- Email: [email protected]
The development of laser spark emission spectroscopy for the characterization of ash deposits
Twenty-Fourth Symposium(International)on Combustion/The CombustionInstitute, 1992/pp. 1579-1585
T H E D E V E L O P M E N T OF LASER SPARK EMISSION SPECTROSCOPY FOR T H E CHARACTERIZATION OF ASH D E P O S I T S DAVID K. OTTESEN Combustion Research Facility Sandia National Laboratories Livermore, CA 94551 USA
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 quantity of the surface material. Optical emission line intensities are then recorded and analyzed to characterize the elemental constituents of the material. Sensitivity and lin~ earity 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.
Introduction The objective of this work is to develop in situ optical diagnostic techniques to investigate in real time the elemental composition of deposits formed on the surfaces of materials exposed to high temperature coal combustion, gasification, and gas cleanup flows. An understanding of the processes that contribute to and control ash deposition is crucial to arresting what is acknowledged to be the principal operational problem in pulverized coal combustion technologies. Ash deposition is directly responsible for decreases in efficiency due to deposit buildup on and corrosion of exposed metal surfaces, while attack by alkali metals in flyash, fume and vapor have been implicated in the corrosion and loss of strength of ceramic filters in hot-gas cleanup devices. A principal motivation for this work is the large body of empirical evidence, accumulated by diesel, gas turbine and boiler manufacturers and users, which identifies the early deposited material as crucial to the composition and rate of buildup of subsequently deposited material. New and better di-
agnostic instrumentation is needed to analyze and quantify these processes, and thus to contribute to improvements in overall operation. We are developing laser spark emission spectroscopy (LASS) to monitor, in situ and in real-time, the elemental composition of inorganic species produced during ash deposition on temperature-controlled, simulated heat transfer and ceramic filter surfaces. Emission spectroscopy of laser-induced plasmas offers the possibility of directly analyzing deposition films without requiring substantial preparation or sampling operations. This work is an extension of our earlier development of LASS as an analytical technique for the elemental composition of individual coal and fly-ash particles. 1,2 LASS is a laser-based diagnostic (also known as laser induced breakdown spectroscopy, or LIBS) which uses a high-power pulsed laser to vaporize a small volume of material, typically 100 Ixm in diameter by a few micrometers thick, from the surface to be analyzed. Time-gated detection is used to observe atomic line emission from the resulting plasma since it is much hotter than the background combustion environment,1'2 and the emission spec-
1579
DIAGNOSTIC METHODS
1580
trum is analyzed to determine the elemental composition of the vaporized material. The results are thus similar to information derived from Auger electron spectroscopy and x-ray fluorescence measurements. This technique, however, has the added advantages of no sample preparation, no high vacuum requirements, and applicability to dielectric materials. Several useful reviews of the technique are found in the recent literature. 3-5 Experimental A more detailed description of the laser spark spectroscopy apparatus may be found elsewhere. 1.2 Samples to be analyzed are mounted on a micrometer-driven XYZ translation stage, and the location of the high power pulsed laser beam on the sample surface is adjusted by means of a HeNe laser alignment beam. A Quanta Ray model DCR-2 Nd:YAG laser is operated in the Q-switched mode, oscillator only, at the frequency doubled wavelength of 532 nm. The nominal energy per pulse is 40 mJ with a pulse width of 7 ns. Ablated areas on steel targets are elliptical in shape with average dimensions of 200 by 800 p~m. The plasma formation is detected by a silicon diode and is used to generate a trigger pulse for the Princeton Instruments model 120 diode array controller. The emitted light from the high temperature plasma is imaged on the entrance slit of a 0.5-m focal length Spex monochromator equipped with either a 300 or 1200 gr/mm ruled grating. The higher dispersion grating is used exclusively in this work. The nominal slit width is 20 p~m. The dispersed radiation is detected using a 1024-element ultraviolet-enhanced diode array detector (Princeton Instruments model IRY-1024). The extremely high plasma temperatures immediately following laser excitation produce highly ionized species which exhibit both line spectra in the far-uv and broad continua emission. For this work we use a time delay of 1 Ixs to allow most of the elemental species in the plasma to cool into the singly-ionized or neutral states. An integration time of 1 ~s is used throughout. Simple, binary deposits of inorganic sulfates are prepared to determine useful analytical wavelengths for various elemental species and to demonstrate sensitivity and linearity for the observed emission intensities. An air brush is used to spray aqueous solutions on heated substrates producing even, thin deposits. This results in the rapid evaporation of the solvent over a large area with little opportunity for thickness or concentration gradients to develop. Deposits prepared by evaporation of thick aqueous films are extremely variable in thickness and composition and not suitable for calibration purposes.
Substrates are in the form of thin, fiat disks of copper and alumina with diameters of 12.7 mm. The amount of solid material deposited on the substrates is less than 0.2 mg, and we estimate that the average film thickness is less than 0.6 Ixm. A series of deposits of sodium and calcium sulfate on copper is prepared with the Na/Ca ratio spanning nearly an order of magnitude (1:1 to 1:8 on an atomic basis for the two extremes in concentration). Alumina substrates of 99% purity were found to contain significant amounts of calcium, magnesium and titanium. Because of the presence of calcium in the substrate material, we chose to prepare binary deposits of sodium and iron sulfate by aspirating aqueous solutions as described above. A pilot-scale pulverized coal combustor at Sandia is used to generate ash deposits from an eastern U.S. coal (Illinois #6, hvC bituminous) and a western U.S. coal (Decker, hvC bituminous) for examination by LASS. These coals were chosen for their substantial differences in ash properties. Deposit samples are prepared in the combustor by exposing air-cooled metallic substrates to flyash generated by the injection and combustion of pulverized coal samples for 30 minutes. Our laser spark emission apparatus currently is located separately from the combustor, and the substrates are subsequently removed for LASS analysis in an off-line fashion. LASS measurements are made at the forward stagnation point (top) and at a point 90~ from this stagnation point (side) of the probes.
