In-situ Measurement of Sodium and Potassium Release during Oxy

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In-situ Measurement of Sodium and Potassium Release during OxyFuel Combustion of Lignite using Laser-Induced Breakdown Spectroscopy: Effects of O2 and CO2 Concentration Yong He,†,‡ Jiajian Zhu,‡ Bo Li,‡ Zhihua Wang,*,† Zhongshan Li,*,‡ Marcus Aldén,‡ and Kefa Cen† †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, 310027 Hangzhou, China Division of Combustion Physics, Lund University, P.O. Box 118, S-22100 Lund, Sweden

‡

ABSTRACT: Laser-induced breakdown spectroscopy (LIBS) was used in this study to measure quantitatively the sodium (Na) and potassium (K) release from burning coal particles under oxy-fuel combustion environments. A specially designed laminar premixed burner was employed to provide a postflame environment with different O2 and CO2 concentrations, in which the effects of O2 and CO2 on the release of Na and K during coal oxy-fuel combustion were studied systematically. For the devolatilization stage, neither O2 nor CO2 had significant influence on the Na and K release. The release of Na and K during the char stage, however, changed significantly at different O2 and CO2 concentrations. Under these experimental conditions, when the O2 concentration increased from 3.9% to 10.6%, the peak concentration of Na at the char stage increased from 15.2 mg/m3 to 33.7 mg/m3, and the maximum concentration of K increased from 6.2 mg/m3 to 11.7 mg/m3. When the CO2 concentration increased from 35.8% to 69.4%, the release of Na and K was inhibited during the char stage, with the peak concentration decreasing from 8.9 mg/m3 to 6.9 mg/m3 for Na and from 3.7 mg/m3 to 2.4 mg/m3 for K. During the ash stage, the release of Na and K decreased with the O2 concentration, whereas it increased with the CO2 concentration.

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INTRODUCTION Coal is still the main energy source for power generation, especially in the developing countries.1 The emission of carbon dioxide (CO2) from coal combustion, as a cause of global warming, has gained great attention in recent years. Oxy-fuel combustion combined with carbon capture and sequestration (CCS) is one of the most promising technologies for CO2 emission control at power plants.2 In the oxy-fuel combustion process, pure oxygen (O2), usually mixed with recycled flue gas, acts as oxidizer for combustion. With N2 absent from the oxidizer, more than 95% of CO2 can be enriched in the final flue gas. After simplified purification, the generated CO2 can be easily compressed and captured. During coal combustion, the released alkali species, mainly sodium (Na) and potassium (K) compounds, may cause severe problems such as fouling and corrosion of the heat transfer surface in industrial furnaces.2−4 Therefore, much work has been undertaken in order to understand the release behavior of Na and K in last decades.5−16 Based on these studies, mechanisms of alkali release have gained significant developments. For example, van Lith et al.15 concluded that the fraction of K and Na released by the vaporization of KCl and NaCl was very limited during wood combustion. Moreover, they mentioned that the release up to 800−850 °C was due to the release of char-bound alkali metals and the release of K and Na above 850 °C was strongly dependent on the inorganic composition of the fuel. Van Eyk et al.16 also proposed a mechanism for sodium release during brown coal combustion. They concluded that the temperature dependent kinetics of sodium release during Loy Yang brown coal char combustion obeyed an Arrhenius expression with pre-exponential and activation energies having average values of 105.2±0.4 s−1 and 214 kJ/mol respectively. These studies improved the understanding © 2013 American Chemical Society

