Desulfurizing method and equipment for exhaust gas from combuster

Abstract: A method and equipment for desulfurizing exhaust gas from a combustor in which the exhaust gas containing SO₂ from the combustor is brought into contact with SO₂ adsor­ bent to remove SO₂ and is blown down out of the system; on the other hand, the SO₂ adsorbent absorbed SO₂ is brought into contact with air to desorb SO₂ and the air containing the desorbed SO₂ is supplied to the combustor as air for combustion to burn to raise the SO₂ concentration in the exhaust gas from the combustor; and the exhaust gas not undergone contact with the adsorbent or a part of the air undergone contact with the SO₂ adsorbent are taken out of the system.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a method and equipment for desulfurizing exhaust gas from combustors such as a boiler, gas turbine, diesel engine, etc. using fuel containing sulfur such as coal and the like. In DE-A-2544676 such a method and equipment is described. In the boiler, diesel engine, gas turbine, etc., due to S content in the fuel, SO₂ is produced in the exhaust gas. If the exhaust gas containing SO₂ is blown down out of the system, it causes environmental pollution such as photochemical smog and acid rain and therefore the blowdown is under a severe restriction. Because of fall in temperature of the exhaust gas occured when heat is exchanged, SO₂ reacts with oxygen and is converted into SO₃. Moreover, since SO₃ reacts with water at less than 200°C to produce sulfuric acid which causes acid corrosion of the materials of the combustor, the heat exchange is very difficult at a temperature of the exhaust gas of less than 200°C. For this reason, a variety of methods for desulfurization of the exhaust have been proposed so far. Among these, the lime- gypsum method, one of the wet-desulfurizing methods, is most widely used owing to (1) its treatment efficiency and (2) economical usability of by-product after treatment. In this method, the exhaust gas, after being cooled to about 70°C by spray of cooled water, thus being lowered to below 200°C, the temperature at which sulfuric acid dew point corrosion is progressed, is introduced into a gas-liquid contact tower where lime water circulates. Thus, SO₂ is removed as CaSO₃ . CaSO₃ is air-oxidized, taken out as lime (CaSO₄) and used for building materials. The purified exhaust gas has a low temperature of 70°C without enough power to go up through a chimney to be discharged and with a large quantity of white smoke and therefore the exhaust gas is burned more to raise its temperature up to about 100°C, and then is blown down out of the system. Thus, the lime-gypsum method provides a very high desulfurizing efficiency; however, if the exhaust gas has a temperature of less than 200°C, the heat collection cannot be made due to sulfuric acid dew point corrosion, and heat energy is required to raise the temperature of the gas after being treated resulting in decrease of process efficiency by 2-4% after all. To sum up, the conventional wet-desulfurizing method, especially the lime-gypsum method has advantages of high desulfurizing efficiency and usability of by-product gypsum as building materials; on the other hand, because the desulfurizing temperature is a low 70°C, there are problems associated with heat balance and sulfuric acid dew point corrosion as listed below; (1) SO₂ in exhaust gas from the combustion is condensed into the combustion materials in the form of H₂SO₄ resulting in sulfuric acid dew point corrosion. (2) Heat collection is difficult when the temperature of exhaust gas is less than 200°C. (3) The exhaust gas after being desulfurized has a low temperature of 70°C and no power to go up through a chimney, with a large quantity of white smoke, and therefore, heat energy must be supplemented to raise the temperature up to about 100°C. OBJECT AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and equipment for desulfurizing exhaust gas from a combustion having a high efficiency and cost effectiveness by overcoming problems of difficulty in heat collection at low temperature, of sulfuric acid dew point corrosion, etc., found in the conventional wet-desulfurizing methods. The present invention provides a method for desulfurizing exhaust gas containing SO₂ from a combustor in which the exhaust gas containing SO₂ from the combustor is first brought into contact with SO₂ absorbent to remove SO₂ and is subsequently discharged to the atmosphere; in which thereafter the SO₂ absorbent absorbed SO₂ is