| Title of Invention | GAS TURBINE APPARATUS, LOW-CALORIE CONTENT GAS FEEDING APPARATUS, AND METHOD OF SUPPRESSING RISE OF CALORIE CONTENT OF THE GAS. |
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| Abstract | A low-calorie gas supply system is provided which is capable of supplying a low-calorie gas as a stabilized fuel for gas turbines by suppressing a steep rise in the calorie of the low-calorie gas. The low-calorie gas supply system includes: low-calorie gas supply piping (3) for supplying a gas turbine (2)with the low-calorie gas; exhaust gas supply piping (4) for supplying an exhaust gas to the low-calorie gas supply piping (3); a first mixer (6) located at a joint between the low-calorie gas supply piping (3) and the exhaust gas supply piping (4); a calorimeter (11) provided to the low-calorie gas supply piping (3) for measuring the calorie of the gas; an oxygen content meter(15) provided to the low-calorie gas supply piping (3) at a location downstream of the first mixer (6); and a control device (100) configured to exert a control based on a result of measurement by the oxygen content meter (15) such that when a value measured by the calorimeter (11) is higher than a reference value, the exhaust gas is supplied from the exhaust gas supply gas supply piping (4) so that the exhaust gas mixing quantity does not fall within the flammability limits of the low-calorie gas. |
| Full Text | 1 DESCRIPTION GAS TURBINE SYSTEM, LOW-CALORIE GAS SUPPLY SYSTEM, AND METHOD OF SUPPRESSING A RISE IN THE CALORIE OF THE GAS Technical Field [0001] The present invention relates to a gas turbine system, a lowcalorie gas supply system and a method of suppressing a rise in the calorie of the gas. More specifically, the invention relates to- a lowcalorie gas supply system configured to supply a gas turbine with a lowcalorie gas in a condition usable as a fuel for the gas turbine; a gas turbine system provided with this lowcalorie gas supply system! and a method of suppressing a rise in the calorific value (or calorie) of the lowcalorie gas used as a gas turbine fuel. Background Art [0002] In the ironmaking field, for example, production of pig iron by the blast furnace process allows blast furnace gas (hereinafter is referred to as "BFG") to be evolved as a byproduct gas. The total calorie of BFG reaches about a half of the calorie of coke used and, therefore, BFG is utilized for various purposes in ironmaking plants in order to lower the production cost of pig iron. BFG is produced in an amount of about 3,000 Nm3 per one ton of 2 coke used, and the composition of BFG typically comprises about 10 to 18 vol% (hereinafter is expressed in "%" simply) of carbon dioxide (CO2), about 22 to 30% of carbon monoxide, about 52 to 60% of nitrogen (N2), about 0.5 to 4% of hydrogen (H2), and about 0.5 to 3% of methane (CH4). [0003] Since BFG contains 2 to 10 g/Nm3 of flue dust in addition to those components, the flue dust content is reduced to about 0.01 g/Nm3 by means of a dust remover so that BFG can be utilized as a fuel gas having a calorie of about 800 kcal/Nm3 for a hot-blast oven, coke oven, heating furnace, boiler or the like. As technical improvements made in recent years have made it possible to combust low-calorie gases in gas turbines, such cases in that electric power generation is conducted by using BFG as a gas turbine fuel are increasing in number. The "lowcalorie gas", as referred to herein, is defined as a gas having a calorie of not more than about 12 MJ/Nm3. Such low-calorie gases include not only blast furnace gas (BFG) but also many other gases, such as converter vessel gas (LDG), and mixed gases thereof. [0004] On the other hand, new ironmaking processes (for example, direct iron reduction processes including FINEX and COREX and others) other than the blast furnace process have been developed. For this reason, a combustion method that is applicable to an effective utilization of by-product gases produced by such new processes is expected to develop. With any of the ironmaking 3 processes, the byproduct gas produced thereby has properties (including gas composition and calorie) which depend upon the processing system and their actual operation. Even with applying the same system, properties of the by-product gas incidentally and inevitably fluctuate moment by moment in accordance with the properties of raw materials and actual chemical reaction occurred in the processing system and thereby the properties are inconstant. [0005] Regarding the calorie of the by-product gas which is the most important property in using the by-product gas as a fuel for gas turbines, when the calorie of the byproduct gas exceeds the upper limit of the allowable calorie fluctuation range (for example, a mean calorific value plus about 10%) inherent to each gas turbine, that is, when the calorie of the byproduct gas rises steeply, the combustion temperature within the combustor of the gas turbine sometimes becomes abnormally high steeply. This might result in harmful effects such that the burners, the stationary blades and moving blades of the turbine are damaged due to such a high temperature and hence shortened lifetimes. In such a case, continuous economical operation of the gas turbine system becomes difficult. [0006] One of the countermeasures is to suppress the calorie fluctuation of a by-product gas by diluting with nitrogen gas (N2) (see patent documents 1 and 2 for example). In the case of diluting a byproduct gas with N2 only, however, some calorie fluctuation 4 levels cannot but require the use of a large amount of an expensive inert gas such as N2. It is very difficult for a large majority of industry to secure continuously a large amount of such an inert gas. Further, for the purpose of supplying the inert gas, various kinds of expensive device, apparatus and piping are needed for storing, supplying and controlling. For these reasons, methods using inert gases worsen the cost efficiency of gas turbine electric power generation and decrease the technical advantage of high-efficient gas turbine. Patent document 1^ Japanese Patent Laid-Open Publication No. 2002-155762 Patent document 2'- Japanese Patent Laid-Open Publication No. HEI 9-317499 Disclosure of Invention Problem to be solved by Invention [0007] The present invention has been made in order to solve the foregoing problems. Accordingly, it is an object of the present invention to provide a lowcalorie gas supply system capable of lessening a rise in the calorie of a lowcalorie fuel gas for gas turbines at low system investment cost and running cost, a gas turbine system provided with this lowcalorie gas supply system, and a method of suppressing a rise in the calorie of a lowcalorie gas used as a gas turbine fuel. Means for solving Problem 5 [0008] In order to attain the foregoing object, the present invention provides a lowcalorie gas supply system comprising'- a lowcalorie gas supply passage for supplying a gas turbine with a lowcalorie gas as a fuel gas," an exhaust gas supply passage connected to the lowcalorie gas supply passage for supplying an exhaust gas produced by a combustion system to the lowcalorie gas supply passage; a calorie measuring device provided on the lowcalorie gas supply passage for measuring a calorie of the lowcalorie gas; and a control device capable of controlling an exhaust gas supply operation of the exhaust gas supply passage based on a result of measuring by the calorie measuring device. [0009] According to this invention, the exhaust gas, which contains large amounts of incombustible gases including N2, carbon dioxide (CO2) and the like and can be procured with extreme ease, is mixed into the lowcalorie gas to dilute the lowcalorie gas, thereby suppressing a rise in the calorie of the lowcalorie gas. The "calorie measuring device", as used herein, is meant to include not only devices of the type configured to measure the calorie of a gas directly (for example, the so-called calorimeter) but also devices of the type configured to determine the content of a combustible component of a gas. [0010] 6 It is possible that the aforementioned control device has stored therein an established maximum allowable calorific value of the fuel gas for the gas turbine and is configured to cause the exhaust gas to be supplied from the exhaust gas supply passage when the calorific value of the low-calorie gas is higher than the established maximum allowable calorific value. [0011] Preferably, the lowcalorie gas supply system has an arrangement wherein^ the lowcalorie gas supply passage is provided with an oxygen density sensing device at a portion thereof located downstream of a joint with the exhaust gas supply passage; and the control device is configured to control the exhaust gas supply operation of the exhaust gas supply passage based on a result of measuring by the oxygen density sensing device. This arrangement is capable of controlling mixing of the lowcalorie gas with the exhaust gas containing oxygen based on the oxygen content of the mixed gas comprising the lowcalorie gas and the exhaust gas in order to prevent the mixed gas from catching fire. [0012] In the system provided with the oxygen density sensing device, the above-described control device may be configured to control the exhaust gas supply from the exhaust gas supply passage by referencing a allowable mixed volume percentage of the exhaust gas which is established based on flammability limit information on the low-calorie gas obtained from a mixing ratio between the low-calorie gas and the exhaust gas. 7 [0013] Alternatively, the above-described control device of the system provided with the oxygen density sensing device may be configured to control an exhaust gas supply operation of the exhaust gas supply passage by referencing a allowable mixed volume percentage of the exhaust gas which is calculated based on an oxygen content ratio from a allowable mixed volume percentage of air established based on flammability limit information on the low-calorie gas obtained from a mixing ratio between the low-calorie gas and air. [0014] Preferably, the system provided with the oxygen density sensing device has an arrangement wherein: the lowcalorie gas supply passage is