On-Line Measurement Of SO₃ In Flue Gas Of Coal Fired Based On Mid-Infrared Laser Absorption Spectroscopy

With the rapid promotion of social productivity,more coal resource is being consumed by humankind.Whereas it brings a serial of issues at the large amount of coal consumption.Fist,the SO2 and SO3 in the exhaust of coal-fired are emitted and pollute the atmosphere due to the formation of smog and acid rain,which is harmful for human health.Additionally,it will increase the risk of corrosion and blockage of instrumentations.It is necessary to monitor the emission of sulfur oxide compound.Currently,there are two approaches to measure SO3,which are chemical and laser spectroscopic methods.However,chemical approach needs to sample and it is difficult to accomplish on-line and in-field measurement.Laser spectroscopy could remedy these corresponding disadvantages.Quantum cascade lasers(QCL)are widely used as the sources of tunable diode laser spectroscopy(TDLAS)for detecting the atmosphere,combustion and medical diagnostics etc.In this thesis,the basic theories of direct absorption,wave-length modulation spectroscopy were briefly introduced.The absorption spectra of SO3 at 1396.889cm-1 and 1365.49cm-1 were simulated based on the analysis of line intensity at different spectral region.In light of interference from other exhaust gases,the v3 band of SO3 were chosen for measurement.The challenges for measuring SO3 are no commercial standard gas available and its strong chemical reactivity.Therefore,a SO3 generator was developed,which includes reaction furnace,crystallizer,reaction chamber and temperature control system.Based on the system,many parameters such as furnace temperature,gas flow and gas ratio were optimized for obtaining the maximum SO3 production.The spectral lines of SO3 and SO2 around 1396.889cm-1(7.16 μm)were measured by employing QCL in conjunction of TDLAS.The relationship between QCL’s wavelength and temperature and current were derived around 7.16 μm.The WMS-2f was used for improving the robustness and sensitivity.The sample cell with small volume can suffer 500℃.The response,Allan deviation and linearity of system were demonstrated by observing the signal of SO2 based on their corresponding relationship.Meanwhile,a temperature calibration was done to suppress the measurement error.The relationship of different noises and integrated time was investigated based on Allan analysis.The SO3 concentration were determined by the relationship between the WMS-2f signal amplitudes and absorption coefficients of SO2 and SO3.An on-line equipment for SO3 was developed based on QCL in combination of TDLAS.The SO3 concentration was implemented calibration-free according to the normalizing WMS-1/f/2f.The instrument includes gas channels,optical part and electronic section.Gas circuit is in charge of generating stably SO3 and keeping air tightness.A QCL at 1365.49cm-1(7.32 μm)was selected as source due to its strong absorption coefficient.The relationship of laser output wavelength and temperature and current was measured.Besides,optical part is composed by HgCdTe detector and absorption cell.The circuit section controls the temperature and current of the QCL driver,signal acquisition and demodulation.The WMS is achieved based on Labview software.WMS-2f/1f is employed to reduce the influence of laser intensity fluctuation,vibration and contamination of the windows.