MXPA98003010A - System and process to treat gas de hum - Google Patents

Abstract

In a human gas treatment system, a cooling tower (21), a reheating section (22) and a fan are arranged in line on a vertical axis, so as to function as at least part of a chimney to emit the gas of flue gas treated in the atmosphere. Also, in a process of treating flue gas, the amount of ammonia injected in the denitration stage (a denitration apparatus (2)] and / or the amount of the ammonia injected at a point downstream of the denitration stage, they are determined so that they are at such an excessive level that the ammonia or an ammonium salt remains in the flue gas introduced in the desulfurization stage [absorption tower (21).) Thus, the size and cost of the equipment is can reduce

Description

SYSTEM AND PROCESS FOR TREATING SMOKE GAS FIELD OF THE INVENTION AND EXHIBITION OF THE RELATED TECHNIQUE This invention relates to a technique for the treatment of flue gas, to carry out at least the denitration and desulfurization of the flue gas. More particularly, it refers to a technique for the treatment of flue gas that makes it possible to reduce the size of the equipment and improve the performance of this equipment. Conventionally, in order to remove nitrogen oxides, sulfur oxides (typically sulfur dioxide) and dust (for example fly ash), present in the flue gas discharged from a boiler of a thermal power plant or the like , an exemplary humer gas treatment system or process, as illustrated in Figures 9 and 10, is widely employed. This humer gas treatment technique is described hereinafter. As illustrated in Figure 9, the untreated flue gas A, discharged from a boiler (not shown in Figure 9) is first introduced into a denier 2, installed in a boiler housing 1, so that the oxides of nitrogen, present in the flue gas, decompose. The denitration apparatus 2 functions to decompose the nitrogen oxides, according to the catalytic method of ammonia reduction, which uses a catalyst. At this stage, it has been a conventional practice to inject the ammonia B into the flue gas, in an amount almost equal to the equivalent amount required by the denitration. The amount of ammonia that slides on the downstream side of the denitration apparatus 2 is as low as about 5 ppm. Next, the flue gas is introduced into the air heater (or heat exchanger) 3, also installed in the boiler housing 1. Thus, the heat is recovered from the flue gas and used to heat the air C supplied to the boiler. Conventionally, a heat exchanger of the type called Ljungstrom, has been used as the air heater 3. The flue gas leaving this air heater 3 is subsequently driven out of the boiler housing 1 by a duct 4 and introduced into a dry electrostatic precipitator 5, installed outside the housing 1 of the boiler. In this electrostatic precipitator 5, the powder, present in the flue gas, is captured and removed.

In the case of a heater that burns oil, the ammonia can also be injected into the flue gas inside the duct 4, so that the sulfur trioxide (SO3), present in the flue gas, can be captured as sulfate. ammonium [(NH4) 2SC > 4] in the electrostatic precipitator 5. On the other hand, in the case of a boiler that burns coal, a large amount of dust, such as fly ash, is present in the flue gas, so that the SO3 present in the The flue gas does not form a harmful fog (submicron particles), but remains in a condensed state on the dust particles and captured in an electrostatic precipitator 5 and an absorption tower 8, which will be described below. Therefore, in the case of a boiler that burns coal, the injection of ammonia into the flue is generally omitted. Next, the flue gas, which leaves the electrostatic precipitator 5, is passed through the duct 6 and enters the heat recovery section (or heat exchanger) 7 of a gas-gas heater, where the heat recover from it Then, the flue gas is introduced into an absorption tower 8, which serves as a desulfurizer. In this absorption tower 8, the flue gas is carried in a gas-liquid contact with an absorbent fluid, having a suspended absorbent (for example, limestone) (hereinafter referred to as the absorbent aqueous paste), so that the SO2 present mainly in the flue gas is absorbed within the absorbent aqueous pulp and, likewise, the residual powder is also captured by the absorbent aqueous pulp. In a tank provided at the bottom of the absorption tower 8, the aqueous paste, having the SO 2 absorbed, is oxidized to form the gypsum as a by-product, according to the following reactions, which include a neutralization reaction.

Absorption Tank SO2 + H2O? H + + HSO3- (1) Tank H + + HSO3- + Y2 O2? 2H + + SO42- (2) 2H + + + SO42- + CaCOß + H2O? CaS? 4-2H2? + CO2 (3) Next, the flue gas from which the SO2 and the like have been removed in the absorption tower 8, which serves as a desulphurizer, is passed to the reheating section 9 of the gas-gas heater, which is heated. at a temperature favorable for emission into the atmosphere, using the heat recovered in the heat recovery section 7. Then, the flue gas is introduced into the lower part of the main body of a chimney 13 by means of a duct 10, a fan 11 and a duct, and finally discharged, as treated flue gas D, from the upper opening. from the main body 13 from the chimney to the atmosphere. The fan 11 functions to deliver the flue gas under pressure, so as to counteract the loss of pressure caused by the equipment and finally allow the flue gas to be emitted into the atmosphere through the main body of the flue 13. Conventionally, a motor lia has been installed separately from the main body of the fan 11. For the same purpose, a similar fan can be installed on the upstream side of the absorption tower 8. Also, in this system, the required height (L) of the main body of the chimney 13, above the ground, is determined only according to the concentrations of the nitrogen oxides and the sulfur oxides, which remain in the treated flue gas D and the concentration of the powder that remains there. , in order to comply with the regulations for the emission into the atmosphere. For example, based on conventional performance (ie, a degree of denitration of just over 80% and a degree of desulphurization of a little more than 80%), a height (L) of about 150 m has been generally required for electric power plants of the 150 MW class. In this case, the floor space required for the installation of the chimney, which includes a frame 14 for supporting and reinforcing the main body of the chimney 13, must usually be in the form of a square, with sides having a length ( W) of around 38 m. Figure 10 is a block diagram illustrating the construction of the system, which extends from the boiler to the air heater 3. In this Figure 10, the boiler is designated by the number a, the denitration catalyst contained in the denitration apparatus 2 by the number 2a and the decomposition catalyst of the ammonia contained in the denitration apparatus 2 by the number 2b. The ammonia decomposition catalyst 2b is used to remove any ammonia that flows downstream. However, when ammonia is injected in ordinary amounts, this catalyst is omitted, because the amount of ammonia that slides is very low. Also, as previously described, a heat exchanger, of the Ljungstrom type, has been used as the air heater 3. Therefore, in this system, a portion (for example of about 5%, based on the volume of the flue gas) of the supplied air C escapes to the side of the flue gas and, at the same time, a portion (per example of 1%) of the flue gas escapes to the side of air C, as shown by the lines interrupted in Figure 10. In the conventional treatment of the flue gas, described above, the large and high-cost equipment It has been disadvantageous. Especially in the markets of developed countries, companies that generate energy on a small scale and similar, a marked reduction in cost has been strongly desired, in addition to a reduction in the installation space and the height of the chimney. Specifically, the arrangement and construction of the conventional system has been such that, between the housing 1 of the boiler and the chimney 13, the electrostatic precipitator 5, the absorption tower 8 and the fan 11., are arranged in a horizontal direction and are connected by the humeros 4, 6, 10 and 12. This requires a large space between the housing 1 of the boiler and the chimney 13 and a plurality of ducts and a large number of supporting elements of them, which results in an increased cost. Also, as previously described, the height and installation space of the chimney are determined only, for example, in accordance with the concentrations of the nitrogen oxides and the sulfur oxides which remain in the treated flue gas D. Consequently , in order to reduce the size of the chimney, it is required lately to increase the performance of the equipment. This has also been difficult in the construction of the conventional system. For example, in order to increase the degree of desulfurization, it is conceivable to increase the gas-liquid contact capacity simply by enlarging the absorption tower 8. However, this is contrary to the desire to reduce in size and thus has certain limits. Likewise, in order to increase the degree of denitration in the denitration apparatus 2, it can be envisaged to achieve this by simply increasing the amount of ammonia injected. In such a case, the conventional system has used ammonia decomposition catalyst 2b, in order to remove any ammonia that slides downstream, which results in a corresponding increase in cost. If the ammonia decomposition catalyst 2b is not used in such a case, the ammonia will slide to the downstream side and exert the following adverse effect. That is, if the ammonia remains in the flue gas, an acidic, highly adherent ammonium sulfate (NH4HSO4) is produced, according to reaction formula (4), given below. The dew point of the acid ammonium sulfate is around 230 ° C, under the ordinary conditions in this type of equipment, while the flue gas is cooled from about 350 to 130 ° C in an ordinary air heater. Consequently, when the ammonia remains in the flue gas leaving the denitration apparatus 2, a large amount of acid ammonium sulfate (NH4HSO4) will be produced, especially in this air heater. An investigation, conducted by the present inventors, has revealed that, in a conventional air heater of the Ljungstrdm type, this acid ammonium sulfate tends to be deposited in the voids of the heat reservoir inside the heater and thus requires frequent operations of maintenance, such as cleaning.

