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1、基于激光技术的天然气微水和微硫的测定Laser Based Trace Moisture and Hydrogen Sulfide Detection in natural gases Xin Zhou (周欣)、Xiang Liu (刘翔) 、 Alfred FeitischSpectraSensors, Inc.11027 Arrow RouteRancho Cucamonga, CA, USA摘要：天然气中水分和硫化氢含量的准确测量具有至关重要的意义。本文介绍可调谐二极管激光吸收光谱技术以及SpectraSensors公司研发的用于测量天然气中微量水分和硫化氢含量的新型检测仪。通过分析水
2、分子和硫化氢分子以及天然气各成分在近红外区域(1-3mm)的吸收光谱，SpectraSensors公司选择了水分子和硫化氢分子在近红外区域中的最佳吸收谱线。这些谱线具有很强的吸收强度以及最少的背景气体的光谱干扰。SpectraSensors检测仪采用波长调制光谱技术和多次反射Herriott 型样品室来提高信号测量的信噪比和测量灵敏度。微量水分检测仪可以准确测量天然气中0-3 ppmv的微量水分含量，测量重复性 (3s) 优于 50 ppbv. 硫化氢检测仪可以准确测量天然气中0-5 ppmv的硫化氢含量，测量重复性 (3s) 优于 500 ppbv.关键词：可调谐二极管激光吸收光谱法、微量水
3、分检测、微量硫化氢检测、天然气、微水、微硫中图分类号：TH83, 文献标识码：B Abstract: Accurate measurements of trace moisture and Hydrogen Sulfide in Natural Gas (NG) are of critical importance. This paper will present the SpectraSensors analyzers developed for the sensitive detection of trace moisture and hydrogen sulfide in natural
4、 gases. Water vapor and hydrogen sulfide absorption spectra have been analyzed in the whole near-infrared range (1-3 mm). The most promising water (H2O) and hydrogen sulfide (H2S) absorption transitions are identified. Both transitions have strong absorption strength and are relatively isolated from
5、 the spectral interferences by other species in natural gases. Wavelength Modulation Spectroscopy (WMS) is utilized to enhance the signal-to-noise ratio (SNR) of the absorption measurements. The sample cell uses a multi-pass Herriott cell design to provide a longer path length and thus better measur
6、ement sensitivity. The demonstration experiments validate that humidity levels of 0-3 ppmv water vapor in natural gases can be accurately measured with repeatability (3s) better than 50 ppbv. Hydrogen sulfide levels of 0-5 ppmv in natural gas can be accurately measured with an excellent repeatabilit
7、y (3s) of 500 ppbv. KEYWORDS：Tunable Diode Laser Absorption Spectroscopy (TDLAS)、Trace Moisture Detection、 Trace Hydrogen Sulfide Detection、 Natural Gases0 简介天然气是当今世界上最重要的能源之一。天然气在向外传输之前需要进行净化处理。水分是天然气中重要的污染物之一，在天然气的净化处理和传输的各阶段都需要实时监测水分含量。过多的水分会导致管道中水化物的形成以及运输管道的腐蚀。这可能导致严重的安全事故以及昂贵的停输成本。过多的水分也会稀释天然气
8、从而降低天然气的热值和品质。由于天然气价格的上涨以及世界各地对天然气需求的快速增长，液化天然气的处理和传输得到了越来越多的关注。天然气中的水分在低温处理设备中结冰会损坏液化和压缩设备，因此在液化天然气的预处理、降温、加压等过程中需要精确地监测及控制水分含量。硫在天然气中以硫化氢的形式存在。天然气中的硫化氢会腐蚀天然气运输管道，从而可能导致天然气泄漏和爆炸事故的发生。并且，硫化氢燃烧后会生成另外一种有毒气体二氧化硫。二氧化硫是酸雨形成的主要原因。因此，准确、灵敏、快速和可靠地测量天然气中的水分和硫化氢含量具有至关重要的意义。传统的水分测量技术主要基于各种接触型传感器例如冷镜、电化学及压电传感器等
