Combustion apparatus

Technical Field

0001The present invention relates to a combustion apparatus such as a commercial boiler, an industrial boiler or the like, and more particularly to a combustion apparatus which produces a reduced amount of unburned combustible, can execute a high-efficiency combustion, and can considerably reduce nitrogen oxides (NOx) or can inhibit nitrogen oxides (NOx) from being generated within a furnace.

Background Art

0002At present, in order to remove the nitrogen oxides (NOx) contained in the combustion gas generated by combustion of a fuel such as coal or the like within a furnace of an industrial boiler for power generation or the like, an NOx removal apparatus is provided at the downstream side of the exhaust gas flow path in which the combustion gas (hereinafter referred to as exhaust gas) discharged from the furnace is circulated. However, on the other hand, in order to save operating costs required for the consumption of ammonia in the NOx removal apparatus, the structure is made such that the amount of NOx generated within the furnace is reduced as much as possible by executing a low NOx combustion in the combustion stage within the furnace.

0003The methods of low NOx combustion include a two-stage combustion method of divisionally supplying the air required for the fuel combustion (hereinafter referred to as combustion air) entirely within the furnace, and a method using a low NOx burner having a low NOx function as a burner, and a low NOx combustion using them together is generally executed.

0004<figref idref="f0016">Fig. 20A</figref> is a schematic front elevational view showing an example of the structure of a combustion apparatus such as a boiler or the like, and <figref idref="f0016">Fig. 20B</figref> is a schematic side elevational view of the combustion apparatus. Three stages of burners 2 and one stage of air ports 3 (hereinafter referred to as after air port (AAP) because the air port is provided at the downstream side of the gas flow as seen from the burner) are attached in the furnace defined and formed by a water wall 1 so as to face each other in four rows. In order to supply the combustion air to each of the burners 2 and the AAP 3, a wind box 4 for the burners and a wind box 5 for the AAP are respectively placed. The burner 2 executes a combustion in which the air ratio (air amount supplied to the burner/theoretical amount of air) is about 0.8. In other words, the NOx generation can be lowered by executing the combustion such that the air is slightly short in comparison with the air amount (the theoretical air amount theoretically required for a complete combustion of the fuel). However, since the rate of unburned fuel (hereinafter referred to as unburned combustible) is inversely increased, the complete combustion is executed by injecting a shortfall air by the AAP 3 in the downstream side.

0005As mentioned above, the two-stage combustion method is an effective method for reducing the generated amount of NOx. In this case, in the low NOx burner, the burner structure is contrived such that a denitration can be executed within a flame formed by the burner, however, a detailed description will be omitted here.

0006A conventional AAP structure is shown in <figref idref="f0017">Fig. 21</figref>. A high-temperature combustion air (hereinafter referred to as high-temperature air) 8 is supplied to the AAP wind box 5 of the AAP 3 attached to the gas flow downstream of the burner 2 of the water wall 1 (the upper side of the burner 2), and the high-temperature air is supplied into the high-temperature combustion gas within the furnace so as to form a jet flow. In this case, the combustion air 8 is supplied to the burner and the AAP after the temperature of the combustion air 8 is increased to about 300°C for improving the power generating efficiency of the plant, generally by maintaining the temperature of the high-temperature combustion gas within the furnace.

0007The combustion region moves into the downstream side of the furnace at the time of employing the two-stage combustion method. Accordingly, if the mixing of the high-temperature combustion gas within the furnace with the high-temperature air flow from the AAP 3 is bad, the high-temperature combustion gas is discharged from the furnace in a state in which the high-temperature combustion gas and the high-temperature air 8 are not sufficiently mixed. Therefore, a lot of unburned combustible (unburned carbon from coal and carbon monoxide in the combustion gas) are contained in the exhaust gas from the furnace. Accordingly, in the furnace of the commercial boiler which has a great combustion efficiency, which has an influence on the economical efficiency, in order to promote the mixing of the air from the AAP3, an AAP having a structure shown in <figref idref="f0017">Fig. 22</figref> is employed (<patcit id="pcit0001" dnum="JP59109714A"><text>JP-A-59-109714</text></patcit> - patent document 1). In this structure, the mixing with the high-temperature combustion gas is promoted by the high-temperature air flow which is supplied from a swirling device 6 and is swirled. At the same time, it is possible to supply the high-temperature air flow to the center portion of the furnace by injecting a straight flow having the flow amount controlled by a damper 7 to the center portion of the swirling flow so as to secure a spray penetration of the jet flow.

0008<figref idref="f0018">Fig. 23</figref> is a schematic view of the outline structure of a combustion apparatus, for example, disclosed in <patcit id="pcit0002" dnum="JP3286906A"><text>JP-A-3-286906</text></patcit> - patent document 2 and <patcit id="pcit0003" dnum="JP1101011U"><text>JP-U-1-101011</text></patcit> - patent document 3. A burner 2, a lower stage port 11 and an upper stage port 12 are placed in a water wall 1. In other words, the AAPs are provided so as to be separated into an upper stage and a lower stage. Exhaust gas or low-temperature air 10 is supplied from the lower stage port 11, and the high-temperature air 8 is supplied from the upper stage port 12.

0009The burner 2 and the upper stage port 12 realize a normal two-stage combustion method. In this case, a high-temperature portion is formed in an upper portion of the burner within the furnace, and the gas temperature becomes too high by supplying the high-temperature air 8, and NOx tends to be generated. Accordingly, in order to temporarily lower the temperature of the gas within the furnace, the exhaust gas or the low-temperature air 10 is supplied from the lower stage port 11, and the NOx is prevented from being generated.

