US20010004516A1 - Method of decreasing nitrogen oxide emissions in a circulating fluidized bed combustion system - Google Patents

Method of decreasing nitrogen oxide emissions in a circulating fluidized bed combustion system Download PDF

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US20010004516A1
US20010004516A1 US09/739,105 US73910500A US2001004516A1 US 20010004516 A1 US20010004516 A1 US 20010004516A1 US 73910500 A US73910500 A US 73910500A US 2001004516 A1 US2001004516 A1 US 2001004516A1
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heat exchanger
hearth
fluidized bed
air
oxygen
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US6415743B2 (en
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Francois Malaubier
Eugene Guilleux
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General Electric Technology GmbH
Alstom Power Boiler SA
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Assigned to ABB ALSTOM POWER COMBUSTION reassignment ABB ALSTOM POWER COMBUSTION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: GUILLEUX, EUGENE, MALAUBIER, FRANCOIS
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Assigned to ALSTOM POWER BOILERS reassignment ALSTOM POWER BOILERS CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: ABB ALSTOM POWER COMBUSTION
Assigned to ALSTOM POWER SYSTEMS SA reassignment ALSTOM POWER SYSTEMS SA MERGER & NAME CHANGE Assignors: ALSTOM POWER BOILERS, ALSTOM POWER CENTRALES
Assigned to ALSTOM TECHNOLOGY LTD reassignment ALSTOM TECHNOLOGY LTD ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ALSTOM POWER SYSTEMS SA
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
    • F23C10/00—Fluidised bed combustion apparatus
    • F23C10/02—Fluidised bed combustion apparatus with means specially adapted for achieving or promoting a circulating movement of particles within the bed or for a recirculation of particles entrained from the bed
    • F23C10/04—Fluidised bed combustion apparatus with means specially adapted for achieving or promoting a circulating movement of particles within the bed or for a recirculation of particles entrained from the bed the particles being circulated to a section, e.g. a heat-exchange section or a return duct, at least partially shielded from the combustion zone, before being reintroduced into the combustion zone

