Fig. 6. Effect of shrinkage on the time vs. residual mass curves in particle models at Tr = 1073 K. (a) Wood logs with the diameter of 14.5 mm, and (b) Hexa His tag pellets with the diameter of 8 mm.Figure optionsDownload full-size imageDownload as PowerPoint slide
The shrinking core model (SCM) in Section 3.2 assumes that devolatilization progresses at an infinite thin reaction zone moving from the surface to the centre of the particle. To investigate the validity of this assumption, Fig. 7 shows the local reaction rate at various overall conversions. For both wood logs and pellets, we can observe the progress of the reaction zone from the surface to the centre. This means that the SCM assumption would be a reasonable assumption to simplify the particle model. However, reaction zone became relatively thick at the late stage of the conversion. For 14.5 mm wood logs, local reaction rate at the centre of the particle stayed below 25% of that at reaction front when the overall conversion was 90%, while the number was 46% for 8 mm wood pellets. This implies that the SCM assumption may cause more significant error to wood pellets than it does to wood logs.