In the Rhenish lignite mining area, the coals extracted daily by high-capacity bucket wheel excavators in two opencast mines (Garzweiler and Hambach) have to be delivered to 30 boiler plants along the North-South railway system (Frimmersdorf, Neurath, Niederaussem, Goldenberg power plants) in qualities ensuring, if possible, controllable deposits in the boiler. With advancing opencast mine operations, marked coal quality changes, involving an increase in the calorific value and sodium content and an associated decrease in the ash content, are occurring today and will occur in the future, in particular in the Hambach opencast mine. These deposit-related changes have an impact on boiler plant operation. A rise in the temperatures at the end of the combustion chamber and shifts in the heat transfer are observed. In combination with certain constituents of the coal, this temperature rise leads to fused mass in the furnaces and fouling in the economisers and air heaters.