EP0088756B1 - Method and installation for decreasing the losses when starting and shutting off a thermal station, and to increase the power available and to improve the control capacity in a thermal station - Google Patents

Abstract

PCT No. PCT/EP81/00204 Sec. 371 Date May 13, 1983 Sec. 102(e) Date May 13, 1983 PCT Filed Dec. 23, 1981 PCT Pub. No. WO83/01090 PCT Pub. Date Mar. 31, 1983.In a method for reducing the start-up and stabilization period losses, for increasing the usable power and to improve the controllability of a thermal power plant, there are integrated into the power plant's steam cycle pressurized heat storage reservoirs which are charged by feeding them with excess heat produced in the said power plant as, for example, during the start-up and load stabilizing periods or during periods of reduced electrical power production and, when there is an increased demand for heat, the said heat storage reservoirs are discharged by the release of stored heat into the water-steam cycle. Control deviations in the electrical power while the power plant is in full operation are counterbalanced by changes in the charging and discharging streams of the pressurized heat storage reservoirs. In the apparatus for carrying out this method, pressurized heat storage reservoirs are connected, on the water-side, to the condensate system and, on the steam side, to the medium pressure of intermediate superheater network of the steam cycle or also to the power plant's medium pressure or low pressure turbine extraction points.

Description

The invention relates to a system for reducing the start-up and shutdown losses, for increasing the usable power and for improving the controllability of a thermal power plant using one or more pressurized heat stores, the input side with steam lines for introducing start-up and shutdown steam or excess steam accumulating during operation are connected.

In conventionally operated thermal power plants, large amounts of steam have to be passed over the condenser bypassing the turbine group during the start-up and shutdown periods. Enormous amounts of heat are released unused into the atmosphere via the condenser cooling water and the cooling tower.

In the case of large power plant units in particular, the start-up and shutdown times are up to 1 hour or more depending on the state of the plant. In addition, many conventional power plant units have to be switched off regularly at weekends and at night, so that the amount of heat emitted unused during these start-up and shutdown periods makes up a significant proportion of the total thermal energy converted.

It is also perceived as unsatisfactory that a power plant block cannot be operated in a controlled manner at its performance limit, in particular the permissible heat output, since a certain power control reserve has to be kept in order to compensate for power fluctuations.

The compensation of control deviations of the electrical power of a conventional power plant block from the power setpoint can only be done with the timing of the steam generation and the limited storage capacity of the steam generator, which decisively determines the control capacity of the power plant block.

In order to compensate for load fluctuations, it is already known to provide a further condensate store in front of the boiler feed pump in addition to the feed water tank, the pressure of these stores being below the pressure of the feed water tank. However, the capacity of these memories is low. The introduction of steam is not possible.

GB-A No. 709888 describes a system in which, in addition to the feed water tank, a second, closed feed water tank is provided, the initially cold condensate filling of which is heated up to operating temperature exclusively during the start-up period by introducing start-up steam while continuously circulating the condensate. After the power plant has started up, the now hot condensate filling of the tank is discharged into the feed water tank and replaced again with cold condensate.

In this system, a small part of the start-up steam can be used to heat additional condensate. However, loading and unloading during load operation and thus an improvement in the control capability is not possible.

From DE-B No. 1128437 a system with a pressure heat accumulator for storing excess heat accumulating in the power plant for a short time is known. The charging current to the storage is either behind the high-pressure preheaters of the steam cycle of the power plant, i.e. H. branched off after the pressure step by the boiler feed pump or, according to a further circuit option, via separate high-pressure preheaters, an additional high-pressure pump being required. In any case, the storage takes place at the highest pressure and temperature level.

The discharge current is returned in the feed water tank, ie at a point with a significantly lower pressure level. This requires a very complex, three-stage relaxation in DE-B No. 1128437. This has poor thermodynamic efficiency.

The storage medium is first pressurized and stored, when it is unloaded it is first relaxed again and then has to be pressurized again. This results - in addition to the poor thermodynamic efficiency - for the high-pressure pump, a higher required output with a larger pressure flow quantity. The excess heat extracted from the medium-pressure withdrawals is first transferred from the extraction steam to the condensate in the high-pressure preheaters, which then flows first into the feed water tank and from there through the high-pressure preheater, where it is heated up again against further extraction steam, into the storage tank. This means a further increase in the flow rate for the high-pressure pump and losses due to the double heat exchange.

The introduction of steam directly to load the store is not provided for in the installation of DE-B No. 1128437.

The object of the present invention is to provide a system which avoids these disadvantages and which makes it possible in a simple manner to reduce the start-up and shutdown losses of a thermal power plant and, at the same time, to increase the usable power during operation and to improve the controllability of the thermal power plant.

This object is achieved according to the invention in a system of the type mentioned at the outset in that the pressure heat accumulators are connected on the water side behind a last medium-pressure low-pressure preheater via a charging line to a condensate line leading to the feed water tank and via a discharge line and a pump to the condensate line or the feed water tank.

By connecting the water-side charging line behind the last medium-pressure low-pressure preheater, i.e. before the pressure step, the pressure level of the heat accumulator can be freely selected within wide limits and only needs to be insignificant Lich above the pressure level of the feed water tank.

