The task of this internship is to be familiar with the thermal energy and power engineering related enterprises, mainly the main thermal system and its layout of thermal power plants. The places visited this time are power plant model room, Nanjing Xiexin Sludge Power Plant and Nanjing Steam Turbine Factory. The purpose is to make students have a systematic and comprehensive understanding of the basic structure, working principle and performance of the main production equipment in thermal power plants during the short internship period, rather than providing necessary perceptual knowledge and basic knowledge for subsequent professional courses.

Thermal power plant is a power plant that uses the chemical energy of coal, oil, natural gas and other fuels to generate electric energy, that is, the chemical energy of fuel → the thermal potential energy of steam → mechanical energy → electric energy. In a boiler, the chemical energy of fuel is converted into the heat energy of steam, in a steam turbine, the heat energy of steam is converted into the mechanical energy of wheel rotation, and in a generator, the mechanical energy is converted into electrical energy. Furnace, engine and electricity are the main equipment of thermal power plant, also known as the three main engines. The equipment that assists the three main engines is referred to as auxiliary engine for short. The main engine, auxiliary engine and their connected pipes and pipelines are called systems. The raw material of Xutang Thermal Power Plant is raw coal. Raw coal is transported to the coal storage yard of the power plant by car or ship (Nanjing Xiexin Sludge Power Plant is transported to the power plant dock by coal truck), and then transported to the coal hopper by coal conveyor belt. Then it falls from the coal hopper, and is sent to the coal mill by the coal feeder to be ground into pulverized coal, and at the same time, hot air is conveyed to dry and convey the pulverized coal. Finally, it is sent to the furnace of the boiler for combustion. The hot air required for fuel combustion is sent to the air preheater of the boiler by the blower for heating. Part of the preheated hot air is sent to the coal mill for drying and pulverized coal transportation through the air duct, and the other part is directly led to the burner and enters the furnace. Under the action of induced draft fan, the high-temperature flue gas generated by combustion first flows through the furnace, water wall tube, superheater, economizer and air preheater in turn along the inverted U-shaped flue of the boiler, and at the same time, the heat energy of the flue gas is gradually transferred to the working medium and air, which becomes low-temperature flue gas itself, and is discharged into the atmosphere after being purified by dust collector and desulfurization device. Among the ash produced after coal combustion, the big ash will be separated from the airflow due to its own weight, settle in the cold ash hopper at the bottom of the furnace to form solid slag, and finally be discharged into the ash ditch by the slag discharge device, and then pumped to the ash yard by the ash. A large number of fine ash particles (fly ash) are carried away with the flue gas and sent to the ash ditch after being separated by the dust collector. The boiler feed water first enters the economizer to be preheated to near saturation temperature, then is heated by the evaporator heating surface into saturated steam, and then is heated by the superheater into superheated steam, also called main steam. Through the above process, the transportation and combustion of fuel, the generation of steam and the treatment and discharge of combustible substances (ash and flue gas) have been completed. The main steam from the superheated steam of the boiler enters the steam turbine through the main steam pipeline to expand and do work, which drives the generator to generate electricity. The exhaust steam discharged from the turbine is discharged into the condenser, where it is condensed and cooled into water, which is called main condensate. The main condensate is sent to the low-pressure heater by the condensate pump, and some steam is pumped out by the steam turbine and enters the deaerator, where various gases (mainly oxygen) dissolved in the water are removed by continuous heating. Make-up water and main condensate treated in chemical workshop are collected in deaerator water tank to become boiler feed water, which is boosted by feed water pump and sent to high-pressure heater. A certain amount of steam is extracted from the high pressure part of the steam turbine for heating, and then sent to the boiler to complete a thermodynamic cycle of the working medium. The circulating water pump sends cooling water (also called circulating water) to the condenser to form a circulating cooling water system. Through the above process, the process of converting the thermal energy of steam into mechanical energy, electrical energy and boiler feed water is completed. Therefore, the thermal power plant is a complex power plant with energy conversion, which consists of furnace, engine, electricity and its corresponding auxiliary equipment and systems.

Two. Boiler parts

1. General overview

Boiler is one of the three main equipments in thermal power plant, and its function is to change water into high temperature and high pressure steam. Boiler is a comprehensive device of fuel combustion, heat transfer and water evaporation.

