TEMPERATURE CONTROL OF THE AIR SUPPLY IN PFBC PLANTS

Abstract: Methods and devices for controlling and limiting the temperature of the air on the inlet and outlet of a high-pressure compressor (3) included in a PFBC plant. The air into the high-pressure compressor passes through an intermediate cooler (5) based on water, which intermediate cooler, via a shunt valve (11) in the water circuit, serves as executive member for the controls. The control loops are otherwise designed with set and actual values as well as controllers (10) which deliver control signals to the shunt valve. Via a selection system, only that control which is most in need of control is active.

Description: Temperature control of the air supply in PFBC plants To ensure satisfactory function of the turbine units in a PFBC plant, it is required that the temperature of the air when it passes through compressors is maintained within certain given limits. If this is not done, compressors may, for example, reach a working range where surge occurs. The present invention comprises methods and devices for controlling and limiting the temperature of the air on the output of the high-pressure compressor, that is, before the air is passed into the pressure vessel and the combustor of the PFBC plant, and also for limiting the temperature of the air on the input of the high-pressure compressor both during stationary operation and during load change . PFBC plants have been described in a number of patent specifications, technical papers etc. In this description, therefore, only the parts of such a plant which are needed to describe the air system will be briefly described. The Swedish patent application SE 9102651-8 entitled "Air flow control system in PFBC plants", filed concurrently herewith, describes in detail, in addition to the PFBC plant as such, how an air system is built up. It also describes the impor¬ tant limitations that such a system entails and how these are overcome with the aid of controls overriding each other. A PFBC plant which includes the air temperature control according to the invention comprises a twin spool gas turbine unit. To the low-pressure shaft there are connected a low-pressure compressor and a low-pressure turbine, and to the high-pressure shaft there are connected a high-pressure compressor and a high-pressure turbine. Air is sucked via the inlet duct into the low-pressure compressor and is passed on via an intermediate cooler to the high-pressure compressor. The air which thereby passes through the high- pressure compressor is pressed into the boiler of the PFBC plant. The return flow from there first passes through the high-pressure turbine and then through the low-pressure turbine and is then released into the ree environment. In order for an air supply system to function satisfacto- rily, certain limitations must be introduced with regard to the temperature of the air, in addition to the limiting controls described in the above-mentioned "Air flow control system in PFBC plants", and the purpose of the invention, as mentioned above, is to maintain the temperature both down- stream and upstream of the high-pressure compressor within permissible limits. In the Swedish patent application dealing with air flow control, it is stated that in the air passage between the low-pressure compressor and the high-pressure compressor there is an intermediate cooler with the implicit but obvious task of cooling the air flow. Swedish patent SE 8602003-9, "A method for operation of a turbine unit", describes both an external intermediate cooler and an internal intermediate cooler arranged in the connec¬ tion between the low-pressure compressor and the high- pressure compressor. These intermediate coolers are arranged for heat exchange between the air in the compressor circuit and the condensate from the condenser (or similar medium) . This patent specification further states that "By the inter¬ mediate cooling the temperature of the air to the high- pressure compressor is reduced while at the same time the condensate to the feedwater tank is heated in the internal intermediate cooler" . However, no temperature control is mentioned in this connection. The maximum permissible temperature of the output of the high-pressure compressor is determined by the maximum permissible temperature of the interdependent systems during stationary operation. To be able to limit this temperature, the air out from the high-pressure compressor is tempera¬ ture-controlled by means of the intermediate cooler. The intermediate cooler may, for example, be of cross-flow type with cooling water on the tube side. A constant water flow is maintained by a circulation pump and the temperature of the water in the intermediate cooler is controlled by a shunt valve. The valve mixes hot water from the outlet of the intermediate cooler with colder cooling water, for example district heating water. If the cooling effect of the intermediate cooler should be reduced, the output power of the gas turbine has to be limited to protect against com¬ pressor surge. The circulation pump maintains the cooling water flow through the intermediate cooler to ensure a uni¬ form cooling of the air through the compressor. Uninten¬ tional shutdown of the circulation pump triggers an alarm signal. During load increase, the pressure ratio across the high- pressure compressor decreases and a corresponding increase of the low-pressure compressor pressure ratio takes place. This means that the temperature of the air at the input of the high-pressure compressor increases and reaches values which may be far above the corresponding steady-state value for that particular load. Because of the high air tempera¬ ture at the input of the high-pressure compressor, the high- pressure compressor suction of the air downstream of the low-pressure compressor deteriorates. Thereby, the low- pressure compressor may be forced towards the surge limit, which may lead to GT trip. To prevent the temperature of the air at the input of the high-pressure compressor from rising above a certain value and thus prevent the occurrence of the above phenomenon, a temperature control is arranged, again with the intermediate cooler as control executing object. Since the intermediate cooler can only function as executing object for one of the above-mentioned controls at a time, the invention comprises the use of a maximum selector which selects the control which is in greatest need of cooling. Introducing temperature control of the air in the manner described entails considerable advantages for the operation of PFBC plants. By limiting and maintaining the temperature on the inlet side of the high-pressure compressor at a con¬ trolled level, a safer and better margin with respect to the surge limit of the low-pressure compressor is obtained. This means that the risk of GT trips is reduced. In addition, the charging capacity will be improved since the power required by the low-pressure compressor during the charging phase is reduced when the pressure after the low-pressure compressor drops. There is also always a risk that the temperature of the gas upstream of the low-pressure turbine may become too high at high outdoor temperatures if no form of limitation of the temperature is provided. The temperature control according to the invention also reduces the gas temperature upstream of the low-pressure turbine. The accompanying Figure 1 shows the air/gas passage through the gas turbine unit and the basic concept of the control systems. Figures 2 and 3 show two alternative embodiments of the control loops, and Figure 4 shows an embodiment of the integration of the control loops with the other operating and protective loops . Figure 1 shows both the low-pressure unit with a low- pressure compressor 1 into which air is sucked and a low- pressure turbine 2, and the high-pressure unit with the high-pressure compressor 3 and the high-pressure turbine 4. The sucked-in air is passed from the low-pressure compressor 1 via an intermediate cooler 5 to the high-pressure com¬ pressor 3 and from there it is passed on to the boiler 6 of the PFBC plant. The air to the boiler 6 and the gas flow from the boiler 6 pass through the intercept valves 7. The return flow is then passed to the high-pressure turbine 4 and is then passed on via the low-pressure turbine 2 out into the free environment. Figure 1 also shows the measuring means 8 for measuring the temperature T3 ahead of the high-pressure compressor and 9 for measuring the temperature T 4 after the high-pressure compressor. Both of these measured values are supplied to the air temperature controller 10 together with the set values of the respective controls, that is, T3 Se a- nci τ 4set ■ The air temperature control actuates the shunt valve 11 which in closed position cuts off all cold water supply to the intermediate cooler. The temperature of the water in the intermediate cooler will thus be raised. The task of the temperature control is to see to it that the water heated in the intermediate cooler is mixed with the colder cooling water from the cooling water network so as to obtain sufficient cooling. This can be done by opening the shunt valve, whereby cold cooling water starts flowing into the intermediate cooler and in this way the cooling effect of the intermediate cooler will increase . In fully open position the water of the intermediate cooler will be returned to the cooling medium network, that is, the bypass section of the shunt valve is closed. The water circulation in the intermediate cooler is per- formed with the aid of a circulation pump 13 driven by a pump motor PM 12. When the shunt valve is closed, the same water will thus circulate in the circuit which comprises the intermediate cooler, the bypass section of the shunt valve, and the circulation pump. To supervise the water flow through the intermediate cooler, the pressure drop across the water side of the intermediate cooler is measured, for example with the difference pressure gauge 14, the measured value DP of which is also supplied to the temperature controller. If the measured value is lower than a certain value, special measures are taken in the temperature control system, which will be described in greater detail under the description of the control system itself. The control principle for temperature control of the air supply in a PFBC plant according to the invention will be clear from Figures 2 and 