Article pubs.acs.org/ac
Direct Determination of Sulfur Species in Coals from the Argonne Premium Sample Program by Solid Sampling Electrothermal Vaporization Inductively Coupled Plasma Optical Emission Spectrometry Daniela Bauer,*,†,‡ Thomas Vogt,†,‡ Mathias Klinger,†,§ Patrick Joseph Masset,∥ and Matthias Otto‡ †
German Centre for Energy Resources, Reiche Zeche, Fuchsmuehlenweg 9, 09596 Freiberg, Germany Institute of Analytical Chemistry, TU Bergakademie Freiberg, Leipziger Straße 29, 09599 Freiberg, Germany § Institute of Energy Process Engineering and Chemical Engineering, TU Bergakademie Freiberg, Fuchsmuehlenweg 9, 09596 Freiberg, Germany ∥ Fraunhofer-Institut für Umwelt-, Sicherheits- und Energietechnik (Fraunhofer UMSICHT), An der Maxhuette 1, 92237 Sulzbach-Rosenberg, Germany ‡
S Supporting Information *
ABSTRACT: A new direct solid sampling method for speciation of sulfur in coals by electrothermal vaporization inductively coupled plasma optical emission spectrometry (ETV-ICP OES) is presented. On the basis of the controlled thermal decomposition of coal in an argon atmosphere, it is possible to determine the different sulfur species in addition to elemental sulfur in coals. For the assignment of the obtained peaks from the sulfur transient emission signal, several analytical techniques (reflected light microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction) were used. The developed direct solid sampling method enables a good accuracy (relative standard deviation ≤ 6%), precision and was applied to determine the sulfur forms in the Argonne premium coals, varying in rank. The generated method is time- and cost-effective and well suited for the fast characterization of sulfur species in coal. It can be automated to a large extent and is applicable for process-accompanying analyses.
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Depending on the way sulfur occurs in energy resources, a strong influence on the operational, environmental, and economic performance of handling and utilizing processes of coal is expected. The knowledge of the sulfur species in coal or its processing products (e.g., pyrolysis oil or ash) is required to determine the suitability of a particular coal treatment or cleaning strategy.6 Furthermore, process issues such as fouling behavior in the boiler, deactivation of catalysts, or promotion of high temperature corrosion of equipment may occur and are strongly related to the sulfur speciation.7 Therefore, the total sulfur content does not provide sufficient information about the quality of coal for the processing industry and the determination of the different sulfur species is usually required. Several instrumental methods for the determination of the total sulfur content and its species has been developed and investigated. A comprehensive review
oal is used as feedstock in different industrial process such as gasification or power generation. The composition of the coal depends on the localization of the mining. Due to its negative impact on system performance, the sulfur content and its distribution represents an important parameter in the evaluation of coal resources. It is well established that sulfur in coal is distributed in three major forms: organic sulfur (Sorg), inorganic sulfides, predominantly pyrite (Spyr), and sulfates (Ssulf).1 The speciation of sulfur in coal has been the subject of many studies. According to these investigations, the presence of elemental sulfur in coal is consistent with its degree of oxidation due to microbial desulfurization or weathering state.2,3 The sulfate form depends mainly on the weathering state of the coal and usually represents only a small amount of the total sulfur content.4 Usually, small quantities of sulfates (e.g., CaSO4· 2H2O) with less than 0.1 wt % occur naturally in coals,5 mainly derived from salts of sea or brackish water flooded peat swamps.2 Consequently, organic and pyritic sulfur (dimorphic with marcasite) represent the main constituents of the total sulfur amount in coal. © 2014 American Chemical Society
Received: July 24, 2014 Accepted: September 22, 2014 Published: September 22, 2014 10380
dx.doi.org/10.1021/ac502823e | Anal. Chem. 2014, 86, 10380−10388
Analytical Chemistry
Article
the standardized procedure are the significant errors.25 The DIN-method also requires a high preparative effort and is very time-consuming. As a result of this, there is a growing need for a rapid-process analysis to assess the sulfur distribution in coal. In this work, an alternative approach for sulfur speciation in coals was developed by using ETV as a kind of solid sample introduction into the ICP OES. Solid samples were analyzed directly, such that the risk of dissolution (e.g., incomplete sample dissolution, element loss or contamination) could be prevented and the possibility of element speciation remains.
