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Characteristics of CaS-CaO Oxidation for Chemical looping combustion with CaSO4-based oxygen carrier Min Zheng, Simei Zhong, Kongzhai Li, Hua Wang, Huili Liu, Yonggang Wei, and Xing Zhu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01771 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 5, 2017
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Energy & Fuels
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Characteristics of CaS-CaO Oxidation for Chemical looping combustion with
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CaSO4-based oxygen carrier
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Zheng Min1,2, Zhong Simei1,2, Li Kongzhai1,2, Wang Hua1,2*, Liu Huili1,2, Wei Yonggang1,2, Zhu
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Xing1,2
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State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of
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Science and Technology, Kun Ming 650093, China 2
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
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*Corresponding author. Tel.: +86-871-65153405 E-mail address:
[email protected];
[email protected] (Wang Hua).
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Abstract
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The oxidation of CaS to CaSO4 oxygen carrier suffers SO2 release. To capture SO2 released, an
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amount of CaO particles were mixed with CaS particles. Isothermal kinetics of CaS-CaO oxidation
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reaction has been conducted in a Thermogravimetric (TG) reactor combined with a Frourier
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transform infrared (FTIR) spectroscopy analyzer. The effects of reaction temperature, O2
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concentration, and molar ratio of CaO to CaS on CaSO4 generation and SO2 emission were
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investigated. The chemical compositions, morphology structures and element distributions on the
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surfaces of solid residuals were characterized by FTIR and SEM-EDS instruments. The results
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show that the oxidation products of CaS species are mainly composed of CaSO4. A small amount of
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SO2 are released from CaS oxidation, causing losses of oxygen carrying capacity of CaSO4-based
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oxygen carrier. Utilizing CaO additive in CaS oxidation process reduces SO2 liberation, and
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improves CaSO4 selective yield, but results in an increase in remaining CaS species in solid
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residues. There is a general positive correlation between the remaining percentage of CaS species
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and the SO2 selective yield. The SO2 liberation varies with reaction temperature, O2 concentration,
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and CaO/CaS molar ratio. At 900 oC and with CaO/CaS molar ratio of 1 and 5% O2, SO2 released
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from CaS oxidation is completely trapped by CaO additive.
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Keywords: Chemical looping combustion; CO2 capture; CaS oxidation; CaSO4 oxygen carrier; SO2
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release; SO2 suppression.
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1. Introduction
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Chemical looping combustion (CLC) is a promising technique for carbonaceous fuel
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combustion with inherent CO2 capture. 1-2 It also has the potential to improve fuel conversion. 3 The
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CLC system generally consists of an air reactor, a fuel reactor, and an oxygen carrier circulating
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between the two reactors. The oxygen carrier transfers oxygen from air in air reactor to fuel in fuel
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reactor. The mixing of air and fuel is avoided. Also the flue gas will not be mixed and diluted by N2
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in the air, which promises a high concentrated CO2 steam.
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The CLC technology has been demonstrated to be feasible. Different sizes of CLC facilities,
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ranging from small-scale pilot units (0.3 - 120 kWth) to pilot scale units (1 - 3 MW), have been
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demonstrated worldwide, and show many promising results. With regard to further
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commercializtion of CLC units, low-cost materials, such as natural ore, cause rising concerns. 4
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CaSO4 species is an available oxygen carrier for large-scale commercial use due to its merits
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of vast reserves, low price and high capacity to transfer oxygen (0.46). 5-7 Figure 1 shows a diagram
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of the carbonaceous-fueled CLC process with CaSO4 oxygen carrier. The general reaction approach
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is described in terms of the main reactions. CaSO4 species is reduced by carbonaceous fuel to CaS
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species in fuel reactor, with the generating of CO2 and H2O products: 4x+y 2x+y CaSO 4 + 2CxHy → CaS + 2 xCO 2 + yH 2 O 4 4
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(R1)
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Then CaS species is oxidized by air to CaSO4 species in air reactor, where molecular oxygen of O2
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in air is migrated in CaSO4 oxygen carrier. θ CaS + 2O2 → CaSO4 ∆H298.15 = −960.89kJ / mol
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(R2)
Having been condensated, the flue gas of the fuel reactor is almost pure CO2. Few work
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was performed on CaS oxidation for CLC with CaSO4 oxygen carrier, although 8-16
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there are a few publications
on disposal of desulfurization product CaS from coal gasification.
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Plenty of CaS species have been generated from sulfur removal in advanced coal power generation
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systems, such as PFBC-CC and IGCC systems, and further oxidized to CaSO4 species as a stable
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product. The oxidation reactions of CaS species are complicated. The oxidation products would be
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CaSO4 or CaSO4 and CaO, depending on specific reaction conditions. For these energy industrial
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applications, the target product of CaS oxidation reaction is just CaSO4 species. However, the
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byproducts CaO and SO2 are formed via the side reactions Reaction 3 and Reaction 4 under some
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conditions. The effects of reaction temperature, O2 concentration, reaction pressure, and particle
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size on CaS oxidation have been investigated in literature. Torres-Ordonez et al.
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SO2 release was ascribed to solid-gas Reaction (R3).
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considered that
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θ 2CaS+3O2=2CaO+2SO2 ∆H 298.15 = −909.94kJ / mol
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(R3)
θ CaS+3CaSO4=4CaO+4SO2 ∆H 298.15 = 1054.03kJ / mol
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(R4)
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Other researchers
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SO2 liberation. The solid-solid reaction proceeded around 850 - 890 oC. 17 The solid-solid Reaction
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R4 in series with Reaction R4 was accounted for SO2 release. Davies’ group
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found that the reaction of CaS oxidation to CaSO4 was of the first order with respect to O2
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concentration (within the O2 concentration range of 1% - 40%). Fuertes’ group found
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the domains of low SO2 release (T
