| Title of Invention | APPARATUS FOR CONTROLLING CALORIFIC VALUE OF GAS |
|---|---|
| Abstract | A gas calorific value control method is provided which is capable of suppressing a calorific value fluctuation of a fuel gas and supplying the fuel gas as a stabilized fuel gas. The gas calorific value control method includes the steps of suppressing a calorific value fluctuation of a fuel gas to be supplied to a combustion system (2) by subjecting the fuel gas to time-lag mixing within a tank (5) formed with a gas inlet (6) and a gas outlet (7) separately; measuring a calorific value fluctuation of the fuel gas having been thus suppressed; and decreasing or increasing a calorie of the fuel gas at a location downstream of the tank (5) so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in the combustion system (2). |
| Full Text | 1 DESCRIPTION GAS CALORIFIC VALUE CONTROL METHOD AND GAS CALORIFIC VALUE CONTROL DEVICE Technical Field [0001] The present invention relates to a gas calorific value control method and a gas calorific value control device. More specifically, the invention relates to: a gas calorific value control method capable of suppressing a calorific value fluctuation of a fuel gas for a combustion system, such as a lowcalorie gas, which is subject to such a calorific value fluctuation; and a gas calorific value control device employing this method. Background Art [0002] In the ironmaking field, for example, production of pig iron by the blast furnace process allows blast furnace gas (hereinafter will be referred to as "BFG") to be evolved as a by- product gas from the blast furnace. The total calorific value of BFG reaches about a half of the calorific value of coke used and, hence, BFG is utilized for various purposes in ironworks in order to lower the cost price of pig iron. The composition of BFG typically comprises 10 to 18 vol% (hereinafter will be expressed in "%" simply) of carbon dioxide (CO2), 22 to 30% of carbon monoxide (CO), 2 52 to 60% of nitrogen (N2), 0.5 to 4% of hydrogen (H2), and 0.5 to 3% of methane (CH4). [0003] Since BFG contains 2 to 10 g/Nm3 of flue dust in addition to the aforementioned components, the flue dust content is decreased to about 0.01 g/Nm3 by means of a dust remover for BFG to be used as a fuel gas having a calorific value of about 800 kcal/Nm3 for a hot-blast oven, coke oven, heating furnace, boiler or the like. [0004] As technical improvements made in recent years have made it possible to combust lowcalorie gases in gas turbines, cases where electric power generation is conducted using BFG as a gas turbine fuel are increasing in number. The "low-calorie gas", as used herein, is defined as a gas having a calorific value of not more than about 12 MJ/Nm3. Such lowcalorie gases include not only blast furnace gas (BFG) but also many other gases, such as converter gas (LDG), and mixed gases thereof. [0005] On the other hand, new ironmaking processes (for example, direct iron reduction processes including FINEX process and COREX process) other than the blast furnace process have been being developed in recent years. For this reason, a combustion method that is applicable to effective utilization of by-product gases produced by such new processes is expected to develop. With any ironmaking process, the by-product gas produced thereby is a low 3 calorie gas having properties (including gas composition and calorie) that depend upon the system used and the contents of operation. Even with the same system, the properties of the byproduct gas fluctuate moment by moment in accordance with the properties of raw materials and the reaction process and hence are inconstant. [0006] Regarding the calorie of the low-calorie gas which is the most important property in using the low-calorie gas as a fuel gas for gas turbines, each gas turbine has upper and lower limit values of an allowable calorific value fluctuation range which is characteristic of the gas turbine. If the calorie of the lovrcalorie gas exceeds the upper limit (for example, an average calorific value plus about 10%), that is, if the calorie of the low-calorie gas rises steeply, the combustion temperature within the combustor of the gas turbine rises to an abnormally high temperature steeply in some cases. This might result in such harmful effects that the burner portion and the stationary blade and moving blade of the turbine are damaged due to such a high temperature and hence suffer from shortened lifetimes. In such a case, economical continuous operation of the gas turbine system becomes difficult. If the calorie of the lowcalorie gas falls below the lower limit (for example, an average calorific value minus about 10%), it is possible that the gas turbine produces an unstable output or a flame-off trip is caused to occur. The "fuel gas", as used in the present Description and Claims, is meant to include the lowcalorie gas. [0007] 4 Such a calorie fluctuation, or increasing and decreasing in calorie, means fluctuation of a physical property related to the calorific value of the fuel gas. Specifically, such physical properties include various physical properties such as calorific value per unit volume (Kcal/Nm3), calorific value per unit weight (MJ/kg), and Wobbe Index (MJ/m3), In the present Description and Claims, the "calorie" and "calorie fluctuation" will be referred to as "calorific value" and "calorific value fluctuation", respectively, as the case may be. [0008] FIG. 11 is a piping diagram schematically illustrating a conventional gas turbine electric power generation system. The conventional art illustrated is configured to increase or decrease the calorific value of a fuel gas fluctuating in calorific value. As shown, in supplying a gas turbine 101 (or a combustion system such as a thermal power boiler) with BFG produced by a fuel gas producing system (blast furnace for example) 100 as a fuel gas, a mixer 102 mixes a calorie decreasing gas or a calorie increasing gas into the fuel gas in order to maintain the calorific value of the fuel gas at an intended value (in terms of average value, fluctuation range or fluctuation speed, which form design preconditions). According to the gas mixing method employed here, calorific value measuring devices 104 and 105 are provided to the upstream side and the downstream side, respectively, of a fuel gas supply passage 103, and a feedforward control using detection signals sent by the calorimeter 104 and a feedback control using detection signals sent by the 5 calorimeter 105 are performed. These detection signals are sent to a controller 106 configured to compare such a detection signal with a predetermined established value 107 which has been previously established to meet the gas turbine 101 used. Subsequently, the controller 106 sends a predetermined control signal to a calorie decreasing flow control valve 108 or a calorie increasing flow control valve 113 via a distributor 115 in order to supply the mixer 102 with the calorie decreasing gas from a calorie decreasing gas supply device 109 through supply piping 110 or the calorie increasing gas from a calorie increasing gas supply device 114 through supply piping 110. Reference characters 111 and 112 denote a gas compressor and an electric generator, respectively. [0009] FIG. 12 is a graph showing one example of a calorific value fluctuation of the fuel gas at measurement points in the gas turbine electric power generation system shown in PIG. 11. The horizontal axis represents time (second) and the vertical axis represents the calorific value (MJ/kg) of the fuel gas. In this figure, the dashed double-dotted line represents a calorific value fluctuation of the fuel gas passing through the lowcalorie gas supply passage 103 and the solid line represents a calorific value fluctuation of the fuel gas at the outlet of the mixer 102 as a result of simulation made under a conventional control. [0010] As can be seen from the figure, the calorific value of the fuel gas supplied from the fuel gas producing system 100 fluctuates 6 largely and irregularly with time as represented by the dashed double-dotted line. In this example, even when the feedforward control and the feedback control are performed over this original fluctuation, the control system is exposed to the calorific value fluctuation of the fuel gas including too large short- and medium- cycle components SO that the control system becomes unstable, which makes it difficult to establish a parameter for the controller. Under a certain condition, this results in responses close to oscillation as shown by the solid line. This figure shows a case where the feedforward control and feedback control over the low- calorie gas fluctuating in calorific value caused oscillation. Such a gas having a calorific value fluctuation like oscillation cannot be used as a fuel gas for gas turbines (combustion systems). [0011] Conventional techniques of this type include, for example, a fuel gas calorific value control device described in patent document 1. If the low-calorie gas is supplied while fluctuating in average calorific value, calorific value fluctuation range and calorific value fluctuation speed, this control device has to select and mix such a proper kind of and amount of gas as to equalize the calorific value of the low-calorie gas to an average value at the moment at which the gas having a fluctuating calorie reaches a mixer in order to suppress such fluctuations. Specifically, when the calorific value of the lowcalorie gas is higher than the average value, the calorie decreasing gas has to be mixed in such an amount as to adjust the calorific value of the lowcalorie gas to the average 7 value. When the calorific value of the lowcalorie gas is lower than the average value, the calorie increasing gas has to be mixed in such an amount as to adjust the calorific value of the lowcalorie gas to the average value. [0012] However, since only a mixed gas control valve serves as an operation device, a possible ill-timed gas mixing operation causes the calorie decreasing gas or the calorie increasing gas to be mixed in an excessive or insufficient amount, thus making it difficult to suppress the calorific value fluctuation accurately. As a result, the calorific value of the lowcalorie gas cannot be suppressed at a required level for the combustion system. For this reason, if the resulting calorific value exceeds the allowable limit for the combustion system, it is required that the combustion system have to stop operating urgently to release the lowcalorie gas into the atmosphere in order to avoid damage to the combustion system. To the contrary, if the resulting calorific value falls below the allowable limit, a flame-out trip may occur in the combustion system. If the calorific value fluctuation speed is increased, it becomes more difficult to perform the gas mixing operation with proper timing, which makes it difficult to perform a reliable control by means of only the mixer and the mixed gas control valve. [0013] If the calorific value fluctuation is of short cycles and has a wide fluctuation range, the mixed gas control valve has to repeat a large stroke action, which might damage the valve and shorten its 8 lifetime. [0014] Further, when mixing of gas is performed by such a large stroke action of the control valve caused by too wide a calorific value fluctuation range, a fluctuation occurs in the pressure at the inlet of a gas Compressor to from a serious disturbance on the fuel supply system associated with the gas turbine. This leads to an unstable operation of the gas turbine. [0015] In mixing different gases which have different specific gravities, such as nitrogen gas and coke oven gas, into a fuel gas having a largely fluctuating calorie, mixing of large amounts of gases is difficult. Further, it is difficult for the mixed gas to be sufficiently suppressed in a short time immediately after the mixing of the gases before supply of the mixed gas to the combustion system because the range of change in mixing rate is large. As a result, nitrogen gas, coke oven gas and a like gas in a non-uniformly mixed condition are present in the fuel gas, thus causing non-uniform combustion to occur within the combustion chamber of the combustion system. Accordingly, it is difficult for the combustion system to operate stably. For this reason, it is difficult to use as a fuel gas such a gas that is subject to a large calorie fluctuation (calorific value fluctuation), like a lowcalorie gas. Patent document 1: Japanese Patent Laid-Open Publication No. 2004-190632 9 Disclosure of Invention Problem to be solved by Invention [00161 From the viewpoints of effective utilization of energy, environment protection, reduction in operation cost and the like, it