Why has the oil cooled permanent magnet governor become the preferred choice for energy-saving renovation of slurry circulation pumps in coal-fired power plants?

The electricity consumption is the largest cost in the operation of the desulfurization system in coal-fired power plants, accounting for more than 80% of the desulfurization system cost. The electricity consumption of the desulfurization system accounts for more than 1% of the total power generation of the entire power plant, which is equivalent to a standard coal consumption of 3-6g/kW · h. Therefore, the optimization research on the operation of the desulfurization system in coal-fired power plants has enormous space and practical significance for improving the economic efficiency of thermal power plant operation and reducing the operation cost of the desulfurization system. As one of the most important equipment in the limestone gypsum wet flue gas desulfurization system, the power consumption of the slurry circulation pump accounts for about 65% -76% of the entire desulfurization system. The power consumption of the slurry circulation pump is relatively high, and the energy-saving and consumption reducing of the slurry circulation pump is of great significance.

Due to the importance of its slurry circulation pump, some new technologies have emerged in recent years to modify it, but each has its own advantages and disadvantages. The following is a comparative demonstration of several configuration schemes for slurry circulation pumps.

Option 1: Ordinary asynchronous motor+reducer+slurry pump

This method is currently the most common and widely used. In this scheme, a common high-voltage asynchronous motor (4-pole motor) is used as the motor, and the slurry pump is driven by a reducer to reduce the speed (to 400rpm~600rpm). The pump is a high flow, low head wear-resistant and corrosion-resistant pump.

Advantages:

The system structure is simple and reliable, with high transmission efficiency. The internal components of the system are all mature products, making it easy to use and maintain.

Disadvantages:

(1) The motor adopts a direct starting method, which has a large impact on the system and can easily cause gear breakage, severe bearing wear, and coupling damage in the gearbox, resulting in a large maintenance workload;

(2) The system keeps running at a constant speed, and when the working conditions change, it can only be met by starting and stopping the number of slurry pumps. Without other adjustment methods, it is impossible to make real-time and precise adjustments according to actual requirements, resulting in a large amount of energy waste and affecting the stability of the slurry in the absorption tower.

Option 2: Low speed motor+slurry pump

This method is currently relatively rare, and some units originally adopted Plan 1, which was modified due to the heavy maintenance workload of the equipment in their system. In this scheme, a low-speed permanent magnet synchronous motor is used as the motor, and the reducer is removed. The slurry pump is directly driven by the motor, and the pump is still the original high flow, low head wear-resistant and corrosion-resistant pump.

Advantages:

(1) Further optimization of the system structure, removal of intermediate reducers and other links, simplification of the transmission chain, simple system composition, reduction of power loss in the transmission chain, and overall efficiency improvement;

(2) High electric starting torque and strong load-bearing capacity; No need to replace gear oil, inspect the gearbox, and reduce maintenance components.

Disadvantages:

(1) The size and weight of the motor are larger than ordinary asynchronous motors, requiring more space, and the related supporting facilities and accessories need to be increased in specifications;

(2) Permanent magnet motors can be started directly, but they have a significant impact on the system, severe equipment wear, and increased maintenance frequency; Generally, in order to reduce the impact on the system and equipment, corresponding frequency converters are required, but additional investment is needed. As high-voltage frequency converters are composed of electronic components, harmonics are generated during operation, and there are many electronic components that age quickly, requiring high environmental requirements. As the service life increases, the failure rate also increases year by year, resulting in poor safety and reliability. In addition, there is a high cost of later use and maintenance. In addition, a dedicated space is needed to place the frequency converter in the distribution room and install air conditioning.

(3) It is understood that the technology of low-speed and low-power permanent magnet synchronous motors is relatively mature at present, but the internal bearings of low-speed and high-power permanent magnet synchronous motors are difficult to establish a lubricating oil film for a long time at low speeds, and the life of the bearings is a major factor restricting the application of high-power low-speed motors; Meanwhile, due to factors such as material and heat dissipation, reliability still needs further verification.

(4) The problem in the same scheme one is that the system has been running at a constant speed, and when the operating conditions change, it can only be met by the number of start stop pumps. Without other adjustment methods, it is impossible to make real-time and accurate adjustments according to actual requirements, resulting in a large amount of energy waste and affecting the stability of the slurry in the absorption tower.

Option 3: Ordinary asynchronous motor+permanent magnet speed controller+reducer+slurry pump

This plan is to install an oil cooled permanent magnet speed controller between the motor and the reducer based on Plan 1. The motor, reducer, and slurry pump are the same as Plan 1 and still use the original equipment.

Advantages:

(1) High power oil cooled permanent magnet governor is a breakthrough new technology developed in recent years, which adopts magnetic non-contact torque transmission. Its technology is advanced and reliable, and there are a large number of mature cases for reference;

(2) It can accurately adjust the speed of the load in real time, meet the process requirements, and achieve the goal of reducing speed and saving energy. Its energy-saving effect is significant, and from a large number of past cases, its energy-saving rate can reach 25-40%. Taking an 800kw motor as an example, the annual energy savings can generally reach over 1.2 million kWh;

(3) The original hard connection between devices has been changed to non-contact soft connection, which solves the problem of installation alignment and reduces equipment vibration; Its buffering start characteristic can reduce impact, improve the reliability of the motor and load, and reduce maintenance workload;

(4) Permanent magnet transmission is a pure mechanical speed control system with high reliability. Its value becomes more apparent when applied in harsh environments such as humidity and high dust. It has a long service life (with an overall lifespan of over 25 years) and is maintenance free during daily operation;

(5) The permanent magnet governor itself does not consume power, does not generate high-order harmonics, and does not cause motor heating during low-speed operation.

Disadvantages:

After installing a permanent magnet speed controller, due to the need to occupy a certain amount of space, the overall length between the shafts of the system needs to be increased by about 1.3m. For renovation projects, installation space needs to be considered and the motor needs to be moved backwards, but this issue does not exist for new construction projects.

By comparing the above three configuration schemes, it can be seen that compared to schemes one and two, although scheme three adds more permanent magnet transmission equipment to the system, which may seem "complex", the permanent magnet governor is a very mature product that does not require maintenance during daily operation, and can reduce vibration and impact. It can also achieve variable speed energy saving through real-time precise adjustment, bringing significant benefits.

At present, there are various technical implementation routes for permanent magnet speed controllers. However, based on the market performance over the years, disc type, symmetrical structure, and oil cooled permanent magnet speed controllers are currently the most practical, high support power (≤ 5000kW), and stable technical schools in terms of supporting high-power loads and operational stability. From the perspective of safety and reliability evaluation, it is recommended to give priority to energy-saving transformation of slurry circulation pumps in coal-fired power plants.