Heavy oil fired boiler exhaust gas treatment apparatus

Abstract: There is provided a heavy oil fired boiler exhaust gas treatment apparatus which removes soot and dust and SO 3 in the heavy oil fired boiler exhaust gas efficiently at a low cost without using an excessively large apparatus. The treatment apparatus comprises a charging device 3 provided at the inlet of an absorption tower of a wet type exhaust gas desulfurizer 4 and neutralizer pouring means A provided on the upstream side of the charging device 3 to pour a neutralizer into exhaust gas and neutralize SO 3 in the exhaust gas.

Description: Background of the Invention The present invention relates to a heavy oil fired boiler exhaust gas treatment apparatus. Conventionally, as shown in FIG. 4, as a treatment method for SO 3 in exhaust gas of a heavy oil fired boiler 401, there has generally been used a method in which a salt such as ammonium sulfate ((NH 4 ) 2 SO 4 ) is yielded by pouring a neutralizer such as ammonia on the upstream side of an electric dust collector 402, and the SO 3 is trapped together with soot and dust by the electric dust collector 402. The exhaust gas passes through a wet type exhaust gas desulfurizer 403 and is discharged from a stack 405. When far higher dust removing performance is required, a wet type electric dust collector 404 is installed on the downstream side of the wet type exhaust gas desulfurizer 403 to effect high-level dust removal. However, the a neutralizing salt yielded by the pouring of a neutralizer consists of fine particles, so that it is difficult to trap the neutralizing salt by the electric dust collector 402 and an absorption tower of the desulfurizer 403. Further, since ammonium sulfate is trapped as part of soot and dust together with soot and duct in the electric dust collector 402, the capacity of the electric dust collector 402 must be increased. Therefore, the conventional method poses a problem in that not only the site area but also the installation cost and operation cost increase. The same applied for another known apparatus for the treatment of exhaust gases from coal fired boilers (US patent No. 5,525,317) using individual units for NO x , SO x and particulate removal and providing ammonia injection to all three units from a single source. The exhaust gas of the boiler passes a elective catalyst reactor, an electrostatic precipitator and an absorption tower upstream of the outlet of which a mist eliminator is provided to which fresh water is supplied. The scrubbed exhaust gas is discharged to a stack. Thereupon, a heavy oil fired boiler exhaust gas treatment apparatus which solves above problems has been desired. Summary of the Invention The present invention has been made in view of the above situation, and accordingly an object thereof is to provide a heavy oil fired boiler exhaust gas treatment apparatus which removes soot and dust and SO 3 in the heavy oil fired boiler exhaust gas efficiently at a low cost without using an excessively large apparatus. A heavy oil fired boiler exhaust gas treatment apparatus solving this problem is defined with its further developments in the appended claims. The heavy oil fired boiler exhaust gas treatment apparatus of the invention comprises a wet type exhaust gas desulfurizer having an absorption tower which receives boiler exhaust gas that has passed a neutralizer pouring means for pouring a neutralizer into the exhaust gas for neutralizing So, therein, and a dust collector provided at the outlet of the absorption tower. According to the invention a charging device including a charging electrode and a dust collecting electrode plate is provided at the inlet of the absorption tower operable to electrically charge soot and dust particles contained in the boiler exhaust gas, a high-pressure water spray (22) is provided immediately behind the charging.device (3), and the dust collector is an accelerating dust collector. A high-pressure spray may be provided immediately behind the charging device and there may be provided a spray for spraying water into the exhaust gas on the upstream side of an accelerating dust collector. Furthermore, there may be provided separator for separating unburned carbon from an absorbent liquid obtained from the absorption tower and also means for separating and recovering a neutralizer in an absorbent liquid drawn from an oxidation tank of the wet type exhaust gas desulfurizer. If an electric dust collector for collecting dust in exhaust gas is provided between the neutralizer pouring means and the charging means (3) dust collection can be effectively enhanced. As is apparent from the above description, according to the present invention, there is provided a heavy oil fired boiler exhaust gas treatment apparatus which removes soot and dust and SO 3 in the heavy oil fired boiler exhaust gas efficiently at a low cost without using an excessively large apparatus. That is, according to the present invention, by installing a charging device, soot and dust (containing a neutralizing salt such as ammonium sulfate) are charged. Further, by installing a high-pressure (pressurized) spray, tiny water drops can be sprayed, by which fine particles