METHOD OF REDUCING SULFUR DIOXIDE CONTENT IN FLUE GAS EMANATING FROM A CIRCULATING FLUIDIZED BED BOILER PLANT

Abstract

A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant. A first stream of sulfur-containing carbonaceous fuel is fed at a first feeding rate to a furnace of the boiler. A second stream of calcium carbonate containing absorbent having a predetermined d50 particle size is fed at a second feeding rate to the furnace. Oxygen containing gas is fed to the furnace for fluidizing a bed of particles forming in the furnace. Fuel is combusted with the oxygen and the sulfur in the fuel is oxidized to sulfur dioxide. The calcium carbonate is calcined to calcium oxide in the furnace. A portion of the calcium oxide is used to sulfate a first portion of the sulfur dioxide to calcium sulfate in the furnace.

Claims

1-10. (canceled)

11. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant, the method comprising the steps of: feeding a first stream of sulfur-containing carbonaceous fuel at a first feeding rate to a furnace of the boiler; feeding a second stream of calcium carbonate containing absorbent having a predetermined d50 particle size at a second feeding rate to the furnace; feeding oxygen containing gas to the furnace for fluidizing a bed of particles forming in the furnace; combusting the fuel with the oxygen, whereby the sulfur in the fuel is oxidized to sulfur dioxide; calcining the calcium carbonate to calcium oxide in the furnace and utilizing a portion of the calcium oxide to sulfate a first portion of the sulfur dioxide to calcium sulfate in the furnace; discharging flue gases, containing a second portion of the sulfur dioxide, and particles, including calcium oxide particles, entrained with the flue gases along a flue gas channel from the furnace; separating a first portion of the entrained particles, including a first portion of the entrained calcium oxide particles, from the flue gases in a particle separator having a cut-off size, and returning at least a portion of the separated particles via a return duct to the furnace; conveying a second portion of the entrained particles, including a second portion of the entrained calcium oxide particles, with the flue gases from the furnace to a semi-dry sulfur-reduction stage arranged downstream of the furnace; and reducing the sulfur dioxide content of the flue gases in the semi-dry sulfur-reduction stage, wherein the predetermined d50 particle size of the calcium carbonate containing absorbent is from 10 ?m to 20 ?m, and smaller than 50% of the cut-off size of the particle separator.

12. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant according to claim 11, wherein the predetermined d50 particle size of the calcium carbonate containing absorbent is smaller than 30% of the cut-off size of the particle separator.

13. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant according to claim 11, wherein more than 80% of the entrained calcium oxide particles are in the second portion of the particles and less than 20% of the entrained calcium oxide particles are in the first portion of the particles.

14. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant according to claim 13, wherein more than 90% of the entrained calcium oxide particles are in the second portion of the particles and less than 10% of the entrained calcium oxide particles are in the first portion of the particles.

15. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant according to claim 11, wherein the step of conveying is performed without diminishing the particle size of the entrained calcium oxide particles.

16. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant according to claim 11, wherein the of conveying is performed without slaking of the second portion of the entrained calcium oxide particles.

17. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant according to claim 11, wherein the step of conveying is performed without any external treatment to alter the physical or chemical characteristics of the particle.

18. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler according to claim 11, wherein a ratio of the first feeding rate to the second feeding rate is such that the molar ratio of calcium in the second stream to sulfur in the first stream (the Ca/S molar ratio) is from 2.5 to 3.5.

19. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler according to claim 18, wherein no additional sorbent is fed to the semi-dry sulfur reduction stage.

20. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler according to claim 15, wherein a ratio of the first feeding rate to the second feeding rate is such that the molar ratio of calcium in the second stream to sulfur in the first stream (the Ca/S molar ratio) is from 1.5 to 2.5.

21. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler according to claim 20, wherein no additional sorbent is fed to the semi-dry sulfur reduction stage.

22. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler according to claim 11, wherein a ratio of the first feeding rate to the second feeding rate is such that the molar ratio of calcium in the second stream to sulfur in the first stream (the Ca/S molar ratio) is from 1.0 to 2.0.

23. A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler according to claim 22, wherein additional sorbent is fed to the semi-dry sulfur reduction stage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings.

[0026] FIG. 1 shows a schematic diagram of a circulating fluidized bed boiler with equipment for reducing sulfur oxide emissions according to an embodiment of the present invention.

[0027] FIG. 2 shows a schematic diagram of a circulating fluidized bed boiler with equipment for reducing sulfur oxide emissions according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 schematically illustrates a CFB boiler system 10 that can be operated by using a method according to an embodiment of the present invention. The boiler system 10 comprises a furnace 12, a particle separator 14 with a return duct 16, and a flue gas channel 18 for directing flue gases discharged from the furnace 12 through a stack 20 to the environment. The boiler system 10 comprises conventional means 22 for feeding fluidizing gas, usually, primary air, to the furnace 12 through a bottom grid 24, and means 26 for introducing secondary air at a higher level of the furnace. The boiler system also comprises conventional means 28 for feeding sulfur-containing fuel, such as coal, petcoke or biofuel and, if needed, also inert bed material, such as sand, to the furnace 12. The fuel and inert bed material feeding means 28 comprise, for example, feed hoppers and feed conveyors, such as feed screws or pneumatic feeding systems.

