GB2541702A

GB2541702A - Steam generation system and method

Info

Publication number: GB2541702A
Application number: GB1515193.9A
Authority: GB
Inventors: John Spalding Douglas; Bardnard Malcolm; Kumar Singh Bavani Vinod
Original assignee: Doosan Babcock Ltd
Current assignee: Altrad Babcock Ltd
Priority date: 2015-08-27
Filing date: 2015-08-27
Publication date: 2017-03-01
Also published as: GB201515193D0; WO2017033028A1; EP3341576A1

Abstract

A thermal power plant comprises a first steam generation module with a first steam superheater 7 connected to a first high pressure turbine, and a first steam reheater 11 connected to a first intermediate pressure turbine. The plant further comprises a second steam generation module 21 comprising a second superheater 24 and a second high pressure turbine HSC HP connected in a bypass line between the first superheater 7 and the first high pressure turbine. There is also a second reheater 27 and intermediate pressure turbine HSC IP in a bypass line between the first reheater 11 and the first intermediate pressure turbine. A selective closure system 1, 2 connects or disconnects the second heaters and turbines. The second steam generation module and turbines may be a Hyper Supercritical augmentation module, selectively incorporated into an Ultra Supercritical system. The Hyper Supercritical module can be isolated, allowing the Ultra Supercritical plant to continue to operate during planned or unplanned maintenance of the higher temperature components. This reduces the commercial risk associated with the relatively unproven materials required for Hyper Supercritical components.

Description

STEAM GENERATION SYSTEM AND METHOD

The invention relates to a steam generation system and method for a thermal power plant. The invention in particular relates to a steam generation system and method capable of operation with steam at temperatures in excess of 700°C and at pressures in excess of SOObar, herein referred to as “hyper-supercritical” steam conditions.

Conventional fossil fuel power plants generate power using the Rankine cycle in which the combustion process is used to heat water to produce steam that powers a turbine, generating electricity. Rankine steam cycles efficiencies have been steadily improved through the 20th century to result in lower fuel costs and lower emissions. Steam conditions have moved from subcritical (e.g. 540°C, 180bar) to Supercritical (e.g. 540°C, 250bar) to Ultra-Supercritical (e.g. 600°C, 280bar). A recognised industry goal is to further improve steam cycle efficiencies by moving to temperatures at or above 700°C. By doing so significant efficiency gains can be achieved with further reductions in fuel costs and emissions. The present application uses the term Hyper-Supercritical (HSC) to describe operational scenarios where the main steam and reheat conditions are at or above 700°C. The term advanced Ultra-Supercritical is also sometimes used.

The main technical challenge posed by such conditions is the requirement for high temperature materials. From a technical perspective the material selection for the furnace waterwalls, superheaters, reheaters and main service pipework are the key risk areas. Therefore the key to achieving Hyper-Supercritical conditions lies in the materials required for both the boiler and turbine. High grade alloy and super alloy materials based on nickel have been developed. These drives plant costs higher and further testing of newer component materials is required before the industry will be able to reduce the risks for proceeding to the full scale plant phase when based upon a conventional design. The additional challenges include a) qualification of existing materials or development of new coatings with service and component optimised properties; b) manufacturing and fabrication technologies and c) development of residual life time concepts. Lack of real time data on thermal fatigue, creep, high temperature corrosion and steam oxidation associated with these alloys have made it difficult to establish material behaviour at elevated temperatures.

As a result, capital cost considerations have to date limited practical implementation of plant capable of exploiting the thermal efficiency advantages of Hyper-Supercritical steam conditions.

In accordance with the invention in a first aspect, a steam generation system for a thermal power plant comprises: a first steam generation module comprising a furnace for the combustion of a fuel and the generation of hot gases to heat water and generate steam, including a first steam superheater disposed to supply superheated steam via a first superheated steam flow conduit to at least one first high pressure turbine and a first steam reheater disposed to supply reheated steam via a first reheated steam flow conduit to at least one first intermediate pressure turbine; a second steam generation module comprising a second hot gas volume supplied with hot gases, a bypass superheated steam flow conduit in fluid communication with the first superheated steam flow conduit disposed to supply superheated steam via a second superheater within the second hot gas volume to at least one second high pressure turbine and a return flow conduit to return steam from the at least one second high pressure turbine to the first superheated steam flow conduit downstream of the bypass superheated steam flow conduit, and a bypass reheated steam flow conduit in fluid communication with the first reheated steam flow conduit disposed to supply reheated steam via a second reheater within the second hot gas volume to at least one second intermediate pressure turbine and a return flow conduit to return steam from the at least one second intermediate pressure turbine to the first reheated steam flow conduit downstream of the bypass reheated steam flow conduit; a selective closure system for example comprising a valve arrangement selectively to close and open a flow passage to divert flow via the respective bypass and return flow conduits.

