年生产能力180万吨矿区型选煤厂设计含8张CAD图
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Low rank coals sulphur functionality study by AP-TPR/TPO coupled with MS and potentiometric detection and by XPSAbstract Atmospheric pressure-temperature programmed reduction (AP-TPR) and X-ray photoelectron spectroscopy (XPS) techniques were applied to low rank coals sulphur study. Coal samples were pyrolysed in a flow of water vapor (WV). It was demonstrated that this treatment influenced mainly aliphatic sulphur. Samples were characterised by two methods and data were interpreted within the limits of the techniques. XPS measurements registered sulphur 2p spectra with two main signals for organic and inorganic sulphur compounds. The AP-TPR set-up, with potentiometric detection of the formed H2S as S2 _ using an ion selective Ag2S-electrode, gives quantitative data about the presence of different sulphur species. The AP-TPR equipment on-line coupled with a mass spectrometer (MS) gives extra qualitative information about different reductive and oxidative organic sulphur forms. Using MS not only H2S but also SO2, COS, CS2, and all other volatile sulphur and organic compounds can be monitored, giving more information for the initial presence of the different sulphur forms and to the mechanisms involved in the pyrolytic process. This AP-TPR-MS experiment is subsequently followed by AP-TPO-MS measurement (in an oxidated atmosphere) to study sulphur presence in the residue (tar and char) in the reactor. Comparing all these AP-TPR profiles results in a better assignment of the different signals to specific sulphur functionalities.1. Introduction The sulphur presence in coal is a serious obstacle for its utilisation since during combustion the sulphur is evolved into the atmosphere mainly as sulphur oxides.Besides the negative effect on human health, the evolution of these gases is a threat to our environment. The chemistry of sulphur has a significant impact on both reactivity and utilisation strategy of fossil fuels. Therefore, accurate information is needed on the distribution and speciation of organic sulphur forms in fuels to improve the understanding of emerging chemical and biological desulphurization processes as well as in predicting its behaviour during pyrolysis, gasification and liquefaction. Temperature programmed reduction at atmospheric pressure (AP-TPR) proves to be an effective technique for the specification of organic sulphur forms in coal 1. This method is based on establishing specific sulphur functional groups hydrogenated/reduced in pure H2 atmosphere to H2S at specific temperatures. Identification of other evolving gases by extended detection techniques makes it possible to obtain more quantitative and qualitative information about sulphur 2,3. By this extension not only H2S can be followed as a function of time/temperature of the reactor but also other volatile sulphur compounds can be recorded. An improved sulphur mass balance after each AP-TPR-MS experiment has been achieved by extra AP-TPO-MS experiment in pure O2 (oxidated atmosphere) where not reduced/not hydrogenated sulphur compounds left in the char or other volatile sulphur compounds captured in the tar can be analyzed 4. Using this APTPO- MS approach also, a difference can be made between the presence of residual different organic and inorganic sulphur compounds. To overcome problems in AP-TPR profile interpretation with the overlapping temperature range of some sulphur groups, a suit of chemical modifications of the samples under study can be applied 57. These modifications lead to transformation of certain sulphur groups into others, easier capable for AP-TPR kinetogram detection and interpretation. Analysis of model compounds as well as selective desulphurization of coal can be useful, in order to interpret the AP-TPR kinetograms 3,8. Therefore, also a mild pyrolysis in a flow of water vapor is applied 9,10. X-ray photoelectron spectroscopy (XPS) is another analytical technique that can be applied to solve the problem of speciating and quantifying sulphur forms in coal 11,12. The limitation of this technique is that it determines 2p signals from sulphur atoms situated only on sample surface. The aim of this study is to obtain new information by the combination of AP-TPR/TPO and XPS techniques with respect to sulphur specification in two low rank and high sulphur coals before and after desulphurization by mild water vapor pyrolysis. 