Waste heat recovery boiler system and method of operating the same

Abstract: A waste heat recovery boiler system has a plurality of waste heat recovery boilers (10,20) each having a main steam generator and a reheater (13,23) capable of generat­ ing main steam and reheat steam by heat derived from a hot waste gas introduced into the waste heat recovery boilers (10 20) . The flows of main steams from the main steam generators are caused to join to form a single flow of main steams to be used in an external installation, and the flows of reheat steam from the reheaters (13,23) are caused to join to form a single flow of reheat steam to be used in the external installation. The steam discharged from the external installation is distributed to the reheaters. Flow rate controllers (51,61) are provided for controlling the flow rates of the steams through the reheaters (13,23) in relation to the hot waste gas supplied to the respective waste heat recovery boilers (10,20).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a waste heat recovery boiler system having a plurality of boilers which generate steam by utilizing heat from a waste gas of a high temperature and which supply the steam to a common system which operates with steam. The invention is also concerned with a method of operating such a waste heat recovery boiler system. Description of the Prior Art Japanese Patent Unexamined Publication No. 59-68503 discloses an electrical power generating combined plant as a typical example of waste heat recovery boiler system. This electrical power generating combined plant has a plurality of combinations of a gas turbine and a waste heat recovery boiler capable of generating steam by the heat recovered from the hot exhaust gas of the gas turbines, and a steam turbine which is supplied with the steam commonly from the boilers. This arrangement is adopted for the following reason. Namely, if only one waste heat recovery boiler is used to receive exhaust gas from a plurality of gas turbines, the operation of the system may seriously be impaired due to backward flow of the exhaust gas which may take place when different gas turbines are operated at different load levels. In order to obviate this problem, it is necessary to take an expensive countermeasure. On the other hand, supply of steam from a plurality of boilers to a common steam turbine offers an advantage that the efficiency of the plant is improved as a result of increase in the size of the steam turbine. A control system for a combined electrical power plant arranged in a similar way and having the characteristics of the precharacterizing part of claim 1 is disclosed in US-A-3 879 616. This control system is divided into independent gas turbine control systems, a steam turbine control system and a combined plant control system. In operation, the latter assigns the combined plant load to the gas turbines alone. When the exhaust temperature of the gas turbines rises, and power can be generated by the steam turbine driven by the exhaust, power demand on the gas turbines is reduced according to the steam turbine output so that the overall generated power corresponds to the desired combined plant load. A power generation plant of the reheater type is described e.g. in EP-A-0 290 220. This plant has a single gas turbine, a steam turbine having different pressure stages and a waste heat recovery boiler into which exhaust gas from the gas turbine is fed, comprising a superheater and two reheater sections. From these sections, vapour is fed into the different pressure stages of the steam turbine. In a combined plant having more than one gas turbine it is economical to provide not one steam turbine associated to each gas turbine, but rather, to use heat recovered from more than one gas turbine to power a single steam turbine, as is disclosed in the cited US-A-3 879 616 for a power plant having heat recovery boilers without reheaters. There is a trend for higher temperature of waste gas (exhaust gas from gas turbine or waste gas from a blast furnace of iron works) as the heat source of waste heat recovery boiler system. It is said that, when the temperature of the waste gas is high, a higher heat recovery efficiency can be obtained by using a combined plant of reheating type. Therefore, in current waste heat recovery system, the waste heat recovery boiler includes a main steam generator for generating main steam and a reheater. On the other hand, steam turbine which is the system which utilizes steam is composed of a plurality of stages including at least a high-pressure steam turbine and a low-pressure steam turbine. The main steam is introduced into and expanded through the high-pressure turbine and the steam from the high-pressure turbine is introduced into the reheater and the steam generated by the reheater is introduced into the low-pressure or reheat steam turbine so as to drive it. The combined plant of the reheating type is actually constructed as a multiple-type unit in which a plurality of waste heat recovery boilers and a common steam turbine are arranged such that streams of main steam from the main steam generators of the boilers and the