WO1991018663A1

WO1991018663A1 - Method of removing nitrogen oxides from combustion exhaust gas

Info

Publication number: WO1991018663A1
Application number: PCT/JP1990/000684
Authority: WO
Inventors: Hirohisa Saito; Etsuo Abe
Original assignee: Fuel Tech Japan Limited; Sanritz Automation Co., Ltd.
Priority date: 1990-05-28
Filing date: 1990-05-28
Publication date: 1991-12-12

Abstract

A method of removing nitrogen oxides formed in an exhaust gas of a combustion apparatus and reducing the residual ammonia concentration, which comprises spraying an aqueous urea solution having controlled hydrogen ion concentration into a combustion chamber or an exhaust passage of the combustion apparatus in accordance with the combustion state of the apparatus. The nitrogen oxides can be removed appropriately in the combustion state of a relatively low temperature by controlling the hydrogen ion concentration of the aqueous urea solution and the quantity of the generated residual ammonia can be minimized.

Description

Specification

Method for removing nitrogen oxides from combustion exhaust

The present invention relates to a method for removing nitrogen oxides in combustion exhaust gas for reducing and removing nitrogen oxides generated in exhaust gas of a combustion device by spraying an aqueous urine solution, and more particularly to a method for removing nitrogen oxides in combustion exhaust gas. The present invention relates to a method for removing nitrogen oxides in combustion exhaust gas that can effectively remove nitrogen oxides even in a low region.

Background art

Conventionally, various methods have been proposed for reducing the concentration of nitrogen oxides (NOx) in exhaust gas in a combustion device as much as possible. Japanese Unexamined Patent Publication (Kokai) No. 63-502 086 states that nitrogen oxides and urea can be reduced by spraying an aqueous urea solution into exhaust gas at a temperature of 2000 ° F (about 114 ° C) or higher. Reacts with each other to decompose into nitrogen gas, carbon dioxide gas and water. According to this method, the removal efficiency of nitrogen oxides in a region where the exhaust temperature is high (hereinafter, referred to as a denitration rate) is achieved by a relatively simple operation of spraying an aqueous urea solution into the exhaust without using a catalyst or the like. ) Can be easily improved. At that time, the smaller the diameter of the liquid droplets to be sprayed, the higher the denitration rate in the region where the exhaust temperature is low.

However, in the above method, even if the diameter of the liquid particles to be sprayed is small (for example, 50 m or less), a desired denitration rate cannot be achieved in a region where the exhaust gas temperature is lower than the above temperature. Therefore, in order to achieve a desired denitration rate even in a low temperature range, urea water It is conceivable to increase the spray amount of the solution. However, if the spray amount is increased, there is a problem that a large amount of harmful ammonia remains in the exhaust gas after denitration.

On the other hand, according to the knowledge of the present inventors, the denitration rate when the urea aqueous solution is sprayed into the exhaust gas as described above is affected not only by the exhaust gas temperature but also by the residual oxygen (O ₂ ) concentration. It has been found that the denitration rate decreases as the oxygen concentration increases. It is well known that the nitrogen oxide concentration increases as the residual oxygen concentration increases. Therefore, even if the exhaust gas temperature is high, even if the residual oxygen concentration becomes relatively high for some reason, the desired denitration rate cannot be achieved in the same manner as described above. 'In this case as well, if the spray amount of the urea aqueous solution is increased, there is a problem of residual ammonia, so the spray amount cannot be increased unnecessarily.

The present invention provides a method for removing nitrogen oxides from combustion exhaust gas, which can achieve a desired denitration rate and easily control the operation even in a region where the exhaust temperature of a flint burning apparatus is low or the residual oxygen concentration is high. It is intended to provide.

Disclosure of the invention

In the present invention, an aqueous urea solution whose hydrogen ion concentration is adjusted according to the combustion state is sprayed into the combustion chamber or the exhaust passage of the baking apparatus. As a result, the temperature is particularly low (650. C.) without increasing the residual ammonium concentration in the exhaust gas.

(About 110 crc) It is not affected by the residual oxygen concentration. At this time, to reduce the hydrogen concentration of the aqueous urea solution, it is preferable to use carboxylic acid such as citric acid.

Further, in the present invention, the injector is provided at each of a plurality of different positions in the combustion chamber or the exhaust passage from upstream to downstream of the exhaust gas, and the hydrogen ion concentration is adjusted according to the state of the exhaust gas at each position. A urea aqueous solution is sprayed from each injector. Thus, the nitrogen oxide concentration can be reduced stepwise according to the state of the exhaust gas, and the denitration rate can be further improved.