0010However, in this combustion apparatus, it is necessary to supply a lot of exhaust gas or low-temperature air 10 for lowering the temperature of the high-temperature combustion gas in the upper portion of the burner within the furnace. Accordingly, the power generation efficiency of the plant is significantly reduced.

0011<figref idref="f0019">Fig. 24</figref> is a schematic view of the outline structure of a combustion apparatus in accordance with further prior art. As shown in the drawing, the burners 2 are arranged so as to face in three stages, and the AAPs 3 are arranged so as to face in one stage. In the drawing, reference numeral 22 denotes an environmental apparatus such as an NOx removal apparatus or the like, reference numeral 23 denotes an opening and closing valve, reference numeral 24 denotes an air preheater, reference numeral 25 denotes a forced blower (FDF), reference numeral 26 denotes a coal pulverizing machine, reference numeral 27 denotes a chimney, reference numeral 28 denotes an exhaust gas recirculation blower (GRF), reference numeral 41 denotes a furnace, reference numeral 43 denotes a combustion air flow path, reference numeral 70 denotes an exhaust gas, reference numerals 71, 72 and 73 denote a heat exchanger tube, and reference numeral 74 denotes a furnace bottom gas supply chamber for supplying the exhaust gas to the bottom portion of the furnace.

0012The distribution of the NOx concentration within the furnace in the combustion apparatus having that structure is shown in <figref idref="f0020">Fig. 25</figref>. The horizontal axis in the drawing shows the NOx concentration, and the vertical axis shows the distance in the furnace height direction.

0013In the case that the flow amount of the air supplied from the burner in accordance with the two-state combustion is less than the theoretical air flow amount as shown in the drawing, the gas within the furnace until the air for the two-stage combustion is mixed constitutes a reducing atmosphere, and the NOx generated in the burner region is gradually lowered. Since the atmosphere is changed to an oxidizing atmosphere by supplying the air for the two-stage combustion by the AAP, the amount of NOx is increased as shown by the solid line for the prior art. The increased NOx is constituted by two kinds of NOx, one being caused by the oxidization of unburned nitrogen compounds contained in the combustion gas, and the other being caused by the oxidization of the nitrogen in the air under the high temperature (thermal NOx). In the pulverized coal firing, the NOx level is widely lowered on the basis of the high development degree of the low NOx combustion technique.

0014Conventionally, the NOx reduction is mainly constituted by reduction of fuel NOx originating from nitrogen contained in the fuel, however, in recent days when the NOx level can be set to be equal to or less than 200 ppm, the existence of the thermal NOx cannot be disregarded. As a result of a combustion simulation, it has been known that the thermal NOx amounts to about one half of the entire NOx generation amount. Further, it has been known that most of the thermal NOx is generated after supplying the air for combustion (which may be called as air for two-stage combustion) from the AAP. Further, it has been known that the unburned combustible existing in the high temperature portion in the upper portion of the burner is heated up to a high temperature locally in the early stage of the combustion caused by the air for two-stage combustion, and thermal NOx is suddenly generated.

0015A description will be given in detail of the phenomenon with reference to <figref idref="f0021">Fig. 26</figref>. This drawing shows an AAP structure in accordance with the prior art provided in the water wall 1, and the mixed state of injected air from the AAP and high-temperature combustion gas within the furnace 41, and the AAP structure is of the type having two flow paths in the case of this example.

0016The air for two-stage combustion (an AAP primary air 105 and an AAP secondary air 106) is injected into the furnace 41 through an AAP primary air flow path 102 at the center side from a two-stage combustion air wind box 101, and an AAP secondary air flow path 103 at the outer peripheral side. A proper swirl is applied to the AAP secondary air 106 by an AAP secondary air register 104. In this case, reference numeral 1000 denotes an opening portion for introducing an AAP primary air flow into the AAP primary air flow path 102 from the two-stage combustion air wind box 101.

0017In the sight of the combustion promotion for improving the power generation efficiency of the plant, the high-temperature air is frequently used for the air for two-stage combustion. In order to reduce the amount of unburned combustible, it is necessary to promote the mixing of the air supplied from the AAP and the high-temperature combustion gas within the furnace. In order to promote the mixing, since it is necessary to make the air jet flow to reach the center portion of the furnace and to widen the width of the jet flow so as to prevent a gap from being generated between the jet flows, there are achieved increasing the spray speed of the air jet flow so as to strengthen the spray penetration of the jet flow, applying the swirl to the air jet flow, and the like. In each of the cases, the intensity or degree of turbulence becomes large in the mixed region between the air and the high-temperature combustion gas. When the intensity or degree of turbulence becomes large, the oxidation reaction in the mixed region is promoted, and the local temperature is increased. Further, since sufficient air is supplied to the mixed region, the oxygen concentration is high. Accordingly, in the mixed region, there is established a condition of high temperature and high oxygen concentration corresponding to the requirement for generating thermal NOx.

0018As a technique of lowering the thermal NOx, an exhaust gas mixing of mixing a part of the exhaust gas with the combustion air is frequently used in oil fired boilers and gas fired boilers. <figref idref="f0022">Fig. 27</figref> shows the outline structure of a combustion apparatus to which the exhaust gas mixing is applied.

