Patent Application
20180171827
This application claims the benefits of the U.S. Provisional Patent Application No. 62/386,908 entitled “Method to integrate regenerative Rankine cycle into combined cycle applications using an integrated heat recovery steam generator” filed on Dec. 16, 2015. This provisional application is incorporated by reference herein in its entirety.
Combined cycle power plants have come of age due to the advances in combustion turbine technology and, most recently, due to the new natural gas recovery technology of “fracking”. Fracking has significantly increased the gas reserves of the United States and has significantly lowered the cost of gas recovery. The state of the combustion turbine technology and the availability of long term and relatively low cost natural gas has made the combined cycle the prominent choice for both future generation needs to serve new loads and to replace coal generation in the near and mid-term future.
Early applications for combustion turbines were aero derivative models which were, essentially, modified jet engines originally designed for aircraft and modified for land base use. However, the design of this type of technology, i.e. combustion turbines, gradually became specific to the needs of the electric utility industry such that by the 1970's specific combustion turbines with characteristics specifically designed to optimize performance in combined cycle operation were commercially available.
A combined cycle can be described in two parts; the “top” cycle which is the combustion turbine utilizing a Brayton cycle, and the “bottom” cycle which is the steam Rankine cycle. Shaft power is initially generated through the use of combustion turbines; the turbine section of the combustion turbine used for land base power generation is designed such that there is no thrust as all developed power is recovered in the shaft; however, there is still significantly high exhaust temperature which, in standalone applications, is wasted. The “bottom” cycle of a combined cycle is a Rankine cycle which uses the waste heat from the combustion turbine. The turbines used in combine cycle applications have not been designed necessarily to be the most efficient in a standalone configuration, but rather to be the most efficient when used in tandem with a bottoming Rankine cycle. Typically, these types of combustion turbines normally have a low pressure ratio which results in a high exhaust temperature. The high exhaust temperature is beneficial to the Rankine cycle which, in tandem use with the combustion turbine, can produce combined overall efficiencies in the 60% range.
Increasing the efficiency of a combustion turbine typically requires higher firing temperatures at the turbine inlet and higher pressure ratio to use the higher thermodynamic availability resulting from the higher firing temperature. However, while increasing the firing temperature without a commensurate increase in the pressure ratio may minimally increase the efficiency of the turbine, the higher exhaust temperature resulting from the higher firing temperature can significantly increase the efficiency of the Rankine cycle. In accordance with the second law of thermodynamics, the efficiency of any heat cycle can be expressed as:
Efficiency=1−(TL/TH)
Where TL is the low temperature of the working fluid, i.e. the low temperature of the steam in the cycle, where heat is exhausted to the heat sink. TH is the high temperature of the working fluid, in our case, steam, and is the point where expansion of the working fluid is used to produce work.
Consequently, it is always thermodynamically preferable to have the working fluid to be expanded at the highest possible temperature. In order to achieve a high steam temperature, typically around 1050 F, a high exhaust temperature is required; this exhaust temperature must be higher than the operating steam temperature in order to affect heat transfer. It is also noted that for a high Rankine cycle efficiency, the steam must be expanded to the lowest possible temperature and pressure. Typically the temperature is around 115 F at about 1.5 psia or so. However, herein lays a problem for an efficient combined cycle.
By expanding and condensing the steam to a low temperature, a regenerative Rankine cycle is not possible for a conventional combined cycle configuration. In order to achieve a low stack gas temperature, low feedwater temperature must be supplied to the waste heat boiler. For example, if the feedwater is heated through regeneration to a temperature of, say, 500 F then it is impossible for the stack gas temperature to be lower than 500 F and, in fact, since a temperature difference must be maintained in order to achieve heat transfer (usually a minimum of 50 F or so), then the stack temperature must exit at around 550 F and this hot gas represents an enthalpy loss to the overall cycle. Therefore, feedwater heating, if any, can only be used sparingly in order to maintain a sufficiently low feedwater temperature in order to ensure there is no unreasonable stack loss.
