High voltage frequency conversion technology in circulating fluidized bed heat source plant

This paper introduces the exploration and experience of Harbin Huaneng Central Heating Co., Ltd. in applying variable frequency speed control equipment, and also gives the technical characteristics of the high-voltage variable frequency device of Oriental Hitachi. Through the comparison of different working conditions, it is shown that the use of high-voltage variable frequency system has significant energy-saving effect and improved the safety and reliability of the system.

1. Process equipment

As a clean and efficient combustion technology, circulating fluidized bed is widely recognized internationally. It has the characteristics of wide combustion adaptability, high combustion efficiency, low nitrogen oxide emission, large load adjustment range, and low pollutant emission. It is an environmentally friendly boiler and a new type of boiler vigorously promoted by the country. The combustion process is shown in Figure 1.

The fuel is delivered to the furnace by the coal feeding system in front of the furnace. The air supply system consists of primary air (blast) and secondary air. The primary air is delivered to the furnace from the lower part of the furnace bed, mainly to ensure the fluidization of the material layer; the secondary air is delivered to the furnace at multiple points along the combustion chamber in stages, mainly to increase the oxygen content in the furnace and play a role in combustion. The burned materials become some smaller particles and enter the separator together with the flue gas. After solid-gas separation, most of the particles are sent back to the furnace by the return device at the bottom of the separator, so that the furnace has a high enough ash content to ensure fluidization. The flue gas is dedusted by the electrostatic precipitator and then extracted by the induced draft fan.

2 System technical solution analysis

The regulation of circulating fluidized bed boilers is mainly the regulation of the wind and coal system. At present, the main method of regulating air volume is to change the air door opening and motor speed, among which the motor variable frequency speed regulation is the best. In the three large fans of the 5 and 6 boilers of the first phase of the project, the chopper internal feedback speed regulation system produced by a domestic manufacturer was used according to the design and selection of the design institute. Through the use in 2004 and 2005, many problems were found, such as small adjustment range (30 50 Hz), high failure rate, large harmonics, full speed must be started before changing to speed regulation, and true soft start cannot be achieved. The most serious problem is that the supporting motor of the system must be a special motor with a rotor with slip rings and carbon brushes. The failure rate is very high during use, and the carbon brushes are seriously worn. On average, the furnace needs to be stopped to replace the slip rings and carbon brushes once a month, which brings many problems to the stability of heating and daily use and maintenance. In order to solve this problem in the second phase of the project, we investigated some professional manufacturers and related users. After analysis, comparison and demonstration, we finally selected 6 high-voltage and high-power inverters produced by Chengdu Dongfang (Hitachi) Electric Control Equipment Co., Ltd. to control the primary fan (1,250 kW/6,000 V), secondary fan (250 kW/6000V) and induced draft fan (800 kW/6000V) of 3 and 4 water heaters respectively.

3. Energy saving and other effect analysis

According to the basic laws of fluid mechanics, fans and pumps are all square torque loads, and their speed n has the following relationship with flow Q, pressure p and shaft power P: Q∝n, P∝n2, P∝n3[2l