Characteristics of biochar and bio-oil produced from wood pellets pyrolysis using a bench scale fixed bed, microwave reactor

Microwave pyrolysis of wood pellets was investigated in a pilot scale fixed bed microwave reactor at various biomass loadings and microwave power levels. A fixed proportion of biochar (10% of biomass loading) was used as a microwave absorber in each test conditions. Effect of biomass loading and the power level on the product yields and on the characteristics of biochar and bio-oil products were examined. While the bio-oil yield decreased when the biomass loading was increased from 1500 g to 3500 g, the biochar and gaseous product yields increased with the biomass loading. However, the microwave power level shows an opposite trend. Biochar exhibited good higher heating value (31 MJ/kg) and possessed a fine pore size (<1 nm), which can be used as a fuel or a source of porous carbon. Higher heating value of bio-oil was found in the range 12-14 MJ/kg. Kinematic viscosities of bio-oils were estimated in the range 1.8-6.1 mm2/s at 40 °C that is similar to the viscosity requirement for the gas turbine applications. Moisture content in bio-oils was found in the range of 57.3-69.3%, which is higher than the upper limit of water content (30% wt.). Only a few chemicals, including furfural, phenol, 3-Methyl-1,2-Cyclipentanedione, 3-Methylphenol, and 4-Methylguaiacol were found in the bio-oils because of the high moisture content. Results of product characterization of biochar and bio-oils confirm that both microwave power level and biomass loading does not have any significant impacts. Further research can be carried out to find out the measures to reduce the moisture content in the bio-oil.