Since biochar properties can vary widely depending on feedstock and processing conditions, making meaningful comparisons between biochar produced by different processes (pyrolysis, gasification, kiln carbonization) and at different locations can be difficult. Biochar characterization methods can provide some useful metrics such as van Krevelen diagrams, fixed carbon to volatiles ratios, and aromatic ring cluster size. One of the key parameters shown to influence biochar properties is the highest heating temperature (HHT) reached during the reaction; clear trends can be observed in the characteristics of slow pyrolysis biochars over the 200-800°C HHT range. These trends do not hold up as well, however, when biochars made at the same HHT by fast pyrolysis or gasification are considered. For low and high temperature gasification, the presence of oxygen in the reaction atmosphere creates biochars with higher O/C molar ratios and higher apparent HHTs than analogous slow pyrolysis reactions. This study used a large sample size thermogravimetric analyzer (TGA) to produce corn stover biochars reaching different HHTs under slow pyrolysis conditions with varying levels of oxygen present in the flow gas. The biochars were characterized by proximate analysis, CHNS elemental analysis and 13C solid-state nuclear magnetic resonance spectroscopy (NMR) to understand the combined effects of HHT and oxygen on biochar properties. The goal of the study is to determine if the presence of oxygen in the reaction atmosphere at a given HHT would be beneficial for the creation of oxygenated functional groups on biochar surfaces similar to biochars that have “aged” in the soil environment.
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