(1) Drawing of Boiler Workers in Nanjing Xiexin Sludge Power Plant

(2) Technical parameters of the boiler

Maximum continuous output of unit boiler Rated output of boiler

Superheated steam flow T/h

Outlet steam pressure MPa

Outlet steam temperature

At the steam flow rate T/h of hot steam

Steam pressure, outlet/inlet MPa

Steam temperature, outlet/inlet

Feed water temperature

2. Boiler system

(1) Steam-water system: equipment for the whole process of water heating, evaporation and overheating. It consists of economizer, steam drum, downcomer, water wall, superheater, reheater and other equipment.

(2) Wind-smoke system: equipment that is heated by wind and burned with fuel to generate smoke, release heat and discharge it into the atmosphere.

(3) Pulverizing system: equipment that grinds raw coal into pulverized coal and then sends it to the powder bin and furnace. The main components are coal mill, coal feeder and pulverized coal separator.

3. Boiler equipment structure

(1) Structure and arrangement of steam drum

Drum (also known as boiler flue) is the ultimate pressure-bearing component of natural circulation and forced circulation boilers. Without drums, there would be no circulation. The main function of the steam drum is: it is the connecting hub of the three processes of heating, evaporation and overheating of the working medium, and it is used to ensure the normal water circulation when crossing the road. Steam-water separator and continuous blowdown device are installed in the drum to ensure the normal water circulation of the boiler. There is a certain amount of water, so it has the ability to store heat, which can slow down the change speed of wind pressure and is beneficial to the operation and regulation of the boiler.

(2) Boiler downcomers, pumps and regular blowdown.

Five downcomer pipe joints are welded at the bottom of the drum, and the downcomers are installed at the bottom of the drum. Its purpose is to make the upper part of the entrance of the downcomer have the maximum water layer height, which is beneficial to the vaporization of the working medium at the entrance of the downcomer and causes the downcomer to carry steam.

(3) Structure, pipe diameter and layout of water cooling wall

The heating surface laid on the furnace wall around the furnace is usually called water cooling wall. The water walls of medium-pressure natural circulation boilers are all evaporative heating surfaces. The water wall of high-pressure, ultra-high-pressure and subcritical pressure boilers is mainly the evaporation heating surface, and a radiant superheater or radiant reheater is usually arranged on the upper part of the furnace. In once-through boiler, the water wall is not only the heating surface of water heating and evaporation, but also the heating surface of superheater, but the water wall is still mainly the evaporation heating surface.

(4) Structure and arrangement of economizer and air preheater

Economizer and air preheater are usually arranged at the end or below the convection flue of boiler. The temperature of flue gas entering these heating surfaces is low, so these two heating surfaces are usually called tail heating surfaces or low temperature heating surfaces.

Economizer is a heat exchange device that uses the heat of flue gas at the tail of boiler to heat feed water. It can reduce the exhaust temperature, improve boiler efficiency and save fuel. Because the feed water is heated in the economizer before entering the boiler evaporation heating surface, the heat absorption of water in the evaporation heating surface can be reduced, and the economizer can replace part of the evaporation heating surface. Moreover, the working medium in the economizer is water, and its temperature is much lower than the saturation temperature under the feed water pressure. In addition, in the economizer, the working medium is forced to flow and the heat transfer coefficient is high. In addition, after the feed water passes through the economizer, the temperature of the feed water entering the drum can be increased, and the temperature difference between the feed water and the drum wall can be reduced, thus reducing the thermal stress of the drum. Therefore, the role of economizer is not only to save coal, but has actually become an indispensable part of modern boilers.

The air preheater can not only absorb the heat in the flue gas, but also reduce the temperature of the flue gas, thus improving the boiler efficiency; Moreover, due to the residual heat of air, the ignition conditions of fuel are improved, the combustion process is strengthened, and the heat loss of incomplete combustion is reduced, which is particularly important for burning anthracite which is difficult to ignite. Using preheated air can increase the furnace temperature, strengthen the radiation heat transfer in the furnace, and reduce the heating surface of water wall that absorbs the same radiation heat. Preheated air with higher temperature is sent to the pulverized coal control system as desiccant. Therefore, the air preheater has also become an indispensable part of modern large-scale boiler units.

Three. Steam turbine

1. Overview of the whole machine

Steam turbine is a kind of rotary thermal machine with steam as working medium. Compared with other prime movers, it has the advantages of high power, high efficiency, stable operation and long service life.