9、。这些传感器的确能在校准后的短时间内提供可靠的测量。但传感器的表面由于暴露在待测流体当中，易被醇、胺、油等杂质所污染，从而导致巨大的测量误差甚至整个传感器的失效。所以这些接触型传感器通常需要频繁的清理甚至更换传感元件，导致大量的维护费用。另外，接触型传感器响应速度慢，通常具有很长的浸润及干燥延迟。 硫化氢的传统测量技术主要有醋酸铅法，宽带非色散式紫外线测光法，气相色谱或电化学法。这类传感器的缺点包括校准漂移，测量响应速度慢，传感器饱和，传感器饱和后恢复时间长，传感器部件易被背景气体污染，等等。而且，这类传感器维护费用高，需要频繁地更换传感元件例如紫外线灯，醋酸铅试纸，气相色谱柱，消耗大量载气和
10、氨水溶液。因此，这些传统的测量方法不适于在线检测和实时的控制天然气中水分和硫化氢含量。可调谐二极管激光吸收光谱技术（Tunable Diode Laser Absorption Spectroscopy, TDLAS）作为一种新型的非接触测量方法，已被广泛应用于各种环境下的气体检测。1-4 它与上述传统技术相比，具有响应灵敏、速度快，测量可靠、精度高，维护成本低，产品寿命长等优点。本文介绍可调谐二极管激光吸收光谱技术以及SpectraSensors公司研发的用于测量天然气中微量水分和硫化氢含量的新型检测仪。该检测仪利用波长调制TDLAS技术，测量透射激光信号在调制频率两倍频处的分量，从而屏蔽在
11、测量带宽范围外来自激光器、光电传感器及气体流动的噪音，显著提高测量的信噪比、敏感度和准确性。通过分析水分子和硫化氢分子以及天然气各成分在近红外区域(1-3mm)的吸收光谱，SpectraSensors公司选择了水分子和硫化氢分子在近红外区域中的最佳吸收谱线。这些谱线具有很强的吸收强度以及最少的背景气体的光谱干扰。SpectraSensors检测仪采用波长调制光谱技术和多次反射Herriott 型样品室来提高信号测量的信噪比和测量灵敏度。微量水分检测仪可以准确测量天然气中0-2 ppmv的微量水分含量，测量重复性 (3s) 优于 100 ppbv。硫化氢检测仪可以准确测量天然气中0-5 ppmv
12、的硫化氢含量，测量重复性 (3s) 可达 500 ppbv。1 可调谐二极管激光吸收光谱技术的基本原理根据激光的调制和信号的检测，可调二极管激光吸收光谱技术可以分为两类：直接吸收光谱技术和波长调制吸收光谱技术。这两种技术的基本原理已经在许多文献中进行了详细的介绍 1-4, 9-14。 本文只概括介绍与开发微量水分和硫化氢新型检测仪相关的基本概念和原理。 可调谐二极管激光吸收光谱技术的基本原理是朗伯-比耳定律(Beer-Lamberts law)。朗伯-比耳定律描述了当单色光穿过均匀气体介质时透射光强和入射光强的关系, 如方程(1)所示 ,(1) P 为气体的压力 T 是样品气体的温度 Xabs
13、 是被测气体在样品气体中的摩尔百分比 (本文中被测气体为水和硫化氢，样品气体为天然气) L 为光路长度 S 为吸收谱线的强度 fn 为吸收谱线的线型函数 光谱吸收率定义为 .(2)吸收谱线的线型函数fn 描述了在特定波长的光谱吸收率与样品气体压力、温度以及被测气体浓度之间的关系。 TDLDETECTOR图 1 波长调制吸收光谱技术的实验装置及基本原理。Figure.1 Schematic of the WMS-2F based TDLAS measurements波长调制吸收光谱技术是一种高灵敏度的吸收光谱技术。通过高频(kHz)调制二极管激光的波长以及测量透射激光信号在调制频率两倍频处(2f
14、)的分量，波长调制吸收光谱技术可以成功的屏蔽在测量带宽范围外来自激光器、光电传感器及气体流动的噪音，显著提高测量的信噪比、敏感度和准确性。9-14 图1显示了波长调制吸收光谱技术的实验装置以及基本原理。二极管激光的输入电流叠加了一个高频f Hz的正弦调制信号, 激光的输出频率n(t) 可以通过方程(3)中的线性频率调制(FM)来描述 ,(3) w = 2pf 是角频率 cm-1 是激光的中心频率 a cm-1 是频率调制幅度 探测器测量的激光透射强度信号It(t)连接到锁相放大器, 用于测量透射激光信号在调制频率两倍频处的分量。特定波长的调制频率两倍频处的分量可以用方程(4) 中数学模型得到1
15、3 ,(4) G 为检测系统的增益 为在频率激光平均强度 i0 为激光强度调制(IM)幅值 Hk 为光谱吸收率傅里叶余弦变换的第k个系数.(5)方程(5)中的叠加项考虑了光谱吸收率来自附近吸收谱线的影响。 在被测气体的浓度很小的情况下，吸收谱线的线型函数不受被测气体的浓度影响，因此2f信号正比于被测气体的浓度。根据2f信号和气体浓度之间的这一线性关系，波长调制吸收光谱技术可以用于各种环境中的微量气体检测。 9-14 2 测量系统和实验装置图2所示为测量系统和实验装置的示意图。测量系统包括样品室、可调二极管激光器、探测器、微处理器电路以及数据分析软件。为了提高系统的信噪比和灵敏度，本文采用多次反
16、射Herriott 型样品室15-16来增加光路长度。Herriott 型样品室由两个平行且同轴的等焦距凹面镜组成。激光束由凹面镜上的一个小孔入射，在腔内经过多次反射后从相同的入射孔射出。激光束在腔内反射的次数以及激光光斑在凹面镜上的分布和形状由凹面镜的曲率半径和两个凹面镜之间的距离来确定。凹面镜的曲率半径和间距采用最优化的设计，因此该Herriott 型样品室具有光路调节方便、体积小和光学干扰小等优点。图2 测量系统和实验装置的示意图Figure.2 Block diagram of the TDLAS analyzer system分布式反馈(DFB)可调二极管激光器波长的扫描通过锯齿波调
