Activated carbon production from algal biochar: Chemical activation and feasibility analysis

Abstract

Chemical activation was employed to convert algal biochar obtained from hydrothermal carbonization of lipid-extracted algae (LEA) to activated carbon. Potassium hydroxide, previously utilized on cellulosic biomass but not on algal biomass, was employed as activating agent and the impact of the activation conditions, namely temperature, activation time, and amount of activating agent, were investigated. The yield of activated carbon from biochar ranged from 28 % to 52% and decreased as the temperature was raised from 400 to 600 °C, the residence time from 30 to 60 min, and the KOH/biochar mass ratio from 0.25 to 1.0. In contrast, surface area increased by 2.1-fold when the activation temperature was raised to 600 °C and by 1.5-fold when the KOH: biochar ratio was raised to 1.0. Maximum BET surface area of 847 m²/g was achieved at 600 °C after 30 min at a mass ratio of 1:1. The integrated hydrothermal carbonization and activation process of LEA was simulated in Aspen Plus® and the technoeconomic feasibility was assessed based on our experimental data at 1,000 and 10,000 acres of cultivation area. For the latter, net present value analysis determined a minimum selling price of $2,200/ton for algal activated carbon with a financial breakeven achieved in 3.5 years. This is cost-competitive with the current price of commercial fossil-derived activated carbon, which is $1,543-$2,645/ton. Sensitivity analysis showed that the minimum selling price is significantly affected by algal biomass yield during cultivation and is more sensitive to the operating expenses than to the capital investment.