Energy & Fuels 1993, 7, 755-760
755
In Situ,Real-Time Characterization of Coal Ash Deposits Using Fourier Transform Infrared Emission Spectroscopy Larry L. Baxter;>+ Galen H. Richards,$David K. Ottesen,+and John N. Harbt Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551-0969, and Advanced Combustion Engineering Research Center, Brigham Young University, Provo, Utah 84602 Received May 21, 1993. Revised Manuscript Received September 10, 199P
In situ Fourier transform infrared (FTIR) emission spectroscopy is used to identify the presence of silica, sulfates, and silicatesas a function of time in coal ash deposits generated in Sandia's multifuel combustor, a pilot-scale reactor. Ash deposits are formed on a cylindrical tube in cross flow under experimental conditions which correspond to convection pass (fouling) regions of a commercial coalfired boiler. The gas temperature, gas composition, particle loading, and extent of particle reaction in the combustor are typical of commercial boiler operation. The major classes of inorganic species deposited on the tube, including silicates and sulfates, are identified using the FTIR emission spectroscopy technique. Post mortem X-ray diffraction and conventional infrared absorption and reflectance analyses on the same deposits are used to corroborate the in situ FTIR emission data. The deposit composition from a western coal changes significantly as a function of both deposition time and combustion conditions. The observed changes include formation of sulfates and silicates. Such changes have implications for deposit properties such as tenacity and strength; the FTIR emission diagnostic shows promise as a method for monitoring such changes in practical systems.
Introduction The design and operation of pulverized-coal-firedutility boilers are significantly influenced by the behavior of the inorganic constituents in the coal. Efficient operation of commercial boilers is becoming more difficult as coal supplies change to help meet recent amendments to the Clean Air Act and other state and federal regulations. The deposition behavior of a particular coal is a function of both the quantity and the mode of occurrence of the inorganic constituents in the individual coal particles as well as the operating conditions of the boiler. Significant attempts are underway to determine the precise dependence of deposit properties on operating conditions and fuel properties.14 However, an increased understanding of the mechanisms governing deposit formation could be obtained by in situ, real-time data describing deposit formation and changes in deposit properties. Many investigatorshave used laboratory and pilot scale furnaces to study the mechanisms governing ash deposition. Typically, ash deposits are grown on probes or surfaces that are inserted in the test furnaces and then Sandia National Laboratories. Brigham Young University. *Abstract published in Advance ACS Abstracts, October 15, 1993. (1) Baxtar, L. L.; DeSollar, R. W. Fuel, in press. (2) Be&, J. M.; Monroe, L. S.;Barta, L. E.;Sarofim, A. F. From Coal Mineral Matter Roperties to Fly Ash Deposition Tendencies;a Modeling Approach; Pittsburgh, PA, 1990. (3) Beneon, 5. A.; Jones, M. L.; Harb, J. N. Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.;Elsevier: New York, 1993. (4) Srinivaenchar, S.; Senior, C. L.; Helble, J. J.; Moore, J. W. A Fundamental Approach to the Redictionof Coal AshDeposit Formation in Combustion S y s t e m 1-in press (The Combustion Institute, The University of Sydney, Australia, 1992). (5) Baxter,L. L.; Dora, L. ASME Paper No. 92-JPGC-FACT-14,1992. (6)Baxter, L. L.; Abbott, M. F:; Douglas, R. E. Dependence of Elemental Ash Deposit Composition on Coal Ash Chemistry and Combustor Environment; The American Society of Mechanical Engineers: Palm Coast, FL, 1991; pp 679-698. f
t
removed from the combustion environment and analyzed.
