Steam Plant With Pressure-fired Boiler

Abstract

A steam power plant providing steam to drive a steam turbine coupled to an electrical generator includes a pressure-fired boiler the fuel flow rate of which is controlled by a livesteam regulator and a pressure-charging set consisting of turbocompressor for providing combustion air and which is driven by a gas turbine powered by the exhaust gas from the boiler and a starter motor, all on the same shaft. The speed of the pressure-charging set is regulated by varying the throughput of the gas turbine and/or the temperature of the gas turbine inlet in such manner that it neither produces surplus power nor requires power from the outside in accordance with variations in the steam power demand from the boiler as the load on the steam turbine changes. The starter motor for the pressure charging set is disconnected after starting and, at full load on the plant, the charging pressure for the combustion air is at least 9 bar, and the exhaust gas at the gas turbine outlet is no higher than 165.degree..

Description

This invention concerns a steam power plant with pressure-fired boiler the fuel flow rate of which is controlled by a livesteam regulator, and which incorporates a pressure-charging set comprising a turbocompressor, a gas turbine connected after the boiler and a starter motor, the speed of the set being variable in order to match the required flow rate of combustion air to the adjusted fuel flow rate.

Pressure-fired boilers for steam power plant have long been known under the name of "Velox" boilers. The pressure-charging group is usually controlled by means of a Ward-Leonard set which is permanently coupled to the pressure-charging group. The fuel flow rate is controlled in terms of steam consumption which at the same time influences the Ward-Leonard set in known manner and thus regulates the speed of the pressure-charging set, or in other words matches the required air flow rate to the fuel flow rate. At approximately three-quarters load the power output of the gas turbine just covers the power requirement of the compressor. At lower partial load the pressure-charging set consumes additional electrical power, while at higher part load, full load and overload surplus power is fed into the network. The Ward Leonard configuration allows speed variation within broad limits, but has the disadvantage that the set runs continuously. It is also costly.

Steam boilers of this kind at full load usually have a charging pressure of only 2 - 3 bar and high gas velocities, and in consequence the gas turbine exhaust temperature is relatively high, thus necessitating a subsequent heating surface which is no longer at the compressor discharge pressure and is therefore very large and expensive. Since for practical reasons only one economiser can be considered for the subsequent heating surface, the feed water temperature must be reduced by bleed steam, which impairs the thermal efficiency of the process. In addition, the condenser must be made larger.

The impaired efficiency of the process due to the lower rise in feed water temperature can be counteracted in known manner by employing a combined process in which the gas turbine set, which at the same time serves to pressure charge the boiler, is operated at the maximum permissible temperature at the gas turbine inlet. This method has the effect of reducing the steam output because the hot gases are cooled to a lesser extent in the boiler, but on the other hand the gas turbine produces useful power by way of a generator. With a combined process the losses due to the smaller rise in feed water temperature can just be overcome, but at the same time the following disadvantages must be taken into account:

Because of the corrosion risk, the high temperature at the gas turbine inlet means that only a very clean, ash-free fuel can be used, which greatly increases the cost of fuel.

The heat exchanger connected after the gas turbine is of very large volume and correspondingly expensive, on the one hand because the higher temperature at the gas turbine inlet also sharply increases the exhaust gas temperature, i.e., a large quantity of heat must be transferred in order to keep the stack losses small, and on the other because the heat transfer rate is less owing to the lower gas pressure and the volume of gas must therefore be large. The heat-exchange area of this non-pressurised heat exchanger is a multiple of those in the pressure-charged boiler. The large volume of the heat exchanger also sharply increases the space required and the cost of the foundation.

The power produced must be divided between two generators, one for the gas turbine and one for the steam turbine, thus increasing the plant costs.

Since the gas turbine has a generator, its speed is constant and at part load the air rate cannot be reduced, or only at great expense, resulting in high exhaust losses. This can in fact be avoided by using a separate power turbine, but again the plant cost would be much higher.

So that no evaporation takes place in the economiser at part load the temperature at the gas turbine inlet must be lowered, thus impairing the thermal efficiency.

The object of the invention is to create a structurally simple steam power plant of the kind mentioned above with good overall efficiency, such that on the gas side no additional heat exchanger is required after the pressure-fired boiler and yet the stack losses remain within the limits customary with such plant.

This object is achieved in that the speed of the pressure-charging set can be regulated by varying the throughput of the gas turbine and/or the temperature at the gas turbine inlet so that it neither produces surplus power nor requires power from outside, the starter motor can be disconnected after starting, at full load the charging pressure is at least 9 bar and the temperature of the gas turbine exhaust is no higher than 165.degree.C.

With this simple, dependable and very economical plant, speed is regulated by means inherent to the boiler, no intervention from outside is necessary and no external power is required.