streams of reheat steam from the reheaters of the boilers are respectively made to join and then introduced to the respective stages of the steam turbine. This system therefore encounters the following technical problems to be solved. Referring first to the reheating system, the steam from the high-pressure steam turbine is introduced in to the reheaters of the respective boilers so that reheated steam is generated in each reheater. Thus, the steam from the high-pressure steam turbine is distributed to a plurality of reheaters and the flow rates of the steam flowing into the reheaters are determined by the pressure drop or resistance across the respective reheaters. Actually, however, the reheating system is designed such that the reheaters produce the same flow resistance so that the flow rates of the steam introduced into the reheaters are substantially equalized. On the other hand, the rates of supply of the waste gases from the sources such as gas turbines to the respective waste heat recovery boilers are not always equal. For instance, it is often experienced that one of the waste heat recovery boilers receives the waste gas at the rated flow rate, while another boiler receives the waste gas at a smaller rate, e.g., 50%, of the rated flow rate. Such condition tends to occur when the level of the load imposed on the whole plant is changed or when the plant is being started up or stopped. In consequence, heat is supplied at different rates to different reheaters, though the reheaters are supplied with steam from the high-pressure steam turbine at an equal rate, resulting in different conditions of reheat steam generated by different reheaters. Such different steam conditions may cause a thermal stress to be generated in the portion of a reheat steam line at which flows of hot reheat steam from all reheaters join into a common pipe, causing a riask for the reheat steam line to be ruptured due to the thermal stress. When one of the heat sources, e.g., a gas turbine, and the associated waste heat recovery boiler are being started while another waste heat recovery boiler is operating, the time required for starting is prolonged due to the fact that the steam from the high-pressure steam turbine flows through the reheater of this boiler at a rate greater than the rate which corresponds to the load on the newly started gas turbine. On the other hand, the flow rate of the steam through the reheater of the operating waste heat recovery boiler is reduced to a level below that corresponding to the load on the gas turbine. In consequence, the operation of the whole plant is disturbed tremendously. Problems are not serious in case of the main steam system, because the water levels in the drums of the respective main steam generators are independently controllable so as to eliminate any large difference of the conditions of the main steam generated by these main steam generators. Namely, when the rate of generation of main steam in a main steam generator is increased due to an increase in the flow rate of the waste gas supplied thereto, the water level in the drum of the main steam generator is lowered and, when the water level has come down to the lower limit level, a boiler control operates to increase the rate of supply of feedwater so as to recover the normal water level in the drum. The increased rate of supply of the feedwater naturally reduces the rate of generation of the steam. Conversely, when the rate of generation of main steam in the main steam generator is decreased as a result of reduction in the flow rate of the waste gas supplied to the main steam generator, the water level in the water drum rises and, when the upper limit level is reached, the boiler control serves to reduce the rate of supply of the feedwater so as to recover the normal water level, thus increasing the rate of generation of the main steam. It is therefore expected that any variation in the waste gas flow rate does not cause serious difference in the conditions of the main steam generated by the main steam generators. Thus, the thermal stress and the condition of operation of the plant are not so seriously affected by difference in the waste gas flow rate between different waste heat recovery boilers. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a waste heat recovery boiler system in which differences in the steam generating rates, steam temperatures and other steam conditions between different waste heat recovery boilers, which may be caused by difference in the condition of supply of the waste gases to different waste heat recovery boilers, are reduced as much as possible so as to ensure stable operation of the plant, while eliminating problems such as the risk of rupture of steam pipes due to thermal stress. To this end, according to the present invention, flow rate control means are provided for controlling the flow rates of the reheat steam in the reheaters of the respective waste heat recovery boilers in accordance with the conditions of supply of the waste gases to the respective waste heat recovery boilers. According to this arrangement, the reheater of each waste heat recovery boiler can receive the steam