Definitions

  • the invention relates to a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed.
  • the fuel which may be crushed coal, is usually injected into the bottom portion of the hearth, where a reducing atmosphere prevails.
  • the fuel is subjected to pyrolysis, with the combustible matter being separated firstly into volatile matter containing a fraction of the nitrogen from the original fuel (volatile-N), and secondly into solid matter containing the remaining nitrogen from the original fuel (coke-N).
  • volatile-N volatile matter containing a fraction of the nitrogen from the original fuel
  • coke-N solid matter containing the remaining nitrogen from the original fuel
  • the coke grains can remain for various lengths of time in the bottom portion of the hearth, where a reducing atmosphere prevails. Under those conditions, the reaction products of the coke-N contain molecular nitrogen rather than the pollutant NOx.
  • the time for which the coke grains remain in the reducing zone depends essentially on the size of each grain. The finest particles leave the zone rapidly and then pass through the top portion of the hearth, where the reactions continue.
  • the flow of solid particles coming from the hearth 1 and collected by a suitable device such as a cyclone 3 is made up not only of inorganic ash but also of coke resulting from the fuel burning incompletely in the hearth. That coke contains the original elements of the fuel, and in particular carbon, sulfur, and nitrogen.
  • Those solid particles are sent back to the hearth via a suitable device such as a recycling loop 4 equipped with a siphon 5 .
  • a suitable device such as a recycling loop 4 equipped with a siphon 5 .
  • FIG. 1 a fraction of the flow of solid particles recycled to the hearth 1 goes through a heat exchanger 7 operating as a fluidized bed.
  • a heat exchanger 7 operating as a fluidized bed is constituted by a casing 71 containing packets of zigzag tubes 72 through which water or steam flows.
  • the flow of solid particles enters via an inlet duct 73 and passes through the heat exchanger by means of a fluidization system and then exits via an outlet duct 74 .
  • the top level of the bed of solid particles in the heat exchanger is indicated by the line L.
  • the heat exchanger 7 is outside the hearth 1 and only a fraction of the flow of solid particles being recycled to the hearth 1 passes through it. It is to be understood that the following description also applies when the heat exchanger 7 is part of the hearth 1 and receives the entire flow of solid particles.
  • the inside of the casing 71 of the heat exchanger 7 is subdivided into a plurality of chambers 75 A, 75 B, 75 C, 75 D, separated by walls 76 .
  • the chambers 75 B and 75 C receive the packets of tubes 72 B, 72 C through which water or steam passes internally.
  • the floor 77 of the heat exchanger is equipped with nozzles 78 making it possible to inject gas for fluidizing the solids.
  • a wind box 79 is placed below the floor 77 that contains the fluidization gas.
  • the wind box 79 may optionally be subdivided into at least as many compartments 79 A, 79 B, 79 C, 79 D as there are chambers inside the casing.
  • the fluidization gases are removed via the outlet duct 74 to the hearth 1 .
  • their temperature is lowered from the temperature they have at the outlet of the cyclone (about 850° C. to 900° C.) to a temperature of about 500° C. to 700° C. depending on the number and the surface area of the tubes 72 inside the casing 71 .
  • the fluidization gas used in the heat exchanger 7 is atmospheric air that is usually at a temperature of in the range 20° C. to 300° C. and at a compression level that is sufficient to enable the solids to flow from the inlet chamber 75 A to the outlet chamber 75 D, for example.
  • the production of nitrogen oxide is much higher in the inlet chambers 75 A, 75 B than in the outlet chambers 75 C, 75 D because of the amount of coke-N consumed in the inlet chambers and, above all, because the temperature of the solids is much higher in the inlet chambers than in the outlet chambers.
  • the temperature of the solids is typically about 850° C. to 800° C. in chamber 75 A, about 650° C. to 800° C. in chamber 75 B, and about 500° C. to 650° C. in chamber 75 C.
  • the nitrogen oxides that are formed in the chambers by reaction between the coke-N and the fluidization air are thus conveyed to the hearth, where they mix with the flue gases produced, thereby participating in the overall emission of pollutant.
  • An object of the invention is to decrease the emission of nitrogen oxides in such an installation.
  • the invention provides a method of decreasing nitrogen oxide emissions in a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed, wherein the heat exchanger is fed with a fluidization gas which is considerably poorer in oxygen than air.
  • the idea behind the invention is thus to control the oxygen partial pressure inside the chambers of the heat exchanger so as to minimize any formation of nitrogen oxides.
  • the oxygen partial pressure is controlled to lie in the range 1% to 4%.
  • the oxygen-poor fluidization gas typically containing less than 12% (molar) of oxygen, is preferably constituted by flue gases preferably taken downstream from a dust filter for removing dust from the flue gases. Dust is thus removed from the flue gases, which dust could otherwise damage, by abrasion or clogging, the fans used to send the fluidization gas under pressure into the heat exchanger.
  • Each heat-exchange chamber of the heat exchanger is fed separately with the oxygen-poor fluidization gas used on its own or as mixed with air.
  • the proportion of air in the mixture it is thus possible to obtain combustion of the carbon in the heat exchanger in the presence of an atmosphere having a low oxygen content, and thus with minimized nitrogen oxide emissions.
  • FIG. 1 shows a combustion installation for implementing the method of the invention.
  • FIG. 2 is a more detailed view of a fluidized bed heat exchanger.
  • FIG. 1 The combustion installation shown in FIG. 1 is described in part above.
  • the flue gases that exit from the top of the cyclone 3 go through an electrostatic or bag dust filter for removing the dust from them before they are removed via a chimney. Downstream from the dust filter 6 , a fraction of the flue gases is taken to serve as fluidization gas for the heat exchanger 7 after being blown by a fan 9 . Air blown by a fan 10 may also be used to fluidize the flow of solid particles in the heat exchanger 7 as shown in FIG. 2.
  • the flue gases on their own or as mixed with air feed the hottest chambers 75 A and 75 B of the heat exchanger 7 so as to fluidize the flow of solid particles.
  • the coldest chambers 75 C and 75 D are preferably fed with air only.
  • the fluidization gas is constituted by air on its own, by flue gases on their own, or by a mixture of flue gases and of air
  • the flow rate of the fluidization gas into each chamber through the wind boxes 79 and the nozzles 78 is adjusted by valves 80 .
  • the quantity of air in the air/flue gas mixture is also adjusted by a valve 81 . It is to be understood that oxygen-poor flue gases could also be injected into all of the chambers of the heat exchanger.

