Abstract:
Power generation plant and process comprising: providing a steam generator; first, second and third steam turbines; a reheater; a gas turbine; and at least one heat exchanger; supplying feedwater bypassing the steam generator to the heat exchanger and heating the feedwater stream therein by supplying the at least one hot exhaust gas stream from the gas turbine to the heat exchanger; and recovering heated steam from the heat exchanger and supplying at least part of the recovered heated steam stream to the second steam turbine to generate power in the second steam turbine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57. 
       BACKGROUND OF THE INVENTION 
       [0002]    Steam turbine power generation plants are one of the oldest power generators and supply most of the world&#39;s power. 
         [0003]    One conventional steam turbine power generation plant operates according to a process in which high pressure saturated steam from a steam generator is fed, directly or indirectly, to a high pressure wet steam turbine and is expanded and cooled therein with the associated generation of power by the turbine. Cooled and expanded steam from the turbine may be supplied to a moisture separator/reheater and then via a low pressure steam turbine to a condenser. Condensed steam from the turbine may be supplied to a de-aerator and returned, generally through a feed pump and feed heaters, to the steam generator. A plant based on such a conventional wet steam cycle is described in ‘Advances in Power Station Construction’, GD&amp;CD, Central Electricity Generating Board published by Pergammon Press 1986. 
         [0004]    Conventionally, the cooled and expanded steam supplied to the moisture separator/reheater is generally separated into two streams. A first stream comprising separated moisture may be supplied to the de-aerator in combination with condensed steam from the turbine. A second stream is reheated and supplied to a low pressure steam turbine for further power generation. Reheating of this stream in the moisture separator/reheater is effected by steam from the steam generator and/or extracted from the high pressure wet steam turbine. 
         [0005]    Steam from the low pressure steam turbine is exhausted to a condenser, from which water is pumped through one or more low pressure feed heaters before being supplied to the de-aerator and thence back to the steam generator. The low pressure feed heaters may be supplied with heating steam extracted from the low pressure turbine. 
         [0006]    Many attempts have been made to improve the efficiency of conventional steam raising plant, in particular nuclear plant, by combining into the steam cycle the exhaust power output from a gas turbine. Examples of such attempts are disclosed in Japanese Laid-open patent publication nos. 2003027906, 11344596, 10089016, 10037717 and 3151505, and in U.S. Pat. No. 5,457,721. 
         [0007]    One hybrid power generation plant disclosed in UK Patent GB 2431968A operates according to a process in which part of the steam from a conventional steam generator is superheated using the heat in the exhaust gases of a gas turbine. The superheated steam is passed directly or indirectly to be expanded in a steam turbine, thereby generating power. 
         [0008]    In this hybrid power generation plant, part of the feedwater to the steam generator is also supplied to an evaporator also heated by the gas turbine exhaust gases. The steam produced in the evaporator is mixed with the steam from the steam generator. Conventionally the exhaust gases of the gas turbine are directed to first heat the steam from the steam generator and then to evaporate additional feedwater in the second heat exchanger. 
         [0009]    Improvements to this power generation plant have been sought to enable the process to maximize steam flow to the steam turbine when little or no steam is available from the steam generator. Such an improvement would allow the power plant to deliver its full capacity when the steam generator was being maintained or when maximum production of power was needed to respond to the demands on the electricity network. 
         [0010]    There is a current and growing need for efficient power generation in many areas of the world to meet energy demands while reducing carbon emissions. The production of low carbon power by many renewable technologies varies over a wide range of timescales according to weather, season or time of day. Power generation from other sources needs to balance this variation while meeting the daily, weekly and seasonal patterns of demand from consumers. There is thus an increasing need for efficient power generation technologies that can deliver different levels of output flexibly when required to balance power demands while minimizing carbon emissions. 
         [0011]    Further, there remains a need to provide an improved process and apparatus for power generation which improves energy efficiency and therefore lowers cost and damage to the environment in relation to conventional power plants. In particular, the use of a combined cycle power plant in conjunction with a nuclear power plant using either the pressurized or boiling water cycles offers opportunities for efficiency improvement. 
         [0012]    However, it has proved difficult in practice to realize such improvements, for example because of the restrictions imposed by nuclear safety requirements and the limitations of electrical transmission network operation. Nuclear safety requirements generally mean that external disturbances to steam flows in the steam generators should be minimized or avoided. The electrical transmission network limitations mean that single breakdowns should not result in losses of generation above a defined maximum value. These restrictions limit acceptable configurations of the combined gas turbine and nuclear steam cycles. In one of its aspects, the present invention comprises a plant configuration that offers the desired high levels of efficiency within these limitations. 
       SUMMARY OF THE INVENTION 
       [0013]    According to one embodiment of the present invention, there is provided a process for power generation comprising: providing a steam generator, first, second and third steam turbines, a reheater, a gas turbine, at least one heat exchanger and a combustion means for burning fuel in hot gas, the process having plural modes of operation. 
