CO2 combustion in circulating fluidized bed

CO2 combustion in circulating fluidized bed

Energy 185 (2019) 254e261 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Transformation characte...

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Energy 185 (2019) 254e261

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Transformation characteristics of sodium, chlorine and sulfur of Zhundong coal during O2/CO2 combustion in circulating fluidized bed Dianbin Liu a, b, Wei Li a, Shiyuan Li a, b, *, Wenhao Song a, b, Daofeng Liu a, b, Runjuan Kong a, b a b

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 May 2019 Received in revised form 27 June 2019 Accepted 6 July 2019 Available online 9 July 2019

Transformation characteristics of sodium, sulfur and chlorine during the high-sodium Zhundong coal combustion were investigated by performing both air combustion and O2/CO2 combustion experiments in a 50 kW circulating fluidized bed testing system. Via analyzing the chemical compositions and crystalline phases of ash samples, the following results were obtained. In comparison with air combustion, sodium release into gas phase was inhibited under O2/CO2 combustion atmosphere at the identical oxygen concentrations. While increasing the inlet oxygen concentration under O2/CO2 combustion could promote the release of sodium. Chlorine almost finished total release from the coal under all experimental conditions. Besides, sulfation reaction affected the transformation of sodium, sulfur and chlorine markedly, for it could alter the chemical form of sodium from chloride to sulfate and further influence the release and condensation process of sodium during the combustion process. In particular, elevating the inlet oxygen concentration could promote the sulfation reaction and reduce the content of chlorine in deposits significantly. © 2019 Elsevier Ltd. All rights reserved.

Keywords: O2/CO2 combustion Circulating fluidized bed High-sodium coal Na/S/Cl transformation

1. Introduction Alkali metals contained in coal can intensify ash-related problems, such as slagging, fouling and corrosion. Originated from the largest integrated coalfield in China, Zhundong coal is characterized with high alkali content. Notably, severe ash-related problems arose during the utilization of high-sodium Zhundong coal in coalfired boilers [1e3]. To reveal the underlying mechanism of such problems and in turn provide guidance for the appropriate utilization of Zhundong coal, extensive researches on the behaviors of alkali metal during coal combustion have been carried out [4e12]. In particular, Li et al. [13] found that chlorine could impose a significant influence on sodium transformation through the laboratory-scale combustion experiments in a tube-quartz reactor. Besides, Nielsen et al. [14] pointed out that Cl-based species accounted primarily for the corrosion during combustion of biomass and some special fuels (such as high-chlorine coal). On the

* Corresponding author. Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (S. Li). https://doi.org/10.1016/j.energy.2019.07.043 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

other hand, Kassman's results [15e18] about SO2/SO3 alleviating the ash-related issues by sulfating the alkali metals during biomass circulating fluidized bed (CFB) combustion provide important references for further investigation on Zhundong coal. Therefore, transformation characteristics of sodium and related elements (including chlorine and sulfur) should be well understood during the combustion process of high-sodium Zhundong coal. Employing a full-scale boiler (350 MW) and a lab-scale fixed bed reactor, Wang et al. [19] investigated Ca/Na/S/Cl transformation during combustion and looked deeply into the ash deposition mechanism. Their results indicated that the formation and condensation of Na2SO4 and CaSO4 aerosols functioned critically upon the ash deposition on convection heating surfaces. Drastic climate change has attracted more and more attention on biomass energy and Carbon Capture and Storage (CCS) technologies [20e22]. Oxy-fuel combustion is performed with pure oxygen diluted by recycled flue gas and can achieve high CO2 concentration in flue gas, making it one of the most promising CCS technologies [23]. Under oxy-fuel combustion atmosphere (inlet oxygen concentration >21%), more fuels would be consumed, while the flue gas volume changed insignificantly, resulting in higher SO2 concentration (in ppm) than air combustion. Once flue gas was

