Unraveling the metallic thermocouple effects during microwave heating of biomass

Real-time temperature measurement during microwave (MW) heating is a significant challenge as metallic thermocouples tend to interact with MW. This paper aims to unravel the mysteries behind metallic thermocouple application during MW heating and present a method to detect a reliable temperature reading during MW processing. Experiments were carried out in a 1.25 kW lab-scale MW system facilitated with a quartz reactor, cavity, and metallic thermocouples placed from the top and the bottom of the MW cavity. The validation of numerical simulation results with an experiment showed the effect of placing the metallic thermocouple on the temperature and electric field distribution. The most striking result was the interaction of the thermocouple with the MW electric field in the top configuration that created localized heating, inaccurate temperature readings (error ± 120 °C), low MW energy absorbance (peak value 91 W), and MW leakage (>10 mW/cm²). Interestingly, a very reliable temperature reading (error ± 20 °C) with high MW power absorption (peak value 273 W) was observed when the metallic thermocouple was placed from the bottom of the cavity. In summary, minimal exposure of metallic thermocouples to MW irradiation would effectively measure accurate temperature.