Patent Application
20120111288
The present invention relates to steam generators endowed with high flexibility, made of materials, also comparable with those used in conventional steam generators. The steam generators of the present invention are capable to substantially expand the flexibility towards low loads (<30%), up to the limit of a night stand-by condition (load at least lower than 10%, preferably higher than or equal to 5%) in constant temperature profile control condition, and ready to rapidly rise up to maximum load according to the requests, even with fuels, as coal, that historically have been confined in continuous (non flexible) production uses.
It is known in the art that the thermal-electrical power production is technologically very diversified along the various types of fuels and the different thermodynamic cycles used.
However, all the technological solutions, both those already known and those still at the development stage, have a conceptually common feature, even if structurally different in the equipments, represented by the thermal recovery operations, under the form of heat, from combustion gas/fumes unsuitable as such to provide mechanical work, towards the operating fluid of a closed cycle which, by exploiting the hot source, is able to produce mechanical work. Generally the most diffused fluid is water/steam, which operates a Rankine cycle (feature always present today) wherein the isoentropic expansion of the steam in a turbine is performed. The thermal recovery equipments are called generators (SG).
The evolution of the heat-recovery steam generators took place according to some guide criteria.
The continuous increase of the cost of fossil fuels and the need to drastically reduce the amount of harmful emissions, comprising recently the “greenhouse” gases, per unit of power produced, they have in fact pushed towards higher and higher yields of thermal power-electric power transformation, even accepting the drawback of more complex and expensive technologies and plants.
As well known, higher cycle yields are associated to water/steam cycles operating at higher pressures and in particular at higher temperatures. By assuming as reference the pressure and temperature steam critical values, i.e. 22.1 MPa (221 bar), and 647 K (374° C.) it has been industrially experienced the move from sub-critical cycles to supercritical (SC) cycles up to the recent ultra-supercritical cycles (USC). Therefore in order to maximize the yields, today USC cycles operating at pressures of 240-280 bar and temperatures of 600-620° C. of the superheated steam are used, wherein the thermal recovery takes place by heating the water fluid without going through the typical two-phase transition state, with the presence of both liquid water and steam at once. The liquid water passes by heating in a continuous manner from the liquid phase to the steam phase, without an intermediate step through the liquid-steam two-phases typical of the steam generators operating under sub-critical conditions. In the USCs one passes from a high density phase (water-like) to a low density phase (steam-like) without the presence of a phase wherein liquid-water and steam-water are contemporaneously present.
The remarkable complexity of the handling of the heat exchange water/steam side has represented the key point in the technological choices for the sub-critical steam generators. In fact, it is important to note that the steam generator:
Therefore, the heat exchange takes place with very different temperature gradients between fumes and water liquid/steam, low in the water liquid preheating zone, high in the evaporation and steam superheating zone, with “pinch” problems (deltaT fumes-water/steam which is restricted to values near to zero of the heat exchange at the boundary between the preheating zone and the evaporation zone.
A system therefore very complex to be designed and operated according to efficiency and handling, which is represented by three well distinct zones, even if physically incorporated in a single equipment body: liquid preheating (ECO), evaporation (mixed liquid and steam phase, EVA), steam superheating (SH), each zone optimized according to specific criteria and controlled according to specific criteria. Each of these zones is thus equipped with different and independent instruments, control units and accessory circuits, i.e. the steam generator is conceptually and really separated into three different operations/equipments.
In particular, established solutions set the evaporation phase (EVA) confined by phase separators and large steam drums for the clear-cut separation of the water from the produced saturated steam, and stabilized through little varied heat exchange and fluid-dynamic conditions of the mixed phase, that is wherein limited amounts of steam are formed in large recirculated water masses.
This solution has been the most preferred, consolidated by its large use and by the appreciable characteristics of great stability in the control, favoured by the inertia given by the large water masses contained in steam drums (large vessels at high temperature and pressure), and appreciated for the large thermal power stations, which have been historically part of the backbone supplying of the continuous stock (i.e the night minimum of EP consumption) of power to the distribution networks.
