7 January 2013 Remote CO2 leakage detection system
Author Affiliations +
Optical Engineering, 52(1), 010502 (2013). doi:10.1117/1.OE.52.1.010502
Abstract
We report our research on the development of a gas detection system for environmental application. The CO2 concentration of a remote site is detected with near-infrared laser spectroscopy. Light from a thermal-tuning DFB laser, whose wavelength is close to 1.57 μm, is delivered to/from the remote gas cell via silica optical fiber. The low transmission attenuation of 0.2  dB/km promises long distance CO2 sensing in the length of the gas cell in our experiment is 20 cm only, and detection accuracy of CO2 is 1%.
Fan, Gong, Dong, and Wang: Remote CO2 leakage detection system

1.

Introduction

Carbon capture and storage have been identified as effective means to control global warming.1 CO2 sequestration is newly developed carbon storage approach, through which CO2 is pumped into the ground for long-term storage. Real-time large-range CO2 monitoring on ground and underground helps to estimate the CO2 movement and leakage, and is essential for CO2 sequestration.2

Traditional electrochemical or semiconductors CO2 sensors are cross sensitive to other chemical species, and cannot work under high temperature and high humidity, hence are not appropriate for CO2 sequestration applications.3,4 Laser spectroscopic sensors measure the gas-specific molecular vibration energy level and are thus crosstalk free. If the laser is delivered via optical fiber, the totally passive configuration of the sensing probe ensures that it can work under critical conductions. CO2 has a strong absorption at middle infrared range, however, the attenuation of middle infrared light in optical fiber is very high and only be delivered to hundred of meters away.5 In our system we selected to use 1.57 μm light, at which CO2 has a weak unique absorption, and the attenuation of such light in silica optical fiber is 0.2dB/km only. The absorption strength around 1.57 μm is so weak that the interaction length of the light and CO2 is usually of dozens of meters or even longer, which is not practical for CO2 sequestration applications. In this letter we use correlation and optimization approaches for data processing, and realize 1% CO2 accuracy for gas cell as short as 20 cm.

2.

Redundant Linear Equations

As shown in Fig. 1, light from a DFB laser passes through a long silica optical single mode fiber (SMF-28), gets modulated by the concentration of CO2 in the gas cell, passes silica fiber again and is then detected by a photo detector. Under the control of a computer, the wavelength λ of the laser is scanned across an absorption line of CO2 around 1572 nm by tuning the working temperature of the laser, and according to the Beer’s law, the light power received by the photo detector can be expressed as

(1)

P(λ)=P0eσ(λ)LρP0P0σ(λ)Lρ,
where P0 is the received power in the absence of gas absorption, L is the optical path length of the gas cell, σ(λ) is the absorption coefficient of CO2, and ρ is the concentration of CO2. For a 20-cm-long gas cell, the absorption is typically less than 103, so we apply the first-order Taylor’s approximation in Eq. (1) for simplicity. During the scan of wavelength, the ratio of the time-varying component over the DC component can be expressed as a function of ρ:

(2)

R[λ(t)]=P0σ(λ)LρP0=σ[λ(t)]Lρ.

Fig. 1

Laser spectroscopy setup for gas sensing.

OE_52_1_010502_f001.png

In our application, the gas pressure is approximately 1 ATM, thus σ(λ) can be regarded as a fixed Lorentzian-like curve. We pre-measured the spectral ratio R100(λ) for 100% CO2, and use it as a reference to solve the optimal ρ solution of a redundant linear equations:

(3)

[R(λ1)R(λ2)R(λN)]=ρ[R100(λ1)R100(λ2)R100(λN)]+A.

There are only two unknown variables in Eq. (3), but N is usually over 1000. This redundant condition helps to improve the detection accuracy of CO2 concentration.

3.

Spectral Shift Correction

Figure 2(a) shows four typical absorption spectral ratio curves we detected under the same condition. They should be the same in theory, however, due to the hysteresis phenomenon of the temperature control, a spectral error up to 0.1 nm was observed among different scans. As Eq. (3) depends not only on the absorption amplitude but also on the spectral information, such wavelength instability will degrade the demodulation accuracy of CO2concentration significantly. A spectrum correlation approach6 is used to solve the problem:

  • 1. For each spectral ratio Rm(λn), m=1,2M, n=1,2,N, its effective center qm is estimated as

    (4)

    qm=1Nn=1Nn*Rm(λn).

  • 2. For m=2,3,M, calculate the cross correlation of Rm and R1 for delay values in the vicinity of qmq1, and find the optical delay τm where the correlation is maximum.