0019A part of the exhaust gas is returned by the gas recirculation blower 28, and a part thereof is supplied into the furnace from the furnace bottom gas supply chamber 74, and is used for controlling the temperature of the reheat steam. Further, a part of the exhaust gas is branched at the outlet of the gas recirculation blower 28 for lowering the NOx so as to be introduced into the combustion air flow path 43 through the gas mixing flow path 29. Reference numeral 30 denotes a gas mixing regulating damper provided on the gas mixing flow path 29.

0020The combustion air with which the exhaust gas is mixed is supplied into the furnace from the burner 2 and the AAP 3. The exhaust gas mixing is a method which can effectively lower the thermal NOx on the basis of the reduction of the combustion temperature and the reduction of the oxygen concentration in the combustion field. This method can be applied with no problems to boilers employing oil or gas as fuel, which have a high combustion speed. However, when applying the exhaust gas mixing to coal fired boilers having a comparatively low combustion speed, the combustion efficiency is largely lowered on the basis of the reduction of the combustion temperature in the entire combustion field and the reduction of the oxygen concentration.

0021Further, in the low NOx coal burner flame, there exists an NOx removing reaction within the flame wherein the temporarily generated NOx is reduced by an intermediate product, however, it is known that the NOx removing reaction within the flame is improved in the NOx removing efficiency if the temperature of the flame becomes high. When the flame temperature is reduced by the exhaust gas mixing, the generated NOx is rather increased on the basis of the reduction of the NOx removing efficiency.

0022As mentioned above, the two-stage combustion method has the NOx reducing effect over the entire furnace, however, the AAP itself has the effect of generating the NOx. The conventional AAP has the disadvantage that in the case of promoting the mixing between the high-temperature combustion gas and the air within the furnace for achieving the complete combustion by reducing the unburned combustible, the NOx generated in the AAP is increased.

0023Further, when applying the exhaust gas mixing for reducing the thermal NOx of the coal firing combustion apparatus as mentioned above, there is the disadvantage that an adverse effect such as the reduction of the combustion efficiency and the reduction of the NOx removing reaction within the flame is generated.

0024<patcit id="pcit0004" dnum="DE19705938A1"><text>DE 197 05 938 A1</text></patcit> discloses a combustion apparatus in which several rows of air ports are arranged on the walls of a furnace downstream of a burner and are injecting recirculated exhaust gas and combustion air into the furnace.

Disclosure of the Invention

0025The underlying problem of the present invention is to provide a combustion apparatus and a wind box which can overcome the disadvantages of the prior art, and can reduce or inhibit the NOx generation in an AAP even in the case of promoting the mixing between a high-temperature combustion gas and the combustion air so as to intend to reduce the amount of unburned combustible.

0026The above problem is solved according to the independent claims. The dependent claims relate to preferred embodiments of the concept of the present invention.

0027In accordance with a first aspect of the present invention, there is provided a combustion apparatus comprising: <ul id="ul0001" list-style="none" compact="compact"><li>a burner burning a fuel within a furnace in the theoretical air ratio or less, and</li><li>a plurality of air ports arranged downstream of the burner and injecting combustion air for the shortfall in the burner into the furnace,</li><li>wherein inhibiting gas supply means for supplying a nitrogen oxide generation inhibiting gas constituted by at least one gas selected from the combustion exhaust gas, a mixed gas of the combustion exhaust gas and air, and low temperature air, inhibiting nitrogen oxides from being generated is provided in the mixing region formed within the furnace by both of the combustion gas generated by burning the fuel by means of the burner and the combustion air injected from the air port or near the mixing region,</li><li>wherein said plurality of air ports are placed along a width direction of the furnace and each of the air ports is provided with said inhibiting gas supply means and with flow rate regulating means for regulating the flow rate of the inhibiting gas wherein said flow rate regulating means is configured so that the inhibiting gas is supplied more to an air port close to the furnace center portion than to an air port close to a furnace side wall in the plurality of air ports (3).</li></ul>

0028In accordance with a second aspect of the present invention, there is provided a combustion apparatus as recited in the first aspect mentioned above, wherein the inner side of the air port is separated into a flow path for injecting the combustion air and a flow path for injecting the nitrogen oxide generation inhibiting gas.

0029In accordance with a third aspect of the present invention, there is provided a combustion apparatus as recited in any one of the first aspect to the third aspect mentioned above, wherein the inhibiting gas is injected into the furnace from an outer peripheral portion side of an air injection port of the air port.

0030In accordance with a fourth aspect of the present invention, there is provided a combustion apparatus as recited in any one of the first aspect to the fourth aspect mentioned above, wherein the inhibiting gas injection port is formed in an annular shape so as to surround the air injection port of the air port.

0031In accordance with a fifth aspect of the present invention, there is provided a combustion apparatus as recited in any one of the first aspect to the fourth aspect mentioned above, wherein a plurality of the inhibiting gas injecting ports are arranged in a peripheral direction so as to surround the air injection port of the air port.

0032In accordance with a sixth aspect of the present invention, there is provided a combustion apparatus as recited in any one of the first aspect to the fourth aspect mentioned above, wherein the inhibiting gas injection port is formed approximately in a circular arc shape so as to surround a part of the air injection port of the air port.

0033In accordance with an seventh aspect of the present invention, there is provided a combustion apparatus as recited in any one of the first aspect to the fourth aspect mentioned above, wherein a plurality of the inhibiting gas injection ports are concentrically arranged in a part of the outer peripheral portion of the air injection port of the air port.

0034In accordance with an eighth aspect of the present invention, there is provided a combustion apparatus as recited in the sixth aspect or the seventh aspect mentioned above, wherein the inhibiting gas injection port is arranged at the burner side of the air injection port of the air port.

