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

Authors: Ellison, Candice; Elkasabi, Yaseen; Mullen, Charles

Submitted to: Thermal Science and Engineering Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2025
Publication Date: 1/6/2026
Citation: Ellison, C.R., Elkasabi, Y.M., Mullen, C.A. 2026. Coupled electromagnetic and thermal analysis of microwave-heated particle beds: Effect of solid volume fraction and particle size. Thermal Science and Engineering Progress. https://doi.org/10.1016/j.tsep.2025.104463.
DOI: https://doi.org/10.1016/j.tsep.2025.104463

Interpretive Summary: Microwave-assisted fast pyrolysis is an advanced method for turning agricultural biomass residues into useful fuels and chemicals quickly and efficiently, offering promising opportunities for decentralized, rural energy production. ARS scientists used computer simulations to study how activated carbon particles absorb microwave energy and heat up, which dominates the heating dynamics during microwave biomass pyrolysis. The results show that the amount of solid material in the reactor has the biggest impact on how well the particles heat by microwaves, while the size of the particles mostly affects how evenly the heat is distributed early on. The simulations showed heating rates as high as 358°C per minute, and lab experiments confirmed many of these results, although some differences appeared at higher power levels due to lack of accurate activated carbon material properties, especially as temperature increases. Overall, the study offers valuable insights that can guide the design of next-generation microwave pyrolysis reactors that can continuously and efficiently convert biomass into clean energy. These advancements could accelerate the development of sustainable, agriculture-based fuel production, particularly in rural communities.

Technical Abstract: 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 dispersed 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 344°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.