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."
