Abstract:
The invention pertains to fluegas recirculation in gas turbines, and specifically to an intake section upstream of the inlet of a compressor of a gas turbine unit with fluegas recirculation. The intake section includes at least one section with a flow path defined by sidewalls in which the fresh airflow of the intake air is flowing along a principal airflow direction, including at least one mixing duct extending into the flow path from at least one sidewall. The mixing duct includes an intake at the at least one sidewall for receiving recirculated fluegas, as well as including at least one outlet opening distanced from said sidewall for blowing recirculated fluegas out of the mixing duct into the airflow.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT/EP2012/050964 filed Jan. 23, 2012, which claims priority to Swiss Application 00115/11 filed Jan. 24, 2011, both of which are hereby incorporated in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to the field of fluegas recirculation in gas turbines, specifically to elements for mixing fluegas with ambient air upstream of the compressor. 
     BACKGROUND 
     There is a proposal to enrich the carbon dioxide concentration at the gas turbine exhaust by means of a fluegas recirculation (FGR) system, in combination with post-combustion carbon dioxide capture. In this respect reference is made to WO-2010072710 disclosing such a setup. Fluegas recirculation is beneficial for the carbon dioxide capture process because both the concentration of carbon dioxide is increased and the overall mass flow to the carbon dioxide capture unit is reduced. These two aspects result in smaller carbon dioxide capture equipment with a more efficient capture process. 
     SUMMARY 
     When using fluegas recirculation in a gas turbine system the carbon dioxide enriched fluegas has to be mixed with ambient air and then supplied to the compressor inlet of the gas turbine. The compressor delivers this carbon dioxide containing mixture to the combustor and into the secondary cooling systems of the gas turbines. 
     The recirculation of fluegas lowers in total the amount of oxygen which is supplied to the combustion process. The remaining oxygen concentration after the combustion process is the limiting parameter for the amount of recirculated fluegas. If the fluegas ratio, defined as the ratio of recirculated fluegas and the gas turbine exhaust mass flow, is too high, the amount of oxygen is less than required for a stoichiometric combustion. In this case incomplete combustion will occur, which leads to high carbon monoxide emissions, unburnt hydrocarbons (UHC), flame instability, and finally the flame can even extinguish. Therefore a good mixing needs to be ensured over the whole operation range. Lowest possible pressure loss needs to be ensured in order not to lose power and efficiency of the power plant. The control of the fluegas recirculation ratio is essential to allow for safe gas turbine operation. A robust design of the mixer is critical to ensure good mixing quality over a wide range of fuel gas ratios and load parameters of the turbine. 
     In other words the recirculated fluegas has to be evenly mixed with the fresh air upstream of the compressor. 
     The gist of the present invention is to install a mixer device in the intake of the gas turbine, wherein this mixer device can for example be upstream or downstream of the filter and/or the silencer in the intake. The mixer can for example be a finger type mixer extending from one single sidewall of the intake unit into the flow path defined thereby. Plates and/or guide vanes can be used to improve mixing in the pipes of the mixer, at the outlet of the mixer or downstream of the outlet of the mixer. 
     Specifically therefore the present invention relates to an intake section upstream of the inlet of a compressor of a gas turbine unit with fluegas recirculation, comprising at least one section with a flow path defined by sidewalls in which the fresh airflow of the intake air is flowing along a principal airflow direction, and comprising at least one mixing duct or mixing finger extending into the flow path from at least one sidewall. This mixing duct preferably comprises an intake at said at least one sidewall for receiving recirculated fluegas, as well as at least one outlet opening distanced from said sidewall for blowing recirculated fluegas out of the mixing duct into the airflow, preferably not too close to a wall region thereof but in a more central region thereof. 
     According to a first preferred embodiment of such an intake section, the mixing duct is attached to a single sidewall only (mixing finger) and freely extends into the flow path. Preferably in this case the at least one outlet opening is located in a tip region of the mixing duct or in proximity thereto, or distributed along the length of the duct. 
     At least two, three, four, or at least five mixing ducts can be arranged in a row, preferably adjacent to each other, said row preferably being aligned essentially along the airflow direction, the most upstream mixing duct defining a leading edge of the row. Preferably the mixing ducts of such a row all have different lengths of extension into the flow path, and preferably the most upstream mixing duct extends most into the flow path, the most downstream mixing duct extends the least into the flow path, and mixing ducts between are of successively decreasing length as a function of their downstream position, wherein further preferably the lengths are regularly decreasing along the row. In case of such a row, but also in case of individual mixing ducts, in a direction perpendicular to the airflow direction at least two, or at least three, or at least four rows/mixing ducts can be arranged distanced from each other, preferably equally distanced from each other, in the flow path. Alternatively or additionally, along the airflow direction at least two or at least three rows/mixing ducts can be arranged distanced from each other, in the flow path. 
     The at least one mixing duct, or the row of mixing ducts, can extend into flow path with its axis essentially perpendicular to airflow direction in the flow path. It can however also be inclined thereto, and for example the leading edge thereof can have a bent or curved shape depending inter alia, on the aerodynamics in the flow path. Preferentially in the flow path the airflow direction is essentially parallel to the sidewalls. 
     The mixing duct can be of tubular design. Alternatively it may comprise or be formed by four, preferably pairwise parallel walls defining the duct. 
     The outlet opening can open in a direction essentially perpendicular to the direction of the airflow or in a direction essentially parallel and concurrent to the direction of the airflow (blowing out of the trailing edge). 
     The at least one outlet opening can open in a direction essentially perpendicular to the direction of the airflow and essentially perpendicular to the axis of the mixing duct (lateral blowing out), wherein preferably two outlet openings are provided for each mixing duct blowing recirculated fluegas in opposite directions into the airflow in the flow path. 
     In the at least one outlet opening or in the region of the at least one outlet opening at least one, rounded or straight guide vane can be located, wherein this guide vane can for example be located essentially at half height of the total height of the outlet opening in the direction along the axis of the mixing duct. 
     The at least one outlet opening according to a further preferred embodiment opens in a direction essentially perpendicular to the direction of the airflow and essentially parallel to the axis of the mixing duct, and a mixing element, preferably an impingement plate, is located downstream, with respect to the flow direction of the fluegas, of the outlet opening, preferably arranged essentially perpendicularly to the flow direction of the fluegas and parallel to the airflow direction. 
     According to yet another preferred embodiment, the mixing ducts are arranged in a region where the flow path is defined by four pairwise parallel sidewalls, and upstream and/or downstream of the mixing duct or of the row of mixing ducts a silencer and/or a filter is arranged in that region or essentially just upstream and/or downstream of that region. 
     For ideal flow conditions and as little pressure loss along the mixing device the mixing duct or in case of a row of mixing ducts the most upstream located mixing duct comprises an aerodynamically optimised leading edge, for example a rounded leading edge. It is also possible to locate turbulators upstream of the openings on the outside of the mixing ducts to initiate turbulences somewhat upstream of the location where the fluegas is introduced into the airflow to further increase the mixing quality. 
     According to one particular preferred embodiment essentially a triangular row of mixing ducts is proposed showing an excellent mixing property. According to this embodiment at least two, or at least three, or at least four, or at least five mixing ducts are arranged in a row, adjacent to each other, said row being alignment essentially along the airflow direction, the most upstream mixing duct defining a (preferably rounded) leading edge of the row, wherein the mixing ducts all have different lengths of extension into the flow path, and wherein the most upstream mixing duct extends most into the flow path, the most downstream mixing duct extending least into the flow path, and mixing ducts between being of successively and preferably regularly decreasing length as a function of their downstream position, wherein the trailing edge of the row is defined by an inclined trailing edge wall and wherein on each lateral side of the row triangular outlet openings open in a direction essentially perpendicular to the direction of the airflow and essentially perpendicular to the axis of the mixing duct. One side of these triangular openings is preferably formed by the inclined trailing edge wall. 
     The present invention furthermore relates to a gas turbine, preferably combined cycle gas turbine, with an intake section as outlined above. 
     In a further embodiment it relates to a plant with flue gas recirculation in combination with post-combustion carbon dioxide capture. In addition to that, the present invention relates to a method for recirculating fluegas to the intake of a gas turbine unit, preferably of a combined cycle, wherein an intake section as outlined above is used for mixing fresh ambient air with recirculated fluegas. According to a preferred embodiment of such a method, flow control elements, controlled based on a mass flow measurement in the recirculation line and/or based on a measurement of the composition of the mixed intake air upstream of the compressor, and/or based on a measurement of the combustion quality, are provided in the mixing ducts or upstream of the mixing ducts, are used for controlling the mass flow of recirculated fluegas. 
