Article pubs.acs.org/EF
XANES Investigation on Sulfur Evolution during Victorian Brown Coal Char Gasification in Oxy-Fuel Combustion Mode Juan Chen, Baiqian Dai, Fiona Low, and Lian Zhang* Department of Chemical Engineering, Monash University, GPO Box 36, Wellington Road, Clayton, Victoria 3800, Australia S Supporting Information *
ABSTRACT: This study has clarified the speciation of sulfur in the pyrolysis char of Victorian brown coal and its evolution during char gasification in oxy-fuel combustion. The synchrotron XANES has been used for sulfur speciation. Coal pyrolysis was first carried out in a lab-scale drop-tube furnace (DTF). The resulting char was then size-segregated and gasified in O2/N2 and O2/CO2 gases to examine the evolution of sulfur as a function of particle residence time and oxygen fraction in bulk gas. The results indicate that thiophenic sulfur is the predominant form for sulfur in raw coal and its pyrolysis char. The substitution of O2/CO2 for O2/N2 slightly increased the rate of mass loss of sulfur upon char gasification, due to thermodynamically viable routes for the decomposition of thiophene into COS and CS2 in CO2, rather than HS in N2. For the small char particle rich in highly reactive alkali and alkaline earth metals, the extra loss of sulfur in CO2 was readily stabilized into sulfate on the local char surface at a relatively long residence time of about 1.6 s in the DTF. The participation of inherent oxygen in char is essential for the sulfation reaction; however, this was inhibited by introducing oxygen into the bulk gas. The rapidly decomposed sulfur present in gaseous oxides preferentially diffused out of the char surface into the bulk gas. The uneven distribution of alkali and alkaline earth metals with respect to char particle size is influential in affecting the sulfation extent on the char surface.
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INTRODUCTION As one of the largest sources for power generation in the world, coal combustion has been facing increased pressure on mitigating its high carbon footprint in both short and long terms. In using a mixture of high-purity oxygen and recirculated flue gas (RFG), Oxy-fuel combustion technology provides an option for direct sequestration of CO2 with minimal preseparation, and it can be easily implemented, either by retrofitting an existing boiler or using a purposely designed plant.1 The substitution of O2/CO2 mixture for air has proven to affect coal combustion performance in a variety of aspects. Apart from the changes on the energy balance (e.g., flame temperature and heat flux) due to the larger specific heat capacity of CO2 than N2, coal burnout rate has been confirmed to be affected significantly by char−CO2 and char−steam gasification reactions;2,3 ash formation pathways and its fouling/slagging characteristics are also potentially changed due to the different temperature profile and chemical reactions (e.g., sulfation and carbonation) in an oxy-firing process.4−6 Changes of the emissions of NOx and SOx have also been confirmed by both lab-scale and pilot-scale tests.7,8 For the former pollutant, the exclusion of air from the boiler helps eliminate thermal NOx. The use of a low-NOx burner can further minimize this pollutant to an environmentally acceptable level. In contrast, due to the smaller volume of oxy-firing flue gas, the SOx emitted has a high concentration relative to air mode. Furthermore, the higher oxygen partial pressure promotes the formation of corrosive SO3 in the superheater and reheater temperature zones. Sulfur in coal and its evolution during pyrolysis and air-firing have been studied comprehensively over the past decades and are summarized in several review papers.9,10 In brief, both © 2012 American Chemical Society
