Research On Ammonia Detection Based On Tunable Diode Laser Spectroscopy Sensing Technique

The atmosphere of nitrogen compounds contain a lot of ammonia. The amount of ammonia is third, only less than that of N2and N2O, and it is one of major pollution sources in the atmosphere. With the improvement in the quality of human life, the requirements of the surrounding living environment are also increased, so research on the ammonia detection in real-time and online is of great significance. However, the electrochemical method is the main detection method of ammonia now. Additionally, the method has a long sampling time, low measurement accuracy, vulnerability to external influences and not easy to achieve on-line monitoring. This paper uses tunable diode laser absorption spectroscopy sensing technique to detect the concentration of ammonia in atmosphere, and the technique has a fast response, high sensitivity, good selectivity as well as real-time on-line continuous monitoring and so on. The success of this study will lay a good theoretical basis and experimental evidence for the detection of other trace gases.This paper describes the tunable diode laser absorption spectroscopy (TDLAS) principles based on Lambert-Beer's law and choice of light absorption of gas molecules. According to the distribution of near-infrared ammonia absorption peak, we studied the distributed feedback (DFB) laser and the relationship of the laser works with the current changes. The DFB laser with the center frequency of1531.14nm is used as light source in experiment. The laser output characteristics are tested by spectrometer. To improve the system detection sensitivity, the paper studied the effects of the system detection sensitivity factors. To avoid low frequency noise interference and improve system detection sensitivity, we proposed wavelength modulation and second harmonic detection methods. And according to the characteristics of an open optical path testing, we designed the optical path based on reflected system. Paper designed the whole hardware circuit system based on detection principle, including DFB laser driver module, DFB laser temperature control module, laser receiver module and data acquisition and serial communication modules. Finally, we tested the TDLAS hardware system and get a good result.In this paper, a qualitative test for the ammonia detection system in three different experimental conditions is made. According to the experimental result, the relationship between test data and the environment, and the relationship between ammonia concentration and the second harmonic signal are analyzed. Experiment results show that the relationship between absorption spectrum of the second harmonic signal peaks and ammonia concentration is linear.