In accordance with the invention in the first aspect, the first steam generation module is in the preferred case envisaged as a conventional thermal boiler of suitable known design, including conventional steam flow conduits such as tubing systems, including furnace tubes within one or more of the furnace walls, at least one steam superheater system to raise the steam temperature to a desired main steam temperature to drive at least one first high pressure (HP) turbine or turbine set, at least one reheater to receive the exhaust steam from the at least one first high pressure turbine and reheat the steam to a desired reheat temperature for supply to at least one first intermediate pressure (IP) turbine or turbine set and other such components as are appropriate for example including at least one low pressure (LP) turbine or turbine set downstream of the IP turbine. The turbines drive one or more first generators.

The invention is distinctly characterised by the provision of a second steam generation module which may be selectively brought into operation as a supplementary steam generator by a selective opening and closing of the selective closure system and thereby a selective opening or closing of a flow passage through each of the respective bypass and return flow conduits. A selective closure system is provided to effect this. A suitable system for example comprises: a first set of closure formations such as a first set of valves comprising at least one closure formation such as at least one valve within each of the bypass superheated steam flow conduit and corresponding return flow conduit and bypass reheated steam flow conduit and corresponding return flow conduit; and additionally a second set of closure formations such as a second set of valves comprising at least one closure formation such as at least one valve within the first superheated steam flow conduit at a location downstream of the bypass superheated steam flow conduit and upstream of the corresponding return flow conduit and at least one closure formation such as at least one valve within the first reheated steam flow conduit at a location downstream of the bypass reheated steam flow conduit and upstream of its corresponding return flow conduit.

Each closure formation is selectively switchable between a first configuration such that it closes the flow conduit within which it is located and a second configuration such that it opens the flow conduit within which it is located. For example each valve is actuatable between a first state such that it closes the flow conduit within which it is located and a second state such that it opens the flow conduit within which it is located.

Thus, the selective closure system comprises a first set of closure formations such as a first set of valves acting selectively to close or open a diverting flow passage via the respective bypass and return flow conduits, and a second set of closure formations such as a second set of valves acting to open and close a flow passage directly through the first superheated steam flow conduit and the first reheated steam flow conduit.

This provides for selection of one of two modes of operation.

In a first mode of operation, the selective closure system is set to close a flow passage which would divert flow via the respective bypass and return flow conduits and to open a flow passage through the respective first flow conduits. For example in the case of the embodiment, the valves in the respective bypass and return flow conduits are closed, and the valves in the respective first flow conduits are open. In this mode of operation, superheated steam flows directly to the first HP turbine or turbine set, reheated steam flows directly to the first IP turbine or IP set, and the second steam generation module is fluidly isolated from the system and not operational. The system operates as a generally conventional boiler.

In a second mode of operation, the selective closure system is set to open a flow passage to divert flow via the respective bypass and return flow conduits and to close a direct flow passage through the respective first flow conduits. For example in the case of the embodiment, the valves in the respective bypass and return flow conduits are open, and the valves in the respective first flow conduits are closed. In this mode of operation, superheated steam passes first via the second superheater to a second HP turbine in the second module, is then returned via the return flow conduit to the first HP turbine, is then returned via the first reheater and the reheater bypass flow conduit to the second reheater to drive the second IP turbine and then returned to the first IP turbine.

In the second mode of operation the second steam generation module is also operational as a supplementary augmentation to the first steam generation module. A particularly advantageous flexibility offered by a steam generation system in accordance with the first aspect of the present invention is that rated steam conditions may be different for each of the two steam generation modules. In particular, rated main and reheat steam temperatures may be different for the first and second modules. Thus, in a particular preferred embodiment, the first steam generation module is rated for operation at a first main and/or reheat steam temperature, the second steam generation module is rated for operation at a second main and/or reheat steam temperature, and the first and second said temperatures are different. In particular, the second temperature is higher.

