Anal. Chem. 1995,67,710-716
Multiwavelength Monitoring of Photofragment Fluorescence after 193 nm Photolysis of NaCl and NaOH: Application to Measuring the Sodium Species Released from Coal at High Temperatures Bruce L. Chadwick* and George Domazetis Generation Victoria, Herman Research Laboratoty, 677 Springvale Road, Mulgrave, Victoria 3 170, Australia
Richard J. S. Morrison Department of Chemistry, Monash University, Clayton, Victoria 3 168, Australia
Excimer laser photodissociation of gas-phase NaCl and NaOH, and monitoring of the subsequent Na photofragment fluorescence, are used to determine the concentration of the species in coal-derivedgaseous environments. Detection limits lower than 1 ppb of NaCl have been achieved under atmospheric conditions using 193 nm photodissociation. It is shown that monitoring two Na emission wavelengths (at 819 and 589 nm) allows speciation between NaOH and NaCl in the gas phase. In particular, emission from the Na 32D levels (at 819 nm) has been unambiguously attributed to photodissociation of NaOH. This emission results from hot-bandabsorption of the excimer laser, and thus its intensity is temperature dependent and weaker than 32P (589 nm)emission. The technique has been applied to the detection of NaCl and NaOH released during the pyrolysis and gasification of samples of Loy Yang (Australian brown) coal. Sodium species monitoring is of particular interest in power generation as many coals contain significant amounts of sodium. The sodium in coal can be a major cause of fouling and slagging in coal gasification and combustion systems. Sodium is also expected to have a detrimental effect on the efficiency of cleancoal technologies, such as coal gasification and coal-fired turbines, where it may cause fouling of gasifier beds and turbine blades. It is thus desirable to develop techniques suitable for in situ determination of Na species concentrations, in particular for the major species NaCl and NaOH. In this paper we report sodium species concentration measurements using the technique of photofragment fluorescence. Sodium species are not amenable for study by the more widely used diagnostic techniques, such as laser-induced fluorescence WF), owing to the dissociative nature of their excited electronic states.' Raman techniques such as coherent anti-Stokes Raman spectroscopy (CARS) are limited in sensitivity by the nonresonant background contribution of the majority species. The application of spectroscopic techniques is further complicated by the small vibrational energies and rotational constants of these molecules. The application of photofragment fluorescence spectro~copy~~~ in
the determination of trace concentrations of alkali halides shows considerable promise in providing a much needed diagnostic technique for these species. In the photofragment fluorescence technique, W radiation is used to photodissociate the sodium species, producing electronically excited sodium which subsequently fluoresces to the ground electronic state? For example, in the case of 193 nm photodis sociation of sodium chloride (NaCl),
NaCl
+h
-
~nm) ( Na* ~ + ~C1- ~Na + C1+ hv(,,,,,,
m)
(1) The photodissociation energy is suflicient to leave the Na* fragment in its 32P electronic state, and the intensity of the subsequent fluorescence, at the characteristic 589 and 589.6 nm wavelengths, can then be related to the concentration of the initial NaC1. The integrated intensity of the fluorescence from the Na photofragments can be related to the concentration of the parent species by the expression W,T)= C W M M A , T ) [ A / C A + Q(T))II
(2)
where S is the integrated fluorescence intensity, 1 is the photodissociation wavelength, T is the temperature, C is a constant relating to the signal collection efficiency and the probe volume, N is the number density of dissociated compound, 0 is the absorption cross section, 4 is the quantum yield of the fragment in the relevant electronic state, A is the emission rate, I is the laser fluence, and Q is the quenching rate of the excited fragment (which depends on both the composition and the temperature of the environment). The relationship shows that the intensity of the excited photofragment fluorescence can be quantitatively related to the concentration of the parent compound. Previous work by Oldenborg, Baughcum, and co-worker~~~~ has shown that the photofragment technique can yield subparts per billion (molar volume units) measurement of NaCl, KCl, and NaOH concentrations. Observation of 32P emission was shown to be independent of the temperature of the compounds involved, indicating dissociation from all rovibrational states of the species.
~~~~
(1) Davidovits, P.; Brodhead, D. C.1.Chem.Phys. 1967,46, 2968-2973. (2) Oldenborg, R C.; Baughcum, S. L Anal. Chem. 1986,58,1430-1436.