of the alkali release mechanism during solid fuel combustion. However, most of the existing studies were carried out under air-fired combustion conditions. The replacement of N2 with CO2 during oxy-fuel combustion has a great impact on the combustion process, including the release of Na and K2. Therefore, it is important to study the release behavior of Na and K during the coal oxy-fuel combustion processes. In previous studies, several methods have been used to measure the Na and K release during coal combustion. For example, Markowski et al.17 and Quann et al.18 studied the Na release by analyzing the bulk composition of fly ash and the ash deposits in both laboratory-scale and full-scale coal fired boilers. Moreover, measurements of metals including alkali species in flue gases of coal combustion using online optical diagnostic methods and mass spectrometry have been reviewed by Monkhouse.19 Although these methods could provide information about the final forms of alkali species and their amounts in the flue gas, they do not reveal the time-resolved release process of Na and K right above the burning coal particle, which is valuable for the model validation and development. To meet this demand, Van Eyk et al. developed a quantitative planar laser-induced fluorescence (PLIF) technique to measure the atomic sodium release history directly from a burning coal particle.12,13 However, it was difficult to obtain the release information of atomic Na during the devolatilization stage because of the strong scattering from soot at this period. Furthermore, sodium compounds such as NaOH, NaCl, and other minor species are also released during coal combustion, which can hardly be measured simultaneously Received: October 29, 2012 Revised: January 20, 2013 Published: January 21, 2013 1123

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this study proved that the delay time of 2 μs was long enough to suppress the continuous emission. Laminar Premixed Burner. Shown in Figure 2 is the laminar premixed burner used in this study, called a multijets burner. The

via the PLIF technique as they are molecular species in which the upper state is dissociative. On the other hand, laser-induced breakdown spectroscopy (LIBS) will detect alkali originating from all relating species (i.e, the total amount of alkali release). LIBS is a type of atomic emission spectroscopy facilitating in situ sensitive elemental analysis in a remote manner. The principle and technical details of LIBS have been reviewed by various authors.20,21 Recently, Hsu et al.14 measured the total amounts of sodium and potassium release simultaneously over burning coal and wood particles using the LIBS technique. With LIBS, the time-resolved release processes of sodium and potassium were obtained not only during the char combustion and ash cooking stages but also at the sooting devolatilization stage. However, studies on the release processes in oxy-fuel combustion environment have seldom been reported. In this study, the LIBS technique was employed to measure quantitatively the sodium and potassium release from coal particles under oxy-fuel combustion environments. Specially, the effects of different O2 and CO2 concentrations were discussed systematically. These data are essential for the model development concerning sodium and potassium release during the oxy-fuel combustion processes of coal.

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Figure 2. Multijets burner: (a) top view for the jets and coflow, (b) overall sketch. burner is made of stainless steel and consists of two main parts, namely jets and coflow. There are 91 jets in total, which are installed evenly in a tube with 69 mm inner diameter. All the jets are connected to a chamber at the bottom of the burner to ensure a uniform gas flow in each jet. A mask is inserted between the outer tube and the jets to form another chamber for the coflow gas. Small holes are drilled in the mask, and each jet is surrounded by six of them, to generate a homogeneous coflow. A cooling water jacket is attached outside the burner. Specifically, a shielding ring is placed on top of the burner to ensure a homogeneous condition of the flue gas while shield the influence of ambient air. A stabilizer is placed 32 mm right above the shielding ring to keep the flow field stable. In this study, a premixed CH4/O2/CO2 gas mixture was employed as the jets flame, which provided the oxy-fuel environment for coal combustion. The overall equivalence ratio of the jets was 0.9, and the unreacted oxygen in the jets flame was used to burn the coal particle. Either CO2/argon (Ar) mixture or O2/Ar mixture was used as the coflow. In order to study the effects of O2 and CO2 concentrations, the flow rate of O2 or CO2 in the coflow was changed systematically while the total flow rate of coflow was kept constant with Ar as the balance gas. Table 1 gives the details of each flame condition. In general, flames 1 and 2 were employed to study the effects of CO2 concentration, while flames 3−5 were used to investigate the influence of O2 concentration. Temperatures at the exit of burner, measured by an R-type thermocouple, were similar in each group of flames. The main gas compositions at the exit of burner, which acted as the environment for coal combustion, were calculated with the EQUIL module of CHEMKIN 3.7. The GRI-Mech 3.0,23 which was originally developed for CH4 combustion, was employed in the calculations. The coal sample used here is a Chinese brown coal, called Zhundong brown coal. Table 2 gives the proximate and ultimate analysis results of the coal sample. In the current experiments, the coal particle, weighing 29 ± 3 mg and ∼3 mm in diameter, was suspended on two quartz rods with 1 mm diameter at the height of 2 mm above the burner exit. For each condition, three individual measurements were carried out to account for the variation among particles. Calibration System. To obtain the concentrations of Na and K from the LIBS measurement, an online calibration experiment was carried out on the multijets burner, as described, before the coal LIBS measurement. A similar calibration method has been described in an earlier work.14 An ultrasonic humidifier was used to produce a fog of sodium chloride (NaCl) and potassium chloride (KCl) solution with known concentrations. The premixed CH4/O2/CO2 jets flow (8.57 SL/min) passed through the fog in the solution bottle to entrain the droplets and introduce the NaCl and KCl into the flame from the burner jets. According to the online LIBS signal during the seeding