SO3 * NH3 + H2O? NH4HSO4 (4) Therefore, a first object of the present invention is to provide a humer gas treatment system in which the arrangement and construction of the equipment is improved, in order to achieve a reduction in the size and cost of the equipment. A second object of the present invention is to provide a treatment process for the flue gas, which makes it possible to increase the treatment performance of the flue gas and thus achieve a reduction in the size of the equipment and the like. A third object of the present invention is to provide a process for the treatment of flue gas that can achieve the aforementioned reduction in the size of the equipment and increase in performance, without impairing its maintenance capacity. A fourth object of the present invention is to provide a process of treatment of flue gas, in which the arrangement and construction of the equipment is improved and the performance of the treatment of the flue gas is increased, for which a marked reduction can be achieved. in the size and cost of the equipment, which includes a reduction in the size of the fireplace. In order to achieve the above objects, the present invention provides a humer gas treatment system, comprising an absorption tower for bringing this flue gas in gas-liquid contact with an absorption fluid, to remove at least the sulfur oxides from the flue gas, by absorption in the absorbent fluid, a reheat section, to heat the flue gas leaving the absorption tower at a favorable temperature for emission to the atmosphere, and a fan for deliver the flue gas under pressure, so as to counteract the loss of pressure caused by the flow path of the flue gas, which includes the absorption tower and the reheat section, in which the absorption tower, the reheat section and the fan are arranged in line on a vertical axis, in order to function as at least a part of a chimney, to emit the treated flue gas into the atmosphere. The present invention also provides a process for the treatment of the flue gas, which comprises the step of denaturing the ammonia injected into the flue gas, which contains at least nitrogen oxides and sulfur oxides, to decompose these nitrogen oxides present in the flue gas. The flue gas, and the desulphurisation step of introducing the flue gas leaving the denitration stage in an absorption tower, is brought into gas-liquid contact with an absorbent fluid to remove at least the sulfur oxides from the flue gas by absorption in the absorption fluid, in which the ammonia is injected into the flue gas, as required, at a point downstream of the denitration stage and the amount of the ammonia injected in the denitration stage and / or the amount of the ammonia injected at the point downstream of the denitration stage are determined so that an excessive level of the ammonia or a salt of master child, remain in the flue gas introduced in the desulfurization stage. In the process of treating the flue gas of the present invention, the amount of the ammonia injected in the denitration stage can be determined so that the concentration of the ammonia remaining in the flue gas leaving the denitration stage will not be lower of 30 ppm. The process of treating the flue gas of the present invention may also include the step of recovering heat from introducing the flue gas, which leaves the denitration stage., in a heat exchanger on the side upstream of the absorption tower and thus recover the heat from the flue gas and a heat exchanger, of the non-exhaust type, of a hull and tube structure, can be used as this heat exchanger. The process of treating the flue gas of the present invention may further include a heat recovery step of introducing the flue gas, which leaves the denitration stage, into a heat exchanger on the side upstream of the tower of absorption and thus recover the heat from the flue gas, and the amount of ammonia injected in the denitration stage and / or the amount of ammonia injected at the point downstream of the denitration stage can be determined so that the concentration of the ammonia remaining in the flue gas introduced into the heat exchanger is in excess of the concentration of SO3 in this flue gas by 13 ppm or more. In the process of treating the flue gas of the present invention, a region, in which a liquid having a higher acidity of the absorbent fluid, is sprayed so as not to allow the ammonia to be easily released in the gas phase and to be created on the downstream side of the region of the absorption tower in which the flue gas is brought into gas-liquid contact with the absorbent fluid, whereby the ammonia, which remains in the flue gas, introduced into the the desulfurization stage is absorbed in the absorption tower without allowing it to remain in the flue gas left by this absorption tower. The process of treating the flue gas of the present invention may also include a first dust removal step of introducing the flue gas into a dry electrostatic precipitator on the upstream side of the absorption tower and thus removing the present dust. in the flue gas, and the second dust removal step of introducing the flue gas into a wet electrostatic precipitator on the downstream side of the absorption tower and thus removing the dust remaining in the flue gas. The present invention also provides a process for treating flue gas to purify the flue gas containing at least nitrogen oxides and sulfur oxides, using a flue gas treatment system including a denitration apparatus, for injecting ammonia. in the flue gas and decompose the nitrogen oxides present there, a heat exchanger to recover heat from the flue gas leaving the denitration apparatus, an absorption tower to carry the flue gas, which leaves the heat exchanger in gas-liquid contact, with an absorbent fluid, to remove at least the sulfur oxides from the flue gas by absorption in the absorbent fluid, a reheat section to heat the flue gas leaving the absorption tower to a favorable temperature for the emission to the atmosphere, using at least a part of the heat recovered in the heat exchanger, and a fan for deliver the flue gas under pressure, in order to counteract the pressure loss caused by the flow of the flue gas, which includes the absorption tower and the reheat section, the absorption tower, the reheat section and the fan are arranged in line on a vertical axis, in order to function as, at least a part, of a chimney, to emit the treated flue gas into the atmosphere, in which the ammonia is injected into the flue gas, as required, in a point downstream of the denitration apparatus, and the amount of ammonia injected into the denitration apparatus and / or the amount of ammonia injected at the point downstream of the denitration apparatus are determined to be at such an excessive level that ammonia or a Ammonium salt remain in the flue gas introduced into the absorption tower. In the humer gas treatment system of the present invention, the absorption tower, the reheating section and the fan are arranged in line on a vertical axis, in order to function as, at least part of, a chimney to emit the treated flue gas into the atmosphere. Thus, all these apparatuses or devices that have been installed conventionally on the outside of the frame for the chimney, are installed in the interior space of the frame for the chimney. As a result, the installation space of all the equipment is significantly reduced, so a marked reduction in the size of the equipment in hontal direction can be achieved. Also, a considerable portion of the conduits and their support elements become unnecessary, and the main body of the chimney becomes much shorter than before. Finally, a marked reduction in the cost of the equipment is achieved. In the process of treating the flue gas of the present invention, the ammonia is injected into the flue gas, as required, at a point downstream of the denitration stage, and the amount of the ammonia injected in the denitration stage. and / or the amount of ammonia injected at the point downstream of the denitration step is determined to be at such an excessive level that ammonia or an ammonium salt will remain in the flue gas introduced in the desulfution step. Consequently, at least the degree of desulfution in the desulfution stage is increased and this ultimately contributes to a reduction in the sizes of the absorption tower and the chimney. In particular, when the amount of ammonia injected in the denitration stage is determined, so that the concentration of the ammonia remaining in the flue gas leaving the denitration stage will not be less than 30 ppm, especially the degree of denitration in the denitration stage is markedly increased and this finally contributes to the reduction in the size of the chimney. Also, when the process of treating the flue gas of the present invention further includes the heat recovery step of introducing the flue gas leaving the denitration stage in the heat exchanger on the upstream side of the absorption tower and thus recover the heat from the flue gas, and a non-exhaust type heat exchanger, of disgust structure and tube, is employed as the heat exchanger, the heat of the flue gas can be effectively used in the conventional way to preheat the air for use in the boiler or reheat the treated flue gas, and the problems due to the formation of costs may decrease. That is, even if the acid ammonium sulfate formed by the reaction of the ammonia injected with the SO3 present in the flue gas, and the sulfuric acid mist formed from the SO3 present in the flue gas, condenses in the heat exchanger mentioned, a non-exhaust type heat exchanger of hull tube structure, is less subject to the decomposition of such materials on heat transfer surfaces and the like or the obstruction therewith, as compared to the conventionally used heat exchanger of the Ljungstrom type. Also, in this case, the heat exchanger does not allow air to escape into the flue gas. This can reduce the amount of flue gas to be treated and thus achieve a reduction in cost.

