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

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Abstract 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−2 at an MW frequency of 2.45 GHz. Cashew nut shells (CNS) or Cassia fistula 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−1 and 39.34–45.91 mg·GAE·g DW−1 for CNS and CFP (both run with 90:10 ratio).