Abstract:
A turbine power generation system is provided for use with an off-gas fuel source. The system comprises first compression means for compressing air; second compression means for compressing off-gas; combustion means for combusting a mixture of said compressed air and a fuel comprising said off-gas; turbine means for converting energy released from combustion into mechanical energy; transduction means for converting the mechanical energy into electrical energy; a shaft linking the first compression means, turbine means, and transduction means, to allow mechanical energy produced by the turbine means to be used by the transduction means and first compression means; and an off-gas heating means for heating the compressed off-gas to a temperature greater than the gas dew point of the off-gas to ensure that no liquids are formed in the fuel gas supply to the combustor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         FEDERAL RESEARCH STATEMENT  
         [0002]    [Federal Research Statement Paragraph] Not applicable.  
         BACKGROUND OF INVENTION  
         [0003]    This invention is generally in the field of turbine power generation systems utilizing off-gas fuels. Combustion turbines are often part of a power generation unit. The components of such power generation systems usually comprise the turbine, a compressor, and a generator. These components are mechanically linked, often employing multiple shafts to increase the unit&#39;s efficiency. The generator is generally a separate shaft driven machine. Depending on the size and output of the combustion turbine, a gearbox is sometimes used to couple the generator with the combustion turbine&#39;s shaft output. Combustion turbines are sometimes recuperated.  
           [0004]    Microturbines are relatively small, multi-fuel, modular, distributed power generation units having multiple applications, such as disclosed in U.S. Pat. No. 4,754,607. Microturbines are a recently developed technology for use in such applications as, without limitation, auxiliary power units, on-site generators, and automotive power plants. Microturbines are normally of single-shaft design and generally use a single stage, radial type compressor and/or turbine with an internal generator directly coupled to the turbine shaft. Microturbines offer the capability to produce electricity remotely, without the necessity of an expensive infrastructure to deliver power to end users, thus providing electricity to remote locations at a lower cost per kilowatt than is available from a traditional centralized power plant with its necessary infrastructure of transmission lines.  
           [0005]    Generally, microturbines and combustion turbines operate in what is known as a Brayton Cycle. The Brayton cycle encompasses four main processes: compression, combustion, expansion, and heat rejection. Air is drawn into the compressor, where it is both heated and compressed. The air then exits the compressor and enters the combustor, where fuel is added to the air and the mixture is ignited, thus creating additional heat. The resultant high-temperature, high-pressure gases exit the combustor and enter the turbine, where the heated, pressurized gases pass through the vanes of the turbine, turning the turbine wheel and rotating the turbine shaft. As the generator is coupled to the same shaft, it converts the rotational energy of the turbine shaft into usable electrical energy. In a single-shaft microturbine, the turbine, the compressor, and the generator share the single shaft, with the components commonly configured with the turbine at one end of the shaft, the compressor in the middle, and the generator at the opposite end of the shaft.  
           [0006]    These microturbine power generation systems can be used to recover energy from off-gas sources. High BTU off-gas is frequently generated as a by-product of processing at oil and gas fields, and low to medium BTU off-gas can be generated from a variety of sources, such as landfills, wastewater treatment facilities, and digesters. Often the cost of recovery and transportation offsite of off-gases would not be economical viable, and the off-gases are simply flared or released into the atmosphere, and the potential energy of the off-gas is lost. Microturbine power generation systems, however, can be used to recover the energy from these high-BTU or low- to medium-BTU off-gases.  
           [0007]    Using these off-gases as a fuel source for microturbine systems can, however, be problematic. In particular, the off-gases often contain condensable components that form liquids during the compression process. These liquids can foul the fuel valve assembly and combustor, leading to poorer system performance and deterioration of fuel and combustion system hardware. In cold climates, the liquid can actually freeze and cause the fuel lines to become blocked, restricting the flow of fuel and inhibiting the microturbine from receiving adequate fuel to operate.  
           [0008]    Off-gases also typically have impurities that can foul and corrode the microturbine. For example, a high-BTU off-gas may contain H 2 S, which can condense and form sulfuric acid, while a low-BTU off-gas may contain CO or CO 2  that can condense as carbonic acid. These acids are corrosive to process equipment in contact with the off-gas, and can increase the maintenance cost and/or shorten the useful operating life of the microturbine power generation system. It would therefore be desirable to provide a system and method for ensuring that no liquids are formed in the fuel gas supply to the combustor of a microturbine or other turbine power generation system. These means desirably would be adaptable to a variety of ambient conditions at the operating site of the turbine power generation system.  
