High flue gas temperature 1

[refiner]

Hi,
Can anybody give me recommendations? We've got a problem with a furnace - very high flue gas temperature at only 70 % heat load. This furnace has no convection tube. Its flue gas duct is connected to others furnace's duct (We have no problem with other furnaces). We have tried :
- increase/decrease excess air
- switch type of fuel : fuel oil and fuel gas
- check our thermocouple
- shutdown and inspect inside : gas duct is overheated, damper damaged, but others seems OK. After start up, the problem still exists.

 

[boilerdoc]

Hi refiner,
can you give some more information about the furnace itself. There are acording to my experience 2 main reasons for high flue gas temp. Increase in energy input like higher flame temp.,increased fuel capacity etc. Or reduction of heat transfer due to tube fouling (deposits), change in the product flow or specification.

 

[Combustor1]

Refiner,

There can only be a few reasons for this.

Could use more info on your furnace :
.If your furnace single pass or has several passes - maybe you could be short circuiting your flue gases.
.If this is an Air heater with recirc/dilution air - check to make sure you have a block in a duct somewhere.
.What was normal stack temps before at Steady State vs. now?
.Do you have a load difference?
.What type of burner and what is Btu/Hr input.
.What kind of fuel are you using?
.Is your Thermocouple & themocouple wire the same type - controller set for same type?
.Do you have a meter on your fuel - is it possible that you are putting more fuel in the furnace for some reason - a control valve problem - linkage slipped?
.Fuel Regulator set with outlet pressure too high?
.Do you have an analyzer to measure O2, CO in your stack
.Any Exhaust fan - Exhaust Fan or damper problem?

Some places to check...

 

[refiner]

Thanks Setra and Combuster,
Here's some more info.
Fuel type : Fuel oil and Fuel gas
Product : heavy naphtha
Design flue gas temp : 890 deg. C (but actual flue gas temp was more than 900 deg C at design feed capacity!)
This furnace has been operating for years but this problem has just happened last month. No change in fuel or product properties ( It is our routine laboratory analysis). Induce draft fans were checked and damper was repaired. On-line analyzer reading is 2 - 3 % excess oxygen.
Tube fouling? I don't think this is the cause since the product is clean but anyway, is there any method to check it without shutdown?
Any suggestion is welcome.

 

[boilerdoc]

You mentioned in your thread that the flue gas duct was overheated and the damper damaged.Was the damper damaged by overheating?
One way to check for tube fouling during operation is to compare product mass flow and inlet/outlet temp with recordings before the problem.

 

[refiner]

Hi setra,
You are right that the damper was damaged by overheating.
Damper material is SS310.

 

[boilerdoc]

Hi refiner,
if the damper was damaged by overheating I am pretty sure that your problem is somewhere at the burner area.I would check again on the fuel supply as mentioned also by combuster.

 

[op9]

Hello Refiner,
A few more responses to the questions from Combuster would help, especially on the heater configuration. Is 2-3% excess O2 always been the case and for both liquid as gaseous fuels? By how much did you try increasing this figure? Do you have a CO reading?

Is the combustion air preheated from conduction within the heater and is it posible that some of this air may be leaking through cracks etc further into the furnace and not all getting to the burner? The burner then will actually be operating rich but your flue gas analysis may not change dramatically as far as excess O2. CO and burner fouling however should show it up.

A reduction in O2 (at the burner) will produce a longer flame and depending on the heater configuration may cause the higher temps. You say no convection tube. So I assume you have a coil in the radiant (furnace) section only. Is there a baffle directly opposite the burner or does it fire directly through the coil with no restriction? Regarding the fitting of a baffle I have experienced the problem mentioned above, where air from the preheated air jacket was leaking into the furnace through the support arrangements for the suspended baffle!!

CO readings would be of interest because some extended burning or reignition may be taking place. Though you may have to pick a suitable sample point as the furthest point when it has actually mostly burnt out may not show anything?
If you can find a sample point closer to the burner and take both O2 and CO readings AND compare them to another similar fired heater it might pick up a difference.

Regards,
Rod Nissen.
Combustion & Engineering Diagnostics
canded@iprimus.com.au

http://www.canded.com.au

 

[refiner]

Hi op9 and Setra,
2 - 3 % excess o2 is same for FO and FG, we used to increase this to 5 - 6 % but does not help much. Unfortunately we don't have CO analyzer. You are right that we have radiant coil only but there is no baffle and no air preheater.
We are trying to "seal" every openings as possible.
Thanks for all suggestions.

 

[Combustor1]

Refiner,

If you damaged your damper it is possible you might have damaged more equipment. I have run onto a few over heating problems before with damaged dampers and worse - exhaust fans coming apart. 890 C is very high outlet temp.

The reason for overheating and damaged damper is probably the same. I would start checking all the instrumentation - if you have independant actuators for both air and gas you might have a controller problem or could have a flow meter problem even and oxygen sensor problem. How is the furnace being controlled - PLC or controller - has someone played with the settings? performed any tuning on sensors or transmitters?

Do you have a high temperature limit to protect against over heating? This is a code requirement! The limit to be set for the lowest temperature rated device - like the exhaust fan.

This does not sound like a difficult problem - you will need to rule out each device if you are not sure.

Good-Luck!

 

[op9]

Refiner,
Try putting your question to the forums at

http://www.r-t-o-l.com

Which is Refinery Technology online, where someone may have more intimate knowledge of your situation, though they will probably ask for plenty of information as we have.
Rod Nissen.
Combustion & Engineering Diagnostics
canded@iprimus.com.au

http://www.canded.com.au

 

[25362]

You are right in concentrating in what may have changed. Yours is a radiant service heater with (my estimate) about 60% of the energy released by radiation.

When working at 70% load the radiation can be reduced -following Hottel- by increasing the wt ratio of air to fuel some 20%. In seteris paris, ie, at the same heat load, added excess air will cool more the combustion gases, and since radiation is a function of T[sup]4[/sup] it is possible that the % radiation drops and the leaving flue gases are somewhat warmer than before. Radiation % absorbed can also be affected by fouling of the tubes (inside and outside) as well as by changes in the process fluid conditions (temperature, evaporation, etc.).

Other factors I can think of that may have changed are the air quality, meaning temperature, humidity, oxygen content; the fuel quality: its H/C ratio, its temperature, the air mixing and atomizing processes. Have you checked all these ?

Besides, how is the damper operated, is there a change of tramp cold air infiltrating or hot gases leaking out because of loss of tightness or sealing efficiency ? Last, but not least, at what location are you measuring the flue gas temperature, is it possible that the sensing element has an error of about 1% in its readings ?

Some lateral thinking may help in this case. Don't you think ?

 

[mickeymouse]

If this is an all radiant vertical heater: do you have a flat roof on the radiant? If you do have a flat roof, and are in an overfired condition, you may not be getting recirculation at the shielded walls. Therefore, the harder you try the worst it gets. If the height of the radiant tubes is short it would magnify the problem. Possible solution: add a cone section replacement for the flat roof during next TAR.