Abstract:
Carbon dioxide recirculating apparatus ( 20, 120 ) is disclosed for use in an arrangement having combination means ( 115 ) and a path for the flow of a gas through the combustion means ( 115 ). The apparatus ( 20, 120 ) comprises extraction means ( 221 ) for extracting carbon dioxide from a first region of the path downstream of the combustion means ( 115 ). It further includes condensing means ( 26, 30 ) for condensing the extracted carbon dioxide, and feed means ( 36, 136 ) for feeding the condensed carbon dioxide to a second region of the path upstream of the combustion means.

Description:
FIELD OF THE INVENTION 
     This invention relates to carbon dioxide recirculating apparatus. More particularly, the invention relates to carbon dioxide recirculating apparatus for heat engines, for example gas turbine engines, or fuel cells. 
     BACKGROUND OF THE INVENTION 
     It is known to inject a water based fog into the compressor region of a gas turbine engine to increase the power of the engine. The water is atomised when it is sprayed into the compressor region and forms a fog. The water droplets forming the fog vaporise and extract latent heat of evaporation from the gases in the compressor, thereby cooling these gases. This has a beneficial affect on the power output of the engine. A disadvantage of such a system is that evaporation of the water droplets is not readily achieved and requires onerous nozzle and spray pressure specifications to achieve the required cooling effect. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this invention, there is provided carbon dioxide recirculating apparatus for an arrangement comprising combustion means and a path for a flow of gas through the combustion means, the apparatus comprising extraction means for extracting gaseous carbon dioxide from a region of the path downstream of the combustion means, condensing means for condensing the extracted carbon dioxide, and feed means for feeding the condensed carbon dioxide to a region of the path upstream of the combustion means. 
     The arrangement preferably comprises a heat engine or a fuel cell. 
     The preferred embodiment of the invention is particularly suitable for use in an arrangement in the form of a gas turbine engine. In this embodiment, the feed means may feed the condensed carbon dioxide to a compressor region of the gas turbine engine. The extraction means may be arrangeable downstream of a turbine arrangement of the gas turbine engine. 
     According to another aspect of this invention there is provided an arrangement comprising combustion means a path for the flow of gas through the combustion means and carbon dioxide recirculating apparatus comprising extraction means for extracting gaseous carbon dioxide from a first region of the path downstream of the combustion means, condensing means for condensing the extracted carbon dioxide, and feed means for feeding the condensed carbon dioxide to a second region of the path upstream of the combustion means. 
     The arrangement may comprise a heat engine or a fuel cell assembly. The heat engine may be a gas-turbine having a compressor region in the path of the gas upstream of the combustion means, and a turbine region in the path of the gas downstream of the combustion means. The compressor means may be a compressor unit. 
     The condensing means may comprise heat removal means to remove heat from the extracted carbon dioxide. The condensing means may include compressor means to compress the extracted carbon dioxide. Preferably, the compressor means is arranged between the extraction means and the heat removal means. In the preferred embodiment, the heat removal means comprises cooling means to cool the carbon dioxide. 
     The feed means may comprise spray means to spray the condensed carbon dioxide into the second region. Where the arrangement comprises a gas turbine engine, the spray means may spray the condensed carbon dioxide into the compressor region of the gas turbine engine, preferably to form a fog of the carbon dioxide. The spray means may comprise atomising means, which may be in the form of a nozzle. Preferably, the atomising means comprises a plurality of atomising nozzles, which may be in the form of an array of nozzles. 
     The extraction means may comprise a recirculating amine based extraction means, and may include cooling and heating units to support the operation of the amine based extraction means. 
     Preferably, the extraction means is arrangeable to extract carbon dioxide from the exhaust gases downstream of the turbine region. 
     The compressor region of the engine may comprise first and second compressors, and the feed means may be arrangeable to feed condensed carbon dioxide to the compressor region between the first and second compressors. Alternatively, or in addition, the feed means may feed the condensed carbon dioxide to the compressor region at an outlet of the compressor region. Alternatively, or in addition, the feed means may be arrangeable to feed the carbon dioxide at an inlet to the compressor region. 
