David K. Ottesen

A laser-based technique to continuously monitor metal aerosol emissions

Abstract We are developing an instrument to continuously monitor metal aerosol emissions in applications including industrial process vents (e.g., exhaust stacks from electroplating baths), waste treatment processes (incinerators), and boilers and industrial furnaces (coal-fired power plants). The monitoring technique is based on laser spark spectroscopy (LASS; also known as laser-induced breakdown spectroscopy - LIBS), in which a pulsed laser is used to rapidly heat a particle and produce a plasma (or laser ‘spark’). The light emission from the spark is spectrally resolved and analyzed to identify the elemental constituents of the particle and quantify the abundance of the measured species. One feature of LASS is that it can measure atomic species embedded in either solid particles or fine liquid droplets, which account for a large percentage of metal emissions from applications of interest. In the initial work described here, we have focused on the application of the LASS technique for measuring chromium emissions from electroplating baths. This paper describes the approach used for measuring the total chromium concentration in laboratory simulations of electroplating aerosols. Chromium concentrations less than 1 mg/scm can be measured. This work forms the basis for future applications to incineration and fossil power plants.

Detection of Contaminants on Electronic Microcircuit Substrates by Laser Spark Emission Spectroscopy

Laser spark emission spectroscopy is used to determine the elemental composition of contaminants found on electronic microcircuits fabricated on alumina substrates. This technique is particularly useful for rapid analyses of dielectric surfaces, and spatially resolved data with some degree of depth profiling information are obtained. Two specific examples are given which illustrate the utility of the method in pinpointing production problems.

The development of laser spark emission spectroscopy for the characterization of ash deposits

Laser spark emission spectroscopy (LASS) is being developed as an in situ , real time monitor for elemental composition of ash deposits formed during coal combustion processes. The analytical technique uses a high-energy pulsed laser to form a high-temperature plasma from a small quanity of the surface material. Optical emission line intensities are then recorded and analyzed to characterize the elemental constituents of the material. Sensitivity and linearity of the method are demonstrated for binary sodium/calcium sulfate deposits prepared on metallic substrates. Observed intensity ratios for sodium and calcium are found to correlate very well with deposit composition. LASS has also been used to qualitatively characterize complex ash deposits prepared by the combustion of pulverized coals in a pilot-scale combustor. Differences in composition related to coal type are monitored by comparing relative emission intensities for calcium, aluminum and iron. Examination of the ash deposit at a location on the probe for two different orientations relative to the main combustion flow reveals changes in composition that can be related to differences in the mechanism of deposition. Several instrument-related issues remain to be addressed in the study of ash deposits including the following: factors which influence the choice of analytical wavelengths; self-absorption of some emission lines; preferential vaporization of atomic species during plasma formation; variations in energy coupling and resultant temperature of the laser-induced plasma; optimum apparatus configuration for detection and resolution of complex emission spectra; and methods of data analysis.

Laser Spark Emission Spectroscopy of Individual Coal Particles

Perhaps one of the most difficult tasks facing the experimenter in coal combustion research is the characterization of solid particulates in a combustion flow. This elusive information would provide crucial insights in the complex mechanisms involved in the chemical reactions of coal during the combustion process. Ultimately this is necessary for any thorough understanding and prediction of such phenomena as mineral matter transformation and the deleterious formation of solid ash deposits on reactor and heat exchanger surfaces.

Diode Laser Diagnostics for Gas Species and Soot in Large Fires: LDRD Project Final Report

The thermal hazard posed by a fire to a weapon or other engineered system is a consequence of combined radiation and convection from high-temperature soot and gases. The development of advanced, predictive models of this hazard requires detailed knowledge of the transient chemical structure and soot distributions within real-scale fires. At present, there are no measurements, and hence limited understanding, of transient gaseous species generation and transport in large, fully turbulent fires. As part of a Laboratory Directed Research and Development (LDRD) project to develop such an experimental capability, near-infrared tunable diode laser absorption spectroscopy (TDLAS) has been identified as the most promising diagnostic technique for making these measurements. In order to develop this capability, significant efforts were applied to choosing optimal species and transitions for detection, to developing an effective multiplexing strategy for several lasers undergoing wavelength modulation spectroscopy with fast laser ramp scans, to developing a methodology for multipassing the TDL beams across a small probe volume, and finally, to designing a water-cooled, fiber-coupled probe for performing these measurements locally within large pool fires. All of these challenges were surmounted during the course of this project, and in the end a preliminary, unique dataset of combined water vapor, acetylene, and soot concentrations was obtained from a 1-m diameter JP-8 pool fire.

