基于波长调制光谱非线性特征的气体浓度宽量程检测方法
Tunable diode laser absorption spectroscopy, a commonly used gas concentration detection technology, has the advantages of non-contact real-time measurement, high sensitivity, and strong selectivity. It includes direct absorption spectroscopy and wavelength modulation spectroscopy. Compared to direct absorption spectroscopy, wavelength modulation spectroscopy technology has a strong anti-interference ability, higher sensitivity, and lower detection limit; it has been widely used in environmental monitoring, industrial gas detection, combustion diagnosis, and other fields. However, real-time wide-range detection of gas concentration has increasingly become a necessity. For example, the volume fraction of methane in coal mines and petrochemical pipelines varies from 0% to 100%, and the water vapor in air fluctuates significantly. Therefore, there is an urgent need for a new method for wide-range detection of gas concentration in petrochemical pipelines, coal mines, and other fields.
To meet the requirements of wide-range detection of gas concentration in many fields, this study utilizes the high sensitivity characteristics of wavelength modulation spectroscopy, examines the nonlinear characteristics of wavelength modulation spectrum (WMS-NL), and then achieves high sensitivity and wide range detection of gas concentration using the wavelength modulation method. According to the principle of laser absorption spectroscopy, the Taylor expansion of the absorption term is analyzed. Specifically, linear approximation and cubic polynomial approximation of the Taylor expansion are adopted at low concentration (low absorbance) and high concentration (high absorbance), respectively. Moreover, methane (CH4) is taken as an example to verify the feasibility of this method in the wide-range detection of gas concentration. Additionally, combined with the three parameters of absorption line intensity, effective optical length, and gas concentration, the specific details of the method are described based on the calculation of the absorbance of CH4.
Based on experimental verification, this method can achieve the detection of CH4 volume fraction in the range of four orders of magnitude (1.5×10-6-10000×10-6). The volume fractions below and above 1000×10-6 (the corresponding integrated absorbance is below and above 0.0236) are detected separately, and there is a good linear correlation between the inverted concentration and the standard concentration. The correlation coefficients in both the low and high concentration ranges are 0.999. In addition, combined with this method, the error, detection limit, and stability of the CH4 detection system are analyzed. In the range where the volume fraction exceeds 1000×10-6, the maximum relative measurement error is 0.93% and the absolute error is 92.1×10-6. Similarly, in the range where the volume fraction is lower than 1000×10-6, the maximum relative measurement error is 4.00% and the absolute error is -34.2×10-6. In addition, CH4 with a volume fraction of 5000×10-6 is measured for a period of time, and then the Gaussian distribution of the inverted concentration is counted. Its half width at half maximum is 15.9×10-6, and the stability of this method is well demonstrated under high concentrations.
The proposed method overcomes the limitation that conventional wavelength modulation spectroscopy can only measure low concentrations, provides a new idea for wide-range detection of gas concentration, and can considerably expand the application ranges of wavelength modulation spectroscopy.
1 引言