A significant understanding of the mechanisms leading to the formation and growth of ash depositshas been obtained by such experiments. Such post mortem analyses are at a disadvantage in identifying chemical species on the deposit surface or resolving changes in this composition with time. The chemical species on the deposit surface may change as the deposit is removed from the combustion environment due to phase changes, crystallization, hydration, and oxidation. In addition, the time required to sample and analyze deposits in this traditional way inhibits its usefulness as a boiler diagnostic. An increased understanding of the mechanisms governing ash adhesion and deposit growth and an ability to use this knowledge as a boiler operational tool require development of an in situ, real-time diagnostic. Fourier transform infrared (FTIR) emission spectroscopy is one technique that provides surface species information at high temperatures for both crystalline and amorphous phases.' Investigators have used this technique to investigate the structure of high temperature inorganic melts819 and the thermal transformations of common coal minerals, including quartz, kaolinite,calcite, and anhydrite.lG12 Emission cells or other controlled environments helped to maximize signal-to-noise ratios and minimize interference from atmospheric or combustion gases in these studies. ~~
(7) Griffiths, P. R.; de Haeeth, J. A. Fourier Transform Infrared Spectrometry; John Wiley & Sons: New York, 1986. (8) Shiraishi, Y.; Kwbiraki, K. High Temp. Sci. 1990,28,67-77. (9) Bates, J. B. Fourier TransformInfrared Spectroacopy; Ferraro,J. R., Baaile, L. J., Eds.;Academic Press: New York, 1978. (10) Baxtar, L. L.; Hardeety, D. R. The Fate ofMineralMatterDuring Pulverized Coal Combustion: Quarterly Report for July-September 1992; Sandia National Laboratories: Livermore, CA, 1993. (11) Vassallo, A. M.; Finnie, K. S. Appl. Spectrosc. 1992, 46,14771482. (12) Vassallo, A. M.; Cole-Clarke, P. A.; Pang, L. 5. K.; Palmisano, A. J. Appl. Spectrosc. 1992,46,73-78.
0887-0624/93/2507-0755$04.00/0 0 1993 American Chemical Society
756 Energy & Fuels, Vol. 7, No. 6, 1993
Table I. Proximate, Ultimate, Heating Value, and Ash Analyses of the Coal Used in This Study. moisture (% ) 24.7 6.04 ash (5% dry) prox (% dafl FC 48.7 VM
Air
-
""""w-1
NaturalGad
Baxter et al.
Gas Bumar
51.3
ultimate (% ) daf C H 0 N S
74.8 5.4 18.4 1.0 0.5 12781
heating value (daf-Btu/lb) ash oxide (96 of ash) Si02
28.8 14.1 5.1 1.4 23.7 4.1 1.3 0.2 1.0 18.8 1.4
A1203
Fen03 Ti02 CaO MgO
NazO K2O p2oS
so3
UNDb
Heatedl Insulated Modules
PCSV b s e r Beam'
?,
1'
]Test section
Probe lo Exhaust
Figure 1. Schematicdiagramof the Sandiamultifuelcombustor.
-
Particle laden Gas Flow
Proximate and heating values are given on a dry, ash-free basis.
* Undetermined.
This paper describes an in situ emission FTIR spectroscopy technique that is being developed a t Sandia National Laboratories to identify chemical species on the surface of ash deposits as they are formed.lQ13-15 The technique has been applied to deposits formed from burning various fuels in the multifuel combustor (MFC). In situ, real-time emission spectra that depend on surface composition are collected from deposits in a pilot-scale combustor. Classes of important chemical species, includingsilica, silicates,and sulfates,can be identified from these spectra. Results are presented for a low-temperature fouling deposit from a western coal.
