Correlations between Coal Compositions and Sodium Release during

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Correlations between Coal Compositions and Sodium Release during Steam Gasification of Sodium-Rich Coals Shuai Guo,†,‡ Yunfeng Jiang,† Jiazhou Li,†,‡ Zhongliang Yu,† Jiantao Zhao,*,† and Yitian Fang† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: Sodium release (SR) during gasification of sodium-rich coals has caused a series of severe operating problems. How to effectively estimate the SR is crucial to design the appropriate controlling technologies. In this work, the detailed correlations between coal compositions and SR, including the underlying mechanisms, were investigated in an atmospheric labscale tube furnace. Six sodium-rich coals were gasified at 900 and 1000 °C under a steam atmosphere. Several coal compositions, such as the content of Na and the mass ratios of Na/Cl, Na/S, Na/ash, and Na/(Si + Al), were correlated with SR. The results indicated that the SR had a close relationship with coal compositions. At both temperatures, the SR presented strongly positive correlations with the Na/(Si + Al) ratio of investigated coals, implying that the inhibiting effect of Si−Al minerals played a dominant role on the SR. However, as a result of the specificity of testing coals and steam atmosphere, the promoting effect of Cl was insignificant. At 1000 °C, it was found that the high content of calcite could notably increase the SR because the generated lime could consume Si−Al minerals, leading to the obvious reduction of captured sodium in the residue. In addition, thermodynamic equilibrium calculations by FactSage have been conducted in comparison to the experimental results. tion.17 Glazer et al. observed that the release of alkali species showed a close relationship with the K/Cl ratio during cocombustion of coal and biomass.19 Except Na and Cl, element S in coal was also deemed as a potentially influential composition on the SR.8,14,21 During coal combustion, Blasing et al. realized that the release of NaCl depended upon not only the Na/Cl ratio but also the S/Cl ratio.14 During cocombustion, Müller et al. noticed that the release of K was obviously inhibited by SO2 through the formation of less volatile K2SO4.21 Beyond the above coal compositions, however, some other researchers emphasized the vital effect of ash on the SR.7,14,16,17,19,21 As was reported, ash and slag had a high potential ability of capturing alkali metals.17 In both coal gasification and combustion, Blasing et al. found that the release of NaCl depended upon negative correlations with Si + Al.14 In the biomass combustion, Glazer et al. observed that the release of K was strongly related with the K/Si ratio as well.19 Even though the correlations between coal compositions and SR have been well-investigated, there still exist some serious debates among the different researchers. By now, the influential extent of each composition on the SR is still unclear. Moreover, for the potentially troublesome sodium-rich coals, the useful data on the SR during steam gasification are extremely lacking, not to mention the correlations. In this work, six sodium-rich coals with significantly different coal compositions were selected as the testing samples. Coal samples were gasified in a lab-scale tube furnace at 900 and 1000 °C. Steam was adopted as the gasification agent because the previous research had verified its important role on the transformations of sodium.22

1. INTRODUCTION Sodium-rich coals are widely distributed around the world, such as Victorian in Australia, North Dakota in the United States, and Zhundong in China.1−3 As a result of the giant reserves and low mining cost, those coals are regarded as a promising energy source. Gasification is considered to be a clean and efficient method to use coal reserves. However, as a result of the high temperature required by coal gasification, sodium in coal can be partially released to the gas phase.2−4 Released sodium could lead to a series of severe problems: (1) bed agglomeration and blockage of the syngas exit in the fixed-bed gasifier, (2) ash deposition and sudden defluidization in the fluidized-bed gasifier, and (3) fouling and blockage of syngas cooler and corrosion of refractory in the entrained-flow gasifier.4,5 The above problems have seriously restricted the utilization of sodium-rich coals.3,4,6 Hence, to develop appropriate control measures and technologies, it is essential to acquire some useful information on the characteristics of sodium release (SR).7 The literature review shows that most researchers focused their interest in the effect of operating conditions on the SR, including the reaction temperature, atmosphere, pressure, and even reactor type.8−13 However, only few researchers reported the crucial role of coal compositions on the SR.14−19 According to the limited reports, the amount of SR strongly depended upon coal compositions under definite conditions. In an earlier study, the researchers found that the water-soluble sodium species, such as NaCl, were more inclined to release compared to ion-exchanged species during combustion.20 This implied that the occurrence modes of sodium in coal might be associated with SR. However, other researchers have highlighted the importance of the Cl content on the SR.7,14−17,21 For instance, Oleschko et al. observed that the release of NaCl depended greatly upon the Cl content during coal combus© XXXX American Chemical Society

