Cone Kiln Biochar Characteristics

If you have been around biochar for awhile, you know that there are many types of biochar with very different characteristics. Biochar characteristics differ depending on the feedstock used, processing temperature and exposure to oxygen. Things to look for in biochar properties are pH, surface area and porosity, ash content, volatile matter content, and the "fixed" carbon content. Fixed carbon refers to the degree that the carbon has been converted to stable benzene rings that do not degrade easily.

Japanese researchers published a paper that compares biochar made in a cone kiln to other biochars: Inoue, Y., Mogi, K., & Yoshizawa, S. (2011). Properties of Cinders from Red Pine, Black Locust and Henon Bamboo (pp. 1–2). Presented at the APBC Kyoto 2011.

Biochar made in the cone kiln from pine, black locust and bamboo, compared favorably with biochar made at fairly high temperatures (600 C) in a retort placed inside a laboratory oven. The cone kiln biochar has as much fixed carbon and surface area as the retort char. The cone kiln char had higher pore volume than the retort chars, and superior water holding capacity. The reason for the difference is that the cone kiln char carbonized more quickly than the retort char (the heat is transferred more quickly since it does not have to pass through a metal retort wall), shrinking less in the process, and maintaining more pore spaces inside.

Other findings from the paper include the efficiency of the cone kiln process: For red pine, about 14% of the feedstock mass (dry basis) was converted to biochar, with about 24% of the carbon retained. Values were higher for black locust and bamboo. The pH was higher in the cone kiln biochar because of the ash produced. This can be altered by rinsing the biochar, or composting with other materials if the high pH is not desired.

The researchers concluded that the cone kiln biochar is "suitable for amendment that would enhance water-holding capacity of soil and neutralize acidic soil."

Report from the Field

Don Morrison reports: "The 55 gal reservoir is just the right volume for quenching the coals from the 5' kiln by raking and spraying. I also liked the ease of tipping the kiln and dumping by one person with cable attached to handle and 4-wheeler.

Flame Carbonizers for Biochar: In Practice and Theory

Here's a presentation I gave at the Char Production Gathering at Aprovecho in August. Tried to compile everything I have learned about Flame Cap Kilns and other simple carbonizers for making biochar.

Checklist for a Biochar Burn

Here's a checklist of things you need to consider when planning a biochar burn:

• You need a water source, first of all, both for safety and for quenching the char. 
• Several shovels and rakes for spreading and cooling the char when it's done.
• Old steel roofing material is helpful for spreading and cooling the char without getting dirt or other contamination.
• Dry wood. A moisture meter is helpful. It's not very efficient to use wood that is more than 20% moisture. 
• Safety - Helpers should all have leather gloves and wear cotton or wool clothing that won't melt. These piles can put out a lot of heat! It is good for at least one person to have a fire helmet and a face shield. 
• You may need to get a burn permit from the local officials. 
• Ignition - I use a propane weed burner type torch for ignition, but all you need is a match if you have plenty of dry kindling. Light it on the top. 
• Make sure the kindling is somewhat densely packed so it will sustain a flame and allow the flame to move down to the lower layers. It seems slow to start but pretty soon you'll find the whole pile is ablaze. 
• Use a wind screen if conditions are breezy. Even if wind is not a factor, a wind screen will help hold in heat for a safer and more efficient biochar burn.

Start Your Flame Cap Kiln with a Top Lit Open Burn

Here is a series of pictures showing how we get a good start in the pyramid kiln by stacking a pile of wood in the kiln and lighting it from the top. It burns down to coals pretty quickly and gets the burn off to a good start by providing a nice, deep bed of coals that can really put out the heat for the first layer of wood.

We call a pyramid, cone, pit or tube an Open Fire Kiln.

We call an open burn pile that you light from the top (so it burns up the smoke and is clean) a Top-Lit Open Burn. You can read my article about open burning of forest fuel load reduction piles here:

Learning to Burn and Make Biochar, Not Smoke. Kelpie Wilson. Tree Care Industry Magazine. February 2014.

Build an open stack of wood with kindling on top. Light the top.

Heat transfers by radiation to the layer of wood below the flame.

Most of the wood in the stack is now charred. It only took about 15 minutes.

All the wood from the stack is now charred, and the pile has collapsed.

Now we have added the first layer on the bed of coals. We will continue adding a new layer as the old one chars.

Top Lit Burn Piles

What is a Top Lit Burn Pile? It is a technique for clean conversion of waste biomass - like forest thinnings - into small amounts of biochar. It is not the most efficient way to produce biochar. However, there are many situations today where "problem" biomass is thinly distributed on the ground and far away from any facility where it could be used to generate energy or used as feedstock in some efficent carbonization machinery to produce biochar.

The Top Lit method can be used in cases where biomass will be burned anyway just to dispose of it, for instance, fuel-load reduction in forests or killing pests in orchard and vineyard prunings. Top Lit Burning will produce some biochar - perhaps about 10% by volume of the original biomass, depending mostly on feedstock moisture - the drier the better. Perhaps more importantly, the Top Lit method drastically reduces the amount of smoke produced, saving our air and our lungs.

Here is a great time lapse video of the Top Lit method as practiced in northern California by Will Emerson, followed by a "how to" video from Steven Edholm and then by a presentation and article I did on our October 2013 Top Lit Burning demonstration project in southern Oregon: 

 

They did a super nice job of laying out my article at Tree Care Industry Association Magazine. It's in the February 2014 issue which you can read for free here: Learning to Burn and Make Biochar, Not Smoke

TCIA is an organization for professional arborists. It's great to see their interest in biochar. I wrote another piece for them back in 2012 (Biochar for Arborists) that highlighted work being done by Bryant Scharenbroch of the Morton Arboretum and Bartlett Tree Services on renewing the health of urban street trees in Chicago.

 

Backyard Biochar in Cave Junction, Oregon

We fired up two of the Oregon Kilns in a suburban setting in Cave Junction last Saturday - real Backyard Biochar! Had lots of help so it was a smooth run. We ran out of feedstock, or we could have made a lot more biochar. Here's a video: