Sodium and Potassium Released from Burning Particles of BrownCoal and Pine Wood in a Laminar Premixed Methane FlameUsing Quantitative Laser-Induced Breakdown Spectroscopy

LI-JEN HSU,* ZEYAD T. ALWAHABI, GRAHAM J. NATHAN, YU LI, Z. S. LI,and MARCUS ALDENSchool of Chemical Engineering (L.-J.H., Z.T.A.), School of Mechanical Engineering (G.J.N.), and Centre for Energy Technology (L.-J.H.,

Z.T.A., G.J.N.), The Environment Institute, University of Adelaide, SA, 5005, Australia; College of Physics, Jilin University, Changchun 130021,China (Y.L.); and Division of Combustion Physics, Lund University, P.O. Box 118, E-22100 Lund, Sweden (Z.L., M.A.)

A quantitative point measurement of total sodium ([Na]total) and

potassium ([K]total) in the plume of a burning particle of Australian Loy

Yang brown coal (23 6 3 mg) and of pine wood pellets (63 6 3 mg) was

performed using laser-induced breakdown spectroscopy (LIBS) in a

laminar premixed methane flame at equivalence ratios (U) of 1.149 and

1.336. Calibration was performed using atomic sodium or potassium

generated by evaporation of droplets of sodium sulfite (Na2SO3) or

potassium sulfate (K2SO4) solutions seeded into the flame. The calibration

compensated for the absorption by atomic alkalis in the seeded flame,

which is significant at high concentrations of solution. This allowed

quantitative measurements of sodium (Na) and potassium (K) released

into the flame during the three phases of combustion, namely devolati-

lization, char, and ash cooking. The [Na]total in the plume released from

the combustion of pine wood pellets during the devolatilization was found

to reach up to 13 ppm. The maximum concentration of total sodium

(½Na�maxtotal) and potassium (½K�max

total) released during the char phase of burning

coal particles for U ¼ 1.149 was found to be 9.27 and 5.90 ppm,

respectively. The ½Na�maxtotal and ½K�max

total released during the char phase of

burning wood particles for U¼ 1.149 was found to be 15.1 and 45.3 ppm,

respectively. For the case of U¼ 1.336, the ½Na�maxtotal and ½K�max

total were found

to be 13.9 and 6.67 ppm during the char phase from burning coal

particles, respectively, and 21.1 and 39.7 ppm, respectively, from burning

wood particles. The concentration of alkali species was higher during the

ash phase. The limit of detection (LOD) of sodium and potassium with

LIBS in the present arrangement was estimated to be 29 and 72 ppb,

respectively.

Index Headings: Brown coal; Pine wood; Alkali elements; Laser-induced

breakdown spectroscopy; Quantitative LIBS; Laminar premixed methane

flame.

INTRODUCTION

Laser diagnostic techniques provide in situ, nonintrusivemeasurements that can be spatially and temporally resolved inmany extreme environments and can be quantitative, forexample, with suitable calibration. Laser diagnostic techniqueshave been employed to measure sodium (Na) and potassium(K) released in various forms during combustion of coal andbiomass. However, these elements exist in various forms andalso vary in space and time through the plume of a burningparticle. Hence, it is difficult to determine the temporal historyof the total amounts of sodium and potassium released duringthe various stages of combustion.

Alkali species, mainly Na and K, released from combustion

of coal and biomass cause substantial problems with thefouling and corrosion of heat exchange surfaces.1,2 Hence,much effort has been invested in understanding the foulingprocesses in order to mitigate these problems. For example, theproduct gases of biomass combustion have been investigatedby applying mass spectrometry and scanning electron micros-copy/energy dispersive X-ray (SEM/EDX) to stable products,such as alkali chloride, alkali hydroxide, and sulfur dioxide.3,4

Ash analysis has also been performed to model combustion ofcoal and biomass.5,6 However, with these methods, it is notpossible to investigate the unstable intermediate alkali species.

