Transformations and Roles of Sodium Species with Different

Aug 13, 2015 - In this work, to better understand the thermal behavior of sodium species in the utilization of Zhundong coal (ZDC) from Xinjiang, nort...
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Transformations and Roles of Sodium Species with Different Occurrence Modes in Direct Liquefaction of Zhundong Coal from Xinjiang, Northwestern China Xiao Li,†,‡ Zong-Qing Bai,*,† Jin Bai,† Yan-Na Han,†,‡ Ling-Xue Kong,† and Wen Li† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

ABSTRACT: In this work, to better understand the thermal behavior of sodium species in the utilization of Zhundong coal (ZDC) from Xinjiang, northwestern China, the transformations of sodium species in direct liquefaction of ZDC were investigated in a bench-scale autoclave. The effects of sodium species with different occurrence modes on the product distributions in ZDC liquefaction were also examined. The results show that the transformations of sodium species strongly depend on the occurrence modes and the liquefaction temperatures. Below 370 °C, ammonium acetate-soluble sodium species (AS-Na) could be partly transformed into water-soluble ones (WS-Na). However, when the temperatures further increased, the reactions between AS-Na and silicates to form hydrochloric acid-insoluble sodium species (HIS-Na) predominated and the residue formed in this process was firmly adhered to the bottom of the autoclave. The retention ratios of sodium species over all temperature ranges are significantly higher (>89 wt %) than those in ZDC pyrolysis, which implies a significant enrichment of sodium species in liquefaction residues. Besides, both WS-Na and AS-Na are verified to be the detrimental species in ZDC liquefaction; especially, the oil generation could be more severely inhibited by AS-Na, and this negative effect could be mitigated by demineralization with hydrochloric acid. the apparatus,18,19 and the mechanistic roles of sodium species in ash-related problems have been probed by a series of work.20−22 For instance, in coal pyrolysis and gasification, the transformations of sodium and potassium species have been investigated by using a sequential dissolution method.23 It is found that the release of alkali metals from coal was the fastest from 400 to 600 °C and most of the volatilized species were water-soluble alkali metals.24 The release of alkali metals from gasification or co-gasification was also investigated, and their release behavior strongly depends on the occurrence modes.25−27 For sodium-rich ZDC, the formation of fine particulates and the behavior of ash deposition during combustion have been examined as well.28 Recently, in coal combustion, the release of sodium species present in ZDC into gas phase was confirmed by a new low-intensity laser-induced breakdown spectroscopy.29 Within the coal flame domain, the spectra of sodium species embedded in the solid phase were clearly observed. Furthermore, in a tubular furnace reactor, sodium species in ZDC were found to release in the form of sodium chloride and no molten matters could be observed when the pyrolysis temperatures were below 900 °C.30 In the meantime, coal blends of a bituminous coal and ZDC were explored in order to extend the utilization of ZDC and better understand the ash properties and their deposition behavior.2 Even though investigations on the efficient utilization of ZDC, such as gasification, pyrolysis, and combustion, have been extensively carried out and the effect of exchangeable sodium

1. INTRODUCTION Recently, the huge unexploited reserve of Zhundong coalfield located in Xinjiang province1 has attracted much coal-related fundamental interest, such as coal pyrolysis, combustion, and gasification, in China. The proven reserve of this newly discovered coalfield is estimated to be over 160 billion tons, and Zhundong coalfield has become the largest intact coalfield in China.2 The typical characteristics of this superhuge reserve are featured by low contents of mineral matters and total sulfur.3 Therefore, Zhundong coal (ZDC) is currently under active mining and viewed as preferred feedstocks for pyrolysis and power generation.4 The detailed structural information on ZDC has been obtained by high-performance liquid chromatography coupled with mass spectrometry,5 and most of the oxygencontaining aromatics extracted from ZDC have been proven to be value-added chemicals.6 Even the fly ashes and slag derived from the utilization of ZDC have high potential uses.7 The coals in Xinjiang are also suggested to contain a high abundance of valuable elements, such as U, Re, and Se.8 In conclusion, the Zhundong coalfield will surely play ever-increasing roles in the energy supply of China in the foreseeable future. However, ZDC is rich in alkali metals (especially sodium species) and similar to the Victorian lignite (Australia) which also features low ash yields and abundant alkali and alkaline earth metals.9,10 On the one hand, alkali metals could act as active catalysts for gasification and pyrolysis of low-rank coals or chars.11−15 The presence of alkali metal is also verified to influence the product distributions from biomass carbonization.16 On the other hand, due to the stronger volatility of sodium species than that of other mineral matters,17 the release of sodium species from coal combustion or pyrolysis could cause serious problems, such as slag, fouling, and corrosion in © XXXX American Chemical Society

