Method of reducing flue gas emissions and a boiler

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

• walls defining a flue gas passage, wherein the flue gas passage includes a furnace zone and a superheater zone;
• a nose 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 fuel is combusted and flue gases 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;
• a feeder extending through the walls at the nose 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
• at least one heat exchanger located at the nose and in the flue gas passage upstream of the feeder, wherein the at least one heat exchanger substantially covers an entire horizontal cross-sectional area of the furnace zone, forming a volume in the boiler defined by the walls of the boiler that is downstream of the heat exchanger 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 exchanger 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 boiler including walls defining a gas path including a furnace zone and a superheater zone downstream of the furnace zone wherein a nose in at least one of the walls separates the furnace zone and the superheater zone, the nose is aligned with a narrowest region of the boiler, the method comprising:

• combusting fuel and air in the furnace zone and generating flue gases from the combustion;
• 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 heat exchanger located at the nose, forming a volume in the boiler defined by the walls of the boiler that is downstream of the heat exchanger and upstream of the superheater zone to introduce and react an agent prior to the flue gas reaching the superheater zone, and covering a horizontal cross section of the furnace zone;
• feeding the agent at the nose and downstream of the heat exchanger to reduce nitrogen oxides into the cooled flue gases while the flue gases are in the furnace zone, 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 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 boiler having a water circulation system comprising:

• walls defining a flue gas passage, wherein the flue gas passage includes a furnace zone and a superheater zone;
• a nose 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 fuel is combusted and flue gases 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;
• a feeder extending through the walls at the nose and configured to introduce a reducing agent into the flue gases in the furnace zone, the feeder is located upstream of the superheater zone, wherein the reducing agent reduces nitrogen oxides in the flue gases;
• at least one heat exchanger at the nose and upstream of the feeder,
• a distance between the heat exchanger and the superheater zone that is confined by the walls of the boiler defines a volume within the furnace zone, in which the reducing agent reacts with the flue gas before the flue gas reaches the superheater zone, and
• wherein the at least one heat exchanger substantially covers an entire horizontal cross-sectional area of the furnace zone, and the at least one heat exchanger 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.