Combustion exhaust gas treatment system

Abstract: Five combustion exhaust gas treatment systems capable of removing dust and selenium (Se) in combustion exhaust gas and making it harmless are proposed: (1) the combustion exhaust gas is cooled to 350°C or less, the dust is separated, the Se is transformed into an insoluble compound, and Se is separated; (2) combustion exhaust gas is cooled to 350°C or less, the dust is separated, Se elution preventive agent is added, and the dust is formed into scale; (3) dust in the combustion exhaust gas is collected by a dry dust collector, the dust is formed into a slurry by making use of part of the circulation liquid in an desulfurization apparatus, and tetravalent Se in the dust slurry is transformed into an insoluble compound, which is separated into solid and liquid; (4) dust is separated from combustion exhaust gas by a dust collector and is heated to gasify Se, and is introduced into desulfurization apparatus, etc., and formed into a slurry, and the Se is transformed into an insoluble compound; and (5) a treating agent for making tetravalent Se insoluble is added to a part of the circulation liquid in a desulfurization apparatus, and is sprayed into combustion exhaust gas to make Se insoluble.

Description: FIELD OF THE INVENTION AND RELATED ARTS The present invention relates to a combustion exhaust gas treatment apparatus and method and more particularly to a combustion exhaust gas treatment apparatus and method capable of easily removing selenium (Se) from combustion exhaust gas containing Se components, and making harmless the Se components. Hitherto, a combustion exhaust gas treatment apparatus installed in a thermal power plant or the like, comprises a dust collector (usually an electrostatic precipitator) for removing fly ash and other dust from the combustion exhaust gas, and combustion exhaust gas desulfurization apparatus for absorbing sulfurous acid in the combustion exhaust gas is generally employed. Moreover, conventionally, as the combustion exhaust gas treatment system installed in a thermal power plant or the like, a combustion exhaust gas treatment system is generally employed which comprises a dry dust collector (usually an electrostatic precipitator) for removing fly ash and other dust from the combustion exhaust gas, and a wet combustion exhaust gas desulfurization apparatus for absorbing sulfurous gas in the combustion exhaust gas by contacting it with an absorbent slurry (for example, slurry containing calcium compound) in an absorption column and separating and recovering gypsum as byproduct from the slurry in the absorption column. Recently, handling of harmful impurities contained in the combustion exhaust gas aside from sulfur oxides is posing a problem. In particular, in the combustion exhaust gas treatment system for coal fired boilers, the harmfulness of selenium (Se) contained at a maximum level of about 10 mg/kg in coal is a problem lately, and its harmless treatment is demanded. Meanwhile, Se exists as tetravalent Se (main form: selenious acid SeO 3 2- ) which is easy to treat by making it insoluble by a treating agent, and hexavalent Se (main form: selenic acid SeO 4 2- ) which is hard to treat by making it insoluble, and in particular the hexavalent Se is high in solubility (solubility at 20°C is 95 %) and is easy to elute. Besides, this Se has a toxicity similar to that of arsenic compound, and disaster cases and emission regulations are known overseas, and it is newly added to the list of regulated items also in Japan, and is controlled by the environmental standard (0.01 mg/liter), discharge standard (0.1 mg/liter), and elution standard in landfill (0.3 mg/liter). FIG. 7 shows an example of prior art of combustion exhaust gas treatment system of this type (an example of combustion exhaust gas treatment system for coal fired boiler). In FIG. 7 and FIG. 8, the combustion exhaust gas A emitted from a coal fired boiler 1 is sent into a denitration apparatus 2 installed downstream of the boiler 1 to be rid of nitrogen oxides (NOx), and passes through an air heater 3 and a heat recovery unit 4 of gas-gas heater (GGH), and is introduced into an electrostatic precipitator (EP) 5, in which fly ash and dust are removed. In succession, the combustion exhaust gas is guided into a wet combustion exhaust gas desulfurization apparatus 7 by a fan 6, and sulfurous gas is removed in this desulfurization apparatus 7, and after passing through a reheater 8 of the gas-gas heater (GGH), it is led into a stack 10 by a fan 9, and is released into the atmosphere through the stack 10 (FIG. 8). On the other hand, fly ash and dust removed in the electrostatic precipitator 5 are discharged from plural hoppers 5a (dust recovery units) formed in the electrostatic precipitator 5, and are conveyed and collected in batch by a conveyor 11. Thus collected dust B is either recycled as cement material or the like, or discarded in an ash disposal yard (FIG. 7). Herein, the desulfurization apparatus 7 comprises an absorption column, for example, in which combustion exhaust gas is introduced, and by contact of combustion exhaust gas with absorbent slurry (usually slurry containing calcium compound) in this absorption column, the sulfurous acid in the combustion exhaust gas is absorbed in wet process, and usually from the slurry in the absorption column, gypsum is separated and collected as byproduct. Incidentally, the heat recovery unit 4 of the gas-gas heater (GGH) may be also disposed immediately before the desulfurization apparatus 7 as shown in FIG. 9. In these combustion exhaust gas treatment systems, most