WO2008014569A1

WO2008014569A1 - Steam generation

Info

Publication number: WO2008014569A1
Application number: PCT/AU2007/001097
Authority: WO
Inventors: Alan Robert Burns
Original assignee: Carnegie Corporation Ltd
Priority date: 2006-08-03
Filing date: 2007-08-03
Publication date: 2008-02-07
Also published as: WO2008014567A1; WO2008014568A1

Abstract

A steam power plant comprising steam generating apparatus (11), a turbine (13) and an electrical generator (15). The turbine being adapted to be driven by a working fluid comprising steam generated by the steam generation apparatus, the steam generation apparatus comprising a combustion zone (22) and a first section (33) and a second section (35) associated with the combustion zone for receiving hot combustion products. The steam generator further comprising a water flow path (165), an outer flow path (163) for hot gas flow therealong, and an inner flow path (161) for hot gas flow therealong, the outer flow path being disposed about the water flow path, and the water flow path being disposed about the inner flow path, the inner and the outer flow paths being in heat exchange relation with the water flow path.

Description

Steam Generation

Field of the Invention

This invention relates to steam generation and more particularly to a steam generator and also a method of generating steam. The invention also relates to a steam power plant. The steam power plant may generate mechanical energy from which shaft power is used for generation of electrical energy by way of a generator.

Background Art

Electricity is traditionally generated by a base load power station in which steam is produced to drive a turbine which in turn drives a generator. The energy for generation of the steam is available from various sources including combustion of fossil fuels such as coal and gas as well as nuclear power.

Where there is a need for decentralised power generation, and also small-scale electricity generation, it is now common to utilise natural gas-fired turbines directly rather than burning the gas in a steam cycle turbine.

There are, however, locations where there is a need for small-scale electricity generation plants but there is a lack of available natural gas for operating the plants.

It would be advantageous for there to be a steam-cycle power generation system that is compact, thermodynamically efficient, environmentally friendly with low or zero emissions and adaptable to combust a variety of fuels.

Steam generation involving a once-through principle would be particularly suited to such a steam-cycle power generation system.

A typical example of a once-through steam generator is the Benson boiler. The Benson boiler as originally proposed utilised a single water tube, with inlet feed water being pressurised and the conversion of steam occurring over a region of the tube without there being a clearly defined water/steam interface. With such an arrangement, there is no need, or provision for, a steam/water separator (typically a drum) as in conventional boilers that operate at lower pressures.

The advantage of such a water tube boiler is that much higher pressures can be obtained because of the smaller pipe diameters involved.

The conventional Benson boiler is vertically oriented with flame going upwards. The steam tubes are all in parallel and fed from a common manifold and are attached to the boiler walls rising upwards either as vertical columns or traverse the walls diagonally, or both. The horizontal cross section of the boiler is typically rectangular and the four vertical side walls are utilised differently with respect to burning different fuels. That is, there are different burner orientations for Anthracite (black coal), Bituminous, Sub-bituminous and Lignite (brown coal). Once of the major problems with the Benson boiler is that it is very difficult to achieve uniform heat transfer to the steam because of the rectangular arrangement; specifically, some tubes are further away from the flame than others and this is further exacerbated if different coals are burnt This requires the steam/water flow to be apportioned so there is greater flow through the tubes closer to the flame. Overall, a boiler must maintain what is called a 'positive flow characteristic'; that is, an increase in steam flow with increasing heat input. Such a state needs to be dynamically maintained in a Benson boiler by, for instance, continually adjusting relative flows in the network of tubes.

It is against this background that the present invention has been developed. In particular, one aspect of the present invention seeks to provide a steam generator suitable for use in a steam-cycle power generation system of the type referred to above. Disclosure of the Invention

According to a first aspect of the invention there is provided a steam generator comprising a water flow path, an outer flow path for hot gas flow therealong, and an inner flow path for hot gas flow therealong, the outer flow path being disposed about the water flow path, and the water flow path being disposed about the inner flow path, the inner and outer flow paths being in heat exchange relation with the water flow path.

With this arrangement, water can enter the water flow path as a liquid and leave as a vapour.

The vapour may comprise wet steam, dry steam or superheated steam.

Preferably, the inner gas flow path is of circular cross section.

Preferably, the outer gas flow path is of annular cross section.

Typically, the inner gas flow path is defined by a duct such as a tube. Further, the outer flow path is typically defined within a casing of circular cross-section. The casing may be formed in two or more longitudinal sections adapted to be coupled together.

Preferably, the water flows between an inlet at one end of the water flow path and an outlet at the other end of the water flow path.

Preferably, the inner flow path has an inlet at one end thereof and an outlet at the other end thereof for hot gas flow therebetween. Similarly, the outer gas flow path preferably has an inlet at one end thereof and an outlet at the other end thereof for hot gas flow therebetween.

