Alkali metal transformation and ash deposition performance of high alkali content Zhundong coal and its gasification fly ash under circulating fluidized bed combustion

Applied Thermal Engineering 141 (2018) 29–41

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Research Paper

Alkali metal transformation and ash deposition performance of high alkali content Zhundong coal and its gasification fly ash under circulating fluidized bed combustion

T

⁎

Shaobo Yang, Guoliang Song , Yongjie Na, Zhao Yang Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

H I GH L IG H T S

(ZD) coal and its gasification fly ash are high alkali fuels. • Zhundong transformation of Na and K during high alkali fuels combustion was studied. • The release of Na during fly ash combustion was less pronounced than it was in the ZD coal. • The in the bed temperature did not greatly affect the conversion of K. • Variation • The ash deposition behavior during ZD coal and fly ash combustion were different.

A R T I C LE I N FO

A B S T R A C T

Keywords: CFB Zhundong coal Gasification fly ash Sodium Ash deposition

Alkali metal transformation and ash deposition during combustion of Zhundong (ZD) coal and its gasification fly ash (ZDf) were studied in a 0.4 t/d circulating fluidized bed. The Na in ZD coal was present primarily as Na2SO4. NaAlSi3O8 and KAlSi3O8 were the main compounds of Na and K in ZDf. Variation in the bed temperature had a significant effect on the transformation of Na, but it did not greatly affect the conversion of K. During combustion of the ZD coal, Na was mainly in flue gas, the Na in fly ash was mainly in the water soluble form, but in bottom ash it was mainly in the insoluble form. During combustion of the ZDf, Na was found mainly in the fly ash and flue gas, insoluble Na and K accounted for above 80.0% of Na and K in ashes. The release of Na during the ZDf combustion was less pronounced than that during the ZD coal combustion. The ash deposition during the ZD coal combustion occurred primarily as a result of the agglomeration and bonding of the ash particles that were rich in Na2SO4, and the deposition propensity was high. However, the ash deposition during the ZDf combustion was primarily caused by the accumulation of fine fly ash particles with a low Na content, the deposition propensity was low.

1. Introduction The growing demand for energy in China has led to an increase in the use of low quality-coal as a fuel. Zhundong (ZD) coal is usually characterized by a high volatile content, good reactivity and low ash and sulfur content, which makes it a promising fuel for power generation [1]. The ZD coal reserve in China has been estimated to be in excess of 3.9 Gt [2]. However, because of the high sodium concentration in ZD coal, a series of ash-related problems including ash deposition and slagging occur during the combustion of ZD coal [3,4]. The sodium content in ZD coal is usually about 0.2–0.9% [1,5]. To decrease the sodium base ratio and mitigate the ash-related problems, at present,

⁎

high sodium ZD coal is utilized by blending it with low sodium coal, but this approach cannot solve the basic ash-related problems [6,7]. Employing low temperatures (850–950 °C) in the gasification/ combustion process in a circulating fluidized bed (CFB) is a good method for the clean use of ZD coal [8,9]. Zhang et al. [10] found that supercritical water gasification was an effective way for the utilization of ZD coal. Alkali metal could not only play a catalytic effect in the gasification process, but also finally enriched in the solid residue in the form of aluminosilicate salt which was stable. Application of CFB gasification to ZD coal would produce a large quantity of gasification fly ash with a high carbon content. Song et al. [9,11–13] conducted a series of gasification experiments using ZD coal in a 0.4 t/d CFB gasifier. Their

Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (G. Song).

https://doi.org/10.1016/j.applthermaleng.2018.05.113 Received 16 March 2018; Received in revised form 26 May 2018; Accepted 28 May 2018 Available online 29 May 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.

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the ash deposition and corrosion of the tail heating surfaces were more pronounced during the combustion of ZD coal. These results suggest that a study of the ash deposition behaviors during combustion of ZDf is needed. The goal of the presently reported work is to clarify the chemical transformation behaviors of sodium and potassium during the combustion of ZD coal and ZDf, and provide the basic data concerning the combustion of high alkali fuels (these are ZD coal and ZDf) in a CFB. The combustion experiments of high alkali fuels were conducted at 860 °C, 910 °C and 960 °C in a 0.4 t/d CFB reactor. The basic properties of the alkali metals in ZD coal as well as ZDf and the effects of the bed temperature on the chemical transformation behaviors of alkali metals during combustion were studied using a series of analytical techniques. The differences in chemical transformation of alkali metals and the ash deposition behaviors during combustion of the ZD coal and ZDf were also discussed.

experimental results showed that the sodium content in ZD coal gasification fly ash (ZDf) was as high as 1.0–4.0%. Therefore, ZD coal and the corresponding ZDf are both high-alkali content fuels, so re-burning of the ZDf is needed to reuse this waste resource [14]. Combustion of ZD coal and ZDf in a CFB can help to reduce the contamination and slagging caused by the sodium in the fuels. Consequently, it is quite important to fully understand the transformation of alkali metals during the combustion of high alkali fuels in a CFB. In recent years, studies of the release and transformation behaviors of alkali metals during thermal conversions of ZD coal have been very popular. Sodium and potassium are the predominant alkali metals in various coals, and sodium is more predominant in ZD coal than potassium [5,15]. In general, sodium in coals can be divided into four categories: water soluble sodium, NH4Ac soluble sodium, HCl soluble sodium and insoluble sodium [10,16]. Water soluble and NH4Ac soluble sodium are easily released and are harmful during combustion [16,17]. During CFB gasification of ZD coal, some of the water soluble sodium released from the coal is condensed and enriched in the fly ash, whereas a portion of sodium can combine with the coal matrix and is preserved in the unburned carbon that is present in the fly ash. The quantities of alkali metals in the ZD coal gasification fly ash are far more concentrated than in the ZD coal. The mass fraction of HCl soluble and insoluble Na in the gasification fly ash are found to increase and the water soluble Na mass fraction usually decreased in comparison to the native ZD coal. Moreover, the gasification fly ash is usually in the form of ultrafine sized particles that contain high concentrations of ash and carbon [14]. Therefore, compared to the normal combustion of ZD coal, the unique properties of ZDf may result in new chemical transformation reactions of sodium during its combustion process. Song et al. [14] studied the release and chemical transformation of sodium in ZDf during its combustion in a vertical tube furnace. These authors found that the sodium in this fly ash was more tightly bound to the fly ash as a result of different chemical reactions that had occurred in the ZD coal during its combustion. Yang et al. [18] studied the sintering and fusion characteristics of anthracite gasification fly ash in a fluidized bed. Their results showed that the slagging tendency of the fly ash was affected by the chemical transformation of the minerals in the fly ash. The content of the fluxing components, including Fe, Ca and Mg, was higher in this fly ash, so that the ash fluid temperature of the fly ash was higher than it was in the raw coal. The chemical transformation reactions of alkali metals in coal are closely related to the operating temperatures of the combustion process. Kosminski et al. [19] found that the combustion temperature directly affected the reactions between the resident Na compounds and the resident SiO2 and Al2O3, which directly affected the Na release process. Zhang et al. [20] reported that when ZD coal was gasified at 850–1050 °C, the sodium retention ratio in the ash initially decreased, but then increased as the temperature of combustion increased. Wang et al. [21] concluded that as the combustion temperature increased from 400 °C to 800 °C, 80% of the sodium was released from the coal. Jiang et al. [22] found that during the pyrolysis of biomass at 900 °C, 53–76% of resident alkali metals escaped from the pyrolyzed char. The operating temperature of the CFB was lower than the coal ash fusion temperature, and many studies have been conducted on the release behaviors of sodium during gasification of ZD coal in CFBs [5,8,9,13]. Unfortunately, little work has been conducted on the chemical transformation characteristics of alkali metals during combustion of high alkali fuels in a CFB. This is especially true in the case of alkali metals during the re-burning of ZDf. Song et al. [8] studied the chemical transformation of sodium in coal under different atmospheres in a 0.25 t/d CFB reactor. These authors found that more sodium was released into the gas phase with an increase in temperature, and the sodium existed primarily as Na2SO4 in the combustion fly ash and as NaCl in the gasification fly ash. Qi et al. [23] found that sodium induced aggregation of ash particles and defluidization in the furnace were more pronounced when ZD coal was gasified in the CFB gasifier. In addition,

