Method of and apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler

Abstract

Sulfurous fuel and CaCO.sub.3-containing sorbent are combusted in a furnace of a circulating fluidized bed boiler. A dry circulating fluidized bed scrubber includes a reactor with water and Ca(OH).sub.2 feeds for converting SO.sub.2 in the exhaust gas to CaSO.sub.3 and CaSO.sub.4 and a dust separator in gas flow connection with the reactor. A discharge removes CaO-containing bottom ash from the furnace. A classifier classifies a portion of the removed CaO-containing bottom ash into coarse and finer portions. A fine ash channel conveys some of the finer bottom ash portion from the classifier to a grinder. A ground ash channel conveys some of the ground bottom ash portion from the grinder to a hydrator to hydrate CaO in the ash to Ca(OH).sub.2. A hydrated ash channel conveys some of the Ca(OH).sub.2 from the hydrator to the dry circulating fluidized bed scrubber as a sorbent.

Claims

1. A method of combusting sulfurous fuel in a circulating fluidized bed boiler, the method comprising the steps of: (a) feeding sulfurous fuel to a furnace of the circulating fluidized bed boiler, combusting the fuel, and generating SO.sub.2-containing exhaust gas; (b) feeding CaCO.sub.3-containing sorbent to the furnace of the boiler for converting the CaCO.sub.3 to CaO and utilizing a portion of the CaO in the furnace to convert a portion of the SO.sub.2 to CaSO.sub.4; (c) conveying the SO.sub.2-containing exhaust gas along an exhaust gas channel from the furnace (i) to a dry circulating fluidized bed scrubber comprising a reactor having feeds for feeding water and Ca(OH).sub.2 for converting SO.sub.2 in the exhaust gas to CaSO.sub.3 and CaSO.sub.4, and (ii) to a dust separator in gas flow connection with the reactor for separating particulate matter from the exhaust gas; (d) removing CaO-containing bottom ash from the furnace; (e) classifying, in a first stage classifier, at least a portion of the CaO-containing bottom ash removed from the furnace into a coarse bottom ash portion and a finer bottom ash portion, wherein the coarse bottom ash portion consists of particles having a particle size larger than a first predetermined limit; (f) removing the coarse bottom ash portion from the furnace as a first combustion product; (g) conveying the finer bottom ash portion from the first stage classifier to a second stage classifier to classify the finer bottom ash portion to a middle size bottom ash portion and a finest size bottom ash portion; (h) removing the finest size bottom ash portion as a second combustion product; (i) conveying the middle size bottom ash portion to a grinder so as to grind the middle size bottom ash portion to a ground bottom ash portion having a median particle size of a second predetermined limit; (j) conveying at least a portion of the ground bottom ash portion from the grinder to a hydrator so as to hydrate CaO in the ground bottom ash portion with a controlled amount of water or steam to produce Ca(OH).sub.2; and (k) conveying at least a portion of the Ca(OH).sub.2 from the hydrator to the dry circulating fluidized bed scrubber as a sorbent.

2. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 1, wherein the second predetermined limit depends on the CaO content of the middle size bottom ash portion conveyed to the grinder.

3. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 2, wherein the second predetermined limit is lesser the lower is the CaO content of the middle size bottom ash portion conveyed to the grinder.

4. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 1, wherein the second predetermined limit depends on the CaO content of the middle size bottom ash portion conveyed to the grinder by a formula 0.43*M3 m, in which M is the CaO content in percent.

5. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 1, further comprising a step of feeding fresh CaO into the hydrator.

6. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 1, further comprising a step of feeding a portion of the separated particulate matter from the dust separator to the hydrator.

7. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 1, further comprising a step of conveying a portion of the ground bottom ash portion from the grinder directly to the furnace.

8. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 1, further comprising a step of feeding a portion of the separated particular matter from the dust separator directly to the furnace.

9. A method of combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 1, further comprising controlling the grinder on the basis of the CaO content of the bottom ash portion conveyed to the grinder.

10. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler, the apparatus comprising: a feed for feeding sulfurous fuel to a furnace of the circulating fluidized bed boiler for combusting the fuel and generating SO.sub.2-containing exhaust gas; a feed for feeding CaCO.sub.3-containing sorbent to the furnace of the boiler for converting the CaCO.sub.3 to CaO and utilizing a portion of the CaO in the furnace to convert a portion of the SO.sub.2 to CaSO.sub.4; an exhaust channel for conveying the SO.sub.2-containing exhaust gas from the furnace to a dry circulating fluidized bed scrubber comprising a reactor having feeds for feeding water and Ca(OH).sub.2 for converting SO.sub.2 in the exhaust gas to CaSO.sub.3 and CaSO.sub.4; a dust separator in a gas flow connection with the reactor for separating particulate matter from the exhaust gas; a discharge for removing CaO-containing bottom ash from the furnace; a first stage classifier for classifying at least a portion of the removed CaO-containing bottom ash into a coarse bottom ash portion and a finer bottom ash portion, wherein the coarse bottom ash portion consists of particles having a particle size larger than a first predetermined limit; a discharge channel for removing the coarse bottom ash portion from the first stage classifier to a hopper; a second stage classifier for classifying the finer bottom ash portion into a middle size bottom ash portion and a finest size bottom ash portion, wherein the finest bottom ash portion consists of particles having a particle size smaller than a second predetermined limit; a discharge channel for removing the finest bottom ash portion from the second stage classifier; a conveyor for conveying the middle size bottom ash portion to a fine ash channel for conveying the middle size bottom ash portion to a grinder so as to grind the bottom ash portion to a ground bottom ash portion having a median particle size of a third predetermined limit; a ground ash channel for conveying at least a portion of the ground bottom ash portion from the grinder to a hydrator so as to hydrate CaO in the ground bottom ash portion with a controlled amount of water or steam to produce Ca(OH).sub.2; and a hydrated ash channel for conveying at least a portion of the Ca(OH).sub.2 from the hydrator to the dry circulating fluidized bed scrubber as a sorbent.

11. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 10, further comprising a controller for controlling the grinder on the basis of the CaO content of the bottom ash portion conveyed to the grinder.

12. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 11, further comprising a conveyor for conveying a portion of the ground bottom ash portion from the grinder to the furnace.

13. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 10, wherein the second predetermined limit depends on the CaO content of the middle size bottom ash portion conveyed to the grinder.

14. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 13, wherein the second predetermined limit is lesser the lower is the CaO content of the middle size bottom ash portion conveyed to the grinder.

15. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 10, wherein the second predetermined limit depends on the CaO content of the middle size bottom ash portion conveyed to the grinder by a formula 0.43*M3 m, in which M is the CaO content in percent.

16. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 10, further comprising a feed for feeding fresh CaO into the hydrator.

17. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 10, further comprising a feed for feeding a portion of the separated particulate matter from the dust separator to the hydrator.

18. An apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler according to claim 10, further comprising a feed for feeding a portion of the separated particular matter from the dust separator directly to the furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram of a circulating fluidized bed boiler with an apparatus for reducing sulfur oxide emissions according to an embodiment of the present invention.

(2) FIG. 2 shows a schematic diagram of a circulating fluidized bed boiler with an apparatus for reducing sulfur oxide emissions according to a second embodiment of the present invention.

(3) FIG. 3 shows a schematic diagram of a circulating fluidized bed boiler with an apparatus for reducing sulfur oxide emissions according to a third embodiment of the present invention.

(4) FIG. 4 shows a schematic diagram of a circulating fluidized bed boiler with an apparatus for reducing sulfur oxide emissions according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) The schematic diagram of FIG. 1 shows a circulating fluidized bed (CFB) boiler (10) with a dry circulating fluidized bed (CFB) scrubber (22) for reducing sulfur oxide emissions from the boiler. The furnace (12) of the boiler comprises feeds for introducing sulfur-containing fuel (14), such as coal, petcoke or biofuel, CaCO3-containing sulfur sorbent (16), such as limestone, and inert bed material (18), such as sand, into the furnace. The fuel is combusted in the furnace with combustion gas, such as air (not shown in FIG. 1) to generate steam (not shown in FIG. 1). During the combustion, 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 sorbent is calcined to CaO, which combines with SO.sub.2 to CaSO.sub.3, which again oxidizes to CaSO.sub.4. Because the sulfation of the CaO particles takes place mainly on the outer surface of the particles, the utilization of the CaO is not perfect. Thus, the combustion process generally produces SO.sub.2-containing flue gases and ash that contains CaO particles covered by a layer of CaSO.sub.4.

(6) In accordance with the teaching of, for example, U.S. Pat. No. 7,427,384, the sulfur reduction process taking place in the furnace (12) is complemented by further sulfur reduction in a dry circulating fluidized bed (CFB) scrubber (22) arranged in the exhaust gas channel (20) of the boiler. Main elements of the CFB scrubber are a reactor (28), a dust separator (30), and a recirculation channel (34) to circulate a portion of ashes separated by the dust separator back to the reactor. The reactor (28) comprises injectors (24, 26) for injecting Ca(OH).sub.2 and water, respectively, to a fluidized bed forming in the reactor (28) so as to cause the SO.sub.2-containing flue gas to react with the Ca(OH).sub.2 to form CaSO.sub.3 and CaSO.sub.4. The exhaust gas and particles entrained therewith are conveyed from the reactor to the dust separator (30), such as a fabric filter, to separate particulate matter (32) from the exhaust gas. Cleaned exhaust gas is conveyed from the dust separator further to be released to the environment. A portion of the particulate matter separated in the dust separator (30) is recycled back to the reactor (28) along a recirculation channel (34), and another portion of the particulate matter is conveyed along a discharge channel (36) to a hopper (38), as an end product of the scrubber.

(7) According to the present invention, CaO-containing bottom ash (40) is conveyed from the furnace (12) to a first stage classifier (42) so as to separate a portion of the bottom ash (44), a so-called coarse bottom ash portion, having a particle size larger than a predefined limit, to be removed from the CFB boiler to another hopper (46), as a first combustion product. The first stage classifier (42) is advantageously a sieve, but it can be of any other suitable type, such as a low efficiency cyclone or a centrifugal separator. When selecting the type of the classifier, it can be taken into account that a sieve classifies particles almost entirely on the basis of their size, whereas for classifying with a cyclone, the result depends also on the density, or mass, of the particles. Thus, a low efficiency cyclone is especially useful for separating particles having a relatively high density, such as silicates, from the bottom ash. By properly selecting the classifying limit, the calcium oxide content of the coarse bottom ash portion can be clearly less than that of the remaining finer bottom ash portion. Due to the relatively low calcium oxide content, the first combustion product can be used for, for example, landfill, without further treatment.

(8) FIG. 1 also shows an optional second stage classifier (42), such as a cyclone, arranged between the first stage classifier (42) and a grinder (50). In the second stage classifier, the finer bottom ash portion is further classified into a finest bottom ash portion and a middle size bottom ash portion. The finest bottom ash portion, which is rich in CaSO.sub.4, is discharged from the boiler together with the first combustion product or separately as a second combustion product. Typically, both the coarse fraction and the finer fraction are 10% to 60% of the total amount of bottom ash. When the system comprises a second stage classifier (42), typically, the middle size fraction is 10% to 60% and the finest fraction 10% to 40% of the total amount of bottom ash.

(9) The finer or middle size bottom ash portion is conveyed from the first or second stage classifier (42, 42) along a fine ash channel (48) to a grinder (50). The grinder is advantageously a hammer mill, but alternatively, it can be of any other suitable type. Because of the classifying of the ash particles upstream of the grinder, the material to be fed into the grinder is not highly erosive, and the grinder can be of a simple and a low-cost design.

(10) The calcium particles are broken during the grinding, and fresh CaO is exposed from behind a layer of CaSO4. The finer or middle size portion of the CaO-containing bottom ash is ground in the grinder so as to have a predefined particle size distribution. It has been found that the higher is the CaO content of the bottom ash portion, the less there is a need for grinding. Thus, because the desired particle size distribution depends on the CaO content of the finer or middle size bottom ash portion, the system advantageously comprises a controller (74) for controlling the grinder on the basis of the measured CaO content of the bottom ash portion. The measurement of the CaO content can be performed by an on-line analyzer, but, in many cases, it is sufficient to frequently analyze the CaO content, or calcium content, of samples taken from the finer or middle size bottom ash portion. The grinder parameters are then adjusted on the basis of the mass flow of the finer or middle size bottom ash portion by taking into account the CaO content of the bottom ash portion.

(11) The ground bottom ash is conveyed from the grinder along a ground ash channel (52) to a hydrator (54), in which the CaO is hydrated, or dry slaked, by a predetermined amount of water or steam (56) to form Ca(OH).sub.2. The hydrator is advantageously a CFB hydrator, but it can alternatively be of any other suitable type. The surface area and moisture content of the produced Ca(OH).sub.2 particles can be varied in a known way by varying the operation parameters in the hydrator. The calcium containing ash particles, especially, particles with a high CaO content, are broken during the hydration to very small particles, because of a volume change associated with the hydration. The hydrated lime is conveyed from the hydrator along a hydrated ash channel (58) to the reactor (28) of the CFB scrubber (22) to be used as a sorbent.

(12) According to a preferred embodiment of the present invention, a stream of fresh CaO (60) is introduced into the hydrator to be dry slaked therein to form sorbent, to be conveyed to the CFB scrubber, in addition to or instead of, sorbent fed with the Ca(OH).sub.2 feed device (24).

(13) The above-described process of desulfurizing exhaust gas may, in some applications, optionally be complemented by further material flows. For example, FIG. 2 shows an embodiment of the present invention, in which a portion of material collected by the dust separator (30) is conveyed along a channel (62) to the hydrator (54). Thereby, CaO particles entrained by the exhaust gas will be activated and converted to Ca(OH).sub.2 by adding water. The activated particles are then conducted along the channel (58) as sorbent to the reactor (28) of the CFB scrubber (22). This feature is especially advantageous when the amount and CaO content of the fly ash entrained with the exhaust gas are high.

(14) FIG. 3 shows an embodiment of the present invention, in which a portion of activated finer or middle size bottom ash portion is conveyed directly from the grinder (50) back to the furnace (12). Because fresh CaO is exposed from behind the CaSO.sub.4 layer during the activation in the grinder, the ground bottom ash portion can act as an efficient sorbent in the furnace. This will reduce the need for feeding limestone or CaCO.sub.3 into the furnace.

(15) FIG. 4 shows an embodiment of the present invention, in which a portion of the material collected by the dust separator (30), arranged downstream of the reactor (28), is conveyed along a channel (70) to the furnace (12) of the CFB boiler. This material flow is especially useful for oxidizing CaSO.sub.3 in the ash collected by the dust separator (30) to CaSO.sub.4, so as to produce a stable combustion product that can be transported to further use without additional treatment.

(16) The main idea of the present invention is to form sorbent to a dry CFB scrubber from the bottom ash of a CFB boiler. This increases the efficiency of the CFB boiler, saves limestone and lime consumption in the process, and provides the ability to obtain useful final products.

(17) 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. It is, for example, possible that the streams of ground or hydrated calcium containing ash which are, in FIGS. 1 to 4, shown as direct injection into the reactor of the CFB scrubber or into the furnace or the hydrator, are optionally, non-direct streams via a corresponding silo. It is also possible to use different material flows shown in FIGS. 2 to 4 in a single application, for example, by combining the activation of material from the dust separator (30) as in FIG. 2 with the conveying of activated ash from the grinder (50) to the furnace, as shown in FIG. 3. Also, it is possible to have a second stage classifier (42) shown in FIG. 1 in the embodiments of the FIGS. 2 to 4 as well.