Production process and operation characteristics of thermal power plants and hydropower plants

Production process and operation characteristics of thermal power plants and hydropower plants
  1. Thermal Power Plant

A thermal power plant is a factory that converts the chemical energy of fuels such as coal, natural gas and heavy oil into electrical energy, among which coal-fired power plants are the main ones. At present, in China’s power system, the installed capacity of thermal power plants accounts for about 70% of the total installed capacity.

The energy conversion process in a thermal power plant is as follows: the demineralized water (soft water) enters the boiler, and the pulverized coal is burned in the boiler to heat the water into steam (fresh steam), thereby converting the chemical energy of the coal into the thermal energy of the steam. The steam converts thermal energy in the steam turbine into mechanical energy (kinetic energy) for the rotation of the steam turbine. The generator rotates under the drive of the steam turbine and converts the mechanical energy into electrical energy. The flow of steam entering the steam turbine is controlled by a regulating valve, which is called a speed regulating valve. The speed control system of the steam turbine operates the speed control valve according to the rotational frequency of the steam turbine to ensure the stability of the rotational frequency of the steam turbine and thus the electrical frequency of the generator (50Hz in China).

The steam (spent steam) after doing work enters the condenser, and the circulating water pump pushes cold water dozens of times of steam into the condenser to absorb the heat of the spent steam (the two are not mixed), cools the spent steam into condensed water, and then It is pumped into the deaerator through the condensate pump. The function of the deaerator is to remove the air dissolved in the water, so as to prevent the oxygen in it from corroding metal equipment at high temperature. The deaerated water is pumped into the boiler by the feed pump. The above is called the steam power cycle of a thermal power plant.

Improving the pressure difference and temperature difference between the fresh steam and the exhausted steam entering and leaving the steam turbine can improve the heat-to-power conversion efficiency of the steam turbine, which is one of the important means to improve the thermal efficiency of thermal power plants. In order to reduce the heat loss in the condenser, a part of steam can be extracted from the middle of the steam turbine (intermediate extraction steam) for the heating of condensed water. heating. Thermal power plants that do not supply heat to off-site users are called condensing thermal power plants. Thermal power plants that supply heat to off-site users are called thermal thermal power plants, or thermal power plants for short. Thermal power plants in cold regions are generally thermal power plants.

The exhaust gas from coal burning in thermal power plants produces the Earth’s greenhouse effect in the atmosphere, hence the name Greenhouse Gas. Limiting the emission of greenhouse gases has also become a major issue for global environmental protection. Therefore, gradually reducing the proportion of thermal power plants in the power system is the direction of power development. The ratio of the annual power generation of the power plant to the rated capacity of the generator is defined as the annual maximum load operating hours, usually denoted as Tmax. Its expression is:

In the formula, Tmax is the annual maximum load operating hours (h); WY is the annual power generation (kW·h); PN is the rated capacity of the generator (kW).

The operating characteristics of thermal power plants are:
(1) Slow start and stop It takes a long time to gradually heat up the boiler ignition furnace and piping equipment, and it takes about 3~6 hours for the thermal power unit to start up to full load, so the thermal power unit cannot start and stop frequently.
(2) There is a minimum load limit. Because the combustion of the boiler is unstable under low load, the exhaust temperature of the steam turbine increases under low load, resulting in tail deformation and vibration. Therefore, it is generally limited to operate under a load of more than 70%, so that the thermal power unit can participate in the operation. The ability of power system load regulation is limited. When the minimum load of power system users is lower than 70%, the pure thermal power generation system is not convenient for operation and scheduling.
(3) High annual maximum load operating hours Because the fuel is not affected by seasonality, its power generation is not limited by seasonality, and can be operated at high load throughout the year, so the annual maximum load operating hours is high, generally 5000h In terms of soil, this is one of the important reasons why thermal power plants have become the main power generation in the power system.

  1. Hydropower plant

A hydropower plant is a plant that converts the potential energy of water into electricity. The water turbine converts the potential energy of the water into kinetic energy, which then drives the generator to rotate and then converts it into electrical energy.
The difference in height between the upstream level and the downstream level (i.e. the tail level) is called the head. The power generated by the hydro-generator is proportional to the water head and flow. Taking into account the energy conversion efficiency, the approximate formula of the generated power is:

It can be seen from equation (1-2) that water head and water flow are two necessary conditions for hydropower generation. According to the different ways of obtaining water head, hydropower plants are divided into two categories: dam type (dams with large height) and water diversion type.

  1. Embankment-filled hydropower plant
    According to the location of the power plant, there are two types of dam-type hydropower plants.
    After the dam – the power plant is behind the dam.
    River bed type – the power plant is on the river bed beside the dam.
    Figures 1-3 show the structure of the post-dam hydropower plant.
Figure 1-3 Schematic diagram of the structure behind the dam
Figure 1-3 Schematic diagram of the structure behind the dam

The rotating part of the turbine is called the runner, the energy released by the pressure water in the runner is converted into the kinetic energy of the unit rotation, and the effluent after power generation is called tail water.
When the runner of the water turbine is moving in water, the runner is damaged due to fluid mechanics, which is manifested as spot-like shedding of the metal on the surface of the runner, which is called cavitation. In order to reduce cavitation, large-capacity turbines bury the runner in the tail water, and the height of the runner centerline below the tail water level is called the siphon height (Hs). Therefore, the water inlet and outlet channels of the turbine are equipped with gates (7 and 8 in Figure 1-3). The gates on both sides should be closed when the turbine is overhauled, and the gates operate slowly (measured in minutes).

