Molecular dynamic simulation of spontaneous combustion and pyrolysis of brown coal using ReaxFF

• Combustion and pyrolysis of brown coal was studied using reactive force field.
• During combustion, the yield of gaseous products were more in fuel lean conditions.
• Potential energy of the system decreases with increase in temperature.
• Effect of temperature was more pronounced as compared to density during pyrolysis process.
• Both combustion and pyrolysis process followed exothermic and endothermic path.

Brown coal is a soft sedimentary organic rock which is complex in nature and is the main source of energy production. In this work, we have studied the combustion and pyrolysis of brown coal using reactive molecular dynamic (MD) simulation. To make the large scale (above 1000 of atoms) reactive system practical, ReaxFF MD system was used which is 100 times faster than the methods of quantum mechanics (QM). To examine the pyrolysis/combustion process and initiation mechanism of brown coal, a fuel lean (ϕ = 2), fuel rich (ϕ = 0.5) and stoichiometric (ϕ = 1) conditions were used in this work. The temperature used was high as per experimental reported condition so as to enable chemical reaction within a computationally affordable time. It was observed that the combustion of brown coal was initialized by thermal degradation subsequently forming small fragments. As the brown coal molecule oxidizes or thermally decomposes, hydrogen is abstracted and reacts with oxygen to form large amount of H2O molecules. Furthermore, the combustion of coal was also studied in the same conditions namely fuel rich, fuel lean and stoichiometric. Potential energy gradually decreases at high temperature while it was the reverse in pyrolysis. It was found that the effects of densities are lesser as compared to temperature. Some important intermediate like formaldehyde (HCHO) generated during the simulation reaction agreed well with the experimental data reported in literature.