Charry Charry Night

So, you might be curious about the topic of this blog post. Here I will talk about a fascinating topic called Biochar.


Bio-what? ... yes, of late I have been fascinated with this whole biochar thing. This is the concept of taking charcoal and putting it in the ground. Very simply, doing this improves the soil's ability to retain moisture and minerals as well as allowing microorganisms to attach to the soil and thrive. You can think of it as turning a desert into an urban megalopolis for microbes, with attached supermarkets! Some tree and vegetation roots have a symbiotic relationship with the microbes, where the roots and microbes mutually benefit from each other. The carbon itself is an inert material (the housing apartment blocks of the microbes), but one that lasts for hundreds or thousands of years.

The natives in the Americas used this very technique many moons ago to great effect, with the added benefit of treating their waste streams to be recycled back into their crop rotations. The rich Terra Preta (black soil) can still be found in places where natives had their settlements. I believe the Maori people of New Zealand also did the same.

When you burn woody biomass, you typically turn this into ash. All (or most) of the carbon molecules at this stage has been converted back into CO2 that was sequestered by the plant during photosynthesis. However when you make Biochar, you "cook" the woody biomass in a kiln and don't let the flames directly contact the material. The heat of this process releases the combustible gasses in the biomass (mostly carbon monoxide, hydrogen, and other volatile tar like compounds), leaving a carbon nano structure that is charcoal. When you subsequently "innoculate" this compound with micro organisms, you have Biochar.
One of the primary benefits of Biochar is sequestering CO2 from the environment, seeing that the carbon is trapped in the soil for a long long time. There is some controversy over this issue, mainly from people who believe that forests should not be cut down and turned into char to sequester CO2. I certainly agree with this - however there is much in the way of biomass that can be turned into Biochar that we currently process in open burns (argicultural waste). This usually creates large plumes of choking smoke that also wastes the embodied energy in the biomass. Additionally this releases all the sequested CO2 back into the environment. This is where I think we can do better. Apart from entrapping the carbon in the soil, Biochar has great potential in improving soil fertility and incresing the yields of crops. 

There is a very active area of research in this area at both grass roots (mind the pun) and acedemia levels. One of the most interesting research of this material is the exploration to use charcoal as a cheaper means to create nano structures that can be used in super capacitors!

I've been so fascinated with all this that I have decided to make myself a Biochar kiln.

The making of Biochar

Biochar/charcoal is created by heating up woody biomass to at least 350-400 degC in a oxygen poor environment. At this point, a process known as pyrolysis occurs, which releases the combustible gasses from the wood.

These combustible gas can be used to facilitate the kiln's operation or used for heat in cooking or space heating. It can also be refined and fed into the manifold of an internal combustion engine to run (say) an electricity generator - a process known as wood gasification. However in this latter application, all the biomass is turned into energy very efficiently where the biomass (including any resulting charcoal) is fully gasified. The people at GEK have a good website on explaining how a wood gasifier works, and the many intricacies in its design.

Gasification is not a new process, it has been around for many years now. It is in fact core to extracting energy from solid fuels/coal. I was drawn to creating a gasification unit as my first project, but was a little daunted by the level of complexity in making such a thing. So I decided to start with a Biochar cooker instead. Such appliances are quite common in poorer regions, where electricity is either unreliable or non-existent, and where LPG or Natural Gas is either expensive or hard to get.

The most common Biochar (or gasification) stove is the TLUD (top lit up draft) stove. You will find many references to this kind of stove, and it certainly quite an efficient and simple design. Its operation is described by the process depicted below (sourced from here):

However the one draw back of this stove that discouraged me is that you have to starve or quench the hot coals if you want the charcoal to work with at the end, or risk having it all go to ash. However as a stove that utilises biomass, it is incredibly useful wood cooking stove and super efficient on fuel. For me, this wasn't great for an unattended outdoor stove. I want an stove that shuts itself down when all biomass was converted to charcoal.

This is where the Anila Stove comes into the picture, which suits my needs better. The diagram below shows how it works.

The stove consists of a central burning shaft surrounded by material that is intended for charring. This stove is also lit from the top, but only the centre cylinder burns. As the heat of the central flame migrates into the charring zone pyrolysis starts to occur. The gasses are generated in an oxygen poor environment and the positive pressure gained from the gaseous release will traverse down to the bottom of the central cylinder and mix in with the incoming air from the hole at the bottom of the stove. This combustible mix then travels up via the central flame and feeds directly into the fire. The flame thus becomes hotter and more energetic, reaching a stage where it is hot enough to crack the tars in the pyrolysis gases, rewarding the stove with a cleaner and more efficient burn. As the charring material comes to the end of its ability to pyrolyse, the central flame migrates downwards towards the air intake - essentially acting as a "candle wick" to the stove. When it reaches the bottom, it extinguishes itself, leaving ash in the middle and charcoal in the external annular volume.

This stove is "charged" up ready for the burn in batches (as are most other simple stoves). I considered making modifications to make it a continuous system, but decided very quickly that was too complex for a first project. However I did modify the stove to instead become an l-shaped burn chamber to see if I could use it more like a rocket stove.