         [0014]    In various embodiments, the present invention provides process and plant for power generation comprising: providing a steam generator; first, second and third steam turbines; a reheater; a gas turbine; and at least one heat exchanger; supplying a first stream comprising steam from the steam generator to the first steam turbine to generate power in the first steam turbine; recovering from the first steam turbine a recovered stream comprising steam and supplying at least a part of the recovered stream to the reheater; supplying a second stream comprising steam from the steam generator to a first zone of the heat exchanger and heating the second stream therein by supplying at least one hot exhaust gas from the gas turbine to the first zone of the heat exchanger; supplying the heated second stream to the second steam turbine to generate power therein; supplying a third stream comprising steam from the steam generator to the reheater to heat the recovered stream from the first steam turbine; recovering from the reheater a heated recovered stream from the first turbine; and supplying at least part of the heated recovered stream from the first turbine to the third steam turbine to generate power therein, wherein an exhaust gas can be obtained from the heat exchanger and an additional source of fuel combusted with the exhaust gas before it is supplied back to the heat exchanger. 
         [0015]    In an embodiment, the first mode of operation is one comprising: supplying a first stream of feedwater to the steam generator and generating a steam output therefrom; supplying a first stream comprising steam from the steam generator to the first steam turbine to generate power in the first steam turbine; recovering from the first steam turbine a recovered stream comprising steam and supplying at least a part of the recovered stream to the reheater; supplying a second stream comprising steam from the steam generator to the heat exchanger and heating the second stream therein by supplying at least one hot exhaust gas from the gas turbine to the heat exchanger; supplying the heated second stream to the second steam turbine to generate power therein; supplying a third stream comprising steam from the steam generator to the reheater to heat the recovered stream from the first steam turbine; recovering from the reheater a heated recovered stream from the first turbine; and supplying at least part of the heated recovered stream from the first turbine to the third steam turbine to generate power therein. 
         [0016]    In an embodiment, the second mode of operation is one comprising: supplying a first stream of feedwater to the steam generator and generating a stream of steam therefrom; supplying a first stream comprising steam from the steam generator to the first steam turbine to generate power in the first steam turbine; recovering from the first steam turbine a recovered stream comprising steam and supplying at least part of the recovered stream to the reheater; supplying a second stream comprising steam from the steam generator to the reheater to heat the recovered steam from the first steam turbine; recovering from the reheater a heated recovered stream from the first steam turbine; and supplying at least part of the heated recovered stream from the first steam turbine to the third steam turbine to generate power in the third steam turbine. 
         [0017]    In an embodiment, the third mode of operation is one comprising: supplying feedwater bypassing the steam generator to the heat exchanger and heating the feedwater stream therein by supplying at least one hot exhaust gas from the gas turbine to the heat exchanger; and recovering heated steam from the heat exchanger and supplying at least part of the recovered heated steam stream to the second steam turbine to generate power in the second steam turbine; 
         [0018]    In an embodiment, the fourth mode of operation is comprising: at least one of two additional modes of operation corresponding to the first and third modes of operation respectively and comprising the additional steps of recovering an exhaust gas stream from the heat exchanger and combusting an additional fuel to reheat the exhaust gas stream before it is supplied back to the heat exchanger. 
         [0019]    At least one embodiment, the present invention therefore provides a more efficient process for generating power, which makes use of any energy that would otherwise be lost in the exhaust gas from the heat exchanger. The heat exchangers used in one embodiment of the present invention are not 100% efficient and so there will be some heat remaining in the exhaust gas. Conventionally, this would be lost as the exhaust gas would be released after it has passed through the heat exchanger, as the temperature of the gas would be too low for it to be used to generate steam. 
         [0020]    However, in at least one embodiment, the present invention utilizes an additional fuel source to raise the temperature of the waste gas, thereby allowing it to be supplied back to the heat exchanger to generate steam, which can then be used to power the steam turbines. The combustion of the additional fuel allows a much higher proportion of the energy in the exhaust gas to be used than is possible in hybrid systems of the prior art. This therefore means that the generation capacity when the steam generator is not available is much increased compared to hybrid plants of the prior art. 
         [0021]    In one embodiment, a preferred process in accordance with the invention the heat exchanger has plural zones, including at least a first zone and a second zone. The plural zones of the heat exchanger may be separate and may comprise separate heat exchangers. 
         [0022]    In this situation, the separate heat exchanger zones may have different roles within the arrangement of at least one embodiment of the present invention. This allows the heat exchanger zones to operate under different conditions and with different sources of energy. It also means that the heat exchanger zones can communicate with each other in a manner that would not be possible if they were interconnected. 
         [0023]    For example, at least one exhaust gas stream may be recovered from the first zone of the heat exchanger and then supplied to the second zone of the heat exchanger. This means that the exhaust stream from the first zone may be used to at least partially supply the energy required to generate steam in the second zone. This therefore reduces the amount of energy that is wasted within the system and thereby improves efficiency. 
         [0024]    If the energy in the exhaust stream of the first zone is not sufficient to supply the energy required to generate steam in the second zone, the additional fuel may be combusted to reheat the exhaust gas before it is supplied to the second zone of the heat exchanger. This will then increase the temperature of the exhaust stream to a level sufficient to generate steam in the second zone. 
         [0025]    The first mode of operation may comprise supplying the second stream comprising steam from the steam generator to a first zone of a heat exchanger, wherein the second stream is heated therein by supplying at least one hot exhaust gas from the gas turbine to the first zone of the heat exchanger. 