2. Experimental 2.1. Experimental system The combustion experiments of ZD coal and ZDf were conducted in a CFB test system, respectively, where the fuel feed rate of the system was 0.4 t per day. A schematic diagram of the system is shown in Fig. 1. The 0.4 t/d CFB test system was comprised of a riser, a cyclone separator and a loop seal. The height of the riser was 4200 mm and the inner diameter was 150 mm. The bottom of the riser was wrapped with a section of heating wire for ignition of the fuel in the initial stage. The air required for the experiment was provided by an air compressor. The flue gas temperatures and wall temperatures were adjusted by the slagging probes (A-G) equipped with a cooling system. In addition, the ash deposition characteristics during ZD coal and ZDf combustion were tested through the slagging probes. The schematic diagram of the slagging probes (A-G) is shown in Fig. 2. The slagging probes were composed of Cr25Ni20 (GB/T208782007). Two thermocouples were set on the outside and inside of the probe end to measure the wall temperature of the heating surface, the wall temperature was calculated from the average temperature of the two thermocouples. The slagging probes were cooled by different mediums (air or water) to adjust the flue gas temperatures and wall temperatures of the heating surfaces, Probe A and Probe B were cooled by water, and the remaining probes were cooled by air. The length of Probe A was 1150 mm, while the length of the remaining probes was 400 mm. The flue gas temperatures were measured with the thermocouples (T8–T13). All of the data, including pressures, temperatures and air volume flow rates, were collected and displayed real-time by a Programmable Logic Controller (PLC) data acquisition system. 2.2. Fuels and operating conditions A typical ZD coal and a sample of gasification fly ash from ZD coal (ZDf) were used as the fuels in the combustion experiments. The ZD coal was obtained from the Xinjiang Zhundong area of China, and ZDf was obtained from the ash collection can from a 0.4 t/d CFB gasifier as shown in Fig. 1, following a 950 °C gasification experiment using ZD coal. ZD coal was crushed and sieved to a size range of 0.1–1.0 mm, and the median particle size of ZDf was about 35 μm . The properties of ZD coal and ZDf were determined based on the Chinese standards including GB/T212-2008, GB/T476-2008, GB/T219-2008, and GB/T1574-2007, etc. As shown in Table 1, the Na2O content in ZD coal ash was as high as 3.92% [5]. The properties of ZDf were significantly different from those of ZD coal, where the volatile matter content and water content were both lower, and the ash content was greater. The Na2O content in ZDf was 2.39%. It should be noted that the K2O content in ZDf ash was as high as 2.29%, and was much higher than it was in the ash from the ZD coal. 30

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C

B A

T6

4.2m 4.0m

T8

T9 Sample 3

T10

D

T11

E

T12

F

T13

G

Bag-filtering Dust Precipitator

3.5m T5

3.0m

Chimney

2.5m Coal Hopper T4

2.0m

T7

1.5m

Loop seal

T3

Ash Collecting Can

1.0m T2

0.5m

T1

Sample 2

0.0m

Air Sample 1 Fig. 1. Schematic diagram of 0.4 t/d CFB experimental system.

1150 mm

Water in

(a) Probe A

Air in

Air out

Next, the riser and bed components were preheated using an external electric furnace. As this occurred, the primary air was gradually pumped through to fluidize the bed materials. When the bed temperature reached 700 °C, the fuel was fed into the riser through a coal hopper. The coefficient of excess air for each condition was maintained at 1.2. The details of the operating conditions for the combustion experiments are shown in Table 2. The temperatures variations at the different positions in the riser (T1−T7) and tail (T8−T13) during combustion are shown in Fig. 3 (a) and (b). It can be seen that the temperature variations in the two ZD coal and ZDf combustions were different. During combustion of the ZD coal, the highest temperature was found at the bottom of the riser (T3), while during the combustion of ZDf the highest temperature was recorded at the upper portion of the riser (T4). In addition, the flue gas temperature at a select same position in the bed during the combustion of ZDf was higher than that during the ZD coal combustion. These results illustrated that the combustion characteristic of ZDf was poorer than that of ZD coal. As shown in Table 3, the ignition temperature and burnout temperature of ZDf were higher than those of ZD coal, which also meant that the combustion characteristic of the ZDf was poorer than that of the ZD coal. Therefore, ZDf particles needed more time to burnout, and primarily burned in the upper region of the riser while the ZD coal particles burned well and were consumed primarily in the bottom portion of the riser. These differences in the distribution of temperature may have affected the distribution of Na and K in the ash product, and the characteristics of ash deposition during the combustion of the ZD coal and ZDf. The carbon content left in the ashes is shown in Fig. 4. As the increase of bed temperature from 860 °C to 960 °C, the carbon content left in fly ash of the ZDf combustion decreased from 4.1% to 1.0%, and the carbon content left in bottom ash decreased from 0.9% to 0.3%. While

Water out

Thermocouples

Ti To 400 mm (b) Other probes

Fig. 2. Schematic diagram of the slagging probes.

