Development of High-Precision
Monitoring Technology forGreen House Gases
KENJI MUTA*1

MASAZUMI TANOURA*1

TOMOYUKI KIKUGAWA*3

KO NAKAYA*4

MASAKI IIJIMA*2

High-precision monitoring technology has been developed to monitor the production of greenhouse gases (CO 2 ,CH 4 , and N 2 O) in the vicinity of industrial plants and in the general atmosphere. This technology is based ontunable diode laser absorption spectroscopy, and, in combination with proprietary wavelength modulation and laser
wavelength stabilization technologies, offers superior sensitivity, responsiveness, and stability.

1. Introduction

2. Measurement principle and characteristics

Reductions in emissions of greenhouse gases consistingof CO2, CH4, and N2O are being sought in the context ofpractical programs aimed at securing both environmental
and economic viability in response to global warming concerns. Greenhouse gas monitoring is, therefore, essential
to the quantitative assessment and verification of the effectiveness of such programs.
The authors of the present report have developed gas
concentration measurement systems using tunable diodelaser absorption spectroscopy (TDLAS) to monitor the production of O2, CO, NOx, and other gases in the furnaces oflarge installations such as power generation boilers andlarge-scale incinerator plants. The effectiveness of thesesystems has been confirmed in the measurement of gas
concentrations in the furnaces of both pilot plants and actual installations(1)-(3), as well as in the measurement ofgas concentrations of CO2 due to automotive exhaust(4).
The method discussed here is not affected by coexistinggases, and enables continuous measurement of gas concentrations with high precision. This method is very close
to practical application as a technology for monitoring theemission and absorption points, as well as the wide-areamonitoring of greenhouse gases. This report presents theprinciples and characteristics of this method, together with
examples of greenhouse gas measurements and discussionof possible applications of the method.

2.1 Measurement principles
The method reported here is based on TDLAS. (Tunable Diode Laser Absorption Spectroscopy) Gaseous
molecules are subject to characteristic absorption causedby molecular vibration in the in the 1-2 u m near-infrared
region.
Table 1 shows the characteristic absorption bands for
CO2, CH4, N2O, etc(5). Each absorption band is made up of
many absorption lines, and when molecules are irradiatedwith light having a wavelength that matches one of theseabsorption lines, the molecules absorb the light and information on molecular concentration can be obtained fromthe absorption intensity.
2.2 Structure and functions of apparatus
Figure 1 presents a block diagram of the apparatus,
while the photograph at the top of this page shows the appearance of the measurement apparatus, which is composedof the following components(6):(1) Light source: Distributed-Feed Back type (MQW-DFB)
InGaAs(P) laser diode (LD) developed by Anritsu Corporation for gas detection applications, featuring astrained multi-quantum well structure for the oscillation of near-infrared region light at room temperature,
and which emits monochromatic light in response to specific species of gas(7);(2) Detector: InGaAs photo-diodes (PD) for the measurement of laser light intensity;(3) Reference cell: Used for calibration and laser wavelengthstabilization, which is done through the introduction ofa standard gas at a known concentration and constantpressure;(4) Measurement unit: Obtains information on concentration by modulating the laser wavelength and separatingout signals synchronized with the frequency of the modulated wavelength from among the optical signals
received; and(5) LD driver unit: Used to set the temperature and current of laser diode, as well as the oscillation wavelengthof the laser.

