A Discrete Distributed Activation Energy Model for Cedar and Polyethylene Fast Heating Pyrolysis Kinetics

The pyrolysis of municipal solid waste (MSW) is an efficient, cost-effective, and environmentally beneficial thermochemical treatment method. A macro thermogravimetric analyzer (Macro TGA) was used to study the pyrolysis behavior of cedar and polyethylene (PE) at slow (10 K/min) and fast (700, 800, and 900 °C) heating rates. For cedar, the pyrolysis rate curve showed multi-peak characteristics at the slow heating rate and single-peak characteristics at the fast heating rate. Conversely, PE exhibited the opposite behavior. At fast heating rate of 700 °C, the pyrolysis rate for cedar increased from 0.685 to 0.847 min−1 as the sample temperature rose by over 100 °C, from 351 to 455 °C. By contrast, for PE, the rate increased from 0.217 to 1.008 min−1 with a smaller temperature rise of less than 30 °C, from 630 to 656 °C. According to the International Confederation for Thermal Analysis and Calorimetry (ICTAC) guidelines for analyzing pyrolysis thermogravimetric data, cedar pyrolysis primarily followed a single-step parallel reaction pathway, while PE exhibited some multi-step parallel reactions. A newly developed discrete distributed activation energy model (DDAEM), along with the traditional iso-conversional model (ICM) and distributed activation energy model (DAEM), were applied to predict pyrolysis characteristics at fast heating rates. For cedar, both DDAEM and ICM provide accurate predictions, with average activation energies calculated by these two models being 48.08 and 66.37 kJ/mol, respectively. For PE, DDAEM demonstrates significantly higher predictive accuracy than ICM, particularly when the conversion is below 0.2. As the pyrolysis conversion of PE increases from 0.25 to 0.65, the average activation energy calculated using ICM was found to be 58.32 kJ/mol. By contrast, for DDAEM, the activation energies for the first and second step reactions were 110 and 60 kJ/mol, respectively. This indicates that ICM can only calculate the activation energy for the final step and not for the rate-limiting step. For both cedar and PE, DAEM fails to provide accurate predictions due to the unsteady heating rate.