Coupled electromagnetic and thermal analysis of microwave-heated particle beds: Effect of solid volume fraction and particle size

• Microwave pyrolysis heats dispersed particles rapidly and efficiently. • Solid volume fraction drives heating behavior in particle beds. • Particle size affects initial thermal uniformity, not energy absorption. • Simulations show heating rates up to 358 °C/min at 1000 W. • Dielectric loss factor is key to heating rate and final temperature. Microwave-assisted fast pyrolysis offers a rapid, energy-efficient route for converting biomass into biofuels and chemicals. This study presents a comprehensive coupled electromagnetic–thermal analysis of microwave-heated particle beds designed for pyrolytic applications, using activated carbon as a model absorber due to its dominant role in energy uptake within biomass/absorber mixtures. Response surface methodology was employed to systematically investigate the effects of particle size and solid volume fraction in disperse particle beds to inform the design of fluidized systems for continuous pyrolysis. Solid volume fraction emerged as the primary factor influencing both electromagnetic and heating behavior, while particle size had minimal impact on energy absorption but significantly affected thermal uniformity during early (sub-second) heating. Extended simulations at 1000 W input power confirmed these trends, with heating rates reaching up to 358 °C/min. Experimental validation showed good agreement at absorbed powers of 20–40 W, with average temperature errors of 14–20 %. Discrepancies at higher powers were attributed to variability in reported material properties and the lack of temperature-dependent data for activated carbon. Sensitivity analysis revealed that dielectric loss factor strongly influences heating rate and final temperature, while density and thermal conductivity play secondary roles. These findings provide new insights into localized electromagnetic and thermal dynamics, supporting the design optimization of scalable, energy-efficient microwave reactors for continuous biomass pyrolysis.