BRIEF DESCRIPTION OF THE FIGURES

1 and 2 are schematic piping diagrams showing a configuration of a boiler device according to a first embodiment to which the present invention is applied.

3 to 6 are flow charts showing the operation of the nitrogen oxide removing device of the boiler device to which the present invention is applied.

FIG. 7 is a graph showing the relationship between the exhaust gas temperature and the boiler load and the denitration rate in the first embodiment.

FIG. 8 is a graph showing the relationship between the exhaust gas temperature and the boiler load in the first embodiment and the residual ammonia gas concentration in the exhaust gas.

FIG. 9 is a schematic piping diagram showing a configuration of a thermoelectric supply device according to a second embodiment to which the present invention is applied.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred practice of the present invention depends on the combustion equipment used. A tank for storing a sufficient amount of the urea aqueous solution, a tank for storing a solution for adjusting the pH of the urea aqueous solution, a mixing tank for mixing these solutions, and a tank for mixing the respective solutions. An injector for spraying the mixed solution toward the combustion chamber or exhaust passage of the combustion device, and the combustion state or exhaust state such as the temperature, oxygen concentration, nitrogen oxide concentration, and ammonia concentration in the load and exhaust of the combustion device It is desirable to use a denitration apparatus which has a sensor for detecting the amount of water and a control means for mixing the above solutions in an appropriate amount at an optimum ratio based on data given from each sensor and controlling the amount of spray from the injector.

Here, the aqueous urea solution in the storage tank may be, for example, a relatively high-concentration aqueous urea solution containing 40% by weight of urea. An organic acid may be used as a solution for adjusting the pH of the aqueous urine solution. Among them, for example, carboxylic acid such as carboxylic acid of an aromatic compound represented by benzoic acid, and fatty acid represented by acetic acid are preferable. A carboxylic acid having a group (one COOH group) is preferably used, and more preferably an oxyacid represented by citric acid and lactic acid.

Hereinafter, in order to explain the present invention in more detail, first and second embodiments will be described with reference to the accompanying drawings.

FIG. 1 shows a configuration of a first embodiment of a nitrogen oxide removing apparatus for a heavy oil fired boiler to which the present invention is applied. A burner 2 is formed inside the boiler 1 and a wrench 3 is open. In addition, water to be heated inside is passed downstream of the combustion and baking chamber 2. Pipes (not shown) are provided. A storage tank 4 for storing an aqueous urea solution (pH 9.9) containing 40% by weight of urea at an arbitrary position away from the boiler 1 and adjusting the pH by adding the aqueous urea solution to the aqueous urea solution And a storage tank 5 for storing a citric acid solution for performing a pumping operation. A lower end of each of the tanks 4 and 5 has a first pump 10 and 11 therein via pipes 6 and 7. Connected to the pump modules 8 and 9. The pump modules 8 and 9 are connected in parallel to the three mixing modules 14 to 16 via pipes 12 and 13. The pipes 12 and 13 are connected to return pipes 17 and 18 for returning excess liquid to the tanks 4 and 5, respectively. In particular, it has been found from experiments by the present inventors that the pH of the aqueous urea solution in the storage tank 4 is lowered by circulating the aqueous urea solution through the pipes 6, 12, and 17, and therefore, it is used for pH adjustment described later. Citrate can be reduced.

Since each of the mixing modules 14 to 16 has a similar structure, only the mixing module 16 will be described with reference to FIG. Inside the mixing module 16, for example, a mixing tank 2, which may have a capacity of 3, is provided. The branch pipes 12a and 13a of the pipes 12 and 13 are opened at the upper part through the solenoid valves 21 and 22. As shown in FIG. 1, the branch pipe 23 a of the plant water supply pipe 23 also opens to the upper part of the mixing tank 20 via the solenoid valve 24. The tubes 1 2a, 1 3a and 23 Filters 79 to 81 are provided at the entrance to the single module 16.

The plant water supply pipe 23 is connected via a solenoid valve 25 to the suction side of a second pump 26 composed of a gear pump. The discharge side of the second pump 26 is connected to each of the injectors 3 provided at a position slightly downstream of the combustion chamber 2 via a pipe 3 1 check valve 32 and a branch pipe 3 la, 31 b, 31 c. 3 to 35 are connected in parallel with each other. On the other hand, in the portion of the plant water supply pipe 23 between the solenoid valve 25 and the second pump 26, a pipe 28 that substantially descends from the bottom of the mixing tank 20 has a check valve 29. Connected through.

