Gas turbine steam addition

Abstract: The output of gas turbines (33) is enhanced by gas moisturization using a water contact tower (51) and a pumped water recirculation loop 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 1724kPa (250 psia) saturated steam fed to a condensing turbine can produce 83.5kW (112 hp), while feeding the same amount of steam to an industrial gas turbine will produce about 67.1kW (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 moisturized 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. 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 in which moisture is added to a waste nitrogen recycle stream of an integrated gas turbine (IGT) air separation system; Fig. 2 is a process flow diagram of an embodiment of said IGT system with means for adding moisture to the waste nitrogen recycle stream; Fig. 3 is a process flow diagram of an embodiment of said IGT system with alternative means for moisture addition to said waste nitrogen recycle system; Fig. 4 is a process flow diagram of an embodiment of the invention for low pressure level addition of steam to a fuel gas. 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, both for gas turbine fuel and for waste nitrogen recycle associated with said integrated gas turbine (IGT) air separation systems. 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. 1724kPa (250 psia) desired to have a moisture content of 10%. For a stream of 226,8Kg/h (500 lb. moles/hr.) of dry gas at 1724kPa (250 psia), approximately 453,6Kg/h (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, 1724kPa (250 psia) is the minimum pressure of saturated steam that can be used to achieve the 10% moisture addition level. Alternatively, the 1724kPa (250 psia) saturated steam could be used in a condensing steam turbine to produce 83.5kW(112 hp). By contrast, the invention enables the use of approximately 276kPa (40 psia) saturated steam to achieve the same moisture level of 10% in the 1724kPa (250 psia) gas stream. The desirability of using 276kPa (40 psia)steam will be apparent since only about 54.4kW (73 hp) can be generated per 453,6Kg/h (1,000 lb/hr) of steam at this pressure level. Consequently, for a highly efficient, intergrated steam system, the equivalent of 29.1kW (39 hp), or nearly 35% 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. As indicated above, the invention can be used for moisturization of gas turbine fuel, as described particularly with respect to the Fig.4 embodiment , or the waste nitrogen return associated with the integrated gas turbine (IGT) air separation system such as that shown in the Olszewski et al. patent, U.S. 4,224,045. In the IGT approach, air is extracted from a gas turbine compressor and passed to an air compressor and passed to an air separation plant. The waste nitrogen from said plant is compressed and returned to the gas turbine either before or after the gas turbine combustor unit. The air separation plant removes about 20% of the volume of said extracted air as product oxygen. As a result, the mass returning to the gas turbine as waste nitrogen is less than the mass of gas extracted from the gas turbine air compressor. In the Fig. 1 embodiment of the invention pertaining to the IGT approach, moisture is added to the waste nitrogen being recycled to the gas turbine through a water recirculation loop as to the fuel gas in the Fig. 4 embodiment . Said waste nitrogen stream is returned to the gas turbine either upstream or downstream of the gas turbine combustor. All or a part of the mass removed from the system in the product gas withdrawn from the air separation system is replaced by moisture provided by low-pressure steam. In the Fig. 1 embodiment, fuel gas is passed through line 30 to gas turbine combustor 31 from which gas is passed in line 32 to gas turbine 33 from which expanded exhaust gas is removed through line 34. Gas turbine 33 is connected by drive shaft 36 to air compressor 35 and by drive shaft 16 to electrical power generator 37. Feed air is passed to air compressor 35 in line 38. A portion of the compressed air from air compressor 35 is passed in line 39 to turbine combustor 31, while another portion thereof is passed in line 40 directly to gas turbine 33. The remaining portion of the compressed air passes in line 41, through heat exchanger 42 and coolers 43 and 44, to air separation system 45, from which a product oxygen stream is recovered through line 46. Said cooler 43 is conveniently operated against boiler feed water from line 47 and passing in line 48 to steam generation means. Cooler 44 is conveniently operated against a source of cooling water. Waste nitrogen from the air separation system is recycled to the gas turbine system, with passage through a saturation tower with associated water recirculation loop as in the embodiments described above. Thus, waste nitrogen from air separation system 45 is passed in line 49 containing expander 50 to the bottom of saturation tower 51. Water is removed from the bottom of said tower 51 through recirculation line 52, containing throttling valve 53 for passage to pump 54 and recirculation line 55 for introduction at the upper portion of said saturation tower 51 for downward passage and countercurrent contact with waste nitrogen therein. As in the earlier embodiments, a small portion of the water flow in recirculation line 52 is removed through line 56 to control harmful salt buildup. Low-pressure steam for replacement purposes is added to recirculation line 52, downstream of valve 53, from steam supply line 57. The thus-treated waste nitrogen stream is removed from the upper portion of tower 51 through line 58 and is heated against compressed