Deposit samples for eight coals were collected on stainless steel probes in reducing and oxidizing regions of a 160kWₜₕ, down-fired, pulverized coal reactor. Firing conditions in the reactor and probe temperatures were controlled to simulate an industrial furnace operating on an advanced ultra-supercritical steam cycle, 500°C in the near burner region and 750°C in the oxidizing region. The samples were analyzed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) to determine particle sizes and elemental compositions. Deposited particles on the top or upstream side of the probes were larger and more irregular in shape. Particles that deposited on the bottom or downstream side of the probes were smaller and more spherical. The SEM–EDS analysis showed each particle consisted of a unique composition of elements. The sulfur concentrations measured in the deposits were averaged for the upstream and downstream side of each specimen and were not found to correlate with sulfur concentrations of their respective coals, but they were strongly correlated with the calcium and iron contents of their respective coals. The high-calcium, sub-bituminous and lignite coals produced high sulfur concentrations in the deposits, particularly in the downstream, oxidizing deposits. The high-iron, bituminous coals produced high sulfur concentrations primarily in upstream, reducing deposits. The relatively low-calcium and low-iron bituminous coals produced low sulfur concentrations in the deposits. Although high in iron, the Mahoning coal deposits contained low sulfur concentrations.