I started with a 60L old steel barrel and attached a piece of 100mm chimney pipe in the centre of it. With some help from the super helpful guys at an engineering shop here in Motueka, the barrel and the pipe became one.

An elbow was created using another steel pipe coming out the side of the bottom of the barrel, shown below. The pieces were initially tack welded in place to get it stable before the full weld was done. This was only going to be a prototype, so the welding was meant only to be functional - sealing the cavity, and not really for amazingly good looks.

The top weld looks like this.

Here is the bottom weld.

This is the stove completed at this stage.

Rob at the Atamai Village workshop scavenged around an old digger and found something that would be suitable as a bottom plate cover. After some work, below is the stove with the bottom plate attached with some brackets, with nuts and bolts - charged up ready for its first light. This is when things started to get interesting.

I managed to attract a modest crowd to the inaugural burn of this stove - the number of people that this project inspires continues to surprise me. I've already started to meet a whole bunch of random and semi-random people who seem to be at least as interested in this field as me, if not more so. It certainly has a buzz about it.

Of course, the burn would not be complete if I didn't at least try to turn weeds into char - there is nothing more poetic than making a weed useful. In this case I decided to use dry Gorse as both the fuel for the burn chamber as well as the bulk material used for charing. My visitors brought humble sacrifices to the Biochar gods, including walnut shells and also some very dry cuttings from a kiwi fruit vine.

The first attempt at lighting  the stove was done as a top-lit process. However this proved ineffective at starting because the draw of air into the burn chamber was somewhat inhibited by the fuel, and we got a very smoky result. The fire in the burn chamber didn't last and basically died out. The Gorse was probably also still a little bit green, I will no doubt try this method again with drier stuff at a later time.

The 2nd attempt was done by removing all the fuel from the burn chamber, and lighting it at the bottom instead. The rocket stove draw effect (and the sound of the rocket) was almost immediate and the stove just took off. I just had to feed fuel into the burn chamber either from the top, or out the elbow. Here is what the flamed looked like after it got hot enough and the pyrolysis gasses were burning - pretty cool huh!! or HOT even!! You will notice that there is hardly any fuel in the main burn chamber, and yet the flame is very much alive. In fact the barrel got so hot that the paint on the outside of the barrel started to discolour - pyrolysis was certainly happening. 

The results?

Here is what I got out at the end when it cooled down.

You can see that I did get some charcoal out of the process, however much of the larger pieces of gorse didn't char too well and remained quite woody still. It was quite difficult to know when the process of pyrolysis was actually complete because the jets of gases aren't visible from within the burn chamber. Also once the fuel in the burn chamber extinguished, the stove cooled down rapidly enough to stop pyrolysis from occuring, and hence shutting down the stove prematurely. I think this is what led to this result.

So I put the whole lot back into the barrel and bottom lit the thing again. This time the result was much better:

This time the yield of charcoal was better, with a much higher proportion of the biomass turned into charcoal. The bits that are still woody will just go back into the stove for the next burn. However even the bits of char on the left still had some proportion of woody mass remaining in them.

A draw back of my modified Anila Stove design is that the central burn chamber accumulates both char and ash from the fuel. This builds up and starts restricting the flow of air into the flue and progressively chokes the fire, reducing the burn temperatures. This in turn restricts the amount of thermal energy delivered to the charring material - particularly if the pieces were themselves large sizes to begin with. A rocket stove designed with a J-tube burn chamber would negate this effect to a degree because the main fuel is combusted away from the central tube using a 90 degree bend riser. The ash from this burn is thus localised outside of the central burn tube and can be easily separated from the main air path. This has another benefit in that the fuel automatically lowers itself when it is consumed, hence further regulating and automating the process.

The side profile would look something like this picture which is a picture of a rocket mass heater:

I would likely also get far better results using charring material that is much smaller in aggregate, such as wood chips/shavings or chopped up gorse. This would increase the surface area which would negate some of the effects of heat shielding of the material, hopefully leading to better charcoal.

The 60L barrel did garner about a 1.5 buckets of charcoal.

One of the by-products of this process was a bituminous tar like substance released during the first burn. I had wondered for some time if this tar (which is common to see in wood gasifier experiments) could be used as a carbon neutral road finishing surface instead of using the more commonly used fossil fuel based one. Watch this space!

After the burn was completed, this is what the inner barrel looks like:

The steel looks like it is holding up ok despite the high temperatures. A few things I would aspire in the future are as follows:

- j-tube rocket design
- straight tube Anila Stove original design (for greatest simplicity and ease of manufacture)
- drill holes in the burn chamber pipe at the top of the flue, so that the pyrolysis gasses can burn at a height where it can be observed.
- place a column of water above this stove to heat and channel into our hydronic underfloor tank. This way to create charcoal and heat our house at the same time!! (my ultimate aim)
- try charring other material like wood shavings, couch grass and other weeds.
- experiment with different inoculants for the char material, my recipe list is growing with each person I talk to!!
- experiment using the inoculated Biochar for growing stuff (my other ultimate aim)

So as Mr. Burns from the Simpsons would say .... "Continue the research" ... ;)

Until next time, take care! 

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