Abstract:
A method for compressing gaseous fuel is disclosed. The method includes, ingesting gaseous fuel into a chamber, ingesting air into the chamber and mixing the gaseous fuel with the air, igniting and partially combusting the resulting mixture of gaseous fuel and air in a confined space such that a predominant fraction of the gaseous fuel is not combusted, causing an increased temperature and therefore an increased pressure of the fraction of the gaseous fuel which is not combusted, and discharging the resulting compressed gaseous fuel. Moreover, a compressor is provided including a casing, a rotor with at least three vanes, an inlet for gaseous fuel, an outlet for gaseous fuel, an air inlet and an igniter. The rotor is placed in the casing such that at least three variable-volume chambers part-bounded by the vanes are formed during a rotor revolution.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2009/050998, filed Jan. 29, 2009 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 08004162.7 EP filed Mar. 6, 2008. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a method for compressing gaseous fuel, to a compressor and to a gas turbine. 
       BACKGROUND OF INVENTION 
       [0003]    Modern gas turbines are evolving towards higher peak cycle pressures in order to increase fuel efficiency. Since the fuel, which is induced at a peak pressure, must be supplied with sufficient overpressure to account for valve losses, ingestion speeds and other flow issues, the fuel supply pressure must also rise. However, gas supply networks and pipelines have been designed for a fixed pressure and the huge infrastructure costs mean that these pressures will stay fixed for a considerable period. 
         [0004]    It is observed that at extreme operating conditions the pipeline may not have sufficient fuel pressure to supply the gas turbine. This difficulty may be increasingly encountered in the further evolution of gas turbine technology. Up to now at off-design conditions either the power output of the engine can be limited, which may cause customer dissatisfaction, or a motor-driven fuel gas compressor can be used, which may add costs and erode efficiency because of the power drain. 
       SUMMARY OF INVENTION 
       [0005]    It is an objective of the present invention to provide an advantageous method for compressing gaseous fuel. It is another objective of the present invention to provide an advantageous compressor. A last objective is to provide an advantageous gas turbine. The first objective is solved by a method for compressing gaseous fuel as claimed in the claims. The second objective is solved by a compressor as claimed in the claims and the third objective is solved by a gas turbine as claimed in the claims. The depending claims define further developments of the invention. 
         [0006]    The inventive method for compressing gaseous fuel comprises the steps: a) ingesting gaseous fuel into a chamber; b) ingesting air into the chamber and mixing the gaseous fuel with the air; c) igniting and partially combusting the resulting mixture of gaseous fuel and air in a confined space such that a predominant fraction of the gaseous fuel is not combusted, causing an increased temperature and therefore an increased pressure of the fraction of the gaseous fuel which is not combusted; and d) discharging the resulting compressed gaseous fuel. This means, that part of the energy of the fuel is used to perform the compression by means of partial combustion. 
         [0007]    The partial combustion the mixture of gaseous fuel and air such that a predominant fraction of the gaseous fuel is not combusted, causes an increased temperature of the combusted fraction which raises the pressure of all the combusted and uncombusted fuel present in the substantially fixed volume chamber. 
         [0008]    For ingesting air, air with a higher pressure than the pressure of the gaseous fuel may be injected into the chamber. Moreover, the volume of the chamber can be decreased for discharging the compressed gaseous fuel. 
         [0009]    Advantageously the steps a) to d) can be repeated after finishing step d). At least one of the steps a) to d) may be performed at least twice before the next step is performed. Moreover, the gaseous fuel may be, for example, introduced into a chamber with an increasing volume. 
         [0010]    The basic principle of the invention is that fuel and air at the same time or afterwards are sucked in, the air is mixed with all or part of the fuel, the mixture is burned or explodes in a confined space and the part-burned fuel at higher pressure is pushed out. 
         [0011]    The gaseous fuel may be compressed to a pressure with a pressure ratio between the compressed gaseous fuel and the uncompressed gaseous fuel of between 1.1:1 and 5:1. Preferably the pressure ratio reaches a value of 2:1. 
         [0012]    The air may be induced such that it is given a swirling or vortex character. This provides a controlled mixture between the induced air and the gaseous fuel. Furthermore, the mixture of gaseous fuel and air can be continuously ignited. Advantageously the mixture of gaseous fuel and air is ignited and partially combusted in a chamber with a constant volume. Moreover, the compressed gaseous fuel may be cooled at constant pressure. 
         [0013]    The inventive compressor comprises a casing, a rotor with at least three vanes, at least one inlet for gaseous fuel, at least one outlet for gaseous fuel, at least one air inlet, and at least one igniter. The rotor is placed in the casing such that at least three variable-volume chambers part-bounded by the vanes are fanned during a rotor revolution. The inlet for gaseous fuel is placed in the casing such that the inlet for gaseous fuel is connected to a first location where a chamber has an increasing volume during a revolution of the rotor. The air inlet and the igniter are placed in the casing in a second location, where a chamber has an increasing, decreasing or constant volume during a revolution of the rotor. The outlet for gaseous fuel is placed in the casing in a third location, where a chamber has a decreasing volume during a revolution of the rotor. Advantageously the air inlet and the igniter can be connected to a second location, where a chamber has a constant or at least nearly constant volume during a revolution of the rotor. 
         [0014]    The rotation axis of the rotor may be eccentrically placed relative to the centreline of the casing. The casing and/or the rotor may have a circular cross section, an elliptical cross section, or a curved cross section with curvature discontinuities or inflexions. The igniter may, for example, be a plasma igniter or a spark plug. Advantageously, the igniter is placed at the air inlet. Generally the igniter can be placed where the mixture between air and fuel has taken place, for example near the air inlet. 
         [0015]    Furthermore, a seal can be placed between the casing and the rotor to provide a seal between a location where a chamber with a decreasing volume is formed and a location where a chamber with an increasing volume is formed. This can minimise leakage and maximise the evacuation of the third chamber through the outlet. 
         [0016]    The compressor may comprise one or more additional sets of three chambers in parallel or series. The sets can be disposed about the rotor periphery so that they at least partly balance out radial and axial forces acting on the rotor. Furthermore, such disposition may be designed to allow some of the pressure produced to be sacrificed as a driving force for the rotor. For example, the different sets of three chambers can be positioned side by side on the same rotor, but with sequences rotated relative to each other. 
         [0017]    The compressor may comprise six or twelve chambers. In these cases the compression can be performed twice or four times during one rotor revolution. An additional advantage of these arrangements is that the radial and axial forces acting on the rotor and the casing are in balance resulting in low bearing reaction forces and hence losses. This arrangement also has a positive impact on vibrations and unbalances generated by the compressor during operation. 
         [0018]    The compressor can comprise a cooling device for cooling the compressed gaseous fuel at constant pressure. Such a cooling device can, for example, be fitted in the exhaust from the compressor where the volume is no longer confined. The benefit of this cooling is for controlling fuels which are sensitive to pre-ignition when mixed with air in the combustor. 
         [0019]    The compressor may further comprise a gaseous fuel supply with an adjustable valve and/or a delivery line with an adjustable valve, the delivery line being connected to the outlet for gaseous fuel. Furthermore, the compressor can comprise an air supply with a non-return valve and/or a gaseous fuel supply with a non-return valve and/or a second gaseous fuel supply with a non return valve, the second gaseous fuel supply being connected to the outlet for gaseous fuel to lead an overrun of compressed gaseous fuel into the chamber with an increasing volume. 
         [0020]    The rotor may comprise a rotor body with slots and at least a portion of each vane is adapted to move in and out of a slot such that the vane is in sliding contact with an inner surface of the casing. The in and out movement provides a simple means for fainting the variable-volume chambers during a rotor revolution. 
         [0021]    Generally each vane may comprise a tip with a seal to provide a seal between the inner surface of the casing and the vane. These seals can be used additionally or alternatively to the previously mentioned seal (stator seal). Moreover, abradable material may be placed between the inner surface of the casing and the vane tip and/or between the stator seal and the vane tip. 
         [0022]    The rotor can be connected to an engine. The engine may comprise a motor or a turbine, for example. 
         [0023]    The inventive gas turbine comprises a compressor as previously described. A typical gas turbine comprises a compressor to compress air, a combustor and a turbine. In the combustor a mixture of air and gaseous fuel can be combusted. The inventive compressor can be used to compress at least part of the gaseous fuel, which may then be combusted in the combustor of the gas turbine. 