Van Eyk et al.7 developed a quantitative planar laser-inducedfluorescence (PLIF) technique providing time-resolved mea-surements and planar distribution of atomic Na released from asingle burning coal particle during the char phase. Saw et al.8

then applied the same technique to measure atomic Na releasedduring black liquor combustion. However, the significantscattering from soot that occurs during the devolatilizationstage leads to low signal-to-noise ratio of the PLIF measure-ment during that stage of combustion. Furthermore, the sodiumis typically distributed in the forms of Na, NaOH, NaCl, andother minor species and the measurement of each species viathe PLIF technique requires a specific excitation wavelength.This is a substantial constraint when seeking to measuremultiple species simultaneously. Indeed it is presentlyunrealistic to resolve simultaneously all of the key forms ofNa and K by PLIF.

Laser-induced breakdown spectroscopy (LIBS)9 is analternative and complementary technique that offers thepossibility of measuring simultaneously the total amounts ofmultiple species samples of solid, liquid, or gas. Yamamotoand co-workers reported a limit of detection (LOD) of theirapparatus for barium (Ba), beryllium (Be), lead (Pb), andstrontium (Sr) in contaminated soil samples of 265, 93, 298,and 42 ppm, respectively.10 Arca et al.11 also quantified Na, K,calcium (Ca), chromium (Cr), and chlorine (Cl) in drinkingwater. The LOD of LIBS applied to a liquid is relatively highdue to the relatively low temperature of the plasma resultingfrom the high heat loss. Dudragne12 reported that LIBS wasapplied to measure quantitatively sulfur (S), fluorine (F), Cl,and carbon (C) in the atmosphere. Other studies13–16 evaluatedC, oxygen (O), hydrogen (H), and nitrogen (N) at variousequivalence ratios of laminar premixed methane flames usingLIBS. Compositions of coal and wood particles have beeninvestigated quantitatively by applying LIBS to solid-fuelparticles.17,18 The LIBS technique has also been employed todetermine the compositions of fly ash19 and other fly ashsamples20 from a coal-fired power plant. It has been employed

Received 30 August 2010; accepted 9 March 2011.

* Author to whom correspondence should be sent. E-mail: li-jen.hsu@cycu.org.tw.

DOI: 10.1366/10-06108

684 Volume 65, Number 6, 2011 APPLIED SPECTROSCOPY0003-7028/11/6506-0684$2.00/0

� 2011 Society for Applied Spectroscopy

to measure multiple elements, including Na and K, at hightemperature in the exhaust gas from industrial furnaces21 and ina laboratory glass furnace.22 However, it has yet to beemployed to investigate alkalis in the plume of burningparticles.

Quantitative LIBS requires calibration based on comparisonof intensities of chosen spectral lines of trace species withstandard samples. It is also necessary to account for theinterferences of self-absorption, spectral overlapping, and thematrix and size effects, which limit the accuracy of quantitativeLIBS. The matrix effect only occurs in solid samples, and socan be neglected in the present flame environment, which isgaseous. The size effect23–25 accompanies the matrix effect inaerosols, such as those generated by a nebulizer for the presentcalibration. However, for the present calibration process, thenebulized salt solution that is seeded into the flame as aerosoldroplets is evaporated upstream of the laser measurementregion. For example, scattering was found to be absent fromthe similar calibration process employed in the PLIF techniquereported by van Eyk7 and Saw.8 Hence, the LIBS method isapplied only to gaseous species, indicating that both the matrixand size effects can be neglected. Furthermore, Diwakar24 et al.also concluded that the size effect can be diminished byapplying a sufficient delay time of approximately 30 to 60 ls.This method was also adopted as a precautionary approach.

The influence of spectral overlap can be reduced byoptimizing gate delay and gate width of a spectrometer.However, self-absorption cannot be avoided and becomessignificant for high concentrations of target species. Neverthe-less, it can be reduced by selecting alternative persistentspectral lines for the calibration instead of the major resonantones. For example, the chosen line for calibration of Fe(I) is404.582 nm instead of 248.328 nm in order to avoid self-absorption in the highly populated level of Fe.26 The LOD forquantitative LIBS can also be improved by choosingappropriate reference wavelengths.

