Numerical simulation of the coupled multiphysics fields and reactions during the microwave pyrolysis of wood particles

Abstract

Microwave heating has gained significant attention as an alternative to conventional methods for biomass pyrolysis. To investigate the electromagnetic-thermal-hydraulic-chemical processes during biomass pyrolysis under microwave irradiation, a three-dimensional comprehensive pyrolysis model was developed. The model numerically described the physicochemical processes of the electromagnetic wave propagation in the reactor, heat transfer and fluid flow inside porous wood particles, pyrolysis reactions, and phase changes. The model was validated by comparing the temperature profiles of biomass samples under varying microwave powers against literature data. The modeling results demonstrated that there was a strong interaction between the distributions of the microwave electromagnetic fields in the cavity and that inside the wood particles; these distributions changed with the size, shape, and location of the biomass sample in the cavity. Using the model, the thermal response of wood particles to microwave radiation was determined. The predicted results showed that a millimeter-sized wood particle was large enough to straddle multiple high-energy microwave regions in the cavity, resulting in more than one intraparticle hotspots. The transient dielectric loss inside wood particles is nonuniform due to the enhancement of the local relative permittivity by the local hotspots. The maximum permeability within the wood particles increases by an order of magnitude during microwave pyrolysis at 1 kW. The hotspots have a considerable influence on the structural homogeneity and stability of biochar due to the formation of large thermal and expansion stresses around the hotspot locations. Moreover, the energy efficiency (ηe) of converting microwave electromagnetic energy into the thermal energy of wood particles is sensitive to the input microwave power and particle shape. For instance, ηe ranges from 12.44% to 36.15% under the microwave powers of 0.8–1 kW for the pyrolysis of a cylindrical wood particle with aspect ratios of 0.25–2. This work provides the fundamental technical support for the industrial application of microwave pyrolysis.