Microwave-assisted catalytic pyrolysis of refuse-derived fuel to improve pyrolysis performance and biochar properties

Abstract

Solid waste disposal and soil contamination remain severe environmental issues in many regions. Biochar can be produced via pyrolysis of solid waste and further applied for soil amendment. Microwave-assisted catalytic pyrolysis (MACP) is an innovative technology to improve pyrolysis performance and biochar quality compared with conventional pyrolysis. This project focuses on investigating the feasibility of using MACP to produce high quality biochar from refuse-derived fuel (RDF), which is generated through pre-processing of municipal solid waste (MSW). Two main catalysts, K₂CO₃ and K₃PO₄, and their combination with bentonite and clinoptilolite, were selected to mix with RDF in a fixed bed reactor exposed to microwave radiation. By comparing heating rate and biochar properties, the optimal catalyst was identified and further evaluated under various operating conditions, i.e., microwave power input, targeted pyrolysis temperature and microwave radiation time, in order to establish the relationship between these parameters and biochar yield and properties. The produced biochar was mainly characterized by specific surface area and pore size distribution based on N₂ adsorption/desorption isotherm. K₂CO₃ showed higher heating rate and larger specific surface area of biochar than that of K₃PO₄ due to its prominent activation effect. Synergistic effect was observed when bentonite or clinoptilolite was added into K₃PO₄ which significantly improved microwave heating rate. The optimal catalyst case was identified as 20% K₂CO₃ + 10% bentonite due to its high heating rate (163 ℃/min) and large specific surface area (206 m²/g) of biochar. Too low a microwave power (600W) could not initiate the reaction. The optimum targeted pyrolysis temperature was determined as 500 ℃ to produce biochar with the highest specific surface area (265 m²/g) and, based on statistical analysis, pyrolysis temperature was the main factor that influenced biochar yield and specific surface area, followed by microwave radiation time. Biochar produced from 30 wt.% K₂CO₃ could act as a precursor of potential adsorbent, while K₃PO₄ remaining in the produced biochar could serve as essential nutrient sources for plant growth and had the potential in adsorbing and immobilizing heavy metal contents in contaminated soil.