Direct measurement of gaseous sodium in flue gas for high alkali coal

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Direct Measurement of Gaseous Sodium in Flue Gas for High-Alkali Coal Jieqiang Ji,† Leming Cheng,*,† Yanquan Liu,† and Li Nie†,‡ †

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State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡ Dongfang Boiler Group Company Limited, Zigong 643001, P. R. China ABSTRACT: A direct measurement technique was developed to detect gaseous alkali metals in this work. It was based on the surface ionization principle. To verify the mechanism, a detector probe was designed and calibrated by NaCl samples. The calibration gives a linear correlation between the detected voltage signals and the vaporizing rates of sodium. Then, the calibrated detector probe was applied to detect the gaseous sodium content in flue gas for the prepared Zhundong coal samples during the heating process. These coal samples contained varied contents of sodium, which were classified as raw coal, washed coal, and coal with sodium additives. Results show that sodium compounds in the raw coal vaporize in the temperature range of 300−600 °C. Its highest release rate appears at 457 °C. For the coal samples with additives of NaCl or Na2SO4, higher addition ratio leads to more released sodium. Compared with SO42−, Cl− has a greater influence on sodium release. This technique may be applied to online measure the gaseous alkali metal content during high-alkali coal combustion in a boiler.

1. INTRODUCTION High contents of alkali and alkaline earth metal elements in coal will cause fouling and deposition problems in coal-fired boilers.1,2 Zhundong (ZD) coal is a typical high alkali coal. It is reported that Na2O proportion in the ZD coal ash is approximately 2−10%.3 Sodium content in the raw ZD coal is about 2260 μg/g of raw coal.4 Sodium compounds in coal play a very important role in the fouling and deposition problems.5−7 They are susceptible to volatilization during the thermal process.8 The released sodium vapors condense on the heat-transfer surface and/or particle surface, resulting in the growth of ash deposition. Investigations on the sodium-release behavior of ZD coal have been carried out in literatures.4,9,10 Li et al. found that most sodium in the form of NaCl was released into gas phase and the remainder was transformed into aluminosilicates with increasing temperature in a laboratory-scale reactor.9 Wang et al. studied the effects of particle sizes, duration time, and atmosphere condition on the sodium release and transformation in their pyrolysis experiments using two ZD coals.10 In Liu et al.’s work, the release characteristic of four types of sodium (H2O-soluble, NH4Ac-soluble, HCl-soluble, and insoluble) were studied.4 However, the gaseous sodium contents were obtained by a conventional offline method in those studies. In our previous work,4 the released sodium content was determined by subtracting the residual content in the ash from the total sodium content based on mass balance analysis. Disadvantages of this method included the relatively long sampling times and the subsequent laboratory work. Any variations and transient effects were ignored, since the results were averaged over a certain period of time.11 Measuring the sodium content in flue gas directly will give better understanding of the sodium-release mechanisms and their effects on fouling and deposition. © XXXX American Chemical Society

There were two main methods used to detect the gaseous alkali metals online12,13 including optical method and ionization method. Many developing techniques were based on laser technology, like planar laser induced fluorescence,14 laser-induced breakdown spectroscopy,15,16 excimer laserinduced fragmentation fluorescence,17,18 tunable diode laser absorption spectroscopy,19 and differential optical absorption spectroscopy.20 These methods were generally based on the mechanism of absorption or scattering of radiation. Alkali atoms could be readily detected by optical methods because the concerned wavelengths of Na and K were accessible with a variety of laser and conventional light sources. However, these methods not only use expensive apparatus but also are hard to realize in in situ measurement, since the devices are sensitive to the surroundings. The ionization method is widely used in gaseous alkali metal detection. Molecular beam sampling combined with molecular beam mass spectrometric (MBMS) detection is one of the typical methods. Bläsing and Müller investigated the release mechanisms of inorganic species and the influence factors using MBMS.21−23 They reported that detailed speciation could be performed by scanning mass spectra. However, the related measurement device has high consumption and poor mobility for in situ detection. Surface ionization (SI) is another practical ionization method.24 Recently, it was mainly used in the online detection of potassium compounds during biomass combustion/gasification.25−29 Considering the advantage of low-cost and easy in situ detection, this principle may be more suitable for online measurement of gaseous alkali metals. However, little Received: February 15, 2019 Revised: March 31, 2019 Published: April 18, 2019 A

DOI: 10.1021/acs.energyfuels.9b00473 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Detector probe measuring gaseous alkali metals.

Figure 2. Ionization unit.

Figure 3. Calibration experiment setup. induct electric signals, which are proportional to the concentrations of the gaseous compounds. By monitoring the electric signals, contents of the specific composition can be detected and determined. The ionization fraction of the specific element depends on its ionization potential and the work function of the Pt surface, according to the Saha−Langmuir eq 1, and the ionization probability may be defined by eq 2.11,24,28 ÄÅ É ÅÅ e(φ − IP) ÑÑÑ g n ÑÑ α = + = + expÅÅÅÅ Ñ ÅÅÇ kBT ÑÑÑÖ n0 g0 (1)

information was reported using this method to detect gaseous sodium during high-alkali coal combustion. In this work, a detector probe was designed and made based on the SI method. To verify the mechanism, the measurement system was first calibrated by NaCl samples. Then, it was applied to measure gaseous sodium in flue gas directly. ZD coal samples containing different contents of sodium were tested to study the sodium-release mechanisms during the heating process.

β = α /(α + 1)

2. EXPERIMENTS

(2)

where α is the ratio of ions to neutrals, g+/g0 is the ratio of the statistical sum of states of the ions and neutrals (0.5 for alkali metals), and e, φ, IP, and kB are the elementary charge, work function, ionization potential, and Boltzmann constant, respectively.

2.1. Experimental Principle and Measurement Technique. Experiment technique is based on the surface ionization principle. Because specific gaseous species may be adsorbed and ionized on a high-temperature platinum (Pt) surface, those ionized elements will B


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Energy & Fuels For most elements, their ionization potential values are higher than the work function of Pt (5.5 eV), leading to an extremely low value of ionization probability (β ≈ 0). However, for alkali metals (Na, K), their ionization potentials (IPNa = 5.14 eV, IPK = 4.34 eV)11 are lower than the work function of Pt. This indicates that alkali metals may be ionized to a high degree. Based on this special ionization characteristic, the alkali metals can be detected by the SI method effectively. Davidsson et al. pointed out that the SI method could give a lower detection limit of the sub-parts per billion level.11 A detector probe was designed to measure gaseous alkali metals directly based on this principle. It includes three parts: ionization unit, voltage booster, and acquisition unit, as shown in Figure 1. The ionization unit consists of a Pt filament (purity of 99.99%), corundum bushing, Ta pellet (purity of 99.95%), and thermocouple, as shown in Figure 2. The Pt filament is made of a twisted Pt wire 0.35 mm in diameter and used as an ion generator, whereas the Ta pellet is used as an ion collector. A direct current (DC)−DC voltage booster is used to generate 400 V potential between the ion generator and ion collector to force the ions to migrate to the collector. Meanwhile, a small current will be generated in the ion collector and then amplified and recorded by an acquisition unit (Agilent 34970). 2.2. Calibration. An online testing system was designed and set up for quantitative calibration experiments to study the correlation between the detector signals and the alkali vaporizing rates. As shown in Figure 3, an alundum tube (inner diameter of 55 mm) was inserted through two closely arranged tube furnaces. The detector probe was put at one end of the alundum tube. Analytical reagent NaCl (Sinopharm Chemical Reagent Co., Ltd.) was selected as the experimental sample during calibration. During the calibration experiment, 1.5 g NaCl samples were put into an alundum boat (length × width × height = 20 mm × 20 mm × 15 mm). A N2 carrier gas flow of 0.25 L/min passed through the alundum tube, which swept away the tube gas through the whole experiment process, including the cooling step. After each test, the alundum tube was taken out and cleaned with hot water. Since the vaporizing rate of NaCl was related to the furnace temperature Ts, two heating processes were carried out to obtain the sodium vaporizing rate Rs at Ts. In step (1), when the NaCl sample and N2 carrier gas was set, furnace B was heated to Ts (heating rate: 15 °C/min) and then turned off immediately. After cooling down, the alundum boat was taken out for weighing. The sample weight loss was recorded as Δm1. In step (2), after the NaCl sample and N2 carrier gas was ready, furnace B was heated to Ts (heating rate: 15 °C/min) and kept at Ts for t seconds then shut down. The alundum boat was taken out and weighed after the cooling process. The decreased sample weight was recorded as Δm2. Thus, the value of Rs at Ts can be calculated using eq 3. Considering the quite lower value of vaporizing rate, the vaporizing time (t) was set as 1800 s to enlarge the vaporizing contents of NaCl samples. By this way, the weighing error could be significantly reduced. It should be noted that both step (1) and (2) were involved in the cooling process. Since the furnace was heated up to the same temperature in step (1) and step (2), the cooling process would proceed in the same way for these two steps. Thus, the sample weight loss of these two steps during the cooling process was considered the same. By subtracting, the error caused by sample loss during the cooling process could be avoided. The operation parameters in the weighing process are shown in Table 1. Three replicate tests were carried out for the same Ts. In this work, the samples were weighed by an electronic balance (Mellter Toledo LE204E). Its measurement accuracy is 0.1 mg. Rs =

