PCT/EP2OU9/U61239 / 2UU/Pibb/6WOAU Description Continuous steam generator The invention relates to a once-though ("continuous") steam generator with a combustion chamber having a number of burners for fossil fuel, downstream of which a vertical gas duct is mounted in an upper region on the hot gas side above a horizontal gas duct, the surrounding wall of the combustion chamber being formed, in a lower region, from evaporator tubes welded together in a gas-tight manner and mounted upstream of a moisture separation system on the flow medium side and, in an upper region, from superheater tubes welded together in a gas-tight manner and mounted downstream of the moisture separation system on the flow medium side. In a fossil fired steam generator, the energy of a fossil fuel is used to produce superheated steam which in a power plant, for example, can then be supplied to a steam turbine for power generation. Particularly at the steam temperatures and pressures prevalent in a power plant environment, steam generators are normally implemented as water tube boilers, i.e. the water supplied flows in a number of tubes which absorb energy in the form of radiant heat of the burner flames and/or by convection from the flue gas produced during combustion. In the region of the burners, the steam generator tubes here usually constitute the combustion chamber wall by being welded together in a gas-tight manner. In other areas downstream of the combustion chamber on the flue gas side, steam generator tubes disposed in the waste gas duct can also be provided.
h(T/EZUUW/UbiZ53 / ZUU/Vi3b/bWUAU 2 Fossil fired steam generators can be categorized on the basis of a large number of criteria: steam generators may in general be designed as natural circulation, forced circulation or once-through steam generators. In a once-through steam generator, the heating of a number of evaporator tubes results in complete evaporation of the flow medium in the evaporator tubes in one pass. Once evaporated, the flow medium - usually water - is fed to superheater tubes downstream of the evaporator tubes where it is superheated. Strictly speaking, this description is valid only at partial loads with subcritical pressure of water (PKri 221 bar) in the evaporator - at which there is no temperature at which water and steam can be present simultaneously and therefore also no phase separation is possible. However, for the sake of clarity, this representation will be used consistently in the following description. The position of the evaporation end point, i.e. the location at which the water content of the flow is completely evaporated, is variable and dependent on the operating mode. During full load operation of a once-through steam generator of this kind, the evaporation end point is, for example, in an end region of the evaporator tubes, so that the superheating of the evaporated flow medium begins even in the evaporator tubes. In contrast to a natural or forced circulation steam generator, a once-through steam generator is not subject to pressure limiting, so that it can be designed for main steam pressures well above the critical pressure of water. During light load operation or at startup, a once-through steam generator of this kind is usually operated with a minimum flow of flow medium in the evaporator tubes in order to ensure reliable cooling of the evaporator tubes. For this PCT/EP2009/061239 / 2U00/Plb6/bWOAU 3 purpose, particularly at low loads of e.g. less than 40% of the design load, the pure mass flow through the evaporator is usually no longer sufficient to cool the evaporator tubes, so that an additional throughput of flow medium is superimposed in a circulatory manner on the flow medium passing through the evaporator. The operatively provided minimum flow of flow medium in the evaporator tubes is therefore not completely evaporated in the evaporator tubes during startup or light load operation, so that unevaporated flow medium, in particular a water-steam mixture, is still present at the end of the evaporator tubes during such an operating mode. However, as the superheater tubes mounted downstream of the evaporator tubes of the once-through steam generator and usually only receiving flow medium after it has flowed through the combustion chamber walls are not designed for a flow of unevaporated flow medium, once-through steam generators are generally designed such that water is reliably prevented from entering the superheater tubes even during startup or light load operation. To achieve this, the evaporator tubes are normally connected to the superheater tubes mounted downstream thereof via a moisture separation system. The moisture separator is used to separate the water-steam mixture exiting the evaporator tubes during startup or light load operation into water and steam. The steam is fed to the superheater tubes mounted downstream of the moisture separator, whereas the separated water is returned to the evaporator tubes e.g. via a circulating pump or can be drained off via a flash tank. Based on the flow direction of the gas stream, steam generators can also be subdivided, for example, into vertical and horizontal types. In the case of fossil fired steam PCT/EP2U9/U61239 / 20U/Plb6/6WOAU 4 generators of vertical design, a distinction is usually drawn between single-pass and two-pass boilers. In the case of a single-pass or tower boiler, the flue gas produced by combustion in the combustion chamber always flows vertically upward. All the heating surfaces disposed in the flue gas duct are above the combustion chamber on the flue gas side. Tower boilers offer a comparatively simple design and simple control of the stresses produced by the thermal expansion of the tubes. In addition, all the heating surfaces of the evaporator tubes disposed in the flue gas duct are horizontal and can therefore be completely dewatered, which may be desirable in frost-prone environments. In the case of the two-pass boiler, a horizontal gas duct leading into a vertical gas duct is mounted in an upper region downstream of the combustion chamber on the flue