air from air compressor 35 in heat exchanger 42 before-passage to the gas turbine system. A portion of the cooled waste nitrogen stream exiting the heat exchanger is passed in line 59 directly to gas turbine combustor 31 for reaction therein. A portion of this waste nitrogen stream, in the illustrated embodiment, is passed in line 60 for combining with compressed air in line 39 and passage of said combined stream in line 61 to said combustor 31. In the practice of the invention, it will be understood that all of the recycled waste nitrogen can be passed to the combustor through either line 59 or line 61 above. In the Fig. 2 embodiment, a heat exchanger is used to provide heat to the water in the water recirculation loop from the compressed air passing to the air separation system. As a result, water, e.g., boiler feed water, can be introduced into recirculation line 52 from line 57, instead of low-pressure steam as in the Fig. 1 embodiment. The recirculating water stream in line 55 downstream of pump 54 is thus passed through heat exchanger 43 for heating against compressed air in line 41 before return to the top of saturation tower 51. The compressed air, upon further cooling in said heat exchanger 43, is passed through cooler 44 to air separation system 45. Fig. 3 illustrates an alternative means for the desired moisture addition to the waste nitrogen stream being recycled from the air separation system to the gas turbine system. In this embodiment, two saturation towers and associated water recirculation loops are employed, with low pressure steam being added to one such loop and boiler feed water being added to the other. The Fig. 3 embodiment is numbered as in the Fig. 2 embodiment except that the waste nitrogen recycled from the top of saturation tower 51 in line 58 does not pass directly to heat exchanger 42 and to the gas turbine system, but is passed to a second saturation tower-water recirculation loop such as is illustrated in the Fig. 1 embodiment of the invention. Thus, waste nitrogen in line 58 passes to the bottom of saturation tower 62 from the top of which a moisturized waste nitrogen stream passes in line 63 to said heat exchanger 42. Water removed from the bottom of said tower 62 is passed in recirculation line 63 containing throttle valve 64 to pump 65, and from said pump through recirculation line 66 to the top of saturation tower 62. As in the other embodiments, a small portion of the water flow in recirculation line 63 is withdrawn through line 66, low-pressure steam is added to said recirculation line from steam supply line 67. With reference to Fig.4 of the drawings, 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. In the Fig. 4 embodiment of the invention, low pressure steam provides the heat of vaporization through a heat exchanger 21, with the resulting condensate being employed to preheat makeup water prior to being returned to the low pressure steam generation system employed to generate the low pressure steam for the system. Thus, low pressure steam from line 10 is passed to heat exchanger 21 to heat water passed thereto through line 7 and pump 11. Low-pressure condensate is passed from heat exchanger 21 through line 10A to heat exchanger 23 from which it is passed in line 24 to said steam generation system, not shown. Makeup water is passed from line 25 to heat exchanger 23 for preheating prior to being passed in line 26, containing pump 22, for introduction into line 7 of the water recirculation loop. 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. The advantage of the Fig. 4 embodiment can be appreciated from a consideration of the relative quality of makeup water for the water recirculation loop and the steam generation system. Typically, deionized/deaerated water is used in steam generation systems to control scaling and corrosion in the steam generation boilers. Makeup water for the water recirculation loop can be any source of relatively clean water, such as cooling water. Cooling water requires substantially less treatment than boiler feed water, and the Fig.4 embodiment enables water suitable for boiler feed purposes to be efficiently used therefor, with less costly cooling water or other such water being used for makeup purposes in the recirculation loop. 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 138kPa to about 689kPa (about 20 psia to about 100 psia), with a steam addition pressure of about 276kPa to about 345 kPa (about 40 to about 50 psia) being generally preferred. Waste nitrogen being recycled for reintroduction to the gas turbine is typically compressed to about 1379kPa (200 psia), although it will be appreciated that the particular pressure level employed for this purpose will depend upon the particular application, the operating characteristics of the gas turbine employed, and the like. The invention is further illustrated by the five case example of the Table below. In this Table representing various IGT operating arrangements, the moisture addition feature is as follows: Case 1 - represents a conventional IGT system in which no moisture is added to the waste nitrogen recycle stream; Case 2 - represents an IGT system in which 250 psia steam is injected directly into the waste nitrogen recycle stream; Case 3 - represents an IGT system operated in accordance with the Fig. 1 embodiment of the invention, with 345kPa (50 psia) steam being introduced into recirculation line 52; Case 4 - represents an IGT system operated in accordance with the Fig. 2 embodiment of the invention, with boiler feed water being introduced into recirculation line 52; and Case 5 - represents an IGT system operated in accordance with the Fig. 3 embodiment of