[0029] When using the CFB boiler system 10, the fuel is combusted by the fluidizing air in a fluidized bed of fuel and inert bed material. Thereby, is formed bottom ash that is discharged from the furnace via conventional means 30 for discharging bottom ash comprising, for example, an ash conveyor and ash cooling means, from the furnace 12 to be disposed or for further use. The fluidizing gas flows upwards in the furnace 12 at a relatively high nominal velocity, typically, 3 m/s to 10 m/s, causing entrainment of bed particles by the flue gases formed in the combustion, which bed particles may then be discharged from the furnace 12. The particle separator 14 arranged in the flue gas channel 18 separates from the flue gas particles larger than a cut-off size, typically, about 50 ?m to 100 ?m, and returns them back to the furnace via the return channel 16.

[0030] The boiler system 10 also comprises means 32 for feeding CaCO.sub.3-containing sulfur reducing additive, such as limestone, into the furnace. The means 32 for feeding the sulfur reducing additive comprise advantageously at least a hopper or bin 34 and a limestone crusher 36 for crushing the particles of the sulfur reducing additive to a desired median particle size (d50). Conventionally, limestone is crushed to a median particle size of about 100 ?m to 300 but, according to the present invention, the limestone is crushed, for reasons explained elsewhere in this description, to very-fine particles having a d50 particle size of 10 ?m to 20 ?m. The crushing of the sulfur reducing additive to the desired median particle size may alternatively take place in a separate plant, before the additive is brought to the CFB boiler plant.

[0031] A typical CFB boiler plant generally also comprises many other elements that are needed, e.g., for steam generation, material handling, and flue gas cleaning. However, because they are not of any particular importance to the present invention, they are not shown in FIG. 1.

[0032] When the sulfur-containing fuel is combusted in the furnace 12, the sulfur in the fuel oxidizes to sulfur oxides, mainly SO.sub.2. In the temperatures prevailing in the furnace of a CFB boiler, typically, from 750? C. to 950? C., the CaCO.sub.3 in the sulfur reducing additive is rapidly calcined to CaO, which then combines with SO.sub.2 to form CaSO.sub.3, which again oxidizes to CaSO.sub.4. Because binding of SO.sub.2 to CaSO.sub.4 is a relatively slow process, limestone is conventionally fed into the furnace in a particle size that is larger than the cut-off size of the particle separator. Thereby, CaO particles entrained with the flue gas are multiple times separated from the flue gas in the particle separator and returned back to the furnace. The present method differs from such a conventional method in that, due to the very-fine particle size, preferably, more than 80%, even more preferably, more than 90% of CaO particles entrained with the flue gas are not separated from the flue gas in the particle separator, but they continue with the flue gas along the flue gas channels.

[0033] The bottom ash removed from the furnace 12 by the means 30 comprises, typically, a first portion of the Ca-containing particles formed in the process. Another portion of the Ca-containing particles is conventionally recycled between the furnace 12 and the particle separator 14 until the particles are pulverized in the fluidized bed to a size smaller than the cut-off size of the particle separator 14, whereafter, the particles are discharged from the furnace 12 as a part of fly ash.

[0034] The sulfation of the CaO particles to CaSO.sub.4 takes place mainly on the outer surface of the particles. Therefore, conventional sulfur reduction in the furnace of a CFB boiler tends to form particles that have a dense layer of CaSO.sub.4 around a core of unreacted CaO. Thus, the utilization of the CaO is not complete, and the fly ash and bottom ash contain unreacted CaO. Because the reactive CaO is harmful in the removed ash, it is often necessary to slake the CaO to Ca(OH).sub.2 before the removed ash is ready for disposal or further use. Due to the relatively low utilization of CaO in the furnace, high sulfur reduction level in the furnace has conventionally been achieved only by feeding CaCO.sub.3 in abundance to the furnace, in a Ca/S ratio much higher than one, such as three to four.

[0035] Because of the difficulties related to the use of a high Ca/S ratio, recently, in some CFB boilers, the sulfur reduction in the furnace has been complemented by further sulfur reduction in a second sulfur reduction stage, such as a spray dryer or a dry CFB scrubber, arranged in the flue gas channel downstream of the furnace. Correspondingly, FIG. 1 shows a dry CFB scrubber 38 arranged in the flue gas channel 18.

[0036] When the sulfur reduction in the furnace is complemented by a second sulfur reduction stage, the Ca/S ratio of calcium fed into the furnace is naturally lower, such as one to two. However, a conventional method of feeding relative large limestone particles in the furnace gives always rise to the above-described Ca-particles with a dense layer of CaSO.sub.4 around an unreacted CaO core, and to the above-mentioned problems.

