10 replies to this topic

[sheiko]

Hello people,

I have noticed what I think is a strange behaviour with the atmospheric crude oil distillation furnace that we have in my plant.

When plotting against time: thermal efficiency and combustibles flowrates (Fuel Gas and Fuel Oil entering the mixed burners), I find that combustibles total flowrate increase (well...FG flowrate increase and FO flowrate decrease but not in the same proportion) while thermal efficiency also increase (because flue gas temperature and % O2 in stack both decrease).

1/ I used to think that more thermal efficiency always meant less need for combustibles... What is your experience/opinion?

2/ I was also wondering if there is a relationship between thermal efficiency and feed (crude oil in my case) flowrate? Could this combustible flowrate increase be due to load increase of the furnace?


Please see attached the plots ("temp fumées" means "flue gas temperature" and "Rdt" means "efficiency").

Thanks

Attached Files

•  Furnace efficiency.xls   69KB   185 downloads

Edited by sheiko, 29 June 2011 - 08:00 PM.

[Zauberberg]

Hi Sheiko,

I would expect such results as there is more efficient convective heat transfer when more fuel is burned, and available heat content of flue gas is utilized more at higher flows of combustion products. It is similar to performance of a heat exchanger at different process flow rates - you'll find that efficiency decreases when hot/cold side flows are reduced.

Significant portion of total heater duty is achieved in the convection section. By approaching to the nameplate capacity of the heater, I think you're just getting higher utilization of heat energy of the flue gas.

I would also try to repeat the same experiment by using more Fuel Oil instead of Fuel Gas, and then see how combustion efficiency affects the overal heater efficiency, also at higher plant rates.

[sheiko]

Hi Sheiko,

I would expect such results as there is more efficient convective heat transfer when more fuel is burned, and available heat content of flue gas is utilized more at higher flows of combustion products. It is similar to performance of a heat exchanger at different process flow rates - you'll find that efficiency decreases when hot/cold side flows are reduced.

Significant portion of total heater duty is achieved in the convection section. By approaching to the nameplate capacity of the heater, I think you're just getting higher utilization of heat energy of the flue gas.

I would also try to repeat the same experiment by using more Fuel Oil instead of Fuel Gas, and then see how combustion efficiency affects the overal heater efficiency, also at higher plant rates.

Thanks Zauber,

It makes indeed sense that increasing the flowrate on one side will increase the heat transfer coefficient of that side and increase the overal heat transfer coefficient.

As for the proposed experiment, I will see if possible, but my first guess is that increasing the proportion of Fuel Oil at the expense of Fuel Gas will decrease the overall Lower Heating Value (LHV) of the combustibles mixture (as FG has a higher LHV than FO), and as a result, it may decrease the thermal efficiency of the furnace at a given combustible total flowrate. Besides, our FG has some Hydrogen in it (from catalytic reformer), that increases again its LHV...

Edited by sheiko, 02 July 2011 - 04:34 PM.

[Technical Bard]

It is also the case that a furnace has the best efficiency at it's design duty, and that efficiency declines with turndown. Therefore, if you are measuring fuel consumption and it is rising towards the original design point, the efficiency should be rising.

Edited by Technical Bard, 30 June 2011 - 09:45 PM.

[sheiko]

It is also the case that a furnace has the best efficiency at it's design duty, and that efficiency declines with turndown. Therefore, if you are measuring fuel consumption and it is rising towards the original design point, the efficiency should be rising.

1/ Yes indeed, I can see on the plot that combustibles flowrate and efficiency goes in the same direction (increase or decrease), except however in the period févr- mars 11 (february to march 2011) on the plot where combustibles rate decrease while efficiency increases. I believe this may be due to the drop in oxygen excess in the stack and flue gas temperature. Do you agree?

Having said that, another question comes to my mind:
If flue gas flowrate = air flowrate + combustible flowrate (by material balance), doesn't this mean that increasing combustible flowrate (thus increasing flue gas flowrate) will also increase heat losses in the flue gas (and thus decreasing the thermal efficiency of the furnace)?

