[Stoves] biochar rocket stove

Tue Sep 18 11:24:02 CDT 2012

Dear Steve

That was an interesting excursion. To be brief, there are a great number of
technical misconceptions contained in the document
http://www.soil-carbon-regeneration.co.uk/biochar/wp-content/uploads/2012/05
/Biochar-Rocket-Stove-building-instructions.pdf so my advice is to try to
gain some knowledge from the general approach and from the unusual layout of
the product but not take it all as literally true. 

It is interesting that anything with a side feed is being termed a Rocket
Stove. That rather undermines the actual Rocket Stove as a unique design, in
my view. I guess people will call it what they want.

I have copied here a paragraph from a the document:

The tlud

Designed by Paul Anderson, the top light upward draught (tlud) gasifying
cook stove works on more than just one level. It is a very efficient cook
stove, producing a lot of heat from a small amount of wood. It is smokeless
and it produces biochar. Paul Anderson has also been instrumental in getting
these stoves distributed and used in developing countries where wood or
charcoal is otherwise used in conventional fires for cooking. By being more
efficient, less wood is needed. By being smokeless, diseases and deaths
caused by smoke in living spaces are reduced. By producing biochar,
subsistence growers are able to [maintain] soil fertility and improve soil
structure, biological activity and moisture holding capacity. Atmospheric
carbon is also being sequestered by the use of these stoves.

So, I have some issues with some of this and because the whole list was
referred to it there is merit in correcting some of the impressions given.

The TLUD was invented centuries ago as a way to burning with little smoke.
The Romans used TLUD fires. A TLUD cooking stove may or may not be
‘efficient’ depending entirely on how well it transfers heat from the flame
to the pot and whether or not the remaining char( if any) is counted as
‘consumed by the stove’ or not. 

The heat produced by a TLUD is no greater than the heat released by burning
the same amount of fuel to the same extent in any other stove. That is, if
you gasify wood in some other device the heat is exactly the same. TLUD’s
are renowned for making lots of smoke when things are not working properly,
like in the beginning and at the end of a burn there can be lots of smoke.
Mitigation of this involves timely intervention by the cook. They are, after
all, smoke producing devices that then burn the smoke. 

You have no doubt seen Paul Anderson throwing a lit match into a smoke bomb
of a stove re-lighting the fire when it has blown out. I have done it myself
dozens of times. When they are running well, especially during the main part
of the burn, they are amazingly smokeless, like any other really good
biomass stove. I hope that the products evolve to the extent that they
really are smokeless.

Now, about ‘being more efficient’ and ‘using less wood’. I have been reading
a range of documents/comments on this matter and it is not at all clear what
is being claimed rather than inferred. If a stove is ‘more efficient’ then
we expect that it will use less input of raw fuel (fewer trees chopped). If
a stove uses the same amount of fuel and cooks while also producing char, it
is not ‘more efficient’ and it is not ‘using less wood’. Only by ‘actually
using less wood’ can a stove claim to be using less fuel, as far as I can
understand it, looking at the forest and counting the trees. We have had
several conversations here about whether a char making stove saves fuel, and
so far there are no clear indications that they do. There is no shortage of
claims that they do. They cook with less energy, but the input of fuel is
about the same because a lot of the energy in the original fuel is still in
the unburned char.

To put some numbers on it (so it is not just a matter of opinion or
prejudice) if a stove produces 20% char, and cooks with the same thermal
efficiency as the baseline stove, then it definitely is using more fuel
because it will consume raw wood to make the char plus the same amount of
fuel to do the cooking.

If a stove uses less energy to do the cooking (because it has a better heat
transfer efficiency) then the fuel saved might equal the fuel needed to
produce the char. If the fuel saved (through better energy delivery
efficiency) is least 40%, and 20% char is produced, that is more efficient
cooking but there is still no fuel saving at all.

If the stove’s cooking efficiency was doubled compared with the baseline
stove, there is some 50% of the fuel left to turn into charcoal. How much?
At the most 45% of the mass. That would be remarkably good. So if half as
much fuel was used to cook and the remainder was turned into char, let’s say
½ of the remaining fuel emerges as char, how much fuel is saved? Zero %. 

Why? Because ½ of the baseline amount of fuel is being used to cook, and the
other ½ is being turned into char. That is not a saving of fuel. It is
clever, it is cooking while making char, but it is not saving fuel.

So how does a TLUD save fuel and produce char at the same time? What are the
real numbers? I think the biochar promoters should lead on this point. They
must marshal their facts put numbers to the claims. If they don’t lead the
field, then those stoves and the char industry will be discredited by these
exaggerated claims.

Suppose a really well designed stove saved 75% of the fuel over a baseline
product. Suppose it also produced 25% char (based on the original dry mass
of fuel). How much fuel, chopped trees or grassy biomass, would be saved
(actual reduction in consumption)? 

If you can calculate the answer to this question you are well on your way to
promoting a ‘fuel saving char making TLUD’ that will be believed. I, for
one, look forward to seeing such a stove.

Best regards

Crispin

HI,
     with regard the quoted exchange in post  

Stoves Digest, Vol 25, Issue 21

"Aron:
>
>        Can you clarify your intended design?  I know of no way that
>     you can turn a rocket stove into a char-making stove (but would
>     love to hear differently).
>
>     Ron."
>

The following link is to an acquaintances site who has adapted the rocket
stove to gasify woody particles within the insulating jacket of the
combustion rocket elbow.
  I would be very interested to hear responses to this design approach.

http://www.soil-carbon-regeneration.co.uk/biochar/biochar-stoves-2/biochar-r
ocket-stoves/

                                             Steve

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