         SUMMARY OF INVENTION  
         [0009]    A turbine power generation system is provided for use with an off-gas fuel source, which comprises a first compression means for compressing air; a second compression means for compressing off-gas; a combustion means, such as a catalytic combustor, for combusting a mixture of said compressed air and a fuel comprising said off-gas; a turbine means for converting energy released from said combustion into mechanical energy; transduction means for converting the mechanical energy produced by said turbine means into electrical energy; a shaft linking said first compression means, said turbine means, and said transduction means, to allow mechanical energy produced by said turbine means to be utilized by said transduction means and said first compression means; and an off-gas heating means for heating said compressed off-gas supplied to said combustion means to a temperature greater than the gas dew point of the off-gas. The off-gas heating means is useful for ensuring that no liquids are formed in the fuel gas supply to the combustor.  
           [0010]    In one aspect, a method is provided for reducing or eliminating condensation of off-gas supplied to a turbine power generation system, which comprises heating off-gas supplied to a combustion means of a turbine power generation system to a temperature greater than the gas dew point of the off-gas, using heat generated by the turbine power generation turbine power generation system or using heat generated by compression of the off-gas for supply to said combustion means.  
           [0011]    In another aspect, a method of generating power from an off-gas fuel source is provided which comprises compressing a quantity of air using a first compression means; compressing a quantity of off-gas using a second compression means; heating said compressed off-gas to a temperature greater than the gas dew point of the off-gas using a heating means; combusting a mixture of said compressed air and said heated compressed off-gas using a combustion means; converting energy released from said combustion into mechanical energy with a turbine means; and converting the mechanical energy produced by said turbine means into electrical energy with a transduction means, said mechanical energy produced by said turbine means also being utilized by said first compression means. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIG. 1 is a process flow diagram of a prior art turbine power generating system, which can be adapted for use with an off-gas fuel source.  
         [0013]    [0013]FIG. 2 is a process flow diagram of an embodiment of a turbine power generating system comprising a system for heating cooled off-gas fuel using heat from the compression of the off-gas.  
         [0014]    [0014]FIG. 3 is a process flow diagram of an embodiment of a turbine power generating system comprising a system for heating off-gas fuel using heat from the compression of the combustion air.  
         [0015]    [0015]FIG. 4 is a process flow diagram of an embodiment of a turbine power generating system comprising a system for heating cooled off-gas fuel using heat from the exhaust air.  
         [0016]    [0016]FIG. 5 is a process flow diagram of an off-gas supply system which comprises means for mixing the off-gas with another fuel gas and means for heating this fuel mixture for supply to a turbine power generating system. 
     
    
     DETAILED DESCRIPTION  
       [0017]    Apparatus and methods have been developed for use in conditioning off-gas fuels used in turbine power generation systems, particularly microturbines, to ensure that no liquids are formed in the fuel gas supply to the combustor. The term “microturbine” generally refers to combustion turbines with an output of between 25 and 1000 kW.  
         [0018]    As used herein, the term “off-gas” or “off-gas fuel” refers to essentially any fuel gas that potentially can form liquids due to the compression process or due to ambient conditions. This includes essentially any high-BTU or low- to medium-BTU fuel gas that is a waste gas or by-product gas from another industrial or biological process. The off-gas typically contains one or more light (C1-C4) hydrocarbons (e.g., methane, propane, n-butane) and may also contain lesser amounts of C5-C7 hydrocarbons (e.g., n-pentane, hexane, heptane). The off-gas typically comprises at least between 20 and 80% methane. Examples of sources of such off-gases include, but are not limited to, oil or natural gas production facilities, landfills, waste water treatment facilities, digesters that convert animal waste (e.g., from cow, pig, duck) into a combustible gas, and coal bed methane (i.e. natural gas that can be found in coal beds). In one embodiment, the off-gas is a bio-based fuel, which typically is corrosive, relatively high in moisture content, and has a relatively low BTU value. The off-gas fuel may also be combined with other fuel gases, such as methane, propane, or other natural or synthetic gases, from non-off-gas sources. For example, commercially supplied natural gas or propane gas may be used to start-up the combustor or to supplement the fuel requirements of the combustor.  
