[Stoves] RE: Henson Center Fiure Burner System; Was: Re: improving charcoal stoves

[Crispin at newdawn.sz]

Dear Kevin and Lanny and All

I mentioned some comments I was finding time to write and it is perhaps time 
to do so.

Two issues: innovation and how Lanny's stove works.

I am not convinced this is a new method of burning.  There are too many 
examples of very similar stoves going back many years.  People have known 
for a long time that having a hole through the fuelbed has certain benefits. 
It was a central hole in the hay that blew off the top of Alex English's 20 
foot high downdraft greenhouse heater so I guess some holes are a 
disadvantage.

The smokeless coal stove on test around Johannesburg in 8 different 
locations uses a central hole to light and closes it once the burn rate is 
acceptable.  One of the first improved stoves I ever heard about (in the 
'70's) worked the same way.  Let's not go overboard on this aspect.  There 
are a LOT of things out there.  It is interesting and we should strive to 
understand it.

I like the wire frame and what may be an lowered CO level due to better 
combustion.  This has not been shown yet though.  If it is a similar burn 
rate and a much lower CO level, the explanation will probably lie in the 
presence of the wire (others use a perforated steel tube) to ignite the 
gases.

This works by having the wire heated by whatever combustion is taking place. 
The CO is drifting in from the fuel packed around the hole.  The air does 
not all pass through the hole, it rises through the charcoal and comes up 
the central space as well.  As the charcoal burns, by radiating heat back 
and forth around the hole, it maintains a high temperature in the wire.

When the fire and fuel arrangement would normally let CO out of the system, 
and given that the wire is hot enough, it will add energy to the gases and 
light the CO which burns at a high temperature.  This in turn maintains the 
high wire temperature which keeps lighting the CO drifting in.

As the supply of CO is highly variable, one cannot always rely on a flame 
being present to light it.  Hence it is good to have something hot around to 
light (increase the energy of) the gases as they leave the fuel bed.  For 
example you can use hot secondary air, a flame from a reliable source (which 
is how Dr Tom's camp stove works, if you think about it), a hot wire (as in 
Lanny's example), a hot tube (as Dean mentioned)  or a hot plate against 
which the gases are directed (as in the case of an FSP stove).

The point is to provide reliable ignition for a variable gas source.  Hot 
metal is a good one.  Their drawback is a short life in most cases.

When I first looked at the pictures of the flame erupting (?) from the 
central hole I was also thinking it is like the holey briquettes and that 
the explanation given for the holey briquette flames some years ago was 
erroneous.  People are impressed by the flame from the biomass briquette 
shooting into the air.  Don't be too impressed too soon.  The reason the 
flame is there is because there isn't enough air in the combustion chamber 
(the air channel inside the briquette) to burn properly.  The only reason 
the flame is shooting out of Lanny's stove is because there isn't enough air 
to burn properly inside it.  It is very likely that there isn't nearly 
enough air to burn, rather than it is a great ignition system.  The only 
role of the wire may be to increase the fuel burn rate.  Once the unburned 
gases (including very hot CO) emerge and gasp for air, they combust. 
Compare that with a short, neat propane flame that completes its burn within 
a few mm of a hole in the jet.

If you were to put a pot on top of the stove in such a way that the flame 
hit the bottom and ran horizontally along it, the combustion would be 
quenched to some extent and the result would be a high CO level.

One way to look at the year it took to develop the FSP stove, is to track 
the gradual reduction of the flame length.  In the beginning I have a flame 
more than 300mm long and masses of black smoke.  In the middle I had a 50mm 
flame with lots of blue in it.  At then end it was 25mm long and a flame 
nearly invisible in daylight.  Why? Because the air supply and ignition 
plate (not a wire in that case though I tried it, and a perforated plate) 
were juxtaposed correctly, with adequare hot primary air.  The combination 
allows the combustion to take place in a small place.  This concentrates the 
heat and give reliable, continuous ignition.

That said I am still thinking about whether Lannys construction is an 
alternative to the layout I described with a cone of hot secondary air 
drafting over a conical pile of charcoal.  Basically it is the same thing 
but inverted.  One is a pile with a CO generating low-air center and hot 
combustion on the outside.  The other is a CO generating clyindrical pile 
with a hot flaming center.  At present it has inadequate air so the 
combustion is taking place well above the fuel.  That could be corrected and 
the role of the hot wire investigated, separately, to confirm its role.  My 
guess is that the conical pile has a higher burn rate for any given amount 
of fuel but I will try to confirm it.

Until we have in Lanny's hand enough equipment to know what the fuel burn 
rate is and the accompanying CO levels, it will be difficult to conclude 
much.  If the wire frame is increasing the burn rate by maintaining the hole 
size (ash is a practical problem with that design) then the burn rate will 
be higher than a carefully stacked pile of briquettes with a central hole 
that nonetheless collapses occasionally.  To show an 'effect' you have to 
control the burn rate and test what the wire does.  It is like varying the 
combustion chamber height in a stove without reproducing the burn rate and 
trying to conclude something about the effect of different heights. 
Different height = different draft = different burn rate + different excess 
air ratio = different heat transfer efficiency.  Not so simple, huh?

Lanny, it would be interesting to know what the effect of adding wires is 
with the following experiment:

1.  Build a stack of briquettes with a central hole (use a stick as a 
hole-mould).  Note its flame and burn time trying to get an overall burn 
rate.

2.  Build a similar fire and put in 2 wires only, one on each side and 
observe the flames and burn time.

3.  Build another similar fire and put in 4 wires as you made it earlier in 
the form of a frame.  Ditto the burn.

4.  Build yet another fire with 8 wires in it and ditto the burn.

5.  Repeat this with a larger central hole, say...double the diameter (4 
times the area).

If you can see a trend towards a shorter flame emerging while having a 
similar overall burn time, then you are tending towards a better combustion 
system.  It indicates a more complete burn without increasing the fuel 
consumption.  It is quite acceptable to choke the air to reduce the burn 
rate, but unless you have two air channels, the result of the choked system 
is likely to be hot unburned CO.  That would be very un-cool!

Best regards
Crispin

PS The first thing you get from a COol fire is CO! 



___________________________________________________________
$0 Web Hosting with up to 200MB web space, 1000 MB Transfer
10 Personalized POP and Web E-mail Accounts, and much more.
Signup at www.doteasy.com