Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

VG04-293

The Evolution and Application of Trace Gas Analyzers based on Tunable Diode Laser

Absorption Spectroscopy

Mickey B. Frish, Richard T. Wainner, Mark G. Allen, William J. Kessler, David M. Sonnenfroh, Shawn D. Wehe,

Matt C. Laderer, and B. David Green

Physical Sciences Inc.Andover, MA 01810

IFPAC 2005Washington, DC12 January 2005

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Outline

• TDLAS Description

• 10 Years of TDLAS Evolution

• Single-Board TDLAS Control Platform

– Standoff Gas Detector/RMLD

– Miniature Stand-Alone TDLAS Package

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TDLAS

• It is highly-selective; generally insensitive to cross-species interference

• It is highly-sensitive, offering sub-ppm detection of many gas species

• It is fast, offering sub-second response time

• It is non-contact; the probe beam need not make contact with the gas stream

• It is configurable as a point, open-path, or standoff sensor

Tunable Diode Laser Absorption Spectroscopy (TDLAS) is an optical method for detecting trace concentrations of one or more selected gases mixed with other gases

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000.0E+0

10.0E-3

20.0E-3

30.0E-3

40.0E-3

50.0E-3

60.0E-3

70.0E-3

760.5 761 761.5 762

Wavelength (nm)A

bsor

banc

e

Absorption Spectroscopy Fundamentals

• Gas molecules absorb light at specific colors (“absorption lines”)

Beer-Lambert law

Iν = Iνo exp [S(T) g(ν - νo) Nl]

Where:ν = optical frequencyνo = line center frequencyg(ν) = lineshape function l = path lengthN = absorbing species number densityS(T) = temperature dependent linestrengthIνo = unattenuated laser intensity

Iν = laser intensity with absorption

Partial O2 Spectrum

Absorbance = - ℓn (Iν /Iνo)

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TDLAS Principles of Operation

• A frequency agile (i.e. tunable) laser beam transits a gas sample• The laser frequency (inverse of wavelength) scans repeatedly across an

absorption line that uniquely identifies the target gas• Absorption of the laser beam by the target gas creates a signal at the

detector, which is processed to provide an output indicating concentration in target gas

– Senses absorbances < 10-5

δ

δ

Wavelength

Laser

λo

ControlSignals

SignalProcessingElectronics

Output

Detector

FocusingOptics

Absorbing Gas

C-5215ez

Tran

smis

sion

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Practical Detection Limits for Some Gases Measured with Near-IR TDLAS

20.0CH3OH

20.0CH3CH2OH10.0CH2Cl2

20.0CH3CHOHCH310.0CH3CN

50.0O20.15HCl0.2NO21.0CH4

30.0NO1.0H2O40.0CO25.0NH3

40.0CO20.0H2S1.0HCN0.2HF

(ppm-m at 1 atm)

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TDLAS System Components

• TDLAS systems typically utilize near-infrared diode lasers that operate at room temperature– these are the same type of solid state lasers that are utilized

for long-distance, high-speed telecommunications

Scan

Generator

LaserCurrent

Controller

LaserTemperature

Controller

LaserTransmitter

OpticalFiber

Laser BeamLaunch Optics

Laser BeamReceive Opticsand Photodetector

ElectricalWires

MeasurementPath

(a) Optional Noise ReducingOptical Comparison Signal (BRD)

(b) Optional Noise ReducingElectronic Comparison Signal (WMS)

SignalProcessor,

Display,Communications

E-5711a

• Wavelength Modulation Spectroscopy (WMS) • Balanced Ratiometric Detection (BRD)

Reference Signal for Noise Reduction

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Wavelength Modulation Spectroscopy (WMS)

• Laser is tuned, via temperature, to the center of the absorption line (νo)• Laser wavelength is repeatedly scanned, via its injection current, across a

portion of an absorption line– Produces an amplitude modulation of the laser power received at the

detector– AM frequency is twice the ωm frequency

• Phase sensitive (lock-in) detection of the small AM signal yields the molecular concentration in the laser path

νo+δνo

νo-δ

Wav

elen

gth

Det

ecte

dLa

ser

Pow

er

Time

δδ

Tran

smis

sion

Wavelength νo C-5215c

2πωm

πωm

3πωm

ν(t) = νo + δsin(ωmt)

A(t) ~ Ao [1-a cos2 ωmt]

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Example TDLAS Waveforms

• Absorbance = ℓn(evacuated/signal)

– common-mode signals suppressed

– waveform is called the 2f signal

• Demodulating 2f signal provides low-noise signal proportional to absorbance

Sign

al (a

u)A

bsor

banc

e

.05

.04

.03

.02

.01

Time (µs)

0

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Low-Noise Demodulation

• Demodulation converts a high frequency oscillation into a low frequency signal with amplitude proportional to the oscillation amplitude

– example: rectification

• Phase-sensitive demodulation employs a “reference” signal to demodulate only input components matching the reference frequency and phase

– example: lock-in amplifier, or mixer (multiplier),and low pass filter– Provides a very-narrowband filter that rejects out-of-band noise

dc

C-5813az

An(t)/2

Multiplier

Reference

cos ωrt

InputΣAn(t)cos (ωnt + φn)n

C-5813z

Low-pass filter

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PSI TDLAS Evolution

• WMS (analog)• Four Measurement

Paths• Industrial I/O Ports

• BRD• Self-Contained

Sampling Cell• Dedicated Industrial

Computer

SpectraScan® (1995)

WaterScan™(1998)

SolventScan™(2003)

RMLD™(2003)

• WMS (digital)• Lightweight, handheld,

battery-powered• Stand-off detection • Embedded

microprocessor

• BRD• Pharmaceutical

Manufacturing• Laptop Computer

GasScan™ (2004)

