Activation of mixed sawdust and spirulina with or without a pre‑carbonization step: Probing roles of volatile-char interaction on evolution of pyrolytic products

Abstract

Volatile-char interaction has been well documented in co-pyrolysis of varied feedstocks, which might also exist in activation process and impact pore evolution of activated carbon (AC). This was investigated herein in activation of mixed sawdust and spirulina with K₂C₂O₄ at 800 °C in two scenarios: one-step direct activation or two-step activation with an intermediate pre‑carbonization. The results showed that volatile-char interaction via cross-polymerisation of the volatiles was more significant in the pre‑carbonization step, forming more biochar/bio-oil while less gases. However, volatile-char interaction was not that important in impacting product yields and evolution of pore structures of AC in activation. This was due to the dominance of gasification/cracking with presence of K₂C₂O₄, minimizing the chances for volatile-char interaction. The in-situ DRIFTS characterization of the activation process showed that removal of oxygen-containing species like CO with K₂C₂O₄ was important for generating pore structures. Comparing with two-step activation, one-step activation of sawdust, spirulina and their mixture without pre‑carbonization all generated the ACs of more developed pore structures and showed higher environmental impact. The pre‑carbonization removed a significant portion of thermally unstable aliphatic structures, enhancing aromatic degree and creating difficulty for generating pores via gasification/cracking in subsequent activation.