In a particularly preferred operational scenario, and a particularly preferred embodiment, the first steam generation module may be rated for operation at Ultra-Supercritical (USC) steam temperatures, for example in that main and/or reheat steam temperatures are rated to be in excess of 580°C and for example in excess of 600°C but not substantially so and for example no higher than 620°C, and the second steam generation module may be rated for operation at Hyper-Supercritical steam (HSC) temperatures, for example in that main and/or reheat steam temperatures are rated to be at least 700°C and for example at least 720°C.

In a most preferred case, the first and second steam generation modules are rated for operation at main steam pressures consistent with Hyper-Supercritical requirements, for example in excess of SOObar, and more preferably yet in excess of 350bar.

In this preferred case, the first steam generation module is thus operating as an Ultra-Supercritical steam generator in a temperature regime at or about 600°C, for which material and component characterisation is well developed, but at a pressure that exceeds normal USC conditions. However, the material requirement posed by this increased pressure is not found to be as significant. Accordingly, it is possible to envisage a first steam generation module operating in these high pressure Ultra-Supercritical (HP-USC) conditions which has essentially a generally conventional design, and in particular which uses essentially conventional materials, for example including known ferritic steels for most of the main service pipework, and known austenitic steels for the superheaters and reheaters.

The second steam generation module operates as a selectively usable augmentation facility which is capable of providing full Hyper-Supercritical (HSC) mode operation when the second steam generation module is brought on stream by appropriate operation of the selective closure system to open a bypass flow passage and divert flow through the second superheater and second reheater. The second superheater and second reheater are configured to raise the temperature of the steam yet further to Hyper-Supercritical conditions, for example at least at least 700°C and for example at least 720°C, respectively to drive a Hyper-Supercritical HP turbine or turbine set and a Hyper-Supercritical IP turbine or turbine set. It is necessary only to use expensive high temperature nickel alloys for the very high temperature operational components (second superheater, second reheater, HSC turbines and associated pipework). A flexible system is offered capable of operating under two different conditions by selective operation of the first module with or without augmentation by the second. In particular, in the preferred embodiment, a first system comprises a high pressure Ultra-Supercritical (HP-USC) fossil fuel fired steam plant designed to operate at USC steam temperatures for which materials and designs are well known (around 600°C for main steam and reheat) but at HSC pressure conditions (over SOObar and for example about 350bar). A flexible Hyper-Supercritical (HSC) augmentation capability is achieved by the co located and integrated second steam generation module with second superheater and reheater capability to elevate steam temperatures for main steam and reheat to HSC conditions in excess of 700°C. A solution is provided that brings the possibility of a move to HSC conditions in a manner that minimises the technical and economic challenges, in that it not only minimises capital cost by limiting nickel alloy use to the HSC module but also manages the risk of failure and the lack of critical redundancy and the effect that these considerations might have on continued operation and project viability. A flexibility of operation is offered that mitigates some of the operational risks associated with conventional HSC steam cycles. In particular, the ability to isolate the HSC augmentation module means that the HP-USC unit can operate completely independently for example if the HSC unit is shut down for repair.

The key characterising feature of the invention is the provision of a second steam generating module which is selectively operational to augment the function of the first steam generation module by appropriate operation of the selective closure system.

In the envisaged embodiments, the first steam generation module comprises a relatively conventional furnace and steam turbine-generator, in particular in the preferred embodiment a relatively conventional Ultra-Supercritical furnace and turbine generator, although optionally rated for operation at higher pressure. Other features of the furnace and steam generator system of the first steam generation module will be readily inferred by the skilled person. In particular, a typical first steam generation modular arrangement can be expected to comprise fluidly in series some or all of the following: a feedwater supply conduit; feedwater heaters; a furnace tubing system within one or more walls of the furnace by means of which hot gases generated by the combustion of a fuel within the furnace may be caused to heat the supplied feedwater and generate steam; a superheater system comprising at least one superheater to heat the steam to a desired main steam temperature; a superheated steam flow conduit; at least one high pressure turbine disposed to drive a generator; a return conduit to return exhaust HP turbine steam to the furnace; a reheater system comprising at least one reheater to heat the steam to a desired reheat temperature; a reheated steam flow conduit; at least one IP turbine driving a suitable generator; low pressure conduits to receive and convey exhaust steam from the IP turbines; at least one low pressure turbine driving a generator; a condenser to receive steam from the low pressure turbine, condense the steam and return to the feedwater supply conduit.

References herein to single turbines, generators, steam flow conduits, heaters etc will be understood to encompass embodiments including plural turbine sets, plural generators, plural parallel conduits and heaters etc. Suitable steam flow conduits comprise steam flow pipes for example.

