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Investigation into the relationship between oxygencontaining groups and the release of Na and Cl during preoxidation and pyrolysis of Na-enriched Zhundong Coal Deng Zhao, Hui Liu, Leixiao Jiang, Jingwei Ge, Lianfei Xu, and Qingxi Cao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02321 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017
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Investigation into the relationship between oxygen-containing groups and the release of Na and Cl during preoxidation and pyrolysis of Na-enriched Zhundong Coal Deng Zhao, Hui Liu,* Leixiao Jiang, Jingwei Ge, Lianfei Xu, Qingxi Cao School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P. R. China ABSTRACT: Following the low-temperature preoxidation of Zhundong coal, the relationship between oxygen-containing functional groups and the release of Na and Cl during pyrolysis was systematically investigated using a one-stage quartz fluidized-bed/fixed-bed experimental system. The different forms of Na were quantified by sequential extraction and inductively coupled plasma optical emission spectrometry (ICP-OES), while the different forms of Cl were quantified using the Eschka method and X-ray photoelectron spectroscopy (XPS). Fourier transform infrared (FTIR) spectroscopy was also employed to characterize the functional groups. During oxidation at 200 ℃, large quantities of Cl were released, and oxygen-containing functional groups were produced. In addition, inorganic Na was converted into organic Na through ion exchange between Na and the oxygen-containing groups. In this study, NaCl was loaded onto the acid-washed coal M1
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sample, but it was found that the release of Na and Cl was not coordinated during pyrolysis. Indeed, the preoxidation of coal effectively inhibited the release of Na and promoted the release of Cl at 500 and 600 ℃, while at 700, 800, and 900 ℃, Cl release was inhibited. Based on the results obtained from sequential extraction and XPS analysis, it was apparent that in preoxidized coal samples, the conversion of inorganic Na/Cl to organic Na and organic Cl was promoted during pyrolysis. In addition, the decrease in Cl content in the gas phase may account for the fixation of Na.
1 INTRODUCTION Zhundong coal is one of the most important energy sources in China because of its huge reserves (i.e., ~390 billion tons) and high reactivity.1, 2 However, Zhundong coal is a typical Na-enriched coal, and the release of Na into the gas phase during combustion reduces the ash melting point, thereby resulting in increased pollution and limiting its application.3,
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To address these issues, a pre-washing method was
proposed to remove Na from the coal.5 However, this is generally undesirable because the Na retained in the pyrolysis char sample is an effective catalyst for the gasification reaction, as it can reduce the ignition point, and thereby improve the gasification reactivity.6,
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As such, fixing Na in the pyrolysis char matrix during the heat
conversion process is of great importance for the effective utilization of Zhundong coal. Three major forms of Na exist in coal, and these can be classified as M2
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water-soluble Na-containing salts, organic Na associated with oxygen-containing functional groups, and insoluble Na in the form of silicates or bonded with coal by coordination bonds.8
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The main volatile forms observed in the thermal conversion
process are water-soluble Na and organic Na.8,
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However, due to its strong
interactions with coal, organic Na is released to a lesser extent than water-soluble Na.11, 12 In addition, it has also been reported that organic Na is converted to inorganic Na prior to evaporation.11, 13 As such, the key to Na volatilization is the release of inorganic Na. In the case of Zhundong coal, the major inorganic Na components are NaCl and Na2SO4,14, 15 where the former is more volatile. In addition, the presence of Cl during the volatilization process is known to influence the release of organic Na.11, 16, 17
The determination of the mechanism of NaCl release during coal pyrolysis is
therefore of particular importance, as the pyrolysis of coal takes place immediately prior to thermal conversion. However, it has been reported that the release of Na and Cl is not coordinated during coal pyrolysis.14, 15, 18 More specifically, at low temperatures (i.e., 200–500 ℃), Cl is released in the form of HCl, while only a small portion of Na is released,14, 18 with the majority remaining in the pyrolysis char in the form of organic Na. Interestingly, Li11 found that the quantity of Na retained in this process is related to the nature of the coal itself. Although it has been hypothesized that this property is related to the oxygen-containing functional groups present in coal, with ion exchange potentially occurring between NaCl and the carboxyl groups,19 such assumptions require further verification. In addition, as the free Cl released from NaCl has a strong M3
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electronegativity and is released in the form of a hot gas at high temperatures, it will attract organic Na upon contact with the pyrolysis char sample and promote the release of Na. This could account for the release of large quantities of NaCl from the coal and the low reactivity of Cl-containing metal salt catalysts in the gasification process.11 As such, the conversion of Cl at high temperatures is one of the key issues that we wish to examine herein. Furthermore, it has recently been reported that at higher temperatures (i.e., 600–900 ℃) HCl can react with the carbon matrix resulting in the formation of organic Cl.20, 21 The formation of organic Cl was also found to be positively correlated with the presence of oxygen-containing groups, and so any active sites formed through the decomposition of oxygen-containing groups could potentially adsorb HCl. The proposed relationship between Na, Cl, and pyrolysis char is therefore summarized in Fig. 1. From previous studies, it appeared that the formation of organic Na takes place at low temperatures, while the transformation from inorganic Cl to organic Cl occurs at higher temperatures, with both processes being closely related to the presence of oxygen-containing functional groups in the pyrolysis char sample. However, to date, no systematic study has been carried out to investigate this relationship.
Figure 1. The reactions taking place between NaCl and coal during the heat treatment process.
