COMBUSTION SYSTEM

Abstract: A combustion system provided with a mixture supply tube through which a mixture of pulverized coal and combustion air flows. The mixture is injected into a furnace through the mixture supply tube, and ignited. A flame holding ring which is flared radially outwardly is provided at the forward end portion of the mixture supply tube. The flame holding ring is held under a reducing atmosphere and exposed to high temperature caused by radiant heat from the furnace. With this arrangement, there are possibilities of burning of the flame holding ring and growth of slag on the flame holding ring. To prevent these problems, a projecting body is extended beyond the flame holding ring in the furnace, radiation of heat to the flame holding ring from the interior of the furnace is suitably shut off and excessive temperature rise is prevented. Combustion air is caused to flow on the surface of the projecting body to thereby keep the projecting body under an oxidizing atmosphere. A pulverized coal/air separating member is extended through the mixture supply tube. Portions where the flows are forcedly delaminated are locally provided on a conical portion at the forward end of the pulverized coal/air separating member. With this arrangement, stabilized combustion in the combustion system as a whole can be attained regardless of the unit capacity or the load of the combustion system.

Description: This invention relates to a combustion apparatus, as disclosed in the first part of claim 1. A prior art apparatus of this kind is known from EP-A-0 280 568. In a pulverized coal firing boiler, a combustion apparatus injects a mixture of pulverized coal and air into a furnace through a mixture feeding pipe. The mixture injected is ignited so as to form a flame in the furnace. As disclosed in USP 4,545,307, a radially outwardly flared flame maintaining ring is provided at an end of the mixture feeding pipe. Vortices of the mixture are formed along the flame maintaining ring so that the pulverized coal is concentrated in the vicinity of the flame maintaining ring. As a result, an ignition takes place from the end portion of the mixture feeding pipe located in the furnace to form a high temperature strong reduction flame, thereby making it possible to suppress the generation of nitrogen oxides NOx. The flame maintaining ring get covered with ashes and is kept under a reduction atmosphere and, further, exposed to high temperatures due to radiant heat from the furnace. These conditions may cause a burnout of the flame maintaining ring or, when the operation is not proper, growth of slag on the flame maintaining ring, that is, promotion of the slagging, under certain circumstances. The burnout of the flame maintaining ring or the growth of the slag results in the deterioration of the effect of the flame maintaining ring, increase of the amount of nitrogen oxides NOx, or the trouble of the apparatus. Accordingly, an object of the present invention is to provide a combustion apparatus capable of effecting a low nitrogen oxide NOx combustion in a stabilised manner regardless of the unit capacity or the operating load of the combustion apparatus. To this end, in the present invention, a radiation from the flame is shut off and one of three factors of occurrence of the slagging (namely, high temperature, reduction and existence of ash) is eliminated. According to the present invention, there is provided a combustion apparatus comprising: a mixture feeding pipe exposed into a furnace for feeding a mixture of powdery fuel and oxygen-containing gas into the furnace; flame maintaining means provided at an exposed peripheral edge portion of the mixture feed pipe; and a gas feeding passage disposed radially outwardly of the mixture feeding pipe; characterised by further comprising a powdery fuel/oxygen-containing gas separating member coaxially disposed inside of the mixture feeding pipe, the separating member including a first portion which cooperates with the mixture feeding pipe to define therebetween a first mixture feeding passage portion, the sectional area of which is constant, and a second portion extending from the first portion toward the downstream side of the flow of the mixture, which second portion cooperates with the mixture feeding pipe to define therebetween a second mixture feeding passage portion,the sectional area of which is increased gradually along the flow of the mixture. Preferably, a projection is also provided to extend into a furnace beyond a flame maintaining means so as to shut off radiation from the inside of the furnace to the flame maintaining means adequately and suppress an excessive temperature rise, thereby restraining the burnout of the flame maintaining means and the occurrence of the slagging on the flame maintaining means. The apparatus in accordance with the present invention and its method of operation will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a sectional view of a combustion apparatus, not within the scope of the present invention; Figure 2 is a front view taken along the lines II-II in Figure 1; Figure 3 is a partly fragmentary sectional view illustrating a projection shown in Figure 1; Figure 4 is a partly fragmentary front view of the projection of Figure 3; Figure 5 is an enlarged fragmentary front view of the projection of Figure 4; Figure 6 is a sectional view taken along the lines VI-VI in Figure 5; Figure 7 is a partly fragmentary front view illustrating a modification of the projection; Figure 8 is a fragmentary sectional view taken along the lines VIII-VIII in Figure 7; Figure 9 is a fragmentary sectional view illustrating another modification of the projection; Figure 10 