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Emissions from coal oxy-fuel combustion are receiving significant attention during the last years. This paper is focused on the analysis of fuel-N emissions in fluidized bed combustion systems. Experiments were carried out in a bubbling fluidized bed unit in the temperature interval 850-950ºC. Different coals (anthracite, bituminous and lignite) were used as fuels and different sorbents were employed for in-bed sulphur retention. In the experiments, NO, N2O and NO2 were measured. NO2 was not detected in any of the operating conditions. The influence of temperature on NO and N2O emissions was the same as in conventional combustion: NO emission increased as temperature increased while N2O emission decreased. Nevertheless, the total fuel-N conversion to nitrogenous species seemed to be lower than in combustion with air. As in air-firing combustion, the highest levels of NO and N2O were registered with the highest rank coals. The Ca-sorbent was found to have a key role on NO reduction via catalytic reaction and this catalytic effect seemed not to be affected by the high CO2 levels present in oxy-fuel combustion. Also, the moisture content in the coal affected the NO formation, which decreased 2 with an increase in the coal moisture content. A similar effect was observed by increasing the oxygen concentration fed to the combustor.

Chemical Looping Combustion (CLC) is an emerging low cost CO2 capture technology for large scale power units. The oxygen needed for combustion is supplied by a solid oxygen carrier circulating between two reactors. Fe-based oxygen carriers have been proposed for CLC of coal due to their low cost. Some of them are minerals or industrial residues which can contain toxic trace elements. After its use, the oxygen carrier should be disposed in a landfill and therefore, the presence of soluble toxic elements in the oxygen carrier should be analyzed. In this study, lixiviation tests were carried out with three different Fe-based oxygen carriers used in coal CLC experiments in a continuous unit: ilmenite, a bauxite waste and an iron ore. All the spent oxygen carriers, except the bauxite waste, can be classified as a stable non-reactive hazardous waste and therefore can be disposed in a landfill for non-hazardous residues. An estimation of the amount of solid waste generated in the process based on the fly ash content of the coal and the oxygen carrier particle lifetime was made.

A Chemical Looping Combustion (CLC) unit for solid fuels has been designed, erected and operated. The design was based on a thermal power of 20 kWth for in-situ Gasification Chemical Looping Combustion (iG-CLC) or 50 kWth for Chemical Looping with Oxygen Uncoupling (CLOU). Fuel and air reactors are two interconnected circulating fluidized beds reactors, with the coal being fed at the bottom of the fuel reactor to maximize the contact between the volatile matter and the oxygen carrier particles. A carbon stripper has been included between fuel and air reactors to increase the CO2 capture rates. In this unit, the char particles are separated from the oxygen carrier particles and recirculated to the fuel reactor. The solids flow exiting from the fuel reactor is split into two different streams by using a double loop seal down the fuel reactor cyclone. One goes to the carbon stripper and the other is recirculated to the fuel reactor. In this way it is possible to have an independent control of solids inventory in the fuel reactor and the global solid circulation flow rate between fuel and air reactors. First operational results at steady state have been obtained with stable operation in iG-CLC mode during combustion of a bituminous coal with ilmenite being the oxygen carrier. A CO2 capture value of 88% at 991 ºC and a total oxygen demand value of 8.5% were obtained with a solids inventory in the fuel reactor of 470 kg/MWth.

CLC technology with solid fuels is being developed mainly by means of two modes: in situ Gasification CLC (iG-CLC) and Chemical Lopping with Oxygen Uncoupling (CLOU). In the iG-CLC concept, coal gasification is an intermediate step taking place inside the fuel reactor, where gasification products (CO+H2) and volatile matter are oxidized by reaction with oxygen carrier particles. The CLOU concept was developed on the strategy of using oxygen carrier materials which can release gaseous oxygen in the fuel reactor environment, and thereby allowing the solid fuel to be quickly burnt with gas phase oxygen. A 50 kWth pilot plant for solid fuels have been designed and constructed to be operated in both iG-CLC and CLOU modes. The design of the concept was based on two interconnected circulating fluidized beds reactors. The coal is fed at the bottom of the fuel reactor to maximize the contact between the volatile matter and the oxygen carrier particles, minimizing the unburnt compounds at the outlet. In iG-CLC mode, residence time of solids in the fuel reactor is low compared to the time to allow complete coal gasification. A carbon stripper has been included between fuel and air reactors to increase the residence time of char particles in the fuel reactor, thus allowing high CO2 capture rates. In this unit, the char particles are separated from the oxygen carrier particles using their different fluidization properties and they are recirculated to the fuel reactor. This unit could be also required for low reactive coals in CLOU mode. Solids circulation flow rate is controlled by using a double loop seal after fuel reactor cyclone. In this device, the solid flow exiting from the fuel reactor splits into two different streams. One goes to the carbon stripper and the other is recirculated to the fuel reactor.

