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Low NOx burners and furnace performance

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athomas236

Mechanical
Jul 1, 2002
607
I am involved in a project with some new 500tonne/hour oil-fired boilers that were designed to the thermal performance standards of an Eastern Block country.

The boilers are not performing as expected and it seems that the reasons for this may be that the furnace performance is not as expected. The supplier is claiming that the fuel oil burned in the boilers is showing different combustion characteristics to those of oils burned on his other projects. These characteristics being less intense combustion with the consequence that the flame is higher in the furnace than expected giving furnace exit temperatures higher than expected.

I think the supplier may be partly right because we have low NOx burners which could result in combustion characteristics different to those of conventional burners which could well be the basis of the thermal performance standards used.

So this is my question. Is there any evidence that suggests that using low NOx burners will give furnace exit gas temperatures that are different to those with conventional burners.

Regards,

athomas236



 
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Different type of Low NOx burner will perform differently.
Output efficiency depends on excess air and some LowNox burners depend on very high excess air thus loosing efficiency of up to 15%
Yes Low-NOX can be different of conventional as conventional burners can be brought to very low O2.
You should contact a reputable Burner Mfr. I will try to get a contact from my supplier maybe they can help. I they do,I will post their web site.
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I remember some type of low NOx burners limiting air to reduce combustion temp. More air was added later ; so "combustion characteristics" were certainly different.
So my suggestion is also to contact a burner specialst.
 
Gentlemen,

Thank you both for your advice.

Best Regards,

athomas236
 
Low NOX burners typically initiate the combustion with lower excess air in order to keep the overall flame temperatures down so that less NOX is formed due to flame temperature. Then air is added later in the combustion process such as overfire air to take it to complete combustion (hopefully). Some burners, especially corner fired burners have such a reducing zone near the burners that there is metal loss from the furnace wall in the vicinity of the burners. I haven't been involved in burners recently so I don't know if they still fight this or not.

The whole purpose of low NOX burners is to keep the ultimate flame temperatures down.

Conventional burners, on the other hand, fog all the air that the flame envelope is going to get right through the registers into the fuel and depend on good mixing to attain complete combustion. When it works well, the flame is very hot.

To me this would result in a hotter flame close to the burner tip, mixed with all the excess air coming through the registers which would pull the flame (and hence the higher heat) in closer to the burners while the low NOX burners would tend to push it out.

Riley Stoker's old turbo furnace design was based on the fact that when you fired downward, the harder you fired, the lower you pushed the flame in the furnace so that when you fired harder for higher load, you didn't overheat the Superheaters.

CE's corner fired units just tilted the burner down for SH control and if they needed to increase the SH temp, they tilted up towards the SH banks.

You may be getting an unintended similar effect. Your low NOX burners should have some type of secondary air addition. You could try upping that to try to cool the combustion products. It would raise your outlet O2, but at that point, low O2 is less about complete combustion and more about total throughput through the boiler banks.

Do you have a combustables analyzer that can monitor your stochiometry if you increase the air and raise the O2?

rmw
 
Ok back in the UK now

The plant I was at when I made the original post was built in three phases and I am working on phase 3 which being commissioned.

The problems that are being experienced with phase 3 boilers are excessive spray water flow (which means four times higher than predicted)and furnace wall vibrations.

As I said earlier the boiler supplier is saying that the fuel oil burned in the boilers is showing different combustion characteristics to those of oils burned on his other projects and this is the reason for the problems. I have tried to suggest to the boiler supplier that maybe the problem is due to the low NOx burners but he will not accept this.

The phase 2 boilers have now been fitted with low NOx burners of the same design and supplier as those for phase 3, guess what, after 10 years operation without any problems, the phase 2 boilers are now experinceing high spray water flows and furnace wall vibrations.