inorganic (e.g., pyrite, sulfate) and organic (i.e., disulfides, thiophene) sulfur are present in coal. The initial pyrolysis of coal favors the decomposition of thermally unstable organic disulfide and a portion of inorganic sulfur such as pyrite, the extent of which are highly dependent on temperature, coal particle heating rate, and gas environment (i.e., N2 versus H2).9,11 The decomposition of thiophene, however, is more difficult, occurring at approximately 1000 °C onward.12,13 Although applicable for air-firing cases, the knowledge gained in the literature has yet to be validated for an oxy-fuel mode in which CO2 and/or steam are predominant in flue gas. On the one hand, these two gases may attack a sulfur-bearing species or its derived fragments/radicals directly, thus affecting the decomposition pathway of sulfur. On the other hand, the char properties and its local gas environment are supposed to be affected by a CO2/steam-gasification reaction, which, in turn, affects the fate of the elements including sulfur hosted within the char. In short, little is known about the evolution of sulfur in a CO2/steam-rich environment. In this paper, a low-rank brown coal (i.e. lignite) named Victorian brown coal and its pyrolyzed char were studied under oxy-fuel mode to clarify the transformation behavior of organic sulfur, particularly thiophene. The thermodynamically unstable disulfidic sulfur is supposed to decompose quickly upon thermal shock, irrespective of the bulk gas environment (i.e., N2 versus CO2) in the reactor. The reaction pathways governing the fragmentation of pyrite in CO2 have also been clarified;9 however, the evolution of thiophene in CO2 has yet to be clarified. Victorian brown coal contains sulfur, which is Received: April 23, 2012 Revised: July 10, 2012 Published: July 11, 2012 4775
dx.doi.org/10.1021/ef300804e | Energy Fuels 2012, 26, 4775−4782
Energy & Fuels
Article
Table 1. Sulfur Content in Raw Coal and its 1000 °C Char Samples by CHNS Analyzera size, μm raw coal char char char char a
106−153 153
avg ± STD, wt %
dibenzyl disulfide, wt % of total S
dibenzothiophene, wt % of total S
RS/sulfate, wt % of total S
± ± ± ± ±
10.0 15.0 22.8 16.2 5.2
88.0 71.3 45.3 72.7 61.2
2.0 13.7 31.9 11.1 33.6
0.662 0.597 0.493 0.518 0.575
0.077 0.019 0.018 0.010 0.018
The components were obtained by curve-fitting of XANES spectra. quenched by dry ice to prevent any secondary reactions of the ash/ char particles. At least three repetitions were done for each condition, resulting in an average standard deviation error of approximately 5%. XANES Speciation on Sulfur. The sulfur K-edge spectra were collected at the National Synchrotron Radiation Research Centre (NSRRC) BL16A1 beamline, in Taiwan. The XAFS spectra were measured at the ratio of the X-ray intensity of the monochromatic incident beam (I0) relative to the fluorescent X-rays emitted by the samples in response to the absorption (If) by Lytle detector. X-ray scan energies (i.e., the energy of the incident photon beam) were 2−8 keV; its resolution is 1.5−2.1 × 10−4 keV. The X-ray beam was diffracted by a fixed-exit double crystal Si (111) monochromators to select X-ray energy. Its spot size achieved is 0.5 mm in height and 0.4 mm in width. The energy scale was calibrated with reference to an elemental sulfur K-edge absorption peak with energy of 2472 eV. The white-line peak position of the elemental sulfur was taken as zero-point of the energy for the sulfur K-edge spectra and all spectra and peak position shown in this study are given relative to this zero-point. Prior to analysis, a solid sample was homogeneously powdered and dispersed as a thin film on a sulfur-free kapton tape, which was subsequently mounted over the window of an aluminum plate for XANES spectrum collection. Considering that the thickness of the powder film on the tape can affect the sulfur XANES spectrum,19−21 the sample was well-ground and dispersed evenly by brushing on the tape to obtain a homogeneous powder film. Several replicas were made for the measurement of each sample to minimize the effect of sample heterogeneity. Intensity of the fluorescent-rays was recorded at each step over a period of two seconds. The step interval varied from 2.0 eV in the pre-edge region to 6 eV over the post-edge region. In the XANES region (−20 to +50 eV), the monochromator was stepped finely over the edge at 0.2 eV/step. A nonlinear least-squares component fitting program in ATHENA was employed to quantify the distribution of sulfur. Numerous standards (listed in Figure S2 in the Supporting Information) including dibenzyl disulfide and dibenzothiophene were chosen for component fitting, whereas the peak located at +9.0 eV was assigned as RS/sulfate, which is an s→p transition energy of organic and inorganic sulfur bonded with oxygen such as sulfate (−SO42−) or resonant scattering (RS) energy triggered by the neighboring elements such as carbon next to sulfur.16 The XANES spectra of model compounds were illustrated in Figure S1 in the Supporting Information. The MBACK background function was implemented to perform background correction and spectrum normalization (−200 to +233 eV). The error resulting from the distortion of the XANES spectra by selfabsorption effect or thickness effect was also corrected. Ultimately, the spectrum of a normalized sample was fit over the range from −20 below to +30 eV above the white-line maxima. The component fitting results indicated that thiophene, disulfide, and RS/sulfate were the predominant sulfur species in all the samples. Prior analysis of the mixture of dibenzyl disulfide and dibenzothiophene by XANES revealed a mass fraction of 80% for dibenzothiophene in the mixture, relative to a pre-determined 72% for this mixture. In light of this, the XANES analysis error for real samples throughout this study is estimated to possess a standard deviation of approximately 10%. Quantification of Sulfur and Metals. Sulfur was measured by a Varia EL-2 CHNS analyzer. Three replicates were performed for each sample, yielding an average standard deviation error of approximately 5%. The major metals including Al, Fe, Ca, Mg, Na, and K were quantified by inductively coupled plasma optical emission spectrosco-
dominated by organic thiophene and in which the interference of inorganic sulfur is negligible.14−16The focus of this paper is to elucidate the evolution of sulfur during the conversion of char, rather than the raw coal during oxy-fuel combustion. Char conversion in oxy-fuel mode is complicated by the coexistence of C−O2 and C−CO2 reactions in the post-flame-zone, which is supposed to affect sulfur evolution in a complex mode in comparison to air-firing. In this regard, the raw coal was first pyrolyzed at an extremely short residence time in a lab-scale drop-tube furnace (DTF). The resulting char was subsequently fed into DTF at various oxygen fractions and particle residence times. Instead of analyzing gaseous species, the solid samples were measured by synchrotron-based X-ray adsorption nearedge spectroscopy (XANES). In addition, HSC Chemistry 7.1 was employed to assess the reaction between thiophenic sulfur and different gases from a thermodynamic equilibrium perspective. This study is complementary to our previous works on elucidating the sulfur evolution upon Victorian brown coal hydrothermal upgrading and mild pyrolysis.16
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EXPERIMENTAL SECTION
Coal Properties. An air-dried Victorian brown coal of 105−153 μm was tested. On the air-dried basis, it is made of 2.56 wt % ash, 42.4 wt % volatile matter (VM), 42.1 wt % fixed carbon (FC), and 13 wt % moisture. The sulfur content is 0.662 wt%. Coal Pyrolysis and Char Oxidation Procedure. Coal pyrolysis was carried out in a lab-scale DTF under the following conditions to generate a char sample: furnace temperature 1000 °C, particle residence time 0.64 s, pure N2 (10 L/min). The schematic of DTF has been described in Figure S1 in the Supporting Information and has been detailed elsewhere.17,18 The coal feeding rate was fixed at 0.5 g/ min, which was entrained into the furnace by the cold primary gas at 1 L/min. Preheated secondary gas was fed into the reactor at 9 L/min to assist coal pyrolysis. The resulting char particles were quenched rapidly by dry ice at approximately 5000 °C/s and were subsequently collected in a 250 mL Erlenmeyer flask and a Waterman silica microfiber thimble filter (Cat. No. 2812259) installed downstream of the reactor. The high-purity thimble filter employed exhibits a capturing capacity of ≥98% of the fine particles down to 0.3 μm. Char yield of 21.8 wt % was achieved under these conditions. Subsequently, the generated char was sieved without grinding into four size ranges: >153 μm, 106−153 μm, 63−105 μm, and
