Optimizing Microwave-Assisted Pyrolysis of Phosphoric Acid-Activated Biomass: Impact of Concentration on Heating Rate and Carbonization Time

Recent studies suggest the use of microwave energy in activated carbon (AC) production emphasizing efficiency specifically in the pyrolysis step as it can significantly reduce heating time compared to conventional furnaces. However, there is no documented effort on the effect of the activating chemical agent on microwave heating dynamics and its impact on pyrolysis time despite its importance for efficiency on both technical and economic aspects of the process. Elucidating the heating rate of H3PO4-activated biomass under microwave energy is one of the objectives of the research while the ultimate goal is to find practical H3PO4 concentration for chemical activation of biomass precursors in obtaining optimum AC yield and textural characteristics. It was found that excessive H3PO4 has a negative effect of slowing down pyrolysis time as it promotes poor microwave absorption on the biomass–H3PO4 complex. In addition, H3PO4 undergoing conversion to its anhydride, P2O5, requires relatively high activation energy, and its conversion may possibly cause extended pyrolysis time. Numerical optimization revealed chemical activation at 56.50% H3PO4, and pyrolysis under 650 W microwave power is a rational balance in terms of maximizing yield and surface area while minimizing activating agents and microwave energy. Under these conditions, a pyrolysis time of around 30 min, yield at around 39.65 wt %, and surface area of 826.38 m2/g can be expected. However, surface area can be as high as 1726.5 m2/g but will require 84.83% H3PO4 on activation, about 803.75 W power and 48 min carbonization time.