Chemical-looping with oxygen uncoupling (CLOU) is an innovative combustion technology with inherent capture of the CO2 and potential to drastically reduce the cost of capture. The process requires two interconnected reactors, between which the oxygen carrier particles are circulated while carrying oxygen from the combustion air to the fuel. In this work, the reactivity and mechanical stability of five materials were studied; three natural ores and two materials which were combinations of an ore and Ca(OH)(2). The overall aim was to investigate the feasibility of making a reactive and mechanically stable material using cheap raw materials and an industrially relevant particle manufacturing process.The two combined materials behaved differently from their natural counterparts. The Brazilian ore + Ca (OH)(2) showed a decrease in reactivity towards methane, but higher reactivity towards syngas in comparison to the pure ore. The South African ore + Ca(OH)(2) showed a major improvement in reactivity towards both syngas and methane, which could be attributed to formation of a perovskite-structure material with significant CLOU properties. A comparison between the pure ores and the combined materials show that the addition of Ca(OH)(2) had generally a beneficial impact on the mechanical stability of the oxygen carriers. As a significant change in the particle size distribution was seen for all oxygen carrier materials after repeated redox cycles with long reduction periods, the particle disintegration was likely caused by the chemical phase transformations occurring inside the particles, rather than by mechanical forces. (C) 2016 Published by Elsevier Ltd.

The influence of ash in chemical-looping combustion (CLC) has been studied in a lab-scale fluidized bed reactor. The oxygen carrier, ilmenite, was investigated with German lignite coal, Chinese bituminous coal, Mexican pet coke, wood char, and methane. Lignite ash and ash from gasified Chinese bituminous coal were compared in this study. Up to 50wt% and 33.3wt% loadings of ash from the combustion of lignite and ash from the gasification of coal were added. The gas conversion was clearly affected by the addition of ash, and very similar results were seen both for methane conversion and for CO conversion in the solid fuel tests. Thus, both ashes showed initial decrease in gas conversion with the addition of some ash due to the presence of inert/deactivating material in the ashes. At higher ash loading, a beneficial effect of ash could be seen. Furthermore, no negative effect of ash addition on the fluidizability of the bed material could be seen.

Chemical-looping combustion (CLC) is a combustion concept with inherent separation of CO2. The process uses a solid oxygen carrier, which consists of metal oxide, to transfer the oxygen from air to fuel. The chemical-looping combustor used in the present experiments features two interconnected fluidized beds; a fuel reactor (FR) and an air reactor (AR). In the FR, fuel is gasified with steam whereupon gasification products react with the oxygen carrier to form, ideally, CO2 and H2O. This study concerns CLC of solid fuels in a continuously operating 10 kW unit using two natural ores as oxygen carrier: (a) ilmenite, an iron-titanium mineral and (b) a manganese ore containing smaller amounts of Fe, Al and Si. The fuel feed was re-designed in order to increase contact between oxygen carrier and fuel. The new in-bed fuel feed was found to significantly improve gas conversion, mainly caused by increased contact between the oxygen carrier and volatile gases released in the fuel chute. Two fuels were used to evaluate the effect of fuel feed; a bituminous coal and a pet coke. The in-bed fuel feed was used when ilmenite and manganese ore were compared. The use of a manganese ore as oxygen carrier was shown to significantly enhance the rate of char gasification and also improve gas conversion. A concern with the manganese ore is the large production of fines. (C) 2012 Elsevier Ltd. All rights reserved.

In chemical looping combustion with solid fuels, the oxygen-carrier lifetime is expected to be shorter than with gaseous fuels. Therefore, it is particularly important to use low-cost oxygen carriers in solid fuel applications. Apart from being cheap, these oxygen carriers should be able to convert the CO and H2 produced from the solid fuel gasification and be sufficiently hard to withstand fragmentation. Several low-cost iron-based materials displayed high conversion of syngas and high mechanical strength and can be used for further development of the technology. These materials include oxide scales from Sandvik and Scana and an iron ore from LKAB. All tested oxygen carriers showed higher gas conversion than a reference sample, the mineral ilmenite. Generally, softer oxygen carriers were more porous and appeared to have a higher reactivity towards syngas. When compared with ilmenite, the conversion of CO was higher for all oxygen carriers and the conversion of H2 was higher when tested for longer reduction times. The oxygen carrier Sandvik 2 displayed the highest conversion of syngas and was therefore selected for solid fuel experiments. The conversion rate of solid fuels was higher with Sandvik 2 than with the reference sample, ilmenite.

