Abstract:
A fuel-conditioning skid for an engine. The fuel-conditioning skid includes an inlet that is connectable to a source to receive a flow of fuel containing undesirable compounds. An outlet is connectable to the engine to deliver a flow of fuel that is substantially free of undesirable compounds. An inlet cleaner is in fluid communication with the inlet and is operable to remove a portion of the undesirable compounds. A compressor is in fluid communication with the inlet cleaner to receive the flow of fuel at a first pressure and discharge the flow of fuel at a second pressure. The second pressure is greater than the first pressure. A purifier is in fluid communication with the inlet cleaner to receive the flow of fuel. The purifier is operable to remove substantially all of the remaining undesirable compounds from the flow of fuel.

Description:
RELATED APPLICATION DATA 
     This application claims benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Patent Application Ser. No. 60/457,072, filed Mar. 24, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     Hydrocarbon-based fuels are available from a variety of sources including biogas from landfills, wastewater treatment plants, agricultural and other digesters, light hydrocarbon gases from oil and gas wells, and various heavier hydrocarbon gases from processes such as refinery processing. These fuels can be used in an engine to generate electricity and heat. These fuels can be combusted either in a gas turbine engine (Brayton cycle), an internal combustion engine (Otto cycle), or other prime-mover engines (e.g., Sterling engines). Any of these engines can be connected to an electric generator to generate electricity or a shaft to provide shaft power for a variety of applications. The electric generator would produce electricity, which could either be exported to the electric utility grid (grid-parallel) or used locally to power various on-site electric loads (grid independent). Waste heat from the combustion process can also be used to heat water or other fluids. 
     The fuels from the above-mentioned sources contain constituents and contaminants that can cause problems in the fuel system, and/or the engine. In addition, most engine combustion systems react fuel and air at elevated pressures. Thus, the fuel gas must be compressed to a higher pressure to be admitted into the engine for combustion. 
     SUMMARY 
     The present invention generally provides a fuel-conditioning skid for an engine. The fuel-conditioning skid includes an inlet that is connectable to a source to receive a flow of fuel containing undesirable compounds. An outlet is connectable to the engine to deliver a flow of fuel that is substantially free of undesirable compounds. An inlet cleaner is in fluid communication with the inlet and is operable to remove a portion of the undesirable compounds. A compressor is in fluid communication with the inlet cleaner to receive the flow of fuel at a first pressure and discharge the flow of fuel at a second pressure. The second pressure is greater than the first pressure. A purifier is in fluid communication with the inlet cleaner (either upstream or downstream of the compressor). The purifier is operable to remove substantially all of the remaining undesirable compounds from the flow of fuel. 
     In another aspect, the invention generally provides a combustion turbine engine comprising a generator operable to produce an electrical output and an air compressor operable to produce a flow of high-pressure air. A combustor receives a flow of fuel and the flow of high-pressure air and combusts the flow of fuel and the flow of high-pressure air to produce a flow of products of combustion. A turbine operates in response to the flow of products of combustion to drive the air compressor and the generator. A fuel-conditioning skid receives a flow of fuel that includes undesirable compounds and delivers the flow of fuel to the combustor. The fuel-conditioning skid includes a compressor that receives the flow of fuel at a first pressure and discharges the flow of fuel at a second pressure, the second pressure being greater than the first pressure. The fuel-conditioning skid also includes a plurality of cooling stages. The flow of fuel enters each stage at an inlet temperature and exits at an outlet temperature that is less than the inlet temperature. A final cooling stage discharges the flow of fuel to the combustor. A plurality of condensate drains, each associated with a cooling stage, operate to drain at least a portion of the undesirable compounds from the flow of fuel. The flow of fuel is substantially free of undesirable compounds at the combustor. 
     In still another aspect, the invention generally provides a method of conditioning a flow of fuel to make the flow of fuel suitable for combustion within an engine. The method includes delivering the flow of fuel to a fuel-conditioning skid, the flow of fuel including undesirable compounds. The method also includes filtering the flow of fuel to remove at least a portion of the undesirable compounds from the flow of fuel and compressing the flow of fuel. The method further includes cooling the flow of fuel in a plurality of cooling stages to condense at least a portion of the undesirable compounds and draining the condensed undesirable compounds from the flow of fuel during each of the plurality of cooling stages. The fuel flow is then reheated to raise the fuel temperature above its dew point to minimize the possibility of condensing liquids between the fuel conditioning skid and the engine. The method also includes directing the flow of fuel to the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description particularly refers to the accompanying figures in which: 
         FIG. 1  is a perspective view of an exemplary microturbine engine system; 
         FIG. 2  is a schematic representation of the turbine section of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a fuel-conditioning skid with deep-chilling technology; and 
         FIG. 4  is a schematic diagram of another fuel-conditioning skid without deep-chilling technology; 
         FIG. 5  is a schematic diagram of another fuel-conditioning skid with deep-chilling technology; and 
         FIG. 6  is a schematic illustration of a portion of the fuel-conditioning skid of  FIG. 3  including first and second stage heat exchangers. 
