxidizing local conditions). I would not be surprised if you find the subject problem to involve problems with suitable distribution of combustion air within the combustion zone. Relatively large amounts of wet steam seems a likely element in the array of possible contributing problems.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[op9]

Actually years ago I did a gas burner commissioning (butane) on a rotary kiln where they made cenospheres out of ash from the nearby powerstation boilers. Apparently it was a quite lucrative industry as long as the product was "white" as they used it for filler in the plastics industry.Being butane they were concerned that any incomplete combustion would disscolour the product.All went well.

In this case these cenospheres appear quite large.Re the possibility they are of mineral composition I would be surprised if there was salt water in the fuel. However it is one possibility I will look at.

Rod Nissen.
Combustion & Engineering Diagnostics