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当一束中心频率为ν0的激光束通过待测气体时,其入射和出射的光强可以用Beer-Lambert定律来描述[8-10, 13-15]:
$ I\left( {{\nu _0}} \right) = {I_0}{{\rm{e}}^ - }^\alpha $
(1) 式中,I(ν0)为经过气体吸收后的光强,I0为进入样品池的激光光强,α为带测量气体的吸收系数。
将频率为f正弦调制信号叠加到一个低频扫描信号作为注入激光器的电流控制信号时,那么出射光的瞬时频率和光强可以表示为:
$ \nu \left( t \right) = {\nu _0} + \alpha {\rm{cos}}\left( {2{\rm{ \mathit{ π} }}ft} \right) $
(2) $ \begin{array}{l} {I_0}\left( t \right) = I[1 + {i_1}{\rm{cos}}(2{\rm{ \mathit{ π} }}ft + {\varphi _1}) + \\ \;\;\;\;\;\;\;\;\;\;\;\;\;{i_2}{\rm{cos}}\left( {4{\rm{ \mathit{ π} }}ft + {\varphi _2}} \right)] \end{array} $
(3) 式中,I为激光平均光强,φ1, φ2, i1以及i2分别为激光器的特征参量。
如果样品吸收度小于5%,那么透过率τ的1阶泰勒级数的展开式表示为:
$ \begin{array}{l} \tau \approx 1 - {p_{{\rm{tot}}}}x{S^*}L\varphi \left( \nu \right) = 1 - \\ {p_{{\rm{tot}}}}x{S^*}L\sum\limits_{k = 0}^\infty {{{\rm{H}}_k}{\rm{cos}}(k2{\rm{ \mathit{ π} }}ft)} \end{array} $
(4) 式中,ptot为气体的总压强,x为带测量气体的体积分数,S*为气体的吸收谱线强度,L为气体吸收池的有效光程,φ(ν)为特征参量,Hk为k次的哈密顿量,可简写为:
$ {{\rm{H}}_k} = \frac{1}{\pi }\int_{ - \pi }^\pi {\varphi (\nu ){\rm{cos}}(k\theta ){\rm{d}}\theta } , (k = 1, 2, 3, \ldots ) $
(5) 令k=0时,那么k次的Hk简写为:
$ {{\rm{H}}_0} = \frac{1}{{2\pi }}\int_{ - \pi }^\pi {\varphi (\nu ){\rm{d}}\theta } $
(6) 式中, θ为相位。在一般情况下,特征参量i2$ \ll $1,那么在吸收谱线中心位置处波长调制的二次谐波信号分量简化为[16]:
$ {S_{2f}} \approx - G{\bar I_0}{p_{{\rm{tot}}}}x{S^*}L{{\rm{H}}_2}/2 $
(7) 式中, G是光电探测系统增益系数,I0为激光器的平均光强,H2为哈密顿的2次的分量。如果G, ptot, x, S*以及L都为常数时,则波长解调信号的各次谐波信号幅值与待测气体体积分数呈正比例关系。
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在燃煤电厂的尾气中含有大量的水和二氧化碳,因此选择吸收谱线时需要选择谱线尽可能强的谱线,同时在此吸收带中的水和二氧化碳的吸收强度相对尽可能的更小。图 1所示为HITRAN2012数据库中的氨气、水和二氧化碳在6524cm-1~6536cm-1范围内的吸收强度分布。由于实验中所采用的分布反馈式(distributed feedback,DFB)激光器输出的波长范围为6529.00cm-1~6529.47cm-1,因此选择中心波数为6529.1901cm-1(图中实线箭头所指)为氨气的吸收谱线。该谱线强度为10-21cm·mol-1,而附近最大的水和二氧化碳谱线强度为10-24cm·mol-1(图中虚线箭头所指),所以水和二氧化碳不会干扰氨气测量。
小型化燃煤电厂逃逸氨气激光测量仪
A miniaturized laser measurement instrument of ammonia escaping from coal-fired power plants
