What Is Biochar?
Biochar is the product of burning any organic material, most often wood, in the absence of oxygen, through a process called pyrolysis. The result is a highly porous and resilient substance that closely resembles charcoal or activated carbon and is usually ground down to be spread over a larger area.
Any carbon based organic material can be turned in to Biochar as long as it is dry enough to burn, but wood (in the form of branches, chips, or sticks) is most commonly used. Biochar’s ability to stay intact in soil for long periods of time (also known as recalcitrance) provides a huge opportunity for its use in the carbon sequestration movement. This is because the wood that is converted into Biochar would otherwise decompose over time and a large component that is let off into the atmosphere is carbon dioxide. By keeping the carbon in a solid state that doesn’t decompose anywhere near as quickly, that off-gassing is prevented, keeping more carbon dioxide out of the atmosphere. Biochar, or “Terra Preta,” that was utilized by the ancient civilizations in the Amazon River Basin can still be found intact in the ground, despite being put there hundreds, if not thousands of years ago.
Coupled with its resilience, Biochar’s microporosity allows it to not only hold five times its weight in water, it also provides massive amounts of surface area for microbial life to grow, and is able to absorb unused/leeched nutrients from its surroundings for later release when plants need them.
How is Biochar Made?
Biochar has not seen a commercial boom as of the time of writing this article, so there isn’t a cemented standard for devices built to produce Biochar. Universities usually produce the Biochar samples they test in a spoon pyrolysis kiln, which, while able to produce consistent results with dialed in parameters, aren’t the most financially accessible option. The commercial producers that are out there usually use one of three major designs, and some add their own unique twist. The major three most common designs are:
Cone/Pyramid Kiln: Being the easiest to produce, the cone or pyramid kiln is a metal kiln in either a cone or pyramid shape flipped upside down and filled with woody material. The top of the pile is lit on fire and as the wood below heats up and releases wood gasses and oils, they are burned off by the fire above. As the fire moves down the pile, a thin layer of pyrolysis creates the Biochar. After burning finishes, the kiln is topped to stop oxygen from entering or the material is quenched with water. These kilns can be scaled up or down relatively easily, but in larger sizes they are limited by footprint.
TLUD: Standing for “Top Lit Updraft,” the TLUD is known technically as a “gasifier kiln.” They are built around a metal container (usually cylindrical), like a barrel or paint can, to which a “crown” is added to the top (usually the bottom of an additional container). The crown has holes cut in the sides and a vent hole in the middle. A tall chimney is added to the top of the crown to increase airflow and raise the temperature of the burn. Holes are punched in the bottom of the main container, so that when the material loaded inside is lit from the top, it burns downward. Air comes in from the bottom, updrafts to the fire and carries the extra oxygen around the fire up and away, leaving a layer of pyrolysis which works its way down. The benefit of this system is that all emissions are carried up to an additional fire occurring at the crown, burning off into less harmful carbon dioxide and similar components than are in regular smoke. Construction of these kilns is relatively easy and scaleable for do-it-yourselfers.
Retort kiln: Retort kilns are most often used in commercial operations for their ability to burn various sized material in large batches. Retorts are built by placing a container with holes in the bottom inside a larger container. The inner container is filled with wood and is usually similar to a TLUD without a crown or chimney, and the larger outer container usually has a chimney placed on top. The larger container’s remaining free space is filled with sacrificial wood that is lit on fire. As the fire burns around the smaller container, it heats up the wood inside which releases its gases/oils. These gases and oils are burned off after escaping from the holes in the bottom of the smaller container and rising to the fire. Like with its TLUD cousin, the emissions in this kiln are forced to burn in direct fire which results in a smokeless burn. Once the fire is finished, the inner container is retrieved and the Biochar is either fully sealed (to cut off oxygen) or quenched with water. Retorts also offer the benefit of being adaptable to heat exchange systems to reduce total energy loss and recycle the heat they give off for things like greenhouse heating or cooking.
Regardless of the methods used, once burning is complete the remaining contents are either fully sealed off from oxygen or quenched with water to cool it off and stop the burning. If the Biochar is allowed to lower in temperature with access to oxygen, ashification occurs and will leave you with a pile of ashes to blow away in the wind. This process is also helpful for knowing when a burn is finished since most kilns burn cleanly while in pyrolysis. Once smoke is visible, its an indication that the fire has cooled down enough to start ashifying and it’s time to quench. Since Biochar is usually ground up before being used, drying is usually necessary after quenching, which can take some time given Biochar’s absorption ability.
Even after all this, the Biochar still isn’t quite ready to be added to your flower garden.
How is Biochar Used?
Biochar can be used for a variety applications around agriculture, but there is one critical thing to note before using it as a soil amendment: Fresh Biochar is extremely absorbent of liquids, nutrients, and microbial life and must be saturated before it can safely be added to soil for growing.
Placing “naked” Biochar in something like a vegetable garden would result in nutrients and microbial life turning their attention to the incredibly vast, empty new space, and neglecting their old plant friends, killing them in the process. The easiest way to saturate Biochar is to add it to something like compost or unused yard space, giving it time to mingle with materials containing enough nutrients and microbes to spare. As the compost breaks down, more microbial life can flourish and eat away at the various wastes because of the great new expanded housing available inside the Biochar, and the resulting nutrients are sucked right up by the Biochar itself.
Targeted supplementing is also possible by adding specific compositions of nutrients to “naked” Biochar and allowing it to absorb and grow. This can be a great way to incorporate Biochar into other Korean Natural Farming processes and use the nutrient-rich solutions like Oriental Herbal Nutrient and Water-Soluble Calcium. Adding Biochar as a soil amendment in varying ratios around 10-25% has shown to produce up to a 15% increase in above-ground vegetative growth.
Biochar is also a fantastic deodorant, which has set it apart as the best bedding available for chicken coops, since it works double duty as a smell suppressor and ammonia absorber. Opportunities for Biochar as a deodorizer seem endless, as it can be used as a filter (like an activated charcoal filter), for runoff reduction on dairy farms, as a topper for composting toilets, and as Bokashi flake for composting, among many others.
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Biochar in Carbon Sequestration as a Positive Climate Change Strategy
Biochar’s Roots in the Amazon: Terra Preta