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The Lurgi gasification technology has attracted significant commercial interest in China owing to its suitability for production of synthetic natural gas (SNG). However, hazardous trace elements (HTEs) in Lurgi gasification ash have the potential risk of contaminating surface and groundwater. In this paper, the HTEs in ash with different particle sizes from Lurgi gasifier and the HTEs concentrations of leachates were investigated by using an ICP-MS (Thermo Fisher, ELEMENT2/XR) and a direct mercury analyzer (DMA 80). Moreover, the mass loading (f(x)) and ecological risk index (RI) was introduced to evaluate the risk level of HTEs in Lurgi gasification ash with different particle sizes. The content of Be, Cr, Ni, Cu, Zn, Cd, Ba, Pb, U and As in fine ash (<3 mm) accounts for 40–55% of the total content. HTEs are more likely to concentrate in the fine ash with a particle size of less than 3 mm and a large specific surface area in the Lurgi gasifier. The pH range of the leachate is from 8.2 to 10.1. As and Cd presented the highest leachability, and more than 85.9% of As and over 54.1% of Cd in the ash were transferred into the leachate. The concentration of As, Cd, Be and Cr in the leachate is negatively correlated with the ash particle size, while Pb is positively correlated. The mass loading index of Lurgi ash with particle size of 3 ~ 6mm is the smallest, less than 10%. And the ecological risk index (RI) of Lurgi gasification ash increased with the decrease of particle size, but all of them were at low-risk level. However, in some cases, the leached concentrations of arsenic exceed the allowable limit of the secondary groundwater standard in China. • Twelve HTEs were evaluated to assess the potential hazard of these solid wastes when disposed. • HTEs are more likely to be concentrated in the fine ash with a particle size of less than 3 mm. • The ecological risk index (RI) was introduced to evaluate the risk level of HTEs in Lurgi gasification ash. • Distribution and leachability of hazardous trace elements (HTEs) in ash with different particle sizes are studied. [ABSTRACT FROM AUTHOR]

Underground coal gasification (UCG) can be adopted to convert unminable coal resources into synthesis gas for use in the production of electricity, fuels, and chemicals. UCG is a promising technology with many health, safety, and environmental advantages over conventional mining techniques. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of the gasification processes and improve the utilization efficiency of coal seams. The presence of coal partings has an important impact on mineral transformation. In this paper, the UCG process has been subdivided into pyrolysis, reduction, and oxidation stages, and the progressive coal conversion products were prepared. Ulanqab lignite was used as the test coal. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and scanning electron microscopy coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic equilibrium simulation software, FactSage 7.2, was used to calculate the relevant phase diagram to assist in understanding the mineral transformations. The experimental results indicate that the different iron-bearing minerals occur in raw coal and non-coal partings which are nontronite (Na 0.3 Fe 2 Si 4 O 10 (OH) 2.4H 2 O) and glauconite (K (Fe, Al) 2 (Si, Al) 4 O 10 (OH) 2) respectively. Pyrrhotite is the main iron-bearing mineral in the pyrolysis products. In the reduction stage, the relative content of Al 2 O 3 is increased due to the presence of non-coal partings, which reacts with FeO to produce hercynite (FeAl 2 O 4) at a temperature above 1100 °C. When the temperature increases to 1300 °C, hercynite is converted to the thermodynamically-stable cordierite (Fe 2 Al 4 Si 5 O 18). In the oxidation stage, mullite (Al 6 Si 2 O 13) is produced when the oxidation temperature reaching 1300 °C, which could be used to reflect the underground reaction conditions in the coal seam. The phase diagram of (SiO 2) 6.2 Al 2 O 3 -CaO-Fe 2 O 3 proves that mullite and anorthite (CaAl 2 Si 2 O 8) are the dominant minerals when the mole fraction of (SiO 2) 6.2 Al 2 O 3 is above 0.7. In addition, the ash melting point rises with an increase of Si and Al content in the presence of non-coal partings. Unlabelled Image • The different iron-bearing minerals occur in raw coal and non-coal partings which are nontronite (Na 0.3 Fe 2 Si 4 O 10 (OH) 2.4H 2 O) and glauconite (K (Fe, Al) 2 (Si, Al) 4 O 10 (OH) 2) respectively. • In the reduction stage, the relative content of Al 2 O 3 is increased due to the presence of non-coal partings, which reacts with FeO to produce hercynite (FeAl 2 O 4) at a temperature above 1100 °C. • In the oxidation stage, mullite (Al 6 Si 2 O 13) is produced when the oxidation temperature reaching 1300 °C, which could be used to reflect the underground reaction conditions in the coal seam. • The phase diagram of (SiO 2) 6.2 Al 2 O 3 -CaO-Fe 2 O 3 proves that mullite and anorthite (CaAl 2 Si 2 O 8) are the dominant minerals when the mole fraction of (SiO 2) 6.2 Al 2 O 3 is above 0.7. [ABSTRACT FROM AUTHOR]