3, which show two different embo- diments. In these diagrams all the limiting and interlocking functions have been omitted. The composition of the compo¬ nents included in the control systems, such as controllers, selector devices, etc., may be varied in numerous ways con- sisting of analog, digital or hybrid analog/digital compo¬ nents in more or less integrated form. This is, of course, also true of the devices and components which are added in connection with the composition of limiting and interlocking functions according to Figure 4, which shows these functions integrated with the control principle according to Figure 2. The position control of the shunt valve may also be designed in several different ways with continuous control, three- position control or the like, and the operation may be elec¬ trical or pneumatic. In Figure 2 the set and actual values for the temperature downstream of the high-pressure compressor, that is, T 4Se t and 4ac ua lr are compared in a first controller 15, and in the same way the set and actual values for the temperature upstream of the high-pressure compressor, that is, T3 Se t and T3 ac tua l / are compared in a second controller 16. The outputs of the controllers are supplied to a maximum selector 17, the output signal of which consists of the greater of the control signals from the two controllers. This signal constitutes the set value S se t for the subsequent position control of the shunt valve. Figure 3 shows another embodiment of the same temperature control. In this embodiment the selection between the two temperatures is done before the temperature control 18 is made. The selection is performed in such a way that T 4Se t and factual are compared in a first summator 19, T3 Se t and τ 3actua l are compared in a second summator 20, whereafter the output signals of the two summators are compared in a third summator 21. The polarity on the output of the latter summator will then determine which of the controls is to be active. The polarity is converted in a limiter 22 to either a "0"- or a "l"-signal, which activates a number of selectors which connect the control loop which is most in need of control into the controller. In the state shown in Figure 3, the output signal from the limiter is equal to zero. The selectors VI and V2 then connect Set and T 4act ual to the controller while at the same time the selectors V3 and V4 connect into the controller the proportional part P 4 and integral part I 4 of that control. In a corresponding way, the temperature control for the temperature before the high-pressure compressor is switched on when the output on the limiter 22 becomes "1". The subsequent position control of the shunt valve is the same as is clear from Figure 2. Figure 4 shows the functional relationship between the con¬ trol loops and the limiting and interlocking loops when the embodiment for the control system is the one shown in Figure 2. The same functional relationship, of course, exists also when the control system has an embodiment according to Figure 3. It is important to prevent large disturbance bumps during the transition between control at T 4 and T3 and vice versa. The set value S se t fo r the position control of the shunt valve, which in normal operation consists of the output of the maximum selector, is therefore allowed to be reduced via a reducing device 23 by a certain percentage a. and the reduced value is allowed to form a follow-up set value F se t which is supplied to the two controllers . These are connec¬ ted such that that controller which is not active receives an output signal which is lower, by the above-mentioned percentage, than the corresponding value of the controller which is active. This means that the signal on the output of the controller 16 in the operating case shown in Figure 4 will have a value somewhat lower than the signal on the output of the controller 15. If, on the other hand, the difference between Ϊ3 S et and T3 ac tua l becomes so great that the temperature control is to be taken over by the T3 control, the controller 15 for the T 4 control will be controlled towards a correspondingly lower value. It is, of course, desirable to be able to control the position of the shunt valve manually, which can be made by activating, by an activating signal "Manual", a selector V5 which connects a desired set value Man set - The activa- tion can be made via an operator or via protection signals. This value now constitutes the set value of the shunt control instead of the control signal from any of the controllers. As mentioned above, there must be a certain minimum flow through the intermediate cooler. If the flow is below a certain limit, information about this is received via the difference pressure gauge 14 according to Figure 1, which measures the pressure drop across the water side of the intermediate cooler. If a pressure drop is lower than the minimum limit DP and the period during which it is lower is 10 seconds or more, which is monitored by the time lag element 24, the selector V6 is activated, which overrides both the control signal from the controllers and the manual operation. A fixed set value DPS se t = 0 is then connected into the control of the shunt valve.

