| 1. |
- Cho, Paul In-Young, 1970, et al.
(author)
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Carbon Formation on Nickel and Iron Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion
- 2005
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In: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 44:4, s. 668-676
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Journal article (peer-reviewed)abstract
- For combustion with CO2 capture, chemical-looping combustion with inherent separation of CO2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g., metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. Carbon deposition on oxygen-carrier particles was investigated to assess whether it could have adverse effects on the process. The oxygen-carrier particles used were based on oxides of nickel and iron and produced by freeze granulation. They were sintered at 1300 degreesC for 4 h and sieved to a size range of 125-180 mum. The study of carbon deposition was performed in a laboratory fluidized-bed reactor, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. The particles with nickel oxide were tested at 750, 850, and 950 degreesC, and the particles with iron oxide at 950 degreesC. On the oxygen carrier with nickel oxide, only minor amounts of carbon formed during most of the reduction. However, when more than 80% of the oxygen available was consumed, significant carbon formation started. The formation of carbon was also clearly correlated to low conversion of the fuel. No carbon was formed on the oxygen carrier based on iron oxide. The interpretation for the actual application of this process is that carbon formation should not be a problem, because the process should be run under conditions of high conversions of the fuel.
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| 2. |
- Cho, Paul In-Young, 1970, et al.
(author)
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Defluidization conditions for fluidized-bed of iron, nickel, and manganese oxide containing oxygen-carriers for chemical-looping combustion
- 2006
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In: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 45:3, s. 968-977
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Journal article (peer-reviewed)abstract
- For combustion with CO2 capture, chemical-looping combustion with inherent separation Of CO2 is a promising technology. Chemical-looping combustion uses oxygen carriers that are composed of metal oxide to transfer oxygen from the combustion air to the fuel. The defluidization of oxygen-carrier particles was investigated to improve the understanding of when particle agglomeration may occur. The study was made in a laboratory fluidized-bed reactor at 950 degrees C, simulating a chemical-looping combustion system by exposing the sample to reducing and oxidizing conditions in an alternating manner. The oxygen-carrier particles used were based on oxides of iron, nickel, and manganese and produced by freeze granulation. For iron oxide particles, there was no defluidization of the bed when the content of available oxygen in the particle was high. The defluidization occurred during the oxidation period after long reduction periods, in which a significant reduction of the magnetite to wustite occurred. This is an important observation, because the reduction to wustite is not expected in chemical-looping combustion with high fuel conversion. Thus, laboratory experiments with iron oxide performed with long reduction times may give an unduly exaggerated impression of the risks of agglomeration. For nickel oxide, the defluidization was dependent on the sintering temperature with no defluidization in experiments conducted with particles sintered at 1300 and 1400 degrees C. The nickel oxide particles that were sintered at 1500 degrees C only defluidized once in a total of 49 cycles, whereas the particles that were sintered at 1600 degrees C defluidized already in the first cycle. For the nickel oxide particles, it was not possible to see any effect of the length of the reducing period on the defluidization. There was no defluidization of the manganese oxide particles. The defluidization of the bed leads to agglomeration for the iron oxide particles, but not for the particles of nickel oxide, where the bed was still loosely packed. Carbon was formed on the particles based on nickel oxide and manganese oxide.
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| 6. |
- Abad, Alberto, 1972, et al.
(author)
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The use of iron oxide as oxygen carrier in a chemical-looping reactor
- 2007
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In: Fuel. - : Elsevier BV. - 0016-2361. ; 86:7-8, s. 1021-1035
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Journal article (peer-reviewed)abstract
- Chemical-looping combustion (CLC) is a method for the combustion of fuel gas with inherent separation of carbon dioxide. This technique involves the use of two interconnected reactors, an air reactor and a fuel reactor. The oxygen demanded in the fuel combustion is supplied by a solid oxygen carrier, which circulates between both reactors. Fuel gas and air are never mixed and pure CO2 can be obtained from the flue gas exit. This paper presents the results from the use of an iron-based oxygen-carrier in a continuously operating laboratory CLC unit, consisting of two interconnected fluidized beds. Natural gas or syngas was used as fuel, and the thermal power was between 100 and 300 W. Tests were performed at four temperatures: 1073, 1123, 1173 and 1223 K. The prototype was successfully operated for all tests and stable conditions were maintained during the combustion. The same particles were used during 60 h of hot fluidization conditions, whereof 40 h with combustion. The combustion efficiency of syngas was high, about 99% for all experimental conditions. However, in the combustion tests with natural gas, there was unconverted methane in the exit flue gases. Higher temperature and lower fuel flows increase the combustion efficiency, which ranged between 70% and 94% at 1123 K. No signs of agglomeration or mass loss were detected, and the crushing strength of the oxygen carrier particles did not change significantly. Complementary experiments in a batch fluidized bed were made to compare the reactivity of the oxygen carrier particles before and after the 40 h of operation, but the reactivity of the particles was not affected significantly.
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| 7. |
- Jerndal, Erik, 1980, et al.
(author)
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Investigation of Different NiO/NiAl2O4 Particles as Oxygen Carriers for Chemical-Looping Combustion
- 2009
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In: Energy & Fuels. - 1520-5029 .- 0887-0624. ; 23:2, s. 665-676
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Journal article (peer-reviewed)abstract
- Chemical-looping combustion is a combustion technology, where CO2 is separated from the rest of the fluegases without an energy-consuming gas-separation process. The combustion is performed in two reactors,with metal oxide particles circulating between them, transferring oxygen from the combustion air to the fuel.Particles of NiO, supported by NiAl2O4, have been reported earlier as excellent oxygen carriers for this process.The aim of the present investigation is to verify that commercially available raw materials can be used toproduce oxygen carrier particles with properties suitable for the technology. A total of 36 oxygen carriermaterials were prepared by freeze granulation and investigated with respect to parameters important for chemical-looping combustion. The reactivity of the particles was investigated in a small fluidized bed reactor by exposing them cyclically to CH4 and 5% O2 in N2, at 950 °C. Although defluidization occasionally occurred for some materials, it was clear that the gas conversion and the reactivity were generally high. An addition of Ca(OH)2 to the oxygen carriers increased the strength and thus reduces the risk of fragmentation and attrition in a chemical-looping combustion device. An addition of MgO enhanced the fuel conversion early in reduction, which seemed to be restricted because of the limited amounts of metallic Ni. An increased sintering temperature generally resulted in harder particles of higher density; however, the risk of defluidization seemed to increase for such particles. Carbon formation was only detected when the oxygen carriers were highly reduced and the fuel conversion was incomplete, i.e., at conditions not expected in a real chemical-looping combustion device.Two of the investigated particles, NOV1T1400 and NOV2T1400, displayed a combination of high reactivityand strength as well as excellent fluidization behavior and should be feasible for use in a chemical-loopingcombustion unit.
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| 8. |
- Jerndal, Erik, 1980, et al.
(author)
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NiO particles with Ca and Mg based additives produced by spray-drying as oxygen carriers for chemical-looping combustion
- 2008
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In: Energy Procedia. - : Elsevier BV. - 1876-6102. ; 1:1, s. 479-486
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Journal article (other academic/artistic)abstract
- Chemical-looping combustion is a two-step combustion process where CO2 is obtained in a separate stream, ready for compression and sequestration. The technique involves two interconnected fluidized bed reactors, with a solid oxygen carrier circulating between them. Results of reactivity experiments with 24 different oxygen carriers, based on NiO with NiAl2O4 and/or MgAl2O4 and produced with spray-drying, are presented. The investigation revealed that oxygen carriers supported by MgAl2O4, or where a small amount of MgO was added, displayed an increased fuel conversion when compared to oxygen carriers of NiO supported by NiAl2O4.
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| 9. |
- Jerndal, Erik, 1980, et al.
(author)
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Thermal analysis of chemical-looping combustion
- 2006
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In: Chemical Engineering Research and Design. - : Elsevier BV. - 0263-8762 .- 1744-3563. ; 84:A9, s. 795-806
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Journal article (peer-reviewed)abstract
- In chemical-looping combustion, a gaseous fuel is burnt with inherent separation of the greenhouse gas CO2. Oxygen is transferred from the combustion air to the fuel by an oxygen carrier, which is usually a metal oxide, and therefore direct contact between the fuel and the combustion air is avoided. Thus, the products of combustion, i.e., CO2 and H2O, are not mixed with the rest of the flue gases and after condensation almost pure CO2 is obtained, without any energy lost for the separation. A thermal analysis of the process using a large number of possible oxygen carriers was performed by simulating reactions using the HSC Chemistry 5.0 software. Three fuels were used in the investigation, CH4, CO and H2. Based on the ability of the oxygen carriers to convert the fuel to the combustion products CO2 and H2O, stability in air and the melting temperatures of the solid material some metal oxides based on Ni, Cu, Fe, Mn, Co, W and sulphates of Ba and Sr showed good ther-modynamic properties and could be feasible oxygen carriers. Only a few of these possible oxygen carrier systems, based on Cu, Fe and Mn, showed complete conversion of the fuel gas, but still the other systems had limited equilibrium restrictions, with only small and acceptable amounts of unreacted CO and H2 released from the fuel reactor. The promising systems were investigated further with respect to temperature changes in the fuel reactor as well as possible carbon, sulphide and sulphate formation in the fuel reactor. For some systems the reactions in the fuel reactor were endothermic, resulting in a temperature drop in the fuel reactor. However, this drop can be limited by applying a sufficient circulation of particles from the air reactor to the fuel reactor. When Ni or Co is used as oxygen carrier the fuel may need to be desulphurized prior to combustion to avoid formation of solid or liquid sulphides or sulphates. On the other hand, to prevent decomposition of the sulphates BaSO4 and SrSO4, in the fuel reactor, to sulphur-containing gases and metal oxides, it is necessary that some sulphur is present in the fuel and that high temperatures are avoided. Formation of carbon should not be a problem as long as the process is run under conditions of high fuel conversion.
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| 19. |
- Johansson, Marcus, 1975, et al.
(author)
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Using continuous and pulse experiments to compare two promising nickel-based oxygen carriers for use in chemical-looping technologies
- 2008
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In: Fuel. - : Elsevier BV. - 0016-2361. ; 87:6, s. 988-1001
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Journal article (peer-reviewed)abstract
- Chemical-looping technologies have obtained widespread recognition as power or hydrogen production units with inherent carbon capture in a future scenario where CO2 capture and storage (CCS) is reality. In this paper three different techniques are described; chemical-looping combustion and two categories of chemical-looping reforming. The three techniques are all based on oxygen carriers that are circulating between an air- and a fuel reactor, providing the fuel with undiluted oxygen. Two different oxygen carriers; NiO/NiAl2O4 (40/60 wt/wt) and NiO/MgAl2O4 (60/40 wt/wt) are compared. Both continuous and pulse experiments were performed in a batch laboratory fluidized bed working at 950 °C using methane as fuel. It was found that pulse experiments offer advantages in comparison to continuous experiments, particularly when evaluating suitable particles for autothermal chemical-looping reforming. Firstly, smaller conversion ranges can be investigated in more detail, and secondly, the onset and extent of carbon formation can be determined more accurately. Of the two oxygen carriers, NiO/MgAl2O4 offers several advantages at elevated temperatures, i.e. higher methane conversion, higher selectivity to reforming and lesser tendency for carbon formation.
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| 20. |
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| 21. |
- Leion, Henrik, 1976, et al.
(author)
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CO2 capture from direct combustion of solid fuels with Chemical-Looping Combustion
- 2008
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In: The proceedings of the 33rd International Technical Conference on Coal Utilization & Fuel Systems.