La combustión indirecta con transportadores sólidos de oxígeno (Chemical Looping Combustion – CLC) es una de las tecnologías más prometedoras de captura de CO2 en el proceso de producción de energía a partir de combustibles fósiles ya que la separación del CO2 generado es inherente al propio proceso y, por tanto, permite reducir en gran medida el coste asociado a la captura de dicho gas. Esta tecnología se fundamenta en la transferencia de oxígeno del aire al combustible por medio de un óxido metálico que actúa como transportador de oxígeno evitando el contacto directo entre el aire y el combustible. Este transportador de oxígeno circula entre dos lechos fluidizados interconectados denominados reactor de reducción y reactor de oxidación. La fase activa de un transportador de oxígeno se compone de un óxido metálico, normalmente óxidos de níquel, cobre, manganeso o hierro., Este trabajo de investigación se ha centrado en la evaluación del comportamiento de transportadores de oxígeno con bajo o nulo contenido en NiO en el proceso CLC cuando se utiliza CH4 u otros gases combustibles en presencia de H2S como impureza. La evaluación de estos transportadores de oxígeno se llevó a cabo en una planta piloto de dos lechos fluidizados interconectados de 500 Wt. Como objetivo final se ha pretendido identificar el transportador de oxígeno más prometedor para su escalado a una planta industrial CLC de gas natural., A partir del estado del arte en el desarrollo de materiales, se escogieron cuatro transportadores de oxígeno con unas propiedades adecuadas para la combustión de gases, tales como gas natural, gas de síntesis o hidrocarburos ligeros mediante el proceso CLC, uno por cada uno de los óxidos metálicos más usados para esta tecnología: materiales basados en níquel, cobre, manganeso y hierro. Los resultados obtenidos con el transportador de oxígeno basado en níquel con un contenido en NiO muy reducido (11 % en peso), y soportado sobre CaAl2O4,demostraron que este material era altamente reactivo con todos los gases empleados obteniéndose altas eficacias de combustión en un rango bien definido de condiciones de operación. Sin embargo, este transportador de oxígeno presentó algunos inconvenientes para su uso en una planta CLC a nivel industrial relacionados con la posible presencia de azufre en el combustible alimentado a dicha instalación así como la necesidad de desarrollar y fabricar un soporte de CaAl2O4 a nivel comercial con mejores propiedades físicas.

The recently developed Chemical Looping Combustion technology (CLC) is nowadays considered an interesting option to capture CO2 at low cost in fossil fuelled power plants. In the past years, significant advances have been achieved in the combustion of both gaseous and solid fuels. Nevertheless, pollutant gas emissions from CLC systems have received little attention. This paper focuses on sulphur, nitrogen and mercury emissions during lignite combustion in a 500Wth CLC unit. Ilmenite was used as oxygen carrier, as it is one of the most common materials used for CLC of solid fuels. The main sulphur species detected in the fuel reactor were H2S and SO2. The amount and proportion depended on the temperature of the fuel reactor. The higher the temperature, the more H2S converted to SO2. In the air reactor, the sulphur in the unconverted char was released as SO2. Regarding the emission of nitrogen in coal, most of the nitrogen was found as N2 at the outlet of the fuel reactor. No NH3 or HCN were registered and only small amounts of NO were detected. The nitrogen contained in the char reaching the air reactor was released as NO. Mercury speciation was also analyzed and the ratio Hg2+/Hg0 determined. In the fuel reactor, the major mercury species was Hg0 and in the air reactor Hg2+. The incorporation of a carbon separation unit between fuel and air reactors would help to reduce the sulphur emissions in the air reactor and comply with the current legislation for power generation systems.

Coal combustion using the Chemical Looping technology can be carried out under different configurations. This paper focuses on the in situ gasification Chemical Looping Combustion (iG-CLC). In this technology, it is especially important the selection of the oxygen carrier as there may be losses in the drainage of coal ashes. Finding low-cost oxygen carriers has become a relevant research focus. Several Fe-based materials have been tested including minerals and industrial residues. In this work, a highly reactive iron ore that had already shown promising characteristics for coal combustion was used in a continuous 500 Wth CLC unit. Its performance in the combustion of anthracite, bituminous coal and lignite was evaluated and compared with the results for other Fe-based materials, such as ilmenite or bauxite waste. The combustion efficiency obtained with the Tierga iron ore was the highest reported to date which makes this carrier a promising candidate for further scale-up. Moreover, the high CaO content of this material led to analyze its relevance for sulphur removal during the first hours of operation. High sulphur retention capacity was observed but this capacity decreased with time as the calcium oxide was both saturated and lost as fines during operation.

This work evaluates the performance of an iron ore, mainly composed of Fe2O3, as an oxygen carrier (OC) for chemical looping combustion (CLC) with gaseous fuels. The OC was characterized by TGA and evaluated in a continuous 500 Wth CLC unit, using CH4, syngas and a PSA off-gas as fuels. The OC was able to fully convert syngas at 880ºC. However, lower conversion rates were observed with methane-containing fuels. The addition of a Ni-based OC was evaluated in order to increase the reactivity of the OC with methane. In spite of this, an absence of catalytic effect was observed for the Ni-based OC. A deep analysis was carried out into the reasons for the absence of catalytic effect of the Ni-based OC. The performance of the iron ore with regard to attrition and fluidization behaviour was satisfactory throughout 50 hours of hot operation in the continuous CLC plant. Thus, this low cost material is a suitable OC for gaseous fuels mainly composed of H2 and CO.