connected to an inert gas supply passage configured to supply an inert gas to the lowcalorie gas supply passage; and the control device is configured to control an inert gas supply operation of the inert gas supply passage based on a result of sensing by the oxygen density sensing device. Even when a rise in the calorie of the lowcalorie gas is suppressed insufficiently due to, for example, restriction on the exhaust gas supply, this arrangement is capable of supplementing the calorie fluctuation suppressing action by supplying the inert gas. A portion of the inert gas supply passage that reaches the mixing portion may form a common passage with the exhaust gas supply passage. [0015] Preferably, the lowcalorie gas supply system has an 8 arrangement wherein: the low-calorie gas supply passage is connected to an inert gas supply passage configured to supply an inert gas to the lowcalorie gas supply passage; and the control device is configured to control an inert gas supply operation of the inert gas supply passage based on a result of measuring by the calorie measuring device under supply of the exhaust gas to the low-calorie gas supply passage by the exhaust gas supply passage. Even if the exhaust gas insufficiently suppresses a rise in the calorie of the lowcalorie gas, this arrangement is capable of a complementing the calorie fluctuation suppressing by supplying the inert gas. [0016] Preferably, any one of the above-described systems has an arrangement wherein the lowcalorie gas supply passage is provided with a first tank for temporarily storing the lowcalorie gas therein, the first tank having an inlet connected to an upstream side of the lowcalorie gas supply passage and an outlet connected to a downstream side of the lowcalorie gas supply passage. All the lowcalorie gas supplied through the lowcalorie gas supply passage is temporarily stored in the first tank in which portions of the low-calorie gas are mixed together to decrease the range of calorie fluctuation of the lowcalorie gas as well as to retard the calorie fluctuation speed. For this reason, the calorie leveling control by dilution with the exhaust gas at a location downstream of the outlet of the first tank can be achieved more easily. The above-described calorie measuring device is provided on the lowcalorie gas supply 9 passage. This feature is meant to include an arrangement wherein the calorie measuring device is fitted on the first tank or a second tank to be described later because the first tank or the second tank forms part of the lowcalorie gas supply passage in the system having the first or second tank provided on the lowcalorie gas supply passage. [0017] Preferably, the lowcalorie gas supply system has an arrangement wherein: the lowcalorie gas supply passage is provided with a second tank for temporarily storing the lowcalorie gas therein! the lowcalorie gas supply passage and the second tank are interconnected through a gas inlet passage for feeding the low-calorie gas from the lowcalorie gas supply passage into the second tank as well as a gas outlet passage for returning the lowcalorie gas from the second tank to the lowcalorie gas supply passage; and the inlet passage is provided with a first gas feeding device for feeding the lowcalorie gas with a pressure toward the second tank. This arrangement can perform an operation similar to that performed by the aforementioned first tank. [0018] Each of the first and second tanks may be either a tank of the invariable type having an invariable internal volume or a tank of the variable internal volume type used as a device (gas holder) for monitoring demand-supply balance of gas in a conventional gas turbine system or the like. Such variable internal volume type tanks include a tank having an airtightly fitted lid 10 member which is vertically slidable in accordance with the internal pressure of the tank, and a tank capable of selecting a tank's internal volume that maximizes the balancing effect by vertically moving a lid member forcibly by means of a driving device. [0019] Preferably, the lowcalorie gas supply system has an arrangement wherein the low-calorie gas supply passage is provided with a return passage for returning a part of the lowcalorie gas to an upstream side of the lowcalorie gas supply passage, the return passage being provided with a second gas feeding device for feeding the lowcalorie gas with a pressure toward the upstream side of the lowcalorie gas supply passage. This arrangement can perform an operation similar to that performed by the aforementioned first tank. [0020] Preferably, the lowcalorie gas supply system has an arrangement wherein the exhaust gas supply passage is provided with an exhaust gas supply shut-off device capable of closing and opening the exhaust gas supply passage and an exhaust gas relief device located upstream of the exhaust gas supply shut-off device. The exhaust gas supply shut-off device may comprise a stop valve or a flow control valve for example. [0021] Preferably, the lowcalorie gas supply system has an arrangement wherein the inert gas supply passage is provided with an inert gas supply shut-off device capable of closing and opening the inert gas supply passage and an inert gas relief device located 11 upstream of the inert gas supply shut-off device. The inert gas supply shut-off device may comprise a stop valve or a flow control valve for example. In cases where a portion of the inert gas supply passage that reaches the lowcalorie gas supply passage forms a common passage with the exhaust gas supply passage which is provided with both of the exhaust gas supply shut-off device and the exhaust gas relief device, it is possible to provide a single supply shut-off device serving as both the inert gas supply shut-off device and the exhaust gas supply shut-off device, or a single relief device serving as both the inert gas relief device and the exhaust gas relief device. [0022] Exhaust gas produced by the aforementioned gas turbine to which the lowcalorie gas supply system supplies the fuel may be utilized as the diluting gas produced by the aforementioned combustion system. The utilization of the exhaust gas produced by the gas turbine includes not only direct utilization of the exhaust gas from the gas turbine but also utilization of the exhaust gas as the diluting gas after utilization of the exhaust gas in a waste heat recovery boiler or the like. [0023] Alternatively, exhaust gas produced by a boiler of a power generation system may be utilized as the exhaust gas produced by the combustion system. Because the exhaust gas produced by the boiler of such a power generation system has a lower oxygen content than the gas turbine exhaust gas, its mixing 12 rate can be increased. [0024] A gas turbine system according to the present invention comprises^ a gas turbine; and a lowcalorie gas supply system for supplying the gas turbine with a lowcalorie gas as a fuel gas, the low-calorie gas supply system being any one of the lowcalorie gas supply systems described above. [0025] In such a gas turbine system, it is possible that: plural gas turbines are provided; each of the gas turbines is provided with a lowcalorie gas supply system; and the combustion system associated with the lowcalorie supply system is any one or more of the plural gas turbines except the gas turbine to which the lowcalorie supply system supplies the fuel gas. That is, exhaust gas produced by any one of the plural gas turbines except the gas turbine to which the lowcalorie supply system supplies the fuel gas may be used as the diluting gas for the lowcalorie gas supply system. [0026] A method of suppressing a rise in a calorie of a low-calorie gas used as a gas turbine fuel according to the present invention comprises the steps of: measuring the calorie of the lowcalorie gas serving as a fuel gas to be supplied to a gas turbine; and mixing an exhaust gas collected from a combustion system into the lowcalorie gas for dilution when a result of the 13 measurement is higher than an established allowable calorific value. [0027] In the calorie rise suppressing method, preferably, the exhaust gas mixing step comprises the steps of: measuring an oxygen density of a mixed gas comprising the low-calorie gas and the exhaust gas; and controlling an exhaust gas mixing quantity so that oxygen density thus measured does not exceed an established allowable value decided from flammability limit information on the low-calorie gas. [0028] Preferably, the calorie rise suppressing method further comprises the step of mixing an inert gas into the low-calorie gas when the calorie thus measured is determined not to fall below the established allowable calorific value under maximum exhaust gas supply. [0029] Preferably, the calorie rise suppressing method including the step of controlling the exhaust gas mixing quantity further comprises the step of supplying an inert gas to the lowcalorie gas while decreasing an exhaust gas mixing quantity when it is decided that decreasing the exhaust gas mixing quantity causes the calorie of the mixed gas to exceed the established allowable calorific value, while increasing the exhaust gas mixing quantity causes an exhaust gas content of the mixed gas to exceed the established allowable exhaust gas content. [0030] 14 In the lowcalorie gas supply system including the tank having the inlet and the outlet which are connected to the upstream side and the downstream side, respectively, of the lowcalorie gas supply passage, it is possible that- the lowcalorie gas supply passage is provided with a return passage interconnecting a portion of the lowcalorie gas supply passage that is located upstream of the tank and a portion of the lowcalorie gas supply passage that is located downstream of the tank; and the return passage is provided with a gas feeding device for feeding the fuel gas with a pressure toward a portion of the lowcalorie gas supply passage that is located upstream of the tank. [0031] In the lowcalorie gas supply system having the inlet passage and the outlet passage which interconnect the lowcalorie gas supply passage and the tank, it is possible that-' the lowcalorie gas supply passage is provided with a return passage interconnecting