Received: March 6, 2017 Revised: April 28, 2017 Published: May 1, 2017 A

DOI: 10.1021/acs.energyfuels.7b00674 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analyses (wt %) of Coal Samplesa proximate analysis

a

ultimate analysis (daf)

sample

Mad

Ad

Vdaf

C

H

Ob

N

St

ZD-1 ZD-2 ZD-3 AU JL HC

10.65 3.93 8.97 12.56 16.01 5.29

3.82 8.17 19.04 27.71 4.41 6.44

28.93 39.67 29.44 52.84 42.33 33.55

82.06 75.88 77.30 67.22 74.48 79.01

3.47 4.87 4.00 5.16 5.18 4.48

13.25 18.02 17.79 21.21 18.36 15.15

0.87 0.91 0.73 0.93 1.16 0.97

0.35 0.32 0.18 5.47 0.82 0.39

ad, air-dried basis; d, dry basis; and daf, dry and ash-free basis. bBy difference.

Table 2. Chemical Compositions in Ash (wt %) sample

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

K2O

Na2O

P2O5

ZD-1 ZD-2 ZD-3 AU JL HC

27.02 31.44 55.39 35.17 25.98 26.09

13.97 6.01 14.34 3.43 9.85 15.58

13.10 32.81 7.02 5.61 9.71 1.50

20.28 8.96 11.20 6.95 18.98 26.03

4.61 3.40 3.57 9.59 5.07 0.01

0.63 0.27 0.64 0.16 0.68 3.45

7.94 11.94 1.35 17.13 22.98 21.95

0.20 0.62 1.25 0.61 0.68 1.31

12.21 4.37 4.64 20.55 4.46 4.27

0.05 0.06 0.19 0.32 0.04 0.03

Figure 1. Schematic diagram of coal steam gasification.

To find out the underlying correlations, several coal compositions, such as the Na content and the ratios of Na/ Cl, Na/S, and Na/(Si + Al), were correlated with SR. The underlying mechanisms were further studied by X-ray diffraction (XRD) analysis and thermodynamic equilibrium calculations. The aim of this work is to find out the underlying correlations between coal compositions and SR and to provide some useful information for predicting the SR during steam gasification of sodium-rich coals.

Table 1, which are in accordance with GB/T 30733-2014 and GB/T 212-2008, respectively. The content of sulfur is determined by GB/T 214-2007. The ash chemical compositions are listed in Table 2, following GB/T 1574-2007. Coal samples were dried under room temperature, then pulverized, and sieved to less than 74 μm. Prior to gasification, coal samples were dried at 110 °C for 4 h to eliminate the moisture disturbance. 2.2. Coal Steam Gasification. Coal gasification is performed in a rapid-heating system with a steam generator and magnetic injection device, as illustrated in Figure 1. The temperatures of the electric furnace and heating tape are displayed and controlled by two temperature controllers. The nitrogen and steam flows are determined by a high-precision mass flowmeter and a double-plunger pump. To avoid the disturbance of air, a magnetic injection device was designed to push or pull the corundum boat into or out of the heating region by a heat-resisting nickel wire. Prior to gasification, the furnace was heated at a rate of 10 °C/min to the preset temperature (900 or 1000 °C) and maintained for 1 h. Meantime, the reactor was purged with 60 vol % H2O and 40 vol % N2

2. EXPERIMENTAL SECTION 2.1. Coal Samples. Six sodium-rich coals with different compositions were selected as coal samples. Among them, ZD-1, ZD-2, and ZD-3 were collected from the Zhundong district (Xinjiang, China), AU was collected from the Loy Yang field in the Latrobe Valley, Victoria, Australia, and JL and HC were collected from the Yili district (Xinjiang, China) and the Yidong district (Inner Mongolia, China), respectively. The proximate and ultimate analyses are listed in B