D. Liu et al. / Energy 185 (2019) 254e261

recycled under oxy-fuel combustion atmosphere, SO2 concentration in flue gas would be further accumulated, usually three to five times that under air combustion atmosphere [24,25], which is conducive to the sulfation reaction. In addition, combustion environment under oxy-fuel combustion is starkly distinct with the one under air combustion, which can make a difference in Na behaviors. Numerous studies have been conducted on the influence of oxyfuel combustion atmosphere on the behaviors of alkali metals. For instance, He et al. [26] investigated the atomic sodium emission during Zhundong coal combustion under low O2 concentration (3.9e10.6%, oxy-fuel combustion) and found the ratio of sodium release from the char was enhanced by O2 but inhibited by CO2. Li et al. [27] further reported that despite that air combustion and oxy-fuel combustion led to alike main mineral phases in fly ash, Na and S contents were reduced under oxy-fuel combustion. Besides, the comparison between sodium conversion in CO2 and in N2 revealed CO2 inhibited the release and the soluble-insoluble conversion of sodium [10,28], while SO2 was also proved inhibitive toward Na and K release during oxy-fuel combustion owing to the formation of stable sulfates [29,30]. Moreover, Ekvall's specialized study on gas phase KeCleS chemistry demonstrated that KCl sulfation was substantially improved under oxy-fuel combustion than under air combustion [31]. CFB combustion, operating at relatively low temperature (850e950  C), may enable the feasible utilization of high-sodium Zhundong coal. However, some of the aforementioned studies were carried out on the pulverized coal combustion systems, which were not applicable to CFB combustion. Together with the insufficient understanding of sodium transformation of Zhundong coal during oxy-fuel combustion, it is urgent to look into the transformation characteristics of sodium and relevant elements so as to popularize the utilization of such high-sodium coal. To provide clues for further investigation on high-alkali fuels under oxy-fuel combustion atmosphere, the present work aimed to clarify the influence of the combustion atmosphere converted from air combustion to oxy-fuel combustion (O2/CO2) on the transformation characteristics of Na/S/Cl. Herein, the transformation characteristics of Na/S/Cl under O2/CO2 combustion atmosphere have been systematically investigated, and the differences between air and O2/CO2 combustion atmospheres were thereby well identified in the present work. The effect of inlet oxygen concentration during O2/CO2 combustion was also included.

2. Experimental

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Table 2 Composition of Shenhua Zhundong coal ash (wt.%). SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

P2O5

K2O

Na2O

17.24

11.90

5.76

28.74

5.34

0.60

19.58

0.05

0.38

3.92

Fig. 1. Schematic diagram of the experimental apparatus.

2.2. Experimental system The 50 kW CFB combustor system employed in the present work could operate under both air and O2/CO2 combustion modes, as shown in Fig. 1. Detailed information of the experimental system can be inquired in previous papers [27,32]. In brief, the experimental system is composed of a CFB test rig, a gas cooling unit, a coal feed unit, a gas supply unit, as well as an integrated unit for measurement and data acquisition. Oxygen and carbon dioxide were supplied by a liquid oxygen tank (O2>99.6%) and cylinder group (CO2>99.5%), respectively. Liquid oxygen was vaporized by a vaporizer, while carbon dioxide was heated by an electric heater. Then pressure reducing valves were used to control the pressure of these two gases. The flow rates of both oxygen and carbon dioxide were controlled by individual mass flowmeters. These gases were thoroughly mixed in a mixer before being introduced into the riser. The inlet oxygen concentration was determined by the following equation:

2.1. Experimental material Shenhua Zhundong coal, a typical high-sodium coal of inferior grade, was selected for experiments, while silicon sand that consisted primarily of SiO2 was used as the bed material. Prior to the experiments, coal and silicon sand were grinded and then sieved individually into size ranges of 0.1 mme1 mm and 0.18 mme0.355 mm. According to the fuel and ash analyses summarized in Tables 1 and 2, resultant ash of such high-sodium coal contained 3.92% Na2O. Meanwhile, the contents of CaO and SO3 account for as high as 28.74% and 19.58%, respectively.

Fig. 2. Schematic diagram of the deposition probe.

Table 1 Fuel analysis of Shenhua Zhundong coal. Proximate analysis (wt.%), ar M 19.60

A 4.79

Ultimate analysis (wt.%), ar V 32.46

FC 43.14

C 51.86

H 1.62

O 21.09

N 0.66

S 0.38

Cl 0.10

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atmospheres, and the inlet oxygen concentration for the latter varied from 21% to 40%. The specific operating parameters under experimental conditions are listed in Table 3. Temperature profiles along the riser height under different combustion conditions were depicted in Fig. 4. The temperature profile of air combustion condition performed a close trend to the 30% O2/CO2 combustion condition, which was higher than 21% O2/ CO2 condition but lower than 40% O2/CO2 condition. On one hand, the heat capacity of CO2 is higher than N2 and CO2 has a cooling effect on the combustion media, accordingly the coal particle ignition and the flame propagation would be retarded in O2/CO2 atmosphere [33]. As a result, the overall combustion rate of carbon particle under 21% O2/CO2 combustion atmosphere was lower than that under air combustion. On the other hand, the combustion of carbon particle can be enhanced with increasing inlet oxygen concentration, leading to a higher combustion temperature. Under the experimental conditions employed in the present work, combustion temperature of 30% O2/CO2 condition was close to that of air condition. Fig. 3. Temperature variation with time for 30% O2/CO2 condition.