In this work, SO2 retention via calcium-based sorbents added in a continuous bubbling fluidized bed combustor (~3 kWth) operating in oxy-fuel combustion mode is analyzed. Tests were performed at different operating temperatures with three sorbents, two limestones and one dolomite, and with three coals, ranging from lignite to anthracite, to analyze the influence of coal rank, type of sorbent, sorbent particle size, and O2/CO2 feeding ratio on the sulfation process It was found that the combustor temperature had a strong influence on the limestones sulfur retention with a maximum at 900-925 ºC. The behavior of the limestones was qualitatively similar with the three coals, attaining the highest sulfur retention values working with the lignite and the lowest working with the bituminous coal. On the contrary, with the dolomite the sulfur retention was hardly affected by the combustion temperature and the sulfur retentions attained were higher than with the limestones. The sulfur retention increased with diminishing the Ca-based sorbent particle size, and it was hardly affected by the O2/CO2 ratio fed into the combustor.

The kinetic of reduction of CuO to Cu2O with N2+O2 mixtures and the oxidation of Cu2O to CuO with O2 of a Cu-based oxygen carrier for the CLOU process has been determined in a TGA. For kinetic determination, the O2 concentrations were varied between 0 and 9 vol.% for reduction, and between 21 and 1.5 vol.% for oxidation reactions; temperature was varied between 1148 and 1273 K for the reduction and between 1123 and 1273 K for the oxidation. The oxygen carrier showed high reactivity both in oxidation and reduction reactions. The nucleation and nuclei growth model with chemical reaction control properly described the evolution of solids conversion with time. The Langmuir-Hinshelwood model was able to describe the effect of oxygen concentration on reduction and oxidation rates. The reaction order was 0.5 for reduction and 1.2 for the oxidation. The kinetic constant activation energies were 270 kJ mol-1 for the reduction and 32 kJ mol-1 for the oxidation. The kinetic model was used to calculate the solids inventory needed in the fuel reactor for complete combustion of three different rank coals. It was possible to use a low oxygen carrier inventory in the fuel reactor (160 kg/MWth) to supply the oxygen required to full lignite combustion. However, to reach high CO2 capture efficiencies (³95%), oxygen carrier inventories in fuel reactor higher than 600 kg/MWth were needed with the lignite.

Se ha demostrado científicamente que la actividad humana está potenciando el efecto invernadero de la atmósfera terrestre [1]. El efecto invernadero se debe a que parte de la radiación emitida por la superficie de la Tierra es absorbida por el CO2, H2O y otros gases existentes en la atmósfera. Los principales gases responsables del efecto invernadero son H2O, CO2, CH4, CFCs, N2O y SF6. El efecto invernadero es el responsable de que las temperaturas medias de la Tierra sean de 15ºC frente a los -15ºC que habría sin la existencia del mismo [1]. En las últimas décadas este efecto está siendo potenciado debido al aumento de las concentraciones de los gases que lo producen. Esta potenciación del efecto invernadero está produciendo un calentamiento del planeta, lo que puede llegar a producir un cambio del sistema climático global [2]. Si se continúa con la tendencia actual en las emisiones de gases de efecto invernadero, se prevén importantes consecuencias medioambientales, para la biodiversidad del planeta, así como en aspectos sociales y económicos.

Economic benefits can be expected in the future if CO2 capture and storage are implemented in energy generation from biomass combustion. The aim of this work is to investigate the combustion of biomass in a chemical looping with oxygen uncoupling (CLOU) process with inherent CO2 separation. The performance of biomass combustion in a continuously operated 1.5 kWth CLOU unit is presented. Particles prepared by spray drying containing 60 wt.% CuO were used as an oxygen carrier. Milled pine wood chips were used as fuel. The work focused on the effect of fuel reactor temperature on the CO2 capture and the combustion efficiency of the CLOU process with biomass. Under CLOU operation, biomass combustion was complete to CO 2 and H2O without the presence of any unburnt material, including tars. Moreover, high carbon capture efficiencies were achieved using very low oxygen carrier inventories and without a carbon separation unit. This is the first time that the CLOU concept has been demonstrated in a continuous CLC unit using biomass as fuel. © 2014 Elsevier B.V.