is important to develop a technique which is capable of efficiently, stably and continuously a low-calorie gas which fluctuate in calorific value incessantly and irregularly or a like gas as a fuel gas, for combustion systems including a gas turbine electric power generation system. [0017] In order to stably use in a combustion system a fuel gas which fluctuate in calorific value incessantly and irregularly, such as the lowcalorie gas, the calorific value fluctuation range has to be decreased to fall within an allowable range where the fuel gas becomes usable in the combustion system. However, there exists no effective method capable of realizing such a technique. [0018] 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 gas calorific value control method which is capable of suppressing a calorific value fluctuation of a gas, such as a low-calorie gas, to be supplied as a fuel gas to a combustion system, thereby making it possible to supply the combustion system with the low-calorie gas or the like as a fuel gas having a stabilized calorific value; and a gas calorific value control device employing 10 this method. Means for solving Problem [0019] In order to attain the foregoing object, the present invention provides a method of controlling a gas calorific value, comprising the steps of: suppressing a calorific value fluctuation of a fuel gas to be supplied to a combustion system by subjecting the fuel gas to time-lag mixing within a tank formed with a gas inlet and a gas outlet separately; measuring a calorific value fluctuation of the fuel gas having been thus suppressed; and decreasing or increasing a calorie of the fuel gas so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in the combustion system. [0020] According to this method, the fuel,gas continuously supplied to the tank through the gas inlet is temporarily stored therein, subjected to time-lag mixing therein and then discharged out of the tank through the gas outlet formed separately from the gas inlet. Even when the calorific value of the fuel gas fluctuates, the time-lag mixing of the fuel gas decreases the calorific value fluctuation range and retards the calorific value fluctuation speed. Subsequently, the calorie of the fuel gas having a calorific value fluctuation thus suppressed is decreased or increased so that the 11 gas calorific value of the fuel gas is adjusted to within the allowable fluctuation range for the combustion system. Thus, the calorific value fluctuation can be controlled easily. The aforementioned "time-lag mixing" means mixing of portions of the fuel gas which consecutively flow into the tank with time-lags with portions of the fufl gas which reside in the tank after having a already flowed thereinto. [0021] In this method, it is possible that: the calorific value fluctuation of the fuel gas having been suppressed is measured at a point on a fuel gas supply passage which is located downstream of the tank; and the calorie of the fuel gas is decreased or increased by a feedback control at a location upstream of the calorific value measurement point on the fuel gas supply passage so that the calorific value fluctuation thus measured falls within the allowable fluctuation range for the fuel gas to become usable in the combustion system. [0022] The "feedback control", as used in the present Description and Claims, means a control performed at a location upstream of a calorific value measurement point based on a value measured at the calorific value measurement point. [0023] In the method, it is possible that: the calorific value fluctuation of the fuel gas having been 12 suppressed is measured at a point on the fuel gas supply passage which is located downstream of the tank; and the calorie of the fuel gas is decreased or increased by a feedforward control at a location downstream of the calorific value measurement point on the fuel gas supply passage so that the calorific value fluctuation thus measured falls within, the allowable fluctuation range for the fuel gas to become usable in the combustion system. [0024] The "feedforward control", as used in the present Description and Claims, means a control performed at a location downstream of a calorific value measurement point based on a value measured at the calorific value measurement point. One feedforward control method includes: calculating a mixing rate of a calorie increasing or decreasing gas that is required for the resulting mixed gas to have an intended calorific value based on the calorific value and flow rate of the fuel gas before mixing with the calorie increasing or decreasing gas; and providing the calculated mixing rate of the calorie increasing or decreasing gas as an instructive value. However, other methods are possible. [0025] In the method, it is possible that: a calorific value fluctuation of the fuel gas is measured at a point on the fuel gas supply passage which is located between the tank and the location on the fuel gas supply passage at which the calorie of the fuel gas is decreased or increased; and 13 the calorie of the fuel gas is decreased or increased by a feedforward control performed in addition to the aforementioned feedback control so that the calorific value fluctuation thus measured falls within the allowable fluctuation range for the fuel gas to become usable in the combustion system. [0026] Another method according to the present invention may comprise the steps of: formulating a simulation model simulating suppression of a calorific value fluctuation to be made by time-lag mixing of a fuel gas in a tank formed with a gas inlet and a gas outlet separately; estimating a calorific value fluctuation of the fuel gas at the gas outlet of the tank from a calorific value fluctuation of the fuel gas measured at a point located upstream of the tank by using the simulation model; and performing a feedforward control for decreasing or increasing a calorie of the fuel gas at a location downstream of the tank so that the calorific value fluctuation of the fuel gas thus estimated falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. [0027] Yet another method may comprise the steps of decreasing or increasing a calorie of a fuel gas at a location upstream of a tank formed with a gas inlet and a gas outlet separately so that a calorific value fluctuation of the fuel gas falls 14 within a predetermined calorific value fluctuation range of the fuel gas when a calorific value fluctuation exceeding the predetermined calorific value fluctuation range is measured at a point located upstream of the tank; and decreasing or increasing the calorie of the fuel gas at a location on a fuel gas supply passage downstream of the tank so t.hat a calorific value fluctuation of the fuel gas at the gas outlet of the tank, which is estimated from the calorific value fluctuation of the fuel gas measured at the point located upstream of the tank by using a simulation model formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas having a decreased or increased calorie in the tank, falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. [0028] Yet another method may comprise the steps of: formulating a simulation model simulating suppression of a calorific value fluctuation to be made by time-lag mixing of a fuel gas in a tank formed with a gas inlet and a gas outlet separately; estimating a calorific value fluctuation of the fuel gas at the gas outlet of the tank from a calorific value fluctuation of the fuel gas measured at a point located upstream of the tank by using the simulation model; performing a feedforward control for decreasing or increasing a calorie of the fuel gas based on the calorie value 15 fluctuation of the fuel gas thus estimated; measuring a calorific value fluctuation of the fuel gas to be supplied to a combustion system at a point on a fuel gas supply passage which is located downstream of the tank; and performing a feedback control, in parallel with the feed forward control, for decreasing or increasing the calorie of the fuel gas at a location upstream of the calorific value measurement point on the fuel gas supply passage so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in the combustion system. [0029] Yet another method may comprise the steps of: decreasing or increasing a calorie of a fuel gas at a location upstream of a tank formed with a gas inlet and a gas outlet separately so that a calorific value fluctuation of the fuel gas falls within a predetermined calorific value fluctuation range of the fuel gas when a calorific value fluctuation exceeding the predetermined calorific value fluctuation range is measured at a point located upstream of the tank; performing a feedforward control for decreasing or increasing the calorie of the fuel gas based on a calorific value fluctuation of the fuel gas at the gas outlet of the tank, which is estimated from the calorific value fluctuation of the fuel gas measured at the point located upstream of the tank by using a simulation model formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel 16 gas having a decreased or increased calorie in the tank; measuring a calorific value fluctuation of the fuel gas to be supplied to a combustion system at a point on a fuel gas supply passage which is located downstream of the tank; and performing a feedback control, in parallel with the feed forward control, for decreasing or increasing the calorie of the fuel gas at a location upstream of the calorific value measurement point on the fuel gas supply passage so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in the combustion system. [0030] It is possible that the above-described simulation model is formulated for a tank having a predetermined volume and supplied with a predetermined flow rate of gas by using the sum total of products obtained by multiplication of signal intensities of respective of plural systems each comprising a first order lag and a dead time by respective constant multiplication factors, wherein a correction equivalent to a time lag essential to a detecting device is made to each of time constants used. [0031] In the above-described gas calorific value control method, an operation of decreasing or increasing the calorie of the fuel gas so that the calorific value fluctuation of the fuel gas falls within the allowable fluctuation range for the fuel gas to become usable in the combustion system, is performed within the tank or at an outside surface of the tank. 17 [0032] In the above-described gas calorific value control method, an average value of the calorific value fluctuation of the fuel gas measured at the point located upstream of the tank and an average value of the calorific value fluctuation of the fuel gas measured at the point on the fuel gas supply passage which is located downstream of the tank are monitored; and when a fixed amount of an average difference is detected between the average values, the calorie of the fuel gas is decreased or increased at a location upstream of the tank so that the calorific value fluctuation of the fuel gas at the point located upstream of the tank approximates to the calorific value fluctuation of the fuel gas at the point on the fuel gas supply passage which is located downstream of the tank. [0033] In order to attain the foregoing object, the present invention also provides a gas calorific value control device comprising: a tank formed with a gas inlet and a gas outlet separately and configured to time-lag mix a fuel gas to be supplied to a combustion system; a first calorific value measuring device for measuring a calorific value fluctuation of the fuel gas which has been suppressed by mixing of the fuel gas within the tank; and a first controller for mixing a calorie decreasing gas or a calorie increasing gas into the fuel gas by means of a mixer so that 18 the calorific value fluctuation measured by the first calorific value measuring device falls within an allowable fluctuation range for the fuel gas to become usable in the combustion system. [0034] It is possible that: the above-described first calorific value measuring device is located downstream of the mixer provided to a fuel gas supply passage at a location downstream of the tank; and the first controller is configured to perform a feedback control for decreasing or increasing a calorie of the fuel gas by means of the mixer so that the calorific value fluctuation measured by the first calorific value measuring device falls within the allowable fluctuation range for the fuel gas to become usable in the combustion system. [0035] Another gas calorific value control device according to the present invention may comprise: a tank formed with a gas inlet and a gas outlet separately and configured to time-lag mix a fuel gas to be supplied