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Sulfur and Chlorine Gas Species Formation during Coal Pyrolysis in Nitrogen and Carbon Dioxide Atmosphere Lorenz Julian Frigge, Ghada Elserafi, Jochen Ströhle, and Bernd Epple Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01080 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 14, 2016
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Sulfur and Chlorine Gas Species Formation during Coal Pyrolysis in Nitrogen and Carbon Dioxide Atmosphere Lorenz Frigge, ∗ Ghada Elsera , Jochen Ströhle, and Bernd Epple Institute for Energy Systems and Technology, Technische Universität Darmstadt, Otto-Berndt-Straÿe 2, 64287 Darmstadt, Germany E-mail:
[email protected] Abstract Coal combustion in CO2 /O2 (oxy-fuel combustion) allows reducing fossil carbon dioxide emissions in the atmosphere. The change of oxidizer from N2 /O2 (air) to CO2 /O2 changes the combustion process, including pyrolysis. The formation of sulfur and chlorine species during temperature-programmed coal pyrolysis in N2 and CO2 atmosphere and with heating rates of 10 and 20 K/min was studied. Two high volatile bituminous coals with dierent sulfur content but almost identical relative distribution of sulfur species (sul de, pyritic, sulfate and organic) were used in the experiments. The release of H2 S, SO2 , COS and HCl was determined using mass spectrometry. Two and three peaks were observed in the hydrogen sul de and sulfur dioxide release, respectively, which are caused by the dierent sulfur species present in the coals. It was found that the release pro le of sulfur dioxide varies for the coals, which means that information about sulfur forms is not su cient to predict SO2 formation during pyrolysis. While the release rate of hydrogen sul de and sulfur dioxide was either similar or lower in the presence of CO2 when compared to pyrolysis in N2 , formation of carbonyl 1
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sul de was only detected in CO2 atmosphere. For hydrogen chloride, two distinct release temperature ranges were identi ed. The predominant HCl formation occurred
at temperatures ranging from 300 to 500 , and is reduced reduced by changing from N2 to CO2 atmosphere. The fate of sulfur and chlorine during pyrolysis of the two coals is very dierent although both coals are very similar regarding sulfur species distribution.
Introduction Coal, being a relatively abundant primary energy source, continues to play a vital role in the world's present energy mix. The concept of carbon capture and storage (CCS) could be employed to mitigate the contribution of coal combustion to climate change. Oxy-fuel combustion is a CCStechnology, which was originally developed to obtain large amounts of CO2 for use in enhanced oil recovery. 1 The technology remains under intensive research and is close to demonstration. Oxy-fuel combustion of coal uses pure oxygen instead of air as oxidant in order to achieve high CO 2 concentrations in the product gas. A certain proportion is subsequently recirculated into the combustion chamber to regulate ame temperature. Accordingly, the resulting gas atmosphere in the combustion chamber comprises mainly CO2 and O2 . Sulfur is found in coal as sul des, sulfate, organic sulfur and trace amounts of elemental sulfur. 2 Each of these forms is released at a dierent temperature during pyrolysis. The typically small amount of chlorine in coal is represented by mineral, organic and sorbed (as anion in pores) chlorine. 3 During pyrolysis, the introductory stage in the process of coal combustion, volatile gases are released from the coal particle. Several sulfur and chlorine compounds are detected among the released volatile gases, depending in species and quantity on coal composition. As a result of ue gas recirculation, all gas components experience an increase in concentration as they re-enter the combustion chamber. Due to the risk of furnace corrosion posed by the presence of sulfur and chlorine species, a comprehensive study investigating the in uence of gas atmosphere (changing coal combustion conditions 2
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from conventional air ring to oxy-combustion) on the evolution of these species during pyrolysis is essential. Earlier investigations on the eect of the atmosphere during coal pyrolysis have agreed that the presence of CO 2 in the atmosphere enhances the release of COS due to the formation of CO at high temperatures. 4,5 A TG-FTIR study performed with a Chinese bituminous coal has found that, in comparison to pyrolysis in N 2 atmosphere, the release of SO 2 and
is suppressed in CO atmosphere. In the case of SO , it was reported that further pyrolysis beyond 800 shows release rates higher than those in N H2 S until approximately 800
2
2
2