The first aim of this paper is therefore to develop a techniquefor real-time quantitative measurement of total sodium([Na]total) and potassium ([K]total) released during combustion

of a particle and to apply it to Australian Loy Yang brown coaland pine wood pellets to provide the temporal release history.The second aim is to evaluate the sensitivity of the LIBSmeasurement and the third aim is to provide the time-resolvedrelease of Na and K for combustion under rich equivalenceratios.

METHODOLOGY

Laminar Premixed Burner. A laminar premixed burner(Perkin Elmer), as shown in Fig. 1, specifically designed forflame atomic emission spectroscopy (AES), was employed inthis work. The burner contains a bottom chamber to premix airand fuel, connected to an upper honeycomb matrix of circularcross-section and diameter of 23 mm. This central matrix wasmounted within a concentric honeycomb 45 mm in diameter.Air and fuel flow rates were controlled by mass flow controllers(MFCs, Bronkhorst). One MFC was utilized to control the flowrate of methane to 0.779 6 0.006 normal liters per minute (mln),and two other MFCs were used to control the main air streamand the seeding air (for calibration). The total flow rates of themain air stream used for the equivalence ratios of 1.149 and1.336 were 6.45 6 0.03 and 5.55 6 0.03 mln, respectively,including 0.300 6 0.015 mln of seeding air during calibration. Anebulizer was employed to produce droplets of salt solution witha nominal diameter of 1 lm,7 which were seeded into the flamethrough the burner for the calibration processes. A bottom outlet,as shown in Fig. 1, was designed to drain away any excess saltsolution to avoid any influence on the mixing chamber.

Laser-Induced Breakdown Spectroscopy. The laser-in-duced breakdown spectroscopy (LIBS) system shown in Fig. 2was applied to measure at a single point on the burner axis thequantitative histories of the elements released into the plume bya single burning particle of coal (23 6 3 mg, diameter of 3mm) or pine wood (63 6 3 mg, diameter of 4 mm) suspendedon a platinum (Pt) wire at a height of 10 mm above the flatflame burner. The terms [Na]total and [K]total represent theinstantaneous concentration of total elemental concentration ofNa or K, respectively, at a single point in the plume. Theconstituents of Loy Yang coal and pine wood pellets shown inTable I were analyzed by HRL Technology Pty Ltd (inAustralia). A Q-switched Nd:YAG laser (Spectra Physics)operating at the fundamental wavelength of 1064 nm (10 Hzrepetition rate and pulse width of 8 ns) and equipped with anattenuator (Iskra electronics, NRC, model: 935-10) to vary thepulse energy was used to provide a laser beam of 240 mJ per

FIG. 1. The Perkin Elmer burner and seeding arrangement used to generate thepremixed laminar flame.

FIG. 2. Experimental arrangement for the LIBS technique and simultaneousatomic emission spectroscopy (SAES). (RaP) right-angled prism; (FL) focallens; (BF) burner and flame; (PL) LIBS plasma; (G) grating; (BS) beamstopper; and (OFD) is optical fiber detector.

APPLIED SPECTROSCOPY 685

pulse. The laser beam was focused to a waist diameter of 1 mmto generate a plasma with an elliptical measurement volume, ata position 10 mm above the burner surface during calibrationor 10 mm above the fuel particles, by a quartz lens with a focallength of 150 mm. The emitted radiation was collected througha quartz lens with a focal length of 150 mm by a spectrometer(Triax series 320, Inc. Edison, NJ) comprising a grating (300grooves/mm) and an intensified charge-coupled device (ICCD)detector (Princeton instruments, model: 7483-0001). A pulsegenerator (Stanford Research Systems, DG535) and anoscilloscope (Tektronix TDS 3054, 500 MHz), respectively,were used to trigger the laser and to optimize the gate delay andgate width, reducing background noise, which was dominatedby emission spectra of atomic Na, atomic K, and water (H2O).The optimized delay time and gate width were experimentallydetermined to be 45 and 5 ls, respectively, which eliminatedline overlapping. This time delay is also a precautionarymeasure to reduce any possible ‘‘size effect’’ due to scatteringfrom any unevaporated salt droplets.24 Here [Na]total and[K]total were not measured simultaneously because of the largewavelength difference between the D1 bands of Na(I) and K(I),which are out of the range covered by the chosen grating. Thisimproves the signal-to-noise ratio and LOD. Nevertheless,commercial spectrometer products, such as the Echellespectrometer,26,27 are capable of measuring multiple elementssimultaneously.