is a sectional view of combustion apparatus according to an embodiment to the present invention; Figure 11 is a side view illustrating a conical portion of a pulverized coal/air separating member shown in Figure 10; Figure 12 is a front view taken along the lines XII-XII in Figure 11; Figure 13 is a side view illustrating the conical portion of another pulverized coal/air separating member; Figure 14 is a front view taken along the lines XIV-XIV in Figure 13; Figure 15 is a side view illustrating the conical portion of still another pulverized coal/air separating member; Figure 16 is a front view taken along the lines XVI-XVI in Figure 15; Figure 17 to 19 are sectional views illustrating other modifications of the conical portion of the pulverized coal/air separating member, respectively; and Figure 20 is a sectional view of a different combustion apparatus. Figures 1 to 9 show a combustion apparatus which is not within the scope of appendant claim 1, but which is useful for understanding the embodiments of the present invention shown in Figures 10 to 20. Referring to Figure 1, a combustion apparatus has a bent mixture feeding pipe 1. The combustion apparatus serves to burn pulverized coal as powdery fuel in air as oxygen-containing gas. The mixture feeding pipe 1 faces at one end thereof into a furnace 2 through an opening 22 formed in a furnace wall 21 of the furnace 2 and communicates at the other end thereof with a coal mill (not shown). A mixture of the pulverized coal and the primary air flows through the mixture feeding pipe 1. The mixture is ignited to form a flame in the furnace 2. A flame maintaining ring 3 having an L-letter form cross-section is provided at the peripheral end portion of the mixture feeding pipe 1. As shown in detail in Figure 2, an annular flow passage 4 is so disposed radially outward of the mixture feeding pipe 1 to be concentrical therewith. Tertiary air is fed into the furnace 2 through the flow passage 4. An annular projection 6 is disposed between the mixture feeding pipe 1 and the flow passage 4. The projection 6 extends into the furnace 2 beyond the flame maintaining ring 3. An outer peripheral wall 61 of the projection 6 extends in parallel with the mixture feeding pipe 1 and an inner peripheral wall 62 thereof expands radially outwardly at its end portion. Both peripheral walls 61 and 62 are terminated with an end disk 63. Referring to Figures 1 and 3, an interior of the projection 6 is divided into two layers by a partition tube 64. Secondary air flows in a zigzag manner through a passage portion defined by the outer peripheral wall 61 of the projection 6 and the partition tube 64, a passage portion defined by the inner peripheral wall 62 of the projection 6 and the partition tube 64 and a passage portion defined by the inner peripheral wall 62 of the projection 6 and the mixture feeding pipe 1, as indicated by arrows, and then flows into the furnace 2. Since the inner peripheral wall 62 of the projection 6 expands radially outwardly at the end portion thereof, the secondary air is reduced at a speed thereof, so that a part of the secondary air can be consumed for maintaining the flame without disturbing the jet of the mixture. This makes it possible to form a high temperature reduction flame in a stabilized manner. In consequence, it is possible to suppress the production of nitrogen oxides NOx. The flame maintaining ring 3 is under a reduction atmosphere, and the pulverized coal is concentrated in the vicinity of the flame maintaining ring due to vortices. Further, the flame maintaining ring 3 is usually exposed to high temperatures attributable to the radiant heat from the furnace as indicated by broken lines in Figures 1 and 3. However, since the projection 6 extends beyond the flame maintaining ring 3 into the furnace 2 to shut off radiation toward the flame maintaining ring moderately, the flame maintaining ring 3 can be prevented from being an excessively high temperature. In consequence, even when the unit capacity of the combustion apparatus is increased (e.g. above 50 MW thermal), the flame maintaining ring 3 can be prevented from being burnt out or suffered from the production of slag. On the other hand, the projection 6 is now brought into the state where it gets covered with ashes and is disposed in the reduction atmosphere and, further, exposed to high temperatures due to the radiant heat from the furnace 2. For this reason, there is a possibility that the projection 6 is suffered from the slagging. To cope with this, in the present invention, the projection 6 is not disposed in the reduction atmosphere but an oxidation atmosphere. By so doing, one of factors of occurrence of the slagging can be eliminated, thereby making it possible to prevent the occurrence of the slagging. To form the oxidation atmosphere, an end disk 63 is provided with a plurality of radial slits 631 which are equiangularly spaced, as shown in Figures 4 to 6. A part of the secondary air is jetted out of these slits 631 and guided by guide plates 632, so that it flows circumferentially on the surface of the projection 6. In consequence, the projection 6 can be kept under the oxidation atmosphere, resulting in the prevention of the production of slag. It is noted in this embodiment that the secondary air cools the projection 6 while it flows through the passage portion defined by the outer peripheral wall 61 of the projection 6 and the partition tube 64, the passage portion defined by the inner peripheral wall 62 of the projection 6 and the partition tube 64 and the passage portion defined by the inner peripheral wall 62 of the projection 6 and the mixture feeding pipe 1. The flow of the secondary air of