Likewise, when the process of treating the flue gas of the present invention further includes a heat recovery step of introducing the flue gas leaving the denitration stage in a heat exchanger on the side upstream of the absorption tower and thus recovering the heat from the flue gas, and the amount injected in the denitration stage and / or the amount of ammonia injected at the point downstream of the denitration stage are determined so that the ammonia concentration remaining in the the flue gas introduced into the heat exchanger will be in excess of the SO3 concentration in this flue gas by 13 ppm or more, the condensation of the acid ammonium sulfate in the said heat exchanger is minimized and a fine powder of neutral ammonium sulfate is mainly produced. Consequently, crusting due to acidic ammonium sulfate is markedly suppressed and this makes maintenance of the heat exchanger very easy. Likewise, when a region in which a liquid, which has an acidity greater than the absorbent fluid, is sprayed so that it does not allow the ammonia to be released easily in the gas phase and is created on the side downstream of the region of the absorption tower, in which the flue gas is brought into gas-liquid contact with the absorbent fluid, whereby the ammonia remaining in the flue gas introduced in the desulphurisation stage is absorbed in the tower. Absorption without allowing remain in the flue gas leaving the tower-absorption, an adverse effect, due to excessive injection of ammonia (ie, the emission of ammonia into the atmosphere can be avoided.) This serves to cope with the standards of future ammonia emissions and also contributes to the further purification of the flue gas, and also when the humer gas treatment process of the present invention also includes the first step of rem tion of dust to introduce the flue gas into a dry electrostatic precipitator on the upstream side of the absorption tower and thus remove the dust present in the flue gas, and the second stage of dust removal to introduce the flue gas. In a humid electrostatic precipitator on the downstream side of the absorption tower and thus remove the remaining dust in the flue gas, the dust separation capacity of the entire system is markedly improved. The other flue gas treatment process of the present invention is one for purifying the flue gas containing at least nitrogen oxides and sulfur oxides, by the use of a flue gas treatment system comprising a flue gas apparatus. denitration, a desulfurizer and the like, and having an absorption tower (which serves as the desulfurizer), an overheating section and a fan arranged in line on a vertical axis, so as to function as at least a part of a chimney, for emitting the treated flue gas into the atmosphere, wherein the ammonia is injected into the flue gas, as required, at a point downstream of the denitration apparatus, and the amount of ammonia injected into the denitration apparatus and / or the amount of ammonia injected at the point downstream of the denitration apparatus is determined to be at an excessive level so that the ammonia or an ammonium salt remains in the gas of humero introduced in the absorption tower. Thus, since the absorption tower, the reheating section and the fan are arranged in line on a vertical axis to form part of the chimney, the installation space of the equipment is markedly reduced. Also, at least the degree of desulfurization is increased due to the excessive injection of ammonia and this ultimately causes a reduction in the height of the chimney. That is, this process can produce excellent effects, such as an increase in equipment performance and a marked reduction in equipment size in both horizontal and vertical directions. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view illustrating a system of humer gas treatment, according to a first embodiment of the present invention; Figure 2 is a schematic view illustrating a flue gas treatment system, according to a second embodiment of the present invention; Figure 3 is a schematic view illustrating the details of the desulfurizer included in the flue gas treatment system of Figure 2; Figure 4 is a schematic view, illustrating a waste water system, without waste, suitable for use with the flue gas treatment system of Figure 2; Figure 5 is a graph showing data demonstrating an effect of the present invention (i.e., an improvement in the degree of desulfurization); Figure 6 is a schematic view illustrating an experimental apparatus for demonstrating an effect of the present invention (ie, minimizing a deposit due to SO3 in the flue gas); Figure 7 is a graph showing the experimental results (changes in gas pressure loss, in the heat exchanger) to demonstrate an effect of the present invention (minimizing a deposit due to SO 3 in the gas of humerus); Figure 8 is a graph showing the experimental results (changes in the overall heat transfer coefficient of the heat exchanger), to demonstrate the effect of the present invention (minimizing a deposit due to SO 3 in the gas of humero); Figure 9 is a schematic view, illustrating a conventional treatment system of the flue gas; and Figure 10 is a schematic view illustrating the denitration apparatus and another apparatus included in the conventional treatment system of the flue gas.

DETAILED DESCRIPTION OF THE PREFERRED ODALITIES Various embodiments of the present invention will be described below with reference to the accompanying drawings.