Here, when the solenoid valve 25 is opened, the plant water supply pipe 23 communicates with the second pump 26, and the check valve 29 is closed by the water pressure. Therefore, by turning on / off the solenoid valve 25, one of the plant water supply pipe 23 and the mixing tank 20 is selectively communicated with the second pump 26, and each of the pumps is connected. The solution mixed in the water or the mixing tank can be selectively supplied to the injectors 33 to 35. A branch pipe 36a of a plant air supply pipe 36 is also connected to these injectors 33 to 35 via a pressure regulating valve 37, a solenoid valve 38, and a check valve 39, and actually, Is ejected from each of the injectors 33 to 35 together with the water or the mixed liquid layer.

Each injector 33 to 35 is connected to flint chamber 2 (or It is guided so that it can come and go freely. Air cylinders 40 to 42 are attached to these injectors 33 to 35, and a pressure adjusting valve 43 is supplied from a plant supply pipe 36 to supply a predetermined pressure into the cylinder. The branch pipes 36 b and 36 c are connected to each other via the switching valve 44. By controlling the switching valve 44, each of the injectors 33 to 35 is protruded into the combustion chamber 2 when used, and is immersed in the boiler wall when not used.

One end of a branch pipe 36 d is connected to the middle of the branch pipe 36 b of the brand air supply pipe 36, and the other is a mixing tank 2 via a flow control valve 45 and a check valve 46. It is open inside 0. The liquid in the mixing tank 20 is agitated by the air ejected from the branch pipe 36d.

Two injectors 50 and 51 are connected to the mixing module 14 instead of three. Otherwise, the structure and configuration of each of the mixing modules 14 and 15 are the same as those of the mixing module 16, and the structure and configuration of each of the indicators 47 to 51 connected to these mixing modules. Are the same as the injectors 33 to 35, and the description thereof is omitted.

On the other hand, as shown in FIG. 1, each of the storage tanks 4 and 5 has a temperature sensor 52 and 53 and a level sensor 54 and 5 for detecting that the temperature and the amount of the stored liquid are within a predetermined range. 5 powers are provided, and the main control It is electrically connected to the mouth (hereinafter referred to as MC) 56. The storage tank 4 is also provided with a pH sensor 64 for measuring the pH of the aqueous urea solution, and is connected to the MC 56.

The MC 56 is electrically connected to a control device (not shown) attached to each of the pump modules 8 and 9 to detect, control, and control the discharge pressure and the like of each of the pumps 10 and 11. . In addition, the MC 56 detects and transmits a fuel flow rate as a load of the boiler 1 and a flow rate sensor 57, and a nitrogen oxide (Ν Ο _χ ), ammonia (Ν Η ₃ ), Measures the temperature inside boiler 1 at each concentration sensor 58 for measuring the concentration of carbon oxide (CO), oxygen ( ₀₉ ), etc., and at the position near each injector 33-35, 47-51 The three temperature sensors 59 and are connected. Further, the MC 56 is a host having an external storage device such as a main unit, a display device, a hard disk, and an input device such as a printer device and a keyboard so that an operator can make various settings and observe the operation status. A computer (hereinafter abbreviated as HC) 60 and a mixing controller (hereinafter abbreviated as MXC) provided to detect the state of each mixing module 14 to 16 and to control a pump, each solenoid valve, and the like. 6) 1 to 63 are connected via a known network system.

As shown in FIG. 2, for example, the MX C 63 has a flow sensor 65-67 attached to each of the pipes 12a, 13a and 23a, and similarly each of the pipes 12a and 13a. , 23a Pressure sensor attached to 68 to 70, the flow sensor 71 attached to the pipe 23, the pressure sensor 72 attached to the pipe 31, the pressure sensor 73 attached to the pipe 36a, and the pipe 31 Connected to the flow sensors 74 to 76 attached to the branch pipes 31a, 31b, 31c branching to the injectors 33 to 35, and the level switch 77 attached to the pipe 28, The flow and pressure of each pipe are monitored. In addition, the MXC 63 is equipped with solenoid valves 21, 22, 24, 2, provided on pipes 12 a, 13 a, 23 a, 23, 36 a, 36, 36 b, 36 c. 5, 38, and switching valve 44 are connected to these for opening / closing and switching control, respectively. Further, the MXC 63 is connected to the pump 26 via a pump driver 78 to control the pump. Since the structure of the MXCs 61 and 62 is the same as that of the MXC 63, the detailed description is omitted.