of Se in coal (Se in combustion exhaust gas) is condensed at the downstream side of the air heater 3 (that is, the position before introduction into the electrostatic precipitator 5), and is removed by the electrostatic precipitator 5 in a state being contained in the dust in combustion exhaust gas, and is directly mixed in the refuse in the ash disposal yard or in the cement material. To render Se harmless by conforming to the elution standard, it requires a complicated and costly aftertreatment of, for example, diluting the ash removed by the electrostatic precipitator 5 in a huge volume of water. British Patent 1,382,232 describes a process for the recovery of selenium from waste combustion gas, particularly the waste gas from a glass melting furnace. Waste gases discharged from such furnaces contain a selenium component in the form of elemental selenium or a compound such as selenium dioxide. The selenium component is toxic and must be separated out and recovered in order to prevent atmospheric pollution. In order to achieve this separation a process is proposed which comprises a first step of contacting the waste gas having been cooled to about 250 - 300°C, with an aqueous absorption solution comprising an alkali metal sulfite and/or an alkali metal bisulfite in an absorption tower so that at least a part of the selenium component is absorbed and the humidity of the gas is increased. The process further comprises a second step of passing the waste gas treated in the first step through a glas fiber filter (demister) kept in a moist condition to collect at least some of the remaining elemental selenium and selenium compound on the filter, and in a third step acid, alpha acid H 2 SO 4 , is added to the absorption solution discharged from the first and second steps so as to reduce the selenium component dissolved in the absorption solution to elemental selenium which is precipitated from the solution and the elemental selenium so precipitated is recovered. A method for separating mercury and selenium from roasting gases from roasting zinc concentrates is described in Erzmetall, 30(1977)12, pp 555-604. The method for separating mercury and selenium from exhaust gas includes introducing the exhaust gas from the roasting furnace for Zn-concentrate to a boiler, cooling and then separating the dust treated with an electric dust collector. For separating selenium calcium carbonate, CaCo 3 , is added into the rinsed precipitate and then mercury is separated and subsequently water, H 2 SO 4 and SO 2 are added. Such exhaust gas from the roasting furnace is different from combustion gas, especially of a thermal power plant. OBJECTS AND SUMMARY OF THE INVENTION In the light of the prior art, it is an object of the invention to present a combustion exhaust gas treatment apparatus and method capable of more easily removing and making harmless the Se components contained in combustion exhaust gas to the extent necessary to meet generally adopted environmental standards for the purity of combustion exhaust gas, in particular of thermal power plants. The invention achieving this object and its preferred embodiments are defined in the appended claims. In the combustion exhaust gas treatment of the invention, most of the Se in the combustion exhaust gas is condensed while being cooled by the cooling means, and is removed by the dust collecting means in a state being contained in the dust. In consequence, treating agent is added by an Se treating means to the dust when separated by the dust collecting means, and the existent form of the Se in the dust is transformed into an insoluble compound. The method of making Se insoluble may be realized in various forms. One representative form includes forming a slurry by adding water to the dust containing Se components separated and recovered by the dust collecting means, adding treating agent to the slurry to make Se insoluble, and separating the slurry into solid and liquid. Another form includes spraying a solution of treating agent to the dust to immerse it uniformly, and making Se insoluble. If the dust is discarded the same way as in the prior art, the Se elution standard is satisfied, and the Se is made harmless easily without requiring complicated after-treatment. By making Se insoluble only in the dust separated and collected from the specific recovery unit at the outlet side of the combustion exhaust gas in the dust collecting means, the required amount of treating agent and capacity of Se treating means can be reduced, and the Se is made harmless easily and at low cost. According to a study made by the present inventors, more Se is contained (deposited) in the smaller particle size dust (ash) separated and collected from the specific recovery unit at the outlet side, and the Se is made harmless on the whole only by applying insoluble treatment on the dust of smaller particle size. Moreover, by the same reason, by making Se insoluble only in the dust of smaller particle size sorted by sorting means, the required amount of treating agent and capacity of Se treating means can be reduced, and the Se is made harmless more easily and economically. Incidentally, in the case of the apparatus for making Se insoluble only in a part of the dust, by further comprising the mixing means, the dust making Se insoluble, and the remaining dust without Se insoluble treatment are mixed so that the moisture content may be 20% or less, and therefore the moisture content in the dust can be easily lowered and handling is made easier in the disposal process of dust, without installing any huge equipment such as solid-liquid separator that requires wastewater (filtrate) treatment. Moreover, by further comprising means for making the dust mixed by the mixing means into scale form, handling in dust disposal process is much easier. The combustion exhaust gas treatment system of the invention is intended to separate Se components contained in the combustion exhaust gas from the combustion exhaust gas together with dust, and making harmless by further making it insoluble. The gas being rid of dust including Se components is further led into the wet combustion exhaust gas desulfurization apparatus, and SO 2 is removed, and is made completely harmless by proper treatment by ordinary method, and is released into the atmosphere. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic explanatory diagram showing a constitution of a combustion exhaust gas passing area of a combustion exhaust gas treatment system in embodiment 1 of the first invention. FIG. 2 is a schematic explanatory diagram showing an essential constitution of the combustion exhaust gas treatment system in FIG. 1. FIG. 3 is a schematic explanatory diagram showing a constitution of a combustion exhaust gas treatment system in embodiment 2 of the first invention. FIG. 4 is a schematic explanatory diagram showing a constitution of a combustion exhaust gas treatment system in embodiment 3 of the first invention. FIG. 5 is a schematic explanatory diagram showing a constitution of a combustion exhaust gas treatment system in embodiment 4 of the first invention. FIG. 6 is a schematic explanatory diagram showing a constitution of a combustion exhaust gas treatment system in embodiment 5 of the first invention. FIG. 7 is a schematic structural diagram showing an example of a conventional combustion exhaust gas treatment system. FIG. 8 is a schematic structural diagram showing other example of a conventional combustion exhaust gas treatment system. FIG. 9 is a schematic structural diagram showing a different example of a conventional combustion exhaust gas treatment system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, embodiments of the invention are described below. (Embodiment 1) FIG. 1 is a schematic explanatory diagram showing a constitution of a combustion exhaust gas passing area in an example of a combustion exhaust gas treatment system of the first invention, and FIG. 2 is a schematic explanatory diagram showing an essential constitution of the combustion exhaust gas treatment system. This combustion exhaust gas treatment system comprises, as shown in FIG. 1, a cooler 121 (cooling means) for cooling combustion exhaust gas A released from a coal fired boiler 120, an electrostatic precipitator 122 (dust collecting means, sorting means) disposed downstream of the cooler 121 for collecting dust such as fly ash in the combustion exhaust gas A and separating from the combustion exhaust gas, and a fan 123 for feeding the combustion exhaust gas (being rid of dust) discharged from the electrostatic precipitator 122 into a later process such as a wet combustion exhaust gas desulfurization apparatus. At the dust discharge side of the electrostatic precipitator 122, as shown in FIG. 2, a dust conveyor 124, an Se treating apparatus 125 (Se treating means) for making Se insoluble, and a solid-liquid separator 126 for separating the dust slurry after Se insoluble treatment into solid and liquid are disposed sequentially. This system may be incorporated, for example, in the conventional combustion exhaust gas treatment system shown in FIGS. 7-9 and connected to the desulfurization apparatus for removing sulfurous acid from the combustion exhaust gas sent by the fan 123. Of course, the conventional air heater or gas heater can be function as the cooler 121. The cooler 121 can set the outlet gas temperature, for example, in a range of 150 to 400°C, and the temperature may be set so that the combustion exhaust gas may be cooled to a temperature sufficient for condensing the Se in the combustion exhaust gas. More specifically, the temperature is set so that the combustion exhaust gas may be cooled to 350°C or less, or preferably 310°C or less. The lower limit of the cooling temperature is not particularly specified, but practically it is about 90°C. The electrostatic precipitator 122 has plural hoppers 131 to 134 (recovery units) for separating and collecting dust, and these hoppers 131 to 134 are formed sequentially from the inlet side (upstream side) to the outlet side (downstream side) of the combustion exhaust gas. In such constitution, dust of larger particle size is collected from the inlet side hopper, and dust of smaller particle size is collected from the outlet side hopper. The conveyor 124 is design to convey dust B1 to B4 collected and discharged from the hoppers 311 to 134 of the electrostatic precipitator 122 into one place in batch (Se treating apparatus 125). The Se treating apparatus 125 has a function of adding water or other liquid to the dust to form into slurry, and adding a treating agent C thereto to mix, and, for example, the repulping tank for forming slurry and the mixing tank for adding and mixing the treating agent C may be separately disposed, or these functions may be realized by a single tank. As the treating agent C, a chemical reacting with SE to make it insoluble is necessary, and if there is at least tetravalent Se (main form: selenious acid SeO 3 2- ) among Se components to be removed, for example, FeCl 3 or Fe 2 (SO 4 ) 3 may be used. If there is hexavalent Se (main form: selenic acid SeO 4 2- ) in the combustion exhaust gas and it is needed to make it insoluble in order to conform to the elution