Balanced or Single Fire for Combo Burner?

[ponderer]

Recently we invited two guests to our refinery for teaching us how to increase the combustion efficiency of our big CDU(Crude Distillation Unit) Furnace.

The burner of our CDU Furnace is Combo type, which can burn both Refinery Fuel Gas and Fuel Oil.

One guest suggest us to burn both Fuel Gas and Fuel Oil at the same time(it is called Balanced Fire) in order to get an optimal efficiency. The recommended excess O2 is under 2%.

The other guest suggest us to burn only one kind of Fuel at one time, in order not to overfire the convection zone.
The recommended excess O2 is 3%.

Which guest's suggestion is correct?

 

[Combustor1]

Ponder,

Consider burning 2 fuels - how do you propose to measure fuel and air flow to know that you have 2% O2 or less. Both fuels will have their own properties, e.g. if your Steady State firing rate is 10 MMBtu/hr - 5 MMBtu/Hr per each fuel what is the stiociometric ratio, of air to fuel - required per fuel - note - it will be different! You may need to have mass flow abilities per each fuel and also air mass flow or have an analyzer in the stack. If you are truely looking for efficiency this will need to be accurate.

The real problem as I see it is safety - how does the flame scanner distinguish between fuels. It is possible that only one fuel would be burning and the other be dumping inside the furnace and then eventually build up and poof - I have known this to happen.

Be safe & good luck!

 

[hacksaw]

on one project, we were burning six different fuels simultaneously. It is not not easy, but it was driven by the economics of the fuels available. Pick your poison...or headache as you see it, and trim the controls (and interlocks) to fit.

As suggested by combuster, don't sacrifice safety. When you get a furnace "puff" due to unstable firing, you'll clear the burner front faster than you thought humanly possible.

 

[NOx]

Hi Ponder,
With regards to your question did either of your guest offer calculations ie fuel gas / fuel oil curves, what the excess air should be for each fuel type and combined, are the burners forced draft or natural draft. Dependant on the type of fuels being fired are pollutants also an issue (depedant on your loaction) I would assume they are.
What burner management system are you using and is there a fuel skid involved if so fuel control should be quite simple and therefore overfiring should not be a problem.
Who's burners are you utilising at this time?
After many site visits myself it seems not enough questions were asked before your visitors arrived to give a beneficial solution.
Have your problems been resolved to your satisfaction.

R
NOx

 

[ponderer]

Dear Sir:

The burners are supplied by John-Zink.

The excess O2 is specified to be:

3% for Fuel Gas
4% for Fuel Oil

The burners are for forced draft heater.

The heater fire Refinery fuel gas with
auxiliary fuel oil.

 

[NOx]

Hi Ponderer,

From the information above this is the industry norm of 3% and 4% for gas and oil respectivly however do you have an O2 analizer at the stack, how old is the furnace could you have a tramp air problem?. If not how is the airflow distributed is it a ring type plenum mount or direct fired into the windbox. Airflow problems could also upset combustion control, what pressure drop are you running at?
Are the burners being fired at the correct heat release rates? do you have the original burner data sheets showing maximum heat release for each fuel type, if dual fuel firing are you exceding the maximum heat release?

As for the burners JZ combination burners are probably the best in the industry and with correct balance should not give any problems.
R
NOx