The production and characterization of magnetic microwave absorbers for microwave-assisted pyrolysis

Abstract

Microwave absorbers (MWAs) are materials with the capability to convert microwaves into heating energy due to their high dielectric permittivity and/or magnetic permeability. Due to these properties, these materials are able to generate heat from microwave energy upon absorption. In this study, various microwave absorbers were used for microwave-assisted pyrolysis of hardwood. The microwave absorbers used in this study originated from commercial activated carbon (Chemviron’s pulsorb WP260 AC) and were modified through magnetite (Fe3O4) addition. Three variables (magnetite addition level: 5 and 20 wt.%, addition method: in-situ co-precipitation and physical blending and washing agent: water and ethanol/water) were tested. The resulting microwave absorbers were evaluated on the pyrolysis performance of hardwood in a Milestone Flexiwave microwave reactor (Power: 400 W; Residence time: 37 min; Pressure: 100 – 500 mbar; Total reactor loading: 20 g) with 10 wt.% MWA addition. The biochar yield varied from 24 wt.% and 89 wt.% and seemed to be significantly influenced by the synthesis method rather than Fe3O4 level. The microwave absorbers with physically blended Fe3O4 resulted in the best pyrolysis performance. All other modified MWAs performed less than the original commercial WP260 AC sample. The resulting biochars from the experiments with physically blended Fe3O4 AC and WP260 were characterized by elemental composition (C,H,N,S,O and ash content) after physical separation from the MWA. Furthermore, the concentration of 16 priority polycyclic aromatic hydrocarbons (PAHs) in the selected biochars were evaluated. The results suggest that physical Fe3O4 blending seems to cause a reduction of total PAHs content, mainly by reduction of the pyrene content in the biochar. Future experiments will involve the study concerning the possibility of magnetic separation of the MWA from the produced biochar. This will provide us more possibilities when scaling up the microwave-assisted pyrolysis process.