cross-posted from: https://slrpnk.net/post/2643215
ABSTRACT
Biochar is not a structured homogeneous material; rather it possesses a range of chemical structures and a heterogeneous elemental composition. This variability is based on the conditions of pyrolysis and the biomass parent material, with biochar spanning the range of various forms of black carbon. Thereby, this variability induces a broad spectrum in the observed rates of reactivity and, correspondingly, the overall chemical and microbial stability. From evaluating the current biochar and black carbon degradation studies, there is the suggestion of an overall relationship in biochar stability as a function of the molar ratio of oxygen to carbon (O:C) in the resulting black carbon. In general, a molar ratio of O:C lower than 0.2 appears to provide, at minimum, a 1000-year biochar half-life. The O:C ratio is a function of production temperature, but also accounts for other impacts (e.g., parent material and post-production conditioning/oxidation) that are not captured solely with production temperature. Therefore, the O:C ratio could provide a more robust indicator of biochar stability than production parameters (e.g., pyrolysis temperature and biomass type) or volatile matter determinations.
I knew some of those words!
This was a fascinating read, even as someone whose own biochar production is not on par with these studies (our “kiln” is an old washing machine drum we pulled from the woods). Anecdotally, it’s reaffirming to know that our approach of mixed feedstock may have an outsized effect when it comes to the heterogeneity of the carbon lattice structures we produce with our efforts. If there’s a hobbyist-level method of testing the O:C ratio of biochars produced at home I think that would be a fun thing with which to experiment, and could lead to higher efficacies for home production.
I find it pretty interesting how things change with temp, and how hotter chars are so much more stable
I feel as though it makes sense intuitively - other refining techniques that drive or cook off impurities also increase the quality of the final product, and seem to expel a higher percentage as the operating temperature rises to optimal given the application.
Still, good science means testing the things one might take for granted, and some of those tables were incredibly linear regardless of whether the char was synthetic or natural.
I used char cooked at about 380C in my MSC. The idea being that the less staw char would be more easily mineralized and improve soils faster than higher temp char. On top of that my soils has pHs of about 7.5 so I really didn’t want a liming effect.
pHs of about 7.5
Yeah, I can’t imagine wanting to drive that higher. Our last tests put us around 5.5 up in the field so a moderate liming effect isn’t unwelcome in those areas.
