This article summarizes the results on the influence of combustion conditions (air, oxy-fuel) on deposit and ash formation and transformation and their implications on plant operation. The investigations involved thermodynamic equilibrium simulations as well as experiments at IFK's 20 kW(th) and 0.5 MWth combustion test rigs and Vattenfall's 30 MWth oxy-fuel pilot plant ''Schwarze Pumpe". Comparative air and oxy-fuel experiments with Lusatian lignite showed a considerable increase of sulfur in ash and deposits and a 12 percentage point-lower sulfur release to SO2 in oxy-firing. One reason for this is the higher SO2 partial pressure in oxy-fuel combustion that stabilizes sulfates. Therefore, sulfates can form at higher temperatures and more extensively. Oxy-fuel deposits may be more sintered by sulfates and therefore more difficult to remove by soot blowing. However, the experimental and theoretical considerations presented here support the conclusion that SO2 levels and therefore sulfate stability in oxy-fuel combustion of low sulfur coals is comparable to conditions experienced in air-firing of high sulfur fuels. Thus, it can be expected that problems associated with sulfatic deposits (e.g.: fouling) in low sulfur coal oxy-fuel combustion can be handled by standard techniques (e.g. soot blowing). For high sulfur fuels with considerable sulfur capture potential (e.g.: high Ca), this may not be true and additional measures may be required for a reliable plant operation. (C) 2015 Elsevier B.V. All rights reserved.