Molecular-Level Kinetic Modeling of the Gasification of Common Plastics

• Published in: Energy & fuels 2016-03, Vol.30 (3), p.1662-1674
• Main Authors: Horton, Scott R, Woeckener, James, Mohr, Rebecca, Zhang, Yu, Petrocelli, Francis, Klein, Michael T
• Format: Article
• Language: English
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.5b02047

A molecular-level kinetic model was developed for the gasification of common plastics, including polyethylene (PE), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polystyrene (PS). Model development was divided into three steps: molecular characterization of the feed, generation of a pathway-level reaction network, and creation of the material balance differential equations (DEs). The structure of all polymers was modeled as linear with known repeat units. For PE, PVC, and PET, Flory–Stockmayer statistics were used to describe the initial polymer size distribution. PS was described using a two-parameter γ distribution. The parameters of all polymer size distributions were tuned using data from the literature. The chemistry of plastic gasification contains depolymerization, pyrolysis, and gasification reactions. The initial depolymerization of PE and PET was modeled using random-scission and Flory–Stockmayer statistics. A statistical method was created extending random scission to a generalized polymer size distribution and applied here to the breakdown of PS. The depolymerization of PVC was modeled as two steps: polyene formation followed by benzene production. Pyrolysis reactions were included on small oligomers and were broken down into two categories: cracking and formation of tar and char molecules. For gasification, incomplete combustion and steam reforming were included to break down oligomers, tar, and char molecules. Also, light gas reactions, e.g., water-gas shift, were added to the network. The final network contained 283 reactions and 85 species. After construction of the material balance DEs, kinetic parameters were tuned using literature data on each plastic. These studies involved gasification, pyrolysis, and thermogravimetric analysis (TGA) experiments each probing different aspects of depolymerization, pyrolysis, and gasification kinetics. Model results matched experimental data well.