El proceso Steam Reforming (SR) es la tecnología más usada hoy en día para producir H2 a partir de CH4 a gran escala, y parece ser que lo seguirá siendo en los próximos años. El principal inconveniente de este proceso es la emisión de CO2 procedente de la combustión de gases combustibles en un horno con el fin de suministrar calor a las reacciones endotérmicas de reformado. Con el fin de mejorar este proceso debería buscarse una forma de proporcionar calor a las reacciones de reformado sin la necesidad de incorporar un sistema de separación de CO2 externo, es decir, buscar una forma de quemar combustible con separación de CO2 inherente. Por ello se ha propuesto en la presente tesis doctoral la integración del sistema convencional SR con un sistema Chemical Looping Combustion, (CLC) actuando esta último como fuente de calor para las reacciones endotérmicas de reformado. En esta tesis se desarrollaron, caracterizaron y evaluaron en una planta en continuo de CLC diferentes transportadores de oxígeno (TO) basados en Fe, tanto sintéticos como naturales de bajo coste, para su uso en un proceso SR-CLC. A su vez, también se llevó a cabo la optimización energética del proceso SR-CLC, con el fin de maximizar la producción de H2, usando TOs de hierro. Inicialmente se desarrolló un transportador sintético de base Fe, La preparación del mismo tuvo lugar por impregnación húmeda incipiente sobre un soporte comercial de alúmina y posterior calcinación. El transportador fue caracterizado textural y estructuralmente, así como por termogravimetría (TGA) y en lecho fluidizado discontinuo. Por último se estudió su comportamiento en la planta CLC en continuo para la combustión de CH4 y PSA-offgas. Los resultados obtenidos con el transportador de hierro se compararon con los obtenidos con mezclas físicas y químicas de óxidos de Fe y Ni. Primero se desarrollaron y caracterizaron transportadores bimetálicos Fe-Ni, obtenidos por impregnación húmeda incipiente, con ambos metales soportados so, Por último se determinaron las condiciones óptimas de funcionamiento del proceso SRCLC con el fin de maximizar la producción de H2, trabajando con un TO de hierro. Para ello se realizaron balances de masa y energía al sistema SR-CLC. En primer lugar se realizó un estudio variando dos parámetros externos al sistema CLC. Una vez determinadas las mejores condiciones de operación, es decir, aquellas que maximizaban la producción de hidrógeno, se procedió al estudio del efecto de dos variables internas del proceso CLC. Además, se estudió el efecto de utilizar Al2O3 como soporte, que permite la formación del compuesto FeO·Al2O3 como producto en la reducción del TO. Finalmente se estudió también la influencia de colocar los tubos de reformado en el reactor de reducción o situarlos en el reactor de oxidación. Las principales conclusiones que se obtuvieron fueron las siguientes: Los resultados obtenidos muestran que el TO de hierro, obtenido por impregnación húmeda incipiente sobre Al2O3, es un transportador adecuado para usarse en un sistema SR-CLC. El inventario de sólidos requerido por este TO para quemar completamente una corriente PSA-offgas o CH4 en la planta en continuo, a 880ºC, es mucho menor que los obtenidos en la literatura hasta la fecha para transportadores de hierro.

In the in situ Gasification Chemical Looping Combustion of coal (iG-CLC), the fuel is gasified in situ in the fuel reactor and gasification products are converted to CO2 and H2O by reaction with the oxygen carrier. This work is the first study on mercury release in Chemical Looping Combustion of coal. The fraction of the mercury in coal vaporized in the fuel reactor depended mainly on the fuel reactor temperature and the coal type. In the fuel reactor, mercury was mainly emitted as Hg0 in the gas phase and the amount increased with the temperature. In the air reactor, mercury was mostly emitted as Hg2+. In a real CLC system, mercury emissions to the atmosphere will decrease compared to conventional combustion as only mercury released in the air reactor will reach the atmosphere. However, measures should be taken to reduce Hg0 in the CO2 stream before the purification and compression steps in order to avoid operational problems.

The objective of this work was to determine the kinetic parameters for reduction and oxidation reactions of a highly reactive Fe-based oxygen carrier for use in chemical looping combustion (CLC) of gaseous fuels containing CH4, CO and/or H2, e.g. natural gas, syngas and PSA-off gas. The oxygen carrier was prepared by impregnation of iron on alumina. The effect of both the temperature and gas concentration was analysed in a thermogravimetric analyser (TGA). The grain model with uniform conversion in the particle and reaction in grains following the shrinking core model (SCM) was used for kinetics determination. It was assumed that the reduction reactions were controlled by two different resistances: the reaction rate was controlled by chemical reaction in a first step, whereas the mechanism that controlled the reactions at higher conversion values was diffusion through the product layer around the grains. Furthermore, it was found that the reduction reaction mechanism was based on the interaction of Fe2O3 with Al2O3 in presence of the reacting gases to form FeAl2O4 as the only stable Fe-based phase. The reaction order values found for the reducing gases were 0.25, 0.3 and 0.6 for CH4, H2 and CO, respectively, and the activation energy took values of between 8 kJ mol-1 (for H2) and 66 kJ mol-1 (for CH4). With regard to oxidation kinetics, the reacting model assumed a reaction rate that was only controlled by chemical reaction. Values of 0.9 and 23 kJ mol-1 were found for reaction order and activation energy, respectively. Finally, the solids inventory needed in a CLC system was also estimated by considering kinetic parameters. The total solids inventory in the CLC unit took a minimum value of 150 kg MW-1 for CH4 combustion, which is a low value when compared to those of other Fe-based materials found in the literature.