2.4. Analysis method

Inlet oxygen concentration ¼ VO2



VO2 þ VCO2



(1)

where VO2 was the flow rate of inlet oxygen, L/min; VCO2 was the flow rate of inlet carbon dioxide, L/min. The fluctuation in oxygen supply was quite small, within 1%, during the experiments. When each experiment was finished under a specific condition, bottom ash, circulating ash and fly ash were separately collected from the riser bottom, the loop seal and the bottom of flue gas cooler (see Fig. 1). Deposits were collected by a deposition probe, of which the schematic diagram is shown in Fig. 2. For the collection of effective deposits, the deposition probe was not inserted into the flue gas (see Fig. 1) until the experimental condition remained stable (variation of temperature and flue gas compositions less than 5%) for over half an hour. Then, more than 3 h’ stable operation under each condition was the key to collection of valid and sufficient ash samples. Temperature variation with time during 30% O2/CO2 experimental condition was illustrated in Fig. 3. Deposition probe was inserted into the flue gas at about 1:00 in Fig. 3. Temperatures at the inner (T1) and outer (T2) end surfaces of the probe were measured by two thermocouples, which were then averaged as the probe temperature. The probe temperature could be adjusted by the cooling media of compressed air under different conditions. While the probe temperature could keep at 800 Ke870 K and the difference between T2 and T1 was within 10 K without cooling under the experimental conditions employed.

Chemical compositions and crystalline phases of the ash samples were analyzed via X-ray fluorescence spectroscopy (XRF, AXIOS-MAX, PANalytical B.V.) and X-ray diffraction (XRD, Empyrean, PANalytical B.V.), respectively, which provided clues for the transformation mechanism of sodium during Zhundong coal combustion. The oxygen concentration of flue gas was measured by a zirconia oxygen analyzer, while other components along with their

2.3. Experimental conditions Experiments were carried out under air or O2/CO2 combustion

Fig. 4. Temperature profiles of the riser under different combustion conditions.

Table 3 Experimental conditions. Condition

Air

21% O2/CO2

30% O2/CO2

40% O2/CO2

Atmosphere Inlet oxygen concentration, % Fluidized velocity, m/s O2 in flue gas, % CO2 in flue gas, % Deposition probe temperature, K In-situ temperature in deposition probe, K

Air 21 2.25 ± 0.1 5.6 ± 0.6 14.5 ± 1.5 805 ± 5 925 ± 5

O2/CO2 21 2.25 ± 0.1 5.8 ± 0.6 91.2 ± 1.6 835 ± 5 955 ± 5

O2/CO2 30 2.25 ± 0.1 6.3 ± 0.6 89.7 ± 2 825 ± 5 955 ± 5

O2/CO2 40 2.25 ± 0.1 6 ± 0.6 90.4 ± 1.8 865 ± 5 975 ± 5

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concentrations of the flue gas were monitored online by a FTIR analyzer (GASMET DX4000) in real time. Under O2/CO2 combustion atmosphere, the higher the inlet oxygen concentration was, the less the flue gas would be generated at identical fuel consumption. To ensure the comparability between different conditions, emissions were expressed as the mass at per unit energy input, with a unit of mg/MJ. 3. Results and discussion 3.1. Distribution of Na, S and Cl among ash samples It was worth noting that bottom ash and circulating ash were mixed with a large amount of bed material (mainly SiO2). Therefore, contents of all the compositions were calculated on a silicafree basis so as to eliminate the effect of bed material, which was similar to that used by Song et al. [7]. Fig. 5 presented Na/S/Cl distribution as well as CO/HCl/SO2 emission concentrations in flue gas under different experimental conditions, through which the significant influence of combustion atmosphere on the transformation of sodium can be clearly observed. As was shown in Fig. 5a, the sodium content in bottom ash and circulating ash was remarkably lower under air combustion condition than that under 21% O2/CO2 condition. Accordingly, sodium release into the gas phase was higher in the former condition than in the latter. This should be ascribed to the inhibitive effect of CO2 on sodium release comparing with N2 [28]. Besides, the inlet oxygen concentration also played a critical part during sodium transformation. It was noticed that with the increase of inlet oxygen concentration, the sodium content declined in bottom ash and circulating ash while an

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opposite tendency occurred for the sodium variation within deposits and fly ash. Also, the ash with the highest sodium content changed gradually from the circulating ash to the deposits. This might be because a higher inlet oxygen concentration could promote the combustion process and thereby conduce to sodium release. Consequently, more sodium would condense on the deposition probe. However, the sodium content in deposits was relatively low in air combustion condition, attributable possibly to the differences in flue gas properties and sodium chemical forms between these two combustion atmospheres. Sulfation of alkali is among the most important reactions affecting sodium transformation in the combustion process. Results in Fig. 5a and Fig. 5b indicated similar distribution patterns of sodium and sulfur under the O2/CO2 combustion atmosphere except that S might be partially captured by Ca, thereby leading to slight differences in their distribution. In view of the powerful influence of chlorine on s release as reported by Li et al. [13], Fig. 5c gave Cl distribution patterns on the silica-free basis under differnent experimental conditions. As can be seen, little amount of chlorine was detected in bottom ash and circulating ash, while great chlorine enrichment was exhibited in the fly ash under air combustion atmosphere. As for the O2/CO2 conditions, chlorine content in deposits and fly ash dropped sharply with the increase of inlet oxygen concentration, which was roughly opposite to the variation of S distribution. The emission concentrations of CO, HCl, and SO2 under different experimental conditions were further summarized in Fig. 5d. Noticeably, no SO2 was detected under the all experimental conditions. From Tables 1 and 2, it can be concluded that S contents in coal and in ash are insignificantly different, which means a large

Fig. 5. Distribution of Na, S and Cl in four ash specimens and emission concentrations of gas compositions CO, HCl and SO2 in flue gas.

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portion of S in Shenhua Zhundong coal exists in the form of sulfate. Combining the high CaO and Na2O content in this coal, it was reasonable that no SO2 was detected. As mentioned above, the combustion rate of coal particle was lower in 21% O2/CO2 atmosphere than that under air atmosphere. A lower combustion rate resulted in higher CO concentration. Also, high CO2 concentration can enhance the reaction between CO2 and char and produce more CO. In addition, elevating inlet oxygen concentration would promote the combustion and reduce the CO concentration markedly. As a result, the CO concentration for 21% O2/CO2 atmosphere was highest of all considered conditions. Moreover, the CO concentration in 30% O2/CO2 condition was close to that in the air condition, which complied with the observation that these two conditions shared alike combustion temperature. On the other hand, HCl emission was enhanced by the increase of inlet oxygen concentration. Together with the distribution of S and Cl, it can be deduced that the higher the inlet oxygen concentration was, the more chlorine in deposits and fly ash was replaced by sulfur. In other words, NaCl sulfation (R2) could be effectively promoted by higher inlet oxygen concentration.

SO2 was detected in the flue gas and that gas analysis was prior to deposit sampling, it could be further concluded that higher inlet oxygen concentration exactly promoted the homogeneous sulfation under the experimental conditions employed.

3.2. Transformation of Na, S and Cl during combustion Crystallographic analysis of the ash samples was realized by XRD for the purposes of determining the reaction process and disclosing the transformation mechanism. As was shown in Figs. 6 and 7, quartz accounted for the major phase of bottom ash and circulating ash, which was consistent with the fact that these two types of ash were mixed with a mass of bed material. Besides, the universal existence of CaSO4 and Fe2O3 in all conditions accorded well with the relatively high amount of CaO, SO3, and Fe2O3 contained in coal. Furthermore, that Na2Al2SiO6 was merely detected under O2/CO2 combustion atmosphere might result from coal itself or be explained by following reaction.