Experimental Method Included in this section is a brief description of the coal, combustionconditions, and laboratory equipment used in this study. A more detailed description of the emission FTIR spectroscopytechnique is also provided. CoalProperties. A western coalfromthe Powder River Basin was used in this study. The properties of the coal are listed in Table I. As indicated in the table, the western coal contains approximately 25% moisture and 6% ash which is comprised mainly of silicon, calcium, and aluminum, with significant quantities of iron and magnesium. The inorganic constituents of the western coalare alsolargelyin the form of silicaand silicates, although approximately 70% of the calcium is atomically dispersed in the coal matrix.5 This and other similar Powder River Basin coals are finding increased use in boilers in the U.S. Utility experience shows that they form sulfate-based deposits in the convection passes of utility boilers, especially the latter section. Experimental Equipment. The multifuelcombustor(MFC) is a down-fired, pilot-scale (0.1 MBtu/h) flow reactor. It is instrumented to simulate the local environment that a coal particle experiencesas it flows through a utility boiler, including the local gas and particle temperature history, composition history, and the overall residence time. It consists of seven (13)Baxter, L.L.; Hardesty,D. R. The Fate of Mineral Matter During Puluerized Coal Combustion;SandiaNational Laboratories: Livermore, CA, in press. (14) Baxter, L. L. Combust. Flame, in press. (15)Baxter,L. L.; Hardesty,D. R. The Fate ofMineralMatter During Puluerized Coal Combustion: Quarterly Report for October-December 1992; Sandia National Laboratories:
Livermore, CA,1993.
Thermocouples
-
Figure 2. Schematic diagram of the deposition probe used to collect ash depositsfor analysis with the emission FTIR system. individualsectionsas illustrated in Figure 1. Thetop sixsections each contain resistance heaters that can be independently controlled to provide the desired temperature profile in the combustor. Each section contains access porta for fuel lances and thermocouples. The open test section at the exit of these modular sections allowsoptical and probe accessto the particleladen combustionflow. A more detailed descriptionof the MFC is available in the literature.l6J' In this study, coal was injected at a rate of 1.7 kg/h at the top of the furnace, resulting in particle residence times of approximately 2 s. The gas temperature at the deposition probe was controlled at 700 "C, with an oxygen concentrationof 4%. The carbon in the particles is approximately99% burned out by the time the particles reach the depositionprobe. These conditions simulate the environment found in the latter portions of the convective passes of commercialboilers. Ash deposits are generated on a horizontal deposition probe (see Figure 2) which is inserted into the test section. The 1.9 cm diameter, air-cooled, steel probe is instrumented with thermocouplesto measureprobe surfacetemperature (at four locations) and gas inlet and outlet temperatures. The probe surface temperature was maintained at approximately450 "C (f10 "C) throughout each of the depositiontests in this study. Deposita are grown for a 3-h period during which emission spectra are frequentlyobtainedfrom the deposit surface, as describedin the following section. Emission FTIR Spectroscopy Technique. The infrared emission from the surface of the deposit is collected by a series of off-axis paraboloidal mirrors and spectrally analyzed using a FTIR interferometer (Figure 3). Reflections of radiation from the furnacewalls and flame are minimizedby collectingemission from the shadowed side of the probe. The probe continuously rotates in the flow, causing the formationof a tangentiallyuniform (16)Baxter, L. L.; Hencken, K. R.; Harding, N. S. The Dynamic Variation of ParticleCapture EfficiencyDuring Ash Depositionin CoalCoal Fired Combustors, Orleans, France. Proceedings of the 23rd Symposium (International)on Combustion;The Combustion Institute: Pittsburgh, 1990,pp 993-999. (17) Baxter, L. L. Combust. Flame 1992,90,174-184.