Δm2 − Δm1 23 × t 58.5

Table 1. Operating Parameters in the Weighing Process number

temperature Ts (°C)

initial samples contents (g)

samples vaporizing time t (s)

N2 flow rate (L/min)

1 2 3 4 5 6

830 850 900 950 1000 1070

1.5 1.5 1.5 1.5 1.5 1.5

1800 1800 1800 1800 1800 1800

0.25 0.25 0.25 0.25 0.25 0.25

the Pt surface temperature at 1000 °C. This temperature is selected for two reasons: (1) enhancing the decomposition of gaseous alkali compounds and (2) preventing the Pt filament from fusing due to its quite thinner diameter (0.35 mm). Taking this temperature into eqs 1 and 2, the theoretical ionization probability of sodium reaches 93%. By changing the vaporizing temperature Ts, a series data of vaporizing rates Rs and voltage signals Vs may be obtained. Figure 4 shows a typical calibration curve, and the standard deviation is given as an error bar. The detected voltage Vs and the

Figure 4. Calibration curve between sodium vaporizing rate and voltage. vaporizing rate Rs shows a linear relationship. As the furnace temperature Ts rises, the NaCl vaporizing content increases, which enlarges the detected voltage signal. This calibration curve can be applied for other sodium compounds according to Jäglid et al.’s work.24 He reported that the calibration curve was independent of the sodium salt species. A mathematical expression of the calibration curve is given by eq 4 Vs = 22347 × R s + 3.927 × 10−4 , R2 = 0.994

(4)

2.3. Coal Sample Preparation. Zhundong (ZD) coal was selected as the test samples in this study due to its high sodium content. The samples were crushed and sieved to a size range of 0.125−0.25 mm. Proximate and ultimate analysis, ash composition, and sodium occurrence are given in Tables 2−4, respectively. Representative coal samples containing different sodium contents were prepared. The raw ZD coal samples were first treated by chemical extraction method3,9 to remove the soluble sodium to obtain the washed coal samples, which were labeled as d-ZD. Then, the d-ZD coal samples were treated by artificially adding Na-based materials using NaCl or Na2SO4.3 With this procedure, the coal samples containing different levels of sodium contents were prepared and labeled as d-ZD-NaCl (Na2SO4)-1% (2%, 3%), which represented the Na/coal mass ratio of 1, 2, and 3%. Testing in the experimental system in Figure 3 with the same procedure as that in the calibration process, the relationship between

(3)