gas side. In said second vertical gas duct, the gas usually flows vertically from top to bottom. Therefore, in the two-pass boiler, multiple flow baffling of the flue gas takes place. Advantages of this design are, for example, the lower installed height and the resulting reduced manufacturing costs. In a steam generator implemented as a two-pass boiler, the walls of the first pass, i.e. the combustion chamber, are usually implemented entirely as an evaporator. The moisture separation system downstream of the evaporator tubes on the flow medium side is accordingly disposed at the upper end of the combustion chamber. However, because of differences both in the geometry of the individual tubes and in the heating thereof, different mass 5 flows and temperatures of the flow medium occur in parallel tubes. These so-called asymmetries must be limited for the following reasons: On the one hand, the evaporator heating surfaces must be sufficiently cooled over the entire load range of the steam generator. The mass flow required for cooling must be reliably supplied to each individual tube. In addition, the stresses occurring due to the thermal expansion of the individual tubes must not exceed the permissible values between adjacent tubes. The temperatures of the flow medium must be limited both in absolute terms and in terms of the difference with respect to the adjacent tubes, as otherwise damage to the combustion chamber wall could arise. To reduce temperature asymmetries in the evaporator tubes, mixing points can for example be installed in the combustion chamber walls configured as evaporators. In this case the flow medium is diverted from the evaporator tubes, mixed and re-distributed to the other evaporator tubes. Such a system must be placed downstream of the mixing point for an even distribution of a water and steam mixture. A design of this kind accordingly involves a high degree of technical complexity and considerably increases manufacturing costs. OBJECT It is the object of the present invention to substantially overcome or ameliorate one or more of the disadvantages of the prior art. SUMMARY The present invention provides a once-through steam generator with a combustion chamber having a number of burners for fossil fuel, downstream of which a vertical gas duct is mounted in an upper region on the hot gas side above a horizontal gas duct, the surrounding wall of the combustion chamber being formed, in a lower region, from evaporator tubes welded together in a gas-tight manner and mounted upstream of a moisture separation system on the flow medium side and, in an upper region, from superheater tubes welded together in a gas-tight manner and mounted downstream of the moisture separation system on the flow medium side, and in which part of the surrounding wall facing the vertical gas duct is inclined 6 inward below the horizontal gas duct, thereby forming with the bottom of the adjacent horizontal gas duct a projection extending into the combustion chamber, wherein the boundary between the regions of the evaporator tubes and the superheater tubes is disposed in an essentially horizontally circumferential manner around the combustion chamber in the region of the bottom of the horizontal gas duct directly above the projection. The invention is based on the ideal that a simple design combined with a comparatively long service life would be achievable if comparatively slight temperature asymmetries in the steam generator tubes were achievable without an additional mixing point being disposed in the evaporator tubes. The moisture separation system present in the steam generator also collects the water exiting the evaporator tubes in circulation mode and separates it from the steam. In once-through operation, the incoming steam is mixed and distributed to the superheater tubes located downstream on the flow medium side. This considerably reduces temperature asymmetries. Based on the knowledge that the moisture separation system thus basically fulfils the function of a mixing point, by placing it lower down, e.g. in the region of the bottom of the horizontal gas duct, this system can therefore be used as a mixing point within the combustion chamber wall, without an additional mixing system being required. In addition, this position of the moisture separation system means that the boundary between the regions of the evaporator tubes and the superheater tubes is disposed in an essentially hori zontally circumferential manner around the combustion chamber in the area of the bottom of the horizontal gas duct. In an advantageous embodiment, the boundary between the regions of the evaporator tubes and the superheater tubes is disposed in an essentially horizontally circumferential manner around the combustion chamber at the level of the edge formed PCT/EP2009/061239 / 2007P15676WOAU 7 by the surrounding wall and bottom of the horizontal gas duct. By means of such an arrangement, all the combustion chamber tubes welded to the tubes of the walls of the horizontal gas duct are likewise designed as superheater tubes. In the existing design with a combustion chamber formed entirely of evaporator tubes, evaporator and superheater tubes were welded in parallel at this point. This creates problems particularly for hot-starting of the steam generator, as the filling of the evaporator tubes with cold flow medium produces considerable temperature differences with respect to the unfilled superheater tubes. Disposing the moisture separation system at the level of the edge formed by the combustion chamber wall and the bottom of the horizontal gas duct ensures that such a vertical interface no longer occurs and altogether more reliable operation of the steam generator can be achieved while at the same time providing a comparatively long service life. In the case of two-pass steam generators, to improve gas flow, a section of the surrounding wall facing the vertical gas duct in inclined inward below the horizontal gas duct, thereby forming, with the bottom of the adjacent horizontal gas duct, a projection extending into the combustion