The evolution of the subcritical to SC steam generators, towards the USC ones, it has from one side partially deprived the meaning of the distinction in three separated distinct zones and of the large water/steam separator systems. However the criterium of the distinction in three zones (ECO, EVA, SH) is still to be maintained, as the partialization of the power load takes place, on the thermal-electrical power conversion machines (turbines), through the sliding pressure concept (reduction of the steam pressure). In fact, the USC steam generators, when the steam generation pressure decreases below the critical pressure, they turn back to the subcritical conditions (appearance of two-phase water and steam, along the heating curve). In other words the power production can be modulated in a continuous way (almost constant temperature profile control) from the nominal value down to the limit of about 30% with respect to the nominal power at constant. Instead, under the 30% load, depending on the various adopted solutions, dedicated starting systems are used.
Lastly, the power generation had to take into account the trends during the whole day of the power consumptions. The evolution of the industrial and consumer system demand has brought in a sensitive increase of power consumption during the day hours, with a ratio between day hours/night hours power demand well above 3, and with abnormal peaks of request with respect to the continuous base consumption (night hours). This is known as (daily) “cycling”.
Production side, the generation of continuous power at full load has historically been a prerogative of the large plants with low variable costs, i.e. the nuclear, and of the thermal plants mainly coal-fired ones, leaving the absorption of day demand and peaks (cycling) to intrinsically quick-responsive technologies for the start up and for the power load increase/decrease with respect to the nominal load, such as the technologies based on turbo-gas cycles. This scheme has been able to absorb the cycling at least until not long ago.
However it has to be remarked that other developing factors create an unbalance:
The cycling requirements exclude for the combined cycles the conventional “steam drum” steam generators, too slow in the load variation, and have given new solutions, of which there is evidence already at least for the so called fast response plants.
All these evolution factors notably pushed towards new solutions, possibly conceived in combination with the new technologies to be developed for near-zero emission target from fossil fuels. As said above, a new solution already apparent today relates to heat recovery steam generators of combined (quick) cycles.
The daily cycling and the quick response to load variations have required to dismiss the use of steam drum, i.e. of the three-phase scheme, and the switching to a much more flexible scheme known as “once through”, literally single-pass water/steam side.
For example, the pure countercurrent scheme has been established, i.e. fluids passing through the equipment in opposite directions, and with contact/exchange, through a wall, between hot fumes and hot steam on one side, throughout to cold fumes in contact with cold water to be preheated, i.e. at minimized heat-exchange deltaT. The equipment is vertical—the fumes rise from the bottom crossing tube banks of horizontal water/steam tubes and water down-comes from the top “once through”.
The flexibility is obtained by:
In this way the problems of slug flow, (plug flow) are overcome. In fact, these problems would arise in the case of upward water/steam flow, for schemes with simple tube passing uninterrupted through the whole steam generator, for all the high water/steam ratios along the evaporation zone.
An example of pure countercurrent scheme, applied at sub-critical conditions, it is the IST one of the AECON group. Specifically it resolves, at high and intermediate water/steam ratio flow, the problems of steam segregation in bubbles from a still low speed water flow, and later on, at lower ratios, of water stratified and wavy flow with superheating of the tube ceiling, followed by projection of water on the tube ceiling (slug flow, plug flow), and subsequent peeling of the metal wall.
However with the load variation, and especially at low loads, in particular lower than about 30%, the problems due to temperature profiles along the water path very different from those of the maximum load are not overcome, and in particular the extension to most of the tube length of temperatures near the temperature of the inletting hot fumes are not overcome. It follows that for most of the exchange surface the tubes must be made of high alloyed materials (alloys with high nickel content, and other valuable metals), with consequent higher costs. The use of high-alloyed materials in the exchange surfaces becomes evident in case of an equipment of this type inserted downstream a carbon combustion reactor of the prior art.
Furthermore, the “once-through” scheme with “downcoming” water requires a vertical installation of the plant. This is a limit of capex relevance particularly for the large power units. Finally, it is worth noticing, apart the pipe high temperature extension mentioned above, that in order to quickly move the load up or down it is necessary that the operations can be carried out with constant temperature profile control (that means for steam generators to maintain the temperature profiles of fumes and water/steam in the same alignment and geometrical position in the steam generator, condition known in the prior art as constant temperature profile control condition, or “profile control”), which is not the case, for the IST boiler, over an ample load interval.
Therefore the undoubted flexibility of this embodiment, that is the quick up and down load variation at constant temperature profile control, it is attenuated until to disappear at loads lower than 30%. In fact, the managing/control of remarkable portions of the steam generator, at various steam/water ratios and at low steam flow rate, owing to the low load, it is no more supported by the sole water downflow and it requires progressively different control strategies and thus not operable in real time.
The concern that the water down-flow by gravity can cause unacceptable risks of turbine damage, in transient condition (start-up/stop) and at low load conditions (<30%), by unacceptable deviation from the steady state (water/steam ratio) of the water/steam flow, and to maintain anyway for a substantial part of the steam generator low deltaTs (for the previously reported reasons), it is apparent in the invention U.S. Pat. No. 5,159,897. In this patent the “once through” scheme, with hot fumes from the bottom and water from the top, it is combined with an intermediate zone wherein the two-phase water/steam fluid (evaporating water) returns to rise (against gravity) in co-current flow with the fumes, delimiting a zone wherein preferably the water to be evaporated is contained, which at low loads would move towards the outlet in non steady conditions. Furthermore, being the water/steam phase transition (in subcritical conditions) an isothermal phenomenon, the entropic inefficiency of the co-current heat exchange results negligible. However at USC full load conditions the entropic inefficiencies come back of relevance and the flexibility at low loads is obtainable only by extending anyway the exchange surface portion made of high-alloyed materials.
The concern of the high deltaT (materials, peeling), and of thermal shock during the quick load variations, it is apparent in U.S. Pat. No. 7,383,791 wherein the “once through” scheme (an uninterrupted single tube from the inlet to the outlet) designs the water path so that the rising flow of hot fumes comes first into contact with water to be preheated, in order to limit the deltaT in the steam generation zone SH (maximum of the fluid temperature to be heated) and the thermal shock risks in the evaporation zone. The water therefore enters from the bottom and is preheated with the hot fumes, outlets and renters at the top in down-flow, countercurrent with the rising fumes for the water/steam evaporation phase and the superheating phase.
Undoubtedly, the deltaT fumes water/steam is more limited with respect to the previous cases (IST), and less valuable materials can be used for a larger portion of the heat exchange surface. However it is apparent this is at the expense of the global yield of the cycle, given the entropy formation associated to the hot fume-water heat exchange in the preheating step.
Although the above described cases introduce, in the operation, flexibility improvements (load variation rate) to the detriment of the efficiency or at the expenses of a larger use of expensive high alloyed materials, for them and for the other consolidated solutions the problem still remains that for loads lower than 30% the steam generator significantly departs from the optimal thermal profile (tube bank temperatures, water/steam and fumes temperature profile) of the full load (deviation from the optimal temperature profile control established for high load). It results therefrom that, for the start up and for the running at load up to 30%, it is necessary to quit the high load control condition and carry out a series of operations with various logics and with the use of accessory circuits/hardware. This implies a tangible penalization in terms of the start-up rate and for the load rising up to 30%, and of the control condition complexity. For power plant types, such as the combined-cycle turbo-gas, which distinguish themselves for quick start and quick rising load performance, the penalization has significant economic impact. Specifically, the steam generator of the combined cycles is the element that determines the start and the load rising rate, that imposes delays of the order of tenths of minutes, up to over one hour.
Various schemes have been studied in order to try to limit the negative impact thereof. One proposes to disconnect the steam generator from the turbogas, by creating a bypass of hot fumes directly sent to the chimney without passing through the steam generator. Another scheme proposes to modulate (by reducing) the turbogas power, via number of revolutions and fuel, by sending all the fumes to the steam generator, with modulation (fumes flow-rate and fumes temperature) based on the startup procedure and on the load rising performance of the steam generator.