the invention, with boiler feed water being introduced into recirculation line 52 and 345kPa (50 psia) steam being introduced into recirculation line 63. CASE Stream kg mole/hr (lb mole/hr) 1 2 3 4 5 (a) compressed air (line 41) 11,700 (25,794) 11,700 (25,794) 11,700 (25,794) 11,700 (25,794) 11,700 (25,794) (b) oxygen product (line 46) 2,463 (5,430) 2,463 (5,430) 2,463 (5,430) 2,463 (5,430) 2,463 (5,430) (c) 345kPa (50 psia) steam addition ― 2,616 (5,767) 2,616 (5,767) 0 1,718 (3,787) (d) waste nitrogen recycle to tower (line 49) 9,137 (20,144) 9,137 (20,144) 9,137 (20,144) 9,137 (20,144) 9,137 (20,144) (e) waste nitrogen recycle to gas turbine (line 39) 9,137 (20,144) 11,450 (25,242) 11,450 (25,242) 11,450 (21,204) 11,450 (25,242) Combined-Cycle Net Power (KW) Base Base + 14,100 Base + 16,900 Base + 12,100 Base + 18,900 Combined-Cycle Heat Rate (Btu/KWH) Base Base - 390 Base - 557 Base - 634 Base - 668 As seen in the Table, the use of moderate pressure, 1724kPa (250 psia) steam, i.e. Case 2, can increase the net power of the gas turbine by 14,100 KW when compared to IGT operation without moisture addition, i.e. Case 1. The mass replacement achieved using direct steam addition also allows the gas turbine to operate more efficiently. Case 3 shows that, by the practice of the invention, an additional 2,800 KW of output improvement can be achieved as contrasted to said direct steam addition. Furthermore, heat rate improvements of 557 and 167 Btu/KWH are obtainable in the practice of the invention as compared to the base case, i.e. Case 1, and the direct steam addition case, i.e. Case 2, respectively. Case 4, using boiler feed water (BFW) instead of 345kPa (50 psia) steam as in Case 3 for desired mass increase, utilizes low-level heat recovery to provide as much moisture as possible from heat obtained from the stream in line 41. The temperature level and heat content of the stream in line 41 limits the moisture content of the stream in line 59 to about 5% by volume. In this case, 12,100 KW of power is available over the base case. However, the equivalent of 670 KW of 862kPa (12.5 psia) steam would be required to replace the BFW heating lost by using the low level heat for moisturizing the gas. The heat rate for Case 4 shows an improvement of 634 Btu/KWH over the base case and of 244 Btu/KWH over Case 2 which uses 1724kPa (250 psia) saturated steam. In Case 5, which employs a combination of moisture addition methods that could be used to moisturize waste nitrogen return for IGT operations, the heat rejected to boiler feed water in the Fig. 1 embodiment of Case 3 is used to provide heat for partial moisturization of the waste nitrogen. Additional moisture is added using low-pressure steam. This embodiment reduces the quantity of low-pressure steam required for moisture addition by about 25%. Integration of the two water recycle loops would provide a substantial capital savings in that only one tower would be required. As shown in the Table, the moisture addition approach of Case 5 results in an 18,900 KW increase in net power over IGT Case 1, without moisture addition, and a 4,700 KW increase over Case 2. The heat rate for the combined-cycle of Case 5 improved by 668 Btu/KWH over the IGT without moisture addition of Case 1 and by 278 Btu/KWH over the IGT with direct steam addition, i.e. Case 2. It will be understood that various other combinations can be employed within the scope of the invention, using direct steam addition and/or recovering heat from hot process streams, for unique advantages in specific applications. The low pressure steam addition of the invention will be understood to comprise the addition of steam at pressures of from about 207 to about 517 kPa (about 30 to about 75 psia), preferably from about 276 to about 414kPa (about 40 to 60 psia), conveniently about 345kPa (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 moisture to the waste nitrogen stream recycled to the gas turbine system of an integrated gas turbine-air separation system comprising: (a) a saturation tower adapted for the contacting of said waste nitrogen stream with a hot water stream for the moisturization of said waste nitrogen; (b) conduit means for passing the waste nitrogen recycle stream from the air separation system to the bottom portion of said saturation tower for upward passage therein; (c) conduit means for passing hot water to the upper portion of said saturation tower for downward passage therein; (d) conduit means for removing moisturized waste nitrogen recycle gas from the upper portion of said saturation tower for recycle to said gas turbine system; (e) conduit means for withdrawing water from the bottom portion of said saturation tower, said conduit means and said conduit means for passing hot water to the saturation tower being in fluid communication and forming a water recirculation loop around the saturation tower; (f) pump means in said water recirculation loop for pumping said water therein for delivery to said saturation tower; and (g) conduit means for passing water to said water recirculation loop, whereby moisturization of said waste nitrogen stream is advantageously achieved. The apparatus of Claim 1 and including: (h) conduit means for passing compressed air from the air compressor of an air compressor-gas turbine system to said air separation system; and (i) heat exchange means positioned in said conduit means for supplying compressed air to the air separation system, said heat exchange means being adapted to heat the moisturized nitrogen recycle gas and the water in said recirculation loop by means of the compressed air in said conduit means. The apparatus of claim 1 adapted for the addition of