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Nitrogen oxides (NOx) are one of the major pollutants in the atmosphere. Among the atmospheric pollutants, more than 90% of nitrogen oxides are derived from the combustion of coal, oil, natural gas and other fuels, of which 70% comes from the combustion of coal, and the fire Coal for power generation in power plants accounts for 70% of coal combustion in the country. With the development of the national economy, the demand for electricity has increased rapidly, coal-fired boilers have continued to expand, and the amount of coal used has increased significantly. The total amount of nitrogen oxides emitted from thermal power plants during the 12th Five-Year Plan period will increase from 10.5 million tons in 2010 to 12 million. Tons of nitrogen oxides will cause serious damage to China's atmospheric environment. Controlling the emission of nitrogen oxides is imminent, and denitrification has also become the focus of work during the 12th Five-Year Plan period.
Based on the requirements of the “Twelfth Five-Year Planâ€, this article describes the existing denitrification technology of thermal power plants and the application of high-voltage frequency inverters from Beijing Leidehuafu Electric Technology Co., Ltd., and describes in detail the characteristics of high-voltage inverters. Out of China's thermal power plant flue gas denitrification is currently in a period of rapid development, so the frequency conversion transformation of denitrification fan market is very large.
Development of China's denitrification business On September 21, 2012, the Ministry of Environmental Protection and the General Administration of Quality Supervision, Inspection and Quarantine jointly issued the new "Emission Standard of Air Pollutants for Thermal Power Plants" (GB13223-2011), requiring new construction from January 1, 2012. The emission of nitrogen oxides from coal-fired power units reaches 100 mg/m3; from July 1st, 2014, except for the special unit emission requirements of 200 mg/m3, the rest are also required to reach 100 mg/m3.
Corresponding to this, "the "12th Five-Year Plan" energy-saving emission reduction plan" clearly specifies the nitrogen oxide emission reduction targets, which, thermal power, cement two major industries, the reduction of nitrogen oxide emissions requirements were 29% and 12%; 2015 In 2004, the construction of denitrification facilities for 400 million kilowatts of active coal-fired units was completed, and the technical transformation of low-nitrogen combustion was implemented for 70 million-kilowatt coal-fired units, and the denitration efficiency of coal-fired units reached over 75%.
The implementation of the new standard brings a huge market for the denitrification industry.
Denitrification process of power plant flue gas introduction The main application of denitrification process is divided into dry flue gas denitrification and wet flue gas denitrification two categories. There are two main methods for denitrification of dry flue gas: selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR).
2.1 comparison of denitrification process 
SCR denitrification chemical reaction principle:
The SCR denitrification technology refers to that under the action of the catalyst, the reducing agent (ammonia gas) reacts with the nitrogen oxides in the flue gas to generate harmless nitrogen and water, thereby removing the NOx in the flue gas. Selectivity means that the reducing agent NH3 reacts with NOx in the flue gas without reacting with the oxygen in the flue gas.
4NH3+4NO+O2---->4N2+6H2O
8NH3+6NO2---->7N2+12H2O
The SCR catalyst generally supports V2O5 (vanadium pentoxide), WO3 (tungsten trioxide) and MoO3 (molybdenum oxide) on a TiO2 (titania) support. The catalyst activity temperature is generally 350-430oC 
SCR flue gas denitration process SCR process system is mainly composed of denitrification reactor, flue system, ammonia storage and supply system, ammonia injection system, soot blowing system.
Liquid ammonia is stored in liquid ammonia storage tanks, vaporized by the evaporator to ammonia gas, sent to the reactor area, mixed with dilution air and sprayed into the denitration system. The well-mixed reducing agent and flue gas react under the action of the catalyst in the SCR reactor to remove NOx.
The process flow chart is as follows: 
Leader Huafu high-voltage frequency converter principle and function characteristics 3.1 High-voltage frequency converter principle HARSVERT-VA series high-voltage frequency conversion speed control system uses the unit series multi-level technology, is the high-voltage source frequency changer, direct 6, 10KV input, direct 6, 10KV high voltage output. The frequency converter is mainly composed of phase-shifting transformers, power modules and controllers. 
The input-side phase-shifting transformer converts the high voltage on the grid side into multiple sets of low voltages on the secondary side. Each secondary winding adopts a delta-side delta connection method during winding, and there is a certain phase difference between them. The multi-stage phase-shift superimposed rectifier method eliminates most of the harmonic currents caused by the independent power unit, greatly improving the current waveform at the grid side, and making the network-side current of the frequency converter approximate to a sine wave; the filter device is not required and can be easily satisfied. The harmonic requirements of the standard; no compensation device, power factor of 0.95 at the grid side.
The power module is a basic AC-DC-AC single-phase inverter circuit, and the rectifier side is a diode three-phase full-bridge. Through sinusoidal PWM control of the IGBT inverter bridge, a single-phase AC output can be obtained. Each power module has the same structure and electrical performance and is interchangeable.
