Design Of High Precision Online Monitoring Instrument For SO₂ And NO_x In Coal-fired Flue Gas

In order to reduce pollutant emissions from coal-fired power plants,and curb air pollution,the National Development and Reform Commission,the Ministry of Environmental Protection,and the National Energy Administration jointly issued the Action Plan for Energy-saving and Emission Reduction and Reconstruction of Coal-fired Power Plants（2014-2020）in 2014.It is required to speed up the ultra-low emission transformation of coal-fired generating units.After transformation,the emissions of pollutants from existing coal-fired power plants,such as SO₂ and NO_X,will be reduced to 35mg/m³ and 50mg/m³ respectively,which will undoubtedly increase the difficulty in accurate monitoring continuous monitoring systems.At present,the range,accuracy and stability of instruments in the field of ultra-low emission monitoring can not meet the monitoring requirements under the new standards.For the problems,this paper will research and design an continuous monitoring system based on DOAS algorithm for ultra-low concentration SO₂ and NO_X.The paper focuses on the design process of the instrument system,including instrument hardware design and software design.Hardware design.The instrument hardware system adopts a modular design to reduce the coupling of various devices inside the instrument,increase the robustness of the instrument,and make maintenance easier.The chassis adopts standard rack-type chassis design,5U and 19-inch instrument chassis to facilitate system integration.The main control module adopts the combination of Industrial Personal Computer（IPC）and Single Chip Micyoco（SCM）,and the IPC is responsible for the calculation of concentration inversion,and the MCU is responsible for the control of internal hardware.Power module adopts switching power supply design,which converts 220VAC into12VDC and 24VDC respectively to supply power to each module of the instrument.Human-computer interaction module adopts serial port screen.Communication module uses RS485 to realize communication between instrument and the data collection instrument,and uses RS232 to realize communication between the IPC and the SCM.In order to solve the problem that the stability of the spectrometer is susceptible to temperature,the temperature control module adopts two-stage PID adjustment design based on the thermoelectric cooler and the fan to effectively control the temperature of the spectrometer.Finally,according to the control requirement,the main control circuit and the temperature control system circuit were designed and the printed circuit board was fabricated.Software design.Software design is divided into PC software design and embedded software design.The PC software adopts Visual Studio and MATLAB cross-platform hybrid programming.The PC software part is completed by Visual Studio,and the algorithm part is written by MATLAB.The two parts are connected by dynamic link library.The embedded software core selects the STM32F103 series chip,and the software framework adopts the layered design method,which is closely integrated with each part of the hardware to realize the coordinated control of the instrument hardware.At present,the monitoring prototype runs well,and the software,hardware and algorithms can cooperate to complete the gas measurement task.The relevant parameters index full-scale error≤2%F.S.,relative error≤10%,reaction time≤30S,etc.all meet the requirements of the project.The whole machine has been verified by the Chongqing Institute of Metrology and Quality Inspection,a third-party testing organization.