8 replies to this topic

[devesh sharma]

how does bridgewall temperature of firebox effects NOx emissions from burners?
which mechanism defines the effect of BWT on NOx of furnace?
is there any temperature correction formula apart from API?

[Padmakar Katre]

how does bridgewall temperature of firebox effects NOx emissions from burners?
which mechanism defines the effect of BWT on NOx of furnace?
is there any temperature correction formula apart from API?

Hi Devesh,
As per my knowledge I don't think there is any relationship between the BWT and Nox formation. If you see the kinetics of NOx formation, there three mechanisms available and those are,
1. Thermal NOx: NO formed from the reaction of N2 and O2 at high temperature is termed as thermal NOx. There are more than 70 steps involved in thermal NOx formation with rate limiting step as rupture of triple bond between Nitrogen by Oxygen. If you see the differential rate equation for thermal NOx, it neither depends on temperature, oxygen concentraion nor time.
2. Prompt NOx formation: This mechanism takes place at flame tip/front. Typical formation of NOx is less than 20 ppm(natural gas fired burners), this mechanism is more sensitive to available oxygen than temperature since the reactions are very fast.
3. Fuel Bound NOx formation: This mechanism takes place due to the fuel bound nitrogen concentraion. When we have low fuel-Nitrogen concentrations, almost all the nitrogen gets converted to NOx.
So the BWT is an indication of how efficient your radiant section performs. If the temperature exceeds, it will lead to substantial decrease in heater thermal efficiency if the convection section could not cater the heat load with the High BWT.
Hope this clears the doubt.

[devesh sharma]

Hi Padmakar,

Thank you for your reply. I completely agree with you on your thoughts that BWT has no relation with NOx formation as this is only a function of flame temperature and other factors as mentioned by you.

But i have created this doubt because burner vendors use BWT somewhere in burner design, so can you please give your views on this?
And also API-535 have a curve which shows some
relation between NOx and firebox temperature.

[S.AHMAD]

Good explanation by Padmakar on the formation of NOx.

High flame temperature increases conversion of N to NOx and bridgewall temperature is related to the flame temperature. Therefore, we can conclude that NOx formation is related to the bridgewall temperature for a specific design (surface area already fixed e.g. existing furnace). Under this situation, when we increase the furnace duty (by firing more) flame temperature will increase and so the bridgewall temperature.

Can we say indirectly related?

[devesh sharma]

see flame temperature is a function of fuel composition and excess air, if you increase firing while keeping excess air same as previous level flame temperature will remain same, however BWT will somwhat increase.
Now, in this situation will NOx emission rise?
If yes, can you please provide some correlation.

[S.AHMAD]

Hi Dev

NOx conversion is an equilibrium reaction of which the equilibrium constant is a function of Temperature. For chemical reaction there a minimum temperature to start the reaction that is correspond to the activation energy required to initiate the reaction. This concept is also apply to NOx conversion. BWT may be too low for such reaction to take place (you may need to check on the minimum temperature - off hand I don't have the figure).

This is the reason that I said the NOx conversion is indirectly related to BWT.

I hope you get the picture.

Edited by S.AHMAD, 11 September 2011 - 07:08 PM.

[Padmakar Katre]

Hi,
BWT is a strong function of the flame length. It does depends upon the fuel composition as the adiabatic flame temperature will be different with different fuels/blends and assuming the radiant sections performance as steady, it may have effect on BWT.
Really tough to find any straight forward technical logic to correlate these two terms, BWT and NOx formation. In case if I come across such information, I will definately update about the same.

[devesh sharma]

Thanks padmakar and ahmad for your views.

[kkala]

Having read this interesting topic, following opinions / notes may be useful.
1. Thermal NOx production rate increases with O2 concentration as well as with temperature, getting doubled for every 90 oC increase beyond ~1900 oC. http://jullio.pe.kr/.../ug/node624.htm.
2. Thus high local temperatures in the combustion chamber will create higer amounts of NOx, compared to gradual combustion in "stages" as in the low NOx burning systems. http://www.combustio...deri/node2.html. Reverse reaction rate is too slow to reduce already created NOx (as equilibrium would dictate), while flue gases are cooled in their way to chimney http://www.cheresour...ox-equilibrium/.
3. Bridge wall temperature (BWT) does not represent maximum flue gas temperature in radiant section, but rather a temperature at its exit, before convection section. This temperature represents the limit of radiative heat transfer and can be estimated by Lobo-Evans method http://www.heattrans....nl/heater.html.
4. So BWT under normal conditions depends on heater thermal design. It would be same for a concrete furnace design, equipped with low NOx burners or not. But in case of low NOx burners local temperatures in radiant section would be lower, so NOx escaping with flue gases would be also lower.
5. Consequently it is not the BWT that affects NOx formation, but the maximum (local) flue gas temperature in radiant section (tmax). High tmax does not necessarily means high BTW among several furnace designs.
6. On the other hand, coke or overload of a concrete furnace will increase both tmax and BWT, as well as NOx formation. In this sense one can say that BWT and NOx emission are “indirectly" related.
7. Any comments / additions are welcomed to the above.

Note on what mainly influence BWT: http://nishilg.blogs...emperature.html.

Edited by kkala, 16 September 2011 - 07:35 AM.