     In one embodiment, the feed means may be arrangeable to feed the condensed carbon dioxide to the outlet of the compressor region, whereby the carbon dioxide thereafter passes into a heat exchanger to exchange heat with gases exiting from the turbine region of the gas turbine engine. Preferably, the heat exchanger comprises a recuperator. 
     The fuel cell assembly may be arranged to receive carbon dioxide from the carbon dioxide recirculating apparatus. The fuel cell assembly may comprise an anode and a cathode. Preferably exhaust from the anode is passed to the carbon dioxide recirculating apparatus. 
     The fuel cell assembly may comprise a compressor for compressing air and other gases to be supplied to the cathode. The fuel cell assembly may comprise a conduit for directing recirculated carbon dioxide to the anode. Alternatively, or in addition, the fuel cell assembly may comprise a conduit for directing carbon dioxide to the compressor for mixing with air compressed by the compressor. The compressor may be a compressor of the compressor region of the gas turbine engine. 
     According to another aspect of this invention, there is provided a method of recirculating carbon dioxide from a flow of gas through an arrangement comprising combustion means and a path for the flow of gas through the combustion means, the method comprising extracting carbon dioxide from a first region downstream of the combustion means, condensing the extracted carbon dioxide and thereafter feeding the condensed carbon dioxide to a second region upstream of the combustion means. 
     Preferably the step of condensing the extracted carbon dioxide comprises providing heat removal means to remove heat from the carbon dioxide and may also include compressing the carbon dioxide prior to removing said heat from the carbon dioxide. 
     The heat removal means may comprise cooling means to cool the carbon dioxide to effect said condensation thereof. 
     The step of feeding the condensed carbon dioxide to the second region of the engine may comprise spraying the condensed carbon dioxide to the second region. The spraying of the condensed carbon dioxide may form a fog of the carbon dioxide in the upstream region of the engine. 
     Preferably the step of feeding the carbon dioxide to the second region of the engine comprises atomising the condensed carbon dioxide. 
     In the preferred embodiment, the engine is a gas turbine engine comprising a compressor region upstream of the combustion means and a turbine region downstream of the combustion means, and the step of extracting carbon dioxide comprises extracting carbon dioxide downstream of the turbine region of the engine. 
     In the preferred embodiment, the step of feeding the carbon dioxide to the second region of the path comprises feeding the carbon dioxide to the compressor region of the engine. The compressor region may comprise first and second compressors arranged in axial flow series in the path and the step of feeding the carbon dioxide to the compressor region may comprise feeding the carbon dioxide between the first and second compressors and/or to an outlet of the compressor region and/or to an inlet of the compressor region. 
     The engine may comprise heat exchange means to exchange heat between gas entering the combustion means and gas exhausted from the combustion means, preferably downstream of the turbine region. The step of feeding the condensed carbon dioxide to the second region may comprise feeding the carbon dioxide to gas entering the heat exchange means upstream of the combustion means preferably at the outlet of the compressor region. The heat exchange means may comprise a recuperator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  shows a sectional view of the upper half of a gas turbine engine; 
         FIG. 2  is a diagrammatic representation of carbon dioxide recirculating apparatus; 
         FIG. 3  is a further diagrammatic representation showing another embodiment of carbon dioxide recirculating apparatus; 
         FIG. 4  is diagrammatic representation of a further embodiment of carbon dioxide recirculating apparatus incorporating fuel cell; and 
         FIG. 5  is a diagrammatic representation of another embodiment of carbon dioxide recirculating apparatus incorporating a fuel cell. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal axis X-X. The engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , a compressor region  113  comprising an intermediate pressure compressor  13 , and a high pressure compressor  14 , combustion means  115  comprising a combustor  15 , and a turbine region  116  comprising a high pressure turbine  16 , an intermediate pressure turbine  17 , and a low pressure turbine  18 . An exhaust nozzle  19  is provided at the tail of the engine  10 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  11  is accelerated by the fan to produce two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which provides propulsive thrust. The intermediate pressure compressor  13  compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
     The compressed air exhausted from the high pressure compressor  14  is directed into the combustor  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbine  16 ,  17  and  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbines  16 ,  17  and  18  respectively drive the high and intermediate pressure compressors  14  and  13  and the fan  12  by suitable interconnecting shafts. 