Energy & Fuels, Vol. 7, No. 6, 1993 757
Characterization of Coal Ash Deposits
optic Table
off-AXIS Paraboloidal Mirrors
Diagnostic Area Exhaust
Figure 3. Optical layout of the emission FTIR system for monitoring chemical species on the surface of ash deposits. deposit on the probe surface. The probe is rotated at 1rpm in one direction for two complete cycles and then returned in the opposite direction for two cycles. (Continuousrotation of the probe in one direction would destroy the thermocouple leads to the probe.) The emission signal is processed by a commercial, rapidscanning interferometer (Biorad FTS-40/60) using a liquidnitrogen-cooled,broad-bandmercury-cadmium-telluride (MCT) detector and a potassium bromide-based beam splitter. The single beam emission spectrum, obtained at a resolution of 0.5 cm-l, is transferred to a personal computer for further analysis. Obtainingmeaningful emission spectrafrom the relativelycool surface (C500 "C) immersed in hot combustion gases ( ~ 7 0 "C) 0 with entrained particles and experiencing flame-temperature radiativefluxes(m1800"C)is achallenge. To increasethe signalto-noise ratio of the system, 64 individualscans are averaged to obtaina singlespectrum. Thetotalcollection time for a spectrum under these conditions is approximately 3 min. The emission signal S measured by the FTIR spectrometer consists of several components as expressed by the following equation S(T,ii) = F(ij)[c,(T,,ij)B(T,,ij) + G(T,ij)+ P(T,,ij)+ I(T,,ij)+ R(T,,ij)]+ N(ij) (1)
whereF representsan instrumentresponse function, represents the emissivityof the surface,B represents a black body function, G representsthe contributionof the gas emission and absorption, P represents signal from the hot particles, I represents signal from the components of the interferometer, R represents reflectionfrom the furnaceand flame,and Nrepresents noise from the detector itself. The components of the signal are a function of temperature (T) and wavenumber (i).Subscriptss, i, f, and g, represent surface, instrument, flame, and gas, respectively. The emissivity is only a weak function of temperature but is a strongfunctionof composition and can changedramaticallywith temperature-inducedcomposition changes. The contribution of reflected furnace radiation to the signal is minimized by collectingthe signal from the bottom side of the depositionprobe. In addition,all equipmentnear the probe has optically absorbing surfaces to minimize the occurrence of multiple reflections. Signal from the components of the spectrometer (beam splitter, aperture, walls, paraboloidal mirrors) that is not accounted for in the instrumentresponsefunction has been measured and represents 3-5 % of signal collectedfrom the combustion environment. The detector is maintained at approximately -195 "C. The spectrometercomponents are maintained at room temperature and nitrogen purged, whereas the probe surface and gases are at approximately 450 and 700 "C, respectively. The relativelycooltemperatureof the spectrometer
components is the primaryreasonthat they contributeonly small amountsto the total signal. Hot fly ash particlesand an occasional burning char particle passing through the optical path of the system add noise to the emission signal. They also can confuse signalcollectionby overwhelmingthe A/D converterin the FTIR spectrometer or distorting the interferogram so bady that the reference point cannot be established. These problems are addressedby (1)operatingwith a high-speeddetector to separate random particlesignalsfrom the coherently-addedinterferogram, (2) conditioning the signal with a high-pass electronic filter (threshold 200 Hz) to attenuate the optical response from the particles (1 kHz),and (3)opticallydesigningthe diagnosticpath to minimize the sampling volume. A combination of particle velocity and diagnostic beam dimensions determines the frequency of the transient particle signals. Under these experimentalconditions, a large fraction of the particle interference could be limited to transient signals with less than 100 Hz frequencies. These measures have been successfulin reducinginterferenceand noise from extraneoussources (other than the gases)to less than 10% of the overall signal. The large contribution due to gas-phase emissioncannot be removed from the signalbefore it reachesthe detector,but much of the gas-phasecontribution can be filtered from the spectrum as illustrated later in this paper. By minimization of the contributions from sourcesother than the surface, the measured signal can be approximated by S(T,ij)4qij)€,(T8,5)B(T*,F)
(2)