Table 1 Measurement wavelength of green house gases
and air pollutants: Absorption Band
1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9m

CO2
CH4
N2O
CONONO2*1 Advanced Technology Research Center, Technical Headquarters*2 Plant and Transportation Systems Engineering &Construction Center*3 Anritsu Corporation*4 Central Research Institute of Electric Power Industry

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

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TDLAS optical bench

Measurement zone

Half mirror

Laser diodemodule

PD2Laser
Background
light

PD1

PD3Ref. cell
MirrorLD Control Signal. Gas conc.
. Particle conc.
PC(. Gas flux)

Reference signal
Control/T
Analysis unit

Fig.1 Schematic diagramof TDLAS monitoringsystem

Background light signal
Measurement signal

Table 2 Detection limits of gases by TDLAS method

Laser light emitted by the LD is divided in two by meansof a half mirror, with one stream passing through the reference cell and received by PD1 for use in gas concentration
calibration and wavelength stabilization. The other streamof laser light is fed through the measurement zone and received by PD2, with the concentration calculated from theamount of light absorbed in the zone. Meanwhile, PD3 isinstalled at a location removed from the optical axis of thelaser light in order to cancel the influence of any light otherthan that originating from the laser (e.g., flame or sunlight).
The measurement unit processes the signals from the threephoto-diodes, and measures the gas concentration.
This technology utilizes LD characteristics that enable
control of the laser oscillation wavelength by means of thetemperature of LD or input current, meaning that the laser wavelength can be modulated by adjusting the input
current to the LD. Only the frequency components, which
are synchronized with the modulation frequencies, can bedetected from among the output signal of PD using synchronous detection. Since the light absorption due to thegas depends on the light wavelength, the wavelength modulation method enables high-precision measurement of thelight absorption.
In addition, using the reference cell, proprietary techniques have been developed to stabilize the measurement
laser wavelength and dual modulation of two modulated
frequencies, thus achieving substantially greater sensitivity than that normally realized using conventional TDLAS
methods. Impressive gains have been achieved in terms oflong-term measurement stability, making it possible to conduct outdoor measurements over long periods of time.
2.3 Characteristics
The TDLAS-based gas monitoring equipment possesses
the following characteristics.
(1) It has a high-sensitivity and real-time measurement capability for gas concentrations at the ppm level and lower.
(2) It is capable of measuring gas and particle concentrations simultaneously, while remaining unaffected bycoexisting substances such as gases, dust, or raindrops.
(3) It is capable of rapid response with no need for sample
intake, and no need for dust filters. It is also maintenance-free for long periods of time.

Combustion

GlobalWarming

Species Sensitivity*1*2
CO2.

3.0

CH4{0.07}

H2O{0.002}(0.73)

O2

9.1

CO

3.6

NO

1.3

NO2{0.01}(2.0){0.006}(0.56)

NH330 m0.06*1 { }: Expected sensitivity, ( ): Present sensitivity*2 Allowable concentration of dust: 10 g/m3N(equivalent to the condition of the following:
the inside of oil-fired boiler furnace
the gas duct of pulverized-coal fired boiler)*3 Measurement of automobile exhaust gas..

Measurement
Length

3.0 ppm mN2OHClLaboratory Furnace: perfomed: notperfomed(4) It is a a compact and low cost apparatus that allows
continuous, automated measurement.
2.4 Greenhouse gases that can be measured
The species of gas that can be measured using theTDLAS method are shown in Table 2
2, together with measurement sensitivities and examples. The unit ofmeasurement sensitivity employed (ppm-m) represents thelower limit of measurement for changes in the concentration of the target gas along the optical path with ameasurement distance of one meter.
For instance, the measurement sensitivity for CO2 is
3 ppm-m, or 3 ppm at a measurement distance of one meter,
or 0.03 ppm at a measurement distance of 100 meters. As
noted in the table, the representative greenhouse gases CO2,CH4, and N2O can all be measured levels of several ppm-m.

