Publikationsserver der RWTH Aachen University
Investigation of thermodynamic properties of alkali metals in oxide systems relevant to coal slags
Abstract
dc:descriptionAlkali release and behaviour of molten coal ashes (slags) are among the main problems in coal combustion and gasification processes. The alkali metals, bonded in the mineral matter of coal, are partly released during coal conversion. This leads to the formation of sticky deposits in boilers or hot corrosion of turbine blades in combined cycle power systems. Therefore, the information about thermodynamic properties of oxide systems containing silica and alumina as well as alkali and alkaline-earth oxides is important. The knowledge of quasi-binary phase diagrams in systems like Alk2O-Al2O3-SiO2 (Alk = Na or K) is essential for thermodynamic modelling of more complex systems like coal slags. The determination of the alkali partial pressures over mixtures of silicon dioxide, aluminium oxide and alkali oxides like Na2O or/and K2O and the alkali oxide activity in these mixtures was carried out. The thermodynamic studies are important because the potential of alkali retention depends on the alkali oxide activity in the slag. The quasi-ternary Na2O-Al2O3-SiO2 and the quasi-quaternary Na2O-K2O-Al2O3-SiO2 systems were investigated by Knudsen Effusion Mass Spectrometry (KEMS). For the investigation of the binary systems the most important part of this work was finding a suitable method of preparing the samples and making the measurements since one component – sodium oxide – is hygroscopic. As one of the most important results, the behaviour of the Na2O-rich parts of the systems Na2O-SiO2 and Na2O-Al2O3 was characterized for the first time. In the quasi-binary Na2O-SiO2 system one eutectic was established at about 861 ± 9°C between 78 and 80 mole percent of sodium oxide. A second eutectic is most likely placed between 85 and 87 mole percent of sodium oxide at a temperature of 845 ± 15°C. In the range of the binary Na2O-SiO2 system, where the phase equilibrium was investigated by Kracek and D'Ans and Loeffler, a generally good agreement between previous data and experiments done in this work was observed. The quasi-binary Na2O-Al2O3 system has a simple eutectic in the high Na2O concentration range of the system. The eutectic is close to pure Na2O at 1093 ± 15°C. The subsystem of the quasi-ternary Na2O-Al2O3-SiO2 system, NaAlSiO4-NaAlO2, has in the low temperature range a few structure changes. No solidus and liquidus lines could be determined in the available temperature range of up to 1550°C of the STA apparatus. Therefore, on one side the phase diagram proposed by Schairer et al. is right, other side it is doubtful if the range between NaAlSiO4 and NaAlO2 is a homogenous solid solution. By vaporization experiments of pure Na2O(s) the vaporization enthalpy of Na2O(s) was calculated by means of the second and third law methods. The results showed that the enthalpies obtained from the third law are in good agreement with data from IVTANTHERMO. The study of vaporization behaviour and the thermodynamic properties of the quasi-ternary Na2O-Al2O3-SiO2 and quasi-quaternary Na2O-K2O-Al2O3-SiO2 system showed that the thermodynamic activity of sodium oxide in the quasi-ternary system increases with increasing sodium oxide content and is in quite good agreement with calculated values for this system (FACT and FZJ database). The comparison of the sodium oxide activity of quasi-ternary and quasi-quaternary samples showed that the existence of potassium oxide increases the activity of sodium oxide in the system.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2007
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Rys, Malgorzata
- Contributors dc:contributor
-
- Singheiser, Lorenz
Subjects
dc:subject × 13Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng