Research On Microwave Characteristics Of Carbon Content In Fly Ash

At present,with the rapid development of new energy power generation,two requirements are put forward for the thermal power units.Large units should further optimize the combustion system to improve combustion efficiency.Small units should be modified for flexible low-load operation.To achieve the above requirements,a reliable combustion control system must be established.The real-time high-precision detection of combustion process parameters is the basis to realize this control system.One of the most important parameters characterizing combustion efficiency is the unburnt carbon content of fly ash.In China,the microwave test method is widely used for the detection of the carbon content of fly ash.However,due to the lack of depth study of the microwave characteristics of fly ash with unburnt carbon,the following problems still exist for the detection devices:Firstly,the determination of the working frequency of the fly ash carbon content test system lacks theoretical and experimental data support,the frequency selection is blind.Secondly,the test accuracy is not good enough.At present,the carbon content of fly ash in large power plants is no more than 2%.Most experiments ignore the research on the accuracy of low carbon range,and lack the precision comparison between different test method.Fly ash concentration is the final problem.In the unburnt carbon content test,the changes of fly ash concentration are difficult to eliminate,which has a serious impact on test accuracy.In view of the above problems,this paper takes the coaxial air line,coaxial probe,free space,and coaxial waveguide four methods to study the microwave characteristics of fly ash with unburnt carbon from the perspective of microwave material testing.The microwave frequency responses of the carbon content in fly ash are studied by testing the dielectric properties,the S21 parameters on a wide frequency band to determine the frequency selection basis.For the accuracy problem,the accuracy index is established to compares the detection accuracy of carbon content under different test methods and test parameters.The low carbon content tests are also performed.Finally,the concentration-independent function is proposed to eliminate the influence of the fly ash concentration,and the validity of the concentration-independent function is verified by test cases.Through the above researches,the following conclusions are obtained.(1)There is no characteristic frequency band of the carbon content test in 2-18 GHz,and the test sensitivity is higher at higher frequencies.(2)To achieve higher test accuracy,the waveguide method is the suitable method,which can achieve low carbon content test within 2%.As for test parameter,S21 phase test results are better than S21 amplitude.(3)The concentration of fly ash and the amplitude(phase)of S21 are linearly related.The influence of the change of fly ash concentration on the test accuracy can be effectively suppressed by the concentration-independent function.Through the above experimental researches,it is possible to provide data support and theoretical basis for the design of the test device in field use,thereby realizing a high-precision fly ash carbon content detecting device.