at a rate which corresponds to the amount of heat possessed by the waste gas supplied to this boiler. In other words, it is possible to control the flow rate of the reheat steam such that all the reheaters operate at a substantially equal ratio between the heat possessed by the reheat steam and the heat possessed by the waste gas, so that the conditions of the steam generated by all the reheaters are substantially equalized. Thus, the flow rates of the steam through the reheaters of the waste heat recovery boilers are controlled in accordance with the conditions of the heat sources associated with the boilers, so that the plant can operate stably and safely. The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments when the same is read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing an embodiment of the waste heat recovery boiler system in accordance with the present invention; Fig. 2 is a diagram showing another embodiment; Fig. 3 is a diagram showing detail of the embodiment shown in Fig. 1; Fig. 4 is an illustration of a control system for controlling the flow rate of reheat steam in the embodiment shown in Fig. 3; Fig. 5 is a diagram showing still another embodiment of the present invention; Fig. 6 is an illustration of a control system for controlling the flow rate of reheat steam in the embodiment shown in Fig. 5; Figs. 7 and 8 are diagrams showing different embodiments of the present invention; Fig. 9 is an illustration of a control system for controlling the flow rate of reheat steam in the embodiments shown in Figs. 7 and 8; Figs. 10 to 13 are diagrams showing different embodiments of the present invention; and Fig. 14 is an illustration of a control system for controlling the flow rate of reheat steam in the embodiment shown in Fig. 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the accompanying drawings. Throughout the drawings, the same reference numerals are used to denote the same or equivalent parts or components. Various forms of reheating multiple combined plant incorporating gas turbines will be described for the purpose of illustration only. Referring to Fig. 1, there is shown a reheating multiple combined plant which has a pair of gas turbines #1GT and #2GT (not shown), a pair of waste heat recovery boilers 10 and 20 for recovering heat wasted from the respective gas turbines, and a single steam turbine 30 which is driven commonly by the steam generated by the waste heat recovery boilers 10 and 20. The waste heat recovery boilers 10 and 20 have, respectively, main steam generators with drums 11 and 12, superheaters 12 and 22 and reheaters 13 and 23. Flows of main steams from the superheaters 12 and 22 are introduced through main steam pipes 14 and 24 which join each other into a common main steam pipe 81 and then led to a high-pressure turbine unit 31 of the steam turbine 30 through the common main steam pipe 81. The steam discharged from the high-pressure steam turbine unit 31 is introduced into the reheaters 13 and 23 of the respective waste heat recovery boilers through cold or low-temperature reheat steam pipes 15 and 25, respectively. Flows of steam reheated by the reheaters 13 and 23 are made to flow into a common hot or high-temperature reheat steam pipe 82 through respective high-temperature reheat steam pipes 16 and 26, and the single flow of the high-temperature reheat steam is introduced into an intermediate-pressure steam turbine unit 32 of the steam turbine 30. Numerals 100, 101 and 102 denote, respectively, a feedwater pump, a condenser and an alternator. According to the present invention, steam flow rate controllers 51 and 61 are provided at intermediate portions of the low-temperature reheat steam pipes 15 and 25, respectively, for the purpose of controlling the flow rates of the reheat steam through the respective reheaters 13 and 23 in accordance with the levels of the load on the respective waste heat recovery boilers 10 and 20, as will be described later. Referring now to Fig. 2 showing another embodiment, the steam flow rate controllers 51 and 61 are provided at intermediate portions of the high-temperature reheat steam pipes 16 and 26, respectively, unlike the first embodiment in which these controllers are provided in the low-temperature reheat steam pipes 15 and 25. Figs. 3 and onwards show practical examples of control means for controlling the steam flow rate controllers 51 and 61. All these control means are applicable equally both to the embodiments of Figs. 1 and 2, though the following description specifically mentions the embodiment of Fig. 1 for the purpose of illustration. Referring to Fig. 3, steam flowmeters 17 and 27 capable of measuring the flow rates R₁ and R₂ of the low-temperature reheat steams flowing into the reheaters of both waste heat recovery boilers 10 and 20 are provided in the low-temperature reheat steam pipes 15 and 16, in addition to the above-mentioned steam flow rate controllers 51 and 61. On the other hand, main steam flowmeters 52 and 62 are provided in intermediate portions of the main steam pipes 14 and 24 for the purpose of measuring the flow rates M₁ and M₂ of the main steams generated by the