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Conference paper (other academic/artistic)abstract
- Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel, thus avoiding direct contact between air and fuel. Thus, the CO2 is inherently separated from the flue gases with a considerable lower energy penalty and cost compared to other techniques for CO2 separation. The oxygen carrier is circulated between two reactors, a fuel and an air reactor, where the flue gas from the air reactor contains only N2 and some unreacted O2 and the flue gas from the fuel reactor contains only CO2 and H2O. The water can easily be condensed and the remaining CO2 can be transported for underground storage. Most of the prior work with CLC has focused on using natural gas and syngas as fuel. However, recent work on adapting the CLC process for solid fuels shows promising results. Two main strategies for achieving this are: 1) using syngas from coal gasification in the fuel reactor and 2) introduction of the coal directly to the fuel reactor where the gasification of the coal and subsequent reactions with the metal oxide particles will occur simultaneously. This paper will focus on this second route, and present results from reactivity investigations in a laboratory fluidized-bed reactor system of a number of different solid fuels, including three types of bituminous coal, petroleum coke, lignite and char from bio fuel. As oxygen carrier the previously investigated natural mineral ilmenite is used. The experiments were conducted at 970°C with 92% steam in the fluidizing gas. The reaction rates were considerably higher compared to investigations using lower steam fractions. The fraction of volatiles in the fuel was found to be important for the conversion rate of the fuel. Furthermore, the presence of an oxygen-carrier was shown to enhance the conversion rate of the intermediate gasification reaction compared with normal gasification performed without the presence of an oxygen carrier.IntroductionChemical-looping combustion (CLC) has been introduced as a technique where the greenhouse gas CO2 is inherently separated during combustion. The CLC-process is composed of two fluidized bed reactors, an air and a fuel reactor. The fuel is introduced to the fuel reactor where it reacts with an oxygen carrier to CO2 and H2O, reaction (1). The reduced oxygen carrier is transported to the air reactor where it is oxidized back to its original state by air, reaction (2). In this paper, when oxidation and reduction is mentioned, it refers to oxidation and reduction of the oxygen carrier.
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| 22. |
- Leion, Henrik, 1976, et al.
(author)
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Combustion of a German lignite using chemical-looping with oxygen uncoupling (CLOU)
- 2008
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In: The Proceedings of 33rd International Technical Conference on Coal Utilization & Fuel Systems.
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Conference paper (other academic/artistic)abstract
- Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn solid fuels in gas-phase oxygen without the need for an energy intensive air separation unit. The carbon dioxide from the combustion is inherently obtained separated from the rest of the flue gases. The technique is based on chemical-looping combustion but involves a completely different reaction mechanism for the fuel oxidation. The process uses three steps in two reactors, one air reactor where a metal oxide captures oxygen from the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen (step 2) and where this oxygen reacts with a fuel (step 3). This means that the char reacts directly with gaseous O2, which is a very fast reaction. In other proposed schemes for using chemical-looping combustion of solid fuels there is a need for an intermediate gasification step of the char with steam or carbon dioxide to form reactive gaseous compounds which then react with the oxygen carrier particles. The gasification reactions are inherently slow, resulting in slow overall rates of reaction. This is solved in the proposed process, since there is no intermediate gasification step needed and the char reacts directly with gas-phase oxygen. Of course this demands another type of oxygen carrier than those normally used in chemical-looping combustion, and a thermal analysis has identified CuO/Cu2O, Mn2O3/Mn3O4 and Co3O4/CoO as potential systems. Thermodynamic calculations indicate that metal sulphates should not be formed in the fuel reactor during normal operation, although they could be formed locally for Co and Mn-based oxygen carriers at lower temperatures. This work presents results from an investigation of the reaction between a Cu-based oxygen carrier with a German lignite in a batch fluidized bed reactor. A ratio of oxygen carriers to fuel of 6 kg/MJ was employed during the combustion and between 30-45 seconds was needed for 95% conversion of the coal in the temperature interval 850-985C. The oxidation was possible at all temperatures, and a substantial part of the oxidation occurred near the equilibrium O2 concentration. No signs of agglomerations of the oxygen carrier particles were found.
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| 23. |
- Leion, Henrik, 1976, et al.