  • 3. Shift Rm(λn) by τm, the spectral instability is then solved.

Figure 2(b) shows the correction results for spectra given in Fig. 2(a).

Fig. 2

Compensate the instability of thermal tuning. (a) original collected spectra for pure CO2 whose absorption lines shift randomly; (b) the shift was compensated by correlation algorithm.

OE_52_1_010502_f002.png

4.

Experimental Results

By changing the operation temperature, the wavelength of the DFB laser was scanned from 1571.0 nm to 1572.5 nm, covering one of the CO2’s absorption lines at 1572.23 nm. CO2 samples whose concentration ranged from 0% to 90%, were tested, and the data were analyzed with correlation and optimization approaches as described in Secs. 2 and 3. Measurement is repeated 10 times to estimate the test stability. Figure 3 shows typical absorption spectra obtained for eight different CO2 concentrations, and the estimated concentration values are given in Fig. 4, where error bars are used to show the repeatability of the results under same test conditions. The error bar demonstrates that the standard deviation is 1%. We hence conclude that the system detection accuracy is 1% and the resolution is higher than 1%.

Fig. 3

Typical measured spectra at different CO2 concentrations.

OE_52_1_010502_f003.png

Fig. 4

Estimated concentrations with error bar.

OE_52_1_010502_f004.png

5.

Conclusion

For CO2 sequestration application, the background (concentration of CO2 in normal air) is around 0.03%, while the signal level (leakage concentration) usually ranges from 1% to 100%. The 1% detection accuracy we have obtained is sufficient for CO2 sequestration monitoring. Moreover, the system is based on near-infrared, the low attenuation and reliable performance of silica fiber ensures the long distance and large range remote sensing.

Acknowledgments

This work was supported by the Chinese National Science Foundation under Grant 51275373 and the Chinese Fundamental Research Funds for the Central University under Grant 2012-IV-019.

References

1. 

S. M. BensonF. M. Orr Jr., “Carbon dioxide capture and storage,” Landsc. Environ. 33, 303–305 (2008).Google Scholar

2. 

R. SteeneveldtB. BergerT. A. Torp, “CO2 capture and storage: closing the knowing-doing gap,” Chem. Eng. Res. Design 84(9), 739–763 (2006).CERDEE0263-8762http://dx.doi.org/10.1205/cherd05049Google Scholar

3. 

G. A. Dawsonet al., “CO2 gas sensing at microelectrodes in nonaqueous solvents,” Electroanal. 12(2), 105–110 (2000).1521-4109http://dx.doi.org/10.1002/(ISSN)1521-4109Google Scholar

4. 

N. Imanakaet al., “CO2 gas sensor with the combination of tetravalent zirconium cation and divalent oxide anion conducting solids with water-insoluble oxycarbonate electrode,” Electrochem. Commun. 3(8), 451–454 (2001).ECCMF91388-2481http://dx.doi.org/10.1016/S1388-2481(01)00201-6Google Scholar

5. 

J. OGormanet al., “Gas sensing using IR laser diode sources,” Proc. SPIE 3105, 301–315 (1997).http://dx.doi.org/10.1117/12.276166Google Scholar

6. 

J. M. Gonget al., “Enhancement of wavelength detection accuracy in fiber Bragg grating sensors by using a spectrum correlation technique,” Opt. Comunm. 212(1–3), 29–33, (2002).OPCOB80030-4018http://dx.doi.org/10.1016/S0030-4018(02)01907-7Google Scholar

Biography

Dian Fan received the PhD degree in communication and information system from Wuhan University of Technology, Wuhan, China, in 2011. She joined the faculty of Optical Fiber Sensing Technology National Engineering laboratory, Wuhan University of Technology as an assistant researcher, after receiving the MS degree in same laboratory in 2005, where she is now an associate professor. Her research interest is optical fiber sensing technology engineering application and optical sensing signal processing.

Biographies and photographs of the authors are not available.

Dian Fan, Jianmin Gong, Bo Dong, Anbo Wang, "Remote CO2 leakage detection system," Optical Engineering 52(1), 010502 (7 January 2013). http://dx.doi.org/10.1117/1.OE.52.1.010502
Submission: Received ; Accepted
JOURNAL ARTICLE
4 PAGES


SHARE
KEYWORDS
Carbon monoxide

Absorption

Gas lasers

Optical fibers

Carbon dioxide

Laser spectroscopy

Signal attenuation

Back to Top