A series of bed temperatures were chosen to study the effect of the CFB bed temperature (the highest temperature of the riser) on the chemical transformation of the alkali metals during the combustion of ZD coal and ZDf. The select bed temperatures were 860 °C, 910 °C and 960 °C. On one hand, the operating temperature of the CFB boiler was generally between 850 and 950 °C. On the other hand, the results of our previous works [5,8,9,11,23] have shown that the tail heating surface was easily corroded, and slagging and defluidization occurred easily in furnace when the bed temperature was far above 950 °C during the combustion of ZD coal, thus the bed temperatures were chosen as 860 °C, 910 °C and 960 °C, and the temperature interval was 50 °C. Before the system was started, 17 kg of quartz sand (the SiO2 content was above 95%) with a particle size range of 0.212–0.425 mm, was added to the riser, and 1 kg of quartz sand was added to the loop seal. 31

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experiment. The ash deposited on the slagging probes were photographed and collected after each experiment. All of the bottom ash, circulating ash, fly ash and deposition ash were collected and weighted. A sequential chemical extraction method was used to determine the composition and content of alkali metals in the samples (fuels, bottom ash, fly ash and deposition ash) [16,17,24,25]. The detailed chemical extraction steps are detailed in our previous work [5,14]. The crystalline phases of the samples (coal, gasification fly ash, combustion bottom ash and fly ash) were determined by X-ray diffraction (XRD, PANalytical, Netherlands). The diffractometer used Cu Kα radiation (λ = 1.5406 Ǻ). The ash compositions were determined by X-ray fluorescence (XRF, PANalytical, Netherlands). The samples (coal and gasification fly ash) were ashed at 575 °C in a muffle furnace before the XRD and XRF tests to burn out the residual carbon in the samples avoiding the loss of sodium during ashing as much as possible [26]. The microstructure and elemental composition of the deposition ash were analyzed using a scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM-EDX, S4800, Hitachi, Japan).

Table 1 Properties of high alkali fuels. ZD

ZDf

Proximate analysis (ad, wt%) Water content Ash Volatile matter Fixed carbon Lower heating value (MJ/kg)

15.64 5.03 34.06 45.27 17.63

3.95 36.64 13.65 45.75 17.45

Ultimate analysis (ad, wt%) C H N O St Cl

54.41 1.70 0.69 22.03 0.40 0.10

54.76 1.16 0.72 0.11 2.67 0.44

Ash fusion temperature (°C) DT ST HT FT

1320 1320 1330 1340

1155 1160 1165 1175

Chemical components in ash (wt%) SiO2 Al2O3 Fe2O3 CaO MgO SO3 P2O5 TiO2 K2O Na2O Others

17.24 11.90 5.76 28.74 5.34 19.58 0.05 0.60 0.38 3.92 6.49

55.13 21.48 3.44 7.66 1.11 4.95 0.27 0.70 2.29 2.39 0.58

3. Results and discussion 3.1. Occurrences of alkali metals in fuels The chemical transformation of alkali metals during combustion are closely related to their presence in the fuels. The compositions of Na and K in ZD coal and ZDf are shown in Fig. 5(a) and (b). As shown in Fig. 5(a), the content of Na in ZD coal was 4.45 mg/g, and the water soluble Na accounted for 81.1% of total Na. Compared with the ZD coal, the quantity and composition of Na in ZDf were quite different. The quantity of Na in ZDf was 2.74 mg/g, insoluble and HCl soluble Na were the predominant species of Na, accounting for 86.5% of the total Na. The fraction of water soluble Na in the ZDf was 12.1%. Researchers have indicated that most of the water soluble Na in ZD coal was released to the gas phase during gasification, and a portion of the remaining water soluble Na was retained in the ash, while the remainder was converted to insoluble Na and HCl soluble Na [5,8,9,13]. The quantity of NH4Ac soluble Na, which was combined organically with the coal matrix, was ignored in the ZDf, because most of the organic matter was destroyed during the gasification process. The presence of K in the ZD coal and ZDf is exhibited in Fig. 5(b). The total content of K in ZD coal was only 0.27 mg/g, and insoluble K accounted for 62.9% of the total K. Compared with the high content of Na in ZD coal (shown in Fig. 5(a)), the concentration of K was quite low, so that the chemical transformation of K can be ignored during the combustion of ZD coal [5,27]. However, the total content of K in ZDf was as much as 4.4 mg/g, and insoluble and HCl soluble K constituted 82.5% of the total K, the high fraction of insoluble K in ZD coal might be responsible for this result. During CFB gasification (about 950 °C) of ZD coal, the volatility of insoluble K was weak, so that most of it was retained in the gasification ash. Compared with the content of Na in ZDf, the concentration of K was also significant, thus the chemical transformation of K during combustion of the ZDf cannot be ignored. Therefore, in this reported study, the chemical transformation of both Na and K during the combustion of ZDf were investigated. But during the combustion of ZD coal, the chemical transformation of Na was studied, and the chemical transformation of K was ignored. The mineral compositions of ZD coal and ZDf were determined using XRD analysis, and the results are shown in Fig. 6. As can be seen from this figure, CaCO3 and CaSO4 were the main mineral compounds present in the ZD coal, and Na was present primarily as Na2SO4 and NaCl. This was consistent with the high content of the water soluble Na content in ZD coal (shown in Fig. 5(a)). However, in ZDf, SiO2 was the predominant mineral phase, and insoluble NaAlSi3O8 was the main form of Na, while KAlSi3O8 was the main compound containing the insoluble K. NaCl was also detected in ZDf, but the height of NaCl

Note: ad-as air dried basis; DT-deformation temperature; ST-softening temperature; HT-hemispherical temperature; FT-flowing temperature. Table 2 Experimental conditions. Fuel

Bed temperature (°C)

Fuel feed rate (kg/ h)

Air flow rate (Nm3/ h)

Excess air coefficient

Superficial gas velocity (m/s)

Working conditions time (h)

ZD

860 910 960 860 910 960

5.2 5.5 5.7 5.5 6.0 6.9

34.3 39.9 46.0 33.3 35.0 40.7

1.2 1.2 1.2 1.2 1.2 1.2

2.1 2.5 3.0 2.0 2.2 2.6

8

ZDf

the carbon content left in fly ash of the ZD coal combustion decreased from 3.2% to 0.9%, and the carbon content left in bottom ash decreased from 0.7% to 0.2%. The results meant that the fuels were completely combusted during the experiments. It also can be found that at the same bed temperature, the carbon content left in ashes of the ZDf combustion was higher than that in ashes of the ZD coal combustion, which further illustrated that the combustion characteristic of the ZDf was poorer than that of the ZD coal. Moreover, all of the experiments were maintained at the excess air ratio of 1.2 (as shown in Table 2), the carbon content left in the bottom ash (Sample 1 in Fig. 1) and fly ash (Sample 2 in Fig. 1) was low (4.1–0.2%, shown in Fig. 4), so that it was a combustion process by air in the riser of CFB. 2.3. Sampling and analysis methods The bottom ash, circulating ash and fly ash (sample 1, sample 2 and sample 3 shown in Fig. 1) were collected during the stable stage of each 32

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S. Yang et al.

