Communication pubs.acs.org/EF
Significant Evolution of Hydrogen Fluoride from Coal Chars after Apparently Complete Release of Carbon Dioxide Naoto Tsubouchi,*,† Yuuki Mochizuki,† Naoyuki Iwabuchi,‡ Yuuki Akama,‡ and Yasuo Ohtsuka‡ †
Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 5, Kita-ku, Sapporo 060-8628, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
‡
Coal usually contains fluorine element in the concentration range of 20−500 μg/g of coal.1,2 It has been reported that fluorine is emitted mainly in the form of HF during pyrolysis, combustion, and gasification1−5 and that HF causes corrosive effects on their facilities and may be involved in the formation of hazardous organic fluorine compounds. There is also further concern that HF not only causes corrosion problems on gas turbine materials in integrated gasification combined cycle (IGCC) but also deteriorates the performance of the fuel cell in integrated gasification fuel cell (IGFC).6 It may thus be important to understand fluorine chemistry for developing superclean coal technologies that should be targeted on zero emissions. However, only quite limited information on this topic has been provided thus far.3−5 In particular, there is a distinct lack of information on the dynamics of HF evolution. In this communication, we examine the dynamics of HF release during coal pyrolysis, investigate the influence of the coexistence of H2O or O2 gas on HF formation from char after devolatilization, and exhibit the significant evolution of HF from char at high burnoff of >90% after the apparently complete release of CO2. For these purposes, HF is determined with an online monitoring method. Three kinds of sub-bituminous coals, AT and DK coals from South Africa and CN coal from Australia, were used in the present study. These samples were air-dried at ambient temperature, ground, and sieved to the particles with a size fraction of 150−250 μm. Their carbon, fluorine, and ash contents were in the range of 78−80 mass % on a dry and ashfree basis (daf), 20−80 μg/g on a dry basis, and 11−13 mass % on a dry basis, respectively. All experiments were conducted with a flow-type fixed-bed quartz reactor under ambient pressure. The details of the apparatus have been described elsewhere.7−9 In a temperature-programmed pyrolysis (denoted as TPP), about 3.0 g of the coal sample was heated at 10 °C/min to 1000 °C in a stream of high-purity N2 (>99.9995%). The resulting char was then exposed to 5000 ppm of H2O/N2 at 1000 °C. It was also subjected to a combustion run, in which the char sample quenched after the TPP was heated in 20% O2/N2 at 10 °C/min up to 1000 °C and soaked for 8 h. In all runs, the evolution of HF was monitored online at intervals of 10 s with a fluoride ion selective electrode. The fluorine content of coal or char was analyzed according to the oxygen bomb method (ASTM D2361).10,11 When the heating run was repeated at least twice, the reproducibility of these fluorine analyses fell within ±3% in all cases. Fluorine conversion to HF or char was calculated on the basis of total fluorine content in the feed sample. The ashes recovered after combustion were also characterized by F 1s X-ray photoelectron spectroscopy (XPS) using Mg Kα radiation. © XXXX American Chemical Society
Figure 1a illustrates the rate profile of HF evolved in the TPP run of AT coal. The evolution of HF started at around 300 °C,
Figure 1. Rate profile for HF formation from AT coal in (a) highpurity N2 and (b) 5000 ppm of H2O/N2 at 1000 °C.
and the profile exhibited a small peak at 400 °C and a large peak at about 680 °C. Similar results were also observed with DK and CN coals. The presence of these peak temperatures indicates different types of HF sources. Fluorine conversion to HF can be estimated by integrating the profile shown in Figure 1a. The conversion up to 1000 °C was as small as ≤5%, irrespective of the kind of coal. On the other hand, fluorine conversion to char was 90−95% with every coal. These results highlight the fact that fluorine in coal is not readily released as HF during pyrolysis but rather retained in the char. We have recently shown that, when 24 coals with carbon contents of 71−92 mass % daf are pyrolyzed in the same manner as above, approximately 50−95% of the chlorine in coal is converted to HCl up to 1000 °C in almost all cases.7,8 Thus, most fluorine appears to be more thermally stable than chlorine with respect to hydrogen halide formation in the TPP. Because some fluorine in coal has been reported to be present as apatite [Ca5(PO4)3F], muscovite [KAl2(Si3Al)O10(OH, F)2], and mica (KMg3AlSi3O10F2),2,5,12 it is of interest to examine the dynamic behavior of HF formation from such compounds. In this work, Ca5(PO4)3F and KMg3AlSi3O10F2 were mixed physically with pure carbon formed by carbonizing powdery phenol resin and then subjected to the same TPP run as the coal shown in Figure 1a. The carbon and fluorine contents of the mixture were 91 and 0.10 mass % on a dry basis, respectively. The mixed Ca5(PO4)3F/carbon and KMg3AlSi3O10F2/carbon samples gave the HF profiles peaking at 400 or 1000 °C, respectively, and the former peak was partly overlaid with the peak observed at 400 °C (actually 380−420 °C) for the three coals (Figure 1a). Part of fluorine in coal may thus be present as apatite, which might Received: March 10, 2016 Revised: April 18, 2016
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DOI: 10.1021/acs.energyfuels.6b00591 Energy Fuels XXXX, XXX, XXX−XXX
Communication
Energy & Fuels react with H2O released in the TPP to form HF. In Figure 1a, the sources of HF observed at 600−900 °C are not clear and should thus be clarified in future work. Figure 1b shows the rate of HF evolved upon H2O exposure of AT char after the TPP. Only a slight amount of HF was detected, and the conversion to HF after 2 h (char conversion, 10−15%) was only 2−3%, irrespective of the kind of feed char. In other words, most fluorine in pyrolyzed char was almost unchanged. When Ca5(PO4)3F and KMg3AlSi3O10F2 samples after heat treatment at 1000 °C were held under the same conditions as the char shown in Figure 1b, it seemed that the coexistence of 5000 ppm of H2O promoted HF formation from the latter compound, although the conversion to HF was only 3%, as compared to the results for apatite and without H2O added. These results show that fluorine in char after pyrolysis and the fluorine compounds are very stable with respect to HF formation even when exposed to steam at 1000 °C. On the basis of thermodynamic calculations, KAl2(Si3Al)O10F2 formed after dehydration of KAl2(Si3Al)O10(OH, F)2 is thermally unstable and might thus be converted to K2SiF6 and AlF3 upon pyrolysis (eq 1).
