| Serious problems such as exhaustion of energy and air pollution are caused by the rapid development of human industry, carbon monoxide produced by the burning of fossil fuels cannot only combine with human hemoglobin which causes hypoxia and poisoning, and emissions to the atmosphere will produce consumption of OH radicals and reduce the self-purification capacity of the atmosphere. CO concentration and temperature can be monitored real-time to improve the combustion efficiency in industrial furnace, and inhibit the exhausted CO concentration. Compared with the traditional measurement method, tunable diode laser spectroscopy technology has advantages in selectivity, strong anti-interference ability, low sensitivity, real-time measuring, can detect gas components concentration, temperature pressure, gas flow rate simultaneously, and is widely used in the industrial field. Combined with wavelength modulation technology can effectively eliminate the white noise generated by TDLAS photoelectric device, which greatly improves the detection limit, and broadens the application of this technology in the field of trace gas concentration detection.In this paper, a set of experimental apparatus is self-built based on wavelength modulation tunable diode laser absorption spectroscopy theory which is used for measuring CO concentration under high temperature conditions, and the following results are obtained:1) By analyzing the mechanism of Gauss, Lorenz, Voigt absorption line shape, it is obtained that Lorenz line shape is the most reasonable one to simulate the actual absorption lines under atmospheric pressure condition;2) After theoretically deducing the nth derivation of Lorenz line shape by Fourier transfer, the second harmonic expression of Lorenz line shape accounting for wavelength modulation and power modulation simultaneously is given, which provides a theoretical basis for the least squares fitting the measured signal on the following.3) Combined with HITRAN database, a simulation about fossil fuel emission gas absorption position is given and finally we choose R14 transition(1563.6nm) of CO in R band as the measuring object for its small interference, high intensity and application of DFB laser.4) A set of apparatus consists of laser modulation system, standard gas preparation system, high temperature system, signal detection and demodulation system and the signal collection and storage system. It is based on wavelength modulation tunable diode laser absorption spectroscopy, and the tunable diode laser is distribute feedback laser(DFB laser), a high frequency signal is added to modulated the injection current of laser, demodulate the signal with a lock-in amplifier, and finally the second harmonic of the detected absorption single will be gotten which is used as quantitative standard.5) It is quantified by external standard method. Eliminate the vapor absorption background interference around CO transition by establishing a vapor absorption background database, and then quantify with a CO standard absorption database.6) We will get the information about water, CO concentration and temperature simultaneously. The linear dynamic range of CO measurement is 0~10%, the detection limit is the order of 100ppm; the temperature measurement range is 300~1000°C; and the measurement error of vapor, CO concentration, temperature is below 5%, 5%, 10% respectively.7) This experimental apparatus is based on wavelength modulation tunable diode laser absorption spectroscopy. It is potential in coal power generation and steel smelting field since it has advantages of anti-interference, low detection limit, real-time online measuring, and is suitable for harsh working conditions. |
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