atmosphere. 4 An investigation carried out using a micro- xed-bed reactor and GC analysis observed dierent behaviors from four Chinese coals (dierent ranks and dierent sulfur contents) regarding H 2 S release. At temperatures below 500
less H S is released from 2
all four coals during slow pyrolysis (5 K/min) in CO 2 atmosphere in comparison with N 2 atmosphere. In a temperature range from 500 to 600
the eect of the CO
2
atmosphere
was dependent on coal type. It was reported that a CO 2 atmosphere promotes the release of H2 S at temperatures greater 650
for all four coals. 5 The in uence of macerals on the
sulfur release behavior during coal pyrolysis was investigated using a xed bed reactor coupled to a gas chromatograph with ame photometric detection 6 or by a TGA-MS setup in combination with xed bed reactor experiments with FTIR spectrometry and X-Ray diractometry. 7 It was found that less sulfur is released during pyrolysis of inertinite-rich coals. A combination of xed bed reactor and gas chromatograph was used to study the eect of adding minerals to coal samples on sulfur retention during pyrolysis. 8 In another study, lignites were demineralized and blended with pyrite and sulfates to study the transformation of the dierent sulfur forms during pyrolysis using TGA, TGA-MS and a xed bed reactor. 9 It was found that added sulfates dier signi cantly from coal-inherent sulfates in their sulfur release behavior. Almost all chlorine present in coal is released in the form of HCl during pyrolysis. 10 Studies on HCl formation during slow pyrolysis (< 20 K/min) in nitrogen used a combination of
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FTIR spectrometry and ion chromatography, 10 mass spectrometry and FTIR spectrometry 11 or NDIR spectrometry. 12 Another study estimated the chlorine volatilization by determining the chlorine present in the char after pyrolysis by potential titration. 13 Although in some studies the behavior of chlorine during gasi cation at high temperatures (>800
) was inves-
tigated, 12,13 so far little research has been conducted regarding the temperature-dependent formation of HCl during coal pyrolysis in CO 2 atmosphere. When red in furnaces or boilers, coal particles will experience high heating rates on the order of 1000 K/s or higher. It was found that sulfur species release behavior diers between low and high heating rate experiments. In a study where experiments were conducted with TGA-MS (low heating rate of 20 K/min) and a coupling of Curie-point pyrolyzer and gas chromatograph (high heating rate of 3000 K/s), the high heating rate caused more sulfur to
be released from the coal samples. 14 A TGA study conducted with heating rates of 20 K/min and 100 K/min found that the higher heating rate caused less of the original coal sulfur to remain in the char. 15 During the pyrolysis of biomass, chlorine release was found to be signi cantly increased at high heating rates when compared to low heating rates. 16 The purpose of this study is to identify the temperature range of the transformation of coal sulfur and chlorine and the evolved gas species during coal pyrolysis. While the release behavior obtained in the experiments is not representative for high heating rate coal combustion processes, it is possible to conjecture on the relevant sulfur or chlorine transformation mechanisms. The focus of this study lies on four prominent sulfur and chlorine species released during coal pyrolysis: H 2 S, SO2 , COS and HCl. The volatile release behavior regarding these compounds during coal pyrolysis in N 2 and CO2 atmosphere during temperature-programmed pyrolysis was studied using mass spectrometry for product gas analysis.
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Experimental The coals used in this study were a US-American high volatile C bituminous coal (U2) with a relatively high sulfur content and a high chlorine content (the chlorine content of U2 coal is one order of magnitude higher than world average 3 ), and a Colombian high volatile C bituminous coal (K1) with a moderate sulfur and low chlorine content. Both coals are similar regarding their proximate analysis. The relative distribution of sulfur species is almost identical for both coals. Pyritic, sulfate and organic sulfur account for approximately 15 %, 16 % and 69 % of total sulfur, respectively, while sul des could not be detected. An analysis regarding the occurrence of dierent chlorine species present in the coal was not performed for this study. The coals were sieved to obtain particle diameters of 90 to 125 µm. The properties of both coals are given in Table 1. Table 1: Properties of studied coals (wt%, dry basis)
a
U2 K1 Proximate analysis Ash 9.33 10.38 Volatiles 37.30 36.89 Ultimate analysis Carbon 72.91 70.25 Hydrogen 4.97 4.77 Nitrogen 1.25 1.24 a Sulfur 3.01 1.03 Chlorine 0.25 0.01 Sulfur forms Sul des
