Article pubs.acs.org/EF
Viscosity Model for Oxide Melts Relevant to Coal Ash Slags Based on the Associate Species Thermodynamic Model Thomas Nentwig,† Alexander Kondratiev,‡ Elena Yazhenskikh,† Klaus Hack,‡ and Michael Müller*,† †
Institute of Energy and Climate Research, Microstructure and Properties of Materials (IEK-2), Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany ‡ GTT Technologies, Kaiserstrasse 100, D-52134 Herzogenrath, Germany ABSTRACT: A viscosity model based on the associate species thermodynamic model is currently being developed for liquid melts of the Al2O3−K2O−Na2O−SiO2−CaO−MgO−FeO−etc. system and subsystems relevant to coal ash slags. In the present publication, we focus on the quaternary subsystem Al2O3−SiO2−Na2O−K2O. The model employs concentrations of the associate species obtained from the recent thermodynamic assessments of these systems. Modified Avramov and Arrhenius approaches were investigated as possible means to describe the viscosity. The original viscosity equations were modified to include the compositional and structural dependence of the viscosity. Associate species introduced in the thermodynamic model were found to provide a reasonable approximation of the melt structure that was confirmed by comparison to available spectroscopic data. A comparison of the two approaches shows their specific applicability with respect to temperature and composition dependence. other Si tetrahedra, thereby forming different structures.6 The structural complexity of silicate melts has a considerable effect on many melt properties, including thermodynamic (e.g., Gibbs free energy) and physical (e.g., viscosity) properties. A number of structure-related thermodynamic models have been developed for silicate melts that take into account the melt structure. First of all, Fincham and Richardson7 introduced the concept of the bridging, non-bridging, and free oxygens (i.e., oxygens connected to two, one, and no Si atoms, respectively), which helped to explain structural aspects of the sulfur behavior in silicate melts. Then, several researchers8,9 proposed models that take into account the nearest neighbors of oxygen anions (e.g., metal cations) to describe the thermodynamics of silicate melts, especially with a highly negative enthalpy of mixing. These nearest neighbor pairs are equivalent to the bridging, nonbridging, and free oxygens only in the binary silicate systems, while they represent a next level of the structural approximation in ternary and higher order systems. An introduction of so-called Q species (Qi, where i = 0, 1, ..., 4), where Q denotes a silicon tetrahedron linked to i neighbor tetrahedra6,10 provided a more complicated structural approximation than previous attempts. For example, it is possible to calculate concentrations of the oxygen species and second nearest neighbor bonds from those of the Q species, but the inverse calculation is not viable, even in the case of binary systems. A procedure to obtain quantitatively the concentrations of Q species directly from spectroscopic data has recently been established.6 Associated species introduced in the associate species thermodynamic model11 are used to describe the Gibbs energy
1. INTRODUCTION The worldwide demand for energy is rapidly increasing. Fossil fuels are the main source in heat and power production. Because of the limitation of fossil fuels, there is a growing interest in using the fuels as efficiently as possible and in using low-grade fuels, such as coals, with higher ash content.1 Furthermore, there is a growing awareness for the environmental protection, so that the development of cleaner and more efficient technologies for energy generation is an important topic in energy research.2 High-temperature gasification systems have been proven to be efficient in energy generation and at minimizing harmful emissions while handling low-grade, high-ash fuels.3−5 One promising high-temperature process is the integrated gasification combined cycle (IGCC) process with an entrained-flow gasifier. State of the art single-stage pressurized entrained-flow gasifiers have an efficiency of 38− 50% lower heating value (LHV).5 To account for the high ash content of low-grade fuels, an entrained-flow gasifier can be equipped with a cooling screen. When the process temperature optimized to the slag melt properties of the specific fuel is maintained, a firmly adhering slag coat is applied to the refractory cladding, protecting the material from abrasion and the reactor from corrosion. The draining slag melt is finally discharged as a solid material, after passing through a water quench. To control slag flow along the cooling screen and out of the reactor, the knowledge of chemical and physical properties of gasifier slags as a function of slag and gas-phase composition, temperature, and pressure is necessary. A general and accurate viscosity model will greatly assist scientists and technologists in process simulation and optimization. It is well-known from spectroscopic studies that silicate melts (such as coal slags) have a complex structure, which varies significantly with the melt composition. The major structural unit of silicate melts is a silicon tetrahedron, which can link to © XXXX American Chemical Society
Received: July 10, 2013 Revised: October 1, 2013
A
dx.doi.org/10.1021/ef401306d | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 1. Associate Species Used for the Binary Silicates in the Al2O3−K2O−Na2O−SiO2 System associate species X(SiO2) 1.00 0.80 0.67 0.50 0.40 0.33 0.00
X(Al2O3)
Al2O3−SiO2
X((K,Na)2O) 0.20 0.33 0.50
0.60
Na2O−SiO2
K2Si4O9 K2Si2O5 K4Si2O6
Na2Si2O5 Na4Si2O6
1.00
K2O
Na8Si2O8 Na2O
Al6Si2O13 0.67 1.00
Al2O3
that mix ideally. However, it appeared that non-ideal interactions between associate species have to be partly taken into account, being added to the molar Gibbs free energy Gm in the form of the Redlich− Kister polynomial
of the liquid phase, which is considered as an ideal mixture of the associated species. The species are normally selected to correspond to major stoichiometric solid compounds existing in a system but can also be related to the Q species and thereby linked to the melt structure. Examples of constructing such a link are given in the present paper. It is a well-known experimental fact (e.g., see ref 12) that the viscosity of coal ash and other silicate melts depends strongly upon the melt structure. As a result, the melt viscosity cannot be calculated over the whole compositional and temperature ranges using a simple regression model. A number of viscosity models13−16 have been developed to date for predicting viscosities of silicate melts relevant to metallurgical or coal ash slags. Most of them are based on an Arrhenian-like equation ⎛E ⎞ η = AT α exp⎜ a ⎟ ⎝ RT ⎠
Gm =
∑ Ea(i)Xi i
∑ XiGi0 + RT ∑ Xi ln Xi i
+
i
∑ ∑ XiXj ∑ (Aij(k) + Bij(k)T )(Xi − Xj)k i