Kinetic data of a promising oxygen carrier of NiO/NiAl2O4 have been established from experiments in a small fluidized bed batch reactor using methane. The particles were prepared by spray-drying using commercially available raw material and selected as the best candidates from an earlier screening study. The particles clearly showed high reactivity, with a maximum gas yield between 86% and 93% in the temperature interval 750 °C to 950 °C when using a bed mass and a gas flow corresponding to only 6 kg/MWfuel. A comparison of the reactivity with data from TGA experiments showed that the reactivity generally was faster in the batch fluidized bed in the investigated temperature interval. A simple reactor model using kinetic data from the batch fluidized bed reactor and the TGA predicted a minimum mass of 9–24 kg/MWfuel of oxygen carrier particles for full gas yield of methane to carbon dioxide in the fuel reactor. Comparison with experiments performed in a 10 and 120 kW CLC reactor with the same type of oxygen carrier showed that even when employing 13 to 50 times the amount of oxygen carrier theoretically needed for complete gas conversion, full gas yield was not obtained in the circulating systems. Hence it is of great importance to consider the fluid dynamics and gas-solid contact when modeling the fuel reactor of a chemical-looping combustor.

Chemical-looping combustion (CLC) is a combustion technique where the CO2 produced is inherently separated from the rest of the flue gases with a considerably low energy penalty. For this reason, CLC has emerged as one of the more attractive options to capture CO2 from fossil fuel combustion. When applying CLC with solid fuels, the use of a low cost oxygen carrier is highly important, and one such low cost oxygen carrier is the mineral ilmenite. The current work investigates the reactivity of several ilmenites, some which are synthetically produced by freeze granulation and two natural minerals, one Norwegian ilmenite and one South African ilmenite.A laboratory fluidized bed reactor made of quartz was used to simulate a two reactor CLC system by alternating the reduction and oxidation phase. The fuel was syngas containing 50% CO and 50% H2. A mixture of 6g of ilmenite with 9g inert quartz of diameter 125–180μm was exposed to a flow of 900mLn/min syngas in the reduction phase. During the oxidation phase, a 900mLn/min flow of 10% O2 diluted in N2 was used.The experimental results showed that all ilmenites give higher conversion of H2 than of CO. Generally, synthetic ilmenites have better CO and H2 conversion than natural ilmenites and synthetic ilmenites prepared with an excess of Fe generally showed higher total conversion of CO than synthetic ilmenites with an excess of Ti. Most synthetic ilmenites and the Norwegian ilmenite showed good fluidization properties during the experiments. However, for two of the synthetically produced materials, and for the South African ilmenite, particle agglomerations were visible at the end of the experiment.