0035In accordance with a ninth aspect of the present invention, there is provided a combustion apparatus as recited in any one of the first aspect to the eighth aspect mentioned above, wherein a system for supplying a part of the recirculated exhaust gas within the furnace as the nitrogen oxide inhibiting gas by branching a part of the exhaust gas recirculated within the furnace is provided.

0036In accordance with a tenth aspect of the present invention, there is provided a combustion apparatus as recited in the ninth aspect mentioned above, wherein a blower exclusive for the inhibiting gas is placed in the inhibiting gas supply system.

0037In order to promote the mixing between the air supplied from the AAP for lowering the amount of unburned combustible and the high-temperature combustion gas within the furnace, there is achieved strengthening the penetration of the jet flow by increasing the injection speed of the air jet flow or applying a swirl to the air jet flow. In both the cases, the non-steady turbulence (intensity of turbulence) in the mixing interface between the high-temperature air and the high-temperature combustion gas shows in <figref idref="f0017">Figs. 21 and 22</figref> becomes strong. In this case, in the conventional AAP, the temperature becomes high and the oxygen concentration becomes high in the mixing interface having a large degree of turbulence. This is because the combustion gas having the high temperature and the air having the high oxygen concentration are directly brought into contact with each other.

0038Under the conditions of the high degree of turbulence, the high temperature and the high oxygen concentration, NOx is generated. When executing the mixing promotion for reducing the unburned combustible by the conventional AAP, the conditions mentioned above are established, so that NOx is generated. In the present invention, since the gas having a low temperature and a low oxygen concentration (the exhaust gas, the mixed gas of exhaust gas and air, the low-temperature air or the like) is supplied to the mixed region (the mixing interface) or the portion near the mixed region, NOx is not generated or the NOx is inhibited from being generated.

Brief Description of the Drawings

0039<ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a schematic view of the outline structure of an AAP in accordance with a first embodiment of the present invention;</li><li><figref idref="f0001">Fig. 2</figref> is a schematic view of the outline structure of an AAP in accordance with a second embodiment of the present invention;</li><li><figref idref="f0002">Fig. 3A</figref> is a schematic view of the outline structure of an AAP in accordance with a third embodiment of the present invention;</li><li><figref idref="f0002">Fig. 3B</figref> is a visual field diagram on the line A-A in <figref idref="f0002">Fig. 3A</figref>;</li><li><figref idref="f0003">Fig. 4A</figref> is a schematic view of the outline structure of an AAP in accordance with a fourth embodiment of the present invention;</li><li><figref idref="f0003">Fig. 4B</figref> is a visual field diagram on the line B-B in <figref idref="f0003">Fig. 4A</figref>;</li><li><figref idref="f0004">Fig. 5</figref> is a schematic view of the outline structure of a combustion apparatus for explaining the route of supplying exhaust gas to an inhibiting gas wind box in accordance with a fifth embodiment of the present invention;</li><li><figref idref="f0004">Fig. 6</figref> is a schematic view of the outline structure of a combustion apparatus for explaining the route of supplying a mixed gas of exhaust gas and air to an inhibiting gas wind box in accordance with a sixth embodiment of the present invention;</li><li><figref idref="f0005">Fig. 7</figref> is a schematic view of the outline structure of a combustion apparatus for explaining the route of supplying a low-temperature air to an inhibiting gas wind box in accordance with a seventh embodiment of the present invention;</li><li><figref idref="f0005">Fig. 8</figref> is a characteristic diagram for explaining the effect obtained by applying the present invention;</li><li><figref idref="f0006">Fig. 9</figref> is a schematic view of the outline structure of a combustion apparatus in accordance with an eighth embodiment of the present invention;</li><li><figref idref="f0007">Fig. 10</figref> is a schematic view of an enlarged structure near a two-stage combustion air wind box in accordance with the embodiment;</li><li><figref idref="f0008">Fig. 11</figref> is an explanatory view showing the flow state of an air jet flow near the wind box, an AAP exhaust gas jet flow and an unburned gas ascending flow from the burner portion side;</li><li><figref idref="f0009">Fig. 12A</figref> is a schematic view of an enlarged structure near a two-stage combustion air wind box in accordance with a ninth embodiment of the present invention;</li><li><figref idref="f0009">Fig. 12B</figref> is a view showing the arrangement of an inhibiting gas injection port;</li><li><figref idref="f0010">Fig. 13A</figref> is a schematic view of an enlarged structure near a two-stage combustion air wind box in accordance with a tenth embodiment of the present invention;</li><li><figref idref="f0010">Fig. 13B</figref> is a view showing the arrangement of an inhibiting gas injection port;</li><li><figref idref="f0011">Fig. 14</figref> is an explanatory view showing the flow state of an air jet flow near the wind box, an AAP exhaust gas jet flow and an unburned gas ascending flow from the burner portion side;</li><li><figref idref="f0012">Fig. 15A</figref> is a schematic view of an enlarged structure near a two-stage combustion air wind box in accordance with an eleventh embodiment of the present invention;</li><li><figref idref="f0012">Fig. 15B</figref> is a view showing the arrangement of an inhibiting gas injection port;</li><li><figref idref="f0013">Fig. 16</figref> is a schematic view of the outline structure of a combustion apparatus in accordance with a twelfth embodiment of the present invention;</li><li><figref idref="f0014">Fig. 17A</figref> is a view showing the distribution state of the gas temperature within a furnace in the width direction within the furnace;</li><li><figref idref="f0014">Fig. 17B</figref> is a view showing the distribution state of the generated NOx concentration in the width direction within the furnace;</li><li><figref idref="f0014">Fig. 18</figref> is a view for explaining the opening degree adjustment of a plurality of adjusting dampers arranged at the downstream side of an AAP exhaust gas recirculation amount adjusting damper placed in the front and rear sides of a can;</li><li><figref idref="f0015">Fig. 19</figref> is a view for explaining an example for setting the flow rate of an AAP recirculation gas in the embodiment of the present invention;</li><li><figref idref="f0016">Fig. 20A</figref> is a schematic front elevational view showing the structure of a boiler combustion apparatus;</li><li><figref idref="f0016">Fig. 20B</figref> is a schematic side elevational view of the combustion apparatus in <figref idref="f0016">Fig. 20A</figref>;</li><li><figref idref="f0017">Fig. 21</figref> is a schematic view of the outline structure of an AAP showing a first prior art;</li><li><figref idref="f0017">Fig. 22</figref> is a schematic view of the outline structure of an AAP showing a second prior art;</li><li><figref idref="f0018">Fig. 23</figref> is a schematic view of the outline structure of a combustion apparatus showing a third prior art;</li><li><figref idref="f0019">Fig. 24</figref> is a schematic view of the outline structure of a combustion apparatus showing a fourth prior art;</li><li><figref idref="f0020">Fig. 25</figref> is a view showing the distribution state within a furnace of the NOx concentration in the combustion apparatus;</li><li><figref idref="f0021">Fig. 26</figref> is a view showing a conventional AAP structure, and the mixing state of injection air from the AAP and a high-temperature combustion gas within the furnace; and</li><li><figref idref="f0022">Fig. 27</figref> is a schematic view of the outline structure of a combustion apparatus showing a fifth prior art.</li></ul>