1000

1000

950

900

Temperature ( C)

Temperature ( C)

900 850 800 750

SH-860 C SH-910 C SH-960 C

700 650

T1

T2

SHf-860 C SHf-910 C SHf-960 C

T3

T4

SH-860 C SH-910 C SH-960 C

SHf-860 C SHf-910 C SHf-960 C

800 700 600 500

T5

T6

400

T7

Position

T8

T9

T10

T11

T12

T13

Position

(a) Riser

(b) Tail

Fig. 3. Bed temperatures distribution at the different positions of the riser and tail.

bottom ash and a decrease in the sodium content of the fly ash. The higher bed temperatures increased the rate of reaction between the sodium and minerals such as SiO2 (mainly in bed material), so that more sodium was retained in the bottom ash. The reduction of sodium content in fly ash with the increase of bed temperature was caused by the decrease in carbon content of fly ash and the decrease in fly ash amount during combustion process. The release of sodium in fly ash was inhibited by the carbon matrix in fly ash. The amount of carbon in fly ash decreased as the bed temperature increased, and more sodium was released from the fly ash to the gas phase. Takuwa et al. [28] also reported an inhibitory effect of carbon on sodium release during coal combustion. Meanwhile, with the increase of bed temperature, the amount of fly ash during combustion process decreased correspondingly. Therefore, the sodium content in fly ash decreased as the temperature increased. In addition, there was a uniform distribution of Na in the bottom ash, circulating ash and fly ash. The Na content of the ash from the combustion of ZDf is shown in Fig. 7(b). As can be seen, the bed temperature had little effect on the Na content of the ash. This may have been the result of the large quantities of HCl soluble and insoluble Na and the low quantity of water soluble Na in the ZDf. There was little water soluble Na in the ZDf, the HCl soluble and insoluble Na were not very volatile, and the insoluble Na was stable at the CFB operating temperatures. Therefore, only a small amount of Na was present in the gas phase during the ZDf combustion. Consequently, the bed temperature had little impact on the Na content of ash. It is worth noting that the Na content in fly ash was much higher than it was in bottom ash and circulating ash. This was due to the small size of the ZDf particles, and the short residence time of the ZDf in the riser, so that most of the ZDf burned in the upper part of the riser, which resulted in a short reaction time for sodium and the bed material. Moreover, some of the small, unburned char particles went directly to the tail, bypassing the circulation process. The collected combustion fly ash was mixed with some of this unburned char that had a high Na content, so most of the Na was retained in the fly ash due to the poor release characteristics of the insoluble and HCl soluble Na. Therefore, more Na was present in ZDf fly ash. By comparing Fig. 7(a) and (b), it can be seen that, under the same conditions, the high sodium content in ZD coal produced ash with a higher Na content than the ash from the ZDf combustion. This contributed to the variation in the ash deposition behavior between the ZD coal and ZDf as shown in Section 3.4. As shown in Fig. 7(c), the distribution of K in the ZDf ashes was similar to the distribution of Na in the ZDf ashes. The K content of bottom ash and fly ash increased slightly as the bed temperature increased, which was caused by the decrease in the carbon content of the ash. Insoluble K is the predominant specie of K in ZDf, and the release

Table 3 Ignition and burnout temperatures of fuels. Fuel

Ignition temperature (°C)

Burnout temperature (°C)

ZD ZDf

376 452

508 617

5

5

ZD

4

4

3

3

2

2

1

1

0

0

-1

860

910

960

-1

Carbon content in bottom ash (%)

Carbon content in fly ash (%)

ZDf

Temperature ( C) Fig. 4. Carbon content in bottom ash and fly ash.

diffraction peak was lower compared with that of NaAlSi3O8 diffraction peak in ZDf. The height of NaCl diffraction peak in ZDf was higher than that of NaCl diffraction peak in ZD coal, which meant that the NaCl content in ZDf was higher than that in ZD coal. The results illustrated that NaCl was enriched in fly ash during gasification of the ZD coal. However, because that most of the NaCl had reacted with the bed material (SiO2 and Al2O3), and a large amount of insoluble NaAlSi3O8 was formed during the gasification of ZD coal, the insoluble NaAlSi3O8 left in ZDf was much more than the water soluble NaCl condensed on ZDf, the high diffraction peaks of SiO2 and NaAlSi3O8 in ZDf also illustrated that.

3.2. Distribution of Na and K in ash The content of Na and K in the bottom ash, circulating ash and fly ash was determined using ICP-AES, and the results are shown in Fig. 7. As shown in Fig. 7(a), during the combustion of ZD coal, the increase in the bed temperature produced an increase in the sodium content of the 33

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5

Water soluble

NH4Ac soluble

HCl soluble

5

Insoluble

NH4Ac soluble

HCl soluble

Insoluble

4

Content of K (mg/g)

4

Content of Na (mg/g)

Water soluble

3 2 1 0

ZD

3 2 1 0

ZDf

ZD

Fuel

ZDf

Fuel

(a) Occurrence of Na

(b) Occurrence of K

Fig. 5. Occurrences of the alkali metals in the fuels.