可调谐半导体激光吸收光谱技术(TDLAS)作为一种常用的气体浓度检测技术,具有非接触实时测量、灵敏度高、选择性强等优点[1-3]。其包含了直接吸收光谱技术(DAS)和波长调制光谱技术(WMS)。DAS具有免标定、电路结构简单的优点,但是容易受到各种噪声的影响,例如激光器光强波动、散粒噪声等[4-5],大大影响了测量精度。相比之下,WMS抗干扰能力强,具有更高的检测灵敏度和更低的检测下限,已经被广泛应用于环境监测[6-7]、燃烧诊断[8]、工业气体检测[9]、同位素检测[10-11]等领域。但是随着工业的发展,气体浓度的实时宽量程检测需求不断增加。例如,煤矿中甲烷的体积分数会在0%~100%范围内变化[12],空气中的水汽含量也会有较大范围的波动[13],石油石化管网的甲烷会发生泄漏等[14]。
通常情况下,当吸收度小于0.05时,波长调制光谱具有良好的线性度[3]。事实上,当吸收度在0.02~0.03范围时,使用WMS进行气体浓度测量就会出现非线性[15]。这就导致了WMS的实际有效测量范围为2~3个数量级。基于以上情况,研究人员针对宽量程气体检测进行了相关研究。庞涛等[16-17]提出了一种WMS和DAS相结合的测量方法,设计了一种宽温紧凑型全量程甲烷传感探头,其检出限为224×10-6,在高浓度下测量误差小于真值的±5%。赵晓虎等[15]利用类似方法对CH4、CO、C2H2气体进行了宽量程检测。另外,Zheng等[18]采用WMS实现了甲烷气体的全量程检测,最低检测限为11×10-6,最大相对误差为7%,但未对高浓度气体检测时出现的非线性进行详细分析。
为了解决以上问题,本文基于波长调制的高灵敏特征[19],通过研究波长调制光谱的非线性特征(WMS-NL),仅利用波长调制方法,实现了对气体浓度的高灵敏和宽范围检测。首先从激光吸收光谱的基本原理出发,分析了气体吸收项泰勒展开式的线性近似和三次多项式近似,并将两者结合,提出了一种波长调制光谱线性和非线性特征相结合的宽量程气体浓度检测方法。该方法在小吸收度时采用线性拟合计算气体浓度,而在大吸收度时将直接吸收项中的指数运算近似为三次多项式。其次,基于积分吸收度的计算,结合气体吸收光程、线强和测量浓度范围三个参数,对该方法的具体细节进行了阐述。同时为了验证该方法的可行性和有效性,搭建了甲烷气体检测系统并进行了实验验证。最后,对实验结果进行了详尽的分析和讨论。
2 理论分析
不同于直接吸收光谱技术,波长调制光谱技术需要在激光器扫描信号上叠加高频的正弦调制信号,那么激光器在某一时刻
式中:
激光吸收光谱技术基于Lambert-Beer定律,待测气体吸收前后的光强变化为
式中:I0为出射激光强度;It为透射激光强度;
将
式中:
通常情况下,当吸收度小于0.05时,也就是α(υ)L<0.05时,可以用气体吸收指数的泰勒展开式的前两项进行近似,此时气体吸收指数被线性近似,那么
但是在煤矿等实际应用场景中,气体吸收度往往会有很大的变化范围,也就意味着气体的吸收度会远远大于0.05,因此此时再使用气体吸收指数的线性近似将得不到理想的测量结果。观察
对比线性近似
3 实验设计
3.1 实验装置及谱线选择
为了验证波长调制光谱的非线性以及利用其进行宽量程测量的可行性,搭建了基于波长调制光谱的CH4检测系统。该实验系统选择中心波长为1653 nm的分布反馈式(DFB)激光器作为光源。实验中通过温度调节和电流调节控制激光器的发射波长在1653.72 nm附近,使得激光器波长扫描范围能够覆盖
以上扫描范围包含了三条甲烷气体的吸收谱线,其相关信息如
表 1. 6047 cm-1附近的CH4吸收谱线参数
Table 1. Absorption line parameters of CH4 around 6047 cm-1
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实验装置原理图如
3.2 标定与计算方法
在小吸收度范围内,标定和计算方法与传统波长调制光谱技术一致,所以这里不再赘述。在大吸收度范围内(即吸收度大于0.0236,对应气体的体积分数大于1000×10-6),由
式中:P为2f信号峰值;a、b、c、d为三次多项式的系数。由此可见至少采用4个标定浓度才可以确定该三次多项式。为了得到更加精确的结果,可以选择更多的标定浓度,本实验采用6个标定浓度来确定三次多项式的系数。在确定了三次多项式的系数之后,便可以根据实时测量得到的2f信号峰值反演气体浓度,而在一般情况下无法计算
4 结果分析与讨论
4.1 三次曲线系数的确定
基于
4.2 宽动态范围检测分析
该方法在小吸收度范围内(即吸收度小于0.0236)对CH4浓度进行检测时,使用线性拟合方法进行浓度反演能够获得良好的结果。在5×10-6~1000×10-6范围内,反演浓度结果及其与标准浓度的线性关系如
图 4. 小吸收度下的反演结果线性度及其误差。(a)反演浓度及线性拟合;(b)绝对误差和相对误差
Fig. 4. Linearity and error of inversion result under low absorbance. (a) Inverted concentration and linear fitting; (b) relative error and absolute error
以上分析了小吸收度范围内(体积分数小于1000×10-6,吸收度小于0.0236)的反演结果线性度及其误差,下面对大吸收度范围内CH4气体的反演浓度结果进行分析。如
图 5. 大吸收度下的反演结果线性度及其误差。(a)反演浓度及线性拟合;(b)绝对误差和相对误差
Fig. 5. Linearity and error of inversion result under high absorbance. (a) Inverted concentration and linear fitting; (b) relative error and absolute error
为了研究该系统的动态测量范围,接下来对其检测下限进行分析。选择小吸收度范围内的2f原始信号进行分析,这是因为在该范围内非线性效应比较微弱,分析得到的检测下限更加精确。本文选择了体积分数为100×10-6的CH4的2f原始信号进行分析,如
图 6. 体积分数为100×10-6的CH4的原始2f信号
Fig. 6. Original 2f signal of CH4 with volume fraction of 100×10-6
根据以上分析,可知该系统实际测量范围为1.5×10-6~10000×10-6,其动态测量范围达到了4个数量级,在没有增加任何硬件的条件下实现了CH4的宽动态范围测量。
表 2. 不同文献中CH4传感器的性能对比
Table 2. Comparison of performance of CH4 sensors in different references
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另外,为了验证该方法测量大吸收度CH4的稳定性,对体积分数为5000×10-6的CH4进行了持续的测量。采集频率为45 Hz,在系统稳定一段时间后,从测量结果中获得了1000个CH4体积分数值,如
图 7. 长时间测量结果及其直方图分布。(a)反演浓度;(b)频率统计直方图分布及高斯拟合
Fig. 7. Long-time measurement result and its histogram distribution. (a) Inverted concentrations; (b) frequency statistics histogram distribution and Gaussian fitting
5 结论
以激光吸收光谱技术中的波长调制光谱作为研究对象,重点研究了其在大吸收度下的非线性特征。基于激光吸收光谱技术的原理,分析了其泰勒展开的线性和三次多项式两种近似形式。并以CH4气体为例,说明了将其应用在气体浓度宽动态范围测量中的有效性和可靠性。经过实验验证,利用波长调制非线性特征能够实现1.5×10-6~10000×10-6范围(对应最大积分吸收度为0.236)内的CH4测量,达到了近4个数量级的动态范围。除此之外,基于该方法还获得了良好的测量结果。在以上检测范围内,不管是在小吸收度还是在大吸收度下,标准浓度和反演浓度的拟合优度都为0.999,证明了该方法在整个检测范围内的线性拟合优度具有一致性。在1000×10-6~10000×10-6对应的大吸收度范围内,最大的测量绝对误差和相对误差仅为-92.1×10-6和0.93%,完全满足油气管网、煤矿等众多工业场景的应用需求。综上所述,该方法利用波长调制光谱的非线性特征实现了气体浓度的宽动态范围测量,大大拓宽了波长调制光谱的应用范围,也进一步拓宽了激光吸收光谱的应用范围。
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