Method of reducing flue gas emissions and a boiler

A steam-generating recovery boiler having a boiler water circulation system comprising:

• walls defining a flue gas passage, wherein the flue gas passage includes a furnace zone and a superheater zone;
• a waste liquor injection nozzle extending through the walls;
• a smelt spout extending through the walls or a bottom of the recovery boiler;
• a bullnose in at least one of the walls between the furnace zone and the superheater zone, the bullnose is aligned with a narrowest region of the flue gas passage;
• heat recovery surfaces in the superheater zone and arranged within the flue gas passage;
• a furnace within which waste liquor, injected through the waste liquor injection nozzle, is combusted, flue gases are generated, and smelt is discharged through the smelt spout, wherein the furnace is within the furnace zone;
• wherein the walls adjacent the furnace zone including water tubes and extend upward and the flue gas passage extends upward through the furnace zone;
• a feeder extending through the walls upstream of the bullnose and configured to introduce a reducing agent into the flue gases in the furnace zone and upstream of the superheater zone, wherein the reducing agent reduces nitrogen oxides in the flue gases, and
• a screen with at least one heat exchanger located in the flue gas passage upstream of the feeder, wherein the screen covers an entire horizontal cross-sectional area of the furnace zone, forming a volume in the recovery boiler defined by the walls of the recovery boiler that is downstream of the screen and upstream of the superheater zone to introduce and react the reducing agent prior to the flue gas reaching the superheater zone, the at least one heat exchanging surface is configured to decrease the temperature of the flue gas to a certain temperature range determined based on a desired flue gas temperature for the introduction of the reducing agent.

A method to reduce nitrogen oxides exhausted from a recovery boiler including a waste liquor nozzle, a smelt spout, and walls defining a gas path including a furnace zone and a superheater zone downstream of the furnace zone wherein a bullnose in at least one of the walls forms a narrowed portion of the gas path and separates the furnace zone and the superheater zone, the method comprising:

• injecting a waste liquor into the furnace zone from the waste liquor nozzle;
• combusting the injected waste liquor and air in the furnace zone and generating flue gases and smelt from the combustion;
• discharging the smelt from the smelt spout;
• as the flue gases flow upward through the furnace zone, cooling the flue gases to a suitable temperature range by passing the flue gases through a screen including at least one heat exchanger, wherein the screen covers an entire cross section of the gas path and is in the furnace zone and upstream of the bullnose,
• introducing an agent into the flue gas in a volume in the recovery boiler between the walls, downstream of the screen, and upstream of the superheater zone;
• shielding the introduced agent from radiant heat from the furnace zone by the screen between the agent and the furnace zone;
• reducing nitrogen oxides in the cooled flue gases flowing through the volume, whereby a reaction between the agent and the cooled flue gases forms flue gases with reduced nitrogen oxides;
• flowing the flue gases with reduced nitrogen oxides through the gas path from the volume in the furnace zone and into the superheater zone;
• flowing the flue gases with reduced nitrogen oxides through a superheater arranged in the superheater zone of the gas path, and
• transferring heat from the flue gases with reduced nitrogen oxides to steam flowing through the superheater.

A steam-generating recovery boiler comprising:

• walls defining a flue gas passage, wherein the flue gas passage includes a furnace zone and a superheater zone;
• a waste liquor nozzle extending through at least one of the walls and configured to inject a waste liquor into the furnace zone;
• a smelt spout at or near a bottom of the recovery boiler;
• superheaters in the superheater zone and in the flue gas passage;
• a bullnose in at least one of the walls between the furnace zone and the superheater zone, the nose is aligned with a narrowest region of the flue gas passage;
• heat recovery surfaces in the superheater zone and arranged within the flue gas passage;
• a furnace within which the waste liquor is combusted and flue gases and smelt are generated, wherein the furnace is within the furnace zone;
• the walls adjacent the furnace zone extend upward and the flue gas passage extends upward through the furnace zone, and the walls include water tubes;
• a feeder extending through the walls upstream of the superheater zone, and configured to introduce a reducing agent into the flue gases in the furnace zone, wherein the reducing agent reduces nitrogen oxides in the flue gases;
• a screen including at least one heat exchanger, wherein the screen covers an entire cross section of the flue gas passage and is upstream of the feeder in the furnace zone, wherein the at least one heat exchanger in the screen is configured to decrease the temperature of the flue gas to a certain temperature range selected for introduction of the reducing agent from the feeder, and
• a circulation system in which water and/or steam flows through the at least one heat exchanger in the screen, the superheaters in the superheater zone and the water tubes in the walls of the recovery boiler.

A method to decrease nitrogen oxides in flue gases flowing through a recovery boiler, wherein the recovery boiler includes a furnace, a waste liquor injection nozzle aligned with the furnace, a smelt spout below the furnace, and a gas path defined by walls of the recovery boiler, the walls including water tubes and the gas path including a furnace zone and a superheater zone separated by a bullnose in the walls from the furnace zone, wherein the bullnose which forms a narrow region of the gas path, the recovery boiler further including a water circulation system comprising superheaters in the superheater zone, at least one heat exchanger in a screen in the furnace zone, and the water tubes in the walls,

• the method comprising:
• injecting a waste liquor from the waste liquor nozzle into the furnace;
• combusting the injected waste liquor and with oxygen in the furnace zone, wherein the combustion generates gases containing nitrogen oxides and smelt;
• discharging the smelt through the smelt spout;
• confining the flue gases to the gas path extending from the furnace zone, past the bullnose, through the superheater zone and exhausts from the recovery boiler,
• cooling the flue gases by passing the flue gases through the screen and the at least one heat exchanger in the screen;
• wherein the flue gases are cooled to temperature within a certain temperate range selected based on the agent to be injected conforms to temperatures suited to the introduction of the agent and the reduction of nitrogen oxides;
• introducing an agent into the furnace zone of the gas path downstream in the gas path of the screen and below or at a common elevation with the bullnose, wherein the introduction of the agent reduces nitrogen oxides in the flue gases;
• shielding the introduced agent from radiant heat from the furnace by the screen between the introduced agent and the furnace;
• moving the flue gases through a portion the gas path downstream of the superheater zone and having a distance selected to promote reactions between the agent and flue gases, wherein the portion of the gas path extends from the at least one heat exchanger to the superheater zone and extends past the bullnose, wherein the distance of the portion of the gas path is greater than a distance between the screen and the introduction of the agent.