Na2 O þ SiO2 þ Al2 O3 /Na2 Al2 SiO6

(3)

Thus, chlorine condensed less on heating surface at higher inlet oxygen concentration. Namely, the corrosion induced by chlorine could be alleviated in this way. In addition, depending on the sequence of reaction and condensation, sulfation reaction can be divided into heterogeneous sulfation and homogeneous sulfation. Compared with heterogeneous sulfation, homogeneous sulfation can finish within several seconds [34]. Combining the facts that no

Via R3, Na could be retained in the bottom ash and circulating ash, which agreed with the inhibitive influence upon sodium release by CO2 in comparison to N2. Wang et al. [10] arrived at a similar conclusion by conducting combustion experiments on a fixed bed. Moreover, Na2SO4 appeared in bottom ash when the inlet oxygen concentration exceeded 30%. Such a result further verified the promotion effect of high inlet oxygen concentration toward sodium sulfation reaction. As was shown in R4, the detected Na2Al2SiO6 could also be produced by the reaction between Na2SO4,

Fig. 6. XRD patterns of bottom ash under different combustion conditions. 1-SiO2, 2CaSO4, 3-Na2Al2SiO6, 4-Fe2O3, 5-Na2SO4.

Fig. 7. XRD patterns of circulating ash under different combustion conditions. 1-SiO2, 2-CaSO4, 3-Na2Al2SiO6, 4-Fe2O3, 5-Na2SO4.

4NaCl þ 2SO2 þ 2H2 O þ O2 /2Na2 SO4 þ 4HCl

(2)

D. Liu et al. / Energy 185 (2019) 254e261

259

SiO2, and Al2O3.

Na2 SO4 þ SiO2 þ Al2 O3 /Na2 Al2 SiO6 þ SO2 þ 0:5O2

(4)

Fig. 8 presented the XRD patterns of deposits yielded from different combustion modes, which demonstrated such main phases as CaSO4, SiO2, CaCO3, NaCl, and Na2SO4. The relative content of mineral phases could be preliminarily estimated from the relative intensity of corresponding XRD peaks [35]. It could be observed that NaCl peak diminished gradually with the increase of inlet oxygen concentration and became undetectable when inlet oxygen concentration reached 40%. Such results confirmed again that high concentration of inlet oxygen could promote NaCl sulfation and further regulate the transformations of sodium, chlorine and sulfur. The fly ash mainly comprised CaSO4, CaCO3, SiO2, MgO, CaO, NaCl and Na2SO4. And little difference was observed for its phase compositions under different experimental conditions, except that the peak intensity of SiO2 declined under higher inlet oxygen concentration (shown in Fig. 9). Similar results have been reported before [27,36,37], and it was because instead of condensation, the fly ash derived majorly from fine particles, whose characteristics were dominated by coal properties. The enhanced combustion at high inlet oxygen concentration would have more mineral species volatilized and produce more solid fine particles composed of nonvolatile mineral species. Consequently, the intensity of SiO2 peak decreased progressively. To further determine the content relationship of sodium with related elements, molar ratios between every two elements were compared in Fig. 10 and Fig. 11. Due to the extremely low content of chlorine in bottom ash and circulating ash, the ratio of Na/Cl was Fig. 9. XRD patterns of fly ash under different combustion conditios. 1-CaSO4, 2-CaCO3, 3-SiO2, 4-MgO, 5-NaCl, 6- Na2SO4, 7-Ca2AlSiO7.

not taken into account. Fig. 10 plotted the molar ratios of Na/S, Na/ Al, and Ca/S, which were found steady under the O2/CO2 combustion atmosphere, implication of close relationships among Na, S, Al, and Ca in these two types of ash. The reason for lower Na/S ratio under the air combustion atmosphere might be that the SO2 capture ability of CaO/CaCO3 was relatively high in this condition. Moreover, the Ca/S ratio was remarkably higher under 40% O2/CO2

Fig. 8. XRD patterns of deposits under different combustion conditions. 1-CaSO4, 2CaCO3, 3-SiO2, 4-MgO, 5-NaCl, 6- Na2SO4, 7-Ca2AlSiO7.

Fig. 10. Mole ratios of Na/S, Na/Al and Ca/S in bottom ash and circulating ash (bbottom ash, c-circulating ash).