         [0026]    In an embodiment, the first mode of the present invention may further comprise the steps of: supplying a second stream of feedwater to the second zone of the heat exchanger; generating a stream comprising steam; and mixing the steam from the second zone of the heat exchanger with the second stream of steam from the steam generator. 
         [0027]    This therefore allows separate functioning of the first and the second zones of the heat exchanger, so that the two can perform different roles in the arrangement of at least one embodiment of the present invention. Specifically, the first zone of the heat exchanger heats the steam required to power the second steam turbine while the second zone creates steam to supplement that provided by the steam generator. 
         [0028]    The features of the steam required for these different functions may also differ and so the two heat exchanger zones should be able to function under different conditions. Additionally, this allows the exhaust gas from the first zone to be used in the second zone, with or without the additional fuel being combusted, as different levels of energy may be required. 
         [0029]    The stream comprising steam created in the second zone of the heat exchanger in the first mode of operation may further comprise water. This stream may therefore be supplied to a separator before the steam in the stream is mixed with the second stream of steam from the steam generator and the water produced in the separator may be recirculated to the second zone or be supplied to the steam generator as at least part of the feedwater supplied thereto. This further improves the efficiency of the arrangement of at least one embodiment of the present invention. 
         [0030]    In the third mode of operation of the plant, the feedwater bypassing the steam generator may be supplied to the second zone of the heat exchanger in which it is heated and at least partially evaporated before the steam stream therefrom is supplied to the first zone of the heat exchanger. This enables the system to generate power in the second steam turbine independently of the operation of the steam generator. 
         [0031]    The first zone of the heat exchanger in the third mode of operation may be heated using at least one hot exhaust gas from the gas turbine. This is a convenient source of energy for the generation of steam. This hot exhaust gas may then be passed to the second heat exchanger, which may operate at a lower temperature than the first. However, if the temperature is too low to create steam, the additional fuel may be combusted. This therefore improves efficiency while allowing the temperatures in the heat exchanger zones to be controlled. 
         [0032]    The third mode of operation may comprise: supplying the at least partially evaporated heated feedwater stream from the second zone of the heat exchanger to a separator; and recovering from the separator a steam stream and supplying said steam stream to the first zone of the heat exchanger. 
         [0033]    As discussed above, the use of a separator may further improve the efficiency of the process. 
         [0034]    Preferably in said first mode of operation of the plant, the second stream from the steam generator is supplied to the first zone of the heat exchanger at a temperature and pressure not substantially below that of the second stream as it is recovered from the steam generator. For example, the pressure of the second stream as it is supplied to the first zone of the heat exchanger is not more than about 15%, preferably not more than about 10%, most preferably not more than about 5% below the pressure of the second stream as it exits the steam generator. 
         [0035]    The output streams from the second and third steam turbines are preferably supplied, in whole or in part to one or more condensers. In one preferred process according to an embodiment of the invention, at least part of the output stream from the second steam turbine condenser is supplied to a third zone of the heat exchanger and heated therein by supplying at least one hot exhaust gas from the gas turbine to the third zone of the heat exchanger. The heated recovered condensate may then be returned to a de-aerator heated with steam extracted from between stages of the second turbine. The part of the output stream from the condenser supplied to the third zone of the heat exchanger may also be passed through one or more low pressure feed heaters. 
         [0036]    The heat exchanger is preferably arranged so that the at least one hot exhaust gas is passed against at least one first heat transfer surface in the first zone of the heat exchanger to heat second stream from the steam generator, so that the at least one hot exhaust gas is passed against at least one second heat transfer surface in the second zone of the heat exchanger to heat the auxiliary heating stream for the reheater, and so that the at least one hot exhaust gas is passed against at least one third heat transfer surface in the third zone of the heat exchanger to heat the recovered condensate stream from the condenser, or part of it. Preferably, the at least one hot exhaust gas from the gas turbine is passed sequentially against the at least one first heat transfer surface, the at least one second heat transfer surface and the at least one third heat transfer surface, becoming progressively cooler from the first to the third zones of the heat exchanger. The thus cooled at least one hot exhaust gas may then be discharged from the plant by any suitable means, such as by means of a stack. 
         [0037]    In one process according to an embodiment of the invention, the first steam turbine is a wet steam turbine and the steam in the first stream from the steam generator is supplied at or at close to a saturated condition. The first steam turbine preferably operates under a high pressure condition, by which is meant by way of example only that the pressure of steam supplied thereto is at least about 40 bar abs. The third steam turbine preferably operates under a relatively low pressure condition, by which is meant by way of example only that the pressure of steam supplied thereto is less than about 10 bar abs. Preferably the second steam turbine is operable at a pressure intermediate between that of the first and third steam turbines, more preferably at a pressure as close as possible to that of the first steam turbine. 
         [0038]    The flow ratio of the stream supplied to the second steam turbine to the first steam stream from the steam generator may be between about 0.05 to about 0.5, preferably between about 0.05 to about 0.2. 
         [0039]    In one preferred process according to an embodiment of the invention, the total enthalpy of the at least one hot exhaust gas stream supplied from the gas turbine is from about 0.05 to about 0.35, preferably from about 0.05 to about 0.25, most preferably from about 0.07 to about 0.15, of the net enthalpy of materials recovered from the steam generator (that is the enthalpy of the first steam stream supplied from the steam generator minus the enthalpy of feedwater stream). 
