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Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Carbon Monoxide vs Burner Clearerance on 04/18/2013 18:28:46 MDT Print View

Hi James

> Well, generally, CO is generated from reburning CO2.
Sorry, but I have to disagree here.
I gave the dynamic chemistry details in Part 1 of the series on CO in the section headed 'Flame Chemistry'.

Basically, burning happens in a series of simple single-molecule steps:
C + O => CO + energy
CO + O => CO2 + energy

First CO is formed, then with extra oxygen CO2 is formed. Quenching the flame by having the pot too close to the burner head interrupts the second step, so that CO escapes. I don't believe that the flame is ever hot enough for CO2 to combine with elemental C to revert to CO - if you have references for this please let us know!

Yes, having lots of oxygen mix with the fuel inside the burner tube is good, but it is not absolutely essential. The FMS-300 stove has tiny air inlet holes, far smaller than normal, but it burns cleanly. It has somewhat different oxygen mixing path and flow behaviour which is explained in the Stove Developments article.

Why does one stove have good CO performance while another very similar-looking stove has higher levels of CO? A tricky question. Does the gauge of the mesh inside the burner head have anything to do with it? (Does the shape of the burner head have anything to do with it?) Yes, in an indirect manner - the size of the holes in the mesh does affect the back pressure inside the burner head. This affects the speed of flow of the fuel/air mix through the burner. The back pressure and the shape of the inside of the burner column affect just how much air gets sucked in, and the speed of flow out of the burner head affects how much extra air (oxygen) gets sucked into the flame.

I did succeed in making a burner head which burnt better when I blocked off half the air inlet holes. Sure, they were big holes, but the air flow inside was all wrong. As I said, tricky!

Cheers

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Re: Carbon Monoxide vs Burner Clearerance on 04/19/2013 08:44:44 MDT Print View

Roger,
Sorry for the delay, GG sent me a new pack and I was going over it...

"Basically, burning happens in a series of simple single-molecule steps:
C + O => CO + energy
CO + O => CO2 + energy"
Yes, this describes the intermediate steps in combustion. I was wrong. I should not have glossed over this. I agree that quenching heat at that point will produce large amounts of CO, regardless of the mixing ratio of the gasses. Rather than leaving unattached ions floating around, they will simply form into the most stable compound available at that moment.

That was not my point. I would note that CO2+CO2+heat->2CO+02, though. Channeling combustion bybroducts (considering only CO2, for this example) into a perfect flame, (such as shown in Hikin Jims picture above) will produce relatively large ammounts of CO, too (measured in ppm.) Many wind screens/heat screens do exactly that by restricting the outflow (or circulation if you will allow such a loose term.) I do not believe you examined this aspect of CO production in your article except in passing. Restricting the outflow of combustion gasses, pretty much regardless of the stove used, will allow increased CO production by recycling exhaust products, CO2 in this case, through a flame and heating it. External oxygen, inflow, may or may not be adequate, it doesn't really matter since the exothermic type reaction will produce an excess.

"I did succeed in making a burner head which burnt better when I blocked off half the air inlet holes. Sure, they were big holes, but the air flow inside was all wrong. As I said, tricky!"
Sorry, I don't follow... Turbulence in the airflow, ie after the jet and air inlet, should be a GOOD thing, it promotes mixing of fuel/air.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Re: Re: Carbon Monoxide vs Burner Clearerance on 04/19/2013 16:47:27 MDT Print View

Hi James

> CO2+CO2+heat->2CO+02
Yes, that is possible of course, but the reaction is hugely endothermic.

For everyone else, that means you have to pump a lot of heat into the reaction to make it happen. Since the reverse reaction of O+CO->CO2 gives off heat, the thermodynamics really drives the chemistry one way - to burn the CO up.

The probability that this disassociation will happen is very low, so the resulting concentration of CO will also be very low. If you suck heat out of the flame there will be even less heat available to drive this reaction. Of course, all this is at equilibrium; a dynamic situation is more complex.

It would be fairly hard to test this experimentally, but my gut feeling is that this reaction path (->CO) is just not going to be significant - especially as the reverse path (-> CO2) can still happen. And especially when there is a pot sitting there waiting to absorb any liberated energy.

Bottom line: allow exhaust fumes from your stove to escape! It's OK to cook in the vestibule of your tent as long as there is ventilation. Many of us have been doing nthat for decades.

