Progress in Reducing Size and Cost of Trace Gas Analyzers Based on Tunable Diode Laser Absorption Spectroscopy

Transcription

1 VG Progress in Reducing Size and Cost of Trace Gas Analyzers Based on Tunable Diode Laser Absorption Spectroscopy M.B. Frish, R.T. Wainner, B.D. Green, J. Stafford-Evans, M.C. Laderer, and M.G. Allen Physical Sciences Inc. Andover, MA Optics East Paper Philadelphia, PA 26 October 2004 Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

2 Outline VG TDLAS Description 10 Years of TDLAS Evolution Single-Board TDLAS Control Platform Standoff Gas Detector/RMLD Miniature Stand-Alone TDLAS Package Oxygen Sensor for Aircraft Fuel Tank Inerting

3 TDLAS 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 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 or long-path sensor VG

4 Absorption Spectroscopy Fundamentals VG Beer-Lambert law I ν = I o ν exp [S(T) g(ν - ν o ) Nl] l = path length N = absorbing species number density S(T) = linestrength I o ν = unattenuated laser intensity I ν = laser intensity with absorption g(ν-ν o ) = lineshape function ν o = line center

5 TDLAS Principles of Operation Laser Control Signals Transmission Absorbing Gas δ δ Focusing Optics Detector Signal Processing Electronics Output VG Wavelength λ o C-5215ez Gas molecules absorb light at specific colors, called absorption lines The laser color or wavelength is scanned or "tuned" to repeatedly cross 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

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 TDLAS System Components Laser Transmitter Laser Temperature Controller Laser Current Controller Scan Generator Laser Beam Launch Optics Optical Fiber Laser Beam Receive Optics and Photodetector Electrical Wires Measurement Path (a) Optional Noise Reducing Optical Comparison Signal (BRD) Reference Signal for Noise Reduction Wavelength (b) Optional Modulation Noise Spectroscopy Reducing (WMS) Balanced Electronic Ratiometric Comparison Detection Signal (BRD)(WMS) VG Signal Processor, Display, Communications E-5711a 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

8 Wavelength Modulation Spectroscopy (WMS) VG Laser is initially tuned via temperature, to the center of the absorption line (ν o ) Laser wavelength is then repeatedly scanned, via its injection current, across a portion of an absorption line, thus producing an amplitude modulation of the laser power received at the detector AM frequency is twice the ω m frequency Radio receiver technology is used to process or demodulate the small AM signal to yield a value of molecular concentration in the laser path Transmission δ δ Wavelength ν o Wavelength Detected Laser Power ν o +δ ν o ν o -δ π ω m Time 2π ω m 3π ω m ν(t) = ν o + δsin(ω m t) A(t) ~ A o [1-a cos 2 ω m t] C-5215c

9 Low-Noise Demodulation VG Demodulation is the process of converting a high frequency oscillation into a low frequency signal having an amplitude that is proportional to the oscillation amplitude example: rectification dc Phase-sensitive demodulation employs a reference signal to demodulate only those input components that have the same frequency and phase as the reference example: lock-in amplifier, or mixer (multiplier),and low pass filter Input ΣA n (t)cos (ω n t + φ n ) n Multiplier Reference 1 2 Σ n An (t) {cosφ sinφ n n [cos( ω [sin( ω n n + + ω ω r r C-5813az ) t + cos( ω ) t + sin( ω n n ω choose ω r = input frequency of interest = ω n, φ n = 0, then multiplier output = A n (f)/2 [1 + cos2ω n t] ω r r )t] )t] } + cos ω r t C-5813z low-pass filter rejects 2ω n component, outputs A n (t)/2

10 PSI TDLAS Evolution SpectraScan (1995) WaterScan (1998) SolventScan (2003) RMLD (2003) VG WMS (analog) Four Measurement Paths Industrial I/O Ports BRD Self-Contained Sampling Cell Dedicated Industrial Computer BRD Pharmaceutical Manufacturing Laptop Computer GasScan (2004) WMS (digital) Lightweight, handheld, battery-powered Stand-off detection Embedded microprocessor Stand-alone detector/alarm Smoke detector style package Configurable as point, open path, or standoff sensor

11 PSI Single-Board WMS Platform VG 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

12 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 VG ppm-m Position (ft) G-6977

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

14 Aircraft Ullage Space O 2 Sensor Concept VG Sensor Head Ullage Intrinsically Safe Wiring Fuel OBIGGS Control Unit Fuel Tank Power (~2W) 2" 10% 10" Cockpit Display 10% 8" O 2 Sensor Control Unit Single-board platform provides a foundation for an airworthy sensor to monitor oxygen in aircraft fuel tanks novel VCSEL laser sources utilized to access oxygen spectral features near 760 nm Used for controlling tank inerting systems that inhibit explosion risk G-3149

15 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 other standoff detection applications Compact packages with low power consumption are expected to find application for distributed fast alarms and oxygen sensing in highly-combustible environments VG

16 Acknowledgments VG US Environmental Protection Agency US Department of Energy/National Energy Technology Laboratory Northeast Gas Association Physical Sciences Inc. Heath Consultants Inc. Public Service Electric and Gas Co.