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Energy & Fuels at a flow rate of 500 mL/min for 15 min to remove the inside air. Subsequently, a corundum boat loaded with about 1.00 g of coal was placed at the cold side of the tube. After 5 min, the boat was instantly pushed into the high-temperature zone. As a result of the steep temperature gradient, the temperature of the coal sample increased violently at a rate of over 1000 °C/min. The residence time was 15 min. After gasification, the boat was pulled out and cooled to room temperature. The residues were collected for the following analysis. 2.3. Solvent Extraction. Sodium species in raw coal can be mainly divided into three occurrence modes: water-soluble sodium (Na-W), ammonium-acetate-soluble but water-insoluble sodium (Na-A/W), and ammonium-acetate-insoluble sodium (Na-I). The occurrence modes of sodium were quantitatively analyzed by the method of solvent extraction based on the different solubilities in the various solvents. The extraction process is as follows: 1.00 g of sample was initially immersed into 100 mL of deionized water or 0.10 mol/L ammonium acetate solution at 60 °C for 1 h with continuous magnetic stirring. It is worth pointing that all of the extraction conditions were optimized by the orthogonal tests. Subsequently, suspension liquid was filtrated and collected for the following analysis. The blank test was performed to decrease the experimental error. 2.4. Sample Characterization. Raw coal was ashed in a lowtemperature asher (EMS1050X, ProSciTech, U.K.), where organic matter of coal could be slowly oxidized by oxygen plasma at such a low temperature of 120−150 °C. Mineral matter could still remain in the original form after ashing treatment. The obtained low-temperature ashes (LTAs) along with high-temperature residues (HTRs) were characterized by an X-ray powder diffractometer (D2 Phaser desktop, Bruker, Germany). The Fourier transform infrared spectroscopy (FTIR) spectra of raw coals were obtained by a Fourier transform infrared spectrometer (iS50, Nicolet, Thermo Fisher Scientific, Waltham, MA, U.S.A.). The content of Na was detected by inductively coupled plasma optical emission spectroscopy (iCAP 6300, Thermo Fisher Scientific, Waltham, MA, U.S.A.). The content of Cl in raw coal was determined by the method of the Eschka mixture in accordance with GB/T 3558-2014. All tests were repeated 3 times. The SR is calculated as follows: SR (%) =

Figure 2. Release of sodium during steam gasification.

biomass, Lith et al. also found that the release of K was dependent upon the temperature as well as fuel compositions.24 3.2. Correlations between Coal Compositions and SR. As a complex mixture, coal contains a variety of elements in different chemical forms, some of which may have a crucial effect on the SR. For instance, it is widely recognized that Cl has a promoting effect on the SR, whereas Si−Al minerals show an inhibiting effect.14,17 Thus, the SR is the result of those two opposite effects. On the basis of the previous studies, we classify the potentially influential compositions into four types: (1) Na, (2) minor elements, such as Cl and S, (3) ash compositions, such as Si and Al, and (4) some other ash elements. The correlations between coal compositions and SR, including the underlying mechanisms, were investigated in the following sections. 3.2.1. Na. The content of Na in the sodium-rich coals is listed in Table 3. It can be found that there is a significant

Wc − Wr × 100% Wc

where Wc is the amount of sodium in per gram of coal (mg, dry basis) and Wr is the amount of sodium in the residue after gasification of 1 g of coal (mg, dry basis). 2.5. FactSage Thermodynamic Equilibrium Calculations. The thermodynamic equilibrium calculations by FactSage “equilibrium module” were carried out to compare the experimental results to the calculated results. Those calculations adopted the principle of the Gibbs free energy minimization of the whole system. Databases for the calculations were Fact PS and FT oxide. Gas-phase species have been handled as ideal gas because of the dilution of gas flow. The elemental concentrations of each coal sample were used as input data for the thermodynamic calculations, including the following elements: C, H, O, N, S, Cl, Si, Al, Ca, Fe, Mg, K, and Na. The given reaction atmosphere was 60% H2O and 40% N2. The input reaction pressure was 1 atm. The input temperatures were 900 and 1000 °C.

