Gas turbine steam addition

Abstract: The output of gas turbines (6) is enhanced by gas moisturization using a water contact tower (2) and a pumped water recirculation loop (3,7) with low pressure steam addition thereto.

Description: Background of the invention Field of the Invention - The invention relates to gas turbines. More particularly, it relates to enhancing the output of gas turbines. Description of the Prior Art - Moisture is often added to gaseous fuels entering combustors as a means of NO x control. For this purpose, water vapor addition to gas streams is generally accomplished through the direct addition of steam to a gas stream. For gas turbine combustors, fuel gas is commonly injected therein, at pressures in excess of 1379 kPa (200 psia), with said moderate pressure steam providing the moisture needed for NO x control. The direct addition of steam to the fuel gas serves to provide mass to the fuel gas, which has a beneficial effect on the net power output from the gas turbine, in addition to providing the moisture needed for NO x control. In many instances, the steam employed for NO x control can be more efficiently used in a steam turbine, producing more power than can be obtained by expanding the steam in a gas turbine. For example, 453,6 kg (1000 pounds) per hour at 1724 kPa (250 psia) saturated steam fed to a condensing turbine can produce 83,5 kW (112 hp), while feeding the same amount of steam to an industrial gas turbine will produce about 67,1 kW (90 hp), because of the lower applicable pressure ratio, and require additional fuel to raise the temperature of the steam to that needed for optimum gas turbine operation. In an alternative approach for adding moisture to a gas stream, a hot water steam is added to the fuel gas by means of a water recirculation loop around a saturation tower from which fuel gas is injected into a gas turbine combustor. The water recirculation loop is heated by recovering heat from a hot fluid by means of a heat exchanger positioned in the loop. The hot water from the loop contacts the process gas stream, i.e., fuel gas, to be moisturised in the saturation tower, which is a packed tower or other device designed to bring the hot water and the process gas stream into intimate contact. In the saturation tower, the heat added in the heat exchanger is removed through vaporization of a portion of the water stream. The vaporized water leaves the top of the saturation tower with the process gas, while the cooled water leaves the bottom of the saturator. A portion of the recirculated water is periodically removed as blowdown to control salt concentrations, and makeup water is added to replace the water lost to the moisturized gas i.e., the fuel gas stream. In the use of such a technique, the fuel gas streams that are to be moisturized need to be located near a source of heat in order to avoid the need for long and expensive water recirculation loops. EP-A-81 996 discloses an apparatus for the addition of low pressure steam to the fuel gas used to drive a gas turbine comprising a saturation tower adapted for the contacting of said fuel gas with a hot water stream for the moisturization of said fuel gas; conduit means for passing fuel gas to the bottom portion of said saturation tower for upward passage therein; conduit means for passing hot water to the upper portion of said saturation tower for downward passage therein; conduit means for removing moisturized fuel gas from the upper portion of said saturation tower for burning in a gas turbine combustor unit to provide the energy to drive a gas turbine; and conduit means for withdrawing water from the bottom portion of said saturation tower, said conduit means and the conduit means for passing hot water to the saturation tower being in fluid communication and forming a water recirculation loop around said saturation tower. While such approaches are useful for achieving a desired moisturizing of gas streams, there remains a desire in the art for further improvements. Thus, the moderate or high pressure steam used in one approach or the long water recirculation loops frequently required in the other, represent elements of expense that desirably should be reduced to achieve the ever-increasing overall requirements for practical gas turbine applications. It is an object of the invention, therefore, to provide an improved apparatus for such gas turbine applications. It is another object of the invention to provide an apparatus for more efficiently operating industrial gas turbines. With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof bring particularly pointed out in the appended claims. Summary of the Invention The output of gas turbines is enhanced using low pressure