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condition than under any other conditions, possibly due to the decomposition of CaSO4. Fig. 11 gave Na/S, Na/Cl, and Ca/S ratios in deposits and fly ash. The Na/S ratio basically kept unchanged and in the same range as that of the bottom ash and circulating ash, so it was reasonably inferred that Na and S have finished sulfation reaction before leaving the cyclone. With respect to Na/Cl, the ratio value rose with increased inlet oxygen whether in the deposits or in the fly ash, suggesting a weaker correlation between Na and Cl under higher inlet oxygen concentration. However, the Ca/S molar ratio herein was roughly twice that in bottom ash and circulating ash. This could be attributed to the absence of SO2 in flue gas and the inability of solid unsulfated Ca-base compounds to absorb SO2 after leaving cyclone. 3.3. Transformation mechanism of Na under different combustion modes The discussion above indicated clearly that no significant difference existed in the underlying mechanisms of sodium transformation between air combustion atmosphere and O2/CO2 combustion atmosphere, and transformation processes under the experimental conditions employed in this work can be essentially described as follows. When migrating outside the coal, sodium partially reacted with SiO2 and Al2O3 to form Na2O$xSiO2$yAl2O3. Together with those held inherently by the coal, most of the Na2O$xSiO2$yAl2O3 produced would remain in the bottom ash and circulating ash, while a little of the residual Na2O$xSiO2$yAl2O3 might escape from the cyclone to be captured by deposits or become fly ash. As for the sodium released from the coal, NaCl was the primary component with Na and Na2O being the secondary [13]. A portion of these Na-base components would be sulfated by SO2 before leaving the cyclone. Na2SO4 produced as liquid form would adhere on the surface of bed material and react partially with SiO2 and Al2O3 again, whilst NaCl, Na2SO4, and HCl in gaseous form integrated into the flue gas and condensed gradually on either the heating surfaces or the ash particles along with the reactions with other minerals. Despite a shared mechanism for sodium transformation in general, the specific processes were largely distinctive among various combustion conditions. These pathways for sodium transformation were illustrated in Fig. 12, with different line types representing the promoted processes by different experimental conditions. Compared with the air combustion, sodium release was

Fig. 12. Transformation pathways of sodium during CFB combustion (solid lines represent the processes promoted by air condition; dash lines represent the processes promoted by O2/CO2 condition which shares the identical inlet oxygen concentration as air condition; dash dot lines represent the processes promoted by O2/CO2 condition which operates at higher inlet oxygen concentration than air condition).

restricted and more sodium was retained in bottom ash and circulating ash under O2/CO2 combustion atmosphere, mainly as Na2Al2SiO6, when the inlet oxygen concentration was same as that of the air condition. Further increase in the inlet oxygen concentration under O2/CO2 combustion would promote the sulfation of sodium-base species, thereby resulting in more Na2SO4 and less NaCl in the deposits.

4. Conclusion Combustion experiments of high-sodium Zhundong coal were carried out in a 50 kW CFB combustion system to investigate the transformation characteristics of Na/S/Cl under both air and O2/CO2 combustion atmospheres. Meanwhile, the effect of inlet oxygen concentration was also discussed. The main conclusions are summarized as follows.

Fig. 11. Mole ratios of Na/S, Na/Cl and Ca/S in deposits and fly ash (d-deposit, f-fly ash).

1) Both combustion atmospheres and inlet oxygen concentration under O2/CO2 combustion could exert great impact on the transformation process of Na/S/Cl.

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2) Compared with air combustion, more sodium was retained in bottom ash and circulating ash, mainly as aluminosilicate, under O2/CO2 combustion with the identical 21% inlet oxygen concentration. Little variation of sulfur content between air and O2/ CO2 combustion atmospheres was observed. Chlorine can achieve almost complete release, mainly as NaCl, under both air and O2/CO2 combustion atmospheres, which would condense gradually in flue gas and be captured by deposit and fly ash. Thus, chlorine was enriched in deposit and fly ash. 3) Elevating inlet oxygen concentration under O2/CO2 combustion could enhance the release of sodium from bottom ash and circulating ash, due to the enhanced coal combustion. While sodium contents in deposit and fly ash increased with elevating inlet oxygen concentration. The variation tendency of sulfur with inlet oxygen concentration was similar to that of sodium. Chlorine content in deposit and fly ash can be effectively reduced by elevating inlet oxygen concentration, which was confirmed to be achieved by the homogeneous suflation of NaCl. Namely, Higher inlet oxygen concentration could promote the homogeneous sulfation of NaCl and ensure less chlorine be captured by deposit and fly ash. This would alleviate the chlorine-induced corrosion during the combustion process.

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Declaration of interests [21]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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[23]

Acknowledgements This study is supported by the National Natural Science Foundation of China (Grant 51706227).

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