         [0024]    Generally the invention has the following advantages: If the fuel conduit from the compressor to the gas turbine is insulated, the heat loss will be low and the fuel energy spent on compression can be recuperated in the turbine during the expansion and in the waste heat recovery unit if one is fitted, for example, in a combined cycle unit. On the other hand if the fuel is cooled down after compression, which is an option with intermediate storage, an energy loss like for conventional gas compression occurs. Due to the highly concentrated heat release provided by the combustion the through-flow capacity of the compressor is much higher for a given geometrical size compared to cycles based on heat exchange across a wall and using an external heat source such as flue gases. 
         [0025]    Moreover, because the rotor is only moving gas around the drive requires far less energy than mechanical compression. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Further features, properties and advantages of the present invention will become clear from the following description of embodiments in conjunction with the accompanying drawings. 
           [0027]      FIG. 1  schematically shows an inventive compressor in a sectional view. 
           [0028]      FIG. 2  schematically shows an inventive compressor, as it is shown in  FIG. 1 , with a cooling device and a pressure compensation pipe. 
           [0029]      FIG. 3  schematically shows an alternative inventive compressor with six chambers in a sectional view. 
           [0030]      FIG. 4  schematically shows an inventive compressor with twelve chambers in a sectional view. 
           [0031]      FIG. 5  schematically shows an alternative inventive compressor with twelve chambers in a sectional view. 
           [0032]      FIG. 6  schematically shows another alternative inventive compressor with twelve chambers in a sectional view. 
           [0033]      FIG. 7  schematically shows two inventive compressors piped in series. 
           [0034]      FIG. 8  schematically shows two inventive compressors piped in parallel. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0035]    A first embodiment of the invention will now be described with reference to  FIGS. 1 and 2 .  FIG. 1  schematically shows an inventive compressor  1  in a sectional view.  FIG. 2  schematically shows an inventive compressor, as it is shown in  FIG. 1 , with a cooling device and a pressure compensation pipe. The compressor  1  in  FIGS. 1 and 2  comprises a casing  2  and a rotor  3 . The rotor comprises a rotor body  4 , three slots  5  and three vanes  6 . The angle between two neighbouring vanes  6  has a value of 120°. The rotor body  4  has a circular cross section. Generally, the rotor body  4  and/or the inner surface  27  of the casing  2  can have an oval, elliptic, circular or another appropriate cross section. 
         [0036]    The rotor  3  is eccentrically placed inside the casing  2 . Each vane  6  is at least partially located inside a slot  5  and protrudes radially outwards from the rotor body  4 . Each vane  6  comprises a portion  28 , which is adapted to move in and out of the slot  5  such that the vane  6  is in sliding contact with the inner surface  27  of the casing  2 . Additionally, each vane  6  may comprise a seal at the tip of the vane  6  to provide a seal between the vane  6  and the inner surface  27  of the casing  2 . 
         [0037]    Between the inner surface  27  of the casing  2  and the rotor  3  three variable-volume chambers  15 ,  16 ,  17  part-bounded by the vanes  6  are formed. During a revolution of the rotor  3  in the direction, which is indicated by an arrow  13 , a chamber assumes an increasing volume  15 , an approximately constant volume  16 , and a decreasing volume  17 . 
         [0038]    The casing  2  comprises an inlet  7  for gaseous fuel, an air inlet  9 , an igniter  14 , and an outlet  11  for gaseous fuel. The inlet  7  for gaseous fuel is located at a position where a chamber assumes increasing volume  15 . The igniter  14  is located at the air inlet  9 , such that it can ignite a mixture between air and fuel. Both are located at a position where a chamber assumes approximately constant volume  16 . The outlet  11  for gaseous fuel is located in the casing  2  at a position where a chamber assumes decreasing volume  17 . 
         [0039]    Between the inner surface  27  of the casing  2  and the rotor body  4  a seal  18  may be placed at a position where the radial distance between the inner surface  27  of the casing  2  and the rotor body  4  is minimal. This means that the seal provides a seal between the chamber  17  with decreasing volume and the chamber  15  with increasing volume. The seal  18  can, for instance, be a brush seal. The seal if used must be so formed that the moving vanes can smoothly travel over it. 
         [0040]    The inventive compressor  1 , as it is shown in  FIGS. 1 and 2 , uses three chambers  15 ,  16 ,  17 . Gaseous fuel is ingested into the chamber  15  with increasing volume through the inlet  7 . The direction of the gaseous fuel flow is indicated by an arrow  8 . During the rotation of the rotor  3  in direction  13  the chamber with gaseous fuel ingested reaches the location  16  where its volume stays constant. Air is induced into the gaseous fuel in the chamber  16  with approximately constant volume through the air inlet  9 , as indicated by the arrow  10 . The pressure of the induced air is higher than the pressure of the gaseous fuel in the chamber  16 . This is achieved principally by the incoming air pressure. The inlet  7  is placed such that the chamber  16  is completely filled before the chamber  16  reaches its maximum volume and before it reaches the igniter  14 . Additionally or alternatively a throttle can be placed at the gas inlet  7  which can be used when necessary, for instance at part load or when low overall pressure boost is required. If no boost is required, it is also possible to throttle the air or to switch off the ignition. 
         [0041]    The induced air is mixed with the gaseous fuel. Advantageously the air is induced such that it is given a swirling or vortex character. The mixture of gaseous fuel and air is ignited and partially combusted in chamber  16 , while it is still in the location where it has a constant or at least a nearly constant volume. The igniter  14  may be a plasma igniter or a spark plug. 
         [0042]    The pressure of the gaseous fuel, which leaves the compressor  1  through the outlet  11 , must be higher than the air pressure to be able to drive the gas turbine combustor, for instance. The pressure rise of the gaseous fuel is achieved by a temperature rise from combusting a small fraction of the gaseous fuel in the chamber  16  with a, at least nearly, constant volume. The part-burned gaseous fuel at higher pressure is pushed out of the chamber  17 , which has a decreasing volume and comprises the outlet  11 . The direction of the compressed gaseous fuel flow is indicated by an arrow  12 . 
         [0043]    After the combustion when the chamber is in the location  16  the uncombusted predominant fraction of the gaseous fuel is further compressed in chamber  17  because of the decreasing volume of chamber  17 . 
         [0044]    Alternatively, the inlet for gaseous fuel can be placed at a position which is indicated in  FIG. 1  by reference numeral  7   a , the air inlet can be placed at a position which is indicated by reference numeral  9   a  and the igniter can be placed at a position which is indicated by reference numeral  14   a.    
         [0045]    The seal  18  near the outlet  11  is used to minimise leakage and maximise the evacuation of the chamber  17 . 
         [0046]    A stratified charge approach, which means that lean burn pressurises the remaining gas, might reduce the pressure ratio significantly but would entail a compromise on the NO x -emission. With a stratified charge approach cavities can be built into the rotor  3 , but more preferably into the casing  2 , to minimise displacement losses during the discharge phase. A stratified charge approach maintains burning with less air but at the expense of achievable pressure rise. 
         [0047]      FIG. 2  schematically shows the inventive compressor  1 , as it is described with reference to  FIG. 1 , but additionally equipped with a cooling device  21  and a pressure compensation pipe  26 . The casing  2  in  FIG. 2  comprises two inlets  7  for gaseous fuel, which are placed at a position where the chamber  15  with increasing volume is located. One inlet  7  for gaseous fuel is connected to a common gaseous fuel supply. This inlet  7  is further equipped with an adjustable valve  19  and with a non-return valve  22 . In the flow direction  8  the gaseous fuel passes at first the adjustable valve  19  and then the non-return valve  22 . The adjustable valve  19  can act as an accelerator when throttled. It can, for instance, be fully closed during start-up and low loads. The adjustable valve  19 , which is the gaseous fuel supply valve, may be ordered by an adjustable governing valve  20 , which is placed at the outlet  11  for compressed gaseous fuel. 
         [0048]    Between the outlet  11  and the adjustable governing valve  20  a pressure compensation pipe  26  is mounted, which leads to the second inlet  7   b  into the chamber with increasing volume  15 . A non-return valve  24  is mounted at the inlet  7   b  of the pressure compensation pipe  26  in chamber  15 . This non-return valve  24  prevents a backflow from chamber  15  into the pressure compensation pipe  26 . During transients such as load shedding the fuel pressure and the fuel flow, which is delivered by the compressor, might be higher than needed. In this case the adjustable governing valve  20  at the outlet  11  is restricting the flow and the overrun of compressed gaseous fuel is routed back to chamber  15  through the pressure compensation pipe  26 . In this case only overrun of gaseous fuel cycles the system, while no ignition and combustion is performed, until the pressure is below the demand of the adjustable governing valve  20 . The direction of the compressed gaseous fuel flow in the pressure compensation pipe  26  is indicated by arrows  25 . 