To get the detector signal Vs at varied Rs, the following step is done. After the NaCl sample and N2 carrier gas were set, the temperature of tube furnace A was increased and kept at 1000 °C, whereas that of tube furnace B increased at a rate of 15 °C/min to a specified temperature Ts. When Ts stayed stable, the voltage signals Vs were recorded continuously, and averaged over a period of time. The temperature of furnace A was set at 1000 °C to heat up and remain C


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Energy & Fuels Table 2. Proximate and Ultimate Analysis proximate analysis (ad, wt %)a M V A FC

ultimate analysis (ad, wt %)

11.17 27.91 6.43 54.49

C H N S O

62.89 3.04 0.55 0.51 15.41

a

ad refers to air-dried basis.

Table 3. Ash Composition Analysis ash composition analysis (wt %) SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

SO3

41.57

11.16

4.79

16.21

6.48

0.51

3.5

8.05

Figure 5. Voltage signal varies with temperature.

higher than that of H2O-soluble potassium (43 μg/g coal) for this coal.4 By subtracting the data of the two curves in Figure 5 and combining with the calibration expression in eq 4, the sodiumrelease rates at different temperatures can be determined and are shown in Figure 6. It can be seen that a sharp increase of

Table 4. Sodium Occurrence in Coal Na occurrence (wt %) H2O-soluble 72.1

NH4Ac-soluble HCl-soluble 13.3

2.8

insoluble

total content (μg/g)

11.8

2260

the detected voltages and furnace temperatures was obtained for each prepared coal sample. The tested coal samples included raw coal, dZD, d-ZD-NaCl-1%, d-ZD-NaCl-2%, d-ZD-NaCl-3%, d-ZD-Na2SO41%, d-ZD-Na2SO4-2%, and d-ZD-Na2SO4-3%. For each test, 1 g coal samples were put into the alundum boat in the furnace. A N2 carrier gas flow was controlled at the rate of 0.25 L/min through the experiment. As the temperature of furnace A was up to 1000 °C, tube furnace B started to heat at a rate of 15 °C/min. The voltage signals were continuously recorded during the heating process. Test for each sample was repeated for three times. The sodium-release behaviors of these coal samples were studied.

3. RESULTS AND DISCUSSION This study focuses on the release behavior of sodium compounds for the raw coal, washed coal, and coal samples with additives. Comparisons of the detected signals from those different samples were carried out. To better show the relationship between the furnace temperature and sodiumrelease rate during the heating process, the X-axis is set as furnace temperature. Time resolution of the recorded data points during each experiment is 5 s. 3.1. Sodium-Release Behavior of the Raw Coal. Experimental results of the raw ZD coal and d-ZD coal samples are discussed in this section. Figure 5 shows the continuous record of the voltage signals from the probe as the furnace temperature increases. During the heating process, voltage signals of both coal samples increased below 475 °C and then decreased. It shows that the voltage values of raw ZD coal are higher than those of the d-ZD coal samples in the range of 300−600 °C in Figure 5. This is caused by the release and ionization of alkali metals. It should be noted that, for the d-ZD coal samples, both soluble potassium and sodium compounds were removed during the extraction process. Hence, the voltage differences between the two curves in Figure 5 correspond to the total alkali metal (K + Na) release rates. However, the voltage signals caused by potassium could be ignored. The reason is that the released alkali metals mainly came from the H2O-soluble form in the raw coal30,31 during the temperature range in Figure 5, whereas the content of H2O-soluble sodium (1630 μg/g coal) is much

Figure 6. Sodium-release rate varies with temperature.

sodium-release rate begins at about 325 °C and then decreases after it reaches the maximum value. The release rate values are fitted well by a quadratic curve in the temperature range of 335−575 °C. Equation 5 gives the expression of the curve. Therefore, the sodium compounds in the raw coal samples vaporize in the temperature range of 300−600 °C, and the highest release rate appears at 457 °C. R s = −1.50 × 10−10 × T 2 + 1.37 × 10−7 × T − 2.89 × 10−5, R2 = 0.943

(5)

