637
spectroscopy (TDLAS) shows the advantages and potential for real time on-line monitoring,
offering high selectivity and sensitivity, long-term stability and excellent reliability.
Because of the stronger absorption of the CO first overtone band, a CO sensor using a
laser diode operating around 2.3 μm can provide sensitive detection. In this work, a sensitive and stable CO monitoring system was developed by using a distributed feedback (DFB)
laser operating at 2.33 μm and a Herriott-type multi-pass gas cell with a 20-m optical length,
taking advantage of the in-situ monitoring, excellent accuracy and simple structure available from direct absorption spectroscopy. The detection accuracy of system was about
±0.1 ppm when as low as 1 ppm CO gas was detected. Further, a long-term continuous
monitoring evaluation has clearly demonstrated the long-term stability and reliability of
the monitoring system. The results obtained have validated the potential use of such a CO
monitoring system in a practical monitoring application, such as the coal spontaneous combustion monitoring.
2 PRINCIPLE
A distributed feedback (DFB) laser operating at a specific wavelength of 2330.18 nm was
employed to target the strong absorption line of CO first overtone band, a wavelength where
the corresponding absorption intensity is 3.39 × 10
−21 cm
−1 /(mol × cm
−2 ). The Herriott-type
multi-pass gas cell (MGC) has an effective optical length of 20 m, with a base volume of
0.57 L. Both sides of the inside of the gas cell were dielectric coated, providing a broadband
transmission over the wavelength range from 2290 nm to 2370 nm. The transmitted laser
beams were detected by using the RT extended-InGaAs photo-detector installed within the
system. Using a current driver and temperature controller, the emitting laser wavelength can
be tuned with a sawtooth wave. The intensity of the laser decreases when the laser light is
passed through CO gas-filled MGC and as a result, the measured small dip in the background line creates the gas absorption signal. According to the Lambert-Beer law, the CO gas
concentration can be obtained using following formula (Paynter, 2010):
C
A
PS L
=
( )
T
(1)
A
I
I
d
=
−
⎛
⎝ ⎜
⎛ ⎛
⎝ ⎝
⎞
⎠ ⎟
⎞ ⎞
⎠ ⎠
−∝
∝
∫ ln
0
I I
λ
(2)
where C is the volume concentration of the measured CO gas sample, I 0 is the initial light
intensity, I is the light intensity due to the absorption, λ is the laser wavelength, L is the length
of the optical path, P is the total pressure of the gas medium and S(T) is the intensity of the
characteristic spectral lines (which show a temperature dependence).
3 RESULTS
In the experiment carried out, the CO gas, at a concentration of 30 ppm, was mixed with pure
nitrogen (99.999% N 2 ) to create various CO gas samples at different nominal concentrations: 3
ppm, 1 ppm, 0.5 ppm, 0.3 ppm, and 0.1 ppm. These gases were successively filled into the gas
cell with a constant flow rate of 1000 mL/min. The monitored raw data were recorded during
the whole process and these are illustrated in Figure 1. At the same time, the measured raw
data were smoothed using a 10-points moving average method, in order to show the trends
more clearly. As depicted, the results show that the CO monitoring system could detect CO
gas samples with excellent precison and stability. And it can readily sense the presence of CO
gas, at a concentration as low as 0.1 ppm. As illustrated in Figure 1, data monitored can be
used to determine that the detection accuracy of system was about ±0.1 ppm when as low as 1
ppm CO gas was detected and the detection limit of system could be estimated to be 0.1 ppm
spectroscopy (TDLAS) shows the advantages and potential for real time on-line monitoring,
offering high selectivity and sensitivity, long-term stability and excellent reliability.
Because of the stronger absorption of the CO first overtone band, a CO sensor using a
laser diode operating around 2.3 μm can provide sensitive detection. In this work, a sensitive and stable CO monitoring system was developed by using a distributed feedback (DFB)
laser operating at 2.33 μm and a Herriott-type multi-pass gas cell with a 20-m optical length,
taking advantage of the in-situ monitoring, excellent accuracy and simple structure available from direct absorption spectroscopy. The detection accuracy of system was about
±0.1 ppm when as low as 1 ppm CO gas was detected. Further, a long-term continuous
monitoring evaluation has clearly demonstrated the long-term stability and reliability of
the monitoring system. The results obtained have validated the potential use of such a CO
monitoring system in a practical monitoring application, such as the coal spontaneous combustion monitoring.
2 PRINCIPLE
A distributed feedback (DFB) laser operating at a specific wavelength of 2330.18 nm was
employed to target the strong absorption line of CO first overtone band, a wavelength where
the corresponding absorption intensity is 3.39 × 10
−21 cm
−1 /(mol × cm
−2 ). The Herriott-type
multi-pass gas cell (MGC) has an effective optical length of 20 m, with a base volume of
0.57 L. Both sides of the inside of the gas cell were dielectric coated, providing a broadband
transmission over the wavelength range from 2290 nm to 2370 nm. The transmitted laser
beams were detected by using the RT extended-InGaAs photo-detector installed within the
system. Using a current driver and temperature controller, the emitting laser wavelength can
be tuned with a sawtooth wave. The intensity of the laser decreases when the laser light is
passed through CO gas-filled MGC and as a result, the measured small dip in the background line creates the gas absorption signal. According to the Lambert-Beer law, the CO gas
concentration can be obtained using following formula (Paynter, 2010):
C
A
PS L
=
( )
T
(1)
A
I
I
d
=
−
⎛
⎝ ⎜
⎛ ⎛
⎝ ⎝
⎞
⎠ ⎟
⎞ ⎞
⎠ ⎠
−∝
∝
∫ ln
0
I I
λ
(2)
where C is the volume concentration of the measured CO gas sample, I 0 is the initial light
intensity, I is the light intensity due to the absorption, λ is the laser wavelength, L is the length
of the optical path, P is the total pressure of the gas medium and S(T) is the intensity of the
characteristic spectral lines (which show a temperature dependence).
3 RESULTS
In the experiment carried out, the CO gas, at a concentration of 30 ppm, was mixed with pure
nitrogen (99.999% N 2 ) to create various CO gas samples at different nominal concentrations: 3
ppm, 1 ppm, 0.5 ppm, 0.3 ppm, and 0.1 ppm. These gases were successively filled into the gas
cell with a constant flow rate of 1000 mL/min. The monitored raw data were recorded during
the whole process and these are illustrated in Figure 1. At the same time, the measured raw
data were smoothed using a 10-points moving average method, in order to show the trends
more clearly. As depicted, the results show that the CO monitoring system could detect CO
gas samples with excellent precison and stability. And it can readily sense the presence of CO
gas, at a concentration as low as 0.1 ppm. As illustrated in Figure 1, data monitored can be
used to determine that the detection accuracy of system was about ±0.1 ppm when as low as 1
ppm CO gas was detected and the detection limit of system could be estimated to be 0.1 ppm