a portion of the lowcalorie gas supply passage that is located downstream of a joint with the outlet passage and a portion of the lowcalorie gas supply passage that is located upstream of a joint with the inlet passage; and the return passage is provided with a gas feeding device for feeding the fuel gas with a pressure toward an upstream side of the lowcalorie gas supply passage. [0032] Preferably, the tank is provided therein with a stirring device for stirring the gas. 15 Advantage of Invention [0033] According to the present invention, a system for supplying a gas turbine with a lowcalorie gas having a variable calorie, such as a by-product gas, is realized at low investment cost and running cost. This is because a great deal of exhaust gas containing large amounts of incombustible gases such as N2 and CO2 can be easily procured in order to suppress a rise in the calorie of the lowcalorie gas that is utilizable as a fuel gas. Brief Description of Drawings [0034] [FIG. l] FIG. 1 is a piping diagram schematically illustrating a gas turbine power generation system including a lowcalorie gas supply system according to one embodiment of the present invention. [FIG. 2] FIG. 2 is a graph plotting flammability limit of a mixed gas comprising a lowcalorie gas and air, wherein the abscissa represents the volume percentage of the low-calorie gas and the ordinate represents the temperature. [FIG. 3] FIG. 3 is a graph showing one example of calorie fluctuation of the lowcalorie gas lessened by passing through a buffer tank shown in FIG. 1. [FIG. 4] FIG. 4 is a graph showing another example of calorie fluctuation of the lowcalorie gas lessened by passing through the buffer tank shown in FIG. 1. [FIG. 5] FIG. 5 is a graph showing yet another example of calorie 16 fluctuation of the low-calorie gas lessened by passing through the buffer tank shown in FIG. 1. [FIG. 6] FIG. 6 is a piping diagram illustrating another example of a buffer tank which can be provided in the gas turbine power generation system shown in FIG. 1. [FIG. 7] FIG. 7 is a piping diagram illustrating yet another example of a buffer tank which can be provided in the gas turbine power generation system shown in FIG. 1. [FIG. 8] FIG. 8 is a piping diagram illustrating yet another example of a buffer tank which can be provided in the gas turbine power generation system shown in FIG. 1. [FIG. 9] FIG. 9 is a graph showing one example of calorie fluctuation of the lowcalorie gas lessened by passing through the buffer tank shown in FIG. 7 or 8. [FIG. 10] FIG. 10 is a piping diagram illustrating another example of calorie fluctuation suppressing means which can be provided in the gas turbine power generation system shown in FIG. 1. [FIG. 11] FIG. 11 is a piping diagram illustrating yet another example of a buffer tank which can be provided in the gas turbine power generation system shown in FIG. 1. [FIG. 12] FIG. 12 is a piping diagram illustrating yet another example of a buffer tank which can be provided in the gas turbine power generation system shown in FIG. 1. [FIG. 13] FIG. 13 is a piping diagram illustrating yet another example of a buffer tank which can be provided in the gas turbine power generation system shown in FIG. 1. 17 [FIG. 14] FIG. 14 is a piping diagram illustrating yet another example of a buffer tank which can be provided in the gas turbine power generation system shown in FIG. 1. Description of reference characters [0035] l...lowcalorie gas supply system 2...gas turbine 3...1owcalorie gas supply piping 4...exhaust gas supply piping 5...N2 supply piping 6...first mixer 7...second mixer 8...diluting gas supply piping 9...dust filter 10...buffer tank 11...calorimeter 12...flowmeter 13...mixed gas supply piping 14...calorimeter 15...oxygen content meter 16...low-pressure compressor 17...high'pressure compressor 18...cooling device 19...combustor 20...flow control valve 21... filter 18 22...electric power generator 23... filter 24...fan 25...check valve 26...stop valve 27...flowmeter 28...flow control valve 29... exhaust gas relief piping 30...flow control valve 31...stop valve 32...flowmeter 33...flow control valve 34...stop valve 35...check valve 36...communication piping 37...flowmeter 38...flow control valve 39...fan 40...gas-balance monitor 41...communication piping (outlet piping) 42...tank 43...lid member 44...balancing weight 45...(upstream-side) inlet piping 46...gas-balance monitor 46a...tank 19 47...pressure measuring device 48...return piping 49...return piping 50... (downstream-side) inlet piping 51... waste heat recovery boiler 52...stack 53...exhaust gas introduction piping 54...cooling device 55...drain piping 56...oxygen content meter 57...stirring device 100...control device S...direct iron reduction equipment Best Mode for Carrying Out the Invention [0036] Hereinafter, embodiments of a lowcalorie gas supply system, a gas turbine system provided with the same and a method of suppressing a rise in the calorie of a lowcalorie gas used as a gas turbine fuel according to the present invention will be described with reference to the attached drawings. [0037] FIG. 1 is a piping diagram schematically illustrating a gas turbine system including a lowcalorie gas supply system 1 according to one embodiment of the present invention. The gas turbine system illustrated is a gas turbine power generation system. 20 As noted earlier, the "low-calorie gas", as referred to herein, is defined as a gas having a calorie of not more than about 12 MJ/Nm3. Most of such low-calorie gases fluctuate in their properties such as calorie. [0038] The lowcalorie gas supply system 1 includes lowcalorie gas supply piping 3 for supplying a gas turbine 2 with a byproduct gas (hereinafter is referred to as "lowcalorie gas") produced by a direct iron reduction equipment S as a fuel, exhaust gas supply piping 4 for supplying a combustion exhaust gas to the lowcalorie gas supply piping 3 to dilute the lowcalorie gas, and inert gas supply piping 5 for supplying an inert gas to the lowcalorie gas supply piping 3. The combustion exhaust gas is mixed into the low-calorie gas to dilute the lowcalorie gas for suppressing a rise in the calorie thereof for the reason that the exhaust gas does not necessarily contain combustible gases though containing large amounts of incombustible gases (N2, CO2 and the like). Since the temperature of the exhaust gas is high, the location for mixing the exhaust gas to the lowcalorie gas supply piping 3 can be selected more flexibly. [0039] The lowcalorie gas supply system 1 is provided with a control device 100 for controlling the operation of the system 1. As the present embodiment uses nitrogen gas (N2) as the inert gas, the inert gas supply piping is referred to as "N2 supply piping 5". The inert gas is not limited to N2, but may be CO2, helium (He) or the 21 like. [0040] In the present embodiment, the exhaust gas supply-piping 4 and the N2 supply piping 5 are connected to a second mixer 7, which in turn is connected to the low-calorie gas supply piping 3 through common piping for the exhaust gas and N2. The common piping is referred to as "diluting gas supply piping 8". The diluting gas supply piping 8 is connected to the lowcalorie gas supply system 1 by a first mixer 6. Though the exhaust gas supply piping 4 and the N2 supply piping 5 may be connected directly to the first mixer 6 without being joined together, it is preferable that the piping 4 and the piping 5 are previously connected to each other as shown reducing system investment cost. Also, the exhaust gas supply piping 4 and the N2 supply piping 5 may be connected directly to each other without use of the second mixer 7. However, the piping 4 and the piping 5 are preferably connected to each other through the second mixer 7 because it is preferable that the first mixer 6 is supplied with a mixed gas in which the exhaust gas and N2 for dilution are uniformly mixed together in advance for the byproduct gas and the diluting gas to be mixed together with high mixability in the first mixer 6. [0041] The aforementioned low-calorie gas supply piping 3 is provided at locations upstream of the first mixer 6 with a dust filter 9 for removing dust from the lowcalorie gas supplied from the direct iron reduction equipment S and a buffer tank 10 for 22 temporarily storing the low-calorie gas therein. The buffer tank 10 has a relatively large volume for mixing the lowcalorie gas that is being stored while fluctuating in its calorie. The effect of such mixing is described later. The lowcalorie gas supply piping 3 is also provided at locations downstream of the buffer tank 10 with a calorie measuring device 11 for measuring the calorie of the lowcalorie gas and a flowmeter 12 for measuring the flow rate of the lowcalorie gas. The location of the flowmeter 12 is not limited to the upstream side of the first mixer 6, but may be on the downstream side of the first mixer 6. For example, the flowmeter 12 may be located between a high-pressure compressor 17 and a combustor 19, which are described later. [0042] The location of the calorie measuring device 11 is not limited to the downstream side of the buffer tank 10, but may be on the upstream side of the buffer tank 10 for example. By measuring a calorie fluctuation of the gas in advance at a location upstream of the buffer tank 10, more reliable calorie control using the first mixer 6 can be realized. If the lowcalorie gas supply piping 3 is provided with two such calorie measuring devices on respective of the upstream and downstream sides of the buffer tank 10, the gas calorie fluctuation suppressing effect of the buffer tank 10 can be monitored by means of the two calorie measuring devices, whereby the total performance of the gas calorie suppressing system integrated with the gas mixing function of the buffer tank 10 can be grasped. Such a calorie measuring device may be fitted directly on 23 the buffer tank 10. In addition to the calorie measuring device 11 provided on the lowcalorie gas supply piping 3, another calorie measuring device may be fitted on the buffer tank 10. [0043] Examples of such a calorie measuring device 11 used here include a so-called calorimeter for directly measuring the calorie of a gas, a device