brought into contact with air to desorb SO₂ and the air containing the desorbed SO₂ is supplied to the combustor as air for combustion to raise the SO₂ concentration in the exhaust gas from the combustor; and in which a part of the exhaust gas prior to contacting the absorbent or a part of the air containing desorbed SO₂ is introduced into a lime-gypsum type desulfurizing equipment to be desulfurized. The present invention furthermore provides a desulfurizing equipment according to claim 2. In the present invention, by absorbing SO₂ in the exhaust gas using absorbent, the SO₂ content in the discharged gas is decreased and, at the same time, the collection of heat is performed even at a low temperature by overcoming the problem of sulfuric acid dew point corrosion and therefore the heat efficiency of the plant can be improved. Moreover, by desorbing SO₂ absorbed by SO₂ absorbent with air and returning the air back to the combustor, and taking out the exhaust gas or a part of the air desorbed SO₂, the SO₂ concentration in the exhaust gas or in the air is raised resulting in a small-sized treatment equipment at the back current side and in a high treatment efficiency. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a conceptual illustration representing an embodiment of the desulfurizing method and equipment provided by the present invention. FIG. 2 is a graph showing the temperature in the SO₂ absorbing tower and the SO₂ concentration at the outlet of the SO₂ absorbing tower in the embodiment illustrated in FIG. 1. FIG. 3 is another graph showing the consumed quantity of SO₂ absorbent and SO₂ concentration at the outlet of the SO₂ absorbing tower in the embodiment illustrated in FIG. 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT In the present invention, by bringing the exhaust gas containing SO₂ from the combustor into contact with the SO₂ absorbent, SO₂ is absorbed in the SO₂ absorbent in proportion to partial pressure in gas phase resulting in removal of SO₂ from the exhaust gas and the purified gas is blown down out of the system. When air is brought into contact with the absorbent absorbed SO₂, because the air contains almost no SO₂, the difference in SO₂ partial pressure between the absorbent and the gas phase reactivates the absorbent from which SO₂ has been desorbed. On the other hand, when the air containing SO₂ is supplied to the combustor as the air for combustion and burned, SO₂ in the air is combined with SO₂ resulted from S content in fuel to raise the SO₂ concentration in the exhaust gas after all. Furthermore, with an increase in the SO₂ concentration in the exhaust gas, the quantity of SO₂ to be absorbed in the SO₂ absorbent is increased, which accordingly raises the SO₂ concentration in the air desorbed SO₂ from the SO₂ absorbent. The exhaust gas or a part of the air in which the SO₂ concentration has been raised is taken out of the system. When attention is paid to the quantity of SO₂, it is confirmed that the quantity of SO₂ in the exhaust gas blown down after contact with the SO₂ absorbent is very small. This means that all the quantity of SO₂ absorbed by the SO₂ absorbent is considered to be desorbed by the air. Then the air is supplied to the combustor, as air for combustion, in which it helps the fuel containing the S content burn. Therefore, in this invention, a closed system regarding SO₂ is formed and only SO₂ contained in the exhaust gas or a part of the air discharged from the system is taken out of the system. The quantity of SO₂ is the value responding to that of the S content in fuel supplied to the combustor and, in a final stationary state, the S content in fuel supplied to the combustor is equal to the S content (SO₂) in the exhaust gas or in a part of the air discharged out of the system; i.e., if the gas flow taken out of the system is represented as G₁ (Nm³/h), the concentration of SO₂ as C₁, and the S content contained in fuel supplied to the combustor as n s (Kg-mol/h), the following formulae are given in the final stationary state. G₁ C₁ = n s x 22.4 C₁ = n s x 22.4 / G₁ The concentration of SO₂ taken out of the system is equal to that of the air supplied to the combustor or blown down from the combustor and therefore the SO₂ concentration in the present invention is represented as n s x 22.4 / G₁ finally. This means that, if 10% of the exhaust gas or the air containing SO₂ is taken out from the SO₂ closed system by the present invention, the concentration of SO₂ in the exhaust gas is raised 10 times than that in the ordinary