Chemical-Looping Combustion, CLC, is one of the most promising processes to capture CO2 at a low cost. It is based on the transfer of the oxygen from air to the fuel by using a solid oxygen carrier that circulates between two interconnected fluidized-bed reactors: fuel and air reactors. The CO2 capture is inherent to this process, as the air does not get mixed with the fuel. In this work two options are evaluated in a 1 kWth continuously operated unit for coal fueled Chemical-Looping. The first one is the gasification of coal in the fuel reactor –in-situ Gasification Chemical- Looping Combustion (iG-CLC)–. Ilmenite or a bauxite waste material is used as oxygen carrier. The second one is the combustion of coal in the fuel reactor by using oxygen carriers which release gaseous oxygen –Chemical-Looping with Oxygen Uncoupling (CLOU)–. A Cu-based material is used in this mode. Coals ranging from anthracite to lignite were used. Complete combustion was always reached in CLOU mode, whereas unburnt compounds were present in iG-CLC. Both in iG-CLC and CLOU processes the CO2 capture increased with temperature and decreased with the coal rank. The highest CO2 capture rate was obtained for lignite, being 93% for iG-CLC and 99% for CLOU, even without a carbon separation system. The key aspects for the good performance of iG-CLC and CLOU processes are analyzed through the performance results obtained with different coals

Oxy-fuel combustion consists in burning a fuel with a mixture of nearly pure oxygen and a CO2-rich recycled flue gas resulting in a product flue gas from the boiler containing mainly CO2 and H2O. In this work, the effect of combustion temperature and flue gas recycle composition on pollutant emissions and on the sulfation process, in order to optimize the SO2 retention in circulating fluidized beds (CFBC), were analyzed. The experimental work was carried out with an anthracite coal in a 3 kWth continuous bubbling fluidized bed (BFB) combustor operating in oxy-fuel conditions. It was found that in oxy-combustion conditions in BFB an increase of combustor temperature produced an increase on NO emission and a decrease in the CO and N2O emissions. The optimum temperature to reach the highest sulphur retention was at 900-925 °C. Flue gas recirculation was simulated by mixing different gases (CO2, SO2, NO, and steam) in different concentrations. It was observed that SO2 recycle produced an increase on the sulfur retention as a consequence of the higher SO2 concentration existing in the bed. About 65% of the recycled NO was reduced to N2, being the NO converted to N2O lower than 5vol.%. Wet recycle produced an important reduction on NO emission in comparison with dry recycle.

This paper is focused on the Chemical Looping Combustion (CLC) of coal. Under this configuration, coal is fed directly into the fuel reactor where coal is gasified and the gaseous products react with the oxygen carrier. In CLC of coal it is especially interesting to consider low cost oxygen carriers as some losses of solid material may occur during the ashes drainage. The performance as oxygen carrier of a Fe-enriched sand fraction (Fe-ESF) generated in the alumina production from bauxite is evaluated in this work. The experiments were carried out in a continuous 500Wth unit using a bituminous coal. Different variables affecting the process were analyzed, such as the temperature in the fuel reactor, coal feeding rate, solid circulation flow and composition of the gasifying gas. High combustion efficiencies in the fuel reactor (around 90%) were obtained at all the temperatures tested. Low oxygen demand (<5%) values were also obtained in the temperature interval tested (880-930°C). Higher temperatures or the use of a carbon separation system would be needed to have high carbon capture efficiencies. Regarding OC performance, good results were found. During 40h of continuous operation, the attrition rate was adequate and no agglomeration problems were detected. The reactivity of the oxygen carrier slightly increased as well as the porosity. Compared to ilmenite, the most tested oxygen carrier material so far for this process, the Fe-ESF material provided lower oxygen demand at the fuel reactor outlet and higher oxygen transfer rates in similar experimental conditions, which makes Fe-ESF an interesting alternative for CLC processes with coal. © 2013 Elsevier Ltd.

Chemical looping combustion (CLC) is a promising CO2 capture technology with a very low energy penalty and cost. Several previous studies have demonstrated the feasibility of this technology using both gaseous and solid fuels. The combustion of biomass using CLC would make the concept of negative-CO2 emission possible. This paper presents a study of biomass combustion in a continuous CLC unit using pine wood as fuel and iron ore as an oxygen carrier and analyses several parameters influencing the CLC process. High carbon captures (>95%) were achieved in the interval 880–915 °C using both steam and CO2 as gasifying agents. Tar compounds were detected at the fuel reactor outlet. After 78 h of continuous operation, no changes were detected in the physical and chemical properties of the oxygen carrier particles.

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a type of Chemical Looping Combustion (CLC) technology that allows the combustion of solid fuels with air, as with conventional combustion, through the use of oxygen carriers that release gaseous oxygen inside the fuel reactor. The aim of this work was to study the behaviour of the sulphur present in fuel during CLOU combustion. Experiments using lignite as fuel were carried out in a continuously operated 1.5kWth CLOU unit during more than 15h. Particles containing 60wt.% CuO on MgAl2O4, prepared by spray drying, were used as the oxygen carrier in the CLOU process. The temperature in the fuel reactor varied between 900 and 935°C. CO2 capture, combustion efficiency and the sulphur split between fuel and air reactor streams in the process were analysed. Complete combustion of the fuel to CO2 and H2O was found in all experiments. Most of the sulphur introduced with the fuel exited as SO2 at the fuel reactor outlet, although a small amount of SO2 was measured at the air reactor outlet. The SO2 concentration in the air reactor exit flow decreased as the temperature in the fuel reactor increased. A carbon capture efficiency of 97.6% was achieved at 935°C, with 87.9wt.% of the total sulphur exiting as SO2 in the fuel reactor. Both the reactivity and oxygen transport capacity of the oxygen carrier were unaffected during operation with a high sulphur content fuel, and agglomeration problems did not occur. Predictions were calculated regarding the use of a carbon separation system in the CLOU process in order to reduce sulphur emissions. Coals with high sulphur content, such as lignite and anthracite, would require a carbon separation system in order to comply with legislation governing sulphur-limits. In conclusion, coals with a high sulphur content can be burnt in a CLOU process using Cu-based material to obtain high carbon capture efficiencies.

