Sodium retention behavior of Xinjiang high-alkali coal in a 20 kW slag-tapping combustor test

Abstract

The most serious ash fouling/slagging problems of ZD coal (Xinjiang Zhundong high-alkali coal) have always been the main reason affecting the long-term safe operation of boilers due to its high content of AAEMs (alkali and alkaline earth metals) in ash. In order to fully use of these kinds of coals, the slagging-type cyclone combustion of ZD coal was induced in this study. The potential of sodium retention as well as the sodium capture mechanism by the molten slag during cyclone combustion has been studied in a 20 kW slag-tapping combustor test facility. The slag and fly ash were sampled and characterized by XRF, XRD, SEM-EDX and 23Na NMR etc. The results show that the molten slag had a higher sodium retention ability than that of un-melted slag, and the sodium in the molten slag was completely insoluble Na. Furthermore, the insoluble Na in the molten slag was 10.5, 4.9 and 4.0 times higher than that in their parent coal ash for the three tested coals, respectively. It seems that the excess insoluble Na was a result of the interaction between the sodium and aluminosilicate slag through strong mixing, coalescence, and collision process during cyclone combustion. Interestingly, the sodium retention potential has a close relationship with the sodium environment in the molten slag. The network formers (Si4+, Al3+, Fe3+ etc.) in slag can attract the O2– in the Na-O pair, prolonging the Na-O distance and inhibiting the release of Na2O out of slag. Whereas the enrichment of network modifiers (Ca2+ etc.) in slag can shorten the Na-O distance, thus facilitating the Na2O release.