In the processes of conversion of solid fuels for their energetic and non-energetic use, the mineral phases of the ashes and their transformation can have a strong influence. This is why it is important to know the temperature-dependent formation and transformation of minerals, both in the optimization processes and in the change / co-utilization of fuels.
Dr. Markus Reinmöller, Dr. Marcus Schreiner, Dr. Stefan Guhl, and Professor Bernd Meyer at TU Bergakademie Freiberg in collaboration with Dr. Markus Neuroth at RWE Power AG in Germany investigated the ashes of fuels, Colombian hard coal, German brown coal, refused-derived fuel and oat husks. To generate the ash, different treatment methods were carried out, in which the mineral phases obtained by the three incineration methods were compared, depending on the temperature, and so their formation and transformation. The research work is now published in Journal, Fuel.
Summarizing the process for the formation and transformation of mineral phases in the four fuels, the research team began by drying the samples of all fuels at 105 ºC, to be able to grind them, and then incinerate them using three methods, namly low temperature ash, medium temperature ash and high temperature ash. In the first method the ash was generated, treating the sample with a low temperature plasma at around 200 °C over a long period. In the second, the ash was generated in an ash furnace under air and a temperature of 450 °C. In the third, the ash was generated under air at a temperature of 815 °C. Subsequently the ashes generated by the second and third methods were ground, and they were used for X-ray fluorescence analyzes, X-ray diffraction at room temperature and ash fusion tests. While the ashes generated by the first method were ground, but only used for X-ray diffraction studies, due to the typically low amount of ash generated by this method.
The authors found that for the Colombian hard coal, the ash obtained by the low temperature ash method is dominated by quartz, corundum, hematite, kaolinite and anhydrite, while by the method of ambient temperature ash the formation of muscovite is achieved by means of the kaolinite previously found. This muscovite is partially broken down by the high temperature ash method, causing an increase in the quartz content. For German brown coal the ash obtained by the low temperature ash method, mainly contains gypsum, anhydrite, thenarditeand quartz, while by the medium temperature ash method there is a lower amount of calcium sulfates, and in the high temperature ashnew Na- and Fe-rich sulfates as well as ferrites and aluminates are formed.
For refused-derived fuel, the ash obtained by the low temperature ash method is composed of carbonates, sulfates, simple oxides and other mineral phases, while by medium temperature ash method, the mineral phase of the calcite is remained stable, and with the high temperature ash method there was an extensive decomposition of the sulphate and carbonate mineral phases together with quartz leading to the formation of various silicates. For the oat husks the ash obtained by the low temperature ash method has contained arcanite, calcite, quartz and phosphates, while, for the high temperature ash method it was recognized, a significant increase of the quartz / cristobalite phases from previously amorphous matter. On the other hand when analyzing the behavior of the fusion of the ashes, it was observed that the ashes of the four fuels show medium or high flow temperatures between 1300 to more than 1600 °C, with a significant decrease of the deformation temperatures for medium temperature ash compared to the one at high temperature.
By virtue of the study carried out, it can be said that, in the resulting mineral phases, a significant influence of temperature is detected, of the three incineration methods, at temperatures of 200, 450 and 815 °C. On the other hand when performing the calculations using the FactSageTM software package, mineral phases are provided for the ash using the medium temperature ash composition, where they, calculated at 815 °C, are shown to coincide as in its majority with X-ray diffraction studies of the high temperature ash, due to phases of identical stoichiometry or similar. Summarizing this study, the mineral phase formation and transformation in ashes of four different fuels up to the entire melting is reproduced by the applied ashing methods and the ash fusion tests in the laboratory, observing the changes in mineral matter that are undergone in real conversion processes for energetic and non-energetic use.
Markus Reinmöller, Marcus Schreiner, Stefan Guhl, Markus Neuroth, Bernd Meyer. Formation and transformation of mineral phases in various fuels studied by different ashing methods. Fuel Volume 202, 2017, Pages 641–649
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