What Is a Thermal Power Plant?

A thermal power plant, referred to as a thermal power plant, is a plant that uses combustibles (such as coal) as fuel to produce electricity. Its basic production process is: when fuel burns water to generate steam, the fuel's chemical energy is converted into thermal energy, the steam pressure drives the turbine to rotate, the thermal energy is converted into mechanical energy, and then the turbine drives the generator to rotate, converting mechanical energy into electrical energy.

A modern thermal power plant is a large and complex plant that produces electrical and thermal energy.

It consists of the following 5

By fuel

1. Compared with hydropower plants and other types of power plants, thermal power plants have the following characteristics:

1. Flexible layout, the size of installed capacity can be determined according to needs.

2. The construction period is short, generally half or even shorter of a hydropower plant. One-time construction investment is small, only about half of the hydropower plant.

3. Coal consumption is large. At present, coal used for power generation accounts for about 25% of the country's total coal production. Adding coal transportation costs and a large amount of water, its production cost is 3-4 times higher than hydropower.

4. There are a lot of power equipment, the control operation of the generator set is complicated, the power consumption and operating personnel of the plant are more than that of the hydropower plant, and the operating cost is high.

5. The process of turning on and shutting down the steam turbine takes a long time and costs a lot of money, so it should not be used as a peaking power source.

6. Great pollution to air and environment. ^[3]

The fuel composition of thermal power plants is determined by national resource conditions and energy policies. After the 1980s, the fuel of China's thermal power plants was mainly coal. In 1987, 87% of the electricity generated by thermal power plants was coal-fired, and the remaining 13% was generated from other fuels. Countries that have bituminous coal resources or rely on imported coal, their thermal power plants mainly use bituminous coal because of its high calorific value and flammability. Other countries with a large proportion of coal types include lignite (Germany, Australia),

Modern thermal power plants consist of a large variety of mechanical devices and electrical equipment. In order to produce electrical and thermal energy, these devices and equipment must coordinate their actions to achieve the purpose of safe and economical production. This work is the operation of thermal power plants. In order to ensure the safe and economic operation of the main equipment such as furnaces, machines, electricity and auxiliary equipment of various systems, a series of operating rules and regulations must be strictly implemented.

The operation of thermal power plants mainly includes three aspects, namely starting and stopping operation, economic operation, failure and countermeasures. The basic requirements for the operation of thermal power plants are to ensure safety, economy and quality of electricity.

1. Before combustion: The quality of coal and the size of particles will affect the combustion efficiency and pollutants generated by combustion, so it needs to be controlled;

2. Combustion chamber: The design of the boiler (such as the way fuel and air enter the boiler) affects the combustion efficiency and the utilization of thermal energy; the material of the boiler affects the heat loss;

3. The design of the circulation system of saturated steam-condensed water vapor affects the efficiency of converting heat into mechanical energy;

4. The design of the engine affects the efficiency of converting mechanical energy into electrical energy;

5. In addition to the above, it is also necessary to consider safety (such as: the center temperature of the combustion chamber can reach 2000 °), and control of pollutants (sulfur oxides and nitrogen oxides generated by combustion). ^[3]