chamber. In steam generators of this kind, the boundary between the regions of the evaporator tubes and the superheater tubes is advantageously disposed in an essentially horizontally circumferential manner around the combustion chamber directly above the projection. In another advantageous embodiment, the bottom of the horizontal gas duct is formed of evaporator tubes welded together in a gas-tight manner upstream of the moisture separation system on the flow medium side. The bottom of the PCT/EP2009/061239 / 2007Plb6~/6WOAU 8 horizontal gas duct is actually suitable to be designed as an additional evaporator heating surface, as its tubes are not welded parallel with the vertically tubed horizontal gas duct walls configured as superheaters and therefore the stresses caused by differential thermal expansion remain comparatively low. The particular advantages of the invention are that dual use of the moisture separation system as a mixing point for reducing temperature differences between parallel tubes is made possible by disposing the boundary between the regions of the evaporator tubes and the superheater tubes in an essentially horizontally circumferential manner around the combustion chamber in the region of the bottom of the horizontal gas duct. In addition, one of the main disadvantages of the two-pass boiler, namely the vertical interface between wall heating surfaces configured as evaporators and those configured as superheaters, are eliminated. Particularly for hot starting of the steam generator, during which high temperature differences and stresses occur at said interface when the evaporator tubes are filled with comparatively cold flow medium, particularly reliable operation and a longer service life of the steam generator can be achieved by avoiding such stresses. The lower positioning of the moisture separation system and therefore of the boundary between evaporator and superheater tubes in the combustion chamber also allows reduced superheating at the moisture separation system and altogether more material-conserving startup of the steam generator, which in turn results in a longer service life of the steam generator and enables less expensive materials to be used for the manufacture thereof.
PCT/EP2UU9/U612J9 / 2UU/Pibb/bWOAU 9 An exemplary embodiment of the invention will now be explained in greater detail with reference to the accompanying drawings in which the figure schematically illustrates a once-through steam generator of two-pass design. The once-through steam generator 1 according to the figure comprises a combustion chamber 2 implemented as a vertical gas duct, downstream of which a horizontal gas duct 6 is disposed in an upper region 4. The horizontal gas duct 6 is connected to another vertical gas duct 8. In the lower region 10 of the combustion chamber 2 a number of burners (not shown in greater detail) are provided which combust liquid or solid fuel in the combustion chamber. The surrounding wall 12 of the combustion chamber 2 is formed of steam generator tubes welded together in a gas-tight manner into which a flow medium - usually water - is pumped by a pump 9 (not shown in greater detail), said flow medium being heated by the heat produced by the burners. In the lower region 10 of the combustion chamber 2, the steam generator tubes can be oriented either spirally or vertically. In the case of a spiral arrangement, although comparatively greater design complexity is required, the resulting asymmetries between parallel tubes are comparatively lower than with a vertically tubed combustion chamber 2. The steam generator tubes in the lower part 10 of the combustion chamber 2 are designed as evaporator tubes. The flow medium is first evaporated therein and fed via pipework 14 to a moisture separation system (not shown in greater detail). In the moisture separation system, not yet evaporated water is collected and drained off. The steam produced is fed PU/EFZUU)/UbiZ.39 / ZUU/F10b/bWUAU 10 into the walls of the combustion chamber 2 and distributed to the superheater tubes disposed in the upper region 4 and in the walls of the horizontal gas duct 6. Such removal of not yet evaporated water is particularly necessary in startup mode when a larger amount of flow medium must be pumped in for reliable cooling of the evaporator tubes than can be evaporated in one evaporator tube pass. To improve flue gas flow, the once-through steam generator 1 shown also comprises a projection 16 forming a direct transition to the bottom 18 of the horizontal gas duct 6 and extending into the combustion chamber 2. In addition, a grid 20 of further superheater tubes is disposed in the transition region from the combustion chamber 2 to the horizontal gas duct 6 in the flue gas duct. Particularly in the case of a vertically tubed combustion chamber 2, temperature differences between parallel evaporator tubes may now occur which can compromise the operation of the steam generator as the result of differential thermal expansion. In order to achieve mixing of the flow medium from different tubes and therefore temperature equalization without using additional components, the boundary 22 between evaporator tubes and superheater tubes is disposed directly above the projection 16 at the level of the bottom 18 of the horizontal gas duct 6. The moisture separation system therefore acts not only as a separator during startup operation, but also as a mixing point in continuous operation, as the entire flow medium from the evaporator tubes is collected, mixed and redistributed to the superheater tubes in the moisture separation system.
11 As now both the upper part 4 of the combustion chamber 2 and the walls of the horizontal gas duct 6 are configured as superheater tubes, there is also no vertical interface in the region of the grid 20 between parallel welded evaporator and superheater tubes. Instead, only the lower part 10 of the combustion chamber 2 and the bottom 18 of the horizontal gas duct are configured as evaporator tubes, as a result of which only superheater tubes are welded together in parallel in this area.