     
    
    
     Before any embodiments of the invention are explained, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof is meant to encompass the items listed thereafter and equivalence thereof as well as additional items. The terms “connected,” “coupled,” and “mounted” and variations thereof are used broadly and encompass direct and indirect connections, couplings, and mountings. 
     DETAILED DESCRIPTION 
     Microturbine engines are relatively small and efficient sources of power. Microturbines can be used to generate electricity and/or to power auxiliary equipment such as pumps or compressors. When used to generate electricity, microturbines can be used independent of the utility grid or synchronized to the utility grid. In general, microturbine engines are limited to applications requiring 2 megawatts (MW) of power or less. However, some applications larger than 2 MW may utilize one or more microturbine engines. 
     With reference to  FIGS. 1 and 2 , the microturbine engine system  10 ′ includes a turbine section  15 ′, a generator section  20 ′, switch gear (not shown), and a control system  30 ′. 
     The turbine section  15 ′, schematically illustrated in  FIG. 2 , includes a gasifier turbine  35 ′, a power turbine  40 ′, a compressor  45 ′, a recuperator  50 ′, and a combustor  55 ′. The turbine section  15 ′ also includes various auxiliary systems such as a fuel supply system or fuel skid  60 ′ and a lubrication system  65 ′. 
     The turbine section  15 ′ is a standard Brayton cycle combustion turbine cycle with a recuperator  50 ′ added to improve engine efficiency. The engine shown is a multi-spool engine (more than one set of rotating elements). However, single spool engines are also contemplated by the invention. The compressor  45 ′ is a centrifugal-type compressor having a rotary element that rotates in response to operation of the gasifier turbine  35 ′. The compressor  45 ′ shown is generally a single-stage compressor, however, multi-stage compressors can be employed where a higher pressure ratio is desired. Alternatively, compressors of different designs (e.g., axial-flow compressors, reciprocating compressors, scroll compressor) can be employed to supply air to the engine. 
     The gasifier turbine  35 ′ is a radial inflow single-stage turbine having a rotary element directly or indirectly coupled to the rotary element of the compressor  45 ′. In other constructions, multi-stage turbines or axial flow turbines are employed as gasifier turbines  35 ′. The rotary element of the power turbine  40 ′ extends out of the turbine section  15 ′ and engages the generator section  20 ′, a gearbox (not shown), or other speed reducer disposed between the turbine section  15 ′ and the generator section  20 ′. 
     The recuperator  50 ′ includes a heat exchanger employed to transfer heat from a hot fluid to the relatively cool compressed air leaving the compressor  45 ′. A recuperator  50 ′ consistent with the turbine section  15 ′ of FIG. 1 is described in U.S. Pat. No. 5,983,992 herein fully incorporated by reference. The recuperator  50 ′ includes a plurality of heat exchange cells stacked on top of one another to define flow paths therebetween. The cool compressed air flows within the individual cells, while a flow of hot exhaust gas passes between the heat exchange cells. 
     During operation of the microturbine engine system  10 ′, the rotary element of the compressor  45 ′ rotates in response to rotation of the rotary element of the gasifier turbine  35 ′. The compressor  45 ′ draws in atmospheric air and increases its pressure. The high-pressure air exits the air compressor  45 ′ and flows to the recuperator  50 ′. 
     The flow of compressed air, now preheated within the recuperator  50 ′, flows to the combustor as a flow of preheated air. The preheated air mixes with a supply of fuel within the combustor  55 ′ and is combusted to produce a flow of products of combustion. The use of a recuperator  50 ′ to preheat the air allows for the use of less fuel to reach the desired temperature within the flow of products of combustion, thereby improving engine efficiency. 
     The flow of products of combustion enters the gasifier turbine  35 ′ and transfers thermal and kinetic energy to the turbine. The energy transfer results in rotation of the rotary element of the turbine and a drop in the temperature of the products of combustion. The products of combustion exit the gasifier turbine  35 ′ as a first exhaust gas flow. 