17、制输入电流实现，波长扫描频率4Hz，扫描范围为目标水分子和硫化氢吸收谱线附近0.3 波数 (cm-1) 。可调二极管激光器的输入电流在上述锯齿波上同时叠加了高频的正弦信号。可调二极管激光器的温度由反馈电路中热电致冷器控制。通过样品气体后的激光器信号经过InGaAs探测器的测量和放大后，输入到电路中的锁相放大器从而得到直流（DC）和2f信号。16位的微处理器用于控制激光器的输入电流，同步采集信号，从测量的DC和2f信号中计算水分子和硫化氢的含量，将计算结果传送到用户的输入输出接口。样品气体首先通过膜分离器去除气体中的杂质，然后流入Herriott 型样品室。气体采样系统固定在绝缘的不锈钢的封闭室
18、中，封闭室的温度控制在500.1 C。恒定的温度控制消除了水分子和硫化氢分子在管路、阀门以及样品室壁面上吸附-脱附效应对测量精度的影响。3 天然气中水和硫化氢含量的测量图3 水分子和甲烷在红外区域1 -10微米的吸收光谱。Figure.3 NIR spectra of H2O and CH4吸收谱线的选择是研发基于可调谐二极管激光吸收光谱技术分析仪的重要环节。选择最佳的吸收谱线可以有助于降低仪器的成本，提高检测的精度和灵敏度。图3所示为HITRAN2004 (HIgh-resolution TRANsmission molecular absorption) 17光谱数据库中水分子和甲烷分子在
19、红外区域的吸收光谱。本文的谱线选择集中于1-3微米的近红外区域，因为这一区域中的激光及光学元件比中红外区域技术上更为成熟而且经济可靠。通过分析水分子和硫化氢分子以及天然气各成分在近红外区域(1-3mm)的吸收光谱，SpectraSensors公司选择了水分子和硫化氢分子在近红外区域中的最佳吸收谱线。CH4H2O+CH4图4 水分子和天然气背景中的DC和2f信号Figure.4 DC and 2f signals for different moisture levels in natural gas background图4所示为0.2-3.0 ppmv 的水分含量在天然气中的吸收光谱DC和2
20、f信号。在DC信号中无法看到水的吸收信号，而在2f 信号中可以明显地看到2f波峰高度随水浓度的变化。这也证明了波长调制吸收光谱技术比直接吸收光谱技术具有更高的测量灵敏度（至少高一个数量级9-14）。SpectraSensors的气体检测仪利用DC信号来规格化2f的峰值，从而消除激光强度变化对测量的影响。通过调节波长调制的幅度，可以将2f信号的峰值最大化并且减小峰值对压力的依赖性14。图5 天然气中微量水分的测量结果Figure.5 Measurement results for trace moisture in natural gas作者设计了一套气体混合系统以获得该分析仪校准所需的标准样气
21、。该气体混合系统的所有部件材质均为电镀不锈钢。整套系统保持绝热恒温（50 C），以防止因水蒸气吸附或释放于气体流道壁面而改变微量水分的含量。样气由多路气流均匀掺混而成。一路为预混的带微量水分的空气或氮气（水分含量已知，通常为50-100 ppmv，由生产商认证）。其余几路 为高纯度的甲烷，乙烷，丙烷，丁烷和二氧化碳。在混合之前，这几路气流通过高效的干燥器以去除其中的未知微量水分。高精度的质量流量控制器精确控制每股气流的流量。使用不同的混合比则会产生带不同水分含量的标准样气。分析仪的读数由一台NIST可追溯的冷镜湿度计来校准。图5所示为微量水分在天然气成分中的测量结果。混合气体成分为CH4 =
22、82%, C2H6=9.7%, C3H8=3.9%, C4H10=1.3%, CO2=3.4% 。水分的浓度为0.2 ppmv 到 3 ppmv。测试结果显示分析仪响应迅速、测量精度高，读数对实际水分含量的线性度优异。在天然气中，分析仪的测量重复性(3s)可达25-50 ppbv。 图6 包含/不包含硫化氢的天然气2f光谱及其差分2f光谱Figure.6 The raw 2f spectra of trace H2S in natural gas and its differential spectra由于硫化氢的的最优谱线仍然受到天然气背景气体(例如甲烷)吸收光谱的严重干扰，作者开发了吸收光
23、谱差分技术以消除背景气体的光谱干扰。简单来说，每隔一段时间，采样的原始气体被切换流经一个高效的硫化氢吸收器（99.9%）以去除其中的硫化氢，从而获得背景气体的2f吸收光谱。如 图6 所示，分析仪分别测量原始样气及去除硫化氢后背景气体的2f吸收光谱，两者之差即为代表硫化氢含量的差分2f光谱。图7 不同浓度硫化氢在天然气中的差分2f吸收光谱。 Figure .7 Differential spectra for different H2S concentration图8 天然气中微量硫化氢的测量结果 Figure.8 Measurement results for trace H2S in nat
24、ural gas图7所示为不同浓度硫化氢在天然气中的差分2f吸收光谱。该图测量中使用的典型天然气成分为85%的甲烷、10%乙烷以及5%的丙烷。即使在1ppmv的微量浓度，硫化氢2f峰值依然具有很高的信噪比。图8所示为微量硫化氢在天然气成分中8.5小时的测量结果。硫化氢的浓度从10 ppmv 递减到 1 ppmv。测试结果显示分析仪响应迅速、测量精度高，读数对实际硫化氢含量的线性度优异。在天然气中，分析仪的测量重复性(3s)可达300-500 ppbv。 4 结论本文介绍可调谐二极管激光吸收光谱技术以及SpectraSensors公司研发的用于测量天然气中微量水分和硫化氢含量的新型检测仪。该分析