Abstract

In a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed, the method of decreasing nitrogen oxide emissions consisting in the heat exchanger being fed with a fluidization gas which is considerably poorer in oxygen than air.

Description

  • The invention relates to a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed. [0001]
    • BACKGROUND OF THE INVENTION
  • In a circulating fluidized bed installation, the fuel, which may be crushed coal, is usually injected into the bottom portion of the hearth, where a reducing atmosphere prevails. [0002]
  • In that zone, the fuel is subjected to pyrolysis, with the combustible matter being separated firstly into volatile matter containing a fraction of the nitrogen from the original fuel (volatile-N), and secondly into solid matter containing the remaining nitrogen from the original fuel (coke-N). The coke grains can remain for various lengths of time in the bottom portion of the hearth, where a reducing atmosphere prevails. Under those conditions, the reaction products of the coke-N contain molecular nitrogen rather than the pollutant NOx. The time for which the coke grains remain in the reducing zone depends essentially on the size of each grain. The finest particles leave the zone rapidly and then pass through the top portion of the hearth, where the reactions continue. [0003]
  • More particularly in FIG. 1, the flow of solid particles coming from the hearth [0004] 1 and collected by a suitable device such as a cyclone 3 is made up not only of inorganic ash but also of coke resulting from the fuel burning incompletely in the hearth. That coke contains the original elements of the fuel, and in particular carbon, sulfur, and nitrogen.
  • Those solid particles are sent back to the hearth via a suitable device such as a recycling loop [0005] 4 equipped with a siphon 5. In FIG. 1, a fraction of the flow of solid particles recycled to the hearth 1 goes through a heat exchanger 7 operating as a fluidized bed. In FIG. 2, a heat exchanger 7 operating as a fluidized bed is constituted by a casing 71 containing packets of zigzag tubes 72 through which water or steam flows. The flow of solid particles enters via an inlet duct 73 and passes through the heat exchanger by means of a fluidization system and then exits via an outlet duct 74. The top level of the bed of solid particles in the heat exchanger is indicated by the line L. In this example, the heat exchanger 7 is outside the hearth 1 and only a fraction of the flow of solid particles being recycled to the hearth 1 passes through it. It is to be understood that the following description also applies when the heat exchanger 7 is part of the hearth 1 and receives the entire flow of solid particles.
  • In FIG. 2, the inside of the [0006] casing 71 of the heat exchanger 7 is subdivided into a plurality of chambers 75A, 75B, 75C, 75D, separated by walls 76. The chambers 75B and 75C receive the packets of tubes 72B, 72C through which water or steam passes internally. The floor 77 of the heat exchanger is equipped with nozzles 78 making it possible to inject gas for fluidizing the solids. Below the floor 77, a wind box 79 is placed that contains the fluidization gas. The wind box 79 may optionally be subdivided into at least as many compartments 79A, 79B, 79C, 79D as there are chambers inside the casing.
  • After passing through the solids from the bottom at floor level to above the top level L of the bed of solid particles, the fluidization gases are removed via the [0007] outlet duct 74 to the hearth 1. As the solids pass through the heat exchanger 7, their temperature is lowered from the temperature they have at the outlet of the cyclone (about 850° C. to 900° C.) to a temperature of about 500° C. to 700° C. depending on the number and the surface area of the tubes 72 inside the casing 71.
