Charcoals and carbonized charcoals (i.e., biocarbons) were prepared from a wide variety of biomass substrates, including pure sugars containing five- and six-membered rings with furanose and pyranose configurations, lignin, agricultural residues (corncob and nut shells), and a hard wood. These biocarbons were subject to proximate and elemental analysis, gas sorption analysis, and analysis by inductively coupled plasma mass spectroscopy (ICP-MS), scanning electron microscopy (SEM), X-ray diffraction (XRD), electron spin resonance (ESR), 13C cross-polarization magic-angle spinning (CPMAS) NMR, and matrix-assisted, laser desorption ionization coupled with time-of-flight mass spectroscopy (MALDI-TOF MS). All the carbonized charcoals contained oxygen heteroatoms, had high surface areas, and were excellent conductors of electricity. Doping the biocarbon with boron or phosphorus resulted in a slight improvement in its electrical conductivity. The XRD analysis indicated that the carbonized charcoals possess an aromaticity of about 71% that results from graphite crystallites with an average size of about 20 Å. The NMR analysis confirmed the highly aromatic content of the carbonized charcoals. The ESR signals indicated two major types of carbon-centered organic radicals. MALDI-TOF spectra of the charcoals and carbonized charcoals greatly differed from those of synthetic graphite. The biocarbons contained readily desorbed discrete ions with m/z values of 317, 429, 453, 465, 685, and 701. These findings were employed to develop a model for the structure of carbonized charcoal that is consistent with the biocarbon's oxygen content, microporosity and surface area, electrical conductivity, radical content, and its MALDI-TOF spectra.

ABSTRACT. Charcoals produced by a modern, efficient method were studied in the kinetic regime, at oxygen partial pressures of 0.2 and 1 bar by thermogravimetric experiments and their reaction kinetic modeling. The charcoals were ground to an average particle size of 5 -13 µm. A partial removal of minerals from the feedstock (corncobs) by an acid-washing procedure resulted in ca. 6 times higher specific surface area in the charcoal. In spite of the increased surface area, this sample evidenced a much lower reactivity. A model based on three reactions gave an adequate description over a wide range of experimental conditions. 38 experiments on 4 charcoal samples were evaluated. The experiments differed in their temperature programs, in the ambient gas composition and in the grinding of the samples. Characteristics of the combustion process were determined, including activation energy values characteristic for the temperature dependence of the burn-off; formal reaction orders characterizing the dependence on the oxygen content of the ambient; and functions describing the conversion dependence of the partial processes.

A half century ago, Rosalind Franklin identified two distinct families of organic materials:  those that become graphitic during carbonization at high temperatures and those that do not. According to Franklin, sucrose-derived biocarbons showed “no trace of homogeneous development of the true graphitic structure, even after heating to 3000 °C” [Proc. R. Soc. A 1951, 209, 196-218]. Franklin concluded that “non-graphitizing” carbons (e.g., sucrose biocarbons) are typically formed from oxygen-rich or hydrogen-poor substances that develop a “strong system of cross-linking, which immobilizes the structure and unites the crystallites in a rigid mass”. In this work, we show that there is a spectrum of non-graphitizing biocarbons ranging from those that release little CO during carbonization at temperatures approaching 1000 °C to those that strongly and persistently emit CO during carbonization at temperatures approaching 1000 °C. Typically, very low-ash biocarbons are not persistent CO emitters, but biocarbons with moderate ash contents can also be a member of this class if their ash lacks the catalytic species K, P, Mg, and/or Na that appear to be responsible for persistent CO evolution at 1000 °C.