         [0040]    The ratio of maximum energy added in additional fuel to energy in exhaust gases of gas turbine may be between 50 to 120%, preferably between 60 and 110% and more preferably between 80 and 100%. 
         [0041]    Preferably the reheater also functions as a moisture separator. Wet steam exhausted from the first steam turbine is passed to the moisture separator/reheater which removes moisture droplets which are returned directly or indirectly as feedwater for the steam generator. The recovered moisture stream may be supplied to the de-aerator separately or together with the part of the recovered stream from the first steam turbine. As with the first steam turbine recovered stream part, the moisture supplied to the de-aerator may be passed to the steam generator via a feed pump and at least one, optionally high pressure, feed heater. 
         [0042]    Preferably the water from the de-aerator is supplied to a feedwater pump which pressurizes it and applies the stream to at least one high pressure feedwater heater. The recovered heated stream from the at least one feedwater heater is supplied to the steam generator. The at least one feedwater heater may be supplied with steam extracted from the first steam turbine to heat the feedwater. 
         [0043]    In another preferred first mode of operation the process comprises: providing feedwater to the second zone of the heat exchanger, the feedwater stream being heated in the second zone of the heat exchanger by the at least one hot exhaust gas; recovering a heated feedwater stream from the second zone of the heat exchanger and supplying the recovered heated feedwater stream to a separator; recovering from the separator the heated feedwater stream and supplying the recovered stream to the steam generator as at least part of the feedwater supplied thereto. 
         [0044]    Conveniently, in said second mode of operation of the plant, the second stream from the steam generator is supplied directly to the heat exchanger, by which is meant in particular that it is not first supplied as input to any steam turbine or heat exchanger. 
         [0045]    Preferably the first steam stream is supplied from the steam generator at a pressure of from about 40 to about 80 bar abs. 
         [0046]    Preferably the temperature and pressure of the second steam stream are substantially the same as the first steam stream. 
         [0047]    Preferably the temperature and pressure of the third steam stream are substantially the same as the first steam stream also. 
         [0048]    Preferably, the first stream comprises the majority of the steam generator output, for example at least about 55% thereof, more preferably at least about 70% thereof. 
         [0049]    In another preferred process the heated recovered condensate from the third zone of the heat exchanger may be supplied to a second deaerator. In this alternative process the separate at least one feed pumps pressurize the water from the second deaerator and deliver low temperature feedwater to the second zone of the gas turbine energy recovery heat exchanger. 
         [0050]    The heat exchanger may be constructed as a single unit with multiple stages therein, or may be constructed as separate units, preferably arranged in series. 
         [0051]    The heat exchange tubes may be of any suitable material, such as the various grades and specifications of steel appropriate to the internal and external conditions and may included extended surfaces such as finning necessary for optimum heat transfer. 
         [0052]    The feedwater used to provide steam to the second steam turbine may be isolated from the feedwater used to provide steam to the first and/or third steam turbines. This may include isolating the stream that originates from the steam generator from that which passes through the heat exchanger. In this embodiment, there are therefore two separated pathways in which water and/or steam can flow. However, the two pathways are thermally connected, with heat passing between the two. 
         [0053]    In one embodiment, the first mode of operation may not require steam to pass directly from the steam generator to the heat exchanger. Instead, the transfer of steam may be indirect. The first mode of operation may further include passing the second stream comprising steam from the steam generator to a steam heated evaporator in which a flow of feedwater that is isolated from the second stream comprising steam from the steam generator is evaporated by at least partially condensing the second stream comprising steam from the steam generator. This isolated stream may be subsequently passed to the heat exchanger and then onto the second steam turbine. 
         [0054]    The first mode of operation may further include passing the condensed water recovered from the second steam turbine through one or more feedheaters in which the condensed water is heated by cooling or at least partially condensing the second stream comprising steam from the steam generator. These feedheaters act to further thermally connect the two pathways and mean that much of the heat from the second stream comprising steam from the steam generator is transferred to the second pathway, which generates power using the second turbine. 
         [0055]    There may be a feedheater directly after the condensed water is recovered. In one embodiment, the condensed water stream is split, with one stream going to the heat exchanger and another going to the feedheater. Preferably, if there are multiple heat exchangers, the condensed water is passed to the second heat exchanger. The two condensed water streams may then subsequently be combined. 
         [0056]    Additionally or alternatively, a feedheater may be present after the condensed water has been passed through the heat exchanger. If there are multiple heat exchangers, the feedwater may pass from the second heat exchanger, to this feedheater, before being passed back to the first heat exchanger. 
         [0057]    According to a second aspect of an embodiment of the present invention, there is provided a power generation plant configured to operate the process described herein. 
         [0058]    In various embodiments, the process of the invention when exemplified in a preferred process in accordance with embodiments of the invention has the following significant advantages: 
         [0059]    It significantly improves the thermal efficiency of the gas turbine and the saturated steam cycles integrated in the hybrid cycle. The net thermal efficiency of the hybrid cycle may for example be about 39%-42% compared with the base saturated steam cycle at 33%. 