         [0019]    [0019]FIG. 1 illustrates a prior art example of a turbine power generating system  10 , which can be adapted to use as off-gas fuel as described herein. The turbine power generating system  10  includes an air compressor  12 , a turbine  14 , and an electrical generator  16 . The electrical generator  16  is cantilevered from the air compressor  12 . The compressor  12 , the turbine  14 , and the electrical generator  16  can be rotated by a single shaft  18 . Although the air compressor  12 , turbine  14 , and electrical generator  16  can be mounted to separate shafts, the use of a single common shaft  18  for rotating the air compressor  12 , the turbine  14 , and the electrical generator  16  adds to the compactness and reliability of the power generating system  10 . The shaft  18  typically is supported by self-pressurized air bearings such as foil bearings.  
         [0020]    Air entering an inlet of the air compressor  12  is compressed. Compressed air leaving an outlet of the air compressor  12  is circulated through cold side passages  20  in a cold side of a recuperator  22 . In the recuperator  22 , the compressed air absorbs heat, which enhances combustion. The heated, compressed air leaving the cold side of the recuperator  22  is supplied to a combustor  24 .  
         [0021]    Fuel is also supplied to the combustor  24 . The flow of fuel is controlled by a flow control valve  26 . The fuel is injected into the combustor  24  by an injection nozzle  28 . The fuel comprises an off-gas as defined above and is supplied as described below.  
         [0022]    Inside the combustor  24 , the fuel and compressed air are mixed and ignited by an igniter  27  in an exothermic reaction. The combustor  24  can be any type of premix combustion system, including, but not limited to, catalytic combustors. In one embodiment, the combustor  24  contains a suitable catalyst capable of combusting the compressed, high temperature, fuel-air mixture at the process conditions. Representative examples of catalysts usable in the combustor  24  include platinum, palladium, as well as metal oxide catalyst with active nickel and cobalt elements.  
         [0023]    After combustion, hot, expanding gases resulting from the combustion are directed to an inlet nozzle  30  of the turbine  14 . The inlet nozzle  30  has a fixed geometry. The hot, expanding gases resulting from the combustion is expanded through the turbine  14 , thereby creating turbine power. The turbine power, in turn, drives the air compressor  12  and the electrical generator  16 .  
         [0024]    Turbine exhaust gas is circulated by hot side passages  32  in a hot side of the recuperator  22 . Inside the recuperator  22 , heat from the turbine exhaust gas on the hot side is transferred to the compressed air on the cold side. In this manner, some heat of combustion is recuperated and used to raise the temperature of the compressed air en route to the combustor  24 . After surrendering part of its heat, the gas exits the recuperator  22 . Additional heat recovery stages could be added onto the power generating system  10 .  
         [0025]    The generator  16  can be a ring-wound, two-pole toothless (TPTL) brushless permanent magnet machine having a permanent magnet rotor  34  and stator windings  36 . The turbine power generated by the rotating turbine  14  is used to rotate the rotor  34 . The rotor  34  is attached to the shaft  18 . When the rotor  34  is rotated by the turbine power, an alternating current is induced in the stator windings  36 . Speed of the turbine can be varied in accordance with external energy demands placed on the system  10 . Variations in the turbine speed will produce a variation in the frequency of the alternating current (i.e., wild frequencies) generated by the electrical generator  16 . Regardless of the frequency of the ac power generated by the electrical generator  16 , the ac power can be rectified to dc power by a rectifier  38 , and then chopped by a solid-state electronic inverter  40  to produce ac power having a fixed frequency. Accordingly, when less power is required, the turbine speed can be reduced without affecting the frequency of the ac output.  
         [0026]    Use of the rectifier  38  and the inverter  40  allows for wide flexibility in determining the electric utility service to be provided by the power generating system of the present invention. Because any inverter  40  can be selected, frequency of the ac power can be selected by the consumer. If there is a direct use for ac power at wild frequencies, the rectifier  38  and inverter  40  can be eliminated.  
         [0027]    The power generating system  10  can also include a battery  46  for providing additional storage and backup power. When used in combination with the inverter  40 , the combination can provide uninterruptible power for hours after generator failure. Additionally, the controller causes the battery  46  to supply a load when a load increase is demanded. The battery  46  can be sized to handle peak load demand on the system  10 .  