Claims: CLAIMS 1. A method for temperature control of the air supply in PFBC plants which comprise a low-pressure unit with a low- pressure compressor (1) and a low-pressure turbine (2) , a high-pressure unit with a high-pressure compressor (3) and a high-pressure turbine (4) , wherein air is sucked into the low-pressure compressor whereupon the air is passed via an intermediate cooler (5) , which is traversed by water, into the high-pressure compressor and is passed from there on to the boiler (6) of the plant, which temperature control is characterized in that it comprises control of the tempe¬ rature of the air both upstream (T3) and downstream (T 4 ) of the high-pressure compressor, whereby the set and actual values of the two controls are compared and that only that of the two controls which is most in need of control will be active, and that the executing object of the temperature control consists of the intermediate cooler, the cooling effect of which is determined with a shunt valve, the con- trol signal (S S et) of which consists of the output signal from the temperature control system (10) . 2. A method for temperature control of the air supply in PFBC plants according to claim 1, which method is characterized in that the set and actual values for the two controls are compared in individual controllers (15, 16) and that the outputs of the controllers are passed to a maximum selector (17), the output signal of which corre¬ sponds to the signal (S se t) from the temperature control system. 3. A method for temperature control of the air supply in PFBC plants according to claims 1 and 2, which method is characterized in that with the aid of a reduced value of the output signal of the temperature control, a follow-up set value (F se t) is formed, which value is fed back to the two controllers which are connected such that the output signal from that controller whose output is not active is controlled towards a reduced signal corresponding to the output signal from the controller which is active. 4. A method for temperature control of the air supply in PFBC plants according to claim 1, which method is characterized in that the set and actual values for the T 4 control are compared in a first summator (19) and that the set and actual values for the T3 control are compared in a second summator (20) , and that the output signals of the two summators are supplied to a third summator (21) , and that that of the controls which is to be active is deter¬ mined by the polarity of the output signal on the third summator via a limiter (22) , whereby also, via polarity- controlled selectors (V1,V2,V3,V4) , the set and actual values for the control which is to be active together with the P- and I-values determined for the respective control are supplied to a common PID-connected controller (18) , whose output signal corresponds to the output signal (S se t) from the temperature control system. 5. A method for temperature control of the air supply in PFBC plants according to claim 1, which method is characterized in that the control signal (S se t) from the temperature control system for control of the shunt valve is replaced by a set value (Man se ) for manual control of the shunt valve. 6. A method for temperature control of the air supply in PFBC plants according to claim 1, which method is characterized in that the control signal (S set ) from the temperature control system for control of the shunt valve is replaced by a set value (DPS S et) for control of the shunt valve when the pressure drop across the water side of the intermediate cooler is below a certain minimum value. 7. A device for carrying out the method for temperature control of the air supply in PFBC plants according to claim 1, which plant comprises a low-pressure unit with a low- pressure compressor (1) and a low-pressure turbine (2) , a high-pressure unit with a high-pressure compressor (3) and a high-pressure turbine (4) and an intermediate cooler (5) located in the air passage between the low-pressure and high-pressure compressors, which device is characterized in that it comprises measuring devices (8,9) for measuring the temperature of the air upstream (T3) and downstream (T 4 ) of the high-pressure compressor as well as control devices for comparison between measured temperatures and corre- sponding set values and for forming a control signal to the intermediate cooler which is arranged as executive member for the controls, the control signal determining the cooling effect of the intermediate cooler via a shunt valve (11) .

Inventors: AHLIN ROGER, KYRKLUND BEN, NOVIN SAIDEH, AHLIN, ROGER, KYRKLUND, BEN, NOVIN, SAIDEH
Applicants: ABB CARBON AB
Application year: 1992
Main classification: F02C7
IPC codes: F01K23, F02C7