[Stoves] [biochar] Charcoal as space filler in TLUD reactors

[Crispin Pemberton-Pigott]

Dear Friends

 

Please consider a lesson learned long ago from Tom Reed which is that if you
regulate the superficial air velocity in a stove you can burn the carbon
completely, partly or not at all. 

 

There may be many people who wish to burn the char chips they have lying
(literally) around the kitchen by placing them into the fuel in a TLUD or
other burner. A TLUD stove that accepts chopped wood (pieces, not chips) can
function much better with the charcoal chips dropped into the spaces between
the pieces of wood. It is worth a try. We cannot rule out whole classes of
combustor because of someone's personal opinion of what other people ought
to do.

 

Regards

Crispin

 

 

 

Dear Paul

 

Thanks very much for your detailed explanation.

 

Fundamentally, a "Perfect" TLUD will work perfectly if it has perfect fuel,
perfect, required air volume, and perfect fan pressure (or stack vacuum) to
deliver the perfect air flow.

 

Given that you have "off-standard" fuel, with greater average void space
diameter, the pressure loss across the fuel bed will be lower. The fan or
stack will thus deliver more air through the bed. Thus, you will get excess
primary air flow, and if your secondary air porting was designed for a bed
with a greater pressure drop, you will get less secondary air delivery; this
will mess up your intended Secondary/Primary air ratio.

 

You can burn virtually any fuel in a TLUd, as long as it is uniform, and the
stove was designed to handle it. Change the fuel significantly, and you need
to change the design, to maintain the desired SA/PA ratio. If you "change
the fuel", but don't change the design, then the only way to restore the
system to "good operation" is to "modify the fuel" to one having similar a
pressure drop across the bed, similar to one for which the system was
designed. This is what you are effectively trying to do.

 

Assume, for example, that you have 4" long pieces of straw as fuel. This
will likely give you all the problems you note. Adding char can, in theory,
help increase the bed pressure drop. However, I am guessing that it will be
a real stinker to get the char particles distributed through the bed. There
will likely be "too much" in one area, and "too little" in another.
Channelling is thus very likely. What you will probably end up with is a
fuel bed having non-uniform flow properties, because of the large
differences between straw properties and char properties. Try mixing various
percentages of char with the straw, in a mixing bucket, then try to take
"mixed fuel" from the "mixing bucket" and place it in the stove. I am
guessing that you should see the non-uniformity of the fuel bed even before
you ignite it.

 

My guess is that your best bet would be to chop the straw, so that it is
free-flowing. This alone will make life very much easier for the operator,
and will enable a greater weight of fuel to be added to the stove. Longer
burn times between re-fuelling. Play with the air flow, simply by
obstructing the fan intake with a piece of paper or cardboard. Then see if
you can get your usual good combustion. If not, then consider re-drilling or
partially plugging, the SA air holes to get back to the correct PA/SA ratio
for good combustion.

 

Is chopping the straw (or whatever the fuel is) an option you can consider?

 

Best wishes,

 

Kevin

 

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