Jonas De Smedt
Chemical activated carbon out of waste from food industry by conversion in molten salts
On an industrial scale, two main routes for the production of activated carbon can be distinguished: physical and chemical activation. The physical activation process of a biomass feedstock includes two steps. Firstly, the precursor undergoes the carbonization process (slow pyrolysis), usually conducted in temperatures between 500 °C and 800 °C in the inert atmosphere. The secondary activation step employs a gaseous activating agent which oxidizes the carbonaceous material mildly and creates additional porous structures. Most commonly used activated agents are steam and carbon dioxide.
Chemical activation does not require a carbonization step, so the feedstock can be processed in raw form. The process starts with the impregnation or preparation of the feedstock mixture and an oxidizing agent in an appropriate ratio. This mixture is thermally treated at temperatures between 400 °C and 1000 °C (by means of pyrolysis). Similarly, to physical activation, upon contact with the feedstock, the activating agent will start to oxidize the carbon contained in the precursors leading to a formation of a porous structure. Most common activated agents for chemical activation are ZnCl2 , H3PO4 , KOH, and NaOH, which have a specific effect on the pore formation and the pore size distribution. In literature, the use of other chloride salts can be found such as FeCl3, KCl, MgCl2, NaCl, and CaCl2 or mixtures of salts.
Chemical activation has its advantages over physical activation such as increased yield, higher surface area, and lower treatment temperature, resulting in lower heat loss and energy requirement. An additional benefit from using the solid activated agents is better heat transfer in the reactor (lower radial gradient) due to higher thermal conductivity in comparison to a gaseous atmosphere. This leads to more homogeneous reaction conditions, hence the properties of the product. Versatile use of specific chemical activating agents allows for better tailoring the development of micro-and mesopores. For example, KCl and NaCl enhance the formation of micropores, whereas ZnCl2 promotes the development of both micro-and mesopores. Especially interesting is an application of the salts containing the potassium ion, which exhibits a catalytic influence on the pathway of biomass devolatilization, hence having a two-fold effect.
Typically used solid activating agents such as KOH (360 °C), NaOH (318 °C), or ZnCl2 (290 °C) have melting temperatures below the process temperature. The melting of the agent before the reaction allows for their better contact with the feedstock particle, intrusion into the precursor’s initial structure, and more efficient activation in the end. Such a situation is favorable for the process and allows for its conduction at relatively low temperatures. Additionally, low melting temperature allows for conducting the discharge, separation, and recovery process at a lower temperature, which translates to significantly lower heat loss. However, activation with other chlorine salts such as NaCl (801 °C) and KCl (770 °C), have relatively high melting temperatures. Therefore, in case of using those salts individually, the reaction temperature must exceed 800 - 850 °C. That poses a severe problem for the post-treatment, especially discharge. In the end, this also severely restricts the use of specific salts on an individual basis as the activation agents, which, on the other hand, could have a very beneficial effect on the pore formation.
One possibility to overcome this issue is to employ combinations of different salts to create a eutectic mixture with a lower melting point and higher viscosity after melting compared to a pure activating agent. This method allows for certain chemicals with high melting points to enhance the interaction with the biomass precursor and have a more profound catalytic effect.