[0037] The present method differs from the above-described conventional practice in that the CaO particles reacting in the furnace can, due to their very-fine particle size, be nearly completely sulfated. Thus, the loss of unreacted CaO to bottom ash is minimized, and the above-mentioned problems are largely eliminated. This leads to improved calcium utilization. It is assumed that, by using the present method, it is possible to decrease overall limestone consumption by at least 10%, and, in some cases, by 20% or even more.

[0038] A CFB scrubber 38 arranged in the flue gas channel 18 comprises a contact reactor 40, a dust separator 42, such as a fabric filter, and a recirculation channel 44 from the bottom of the dust separator 42 back to the contact reactor 40. The CFB scrubber 38 also comprises means 46 for injecting water to the fluidized bed forming in the contact reactor 40 for humidifying and cooling down the fluidized bed. A conventional SCB scrubber also comprises means 48 for injecting second sulfur reducing additive, usually, Ca(OH).sub.2, to the contact reactor 40. The SO.sub.2 in the flue gas then combines with the Ca(OH).sub.2 in the contact reactor 40 to form CaSO.sub.3 and CaSO.sub.4. The means for injecting Ca(OH).sub.2 and water are usually arranged in the contact reactor 40, but in some case, they may alternatively be arranged, for example, in the recirculation channel 44.

[0039] The present method differs from the above-described conventional use of a supplementary CFB scrubber in that due to the very small particle size of the absorbent fed into the furnace, the flue gas entering into the CFB scrubber carries a considerable amount of very-fine CaO particles, which can readily be used as a sorbent in the CFB scrubber. Due to the small size of the CaO particles, there is no need to separate the particles from the flue gas stream upstream of the CFB scrubber and alter their chemical or physical characteristics, i.e., to slake the CaO particles and/or to diminish their particle size, before the particles are used in the CFB scrubber. By utilizing the very-fine CaO particles originating from the furnace as a sorbent in the CFB scrubber, it is possible to eliminate or at least to considerably decrease the need for injecting separate sulfur reducing additive to the CFB scrubber.

[0040] Particles, including fly ash, reaction products formed in the contact reactor 40 and unreacted reagent, are separated from the flue gas in the dust separator 42 downstream of the contact reactor 40. In order to maintain a sufficient particle bed in the contact reactor 40, and also to improve utilization of the reagent, a portion of the particles separated in the dust separator 42 is continuously recycled through the recirculation channel 44 back to the contact reactor 40. Another portion of the particulate matter separated in the particle separator 42 is discharged from the CFB scrubber along a discharge channel 50. Cleaned flue gas is conveyed from the dust separator further to a stack 20 to be released to the environment.

[0041] FIG. 2 shows another preferred embodiment of the present invention, which differs from the embodiment shown in FIG. 1 in that the second sulfur reduction stage comprises a spray dryer 52 instead of a CFB scrubber 38. All elements in FIG. 2 that correspond to the same or similar element in the embodiment shown in FIG. 1 are numbered by the same reference number as in FIG. 1.

[0042] In the embodiment of FIG. 2, the flue gas duct 18 leads to an upper portion of the reaction chamber 54 of the spray dryer 52. In the reaction chamber 54, the sulfur dioxide containing flue gas contacts with humidified sulfur reducing sorbent. Unreacted sorbent, reaction products, and fly ash are conveyed with the flue gas from the reaction chamber 54 of the spray dryer 52 to a dust separator 42, preferably, a fabric filter. Cleaned flue gas is discharged from the fabric filter via a stack 20 to the environment, and fly ash, reaction products, and possible unreacted sorbent are collected via a discharge duct 60 from the bottom of the dust separator 42.

[0043] The humidified sorbent may, in some cases, consist solely of CaO particles entrained to the reaction chamber 54 with the flue gas and water injected with a water feed 56 to the upper portion of the reaction chamber 54. However, usually, the humidified sulfur reducing sorbent comprises, in addition to the CaO particles entrained with the flue gas, sorbent slurry formed in a slurry preparation vessel 64 and injected as small droplets to the reaction chamber 54 through a slurry feed 58. The slurry is usually made by adding water via a water feed 66 to Ca(OH).sub.2 or CaO, to slake the CaO to Ca(OH).sub.2, in a slurry preparation vessel 64. The Ca(OH).sub.2 or CaO may at least partly be collected by the discharge duct 60 from the bottom of the dust separator 42 and also by a discharge duct 62 from the bottom of the reaction vessel 54.

[0044] As explained above, the sulfur reducing sorbent used in the semi-dry sulfur reduction stage may, based on case specific reasons, both in the embodiment shown in FIG. 1 and in the embodiment shown in FIG. 2, originate solely of CaO particles formed in the furnace 12 of the boiler system 10, or of CaO particles formed in the furnace 12 of the boiler system 10, together with additional sorbent originating from a separate supply. However, in all cases, the very-fine limestone particles fed into the furnace give rise to very-fine CaO particles entrained with the flue gas, which minimize, i.e., decrease or totally eliminate, the need for feeding additional sorbent directly to the semi-dry sulfur reduction stage.

[0045] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such a combination is technically feasible.