standard, as the treating agent, a reducing agent for transforming hexavalent Se into tetravalent Se (e.g. Na 2 SO 3 ) and the above chemical may be charged. In this case, a pretreatment tank for reduction reaction of hexavalent Se into tetravalent Se may be provided aside from the mixing tank as the constitution of the Se treating apparatus 125. In this Se treating apparatus 125, if it is necessary to repulp the dust, the circulation water of the desulfurization apparatus may be used as the solvent. The charging amount of the treating agent C may be set slightly more than the stoichiometric equivalent for making Se in the dust completely insoluble determined from the reaction mentioned below (reaction formulas 1, 2, or 3, 4). In thus constituted combustion exhaust gas treatment system, dust removal treatment and harmless treatment of Se contained much in the dust are performed as follows. That is, the combustion exhaust gas A leaving the boiler 120 is first cooled by the cooler 121 to 350°C or less, and therefore at least at the downstream side of the cooler 121, the Se in the combustion exhaust gas is condensed and the majority deposits on the ash which composes the dust. This dust is separated and collected by the electrostatic precipitator 122, and all the separated and collected dust is fed into the Se treating apparatus 125 in batch by the conveyor 124. In the Se treating apparatus 125, the tetravalent Se (main form: selenious acid SeO 3 2- ) contained in the slurry formed by repulping of the supplied dust (hereinafter called dust slurry) reacts with the treating agent (FeCl 3 or Fe 2 (SO 4 ) 3 ) as shown in the following reaction formulas 1, 2, or 3, 4, and is made insoluble in a form of iron selenite (Fe 2 (SeO 3 ) 3 ). When hexavalent Se (main form: selenic acid SeO 4 2- ) is present, the reducing agent as mentioned above is charged, and this hexavalent Se reacts with the reducing agent to be tetravalent Se, which similarly reacts as shown in the following reaction formulas 1, 2, or 3, 4 to be insoluble. FeCl 3 → Fe 3+ + 3Cl - 2Fe 3+ + 3SeO 3 2- → Fe 2 (SeO 3 ) 3 ↓ or Fe 2 (SO 4 ) 3 → 2Fe 3+ + 3SO 4 2- 2Fe 3+ + 3SeO 3 2- → Fe 2 (SeO 3 ) 3 ↓ Accordingly, when the dust slurry making Se insoluble is separated into solid and liquid in the solid-liquid separator 126, most of Se is separated to the solid-phase side as iron selenite, and is mixed in insoluble form into the dust cake D discharged from the solid-liquid separator 126. Therefore, if the dust cake D is directly discarded in the ash disposal yard, it conforms to the elution standard. Incidentally, since the filtrate E from the solid-liquid separator 126 is extremely low in Se concentration, it can be easily treated as wastewater. Or it may be used as addition water for dust slurry, or returned to the desulfurization apparatus as circulation water for absorbent slurry of the desulfurization apparatus. The result of combustion exhaust gas treatment experiment by the test apparatus constructed as shown in FIGS. 1 and 2 is explained below. In the experiment, coal containing 3 mg/kg of Se was supplied into a combustion furnace at a rate of 25 kg/h, and combustion exhaust gas of 200 m 3 N/h discharged from the combustion furnace was cooled to 150°C, and fed into an electrostatic precipitator. As a result, more than 99% of dust was captured by the electrostatic precipitator (Se capturing rate about 99.4%), and the amount of dust collected by the conveyor (the total collected from the hoppers) was 3.4 kg/h. As a result of analysis of the Se concentration in the dust (B1 to B4) collected by the conveyor according to the elution test and atomic absorption method by hydrogen compound generating method conforming to the ordinance No. 13 of Environmental Agency of Japan, the concentration (Se components contained in the dust and eluting by elution test) was 0.33 mg/liter, which was over the landfill elution standard (0.3 mg/liter). However, in 30 minutes after mixing by adding treating agent C (using FeCl 3 ) in the Se treating apparatus 125, by separating into solid and liquid by the solid-liquid separator 126, the Se concentration was measured in the solid-phase side (dust cake D) and liquid phase side (filtrate E), and the results are shown in Table 1, which sufficiently satisfied the elution standard (0.3 mg/liter). That is, the eluting Se concentration in the solid-phase side (Se components contained in the solid phase and eluting by the elution test) was 0.04 to 0.1 mg/liter, and the eluting Se concentration in the liquid-phase side was all 0.01 mg/liter or less. Slurry concentration (wt.%) FeCl 3 concentration (wt.%) Eluting selenium concentration from solid phase after solid-liquid separation (mg/liter) Selenium concentration in liquid phase after solid-liquid separation (mg/liter) 25 0.1 0.10 25 0.25 0.08 25 0.5 0.07 25 1.0 0.05 25 2.5 0.04 All 0.01 or less 25 5.0 0.04 50 0.5 0.08 50 1.0 0.06 50 5.0 0.05 As described herein, according to the combustion exhaust gas treatment system of the embodiment, the Se in the combustion exhaust gas is removed together with the dust, without requiring the complicated aftertreatment needed in the prior art, and is finally contained in the dust cake D in insoluble form, so as to be discarded directly. In this combustion exhaust gas treatment system, only by adding the Se treating apparatus 125 and solid-liquid separator 126 to the conventional combustion exhaust gas treatment system shown in FIGS. 7-9, the combustion exhaust gas treatment system incorporating the system of the invention can be realized, and the Se in the combustion exhaust gas is made