Fluid Heating Device And Exhaust Gas Purifying Apparatus

Abstract

A fluid heating device includes a heating part and a plurality of heat transfer sections. The heating part is disposed in a fluid stored in a container and is elongated in an axial direction for heating the fluid between a lower portion and an upper portion of the container by being supplied with electricity. The heat transfer sections arranged along the axial direction of the heating part. Each of the heat transfer sections has a plate shape extending from the heating part to a radial outside of the heating part approximately perpendicularly to the axial direction of the heating part.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on Japanese Patent Application No. 2007-110689 filed on Apr. 19, 2007, the content of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a fluid heating device and an exhaust gas purifying apparatus including the fluid heating device.

[0004] 2. Description of the Related Art

[0005] Conventionally, an exhaust gas purifying apparatus using a urea reducing catalyst, such as a urea SCR (selective catalytic reduction) system, is suitably used for reducing nitrogen oxide (NOx) in exhaust gas from a vehicle engine. In an example of the conventional urea SCR system 9 shown in FIG. 9, a SCR catalyst 620 is disposed in an exhaust pipe 60. The SCR catalyst 620 reduces urea selectivity by an action of a reducing agent. An injection valve 40 is disposed on an inlet side of the SCR catalyst 620, for supplying a urea aqueous solution 20 from the tank 900. The urea aqueous solution 20 supplied into the exhaust pipe 60 is thermally decomposed and hydrolyzed, and thereby ammonia is generated. The generated ammonia removes NOx at the SCR catalyst 620.

[0006] The urea aqueous solution 20 as a reducing agent is stored in the tank 900, and is supplied via a supply passage 904 to the injection valve 40 after passing through a filter (not shown). The urea aqueous solution 20 is harmless and is easy to handle compared with ammonia. Thus, the urea aqueous solution 20 can be suitably used for the urea SCR system 9. Specifically, about 32.5% urea aqueous solution is mainly used because it has the lowest frozen temperature (i.e., about -11.degree. C.).

[0007] However, when the urea SCR system 9 is used at an extremely low temperature environment, e.g., a cold region or midwinter, a temperature of the urea aqueous solution 20 may decrease under the frozen temperature (about -11.degree. C.) at portions adjacent to a bottom and a wall of the tank 900. Thus, the urea aqueous solution 20 may be frozen locally or wholly in the tank 900. Therefore, it is required for restricting the freezing of the urea aqueous solution 20 at a low temperature.

[0008] For example, an electric heater 910 may be disposed in the tank 900 for heating the urea aqueous solution 20. However, the electric heater 910 can melt only a part of the frozen urea aqueous solution 20 that is located at a position near the electric heater 910.

[0009] US 2007/0059222 A (corresponding to JP-2005-351253A) discloses an exhaust gas purifying apparatus in which a heat transfer medium heated by an engine is circulated and heat exchanges with a liquid reducing agent stored in a container and thereby the liquid reducing agent is restricted from freezing. When a vehicle is running stably, heat from the engine is sufficient to restrict the liquid reducing agent from freezing. However, just after starting the engine, a temperature of the engine as a heat source is low. Thus, a temperature of a coolant as the heat transfer medium is also low and may not have enough heat quantity to melt the liquid reducing agent.

[0010] Alternatively, US 2007/0035832 A (corresponding to JP-2005-282413A) discloses an exhaust gas purifying apparatus that has a main tank having a urea aqueous solution therein and a sub tank having a smaller capacity than the main tank and disposed in the main tank. The sub tank stores the urea aqueous solution supplied from the main tank and melts a frozen urea aqueous solution by using an electric heater. The frozen urea aqueous is rapidly melted in the small sub tank. However, the exhaust gas purifying apparatus has a complicated structure. Additionally, the urea aqueous solution in the sub tank may have different concentration with that in the main tank, and a concentration of the urea aqueous solution supplied to exhaust gas may be unstable.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a fluid heating device that has a high thermal efficiency with a simple structure. Another object of the invention is to provide an exhaust gas purifying apparatus for purifying exhaust gas from an engine that can stably supply a reducing agent to exhaust gas even just after the engine is started in a low temperature environment.

