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Environmental and Industrial Measurements with Practical Laser Diagnostics

Environmental and Industrial Measurements with Practical Laser Diagnostics. Dr. Steven G. Buckley University of Washington Energy Seminar November 28, 2007. photon machines inc. Making light work ™. About Photon Machines. Founded in 2006 Specializing in Laser Ablation

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Environmental and Industrial Measurements with Practical Laser Diagnostics

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  1. Environmental and Industrial Measurements with Practical Laser Diagnostics Dr. Steven G. Buckley University of Washington Energy Seminar November 28, 2007 photonmachines inc. Making light work™

  2. About Photon Machines • Founded in 2006 • Specializing in • Laser Ablation • Laser Micromachining • TDL spectroscopy • LIBS • Making Light Work™ • Steve Buckley • Assoc. Prof., UC San Diego • Center for Energy Research • Formerly UMCP, Sandia • LIBS, TDLs, aerosols • R&D background John Roy - Founder, Merchantek (1994) - V.P., New Wave Research (- 2006) - Laser ablation, laser heating systems, LIBS - Operations and industry knowledge

  3. How are sensors related to energy? • An energy and materials analysis of many industrial processes shows that there is inadvertent waste • Tighter control of a) temperatures and b) concentrations can improve product yields and reduce energy usage • Real-time monitoring of emissions aids in process improvement

  4. What would the ideal sensor look like? • Real-time data reporting • In situ, so no probe effects • Small size and weight • Infinitely durable • Versatile in application • Etc…

  5. Focus today: two diagnostic methods that can / could be used in harsh environments • Molecular absorption for small molecule sensing – tunable diode laser(TDL) spectroscopy • Atomic emission for elemental analysis of gases, liquids, solids – laser-induced breakdown spectoscopy(LIBS)

  6. Photon Machines’ diagnostics focus Real-time diagnostics for industrial and environmental measurements Plasma Spectra Concentration/classification Laser-Induced Breakdown Spectroscopy for elemental analysis Absorption Concentration / temperature Tunable Diode Laser Absorption for molecular concentration and temperature High-speed multiplexed measurements

  7. Big Picture Goal: Develop and deploy broadband optical diagnostics • “Broadband optical diagnostics” capable of returning several parameters of interest • E.g. temperature and species, several species, equivalence ratio / mixture fraction • Diagnostics of interest • Tunable Diode Laser Absorption Spectroscopy (TDLAS) • Temperature, species concentrations • Laser-Induced Breakdown Spectroscopy (LIBS) • Trace concentrations, equivalence ratio, mixture fraction, particle composition • Ultimate deployment – engines / turbines, industrial processes

  8. The mission of my seminar… • Initial assumptions: • Maybe you’ve been exposed to spectroscopy, maybe not … • Experts in TDL spectroscopy and LIBS are few and far between • Teaching the basics of TDL and LIBS diagnostics may encourage further investigation …  Provide an appreciation for these practical measurement techniques, as well as guidance on implementation

  9. Tunable Diode Laser Spectroscopy for molecular species concentration analysis

  10. Benefits to tunable diode laser approach • Near-infrared tunable diode lasers have been developed by large telecommunications investment • Relatively inexpensive hardware • Durable hardware • Direct optical measurements are obtained in real-time without perturbations to concentrations that might occur in a sampling system • Measurements can easily be performed at several hundred Hz • Laser light can be multiplexed and fiber-coupled • Multiple species can be simultaneously measured with one detection system Scale: White portion of laser approximately the size of your thumbnail

  11. The spectral landscape of the near-IR

  12. Familiar ground: Absorption spectroscopy • Beer’s law • Concentration measurements • Integrated absorption of spectral line • Calibration or

  13. Typical detection limits for IR absorption

  14. Optical intensity Current Time Time Output optical signal from the laser Input current sweep signal to the laser Intensity (I0) Absorbance Frequency (n) Unperturbed Signal (I0) Intensity (I) Frequency (n) Calculated Absorbance, ln(I0/I), from measured I and I0 Frequency (n) Absorption Signal (I) Basic absorption spectroscopy with TDLs