Simultaneous Atomic Emission Spectroscopy. A spec-trometer (Ocean Optics, USB 2000) was employed to measuretime-resolved atomic Na or K emission during combustion,simultaneously with the LIBS measurement, termed assimultaneous atomic emission spectroscopy (SAES), as shownin Fig. 2. This compensates the present arrangement of LIBSmeasurement in which the Na and K were not measuredsimultaneously. Hence, SAES can be used to investigate thedifference between atomic Na and K released from burningsolid fuel and the particle-to-particle variability.

Calibration Method. The experimental measurements ofthe concentrations of Na ([Na]) and K ([K]) using LIBS werequantitatively converted by applying a calibration process.Unlike conventional calibration methods reported in theliterature27,28 based on the relative intensities of the spectrallines of the reference and trace elements, the present calibrationcurves were based on a known quantity of seeded Na and K.

Various concentrations of potassium sulfate (K2SO4) andsodium sulfite (Na2SO3) were seeded into the flame. Thedistribution of alkali salt in the flame was assumed to beuniform. Mass flow rate (tm) was calculated as the product ofthe concentration of alkali salt (Cs) and the consumption rate ofsalt solution (ts), as shown in Eq. 1a. In order to calculate ts,the volumes of salt solutions

tm ¼ Cs 3 ts ð1aÞ

were averaged over a 4 hour consumption period, with aconstant seeding air flow rate of 0.300 6 0.015 mln (describedabove). Condensed salt solution was taken into account byfeeding it back to the nebulizer system directly. Hence themolar flow of alkali salts in the flame (tms) was obtained, fromEq. 1b:

tms ¼ am � tm=M ð1bÞ

where M is molar mass of salt and am is molar ratio of alkaliions in the salt. It can be seen from Eq. 1c that the total gasflow rate (tf):

tf ¼ ðtg þ taÞ3 Tf=Tr ð1cÞ

at flame temperature (Tf), is a function of tg (flow rate ofmethane), ta (flow rate of air), and Tr (room temperature).Then, using the ideal gas law, the molar flow rate through theflame is obtained, as presented in Eq. 1d:

tmf ¼ P � tf=R � Tf ð1dÞ

The concentrations of Na or K (Cseeding) in the flame arerepresented in parts per million (ppm), as calculated in Eq. 1e:

Cseeding ¼ tms=tmf ð1eÞ

From Eqs. 1a through 1e, two linear calibration equations ofthe concentrations of sodium and potassium were thusobtained. Moreover, the total seeded amounts of Na and K,indicating the number density of target species (ns: atoms/m3),were obtained by dividing Eq. 1b with Eq. 1c.

RESULTS AND DISCUSSION

Figure 3 illustrates the process by showing the flame duringthe various stages of the experiment. The orange color of thesodium is evident in the seeded flame used for calibration (Fig.3a), the presence of soot during devolatilization is evident fromthe white orange (Fig. 3b), but not in the other stages, and theelliptically shaped plasma generated by the LIBS is evident inall stages. The visually distinct features of each stage ofcombustion were used to identify them following earlierwork,7,8 to confirm direct measurements and to estimate theduration of each phase, namely devolatilization (sd), char (sc),and ash cooking (sa) phases.

The fundamental equation widely used in the quantitativeinductively coupled plasma (ICP) method to correlate thenatural emission intensity with the concentration of targetspecies is shown in Eq. 2:

lnðIemissionÞ ¼ lnðAÞ þ b � lnðCseedingÞ ð2Þ

where Iemission is the intensity of Na or K measured using LIBS,b is the coefficient of self-absorption, Cseeding is the

TABLE I. Composition of Loy Yang coal and pine wood.