2/ About my 2nd query. I think the answer is finally: yes. An increase of feed flowrate shall obviously require more combustibles, all other things remaining equal.

3/ Now I have a naive question: If more combustibles means better efficiency, how can we reduce the amount of combustibles consumed in our furnaces without lowering their efficiencies?

Edited by sheiko, 02 July 2011 - 09:38 PM.

[T.S.MURALI]

Efficiency :Thermal output/thermal input. All that you have to do is to know how much heat is lost -Radiation convection, unburnt and exitgas/stack losses.

If you increase the Input enthalpy of air and fuel by preheating and recover waste heat enrergy as well to the practicable level/extent then you will need less fuel.

You can contain the losses by judicious use of waste heat, fuel control with ratio controller, efficient burning ,reducing air ingress etc, good thermal lining and insulation etc

[S.AHMAD]

1. Efficiency is a function of flowrate/throughput for furnace as well as for boiler. For a specific % excess air, there is an optimum (max eff) point normally very close to the design capacity.
2. Efficiency is also a function of excess air (related to % O2 in flue gas) therefore for a specific throughput, there is an optimum point
3. Efficiency is also a function of type of fuel, normally higher for fuel gas
4. Combining all the above factors, there is always an optimum point where efficiency is maximum.

[Robert Montoya]

Hi, the attachment file could help you

Attached Files

•  Thermal efficiency fired heater.pdf   281.44KB   136 downloads

[sheiko]

Thanks Robert.
I already have it.

[kkala]

Having considered the interesting previous posts, following views on the diagrams (presented in "Furnace efficiency.xls", post No 1) could be useful as a try for interpretation (comments welcomed). Points are assumed to represent "experiments" in steady conditions for a few hours (not e.g. daily averages); while points of Jan-11 (being on y axis) have been omitted.
1. In Jan - Feb - Mar 11 measurements, fuel is about 50% fuel oil (FO) and 50% fuel gas (FG), w/w, while total fuel burning (kg/h) seems more or less same (~1650 kg/h). Flue gas temperature (understood as exit temperature) decreases, so furnace load increases and efficiency had to increase. This has not been realized on Feb 11 due to higher excess air (3.6% O2 in flue gases).
Note: What does "flue gas temperature" represent? It seems high for flue gas exit to atmosphere, it might concern exit from radiant section or just upstream e.g. a steam heating coil at end of convection section. Efficiency in the diagrams is understood to refer to part of the furnace, from which flue gases come out with reported temperature.
2. On May 11 total fuel burning (kg/h) has practically remained as before (very light increase), but weight ratio FG/FO increased. Efficiency jumped to 71%, as also indicated by the decrease of "flue gas temperature". This was due to lower excess air and some increase of furnace load (approaching normal capacity), since FG has reportedlly higher LHV than FO (on weight basis, post No 3).
Note: NHV (per kg) of FO / FG could make things a bit clearer.
3. On Jun 11 FG burning was same as May 11, but FO burning was sensibly reduced further. Decrease in efficiency is due to reduction in furnace load (going far from normal), also to excess air increase.

Higher efficiency means less fuel energy (that is LHV) at constant furnace load. As mentioned in posts, best efficiency is for normal capacity and conditions (where performance guarantees are based on).Modern burners can burn even heavy FO using only 5% excess air, higher than required excess air would reduce efficiency (actually seen this in steam boiler proposals).
Note: Substituting all FO with FG does not necessarily mean increase of efficiency, since flame will get much less luminous (Nelson, Petroleum Refinery Engineering, Chapter on Tubestill Heaters).

Edited by kkala, 16 November 2012 - 10:13 AM.

[Ahsan67]

can u please eloborate what method you used while calculating efficiency eihter API-560 or other method & kindly share how u calculated amount of BTU's going from stack with flue gases(e.g,CO2,N2,O2,Excess Air)


Regards
Ahsan