First Mode A first embodiment of the present invention is described with reference to Figure 1-. The same elements as included in the conventional system of Figure 9, are designated by the same reference numbers, and their duplicate explanation will be omitted. The humer gas treatment system of this embodiment is characterized in that, below the main body 13a of a chimney, an absorption tower 21, the reheating section 22 of a gas-gas heater, and a fan 23 are arranged in line on the vertical axis of the chimney, to be part of the chimney. The heat recovery section 24 of the gas-gas heater is installed in a position which is in the path of the conduit 25, which connects a dry electrostatic precipitator 5 with the absorption tower 21 and inside the frame 14 for the chimney. Finally, the entire absorption tower 21, the heat recovery section 24 and the reheating section 22 of the gas-gas heater, and the fan 23 are all installed in an unoccupied space inside the framework 14 for the chimney . In this embodiment, the absorption tower 21 is such that the flue gas is introduced there through an inlet, formed in its lower lateral part, through gas-liquid contact with an absorbent fluid, in a countercurrent manner. , to remove at least the sulfur oxides from the flue gas by absorption in the absorbent fluid, and discharged from an outlet formed at its upper end. In the manner previously described in connection with the conventional system, gypsum is formed as a by-product, using, for example, limestone as the absorbent.

The reheating section 22 of the gas-gas heater is directly connected to the upper end of the absorption tower 21. Thus, the flue gas discharged from the upper outlet of the absorption tower 21 is introduced into the reheating section 22 from the bottom side, heated to a favorable temperature for emission into the atmosphere, using the heat recovered in the heat recovery section 24, and discharged from the top side. In this case, a gas-gas heater of the circulating type of the heating medium is used, and this heat recovery section 24 and the reheating section 22 comprise non-exhaust type heat exchangers of shell and tube structure. These non-exhaust type heat exchangers are advantageous in that, even if the SO3 were present in the flue gas, it reacts with the ammonia present there, according to the aforementioned reaction formula (4), to produce the acid sulfate of ammonium (NH4HSO4) and are less subject to the deposition of this ammonium acid sulfate, which tends to be responsible for crusting. The fan 23 is a fan of axial flow installed above the reheating section 22 mentioned and which functions to suck the flue gas from the bottom side and discharge it from the upper side. The motor is arranged on its internal axis. In the arrangement and construction of the humer gas treatment system, according to this embodiment, the installation space of the total equipment is significantly reduced in comparison with the conventional system illustrated in Figure 9, so that a marked reduction in the Equipment size, in horizontal directions, can be achieved. Specifically, all the space required for the installation of the absorption tower 8, the fan 11 and the ducts 6 t 10, in the system of Figure 9, becomes unnecessary. Also, the installation space of the chimney is the same as before, due to the space allowed by the absorption tower 21 and the like when installed, which has been conventionally left in the frame 12 for the chimney. Likewise, the conduits 6 and 10, by themselves and their support elements, become unnecessary, and the main body of the chimney 13a becomes much shorter than before. Finally, a marked reduction in the cost of the equipment is achieved.

Second Mode Next, a second embodiment of the present invention is described with reference to Figure 2. The same elements as those included in the first embodiment are designated by the same reference numerals and their duplicate explanation will be omitted. This embodiment is characterized in that it is equipped with an air heater (or heat exchanger) 31 comprising an exhaust type heat exchanger of hull and tube structure, and a wet electrostatic precipitator 32 is installed between the absorption tower 21a and the reheating section 22. In this embodiment, the mentioned air heater 31 and the heat recovery section 24, previously described, of the gas-gas heater constitutes a heat exchanger to carry out the heat recovery stage of the gas heater. the present invention. Likewise, the dry electrostatic precipitator 5, described in relation to the conventional system, serves to carry out the first powder removal step of the present invention and the aforementioned wet electrostatic precipitator 32 serves to carry out the second stage of removal of dust. powder of the present invention. The construction, described above, has the following advantages. First of all, since the air heater 31 comprises a non-exhaust type heat exchanger, the amount of deposited crusts is relatively small, even if acid ammonium sulfate is produced in the flue gas introduced into the air heater 31, as previously described. This is very advantageous from the point of view of maintenance. More specifically, as previously described, when the ammonia remains in the flue gas leaving the denitration apparatus 2, the acid ammonium sulfate is produced especially in the air exchanger 31. According to an investigation conducted by the present inventors, it has been found that, in the case of a conventional air heater, of the Ljungstrom type, such acid ammonium sulfate tends to deposit in the voids of the heat reservoir inside the heater of air, and requires frequent maintenance operations, such as cleaning. However, an investigation conducted by the present inventors has revealed that the non-exhaust type heat exchangers of hull and tube structure are less subject to such deposition of, or obstruction with, the acid ammonium sulfate. It has also been known that the problem concerning the production of acidic ammonium sulfate and its deposit on the internal surfaces of the heat exchanger can also be remedied by the injection of an excessive amount of ammonia into the flue gas, and this will be described specifically below. Also, when the are heater 31 comprises a non-exhaust type heat exchanger, the air C fed to the boiler does not escape inside the flue gas. This causes a decrease in the flow regime of the flue gas being treated and thus a corresponding reduction in the capacities of the fan 23 and the flue gases in the energy consumption. Also, since the electrostatic precipitator 32 is installed, fine dust and other foreign matter, which was not captured in the absorption tower 21a, can be removed. Thus, the concentration of residual dust in the treated flue gas D is reduced. This increases the performance of the equipment in that aspect and also contributes to a reduction in the height of the chimney. Next, the construction and effects of the characteristic parts of the humer gas treatment process according to the present invention is described, which is carried out using the humer gas treatment system of the above-described embodiment. According to this process, instead of using an ammonia decomposition catalyst, the amount of ammonia B injected into the denitration apparatus 2 is determined to be at an excessive level compared to a large amount of ammonia or a salt of ammonia. ammonium remaining in the flue gas introduced into the absorption tower 21a. If the ammonia or an ammonium salt remains in the flue gas introduced in the absorption tower 21a, this ammonia or ammonium salt is dissolved in the aqueous paste within the absorption tower 21a as a result of the gas-liquid contact between the flue gas and the absorbent aqueous paste. This raises the concentration of the ammonium salt (in other words, the concentration of the ammonium ion) in the liquid phase of the aqueous paste flowing through the absorption tower 21a. An investigation conducted by the present inventors has revealed that, when the concentration of the ammonium salt (or the concentrations of ammonium ions) in the circulation fluid of the absorption tower is increased to 150 mmoles / liter or more, the degree The removal of sulfur dioxide from the flue gas (ie, the degree of desulfurization) in the absorption tower is raised to the neighborhood of 95%, k as shown in Figure 5, even if the other conditions remain constant. . Accordingly, according to this embodiment, in which the amount of ammonia B injected into the denitration apparatus 2 is determined, to be at an excessive level, as described above, the size of the absorption tower 21a can be reduced in size. comparison with the prior art. Also, the concentration of the sulfur oxides (typically the sulfur dioxide) remaining in the treated flue gas D, can be further reduced and, therefore, the height of the chimney can be reduced. Also, since the amount of ammonia B injected is naturally in excess of the equivalent amount required for denitration, the denitration capacity of the denitration apparatus 2 (or the denitration stage) is increased. According to an investigation conducted by the present inventors, it has been found that, if the amount of ammonia B injected is determined to be in excess of the equivalent amount required for the denitration and also the ammonia concentration remaining in the the flue gas leaving the denitration stage (ie, the sliding ammonia) will not be less than 30 ppm, the degree of denitration is increased from the conventional level of approximately 80 to 90% and the concentration of the oxides of Nitrogen remaining in the treated flue gas D can be reduced by half.