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摘要: 为了准确测量高温下逃逸氨的体积分数, 采用可调谐半导体激光吸收光谱技术、波长调制光谱技术(WMS)和长光程技术, 开发了一套高温小型化的逃逸氨气测量仪; 为了减小逃逸氨气的吸附效应和提高探测灵敏度, 研制了新型高温长光程样品吸收池。在前期研制的激光驱动模块基础上, 采用74HC4046锁相环芯片作为可调正弦调制信号源, 以EPM7064为移相和倍频逻辑控制芯片, 同时采用两片AD630作为一次解调(WMS-1f)和二次解调(WMS-2f)同步解调乘法器, 实现了吸收信号的1f和2f同步解调。此外, 以STM32F429为主控制器, 将解调滤波后的信号输入到AD7606进行模数转换, 并进行数字滤波和体积分数的反演。结果表明, 氨气体积分数与WMS-1f幅值、WMS-2f幅值以及同步解调WMS-2f/WMS-1f归一化幅度值的线性拟合系数分别为0.998, 0.997以及0.998;Allen方差表明在优化时间228s时, 其测量的体积分数最低为0.496×10-6, 在体积分数为20×10-6~100×10-6范围内测量误差小于±2%。该测量仪可以为燃煤电厂氨逃逸的高温测量提供高精度的原始数据。Abstract: In order to accurately measure volume fraction of the escaping ammonia at high temperature, tunable diode laser absorption spectroscopy, wavelength modulation spectroscopy (WMS) and long-path technology were used. A set of miniaturized measurement instrument for escaped ammonia gas at high temperature was developed. In order to reduce the adsorption effect of escaping ammonia and improve the detection sensitivity, a new type of sample absorption cell with high temperature and long optical path was developed. On the basis of the laser driving module developed earlier, phase-locked loop chip 74HC4046 was used as an adjustable sinusoidal modulation signal source. EPM7064 was used as phase shifting and frequency doubling logic control chip. At the same time, two AD630 chips were used as simultaneous demodulation multiplier of the first demodulation (WMS-1f) and the second demodulation (WMS-2f). The synchronous demodulation of 1f and 2f absorption signals was realized. In addition, STM32F429 was used as main controller and the demodulated and filtered signal was input to AD7606 for analog-to-digital conversion. Digital filtering and volume fraction inversion were performed. The results show that linear fitting coefficients of volume fraction of ammonia gas with WMS-1f amplitude, WMS-2f amplitude and the normalized amplitude of WMS-2f/WMS-1f are 0.998, 0.997 and 0.998, respectively. Allen variance shows that when the optimization time is 228s, the lowest volume fraction is 0.496×10-6. In the range of volume fraction 20×10-6~100×10-6, the measurement error is less than ±2%. The instrument can provide high precision raw data for high temperature measurement of ammonia escaping from coal-fired power plants.