to a combustion system; a second calorific value measuring device for measuring a calorific value fluctuation of the fuel gas which has been suppressed by mixing of the fuel gas within the tank; and a second controller configured to perform a feedforward control for mixing a calorie decreasing gas or a calorie increasing gas into the fuel gas by means of a mixer so that the calorific value 19 fluctuation measured by the second calorific value measuring device falls within an allowable fluctuation range for the fuel gas to become usable in the combustion system. [0036] The calorific value control device may further comprise: a second calorific value measuring device, located at a point on the fuel supply passage which is located between the tank and the mixer, for measuring a calorific value fluctuation of the fuel gas; and a second controller configured to perform, in addition to the feedback control for decreasing or increasing the calorie of the fuel gas by means of the mixer, a feedforward control for decreasing or increasing the calorie of the fuel gas so that a calorific value fluctuation of the fuel gas falls within the allowable fluctuation range for the fuel gas to become usable in the combustion system, based on the calorific value fluctuation measured by the second calorific value measuring device. [0037] Yet another a gas calorific value control device may comprise: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas at a point located upstream of a tank formed with a gas inlet and a gas outlet separately; and a second controller configured to estimate a calorific value fluctuation of the fuel gas at the gas outlet of the tank by using a simulation model which is formulated so as to simulate 20 suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in the tank and perform a feedforward control for decreasing or increasing a calorie of the fuel gas so that the calorific value fluctuation of the fuel gas thus estimated falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. [0038] Yet another a gas calorific value control device may comprise: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas which exceeds a predetermined calorific value fluctuation range at a point located upstream of a tank formed with a gas inlet and a gas outlet separately; a third controller for decreasing or increasing a calorie of the fuel gas at a location upstream of the tank so that the calorific value fluctuation of the fuel gas falls within the predetermined calorific value fluctuation range when the calorific value fluctuation exceeding the predetermined fluctuation range is measured by the third calorific value measuring device; and a second controller configured to perform a feedforward control for decreasing or increasing the calorie of the fuel at a location on a fuel gas supply passage downstream of the tank so that a calorific value fluctuation of the fuel gas at the gas outlet of the tank, which is estimated from the calorific value fluctuation of the fuel gas measured at the point located upstream of the tank by 21 using a simulation model formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in the tank, falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. [0039] Yet, another gas calorific value control device may comprise: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas at a point located upstream of a tank formed with a gas inlet and a gas outlet separately; a second controller configured to estimate a calorific value fluctuation of the fuel gas at the gas outlet of the tank by using a simulation model which is formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in the tank and perform a feedforward control for decreasing or increasing a calorie of the fuel gas based on the calorific value fluctuation of the fuel gas thus estimated; a mixer provided to a fuel gas supply passage; a first calorific value measuring device located downstream of the mixer for measuring a calorific value fluctuation of the fuel gas to be supplied to a combustion system; and a first controller configured to perform, in parallel with the feedforward control, a feedback control for decreasing or increasing the calorie of the fuel gas by means of the mixer so that the calorific value fluctuation measured by the first calorific value measuring device falls within an allowable fluctuation range for the 22 fuel gas to become usable in the combustion system. [0040] Yet another gas calorific value control device may comprise: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas which exceeds a predetermined fluctuation range at a point located upstream of a tank formed with a gas inlet and a gas outlet separately? a third controller for decreasing or increasing a calorie of the fuel gas at a location upstream of the tank so that the calorific value fluctuation of the fuel gas falls within the predetermined fluctuation range when the calorific value fluctuation exceeding the predetermined fluctuation range is measured by the third calorific value measuring device; a second controller configured to perform a feedforward control for decreasing or increasing the calorie of the fuel at a location on a fuel gas supply passage downstream of the tank so that a calorific value fluctuation of the fuel gas at the gas outlet of the tank, which is estimated from the calorific fluctuation of the fuel gas measured at the point located upstream of the tank by using a simulation model formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in the tank, falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system; a mixer provided to the fuel gas supply passage; and a first calorific value measuring device located 23 downstream of the mixer for measuring a calorific value fluctuation of the fuel gas to be supplied to the combustion system; and a first controller configured to perform, in parallel with the feedforward control, a feedback control for decreasing or increasing the calorie of the fuel gas by means of the mixer so that the calorific value fluctuation measured by the first calorific value measuring device falls within the allowable fluctuation range for the fuel gas to become usable in the combustion system. [0041] In the above-described gas calorific value control device, it is possible that: the mixer is located within the tank or at an outside surface of the tank for decreasing or increasing the calorie of the fuel gas so that the calorific value fluctuation of the fuel gas falls within the allowable fluctuation range for the fuel gas to become usable in the combustion system; and the calorie of the fuel gas is decreased or increased within the tank or at the outside surface of the tank by means of the mixer. [0042] In the above-described gas calorific value control device, it is possible that: the mixer is located on the fuel gas supply passage upstream of the tank; a monitoring controller is provided for monitoring an average value of a calorific value fluctuation of the fuel gas 24 measured at a point located upstream of the mixer and an average value of the calorific value fluctuation of the fuel gas measured at the point located downstream of the mixer; and the monitoring controller is imparted with a function of decreasing or increasing the calorie of the fuel gas by means of the mixer located upstream of the-tank so that the calorific Value fluctuation of the fuel gas at the point located upstream of the tank approximates to the calorific value fluctuation of the fuel gas at the point located downstream of the tank when a fixed amount of an average difference is detected between the average values. Advantage of Invention [0043] According to the present invention, in supplying a combustion system, such as a gas turbine, with a low-calorie gas having a fluctuating calorific value or a like gas as a fuel gas, time- lag mixing of the fuel gas makes it possible to suppress (or lessen) a calorie value fluctuation of the fuel gas to a calorific value fluctuation level meeting the combustion system. Thus, the calorie of the fuel gas can be decreased or increased easily. That is, the present invention makes it possible to lessen the amplitude of the calorific value fluctuation by means of a tank, thereby suppressing short-cycle and medium-cycle fluctuations to leave long-cycle fluctuations mostly. For this reason, the present invention makes it possible to bring the fuel gas having a fluctuating calorific value into a stabilized fuel gas for use in the combustion system easily by decreasing or increasing the calorie of the fuel gas. Thus, the 25 present invention can provide for a combustion system capable of a stabilized continuous operation by decreasing the calorific value fluctuation of the gas to within an allowable fluctuation range for the fuel gas to become usable by the combustion system. Brief Description of Drawings [0044] [FIG. 1] FIG. 1 is a piping diagram schematically illustrating a gas turbine electric power generation system including a gas calorific value control device according to a first embodiment of the present invention. [FIG. 2] FIG. 2 is a graph plotting a calorific value fluctuation of a gas which was suppressed by the gas calorific value control device shown in FIG. 1. [FIG. 3] FIG. 3 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a second embodiment of the present invention. [FIG. 4] FIG. 4 is a graph plotting a calorific value fluctuation of a gas which was suppressed by the gas calorific value control device shown in FIG. 3. [FIG. 5] FIG. 5 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a third embodiment of the present invention. [FIG. 6] FIG. 6 is a block diagram illustrating one example of a 26 simulation model used in the gas calorific value control device according to the third embodiment shown in FIG. 5. [FIG. 7] FIG. 7 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a fourth embodiment of the pesent invention. [FIG. 8] FIG. 8 is a graph plotting a calorific value fluctuation of a gas which was suppressed by the gas calorific value control device shown in FIG. 7. [FIG. 9] FIG. 9 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a fifth embodiment of the present invention. [FIG. 10] FIG. 10 is a block diagram illustrating one example of a simulation model used in the gas calorific value control device according to the fifth embodiment shown in FIG. 9. [FIG. 11] FIG. 11 is a piping diagram schematically illustrating a conventional gas turbine electric power generation system. [FIG. 12] FIG. 12 is a graph plotting a calorific value fluctuation in the gas turbine electric power generation system shown in FIG. 11. Description of Reference Characters [0047] l...gas calorific value control device 2... gas turbine 3...fuel gas supply passage 4...fuel gas producing system 27 5...tank 6...gas inlet 7...gas outlet 8...mixer 9...gas compressor 10...electric generator 11...control gas supply piping 12...calorie decreasing flow control valve 13...calorie decreasing gas supply device 14...calorie increasing flow control valve 15...calorie increasing gas supply device 16...calorific value measuring device 17...input path 18...established value 19...first controller 20...output path 21... distributor 22...flow rate 23...second calorific value measuring device 24...input path 25...second controller 26...output path 27...gas calorific value control device 29...third calorific value measuring device 30...input path 31...simulator 28 32...path 33... gas calorific value control device 35...gas calorific value control device 37...second mixer 38...third controller (monitoring controller) 39...output path 40... distributor 41...calorie decreasing flow control valve 42...calorie decreasing gas supply device 43...calorie increasing flow control valve 44...calorie increasing gas supply device 45...second control gas supply piping 46...gas calorific value control device S...gas turbine electric power generation system Best Mode for Carrying Out the Invention [0046] Hereinafter, a gas calorific value control device and a gas calorific value control method using the same according to the present invention will be described with reference to the attached drawings. In the following description, a gas turbine will be illustrated as a combustion system. Embodiments to be described below are each configured to be capable of decreasing or increasing the calorific value of a fuel gas. [0047] FIG. 1 is a piping diagram schematically illustrating a 29 gas turbine electric power generation system S including a gas calorific value control device 1 according to a first embodiment of the present invention, wherein the gas calorific value control device 1 of the present invention is provided to a fuel gas supply passage 3 configured to supply the fuel gas to a gas turbine 2. [0048] The fuel gas supply passage 3 is configured to supply the gas turbine 2 with a low-calorie gas or the like (hereinafter will be referred to as "fuel gas") produced by a fuel gas producing system 4 (for example a blast furnace) as a fuel. The fuel gas supply passage 3 is provided with a tank 5 formed with a gas inlet 6 and a gas outlet 7 separately. [0049] The tank 5 is formed to have a predetermined volume and configured to allow the fuel gas passing through the fuel gas supply passage 3 to flow into the tank 5 through the gas inlet 6 and then flow out of the tank 5 through the gas outlet 7. The tank 5 functions as a buffer tank which allows portions of the fuel gas which continuously flow thereinto through the gas inlet 6 while fluctuating in calorific value to be time-lag mixed with portions of the fuel gas which reside in the tank 5 after having already flowed thereinto and then to be discharged out of the tank 5 through the separate gas outlet 7. Even when the fuel gas has a fluctuating calorific value, the time-lag mixing makes it possible to decrease the range of the calorific value fluctuation of the fuel gas and lower the calorific value fluctuation speed. 