3.2 Characterization of SCG
The influence of the heat treatment atmosphere on the color and microstructure of SCG was investigated. The digital images of SCG, NSCG, and ASCG are shown in Figure 2 (a-c). The colors of the SCG, NSCG, and ASCG are light brown, dark brown, and light black, respectively. The blackening degree of the ASCG is darker than that of the NSCG. The color shifts indicate that SCG will be carbonized to some degree at the same temperature, regardless of the atmosphere. In addition, the SEM images corresponding to the three samples are shown in Figure 2 (d-f). It can be found that they are agglomerate. But after heat treatment, the surface of particles becomes fluffy and the adhesion degree of particles decreases markedly. It is the result of partial volatilization of small molecular weight materials and low molecular weight hemicellulose (van der Stelt et al., 2011).