13. Appendix: Ash and Slag Sample Preparation
Ashing process started with extraction of coal samples from transport containments by a standard sampling method. Proximate analyses were carried out to detect the ash content of rare coal. When the ash content is known, an adequate amount of coal was ashed to receive enough products for further preparation steps. Coal was ashed under air for 36 h according to DIN 51719. If necessary, coal was milled to a grain size <10 mm to improve ashing success within THERCONCEPT furnace (THERMCONCEPT Dr. Fischer GmbH & Co. KG, Bremen), Figure 89. Ultimate analysis and X-ray fluorescence (XRF) of melted bead were applied to estimate ash composition.
Figure 89: Ashing furnace chamber.
Slagging under air was the second process step. After XRF analysis of ash, the liquidus temperature was estimated by thermoequilibrium software FactSage 6.4TM. Liquidus temperature prediction was based on the composition of metal oxides. The use of phase diagrams is also possible but limited due to the availability of certain data for multicomponent ashes. All ash sample was filled into a platinum-gold (Pt95-Au5, mass- related) dish and slagged under air with at least 50 K more than calculated liquidus temperature. A Carbolite 1500 RHFTM chamber furnace (Carbolite Gero GmbH & Co. KG, Neuhausen, Germany) was used for samples with liquidus temperature below 1450 °C. In case of ashes with liquidus temperatures from 1450-1700 °C, the TOMMI plusTM furnace (Fraunhofer Institut für Silicatforschung, Bronnbach, Germany) was engaged, Figure 90 a) and b). Initial temperatures were 600 °C.
13. Appendix: Ash and Slag Sample Preparation 156
Figure 90: High-temperature furnace a) of Carbolite and b) TOMMI plus.
Heating rates varied from 1-10 K/min depending on the swelling or bubbling behavior. The swelling behavior was not in agreement with the ash composition, i.e. ashes with high sulfur, sodium or other contents were similarly bubbling than ashes with a moderately amount of volatiles. The melting state has to be eye-proofed due to the non- predictability of bubbling behavior. In summary, a heating rate of 2 K/min is recommended. A boil over of ash/slag can cause serious damages on the furnace, Figure 91. When the liquidus temperature was passed by 50-100 K, the sample was hold for 1-3 h and cooled down with 10 K/min afterwards to room temperature. The remaining slag was removed from the Pt95-Au5 crucible and milled to powder in a swinging disc mill (Herzog HSM, HERZOG Maschinenfabrik GmbH & Co. KG, Osnabrück, Germany). Milling is required for further XRF-analysis and to provide a well homogenized slag powder.
Figure 91: Damages caused by slag boil over within a high temperature furnace.
Slagging under reducing conditions was done in a vertical furnace with atmospheric control, Reetz VRO 1650 (HTM Reetz GmbH, Berlin, Germany), Figure 92. Preparation of slag samples under reducing conditions was introduced before viscosity measurements were carried out under reducing conditions. This ensures a defined sample condition in the beginning of viscosity measurements.
13. Appendix: Ash and Slag Sample Preparation 157
Figure 92: Reetz vertical tube furnace 1650 to adjust oxidation state.
Slag powder prepared under air was divided into two equal portions. One half was taken for reducing conditions preparation. The powder was filled in a Platinum-Rhodium (Pt80-Rh20, mass-related) crucible and inserted within the furnace. A heating rate of 10 K/min was achieved up to 50-100 K above liquidus temperature predicted for reducing conditions by FactSage 6.4TM. From startup to 1000 °C, the furnace tube was purged by argon gas (ALPHAGAZTM 1, AIR LIQUIDE Deutschland GmbH, Düsseldorf). At 1000 °C, a mixture of carbon monoxide (CO) and carbon dioxide (CO2) was achieved to maintain a reducing atmosphere. Primary goal was to change all Fe3+ into Fe2+. Depending on temperature and slag composition, the CO:CO2 ratio was set to 6:4 and in later experiments 7:3. This causes partial oxygen pressures pO2 in the range 1.10-7 to 1.10-15 atm in the investigated temperature ranges, Figure 93 a). The ratio Fe2O3/FeO is shifted to lower values, more FeO is formed, Figure 93 b). Values are taken from FactSage 6.4TM calculations with FactPS, FToxid, FTmisc, pure solids, liquids and gases whereas real gas behavior is switched on. The mechanism of iron reducing can be related to
o thermochemical equilibrium and/or to
o chemical reaction, e.g. by free C from the Boudouard reaction or directly by CO from the gas atmosphere.
A detailed discussion on the impact of thermochemical equilibrium and chemical reaction in view of iron reducing will be not given.
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