Abstract: A method of and control apparatus for operation of a boiler plant are described. The boiler plant has a furnace volume, an oxyfuel firing system for oxyfuel combustion of fuel in the furnace volume, and a compression system for compression of gases exhausted from the furnace volume after combustion. The method and control apparatus are characterized by the step of controlling mass flow of gases through the compression system as a means to control pressure within the furnace volume. This invention relates to both single and multi unit arrangements.