Preferably, the various flow paths are configured such that gases flowing in the two gas flow paths are in counter-flow to water flowing in the water flow path. In one arrangement, the water flow path may comprise a direct flow path; that is, there is direct flow between the inlet and the outlet. In other words, the length of the water flow path corresponds to the linear distance between the water inlet and water outlet.

In another arrangement, the water flow path comprises an indirect flow path between the inlet and the outlet. With this arrangement, the water flow path has a longer path length than the linear distance between the inlet and outlet. Such an arrangement enhances thermal transfer to water flowing along the water flow path.

The indirect flow path may, for example, comprise a spiral flow path.

The indirect flow path may be configured to optimise, or at least enhance, the hydrodynamic flow of the water as its properties change under the influence of heating.

Where the indirect flow path comprises a spiral flow path, such a configuration may be achieved by varying the pitch of the spiral flow path along the length thereof.

The inner gas fjow path may comprise a direct flow path or an indirect flow path. In the latter case, the indirect nature of the inner gas flow path may be achieved by provision of flow diversion elements such as baffles within the flow path.

Similarly, the outer gas flow path may comprise a direct flow path or an indirect flow path. Again, in the latter arrangement, the indirect nature of the outer gas flow path may be achieved by provision of flow diversion elements such as baffles within the flow path.

The water flow path may be defined between the duct defining the inner gas flow path and a tubular element disposed adjacent the duct. In one arrangement, the duct and the tubular element may be in spaced apart relationship to define an annular water flow path therebetween.

In another arrangement, the duct and the tubular element co-operate to define the indirect water flow path. Where, for example, the indirect flow path is of spiral configuration, the flow path may be defined within a spiral formation at the interface between the inner duct and the tubular element.

The steam generator according to the invention is typically of a co-axial construction, in that the inner gas flow path is defined by the inner duct of circular cross-section and the outer gas flow path is of annular cross-section defined within the outer casing also of circular cross-section, with the inner duct and the outer casing being in concentric relation. With this arrangement, there is circular symmetry which is useful in imparting uniform radial heating to the water. This is beneficial in that it assists in maintaining a positive flow characteristic in the steam generator.

Further, the co-axial construction of the steam generator assists in withstanding high internal fluid pressures required to achieve super-critical steam conditions.

Still further, the tubular nature of the componentry facilitates a construction using materials of lesser wall thickness than would otherwise be appropriate in circumstances where the configuration was not tubular.

According to a second aspect of the invention there is provided a method of generating steam comprising: causing water to flow along a water flow path; causing a hot gas to. flow along an inner gas flow path about which the water flow path is disposed in heat exchange relation therewith; and causing a hot gas to flow along an outer gas flow path disposed about the water flow path in heat exchange relation therewith.

Preferably, the water is caused to flow in a direction counter to the direction of flow of the two hot gas flows. According to a third aspect of the invention there is provided a steam power plant comprising apparatus for generating steam and a turbine, the turbine being adapted to be driven by a working fluid comprising steam generated by the steam generation apparatus, the steam generation apparatus comprising a combustion zone and first and second sections associated with the combustion zone for receiving hot combustion products.

In one arrangement, the steam generation apparatus further comprises a flow path in heat exchange relationship with the first and second sections, wherein wet steam flowing along the flow path is converted into dry steam in the first section and thereafter the dry steam is converted into superheated steam in the second section.

The first and second sections may receive not only the combustion products but also flame from the combustion process.

The first and second sections may form part of the combustion zone or may be separate from (but in communication with) the combustion zone.

Means may be provided for regulating the separation of the combustion products, and also the flame, into the first and second sections.

The steam power plant may further comprise a boiler at which feed water (which may be preheated) is converted into wet steam for delivery to the first section at which the wet steam is converted into the dry steam.

The boiler may comprise a heat exchanger for transferring heat from the combustion products to the water for conversion of the water into wet steam.

The boiler may comprise two flow paths, one for receiving combustion products from the first section and the other for receiving combustion products from the second section. In another arrangement, the steam generation apparatus further comprises a boiler at which feed water (which may be preheated) is converted into superheated steam. The boiler may comprise two flow paths, one receiving hot combustion products from the first section and the other for receiving hot combustion products from the second section.

