Microwave-assisted Carbonization of Phosphoric Acid Activated Biomass

Abstract. Biomass as a feedstock for activated carbon (AC) production is attractive because they are available and renewable. Greatest challenge, as in any biomass conversion technologies, is the technological and economic efficiency. In context of controlling process parameters to obtain a desired textural quality of AC, chemical activation is favored over physical activation because process variables such temperature and oxidizing agent of the physical activation process show minimal influence on the final AC textural quality. Recent studies suggest use of microwave in activated carbon production emphasizing efficient use of energy in the process. Specifically as mode of heating in carbonization step, it can promote significantly reduced heating time compared to conventional furnaces. Main goal of the study is to find the optimum phosphoric acid concentration for chemical activation of biomass in obtaining desirable AC yield and characteristics while elucidating the microwave heating mechanism on the carbonization step of the AC production. It was found that residual H₃PO₄ has negative effect of slowing down pyrolysis as it decreases microwave absorption capacity of the biomass-phosphoric acid complex. In addition, residual H₃PO₄ undergoing conversion to its anhydride, P₂O₅, requires additional energy and conversion time extending the overall carbonization time. Chemical activation by 60% phosphoric acid is practically rational as carbonization time is just about half of that of 85% while having similar textural characteristics and slightly higher yield compared to other concentrations used. Textural characterization of AC derived from 60% phosphoric acid activated Douglas fir show S_BET of 1285 m²/g and mesoporous nature.