Method of reducing flue gas emissions and a boiler

Patent metadata

Field	Value
Publication number	US-10443839-B2
Application number	US-201615094154-A
Country	US
Kind code	B2
Filing date	Apr 8, 2016
Priority date	Apr 15, 2009
Publication date	Oct 15, 2019
Grant date	Oct 15, 2019

Abstract

A method and a boiler for decreasing the amount of nitrogen oxides in flue gases of a boiler, which flue gases are generated in the combustion of fuels and air. The boiler has a water circulation system comprising superheaters and a furnace for combustion of fuel and for generating flue gases that contain nitrogen oxides, which flue gases mainly flow upwards in the furnace and further to the superheater zone and via other heat recovery surfaces of the boiler out of the boiler, and a nitrogen oxides reducing agent is introduced into the flue gases. The nitrogen oxides reducing agent is introduced into the flue gases prior to the superheater zone, after the temperature of the flue gases is decreased by at least one heat exchanger that is located upstream of the nose of the boiler and upstream of the introduction of the nitrogen oxides reducing agent.

First claim

The invention is:

1. A method to decrease nitrogen oxides in flue gases flowing through a recovery boiler, wherein the recovery boiler includes a smelt spout at or near a bottom of the recovery boiler, a waste liquor injection nozzle aligned with a furnace zone above the bottom, and walls extending upward from the bottom, wherein the walls include water tubes, and a gas path is defined by the walls, and the walls include a bullnose section forming a narrowed section of the gas path,
wherein the gas path includes the furnace zone and extends upwards towards the bullnose, the narrowed section formed by the bullnose, and a superheater zone extending upward from the bullnose, the recovery boiler further including a water circulation system comprising the water tubes in the furnace zone, and superheaters in the superheater zone,
wherein a screen in the furnace zone of the recovery boiler and covers an entire cross section of the gas path below the bullnose, and the screen includes a heat exchanger;
the method comprising:
injecting into the furnace zone through the waste liquor injection nozzle a waste liquor;
combusting the waste liquor and oxygen in the furnace zone, wherein the combustion generates flue gases containing nitrogen oxides;
discharging smelt formed during the combustion from the smelt spout;
confining the flue gases to the gas path such that the flue gases flow from the furnace zone, through the narrowed section at the bullnose, through the superheater zone and exhausts from the recovery boiler,
introducing an agent into the furnace zone of the gas path, above the heat exchanger and below the bullnose, wherein the introduction of the agent reduces nitrogen oxides in the flue gases while the flue gases are in the furnace zone,
while the flue gases are in the furnace zone, and upstream of the introduction of the agent, cooling the flue gases by passing the flue gases through the heat exchanger of 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,
suppressing radiant heat from heating the introduced agent by the screen which separates the agent from the furnace, and
moving the flue gases a distance above the heat exchanger and along the gas path selected to promote reactions between the agent and flue gases, wherein the distance is downstream of the heat exchanger and upstream of the superheater zone.

2. The method according to claim 1 , wherein, in the heat exchanger in the screen, heat is recovered from the flue gases for superheating steam.

3. The method according to claim 1 , wherein, in the heat exchanger in the screen, heat is recovered from the flue gases and used to evaporate boiler water.

4. The method according to claim 1 , wherein, in the heat exchanger in the screen, heat is recovered from the flue gases and used to preheat boiler feed water.

5. The method according to claim 1 , wherein, in the heat exchanger of the screen, heat is recovered from the flue gases and used to heat combustion air for the recovery boiler.

6. The method according to claim 1 , wherein the agent is conveyed by a medium into the flue gas flow.

7. The method according to claim 6 , wherein the agent is conveyed by air to the flue gas flow.

8. The method according to claim 6 , wherein the agent is conveyed by flue gas to the flue gas flow.

9. The method according to claim 1 , wherein the agent for reducing nitrogen oxides is ammonia, urea or a precursor producing ammonia.

10. The method according to claim 1 , further comprising combusting black liquor in the furnace.

11. 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.

12. The recovery boiler according to claim 11 , wherein said at least one heat exchanger in the screen is connected to the boiler water circulation system and steam flowing in the system is superheated in the heat exchanger which receives heat energy from the flue gases.

13. The recovery boiler according to claim 11 , wherein the feeder for the reducing agent is connected to a boiler combustion air system or a flue gas discharge system such that combustion air or circulated flue gas is a carrier gas for the introduction of the reducing agent.

14. The recovery boiler according to claim 11 , wherein the feeder for the reducing agent is connected to a gas source which supplies a carrier gas for the introduction of the reducing agent.

15. The recovery boiler according to claim 11 , wherein the feeder for the reducing agent is connected to the flue gas discharge system of another boiler for using circulated flue gas as carrier gas in the introduction of the reducing agent.

16. The recovery boiler according to claim 11 , wherein the recovery boiler is a chemical recovery boiler of a chemical pulp mill.

17. 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

Assignees

• Andritz Oy

Inventors

• Miikkulainen Pasi
• Pakarinen Lauri

Classifications

• F22B21/40

  built-up from water tubes arranged in a comparatively long vertical shaft, i.e. tower boilers · CPC title

• B01D2251/2062

  Ammonia · CPC title

• B01D2251/2067

  Urea · CPC title

• F23J2215/101

  Nitrous oxide (N2O) · CPC title

• B01D53/56

  Nitrogen oxides (B01D53/60 takes precedence) · CPC title