Biochar
Use our Portable Kiln Method to make your own biochar.
First off, what exactly is Biochar?
Biochar* is charcoal created by pyrolysis of biomass, and differs from charcoal only in the sense that its primary use is not for fuel, but for biosequestration or atmospheric carbon capture and storage. Charcoal is a stable solid, rich in content, and thus, can be used to lock carbon in the soil. Biochar is of increasing interest because of concerns about climate change caused by emissions of carbon dioxide (CO2) and other greenhouse gases (GHG). Carbon dioxide capture also ties up large amounts of oxygen and requires energy for injection (as via carbon capture and storage), whereas the biochar process breaks into the carbon dioxide cycle, thus releasing oxygen as did coal formation hundreds of millions of years ago. Biochar is a way for carbon to be drawn from the atmosphere and is a solution to reducing the global impact of farming (and in reducing the impact from all agricultural waste). Since biochar can sequester carbon in the soil for hundreds to thousands of years, it has received considerable interest as a potential tool to slow global warming. The burning and natural decomposition of trees and agricultural matter contributes a large amount of CO2 released to the atmosphere. Biochar can store this carbon in the ground, potentially making a significant reduction in atmospheric GHG levels; at the same time its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity and reduce pressure on old-growth forests.
Preparation of biochar
After browsing through many resources on techniques to prepare your biochar before mixing it with the soil we found Peter Hirst (New England Biochar) gave the best instructions: “The biochar has a micro-porous structure and it also chemically attracts organic molecules and water and tends to hold them in place. To make them available for the plants it has to be soaked or saturated with all of those elements before it goes in the ground otherwise it will be competing with the plants for the water, the microbes and the nutrients and it can actually harm the plants if it goes in raw. So we advocate mixing the biochar with soil or a good active organic compost before it goes in the soil so it soakes up its full compliment of water and nutrients and microbes so that it can make those available immediately to the plants as soon as it is added to the soil. Ultimate concentrations after some time of repeated applications of biochar will work up to about 8 to 10% biochar by weight of the soil content.” Since this system produces approximately 265 lb (120 kg) of biochar, you would eventually be able to enrich an area of approximately 250 ft2 (23 m2) or 2860 lb (1300 kg) of topsoil.
To see more benefits of biochar as a soil amendment see this post: http://vuthisa.com/2011/01/09/biochar-as-a-soil-amendment-and-carbon-sequestering-tool
Make your own biochar
Herewith a step-by-step guide then to making biochar with the Portable Metal Kiln Method. This specific design has been used extensively since 1996 to clear Namibian encroachment bush. Having tested this system recently we see no reason why this technique cannot be applied to turn yard waste, trees or any unwanted biomass (including corn cobs) into biochar (charcoal). What makes this design unique is that it can be used for extended periods, due to its durable design AND its portability – you simply have to roll the drum to where the burn material is located. Even though this is a direct method of making biochar, it does NOT require complicated opening and closing of air vents or chimneys, due to the vertical stack effect of the narrow drum. If you’re looking for ways to transform biomass into a fuel briquettes (with or without charring), visit our Fuel briquettes page.
A single burn will produce approximately 120 to 160 kg (350 lb) of biochar from 0.7 m3/1000 kg (25 ft3/2200 lb) of medium-sized hardwood, but yields are dependent on many variables, such as your geographic location, moisture content-; type-; size of material and the experience of the operator, to name a few. It is possible to char wet timber with this system, but we don’t advocate this due to the increased time it takes to vent the extra water vapor. Colder, wetter climates yield less biochar. In the test described below we achieved a yield of 16% biochar, a conversion ratio of approximately 6:1 (6 weeks air-dried). By fine tuning your burn method and identifying the visual clues more accurately, yields closer to 30% can easily be obtained. Instead of increasing your carbon footprint by transporting large amounts of timber to the kiln (as with large industrial kilns) the kiln is simply rolled (by one person) to the already prepared and air-dried brush-piles. What you will need:
- A kiln consisting of a cylindrical drum and lid
- 120 kg of river grade sand (optional)
- General purpose soft potters-clay
- Industrial purpose gloves
- Large polypropylene bags or paper charcoal bags
- Small ladder
- Wheelbarrow
- Water or Fire extinguisher
- Shovel (optional) and Spade
- A few bricks or loose soil to weigh down the lid
- Garden fork
- Tinder and matches
- Axe and/or bow saw and/or chainsaw
- A stockpile of yard waste, preferably within wheelbarrow distance of burn site
Kiln construction
You need to construct this kiln with the help of your local light engineering works. Expect to pay between $188 USD and $350 USD which includes the cost of labor, so get more than one quotation. It can be a D.I.Y. job if you have good welding and bending skills. Otherwise consider having the kiln built in three sections, which is simply bolted together, no welding required and it is easier to transport. The plans for both types of kilns are contained in our Portable Kiln Construction Plans and User Manual. Click here to download the plans. We have also launched the Portable Kiln Google Group. Joining it is optional, and is the exclusive domain of Portable Kiln Owners for sharing and discussing their experiences OR enter our DESIGN CHALLENGE to design a biochar producing kiln that emits less greenhouse gases. Just print out the plans and hand it over to your local engineering works.