Thermal Power Plant Pollutants

I. Smoke

The boiler flue gas is discharged into the atmosphere through a high chimney. The main pollutants are smoke, SO _X , NO _X and CO ₂ .
(1) Soot refers to dust that has not been captured by a dust collector. Among them, the particle size is less than 10 m, which is called floating dust (TSP). It can float in the atmosphere for a long time and be transmitted for a long distance. . The interior of the dust particles is an iron / aluminosilicate glass body, and sulfates are deposited on the surface, and a variety of trace metal elements and organic compounds are enriched, thereby increasing the harm of soot.
(2) Sulfur dioxide, coal contains combustible sulfur, such as organic sulfur and pyrite sulfur, both of which can be burned. In general, the combustion product is SO ₂ . In a proper temperature range, SO ₃ can be generated under the catalysis of a certain excess of oxygen and metal oxides (such as V ₂ O ₅ ) in the flue gas. They dissolve in water to form sulfurous acid and sulfuric acid, respectively, and have strong corrosive effects. When they are discharged into the atmosphere, they will corrode machinery and equipment and buildings. They are collectively referred to as SO _X. SO _X is harmful to human health, causing bronchitis, asthma, pulmonary heart disease, and even death. In addition, it can reduce crop yields or wither vegetation.
(3) Nitrogen oxides (NO _X ). There are six kinds of nitrogen oxides. Nitrogen exists in the air. Usually, coal also contains a small amount of nitrogen about 0.5% to 2%. NO _X is a collective name for NO and NO ₂ .
During the combustion process, the nitrogen oxides produced by the combination of nitrogen and oxidation in the fuel are called fuel NOx. In addition, the nitrogen contained in the air sent by the fuel into the furnace during the combustion process will also generate a part of NOx under the high temperature conditions of combustion, which is called high-temperature NOx. It mainly contains NO. It will be oxidized to NO ₂ in the atmosphere quickly, which is 4 to 5 times more toxic than NO.

NO is highly toxic. When its content in the atmosphere reaches 4ppm, it can cause death within 5 minutes. NO ₂ reacts with OH ^- ions in the atmosphere to form nitrous acid and nitric acid, which are highly toxic and extremely corrosive. The nitrite formed by it is a carcinogen. NO ₂ can accelerate the conversion of SO ₂ to SO ₃ and can form an "aerosol" with SO _{3 to} cause photochemical smog, damage the central nervous system of humans, and the toxicity and corrosiveness of NO _X Stronger than SO _X. The harm of nitrogen oxides is that it will destroy the ozone layer in the atmosphere, which will cause ultraviolet rays to radiate the human body through the atmosphere and increase the incidence of skin cancer. NO _X is also one of the culprits causing acid rain.
(4) Carbon oxide (CO ₂ , CO)
CO ₂ , the main product of fossil fuel combustion, has always been considered to be harmless to the environment. Plant photosynthesis will absorb CO ₂ , synthesize energy, and release oxygen to form a carbon cycle. In recent years, due to excessive burning of fossil fuels and destruction of forests and vegetation, the concentration of CO _{2 in the} atmosphere has increased. In the most affected surface heat dissipation, the greenhouse effect is formed, and the atmospheric temperature rises abnormally. CO ₂ is also considered as a greenhouse gas and is harmful. In addition, the incomplete combustion product of carbon, CO, is a lethal gas.
Dust and SO _X , NO _X , N ₂ O, CO ₂ and CO are carried out by the flue gas, causing environmental pollution in the area emitted by thermal power plants. ^[4]

Second, dust

Productive dust refers to solid particles that are formed during production and can float in the air of the workplace for a long time. For thermal power plants, there are mainly coal dust floating in the working place of coal transportation system, boiler dust generated during boiler operation and contact during boiler maintenance, dry dust collector operation, dry ash conveying system, and dust in the comprehensive utilization of fly ash. , Welding dust generated by the welding operation, the lime, limestone dust and gypsum drying system produced by the wet and dry desulfurization milling and pulping system, the dust generated by the desulfurization waste slag utilization and disposal system

Silica dust generally refers to the dust of free silica. Silica dust represented by quartz is the most harmful and widely harmful occupational harmful factor in the power industry. Coal dust in thermal power plants is generally dust containing less than 10% free silica (the maximum allowable emission mass concentration specified by the state is 10 mg / m ³ ). The dust particles have a high degree of dispersion, with 73% of the diameter being less than 5 m. Boiler dust is generally dust containing 10% to 40% free SiO ₂ (the maximum allowable emission mass concentration specified by the state is 2 mg / m ³ ), and the dust particles have a high degree of dispersion, with 73% of the diameter being less than 5 m. Welding dust is a very fine aerosol formed during welding due to the high temperature melting and evaporating of the flux, the welding core and the material to be welded, and oxidized and condensed in the air. The welding aerosol re-condenses to form an extremely fine dust Particles, among which dust particles below 1 m account for more than 90%. Electric welding dust is mainly composed of iron oxides. When high manganese electrodes are used, the content of manganese dioxide in the air far exceeds the content of iron oxide. Dust from dust collectors, dry ash conveying systems, and fly ash, etc., are also dust containing 10% to 40% free SiO _2. The particle size is generally below 15 m, and a significant share is below 5 m. The dust in the desulfurization system's pulverizing system is generally dust containing less than 10% free SiO ₂ . Its main component is CaO, CaCO ₃ or other desulfurizing agents (the desulfurizing agent generally requires purity of 90% or 95%). The dust in the gypsum treatment or waste residue treatment system of the desulfurization device is generally dust containing less than 10% free SiO ₂ , and its main component is CaSO ₄ .H ₂ O or other desulfurization waste residue.

