Characterization of White Charcoal
23 November 2011
White charcoals, also known as shiro-zumi or binchotan, are synthesized by pyrolysing wood at a relatively low temperature (~200-300ºC) for a period, typically of a few days, and then raising the kiln temperature to ~1000ºC towards the end of the carbonization process. The hot charcoal is then withdrawn and smothered with a moistened mixture of earth, sand and ash. The term white charcoal arises from the use of ash to quench the material, which gives a whitish hue to the charcoal surface. White charcoal has a higher carbon content, density, hardness and ignition point compared to conventional charcoals. It has found many uses, especially in Japan, where it is used in diverse applications such as removing unpleasant odours, assisting and in the preparation of food, as well as in furnaces and musical instruments. It is believed that the pore structure in white charcoal exerts significance influence on behaviour of these materials. However, there have been no detailed studies of its microstructure.
White charcoal particles were examined using a range of analytical tools including scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectroscopy (EDS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), x-ray photoelectron spectroscopy (XPS), attenuated total reflectance (ATR) and gravimetric adsorption capacity scan (GACS). Initial characterization studies reveal a microstructure containing a bimodal distribution of pores. This includes macro-pores, often several hundred microns in diameter, which arise from the cellulose structure of the feedstock. These are often partly filled with micron-sized Ca-rich ash particles (visible as the white phase in the SEM images shown). In addition, the white charcoals contain very high densities of micron-scale pores. It is believed that these finer pores exert significance influence on the properties of this material. Initial NMR spectroscopy studies have shown that white charcoal is made up of very condensed aromatic rings with minimal aliphatic groups.