The leaving from the temperature profile control condition forcedly takes place also because the heat exchange flux at high temperature is not based on a single well known mechanism (forced convection), but on two:
Depending on the upstream fumes generating plant (combustion, hot fume generator) one will have:
Therefore, in the operations below 30% load, the temperature profile control cannot be maintained and different control logics, the more different the more the load decreases, are to be progressively taken, and often with the use of accessory circuits (external recirculations, water injection-modulation into steam) which interrupt the single tube path. That is, the steam generator cannot be operated extending the automatic temperature profile control to the whole range below 30% load (both in rising and in descent) as well as in the start/stop phases.
The need was therefore felt to have available steam generators having the following combination of properties
It is to be remembered in fact that, for the characteristics of the turbines, the specific yield to produce power for fuel unit (kWhr produced/Kjoule heat of combustion) significantly decreases as the load decreases, up to unacceptable values (about 15%) at plant loads of 30%, i.e. at the lower load limit suitable to temperature profile control.
The Applicant has surprisingly and unexpectedly found a steam generator solving the above described technical problem and capable to satisfy the high efficiency and cycling requirements, and of reduced costs (conventional materials of the prior art).
It is an object of the present invention a steam generator comprising
The water/steam tubes preferably pass through the steam generator from the water input to the superheated steam output preferably without intermediate inlets and outlets, more preferably without interruption. The water-steam tubes can be made of materials normally used in conventional USC steam generators.
Generally the used materials vary depending on the operating temperature to which they are subjected along the steam generator axis. In the steam generator of the invention the high-alloyed material section is only that corresponding to the last part wherein the final steam superheating is performed. For example, if the steam outlets at 605° C. and at a pressure of 240-280 bar, the length of this part corresponds to about 10% of the tube length. After the first part in high-alloyed material, there is in sequence a cascade of materials preferably comprising chromium steels, the most of the tube length (about 60%) preferably made of carbon steel.
The water/steam tubes arranged in flat banks, perpendicularly crossed by fumes, have preferably a relatively limited rectilinear horizontal tube length, generally preferably lower than 12 meters, still more preferably lower than 6 meters.
These dimensions are used to avoid too long rectilinear horizontal sections, which favor the appearance of periodic water accumulation and plug flow (or slug flow) propagation. Therefore, although the minimum operating load of the tube is about 30%, in the steam generators of the invention shorter lengths, as said, are preferred, followed by remixing (curves, more frequent ascents) in order to avoid plug flow phenomenon and its propagation. When ribbed tubes are used, see below, the tube length can be even longer, for example of 20 meters.
The tubes ascending with an oblique path between a tube bank and the other one are described in detail later on. The water/steam tubes are divided in two or more separate branches, separately fed, as described in detail hereinafter.
The headers are preferably positioned according to criteria described in detail afterwards.
The steam generator of the invention is once through vertical in pure countercurrent, preferably with fume inlet from the top and water inlet from the bottom.
Preferably, the “once-through” pure countercurrent steam generator of the invention is horizontal. In this way the industrial installation is simplified and thus a substantial reduction of the installation costs is achieved. This point is more widely illustrated later on.
The temperature modulation of the inletting hot fumes is preferably operated by recycling cold fumes after recovery, as described afterwards when the advantages concerning the superheated steam control and pinch elimination are illustrated.
It is a further object of the invention a process for operating the steam generator of the invention in sliding pressure modality, with water/steam always in supercritical conditions at 100% load (
Optionally the steam generator can be operated in constant pressure modality, with the water/steam in the steam generator always at supercritical conditions for all the loads (from 100% to 30% load) and final lamination before injection into a turbine (
It is a further object of the invention a process for operating the steam generator of the invention at loads from 5-10% to 100% comprising the following steps:
The preferred solution for the maintenance of the temperature profile is the use of the above mentioned steps b) and c).
Optionally the process of the invention comprises the following step e):
The step of the heat exchange surface choking, when operating at low loads, it is described in detail hereinafter.
The feedback control step c), of the produced steam temperature at any load, by modulating the hot fume temperature, is dealt with further on, where how to maintain the superheated steam temperature, and to avoid pinch phenomena, is reported.