We are bound by a license agreement for use of the kiln in Southern Africa, so if you are from the southern part of Africa and you wish to use the Portable Kiln commercially please click on the commercial license order form. For non-commercial use options and backyard trials click here.
The plans will outline the exact dimensions- and type of materials to use as well as the guidelines set out below. The kiln basically consists of a:
a) Lid. The lid sits loosely on top of a cylindrical drum.
b) Open ended cylindrical drum/barrel. A length of mild steel sheet, rolled into a drum shape and welded on the seam. Please note: an ordinary 200 litre (55 gal) oil drum will not work – or last! This drum has a 1400 litre (370 gal) capacity and weighs 37kg (81 lb).
**Update**
We’ve realized that to manufacture biochar one has to limit the emissions to best practice standards and at the same time increase yields that provide high quality biochar. With this in mind we have embarked on trials whereby 2 x 55 gal drums are placed inside the large 1400 litre drum and filled with feedstock. The reduction in feedstock size (capacity of 2 x 55 gal drums) is nullified by the increased biochar yield, typical of this method. Trials are continuing and the User Manual will be augmented in due course. The large outer drum can be made oval to accommodate the 2 x 55 gal drums. A slightly larger lid will be needed. Watch this space…
Preparation of burn material
The thickness of the branches or stems that you use need to be more than 30 mm (1-1/4 inch), but preferably not more than 120 mm (7-3/4 inch). The larger pieces tend to not pyrolyze completely in the center.
Organize the waste into a separate pile. The volume of waste is not important, although you have to try and fill the drum up with as much yard waste as possible, but typically one should have more than 1 m3 (35 ft3) of waste ready. The kiln can accept 0.7 m3 (25 ft3) of carbonous brush. The branches and stems must be cut in lengths not exceeding 1 m (3.2 ft) long with the use of an axe, bow-saw or chainsaw.
Site preparation
Before anything else, carefully decide on the location of the kiln. Make sure there are no overhead hanging branches or power- or telephone wires nearby. Have running water (preferably a hose pipe) available or if it’s in a remote area, bring a fire extinguisher along, in case of unforeseen emergencies. The kiln is portable in the sense that it can be rolled into position. Care have to be taken not to position the kiln near the brush pile. Choose a level site and clear an area of approximately 3 x 3 metres (10 x 10 ft) of any flammable material.
If you don’t intend moving the kiln to other brush piles we recommend pouring river grade sand evenly over the site, but make sure it is at least 10 cm (4 inches) thick. The sand insulates the ground, but also allows one to dig vent holes underneath the kiln. If you find you don’t have sand or enough volume of sand, you can place 3 wooden pegs underneath the kiln, each approximately 100 mm (4 inches) in thickness, evenly spaced, to keep the kiln slightly elevated. This will allow for enough ventilation or allow one to create the holes in the sand more easily.
Making biochar
(Also see alternative method posted as a blog comment)
- Burn “out of the fire season” and if you feel it necessary alert the neighbors if they are the jittery type – it is just common courtesy to respect other people’s rights.
- Pick a clear morning or a quiet windless afternoon. This is very important because in the first hour you are venting copious amounts of smoke and the neighbors will learn to hate you all over again! Theoretically, strong winds can also cause tiny pieces of burning material/embers to start spot fires far away from the kiln. I don’t believe this to be a serious possibility, but it’s always better to be on the safe side.
- The burn site must be cleared of dry grass and any other flammable material. A normal fire must be lit.
- The kiln is then rolled into position over the fire. If you’re using pegs – insert them now. Make sure the drum is completely round, not oval, as the lid will not be able to close up the opening at the top. Four air vents are then dug under the bottom of the kiln with the use of a spade.
- The air holes will provide much-needed draft through the kiln. Make sure holes are dug at least 30 cm (1 ft) across, even up to 50 cm (1.6 ft) to allow as much air in as possible. The kiln is then fed with material until the burning slash-pile reaches the top. The general rule is to keep adding timber until it no longer sags below the rim of the kiln. Be careful not to have pieces hang over the edge. In this test we only used 93 kg (205 lb/) of 6 weeks air-dried firewood.
- Soon, all the wood/brushwood in the kiln will be ablaze and the smoke issuance will cease almost immediately. The temperature in the kiln will consolidate (and sometimes fluctuate) at between 380 to 400°C (716 to 752°F) anything from 1 hour to 1.5 hours after the last piece of timber has been loaded.
- Once white ash is seen spilling out of the vent holes (four openings under kiln), they must be sealed, by simply kicking them closed with the surrounding sand or by removing the wooden pegs. Blue/grey smoke will start appearing which means that the alcohols and phenols are burning off. This phase is closely followed by the tar producing phase whereby yellow flames are now visible. We advise that the lid be closed before the end of this phase as the tar given off assists in sealing off the lid against the drum. This phase can last for up to 30 minutes. Waiting too long before closing the lid can reduce the smaller sized wood to ash and closing the lid too quickly, won’t allow larger pieces of wood to pyrolyze completely on the inside. One is usually left with a small proportion of semi-charred pieces, but these can simply be added to the next load waiting to be turned to biochar. Seal off the remaining openings with clay or strategically placed bricks, where one can see smoke spilling out.