Figure 2. Concentration of HF or CO2 formed (a) during combustion of AT char and (b) after a significant decrease in the CO 2 concentration.
chars. Figure 2b also suggested a similarity between the evolution of CO2 and HF. Further, the XPS analyses of the ashes recovered after 8 h of combustion showed the presence of the distinct F 1s peaks, which are mainly associated with organic fluorine. On the basis of the above-mentioned results, it is possible that fluorine in coal exists predominantly as thermally stable included minerals in the carbon matrix, and thus, most fluorine is evolved significantly at high carbon burnoff of >90% after the apparently complete release of CO2, with the rest being retained at less reactive carbon in ash after combustion. It has been reported that fly ashes formed during pulverized coal combustion contain fluorine element and that fluorine is enriched at the ash surface and present dominantly as organic C−F forms.10 Surface oxygen functional groups produced during char combustion may play an important role in the formation of organic fluorine, because it has been widely accepted that such oxygen species exist in coal chars and can work as reactive sites with O2, H2O, and CO2.13,14 In conclusion, HF formation from char proceeds during combustion at 1000 °C, and the concentration increases considerably after the apparently complete release of CO2. Fluorine in the ashes after combustion is present mainly as organic C−F forms.
KAl 2(Si3Al)O10 F2 → 1/6K 2SiF6 + 1/3AlF3 + 2/3KAlSi3O8 + Al 2O3 + 5/6SiO2
(1)
The standard Gibbs free energy changes (ΔG) for eq 1 are estimated to be −42 and −48 kJ/mol at 800 and 1000 °C, respectively. In addition, equilibrium conversion for the reaction of 5000 ppm of H2O with K2SiF6 (eq 2) or AlF3 (eq 3) at 1000 °C is 77 or 74%, respectively. Thus, part of HF observed in Figure 1b might also be formed via eqs 1−3. K 2SiF6 + 3H 2O → 6HF + K 2O + SiO2
(2)
AlF3 + 3/2H 2O → 3HF + 1/2Al 2O3
(3)
On the other hand, the formation of HF by the reaction (eq 4) of H2O with Ca5(PO4)3F is unfavorable thermodynamically under the present conditions, because the ΔG value is +125 kJ/ mol at 1000 °C. Ca5(PO4 )3 F + H 2O → HF + Ca5(PO4 )3 OH
(4)
It is therefore likely that Ca5(PO4)3F, if present actually, is retained in solid phase even in the coexistence of H2O at 1000 °C. Figure 2a presents the concentration of HF or CO2 evolved in the combustion process of AT char after the TPP run. CO2 formation started at around 350 °C, and the concentration increased with increasing temperature but decreased while holding the sample at 1000 °C. These trends were also observed with DK and CN chars. Total amounts of released CO2 corresponded to about 90−95% of carbon present in the feed chars. On the other hand, HF formation took place mainly after the apparently complete release of CO2, and the conversion to HF after soaking for 8 h at 1000 °C was estimated to be approximately 40−50% in every case. It is of interest to compare the dynamics of HF and CO2 released after carbon conversions of 90−95%, that is, at the latter stage of the combustion. The results for AT char are shown in Figure 2b. A significant amount of CO2 was detectable, and the concentration lowered gradually with increasing time on stream. Similar results were also observed for DK and CN chars. These observations point out the occurrence of oxidation reactions of less reactive carbon in the
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +81-11-706-6850. Fax: +81-11-726-0731. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The present study was supported in part by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors gratefully acknowledge the assistance of Hiroyuki Hashimoto and Megumi Nishio in carrying out experiments.
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b00591 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.6b00591 Energy Fuels XXXX, XXX, XXX−XXX
