Is "Stoichiometric" combustion possible?

Tim McElwain

I just finished a three day combustion class at my training center. There was some discussion about reaching "perfect combustion" which is what stoichiometric is all about.



That would be for gas:



CO2 at Ultimate 11.7%



O2 would be zero



CO would be zero



Excess air zero



Efficiency 100%



Flame temp 3495°



Dew point 140°



What do you think? Has anyone ever experienced this condition on Gas or oil for that matter?



With Mod/Cons what are you getting for combustion readings?



Can I have zero CO and still not be at stoichiometric?



I am interested to see what comes up with this posting. I will give you what my experience has been in over 55 years of testing with both "wet kits" and electronics.

Steamhead

Not in the real world

even under the best conditions, when you get down to 20% or so excess air you start getting CO, and on oil you get smoke also. And of course one should never leave a unit on the bleeding edge like that, since air and fuel characteristics can and do change over the course of a season. 

Mark Eatherton

Been close, but never at Stoichiometric...

I've seen 2 burners, both atmospheric with ZERO ppm AFCO, but have never seen perfect stoichiometric conditions, even with modcons.



With a mod con, I'm thinking that the flue gas temperature would be dictated by the operating conditions of the appliance, and that can be all over the place.



I have seen zero O2 one time, but it was burning so rich that the CO was off the charts (think overpowered LP being drawn in due to restriction on the inlet air acting as a choke.)



ME

Tim McElwain

Have had Zero CO

on many occasions because I was using a Testo 325 or 330 tester which filters out the NoX in the Air free CO so you can see a Zero reading. This usually occurs when O2 is around 2% to 4% which gives less than 25% excess Air.



I have had some interesting fun with using a restrictor on the intake air on a couple of Mod/Con furnaces and boilers with seeing O2 around 2% or less.



What are most of you seeing with Mod/Cons for combustion efficiency. I am not a big fan of talking calculated efficiency which you get on testers but I am curious to see what readings are being obtained.

Jamie Hall

In principle...

sure, why not?  In practice -- no.  This is true whether it's an atmospheric burner or a mod con -- or a $100,000 automobile racing engine or a multi-million dollar aircraft gas turbine.  The problem is that to get perfect stoichiometric combustion, you would have to have perfect conditions everywhere.  In practice, you have all kinds of problems with varying fuel/air concentrations throughout the flame front and in the firebox.  The big boys in the engine field go to truly incredible lengths to get those variations sorted out -- and keep them sorted out.  In our real world of boilers in the basement... it just ain't worth the effort.

Charles Johnson

Stoichiometric Combustion

If you could feed the burner with 100% pure O2 you would be able to achieve it.

But its not possible in the real world.

Jean-David Beyer

If you had excess money,

you should be able to feed your gas burner with pure oxygen. I am not a combustion expert, but it seems to me that you might get too hot a flame if you do that, and risk harming the heat exchanger. If you lower the gas and oxygen delivery, you might be able to correct that. Someone here might know what a bottle of oxygen costs, and how many bottles a month you would need to burn the natural gas. I cannot imagine any way to save money doing this, though.

Charles Johnson

Stoichiometric

As said above the hottest the flame would get would be around 3500°F on natural gas.   That would not likely be a problem for the heat exchanger if you have enough fluid (air or water) flow to keep moving the heat away.  An ideal O2 level for atmospheric gas burners, is 6% which gives you a flame temp of 2800°F.

Charles Johnson

Is it possible?

You asked Tim if it was possible to see zero CO and not be at Stoichiometric combustion.

Based on my experience I would say it is.  My analyzer has a NOx filter and I quite often see zero CO on oil and sometimes on gas.  Or is my CO sensor not calibrated correctly??

The lowest O2 level I have seen on oil with zero smoke was 2.5%.

Mark Eatherton

Numbers from my lil Munchkin...

Tim, I drained my DHW tank and fired it up and tested it.



O2 = 5 %

CO = 86 ppm

CO2 = 9 %

Stack temperature (inside) 114 degrees F

Stack temperature (outside) 100 degrees F

Incoming air temp = 69 deg F

excess air = 27.7 %

Air Free CO = 113 ppm

Boiler inlet temperature = 108.3

Boiler outlet temperature = 139.1

Flow rate thru boiler = 3.5 GPM



Of interesting note, the fluid temperature is significantly higher than the flue gas temperature. I let it run for 10 minutes before I recorded the parameters, so it can't be charged off to a fluke condition.



This boiler has been in operation for about 6 years now, on PVC venting, and there are NO signs of any issues with the venting system.



Attached is a screen shot of the boilers operating parameters for those who are interested.



What do you think?



ME

bob_46

Stoker-metric

I saw a lot of stokers in my day but never saw a stoichio. All the fellows I worked with called it stokermetrics kind of like the psychiatric chart. I am not sure if it is still relevant but

some large municipalities specified in their code maximum CO2 percentages for nat.gas.

If I remember 10% was max and some were even lower. They didn't want any burners set on the knifes edge. bob

Tom Blackwell_2

Research

All joking aside, it's theoretically possible, but not even in a lab can it be done. To have a purely stoichometric burn, it would require each fuel molecule to combine with an oxygen molecule. Not going to happen in "real life". The closest I have seen on large boilers is O2 at 2.5%, before making copious amounts of CO. The new Miura burner design comes close, with mixing vanes in the gas/air duct ahead of the combustion zone. Oddly enough, duct burners utilizing fresh air firing with O2 at more than 10% make lots of CO because of flame cooling caused by too much excess air. I have learned that it's best to leave the setup on the lean side, so changes over time don't cause a dangerous condition. The old fan/premix burners should have come close, but again suffered from flame conditions and required much excess air to burn without making CO. All that being said-much progress has been made for efficiency. The trade-off is higher complexity and shorter lifespan.