3. Examples of Measurement of Greenhouse Gases
3.1 Measurement of N2O concentrations in an incinerator
The TDLAS method was applied to measure the concentration of N2O present in exhaust gas at the furnace
outlet of an actual toxic waste incinerator.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

2As shown in Fig
Fig.. 2 (a), optical windows were installedon either side in the vicinity of the furnace outlet, with thelight source placed on one side and the receptor on the other,and a laser operating in between for the continuous measurement of N 2 O concentrations. For the purpose ofcomparison, a gas sampling type N2O meter was placed atnearly the same location. The measurement results arepresented in Fig. 2 (b).
Note that the results for the TDLAS method show theaverage concentration of N 2O across the entire furnace
width (90 cm), while the results for the sampling type meter
represent the local concentration at the sampling location.
The average values for both sets of measurements are inagreement at about 60 ppm. In addition, it was also possible to ascertain any sudden changes in concentration thatwould occur due to fluctuations in the amount of fuel, etc.,
in real time using the TDLAS method, as opposed to a delay of about two minutes using the conventional sampling
type concentration meter. The real-time capabilities of theTDLAS method were accordingly confirmed.
3.2 Measurement of CO2 concentrations in the atmosphere
Preparatory consideration is being given to applying thistechnology to monitoring CO2 in underground CO2 processing plants, and for use in monitoring the amount of CO2
absorption in forests. Figure 3 shows the TDLAS measurement results obtained for CO2 in the atmosphere at ameasurement distance of 25 m. Measurement of CO2 in
the atmosphere was performed using a light source placedat ground level, and with the receptor 25 m distant. Also,Light
SourceControlsignalPC
Controller
analyzer

Sludge incineratorFree BoardSandSignal

Fuel200150PC

Contol &analysis
unit

1' 25"
0
13:30

Detector

Gas sampling
250 mmConventional
CO2 analyzer

Pretreatment

14:00

Time (HH:MM)

14:30

Fig. 2 In-furnace measurement of N2O(B) Example of measurement results

25 m(Atmospheric air)Laser

1' 38"

50

Wind direction

TDLAS
optics: TDLAS: N2O analyzer

100

Air
(a) Schematic diagram of N2O measurement by TDLAS: Signal: Laser: Gas (air)Figure 4 shows candidate areas for the applied development of this technology. Various applications areanticipated for industrial areas, urban environments, andthe natural world.
4.1 Wide area monitoring
Applications of the TDLAS-based gas monitoring method
are being considered for measuring CO2 levels in underground storage systems, the detection of CH 4 leaks innatural gas plants and pipelines, and for various other applications including urban environmental monitoring intunnels and at intersections.
Real-time measurement can be
performed by TDLAS.

WindowDetector

Ref.
signal4. Future Application and Issues

150 mmAtmospheric CO2 conc. (ppm)[Gas sampling]
Pretreatment
Flue Gas

N2O Concentration (ppm)

N2O
analyzerin order to verify the appropriateness of the concentration
measurement values obtained, measurement was also performed using a gas sampling type CO2 concentration meterat the center of the measurement optical path.
Examples of the measurement results are presented inFig. 3 (b). The average values recorded by the TDLAS andgas sampling methods were in agreement. It is noteworthy that the noise level associated with the TDLAS
measurement data was 1/100th of the TDLAS measurement data shown in Fig. 3(b). The data in Fig. 3 (b) reflectsfluctuations in the actual CO2 concentration measured.
This shows that the TDLAS method is capable of detecting atmospheric changes and the effects on thesurrounding environment from passing vehicles, demonstrating the possibility of application in real-timemonitoring of actual phenomena.

600
500
400
300

TDLAS

200

Conventional
CO2 analyzer

100
0
11:00

Ground(a) Schematic diagram of CO2 measurement

Interrupt ofTDLAS optical A

15:00
13:00
14:00
Time (H . M)(b) Example of measurement results
12:00

Fig. 3 Measurement of atmospheric CO2

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

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Sky Monitoring of greenhouse gases
and air polluting gases by airshipand aircraft (CO2, NOx)

VolcanoMonitoring of H2S,HF....etc.