25、仪利用水分子和硫化氢分子在近红外区域中的最佳吸收谱线来实现水分和硫化氢含量的测量。SpectraSensors检测仪采用波长调制光谱技术和多次反射Herriott 型样品室来提高信号测量的信噪比和测量灵敏度。并采用差分吸收光谱技术来消除天然气成分吸收光谱对硫化氢测量的干扰。微量水分检测仪可以准确测量天然气中0-3 ppmv的微量水分含量，测量重复性 (3s) 优于 50 ppbv. 硫化氢检测仪可以准确测量天然气中0-5 ppmv的硫化氢含量，测量重复性 (3s) 优于 500 ppbv.致谢作者感谢Spectrasensors 的全体同事对开发这一新型分析仪所提供的所有支持和帮助。参考文献1
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34、ner, “Off Axis Paths in Spherical Mirror Interferometers,” Appl. Opt. 3, 523-526 (1964).16. Herriottt and Harry J. Schulte, “Folded Optical Delay Lines,” Appl. Opt. 4, 883-887 (1965). 17. Available at http: /cfa-www.harvard.edu/hitran/. Editors note: Judson Jones is a meteorologist, journalist and p
35、hotographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radio and through the crackling static from space hear the faint beeps of the worlds
36、 first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug on the shuttle program I was heartbr
37、oken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. Im anxious for the next one: a space
38、capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from the edge of space - live.The (lack of) air up there Watch man jump from 96,000 f
39、eet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a vertical straight line we would be go for launch.I feel this mission was created fo
40、r me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a level I will never reach. However, it did not s
41、top me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia
42、almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communications), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the disappointing news: Mission aborted.Th
43、e supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through
44、the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he
45、 crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I would assume, he will slowly step out onto som
46、ething resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell be traveling fast, so despite the distance, it will not be like diving into the deep end of a pool. It will be like he is diving into t
47、he shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he approaches the more dense air closer to Earth. But this will not be enough to stop h
48、im completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Instead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute tha
