Abstract:
Disclosed is an ammonia adsorption apparatus for removing ammonia contained in a treated gas discharged from a denitrator for decomposing nitrogen oxides present in exhaust gas using ammonia as a reducing agent, comprising at least two adsorption towers into which ammonia-containing exhaust gas is introduced in a vertical direction for removing the ammonia by adsorption, wherein, during regeneration of each adsorption tower, hot gas used for desorption purposes flows from a top toward a bottom of the adsorption tower and cooling gas flows from the bottom toward the top of the adsorption tower.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to ammonia adsorption apparatus to be disposed downstream in the flow of exhaust gas from a denitrator which removes nitrogen oxides present in exhaust gas using ammonia as a reducing agent. 
     2. Description of the Related Art 
     FIG. 4 is a flow diagram illustrating a NO x  removal system provided with conventional ammonia adsorption apparatus. In this conventional system, exhaust gas emerging from a gas turbine 1 is conducted through a flue and introduced into a waste heat recovery boiler 2, where it is mixed with ammonia injected by an ammonia injector 3 installed therein. The resulting mixture is then introduced into a denitrator 4 installed downstream, where nitrogen oxides (NO x ) present in the exhaust gas are decomposed into innoxious nitrogen and water. Subsequently, the exhaust gas is introduced into one of two horizontal adsorption towers 5 and 5 disposed in parallel by way of the corresponding exhaust gas duct 7A or 7B, and freed of any residual ammonia by adsorption. Thereafter, the exhaust gas is conducted through an exhaust gas duct 8 and discharged from a stack 6 into the atmosphere. Although each adsorption tower 5 is packed with an ammonia adsorbent, further adsorption cannot be effected after a certain amount of ammonia has been adsorbed. Accordingly, the dampers for adsorption tower 5 are adjusted so that desorption gas comprising a portion of the hot exhaust gas emerging from gas turbine 1 is conducted through a desorption gas duct 9 and introduced into adsorption tower 5 in order to raise the temperature of the ammonia adsorbent and desorb ammonia therefrom. Thereafter, the desorption gas is discharged from stack 6. 
     After ammonia has been fully desorbed from one adsorption tower 5, this adsorption tower 5 is cooled in preparation for the next adsorption step. This can be accomplished by supplying cold gas to adsorption tower 5 through a cooling gas duct (not shown) and making it flow from the top to the bottom thereof, or by blowing air into the top of adsorption tower 5 by means of a fan (not shown). The above-described steps are repeatedly performed in each adsorption tower. In FIG. 4, while one adsorption tower 5 is engaged in the adsorption step, the other adsorption tower 5 is engaged in the desorption step. Thus, these steps are alternately and repeatedly performed in the two adsorption towers 5 and 5. 
     Where the adsorption towers are of the horizontal type as seen in the above-described conventional ammonia adsorption apparatus, the desorption of ammonia is effected by using hot exhaust gas for heating purposes in a volume equal to 1/50 of that of the main exhaust gas and passing it through an adsorption tower in the same direction as for the adsorption of ammonia. However, the small volume of gas tends to cause channeling and fails to raise the temperature of the adsorption tower uniformly. Accordingly, the ammonia adsorbed on the adsorbent cannot be fully desorbed within the heating/desorption period, resulting in a decrease in the amount of ammonia adsorbed in the next adsorption step. Moreover, since cooling is effected by making cold gas flow from the top to the bottom of the adsorption tower, the cold gas passes through the adsorption tower locally. Thus, the adsorption tower is not cooled uniformly as a whole and requires a long cooling time. For these reasons, the operation of this apparatus involves various problems including readjustment of the adsorption/desorption time schedule, reconsideration of the number of adsorption towers, and complication of the adsorption/desorption control. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide ammonia adsorption apparatus which can solve the above-described problems encountered in the conventional ammonia adsorption apparatus. 