combustion. Thus, in the present invention, the exhaust gas containing almost no SO₂ owing to absorption by the absorbent and the condensed exhaust gas or air, each having high SO₂ concentration, can be obtained. As explained above, in the present invention, the discharged exhaust gas contains almost no SO₂ because SO₂ has been absorbed in the absorbent and therefore there is no problem associated with sulfuric acid dew point corrosion; and the collection of heat can be made even while the temperature of the discharged exhaust gas is low resulting in a great improvement of plant efficiency and, moreover, the SO₂ concentration of the discharged exhaust gas is reduced to a level satisfying regulations. On the other hand, when attention is paid to the exhaust gas or a part of air, each having high SO₂ concentration, as the quantity of the gas taken out is small and the speed of absorption rises with an increase of SO₂ concentration, the quantity of gas treated by the desulfurizing equipment is increased and the cost for the equipment is greatly reduced. Furthermore, when SO₂ is desorbed from the SO₂ absorbent, if the air with almost the same quantity as or more than the exhaust gas is brought into contact with the SO₂ absorbent, the required quantity of air for combustion is obtained. Acid-resisting alumina, zeolite with high silica alumina base, etc. are suitable for the SO₂ absorbent to be used in the present invention. In order to avoid the effect of pore closing of the absorbent due to existence of sulfuric acid at a low temperature and prohibit a decrease of absorbing capability at a high temperature, the desired temperature of the exhaust gas at the time of contact with the SO₂ absorbent is 150° - 600° C. The embodiment of the desulfurizing method and equipment provided by the present invention is shown by FIG. 1. The combustor 1 is a heavy oil exclusive burning boiler which produces exhaust gas of 3,000 Nm³/h and to which C heavy oil 3 as fuel is supplied from the fuel line 2 and air for combustion is sent from the passage 4 by premixing method, where the C heavy oil 3 is burnt to produce the exhaust gas containing SO₂ of 800ppm. The exhaust gas is sent through the passage 5 to the heat exchangers 6, where the collection of heat is made, and through the passage 7 to the absorbing towers 8A and 8B. Each of the absorbing towers 8A and 8B is filled with the acid-resisting absorbents 10A and 10B such as active alumina, zeolite with high silica alumina base, etc. The bottoms of the absorbing towers 8A and 8B are connected through the valves 9A and 9B to the above-mentioned passage 7. Moreover, the tops of the absorbing towers 8A and 8B are connected through the valves 11A and 11B to the passage 12 which connects with the chimney 20 and also through the valves 15A and 15B to the passage 14 which connects with the push fan 13. The heat exchanger 21 is equipped between the above-mentioned passage 12 and 14. The bottoms of 8A and 8B are connected through the valves 16A and 16B to the passage 4. The passage 17 diverges from the passage 4 and is connected to the lime-gypsum type desulfurizing equipment 18, the outlet of which is connected through the passage 19 to the lower current side of the heat exchanger 21 which connects with the passage 12. Here, it is assumed that the SO₂ absorbing tower 8A is in the process of absorbing SO₂ with the valves 9A and 11A open and with the valves 15A and 16A closed, and the SO₂ absorbing tower 8B is in the process of reactivation with the valves 9B and 11B closed and with the valves 15B and 16B open. In the embodiment constructed as above, the exhaust gas containing SO₂ from the combustor 1 is introduced through the valve 9A to the SO₂ absorbing tower 8A, where the SO₂ content of the gas is absorbed and removed by the SO₂ absorbent, and then is sent from the top of the tower, through the valve 11A and the passage 12, to the chimney 20, from which it is blown down out of the system as cleaned exhaust gas. At this time, the temperature within the absorbing tower 8A is maintained between 150°C and 600°C. At the same time, air of almost the same quantity as the exhaust gas, pressurized down to about 9,8·10³ Pa (1,000 mmAq) by the push fan 13, through the passage 14 and the valve 15B, flows and passes countercurrently in the SO₂ absorbing tower 8B. Moreover, this air, desorbing SO₂ absorbed by the absorbent 10B into the gas phase, continues to flow and pass to reactivate the absorbent 10B and is supplied, as the air containing SO₂, through the valve 16B and