in regionswhere thegas phase interferenceis small. (Treatment of the gas emission/absorptionlinesis discussedseparatelylater.) This is the working equation for data analysis. An unknown emissivitycan be measured by means of a suitable known reference emissivity if both are collected in the same environment as follows: (3) The emission signal from the clean probe, $1, is recorded at the beginning of each experiment. A relative surface emissivity, €2, is calculatedfrom the depositemission signaland the clean probe emission signal and emissivity using eq 3. A temperaturecontrolled black-body reference is used either to characterize the emissivity of the clean surface or to act directly as the reference. The ratio of the black body functions in eq 3 will correct the emissivity if the deposit surface temperature (T2) is different However, in the than the temperature of the clean probe (TI). wavenumber range of interest (1300-450cm-l), a 50 "C difference in the temperature at which the two signals are collected will affect the overallmagnitudeof the emissivityby about 20 % but not the location of the individual emission peaks. The probe surfacetemperatureis typically controlledwithin *lO "C during an experiment, although the deposit surface temperature will increase with increasing deposit thickness. In this study we are seeking chemical species information in the spectra. This is related to wavenumber location and relative height of peaks but not to their absolute magnitude. Figure 4 illustrates a typical spectrum obtained from an ash deposit using the emission FTIR spectroscopytechnique. This spectrum was obtained after the deposition probe had been exposed to the particle-laden, vitiated gas for 10 min. The dominant features of the single-beam spectrum are gas-phase absorptionand emission lines. Sincethe gas phase emission lines are much narrower (m0.05cm-I at this temperature) than the emission bands of the condensed phase inorganics (=20 cm-I), it is possible to filter some of their influencefrom the spectrum. An algorithm to clip the narrow emission and absorption peaks from the broad baseline was developed for this purpose. The spectrum is convolved with a low-pass filtering function and is then transformed to provide a smoothed spectrum. The low pass filtering is accomplished by multiplying the Fourier transform of the spectrumby a sinc(ax)= sin(ax)/(ax)function,where
758 Energy & Fuels, Vol. 7, No. 6, 1993
Baxter et al.
:
I q y . . :...".... ......
*.Et,, , ,
I , ,
1200
,,I,,,,
I , ,
43.2
,,I , ,,,I,,,,
1100
0.84
1000
,,,
~
42.6 900
V [cm"] Figure 4. Raw and filtered single beam spectrum from an ash deposit after 10 min of deposition. The filtered spectrum has been offset -0.15 V.
Wavenumbers, 0 [cm-'1
Wavenumbers,
the parameter a defines the threshold frequency at which the interferogram is filtered. Applying an inverse transform results in transformingspectral features with widths below the threshold into square peaks with the same area and widths given by the threshold value. The positive difference between the initial spectrumand the modified spectrumis subtracted from the initial spectrum to produce a new spectrum with a much smaller complement of narrow peaks but no alteration to peaks broader than the threshold value. The details of this filtering technique, which extends standard Fourier filtering,are discussed elsewhere (ref 18). This process is repeated until the narrow peaks are eliminated entirely. Negative-goingpeaks caused by absorption of the signalby gas phase speciescan also be removed by a similar strategy. The filtered spectrum after 10 min can also be seen in Figure 4. Note that the filtered spectrum has been offset -0.15 V so that it can be more clearly seen. This filtering technique is effectiveso long as the gas-phaselines are not tightly grouped.
Results and Discussion Emission spectra for a number of reagent-grade inorganic species were collected from the probe to validate the ability of technique to measure chemical composition and to act as standards. These reference spectra, together with available literature standards collected under a variety of conditionslS2l were used to evaluate the chemical composition changes indicated by the data. The reagenbgrade inorganic materials were pulverized, mixed with water, and either sprayed or painted on the surface of the deposition probe. Reference emission spectra were collected in both air and combustion environments. In the former, electrically heated air passing over the probe maintained a surface temperature of 120 "C. Emission standards in the combustion environment were collected at a surface temperature of 450 "C obtained by exposure to the vitiated flow in the test section of the MFC. Figure 5 illustrates the relative emissivity of an ash deposit from the western coal after 10 and 45 min of deposition. The wavenumber-dependent temperature and emissivity corrections indicated in eq 3 are not applied to these data. No temperature correction is made and the (18) Baxter, L. L.; Hardesty, D. R. TheFate ofMineral Matter During Pulverized Coal Combustion: QuarterlyReportfor Januury-March1993; Sandia National Laboratories: Livermore, CA, 1993. Baxter, L. L.; DeSollar, R. W. Fuel, in press. (19) The Infrared Spectra Handbook oflnorganic Compounds;Sadtler Research Laboratories, a Division of Bio-Rad Laboratories, Inc.: Philadelphia, PA, 1984. (20)Ferraro,J.R.TheSadtlerInfraredSpectraHandbookofMinerals and Clays; Sadtler Research Laboratories, a Division of Bio-Rad Laboratories, Inc.: Philadelphia, PA, 1982. (21) Nyquist, R. A.; Kagel, R. 0. Infrared Spectra of Inorganic Compounds; Academic Press, Inc.: San Diego, CA, 1971.
