Activation of biomass with volatilized KOH

Abstract

In the chemical activation of biomass, it is a common practice to directly mix an activating agent, such as KOH, and feedstock for production of activated carbon (AC). Nevertheless, this generates AC with a high ash content and the solid–solid contact may not be sufficient. Herein, the activation of biomass (cellulose, lignin, and sawdust) with volatilized KOH at 900 °C was proposed by placing feedstock and KOH in two beds with up-down configurations. The volatilized KOH was expected to penetrate and activate the biomass in the upper bed. The results confirmed the hypothesis and a higher yield of AC was obtained from the activation with volatilized KOH than the direct activation of the mixture. The activation with volatilized KOH resulted in lignin-derived AC with a specific surface area of 652.8 m² g⁻¹, which was significantly higher than that from heating without an activator (262.2 m² g⁻¹). The activation with volatilized KOH was also effective for creating micropores in the sawdust- and cellulose-derived AC. Notably, the activation reduced residual KOH by >90% in the cellulose-derived AC, which made recycling of the unused KOH possible and remarkably reduced the cost for post-pretreatment of the AC for removing the inorganics. Besides, the activation with volatilized KOH enhanced the carbonization and thermal stability of the AC. In addition, the intensive cracking reactions in the direct activation destroyed the fibrous biological structure of the biomass in the AC, which, however, could be retained in the activation with volatilized KOH. Furthermore, the activation with volatilized KOH also showed a lower environmental impact and higher material efficiency than the direct activation.