EXPERIMENTAL SETUP

LIBS Measurement System. To measure the sodium and potassium release during coal combustion, a LIBS system was employed in this work, as shown in Figure 1. An Nd:YAG laser

Figure 1. Experimental setup of LIBS measurement during coal combustion.

(Brilliant B, Quantel) with a third harmonic package was adopted to generate a 355 nm laser beam (repetition rate of 10 Hz and pulse duration of 5 ns). The energy of the laser pulse used was 185 mJ, and the diameter of the laser beam was about 6 mm, which was focused by a quartz lens ( f = 200 mm) into the gas plume right above the burning coal particle, generating the LIBS signal of elements (e.g., sodium and potassium) simultaneously. The measurement point was set at a height of 10 mm above the particle. A small spectrometer (Ocean Optics, USB2000+) was adequate in recording the LIBS signal. The signal was collected by a BK 7 glass lens with focal length 100 mm and diameter 125 mm and then focused into an optical fiber that was connected to the input slit of the spectrometer. The BK 7 glass lens can block the laser scattering at 355 nm and transmit the emission signal in the visible region. In the spectrometer, the signal was dispersed by a 600 groove/mm grating and recorded on a 2048 pixels CCD chip. The spectrometer was synchronized with the laser by a pulse generator (Stanford Research System, DG535). The optimized delay time and exposure time were determined to be 2 μs and 1 ms, respectively, to reduce the continuous emission mostly at the beginning of the plasma and suppress the spontaneous emission of the flame as well. As discussed by Yueh et al.,22 emission lines from atoms can be found after 300 ns delay and start to decay after 10 μs delay, and a good signal-to-background ratio was usually achieved with several microseconds delay. On the other hand, the observed LIBS spectrum with low continuous emissions in 1124

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Table 1. Flame Conditions for Coal Combustion (SL = liter at Standard Temperature and Pressure) jets flow rate (SL/min) flame flame flame flame flame

1 2 3 4 5

gas composition for coal combustion (mole fraction, %)

co-flow (SL/min)

CH4

O2

CO2

O2

CO2

Ar

temp. at burner exit (K)

O2

CO2

H2O

Ar

1.77

3.94

2.86

1.77

3.94

2.86

0 0 0.1 0.5 0.9

4 0 0 0 0

0 4 3.9 3.5 3.1

1587 1584 1655 1641 1631

3.1 3.1 3.9 7.3 10.6

69.4 35.8 35.8 35.8 35.8

27.5 27.5 27.5 27.5 27.5

0 33.6 32.8 29.4 26.0

Table 2. Properties of Zhundong Coal proximate analysis (%, dry basis) fixed carbon

volatile matter 30.86

ash

64.79 ultimate analysis (%, dry basis)

C

H

75.39

3.48

N

4.34 S

O (by difference)

1.19 0.42 alkali concentrations in coal (%, dry basis)

15.19

Na total water soluble

K

0.219 0.131 ash composition (%)

0.044 0.012

SiO2

Al2O3

CaO

MgO

Fe2O3

Na2O

K2O

TiO2

SO3

MnO

10.79

9.62

36.83

9.21

3.95

3.42

0.41

0.68

24.74

0.35

process, the seeding was stable under this gas flow rate. Based on the mass change before and after the seeding, the average seeding rate of the salt solution was calculated to be 0.226 g/min in this study. To obtain various concentrations of Na and K in the flame, a series of concentrations of NaCl and KCl solutions were prepared, and the distribution of Na and K in the flame was assumed to be uniform.