Optical Sensors for Post Combustion Control in Electric Arc Furnace Steelmaking (TRP 9851)

Working in collaboration with Stantec Global Technologies, Process Metrix Corporation, and The Timken Company, Sandia National Laboratories constructed and evaluated a novel, laser-based off-gas sensor at the electric arc furnace facility of Timkens Faircrest Steel Plant (Canton, Ohio). The sensor is based on a mid-infrared tunable diode laser (TDL), and measures the concentration and temperature of specific gas species present in the off-gas emanating from the EAF. The laser beam is transmitted through the gas stream at the fourth hole of the EAF, and provides a real-time, in situ measurement that can be used for process optimization. Two sets of field tests were performed in parallel with Stantecs extractive probe off-gas system, and the tests confirm the TDL sensors operation and applicability for electric steel making. The sensor measures real-time, in situ line-of-sight carbon monoxide (CO) concentrations between 5% and 35% CO, and measures off-gas temperature in the range of 1400 to 1900 K. In order to achieve commercial-ready status, future work is required to extend the sensor for simultaneous CO and CO{sub 2} concentration measurements. In addition, long-term endurance tests including process optimization must be completed.

PPLN laser-based system for chemical imaging

An infrared-imaging instrument is being developed to provide in situ qualitative and quantitative assessment of hydrocarbon contaminants on metallic surfaces for cleaning verification. A continuous-wave infrared optical parametric oscillator (OPO), based on the quasi-phasematched material periodically poled lithium niobate (PPLN), is interfaced with an InSb focal plane array camera to perform fast, non-invasive analysis by reflectance spectroscopy. The period range of the designed fan-out PPLN crystal determines the range of the output wavelength of the light source. It is able to scan hundreds of wavenumbers positioned in the range of 2820 - 3250 cm-1, which is sufficient to detect functional groups of common organic compounds (-CH, -OH, and -NH). The capability of the instrument has been demonstrated in a preliminary investigation of reflectance measurements for hydrocarbon solvents (methanol and d-limonene) on an aluminum surface. A substantial difference in absorption is obtained for the two solvents at two different laser-illumination wavelengths, thus permitting hydrocarbon detection and molecular species differentiation. Preliminary reflectance spectra of a mixture of aliphatic hydrocarbon lubricants and drawing agents on an aluminum panel are also presented. The relative thickness of the hydrocarbon thin film is determined by the intensity ratio of images acquired at two different laser illumination frequencies.

Laser diagnostics for research in coal combustion

Recent results are presented for the application of laser spark emission spectroscopy in the determination of elemental composition of particles in the combustion zone of a pulverized coal flame. The technique is particularly sensitive to metallic elements and may be very useful in the study of mineral matter transformations during combustion. High energy laser pulses were used to form plasmas on single coal particles and timeresolved spectra of the optical emission was collected using a linear diode array detector. The various transitions were assigned and qualitative trends in elemental composition for wellcharacterized coals were observed to be in agreement with other analytical results. Semiquantitative elemental concentrations were calculated and are presented.

Detection Of Contaminants In Steel Tubing Using Infrared Reflection Spectroscopy

We have used Fourier transform infrared (FT1R) spectroscopy to evaluate the contamination of stainless steel tubing with inner diameters of less than one mm, and lengths of 25 to 100 mm. We will discuss the development of a non-destructive analytical tech-nique which is sensitive to both organic and inorganic contaminants which may arise during fabrication processes. Experimental results will be compared with theoretical calculations for thin films of hydrocarbon lubricants and metal oxides on metallic surfaces. The main advantages of infrared measurements are their non-destructive nature, ease of application, short measurement times, and the ability to interface the necessary auxilliary optics with existing FTIR instruments. The sensitivity of the technique and its application to the evaluation of current cleaning processes will be presented.

AISI/DOE Advanced Process Control Program Vol. 1 of 6: Optical Sensors and Controls for Improved Basic Oxygen Furnace Operations

The development of an optical sensor for basic oxygen furnace (BOF) off-gas composition and temperature in this Advanced Process Control project has been a laboratory spectroscopic method evolve into a pre-commercialization prototype sensor system. The sensor simultaneously detects an infrared tunable diode laser ITDL beam transmitted through the process off-gas directly above the furnace mouth, and the infrared greybody emission from the particulate-laden off-gas stream. Following developmental laboratory and field-testing, the sensor prototype was successfully tested in four long-term field trials at Bethlehem Steels Sparrows Point plant in Baltimore, MD> The resulting optical data were analyzed and reveal correlations with four important process variables: (1) bath turndown temperature; (2) carbon monoxide post-combustion control; (2) bath carbon concentration; and (4) furnace slopping behavior. The optical sensor measurement of the off-gas temperature is modestly correlated with bath turndown temperature. A detailed regression analysis of over 200 heats suggests that a dynamic control level of +25 Degree F can be attained with a stand-alone laser-based optical sensor. The ability to track off-gas temperatures to control post-combustion lance practice is also demonstrated, and may be of great use in optimizing post-combustion efficiency in electric furnace steelmaking operations. In addition to the laser-based absorption spectroscopy data collected by this sensor, a concurrent signal generated by greybody emission from the particle-laden off-gas was collected and analyzed. A detailed regression analysis shows an excellent correlation of a single variable with final bath turndown carbon concentration. Extended field trials in 1998 and early 1999 show a response range from below 0.03% to a least 0.15% carbon concentration with a precision of +0.0007%. Finally, a strong correlation between prolonged drops in the off-gas emission signal and furnace slopping events was observed. A simple computer algorithm was written that successfully predicts furnace slopping for 90% of the heats observed; over 80% are predicted with at least a 30-second warning prior to the initial slopping events,