Advanced Search

ISSN 1001-3806 CN 51-1125/TN Map

Volume 43 Issue 5
Sep.  2019
Article Contents
Turn off MathJax
Article Navigation >  LASER TECHNOLOGY  > 2019 >  43(5): 697-701

Citation:
shu

A miniaturized laser measurement instrument of ammonia escaping from coal-fired power plants

  • 1.

    School of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China

  • 2.

    Key Laboratory of Modern Preparation of Traditional Chinese Medicine, Ministry of Education, Jiangxi University of Traditional Chinese Medicine, Nanchang 330004, China

  • Received Date: 2018-10-11
    Accepted Date: 2018-11-19
  • 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.
  • 加载中
  • [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.
  • 加载中
通讯作者: 陈斌, bchen63@163.com
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

  1. 本站搜索
  2. 百度学术搜索
  3. 万方数据库搜索
  4. CNKI搜索

Figures(9)

Get Citation
PDF
XML

Article views(5413) PDF downloads(25) Cited by()

Proportional views

A miniaturized laser measurement instrument of ammonia escaping from coal-fired power plants

  • 1. School of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China
  • 2. Key Laboratory of Modern Preparation of Traditional Chinese Medicine, Ministry of Education, Jiangxi University of Traditional Chinese Medicine, Nanchang 330004, China
  • Received Date: 2018-10-11
  • Accepted Date: 2018-11-19
  • Available Online: 2019-09-25

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.

    HTML

引言

  • 氨气是一种具有强烈吸附性和刺激性的无色气体，也是制造硝酸、化肥、炸药等化工和工业中的重要原料之一[1]。特别是在燃煤电厂中，尿素(液态氨)是选择性催化还原反应中的还原剂[2]，将燃煤烟气污染排放物中的氮氧化物NOx(如NO, NO2等)还原成氮气。在催化还原反应中，当氨气偏小，使得NOx反应不充分而造成脱硝不彻底；反之则造成氨逃逸，腐蚀烟气管道和测量仪器，同时排放到大气中成为大气污染物[3-4]。

    近年来，氨逃逸造成的环境污染问题已经引起了国内外学者的关注。由于半导体可调谐激光吸收光谱(tunable diode laser absolption spectrosc, TDLAS)技术具有选择性强、速度快、灵敏度高、原位测量等优点[5-7]，已经成为了燃煤电厂中的烟气大气污染物在线测量的首要方法之一[8]。斯坦福大学的HANSON小组对燃煤电厂污染排放物中的NH3在2.25μm附近的高温光谱进行了实验测量[9-10]。中国科学院安徽光学精密机械研究所LIU院士团队测量了氨气在1.5μm附近的光谱参量，包括修订常温下和高温下的线强以及高温下的体积分数反演算法[11-12]。ZHANG等人在2.25μm波段附近采用直接吸收和二次解调方法实现了逃逸氨气对不同温度下超低体积分数高分辨率快速测量，其测量的体积分数最低为0.496×10-6 [13]。

    由于1.5μm附近的激光器具有高性价比和稳定性优点，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作为波长调制光谱技术(wavelength modulation spectroscopy, WMS)一次解调(WMS-1f)和二次解调(WMS-2f)同步解调乘法器，实现了WMS-1f和WMS-2f同步解调(WMS-1f/WMS-2f)。同时为了解决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，实现了小型化的亚百万分之一量级的逃逸氨气在线测量。

1.   测量原理

    1.1.   波长调制原理

  • 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]：

    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：

    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：

    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:

    令k=0时，那么k次的Hk简写为：

    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]：

    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。

    • 1.2.   谱线选择

  • 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。

    Figure 1.  Absorption lines strengths for ammonia, H2O and CO2 based on HITRAN12 database

2.   系统装置研制

    2.1.   系统结构

  • 图 2为基于TDLAS技术新型高温氨气测量系统原理框图。其主要由DFB激光器、激光器驱动板、新型高温光程池、光电探测放大器、同步模拟解调锁相放大器(analog lock-in amplifier, ALIA)以及基于STM32F429的数据采集板等组成。数据采集板输出的扫描信号叠加上解调板的正弦信号后通过激光器驱动板对DFB激光器的注入电流进行光强调制，同时可以稳定激光器的工作温度。带尾纤输出的DFB激光器产生的激光束(中心波数为6529.1901cm-1)经光隔离器(40dB)隔离后由格林透镜进行准直，然后耦合进入高温Herriott型长光程吸收池中。

    Figure 2.  Schematic of ammonia measurement

    在吸收池内经多次反射，出射光经透镜汇聚到InGaAs光电探测器，实现光电信号转换，探测器信号输入到ALIA，同时解调得到一次谐波和二次谐波信号。最终根据一次和二次谐波信号幅值以及归一化比值反演得到气体体积分数。

    • 2.2.   激光器驱动[17]

  • 图 3为激光驱动的实物图和原理框图。该系统分为电源、温度控制单元和恒流驱动三部分。电源提供超稳定、低纹波(小于5mV)的12V电压，可驱动高电压激光二极管，比如蓝光激光器(工作电压大于5V)。温度控制电路采用模拟公司的ADN8834芯片(工作电压为5V，由LM2596转换芯片得到)。ADN8834可以为半导体热电制冷器(thermal electronic cooler，TEC)提供±1.5A的最大电流，这足以驱动几乎所有DFB激光器。恒流驱动电路基于Hall-Libbrecht结构设计，可以输出最大250mA的电流。

