A method of operation of a thermal power plant having an air separation system with a plurality of air storage unit (ASU) compressors and a liquid oxygen / liquid air (LOX/ LA) storage facility for oxyfuel firing of fossil fuel and a power plant having a control system to perform the same are described. The method is characterized by the step of controlling the net power output of the plant in response to short term variations in grid demanded net plant output by dynamically adjusting the works power of the ASU compressors preferably in conjunction with co-ordinated changes in firing demand. The method is in particular a method to produce an improved primary and secondary response to transient changes in grid demand and to provide accurate response to load dispatch ramps.