Thermogravimetric kinetic analysis of catalytic and non-catalytic pyrolysis of simulated municipal solid waste

Abstract

Municipal solid waste (MSW) disposal through landfill and incineration represents a costly and hazardous challenge to global health, and loss of underutilised resource for the circular economy. Thermal pyrolysis of the organic components of MSW offers a scalable route to liquid fuels, but process optimisation requires deep insight into the associated thermochemistry and kinetics. Here we apply thermogravimetric analysis (TGA) and kinetic modelling to the (catalytic) pyrolysis of a model MSW feedstock comprising cellulose, sucrose, sugarcane bagasse and low-density polyethylene (LDPE). Co-pyrolysis of biomass and plastic waste, with or without Al-SBA-15, MM-Al-SBA-15 or HZSM-5 catalysts, occurs in three main stages, namely the melting and concomitant evaporation of volatiles, and subsequent decomposition of carbohydrate, then LDPE and finally lignin components. The rate of non-catalytic MSW pyrolysis increased with sample heating rates (from 20 to 120 °C /min) but had little impact on the temperature or mass loss during each stage. Catalytic pyrolysis is sensitive to the catalyst:feedstock mass ratio; increasing this ratio from 1 to 5 increased mass losses during the removal of volatiles and three subsequent decomposition stages, while lowering the temperature of LDPE decomposition. Kinetic analysis by Friedman’s isoconversional method reveals a monotonic increase in activation energy for each stage of (non-catalytic and catalytic) MSW pyrolysis. However, energy barriers for individual stages decreased with increasing catalyst:feedstock ratio and were sensitive to the catalyst type: Al-SBA-15 offers the largest decrease in barriers for carbohydrate (−86 kJ/mol) and LDPE decomposition (−92 kJ/mol), and similar promotion to HZSM-5 for lignin decomposition (∼100 kJ/mol), at the highest catalyst content. TGA is an effective tool for catalyst selection and process optimisation provided heating rates < 60 °C /min are employed, above which pyrolysis appears heat transfer limited.