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NIRE Annual Report
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 1999

Computer Simulation Techniques for the Elucidation of Coal Structure - Property Relationship

Fundamental Research Division
Energy Resources Department
Objectives
Coal has a complex chemical structure consisting of various bond types and the details are still not well understood. Knowledge of coal's molecular structure is important for the understanding of various coal properties and reactivities under several conditions. The objective of this study is to elucidate the structure - property relationship using the computer simulation techniques.
Results
A molecular mechanics-molecular dynamics computer simulation method was used to model the swelling behavior of a coal extract fraction. Methanol molecules were successively added to a model compound for the coal fraction under periodic boundary conditions. After each addition, the energy of the system was minimized using molecular mechanics methods, and then molecular dynamics computations were performed.
The total energy, volume and mass in the model-methanol system changed as successive methanol molecules were added. When a methanol molecule is introduced into the stable coal model unit cell, the interaction energy (negative) between the methanol and model molecule depends on the affinity between the solvent and model structure and corresponds to the mixing energy between them. The unit cell is extended by introduction of the methanol and leads to an increase in total energy of the original model structure, which corresponds to the elastic energy of the original structure. When the difference in energy between structures with the successive addition of methanol molecules is negative, the additional methanol molecule is interacting favorably with the preceding structure. Changes in the total energy of the system and the volume of cell as a function of the number of methanols added are shown in Fig. 1. The total energy of the system decreased monotonically until 14 molecules of methanol had been added, after which there was essentially no change. The stable structure of the model including 14 molecules of methanol (represented as spheres) is shown in Fig. 2. An interaction of methanol molecules with several polar sites in the coal model can be seen readily. The swelling behavior was estimated from the incremental volume changes of the model compound and added methanol. The swelling ratio obtained from the simulation, 1.20, was in excellent agreement with the experimentally obtained value of 1.19. The simulation method proposed here appears to be a valid representation of swelling behavior.
Fig.1. Changes in total energy and volume of the coal-methanol system
Fig.1. Changes in total energy and volume of the coal-methanol system

Fig.2. The equilibrium state of swelling of the coal-methanol system
Fig.2. The equilibrium state of swelling of the coal-methanol system
Selected Publications
1) Takanohashi, T., Iino, M. and Nakamura, K., Energy Fuels, 12, 1168-1173 (1998).
2) Takanohashi, T. Nakamura, K. and Iino, M., Energy Fuels, in press (1999).
3) Takanohashi, T., Terao, Y. and Iino, M. Fuel, in press.
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