6000

e

released into gas phase at lower bed temperatures (< 860 °C). Wang et al. [21] reported the similar results. The fraction of Na in bottom ash increased from 1.3% to 9.5%, which illustrated that the reactions between Na and bed material were improved by higher bed temperatures and more Na was retained in the bottom ash as the bed temperature increased. The fraction of Na in fly ash decreased from 10.8% to 3.3%. However, in the ZDf combustion (shown in Fig. 8(b)), Na was present mainly in the fly ash and flue gas, and a small amount of Na was present in the bottom ash. As the bed temperature increased from 860 °C to 910 °C, the fraction of Na in flue gas increased markedly from 32.8% to 54.2%, but when the bed temperature increased to 960 °C, the fraction of Na in flue gas dropped by 4.0%. These results showed that most of the Na in ZDf combustion was released in the flue gas at higher bed temperatures (> 910 °C). As the bed temperature increased from 860 °C to 960 °C, the fraction of Na in the fly ash decreased from 55.7% to 40.7%. At lower bed temperatures, the efficiency of the ZDf combustion decreased, so that more Na was retained in the unburned char in the fly ash. When the bed temperature increased, more carbon in fly ash was consumed, and more Na was released from the fly ash to the flue gas. The fraction of Na distributed in the bottom ash varied little with the bed temperature. It was evident from these results that the distribution of Na during the ZD coal combustion was quite different than that during the ZDf combustion. That was to say that the Na was found primarily in the flue gas in the ZD coal combustion, while during the ZDf combustion, it was found mainly in the fly ash and flue gas. These results can be explained by the following observations. Insoluble and HCl soluble Na were not very volatile, and they were the main species of Na in ZDf, but the easily released water soluble Na was the primary specie of Na found in the ZD coal. Furthermore, the ZDf had a short residence time in the furnace, so that large quantities of ash with a high Na content proceeded to the tail. Therefore, most of the Na was released to the flue gas during the combustion of ZD coal, while most of the Na in the ZDf combustion was found in the flue gas and fly ash. Moreover, most of the Na in ZD coal was released to the gas phase at lower bed temperatures (< 860 °C), while most of the Na in ZDf was released to the flue gas at higher bed temperatures (> 910 °C). The release of Na during combustion of the ZDf was less pronounced than it was in the ZD coal. This was because the Na in ZDf had completely reacted with the other minerals during the gasification of ZD raw coal, and the surface of ZDf was covered with a layer of oxides and sulfides of Ca [14]. This coating inhibited further release of Na from the ZDf during its combustion. The distribution of K in flue gas and solid phase during the ZDf combustion is shown in Fig. 8(c). As shown, at 860 °C, the K was found mainly in the fly ash, where the fraction of K was 45.8%, the fraction of

ZD

5000

a 4000 3000

Diffracted intensity (cps)

2000

b d c e e e ee aa c a e

1000 0 12000

d ZDf

10000 8000 6000 4000 2000 0 10

g f d a a f c dh dc d d d 20

30

40

50

60

70

80

90

2 () Fig. 6. XRD patterns of ZD coal and ZDf.

insoluble K was low at the experimental test temperatures, but as the bed temperature increased, there was less carbon in ashes, so there was a corresponding increase of K content in ashes. In addition, these same factors caused the K content in ashes to be greater than Na content. The distributions of Na and K in the gas phase and solid phase are presented in Fig. 8(a)−(c). These Na and K distributions were calculated based on the mass balance during the experiments, and the detailed calculation method was reported in our previous work [13]. During ZD coal combustion (shown in Fig. 8(a)), most of the Na was released into gas phase, because of the high content of water soluble Na in ZD coal. As the bed temperature increased from 860 °C to 960 °C, the fraction of Na in flue gas initially increased from 87.9% to 91.4%, and then decreased to 87.2%. This meant that most of the Na had been 34

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860 C

910 C

14

960 C

12

12

10

10

Content of Na (mg/g)

Content of Na (mg/g)

14

8 6 4 2 0

860 C

910 C

960 C

8 6 4 2

Bottom ash

Circulating ash

0

Fly ash

(a) Content of Na in ash produced by ZD 24

Bottom ash

Circulating ash

Fly ash

(b) Content of Na in ash produced by ZDf

860 C

910 C

960 C

21

Content of K (mg/g)

18 15 12 9 6 3 0

Bottom ash

Circulating ash

Fly ash

(c) Content of K in ash produced by ZDf Fig. 7. Distribution of Na and K in ash.

K in the flue gas and bottom ash was 28.7% and 25.5%, respectively. However, when the bed temperature increased to 960 °C, the fraction of K in the flue gas, fly ash and bottom ash was the same, about 33.0%. The distribution of K in the various combustion products varied little with the increase in bed temperature. This was because the insoluble and HCl soluble K accounted for 82.5% of the total K in the ZDf (see Fig. 5(b)), so that a large amount of K was retained in the ashes during the ZDf combustion, so there was little difference in the K fraction in fly ash, flue gas and bottom ash.

the bed temperature increased, the reactions between the Na and the materials in the bed became more intense, so that more HCl soluble Na was converted into insoluble Na [27]. However, the water soluble Na accounted for a large proportion of the Na in fly ash. When the bed temperature increased from 860 °C to 960 °C, the fraction of water soluble Na increased from 18.6% to 43.0%. Song et al. [5,14] reported the similar results. In the case of ZDf, the fractions of various forms of Na in bottom ash and fly ash were similar, and the bed temperature had a marked effect on the quantity of Na in the ashes. At 860 °C, water soluble and NH4Ac soluble Na accounted for 13.6–26.3% of the Na in ash, and the fraction of HCl soluble Na ranged from 18.4% to 24.5%, while the insoluble Na accounted for 48.7–68.0% of the Na in ashes. When the bed temperature increased to 960 °C, the water soluble and NH4Ac soluble Na accounted for only 1.4–5.5% of the total Na in the ashes, while the fraction of HCl soluble Na decreased to 13.4–15.0%, and the fraction of insoluble Na increased to 79.3–85.1%. These results confirmed that, as the bed temperature increased, a portion of the water soluble and NH4Ac soluble Na were released into the flue gas, while another portion was transformed into insoluble Na. Part of the HCl soluble Na was also converted into insoluble Na. When Fig. 9(a) is compared to and Fig. 9(b), it can be seen that, during the ZDf combustion at 860 °C, the fractions of various types of Na in bottom ash and fly ash were similar to the fractions of various types of Na in the original ZDf (shown in Fig. 5(a)). This meant that the Na in ZDf did not readily react with bed material during combustion so

3.3. Chemical transformation characteristics of Na and K 3.3.1. Occurrence of Na and K in the ash The presences of Na in bottom ash (BA) and fly ash (FA) for various combustion bed temperatures for the ZD coal and ZDf are shown in Fig. 9(a) and (b). In the case of ZD coal, the content of Na in bottom ash was remarkably different from the Na content in fly ash (shown in Fig. 9(a)) and the bed temperature greatly affected on the amount of Na found in ashes. Insoluble and HCl soluble Na accounted for 93.5% of the total Na in the bottom ash. As the bed temperature increased from 860 °C to 960 °C, the fraction of insoluble Na in bottom ash increased from 64.7% to 81.1%, while the fraction of HCl soluble Na decreased from 29.2% to 13.9%, and little water soluble Na was present in bottom ash. These results illustrated the degree of chemical transformation of the various Na species that occurred during the combustion of ZD coal, as most of the water-soluble Na had been released to the gas phase. As 35