The feedback control step b) of the fed water flow rate at any load by maintaining the temperature flex in supercritical conditions, or of the vaporization isotherm at sub-critical conditions (in sliding pressure) is treated in detail afterwards.
Optionally the process of the invention comprises the optional lamination step e), which may be of interest for horizontal installations in case of high capacity combined cycle plants.
The steam generator of the invention, operated with the above described process, unexpectedly and surprisingly, it is able to offer the above mentioned high performances without significant cost increase. The steam generator of the invention meets the cycling from 5-10% to 100% load, it has a high efficiency and it works without necessarily requiring high alloyed materials for most of the heat exchange (wall) surface.
The present invention makes therefore available steam generators having high flexibility, made of materials of a quality comparable to those of conventional steam generators, able to operate also at very low loads, of the order of 5-10%, working under constant operation and temperature profile control condition, and able to rapidly rise again to the maximum load, also when using solid fuels such as coal.
The steam generator of the invention, with the above mentioned characteristics, shows furthermore the following properties:
With the above mentioned characteristics, the following desired performances are obtained:
The principle scheme of the invention is simple, similar to an heat exchanger in pure countercurrent, as shown in
The effect of combining the inlet fume temperature modulation with the poly-partition into branches on the maintenance of the temperature profiles at low loads, and on the use of standard materials, it is evident by comparing (same boundary condition for both) the temperature profile of the water/steam and of the fumes along the steam generator axis, in case of no partition (
The development of each single heat exchange tube preferably without interruptions from the water inlet to the superheated steam outlet, and the partition into more branches, allows the perfect distribution of the flow rate on each single tube by simple orifices (localized head losses), without energy penalizations for excessive load losses at full capacity or uneven distributions due to insufficient head loss at low loads (5-10%), the minimum load of the operating branch being 30% for achieving the desired total load of 5-10%.
As said, the water/steam is divided in branches, at least branches, preferably 3 branches, still more preferably from 4 to 6 branches. In order to maintaining the desired temperature profile (fume side and water/steam side) when one or more are put out of service, one tube is taken from the header of each branch to form couples, terns, sets of four groups (and so on), so that the branch tubes are always contiguously grouped. See
Always for obtaining the above indicated results, the tube, after having passed through an horizontal tube bank rises obliquely towards the next tube bank for avoiding to form unbalanced fume and water/steam paths and for improving uneven distribution of the fumes, always present in any geometry configuration and the steam generator design (see
As said, the surface choking allows to maintain constant the fumes temperature decrease profile, thanks to the fact that one or more branches are excluded from the operation, for example by excluding the water feeding and/or by closing the outlet towards the high pressure superheated steam. By keeping in place the fumes temperature profile, it is obtained furthermore that the out of service branch is brought at most up to the fumes temperature pertaining to the axial position, along the steam generator axis. Furthermore, thanks to hot fumes temperature tuning, via recycled cold fumes admixing, and the superheated steam temperature control linked to the inlet temperature, the deltaT (between fumes and water/steam) of the obtained profile is always very small, including the hot zone. Therefore excessive overheating of out of service tubes, in respect to design operating condition, is excluded; thus, upgrading of the materials, in comparison with the traditionally established sequence of materials used in USC boilers, is not needed.
In
On the contrary the fumes temperature profile, obtained by operating with one branch (out of the three in the proposed example,
In the steam generator of the invention, the maintaining/control of temperature profile water/steam side, from USC conditions at maximum load downward to lower load by pressure decrease to subcritical conditions (sliding pressure) up to a limit of 30% on one branch or on more branches, it is performed by maintaining the geometrical position, along the steam generator axis, of the temperature inflection point in supercritical conditions, or of the isothermal vaporization temperature in subcritical conditions. The position is sensored by temperature measurements of the water/steam flow. They detect the inflection position or the isothermal vaporization position, and precisely upstream and downstream of the plateau wherein the positive and negative temperature shift from the inflection, or from the isothermal vaporization, takes place. In fact, it has been noticed that the supercritical conditions, though the two-phase isothermal vaporization is absent, correspondingly show a marked temperature inflection point (quasi-isothermal), and of pronounced density and enthalpy variation. More precisely, there is a continuity of temperature profile “shape” from subcritical to supercritical, and for the above mentioned parameters. Therefore, with a single logic, the feed-back regulation, operating on the inlet water flow rate, maintains the position of the isothermal, or quasi-isothermal portion, in place, and consequentially the desired temperature profile, that is it maintains heat exchange characteristics and typology.
