Catalytic Microwave Pyrolysis of Plastic Waste Using Metallic Biochar Derived from Oil Palm Waste: Production of Hydrogen, Gaseous Hydrocarbons and Energy Efficiency

Microwave pyrolysis was performed on plastic waste pre-mixed with metallic biochar synthesized from similar pyrolysis technique. The metallic biochar, possessing a highly porous structure with high surface area (410 m²/g), acted initially as ‘adsorbent’ by providing many active sites for metals, metal oxides, and plastic waste to be attached onto. The biochar then acted as microwave absorbent, which absorbed microwave radiation and heated up to high temperature (650 °C) in a short time, observed to generate arcing and sparks intermittently, and in turn transformed into high temperature hot spots and acted as catalyst that promoted the pyrolysis cracking of plastic waste. This pyrolysis approach provided a fast heating rate (50 °C/min) and short process time (35 min) to generate higher yields of light C₅-C₁₀ hydrocarbons (up to 75 wt%), H₂ and CO gases (up to 40 vol%) in the pyrolysis products. The pyrolysis-gas (up to 35 wt%) were dominated by up to 70 vol% of C₁-C₆ hydrocarbons and 40 vol% of H₂ and CO, showing great promise for use as gaseous fuel or to be upgraded to produce more hydrogen as second-generation fuel. Up to 65 wt% yield of liquid oil was also obtained, detected to have high energy content (49 MJ/kg), C₁₃–C₂₄ hydrocarbons within the hydrocarbon range of diesel fuel, and promising green features for use as a potentially ‘cleaner’ fuel with low oxygen and nitrogen content, and free of sulphur. Energy balance analysis showed that this catalytic pyrolysis approach is potentially energy efficient to produce fuel products at a positive energy ratio of 8 (energy content of fuel products/electrical energy supplied for microwave heating) and a net energy output of about 160 MJ/h; this is supported by the high heating rate and short process time recorded by this approach, hence the lower power and energy consumption.