BOILER SINK

Description

[0001] The invention relates to a method for rinsing a waste heat steam generator of a power plant.

[0002] Stationary gas turbines and methods for operating them are extensively known from the available state of the art. Modern gas turbines used for generating electricity typically have an axially flowing compressor, one or more combustion chambers, and a turbine unit. During operation, fuel supplied to the combustion chamber is combusted with the ambient air compressed by the compressor to produce a hot gas, which expands in the turbine unit at the rotor of the gas turbine, performing work. The rotor then drives a generator, which converts the mechanical energy into electrical energy with minimal loss and feeds it into a power distribution network.

[0003] As is well known, short start-up times of gas turbines, especially in power plants for covering peak loads, are playing an increasingly important role in the gas turbine market, as short start-up times are financially relevant for power plant operators (availability). EP 2 644 839 A1 discloses an example of a method for fast gas turbine start-up, according to which, during start-up of the gas turbine, the generator is connected to the electrical grid via the frequency converter before the gas turbine reaches operating speed, with the frequency converter being controlled in such a way that it generates an output current at the grid frequency.

[0004] In order to utilize the hot exhaust gases of the gas turbine, a waste heat steam generator (boiler) is connected downstream of the gas turbine in a gas and steam turbine plant, the heating surfaces of which are connected in the water-steam cycle of a steam turbine.

[0005] For such plants, it is typically necessary to perform a so-called boiler purge before igniting the gas turbine. During this process, the gas turbine is brought up to a specific purge speed and purged with fresh air until it is ensured that all explosive fuel mixtures have been flushed from the heat recovery steam generator. The boiler purge process is also referred to as ventilation or pre-ventilation, but the term purge is more accurate. Explicit specifications and regulations exist for this procedure. In particular, after a trip (unplanned or emergency shutdown) has been triggered, the exhaust system and heat recovery steam generator must be purged before the next gas turbine start. German patent DE 199 59 342 A1 discloses a combined cycle power plant in this context and states that it is common practice and mandatory for safety reasons to purge the heat recovery steam generator with air before restarting the plant after an operational interruption.Such a boiler purge, including acceleration and shutdown, takes approximately 15 minutes, depending on the boiler volume. As with pure gas turbine plants, short start-up times are also financially relevant for combined cycle gas turbine plants. Document US2018 / 058337 A1 discloses a method for improving the shutdown purge flow in a gas turbine system.

[0006] Another aspect of the invention is that after a gas turbine shutdown from load at rated speed, the exhaust gas temperature drops with a very steep gradient. If this is a trip, boiler purging is mandatory before the next gas turbine start. In the case of a hot start, this increases the stresses in the gas turbines and on the thick-walled components of the heat recovery steam generator because relatively cold ambient air is drawn in for purging and blown through both the gas turbine and the heat recovery steam generator. Furthermore, the gas turbine casing cools down faster than the rotor. This leads to smaller gaps in the gas turbine, which can result in clearance bridging (rubs). In the medium term, this impairs the gas turbine's sealing properties due to gap enlargement, and a loss of efficiency is to be expected.This problem could be solved by increasing the cold gap in the gas turbine, but this would lead to lower efficiency. Furthermore, the waste heat steam generator would have to be designed with higher-quality materials and therefore be more expensive.

[0007] The object of the invention is to provide a method for rinsing a waste heat steam generator of a power plant.

[0008] The invention solves this problem by providing that, in such a method for purging a waste heat steam generator of a power plant with a gas turbine comprising a rotor, a generator coupled to the gas turbine, and a starting converter for converting an alternating current of any frequency into an alternating current of a predetermined frequency, wherein the starting converter is connected to an output of the generator at which the alternating current of any frequency is present, when the gas turbine is shut down, the rotor is stopped at a boiler purge speed by means of the starting converter, and the waste heat steam generator is purged until the purge parameters are met. The boiler purge speed is to be understood as the speed at which the gas turbine is operated during purge operation, i.e.,The rotational speed is sufficiently high to ensure the required volume exchange in the waste heat steam generator, as stipulated by standards. This ensures the safe removal of any remaining unburned fuel and prevents, for example, unwanted auto-ignition. In other words, this means that the rotor, starting from a rated speed, gradually slows down as the gas turbine shuts down, without falling below the boiler purge speed for a predetermined duration. Instead of the conventional boiler purge after a trip and before restarting, it is possible to utilize the purge volume during the gas turbine shutdown (starting from rated speed) under the following conditions: Due to relevant safety regulations, the fuel system typically includes a gas lock. A gas lock comprises two fittings, such as ball valves, which open or close to allow gas flow. An intermediate relief valve or pressure line is connected between these two fittings. This gas lock is closed (this happens after a few seconds). The gas turbine is running above the boiler purge speed. The air intake and chimney damper are open.