     The power turbine  40 ′ receives the first exhaust flow and discharges a second exhaust flow. The rotary element of the power turbine  40 ′ rotates in response to the flow of exhaust gas therethrough. The rotary element of the power turbine  40 ′ is preferably connected through a gearbox to the rotary element of the device to be driven, in the case of  FIG. 1 , the generator section  20 ′. The power turbine  40 ′ of  FIG. 1  drives the generator section  20 ′ at a fixed speed to produce the desired electrical output (e.g., 3600 or 1800 RPM for a 60 Hz system, 3000 or 1500 RPM for a 50 Hz system). In other constructions, a permanent magnet, or other non-synchronous generator may be used in place of the described synchronous generator. 
     The second exhaust flow enters the flow areas between the heat exchange cells of the recuperator  50 ′ and transfers excess heat energy to the flow of compressed air. The exhaust gas then exits the recuperator  50 ′ and is discharged to the atmosphere, processed, or further used as desired (e.g., cogeneration or heat recovery). 
     Radial inflow turbines of the type discussed herein operate most efficiently at very high speeds relative to the equipment they potentially drive (e.g., generators, screw-pumps, gear-pumps, etc.). For example, a gasifier turbine  35 ′ may operate at 50,000 RPM or higher, while a synchronous generator operates at no more than 3600 RPM (to produce a 60 Hz output) and screw-pumps generally operate at about 15,000 RPM. These large speed differentials make multi-spool turbine systems desirable. The gasifier turbine  35 ′ is able to operate at a very efficient speed, while the power turbine  40 ′ operates at the speed needed by the equipment it is driving or at a speed necessary to drive a speed-reducing device. 
     In another construction, a single radial turbine rotates to drive both the compressor and the electrical generator  20 ′ simultaneously. This arrangement has the advantage of reducing the number of turbine wheels. 
     A plurality of bearings support the rotary elements of the turbines  35 ′,  40 ′, the compressor  45 ′, and the generator  90  for rotation. The lubrication system includes a lube oil pump  95  that provides a flow of lubricating oil to the bearings to reduce friction and wear, and to cool the bearings. While oil is generally used as the lubricating fluid, other fluids may be used to lubricate and cool the engine components. 
     While the constructions described in connection with  FIGS. 1-2  include a microturbine engine, the system  10 ′ is not limited to a microturbine engine for some aspects of the invention. Other prime movers (e.g., Otto cycle engines, Sterling engines, etc.) can be used in place of the turbine section  15 ′ with some of the elements or components described herein. 
     Many engines combust natural gas to produce electricity. Natural gas is readily available in a clean high-pressure state (pipeline quality), which is easily deliverable to the combustion systems of these engines. In addition to natural gas, other fuels, such as biogas from landfills, are available and combustible by engines such as microturbine engines to produce electricity. However, before these fuel sources can be used, the fuel is generally “conditioned” (i.e., filtered, dried, cleaned of certain contaminants, and compressed). Fuels that are not conditioned sufficiently (e.g., inadequate filtering, drying, or removal of contaminates) can cause short-term and long-term damage to the microturbine fuel system, combustor, and downstream flow path components (e.g., turbine, recuperator, housings, ducts, and the like). 
     Conditioning involves several basic processes including but not limited to superheating the fuel to provide dewpoint suppression and removing unwanted elements or undesirable compounds in the fuel stream (e.g., solid particulates, water, heavier hydrocarbons that can condense in fuel lines, hydrogen sulfide and other halogenated compounds, siloxanes, and the like). 
     A fuel-conditioning skid  60 ′, illustrated schematically in  FIG. 3 , conditions the fuel as described above and increases the delivery pressure of the fuel to a level suited to the particular engine to which the fuel skid is supplying fuel. The fuel-conditioning skid may be individually packaged as its own platform or skid, or it may be combined with one or more microturbines on a common platform. Combining the fuel-conditioning equipment with the microturbines provides a total system that will process and burn gases other than, and in addition to, pipeline quality natural gas to produce electric (or shaft) power and cogenerative heat. 
     The fuel-conditioning skid (FCS)  60 ′, in which various components are packaged together onto a single, movable platform, is used to condition various types of fuels for subsequent supply to small turbine engines, internal combustion engines, and/or other prime movers. These engines can be used to drive generators and produce electricity and/or heat. 