In this regard, the test calculations made by the present inventors with respect to a power plant of class 150 MW, indicate that, if the degree of desulfurization and the degree of denitration are increased, as described above, and the The degree of dust removal is also increased by the installation of the wet electrostatic precipitate 32, the height of the chimney (Ll) can be markedly reduced from the conventional value of approximately 150 m to 90 m. Also, as a consequence thereof, the width (Wl) of the installation space of the frame 14b for the chimney can be markedly reduced from the conventional value of about 38 m to about 25 m. In this case, the particular amount of ammonia B injected must be determined not only to be in excess of the equivalent amount required for denitration, but also considering the concentration of SO3 in the flue gas. Specifically, at least a portion of the ammonia remaining in the flue gas, which leaves the denitration apparatus 2 (or the denitration stage), reacts with the SO 3 present in this flue gas, to form ammonium salts, such as ammonium sulfate and ammonium acid sulfate, described above. In this case, most of these ammonium salts are captured by the electrostatic precipitator 5. Consequently, of the ammonia gas remaining in the flue gas leaving the denitration apparatus 2, only that portion of the ammonia gas that is in excess of the equivalent amount for SO3, remains in the flue gas introduced in the absorption tower 21a. More specifically, it is convenient to determine the amount of the ammonia injected so that the concentration of this ammonia remaining in the flue gas introduced into the air heater 31 and the heat recovery section (or heat interwarmer) 24 of the heater Gas-gas will be in excess of the concentration of SO3 in this flue gas by 13 ppm or more. As demonstrated by the experiments that will be described below, this makes it possible to suppress the deposit of ammonium acid sulfate which condenses in the mentioned heat exchanger. Thus, the formation of a deposit (or crust) on the heat transfer surfaces and other internal surfaces of the heat exchanger becomes slight and facilitates the maintenance of the heat exchanger. That is, in the conventional system, illustrated in Figure 9, the concentration of the ammonia remaining in the flue gas, introduced into the air heater 3, is as low as about 5 ppm, so that the acid sulfate of Ammonium is produced in a greater amount than the common ammonium sulfate. This acid ammonium sulfate tends to condense especially in the air heater 3 and crusts there. However, if this concentration of ammonia is in excess of the concentration of SO3 in the flue gas by 13 ppm or more, most of the SO3 present in the flue gas is converted to a fine powder of neutral ammonium sulfate, which contains (NH4) 2SC > 4 and the production of highly adherent ammonium sulfate, which tends to form scabs, becomes relatively low. Also, in accordance with this embodiment using the air heater 31 of the hull and tube structure, the problems due to crust formation are minor compared to the conventional system using an Ljungstrom type air heater. Finally, the problem of scab formation, due to ammonium acid sulfate, can be solved practically and the denitration apparatus need not be provided with a decomposition catalyst of ammonia. In this embodiment, an excessive amount of the ammonia is injected positively, so that a large amount of ammonia or an ammonium salt remains in the flue gas introduced into the absorption tower 21a. Therefore, the arrangement of the ammonia absorbed in the aqueous paste within the absorption tower 21a and the ammonia escaping in the treated flue gas D, they have problems. However, these problems can be solved by using the water disposal technician without existing waste (named AWMT) in which the ammonia is recovered and reused by mixing the powder from the electrostatic precipitator with the waste water from the desulfurizer, or a ammonia absorption, recently devised by the present inventor-is. Some embodiments of these techniques are described below with reference to Figures 3 and 4. Figure 3 is a schematic view showing, in particular, the detailed construction of a desulfurizer suitable for use in the flue gas treatment system of this embodiment, as illustrated in Figure 2, and Figure 4 is a schematic view showing the construction of an exemplary water disposal system without waste, suitable for use with the flue gas treatment system of this embodiment (in the case of a boiler that burns oil). In this case, as illustrated in Figure 3, the absorption tower 21a, which serves as a desulphurizer, is an absorption tower of the liquid column type, which is provided at the bottom with a tank 41 for retaining a fluid E of absorption, having an absorbent (for example limestone) suspended therein (hereinafter referred to as the absorbent aqueous paste E) and having a gas-liquid contact region, which extends above the tank 41 and which serves for bringing the flue gas into gas-liquid contact with the slurry within the tank 41. This absorption tower 21a is a so-called counterflow absorption tower, in which an inlet section 42 of the flue gas, to introduce This flue gas is formed in its lower part and an exit section 43 of the flue gas to discharge the desulfurized flue gas Al, is formed at its upper end, so that the flue gas enters from the lower part of the flue gas. the absorption tower and flow up. The mist eliminator 43a is installed in the flue gas outlet section 43. This mist eliminator 43a serves to pick up any fog produced as a result of gas-liquid contact and entrained by the flue gas, so that a large amount of sulfur dioxide containing mist, ammonia and the like, may not be discharged together with the desulfurized flue gas Al. In this embodiment, the mist collected by this mist eliminator 43a is allowed to flow down from its lower end and return directly to the tank 41.

