Spark plasma sintering for ultrafast lignin carbonization

Biobased carbon materials are emerging as alternatives to fossil-based carbons, but the slow carbonization process remains a major bottleneck for scalability. In this study, spark plasma sintering (SPS) is explored for direct and ultrafast carbonization of softwood kraft lignin. The carbonization was performed using SPS in vacuum where the die acts as a radiation heating element. The heating program employed variable heating rates ranging from 15 to 1000 °C min -1 up to a maximum temperature of 1000 °C, followed by an isothermal hold for three minutes. Consequently, excluding instrument cooling, the total carbonization time ranged from 4 to 70 minutes. Despite markedly shorter process time, the lignin-based carbons had similar properties to what can be produced with a conventional carbonization process at a slower heating ramp at similar final temperatures. The lowest energy consumption (kWh) was observed at the highest heating rate (0.75 ± 0.09 kWh at 1000 °C min -1 ), and a decrease in the heating rate increased the energy consumption. This study therefore shows that SPS can be a useful carbonization method for producing lignin-based carbon materials in a more time- and energy-efficient manner compared to conventional furnace processes.