Applied Thermal Engineering 141 (2018) 29–41

100

100

80

80 Fly ash

Bottom ash

Flue gas

Fracton of sodium (%)

Fraction of sodium (%)

S. Yang et al.

60 40 20 0 -20

860

910

Bottom ash

Flue gas

60 40 20 0 -20

960

Fly ash

860

910

Temperature ( C)

960

Temperature ( C)

(a) Distribution of Na during ZD combustion

(b) Distribution of Na during ZDf combustion

100 Fly ash

Bottom ash

Flue gas

Fraction of potassium (%)

80 60 40 20 0 860

910

960

Temperature ( C)

(c) Distribution of K during ZDf combustion Fig. 8. Distribution of Na and K at different bed temperatures.

bottom ash and fly ash changed considerably (see Fig. 5(a)). This meant that the low bed temperature combustion of the ZD coal caused most of the Na in bottom ash and fly ash to be released or chemically

that not much of the Na was released during the low bed temperatures combustion of the ZDf. However, in contrast to the ZDf case, during the ZD coal combustion at 860 °C, the fractions of various Na species in 120

120 NH4Ac soluble

HCl soluble

Insoluble

Water soluble

100

100

80

80

Content of Na (%)

Content of Na (%)

Water soluble

60 40 20 0

NH4Ac soluble

HCl soluble

Insoluble

60 40 20

BA-860 BA-910 BA-960

0

FA-860 FA-910 FA-960

Temperature ( C)

BA-860 BA-910 BA-960

FA-860 FA-910 FA-960

Temperature ( C)

(a) Combustion of ZD coal

(b) Combustion of ZDf

Fig. 9. Occurrences of Na in ashes during combustion of ZD coal and ZDf. 36

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120 Water soluble

NH4Ac soluble

HCl soluble

Fig. 9(a)). There was no NaCl detected in the fly ash. Researchers have found that, during combustion, some NaCl can be released to the gas phase, and some NaCl can be sulfurized by SO2 in flue gas to form stable Na2SO4 as shown in reaction (1) [8,29,30]. In addition, water soluble Na2SiO3 and insoluble NaAlSiO4 were present as a result of the lower bed temperatures (860 °C, 910 °C), and insoluble NaAlSi3O8 was found at 960 °C. During CFB combustion, Na in the fuels reacted with the SiO2 and Al2O3 in the bed material and the fuel, and these reactions were enhanced by the increase in bed temperature [5,19]. The reactions were as follows:

Insoluble

Content of K (%)

100 80 60 40 20 0

BA-860 BA-910 BA-960

FA-860 FA-910 FA-960

Temperature ( C)

4NaCl + 2SO2 + O2 + 2H2 O= 2Na2SO4 + 4HCl

(1)

Na2 O+ SiO2 + H2 O= Na2SiO3 + H2 O

(2)

Na2 O+ Al2 O3 + 2SiO2 = 2NaAlSiO4

(3)

Na2 O+ Al2 O3 + 6SiO2 = 2NaAlSi3O8

(4)

KAlSi3O8 was also found in the fly ash. In the case of the fly ash from ZDf combustion, SiO2 was found to be the predominant mineral in the fly ash, and Na was present mostly as insoluble NaAlSi3O8, while K was present as insoluble KAlSi3O8. These results coincided with the previous analyses concerning the presence of Na and K in the fly ash from the ZDf combustion (shown in Figs. 9 and 10). Furthermore, the mineral phases in fly ash varied somewhat in comparison to the minerals found in ZDf as shown in Fig. 5. Here it can be seen that most of the Na species in ZDf were not as reactive, so most of the Na compositions were stable and were retained in ash during combustion of the ZDf. The Na species found in the ZDf reacted almost completely with other minerals during the gasification of ZD coal, and the content of insoluble Na in ZDf was high. Therefore, the Na in ZDf was not as easily released or transformed during its combustion as the Na in ZD coal during combustion [14].

Fig. 10. Occurrences of K in ashes during ZDf combustion.

transformed. These phenomena illustrated that the release of Na from the ZDf during combustion was inferior to that from the ZD coal combustion, which coincided with the analytical results shown in Fig. 8 [14]. Due to the low quantity of water soluble Na in ZDf and low gaseous Na content during the ZDf combustion, the mass fraction of water soluble Na in ZDf combustion fly ash was much lower than that in ZD coal combustion fly ash. The occurrence of K in bottom ash and fly ash as a function of various ZDf combustion bed temperatures is shown in Fig. 10. As shown, the insoluble K represented 85.6–94.8% of the total K in bottom ash and fly ash, and the fraction of insoluble K varied little as the bed temperature increased. Insoluble and HCl soluble K were the main chemical species of K in ZDf (as shown in Fig. 5(b)), and part of the HCl soluble K had been converted into insoluble K. At the experimental temperatures, insoluble K was stable and its reactivity was low; therefore, there was little release or chemical transformation of K during the combustion of ZDf, and the bed temperature had little effect on the presence of K in the ashes. However, the NH4Ac-soluble K was still found in combustion residues, It might be because that the combustion characteristic of Zhundong coal gasification fly ash was poor, and some of the carbon still left in the combustion residues (shown in Fig. 4). Furthermore, Fig. 10 shows the fractions of different occurrences of K in ashes, although NH4Ac-soluble K accounted for a certain proportion of total K, the absolute content of NH4Ac-soluble K was still negligible because the overall K content in ashes was very low.

3.4. Ash deposition behavior during combustion 3.4.1. Deposit propensity Two parameters are used to quantify the deposition of ash, the deposition rate (DR, g/h) and the deposition propensity (DP, %), which are defined by formulas (5) and (6) below [31]:

DR =

md td

(5)

DP =

DR × 100% ra

(6)

where md represents the mass of the deposited ash, g, td represents the deposition time, h, and ra represents the coal feed ash content, g/h. The full combustion of ZD coal and ZDf required variation of the coal feed rates, so that the quantity of ash produced by the ZD coal and ZDf was different. For these reasons, the deposition propensity was introduced as a parameter that could eliminate the effects of the varying quantities of ash. Fig. 12 (a) and (b) show the deposition rates of ash on different probes and the corresponding flue gas temperatures at these probes during the combustion of ZD coal and ZDf at 910 °C. As shown in Fig. 12(a), the ash deposition rate on the various positioned probes varied greatly. The ash deposition rate on probe D was the highest during combustion of ZD coal and ZDf, achieving 0.56 g/h and 0.59 g/ h, respectively. Furthermore, the ash deposition rate during the combustion of ZDf was higher than that during combustion of the ZD coal, which may have resulted from the larger quantity of ash present during the ZDf combustion. As listed in Table 1, the ash content in ZDf combustion was as high as 29.17%, and most of the ash went into the tail during the ZDf combustion, because of the fine particle size of the ZDf. The ash content in the ZD coal combustion was only 5.03%, which meant that the ash yield was low during the ZD coal combustion. The larger amount of ash during combustion of the ZDf produced a higher ash deposition rate on the slagging probe. However, there was not much difference in the ash deposition rate between the two materials.