  • In the state of the art, the fluidization gas used in the [0008] heat exchanger 7 is atmospheric air that is usually at a temperature of in the range 20° C. to 300° C. and at a compression level that is sufficient to enable the solids to flow from the inlet chamber 75A to the outlet chamber 75D, for example. An atmosphere that is strongly oxidizing, or, in other words, an atmosphere that has a very high oxygen partial pressure, prevails in each chamber. That technique suffers from the drawback that nitrogen oxides form by reaction between the above-mentioned coke-N and the fluidization air of the heat exchanger 7. The production of nitrogen oxide is much higher in the inlet chambers 75A, 75B than in the outlet chambers 75C, 75D because of the amount of coke-N consumed in the inlet chambers and, above all, because the temperature of the solids is much higher in the inlet chambers than in the outlet chambers. By way of example, for a heat exchanger having four chambers as shown in FIG. 2, the temperature of the solids is typically about 850° C. to 800° C. in chamber 75A, about 650° C. to 800° C. in chamber 75B, and about 500° C. to 650° C. in chamber 75C. The nitrogen oxides that are formed in the chambers by reaction between the coke-N and the fluidization air are thus conveyed to the hearth, where they mix with the flue gases produced, thereby participating in the overall emission of pollutant.
    • OBJECTS AND SUMMARY OF THE INVENTION
  • An object of the invention is to decrease the emission of nitrogen oxides in such an installation. [0009]
  • To this end, the invention provides a method of decreasing nitrogen oxide emissions in a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed, wherein the heat exchanger is fed with a fluidization gas which is considerably poorer in oxygen than air. [0010]
  • The idea behind the invention is thus to control the oxygen partial pressure inside the chambers of the heat exchanger so as to minimize any formation of nitrogen oxides. The oxygen partial pressure is controlled to lie in the range 1% to 4%. The oxygen-poor fluidization gas, typically containing less than 12% (molar) of oxygen, is preferably constituted by flue gases preferably taken downstream from a dust filter for removing dust from the flue gases. Dust is thus removed from the flue gases, which dust could otherwise damage, by abrasion or clogging, the fans used to send the fluidization gas under pressure into the heat exchanger. [0011]
  • Each heat-exchange chamber of the heat exchanger is fed separately with the oxygen-poor fluidization gas used on its own or as mixed with air. By adjusting the proportion of air in the mixture, it is thus possible to obtain combustion of the carbon in the heat exchanger in the presence of an atmosphere having a low oxygen content, and thus with minimized nitrogen oxide emissions. [0012]
  • To avoid recycling too much flue gas into the heat exchanger, it is advantageous to feed the coldest chambers of the heat exchanger with air not mixed with the flue gases because nitrogen oxide formation is low in those chambers. [0013]
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • FIG. 1 shows a combustion installation for implementing the method of the invention. [0014]
  • FIG. 2 is a more detailed view of a fluidized bed heat exchanger. [0015]
    • MORE DETAILED DESCRIPTION
  • The combustion installation shown in FIG. 1 is described in part above. The flue gases that exit from the top of the [0016] cyclone 3 go through an electrostatic or bag dust filter for removing the dust from them before they are removed via a chimney. Downstream from the dust filter 6, a fraction of the flue gases is taken to serve as fluidization gas for the heat exchanger 7 after being blown by a fan 9. Air blown by a fan 10 may also be used to fluidize the flow of solid particles in the heat exchanger 7 as shown in FIG. 2.
  • In FIG. 2, the flue gases on their own or as mixed with air feed the [0017] hottest chambers 75A and 75B of the heat exchanger 7 so as to fluidize the flow of solid particles. The coldest chambers 75C and 75D are preferably fed with air only. Whether the fluidization gas is constituted by air on its own, by flue gases on their own, or by a mixture of flue gases and of air, the flow rate of the fluidization gas into each chamber through the wind boxes 79 and the nozzles 78 is adjusted by valves 80. The quantity of air in the air/flue gas mixture is also adjusted by a valve 81. It is to be understood that oxygen-poor flue gases could also be injected into all of the chambers of the heat exchanger.

