Characteristics of the power systems in coal chemical enterprises

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Coal chemical industry: The coal chemical industry uses coal as a raw material and, through chemical processing, converts it into gases, liquids, solids, and chemicals. This involves the production of olefins, synthetic ammonia, methane gas, dimethyl ether, ethylene glycol, coal-based gasoline, coal-based aromatics, etc.; these products are then used as raw materials to manufacture thousands of other chemicals that are utilized in various sectors of the national economy. The core process units in coal chemical enterprises are coal gasification, coal liquefaction, and methanol-to-olefins. Compared with traditional petrochemical industries, coal chemical industries have longer production process routes and more complex production processes, with high consumption of utilities such as water, electricity, and gas, requiring the installation of power stations (on-site power plants). Large coal chemical enterprises have high electricity consumption and require high reliability of power supply, necessitating the installation of emergency power sources. The power supply requirements for coal chemical enterprises are such because coal chemistry involves continuous production processes that handle flammable and explosive hazardous materials in solid, gas, and liquid forms; a power outage would disrupt production, resulting in significant economic losses. Improper handling of these situations can also lead to serious damage to equipment and pose threats to human safety. Therefore, the main pumps of the process equipment, as well as the electrical load of the utility systems that directly affect the continuous operation of the facility, are classified as first- and second-class loads; dual power supplies must be used to ensure the reliability and continuity of normal production. In addition to the regular power supply, an emergency (backup) power supply is also required so that it can be activated in the event of a complete outage of the regular power supply, thereby supplying power to the facility’s emergency loads and fire protection systems and ensuring that the facility can shut down safely. Characteristics of the power systems in coal chemical enterprises: 1. Requirement for power supply reliability. Long production processes, complex technology, and high electricity consumption are the main characteristics of coal chemical enterprises, and these factors are also the reasons why such enterprises require a high level of power supply reliability. Therefore, it is recommended to conduct tests for partial discharges under live conditions; this can effectively improve power supply reliability, help monitor the development trend of partial discharges, and provide information on the service life of equipment. This is also considered the most effective method currently recommended by the International Electrotechnical Commission. 2. Voltage level of the external power supply: A significant feature of coal chemical industries is their high electricity consumption; therefore, the voltage of the external power grid supplying power to these industries is 110 kV, 220 kV, or 330 kV. Compared to traditional petrochemical or refining plants, coal-based plants with an annual production capacity of 1.2 million tons of coal-derived olefins or 4 million tons of coal-derived oil require a power consumption of around 400 MW, which is much higher than that of petrochemical facilities or refineries of similar scale that use oil as raw material. The supply voltage for external power sources in such plants is 220 kV or 330 kV. 3. On-site power plants: Since coal chemical enterprises need to produce large amounts of syngas through gasification units, these units require a significant amount of oxygen during operation; therefore, multiple large-scale air separation units are necessary to meet their demands. The air compressors and boosters in air separation units are typically driven by turbines, which require large amounts of high-temperature, high-pressure steam. As a result, the on-site power plant must be equipped with multiple high-temperature, high-pressure boilers, and the capacity of the turbines and generators as well as the configuration of the units are determined based on the demand for steam at various levels throughout the facility and the requirements for regulation. When the total power generation capacity of the on-site power plant reaches or approaches the factory’s total power consumption, it can be considered as the primary power supply. Furthermore, a large amount of waste heat steam is generated during the coal chemical industry’s production process, and it is common to use this waste heat steam for power generation. However, since the capacity of gensets powered by humans is relatively small, they are susceptible to changes in steam conditions under different production scenarios and in various seasons; as a result, the power generation output of waste heat recovery units also changes, making them unsuitable as primary power sources. 4. Plant power supply system: I. In most coal-to-methanol, coal-to-ethylene glycol, and coal-to-oil plants with a capacity of less than 1 million tons, the electrical load does not exceed 100 MW. These plants typically use 110 kV power supplies, and an 110 kV main substation is installed. Depending on the electrical load of the individual substations within the plant, as well as whether motors are used (i.e., line-to-transformer connection), 110/35 kV or 110/10 kV substations are established. II. Enterprises producing coal-based olefins in quantities of over 600,000 tons and coal-based oils in quantities of over 1 million tons generally use 220kV or 330kV power supply lines. They have main substations of 220(330)/110kV or 220(330)/110/35kV, and 35kV voltage is used to supply power to the various plant substations. III. The generator will be selected based on its capacity, installation location, and the chosen connection point. 5. Emergency power supply system: In the event of a widespread power outage in coal chemical enterprises, fires and explosions are likely to occur, and the main production facilities can suffer serious equipment failures as well as lead to casualties. Therefore, an emergency power supply must be installed in accordance with the enterprise’s power requirements to ensure that the equipment can shut down safely in the event of a failure of the normal power supply. Furthermore, in the main substation of coal chemical plants, circuits such as 35kV circuits require the installation of low-frequency load shedding devices to enhance the stability of engine operation in case of an accident, thereby enabling island operation of the generator in the on-site power station. When the external power supply is interrupted, the generator can operate in island mode, allowing some of the facilities and utility systems in coal chemical plants to continue operating for a short period of time or to shut down safely in an orderly manner. This helps to minimize downtime and economic losses, enabling the plants to resume normal operations as soon as possible. When the external power supply is interrupted, and if the generators in the on-site power station cannot operate, emergency diesel generators must be brought online to supply power for the emergency loads of the production facilities as well as the fire protection system, thereby ensuring that the facilities can shut down safely.