Brief Description of the Drawings

The invention will be better understood by reference to the following description of several specific embodiments thereof as shown in the accompanying drawings in Which:

Figure 1 is a schematic diagram illustrating a steam power plant according to a first embodiment;

Figure 2 is a schematic perspective view of the steam power plant according to the first embodiment;

Figure 3 is a plan view of the steam power plant;

Figure 4 is side elevational view of the steam power plant;

Figure 5 is a further side elevational view of the steam power plant;

Figure 6 is a fragmentary perspective view of the steam power plant;

Figure 7 is a fragmentary plan view illustrating in particular the burner of the steam power plant;

Figure 8 is a fragmentary plan view (on an enlarged scale) of a part of the burner;

Figure 9 is a fragmentary perspective view of a part of the burner;

Figure 10 is a fragmentary elevational view of the steam power plant; Figure 11 is a fragmentary perspective view, partly in section, showing the turbine, generator and boiler of the steam power plant;

Figure 12 is a partly sectioned perspective view of the turbine and the generator;

Figure 13 is a side view of the turbine (in part section) and the generator;

Figure 14 is a fragmentary side view, in part section and on an enlarged scale, of the turbine;

Figure 15 is a schematic fragmentary view of part of the turbine, illustrating in particular the nozzle system and the rotor;

Figure 16 is a further fragmentary view of the turbine rotor and the nozzle system, again on an enlarged scale;

Figure 17 is a schematic fragmentary side view of the nozzle system and the rotor of the turbine;

Figure 18 is a plan view of the turbine, showing in particular the nozzle system and the rotor;

Figure 19 is a schematic view illustrating steam issuing from a nozzle onto the rotor;

Figure 20 is a fragmentary perspective view at the lower end of the boiler;

Figure 21 is a schematic perspective view of the boiler;

Figure 22 is a schematic sectional view of the boiler;

Figure 23 is a cross-sectional view of the boiler;

Figure 24 is a fragmentary view of the boiler shown in section; Figure 25 is a view of part of the boiler shown in Figure 24 but on an enlarged scale;

Figure 26 is a view similar to Figure 24 but again on a further enlarged scale;

Figure 27 is a fragmentary perspective view of part of the boiler;

Figure 28 is a schematic perspective view of a baffle incorporated in the boiler;

Figure 29 is a fragmentary schematic view of a spiral water flow path within the boiler,

Figure 30 is a fragmentary view of a tubular element used to form the spiral water path;

Figure 31 is a schematic sectional view of the boiler;

Figure 32 is a fragmentary view of a boiler for a steam power plant according to a second embodiment;

Figure 33 is a fragmentary view, on an enlarged scale, of one end of the boiler of Figure 32;

Figure 34 is a fragmentary view of a boiler for a steam power plant according to a third embodiment;

Figure 35 is a schematic diagram illustrating a steam power plant according to a fourth embodiment;

Figure 36 is a schematic perspective view of a steam power plant according to a fourth embodiment; and Figur© 37 is a fragmentary side view of the steam power plant according to the fourth embodiment, illustrating the relationship between the boiler and a burner for generating combustion products for use in the boiler.

Best Mode(s) for Carrying Out the Invention

Referring to Figures 1 and 2 of the drawings, the steam power plant 10 according to the first embodiment is used for generation of electricity. The steam power plant 10. comprises steam generation apparatus 11 , a steam turbine 13 and an electrical generator 15.

The steam generation apparatus 11 generates superheated steam from feed water. Energy within the superheated steam (in the form of fluid pressure) is converted directly to mechanical energy in the turbine 13. Mechanical energy from the turbine 13 is transmitted by a rotary shaft to the electrical generator 15 for generation of electricity. Remnant heat recovered from the turbine is used for pre-heating feed water, as will be explained later.

The steam generation apparatus 11 comprises a burner 21 having a combustion zone 22 for generating hot combustion products (gases) for production of steam. The burner 21 is adaptable to combust a variety of fuels, including coal, oil and gas. In this embodiment, the burner 21 receives fuel from a fuel tank 23 by way of a feed line 25 incorporating a fuel delivery pump 27. Combustion air is delivered to the combustion zone 22 of burner 21 via an air intake 29 incorporating an air blower 31.

From the combustion zone 22, the combustion flame and also hot combustion products are directed into two heating zones, being a first heating zone 33 and a second heating zone 35. In this embodiment, the flame and the hot combustion products are split into two portions, one of which flows through the first heating zone 33 and the other of which flow through the second heating zone 35, as will be explained in more detail later. As also will be explained later, wet steam generated from feed water passes in heat exchange relationship with the hot combustion products in the first heating zone 33 to be converted to dry steam, and the dry steam passes in heat exchange relationship with the hot combustion products in the second heating zone 35 to be converted into supertieated steam. The superheated steam is superheated to a level to constitute supercritical steam.

From the two heating zones 33, 35, the hot combustion products are passed through in heat exchange relationship with the feed water in a boiler 41 .to generate the wet steam. The combustion products from the first and second zones 33, 35 have separate flow paths 43, 45 passing through the boiler 41. The two flow paths 43, 45 communicate with the common outlet flow path 47 which terminates at a flue 49 at which the combustion products are discharged to atmosphere. A blower 51 is incorporated in the flow path 47 for delivery of the combustion products to the flue 49. White not shown in the drawings, there may be treatment means for treating the combustion products prior to discharge thereof to atmosphere.

