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"refractory Coating And Fuel Saving"

fire heater emissivity radiation refractory coating furnance firebox fuel saving

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#1 obbchao

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Posted 22 October 2016 - 12:30 PM

Hello all,

 

Does anyone know how to calculate fuel saving in fire heater after perform firebox refractory coating?

 

I saw many vendor claim that after coating, fuel gas will reduce by 6-10%.

In my understand coating material are change the emissivity of refractory  from 0.7 to around 0.9 but as I try to simulate in Aspen EDR, when I change emissivity of refractory fuel gas are reduce only 0.03% (very less!!).

 

If someone would show me detail to evaluate fuel saving and emissivity effect in firebox would be great.

 

 

Thanks in advance.

 

 

OBB

 

 

 

 

 



#2 Saml

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Posted 22 October 2016 - 08:42 PM

The coating works by absorbing  and radiating in the whole frequency closer to the distribution of a black body, While the uncoated refractory reflects part of the incoming radiation and this radiation is re-absorbed by the combustion gases.

 

Take a look at the article "High-Emissivity Coatings on Reactor Tubes and Furnace Walls in Steam Cracking Furnaces" Chemical Engineering Journal 137 (2008) 411–421.

Textually from that article: "The effect of high-emissivity furnace wall coatings on the furnace thermal efficiency is explained on the basis of the fundamental difference between radiation in gases and radiation from surfaces. Surfaces absorb and emit radiation at all frequencies.Gases absorb and emit radiation at certain discrete frequencies"

 
The net effect is that the radiation reaching the tubes is higher. There is a caveat however: flue gas from the radiant section will be cooler. So the saving is true as long as you are not using the convective section to preheat you furnace feed or for other use that requires that you burn gas elsewhere.
You will achieve a saving if the flue gas at the stack is cooler or you can tolerate a cooler convective section: i.e.  you are generating steam you can reduce by replacing a turbine for an electric motor.
 
What the coating do is to change the balance between radiant and convective section. But the energy balance from the gas supply to the flue stack is the same. The amount of gas will be determined by the heat load needed, the excess air and the flue gas temperature (and air preheating temperature if you have it)
 
So take a look at Aspen EDR to see if it is reflecting this change (I don't know if that is possible at all, I have no worked with that software). 
 
Anyway, the coating supplier should be able to explain where the savings are and what you can expect both in the radiant and the convective sections of your fired heater.


#3 obbchao

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Posted 24 October 2016 - 07:38 PM

Thanks Saml for explanation, even I try to study the theory of heat transfer base on your information but I still would like to know more about the magnitude of fuel saving after wall emissivity change.

 

I still searching for method to calculate that data.

 

 

regards,

 

Obb



#4 Saml

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Posted 27 October 2016 - 06:40 PM

You may already have a method if Aspen EDR has this into account.

 

When you go to older methods (like the Lobo and Evans in Kern) you see that they consider all the radiation reaching the refractory as re-emmited, but they make no difference on whether it is by reflection or absorption-radiation.

 

Aspen EDR may have this calculation sophistication already built in (that is, that the gas is more transparent to absorbed-re-radiated radiation than to reflected one).

 

The way to know is to look at the radiant section results, or to simulate a furnace without convective section.  

 

The total gas needed may be kept constant by the simulator either:

a-) because Aspen EDR does not considers the effect  of wall emmissivity (then you have no way to calculate with it) or

b-) because it does, but the specs are such that in the second case you have more heat transferred in the radiant section and less in the convective but overall the same.

 

In the case I was involved, we relied on the supplier calculation. They were accurate. There was no fuel saving, by the way. The main benefit for us was lower tube metal temperature, that was a bottleneck. The heat in the radiant section of the reformer increased by 4% and the flue gas temperature leaving the radiant section was reduced initially by about 40°C. Then we had to change the firing profile to return the flue gas to the previous temperature (we needed the heating in the convective section), but this resulted in a reduction in the peak tube metal temperature.


Edited by Saml, 27 October 2016 - 06:42 PM.





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