         [0049]    The air inlet  9  of the compressor  1  in  FIG. 2  comprises a non-return valve  23 , which prevents a backflow. When the combustion commences the pressure might rise above the air supply pressure and in this case a backflow must be prevented. 
         [0050]    The compressor  1 , which is shown in  FIG. 2 , further comprises a cooling device  21 , which cools the compressed gaseous fuel at constant pressure. The direction of the compressed gaseous fuel flow is indicated by the arrows  12 . 
         [0051]    Now a second embodiment will be described with reference to  FIG. 3 . Elements corresponding to elements of the first embodiment will be designated with the same reference numerals and will not be described again.  FIG. 3  schematically shows an alternative inventive compressor  101  with six chambers  15 A,  15 B,  16 A,  16 B,  17 A,  17 B in a sectional view. 
         [0052]    Differing from  FIGS. 1 and 2  the compressor  101  in  FIG. 3  comprises a casing  2 , which has an inner surface  27  with an elliptical cross section. The rotor body  4  of the rotor  3  has a circular cross section, as in the first embodiment. The rotor  3  is concentrically placed inside the casing  2 . The rotor  4  further comprises six vanes  6 , which are located in slots  5 , as described in the first embodiment. 
         [0053]    The compressor  101  in  FIG. 3  comprises six variable-volume chambers  15 A,  15 B,  16 A,  16 B,  17 A,  17 B part-bounded by the vanes  6 . The vanes  6  are arranged in the rotor  3  such that the angle between two neighbouring vanes  6  has a value of 60°. The chambers  15 A,  15 B,  16 A,  16 B,  17 A,  17 B are formed such that in rotation direction  13  a chamber assumes increasing volume  15 A followed by an at least nearly constant volume  16 A, followed by a decreasing volume  17 A, followed by an increasing volume  15 B and so forth. This means that the two chambers with increasing volume  15 A and  15 B are situated opposite to each other regarding the centre of the rotor body  4 . The chambers with constant volume  16 A and  16 B as well as the chambers with decreasing volume  17 A and  17 B are also situated opposite to each other. 
         [0054]    The compressor  101  comprises two seals  18 . Each seal  18  is mounted between a chamber with decreasing volume  17 A or  17 B and a chamber with increasing volume  15 B or  15 A. The seals  18  are formed such that the moving vanes can smoothly travel over it. The casing  2  of the compressor  101  comprises two inlets  7  for gaseous fuel, which are located opposite to each other at positions where chambers with increasing volume  15 A and  15 B are located. The casing  2  further comprises two outlets  11  for compressed gaseous fuel, which are located opposite to each other at positions where chambers with decreasing volume  17 A and  17 B are formed. Moreover, the casing  2  comprises two air inlets  9  and two igniters  14 , which are located at positions where chambers with constant volume  16 A and  16 B are formed. This means that the igniters  14 , as well as the air inlets  9  are situated opposite to each other regarding the rotation axis of the rotor  3 . 
         [0055]    In the compressor  101  of  FIG. 3  the compressing process is performed twice during one revolution of the rotor  3 . 
         [0056]    Now a third embodiment of the present invention will be described with reference to  FIGS. 4 to 6 . Elements corresponding to elements of the previous embodiments will be designated with the same reference numerals and will not be described again. 
         [0057]      FIG. 4  shows an alternative inventive compressor  201  with twelve chambers in a sectional view. The casing  2  and the rotor body  4  of the compressor  201  in  FIG. 4  have the same shape as in the compressor  101 , which is described in the second embodiment. In contrast to the second embodiment, the rotor  3  in the  FIGS. 4 to 6  comprises twelve vanes  6  which form twelve chambers  29 A,  29 B,  30 A,  30 B,  31 A,  31 B,  32 A,  32 B,  33 A,  33 B,  34 A,  34 B. The angle between two adjacent vanes  6  has a value of 30°. Two similar chambers  29 A,  29 B,  30 A,  30 B,  31 A,  31 B,  32 A,  32 B,  33 A,  33 B,  34 A,  34 B are located opposite to each other relating to the rotation axis of the rotor  3 . In the direction of rotation  13  chamber  29 A is followed by chamber  30 A, which is followed by chambers  31 A,  32 A,  33 A, and  34 A. Chamber  34 A is then again followed by the second chamber  29 B and so forth. 