A model based on the shrinking core model is proposed and evaluated to predict the behavior of limestones during the sulfation reaction under oxy-fuel firing mode in calcining conditions at typical time scales and particle sizes of circulating fluidized bed combustors (CFBCs). This model is chosen due to its simplicity in order to incorporate it in an easy way into available global CFBC models. It is observed that the model predicts properly the evolution of the sorbent sulfation conversion assuming that the first step of the sulfation reaction is controlled by gas diffusion through the porous system of the particle and the second step is controlled by gas diffusion through the CaSO4 product layer. The kinetic parameters needed in the model are determined for two limestones. For that, sulfation reaction tests were carried out in a thermogravimetric analyzer and in a batch fluidized bed reactor.

Different bimetallic Fe–Ni-based OCs have been prepared and evaluated in a TGA, a batch fluidised bed reactor, and a continuous CLC unit in order to analyse the effect of NiO content on the CLC performance when CH4 or a PSA-offgas was used as fuel. A set of experiments was conducted in continuous operation in a 500 Wth CLC unit, firstly working with a chemically mixed OC, with the iron and nickel oxides impregnated over the same alumina particle, and secondly working with a physical mixture of two impregnated Fe- and Ni-based OCs. The results were also compared with those obtained with an unmixed Fe-based OC. The effect on the combustion efficiency of different operating conditions, such as fuel composition, oxygen carrier to fuel ratio and fuel reactor temperature has been determined in the continuous unit. It was found that the use of a chemically mixed OC had a negative effect on the combustion efficiency since the formation of Fe–Ni compounds reduced the catalytic effect of Ni addition. On the other hand, a physically mixed OC with 2% of NiO increased significantly the combustion efficiency at low temperatures.

The chemical looping combustion of solid fuels by in situ gasification (iG-CLC) has great potential to reduce the economical and energetic cost of CO2 capture for generating energy from coal. Previous studies have highlighted that a high CO2 capture rate can be reached, but incomplete combustion is predicted by theoretical models or obtained during experimental work. In this paper, a mathematical model for the fuel reactor and carbon stripper, validated through experimental results, is adapted to evaluate the relevance of several technological improvements in order to increase the combustion efficiency of the iG-CLC process. The technological options evaluated include increasing the gas–solid contact in the fuel reactor, incorporating a secondary fuel reactor, re-circulating exhaust gases to the fuel reactor or the carbon stripper, or feeding coal into the carbon stripper instead in the fuel reactor. Model simulations showed that the use of a secondary fuel reactor has the major impact by reducing the unburnt compounds in the CO2 stream. The origin of the unburnt compounds is determined from a thorough analysis of the results obtained during the evaluation of these technological options. Thus, a new arrangement of the iG-CLC reactors is proposed, one which would minimise the presence of unburnt compounds. In this concept, exhaust gases from the fuel reactor are sent to the carbon stripper. The oxygen demand for this concept is predicted to be XT= 0.9%.

The chemical looping with oxygen uncoupling (CLOU) process is a chemical looping combustion (CLC) technology that allows for the combustion of solid fuels with inherent CO2 separation. As in CLC technology, in the CLOU process, the oxygen necessary for fuel combustion is supplied by a solid oxygen carrier, which is moving between two reactors: the fuel and air reactors. The CLOU technology uses the property of some metal oxides (CuO, Mn2O3, and Co3O4), which can generate gaseous oxygen at high temperatures. The oxygen generated by the oxygen carrier reacts directly with the solid fuel, which is mixed with the oxygen carrier in the fuel reactor. The reduced oxygen carrier is transported to the air reactor, where it isoxidized by air. In this work, a material prepared by spray drying containing 60 wt % CuO and 40 wt % MgAl2O4 as supporting material was evaluated as an oxygen carrier for the CLOU process using different installations. First, the oxygen release rate and the fluidization behavior, with regard to the agglomeration and attrition rate, were analyzed in a thermogravimetric analyzer (TGA) and in a batch fluidized bed, respectively. Then, the effects of the main operating conditions, such as the temperature, solids flow rate, and gas velocity in the fuel reactor, on the oxygen-carrier capability to release gaseous oxygen were analyzed in a continuous CLOU unit using N2 and CO2 as fluidization media. In addition, the effect of the oxygen concentration in the air reactor on the capability of the oxygen carrier to be regenerated was evaluated. The results obtained showed that this oxygen carrier has suitable characteristics for the CLOU process. Nevertheless, after 40 h of continuous operation at high temperatures, the particle integrity decreased significatively, indicating the need to improve the lifetime of this kind of material for use in an industrial CLOU process.