about the methods for the identification and the quantification of sulfur and its various chemical forms is provided by Chou2 and Speight.7 Some of these analytical methods are based on a working principle similar to the electrothermal vaporization inductively coupled plasma optical emission spectrometry (ETV-ICP OES) (e.g., by measuring the element release as a function of the temperature). The sulfur compounds in coal and their kinetic decomposition have been the object of many studies, due to the different sulfur forms in terms of processing.8−11 For example, Bolin12 and Kelemen et al.13,14 provide quantitative results for sulfur species in coal or kerogen by using the sulfur X-ray near edge absorption spectroscopy (S-XANES) third-derivative analysis method. Among other things, they investigate the thermal transformation of organic and inorganic sulfur in energy resources, with the use of this direct, nondestructive characterization technique. Yperman’s research group published the results of the coal decomposition by using atmospheric pressure temperature-programmed reduction (AP-TPR) and thermogravimetric analysis (TGA).10,15 The behavior of coal subjected to a linear increase of the temperature up to 1000 °C in an inert atmosphere as well as in a pure H2 gas flow were discussed. According to their research most of the organic sulfur compounds, except some thiophenes, were released in a temperature range between 180 and 630 °C.10,15 Pyrite, the predominant sulfide mineral in coal, is known to decompose in nonoxidizing atmospheres in a multistep process in the sequence of pyrite (FeS2) to pyrrhotite (Fe(1−x)S; x = 0− 0.2) to troilite (FeS) to iron with the respective elimination of sulfur.2,16 Boyabat et al. investigated the thermal decomposition of pyrite during its pyrolysis in a temperature range of 400−700 °C.9 Maes et al. concluded, based on measuring separated pyrite from coal in an argon atmosphere with TGA, that the thermal decomposition of pyrite through pyrrhotite proceeds in a temperature range of about 550−620 °C.5 The TG/DTA of pyrite, heated in N2 atmosphere, show the pyrite decomposition in a temperature range between 570 and 700 °C.17 However, the thermal stability of ferrous sulfide is higher and so no decomposition of the chemical compound will take place before 1300−1400 °C.18 The reduction of ferrous sulfide to free iron and sulfur will be completed at 1700 °C.19 Yani et al. confirmed this statement based on investigations about the complete transformation process of pyrite during pyrolysis by using TGA, infrared spectroscopy (FTIR), and microscopy techniques.20 Reflected light microscopy (RLM), scanning electron microscopy (SEM) in backscattering electron (BSE) mode or energy dispersive X-ray spectroscopy (EDX) combination, and X-ray diffraction (XRD) were found to be suitable analysis methods for keeping track of the decomposition of pyrite in coal. With the latter two techniques, it enables us to distinguish pyrite from pyrrhotite and troilite (pyrite or marcasite, undifferentiated).21 In the case of determination by RLM, the shape, color, and the anisotropic behavior in polarization mode allows for a distinction between pyrite and pyrrhotite but not between pyrrhotite and troilite.22 In contrast to instrumental methods for the determination of the different sulfur forms, traditional wet chemistry method according to DIN 51724-2,23 ISO 157,23 or ASTM D-249224 was established. Sulfates and pyrite are dissolved selectively in a sequential digestion process, which uses various chemicals. The organic sulfur value, which may also include any elemental sulfur if present, is calculated as the difference between the total sulfur and the sum of the other two. The main disadvantages of
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EXPERIMENTAL SECTION Instrumentation. Solid sample analyses were performed by means of a commercially available ETV unit (ETV 4000c AD50; Spectral Systems) equipped with a solid sampling autosampler for the introduction of 50 graphite boats. The temperature profile was adjusted in the range from 20 to 3000 °C and regulated by an internal furnace pyrometer, which is calibrated every measuring day with an external pyrometer (PL31 AF4, Keller MSR) to ensure the accurate furnace temperature. The analog signal of the internal pyrometer was logged during the measurement. The respective ETV solid sampling system was interfaced to an axial viewing ICP OES instrument via polytetrafluoroethylene (PTFE) transport tubing. In the case of axial viewing, the plasma was operated in a horizontal orientation and the analytical zone was observed from the end of the plasma (EOP). For development of the analytical procedure, the measurements were carried