30 [0050] Specifically, portions of the fuel gas which have flowed into the tank 5 at a time provide such a distribution as to allow some portions to flow out through the gas outlet 7 relatively shortly and cause some portions to reside within the tank 5 for a relatively long time. Since fresh portions of the fuel gas newly flow into the tank 5 through the gas inlet 6 consecutively, the portions of the fuel gas which flowed into the tank 5 in the past and those that have newly flowed thereinto are mixed together incessantly. In this way, portions of the fuel gas continuously flowing into the tank 5 with a fluctuating calorific value are mixed with time-lags within the tank 5. In the present Description and Claims, this operation is referred to as "time-lag mixing". The tank 5 performing the time-lag mixing operation functions to suppress the calorific value fluctuation of the fuel gas. As a result, the fuel gas flowing out of the tank 5 through the gas outlet 7 has a decreased calorific value fluctuation range and a lowered fluctuation speed. That is, the calorific value fluctuation is largely suppressed (or lessened). [0051] Assuming that: the calorific value fluctuation of the fuel gas at the gas inlet 6 is represented by a sine curve of an angular velocity w; mixing within the tank 5 is complete mixing; and the time constant is T, the buffering effect of the tank 5 is expressed by: outlet fluctuation amplitude/inlet fluctuation amplitude == Gain = 1/(1+w2-T2)1/2. Accordingly, as GO or T (i.e., the volume of the tank) increases, Gain decreases, namely, the calorific value fluctuation 31 range of the fuel gas at the outlet decreases. Thus, the fluctuation suppressing effect can be obtained. The diagrammatic representations, which are put above the fuel gas supply passage 3 in the figure, each represents a calorific value fluctuation of the fuel gas schematically. [0052] The time-lag mixing by the tank 5 is an important feature of the present invention. Since the calorific fluctuation is thus lessened in advance by time-lag mixing of the fuel gas within the tank 5, the calorific value fluctuation of the fuel gas flowing on the downstream side of the tank 5 can be stably controlled so as to fall within an allowable gas property fluctuation range which is characteristic of the combustion system by decreasing or increasing the calorie of the fuel gas. [0053] There is no limitation on the structure of the tank 5 as long as the tank 5 has the predetermined volume. For example, the tank 5 may be either a tank of the invariable internal volume 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 member which is vertically slidable in accordance with the internal pressure of the tank. Such a tank can be diverted to the tank 5 which can exercise the effect of suppressing the calorific value fluctuation of the fuel gas. It is 32 possible to provide plural tanks 5 arranged in series or parallel. [0054] For the time-lag mixing of fuel gas to be achieved more effectively within the tank 5, the tank 5 may be provided with a stirring device for stirring and mixing the fuel gas having flowed tereinto through the gas inlet 6 or a perforated plate or the like incorporated therein to allow the fuel gas having flowed thereinto through the gas inlet 6 to pass through multiple perforations in order to mix the fuel gas. [0055] On a portion of the fuel gas supply passage 3 which is located downstream of the tank, there is provided a mixer 8 for mixing a calorie decreasing or increasing gas into the fuel gas in order to suppress the calorific value fluctuation of the fuel gas having flowed out of the tank 5. On the downstream side of the mixer 8, there are provided a gas compressor 9 configured to compress the fuel gas and the gas turbine 2 configured to combust the fuel gas compressed by the gas compressor 9 in order to drive an electric generator 10. [0056] The mixer 8 is connected to control gas supply piping 11 configured to supply the calorie decreasing or increasing gas to the mixer 8. The control gas supply piping 11 is provided with a calorie decreasing gas supply device 13 connected thereto through a calorie decreasing flow control valve 12 configured to control the flow rate of the calorie decreasing gas and a calorie increasing gas supply 33 device 15 connected thereto through a calorie increasing flow control valve 14 configured to control the flow rate of the calorie increasing gas. [0057] Examples of such calorie decreasing gases which can be employed include inert gases, air, steam, waste nitrogen and exhaust gases from combustion systems and the like. Nitrogen gas (N2) can be suitably used as an inert gas. However, it is needless to say that the inert gas is not limited to N2, but may be carbon dioxide (CO2), helium (He) or the like. Examples of such calorie increasing gases which can be employed include medium-calorie gases and high-calorie gases such as natural gas and coke oven gas (COG). [0058] A calorific value measuring device 16 is provided to the fuel gas supply passage 3 at a location downstream of the mixer 8 for measuring the calorific value of the fuel gas passing through the fuel gas supply passage 3. The calorific value measuring; device 16 may comprise a so-called calorimeter for directly measuring the calorific value of a gas, a device for determining the content (density) of a flammable component, and a like device. In cases where importance is attached to the detecting speed, use of a flammable gas density detecting device is presently preferable. A density detecting device meeting the kind of a major flammable component contained in the fuel gas to be used or the kind of a flammable component which is responsible for a major calorific value fluctuation may be used for detecting the density of such a 34 component. [0059] The calorific value of the fuel gas flowing on the downstream side of the mixer 8 is monitored by means of the calorific value measuring device 16. A value measured by the calorific value measuring device 16 is transmitted as an input signal to a first controller 19 through an input path 17, the first controller 19 being configured to compare the measured value with a predetermined established value 18 previously established to meet the gas turbine 2 (combustion system) used. The first controller 19 is a PI controller. The first controller 19 takes charge of feedback control. A control signal for decreasing or increasing the calorie of the fuel gas passing through the fuel gas supply passage 3, which is generated as a result of comparison made by the first controller 19, is transmitted as an output signal to the calorie decreasing flow control valve 12 or the calorie increasing flow control valve 14 through an output path 20 via a distributor 21. In this way, the feedback control is performed. [0060] The following description is directed to a control over the fuel gas passing through the fuel gas supply passage 3 with a fluctuating calorific value by the gas calorific value control device 1 thus configured according to the first embodiment for adjusting the calorific value fluctuation range of the fuel gas to within an allowable fluctuation range for the fuel gas to become usable as a stabilized fuel gas for the gas turbine 2 (combustion system). 35 [0061] The fuel gas supplied from the fuel gas producing system 4 through the fuel gas supply passage 3 is introduced into the tank 5 through the gas inlet 6 and then subjected to time-lag mixing within the tank 5. As already described, the tank 5 allows the fuel gas to flow therein to continuously, and portions of the fuel gas which have flowed into the tank 5 at a time provide such a distribution as to allow some portions to flow out through the gas outlet 7 relatively shortly and cause some portions to reside within the tank 5 for a relatively long time. Fresh portions of the fuel gas newly flowing into the tank 5 consecutively and the portions of the fuel gas which flowed into the tank 5 in the past are mixed together incessantly. Thus, calorific value fluctuations of the fuel gas supplied from the fuel gas producing system 4 which have large fluctuation ranges are suppressed. As a result, the fuel gas flows out of the tank 5 through the gas outlet 7 with its large calorific value fluctuations suppressed (or lessened). [0062] In this way, large calorific value fluctuations of the fuel gas are suppressed by the tank 5 which is provided to the fuel gas supply passage 3 and is capable of realizing time-lag mixing of the fuel gas. As a result, the tank 5 facilitates a control for mixing the calorie decreasing or increasing gas into the fuel gas on the downstream side of the tank 5 while suppressing the calorific value fluctuation of the fuel gas. [0063] 36 FIG. 2 is a graph plotting a calorific value fluctuation which was suppressed (or lessened) by the gas calorific value control device of FIG. 1. Specifically, FIG. 2 shows a result of simulation of a calorific value fluctuation which was suppressed (or lessened) under the conditions that: the tank 5 shown in FIG. 1 had a volume of 40,000 m3; and the fuel gas fluctuating in its calorie was supplied at a flow rate of 280,000 Nm3/hr. The horizontal axis represents time (in seconds) and the vertical axis represents the gas calorific value (MJ/kg) of the fuel gas. [0064] As can be seen from this example of a calorific value fluctuation range lessened by the tank 5 shown in the figure, a calorific value of the fuel gas supplied from the fuel gas producing system 4 (original fluctuation) fluctuates greatly at the inlet of the tank as represented by the dashed double-dotted line in the figure, while, in contrast, a calorific value fluctuation of the fuel gas at the outlet of the tank after time-lag mixing within the tank 5 (suppressed fluctuation) was in a condition represented by the broken line in which large calorific value fluctuations were suppressed. Specifically, the fuel gas before flowing into the tank 5 had a gas calorie fluctuating from about 5.3 MJ/kg to about 8.8 MJ/kg, while the fuel gas flowing out of the tank 5 had a gas calorie fluctuating from about 5.8 MJ/kg to about 6.8 MJ/kg. That is, the fluctuation range was decreased largely. With respect to fluctuation cycles, short-cycle fluctuations and medium-cycle fluctuations were eliminated and long-cycle fluctuations remained 37 mostly as shown. This effect tends to become more conspicuous as the volume of the tank 5 increases relative to the supply flow rate of the fuel gas. In cases where the original fluctuation has a relatively short fluctuation cycle and a relatively small fluctuation range, the tank 5, even when decreased in its volume from the economical point of view, is still effective. [0065] The calorific value measuring device 16 located on the downstream side of the mixer 8 measures the calorific value of the fuel gas passing through the fuel gas supply passage 3. A value measured by the calorific value measuring device 16 is transmitted as an input signal to the first controller 19 through the input path 17. The first controller 19 compares the measured value transmitted from the calorific value measuring device 19 with the predetermined established value 18 previously established to meet the gas turbine 2 used. [0066] If it is decided from the result of comparison that the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be lowered from the value measured by the calorific value measuring device 16, a control signal is transmitted to the calorie decreasing flow control valve 12 through the output path 20 