Since the heating rate of furnace B remains constant (15 °C/ min), the abscissa parameter (temperature) in Figure 6 may also represent the experiment time. By integrating over the curve at a certain experiment period, the total release contents of sodium can be obtained. Figure 7 gives the comparisons of the sodium-release ratios (mR,Na/mT,Na) of ZD coal among the results in this work and those in literature32 at five temperatures (400, 450, 500, 550, and 600 °C), where mT,Na is the total sodium content. Similar tendency of the sodiumrelease ratio variation is observed. Higher temperature leads to more released sodium. D


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Energy & Fuels

Figure 7. Comparisons of the sodium-release ratio results with literatures.

Figure 9. Voltage signals for Na2SO4 addition samples.

sodium-release rate for NaCl addition samples is higher than that for Na2SO4 addition samples. Using eq 4, the sodium-release rates of the above coal samples are calculated based on the data in Figures 8 and 9. As shown in Figures 10 and 11, the release rates experience first

Generally, sodium in coal was classified as H2O-soluble, NH4Ac-soluble, HCl-soluble, and insoluble.3,4,9 van Eyk et al.31 proposed that portion of H2O-soluble sodium was released from coal during the devolatilization stage, whereas negligible organically bound sodium was released during this period. The possible release mechanism of H2O-soluble sodium is the direct vaporization of NaCl (eq 6).30 Comparing the result in Figure 7 with the H2O-soluble content in Table 3, it is found that some of the H2O-soluble sodium still remains in the coal particle at 600 °C. It is suggested that a possible mechanism is the reaction between carboxylic acid groups and NaCl (eq 7).33 NaCl → NaCl(g)

(6)

−(COOH) + NaCl → −(COONa) + HCl

(7)

3.2. Sodium-Release Behavior of the Coal Samples with Additives. The direct measurements on coal samples with NaCl/Na2SO4 addition are carried out. As shown in Figures 8 and 9, the signal curves of different coal samples Figure 10. Sodium-release rate for NaCl addition samples.

Figure 8. Voltage signals for NaCl addition samples.

Figure 11. Sodium-release rate for Na2SO4 addition samples.

increased first and then decreased in 350−600 °C. The detected voltages of the coal samples with additives are higher than those of washed coal samples (d-ZD), indicating that the added H2O-soluble sodium in coal samples are released during the heating process. Higher addition ratio results in higher voltage signals for both NaCl and Na2SO4 addition samples. However, comparing the results of different additives at the same addition ratio (e.g., 3%), the peak voltage value in Figure 8 is higher than that in Figure 9. It indicates that the maximum

an increase and then a decrease for these coal samples. Some difference is found for the coal samples with 3% Na addition: (1) the maximum sodium-release rate shifts to left side for both NaCl and Na2SO4 addition samples and (2) for lower adding ratio (1 or 2%), the sodium-release rates increase rapidly at about 450 °C, whereas this temperature shifts to about 400 °C for higher addition ratio (3%). The reasons of this phenomenon are as follows. E


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Energy & Fuels The previous literature showed that some of the alkali species in coal would be bounded into the char matrix during coal pyrolysis at a slow heating rate,34,35 as shown in reactions 7−9, where CM denotes the char matrix. The char-bonded sodium (CM-Na) was hard to vaporize35 and would be retained in char. However, Quyn et al.36 found that the “Na holding capacity” of the char was limited. In his study, the Na content in the resulting char was not changed, as the NaCl loading increased from 1.48 to 2.09 wt % during the pyrolysis process.36 NaCl‐loaded or Na‐form coal → CM − Na + Na + volatiles

(8)

CM − COONa → CM − Na + CO2

(9)

