Numerical simulation of heating behaviour in biomass bed and pellets under multimode microwave system

Abstract

Domestic multimode microwave (MW) systems have been extensively used to process biomass materials for proof of concept. However, one of the major drawbacks for these systems is the non-uniform heating. Therefore, the aim of this article was to predict the heating behavior of empty fruit bunch (EFB) biomass in both bed and pellet form using finite element based COMSOL Multiphysics software. The temperature data from a modified domestic multimode MW system at 2.45 GHz frequency was used. Quantitative validation of 10 s heating profile was performed by comparing the simulated temperature profile with the experimental temperature. The agreement of simulated temperature profiles depended on various factors such as biomass loading bed height, defining specific heat capacity value and form of biomass shape (bed or pellet). Interestingly, the location of local hot spots during MW heating of EFB bed and pellets were almost close enough in both simulation and experimental study. An optimal biomass loading height was found whereby maximum MW energy is absorbed by the sample. The effect of biomass loading height on the distribution of electromagnetic fields is discussed in the paper. This study provides a framework and required model parameters to predict temperature and optimum biomass loading for a specific geometry of MW cavity. Further, the model can be effectively used to identify hot and cold spots in the biomass material during MW heating and thereby help to design and optimize the MW applicators in terms of heating uniformity. The proposed model can also be useful to identify the electromagnetic field distribution inside the cavity.