Received: May 21, 2015 Revised: July 16, 2015

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DOI: 10.1021/acs.energyfuels.5b01138 Energy Fuels XXXX, XXX, XXX−XXX

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

a

ultimate analysis (daf)

sample

Mad

Ad

Vdaf

C

H

N

S

Ob

raw coal De-H Na-9.0 Im-Na

10.65 11.66 7.06 6.55

3.82 1.68 3.72 2.34

28.93 28.22 26.07 27.34

82.06 82.29 79.20 81.64

3.47 3.25 3.71 3.70

0.87 0.92 0.80 0.83

0.35 0.24 0.24 0.22

13.25 13.30 16.05 13.61

Mad, moisture content, air-dried base; Ad, ash yield, dry base; Vdaf, volatile matter, dry and ash-free base. bBy difference. in residues and leachates are also detected by ICP-AES (Thermo iCAP6300). 2.3. Sodium-Loading Treatment. In order to examine the transformations and roles of sodium species with different occurrence modes in direct ZDC liquefaction, raw coal was initially demineralized with 2 mol/L hydrochloric acid in accordance with our previous work.35 In each experiment, 400 mL of hydrochloric acid was added to 50 g of raw coal and the mixtures were magnetically stirred at 60 °C for 4 h in a beaker. After that, the slurries were cooled to room temperature, followed by filtering and washing with adequate distilled water. Finally, the residue was dried at 70 °C for 12 h under vacuum before sodium-loading treatment. A coal sample treated by hydrochloric acid is denoted as De-H in this work. As−Na were loaded into coals by ion-exchanged treatment as described in our previous work.36,37 In brief, 100 g of De-H was mixed with 200 mL of 1 mol/L sodium acetate solution in a beaker and the mixtures were magnetically stirred at room temperature, in which process the pH values of the mixtures were continuously monitored and adjusted between 8.9 and 9.0 until no obvious variation was observed within 6 h.37 After that, the mixtures were carefully filtrated and rinsed with adequate distilled water. The sample obtained from ion-exchanged treatment is denoted as Na-9.0. In the meantime, a NaCl-loading sample was prepared by immersing De-H in sodium chloride solution (0.2 mol/L). The mixtures were magnetically stirred for 6 h and subsequently dried at 70 °C to obtain the “‘NaCl-loading coal”’ which is labeled as Im-Na in this work. The amount of sodium species in raw coal and pretreated samples was also detected by ICPAES, and the results are listed in Table 2.

species on direct liquefaction of low-rank coals has also been examined,31 relatively little is known about the roles of different sodium species in direct ZDC liquefaction and insufficient work has emphasized the transformations of sodium species with different occurrence modes in direct liquefaction. As a potential feedstock, the knowledge for the transformations and roles of sodium species in direct liquefaction is necessary for better utilization of ZDC. In this work, a sodium-rich ZDC was liquefied in a laboratory autoclave from 340 to 460 °C. In order to precisely investigate the influences of sodium species on direct ZDC liquefaction, sodium species with different occurrence modes were loaded into ZDC by ion-exchanged or immersed method, followed by liquefaction with the same procedure. In situ diffuse reflectance infrared Fourier transformation spectra (DRIFT) were used to characterize the dynamic variations of -COO− as a function of temperature. The sequential chemical extraction was employed to determine the different chemical forms of sodium species in raw coal and liquefaction residues. The liquefaction residues obtained from different temperatures were also compared by X-ray powder diffractometer (XRD) to probe the agglomeration behavior in ZDC liquefaction. This work might provide a reference for better understanding on the mechanistic roles of sodium species involved in the efficient utilization of sodium-rich coals.