Feed water for the production of steam is delivered to the boiler 41 from a feed water tank 53 by way of a feed water line 55 incorporating a feed water pump 57.

Prior to being delivered to the boiler 41 for conversion to wet steam, the feed water is preheated using heat extracted from spent steam in the turbine 13. For this purpose, the turbine 13 has a" rotor 61 which incorporates a heat exchanger 62 through which the feed water passes in heat exchange relation for preheating.

Wet steam from the boiler 41 is conveyed to heat exchanger 63 associated with the first heating zone 33 by wet steam line 64. The heat exchanger 63 converts the wet steam into dry steam.

Dry steam from the heat exchanger 63 is conveyed to heat exchanger 65 associated with the second heating zone 35 by dry steam line 66. The heat exchanger 65 converts the dry steam into superheated steam.

Superheated steam from heat exchanger 65 is conveyed to the turbine 13 by superheated steam line 67. The superheated steam drives the turbine 13, as will be explained in more detail later. The spent steam from the turbine 13 condenses and is conveyed to a first return water tank 71 via a return line 73 incorporating a return pump 75. A steam trap 77 is incorporated in the return line 73 upstream of the return pump 75.

The first return water tank 71 is coupled to a second return water tank 72 by way of a water line 74. The Water line 74 passes through a heat exchanger 76 in which the return water flowing from the first return waW tank 71 to the second return water tank 72 passes in heat exchange relationship with air delivered to the burner 21 via air intake 29. With this arrangement, remnant heat contained in the water condensed from the steam can be utilised for preheating the combustion air.

Referring now to Figures 11 to 19, the turbine 13 comprises the turbine rotor 61 and a nozzle system 80 for directing the superheated steam onto the rotor 61 to cause rotation thereof.

The rotor 61 comprises a rotor body 83 mounted onto a rotor shaft 85. In this embodiment, the axis of rotation of the rotor shaft 85 is substantially vertical. The rotor shaft 85 is rotatably supported in bearings 86 and one end of the shaft is drivingly coupled to the generator 15 to transmit shaft power thereto.

The rotor body 83 comprises a plurality of tubes 87 which provide parallel flow passages within the rotor 61 for preheating the feed water. With this arrangement, the tubes 87 are incorporated in the rotor 61 and constitute part of the heat exchanger 62.

In this embodiment, the tubes 87 are disposed circumferentially around the periphery of the rotor 61 and are supported on two supports 90 mounted on the rotor shaft 85. Each support 90 comprises a disc structure 91 having an outer periphery 94 incorporating recesses 95 in which the tubes 87 are accommodated.

A casing 93 surrounds the tubes 87 to define the radial outer periphery of the rotor 61. The casing 93 is thermally isolated from the tubes 87 for the purposes of minimising heat loss from the tubes through the casing. Specifically, each tube 87 is attached to the casing 93 by way of a mounting 95 configured as an elongate cowling 97. The cowling 97 is positioned longitudinally adjacent the tube 87 on the trailing side thereof with respect to the direction of rotation of the rotor 61. The cowling 97 comprises a central web 99, an inner flange 101 on one side of the central web and an outer flange 103 on the other longitudinal side of the central web, with the two flanges 101 , 103 extending to opposed sides of the web. The cowling 97 is attached at the outer flange 101 to the casing 93, and the respective tube 87 rests against the central web 99 and the inner flange 103, as best seen in Figure 19. With this arrangement, the cowling 97 provides the connection between its respective tube 87 and the casing 93. There is no direct connection between the tube 87 and the casing 93, and in fact there is a gap therebetween (which is not apparent in the drawings) to provide the thermal isolation referred to previously.

The cowlings 97 provide an enhanced surface area for the impinging steam to impart momentum to the rotor 61.

Each tube 87 has an exposed surface 111 against which steam issuing from the nozzle system 80 is directed in order to cause rotation of the rotor 61. Each exposed surface 111 is so configured with respect to the direction of incident steam such that a portion of the steam flow attaches itself to the surface by virtue of the Coanda effect. This is illustrated schematically in Figure 19 in which the incident flow of steam is identified by reference numeral 112 and a portion 112a thereof is shown attaching itself to the surface 111. As a consequence of such attachment, the steam flow exerts an attractive force on the exposed surface in the direction of rotation of the rotor. In this way, rotational effects imparted to the rotor 61 by virtue of the steam are enhanced; that is, rotation is imparted to the rotor 61 not only as a reaction to the impingement of the steam on the rotor 61 (and more particularly the cowling 97 on the tubes 87) but also by virtue of the Coanda effect arising through attachment of the steam flow to the surface 111. While the tubes 87 in this embodiment are disposed in a circumferential arrangement adjacent the outer periphery of the rotor 61 , other arrangements are of course possible. Tubes may, for example, be disposed longitudinally within the rotor 61 at various radial distances from the axis of rotation of the rotor.