Also, in the absorption tower 21a, a plurality of spray tubes 44 are arranged in parallel. In these spray tubes 44, a plurality of nozzles (not shown) for injecting the aqueous paste into the tank 41 upwards in the form of liquid columns, are formed. Also, a circulation pump 45, for removing and raising the absorbent aqueous paste inside the tank 41 is installed on the outside of the tank 41. Thus, the aqueous paste is fed to the spray tubes 44 through a circulation line 46. In the embodiment illustrated in Figure 3, the tank 41 is provided with an element for blowing air F stops the use of oxidation in the form of fine bubbles, while stirring the aqueous paste within the tank 41. This element comprises a stirrer 47 and a supply tube 48 of air for blowing air F into the aqueous paste, in the vicinity of the stirring blades of the agitator 47. Thus, the absorbent aqueous paste having the absorbed sulfur dioxide, is brought into efficient contact with the air in the tank 41 and there it is completely oxidized to form the gypsum. More specifically, the absorbent aqueous paste injected from the spray tubes 44 into the absorption tower 21a flows downwards, while the absorption sulfur dioxide and the powder (which contains ammonium salts, such as ammonium sulfate) and also, the ammonia gas, as a result of gas-liquid contact with the flue gas, and enters the tank 41 where it is oxidized by contact with a large number of air bubbles there blown, while being stirred by means of of the agitator 47 and the air supply tube 48 and then subjected to a neutralization reaction to become an aqueous paste containing gypsum with high concentration. The dominant reactions that occur in the course of these treatments are represented by reaction formulas (1) to (3), mentioned above. Thus, a large amount of gypsum, a small amount of limestone (used as the absorbent) and a slight amount of dust and ammonia, collected from the flue gas, are suspended or dissolved stably in the aqueous paste within the tank 41. In this embodiment, the slurry within the tank 41 is removed and fed to a solid-liquid separator 49, via a pipe line 46a branching from the circulation line 46. The slurry is filtered on a separator 49. of solid-liquid, so that gypsum G, which has a low water content, is recovered. By another art, a portion Hl of the filtrate from the solid-liquid separator 49 is fed to an aqueous paste preparation tank 52 as water constituting the absorbent aqueous paste E and the remainder is discharged as desulfurization waste water H2, in order to prevent the accumulation of impurities. Since ammonia and ammonium salts (such as ammonium sulfate) absorbed from the flue gas have high solubilities, most of them are contained in the liquid phase of the aqueous paste E and finally discharged together with H2 water. of desulphurisation waste. In this embodiment, an aqueous slurry containing the limestone as the absorbent is fed from the tank 52 for preparing the slurry to the tank 41 during the operation. This aqueous paste preparation tank 52 is equipped with an agitator 53 and serves to prepare the absorbent aqueous paste E, by mixing the powdered limestone I introduced from a silo (not shown) with the filtrate Hl fed as described above, and stirring this mixture. The absorbent aqueous paste E inside the tank 52 for preparation of this aqueous paste is suitably fed to the tank 41 by means of a pump 54. Likewise, in order to replenish the water, it is gradually lost due to the evaporation in the absorption tower 21a or similar, this replenishment water (such as industrial water) is suitably supplied, for example, to tank 41 or water slurry preparation tank 52. During the operation, the flow regime of the aforementioned spare water, supplied to the tank 41, the flow rate of the aqueous pulp withdrawn through the pipe line 46 °, and the like, are suitably controlled. Thus, tank 41 is maintained in such a state that the aqueous paste containing gypsum and absorbent at predetermined concentrations is always stored at a level within certain limits. Also during the operation, in order to maintain the degree of desulfurization and the purity of the system at a high level, the boiler load (ie, the flow rate of flue gas A), the concentration of the sulfur dioxide in the the flue gas introduced into the absorption tower 21a, the pH and the concentration of the limestone of the absorbent aqueous paste within the tank 41, and the like, are detected with sensors. Based on the detection results, the limestone feed regime to tank 41 and other parameters are adequately controlled by means of a controller (not shown). Conventionally, the pH of the absorbent aqueous paste within the tank 41 is usually adjusted to approximately 6.0, so that highly pure gypsum can be formed by accelerating the oxidation reaction described above, while maintaining the high capacity to absorb the sulfur dioxide. Also, as a remedy to prevent the excess injected ammonia from remaining in the desulfurized flue gas Al, spray tubes 55 are installed above the spray tubes 44. These spray tubes 55 serve to inject a liquid J (which may be in the form of an aqueous paste) having a lower pH value than the aqueous paste within the tank 41 in the absorption tower 21a and thus creating, in the upper part of the absorption tower 21a, a region which it does not allow ammonia to be easily released into the gas phase. For example, the liquid J injected leaves the tubes 55 spray comprises a dilute solution of sulfuric acid, and its pH is adjusted to a value (for example 4.0 to 5.0) in which the ammonia is not easily released into the flue gas. In the above-described construction, the flue gas introduced into the absorption tower 21a, through the inlet section 42 of the flue gas, first contacts gas-liquid with the aqueous paste injected from the tubes 44 of sprayed in the form of columns of liquid, and then in gas-liquid contact with the liquid injected from the spray tubes 55. Thus, dust and ammonia together with sulfur dioxide, are absorbed or captured. During this process, the liquid injected from the spray tubes 55 into the outlet section (or upper end) of the absorption tower 21a is adjusted to a pH value at which the ammonia is not easily released into the flue gas . Consequently, the partial pressure of the ammonia is decreased in the upper part of the absorption tower 21a, so the phenomenon in which the ammonia, once dissolved in the liquid phase of the aqueous paste, is released inversely in the gas of Smoke at the top of the absorption tower is avoided. Thus, the desulfurized flue gas Al, which has very low concentrations of sulfur dioxide, dust and ammonia, is finally discharged from the flue gas outlet section 43, formed at the upper end of the absorption tower 21a. In this case, the calculations made by the present inventors have revealed that the degree of ammonia removal is approximately 90%. Consequently, in spite of the construction in which an excessive amount of ammonia is injected positively, little ammonia is contained in the treated flue gas D (Figure 2) and the problem with the emission of ammonia into the atmosphere does not arise.

However, it is convenient from the point of view of the prevention of air pollution, to minimize the concentration of ammonia in the treated flue gas D, emitted into the atmosphere. Therefore, there is a demand for a flue gas treatment technique that can achieve a reduction in the size of the equipment and a high degree of desulfurization and, also, the amount of ammonia emitted can be minimized. Next, the construction of the exemplary water disposal system without waste is described, illustrated in Figure 4. This is an example related to the treatment of flue gas from a boiler that burns oil. In this case, the powder K collected from the flue gas by the electrostatic precipitator 5, illustrated in Figures 2 and 3, contains, in addition to the unburnt carbon, constituting its main component, impurities, such as vanadium (which is a toxic heavy metal) and magnesium, ammonium sulfate formed from the injected ammonia and SO3 present in the flue gas, and the like. In this system, as illustrated in Figure 4, the H2 water of desulphurisation waste, shown in Figure 3, is first introduced into a mixing tank 61, where it is agitated and mixed with the powder K fed from the dry electrostatic precipitator. 5, to form a mixed aqueous slurry Sl. In this step, the ammonia and the ammonium sulfate contained in the powder K, are dissolved in the liquid phase of the slurry Sl, and most of them exist as sulfate ions or ammonium ions, similarly to those contained in the water H2 waste. Then, the mixed slurry Sl is transferred to a pH adjustment / reduction tank, where an L acid (for example sulfuric acid (H2SO4)) is added. Thus, the mixed slurry Sl is adjusted to a pH value (of about 2 or less) that allows the reduction of vanadium. Likewise, a reducing agent M (for example sodium sulfite (Na2SC > 3)) is added and mixed with the aqueous paste. Thus, the pentavalent vanadium present in the aqueous paste is reduced to its tetravalent subtract, according to the following reaction formula (5), so that the vanadium is dissolved in the liquid phase. 2V? 2+ + SO32- + 2H +? 2V02 + + SO42- + H20 (5) Next, the mixed aqueous slurry S2, which has to undergo the vanadium reduction, is transferred to the precipitation tank 63, where the ammonia B3 (to be described later) is added to and mixed with the slurry. In this step, the tetravalent vanadium, present in the aqueous paste *, reacts with the ammonia, according to the following reaction formula (6) and precipitates the resulting product.

VQ2 + + 2NH4OH - »VO (OH) 2 + 2NH4 + (6) After being treated for the precipitation of the vanadium, the mixed aqueous slurry S3 is removed from the precipitation tank 63 and transferred to a solid-liquid separator 65, comprising a flocculation settler and / or a vacuum-type band filter, by means of a pump 64 of aqueous paste. Thus, the solid matter N is separated from it in the form of mud or a mass. The separated solid matter N consists essentially of unburned carbon, present in the powder K and additionally contains the precipitated vanadium. Next, the waste liquor S4 from which the solid matter, which contains the vanadium, has been removed, transferred to a neutralizing tank 66, where a chemical agent O (for example, slaked lime (Ca (OH) 2 )) and the return waste P liquor (which will be described later) are added with agitation. Thus, the sulfur and ammonium ions present in the waste liquor are converted to gypsum or ammonium hydroxide.

