Gasification characteristic of large wood chars with anisotropic structure

An experimental study was performed to investigate pyrolysis products, char morphology, and reactivity of char derived from large wood cylinders. Wood samples with various sizes (i.e. sawdust with particle size <1 mm and wood cylinders with a diameter of 8 mm) were pyrolyzed under an argon atmosphere to produce char samples using a thermobalance. The final pyrolysis temperature was 1173 K and the heating rate was set to 1 or 30 K/s. Then, the reactivity of char samples was measured using an isothermal method at various gasification temperatures from 673 to 1173 K under an oxygen/argon atmosphere. Scanning electron microscopy (SEM) and X-ray computed tomography (CT) were used to observe the char morphology. Quantitative measurements of pore size and shape were made using the digital image analysis. As for the pyrolysis, the effect of intraparticle secondary reactions of tar became significant for the large wood cylinder, resulting in the decrease of tar yield and the increase of char yield. Heating rate during the pyrolysis influenced the pore structure, the radial shrinkage and swelling of the wood cylinder. As for the char gasification, the char reactivity was significantly decreased with increasing cylinder length in the diffusion-controlled zone. In addition, it was found that the char gasification mainly progressed in the direction of pores when the gasification rate was controlled by the diffusion process, indicating the anisotropic evolution of particle shape during the gasification.