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Saved by Philip Small
on May 22, 2008 at 7:58:51 pm

Welcome to a Gardening with Biochar FAQ!

... a work in progress...


When gardeners add biochar to garden soil, we are, in effect attempting to follow in the footsteps of the originators of Terra Preta. Because we don't know exactly how that process worked, nor how we can best adapt it outside its area of origin, we are left to discover much of this by experimenting with our own gardens and comparing observations within our own communities.


1.0 What is Biochar?

Biochar is charcoal formed by low temperature pyrolysis.  Medium temperature pyrolysis produces a more traditional charcoal, high temperature pyrolysis produces activated charcoal.  Ideally biochar is made in a way that achieves maximal woodgas condensate retention.


1.01 How does biochar relate to agrichar and to Terra Preta?

Agrichar is a synonym for biochar. This material was fundamental to the creation of Terra Preta de Indio, as it is to creating its modern equivalent, Terra Preta Nova. Terra Preta "Classic" was made by adding charcoal, broken pottery shards, along with organic fertilizer amendments. This, in conjunction with the microbial ecology occurring in these soils, resulted in an incredibly fertile soil, and a reputation for self-regeneration.


1.02 What is pyrolysis?

Pyrolysis is the chemical decomposition of organic materials by heating in the absence of oxygen.  This yields combustible gases (called syngas), tars and charcoal. The charcoal produced is a combination of black carbon, along with small amounts of condensed tars and ash.


1.03 What temperature range is considered "low temperature" in the context of biochar?

The theoretical low end of the range approaches 120 deg C, the lowest temperature at which wood will char, (Reference) thus the temperature at the pyrolysis front.  A more practical low end is to use the piloted ignition temperature of wood, typically 350 deg C. (Reference) The theoretical high end, between biochar and more traditional charcoal, depends on the process and feedstock used, but is seldom indicated in excess of 600 deg C.


1.04 Can I substitute other forms of charcoal for biochar?

Yes, up to a point. The woodgas condensates in biochar give it considerable value, but that is not to imply that using simple charcoal, or charcoal made from other than plant materials, won't produce some, and even most, of the same benefits.  It is normally adviseable to avoid charcoal briquetttes because the binders used during manufacture can add undesireable constituents.


1.05 Why are the condensates valuable?

Charcoal manufactured at higher temperature does not produce as much of an observed beneficial effect as lower temperatur charcoal. The primary material difference is the ...


1.06 Is biochar made from hardwood best?

Biochar made from hardwood is richer in condensates when compared to biochar made from softer wood, from bamboo and from less woody vegetation.  The fact that hardwoods were readily available to the originators of Terra Preta de Indio has not escaped the attention of Terra Preta enthusiasts.


1.07 Where can I join in with this community of Terra Preta enthusiasts?

  1. Bioenergy lists: Terra Preta: the intentional use of charcoal in soils.
  2. Bioenergy lists: Terrapreta -- Discussion of terra preta, the intentional placement of charcoal in soil.
  3. Hypography Science Forums: Terra Preta


2.0 How do I Get Biochar?

You can purchase charcoal from a biochar manufacturer, purchase a charcoal substitute, or you can make charcoal yourself. Hopefully when you do, you can pick up the knack of making charcoal which retains that condensate goodness.


2.01 Where can I purchase biochar?

Currently manufactured biochar is in short supply and is needed for research projects. The alternative is to purchase charcoal and use it as a biochar substitute.

In Britain charcoal is widely available in nurseries.   Cowboy brand hardwood charcoal is available in the United States in 20 pound bags by the pallet, about 600 pounds, for less than US $ 0.7/lb.  For larger amounts, as in a shipping container, consider coconut shell charcoal, available for less than US $ 300/mt. Worth repeating: It is normally advisable to avoid charcoal briquettes because the binders used during manufacture can add undesirable constituents.


2.02 What can I grow to make my own charcoal?

In Britain commercially available charcoal and is made from fuel produced by "coppicing" as has been done in British forests for more than 2,000 years. This is an ecologically sustainable use of forests and may contribute to the health and longevity of some British  forests.


2.03  Can I burn to bones to make chacoal for my garden?

Yes.  It appears that char derived from bones, along with char derived from other types of food wastes, was a component in Terra Preta de Indios.


2.04 How do I make my own charcoal?

While colliers the world over normally use either a covered pit or a covered mound (earth kiln) to make charcoal, most gardeners will want to start with an easier method that works at a smaller scale.  Home pyrolysis is pretty easy to accomplish and a simple burn barrel is a common starting point. A bottom ventilated, bottom lit burn barrel is a popular variation. These approaches can produce a fair amount of smoke and partially combusted gases.  Out of concern for air quality, many gardeners may prefer a less smokey approaches.  

Covered pit: [Example1], [Example2]

Covered mound: [Example1], [Example2]


2.05 What are some less smokey approaches to making charcoal for the gardener?

Choose your feedstock wisely. No matter what technique you use to make charcoal, choosing uniformly sized, dry woody material produces the highest yields. Uniformity is one reason that colliers will routinely use coppiced hardwoods

Inverted Downdraft Gassification. For a cleaner burning configuration, consider a Top Lit Updraft (TLUD) technique, also referred to as an inverted downdraft gassification.  The technique looks simple but in reality it involves some fairly sophisticated physics (PDF). That doesn't prevent success using common materials and dead simple design. Take that same open barrel configuration, tweak the design per the afformentioned physics involved, and now light it from the top instead of the bottom.  This takes a different skill set than lighting from the bottom but its also not that difficult to master.  A little vaseline or ethanol on a cotton ball can work wonders for starting up. Once the fire gets going, the top layer of wood burns, creating charcoal, naturally. The heat from the charcoal layer burning heats the wood below it, and ignites it, but at a lower temperature sufficient for pyrolysis. The gases released by pyrolysis (carbon dioxide and water) flow through the charcoal layer. Glowingly hot charcoal has a wonderous ability to strip oxygen molecules from of anything that passes over it, so it converts the water into hydrogen, and the carbon dioxide into carbon monoxide. These two gases are flammable and they are ignited once mixed with air coming into the top of the open barrel above the charcoal layer. The result is a scrubbed gas-fed flame that is much more controlled, and which burns substantially cleaner and hotter than can be achieved with the bottom lit burn barrel. (Source). The lack of oxygen below the combustion zone is impedes loss of the charcoal despite the high temperature flame immediately above it.  This alows charcoal to build up faster than it is consumed, at least until the pyrolysis zone reaches the bottom of the fuel column.  