The high-pressure pump and high-pressure preheater are not touched directly by the storage system. Extraction steam as well as start-up and shutdown steam are immediately, ie. H. Heat and heat transfer medium, introduced into the storage, without intermediate heat exchange.

The storage of hot condensate is discharged forward in the direction of flow of the steam cycle and in particular without substantial relaxation and the associated thermodynamic losses.

The pressure heat accumulators are charged with start-up steam or shutdown steam of the power plant during the start-up and shutdown processes. During periods of high load or periods of increased power demand, they return their charging energy to the steam cycle of the power plant.

By changing the charging and discharging current of the pressure heat accumulator, control deviations in the electrical power can be compensated, at least in part. As a result, the power reserve of a power plant block that is necessarily to be maintained can be reduced by the regulating capacity of the pressure heat accumulator and the nominal block power can be increased accordingly.

A pressure relief vessel is advantageously connected between the pressure heat accumulator and the feed water container, in which, if the heat accumulator is operated at a higher pressure than the feed water container, the storage medium is expanded to the pressure of the feed water container and the same thermodynamic states of discharge current and feed water container content are set.

If the discharge current with the enthalpy of the storage content is introduced directly into the feed water tank or into the condensate line leading to it, then the discharge current and thus the improvement of the control capacity of the power plant are limited due to the different thermodynamic states of the discharge current and the feed water tank content.

Further explanations of the invention can be found in the exemplary embodiment shown schematically in the figure. In the power plant block shown by way of example in the figure, the steam flows successively through a high-pressure turbine 31, an intermediate superheater 34, a medium-pressure turbine 32 and a double-flow low-pressure turbine 33 passed and from there via a feed water pump 7 back into the steam generator. 3 designates a shunt-type condensate store. A pressure heat accumulator 21 is connected to the condensate system on the water side via lines 23, 26 and a pump 22 in a shunt. In the example shown, a pressure line after the discharge pump 22 opens into a condensate line 30 between the last medium-pressure low-pressure preheater 4n and upstream of the feed water tank 6. The pressure line can, however, also lead directly into the feed water tank 6.

On the steam side, the pressure heat accumulator 21 is once via a line 27 with the medium pressure or reheater network of the power plant block and / or with other, economically suitable steam networks and steam systems with a higher steam pressure than that prevailing in the pressure heat accumulator 21, e.g. B. with a removal 28, which also supplies the feed water tank 6 with steam. To charge the pressure heat accumulator 21 during arrival or departure, steam from the medium-pressure reheater network is introduced via line 27, possibly with the interposition of a reducing station, into the pressure heat accumulator 21, which is pre-filled with cold condensate, and the condensate filling is heated.

In the power range, in low or partial load periods, the pressure heat accumulator 21 is charged with hot condensate via the low-pressure medium-pressure preheaters 4a to 4n, and the hot condensate stream from the same withdrawal 28, which also supplies the feed water tank 6 with steam, in a mixed preheating unit, not shown in the figure. and degassing stage immediately before the pressure heat accumulator 21 warmed up.

To discharge, hot condensate from the pressure heat accumulator 21 is mixed via the line 26, the expansion vessel 24 and the discharge pump 22 with the condensate flowing in the line 30 to the feed water tank 6.

If the pressure heat accumulator 21 is operated at times with increased pressure relative to the feed water container 6, the hot accumulator discharge current in the expansion vessel 24 can be expanded to the pressure in the feed water container 6 and introduced into the condensate line 30.

The flash steam is led via a line 35 directly into the feed water tank 6 or into a steam line 25 leading to the feed water tank 6.

In this way, the same thermodynamic states of discharge current and feed water tank content are achieved.

In a simplified circuit, the expansion vessel 24 and the line 35 can be omitted, and the discharge current can be conducted directly into the condensate line 30 with the enthalpy of the pressure heat storage content. However, this limits the discharge current in the lower load range, since the pressure in the pressure heat accumulator 21 is greater than in the feed water tank 6. In this simplified thermal circuit, a control safety circuit is therefore necessary which prevents evaporation in the condensate line 30 and at the feed water tank inlet.

Through the use of the loading and unloading currents of the pressure heat accumulator 21 as control currents in a power control, the lei control operation of the power plant occurring control deviations of the electrical power from the power setpoint in the power control range offered can be easily and quickly corrected.

Claims (2)

1. An installation for decreasing the losses when starting and shutting down a thermal power station and for increasing the power available and improving the control capacity of a thermal power station, using one or more pressure heat regenerators, which at the input end are connected to steam pipes for the introduction of starting and shutting- down steam or surplus steam occurring during operation, characterised in that at the water side, following a final intermediate pressure/low-pressure preheater (4n), the pressure heat regenerators (21) are connected to a condensate pipe (30) leading to the feedwater tank (6) through a charging pipe (23) and are connected to the condensate pipe (30), or the feedwater tank (6), through a discharge pipe (26) and a pump (22).

2. An installation as claimed in Claim 1, characterised in that an expansion tank (24) is inserted into the discharge pipe (26) and in that, at the steam side, the expansion tank is connected to the vapour space of the feedwater tank (6), or to a vapour connection pipe (25) which leads to the feedwater tank (6).

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