FTIR固定污染源VOCs在线监测系统

挥发性有机物(VOCs)造成了全球环境污染，给人们日常生活工作带来不利影响。对挥发性有机物进行高效准确监测成为我国大气环境治理的热点。与其他污染物气体相比，VOCs更易挥发并可以和其他污染物发生反应，其物理化学性质的复杂性对已有的检测方法提出了很高的要求。在众多的气体检测方法中，光谱检测技术以其方便快捷、检测准确等优点得到了广泛应用。傅里叶变换红外光谱(FTIR)作为光谱检测技术中重要的一员，不仅可以多通道快速检测，还可以分析上百种污染物种类并实时计算污染物浓度，解决了VOCs气体性质复杂带来的困扰。开展了固定污染源VOCs在线监测系统的研制，整套系统基于傅里叶变换红外光谱，干涉仪出射的红外干涉信号被10 m光程的气体池中的目标气体吸收后进行傅里叶变换，得到含有气体特征吸收峰的红外光谱；将红外光谱与标准谱库的数据进行对比分析即可实现对目标气体的种类鉴定和浓度测量。系统覆盖650～4 000 cm-1光谱范围，由于大多数VOCs在中红外指纹区具有相对独立的吸收峰，因此可实现对多种气体的分析检测。光谱分辨率为1 cm-1，浓度检测范围为1.6～319.47 mg·m-3(以苯为例)。系统对甲苯、丙酮、乙酸乙酯等十几种VOCs进行分析测试，得到不同气体的红外光谱图，与标准数据库吻合得很好，并且可以根据不同气体的吸收峰对其进行区分。为了得到气体的准确浓度，需要对仪器进行标定。为降低气体在气体池内腔和反射镜上的吸附并控制水蒸气含量，加入温控系统对气体池温度进行实时监测。实验中通入不同浓度的二甲苯标准气体，利用五点标定法得到分析浓度与标准浓度之间的关系，分析浓度的相对偏差小于0.06%。为验证系统在实际工作场景下的性能，选取某喷涂车间，对喷漆过程中溶剂和稀释剂挥发形成的VOCs污染进行一周的监测，得到苯、甲基乙基酮、异丙醇以及乙酸乙酯四种主要污染气体的浓度变化。设定浓度安全阈值为安全作业提供参考。从长时间测试数据分析，系统平均无故障时间(MTBF)长达1 000 h，可长时间稳定可靠地实时监测。

The Online Monitoring System of VOCs Emitted by Stationary Pollution Source Based on FTIR

VOCs (Volatile organic compounds) not only cause global environmental pollution, but also have negative impact on people’s daily life. Efficient and accurate monitoring of VOCs has become a hot issue in China’s atmospheric environment governance. Compared with other gaseous pollutants, VOCs are more volatile and easily react with other gaseous pollutants. The complexity of their physical and chemical characteristics sets higher requirements on existing detection methods. Among various gas detection methods, spectral detection technology has been widely used due to its advantages of convenience, rapidity, and accuracy. As an important spectrum detection technology, FTIR (Fourier transform infrared spectroscopy) is multi-channel, which could analyze hundreds of pollutants and calculate real-time pollutant concentrations as well, solving the problems caused by the complex nature of VOCs gas. This paper has tested the online monitoring system of VOCs emitted by stationary pollution source, which is based on FTIR. The infrared interference signal emitted by the interferometer is absorbed by the target gas in the 10-m path length gas pool and subjected to Fourier Transform to obtain the infrared spectrum containing the characteristic absorption peak of the gas. In addition, the comparison between the infrared spectrum and standard database helps with identification of the target gas and concentration measurement. This system covers a spectral range of 650～4 000 cm-1. Since most VOCs have relatively independent absorption peaks in the mid-infrared fingerprint area, the analysis of multiple gases can be completed with a spectral resolution of 1 cm-1 and a concentration detection range of 1.6～319.47 mg·m-3 (Take Benzene as an example). What’s more, the system analyzes and tests dozens of VOCs, such as toluene, acetone, and ethyl acetate, obtaining infrared spectra of different gases, which coincides well with the standard database and can be differentiated according to the absorption peaks of different gases. In order to obtain the exact gas concentration, instruments need calibration as well as to reduce the adsorption of gas in the inner chamber and the mirror and control the water vapor content, a temperature control system is added to monitor the temperature of the gas pool in real time. Meanwhile, with xylene standard gas with different concentrations inflated, this experiment uses the five-point calibration method to obtain the relationship between the analytical concentration and the standard concentration, leading to a relative deviation of the analytical concentration less than 0.06%. To verify the performance of the system in actual working scenario, this paper selects a coating workshop to monitor the VOCs pollution caused by volatilization of solvents and diluents in coating process for one week obtaining concentration changes of benzene, methyl ethyl ketone, isopropyl alcohol and ethyl acetate. Concentration safety threshold is set to guarantee safe operations. From the long-time data analysis, the system MTBF (Mean Time Between Failure) is as long as 1 000 h, which provides a lasting, stable and reliable real-time monitoring.