Abstract: The present disclosure relates to a method of operating a boiler system that includes an oxyfuel boiler in which an oxygen stream and a fuel stream are combusted to generate a stream of flue gas, an oxygen gas source producing the stream of oxygen for the boiler, and a gas processing unit for cleaning and compressing at least a portion of the stream of flue gas generated in the boiler for producing a stream of pressurized fluid comprising carbon dioxide. The method includes operating the boiler system, at least for a period of time, in an evaporation mode, in which a stream of oxygen from the oxygen gas source in liquid form is evaporated prior to being introduced in the boiler by transferring heat energy to the stream of liquid oxygen from a first stream of carbon dioxide of the gas processing unit. The present disclosure further relates to a boiler system for an oxy-fuel process as well as to a power plant comprising such a system.