With millions of tons of plastic produced annually, plastic-related wastes have significantly increased, becoming a great threat to the environmental system. To this end, recycling and utilization of plastic, especially those of high calorific value, has been advocated for as an effective approach to reducing plastic waste. Thermochemical transformation technologies, especially gasification technology, are a prominent and highly flexible method for processing and treating mixed feedstock to produce clean CO and H2-rich syngas.
Steam gasification has drawn significant research attention owing to the high H2 yield. The influence of different factors like water content, temperature and plastic-type on the reaction rate and gas component have been extensively studied to improve H2 yield. However, the reaction mechanism involved in the steam gasification of plastic and the corresponding syngas formation mechanism is still unclear. This is because most existing experimental methods fail to accurately capture the structural changes at the molecular level during steam reaction and polymer dissociation.
Lately, ReaxFF-MD simulation method has emerged as an effective method for studying reaction mechanisms owing to its high accuracy and remarkable visualization at miscroscpic level. It does not require prior definition of the reaction path, and the accompanying chemical process can be defined from the bond breakdown and formation perspectives. It has been widely employed to study the mechanisms of various complex macromolecular systems, and its effectiveness in revealing the chemical reaction mechanism at molecular level has been demonstrated in many studies.
On this account, Professor Weiwei Xuan, Dr. Hailun Wang, Dr. Shuo Yan and Professor Dehong Xia from the University of Science and Technology Beijing studied the chemical reaction process of polyethylene (PE) steam gasification and the subsequent syngas, especially hydrogen, formation mechanism using a combination of ReaxFF-MD simulation and DFT calculation methods. The evolution of the radicals at the micromolecular level was explored and the reaction paths were analyzed through energy barriers. Additionally, the influences of water content and temperature on the distribution of the products were investigated. Their work is currently published in the journal, Fuel.
The authors divided the whole gasification process into two overlapping stages: PE thermal depolymerization (the first stage) and water reforming rection (the second stage). The depolymerization mainly involved terminal and intermediate C – C breaking, which facilitated the continuous depolarization of PE chain into smaller hydrocarbons until C1 – C4 chains. This explains the formation of many ethylene monomers in the process. The process also involved the generation of different R⋅ radicals and ⋅R⋅ double radicals fragments and the resulting free H⋅ radicals produced through dehydrogenation and H-transfer reaction could significantly reduce the energy barrier of subsequent water decomposition reaction.
H2 production path came from water reformation reaction with H⋅ radicals, which account for about 70% of the total production. Consequently, the production of water mainly stemmed from the reforming reaction of water with free H⋅radicals and the accompanying ⋅OH radical actively facilitated the dehydrogenation process to form CO. An increase in the temperature and water content could accelerate the reaction process to improve H2 and CO yields, though the improvement decreased gradually. For example, a temperature increase from 2500 to 3500K increased the amount of H2 yield by 20%, but the H2 volume proportion in the resulting product gas remained relatively constant (about 60 – 70%). For steam plastic ratio (S/P) below 1.5, an increase in the S/P improved the CO and H2 yield because of water. The water effect increased H2 yield by 2.5 times that without water.
In summary, PE steam gasification mechanism was explored in this study. The study provided valuable information regarding radical transformation and integration that would contribute to appropriate catalyst selection and reaction adjustments. In a statement to Advances in Engineering, Professor Weiwei Xuan stated that their findings would advance the development of steam gasification technology to address the global plastic waste crisis through recycling.
Reference
Xuan, W., Wang, H., Yan, S., & Xia, D. (2022). Exploration on the steam gasification mechanism of waste PE plastics based on ReaxFF-MD and DFT methods. Fuel, 315, 123121.