In a first mode of operation, this generally conventional system, for example in the preferred embodiment comprising a HP-USC furnace and generator set, is operated in generally conventional manner. In the second mode of operation, it is operated in fully integrated manner with a second steam generation module, for example rated at the higher temperature, and in the preferred case at HSC steam conditions.

Elevated temperature operation in the second steam generation module, and in the preferred case HSC main and reheat steam conditions, are achieved in that a second superheater system and a second reheater system are provided within a second hot gas volume. The second hot gas volume is supplied with hot gases to heat steam in the second superheater and second reheater systems to a desired main and reheat steam temperature when the second module is in operation.

In a preferred embodiment, the second hot gas volume is supplied with hot gases generated by combustion of a fuel in the first steam generation module.

For example, the second hot gas volume comprises a flue gas recirculation duct through which a proportion of the combustion gases from the furnace volume of the first steam generation module may be diverted.

Preferably, selective dampers operate to supply hot gases to the second hot gas volume, and for example to the flue gas recirculation duct, only when the selective closure system is operated to open a flow passage to divert flow via the respective bypass and return flow conduits of the second steam generation module so that it is operational. Preferably these dampers then act to fluidly isolate the second hot gas volume from the supply of hot gas when the second steam generation module is not operational.

More completely the system comprises a first generator disposed to be driven by the first HP and IP LP turbines to generate electricity, and a second generator disposed to be driven by the second HP and IP turbines to generate electricity.

More completely the system comprises a fuel supply and a comburant gas supply to supply fuel and comburant gas to the furnace of the first steam generation module.

In accordance with the invention in a further aspect, a method of steam generation for a thermal power plant comprises: providing a first steam generation module comprising a furnace for the combustion of a fuel and the generation of hot gases to heat water and generate steam, including a first steam superheater disposed to supply superheated steam via a first superheated steam flow conduit to at least one first high pressure turbine and a first steam reheater disposed to supply reheated steam via a first reheated steam flow conduit to at least one first intermediate pressure turbine; operating the first steam generation module by combustion of fuel to cause the generation of steam; selectively switching between a first mode of operation in which steam from the first steam superheater is supplied directly to the first high pressure turbine and steam from the first steam reheater is supplied directly to the first intermediate pressure turbine and a second mode of operation in which steam from the first steam superheater is diverted upstream of the first high pressure turbine to a second steam superheater in a second steam generation module and caused to drive a second high pressure turbine and then returned to the first high pressure turbine and steam from the first steam reheater is diverted upstream of the first intermediate pressure turbine to a second steam reheater in a second steam generation module and caused to drive a second intermediate pressure turbine and then returned to the first intermediate pressure turbine.

Conveniently the second mode of operation is effected by: providing a second steam generation module comprising a second hot gas volume supplied with hot gases, a bypass superheated steam flow conduit in fluid communication with the first superheated steam flow conduit disposed to supply superheated steam via a second superheater within the second hot gas volume to at least one second high pressure turbine and a return flow conduit to return steam from the at least one second high pressure turbine to the first superheated steam flow conduit downstream of the bypass superheated steam flow conduit, and a bypass reheated steam flow conduit in fluid communication with the first reheated steam flow conduit disposed to supply reheated steam via a second reheater within the second hot gas volume to at least one second intennediate pressure turbine and a return flow conduit to return steam from the at least one second intermediate pressure turbine to the first reheated steam flow conduit downstream of the bypass reheated steam flow conduit; and a selective closure system for example comprising a valve arrangement selectively to close and open a flow passage to divert flow via the respective bypass and return flow conduits; operating the selective closure system to open a flow passage to divert flow via the respective bypass and return flow conduits.

Thus the method of the second aspect is preferably a method of operation of the system of the first aspect and preferred features will be understood by analogy and from the foregoing description of operation of the system.

In particular, the method comprises operating the first superheater to raise steam to a first main steam temperature and operating the first reheater to a raise steam to a first reheat steam temperature; and in the second mode of operation operating the second superheater to raise steam to a first main steam temperature and operating the second reheater to raise steam to a second reheat steam temperature; wherein the second main and/or reheat steam temperature is different from and in the usual case higher than the first main and/or reheat steam temperature.

Preferably, the first superheater and the first reheater are operated to raise steam to Ultra-Supercritical (USC) steam temperatures for example in excess of 580°C and for example in excess of 600°C but not substantially so and for example no higher than 620°C.