The flow of water into the runner is also controlled by a regulating valve called a guide vane (not shown in Figures 1-3). The opening of the guide vane is continuously adjustable and the action speed is fast (measured in seconds). The function of the water guide vane is similar to that of the speed regulating valve in the steam turbine: the speed regulating system of the water turbine operates the water guide vane according to the rotational frequency of the water turbine to ensure the stability of the rotational frequency of the water turbine, thereby ensuring the electrical frequency of the generator. Stablize.

  1. Diversion type hydropower plant
    Figure 1-4 is a schematic diagram of the water flow system of the diversion type hydropower plant, which uses the slope of the river to obtain the water head. The main function of the dams in this system is to facilitate the intake of water from the diversion channels, not to capture the water head, so the height is low.
Figure 1-4 Schematic diagram of the water flow system of the diversion type hydropower plant
Figure 1-4 Schematic diagram of the water flow system of the diversion type hydropower plant

For the atmosphere, the water diversion channel is open, and it diverts water downstream with a small slope (about 1% to 2%). The pressure water pipe is closed, and the function of the pressure front pool is to ensure that the water in the pressure water pipe does not mix with air, so as to avoid water impact. Compared with the power plant on the bank of the river, the water in the pressure forepool has a larger potential energy, and is led to the turbine to generate electricity through the pressure pipeline.

Compared with thermal power plants, hydropower plants have large hydraulic construction projects, long station construction time, large investment per kilowatt, but low power generation costs.
On a river, multiple hydropower stations are often built, which is called cascade development. The cascade power stations are sorted from top to bottom. The tail water of the upper power station is the main water source of the lower power station. How to make the best operation of the cascade power station considering the comprehensive benefits of shipping and power generation is a very important technical and economic issue, which is called Operation scheduling optimization of cascade power stations.

In contrast to thermal power plants, the operational characteristics of hydropower plants are:
1) Start and stop quickly. It only takes a few minutes for the hydroelectric unit to start up to full load. The speed of the emergency shutdown of the hydroelectric unit depends on the additional force induced in the pressurized water pipe when the water flow is shut off. The faster the closing, the greater the additional force, and the minimum closing time should be calculated by the hydraulic staff.
2) No minimum load limit. Therefore, it is often used to balance the changing part of the load, even out of operation when the load is low, and put into operation at the peak, which is called peak shaving.
3) Since the incoming water is affected by the season, the power generation is limited by the season, which can be divided into the wet season and the dry season. The power generation in the dry season is about 30% of that in the wet season, and the maximum load operating hours throughout the year is low, generally 1500~3000h. The designed installed capacity of a few runoff power stations on large rivers is often small, so it can reach 5000h.

The main body that determines the operating state of the power system is the electricity load, and the power generation must track the change of the electricity load power at any time to maintain the stability of the system frequency and voltage. Obviously, the power generation system with both water and fire has good tracking ability, so it is convenient for operation and scheduling. Generally, the installed capacity of hydropower should account for about 30% of the total system capacity.

In the power grid with a small proportion of hydropower installed capacity and insufficient adjustment capacity, a pumped storage power station can be built. The difference between a pumped-storage power station and an ordinary hydropower station is:
1) In addition to the upstream reservoir in the hydraulic building, the downstream reservoir is also built to store tail water.
2) In a large pumped storage power station, the pumping unit and the generator are combined into one. That is, the prime mover is both a turbine and a pump, and the motor is both a generator and an electric motor.
There are two modes of operation of a pumped-storage power station:

(1) Power generation method The water flows from the upper reservoir to the lower reservoir, and the unit rotates forward. As a hydro-generator unit, the potential energy of the water in the upper reservoir is converted into the electrical energy output by the generator.
(2) Water-lifting method The motor absorbs the power of the grid, and the unit is reversed. As an electric water aggregation unit, the water is lifted from the lower reservoir to the upper reservoir, and the electric energy absorbed by the motor is converted into the potential energy of the water flowing from the lower reservoir to the upper reservoir. A pumped-storage power station can be regarded as a large-capacity battery made of the potential energy of water.

The water of the pumped storage power station is reused, and the natural inflow water is only used to supplement the loss of water. Therefore, it is less affected by hydrological conditions, and is generally built near the load center or nuclear power plant to reduce the loss of power transmission.

The construction of a pumped-storage power station can achieve both technical and economic benefits:
(1) Technical benefits It is convenient for the dispatching and operation of the power system: generating electricity in the peak load period to ensure that users do not have power outages; pumping water in the low load period to ensure that the thermal power plant does not operate. Due to the large capacity of nuclear power plants, in order to ensure their safe and stable operation, the power generation is generally kept constant, so it is often necessary to build a pumped storage power station.

(2) Economic benefits In the electricity market, the electricity price (peak-valley electricity price) between the peak load period and the trough period is doubled. The pumped storage power station buys at the trough and sells it at the peak, which can achieve great benefits. economic benefits. The efficiency of the pumped storage power station is about 65%, that is, the water pumped with 1 kWh of electricity can generate 0.65 kWh of electricity, and the peak-to-valley electricity price difference is much higher than this ratio.

Read more: Transmission layer and substation layer of power transmission system

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