The resulting aqueous paste S5, which now contains the gypsum as the solid component and the ammonium hydroxide, is then transferred to a primary concentrator 67, where the ammonia Bl is separated therefrom by evaporation. The resulting aqueous paste S6 containing the gypsum and other solid matter in high concentrations is removed therefrom by means of an aqueous paste pump 68. The primary arranger 67 consists of an evaporator 67a, a heater 67b and a circulation pump 67c, and functions to heat the aqueous slurry with hot steam Wl, generated, for example, in a boiler of an electric power plant and thus evaporate the Bl water that contains ammonia. In addition to the gypsum, the solid matter contained in the aqueous paste S6 includes mainly the magnesium hydroxide (Mg (OH) 2) - This magnesium hydroxide is formed by the combination of the magnesium present in the powder K as an impurity with the ions of hydroxide present in the aqueous paste. • Subsequently, this paste S6 is introduced into a solid separator 69, comprising a cyclone or a settling centrifuge, where the aqueous paste S6 is separated into an aqueous paste S7 containing mainly gypsum (particulate solid matter in coarse form) and a aqueous paste S8, which contains solid matter in fine particles (which mainly includes the magnesium hydroxide described above). The slurry S5 is returned to the tank 41 of the absorption tower constituting the desulphuriser illustrated in Figure 3. On the other hand, a portion of the slurry S8 is dehydrated in a dehydrator (or secondary concertator) 70, and the material The resulting solid, which contains mainly the magnesium hydroxide, is discharged as a slurry Q. From the slurry S8, the remaining portion not fed to the dehydrator 70 is returned to the neutralization tank 66 as a waste return liquor P. The ammonia water Bl, produced by evaporation in the primary pairing 67, is cooled and condensed in a cooler 71, using the cooling water W2 as the refrigerant, and stored in a storage tank 72. Since the ammonia water Bl, inside the storage tank 72, it usually has a low concentration of about 3-6%, it is fed to an ammonia binder 74 by means of a pump 73 and is concentrated to supply ammonia water having a concentration of 10 to 20% . A portion of this ammonia water is gasified in a vaporizer 75 and the resulting ammonia B2 is injected into the flue gas in the denitration apparatus 2., previously described, as ammonia B containing water vapor W3. The remaining portion of the ammonia water is fed to the above-mentioned precipitation dam 63, such as ammonia B3. In the waste water disposal system described above, the H2 water of desulphurisation waste resulting from the desulfurization operation is treated by mixing it with the removed dust K, so that an improvement in the handling capacity is achieved. The resulting mixed aqueous paste is subjected to a series of treatments for the reduction, precipitation and solid-liquid separation of the vanadium present there, and the separated vanadium is discharged as a slurry. Likewise, the mixed aqueous paste concentrated and treated in such a way that the gypsum, water and ammonia are finally returned to the flue gas or to the side upstream of the system (for example, the absorption tower that constitutes the desulfurizer). Thus, it becomes possible to use ammonia in a circulation way and to realize a closed system named water without waste, in which no waste water produced will be discharged. This eliminates the need for wastewater treatment before discharge and enables the effective use of ammonia.

Experiments Now, some experiments carried out by the present inventors are described below. The purpose of these experiments is to demonstrate that the formation of crusts on the internal surfaces (for example, the heat transfer surfaces) of the heat exchanger for the heat recovery stage, due to ammonium acid sulfate, is suppressed when inject an excessive amount of ammonia, in accordance with a feature of the present invention and a leak-free type heat exchanger of shell and tube structure is used as the heat exchanger. First of all, an experimental apparatus is used, illustrated in Figure 6. Specifically, an air heater 82 and a cooler 83 are installed downstream of a combustion furnace 81. Also, a cyclone 84 for separating and removing dust, such as unburnt carbon, is installed downstream of them. The flue gas leaving this cyclone 84 was introduced into the heat exchanger 85, of leak-free type, of hull and tube structure. In this case, the flue gas was passed through the heat exchanger 85 on its side of the hull (ie, on the outside of the heating tubes), while the heating means is passed through the heating tubes of the heat exchanger 85. The heating medium, heated by the heat of the flue gas in the heat exchanger 85, was cooled and regenerated in a cooler 86 using the cooling water. The concentration of SO3 in the flue gas was regulated by injecting the SO3 into the flue gas at a position downstream of the air heater 82 and upstream of the cooler 83. Also, the concentration of the ammonia in the flue gas is regulated by the injection of the ammonia into the flue gas at a position downstream of the cyclone 84 and upstream of the heat exchanger 85. This heat exchanger 85 is capable of so-called cleaning of steel balls, by spreading steel balls continuously in its side of the hull, and is subjected to the cleaning tests of the steel balls, as required. Other experimental conditions are as follows: Type Fuel. Fuel oil A Burning rate: 15 liters / hour. Flue Gas Flow rate: 200 m3N / h SO3 concentration: 25 ppm NH3 concentration: 63 ppm. Temperature at cyclone outlet: 170 ° C Heat exchanger inlet / outlet temperature: 130/90 ° C. Heating Medium Inlet Temperature: 75 ° C Steel Balls Dissemination Regime: 2,800 kg / m2 •] - .. In this case, the concentration of ammonia in the flue gas was determined to be 63 (= 50 + 13 ) ppm, so it will be in excess of the equivalent amount required for the formation of ammonium sulfate [(NH4) 2SO4], as a result of the reaction with SO3, by 13 ppm. The equivalent amount, mentioned above, is equal to twice the number of moles of SO3 and, in this case, corresponds to a concentration of 50 (= 25x2) ppm. Under the conditions described above, the experimental apparatus was continuously operated for 83 hours without the cleaning of steel balls. Next, the cleaning of steel balls was carried out for 2 hours. Figure 7 shows the results of the actual measurement of the changes in the gas pressure loss in the heat exchanger 85, and Figure 8 shows the results of the actual measurement of the changes in the overall heat transfer coefficient of the heat exchanger. heat exchanger 85, which serves as an indication of its heat transfer capacity. As can be seen from these results, the changes in the gas pressure loss and the overall heat transfer coefficient are relatively slight even after a continuous operation of 83 hours. Likewise, they can be completely restored to their initial levels by cleaning steel balls. Similarly, when the surfaces of the heating tubes of the heat exchanger 85, after 83 hours of continuous operation were photographed and visually observed with the naked eye, the accumulation of deposits was slight. In the analysis, this deposit mainly includes an ammonium sulfate type compound, which has a molar ratio of NH4 / SO4 of 1.5 to 1.9. Thus, it can be seen that, if the ammonia is injected in such an amount as to be in excess of the SO3 concentration by 13 ppm or more, the production of the ammonium acid sulfate is suppressed and this greatly facilitates the operation to remove a deposit. . It will be understood that the present invention is not limited to the modalities described above, since it can also be practiced in several other ways.

