Answer
Combustion of the fuel and subsequent heat transfer from hot combustion gases to the cycle working fluid.
Work Step by Step
With respect to the Boiler the most significant source of irreversibility by far for a fossil-fueled vapor power plant is associated with combustion of the fuel and subsequent heat transfer from hot combustion gases to the cycle working fluid. As combustion and subsequent heat transfer occur in the surroundings of subsystem, they are classified here as external.