The higher the degree of dispersion of the dust, that is, the smaller the dust particle size, the higher its stability in the air, the longer the suspension in the air, the more opportunities for workers to inhale, and the greater the harm to the human body. Respirable dust can settle on the respiratory bronchial and alveolar walls. Long-term inhalation of productive dust is prone to cause systemic diseases that are mainly fibrosis of the lung tissue, namely pneumoconiosis, which is a national legal occupational disease. Among them, silicosis, coal pneumoconiosis, electric welder pneumoconiosis, asbestos lung, and cement pneumoconiosis are all pneumoconiosis mainly composed of collagen fiber hyperplasia. Long-term high-concentration inhalation of free SiO ₂ dust (that is, silicon dust) by employees may cause silicosis. Collagen fibrosis of the lung tissue is an irreversible and destructive pathological change. There is currently no way to eliminate it. Regarding the treatment of this kind of pneumoconiosis, especially silicosis, it is mainly symptomatic treatment and active prevention and treatment to reduce the patient's pain, delay the development of the disease, and strive to extend its life. More than 73% of productive dust in thermal power plants is respirable dust with a particle size of less than 5 m. Therefore, we must pay attention to the seriousness of the consequences of dust hazards, do a good job of dust prevention, prevent the occurrence of pneumoconiosis, and protect the health of employees. ^[1]

Third, waste residue

Coal-fired power plants emit a large amount of ash, which consists of several parts. Most of them are fine particles collected by the dust collector, called fly ash, and the other part is the slag collected at the bottom of the boiler combustion chamber. The two are sent to the ash yard by the mortar pump through the pressure ash removal pipe. The coal ash slag of various boilers mainly contains silicon oxide, aluminum oxide and iron oxide, and contains trace elements such as arsenic, cadmium, aluminum, and selenium.A large area of ash will occupy a large area of farmland, and it will The accumulation of dust in the field has risen and secondary pollution has occurred. Trace elements and radioactive elements can cause human body poisoning and even cause cancer. ^[4]

Fourth, wastewater

Waste water discharged from thermal power plants contains acids and bases, oils and fats, suspended matter, organic matter, eutrophication and trace elements. The sources of wastewater include chemical wastewater, wastewater containing wastewater, ash washing water and domestic sewage. Acid and alkali make the water quality of the water body gradually acidify or alkalinize, reducing the self-purification ability of the water body; the inclusion of waste water reduces the dissolved oxygen in the water body and causes fish death; the suspended matter in the ash flushing water is mainly coal ash and insoluble salts, which make the water turbid Increased degree, sedimentation in the bottom of the silt channel; organic pollutants reduce the dissolved oxygen in the water, affecting fish survival. ^[4]

Protection and operation of thermal power plant

Environmental protection facilities of power plants should include dust removal, desulfurization, denitration equipment, chimneys, fly ash anti-ash, anti-seepage facilities, wastewater treatment, recycling systems, mufflers, greening facilities, environmental monitoring systems and environmental monitoring stations. These facilities must be designed, constructed and put into operation at the same time as the main project. New and expanded power plants during the Eighth Five-Year Plan period and beyond have basically been equipped with other environmental protection facilities except for desulfurization and denitrification. Environmental protection facilities accounted for more than 10% of the total investment in the project. Is great. In power companies, the operation of major environmental protection facilities is basically synchronized with the unit, with an input rate of more than 95%.

The large investment in environmental protection facilities of coal-fired power plants has achieved significant results in pollution control, and various pollution indicators such as smoke, waste water, and noise basically meet the national emission standards.