As mentioned above, the tubes 87 constitute part of the heat exchanger 62. The tubes 87 are connected at each end to a common manifold 88 which is connected into the feed water line 55 by a rotary coupling 89. In this way, the feed water flow path is incorporated into the rotating rotor 61.

The nozzle system 80 comprises a plurality of nozzles 121 each of which receives superheated steam from the superheated steam line 67. The superheated steam line 67 has a plurality of branch lines 123 each of which terminates at one of the nozzles 121. The branch lines 123 extend into the confines of the rotor 61 though the upper end thereof and the nozzles 121 are disposed within the confines of the rotor to direct superheated steam onto the tubes 87 and associated cowlings to impart momentum and thereby generate motive torque on the rotor, as previously described.

The nozzles 121 are arranged at various spacings axially along the rotor 61. The nozzles 121 each comprise a Laval nozzle or similar nozzle.

The superheated steam undergoes collapse or implosion upon contact with the tubes 87 and cowlings 97, imparting thermal energy thereto and thus heating the feed water flowing along the tubes 87.

The collapsed steam is confined within the rotor 61 by the casing 93. Because of the vertical orientation of the rotor 61, spent steam and its condensate descend within the rotor 61 and exits through the lower end thereof. The exiting spent steam and condensate communicate with return line 73 which incorporates the steam trap 77 as previously described. As mentioned earlier, the combustion flame and also hot combustion products from the combustion zone 22 are separated into two heating zones, being the first heating zone 33 and the second heating zone 35.

The first heating zone 33 comprises a passage 131 defined by a first flame tube 133, and the second zone 35 comprises a passage 135 defined by a second flame tube 137. The two flame tubes 133, 137 e>ctend from the combustion zone

22 and are in communication with each other at their inner ends, with the first flame tube 133 branching from the second flame tube, as best seen in Figure 8.

A control device such as an adjustable butterfly valve 141 is provided for selectively varying the extent of separation of the combustion flame and the combustion products between the two flame tubes 133, 137.

An outer tube 143 surrounds the first flame tube 133 in spaced apart relation therewith. A longitudinal baffle structure 145 is provided in the annular space 147 defined between the two tubes 133, 143 to form a steam flow path 149. With this arrangement, the steam flow path 149 is in heat exchange relation with the flame and the hot combustion products contained within the flame tube 133. The spacing of the baffles 145 is set according to the heat transfer necessary to achieve conversion of the wet steam into dry steam as it flows along the steam flow path 149.

Similarly, an outer tube 153 surrounds the second flame tube 137 in spaced apart relation therewith. A longitudinal baffle structure 155 is provided in the annular space 157 defined between the two tubes 137, 153 to form a steam flow path 159. With this arrangement, the steam flow path 159 is in heat exchange relation with the flame and the hot combustion products contained within the flame tube 137. The pitch of the baffle structure 155 is set according to the heat transfer necessary to achieve conversion of the dry steam into superheated steam as it flows along the steam flow path 159.

As mentioned above, the boiler 41 has separate flow paths which receive hot combustion products from the first and second heating zones 33, 35. The boiler 41 comprises an inner tube 161 , an outer tube 163 spaced from the inner tube to define an annular space 164 therebetween, and a spiralling water path 165 accommodated within the inner tube 161 in close proximity to the sidewall thereof. The pitch of the spiral can be selected according to the characteristics of the wet steam required to be generated in the boiler 41, along with other parameters of the boiler.

The outer tube 163 is configured as a casing 166. The casing 166 comprises two longitudinal sections 166a, 166b adapted to be releasably secured together in order to facilitate access to the interior of the boiler.

With this arrangement, the inner tube 161 defines inner gas flow path 168 and the annular space 164 defines outer gas flow path 169. The inner gas flow path 168 has an inlet 172 and an outlet 174, Similarly, the outer gas flow path 169 has an inlet 176 and an outlet 178.

In the arrangement shown, the spiralling water path 165 is defined between the inner tube 161 and a tubular element 170 disposed within the inner tube 161. The tubular element 170 incorporates a spirally wound rib 171, the radially outer edge of which is adapted to engage the interior wall of inner tube 161, a$ best seen in

Figure 29. With this arrangement, the inner tube 161 and the inner tubular element 170 co-operate to define the spirally wound water path 165. Typically, the tubular element 170 is a friction fit within the inner tube 161 such that frictional engagement between the radially outer edge of the rib 171 establishes a fluid seal therebetween.