Table 3. Content of Sodium in Coal sample

ZD-1

ZD-2

ZD-3

AU

JL

HC

Na (mg/g)

3.84

2.88

8.01

64.41

1.35

1.79

difference in the Na content. Among them, the content of Na in AU is much higher than that in other coals, probably as a result of the great difference in the coal-forming plants and geological conditions.1,25 During steam gasification of Victorian coal, Tanner et al. found that the SR was related to inherent Na in coal.26 This means that a higher content of Na in coal may lead to a larger SR. The relationships between the content of Na in coal and SR were investigated, as shown in Figure 3. Unexpectedly, the SR shows no obvious positive correlations with the Na content at both temperatures. During co-combustion of coal and straw, Müller et al. also observed that the release of K depended upon the Cl content rather than the K content.21 During gasification and combustion of Rhenish lignite, Blasing et al. found that the release of NaCl depended with negative correlations upon the Si + Al content.15 In view of this, it can be inferred that the SR may be controlled by Na-releasing and Na-capturing reactions rather than inherent Na. The occurrence modes of sodium in coal are shown in Figure 4. It indicates that the distribution of sodium species is different

3. RESULTS AND DISCUSSION 3.1. Release of Sodium during Steam Gasification. Figure 2 shows the release of sodium during gasification. For all of the coals, more sodium is released at 1000 °C compared to that at 900 °C. For instance, the SR of HC is 17.32% at 1000 °C, while that of HC is only 1.12% at 900 °C. This further verifies the temperature dependence of SR.7,8,23 Besides, a clear difference in SR among coals is observed after gasification. For instance, the SR of AU is as high as 53.2% at 1000 °C, while that of ZD-3 is only 0.9%. This implies that not all sodium-rich coals show an intense SR.16 Actually, the SR has a close correlation with coal compositions. During combustion of C


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Figure 5. Correlations between the percentage of Na-W in coal and SR.

Figure 3. Correlations between the content of sodium in coal and SR.

mostly upon the Cl content.16 Moreover, chemical equilibrium calculations by FactSage also showed that the SR depended linearly upon the Cl content under combustion conditions.28 Thus, the relationships between Na/Cl and SR were investigated, as shown in Figure 6. Beyond expectation, the

Figure 4. Occurrence modes of sodium in coal.

among six coals. For instance, Na-A/W accounts for 40.3% of total sodium in ZD-2, while that in HC is only 3.6%. However, it should be noticed that, for the vast majority of coals, Na-W is the predominant form, while Na-I is almost negligible (except ZD-3). Na-A/W differs greatly among coals. As mentioned above, the SR can also be affected by the occurrence modes of Na in coal. During coal devolatilization, Eyk et al. observed that about 33% water-bound Na was released, whereas released organically bound Na was almost negligible.27 Thus, the relationships between Na-W and SR were investigated, as shown in Figure 5. However, the SR shows no obvious correlation with Na-W at both temperatures. For instance, the values of SR vary greatly from 2.2 to 53.2% during gasification at 1000 °C, although AU, JL, and HC have almost the same percentages of Na-W. Thus, the occurrence modes of sodium are not the release-determining property for the SR. Some other coal compositions still need to be further investigated. 3.2.2. Cl. Considerable researchers have highlighted the importance of Cl on the release of alkali metals. For instance, during combustion of five hard coals, Blasing et al. found that the release of NaCl presented a good linear correlation with the Na/Cl ratio (R2 = 0.95).14 During combustion of brown coals, Oleschko et al. also found that the release of NaCl depended

Figure 6. Correlations between the mass ratio of Na/Cl and SR.

SR shows no clear correlations with the Na/Cl ratio of investigated coals at both temperatures. This indicates that the promoting effect of Cl on the SR is insignificant during steam gasification of sodium-rich coals, which is not in accordance with the pervious findings on the German hard coals.14 The possible explanations for this difference could be ascribed to two main aspects: the specificity of sodium-rich coals and the steam atmosphere. First, for the specificity of sodium-rich coals, the Na surplus with the respect to Cl is found in the sodium-rich coals. As listed in Table 4, all of the molar ratios of Na/Cl of investigated coals are greater than 1, even up to 18.9 for ZD-3 coal. This characteristic is very different from other investigated coals, Table 4. Content of Cl and Molar Ratio of Na/Cl

D

sample

ZD-1

ZD-2

ZD-3

AU

JL

HC

Cl (mg/g) Na/Cl (mol/mol)

0.45 13.1

0.36 12.3

0.66 18.9

89.49 1.1

0.49 4.3

0.23 12.2


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Energy & Fuels such as the German hard coals.17 As reported, the lack of Cl in coal meant that released Na was more likely in the form of NaOH rather than NaCl.7,14 Furthermore, the vast majority of sodium-rich coals are assigned to low-rank coal, in which the hydrogen-donating groups, such as carboxyl and hydroxyl, are relatively abundant (Figure 7). During the fast heating process,

Figure 8. Correlations between the mass ratio of Na/S and SR.

are observed at both gasification temperatures. One possible explanation is that, as a result of the lack of O2, S was mainly released in the form of H2S.14 Thermodynamic calculations have verified that Na2S was not as stable as NaCl and NaOH.15 Hence, the formation of less volatile Na2S might be quite limited. During gasification of high sulfur and sodium coal, Kosminski et al. had not found any evidence of Na2S in the residue.22 Equilibrium calculations further verified that the effect of S on the SR was minimal under gasification conditions.28 This agreed well with the findings in the work. Hence, it can be inferred that S in coal has a negligible effect on the SR during steam gasification. Some other coal compositions, such as ash compositions, may play a dominant inhibiting effect on the SR. 3.2.4. Si and Al. The importance of Si and Al on the SR during coal thermal conversion has been verified by many researchers.7,14,16,17 At high temperatures, some minerals in ash, such as quartz and kaolin, have the ability to capture sodium species and retain them in the residue rather than release them to the gas phase.4,30,31 During gasification and combustion of Rhenish lignite, it was observed that the release of NaCl depended with negative correlation upon the sum of the Si and Al content.15 For this reason, the Si−Al minerals are frequently used as additives to alleviate the ash-related problems during combustion of sodium-rich coal.32 The relationships between Na/(Si + Al) and SR were investigated, as shown in Figure 9. As expected, the SR shows a strongly positive correlation with the ratio of Na/(Si + Al) at both temperatures. If we linearly fit those data points, R2 can be up to 0.998 at 900 °C and 0.997 at 1000 °C (except HC). This further confirmed that the inhibiting effect of Si−Al minerals plays a dominant role on the SR. To better understand the Na-capturing mechanisms of Si−Al minerals, detailed investigations on the mineral compositions of LTAs and HTRs were conducted by the XRD analysis, and the results are displayed in Figures 10−12. To more visually find out the transformation behaviors of minerals, the main mineral compositions of LTAs and HTRs are listed in Table 5. As seen in Figure 10, the Si−Al minerals in coal are mainly in the form of quartz, kaolin, and boehmite. During gasification, kaolin, as an example, would react with the sodium species. The possible mechanisms are listed in reactions 4−6. Highly volatile NaCl and NaOH were transformed to the stable Na aluminosilicates, such as nepheline.33 Those transformations

Figure 7. FTIR spectra of coals (air-dried).

part of NaCl could react with those groups (reactions 1 and 2), where Na replaced H in the coal structure, further forming active Na2CO3.22 This mechanism has been verified by Kosminski et al. and Li et al.2,22 In comparison to NaCl, highly reactive NaOH and Na2CO3 are more easily captured by the Si−Al minerals.15,29 In addition, part of Cl would be released as HCl rather than NaCl.14,15 Thus, the inhibiting effect of Si−Al minerals on the SR was remarkably strengthened, whereas the promoting effect of Cl was largely weakened. CM−OH + NaCl → CM−ONa + HCl

(1)

CM−COOH + NaCl → CM−COONa + HCl

(2)

Second, for the steam atmosphere. steam was used as the gasification agent, while most previous researchers adopted the low concentration of oxygen. Abundant H2O could shift the gas-phase equilibrium of NaCl and NaOH to the production side, as listed in reaction 3.7,14,26 A higher proportion of Na would be released in the form of NaOH rather than NaCl. With the increase of NaOH, the inhibiting effect of Si−Al minerals on the SR was further strengthened, whereas the promoting effect of Cl was largely weakened. Hence, it can be inferred that the SR was possibly controlled by the capturing reactions of Si−Al minerals. NaCl + H 2O ⇌ NaOH + HCl