Preferably, the second superheater and the second reheater are operated to raise steam to Hyper-Supercritical steam (HSC) temperatures for example at least 700°C and for example at least 720°C.

Preferably main steam from the first superheater and main steam from the second superheater are both supplied at a pressure consistent with Hyper-Supercritical requirements, for example in excess of SOObar, and more preferably yet in excess of 350bar.

Thus in the particularly preferred operational scenario the first steam generation module is operating as a generally conventional Ultra-Supercritical steam but at a pressure that exceeds normal USC conditions. The second steam generation module operates as a selectively usable augmentation facility which is capable of providing full Hyper-Supercritical (HSC) mode operation when the second steam generation module is brought on stream.

In a first mode of operation, this generally conventional system is operated in generally conventional manner. In the second mode of operation, it is operated in fully integrated manner with HSC steam conditions.

Preferably, in the second mode of operation, the second superheater and second reheater are supplied with hot gases generated by combustion of a fuel in the first steam generation module to raise the steam temperature to the desired second main and reheat steam temperatures.

For example the second superheater and second reheater are supplied with a proportion of the combustion gases from the furnace volume of the first steam generation module via a flue gas recirculation duct.

The invention will now be described by way of example only with reference to figures 1 and 2 of the accompanying drawings, in which:

Figure 1 is a schematic representation of a possible embodiment of the invention, in which a first module comprising a high pressure Ultra-Supercritical (HP-USC) power plant is augmented by a second module operating at Hyper-Supercritical (HSC) steam conditions;

Figure 2 is a more detailed illustration of the flue gas recirculation duct by means of which the required additional heat is supplied to the HSC module of figure 1.

Referring first to figure 1, an embodiment of the invention is shown in which a first steam generation module comprising an Ultra-Supercritical fossil fuel fired steam plant designed to operate at USC steam temperatures (circa 600°C for main steam and reheat), but at HSC pressure conditions, for example at around 350bar, is provided with a co-located second steam generation module to give a Hyper-Supercritical augmentation capability.

The first steam generation module of the embodiment of the invention thus constitutes the relatively conventional DSC power plant located below and right of the broken line in figure 1, and the second steam generation module comprises the augmentation operating at HSC temperatures above and to the left of the broken line in figure 1.

The illustrated embodiment is capable of operating in USC mode of operation with the second augmentation module fluidly isolated and idle, or with the integrated HSC augmentation capability brought in stream to operate in conjunction with the HP-USC plant. A strength of this arrangement is that the USC plant itself can be of relatively conventional design, using conventional established steels. Although in the embodiment it is envisaged that the Ultra-Supercritical module will operate at a higher pressure than is typical in the prior art, compatible with the HSC module at around 350bar, it is rated to operate at USC temperatures. The temperature is the most critical engineering consideration, and the one that has mandated use of expensive nickel alloys in conventional design which seek to move to Hyper-Supercritical conditions. Accordingly, significant potential advantages accrue in that the main plant is operating at USC temperatures.

The HP-USC boiler 30 is thus of generally conventional design. Feedwater is supplied via feedwater conduits 3, 4 having been preheated in feedwater heaters 5 in generally familiar manner. Feedwater is preferably preheated to at least 300°C. The boiler 30 defines a combustion chamber for the combustion of carbonaceous fuel in a generally conventional manner.

Water is passed through an arrangement of furnace tubing within one or more of the furnace walls of the boiler in a suitable arrangement represented purely schematically in the figure, so that thermal energy from the hot combustion gases generated by burning the fuel is transferred to the feedwater to generate steam. The arrangement includes one or more superheaters 7 and reheaters 11, again represented purely schematically in the figure. Suitable designs will be familiar to the skilled person. A superheated steam conduit 8 supplies main steam at a DSC main steam temperature from the superheater to the HP turbine in the DSC steam turbine generator set which is then recirculated via the conduit 9 back to the boiler to be reheated in the reheater 11 to a DSC reheat steam temperature and supplied via the reheater conduit 12 to the IP turbine in the USC steam turbine generator set and subsequently to the LP turbine. The turbines of the USC steam turbine generator set drive the USC generator 13. LP exhaust steam passes through the condenser 15 to be recirculated to the feedwater conduit 3.

An envisaged preferred design for a first steam generation module is based on current state of the art USC thermal power plants, and is envisaged to employ a once through supercritical boiler using single reheat or double reheat turbine configurations focused on application of conventional Rankine steam cycle processes and established design concepts. Such boiler designs are established and represent known technical and commercial risk to the operator, so that a major part of the plant is based on known technology. An example is represented by the arrangement to the right and below the broken line of figure 1 but the skilled person will readily be able to match know designs to requirements.