Chemical-looping combustion (CLC) has been emerging in recent years as a promising CO 2 capture technology. In CLC, the oxygen needed in the combustion is supplied by an oxygen carrier (OC), normally a metal oxide, which circulates between fuel and air reactors. In the fuel reactor, the fuel is oxidized to CO 2 and H 2O, which facilitates CO 2 capture once the water has been condensed. The reduced OC is then transported to the air reactor, where it is reoxidized before the start of a new cycle. Gaseous fuel combustion has been extensively studied to date, and now attention has been directed to solid fuels, such as coal. In this case, the OC should be inexpensive, as some losses are expected with the ashes. A bauxite waste from alumina production, containing Fe 2O 3, could fulfill the requirements for CLC of solid fuels. In this work, the reactivity of this waste material to different gases present in coal combustion was studied. Thermogravimetric analysis (TGA) together with experiments in a batch fluidized-bed reactor (FB) were performed in the 1100-1223 K temperature range using H 2, CO, H 2/CO mixtures, and CH 4 as reducing gases. Similar to other natural minerals or waste materials, a gain in the bauxite residue reactivity with the number of cycles in TGA was observed. TGA experiments at 1173 K showed that the bauxite waste samples reacted more rapidly with H 2 than with CO and CH 4. The reactivity of this material was found to be comparable to those of other Fe-based synthetic materials and even higher than those of some minerals, such as ilmenite. In batch FB experiments, H 2 and CO were completely converted. CH 4 was partially converted at all temperatures and reducing times tested. With regard to the fluidization behavior of this waste, the material presented moderate attrition rates and no defluidization problems were observed during operation in the fluidized bed after >40 h. Therefore, this material has suitable properties to be considered as an OC for CL

Oxy-fuel combustion is one of the leading options for power generation with CO2 capture. The process consists of burning the fuel with a mixture of nearly pure oxygen and a CO2-rich recycled flue gas, resulting in a product flue gas from the boiler containing mainly CO2 and H2O. Among the possible boiler types, fluidised bed combustors are very appropriate for the oxy-fuel process because they allow the in situ des- ulphurisation by feeding Ca-based sorbents into the combustor. In this work, the effect of the temperature of the combustor on the retention of the SO2 generated in the combustion of two coals with very different sulphur content (a lignite and an anthracite) has been stud- ied. The experimental facility used was a bubbling fluidised bed (BFB) combustor of 3kWth. Tests were conducted under oxy-fuel combustion mode and also under enriched-air combustion mode for compar-ison reasons. A Spanish limestone ‘‘Granicarb’’ was used as Ca-based sorbent for sulphur retention. The temperatures tested were between 800 and 970 C using Ca/S molar ratios between 0 and 3. It was found that in BFB combustors operating under oxy-fuel combustion conditions the optimum temperature to achieve the highest sulphur retention was 900–925 C, whereas operating with enriched air the optimum combustion temperature was 850–870 C. Working at the optimum temperature, the SO2 retentions were lower in oxy-fuel combustion than in enriched air combustion conditions. It was also observed that working with lignite there was 10–15% of sulphur retention by coal ashes, however, working with anthracite the sulphur retention by coal ashes was negligible. This finding was independent of the combustion mode used, oxy-fuel or enriched air.

In order to compare potential oxygen carriers for the iG-CLC with coal a methodology was developed from results obtained in a batch fluidized reactor. The methodology is based on the solids inventory in the fuel reactor. The solids inventory values will be used for comparison purposes among different materials rather than for design purposes. In a companion paper, experiments with different types of Fe-based oxygen carriers were presented. Based on these experimental results, the present paper proposes two theoretical approaches for a fast evaluation of different materials as potential oxygen carriers for the iG-CLC process with different fuels. The methodology is focused on the study of the gasification of the char generated in the process after the devolatilization of the corresponding solid fuel and the subsequent combustion of the gasification products. Considering the results of carbon capture and combustion efficiency, the adequacy of the material can be evaluated. Four different Fe-based materials are tested in this work: ilmenite, bauxite waste, a mineral based on hematite, and a synthetic material. Based on the results obtained, the bauxite waste and the hematite mineral present the best performance for iG-CLC. As a rough estimation, an inventory around 1600 kg/MWth will be needed using any of the above materials to achieve carbon captures above 0.93 and combustion efficiencies of 0.99 at 1000 °C.

