Combined ReaxFF and Ab Initio MD Simulations of Brown Coal Oxidation and Coal-Water Interactions.

Guoguang Wu,Xiao Li,Ruizhi Chu,Xianliang Meng,Shi Yu

doi:10.3390/e24010071

Abstract

In this manuscript, we use a combination of Car–Parrinello molecular dynamics (CPMD) and ReaxFF reactive molecular dynamics (ReaxFF-MD) simulations to study the brown coal–water interactions and coal oxidation. Our Car–Parrinello molecular dynamics simulation results reveal that hydrogen bonds dominate the water adsorption process, and oxygen-containing functional groups such as carboxyl play an important role in the interaction between brown coal and water. The discrepancy in hydrogen bonds formation between our simulation results by ab initio molecular dynamics (CPMD) and that by ReaxFF-MD indicates that the ReaxFF force field is not capable of accurately describing the diffusive behaviors of water on lignite at low temperatures. The oxidations of brown coal for both fuel rich and fuel lean conditions at various temperatures were investigated using ReaxFF-MD simulations through which the generation rates of major products were obtained. In addition, it was observed that the density decrease significantly enhances the generation of gaseous products due to the entropy gain by reducing system density. Although the ReaxFF-MD simulation of complete coal combustion process is limited to high temperatures, the combined CPMD and ReaxFF-MD simulations allow us to examine the correlation between water adsorption on brown coal and the initial stage of coal oxidation.

Highlights

Coal continues to be the second largest fuel, which supplies 27% of global primary energy needs [1]
Note that the van der Waals radius of hydrogen atom is 1.06 Å and the van der Waals radius of oxygen atom is 1.42 Å [40,55]. This observation does not agree with the recent simulation results of CPMD with Vanderbilt pseudopotentials obtained by Gao et al [40], since Gao et al observed replacement phenomenon of the water during their 30-ps Car–Parrinello molecular dynamics simulations
Our CPMD simulation results demonstrate that hydrogen bonds formed between water and oxygen-containing polar functional groups contained in lignite, especially carboxyl, dominate the interaction between water and coal

Summary

Introduction

Coal continues to be the second largest fuel, which supplies 27% of global primary energy needs [1]. Revealing the relationship between brown coal chemical structures and its oxidation at various temperatures can throw some light on brown coal spontaneous combustion and gasification [6] Those polar functional groups play an important role in water adsorption [2,3,7]. Examined the interactions between water molecules and oxygen/nitrogen atoms in a lignite model molecule with a DFT and AIM study, and they demonstrated that water clusters play an important role in water–lignite interaction Such ab initio studies of brown coal–water interaction provide detailed structural information about water adsorption on lignite, it is still challenging to access the dynamical information about water transportation within brown coal under different conditions. Enough heat accumulation (e.g., 100 degrees temperature increase) is only possible for large systems which can hardly be studied by molecular dynamics simulations Our strategy in this manuscript is to simulate the brown coal–oxygen/water systems under different conditions.

Simulation Models and Methods

Results and Discussion

Product Analysis of ReaxFF-MD Simulation Results—Fuel Rich

Product Analysis of ReaxFF-MD Simulation Results—Fuel Lean

Conclusions