The novelty of the design lies in the HSC augmentation, in the case of the embodiment provided a co-located ground level HSC augmentation capability, represented by the arrangement to the left and above the broken line of figure 1. A bypass superheated steam conduit 23 forms a junction with the main steam conduit 8 upstream of the HP turbine in the USC steam turbine generator set through which superheated steam may be diverted to the HSC superheater 24 to be heated further to HSC main steam conditions for example in excess of 700°C, and fed to the HP turbine of the HSC steam turbine generator set to drive the HSC steam turbine generator 29.

Steam exhausted by the HSC HP turbine returns via the return conduit 25 to a junction with the main superheated steam conduit 8 downstream of the position of the junction formed by the bypass conduit 23 but upstream of the HP turbine of the use steam turbine generator set and thereby supplied to the HP turbine of the USC steam turbine generator set.

In similar manner, a bypass reheated steam conduit forms a junction with the reheated steam conduit 26 of the HP-USC plant upstream of the IP turbine of the HSC steam turbine generator set and fluidly connects a bypass flow to a HSC reheater 27 to be heated further to HSC reheat steam conditions for example in excess of 700°C and then fed to the IP turbine of the HSC steam turbine generator set to drive the HSC steam turbine generator 29. The exhaust of the HSC IP turbine is then fed by the return conduit 28 to the IP turbine of the USC steam turbine generator set.

The provision of the two sets of control valves, respectively labelled 1 and 2, permits selective operation either in a HP-USC mode with just the HP-USC module on line or in an integrated HSC mode with HSC augmentation capability brought on line.

In HSC mode, valves 1 are open and valves 2 closed. In this mode of operation, superheated steam from the HP-USC boiler 30 is diverted first via the HSC superheater 24 and HSC HP turbine to drive the HSC generator 29, and then subsequently passed to the USC HP turbine to drive the generator 13. Similarly, reheated steam from the HP-USC boiler 30 is diverted via the HSC reheater 27 to drive the HSC IP turbine and the generator 29 before being passed to the USC IP and LP turbines.

In USC mode with valves 1 closed and valves 2 open, the HSC module is fluidly isolated, main steam from the boiler 30 flows directly to the USC HP turbine, and reheated steam from the boiler 30 flows directly to the USC IP turbine.

The HSC superheater and reheater are contained in a second hot gas chamber to be supplied with a second source of hot gas to raise the steam to the desired HSC temperatures. In the illustrated embodiment, this is provided in the form of a flue gas recirculation duct 21. This is illustrated in more detail in figure 2.

When the system is operating in HSC mode isolation dampers 31 are opened and flue gas from the HP-USC furnace 30 is used to supply thermal energy to the HSC superheater 24 and reheater 27. With the valves 1 open and the valves 2 closed in this mode, the flue gas from the HP-USC furnace is thus used to heat the diverted flow of HP-USC main and reheat steam up to HSC steam conditions. Flue gas is then injected back into the furnace, for example with the use of recirculation fan or by natural draft circulation. When the HSC capability is not in use it can be isolated by closing the dampers 31 in the recirculation duct, as well as closing valves 1 and opening valves 2 on the steam side. This would allow the plant to continue in operation in USC mode.

Although the flue gas recirculation duct represents a convenient means to supply additional heat to the HSC superheater 24 and reheater 27, the invention is not limited to such recirculation. An alternative means to supply combustion gases from the main furnace 30, or an alternative fired heater, for example comprising a coal, gas, oil or biomass fired heater and hot box, may be considered.

The HP-USC unit with HSC augmentation module concept provides a number of advantages over a more conventional approach to uprate the entire plant towards HSC capability.

The concept introduces a novel ‘flexible operation’ capability, where the HP-USC unit can operate completely independent of the HSC heater section and turbine set. This capability delivers significant commercial advantages over conventional HSC steam cycles. The isolation steam valves between HP-USC and HSC units allows the HP-USC unit continued operation should the HSC unit be shut down for example for repair. Providing shut off dampers on the flue gas recirculation duct extraction and injection points at the HP-USC boiler ensures total isolation between the two systems.

The configuration restricts the application of Ni based alloys to the highest temperature region of the plant, the HSC Augmentation facility, where co-locating the flue gas recirculation duct with the HSC turbines significantly minimises technical risk. There is also significantly reduced initial capital outlay by reducing the requirement for Ni based alloys. The main steam pipes connecting USC and HSC units may be manufactured from Fe based material.