Accordingly, the boiler 41 comprises an inner gas flow path 168, an outer gas flow path 169 and a water flow path 165 disposed between the two gas flow paths 168, 169.

The water flow path 165 has an inlet 181 through which feed water from feed water line 55 enters the boiler 41 under pressure and an outlet 183 through which wet steam generated in the boiler 41 exits to flow along wet steam line 64. ^■ Thβ pitch of the spiralling water path 165 varies at intervals along the length of the water path. This is for the purpose of optimising the hydrodynamic flow of the water as its properties change under the influence of heating within the boiler. As best seen in Figure 31, the section 165a of the spiralling water path 165 has a larger pitch in the region thereof towards the feed water inlet 181 as compared to the pitch of the section 165b in the region thereof adjacent the steam outlet 183.

The annular $pace 164 accommodates a plurality of baffles 167 disposed at intervals along the space. Each baffle 167 is configured as an annular disc having a radially inner end 171 which engages the outer periphery of the inner tube 161 and a radially outer end 173 which- engages the inner periphery of the outer tube 163. Each baffle 167 has a plurality of flow openings 175 formed therein, there being four such opening in each baffle in this embodiment. Each flow opening 175 is formed by cutting a flap 177 in the disc, with the cuts forming the sides of the flap extending inwardly from the radially inner end 171 of the disc, and then bending the flap to extend normally from the plane of the disc. The area of the disc from which each flap 177 provides the respective flow opening 175. The baffles 167 are butted one against another along the space 165, with the flaps 177 function as spacers between the baffles. The spacings betweens the baffles 167 and the flow openings in the baffles cooperate to form a tortuous flow passage 179 which extends along the annular space 164. Further, the flaps 177 function as turbulators for generating turbulence in flow along the flow path 179. The flow passage 179 is incorporated in flow path 43 and as such receives hot combustion products from the first heating zone 33.

The inner tube 161 is incorporated in flow path 45 and as such the inner gas flow path 168 receives hot combustion products from the second heating zone 35. The inner tube 161 has a plurality of baffles 191 provided along its length to assist in heat transfer to the surrounding water flow path 165. The baffles are accommodated within the confines of the inner tubular element 170. The baffles

191 each comprise a plate 193 having a plurality of outer flow openings 197 formed by bending portions 198 at the outer periphery to farm flaps 199. The flaps

199 function as spacers between the baffles 191 and also as turbulators to assist heat transfer to the water flow path 165 also accommodated within the inner tube 161.

Accordingly, there is heat transfer to the feed water from hot combustions products derived from both the first and second heating zones 33, 35. The combustion products from the second zone 35 are hotter and are therefore more effective in the heat transfer process for generation of the wet steam from the feed water.

The flow direction of the feed water is in counter flow to the flow directions of the hot combustion products in the flow paths 43, 45 in order to optimise heat transfer.

It is a feature of the boiler 41 that feedwater is pressurised and heated in a single process for conversion to steam.

The boiler 41 of a co-axial construction, in that the inner gas flow path 168 is defined by the inner tube 161 of circular cross-section and the outer gas flow path 169 is of annular cross-section defined within the outer casing 167 also of circular cross-section, with the inner duct and the outer casing being in concentric relation. With this arrangement, there is circular symmetry which is useful in imparting uniform radial heating to water flowing along the water flow path 165. This is beneficial in that it assists in maintaining a positive flow characteristic in the boiler 41.

Further, the co-axial construction of the boiler 41 assists in withstanding high internal fluid pressures.

Still further, the tubular nature of the componentry facilitates a construction using materials of lesser wall thickness than would otherwise be appropriate in circumstances where the configuration waβ not tubular.

In this embodiment, the regions of highest steam pressures are associated with the heat exchangers 63, 65. This provides an opportunity for cost savings in relation to construction materials, as only regions of highest pressure require use of higher strength alloys such as Inconel, whereas the boiler 41 can be constructed of less costly lower tensile alloys such as appropriate grades of stainless steel.

While in the embodiment described the inner gas flow path 168 and the outer gas flow path 169 have each comprised an indirect flow path (in that there are elements such as baffles incorporated therein to divert the gas flow as it moves between the inlet and the outlet thereof), there may be circumstances where either one or both of the gas flow paths are not indirect but rather are direct in that gas can flow between the inlet and outlet without diversion.