Results and Discussion The intent of this work is to investigate the feasibility of LASS as an analytical technique for the elemental composition of ash deposits, and to determine critical instrument parameters which limit the quantitative accuracy of the results. Accordingly, simple binary deposits of inorganic sulfates prepared on metal substrates are examined to assess the effects of plasma formation, and to demonstrate the sensitivity and linearity of the method for the metallic constituents of the deposit. Sodium and calcium sulfate are used since sodium, and to a lesser extent, calcium, are of primary importance in the fouling and slagging properties of ash deposits. Samples of complex ash deposits are also examined in this early stage of instrument development since the presence of many diverse atomic constituents has a large effect on the resulting plasma emission spectra. While the present results are obtained off-line and are qualitative only, they serve to direct future research and illustrate the utility of the method in detecting trends in deposit compo-
LASER SPARK SPECTROSCOPY FOR ASH DEPOSITS sition as a function of coal type and deposition mechanism.
8000
Analysis of Binary Deposits: LASS data are taken for binary sodium/calcium sulfate deposits on copper substrates prepared from solutions containing sodium to calcium ratios of 1 : 1, 3:1, and 8: i on an atomic basis. In several locations a series of five successive laser pulses were taken to assess the extent of material ablation during plasma formation. Sodium and calcium emission signals are obtained from the first laser pulse and decrease by an average of 80% on the second pulse indicating that most of the deposit is removed on the first shot. The energy distribution in the Nd:YAG laser beam is not uniform, and we speculate that the line emission observed on the second pulse is due to the small amount of deposit left unvaporized at the periphery of the laser focal spot after the first shot. Deposit emission intensities are reduced to less than 5% of the original on the third and succeeding laser pulses. The very strong copper doublet at 325 and 327 nm is also observed for all laser pulses indicating that the entire sulfate layer is being sampled in addition to substrate material at the center of the laser focal spot. This is essential for the quantitative interpretation of laser spark emission spectra, since it implies that all of the deposit material in the center of the beam is completely vaporized in a single laser pulse. This limits selective vaporization of the more volatile species in complex deposits (matrix effects) to the small amount of material at the edges of the beam focal spot. LASS data were taken across the sample to quantitatively assess the composition and spatial uniformity of the deposit. We acquired data for fifty laser shots for each sample with a spatial interval between successive shots of 1 mm. An example is shown in Fig. 1 where all unmarked lines are due to neutral Cu I transitions. The singly-ionized Ca II lines at 315.8 nm and 317.9 nm are quite strong for most samples. The observed intensity ratios of this doublet are near 1:2 which is expected for optically thin transitions, and indicates that these lines might be analytically useful for very thin films. The unresolved neutral Na I transitions at 330.2 nm (Fig. 1) are observed for all but a few shots at the lowest sodium concentration, and are adjacent to a Cu I line at 330.8 nm as seen in Fig. 1. Line intensities for Na I and Ca II were measured using transitions at 330.2 mn and 315.8 nm, respectively, and intensity ratios were formed for individual shots for each of the samples examined. Ratios are used in order to compensate for variations in absolute line intensities caused by geometric factors, such as variations in the position of the laser-induced plasma image on the spectrome-
1581
Ca II
~2 Na t
310
320
340
330
Wavelength (nm) FIG. 1. Laser spark emission spectrum of a binary sodium/calcium sulfate deposit on a copper substrate. Unlabeled lines are due to Cu I transitions. ter entrance slit. A plot of these peak height ratios is shown in Fig. 2. Arithmetic means of the sodium and calcium intensity ratios for these three data sets are 0.0883/ 0.0333/0.0079. These values may be compared to a set of Na/Ca molar ratios of 0.0888/0.03:33/0.0111 in the deposits based on the respective sodium and calcium concentrations in the preparation solutions, and normalized by an empirical scaling factor of 0.0111. The agreement between observed emission intensity ratios and solution concentration ratios is excellent for the two deposits with the largest sodium concentration. The significant difference between these ratios for the most dilute sodium deposit is most likely due to errors in measuring the intensity of the weak sodium emission lines. The same empirical scaling factor is used to re-
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0.001
0
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I
t
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20
30
40
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Shot Number
FIG. 2. Sodium/calcium emission intensity ratios measured by LASS for three binary sulfate deposits on copper substrates. Sodium/calcium concentration ratios for starting solutions are: 8:1 (O), 3:1 (~), and 1:1 (O).
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DIAGNOSTIC METHODS
late observed line intensity ratios to preparative solution concentration ratios. It incorporates differences in transition probability, upper state energy and multiplicity, and degree of ionization for the two lines. Quantitative determinations of this factor are possible under the assumption of local thermal equilibrium in the plasma if both the effective plasma temperature and electron density are known. While we have calculated plasma temperatures for several of these shots, as described below, the plasma electron density has not been determined and remains for future work. Although our average emission intensity ratios are in very good agreement with preparative solution concentration ratios, there is a significant amount of scatter in the data. This is not unexpected for the deposit most dilute in sodium; however, one standard deviation in the Na/Ca intensity ratio for the other two cases is approximately one-third of the mean value. Further work is necessary on a larger number of samples which have been characterized by an independent method of analysis for spatial variation in elemental composition in order to firmly establish the accuracy and reproducibility of our intensity ratio measurements. Changes in excitation conditions that produce a variation in effective plasma temperature will affect the observed Na/Ca intensity ratios. The sodium and calcium transitions that we observe originate from upper energy levels of 30,273 and 56,839 cm -1, respectively. Thus, higher plasma temperatures will increase the relative calcium transition intensity for a given concentration ratio. The dominant factors that might affect plasma temperature are changes in deposit thickness, composition, and morphology, for a given incident laser energy density. We were able to calculate plasma temperatures for about half of the laser shots in this series of samples using peak intensities of two Cu I emission lines (319.41 and 336.78 nm) for which upper state energies, multiplicities, and transition probabilities are known. ~ The arithmetic average for these shots is 9500 K with a standard deviation of 2500 K. While the average temperature agrees well with our previous work on single coal particles, t'2 the standard deviation is about three times larger. Pulse-to-pulse incident laser energy was within 5% of the nominal 40 mJ and is unlikely to be the source of the observed variations in plasma temperature. Some of the variation may be attributed to inaccuracies in measuring line intensities. The two Cu I transitions are quite weak, and background detector noise is on the order of 5-20% of the peak intensity. Furthermore, the Cu I line at 319.41 nm is located on the wing of the strong Ca II line at 317.9 nm, and an accurate determination of the appropriate baseline is often difficult. Integrated intensities from a modified spectral analysis code are expected to improve these results.