Plant can be operated in either USC or HSC mode

The fuel system for the HSC unit in the embodiment is based on the same fuel used for HP-USC as it utilises only flue gas generated by HP-USC for HSC steam conditions.

The concept reduces commercial risk in plant project viability and operation. In project viability terms, the concept has the potential to deliver higher plant availability at the same plant efficiency levels but at significantly lower capital cost than might be the case if a conventional two pass opposed wall fired conventionally configured plant was uprated fully to HSC. As the HP-USC plant is self-contained the project can achieve financial closure in steps. The HP-USC coal plant can proceed as a state of the art USC plant with only minimal additional investment to achieve the higher pressure ratings. The HSC Augmentation flue gas recirculation duct facility can achieve financial closure separately and be retrofitted at a later date if required.

From an operational perspective the separation of the HSC Augmentation flue gas recirculation duct facility allows continued operation of the HP-USC plant in the event of planned or unplanned maintenance and offsets the commercial risk of defect management or failure of any of the HSC pressure parts or flue gas recirculation duct facility.

Increasing the pressure above SOObar allows the HP-USC and HSC units to operate at optimum efficiency. Also increasing the feed water above 300°C reduces the fuel required to generate HSC steam thereby increasing overall plant efficiency.

Claims

1. A steam generation system for a thermal power plant comprising; a first steam generation module comprising a furnace for the combustion of a fuel and the generation of hot gases to heat water and generate steam, including a first steam superheater disposed to supply superheated steam via a first superheated steam flow conduit to at least one first high pressure turbine and a first steam reheater disposed to supply reheated steam via a first reheated steam flow conduit to at least one first intermediate pressure turbine; a second steam generation module comprising a second hot gas volume supplied with hot gases, a bypass superheated steam flow conduit in fluid communication with the first superheated steam flow conduit disposed to supply superheated steam via a second superheater within the second hot gas volume to at least one second high pressure turbine and a return flow conduit to return steam from the at least one second high pressure turbine to the first superheated steam flow conduit downstream of the bypass superheated steam flow conduit, and a bypass reheated steam flow conduit in fluid communication with the first reheated steam flow conduit disposed to supply reheated steam via a second reheater within the second hot gas volume to at least one second intermediate pressure turbine and a return flow conduit to return steam from the at least one second intermediate pressure turbine to the first reheated steam flow conduit downstream of the bypass reheated steam flow conduit; a selective closure system for example comprising a valve arrangement selectively to close and open a flow passage to divert flow via the respective bypass and return flow conduits.

2. A steam generation system in accordance with claim 1 wherein the selective closure system is configured such that the second steam generation module which may be selectively brought into operation as a supplementary steam generator by a selective opening or closing of the selective closure system and thereby a selective opening or closing of a flow passage through each of the respective bypass and return flow conduits.

3. A steam generation system in accordance with claim 1 or claim 2 wherein the selective closure system comprises; a first set of closure formations comprising at least one closure formation within each of the bypass superheated steam flow conduit and corresponding return flow conduit and bypass reheated steam flow conduit and corresponding return flow conduit; and additionally a second set of closure formations comprising at least one closure formation such as at least one valve within the first superheated steam flow conduit at a location downstream of the bypass superheated steam fbw conduit and upstream of the corresponding return flow conduit and at least one closure formation such as at least one valve within the first reheated steam flow conduit at a location downstream of the bypass reheated steam flow conduit and upstream of its corresponding return flow conduit.

4. A steam generation system in accordance with claim 3 wherein each closure formation is selectively switchable between a first configuration such that it closes the flow conduit within which it is located and a second configuration such that it opens the flow conduit within which it is located.

5. A steam generation system in accordance with claim 3 or claim 4 wherein each closure formation is a valve actuatable between a first state such that it closes the flow conduit within which it is located and a second state such that it opens the flow conduit within which it is located.

6. A steam generation system in accordance with any preceding claim wherein the first steam generation module is rated for operation at a first main and/or reheat steam temperature, the second steam generation module is rated for operation at a second main and/or reheat steam temperature, and the first and second said temperatures are different.

7. A steam generation system in accordance with claim 6 wherein second temperature is higher.

8. A steam generation system in accordance with claim 7 the first steam generation module is configured for operation at main and/or reheat steam temperatures in excess of 580°C and for example in excess of 600°C but not higher than 620°C, and the second steam generation module is configured for operation at main and/or reheat steam temperatures of at least 700°C and for example at least 720°C.

