The first hard fact that we can accept is that the best process temperature for agricultural biochar is unsurprisingly low as was certainly the case with classic terra preta. Most literature suggested that 350C degrees were about right. In fact the nature of an earthen kiln as certainly was used would suggest that a combustion front would pass through the bulk of the material and that the material would be hit with a peak temperature generated by the flame.
Remaining moisture would dampen the final temperature somewhat and perhaps provide a little measure of product control.
We also discover that the agricultural characteristics are markedly superior at the lower temperature. This is important because the tendency with metal kilns is to run to higher temperatures in order to speed the process. That earthen kiln methodology looks more foolproof by the day.
Certainly my intuition told me that an earthen kiln approach was likely the best method and that it was likely to remain a very good option. Once you are into metal, you are no longer losing surplus heat into the atmosphere and you really then need to draw off the volatiles some other way as the pyrolysis boys are trying to do.
The earthen kiln allows all the volatiles to be burned while using enough heat to char out the feedstock only. This is eminently practical for the agricultural industry from the subsistence farmer up. Corn culture makes it practical for the subsistence farmer as does elephant grass in Africa. The simple creation of shallow trench by removing top soil with a blade should allow any other form of farm waste to be packed and enclosed in dirt to form a similar kiln. And bales can be set on end, wrapped in a metal sheet and covered with a layer of dirt before set afire.
The importance of the dirt is that it will smother the red hot char as it loses structural integrity. You want it burning to that point at which you need to stop the process as fast as possible.
EFFECT OF LOW TEMPERATURE PYROLYSIS CONDITIONS
ON BIOCHAR FOR AGRICULTURAL USE
J. W. Gaskin, C. Steiner, K. Harris, K. C. Das, B. Bibens
ABSTRACT. The removal of crop residues for bio‐energy production reduces the formation of soil organic carbon (SOC) and therefore can have negative impacts on soil fertility. Pyrolysis (thermoconversion of biomass under anaerobic conditions) generates liquid or gaseous fuels and a char (biochar) recalcitrant against decomposition. Biochar can be used to increase SOC and cycle nutrients back into agricultural fields. In this case, crop residues can be used as a potential energy source as well as to sequester carbon (C) and improve soil quality. To evaluate the agronomic potential of biochar, we analyzed biochar produced from poultry litter, peanut hulls, and pine chips produced at 400°C and 500°C with or without steam activation. The C content of the biochar ranged from 40% in the poultry litter (PL) biochar to 78% in the pine chip (PC) biochar. The total and Mehlich I extractable nutrient concentrations in the biochar were strongly influenced by feedstock. Feedstock nutrients (P, K, Ca, Mg) were concentrated in the biochar and were significantly higher in the biochars produced at 500°C. A large proportion of N was conserved in the biochar, ranging from 27.4% in the PL biochar to 89.6% in the PC biochar. The amount of N conserved was inversely proportional to the feedstock N concentration. The cation exchange capacity was significantly higher in biochar produced at lower temperature. The results indicate that, depending on feedstock, some biochars have potential to serve as nutrient sources as well as sequester C. Keywords. Agricultural residues, Biochar, Bioenergy, Black carbon, Carbon sequestration, Charcoal, Plant nutrition, Pyrolysis, Soil fertility, Soil organic carbon.