     A schematic of the fuel conditioning apparatus of the FCS  60 ′ is given in  FIG. 3 . A source of gaseous fuel (landfill, anaerobic digester, wellhead, refinery process, etc.) is supplied to the fuel-conditioning skid  60 ′ at a skid inlet  1 . The fuel can be either low pressure (below atmospheric pressure) or high pressure, in which case a pressure regulator  3  could be placed downstream of a strainer  2  to reduce the supply gas pressure. The fuel flows through the inlet strainer  2  to remove large particulates, including rust, weld slag, filings, and other debris. A purge assembly, consisting of a tee  4 , a manual valve  5 , and a check valve  6  can be used to initially purge the line to remove entrapped gas (air) and vent it to an outlet  7 . This purge outlet  7  can also be used to adjust the FCS inlet pressure regulator  3  on skids  60 ′ that include the regulator  3 . A pressure gauge may be attached to the FCS  60 ′ downstream of the pressure regulator  3  to indicate the gas pressure. An inlet solenoid valve  8  may be used to externally shut off the gas and isolate the FCS  60 ′. In addition, a back-up manual valve  9  may be provided to isolate the FCS  60 ′ from gas flow if desired. 
     An inlet liquid separator and filter tank  10  removes (i.e., knocks out) at least a portion of the liquid components from the incoming gas, and also filters out particulates, typically in the 1 to 10 micron size range. A coalescer filter could also be used in this device for liquid separation if desired. 
     At this point in the FCS  60 ′, it is possible that the fuel pressure is below atmospheric pressure. A liquid drain arrangement consisting of two solenoid valves  11 ,  13  and a liquid reservoir  12  can be used to drain liquid from the sub-atmospheric separator tank  10 . During a drain cycle, valve  11  would be opened to permit liquid to fill the reservoir  12 . Valve  11  then closes and valve  13  opens to drain liquid out through a check valve  14  to the drain system  15 . This system reduces the amount of air that is admitted into the separator tank  10  during a drain cycle. Valve  13  then closes and the cycle is repeated periodically to keep the inlet separator liquid level below a predetermined level. Other methods of draining the inlet separator tank  10  may include, but are not limited to, a pump system that removes liquids, or an eductor system. The eductor system uses a flow of high-pressure fuel, or air, at high velocities through a throat to depress the local static pressure below that of the inlet separator  10 . The eductor removes liquids entrained within this flow, in much the same manner as an automobile carburetor draws in fuel. A float switch and/or a pressure switch  17  may be used to send an alarm or stop signal to the FCS control to reduce the likelihood of liquid slipping past the separator  10  in the event that the drain system fails to remove sufficient moisture from the separator tank  10 . 
     Gaseous fuel exits the inlet filter/separator  10  and passes through a check valve  18  which ensures that gas, which could potentially be at a higher pressure than the source gas, does not flow backward into the fuel supply lines. The fuel then feeds into an inlet valve  19  that controls the flow rate to a compressor such as an oil-flooded screw-type compressor  20 . The compressor  20  boosts the fuel pressure to a predetermined level suitable for engine operation. The screw compressor  20  is driven by an electric motor  21  through a gearset/adapter/coupling arrangement. The electric motor  21  is driven by a variable frequency drive (VFD)  22 , sometimes also referred to as a variable speed drive. The VFD  22  varies the motor speed in response to a measured parameter, such as a fuel pressure, to maintain the parameter (fuel delivery pressure) within a desired range. Fuel temperature sensors and/or switches measure and ensure proper gas conditions from the compressor  20 . In other constructions, other parameters such as a turbine or engine temperature, an engine pressure, or engine power output, are used to control the motor speed. One of ordinary skill will realize that many parameters are available that can be used to control the motor speed. 
     Rather than using an oil-flooded screw compressor  20 , the FCS  60 ′ could use other types of gas compressors, including positive displacement rotary and piston-type compressors, scroll compressors, or other devices. The oil-flooded screw-type compressor  20  uses mechanical seals to prevent fuel leakage from the compressor case. In another construction, a semi-hermetically sealed motor and air end (screw compressor) such as that described in U.S. patent application Ser. No. 10/627,212 herein fully incorporated by reference could also be used. Similarly, a magnetic coupling could be used with a semi-hermetically sealed air end compressor to compress the gaseous fuel/oil mixture. 
     Following compression, a compressed fuel/oil mixture flows through tubing/piping to a gaseous fuel/oil separator tank  23 . The separator tank  23  uses a coalescer filter or other known means to separate the compressed gas from the oil. Following separation, the oil circulates through a liquid oil line to a temperature regulator valve  24 . The valve  24  diverts oil through an air/oil heat exchanger  25  in response to the measured oil temperature to maintain the temperature of the oil returning to the screw compressor  20  within a desired temperature range. The temperature range should be high enough to inhibit the condensation of water in the oil and low enough to inhibit degradation of the oil. Thus, the valve  24  functions to maintain the oil temperature above a predetermined temperature and below a second predetermined temperature. This air/oil heat exchanger  25  has a fan-driven cooling system. In addition to the separator tank  23 , an oil filter  26  may be used to aid in the removal of particulates from the flow of oil. 