710 Analytical Chemistry, Vol. 67, No. 4, February 15, 1995
(3) Oldenborg, R C.; Baughcum, S. L.; Hof, D. E.;Winn, IC R PYOC. SPIE-Int. SOC.Opt. Eng. 1985,540, 339-346. 0003-2700/95/0367-0710$9.00/0 0 1995 American Chemical Society
In addition, these authors have shown that variation of the UV excitation wavelength allows some speciation between compounds within the same alkali group; in particular, NaCl and NaOH were able to be distinguished. Helble et d4have applied the photofragment technique to coals using a droptube furnace, where the 589 nm fluorescence was attributed to NaC1, the major Na species calculated by kinetic modeling. Their work provided insight into relative concentrations of Na species released at elevated temperatures from d ~ e r e n t types of coal; however, exact gas-phase concentrations were not measured owing to the effects of fluorescence quenching by the majority species present. Interference from other possible fluorescence sources (burning char particles, etc.) was also assumed negligible in the dilute postaame environment of the droptube furnace. Assignment of 589 nm fluorescence solely to NaCl would not be valid under the reducing conditions utilized in the gasification of coal. The presence of other Nacontaining species, in particular NaOH, prevents accurate determination of gas-phase concentrations. To optimize the performance of cleancoal technologies, strategies will need to be developed to remove NaCl and NaOH prior to the coal-derived gases entering a turbine. It is therefore necessary to distinguish the two species and accurately know their concentrations so that an optimal chemical or physical cleanup process can be developed. In this paper we show that NaCl and NaOH concentrations may be distinguished when two Na emission wavelengths are monitored. The importance of fluorescence quenching and radiation trapping are also considered. Detailed attention is given to calibration so that absolute concentrationsare measured under the different conditions utilized. The ability to discriminate between NaCl and NaOH is obtained in an experimental arrangement using the single frequency output of an ArF excimer laser; more complex tunable laser systems are not required. The efficacy of the technique is demonstrated in measurements of gaseous NaCl and NaOH species during the pyrolysis and gasification of Australian brown coal. Speciation between NaCl and NaOH. Under the temperature and pressure conditions occurring in coal gasification plants, the major gas-phase Na species, predicted by thermodynamic calculations to be present, are NaCl and NaOH. The reliable detection of these two species is the primary purpose of this current work. The energies and threshold wavelengths of the lowest 32P state production from photodissociation of NaCl and NaOH are r e p ~ r t e das ~~~,~
NaCl
-
NaOH
Na*
-
+ C1
AH = 145.9 kcal/mol, threshold il= 195.9 nm (3)
Na*
+ OH
AH = 130.0 kcal/mol, threshold A = 219.8 nm (4)
It is possible, therefore, to photodissociate both NaCl and NaOH using radiation of 193 nm wavelength. The amount of Na* produced from a mixture of gaseous NaCVNaOH will depend on the laser intensity, the molecular absorption cross sections, and the concentrations of each species (see eq 2). (4) Helble, J. J.; Srinivasachar, S.; Boni, A A; Charon, 0.;Modestino, A Combust. Sci. Technol. 1992, 81, 193-205. (5) Dab, S.; Smith, W. T.; Taylor, E. H. /. Chem. Phys. 1961,34,558-564.
ENERGY
PHOTO-
I FRAGMENT
(cm-') 2 '1/2,3/2
2S,/2
40 000
5d 5P
5s
30 000
ENERGY
2D3/2,5/2
1
Od
40
4s
568.8 20 000
819.5
330.3
1
10 000
0 Figure 1. Energy of NaCl and NaOH photofragments (after 193 nm dissociation) illustrated against the partial energy diagram for atomic Na. The energy of the photofragments assumes photodissociation of ground state molecules. Only molecules with an additional amount of internal energy (A&, in the case of NaOH) are able to produce Na atoms in the 32D electronic state. The most prominent Na fluorescence transitions and wavelengths are also shown. Na 4'P-> 3s
t
OH
L
I
290
*
I
300
1
A->X
B
1
s
'
'
I
'
310 320 330 340 WAVELENGTH (nm)
l
'
l
J
350
Figure 2. Emission spectrum, in the wavelength range 290-350 nm, resulting from 193 nm photodissociation of NaOH.