for determining the content (density) of a combustible component, and like devices. The above-described calorie measuring device 11 may be fitted directly on the buffer tank 10. In addition to the calorie measuring device 11 provided on the lowcalorie gas supply piping 3, another calorie measuring device may be fitted on the buffer tank 10. In present cases where the sensing speed is important, it is preferable to use of a combustible gas density sensor. A density sensor meeting the kind of a major combustible component contained in the lowcalorie gas or the kind of a combustible component generating a major density fluctuation (for example, carbon monoxide among by-product gases produced by the direct iron reduction process) may be used for sensing the density of such a combustible component. These calorie measuring devices will generally be represented as "calorimeter" herein. [0044] Since the portion of the lowcalorie gas supply piping 3 which extends downstream of the first mixer 6 can supply the gas turbine 2 with the lowcalorie gas in a condition mixed with the exhaust gas and/or N2, this portion of the piping is referred to as mixed gas supply piping 13. The mixed gas supply piping 13 is 24 provided with a calorimeter 14 and an oxygen content meter 15 for measuring the oxygen density of the mixed gas. On the downstream side of the oxygen content meter 15 there are provided a low-pressure fuel gas compressor (hereinafter is. referred to as "low-pressure compressor") 16 and a high-pressure fuel gas compressor (hereinafter is referred to as "high-pressure compressor) 17 in this order, which form part of the gas turbine 2. A cooling device 18 is located intermediate the two compressors 16 and 17 for cooling the mixed gas as a fuel gas. Fuel piping 13a between the high-pressure compressor 17 and the combustor 19 of the gas turbine 2 is provided with a flow control valve 20 for controlling the gas turbine output. Reference character 21 denotes a filter provided on piping which supplies exhaust gas to the combustor 19. The gas turbine 2 is coupled to an electric power generator 22. [0045] While both of the two compressors 16 and 17 shown in FIG. 1 are of the type configured to be driven by the turbine 2, there is no limitation to this type. It is possible that the two compressors 16 and 17 are not coaxially coupled to the turbine 2, but configured to be driven by respective motors connected thereto. Also, the gas turbine power generation system may be provided with a waste heat recovery boiler 51 configured to utilize exhaust gas produced by the gas turbine 2, as shown. Steam generated by the waste heat recovery boiler 51 can be used as process steam. Though not shown, a steam turbine may be provided for electric power generation by using steam. The waste heat recovery boiler 51 is provided with a 25 stack 52 for dissipating the exhaust gas. The exhaust gas is not only diffused into the air but also utilized to dilute the lowcalorie gas serving as the fuel for the gas turbine 2 as will be described below. [0046] Description will be made of the exhaust gas supply system. The exhaust gas supply piping 4 is connected to exhaust gas introduction piping 53 configured to feed the exhaust gas from the waste heat recovery boiler 51 into the exhaust gas supply piping 4. The exhaust gas introduction piping 53 is provided with a cooling device 53 which can cool the exhaust gas to a temperature suitable for mixing with the lowcalorie gas and remove the moisture from the exhaust gas as much as possible through drain piping 55. In the present embodiment, the exhaust gas to be supplied as the diluting gas to the lowcalorie gas supply piping 3 gas is the exhaust gas produced by the gas turbine 2, to which the lowcalorie gas is supplied by the lowcalorie gas supply piping 3. However, the present invention is not limited to such a feature. In cases where, for example, a power generation system is provided near the lowcalorie gas supply system or in like cases, it is possible to utilize the exhaust gas produced by a boiler of a power generation system. In cases where plural gas turbine power generation systems each comprising the combination of lowcalorie gas supply system 1 and gas turbine 2 as shown in FIG. 1 are provided, the exhaust gas produced by the gas turbine of one system can be utilized as the diluting gas for another system. With such a feature, 26 even when an abnormal condition occurs in the exhaust gas introduction piping 53 or the cooling device 54 of one system thereby and can not supply the exhaust gas to the low-calorie gas supply system, it is possible to continue operation without stopping the gas turbine in trouble. The exhaust gas from the gas turbine or the boiler of another system is also supplied to the exhaust gas supply piping 4. [0047] A portion of the exhaust gas supply piping 4 that is joined to the exhaust gas introduction piping 53 is branched and provided with two fans 24 for suctioning in the exhaust gas from the exhaust gas introduction piping 53 and feeding the exhaust gas with a pressure through the exhaust gas supply piping 4, the fans 24 being positioned in parallel for convenience of maintenance. A check valve 25 is located downstream of each fan 24 for preventing the exhaust gas from flowing back toward the fan 24 inlet side. The branch portions of the exhaust gas supply piping 4 are unified again at a location downstream of the two check valves 25. The unified portion extending downstream of the check valves 25 is provided with a stop valve 26, flowmeter 27, flow control valve 28 and oxygen content meter 27 in this order and connected to the second mixer 7. Exhaust gas relief piping 29 is joined to the exhaust gas supply piping 4 at a location intermediate the aforementioned stop valve 26 and flowmeter 27 for releasing the exhaust gas into the atmosphere. The exhaust gas relief piping 29 is provided with a flow control valve 30. 27 [0048] The following description is directed to the N2 supply system. The N2 supply piping 5 has a stop valve 31, flowmeter 32 and flow control valve 33 in this order from the upstream side thereof and is connected to the second mixer 7. The diluting gas supply piping 8 extending from the second mixer 7 to the first mixer 6 is provided with a stop valve 34 and a check valve 35 in this order from the upstream side thereof. The check valve 35 functions to prevent the low-calorie gas from flowing back to the diluting gas supply piping 8. A portion of the diluting gas supply piping 34 that is located upstream of the stop valve 34 and a portion of the aforementioned exhaust gas relief piping 29 that is located downstream of the flow control valve 30 are interconnected through a communication piping 36 for releasing the mixed gas of the exhaust gas and N2 into the atmosphere through the exhaust gas relief piping 29. The communication piping 36 is provided with a flowmeter 37 and a flow control valve 38. In cases where the exhaust gas supply piping 4 and the N2 supply piping 5 extend separately to the first mixer 6, each of the piping 4 and the piping 5 is provided with a stop valve and a check valve and the communication piping 36 eliminated. Instead of the communication piping 36, N2 relief piping provided with a flow control valve is joined to the N2 supply piping 5 at a location intermediate the stop valve 31 and the flowmeter 32. [0049] The following description is directed to one example of 28 operation control over this system by the control device 100. Initially, the control device 100 causes the lowcalorie gas to be fed with a pressure toward the gas turbine 2 while monitoring the calorimeter 11 and flowmeter 12 of the lowcalorie gas supply piping 3. At that time, the fans 24 are already operating under the conditions that-' the stop valve 26 and the flow control valve 28 of the exhaust gas supply piping 4 are open and closed, respectively; the flow control valve 38 of the communication piping 36 closed; and the flow control valve 30 of the exhaust gas relief piping 29 open. In short, the exhaust gas is introduced by suction and released into the atmosphere from a non-illustrated stack through the exhaust gas relief piping 29. The flow control valve 33 of the N2 supply piping 5 is closed. Both of stop valves 31 and 34 are open. [0050] The control device 100 has stored therein an established allowable calorie range of the fuel gas to be used for the gas turbine 2. Specifically, the allowable calorie range is determined from a reference calorific value (1600 kcal/Nm3 for example) and a fluctuation range (±10% of the reference calorific value). When the calorific value of the lowcalorie gas is higher than the upper limit calorific value of the allowable fluctuation (for example, +10%, namely 1760 kcal/Nm3), the control device 100 adjusts the flow control valve 30 of the exhaust gas relief piping 29 toward the valve closing position so that the flow control valve 28 of the exhaust gas supply piping 4 becomes open. By so doing, the lowcalorie gas is mixed with the exhaust gas in order for the calorific value thereof to 29 be lowered to within the allowable range. In supplying the exhaust gas and N2 as described later, the control device 100 monitors the calorimeter 14 of the mixed gas supply piping 13 to determine whether or not a final calorific value is proper in addition to the monitoring of the calorimeter 11 and the flowmeter 12. In supplying the exhaust gas, the control device 100 monitors the oxygen density of the fuel gas by means of the oxygen content meter 15 of the low-calorie gas supply piping 3, as will be described later. [0051] If it is determined from the result of measurement by the calorimeter 14 of the mixed gas supply piping 13 that the calorific value of the fuel gas does not fall within the allowable range even under maximum supply of the exhaust gas, the control device 100 causes the flow control valve 33 to open while monitoring the flowmeter 32 of the N2 supply piping 5, thereby mixing the low-calorie gas with N2 to lower the calorific value of the fuel gas into the allowable range. At that time, the control device 100 adjusts the flow control valve 30 of the exhaust gas relief piping 29 toward the valve open position