Loy Yang coal Pine wood

% (Dry ash free basis)C 67.80 51.30H 5.20 6.00N 0.57 , 0.01S 0.24 0.02Cl 0.06 0.09O (By difference) 26.13 42.59

% (Ash basis)SiO2 12.90 43.30Al2O3 31.40 0.97Fe2O3 6.70 0.82TiO2 0.70 0.15K2O 0.77 5.70MgO 12.20 8.30Na2O 11.30 4.90CaO 5.60 24.8SO3 16.90 3.90

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concentration of alkalis in the seeded flame, and A is anempirical constant. Calibration curves are linear while self-absorption in the plasma is very small (b¼ 1). Hence, a linearrelationship between Iemission and Cseeding can be obtained, asshown in Eq. 3. Given the empirical equation

Iemission ¼ A � Cseeding ð3Þ

calibration curves for [Na] and [K] can be obtained. The LIBSdata, collected at different seeding concentrations, are plottedin Fig. 4. It is evident from Fig. 4 that, at high seedingconcentrations, significant absorption is present, leading tononlinear calibration curves. The absorption effect in conven-tional LIBS measurements can be considered as a result fromself-absorption, which is not avoidable. The chosen wave-lengths of atomic Na and K for calibration in Fig. 4 are 589.592and 769.896 nm (D1 lines), respectively. These are the mainresonant lines and can be expected to lead to significant self-absorption where high concentrations of alkali species arepresent. Self-absorption is an intrinsic property of plasma-based techniques. The LIBS signal is dynamically varyingowing to the use of a pulsed laser, in which the localthermodynamic equilibrium (LTE) is typically reached afterabout 1 ls from the generation of the LIBS plasma. Elementsin the center of the plasma are in a range of higher energystates, owing to their higher temperature, while those in thelower temperature region outside the plasma tend to stay in themajor electronic energy states, which emit the main resonantspectral lines. The radiation emitted from the center of theplasma will thus be re-absorbed by the atoms in the outerregions of the plasma. Therefore, significant absorption of theemission from the target elements necessarily occurs in thehigh population levels at the edge of the plasma, leading tononlinearity of the calibration curve. However, self-absorptionin the present experiment is not the only source of nonlinearity.

Due to the configuration of the present calibrationexperiment, shown in Fig. 5, the LIBS plasma was located inthe core of a much larger seeded region. The LIBS signal istherefore subject to absorption from the atomic Na and K in

outer seeded region of the flame. This absorption (defined as

signal trapping) should be considered prior to self-absorption

of LIBS plasma because the flame absorption intrinsically

occurs before radiation is collected. This also explains why the

weak persistent lines were not detectable, owing to the strong

signal trapping for high concentrations of alkali. Therefore the

D1 lines of Na and K were chosen instead of other persistent

lines.

For the present experimental arrangement, the absorption

(signal trapping) was taken into account in Eq. 3 using the

Beer–Lambert law to obtain Eq. 4, which was applied to fit the

FIG. 3. Photographs of the flame showing the LIBS induced plasma during thestages of (a) calibration with a seeded species, (b) devolatilization, (c) char, and(d) ash, with the particle suspended on a platinum wire.

FIG. 4. Best-fit curves of calibration obtained for [Na]total (solid line) and[K]total (dashed line) in the seeded flame using LIBS; (A) and (*) indicate theexperimental data of atomic Na and K, respectively.

FIG. 5. Signal trapping, due to atomic Na or K seeded into the outer region ofthe flame. The effective absorption volume is considered as a cone (hcone is thedistance from the LIBS plasma to the edge of the seeded flame and rcone is theradius of the base circle of the cone).