can be agglomerated and enlarged. Thereby, soot and dust can be trapped in the absorption towers with high efficiency. When an accelerating dust collector is installed, soot and dust can be recovered efficiently, so that an electric dust collector, which requires a high cost, can be omitted or made compact. Further, when a liquid cyclone and means for separating and recovering a neutralizer are installed, unburned carbon and neutralizer trapped in the absorption towers can be recovered, so that the usage can be reduced. Therefore, the heavy oil fired boiler exhaust gas treatment apparatus can be improved as a whole. Brief Description of the Drawings Fig.1 is a block diagram for illustrating one embodiment of a heavy oil fired boiler exhaust gas treatment apparatus in accordance with the present invention; Fig.2 is a schematic view for mainly illustrating a charging device, high-pressure spray, and wet type exhaust gas desulfurizer for one embodiment of a heavy oil fired boiler exhaust gas treatment apparatus in accordance with the present invention; Fig.3 is a schematic view for mainly illustrating a liquid cyclone and means for evaporating, separating and recovering ammonia for one embodiment of a heavy oil fired boiler exhaust gas treatment apparatus in accordance with the present invention; and Fig.4 is a block diagram for illustrating one mode of a conventional heavy oil fired boiler exhaust gas treatment apparatus. Detailed Description of the Preferred Embodiment An exhaust gas to which the present invention is applied is the exhaust gas discharged from a heavy oil fired boiler. The heavy oil fired boiler is a boiler which uses heavy oil as a main fuel. As a heavy oil, three types specified in JIS K-2205-1980, which is the standard for heavy oil, are particularly typical. A "heavy oil" includes residual oils obtained by distilling a crude oil, petroleum residues obtained by extracting a crude oil with a solvent, and tar pitch obtained as a by-product in the process of decomposition reaction of crude oil such as thermal and catalytic decompositions. In addition, coal based oils such as liquefied coal oils and solvent extracts from coals and bitumens such as sand oil and shale oil are also included. These materials are characterized in common by high boiling points, appearance as blackish brown or black liquid, semisolid or solid in some cases, and ability of being dissolved in strong solvents such as pyridine and quinoline, by which they are differentiated from a coal. Since the exhaust gas contains SO 3 , it is necessary to perform treatment therefor. FIG. 1 schematically shows one embodiment of a heavy oil fired boiler exhaust gas treatment apparatus in accordance with the present invention. In this embodiment, ammonium sulfate ((NH 4 ) 2 SO 4 ) is yielded by pouring ammonia as a neutralizer into an exhaust gas of a heavy oil fired boiler 1 through a line A on the upstream side of an electric dust collector 2, and the ammonium sulfate is trapped together with soot and dust by the dry type electric dust collector 2. The exhaust gas is fed into a later-stage wet type exhaust gas desulfurizer 4 through a charging device 3. The exhaust gas having passed through the wet type exhaust gas desulfurizer 4 passes through an accelerating dust collector 5, and is discharged from a stack 6. FIG. 2 schematically shows the charging device 3 and the wet type exhaust gas desulfurizer 4. This wet type exhaust gas desulfurizer 4 is a device using a parallel counterflow type gas-liquid contact device. This desulfurizer 4 includes an oxidation tank 11 supplied with an absorbent liquid in which an absorbent consisting of limestone is suspended (hereinafter called absorbent slurry), a liquid column type inlet-side absorption tower 12 which is provided extendedly upward from one side of the oxidation tank 11 to effect gas-liquid contact between untreated exhaust gas and the absorbent slurry in the oxidation tank 11, and a liquid column type outlet-side absorption tower 13 which is provided extendedly upward from the other side of the oxidation tank 11 to again effect gas-liquid contact between the exhaust gas led from the inlet-side absorption tower 12 and the absorbent slurry in the oxidation tank 11. The inlet-side absorption tower 12 is a so-called parallel flow type absorption tower in which untreated exhaust gas is led from the above and flows downward. Also, the outlet-side absorption tower 13 is a so-called counterflow type absorption tower in which the exhaust gas flows upward. The absorption towers 12 and 13 are provided with a plurality of spray pipes 14 and 15 arranged in parallel, respectively. These spray pipes 14 and 15 each are formed with a plurality of nozzles for injecting the absorbent slurry upward in a liquid column form. On the outside of the oxidation tank 11, there is provided a circulating pump 16 for sucking the absorbent slurry in the oxidation tank 11, by which the absorbent slurry is fed to the spray pipes 14 and 15 via circulating lines 17 and 18, respectively, and is injected upward from the nozzles. Although not shown in the figure, the oxidation tank 11 can be provided with an arm rotating type air