基于可调谐激光吸收光谱技术的脱硝过程中微量逃逸氨气检测实验研究

为了对电厂脱硝过程中逃逸的微量氨气进行在线检测，实验室采用可调谐激光吸收光谱技术对常温常压下以及不同温度下的低浓度氨气进行了测量试验，其中电厂逃逸氨气检测处温度约为650 K。通过分析近红外波段的氨气吸收谱线，并考虑实际测量环境H₂O和CO₂等浓度很大的气体吸收谱线的干扰，实验选取2.25 μm附近的ν₂+ν₃谱线作为浓度检测谱线。为了验证所选谱线对低浓度NH₃的测量能力，实验对H₂O，CO₂和NH₃的吸收谱线进行模拟，发现低浓度NH₃受较大浓度的H₂O和CO₂谱线的干扰较小，尤其是CO₂谱线的干扰可以忽略不计，且2.25 μm处谱线强度远远大于通讯波段1.53 μm处的谱线。基于新型Herriott池以及高温管式炉，结合可调谐激光吸收光谱中的直接吸收技术和波长调制技术，实现了对不同温度下超低浓度NH₃的高分辨率快速检测。常温常压下其线型函数可以利用洛伦兹线型来近似描述，直接吸收测量技术可以使探测极限降低到0.225×10-6。通过采用简单降噪处理技术如多次平均、简单小波分析等，得到不同温度下的谐波信号与浓度具有良好的线性关系，为采用可调谐激光吸收光谱技术进行现场低浓度逃逸氨气检测提供了很好的依据。

The Research for Trace Ammonia Escape Monitoring System Based on Tunable Diode Laser Absorption Spectroscopy

In order to on-line measure the trace ammonia slip of the commercial power plant in the future, this research seeks to measure the trace ammonia by using tunable diode laser absorption spectroscopy under ambient temperature and pressure, and at different temperatures, and the measuring temperature is about 650 K in the power plant. In recent years lasers have become commercially available in the near-infrared where the transitions are much stronger, and ammonia’s spectroscopy is pretty complicated and the overlapping lines are difficult to resolve. A group of ammonia transitions near 4 433.5 cm-1 in the ν₂+ν₃ combination band have been thoroughly selected for detecting lower concentration by analyzing its absorption characteristic and considering other absorption interference in combustion gases where H₂O and CO₂ mole fraction are very large. To illustrate the potential for NH₃ concentration measurements, predictions for NH₃, H₂O and CO₂ are simultaneously simulated, NH₃ absorption lines near 4 433.5 cm-1 wavelength meet weaker H₂O absorption than the commercial NH₃ lines, and there is almost no CO₂ absorption, all the parameters are based on the HITRAN database, and an improved detection limit was obtained for interference-free NH₃ monitoring, this 2.25 μm band has line strengths several times larger than absorption lines in the 1.53 μm band which was often used by NH₃ sensors for emission monitoring and analyzing. The measurement system was developed with a new Herriott cell and a heated gas cell realizing fast absorption measurements of high resolution, and combined with direct absorption and wavelenguh modulation based on tunable diode laser absorption spectroscopy at different temperatures. The lorentzian line shape is dominant at ambient temperature and pressure, and the estimated detectivity is approximately 0.225×10-6 (SNR=1) for the directed absorption spectroscopy, assuming a noise-equivalent absorbance of 1×10-4. The heated cell experiments with controlled the temperature were performed to validate the sensing strategy. Here the Wavelength Modulation Spectroscopy (WMS) strategy was usually used to measure lower gas concentration for high noise immunity to the non-absorption transmission losses. The great agreement 2f signal with the calibrated concentration is within the uncertainty at different temperatures by using simple digital signal processing such as multiple averages, wavelet analysis and so on. The denoise processing has a great advantage in application and implementation over other noise suppression techniques. The result provided a good basis for trace ammonia escape detection based on tunable diode laser absorption spectroscopy.