harmless, and modification of the existing combustion exhaust gas treatment system is easy, and when newly installing this system, the conventional design or equipment may be used as it is. (Embodiment 2) Fig. 3 is a schematic explanatory diagram showing a constitution of a second example of a combustion exhaust gas treatment system of the invention. Same constituent elements as in embodiment 1 are identified with same reference numerals, and their explanations are omitted. This combustion exhaust gas treatment system possesses, as shown in FIG. 3, a conveyor 141 for feeding only dust B3, B4 separated and collected from specific hoppers 133, 134 at the outlet side of the combustion exhaust gas, out of plural hoppers 131 to 134 in the electrostatic precipitator 122, into the Se treating apparatus 125, and a conveyor 142 for conveying and treating the dust B1, B2 separated and collected from specific hoppers 131, 132 at the inlet side of the combustion exhaust gas, separately from the dust B3, B4, and is characterized by making Se insoluble only in part of the dust B3, B4, and discarding the remaining dust B1, B2. The electrostatic precipitator 122 in this embodiment function as the dust collecting means and the sorting means in the invention. In this case, only the dust B3, B4 separated and collected from the specific hoppers 133, 134 at the outlet side of the combustion exhaust gas undergoes Se insoluble treatment, and therefore the required amount of the treating agent C and the required capacity of the Se treating capacity 125 can be decreased, and the Se is made harmless more easily and inexpensively. That is, according to the study by the present inventors, it is known that more Se is contained (deposited) in the smaller particle size dust (ash) separated and collected from the specific recovery unit at the outlet side, and the Se is made harmless on the whole only by applying insoluble treatment on the dust of smaller particle size, thereby contributing to reduction of facility cost and running cost. The result of combustion exhaust gas treatment experiment by the test apparatus in FIG. 3 is described below. In the experiment, coal containing 3 mg/kg of Se was supplied into a combustion furnace at 4 rate of 25 kg/h, and the flue of 200 m 3 N/h released from the combustion furnace was cooled to 150°C, and fed into an electrostatic precipitator. In this case, more than 99% of the dust was captured by the electrostatic precipitator (capturing rate of Se about 99.4%), and the amount of dust collected by the conveyors (the total collected from the hoppers) was 3.4 kg/h. The discharge amount of dusts (collected ash) conveyed and collected by the conveyors 141, 142, the mean particle size, and eluting Se concentration (Se components contained in the collected ash and eluting by the elution test) were as shown in Table 2. Collected ash Item Collected ash discharged from conveyor 142 (combustion exhaust gas inlet side) Collected ash discharged from conveyor 141 (combustion exhaust gas outlet side) Discharge amount kg/h 2.27 1.14 Mean particle size of collected ash µm 12 5 Eluting selenium concentration in collected ash mg/liter 0.20 0.49 More specifically, the discharge amount of the dust B3, B4 separated and collected from the hoppers 133, 134 at the outlet side of the combustion exhaust gas was slight, 1.14 kg/h, but the eluting Se concentration was 0.49 mg/liter, high above the standard. On the other hand, the discharge amount of the dust B1, B2 separated and collected from the hoppers 131, 132 at the inlet side of combustion exhaust gas was large, 2.27 kg/h, but the eluting Se concentration was 0.20 mg/liter, far below the standard. Accordingly, it is known that the dust B1, B2 separated and collected from the hoppers 131, 132 at the inlet side of the combustion exhaust gas can be directly discarded. That is, Se insoluble treatment is not needed in the dust at the inlet side of the combustion exhaust gas which is about twice larger in the discharge amount, and hence it is evident that the required amount of the treating agent C and the required capacity of the Se treating apparatus 125 can be substantially saved. Incidentally, such difference in eluting Se concentration is regarded to be due to the particle size of dust (ash). That is, when gaseous Se (SeO 2 ) is condensed and deposits on the surface of the ash forming the dust, ash of smaller particle size is greater in the specific surface area per unit weight, and hence more Se deposits. On the other hand, in the dust collector such as the electrostatic precipitator mentioned above, coarse ash particles are likely to be captured at the inlet side of the combustion exhaust gas, and fine ash particles are likely to be captured at the outlet side of the combustion exhaust gas, and in other words there is a sorting function, and it is hence considered that the eluting Se concentration is high in the dust captured at the outlet side of the combustion exhaust gas. The dust B3, B4 separated and collected from the hoppers 133, 134 at the outlet side of the combustion exhaust gas were fed into the Se treating apparatus 125 by the conveyor 141, and the treating agent C (using FeCl 3 ) was added and mixed, and 30 minutes later, it was separated into solid and liquid in the solid-liquid separator 126, and the Se concentration was measured in the solid-phase side (dust cake D) and liquid-phase side (filtrate E), and the measurements as shown in Table 3 were obtained, which sufficiently satisfied the elution standard (0.3 mg/liter). That is, the eluting Se concentration in the solid-phase side (Se components contained in the solid phase and eluting by the elution test) was 0.05 to 0.09 mg/liter, and the eluting Se concentration in the liquid-phase side was 0.01 mg/liter or less. In embodiments 1 and 2, at the same slurry concentration and treating agent (FeCl 3 ) concentration, the consumption was the treating agent is compared in Table 4, which shows that it is decreased to about 1/3 in embodiment 2 as compared with embodiment 1. Slurry concentration (wt.