Inherent irreproducibilities in the plasma excitation process may prove not to seriously affect the quantitative accuracy of the technique if an effective plasma temperature can be calculated and incorporated into the analysis. Potentially more limiting are matrix effects which cause variations in the evaporation rates of elemental species during plasma formation. Matrix effects can substantially alter the empirical scaling factors used to correlate emission intensity with species concentration, and these variations must be evaluated for the range of deposit composition to be examined. These effects are more likely to occur in thicker films, and are discussed below for the analysis of complex ash deposits. We have also investigated deposits on ceramic materials by laser spark spectroscopy. LASS data are acquired in the region of 330 nm for binary deposits of sodium and iron sulfate on alumina. These spectra are simpler to interpret due to the much less complex emission spectrum of the substrate material compared to metallic substrates. Sodium and iron lines are clearly observed without interference, and quantitative calibration studies on these systems are underway.
Analysis of Complex Ash Deposits: The intelligent choice of analytical wavelengths, methods of calibration, and instrument design for laser spark spectroscopy all depend critically on the nature of real coal ash deposits. We also expect that a qualitative analysis of such spectral data will reveal variations in deposit composition due to the characteristics of the parent coal and the mechanism by which the ash deposit was formed. In order to explore the method's potential and limitation in these applications, we collected LASS data for ash deposits from representative coals on metallic substrates as described above. We selected copper as a substrate, due to its simple emission spectrum, in a parallel effort to our work on binary sulfate deposits prepared on Copper discs. While the spark emission spectra from ash deposits resemble data obtained for single coal particles observed earlier, l'z many of the transitions used for quantitative analysis for particulates do not appear to be well suited for use in the analysis of deposits. Furthermore, the complexity in the observed emission spectra due to iron in the deposits necessitates the use of a higher dispersion diffraction grating. We employ a 1200 gr/mm grating that offers a spectral window slightly greater than 40 nm, in contrast to the 160-nm window used in our work on particulates. Our first concern is to demonstrate the simultaneous detection of as many elemental species as possible within our experimental bandwidth. Of particular concern is the detection of sodium, which is of primary importance in the fouling and slagging
LASER SPARK SPECTROSCOPY FOR ASH DEPOSITS properties of ash deposits. The use of the familiar sodium doublet lines near 589 nm does not seem attractive for two reasons: first, these transitions are extremely intense and have the ground state as a lower energy level which causes a considerable amount of self-absorption in the plasma; secondly, they are quite remote from emission lines of other species of interest given our experimental spectral window. Consequently we are directing our initial studies to the region surrounding the much weaker unresolved sodium lines near 330 nm. Although these transitions also end on the ground state, their transition probabilities are a factor of 20 lower than the lines near 589 nm. ~ The problem of self-absorption is greatly mitigated as a result, and indeed we do not observe any line-broadening beyond our instrumental line-width for the deposits studied thus far. The 330-nm sodium lines are also very convenient since it is possible to observe simultaneously emission lines from Ca, AI, Fe and Ti in the wavelength interval from 305 to 345 nm. The Ca II doublet at 315.9 and 317.9 nm that was used successfully in the analysis of binary sulfate deposits is broadened in all of our spark spectra on ash deposits. The full-width at half-maximum of these lines is quite variable, but is generally several times the instrumental linewidth of 0.22 nm. The Ca II doublet at 393.4 and 396.8 nm is also broadened, and this is shown in Figure 3 for two ash deposits from Decker and Illinois #6 coals. These spectra are normalized with respect to the Ca II doublet. Another test for the presence of selfabsorption is the relative intensities of multiplets. The two Ca II lines in Figure 3 should show an intensity ratio of 2:1 for an optically thin plasma, while we observe a ratio much closer to 1.3:1. Several Ca I lines are available between 320-335 nm and 425-435 nm with much smaller transition probabilities and are not obscured by emissions from the copper substrate. These features do not appear to be broadened beyond the ~instrumental line-width and may prove to be useful for quantitative applications in thicker surface deposits. The use of transitions that either end on levels significantly above the ground state or are inherently very weak has been shown to be a very useful approach in the quantitative analysis of iron ores by LASS. v Aluminum is also a common constituent in ash deposits observed in our laser-induced emission spectra. Less broadening of the A I I doublet lines near 394 nm is observed in Figure 3, although the relative intensity ratio of these transitions indicates that self-absorption is still a problem. Still, some semi-quantitative information may be available from these features. When normalized for the Ca II emission intensities, the LASS data in this figure indicate a substantial enrichment in both aluminum and iron for the Illinois # 6 deposit relative to that
8000
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1583
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392 394 396 398 400 402 404 Wavlength ( nrn )
Flc. 3. LASS spectra of coal ash deposits for two different coals illustrating qualitative changes in elemental composition as a function of coal type (Decker--solid, Illinois #6--dash). of the Decker deposit. This is in good agreement with the analysis of the coal ash. Large variations in the qualitative appearance of the spectra are observed for the initial laser pulse at three different axial locations for the thick, top deposits. This can be largely explained by the porous, fragile nature of the deposits. Although the technique could conceivably be used to measure deposit thickness by material ablation from multiple laser shots, careful calibration would be necessary for different ash morphologies and compositions, and material loss through mechanical shock during plasma excitation could significantly affect the results. The spectra from different locations become much more similar after several successive laser shots have ablated most of the outer material, and copper lines from the underlying substrate begin to appear in the emission spectra. This is also accompanied by a general increase in emission intensity for successive shots due to improved coupling of the laser energy into the cavity formed in the deposit and the dense underlying copper substrate. Excitation characteristics for laser-induced plasmas then depend primarily on the dense substrate material that should provide a reasonably constant matrix for the ablation process. These observations generally reinforce our opinion that laser spark spectroscopy will be most quantitatively accurate for elemental composition in thin deposits during the initial stages of growth. As a final illustration of our initial work on complex ash deposits, we have used laser spark emission spectroscopy to distinguish between relative compositions of deposits formed in a given region of the probe at different angular orientations to the main combustion flow. This is demonstrated in Figure 4 for a Decker ash deposit. We have normalized two emission spectra for the Ca II doublet near
1584
DIAGNOSTIC METHODS
8000
I
Ca II
<~
1600
31,
3~3
3~2
3~8
3~o
Wavelength ( nm )
FIG. 4. LASS spectra of coal ash deposits for two locations on probe relative to combustion flow (top deposit--solid; side deposit--dash). Changes in elemental composition are interpreted as differences in deposition mechanism.
