Characterization of biochar from fast pyrolysis and gasification systems

Abstract

Thermochemical processing of biomass produces a solid product containing char (mostly carbon) and ash. This char can be combusted for heat and power, gasified, activated for adsorption applications, or applied to soils as a soil amendment and carbon sequestration agent. The most advantageous use of a given char depends on its physical and chemical characteristics, although the relationship of char properties to these applications is not well understood. Chars from fast pyrolysis and gasification of switchgrass and corn stover were characterized by proximate analysis, CHNS elemental analysis, Brunauer‐Emmet‐Teller (BET) surface area, particle density, higher heating value (HHV), scanning electron microscopy, X‐ray fluorescence ash content analysis, Fourier transform infrared spectroscopy using a photo‐acoustic detector (FTIR‐PAS), and quantitative ¹³C nuclear magnetic resonance spectroscopy (NMR) using direct polarization and magic angle spinning. Chars from the same feedstocks produced under slow pyrolysis conditions, and a commercial hardwood charcoal, were also characterized. Switchgrass and corn stover chars were found to have high ash content (32–55 wt %), much of which was silica. BET surface areas were low (7–50 m²/g) and HHVs ranged from 13 to 21 kJ/kg. The aromaticities from NMR, ranging between 81 and 94%, appeared to increase with reaction time. A pronounced decrease in aromatic CH functionality between slow pyrolysis and gasification chars was observed in NMR and FTIR‐PAS spectra. NMR estimates of fused aromatic ring cluster size showed fast and slow pyrolysis chars to be similar (∼7–8 rings per cluster), while higher‐temperature gasification char was much more condensed (∼17 rings per cluster). © 2009 American Institute of Chemical Engineers Environ Prog, 2009