     If oil permeates the coelescer filter, a scavenge line may be used to direct the oil back to the screw compressor  20  through a check valve/filter/orifice  27 . A pressure relief valve  28  and a drain valve  29  may also be provided on the separator tank  23  to protect against overpressure and to allow for periodic maintenance and oil replacement. A pressure switch  30  could also be used to determine and monitor the extent of coalescer filter fouling. 
     Compressed fuel leaves the separator tank  23  and is regulated using a minimum pressure check valve  31 . The minimum pressure check valve  31  permits the fuel pressure to build up to a certain level before proceeding through the rest of the FCS  60 ′. Thus, only gas above a predetermined pressure passes through the check valve  31 . Alternative constructions may use a backpressure regulator or other device rather than a minimum pressure check valve  31 . 
     Under certain operating conditions, the desired fuel flow rate as required by the engine (e.g., microturbine) is less than the fuel flow rate delivered by the compressor  20 , while operating at the motor&#39;s minimum speed. A fuel bypass control system may be employed to further reduce the fuel flow rate. The fuel bypass control system uses bypass fuel to control and reduce the fuel flow rate. To reduce the fuel flow rate, the bypass line with solenoid valves  34  and  37  are opened, while valve  36 ,  38 , and  41  are closed. The modulating valve  35  partially or fully opens the inlet valve  19  using recirculated fuel pressure. The inlet valve then “throttles” the inlet fuel flow going to the inlet side of the gas compressor  20 . By effectively reducing the inlet pressure to the compressor  20 , the amount of flow pumped to the rest of the FCS  60 ′ is reduced. 
     If further gaseous fuel flow reductions are necessary, a fuel bypass line can be employed. To open the bypass line, solenoid valves  37  and  34  are closed, and valve  36  is opened. This configuration closes the inlet valve, but allows compressed gas to recirculate back through a solenoid valve  38 , to the inlet of the inlet valve  19 . A check valve  40  prevents backflow of fuel from the inlet valve into the recirculation line. A pressure drop device  39  is used to reduce the pressure to the level of the screw compressor inlet. The pressure drop device  39  can be an orifice, a manually adjustable needle valve, a regulator, or an automated valve. The solenoid valve  38  ensures no fuel is recirculated when not required. 
     Compression of the fuel by the compressor  20  adds heat, thereby raising the fuel temperature above the level at which it entered the FCS. A fan-cooled aftercooler  32  or heat exchanger may be used to cool the compressed fuel before the fuel enters a liquid separator device  33 . A variable frequency drive (VFD)  42  (shown in  FIG. 5 ) could be used to drive a fan  58  to more accurately control the temperature of the fuel leaving the aftercooler  32 . The cooling may cause some undesirable compounds to condense out of the flow of fuel. These undesirable compounds drain from the liquid separator device  33 , thus further conditioning the fuel. 
     Once any condensate is drained from the liquid separator device  33 , the fuel is fed to a first cooling stage including a gas-to-gas heat exchanger  44  for cooling. The fuel is chilled to a suitable temperature (e.g., 55-65 degrees F.) in the first cooling stage. During this chilling process, more undesirable compounds (including water and heavy hydrocarbons) condense out of the fuel and are drained from the heat exchanger via a drain line  45 . The chilled fuel leaves the first heat exchanger and flows to a coalescing separator  46  where additional undesirable compounds may be separated and drained. 
     Following the first cooling stage, the compressed fuel flows to one of two similar second-stage refrigerant-to-gas heat exchangers  47 ,  48 . Each refrigerant-to-gas heat exchanger  47 ,  48  has sufficient capacity to cool the flow of compressed fuel to a desired temperature. Thus, at any given time (during non-transient steady-state operation), the flow of compressed fuel is directed to one of the two refrigerant-to-gas heat exchangers  47 ,  48 . With no flow in the other of the two heat exchangers  47 ,  48 , a defrost cycle, or other maintenance can be performed on the idle heat exchanger  47 ,  48  without interrupting the flow of fuel. Solenoid valves  53 ,  54  are used to control which of the second-stage heat exchangers  47 ,  48  is used to cool the gas. This cycling of the second stage heat exchanger reduces the likelihood of tubes freezing and improves the heat exchanger efficiency by periodically removing any frost build-up. As one of ordinary skill will realize, more than two heat exchangers could be employed and more than one heat exchanger could receive the flow at any given time. For example, in one construction, three heat exchangers are employed with two being active at any given time. 