(3)

3.2.3. S. Except Cl, minor element S in coal may also play an important role on the SR as well. During combustion of coal and straw, the presence of SO2 obviously suppressed the release of Na and K via the formation of less volatile sulfates.8 Besides, by means of online analysis, the release of NaCl was found to show a negative correlation with the S/Cl ratio during combustion of Rhenish lignite.15 Thus, the relationships between Na/S and SR were investigated, as shown in Figure 8. No obvious correlations E


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Figure 9. Correlations between the mass ratio of Na/(Si + Al) and SR.

Figure 11. XRD patterns of gasification residues at 900 °C [1, nepheline (NaAlSiO4); 2, magnetite (Fe3O4); 3, hematite (Fe2O3); 4, calcite (CaCO3); 5, larnite (Ca2SiO4); 6, lime (CaO); 7, periclase (MgO); 8, halite (NaCl); 9, srebrodolskite (Ca2Fe2O5); 10, calcium ferrite (CaFe2O4); 11, anorthite (CaAl2Si2O8); 12, quartz (SiO2); 13, albite (NaAlSi3O8); 14, anhydrite (CaSO4); 15, tricalcium aluminate (Ca3Al2O6); and 16, gehlenite (Ca2Al2SiO7)].

amorphous structure, the corresponding productions could not be detected by XRD analysis. Al 2O3·2SiO2 ·2H 2O → Al 2O3·2SiO2 + 2H 2O

(4)

2NaCl + Al 2O3·2SiO2 + H 2O → 2NaAlSiO4 + 2HCl (5)

2NaOH + Al 2O3·2SiO2 → 2NaAlSiO4 + H 2O

(6)

It should be noticed that the SR of HC is much higher than the regression value during gasification at 1000 °C. This unusual behavior strongly reveals that, except the inhibiting effect of Si and Al, some other ash compositions might play an oppositely promoting effect on the SR. As shown in Figure 10, the diffraction peaks of calcite in HC are extremely intense. The comparison of ash compositions in Table 2 also shows that the ratio of CaO in HC is up to 26.03%, much higher than that in other coals. At a high temperature of 1000 °C, most calcite was decomposed to lime, as listed in reaction 7. The signals of lime were identified in HTAs. As an active alkaline earth oxide, a notable amount of lime could take part in the reactions with Na-capturing Si−Al minerals, such as quartz, boehmite, and kaolin.34 The corresponding productions of gehlenite, larnite, hatrurite, and calcium aluminum oxide were identified in the

Figure 10. XRD patterns of low-temperature coal ashes {A, albite (NaAlSi3O8); B, boehmite [AlO(OH)]; C, calcite (CaCO3); D, dolomite [CaMg(CO 3 ) 2 ]; H, halite (NaCl); G, gehlenite (Ca2Al2SiO7); K, kaolinite [Al2Si2O5(OH)4]; N, nitratine (NaNO3); P, pyrite (FeS2); Q, quartz (SiO2); and S, siderite (FeCO3)}.

had been verified by XRD analysis (Figures 11 and 12), in which the Na−Si−Al−O minerals frequently occurred in HTAs. Meanwhile, a certain amount of Na−Si−O minerals might also be generated because the peak intensity of quartz obviously reduced after gasification. However, as a result of the F


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Energy & Fuels 2CaO + SiO2 → Ca 2SiO4

(10)

As listed in reactions 8−10, a substantial fraction of the Si−Al minerals in HC were consumed by lime, leading the obvious reduction of available Si−Al minerals for capturing sodium. Accordingly, retained Na decreased greatly in the residue. Thus, it can be reasonable to speculate that calcite shows a promoting effect on the SR. A further comparison indicates that the Ca/ Na ratio in HC is largest among coals (Table 6). This means that the effect of calcite in HC is most significant; therefore, its SR is much higher than the regression value. Table 6. Mass Ratio of Ca/Na in the Sodium-Rich Coals

residue. According to the distribution of Ca-containing minerals, the possible mechanisms are listed in reactions 8−10. (7)