To date, turbine manufacturers have concentrated on increasing firing temperatures of the combustion turbines for increased efficiencies; but high firing temperature requires enormous research and development costs as well as costly material and blade cooling methods. The novelty proposed herein goes back to the basics and proposes an alternative that increases the efficiency of the Rankine cycle not through higher operating working fluid temperatures but employing regenerative heating to increase the Rankine cycle efficiency.
Overall, the energy consumption in the United States has declined slightly over the last 5 years and much of this decline can be attributed to the overall economic decline of the past several years. However, domestic production has still increased by about 3% per year due to a decrease in the importation of electricity from Mexico and Canada. Overall, in the next ten years, electric consumption in the United States is expected to grow incrementally at about 1 to 1.5% per year. Even though this is a small number, the total installed capacity in the United States in 2010 was about 1,140 GW's. Therefore, even a 1% increase would require construction of about twenty 500 MW power plants every year. And this does not include the replacement capacity due to aging plants, and, in particular, aging coal plants.
There is a significant market driven by the aging coal plants in this country. Over the next 10-15 years, dozens of coal units will be replaced with gas-fueled combined cycle units. It is unlikely that the power plant operators will walk away from an existing power plant site which has high value infrastructure including transmission and water rights as well as a certain ease of permitting since development would occur on an already despoiled plant site. There is significant difficulty in developing a new coal plant since coal has increased in price and natural gas has decreased. In addition, the combined cycle is about 40-45% more efficient than a coal plant and the capital cost is about ⅓ the cost of a coal plant. And this price differential does not include the cost of greenhouse gas (CO2) clean up which would add considerably to the cost of coal generation.
Greenhouse gases will be a significant driver not only for renewable energy resources but also for combined cycle plants as well. Combined cycle plants emit less than 50% greenhouse gas than a similar size coal plant operating at the same capacity factor. Green house gas reduction is a significant driver for the construction of combined cycle power plants. Consequently, a low cost and highly efficient Regenerative cycle integrated in a combined cycle novelty will be received favorably in the commercial markets.
Using the concept of a combined cycle, this novelty creates a separate and designated stream of preheated feedwater mass flow rate by bifurcating the flow from the condenser hotwell (condensate/feedwater) to allow harvesting of additional enthalpy resulting from duct firing. One stream is the traditional low temperature condensate/feedwater (at condenser saturation pressure) that is fed directly to the Heat Recovery Steam Generator (HRSG) and the other is a separate preheated condensate/feedwater that is fed into common heating elements of the HRSG. This preheated condensate/feedwater is generated through steam extractions thereby creating a separate regenerated Rankine cycle within the combined cycle. This is differentiated from the traditional method of duct firing whereby the increased low temperature feedwater flow is added to the existing feedwater flow from the condenser and fed directly to the HRSG without regeneration. Consequently, in a traditional arrangement of feedwater flow to the HRSG, the feedwater flow must be kept at low temperature prior to entering the HRSG in order to ensure that the stack gas temperature does not rise.
By having the additional and differentiated feedwater mass flow rate in the cycle, which has been preheated through regeneration and delivered by a separate feed, the preheated feedwater can be further heated through common heating elements and the addition of one or more duct firing arrays in a modified design of a conventional Heat Recovery Steam Generator (HRSG). In this application, the term “common heating elements” is defined as those pipes, headers, drums and other associated heating elements and components which are co-shared with the flows of a traditional combined cycle in a non-regenerative Rankine cycle and those flows resulting from a separately generated regenerative Rankine cycle. It is important to note and differentiate the primary difference between this novelty and the previously submitted concept is that this novelty co-shares heating elements, tubes, headers and drums that are common to a traditional Heat Recovery Steam Generator (HRSG). In other words, the heated feedwater flow that has been pre-heated through regeneration is continued to be heated in tubes, heating elements and flows through headers and drums that are also utilized in the production of steam common to a non-regenerated steam cycle. In this manner, less exotic tube material that is less expensive is used and the design is also simplified. In the previous design, which incorporated direct duct firing on separate heating elements, high temperature dictated a costly design and expensive tube material.