Environmental protection measures for thermal power plants

(1) Strengthen the maintenance and management of dust-proof equipment

Improve the quality of maintenance of ash removal system and powder making system to prevent ash leakage and powder leakage. Regularly check the dust mass concentration and find that it exceeds the standard. Measures must be taken. Invest in funds to carry out technological transformation to improve the investment rate and dustproof efficiency of dustproof equipment. Maintain and manage the existing dust-proof equipment to ensure the normal investment of the equipment and play the role of dust-proof.

(2) Improve operation automation

In places where the dust is hazardous, robots or automatic control are used to achieve unattended or less-attended duty, reducing the contact time between workers and dust.

(3) Enhance self-protection awareness

Strengthen the safety education and training of workers on dust prevention knowledge, increase workers 'awareness of dust hazards and prevention and control knowledge, and enhance workers' awareness of self-protection. Power plants should purchase qualified, efficient, practical and convenient dust-proof personal products for workers. When working in a work place where there is a dust hazard, workers must properly wear dust-proof personal products in accordance with regulations and develop good habits like wearing a safety helmet.

(4) Reasonably arrange working procedures

For example, when the boiler is overhauled, it must be fully protected against dust before entering the furnace and pipeline work.

(5) Technical transformation of fly ash dry ash system

If there is a leak in the dry ash collection system of the electrostatic precipitator, it is silicon dust, which is very harmful to the human body. Measures should be taken to gradually transform simple ash extraction into mechanized and automated operations. The new plant should consider the comprehensive utilization of fly ash when designing the project, and create the necessary conditions for ash supply and ash in terms of investment, equipment purchase, site use, etc., such as coarse subdivision installation, equipped with a good storage and transportation system , Transportation vehicles, and roads for ash transportation.

(6) Do a good job of management of coal yard and ash yard

Do a good job of spraying and rolling of dry ash storage sites. The ash yards that have been filled should be rehabilitated in time, and the plant area (including around the coal yard) should be well-greened, civilized and reduced dust.

(7) Strengthen the dustproof work of the desulfurization system

As the desulfurization project is launched, the dust prevention problem of the pulverizing system, gypsum or waste residue treatment system of the desulfurization project should be considered. The limestone powder silo must have a dust collector, and the pipeline valves must be tight and leak-free. For example, the wet ball milling process can be used to make limestone slurry, which can directly make large limestone material less than 200 mm through wet milling to make limestone slurry to reduce dust pollution. ^[1]

(8) Improve coal utilization efficiency

There are three types of energy conversion processes in thermal power plants: the chemical energy of coal in a boiler is converted into thermal energy; the thermal energy is converted into mechanical energy in a steam turbine; and the mechanical energy is converted into electrical energy in a generator. The main equipment for energy conversion-boilers, turbines and generators, are known as the three main engines of thermal power plants, and boilers are the most basic energy conversion equipment among the three main engines.

The pulverized coal used for boiler combustion is irregular fine coal particles ground by a coal mill. The average particle size is 0.05 to 0.01 mm, and the majority of particles are less than 20 to 50 m (microns). Because the pulverized coal particles are small and the surface is large, they can absorb a large amount of air, and have properties that are not found in ordinary solids-fluidity. From the aspect of the pulverizing system, it is desired that the pulverized coal be ground coarsely, thereby reducing the power consumption and metal consumption of the pulverized coal. Therefore, when selecting the fineness of pulverized coal, the sum of the above losses should be minimized. The fineness of pulverized coal with a small total loss is called "economic fineness". It can be seen that the fineness of pulverized coal can be appropriately larger for coal types with higher volatile and flammable coal, or for pulverizing equipment with more uniform grinding of pulverized coal particles, and some intensified combustion boilers to save grinding Coal energy consumption and increase coal utilization.