A handy TLUD fired Retort. The retort process works by restricting the air supply to the target feed stock for the duration of the burn.  An outside heat source pyrolizes the retort contents, small openings in the retort allow wood gas to escape, but restrict the flow of oxygen in.  While capable of very high yield efficiency, the open flame used to fire the retort is not as clean as can be achieved with an inverted downdraft gassifier.  A common further inefficiency with smaller retorts is that much of the wood gas generated from the retort can end up not being burned. Folke Günther's hybrid TLUD/retort has developed an elegant configuration that neatly addresses these concerns.




2.06 What are some higher volume but less smokey approaches to making charcoal for the garden?

While TLUD's can get fairly large [Link needed], a large TLUD/Retort is less practical than a large drum retort.


A Large Drum Retort. [Expand] Use a drum with a fairly tight lid.  Place it on a stand over the hearth, and run a piece of perforated pipe from the drum to use the volatile gasses take over the initial firing in the firebox underneath [Example1] [Example2] [Example3]


The Wood Vinegar Kiln. [Expand]


2.07 How much charcoal yield can I expect?

On a dry matter weight basis, as well as an energy basis, between 20 percent, for the top lit open barrel approach, and 60 percent, for a retort under ideal conditions. 50 percent is a reasonable goal. [Sources needed] 


2.08 What refractory materials can I use to make a kiln? a retort?

2.09 What gases does pyrolysis produce?

The dominant combustible gases produced are carbon monoxide and hydrogen, along with a small amount of methane.  Carbon dioxide is also produced, especially with higher fuel moisture content. (Source)


2.10 How much heat does pyrolysis produce?

Pyrolysis itself is endothermic, thus requires an input of heat to be sustained. Heating value of the gas produced is 5000 - 5900 kJ/m³. (Source)


2.11 Is charcoal worth more as a fuel than as a soil amendment?

In most cases this appears to be true. A basic spreadsheet can help in evaluating this.


2.12 Is charcoal worth more as a fuel than its value for offsetting greenhouse gases?


3.0 How do I prepare the biochar once I've made it?

You can use it as is, especially if it is a small amount. For larger amounts, the choices are to crush, screen, add liquids, add dry materials, and to compost it.

3.01 Why would I need to prepare the biochar, as opposed to applying it as is?

There are several reasons that might apply to your situation. [Expand, obviously]

3.02 What size should the biochar be?

3.03 What are some ways to crush and screen biochar?

[For crushing, I am leaning to a mortor and pestle approach: a 5 cm dia hardwood trunk 2 m long and a 20 liter bucket with a plywood insert in the bottom.

For screening, I think a sloped screen works better than a horizontal screen for higher volumes.]

3.04 What can I do to make the biochar easier to crush?

Wetting and drying it seems to help.  Crushing it with a little moisture in it helps to control dust.

3.05 Besides water, what else can I soak the biochar in?

Yes.  Compost tea, MiracleGro (TM), fish emulsion, urine,  ....

3.06 Can I add biochar to compost?

Yes. This will help temper the biochar. For the added benefit of odor control, consider topping off each addition to the household kitchen scrap collector with a healthy layer of biochar.

3.07 Will biochar affect the compost process?

Casual observation indicates that adding fine, untempered biochar may accelerate the composting process.

3.05  Will biochar harm the worms in my compost?

Composting worms have been observed to be unaffected below 50% charcoal content, above which reduced worm activity could occur.

3.08  Can I use biochar in my composting toilet?

Yes. Again, the added benefit of odor control is compelling.



4.0 How do I apply Biochar?

4.01 What is the maximum reasonable application rate?

The maximum rate application rate of biochar determined to be of benefit so far is 50 Mg C ha -1. This rate was on low fertility, low organic matter tropical soils, and works out to a loose charcoal depth of about 5 cm. (Calculation)

4.01 What materials combine well with biochar for application?

4.02 How is biochar generally used?

4.03 What is the normal application rate for biochar?

4.04 Are there benefits to deeper placement?

4.05 Are there benefits to using biochar as a mulch?



5.0 What happens after biochar is in the soil?

5.01 Does biochar affect soil pH?

5.02 Does biochar increase soil CEC and Base Saturation?

5.03 Does biochar improve soil moisture characteristics?

5.04 Can biochar have a harmfull effect on my soil or on my garden?

5.05 Does biochar affect soil ecology?

5.06 Does biochar improve plant growth?

5.07 How much improved plant growth can I expect?

5.08 How much carbon dioxide does sequestered biochar offset?

5.09 How much nitrous oxide formation does biochar prevent?

Soil scientist Lucas Van Zweiten has observed a 5 to 10 fold reduction in nitrous oxide emmissions with some of the biochars he is working with in an agricultural setting. Generally, soil with elevated soil nitrate levels in the presence of sufficient moisture and robust soil organic matter will have higher nitrous oxide production, and thus will be more likely to benefit at the levels observed by Van Zweiten.





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