Electrical conductivity of beech sawdust using graphite catalytic coating: unlocking the microwave-assisted thermolysis efficiency of lignocellulosic biomass

The coating of the beech sawdust using a catalytic amount of graphite (as low as 0.25 wt%) allowed a step improvement in the microwave-assisted thermolysis. Results demonstrated that the pyrolysis performance was linked to an electrical conductivity threshold of the coated samples rather than a gradual increase. With as low as 0.13 mS m−1 of electrical conductivity, the 0.75 wt% graphite coated sawdust (250–500 μm) was efficiently gasified with up to 43 wt% of gas (30 wt% of carbon monoxide, 25 vol% of hydrogen). Initial particle size impacted the thermolysis performance where optimal size (250–500 μm) provided high heat homogeneity due to efficient graphite coating and low temperature gradient between the outer and inner part of the sawdust. The small initial particle size (75–250 μm) was unsuitable for microwave pyrolysis, exhibiting a too large surface area for efficient coating with 0.75 wt% of graphite which was confirmed by the absence of electrical conductivity (<0.003 mS m−1). The electrical conductivity can be used as a marker to evaluate the suitability of the sample for microwave-assisted pyrolysis. Unlike simple graphite mixing, the mechanical coating allowed more than 20-fold decrease of susceptor quantity, providing more homogeneous samples with higher reproducibility.