Figure 5. Relative emissivity of an ash deposit from a western coal after 10 min (left ordinate) and 45 min (right ordinate) of deposition.
Y
.-
0.95
m
0.90
Wavenumbers, C [cm"]
Figure 6. Emission spectra of reagent-grade silica in air and in a combustion environment. relative emissivity data are normalized to a value of 0.75, meaning that the results would be a horizontal line a t a value of 0.75 if nothing deposited on the probe. Figure 5 indicates an overall increase in the relative emissivity with time due to emission from the deposit and increase in deposit surface temperature relative to the probe surface temperature. In an effort to simulate realistic deposit behavior, the probe surface temperature was held constant within a few degrees. The deposit surface temperature increases with increasing deposit thickness. Surface temperature changesresult in an essentially constant offset in the data with no spectral features and a very small slope. Note that the spectrum a t 10 min is plotted on the left ordinate while the spectrum after 45 min is plotted on the right. The other major difference between the spectra is the increase in emission that occurs between 1150 and 1200 cm-l. Some residual water peaks are evident in the spectra above 1200 cm-l and below 800 cm-l. Interpretation of the data in Figure 5 and comparisons with reference data and supplementary analyses is discussed below for silica, silicates, and sulfates. Mixtures of these three major components of the deposit are also discussed. Silica. The a form of quartz, which is stable a t room temperature, has emission peaks a t 690,778,795, and 1055 cm-I.l2 The emission peak a t 1055cm-' is quite broad and is the dominant feature of the spectrum. Figure 6 illustrates the emission spectra for silica obtained in both the air and combustion environments. Note that all spectra obtained in the combustion environment are filtered using the clipping routine previously described in this paper. The emission spectrum obtained in the air
Energy & Fuels, Vol. 7, No. 6,1993 759 Kaolinite
z .-E
0.95
I,.* 1 .o
Omok'
I
;"do
3
'*
1000 I a '
a
'
'
I
800 I
8
n
8
I
I
'
a
6 0I 0
l4
F [cm-'I Figure 7. Infrared absorption (left ordinate) and diffuse reflectance (right ordinate) spectra of the ash deposit.
Wavenumbers, t [cm"] Figure 8. Emission spectra of reagent-gradekaolinite in air and in a combustion environment.
environment is very similar to reference spectra from the literature. The emission spectrum obtained in the combustion environment is also very similar to literature standards with the exception of the peak at 690 cm-1. The emission peak at 690 cm-' appears to have shifted to 670 cm-l, although there is strong C02 interference a t approximately 670 cm-l and the silica peak may be partially obscured by the dominant gas-phase emission in this region. The emission spectrum of quartz changes with temperature. At a temperature of 574 "C, the a form of quartz is transformed to the 0 form and the two peaks a t 778 and 795 cm-l coalesce to form a single peak a t about 792 cm-1 and the peak a t 690 cm-' disappears.12 The emission spectrum of the ash deposit (Figure 5) shows little evidence of the presence of silica which is a major component of the fly ash. There are no strong emission features a t either 1060 cm-l or near 800 cm-' to suggest that silica is one of the dominant species, although there is emission between 1080 and 1000 cm-l. Three post mortem analyses were performed to corroborate the emission FTIR data: X-ray diffraction, infrared absorption, and diffuse reflectance spectroscopies. The X-ray diffraction results indicate quartz is the major crystalline species in the deposit and that a large fraction (>90%)of the deposit is crystalline. Trace amounts of anhydrite were also detected. The infrared absorption analysis, performed on finely ground samples of the deposit pressed into a KBr pellet, also indicate that quartz is a major component of the deposit. Diffuse reflectance measurements, which are performed without significant disruption of the deposit structure, indicate very little quartz. The spectra from the infrared absorption and diffuse reflectance techniques are shown in Figure 7. These results are more consistent than they at first appear. Both the X-ray diffraction and the infrared absorption analyses are sensitive to bulk deposit composition and both are collected from room temperature samples. The emission FTIR and the diffuse reflectance techniques are both sensitive to surface composition, the former being collected in situ and the latter also collected a t room temperature. The thickness of material contributing to the emission signal of the deposit is a function of wavelength and the optical properties of the material. For wavelengths where a material is strongly emitting, the thickness may be just a fraction of a wavelength. The implication of the results is that the silica-based deposit is coated with other species (see below) such that surfacesensitive techniques cannot resolvedthe bulk composition. This is consistent with chemicaltransformationsthat occur during deposit formation. Condensates coat particles and