about 300°C makes the projection be 950°C or below, at which temperature any slag is hardly produced. In consequence, it becomes possible to make it harder for the slagging to occur in the projection 6 as well as to make the lifetime of the projection longer. On the other hand, since the temperature of the secondary air is increased by about 40°C due to the radiant heat from the furnace 2, the combustion efficiency can be improved. In a modification shown in Figures 7 and 8, a plurality of circumferential slits 633 are provided equiangularly in the end disk 63, so that a part of the secondary air is guided by a guide plate 634 to flow radially outwardly on the surface of the projection 6. As a result, production of slag can be prevented like the above embodiment. In another modification shown in Figure 9, the end disk 63 is partially cut off and inclined. Figure 10 shows a combustion apparatus which is an embodiment of the present invention and in which, in order to make the concentration of the mixture around the mixture feeding pipe 1 higher, a pulverised coal/air separating rod member 7 is disposed inside of the mixture feeding pipe 1 coaxially. The separating member 7 is attached to the mixture feeding pipe 1 at a stem portion 71 thereof. The separating member 7 also has a flare portion 72 which defines a throat portion in cooperation with a projective member 11 provided in the mixture feeding pipe 1. At the throat portion, the mixture is reduced at a speed thereof. Further, the separating member 7 comprises a right circular cylindrical portion 73 and a conical portion 74 which extends from the right circular cylindrical portion so as to be tapered toward the downstream side of the flow of the mixture. The right circular cylindrical portion 73 cooperates with the mixture feeding pipe 1 to define therebetween a mixture passage portion I the sectional area of t which is held constant. The conical portion 74 cooperates with the mixture feeding pipe 1 to define therebetween a mixture passage portion II the sectional area of which is increased gradually along the flow of the mixture. The mixture is increased at a speed thereof in the passage portion I. When the mixture flows through the passageway portion II, the pulverized coal is separated from the mixture due to its inertia and then flows radially outwardly. As a result, the pulverized coal is concentrated in the vicinity of the flame maintaining ring. Therefore, even if the load of the combustion apparatus is reduced (down to about 30% of the load of the mill, for example), it is possible to effect a highly efficient combustion with the less amount of nitrogen oxides NOx produced. However, if the conical portion 74 is tapered uniformly, there is a possibility that the mixture may separate from the conical portion. Once the separation occurs, the pulverized coal once concentrated in the vicinity of the flame maintaining ring is brought back radial inwardly due to separated flow, resulting in the possibility that the concentration of pulverized coal in the vicinity of the flame maintaining ring is lowered. Further, it is impossible to specify the location where such separation is caused. For this reason, it is designed in this embodiment that the separation of the flow occurs exactly or forcibly at the predetermined portions on the conical portion. In addition, these portions where the separation is occurred are circumferentially located. In other words, the portions where the separation is prevented from occurring are circumferentially equiangularly located as well. In consequence, the concentration of the pulverized coal in the vicinity of the flame maintaining ring becomes circumferentially uniform, and therefore, it is possible to effect a stabilized combustion. To this end, in the present embodiment, the conical portion 74 consists of portions 741 each making a tapering angle  1 with respect to the axial direction and portions 742 each making a tapering angle  2 (>  1 ) with respect to the axial direction, which portions 741 alternate with the portions 742, as shown in Figures 11 to 14. The tapering angle  1 is in the range of 5° to 15°, and the tapering angle  2 is in the range of 25° to 65°. The separation occurs in the portions 742 but it does not occur in the portions 741. Further, the area occupied by the portions 741 is made larger than that occupied by the portions 742. In consequence, the effect of the separation can be minimized, thereby enhancing a stabilized combustion. The portions 741 and 742 may be connected smoothly (as shown in Figure 12) or steeply (as shown in Figure 14). The tapering angle  2 of the portion where the separation is occurred is not limited to be in the range of 25° to 65°. Even when the tapering angle 2 is 90°, that is, even when the portion 742 is a slit as shown in Figures 15 and 16, the same effect can be obtained. Further, as shown in Figures 17 to 19, the portions 741 and 742 may be arranged asymmetrically. Incidentally, although the projection and the pulverized coal/air separating member coexist in this embodiment, these can be provided separately. In addition, the present invention is also applicable to a pulverized coal combustion apparatus shown in Figure 20 which is equipped with a start-up oil burner 8 and an auxiliary gas burner 9. The oil burner 8 extends through within the separating member 7 to the tip end of the conical portion 74. The gas burner 9 extends through the inner peripheral wall 62 into the furnace 2 to the extent that it can be prevented from being exposed to the radiation from the inside of the furnace 2. The present invention can be used in the combustion apparatus of a pulverized coal boiler, for example.