Next, the operation of the HC 60, the MC 56, and the MX Cs 61 to 63 will be described in detail with reference to the flowcharts of FIGS. 3 to 6.

Fig. 3 is a flowchart showing the operation procedure of HC60. First, after performing the initialization processing in step S1, it is determined in step S2 whether or not the screen of the display device of the HC 60 needs to be changed, and if not, step S4 is performed. If it is necessary to change, the process proceeds to step S4 with only the fixed display portion of the screen displayed in step S3. Then, in step S4, it is determined whether or not the data to be displayed already exists in the HC 60, and if so, the process proceeds to step S6. If there is no data to be displayed, the process proceeds to step S5. After receiving the display data by the process of step S33 in the flowchart (FIG. 4) of the MC 56, the process proceeds to step S6. In step S6, the display data stored in the HC 60 or the display data received from the MC 56 in step S5 is made to correspond to the fixed display portion as a variable display portion on the display device screen. To display. Then, proceed to step S7, enter the operation mode of each MXC 61 to 63, the flow rate information of various liquid materials (during manual operation), etc., and in step S8 the operation table used by MC 56. It is determined whether or not it is necessary to change the value. If no change is required, the process proceeds to step S10. If the value is to be changed, the table to be changed to MC35 is transmitted in step S9, and then the step S Go to 10. Here, the above operation table shows that each mixing module 30 to 32 corresponds to the pH of the urea aqueous solution in the storage tank 4, the temperature near each injector, and the load (fuel flow rate) of the boiler 1. This is a sample for determining the ratio and concentration of each liquid agent to be mixed and the injection amount of each mixed solution in each injection. With this table, the concentration of the urea aqueous solution and i> H are adjusted for each mixing module, and the mixed solution is sprayed according to the exhaust state at the position where each injector is arranged. In steps S10 and S11, if it is time to collect data from each MXC 61 to 63 and each sensor and switch and receive the data edited by MC 56, perform communication processing. In steps S12 and S13, if the operator requests a report output such as the amount of each liquid used, the report is output from the printer and the process returns to step S2. Then, the processing of steps S2 to S13 is repeated.

FIG. 4 is a flowchart showing the operation procedure of the MC 56. When the display device, the input device, and the like are connected, the MC 56 can display the same information as that of the HC 60 and can input each data. First, after the initialization process in step S21, the process proceeds to step S22, in which the time synchronization with the HC 60 when the system is started, various programs, data, tables, etc. are stored in the HC 60 and Z or each MXC. 6 Determine whether it is necessary to receive from 1 to 63, that is, at the time of system startup, or whether there is any change in various programs, data, tables, etc. already stored in the MC 56. If it is not necessary, the process proceeds to step S24. If necessary, the data is received and edited from the HC 60 or each of the MXCs 61 to 63 in step S23, and the process proceeds to step S24. In step S24 and step S25, the furnace information and the current data of each of the MXCs 61 to 63 are received. Then, in step S26, it is determined whether or not to stop the operation of the nitrogen oxide removing device. Proceed to step S27 if it is to be operated.

In step S27, it is determined whether the operator has set the operation mode to manual or automatic with the HC 60. If the operation mode is manual, the flow information is determined from the key input information in step S29. create. If it is auto, the flow rate is calculated from the current load and the operation table received in step S24 in step S28, and flow rate information is created in step S29. Then, proceeding to step S30, it is determined whether or not the flow rate information created in step S29 has changed from the previous time, and if there is a change, the flow rate information is sent to MXC 61 in step S31. Is transmitted, and if there is no change, the process proceeds to step S32 without transmitting.

In step S32, it is determined whether or not there has been a data request from the HC 60. If there is a request, the current data is transmitted in step S33, and the process returns to step S22. Then, the processing of steps S22 to S33 is repeated.