2. Experimental The coal samples were collected from Elhovo and Katrishte deposits in Bulgaria. Typical for Elhovo is surface location of the coal strata. The samples freshly obtained from the mines above were stored under argon atmosphere before analyses. The proximate and ultimate analyses (wt.%) are listed in Table 1. The initial coal samples were treated with mineral diluted acids to remove inorganic sulphur and carbonate salts, because these compounds influenced the TPR profiles 2,13. Samples were ground 60 Am and treated consecutively by HCl solution (5%, 1 h, 80 jC) and HNO3 acid (12%, 3 h, ambient temperature, shaker), followed by a hot distillated water washing and dried under vacuum at 60 jC for 48 h. The lack of pyrite in treated samples was checked by X-ray diffraction spectroscopy. In a flow of water vapor (WV) at atmospheric pressure, demineralized coals (2 g) were subjected to a pyrolysis at 450 jC at a heating rate of 510 jC/min. The samples were kept at the final temperature for 2 h. XPS sulphur 2p binding energy provides a sensitive measure for the electronic character of sulphur within a molecule. The XPS measurements were carried out in the analysis chamber of the electron spectrometer ESCALAB-MkII (VG Scientific) at a base pressure of 1_10_ 8 Pa. The spectra were excited with an Mg-Ka radiation at an instrumental resolution of c0.9 eV. The C 1s line had a binding energy of 284.8F0.1eV and no charge effects were observed. The acquisition time for S 2p spectra was over 3 h at 200W X-ray power (20 mA/10 kV). The AP-TPR study performed by heating the sample in a reducing atmosphere is described previously 1. Briefly, at discrete temperatures specific sulphur functionalities are reduced/hydrogenated to H2S, which can be subsequently detected. The data obtained by potentiometric detection system were supplemented with MS results by coupling the AP-TPR reactor on-line with a mass spectrometer (FISONS-VG Thermolab MS) through a fused silica capillary heated at 135 jC. The mass spectrometer equipped with a quadrupole analyzer was set at an ionizing voltage of 70 eV. The AP-TPO technique was applied to check if any sulphur is left in the residue after AP-TPR (char and tar). The reactor set-up was the same as in an AP-TPR analysis but hydrogen was replaced by oxygen. The flow rate of the pure oxygen gas was set at 100 ml/ min. The SO2 evolution was monitored during reactor heating at 20 jC/min from 25 up to1100 jC.3. Results and discussion The consecutive treatments by diluted acids and steam pyrolysis decreased the sulphur contents of coal samples under study (Table 2). For Elhovo coal the used procedure seems to be more effective, 82% degree of sulphur content decrease compared to 59% for Katrishte coal. The treatment with mineral acids at applied conditions affected not only the mineral sulphur, resulting in its disappearance, but also removed and degraded parts of the easier accessible organic sulphur compounds. Respectively, total sulphur (St) will be equal to organic sulphur (So). The determination of So species were checked by XPS and TPR analyses. XPS spectra were interpreted by using a curve resolution method. The signal of a sulphur single species was composed by two peaks representing 2p3/2 and 2p1/2 components having a 2:1 relative intensity and separated in energy by 1.2 eV. We performed peak synthesis for S 2p by mixed Gaussian and Lorentzian line shapes with full width at half maximum of 2.60 eV for each sulphur species. XPS spectra obtained by a deconvolution procedure are presented in Fig. 1 while their XPS data are included in Table 3. Differences between the (S/C) atomic ratios for the studied samples, determined by the XPS technique and elemental analyses, in absolute value were in the range of 0.001 to 0.003 atomic ratio units, or expressed in %, in the range of 3.6% to 17.7%. In all cases, the S/C ratios determined by XPS were smaller than the corresponding values calculated from elemental analyses. This indicates some systematic differences between bulk and surface S/C ratios and thus some differences in organic species distribution. Subsequently, the XPS results becomes very imprecise, if the amounts of sulphur in different forms are close to the detection limits (0.1 atomic %) 14. This is the reason why a great difference is registered between the ultimate analysis and the XPS results, certainly in the case of the determined amount of sulphatic sulphur (Ss). The relative proportions of sulphur functionality concentrations were calculated from the peak area ratios of the XPS spectra. The assignments of the sulphur forms were based on reference data 11,12 and our own analyses. Their relative concentrations in XPS spectra were determined using peak values fixed at 163.3, 164.1, 168.4 and 170.4 eV of binding energy for sulphidic, thiophenic, sulphonic and sulphatic sulphur forms, respectively. The range with peak at 163.3 eV binding energy comprises organic and inorganic (pyritic) sulphidic sulphur compounds in the native coals. This range is addressed only to organic sulphidic sulphur compounds after treatment with mineral acids. Data in Table 3 depict a strong decrease in sulphate content after treatment with mineral acids. A small increase in sulphones was noticed, possible due to the nitric acid treatment. An increase in sulphidic and thiophenic species for both samples was registered after demineralization. WV treatment results in a spectacular decrease of sulphidic and sulphone forms in favour of the thiophenic forms. This should be considered only relatively because a further desulphurization has proceeded. For the Elhovo coal, an increase in the sulphate content was observed after WV pyrolysis. This fact could be explained by some sulphur transformation (possibly a mineralization) from organic to inorganic forms in the WV heating procedure. The H2S kinetograms of AP-TPR with potentiometric registration of native and treated coals are quite similar to the ones with MS detection set-up (sum of m/z 34 and m/z 33 fragments). The last ones are therefore shown by Figs. 2 and 3, accompanied by the curves for m/z 48 (SO) and 64 (SO2). AP-TPR H2S/HS profiles for the two raw coals are rather different from each other. Katrishte coal kinetogram (Fig. 2a) is dominated by two separated peaksone with a maximum around 445 jC (starting at 300 jC) and the second one around 685 jC. For Elhovo coal (Fig. 3a) in the kinetogram one global and complex signal is registered with two main maxima, one f325 jC (starting at 200 jC) and the other f645 jC, as well as a number of small shoulders/peaks at 435, 505, 540, 565, 605 and 740 jC. The first peak in Katrishte coal profile could be attributed to a wide range of nonthiophenic sulphur groups, precisely the lower temperatureto aryl thiols and the higher temperatureto dialkyl and mixed arylalkyl sulphides. The second peak refers to the presence of pyrite and diaryl sulphides, and thiophenic structures. In the case of Elhovo coal, the peak at 325 jC corresponds to the presence of alkyl and aryl thiols, the small peak around 435 jC to dialkyl and arylalkyl sulphides. The complex huge signal maximizing at 645 jC could be related to pyrite, diaryl sulphides and to less complex thiophenic structures. The shoulder around 740 jC could be attributed to more complex thiophenic compounds. Above 950 jC the signal starts again to increase, an indication for the beginning of the reduction of inorganic sulphate groups. A sulphur recovery of 61% is calculated for Elhovo coal by potentiometric detection while this value is 89% for Katrishte coal, both values for native coals (Table 4). The recovery for organic sulphur must be a little higher because sulphatic sulphur is almost not reduced into H2S and thus not detectable by potentiometric detection set-up 3,8. For Elhovo the amount of Ss is quit high, i.e. 37.7% of the total amount of sulphur originally present and for Katrishte it is 10.8%. So the recorded potentiometric quantities are in agreement with this amount, i.e. there is low sulphur recovery for Elhovo because only around 62.3% of the total amount of sulphur could be theoretically found by AP-TPR if it is 100% efficient. All organic and pyritic sulphur forms are then reduced/hydrogenated into H2S surrounding and if only minor amounts of oxidized organic sulphur forms are present in these coals asindicated by the m/z 64 and m/z 48 (Figs. 2a and 3a). This higher sulphur recovery forKatrishte will be close to the theoretically 89.2% if AP-TPR is again 100% efficient. For m/z 64 curve a contribution of an alkyl fragment C5H4 + can interfere 3. It can be concluded out of these profiles, that some small amount of organic sulphonic acids is also present in the case of Elhovo coal (broad maximum f315 jC in the SO kinetogram) and less in the case of Katrishte coal (maxima at f320 jC and 400 jC) together with some sulphones (675 jC for Elhovo and Katrishte coals). Normally, SO2 and SO profiles overlap when they only correspond to oxidized sulphur forms 3,8. Additionally, oxidized sulphur groups are also partly reduced in AP-TPR media 8. The