         [0028]    During operation of the power generating system  10 , heat is generated in the electrical generator  16  due to inefficiencies in generator design. In order to extend the life of the electrical generator  16 , as well as to capture useful heat, compressor inlet air flows over the generator  16  and absorbs excess heat from the generator  16 . The rectifier  38  and the inverter  40  can also be placed in the air stream. After the air has absorbed heat from the aforementioned sources, it is compressed in the compressor  12  and further pre-heated in the recuperator  22 .  
         [0029]    A controller  42  controls the turbine speed by controlling the amount of fuel flowing to the combustor  24 . The controller  42  uses sensor signals generated by a sensor group  44  to determine the external demands upon the power generating system  10 . The sensor group  44  could include sensors such as position sensors, turbine speed sensors and various temperature and pressure sensors for measuring operating temperatures and pressures in the system  10 . Using the aforementioned sensors, the controller  42  controls both startup and optimal performance during steady state operation. The controller  42  can also determine the state of direct current storage in the battery  46 , and adjust operations to maintain conditions of net charge, net drain, and constant charge of the battery.  
         [0030]    A switch/starter control  48  can be provided off-skid to start the power generating system  10 . Rotation of the compressor  12  can be started by using the generator  16  as a motor. During startup, the switch/starter control  48  supplies an excitation current to the stator windings  36  of the electrical generator  16 . Startup power is supplied by the battery  46 . In the alternative, a compressed air device could be used to motor the power generating system  10 .  
         [0031]    [0031]FIG. 2 illustrates the power generation system  10  operably connected to off-gas conditioning sub-system  50 . The off-gas conditioning sub-system  50  includes an off-gas compressor  52  to increase the pressure of the off-gas from its low supply pressure to a pressure suitable for delivery to the combustor  24 . The off-gas compressor can be, for example, a sliding vane (i.e. rotary) compressor, a reciprocating compressor, or a rotary screw compressor. The sub-system further includes an off-gas cooler  54 , to cool the compressed off-gas, and a mix tank  56 , where a supplemental or start-up fuel gas can be mixed with the off-gas. The off-gas cooler  54  can be, for example, an air-cooled heat exchanger or a cooling water cooled heat exchanger. In addition, condensed components in the off-gas are separated from the off-gas in the mix tank  56 , and can be drained through a drain (not shown) in the mix tank  56 . The primary functions of the mix tank  56  are to provide a location to collect and drain any liquids that form and to serve as a point to inject a starter fuel if necessary.  
         [0032]    The off-gas conditioning sub-system  50  also includes an off-gas heater  58  for reheating the cooled off-gas that is discharged from the mix tank  56 . The off-gas heater  58  includes a gas-to-gas heat exchanger, such as a shell and tube heat exchanger, having a hot side flow path and a cold side flow path adjacent and thermally coupled to the hot side flow path. The off-gas discharged from compressor  52  flows through the hot side flow path to cool the off-gas to a first temperature and then flows through off-gas cooler  54  where the off-gas is cooled to a second temperature that is lower than the first temperature. The off-gas from the mix tank  56  then flows through the cold side flow path of the gas-to-gas heat exchanger of the off-gas heater  58 , thereby reheating the off-gas to a temperature at least greater than the gas dew point of the off-gas, to ensure that no liquids are formed in the fuel gas supply to the combustor  24 . Optionally, the off-gas can be passed through a scrubber  60  before being fed to the inlet of the off-gas compressor, for example to separate liquids or solid particulate matter entrained in the off-gas.  
         [0033]    The values that follow are provided as an example. Off-gas is compressed in a single stage rotary compressor  52  to between 5.9 and 6.9 bars (85 and 100 psig). (It should be noted that multi-stage compressors also could be used, with any liquid formed during the interstage cooling process being removed prior to entering the next compression stage.) The compressed off-gas leaving the cold side of the off-gas heater  58  is between 40 and 65° C. (100 and 150° F.). The pressure drop between the fuel gas compressor discharge and the inlet of the microturbine typically would be less than 3% of the compressor discharge, e.g., a pressure drop of 1.25% each for the offgas cooler and the offgas heater, and 0.5% for the mixing tank. The temperature of the fuel gas discharged from the fuel gas compressor would be approximately 200° C. (400° F.). The off-gas cooler discharge temperature and the mixing tank temperature should be approximately ambient.  