The main purpose of steam turbine is as the prime mover of power generation. Steam turbine must be combined with boiler, generator, condenser, heater, pump and other mechanical equipment into a complete set of devices to work together. Steam with a certain pressure and temperature comes from the boiler, enters the steam turbine through the main air valve and the regulating air valve, flows through a series of annular nozzle grids and moving cascade at one time and expands to do work, and its heat energy is converted into mechanical work to drive the rotor of the steam turbine to rotate, and other machinery is driven through the coupling, that is to say, the generator does work here. In the thermal power plant, the expanded steam is introduced into the condenser by the exhaust part of the steam turbine and condensed due to the heat release of cooling water. The condensed water is pumped to the heater for heating, and then used as boiler feed water for circulation.

Steam turbines are divided into two categories according to their working principles: impulse steam turbines and reactive steam turbines.

The nozzle grid and its matching moving cascade constitute the most basic working unit "stage" in the steam turbine, and different stages are connected in series to form a multi-stage steam turbine. Steam is converted into energy in different ways in the stage, and then steam turbines with different working principles are formed, namely impulse steam turbines and reactive steam turbines.

(1) impact turbine. It is mainly composed of pulse stage, in which steam expands basically in nozzle cascade and only a little in moving cascade.

(2) reaction turbine. It is mainly composed of reaction stages, and steam has moderate expansion in both static cascade and moving cascade of steam turbine.

2. Composition and function of rotor, stator and other parts

The rotating part of steam turbine is called rotor, which is one of the most important parts of steam turbine and undertakes the task of energy conversion and torque transmission of working fluid. The working conditions of the rotor are quite complicated. It is in a high-temperature working medium and rotates at a high speed, so it bears huge profits from centrifugal force of blades, impellers and spindles and thermal stress caused by uneven temperature distribution. On the other hand, the moment of steam acting on the moving cascade is transmitted to the motor through the impeller, spindle and coupling of the rotor.

The cylinder is the shell of the steam turbine. Its function is to isolate the flow passage of the steam turbine from the atmosphere. So as to seal the air chamber by converting the thermal energy of the steam into mechanical energy. The cylinder is equipped with nozzle (stationary blade), baffle, baffle sleeve (stationary blade retaining ring), air seal and other components. They are collectively called Shizuko.

When the steam turbine is running, it rotates at high speed, and the cylinder, baffle and other static bodies are fixed, so it is necessary to leave a proper gap between the rotor and stator to avoid collision. However, the existence of the gap will lead to air exposure, which will not only reduce the efficiency of the unit, but also affect the safe operation of the unit. In order to reduce steam leakage and prevent air leakage, it is necessary to have a sealing device, usually called air seal. According to different installation positions, gas seals can be divided into circulating part gas seal, baffle gas seal and shaft end gas seal. The reactive turbine is also equipped with high, medium and secondary balance piston gas seals and low pressure balance piston gas seals.

3. Condenser and heater

The condenser uses circulating cooling water to condense the steam discharged from the turbine, establishes and maintains the required vacuum in the exhaust space of the turbine, and recovers pure condensate for boiler feed water, thus improving the thermal efficiency of the unit.

High-pressure heater uses steam extraction from steam turbine to heat boiler feed water, so as to increase feed water temperature and improve thermal economy of the unit. The high-pressure heater consists of shell, tube sheet, tube bundle, baffle and other parts. The high-pressure feed water heater is a single-row horizontal surface condensation heat exchanger, and the water chamber adopts self-sealing structure.

The high-pressure shell is of fully welded structure and consists of welded steel plates. In order to facilitate the disassembly of the shell, lifting lugs and shell rollers are installed, which can expand freely during operation. In order to prevent shell deformation, each heater with superheated steam cooling section is equipped with protective cover and baffle. All steam inlets and drain inlets (in the shell) of the heater are equipped with stainless steel impact plates to prevent the pipeline from vibration and corrosion caused by direct impact of steam and water.