respective waste heat recovery boilers 10 and 20. These flowmeters produce reheat steam flow rate signals (R₁, R₂) and main steam flow rate signals (M₁, M₂) indicative of the respective components of the reheat steam and main steam. These signals are delivered to a control unit 40 which is capable of computing the flow rates of the reheat steams into the respective waste heat recovery boilers 10 and 20 and operating the steam flow rate controllers 51 and 61. A practical example of the controlling method will be described with reference to Fig. 4. The aforementioned reheat steam flow rate signals (R₁, R₂) and main steam flow rate signals (M₁, M₂) indicative of the respective components of the reheat steam and main steam are delivered to reheat steam flow rate setting devices 41 and 42 which are capable of computing and setting the optimum flow rates of reheat steams to be supplied to the respective waste heat recovery boilers. For instance, the flow rate RS₁ of the reheat steam to be supplied to the waste heat recovery boiler 10 is determined in proportion to the ratio between the main steam flow rates M₁ and M₂, in accordance with the following formula: The thus computed set value RS₁ of the reheat steam to be supplied to the waste heat recovery boiler 10 is compared with the actually measured flow rate R₁. A control device 43 operates the steam flow rate controller 51 such that the actual flow rate R₁ coincides with the set value RS₁ of the reheat steam. The flow rate of the reheat steam to be supplied to the other waste heat recovery boiler 20 is also controlled in the same manner as that described above. Namely, the flow rate RS₂ of the reheat steam to be supplied to the waste heat recovery boiler 20 is determined in accordance with the following formula: The thus computed set value RS₂ of the reheat steam to be supplied to the waste heat recovery boiler 20 is compared with the actually measured flow rate R₂. A control device 44 operates the steam flow rate controller 61 such that the actual flow rate R₂ coincides with the set value RS₂ of the reheat steam. Thus, in this embodiment, the flow rates of the main steam generated by the respective waste heat recovery boilers 10 and 20 are used as indices of the levels of the load on the respective waste heat recovery boilers 10 and 20, and the flow rates of the reheat steam through the reheaters 13 and 23 of the respective waste heat recovery boilers 10 and 20 are controlled such that the ratio between the flow rates of the reheat steam equals to the ratio between the flow rates of the main steam, whereby a stable operation of the plant is ensured. Fig. 5 shows still another embodiment, in which the flow rates of the reheat steam to be supplied to the reheaters 13 and 23 of the respective waste heat recovery boilers 10 and 20 are determined on the basis of the total flow rate R F of the reheat steam which is measured by means of a steam flowmeter 71 which is provided in a common portion 83 of the low-temperature reheat steam pipes. This is contrasted to the embodiment of Fig. 3 in which the flow rates of reheat steam to respective reheaters 13 and 23 are determined on the basis of the total reheat steam flow rates (R₁ + R₂) which are calculated from the actually measured values R₁ and R₂ of the reheat steam. The control of the flow rates of the reheat steam in this embodiment is conducted in a manner shown in Fig. 6. Figs. 7 and 8 show further embodiments of the invention. Unlike the embodiment of Fig. 5 in which the total flow rate R F of the reheat steams is measured by means of the steam flowmeter 71 provided in a common portion 83 of the low-temperature reheat steam pipes, the embodiment shown in Fig. 7 measures the pressure of the low-temperature reheat steam by means of a pressure detector 72. Similarly, the embodiment of Fig. 8 measures the pressure of the high-pressure reheat steam by means of a pressure detector 73. As shown in Fig. 9, the total flow rate R F of the reheat steams is predicted as an output from a function generator 103 which is capable of generating, upon receipt of the steam pressure signal, a signal representative of the steam flow rate corresponding to the input steam pressure signal. Then, the flow rates of the reheat steam to be supplied to the reheaters 13 and 23 of the respective waste heat recovery boilers 10 and 20 are controlled in accordance with the thus predicted total flow rate R F of the reheat steams. Fig. 10 shows a still further embodiment which makes use of the rates of supply of feedwater to the respective waste heat recovery boilers 10 and 20 as indices of the levels of the load applied to the waste heat recovery boilers 10 and 20, in contrast to the foregoing embodiments in which the flow rates M₁ and M₂ of the main steam from the respective boilers 10 and 20 are used as the indices. More specifically, in the embodiment shown in Fig. 10, the rates of supply of feedwater to the drums 11 and 21 of the respective waste heat recovery boilers 10 and 20 are measured by means of feedwater flowmeters 53 and 63 provided in the feedwater pipes 84 and 85 and the signals representative of the measured flow rates of