out with the spectrometer (ARCOS EOP) from SPECTRO Analytical Instruments. The combination of an equal ETV system with another optical emission spectrometer (iCAP Duo 6500) from Thermo Scientific was used to check the reproducibility. Detailed information about the instruments and the procedure of electrothermal vaporization coupled with optical emission spectrometer are reported in previous studies.26,27 The instrumental operating conditions and wavelengths used for the experiments are summarized in Table S-1 in the Supporting Information. In addition to the ETV-ICP OES measurements, two different microscopy techniques and X-ray diffraction were used for the identification of the decomposition state of pyrite in the samples. Therefore, a scanning electron microscope (Quanta 250 FEG, FEI) fitted with a 30 mm2 silicon drift detector EDX analyzer (Octane Plus, EDAX/AMETEK) was used to study the local distribution of ash particles (especially pyrite and pyrrhotite) in the organic matrix, with an accelerating voltage of 20 kV. Measurements by reflected light microscopy (RLM) at 500-fold magnification were performed with a microscope (DM 4000 Pin, Leica) with an integrated camera (DFC 295, Leica) in bright field and polarization modes. The measurements by an X-ray diffractometer (D8A25 Discover, Bruker) with an X-ray tube (Co1.78897 Å) were done by using a 35 kV accelerating voltage and a 40 mA beam current. The powdered samples were scanned from 10 to 80° in 2θ range with a 0.01° step interval and 1 s/step counter-time. Standard Reference Material and Samples. The sample identifiers of all used samples and their corresponding characteristics (seam, rank, total sulfur content, sulfur distribution) are summarized in Table S-2 in the Supporting Information.28,29 Additionally, the standard reference material (SRM) 1632d, purchased from the National Institute of Standards and Technology (NIST), was used to develop the 10381
dx.doi.org/10.1021/ac502823e | Anal. Chem. 2014, 86, 10380−10388
Analytical Chemistry
Article
refractive index, air/material) and increasing the optical resolving power. For measurements by X-ray diffraction, the SRM 1632d samples were additionally thermally treated by ETV up to temperatures of 1100, 1500, and 1800 °C, respectively. Measurement Procedure ETV. Before each measurement series, the empty graphite boats were cleaned by a thermal cleaning run up to a temperature of 2400 °C. It was necessary to use a halogenated reaction gas (CCl2F2), which was added to the inner gas phase of the oven to ensure the volatilization of carbideforming elements by formation of volatile halides.26 The heating program of the vaporization unit was optimized by working with the SRM 1632d in a pure argon atmosphere and by measuring the emission of sulfur. Different temperature programs were tested and the one with the ideal release of sulfur for speciation, recorded in the transient emission signal, was used for the following work. Although they focus on different heating rates and times, all temperature programs share the final temperature, which was limited up to 2350 °C with respect to wear of the ETV graphite boats and tubes. According to the optimized temperature program, about 3 mg of the SRM, the APC, and the prepared samples were weighed into the graphite boats and were measured in triplicate, respectively. To determine the repeatability of sulfur speciation by ETV-ICP OES, the SRM was measured six times. Data Processing. To draw conclusions about the distribution of sulfur species in the different coal samples, by measuring the element release as a function of temperature, deconvolution of the transient signals was needed. Therefore, the obtained data were exported to OriginPro 8G for further signal processing. To detect the peak positions quite accurately in addition to the first derivatives of the smoothed emission signals, the second derivatives were used to find hidden peaks. The defined respective fitting criteria were Gaussian peak shape and 50 iterations.
sulfur speciation method. The distribution among the three major forms is only a result of the round robin study CANSPEX 2009-1, conducted by Quality Associates International, Ltd. and cannot be used as certified values.28 For validation of the developed method for sulfur speciation by ETV-ICP OES, the complete collection from the Argonne Premium Sample Coal Bank, covering the rank range from lignite to semianthracite, was analyzed. The eight Argonne premium coals (APC) were collected, prepared, and stored under controlled conditions and have been used by researchers worldwide in numerous coal studies, so the values could be verified. Sample Preparation. Prior to the determination of the sulfur distribution, the APC (