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie decreasing gas from the calorie decreasing gas supply device 13. On the other hand, if it is decided that the calorie of the fuel gas has to be increased, a control signal is transmitted to the 38 calorie increasing flow control valve 14 through the output path 20 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie increasing gas from the calorie increasing gas supply device 15. In this way, the calorific value fluctuation range of the fuel gas to be supplied to the gas turbine 2 is controlled so as not to exceed the allowable fluctuation range for the fuel gas to become usable in the gas turbine 2. As can be seen from FIG. 2, the calorific value fluctuation having controlled by the mixer 8, which is represented by the solid line, comprised long-cycle and small-range calorific value fluctuations as a result of suppression of large calorific value fluctuations. Thus, the fuel gas was brought into a stabilized fuel gas having a calorific value fluctuation lessened to within the allowable fluctuation range for the fuel gas to become usable in the gas turbine 2. [0067] In decreasing or increasing the calorie of the fuel gas by means of the mixer 8, the gas calorific value control device 1 according to the first embodiment performs the feedback control for mixing the calorie decreasing or increasing gas into the fuel gas of which large calorific value fluctuations have been suppressed by the tank 5 as described above, based on the calorific value fluctuation of the fuel gas flowing on the downstream side of the mixer 8, thereby making it possible to stabilize the calorific value of the fuel gas easily. In addition, since this control is performed over the fuel gas with its large calorific value fluctuations suppressed, it is possible to decrease the required amount of the calorie decreasing or 39 increasing gas to be supplied to the fuel gas. [0068] If the established calorific value fluctuation range of the fuel gas for the gas turbine 2 is ±10% of a reference calorific value (average value) for example, the control gas has to be simply supplied at a constant rate on the downstream side of the tank 5 in some cases, provided the tank 5 has such a volume as to equalize the average calorific value of the fuel gas flowing on the downstream side of the tank 5 to the reference calorific value established for the gas turbine 2. [0069] While this embodiment is configured to decrease or increase the calorific value of the fuel gas, a control such as to supply the calorie decreasing gas and the calorie increasing gas at the same time may be performed under some condition. If the fuel gas and the combustion system meet certain conditions, the control device 1 may be configured to perform only one of the calorie decreasing operation and the calorie increasing operation, thereby suppressing the calorific value fluctuation of the fuel gas by using only one of the calorie decreasing gas and the calorie increasing gas. [0070] The mixer 8 may be located within the tank 5 or at an outside surface of the tank 5 for decreasing or increasing the calorie of the fuel gas at that location. Such a structural feature can render the system compact. [0071] 40 FIG. 3 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a second embodiment of the present invention. The second embodiment is configured to perform a feedforward control based on a measured calorific value fluctuation of the fuel gas-passing through a portion of the fuel gas supply passage 3 which extends between the tank 5 and the mixer 8, in addition to the feedback control based on a signal from the calorific value measuring device 16 located downstream of the mixer 8 according to the foregoing first embodiment. Like reference characters are used to denote like parts throughout the first and second embodiments for the purpose of omitting the description thereof. [0072] As shown, a portion of the fuel gas supply passage 3 which extends between the tank 5 and the mixer 8 is provided with a second calorific value measuring device 23. The calorific value of the fuel gas passing through that portion of the fuel gas supply passage 3, which is measured by the second calorific value measuring device 23, is sent to a second controller 25 through an input path 24. In the second embodiment, the second controller 25 outputs a control signal for controlling over a residual calorific value fluctuation remaining in the fuel gas of which the calorific value fluctuation has been suppressed by the tank 5. The second controller 25, which takes charge of the feedforward control, may be integrated with the aforementioned first controller 19. 41 [0073] As in the first embodiment, the calorific value measuring device 16 located downstream of the mixer 8 measures the calorific value of the fuel gas passing through the fuel gas supply passage 3 and the calorific value thus measured is sent to the first controller 19 through the input path 17.The first controller 19 compares the measured value with the predetermined established value 18 previously established to meet the gas turbine 2 used. When the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be decreased, a calorie decreasing control signal is sent to the output path 20. On the other hand, when the calorie of the fuel gas has to be increased, a calorie increasing control signal is sent to the output path 20. [0074] Subsequently, the calorie decreasing or increasing control signal is corrected by the calorie decreasing or increasing signal outputted from the second controller 25 through an output path 26. If it is decided from the result of correction made to the control signal from the first controller 19 by the control signal from the second controller 25 that the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be decreased, a control signal is transmitted to the calorie decreasing flow control valve 12 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie decreasing gas from the calorie decreasing gas supply device 13. On the other hand, if it is decided that the calorie of the fuel gas has to be increased, a control signal is transmitted to the 42 calorie increasing flow control valve 14 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie increasing gas from the calorie increasing gas supply device 15. [0075] In decreasing or increasing the calorie of the fuel gas by means of the mixer 8, the gas calorific value control device 27 according to the second embodiment performs the feedforward control for mixing the calorie decreasing or increasing gas into the fuel gas based on residual calorific value fluctuation remaining in the fuel gas flowing on the upstream side of the mixer 8 after having flowed out of the tank 5 in order to adjust the calorific value of the fuel gas, in addition to the feedback control for mixing the calorie decreasing or increasing gas into the fuel gas of which large calorific value fluctuations have been suppressed by the tank 5 based on the calorific value fluctuation of the fuel gas flowing on the downstream side of the mixer 8 in order to adjust the calorific value of the fuel gas as in the first embodiment. As compared with the first embodiment, the second embodiment thus configured is capable of following up more a rapid calorific value fluctuation, thereby stably adjusting the calorific value of the fuel gas to within the predetermined fluctuation range. Since the second embodiment controls the fuel gas with its large calorific value fluctuations suppressed like the first embodiment, it is possible to decrease the required amount of the calorie decreasing or increasing gas to be supplied to the fuel gas. [0076] 43 As described above, the second embodiment is configured to perform the feedforward control in addition to the feedback control. However, since the calorific value fluctuation range of the fuel gas flowing on the downstream side of the tank 5 has been decreased by the tank 5, the second embodiment may be configured such that: the second calorific value measuring device 23 measures the calorific value of the fuel gas passing through the fuel gas supply passage 3 and inputs the calorific value thus measured to the second controller 25; the second controller 25 compares the measured value with the predetermined established value 18 previously established to meet the gas turbine 2 based on the calorific value thus measured and the flow rate of the fuel gas passing through the fuel gas supply passage 3; and the second controller 25 outputs either a calorie decreasing control signal to the output path 20 when the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be decreased or a calorie increasing control signal to the output path 20 when the calorie of the fuel gas has to be increased. In this case, the feedforward control is such that: a mixing rate of the calorie decreasing or increasing gas which is required to adjust the calorific value of the resulting mixed gas to a predetermined value is calculated based on the calorific value and the flow rate of the fuel gas before mixing with the calorie decreasing or increasing gas; and a required mixing rate control using the calculated mixing rate as a mixing value is performed to achieve accurate mixing. This control is included in the feedforward control performed by the second controller 25 only. 44 [0077] FIG. 4 is a graph plotting a calorific value fluctuation of a gas which was lessened by the gas calorific value control device 27 shown in FIG. 3. Specifically, FIG. 4 shows a result of simulation of a calorific value fluctuation made under the same condition as in FIG. 2. According to the second embodiment, the calorific value fluctuation controlled by the mixer 8, which is represented by the solid line in the figure, comprised long-cycle calorific value fluctuations having smaller fluctuation ranges than in FIG. 2 as a result of suppression of large calorific value fluctuations. Thus, it is possible to bring the fuel gas into a stabilized fuel gas having an allowable fluctuation range for the gas turbine 2. [0078] FIG. 5 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a third embodiment of the present invention. The third embodiment is configured such that: a simulation model is previously formulated so as to estimate a calorific value fluctuation at the gas outlet 7 from a calorific value fluctuation at the gas inlet 6 of the tank 5; the calorific value fluctuation at the outlet of the tank 5 is estimated from a signal indicative of a calorific value of the fuel gas measured by a third calorific value measuring device 29 provided to the upstream side of the tank 5 by using the simulation model; and a feedforward control for decreasing or increasing the calorie of the fuel gas by means of the mixer 8 is performed based on the calorific value fluctuation 45 thus estimated. Like reference characters are used to denote like parts throughout the second and third embodiments for the purpose of omitting the description thereof. [0079] As shown, a portion of the fuel gas supply passage 3 which is located upstream of the tank 5 is provided with the third calorific value measuring device 29. The calorific value of the fuel gas passing through the fuel gas supply passage 3 which is measured by the third calorific value measuring device 29 is sent to a simulator 31 through an input path 30, the simulator 31 incorporating the simulation model therein. [0080] The simulation model previously incorporated in the simulator 31 comprises simulation models each formulated so as to meet a buffer tank to be used based on the result of simulation of a buffer tank model by the finite element method or the like. For example, such a simulation model is formulated for the tank 5 having a predetermined volume and supplied with a predetermined flow rate of gas by using the sum total of products obtained by multiplication of signal intensities of respective of plural systems each comprising a first order lag and a dead time by respective constant multiplication factors, wherein a correction equivalent to a time lag essential to a detecting device is made to each of time constants used. [0081] FIG. 6 is a block diagram showing one example of a 46 simulation model used in the gas calorific value control device according to the third embodiment shown in FIG. 5. The above- described simulation model is formulated by using the sum total of products obtained by multiplication of signal intensities of respective of plural systems each comprising a first order lag and a dead time by respective constant multiplication factors, For the convenience of illustration, the example shown is based on three systems. Specifically, a measure signal outputted by the third calorific value measuring