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Effects of HS on the reactivity of ilmenite ore as chemical looping combustion oxygen carrier with methane as fuel Yewen Tan, Zhenkun Sun, Arturo Cabello, Dennis Y. Lu, and Robin Hughes Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03012 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018
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Energy & Fuels
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Effects of H2S on the reactivity of ilmenite ore
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as chemical looping combustion oxygen carrier
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with methane as fuel
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Yewen Tan1, Zhenkun Sun, Arturo Cabello, Dennis Y. Lu, Robin W. Hughes
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Natural Resources Canada, CanmetENERGY-Ottawa
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1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1
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KEYWORDS
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chemical looping combustion; ilmenite; sulfur; thermogravimetric analysis
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ABSTRACT
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A series of experiments were carried out in a pressurized thermogravimetric analyzer
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(PTGA) to study the effects of H2S on the reactivity of ilmenite oxygen carrier under
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pressurized conditions. The total pressure was maintained at 0.4 MPa(absolute). Methane
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was used as fuel and its partial pressure varied from 0.08-0.119 MPa. The concentration of
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H2S varied from 2020 ppm to 3030 ppm. The test temperature ranged from 1073-1173 K.
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The results showed that the reactivity of the ilmenite oxygen carrier improved with the
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presence of H2S, for both the reduction and the oxidation steps. Scanning electron
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microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron
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spectroscopy (XPS) analyses were conducted to assess sulfur deposition on the reduced
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ilmenite samples. Thermodynamic analyses were performed to help interpret the test
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Corresponding author,
[email protected] 1 ACS Paragon Plus Environment
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results. A kinetic model was developed based on the experimental results. The model
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showed an activation energy for ilmenite reduction with H2S at ~79 kJ mol-1 when H2S was
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present and 104 kJ mol-1 for pure methane.
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1. Introduction
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Chemical-looping combustion (CLC) has been promoted as a promising technology for
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reducing CO2 emissions from the combustion of fossil fuels. As a result, much attention has
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been given to develop the CLC technology for both gaseous (natural gas, syngas, etc.) and
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solid (coal, coke, biomass) fuels and comprehensive reviews on the CLC technology have
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been published.1-2 Recently, pressurized CLC technology (PCLC) has attracted attention due
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to its higher efficiency compared to the atmospheric pressure CLC technology3-8 and
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CanmetENERGY-Ottawa has been conducting research of the PCLC technology since 2010.9-
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Sulfur is a common component in many fuels, both gaseous and solid ones. Even with the
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desulfurization process, some sulfur can still be present in the fuel and this presence of
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sulfur can have an impact on the oxygen carrier used in both the CLC and PCLC processes. In
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a thermodynamic study, Wang et al. investigated the effect of sulfur on various oxygen
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carriers for CLC of syngas.13 They concluded that carbon and sulfur deposition on the
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oxygen carriers were enhanced by increasing pressure. They also found that the presence of
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H2S led to more carbon deposition. According to them, the most probable solid sulfur
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compounds deposited on oxygen carriers were Ni3S2, Fe0.84S, MnSO4 and CoS0.89 for NiO,
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Fe2O3, MnSO4 and CoO oxygen carriers, respectively.
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Jerndal et al. performed a thermal analysis of CLC with CH4 as fuel and H2S as a
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contaminant.14 The study concluded that, for several pairs of oxygen carriers including 2 ACS Paragon Plus Environment
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Energy & Fuels
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Fe2O3/Fe3O4, H2S in the fuel was converted to SO2 with 99.3 to 100% efficiency, for
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temperatures between 873 and 1473 K. They also showed that, with Fe2O3/Fe3O4, there was
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no risk of sulfide or sulphate formation under the conditions investigated in their work.
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Experimental research activities have also been carried out to study the effect of sulfur on
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the performance of the oxygen carriers with a variety of fuels such as syngas, methane, sour
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gas,15-19 acid gas,20 kerosene21 and even elemental sulfur.22 In a thermogravimetric analyzer
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(TGA) study with syngas-H2S (4042 ppm) using bentonite supported iron oxide oxygen
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carrier, Tian et al. found that H2S decreased the reaction rates of both reduction and
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oxidation by 1.5 times although the reduction capacity was unaffected.23 They also observed
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that direct reduction of Fe2O3 by H2S was negligible and that the sulfidation reaction
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between H2S and oxygen carrier occurred after the initial oxygen carrier was first reduced
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by CO and H2. X-ray photoelectron spectroscopy (XPS) analyses showed the presence of FeS
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and FeS2 on the reduced samples.
