Steam activation and mild air oxidation of vacuum pyrolysis biochar

Abstract

The objective of this paper was the investigation of steam activation and mild air oxidation of biochar produced by using the Pyrovac Inc. vacuum pyrolysis for a biomass feedstock composed of a 20% volume fraction of (spruce and fir) with pine (stem of Pinus strobus without bark), with a moisture mass fraction of 10%. The biochar was activated at 900 °C under a steam partial pressure of 53 kPa over 60 min. The steam activated sample was then submitted to a mild oxidation process through a reactor fed with 164 cm3 min−1 of air at 200 °C under 100 kPa total pressure for 60 min. Biochar was also submitted to this mild oxidation process under the same conditions except for the oxidation time which was either 30 or 60 min. The samples were analyzed using techniques including proximate and elemental analysis, Brunauer-Emmett-Teller (BET) analysis for surface area and pore size distribution by non-local density functional theory (NLDFT), scanning and transmission electron microscopy (SEM/EDX and TEM), Boehm titration, Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). The BET surface area was increased from 50 to 1025 m2 g−1 upon steam activation while the concentration of functional groups was extensively decreased. Upon mild air oxidation, the concentration of functional groups in biochar was increased from 44 to 104.6 μmol m−2 in oxidized biochar. Combined elemental analysis and XPS results indicated that mild air oxidation generated a more uniform spatial distribution of oxygenated functional groups than in the biochar.