     Referring to  FIG. 2 , there is shown a schematic representation of carbon dioxide recirculating apparatus  20  for use in a gas turbine engine.  FIG. 2  shows the compressor region  113 , the combustion region  115 , and the turbine region  116 , of the engine  10  as described above.  FIG. 2  also shows the carbon dioxide recirculating apparatus  20  comprising extraction means  22  arranged downstream of the turbine region  116  in the main flow of gas to be exhausted from the engine  10 . The extraction means  22  may comprise any suitable known carbon dioxide extraction arrangement. An example of such an arrangement is a recirculating amine based unit in which amine solvents such as diethandamine can be employed to remove the carbon dioxide to remove carbon dioxide from the gas in the main flow downstream of the turbine region  116 . Carbon dioxide extracted from the exhaust gases is passed via a line  24  to compressor means in the form of a carbon dioxide compressor unit  26 . The resulting carbon dioxide depleted exhaust is passed to the exhaust nozzle  19 . 
     The compressor unit  26  compresses the extracted carbon dioxide, which is passed via a line  28  to a cooler  30  which condenses the compressed carbon dioxide, and produces pressurized liquid carbon dioxide at or near ambient temperature. The pressurized liquid carbon dioxide is then passed via a line  32  to feed means in the form of a feed nozzle arrangement  36 . The pressurized liquid carbon dioxide is fed by the nozzle arrangement  36  to the compressor region  113  of the gas turbine engine  10 . Specifically, the liquid carbon dioxide is fed to the main duct designated  34  (shown schematically in  FIG. 2 ) between the intermediate pressure compressor  13  and the high pressure compressor  14 . 
     The feed nozzle arrangement  36  comprises an array of atomising nozzles to spray the liquid carbon dioxide into the main duct  34  between the intermediate and high pressure compressors  13 ,  14 . 
     As the liquid carbon dioxide is atomised into the duct  34 , it partly flashes to the vapour phase. Carbon dioxide which does not vaporise partly solidifies. This results in a mixture of solid, liquid and gaseous carbon dioxide. The solid carbon dioxide then sublimes to the vapour phase, and the remaining liquid carbon dioxide then vaporises to the vapour phase. This results in a carbon dioxide fog forming in the duct  34 . The sublimation and vaporisation of the carbon dioxide absorbs latent heat of sublimation and vaporisation from the gases in the duct  34  thereby cooling these gases. 
       FIG. 3  shows a further embodiment, which has many of the features of  FIG. 2 , and these features have been designated with the same reference numeral. In  FIG. 3 , the carbon dioxide recirculating apparatus is designated  120  and the feed means is designated  136  and comprises a first feed nozzle arrangement  136 A and a second feed nozzle arrangement  136 B. 
     The line  32  carrying the pressurised liquid carbon dioxide at ambient temperature splits into a first line  132 A leading to the first feed nozzle arrangement  136 A and a second line  132 B leading to the second feed nozzle arrangement  136 B. The first feed nozzle arrangement  136 A atomises the liquid carbon dioxide so that it is sprayed into the duct  34  between the intermediate and high pressure compressors  13 ,  14 . This spraying of the carbon dioxide has the same effects upon it as described above, with reference to the spraying of the carbon dioxide into the duct  34  in  FIG. 2 . The second feed nozzle arrangement sprays the liquid carbon dioxide into a duct  38  downstream of the high pressure compressor  14 , and upstream of the combustor region  115 . Again the carbon dioxide sprayed into the duct  38  undergoes the same phase changes as described above. 
