Using Solid-State ¹³C NMR to Study Pyrolysis Final Temperature Effects on Biochar Stability

Abstract

Recent results in biochar research show that it is not only composed of stable carbon, since a portion of these materials is degraded relatively easily once applied to soil, and this condition is most dependent on pyrolysis conditions, especially the final temperature. Thus, the aim of this study was to evaluate pyrolysis final temperature effects on the stability of biochar produced from forest residues using solid-state ¹³C NMR. Pyrolysis was performed at a heating rate of 10°C·min⁻¹ up to the final temperature of interest (350, 450 and 550°C), maintaining this temperature for 60 min. Solid-state ¹³C NMR spectra were obtained on a Varian 500-MHz spectrometer for fresh wood and biochars produced at 350, 450 and 550°C for Eucalyptus dunnii (DUN) and Pinus caribaea (CAR). Comparing fresh samples with their respective biochars, regardless of the botanical group, after pyrolysis, carbohydrates are degraded, and there is a change in the structure of the materials, with a predominance of aromatic structures, which are more resistant to degradation, therefore reflecting in the increased stability of these materials. For 350°C, it is still possible to observe signals related to lignin indicating that up to this temperature it has not been completely degraded. The spectra of biochars produced at 450 and 550°C are very similar, indicating that there is no need to produce biochars at very high final temperatures, since the structure of these materials obtained at 550°C slightly altered as of 450°C, keeping the predominance of aromatic structures.