3CaO + 2AlO(OH) → Ca3Al 2O6 + H 2O

(8)

ZD-1

ZD-2

ZD-3

AU

JL

HC

1.44

1.82

1.90

0.21

4.44

6.72

During gasification at 900 °C, the promoting effect of calcite is not as significant as that at 1000 °C. However, the SR still shows a strongly positive correlation with the Na/(Si + Al) ratio of all of the investigated coals. This is because most calcite was still not decomposed at 900 °C (Figure 11). Moreover, the mineral reactions between Si−Al minerals and lime might be restrained by kinetics. Thus, only a few Ca−Si−O, Ca−Al−O, or Ca−Si−Al−O minerals were generated in HTAs after steam gasification at 900 °C. 3.2.5. Other Ash Elements. For another alkaline earth metal Mg, it is mainly in the form of dolomite in coal. During gasification, dolomite would be decomposed to lime and periclase, as listed in reaction 11. Similar to lime, periclase was also involved into the reactions with the original Si−Al minerals at 1000 °C. The productions of forsterite and akermanite were detected out in HTAs. According to the distribution of Mgcontaining minerals, the possible mechanisms are listed in reactions 12 and 13. A small proportion of Si−Al minerals was consumed by periclase, resulting in the reduction of available Si−Al minerals for capturing sodium. However, in comparison to Ca, Mg has a less promoting effect on the SR because, for most coals, the content of Ca is much higher than that of Mg (Table 2). XRD analysis also confirmed this because Ca−Si−O and Ca−Si−Al−O minerals frequently occurred in HTAs at 1000 °C, whereas Mg−Si−O minerals rarely appeared.

Figure 12. XRD patterns of gasification residues at 1000 °C [1, hematite (Fe2O3); 2, gehlenite (Ca2Al2SiO7); 3, magnetite (Fe3O4); 4, sodium aluminum silicate (Na1.45Al1.45Si0.55O4); 5, calcium silicate (CaSi2O5); 6, srebrodolskite (Ca2Fe2O5); 7, calcium aluminum oxide (CaAl2O4); 8, calcite (CaCO3); 9, lime (CaO); 10, quartz (SiO2); 11, albite (NaAlSi3O8); 12, anorthite (CaAl2Si2O8); 13, halite (NaCl); 14, periclase (MgO); 15, forsterite (Mg 2 SiO 4 ); 16, akermanite (Ca2MgSi2O7); 17, nepheline (NaAlSiO4); 18, calcium aluminum oxide (Ca3Al2O6); 19, calcium iron oxide (CaFeO3/CaFe3O5); 20, larnite (Ca2SiO4); and 21, hatrurite (Ca3SiO5)].

CaCO3 → CaO + CO2

sample Ca/Na

CaMg(CO3)2 → CaO + MgO + 2CO2

(11)

2CaO + MgO + 2SiO2 → Ca 2MgSi2O7

(12)

2MgO + SiO2 → Mg 2SiO4

(13)

Transition metal Fe is mainly in the form of siderite and pyrite in ash. During gasification, most siderite and pyrite were transformed to magnetite or hematite, as shown in Table 5. Meanwhile, a portion of them could react with lime and form

2CaO + Al 2Si 2O5(OH)4 → Ca 2Al 2SiO7 + SiO2 + 2H 2O (9)

Table 5. Distribution of Na, Ca, Mg, and Fe Species in Raw Coals and Gasification Residues gasification residue element Na Ca Mg Fe

raw coal NaCl, NaNO3, and NaAlSi3O8 CaCO3, CaMg(CO3)2, and Ca2Al2SiO7 CaMg(CO3)2 FeCO3 and FeS2

900 °C

1000 °C

NaCl, NaAlSiO4, and NaAlSi3O8

NaCl, NaAlSiO4, NaAlSi3O8, and Na1.45Al1.45Si0.55O4

CaCO3, CaO, CaSO4, Ca3Al2O6, CaAl2Si2O8, Ca2SiO4, Ca2Al2SiO7, Ca2Fe2O5, and CaFe2O4