Combustion Equipment
Forest Monitoring of CO2
uptake by forests
Diagnostics for combustion in incinerators Field(NOx,,CO,N2O)
Monitoring of N2O and CH4 from riceSeafield and livestock production facilities
Monitoring of CO absorption
2in the oceanPM

LNG plants
Monitoring ofCH4 Ieakage

Transportation
Monitoring of air polluting gases in theunderground town and tunnels (NOx,CO)CO2 Storage
Monitoring of CO2 leakage

Longer measurement distances are required for wide
area monitoring, and current development work is aimed
at achieving a distance of 1 km through the developmentof automatic control mechanisms for the laser optical axis,
used in combination with convergent optical systems. The
addition of laser scanning functions would enable area
measurements, and consideration is being given for plans
for the application of gas leakage monitoring over wide
areas of one square kilometer (Fig. 4).
4.2 Flux measurement
Technology for the measurement of gas fluxes (theamount of perpendicular shift in the measurement targetgas) is being developed through deployment of TDLASbased gas concentration measurement. When theatmosphere along a laser optical path is in a turbulent state,
the refractive index along the path changes irregularly, andthe laser intensity received also correspondingly fluctuates
irregularly. Accordingly, the kinetic and thermal flux along
the optical path can be measured from the amount of fluctuation (this is known as the scintillation method).
Technology for the measurement of gas flux is currently
being developed through the combination of this methodwith TDLAS-based concentration measurement. The scaleof this technique would be between that of the localized
flux measurement method known as vortex correlation andthat of remote sensing from observation satellites, suchthat integration and analysis of the resulting data could
enable the construction of wide area monitoring networks
covering several hundred square kilometers.

measurement without sampling. Moreover, as a result of theproprietary development of wavelength modulation and laser wavelength stabilization techniques, high measurement
sensitivity, responsiveness, and stability have been achieved
without any influence from coexisting substances.
Future applications include underground monitoring of
CO2 storage systems and gas leakages, as well as the simultaneous promotion of technologies for wide area gas
concentration monitoring and gas flux measurement. Suchtechnology could be employed for use in investigation andverification as required for emissions rights trading, including the measurement of the amount of CO2 absorbed by forests
or estimating the amount of CH4 released by landfills.
The authors wish to express their sincere appreciation
to Dr. Koichi Yamada of the Research Institute for Environmental Management Technology of the National Instituteof Advanced Industrial Science and Technology (AIST), for
his guidance on the evaluation of the temperature dependence of the absorption strength of N2O molecules.
References(1) Matsudera et al., Application of TDLAS for Flue Gas Measurement in the Sludge Incinerator, Proceeding of 40th Meeting of Japan Sewage Works Association (2003), p.976(2) Muta K, et al., Simultaneous Detection of oxygen moleculeand soot particles by visible diode laser absorption spectroscopy, European Sensing Symposium (1997)(3) Muta K, et al., In-situ Measurement of CO by Tunable DiodeLaser Absorption Spectroscopy in a Large Scale Waste Test
Furnace, International Laser Sensing Symposium (1999)(4) Tanoura M, et al., Tunable diode laser measurements of nitrous oxide and carbon dioxide at 1.5 ƒÊ m, Conference onTunable Diode Laser Spectroscopy (2001)(5) Tanoura M, et al., Detection of nitrogen oxides by high resolution near-infrared absorption spectroscopy, European Sensing Symposium (1997)(6) Tanoura M, et al., An In-situ Gas Analyzer Based on Tunable
Diode Laser Absorption Spectroscopy, International Laser
Sensing Symposium (1999)(7) Tohmon, R. et al., Distributed Feedback Laser Diodes forLaser Gas Sensing, Anritsu Technical Bulletin No.76 (1998)

5. Conclusion
Technology has been developed for the high-precision monitoring of greenhouse gases such as CO2, CH4, and N2O. Basedon the TDLAS method, this technology allows automatedKenji Muta

Masazumi Tanoura

Masaki Iijima

Fig. 4 Application of awide-area monitoringof green house gases
and air pollutions

Tomoyuki Kikugawa

Ko Nakaya

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

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