0040<figref idref="f0001">Fig. 1</figref> is a schematic view of the outline structure of an AAP in accordance with a first embodiment. An AAP wind box 5 is placed in a water wall 1, and an NOx generation inhibiting gas wind box 9 is placed in the inner portion thereof so as to form a double structure. An inhibiting gas injection port 9a formed in a furnace side of the inhibiting gas wind box 9 is provided annularly at the outer peripheral portion side of an AAP air injection port 5a formed in the furnace side of the AAP wind box 5.

0041The high-temperature air flow 8 is introduced to the AAP wind box 5, and is injected into the furnace linearly from the AAP air injection port 5a. The NOx generation inhibiting gas 10 constituted by the exhaust gas is introduced into the inhibiting gas wind box 9, and is injected into the furnace toward the periphery of the high-temperature air jet flow, that is, the mixing region of the both (the wavy line portion in the drawing) formed by the high-temperature combustion gas and the high-temperature air (the air for combustion) within the furnace or a portion near the same.

0042As mentioned above, the AAP is constituted by the double structure, and the high-temperature air 8 is supplied into the furnace from the center portion, and the inhibiting gas 10 is supplied into the furnace from the outer peripheral portion thereof. In this case, if the injection speed of the high-temperature air jet flow is increased for promoting the mixing of the high-temperature air 8, the inhibiting gas 10 constituted by the exhaust gas having the low temperature and the low oxygen concentration is supplied to the mixing region of the high-temperature air 8 and the high-temperature combustion gas or the portion near the same.

0043Accordingly, the generation of NOx is inhibited.

0044In this case, when mixing the exhaust gas with the high-temperature air supplied from the AAP, it is possible to inhibit the generation of NOx due to the reduction of the oxygen concentration and the reduction of the gas temperature on the basis of the dilution, however, this is not preferable because the efficiency of the power generation plant is lowered on the basis of the recirculation of a large amount of exhaust gas. Since it is possible to inhibit the generation of NOx by supplying a small amount of inhibiting gas 10 only to the mixing region of the high-temperature air and the high-temperature combustion gas corresponding to the portion in which the thermal NOx is generated, the efficiency of power generation is not lowered.

0045In the present embodiment, the exhaust gas is used as the inhibiting gas 10, however, the same effect can be obtained by supplying the mixed gas of the exhaust gas and the air or the low-temperature air from the inhibiting gas injection port 9a in the outer peripheral side of the AAP flow path.

0046<figref idref="f0001">Fig. 2</figref> is a schematic view of the outline structure of an AAP in accordance with a second embodiment. In the present embodiment, the structure is made such that an inhibiting gas 10 constituted by a high-temperature air straight flow having the flow rate controlled by a damper 7, a high-temperature air swirling flow passing through a swirling device 6 and an inhibiting gas 10 constituted by the exhaust gas is supplied into the furnace. In other words, the AAP is formed in a multiple structure (a triple structure in the present embodiment), and inhibits the generation of NOx by supplying the inhibiting gas 10 from the outermost periphery.

0047In the combustion apparatus in which a lot of AAPs 3 are arranged, the arrangement of the AAP wind box 5 and the inhibiting gas wind box 9 comes into question. Third and fourth embodiments are shown in <figref idref="f0002">Figs. 3A, 3B</figref>, <figref idref="f0003">4A and 4B</figref>.

0048<figref idref="f0002">Fig. 3B</figref> is a visual field diagram on the line A-A in <figref idref="f0002">Fig. 3A</figref>. In this third embodiment, a plurality of AAPs 3 are placed in the water wall 1, however, the high-temperature air flow 8 is supplied from the common AAP wind box 5. The common inhibiting gas wind box 9 is placed in an inner portion of the AAP wind box 5, and supplies the inhibiting gas 10 through the common inhibiting gas wind box 9.