%) FeCl 3 concentration (wt.%) Eluting selenium concentration from solid phase after solid-liquid separation (mg/liter) Selenium concentration in liquid phase after solid-liquid separation (mg/liter) 25 0.5 0.08 All 0.01 or less 25 1.0 0.06 25 2.5 0.05 50 0.5 0.09 50 1.0 0.07 50 5.0 0.05 Embodiment 1 Embodiment 2 Dust amount (kg/h) 3.4 1.14 Slurry concentration (wt.%) 25 25 FeCl 3 concentration (wt.%) 0.5 0.5 Consumption of treating agent (kg/h) 0.051 0.017 (Embodiment 3) FIG. 4 is a schematic explanatory diagram showing a constitution of a third example of a combustion exhaust gas treatment system of the invention. Same constituent elements as in embodiment 1 are identified with same reference numerals and their explanations are omitted. This combustion exhaust gas treatment system, as shown in FIG. 4, comprises a sorter 151 (sorting means) for classifying the dust B1 to B4 captured by the electrostatic precipitator 122 and conveyed in batch by the conveyor 124 into large particle size (coarse ash) B5 and small particle size (fine ash) B6, and only fine ash B6 sorted by the sorter 151 is captured by a fine particle capturing apparatus 152, and fed into the Se treating apparatus 125 to make Se insoluble. Meanwhile, in this case, the dust B1 to B4 conveyed in batch by the conveyor 124 are conveyed by air F and led into the sorter 151. The sorter 151 may be constituted by, for example, a cyclone, and it is convenient when it is designed to adjust the degree of sorting. As the fine particle capturing apparatus 152, in this case, a bag filter is used. In this case, only the fine ash B6 is subjected to Se insoluble treatment, and same as in embodiment 2, therefore, the required amount of treating agent C and required capacity of Se treating apparatus 125 can be reduced, so that the Se may be made harmless more easily and inexpensively. In this combustion exhaust gas treatment system, only by adding the Se treating apparatus 125 and sorter 151 to the conventional combustion exhaust gas treatment system shown in FIGS. 7-9, the combustion exhaust gas treatment system incorporating the system of the invention can be realized, while the conveyor and other structures may be the same, and the Se in the combustion exhaust gas is made harmless, and modification of the existing combustion exhaust gas treatment system is easy, and when newly installing this system, the conventional design or equipment may be used as it is. The result of combustion exhaust gas treatment experiment by the test apparatus shown in FIG. 4 is described below. In the experiment, coal containing 3 mg/kg of Se was supplied into a combustion furnace at a rate of 25 kg/h, and the combustion exhaust gas exhausted from the combustion furnace at a rate of 200 m 3 N/h was cooled to 150°C and fed into the electrostatic precipitator. In this case, more than 99% of the dust was captured by the electrostatic precipitator, and the amount of dust collected by the conveyor (the total collected from the hoppers) was 3.4 kg/h. The capturing amount of coarse ash B5 and fine ash B6, mean particle size, and eluting Se concentration (Se components contained in captured ash and eluting by the elution test) are shown in Table 5. Collected ash Item Coarse ash B5 Fine ash B6 Capturing amount kg/h 2.05 1.30 Mean particle size of captured ash µm 13 5.4 Eluting selenium concentration in captured ash mg/liter 0.26 0.36 That is, in the fine ash B6, the mean particle size was 5.4 µm, and the capturing amount was small, 1.30 kg/h, but the eluting Se concentration exceeded the standard, 0.36 mg/liter. In coarse ash B5, the mean particle size was 13 µm, the capturing amount was large, 2.05 kg/h, but the eluting Se concentration was below the standard, 0.26 mg/liter. Accordingly, it is known that the coarse ash B5 can be directly discarded. Hence, Se insoluble treatment is not needed in the coarse ash B5 which is very large in output, and it is evident that the required amount of treating agent C and required capacity of Se treating apparatus 125 can be saved substantially. The fine ash B6 was fed into the Se treating apparatus 125, and blended with the treating agent C and mixed for 30 minutes, and it was separated into solid and liquid by the solid-liquid separator 126, and the Se concentration was measured in the solid-phase side (dust cake D) and liquid-phase side (filtrate E), and the same results as in embodiment 2 shown in Table 3 were obtained, and the elution standard (0.3mg/liter) was sufficiently satisfied. (Embodiment 4) FIG. 5 is a schematic explanatory diagram showing a constitution of a fourth example of a combustion exhaust gas treatment system of the invention. Same constituent elements as in embodiment 2 in FIG. 3 are identified with same reference numerals and their explanations are omitted. This combustion exhaust gas treatment system is, as shown in FIG. 5, characterized by comprising a mixer 161 (mixing means) for mixing the dust G undergoing Se insoluble treatment as being fed into the Se treating apparatus 125a from the conveyor 141 after being separated and collected by the hoppers 133, 134 at the outlet side of the combustion exhaust gas, and the other dust B1, B2 not undergoing Se insoluble treating after being separated and collected by the hoppers 131, 132 at the inlet side of the combustion exhaust gas, so that the water content may be 20% or less (preferably 10% or less). The Se treating means is an Se treating apparatus 125a capable of inducing Se insoluble reaction (for example, reaction formulas 1, 2, or 3, 4), by spraying a liquid containing a treating agent such as FeCl 3 or Fe 2 (SO 4 ) 3 (concentration about 0.5 to 5.0 wt.