Kefeng Yan

Reactive molecular dynamics simulations of the initial stage of brown coal oxidation at high temperatures

To investigate the detailed mechanisms for brown coal oxidation at high temperatures, a ReaxFF reactive forcefield was used to perform a series of molecular dynamics simulations from 1000 K to 2500 K. Analyses indicated that the chemical system tend to be more reactive with increasing temperature. It was found that the oxidation process of brown coal primarily initiates from hydrogen abstraction reactions by O2 and related oxygenated radicals from phenolic hydroxyl groups, methyl groups, especially carboxyl groups in lower temperature to form peroxygen species, or by either thermal decomposition of brown coal backbone in higher temperature. These peroxygen species usually could chemically adsorb on the C-centered radicals of brown coal backbone. The weak O–O bond in peroxygen makes them easier to break into oxygenated radical, which could also chemically adsorb on the C-centred radical to form hydroxyl group and other oxygenated compounds. In the oxidation process of brown coal, the decomposition and oxidation of aliphatic chain is easier than aromatic ring. The chemisorption of peroxygen radical induces the breakage of aromatic ring and accelerates the depth oxidation of brown coal. An increasing number of products are observed with increasing temperature.

Molecular dynamics simulation of methane hydrate dissociation by depressurisation

Methane (CH4) hydrate dissociation and the mechanism by depressurisation are investigated by molecular dynamics (MD) simulation. The hydrate decomposition processes are studied by the ‘vacuum removal method’ and the normal method. It is found that the hydrate decomposition is promoted by depressurisation. The quasi-liquid layer is formed in the hydrate surface layer. The driving force of dissociation is found to be controlled by the concentration gradient between the H2O molecules of the hydrate surface layer and the H2O molecules of the hydrate inner layer. The clathrates collapse gradually, and the hydrate decomposes layer by layer. Relative to our previous MD simulation results, this study shows that the rate of the hydrate dissociation by depressurisation is slower than that by the thermal stimulation and the inhibitor injection. This study illustrated that MD simulation can play a significant role in investigating the hydrate decomposition mechanisms.

ReaxFF molecular dynamics simulations of non-catalytic pyrolysis of triglyceride at high temperatures

In order to investigate the initiation mechanisms associated with the pyrolysis of triglyceride that could potentially be used as petrochemical replacements, we carried out 500 ps molecular dynamics simulations employing the ReaxFF reactive force field using tripalmitin as the model molecule at 1500 and 2000 K. We find that the primary decomposition reactions of tripalmitin initiate with the successive scission of the alkyl-oxygen bond to form three straight chain C16H31O2˙ (RCOO˙) radicals and C3H5˙ radical. The deoxygenated alkyl chain is produced through the decarboxylation of the RCOO˙ radical with concurrent production of CO2. The resulting alkyl and C3H5˙ radicals further undergo recombination and decomposition to yield mainly alkanes and alkenes, with the actual product distribution being dependent on reaction temperature. β-Scission plays an important role in alkyl chain decomposition with a concomitant release of C2H4. Compared to 1500 K, this reaction is accelerated at 2000 K. In addition, the formation of cyclic hydrocarbon is also observed at 2000 K. As opposed to previous proposed Diels–Alder reactions or intramolecular cyclizations of alkenyl radicals mechanisms, it is found that cyclopentane could be produced by intramolecular cyclization of a biradical.

Molecular Dynamics Simulation of the Crystal Nucleation and Growth Behavior of Methane Hydrate in the Presence of the Surface and Nanopores of Porous Sediment

The behavior of hydrate formation in porous sediment has been widely studied because of its importance in the investigation of reservoirs and in the drilling of natural gas hydrate. However, it is difficult to understand the hydrate nucleation and growth mechanism on the surface and in the nanopores of porous media by experimental and numerical simulation methods. In this work, molecular dynamics simulations of the nucleation and growth of CH4 hydrate in the presence of the surface and nanopores of clay are carried out. The molecular configurations and microstructure properties are analyzed for systems containing one H2O hydrate layer (System A), three H2O hydrate layers (System B), and six H2O hydrate layers (System C) in both clay and the bulk solution. It is found that hydrate formation is more complex in porous media than in the pure bulk solution and that there is cooperativity between hydrate growth and molecular diffusion in clay nanopores. The hydroxylated edge sites of the clay surface could serve as a source of CH4 molecules to facilitate hydrate nucleation. The diffusion velocity of molecules is influenced by the growth of the hydrate that forms a block in the throats of the clay nanopore. Comparing hydrate growth in different clay pore sizes reveals that the pore size plays an important role in hydrate growth and molecular diffusion in clay. This simulation study provides the microscopic mechanism of hydrate nucleation and growth in porous media, which can be favorable for the investigation of the formation of natural gas hydrate in sediments.

