Research Of In-situ Ammonia Online Measurement System Based On TDLAS

With the trend that more and more people focus on the problem of environmental pollution, the emission standard of polluting gas is increasingly severe, and the measurement of trace gas is already an inevitable trend. Ammonia, a kind of polluting gas, as the reducing agent, is applied to desulfurization and denitrification in power plant, so the concentration detection of escaping ammonia is very important. Normally, the concentration of escaping ammonia is limited to 3 ppm or less, it's very low and the signal is very weak, so the requirements for measurement system precision and stability is much higher.To solve the difficulties of escaping ammonia detection, the paper designs an in-situ online measurement system for escaping ammonia, which has a high-precision and a good stability. The system, which is based on TDLAS theory, uses STM32 as the MCU and DFB-LD as the laser source, and takes wavelength-modulation spectroscopy and second harmonic detection as the measurement method of small signal. After sampled by 24-bits resolution AD converter and processed through digital filtering, the calculated result of ammonia concentration is sent to master computer by MCU. The system designs the in-situ cube-corner prism system to increase absorption path, improve detection precision, reduce difficulties of assembling and debugging and enhance system stability. In terms of signal processing, the system optimizes the spectrum by deducting the saw wave background signal caused by incident light intensity to reach high-precision and high-accuracy. By analyzing the result of experiment, the resolution of the system can reach 0.1ppm.The paper is divided into three parts---theory, system design and experiment. The part of theory includes the principle of Flue Gas Denitrification, the principle of DFB laser, the TDLAS theory and the deduction of second harmonic coefficient. About the second part, the paper describes the overall scheme, the selection of measuring method, the design of in-situ cube-corner prism system, optical path and air chambers, and the development of hardware and salve computer software. The part of experiment involves the optimization of modulation parameter, the standardization of detection signal and the analysis of detection precision.