3.3.2. XRD analysis The crystalline phases of bottom ash and fly ash as a function of bed temperature during combustion were determined using XRD analysis, and the results are shown in Fig. 11. As can be seen, in the temperature ranges from 860 °C to 960 °C, the bed temperature had little effect on the crystalline phases of the products, but it did affect the quantity of crystalline material. As shown in Fig. 11(a) and (b), it appeared that SiO2 was the main mineral phase in the bottom ash from the ZD coal and ZDf combustion, which was because that the bed material appeared to be composed primarily of SiO2. A small amount of insoluble KAlSi3O8 was found in the bottom ash, which coincided with the high fraction of insoluble K in ZD coal and ZDf (shown in Fig. 5(b)). It can be seen from Fig. 11(c) and (d) that the crystalline phases present in the fly ash were more complex than those found in the bottom ash. For the ZD coal combustion, the mineral compounds in fly ash were more varied than those found in ZD coal as shown in Fig. 6. SiO2 and CaSO4 were the predominant materials, and Na compounds in the fly ash were found to be mostly water soluble Na2SO4, which agreed with the high quantity of water soluble Na in the fly ash (shown in 37

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55000

70000 860 C

a

860 C

a

50000 65000

45000 40000

60000 10000

a

5000

a a

bc

a a

a

5000

a

c

0 100000

c

a

a

aa

a

a a a

a

0 60000 910 C

a

80000 20000

a a

10000

c

b

a aa a

a a

a

910 C

a

55000

90000

Diffraction intensity (cps)

Diffraction intensity (cps)

a

a

50000 45000

a

15000 10000 5000

dc

a

a

a

a

a

a

a

0 100000

0 50000 960 C

960 C

a

90000 45000

a

80000

40000

15000 10000

a

10000

a b

c

f a aa a

a

a

a

a

5000

a

c c

a

a

a

a

a

a

0

0 10

20

30

40

50

60

70

80

90

10

20

30

40

2 (°)

50

60

70

80

90

2

(a) Bottom ash of ZD combustion

(b) Bottom ash of ZDf combustion

Fig. 11. XRD patterns of ash produced at different bed temperatures.

firmly attached to the probe surface, and the ash particles from the inner surface layer were smaller than the ash particles in the outer surface layer. However, by comparison, the ash particles on the probe from the ZDf combustion were smaller and there was more ash accumulated on the probe. In addition, the bond between the ash particles and the probe surface was loose so that the ash particles were easily removed. These results also illustrated that the ash deposition rate from the ZD coal was lower, but the deposition propensity was much higher (shown in Fig. 12). Wei et al. [33] found that the small ash particles with a high Na content adhered to the deposit surface and formed melted surface, which played the key role in the ash deposition during the combustion of ZD coal. In order to further analyze the deposition behavior of the ash particles during the combustion of the ZD coal and ZDf, SEM-EDX was used to determine the microstructures and elemental compositions of the ash particles removed from slagging probe E, and the results are shown in Fig. 14 (a) and (b) and listed in Table 4. It can be seen from Fig. 14(a) that agglomeration and bonding of the ash particles evidently occurred during the ZD coal combustion. The elemental compositions of surface 1 and surface 2 listed in Table 4 show that ash particles were enriched with Na, S, Ca and O, which meant that Na2SO4 and CaSO4 were probably the primary compositions of the minerals in the ash particles.

The deposition propensity during combustion of the ZD coal and ZDf is shown in Fig. 12(b). Although the deposition rate from the ZD coal combustion was lower than that from the ZDf combustion, the ash deposition propensity was much higher, which meant that the serious ash deposition and slagging problems were more likely to occur during the ZD coal combustion. The deposition propensity was closely related to the sodium content found in the ash particles, that was to say that the higher the Na content in ash, the higher the ash deposition propensity [11,12,21]. Gao et al. [32] have found that the ash deposits from the combustion of ZD coal was composed mainly of Na, S and Cl. The content of Na in fly ash and flue gas during ZD coal combustion was much higher than that from the ZDf combustion (as shown in Fig. 7 and Fig. 8), thus the Na content in deposition ash of the ZD coal combustion was higher, and the deposition propensity of the ZD coal combustion was much higher than that of the ZDf combustion. 3.4.2. Deposit morphology The ash deposition behaviors of the two materials were closely related to the slagging, fouling and corrosion problems [32,33]. The morphologies of the deposition ash found on slagging probe E during the combustion of ZD coal and ZDf at 910 °C are shown in Fig. 13 (a) and (b). In the case of the ZD coal deposition ash, the ash particles were 38

Applied Thermal Engineering 141 (2018) 29–41

S. Yang et al.

9000

14000

860 C

a 7500

860 C

10000

6000

b

8000

4500

a e

3000 1500

i

f c k

c a

0 12000

a

bb

6000 4000

a b b

0 10000

a

Diffraction intensity (cps)

6000 4000

a

f c a b ba i hc c e k

2000 0 40000

a

a a

a

910 C

8000 6000 4000

a bc n nn cbf a ab

2000

a

a

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a

960 C

12000 30000

a

0 14000

960 C

a

a

910 C

b

8000

b a c nb nn c f a a b

2000

10000

Diffraction intensity (cps)

a

12000

10000 8000

20000

6000 10000

4000

a b n c f

a

a

a

0 10

20

30

40

50

c a b b nn nf oa a a o

2000 60

70

80

0 10

90

20

30

a

40

a

50

2

60

a 70

80

90

2

(c) Fly ash of ZD combustion

(d) Fly ash of ZDf combustion Fig. 11. (continued)

ZDf

ZD

900

0.25

SH

600 450

0.4

300 0.2 150 0.0

A

B

C

D

E

F

G

Deposition propensity (%)

SHf

Flue gas temperature ( C)

Deposition rate (g/h)

750 0.6

0

SHf

SH

600

0.15 450 0.10 300 0.05

150

A

B

C

D

E

F

(b) Deposition propensity

Fig. 12. Ash deposition rate and propensity along the flue gas flow direction. 39

900 750

Slagging probe

(a) Deposition rate

ZD

0.20

0.00

Slagging probe

ZDf

G

0

Flue gas temperature ( C)

0.8

Applied Thermal Engineering 141 (2018) 29–41

S. Yang et al.

(a) Deposition ash of ZD

(b) Deposition ash of ZDf

Fig. 13. Morphology of the deposition ash on slagging probe E.