[0009] Preferably, once a boiler purging speed of the gas turbine is reached, the gas turbine speed is repeatedly ramped up and down, but without falling below the boiler purging speed. This avoids the gas turbine remaining in critical frequency ranges (natural frequencies) for extended periods.

[0010] Preferably, the gas turbine speed is repeatedly ramped up and down until a minimum duration for a purging cycle is reached. It is advisable to comply with relevant (international) standards, such as the minimum purging cycle duration, which is typically 5 minutes, as a certain volume exchange in the boiler is necessary for successful completion of the purging process (for example, the NFPA 85 standard specifies the requirement of "at least 5 minutes"). To ensure this, when the gas turbine is shut down, the rotor can be stopped at the boiler purging speed using the start-up inverter and purged until the aforementioned requirements are met.

[0011] It is advisable to keep the gas turbine speed above the boiler purging speed, as it too is standardized for the purging process and is typically 8% of the rated speed.

[0012] The invention utilizes the shutdown of the gas turbine as a boiler purge. Furthermore, the start-up converter is used during the gas turbine shutdown to design the boiler purge in such a way that all standard requirements are met in the shortest possible time. In addition, the inventive method avoids purging still-hot components of the gas turbine and heat recovery steam generator with comparatively cold ambient air.

[0013] This method improves start-up availability, extends component lifespan and provides the power plant with a performance advantage through smaller design gaps or fewer "grazing" incidents in the gas turbine.

[0014] The invention is explained in more detail using the figures as examples. They are shown schematically and not to scale: Figure 1 : a gas turbine connected to an electrical supply network and Figure 2: the gas turbine speed during the shutdown process according to the invention.

[0015] The Figure 1 Figure 1 shows a gas turbine 1 with compressor 2, combustion chamber 3 and turbine 4 with rotor 12. In the exemplary embodiment of the Figure 1 Compressor 2 and turbine 4 are arranged on a common shaft 5. Also arranged on the shaft 5 is a generator 6, which is connected to a power supply network 8 via a starting converter 7.

[0016] Figure 2 schematically and exemplarily shows the gas turbine rotational speed over time when shutting down a gas turbine 1 according to the inventive method.

[0017] After combustion is shut down, the rotational speed of gas turbine 1 decreases, with the rate of change of rotational speed also decreasing over time. Without further external intervention, a lower rotational speed limit 9 relevant for scavenging processes is reached, which is specified in the Figure 2The value shown is at 8% of the rated speed, is undershot at approximately 300s and continues to fall (curve with reference numeral 10).

[0018] Another criterion for sufficient purging is its duration at a rotational speed above the lower speed limit 9. This duration is 5 minutes. However, since the start of purging is not immediately visible at t=0s, but a few seconds later (the figure shows a rather conservative assumption), because a gas valve in the fuel supply must first be closed, the conditions (intensity and duration) for successful purging are not met at time t=300s.

[0019] This is where the invention comes into play, using the start-up converter 7 to stop the gas turbine 1 and maintain the rotational speed above the lower speed limit 9 relevant for the purging process. Figure 2 This is shown in curve 11 for an ideal rinsing process with start-up converter 7.

[0020] The rotational speed of gas turbine 1 is maintained above the lower speed limit 9 until a minimum duration for the purging process is reached. In the Figure 2 This minimum duration is defined by the times t1 and t2. The area under curve 11 and between t1 and t2 represents the flushing volume.

[0021] The Figure 2 The figure further shows that the rotational speed of gas turbine 1 is modulated (by the starting converter 7). This is done to prevent natural frequencies from building up and damaging the system.

Claims

1. Method for flushing a heat recovery steam generator of a power plant with a gas turbine (1), comprising a compressor (2), a combustion chamber (3) and a turbine (4) with a rotor (12), and with a generator (6) coupled to the gas turbine (1) and a start-up converter (7) for converting from an alternating current of any frequency into an alternating current of a specified frequency, wherein the start-up converter (7) is connected to an output of the generator (6) at which the alternating current of any frequency is applied, characterized in that, when the gas turbine (1) is run down, the rotor (12) is supported with the aid of the start-up converter (7) at a boiler flushing speed and the heat recovery steam generator is flushed further until the specifications for flushing the heat recovery steam generator are satisfied.

2. Method according to Claim 1, wherein, after reaching a boiler flushing speed of the gas turbine (1), a gas turbine speed is repeatedly raised and lowered.

3. Method according to Claim 2, wherein the gas turbine speed is repeatedly raised and lowered at least until a minimum duration for a flushing process has been reached.

4. Method according to Claim 3, wherein the minimum duration is 5 minutes.

5. Method according to one of the preceding claims, wherein the boiler flushing speed is 8% of a nominal speed of the gas turbine.