The speed, and hence also the air flow rate, is continuously matched to the fuel flow rate so that excess air in the boiler remains practically constant under all operating conditions and is restored immediately after load changes. The high pressure ratio of the pressure-charging set means that the temperature of the compressed combustion air is high, allowing even heavy oil to be burned without difficulty and without the need for additional preheating of the air. Because of the high pressure ratio the dimensions of the steam generator can be small, allowing it to be transported in the assembled condition (packaged unit), and also the exhaust temperatures are low -- without the pressure-charging set sending out power -- so that no feed water heating by the exhaust gases is necessary and the thermal process can be optimised by heating the feed water with bleed steam.

The schematic, simplified drawing shows an example of the invention. The steam power plant consists essentially of a high-pressure turbine 1 and a low-pressure turbine 2, which together drive the electric generator 3, of a condenser 4, condensate pump 32, feed water heaters 5, heated by bleed steam, and steam boiler 6 which includes a vaporizer section a and an economizer section 31 for pre-heating the returning feed water. The boiler is pressurised, i.e. brought to the required gas-side pressure, by means of the pressure-charging set which basically comprises a turbocompressor 7, gas turbine 8, starter motor 9 and hydraulic coupling 10. All components of the pressure-charging set are mounted on the same shaft.

Gas turbine 8 is driven by the hot combustion gases discharged from the steam boiler 6, the temperature of the combustion gas delivered to the turbine inlet being controlled by a valve 29 which controls the amount of combustion gas permitted to by-pass the economizer section 31 through a by-pass duct 30. That is to say, when valve 29 is open part of the hot combustion gas exiting from the boiler by-passes the economizer section 31 by direct flow through duct 30 and the remaining part of the combustion gases pass through the economizer section 31 and are partially cooled off. The two partial gas flows mix and are thus delivered to the gas inlet of turbine 8. When valve 29 is closed, all of the combustion gas is forced to pass through the economizer section 31 and cooled off with the result that the temperature in front of turbine 8 will be correspondingly lower. A rise in temperature of the combustion gas exiting from the steam generator results in a higher turbine speed and hence a higher speed for the compressor 7 driven by it and a greater amount of combustion air delivered to the steam generator. Conversely a decrease in temperature of the combustion gas exiting from the steam generator results in a decrease in speed of the charging group 7, 8 and hence a decrease in combustion air from compressor 7.

The control system of the plant includes the following individual components of interest in the present context: the steam pressure or flow rate in the live steam line 11 influence the live steam regulator 12 which controls drain 13 on pressure-oil line 14 of the primary system, this line being fed at 15 by way of throttle 16. Pressure-oil line 14 is connected at one end to servo 17, which regulates the fuel supply (not shown) via fuel nozzles 18 in the boiler, and at the other end to servo 19 which moves the cylinder 20 of centrifugal governor 21 of the pressure-charging set, whereupon the oil pressure in the pressure-oil line 23 of the secondary system is varied by means of drain 22. Pressure-oil line 23 is fed at 24 by way of throttle 25 and leads to valve 26 on the bypass line 27 round gas turbine 8 and to the servo 28 which actuates valve 29, which in turn influences bypass 30 for the economizer section 31. The hydraulic control system can also be replaced by an electrical control system with the same functions.

The interaction between the individual parts and the method of controlling the plant are described below.

At full load the turbocompressor 1 compresses the combustion air to at least 9 bar, thus heating it to approx. 330.degree.C, which facilitates the burning of heavy oil. The boiler 6 is so designed that the gas temperature at the outlet is approx. 430.degree.C, which is also the inlet temperature to the gas turbine 8. In association with the high pressure ratio, this results in an exhaust gas temperature after the gas turbine, and hence a stack temperature, of only 150.degree.C. The exhaust loss can thus be kept very low and a bulky extra heating surface avoided. The turbocompressor has no form of cooling and thus the feed water does not have to be heated either by the compressed air or by the exhaust gas, which allows optimum heating by means of bleed steam.

The temperature at the gas turbine inlet is kept down, by the control system described below, to a value which just allows the gas turbine to drive the turbocompressor of the pressure-charging set. As a result, with the exception of the bearing and radiation losses, the entire heat of expansion of the gas turbine passes to the compressor so that the pressure-charging set has the effect of the exhaust-gas-heated air heater required with known kinds of plant. At the same time, however, it generates a high pressure and thus reduces the boiler heating surface area to a fraction of that in a non-pressurised boiler.

In order to be able to reduce the speed of the pressurecharging set at partial load -- which has the advantage that the combustion air rate can at any time be matched to the boiler load or fuel rate -- starter motor 9 is disconnected from the pressure-charging set by disengaging coupling 10, and stopped as soon as the pressure-charging set, after the combustor has ignited, reaches the speed required for equilibrium between compressor power and gas turbine power. This is then subject to the action of the control system. In this way the combustion air flow rate can be reduced to some 55 percent at a boiler load of approx. 50 percent, so that the excess air factor in the boiler can be kept practically constant between full and half load. This is important with respect to clean combustion and correspondingly less atmospheric pollution.