316 and 318 nm, and observe a marked enhancement in sodium emission intensity for the deposit formed on the substrate tangent to the combustion flow relative to the deposit formed normal to the combustion flow. Other unlabdled transitions in the figure are due to Cu emission lines from the substrate. The dominant mechanisms for deposit formation are: impaction for the top deposit (which grows on a surface normal to the gas flow), and thermophoresis and condensation for the side, or tangential, deposit. It is thus expected that deposits on the side of the probes would be preferentially enhanced in volatile species, such as sodium, relative to the bulk deposit formed on the top of the probe by impaction of flyash particles. Our laser spark emission results are clearly in agreement with this interpretation. Conclusions Laser spark spectroscopy (LASS) is being developed as an in situ, real-time diagnostic for determining the elemental composition of complex coalash deposits. Thin, binary sulfate deposits on metal substrates are prepared in order to quantify the method. Sodium and calcium emission line intensity ratios from laser-induced plasmas show good correlation with average deposit compositions as determined from the concentrations of preparative aqueous solutions. The technique is also demonstrated for the detection of metal constituents in thin binary sulfate deposits on ceramic substrates. Ash deposits are prepared on metal substrates
from coals of varying rank and mineral matter content. LASS analysis reveals qualitative trends in deposit composition that parallel the composition of mineral matter of the parent coal. Analysis of the deposit formed at different angular orientations of the substrate relative to the main combustion flow show differences in composition that are consistent with distinct mechanisms of formation. LASS data from these thicker deposits reveal strong broadening in several emission lines of AI and Ca due to self-absorption in the plasma, although weaker optically-thin lines of Ca exist that may be useful in quantitative applications. Of greater concern are matrix effects resulting in preferential vaporization of the more volatile atomic species when only a portion of a thick deposit is ablated during plasma formation. Although this effect can be evaluated under certain conditions (for instance, our earlier work on the mineral matter content of individual coal particles), 1'2 it may ultimately limit the quantitative accuracy of LASS in the analysis of thick ash deposits in practical combustion systems. Limitations on the technique due to matrix effects must be evaluated in future work on thick, well-characterized deposits; our initial work suggests that LASS will be most useful to quantitatively determine the elemental composition of thin films on surfaces during the early stages of deposit formation. Other instrument-related issues to be addressed in the study of ash deposits include: factors which influence the choice of analytical wavelengths; 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.
Acknowledgements This work was performed for the United States Department of Energy, Office of Fossil Energy, Morgantown Energy and Technology Center. The assistance of Eric Harwood in the preparation of ash deposits, of Howard Johnsen in preparing sodium/ calcium sulfate deposits, and of Jane Burrows in the collection and analysis of LASS data is gratefully acknowledged. We would also like to thank Larry Baxter for his helpful discussions and guidance in the preparation of ash deposits. REFERENCES 1. OrrESEN, D. K., WANG, J. C. F. AND RaDZmMSKI, L. J.: Appl. Spectrosc. 43, 967 (1989). 2. OTTESEN, D. K., BAXTER, L. L., RADZIEMSKI,L. J. AND BURROWS, J. F.: Energy and Fuels 5, 304 (1991). 3. CREMEBS, D. A. AND RADZ1EMSKI, L. J.: Laser
LASER SPARK SPECTROSCOPY FOR ASH DEPOSITS Spectroscopy and its Applications (Radziemski, L. J., Solarz, R. W. and Paisner, J. A., Eds.), Marcel Dekker, 1987. 4. ADaAIN, R. S. AND WATSON, J. : J. Phys. D: Appl. Phys. 17, 1915 (1984). 5. RADZIEMSKI, L. J. AND CREMERS, D. A.: LaserInduced Plasmas and Applications (Radziemski, L. J., and Cremers, D. A., Eds.), Marcel Dekker, 1989.
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6. WIESE, W. L. AND MARTIN, G. A.: Wavelengths and Transition Probabilities for Atoms and Atomic Ions; Part II, National Institute of Standards and Technology: Gaithersburg, MD~ NSRDS-NBS68, December, 1980. 7. GRANT, K. J.: Laser-lnduced Breakdown Spectroscopy of Iron Ore, Ph.D. Thesis, University of New South Wales, Kensington, Australia, 1988.
COMMENTS Brad Williams, Cornell University, USA. Did you use the second harmonic of the ND:YAG laser in your studies? Author's Reply. Yes, we did. This was for the sake of convenience in alignment for dilute laboratory flow experiments. W h e n dealing with high particleloadings in process applications we will probably utilize the 1.064 Ixm fundamental to minimize particle-beam interactions. o
John W. Daily, University of Colorado at Boulder, USA. Have you studied the effect of varying the amount of laser energy deposited?
Author's Reply. We have looked at this in our previous work on coal and ash particle characterization, but have not as yet carried out a systematic variation of incident laser energy in our current investigations of surface deposition. Based on our earlier work on particles, energy transfer from the laser pulse to the solid seems to be limited primarily by the thermal conductivity of the material during the short pulse width. After a small amount of material
is vaporized, the remainder of the laser pulse is absorbed by the optically dense plasma, and increasing the incident energy merely serves to increase the plasma temperature rather than vaporizing more material from the solid. We plan to investigate this phenomenon in our studies on ash deposition during the coming year.
C. L. Senior, PSI Technology Company, USA. Can sulfur be resolved with this technique? The ability to measure sodium to sulfur ratio, for example, would be useful in characterizing deposits. Author's Reply. We have looked for sulfur emissions lines in our work on coal and ash particle characterization as well as the current study of ash deposition. Tile strongest emission transitions for sulfur lie in the vacuum ultraviolet and the nearinfrared regions, and we have been unable to observe any lines for even quite concentrated sulfurcontaining solids (such as inorganic sulfates). It is possible that optimization of the optical equipment specifically for wavelength regions containing these emission lines could yield usable analytical results.