     Further embodiments of the invention are laid down in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings, 
         FIG. 1  shows a schematic representation of a combined cycle with fluegas recirculation; 
         FIG. 2  in a) shows a perspective view onto an intake housing with three mixing fingers each with three ducts, in b) shows a rows of mixing fingers in a lateral view with fluegas introduction in axial direction with respect to the axis of the fingers; and in c) shows a row of mixing fingers in a lateral view with fluegas introduction in a direction parallel to the airflow; 
         FIG. 3  in shows mixing finger with impingement plates and fluegas introduction in axial direction with respect to the axis of the finger, wherein a) shows a perspective view, b) a view opposite to the airflow direction, c) a lateral view onto an embodiment with straight leading edge, and d) a lateral view onto an embodiment with bent leading edge; e) shows a lateral view onto the horizontal section of an intake housing with two successive lines of mixing fingers and the corresponding introduction elements for fluegas, and f) a top view onto the horizontal section according to e); 
         FIG. 4  shows an embodiment with 16 mixing finger ducts wherein the groups are arranged in one line of four rows of four ducts and wherein the mixing fingers are arranged downstream of the silencer, wherein a) shows a perspective view, b) shows a lateral view, c) shows a view opposite to the airflow direction, d) shows a top view; 
         FIG. 5  shows in a) the horizontal and bent section of an embodiment with rounded guide vanes, in b) a detailed view opposite to the airflow direction onto the rounded guide vanes, in c) a general view opposite to the airflow direction onto this embodiment, in d) a lateral view and in e) a top view; 
         FIG. 6  shows an embodiment with straight guide vanes, wherein in a) a perspective view is shown, in b) a lateral view is shown, in c) a view opposite to the airflow direction is shown; and 
         FIG. 7  shows an embodiment with triangular groups of finger ducts, wherein in a) a perspective view is shown, in b) the lateral view, in c) a lateral view with the airflows indicated, in d) a view opposite to the airflow direction, in e) a top view. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a general schematic set up of a combined cycle power plant with flue gas recirculation. In a compressor  1  intake air is compressed and introduced to a first combustor  4 , supplied with fuel  3 , and the resulting combustion air passes a high pressure turbine  5 , downstream of which a second combustor  6  is located, downstream of which in a low pressure turbine  7  the exhaust gases are expanded as much as possible. Downstream of the low pressure turbine  7  a heat recovery steam generator  8  is located by means of which, using the heat in the exhaust gases, steam is generated for driving steam turbines. The steam is expanded in a first stage in a high pressure steam turbine  9 , followed by an intermediate pressure steam turbine  10  and subsequent to this by a low pressure steam turbine  11 , where usually means are provided for by-passing the low pressure steam turbine  11 . Downstream of the low pressure steam turbine  11  a condenser  12  condenses the steam to water, which is subsequently pumped by a pump  13  into the heat recovery steam generator unit  8 , where it is converted in counter flow to the flow of the exhaust gases to steam again. Normally the steam cycle is an essentially closed cycle. 
     Downstream of the heat recovery steam generator unit  8  the exhaust gases, now cooled down to a certain extent, usually pass a diverter  17 , where depending on the mode of operation a fraction of the exhaust gases or the totality thereof can be guided to a stack  14  or to a downstream flue gas recirculation system and/or carbon capture system. 
     The fraction guided to the flue gas recirculation system usually first passes a direct contact cooler  21   a , downstream of this a diverter  17  is provided which, again depending on the mode of operation and specific parameters measured in the unit, a fraction of the exhaust gases is fed to the carbon capture unit, and the other fraction is recirculated in the actual flue gas recirculation piping, which usually also comprises at least one blower  15  as well as a mass flow measurement device  16  in order to control, for the reasons outlined above, the amount of recirculated flue gas in order to keep the combustion conditions in an optimum window. For flue gas recirculation these exhaust gases are mixed with ambient air in a mixer  20  which is typically located upstream of the intake housing which is essentially adjacent and upstream of the intake of the compressor  1 . In this example the direct contact cooler  21   a  comprises a water spray, which is injected into the flue gas. The water droplets are collected and re-cooled before they are re-injected. 
     The fraction of the exhaust gases not recirculated and not specifically used in the flue gas recirculation system  18  are introduced into the carbon capture system  19 , if necessary assisted by a blower  15 , where in a carbon dioxide absorption unit  19   a  the carbon dioxide is extracted from the gases and taken out of the system and the remaining gas is fed to a stack  14 . 
