System for producing energy at a pulp mill

A liquor recovery boiler system comprising:

• a boiler further comprising at least one wall defining a furnace, at least one liquor injector arranged to inject liquor into the furnace, and at least one superheater arranged in a flue gas passage for combustion gases generated in the furnace,
• a plurality of water cooled tubes arranged in the at least one wall defining the furnace;
• at least one cavity separate from the furnace and having walls formed of water cooled tubes, wherein fluid flowing through at least one of the water cooled tubes of the at least one wall defining the furnace flows through at least one of the water cooled tubes of the cavity, and wherein at least a portion of the at least one of the water cooled tubes of the cavity are formed of at least one of the water cooled tubes of the walls of the boiler such that water or steam flows from the at least one of the water cooled tubes of the walls of the boiler to the at least one of the water cooled tubes of the cavity and to the at least one superheater in the flue gas passage in the furnace;
• a fuel combustor arranged in the at least one cavity;
• at least one outlet for discharging combustion gases from the cavity to the boiler;
• an interior of the at least one cavity being provided with a heat exchanger which receives superheated steam from the at least one superheater of the boiler, wherein the heat exchanger (i) is below the fuel combustor of the at least one cavity and above the at least one outlet, and (ii) receives steam from the at least one superheater in the flue gas passage of the boiler, and
• a gasifier for gasifying a biomass material and said gasifier produces combustion gas provided to the fuel combustor of the cavity.

A method for superheating steam in a recovery boiler having at least one wall defining a furnace and a separate combustion cavity, said method comprising:

• a. injecting spent liquor in the furnace to generate hot combustion flue gases in the furnace;
• b. cooling the at least one wall of the furnace by flowing cooling fluid through the wall;
• c. generating hot combustion gases in the cavity, wherein the combustion gases in the cavity are generated in a gasifier gasifying a biomass material and the combustion gases are generated in an upper portion of the cavity;
• d. cooling a cavity wall of the cavity by flowing the cooling fluid from the at least one wall of the furnace through the cavity wall, wherein the cavity wall is at least partially formed of the at least one wall of the furnace;
• e. passing cooling fluid from the wall of the cavity to a superheater arranged in a flue gas passage of the boiler;
• f. passing cooling fluid from the superheater in the gas passage of the boiler directly to a superheater in the cavity, wherein the superheater in the cavity is below the upper portion of the cavity and above a combustion gas outlet between the cavity and the at least one wall of the furnace;
• g. passing the combustion gases in the cavity downward from the gasifer, over the superheater in the cavity and to the outlet to mix the combustion gases with the flue gases in the furnace.

A method for superheating steam in a recovery boiler having a furnace and a separate combustion cavity, wherein said furnace and separate combustion cavity include walls formed of common water cooled tubes in fluid communication, said method comprising:

• a. injecting spent liquor in the furnace to generate hot combustion flue gases in the boiler;
• b. directing the flue gases through a flue gas passage of the furnace;
• c. generating hot combustion gases in an upper region of the cavity;
• d. flowing cooling fluid through the common water cooled tubes from the wall of the furnace to the wall of the cavity;
• e. injecting the hot combustion gases from the cavity into the furnace at an opening between the cavity and a front wall of the furnace and opposite to a bullnose section of a back wall of the furnace;
• f. the injection of the hot combustion gases from the cavity mixes with the flue gas in the flue gas passage;
• g. flowing the cooling fluid from the water cooled tubes of the cavity to a superheater in a portion of the flue gas passage of the furnace downstream of the mixing of the hot combustion gases from the cavity and the flue gases in the gas passage, and
• g. directing the cooling fluid from the superheater in the flue gas passage to a superheater in the cavity between the upper region of the cavity and the opening between the cavity and the furnace, wherein the steam is directed through a conduit external to the flue gas passage.