sparger. This air sparger blows air for oxidation as minute bubbles into the absorbent slurry in the oxidation tank 11 while agitating the absorbent slurry. Thus, the absorbent slurry having absorbed SO 2 and the air are brought into contact with each other efficiently in the oxidation tank 11, whereby the whole quantity is oxidized to obtain gypsum. In this wet type exhaust gas desulfurizer 4, the absorbent slurry is injected from the spray pipes in the absorption towers 12 and 13, and flows down while absorbing SO 2 by the gas-liquid contact with exhaust gas and also while trapping soot and dust (containing ammonium sulfate). The absorbent slurry comes into contact with many air bubbles generated as described above, being oxidized, and further causes neutralization reaction to yield a slurry containing much gypsum. In this embodiment, the flow path cross-sectional area of the outlet-side absorption tower 13 is set relatively larger than that of the inlet-side absorption tower 12. Thereby, in the inlet-side absorption tower, a high flow velocity of exhaust gas is achieved, so that soot and dust are trapped effectively. On the other hand, in the outlet-side absorption tower 13, a high gas-liquid contact capacity is assured, so that SO 2 is absorbed especially effectively. The charging device 3, which offers one feature of the present invention, is installed on the upstream side of the above-described wet type exhaust gas desulfurizer 4. In this embodiment, as shown in FIG. 2, the charging device 3 includes a charging electrode 20 and a dust collecting electrode plate 21. Under the experimental conditions given in Table 1, a negative voltage is applied to the charging electrode 20 so that the current density of the dust collecting electrode plate 21 is 0.5 to 6 mA/m 2 by using a power source 19. Thereby, soot and dust are trapped on the dust collecting electrode plate 21. The agglomerated and enlarged soot and dust are knocked out of the dust collecting electrode plate 21 to the absorption towers 12 and 13 by hammering. Because the particle size has been increased, the soot and dust can be trapped easily by the absorbing section. Treatment gas flow rate 5,000-10,000 m 3 N/h Exhaust gas temperature 105-120 °C Water content 7-11 vol% Concentration of soot and dust 20-70 mg/m 3 N(d) Pressurized spray flow rate 0-520 l/h Circulation flow rate (parallel flow) 0-165 m 3 N/h Circulation flow rate (counterflow) 0-224 m 3 N/h On the other hand, the minute soot and dust having not been trapped on the dust collecting electrode plate 21 are also charged by the charging device 3. Tiny water drops (0.1 to 0.5 mm) are sprayed by using a high-pressure (pressurized) spray 22 (d = about 100 µm) installed immediately behind the charging device 3. Thereby, the charged soot and dust are trapped by the tiny water drops by being subjected to an electrical attracting force caused by the image charge of tiny water drops. The tiny water drops having trapped soot and dust are trapped easily with high efficiency in the absorption towers 12 and 13 by inertial collision and interruption effect. Thereby, the soot and dust which have not been removed by the dry electric dust collector 2 can be trapped by the absorption towers 12 and 13 with high efficiency. Since ammonium sulfate, which constitutes part or sometimes most of the soot and dust, is water-soluble, it dissolves in the absorbent slurry in the oxidation tank 11, and is ionized to ammonium ion and SO 4 ion. Thus, in the oxidation tank 11, a large quantity of gypsum, a small quantity of limestone, which is an absorbent, and soot and dust (containing ammonium ions and SO 4 ions produced by the dissolution of ammonium sulfate) trapped from the exhaust gas are normally suspended or dissolved. The absorbent slurry in the oxidation tank 11 is sent to the later-described solid-liquid separation side through a piping line 30. During the operation, the oxidation tank 11 is supplied with limestone, which is an absorbent, as slurry from a slurry regulating tank (not shown). The absorbent is supplied appropriately to the oxidation tank by using a slurry pump or the like. Also, the oxidation tank 11 is supplied with make-up water appropriately, by which the water decreased gradually by evaporation in the absorption towers 12 and 13 is supplemented. During the operation, the flow rate of the make-up water to the oxidation tank 11, the flow rate of the slurry drawn through the piping line 30, etc. are regulated, whereby the slurry containing a predetermined concentration of gypsum and absorbent is accumulated in the oxidation tank 11 within a level of a fixed range. Also, during the operation, in order to keep the efficient SO x removal and to keep high gypsum purity, the boiler load (flow rate of exhaust gas), the SO 2 concentration in untreated exhaust gas, the pH in the oxidation tank 11, the concentration of limestone, etc. are detected by using sensors. Based on these detection results, the supply amount of limestone to the oxidation tank 11 and other factors are appropriately controlled by a controller (not shown). In this embodiment, mist flowing out from the absorbing section is removed by a mist