Water circulation system for stable operation of low load of once-through boiler

Abstract

The application discloses a water circulation system for stable operation of a once-through boiler at low load, which comprises a steam-water separator, a water storage tank, a recirculation pump, a high-pressure heater, a feedwater pump, an economizer, a water-cooled wall, a superheater, a steam-water heat exchanger, a steam turbine and a reheater; the steam-water separator is connected with the water storage tank and the superheater respectively, the water storage tank is connected with the recirculation pump, the feedwater pump is connected with the high-pressure heater, the economizer is connected with the water-cooled wall, and the water-cooled wall is connected with the steam-water separator; the steam-water heat exchanger is connected with the recirculation pump, the high-pressure heater, the economizer and the steam-water separator respectively; the steam turbine is connected with the superheater; and the reheater is connected with the steam turbine and the steam-water heat exchanger respectively. The steam-water heat exchanger is used for heating the feedwater after the recirculation water is mixed with the feedwater, and the inlet temperature of the water-cooled wall is increased after the recirculation flow is increased, so that the problem of the water power multi-value of the water-cooled wall is solved, and the water power stability of the once-through boiler at low load is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power generation, and in particular to a water circulation system for stable operation of a once-through boiler at low load. BACKGROUND

[0002] To achieve the strategic goal of "carbon peak and carbon neutrality", green new energy such as wind energy and solar energy has developed rapidly in recent years. The proportion of new energy power generation in the total power generation in China is increasing year by year. As of 2021, wind power and solar power generation increased by 16.6% and 20.9% respectively. However, new energy power generation mainly based on wind energy and solar energy has problems such as intermittency, randomness and volatility. In order to ensure the safety and stability of the power grid, thermal power generation needs to be deeply peaking to reduce the impact of new energy power generation on the power grid. The existing thermal power generating units in China are mainly supercritical boilers. And the supercritical once-through boiler has the advantages of small inertia, advanced automatic control system, etc. compared with the natural circulation drum boiler. This makes the once-through boiler become the main unit in the deep peaking process. Therefore, the once-through boiler is required to work stably at low load. However, the once-through boiler has water power safety problems at low load, which may cause pipe explosion and other safety problems.

[0003] The invention patent with publication number CN109654471A improves the mass flow of the water-cooled wall by increasing the recirculation pump flow under the premise of keeping the feed water pump flow constant, to solve the water power problem of low mass flow and uneven heat load distribution of the water-cooled wall at low load of the once-through boiler. However, this will cause the problem of decreasing the dryness at the inlet of the water-cooled wall. At the same time, the patent CN109654471A replaces the traditional intermediate header with the intermediate header with a flute-shaped pipe in the patent CN108613159A to solve the problem of uneven flow distribution caused by the decrease of dryness. However, this method does not solve the problem of decreasing the temperature at the inlet of the water-cooled wall, which will increase the length of the heating section and reduce the length of the evaporation section, resulting in the decrease of the average specific volume of steam and water, thereby causing the water power multi-value problem. SUMMARY

[0004] The purpose of the present application is to provide a water circulation system for stable operation of a once-through boiler at low load, which improves the temperature at the inlet of the water-cooled wall to solve the water power multi-value problem and ensure the stability of the water power of the once-through boiler at low load.

[0005] The technical scheme of the present application is: a water circulation system for stable operation of a once-through boiler at low load, comprising a steam-water separator, a water storage tank, a recirculation pump, a high-pressure heater, a feedwater pump, an economizer, a water-cooled wall, a superheater, a steam-water heat exchanger, a steam turbine, and a reheater; the steam outlet of the steam-water separator is connected with the water storage tank, the steam outlet of the steam-water separator is connected with the steam inlet of the superheater, the water storage tank is connected with the water inlet of the recirculation pump, the water outlet of the feedwater pump is connected with the water inlet of the high-pressure heater, the water outlet of the economizer is provided with the water-cooled wall, the wet steam outlet of the water-cooled wall is connected with the wet steam inlet of the steam-water separator; the water inlets of the steam-water heat exchanger are respectively connected with the water outlet of the recirculation pump and the water outlet of the high-pressure heater, the water outlet of the steam-water heat exchanger is connected with the water inlet of the economizer, and the steam outlet of the steam-water heat exchanger is connected with the wet steam inlet of the steam-water separator; the high-pressure cylinder steam inlet of the steam turbine is connected with the steam outlet of the superheater; the steam inlet of the reheater is connected with the high-pressure cylinder steam outlet of the steam turbine, and the steam outlet of the reheater is connected with the steam inlet of the steam-water heat exchanger.

[0006] Preferably, the water outlet of the steam-water heat exchanger is provided with an enthalpy difference measuring module, the enthalpy difference measuring module comprises a thermometer and a pressure gauge, and the steam outlet of the reheater is provided with a reheated steam extraction regulating valve.

[0007] Preferably, the water outlet of the steam-water heat exchanger is provided with a first isolation valve, and the reheater and the reheated steam extraction regulating valve are respectively provided with a second isolation valve and a temperature and pressure reducing valve.

