JP5225469B2 - Once-through boiler - Google Patents

Description

  The present invention has a combustion chamber provided with a plurality of burners for fossil fuels, a vertical flue is post-connected via a horizontal flue in the upper part of the hot gas flow path, and the surrounding walls of the combustion chamber are mutually connected in the lower part. A plurality of evaporator tubes welded hermetically and pre-connected to the air / water separator in the flow medium flow path, and hermetically welded to each other in the upper part and connected to the air / water separator in the flow medium flow path The present invention relates to a once-through boiler formed of a superheated tube.

  In a fossil fuel boiler, the energy of the fossil fuel is used to generate superheated steam, which is then led to a steam turbine for power generation, for example at a power plant. At normal steam temperatures and pressures, especially in power plant environments, the boiler is usually designed as a water tube boiler, i.e., the supplied water flows through a number of tubes, which transfer energy to the burner flame. Absorbs in the form of radiant heat and / or by convection of combustion gases produced during combustion.

  In the region of the burner, the steam generation pipe generally forms a combustion chamber wall by hermetically welding a plurality of steam generation pipes to each other. A plurality of steam generation pipes can be arranged in the exhaust gas passage in other regions connected downstream of the combustion chamber in the combustion gas passage.

Fossil fuel boilers can be classified according to many criteria. Boilers can generally be designed as natural circulation, forced circulation or once-through boilers. In the case of a once-through boiler, heating of the plurality of evaporator tubes causes the flow medium to completely evaporate with a single once-through in these evaporator tubes. The flow medium (usually water) is led to a superheater tube connected downstream of the evaporation tube after its evaporation, and is heated there. This explanation is only true when the evaporator is partially loaded with subcritical pressure of water (P _Kri ? 221 bar) (at low temperatures, water and steam are present simultaneously and phase separation is not possible). However, for the sake of clarity, this description is used consistently in the following description. In that case, the position of the evaporation end point, i.e. where the components in the flow are completely evaporated, varies and depends on the operating conditions. During full-load operation of such once-through boilers, the evaporation end point is located, for example, at the end of the evaporation tube, so that the superheating of the evaporated flow medium already begins in the evaporation tube.

  A once-through boiler is not subject to pressure limitations, unlike a natural circulation boiler or a forced circulation boiler, and can therefore be designed to be significantly higher than the critical pressure of water for the live steam pressure.

  Such once-through boilers are usually operated during low load operation or at start-up with a minimum flow medium flow rate in the evaporator tube to ensure reliable cooling of the evaporator tube. For this reason, pure flow-through mass flow through the evaporator, especially at low loads, for example, less than 40% of the design load, is usually inadequate for cooling of the evaporator tube, and thus for the flow medium through-flow through the evaporator. Additional flow medium throughflow is superimposed. Therefore, during start-up or during low-load operation, the minimum flow medium flow rate in the evaporator tube that is scheduled for operation is not completely evaporated in the evaporator tube, so that during such operating conditions, there is still no evaporation at the end of the evaporator tube. There is a flowing medium, especially water, steam and mixtures.

  A once-through boiler is usually used during start-up and during low-load operation, since a superheater tube connected downstream of the once-through boiler wall after the combustion chamber wall is generally not designed for the flow of un-evaporated flow medium. It is also designed to ensure that water does not flow into the superheated tube. For this purpose, the evaporation pipe is usually connected to a superheater pipe connected downstream of it through a steam / water separator. The steam separator functions to separate water / steam / mixture from the evaporator tube into water and steam at start-up or during low-load operation. The steam is led to a superheater pipe connected downstream of the steam separator, whereas the separated water is led again to the evaporator pipe, for example via a circulation pump, or discharged via an expander. can do.

  Boilers can be divided into vertical and horizontal structures based on the direction of gas flow. In the case of vertically structured fossil fuel boilers, a distinction is generally made between single and double flue boilers.

  In a single flue boiler or tower boiler, the combustion gas generated by combustion in the combustion chamber always flows vertically from bottom to top. All heat transfer surfaces arranged in the combustion gas passage are located above the combustion chamber in the combustion gas passage. Tower boilers offer a relatively simple structure and simple control of the stresses caused by the thermal expansion of the tubes. Also, all the heat transfer surfaces of the boiler located in the combustion gas path are horizontal and can therefore be drained completely, which is desirable in environments where there is a risk of freezing.

  In the case of a two-fluid boiler, a horizontal flue is connected downstream of the combustion chamber in the upper part of the combustion chamber, and this horizontal flue opens to a vertical flue. In this second vertical flue, the gas usually flows vertically from top to bottom. That is, the direction of the combustion gas is changed several times in the double flue boiler. The advantage of this structure is, for example, the low structure height and the resulting low manufacturing costs.

  In the case of a boiler designed as a double flue boiler, the wall of the first flue, i.e. the wall of the combustion chamber, is usually completely piped as an evaporator. A steam separator connected downstream of the evaporation pipe in the flow medium flow path is accordingly arranged at the upper end of the combustion chamber.

  However, in that case, different mass flow rates and temperatures of the flow medium occur in the plurality of parallel pipes based on the differences in the shapes of the individual pipes as well as in their heating.

  On the one hand, the evaporator heat transfer surface must be sufficiently cooled over the entire load range of the boiler. The mass flow required for cooling must be reliably supplied to each individual pipe. Also, the stress between adjacent tubes caused by the thermal expansion of individual tubes must not exceed the allowable value. The temperature of the flow medium must be limited in absolute values as well as in the differences between adjacent tubes. Otherwise, the combustion chamber wall will be damaged.

  In order to reduce the temperature gradient in the evaporator tube, for example, a plurality of mixing devices can be installed on the combustion chamber wall connected by piping as an evaporator. There, the flow medium is discharged from the evaporator tube and mixed and then distributed again to the other evaporator tubes. Such systems must be designed to provide a uniform distribution of water, steam and mixture downstream of these mixing devices. Such a structure is accordingly technically expensive and results in a considerable increase in manufacturing costs.

  The object of the present invention is to provide a once-through boiler of the type described above which has a relatively simple structure and a particularly long life.

  This problem is solved according to the invention by the fact that the boundary between the range of the evaporator tube and the superheater tube is arranged almost horizontally around the combustion chamber in the bottom part of the horizontal flue.

  The invention starts from the idea that a relatively long lifetime is obtained under a simple structure when a relatively small temperature gradient is achieved without placing additional mixing devices in the evaporator tube. .

  The steam-water separator existing in the boiler also collects water from the evaporator pipe during the circulation operation and separates it from the steam. In the flow-through operation, the incoming steam is mixed and distributed to the superheater pipes connected downstream in the flow medium flow path. In that case, the temperature gradient is considerably reduced. Therefore, starting from the recognition that the steam separator basically fulfills the function of the mixer, the steam separator is additionally mixed, for example by installing it in the lower part of the horizontal flue. It can be used as a mixing device inside the combustion chamber wall without the need for a device.

  In addition, its position of the steam separator achieves that the boundary between the range of the evaporator tube and the superheater tube is arranged almost horizontally around the combustion chamber at the bottom part of the horizontal flue.

  In an advantageous embodiment, the boundary between the range of the evaporator tube and the superheater tube is arranged around the combustion chamber approximately horizontally at the edge height formed by the surrounding wall and the bottom of the horizontal flue. With this arrangement, all the tubes of the combustion chamber welded to the tubes of the horizontal flue wall are likewise designed as superheated tubes. In the case of a conventional structure having a combustion chamber formed entirely of a plurality of evaporator tubes, a plurality of evaporator tubes and a plurality of superheated tubes are welded in parallel instead. This is a problem, in particular, because during the hot start-up of the boiler, a considerably large temperature difference is produced with respect to the unfilled superheated tube due to filling of the evaporator tube with a cold flow medium. Due to the arrangement of the air / water separator at the edge height formed by the combustion chamber wall and the bottom of the horizontal flue, such vertical separation does not occur, and overall reliable operation of the boiler is achieved in a relatively long life. it can.

  In the case of a dual flue boiler, to improve gas flow, the vertical flue side portion of the enclosure can be tilted inward on the underside of the horizontal flue so that the bottom of the adjacent horizontal flue At the same time, a nose protruding into the combustion chamber is formed. In such a boiler, the boundary between the range of the evaporator tube and the superheater tube is preferably arranged almost horizontally around the combustion chamber just above the nose.

  In another advantageous embodiment, the bottoms of the horizontal flues are formed by evaporation tubes which are hermetically welded together and pre-connected to the steam separator in the flow medium flow path. That is, the bottom of the horizontal flue is suitable for designing as an auxiliary evaporator heat transfer surface. This is because they are not welded in parallel to the wall of a horizontal flue that is laid vertically and piped as a superheated tube, and therefore the stress due to different thermal expansion is relatively small. .

  The advantage obtained by the present invention is in particular connected in parallel by the boundary between the range of the evaporator tube and the superheater tube being arranged almost horizontally around the combustion chamber in the region of the bottom of the horizontal flue. In order to reduce the temperature difference between the tubes, the steam-water separator can be used as a mixing device. One of the main drawbacks of the two-fluid boiler is that the vertical separation between the wall heat transfer surface connected by piping as an evaporator and the wall heat transfer surface formed as a superheater is eliminated. As a result, when the boiler is heated at a high temperature, a high temperature difference and stress are generated particularly when the evaporator tube is filled with a relatively low-temperature flow medium at the separation point. Life is obtained.

  Due to the lower position of the steam separator, and hence the lower position of the boundary between the evaporator and superheater tubes in the combustion chamber, there is less superheat in the steam separator and overall boiler. This makes it possible to start up the material more easily, which also increases the life of the boiler and allows the use of lower cost materials for its manufacture.

  Embodiments of the present invention will be described in detail with reference to the drawings.

The schematic block diagram of a two flue type once-through boiler.

  The once-through boiler 1 in the figure has a combustion chamber 2 formed as a vertical flue, and a horizontal flue 6 is connected downstream of the combustion chamber 2 in the upper part 4. This horizontal flue 6 is followed by another vertical flue 8.

  A plurality of burners (not shown in detail) are provided in the lower part 10 of the combustion chamber 2, and these burners burn liquid fuel or solid fuel in the combustion chamber. The surrounding wall 12 of the combustion chamber 2 is formed of a plurality of steam generation pipes hermetically welded to each other, and a flow medium (usually water) is pumped to these steam generation pipes by a pump not shown in detail. The medium is heated by the heat generated by the burner. The steam generation tubes are aligned spirally or vertically in the lower part 10 of the combustion chamber 2. In the case of the spiral arrangement structure, a relatively high structure cost is required, but instead, the heating difference generated between the pipes connected in parallel is smaller than that in the case of the combustion chamber 2 of the vertical pipe laying structure.

  The plurality of steam generation tubes in the lower part 10 of the combustion chamber 2 are designed as evaporation tubes. The flow medium is first evaporated in them and is led via pipe 14 to an air / water separator not shown in detail. Water that has not yet been evaporated is collected and discharged into the steam separator. The generated steam is guided into the wall of the combustion chamber 2 and distributed to superheater tubes arranged on the top 4 and the wall of the horizontal flue 6. Such separation of water that has not yet been evaporated is necessary, especially during start-up, when more flow medium has to be pumped than is evaporated in the through-flow of the evaporator to ensure reliable cooling of the evaporator. .

  The illustrated once-through boiler 1 also has a nose 16 that transitions directly into the bottom 18 of the horizontal flue 6 and protrudes into the combustion chamber 2 to improve combustion gas guidance. Further, a grid 20 made of another superheater tube is arranged in the transition range from the combustion chamber 2 to the horizontal flue 6 in the combustion gas path.

  In particular, in the case of the combustion chamber 2 having a vertical pipe laying structure, a temperature difference may occur between the evaporation pipes connected in parallel, and this temperature difference may endanger the operation of the boiler due to different thermal expansion. In order to mix the flow media from the various tubes and thereby achieve temperature equilibrium without using additional components, the boundary 22 between the evaporator tube and the superheated tube is connected to the horizontal flue 6. Located just above the nose 16 at the height of the bottom 18. As a result, the steam-water separator not only functions as a separator during start-up operation, but also functions as a mixing device during a once-through operation. This is because in this steam separator, all the flow medium from the evaporator tube is collected, mixed and redistributed to the superheated tube.

  Since the upper part 4 of the combustion chamber 2 and the wall of the horizontal flue 6 are now connected as superheater pipes, there is no vertical separation between the evaporation pipes and superheater pipes welded in parallel in the lattice 20 part. . The lower part 10 of the combustion chamber 2 and the bottom 18 of the horizontal flue are simply connected by pipes as evaporation pipes, whereby only a plurality of superheated pipes are welded in parallel with each other in that part.

  1: once-through boiler, 2: combustion chamber, 6: horizontal flue, 8: vertical flue, 12: enclosure, 16: nose, 22: boundary.

Claims (3)

Combustion chamber (2) provided with a plurality of fossil fuel burners, and vertical flue (8) is connected downstream from the upper part (4) of the hot gas flow path via horizontal flue (6) for combustion The enclosure (12) of the chamber (2) is hermetically welded to each other in the lower part (10) and is formed by a plurality of evaporation pipes connected in front of the air / water separator in the flow medium flow path. Are formed by a plurality of superheater pipes connected to the air / water separator in the flow medium flow path, and the portion of the surrounding wall (12) on the horizontal flue (6) side is below the horizontal flue (6). Inclined inwardly on the side, thereby forming a nose (16) protruding into the combustion chamber with the bottom (18) of the adjacent horizontal flue (6),
Boundary between the extent of the evaporation tube and the superheating tube (22), once-through boiler in which the combustion chamber in the region of the bottom (18) of the horizontal flue (6) (2) are arranged around the horizontal. The boundary (22) between the range of the evaporator tube and the superheater pipes the combustion chamber (2) to the height of the edge formed by the enclosure (12) and the bottom (18) of the horizontal flue (6). The once-through boiler according to claim 1, which is arranged in a flat manner. The once-through boiler according to claim 1 or 2 , wherein the bottom (18) of the horizontal flue (6) is formed of a plurality of evaporation pipes hermetically welded to each other and connected in advance to a steam separator.

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