device 29 is corrected by a lag compensation factor (l+Ta*s)/(l+Tb*s) appearing on the upstream side of the simulation model in the figure for compensating for a first order lag of the measuring device. A calorific value fluctuation at the gas outlet 7 of the tank 5 is estimated from the signal thus corrected by using the sum total of products obtained by multiplication of signal intensities of respective of plural systems each comprising a first order lag and a dead time by respective constant multiplication factors. In FIG. 6, s represents a Laplace transformation parameter; Ta and Tb represent lag compensating constants; T1, T2 and T3 represent first order lags; L1; L2 and L3 represent dead times; and G1, G2 and G3 represent are constant multiplication factors. [0082] A signal indicative of the calorific value fluctuation at the gas outlet 7 of the tank 5 estimated by such a simulation model is sent to the second controller 25 through a path 32. The second controller 25, in turn, compares the signal with the predetermined 47 established value 18 previously established to meet the gas turbine 2 in view of the flow rate 22. [0083] If it is decided from the result of comparison that the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be lowered from the value measured by the third calorific value measuring device 29, a control signal is transmitted to the calorie decreasing flow control valve 12 through the output path 20 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie decreasing gas from the calorie decreasing gas supply device 13. On the other hand, if it is decided that the calorie of the fuel gas has to be increased, a control signal is transmitted to the calorie increasing flow control valve 14 through the output path 20 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie increasing gas from the calorie increasing gas supply device 15. [0084] Even when a rapid calorific value fluctuation occurs in the fuel gas, the gas calorific value control device 33 according to the third embodiment measures the rapid calorific value fluctuation at a point located upstream of the tank 5 and then performs a control meeting the calorific value fluctuation by using the simulation model, thereby making it possible to suppress the calorific value fluctuation with a high follow-up capability. Further, the gas calorific value control device 33 according to the third embodiment is capable of accommodating such a situation that a 48 time lag would occur in the measurement by the second calorific value measuring device 23 of the second embodiment. [0085] FIG. 7 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a fourth embodiment of the present invention. The fourth embodiment is configured to perform the feedback control according to the first or second embodiment in addition to the feedforward control according to the third embodiment. Like reference characters are used to denote like parts throughout the second to fourth embodiments for the purpose of omitting the description thereof. [0086] As shown, the fourth embodiment includes, in addition to the configuration shown in FIG. 5, a configuration wherein: the calorific value measuring device 16 located downstream of the mixer measures the calorific value of the fuel gas passing through the fuel gas supply passage 3 and inputs the calorific value thus measured to the first controller 19; the first controller 19 compares the measured value with the predetermined established value 18 previously established to meet the gas turbine 2; and when the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be decreased, a calorie decreasing control signal is outputted to the output path 20, or when the calorie of the fuel gas has to be increased, a calorie increasing control signal is outputted to the output path 20. 49 [0087] Subsequently, the calorie decreasing or increasing feedback control signal is corrected by the calorie decreasing or increasing feedforward control signal outputted from the second controller 25. If it is decided from the result of correction made to the control signal from the first controller 19 by the control signal from the second controller 25 that the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be decreased, a control signal is transmitted to the calorie decreasing flow control valve 12 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie decreasing gas from the calorie decreasing gas supply device 13. On the other hand, if it is decided that the calorie of the fuel gas has to be increased, a control signal is transmitted to the calorie increasing flow control valve 14 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie increasing gas from the calorie increasing gas supply device 15. [0088] FIG. 8 is a graph plotting a calorific value fluctuation of a gas which was suppressed (or lessened) by the gas calorific value control device shown in FIG. 7. As represented by the solid line, the calorific value fluctuation, which had been controlled by the feedforward control for controlling the calorific value fluctuation of the fuel gas flowing on the upstream side of the tank 5 by using the simulation model as well as by the feedback control based on the calorific value fluctuation of the fuel gas flowing on the downstream 50 side of the mixer 8, comprised long-cycle calorific value fluctuations having small fluctuation ranges as a result of suppression of large calorific value fluctuations. Thus, the gas calorific value control device of this embodiment makes it possible to bring the fuel gas into a stabilized fuel gas having a calorific value fluctuation within the allowable fluctuation range for the gas turbine 2, [0089] Even when a rapid calorific value fluctuation occurs in the fuel gas, the gas calorific value control device 35 according to the fourth embodiment measures the rapid calorific value fluctuation at a point located upstream of the tank 5 and then performs a control meeting the calorific value fluctuation by using the simulation model, thereby making it possible to suppress the calorific value fluctuation with a high follow-up capability. Further, the gas calorific value control device 35 according to the fourth embodiment is capable of accommodating such a situation that a time lag would occur in the measurement by the second calorific value measuring device 23 of the gas calorific value control device 27 according to the second embodiment. [0090] Further, this embodiment employs the feedforward control using the simulator 31 incorporating therein the simulation model which is capable of simulating the characteristics of a real tank to estimate the calorific value of the fuel gas at the tank outlet highly accurately while compensating for a time lag in measurement by making good use of the residence time of gas within the tank 51 which is longer than a measurement time of the third calorific value measuring device 29 located upstream of the tank 5. Thus, the present embodiment makes it possible to realize a satisfactory follow-up capability based on the feedforward control, thereby decreasing the range of residual calorific value fluctuation of the fuel gas flowing on the inlet side of the gas turbine 2 to within the allowable limits in order for the system to realize a stabilized continuous operation. [0091] FIG. 9 is a piping diagram schematically illustrating a part of a gas turbine electric power generation system including a gas calorific value control device according to a fifth embodiment of the present invention. The fifth embodiment includes, in addition to the configuration of the fourth embodiment, a configuration capable of controlling the calorific value fluctuation of the fuel gas even when the average value of the calorific value fluctuation increases or decreases by a fixed range as schematically represented by a diagrammatic representation put above the fuel gas supply passage 3 in the figure. Like reference characters are used to denote like parts throughout the fourth and fifth embodiments for the purpose of omitting the description thereof. [0092] As shown, a portion of the fuel gas supply passage 3 which is located upstream of the tank 5 is provided with a second mixer 37, while a portion of the fuel gas supply passage 3 which is located upstream of the second mixer 37 provided with a third 52 calorific value measuring device 29. The calorific value of the fuel gas passing through the fuel gas supply passage 3 which is measured by the third calorific value measuring device 29 is sent to a third controller 38 through an input path 30. The calorific value of the fuel gas which is measured by the calorific value measuring device 16 located downstream of the mixer 8 on the downstream side of the tank 5 is also sent to the third controller 38 through the input path 17. The third controller 38 monitors an increase or decrease in the average value of calorific value fluctuation of the fuel gas flowing on the upstream side of the tank 5 by a fixed range relative to the average value of calorific value fluctuation of the fuel gas flowing on the downstream side of the mixer 8. Thus, the third controller 38 serves as a monitoring controller. [0093] When the fixed range of an increase or decrease is measured in the average value of calorific value fluctuation of the fuel gas flowing on the upstream side of the tank 5, the third controller 38 outputs either a calorie decreasing control signal to a calorie decreasing flow control valve 41 through an output path 39 via a distributor 40 in order to supply the second mixer 37 with a required amount of the calorie decreasing gas from a calorie decreasing gas supply device 42 through second control gas supply piping 45 when the calorie of the fuel gas has to be decreased, or a calorie increasing control signal to a calorie increasing flow control valve 43 through the output path 39 via the distributor 40 in order to supply the second mixer 37 with a required amount of the calorie 53 increasing gas from a calorie increasing gas supply device 44 through the second control gas supply piping 45 when the calorie of the fuel gas has to be increased. [0094] The calorific value of the fuel gas passing through the fuel gas supply passage 3 which is measured by the third calorific value measuring device 29 is inputted to the simulator 31 incorporating a simulation model therein. The control signal outputted from the third controller 38 to the output path 39 is also sent to the simulator 31. [0095] FIG. 10 is a block diagram showing one example of a simulation model incorporated in the simulator 31 of the gas calorific value control device 46 according to the fifth embodiment. With reference to this figure, description will be made of a control for decreasing the calorie of the fuel gas having a high average value of calorific value fluctuation as represented by the diagrammatic representation put on the upstream side of the tank 5 in FIG. 9. In this case, the calorie decreasing flow control valve 41 shown in FIG. 9 is controlled so as to supply the second mixer 37 with the calorie decreasing gas from the calorie decreasing gas supply device 42. Like the foregoing simulation model, the simulation model used in this example is formulated by using the sum total of products obtained by multiplication of signal intensities of respective of plural systems each comprising a first order lag and a dead time by respective constant multiplication factors. For the 54 convenience of illustration, the simulation model shown is based on three systems. [0096] In the case of this simulation model, when a difference occurs in the average value of calorific value fluctuation of the fuel gas flowing on the upstream side of the tank 5, a correction is made at a location upstream of the simulation model. Specifically, a measure signal outputted by the third calorific value measuring device 29 is corrected by a lag compensation factor (l+Ta*s)/(l+Tb*s) appearing on the upstream side of the simulation model in the figure for compensating for a first order lag of the measuring device. The signal thus corrected is further corrected by the third controller 38 after supply of the calorie decreasing gas in order to correct the difference in the average value of calorific value fluctuation. A calorific value fluctuation at the gas outlet 7 of the tank 5 is estimated from the signal thus corrected by using the sum total of products obtained by multiplication of signal intensities of respective of plural systems each comprising a first order lag and a dead time by respective multiplication factors. The signs used in FIG. 10 are same as those used in FIG. 6; specifically, s represents a Laplace transformation parameter; Ta and Tb represent lag compensating constants; T1, T2 