For the coal samples in this work, it could be inferred that as the sodium addition ratio increases from 2 to 3%, the Naholding capacity of char is saturated. Hence, the contents of the char-bonded sodium (CM-Na) are not changed, and more H2O-soluble sodium will release directly from the coal during the heating process. That is the reason why the highest release rate is significantly shifted to lower temperature for the samples with 3% sodium addition. For coal samples with lower adding ratio (1 or 2%), the Naholding capacity of char is not saturated. Since the reactivity of reactions 8 and 9 is decreased with increasing temperature,36 the H2O-soluble sodium starts to vaporize at a higher temperature. As the sodium addition ratio increased from 0 to 2%, the contents of H2O-soluble sodium are increased, which will enlarge the sodium-release rate and extend the vaporizing time. That is the reason why the highest sodiumrelease rates shift to higher temperature. The sodium-release contents of the above coal samples (mR,Na) at five temperatures (400, 450, 500, 550, and 600 °C) are calculated and shown in Figure 12. It shows that higher

Figure 13. Sodium-release ratios for different coal samples.

of sodium in the coal was an important factor influencing the volatilization of sodium. However, it is difficult to conclude that Cl− has a greater effect on the sodium release based on the results in Figures 12 and 13, since the contents of Cl− and SO42− are different at the same sodium addition ratio. Hence, the expression of mR,Na/mCl−(SO2− 4 ) is used to represent the sodium-release content per gram of anions (Cl−, SO42−). As shown in Figure 14, the values of mR,Na/mCl− are higher than

Figure 14. Sodium-release content per gram of anions.

those of mR,Na/mSO42− at the same addition ratio, which indicates that Cl− has a greater influence on sodium release. Hence, more attention should be paid to those coal with higher contents of Cl and Na. Figure 12. Sodium-release contents for different coal samples.

4. CONCLUSIONS An alkali metal detection probe was designed to measure gaseous alkali metals directly based on the SI method. A linear correlation between the detected voltage signals and the sodium vaporizing rates is obtained by calibration experiments. Using the alkali metals detection probe, the release behavior of sodium compounds in raw coal, washed coal, and coal samples with additives was tested. Results show that the sodium compounds in raw coal vaporize in the temperature range of 300−600 °C and the highest release rate occurs at about 457 °C. Higher addition ratio may result in more released sodium for both NaCl and Na2SO4 addition samples.

temperatures lead to more released content of sodium. The total release content increases as the addition proportion rises from 1 to 3%. Figure 13 gives the sodium-release ratio (mR,Na/ mT,Na) for different coal samples. At the same addition ratio, the NaCl addition samples have higher sodium-release ratios than those of Na2SO4 addition samples. As shown in Figures 12 and 13, different anions (Cl−, SO42−) result in different sodium-release contents at the same addition ratio. Experiments reported by Kosminski et al.8 and Quyn et al.34 also showed that chemical and/or physical form F


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Energy & Fuels Compared with SO42−, Cl− has a greater influence on sodium release. This work verified the availability of the alkali metal detection probe. It may be applied to measure the gaseous alkali metals contents online during high-alkali coal combustion in a boiler.

