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General Knowledge of Thermal Power Difference between Cogeneration and Thermal Power Plant

Number of visits: Date:2019-01-10
The so-called thermal power plant is a power plant with the main purpose of heat supply. Its downstream customers are thermal companies and thermal enterprises. The downstream customers of thermal companies are mainly urban residents.
The installed capacity of thermal power plant is restricted by the size and nature of heat load, and the unit scale is smaller than the main units of thermal power plant at present. Because of both power generation and heat supply, the capacity of boilers is larger than that of thermal power plants of the same scale. Thermal power plants have more boiler capacity than general thermal power plants for reserve, and the amount of water treatment is also larger. Thermal power plants are generally close to heat load centers, often urban centers in densely populated areas. Their water use, land acquisition, demolition and environmental protection requirements are much higher than those of thermal power plants with the same capacity. At the same time, thermal pipe networks need to be built.
The downstream customers of thermal power plants in the northern region are thermal companies and thermal enterprises. The downstream customers of thermal companies are mainly urban residents. Therefore, the customer source is stable and there is basically no market competition. Secondly, the price of heat and power is fixed by the government, the operation is stable, and the development trend of the heat and power industry is favorable.
Generally, condensing units are used in power plants, which only produce electricity to supply power to users. Industrial production and people's daily heat are supplied separately by special industrial boilers and heating boilers. This mode of energy production is called thermal power division. Thermal power plants use heat-supply units, in addition to supplying electric energy, but also use steam extraction or exhaust of steam turbines that do overwork (i.e. generate electricity) to meet the production and life needs of heat. This mode of energy production is called cogeneration.
Considering the effect of energy utilization, the division of heat and power production is unreasonable for energy use: on the one hand, the process of thermal power conversion (condensing unit power generation) inevitably produces low-grade heat loss (steam exhaust heat in the cold source), on the other hand, high-grade heat energy (steam heat provided by boiler) is devalued for low-grade heat supply. In cogeneration, fuel chemical energy is converted to high-grade heat energy, which is used to generate electricity (high-grade heat energy) first, and then to heat users with low-grade heat energy that has worked, which conforms to the principle of using energy according to quality and comprehensive energy. Therefore, the characteristics of thermal power plants are that the primary energy is used reasonably, so as to supply energy according to quality, step by step, and make the best use of it, so that the whole energy supply system in the region saves energy.
Considering from the angle of heating mode, there are two kinds of heating modes: decentralized heating and centralized heating. Due to the limitation of heating scale, small boilers with low thermal efficiency and low parameters can only be used for decentralized heating at user locations (thermal efficiency is about 50%). However, district central heating is formed by heating from thermal power plants. Because of the large scale of heating, large-scale boilers with high parameters and high efficiency (thermal efficiency is more than 85%) can be used, which greatly improves the efficiency of energy utilization and saves fuel.
In the aspect of environmental pollution, because of the central heating of thermal power plants, the use of coal is reduced, the discharge of sewage is also reduced, the trouble of coal transportation and ash removal is also reduced, and large capacity boilers are equipped with efficient dust collector equipment and high chimney, which greatly reduces the degree of environmental pollution.
II. Cogeneration of Heat and Power
Cogeneration of heat and power refers to the production mode in which power plants not only produce electricity, but also use steam that has worked by turbogenerators to heat users, i.e. the process of simultaneously producing electricity and heat energy, which saves fuel compared with the way of producing electricity and heat energy separately. The steam source for external heating is the adjusted exhaust steam of the exhaust steam turbine or the exhaust steam of the back steam turbine. The pressure is usually divided into 0.78-1.28 MPa and 0.12-0.25 MPa. The former is for industrial production and the latter for civil heating. Cogeneration steam has no loss of cold source, so it can improve thermal efficiency to 85%, much higher than large condensing units (thermal efficiency up to 40%).
Cogeneration of heat and power not only saves a lot of energy, but also improves the environmental conditions and the living standards of residents. However, cogeneration of heat and power closely links the power generation of power plants with the user's heat consumption, reduces flexibility, and increases the investment of power plants. Therefore, only by making overall arrangements for urban planning and central heating zones, and under the condition of fully guaranteeing heat load, can a reasonable construction plan be determined, can a good comprehensive benefit be achieved.
Cogeneration of heat and power requires that thermal power plants should be centralized with relevant factories and urban residential buildings in a certain area to achieve maximum economic benefits of energy utilization. The development of cogeneration in Western and Eastern European countries has reached a high level. The installed capacity of thermal power plants accounts for 30% of the total installed capacity of electricity, and half of them are used for industrial production and District Central heating. Paper, steel and chemical (including petroleum chemistry) industries are the main users of cogeneration. They are not only large users of electricity and heat consumption, but also waste materials and exhaust gases (such as blast furnace gas) discharged in the production process can be used as fuel for cogeneration units. Urban industrial zones and densely populated areas are also the main targets for the development of cogeneration, but attention should be paid to the analysis of local heat load. Generally, the thermal coefficient should not be less than 0.5 (the annual utilization hours of industrial heat load are more than 3500 hours, and the winter heating of residents is not less than 3 months). The heating distance of thermal power plants usually does not exceed 5-8 kilometers. The fuel quality (mainly sulphur and phosphorus) of cogeneration has higher requirements. At the same time, the plant site should be chosen in the downwind direction of the prevailing urban wind to avoid pollution to the urban environment.

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