0049<figref idref="f0003">Fig. 4B</figref> is a visual field diagram on the line B-B in <figref idref="f0003">Fig. 4A</figref>. In this fourth embodiment, the individual inhibiting gas wind box 9 is placed in an inner portion of the common AAP wind box 5.

0050Next, a description will be given of the supply path of the exhaust gas corresponding to the inhibiting gas and the air on the basis of embodiments shown in <figref idref="f0004 f0005">Figs. 5 to 7</figref>. In these drawings, reference numeral 13 denotes a first blower, reference numeral 14 denotes a heat exchanger, and reference numeral 15 denotes a second blower. In a fifth embodiment shown in <figref idref="f0004">Fig. 5</figref>, the inhibiting gas constituted by the exhaust gas is supplied to the inhibiting gas wind box 9 by the second blower 15. The gas temperature of the inhibiting gas is between about 250 and 350°C, and the oxygen percentage content thereof is between about 2 and 6%.

0051In a sixth embodiment shown in <figref idref="f0004">Fig. 6</figref>, the combustion air passing through the heat exchanger 14 and the exhaust gas from the second blower 15 are mixed at a proper rate, and the inhibiting gas constituted by the mixed gas is supplied to the inhibiting gas wind box 9. For example, in the case of mixing the combustion air at about 10%, the gas temperature of the inhibiting gas is between about 250 and 350°C, and the oxygen percentage content is between about 5 and 9%.

0052In a seventh embodiment shown in <figref idref="f0005">Fig. 7</figref>, the low-temperature air from the first blower 15 is directly supplied as the inhibiting gas to the inhibiting gas wind box 9 without passing through the heat exchanger 14. The gas temperature of the inhibiting gas is approximately equal to the atmospheric temperature, and the oxygen percentage content is about 20%.

0053Since the mixing between the combustion gas and the air is promoted by increasing the after air injection speed, the unburned combustible amount is lowered as shown in <figref idref="f0005">Fig. 8</figref>. On the other hand, the exhaust amount of the NOx is increased in accordance with the increase of the after air injection speed. This is because the oxidation of the nitrogen content is generally promoted and NOx is generated in the case that the combustion corresponding to the fuel oxidation reaction is promoted. In the prior art, it is impossible to achieve the simultaneous reduction of the unburned combustible and the NOx. However, since the generation of NOx can be inhibited in the mixing region of the high-temperature air and the high-temperature combustion gas, the generation of NOx in the case of increasing the after air injection speed is less in comparison with the prior art as shown in <figref idref="f0005">Fig. 8</figref>.

0054<figref idref="f0006">Fig. 9</figref> is a schematic view of the outline structure of a combustion apparatus in accordance with an eighth embodiment. In the present embodiment, a part of the exhaust gas is supplied to a furnace 41 from a furnace bottom gas supply chamber 74 by a gas recirculation blower 28, and is devoted to the adjustment of the heat transfer amount in heat transfer devices 71, 72 and 73 on the basis of a convective heat transfer, that is, a control of the steam temperature. Further, the other part of the exhaust gas is increased in pressure by an AAP exhaust gas recirculation blower 37 so as to form an NOx generation inhibiting gas, and is injected into the furnace from an AAP 5 through an AAP exhaust gas recirculation flow path 31.

0055In the present embodiment, since the AAP exhaust gas recirculation blower 37 is placed for an exclusive use therefor, it is possible to easily set an appropriate condition for the AAP exhaust gas recirculation regardless of the condition of the exhaust gas recirculation from the furnace bottom portion used for controlling the steam temperature.

0056<figref idref="f0007">Fig. 10</figref> is a schematic view of an enlarged structure near a two-stage combustion air wind box in accordance with the embodiment, and <figref idref="f0008">Fig. 11</figref> is an explanatory view showing the flow state of an air jet flow near the wind box, an AAP exhaust gas jet flow and an unburned gas ascending flow from the burner portion side.

0057In these drawings, reference numeral 1 denotes a water wall, reference numeral 32 denotes an AAP exhaust gas recirculation amount adjusting damper, reference numeral 33 denotes an AAP exhaust gas supply tube, reference numeral 34 denotes an AAP exhaust gas supply ring, reference numeral 35 denotes an AAP exhaust gas supply flow path, reference numeral 36 denotes an AAP exhaust gas jet flow, reference numeral 38 denotes an unburned gas ascending flow, and reference numeral 41 denotes a furnace. Further, reference numeral 101 denotes a two-stage combustion air wind box, reference numeral 102 denotes an AAP primary air flow path, reference numeral 103 denotes an AAP secondary air flow path, reference numeral 104 denotes an AAP secondary air register, reference numeral 105 denotes an AAP primary air, reference numeral 106 denotes an AAP secondary air, and reference numeral 107 denotes an air jet flow. In this case, reference numeral 1000 denotes an opening portion for introducing the AAP primary air 105 to the AAP primary flow path 102 from the two-stage combustion air wind box 101.

0058In the case of the present embodiment, the inhibiting gas injection port 9a of the AAP exhaust gas supply flow path 35 is arranged so as to surround the entire air injection port 5a of the AAP air flow paths 102 and 103. The recirculation gas, the flow rate of which is adjusted to a predetermined flow rate by the AAP exhaust gas recirculation amount adjusting damper 32, is introduced to the AAP exhaust gas supply ring 34 through the AAP exhaust gas supply tube 33,passes through the AAP exhaust gas supply flow path 35 as shown in <figref idref="f0008">Fig. 11</figref>, and is injected as the AAP exhaust gas jet flow 36 to the outer peripheral portion of the air jet flow 107 from the inhibiting gas injection port 9a.