Low-temperature oxidation has also received significant attention,22-24 where M4
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either air or oxygen is employed for treatment at ~200 ℃ over a range of residence times. During this oxidation process, a large number of oxygen-containing functional groups are produced without destroying the carbon matrix, and these can potentially be retained in the char samples during pyrolysis. Thus, we herein report our attempt to regulate the morphology and volatilization characteristics of Na and Cl through variation in the number of oxygen-containing functional groups under low-temperature oxidation conditions. As such, we report the low-temperature oxidation of Zhundong coal followed by pyrolysis, where the release of Na and Cl is quantified by inductively coupled plasma optical emission spectrometry (ICP-OES) to determine the influence of the oxygen-containing functional groups. In addition, a sequential extraction procedure and X-ray photoelectron spectroscopy (XPS) measurements are conducted to characterize the different forms of Na and Cl present during these procedures. Finally, the relationship between the oxygen-containing functional groups and the release of Na and Cl during the pyrolysis process is investigated systematically.
2 EXPERIMENTAL SECTION 2.1 Sample Preparation. The coal sample employed herein is Zhundong Wu Caiwan sub-bituminous coal. After drying the coal samples under reduced pressure at 85℃ for 12h, they were ground and sieved to a size of 109–180µm. Ultimate analysis of the coal sample gave the composition: C, 73.52; H, 6.55; O (diff), 18.51; S, 0.51 wt%. Following coal demineralization using 0.1M sulfuric acid, solid NaCl solid was M5
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loaded onto the sample by impregnation to obtain a Na content of 2wt%. During impregnation, 15 wt% nitric acid was employed to maintain a pH of 1 and to prevent ion exchange between NaCl and the organic structure.16 The raw coal, acid-washed coal, and Na-loaded coal samples were designated the following labels: S, S-H, and S-H-Na, respectively.
2.2 Preoxidation and Pyrolysis Experiments. The preoxidation and pyrolysis of the samples were carried out in a one-stage quartz fluidized-bed/fixed-bed reactor, which is depicted in Fig. 2.18 In this system, the fluidized bed aids to dissipate heat and control the temperature during the exothermic coal oxidation process. In addition, the samples are heated rapidly at a rate >103–104 Ks−1, which is comparable to the conditions employed in industry. The temperature control system consists of four thermocouples, i.e., three in the oven and one inside the reactor. Thus, the temperature was controlled precisely at 200±1 ℃ throughout the process. A water-cooling system was also employed to ensure that the sample would not undergo oxidation prior to its addition to the reactor.
Figure 2. Schematic representation of the one-stage quartz fluidized-bed/fixed-bed reactor experimental system employed for the oxidation and pyrolysis procedures. (a) is the temperature control oven, and (b) is the reactor. M6
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Initially, the reactor was heated to 200 ℃ under pure air (21% O2, 79% N2) at a flow rate of 1.5 Lmin−1 to promote formation of the oxygen-containing functional groups,23 and the three coal samples (i.e., raw coal, acid-washed coal, and Na-loaded coal) were sent separately to the reactor using a micro-coal feeder. The oxidation reaction was then allowed to proceed over a range of residence times. During this preoxidation process, the oxidation atmosphere affects the conversion of Na and Cl, while heating promotes their conversion. A control experiment was also carried out by treating a raw coal sample in pure Ar gas to determine the effect of the heating process alone. All other experimental conditions were maintained constant. Fast pyrolysis was conducted between 500 and 900 ℃ over 30 min using oxidized and non-oxidized samples for comparison. Prior to each pyrolysis reaction, the sample was dried in a vacuum oven at 105 ℃ over 2 h.
2.3 Sample codes. As mentioned above, the raw coal, acid-washed coal, and Na-loaded coal samples were assigned the labels S, S-H, and S-H-Na. In addition, “O” and “Na” are used to represent the “oxidized” samples and the Na-impregnated samples. The order of “O” and “Na” in these abbreviations represents the order of the oxidation and impregnation operations. For example, the abbreviation “S-H-O-Na” indicates that the acid-washed coal sample was oxidized prior to impregnation with Na. The letter “I” indicates that the sample was treated with the inert gas, Ar.
2.4 Determination of Na and Cl. The Na contents of the samples both before and after the preoxidation and/or pyrolysis treatment stages were determined by high-pressure closed digestion and ICP-OES, as detailed below. For analysis by M7
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ICP-OES (Optima 5300 DV ICP-OES), the samples were pumped into the instrument at an atomizer flow rate of 0.7 Lmin−1, and the plasma was operated in a vertical orientation. Digestion was carried out using a mixture of HNO3/30 % H2O2/HF (10:6:4 mL) at 210 ℃ over 4 h. The Eschka method25 (AS 1038.8.1-1999, ISO-587) was employed to determine the total Cl content in each sample, which was then confirmed by ion chromatographic quantification (ICS-3000). The Na and Cl contents in the volatile (Vx) fraction released during pyrolysis were calculated according to equation (1):
Vx a (cx1 cx 2
M ) 100% m
(1)
where x represents Na or Cl; cx1 and cx2 are the concentrations of Na or Cl before and after pyrolysis; M and m are the mass of coal and its pyrolysis char; and “a” represents the volume of solution (unit: L) obtained from the digestion or Eschka experiment, where a value of 0.1 was employed herein. Each quantification experiment was repeated in triplicate, and the average values are quoted (error