T H E D E V E L O P M E N T OF LASER SPARK EMISSION SPECTROSCOPY FOR T H E CHARACTERIZATION OF ASH D E P O S I T S DAVID K. OTTESEN Combustion Research Facility Sandia National Laboratories Livermore, CA 94551 USA
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 quantity of the surface material. Optical emission line intensities are then recorded and analyzed to characterize the elemental constituents of the material. Sensitivity and lin~ earity 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.
Introduction The objective of this work is to develop in situ optical diagnostic techniques to investigate in real time the elemental composition of deposits formed on the surfaces of materials exposed to high temperature coal combustion, gasification, and gas cleanup flows. An understanding of the processes that contribute to and control ash deposition is crucial to arresting what is acknowledged to be the principal operational problem in pulverized coal combustion technologies. Ash deposition is directly responsible for decreases in efficiency due to deposit buildup on and corrosion of exposed metal surfaces, while attack by alkali metals in flyash, fume and vapor have been implicated in the corrosion and loss of strength of ceramic filters in hot-gas cleanup devices. A principal motivation for this work is the large body of empirical evidence, accumulated by diesel, gas turbine and boiler manufacturers and users, which identifies the early deposited material as crucial to the composition and rate of buildup of subsequently deposited material. New and better di-
agnostic instrumentation is needed to analyze and quantify these processes, and thus to contribute to improvements in overall operation. We are developing laser spark emission spectroscopy (LASS) to monitor, in situ and in real-time, the elemental composition of inorganic species produced during ash deposition on temperature-controlled, simulated heat transfer and ceramic filter surfaces. Emission spectroscopy of laser-induced plasmas offers the possibility of directly analyzing deposition films without requiring substantial preparation or sampling operations. This work is an extension of our earlier development of LASS as an analytical technique for the elemental composition of individual coal and fly-ash particles. 1,2 LASS is a laser-based diagnostic (also known as laser induced breakdown spectroscopy, or LIBS) which uses a high-power pulsed laser to vaporize a small volume of material, typically 100 Ixm in diameter by a few micrometers thick, from the surface to be analyzed. Time-gated detection is used to observe atomic line emission from the resulting plasma since it is much hotter than the background combustion environment,1'2 and the emission spec-
1579
DIAGNOSTIC METHODS
1580
trum is analyzed to determine the elemental composition of the vaporized material. The results are thus similar to information derived from Auger electron spectroscopy and x-ray fluorescence measurements. This technique, however, has the added advantages of no sample preparation, no high vacuum requirements, and applicability to dielectric materials. Several useful reviews of the technique are found in the recent literature. 3-5 Experimental A more detailed description of the laser spark spectroscopy apparatus may be found elsewhere. 1.2 Samples to be analyzed are mounted on a micrometer-driven XYZ translation stage, and the location of the high power pulsed laser beam on the sample surface is adjusted by means of a HeNe laser alignment beam. A Quanta Ray model DCR-2 Nd:YAG laser is operated in the Q-switched mode, oscillator only, at the frequency doubled wavelength of 532 nm. The nominal energy per pulse is 40 mJ with a pulse width of 7 ns. Ablated areas on steel targets are elliptical in shape with average dimensions of 200 by 800 p~m. The plasma formation is detected by a silicon diode and is used to generate a trigger pulse for the Princeton Instruments model 120 diode array controller. The emitted light from the high temperature plasma is imaged on the entrance slit of a 0.5-m focal length Spex monochromator equipped with either a 300 or 1200 gr/mm ruled grating. The higher dispersion grating is used exclusively in this work. The nominal slit width is 20 p~m. The dispersed radiation is detected using a 1024-element ultraviolet-enhanced diode array detector (Princeton Instruments model IRY-1024). The extremely high plasma temperatures immediately following laser excitation produce highly ionized species which exhibit both line spectra in the far-uv and broad continua emission. For this work we use a time delay of 1 Ixs to allow most of the elemental species in the plasma to cool into the singly-ionized or neutral states. An integration time of 1 ~s is used throughout. Simple, binary deposits of inorganic sulfates are prepared to determine useful analytical wavelengths for various elemental species and to demonstrate sensitivity and linearity for the observed emission intensities. An air brush is used to spray aqueous solutions on heated substrates producing even, thin deposits. This results in the rapid evaporation of the solvent over a large area with little opportunity for thickness or concentration gradients to develop. Deposits prepared by evaporation of thick aqueous films are extremely variable in thickness and composition and not suitable for calibration purposes.
Substrates are in the form of thin, fiat disks of copper and alumina with diameters of 12.7 mm. The amount of solid material deposited on the substrates is less than 0.2 mg, and we estimate that the average film thickness is less than 0.6 Ixm. A series of deposits of sodium and calcium sulfate on copper is prepared with the Na/Ca ratio spanning nearly an order of magnitude (1:1 to 1:8 on an atomic basis for the two extremes in concentration). Alumina substrates of 99% purity were found to contain significant amounts of calcium, magnesium and titanium. Because of the presence of calcium in the substrate material, we chose to prepare binary deposits of sodium and iron sulfate by aspirating aqueous solutions as described above. A pilot-scale pulverized coal combustor at Sandia is used to generate ash deposits from an eastern U.S. coal (Illinois #6, hvC bituminous) and a western U.S. coal (Decker, hvC bituminous) for examination by LASS. These coals were chosen for their substantial differences in ash properties. Deposit samples are prepared in the combustor by exposing air-cooled metallic substrates to flyash generated by the injection and combustion of pulverized coal samples for 30 minutes. Our laser spark emission apparatus currently is located separately from the combustor, and the substrates are subsequently removed for LASS analysis in an off-line fashion. LASS measurements are made at the forward stagnation point (top) and at a point 90~ from this stagnation point (side) of the probes.