     Thus, the pressurised liquid carbon dioxide sprayed into the ducts  34  and  38  via the respective arrays of nozzles  136 A and  136 B form a carbon dioxide fog in the ducts  34  and  38 . 
     A heat exchanger in the form of a recuperator  40  is provided in the embodiment shown in  FIG. 3 , to exchange heat between gases exiting the compressor region  113  and the gases exiting the turbine region  116 . The carbon dioxide fed by the second feed nozzle arrangement  136 B into the main flow of gas in the duct  38  flashes to the vapour phase, solidifies, sublimes and vaporises in the same way as described above with reference to  FIG. 2 . The carbon dioxide, along with other gases in the duct  38  is passed, as indicated by the arrow  39 , to one side of a recuperator  40  to exchange heat with exhaust gases from the turbine region  116  passed to the other side of the recuperator  40  as indicated by the arrow  42 . The recuperator  40  is provided to increase the heat in the gases entering the combustor  15 , which also has the effect of cooling the gases exiting from the turbine region  116  upstream of the extraction means  22 . 
     The exhaust gases from the turbine region  116  exit the recuperator  40  and are then passed to the carbon dioxide extraction means  22  via the main duct, as indicated by the arrow  44 . 
       FIG. 4  is a diagrammatic representation of carbon dioxide recirculating apparatus  20  incorporating a fuel cell  50 . The carbon dioxide recirculating apparatus  20  in  FIG. 4  is shown in use in a gas turbine engine  10 . The features of the gas turbine engine  10  and the carbon dioxide recirculating apparatus  20  are given the same reference numerals as in  FIG. 2 . 
     The fuel cell  50  is, in the embodiment shown, a fuel cell of a type known generally as a solid oxide fuel cell. The fuel cell  50  comprises an anode  52  and a cathode  54 . 
     The compressor region  113  of the gas turbine engine  10  supplies compressed air to the cathode  54  via a recuperator  140  along a line  56 . In the anode  52 , the hydrogen in the fuel reacts with oxygen ions produced at the cathode  54  (as explained below) to produce water molecules and electrons creating an electric current. This is an exothermic reaction and the heat generated is transferred to the incoming compressed air in the line  56  in the recuperator  140 . The output from the cathode  54  is passed along a line  58  (via the recuperator  140 ) to a combustor  160 . If desired the combustor  160  can be the combustor  15  of the engine  10 . 
     A fuel mixture (labelled FUEL in  FIG. 4 ) is supplied to a supplementary compressor  62  which also received recirculated carbon dioxide from the carbon dioxide recirculating apparatus  20  (as explained below). The fuel mixture comprises fuel, hydrogen, carbon dioxide, carbon monoxide and hydrocarbons and is compressed by the supplementary compressor  62  and fed via a line  64  to the anode  52  of the fuel cell. 
     The oxygen in the compressed air in the cathode  54  is electrically charged to provide oxygen ions. The oxygen ions pass through/across the solid oxide electrolyte membrane in the fuel cell  50  between the cathode  54  and the anode  52 , to react with the hydrogen in the anode  52  (as described above). 
     The exhaust products from the anode  52  are fed via a line  66  to the carbon dioxide extraction means  22 . The carbon dioxide is extracted from the exhaust products and passed via the line  24  to the carbon dioxide compressor unit  26  to be recirculated, via the cooler  30 , back to the supplementary compressor  62 . 
     The remaining exhaust products entering the carbon dioxide extraction means are passed to the combustor  160  along a line  68  and fed back to the turbine arrangement  116  of the engine  10  along a line  70 . This powers the turbine arrangement to the drive the compressor arrangement  113 . The exhaust from the turbine arrangement  116  is exhausted to atmosphere via the exhaust nozzle  19 , (labelled EXHAUST). 
     As an alternative, or in addition, as shown in broken lines, the recirculated carbon dioxide flowing along the line  28  could be fed via a line  128  to the compressor arrangement  113 . 