(9) Purification technology in combustion

Clean coal technology for coal-fired power plants refers to the high-tech term for processing, conversion, combustion and pollution control in the whole process of coal from development to utilization, which aims to reduce pollution emissions and improve utilization efficiency. Purification technology during combustion refers to the combustion of fuel during combustion. The technology of improving efficiency and reducing pollution emissions in the process is an important part of clean coal technology and consists of five technologies. Advanced burner improves boiler design, adopts advanced burner to reduce pollution emissions and improve boiler efficiency. Today, there are low NO ₂ burners. During the combustion process, fuel and air are gradually mixed to reduce the flame temperature and thereby reduce NO ₂ generation; or the mixture ratio of fuel and air is adjusted to provide only enough oxygen for fuel combustion, but insufficient. Combines with nitrogen to produce NO ₂ . There are also limestone multistage burners, natural gas reburners, and furnace desulfurization technologies.

(10) Electron beam desulfurization process for coal-fired power plants

The process consists of pre-dust extraction, smoke cooling, ammonia addition, electron beam irradiation, and by-product capture. The flue gas discharged from the boiler is pre-dusted through the coarse filter treatment of the dust collector and enters the cooling tower. Under the cooling of the injected cooling water, the flue gas temperature is reduced to a temperature suitable for desulfurization and denitrification. The cooled flue gas flows into the reactor, and a certain mixture of ammonia and compressed air is sprayed into the reactor inlet. After being irradiated by the electron beam, SO ₂ and NO ₂ in the flue gas react with ammonia under the action of free radicals. The neutralization reaction produces a mixed powder of ammonium sulfate and ammonium nitrate, and this by-product is chemical fertilizer. Some of these powdery particles are deposited at the bottom of the reactor and discharged, and the rest is captured by the dust collector. After purification, the flue gas is discharged from the chimney into the atmosphere through a desulfurization fan to reduce pollution. This method is currently the most widely used and largest desulfurization method in the world, with a desulfurization efficiency of more than 95% and an absorbent utilization rate of more than 90%. ^[1]

(11) Focus on solving the problem of sulfur dioxide emission pollution. China s sulfur dioxide pollution is particularly serious. The area with an average annual precipitation pH 5.6 already accounts for 40% of the country's area, and it is increasing year by year. The Chinese government attaches great importance to the pollution control of sulfur dioxide. First, source control, burning low-sulfur coal, strictly controlling the use of high-sulfur coal, coal with higher sulfur content can be washed first, and appropriate reduction of sulfur dioxide emissions. The proportion of high sulfur coal to sulfur dioxide pollution is very large, and must be strictly controlled from the source. Second, the end treatment, that is, flue gas desulfurization. In China, there are already some relatively mature technologies for flue gas desulfurization, which have been applied in some power plants, mostly limestone-gypsum method. Regardless of the source or terminal desulfurization, its equipment costs and operating costs are very high. Therefore, power plants can try to adopt some high-tech desulfurization technologies that are gradually being promoted.
(12) Low NO _X pulverized coal combustion technology In the boiler combustion process, NO _X is mainly generated in two forms, namely high temperature NO _X and fuel NO _X. High-temperature NO _X is produced by the oxidation of nitrogen in the air at high temperatures. A large amount of NO _X is generated only when the furnace temperature is higher than 1 500 ° C. The temperature of the solid-state slagging pulverized coal furnace is generally lower than 1500 ° C, and the proportion of high-temperature type NO _X produced by the combustion of pulverized coal is not large. Generally, high-temperature type NOX only accounts for 10% of the total amount of NO _X produced. 20%, and the remaining 80% to 90% are fuel-based NOx, so the main problem is to reduce fuel-NOx emissions.
(13) Prevention and control of wastewater pollution Promote closed-loop recycling of grey water to save water and reduce sewage discharge; strengthen governance to ensure that grey water is discharged to standards; use pneumatic ash removal to enhance utilization of dry ash, and at the same time eliminate grey water pollution. Because the production of industrial wastewater containing oil, acid, alkali, suspended solids and other pollutants is intermittent, and the amount of water is not large, it can be considered to adopt the first centralized treatment method, which can improve the treatment efficiency and reduce the operating cost. For domestic sewage, it is more economical and reasonable to send it to the urban domestic sewage treatment plant where conditions permit. At the same time, comprehensive prevention and control work should be strengthened, and the use of watercourse engineering technology should be sought to reuse water resources as many times as possible to save water and energy and reduce pollution. ^[4]