Results and Discussion The intent of this work is to investigate the feasibility of LASS as an analytical technique for the elemental composition of ash deposits, and to determine critical instrument parameters which limit the quantitative accuracy of the results. Accordingly, simple binary deposits of inorganic sulfates prepared on metal substrates are examined to assess the effects of plasma formation, and to demonstrate the sensitivity and linearity of the method for the metallic constituents of the deposit. Sodium and calcium sulfate are used since sodium, and to a lesser extent, calcium, are of primary importance in the fouling and slagging properties of ash deposits. Samples of complex ash deposits are also examined in this early stage of instrument development since the presence of many diverse atomic constituents has a large effect on the resulting plasma emission spectra. While the present results are obtained off-line and are qualitative only, they serve to direct future research and illustrate the utility of the method in detecting trends in deposit compo-
LASER SPARK SPECTROSCOPY FOR ASH DEPOSITS sition as a function of coal type and deposition mechanism.
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Analysis of Binary Deposits: LASS data are taken for binary sodium/calcium sulfate deposits on copper substrates prepared from solutions containing sodium to calcium ratios of 1 : 1, 3:1, and 8: i on an atomic basis. In several locations a series of five successive laser pulses were taken to assess the extent of material ablation during plasma formation. Sodium and calcium emission signals are obtained from the first laser pulse and decrease by an average of 80% on the second pulse indicating that most of the deposit is removed on the first shot. The energy distribution in the Nd:YAG laser beam is not uniform, and we speculate that the line emission observed on the second pulse is due to the small amount of deposit left unvaporized at the periphery of the laser focal spot after the first shot. Deposit emission intensities are reduced to less than 5% of the original on the third and succeeding laser pulses. The very strong copper doublet at 325 and 327 nm is also observed for all laser pulses indicating that the entire sulfate layer is being sampled in addition to substrate material at the center of the laser focal spot. This is essential for the quantitative interpretation of laser spark emission spectra, since it implies that all of the deposit material in the center of the beam is completely vaporized in a single laser pulse. This limits selective vaporization of the more volatile species in complex deposits (matrix effects) to the small amount of material at the edges of the beam focal spot. LASS data were taken across the sample to quantitatively assess the composition and spatial uniformity of the deposit. We acquired data for fifty laser shots for each sample with a spatial interval between successive shots of 1 mm. An example is shown in Fig. 1 where all unmarked lines are due to neutral Cu I transitions. The singly-ionized Ca II lines at 315.8 nm and 317.9 nm are quite strong for most samples. The observed intensity ratios of this doublet are near 1:2 which is expected for optically thin transitions, and indicates that these lines might be analytically useful for very thin films. The unresolved neutral Na I transitions at 330.2 nm (Fig. 1) are observed for all but a few shots at the lowest sodium concentration, and are adjacent to a Cu I line at 330.8 nm as seen in Fig. 1. Line intensities for Na I and Ca II were measured using transitions at 330.2 mn and 315.8 nm, respectively, and intensity ratios were formed for individual shots for each of the samples examined. Ratios are used in order to compensate for variations in absolute line intensities caused by geometric factors, such as variations in the position of the laser-induced plasma image on the spectrome-
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Ca II
~2 Na t
310
320
340
330
Wavelength (nm) FIG. 1. Laser spark emission spectrum of a binary sodium/calcium sulfate deposit on a copper substrate. Unlabeled lines are due to Cu I transitions. ter entrance slit. A plot of these peak height ratios is shown in Fig. 2. Arithmetic means of the sodium and calcium intensity ratios for these three data sets are 0.0883/ 0.0333/0.0079. These values may be compared to a set of Na/Ca molar ratios of 0.0888/0.03:33/0.0111 in the deposits based on the respective sodium and calcium concentrations in the preparation solutions, and normalized by an empirical scaling factor of 0.0111. The agreement between observed emission intensity ratios and solution concentration ratios is excellent for the two deposits with the largest sodium concentration. The significant difference between these ratios for the most dilute sodium deposit is most likely due to errors in measuring the intensity of the weak sodium emission lines. The same empirical scaling factor is used to re-
c
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0.001
0
9 oeoe
ee
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I
t
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20
30
40
50
Shot Number
FIG. 2. Sodium/calcium emission intensity ratios measured by LASS for three binary sulfate deposits on copper substrates. Sodium/calcium concentration ratios for starting solutions are: 8:1 (O), 3:1 (~), and 1:1 (O).
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DIAGNOSTIC METHODS
late observed line intensity ratios to preparative solution concentration ratios. It incorporates differences in transition probability, upper state energy and multiplicity, and degree of ionization for the two lines. Quantitative determinations of this factor are possible under the assumption of local thermal equilibrium in the plasma if both the effective plasma temperature and electron density are known. While we have calculated plasma temperatures for several of these shots, as described below, the plasma electron density has not been determined and remains for future work. Although our average emission intensity ratios are in very good agreement with preparative solution concentration ratios, there is a significant amount of scatter in the data. This is not unexpected for the deposit most dilute in sodium; however, one standard deviation in the Na/Ca intensity ratio for the other two cases is approximately one-third of the mean value. Further work is necessary on a larger number of samples which have been characterized by an independent method of analysis for spatial variation in elemental composition in order to firmly establish the accuracy and reproducibility of our intensity ratio measurements. Changes in excitation conditions that produce a variation in effective plasma temperature will affect the observed Na/Ca intensity ratios. The sodium and calcium transitions that we observe originate from upper energy levels of 30,273 and 56,839 cm -1, respectively. Thus, higher plasma temperatures will increase the relative calcium transition intensity for a given concentration ratio. The dominant factors that might affect plasma temperature are changes in deposit thickness, composition, and morphology, for a given incident laser energy density. We were able to calculate plasma temperatures for about half of the laser shots in this series of samples using peak intensities of two Cu I emission lines (319.41 and 336.78 nm) for which upper state energies, multiplicities, and transition probabilities are known. ~ The arithmetic average for these shots is 9500 K with a standard deviation of 2500 K. While the average temperature agrees well with our previous work on single coal particles, t'2 the standard deviation is about three times larger. Pulse-to-pulse incident laser energy was within 5% of the nominal 40 mJ and is unlikely to be the source of the observed variations in plasma temperature. Some of the variation may be attributed to inaccuracies in measuring line intensities. The two Cu I transitions are quite weak, and background detector noise is on the order of 5-20% of the peak intensity. Furthermore, the Cu I line at 319.41 nm is located on the wing of the strong Ca II line at 317.9 nm, and an accurate determination of the appropriate baseline is often difficult. Integrated intensities from a modified spectral analysis code are expected to improve these results.