  15. Key: Wavelength Modulation Spectroscopy (WMS) • Attributes of WMS • Increases sensitivity by 2-3 orders of magnitude • Eliminates most flow-related sources of noise • Allows frequency-domain multiplexing and demodulation Problem: difficulty in detecting a small dip in a large signal, particularly with a fluctuating background

  16. I / I0= exp(- STg(- 0)LPabs) Direct absorption 50 - 1000 Hz sweep of laser current Beer’s law absorption Wavelength modulation + 50 - 1000 Hz sweep of laser current kHz rates modulation Direct absorption vs. “Wavelength modulation” Typical absorption line shape • Function of collisional and Doppler broadening • (Lorentzian shape) + (Gaussian shape) Intensity Frequency Problem: measurement of small signal on large background • Modulation rejects flow-related noise, electrical noise, etc., typically 1/f • Detection of 1f, 2f, etc. signals with lock-in amplifier

  17. Generate driver signal Laser controller modulates laser drive current Laser frequency is modulated A lock-in amplifier compares detected signal with driver signal Detector catches light Light is absorbed How WMS works • Harmonic signals look like derivatives of the absorption line, hence sometimes called “derivative spectroscopy”

  18. How is the signal processed? I2f = I0H2(-0)STg(-0)LPabs Pressure of the absorbing species – what we want to find out Absorption coefficient has two parts – a temperature- dependent line strength ST, and a line shape function g Measured signal Initial intensity Fourier coefficient (known function of amplitude of modulation and line width) Path length (known) Line strength and line shape are functions of T and P, must be known to quantify WMS signal…

  19. Experiments measure the temperature and pressure broadening characteristics of each absorption line • Schematic of the experimental apparatus for line strength and pressure broadening measurements (at different temperatures) is shown below

  20. Example: Line strength measurements as a function of temperature • Measurements of R(4) CH4 transition are fit to model to derive line strengths as a function of temperature M. Gharavi and S.G. Buckley, “Diode Laser Absorption Spectroscopy Measurement of Line Strengths and Pressure Broadening Coefficients of the Methane 23 Band at Elevated Temperatures,” Journal of Molecular Spectroscopy 229 pp 78-88 (2005).

  21. TDL H2O / Temperature measurement Measured absorbance of two H2O transitions near 1478 nm at two different temperatures At T1=468 K At T2=997 K M. Gharavi and S.G. Buckley, “A Single Diode Laser Sensor for Wide Range Temperature and H2O Concentration Measurements,” Applied Spectroscopy 58 (4) pp 468-473 (2004).

  22. TDL H2O / Temperature measurement Measured absorption line strengths of the selected H2O transitions vs. temperature M. Gharavi and S.G. Buckley, “A Single Diode Laser Sensor for Wide Range Temperature and H2O Concentration Measurements,” Applied Spectroscopy 58 (4) pp 468-473 (2004).

  23. TDL H2O / Temperature measurement Variation of absorption ratio vs. temperature Claim: optical temperature 400-2000 K within ± 15 deg. M. Gharavi and S.G. Buckley, “A Single Diode Laser Sensor for Wide Range Temperature and H2O Concentration Measurements,” Applied Spectroscopy 58 (4) pp 468-473 (2004).

  24. Overall conceptual sensor architecture

  25. Concentration example: complicated NH3 spectrum

  26. Accurate modeling allows determination of concentration of NH3 • Model agrees at room temperature and high temperature, and as pressure increases • Procedure is similar for all gases

  27. Application: Gas turbine oscillation measurements • Use TDL CH4 sensor to determine … • Sensitivity • Response time • At pressure • Feasibility • Apparatus to investigate modulated CH4 jet in co-flow of N2 • Short pathlength • Portable DAQ system Bandwidth

  28. Phase I Hardware

  29. Phase I Sample Results • Left = 1 atm, ~ 950 Hz measurement • Right = ~ 9 atm, ~ 200 Hz measurement  Success in the laboratory led to full-scale turbine experiments. Results: detection of oscillations at operational temperature down to 50 ppm absolute fluctuation in CH4!