Any experts in flame chemistry around?


> Restricting the outflow of combustion gasses, pretty much regardless of the stove
> used, will allow increased CO production by recycling exhaust products,
I agree, which is why I always say you should leave a 10 - 20 mm gap around the pot.
But inadequate oxygen availability also generates CO, and that is what often happens with fuels based on heavier carbon chains (eg white gas and kero).


> Turbulence in the airflow, ie after the jet and air inlet, should be a GOOD thing,
> it promotes mixing of fuel/air.
True, very true ... AFTER the jet and air inlet.
However, it is possible to design a burner tube so that the turbulence happens at the air inlets, blocking the inflow of air! Some (a few) commercial burner columns seem to me to be very badly designed.

Cheers

Edited by rcaffin on 04/19/2013 16:53:49 MDT.

Hikin' Jim
(hikin_jim)

Locale: Los Angeles, CA, USA
Carbon Monoxide vs Burner Clearance on 04/19/2013 19:05:17 MDT Print View

Roger,

Thanks for an excellent discussion. Thanks also for the link to the CO article. It's been a while since I read it, but it's good to have a refresher. Extremely helpful and informative.

HJ
Adventures In Stoving

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Re: Re: Re: Carbon Monoxide vs Burner Clearerance on 04/19/2013 19:20:54 MDT Print View

Roger,
Yes, quite correct.
"The probability that this disassociation will happen is very low, so the resulting concentration of CO will also be very low. If you suck heat out of the flame there will be even less heat available to drive this reaction. Of course, all this is at equilibrium; a dynamic situation is more complex."
Sure, but the actual toxicity of CO2 is low in comparison to CO. Generally measured in PPM. I cannot remember the math to calculate the exact equilibrum equation for this but I am sure it results in a significant increase in CO production. 40 years stale, I am afraid. Anyway, since the actual heat loss is directly proportional to the CO increase, I really doubt that a few parts per million in heat loss would be be noticable. The biggest problem is it's effective toxicity as outlined in "Carbon Monoxide Toxicity", David Penney, et al. Basically, it binds permanently with hemoglobin. But the text is geared more to fire fighters.

Got it...Just for grins, I looked up CO2 production in flames and found many references. Here is a fair starter. I read through this one several years ago when I was working on alcohol stoves and describes several. But, I quickly found out that the fuel was so-so and not real great. Just not enough heat. Canister gas could be great, except for the weight of the cans. WG/Kero would be great except for the weight of the stove. Ahh well, it ain't a perfect world. Anyway, this has the equations for the equilibrum reactions and can be integrated over the temperature ranges as you were looking for. Though I sort'a doubt that's really necessary. At STP and max temp of isobutane should give you a ballpark. Most alkanes are within a couple hundred degrees.

http://books.google.com/books?id=oiPLRWMPL4UC&pg=PR15&dq=CO2+production+in+flames&hl=en&sa=X&ei=CudxUeODC5PF4AOmjYCYBA&ved=0CF0Q6AEwCDgy#v=onepage&q=CO2%20production%20in%20flames&f=false

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Re: Re: Re: Re: Carbon Monoxide vs Burner Clearerance on 04/20/2013 01:37:38 MDT Print View

Hi James

The toxicity of CO was of course the entire point of the 6-part series onCarbon Monoxide and Stoves. No arguments there. Ventilation!

"Combustion and Flames" - sigh, I know. Many text books on flames, especially on large industrial burners. I hadn't read that one. And let me cheerfully admit that thermodynamics was never my favourite subject at Uni.

"For the propane/oxygen stoichiometric mixture, at constant pressure, a temperature increase of 6340 K is calculated if it is assumed that the final mixture only contains CO2 and H2O, and a value of 3801 K if all the possible equilibria are taken into account, ie a final mixture which additionally contains CO, H2, O2, H, O, OH, etc"
"Combustion and Flames", p42

But that quote is for pure oxygen, not for air. Good sources suggest a flame temperature for propane/butane mixes in air (not oxygen) of 1970 C or over 2200 K, but that is the theoretical maximum. That is almost 1000 C/K lower than for oxygen. When you stick a pot on top the flame temperature is going to be hundreds of degrees lower. In fact, some sources suggest that propane/butane in air will struggle to get much over 1200 C.