0059As mentioned above, the exhaust gas supply flow path 35 is placed at the outer side in the diametrical direction of the AAP secondary air flow path 103, and the exhaust gas is supplied so as to surround the two-stage combustion air (the air jet flow 107). In accordance with the present structure, as shown in <figref idref="f0008">Fig. 11</figref>, it is possible to supply the exhaust gas to the mixing region in which the unburned combustible in the unburned gas ascending flow 38 from the burner side starts combustion by the two-stage combustion air or (and) the portion near the same.

0060<figref idref="f0009">Figs. 12A and 12B</figref> are schematic views of an enlarged structure near a two-stage combustion air wind box in accordance with a ninth embodiment, in which <figref idref="f0009">Fig. 12A</figref> is a schematic view of the entire structure near the wind box, and <figref idref="f0009">Fig. 12B</figref> is a view showing the arrangement of the inhibiting gas injection port.

0061In the case of the present embodiment, the AAP exhaust gas supply flow path 35 is formed by a plurality of exhaust gas supply nozzles, the exhaust gas supply nozzles are placed in the outer peripheral portion in the AAP secondary air flow path 103, and a plurality of inhibiting gas injection ports 9a of the AAP exhaust gas supply flow path 35 are placed in the peripheral direction as shown in <figref idref="f0009">Fig. 12B</figref>.

0062In accordance with this structure, the exhaust gas is mixed so as to surround the two-stage combustion air. It is possible to supply the exhaust gas to the region in which the unburned component ascending from the burner starts combustion on the basis of the two-stage combustion air, in the same manner as the embodiment in <figref idref="f0008">Fig. 11</figref>. In the present embodiment, it is possible to place the exhaust gas supply nozzle by applying a comparatively simple modification to the existing two-stage combustion air port.

0063<figref idref="f0010">Figs. 13A and 13B</figref> are schematic views of an enlarged structure near a two-stage combustion air wind box in accordance with a tenth embodiment, in which <figref idref="f0010">Fig. 13A</figref> is a schematic view of the entire structure near the wind box, and <figref idref="f0010">Fig. 13B</figref> is a view showing the arrangement of the inhibiting gas injection port. <figref idref="f0011">Fig. 14</figref> is an explanatory view showing the flow state of an air jet flow near the wind box, an AAP exhaust gas jet flow and an unburned gas ascending flow from the burner portion side.

0064In the case of the present embodiment, as shown in <figref idref="f0010">Fig. 13B</figref>, the inhibiting gas injection port 9a of the exhaust gas supply nozzle formed in a semi-ring shape (a circular arc shape) is placed only in the lower side of the outer peripheral portion in the AAP secondary air flow path 103, and the AAP exhaust gas jet flow 36 is injected from here (refer to <figref idref="f0011">Fig. 14</figref>). As shown in <figref idref="f0011">Fig. 14</figref>, since the AAP exhaust gas jet flow 36 can be formed only at the lower side of the AAP air jet flow 107 in which the unburned component ascending from the burner portion side starts combustion on the basis of the two-stage combustion air, it is possible to obtain the same NOx reduction effect by a small amount of recirculation gas.

0065<figref idref="f0012">Figs. 15A and 15B</figref> are schematic views of an enlarged structure near a two-stage combustion air wind box in accordance with an eleventh embodiment, in which <figref idref="f0012">Fig. 15A</figref> is a schematic view of the entire structure near the wind box, and <figref idref="f0012">Fig. 15B</figref> is a view showing the arrangement of the inhibiting gas injection port.

0066In the case of the present embodiment, a plurality of inhibiting gas injection ports 9a of the exhaust gas supply nozzle are placed concentrically only at the lower side of the outer peripheral portion of the AAP secondary air flow path 103. In this case, in the same manner as <figref idref="f0011">Fig. 14</figref>, since it is possible to form the AAP exhaust gas jet flow only at the lower side of the AAP air jet flow in which the unburned component ascending from the burner starts combustion on the basis of the two-stage combustion air, the same NOx reduction effect can be obtained by a small amount of recirculation gas.

0067In the case of the tenth and eleventh embodiments, the AAP exhaust gas supply ring 34 may be actually formed in a semi-ring shape in correspondence to the inhibiting gas injection port 9a in place of a complete ring shape.

0068<figref idref="f0013">Fig. 16</figref> is a schematic view of the outline structure of a combustion apparatus in accordance with a twelfth embodiment. In the case of the present embodiment, since the low-temperature exhaust gas in which the heat is recovered through the heat exchanger such as the air preheater 24 or the like is supplied to the AAP wind box 5, there is obtained the effect of reducing the thermal NOx on the basis of the temperature reduction.

0069<figref idref="f0014">Figs. 17A and 17B</figref> are views showing the distribution of the gas temperature within a furnace in the width direction within the furnace, and generated NOx concentration in the width direction within the furnace. As shown in <figref idref="f0014">Fig. 17A</figref>, the gas temperature within the furnace is lower in the portion close to the side wall of the furnace (the portion close to right and left ends in the direction of the drawing), and is higher in the center portion of the furnace. Accordingly, as shown in <figref idref="f0014">Fig. 17B</figref>, the concentration of generated NOx becomes higher in the center portion of the furnace in which the temperature is high. In the case of the invention that a plurality of AAPs are provided in the width direction of the furnace, it is possible to effectively reduce the NOx by supplying more exhaust gas to the center portion of the furnace than to the portion close to the side wall of the furnace.