Referring now to Figures 32 and 33, there is shown part of a boiler for a steam power plant according to a second embodiment. The steam power plant according to the second embodiment is similar in many respects to the steam power plant according to the first embodiment and so corresponding referenced numerals are used to identify similar paths. In this embodiment, the boiler 41 incorporates a water path 165 which is direct, rather than being indirect as was the case with the previous embodiment. In the arrangement shown, the water path 165 is annular in cross-section and also of substantially constant cross- section throughout its length. In the arrangement shown, the water flow path 165 is defined between the inner tube 161 and an inner tubular element 170 disposed within the inner tube 161 in spaced apart relationship therewith to define an annular space 201 which provides the water flow path 165. Support elements 203 at the end of the tubular element 170 support the latter in the space relationship with the inner tube element 170, as best seen in Figure 33. Further, support elements, such as radial struts (not shown), are provided at intervals along the annular space 201 between the inner and outer tubes 161 , 163. The support elements are sized and shaped so as not to adversely affect the direct nature of the water flow.

In the boiler 41 according to this embodiment, the inner gas flow path 168 and the outer gas flow path 169 each comprise direct flow paths (in that there are no flow diverters disposed along the length thereof). Referring now to Figure 34, there is shown a boiler for a stearn power plant according to a third embodiment The steam power plant according to the third embodiment is similar in many respects to the steam power plant of the previous embodiment and so corresponding reference numerals are used to identify corresponding paths. More particularly, the boiler 41 of this embodiment is similar to the boiler of the previous embodiment in that the water flow path 165 comprises a direct flow path. However, in this embodiment, the inner gas flow path 168 and the outer gas flow path 169 each comprise indirect flow paths. Specifically, inner gas flow path 168 has baffles 191 provided therein and outer gas flow path 169 has baffles 167 provided therein.

In the first embodiment, the boiler 41 generated wet steam which was converted to dry steam in heat exchanger 63 and the dried steam was converted into superheated steam in heat exchanger 65. In other arrangements it is possible for such a boiler to be constructed and operated to generate dry steam which can subsequently be converted to superheated steam for delivery to the turbine 13, or alternatively to generate superheated steam which can then be delivered to the turbine 13.

Referring now to Figures 35 to 37, there is shown a steam power plant according to a still further embodiment which incorporates a boiler that can convert feedwater directly into superheated steam. The steam power plant according to this still further embodiment is similar in many respects to the steam power plant according to the first embodiment and so corresponding reference numerals are used to identify corresponding parts.

As alluded to above, conversion of feed water into super-heated steam in this further embodiment is performed entirely within the boiler 41 , and consequently, there is no need for the heat exchanger 63, 65.

In particular, in this further embodiment, the steam generation apparatus 11 has the combustion zone 22 for generating hot combustion products (gases). From the combustion zone, the combustion flame and also hot combustion products are split into two portions, one of which is delivered to the inner gas flow path 168 of the boiler 41 and the other of which is delivered to the outer gas flow path 169. In this regard, ducting 210 extends between the combustion zone 22 and the boiler 41. The ducting 210 incorporates a first duct section 211 which communicates with the inner gas flow path 168 and a second duct section 212 which branches outwardly and communicates with the outer gas flow path 168, as best seen in Figure 37. As with the first embodiment, a control device such as an adjustable butterfly valve may be provided for selectively varying the extent of separation of the combustion flame and the combustion products into the two ducts 211 , 212.

It is a feature of this embodiment that the co-axial construction of the boiler 41 assists in withstanding high internal fluid pressures required to achieve supercritical steam conditions within the boiler rtself.

By having such a boiler 41 that can convert feedwater into superheated steam, the steam generation apparatus 11 may be of more compact construction, as there is no requirement for the heat exchangers 63, 65. Additionally, the construction may provide easier access to the burner 21 for it to be exchanged.- As alluded to earlier, the steam power plant has been devised to combust different types of fuel. A burner designed for combusting one type of fuel may be very different from that designed to combust another type of fuel. Accordingly, it is advantageous to have a construction which allows the burner to be readily exchanged.

From the foregoing, it is evident that the embodiments can each provide a steam power plant which is particularly, although not necessarily exclusively, suitable as a relatively small scale, decentralised power plant that can use a variety of fuels including in particular coal. It is a feature of the power plant that it is designed to be entirely modular and readily transportable. As such, all components of the steam power plant are designed such that the whole unit, when disassembled, can be transported in one or more sea containers. The boiler utilised in the steam power plant of each embodiment has been developed in order to meet this requirement. It should be appreciated that the invention is not limited to the embodiments described and that various alterations and modifications may be made without departing from the scope of the invention. Further, the boilers described in relation to the various embodiments of the steam power plant may have applications in areas other than steam power plants.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers

Claims

The Claims Defining the Invention are as Follows:

1. A steam generator comprising a water flow path, an outer flow path for hot gas flow therealoπg, and an inner flow path for hot ga$ flow therealong, the outer flow path being disposed about the water flow path, and the water flow path being disposed about the inner flow path, the inner and outer flow paths being in heat exchange relation with the water flow path.