So we may expect that the actual flame temperature for a typical canister stove is often below 1800 K, and the graph on p43 suggests that CO production at that temperature is going to be very small. That sort of temperature is not really hot enough for the reverse reactions to be significant (fortunately).

Cheers

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Re: Re: Re: Re: Re: Carbon Monoxide vs Burner Clearerance on 04/20/2013 03:54:12 MDT Print View

Roger, yupper. I seem to remember about 2000C but not an exact number for most alkanes. Chances are, at 6400K they were looking at nitrogen compounds, too...probably why they used pure O2.

Yes, I agree. "very small" includes PPM though, unfortunatly. Not deadly by any means. But cumulative effects could start showing up after a couple weeks in a constrained system with lower pot stands. This worst case could prove to be bad on a camping trip, again, I don't think it matters which type of fuel is in use. But it could matter more which type of stove is in use. Example: The FMS300T vs the Kovea Spider. These are two types of burners that will have different responses to wind screens based on CO production. Other burner types are certainly possible such as MSR's IR stove, Reactor.

Hikin' Jim
(hikin_jim)

Locale: Los Angeles, CA, USA
Re: Carbon Monoxide vs Burner Clearerance on 04/20/2013 09:48:43 MDT Print View

... it could matter more which type of stove is in use. Example: The FMS300T vs the Kovea Spider. These are two types of burners that will have different responses to wind screens based on CO production. Other burner types are certainly possible such as MSR's IR stove, Reactor.
I think you've hit the nail on the head, James.

Look back through Roger's Carbon Monoxide series. The Reactor (note however that Roger had a prototype) had scandalously high CO when run at low flame. The Reactor is one of the hardest stoves to get to simmer. Maybe that's by design. If you don't run the stove hot enough, the airflow cycle stalls, and you get a lot of CO.

The other intresting one out there is the Crux vs. the FMS-116t. The burner heads are outwardly identical but the CO output is far less with the FMS-116t.

HJ
Adventures In Stoving

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Carbon Monoxide vs Burner Clearerance on 04/20/2013 10:31:38 MDT Print View

Jim,
I had forgotten that was part of the series, my mistake, but thanks for straightening me out. They do say that short term memory is the first to go...

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Carbon Monoxide vs Burner Clearerance on 04/20/2013 18:36:28 MDT Print View

> The Reactor (note however that Roger had a prototype)
AND a production model later. The proto was XS bad, the prod model was not quite as bad - but still very very bad.

Yes, they need a blower under it for air flow!

> The other intresting one out there is the Crux vs. the FMS-116t. The burner heads
> are outwardly identical but the CO output is far less with the FMS-116t.
That is purely the outwards appearance. Internally - very different.

Cheers

Hikin' Jim
(hikin_jim)

Locale: Los Angeles, CA, USA
Re: Re: Re: Carbon Monoxide vs Burner Clearerance on 04/20/2013 21:26:22 MDT Print View

That is purely the outwards appearance. Internally - very different.
Indeed, it must be internal for I can discern nothing from the outside. Well, the air inlets are a couple of cm closer to the burner head on the FMS-116t, but that's about it. Maybe the mesh inside the burner head is a bit finer on the Crux which might create back pressure (similar to the Snow Peak LiteMax vs. Kovea Supalite). Interesting subtleties.

HJ
Adventures in Stoving

Stuart R
(Scunnered) - F

Locale: Scotland
Flame chemistry on 04/21/2013 03:22:24 MDT Print View

The details of flame chemistry are extremely complex and my lmited knowledge barely scratches the surface:

Step 1: Formation of radicals and breakdown of fuel from radical attack. The radicals strip off protons and break C-C bonds eg.
O2 + H -> OH + O
C2H6 + O -> C2H5 + OH

This continues with dozens of different reactions at different rates until all the fuel has been converted to CO and H2

Step 2: Final oxidation of CO and H2
H2 + OH -> H2O + H
H + OH -> H2O
CO + OH -> CO2 + H

The final step can be inhibited by quenching the flame, such as by a pot, which can result in CO being present.

There are other reactions possible between CO and CO2, such as the Boudouard Reaction, which can happen when CO in a chimney stack is cooled, or can happen in reverse in a blast furnace. This does not happen in an open flame:
2CO <-> CO2 + C

>>> CO2+CO2+heat->2CO+02
Just doesn’t happen

I'm sure David will chime in...