[0008] Preferably, the water outlet of the recirculation pump is provided with a first regulating valve, and the water outlet of the feedwater pump is provided with a second regulating valve.

[0009] Compared with the prior art, the present application has the following beneficial effects:

[0010] 1. The present application uses the steam-water heat exchanger to heat the feedwater mixed with the recirculated water, and after the recirculation flow rate is increased, the inlet temperature of the water-cooled wall is increased, thereby solving the problem of multiple values of the water power of the water-cooled wall and ensuring the stability of the water power of the once-through boiler at low load.

[0011] 2. The present application uses the deviation of the enthalpy value of the inlet feedwater of the economizer from the target enthalpy value to control the extraction amount of the reheated steam, thereby improving the efficiency and flexibility of the system. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 The figure is a structural schematic diagram of the present application.

[0013] Mark explanation:

[0014] 1, steam-water separator; 2, water storage tank; 3, recirculation pump; 4, first regulating valve; 5, second regulating valve; 6, high-pressure heater; 7, feed water pump; 8, steam-water heat exchanger; 9, enthalpy difference measurement module; 10, first isolation valve; 11, coal economizer; 12, water-cooled wall; 13, superheater; 14, steam turbine; 15, reheater; 16, second isolation valve; 17, temperature and pressure reducing valve; 18, reheated steam extraction regulating valve. DETAILED DESCRIPTION

[0015] The specific embodiments of the present application will be described below in conjunction with the accompanying drawings. Figure 1 The specific embodiments of the present application will be described below in conjunction with the accompanying drawings.

[0016] The terms "first", "second", "third", etc. are used only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features; in the description of the application, unless otherwise specified, the meaning of "a plurality of" is two or more.

[0017] Example 1

[0018] As Figure 1As shown, the embodiment of the present application provides a water circulation system for stable operation of a once-through boiler at low load, which comprises a steam-water separator 1, a water storage tank 2, a recirculation pump 3, a high-pressure heater 6, a feedwater pump 7, an economizer 11, a water-cooled wall 12, a superheater 13, a steam-water heat exchanger 8, a steam turbine 14 and a reheater 15. The drain outlet of the steam-water separator 1 is connected with the water storage tank 2, the steam outlet of the steam-water separator 1 is connected with the steam inlet of the superheater 13, the water storage tank 2 is connected with the water inlet of the recirculation pump 3, the water outlet of the feedwater pump 7 is connected with the water inlet of the high-pressure heater 6, the water outlet of the economizer 11 is provided with the water-cooled wall 12, and the wet steam outlet of the water-cooled wall 12 is connected with the wet steam inlet of the steam-water separator 1. The water inlets of the steam-water heat exchanger 8 are respectively connected with the water outlet of the recirculation pump 3 and the water outlet of the high-pressure heater 6, the water outlet of the steam-water heat exchanger 8 is connected with the water inlet of the economizer 11, and the steam outlet of the steam-water heat exchanger 8 is connected with the wet steam inlet of the steam-water separator 1. The high-pressure cylinder steam inlet of the steam turbine 14 is connected with the steam outlet of the superheater 13. The steam inlet of the reheater 15 is connected with the high-pressure cylinder steam outlet of the steam turbine 14, and the steam outlet of the reheater 15 is connected with the steam inlet of the steam-water heat exchanger 8.

[0019] Further, the water outlet of the recirculation pump 3 is provided with a first regulating valve 4, and the water outlet of the feedwater pump 7 is provided with a second regulating valve 5.

[0020] The working principle is as follows. When the once-through boiler is reduced to below 30% BMCR, the once-through boiler will be converted from dry state to wet state. At this time, the steam-water separator 1 will not only be a channel, but also perform steam-water separation. The qualified steam separated by the steam-water separator 1 is heated by the heater and sent to the steam turbine 14 to do work, and the drain will flow to the water storage tank 2, and then to the economizer inlet through the recirculation pump 3, so as to make the recirculation pump 3 and the feedwater pump 7 coordinate with each other and maintain the mass flow of the water-cooled wall. If the once-through boiler is to be stably operated at low load, the capacity of the recirculation pump 3 needs to be increased, the recirculation flow needs to be increased, and then the mass flow of the water-cooled wall needs to be increased.

[0021] The increase of the recirculation flow will cause the decrease of the water-cooled wall inlet temperature, which is easy to cause the hydrodynamic multi-value, and cause hidden troubles for the low load operation of the once-through boiler. In the embodiment, the steam-water heat exchanger 8 is added to increase the water-cooled wall inlet temperature, so as to solve the problem of hydrodynamic multi-value of the water-cooled wall and ensure the stability of the hydrodynamic of the once-through boiler at low load. In the embodiment, the steam on the steam side of the steam-water heat exchanger 8 comes from the reheater 15, the water side is the mixture of circulating water and feedwater, the steam on the steam side is changed into steam-water mixture after heat exchange in the steam-water heat exchanger 8, the steam-water mixture is separated by the steam-water separator 1, and the water side is heated and then enters the economizer 11.

