Compact Sensor Heads for In-situ and Non-Contact Standoff Gas Sensing using TDLAS in Harsh Environments

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1 VG Compact Sensor Heads for In-situ and Non-Contact Standoff Gas Sensing using TDLAS in Harsh Environments Mickey B. Frish, Richard T. Wainner, Matt C. Laderer Physical Sciences Inc. Andover, MA IFPAC 2007 Washington, DC 31 January 2007 Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

2 Outline VG TDLAS Technology Description TDLAS Gas Analyzer Application Examples Single-Board TDLAS Control Platform Handheld TDLAS for Standoff Gas Detector/RMLD Compact Standoff Sensor Head Configuration and Application Examples

3 TDLAS VG Tunable Diode Laser Absorption Spectroscopy (TDLAS) is an optical method for detecting trace concentrations of one or more selected gases mixed with other gases It is highly-selective; generally insensitive to cross-species interference It is highly-sensitive, offering sub-ppm detection of many gas species Wavelength Modulation Spectroscopy (WMS) detection technique rejects noise to enhance sensitivity 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

4 Absorption Spectroscopy Fundamentals Gas molecules absorb light at specific colors ( absorption lines ) Beer-Lambert law I ν = I νo exp [S(T) g(ν - ν o ) Nl] where: ν = optical frequency ν o = line center frequency g(ν) = lineshape function l = path length N = absorbing species number density S(T) = temperature dependent linestrength I νo = unattenuated laser intensity Absorbance 70x x x x x x x10-3 Absorbance = - ln (I ν /I νo ) VG Partial O 2 Spectrum Wavelength (nm) H-4729 I ν = laser intensity with absorption

5 Wavelength Modulation Spectroscopy (WMS) VG Transmission δ δ Wavelength ν o Figure 1 - TDLAS gas detector system A frequency agile (i.e. tunable) laser beam transits a gas sample typically fiber-coupled telecom-style low power (10 mw) near-infrared roomtemperature diode lasers The laser wavelength scans repeatedly across an absorption line unique to the target gas Absorption of the laser beam by the target gas produces an amplitude modulation of the laser power received at the detector Phase sensitive (lock-in) detection of the small AM signal yields the target gas concentration in the laser path senses absorbances < 10-5

6 Practical Detection Limits for Some Gases Measured with Near-IR TDLAS VG (ppm-m at 1 atm) HF 0.2 HCN 1.0 H 2 S 20.0 CO 40.0 NH CO H 2 O 1.0 NO 30.0 CH NO HCl O 2 CH 3 CN 10.0 CH 3 CHOHCH CH 2 Cl CH 3 CH 2 OH 20.0 CH 3 OH 20.0

7 Open Path Alarm Installation Example (c. 1995) VG Passive Retroreflector Processing Area Electronics Console in Control Room or Instrumentation Shack Laser Beam Laser Transceiver 200 Meters D-1058z Optical Fiber and Interface Cable (Up to 1000m Cable Length)

8 In-Situ Optical Measurement Cell Transceiver Section Measurement Section VG Pointing Mirror Detector/ Pre-Amp Optical Fiber Window Mounting Flange O-ring Seal Front Mirror 25 cm Support Structure Rear Mirror Optical Bench D-1896 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

9 Multipass Cell in Close-Coupled Heated Flue Gas Extraction Probe ~1m VG Optical Fiber Laser Beam Cell Cover Window Pressure Reducer Heated Measurement Cell Flange Boiler Wall Port Probe Support Tube Purge Line Insulation Dust Cover Optical Bench Detector/ Amplifier Detector Power Supply AC Filter Pump Valve Removable Herriot Cell Optics Purge Valve Calibration and Zero Gas Valves Probe Heaters Computer Controlled Valve Bank Calibration Gas Line Gas Sampling Line Purgeable Gas Intake Filter (0.5 µm) C-7984 Ammonia (NH 3 ) Monitor for NOx reducing SCR/SNCR systems Probe Installed in Ceramic Manufacturing Kiln Exhaust Duct F-9695 F-9696