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RESULTS AND DISCUSSION The aim of this work is to measure the release of Na and K in situ from burning coal particles under oxy-fuel environments using the LIBS technique. To achieve quantitative measurements, an online calibration of the adopted LIBS system was performed in the same flame as for coal particle burning but seeded with different concentrations of Na and K. Then, the LIBS system was employed to measure the Na and K concentrations right above the coal particles burning in the postflame environments with different O2 and CO2 concentrations. Based on these measurements, the effects of O2 and CO2 concentrations on the Na and K release during the oxyfuel combustion of coal were investigated. Quantitative Calibrations. Figure 3 shows a typical LIBS spectrum obtained in a calibration flame. Individual atomic lines could be identified using the NIST Atomic Spectra Database.24 The major elements in the calibration flame were clearly identified (e.g., the hydrogen Balmer lines at 486.1 nm/ 656.3 nm and the oxygen atomic line at 777.2 nm). The atomic carbon lines were not clearly observed due to the low transmission of the collecting lens in the corresponding emission region (around 400 nm). The atomic lines of Na and K were zoomed-in and are shown as insets in Figure 3. In this study, the unresolved doublet of Na (588.995 and 589.592 nm) and D2 line of K (766.490 nm) were chosen to calculate the emission intensities of Na and K, respectively. The choice of D2 line of K other than D1 line of K (769.897 nm) could eliminate the interference from the line of argon atoms

Figure 3. LIBS spectrum in the calibration flame (especially the zoomed-in views of Na and K signals).

(772.376 nm) when measuring the K release during the coal combustion. The measured LIBS intensities versus concentrations of Na and K in calibration flames are given in Figure 4. In principle, there should be a linear relationship between the LIBS signal and the concentration of measured species (e.g., Na or K). However, a significant self-absorption was clearly observed from Figure 4, especially at the high concentrations, leading to a nonlinear relationship between the measured LIBS signal and the seeding concentration of Na and K. At low concentrations, the self-absorption was weaker than that at higher concentrations, and the data scattering was larger on the other hand due to the weaker signal, which resulted in the observed trapping effect being not so obvious as that at the higher concentrations. In general, the self-absorption effect in this calibration process (i.e., the absorption of the emission by Na 1125

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The directly measured LIBS signal is shown as square scatters in Figure 4. Each result is an average of 300 measurements, and the error bar indicates the standard deviation of these measurements. The raw intensity versus the seeding concentration was fitted as follows so that the selfabsorption effect can be accounted for in the next step: INa,received = 457.5C Na e−0.0468C Na

(1)

IK,received = 83.04C K e−0.0356CK

(2)

where Ireceived is the received LIBS signal and C is the concentration of elemental Na or K in calibration flames. The fitted curves of raw experimental data are shown as dash lines in Figure 4. Then, each signal was corrected to get the real intensity (with self-absorption compensated) based on the Beer−Lambert law, and finally, a linear relationship was obtained between the LIBS intensity and the concentration of Na or K. For the details of this correction process, the reader is referred to ref 14. For the current LIBS system, the final fitted linear equations that give the relation between the corrected LIBS intensity and the concentration of Na and K are shown as follows (as shown in solid lines in Figure 4): ILIBS,Na = 333.7C Na ,

ILIBS,K = 60.26C K ,

R2 = 0.97

R2 = 0.96

(4) (5)

As mentioned, the self-absorption from the outer region of the flame dominated the signal trapping in this setup, so the selfabsorption effect during the LIBS measurement of Na and K right above the coal particle was expected to be small due to the fact that there were negligible amounts of Na or K atoms existing in the outer flame. Therefore, the calibration results in eqs 4 and 5 were used to calculate the concentrations of Na and K released from the burning coal particles. Effects of O2 Concentration. Figure 5 shows the photographs of burning coal particles at different combustion stages with LIBS plasma sparks. The bright orange emission from soot was evident during the devolatilization stage (Figure 5a) but disappeared in the char and ash stages (Figure 5b and c). However, the yellow light emitted from atomic sodium was obvious at all these three stages. The visually distinct features of different combustion stages were used to identify each stage and to estimate the duration of the three stages, namely