[0012] According to a first aspect of the invention, a fluid heating device includes a heating part and a plurality of heat transfer sections. The heating part is disposed in a fluid stored in a container and is elongated in an axial direction for heating the fluid between a lower portion and an upper portion of the container by being supplied with electricity. The heat transfer sections arranged along the axial direction of the heating part. Each of the heat transfer sections has a plate shape extending from the heating part to a radial outside of the heating part approximately perpendicularly to the axial direction of the heating part.

[0013] When the heating part is supplied with electricity, heat of the heating part is transferred to the heat transfer sections. Thus, the fluid is heated by the heat transfer sections in addition to the heating part, and thereby a frozen fluid in the container can be melted rapidly.

[0014] According to a second aspect of the invention, a fluid heating device includes a heating part and a heat transfer section. The heating part is disposed in a fluid stored in a container and is elongated in an axial direction for heating the fluid between a lower portion and an upper portion of the container by being supplied with electricity. The heat transfer section is located on the heating part and has a spiral shape extending along the axial direction of the heating part.

[0015] When the heating pad is supplied with electricity, a flow of the fluid is generated due to a thermal conviction, and the fluid flows spirally along the spiral shape of the heat transfer section. Thus, the fluid in the container is stirred and a frozen fluid is melted effectively.

[0016] According to a third aspect of the invention, an exhaust gas purifying apparatus includes a reducing catalyst, a reducing agent supplying device, a tank for storing the reducing agent therein, a supply passage connecting the reducing agent supplying device and the tank, and one of the above-described fluid heating devices for heating the fluid in the tank. The reducing catalyst is disposed in the exhaust pipe, and the reducing agent supplying device is arranged to extend into the exhaust pipe on an upstream side of a flow of exhaust gas with respect to the reducing catalyst, for supplying a reducing agent into the exhaust pipe.

[0017] The exhaust gas purifying apparatus can stably supply the reducing agent to exhaust gas even just after the engine is started in a low temperature environment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments when taken together with the accompanying drawings. In the drawings:

[0019] FIG. 1 is a partially sectional view of a heating device according to a first embodiment of the invention;

[0020] FIG. 2 is a schematic diagram of an exhaust gas purifying apparatus according to the first embodiment;

[0021] FIG. 3 is a schematic diagram showing a state of a urea aqueous solution in the heating device when the urea aqueous solution is almost frozen;

[0022] FIG. 4 is a schematic diagram showing a state of the urea aqueous solution in the heating device after the whole urea aqueous solution is melted;

[0023] FIG. 5 is a perspective view of a heat generator according to a second embodiment of the invention;

[0024] FIG. 6 is a schematic diagram showing a state of the urea aqueous solution in a heating device according to the second embodiment;

[0025] FIG. 7 is a cross-sectional view of a heating device according to a third embodiment of the invention;

[0026] FIGS. 8A and 8B are schematic diagrams showing a heating device according to a fourth embodiment of the invention; and

[0027] FIG. 9 is a schematic diagram of an exhaust gas purifying apparatus according to a related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

[0028] Referring to FIG. 1, a heating device 10 according to a first embodiment of the invention will be now described. The heating device 10 is suitably used for heating a urea aqueous solution 20, for example. The heating device 10 includes a tank 100 and a heat generator 110. The heat generator 110 includes a heating part 115 having a bottomed cylindrical shape, an insulating member 114 disposed in the heating part 115, a heat-generating element 111 having a rod shape and disposed in the heating part 115 through the insulating member 114, and a plurality of heat transfer sections 116. Each of the heat transfer sections 116 has a plate shape extending from an outer peripheral side of the heating part 115 to a radial outside of the heating part 115. A length of the heat generator 110 is set so that the heat generator 110 can heat the urea aqueous solution 20 between an upper portion and a lower portion of the tank 100. Each of the heat transfer sections 116 has a plurality of opening portions 117.

[0029] The heat transfer sections 116 are plates, for example, arranged in an axial direction of the heating part 115 in plural layers to be parallel with each other. For example, a lower one of the heat transfer sections 116 may have the larger outside diameter. Alternatively, the lower one of the heat transfer sections 116 may have the more opening portions 117. The numbers of the heat transfer sections 116 and the opening portions 117 are not limited to those shown in the example of FIG. 1. The plural heat transfer sections 116 can be separated from each other in the axial direction at equal distance or different distances.