CaCO3, CaO, Ca2SiO4, Ca3SiO5, CaSi2O5, Ca3Al2O6, CaAl2O4, Ca2Al2SiO7, CaAl2Si2O8, CaFeO3, CaFe3O5, Ca2Fe2O5, and Ca2MgSi2O7

MgO Fe2O3, Ca2Fe2O5, Fe3O4, and CaFe2O4

MgO, Mg2SiO4, and Ca2MgSi2O7 Fe2O3, Fe3O4, CaFeO3, CaFe3O5, and Ca2Fe2O5 G


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This finding provides a reasonable explanation for the nicely positive correlation between Na/(Si + Al) and SR in section 3.2.4. However, it should be noticed that, for AU coal, there exists a significant variation between the experimental value and calculated value, especially gasification at 900 °C. Moreover, the predicted SR of HC is far less than the experimental result at 1000 °C, whereas it agreed well with the result at 900 °C. This phenomenon is also found in the correlation analysis. The above variations can be explained by the great difference between the experiment and equilibrium calculation.14 Actually, no equilibrium state is achieved in the actual experiment. On one hand, the release of sodium may be a time-consuming process, which could not be finished in a short time. For AU coal, crystalline NaCl was still detected out in the gasification residues. On the other hand, coal is a heterogeneous mixture. The concentration gradient of mineral matter exists in any direction of particle. During the heating process, mineral reactions possibly emerged with a certain degree of randomness. For HC coal, although sodium species may more easily react with the Si−Al minerals, the plentiful lime could react with the adjacent Si−Al minerals to form the stable Ca−Si−O or Ca−Si−Al−O minerals. In view of this, the assumption of thermodynamic equilibriums possibly gives the wrong results for AU and HC coal gasification.17

some Ca−Fe−O minerals, such as srebrodolskite. The formations of Ca−Si−O and Ca−Si−Al−O minerals suppress the promoting effect of CaO on the SR. This can give a reasonable explanation of why JL coal presented a low SR despite the high CaO content in ash. Thus, it can be concluded that the Fe-containing minerals in ash could suppress SR to some extent. Although the chemical property of K is similar to that of Na, however, the effect of K on the SR is negligible. Two possible reasons can explain this. On one hand, the content of K is far less than that of Na in ash (Table 2). On the other hand, most K is in the form of non-volatile K aluminosilicates. At high temperatures, K in K aluminosilicates could be replaced by Na (reaction 14) and then released to the gas phase in the form of KCl.14 Furthermore, K-containing minerals were not identified in both LTAs and HTRs, further confirming that K in ash has a negligible effect on the SR. mK 2O·xSiO2 ·y Al 2O3 + NaCl → (m − 1)K 2O·Na 2O·xSiO2 ·y Al 2O3 + 2KCl

(14)

3.3. Comparison to Thermodynamic Calculations. Thermodynamic calculations for SR are shown in Figure 13

4. CONCLUSION With a special focus on sodium-rich coals, the correlations between SR and coal compositions, including the underlying mechanisms, have been investigated during steam gasification. The following conclusions can be drawn: (1) The promoting effect of Cl on the SR was insignificant as a result of the specificity of sodium-rich coals and steam atmosphere. (2) The inhibiting effect of Si−Al minerals played a dominant role on the SR. At both temperatures, the SR showed a strongly positive correlation with Na/(Si + Al). (3) At 1000 °C, the high content of calcite in ash could notably increase the SR because the generated lime could consume Si−Al minerals, leading to the obvious reduction of capturing sodium in the residue. (4) For most coals, SR can be well-predicted by thermodynamic equilibrium calculations at both gasification temperatures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuai Guo: 0000-0002-2186-0443 Jiantao Zhao: 0000-0002-8353-2584 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07050100) and the National Natural Science Foundation of China (21576276 and 21506241).

Figure 13. Comparison of experimental and calculated results of SR during steam gasification (A, 900 °C; B, 1000 °C).

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in comparison to the experimental results. At both temperatures, the predicted SR of ZD-2 and ZD-3 is in perfect accordance with the experimental results; meanwhile, that of ZD-1 and JL is in moderate agreement. This suggests that, for most sodium-rich coals, the SR is controlled by the thermodynamic equilibrium under the experimental conditions.

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Correlations between Coal Compositions and Sodium Release during

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