Flue Gas Recirculation

Hmmm...only ever saw gas recirc used as a means of superheat/reheat steam temperature control; would not the boiler's/steam generator's thermal profile, pinch points, economozer sizing and duty, etc., be so designed as to capture/"recover" heat right from the get-go?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

Hi crshears.

It's not a boiler or a steam generator. In an ideal world, yes the system original design should have considered this but unfortunately that was not the case. Now I retrofit to improve.

Regards,
PR

The scientist describes what is; the engineer creates what never was.

I saw this post in the Chemical Plant Design and Operations forum first, and responded there. Double posting is discouraged; I recommend you flag that post for deletion.

I've copied my response below:

On fired boilers, I typically see flue gas recirculation into the inlet of the forced draft fan utilized as a means to lower NOx production in the furnace - you add in the necessary turbulence for mixing the air and fuel without adding in more oxygen than you require for complete combustion. Heating the incoming air is somewhat of a side effect. If you're concerned about the temperature of your combustion air, you'd be better off with a steam coil air heater, or something similar. I've not heard of flue gas recirculating directly into the furnace or windbox - and would not recommend doing so as it would disrupt the uniformity of your air flow and destabilize your flame.

If you utilize flue gas recirculation, you'll need to retune your burner(s) to account for the impact to your air flows and O2.

P.S. Since it isn't a boiler or steam generator, can you give us an idea of what application you're dealing with?

Hi KoachCSR,

Apologies, I didn't know about the double post faux pas, thought I might reach a different audience there. Deleted now.

Thanks for your response here, interesting and helpful.

The reason we have a problem with high gas consumption is that we are using ambient 'dilution' air to create the velocities needed in the furnace chamber for heat transfer. My concept is to replace the ambient dilution air hot exhaust gas, therefore reducing the heat required to raise the temperature. To be honest, flame stabilisation is my main concern but I plan on taking care of that by feeding the exhaust gas into a mixing chamber downstream of the burner & combustion chamber and upstream of the furnace. There should be sufficient distance between exhaust gas entry and flame to negate the chance of a problem, and anyway we would only be replacing an existing flowrate for a new one albeit at a higher temperature.

I can't/won't give anymore detail on my application and don't think it is appropriate. Purely looking for sound advice based on logic and/or experience.



The scientist describes what is; the engineer creates what never was.

Keep in mind that you wouldn't simply be replacing one flow rate with another, you'd also be changing the amount of O2 introduced to the flame when you switch to flue gas recirculation. With the oxygen supplied to the flame altered, you'd need to re-tune your control system to account for this change, as well as verify that your burner can still perform properly at these new conditions. If you simply found a way to pre-heat the "dilution" air you're currently using, you could avoid this complication. You'll need to compare the economics of your options to determine your best path forward.

The oxygen supplier to the flame per moles of natural gas won't be changing as the re-circulation stream will enter downstream of the flame. I must not have been so clear in my description. Yes we will need to re-tune the furnaces to a certain degree, hopefully only on the automation side.

Thanks for your time. My takings from this is that it is a novel application and should be handled with care. We start with an R&D trial only and judge results before proceeding.

Thanks

The scientist describes what is; the engineer creates what never was.

Recirculating flue gas for additional heat extraction sounds like perpetual motion. Gas recirculation is used to lower flame temperature due to lower O2. A recuperator or regenerator can be used to heat the inlet air through heat exchange.

Hi Compositepro,

Not perpetual motion, as heat & flow will be added from the burner, just less than at present. And excess flue gas will be bled through the chimney. I'll look into those options.

Mine's is an unconventional application, and as such I don't have the luxury to use typical heat recovery options which is why I'm exploring this. I must say I can't see any firm reason from the comments as to why this won't work.

The scientist describes what is; the engineer creates what never was.

The energy in the the flue gas you are putting back into the combustion chamber just came out of the combustion chamber. Net effect is zero except for the displacement of oxygen in the feed, which is required for combustion. This idea is silly. You will eventually realize it.

Why can't you use air preheaters? How can you avoid quenching of flame by CO2 in flue gas?

Compositepro,

Gas leaves furnace @ 400C>
Gas blown back into mixing chamber upstream of furnace @ 400C
ENERGY ADDED via combustion of SEPARATE fuel and air stream to raise mixed gas to 600C
Gas leaves furnace @ 400C
Repeart
Please tell me how that is silly? It's using the recirculated gas as a medium which is heated in order to achieve velocity required for heat transfer.

Your statement does not make sense, perhaps you have not understood the thermodynamics of the problem. Calling an idea "silly" without offering logic or a well reasoned argument is not productive.

Hi Quark,

I'm looking at air preheaters too but cost is much higher per energy saving as per my calculations. The flue gas will enter a mixing chamber downstream of the burner, this will be designed to prevent flame quenching.


The scientist describes what is; the engineer creates what never was.

Gas recirc had been used for NOx control ( lowers flame temp) , control steam temperature, and reduce furnace flame bouyancy effects. Recent oxi-combustion units will recirc gas to lower flame temperature as well. Historically gas recirc fans that operate above 750 F have had very low availability and tend to self-destruct after a few years of creep damage under centrifugal forces. There are alternative means to solve flame issues now that 3d fluid dynamci softeware is available to model the flame zone and the furnace.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick