AU2010316285A1 - Method and apparatus for treatment of exhaust gas - Google Patents

Abstract

Disclosed is an apparatus for treating SO contained in a combustion gas more readily with high efficiency and at lower cost. Specifically disclosed is an exhaust gas treatment facility (100) comprising a desulfurizing agent supply unit (10), a combustion furnace (20), an exhaust gas temperature decreasing facility (30), and an electrical dust collector (40). The desulfurizing agent supply unit (10) enables the introduction of a desulfurizing agent into a position located adjacent to an upper nose section (21) in the combustion furnace (20) through a desulfurizing agent inlet (14). The facility (100) may be so adapted that an exhaust gas is supplied into the exhaust gas temperature decreasing facility (30) through a smoke path (22), a dust is removed by means of the electrical dust collector (40) while decreasing the temperature of the exhaust gas, and the dust is discharged into air through a smoke stack (49).

Description

DESCRIPTION METHOD AND APPARATUS FOR TREATING EXHAUST GAS Technical Field [0001] The present invention relates to a method and an apparatus for treating exhaust gas in a facility comprising a combustion furnace such as a boiler that burns fuel which includes sulfur component. Background Art [0002] In a coal-fired thermal power station, generation of electricity is performed by burning fuel such as pulverized coal or heavy oil, petroleum coke, and so on, in a combustion furnace such as a boiler. Accordingly, when sulfur component is included in these fuels, sulfur dioxide (SO 2 ) is included in exhaust gas after fuel is burned, and a portion of the SO 2 is oxidized to become sulfur trioxide (SO 3 ). [0003] The exhaust gas from the combustion furnace is normally treated in an exhaust gas treatment unit such as a denitration unit, a gas air heater, an electric dust collector, and a desulfurizing unit provided in a stage subsequent to the combustion furnace. In this exhaust gas treatment unit, when a temperature of the exhaust gas falls to below an acid dew point, SO3 in the exhaust gas ends up condensing as sulfuric acid (H 2

SO

4 ), which results in a gas duct, various kinds of devices, and so 1 on, being corroded. [0004] Known as methods for removing such acidic material,

SO

3 , and so on, in the exhaust gas are a drying type desulfurizing method using ultrafine particles (refer to, for example, Patent Literature 1 (pages 2-4, Figs. 1-4)), a method for removing SO 3 in the exhaust gas (refer to, for example, Patent Literature 2 (pages 1-3, Figs 1-4)), and so on. In the drying type desulfurizing method disclosed in Patent Literature 1, ultrafine particles of calcium oxide (CaO) are injected into an inside of a furnace generating the exhaust gas and/or into a gas duct to adsorb acidic material. Moreover, in the removing method disclosed in Patent Literature 2, S03 is treated, for example, by injecting ammonia between a gas air heater and an electric dust collector in an exhaust gas treatment unit. [Related Art Document] [Patent Literature] [0005] [Patent Literature 1] Japanese Patent Application Laid Open No. H5-269341 [Patent Literature 2] Japanese Patent Application Laid Open No. H10-230130 Summary of the Invention Problem to be Solved by the Invention [0006] In the conventional drying type desulfurizing 2 method disclosed in the aforementioned Patent Document 1. The ultrafine particles are supplied to the inside of the combustion furnace from an ultrafine particle injecting inlet provided in the combustion furnace. However, it is difficult to remove the acidic material with high efficiency, depending on where the injecting position is located. This is the problem of the method disclosed in Patent Document 1. Moreover, in the conventional removing method disclosed in Patent Document 2, since it is required to inject ammonia for treatment of SO 3 , it is difficult to remove SO 3 in the exhaust gas more cheaply and easily. This is the problem of the method disclosed in Patent Document 2. [0007] The present invention has been made in order to solve the above-described problems of the conventional technology, and has an object of providing a method and an apparatus for treating exhaust gas capable of treating S03 in combustion gas with high efficiency and more cheaply and easily. Means for Solving the Problem [0008] In order to solve the above-described problems and achieve the above-described object, a method for treating exhaust gas according to the present invention, where fuel including sulfur component is burned inside a combustion furnace to emit combustion gas from the combustion furnace as the exhaust gas, wherein the 3 combustion furnace includes an upper nose section in an upper side of an interior of the combustion furnace, the upper nose section being configured to narrow an internal space of the combustion furnace, and injecting desulfurizing agent into a vicinity of the upper nose section in the combustion furnace that emits the exhaust gas, using a desulfurizing agent injecting means. [0009] In addition, the method for treating exhaust gas according to the present invention may be configured to, after cooling the exhaust gas emitted from the combustion furnace to a temperature of from 900C to 1200C by an exhaust gas temperature lowering means, supply the exhaust gas to an electric dust collector. [0010] Furthermore, it is preferable that the desulfurizing agent is calcium compound, and the calcium compound includes cement plant dust containing calcium carbonate (CaCO 3 ). [0011] Moreover, the method for treating exhaust gas according to the present invention may be configured by a method where the exhaust gas temperature lowering means cools the exhaust gas indirectly by a gas-water heat exchanging means, or a method where the exhaust gas temperature lowering means cools the exhaust gas directly by spraying water into the exhaust gas. [0012] 4 The vicinity of the upper nose section is, for example, a range of a height direction defined by a base of a triangle of the nose section. [0013] In addition, for example, the desulfurizing agent injecting means includes a pipe, and the pipe is a protruding pipe connected to the combustion furnace and protruding in a horizontal direction toward an interior of the combustion furnace. [0014] A protruding length of the protruding pipe into the interior of the combustion furnace is preferably greater than 0 mm and less than or equal to 600 mm. [00151 An apparatus for treating exhaust gas according to the present invention comprises: a combustion furnace for burning fuel and having an upper nose section formed in an upper side of an interior of the combustion furnace, the upper nose section being configured to narrow an internal space of the combustion furnace; and a desulfurizing agent injecting means for injecting desulfurizing agent into a vicinity of the upper nose section inside the combustion furnace. [0016] The apparatus for treating exhaust gas according to the present invention may be configured to further comprise: an exhaust gas temperature lowering means for lowering a temperature of the exhaust gas from the 5 combustion furnace; and a dust collector for removing dust in the exhaust gas from the exhaust gas temperature lowering means, and such that the exhaust gas temperature lowering means cools the exhaust gas emitted from the combustion furnace indirectly by a gas-water heat exchanging means or directly by a water-spray device. Effects of the Invention [0017] According to the present invention, SO 3 in combustion gas can be treated with high efficiency and more cheaply and easily. Brief Description of the Drawings [0018] FIG. 1 is a schematic system diagram showing an example of an exhaust gas treatment facility for carrying out a method for treating exhaust gas according to an embodiment of the present invention. FIG. 2 is a horizontal cross-sectional view of an injecting position of desulfurizing agent at 0.8 M above and 0.4 L below in a combustion furnace in an embodiment of the present invention. FIG. 3 is a simulation result concerning a supply position of the desulfurizing agent in the method for treating exhaust gas according to the embodiment of the present invention. FIG. 4 is a simulation result concerning a protruding pipe in the method for treating exhaust gas according to the embodiment of the present invention. 6 Best Mode for Carrying Out the Invention [0019] Preferred embodiments of a method and an apparatus for treating exhaust gas according to the present invention will be described hereafter, with reference to the accompanying drawings. [0020] FIG. 1 is a schematic system diagram showing an example of an exhaust gas treatment facility for carrying out the method for treating exhaust gas according to an embodiment of the present invention. As shown in FIG. 1, an exhaust gas treatment facility 100 comprises: a desulfurizing agent supply unit 10 for storing desulfurizing agent transported by a truck 90 or the like and supplying the desulfurizing agent to a combustion furnace 20 such as a boiler; and the combustion furnace 20 for burning fuel such as pulverized coal, heavy oil, and petroleum coke. A type of the combustion furnace 20 is not limited particularly. A boiler, particularly a pulverized coal burning boiler, is preferably used as the combustion furnace 20. [0021] In addition, the exhaust gas treatment facility 100 comprises: an exhaust gas temperature lowering unit 30 for lowering a temperature of the exhaust gas emitted from the combustion furnace 20; and an electric dust collector 40 for capturing dust in the exhaust gas emitted from the exhaust gas temperature lowering unit 30. 7 Further, exhaust gas emitted from this electric dust collector 40 is transported by a blower 48 to be emitted to the atmosphere via a chimney 49. [0022] The desulfurizing agent supply unit 10 is configured comprising a storage tank 11 for storing the transported desulfurizing agent, and a quantitative discharging mechanism 12 for appropriately supplying the desulfurizing agent stored in the storage tank 11 to the combustion furnace 20, and a blower 13. The desulfurizing agent transported from the storage tank 11 by the quantitative discharging mechanism 12 and the blower 13 is injected into the combustion furnace 20 via, for example, a desulfurizing agent supply pipe (a pipe) not shown which is connected to a desulfurizing agent injecting inlet 14 provided in a wall section 20a of the combustion furnace 20. The desulfurizing agent injecting inlet 14 is preferably disposed in a side wall of the combustion furnace 20. [0023] Note that the desulfurizing agent injecting inlet 14 in the desulfurizing agent supply unit 10 is formed to enable injection of desulfurizing agent 15 into a vicinity position of a nose section 21 (also sometimes referred to as upper nose section 21) formed upwardly in the combustion furnace 20. The nose herein is a protruding object provided in a furnace and functioning to divert combustion gas to prevent the combustion gas 8 from flowing through a short path, but cause it to pass through a superheater, thereby securing a retention time of the combustion gas. [0024] In addition, "the vicinity position of the nose section 21" is a portion shown by H (or L+M) in FIG. 1. That is, "the vicinity position of the nose section 21" is included in a range of a height direction defined by a base of a triangle of the nose section 21. Also, "the vicinity position of the nose section 21" is a space in the combustion furnace included in the range of the height direction, but a space where a superheater 20b is not present. The superheater 20b extends from an upper side of the nose section 21 to the space of the nose section 21. The desulfurizing agent is supplied to that space. [0025] The number of the desulfurizing agent injecting inlets 14 is one, or two or more. Among, these numbers in view of appropriately dispersing the desulfurizing agent in the combustion furnace 20, two or more, particularly four to six, is preferable. If a position of the desulfurizing agent injecting inlet 14 is in the previously mentioned range of "H" of the nose section, a plurality of the desulfurizing agent injecting inlets 14 may be provided in the height direction. [0026] Calcium compound is preferable as the desulfurizing 9 agent 15. Calcium hydroxide, calcium oxide, or calcium carbonate are preferable, More preferably, the desulfurizing agent 15 is cement plant dust comprising calcium carbonate as a main component. [0027] Cement plant dust is, for example, recovered from exhaust gas of a process for manufacturing cement raw material. It has a particle diameter of about two microns, and is available at extremely low cost and in large quantities. Injecting such a desulfurizing agent 15 into the vicinity position of the upper nose section 21 in the combustion furnace 20 causes the injected desulfurizing agent 15 to capture SO 3 generated by combustion of fuel more optimally and with higher efficiency. Examples of the cement plant dust include dust recovered from a cement raw material pulverizing process or dust recovered from cement calcination exhaust gas. [0028] Specifically, for example, when calcium carbonate is used as the desulfurizing agent 15, a decarboxylation reaction causes the calcium carbonate to become calcium oxide (CaCO 3 -CaO), and a desulfurizing reaction causes this calcium oxide CaO to react with sulfur dioxide SO 2 to become calcium sulfate (CaO+SO 2 +0.50 2 -CaSO 4 ). In addition, the calcium oxide CaO after the decarboxylation reaction captures SO 3 . It has been confirmed by the inventors of the present invention that 10 injecting the desulfurizing agent 15 into the vicinity position of the upper nose section 21 of the combustion furnace 20 results in this kind of desulfurizing reaction being most activated. [0029] A desulfurizing capacity of the desulfurizing agent increases as specific surface area of the desulfurizing agent increases. Described herein is the case where cement plant dust is used as the desulfurizing agent 15. Physical property values when cement plant dust (including 75 percent by mass of calcium carbonate, 13 percent by mass of silica, 7 percent by mass of alumina, 2 percent by mass of iron oxide, and 3 percent by mass of others) is calcinated are shown in Table 1. [0030] [Table 1] Calcination Conditions Uncalcinated 10000C 12000C 1400'C Average Pore nm 20.1 19.5 21.5 21 Diameter Total Pore Volume ml/g 0.0325 0.0557 0.0485 0.0339 Multipoint BET Specific Surface Area m 2 /g 6.48 11.4 9.04 6.44 The specific surface area of the cement plant dust, when calcinated at 10000C, increases to 1.8 times that before calcination. On the other hand, when a calcination temperature is raised to 1200*C and 1400'C, 11 the specific surface area lowers. [0031] Moreover, as a result of TG-DTA measurement of the cement plant dust, the decarboxylation reaction in the cement plant dust begins from around 700*C and shows a maximum peak at 741'C. When heat is further applied, structural change accompanied by heat generation is confirmed from around 12000C. A peak temperature of that change is 1288*C. [0032] It is inferred from these findings that the cement plant dust, when it reaches a temperature of 12000C or more, undergoes structural change whereby its specific surface area lowers and its desulfurizing capacity lowers. Therefore, a temperature of under 12000C is preferable in desulfurization in a furnace, when cement plant dust is used as the desulfurizing agent. [0033] Regarding amount of supply of desulfurizing agent with respect to fuel, a molar ratio (Ca/S) of calcium (Ca) to sulfur portion (S) in the desulfurizing agent provided in the fuel is preferably between 0.5 and 3, more preferably between 1 and 2.5. If the molar ratio exceeds 3, the amount of dust increases. [00341 That is, experiments performed by the inventors of the present invention have made it clear that when the desulfurizing agent 15 is injected into a position lower 12 than the upper nose section 21 of the combustion furnace 20, CaO is used for reformation of coal ash or the like because since temperature in the furnace is high, and there may occur also a reverse reaction to desulfurization. The experiments have also made it clear that when the desulfurizing agent 15 is injected into a position higher than the upper nose section 21 of the combustion furnace 20, capture of SO 3 becomes insufficient because since temperature in the furnace is low. [0035] On the other hand, as described above, the desulfurizing agent 15 is injected into a vicinity position of the upper nose section 21. This allows appropriate time of contact between CaO and S03 to be obtained. Also, CaO is dispersed effectively in a layer of gas in the combustion furnace 20, thereby capturing

SO

3 . Accordingly, it is expected that desulfurizing reaction becomes more active. This makes it possible to prevent a concentration of SO 3 being raised locally in the combustion furnace 20 causing condensation that results in sulfuric acid being generated and corroding places to which the sulfuric acid attaches. [0036] Now, in order to evaluate a dispersion state of dust in the nose section, a simulation of particle dispersion was performed. Results of the simulation are indicated below. 13 STAR-CD (registered trademark) Version 3.26 was used as simulation software. Simulation conditions were as follows. (1) Combustion conditions Properties of coal used in combustion calculations were set to those of generally-used coal as conditions. (i ) 'Air ratio: 1.17 (H ) 'Total moisture of coal [percent by mass as received basis]: 9 percent by mass (1) -Industrial analysis of coal [percent by mass air dried basis]: 'water 3 percent by mass, -ash 13 percent by mass, -volatile component 34 percent by mass, -fixed carbon 50 percent by mass. (iv) *Elemental analysis of coal [percent by mass dry ash-free basis]: -carbon 83 percent by mass, -hydrogen 5 percent by mass, -oxygen 9 percent by mass, -nitrogen 2 percent by mass, 'sulfur 1 percent by mass. (2) Calculation models The following calculation models were used in combustion, heat transfer and fluid calculation. -Two-phase flow model: Lagrangian two-layer flow model -Turbulence model: k-c model -Volatile component emission model: overall first-order 14 reaction model -Gas combustion model: Eddy-breakup model -Char combustion model: shrinking core first-order reaction model *Radiant heat transfer model: Discrete transfer method (3) Boundary conditions The following conditions were set as boundary conditions. 'Boiler exit target oxygen concentration: 3 percent by volume Boiler exit target temperature: 3700C [0037] It was confirmed that a temperature simulation result matches well with actual measured values of temperature in the furnace in the actual apparatus. A gas temperature indicated a maximum of about 18000C at a burner vicinity, and was approximately 1200*C at a nose section entrance, 10000C at a nose section exit, and 7000C at a superheater exit. It is known that regarding a particle having a particle diameter of several tens of pm or less, temperature of the particle becomes identical to the gas temperature in no more than 0.1 seconds. Therefore, the temperature of particles injected into the furnace may be regarded as being equal to the gas temperature. [0038] FIG. 3 shows results of simulating a dispersion situation of particles and a temperature history of 15 particles when the desulfurizing agent is injected to the upper side of the nose section, to the nose section, and to the lower side of the nose section. As shown in FIG. 3(a), when the desulfurizing agent is injected to the upper side of the nose section, there is little disturbance in a gas flow, and the particles flow straight in a gas flow direction from an injecting inlet. As a result, variations in particle temperature do not occur, and a dispersion state is not good. Moreover, as shown in FIG. 3(b), when the desulfurizing agent is injected to the nose section, there is a disturbance in gas flow in the nose section, hence the particles are also drastically disturbed back and-forth and from left to right to be dispersed overall. As a result, a wide-ranging distribution occurs in particle temperature at the nose section exit. The disturbance causes a retention time of the particles to be lengthened. [0039] Furthermore, as shown in FIG. 3(c), when the desulfurizing agent is injected to the lower side of the nose section, the particles, after once having their temperature raised to 1200*C or more, enter the nose section to be drastically mixed in the nose section and dispersed overall. As previously mentioned, in a lower region of the nose section, a structural change of the particles occurs, whereby reaction activity of the desulfurizing agent lowers. These results make it clear 16 that the desulfurizing agent must be injected to the nose section. [0040] A pipe acting as a desulfurizing agent injecting means is connected to the combustion furnace 20, whereby the desulfurizing agent is supplied to an interior of the combustion furnace. The pipe may have a structure not protruding into an interior of the combustion furnace 20 (protruding length is zero), but is preferably a protruding pipe protruding in a horizontal direction toward an interior of the combustion furnace. This enables dispersion of the desulfurizing agent to be improved and desulfurizing rate to be improved. A protruding length of the protruding pipe into the interior of the combustion furnace is greater than zero (0) and not more than 600 mm, and, preferably, 100-500 mm. If the protruding length is too long, the protruding pipe becomes uneconomical and also difficult to construct. The protruding length herein is a distance from an inner wall of the combustion furnace 20 to a tip of the protruding pipe. [0041] A simulation result according to presence/absence of the protruding pipe is shown herein. Simulation conditions are as mentioned above. The amount of injected dust was set to 243 kg/hour, the injection air speed was set to 65 m/s, the temperature of injected air was set to 25*C, the diameter of desulfurizing agent 17 injecting pipe was set to 932, and the injecting position of the desulfurizing agent was set to 15 m from a bottom of the boiler. The case where a length of the protruding pipe was set to 500 mm and the case where there is no protruding pipe were evaluated. Results of this evaluation are shown in FIG. 4. [0042) FIG. 4 shows a locus of dust particles. It is clear from FIG. 4(a) that particles are more widely dispersed in the case where there is a protruding pipe than in the case where there is no protruding pipe (protruding length is 0 mm) shown in FIG. 4 (b). In the case where there is no protruding pipe, the particles gravitate toward the wall of the combustion furnace. This result shows that in the case where there is a protruding pipe, the desulfurizing agent is more dispersed and moreover has a longer retention time. This allows an improvement in desulfurizing rate to be achieved. Reasons for advantages resulting from the protruding pipe are not clear, but it is assumed to be because a circling flow of the gas inside the furnace is stronger at a wall of the furnace than in a center of the furnace and that in order to disperse the dust particles widely, this strong flow should be avoided. [0043] Note that a preferred internal furnace temperature of the combustion furnace 20 when injecting the aforementioned desulfurizing agent 15 into the vicinity 18 position of the upper nose section 21 is in a range of 1050*C-1150 0 C. Then, the exhaust gas having S02 or SO 3 removed inside the combustion furnace 20 in this way is emitted from the combustion furnace 20 via a gas duct 22 to be supplied to the exhaust gas temperature lowering unit 30 of a subsequent stage. [0044] Since the desulfurizing reaction in the furnace takes place in a high temperature atmosphere, a CaSO 4 formation reaction caused by a reaction of CaO after the decarboxylation reaction with SO 2 and 02, and a decomposition reaction (reverse reaction) of the CaSO 4 occur simultaneously. A reaction rate constant of the formation reaction can be expressed by an Arrhenius equation. It has been confirmed by results of experiments by the inventors that the reaction rate constant can be expressed by Ks=7.7x0 3 exp(-67000/RT). [0045] According to this expression, the higher the temperature, the more the formation reaction proceeds. On the other hand, the decomposition reaction is dominated by an equilibrium reaction. As a result of calculating existence conditions of CaSO 4 and SO 2 , it was confirmed that the decomposition reaction occurs at 1050 0 C and above, and that, at 11500C and above, almost the entire amount is decomposed. It was judged from the above results that, in view of the formation reaction and the decomposition reaction, 19 the desulfurizing reaction preferably has a temperature range of from 1050'C to 1150*C. [0046] The exhaust gas temperature lowering unit 30 is configured from, for example, a gas air heater 31 and a gas-water heat exchanger 32 or water-spray device 33. Considered here as methods for lowering a temperature of the exhaust gas are the three methods: (1) improving performance of the gas air heater; (2) indirect cooling; and (3) direct cooling. In the case of (3) direct cooling (that is, cooling by, for example, spraying water into the exhaust gas), there is a possibility that dust included in the exhaust gas attaches to a device interior in the exhaust gas temperature lowering unit 30 thereby causing a clogging, and so on. Therefore, in the present embodiment, although (3) direct cooling may also be adopted, (2) indirect cooling is preferably adopted. [0047] Specifically, heat of the exhaust gas undergoes heat exchange, by means of circulating water in the gas water heat exchanger 32 disposed on a downstream side of the gas air heater 31. The circulating water is used for preheat of water supplied to the combustion furnace 20. Conventionally, the amount of SO 3 in the combustion furnace 20 is approximately 1% of that of S02, and an acid dew point thereof is about 120*C-130*C. Accordingly, heat recovery from the exhaust gas is limited to before the temperature of the exhaust gas is lowered to a 20 temperature of about 150*C. [0048] In contrast, in the method for treating exhaust gas according to the present embodiment, since SO 3 in the exhaust gas is removed beforehand by the desulfurizing agent 15 injected into the vicinity position of the upper nose section 21 in the combustion furnace 20, it has become possible to lower the acid dew point significantly. Specifically, this allows the exhaust gas to be cooled such that the temperature of the exhaust gas is lowered, for example, to approximately 1000C. It has become clear that resultant increase in amount of heat recovery allows energy efficiency to be significantly improved. [0049] Moreover, incidentally to this, it is no longer required to configure various devices provided in the exhaust gas temperature lowering unit 30 by expensive corrosion-resistant material. For example, a material of regions in contact with the exhaust gas of the gas-water heat exchanger 32 can be configured as a low-cost carbon steel material. Note that lowering the temperature of the exhaust gas by this kind of exhaust gas temperature lowering unit 30 is understood to greatly affect maintenance and improvement of dust collecting performance of the electric dust collector 40 in a next stage. [0050] That is, in the exhaust gas treatment facility 100 21 according to the present embodiment, removing SO 3 in the exhaust gas by the desulfurizing agent 15 inside the combustion furnace 20 allows the problem of corrosion and so on to be solved to a certain extent. However, removing SO 3 from the exhaust gas leads to dust collecting performance of the electric dust collector 40 lowering significantly. [0051] Generally, dust collecting performance of the electric dust collector 40 is assumed to be determined by each of the following elements: (1) temperature of the exhaust gas; (2) velocity of the exhaust gas (flow velocity); and (3) concentration of SO 3 . It is assumed that the higher is (3) concentration of SO 3 , the more improved is the dust collecting performance. In the exhaust gas treatment facility 100 according to the present embodiment, since SO 3 is removed by the desulfurizing agent 15 injected into the vicinity position of the upper nose section 21 in the combustion furnace 20. When the exhaust gas in a state of low S03 concentration is supplied to the electric dust collector 40, a desired dust collecting effect can no longer be obtained. [0052] Therefore, by providing the exhaust gas temperature lowering unit 30 between the combustion furnace 20 and the electric dust collector 40 and thereby lowering the temperature of the exhaust gas emitted from the 22 combustion furnace 20, volume of the exhaust gas is reduced and flow velocity of the exhaust gas is lowered. As a result, the concentration of SO 3 in the exhaust gas attains a level having no effect on dust collecting performance of the electric dust collector 40, whereby dust collecting performance can be maintained and improved. Examples [0053] The method for treating exhaust gas according to the present invention is described specifically below by way of examples. The test machine used in the examples is an 80 tons of steam/hour boiler in an electric power generating facility shown in FIG. 1. Pulverized coal and air were supplied to the pulverized coal burning boiler. [0054] The desulfurizing agent used is the aforementioned cement plant dust recovered from cyclone exhaust gas in the raw material pulverizing process of a cement plant. A chemical composition of the cement plant dust was measured by fluorescent X-ray analysis. The result is that CaO has 60.6 percent by mass, SiO 2 has 20.8 percent by mass, and A1 2 0 3 has 10.3 percent by mass. In addition, the weight-average of the particle diameter of the cement plant dust used was about 2 microns. [0055] Injecting positions in examples 1 and 2 shown below are in-furnace a, and injecting positions in example 3 23 are in-furnace P. The injecting positions of in-furnace a are four places A, B, C, and D at a height 0.8 M above a vertex of the nose section 21 shown in FIG. 2(a) (a vertex of a triangle of the nose section 21 in FIG. 1), and three places E, F, and G at a height 0.4 L below the vertex of the nose section 21 shown in FIG. 2(b) (total of seven places). [0056] On the other hand, the injecting positions of in furnace D are the three places E, F, and G at a height 0.4 L below the vertex shown in FIG. 2(b). In FIG. 2(a), the desulfurizing agent is supplied avoiding positions where the superheater 20b is present. B and C in FIG. 2(a) are positioned intermediate between a central point and end points of a side surface. Moreover, E, F, and G in FIG. 2(b) are positioned at central line portions of each of side surfaces of the combustion furnace. [0057]

SO

3 measurement results obtained in the examples are shown in Table 2. Note that measurement of SO 3 was performed at an inlet of the electric dust collector. [0058] [Table 2] 24 Desulfurizing Agent SOx S03 Concentration Concentration Type Injecting Ca/S (ppm) (ppm) Position Example 1 0.93 200 Less Than In-Furnace 0.05 Example 2 Cement 2.06 180 Less Than Plant Dust 0.05 Example 3 In-Furnace 2.92 150 Less Than /3 0.05 (Example 1) In example 1, the cement plant dust was injected into the furnace such that an SOx concentration of S0 2

+SO

3 within the furnace was 200 ppm and a molar ratio of Ca/S was 0.93. The result was an SO 3 concentration of less than 0.05 ppm. (Example 2) In example 2, when SOx concentration within the furnace was 180 ppm and molar ratio of Ca/S of cement plant dust injected into the furnace was 2.06, an SO 3 concentration was less than 0.05 ppm similarly to in example 1. (Example 3) In example 3, when SOx concentration within the furnace was 150 ppm and molar ratio of Ca/S of cement plant dust injected into the furnace was 2.92, an SO 3 concentration was less than 0.05 ppm similarly to in examples 1 and 2. Note that in examples 1, 2, and 3, the SOx 25 concentrations in cases where cement plant dust was not injected were the same as respective concentrations prior to desulfurizing. [0059] On performing an estimate of heat balance, it has become clear that the exhaust gas treatment facility 100 according to the present embodiment allows the acid dew point of the exhaust gas to be set lowered from 1260C to less than 88 0 C. According to this, even when the gas water heat exchanger 32 on a downstream side of the gas air heater 31 in the exhaust gas temperature lowering unit 30 collects heat of about 500C from the exhaust gas with a temperature of about 150*C on passing through the gas air heater 31, and use this heat as a preheat source for water supplied to the combustion furnace 20, corrosion due to condensation of SO 3 may be prevented. Specifically, the electric power-generating facility of examples 1-3 has made it possible for steam for water supply heating in the combustion furnace to be reduced by 2.5 ton/hour, thus enabling energy efficiency to be improved by about 3 percent. [0060] As mentioned above, the exhaust gas treatment facility 100 according to the present embodiment allows

SO

3 in exhaust gas to be treated with high efficiency and more cheaply and easily, and makes it possible to use thermal energy with high efficiency while preventing corrosion and so on of the facility. 26 Description of reference numerals [0061] 10 desulfurizing agent supply unit 11 storage tank 12 quantitative discharging mechanism 13 blower 14 desulfurizing agent injecting inlet 15 desulfurizing agent 20 combustion furnace 20a wall section 20b superheater 21 nose section 22 gas duct 30 exhaust gas temperature lowering unit 31 gas air heater 32 gas-water heat exchanger 33 water-spray device 40 electric dust collector 48 blower 49 chimney 100 exhaust gas treatment facility 27

Claims (9)

1. A method for treating exhaust gas, where fuel including sulfur component is burned inside a combustion furnace to emit combustion gas from the combustion furnace as the exhaust gas, wherein the combustion furnace includes an upper nose section in an upper side of an interior of the combustion furnace, the upper nose section being configured to narrow an internal space of the combustion furnace, and the method includes injecting desulfurizing agent into a vicinity of the upper nose section in the combustion furnace that emits the exhaust gas, using a desulfurizing agent injecting means.

2. The method for treating exhaust gas according to claim 1, further comprising: after cooling the exhaust gas emitted from the combustion furnace to a temperature of from 90*C to 120'C by an exhaust gas temperature lowering means, supplying the exhaust gas to an electric dust collector.

3. The method for treating exhaust gas according to claim 1 or 2, wherein the desulfurizing agent is calcium compound, and the calcium compound includes cement plant dust containing calcium carbonate (CaCO 3 ). 28

4. The method for treating exhaust gas according to claim 2 or 3, wherein the exhaust gas temperature lowering means cools the exhaust gas indirectly by a gas-water heat exchanging means or directly by a water-spray.

5. The method for treating exhaust gas according to any one of claims 1 to 4, wherein the vicinity of the upper nose section is a range of a height direction defined by a base of a triangle of the nose section.

6. The method for treating exhaust gas according to any one of claims 1 to 5, wherein the desulfurizing agent injecting means includes a pipe, and the pipe is a protruding pipe connected to the combustion furnace and protruding in a horizontal direction toward an interior of the combustion furnace.

7. The method for treating exhaust gas according to claim 6, wherein a protruding length of the protruding pipe into the interior of the combustion furnace is greater than 0 mm and less than or equal to 600 mm.

8. An apparatus for treating exhaust gas, comprising: 29 a combustion furnace for burning fuel and having an upper nose section formed in an upper side of an interior of the combustion furnace, the upper nose section being configured to narrow an internal space of the combustion furnace; and a desulfurizing agent injecting means for injecting desulfurizing agent into a vicinity of the upper nose section inside the combustion furnace.

9. The apparatus for treating exhaust gas according to claim 8, further comprising: an exhaust gas temperature lowering means for lowering a temperature of the exhaust gas from the combustion furnace; and a dust collector for removing dust in the exhaust gas from the exhaust gas temperature lowering means, and wherein the exhaust gas temperature lowering means cools the exhaust gas emitted from the combustion furnace indirectly by a gas-water heat exchanging means or directly by a water-spray device. 30