Surface Functionality and Carbon Structures in Lignocellulosic-Derived Biochars Produced by Fast Pyrolysis

Switchgrass- and pine wood-derived biochars produced by fast pyrolysis were characterized to estimate the degree of thermochemical transformation and to assess their potential use as a soil amendment and to sequester carbon. The feedstocks were pyrolyzed to biochars in an auger reactor at 450, 600, and 800 °C with a residence time of 30 s. Ash contents of switchgrass and pine wood biochars varied from 13 to 22% and from 1.3 to 5.2%, respectively. Nutrients, such as N, P, K, S, Mg, and Ca, in switchgrass biochars ranged from 0.16 to 1.77%. Under combustion conditions, switchgrass chars were decomposed at lower temperatures than pine wood biochars because of the structural differences between the two feedstocks. Principal component analysis of the Fourier transform infrared (FTIR) spectra allowed for the discrimination of all biochars by significant contributions of cellulose-derived functionality at low pyrolysis temperatures, while the same analysis of the Raman spectra presented apparent separation of all biochars by two broad bands at 1587 and 1350 cm–1. These two broad peaks were deconvoluted into pseudo-subpeaks, which revealed that the number of aromatic rings linearly increased with the pyrolysis temperature. Cross-linkages between aromatic rings were also found to increase with thermal treatment, and switchgrass biochars contained a higher number of aromatic rings and cross-linkages than pine wood biochars, which was consistent with turbostratic carbon crystallites in the X-ray diffraction (XRD) pattern.