[Stoves] TLUD bottom wet wood layer

[neiltm at uwclub.net]

Breakfast this morning marked a further improvement in the usefulness of 
the stratified batch loading, and a tentative conclusion that there is a 
useful general principle to be taken from this.

This morning's breakfast consisted of lightly frying smoked mackeral in 
butter, followed by putting on a kettle for coffee, followed by toast 
made over the char at the end of the burn with the concentrator disk and 
pot support removed.  This has been one of our breakfast routines for 
years, and something the Reed woodgas campstove LE did well.

It was freezing hard this morning and so the outside wood was not just 
wet, but frozen solid.

I put a generous layer of just the frozen wood in the bottom, half 
wondering if I might be pushing it a bit for expecting it not to struggle 
with hitting that layer, albeit right at the bottom.

After eating the fish, (indoors!), I came out to a half filled kettle 
boiling its head off, in contrast to it more usually needing a bit of 
extra wood to bring less water to the boil.

When I took the kettle off, the flame was lovely.  Mostly the char 
burning vigorously, but very noticeably augmented by the obviously still 
pyrolising bottom layer which was streaking the blue flame with red, but 
not smoking. (I'll try and take a picture of this to share and get some 
timings). The combined combustion looked really nice and seemed obvious 
to me that the bottom layer was maintaining a much more vigorous fire 
than would have been the case with just the char burning off.  Not only 
that, but it continued for some time after I had made the toast, which 
was really quick.  So often the toast stage has been a bit of a last gasp 
barely doing the job and a bit rushed to get to it before it disappears, 
but not this morning.  And this was all off one batch loading, no feeding 
at all.  This was a far more useful fire than I would normally get from a 
batch loading in this stove.

I wouldn't want to try to claim that more heat was output from the stove 
with the wet wood layer, but clearly more *useful* (to me) heat was, and 
that's my point, this simple technique is a great control over getting 
the fire you want when you want it.  This is no longer merely a way to 
usefully burn wet found wood where 'better' dry fuel might be 
unavailable, it occurs to me that a bottom layer of deliberately soaked 
wood might, in simple TLUDs, prove advantageous for control where there 
is no naturally found damp wood.

Instead of an unwanted flare up at the end of the burn as the chamber 
floor reflects heat back into the bottom fuel, this tendency is instead  
taken advantage of by modifying the fuel at the bottom, and optionally 
variably above it, to control the rate of combustion, potentially greatly 
extending it.  For my cooking purposes this morning I probably did overdo 
the frozen wood layer in that the fire lasted longer than I wanted it, 
and in the process of waiting for pyrolysis to finish I lost some of the 
remaining char which was still much more than usually remains from that 
cooking routine, the toast stage usually pretty much consuming it.

I have found that modifying the wood chunk size makes far less difference 
to the size of the flame than you might think.  With chainsaw dust it has 
to get almost that small before it exercises enough control over the 
primary air to give a long moderate or simmer heat, but then the stove is 
useless once the batch is burned and cannot be revived.  Ditto for mixed 
woodchip including a lot of fines.  With large chunks the stove is 
easiest to keep going indefinitely by further feeding.  I am using very 
large chunks of dried hardwood that was felled green only a few weeks ago 
and dried on top of our central heating boiler, but these do seemingly 
nothing to slow the rate of burn down.  There is so much more control, 
and potentially really fine control over the heat by varying the moisture 
content in combination with stratifying to produce wanted change over 
time.  There is a simplicity in this approach that might have the 
potential to further improve and refine the turn down/up control sought 
through variable air supplies in more sophisticated NDTLUDs?  

I guess it is one thing not to expect a biomass cook to naturally invent 
a TLUD, or any kind of improved stove necessarily, but to what extent do 
cooks commonly and naturally without necessarily thinking about it, 
intuitively as it were, modify how they make and tend a fire, select and 
prepare fuel to the extent that, just like me they may come up with all 
manner of refined ways to use simple stoves to get better control over 
the fire they want?   I have never seen that Q discussed, and would be 
interested in thoughts and observations on the Q.  If that potential 
proved to be a commonplace human attribute, then controlling the fuel in 
combination with good stove design surely ought to be able to achieve 
better results?  Is there even a danger in not doing so that the cook is 
being de-skilled by being handed an LPG stove, or improved biomass 
cookstove with processed biomass fuel that aims at LPG levels of skill to 
operate?  I don't know the answers to these questions, but I'm hoping 
those working in the field who have observed fire making and using 
skills, especially with simple stoves, might enlighten me.  Some and some 
I expect, human aptitudes being usually highly variable.  My own laziness 
to prepare TLUD fuel has impelled experiments in 'how large can the 
chunks be?' for eg.

Neil Taylor