Figure 4. LIBS calibration results for (a) Na and (b) K.

and K atoms themselves) includes two parts: the selfabsorption by the Na or K atoms in the region of the plasma and the self-absorption by these atoms in the calibration flame. Based on the discussion by Hsu et al.,14 the self-absorption by Na or K atoms in the outer region of the flame dominated this nonlinear response and this effect could be corrected using the Beer−Lambert law.

Figure 5. Photographs of a burning coal particle at different combustion stages with LIBS plasma sparks: (a) devolatilization, (b) char, and (c) ash. 1126

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corresponding time for maximum Na release at this stage were ∼11 mg/m3 and ∼15 s, respectively, at all three O2 concentrations. According to the study by van Eyk et al.,13 the Na release at the devolatilization stage comes mostly from the vaporization of the water-bound sodium (e.g., NaCl) at high temperatures. As shown in Table 1, the temperature for the coal combustion was rather constant among these three O2 concentrations. Therefore, the vaporization rates of sodium compounds were hardly affected, resulting in the similar Na release profiles at different O2 concentrations in the range studied. However, the release process at the char stage was significantly affected by the O2 concentration in the flame. In the current study, when the O2 concentration increased from 3.9% to 10.6%, the duration of the char stage decreased from ∼250 s to ∼90 s. Meanwhile, the peak concentration of Na release increased from 15.2 mg/m3 to 33.7 mg/m3 and the corresponding time for maximum Na release shifted from ∼254 s to ∼97 s. Lots of studies in the past as reviewed by Laurendeau et al.26 have demonstrated that the char combustion is a boundary layer diffusion-controlled process when the particle size is over 200 μm. This implies that the char combustion in current experiments is predominantly dependent on the diffusion rate of oxidant (e.g., O2) to the particle. A higher O2 concentration in the flame increased the diffusion rate of O2 to the char surface and also into the pores inside the char particle, resulting in an increase of char combustion rate and hence a shorter char burn-out time. On the other hand, the particle temperature increased with O2 concentration owing to the increase of char combustion rate. As discussed by van Eyk et al.,16,27 particle temperature is the most important factor in the release of sodium. Therefore, the increase of O 2 concentration enhanced the char combustion rate and the particle temperature, which further increased the release rate of Na remained in the char, making that the peak concentration of Na increased with the O2 concentration and the ratio released at this stage increased as well, as shown in Figures 6 and 7. The sodium compounds retained after char combustion will continue to be released at the ash stage. The possible sodium compounds formed during char combustion include sodium sulfate (Na2SO4), sodium carbonate (Na2CO3), sodium oxide (Na2O), sodium hydroxide (NaOH), sodium chloride (NaCl), and metallic sodium.13 At higher O2 concentrations, less sodium was retained due to a larger release during the char stage (see Figure 7). Hence, the concentration of Na released from the ash was lower compared with that at lower O2 concentration, as observed in Figure 6. Similarly, the simultaneously measured release histories of K at different O2 concentrations are given in Figure 8. Figure 9 shows the ratio of K released at each stage. The K release at each stage can also be clearly identified from the measured curves of Figure 8. Although the measured concentration of K was much lower than that of Na because of a lower content of K than Na in the parent coal (see Table 2), it showed a similar variation between the release of K and Na at different O2 concentrations. The K release at the devolatilization stage was almost independent of the O2 concentration, resulting in a peak concentration and a ratio of the released K that were almost the same at the three different O2 concentrations. However, O2 concentration had a significant effect on the K release during the char stage. Both the peak concentration and the ratio of K released at this stage increased significantly with the O2 concentration, which was also attributed to the increase of

devolatilization (td), char combustion (tc), and ash stage (ta), as employed in previous studies.13,14 The measured release histories of Na from burning coal particles at different O2 concentrations are given in Figure 6.

Figure 6. Release histories of Na at different O2 concentrations.