    Figure 3.  Physical diagram and schematic diagram of laser driver

    • 2.3.   同步模拟锁相放大器

  • 测量仪采用自主研制的模拟锁相放大器来实现信号解调，主要分为时钟和参考信号的产生、信号放大电路以及信号调制和解调三部分，其实物和原理图见图 4。

    Figure 4.  Picture and schematic diagram of ALIA

    测量仪采用74HC4046锁相环芯片作为可调正弦调制信号源，以EPM7064(complex programmable logic device，CPLD)为移相和倍频逻辑控制芯片，同时采用两片AD630作为WMS-1f和WMS-2f同步解调乘法器，实现了WMS-1f和WMS-2f同步解调。本测量仪的参考信号和调制信号为5kHz的正弦波。

    锁相放大器采用模拟公司的AD630作为同步解调乘法器。该芯片是基于开关式相敏解调器，集成输入放大器、电子开关、输出放大器等功能，单芯片的同步解调方案，可以从100dB干扰噪声中恢复原始信号。光电探测器输出信号采用跨阻放大电路将探测器的微弱电流信号转换为电压信号，同时采用PGA202对输入信号进行程控放大。PGA202不仅可以外接控制器对输入放大器的放大倍数进行自动化的设置，还能通过外接电容来设置该放大器输出信号的截止频率。被放大和滤波后的信号输入到两片AD630进行同步解调后经过积分电路和放大电路从而得到WMS-1f和WMS-2f。

    以STM32F429为主控制器，将谐波成分经过低通滤波器(low pass filter，LPF)滤波后的信号输入到AD7606进行模数转换，转换后的数据通过串行外设接口(serial peripheral interface, SPI)传输至主控制器中进行数字滤波和体积分数的反演。主控制器将得到的信号波形通过并行接口(parallel interface，PI)方式显示在4.7in的液晶显示(liquid crystal display, LCD)上，同时经串口(universal synchronous/ asynchronous receiver/transmitter，USART)将数据发送到计算机进行存储。

    • 2.4.   样品吸收池设计

  • 由Beer-Lambert定律可知，当经过吸收气体的激光光程变大，则探测灵敏度也会提高，探测极限会降低。该仪器中设计了一种新型高温Herriott型多光程吸收池，其基长为0.3m，反射次数为52次，有效光程为15.6m。该吸收池通过控制流过两根加热棒的电流，来使加热棒的热量传导到光程池的铁壳上。高温光程池的工作温度范围可达293K~523K，温度控制精度为±1K，其实物图如图 5所示。

    Figure 5.  Picture of new type high temperature multiple-pass Herriott cell

3.   结果与分析

  • 实验中，温度控制器通过动态调整加热棒的电流来实现光程池中的温度保持在453K，待炉膛温度稳定后(10min)，再分别充入不同体积分数的氨气标准气(压强为6800Pa，精度为1Pa)。实验前都需充入高纯干燥氮气吹扫，防止气体残留。

    图 6中给出了压强为6800Pa、温度为453K、体积分数20×10~6~100×10-6的氨气标准气体的光谱信号，其中信号强度用电压峰峰值表示，单位为V。从图中可得随着体积分数的增加，WMS-1f和WMS-2f信号的幅值随之增加。为了得到检测系统的谐波信号与气体体积分数的关系，将各体积分数和其对应的谐波信号幅值可得图 7。从图 7可以看出，氨气体积分数与WMS-1f和WMS-2f幅值呈良好的线性关系，拟合后得到：(1)WMS-1f其线性相关系数为0.998，拟合误差在±2%以内；(2) WMS-2f其线性相关系数为0.997，拟合误差在±2%以内。图 8给出了压强为6800Pa、温度为453K实验条件下的同步解调WMS-2f/WMS-1f归一化的信号幅值，伴随着氨气体积分数变大的情况。从图中可以看出良好的线性关系，其线性相关系数为0.998。

    Figure 6.  Harmonic signals of ammonia with volume fraction of 20×10-6~100×10-6

    Figure 7.  Relationship between amplitude and volume fraction

    Figure 8.  The WMS-2f/WMS-1f normalized peak amplitudes vs. volume fraction

    为了进一步分析新型氨气体积分数测量系统的稳定性，采用Allan-Werle方差对数据中噪声进行量化处理[18-20]，如图 9所示。从图 9可以看出，Allan-Werle方差伴随着时间增长，呈现减小后增大的趋势，当积分时间为228s时，探测最小体积分数为0.496×10-6。

    Figure 9.  Allan-Werle variance for ammonia with volume fraction of 20×10-6

4.   结论

  • 由TDLAS波长调制技术原理，针对燃煤电厂尾气高温环境条件研制了新型氨气检测系统。通过采用新型高温多光程和模拟一次和二次谐波信号同步解调相结合，可大大提高高温下逃逸氨气的探测灵敏度。实验结果显示，各次谐波信号幅值(WMS-1f，WMS-2f以及WMS-2f/WMS-1f)与体积分数具有相关系数大于0.997，Allen-Werle方差表明：在优化时间228s时，其探测最小体积分数为0.496×10-6。下一步实际的测量中，可以将待测氨气通过旁路管道引入到开发的传感器中进行实际测量，此外烟气管道中的实际压强带来的影响也需要进行校准, 以便进行更为精确的测量。

Reference (20)

Catalog

    Copyright © Laser Technology. 蜀ICP备10038222号-1

    Address：No. 7, Section 4, Renmin South Road, Chengdu, Sichuan Province, China Postal Code：610041  Telephone：028-68011091/68011046  Email： jgjs@vip.163.com,  jgjs@sina.com

    Supported by: Beijing Renhe Information Technology Co. Ltd  support: info@rhhz.net

    /

    DownLoad:  Full-Size Img  PowerPoint
    Return
    Return