Application And Measurement Uncertainty Study Of Ultra-low Emission CEMS In Thermal Power Plants

Combined heat and power companies in Hangzhou are undertaking the task of the central heating.They are the important infrastructures for social and economic development and parts of the major industries in coal consumption and air pollutant emissions.The energy-saving and emission-reduction transformation of co-generation is a necessary way to improve energy efficiency and reduce air pollutant emissions.At present,Hangzhou government requires all the coal-fired thermal power plants to reach the emission limit of the‘Emission Standard for Air Pollutants of Thermal Power Plants’,that is,under the condition of the reference oxygen content of 6%,the particles concentration can not exceed 5 mg/Nm³,the SO₂concentration can not exceed 35 mg/Nm³,the NO_X concentration can not exceed 50mg/Nm³.Continuous Emission Monitoring Systems（CEMS）networked with the monitoring stations have become important facilities for supervising and inspecting whether the ultra-low emissions meet the standards.Therefore,the application of CEMS of ultra-low emissions in thermal power plants and the study of measurement uncertainty is of great significance to improve the reliability and accuracy of monitoring datas.In the paper,CEMS installed in several representative thermal power plants in Hangzhou were taken as research objects,based on‘Guide to the Expression of Uncertainty in Measurement’（GUM）,the measurement results uncertainties of CEMS were evaluated,Monte Carlo Method（MCM）was introduced to analyze the model and explore the differences between the two methods.Firstly,the paper analyzed the influencing factors of the measurement results uncertainty in CEMS,including zero drift,span drift,measurement error,measuring repeatability,standard gas species concentration.Calibration tests,reference tests andrepeatability measurement experiments were performed on the gaseous pollutant CEMS,the uncertainty components introduced by each influencing factor and the combined standard uncertainty were calculated,the influences of uncertainty components on combined standard uncertainty were investigated.The results show that,for the test results of SO₂,NO in CEMS,the influence of measurement error plays a decisive role.The stability of the flue gas,the differences of actual concentration of the flue gas and the differences in the analysis principle and measurement accuracy of the portable flue gas analyzers,all affect the uncertainty component introduced by measurement error;for test results of NO₂ in CEMS,the uncertainty component introduced by zero drift and range drift accounts for the main impact,and the four thermal power plants have certain stability and reliability for concentration measurement of NO₂.However,the order of magnitude of the uncertainty is smaller than the order of magnitude of the measurement result uncertainty of NO,for the NO_X concentration datas displayed by the analyzer,the measurement result uncertainty of NO plays a major role.Secondly,MCM was introduced to evaluate the uncertainties of CEMS in thermal power plants,the probability distribution of two methods was expressed based on Matlab.The two types of evaluation methods were compared through the numerical tolerance method.For some thermal power plants,the reasons why GUM and MCM have a certain degree of deviation in measurement uncertainty of CEMS for some gaseous pollutants are disscussed,the results show that,GUM analyzes the uncertainty source of experimental process,rather than the sampling analysis of large data samples,for the uncertainty evaluation of complex systems,GUM has limitations;MCM does not need to consider the sensitivity of the measurement model;GUM ignores the degrees of freedom of the uncertainty components,and considers that the measurement model is linear and has no correlation.Finally,particle concentration analyzer in CEMS were taken as the research objects,the measurement results of the particles concentration were mainly detected and calibrated.After zero point and span calibration,by comparing the manual weighting method with the CEMS measurement,the relevant calibration curves,confidence intervals and allowable intervals were analyzed,and the results show that CEMS in thermal power plants meet the standard in particles tests,the measurement results are accurate and reliable.