The data were recorded once per second as an average of 10 pulses, and each curve was an average of three individual measurements. The signals during the initial 400 s were zoomed-in and shown as an inset to show the variations at the devolatilization and char stages. The three combustion stages can also be clearly identified from the release curve itself, which agreed well with the visual observation. An example is given in Figure 6 to show the identification of the Na release at each stage, with two peaks corresponding to the devolatilization and char stages, respectively, and a decay process at the ash stage. As can be seen, the devolatilization and the char stage overlapped, and the extent of the overlapping increased with the O2 concentration in the flame, which was consistent with the observation by Gurgel Veras et al.25 Figure 7 further gives

Figure 7. Ratios of Na released at each stage for different O2 concentrations.

the ratio of Na released at each stage for different O2 concentrations. The ratios were calculated from the integration of the release curve at each stage compared with the whole combustion process. From Figure 6, it can be seen that the Na release at the devolatilization stage had only a weak dependence on the O2 concentration in the flame. The peak concentration and 1127

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three individual measurements. The release processes at the initial 700 s were zoomed-in to show the variations at the devolatilization and char stages. Figure 11 further gives the ratio

Figure 8. Release histories of K at different O2 concentrations. Figure 11. Ratios of Na released at each stage for different CO2 concentrations.

of Na released at each stage for different CO2 concentrations. It can be seen that, similar to the O2 cases, different CO2 concentrations had also no significant effect on the Na release at the devolatilization stage (e.g., the maximum concentrations were ∼6 mg/m3 at both concentrations). Moreover, as shown in Figure 11, the ratio of Na released at this stage was similar at these two conditions. As discussed, the Na released at the devolatilization stage comes mainly from the vaporization of water-bound sodium at high temperatures, which, therefore, may not be influenced by the atmosphere in the flame (e.g., the O2 concentration discussed above and the CO2 concentration discussed here). For the char stage, however, there was an obvious variation at these two CO2 concentrations. Different from the dependence on O2, the duration of the char stage increased from ∼380 s to ∼430 s when the CO2 concentration increased from 35.8% to 69.4%. Meanwhile, the peak concentration of Na decreased from 8.9 mg/m3 to 6.9 mg/m3 and the corresponding time for maximum Na release was delayed from 361 to 391 s, as shown in Figure 10. The char combustion in oxy-fuel environments may include two types of reactions: char−O2 reaction and char−CO2 reaction. However, the reaction rate of char−CO2 is almost 2 orders of magnitude slower than that of char−O2 within the flame temperature range.28 Therefore, the char combustion rate in current experiments was dominated by the char−O2 reaction. The increase of CO2 concentration in the surrounding may enhance the char−CO2 reaction, but the lower diffusivity of O2 in the higher CO2 atmosphere2,29 reduced the rate of char−O2 reaction, and hence, the overall reaction rate of char combustion decreased with the CO2 concentration. Accordingly, the particle temperature dropped due to the decrease of the char combustion rate. At the same time, the particle temperature could further decrease because of the enhancement of the endothermic char−CO2 gasification reaction. As mentioned before, particle temperature dominated the release rate of sodium.16,27 Therefore, the release rate of Na remained in the char dropped and the peak concentration of Na became lower at higher CO2 concentration (see Figure 10). Moreover, as shown in Figure 11, the ratio of Na released at the char stage also decreased with the CO2 concentration, resulting in more Na that was retained in the ash and a higher

Figure 9. Ratios of K released at each stage for different O2 concentrations.

char combustion rate and the particle temperature. Due to the larger release of K during the char stage at higher O2 concentration, the K released at the ash stage decreased and a commensurate drop in the release ratio was observed. Effects of CO2 Concentration. Figure 10 shows the measured Na release from the burning coal particle versus the combustion time at two flames with different CO2 concentrations. These results were also obtained from the average of