Wang et al. [21] found that the condensation and deposition of sulphates of Na and Ca played an important role in the deposition and slagging on heat transfer surfaces in a combustion bed. As shown in Fig. 14(b), the deposition ash particles from the ZDf combustion were smaller and looser, in nature than the ash particles from the ZD coal combustion. The element analyses of surface 3 and surface 4 showed that S, Ca, Si and O were the main elements present in the ash particles, which meant that CaSO4 and SiO2 were the predominant minerals in the ash particles. Gao et al. [32] found that these fine ash particles adhered well to the bed wall as a result of thermophoresis and inertial forces. Therefore, the ash deposition during ZD coal combustion resulted from the deposition of ash particles rich in Na2SO4 and CaSO4, and the deposition ash was hard and harmful to the bed wall. However, the ash deposition during ZDf combustion was due mainly to the accumulation of fine fly ash particles with a low Na content, and the ash particles were not compact. Therefore, the deposition of ash in the ZDf combustion was not as significant, so more attention should be given to the problem of ash deposition during the ZD coal combustion. The content of Na and K in the ashes deposited on slagging probes (A−E) that were located along the direction of the flue gas flow, and the corresponding flue gas temperatures during the ZD coal and ZDf combustion at 910 °C are shown in Fig. 15. It can be seen from these results that during ZD coal combustion, the Na content in deposition ash decreased with the decrease in flue gas temperature. The Na content in ash deposited on probe A was the highest, reaching 59.9 mg/g. However, during combustion of the ZDf, the Na and K content in the deposited ash varied little as the flue gas temperature deceased. The Na content in ZDf deposition ash was much lower than that in ZD coal deposition ash, which coincided with the SEM-EDX analyses (shown in Fig. 14). Song et al. [5] found that during combustion of ZD coal, the alkali metals in deposition ash consisted primarily of the alkali metals that were present in the fly ash and the alkali metal gaseous condensate. The content of gaseous Na compounds was high during the ZD coal

Table 4 Element contents in deposition ash (wt%). Position

O

Na

K

Cl

S

Mg

Al

Si

Ca

1 2 3 4

44.82 37.58 52.44 59.84

0.79 12.89 0.29 0.66

0.29 1.82 0.26 0.80

0.26 − 0.10 0.26

21.97 8.37 2.28 8.91

0.41 1.76 0.39 0.08

1.40 2.87 12.73 5.34

0.38 1.40 16.02 10.47

29.71 4.48 10.25 13.62

880

80 ZD

60

660

40 440 20 220

0 -20

Na in deposition ash of ZD Na in deposition ash of ZDf K in deposition ash of ZDf

A

B

C

Flue gas direction

D

E

Flue gas temperature ( C)

Alkali metal content (mg/g)

ZDf

0

Slagging probe Fig. 15. Contents of Na and K in deposition ashes at 910 °C.

combustion (shown in Fig. 8), and these gaseous Na compounds was easily condensed and enriched on the surface of the cooled slagging probes. This was responsible for the resulting high Na content in the deposition ash of the ZD coal combustion. As the flue gas temperature dropped off, the quantity of gaseous Na compounds decreased, and the

(a) Deposition ash of ZD

(b) Deposition ash of ZDf

Fig. 14. Microstructures of ash deposition on slagging probe E. 40

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S. Yang et al.

quantity of gaseous Na compounds that condensed on the probe surface was also less, so that the Na content in the deposition ash decreased. While the insoluble Na and insoluble K accounted for most of the alkali metals in combustion fly ash of the ZDf (shown in Figs. 9 and 10), and the quantity of gaseous alkali metals compounds was low, some gaseous alkali metals compounds were condensed on the deposition ash during the ZDf combustion. Therefore, the alkali metal content in the ZDf deposition ash was low and varied little with the decrease in flue gas temperature. The difference in the concentration and presence of the alkali metals in deposition ash during the combustion of ZD coal and ZDf was a cause of the variation in morphology of the deposits and the deposition mechanism.

[5]

[6]

[7]

[8]

[9]

4. Conclusions [10]

The chemical transformation of Na and K and the ash deposition during the combustion of Zhundong (ZD) coal and its gasified fly ash (ZDf) were studied in a CFB test system. The main conclusions were as follows:

[11]

[12]

(1) Water soluble Na accounted for 81.1% of the total Na in the ZD coal, and the Na was present primarily as Na2SO4 and NaCl. However, insoluble and HCl soluble Na accounted for 86.5% of total Na in ZDf, where NaAlSi3O8 was the main compound of Na. The K was present primarily as insoluble KAlSi3O8, and the transformation of K during the ZDf combustion cannot be ignored, because there was significant quantities of K in the ZDf. (2) Variation in the bed temperature had a significant effect on the release and transformation of Na, but it did not greatly affect the conversion of K. During combustion of the ZD coal, most of the Na was released to the gaseous phase at low temperatures, the Na in fly ash was mainly in the water soluble form, but in bottom ash it was mainly in the insoluble form. During combustion of the ZDf, Na was found mainly in the fly ash and flue gas, while the fractions of K in flue gas, fly ash and bottom ash were essentially the same, insoluble Na and K accounted for above 80.0% of Na and K in ashes respectively. There was not much chemical transformation of Na during the ZDf combustion in comparison to what occurred to the Na during the ZD coal combustion. (3) The ash deposition during the ZD coal combustion occurred primarily as a result of the agglomeration and bonding of the ash particles that were rich in Na2SO4 and CaSO4, and the ash deposition propensity was high. However, the ash deposition during the combustion of ZDf resulted primarily as a result of the accumulation of fine fly ash particles with a low Na content, and the ash deposition propensity was low. These observations and results suggested that the deposition problems that occurred during the combustion of ZD coal should be receive greater attention.

[13]

[14]

[15]

[16] [17]

[18] [19] [20] [21]

[22]

[23] [24]

[25] [26] [27]

Acknowledgements

[28]

This work was financially supported by the National Key Research & Development Program of China, Grant NO. 2018YFB0604104.

[29]

[30]

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