Fig. 5 and Fig. 6 are flowcharts showing the operation procedure of each MXC61-40. Each MXC has a main body having a communication function with the MC 56, and when a display device is connected, information on the connected MXC can be displayed. First, after the initialization processing in step S41, it is determined whether or not there is data to be received in step S #. If so, the analysis processing is performed after receiving the data in step S43. Then, in step S44, this cycle If this is not the first time, collect the detection results of boiler 1 load, exhaust temperature, ammonia concentration, nitrogen oxide concentration, oxygen concentration, carbon monoxide concentration, flow rate of each pipe, pressure, etc. And memorize it. Next, proceeding to step S45, the flow rate of each liquid material is adjusted by controlling the pumps 10 and 11 in the pump modules 8 and 9 based on the flow rate information received from the MC 56. Adjust the mixing ratio of each solution in each mixing tank in each of the mixing modules 14 to 16. Then, proceeding to step S46, it is determined whether or not to perform pulse control described later. If pulse control is to be performed, step S47 (subroutine shown in FIG. 6) is performed. If pulse control is not to be performed, Go to step S48.

In the following steps S48 to S53, the mixing module 16 shown in FIG. 2 will be mainly described, and the description of the mixing modules 14 and 15 having the same structure will be omitted. In step S48, the supply amount L1 to each of the injectors 33 to 35 at the unit time t is set, and in step S49, the solenoid valve 25 of the plant water supply pipe 23 is closed. (Figure 2). Then, if there is a liquid in the mixing tank 20, it flows into the pipe 23 via the pipe 28 and is supplied to the pump 26. At about the same time, in step S50, the solenoid valves 21, 22, and 24 of the pipes 12a, 13a, and 23a are opened, and each solution is continuously replenished to the mixing tank 20. 5 Continuous mixing tank for unit time t at 1 Only continuous from 20 side And supply the mixed solution to the injector. Then, in step S52, the solenoid valves 21, 22, and 24 are closed to stop the supply of the solution, and in step S53, the current data of each sensor is transmitted to the MC 56, and step S53 is performed. 4 Return to 2.

On the other hand, if it is determined in step S46 that pulse control is to be performed, the flow advances to step S47 to perform the subroutine shown in FIG. That is, first, in step S61, the injection amount L2 in one cycle is set, and in step S'62, it is determined whether or not the pulse control routine is the first time. If not, the process proceeds to step S64, and if it is the first time, it is determined in step S63 whether or not the level switch 77 of the mixed ink 20 is on. If it is off, that is, if liquid remains inside, step S63 is repeated until the mixing tank 20 is empty and the level switch 77 is turned on. Proceed to 4. If the level switch 77 is not turned on for a predetermined time, an error process such as displaying on the display of the HC 60 is performed. In step S64, the cycle clock T is reset, and the process proceeds to step S65.

In step S65, the supply amount L3 of the brand water to each injector is calculated from the above-described amount L2 of the supplied injector and the concentration of the mixed solution in the mixing tank 20, and the flow proceeds to step S66. Open solenoid valve 25 of plant water supply pipe 23. Then, the check valve 29 of the pipe 28 closes due to the pressure of the plant water, and the mixing tank 2 The bottom of 0 is closed. At approximately the same time as Step S66, in Step S67, the liquid tank 20 is replenished with each liquid material calculated from the set supply amounts L2 and L3.

Next, in step S68, it is determined whether or not the cycle clock T has reached a predetermined time t1, which may be, for example, 10 seconds, and if not, repeat step S68 if t1 has not been reached. When it reaches 1, the process proceeds to step S69. In this step, it is determined from each flow sensor 71, 74-76 whether or not the supply amount L3 calculated in step S64 has been reached, and if not, an error has occurred in step S70. After performing the processing, the process returns to step S42 of FIG. 5, and if it is L3, the process proceeds to step S71.

In step S71, the solenoid valves 21, 22, 24 are closed to stop the supply of the liquid materials, and the process proceeds to step S72. In step S72, the mixture in the mixing tank 20 is supplied to the pump 26 by closing the solenoid valve 25. Then, proceeding to step S73, it is determined whether or not the cycle clock T has reached a predetermined time t2, and if it has not reached t2, the step S73 is repeated. Proceed to 4.

In step S74, it is determined whether or not the level switch 77 is turned on. If the switch is turned on, the process returns to step S42 in FIG. 5; if the switch is turned off, error processing is performed. Return to S42. Then, it is determined whether or not the control contents have been changed in the processing in steps S42 and S46. The determination is made, and the processing from step S48 to step S52 or step S61 to step S74 is repeated again according to the content. In this embodiment, the force (MC) for confirming whether or not the control information from the MC 56 for each of the MXCs 61 to 63 changes every cycle (steps S61 to S74). In step S42), multitask processing is actually performed, and it is always checked whether there is a change in the control information. If there is a change, every half cycle (step S61 to step S68 and step S68) Tl, t2, L2, L3, etc. may be changed from S69 to S74).