9. A steam generation system in accordance with claim 8 wherein both the first and second steam generation modules are configured for operation at main steam pressures consistent in excess of SOObar, and more preferably in excess of 350bar.

10. A steam generation system in accordance with any preceding claim wherein the second hot gas volume is provided with a supply conduit for the supply of hot gases derived from the hot gases generated by combustion of a fuel in the first steam generation module.

11. A steam generation system in accordance with claim 10 wherein the second hot gas volume comprises a flue gas recirculation duct through which a proportion of the combustion gases from the furnace volume of the first steam generation module may be diverted.

12. A steam generation system in accordance with claim 10 or 11 wherein the second hot gas volume includes selective dampers operable to fluidly isolate the second hot gas volume from the supply of hot gas.

13. A method of steam generation for a thermal power plant comprises the steps of: providing a first steam generation module comprising a furnace for the combustion of a fuel and the generation of hot gases to heat water and generate steam, including a first steam superheater disposed to supply superheated steam via a first superheated steam flow conduit to at least one first high pressure turbine and a first steam reheater disposed to supply reheated steam via a first reheated steam flow conduit to at least one first intermediate pressure turbine; operating the first steam generation module by combustion of fuel to cause the generation of steam; selectively switching between a first mode of operation in which steam from the first steam superheater is supplied directly to the first high pressure turbine and steam from the first steam reheater is supplied directly to the first intermediate pressure turbine and a second mode of operation in which steam from the first steam superheater is diverted upstream of the first high pressure turbine to a second steam superheater in a second steam generation module and caused to drive a second high pressure turbine and then returned to the first high pressure turbine and steam from the first steam reheater is diverted upstream of the first intermediate pressure turbine to a second steam reheater in a second steam generation module and caused to drive a second intermediate pressure turbine and then returned to the first intermediate pressure turbine.

14. The method of claim 13 wherein the second mode of operation is effected by: providing a second steam generation module comprising a second hot gas volume supplied with hot gases, a bypass superheated steam flow conduit in fluid communication with the first superheated steam flow conduit disposed to supply superheated steam via a second superheater within the second hot gas volume to at least one second high pressure turbine and a return flow conduit to return steam from the at least one second high pressure turbine to the first superheated steam flow conduit downstream of the bypass superheated steam flow conduit, and a bypass reheated steam flow conduit in fluid communication with the first reheated steam flow conduit disposed to supply reheated steam via a second reheater within the second hot gas volume to at least one second intermediate pressure turbine and a return flow conduit to return steam from the at least one second intermediate pressure turbine to the first reheated steam flow conduit downstream of the bypass reheated steam flow conduit; and a selective closure system for example comprising a valve arrangement selectively to close and open a flow passage to divert flow via the respective bypass and return flow conduits; operating the selective closure system to open a flow passage to divert flow via the respective bypass and return flow conduits.

15. The method of claim 13 or 14 performed by operating a system in accordance with one of claims 1 to 12.

16. The method of one of claims 13 to 15 comprising operating the first superheater to raise steam to a first main steam temperature and operating the first reheater to a raise steam to a first reheat steam temperature; and in the second mode of operation operating the second superheater to raise steam to a first main steam temperature and operating the second reheater to raise steam to a second reheat steam temperature; wherein the second main and/or reheat steam temperature is different from the first main and/or reheat steam temperature.

17. The method of claim 16 wherein the second main and/or reheat steam temperature is higher than the first main and/or reheat steam temperature and in the usual case higher than the first main and/or reheat steam temperature.

18. The method of one of claims 16 to 17 wherein the first superheater and/or the first reheater are operated to raise steam to steam temperatures in excess of 580°C and for example in excess of 600°C but no higher than 620®C and the second superheater and/or the second reheater are operated to raise steam to temperatures of at least 700°Ο and for example at least 720°C.

19. The method of claim 18 wherein main steam from the first superheater and main steam from the second superheater are both supplied at a pressure in excess of 300bar, and more preferably in excess of 350bar.

20. The method of one of claims 13 to 19 wherein the second superheater and second reheater are supplied with a proportion of the combustion gases from the furnace volume of the first steam generation module.

21. The method of claim 20 wherein the second superheater and second reheater are supplied with a proportion of the combustion gases from the furnace volume via a flue gas recirculation.