     According to the present invention, there is provided ammonia adsorption apparatus to be installed downstream of a denitrator for decomposing nitrogen oxides present in exhaust gas by using ammonia as a reducing agent, characterized in that said ammonia adsorption apparatus includes at least two adsorption towers into which ammonia-containing exhaust gas is introduced in a vertical direction for the purpose of removing the ammonia by adsorption and in that, during regeneration of each adsorption tower, hot gas used for desorption purposes is made to flow from the top toward the bottom of the adsorption tower and cooling gas is made to flow from the bottom toward the top of the adsorption tower. 
     After ammonia has been fully adsorbed in an adsorption tower during the adsorption period, hot gas used for desorption purposes is supplied to the adsorption tower (usually in a volume equal to 1/50 of that of the main exhaust gas) and made to flow from the top toward the bottom thereof. Thus, in spite of its small differential pressure, the gas flows through the adsorbent bed and forms layered flow paths among the absorbent layers, and thus it raises the temperature of the adsorption tower uniformly, allowing the desorption of ammonia to proceed smoothly. Moreover, since cold cooling gas is made to flow from the bottom toward the top of the adsorption tower, this cooling gas passes through the whole of the adsorption tower, making it possible to cool the entire adsorption tower in a short period of time. Thus, adsorption/desorption cycles can be performed smoothly and their control can be simplified. 
     The present invention can achieve the following advantageous results. 
     (1) Since vertical adsorption towers are employed and hot desorption gas flows from the top toward the bottom of each adsorption tower, even a small volume of gas can raise the temperature of the adsorption tower sufficiently. Moreover, since the gas descends through the adsorption tower while forming a gas layer, ammonia can be desorbed uniformly and the desorption of ammonia can be completed in a short period of time. 
     (2) Since cooling gas flows from the bottom toward the top of the adsorption tower, the cooling gas ascends through the adsorption tower without causing channeling. Thus, the cooling step can be completed in a short period of time. 
     (3) Since the desorption/cooling step has a good margin as described above, the adsorption/desorption cycles are stabilized. Moreover, this adsorption/desorption system can be designed to have a dual construction capable of achieving the highest operating efficiency, and can hence be efficiently operated by using a small amount of catalyst and a small number of adsorption towers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram illustrating one embodiment of the present invention; 
     FIGS. 2(a), 2(b) and 2(c) are schematic views showing the flow of gas within an ammonia adsorption tower in accordance with the embodiment of FIG. 1; 
     FIG. 3 is a schematic diagram showing the changes with time of the top and bottom temperatures of an ammonia adsorption tower; and 
     FIG. 4 is a flow diagram illustrating conventional ammonia adsorption apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of the present invention is described with reference to FIG. 1. In FIG. 1, numeral 1 designates a gas turbine. Exhaust gas from gas turbine 1 is introduced into a waste heat recovery boiler 2. An ammonia injector 3 is installed within waste heat recovery boiler 2. A denitrator 4 is installed downstream of ammonia injector 3. Two adsorption towers 5A and 5B are disposed in parallel on the downstream side of denitrator 4 and packed with an ammonia adsorbent. A stack 6 is installed downstream of adsorption towers 5A and 5B. The adsorption towers 5A and 5B are of the vertical type and, as will be described later, ammonia-containing exhaust gas flows through adsorption towers 5A and 5B as a vertical, downward stream. 
     Two branched exhaust gas ducts 7A and 7B for conveying the exhaust gas leaving denitrator 4 are connected to the tops of adsorption towers 5A and 5B, respectively. These exhaust gas ducts 7A and 7B are provided with dampers D 1  and D 6 , respectively. The bottoms of adsorption towers 5A and 5B are connected to a stack 6 by exhaust gas ducts 8A and 8B having dampers D 2  and D 7 , respectively. 
     A duct 12 connects the downstream side of damper D 1  of the aforesaid exhaust gas duct 7A to the downstream side of damper D 6  of the aforesaid exhaust gas duct 7B, and is provided with dampers D 3  and D 8 . A desorption gas duct 9 for allowing a portion of the exhaust gas from gas turbine 1 to diverge from a point between gas turbine 1 and waste heat recovery boiler 2 is provided with a damper D 13 . Moreover, a duct 11 for allowing a portion of the exhaust gas to diverge from the upstream side of stack 6 is provided with dampers D 14  and D 15 . These ducts 9 and 11 are combined with each other and then connected to the aforesaid duct 12 between dampers D 3  and D 8 . Thus, the flow rates of the exhaust gases in ducts 9 and 11 are controlled by dampers D 13  and D 15  so as to obtain a predetermined temperature and the resulting gas mixture is supplied to duct 12. 