(author)
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Effects of steam and CO2 in the fluidizing gas when using bituminous coal in chemical-looping combustion
- 2009
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In: Proceedings of the 20th International Conference on Fluidized Bed Combustion. ; :1, s. 608-611
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Conference paper (peer-reviewed)abstract
- Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel in order to avoid direct contact between air and fuel. Thus, the CO2 is inherently separated from the flue gases with a potential for considerably lower energy penalty and cost compared to other techniques for CO2 separation. The oxygen carrier is circulated between two reactors, a fuel and an air reactor, where the flue gas from the air reactor contains oxygen depleted air and the flue gas from the fuel reactor contains mainly CO2 and H2O. The water can easily be condensed and the remaining CO2 can be transported for underground storage. Most of the prior work with CLC has focused on using natural gas and syngas as fuel and oxygen carrying material normally produced from pure chemicals. However, recent work on adapting the CLC process for solid fuels with ores and natural minerals as oxygen carrier shows promising results. This paper will present results from reactivity investigations in a laboratory fluidized-bed reactor system using previously investigated natural mineral ilmenite as oxygen carrier and a bituminous Colombian coal as fuel. Experiments were conducted at a temperature of 970°C with N2, steam, and/or CO2 in the fluidizing gas. Synergy effects between steam and CO2 on fuel conversion was noted. The results show that the fuel conversion was a roughly a factor 5 faster with steam as compared to CO2 in the fluidizing gas.
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| 24. |
- Leion, Henrik, 1976, et al.
(author)
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Solid fuels in chemical-looping combustion
- 2007
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In: International Journal of Greenhouse Gas Control. ; 2, s. 180-193
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Journal article (peer-reviewed)abstract
- The feasibility of using a number of different solid fuels in chemical-looping combustion (CLC) has been investigated. A laboratory fluidized bed reactor system for solid fuel, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions, was used. In each reducing phase 0.2 g of fuel in the size range 180–250 μm was added to the reactor containing 40 g oxygen carrier of size 125–180 μm. Two different oxygen carriers were tested, a synthetic particle of 60% active material of Fe2O3 and 40% MgAl2O4 and a particle consisting of the natural mineral ilmenite. Effect of steam content in the fluidizing gas of the reactor was investigated as well as effect of temperature. A number of experiments were also made to investigate the rate of conversion of the different fuels in a CLC system. A high dependency on steam content in the fluidizing gas as well as temperature was shown. The fraction of volatiles in the fuel was also found to be important. Furthermore the presence of an oxygen carrier was shown to enhance the conversion rate of the intermediate gasification reaction. At 950 °C and with 50% steam the time needed to achieve 95% conversion of fuel particles with a diameter of 0.125–0.18 mm ranged between 4 and 15 min depending on the fuel, while 80% conversion was reached within 2–10 min. In almost all cases the synthetic Fe2O3 particle with 40% MgAl2O4 and the mineral ilmenite showed similar results with the different fuels.
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| 25. |
- Leion, Henrik, 1976, et al.
(author)
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Solid fuels in chemical-looping combustion using a NiO-based oxygen carrier
- 2009
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In: Chemical Engineering Research and Design. - : Elsevier BV. - 0263-8762 .- 1744-3563. ; 87:11, s. 1543-1550
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Journal article (peer-reviewed)abstract
- The feasibility of using three different solid fuels in chemical-looping combustion (CLC) has been investigated using NiO as oxygen carrier. A laboratory fluidized-bed reactor system for solid fuel was used, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. In each reducing phase 0.2 g of fuel was added to the reactor containing 20 g oxygen carrier. The experiments were performed at 970°C. Compared to previously published results with other oxygen carriers the reactivity of the used Ni-particles was considerably lower for the high-sulphur fuel and higher for the low-sulphur fuel. Much more unconverted CO was released and the fuel conversion was much slower for high-sulphur fuel such as petroleum coke, suggesting that the nickel-based oxygen carrier was deactivated by the presence of sulphur. The NiO particles also showed good reactivity with methane and a syngas mixture of 50% H2 and 50% CO. For all experiments the oxygen carrier showed good fluidizing properties without any signs of agglomeration.
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