In this way, the on-off valves, switching valves, pumps, etc. are controlled by feedback control according to the combustion state of the boiler 1 by the sensors 57 to 59 and the like by the HC 60, MC 56, and MX C 61 to 63. The urea aqueous solution concentration and pH are appropriately controlled so that the nitrogen oxide concentration, ammonia concentration and the like fall within predetermined ranges. Also, if necessary, each control can be manually set by remote control at the HC 60 or the MC 56, and the control status can be output from the HC 60 or the MC 56.

Actually, the denitration rate and residual ammonia concentration were compared between an aqueous urea solution (U + C) whose pH was adjusted by manual setting and an aqueous urea solution (U) whose pH was not adjusted using the above equipment. These are shown in Table 1 below and in Figs. 7 and 8. The boiler used here has a steam volume of 30 t h. Fuel consumption at 100% load 2400 £ Zh, and exhaust gas volume at that time 26 300 7

It is a heavy oil fired boiler with N m ³ / h (dry). In Table 1, L 0, L 1, and L 2 indicate the installation positions of the injectors 33 to 35, the injectors 49 to 51, and the injectors 47, 48. 33 to 35 (L 0) are provided near the burner 4 and spray a urea aqueous solution or the like toward the middle of the flame, and the injectors 49 to 51 are provided in the combustion chamber 2. Urea solution is sprayed from the front end of the flame to the exhaust passage, and the injectors 47 and 48 are installed upstream of the exhaust passage. And sprays urea aqueous solution and the like toward the exhaust gas in the exhaust passage.

The base NO x in Table 1 is the nitrogen oxide concentration when no aqueous urea solution is sprayed, and the NSR is the mole of urea used relative to the mole of base NO x in the present invention. It is the number X 2 (the number of amino groups) (for example, when 1 mole of urea is 2 moles of N 0 X, NSR is 4).

Fig. 7 is a graph showing the relationship between the exhaust gas temperature and the denitration rate. The solid lines A, C and E show the results when the urea aqueous solution (U) whose pH was not adjusted was sprayed, and the solid lines B, D and F show the values of p. This is the case where the urea aqueous solution (U + C) adjusted to H is sprayed. According to this graph, in this embodiment, when the urea aqueous solution whose pH was adjusted was used, compared with the case where the urea aqueous solution whose pH was not adjusted was used, the exhaust gas in the high temperature range was used. Although the denitration rate is slightly lower (solid lines A and B), the denitration rate from low to medium temperature range is significant. (Solid lines (:, D, E, F).) In addition to Table 1, when the residual oxygen concentration is high when the pH adjusted urea aqueous solution is In the example, it is clear that the denitration rate tends to be improved particularly in the case of low load operation.

Here, as described above, when the urea aqueous solution whose pH is not adjusted is used, the denitration rate is affected by the residual oxygen concentration other than the exhaust gas temperature, but the urea aqueous solution whose pH is adjusted is used. The denitration rate in this case is no longer affected by the residual oxygen concentration. This means that even when the temperature is high, if the residual oxygen concentration is high for some reason, a desired denitration rate can be easily obtained by using an aqueous urea solution whose pH has been adjusted.

Therefore, for example, in this embodiment, when the boiler operation gradually shifts from a low load operation to a high load and the exhaust gas temperature gradually shifts from a low temperature to a high temperature, the pH of the urea aqueous solution is optimized. By adjusting to, the denitration rate can be improved over the entire exhaust temperature range.

Figure 8 is a graph showing the relationship between the exhaust gas temperature and the ammonia gas concentration. The solid lines G, I, and K show the solid lines H, J when the urea aqueous solution (U) whose pH was not adjusted was sprayed. L shows the case where the urea aqueous solution (U + C) whose pH was adjusted was sprayed. Paying attention to this graph, it can be seen that the urea aqueous solution whose pH has been adjusted is used especially at low load from low to medium temperature range. In this case, it can be seen that the concentration of ammonia gas is extremely low as compared with the case where an aqueous urea solution whose pH is not adjusted is used (solid lines G and H).

