Abstract : Continuous Emission Monitoring System (CEMS) for thermal power plants is used for online automatic monitoring of particulate pollutants, gaseous pollutants, and total emissions from flue gas emitted by coal-fired power plants. It provides a scientific basis for environmental management of thermal power plants and the safe operation of dust removal and desulfurization equipment, and also provides a basis for national environmental management departments to determine whether pollution source emissions meet standards. This paper describes a CEMS system developed using an S7-200 series PLC and configuration software, replacing the traditional "industrial computer + board" model. The paper focuses on the hardware and software configuration of this control system, as well as the design of typical analog signal acquisition and sequential control (SCS). Actual production operation proves that the system design is reasonable, the operation is stable and reliable, and it meets the technical indicators and requirements of the CEMS system. Keywords : PLC; configuration software; CEMS 1. Introduction With the rapid growth of the national economy, the power industry has developed rapidly, and the air pollution problem caused by coal combustion has become increasingly serious. In 2007, the China National Environmental Monitoring Center, the Shanghai Environmental Monitoring Center, and the Information Center of the State Environmental Protection Administration jointly drafted and approved the "HJ T 76-2007 Technical Requirements and Testing Methods for Continuous Emission Monitoring of Stationary Pollution Sources" [1], which was approved by the State Environmental Protection Administration, clearly stipulated the main technical indicators, testing items, testing methods, and quality assurance during testing of CEMS. Continuous Emission Monitoring System (CEMS) is a system that uses online automatic monitoring instruments as the core and is supplemented by related hardware and software to realize on-site pollution source monitoring. It mainly monitors the concentration of gaseous pollutants (SO2, NOx), auxiliary parameters (flue gas temperature, flow rate, oxygen content, humidity, pressure) and particulate matter concentration (or turbidity) in flue gas. There are three main methods for monitoring the concentration of gaseous pollutants: complete extraction method, dilution method, and electrochemical method. There are three methods for monitoring the concentration of particulate matter: laser transmission method, laser backscattering method, and charge induction method. In this system, the concentration of gaseous pollutants is monitored by complete extraction infrared measurement, and the particulate matter is monitored by laser backscattering method. 2. System Composition and Functions System Composition After the boiler flue gas is desulfurized and dust removed by the system, it is discharged through the flue. Therefore, a standard CEMS system consists of the following parts: sampling probe, flue gas pretreatment device, gaseous pollutant monitoring subsystem, particulate pollutant monitoring subsystem, flue gas emission parameter monitoring subsystem, system control and data acquisition and processing (DAS) subsystem, and auxiliary systems such as power supply and communication [2]. The working principle diagram of the system is as follows: CEMS system based on S7-200 PLC and configuration software full text download