Tracing the Physico-chemical Evolution of Soot Primary Particles via Molecular Dynamics Simulations of Acetylene Pyrolysis

EVAN GVENTER, Khaled Mosharraf Mukut, Eirini Goudeli, Georgios A. Kelesidis, Somesh Roy, Marquette University

     Abstract Number: 251
     Working Group: Combustion

Abstract
Soot is a carbonaceous particulate formed during combustion that poses significant environmental and public health risks. It is challenging to accurately characterize properties of soot due to the complexity of hydrocarbon combustion systems and the range of scales involved. This study is an extension of our previous exploration of chemical, physical, and morphological properties of soot primary particles using reactive molecular dynamics (MD) simulations of acetylene pyrolysis. Using the ReaxFF forcefield, a series of reactive MD simulations of acetylene pyrolysis were performed to capture the inception of soot primary particles. The primary particles thus obtained were then separated and put into a simulation box with more acetylene molecules in another set of reactive MD simulations to see how their characteristics change as they grow. Several different growth patterns of incipient particles at four different temperatures and densities were simulated. During this simulated growth process, each incipient soot primary particle grew from clusters of around 1400 carbon atoms to more than 4000 carbon atoms. An initial analysis reveals the growth of incipient particles takes place in stages – a stage of mass growth, usually in conjunction with an increase in aliphatic carbons, followed by a stage of chemical reorganization, where aliphatic carbons gradually form aromatic bonds. These observations indicate that primary particles appear to go through multiple mass accumulation and chemical restructuring periods as they mature. While the mass of the particles grew from around 17000 amu to more than 50000 amu, their radius of gyration grew from around 1.6 nm to 2.3 nm. During this process, the C/H ratio of the particles also increased, albeit slightly. This work indicates that during their growth, incipient soot particles go through a complex evolution of their physicochemical properties, providing valuable insight into the pathways of soot formation.