steam. A pumped water circuit and water contact tower are used to achieve desired gas moisturization using such low pressure steam. Brief Description of the Drawings The invention is hereinafter described with reference to the accompanying drawings in which: Fig. 1 is a process flow diagram of an embodiment of the invention for low pressure level addition of steam to a fuel gas; Fig. 2 is a process flow diagram of another embodiment of the invention illustrating said low pressure steam addition with heat exchange; Detailed Description of the Invention The objects of the invention are accomplished by employing means to enhance the output of a gas turbine using low pressure steam, thereby enabling energy to be advantageously conserved. By the use of a pumped water circuit and a water contact tower, gas moisturization can be achieved using low pressure steam, for gas turbine fuel. In the practice of the invention, a water recirculation loop similar to that used in recovering heat from a hot process stream is employed. The heat required for moisturizing the process gas stream, e.g. fuel gas to a gas turbine, is provided either by direct injection of steam into the recirculation loop water or by heat exchange with steam. In either case, the steam will have a pressure only slightly above the partial pressure of water vapor in the moisturized gas stream. The benefits derived from the invention are illustrated by consideration of a moderate pressure gas stream, e.g. 1724 KPa (250 psia), desired to have a moisture content of 10%. For a stream of 226.8 kg/hr (500 lb. moles/hr.) of dry gas at 1724 kPa (250 psia), approximately 453,6 kg/hr (1,000 lb/hr) of steam is needed to produce a gas mixture containing 10 mol of water vapor. If direct steam addition to said gas stream is employed, 1724 kPa (250 psia) is the minimum pressure of saturated steam that can be used to achieve the 10% moisture addition level. Alternatively, the 1724 kPa (250 psia) saturated steam could be used in a condensing steam turbine to produce 83,5 kW (112 hp). By contrast, the invention enables the use of approximately 276 kPa (40 psia) saturated steam to achieve the same moisture level of 10% in the 1724 kPa (250 psia) gas stream. The desirability of using 276 kPa (40 psia) steam will be apparent since only about 54,4 kW (73 hp) can be generated per 453,6 kg/hr (1,000 lb/hr) of steam at this pressure level. Consequently, for a highly efficient, intergrated steam system, the equivalent of 29,1 kW (39 hp), or nearly 35% 25 less useable energy, is required to moisturize the gas stream by use of the invention as compared to the requirements of direct steam addition to the gas stream. With reference to Fig. 1 of the drawings, 30 the low-level steam moisture addition of the invention is shown in conjunction with a gas turbine employed for electric power generation. In this illustrated embodiment, fuel for driving a combustion turbine is passed in line 1 to the bottom of saturation tower 2 in which it is heated and its moisture level is raised to 10% in countercurrent contact with hot water passing to the upper, vapor portion of said tower from recirculation line 3. The thus-treated fuel gas removed from the upper portion of tower 2 is passed in line 4 for burning in gas turbine combustor unit 5 to provide the energy to drive gas turbine 6. The somewhat cooler water removed from the bottom of said tower 2 in line 7 is throttled across valve 8 to a pressure sufficiently low to permit injection therein of low pressure steam. A small portion of the water flow in recirculation line 7 is removed through line 9 to control the buildup of harmful salts in the recirculation loop comprising said recirculation lines 3 and 7. Low-pressure steam for replacement purposes is added to recirculation line 7 from steam supply line 10. Such steam addition serves to raise the temperature of the water in the recirculation loop and provide the mass of water needed to sustain the moisturization operation. The hot water in recirculation line 7 downstream of the steam addition point is pumped by means of pump 11 at a pressure sufficiently high for delivery in line 3 to tower 2. Gas from gas turbine combustor 5 is passed in line 12 to gas turbine 6 connected by drive shaft 13 to air compressor 14 to which feed air is passed in line 15. A portion of the compressed air removed from air compressor 14 is passed in line 16 to said combustor 5, while the remaining portion of said compressed air is passed in line 17 to the inlet end of gas turbine 6. Exhaust gas from the gas turbine is removed in line 18. The power generated by gas expansion in said gas turbine is also used to generate electrical power in generator 19 driven by shaft 