     The present invention pertains to a specific device for mixing re-circulated flue gas with newly aspired ambient air  21 , and the idea is to locate the corresponding mixing device in the actual intake section or intake housing  2  upstream of the compressor inlet. 
     A corresponding device is illustrated in  FIG. 2 a    in a perspective view. Such an intake housing  2  usually comprises a wide aspiration section  25  into which the ambient air flow  27  is flowing. Downstream of this wide aspiration section  25  or within this wide aspiration section  25 , typically the flow cross section reduces and is followed by a typically horizontal section  22  of constant flow cross section. Downstream of this section  22  there is typically provided a bent section  24  diverting the airflow to a vertical direction and into a vertical section  23  of the intake housing so that the airflow  26  can enter the intake of the compressor  1 , and can be compressed in the compressor  1 . 
     In accordance with the invention, mixing ducts  32  are located in section  22  in the form of a multitude of fingerlike elements protruding from one of the side walls of the section  22  into the flow path  31 , typically in a direction essentially perpendicular to the airflow direction  33  in the section  22 . The mixing ducts  32  are mounted on one of the side walls (or on several of the side walls), and where they are fixed to the side walls there is provided an inlet  34  for the intake of re-circulated flue gas  41 . 
       FIG. 2 b    illustrates a first embodiment of such mixing ducts. In this case a row of three mixing ducts  32  in the form of tubular elements with rectangular or square cross section are arranged adjacent to each other. Of the three mixing ducts  32 , the one located most upstream, is the longest one, therefore penetrates the most into the flow path  31 , and it also forms the leading edge  35  of the row as the subsequent mixing ducts are arranged in a downstream direction thereof. Within the row the mixing ducts are of regularly decreasing lengths such that the mixing duct located most downstream is the shortest one and penetrates the least into the flow path  31 , and essentially forms the trailing edge  36  of the row of mixing ducts. Like that the row of mixing ducts provides for a structure where, as at the end of each mixing duct there is an opening  37  through which flue gas taken in via inlets  34 , is blown out as illustrated with the arrows  39 , and introduces the re-circulated flue gas in a distributed manner over the flow cross section of the airflow  33 . 
     An alternative embodiment is illustrated in  FIG. 2 c   . In contrast to the embodiment according to  FIG. 2 b   , where essentially the tubular mixing ducts  32  are not provided with a bottom wall thereby providing the openings  37  there, in this case the tubular elements  32  are closed towards the bottom but on each tip portion on the corresponding trailing side of each mixing duct there are provided outlet openings  37  through which the flue gas exits the mixing ducts essentially in a direction parallel to the airflow direction  33 . 
     In  FIG. 3  yet another embodiment of a mixing duct  32  is illustrated. In this case there are not provided a row of mixing ducts but there is provided one mixing duct with several blow out openings  36  distributed over the length of the mixing duct. The mixing duct is of a stepped design and at each step there is provided an outlet  37  blowing flue gas in a direction similar to the one as illustrated in  FIG. 2 b    into the airflow. In this case the airflow  39  right downstream of the corresponding opening  37  impinges onto an impingement plate  38  which is arranged essentially perpendicular to the direction of the airflow  39  and in a direction parallel to the airflow  33  of the ambient air flowing in the flow path  31 . These impingement plates  38  are mounted on trailing edge side walls of the mixing duct  32 . The impingement plates  38  in this case laterally on both sides protrude beyond the side walls of the mixing duct, however it is also possible that the impingement plates  38  do not extend beyond the side walls. 
     Generally speaking the mixing ducts  32 , be it in a row as illustrated in  FIG. 2  or structured with several openings  37  distributed along their length as illustrated in  FIG. 3 , can be built of metal sheet elements. They can also be built of tubular pipe-like elements. In order provide as little resistance to the airflow  33  at the leading edge  35  as possible, as illustrated in  FIG. 3 , this leading edge  35  is preferably rounded or can have a sharp leading edge. Preferably a flat leading edge with a surface essentially perpendicular to the direction  33  should be avoided. 
     The leading edge  35  is normally straight in a direction parallel to the axis of the duct as illustrated in  FIG. 3 c   . It is however also possible, depending on the flow circumstances in the flow path  31 , to have a shaped leading edge, as for example illustrated in  FIG. 3   d.    
