Revealing the initial pyrolysis behavior of municipal solid waste by ReaxFF molecular dynamics simulation

Abstract

Context

Integrating municipal solid waste (MSW) treatment with chemical looping combustion technology offers a promising strategy for energy recovery and pollution/carbon reduction. While pyrolysis serves as the crucial first step in this process, its fundamental reaction mechanisms remain incompletely understood. This study employs ReaxFF molecular dynamics simulations to investigate early-stage pyrolysis behaviors of MSW, focusing on the effects of temperature and H₂O/CO₂ additives on pyrolysis characteristics and nitrogen transformation pathways. The results indicate that inorganic gas yields increase with temperature, while among organic gases, C₂H₄ demonstrates both the earliest formation and the highest yield. The maximum gas yield (60.4%) and light tar production (32.9%) occur at 2500 K. 10 wt% CO₂ and 10 wt% H₂O enhance organic gas production. The promoting effect of H₂O is more pronounced, increasing the output of organic gases by 4.9% while promoting the decomposition of heavy oil and char. Nitrogen migration analysis reveals a progressive transformation from char-N to gas-N with increasing temperature. Under continuous high-temperature conditions, these N compounds further convert into NH₃ and CH₃N. This atomic-level investigation provides insights into the pyrolysis behavior of multi-component waste, offering theoretical support for further studies on the interaction between pyrolysis products and oxygen carriers.

Methods

In the Forcite module of Materials Studio, the COMPASS II force field is employed to perform geometric optimization and annealing for the construction of the MSW models. ReaxFF MD calculations are conducted using the ReaxFF module within the Amsterdam Modeling Suite computational platform. Force field parameters for H/C/O/N/S/B are adopted, and temperature is controlled via the Berendsen thermostat.