while monitoring the flowmeter 15 of the mixed gas supply piping 13 also, thereby releasing the exhaust gas in order to decrease the mixing quantity of the exhaust gas. The control operation thus described prevents the calorie of the low calorie gas from exceeding the allowable upper limit value. [0052] The following description is directed to the arrangement wherein the diluting gas supply piping 8 and the exhaust gas relief 30 piping 29 are interconnected through the communication piping 36 to allow N2 or the mixed gas of N2 and exhaust gas (diluting gas) to be released into the atmosphere. Usually, the flow rate of the diluting gas is controlled by means of the flow control valves 28 and 33. If there is a case where the calorific value measured quickly and enormously by the calorimeter 11 of the lowcalorie gas supply piping 3 drops steeply, the control relying upon the flow control valves 28 and 33 might encounter a problem with the responsiveness of the flow control valves 28 and 33 not to respond the drop. In such a case, the diluting gas is partially released into the atmosphere by means of the flow control valve 38 of the communication piping 36 to decrease the supply of the diluting gas quickly, thereby coping the steep drop in the calorific value. [0053] Since the exhaust gas produced by the gas turbine 2 naturally contains oxygen of about 10% to about 13%, mixing of the exhaust gas into the lowcalorie gas causes the oxygen content (oxygen density) of the mixed gas to increase. When a combustible gas contains oxygen in a predetermined proportion, the combustible gas theoretically falls within the flammability range at a predetermined temperature. The supply of the exhaust gas has to be limited before such a flammability condition is reached. If the calorific value of the lowcalorie gas still has to be lowered at that time, N2 is supplied to and mixed with the lowcalorie gas, while the mixing quantity of the exhaust gas into the lowcalorie gas is being lowered. 31 [0054] Different exhaust gases have different oxygen contents. Combustion gas produced by a gas turbine using the lowcalorie gas as a fuel having an oxygen content (volume percentage) of about 10 to about 13%, while exhaust gas produced by a boiler used in an electric power generation system contains no more than about 3 to about 6% of oxygen. For this reason, it is possible that for each of exhaust gases to be used, the flammability limits of a mixed gas comprising the exhaust gas and the low-calorie gas are decided in terms of the volume percentage of the lowcalorie gas or the exhaust gas and then a maximum allowable mixing rate of the exhaust gas is established based on the data thus obtained. [0055] On the other hand, air contains a constant volume percentage of oxygen (about 21%, which is invariable). For this reason, a convenient way of establishing a maximum allowable mixing rate of the exhaust gas includes: deciding the flammability limits of a mixed gas comprising air and the low-calorie gas in terms of the volume percentage of the lowcalorie gas or air,' establishing a maximum allowable mixing rate of air based on the data thus obtained; and calculating the maximum allowable mixing rate of the exhaust gas based on the data thus obtained and the oxygen content ratio between air and the exhaust gas. For example, the maximum allowable mixing rate of air is multiplied by the ratio of the oxygen content of air (21%) to the oxygen content of the exhaust gas (about 10 to about 13% in the case of the combustion gas produced by a gas 32 turbine). The details are described herein after. [0056] FIG. 2 plots the flammability limits of a mixed gas comprising the lowcalorie gas and air in relation to the volume percentage of the lowcalorie gas and the temperature. In this figure, the curve on the left-hand side shown by black circles indicates a minimum volume percentage of the lowcalorie gas (i.e., a maximum volume percentage of air) within the flammability range of the mixed gas. The curve on the right-hand side shown by black squares indicates a maximum volume percentage of the lowcalorie gas (i.e., a minimum volume percentage of air) within the flammability range of the mixed gas. The range defined between the two curves is the flammability range of the mixed gas. Since the calorific value of the lowcalorie gas fluctuates, the two curves fluctuate also. Therefore, the control device 100 has stored therein a maximum allowable volume percentage of air for mixing which is established based on such data in consideration of a safety factor. Such a maximum allowable volume percentage of air for mixing is, for example, an air volume percentage of 20% (i.e., a lowcalorie gas volume percentage of 80%) indicated in FIG. 2. The maximum allowable volume percentage of air (20%) is established so as to be lower than the minimum volume percentage of air indicated by the right-hand side curve shown by black squares. This value is only illustrative. [0057] The oxygen content ratio between air and the gas turbine 33 exhaust gas is 21/13. Here, the oxygen content of the exhaust gas is set to 13%, which has a margin. Since the maximum allowable volume percentage of air is 20%, the maximum allowable volume percentage of the exhaust gas is 20%x21/13 = 32%. The maximum allowable volume percentage of the boiler exhaust gas is 20%x21/6 ^ 70%. The control device 100 has stored therein the maximum allowable volume percentages of respective of these exhaust gases, each of which is a maximum allowable volume percentage derived from the flammability limits of the mixed gas. When the volume percentage of the gas turbine exhaust gas exceeds, for example, 25%, which is slightly less than the upper limit value (32%), the mixing quantity of the exhaust gas is decreased as described above with optional by supplying N2. This control operation is conducted based on results of measurement by the flowmeter 12 of the lowcalorie gas supply piping 3 and the flowmeter 27 of the exhaust gas supply piping 4. Further, the aforementioned oxygen content meters 15 and 56 constantly monitor both the oxygen content of the mixed gas and that of the exhaust gas in order to cope with unexpected fluctuation of the oxygen density. [0058] The following description is directed to the operation and effect of the buffer tank 10 shown in FIG. 1. As shown, the buffer tank 10 is formed with an inlet 10a connected to an upstream-side portion of the lowcalorie gas supply piping 3 and an outlet 10b connected to a downstream-side portion of the lowcalorie gas supply piping 3. Accordingly, all of the lowcalorie gas generated and 34 supplied is introduced into the buffer tank 10. While the buffer tank 10 shown in FIG. 1 has the inlet 10a and the outlet 10b which are formed adjacent to the lower edge of the tank peripheral wall, the inlet and the outlet may be formed, for example, in a central or upper portion of the tank peripheral wall, in a bottom portion of the tank, or in a like portion without particular limitation to the aforementioned locations of the inlet 10a and outlet 10b. [0059] The buffer tank 10 has a large volume. Usually, the buffer tank 10 has a volume of about 20,000 to about 200,000 m3 relative to the low-calorie gas supply piping 3 having a diameter of about 2 to about 3 m. The low-calorie gas being supplied to the buffer tank 10 with its calorie fluctuating moment by moment is subjected to mixing within the buffer tank 10. The expression "mixing of gas within the tank", as used in the present Description, means, so to speak, mixing with time-lags. Specifically, portions of the low-calorie gas that flow into the buffer tank 10 at a certain point of time provide such a distribution as to allow some portions to flow out through the outlet 10b in a relatively short time and cause some portions to reside within the buffer tank 10 for a relatively long time. Since fresh portions of the low-calorie gas newly flow into the buffer tank 10 through the inlet 10a consecutively, the portions of the lowcalorie gas that flowed into the buffer tank 10 in the past and those that have newly flowed thereinto are mixed together incessantly. This operation is referred to as "time-lag mixing". As a result of time-lag mixing, the calorie fluctuation of 35 the low-calorie gas flowing out of the buffer tank 10 through the outlet 10b is weakened and the fluctuation speed lowered. That is, the calorie fluctuation is largely lessened (or suppressed). If the calorie fluctuation is thus lessened in advance, it becomes very easy to perform the suppressing control of a rise in calorie by dilution with the exhaust gas or the like at a location of downstream. This phenomenon is described with reference to FIGs. 3 to 8 and 11 to 14. [0060] FIG. 3 is a graph showing a result of simulation of a calorie fluctuation that was lessened (suppressed) under the conditions that: the buffer tank 10 of FIG. 1 had a volume of 200,000 m3 was provided; and the lowcalorie gas was supplied at a flow rate of 500,000 Nm3/hr. The abscissa represents time (minute) and the ordinate represents the gas calorific value (kcal/ Nm3) as calories of the lowcalorie gas. The curve drawn by dotted line in this figure shows a calorie fluctuation of the lowcalorie gas reaching the buffer tank 10 (original fluctuation). This is an actually measured sample. The curve drawn by solid line shows a calorie fluctuation of the lowcalorie gas flowing out of the buffer tank 10 (suppressed fluctuation). As can be seen from the figure, the lowcalorie gas before flowing into the buffer tank 10 had a calorie fluctuating from a minimum of about 1,530 kcal/ Nm3 to a maximum of about 2,360 kcal/ Nm3. That is, the fluctuation range was about ±21% of a mean value of these two values (hereinafter is referred to as "mean value" simply). According to a theoretical calculation of the calorie fluctuation of the lowcalorie gas flowing out of the buffer tank 10, the lowcalorie 36 gas had a calorie fluctuating from a minimum of about 1,780 kcal/ Nm3 to a maximum of about 1,960 kcal/ Nm3. That is, the fluctuation range was reduced to about ±5% of the mean value. As shown, short cycle components and medium cycle components of the fluctuation cycle were also