the feedwater are supplied to the control unit 40. In general, water flowmeter exhibits a higher accuracy than steam flowmeters, so that the embodiment shown in Fig. 10 can offer a more appropriate determination of the reheat steam flow rates than other embodiments which rely upon steam flow meters. Fig. 11 shows a still further embodiment in which the flow rates of the reheat steam to be supplied to the respective waste heat recovery boilers 10 and 20 are determined on the basis of the loads applied to the gas turbines 1 and 2 which are the sources of the waste heat, in contrast to the foregoing embodiments which employ the levels of the load on the waste heat recovery boilers 10 and 20. Thus, in the embodiment shown in Fig. 11, the output signals from alternators 54 and 64 driven by the gas turbines 1 and 2 are delivered as values Ma and Mb which represent the levels of the load applied to the gas turbines 1 and 2, and these signals Ma and Mb are delivered to the control unit 40. This embodiment can provide substantially the same advantage as those provided by the foregoing embodiments, provided that the waste heat is derived from the gas turbines alone. Fig. 12 shows a modification to the embodiment shown in Fig. 11. In this modification, the rates of supply of fuel to the gas turbines 1 and 2 are used in place of the output signals Ma and Mb used in the embodiment shown in Fig. 11. Thus, fuel flow rate signals produced by fuel flowmeters 55 and 56 provided in fuel lines 86 and 87 leading to the gas turbines 1 and 2 are delivered to the control unit 40. In the embodiments described hereinbefore, the flow rates of the reheat steam to be supplied to the reheaters of the respective waste heat recovery boilers are determined by the control unit 40 which receives signals indicative of the measured values of flow rates of the reheat steams and the levels of the loads on the respective waste heat recovery boilers or gas turbines. In contrast, a still further embodiment shown in Fig. 13 is capable of operating the steam flow rate controllers 51 and 61 such that the flow rates of the reheat steams flowing through the reheaters 13 and 23 of the respective waste heat recovery boilers 10 and 20 are substantially equalized to the flow rates of the main steams of the respective waste heat recovery boilers. The control is practically carried out in a manner shown in Fig. 14. It will be seen that the embodiment shown in Fig. 13 offers substantially the same effect as those produced by other embodiments. It is to be noted, however, that the embodiment shown in Figs. 13 and 14 is based on an assumption that suitable control systems (not shown) are used to maintain a constant water level or to keep the water level within a predetermined range in the drums 11 and 21 of the respective waste heat recovery boilers 10 and 20, so that the flow rates of the main steams from the respective waste heat recovery boilers 10 and 20 well represent the levels of the loads applied to these boilers. Flow rate detectors 52 and 62 measure the flow rates of the main steams from the respective waste heat recovery boilers 10 and 20 and produce signals indicative of these flow rates. These signals are supplied to arithmetic units 107 and 108 which are capable of producing command values of the flow rates of the reheat steams to be supplied to the reheaters 13 and 23 of the respective waste heat recovery boilers 10 and 20. Then, the control devices 45 and 46 control the steam flow rate controllers 51 and 61 so that the flow rates of the reheat steams through the waste heat recovery boilers are controlled substantially in conformity with the levels on the respective waste heat recovery boilers 10 and 20. Although the invention has been described with reference to a reheating multiple combined plant having two gas turbines, two waste heat recovery boilers coupled to the gas turbines, and a single steam turbine, this is not exclusive and the invention can be applied to plants having three or more gas turbines and boilers. The waste heat for the waste heat recovery boiler system of the present invention may be derived from various kinds of waste heat sources such as gas turbines, blast furnaces of iron works, and so forth. Various systems can utilize the steam produced by the waste gas recovery boiler system of the invention. For instance, the steam may be used for driving an electrical power generating steam turbine or for various other industrial purposes. The waste heat recovery boiler system of the present invention can also be used as a system which produce and supply both heat and electric power. In the described embodiments, the flow rates of the reheat steams to be supplied to the reheaters of the respective waste heat recovery boilers are controlled by independent flow rate controllers provided in respective reheat steam lines. This, however, is only illustrative and a simpler system can be realized within the scope of the invention by using only one controller which is associated with one of the waste heat recovery boilers so as to control the flow rates of the reheat steams to be supplied to both waste heat recovery boilers.