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Gu et al. studied the effect of sulfur on iron ore oxygen carrier in a TGA using CO as fuel with
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3000-6000 ppm of H2S under both atmospheric and pressurized conditions.24 They found
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that the initial reduction of iron ore was not affected by H2S due to the much higher
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concentration of CO. However, after about 2000 s, the weight loss rate was lower compared
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to the case without H2S addition. When the reaction time was long enough, a weight gain
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was observed, probably due to sulfide deposition. Gu et al. observed that as the iron ore was
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progressively reduced to Fe3O4, FeO and Fe, the reactions between iron ore and CO became
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slower so that the sulfidation rate became competitive with the reduction rate of Fe3O4 and
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FeO by CO.
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Forero et al. studied the effect of H2S in CLC with copper-based oxygen carriers using CH4 as
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fuel in a bench scale fluidized bed.16 They found that while H2S had a negative effect on the
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oxygen carrier reactivity, this negative effect did not happen until oxygen carrier to fuel
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ratio dropped to below 1.5. No deactivation was observed when the ratio was above 1.5.
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Moldenhauer et al. used ilmenite as an oxygen carrier with both sulfurous and sulfur-free
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kerosene in a 300 W CLC reactor.21 They found that sulfur had a positive effect on ilmenite
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reactivity and there was no evidence of sulfur poisoning or deactivation of the ilmenite
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oxygen carrier.
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Literature studies cited above showed that sulfur deposits on iron-based oxygen carriers
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could be completely removed with air and the regenerated oxygen carrier maintained its
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reactivity. This is not the case for non iron-based oxygen carriers. For example, in the case
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of Ni-based OCs, this type of materials were deactivated by the presence of sulfur and fuels
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with sulfur contents above 100 ppm H2S were not adequate to be used in CLC units.15 The
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material was only regenerated when the H2S feeding was stopped. Other type of promising
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oxygen carriers that cannot be directly regenerated with air are the CaMnO3-based
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perovskites because sulfur reacts with the calcium present in the perovskite forming
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compounds such as CaSO4 and/or CaS at conditions which could lead to the damage of the
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perovskite structure.17, 25-26 This shows that one of the advantages with iron-based oxygen
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carriers is its resistance to sulfur poisoning.
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In summary, most of the studies used syngas as fuel to study the effect of sulfur in CLC and
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these studies showed some conflicting observations. Very few studies used methane as fuel,
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especially under pressurized conditions, and we aimed to fill this gap with this work.
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Energy & Fuels
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This paper presents a TGA investigation on the effect of sulfur on the reactivity of an
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ilmenite ore oxygen carrier using CH4 as fuel under both atmospheric and pressurized
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conditions. SEM-EDX and XPS analyses were conducted to understand sulfur deposition.
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With this study, we aim to gain further insights into sulfur’s effects on the kinetics as well as
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the oxygen carrying capacity of ilmenite ore when used in CLC and PCLC processes for CH4
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as fuel.
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2. Experiment
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Our previous publications described the pressurized TGA (PTGA) used in this work.11-12
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The PTGA can operate from atmospheric pressure to 10.1 MPa at sustained temperatures
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of 1673 K. The reaction gases are provided by the gas cylinders and enter the top of the
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PTGA. Nitrogen is used to purge the balance during the operation. The test data are
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acquired at 1 s intervals. Previous studies showed that the data repeatability of the PTGA
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was very good.11-12
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Ventilation system, stationary alarm system and a portable monitor for H2S and CO with
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alarm were used to ensure operational safety.
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2.1 Sample preparation and fuel mixture
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The fuel mixture used for this work was provided by BOC with a certified gas composition
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of 98.9% CH4 and 1.1% H2S. In industrial operations, it may be more practical to use
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recycled flue gas (mainly CO2) or a mixture of CO2 and steam to fluidize the fuel reactor
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instead of natural gas, so carbon dioxide was used in this work to simulate a more realistic
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operating environment.
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Rio Tinto Canada provided the ilmenite ore used as oxygen carrier (OC) in this work. The
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ilmenite ore was crushed and sieved to 106-212 µm and calcined in a muffle furnace for 2 5 ACS Paragon Plus Environment
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hours at 1173 K before being used for testing in the PTGA. Its composition was analyzed
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using X-ray fluorescence and is presented in Table 1.
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Table 1. UKTO ilmenite ore composition (wt.%)
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XRF composition analysis of the UKTO ilmenite ore Component
Average (wt.%)
SiO2 (wt.%)
