Crude Furnace Excess O2

[JonnyOnTheSpot]

I have a problem with my crude furnace. The excess O2's are a little too high, currently around 7-8%. I would like to work on lowering them, but operations doesn't like the idea because they say the furnace cokes up faster when the O2's are lowered. This does not make sense to me. I would think that with higher O2's, you would have to fire the furnace harder to get the same transfer temp or outlet temp from the furnace. In their experience though, the furnace just cokes up faster. Does anyone have any ideas about this phenomenon?

 

[lt5q5p]

Check to see If your heater box temperature is tied to your fuel gas. If it is, as you reduce your O2 you may bog the heater which will cool the heater box off and the fuel gas usage will increase. This would only be an issue if you had flame inpingment on the heater tubes. Other than that your operaters may be referring to the flame temperature cooling off as you increase the air. Again this should only be an issue if you have serious flame impingment.

 

[ash9144]

High excess O2 carries less heat out of the firebox. This could result in higher tube temperatures which may result in coking. Though it seems odd to me that your process fluid would coke up at the furnace temperatures since it was designed as direct fire heater.

 

[ash9144]

Got that backwards. Higher O2 carries more heat out of firebox. So reducing O2 could increase tube temps.

 

[rmw]

If your burners aren't up to the job, lowering the O2 will increase the unburned hydrocarbons which is what is coking up the tubes. Before attempting to lower the O2, be sure to tune up the burner to get the best possible combustion. If the burner is not able to be fine tuned, then you will have to remain with high O2 levels.

rmw

 

[mbt22]

It may be that the measure excess O2 is isn't real. If it the furnace isn't in great condition, air leakage in could be giving you a falsely high reading. When you try to reduce O2, you might actually be taking it too low.

Matt

 

[25362]

Radiation [R], as a fraction of the total heat released by combustion [Q] in british units, relates to the mass ratio of air to fuel [G] by Hottel's equation:

R = 1/(1+a.G[√]Q)​

where [a], is a factor that depends on the geometry of the firing chamber and the heated tubes projected surface.

For a given heater, just by reducing [G] the radiation % [R] would increase.

Hottel's equation was developed for pulverized solid (and liquid) fuels, but it is generally applied, w/o great error, also to gaseous hydrocarbon fuels.

In an old example I found in my notes for a cracking furnace fired with refinery gas, a reduction of [G] from 20.5:1, to 20:1 (a drop of 2.44%), resulted in an increase in [R] from 0.414 (41.4%) to 0.421 (42.1%).