low pressure steam to the waste nitrogen recycled to the gas turbine system of an integrated gas turbine-air separation system further comprising: (a) conduit means for passing compressed air from the air compressor of an air compressor-gas turbine system to an air separation system; (b) an air separation system adapted to recover product oxygen and a waste nitrogen recycle stream from said compressed air; (c) valve means in said water recirculation loop upstream of said pump means for throttling the pressure of water withdrawn from the saturation tower to a pressure such as to enable low pressure steam to be injected therein; and (d) steam supply conduit means for the addition of low pressure steam to the water recirculation loop at a point intermediate said pump means and said valve means, whereby moisturization of the waste nitrogen recycle stream is advantageously achieved using low pressure steam. The apparatus of Claim 1 or 3 and including conduit means for removing a portion of the water in said water recirculation loop. The apparatus of Claim 3 and including heat exchange means positioned in said conduit means for supplying compressed air to said air separation system, said heat exchange means being adapted to heat the moisturized nitrogen recycle gas by means of the compressed air in said conduit means. The apparatus of Claim 3 and including: (a) a second saturation tower positioned for the passage of said waste nitrogen recycle stream thereto prior to its passage to said saturation tower; (b) conduit means for passing the waste nitrogen recycle stream from the air separation system to the bottom portion of said second saturation tower for upward passage therein; (c) conduit means for passing hot water to the upper portion of said saturation tower for downward passage therein; (d) conduit means for removing moisturized waste nitrogen recycle gas from the upper portion of said second saturation tower, said conduit means comprising said conduit means for passing the waste nitrogen stream to the bottom portion of said saturation tower; (e) conduit means for withdrawing water from the bottom portion of said saturation tower, said conduit means and said conduit means for passing hot water to the saturation tower being in fluid communication and forming a second water recirculation loop around said second saturation tower; (f) pump means in said second water recirculation loop for pumping said water therein for delivery to said second saturation tower; and (g) conduit means for passing water to said water recirculation loop. The apparatus of Claim 6 and including heat exchange means positioned in said conduit means for supplying compressed air to the air separation system, said heat exchange means being adapted to heat the moisturized nitrogen recycle gas and the water in said second recirculation loop by means of the compressed air in said conduit means. The apparatus of Claim 6 and including conduit means for removing a portion of the water in said water recirculation loop and said second water recirculation loop. The apparatur of Claim 1 adapted for the moisturization of the waste nitrogen stream recycled to the gas turbine system of an air compressor-gas turbine system comprising: (a) first and second saturation towers adapted for contacting said waste nitrogen stream with a hot water stream for the moisturization of said waste nitrogen; (b) conduit means for passing the waste nitrogen stream to the bottom portion of each saturation tower for upward passage therein, the conduit means for passing said nitrogen stream to said first saturation tower comprising conduit means for recovering said nitrogen stream from the second saturation tower; (c) conduit means for passing hot water to the upper portion of each said saturation tower for downward passage therein; (d) conduit means for removing a moisturized nitrogen stream from the upper portion of each said saturation tower, said conduit means from the first saturation tower comprising conduit means for passing said nitrogen stream to the gas turbine system; (e) conduit means for withdrawing water from the bottom portion of each saturation tower, said conduit means and said conduit means for passing hot water to each saturation tower being in fluid communication and forming first and second water recirculation loops around said first and second saturation towers; (f) pump means in each said water recirculation loop for pumping said water therein to said saturation towers; (g) valve means in said first water recirculation loop for throttling the pressure of water withdrawn from the first saturation tower to a pressure such as to enable low pressure steam to be injected therein; (h) steam supply conduit means for the addition of low pressure steam to the first water recirculation loop at a point intermediate said pump means and said valve means; and (i) conduit means for passing water to said water recirculation loop, whereby moisturization of said nitrogen stream is advantageously achieved. The apparatus of Claim 9 and including conduit means for passing compressed air from the air compressor to a gas expander unit. The apparatus of Claim 10 and including heat exchange means positioned in said conduit means for passing compressed air to said gas expander unit, said heat exchange means being adapted to heat the moisturized nitrogen stream removed from said first saturation tower and said water in the second water recirculation loop by means of the compressed air in said conduit means. The apparatus of Claim 9 and including conduit means for removing a portion of the water in each water recirculation loop.

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