2. A steam generator according to claim 1 wherein the inner gas flow path is of circular cross section.

3. A steam generator according to claim 1 or 2 wherein the outer gas flow path is of annular cross section.

4. A steam generator according to claim 1 , 2 or 3 wherein the inner gas flow path is defined by a duct such as a tube.

5. A steam generator according to any one of the preceding claims wherein the outer flow path is defined within a casing of circular cross-section.

6. A steam generator according to any one of .the preceding claims wherein the water flows between an inlet at one end of the water flow path and an outlet at the other end of the water flow path.

7. A steam generator according to any one of the preceding claims wherein the inner flow path has an inlet at one end thereof and an outlet at the other end thereof for hot gas flow therebetween, and the outer gas flow path has an inlet at one end thereof and an outlet at the other end thereof for hot gas flow therebetween.

8. A steam generator according to claim 7 wherein the various flow paths are configured such that gases flowing in the two gas flow paths are in counter- flow to water flowing in the water flow path.

9. A steam generator according to any one of the preceding claims wherein the water flow path comprises a direct flow path between the water inlet and water outlet.

10. A steam generator according to any one of claims 1 to 8 wherein the water flow path comprises an indirect flow path between the inlet and the outlet.

11. A steam generator according to claim 10 wherein the indirect flow path comprise a spiral flow path.

12. A steam generator according to claim 10 or 11 wherein the indirect flow path is configured to optimise, or at least enhance, the hydrodynamic flow of the water as its properties change under the influence of heating.

13. A steam generator according, to claim 12 when dependant on claim 11 wherein the spiral flow path has a pitch which varies at intervals along the length of the flow path.

14. A steam generator according to any one of the preceding claims wherein the inner gas flow path comprises a direct flow path between an inlet and outlet thereof.

15. A steam generator according to any one of claims 1 to 13 wherein the inner gas flow path comprises an indirect flow path between an inlet and outlet thereof:

16. A steam generator according to any one of the preceding claims wherein the outer gas flow path comprises a direct flow path between an inlet and outlet thereof

17. A steam generator according to any one of claims 1 to 15 wherein the outer gas flow path comprises an indirect flow path between an inlet and outlet thereof.

18. A steam generator according to any one of the preceding claims wherein the water flow path is defined between the duct defining the inner gas flow path and a tubular element disposed adjacent the duct.

19. A steam generator according to claim 18 wherein the tubular element and the duct are in spaced apart relationship to define an annular water flow path therebetween.

20. A steam generator according to claim 18 wherein the duct and the tubular element co-operate to define the indirect water flow path.

21 - A steam generator according to acclaim 20 wherein the flow path is defined within a spiral formation at the interface between the inner duct and the tubular element.

22. A method of generating steam using a steam generator according to any one of the preceding claims.

23. A method of generating steam comprising: causing water to flow along a water flow path; causing a hot gas to flow along an inner gas flow path about which the water flow path is disposed in heat exchange relation therewith; and causing a hot gas to flow along an outer gas flow path disposed about the water flow path in heat exchange relation therewith.

24. A method according to claim 23 wherein the water is caused to flow in a direction counter to the direction of flow of the two hpt gas flows.

25. A steam-cycle power generation system comprising a steam generator according to any one of claims 1 to 21.

26. A steam power plant comprising a steam generator according to any one of claims 1 to 21.

27. A steam power plant comprising apparatus for generating steam and a turbine, the turbine being adapted to be driven by a working fluid comprising steam generated by the $team generation apparatus, the steam generation apparatus comprising a combustion zone and first and second sections associated with the combustion zone for receiving hot combustion products.

28. A steam power plant according to claim 27 wherein the steam generation apparatus further comprises a flow path in heat exchange relationship with the first and second sections, wherein wet steam flowing along the flow path is converted into dry stearn in the first section and thereafter the dry steam is converted into superheated steam in the second section.

29. A steam power plant according to claim 28 further comprising a boiler at which feed water is converted into wet steam for delivery to the first section at which the wet steam is converted into the dry steam.

30. A steam power plant according to claim 29 wherein the boiler comprises two flow paths, one for receiving combustion products from the first section and the other for receiving combustion products from the second section.

31. A steam power plant according to claim 27 wherein the steam generation apparatus further comprises a boiler at which feed water is converted into super-heated steam.

32. A steam power plant according to claim 31 wherein the boiler comprises two flow paths, one receiving hot combustion products from the first section and the other for receiving hot combustion products from the second section.

33. A steam generator substantially as herein described with reference to the accompanying drawings.

34. A method of generating steam substantially as herein described.

35. A steam power plant substantially as herein described with reference to the accompanying drawings.