suppressed considerably. This effect tends to become more conspicuous as the volume of the buffer tank increases in relation to the flow rate of the lowcalorie gas supplied. In cases where the original fluctuation has a relatively small fluctuation range, the buffer tank having its volume is reduced from an economical point of view is effective. [0061] FIG. 4 shows a calorie fluctuation that was lessened under the conditions that: the volume of the buffer tank 10 was 100,000 m3, which was a half of the aforementioned volume; and the flow rate of the lowcalorie gas was 500,000 Nm3/hr, which remained at the same level as above. In this case, the calorie fluctuation was suppressed to within a range from 1,700 kcal/ Nm3 to 2,040 kcal/ Nm3 by the buffer tank 10. The fluctuation range was about ±9% of the mean value (l,970kcal/ Nm3). [0062] FIG. 5 shows a calorie fluctuation that was lessened in a system in which the lowcalorie gas was supplied at a flow rate of 200,000 Nm3/hr and the buffer tank 10 had a volume of 50,000 m3. In this case, the calorie fluctuation was suppressed to within a range from 1,740 kcal/ Nm3 to 2,010 kcal/ Nm3 by the buffer tank 10. The fluctuation range was about ±7.2% of the mean value 37 (l,875kcal/Nm3). [0063] Though not shown, with a system in which the low-calorie gas was supplied at a flow rate of 200,000 Nm3/hr as in the case described above and the buffer tank 10 had a volume of 25,000 m3, which was a half of the aforementioned volume, the resulting fluctuation range was about ±12% of the mean value (I,875kcal/Nm3). [0064] FIG. 6 shows a system in which the low-calorie gas is supplied at a flow rate of 200,000 Nm3/hr and two buffer tanks 10 each having a volume of 25,000 m3 are arranged in parallel. With this system, it is possible to operate the two buffer tanks 10 in such a manner that both of the two buffer tanks 10 are used in a normal operation and only one buffer tank 10 used at periodical inspection, malfunction or the like. [0065] By thus simply providing the buffer tank, the calorie fluctuation of the low-calorie gas can be largely suppressed without active controls. As a result, mixing of the exhaust gas or the inert gas with the lowcalorie gas, which is performed on the downstream side, can be controlled very easily. If the calorie fluctuation range of the fuel gas for the gas turbine 2 is ±10% of the reference calorific value (mean value) for example, it is sufficient that the buffer tank has such a volume as to equalize the mean value of calorie fluctuating on the downstream side of the buffer tank to the 38 reference calorific value established for the gas turbine 2 while the exhaust gas supplied at a constant rate on the downstream, side. Thus, the exhaust gas supply operation can be performed without considering different forms of calorie fluctuation of the lowcalorie gas. [0066] In extreme cases, if the mean value of fluctuating calorie of the lowcalorie gas having passed through the buffer tank 10 is substantially equal to the reference calorific value established for the gas turbine 2, there is no need to provide the exhaust gas supply system or the inert gas supply system. Even with these two systems provided, the gas turbine system can simply be operated with the stop valve 34 of the diluting gas supply piping 8 of FIG. 1 closed. It is needless to say that if the calorie fluctuation of the lowcalorie gas produced is inherently small, provisions of only the exhaust gas supply system and the inert gas supply system can accommodate such a calorie fluctuation without providing the buffer tank. [0067] FIG. 7 shows another buffer tank 42. This buffer tank 42 is of the type sometimes used in a conventional gas turbine system and is included in a device 40 configured to monitor the gas balance. The gas-balance monitor 40 serves to balance the volume of the lowcalorie gas fed from the upstream side with the volume of the gas required by the gas turbine. When there are fluctuations in the gas supply or the load on the gas turbine, it is necessary to 39 balance the gas supply with the gas consumption. Specifically, when the gas supply becomes excessive unexpectedly, the excess gas may be released into the atmosphere. When the gas supply is short, the load on the gas turbine may be lowered or gas turbine may be stopped. [0068] The gas-balance monitor 40 includes a tank 42 connected to the lowcalorie gas supply piping 3 through a communication piping 41, a lid member 43 which is capable of airtightly closing the open upper end of the tank 42 and is vertically slidably fitted in the tank 42, and a balancing weight 44 provided on the lid member 43. The lid member 43 slides vertically within the tank 42 to balance the sum total of its own weight, the weight of the balancing weight 44 and the depressing force of the atmospheric pressure with the raising force of the internal pressure of the tank 42. Accordingly, the lid member 43 slides vertically as the balance changes between the supply of the lowcalorie gas and the gas consumption changes. An action to release the gas into the atmosphere or lower the gas turbine load or other measures are taken with the vertical slide of the lid member 43 being monitored. [0069] In the present invention, the gas-balance monitor 40 is utilized to suppress calorie fluctuation. The tank 42 is connected to inlet piping 45 communicating with the lowcalorie gas supply piping 3 separately from the aforementioned communication piping 41. The inlet piping 45 is provided with a fan 39 for feeding the 40 low-calorie gas into the tank 42. As the inlet piping 45 is joined to the lowcalorie gas supply piping 3 at a location upstream of the joint between the communication piping 41 and the low-calorie gas supply piping 3, the fan 39 can be omitted by designing the pipings in consideration of pressure loss. This is also applied to the upstream-side inlet piping 45 shown in FIG. 8 or 12. Apart of the lowcalorie gas supplied is introduced into the tank 42 through the inlet piping 45 and subjected to mixing within the tank 42, and then an equal amount of the low-calorie gas returns to the lowcalorie gas supply piping 3 through the aforementioned communication piping 41. In this case, the communication piping 41 can otherwise be called "outlet piping". Since the buffer tank 42 is connected to the inlet piping 45 and the outlet piping 41, which form bypass piping connected to the lowcalorie gas supply piping 3, the buffer tank 42 is positioned relative to the lowcalorie gas supply piping 3 in, so to speak, a parallel arrangement. [0070] FIG. 8 shows another gas-balance monitor 46 which can be utilized as calorie fluctuation suppressing means. This gas-balance monitor 46 has a more economical construction having an airtight tank 46a in communication with the lowcalorie gas supply piping 3 through the communication piping 41. The tank 46a is provided with a pressure measuring device 47 for constantly monitoring the internal pressure of the tank 46a. When the measured pressure reaches the upper limit zone, the control device 100 provides an instruction to increase the gas consumption of the 41 system thereby to make a balance between demand and supply of the gas. Other features of the monitor 46 are similar with the features of the aforementioned monitor 40 (FIG. 7), and the monitor 46 can be used satisfactorily as the calorie fluctuation suppressing means. [0071] FIG. 9 shows a calorie fluctuation that was lessened in such a system as the low-calorie gas was supplied at a flow rate of 500,000 Nm3/hr, the tank 42 (46a) of FIG. 7 or 8 had a volume of 200,000 m3 and a part of the gas was supplied to the tank 42 at a flow rate of 200,000 Nm3/hr. The curve drawn by dotted line in this figure shows a calorie fluctuation of the lowcalorie gas supplied from the direct iron reduction equipment S (original calorie fluctuation). This is the aforementioned actually measured sample. The curve drawn by dashed double-dotted line shows a result of simulation of a calorie fluctuation of the low-calorie gas passing through the above-described communication piping 41 from the tank 42 (transitional calorie fluctuation). The curve drawn by solid line shows a calorie fluctuation of the lowcalorie gas passing through a portion of the lowcalorie gas supply piping 3 that extends downstream of the communication piping 41 to the first mixer 6 (suppressed calorie fluctuation). As has been already noted, the lowcalorie gas before flowing into the tank 42 (46a) had a calorie fluctuation range of about ±21% of the mean value (1,945 kcal/ Nm3). In contrast, the calorie fluctuation of a portion of the lowcalorie gas having passed through the communication piping 41 and joined with 42 the lowcalorie gas passing through the low-calorie gas supply piping 3 ranged from 1,690 kcal/ Nm3 to 2,100 kcal/ Nm3. That is, the calorie fluctuation range was reduced to about ±11% of the mean value (1,895 kcal/ Nm3). This value is only illustrative. [0072] Thus, it is possible to suppress gas calorie fluctuation by utilizing an existing system having the tank 42 (46a). Also, dilution of the lowcalorie gas with the exhaust gas at a location downstream of the tank 42 can be achieved easily. While the inlet piping 45 shown in FIG. 7 or 8 for introducing the low-calorie gas into the tank 42 (46a) is joined to the lowcalorie gas supply piping 3 at a location upstream of the joint between the outlet piping 41 and the lowcalorie gas supply piping 3, there is no particular limitation to this arrangement, but the inlet piping 45 may be joined to the lowcalorie gas supply piping 3 at a location downstream of the outlet piping 41. Further, each of the outlet piping 41 and the inlet piping 45 may comprise plural pipings. [0073] FIG. 10 shows other calorie fluctuation suppressing means. The means comprises return piping 48 is connected to the lowcalorie gas supply piping 3 for returning a part of the lowcalorie gas toward the upstream side of the lowcalorie gas supply piping 3. The return piping 48 is provided with a fan 49 for feeding the lowcalorie gas with a pressure toward the upstream side. The return piping 48 branches into plural branch pipings 48a from a single suction part for returning the lowcalorie