Best Regards,

athomas236
 
Funny how things like that work out. Low NOX burners were just appearing in the latter days of any serious work I did with large utililty boilers, but they were doing numbers on furnace walls in lots of situations where they were used that I was involved with.

rmw
 
Low NOx burners were not in my time either. I stopped being a boiler designer in 1977. Back in those days we were still trying to build 660MW oil fired boilers in the UK.

athomas236
 
There are different types of Lo Nox combustion systems. Lowering flame temperature is one method of reducing thermal nox, but deliberately generating CO also causes the CO to later reduce the NOx , so substochiometric combustion in the lower furncace followed by combustion completion with overfire air allows the CO to remove radicals such as Nox.

One means of predicting furnace heat absorption and predicting FEGT funace exit gas temperature is the "russian Normative Method" published by a.G. Blokh circa 1988. Using that method and adjusting for the elevation at which complete combustion occurs should provide an estimate of teh effect of Lo Nox burners and/or staged combustion with overfire air.
 
Gentlemen,

Thanks again for your replies.

I have searched with Google for the "Russion Normative Method" but without success, Any suggestions on how to find this method would be appreciated.

Best Regards,

athomas236
 
athomas236:

The book title is "Heat Transfer in Steam Boiler Furnaces " by A.G. Blokh, published in 1987,used copies may be found at amazon.com. Please not that many typographical errors are found in the english translation.

Another version of the same method can be found in S.Kakac's book "boilers , evaporators and condensers" in the chapter by the chinese boiler designer ( I forgot his name)
 
Thanks for the information.

I have searched for both books and cannot locate the Blokh book but I can locate Kakac's book which can be bought from Amazon for £190 (new and used are same price). So I have asked my local library to request the books from the British Lending Library at the great cost of £2.50 each.

Just have to wait and see what happens, might try I.Mech.E library.

Have been asked to chair meetings with EPC, boiler and burner contractors together with client starting in two weeks. Should be fun.

Best regards,

athomas236
 
Athomas236,

I don't think low NOx burners is the issue. Flame could be a little bit longer, but not enough to change FEGT enough to affect heat transfer in the superheater. Low NOX burners lower the peak flame temperature, but not the average flame temperature. Is thera an OFA installed? FGR?

In any case, any of your supervisors could check flames through sight ports in the furnace wall and see the shape of the flame.

What does the supplier say about the fuel? Why they say it is nt the same? Has asphaltenes content and/or Conradson residue change a whole lot? IS the furnace clean?

Most probably the supplier has been conservative and superheater surface is too large.

Regards,

Darshiva
 
Darshiva,

There is no FGD or OFA.

Regards,

athomas236
 
Athomas236,

If there is no OFA or external FGR, most probably, oil low NOX burners are based on air staging. Typically burner will be of Venturi style or double register style. It will have a central priamry/secondary air stream going through an axial swirler and a tertiary air stream enveloping the flame. If dampers for controlling the amount of air going to each zone exist, your supervisors can adjust them for shaping the flame -eg: closing the tertiary damper will drive more air to the core of the flame, enhancing NOX generation and shortening the flame. See if this fact impacts your furnace temperature profile.

Other thing you can try is to change the amount of atomization steam, by setting the DP steam-FO in the DCS. This could help to alleviate furnace vibration, if the flame is pulsating.
 
darshiva,

So far the contractor has made 25 tests to establish the conditions that minimise the possibility of vibration. These include the following suggestions:

"The probabilities of vibration occurring are lowest when:
? the fuel oil pressure is more than 2 bar greater than the atomising steam pressure,
? the fuel oil flow per burner is above 2.5 tonne/hour (which requires the operation of the fewest number of burners whilst not exceeding the rated capacity of each burner) and,
? all burner dampers are open."

Best regards,

athomas236
 
So these boilers are firing what we in the US would call #6 Fuel oil? i.e., a heavy residual oil?
 
A number of oils are being burned and these include a 1500sec and 3500sec Redwood.

Regards,

athomas236
 
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