Abstract
A steam generator comprising water/steam tubes passing through the steam generator from the water inlet to the superheated steam outlet, horizontally arranged in tube banks, preferably flat tube banks, perpendicularly crossed by the fumes, the tubes ascend along the steam generator axis from one tube bank the other, with an oblique path so to expose the tube to the fume flow in different positions at each tube bank, the tubes are divided into two or more separate branches, each branch fed by a header distinct from the others, the steam generator being once-through in pure counter-current, vertical or horizontal, the headers of the outlet superheated steam are grouped at direct contact in a bundle, and they are thermally insulated from the outside.
Claims
1. A process for operating a steam generator at loads from 5% to 100%, the steam generator comprising: a cylindrical vessel having an axis; an inlet for water, an outlet for a superheated steam, a fumes inlet and a fumes outlet: water/steam tubes passing through the cylindrical vessel from the inlet for water to the outlet for superheated steam; and headers for the superheated steam: wherein: the water/steam tubes are arranged in flat tube banks, and the flat tube banks form a sequence of flat tube banks that extend along a direction of the vessel axis, wherein the flat tube banks are crossed perpendicularly by a fumes flow that passes through the vessel from the fumes inlet to the fumes outlet; the water/steam tubes are arranged horizontally and contiguous within the flat tube banks and pairs of flat tube banks are connected to one another by obliquely extending tubes such that water/steam tubes in each pair of flat tube banks are exposed to fumes at different positions along the direction of the vessel axis; the water/steam tubes arranged in the flat tube banks are divided into two or more separate branches, wherein each branch is fed by a separate header; the headers for the superheated steam are grouped together in a bundle, and the bundle of headers is thermally insulated from a region external to the steam generator; and the steam generator is a once-through type steam generator with fumes flow in counter-current with respect to water flow; the process for operating the steam generator comprising steps of: feeding water into the water/steam tubes and fumes through the fumes inlet; flowing the fumes in counter-current with respect to water flowing inside the water/steam tubes thereby forming water/steam, the fumes and the water/steam exchanging heat through a heat exchange surface of the steam generator so that the fumes and the water/steam have respective temperature profiles along the vessel axis; maintaining, in the steam generator, the temperature profiles of the fumes and the water/steam in a common position along the vessel axis; and choking the heat exchange surface of the steam generator so that operation at loads lower than 30% takes place by excluding and then maintaining in a dry condition one or more branches of the water/steam tubes, up to having only one operating branch.
2. The process according to claim 1, wherein the temperature profile of the fumes and the water/steam are maintained at a constant position along the vessel axis by performing two or more steps of: a) for loads lower than 30%, choking the heat exchange surface by excluding and then maintaining in the dry condition one or more branches of the water/steam tubes, up to have only one operating branch of the water/steam tubes; b) feedback control of feed water flow-rate at loads from 5% to 100% so that under loads requiring supercritical conditions which involve production of a supercritical fluid, the supercritical fluid is maintained in a first position along the vessel axis and under loads requiring subcritical pressure conditions which involve the production of a two-phase water/steam mixture, the two-phase water/steam mixture is maintained in a second position along the vessel axis; c) feedback control of the temperature of the superheated steam at loads from 5% to 100% by inlet fume temperature tuning, by recycling fumes exiting from the fumes outlet of the vessel to a combustor operated with solid fuels; d) feedback control of the temperature of the fumes at the fumes outlet of the vessel via feed water pre-heating.
3. The process according to claim 2, wherein the temperature profile of the fumes and the water/steam are maintained by carrying out steps b) and c).
4. The process according to claim 2, further comprising step e): maintaining the steam generator under supercritical pressure conditions to produce a supercritical fluid at loads higher than 5% up to 100%, lamination of the supercritical fluid, and transformation of the supercritical fluid into steam only.
5. The process according to claim 1, wherein a feed-forward control is performed by increasing or decreasing the load of the steam generator.
6. The process according to claim 1, wherein a minimum load of the steam generator for achieving a temperature profile control condition is 5% load.
7. The process according to claim 1, wherein a minimum load of the one or more operating branch of water/steam tubes for achieving a load of 5-10% in the steam generator is 30%.
8. The process according to claim 1, wherein one water steam tube from the header of each of the one or more branches of water steam tubes form couples, terns or sets of four groups of contiguously grouped water/steam tubes.
9. The process according to claim 1, wherein in each pair of flat tube banks a water/steam tube reaches one end of the vessel and turns and extends to an opposite end of the vessel.
