Book/Dissertation / PhD Thesis

FZJ-2020-04113

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Crystallisation of Oxidic Gasifier Slags

Schupsky, J. P. (Corresponding author)FZJ*

2020
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-506-2

Please use a persistent id in citations: http://hdl.handle.net/2128/26325  urn:nbn:de:0001-2020120102

Abstract: Gasification is a flexible technology that has the potential to support the transition from a fossil fuelled to decarbonised energy supply system. During gasification, an oxidic residue is produced thatis named as slag. Slag flow needs to be constant in a gasifier, but a high viscosity impairs the flow behaviour. The phenomenon of crystallisation has such effect, yet is only barely understood. Viscosity of slag can be modelled and therefore predicted. However, no sufficient model exists to calculate the viscosity of partly crystallised slags. A sufficient data source of crystal morphology data is required to enable the application of an improved viscosity model for partly crystallised slags. In this study, the crystallisation characteristics of four synthetic gasifier slags (ST-D-2, HKT, SOM-1,and HKR) have been investigated. Initially, the synthetic slag systems have been blended from high purity compounds. Their compositions are mostly based on real coal slags that have been investigated in previous studies. The slag systems were analysed on their melting and solidification behaviour and so, the temperature parameters for the following experiments were set. Equilibrium calculations were conducted with FactSage Equilib programme to predict solidus and liquidus temperatures, as well as the crystallised phases. As a first experiment, high temperature viscosimetry was performed on the four slag systems. ST-D-2 and HKT slags were identified as high viscous, HKR as a low viscous and SOM-1 as an intermediately viscous slag. Viscosity measurements revealed non-Newtonian behaviour and the presence of crystals in the slag. The evolution of crystallisation was analysed by quenching and CLSM (confocal laser scanning microscopy) experiments. The resulting slag samples were analysed via microscopy, X-ray diffraction and SEM (scanning electron microscopy). ST-D-2, HKT, and SOM-1 slag displayed significant growth of anorthite (CaAl$_{2}$Si$_{2}$O$_{8}$)crystals. Several more phases crystallised in these slags, such as cristobalite (SiO$_{2}$), clinopyroxene((Ca,Mg,Fe)(Si,Al)$_{2}$O$_{6}$), and olivine ((Ca,Mg,Fe)$_{2}$SiO$_{4}$). In the HKR slag, melilite (Ca$_{2}$(Al,Mg)(Si,Al)$_{2}$O$_{7}$)was the dominating crystal phase, followed by olivine and spinel ((Mg,Fe)AlO$_{4}$). For the high viscous slags, time-temperature-transformation (TTT) diagrams indicated an incubation time of single hours. SOM-1 slags incubation time varied between double digit minutes to single hours and lastly HKR slag displayed very rapid, partly instant crystallisation. The crystal morphologies of anorthite, spinel, olivine, and melilite could be defined based on the sample analysis of quenching and CLSM experiments. Anorthite was defined as a tetragonal prism with variation in the elongation, spinel formed idiomorphic octahedrons, olivine crystallised aselongated, equiaxed bipyramids and melilite formed large rectangular bipyramids. To gather relevant morphology data, the crystals were measured on their length and width to quantify their presence with respect to the applied temperature. In total, 1022 individual crystals were measured for the quantification. Generally, the crystals tend to grow larger at higher temperatures, which is in agreement with the crystallisation theory. The morphology was compared with crystallisation of real slag samples from PiTER reactor (TU Munich, HotVeGas project). Anorthite and spinel were found in PiTER slag with identical morphologies, as provided for the synthetic slag samples. This accordance is an application-related proof that the investigations on crystal morphologies performed in this study are reproducible and realistically display crystallisation processes in gasifiers. Therfore, they are highly applicable in viscosity models for partly cristallised slags.

Note: RWTH Aachen, Diss., 2020

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Contributing Institute(s):

1. Werkstoffstruktur und -eigenschaften (IEK-2)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

Appears in the scientific report 2020

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Database coverage:
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