China is the largest coal gasification market in the world. The gasifiers have been commercialized for producing syngas for coal-to-liquids, coal-to-olefin, and coal-to-substitute natural gas. During coal gasification at high temperature and pressure, the differences in reactivity of organic matter in coal are almost negligible; however, the transformation behaviour of minerals is important to the stability of the process. Detailed knowledge of the mineralogical properties of ash and slag is essential for optimizing the operational parameters. It is also significant for determining the influence of the solid wastes on the environment: for example, trace elements carried in bottom slag, fly ash, condensate water, and gaseous products migrate to natural water and soil and into the atmosphere. With this in mind, the mineralogy of ash and slag from typical coal gasification processes in China are reviewed, including the transformation of minerals typically found in coal, and the effects of operating conditions on mineral transformation and melting/slagging behaviour of minerals during gasification process. In addition, trace element migration behaviour in commercialized gasifiers is discussed. [ABSTRACT FROM AUTHOR]

Detailed mineralogical information from underground coal gasification (UCG) is essential to better understand the chemical reactions far below the surface. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of gasification processes and improve the utilization efficiency of coal seams. The transformation of iron-bearing minerals has the typical characteristics during the UCG process and is expected to indicate the process parameters. In this paper, UCG progress was subdivided into pyrolysis, reduction and oxidation stages, and the progressive coal conversion products were prepared. Two types of lignite with different iron contents, Ulankarma and Ulanqab coals, were used in this study. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation performed using the phase diagram of FactSage 7.1 was used to help to understand the transformation of minerals. The results indicate that the transformation behavior of iron-bearing minerals in the two lignites are similar during the pyrolysis process, in which pyrite (FeS2) in the raw coal is gradually converted into pyrrhotite (Fe1-xS). In the reduction stage, pyrrhotite is transformed into magnetite (Fe3O4) and then changes to FeO. The reaction of FeO and Al2O3 in the low iron coal produces hercynite above 1000 °C because of the difference in the contents of Si and Al, while in the high iron coal, FeO reacts with SiO2 to generate augite (Fe2Si2O6). When the temperature increases to 1400 °C, both hercynite and augite are converted to the thermodynamically-stable sekaninaite. [ABSTRACT FROM AUTHOR]

Coal gasification plays an irreplaceable role in promoting the sustainable development of coal chemical industry. The characteristics of ash have become the key factor to reflect the status and influence of the stable operation of coal gasification process. Introduce the current development and working principle of traditional characterization analysis of the composition, structure and morphology in gasification ash. Investigate the application of some new technologies, such as HTXRD, which can directly achieve the diffraction pattern of minerals at different temperature. Mössbauer spectroscopy can determine the iron materials in coal and ash through the qualitative and quantitative analysis, it also can test Fe2+/Fe3+, crystal structure parameters, solid solution types and content. Microanalysis can test the micro-constitution of minerals using SEM and X-ray. The choice of analysis technologies should depend on the composition features of ash. [ABSTRACT FROM AUTHOR]

Calcium-bearing minerals are one of the main typical minerals in coal and coal ash. In the process of coal thermal conversion, calcium-bearing minerals undergo different morphological transformation in which the reaction temperature, pressure, and atmosphere are important factors affecting their transformation. The reaction process of underground coal gasification (UCG) could be clearly divided into pyrolysis, reduction, and oxidation and the typical calcium-bearing minerals are expected to indicate the actual reaction conditions of UCG. A high-calcium coal, Zhundong coal, was used in this research. The products of UCG were prepared and the minerals were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation was used to assist in understanding the transformation behaviors of calcium-bearing minerals. The experimental results show that the calcium-bearing mineral is gradually converted from gypsum (CaSO4·2H2O) in the raw coal into anhydrite (CaSO4) during the pyrolysis process. In the reduction stage, anhydrite reacts with the reducing gas (CO) to produce oldhamite (CaS), and the oldhamite is stably present in the reduction ash. During the oxidation process, oldhamite is first transformed into CaSO4, and then CaSO4 is converted into CaO. Finally, CaO reacts with Al2O3 and SiO2 to produce gehlenite (Ca2Al2SiO7) at 1100 °C. As the oxidation temperature rises to 1400 °C, gehlenite is transformed into the thermodynamically stable anorthite (CaAl2Si2O8). With the further progress of the reaction, anorthite will co-melt with iron-bearing minerals above 1500 °C. The ternary phase diagram of SiO2–Al2O3–CaO system proves that anorthite and gehlenite are the typical high-temperature calcium-bearing minerals when the mole fraction of SiO2 is higher than 0.6. Moreover, the gehlenite is converted to anorthite with the temperature rise, which is consistent with experimental results. This study provides a scientific basis for understanding the UCG reaction conditions. [ABSTRACT FROM AUTHOR]