10. The process according to claim 2, wherein the steam generator is positioned downstream a combustor operated with solid fuels and the feedback control of the temperature of the superheated steam is carried out by modulating the temperature of fumes recycled from the fumes outlet and fed to the fumes inlet of the steam generator.
11. The process according to claim 1, wherein the headers for superheated steam are positioned in the fumes flow, said headers having piping outside the vessel collected in a bundle, wherein a thermal insulation is disposed around the bundle.
12. The process according to claim 1, wherein fumes entering the fumes inlet are under pressure.
13. The process according to claim 4, wherein operating the steam generator comprises a start-up phase wherein step e) of the process is carried out.
14. The process according to claim 13, wherein: the start-up phase is carried out by selecting a steam generator operating pressure so that at first water exiting the vessel of the steam generator is sub-cooled and then without forming a two-phase liquid water/steam mixture, pressure is made supercritical, steam is superheated; in the start-up phase a supercritical fluid is generated and the fluid is laminated and conveyed to a flash tank; and when water exiting the vessel of the steam generator has an enthalpy of about 150 kJ/kg higher than a saturated steam enthalpy at an admission pressure into a turbine, it is introduced into a start-up circuit of the turbine.
15. The process according to claim 13, wherein the start-up phase comprises: heating of dry water/steam tubes of all the branches, feeding of the water/steam tubes of one branch with water at the supercritical pressure of 240-280 bar, heating the fumes, fluid lamination when water at the outlet of the steam generator has an enthalpy of about 150 kJ/kg higher than a saturated steam enthalpy at the inlet pressure of a turbine, or heating the fluid so that lamination produces superheated steam; and when a load equal to 30% of a fed one branch is reached, step b), step c) and step d) of claim 2 are carried out.
Description
(1) The above mentioned Figures are described more in detail hereinafter.
(2) FIG. 1 is a perspective view from the top of the tube course in a vertical steam generator of the invention.
(3) FIG. 2 represents the course of a tube in a vertical steam generator of the invention.
(4) FIG. 3 is a front view of the steam generator of FIG. 1.
(5) FIG. 4 is a front view of the tube of FIG. 2.
(6) FIG. 5 shows the independent branches feeding in an embodiment of the steam generator of the invention. In the case exemplified in the Figure three independent circuits are shown.
(7) FIG. 6 schematically represents a steam generator according to the invention with pure countercurrent heat exchange with fumes entering from the top and water fed from the bottom.
(8) FIG. 7A is a diagram pressure-temperature-enthalpy showing the heating in supercritical conditions of the water/steam fluid at a 100% load.
(9) FIG. 7B shows in a diagram pressure-temperature-enthalpy the heating in subcritical conditions of the water/steam fluid at a 50% load, representative for the partial loads of a steam generator.
(10) FIG. 7C shows in a diagram pressure-temperature-enthalpy the heating in supercritical conditions of the water/steam fluid at a 50% load (representative for the partial loads of a steam generator), and the subsequent lamination at the steam turbine inlet.
(11) FIG. 7D shows in a diagram pressure-temperature-enthalpy the heating in supercritical conditions of the water/steam fluid, the subsequent pressure decrease by lamination of the fluid itself without formation of bi-phase water/steam mixture, and superheating of the subcritical steam.
(12) FIG. 8 represents a plot of the temperature of: the fumes, the water/steam fluid at a 100% load as a function of the heat exchange surface of the steam generator.
(13) FIG. 9 comparative, it represents a plot of the temperature of: the fumes, the water/steam fluid as a function of the heat exchange surface at a reduced load in the case of the prior art without choking and partial exclusion of the heat exchange surface.
(14) FIG. 10 shows a plot in a steam generator of the invention of the temperature of: the fumes and the water/steam fluid at a 100% load as a function of the heat exchange surface at a reduced load with surface tri-partition choking and with one branch in service only.
(15) FIG. 11 is a perspective view showing the course of the tubes in an horizontal steam generator according to the present invention.
(16) FIG. 12 shows the course of a tube in an horizontal steam generator according to the invention.
(17) FIG. 13 is a front view of the steam generator of FIG. 11.
(18) FIG. 14 is a front view of the tube of FIG. 12.
(19) FIG. 15 shows in a diagram pressure-temperature-enthalpy the start up zone of the steam generator of the invention with fluid at the steam generator outlet in single-phase conditions.
(20) FIG. 16 shows in a diagram pressure-temperature-enthalpy the preferred start up method of the steam generator of the invention by maintaining the fluid always in supercritical conditions and fluid lamination at an enthalpy value such as to obtain only steam in conditions for admission into the turbine.
(21) The following Figures are described in detail.
(22) FIG. 1 is a tridimensional picture of tube banks (2) of a vertically arranged steam generator of the invention, with water feeding from the bottom and fumes 16 entering from the top (fume outlet 16A). The single exchange tubes, see for example tube 13, by turning after an horizontal rectilinear part, not only shift from a plane to the upper one, for example from the plane 11 to the upper plane 12 of the figure, but at once they also shift laterally towards the left. Once arrived to the limit of the fumes containing vessel (not shown in the figure) at the extreme left of the Figure, the tube at position 14 turns and, crossing the tube bank, takes the place 15, at the right end of the vessel.
(23) FIG. 2 represents an extract of FIG. 1 wherein only tube 13 is represented. 17 is the water inlet in the lower part of the tube bank and 18 represents the outlet of the fluid in the upper part of the tube bank.
(24) FIG. 3 shows a front view of a tube bank of a vertical steam generator with water feeding from the bottom already described in FIG. 1. The single heat exchange tube, for example tube 13, by turning, not only it shifts from a plane to the upper one (for example from plane 11 to the upper plane 12), but it also shift laterally towards the left (FIG. 2). Once arrived to the limit of the fume containing vessel (not shown in the figure) at the extreme left of the Figure, the tubes turn at position 14 and, crossing the tube bank, insert at position 15, at the right end of the vessel.
(25) FIG. 4 shows, in the same front view of FIG. 3, only tube 13 isolated from the remaining part of the tube bank, as described in FIG. 1 and FIG. 2. The heat exchange tube by turning, shifts from a plane to the upper one and also laterally to the left. Once arrived to the limit of the fume containing vessel (not shown in the figure) at the extreme left of the Figure, the tube turns at position 14 and, by crossing the tube bank, takes position 15, at the right end of the vessel.
(26) FIG. 5 shows one tube bank of the type described in FIG. 1, in a front view as in FIG. 3, formed of 30 tubes in the horizontal plane. The 30 tubes are alternately fed by three separate headers through the opening of valves 531, 532, 533. There are therefore three separate circuits, each formed of 10 tubes (fed in parallel). The tubes 51, 54, 57, 510, 513, 516, 519, 522, 525, 528, wherein passes water/steam when valve 531 is open, belong to the first circuit. In the second circuit there are tubes 52, 55, 58, 511, 514, 517, 520, 523, 526, 529, fluxed water/steam when the valve 532 is open. In the third circuit there are, of the remainder branch, tubes 53, 56, 59, 512, 515, 518, 521, 524, 527, 530 with the related valve 533 that regulates the flow thereof with water/steam. In the figure there is a schematic representation of the separate feeding system for each circuit, with the flow metering valves of each circuit. As an example, with the valve 531 open and the valves 532 and 533 closed, only in the tubes of the first circuit (tubes 51, 54, 57, 510, 513, 516, 519, 522, 525, 528) there is water/steam flow. With the tubes of the different circuits assembled together and arranged for the oblique tube bank rise, there is an uniform absorption of heat flux in the various circuits when all the circuits are fed. When one or more branches are without feeding, the temperatures reached by their tubes are limited to the average fumes temperature, by the near tubes of the circuits in operation (one or more). In fact the fed circuits locally keep fumes, which come into contact also with the tubes of the non operating circuits, at the optimal design temperature profile. FIG. 6 represents one type of steam generator of the invention with vertical arrangement, with fumes 61 entering from the top (and outlet 61A) and water entering from the bottom (through the headers 62, 63, 64). The heat exchange scheme is that of pure countercurrent. Therefore three separate circuits 65, 66, 67 are represented, each set up with one inlet header (in the Figure, header 62 feeds circuit 65, header 63 circuit 66, header 64 feeds circuit 67), heat exchange tubes (in the Figure it is reported one heat exchange tube for a circuit) and steam outlet headers (in the Figure header 68 for steam extraction from circuit 65, header 69 for circuit 66, header 610 for circuit 67). Headers 68, 69, 610 can be positioned both outside the fumes containing vessel 611, (option not reported in the figure), and in the fumes themselves in a position wherein the fumes temperature is near that of steam (preferred option, shown in the figure).
(27) 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.