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[1] ZOU D B, CHEN W L, DU Zh H, et al. Selection of digital filtering in the escaping ammonia monitoring with TDLAS[J].Spectroscopy and Spectral Analysis, 2012, 32(9): 2322-2326(in Chinese). [2] LIU F D, SHAN W P, SHI X Y, et al. Reseach progress in vanadium-free catalysts for the selective catalytic reduction of NO with NH3. Chinese Journal of Catalysis, 2011, 32(7): 1113-1128(in Chinese). [3] MA F N, CHENG W Q. The discharge status and controlling measures of nitrogen oxides of thermal power plants in China[J]. Guangzhou Chemical Industry, 2011, 39(15):57-59(in Chinese). [4] SRIVASTAVA R K, HALL R E, KHAN S, et al. Nitrogen oxides emission control options for coal-fired electric utility boilers[J]. Air Repair, 2005, 55(9):1367-1388. [5] ZHANG X, CAO Sh Y, GUO T X, et al. Research of methane volume fraction field reconstruction based on tunable diode laser absorption spectroscopy detection technology[J]. Laser Technology, 2018, 42(4):577-582(in Chinese). [6] YANG B, HE G Q, LIU P J, et al. Research progress of tunable di-ode laser absorption spectroscopy for combustion diagnostics[J]. Laser Technology, 2011, 35(4):503-510(in Chinese). [7] DONG Y H, WU Sh Q, ZHAI W, et al. Effect of modulated phase difference on TDLAS signal-to-noise ratio[J]. Laser Technology, 2013, 37(4):498-502(in Chinese). [8] SUN K, SUR R, CHAO X, et al. TDL absorption sensors for gas temperature and concentrations in a high-pressure entrained-flow coal gasifier[J]. Proceedings of the Combustion Institute, 2013, 34(2):3593-3601. doi: 10.1016/j.proci.2012.05.018 [9] CHAO X, JEFFRIES J B, HANSON R K. Development of laser absorption techniques for real-time, in-situ, dual-species monitoring (NO/NH3, CO/O2) in combustion exhaust[J]. Proceedings of the Combustion Institute, 2013, 34(2):3583-3592. doi: 10.1016/j.proci.2012.05.024 [10] CHAO X, JEFFRIES J B, HANSON R K. In situ absorption sensor for NO in combustion gases with a 5.2μm quantum-cascade laser[J]. Proceedings of the Combustion Institute, 2011, 33(1):725-733. doi: 10.1016/j.proci.2010.05.014 [11] YING H E, ZHANG Y J, YOU K, et al. Study on hydrogen fluoride at high temperature detection method with temperature correction based on laser technology[J]. Spectroscopy & Spectral Analysis, 2017, 37(3):964-970.. [12] NIE W, KAN R F, XU Zh Y, et al. Measuring spectral parameters of water vapor at low temperature based on tunable diode laser absorption spectroscopy.Acta Physica Sinica, 2017, 66(20):91-96. (in Chinese). [13] ZHANG L F, WANG F, YU L B, et al. The research for trace ammonia escape monitoring system based on tunable diode laser absorption spectroscopy[J].Spectroscopy and Spectral Analysis, 2015, 35(6): 1639-1642(in Chinese). [14] MA Y, HE Y, ZHANG L, et al. Ultra-high sensitive acetylene detection using quartz-enhanced photoacoustic spectroscopy with a fiber amplified diode laser and a 30.72kHz quartz tuning fork[J]. Applied Physics Letters, 2017, 110(3):031107. doi: 10.1063/1.4974483 [15] TU X H, LIU W Q, ZHANG Y J, et al. Second-harmonic detection with tunable diode laser absorption spectroscopy of CO and CO2 at 1.58μm[J]. Spectroscopy and Spectral Analysis, 2006, 26(7): 1190-1194(in Chinese). [16] LI C L, WU Y, QIU X B, et al. Pressure-dependent detection of carbon monoxide employing wavelength modulation spectroscopy using a herriott-type cell[J]. Applied Spectroscopy, 2017, 71(5): 809-816. doi: 10.1177/0003702816682194 [17] LI N, QIU X B, WEI Y B, et al. A portable low-power integrated current and temperature laser controller for high-sensitivity gas sensor applications[J]. Review of Scientific Instruments, 2018, 89(10): 103103. doi: 10.1063/1.5044230 [18] YE W L, ZHENG Ch T, WANG Y D. Stability measurement and temperature compensation of mid-infrared methane detection device[J]. Acta Optica Sinica, 2014, 34(3): 0323003(in Chinese). doi: 10.3788/AOS201434.0323003 [19] ALLAN D W. Statistics of atomic frequency standards[J].Proceedings of the IEEE, 1966, 54(2): 221-230. doi: 10.1109/PROC.1966.4634 [20] DANG J M, HU H Y, SONG F, et al. An early fire gas sensor based on 2.33μm DFB laser[J]. Infrared Physics and Technology, 2018, 92(5):84-89.