gas to the low 43 calorie gas supply piping 3. However, the return piping 48 may comprise a single return piping. Otherwise, the lowcalorie gas supply piping 3 may be provided at their different portions thereof with individual return pipings which are independent from each other. Such means causes a part of the lowcalorie gas that has returned to the upstream side of the lowcalorie gas supply piping 3 to mix with a newly coming part of the lowcalorie gas, thereby lessening the calorie fluctuation of the lowcalorie gas. In order to enhance this function, the return piping 48 is simply lengthened to increase the volume ratio of returned gas to supplied gas. [0074] Though not shown, the buffer tank 42 of the gas-balance monitor 40 shown in FIG. 7 may be replaced with a buffer tank of the invariable internal volume type shown in FIG. 1. That is, the buffer tank 42 shown in FIG. 7, which is connected to the inlet piping 45 and communication piping 41 forming the bypass piping of the lowcalorie gas supply piping 3, may be connected directly to the lowcalorie gas supply piping 3. Specifically, the buffer tank 42 is formed with inlet and outlet which may be connected directly to the upstream side and the downstream side, respectively, of the lowcalorie gas supply piping 3. [0075] FIG. 11 shows a form of piping which is substantially the same as the above-described form of piping. The buffer tank 42 shown in FIG. 11 is the same as the buffer tank 42 of the gas-balance monitor 40 shown in FIG. 7. The difference resides in the 44 form of piping connecting the buffer tank 42 to the lowcalorie gas supply piping 3. The form of piping shown in FIG. 11 is obtained by eliminating the portion of the lowcalorie gas supply piping 3 that extends from the joint with the inlet piping 45 to the joint with the communication piping 41 as well as the fan 39 provided on the inlet piping 45. That is, an upstream-side portion of the lowcalorie gas supply piping 3 is connected to an inlet 42a of the buffer tank 42, while a downstream-side portion of the lowcalorie gas supply piping 3 connected to an outlet 42b of the buffer tank 42. Stated otherwise, the buffer tank 10 shown in FIG. 1 is replaced with the tank 42 of the gas-balance monitor 40. Such a form of piping can be easily modified from the piping associated with the existing gas-balance monitor 40 in utilizing the gas-balance monitor 40 as the gas calorie fluctuation suppressing device. [0076] Further, the tank 42 may be provided therein with a stirring device 57, such as a fan, for stirring the gas, as shown in FIG. 11. The stirring device 51 facilitates mixing of the gas within the tank thereby to realize more effective suppression of calorie fluctuation. It is preferable from the viewpoint of effective mixing of gas that the stirring device 51 is mounted adjacent to the outlet 42b in such a position as to allow the gas to flow inwardly of the tank from a location adjacent to the outlet 42b. The stirring device 51 can be provided not only in the tank 42 shown in FIG. 11 but also in any one of the tanks 10, 42 and 46a shown in other figures as well as in any other tank which is capable of exhibiting the calorie 45 fluctuation suppressing effect. A driving device for rotating the stirring device 51, such as an electric motor 57a, is preferably located exteriorly of the tank. [0077] FIG. 12 also shows the buffer tank 42 positioned in parallel connection with the low-calorie gas supply piping 3, like the tank shown in FIG. 7. As shown, the outlet piping 41 interconnects the outlet 42a of the tank 42 and the low-calorie gas supply piping 3, while the inlet piping 45 interconnects the inlet 42a of the tank 42 and a portion of the low-calorie gas supply piping 3 that is located upstream of the joint with the outlet piping 41. For this reason, the inlet piping 45 is referred to as "upstream-side inlet piping 45". The tank 42 is formed with another inlet 50a connected to inlet piping 50 which is joined to a portion of the low-calorie gas supply piping 3 at a location downstream of the joint with the outlet piping 41. The inlet piping 50 is referred to as "downstream-side inlet piping 50". The inlet piping 45 and the inlet piping 50 are provided with respective fans 39 for feeding the low-calorie gas into the tank 42. As shown, the locations (inlets 42a and 50a) at which the upstream-side inlet piping 45 and the downstream-side inlet piping 50 are joined to the tank 42 are close to each other. [0078] With this arrangement, a part of the low-calorie gas is fed to the tank 42 with a pressure from the upstream side of the low-calorie gas supply piping 3 through the upstream-side inlet piping 45, while at the same time another part of the lowcalorie gas 46 fed to the tank 42 with a pressure from the downstream side of the low-calorie gas supply piping 3 through the downstream-side inlet piping 50. These parts of the lowcalorie gas are mixed together in the tank 42 and then flow out from the outlet 42b into the outlet piping 41. That is, the lowcalorie gas having a suppressed calorie fluctuation is partially circulated to realize time'lag mixing over a longer span of time within the tank. As the length of the downstream-side piping 50 increases, the residence time of the gas to be mixed becomes longer, which results in more preferable mixing. While the downstream-side inlet piping 50 interconnects the inlet 50a of the tank 42 and the downstream side of the lowcalorie gas supply piping 3, the downstream-side inlet piping 50 may interconnect the downstream side of the lowcalorie gas supply piping 3 and a portion of the lowcalorie gas supply piping 3 that is located upstream of the joint with the upstream-side inlet piping 45. [0079] FIG. 13 also shows the buffer tank 42 positioned in parallel connection with the lowcalorie gas supply piping 3. As shown, the tank 42 and the lowcalorie gas supply piping 3 are interconnected through the aforementioned communication piping 41 serving as the outlet piping as well as through the downstream-side inlet piping 50. The downstream-side inlet piping 50 is provided with the fan 39 for feeding the lowcalorie gas into the tank 42. [0080] With this arrangement, even though the downstream-side 47 inlet piping 50 is connected to a portion of the low-calorie gas supply piping 3 that is located downstream of the joint with the outlet piping 41, the lowcalorie gas is fed into the tank 42 through the downstream-side inlet piping 50 by means of the fan 39, subjected to mixing and then flows out from the outlet 42 into the outlet piping 41. That is, the lowcalorie gas having a suppressed calorie fluctuation is partially circulated to realize effective mixing. As the length of the downstream-side piping 50 increases, mixing of gas within the tank for a longer span of time can be realized. [0081] FIG. 14 has two inlets 42a and 49a. One inlet 42a is connected to upstream-side lowcalorie gas supply piping 3, while the outlet 42b connected to downstream-side lowcalorie gas supply piping 3. Further, the other inlet 49a is connected to return piping 49, which in turn is connected to the downstream-side lowcalorie gas supply piping 3. The two inlets 42a and 49a are located close to each other. The return piping 53 is provided with the fan 39 for feeding the lowcalorie gas into the tank 42. [0082] With this arrangement, the lowcalorie gas having a calorie fluctuation having been suppressed in the tank 42 is partially returned to the tank 42 and mixed again therein, which results in more preferable mixing. As the length of the return piping 49 increases, the residence time of the gas to be mixed becomes longer. While the return piping 49 interconnects the inlet 49a of the tank 42 and the downstream side of the lowcalorie gas 48 supply piping 3, the return piping 53 may interconnect the downstream side of the lowcalorie gas supply piping 3 and a portion of the low-calorie gas supply piping 3 that is located upstream of the tank. [0083] While any one of the foregoing embodiments uses the byproduct gas produced from the direct iron reduction process as an example of a low-calorie gas to be used, there is no limitation to this gas. Examples of such low-calorie gases include blast furnace gas (BFG), converter vessel gas (LDG), coal mine gas (CMG) contained in coal strata, byproduct gas produced from the smelting reduction ironmaking process, tail gas produced from the GTL (Gas-to-Liquid) process, by-product gas produced from an oil refining process for refining oil from oil sands, gas produced during waste incineration using plasma, methane gas (Landfill gas) produced during fermentation and decomposition of waste including garbage in landfills, and by-product gases produced through chemical reaction of other similar materials . It is needless to say that such low-calorie gases can be used either alone or as mixed gases having calorific values of not more than about 12 MJ/Nm3 resulting from mixing of different types of gases, such as the mixed gas of BFG and LDG. Industrial Applicability [0084] According to the present invention, a lowcalorie gas 49 having its calorie fluctuating moment by moment is diluted with an exhaust gas having a low oxygen density, which is available in a large amount and easy to collect, whereby an abnormal rise in combustion temperature can be suppressed to allow stabilized combustion to proceed continuously. This effect can be obtained at low system investment cost and running cost. 50 CLAIMS [l] A lowcalorie gas supply system comprising-' a lowcalorie gas supply passage for supplying a gas turbine with a lowcalorie gas as a fuel gas! an exhaust gas supply passage connected to said lowcalorie gas supply passage for supplying an exhaust gas produced by a combustion system to said lowcalorie gas supply passage; a calorie measuring device provided on said lowcalorie gas supply passage for measuring a calorie of the lowcalorie gas; and a control device capable of controlling an exhaust gas supply operation of said exhaust gas supply passage based on a result of measuring by said calorie measuring device. [2] The lowcalorie gas supply system according to claim 1, wherein said control device has stored therein an established maximum allowable