For example, injection of ammonia need not only be carried out in the denitration apparatus (or the denitration stage), but also be carried out at any point downstream of the denitration apparatus and upstream of the absorption tower . By way of example, in the system of Figure 2, ammonia can be injected into the flue gas in line 4 for the purpose of capturing SO 3 and increasing the desulphurisation capacity, or the ammonia can be injected into the gas of flue in conduit 25 (on the downstream side of electrostatic precipitator 5), in order to increase the desulphurisation capacity and the like. In this case, in order to increase the desulfurization capacity of the absorption tower, the amount of ammonia injected in the denitration stage and / or the amount of ammonia injected at the point downstream of the denitration stage, can be determined so that it is in excess of the equivalent amount required for denitration or the equivalent amount for SO3 and, therefore, ammonia or ammonium salt (eg ammonium sulfate) will remain in the introduced flue gas in the desulfurization stage. Also, in order to completely suppress the formation of crusts due to SO3 in the air heater 31 and the heat recovery section 24 of the gas-gas heater, the amount of the ammonia injected in the denitration stage and / or the The amount of ammonia injected at the downstream point of the denitration stage can be determined to be at such an excessive level, so that the ammonia concentration remaining in the flue gas introduced into these heat exchangers is greater than the SO3 concentration in this flue gas by 13 ppm or more. Similarly, the air heater 31 and the section 24 of heat recovery of the gas-gas heater are installed separately, for example, in the previously described embodiment of Figure 2. However, they can be combined in a single unit. That is, the system can be constructed in such a way that the air C fed to the boiler is powered by the heat recovered in the installed heat exchanger, for example, in the position of the air heater 31 (Figure 2) and a portion of the heating medium is conducted to the reheating section 22 of the gas-gas heater and used to heat the treated flue gas D. Even when the air heater and the heat recovery section of the gas-gas heater are installed separately, the heat recovery section of the gas-gas heater can be installed in a position upstream of the electrostatic precipitator 5. In this aspect, if the heat recovery from the flue gas is carried out completely on the current side above the electrostatic precipitator 5 and the temperature of the flue gas introduced into the electrostatic precipitator 5 is subsequently reduced, this is especially advantageous in the case or of the flue gas from a coal burning boiler, due to the degree of dust removal (eg fly ash) in the electrostatic precipitator 5 is markedly improved based on its increased resistivity.

Claims (8)

1. A flue gas treatment system, which comprises an absorption tower, for carrying the flue gas in a gas-liquid contact with an absorption fluid, to remove at least the sulfur oxides from the flue gas by absorption inside the absorbent fluid, a reheating section, to heat the flue gas, which leaves the absorption tower, at, a favorable temperature for emission to the atmosphere, and a fan to deliver the flue gas under pressure, for thus counteracting the pressure loss caused by the flow path of the flue gas, which includes the absorption tower and the reheat section; wherein the absorption tower, the reheating section and the fan are arranged in line on a vertical axis, so that it functions, as at least a part of a chimney, to emit the treated flue gas into the atmosphere.

2. A process for the treatment of the flue gas, which comprises the denitration stage of injecting ammonia into the flue gas, which contains at least nitrogen oxides and sulfur oxides, to decompose the oxides of nitrogen present in the flue gas , and the desulfurization step of introducing the flue gas leaving the denitration stage in an absorption tower, where it is put in gas-liquid contact with an absorbent fluid, to remove at least the sulfur oxides from the gas of humero for absorption within the absorbent fluid; wherein the ammonia is injected into the flue gas, as required, at a point downstream of the denitration stage; and the amount of the ammonia injected in the denitration step and / or the amount of ammonia injected at the point downstream of the denitration step, are determined so that an excessive level of ammonia or an ammonium salt remains in the flue gas introduced in the desulfurization stage.

3. A process for treating the flue gas, as claimed in claim 2, wherein the amount of the ammonia injected in the denitration stage is determined so that the concentration of this ammonia, which remains in the flue gas, which leave the denitration stage, do not be less than 30 ppm.

4. A process for the treatment of the flue gas, as claimed in claim 2, which also includes the heat recovery step of introducing the flue gas, which leaves the denitration stage, in a heat exchanger, on the side upstream of the absorption tower and thus recover the heat from the flue gas and in which a heat exchanger, of leak-free type, of hull and tube structure, is employed as the mentioned heat exchanger.

5. A process for the treatment of the flue gas, as claimed in claims 2 or 4, which further includes the heat recovery step of introducing the flue gas, which leaves the denitration stage, in a heat exchanger in the side upstream of the absorption tower and thus recover the heat from the flue gas and where the amount of the ammonia injected in the denitration stage and / or the amount of ammonia injected at the point downstream of the denitration stage are determined , so that the concentration of ammonia, which remains in the flue gas, introduced into the heat exchanger, will be in excess of the concentration of SO3 in this flue gas by 13 ppm or more.

6. A process for the treatment of the flue gas, as claimed in any of claims 2 to 4, in which a region is created in which a liquid is sprayed, which has a higher acidity than the absorption fluid, to allow the ammonia is easily released in the gaseous phase, on the downstream side of the region of the absorption tower in which the flue gas is brought into gas-liquid contact with the absorbent fluid, whereby the remaining ammonia in the flue gas introduced in the desulfurization stage, it is absorbed in the absorption tower, without allowing it to remain in the flue gas leaving the absorption tower.

7. A process for the treatment of the flue gas, as claimed in any of claims 2 to 4, which further includes the first stage of dust removal of introducing the flue gas into a dry electrostatic precipitator on the upstream side of the absorption tower and thus remove the dust present in the flue gas, and the second stage of dust removal to introduce the flue gas in a wet electrostatic precipitator on the downstream side of the absorption tower and thus remove the dust which remains in the flue gas.

8. A process for treating flue gas, for purifying this flue gas containing at least nitrogen oxides "and sulfur oxides, by the use of a flue gas treatment system, comprising a denitration apparatus, for injecting ammonia in the chimney gauze to decompose the nitrogen oxides present there, a heat exchanger, to recover the heat from the flue gas, which leaves the denitration apparatus, an absorption tower to put the flue gas, which leaves the heat exchanger, in gas-liquid contact with an absorption fluid, to remove at least the sulfur oxides from the flue gas by the absorption in the absorbent fluid, a reheat section to heat the flue gas leaving the absorption tower, at a favorable temperature for the emission to the atmosphere using at least part of the heat recovered in the heat exchanger, and a fan, to deliver the humer gas or under pressure, in order to counteract the loss of pressure caused by the flow path of the flue gas, which includes the absorption tower and the reheat section; this absorption tower and the reheating section and the fan are arranged in line on a vertical axis, in order to function, as at least a part of the chimney to emit the treated flue gas into the atmosphere; wherein the ammonia is injected into the flue gas, as required, at a point downstream of the denitration apparatus; and the amount of the ammonia injected into the denitration apparatus and / or the amount of ammonia injected at the point downstream of the denitration, are determined to be at such an excessive level that ammonia or an ammonium salt remains in the flue gas introduced in the decomposition tower.