[0022] Embodiment 2

[0023] This embodiment, based on Embodiment 1, aims to improve system efficiency and flexibility by controlling the reheat steam extraction rate using the deviation between the economizer inlet feedwater enthalpy and the target enthalpy. Therefore, an enthalpy difference measurement module 9 is installed at the outlet of the steam-water heat exchanger 8, and a reheat steam extraction regulating valve 18 is installed at the steam outlet of the reheater 15. The enthalpy difference measurement module 9 includes a thermometer and a pressure gauge. Its operation is as follows: First, the target enthalpy of the feedwater is determined based on the boiler's operating status to prevent hydrodynamic variability. Then, the temperature and pressure of the feedwater after heat exchange in the steam-water heat exchanger 8 are measured using the thermometer and pressure gauge. The measured enthalpy of the feedwater is then calculated online using an IAPWS-IF97 system. Finally, the enthalpy difference is obtained from the target enthalpy and the measured enthalpy. The opening of the reheat steam extraction regulating valve 18 is controlled by the enthalpy difference measurement module 9. When the target enthalpy is greater than the measured enthalpy, the opening of the reheat steam extraction regulating valve 18 is increased; when the target enthalpy is less than the measured enthalpy, the opening of the reheat steam extraction regulating valve 18 is decreased. In this way, the enthalpy of the economizer feedwater is always kept near the target enthalpy.

[0024] Furthermore, a first isolation valve 10 is provided at the outlet of the steam-water heat exchanger 8, and a second isolation valve 16 and a cooling and pressure reducing valve 17 are respectively provided between the reheater 15 and the reheat steam extraction regulating valve 18.

[0025] In the event of a safety incident in the system, the first isolation valve 10 and the second isolation valve 16 can cut off the working fluid and prevent further danger. The cooling and depressurization valve 17 can cool and depressurize the reheat steam, making it easier to use the reheat steam as a heat exchange source.

[0026] The above-disclosed embodiments are merely preferred embodiments of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A water circulation system for stable low-load operation of a once-through boiler, comprising a steam-water separator (1), a water storage tank (2), a recirculation pump (3), a high-pressure heater (6), a feedwater pump (7), an economizer (11), a water-cooled wall (12), and a superheater (13), wherein the condensate outlet of the steam-water separator (1) is connected to the water storage tank (2), the steam outlet of the steam-water separator (1) is connected to the steam inlet of the superheater (13), the water storage tank (2) is connected to the inlet of the recirculation pump (3), the outlet of the feedwater pump (7) is connected to the inlet of the high-pressure heater (6), the outlet of the economizer (11) is provided with a water-cooled wall (12), and the wet steam outlet of the water-cooled wall (12) is connected to the wet steam inlet of the steam-water separator (1), characterized in that, Also includes: The water inlet of the steam-water heat exchanger (8) is connected to the outlet of the recirculation pump (3) and the outlet of the high-pressure heater (6), respectively. Its outlet is connected to the inlet of the economizer (11), and its steam outlet is connected to the wet steam inlet of the steam-water separator (1). The steam turbine (14) has its high-pressure cylinder steam inlet connected to the steam outlet of the superheater (13); The reheater (15) has its steam inlet connected to the high-pressure cylinder steam outlet of the steam turbine (14) and its steam outlet connected to the steam inlet of the steam-water heat exchanger (8). The first regulating valve (4) is located at the outlet of the recirculation pump (3); The second regulating valve (5) is located at the outlet of the water supply pump (7); An enthalpy difference measurement module (9) is provided at the outlet of the steam-water heat exchanger (8). The enthalpy difference measurement module (9) includes a thermometer and a pressure gauge. A reheat steam extraction regulating valve (18) is provided at the steam outlet of the reheater (15). The target enthalpy value of the feedwater is determined according to the boiler operating status. The temperature and pressure of the feedwater after heat exchange by the steam-water heat exchanger (8) are measured by the thermometer and pressure gauge respectively. The measured enthalpy value of the feedwater is calculated online by IAPWS-IF97. The enthalpy difference is obtained from the target enthalpy value and the measured enthalpy value of the feedwater. The opening of the reheat steam extraction regulating valve (18) is controlled by the enthalpy difference measurement module (9). When the target enthalpy value is greater than the measured enthalpy value, the opening of the reheat steam extraction regulating valve (18) is increased. When the target enthalpy value is less than the measured enthalpy value, the opening of the reheat steam extraction regulating valve (18) is decreased.

2. The water circulation system for stable low-load operation of a once-through boiler according to claim 1, characterized in that, The outlet of the steam-water heat exchanger (8) is provided with a first isolation valve (10), and a second isolation valve (16) and a cooling and depressurizing valve (17) are respectively provided between the reheater (15) and the reheat steam extraction regulating valve (18).

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