     The upstream side of damper D 2  of the aforesaid exhaust gas duct 8A and the upstream side of damper D 7  of the aforesaid exhaust gas duct 8B are connected by a duct 14 having dampers D 4  and D 9 . A duct 10 branches from this duct 14 between dampers D 4  and D 9  and is connected to waste heat recovery boiler 2 between ammonia injector 3 and denitrator 4. 
     The downstream side of damper D 1  of the aforesaid duct 7A and the downstream side of damper D of the aforesaid duct 7B are connected by a duct 15 having dampers D 11  and D 12 . A duct 16 branches from this duct 15 at a point between dampers D 11  and D 12  and is connected to stack 6. Moreover, the upstream side of damper D 2  of the aforesaid duct 8A and the upstream side of damper D 7  of the aforesaid duct 8B are connected by a duct 17 having dampers D 5  and D 10 , and a cooling gas duct 13 branching from the aforesaid duct 11 at a point between dampers D 15  and D 14  is connected to this duct 17 between dampers D 5  and D 10 . 
     In this embodiment, NO x  -containing hot exhaust gas emerging from gas turbine 1 is introduced into waste heat recovery boiler 2 installed downstream thereof, where it is cooled by heat exchange in a heat exchanger (not shown) and mixed with ammonia injected by ammonia injector 3. The resulting mixture is then introduced into denitrator 4, where the NO x  is decomposed into innoxious nitrogen and water In this process, ammonia is injected in an amount in excess of that required to react with the NO x  so that a high degree of denitration may be achieved. Accordingly, residual ammonia which has not been used for reaction in denitrator 4 remains in the exhaust gas leaving denitrator 4. 
     By opening dampers D 1  and D 2  associated with one adsorption tower 5A, the above-described exhaust gas containing ammonia injected in excess by ammonia injector 3 is introduced into the top of adsorption tower 5A installed downstream of denitrator 4. 
     Thus, the ammonia-containing exhaust gas introduced into one adsorption tower 5A flows through adsorption tower 5A as a vertical, downward stream, and ammonia present in the exhaust gas is adsorbed by the adsorbent packed in adsorption tower 5A. Thereafter, the exhaust gas is conducted through exhaust gas duct 8A and discharged from stack 6 into the atmosphere. 
     During this period, dampers D 13 , D 14  and D 15  are opened, so that a portion of the hot exhaust gas from gas turbine 1 is mixed with a portion of the cold exhaust gas entering stack 6. The resulting gas mixture, which is used as a desorption gas, is conducted through open damper D 8  in duct 12 and introduced into the top of the other adsorption tower 5B. This desorption gas flows through adsorption tower 5B as a vertical, downward stream to heat the adsorbent packed in adsorption tower 5B and thereby desorb ammonia from the adsorbent. The desorption gas containing the desorbed ammonia is withdrawn from the bottom of adsorption tower 5B, conducted through exhaust gas duct 8B, duct 14 having the opened damper D 9 , and duct 10, and then introduced into waste heat recovery boiler 2. Thus, the desorbed ammonia is recycled and utilized. As the aforesaid desorption gas, a portion of the exhaust gas from gas turbine 1 is used in a volume equal to about 1/50 of that of the main exhaust gas introduced into waste heat recovery boiler 2. 
     After ammonia has been fully desorbed from adsorption tower 5B in the above-described manner, dampers D 13  and D 15  are closed, dampers D 14  and D 10  are opened, and other dampers are suitably controlled. Then, cold gas is introduced into the bottom of adsorption tower 5B by way of duct 11, cooling gas duct 13, duct 17, and exhaust gas duct 8B. This cooling gas ascends through adsorption tower 5B to cool the adsorbent packed therein. Thereafter, the cooling gas is withdrawn from the top of adsorption tower 5B and conducted to stack 6 by way of exhaust gas duct 7B, duct 15 and duct 16. 