In addition, another strong advantage of this novelty's design over the predecessor is the reduction of cooling flow required. In this novelty's design, where common flow is shared in the same tubes, headers, and heating elements, cooling is reduced to near zero. The previous design required cooling flows during those periods when no pre-heated feedwater, heated by extraction steam, was available. Since the previous design had separate and flow dedicated heating elements in the exhaust gas flow prior to the HRSG, these forward located heating elements, located immediately downstream from the duct firing arrays, had to be cooled. Consequently, the cooling requirement resulted in a heat penalty attributed due to the overall system heat rate.
The additional heat added to the separate preheated feedwater mass flow results in the production of additional main and reheat steam flow produced by the common heating elements used in combination with one or more duct firing arrays integrated with the conventional combined cycle production of main and reheat steam for generation of energy in a Rankine non-regenerative system. The main and reheat steam produced by the preheated regenerative feedwater mass flow rate is thermodynamically compatible with the main and reheat steam produced by the conventional combined cycle and these steam flows are combined prior to steam turbine entry.
In this manner, the steam turbine serves as the primary mover for both the steam extraction regenerative Rankine cycle resulting from the additional heat that is added and the non-regenerative or straight through non-regenerative Rankine cycle resulting from the traditional combined cycle. This additional heating of the regenerated preheated feedwater is through an integrated design of the HRSG which allows heating of both the non-preheated condensate/feedwater and the separately fed preheated condensate/feedwater. The co-sharing of flows, one flow generated by extraction steam and the other flow generated by the once through cycle of the HRSG, allows for a simple and cost effective design of the HRSG. The additional heating required due to the additional flow produced by the heated feedwater, is performed with one more duct firing arrays within the HRSG such that there is no or minimal increase in the stack temperature. By firing the added duct burner array, there is additional enthalpy provided to the feedwater, main steam and reheat steam that is the result of the added regenerative cycle.
The integrated design of the HRSG that is capable of heating both the preheated and non-preheated condensate will require larger piping diameters in order to optimize the overall heat absorption in the HRSG. The co-sharing of the tubes is only necessary for the evaporator, superheating and reheating portion of the HRSG. However, some economizer heating, feedwater preheating and low pressure steam generation may be required since additional enthalpy supplied by the duct firing must also be absorbed in the low temperature end of the HRSG. The amount of enthalpy that must be absorbed to preclude a rise in stack temperature is dependent on the overall HRSG design and at what temperature the pre-heated feedwater is brought to the HRSG. The additional enthalpy in the back end of the HRSG can be used for low pressure steam generation and also used for steam dearating purposes of the overall cycle.
Typically, the mass ratio of the total steam extraction flows to the main throttle steam flow is in the order of 0.35 or so to fully utilize regeneration and to pre-heat the feedwater as much as possible. This ratio assumes that the amount of main steam throttle flow is essentially the same as the condensate/feedwater flow rate as there are practical considerations regarding the amount of heat that can be transferred from the steam to the condensate/feedwater. However, in this case, since the feedwater heating only applies to that amount of additional flow attributed to duct firing, this dedicated flow of preheated feedwater could be raised close to or even to the saturation temperature of the operating pressure of the waste heat boiler. Limitations would ensue based on amount of total flow of main steam throttle flow to the dedicated feedwater flow for duct firing. In traditional regenerative cycles, regeneration is normally limited by the amount of heat that can be transferred from the main throttle steam flow; in this invention, the limitation can be the amount of heat absorbed by the feedwater stream. In any case, the heating of the independent preheated feedwater flow in an integrated designed HRSG will result in a significant gain in thermodynamic efficiency.