Inherent irreproducibilities in the plasma excitation process may prove not to seriously affect the quantitative accuracy of the technique if an effective plasma temperature can be calculated and incorporated into the analysis. Potentially more limiting are matrix effects which cause variations in the evaporation rates of elemental species during plasma formation. Matrix effects can substantially alter the empirical scaling factors used to correlate emission intensity with species concentration, and these variations must be evaluated for the range of deposit composition to be examined. These effects are more likely to occur in thicker films, and are discussed below for the analysis of complex ash deposits. We have also investigated deposits on ceramic materials by laser spark spectroscopy. LASS data are acquired in the region of 330 nm for binary deposits of sodium and iron sulfate on alumina. These spectra are simpler to interpret due to the much less complex emission spectrum of the substrate material compared to metallic substrates. Sodium and iron lines are clearly observed without interference, and quantitative calibration studies on these systems are underway.
Analysis of Complex Ash Deposits: The intelligent choice of analytical wavelengths, methods of calibration, and instrument design for laser spark spectroscopy all depend critically on the nature of real coal ash deposits. We also expect that a qualitative analysis of such spectral data will reveal variations in deposit composition due to the characteristics of the parent coal and the mechanism by which the ash deposit was formed. In order to explore the method's potential and limitation in these applications, we collected LASS data for ash deposits from representative coals on metallic substrates as described above. We selected copper as a substrate, due to its simple emission spectrum, in a parallel effort to our work on binary sulfate deposits prepared on Copper discs. While the spark emission spectra from ash deposits resemble data obtained for single coal particles observed earlier, l'z many of the transitions used for quantitative analysis for particulates do not appear to be well suited for use in the analysis of deposits. Furthermore, the complexity in the observed emission spectra due to iron in the deposits necessitates the use of a higher dispersion diffraction grating. We employ a 1200 gr/mm grating that offers a spectral window slightly greater than 40 nm, in contrast to the 160-nm window used in our work on particulates. Our first concern is to demonstrate the simultaneous detection of as many elemental species as possible within our experimental bandwidth. Of particular concern is the detection of sodium, which is of primary importance in the fouling and slagging
LASER SPARK SPECTROSCOPY FOR ASH DEPOSITS properties of ash deposits. The use of the familiar sodium doublet lines near 589 nm does not seem attractive for two reasons: first, these transitions are extremely intense and have the ground state as a lower energy level which causes a considerable amount of self-absorption in the plasma; secondly, they are quite remote from emission lines of other species of interest given our experimental spectral window. Consequently we are directing our initial studies to the region surrounding the much weaker unresolved sodium lines near 330 nm. Although these transitions also end on the ground state, their transition probabilities are a factor of 20 lower than the lines near 589 nm. ~ The problem of self-absorption is greatly mitigated as a result, and indeed we do not observe any line-broadening beyond our instrumental line-width for the deposits studied thus far. The 330-nm sodium lines are also very convenient since it is possible to observe simultaneously emission lines from Ca, AI, Fe and Ti in the wavelength interval from 305 to 345 nm. The Ca II doublet at 315.9 and 317.9 nm that was used successfully in the analysis of binary sulfate deposits is broadened in all of our spark spectra on ash deposits. The full-width at half-maximum of these lines is quite variable, but is generally several times the instrumental linewidth of 0.22 nm. The Ca II doublet at 393.4 and 396.8 nm is also broadened, and this is shown in Figure 3 for two ash deposits from Decker and Illinois #6 coals. These spectra are normalized with respect to the Ca II doublet. Another test for the presence of selfabsorption is the relative intensities of multiplets. The two Ca II lines in Figure 3 should show an intensity ratio of 2:1 for an optically thin plasma, while we observe a ratio much closer to 1.3:1. Several Ca I lines are available between 320-335 nm and 425-435 nm with much smaller transition probabilities and are not obscured by emissions from the copper substrate. These features do not appear to be broadened beyond the ~instrumental line-width and may prove to be useful for quantitative applications in thicker surface deposits. The use of transitions that either end on levels significantly above the ground state or are inherently very weak has been shown to be a very useful approach in the quantitative analysis of iron ores by LASS. v Aluminum is also a common constituent in ash deposits observed in our laser-induced emission spectra. Less broadening of the A I I doublet lines near 394 nm is observed in Figure 3, although the relative intensity ratio of these transitions indicates that self-absorption is still a problem. Still, some semi-quantitative information may be available from these features. When normalized for the Ca II emission intensities, the LASS data in this figure indicate a substantial enrichment in both aluminum and iron for the Illinois # 6 deposit relative to that
8000
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1583
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392 394 396 398 400 402 404 Wavlength ( nrn )
Flc. 3. LASS spectra of coal ash deposits for two different coals illustrating qualitative changes in elemental composition as a function of coal type (Decker--solid, Illinois #6--dash). of the Decker deposit. This is in good agreement with the analysis of the coal ash. Large variations in the qualitative appearance of the spectra are observed for the initial laser pulse at three different axial locations for the thick, top deposits. This can be largely explained by the porous, fragile nature of the deposits. Although the technique could conceivably be used to measure deposit thickness by material ablation from multiple laser shots, careful calibration would be necessary for different ash morphologies and compositions, and material loss through mechanical shock during plasma excitation could significantly affect the results. The spectra from different locations become much more similar after several successive laser shots have ablated most of the outer material, and copper lines from the underlying substrate begin to appear in the emission spectra. This is also accompanied by a general increase in emission intensity for successive shots due to improved coupling of the laser energy into the cavity formed in the deposit and the dense underlying copper substrate. Excitation characteristics for laser-induced plasmas then depend primarily on the dense substrate material that should provide a reasonably constant matrix for the ablation process. These observations generally reinforce our opinion that laser spark spectroscopy will be most quantitatively accurate for elemental composition in thin deposits during the initial stages of growth. As a final illustration of our initial work on complex ash deposits, we have used laser spark emission spectroscopy to distinguish between relative compositions of deposits formed in a given region of the probe at different angular orientations to the main combustion flow. This is demonstrated in Figure 4 for a Decker ash deposit. We have normalized two emission spectra for the Ca II doublet near
1584
DIAGNOSTIC METHODS
8000
I
Ca II
<~
1600
31,
3~3
3~2
3~8
3~o
Wavelength ( nm )
FIG. 4. LASS spectra of coal ash deposits for two locations on probe relative to combustion flow (top deposit--solid; side deposit--dash). Changes in elemental composition are interpreted as differences in deposition mechanism.