the passage 4 to the combustor 1 to burn the fuel 3. The SO₂ content in the air is mixed with SO₂ produced from the S content in the fuel resulting in an increase of the SO₂ concentration in the exhaust gas. On the other hand, a part of the air (10%) containing SO₂ is sent from the passage 4 through the passage 17 to the lime-gypsum type desulfurizing equipment 18 to produce gypsum. The purified gas after being treated is blown down from the chimney 20 through the passages 19 and 12. When the absorbing tower 10A reaches the state of saturation as a result of gradual absorption of SO₂ and the reactivation of the absorbing tower 10B is completed, the valve is switched to reactivate the absorbing tower 10A in turn and to put the absorbing tower 10B in the process of SO₂ absorption, thus achieving consecutive combustion and absorption and desorption of SO₂. In this embodiment, the quantity of SO₂ in the exhaust gas discharged from the absorbing towers 10A and 10B is very small and can be negligible; therefore, a closed system is formed because all amounts of SO₂ absorbed in the absorbing tower are returned back to the combustor and SO₂ taken out of the system is limited to SO₂ to be supplied through the passage 17 to the lime-gypsum type desulfurizing equipment 18. Accordingly, in a final stationary state, the quantity of SO₂ is equivalent to that of SO₂ produced from the S content contained in fuel to be supplied to the combustor 1 is considered to be taken out of the system through the passage 17. For this reason, as explained above, the SO₂ concentration in a stationary state in the air within the passage 17, i.e., the SO₂ concentration in the stationary state in the air within the passage 14 is given as 22.4n s /G₁ and, as a result, the SO₂ concentration in the exhaust gas in the combustor is raised accordingly. In the present embodiment, by taking out 10% of the exhaust gas from the diverged passage 17, the SO₂ concentration in the exhaust gas staying between the combustor 1 and the SO ₂ absorbing towers 8A and 8B is maintained at a definite value of 8,000 ppm after all. On the other hand, the SO₂ concentration in the air diverged by the passage 17 is also 8,000 ppm. The air with a flow rate of 300Nm³/h, after its SO₂ content is removed in the lime-gypsum type desulfurizing equipment 18, is blown down, as purified gas, from the chimney 20. In view of the equipment, in the equipment to treat the exhaust gas of 300Nm³/h, when compared with that of 3,000Nm³/h type, the scale of the equipment is reduced to one fifth and, owing to a high SO₂ concentration of the gas treated, SO₂ is absorbed with high efficiency. Moreover, in the present embodiment, as described above, the heat exchange is made between the exhaust gas from which SO₂ content is absorbed in the absorbing towers 8A and 8B and the air in the heat exchanger 21, and the gas is preheated as a result. However, this exhaust gas contains almost no SO₂, there is no troublesome problem such as sulfuric acid dew point corrosion issue. Therefore, the heat exchange can be made until the temperature of the exhaust gas lowers to less than 200°C resulting in improvement of heat efficiency of the plant and the exhaust gas can be discharged without additional burning. In the present embodiment, no power generation is made; however, when a trial calculation is made by scaling up the embodiment to a plant to treat the exhaust gas of 1 million Nm³/h by 350,000 KW, it is confirmed that, because the collection of heat is performed at a low temperature of the exhaust gas and the cost of heat source for raising the temperature of the exhasut gas after being desulfurized is reduced, the plant efficiency is improved by 3% in the end. Moreover, the utility of the SO₂ absorbing tower is required only for the power to raise pressure equivalent to pressure loss of about 4.9·10³ Pa (500 mmAq) and the normal utility for the combustor is also usable for other operations at the same time. In the present embodiment, although the SO₂ concentration in the exhaust gas is 800 ppm in the ordinary burning in the combustor, because SO₂ from the SO₂ absorbing tower joins in the combustor, the SO₂ concentration in the exhaust gas is raised up to 8,000 ppm. FIG. 2 represents the relation between the temperature within the tower of the SO₂ absorbing equipment in the present embodiment and the SO₂ concentration at the outlet of the absorbing tower when 1 ton of the SO₂ absorbent is used. A full line shows the result obtained from the case where active alumina was