[0030] The heat-generating element 111 is fixed in the heating part 115 by a sealing member 113. The heat-generating element 111 is connected with a control device (ECU) 50 in FIG. 2 through a pair of conductive wires 112.

[0031] At upper portions of the tank 100, an opening part 103 and an inlet part 101 are provided. The heat generator 110 is inserted in the tank 100 through the opening part 103 and is fixed to a cover 118. The cover 118 seals the opening part 103 through a sealing member 120 by using bolts 119, after the heat generator 110 is inserted into the tank 100.

[0032] The urea aqueous solution 20 is filled in the tank 100 through the inlet part 101. The inlet part 101 is sealed by a cap 102 having a vent 121. The vent 121 has a check valve 122 for providing ventilation into the tank 100. At a lower portion of the tank 100, a first supply passage 104 for supplying the urea aqueous solution 20 to an injection valve 40 is disposed. It is preferred that the first supply passage 104 is disposed adjacent to the heat generator 110.

[0033] The tank 100, the heating part 115, and the heat transfer sections 116 may be made of a material having a corrosion resistance to the urea aqueous solution, e.g., stainless. Alternatively, the heat transfer sections 116 may be made of a ceramic that has a high thermal conductivity in addition to a corrosion resistance, e.g., alumina or aluminum nitride.

[0034] An exhaust gas purifying apparatus 1 according to the first embodiment will now be described with reference to FIG. 2. The exhaust gas purifying apparatus 1 may be suitably used for purifying exhaust gas from an engine, e.g., a multicylinder engine (not shown).

[0035] Exhaust gas from the engine passes through the exhaust gas purifying apparatus 1 disposed at an exhaust pipe 60 and flows out to an outside of the vehicle. The exhaust gas purifying apparatus 1 has a first oxidization catalyst 610, a urea reduction catalyst (SCR catalyst) 620, and a second oxidization catalyst 630, for purifying NOx in exhaust gas. The first oxidization catalyst 610, the SCR catalyst 620, and the second oxidization catalyst 630 are arranged in the exhaust pipe 60 in this order from an upstream side of a gas flow. The first oxidization catalyst 610 oxidizes nitrogen monoxide (NO) in exhaust gas into nitrogen dioxide (NO.sub.2) for increasing NO.sub.2 ratio in NOx and facilitating a reduction reaction of NOx by the SCR catalyst 620. Additionally, the first oxidization catalyst 610 oxidizes a carbon hydride (HC) and carbon monoxide (CO). The SCR catalyst 620 and the second oxidization catalyst 630 may be formed integrally or separately.

[0036] The SCR catalyst 620 reduces and purifies NOx by using a reducing agent. Thus, an injection valve 40 for supplying the reducing agent to the SCR catalyst 620 is arranged to extend into the exhaust pipe 60 at a position between the first oxidization catalyst 610 and the SCR catalyst 620.

[0037] In the exhaust gas purifying apparatus 1, urea as a precursor of ammonia is used as the reducing agent. Thus, the urea aqueous solution 20, which is easy to handle compared with urea, is supplied into the exhaust pipe 60 from the injection valve 40. The second oxidization catalyst 630 oxidizes and purifies ammonia generated from urea and passing through the SCR catalyst 620 without reacting with NOx, and thereby ammonia is not discharged to the outside of the vehicle.

[0038] The urea aqueous solution 20 to be supplied to the injection valve 40 is stored in the tank 100. The tank 100 and the injection valve 40 are connected through the first supply passage 104, a pump 30, and a second supply passage 106. The urea aqueous solution 20 is drawn from the tank 100 by an operation of the pump 30, which is disposed on a downstream side of a flow of the urea aqueous solution 20, and is supplied to the injection valve 40 through a filter (not shown) disposed at the second supply passage 106.

