(82b) Reactive Molecular Dynamics Simulations of Cellulose Gasification Chemistry

Two problems preventing the commercial viability of biomass gasification technologies are low product yields and high tar production. Both of these stem from a lack of understanding about the underlying chemistry. Detailed reaction mechanisms have not been developed because of the complexity of the thermal decomposition process and the difficulty observing all the short-lived intermediates in experiments. Computational methods, until recently, have been too either computationally expensive to model polymers or incapable of accurately describing bond breakage and formation. Reactive molecular dynamics may be a tool that will simulate biomass decomposition accurately enough to give insight into the reaction mechanisms. The ReaxFF force field [1] changes classical molecular dynamics by evaluating each pair of atoms at each timestep to determine the bond order from internuclear distance based on empirical relationships. Using empirical data decreases computation time by a factor of 10^3 over ab initio methods and also allows for molecular decomposition.

The current study has attempted to validate this approach by reproducing experimentally observed product distributions. These final product distributions have been well documented in the experimental literature. We expect to see five- to six-carbon rings early in the decomposition process followed by the production of gas phase molecules like hydroxyacetaldehyde, carbon oxides, water, and molecular hydrogen [2].

Simulations have been run on cellulose structures of 30 glucose base units for 1 ns in 0.2 fs timesteps. Temperature was varied between 900K and 2000K in constant temperature and constant energy simulations. Simulations in vacuum, nitrogen, and steam were considered. Results show production of physically realistic products including hydroxyacetaldehyde and water. However, depolymerization is not observed despite being widely evident in experiments. Further work will consider whether ReaxFF can account for unimolecular electron transfer reactions.

References: 1. Van Duin, A. C. T., et al., (2001). "ReaxFF: A Reactive Force Field for Hydrocarbons." J. Phys. Chem. A 105: 9396-9409. 2.Lin, Y.C., et al. (2009) ?Kinetics and mechanism of cellulose pyrolysis? J Phys Chem C 113: 20097-20107.