Carbon capture and storage has the potential of reducing emissions of CO2, generated by combustion of fossil fuels. Chemical-looping combustion is a method intended to capture CO2 without an energy consuming gas separation process. Gas separation is avoided by using circulating oxygen carriers to transfer oxygen from an air reactor to a fuel reactor. Thus, the fuel and the combustion air are never mixed. A thermal analysis of the process identified several oxygen carrier systems with properties suitable for chemical-looping combustion applications. Such properties include high ability to convert different fuels, stability in air and sufficiently high melting temperature. Systems fulfilling these criteria were; metal oxides based on Ni, Cu, Fe, Mn, Co, W and sulphates of Ba, Sr and Ca. In the experimental part of this work, Ni- and Fe-based particles were analyzed with respect to both chemical and physical properties important for oxygen carriers in chemical-looping combustion, as well as gas conversion in a fluidized bed. Oxygen carriers of NiO, supported by NiAl2O4, are suitable for converting gaseous fuels with a high content of CH4. The main reasons are their high reactivity and high melting temperature. The Ni-based oxygen carriers investigated here were prepared from commercially available raw materials in contrast to the pure chemicals which have generally been used before. Oxygen carriers prepared by spray-drying, a production method suitable for large-scale particle preparation, displayed similar properties as oxygen carriers produced by the small-scale freeze-granulation method. Thus, up-scaling of particle production is not expected to present any difficulties. To reduce the risk of fragmentation and attrition of Ni-based oxygen carriers in a circulating chemical-looping combustion system, the strength can be improved by an addition of Ca(OH)2, by increasing the sintering temperature or by extending the sintering time. Materials with MgO added during particle preparation or with MgAl2O4 as supporting agent resulted in a considerably increased CH4 conversion. All oxygen carriers showed high reactivity with CH4 and O2 and a for a promising oxygen carrier of NiO/NiAl2O4, it was concluded that in an ideal reactor without gas solid-phase mass transfer limitations, full CH4 yield should be reached with a solids inventory in the fuel reactor of less than 10-20 kg/MW at 950°C. Fe-based oxygen carriers are cheap, abundant and environmentally sound and therefore well suited for chemical-looping combustion with solid fuels, where the expected lifetime of the oxygen carriers is comparatively short. Several industrial iron-based materials and a natural iron ore with properties well suited for chemical-looping combustion with solid fuels were identified. Generally, these materials displayed a high conversion of syngas, the main intermediate when solid fuels are gasified by steam, in combination with a high mechanical strength. The investigation of synthetically produced ilmenites, FeTiO3 in their reduced form, revealed that an increased Fe:Ti ratio generally improves the total conversion of CO although the initial maximum conversion is relatively constant.

Chemical-looping combustion, CLC, is a novel combustion concept with inherent separation of CO2. This study evaluates the performance of spray-dried nickel-based oxygen-carrier particles prepared from commercially available materials. The possibility to optimize the methane conversion while retaining the oxygen transport capacity by mixing different NiO-based oxygen carriers was evaluated, and the results showed that such optimization was indeed possible. Experiments were carried out in a batch reactor as well as in a continuous 300-W unit. Experiments in the batch reactor evaluated the performance of two different spray-dried particles, individually and mixed, at two different temperatures, 850°C and 950°C. The reference particle, referred to as N-VITO in this study, contained only NiO and NiAl2O4 while the other spray-dried particle was similar to the reference but contained a small amount of MgO as additive in the starting material. It was found that the reference particle had good oxygen-transport characteristics, but that methane conversion left room for improvement. The particle with MgO addition, on the other hand, showed excellent methane conversion, but poor oxygen transport capability, especially at the lower temperature. The 50/50mass-mixture of the two particles resulted in a potent oxygen-carrier batch with the desired qualities. Three experimental series were conducted in the 300-W CLC-unit: (i) using only the reference particle, (ii) using a mixture of the reference particle and the particle with MgO-addition, and (iii) using the previous mixed oxide system together with a small quantity of a high-surface impregnated oxygen-carrier based on NiO and Al2O3. The methane conversion to CO2 was found to depend not only on the solids flux in the reactor system, but also on the temperature in the fuel reactor. Results showed that the operation was more stable and that it was possible to obtain better fuel conversion when the mixtures of two or three different oxygen-carrier particles were used. With these mixtures, the methane fraction could be brought down to <0.1% while still maintaining a low CO fraction.

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.

Chemical-looping combustion is a novel combustion technology with inherent separation of the greenhouse gas CO2. The technology uses circulating oxygen carriers to transfer oxygen from the combustion air to the fuel. In this paper, oxygen carriers based on commercially available NiO and α-Al2O3 were prepared using the industrial spray-drying method, and compared with particles prepared by freeze-granulation. The materials were investigated under alternating oxidizing and reducing conditions in a laboratory fluidized bed, thus simulating the cyclic conditions of a chemical-looping combustion system. The particles produced by spray-drying displayed a remarkable similarity to the freeze-granulated oxygen carriers, with high reactivity when the bed was fluidized and similar physical properties when sintered at the same temperature. This is an important result as it shows that the scaling-up from a laboratory production method, i.e. freeze-granulation, to a commercial method suitable for large-scale production, i.e. spray-drying, did not involve any unexpected difficulties. A difference noticed between the spray-dried and freeze-granulated particles was the sphericity. Whereas the freeze-granulated particles showed near perfect sphericity, a large portion of the spray-dried particles had hollow interiors. Defluidization was most likely to occur for highly reduced particles, at low gas velocities. The apparent density and crushing strength of the oxygen carriers could be increased either by increasing the sintering temperature or by increasing the sintering time. However, the fuel conversion was fairly unchanged when the sintering temperature was increased but was clearly improved when the sintering time was increased.