In the case of installation of the steam generator of the invention downstream a combustors operating with solid fuels, preferably the superheated steam temperature control takes place by modulating the inlet fumes temperature, by recycling cold fumes outletting the steam generator. It has been unexpectedly and surprisingly found that by this control procedure the above mentioned pinch problems can avoided, also. In fact, as said, in any steam generator, heat exchange takes place with very large deltaT (between fumes and water/steam) variations, i.e very low deltaT in the water preheating zone, and very high in the EVA and SH zones, with pinch problems (deltaT which shrinks to values that almost nullify the heat flux) at the boundary between the ECO and EVA zones, every time even limited fluctuations (oscillations) take place (at an apparently constant load), implying unbalances between ECO and the other zones.
On the contrary, in the steam generators of the invention, when the recycle/addition of cold fumes to hot fumes is applied (notice: the hot-cold recycled fumes mixing does not alter the enthalpy balance of thermal recovery), the following conditions are achieved:
It is thus apparent that the flow rate/temperature couple allows to shift the load among the various zones so as to provide always the requested deltaT at the boundary ECO-zone-EVA zone (deltaT is never reduced to unacceptable values), the typical heat exchange surface for the various zones being assured by regulating the previously described inflection point position. It has been surprisingly and unexpectedly observed that the above pinch regulation is converging with the temperature regulation of the produced superheated steam temperature.
In the steam generator of the invention the steadiness of the temperature profiles in a very wide range allows to reach a good solution also for the collecting headers of the superheated steam.
It is well known in the art that the tube collecting headers have a high thickness due to the larger diameter and to the high design temperature. When they are subjected to sudden temperature shock, they are subjected also to radial differential thermal expansion stress in the wall thickness, which is additive to the stress of continuous working conditions, generating oligo-cyclic (low cycle number) and yet relevant fatigue. This implies limitation of the speed of load increase and consequent limitation of the cycling capability.
The risk of thermal shock, which must be avoided, represents therefore one of the additional elements limiting the quick response to load variations.
In the steam generator of the invention, the maintaining of temperature profiles over a wide operating range (5-10% up to 100% load) allows to identify an axial position along the fumes pathway wherein the temperature of the fumes is kept at about the temperature of the superheated steam (for example about 600° C.). It has been found that by bending down the tubes at the end of the exchange path, aside the tube banks down to the above mentioned point, and preferably by positioning the steam outlet headers in the fume flow (
One of the preferred embodiments of the steam generator of the invention is the horizontal arrangement, as represented in
In U.S. Pat. No. 7,406,928 the horizontal arrangement of the steam generator is obtained by arranging an horizontal coil with straight ascending and descending tubes (raiser and downcomer in series). Furthermore also a preheating zone of the inlet water with hot fumes (with high heat flux) is set out for assuring a rapid heat transfer rate, so that at the first downcomer there is a sufficient two-phase fluid flow rate, capable to enhance the water carryover of vaporized steam bubbles. The rising/downcoming of the tube prevents the establishing of unsteady conditions (water still present far ahead along the steam generator) of water-steam side, a sufficient bi-phase volume fluid flow rate being possibly assured in the part of incipient vaporization in order to avoid water segregation out of the flow and the plug flow.
The implementation of the horizontal arrangement does not however change what observed above for U.S. Pat. No. 5,159,897 and U.S. Pat. No. 7,383,791, and at most it introduces a further critical element of the plant when it is operated at low loads.