APPLIED SPECTROSCOPY 687

nonlinear calibration curves, as indicated in Fig. 4:

ILIBS ¼ C1 � ½X�total � e�C2�½X�total ð4Þ

where ILIBS is the raw intensity of [X]total (X represents Na orK), C1 and C2 are constants, and C1�[X]total is the LIBSintensity in the absence of absorption introduced in Eq. 3. Thebest-fit curves for [Na]total and [K]total in Fig. 4 exhibited goodagreement with the experimental results (R2 ; 0.94), indicatingthat the LIBS measurement in the present arrangement isreliable. Nevertheless, this is not sufficient for quantitativeLIBS measurements due to the nonlinear calibration curves.The constant, C2, presented in Fig. 4, is related to theabsorption cross-section and absorption path, which aredescribed in the Beer–Lambert law.

To correct for signal trapping and to provide absoluteconcentrations of trace species, the Beer–Lambert law, aspresented in Eq. 5, has been widely used:7,8,29

lnðIreal=ILIBSÞ ¼ aðxÞ � x ð5aÞ

aðxÞ ¼ ns � raðxÞ ð5bÞ

where Ireal and ILIBS are respectively the actual (with signaltrapping compensated) and raw (with signal trapping affected)intensities of Na and K using LIBS. Here a(x) is the absorptioncoefficient at angular frequency x, x is the absorption pathwithin the flame, ns is the number density introduced in theCalibration Method subsection above, and ra(x) is theabsorption cross-section of the target species at angularfrequency x. It is well established that the coefficient ofabsorption can be determined by introducing the Einsteincoefficient for target species in the specific state.29 Theconventional application of the Beer–Lambert law onlyconsiders two-dimensional absorption. Here the volumetricnative of the absorption should be taken into account.Therefore, Eq. 5 needs to be modified, as shown in Eq. 6:

lnðIreal=ILIBSÞ ¼X

aðxÞ �X

xi ð6aÞ

XaðxÞ ¼ e �

X½ns � raðxÞ� ð6bÞ

where R indicates volumetric calculations of a(x), x, and ra(x)described previously in Eq. 5. Here, Rxi and R[ns�ra(x)] wererespectively considered as the volume of a cone and thesummation of absorption cross-section within the effectiveabsorption volume. Here Rxi was calculated, from Eq. 7:

Dm ¼ 1

3� p � r2

cone � hcone ð7Þ

where rcone and hcone are shown in Fig. 5. In addition, theabsorption, due to the population level of atomic Na or K in theflame, can be obtained in terms of the ratio (the percentage ofatomic Na or K in the flame), e, estimated based on flametemperature (;1500 K), multiplied by the total seeded ns.Therefore, a modified Beer–Lambert law was obtained, asdescribed in Eq. 8:

Ireal ¼ ILIBS � ee�P½ns�raðxÞ��Dm ð8Þ

The factor e was estimated to be 0.020 and 0.018 for atomic Naand K, respectively. For this experimental arrangement, theconstant, C2, in Eq. 4 was substituted into Eq. 8 to obtainP

ra(x)�Dm. Equation 8 was used to correct the rawexperimental intensities of total Na and K, to obtain twomodified calibration curves, which are linear, for both [Na]total

and [K]total, as shown in Fig. 6. The best-fit curves of thecalibration equations are shown in Eqs. 9 and 10:

ILIBS;Na ¼ 426:674 � CNa ð9Þ

ILIBS;K ¼ 219:500 � CK ð10Þ

where ILIBS,Na and ILIBS,K are emission intensities of Na(I) andK(I) in the D1 bands with LIBS, respectively. Here CNa and CK

are the molar concentrations in the seeded flame obtained fromEq. 1 and were utilized to quantitatively convert the time-resolved profiles of [Na]total and [K]total released from burningparticles of coal and wood.

The corrected linear calibration curves display goodcorrelation with the experimental data, indicating that thesignal trapping dominated over the self-absorption. Hence,while self-absorption indeed needs to be taken into account inconventional quantitative LIBS measurements, for the presentexperiment the good linearity without considering self-absorption indicate that it is not significant. This demonstratesthe feasibility of quantitative measurement of alkali speciesreleased from burning solid-fuel particles using LIBS.Nevertheless, further correction for self-absorption could becarried out by employing very low concentrations of seededsalt solutions to further improve the accuracy of the LIBSmeasurement.