That is, in the boiler of the present embodiment, the denitration rate is remarkably improved by adjusting the pH at a low load and a medium load from a low temperature range to a medium temperature range of the exhaust gas temperature, and ammonia gas is generated. Can be suppressed. This not only reduces the amount of chemicals such as urea used to obtain the desired denitration rate, but also reduces operating costs and increases the amount of other harmful components in the exhaust gas. This means that exhaust gas purification is promoted.

As a side effect of adding the citric acid solution to the aqueous urea solution, the mixed liquid flowing through tubes 28, 31, 31, 31a, 31b, 31c, etc. is frozen in winter etc. It is possible to simplify the structure of the nitrogen oxide removing apparatus by eliminating the necessity of separately providing a heater and a heat retaining structure. Also, it is mentioned that deterioration of the ink can be prevented by softening water as a solvent of the urea aqueous solution with citric acid.

(Hereinafter the margin)

One 0 to

IDd £ 9981/16 O FIG. 7 is a schematic configuration diagram showing a second embodiment to which the present invention is applied. In the present embodiment, the method for removing nitrogen oxides from the combustion exhaust gas is applied to a known thermoelectric supply device (cogeneration). In this device configuration, a gas turbine 91 is used as a combustion engine. An output shaft 92 of the gas turbine 91 is connected to a generator 94 via a reduction gear 93. On the other hand, an exhaust passage 95 of the gas turbine 91 is connected to an exhaust boiler 97 via a reheating device 96. The exhaust gas passing through the exhaust boiler 97 is discharged from a chimney 99 via a pipe 98. In addition, water is supplied to the exhaust boiler 97 from the tank 101 via the pump 102 and the pipe 103, and the water is heat-exchanged to form steam, and the pipe 10 Being used by various organizations via 4 0

Here, the nitrogen oxide removing device 105 to which the present invention is applied is provided in a portion of the pipe 95 between the reheating device 96 and the exhaust boiler 97, and the nitrogen oxide removing device 105 is provided inside the pipe. An injector similar to that of the first embodiment appears and disappears. The nitrogen oxide removing device, the removing method, and the control method are the same as in the first embodiment.

In this embodiment, even if the temperature of the exhaust gas from the combustion engine is relatively low, the exhaust gas is heated to, for example, 850 or more by the reheating device 96, and thus the nitrogen oxide removing device 10 to which the present invention is applied is used. 5 makes it possible to remove nitrogen oxides ing. Needless to say, the same effect can be obtained by using other engines such as a diesel engine and a gasoline engine as the combustion engine.

Industrial applicability

INDUSTRIAL APPLICABILITY As described above, the method for removing nitrogen oxides from combustion exhaust gas according to the present invention is useful as an exhaust treatment device for a heavy oil fired boiler, other various boilers, a heat transfer supply diesel engine, and a gas turbine. .

Claims

The scope of the claims

(1) In order to remove nitrogen oxides (NO _x ) generated in the exhaust gas of the combustion device, nitrogen oxides in the combustion exhaust gas are sprayed with an injector into a combustion chamber or an exhaust passage of the combustion device with an injector. An object removal method,

A method for removing nitrogen oxides from combustion exhaust gas, characterized in that a hydrogen ion concentration of an aqueous urea solution sprayed into the combustion chamber or the exhaust passage is adjusted according to a combustion state of the combustion device.

(2) The method for removing nitrogen oxides from combustion exhaust gas according to claim 1, wherein a carboxylic acid having a carboxyl group is added to the urea aqueous solution to adjust the hydrogen ion concentration of the aqueous urea solution. .

(3) The method for removing nitrogen oxides from combustion exhaust gas according to claim 2, wherein the carboxylic acid includes citrate.

(4) Indices are provided at a plurality of different positions from the upstream side to the downstream side of the exhaust of the combustion chamber or the exhaust passage of the combustion device, respectively, and the hydrogen ion concentration is determined according to the state of exhaust at each of the positions. 2. The method for removing nitrogen oxides from combustion exhaust gas according to claim 1, wherein the urea aqueous solution having the adjusted pressure is sprayed from each of the injectors.

(5) Removal of nitrogen oxides in combustion exhaust gas according to claim 4, wherein a carboxylic acid having a carboxyl group is added to the urea aqueous solution to adjust a hydrogen ion concentration of the aqueous urea solution. Method. (6) The method for removing nitrogen oxides from combustion exhaust gas according to claim 5, wherein the carboxylic acid has citrate.