The steam generator of the invention, with an horizontal arrangement not only introduces the above advantages (accessibility and reduced steel-work), but maintains unaltered the above cited advantages of the vertical arrangement for loads from 5-10% to 100%.
It has been surprisingly and unexpectedly found that the conception of the raising obliquely tube is valid also for the horizontal arrangement. In fact, the steam generator rotation of 90° in horizontal position, made by horizontally maintaining the bank tubes, it finds the oblique rise of each tube rotated of 90°, anyway oblique. Or better, an embodiment can be implemented which maintains the desired oblique angle, providing therewith a rise, this time in a direction orthogonal to the steam generator axis, which in all the aspects corresponds to the rise obtained in the vertical arrangement by crossing from the left to the right (or viceversa) along the steam generator axis.
Observed from a side view, the development of the single tube in the connecting elbows among the horizontal parts, follows, along the steam generator axis, a saw-toothed path (it raises obliquely to the end of the fume containment and then downcomes by taking again the lowest position at the other end of the containment; see
As step e) is concerned, that is the maintaining, in all the pressure conditions of the produced steam, of a first part, or all, of the steam generator in supercritical pressure conditions followed by lamination when the fluid enthalpy allows downstream of the lamination the direct transfer of the supercritical fluid to steam phase without crossing the water/steam two-phase fluid area (
In particular it has been surprisingly and unexpectedly found by the Applicant that the modalities of step e) can be preferably used also in the start up phase of the steam generator. In fact a particularly rapid and highly desired procedure from an industrial point of view has been found out. The start up procedure comprises the following process steps:
The advantages of this start-up procedure are the very fast load feeding, the production of only steam, the control of the interval from 0 to 30% load of the branch with a different (from temperature profile control) and yet very simple regulation logic, i.e. with steam temperature controlling the final lamination valve, anticipated set up of the feedback regulation control devices. The profile control conditions are exceptionally fast.
The above mentioned Figures are described more in detail hereinafter.
The following Figures are described in detail.
It is noticeable that tubes are uninterrupted, from the inlet headers to the outlet headers. Alternatively, (embodiment not shown in the figure), intermediate headers can be made available (suitably positioned before and/or after the evaporation or pseudo evaporation zone). Alternatively, (embodiment not shown in the figure), re-heating stages of intermediate pressure steam spilled from the turbine, or more steam re-heating stages at a different pressure, can be made available. Alternatively, (embodiment not shown in the figure), de-superheating stages can be arranged.
In
In
In
In
The steam generator of the invention, allows, as said above, to solve the problem of “cycling”, as it is very quick in the start up and in the power load increase/decrease within the nominal capacity.
The steam generators of the invention quickly reacts to load variations, and especially at low loads, and in particular lower than about 30%, because it overcomes the problems due to wide temperature profiles, along the water/steam pathway, deviation from those of maximum load. The steam generator of the invention can withstand the extension, towards a very large portion of the tube pathway, of temperatures close to the temperature of the incoming hot fumes. For this reason, the use, for a large portion of the heat exchange surface, of high alloyed materials for tubes (alloys with a high content of nickel, and other valuable metals) is not necessary. In this way the cost of the steam generator of the present invention is lower in comparison with other prior art steam generators.
In fact, in the steam generators of the invention:
In the operations under the limit of about 30% load in the steam generators of the invention the profile control is maintained and the steam generator can be operated in automated temperature profile control, constant over the whole range lower than 30% load, both in rising and in decreasing, in addition to quick start-up and downs.
Therefore the steam generators of the invention show high flexibility and can be made of materials even of a quality comparable to those used in traditional USC steam generators, that is the portion of tubes length in high alloyed materials is very limited. Besides, the steam generators of the invention are able to expand the flexibility towards the low loads (<30%), down to the limit close to an economically acceptable night stand-by condition (load at least below 10%, preferably higher than or equal to 5%), in a constant temperature “profile” control modality, ready to quickly raise to maximum load according to the requirements, also with fuels, as coal, which historically have been limited to power stations servicing the continuous production close to capacity.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2009A001336 | Jul 2009 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/060558 | 7/21/2010 | WO | 00 | 1/12/2012 |