20 connected to said gas turbine 6. In the alternative embodiment of the invention shown in Fig. 2, the system as shown in Fig. 1 is employed with the desired heat of vaporization being supplied to the recirculating water by low-pressure steam across a heat exchanger rather than by direct injection. Thus, low-pressure steam in line 10 passes to heat exchanger 21, with the low-pressure condensate formed therein being pumped through line 10A, containing pump 22, for injection into line 7 of the water recirculation loop. Upon passing to pump 11, the water in said line 7 is heated in said heat exchanger 21 before being recycled in line 3 to saturation tower 2. In this embodiment, the water recirculation loop will be seen to perform as in the Fig. 1 embodiment except with respect to pump 11. Because there is no need to reduce the pressure on the recirculation loop to allow low-pressure steam injection therein, since said steam is condensed in heat exchanger 21, pump 11 is needed only to raise the pressure of the recirculating water in line 7 a small amount to overcome the pressure drop in the water recirculation loop. Consequently, the energy used by pump 11 will be much less in the Fig. 2 embodiment than in the Fig. 1 embodiment. Those skilled in the art will appreciate that variations can be made in the details of the invention as carried out in practice without departing from the scope of the invention as set forth in the appended claims. For purposes of the invention, low pressure steam addition to the water in the recirculation loop will be understood to be such addition at a pressure of from about 138 kPa about 690 kPa (about 20 psia to about 100 psia), with a steam addition pressure of preferably from about 276 kPa to 414 kPa (about 40 to 60 psia), conveniently about 345 kPa (50 psia), although low pressures outside such ranges can also be employed in the practice of the invention. The invention will be seen to represent a significant advance in the field of gas turbine operations. The use of low pressure steam to enhance the output of a gas turbine advantageously enables desirable energy conservation to be achieved.

Claims: An apparatus for the addition of low pressure steam to the fuel gas used to drive a gas turbine comprising: (a) a saturation tower (2) adapted for the contacting of said fuel gas with a hot water stream for the moisturization of said fuel gas; (b) conduit means (1) for passing fuel gas to the bottom portion of said saturation tower (2) for upward passage therein; (c) conduit means (3) for passing hot water to the upper portion of said saturation tower (2) for downward passage therein; (d) conduit means (4) for removing moisturized fuel gas from the upper portion of said saturation tower (2) for burning in a gas turbine combustor unit (5) to provide the energy to drive a gas turbine (6); (e) conduit means (7) for withdrawing water from the bottom portion of said saturation tower (2), said conduit means and the conduit means (3) for passing hot water to the saturation tower being in fluid communication and forming a water recirculation loop around said saturation tower; characterized in that the hot water enters the saturation tower (2) in liquid form; and in that the apparatus further comprises: (f) pump means (11) in the water recirculation loop for pumping said water therein for delivery to the saturation tower (2); (g) valve means (8) in said water recirculation loop upstream of said pump means (11) for throttling the pressure of the water withdrawn from the saturation tower (2) to a pressure such as to enable low pressure steam to be injected therein; and (h) steam supply conduit means (10) for the addition of low pressure steam to the water recirculation loop at a point intermediate said pump means (11) and said valve means (8); whereby moisturization of the fuel gas for a gas turbine is advantageously achieved using low pressure steam. The apparatus of claim 1 and including conduit means (9) for removing a portion of the water in said water recirculation loop. The apparatus of claim 1 and including heat exchange means (21) positioned in said water recirculation loop downstream of said pump means (22), said heat exchange means adapted to heat the water in the water recirculation loop by means of said low pressure steam from said steam supply conduit means (10), and pump means (22) for pumping low-pressure condensate formed upon cooling of said low pressure steam to said water recirculation loop at a point intermediate said pump means and said valve means (8).

Inventors: DRNEVICH RAYMOND FRANCIC, DRNEVICH, RAYMOND FRANCIC
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1992
Main classification: F02C3
IPC codes: F02C3, F01K21