316 and 318 nm, and observe a marked enhancement in sodium emission intensity for the deposit formed on the substrate tangent to the combustion flow relative to the deposit formed normal to the combustion flow. Other unlabdled transitions in the figure are due to Cu emission lines from the substrate. The dominant mechanisms for deposit formation are: impaction for the top deposit (which grows on a surface normal to the gas flow), and thermophoresis and condensation for the side, or tangential, deposit. It is thus expected that deposits on the side of the probes would be preferentially enhanced in volatile species, such as sodium, relative to the bulk deposit formed on the top of the probe by impaction of flyash particles. Our laser spark emission results are clearly in agreement with this interpretation. Conclusions Laser spark spectroscopy (LASS) is being developed as an in situ, real-time diagnostic for determining the elemental composition of complex coalash deposits. Thin, binary sulfate deposits on metal substrates are prepared in order to quantify the method. Sodium and calcium emission line intensity ratios from laser-induced plasmas show good correlation with average deposit compositions as determined from the concentrations of preparative aqueous solutions. The technique is also demonstrated for the detection of metal constituents in thin binary sulfate deposits on ceramic substrates. Ash deposits are prepared on metal substrates
from coals of varying rank and mineral matter content. LASS analysis reveals qualitative trends in deposit composition that parallel the composition of mineral matter of the parent coal. Analysis of the deposit formed at different angular orientations of the substrate relative to the main combustion flow show differences in composition that are consistent with distinct mechanisms of formation. LASS data from these thicker deposits reveal strong broadening in several emission lines of AI and Ca due to self-absorption in the plasma, although weaker optically-thin lines of Ca exist that may be useful in quantitative applications. Of greater concern are matrix effects resulting in preferential vaporization of the more volatile atomic species when only a portion of a thick deposit is ablated during plasma formation. Although this effect can be evaluated under certain conditions (for instance, our earlier work on the mineral matter content of individual coal particles), 1'2 it may ultimately limit the quantitative accuracy of LASS in the analysis of thick ash deposits in practical combustion systems. Limitations on the technique due to matrix effects must be evaluated in future work on thick, well-characterized deposits; our initial work suggests that LASS will be most useful to quantitatively determine the elemental composition of thin films on surfaces during the early stages of deposit formation. Other instrument-related issues to be addressed in the study of ash deposits include: factors which influence the choice of analytical wavelengths; 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.
Acknowledgements This work was performed for the United States Department of Energy, Office of Fossil Energy, Morgantown Energy and Technology Center. The assistance of Eric Harwood in the preparation of ash deposits, of Howard Johnsen in preparing sodium/ calcium sulfate deposits, and of Jane Burrows in the collection and analysis of LASS data is gratefully acknowledged. We would also like to thank Larry Baxter for his helpful discussions and guidance in the preparation of ash deposits. REFERENCES 1. OrrESEN, D. K., WANG, J. C. F. AND RaDZmMSKI, L. J.: Appl. Spectrosc. 43, 967 (1989). 2. OTTESEN, D. K., BAXTER, L. L., RADZIEMSKI,L. J. AND BURROWS, J. F.: Energy and Fuels 5, 304 (1991). 3. CREMEBS, D. A. AND RADZ1EMSKI, L. J.: Laser
LASER SPARK SPECTROSCOPY FOR ASH DEPOSITS Spectroscopy and its Applications (Radziemski, L. J., Solarz, R. W. and Paisner, J. A., Eds.), Marcel Dekker, 1987. 4. ADaAIN, R. S. AND WATSON, J. : J. Phys. D: Appl. Phys. 17, 1915 (1984). 5. RADZIEMSKI, L. J. AND CREMERS, D. A.: LaserInduced Plasmas and Applications (Radziemski, L. J., and Cremers, D. A., Eds.), Marcel Dekker, 1989.
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6. WIESE, W. L. AND MARTIN, G. A.: Wavelengths and Transition Probabilities for Atoms and Atomic Ions; Part II, National Institute of Standards and Technology: Gaithersburg, MD~ NSRDS-NBS68, December, 1980. 7. GRANT, K. J.: Laser-lnduced Breakdown Spectroscopy of Iron Ore, Ph.D. Thesis, University of New South Wales, Kensington, Australia, 1988.
COMMENTS Brad Williams, Cornell University, USA. Did you use the second harmonic of the ND:YAG laser in your studies? Author's Reply. Yes, we did. This was for the sake of convenience in alignment for dilute laboratory flow experiments. W h e n dealing with high particleloadings in process applications we will probably utilize the 1.064 Ixm fundamental to minimize particle-beam interactions. o
John W. Daily, University of Colorado at Boulder, USA. Have you studied the effect of varying the amount of laser energy deposited?
Author's Reply. We have looked at this in our previous work on coal and ash particle characterization, but have not as yet carried out a systematic variation of incident laser energy in our current investigations of surface deposition. Based on our earlier work on particles, energy transfer from the laser pulse to the solid seems to be limited primarily by the thermal conductivity of the material during the short pulse width. After a small amount of material
is vaporized, the remainder of the laser pulse is absorbed by the optically dense plasma, and increasing the incident energy merely serves to increase the plasma temperature rather than vaporizing more material from the solid. We plan to investigate this phenomenon in our studies on ash deposition during the coming year.
C. L. Senior, PSI Technology Company, USA. Can sulfur be resolved with this technique? The ability to measure sodium to sulfur ratio, for example, would be useful in characterizing deposits. Author's Reply. We have looked for sulfur emissions lines in our work on coal and ash particle characterization as well as the current study of ash deposition. Tile strongest emission transitions for sulfur lie in the vacuum ultraviolet and the nearinfrared regions, and we have been unable to observe any lines for even quite concentrated sulfurcontaining solids (such as inorganic sulfates). It is possible that optimization of the optical equipment specifically for wavelength regions containing these emission lines could yield usable analytical results.