[0039] At an upper portion of the tank 100, a first return passage 105 is connected. When a supply pressure of the injection valve 40 is higher than a predetermined pressure, the injection valve 40 is opened and a surplus urea aqueous solution 20 flows back to the tank 100 through a second return passage 107 and the first return passage 105. The injection valve 40 may have an air-assist type valve structure, for example. When the injection valve 40 has the air-assist type valve structure, the injection valve 40 is connected with the second supply passage 106 and an air supply passage (not shown). The injection valve 40 is supplied with air from the air supply passage, and a nozzle portion at an end of the injection valve 40 is opened or closed for supplying the urea aqueous solution 20 into the exhaust pipe 60.

[0040] As shown in FIG. 2, the injection valve 40 is arranged to incline with respect to the exhaust pipe 60. Thus, an injection direction of the nozzle portion, which extends into the exhaust pipe 60, is approximately parallel to a direction of the gas flow, so that the urea aqueous solution 20 can be supplied to the whole surface of the SCR catalyst 620 on an inlet side. The injected urea aqueous solution 20 is thermally decomposed and hydrolyzed by heat of exhaust gas, and thereby ammonia is generated as shown in formulas (1) and (2).

(NH.sub.2).sub.2CO+H.sub.2O.fwdarw.NH.sub.3+NHCO (1)

NHCO+H.sub.2O.fwdarw.NH.sub.3+CO.sub.2 (2)

[0041] The generated ammonia functions as the reducing agent for NOx at the SCR catalyst 620 and promotes a reduction reaction shown in formula (3).

NO+NO.sub.2+2NH.sub.3.fwdarw.N.sub.2+3H.sub.2O (3)

[0042] Ammonia passing through the SCR catalyst 620 without reacting with NOx is removed at the second oxidation catalyst 630 as shown in formula (4).

4NH.sub.3+3O.sub.2.fwdarw.2N.sub.2+6H.sub.2O (4)

[0043] The tank 100 is a closed container having a predetermined capacity, and having therein the urea aqueous solution 20 as a precursor of ammonia. As the urea aqueous solution 20, about 32.5% urea aqueous solution, which has the lowest frozen temperature (about -11.degree. C.), is generally used. When the exhaust gas purifying apparatus 1 is used in an extremely low temperature environment, e.g., a cold region, a temperature of the urea aqueous solution 20 may decrease under the frozen temperature, and a part of the urea aqueous solution 20 may be frozen. Thereby, a concentration of the urea aqueous solution 20 supplying to the injection valve 40 may be unstable. In the low temperature environment, a temperature of the urea aqueous solution 20 is decreased from portions adjacent to a bottom surface and a wall surface of the tank 100, which are exposed to an outside air, and solid urea may be generated due to freezing or uneven temperature. In this case, a concentration of an unfrozen urea aqueous solution 20 drawn from the tank 100 may be higher than a predetermined concentration. In contrast, when the whole urea aqueous solution 20 is melted, the concentration may be lower than the predetermined concentration.

[0044] Thus, the heating device 10 is provided for heating urea aqueous solution 20 in the tank 100. The control device 50 controls electricity supplied to the heat generator 110 based on a freezing monitoring information of the urea aqueous solution 20, so that the urea aqueous solution 20 is not frozen in the tank 100. As a power source of the heating device 10, a vehicle battery (not shown) or an alternator (not shown) may be used.

[0045] For example, a freezing monitor for monitoring a freezing state of the urea aqueous solution 20 may be a temperature detector (not shown) for detecting the temperature of the urea aqueous solution 20 at a lower portion of the tank 100 and the detected temperature may be output to the control device 50. Alternatively, the freezing monitor may be another temperature detector (not shown) for detecting a temperature of outside air, and the detected temperature may be output to the control device 50.

[0046] The heat generator 110 is supplied with electricity for heating the urea aqueous solution 20 in the tank 100 when the control device 50 determines that the urea aqueous solution 20 has a possibility of freezing and an operation of the heating device 10 is required, based on the detected temperature of the urea aqueous solution 20 or the outside air.

[0047] As shown in FIG. 3, when the urea aqueous solution 20 in the tank 100 is almost frozen and then the heat generator 100 is supplied with electricity, a frozen urea aqueous solution 21 is widely melted by receiving heat from a surface of the heating part 115 and surfaces of the heat transfer sections 116 extending radially outwardly from the heating part 115. Additionally, the dimension of the lower one of the heat transfer sections 116 is larger than that of the upper one of the heat transfer sections 116 in the radial direction. When the urea aqueous solution 20 heated by the lower one of the heat transfer sections 116 convects upward, the urea aqueous solution 20 is not interrupted by the upper one of the heat transfer sections 116. Thus, the urea aqueous solution 20 is stirred due to a thermal conviction from the lower portion to the upper portion of the heating part 115, and unevenness in the temperature and concentration of the urea aqueous solution 20 can be reduced. As a result, the urea aqueous solution 20 is melted easily and is restricted from refreezing.