0070<figref idref="f0014">Fig. 18</figref> is a view for explaining the opening degree adjustment of a plurality of adjusting dampers 32a to 32h arranged at the downstream side of the AAP exhaust gas recirculation amount adjusting damper 32 placed at the front and rear sides of a can. As shown in the drawing, the adjusting dampers 32a to 32d are individually placed at the downstream side of the AAP exhaust gas recirculation amount adjusting damper 32X in the front side of the can in correspondence to the respective AAP (not shown) arranged in the width direction of the furnace. The adjusting dampers 32e to 32h are placed at the downstream side of the AAP exhaust gas recirculation amount adjusting damper 32Y at the rear side of the can in the same manner.

0071As is apparent from the result of <figref idref="f0014">Figs. 17A and 17B</figref> mentioned above, the gas temperature within the furnace is lower in the portion close to the side wall of the furnace and is higher in the center portion. Accordingly, the concentration of generated NOx becomes higher in the center portion of the furnace in which the temperature is higher. A large amount of exhaust gas is supplied to the center portion of the furnace having a large amount of generated NOx, by setting the damper opening degree of the adjusting dampers 32a, 32d, 32e and 32h placed in the portion close to the side wall in both the front and rear sides of the can small and setting the damper opening degree of the adjusting dampers 32b, 32c, 32f and 32g placed in the center portion side within the furnace large in correspondence to the condition of the gas temperature within the furnace as mentioned above.

0072The higher the boiler load is, the higher the gas temperature within the furnace becomes. As a result, the higher the boiler load is, the higher is thermal NOx. <figref idref="f0015">Fig. 19</figref> is a view for explaining an example for setting the flow rate of the AAP recirculation gas, in which the horizontal axis shows the boiler load and the vertical axis shows the AAP exhaust gas recirculation ratio. In this case, the AAP exhaust gas recirculation ratio is a numeric value determined by the following expression. <maths id="math0001" num=""><math display="block"><mi>AAP exhaust gas recirculation ratio</mi><mo>=</mo><mfenced><mi>AAP exhaust gas recirculation flow rate</mi></mfenced><mo>/</mo><mfenced><mi>combustion gas flow rate</mi></mfenced><mo>×</mo><mn>100</mn><mspace width="1ex" /><mfenced><mi>%</mi></mfenced><mo>.</mo></math><img file="EP1580485B1_D0001.tif" /></maths>

0073In the present example, the AAP exhaust gas is supplied between the boiler load of 75 and 100% largely affected by the thermal NOx, the AAP exhaust gas recirculation ratio in the boiler load of 100% is set to about 3%, and the exhaust gas supply is stopped in the low load region (less than 75% in the present example). The reduction of combustion efficiency is inhibited by stopping the exhaust gas supply in the low load region having no problem in NOx.

0074In the case that the inhibiting gas such as the exhaust gas or the like is supplied to a plurality of air ports, it is preferable that the total supply flow rate of the inhibiting gas is variable as mentioned above in correspondence to the load of the combustion apparatus, and the total supply flow rate of the inhibiting gas is variable in correspondence to the nitrogen oxide discharge concentration of the combustion apparatus.

0075In view of the fuel, there is a case that there is no NOx problem without the AAP exhaust gas supply. In such a case, an operation giving priority to a high efficiency is desirable, without supplying the AAP exhaust gas. In other words, an optimum operation can be achieved by making the total recirculation gas supply amount variable in correspondence to the NOx discharge concentration.

0076Since the nitrogen oxide generation inhibiting gas is supplied only to the local high-temperature portion in the mixing region of the air port air and the high-temperature combustion gas controlling the thermal NOx, it is possible to effectively reduce concentration of generated NOx while inhibiting the entire temperature reduction within the furnace so as to maintain the combustion efficiency. An example of reducing the NOx is shown by the dotted line in <figref idref="f0020">Fig. 25</figref>. As is apparent from this result, it is possible to inhibit the NOx generation at the downstream side of the AAP changing to the oxidizing region, and it is finally possible to widely reduce the NOx in the outlet of the furnace.

0077In accordance with the means described in the second aspect, since the combustion air flow path and the inhibiting gas flow path are provided within the air port so as to be separated, it is possible to avoid an enlargement of the structure.

0078It is possible to employ various gases as the inhibiting gas.

0079In accordance with the means described in the third, fourth and fifth aspects, it is possible to cover the entire of the outer peripheral portion of the air port air flow by the inhibiting gas flow, and the NOx reducing effect is great.

0080In accordance with the means described in the sixth, seventh and eighth aspects, it is possible to obtain an improved NOx reducing effect by a small amount of inhibiting gas.

0081In accordance with the means described in the ninth aspect, it is possible to effectively utilize the exhaust gas as the inhibiting gas, and it is unnecessary to peculiarly prepare the inhibiting gas.

0082In accordance with the means described in the tenth aspect, it is possible to easily set the optimum condition for the NOx generation inhibiting gas regardless of the condition of the exhaust gas recirculation used for controlling the steam temperature of the heat transfer device.

Industrial Applicability

0083The combustion apparatus in accordance with the present invention can inhibit the NOx generation even if the mixing between the high-temperature combustion gas and the air is promoted so as to reduce the unburned combustible.