Method for upgrading a boiler

Method for upgrading a boiler, wherein the boiler comprises in an upper part of a furnace:

• a nose arch arrangement in which
• a lower wall is directed from a rear wall of the boiler towards the furnace of the boiler, wherein the lower wall is provided with a tip which includes
• an upper wall directed from the tip of the nose arch arrangement diagonally upwards away from a front wall of the boiler, wherein the nose arch arrangement ends in a grid-tube system extending vertically from an upper edge of the upper wall of the nose arch arrangement substantially to roofing of the boiler, forming a flue gas outlet from the boiler;
• a basic superheater arrangement placed substantially between the upper wall of the nose arch arrangement and the roofing,
• an open space between a front edge of the basic superheater arrangement and the front wall of the boiler, being in flow communication of the flue gases to the furnace through an opening between the front wall of the boiler and the tip of the nose arch arrangement, and wherein the boiler further comprises:
• a boiler drum, connecting pipes including downcomer tubes and screen tubes connected to the boiler drum, riser tubes placed after a grid-tube system in the flow direction of the flue gases, in the area of a boiler bank, connected to the connecting pipes, and upper circulation pipes connecting an upper part of the riser tubes and the boiler drum, the method comprising the steps of:
• using the boiler at a first boiler capacity by:
• keeping the open space in radiation contact with the furnace and empty, and using in connection with the riser tubes a first set of the water circulation system connected to the boiler drum, using the boiler at a least one other boiler capacity exceeding the first boiler capacity, by placing at least one part expanding the basic superheater arrangement in the open space.