[0048] Additionally, each of the heat transfer sections 116 has the plural through holes 117. Thus, a surface area of the heat transfer section 116 increases and a melting rate of the urea aqueous solution 20 further increases. In addition, the urea aqueous solution 20 passes through the trough holes 117, and thereby the urea aqueous solution 20 convicts rapidly and a melting rate of the urea aqueous solution 20 further increases. Furthermore, because the first supply passage 104 is provided just under the heat generator 110, the urea aqueous solution 20 can be supplied even just after the engine is started.

[0049] When the whole urea aqueous solution 20 in the tank 100 is melted, as shown in FIG. 4, a flow of the urea aqueous solution 20 is generated by the thermal conviction. Thus, the unevenness in the temperature and concentration of the urea aqueous solution 20 is further reduced, and the urea aqueous solution 20 is restricted from freezing.

[0050] The tank 100 may be covered by a thermal insulation member (not shown) so that the temperature in the tank 100 is restricted from decreasing.

Second Embodiment

[0051] In the heat generator 110 in FIG. 1, the plural heat transfer sections 116, which extend from the heating part 115 to the radial outside of the heating part 115, are arranged to be parallel to each other along the axial direction of the heating part 115. Alternatively, the plural heat transfer sections 116 may be located to be connected without being limited to the shape shown in the example of FIG. 1.

[0052] In a heat generator 110a of a second embodiment of the invention shown in FIG. 5, a heat transfer section 116a has a single spiral plate shape located on the heating part 115 and extending along the axial direction of the heating part 115.

[0053] The heat transfer section 116a extends continuously in a spiral shape to have an upper part and a lower part in the axial direction of the heating part 115. The upper part of the heat transfer section 116a may have a dimension smaller than that of the lower part, in the radial direction of the heating part 115. The heat transfer section 116a may have a plurality of through holes 117.

[0054] As shown in FIG. 6, a heating device 10a according to the second embodiment includes the heat generator 110a instead of the heat generator 110 in FIG. 1. When the heating device 110a is operated, the flow of the urea aqueous solution 20 is generated due to the thermal conviction, and the urea aqueous solution 20 flows spirally along the shape of the heat transfer section 116a. Thus, the urea aqueous solution 20 in the tank 100 is stirred, and the temperature and concentration of the urea aqueous solution 20 are homogenized.

[0055] The heat transfer section 116a may be divided into plural parts each having a spiral shape extending along the heating part 115, and separated from each other in the axial direction of the heating part 115.

Third Embodiment

[0056] In a heating device 10b in a third embodiment of the invention, a heat generator 110b are disposed at a bottom portion of a tank 100b, as shown in FIG. 7. That is, the heat generator 110b is inserted into the tank 100b via the bottom portion, and is fixed to the bottom portion. A cover 118b, bolts 119b, a sealing member 120b, and a first supply passage 104b in FIG. 7 are similarly with the cover 118, bolts 119, the sealing member 120, and the first supply passage 104 of the heating device 10 in FIG. 1, respectively. The heating device 10b has similar effects with those of the heating device 10.

Fourth Embodiment

[0057] A heating device 10c according to a fourth embodiment of the invention includes a heat generator 110c having a plurality of heat-generating elements 111c and a level sensor 130 for monitoring a volume of the urea aqueous solution 20. The level sensor 130 outputs a detected volume to a control device 50c, and the control device 50c controls electricity supplied to the heat generator 110c so that a thermal quantity is appropriate for the volume of the urea aqueous solution 20. For example, when a level of the urea aqueous solution 20 is high, the control device 50c controls the electricity supply so that the heat generator 110c generates heat from an upper portion to a lower portion. In contrast, when the level of the urea aqueous solution 20 is low, the control device 50c controls the electricity supply so that the heat generator 110c generates heat only at the lower portion. In this case, an energy efficiency of the heating device 110c is improved, a surplus heating can be prevented when the level of the urea aqueous solution 20 is low, and a precipitation of urea is restricted.