Figure 10. Release histories of Na at different CO2 concentrations. 1128

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and K seeded flames. The effects of different O2 and CO2 concentrations in the atmospheres were studied systematically. Such data could be useful for the model validation and development about the Na and K release during the oxy-fuel combustion of coal. The main results are summarized as follows: (1) The O2 concentration in the flame has a similar effect on the release of Na and K. At the devolatilization stage, the release of Na and K was not significantly influenced by the O2 concentration. However, the release at the char stage increased significantly with the O2 concentration. At the ash stage, the release of Na and K decreased at higher O2 concentration. (2) The CO2 concentration in the flame has also a similar influence on both Na and K release. The CO 2 concentration did not have obvious effects on the release at the devolatilization stage. At higher CO2 concentration, the release of both Na and K decreased at the char stage, but it increased at the ash stage.

concentration of Na that released at the ash stage at higher CO2 concentration. Due to the lower release of Na during the char stage, the ratio of Na released at the ash stage increased with the CO2 concentration, as shown in Figure 11. Finally, the simultaneously measured release histories of K and the ratio of K released at each stage for different CO2 concentrations are given in Figures 12 and 13, respectively. As

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +46-46 222 98 58. Fax: +46-46 222 45 42. E-mail: [email protected]. *Phone: +86-571 8795 3162. Fax: +86-571 8795 1616. E-mail: [email protected].

Figure 12. Release histories of K at different CO2 concentrations.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financed by the Swedish Energy Agency through CECOST (Center for Combustion Science and Technology), Swedish−Chinese collaboration project between ZJU-LU (project No. 33305-1), VR (Swedish Research Council) and the European Research Council advanced grant DALDECS. Yong He would also like to acknowledge the support from Erasmus Mundus Europe−Asia project, China Scholarship Council, and National Basic Research Program of China (2012CB214906).

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Figure 13. Ratios of K released at each stage for different CO2 concentrations.

REFERENCES

(1) International Energy Agency. World Energy Outlook 2010; IEA: Paris, 2010. (2) Toftegaard, M. B.; Brix, J.; Jensen, P. A.; Glarborg, P.; Jensen, A. D. Prog. Energy Combust. Sci. 2010, 36 (5), 581−625. (3) Neville, M.; Sarofim, A. F. Fuel 1985, 64 (3), 384−390. (4) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22 (1), 29−120. (5) Wibberley, L. J.; Wall, T. F. Fuel 1982, 61 (1), 87−92. (6) Lindner, E. R.; Wall, T. F. Proc. Combust. Inst. 1991, 23 (1), 1313−1321. (7) Manzoori, A. R.; Agarwal, P. K. Fuel 1992, 71 (5), 513−522. (8) Gallagher, N. B.; Peterson, T. W.; Wendt, J. O. L. Proc. Combust. Inst. 1996, 26 (2), 3197−3204. (9) Gottwald, U.; Monkhouse, P.; Bonn, B. Fuel 2001, 80 (13), 1893−1899. (10) Oleschko, H.; Schimrosczyk, A.; Lippert, H.; Müller, M. Fuel 2007, 86 (15), 2275−2282. (11) Schürmann, H.; Monkhouse, P. B.; Unterberger, S.; Hein, K. R. G. Proc. Combust. Inst. 2007, 31 (2), 1913−1920. (12) van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J. Combust. Flame 2008, 155 (3), 529−537. (13) van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J. Combust. Flame 2011, 158 (6), 1181−1192.

expected, CO2 had a similar effect on the release of K as that of Na. The K release at the devolatilization stage was also independent of the CO2 concentration in the flame, resulting in a peak concentration and a ratio of released K that were almost the same for these two CO2 concentrations. However, the release of K during the char stage was affected significantly. Both the peak concentration and the ratio of K released at this stage decreased with the CO2 concentration because of the reduction of char combustion rate and the particle temperature at higher CO2 concentration. Furthermore, due to the lower release of K during the char stage at higher CO2 concentration, a slight increase of K release at the ash stage was observed, as shown in Figures 12 and 13.

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CONCLUSIONS In this study, the release of Na and K from burning coal particles under oxy-fuel combustion environments was measured simultaneously using the LIBS technique. Quantitative results were obtained based on an online calibration in Na 1129

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dx.doi.org/10.1021/ef301750h | Energy Fuels 2013, 27, 1123−1130