     Another embodiment of a carbon dioxide recirculating apparatus  20  incorporating a fuel cell  50  is shown in  FIG. 5 . The carbon dioxide recirculating apparatus  20  is shown in use in a gas turbine engine  10 . The features of the gas turbine engine  10  and the carbon dioxide recirculating apparatus are given the same reference numerals as in  FIG. 2 . 
     The fuel cell  50  shown in  FIG. 5  is of a type known as a molten carbonate fuel cell. In such a fuel cell  50  there is a requirement for carbon dioxide on the air/oxygen side of the fuel cell  50 , i.e. the cathode  54 . 
     The compressor region  113  receives recirculated carbon dioxide via line  128 A (as explained below), in addition to air. The compressed air and carbon dioxide is passed via the line  56  to the cathode  54  of the fuel cell  50 . Some of the reaction products from the cathode are passed via the line  58  to the combustor  160 . In some embodiments, the combustor  160  could be the combustor  15  of the gas turbine engine  10 . 
     The remainder of the reaction products from the cathode  54  are passed via a line  70  to the turbine arrangement  116 , which drives the compressor arrangement  113  as explained above. The exhaust  72  from the turbine arrangement  116  drives a free power turbine  74 , which, in turn, drives a further compressor  76  via a shaft  78 . 
     The exhaust from the free power turbine  74  is passed via a line  90  to a heat exchanger or recuperator  92  where heat is exchanged with compressed gases exiting from the compressor arrangement  113  prior to entering the cathode  54 . 
     After exiting the recuperator  92 , the gases from the free power turbine  74  are exhausted to atmosphere via the exhaust nozzle  19 . 
     The combustion products from the combustor  160  are passed via a line  80  to the further compressor  76 . The further compressor  76  also receives recirculated carbon dioxide (as explained above) via a line  128 B. The compressed combustion products and carbon dioxide are passed to the cathode  54  via a line  82 . 
     The anode  52  receives fuel via the line  64 . The reaction products pass from the anode  52  by a line  66 . Some of the reaction products may be recirculated via a line  84  and a supplementary compressor  86  to be passed back into the anode  52 . 
     The reaction products from the anode  52 , which are not recirculated, are split into two. Some of the reaction products from the anode  52  are passed via a line  66 A to the combustor  160  and are mixed with the incoming reaction products from the cathode  54  and combusted. The remainder of the reaction products from the anode  52  are passed via a line  66 B to the carbon dioxide extraction means  22 . 
     The carbon dioxide is extracted and passed via the line  24  to the carbon dioxide compressor unit  26  and then cooled by the cooler  30 . The cooled carbon dioxide exits the cooler  30  via the line  128 . Some of the cooled carbon dioxide is passed via the line  128 A to the compressor arrangement  113 . The remainder of the cooled carbon dioxide is passed via the line  128 B to the compressor  76 , as described above. 
     The remaining cathode reaction products in the carbon dioxide extraction means  22  are passed via the line  68  to the combustor  160  to be combusted. 
     The above described embodiments have the advantage that the fog sprayed into the compressor region is formed from liquid carbon dioxide that flashes under more favourable and more easily achieved conditions than water. As a result, a fog with a small droplet size is generated more readily than with water. This results in there being less demanding nozzle and spray pressure specifications than are necessary with water. 
     Further, the above described embodiments have the advantage that the use of carbon dioxide means that complete evaporation of the fog can be more easily achieved than with water, due to the high saturated vapour pressure of the carbon dioxide at ambient temperatures. This permits the use of carbon dioxide based fog cooling to be used in conditions that would be too confined in length to achieve adequate evaporation with a water based fog. A further benefit is that carbon dioxide recirculation is achieved with less compression in the recirculating system than with a water based recirculation. It is possible thus to enhance efficiency and power of the engine more substantially than with the use of water. 
     The use of carbon dioxide has the further advantage that cooling could be carried out at the inlet of the compressors of a gas turbine engine, and could also be used in other engines, for example reciprocating engines. Indeed, the invention could be applied to a wide range of cycles using heat engines and/or fuel cells as the primary source of carbon dioxide. 
     Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.