     The refrigerant system typically uses R22, freon, R-134a, or another suitable refrigerant to chill and defrost the second-stage heat exchangers. In another construction, the refrigeration (vapor-compression) cycle is replaced by an ammonia absorption chiller system. The ammonia absorption chiller system uses the hot engine exhaust gas to drive the system rather than an electrically operated compressor as is used in the refrigeration cycle. High-pressure ammonia is expanded to provide cooling for the flow of fuel, thus achieving the desired level of cooling (approximately −20 degrees F.) as described with regard to the construction of  FIG. 3 . 
     The fuel makes at least one pass through the active second-stage heat exchanger  47 ,  48  and is cooled to a temperature below the freezing point of water. Each circuit in the heat exchanger has a liquid drain  49 ,  50  to remove liquids (undesirable compounds) that may condense. An additional liquid coalescer separator  51 ,  52  may also be employed to remove additional liquid materials. 
     The deep chilling of the fuel (nominally −20 F.) not only removes water and heavier hydrocarbons, but also removes siloxanes, halogenated compounds, and other contaminants from the fuel system. The deep chilling also removes some hydrogen sulfide (contaminant). The final chilled fuel leaving the second stage heat exchanger  47 ,  48  has a saturation temperature (liquid hydrocarbons, water) approximately equal to the exhaust temperature (nominally −20 F.). 
     The chilled, saturated fuel leaving the second stage heat exchanger  47 ,  48  re-enters the first gas-to-gas heat exchanger  44  where it is used to chill the incoming fuel. This also serves to heat the chilled fuel from the very cold condition (−20 F.) to a higher temperature. This produces a flow of fuel with a −20 F. dewpoint and 50-100 F. of superheat (dewpoint suppression) at location  55 . Of course, more or less superheat can be achieved by varying the operating parameters or ambient conditions. The fuel exiting the gas-to-gas heat exchanger is then supplied to one or more carbon-absorber tanks  56  (arranged in parallel and/or series) to remove additional undesirable compounds (e.g., H 2 S, HCl) that may not have been removed in the deep-chilling process. 
     A manual valve  57  is supplied to purge the carbon-absorber tank(s) for servicing. The deep-chilling process removes significant quantities of undesirable compounds, thereby reducing the service and maintenance requirements of the carbon media used in the absorber tank  56 . A final filter  59  may be used to capture any carbon particulates that slip through the carbon-absorber  56 . It should be noted that many other filters and filter media are available other than carbon (e.g., iron sponge, regenerative silica gel, absorption-de-absorption systems, Sulfatreat, etc.). As such, the invention should not be limited to carbon-absorbers alone. 
     The fuel is now conditioned and ready for supply to one or more microturbines via a fuel-conditioning skid outlet  60 . A purge assembly consisting of a tee  61 , a manual valve  62 , and a check valve  63  can be used to purge air or other gases out of the FCS  60 ′ prior to start-up. Gaseous vent  64  can be connected to vent  43  or to other vents if desired. 
     In another construction, the manual valve  62  is replaced with a solenoid valve so that if the fuel heating content (energy per cubic foot of gas) drops below a predetermined value, the fuel can be diverted to a flare or other outlet until the fuel quality improves. For example, if a particular application requires at least 35% methane in the fuel to operate properly, the system could divert the fuel through the solenoid valve if the methane content dropped below 35%. In some constructions, this gas is diverted to a flare or back to the source of the fuel. In yet another construction, the solenoid valve would admit high-quality (pipeline) gas to supplement the fuel supply and allow for the continued operation of the system. 
     Not shown in  FIG. 3  is an optional receiver tank which could be used to store compressed, conditioned fuel for periods of high demand (start-up) and to dampen out pressure fluctuations due to fuel supply and demand issues. The receiver tank would preferentially be located between the FCS outlet  60  and the microturbine(s)  10 ′ or other engines that use the fuel. A fuel solenoid could be used to ensure that when the FCS  60 ′ is depressurized for service or maintenance, the receiver tank remains pressurized. Alternatively, if low heat content fuel were the cause of a shutdown, the receiver tank could bleed down its fuel to a flare or other on-site system. 