     As soon as the above-described adsorption in adsorption tower 5A and the above-described desorption, including cooling by cooling gas, in adsorption tower 5B are completed, the dampers are controlled in such a way that, conversely, the desorption step is performed in adsorption tower 5A, and the adsorption step is performed in adsorption tower 5B. Subsequently, adsorption and desorption steps are alternately repeated in each of adsorption towers 5A and 5B. In the above description of these steps, only the opening and closing of main dampers are explained. It is to be understood that other dampers are suitably controlled so as to allow the aforesaid steps to be performed. 
     The changes with time of the flow of desorption gas within a vertically disposed adsorption tower 5A or 5B are explained with reference to FIGS. 2(a), 2(b) and 2(c). A small volume of hot desorption gas is introduced into the top of adsorption tower 5A or 5B by way of desorption gas duct 9, duct 12 and exhaust gas duct 7A or 7B, and flows through adsorption tower 5A or 5B as a downward stream. Since the desorption gas is small in volume and has a low differential pressure, it does not pass directly to the bottom, but descends slowly through adsorption tower 5A or 5B while forming layered flow paths  FIG. 2(a) to FIG. 2(b)!. As a result, adsorption tower 5A or 5B is gradually and uniformly heated to the ammonia desorption temperature, whereupon the desorbed ammonia is discharged from the bottom by way of exhaust gas duct 8A or 8B, duct 14 and duct 10. Thus, by heating adsorption tower 5A or 5B uniformly with the aid of hot desorption gas flowing from the top toward the bottom thereof while forming layered flow paths, the temperature of adsorption tower 5A or 5B can be sufficiently raised using a small volume of desorption gas and the uniform desorption of ammonia can be achieved in a short period of time. 
     Changes in time of exhaust gas temperature at the top and bottom of an ammonia adsorption tower during its operation are explained with reference to FIG. 3. In this figure, there are shown two cases: (1) the case in which the cooling gas flows from the bottom to the top of the adsorption tower according to the above-described embodiment of the present invention (i.e., cooling gas ascending mode), and (2) the case in which the cooling gas flows reversely from the top to the bottom of the adsorption tower (i.e., cooling gas descending mode). In case (1), the full line and the broken line represent the top temperature of the adsorption tower and the bottom temperature of the adsorption tower, respectively. In case (2), the single and double dotted lines represent the top temperature of the adsorption tower and the bottom temperature of the adsorption tower, respectively. An adsorption step is commenced at the starting point of adsorption and completed after ammonia has been fully adsorbed. Then, a heating/desorption step is performed by introducing hot exhaust gas into the top of the adsorption tower in order to heat the adsorption tower. In this step, the temperature of the adsorption tower rises initially at the top thereof and later at the bottom thereof. After the desorption step is continued until the ammonia present in the adsorption tower is fully desorbed (i.e., until the ammonia concentration at the bottom is reduced to about 0 ppm), the adsorption tower is cooled in preparation for the next adsorption step. In the cooling gas ascending mode (1), the temperature of the adsorption tower begins to fall at the bottom thereof and all the steps are completed at the end point of this adsorption/desorption cycle, so that the next cycle can be commenced smoothly. In the cooling gas descending mode (2), the temperature of the adsorption tower begins to fall at the top thereof and causes the interior of the adsorption tower to be cooled gradually. However, an uneven temperature distribution is created within the adsorption tower because the cold cooling gas flows locally through paths having lower pressure losses. Accordingly, the time required to spread the cooling gas throughout the adsorption tower and cool it to a temperature suitable for the next adsorption step is about 1.5 times as long as that observed in the cooling gas ascending mode (1). As a result, the adsorption/desorption cycle time varies from cycle to cycle, and mode (2) is undesirable from a practical point of view. 
     Although exhaust gas from a gas turbine is used as desorption gas in the above-described embodiment, it is also possible to use any other hot gas.