Claims: A combustion apparatus comprising: a mixture feeding pipe (1) exposed into a furnace (2) for feeding a mixture of powdery fuel and oxygen-containing gas into said furnace (2); flame maintaining means (3) provided at an exposed peripheral edge portion of said mixture feed pipe (1); and a gas feeding passage (4,5) disposed radially outwardly of said mixture feeding pipe (1) for feeding an oxygen-containing gas into said furnace (2); characterised by further comprising a powdery fuel/oxygen-containing gas separating member (7) coaxially disposed inside of said mixture feeding pipe (1), said separating member (7) including a first portion (73) which cooperates with said mixture feeding pipe (1) to define therebetween a first mixture feeding passage portion (I), the sectional area of which is constant, and a second portion (74) extending from said first portion (73) toward the downstream side of the flow of the mixture, which second portion (74) cooperates with said mixture feeding pipe (1) to define therebetween a second mixture feeding passage portion (II), the sectional area of which is increased gradually along the flow of the mixture. A combustion apparatus according to claim 1, wherein said first portion is circular cylindrical and said second portion is conical and tapers toward the downstream side of the flow of the mixture. A combustion apparatus according to claim 1 or 2, wherein said second portion has a portion where separation of the flow is caused to occur and another portion where separation of the flow is not caused to occur, which portions alternate circumferentially. A combustion apparatus according to claim 3, wherein the circumferential dimension of said portion where separation of the flow is caused to occur is smaller than that of said another portion where separation of the flow is not caused to occur. A combustion apparatus according to Claim 3 or 4 when dependent on claim 2, wherein said conical portion of said powdery fuel/oxygen-containing gas separating member is provided at a peripheral surface thereof with portions the tapering angles of which are different from each other with respect to the axis of said conical portion. A combustion apparatus according to any one of claims 1 to 6, further comprising: projection means (6) disposed radially between said gas feeding passage (4) and said mixture feeding pipe (1) with and extending into said furnace (2) beyond said flame maintaining means (3) so as to shield said flame maintaining means (3) from radiation from the inside of said furnace. A combustion apparatus according to claim 6, wherein a second gas feeding passage (5) in disposed radially between said first mentioned gas feeding passage (4) and said mixture feeding pipe (1) for feeding an oxygen-containing gas into said furnace (2). A combustion apparatus according to claim 6 or 7, further comprising: means (631,632; 633,634) for forming an oxidising atmosphere around a surface (63) of said projection (b) which is exposed into said furnace. A combustion apparatus according to claim 8 when dependent on claim 7, wherein said oxidising atmosphere is formed by the oxygen-containing gas flowing through said second gas feeding passage (5).

Inventors: BABA AKIRA, HODOZUKA KUNIO, ISHII KEIJI, JIMBO TADASHI, KOBAYASHI HIRONOBU, KURAMASHI KOUJI, MORITA SHIGEKI, NAKASHITA SHIGETO, BABA, AKIRA, HODOZUKA, KUNIO, ISHII, KEIJI, JIMBO, TADASHI, KOBAYASHI, HIRONOBU, KURAMASHI, KOUJI, MORITA, SHIGEKI, NAKASHITA, SHIGETO
Applicants: BABCOCK HITACHI KK, BABCOCK-HITACHI KABUSHIKI KAISHA
Application year: 1991
Main classification: F23D1
IPC codes: F23D17, F23D1, F23C99