Effect of feedstock and microwave pyrolysis temperature on physio-chemical and nano-scale mechanical properties of biochar

Biochars were produced from softwood chips (spruce-fir mix) and hemp stalk biomasses in an in-house-developed microwave pyrolysis reactor. A kilogram batch raw biomass mixed with 10 wt% microwave absorber was pyrolyzed at 60-min residence time. Microwave power levels were set at 2100, 2400, and 2700 W with optimum heating rates ranging 25-50 degrees C/min. The proximate analysis indicated a progressive gain in biochar carbon content with power level increase. Both biochars showed a H:C ratio of < 1.2 with a graphite-like structure, which is an important observation for their potential use as a filler in bio-composites structural strength increase. Fourier Transfer Infrared (FT-IR) spectra showed a major loss of functional groups as the power level increased. Brunauer-Emmett-Teller (BET) surface area and porosity distribution contained higher volume of smaller pores in the hemp biochar. The char hardness and Young's modulus, obtained via nanoindentation technique and load-depth curve analysis, indicated that hemp biochar possessed a higher Young's modulus and lower hardness than softwood chip biochar.