Boiler efficiency after fuel conversion 1

[mfqd13]

Hi,

I would like to know if there is any effective improve (or not) in a boiler efficiency after a conversion from fueloil to natural gas.

Thanks

 

[macmet]

yes

 

[mfqd13]

There are other energy saving regarding this like: fuel pumps OFF, energy for heating fuel tanks, etc.
But, looking only to boiler efficiency and not the overall system, the improvement is related to the energy loss in the flue gas?? Because of the excess air reduction??

But how much is the improvement? Is there any know values or any way to determine it through calculations?

Thanks

 

[macmet]

Did you try Googling this? I'm sure B&W Steam or Engineering Toolbox or something similar would have typical efficiency values.

Excess air will play a factor in efficiency as will fuel chemistry and water production.

 

[rmw]

Was the boiler designed day one to burn fuel oil? Oil flames and gas flames are different with respect to heat release rates and design heat release volume in the furnace. Radiant heat value (to the furnace wall) differences also dictate furnace design parameters. I suspect if it was originally an oil design and it is being converted to nat gas, you will have some efficiency differences and some heat transfer differences in various zones of the boiler - furnace, convection passes, superheaters and/or reheaters (if any) that have to be accounted for.

The recommendation to read B&W Steam or if you can get a copy of Combustion Engineering's equivalent was a good one.

rmw

 

[davefitz]

The indicated boiler efficiency will decrease if based on fuel HHV, but may increase if based on fuel LHV.

If you are also going to change the pressure part configuration to better deal with the changes heat transfer and corrosion characteristics, then you can increase the boiler efficiency- natural gas boiler may be permited to have a lower stack gas temperature than a fuel oil fired unit, but may require changes to the economizer and air heater to realize that improvement. The fuel gas flame is less radiant and a lower temperature flame than that of fuel oil, so the furnace water wall heat absorption will drop and the heat available for superheating steam increases. The back end velocitywill increase and ther may be tube vibration issues.

 

[mfqd13]

Thanks for the replies and opinions that i've registered.

I didn't understand why the efficiency will decrease with HHV and increase with LHV...
Anyway, i've looked in STEAM a i didn't find a value...i will keep looking for it...

Thanks

 

[macmet]

I wasn't trying to say that there would an efficiency listed in Steam. What I meant is you can read up on how efficiency is measured and other factors that affect the efficiency and it may help answer your own question. There have been some good comments made since I last posted.

Are you just looking for standard efficiency of a NG boiler vs. oil?

 

[mfqd13]

Yes, it would be a starting point.

 

[davefitz]

marcosdia,

The HHV efficiency will be reduced due to the higher value of the "water vapor loss" term- natural gas will gnerator a lot more water vapor in the exhaust gas than fuel oil .

LHV efficiency ( and LHV fuel heating values) deducts the heat of vaporization of the generated steam vapors from the heating value of the fuel , so there is no significant water loss term in the efficiency calculation. As I recall , there have been a few published values of LHV efficiencies greater than 100% in the case of a condensing exhaust.