and T3 represent first order lags; L1, L2 and L3 represent dead times; and G1, G2 and G3 represent constant multiplication factors. The expression appearing in the second controller 25 is a differential expression of a fluctuation. [0097] 55 A signal indicative of the calorific value fluctuation at the gas outlet 7 of the tank 5 estimated by such a simulation model is sent to the second controller 25 through the path 32. The second controller 25, in turn, outputs a control signal for controlling a residual calorific value fluctuation estimated to remain in the fuel gas haying been subjected to suppression of the average difference in calorific value fluctuation by the tank 5. [0098] On the other hand, as in the fourth embodiment, the calorific value measuring device 16 located downstream of the mixer 8 measures the calorific value of the fuel gas passing through the fuel gas supply passage 3 and inputs the calorific value thus measured to the first controller 19; the first controller 19 compares the measured value with the predetermined established value 18 previously established to meet the gas turbine 2; and when the calorie of the fuel gas passing through the fuel gas supply passage 3 has to be decreased, a control signal for decreasing the calorie of the fuel gas is outputted to the output path 20. [0099] Subsequently, the calorie decreasing control signal is corrected by the control signal outputted from the second controller 25 to the output path 26. If it is decided from the result of correction made to the control signal from the first controller 19 by the control signal from the second controller 25 that the calorie of the fuel gas passing.through the fuel gas supply passage 3 has to be decreased, a control signal is transmitted to the calorie decreasing 56 flow control valve 12 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie decreasing gas from the calorie decreasing gas supply device 13. In this way, the calorific value of the fuel gas is adjusted to within the allowable fluctuation range. [0100] The control operation for decreasing the calorie of the fuel gas has been described here. However, when the calorie of the fuel gas has to be increased, a control signal is transmitted to the calorie increasing flow control valve 14 via the distributor 21 in order to supply the mixer 8 with a required amount of the calorie increasing gas from the calorie increasing gas supply device 15. [0101] Even when the average value of calorific value fluctuation of the fuel gas increases or decreases by a fixed amount, the gas calorific value control device 46 according to the fifth embodiment is capable of suppressing the increase or decrease in the average value of calorific value fluctuation at a location upstream of the tank 5. Thus, the gas calorific value control device 46 can perform a stabilized control such as to adjust the calorific value fluctuation of the fuel gas to within the allowable fluctuation range for the fuel gas to become usable in the gas turbine 2 even when the average value of calorific value fluctuation of the fuel gas is changing. [0102] Like the other embodiments, this embodiment may be 57 configured to perform a control such as to supply the calorie decreasing gas and the calorie increasing gas at the same time under some condition. If the fuel gas meets a certain condition, the control device 46 may be configured to perform only one of the calorie decreasing operation and the calorie increasing operation in order to suppress the average value of calorific value fluctuation of the fuel gas by means of only one of the calorie decreasing gas and the calorie increasing gas. [0103] Use of the gas calorific value control device (or method) according to any one of the foregoing embodiments makes it possible to stabilize the calorific value fluctuation of each of various fuel gases by lessening the calorific value fluctuation to within the allowable fluctuation range for a combustion system reliably, thereby utilizing such fuel gases effectively and efficiently. [0104] While any one of the foregoing embodiments includes the gas turbine as a combustion system, the combustion system according to the present invention is not limited to any gas turbine. The foregoing gas calorific value control devices are applicable to other combustion systems including a boiler, heating oven and incinerator for example. [0105] While any one of the foregoing embodiments is configured to decrease or increase the calorific value of a fuel gas, an embodiment configured to perform only one of the calorie decreasing 58 control and the calorie increasing control is possible. Alternatively, an embodiment configured to perform both of the calorie decreasing control and the calorie increasing control at the same time is possible. [0106] While any one of the foregoing embodiments includes controllers having respective individual functions, such controllers may be integrated together as desired. [0107] Examples of fuel gases for use in the present invention include lowcalorie gases such as blast furnace gas (BFG), converter gas (LDG), coal mine gas (CMG) contained in coal seams, by-product gases produced by the direct iron reduction process and the smelting reduction ironmaking process, tail gas produced by the GTL (Gas-to- Liquid) process, byproduct gas accompanying an oil refining process for refining oil from oil sands, gas produced during rubbish incineration using plasma, methane gas (Landfill gas) produced during fermentation and decomposition of general waste including kitchen garbage in a landfill, and by-product gases accompanying chemical reaction between other similar materials. Such fuel gases include not only such lowcalorie gases but also medium-calorie gases and high-calorie gases. It is needless to say that the present invention can use the aforementioned gases either as a single gas or as mixed gases resulting from appropriate mixing of two or more different types of such gases. [0108] 59 Since any one of the foregoing embodiments is illustrated in a conceptual figure for description of basic functions, associated auxiliary units or devices and mounted components (including, for example, valve, starting device, transformer, shut-off device, tanks and the like), which are actually provided, are omitted from the figures and desrription. Industrial Applicability [0109] The present invention is useful as a gas calorific value control device which is capable of suppressing a calorific value fluctuation of a gas having fluctuating gas properties thereby bringing the gas into a stabilized fuel gas to be supplied to a combustion system such as a gas turbine, boiler, heating oven or incinerator. 60 CLAIMS [1] A method of controlling a gas calorific value, comprising the steps of: suppressing a calorific value fluctuation of a fuel gas to be supplied to a combustion system by subjecting the fuel gas to time-lag mixing within a tank formed with a gas inlet and a gas outlet separately; measuring a calorific value fluctuation of the fuel gas having been thus suppressed; and decreasing or increasing a calorie of the fuel gas so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in said combustion system. [2] The method according to claim 1, wherein: the calorific value fluctuation of the fuel gas having been suppressed is measured at a point on a fuel gas supply passage which is located downstream of said tank; and the calorie of the fuel gas is decreased or increased by a feedback control at a location upstream of the calorific value measurement point on said fuel gas supply passage so that the calorific value fluctuation thus measured falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system. 61 [3] The method according to claim 1, wherein.: the calorific value fluctuation of the fuel gas having been suppressed is measured at a point on said fuel gas supply passage which is located downstream of said tank; and the calorie of the fuel gas is decreased or increased by a feedforward control at a location downstream of the calorific value measurement point on said fuel gas supply passage so that the calorific value fluctuation thus measured falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system. [4] The method according to claim 2, wherein: a calorific value fluctuation of the fuel gas is measured at a point on said fuel gas supply passage which is located between said tank and the location on said fuel gas supply passage at which the calorie of the fuel gas is decreased or increased; and the calorie of the fuel gas is decreased or increased by a feedforward control performed in addition to said feedback control so that the calorific value fluctuation thus measured falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system. [5] A method of controlling a gas calorific value, comprising the steps of: formulating a simulation model simulating suppression of a calorific value fluctuation to be made by time-lag mixing of a 62 fuel gas in a tank formed with a gas inlet and a gas outlet separately; estimating a calorific value fluctuation of the fuel gas at said gas outlet of said tank from a calorific value fluctuation of the fuel gas measured at a point located upstream of said tank by using said simulation model; and performing a feedforward control for decreasing or increasing a calorie of the fuel gas at a location downstream of said tank so that the calorific value fluctuation of the fuel gas thus estimated falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. [6] A method of controlling a gas calorific value, comprising the steps of: decreasing or increasing a calorie of a fuel gas at a location upstream of a tank formed with a gas inlet and a gas outlet separately so that a calorific value fluctuation of the fuel gas falls within a predetermined calorific value fluctuation range of the fuel gas when a calorific value fluctuation exceeding said predetermined calorific value fluctuation range is measured at a point located upstream of said tank; and performing a feedforward control for decreasing or increasing the calorie of the fuel gas at a location on a fuel gas supply passage downstream of said tank so that a calorific value fluctuation of the fuel gas at said gas outlet of said tank, which is estimated from the calorific value fluctuation of the fuel gas 63 measured at the point located upstream of said tank by using a simulation model formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas having a decreased or increased calorie in said tank, falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. [7] A method of controlling a gas calorific value, comprising the steps of: formulating a simulation model simulating suppression of a calorific value fluctuation to be made by time-lag mixing of a fuel gas in a tank formed with a gas inlet and a gas outlet separately; estimating a calorific value fluctuation of the fuel gas at said gas outlet of said tank from a calorific value fluctuation of the fuel gas measured at a point located upstream of said tank by using said simulation model; performing a feedforward control for decreasing or increasing a calorie of the fuel gas based on the calorific value fluctuation of the fuel gas thus estimated; measuring a calorific value fluctuation of the fuel gas to be supplied to a combustion system at a point on a fuel gas supply passage which is located downstream of said tank; and performing a feedback control, in parallel with said feedforward control, for decreasing or increasing the calorie of the fuel gas at a location upstream of the calorific value measurement 64 point on said fuel gas supply passage so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in said combustion system. [8] A method of controlling a gas calorific value, comprising the steps of: decreasing or increasing a calorie of a fuel gas at a location upstream of a tank formed with a gas inlet and a gas outlet separately so that a calorific value fluctuation of the fuel gas falls within a predetermined calorific value fluctuation range of the fuel gas when a calorific value fluctuation exceeding said predetermined calorific value fluctuation range is measured at a point located upstream of said tank; performing a feedforward control for decreasing or increasing the calorie of the fuel based on a calorific value fluctuation of the fuel gas at said gas outlet of said tank, which is estimated from the calorific value fluctuation of the fuel gas measured at the point located upstream of said tank by using a simulation model formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas having a decreased or increased calorie in said tank; measuring a calorific value fluctuation of the fuel gas to be supplied to a combustion system at a point on a