calorific value of the fuel gas for said gas turbine and is configured to cause the exhaust gas to be supplied from said exhaust gas supply passage when the calorific value of the lowcalorie gas is higher than the established maximum allowable calorific value. [3] The lowcalorie gas supply system according to claim 1, wherein^ said lowcalorie gas supply passage is provided with an 51 oxygen density sensing device at a portion thereof located downstream of a joint with said exhaust gas supply passage; and said control device is configured to control the exhaust gas supply operation of said exhaust gas supply passage based on a result of sensing by said oxygen density sensing device. [4] The lowcalorie gas supply system according to claim 3, wherein said control device is configured to control an exhaust gas supply from said exhaust gas supply passage by referencing a allowable mixed volume percentage of the exhaust gas which is established based on flammability limit information on the low-calorie gas obtained from a mixing ratio between the lowcalorie gas and the exhaust gas. [5] The lowcalorie gas supply system according to claim 3, wherein said control device is configured to control an exhaust gas supply operation of said exhaust gas supply passage by referencing a allowable mixed volume percentage of the exhaust gas which is calculated based on an oxygen content ratio from a allowable mixed volume percentage of air established based on flammability limit information on the lowcalorie gas obtained from a mixing ratio between the lowcalorie gas and air. [6] The lowcalorie gas supply system according to any one of claims 3 to 5, wherein: said lowcalorie gas supply passage is connected to an 52 inert gas supply passage configured to supply an inert gas to said lowcalorie gas supply passage! and said control device is configured to control an inert gas supply operation of said inert gas supply passage based on a result of sensing by said oxygen density sensing device. [7] The lowcalorie gas supply system according to claim 1, wherein^ said lowcalorie gas supply passage is connected to an inert gas supply passage configured to supply an inert gas to said lowcalorie gas supply passage; and said control device is configured to control an inert gas supply operation of said inert gas supply passage based on a result of measuring by said calorie measuring device under supply of the exhaust gas to said lowcalorie gas supply passage by said exhaust gas supply passage. [8] The lowcalorie gas supply system according to claim 1, wherein^ said lowcalorie gas supply passage is provided with a first tank for temporarily storing the lowcalorie gas therein; and said first tank having an inlet connected to an upstream side of said lowcalorie gas supply passage and an outlet connected to a downstream side of said lowcalorie gas supply passage. [9] The lowcalorie gas supply system according to claim 1, 53 wherein^ said lowcalorie gas supply passage is provided with a second tank for temporarily storing the lowcalorie gas therein,' said lowcalorie gas supply passage and said second tank are interconnected through a gas inlet passage for feeding the lowcalorie gas from said lowcalorie gas supply passage into said second tank as well as a gas outlet passage for returning the lowcalorie gas from said second tank to said lowcalorie gas supply passage,' and said inlet passage is provided with a first gas feeding device for feeding the lowcalorie gas with a pressure toward said second tank. [10] The lowcalorie gas supply system according to claim 1, wherein said lowcalorie gas supply passage is provided with a return passage for returning a part of the lowcalorie gas to an upstream side of said lowcalorie gas supply passage, said return passage being provided with a second gas feeding device for feeding the lowcalorie gas with a pressure toward the upstream side of said lowcalorie gas supply passage. [ll] The lowcalorie gas supply system according to claim 1, wherein said exhaust gas supply passage is provided with an exhaust gas supply shut-off device capable of closing and opening said exhaust gas supply passage and an exhaust gas relief device located upstream of said exhaust gas supply shut-off device. 54 [12] The lowcalorie gas supply system according to claim 6, wherein said inert gas supply passage is provided with an inert gas supply shut-off device capable of closing and opening said inert gas supply passage and an inert gas relief device located upstream of said inert gas supply shut-off device. [13] The lowcalorie gas supply system according to claim 1, wherein said combustion system is said turbine. [14] The lowcalorie gas supply system according to claim 1, wherein said combustion system is a boiler of a power generation system. [15] A gas turbine system comprising'- a gas turbine,' and a lowcalorie gas supply system for supplying said gas turbine with a lowcalorie gas as a fuel gas, said lowcalorie gas supply system being a lowcalorie gas supply system as recited in any one of claims 1 to 14. [16] The gas turbine system according to claim 15, wherein- plural gas turbines are provided; each of said gas turbines is provided with a lowcalorie gas supply system; and said combustion system associated with said lowcalorie 55 supply system is any one or more of said plural gas turbines except the gas turbine to which said lowcalorie supply system supplies the fuel gas. [17] A method of suppressing a rise in a calorie of a low- calorie gas used as a gas turbine fuel, comprising the steps of: measuring the calorie of the lowcalorie gas serving as a fuel gas to be supplied to a gas turbine! and mixing an exhaust gas collected from a combustion system into the lowcalorie gas for dilution when a result of the measurement is higher than an established allowable calorific value. [18] The method according to claim 17, wherein said exhaust gas mixing step comprises the steps of: measuring an oxygen density of a mixed gas comprising the lowcalorie gas and the exhaust gas; and controlling an exhaust gas mixing quantity so that a result of the measurement does not exceed an established allowable value decided from flammability limit information on the lowcalorie gas. [19] The method according to claim 17, further comprising the step of mixing an inert gas into the lowcalorie gas when the calorie thus measured is decided not to fall below the established allowable calorific value under maximum exhaust gas supply. 56 [20] The method according to claim 18, further comprising the step of supplying an inert gas to the lowcalorie gas and simultaneously decreasing an exhaust gas mixing quantity when it is decided that decreasing the exhaust gas mixing quantity causes the calorie of the mixed gas to exceed the established allowable calorific value, while increasing the exhaust gas mixing quantity causes an exhaust gas content of the mixed gas to exceed an established allowable exhaust gas content. [21] The lowcalorie gas supply system according to claim 8, wherein: said lowcalorie gas supply passage is provided with a return passage interconnecting a portion of said lowcalorie gas supply passage that is located upstream of said tank and a portion of said lowcalorie gas supply passage that is located downstream of said tank; and said return passage is provided with a gas feeding device for feeding the fuel gas with a pressure toward the portion of said lowcalorie gas supply passage that is located upstream of said tank. [22] The low-calorie gas supply system according to claim 9, wherein said lowcalorie gas supply passage is provided with a return passage interconnecting a portion of said lowcalorie gas supply passage that is located downstream of a joint with said outlet passage and a portion of said lowcalorie gas supply passage that is 57 located upstream of a joint with said inlet passage; and said return passage is provided with a gas feeding device for feeding the fuel gas with a pressure toward an upstream side of said lowcalorie gas supply passage. [23] The low-calorie gas supply system according to claim 8 or 9, wherein said tank is provided therein with a stirring device for stirring the gas. A low-calorie gas supply system is provided which is capable of supplying a low-calorie gas as a stabilized fuel for gas turbines by suppressing a steep rise in the calorie of the low-calorie gas. The low-calorie gas supply system includes: low-calorie gas supply piping (3) for supplying a gas turbine (2)with the low-calorie gas; exhaust gas supply piping (4) for supplying an exhaust gas to the low-calories gas supply piping (3); a first mixer (6) located at a joint between the low-calories gas supply piping (3) and the exhaust gas supply piping (4); a calorimeter (11) provided to the low-calorie gas supply piping (3) for measuring the calorie of the gas; an oxygen content meter(15) provided to the low-calorie gas supply piping (3) at a location downstream of the first mixer (6); and a control device (100) configured to exert a control based on a result of measurement by the oxygen content meter (15) such that when a value measured by the calorimeter (11) is higher than a reference value, the exhaust gas is supplied from the exhaust gas supply gas supply piping (4) so that the exhaust gas mixing quantity does not fall within the flammability limits of the low-calories gas. |
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| Patent Number | 271511 | |||||||||
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| Indian Patent Application Number | 615/KOLNP/2007 | |||||||||
| PG Journal Number | 09/2016 | |||||||||
| Publication Date | 26-Feb-2016 | |||||||||
| Grant Date | 24-Feb-2016 | |||||||||
| Date of Filing | 20-Feb-2007 | |||||||||
| Name of Patentee | KAWASAKI JUKOGYO KABUSHIKI KAISHA. | |||||||||
| Applicant Address | 1-1,HIGASHIKAWASAKI-CHO 3-CHOME, CHUO-KU, KOBE-SHI, HYOGO 650-8670. | |||||||||
Inventors:
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| PCT International Classification Number | F02C 9/40 | |||||||||
| PCT International Application Number | PCT/JP2005/004101 | |||||||||
| PCT International Filing date | 2005-03-09 | |||||||||
PCT Conventions:
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