By switching “off” this novelty's concept, the HRSG can still be operational in a “normal mode” though a small incremental amount of duct firing may be required during normal mode operation; additional duct firing may be necessary in order to keep the regenerative portion of the heating elements “hot”. This flow would be a nominal few percent of maximum flow to eliminate possible thermal shock and to ensure that water is not transported back to the turbine via the extraction piping. In this mode of operation, the overall capacity would be reduced since there is minimal or no additional preheated feedwater being delivered to the HRSG; however, the overall efficiency would be improved since a combined cycle operation will normally have a higher efficiency when compared to a steam regenerative Rankine cycle. Alternately, when high capacity is preferred, the plant can be operated in the enhanced regenerative mode as described herein; however, the overall efficiency may be slightly lower when the steam regenerative Rankine cycle is averaged with the combined cycle efficiency. When higher capacity is required the switch can be made to this novelty of increasing capacity through the addition of regenerative Rankine generation. Depending on need and design, the added amount of generating capacity resulting from the added regenerative cycle is significantly higher than can be achieved when using traditional duct firing and merely increasing the low temperature feedwater flow into the traditional combined cycle.
The reheating elements for the production of intermediate pressure steam or hot reheat required for the regenerative steam cycle within the HRSG would also be integrated and combined with the reheating of the combined cycle steam cycle. In this manner, an integrated design of the HRSG heating elements serves both the needs of the combined cycle steam production, both main steam and reheat steam, and the regenerative Rankine steam production, both main steam and reheat steam.
Consequently, the integrated HRSG design produces main and reheat steam at the same pressure since this steam is produced by co-sharing the same tubes, headers, drums and overall heating elements as the steam produced by the once through steam cycle typical with standard combined cycle operation. Additional duct firing would be required to provide the necessary enthalpy to create steam from the preheated condensate and to increase the reheat steam temperature. This technique allows for reheating back to the original main steam temperature without impacting the stack gas temperature.
It is noted that this novelty can be applied to new installation or to existing regenerative Rankine cycle installations. In particular, coal plants that are near end of life operation could be repowered utilizing the existing steam turbine generator, feedwater train and associated piping, and equipment as well as the indigenous infrastructure such as site and transmission. In this embodiment, at least one combustion turbine with at least one HRSG could be used to incorporate the existing coal plant's equipment.
Drawing 1 is a sketch that diagrammatically shows the proposed concept. The drawing shows an inner feedwater loop, shown in dotted lines, employing feedwater heaters supplying additional feedwater flow in a designated flow path such that common heating elements and added duct firing results in a separate regenerative Rankine cycle. The duct firing shown is an added array and is not to be confused with conventional duct firing used to increase the steaming capacity of the HRSG. The novelty's proposed additional duct firing does not increase the feedwater flow rate from the condenser directly to the HRSG to produce more steam; this novelty proposes a separate loop method allows the feedwater to be preheated in a separate loop using extraction flows from the steam turbine with additional enthalpy added for steam production using a dedicated duct burner array. The novelty's added duct firing precludes the installation of a conventional duct firing array but does not impact or impede the operation of using the new array for conventional duct firing and can be used in tandem with the proposed novelty.
Drawing 2 is similar to Drawing 1 but shows the additional embodiment of reheating that would be available, if deployed, under this novelty. In this scheme, the cold reheat steam is bifurcated with the majority of steam flowing to the common and co-shared heating elements and the remaining steam flow used for regenerative heating in the first point heater.
The numbers and data shown are general approximations only in order to more fully delineate the principles of the proposed novelty and the overall flow schematic should not be construed as a final thermodynamic analysis. Referring to Drawing 1, if we assume a closed operating Rankine cycle, condenser 1 condenses the steam flow 18 from the low pressure steam turbine 12. This novelty separates that amount of condensate into two streams 2 and 3 where stream 2 is the additional mass flow rate used for regeneration and absorbs the heat from steam extractions from appropriate ports in the extraction turbine. In practice, the fraction dedicated to the regenerative portion of the condensate flow 2 from the condenser is, typically, about 40-45% of total condensate flow. However, these values can be adjusted for cycle optimization. The pre-heated feedwater 7 is shown in Drawing 1 as a dedicated feed to the co-shared heating elements 8. The amount of condensate 3 used for non-regenerative cycle operation is fed directly to the HRSG 9 for feedwater heating, evaporating and superheating and then directed to the High Pressure (HP) steam turbine 11. Condensate 2 flows through the regenerative heater #34, then through heater #25 and then completes its pre-heating through heater #16. Typically, in traditional Rankine regenerative reheat cycles that are non-critical, the first point heater (heater #1) 6 receives steam extraction from the cold reheat line; this embodiment of reheat is described further in Drawing 2. The herein embodiment description assumes that the first point heater 6 receives its extraction flow from the cold reheat line from the HP turbine 11. For simplicity, boiler feed pumps and other associated flow lines, such as feedwater drip lines, have not been shown.