Our strategy for discrimination between the two compounds is best qualitatively illustrated by considering the partial energy level diagram for atomic Na (Figure 1) compared to the energy of the NaCl and NaOH photofragments after 193 nm photodissaciation. For both NaOH and NaCl, there is suf6cient excess energy to yield Na in its 32P electronic state, resulting in 589 nm fluorescence. In the case of NaOH, the lower bond dissociation energy results in a larger excess energy after dissociation (23 300 cm-I) than is the case for NaCl. With the aid of some internal vibrational energy (-6OOO cm-l) ,a certain proportion of the NaOH molecules have sufficient excess energy to produce a Na fragment in either the 32D, 42P, or 4% electronic states, resulting in fluorescence at other wavelengths. Thus, hot-band absorption of 193 nm radiation by NaOH is expected to yield Na fluorescence at wavelengths other than the well-characterized2~3 589 nm. Evidence of this hot-band absorption and photodissociation can be observed in the W emission spectrum resulting from 193 nm dissociation of NaOH. This is shown in Figure 2, where OH fluorescence is observed in addition to Na 42P 3 s fluorescence at 330 nm. The OH fluorescence is remarkably strong and perhaps results from a multiple absorption process. The hot-band absorption process results in observed Na fluorescence at 330 (42P 3S), 340 (32D 3 3 , and 819 nm (32D 32P). Other possible
-
-
-
-
Analytical Chemistry, Vol. 67, No. 4, February 15, 1995
711
10-1 10-2 v,
lo3
W
2
104
0
Y io5
0
=
10-6
Bz
IO7
6
IO8
0
2
10-9
600
800
1000
1200
1400
1600
TEMPERATURE (K)
Figure 3. Population fraction versus temperature of molecules having sufficient vibrational excitation to populate the Na 32D electronic state, after photodissociationat 193 nm, for (a) NaOH and (b) NaCI. The NaCl data are calculated using the molecular constants W e = 363.62 cm-l and U e X e = 1.72 cm-1.8
-
fluorescence lines include the 1140 nm 42S 32P transitions. These additional fluorescence lines are observed only for NaOH dissociation (not NaC1) and thus may be used as an analytical tool for establishing NaOH concentrations in environments containing these two species. The relative intensities of the Na fluorescence lines resulting from hot-band dissociation are determined from a combination of the quantum yield of the photofragment in the relevant excited state (6 in eq 2) with the weighted oscillator strength of the transition (gfvalue6). It has been found experimentally that the 819.5 nm (32D5/2 32P3/2)emission line yields the best analytical sensitivity for NaOH detection - which is consistent with its relatively high gfvalue of 3.62.7 A quantitative measure of the extent of hot-band absorption in NaOH can be obtained by considering the NaOH density of states. NaOH is a linear molecule with C ,, symmetry, and its ground-state vibrational frequencies and degeneraciess are 431 (u), 337 (n),and 3650 cm-l (a). The fraction of NaOH molecules having 5900 cm-l or more of vibrational energy (Le., sufficient energy to produce the Na 32D fragment) is shown as a function of temperature in Figure 3a. At relatively low temperatures (-600 K), only 1 in lo5 of the NaOH molecules have sufficient energy to produce a Na fragment in the 32D electronic state; at higher temperatures a much larger fraction of molecules will have the required energy. For example, at 1100 K, 1 in lo2 of the NaOH molecules may dissociate and produce a Na fragment in the 32D electronic state. Hence, we can expect 819/340 nm fluorescence emission to accompany NaOH 193nm photodissociation,with the intensity of the observed fluorescence increasing with temperature. In the case of NaCl, the energy deficit for 32D Na fragment production is 9400 cm-l (cf. 5900 cm-1 in the case of NaOH). This additional energy deficit, combined with a lower density of states in NaC1, leads to a very poor yield of 32D Na after photodissociation, as shown in Figure 3b. In fact, we have not been able to
-
(6) Alkemade, C. Th. J.; Hollander, 3.; Snelleman, W.; Zeegers, P. J. Th. Metal Vapours in Flames; Pergamon Press: New York, 1982; Chapter 2. (7) CRC Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Boca Raton, FL, 1993; pp 10-169. (8) J. Phys. Chem. Ref: Data 1985, 14, Suppl. 1.