         [0060]    When the improvements are attributed to the addition of the gas turbine cycle, the efficiency of gas to additional power compared with the original saturated steam cycle is substantially higher than can be realized by other means, permitting 60-65% net conversion efficiency to be achieved. 
         [0061]    The specific capital cost of the additional capacity of the hybrid plant is comparable with that for a combined cycle gas turbine rather than a conventional power plant or nuclear plant. 
         [0062]    The specific operating and maintenance costs for the cycle are lower than for a comparable combined cycle gas turbine plant as the net capacity is significantly increased for the same gas turbine maintenance costs. 
         [0063]    The higher fuel conversion efficiency and lower specific capital and operations and maintenance costs of the generating capacity enables power to be generated from gas at a significantly lower cost than any available alternative technologies, typically offering output at about 90% of the cost of a conventional combined cycle gas turbine plant with the same cost of fuel. 
         [0064]    Configuration of the integrated steam cycle minimizes the impact of disturbances in the gas turbine cycle, such as gas turbine shutdowns, on the saturated steam plant and enables the saturated steam generator to continue to function normally despite such disturbances. The small effects on the steam generator mean that safety issues related to any nuclear primary circulation through the steam generator are minimized. 
         [0065]    The peak power available from the gas turbine and second steam turbine by the use of additional fuel to reheat the exhaust gases can be delivered independently of operation of the steam generator. 
         [0066]    While outstanding efficiency of the gas turbine cycle is achieved in integrated operation with the steam generator, operation of the gas turbine cycle is maintained at reasonably high efficiency while the steam generator is out of service. Thus the high availability of the gas turbine cycle contributes to revenues while the steam generator is shutdown, e.g. for nuclear plant refueling. 
         [0067]    Start-up and shutdown of either the (nuclear) steam generator or the gas turbine can be accomplished flexibly, simply and with minimum mutual interference, maintaining safety provisions for the nuclear steam system while permitting flexible dispatch of the gas turbine cycle capacity. 
         [0068]    The additional capacity from the gas turbine cycle can be dispatched flexibly according to power demand without significantly affecting the saturated steam plant. 
         [0069]    Breakdown of either the gas turbine plant or the heat supply to the steam generator do not result in a total loss of generated output. The breakdown cases have a predictable loss of output to the electrical transmission network which are the same as the values for the gas turbine, energy recovery heat exchanger and second steam turbine or conventional nuclear plant independently. 
         [0070]    The improved efficiency of fuel conversion results in environmental benefits including reductions of around about 10% of emissions per unit of energy delivered of carbon, sulfur and nitrogen oxides and lower thermal discharges to the environment compared with the best available fossil fueled plant. The additional lower cost generating capacity will displace older more expensive plant with higher emissions, further reducing the overall discharges to the environment. 
         [0071]    The concept can be applied to new power plant or to existing saturated steam cycle plant with similar benefits. 
         [0072]    The design of the heat exchanger zones and the separator are conventional for energy recovery heat exchangers in combined cycle gas turbine power plant so that costs of construction are minimized. 
         [0073]    The enhanced robustness of the gas turbine cycle operation on shutdown of the (nuclear) steam generator increases the integrity of power generation available to support safe reactor operation during the critical shutdown period. 
         [0074]    Transitions of conditions in the heat exchanger are smooth and self-regulating so that operation is simplified and cycle behavior is tolerant of changes in steam cycle or gas turbine conditions. 
         [0075]    The construction of the gas turbine and nuclear plants can be undertaken at different times while permitting operation at up to full capacity but at reduced efficiency prior to completion of the hybrid cycle. 
         [0076]    The design of the steam and water cycle associated with the gas turbine can be designed for maximum independence from the nuclear steam cycle so that interfaces for a retrofit can be minimized and any potential safety case impacts reduced to the lowest possible level. 
         [0077]    The process can deliver a significantly higher fuel efficiency than a conventional combined cycle using the same gas turbine, offering reduced carbon and other gaseous emissions per unit energy production. 
         [0078]    The additional power available to meet demand by the combustion of supplementary fuel is a much larger proportion of gas turbine power than can be achieved by a conventional high efficiency combined cycle plant. 
         [0079]    The specific cost of capacity for the improved process is significantly lower than for the disclosed process in UK Patent GB 2431968A and is lower than or similar to the specific costs applicable to conventional combined cycle power plant. 
         [0080]    The generation capacity of the plant when the steam generator is not available is much increased compared with that offered by a hybrid plant as disclosed in UK Patent GB 2431968A. 
         [0081]    When the steam generator supplies steam at a substantially constant pressure and temperature, the hybrid combined cycle plant can change output upwards or downwards at a rate comparable with gas turbine alone, which is typically an order of magnitude faster than conventional combined cycle power plant is able to offer, in both cases within the permissible rates of change of conditions for the plant components without cyclic life reduction; 
         [0082]    The additional flexibility of the improved process increases the range of roles a plant embodying the process can fulfil within a power system, offering advantages for the system operator and improved opportunities for plant owner to raise revenues for providing additional services to the power system; 
         [0083]    The additional flexibility of the improved process would enable an electricity system to which a plant embodying the process was connected to include a higher proportion of intermittent renewable generation than would be feasible with a conventional combined cycle power plant, thereby reducing carbon dioxide and other gaseous emissions more significantly that for the power plant alone or if a combined cycle plant was used for this duty. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0084]    Embodiments of the invention will now be more particularly described with reference to the following drawings, in which: 