%) uniformly into the dust. In this example, the dust G undergoing Se insoluble treatment, and the other dust B1, B2 not undergoing Se insoluble treatment are mixed by the mixer 161, and a harmless dust cake H (mixed ash) of low water content is formed. Accordingly, without using large-scale equipment such as solid-liquid separator that requires wastewater (filtrate) treatment, the water content in the dust can be easily lowered, and the harmless dust can be handled easily in waste disposal. In the constitution of the embodiment, the water content of the mixed ash H and eluting Se concentration (Se components contained in mixed ash and eluting by the elution test) were measured in the same conditions as in embodiment 2, of which results are shown in Table 6. FeCl 3 concentration in spray liquid (wt.%) Spray flow rate of FeCl 3 solution (g/h) Water content in mixed ash (wt.%) Eluting selenium concentration in mixed ash (mg/liter) 0.5 400 9.7 0.18 1.0 400 10.5 0.15 2.5 400 10.6 0.14 5.0 400 9.8 0.16 5.0 600 15.4 0.13 5.0 800 19.9 0.11 (Embodiment 5) FIG. 6 is a schematic explanatory diagram showing a constitution of a fifth example of a combustion exhaust gas treatment system of the invention. Same constituent elements as in embodiment 4 in FIG. 5 are identified with same reference numerals and their explanations are omitted. This combustion exhaust gas treatment system is, as shown in FIG. 6, characterized by comprising a scale forming apparatus 171 (scale forming means) for compacting the dust H mixed by the mixer 161 to form into scale. In this case, the scale forming apparatus 171 forms the dust H into scale dust (scale ash) I, and the volume of the dust H is further reduced, and handling in waste disposal is much easier. Besides, the water content of the dust H is kept under 20% (preferably under 10%) by the mixer 161, which is effective for making it easier to operate to compact by the scale forming apparatus 171. As a result of experiment in the same running conditions as in embodiment 4, the bulk density of the dust cake H (mixed ash) mixed by the mixer 161 was 0.8 g/cc, whereas the bulk density of the dust (scale ash) I formed into scale by the scale forming apparatus 171 was 1.5 g/cc. The invention may be realized also in other various forms than the illustrated embodiments. For example, as the treating agent for making Se insoluble, aside from FeCl 3 or Fe 2 (SO 4 ) 3 , chelating agents (e.g. Miyoshi Resin Epolus MX-7), and high polymer heavy metal capturing agents (e.g. Miyoshi Resin Epofloc L-1) can be used. The dust collecting means and sorting means of the invention are not limited to the electrostatic precipitator and single cyclone connected thereto, but the dust collecting means and sorting means of the invention may be realized, for example, by multiple cyclones. The combustion exhaust gas treatment system of the first invention may be incorporated in part of the combustion exhaust gas treatment system having a conventional desulfurization apparatus. For example, in embodiment 1, the desulfurization apparatus for removing sulfurous acid from the combustion exhaust gas sent by the fan 123 may be provided, for example, as in the prior art shown in FIGS. 7 and 8. Besides, the conventional air heater or gas-gas heater may be used as the cooling means of the invention. (Effects of the Invention) According to the combustion exhaust gas treatment system of the invention, most of Se in combustion exhaust gas is cooled by the cooling means and condensed, and is removed by the dust collecting means in a state being contained in the dust. To the dust separated by the dust collecting means, a treating agent is added by the Se treating means, and the existent form of Se in the dust is transformed into an insoluble compound. Accordingly, if the dust is discarded same as in the prior art, the Se elution standard is satisfied, and Se is made harmless easily without requiring complicated after treatment. By applying Se insoluble treatment only on the dust separated and collected from the specific recovery unit at the outlet side of combustion exhaust gas in the dust collecting means, the required amount of treating agent and required capacity of the Se treating means can be reduced, and the Se is made harmless more easily and economically. Also by applying Se insoluble treatment only on the dust of small particle size sorted by the sorting means, the required amount of treating agent and required capacity of the Se treating means can be reduced, and the Se is made harmless more easily and economically. By installing mixing means, when the dust undergoing Se insoluble treatment and remaining dust not undergoing Se insoluble treatment are mixed so that the water content may be 20% or less, the water content of the dusts may be easily lowered without using large-scale equipment such as solid-liquid separator that requires treatment of wastewater (filtrate), and therefore handling in disposal of dust may be easier. By installing scale forming means, when the dust mixed by the mixing means is formed into scale, handling in disposal of dusts may be much easier.