     During low-flow operating conditions (e.g., part load operation, skid start-up, and the like) the desired compressor output may fall below a predetermined level at which control of the compressor  20  becomes difficult or unstable. To avoid this scenario, the FCS  60 ′ includes a fuel bypass line  180  that includes a bypass valve  185  that selectively redirects a portion of the fuel, after the removal of some or all of the undesirable compounds, back to the compressor  20 . In the construction of  FIG. 3 , fuel is diverted from the outlet of the second stage heat exchanger  47 ,  48  to the compressor inlet valve  19 . By diverting a portion of the flow of fuel, the flow into the compressor  20  can be maintained above a predetermined value. For example, one compressor must operate at 50 percent flow capacity to be controllable. During certain operating conditions, it may be desirable to deliver only 25 percent of the compressor&#39;s capacity to the engine. In this situation, the compressor operates at 50 percent (or higher) and the fuel in excess of the 25 percent needed by the engine is diverted back to the compressor. In addition to the foregoing example, the fuel bypass line  180  allows the FCS  60 ′ to start before the engine or engines start. All of the fuel is bypassed to the compressor  20  until the heat exchangers  47 ,  48  reach their desired operating temperatures. This allows the FCS  60 ′ to deliver properly conditioned fuel to the engine immediately upon engine start. In addition to the bypass valve  185 , the fuel bypass line  180  may include a check valve  190  and a pressure-reducing element  195  (e.g., orifice, needle valve, pressure-reducing regulator, and the like). In some constructions, the bypass valve  185  includes a pressure regulating valve that limits the outlet pressure from the FCS  60 ′ and a backpressure regulator valve that maintains the inlet pressure at the compressor below a predetermined value. Thus, when the FCS exit pressure exceeds a predetermined value, the valves direct a portion of the flow from the FCS outlet back to the compressor inlet. 
     To purge the skid  60 ′ of high-pressure gas, solenoid valve  8  closes, and valves  66 ,  38  and  41  open. The minimum pressure check valve  31  inhibits the backward flow of fuel in the system through to the vent  43 . The high-pressure fuel downstream of the minimum pressure check valve  31  flows through the skid  60 ′ to location  61 . The dry, clean gas then flows back to the entrance of the inlet valve  40  through the line with valve  66 . The gas then flows from the inlet valve  40 , through the compressor  20 , separator tank  23 , out to the bypass line  38 , and eventually out of the vent  43 . This method of purging will preferentially first purge the higher-moisture content gas from the compression end of the FCS  60 ′ and replace it with dry, clean gas. This purge system reduces the risk of liquid condensation within the skid  60 ′, which if present, could induce corrosion or freezing during long periods of shutdown. 
     The FCS  60 ′ is designed in a modular fashion to supply a number of different fuel flow rates and pressures. The FCS  60 ′ provides conditioned fuel for an SCFM (standard cubic feet per minute) range of 18-4300 SCFM using a plurality of sized products with higher or lower values possible. The fuel pressures output from the FCS  60 ′ range from about 60 psig through about 120 psig, but could be either higher or lower than this if desired. Each FCS  60 ′ can address a range of hydrogen sulfide (H 2 S) gas contaminate levels. While not intended to remove H 2 S other than what is removed during the deep-chilling process, different levels of removal (e.g., 300 ppmV max, 1000 ppmV max, 3000 ppmV max) can be achieved by varying the amount of carbon absorbent material. This will entail sizing the carbon-absorber tanks  56  of the FCS  60 ′ to address the different levels. The carbon-absorber tank(s)  56  allow easy removal and exchange of the carbon media. 
     Another construction of the FCS  60 ″ excludes the deep-chilling system shown in  FIG. 4 . With the exception of the exclusion of the deep-chilling system, the constructions of  FIG. 3  and  FIG. 4  are substantially similar. In the FCS  60 ″ of  FIG. 4 , the minimum gas temperature remains above the freezing point of water at the coldest part of the fuel skid  60 ″ (adjacent the condensate drain  51 ). Because subfreezing temperatures are avoided, only one second-stage heat exchanger  47  is needed and no defrost cycle is required. The purpose of this FCS  60 ″ is to avoid condensing out beneficial hydrocarbons (propane, butane, and the like) at very cold temperatures. The carbon absorber tank  56  will still remove some undesirable compounds. Again, by flowing gas back through the first stage gas-to-gas heat exchanger  44 , dewpoint suppression is achieved to ensure condensation in downstream fuel lines is inhibited. 
       FIG. 5  illustrates another construction of the fuel-conditioning skid  60 ′″. The skid  60 ′″ is similar to the skid  60 ′ of  FIG. 3 .  FIG. 5  illustrates the various system boundaries within the FCS  60 ′″. A chiller boundary  200  surrounds the chiller portion and in some constructions defines a housing or enclosure boundary. Similarly, a compressor module boundary  205  surrounds the compressor module and in some cases defines the location of a compressor housing or an enclosure. 
       FIG. 6  is a more detailed view of the interaction between the first stage heat exchanger  44  and the second stage heat exchanger  47 ,  48  and the refrigeration cycle components. The refrigeration cycle components include a compressor  210 , a condenser  215 , and an expansion valve  220 . In addition,  FIG. 6  illustrates a condensate drain system that includes a drain valve  225  at the outlet of the second stage heat exchanger  47 ,  48  to drain condensed undesirable compounds. 