Chemical-looping combustion (CLC) is a novel technology that can be used to meet demands on energy production without CO2 emissions. The CLC-process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N-2 from the air with CO2 from the combustion. The combustion gases consist almost entirely Of CO2 and H2O. Therefore, the technique reduces the energy penalty that normally arises from the separation Of CO2 from other flue gases, hence, CLC may make capture of CO2 cheaper.Iron ore and oxide scale from steel production were tested as oxygen carriers in CLC batch experiments with solid fuels. Petroleum coke, charcoal, lignite and two bituminous coals were used as fuels.The experiments were carried out in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction phases. The exhaust gases were led to an analyzer where the contents Of CO2, CO, CH4 and O-2 were measured. Gas samples collected in bags were used to analyze the content of hydrogen in a gas chromatograph.The results showed that both the iron ore and the oxide scale worked well as oxygen carrier and both oxygen carriers increased their reactivity with time. (c) 2009 Elsevier Ltd. All rights reserved.

Chemical-looping combustion is a combustion technology, where CO2. is separated from the rest of the flue gases 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 to produce oxygen carrier particles with properties suitable for the technology. A total of 36 oxygen carrier materials 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% O-2 in N-2, at 950 degrees 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 reactivity and strength as well as excellent fluidization behavior and should be feasible for use in a chemical-looping combustion unit.

Chemical-looping combustion is a combustion technology, where CO2 is separated from the rest of the flue gases 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 to produce oxygen carrier particles with properties suitable for the technology. A total of 36 oxygen carrier materials 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 reactivity and strength as well as excellent fluidization behavior and should be feasible for use in a chemical-looping combustion unit.

The reduction and oxidation behaviour of oxygen carrier particles of NiO and NiAl2O4 has been investigated in a fluidized bed reactor as well as a thermogravimetric analyzer (TGA). The particles showed high reactivity and gas yield to CO2 with methane in the temperature interval 750-950 degrees C. In the fluidized bed the yield to CO2 was between 90 and 99% using bed masses corresponding to 16-57 kg/MWfuel. Complementary experiments in a TGA at 750 and 950 degrees C showed a clear reaction of the NiAl2O4 with CH4 at the higher temperature. There was methane released from the reactor at high degrees of solid oxidation, which is likely associated with the lack of Ni-sites on the particles which can reform the methane. There was some carbon formation during the reduction, although the amount was minor when the gas yield to carbon dioxide and degree of oxidation of the solid was high. A simple reactor model using kinetic data from a previous study predicted the gas yield during the reduction in the fluidized bed experiments with reasonable accuracy. The oxygen carrier system investigated in this work shows high promise for use in a real CLC system, provided that the particle manufacturing process can be scaled up with reasonable cost.

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 H-2. 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 thermodynamic 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 H-2 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.

The work presented in this chapter demonstrates the economical feasibility of making a good oxygen carrier from commercial raw materials using a commercial production method, i.e. spray-drying. A batch fluidized-bed reactor was used for an extensive screening of many NiO-based oxygen carriers. This screening process led to the production of two major particle batches, which were used in continuous chemical-looping experiments. High-temperature experiments in a batch-fluidized bed verified the thermal durability of the particles. The most important result presented here concerns long-term operation (> 1000 h) of a 10-kWth chemical-looping combustor using spray-dried NiO-based oxygen carriers. Conversion of the fuel was good, and increased with (a) decreased circulation, and (b) increased fuel-reactor temperature. Combustion efficiency close to 99% was accomplished using these spray-dried particles. At the end of the test series, the continuous loss of fine material was 0.003%/h, which corresponds to a particle life time of 33000 h. No decrease in reactivity was seen during these long-term tests. The fuel used in the experiments was natural gas and methane.