The amount of reheating, and the number of feedwater heaters, is an economic evaluation whereby the cost of preheating is evaluated against the gain in efficiency; typically large coal plants use 7 or 8 heaters; if a new facility is used, an economic evaluation will determine the number of feedwater heaters used. Drawing 1 shows only three for simplicity. While this novelty permits heating close to the saturation point, it is assumed here for illustrative purposes that the pre-heated feedwater 7 is heated to approximately 500 F. Heating elements 8 provide sensible heating, evaporation and superheating required for production of main steam.
While the exhaust of the combustion turbine 13 is shown as 1160 F, the additional duct firing 14 adds heat such that the overall gas temperature is now 1540 F. The amount of heat required to evaporate and superheat the main steam and to reheat the steam from the feedwater 7 would then bring down the combustion turbine's exhaust gas temperature as the gas flow travels from the high temperature heating elements to the lower heating elements (feedwater heating and economizers). Since there would be excess heat in the lower temperature end of the HRSG due to the duct firing and heating the 500 F preheated feedwater, excess enthalpy is used for lower steam pressure generation and to preheat the steam used for dearation. In this manner, any increase in the stack temperature, as compared to the stack temperature when no regenerative steam is being produced and there is no duct firing, can be held to a minimum
Referring again to Drawing 1, the feedwater 7 is heated in the co-shared heating elements used for production of steam and reheat steam in the non-regenerative combined cycle, the feedwater stream which is now superheated steam 10 is directed to the inlet of the HP steam turbine 11 where it is mixed with the main steam produced by the CT exhaust flow in the HRSG 9 at the same pressure and enthalpy for expansion in the HP turbine 11. A separate line 10, as shown in Drawing 1 may be necessary depending on the design of the existing turbine; otherwise, the steam is fed to the turbine in a common header. It is noted that this example depicts a three pressure combined cycle and that the low pressure steam 15, and the intermediate pressure steam 16 are directed to the IP/LP steam turbine 12 as appropriate. The main steam 17, the intermediate pressure steam 16 and the low pressure steam 15 have all been generated with minimal changes to the HRSG 9. The primary design parameter proposed in this novelty is that the heating of the separated and designated regenerative feedwater 7 is performed by integrating with the heating elements required for the combined cycle although larger carrying capacity is required. These co-shared heating elements 8 and the added duct firing 14 in the duct upstream of the conventionally designed HRSG 9 where the said HRSG design is, essentially, unaltered and the stack temperature 23 remains, essentially, unchanged.
Referring to Drawing 2, the addition of a reheat section is shown in conjunction with the production of main steam produced by the previously described regenerative Rankine cycle in Drawing 1. The cold reheat working fluid 24 is a separate loop used to reheat that portion of the main steam that has been generated through a regenerative Rankine cycle. It is noted that the main steam produced by the HRSG using solely the waste heat of the CT 13 is reheated through the HRSG operation only. Drawing 2 is the same as Drawing 1 except for the addition of the specific equipment and lines required for reheating of the main steam. In Drawing 2, we follow the assumption that most non-critical Rankine cycles take the first point heater steam extraction 22 from the cold reheat line 19. The remaining fraction of the cold reheat 24 is then directed to a co-shared reheater 21 used by the traditionally designed combined cycle. The reheated steam 20 is directed to the intermediate steam line 16 and mixed with the combined cycle's production of intermediate steam and directed to the IP/LP steam turbine 12. Although a separate line is shown, the delivery of the hot reheat may also use a co-shared header, drum and other heating elements. The reheating process does not impact the stack temperature 23.