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Flame chemistry on 04/21/2013 07:02:30 MDT Print View

There are a lot of differing sub-optimal reactions that could and do occur with lower probababilities. In a perfect world, these would not occur. Taking your above example there is no accouning for the pure soot (C) which can be induced out of a flame. Such things are small parts of a reaction, such as NO. (BTW: Roger, in the text "Cumbustion and Flames" the chart was on p43, and the calculations *were* done in air, not pure oxygen, note the NO on the bottom of the chart. Though it seems that NO breaks down at the higher heats rather than being produced, I was wrong.)

Anyway, getting back to the discussion of the primary path way of hydrocarbon combustion, you can ALWAYS compute an equilibrum component. Again the stoichioistic equations gloss over any intermediates. Roger's reference was a fairly good one and typical of such texts. Overall, chemical reactions are ALWAYS are two way. This is in itself, a simplified statement of the actual events. Isomeres, sterioisomeres, and lots of other examples can be looked up, I am sure. This is also why some chemicals can be built up and some broken down in chemical plants. The conditions for a reaction usually decide which will occur most frequently, but this is NOT saying less probable reactions will NOT occur. Indeed, often the smaller fraction is what we are after, but that is seperation. Hmm, as I remember, Miller/Urey passed an electrical current through methane,and other gasses to produce all sorts of organic compounds. How? If not for reverse reactions? This seems to go against simplistic chemistry, but pchem/thermodynamics shows it does not. I am not defending such a silly thing as the equation. It is NOT an absolute description, rather an easy way to comunicate the thought to Roger and others. Do not take it lterally. Again, this is just one part of a rather complex ball wax, if you will. But one that sheds light on circulation, gas mixing, and combustion generally, that might help boil a cup of water out camping. Waste of time? Likely, hey, ha...You do not need to know that crap to boil water without breathing the exhaust fumes. Just stuff that was plugged into my head 40teen years ago.

Jeffs Eleven
(WoodenWizard) - F

Locale: Greater Mt Tabor
Re: Re: Flame chemistry on 04/21/2013 11:32:28 MDT Print View

"Again the stoichioistic equations gloss over any intermediates"

I don't know if all this is a stoichioistic equation or not, but i must be an intermediate, cause it glossed ME over.

Alls I'm sayin is if y'all figure something out let the rest of us know!

:)

Jim Sweeney
(swimjay) - MLife

Locale: Northern California
stochastic or stoichiometry (or stoichiometric) on 04/21/2013 15:51:18 MDT Print View

Probably one of these words is meant?

Edited by swimjay on 04/21/2013 15:51:51 MDT.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Flame chemistry on 04/21/2013 16:22:31 MDT Print View

> accounting for the pure soot (C) which can be induced out of a flame

It sticks in my mind that it is easier to strip a H off a C chain so it can be oxidised than it is to oxidise the C chain. I forget the details, but I think it may be the higher energy required to break a C=C bond than to break a C-H bond. Anyhow, what that means is that something like C=C-H + O -> C=C + OH is going to happen - giving soot.

Translation: that's why a COOL candle flame glows brightly and makes soot: all those bits of C+C floating around unburnt and radiating, while a gas (canister) stove makes a far hotter blue flame with little soot.

From memory ...

Cheers

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: stochastic or stoichiometry (or stoichiometric) on 04/21/2013 18:53:50 MDT Print View

Ha, ha...yeah, something like that... Rather like the compression of Beer Ball and Bear Canister -> Bear Ball. (Inside joke, I guess...ya had to be there.)

Edited by jamesdmarco on 04/21/2013 19:00:32 MDT.

Gregory Allen
(Gallen1119) - M

Locale: Golden, CO
Waiting for more on 04/21/2013 19:32:48 MDT Print View

Roger,

You tease in the article and in some subsequent posts than more is coming on new stoves. When do we get more?

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Waiting for more on 04/22/2013 04:08:52 MDT Print View

> more is coming on new stoves. When do we get more?
Yeah, sorry about that.
I would love to think 'in a few months'.
But no promises in advance.

Cheers

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Waiting for more on 04/22/2013 08:13:32 MDT Print View

Yes, Roger, please do. I look forward to the well written, well researched articles you present that have propelled you, and BPL, to the top of the camping comunity. Again, Well Done!