Understanding the mechanism of two-step, pyrolysis-alkali chemical activation of fibrous biomass for the production of activated carbon fibre matting

Pyrolysis of fibrous biomass followed by alkali (KOH and K2CO3) chemical activation of the pyrolysis char for the production of activated carbon fibre matting material has been investigated. For KOH, activation initially involved the melting of the KOH and reaction with the disorganised/volatile material of the char, producing hydrogen gas and K2CO3 and further developing the existing pore structure of the char. At higher temperatures (>700 °C), pore widening occurs due to physical activation with less reaction of the alkali molten phase with the char. Metallic potassium or other potassium species may also contribute to the pore widening process. Chemical activation with K2CO3 occurs at higher temperatures (>600 °C) and is controlled by the decomposition of K2CO3 to K2O and CO2. The development of surface area and porosity occurs through reaction of the pyrolysis char with K2O and CO2. The production of CO2, enhances the physical activation of the pyrolysis char, leading to widening of pores. KOH activation results in the development of micropores at low temperatures, followed by pore widening at higher temperatures. For K2CO3 activation, pore creation and pore widening occur simultaneously at higher temperature, resulting in a lower micropore volume and a wider pore size distribution.