Claims: Combustion gas treatment apparatus for treating a combustion exhaust gas (A) containing dust and Se components, comprising cooling means (121) for cooling the combustion exhaust gas to 350°C or less, dust collecting means (122) for separating dust from the combustion exhaust gas, dust conveying means (124, 141), and Se treating means (125, 125a) for transforming the form of Se existent in the dust into an insoluble compound, said dust conveying means (124, 141) conveying dust separated by the dust collecting means (122) to the Se treating means (125, 125a), and said Se treating means (125, 125a) having means for adding water and treating agent (C) to the dust separated by the dust collecting means (122). Combustion gas treatment apparatus according to claim 1, wherein said dust collecting means (122) comprises a plurality of recovery units (131, 132, 133, 134) arranged in the combustion exhaust gas duct from the inlet side to the outlet side for separating and collecting dust fractions, and said conveyor (141) is adapted to introduce only the dust collected by the recovery units (B3, B4) at the outlet side of said collecting means (122) into the Se treating means (125, 125a). Combustion gas treatment apparatus according to claim 1, wherein said dust collecting means (122) comprises sorting means (151) for sorting the dust separated into a large particle size fraction (B5) and a small particle size fraction (B6), and said conveyor (141) is adapted to introduce only the small particle size dust fraction (B6) into the Se treating means (125) to make the Se insoluble. Combustion gas treatment apparatus according to any of claims 1 to 3, further comprising solid-liquid separating means (126) for separating a slurry of solid and liquid containing the insoluble Se compound discharged from the Se treating means (125), into solids and liquid. Combustion gas treatment apparatus according to claim 2, further comprising mixing means (161) for mixing the dust fractions (B3, B4) in which Se is made insoluble by the Se treating means (125a), and the remaining dust fractions (B1, B2) in which Se is not made insoluble, such that the moisture content is 20 %, preferably 10 % or less. Combustion gas treatment apparatus according to claim 5, further comprising scale forming means (171) for compacting moist dust (H) mixed by the mixer (161) to form it into scale (I). Method for treating combustion exhaust gas containing dust and Se components, comprising the steps of cooling the combustion exhaust gas to.350°C or less, collecting dust particles on which Se compounds are deposited, treating the form of Se existent in the dust to transform it into an insoluble compound state by adding water and a treating agent to the collected dust, and discarding the treated dust. Method for treating combustion exhaust gas according to claim 7, further comprising the step of discharging slurry obtained by the Se treating step and separating the solid component containing the insoluble Se compound from said slurry prior to its discarding.

Inventors: HARUKI TAKASHI, HINO MASAO, KIMURA KAZUAKI, OCHI EIJI, OKAZOE KIYOSHI, OKINO SUSUMU, SHIMIZU TAKU, TAKASHINA TORU, TATANI ATSUSHI, UKAWA NAOHIKO, HARUKI, TAKASHI, HINO, MASAO, KIMURA, KAZUAKI, OCHI, EIJI, OKAZOE, KIYOSHI, OKINO, SUSUMU, SHIMIZU, TAKU, TAKASHINA, TORU, TATANI, ATSUSHI, UKAWA, NAOHIKO, Haruki, Takashi, c/o Mitsubishi Jukogyo K.K., Hino, Masao, c/o Mitsubishi Jukogyo K.K., KIMURA, KAZUAKI, C/O MITSUBISHI JUKOGYO K.K., OCHI, EIJI, C/O MITSUBISHI JUKOGYO K.K., Okazoe, Kiyoshi, c/o Mitsubishi Jokogyo K.K., OKINO, SUSUMU, C/O MITSUBISHI JUKOGYO K.K., SHIMIZU, TAKU, C/O MITSUBISHI JUKOGYO K.K., Takashina, Toru, c/o Mitsubishi Jukogyo K.K., TATANI, ATSUSHI, C/O MITSUBISHI JUKOGYO K.K., Ukawa, Naohiko, c/o Mitsubishi Jukogyo K.K.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1996
Main classification: B01D53
IPC codes: F23J15, B01D53