           [0085]      FIG. 1  shows a flow diagram of a hybrid power generation plant arranged to operate according to an embodiment of a first process of the invention; 
           [0086]      FIG. 2  shows a flow diagram of a hybrid power generation plant arranged to operate according to an embodiment of a first process of the invention, which corresponds to the data in Table 1; and 
           [0087]      FIG. 3  shows a flow diagram of a hybrid power generation plant arranged to operate according to an embodiment of a second process of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0088]    Referring to  FIG. 1 , gas turbine  150  produces hot exhaust gases which are passed in line  151  through energy recovery heat exchanger  152 . The exhaust flows in turn over heat transfer surfaces with their outlet passes at the side facing the incident hot gases to maintain a near constant temperature difference between the external hot gases and the internal process fluid. The first zone of the heat transfer surface, in contact with the hottest gases, is steam superheater  153 . On leaving the first zone the exhaust gases may be reheated by combustion of a fuel in the burners  157  before flowing through the second zone of the energy recovery heat exchanger  154 . 
         [0089]    The intermediate section of the heat exchanger  154  acts as an evaporator through which water is circulated by pumps (not shown) or by natural convection from separator  135  via lines  133  and  134 . The final heat exchange surface is economizer section  155  which is in two parts. The first heats water directly from condenser  127  of steam turbine  126 , or after passage through one or more feedheaters (not shown), delivering it to deaerator  136 . 
         [0090]    Dissolved gases are removed by vigorous direct contact heating of water droplets by steam in the deaerator  136 . Steam extracted from between stages of steam turbine  126  or other steam source (not shown) is used to heat deaerator  136 . 
         [0091]    One or more pumps  137  delivers feedwater from deaerator  136  at high pressure to the second part of economizer  155  which heats water to flow in part to separator  135  via line  138  with the balance via one or more pumps  136  (optional, according to design) and line  160  to mix with feedwater in line  124  to the steam generator  100 . 
         [0092]    The cool exhaust gases from the energy recovery heat exchanger are finally discharged via stack  156 . 
         [0093]    The steam flows in the cycle are integrated with the conventional steam turbine cycle for steam at close to saturated conditions as follows. Steam at near saturated conditions from steam generator  100  is supplied in line  101  and is divided into three, with a large part flowing in line  102  to wet steam turbine  103 , another part passing in line  125  to mix with steam from separator  135  in line  139  to superheater  153  for heating, while the balance flows in line  108  to moisture separator and reheater  105 . 
         [0094]    The steam heated in superheater  153  in gas turbine heat energy recovery exchanger  152  is delivered at high temperature to secondary steam turbine  126  which exhausts into condenser  127  via line  128 . Condensed water is recovered via line  130  and pump  131  with part flowing via line  132  to mix with the condenser flow from the low pressure steam turbine  111  in line  116 . The balance of the condensed steam flow from pump  131  is delivered to the cold end of the economizer  155 . 
         [0095]    Steam flow through steam turbine  126  is set by inlet valve  129  and is preferably controlled to maintain a constant steam temperature at the outlet of superheater  153 . 
         [0096]    Steam at near saturated conditions flows though the high pressure turbine  103  which exhausts wet steam in line  104  to moisture separator/reheater  105 . Moisture separator  105  removes most of the entrained water droplets, draining them in line  107  to deaerator  106  (via a link not shown in  FIG. 1 ), and the steam flow remaining in moisture separator/reheater  105  is reheated. The steam flow through reheater  105  is heated by saturated steam in line  108  from steam generator  100  and/or with bled steam (not shown) from high pressure turbine  103 . The heating steam is condensed in the reheater and the condensed water is returned to the condensate of system high pressure feedheaters  109  via a link not shown in  FIG. 1 . 
         [0097]    The steam entering the reheater flows in turn over heat transfer surfaces with their outlet passes at the side receiving the highest temperature fluid from the heat exchanger to maintain a near constant temperature difference between the external steam and internal process fluid. 
         [0098]    Reheated steam from moisture separator/reheater  105  is recovered in line  110  and expanded through low pressure steam turbine  111 . The steam from turbine  111  passes in line  112  to condenser  113  and the condensed water is recovered in line  114  and pumped by pump  115  through one or more low pressure feedheaters  117  to deaerator  106 . Steam extracted from between stages of the steam turbine is used to supply heat to the feedheaters. The water condensed in the feedheaters is cascaded (not shown) to a feedheater at lower temperature or discharged into condenser  113 . 
         [0099]    Dissolved gases are removed by vigorous direct contact heating of water droplets by steam in the deaerator  106 . The heating steam for deaerator  106  is taken either from the exhaust or from between stages of high pressure steam turbine  103 . The water from deaerator  106  is pumped to high pressure by one or more feed pumps  121  and further heated by one or more high pressure feedheaters  109  to a temperature suitable for return to the steam generator  100  in line  124 . 
         [0100]    The high pressure feedheaters are heated with steam extracted from between stages of the steam turbine  103  and with hot water from the condensed heating steam flows to reheater  105 . The steam condensed in the feedheaters and the water flows are cascaded (not shown) to a feedheater at lower pressure and/or to the deaerator  106 . 
         [0101]      FIG. 2  shows a hybrid power generation plant arranged to operate in the same way as described above in connection with  FIG. 1 . The reference numerals in  FIG. 2  correspond to the points at which the data outlined in Table 1 below are obtained. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Reference 
                 With additional 
               