[0058] In the above-described embodiments, the heating devices 10, 10a, 10b, and 10c are typically used for the exhaust gas purifying apparatus, such as the urea SCR system shown in FIG. 2. However, the heating devices 10, 10a, 10b, and 10c can be used for any type SCR system.

[0059] The heat transfer section 116a described in the second embodiment can be also used for the heat generator 110b or 110c.

[0060] Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.

Claims

1. A fluid heating device comprising: a heating part disposed in a fluid stored in a container and elongated in an axial direction for heating the fluid between a lower portion and an upper portion of the container by being supplied with electricity; and a plurality of heat transfer sections arranged along the axial direction of the heating part, wherein each of the heat transfer sections has a plate shape extending from the heating part to a radial outside of the heating part approximately perpendicularly to the axial direction of the heating part.

2. The fluid heating device according to claim 1, wherein: one of the heat transfer sections extending from a lower portion of the heating part has a dimension larger than that of another one of the heat transfer sections extending from an upper portion of the heating part, in a radial direction of the heating part.

3. The fluid heating device according to claim 1, wherein, each of the heat transfer sections has a plurality of through holes.

4. The fluid heating device according to claim 1, wherein: the heat transfer sections are made of a material having a high thermal conductivity.

5. The fluid heating device according to claim 1, further comprising: a tank for defining the container, the tank having an inlet portion from which the fluid is introduced, and an outlet portion from which the fluid flows out, wherein: the heating part is located at a position near the outlet portion.

6. The fluid heating device according to claim 1, wherein: the heating part has an approximately cylindrical shape elongated in the axial direction; and the heat transfer sections are separated from each other in the axial direction.

7. An exhaust gas purifying apparatus for purifying exhaust gas passing through an exhaust pipe from an engine comprising: a reducing catalyst disposed in the exhaust pipe; a reducing agent supplying device disposed to extend into the exhaust pipe on an upstream side of a flow of exhaust gas with respect to the reducing catalyst, for supplying a reducing agent into the exhaust pipe; a tank for storing the reducing agent therein; a supply passage connecting the reducing agent supplying device and the tank; and the fluid heating device according to claim 1 disposed in the tank to heat the reducing agent.

8. The exhaust gas purifying apparatus according to claim 7, wherein: the reducing agent includes a urea aqueous solution.

9. A fluid heating device comprising: a heating part disposed in a fluid stored in a container and elongated in an axial direction for heating the fluid between a lower portion and an upper portion of the container by being supplied with electricity; and a heat transfer section located on the heating part and having a spiral shape extending along the axial direction of the heating part.

10. The fluid heating device according to claim 9, wherein: the heat transfer section has an upper part and a lower part in the axial direction of the heating part and the upper part has a dimension smaller than that of the lower part in a radial direction of the heating part.

11. The fluid heating device according to claim 9, wherein: the heat transfer section has a plurality of through holes.

12. The fluid heating device according to claim 9, wherein: the heat transfer section is made of a material having a high thermal conductivity.

13. The fluid heating device according to claim 9, further comprising: a tank for defining the container, the tank having an inlet portion from which the fluid is introduced, and an outlet portion from which the fluid flows out, wherein: the heating part is located at a position near the outlet portion.

14. The fluid heating device according to claim 9, wherein: the heat transfer section continuously extends in the spiral shape along the axial direction.

15. An exhaust gas purifying apparatus for purifying exhaust gas passing through an exhaust pipe from an engine, comprising: a reducing catalyst disposed in the exhaust pipe; a reducing agent supplying device disposed to extend into the exhaust pipe on an upstream side of a flow of exhaust gas with respect to the reducing catalyst, for supplying a reducing agent into the exhaust pipe; a tank for storing the reducing agent therein; a supply passage connecting the reducing agent supplying device and the tank; and the fluid heating device according to claim 9 disposed in the tank to heat the reducing agent.

16. The exhaust gas purifying apparatus according to claim 15, wherein: the reducing agent includes a urea aqueous solution.