AP-TPO-MS profiles of these native coals (Fig. 4) demonstrate that after the reduction experiment some organic sulphur compounds are left in the char or captured in the tar AP-TPR H2S/HS profiles after demineralization are more influenced in the case of Elhovo coal compared to Katrishte coal. If pyritic and sulphatic sulphur are completely removed and only organic sulphur is present, then it is evident from data (Tables 1 and 2) that in the case of Elhovo coal also the organic sulphur compounds are more degraded and removed than in the case of Katrishte coal. The low sulphur recovery for Elhovo coal (43%) compared to the amount determined for Katrishte coal (64%) could support the abovementioned assumption. Figs. 2b and 3b, profiles for SO (m/z 48) and m/z 64, are pointing into the same direction, i.e. for Elhovo a relative high intensity for SO and for m/z 64 are found if both are compared with the H2S/HSkinetogram and with the kinetograms for Katrishte. In both coals, the signals in the SO+ profile f225 jC for Elhovo coal and 240 and 290 jC for Katrishte coal refer to newly formed organic sulphonic acids groups during demineralization treatment. AP-TPOMS (Fig. 4) results prove that some organic sulphur compounds were still left in the char or incorporated in the tar. AP-TPR-MS profiles (Figs. 2c and 3c) demonstrate that at the experimental conditions of water vapor pyrolysis for both coals almost complete removal of nonthiophenic sulphur compounds is observed. In both coals only one huge peak mostly for thiophenic sulphur around 650695 jC for Katrishte coal and around 595695 jC for Elhovo coal is found. In the two coals, the lower temperature region of this single peak, points to the presence of mixed alkylaryl sulphides. The fact of the almost complete removal of non-thiophenic sulphur was also confirmed by using the reducing solvent mixtures in an AP-TPR experiment with potentiometric detection system 1,6,7. The use of the reducing solvent mixture in this AP-TPR experiment makes (up to 300 jC) sulphur compounds reduced/hydrogenated in the lower temperature range more visible 1, if present. However, no such experimental evidence was found, indicating the absence of pure alkyl sulphides or thiols. APTPR- MS also showed that in this WV pyrolysis samples no longer organic sulphonic acids are present (no SO signal in the temperature range 200400 jC). Only some small amounts of sulphones/sulphoxides are still left (maximum f640 jC for Elhovo coal and f675 jC for Katrishte coal). Again a better sulphur recovery using AP-TPR with potentiometric detection compared with the demineralized samples is found. This indicates an improved accessibility towards the reducing/hydrogenating possibilities and the presence of less oxidized sulphur compounds. AP-TPO-MS proved once again that some sulphur compounds are left in the char fraction or trapped by the tar fraction in the reactor 4. Conclusions XPS and TPR analyses proved a complete removal of non-thiophenic sulphur by WV pyrolysis. XPS and AP-TPR techniques indicate an increase in oxidized sulphur compounds after treatment with mineral acids. AP-TPR with potentiometric detectionand AP-TPR-MS profiles demonstrate that organic sulphur groups present in both samples were very different from each other: Elhovo coal contains more aliphatic sulphur compounds while Katrishte coal is enriched in more complex thiophenic structures. Acknowledgements The authors are grateful to Lic. Sc. G. Reggers, Mr. J. Kaelen and Mr. K. Van Vinckenroye for their technical assistance during the AP-TPR experiments. This study was supported by FWO-Flanders and BAS-Bulgarian bilateral project.References1 J. Yperman, I.I. Maes, H. Van den Rul, S. Mullens, J. Van Aelst, D.V. Franco, J. Mullens, L.C. Van Poucke,Sulphur group analysis in solid matrices by atmospheric pressure temperature programmed reduction,Anal. Chim. Acta 395 (1999) 143155.2 P. Rutkowski, S. Mullens, J. Yperman, G. Gryglewicz, AP-TPR investigation of the effect of pyrite removalon the sulphur characterization of different rank coals, Fuel Process. Technol. 76 (2002) 121138.3 S. Mullens, J. Yperman, G. Reggers, R. Carleer, A.C. Buchanan III, P.F. Britt, P. Rutkowski, G. Gryglewicz, A study of the reductive pyrolysis behavior of sulphur model compounds, J. Anal. Appl. Pyrolysis (2003) (in press).4 S. Mullens, PhD thesis, Limburgs Universitaire Centrum, Diepenbeek, 2000.5 I.I. Maes, S.C. Mitchell, J. Yperman, D.V. Franco, S.P. Marinov,
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