The layered structure of Ash deposition in a 350 MW PC furnace burning high sodium–calcium lignite

The high content of sodium and calcium in Xinjiang lignite induces severe slagging and ash deposition in pulverized coal furnaces. In the present study, kaolin was co-fired with high sodium–calcium lignite (HSCL) in a 350 MW PC furnace for 9 months to inhibit slagging and deposition, and the slags and deposits along the furnace were collected and analyzed by using EDS and XRD. The results showed that when average sodium content in ash was controlled below 2.1 %, there was no severe slagging observed. The slagging showed distinguishing structure and composition on different heating surfaces. In furnace chamber, slags grew windward because molten or soften ash moved with the tangential gas flow and adhered on the cooled water wall. In the lower temperature region below burners, amounts of sulfur and ferrum was found, indicating significant sulfate condensing and molten-salt corrosion; while in the water wall of higher temperature region around burners, no sulfate was found in slags which were porous and mainly contained aluminosilicates. A layered structure of ash deposition was observed on the heating surfaces of medium gas temperature below 1000 °C. Calcium sulfates were enriched in the bottom layer adhering on tube surfaces and should play an important role on the initial layer formation. With the growth of ash deposition, the surface temperature of ash layer remarkably increased and the capture of gaseous sodium by ash layer was significantly enhanced at such high temperatures, resulting in a higher sodium content of outer layer than that of inner layer.