used as the absorbent and a dashed line shows the case where zeolite with high silica alumina base. As shown in FIG. 2, when the temperature within the tower is less than 150°C, the SO₂ concentration at the outlet is raised rapidly and when it is more than 600°C, the concentration is raised as well. FIG. 3 shows the relation between the SO₂ concentration of the exhaust gas at the outlet of the absorbing tower and the consumed quantity of the absorbent with the temperature within the absorbing tower set at 500°C in the embodiment. FIG. 3 shows that, by using 1 ton of the absorbent, the SO₂ concentration of the exhaust gas at the outlet of the absorbing tower is lowered to 1 ppm. As described above, in the present embodiment, almost all SO₂ can be removed from the exhaust gas discharged and SO₂ is changed to gypsum efficiently by small-sized lime-gypsum desulfurizing equipment, and furthermore, the heat efficiency of the plant can be improved by success in overcoming the problem of sulfuric acid dew point corrosion. Although a part of the air containing SO₂ desorbed from the SO₂ absorbent is taken out and treated by the lime-gypsum desulfurizing equipment in the present embodiment, the system can be designed so that a part of the exhaust gas staying between the combustor and the absorbing tower can be taken out, or a part of the exhaust gas or air, each having high SO₂ concentration, can be treated by other equipment than the lime-gypsum desulfurizing equipment. Moreover, although the consecutive burning of the combustor is performed by using two SO₂ absorbing towers in this embodiment, the system can be designed so that, by using only one SO₂ absorbing tower, the burning of the combustor, absorption of SO₂ from the exhaust gas and desorption of SO₂ by air can be achieved intermittently.

Claims: A method for desulfurizing exhaust gas containing SO₂ from a combustor in which the exhaust gas containing SO₂ from the combustor is first brought into contact with SO₂ absorbent to remove SO₂ and is subsequently discharged to the atmosphere; in which thereafter the SO₂ absorbent absorbed SO₂ is brought into contact with air to desorb SO₂ and the air containing the desorbed SO₂ is supplied to the combustor as air for combustion to raise the SO₂ concentration in the exhaust gas from the combustor; and in which a part of the exhaust gas prior to contacting the absorbent or a part of the air containing desorbed SO₂ is introduced into a lime-gypsum type desulfurizing equipment to be desulfurized. A desulfurizing equipment for desulfurizing exhaust gas containing SO₂ comprising at least one SO₂ absorbing tower (8) in which the SO₂ absorbent (10) is filled; a combustor (1) from which an exhaust gas passage (5,7) comprising a valve (9) is connected which can be switched to the SO₂ absorbing tower (8); a passage (14) comprising a valve (15) to supply the air for desorbing the SO₂ absorbant which is connected with a push fan (13) and can be switched to the abovementioned SO₂ absorbing tower (8); a passage (12) comprising a valve (11) to discharge the exhaust gas from which SO₂ is removed in the SO₂ absorbing tower (8) which is connected with the abovementioned SO₂ absorbing tower and can be switched to a chimney (20); a passage (4) comprising a valve (16) to supply the air containing SO₂ desorbed from the absorbent (10) in the tower (8) as combustion air to said combustor (1); a passage (17) to take out a part of the exhaust gas prior to contacting the absorbent or the air containing desorbed SO₂ from the exhaust gas passage or the abovementioned passage (4) to supply the air for combustion containing SO₂; and a lime-gypsum type desulfurizing equipment (18) which is connected with the abovementioned passage (17) to take out a part of the exhaust gas or the air and a passage (19) connected to the lime-gypsum type desulfurizing equipment (18) from which passage a part of the exhaust gas or the air is discharged to the atmosphere after being treated in the lime-gypsum type desulfurizing equipment(18). A desulfurizing equipment for the exhaust gas from the combustor (1) as in claim 2 wherein the SO₂ absorbing tower (8) is of two-tower type (8A, 8B).

Inventors: IZUMI JUN C O NAGASAKI, OSHIMA KAZUAKI C O MITSUBISHI, TANABE SEEIICHI C O MITSUBISHI, TSUTAYA HIROYUKI C O NAGASAKI, IZUMI, JUN, C/O NAGASAKI, OSHIMA, KAZUAKI, C/O MITSUBISHI JUKOGYO K.K., TANABE, SEEIICHI, C/O MITSUBISHI JUKOGYO K.K., TSUTAYA, HIROYUKI, C/O NAGASAKI
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: B01D53
IPC codes: B01D53