Exploring depolymerization mechanism and complex reaction networks of aromatic structures in chemical looping combustion via ReaxFF MD simulations

Abstract

Uncovering the reaction mechanism of various fuels to chemical looping combustion (CLC), especially depolymerization of solids in a complex environment, is an important scientific issue. Aromatic structures are the backbone of solid fuels such as coal and biomass. However, their depolymerization mechanisms during CLC remain unclear, and few complex reaction networks have been established to date. This work presents molecular insights into reaction behaviors and mechanism for several aromatic structures in the fuel reactor of CLC. The distribution and evolution of reactants, important intermediates, and products in the CLC are studied in details by means of reactive force field molecular dynamics (ReaxFF MD) simulations. It is found that the reaction process of aromatic fuels reacted with Fe2O3 as oxygen carriers in CLC can be divided into four stages. The decomposition rate of fuel molecules is relevant to the reaction temperature and aromatic sizes. The higher of reaction temperature and the smaller aromatic sizes, the faster the conversion of fuel molecules. The amount of CO and H2O produced gradually increases during CLC process, and however shows a decreasing trend in the later stages. The higher the CLC temperature is, the more favorable the oxygen releases from the oxygen carrier lattice are, so that the fuel molecules can burn fully to produce more CO2. The fuel molecular structure has less effect on generation time of CO molecule, while more on that of H2O molecule. The initial reactions of aromatic structures in CLC involve competition between thermal cracking and oxidation reactions. Fuel molecules generally go through a thermal ring-opening step first, followed by continuous bonding dissociation and oxidation to produce numerous structural fragments. The reactions to aromatic structures become more complex with an increasing number of aromatic rings and a wider variety of intermediates. By visualizing the ReaxFF MD results, complex reaction networks of different aromatic molecules in CLC are systematically established.