712 Analytical Chemistry, Vol. 67, No. 4, February 75, 7995
observe any 32D Na fluorescence (either 819 or 340 nm) after 193 nm photodissociation of NaC1. At high temperatures (we estimate '1500 K), detectable 819/340 nm fluorescence is expected from NaCl as well as NaOH. However, under most practical circumstances, where there is a mixture of gas-phase NaCl and NaOH, any 819/340 nm fluorescence is attributable only to NaOH. Conversely, 589 nm fluorescence is attributable to both species, and when measured in addition to 819 nm fluorescence, the concentration of NaCl can be obtained once the concentration of NaOH has been determined. The monitoring of two emission wavelengths in tandem should unambiguously lead to the determination of both NaCl and NaOH concentrations in environments containing a mixture of these two species. EXPERIMENTAL SECTION It is our aim to reproduce the conditions found in coal gasification systems as closely as possible. Fluorescence signals obtained using the photofragment technique can be strongly affected by experimental conditions such as the gas mixture and pressure, owing to collision quenching by the majority species. Results taken in evacuated cells are of limited value as they are dflcult to extrapolate to "real" systems. An atmospheric pyrolysis and gasification reactor was therefore constructed to more closely emulate gasification conditions. The apparatus consists of a tubular furnace into which a hightemperature steel reaction vessel was mounted. The reaction vessel allows for the containment of coal samples between fine (30 pm) mesh screens. The steel mesh screens confine the coal/ ash particles while allowing the gas-phase products to exit the coal bed and be analyzed using the photofragment technique. Coal samples were obtained from the Loy Yang (Australia) open cut mine. The sample of coal was thoroughly mixed, dried, and pulverized using a laboratory mill. The crushed coal was elutriated to remove tines and sieved to obtain uniform particle sizes in the range 90-106 pm. Gases, typically Nz (SO-lOO%), 02 (0-20%), and HzO (0-2%), enter the bottom of the vessel through the heated bed of coal. The gas composition and flow are then adjusted to emulate speciiic gasitication conditions. For calibration and testing purposes, platinum crucibles containing NaCl or NaOH are suspended in the middle of the reaction vessel, below three optical ports containing W silica windows which allow transmission of the laser radiation and side-on fluorescence detection. The optical excitation and detection arrangement is illustrated in Figure 4. A Lumonics (Model TE-86@3)ArF excimer laser is used as the photodissociation radiation source. Typical laser energy employed was 5 mJ of 193 nm radiation, in a 20 ns pulse, focused to a spot size of 1 mm x 3 mm, which yields a laser fluence of 150 mJ/cm2. No evidence of optical saturation was observed under the atmospheric conditions used in this work. A variety of excitation geometries were investigated, and the single lens focusing method was found to give the best fluorescence signal. This is consistent with the combined effects of the fluorescence signal being proportional to laser power (from eq 2) and radiation trapping increasing with larger excitation volumes. The side-on fluorescence monitoring system consists of collection optics and a monochromator equipped with a photomultiplier tube (Spex 500M and Hamamatsu R928, respectively).The transient fluorescence signals are integrated with a boxcar averaging system (Stanford Research Model SRS 250/280), typically set with a 60 ns integrating gate width. For multiwave-
FURNACE
LASER ENERGY
f
ArF EXCIMER
3
1 61
589 nm FILTER PC CONTROL
BEAM-, SPLITTER
1
NaD2 e .. * ..'*
Na D,
m 0:
9
$
.,*
W
0
BOXCAR AVERAGER
MONOCHROMATOR
z w
>.a8
. ..
w-
;e
.
.*
-
3
8
588.5
589.0
589.5
590.0
WAVELENGTH (nm)
length monitoring, a beam splitter is inserted so as to pass a small portion of the fluorescence (-10%) onto a narrow band-pass filter (589 nm peak, 10 nm fwhm) and photomultiplier tube. In the multiwavelength experiments, the monochromator is typically set to observe 819.5 nm (i.e., Na 3 9 ) fluorescence,while the second photomultiplier monitors 589 nm m a 32P) fluorescence. The use of the monochromator instead of a second interference filter greatly reduces (to a negligible extent) interference between thermal background radiation and the observed fluorescence.The monochromator's resolution is typically adjusted to be 0.3 nm in the experiments. Transmitted laser energy is monitored during the course of an experiment so that both laser energy fluctuations, and the presence of molecular species which strongly absorb 193 nm radiation, can be taken into account. For calibration purposes, platinum crucibles containing NaCl Werck 99.5%)or NaOH (Ajax 97%) may be lowered into the reaction vessel and situated a few millimeters below the optical axis. The fluorescence signal intensity and the temperature of the crucible are simultaneouslyrecorded to form a signal intensity versus temperature plot as the temperature of the system is ramped. This plot is then converted to a calibration curve by using the equilibrium concentration of NaCl or NaOH vapor, calculated using the CSIRO Thermochemistry System? for the temperature range of the calibration experiment. Calibration of the system was performed after each coal experiment by lowering the crucibles into the reaction vessel and performing a single-point calibration measurement (at -600 "C) in an atmosphere of composition similar to that of the coal experiment. RESULTS AND DISCUSSION
The excellent sensitivity of the photofragment technique is best illustrated by the photofragment fluorescence spectrum of a dilute concentration of NaC1. The spectrum obtained after photofragmentation of 15 ppb (1.4 x 10" molecules/cm3) of NaCl in an atmosphere of Nz is shown in Figure 5. Here, the intensities of the clearly discernible Na Dlines indicate an ultimate detection limit of