10 Measurement Example Analyte gas Sample gas mix Measurement ranges Detection limit Repeatability Cell pressure range Wetted materials Response time (t 90 ) Dynamic range H 2 O NH 3, BCl, DCS, SiH 4, PH 3, AsH 3, B 2 H 6, N 2, Ar, H 2, He, O 2, Kr, Ne, Xe, CH 4, C 2 H 6, C 2 H 4, C 3 H 8, C 3 H 6 0 to 0.1; 0 to 1; 0 to 10 ppm 50 ppb (S/N=1) ±5% of full scale or 50 ppb 0.1 mm Hg to 1.5 atm 316L stainless steel, MgF, Viton 1 Hz, time between zero and 90% of final concentration 50 ppb to >10 ppm [ H 2 O ] ( B R D ) Neat H 2 OVapor ppb at STP [H 2 O] cm -3 (Baratron Measurement) Comparison with Capacitance Manometer 85m optical pathlength extractive multi-pass cell VG ppm at STP D-9046a

11 Example Data using Extractive Sampling VG Solvent Drying in Fluidized Bed Extractive flow through compact non-incendive multipass cell ppm Industrial Emission Control System Time Injected Reagent TDLAS Control SIgnal Time (h) F-9697 Extractive heated close-coupled cell in highly-reactive process F-9697

12 Portable Standoff TDLAS: The Remote Methane Leak Detector (RMLD) VG Developed for manual pipeline leak surveying Like a flashlight, laser beam Illuminates a surface up to 100 ft distant Senses target gas between surveyor and illuminated surface

13 RMLD Internals Single-Board Control Platform Complete WMS system 10 khz modulation 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 Transceiver lightweight, compact, rugged handheld unit co-linear laser transmitter and receiver rejects sunlight integrated visible pointing laser User Interface visual: LCD display in controller unit audio variable tone: frequency = 10 x methane concentration fluctuation algorithm: leaks indicated by rapid concentration changes VG

14 Compact Configurable Standoff Sensor Heads VG ~ 20 cm ~ 8 cm ~ 2 cm Standard Transceiver Module Laser launch Mirror Example Sampling Section Configurable and Replaceable Measurement Module Gas inlet Detector Window Gas outlet

15 Sensor Head Construction VG S e a l e d W i n d o w Sealed Window

16 Application as In-situ Sensor Tank Penetration Hole 0.55 inch VG Example uses O 2 analysis within fuel or chemical storage tanks for explosion protection Measurement of contaminants or reactive gases (e.g H 2 O, HCl) in pipelines or reactive chambers (e.g CVD) for quality control Demonstrated operation over process and ambient temperatures of -50 to +65C

17 Application as Non-Contact Process Analyzer VG Compact Sensor Head for standoff detection of process gases observed through a single window True non-contact method Example Method of Use Fuel Cell Gas Analysis Position sensor head alignment tube against window 1 mm diameter laser beam enters fuel cell channel through window Laser beam scatters from surface at channel rear Sensor head collects scattered light and focuses it on a photodetector Control Unit interprets photodetector signals and computes path-integrated concentration Data displayed on personal computer

18 Measurement Examples VG H 2 O Concentration (ppm-m) Avg = 2050, RMS = 24.1 Begin N2 Purge Noise-equivalent detection limit Avg = 0.106, RMS = Time (sec) Avg = 79734, rms = 351 O 2 Avg = 359, rms = 122

19 Miniature Ambient Gas Sensor VG Self-contained sensor head Example Applications Industrial/Commercial Toxic Gas Alarms On-line trace gas process monitoring and control Environmental monitoring Combustible Gas Alarms Combustion Gas (Fire) Sensors

20 Summary VG Commercial TDLAS sensors are now accepted as rugged, reliable industrial gas analyzers Battery-operated, hand-portable Standoff TDLAS sensors are gaining acceptance for natural gas pipeline leak surveying and other standoff detection applications Compact standoff sensor heads combined with battery-operated control units provide true non-contact sensing of processes observed through a single window Configurable compact low-cost standoff sensor heads simplify adapting TDLAS to a wide variety of applications