- Care must be taken against accidental skin burns when applying the clay, as temperatures inside the kiln can exceed 500°C (935°F). Absolutely ALL the air supply to the kiln must be choked off, otherwise you’ll end up with too much ash.
- No further oxygen is therefore allowed into the kilns and the cooling down period commences. Under these conditions hydro-carbons are absorbed by the biochar. In larger horizontal type kilns, temperatures remain hot for almost 72 hours and larger pieces of timber (usually the trunks) are able to sufficiently “cook” during this period. The same cannot be said for large pieces of wood contained in our kiln. On the positive side that is why this kiln works so well with small sticks and twigs. The kiln must only be opened once the metal sides are cool to the touch, usually the next morning (24 hours later).
- Charcoal is then sieved and smaller pieces are separated from the larger pieces by use of a wire mesh screen or simple garden fork and placed into polypropylene bags. In this exercise we produced 16 kg (35 lb) of biochar which amounts to a conversion factor of 5.5 : 1. If you are planning to combust some of the biochar as charcoal, store this batch in a dry environment as moisture can lower the calorific content (heating value) of charcoal. The average heating value of charcoal is around 29 MJ/kg, almost twice that of wood. We used logs with an average diameter of approximately 100 to 120 mm (4 to 4-3/4 inch) and as a result produced very little fines and ash.
- Remember to crush the biochar post-pyrolysis and condition the biochar by (1) charging the biochar with plant nutrients and (2) inoculating the biochar with a combination of living organisms by composting the biochar with organic material and treat it as a compost ingredient. Just add the crushed biochar into existing composting systems or mix it 50/50 with existing compost. Unconditioned biochar can be added to the soil, but this is risky and may cause unwanted changes in moisture, pH, or nutrient availability.
The process – explained
Now that we know how to make biochar, what thermodynamic reactions are actually taking place during the different stages of biochar making? Knowing this might help you to gauge your own operation and fine tune it. It is a good idea to invest in an industrial purpose, digital thermometer and Type K thermocouple rod ±0.5 metres (1.6 ft) long. This will cost approximately $200 (USD).
The pyrolysis of wood in such apparatus basically resolves itself into 4 different stages:
- Evaporation of the moisture present in the wood takes place as the initial phase, up to an average temperature of 170 °C (338°F). During this period scarcely any gas is produced.
- As the fire builds up inside the kiln, the temperature rises and at approximately 280 °C (536°F) the exothermic reaction begins to take place. Gas consisting almost entirely of carbon monoxide and dioxide is evolved and a certain amount of acetic acid is formed together with small quantities of wood-naphtha and tar.
- The exothermic reaction then continues during which concentration of carbon in the biochar takes place. Large quantities of hydro-carbons, acetic acid and wood-naphtha and tar are produced while the temperatures rise to 380 to 400 °C (716 to 752°F).
*As per Terms and Conditions of Wikipedia Encyclopdia we attribute credit to the authors on Biochar by listing their URL and hyperlink here: http://en.wikipedia.org/wiki/Biochar
Make charcoal in your own backyard « said,
June 12, 2010 @ 9:23 am
[...] Biochar [...]
Is it better to burn wood or charcoal? « said,
September 5, 2010 @ 12:28 am
[...] Biochar [...]
Welcome « said,
October 28, 2010 @ 3:36 pm
[...] Biochar [...]
greenpower said,
December 28, 2010 @ 5:12 pm
I agree “The Biochar Revolution” with “The Biochar Solution”
http://biochar-books.com/ is pretty cool.
The Biochar Revolution collects the results and best practical advice that these entrepreneurs have to offer to the biochar community. When practice and theory advance to the point where they meet in the middle, then we will truly see a biochar revolution.
Biochar as a soil amendment and carbon sequestering tool « said,
January 9, 2011 @ 9:40 pm
[...] Vuthisa investigated the Portable Metal Kiln for biochar production, which differs from conventional open burn methods in five ways: (1) The steel plate is very thick and not only lasts longer than 55 gallon drums, but allows the contents of the kiln to “cook” in the absence of charcoal by means of a tight sealing lid (2) The drum can be made oval to fit in two seal-able 55-gallon drums, yet also allow burning material to be added to heat the drums (3) Small sized feedstock, typically found in yard waste, such as twigs and branches are ideal for this system as it will not turn to ash, as with larger horizontal type kilns that can take up to 3 days to cool down, reducing small diameter feedstock to ash (4) Large quantities of biochar can be produced as opposed to the 55/30 type closed retort that produces small quantities (5) The kiln can be rolled long distances without any wear and tear to adjacent feedstock sites. More information and detailed construction plans can be found here: http://www.vuthisa.com/biochar [...]