fuel gas supply passage which is located downstream of said tank; and performing a feedback control, in parallel with said feedforward control, for decreasing or increasing the calorie of the 65 fuel gas at a location upstream of the calorific value measurement point on said fuel gas supply passage so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in said combustion system. [9] The method according to any one of claims 4 to 8, wherein said simulation model is formulated for a tank having a predetermined volume and supplied with a predetermined flow rate of gas by using the sum total of products obtained by multiplication of signal intensities of respective of plural systems each comprising a first order lag and a dead time by respective constant multiplication factors, wherein a correction equivalent to a time lag essential to a detecting device is made to each of time constants used. [10] The method according to claim 2 or 5, wherein an operation of decreasing or increasing the calorie of the fuel gas so that the calorific value fluctuation of the fuel gas falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system, is performed within said tank or at an outside surface of said tank. [11] The method according to claim 7 or 8, wherein: an average value of the calorific value fluctuation of the fuel gas measured at the point located upstream of said tank and an average value of the calorific value fluctuation of the fuel gas 66 measured at the point on said fuel gas supply passage which is located downstream of said tank are monitored; and when a fixed amount of an average difference is detected between said average values, the calorie of the fuel gas is decreased or increased at a location upstream of said tank so that the calorific value fluctuation of the fuel gas at the point located upstream of said tank approximates to the calorific value fluctuation of the fuel gas at the point on said fuel gas supply passage which is located downstream of said tank. [12] A gas calorific value control device comprising: a tank formed with a gas inlet and a gas outlet separately and configured to time-lag mix a fuel gas to be supplied to a combustion system; a first calorific value measuring device for measuring a calorific value fluctuation of the fuel gas which has been suppressed by mixing of the fuel gas within said tank; and a first controller for mixing a calorie decreasing gas or a calorie increasing gas into the fuel gas by means of a mixer so that the calorific value fluctuation measured by the first calorific value measuring device falls within an allowable fluctuation range for the fuel gas to become usable in said combustion system. [13] The gas calorific value control device according to claim 12, wherein; said first calorific value measuring device is located 67 downstream of said mixer provided to a fuel gas supply passage at a location downstream of said tank; and said first controller is configured to perform a feedback control for decreasing or increasing a calorie of the fuel gas by means of said mixer so that the calorific value fluctuation measured by said first calorific value measuring device falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system. [14] A gas calorific value control device comprising: a tank formed with a gas inlet and a gas outlet separately and configured to time-lag mix a fuel gas to be supplied to a combustion system; a second calorific value measuring device for measuring a calorific value fluctuation of the fuel gas which has been suppressed by mixing of the fuel gas within said tank; and a second controller configured to perform a feedforward control for mixing a calorie decreasing gas or a calorie increasing gas into the fuel gas by means of a mixer so that the calorific value fluctuation measured by said second calorific value measuring device falls within an allowable fluctuation range for the fuel gas to become usable in said combustion system. [15] The gas calorific value control device according to claim 13, further comprising: a second calorific value measuring device, located at a 68 point on said fuel gas supply passage which is located between said tank and said mixer, for measuring a calorific value fluctuation of the fuel gas; and a second controller configured to perform, in addition to said feedback control for decreasing or increasing the calorie of the fuel gas by means of said mixer feedforward control for decroasing or increasing the calorie of the fuel gas so that a calorific value fluctuation of the fuel gas falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system, based on the calorific value fluctuation measured by said second calorific value measuring device. [16] A gas calorific value control device comprising: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas at a point located upstream of a tank formed with a gas inlet and a gas outlet separately; and a second controller configured to estimate a calorific value fluctuation of the fuel gas at said gas outlet of said tank by using a simulation model which is formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in said tank and perform a feedforward control for decreasing or increasing a calorie of the fuel gas so that the calorific value fluctuation of the fuel gas thus estimated falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. 69 [17] A gas calorific value control device comprising: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas which exceeds a predetermined fluctuation range at a point located upstream of a tank formed with a gas inlet and a gas outlet separately; a third controller for decreasing or increasing a calorie of the fuel gas at a location upstream of said tank so that the calorific value fluctuation of the fuel gas falls within said predetermined calorific value fluctuation range when the calorific value fluctuation exceeding said predetermined fluctuation range is measured by said third calorific value measuring device; and a second controller configured to perform a feedforward control for decreasing or increasing the calorie of the fuel at a location on a fuel gas supply passage downstream of said tank so that a calorific value fluctuation of the fuel gas at said gas outlet of said tank, which is estimated from the calorific value fluctuation of the fuel gas measured at the point located upstream of said tank by using a simulation model formulated to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in said tank, falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system. [18] A gas calorific value control device comprising: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas at a point located upstream of a tank formed with a gas inlet and a gas outlet separately; 70 a second controller configured to estimate a calorific value fluctuation of the fuel gas at said gas outlet of said tank by using a simulation model which is formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in said tank and perform a feedforward control for decreasing or increasing a calorie of the fuel gas based on the calorific value fluctuation of the fuel gas thus estimated; a mixer provided to a fuel gas supply passage; a first calorific value measuring device located downstream of said mixer for measuring a calorific value fluctuation of the fuel gas to be supplied to a combustion system; and a first controller configured to perform, in parallel with said feedforward control, a feedback control for decreasing or increasing the calorie of the fuel gas by means of said mixer so that the calorific value fluctuation measured by said first calorific value measuring device falls within an allowable fluctuation range for the fuel gas to become usable in said combustion system. [19] A gas calorific value control device comprising: a third calorific value measuring device for measuring a calorific value fluctuation of a fuel gas which exceeds a predetermined fluctuation range at a point located upstream of a tank formed with a gas inlet and a gas outlet separately; a third controller for decreasing or increasing a calorie of the fuel gas at a location upstream of said tank so that the calorific value fluctuation of the fuel gas falls within said predetermined 71 fluctuation range when the calorific value fluctuation exceeding said predetermined fluctuation range is measured by said third calorific value measuring device; a second controller configured to perform a feedforward control for decreasing or increasing the calorie of the fuel at a location on a fuel gas supply passage downstream of said tank so that a calorific value fluctuation of the fuel gas at said gas outlet of said tank, which is estimated from the calorific value fluctuation of the fuel gas measured at the point located upstream of said tank by- using a simulation model formulated so as to simulate suppression of a calorific value fluctuation to be made by time-lag mixing of the fuel gas in said tank, falls within an allowable fluctuation range for the fuel gas to become usable in a combustion system; a mixer provided to said fuel gas supply passage; a first calorific value measuring device located downstream of said mixer for measuring a calorific value fluctuation of the fuel gas to be supplied to said combustion system; and a first controller configured to perform, in parallel with said feedforward control, a feedback control for decreasing or increasing the calorie of the fuel gas by means of said mixer so that the calorific value fluctuation measured by said first calorific value measuring device falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system. [20] The gas calorific value control device according to claim 72 13 or 16, wherein; said mixer is located within said tank or at an outside surface of said tank for decreasing or increasing the calorie of the fuel gas so that the calorific value fluctuation of the fuel gas falls within said allowable fluctuation range for the fuel gas to become usable in said combustion system; and the calorie of the fuel gas is decreased or increased within said tank or at said outside surface of said tank by means of said mixer. [21] The gas calorific value control device according to claim 18 or 19, wherein: said mixer is located on said fuel gas supply passage upstream of said tank; a monitoring controller is provided for monitoring an average value of a calorific value fluctuation of the fuel gas measured at a point located upstream of said mixer and an average value of the calorific value fluctuation of the fuel gas measured at the point located downstream of said mixer; and said monitoring controller is imparted with a function of decreasing or increasing the calorie of the fuel gas by means of said mixer located upstream of said tank so that the calorific value fluctuation of the fuel gas at the point located upstream of said tank approximates to the calorific value fluctuation of the fuel gas at the point located downstream of said tank when a fixed amount of an average difference is detected between said average values. A gas calorific value control method is provided which is capable of suppressing a calorific value fluctuation of a fuel gas and supplying the fuel gas as a stabilized fuel gas. The gas calorific value control method includes the steps of suppressing a calorific value fluctuation of a fuel gas to be supplied to a combustion system (2) by subjecting the fuel gas to time-lag mixing within a tank (5) formed with a gas inlet (6) and a gas outlet (7) separately; measuring a calorific value fluctuation of the fuel gas having been thus suppressed; and decreasing or increasing a calorie of the fuel gas at a location downstream of the tank (5) so that the calorific value fluctuation thus measured falls within an allowable fluctuation range for the fuel gas to become usable in the combustion system (2). |
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| Patent Number | 270533 | ||||||||||||
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| Indian Patent Application Number | 3180/KOLNP/2007 | ||||||||||||
| PG Journal Number | 01/2016 | ||||||||||||
| Publication Date | 01-Jan-2016 | ||||||||||||
| Grant Date | 29-Dec-2015 | ||||||||||||
| Date of Filing | 29-Aug-2007 | ||||||||||||
| Name of Patentee | KAWASAKI JUKOGYO KABUSHIKI KAISHA | ||||||||||||
| Applicant Address | 1-1, HIGASHIKAWASAKI-CHO 3-CHOME,CHUO-KU, KOBE-SHI HYOGO 6508670 | ||||||||||||
Inventors:
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| PCT International Classification Number | F23K 5/00, F02C 7/22 | ||||||||||||
| PCT International Application Number | PCT/JP2005/002606 | ||||||||||||
| PCT International Filing date | 2005-02-18 | ||||||||||||
PCT Conventions:
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