The controller consists of a high-speed single-chip processor, human-machine interface, and PLC. The man-machine interface solves the problems of the high-voltage variable-frequency speed control system itself and the user's on-site interface, and provides a friendly Chinese-language monitoring interface, which is convenient and quick to use, and can realize remote monitoring and networked control at the same time. The built-in PLC is used for the logic processing of the switch signal inside the cabinet, and can be flexibly interfaced with the user's site to meet the special needs of the user.
3.2 Features and Benefits of Beijing Leader Huafu High-voltage Inverter Introduction The following four aspects describe the functional characteristics of the HARSVERT-VA series of high-voltage frequency inverters from Beijing Leide Huafu Electric Technology Co., Ltd.:
reliability:
The use of DSP speed sensorless vector control technology allows for more precise control and enables low-speed, high-torque loads to be started and run.
With the function of re-start in rotation, the inverter can automatically search and track the speed of the motor to resume normal operation (ie, speed tracking technology) according to the set acceleration/deceleration time to ensure that the unit does not trip in safe operation.
Strong anti-grid power fluctuation, -35% - +15% U maintains operation without stopping;
Frequency conversion fault classification protection, with perfect motor and frequency converter protection function;
The high and low pressure are completely isolated and the design is safe and reliable;
Control redundant power supply design to ensure stable operation of the inverter.
Efficient use of multi-level series units, THDI <3%, higher than the national standard, IEC standards;
Dv/dt is small, no output filter is required, and long-distance output is possible;
Inverter network side power factor > 0.95, without compensation;
The overall efficiency is ≥96% (including transformers).
Convenient frequency front maintenance structure, module push-pull structure, control panel box structure, easy maintenance;
7-inch color touch screen, full Chinese interface, fault location, with automatic scheduling function;
Data and operation record recall function.
Flexibility enables on-site and remote monitoring;
Support multiple communication protocols Modbus, TCP/IP, Profibus;
Can customize a variety of power frequency bypass switch cabinet, to the greatest extent meet the user's production needs;
Built-in PLC, with PID adjustment function, can automatically adjust the speed of the inverter through the pressure of the denitrated air duct;
Modular design, professional frequency conversion system solutions, such as air-cooled cooling overall solution.
Application of high-voltage frequency converter in denitrification transformation of power plant 4.1 Application of high-voltage frequency converter in denitrification reform In order to overcome the pressure of newly added denitrification equipment, the output of the system fan must be increased (approximately 0.8-1 KPa increase). The following are the three commonly used methods for denitrification reforming high-voltage inverters:
1) Cases of Induced Draft Fan Expansion 1. Guizhou West Yunnan Zhongshui Power Generation (4×300MW) 
The following are the boiler auxiliary machine configuration parameters for each generator set: 
2). Another increase in denitrification booster fan This method requires a total of 3 fans of the induced draft fan, desulfurization booster fan, and denitrification booster fan. The efficiency is very low, and the area is large and not commonly used.
3), using three-in-one (draft fan, desulfurization booster fan, denitration booster fan), configuration of high-power combined fan case, Shaanxi Luohe Power Plant 300MW combined fan a 7100KW6KV high-voltage inverter 
4) Comparison of denitrification transformation methods 
4.2 Contrast of the type of combined fan selection There are currently two methods of retrofitting a combined fan: the stationary blade with adjustable axial fan + frequency conversion or the use of adjustable blade fan, these two options have their own advantages.
The following is a detailed comparison of these two options: 
In a word, the Leader high voltage frequency converter can be vigorously promoted in the denitrification fan system, and has good benefits in terms of safety, reliability, equipment maintenance, and energy-saving effects.
Application of High Voltage Frequency Converter in Denitrification Transformation of Power Plant
Foreword: Selective catalytic reduction of flue gas denitration technology (SCR) due to the high denitration rate, low NH3 escape rate, high efficiency of equipment, basically no secondary pollution characteristics, it has become the mainstream of denitrification technology, the market share in the global gas denitrification field The rate is as high as 98%. Conclusion In the past one or two years, the project of flue gas denitrification in thermal power plants in China will be in a period of rapid development. Through the analysis and demonstration of denitrification process technology and high pressure inverters from Lederhwaf, we can conclude that the denitrification fan is frequency-converted using high-voltage inverters. It is feasible to change the traditional damper damper adjustment to the motor rotational speed adjustment, which greatly improves the degree of automation of the system, realizes the soft start of the motor, and prolongs the motor life, which not only meets the production requirements, but also saves energy and reduces plant power consumption. The purpose of the rate is to reduce the economic losses caused by baffle and pipe wear due to the adjustment of the baffle plate and frequent downtime maintenance, and at the same time greatly reduce maintenance.