| More than 70%of China's coals are consumed in coal-fired power generation.Nitrogen oxides?NOX?and sulfides in exhaust from coal-fired power plants are the main precursors in the formation of acid rain and photochemical smog.At present,NH3 is mainly used as a reducing agent in a selective catalytic reduction reaction?SCR?to reduce NOX in the flue gas to nitrogen.The concentration and temperature of NH3 during SCR not only affect the removal rate of NOX,but the escaped NH3 may also corrode pipelines and testing equipment.In recent years,the laser absorption spectroscopy technology?LAST?has been more and more widely used in coal-fired flue gas detection due to its advantages of non-intrusion,fingerprint recognition,real-time and online,simultaneous measurement of temperature and concentration and so on.A set of NH3 concentration and temperature simultaneous measurement devices for SCR in flue gas has been developed in this paper by using LAST.The main works are as following:Firstly,the measurement principle of LAST is analyzed.Starting from Lambert-Beer's law,the principle of line shape and line width of molecular absorption line is introduced.The gas concentration and temperature measurement formulas based on direct absorption spectroscopy are derived,which provides a theoretical basis for the concentration and temperature measurement of NH3.Secondly,a series of NH3 concentration and temperature miniaturized measurement circuits for industrial sites have been developed,including:low ripple linear regulated power supply,modulation module,signal generation circuit,180°phase shifter circuit,optical-electronic detector,amplifier and filter circuit,and high temperature tube furnace control circuit.Thirdly,high temperature multi-pass cell and automatic temperature heating control system were built,and the temperature measurement of the escape NH3was realized by LAST.The experimental results show that the temperature sensor has good linearity?>0.99?and accuracy?error<2%?when the NH3 concentration is less than 5%in the range of 295.15?653.15 K.Finally,based on wavelength modulation spectroscopy,a home-made high-temperature multi-pass cell and temperature automatic heating control system are adopted to simultaneously measure the first harmonic and second harmonic signals of the concentration of escaped ammonia combined with the new analog lock-in circuit.The experimental results show that the correlation coefficients between the amplitudes of the harmonic signals and the ammonia concentrations are greater than 0.997,and the minimum detection volume fraction of ammonia is 0.496×10-6,which provides a new method for the concentration measurement of NH3 in the exhaust gas for coal-fired power plants. |
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