2. EXPERIMENTAL SECTION

Table 2. Content of Na+ (wt %, ad) Determined by ICP-AES

2.1. Material. A sodium-rich ZDC collected from the faces of the mined coal seams in the Sandaoling mine, Zhundong district, was used in this work. The method to collect the bench sample accords with the Chinese standard method, “Sampling of coal seams” (GB/T 4822008)”. Its proximate and ultimate analyses are listed in Table 1. The procedures for proximate and ultimate analyses of samples are in accordance with GB/T 30733-2014 and GB/T 476-2008, respectively. The sulfur content was determined with the method GB/T 214-2007. Raw coal was initially stored in a sealed desiccator at room temperature in order to avoid oxidation. After being pulverized to less than 150 μm, raw coal was dried at 60 °C for 4 h under vacuum to avoid any decarboxylation and resultant structural changes. 2.2. Sequential Chemical Extraction. The occurrence modes of sodium species in raw coal and liquefaction residues are determined by sequential chemical extraction. The procedure used in this work is a simplified one of the traditional method,32 and the detailed information could be found elsewhere.23,33 Briefly, sodium species are divided into four categories: water-soluble portion (WS-Na), ammonium acetate-soluble portion (AS-Na), hydrochloric acid-soluble portion (HS-Na), and hydrochloric acid-insoluble portion (HIS-Na). The extraction process is as follows: 1 g of raw coal or liquefaction residue is initially immersed in 500 mL of distilled water at 60 °C for 6 h. The high ratio of solution to solid sample used in this work is to diminish the overestimation of WS-Na, which might be caused by water extraction.34 Then, the slurries are centrifuged, separated, and sequentially immersed in 50 mL of 0.2 mol/L CH3COONH4 solution. Finally, the residues are separated by leaching with 50 mL of 0.5 mol/ L HCl solution at the same condition. The amounts of sodium species

sample

content

raw coal De-H Na-9.0 Im-Na

0.34 89 wt %) than those in chars (ca. 60 wt % at 800 °C) from ZDC pyrolysis.4 The comparison indicates that temperatures have remarkable effects on the release of sodium species in the thermal conversion of coals. When the liquefaction temperatures were above 340 °C, the retention ratios stayed nearly unchanged and a flat shoulder could be observed on the curve. The limited volatilization of sodium species implies enrichment of these alkali metals in C

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exchangeable sodium species are highly dispersed. Moreover, the contents of sodium species are very similar to that listed in Table 2 if the interferance of other elements, such as hydrogen and silicon, is taken into account. These quantitative analyses indicate the high dispersion homogeneity of AS-Na in Na-9.0. Figure 5 clearly shows that AS-Na could increase THFISP and inhibit oil generation in direct liquefaction compared with De-H. The adverse effect on oil yield is more severe at low temperatures and could be mitigated by high temperatures (>400 °C) to some extent. These results differ from a previous study31 and indicate that temperatures significantly influence the roles of AS-Na in direct liquefaction. The mechanistic roles of exchangeable sodium species might be related to free radicals, and the detailed mechanisms are currently investigated by using model compounds and electron paramagnetic resonance. Compared with AS-Na, WS-Na show less detrimental effects on direct ZDC liquefaction (Figure 5). Though THFISP of Na-9.0 and Im-Na are similar at high temperatures, oil yields are more severely lowered by AS-Na at all liquefaction temperatures. This phenomenon implies that oil generation in direct ZDC liquefaction is mainly suppressed by AS-Na. To preliminarily investigate the possible mechanistic roles of AS-Na, in situ DRIFT were conducted and the spectra have been shown in Figure 6. The numbers above the curves stand for the sample temperature in the in situ reaction cell. With the increase of temperature, the band intensity of 1255 cm−1 (assigned to vibration of −COO−) of Na-9.0 decreased gradually, indicating the decomposition of carboxylate. The breakages of -COO− are accompanied by the formation of abundant free radicals, which would subsequently break the balance between the generation rate of free radicals and the rate of their combination with hydrogen radicals, and inevitably result in formation of THFISP. On the contrary, the corresponding peaks of Im-Na at different temperatures show no clear regularity, indicating the complex roles of WS-Na in direct ZDC liquefaction. 3.3. Transformations of Different Sodium Species in Direct Liquefaction. Figure 7 shows that the volatility of WSNa is stronger than that of AS-Na. For instance, significant amounts of sodium species (ca. 40 wt %) in Im-Na were released from the solid phase at 400 °C in the presence of H2 and tetralin. This result agrees with the observations that sodium species in the form of NaCl could volatilize more easily than those in the form of carboxylate,49,50 probably because the chemical bonds formed between the sodium and coal matrix would be thermally broken prior to the release of sodium species. However, the retention ratios of sodium species in ImNa slightly changed as the temperature further increased to 460 °C, indicating that hydrogen-donor solvents or H2 might influence the behavior of WS-Na in direct ZDC liquefaction at high temperatures. Clear differences between transformations of WS-Na and that of AS-Na could be observed from the evolution of the amounts of HIS-Na and HS-Na. At high temperatures, significant amounts of AS-Na in Na-9.0 would transform into HIS-Na and the variation of the amount of HS-Na is nearly negligible. On the contrary, considerable amounts of WS-Na in Im-Na could transform into HS-Na even at low temperatures. Above 400 °C, a noticeable increase in the amount of HIS-Na could be also identified in the residues. XRD patterns of residues obtained from liquefaction of Na9.0 are similar to those of raw coal, as signals of portil are also noticeable above 370 °C (Figure 8A). For Im-Na, only the

