Advanced coal-fired combined-cycle power systems under development and demonstration have the potential to increase generating efficiency to approach 50%, reduce the cost of electricity by up to 20%, and meet stringent standards on emissions of SOx, NOx, fine particulates, and air toxic metals. Integrated gasification combined-cycle, pressurized fluidized-bed combustion, and externally fired combined-cycle systems rely on different high-temperature combinations of heat exchange, gas filtration, and sulfur capture to meet these requirements. The success of these systems when operated on low-rank coals depends importantly on the behavior of the ash. Deposits were analyzed from the riser and disengager sections of an experimental transport reactor development unit (TRDU) operating on a high-calcium Belie Ayr, Wyoming, subbituminous coal under gasification conditions for 107 cumulative hours, with short periods at oxidizing conditions during start-up and system upsets. Deposits appeared to have formed by the cementing action of low-melting-point calcium magnesium aluminosilicates derived from the finer coal ash. Depending an deposit location, aerodynamic classification appeared to have affected the proportion of finer material derived from coal ash to coarser particles from the zeolite-based start-up bed material. Bridging deposits from a test candle filter were similar in chemical composition to the finer ash.