Molecular dynamics simulation of the intercalation behaviors of methane hydrate in montmorillonite

The formation and mechanism of CH4 hydrate intercalated in montmorillonite are investigated by molecular dynamics (MD) simulation. The formation process of CH4 hydrate in montmorillonite with 1 ~ 8 H2O layers is observed. In the montmorillonite, the “surface H2O” constructs the network by hydrogen bonds with the surface Si-O ring of clay, forming the surface cage. The “interlayer H2O” constructs the network by hydrogen bonds, forming the interlayer cage. CH4 molecules and their surrounding H2O molecules form clathrate hydrates. The cation of montmorillonite has a steric effect on constructing the network and destroying the balance of hydrogen bonds between the H2O molecules, distorting the cage of hydrate in clay. Therefore, the cages are irregular, which is unlike the ideal CH4 clathrate hydrates cage. The pore size of montmorillonite is another impact factor to the hydrate formation. It is quite easier to form CH4 hydrate nucleation in montmorillonite with large pore size than in montmorillonite with small pore. The MD work provides the constructive information to the investigation of the reservoir formation for natural gas hydrate (NGH) in sediments.

Molecular dynamics simulation of oxygen diffusion in dry and water-containing brown coal

Moisture content of brown coal has a pronounced impact on O2 diffusion during coal oxidation. The transport behaviour of O2 in dry and hydrated brown coal matrix with 10, 20, and 30 wt% moisture contents have been studied by molecular dynamics (MD) simulations at 298.15 K and 0.1 MPa. By analysing the diffusion characteristics of O2, it is found that the diffusion process results from jumps of O2 molecules between adjacent cavities in the coal matrix. With the increase of moisture content, the swelling extent of coal matrix increases in the range of 10–30 wt%, resulting in increased cavity volume, cavity interconnectivity, and mobility of coal macromolecule. These factors are all beneficial to the transport process of O2 molecule. In addition, O2 molecule prefers to reside in the available water-poor coal cavity and unoccupied region of water-rich cavity according to the estimated binding energies.

Raman Spectroscopic Analysis on the Hydrate Formed in the Hydrate-Based Flue Gas Separation Process in Presence of Sulfur Dioxide and Tetra-n-butyl Ammonium Bromide

In this work, Raman spectroscopic analysis was applied to determine the structures and cage occupancies of the hydrates that formed from the system of flue gas (simulated by carbon dioxide–nitrogen–sulfur dioxide)–sulfur dioxide aqueous solution, and from the system of flue gas–sulfur dioxide containing tetra-n-butyl ammonium bromide (TBAB) aqueous solutions (sulfur dioxide mass concentration 0, 1.0, and 7.0 wt%). Comprehensive TBAB (solid, aqueous, and hydrate) Raman spectra were also obtained. The results reveal that when TBAB is used as promoter, both sulfur dioxide and carbon dioxide are encaged in the hydrate from systems of flue gas-TBAB solution with low sulfur dioxide concentration (0, 1.0 wt%), whereas in the hydrate from the system of flue gas-sulfur dioxide highly concentrated (7.0 wt%) TBAB solution, sulfur dioxide will be the sole gas guest encaged in the semi-clathrate hydrate. This suggests the sulfur dioxide concentration significantly influences the hydrate cage occupancies and separation selectivity of the hydrate-based technology. A two-stage hydrate-based flue gas purification process is proposed: one aims at desulfurization when sulfur dioxide concentrates to a relatively high level with the solutions recycling and in the other we can remove the sulfur dioxide and carbon dioxide simultaneously.

Quantum chemical investigation of the primary thermal pyrolysis reactions of the sodium carboxylate group in a brown coal model.

The primary pyrolysis mechanisms of the sodium carboxylate group in sodium benzoate—used as a model compound of brown coal—were studied by performing quantum chemical computations using B3LYP and the CBS method. Various possible reaction pathways involving reactions such as unimolecular and bimolecular decarboxylation and decarbonylation, crosslinking, and radical attack in the brown coal matrix were explored. Without the participation of reactive radicals, unimolecular decarboxylation to release CO2 was calculated to be the most energetically favorable primary reaction pathway at the B3LYP/6-311+G (d, p) level of theory, and was also found to be more energetically favorable than decarboxylation of an carboxylic acid group. When CBS-QBS results were included, crosslinking between the sodium carboxylate group and the carboxylic acid and the decarboxylation of the sodium carboxylate group (catalyzed by the phenolic hydroxyl group) were found to be possible; this pathway competes with unimolecular decarboxylation of the sodium carboxylate group. Provided that H and CH3 radicals are present in the brown coal matrix and can access the sodium carboxylate group, accelerated pyrolysis of the sodium carboxylate group becomes feasible, leading to the release of an Na atom or an NaCO2 radical at the B3LYP/6-311+G (d, p) or CBS-QB3 level of theory, respectively.