alkali species react on the surfaces of silica to form silicates. In general, there should be better agreement between the surface-sensitive techniques than between the surface techniques and the bulk analyses, as is indicated by our data. Silicates, Common silicates found in coal include the clay minerals kaolinite, illite, and montmorillonite. These minerals often melt during the combustion process and may also incorporate alkali or alkaline earth elements to form other complex silicate phases. The emission spectra of kaolinite (A12SizO&(OH)4)available in the literature indicate strong emission features a t 1105,1040,1007,910, 695, and 540 cm-l.12 At temperatures between 400 and 600 "C kaolinite transforms to metakaolinite, an anhydrous and largely amorphous form of kaolinite. Upon dehydration, the emission peak a t 910 cm-l (due to the A1-OH stretch) disappears and the band a t approximately 1100 cm-' is significantly broadened. Figure 8 illustrates the emission spectra obtained for reagent-grade kaolinite. The emission peak near 690 cm-l is obscured significantly in the vitiated gases by intense C02 emission in this region. Most silicate species contain strong emission features a t approximately 1000 cm-l due to the Si-0 stretch. The position of this strong feature will vary by a few wavenumbers, although it typically occurs below 1100 cm-1. Additional features occur between 800 and 1000cm-l that vary significantly with silicate composition. The increased emissivity of the ash deposit in the region between 1050 and 950 cm-1, as shown in Figure 5, is an indication of the formation of silicates on the probe surface. Many silicates are amorphous so they are not detectable using X-ray diffraction. The infrared absorption and diffuse reflectance analyses are better suited to corroboration of the presence of silicates. Both analyses indicate the presence of silicates, with a much stronger indication suggested by the surface-sensitive reflectance data. Sulfates. The emission spectra of calcium sulfate obtained in the air and combustion environments are shown in Figure 9. The spectrum for calcium sulfate, with strong emission features a t 1130, 670, and 595 cm-1, is similar to emission spectra of gypsum (a hydrated form of calcium sulfate) at the 5ame temperature.loJ1 The emissionspectrum of anhydrite is similar to that of gypsum with emission peaks a t 1130 and 675 cm-l. Sodiumsulfate also has strong emission features above 1100 cm-1. More complex sulfate species containing, for example, iron and magnesium,have emissionfeatures a t higher wavenumbers still (up to about 1250 cm-l). In general, sulfates can be distinguished from silicates in that sulfates have emission peaks above 1100 cm-l, extending up to 1200 cm-1.
Wavenumbers,
Baxter et al.
760 Energy & Fuels, Vol. 7, No. 6,1993 Calcium Sulfate in air ...... ,.. in combustion environment Y
t 0.75""1"""'"1"""""""""'""''a 1200 1000 800 600
Wavenumbers, P [cm-'1 Figure 9. Emission spectra of reagent-gradecalcium sulfate in air and in a combustion environment.
interactions with other species near the same location (matrix effects). These effects are particularly pronounced when a material is surrounded by chemically similar species. Complex morphologiesalso increase the difficulty of measuring deposit composition. As deposits become increasingly porous, their emission approaches that of a black or gray body regardless of their composition. The challenge then becomes that of resolving specific species identities in chemically and morphologically complex mixtures. Spectral features with the presence of specific species have been observed for a number of deposits under a variety of ~onditions.~OJ5J8 The presence of these species is also consistent with the chemistry of ash deposit formation. However, the complexities of gas-phase interference and matrix effects are large and further work is needed before definitive or quantitative species assignments can be made.