(28) FIG. 7A represents, in a diagram pressure-temperature-enthalpy for water in supercritical conditions, the heating pathway from water at high density (water-like) to a fluid at lower density (steam-like), called superheated supercritical steam, at a 100% load. This transition takes place in one of the steam generator embodiments of the invention. In the diagram, four zones (or regions) can be identified, indicated in the figure with 71, 72, 73 and 74. Zone 71 represents the sub-cooled water; it is represented by the tract below the evaporation area (zone 72), when the pressure is lower than the critical pressure (around 221 bar). Zone 72, called evaporation zone, is the region, for a pressure below critical value, wherein liquid water and steam are both present. Above zone 72 (always pressures below critical pressure) only steam (zone 73) is present. Zone 74 comprises water in conditions above the critical pressure. Water at low enthalpy and high density (water like) in the conditions represented by point 75, undergoes a pseudo evaporation (state transition in the absence of formation of the liquid/steam mixture) represented by the points of the line comprised between points 75 and 76. At point 76 water has high enthalpy and low density (steam like), so that to be fed to the turbine.
(29) FIG. 7B represents, in a diagram pressure-temperature-enthalpy for water, the heating from sub-cooled water at subcritical conditions to superheated subcritical pressure steam at a 50% load (partial load). This transition takes place in one of the steam generator embodiments of the invention, being the load variation operated in sliding pressure modality. In the diagram four zones (or regions), indicated in the figure with 71, 72, 73 and 74 and described in FIG. 7A, are shown. The sub-cooled water at the conditions represented by point 77, undergoes the evaporation (state transition by formation of the liquid/steam mixtures) represented by the points of the line comprised between points 77 and 78. In 78 the superheated steam at subcritical pressure is in the conditions for feeding the turbine.
(30) FIG. 7C represents, in a diagram pressure-temperature-enthalpy for water, the heating from sub-cooled water at supercritical condition to superheated supercritical steam at a 50% load (partial load). This transition takes place in one of the steam generator embodiments of the invention operated in constant pressure modality. In the diagram, four zones (or regions) are shown, indicated in the figure with 71, 72, 73 and 74 and described in FIG. 7A. The sub-cooled water, in the conditions represented by point 79, undergoes the pseudo evaporation (it corresponds to the above state transition, but without formation of the liquid/steam mixture) represented by the points of the line comprised between points 79 and 710. In 710 the superheated steam, at supercritical pressure, outlets the steam generator and it is laminated (lamination from point 710 to point 711) in order to have in 711 the suitable pressure conditions for admission into the turbine.
(31) FIG. 7D represents, in a diagram pressure-temperature-enthalpy (H-T-p) for water, the heating pathway from water at high density (water like) in supercritical conditions to a fluid at lower density (steam like), called superheated subcritical steam, and the successive pressure decrease by lamination of the steam without formation of a water/steam two-phase mixture. These transitions (heating and lamination) take place in one of the steam generator embodiments of the invention. In the diagram four zones are shown, indicated in the figure with 71, 72, 73 and 74 and described in FIG. 7A. The low enthalpy and high density water (water like) in the conditions represented in point 712, undergoes the pseudo evaporation (state transition without formation of the liquid/steam mixture) represented by the tract comprised between points 712 and 713. In 713 the water has high enthalpy and low density (steam like). Through lamination (transition represented by the points comprised between 713 and 714, through one or more valves, the water pressure is decreased without having the liquid/steam mixture formation typical of zone 72, but belonging to zone 73 of superheated steam. The transformation represented by the tract between 714 and 715 is the superheating of subcritical steam, taking place in the terminal part (terminal part along the water/steam path) of the steam generator.
(32) In FIG. 8 it is shown, at 100% of the steam generator load and at supercritical conditions of the water/steam fluid, the plot of the temperature of: the fume (curve 81) and of the water/steam (curve 82), as a function of the heat exchange surface. In the figure, three zones are represented: the first one, from the left, includes the heat exchange surface wherein the fluid superheating takes place (zone 83). Zone 84 is the heat exchange surface wherein pseudo evaporation takes place. Zone 85 represents the zone wherein there is the heat exchange surface for the fluid preheating (ECO). The straight-broken curve 86 is the envelope of the design temperatures of the various sections of the heat exchange surface of the steam generator.
(33) In FIG. 9 it is represented, at a partial load (about 10% of the maximum load) of the steam generator in sub-critical conditions, the plot of the temperature of: the fumes (curve 91) and of the water/steam (curve 92) as a function of the exchange surface. The steam generator is not operated with exchange surface partition by exclusion of branches, as described in FIG. 5. In the figure the three zones (83, 84, 85) described in FIG. 8 are reported. It is noticeable the effect of the heat exchange surface overabundance; it causes, at a partial load, a shift of the EVA zone towards the ECO zone 85, wherein less expensive and less resistant to high temperature materials are used in USC boiler of the art. The straight-broken curve 86 is the envelope of the design temperatures, defined for the full load, of the various sections of the heat exchange surface. It is noticeable as well how the water/steam temperature (curve 91) reaches the same values of the fumes temperature (curve 92) for most of the heat exchange surface. Furthermore the water/steam curve 91 approaches and also goes over curve 86 of the design temperatures for materials of the art.