The overall temporal histories of alkali species released fromburning coal and wood using LIBS and SAES underequivalence ratios of 1.149 and 1.336 are shown in Fig. 7.Figures 7c and 7d reveal the variability in the time histories ofatomic alkali species released from two different solid-fuelparticles. Given that combustion conditions dominate thecombustion rates of solid-fuel particles,30 Figs. 7a and 7breveal a consistent behavior that the char stage is longer at theequivalence ratio of 1.336 than at 1.149, as expected. Theeffect of equivalence ratio was also observed using SAES, as

FIG. 6. Final calibration curves for the measurement of [Na]total and [K]total

using LIBS, corrected for signal trapping, as shown in Eqs. 9 and 10; the errorwas multiplied by 10.

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shown in Fig. 7c and 7d. Again the char combustion occursfaster at the equivalence ratio of 1.149 than at 1.336, while thedevolatilization time for coal and wood are 12 and 22 seconds,respectively, for both equivalence ratios. Moreover, the ashstage was found to be longer for the richer equivalence ratio.The combustion stages of solid-fuel particles are summarizedin Table II. The maximum concentration of total sodium(½Na�max

total) and potassium (½K�maxtotal) released during the char phase

of burning coal particles for U ¼ 1.149 was found to be 9.27and 5.90 ppm, respectively. The ½Na�max

total and ½K�maxtotal released

during the char phase of burning wood particles for U¼ 1.149was found to be 15.1 and 45.3 ppm, respectively. Moreover,for the case of U¼ 1.336, the ½Na�max

total and ½K�maxtotal were found to

be 13.9 and 6.67 ppm from burning coal particles, respectively,

and 21.1 and 39.7 ppm, respectively, from burning woodparticles. The detailed ½Na�max

total and ½K�maxtotal are shown in Table

III. In addition, SAES also demonstrated that the peaks of bothatomic Na and K occur at the end of the char stage of solid-fuelparticle combustion. The condition U¼ 1.336 was observed toinhibit the release of alkali species during the char stage so thatalkali species during the ash stage are significantly increased,as shown in Table IV. Although the alkali species in each solid-fuel particle are different, combustion conditions still dominatethe fundamental mechanisms for alkali species released duringcombustion, as expected.30

Figure 8 displays the concentrations of alkali species in theprobe volume released during the devolatilization stage ofburning coal and wood particles using LIBS and SAES.

FIG. 7. Overall temporal release of Na and K at three phases (sd, sc, and sa) with U¼ (a) 1.149 and (b) 1.336. ½Na�maxtotal and ½K�max

total in (a) and (b) released from coaland wood using LIBS are presented numerically. The SAES technique was simultaneously performed with LIBS providing the instantaneous measurement of atomicNa and K release for U¼ (c) 1.149 and (d) 1.336. The solid and symbol lines in (c) and (d) represent profiles of alkali species released from burning particles of woodand coal, respectively.

TABLE II. Duration of the three combustion stages as determined fromSAES for two stoichiometries, U¼ 1.149 and 1.336.

U sd sc sa

Coal 1.149 12 ;450 ;7501.336 ;650 ;1350

Wood 1.149 22 ;650 ;5501.336 ;1000 ;1000

TABLE III. [K]maxtotal/[Na]max

total during the devolatilization and char phasesas mentioned using LIBS under U¼ 1.149 and 1.336.

U Devolatilization Char

Coal 1.149 3.53 / 3.06 5.90 / 9.271.336 3.03 / 6.35 6.67 / 13.9

Wood 1.149 5.12 / 12.8 45.3 / 15.11.336 5.22 / 11.6 39.7 / 21.1

APPLIED SPECTROSCOPY 689

Generally the total release of atomic alkali species duringdevolatilization is negligible compared with the entire release.However, as seen in Table IV, [Na]total and [K]total releasedfrom the devolatilization of burning wood and coal particles,respectively, are significant. Unlike [K]total released from coalcontaining low K species, ½Na�max

total during the devolatilization ofburning wood particles was found to be 13 ppm, as strong asthat at the end of the char stage. Hence, the release of Naduring the devolatilization of burning wood particles should beconsidered when seeking to mitigate slagging and fouling.