Conclusions
c1 Y
1200
1000
800
600
Wavenumbers, ? [cm"]
Figure 10. Emission spectra of mixtures of silica, silicates,and sulfates. Mix 1 contains equal weights of silica and calcium sulfate. Mix 2 contains 30% each of silica and kaolinite and 15% each of calcium and sodium sulfate. The emission spectrum from the ash deposit after 45 min of deposition, presented in Figure 5, shows strong indications of sulfates (features above 1100cm-l). Sulfates are also noted as a trace species in the X-ray diffraction results. The infrared absorption and diffuse reflectance data also indicate a significant presence of sulfates, with the reflectance data most closely resembling the emission data. Mixtures. Figure 10 illustrates the emission spectrum of a mixture of equal weights of silica and calcium sulfate (mix 1)and a mixture of 30% each of silica and kaolinite and 15% each of calcium and sodium sulfate (mix 2). These spectra contain features that can be identified with specific species, but the species become increasingly difficult to distinghish as the mixture becomes more complex. The presence of major classes of species is clearly evident, however, by changes in emission spectra in the ranges of 1100-1200 cm-l (indicating sulfates) and 800-1000 cm-l (indicating silicates). The large emission peak which appears a t 615 cm-l in the spectrum from mix 2 is due to the presence of sodium sulfate. This highlights one of the principal remaining challenges in development of the FTIR emission diagnostic. The tests with standards have demonstrated a clear ability to detect emission over a broad range of wavenumber (1300500 cm-l) in the presence of a particle-laden, vitiated flow except where interfering gases are optically thick over a broad wavenumber range (near 670 cm-1, for example). As the complexity of the deposit increases, the features become increasingly difficult to distinguish. In addition, the wavenumber location of the features may shift due to
Fourier transform infrared emission spectroscopy was used to identify the presence of sulfates, silicates, and silica, on the surface of deposits formed from a western coal. The diagnostic was successfully demonstrated in an environment which simulates that of commercial pulverized coal-firedpower stations. The in situ, real-time nature of the measurements illustrates the time development of deposit compositionunavailable by traditional means. The technique has potential use as a boiler diagnostic to distinguish chemical transformations on deposit surfaces. The results are in general agreement with post mortem analyses of the same deposits. Diffuse reflectance spectroscopy, a surface-sensitive analysis technique, resulted in similar spectral results. Bulk analysis techniques, infrared absorption spectroscopy, and X-ray diffraction identified the same species but in different quantities. This is expected based on known mechanisms of ash deposition (surface composition is enriched in silicates and sulfates relative to bulk composition). Progress in making quantitative analyses and identifying specific chemical species will depend on developing methods of distinguishing features from chemically and morphologically complex deposits.
Acknowledgment. The advice and guidance of Don Hardesty, manager of Sandia's Combustion Research Department, is gratefully acknowledged. The authors also wish to thank Eric Harwood, a senior in chemistry a t UC Davis, for his assistance in performing the initial experiments to help develop the emission FTIR technique. The assistance of Gian Sclippa, the laboratory technician responsible for the MFC, is also greatly appreciated. This work was conducted a t the Combustion Research Facility, Sandia National Laboratories, Livermore, California, and sponsored by the U.S. Department of Energy through the Pittsburgh Energy TechnologyCenter's Direct Utilization Advanced Research and Technology Development Program under the direction of James Hickerson and Phillip Goldberg. Additional financial support for one of the authors, G. H. Richards, was also provided by the Advanced Combustion Engineering Research Center (ACERC). Funds for ACERC are received from the National Science Foundation, the State of Utah, 29 industrial participants, and the U.S. Department of Energy.
