Viscosity temperature properties from molecular dynamics simulation: The role of calcium oxide, sodium oxide and ferrous oxide

Abstract

• For the long-term stable operation of the entrained flow gasifier in the coal chemical industry, the flux agents are generally adopted to adjust the fusibility of coal ash. Therefore, it is necessary to understand the underlying mechanism for the impact of typical oxide flux agents on viscosity temperature properties of coal ash from structures and thermodynamics. In this work, the role of calcium oxide, sodium oxide and ferrous oxide on viscosity temperature properties is investigated by a combination of molecular dynamics simulations and thermodynamic calculations. The variations of viscosity and temperature of critical viscosity are obtained for different ternary coal ash systems by thermodynamic calculation. Ternary phase diagrams are applied to evaluate the effect of different flux agents, which are also found to cause mineral transformation from high-temperature minerals to low-temperature minerals. Oxygen bond species are employed as the indicator of the structural evolution originating from addition of different flux agents. The sodium atoms may more readily weaken the tricluster oxygen bonds than calcium or ferrous atoms according to the results. Higher content of bridging oxygen bonds in the sodium oxide ternary coal ash system can enhance the stability of the structures and induce higher viscosity. Stability coefficients are defined here and a function to describe the relationship between the viscosity and flux agent content is established. The results from the current work are expected to provide new clues to find strategies controlling the fusion behaviour of coal ash systems.

Introduction

The viscosity temperature properties are significant for the long-term stable operation of the entrained flow gasifier [1]. Generally, the expected viscosity range is 2.5 Pa·s–25 Pa·s under the operation temperature of an entrained-flow gasifier. Viscosity higher than that range may cause blockage at the reactor bottom near the slag tapping hole, and consequently, lead to unscheduled emergency shutdown of the process [2]. When the viscosity is lower than 2.5 Pa·s, however, the problem of refractory wear may arise [3], [4]. Therefore, the adjustment of the fusibility, by addition of flux agents to the coal ash system or coal blending, to control viscosity in the appropriate range is one of the most important aspects for running the entrained flow gasifier in both theory and practice. To date, limitations in the coal blending scheme remain such as restrictions in the availability of suitable coal in the neighbouring region. Thus, the method using flux agents is most commonly adopted under the gasification condition. Accordingly, it is essential to understand the impact of flux agents at high temperatures.

Calcium oxides and ferrous oxides are major flux agents widely chosen in the entrained-flow gasifiers. Besides, there is high content of sodium oxide in the Zhundong coal, which is the largest intact coal field in the world [5]. Much research has been conducted to investigate the mechanism by which these flux agents affect viscosity [6], [7], [8]. The effect of calcium oxide on the pattern of viscosity-temperature was investigated macroscopically by varying the amount of solid minerals in the slag [6]. The role of ferrous oxides in the viscosity varies depending on the chemical valences of iron [9]. The analyses of structural characteristics of molten slags reveal sodium oxide plays a role on the viscosity-temperature behavior of coal ash slag [4], [8]. Although the general effect of fluxing agents on viscosity has been reported, the underlying fluxing mechanism for a complicated coal ash system remains unaddressed due to insufficient knowledge of the detail of structural variation in the molecular scale [10]. Moreover, it is meaningful to recognize the difference in the fluxing mechanisms among ferrous oxide, calcium oxide and sodium oxide since these flux agents may coexist in the coal ash. Therefore, it is highly important to understand the fluxing mechanism from the microscopic perspective.

In practice, the influence of flux agents on the chemical and mineral composition can be analyzed by the spectroscopies of X-ray diffraction (XRD), X-ray photoelectron, FTIR and Raman. FactSage is a software package that consists of a thermodynamic database, and various calculation and manipulation modules which enables one to perform thermodynamic calculations using the databases of pure substances and solutions [11]. The ternary phase diagram, predicted by FactSage, is frequently used to investigate the thermodynamics and mineral transformation [11], [12], [13]. Molecular dynamics (MD) simulation has been widely applied to investigate the variations of mineral compositions and structures microscopically, which helps illustrate the nature of physical properties of materials in-depth [14], [15], [16], [17]. The structural features, such as the radial distribution functions, coordination numbers, mean square distances and oxygen bond species, are commonly considered as the key to understand the mechanism of microscopic viscosity variation [16]. Li et al. uncovered the mechanism of the coke reaction at high temperatures by analyses of the structural evolution with the SiO₂ content [14]. The role played by alkalis was investigated by the radial distribution function and oxygen bond species. Both Si-O and Al-O networks de-polymerize into simple structures with increasing basicity, causing increased atomic diffusion coefficients and decreased viscosities [17]. The alkalis show different influence on the total diffusivity for various ions in K₂O and Na₂O bearing systems, leading to distinctive trends in viscosities [15]. Furthermore, mineral transformation in the high-temperature or high-pressure process can also be understood by MD simulation. Molten anorthite was studied from first-principles calculations on the diffusion and viscosity as a function of pressure and temperature [18].

In this study, thermodynamic calculations combined with MD simulations were conducted on the comparison of the roles of calcium oxide, ferrous oxide and sodium oxide on the behavior of the coal ash, especially their influence on viscosity temperature properties. The current work aims to reveal the viscosity variation mechanism for various flux agents and quantify the corresponding dependence of viscosity temperature properties on calcium oxide, ferrous oxide and sodium oxide contents.