Claims: A reheating multiple combined electrical power generating plant comprising a plurality of sets of gas turbines (1, 2) and generators (54, 64) operable thereby, a plurality of waste heat recovery boiler units (10, 20) associated with the respective gas turbines (1, 2) to generate flows of steam, a steam turbine unit (30) to be driven by the flows of the steam from said boiler units (10, 20), and an additional generator (102) to be driven by said steam turbine unit (30), characterized in that each of said waste heat recovery boiler units (10, 20) includes a main steam generator (11 or 21) operable by the heat derived from the waste gas from the associated gas turbine (1 or 2) to generate a flow of main steam and a reheater (13 or 23) operable also by said heat of the waste gas to generate a flow of high-temperature reheat steam; and that said plant is provided with: a main steam common pipe (81) for collecting the flows of the main steam generated by said main steam generators (10 and 20) and introducing the thus collected main steam into said steam turbine unit (30); a high-temperature reheat steam common pipe (82) for collecting the flows of the high-temperature reheat steam generated by said reheaters (13 and 23) and flowing therefrom through high-temperature reheat steam branch pipes (16 and 26) and for introducing the thus collected high-temperature reheat steam into said steam turbine unit (30); low-temperature reheat steam branch pipes (15, 25) through which low-temperature reheat steam discharged from said steam turbine unit (30) is distributed to said reheaters (13 and 23); and control means (15, 61) provided on said high-temperature reheat steam branch pipes (16, 26) or said low-temperature reheat steam branch pipes (15, 25) to control the distribution of said low-temperature reheat steam to respective reheaters (13 and 23). A reheating multiple combined electrical power generating plant according to claim 1, wherein said steam turbine unit (30) has at least one high-pressure turbine (31) and at least one low-pressure turbine (32), and wherein said main steam is introduced through said main steam pipe (81) into said high-pressure turbine (31), said high-temperature reheat steam is introduced through said high-temperature reheat steam common pipe (82) into said low-pressure turbine (32), and said low-temperature reheat steam is returned through said low-temperature reheat steam branch pipes (15, 25) to said reheaters (13 and 23). A method of operating a reheating multiple combined electrical power generating plant defined in claim 1, characterized in that the rate of the flow of the reheat steam through each reheater (13 or 23) is controlled in accordance with either the level of the load on the gas turbine associated with the reheater, the heat energy possessed by the waste gas introduced into an associated waste heat recovery boiler unit (10 or 20) or a process amount determined by said load level or by said heat energy. The method according to claim 3, wherein, when an unbalance is produced between different waste heat recovery boilers (10 and 20), the flow rate of the steam flowing through each reheater (13 or 23) is controlled in accordance with either the level of the load on the gas turbine associated with the reheater, the heat energy possessed by the waste gas introduced into the associated waste heat recovery boiler unit (10 or 20) or the process amount determined by said load level or said heat energy. The method according to claim 3 or 4, wherein the flow rate of the main steam from the associated main steam generator is used as said process amount. The method according to claim 3 or 4, wherein the flow rate of the feedwater supplied to the associated main steam generator is used as said process amount. The method according to claim 3 or 4, wherein the output power of an alternator driven by the associated gas turbine is used as said process amount. The method according to claim 3 or 4, wherein the rate of supply of the fuel to the associated gas turbine is used as said process amount.

Inventors: HOIZUMI SHINICHI, TERANISHI TSUGUTOMO, HOIZUMI, SHINICHI, TERANISHI, TSUGUTOMO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: F01K23
IPC codes: F01K7, F01K23