[Stoves] biochar rocket stove

Tue Oct 16 20:09:12 CDT 2012

Dear Ed

I first thank you for posting your comments to the Stoves groups and second for detailing the various points. I promise not to take anything too seriously or literally.

You have mentioned efficiency in a number of ways and it is important that readers understand the different ways the word can be intended.

“Obviously these stoves lose efficiency compared to similar stoves which burn the fuel to ash.”

This is the one I was referring to. Combustion efficiency is usually a measure of the completeness of a carbon burn, or the ratio of the CO to CO2 in the gas stream. However for a stove that is burning either a low carbon fuel or deliberately not burning carbon in the fuel that is available for other reasons (in this case to retain the char) other metrics might be relevant such as the H2/H2O ratio because that is more plentiful.

The heat transfer efficiency is of course related to the stove structure more than combustion efficiency (save in really poor flame conditions).

Ed, the problem the stove group faces is that claims are being made, sometimes in error and sometimes craftily, that this or that stove can perform miracles – kind of like cars that get 200 MPG and 0-60 in 10 seconds.

I would really appreciate it if your website helped the readers who will no be many, to keep in mind the way various efficiencies add up to the total fuel or energy. With respect to char-making stoves, there are two notable issues which I wll not detail by describe a little so you are clear what I mean.

The first are claims that a stove making char (whether a TLUD or other type) both ‘saves fuel’ and ‘yields char’. It is easy to calculate that this is very unlikely. Gasifiers and pyrolysers make char by not burning the fuel so the energy released os lower, but that does not necessarily mean it uses less raw fuel.

The second is that the common test used for calculating fuel consumption, the WBT’s in their many forms, do not actually measure fuel consumption, they calculate a ‘specific fuel consumption’ figure. It is not the specific fuel consumption, it is a calculated number given the name ‘specific fuel consumption’ which is a different kettle of fish. The SFC is a re-expression of the energy released to make the thermal efficiency calculation – by its definition not the sum of the fuel burned or remaining unburned in the stove.

You have confirmed that you understand this point but many do not.

So the usual claim to being ‘fuel efficient’ is often (usually) rooted in the SCF number and is not a measure of the fuel consumed. It may well use less energy to cook (usually does) but it has a second purpose. Thus saying it is more energy efficient should also in fairness to the uninitiated reader, include a caveat that the remaining energy remains in the unburned carbon, not unchopped trees.

Thanks

Crispin 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Edward Revill
Sent: Wednesday, October 17, 2012 5:01 AM
To: stoves at lists.bioenergylists.org
Subject: [Stoves] biochar rocket stove

Hi, I'm sorry to come in with a slow reply to Crispins comments, I was only recently made aware this comment thread.

May I introduce myself to this group, my name is Ed and I am responsible for the pdf document 'biochar rocket stove' and for the web page on which it appears. 

I find it strange, Crispin, that you tell people to dismiss the idea of using a biochar producing stove which can easily be brought indoors and left permanently in place under a flue pipe without the need for removing the stove to empty the biochar (unlike tlud or anila stoves) because you do not like the way I tried to write a general, brief introduction to the work which inspired me to build such stoves. Surely that is a bit like judging a song by the record sleeve. (do they still have record sleeves?)

Crispin, you say not to take this product as literally true. 

Steve you are very welcome to come and see this product working for yourself, I will be very happy to feed you with a CO2 negative meal grown in biochar and cooked indoors in a biochar stove powered oven/hotplate that does not need feeding, emptying and lighting from the top and so is a permanent feature of my home, built into a cob wall. You can even have a CO2 negative shower! 

I suggest you do not do not take Crispins advice as literally true because he has probably never used one before. 

The point I am making is that I have been producing biochar in my home each time I cook, generate hot water or heat without needing to quench biochar or remove the stove unit to fill or empty it, using a simple system. You recommend that people dismiss this on a completely irrelevant point. I am writing this in the hope that anybody who read your recommendation for dismissal will look again. This anilaXrocket hybrid has a lot of advantages over both anila stoves and rocket stoves. This is the point I was trying to make in the pdf. I have been improving the design and running workshops (here on my market garden and at various other events) for over two years now and I am always pleased to demonstrate the various biochar stoves and heat capture devices I have made.

Just this afternoon I demonstrated a shower and cooker heated by a 'modified biochar producing anila X rocket stove hybrid' (does that sound better?) at the Swansea community farm. It is very efficient, it works, it produces biochar and can be used indoors because it remains under a flue pipe and does not need moving to feed, empty and light it.

I am not sorry if you do not like the name 'biochar rocket stove', feel free to call it what you want but again poor grounds to suggest that people to dismiss the idea.

With regard to the criticisms you make, although they are not points central to the stove primarily described in the pdf, I am aware that there are a few sentences which could be written more precisely. I wanted to keep the wording brief and not use any jargon. The point I made about tlud stoves being more efficient than open fires of wood or charcoal I could have expanded to say;

1 that effeciency depends on many variables and so I obviously made a generalisation.

2 That tluds are generally more efficient than open fires of wood or charcoal for many reasons;

     i, the process of separating gases from the char and then blowing hot air into these gases at the point of combustion usually results in more efficient combustion than is found in most open fires.

    ii, these stoves allow for more efficient heat capture and retention than open fires as the flue pipes can be run horizontally through a thermal mass.
    iii, they allow for more efficient heat capture because the heat source is very close to where it is required, i.e. at the top of the stove
    iv, they allow for more efficient heat capture because the stove contains and channels most of the heat upwards to where it is required. (The heat from open fires is more dissipated.)

    v, by burning the wood gases which are otherwise released as pollution or occasionally burnt in the charcoal making process, they are more efficient than charcoal burners.
    vi, they can be built to the scale required according to the amount of heat needed.

However, for the sake of brevity, I will leave the sentence as it is for now as it is not inaccurate and I hope to go into these points in greater detail at various points in the web page.

Obviously these stoves lose efficiency compared to similar stoves which burn the fuel to ash. 

This pdf is part of a web site I am in the process of creating with a view to generate interest in growers and permaculture practitioners in the idea producing biochar from their domestic heating systems. Before you go through this web page listing all the small errors, could you kindly wait until after the winter. I use it to promote my business of running a market garden and of maximising soil carbon. I am very busy and hope to find time during the quieter winter months to complete, update and improve the site. However, I am always happy to receive suggestions of how it may be improved. 

Best wishes, Ed.

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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