Claims (4)

1. A method of decreasing nitrogen oxide emissions in a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed, wherein the heat exchanger is fed with a fluidization gas which is considerably poorer in oxygen than air.
2. A method according to
claim 1
, in which the combustion installation includes a dust filter for removing the dust from the flue gases, and in which the oxygen-poor fluidization gas is constituted by the flue gases taken downstream from the dust filter.
3. The method according to
claim 1
, in which the heat exchanger is subdivided into a plurality of heat-exchange chambers through which the flow of solid particles passes, and each chamber is fed separately with the oxygen-poor fluidization gas used on its own or as mixed with air.
4. A method according to
claim 3
, in which one or more of the heat-exchanger chambers are fed with the oxygen-poor fluidization gas used on its own or as mixed with air, and the other heat-exchanger chamber(s) is/are fed with air.
US09/739,105 1999-11-22 2000-12-19 Method of decreasing nitrogen oxide emissions in a circulating fluidized bed combustion system Expired - Lifetime US6415743B2 (en)

Applications Claiming Priority (2)

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FR9916244 1999-12-22
FR9916244A FR2803020B1 (en) 1999-12-22 1999-12-22 PROCESS FOR REDUCING NITROGEN OXIDE EMISSIONS IN A FLUIDIZED BED COMBUSTION PLANT

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US20010004516A1 true US20010004516A1 (en) 2001-06-21
US6415743B2 US6415743B2 (en) 2002-07-09

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US (1) US6415743B2 (en)
EP (1) EP1111301B1 (en)
CN (1) CN1145755C (en)
DE (1) DE60015908T2 (en)
ES (1) ES2232398T3 (en)
FR (1) FR2803020B1 (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20090120379A1 (en) * 2007-11-14 2009-05-14 Alstom Technology Ltd. Boiler having an integrated oxygen producing device
CN107327839A (en) * 2017-08-16 2017-11-07 华能白山煤矸石发电有限公司 A kind of CFBB drop oxygen suppression nitrogen system and control method

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Publication number Priority date Publication date Assignee Title
FR2848641B1 (en) * 2002-12-11 2005-12-16 Alstom Switzerland Ltd INDIRECT HEATING SYSTEM WITH VALORIZATION OF ULTRA FINE FUEL PARTICLES
DE102007062390B3 (en) * 2007-12-22 2009-04-02 Michael Kaden Fluidized-bed furnace for combustion of fuel, has fluid bed, where heat is received from fluid bed such that surplus bed material overflows from fluid bed and cooled bed material in fluid bed is recycled to fluid bed
FI123853B (en) * 2009-03-06 2013-11-15 Metso Power Oy A method for reducing nitrogen oxide emissions from oxygen combustion
FR2975094B1 (en) * 2011-05-12 2013-05-03 Lafarge Sa DECARBONATION PROCESS
CN105180159B (en) * 2015-08-26 2018-10-09 中国科学院工程热物理研究所 Mend the circulating fluidized bed combustion method of air cooling

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US4257478A (en) * 1979-06-27 1981-03-24 Stal-Laval Apparat Ab Gaseous media heat exchanger
DE3922765A1 (en) * 1989-07-11 1991-01-17 Babcock Werke Ag BURNING, ESPECIALLY FLUIDIZED BURNING
US5218932A (en) * 1992-03-02 1993-06-15 Foster Wheeler Energy Corporation Fluidized bed reactor utilizing a baffle system and method of operating same
US5341766A (en) * 1992-11-10 1994-08-30 A. Ahlstrom Corporation Method and apparatus for operating a circulating fluidized bed system
US5406914A (en) * 1992-11-10 1995-04-18 A. Ahlstrom Corporation Method and apparatus for operating a circulating fluidized bed reactor system
US5339774A (en) * 1993-07-06 1994-08-23 Foster Wheeler Energy Corporation Fluidized bed steam generation system and method of using recycled flue gases to assist in passing loopseal solids
FI102316B (en) * 1996-06-05 1998-11-13 Foster Wheeler Energia Oy Method and apparatus for reducing corrosion caused by harmful components of solid state suspensions on heat transfer surfaces
FR2775061B1 (en) * 1998-02-16 2000-03-10 Gec Alsthom Stein Ind CIRCULATING FLUIDIZED BED BOILER WITH IMPROVED NITROGEN OXIDE REDUCTION

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20090120379A1 (en) * 2007-11-14 2009-05-14 Alstom Technology Ltd. Boiler having an integrated oxygen producing device
US7954458B2 (en) 2007-11-14 2011-06-07 Alstom Technology Ltd Boiler having an integrated oxygen producing device
CN107327839A (en) * 2017-08-16 2017-11-07 华能白山煤矸石发电有限公司 A kind of CFBB drop oxygen suppression nitrogen system and control method

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CN1145755C (en) 2004-04-14
ES2232398T3 (en) 2005-06-01
FR2803020B1 (en) 2002-04-12
DE60015908D1 (en) 2004-12-23
US6415743B2 (en) 2002-07-09
FR2803020A1 (en) 2001-06-29
EP1111301B1 (en) 2004-11-17
EP1111301A1 (en) 2001-06-27
CN1300917A (en) 2001-06-27
DE60015908T2 (en) 2005-11-03

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