It should be noted that the maximum concentration of thecalibration was about 10 ppm while the maximum measuredvalues reach 45 ppm. For the present calibration method, it is

reasonable to extrapolate over this concentration range because

Eqs. 9 and 10 are linear. Although the self-absorption will

increase with concentration, it is still negligible compared with

the signal trapping. In a further investigation, a correction for

self-absorption performed under low concentrations may be

warranted.

Another property of the LIBS technique, the limit of

detection, can be calculated by Eq. 11:31,32

LOD ¼ 3r=S ð11Þ

where r is the mean relative standard deviation (RSD) and S is

the slope of the calibration equation. The values of r for

sodium and potassium in the present arrangement were 4.125%and 5.268%, respectively. In addition, the values of S for

sodium and potassium are 426.674 and 219.500, respectively.

Hence, the LOD for Na and K in the present setup was

respectively estimated to be 29 and 72 ppb, which is much

lower than those of other elements in solid and liquid samples

reported in the literature10,11,26 because the premixed flames

provide a high-temperature environment from which the alkali

species are vaporized, reducing the energy required to

TABLE IV. Ratios of the integral release of [Na]total and [K]total duringthe three combustion stages using LIBS for U¼ 1.149 and 1.336.

U sd (K% / Na%) sc (K% / Na%) sa (K% / Na%)

Coal1.149 5.97 / 0.79 49.59 / 46.41 44.44 / 52.801.336 6.92 / 0.94 43.64 / 37.16 49.44 / 61.90

Wood1.149 1.46 / 4.93 39.00 / 38.30 59.54 / 56.781.336 1.52 / 3.42 38.54 / 31.75 59.95 / 64.83

FIG. 8. Temporal release of [Na]total and [K]total using LIBS during the devolatilization of coal and wood lasts for 12 and 22 seconds, respectively, for both U¼ (a)1.149 and (b) 1.336. The SAES was performed simultaneously with LIBS, providing the instantaneous measurement of atomic Na and K for U¼ (c) 1.149 and (d)1.336. [Na]total using LIBS was found to be significant around 13 ppm. The solid and symbol lines in (c) and (d) represent the temporal histories of atomic alkalispecies released from burning particles of coal and wood, respectively.

690 Volume 65, Number 6, 2011

dissociate alkali molecules in flames relative to that required inthe solid state.

CONCLUSION

A novel application of LIBS has been developed to providequantitative measurements of Na and K released from burningparticles of brown coal and pine wood. Quantification has beenachieved by developing a calibration procedure in whichatomic Na or K is seeded into the flame by an evaporated mistof salt solution. To correct for signal trapping from the atomicspecies in the seeded zone surrounding the LIBS plasma, amodified Beer–Lambert’s law has been employed. Thecalibration is linear, allowing reliable measurement of [Na]total

and [K]total conducted in the flame environment. The release ofalkali species was observed to be inhibited by a richerequivalence ratio, which also lengthens the stages of charand ash combustion. The maximum concentration of totalsodium (½Na�max

total) and potassium (½K�maxtotal) released into the

plume during the char phase of burning coal particles for U¼1.149 was found to be 9.27 and 5.90 ppm, respectively. The½Na�max

total and ½K�maxtotal released during the char phase of burning

wood particles for U ¼ 1.149 was found to be 15.1 and 45.3ppm, respectively. For the case of U ¼ 1.336, the ½Na�max

total and½K�max

total was found to be 13.9 and 6.67 ppm from burning coalparticles, respectively, and 21.1 and 39.7 ppm, respectively,from burning wood particles. The LOD of Na and K was foundto be 0.029 and 0.072 ppm, respectively.

ACKNOWLEDGMENTS

The work was supported by the Centre for Energy Technology, TheEnvironment Institute of Adelaide University, and Division of CombustionPhysics, Lund University, Sweden.

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APPLIED SPECTROSCOPY 691