residues, which might bring about agglomeration issues in large-scale productions. Figure 2 also shows that WS-Na and AS-Na, regarded as the most harmful species,33 are the predominant chemical forms (ca. 90 wt %) in ZDC, and the amount of HIS-Na is almost negligible (ca. 2.5 wt %). The interconversions of different chemical forms of sodium species in direct liquefaction could be also observed in Figure 2. At low temperatures (340 and 370 °C), the amounts of HS-Na and HIS-Na were nearly kept constant and the main transformations occurred between AS-Na and WS-Na with synchronous decreases in their amounts. The amount of sodium species in Na-9.0 is 69 mg in total and is all AS-Na. However, as Figure 7 shows, ca. 10 mg (15 wt %) WS-Na is detected in the residues from liquefaction of Na-9.0 at 340 or 370 °C. Generally, WS-Na could be volatilized more easily than AS-Na in heat treatment of coals.47 Therefore, it might be reasonable to speculate that, under thermal conditions, a slight amount of AS-Na would be first transformed into WS-Na, part of which would be volatilized into the gas phase simultaneously. However, the interconversions showed significant complexity with the increase of temperature. At higher temperatures, reactions between sodium species and silicates prevailed and the amount of HIS-Na increased sharply, which accorded finely with the XRD patterns in Figure 3. Above 370 °C, the signals of

Figure 3. XRD patterns of liquefaction residues obtained from raw coal liquefaction.

silicate are clearly strengthened and the peaks of amorphous carbon are gradually weakened. The formation of portil might account for the agglomeration behavior at high temperatures. These results are consistent with the observation that sodium species could be captured by silicates and retained in the bottom ashes during co-combustion of straw and coal.18 3.2. Roles of Different Sodium Species in Direct Liquefaction. Because raw coal contains sodium species with different occurrence modes, De-H is therefore selected as the control sample.36 Figure 2 and previous work48 have shown that most of the sodium species in ZDC exist in the forms of WS-Na and AS-Na; hence their effects on the product distributions in ZDC liquefaction were examined. First, in order to know whether AS-Na were highly dispersed within coal, the SEM-EDX was conducted and two areas were randomly selected. Figure 4 shows that strong signals of sodium species could be observed in both areas, indicating that D

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Figure 4. SEM image (secondary electron image) of Na-9.0 and in situ analyses of two selected areas.

Figure 5. Oil yields and THFISP of De-H, Na-9.0, and Im-Na at elevated temperatures.

Figure 6. In situ DRIFT spectra of Na-9.0 and Im-Na as a function of temperatures.

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DOI: 10.1021/acs.energyfuels.5b01138 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 8. XRD patterns of residues obtained from (A) Na-9.0 and (b) Im-Na.

4. CONCLUSIONS In this work, the transformations and roles of sodium species in direct liquefaction of ZDC were investigated. The results show that WS-Na and AS-Na are the main chemical forms in ZDC and the amount of HIS-Na is almost negligible. Below 370 °C, AS-Na could be partly transformed into WS-Na whose volatility is also a little stronger. With the temperatures further increasing, the reactions between AS-Na and silicates predominated, which might cause severe agglomeration in the reactor. Over all of the temperature range, the retention ratios of sodium species in liquefaction residues are significantly higher than those in pyrolysis (ca. 60 wt % at 800 °C), which indicates an enrichment of sodium species in liquefaction residues. Besides, both WS-Na and AS-Na are verified to be detrimental to the liquefaction process, and especially AS-Na could severely inhibit the oil generation and this negative effect could be mitigated by hydrochloric acid treatment.

Figure 7. Volatility and transformations of AS-Na and WS-Na at different temperatures.

signal of NaCl could be observed (Figure 8B). These results could be ascribed to the higher reactivity of AS-Na and the reactions between aluminosilicates embedded in coal and ASNa. Previous work has revealed that, with the addition of silicon powder into coal, the sodium fixation rate would rise accordingly.51 This phenomenon might provide a potential way to regulate the transformations of sodium species in direct ZDC liquefaction. Moreover, though significant amounts of HS-Na existed in the residues obtained from liquefaction of ImNa, no peaks could be observed in the XRD patterns except for silica and NaCl, which indicates that most of HS-Na are present in the form of noncrystalline mineral matters.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work was financially supported by the Joint Foundation of the Natural Science Foundation of China and Shenhua Group Corp. Ltd. (Grant No. U1261209), the National Basic Research Program of China (Grant No. 2011CB201401), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA07060100).

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DOI: 10.1021/acs.energyfuels.5b01138 Energy Fuels XXXX, XXX, XXX−XXX