         [0034]    In alternative embodiments, the off-gas heater is adapted to extract heat from other process gases flowing in the system. For example, the heat could be from the compressed combustion air, e.g., between the discharge of the air compressor  12  and the inlet of the combustor  24 , or from the exhaust air, e.g., after the discharge of the turbine  14 , before or after the recuperator  22 . FIG. 3 illustrates an embodiment in which an off-gas heater  70  transfers heat to the off-gas from the compressed air flowing between the discharge of the compressor  12  and the combustion air inlet of the recuperator  22 . FIG. 4 illustrates an embodiment in which an off-gas heater  80  transfers heat to the off-gas from the exhaust air discharged from the recuperator  22 . Off-gas heater  70  and off gas heater  80  each can be a gas-to-gas heat exchanger, such as of a shell and tube design.  
         [0035]    [0035]FIG. 5 shows another embodiment of an off-gas supply and conditioning system for a turbine power generation system. The off-gas is compressed by compressor  52  and passes through off-gas heater  58  (hot side) and then through off-gas cooler  54 . The off-gas then flows into mixing device  90  where it can be mixed with another fuel gas, which supplements the off-gas as needed. An example of a suitable mixing device is a static mixer, as known in the art. Control valves  92  and  94  can be used to control the ratio of off-gas to other fuel gas, as well as controlling the total flow of the gas mixture to the combustor. The gas mixture then flows from mixing device  90  and through off-gas heater  58  (cold side) and to turbine power generation system  10 .  
         [0036]    The power generating system  10  operates on a conventional recuperated Brayton cycle. The Brayton cycle can be operated on a relatively low pressure ratio (e.g., 3.8 bars) to maximize overall efficiency; since, in recuperated cycles, the lower the pressure ratio, the closer the turbine exhaust temperature is to the inlet temperature. This allows heat addition to the cycle at high temperature and, in accordance with the law of Carnot, reduces the entropic losses associated with supplying heat to the cycle. This high temperature heat addition results in an increased overall cycle efficiency.  
         [0037]    The values that follow are provided as an example. Air is compressed in a single stage radial compressor  12  to 3.8 bars. The compressed air can be directed to the recuperator  22  where the temperature of the compressed air is increased using the waste heat from the turbine exhaust gas. The temperature of the exhaust gas from the turbine is limited to about 700° C. (1,300° F.), in order to help extend the life of the recuperator  22 . For exhaust gas temperatures above 700° C. (1,300° F.), the recuperator  22  can be made of super alloys instead of stainless steel. The recuperator  22  can be designed for either 85% or 90% effectiveness depending on the economic needs of the customer. In the most efficient configuration, and using the 90% recuperation, the overall net cycle efficiency is 30%, yielding a high heating value heat rate of approximately 12,500 kJ/kWh (11,900 BTU/kWh) on diesel.  
         [0038]    After being heated in the recuperator  22 , the compressed air is directed to the combustor  24 , where additional heat is added to raise the temperature of the compressed air to 900° C. (1,650° F.). A combustor  24  designed according to a conventional design can yield a NO x  level of less than 25 ppm, and a combustor  24  using a catalyst can yield a NO x  rate that is virtually undetectable (commercial NO x  sensors are limited to a 2 to 3 ppm detection range). The high enthalpic gas is then expanded through the turbine  14 . The impeller, the turbine wheel, the rotor, and the tie shaft the moving parts in the engine core spin as a single unit at high speeds of approximately 60,000 rpm or more. The resulting generator output frequency of around 1,200 Hz is then reduced by the inverter  40  to a grid-compatible 50 or 60 cycles. Resulting is a high power density typified by low weight (about a third of the size of a comparable diesel generator) and a small footprint (for example, approximately 0.9 m by 1.5 m by 1.8 m high). The high power density and low weight of the technology is made possible through the high speed components which permits large amounts of power using a minimum of material.  
         [0039]    Potential applications for the power generating system  10  are many and diverse. Applications include use in off-grid applications for standalone power, on-grid applications for peak shaving, load following or base load service, emergency back-up and uninterruptible power supply, prime mover applications (e.g., pump, air conditioning) and automotive hybrid vehicles.  
         [0040]    The invention is not limited to the specific embodiments disclosed above. For example, the present invention could be configured without the electrical generator  16 . Turbine power would be transmitted and applied directly, as in the case of a mechanically driven refrigeration system.  
         [0041]    Modifications and variations of the methods and devices described herein will be obvious to those skilled in the art from the foregoing detailed description. Such modifications and variations are intended to come within the scope of the appended claims.