               
                   
                 Condition 
                 heat input 
               
             
          
           
               
                   
                 Ref. 
                 kg/s 
                 bara 
                 C. 
                 kg/s 
                 bara 
                 C. 
               
               
                   
                   
               
             
          
           
               
                 Steam generator output 
                 1 
                 1503 
                 66.3 
                 282 
                 1518 
                 66.8 
                 283 
               
               
                 Net main steam flow 
                 3 
                 1291 
                 66.1 
                 280 
                 1501 
                 66.8 
                 283 
               
               
                 NuGas ST steam 
                 4 
                 224 
                 60.5 
                 566 
                 224 
                 60.5 
                 566 
               
               
                 HRSG evaporator outlet 
                 5 
                 12 
                 65.1 
                 281 
                 206 
                 65.1 
                 281 
               
               
                 Evaporator feed 
                 6 
                 12 
                 65.1 
                 278 
                 206 
                 65.1 
                 256 
               
               
                 Economiser outlet 
                 7 
                 112 
                 65.1 
                 278 
                 214 
                 65.1 
                 256 
               
               
                 Economiser inlet 
                 8 
                 103 
                 3.5 
                 30 
                 184 
                 2.9 
                 29 
               
               
                 NuGas ST exhaust 
                 9 
                 222 
                 0.04 
                 30 
                 202 
                 0.04 
                 29 
               
               
                 Nuclear ST exhaust 
                 10 
                 627 
                 0.04 
                 27 
                 758 
                 0.04 
                 30 
               
               
                 Steam generator feedwater 
                 11 
                 1503 
                 73 
                 222 
                 1519 
                 73 
                 226 
               