     The design of the control system is such that the FCS  60 ′,  60 ″,  60 ′″ can operate independent of the microturbines or other subsequent fuel users. Energy to start the FCS  60 ′,  60 ″,  60 ′″ can be supplied either from the existing electrical grid or from a stored energy source such as a battery. Alternatively, a supply of high-pressure, conditioned, stored fuel, such as propane, can be supplied directly to the microturbine  10 ′ to start the engine and subsequently supply power to start and operate the FCS  60 ′,  60 ″,  60 ′″. 
     A programmable logic controller (PLC) or other microprocessor-based control unit performs protective functions (alarms, trips, interlock/permissives), control functions (sequencing, logic, PID control), and information functions (operator interface, interface to other systems). Once started, the PLC controls the FCS exhaust flow rate and pressure. A pressure sensor  65  located near the outlet  60  of the FCS  60 ′,  60 ″,  60 ′″ senses the FCS outlet pressure. A proportional-integral-derivative (PID) control algorithm is used to adjust the gas compressor speed by adjusting the variable frequency drive device  22 . A person of ordinary skill in the art will realize that many other control systems and algorithms may be used (e.g., proportional-integral, fuzzy, etc.). The PLC also monitors gas train parameters, refrigeration cycle parameters, and the like and performs startups, shutdowns, cycles the second-stage heat exchangers, and communicates via a modem (RS232 or RS485) or other communication protocol. The FCS  60 ′,  60 ″,  60 ′″ may also include fuel flow measurement sensors and/or gas monitoring equipment, such as a methane detector. The methane detector or flow rate measurement sensor could be used to determine when the source gas methane content is becoming too low for proper microturbine operation. A gas mixing system could then be used to blend pipeline natural gas, or other available gas (e.g., propane), with the biogas flow stream to improve the heat content of the fuel or simply augment the fuel flow during a start-up or during cold day operation. 
     The FCS  60 ′,  60 ″,  60 ′″ provides enough gaseous fuel flow to start one or more microturbines  10 ′. During start-up, recuperated microturbines  10 ′ and other prime movers require an increased amount of fuel flow to warm up the equipment. The design of the gas capacity of the FCS  60 ′,  60 ″,  60 ′″ provides sufficient flow for this start-up. The FCS  60 ′,  60 ″,  60 ′″ will also supply the higher steady-state fuel flows required during cold days. On cold days, microturbines  10 ′ and other generators can increase the amount of energy that they produce. 
     The fuel-conditioning skids  60 ′,  60 ″,  60 ′″ may include weatherproof enclosures for outdoor use. The FCS  60 ′,  60 ″,  60 ′″ can also be used in environments where gaseous fuel could be present if a significant leak or rupture of the fuel line occurs (Class 1, Division 2 of the National Electric Code—NFPA 70). The skids operate in ambient temperatures from about −10 F. to about 115 F. and at altitudes from about sea level (0 ft) to about 6000 ft. Higher and lower temperature as well as altitude ranges can also be achieved if desired. 
     In applications where both a FCS  60 ′,  60 ″,  60 ′″ and microturbine(s)  10 ′ are packaged on the same skid, piping of fuel and scavenge lines between units are integrated into the apparatus to minimize installation effort after leaving the factory. A significant advantage of combining the FCS  60 ′,  60 ″,  60 ′″ and microturbine(s)  10 ′ on the same skid is that each microturbine  10 ′ does not need its own gaseous fuel compression system. Since a source of high-pressure, clean gas is supplied to each microturbine  10 ′ from the FCS  60 ′,  60 ″,  60 ′″, a fuel control valve can be used in each microturbine  10 ′ to modulate the supply of fuel. This leads to a more efficient energy package, by leveraging the gas compression in a single, larger, more efficient location (FCS  60 ′,  60 ″,  60 ′″). Also, the fuel control valve modulation of fuel to each microturbine  10 ′ provides for a faster response of the microturbine  10 ′ to a change in power requirements as compared with a system where an on-board gas compressor must change its operating condition. 
     The various constructions described herein are capable of removing a substantial portion of the undesirable compounds from a stream of fuel to produce a stream of fuel that is substantially free of undesirable compounds. For example, the constructions of  FIG. 3 and 4  are capable of removing 90 percent of the undesirable compounds from a stream of fuel. Of course, the amount of undesirable compounds removed is a function of the quantity and type of compounds initially present. As such, systems that remove less than 90 percent and systems that remove more than 90 percent are also possible. However, to provide for proper operation of a prime mover, such as a microturbine engine, it is desirable to remove at least 98 percent of the undesirable compounds including 98 percent of the siloxanes present in the initial stream of fuel. 
     Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.