Patent Application
20110162592
The invention relates to a once-through (“continuous”) steam generator comprising a number of burners for fossil fuel, the surrounding wall thereof being completely or partially formed from steam generator tubes welded together in a gas-tight manner. Said burners are disposed in a combustion chamber downstream of which a vertical gas duct is mounted above a horizontal gas duct on the hot gas side, a first part of the steam generator tubes being implemented as a system of evaporator tubes mounted upstream of a moisture separation system on the flow medium side, and a second part of the steam generator tubes being implemented as a system of superheater tubes 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 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 are usually welded together in a gas-tight manner to form the combustion chamber wall. 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.
Fossil fired steam generators can be categorized on the basis of a large number of criteria: based on the flow direction of the gas stream, steam generators can be subdivided, for example, into vertical and horizontal types. In the case of fossil fired steam 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 located above the combustion chamber. 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.
Steam generators may also 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 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 circulating 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 not receiving flow medium until 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.
However, particularly in startup mode, the above mentioned concept causes high temperature differences between evaporator tubes and superheater tubes: during cold starting, as yet unevaporated flow medium flows in the evaporator tubes at saturation temperature, while steam at higher temperature is still present in the superheater tubes. During hot starting, on the other hand, the evaporator tubes are filled with cold feedwater, while the superheater tubes are still at operating temperature level. This can result in overloading and damage of the materials due to the differential thermal expansion.
The object of the invention is therefore to specify a once-through steam generator of the above mentioned type requiring comparatively low repair costs and having a comparatively long service life.
This object is achieved according to the invention by mounting superheater tubes in parallel contiguity with evaporator tubes immediately downstream of the moisture separation system on the flow medium side.
The invention is based on the idea that it would be possible to reduce repair costs and increase the service life of the once-through steam generator if damage caused by differential thermal expansion of welded-together steam generator tubes could be minimized. The differential expansion is the result of high temperature differences between the steam generator tubes. Said temperature differences are caused by differential cooling of the steam generator tubes and different temperatures of the flow medium flowing therein and therefore occur in particular at the interface between welded-together evaporator and superheater tubes, as these exhibit a different throughput of flow medium with different temperatures through the intervening moisture separation system particularly during cold and hot starting.
Particularly in the case of once-through steam generators of the two-pass type, the design means that an interface between parallel-welded evaporator and superheater tubes is typical. In order to minimize as far as possible the temperature differences between evaporator and superheater tubes, the steam temperature in the superheater tubes welded parallel with the evaporator tubes must be minimized. This can be achieved by mounting said superheater tubes immediately downstream of the moisture separation system, so that there is no increase in the temperature of the flow medium flowing therein due to additional intervening superheater tubes. This consistently minimizes temperature differences as a cause of damage at the interface.
In an advantageous embodiment, the combustion chamber wall of the once-through steam generator is formed from evaporator tubes and a sidewall of the horizontal gas duct is formed from superheater tubes, the superheater tubes adjacent to the combustion chamber being mounted directly downstream of the moisture separation system on the flow medium side. This effectively minimizes the temperature differences at the vertical interface between evaporator tubes of the combustion chamber and superheater tubes of the horizontal gas duct in the case of the two-pass boiler.
Advantageously, the top of the once-through steam generator is found from superheater tubes which are disposed immediately downstream of the moisture separation system on the flow medium side. This means that the superheater tubes of the top are mounted parallel with other superheater tubes adjacent to the evaporator tubes. Due to the paralleling of the heating surfaces, such an arrangement is advantageous in respect of the pressure loss to be expected.
In a once-through steam generator in which superheater tubes in parallel contiguity with evaporator tubes are disposed vertically, these are advantageously designed such that the flow medium flows through the superheater tubes from top to bottom. This means that, in the event of overfeeding of the moisture separation system resulting in unevaporated flow medium being applied to the superheater tubes, this can be drained off at the outlet header of the superheater tubes, thereby enabling any flow stagnation to be effectively prevented.
The advantages achieved with the invention are in particular that by mounting superheater tubes in parallel contiguity with evaporator tubes immediately downstream of the moisture separation system on the flow medium side, the temperature differences between said tubes are consistently minimized. As a result, the differential thermal expansion is minimized and damage and overloading are prevented, in turn resulting in fewer repairs and a longer service life of the once-through steam generator.
Such an arrangement is advantageous particularly in the case of once-through steam generators without circulating pump. The absence of circulation results in lower inlet temperatures to the evaporator, smaller steam mass flows and an increase in the firing capacity required at startup. Simulations have shown that particularly for these systems, impermissible temperature differences can occur at the interface between evaporator and superheater tubes if—as hitherto usual—the superheater tubes at the interface are mounted downstream of other superheater tubes, e.g. of the top. Mounting said superheater tubes directly downstream of the moisture separation system effectively prevents these temperature differences.
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 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 (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 heating differences between parallel tubes are comparatively lower than with a vertically tubed combustion chamber 2.
To improve flue gas flow, the once-through steam generator 1 shown also comprises a projection 14 forming a direct transition to the bottom 16 of the horizontal gas duct 6 and extending into the combustion chamber 2.
The steam generator tubes of the combustion chamber 2 are designed as evaporator tubes. The flow medium is first evaporated therein and fed via outlet headers 20 to the moisture separation system 22. In the moisture separation system 22, not yet evaporated water is collected and drained off. This is particularly necessary in startup mode when a larger amount of flow medium must be pumped in to ensure reliable cooling of the evaporator tubes than can be evaporated in one evaporator tube pass. The steam produced is fed to the inlet headers 24 of the downstream superheater tubes which form the top 26 of the once-through steam generator 1 and the walls of the horizontal gas duct 6. The transition from the sidewalls of the vertical gas duct to the sidewalls of the horizontal gas duct 6 constitutes the interface 18 between evaporator tubes of the combustion chamber wall 12 and superheater tubes in the walls of the horizontal gas duct 6.
In addition to the two-pass boiler shown in the FIGURE, other configurations for fossil fired boilers are self-evidently also possible.
In order to prevent damage due to differential thermal expansion caused by temperature differences at the interface 18 between the evaporator tubes of the combustion chamber wall 12 and the superheater tubes in the walls of the horizontal gas duct 6, these superheater tubes are mounted directly downstream of the moisture separation system 22 via a connecting line 28. As a result, said superheater tubes are only subject to saturated steam and not higher-temperature superheated steam, thereby reducing the temperature.
The superheater tubes in the walls of the horizontal gas duct 6 are parallel to those of the top 26 and are flowed through from top to bottom. Thus, in the event of overfeeding of the moisture separation system 22, unevaporated flow medium in the outlet headers 30 of the superheater tubes can be drained off and flow stagnation cannot occur.
The arrangement described minimizes the temperature differences at the interface 18 between the evaporator tubes of the combustion chamber wall 12 and the superheater tubes in the walls of the horizontal gas duct 6, thereby enabling damage to be effectively prevented. This results in comparatively fewer repairs and a longer service life of the once-through steam generator 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08015871.0 | Sep 2008 | EP | regional |
This application is the US National Stage of International Application No. PCT/EP2009/061468, filed Sep. 4, 2009 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 08015871.0 EP filed Sep. 9, 2008. All of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/061468 | 9/4/2009 | WO | 00 | 3/8/2011 |