     The mixing ducts, arranged in rows, can be, as illustrated in  FIG. 2 a   , located in a series distanced from each other in a direction perpendicular to the flow direction  33 . As illustrated in  FIG. 3 e   , it is also possible to have several mixing ducts or rows of mixing ducts arranged following each other in the direction  33 , so to have an upstream mixing duct  32  or row of mixing ducts  32 , and a downstream mixing duct  32  or row of mixing ducts. Each of these mixing ducts can be supplied with re-circulated flue gas with individual ducts  45  and  46 , as illustrated in  FIG. 3 e   , or by using the same duct coupled to both rows. In the embodiment according to  FIGS. 3 e  and  f   , actually the arrangement of mixing ducts is such that two upstream mixing ducts  32  are located laterally displaced with each other by a long distance, so close to the side walls of the section  22 , in the flow path and are supplied with flue gas via duct  45 . Downstream of this pair of mixing ducts there is provided a second group of mixing ducts  32 , also displaced in a direction perpendicular to the airflow direction  33  but closer to each other so essentially filling the gap between the two upstream mixing ducts. 
     Yet another embodiment is illustrated in  FIG. 4 . Here one can see that downstream of the wide aspiration section  25  there is first located a silencer  40 . There can also be provided in addition to that or replacing the silencer a filter in this position, a filter may also be located within section  25 . 
     Downstream of the silencer  40  within section  42  there are provided four laterally displaced rows of mixing ducts  32  wherein again each row comprises in this case four mixing ducts of regularly decreasing lengths in a downstream direction. In this case each of the mixing ducts, in its terminal tip portion, comprises a bottom wall and at both lateral side a lateral opening  37  is provided through which the flue gas  41 , as illustrated in  FIG. 4 b   , passing through the tubular section of each mixing duct, is then blown out in a direction perpendicular to the flow direction  33 , essentially in horizontal direction, as illustrated in  FIG. 4 c   . Correspondingly the flue gas introduced into the airflow  33  is well distributed over the flow cross section thereof, and this in a vertical direction, as illustrated in  4   c , and in horizontal direction, as illustrated in  FIG. 4 d   , but also the mixing introduction takes place over a certain length of the flow path, as one can also see in  FIG. 4   d.    
     Yet another embodiment is shown in  FIG. 5 a   . Essentially in this embodiment the same structure of sixteen mixing ducts arranged in four laterally displaced groups of four mixing ducts each is provided as in  FIG. 4 . However in this case the lateral openings  37  provided on each side of each mixing duct in the tip portion thereof with a height h as illustrated in  FIG. 4 b    is provided with a rounded guide vane  42 , the function of which is best illustrated by  FIG. 5 b   . These rounded guide vanes make sure that the flue gas  41  passing through the channel of the mixing duct  32  in the region of the lateral openings  37  is exiting through these openings in a well distributed manner so not only under high speed and high pressure in the bottom region thereof but also in the top region thereof. This leads to a lower pressure loss in the mixing device and to an even more homogenous distribution and mixing. 
       FIG. 6  shows a similar embodiment; in this case however, the guide vanes are not rounded but are provided as straight plates arranged essentially perpendicular to the main axis direction of the respective mixing ducts. 
     The vanes are typically arranged in or very close to the actual opening  27 . As illustrated in  FIG. 5 b   , the height of the vane is preferably chosen so as to be essentially at half height of the total height h of the opening  37 . The width of the corresponding vane is preferably chosen to be about one fourth of the total lateral width of the mixing duct, so that one half of the airflow  41  is so to speak captured by the guide vanes and the other half can pass between them and exit via the part of the opening  37  located below the vane  42 / 43 . 
     A final embodiment is shown in  FIG. 7 . In this case, where 20 mixing ducts are arranged in four groups of five mixing ducts each are of triangular shape. The trailing edge of this triangular group is formed by an inclined trailing edge wall  44 . Due to this trailing edge wall, which is basically attached to a structure similar to the one illustrated in  FIG. 2 b   , leads to triangularly shaped openings  37 , as can be best viewed in  FIGS. 7 b  and  c   . These triangularly shaped lateral openings lead to an even better distribution of the blowing out of the flue gas and in this case, as one can see from  FIGS. 7 b  and  c   , the length of the triangular row of mixing ducts is such that it completely bridges the flow path section  22 , allowing for attachment of the mixing element on to opposite side walls in this section thereby increasing stability of the whole set up.