[Stoves] Blown charcoal is the base of maintained good wood-burning

[Boll, Martin Dr.]

Andrew,
I like your answers. They add very good and interesting points.
Within the text I will follow your ideas.

> 
> Date: Sat, 19 Jul 2014 21:01:06 +0100
> From: ajheggie at gmail.com
> 
> [Default] On Sun, 13 Jul 2014 01:09:31 +0200,"Boll, Martin Dr."
> <boll.bn at t-online.de> wrote:
> 
>> -but first I give the additional result of my very simple tests told some days ago: A piezo-spark does not light some very-light-cotton cloud, containing (spark-incended) motor-fuel.
>> It needs the "very fitting fuel-air-ratio".
> 
> Martin I shall try this with a piezo spark  but due to the
> contrariness of chance all my lighters are full, normally when I need
> one it is empty. I find the good old flint ones give a better spark.
Andrew, the flint-spark does light the old-fashioned Benzin ( normal car-fuel, no diesel) we all know.
You must not look for an old flint-lighter: I tested it: A very light "cloud" of medical cotton put into the "spark-region" of a lighter gets ignited after some tries.

The "flint-spark" releases an amount of chemical-bound energy, which is more than the mechanical energy used to create the spark.
The piezo-spark contains only the mechanical energy made by pressing the button: Even when I used a "multi-spark" (several one after the other) piezo-lighter, that does not work.
Looking to the electrical flame-bow of the lighter, it must be very hot because of the blue light, but it is very thin/small.
- Would be interesting to calculate the energy of a burning match as a tiny log (aware, but omitting that it must contain e.g. some wax to burn properly.  The match is somehow a candle, not a log)
- The lost energy of a flame by radiation is so important that a single log needs energy-"reflection" from neighbor-logs. -Lenny Henson talked about the minimal amount of logs burning properly together. - Have fun to observe some thin wood-spoons burning together in different positions.
> As you say most gases and vapours have a small range over which the
> premixed fuel and air will ignite, I suppose this is something to do
> with the chances of a fuel and oxygen molecule being close enough and
> the spark having enough energy to dissociate the molecules, or part of
> them. hydrogen and acetylene have a very wide ratio over which they
> will ignite and methane is particularly narrow a range.
 There is mostly burning no pure hydrogen and no acetylene. 
But when water is broken into H2 and O by high temperature, ( few water in the wood) that would widen the working ratio by hydrogen.  Some water in wood-fuel makes a better burn, I remind e.g. a Chinese stove adding with success some H2O to the flames.
> A diffuse flame is a bit different as there will be an area at the
> interface of gas and air where the conditions will be right as the two
> gases diffuse into each other. I think this is why Bunsen burners are
> lit with the air holes closed.
 Interesting reminding!  The closed air holes cause a "fat" mixture. That is easier to light and for continuous (not qualitative better)  burning.
The air-premixed Bunsen-flame, I think,  is the narrow path between out-blown flame and retrograding flame.
- The difficulty  to control as well the flame-speed and the right fuel-air-mixture so narrowly as needed to burn clean; - whatever is burnt.

>> 
>> The low self-ignition temperature of charcoal, is simple to maintain. ( 300?C ; but charcoal-gasification is not below 400-600?C, * P.S. ) Charcoal burns properly (during some time) just beside some fresh air,  ( one cannot blow it out under most conditions )
>> The high self-ignition temperature of woodsmoke is higher, (otherwise the smoke would not exist, but self-ignite) ( start about 400?C sure above 550?C, * P.S. )
>> My own can-burn-observations are, that a dark red color does not fit to reignite smoke, so I guess, that, to be safe, the temperature must be above the charcoal-gasification temperature, which is, compared by the rates of wikipedia above the self-incending temperature of wood-gas. 
>> - Out of that I followed: The C in gas-form is burning and maintains, or re-ignites the woodgas-flame. -I guess, It is at least the 730?C, bright red, slightly orange ( in terms of Tom's given scale)
> 
> We see this when we blow on a smouldering fire, the char changes from
> a dull red to a brighter red and eventually its temperature gets above
> the auto ignition point of one of the components of the offgas which
> then propagates a flame.
That is the old manner to start a fire with first relative big amount of low temperature that ignites charcoal then the blow to rise temperature to self-igniting
The modern way is, to make a tiny spark, with high temperature to ignite right-rated gas-air, which needs high temperature, but least warmth-energy of the spark (piezo-spark works)
> If the calorific value of the offgas is too
> diluted by steam or carbon dioxide a flame never develops.
By starting the fire, the somehow _overflow-amount_ of right-sized kindling, followed by the right dimension and amount of fuel to get a quick-working fire-cascade, to avoid smoke and steam.
- How important is preheating of secondary air, to maintain the flame ?
- How to get then heat down from the flame to the secondary air?   

When there is gasified C by higher temperature (brighter glow) that would help to re-ignite? 
>> 
>> I smile about myself, because it took years for me to get that "really" aware; -So much aware, that I came, for me,  to the simplest and most rational conclusion, that there is no stable burn below the self-ignition-temperature of all burning "stuff". 
> 
> I see where you are coming from but I think you miss the phenomenon of
> "flame holding", if the flame can stay attached to something then it
> remains stable without the containment getting particularly warm as
> most of the heat from the flame is convected upwards.
I did not miss the phenomenon of "flame holding" ,( you think e.g. about a "Prallplatte" = bafffle-plate, yes I totally agree.)  But I think, in a lot of cases, it is easier, to avoid burn-failure by keeping the temperature on some charcoal-areas locally high, then to be able to hold the flame by e.g. a baffle-plate.
- With low-dense-energy fuel the chance is bigger for the flame to die by temperature-loss. therefore I think to prefer a blow. 
- Should we think about experimenting with baffle-plates by TLuds?

- I think of the baffle-plate by kerosene-burners; which prevents, that the gas-stream blows out itself. It creates  areas (continuous and close together) with different fuel/air mixture and different gas-speeds; and so _somewhere_, within the turbulence, is always an area which has the _right_ conditions to hold the flame.

- A small "blow downward" in a TLud would certainly warm up the containment (- bad for the containment,) and consume some created char (but good for _making_ char).
 Practice should show, if a "blow downward" would be agreeable simple and would make agreeable burning (and char-making) conditions.

I suspect, that in general (not only stoves), conditions can be so "fine-tuned" that a system "likes" to fail. -That there is nearly no transition between excellent work and failure.
Somehow like the difference between analog and digital :-) 
> 
> What I found interesting in the tincanium vortex burners I played with
> is as things got hot the flame lifted off and would be suspended in
> the centre of the vortex as a purple flame. I took this to be because
> the hot central are was above the auto ignition point of the woodgas
> and if I could maintain 800+ degrees C then there would be no visible
> smoke.
When you would salt the fuel with NaCl, would there be a yellow color in the off-lifted part??
 
> AJH

Regards
Martin