If the live steam pressure falls, for example, because the steam turboset requires a higher steam rate owing to a prolonged increase in output, the live steam regulator 12 closes drain 13 on pressure-oil line 14. The pressure in the primary system then rises and, by way of servo 17, opens the fuel nozzles 18 in the boiler. At the same time servo 19 moves the cylinder 20 of centrifugal governor 21 so that drain 22 is closed, thus raising the oil pressure in pressure-oil line 23 of the independent secondary system. The result of this is that valve 26 in bypass line 27 round the gas turbine 8 closes, and servo 28 causes valve 29, and hence bypass 30 on the gas exit side of the boiler, to open. In consequence, the flow rate through the gas turbine increases and the temperature at the gas turbine inlet is raised; thus the speed of the pressure-charging set, or the combustion air flow rate, is adapted to the new, higher fuel rate.

In the event of temporarily rising live steam pressure, i.e. falling load, the control process takes place in a similar manner, but in the opposite sense.

From the standpoint of efficiency it is of advantage not to actuate valves 26 and 29 simultaneously, but in the case of increasing demand for combustion air first to close bypass 27 round the gas turbine 8 and only then to open bypass 30 on the gas exit side of the boiler, and in the case of decreasing combustion air demand first to close 30 on the gas exit side of the boiler and then to open bypass 27 round the gas turbine 8. In this event the two control procedures can overlap to some degree, but it is also possible to leave a small neutral zone between them. These various possibilities can easily be achieved by suitably selecting the spring forces of valve 26 and servo 28, in which case the spring of the servo must be the stronger.

With this arrangement it may be most efficient to select the spring forces so that at full load both bypasses are closed. If the actual load exceeds the full load, then bypass 30 on the gas exit side of boiler 6 opens at least temporarily in order to provide the combustion air flow needed for the higher fuel flow rate as quickly as possible, and closes again when the actual load decreases. If the actual load is less than full load, bypass 27 round gas turbine 8 opens temporarily in order to match the speed of the pressure-charging set as quickly as possible to the lower fuel rate. In this way the speed or combustion air rate can be matched as quickly as possible to the fuel rate.

These sequences of events can be accelerated by using proportional controllers which respond to the rate of change, instead of ordinary controllers.

It is also possible for the pressure-charging set to alter its speed with the live steam regulator 12 at a constant setting, i.e., at constant load. This happens if the outside air temperature fluctuates very widely, for example. In this case, too, the control system performs its function in full.

If, under otherwise constant conditions the outside air temperature rises, the weight of air discharged by the compressor 7 decreases, whereupon the pressure-charging set is no longer in equilibrium, and its speed begins to fall. Oil drain 22 is closed by centrifugal governor 21 and the pressure in oil line 23 of the secondary system starts to rise. As a result, bypass valve 26 is closed and/or valve 29 is opened, whereupon the flow rate through the turbine and/or the temperature at the gas turbine inlet rises until the desired speed is regained.

The entire control system can also be so designed that the pressure in pressure-oil lines 14 and 23 decreases when the live steam pressure falls, but the effects remain the same.

Claims

I claim:

1. A combined steam and gas turbine plant comprising a pressure-fired boiler providing steam for driving a steam turbine coupled to a power consumer such as an electrical generator, means including a condenser at the discharge side of said steam turbine for converting the discharged steam into feed water and a return line for the feed water to the inlet side of said boiler for re-cycling, a heat exchanger for pre-heating the returned feed water by heat exchange with combustion gas at the gas discharge side of said boiler, a by-pass for the combustion gas passing through said heat exchanger, valve means for controlling said by-pass, the respective portions of the combustion gas put through said by-pass and through said heat exchanger being combined at the outlet thereof and delivered to the inlet of a gas turbine at a temperature variable in accordance with the portion of the gas put through said by-pass and constituting the sole motive fluid for driving said gas turbine, a live-steam regulator responsive to a change in load on the power plant for correspondingly regulating the fuel supply to said boiler and for regulating said valve means which controls the by-pass to said feed water heat exchanger thereby to increase or decrease, respectively the temperature of the discharged combustion gas and thereby increase or decrease, respectively the speed of the gas turbine and air compressor coupled thereto in dependence upon the sense of the change in load on the power plant so as to maintain a match between the required power input to said gas turbine and the required pressurized air output from said compressor.

2. A power plant as defined in claim 1 and which further includes an additional by-pass provided between the inlet and outlet sides of said gas turbine, and valve means for controlling said additional by-pass and which are likewise regulated by said live-steam regulator in such sense as to move said valve means to a more closed or open position respectively in response to an increase or decrease respectively in load on the power plant, said valve means for controlling the amount of combustion gas put through the by-pass for the heat exchanger being moved to a more open position followed by a movement of said valve means for controlling the amount of combustion gas by-passed between the inlet and outlet sides of said gas turbine to a more closed position in the event of an increase in load on the power plant, and said valve means for controlling the amount of combustion gas put through the by-pass for the heat exchanger being moved to a more closed position followed by movement of said valve means for controlling the amount of combustion gas by-passed between the inlet and outlet sides of said gas turbine to a more open position in the event of a decrease in load on the power plant.

3. A power plant as defined in claim 2 wherein both of said valve means for controlling said by-passes are closed when said power plant is operating at its full load rating.