  30. Application: Real-time gasifier / combustor measurements for control • Integrated high-speed sensor that can measure concentration of important major and minor species, plus temperature, in real time and in situ. • Targeted species: CH4, H2O, CO, CO2, NH3, H2S • Challenges • High, variable temperature • High, variable pressure • Sensitivity with short path length • Rugged design

  31. Laser-Induced Breakdown Spectroscopy for Elemental Composition

  32. Laser-induced plasma . Pulsed laser Atomic emission collection Fiber optic Spectrometer Detector Laser-Induced Breakdown Spectroscopy (LIBS) uses a laser microplasma to excite atomic emission • Line position provides species identification. • Line intensity provides species concentration. F. Ferioli and S.G. Buckley, “Measurements of Hydrocarbons using Laser-Induced Breakdown Spectroscopy,” accepted, Combustion and Flame, available online Dec 27, 2005..

  33. Elements in RED have been measured with LIBS and reported in the literature (courtesy ARL)

  34. Example: LIBS spectra of different coals

  35. Many applications for LIBS have been developed in the last twenty years • Analysis of archeological samples. • Analysis of metal alloys. • Measurement of incinerator exhaust. • Aerosol analysis. • Direct combustion analysis • Etc.! LIBS is planned as a diagnostic for a Mars Rover expedition set to launch in 2012 • One of my favorite things…

  36. Practical uses abound: Measurements of ambient aerosols in Pittsburgh, Pennsylvania • Weekly average concentration (ng m-3) and mass detection limits (fg) • Corresponding sizes if particles were pure oxides (examples): • 342 nm CaO and 360 nm MgO G.A. Lithgow, A.L. Robinson, and S.G. Buckley, “Ambient Measurements of Metal-Containing PM 2.5 in an Urban Environment Using Laser-Induced Breakdown Spectroscopy,”Atmospheric Environment 38(20) pp 3319-3328 (2004)..

  37. Time series measurements of ambient aerosol – Ca hourly averages G.A. Lithgow, A.L. Robinson, and S.G. Buckley, “Ambient Measurements of Metal-Containing PM 2.5 in an Urban Environment Using Laser-Induced Breakdown Spectroscopy,” Atmospheric Environment38 (20) pp 3319-3328 (2004).

  38. Multi-Element Particles • Several multi-element particles were observed – may show relationships between the elements and/or fingerprints from sources Ca, Mn, Cr Cu, Fe, Cr G.A. Lithgow, A.L. Robinson, and S.G. Buckley, “Ambient Measurements of Metal-Containing PM 2.5 in an Urban Environment Using Laser-Induced Breakdown Spectroscopy,”Atmospheric Environment 38(20) pp 3319-3328 (2004)..

  39. Measurements of Bioparticles: Collaboration with Dr. John Hybl, MIT Lincoln Labs • Basic discrimination is possible • Other orthogonal techniques might be used in conjunction • Sensitivity is low overall J.D. Hybl, G.A. Lithgow, S.G. Buckley, “Laser-Induced Breakdown Spectroscopy Detection and Classification of Biological Aerosols” Applied Spectroscopy57 (10), pp 1207-1215, (2003).

  40. 190 individual particle hits of 930 nm polystyrene particles, 65% Fe3O4 by mass Standard deviation 30% of mean David W. Hahn, Applied Physics Letters 72 (23), 1998. Absolute mass detection limits < 100 fg for most elements Particles < 300 nm can be measured in many cases Multiple species can be measured simultaneously Problem and promise with LIBS measurements of single particles: an example YET Promise!

