Multi-methodological characterisation of Costa Rican biochars from small-scale retort and top-lit updraft stoves and inter-methodological comparison

We applied common (pH, elemental analysis, thermogravimetry) and less-common (infrared spectroscopy, GACS adsorption test, pyrolysis-GC-MS, hydrogen pyrolysis) analytical procedures to a set of biochars from Costa Rica (bamboo stalk, cacao chaff, sawmill scrap, coconut husk and orchard prunings feedstocks). The biochars were produced by high temperature combustion in a top-lit updraft stove (TLUD) and low temperature anaerobic charring in a retort (RET), the latter of which was heated by the gas that evolved from the TLUD. The RET biochars exhibit a smaller adsorption capacity, higher molecular diversity and larger proportion of thermolabile materials, because of the lower degree of thermochemical alteration (DTA) and therefore limited formation of the microporous polycondensed aromatic matrix typical of the TLUD biochars. Multivariate statistics showed that DTA, not feedstock composition, controls biochar organic chemistry. The TLUD biochars might be better candidates for soil amendment because of their adsorption capacities and will probably exert a more prolonged effect because of their chemical stability. The cross-comparison of the methods showed the complementarity of especially elemental analysis, GACS, thermogravimetry, hypy and pyrolysis-GC-MS