(34) In FIG. 10, at a partial load (about 10% of the maximum load, the same considered in FIG. 9) of the steam generator, in subcritical conditions, a plot, as a function of the heat exchange surface available, of fumes temperatures (curve 101), of the water/steam of the circuit in operation (curve 102), and of the water/steam in the two dry circuits (curve 103) are represented. The steam generator is in fact operated with surface partition by exclusion of some circuits or branches. In the example of the figure there are three circuits (as shown also in FIG. 5), of which only one is fed. In the Figure, the three zones (83, 84, 85) described in FIG. 8 are present. It is worth noticing how the exclusion of a part of the surface (in the example two thirds of the total surface is excluded) causes, also at a partial load, the two-phase transition zone of the running circuit to stay in zone 84, wherein also at full load the pseudo evaporation takes place. In the segmented curve 86, as in FIG. 8, there is the envelope of the mechanically admissible (design) temperatures of the various sections of the heat exchange surface. The temperatures of the two excluded (non operative) circuits are close to the fumes temperature, condition shown in the figure by the overlapping of the curves 101 (fumes) and 103 (water/steam in the dry circuits). Both fumes temperatures (curve 101) and those of the water/steam of the three circuits (curves 102 and 103) are lower than the design temperatures of the curve 86. In other words the running circuit keeps the fumes temperature profile in place and protects the non-operative circuits from metal overheating above design temperatures. The fumes temperature plot and the water/steam one is similar to the plot of the same parameters reported in FIG. 8.
(35) FIG. 11 represents, by a tridimensional picture with bottom-up view, the path of the tubes in a tube bank, in the horizontal arrangement. The fumes 116 flow through the tube bank from the right to the left (fume outlet 116A). It is worth noticing that the tubes (for example the black-color tube 113 for better following the path thereof), after an horizontal rectilinear part, end up with curves which shift them in the successive plane, but also towards the upper end of the tube bank. The tubes describe a saw-toothed path.
(36) FIG. 12 represents a particular of FIG. 11, wherein only the tube 113 is represented. The water inlet 117 and the water/steam outlet 118 are shown.
(37) In FIG. 13 a front view of the steam generator described in FIG. 11, is shown. The single heat exchange tube, for example the mentioned tube 113 (black-color to be better evidenced), by bending, not only shift from a plane to the following one (for example from plane 111 to plane 112), but it also shifts towards the upper part of the steam generator. Once arrived to the limit of the fumes containing vessel (not shown in the figure), the tube bends at position 114 and, by crossing the tube bank, takes the opposite position 115, at the lower end of the body.
(38) FIG. 14 shows, in the same front view of FIG. 13, only tube 113 of FIG. 12, blanketing all the other tubes.
(39) FIG. 15 represents, in the diagram H-T-p already described in FIG. 7, the straight-broken curve passing from points 151, 152, 153, 154, 155, 156. The position on the graph of these points is to be intended as an example and not as a precise indication of the limits of the broken curve crossing them. The points of this curve (developed around the evaporation area of the two-phase mixture 157), those to the right of the curve and over points 155 and 156 represent the acceptable conditions of the water/steam outletting the circuit when the steam generator starts-up, as the described start up modality foresees at the steam generator outlet only single-phase fluid.
(40) FIG. 16 represents, in a H-T-p diagram (see FIG. 7) with the start up zones indicated by the segmented curve passing trough points 151, 152, 153, 154, 155, 156 of FIG. 15, one of the preferred start up modality of the steam generator of the invention, by maintaining the fluid always in supercritical conditions up to an enthalpy level, so that fluid lamination produces only steam, with characteristics suitable for direct admission into the turbine. Water in supercritical conditions at low temperature (point 158) is heated up to point 159. In 159 the water has an enthalpy such that, after lamination (transformation between point 159 and 156), the evaporation zone 157 is avoided.
(41) 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.
(42) 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.
(43) In fact, in the steam generators of the invention:
(44) The load can be quickly moved upward or downward in an wide load interval with operations carried out at constant control logic, that for the steam generators means to maintain the temperature profiles of fumes and of the water/steam, i.e. in the same alignment and geometrical position in the steam generator, condition known in the prior art as constant temperature profile control condition, or as profile control. The flexibility of this embodiment, meant as quick load move upward or downward, with regulation systems operating at constant regulation logic, takes place also for loads lower than 30%.
(45) 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.
(46) 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.