catcher 23. Further, in this embodiment, an accelerating dust collector 5 is installed additionally at the outlet of the absorption tower. The accelerating dust collector 5 narrows the ordinary mist catcher passage width, by which the inertia force of mist is increased by the increased fluid flow velocity to enhance the collision dust removal performance. In this accelerating dust collector 5 as well, the mist containing the untrapped soot and dust is caught. Further, by spraying water into the exhaust gas on the upstream side of the accelerating dust collector 5, water saturated gas is made in a supercooled state, whereby the water in excess gas is condensed around fine particles to enlarge minute soot and dust. Therefore, the dust removal performance can be enhanced by the treatment using a higher inertia force. The spray water is supplied from a spray water tank 24. By adding this accelerating dust collector 5, dust removal performance equivalent to that in the case where a wet type electric dust collector is installed can be obtained. Thereby, high dust removal performance can be obtained without the installation of a wet type electric dust collector. Also, the electric dust collector 2 can be omitted or made compact. In this embodiment, the absorbent slurry is drawn to a treatment system as shown in FIG. 3 and treated. The absorbent slurry is drawn by a pump 31, and floatation separation of unburned carbon from the absorbent slurry is effected by a liquid cyclone 32, which is a separator. The absorbent slurry from which unburned carbon has been separated is separated into gypsum and liquid component by a belt filter 33. The gypsum is recovered by a recovery conveyor 34. The liquid component is stored in a recovery tank 35, and is sent to an evaporator 37 by using a pump 36. The liquid component is drawn by using a pump 38 and heated by a heater 39, whereby the contained ammonia is separated by evaporation and recovered. The recovered ammonia is reused as ammonia for pouring. Thus, by effecting the floatation separation of unburned carbon from the absorbent slurry by the liquid cyclone 32, the gypsum purity can be ensured. Also, by reusing the recovered unburned carbon, the amount of fuel usage can be reduced. Further, the ammonia in the absorbent slurry is separated by evaporation by an evaporator 37 or the like equipment and recovered, by which the consumption of ammonia can be kept low. Other embodiments Each of the aforementioned absorption tower is not necessarily limited to a liquid column type absorption tower. For example, a simple spray type or fill type absorption tower may be employed. However, the liquid column type absorption tower, which has appreciably high dust removal performance, is superior to other types in the case where large quantities of soot and dust are contained in the exhaust gas. As the neutralizer, NaOH, calcium carbonate, or the like can be used besides ammonia. The liquid cyclone 32 may be any other device for separating unburned carbon.

Claims: A heavy oil fired boiler exhaust gas treatment apparatus comprising a wet type exhaust gas desulfurizer (4) having an absorption tower (12, 13) which receives boiler exhaust gas that has passed a neutralizer pouring means for pouring a neutralizer into the exhaust gas for neutralizing SO 3 therein, and a dust collector provided at the outlet of the absorption tower (13), wherein a charging device (3) including a charging electrode (20) and a dust collecting electrode plate (21) is provided at the inlet of the absorption tower (12) operable to electrically charge soot and dust particles contained in the boiler exhaust gas, a high-pressure water spray (22) is provided immediately behind the charging device (3), and the dust collector is an accelerating dust collector (5). The heavy oil fired boiler exhaust gas treatment apparatus according to claim 1, wherein there is provided means for spraying water into the exhaust gas on the upstream side of the accelerating dust collector (5). The heavy oil fired boiler exhaust gas treatment apparatus according to any of claims 1 to 2, wherein there is provided a separator (32) for separating unburned carbon from an absorbent liquid obtained from said absorption tower (12, 13). The heavy oil fired boiler exhaust gas treatment apparatus according to claim 3, wherein there is provided means (37) for separating and recovering a neutralizer in an absorbent liquid drawn from an oxidation tank (11) of the wet type exhaust gas desulfurizer (4). The heavy oil fired boiler exhaust gas treatment apparatus according to any of the preceding claims, wherein an electric dust collector (2) for collecting dust in exhaust gas is provided between the neutralizer pouring means and the charging means (3).

Inventors: OCHI EIJI, ONIZUKA MASAKAZU, SHINODA TAKEO, TAKASHINA TORU, OCHI, EIJI, ONIZUKA, MASAKAZU, SHINODA, TAKEO, TAKASHINA, TORU, Ochi, Eiji Mitsubishi Heavy Ind., Ltd, Onizuka, Masakazu Hiroshima R&amp;D, Mitsub. Heavy Ind, Shinoda, Takeo Mitsubishi Heavy Ind., Ltd, Takashina, Toru Hiroshima R&amp;D, Mitsub. Heavy Ind.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 1998
Main classification: B01D53
IPC codes: B01D53