Characterization of bio-oil production by microwave pyrolysis from cashew nut shells and Cassia fistula pods

In this study, the production of bio-oil by pyrolysis with microwave (MW) heating was tested experimentally. Two magnetrons with a total MW power of 1,600 W were used in the pyrolysis setup. The electric field strength was 185.38 V·m at an MW frequency of 2.45 GHz. Cashew nut shells (CNS) or pods (CFP) were pyrolyzed at 400°C, 500°C, or 600°C, with biomass-to-activated carbon ratio set at 70:30, 80:20, or 90:10. The largest yield of bio-oil was found for CNS at 600°C, and for CFP at 500°C, both with 90:10 ratio, achieving, respectively, 20.0% and 15.8% yields. When the bio-oil yields from CNS and CFP at 90:10 ratio and 400–600°C were analyzed with gas chromatograph-mass spectrometer, the components found included acids, esters, ketones, furans, pyrans, guaiacol, syringol and phenols, and phenolic derivatives were the dominant type of compounds. There were 23.56% and 13.23% phenolic derivatives, respectively, in the bio-oils from CNS (at 500°C) and from CFP (at 400°C). An analysis with Folin–Ciocalteu reagent of the phenolic contents in bio-oils gave the respective ranges 146.83–164.83 mg·GAE·g·DW and 39.34–45.91 mg·GAE·g DW for CNS and CFP (both run with 90:10 ratio).