Microwave Co-pyrolysis on Plastic and Sludge Waste: Effect of Heating Insulation on Product Yield

Abstract

The surge in global waste production strains waste management systems, where plastics made from polyethylene terephthalate (PET) and sludge waste pose significant environmental threats. Microwave pyrolysis offers a promising solution by converting waste into valuable products. The main advantages and disadvantages of microwave irradiation are the rapid heating during operation and significant heat loss during idle conditions. Microwave rays emitted by a magnetron undergo pulse operation on an on-off basis. The capability of the reactor setup and feedstock properties to retain and prevent heat loss during idle conditions is important to ensure effective thermal decomposition. Statements regarding the energy efficiency of microwave heating may be subject to scrutiny if the microwave needs to be operated for extended durations to offset heat dissipation. In this work, a microwave-powered pyrolysis system is designed with and without insulation. A feedstock mixture comprised of PET and sludge (1:1 mass ratio) is being thermally degraded and the degree of decomposition will be measured through the bio-char yield, which will be compared with the mass loss data from the thermogravimetric analysis (TGA). The discrepancies between both values indicate the possibility of incomplete decomposition with speculations of 1. Insufficient operation period and/or 2. Ineffective heat insulation. Results reveal the insulation of having a ceramic blanket wrapping the entire reactor during operation (Setup 2) was the most effective as it can achieve the highest temperature (557.19 °C) and heating rates (28.53 °C/min) with minor heat loss (29 °C/min), resulting in a more stable temperature profile. Setup 1 (with only fibreglass insulation) and Setup 3 (without insulation) report lower maximum temperatures (470.57 °C and 391.11 °C), lower heating rates (~23.9 °C/min) and unstable temperature profiles as compared to Setup 2. The average solid yield obtained at a pyrolysis temperature of 500 °C is around 39.05 wt.%, higher than those reported from TGA (26.01 wt.%) indicating that the current setup still requires optimization. Setup 2 presents the lowest solid yield at an improved decomposition, owing to the better insulation setup. This study reveals that despite the good performance of microwave heating, insulation also plays a vital role in ensuring effective decomposition.