• Stand-alone detector/alarm• 7”x 6” x 1.5” package• Configurable as point, open

path, or standoff sensor

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Measurement Example

50 ppb to >10 ppmDynamic range

1 Hz, time between zero and 90% of final concentration

Response time (t90)

316L stainless steel, MgF, Viton

Wetted materials

0.1 mm Hg to 1.5 atmCell pressure range

±5% of full scale or 50 ppb

Repeatability

50 ppb (S/N=1)Detection limit

0 to 0.1; 0 to 1; 0 to 10 ppm

Measurement ranges

NH3, BCl, DCS, SiH4, PH3, AsH3, B2H6, N2, Ar, H2, He, O2, Kr, Ne, Xe, CH4, C2H6, C2H4, C3H8, C3H6

Sample gas mix

H2OAnalyte gas1016

[H2O] cm-3 (Baratron Measurement)

[H2O

](BR

D)

1015

1014

1013

1012

1012 1013 1014 1015 1016

374 ppb at STP

D-9046a

Neat H2O Vapor

37 ppm at STP

• Comparison with Capacitance Manometer–85m optical pathlength extractive multi-pass cell

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In-Situ Optical Measurement Cell

• Multi-pass optics improve sensitivity by providing a 10 m optical path in 25 cm physical length

• Measurement section may be heated to 250 C with no performance loss

PointingMirror Optical

Fiber

Detector/Pre-Amp

Window

OpticalBench

MountingFlange

O-ringSeal

FrontMirror

SupportStructure

RearMirror

25 cm

D-1896

Transceiver Section Measurement Section

25 cm

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Multipass Cell in Close-Coupled HeatedFlue Gas Extraction Probe

Ammonia (NH3) Monitor for NOx reducing SCR/SNCR

systems

F-9695 F-9696

Probe Installed in Ceramic Manufacturing Kiln Exhaust

Duct

LaserBeam

OpticalFiber

Window

CellCover Heated

MeasurementCell

Insulation

Flange Boiler WallPort

ProbeSupportTube Purge

Line

PurgeableGas IntakeFilter (0.5 µm)

GasSamplingLine

CalibrationGas Line

ProbeHeaters

Computer ControlledValve Bank

RemovableHerriot Cell Optics

Detector/Amplifier

OpticalBenchDust

Cover

DetectorPowerSupply

ACFilter

C-7984

PressureReducer

~1m

Pur

ge V

alve

Pum

p Va

lve

Cal

ibra

tion

and

Zero

Gas

Val

ves

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• Extractive flow through compact non-incendivemultipass cell

Solvent Drying in Fluidized Bed

Example Data using Extractive Sampling

Industrial Emission Control System

• Extractive heated close-coupled cell in highly-reactive process

Timepp

m

InjectedReagent

TDLASControlSIgnal

908070605040302010

0

F-9697

0 2 4 6 8 10 12 14 16 18 20 22Time (h) F-9697

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Multipass Cells in CO2 Eddy CovarianceFlux Measurement System

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Installation Example:Control of Oxygen-Enriched Furnaces

Temperature

Heat Flux

Natural Gas

Oxygen

Air

Water

Water

Exhaust Gas

D-5098z

TDL Sensorfor CO, O2

5.0x1015

1.0x1016

1.5x1016

2.0x1016

2.5x1016

3.0x1016

200 210 220 230 240 250Time (sec)

CO

Num

berD

ensi

ty(c

m-3

)

FuelFuel --richrich

0

Fuel-Rich

Fuel-Lean F-9691

• In-situ cross-duct configuration• Pulsed-fuel operation mode

– 0.5 Hz oscillation

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Open Path Alarm Installation Example

Electronics Consolein Control Room orInstrumentation Shack

LaserTransceiver

Optical Fiber andInterface Cable(Up to 1000m Cable Length)

LaserBeam

PassiveRetroreflector

D-1058z

Processing Area

≤ 200Meters

One day’s data

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PSI Single-Board WMS Platform

• Incorporates laser control and data processing on battery-operated board• Digital signal processor for high-speed data acquisition and processing• Embedded microcontroller for laser operation, data reduction,

communication• Serial (RS-232) data output stream and setup interface • SPI communication available for interface with other microcontrollers

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Portable Standoff Hazardous Gas Detector

• Senses target gas along path between transceiver and a surface up to 30 m (100 ft) distant

• No cross-species interferences

• Shoulder-mounted control unit; handheld transceiver

• Total weight < 6 lb• Eye safe• Battery-operated, > 8 hours

between charges TDLAS vs Position at Municipal Gas Leak

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100Position (ft)

ppm

-m

G-6977

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Miniature Ambient Gas Sensor

• Instant (< 1 s) response• No cross-species interferences• Audible alarm• Serial data port• AC or Rechargeable Battery Power • Internal or Remote Sensor Head• Configurable as Point, Open Path, or

Standoff Sensor

Applications• Industrial/Commercial Toxic Gas Alarms• On-line trace gas process monitoring and control• Environmental monitoring• Combustible Gas Alarms• Combustion Gas (Fire) Sensors

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Summary

• TDLAS has evolved over the past decade from a laboratory specialty to rugged, reliable commercial industrial instrumentation

• Current applications for permanently-installed systems abound in industries that include: petrochemical and chemical processing, aluminum smelting, energy production, pharmaceutical and ceramics manufacturing, agriculture, and medicine

• Novel, battery-operated, hand-portable TDLAS systems will soon be used widely for natural gas pipeline leak surveying and otherstandoff detection applications

• Compact packages with low power consumption are expected to find application for distributed fast alarms and oxygen sensing in highly-combustible environments