Nonequilibrium Molecular Dynamics Simulations of Coal Ash

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

doi:10.3390/en14010011

Abstract

Both molecular dynamics (MD) and nonequilibrium molecular dynamics (NEMD) simulations were performed to simulate coal ashes using the Guillot-Sator model in this work. The structural and transport properties of coal ashes at high temperatures have been obtained. Superheating of coal ash system with anorthite crystal structure initial configuration has been observed for MD simulation which explains the discrepancy between previous MD simulation results and FactSage thermochemical calculations. The fluxing effects of both calcium oxide and sodium oxide have been investigated systematically through MD and NEMD simulations. Moreover, the viscosities of coal ash systems have been computed by two methods: (1) Stokes-Einstein equation; (2) NEMD simulations. Estimations of viscosities for various coal ash systems based on Stokes-Einstein equation exhibit a strong temperature dependence of viscosity, which agrees with previous experimental results. On the other hand, NEMD simulation results that showed a strong shear-thinning feature, failed to reproduce this strong temperature dependence of viscosity, possibly due to the short simulation time. Nevertheless, NEMD simulations not only provide us detailed information about atoms dynamics under shear, but also allow us to model the coal ash system far from equilibrium which cannot be accessed by thermodynamics calculation using software like FactSage.

Highlights

IntroductionThe mineral formation (e.g., reaction among silica melt, CaO and Al2 O3 ) during coal combustion took place due to heterogeneous nature of feeding coals [1,2,3,4,5,6,7,8]
Since the strict thermodynamics definition of melting point is the temperature at which the free energy of crystal phase is equivalent to the free energy of liquid phase, a bunch of methods involving free energy calculation, e.g., Hoover and Ree’s single-occupancy cell method [46], Frenkel and Ladd’s Einstein crystal method [47], and λ-integration method developed by Grochola [48] and extended by Maginn et al [49], have been developed which can predict the melting temperature even more accurately as those methods allow researchers to minimize the effects of hysteresis phenomenon
This hysteresis phenomenon explaines the overestimation of melting point of anorthite by molecular dynamics (MD) simulations against FactSage thermochemical calculations which has been observed by Dai et al [29], as demonstrated below

Summary

Introduction

The mineral formation (e.g., reaction among silica melt, CaO and Al2 O3 ) during coal combustion took place due to heterogeneous nature of feeding coals [1,2,3,4,5,6,7,8]. These reactions could cause slagging and fouling problems in coal fired power plants which caused reduction in combustion efficiency [9,10,11,12,13]. E.g., slag viscosity measurement [17,18], XRD [19], FTIR [20], SEM [20], and thermomechanical analysis [21], have recently been performed to investigate the compositions and fusibility of coal ashes.

Methods

Results

Conclusion