The high-pressure heater consists of superheated steam cooling section, condensing section and hydrophobic cooling section. The superheated steam cooling section uses part of sensible heat of superheated steam extracted from the steam turbine to raise the feed water temperature. It is located at the flow side of the feed water outlet and sealed by the cover plate. Under the guidance of a group of baffles, superheated steam flows evenly through the pipeline at an appropriate linear speed and mass speed, and the steam is allowed to have enough superheat to ensure that the steam is dry when it leaves the section, so that when it leaves the section and enters the condensation section, it can prevent the damage caused by wet steam erosion and water erosion. In the condensation section, the latent heat of steam condensation is used to heat the feed water, and a group of baffles make the steam evenly distributed along the length of the heater to support the heat transfer tube. According to the principle of gas cooling, the steam entering this section will be automatically balanced until the saturated steam condenses into saturated condensate, which is collected at the tail or bottom of the heater. The exhaust pipe for collecting non-condensable gas must be placed at the lowest pressure of the tube bundle, and at the place where it is easy to collect non-condensable gas in the shell. The accumulation of non-condensable gases will affect effective heat transfer, thus reducing efficiency and leading to corrosion. The hydrophobic cooling part transfers the hydrophobic heat leaving the condensing part to the feed water entering the heater, thus reducing the hydrophobic temperature below the saturation temperature. The hydrophobic cooling section is located at the process side of the feed water inlet and sealed by the cladding plate. When the hydrophobic temperature decreases and flows to the next heater with lower pressure, the tendency of vaporization in the pipeline will be weakened. The cladding plate is internally separated from the integral part of the heater shell side, and a certain hydrophobic water level is maintained from the end plate and the suction or inlet end, so that this part is sealed. The hydrophobic water enters the section, is guided by a group of baffles, and is output from the hydrophobic outlet pipe.

Four. Systems and auxiliary equipment

1. pump

Pump is a kind of power equipment that converts mechanical energy into potential energy and kinetic energy of liquid pressure. It is an indispensable equipment to maintain the steam power cycle in thermal power plants, and it is also one of the main auxiliary equipment in thermal power plants. There are many places where pumps are used in thermal power plants, such as feed pumps to supply water to boilers, condensate pumps to extract condensate from condenser hot wells, and circulating pumps to provide cooling water to condensers. The water pump in thermal power plant directly or indirectly participates in the production process, and its safety directly affects the production safety of thermal power plant.

2. Fan

Fan is a kind of power equipment that converts mechanical energy into gas pressure potential energy and kinetic energy, and it is one of the main auxiliary equipment in thermal power plant. Fans in thermal power plants are mainly used in the flue gas system and pulverizing system of boilers to transport air, flue gas and wind powder mixture. , mainly including blower, induced draft fan, primary fan and powder exhaust fan.

These fans in thermal power plants directly participate in the production process, and their safety and reliability directly affect the safe production of thermal power plants. These fans also consume a lot of electricity, ranging from several hundred kilowatts to several thousand kilowatts under their shaft power, which is roughly equivalent to the water pump in thermal power plants. Therefore, we must pay enough attention to the safe and economical operation and maintenance of fans to ensure the overall safety and economy of the power plant.

Verb (short for verb) experience

Short-term cognitive practice, the school department teaches us theoretical knowledge. After the teacher's explanation and watching relevant video pictures, we have a more comprehensive understanding of the production process of boilers, steam turbines, auxiliary machines and power plants. On the morning of September 6th, we first visited the power plant model and various equipment models in the school laboratory. Then they came to Nanjing Xiexin Sludge Power Plant with small installed capacity in droves. After safety education, they were grouped. Finally, they and the master on duty started their internship seriously. Everyone abides by the rules and regulations of the power plant and the requirements put forward by the teacher. If you don't understand anything, please consult the master who took us humbly, and the masters are also very enthusiastic to answer for us. Through this internship, we not only combine the theoretical knowledge during the school with the specific production practice, but also have a deeper understanding of the production process, water treatment, coal treatment and desulfurization and dust removal process of the power plant through the explanations of the masters. Through the visit to Nanjing Xiexin Sludge Power Plant and the teachers' detailed explanation, we have a further understanding of the power generation process of thermal power plant.

In this internship, I learned a lot of knowledge that can only be obtained in practice, and learned the general situation and operation process of thermal power plants. Today, with the rapid economic development in China, electricity plays an unshakable role. Production practice is an important practical link in the university stage, and every college student should participate in it. This internship lays a foundation for better theoretical study in the future, further realizes the importance of electric power production, and fully embodies the practice-oriented characteristics of thermal energy specialty in our college.