         [0058]    The chambers  29 A,  29 B,  30 A,  30 B,  31 A and  31 B are in a state of rotation where they have an increasing volume. The chambers  32 A,  32 B,  33 A,  33 B,  34 A and  34 B are in a state where they have a decreasing volume. At the location of the chambers  30 A and  30 B inlets  7  for gaseous fuel are provided. The chambers  31 A,  31 B,  32 A and  32 B comprise an air inlet  9  and an igniter  14 , which have the characteristics as described in the previous embodiments. The chambers  33 A,  33 B,  34 A and  34 B have a decreasing volume. The chambers  34 A and  34 B comprise an outlet  11  for gaseous fuel. In the compressor  201 , as it is shown in  FIG. 4 , the temperature rise and the pressure rise is divided into two stages. Thus the control of the combustion can more accurately lead to an improved combustion performance with lower emissions. 
         [0059]      FIG. 5  schematically shows a compressor  301 , which is a variation of the compressor  201 , which is shown in  FIG. 4 . In contrast to  FIG. 4 , the compressor  301  in  FIG. 5  comprises two inlets  7  and two outlets  11  for gaseous fuel per each half revolution of the rotor  3 , this means per compression cycle. 
         [0060]    At the location of the chambers  29 A and  29 B and the chambers  30 A and  30 B the compressor  301  comprises an inlet  7  for gaseous fuel and at the location of the chambers  33 A and  33 B and the chambers  34 A and  34 B the compressor  301  comprises an outlet  11  for compressed gaseous fuel. In this case the advantage is that the chambers  29 A,  29 B,  30 A,  30 B,  33 A,  33 B,  34 A and  34 B can be gradually filled and gradually emptied. In particular in the discharge sector, which is defined by the chambers  33 A,  33 B,  34 A and  34 B, this has the advantage that the mechanical compression work due to volume change is reduced. The ingestion, which takes place in the chambers  29 A,  29 B,  30 A and  30 B, as well as the discharge, which takes place in the chambers  33 A,  33 B,  34 A and  34 B, does not have to be arranged in discrete locations as shown in  FIG. 4 . It can also be arranged as slots between the two chambers  29 A and  30 A as well as between the two chambers  29 B and  30 B and/or between the chambers  33 A and  34 A and/or between the chambers  33 B and  34 B. Thereby a more continuous ingestion and discharge can be achieved. 
         [0061]    Another alternative compressor  401  is schematically shown in  FIG. 6 . In contrast to  FIGS. 4 and 5  the compressor  401  comprises an outlet  11  for gaseous fuel at the location of the chambers  32 A and  32 B. Moreover, at the location of the chambers  33 A and  33 B the compressor  401  comprises an air inlet  9  and an igniter  14 . This means that a combustion zone is located between two discharge sectors, which are defined in this variation by the locations of the chambers  32 A,  32 B,  34 A and  34 B. By dividing the discharge into two sectors, i.e. the locations of the chambers  32 A,  32 B and  34 A,  34 B, two different pressure levels of fuel can be delivered without raising the pressure to the highest of the two discharge levels. This may, for example, be used if a gas turbine has two combustion chambers operating in series with an intermediate turbine stage, thus having two air pressure levels. The same arrangement can also be used if two different pressure levels are required in one combustor, for example a higher pressure for a pilot burner compared to a main burner. The difference in fuel composition for the two discharges can also be used to operate the pilot burner and the main burner in advantageous ways for emission control. In this case the compressed gaseous fuel, which leaves the compressor  401  at the locations of the chambers  34 A and  34 B and has been ignited twice, can be used as pilot fuel. 
         [0062]    Obviously two or more inventive compressors  1 ,  101 ,  201 ,  301 ,  401 , as described in the embodiments, can be arranged to operate in series, with or without inter cooling. This is schematically shown in  FIGS. 7 and 8 .  FIG. 7  schematically shows two inventive compressors  501 ,  601  piped in series. The outlet for gaseous fuel  11  of the compressor  601  is connected to the inlet for gaseous fuel  7  of the compressor  501 .  FIG. 8  schematically shows two inventive compressors  701 ,  801  piped in parallel. 
         [0063]    Generally, the compressors  1 ,  101 ,  201 ,  301 ,  401 ,  501 ,  601 ,  701 ,  801  as described in the embodiments, can be actuated by means of an engine, for example by means of a motor or a turbine. Because the rotor is only moving gas around the drive requires far less energy than mechanical compression.