               
                 Nuclear DA outlet 
                 12 
                 1437 
                 4.7 
                 152 
                 1546 
                 5.6 
                 158 
               
               
                 Nuclear DA inlet condensate 
                 13 
                 934 
                 7.0 
                 121 
                 978 
                 7.9 
                 127 
               
               
                 Nuclear ST condensate 
                 14 
                 864 
                 14 
                 27 
                 903 
                 15 
                 30 
               
               
                 NuGas ST condensate 
                 15 
                 222 
                 3.6 
                 30 
                 202 
                 2.9 
                 29 
               
               
                 Nuclear ST MSR Outlet 
                 16 
                 787 
                 4.7 
                 274 
                 933 
                 5.5 
                 272 
               
               
                 Nuclear ST HP inlet 
                 17 
                 1222 
                 64.2 
                 280 
                 1429 
                 64.2 
                 280 
               
               
                 MSR inlet 
                 18 
                 911 
                 4.9 
                 151 
                 1077 
                 5.8 
                 157 
               
               
                 NuGas condensate to Nuclear 
                 19 
                 120 
                 14 
                 30 
                 18 
                 15 
                 30 
               
               
                 GT exhaust 
                 20 
                 537 
                 1.04 
                 610 
                 537 
                 1.04 
                 610 
               
               
                 HRSG stack 
                 21 
                 537 
                 1.02 
                 88 
                 549 
                 1.02 
                 88 
               
               
                 MSR heating steam 
                 22 
                 69 
                 64.2 
                 280 
                 72 
                 64.2 
                 280 
               
               
                 HRSG superheater inlet 
                 29 
                 224 
                 65.1 
                 281 
                 224 
                 65.1 
                 281 
               
               
                 Nuclear to NuGas transfer steam 
                 31 
                 212 
                 65.5 
                 281 
                 18 
                 66.8 
                 283 
               
               
                 HRSG firing gas 
                 32 
                 0 
                 — 
                 — 
                 7.2 
                 2 
                 25 
               
               
                 Generator Outputs (MW) 
                 A 
                 808 
                   
                   
                 932 
               
               
                   
                 B 
                 227 
                   
                   
                 227 
               
               
                   
                 C 
                 291.5 
                   
                   
                 283 
               
               
                   
               
             
          
         
       
     
         [0102]    Referring to  FIG. 3 , the cycle applied to the plant follows the process and references as described for  FIG. 1  except as follows: 
         [0103]    The second stream from steam generator  500  is carried by line  525  to steam heated evaporator  560 . Secondary steam is generated by evaporating the feedwater delivered in line  561  by condensing the incoming flow from line  525 . The secondary steam is supplied by line  562  to the superheater  553  for heating and is delivered at high temperature to steam turbine  526  which exhausts into condenser  527 . Condensed water is recovered via line  530  and delivered by pump  531  as a stream which is divided into two parallel streams for heating and delivery to second deaerator  564 . The first stream is delivered to feedheater  563  while the second part is delivered to the first section of economizer  555  in energy recovery heat exchanger  552 . The heated recovered streams from the feedheater and economizer are mixed and delivered to the second deaerator  564  in line  565 . 
         [0104]    The second deaerator  564  removes dissolved gases from the condensed water using vigorous direct contact heating with steam supplied to the deaerator from the separator  535  or steam turbine extraction (connections not shown for clarity). The resulting hot water, pumped to high pressure by the one or more feed pumps  566 , is divided into two streams. The first stream is delivered to the second section of economizer  555  of the gas turbine energy recovery heat exchanger  552 . The heated recovered stream is further split into a stream to separator  535  via line  538  and a stream in line  536  to optional pump  539  for delivery as feedwater into line  561 . The second stream delivered by the one or more feed pumps  566  is heated in the water to water feedheater  567  and recovered into line  561 . The mixed heated water flow in line  561  is delivered to the steam heated evaporator  560  to generate secondary steam. 
         [0105]    Feedwater supplied to separator  535  is circulated by convection or by pump(s) (not shown) through evaporator section  554  of the energy recovery heat exchanger  552  via lines  533  and  534 . The steam stream from separator  535  is mixed with secondary steam from steam heated evaporator  560  in line  540 . 
         [0106]    The steam flow from line  525  condensed in the steam heated evaporator is recovered via line  568  and divided into two parts. The first part is delivered by pump  569  to mix in line  524  with the heated feedwater from feedheaters  509  to be supplied to the steam generator  500 . The second part is delivered to water to water feedheater  567  where it heats part of the stream from the one or more feed pumps  566 . The recovered cooled part is delivered to feedheater  563  where it heats part of the condensate pumped from the condenser  527 . The cooled condensed stream is reduced in pressure in valve  570  and returned to the main cycle in line  516 .