  41. Need: Better understanding of the LIBS plasma for improved quantification • Plasma structure • Time / temperature history • Hydrodynamics • Directly influences emission • Interaction of plasma with materials • Nanoparticles • Matrix effects • Interpretation of spectra • Optimization of optics • Statistical methods We have examined these

  42. General Plasma Characteristics • At short times (< 100 ns) significant thermal disequilibrium exists • Electron temperatures significantly higher than gas temperatures • At longer times (> 1 s) thermodynamic equilibrium • Cascade of ionic (0.5-3 s), atomic emission (2-50 s), molecular recombination (3 -100 s) After Ferioli and Buckley, to be submitted, Physics of Fluids

  43. Characteristics of the LIBS plasma Ionic emission 0.25-3s Atomic emission 0.5-50 s • Amount of energy coupled into the plasma influences initial size greatly, but initial temperature only slightly • Losses from plasma determine evolution of temperature with time • This directly influences evolution of emission with time Molecular recombination 2-100 s Time 0 1 s 10 s Local thermodynamic Equilibrium Emission intensity I of ion state k to state i:

  44. Effect of spatial location on particle detection • Obviously spatial inhomogeneity is expected in the plasma • How does this affect nanoparticle measurements? • How does this influence repeatability? Typical measurement configuration in a practical device

  45. What happens during detection of a single nanometer-sized particle? • Scale difference = 104 • Plasma: 5 mm • Particle: 500 nm • How does this influence measurement? Dispersed Emission Localized Emission ? ? ? Spatial Effects

  46. f = 7.5 cm UVFS Bifurcated optical fiber bundle, UVFS f = 7.5 cm Fused Silica Pierced Mirror f = 7.5 cm UVFS ICCD camera 0.3 m Czerny-Turner Spectrometer Nd:YAG laser 275 mJ per pulse 10 mm beam diameter Investigation of inherent LIBS variabilityBifurcated Fiber Experiments • Each bundle leg- 7 fibers, 200μm core • Acton 0.3 m imaging Spectrometer • Roper PI-Max 1024x256 pixel intensified CCD • Each leg of fiber bundle binned on chip - 85 pixels • Dilute, MgCl2 aerosol produced by nebulizer and size-selected by differential mobility analyzer (DMA) Lithgow and Buckley, “Effects of focal volume and spatial inhomogeneity on uncertainty in single-aerosol laser-induced breakdown spectroscopy measurements,” Appl. Phys Lett.87, 011501-1 (2005)

  47. Simultaneous backwards and side collection with bifurcated fiber Bulk distributions appear to be similar G.A. Lithgow and S.G. Buckley, “Influence of particle location within plasma and focal volume on precision of single-particle laser-induced breakdown spectroscopy measurements,” iSpectrochimica Acta B60 (7-8) (2005) p 1060.

  48. Each point corresponds to a single particle simultaneously captured in both fibers Ideally, the two signals should be exactly correlated  Difference in signals attributable only to particle location relative to focal volumes Poor correlation between backwards-and side-collected signal G.A. Lithgow and S.G. Buckley, “Effects of focal volume and spatial inhomogeneity on uncertainty in single-aerosol laser-induced breakdown spectroscopy measurements,” Appl. Phys Lett.87, 011501-1 (2005)

  49. Side Only 125 Back Only 177 Both 220 Significant number of particles detected by one fiber are missed by other fiber • Total Hits = 522 • 24% detected from side only • 34% detected from back only • 42% detected by both Particles seen only by side-collection Particles seen only by back-collection G.A. Lithgow and S.G. Buckley, “Influence of particle location within plasma and focal volume on precision of single-particle laser-induced breakdown spectroscopy measurements,” iSpectrochimica Acta B60 (7-8) (2005) p 1060.

  50. Side Only 298 Back Only 33 Both 353 Dual Spectrometer ResultsLinear Fiber Array 684 Total Hits 43% Detected From Side Only 52% Detected By Both Channels 5% Detected From Back Only G.A. Lithgow and S.G. Buckley, “Influence of particle location within plasma and focal volume on precision of single-particle laser-induced breakdown spectroscopy measurements,”Spectrochimica Acta B60 (7-8) (2005) p 1060.

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