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(9) Li, G.; Wang, C.; Yan, Y.; Jin, X.; Liu, Y.; Che, D. Release and transformation of sodium during combustion of Zhundong coals. J. Energy Inst. 2016, 89, 48−56. (10) Wang, C. A.; Jin, X.; Wang, Y.; Yan, Y.; Cui, J.; Liu, Y.; Che, D. Release and Transformation of Sodium during Pyrolysis of Zhundong Coals. Energy Fuels 2015, 29, 78−85. (11) Davidsson, K. O.; Engvall, K.; Hagström, M.; Korsgren, J. G.; Lönn, B.; Pettersson, J. B. C. A Surface Ionization Instrument for OnLine Measurements of Alkali Metal Components in Combustion: Instrument Description and Applications. Energy Fuels 2002, 16, 1369−1377. (12) Monkhouse, P. On-line diagnostic methods for metal species in industrial process gas. Prog. Energy Combust. Sci. 2002, 28, 331−381. (13) Monkhouse, P. On-line spectroscopic and spectrometric methods for the determination of metal species in industrial processes. Prog. Energy Combust. Sci. 2011, 37, 125−171. (14) van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J. Quantitative measurement of atomic sodium in the plume of a single burning coal particle. Combust. Flame 2008, 155, 529−537. (15) He, Y.; Zhu, J.; Li, B.; Wang, Z.; Li, Z.; Aldén, M.; Cen, K. Insitu Measurement of Sodium and Potassium Release during Oxy-Fuel Combustion of Lignite using Laser-Induced Breakdown Spectroscopy: Effects of O2 and CO2 Concentration. Energy Fuels 2013, 27, 1123−1130. (16) Fatehi, H.; He, Y.; Wang, Z.; Li, Z. S.; Bai, X. S.; Aldén, M.; Cen, K. F. LIBS measurements and numerical studies of potassium release during biomass gasification. Proc. Combust. Inst. 2015, 35, 2389−2396. (17) Erbel, C.; Mayerhofer, M.; Monkhouse, P.; Gaderer, M.; Spliethoff, H. Continuous in situ measurements of alkali species in the gasification of biomass. Proc. Combust. Inst. 2013, 34, 2331−2338. (18) van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J. Simultaneous measurements of the release of atomic sodium, particle diameter and particle temperature for a single burning coal particle. Proc. Combust. Inst. 2009, 32, 2099−2106. (19) Schlosser, E.; Fernholz, T.; Teichert, H.; Ebert, V. In situ detection of potassium atoms in high-temperature coal-combustion systems using near-infrared-diode lasers. Spectrochim. Acta, Part A 2002, 58, 2347−2359. (20) Li, B.; Sun, Z.; Li, Z.; Aldén, M.; Jakobsen, J. G.; Hansen, S.; Glarborg, P. Post-flame gas-phase sulfation of potassium chloride. Combust. Flame 2013, 160, 959−969. (21) Bläsing, M.; Müller, M. Influence of pressure on the release of inorganic species during high temperature gasification of coal. Fuel 2011, 90, 2326−2333. (22) Bläsing, M.; Müller, M. Investigations on the influence of steam on the release of sodium, potassium, chlorine, and sulphur species during high temperature gasification of coal. Fuel 2012, 94, 137−143. (23) Bläsing, M.; Müller, M. Mass spectrometric investigations on the release of inorganic species during gasification and combustion of Rhenish lignite. Fuel 2010, 89, 2417−2424. (24) Jäglid, U.; Olsson, J. G.; Pettersson, J. B. C. Detection of sodium and potassium salt particles using surface ionization at atmospheric pressure. J. Aerosol Sci. 1996, 27, 967−977. (25) Davidsson, K. O.; Pettersson, J. B. C.; Nilsson, R. Fertiliser influence on alkali release during straw pyrolysis. Fuel 2002, 81, 259− 262. (26) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Jäglid, U. The effects of fuel washing techniques on alkali release from biomass. Fuel 2002, 81, 137−142. (27) Tran, K.; Iisa, K.; Steenari, B.; Lindqvist, O. A kinetic study of gaseous alkali capture by kaolin in the fixed bed reactor equipped with an alkali detector. Fuel 2005, 84, 169−175. (28) Tran, K.; Iisa, K.; Hagström, M.; Steenari, B.; Lindqvist, O.; Pettersson, J. B. C. On the application of surface ionization detector for the study of alkali capture by kaolin in a fixed bed reactor. Fuel 2004, 83, 807−812. (29) Svane, M.; Hagström, M.; Pettersson, J. B. C. Online Measurements of Individual Alkali-Containing Particles Formed in

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Leming Cheng: 0000-0003-3643-2617 Yanquan Liu: 0000-0002-4972-9420 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2018YFB0605403). NOMENCLATURE mR,Na = sodium-release content (g) mT,Na = total sodium content in coal (g) MBMS = molecular beam mass spectrometric Pt = platinum R2 = coefficient of determination Rs = sodium vaporizing rate (g/s) SI = surface ionization t = time interval (s) Ta = tantalum Ts = furnace temperature (°C) Vs = detected voltage signal (V) ZD = Zhundong

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Direct measurement of gaseous sodium in flue gas for high alkali coal

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