A Fe-based oxygen carrier was evaluated for H2 production with CO2 capture by coupling steam reforming of methane and chemical–looping combustion processes (SRCLC). In this process hydrogen is obtained by conventional steam reforming of methane (SR), and the heat necessary to carry out the endothermic reformed reactions is supplied by a chemical-looping combustion (CLC) system with inherent CO2 capture The tail gas from the PSA unit, a mixture of CH4, H2, CO and CO2, is used as fuel in the CLC system. The Fe-based oxygen carrier was made by incipient hot impregnation using Al2O3 as support and was first characterized by TGA and batch fluidized bed reactor in order to determine the reactivity for CH4, CO and H2 and the fluidization behaviour. The oxygen carrier behaviour with respect to gas combustion was evaluated in a continuous CLC unit using a simulated PSA off-gas stream as fuel. Methane was also used as fuel for comparison purposes. During 46 h of continuous operation, the OC never showed agglomeration problems or carbon deposition, and the loss of fines due to attrition was moderate. It was found that the fuel composition has low effect on the combustion efficiency and full conversion can be obtained at 880ºC with an oxygen carrier-to-fuel ratio > 1.5. The solids inventory needed to fully convert methane or PSA-offgas was found to be lower than those found with other Fe-based materials, both natural and synthetic. These results were due to the important positive effect of using alumina as support for impregnated Fe-based OC on the oxygen carrier reactivity. Therefore, the Fe2O3/Al2O3 oxygen carrier prepared by impregnation is suitable for burning a PSA tail gas for the H2 production with CO2 capture in the SR-CLC process.

Chemical-Looping with Oxygen Uncoupling (CLOU) process is a Chemical- Looping Combustion (CLC) technology that allows the combustion of solid fuels using oxygen carriers with inherent CO2 separation The oxygen necessary for the fuel combustion is supplied by a solid oxygen carrier, which contains a metal oxide. The oxygen carrier circulates between two interconnected fluidized reactors: the fuel and the air reactor. In the CLOU process, the oxygen carrier releases gaseous oxygen in the fuel reactor which burns coal as in common combustion with air, so the CO2 generated is undiluted with N2. The reduced oxygen carrier is oxidized by air to the initial metal oxide in the air reactor, then being ready to start a new cycle. The aim of this work is to study the performance of the CLOU process using coals of different rank. Experiments were carried out in a continuously operated 1.5 kWth unit. Particles prepared by spray drying containing 60 wt.% CuO were used as oxygen carrier. Four coals of different rank (anthracite, low volatile bituminous, medium volatile bituminous and lignite) were used as fuel. Besides, the temperature in the fuel reactor was varied between 900 and 950 ºC. In all the experiments there was complete combustion of the coal to CO2 and H2O, without any unburnt product. The carbon capture efficiency greatly depends on the coal rank and fuel reactor temperature. High carbon capture efficiencies were obtained for Lignite and Medium Volatile Bituminous coals. The maximum capture efficiency was 99.3% at 950 ºC with Lignite. The analysis of the experimental results was used to evaluate the effect of the coal rank in a CLOU system when a carbon separation system is included. At 925 ºC, the solid inventory needed to reach 95 % of CO2 capture efficiency with a carbon separation system of 90 % of efficiency is 45 kg/MWth using Lignite, 85 kg/MWth using MV Bituminous, 140 kg/MWth using LV Bituminous and 490 kg/MWth using Anthracite. It must be pointed out the low

Different low-cost materials, especially Fe-based minerals or industrial residues, have been tested as carriers for iG-CLC. This work presents a methodology for the comparison of potential oxygen carriers. The methodology is based on results obtained in a batch fluidized bed reactor. The experiments were designed so that estimated solids inventory in the fuel reactor could be calculated for each oxygen carrier at different conditions and later compared. In the present paper, which is the first of a series of two, experimental results for different Fe-based materials were obtained. The second paper focuses on the evaluation and the comparison of the carriers based on these experimental results. In the present work, the performance of two iron minerals (hematite based and copper ferrite) is analyzed and compared to the behavior of ilmenite and a bauxite waste. The performance of a highly reactive synthetic Fe-based material is evaluated for comparison. In the TGA, the reactivity of the different materials to H2, CO, CH4, and O2 was studied. In the experiments in a fluidized bed, several O/C molar ratios were used to evaluate its effect on char gasification rate and combustion efficiency of the gasification products. The char gasification rate decreased when the O/C molar ratio decreased as the concentration of H2 and CO in the bed increased and high levels of these gases inhibit char gasification. With no inhibitory effect, the char gasification rate was very similar. Both the bauxite waste and the hematite based mineral were able to burn the gasification products with high combustion efficiencies, comparable to those obtained in the same conditions using the synthetic Fe-based material. © 2012 American Chemical Society.

In this work, pollutant emissions in a continuous bubbling fluidized bed combustor (∼3 kWth) at oxy-firing conditions were measured. An anthracite coal was used as fuel and a limestone was added for sulfur retention. Flue gas recirculation was simulated by mixing different gases (CO2, SO2, steam, and NO) and the effect of varying the gas composition of the recycled flow on the pollutant emissions was analyzed. It was observed that the most important effect of CO2 recirculation was the increase of the optimum temperature for SO2 retention from ∼850 °C (conventional air combustion) to 900-925°C. SO2 recirculation increased the Ca-based sorbent utilization and did not affect the N2O emissions at any temperature. In addition, at 850°C the CO emission increased and the NO emission decreased, however, at 925°C the CO variation was negligible and the NO reduction was low. About 60-70% of the recycled NO was reduced to N2 and N2O, being the NO converted to N2O lower than 5%. Steam recirculation mainly led to a sharp decrease in NO emission. A synergetic effect among the different recycled gases was not found in any case.

La presente invención se refiere a un material transportador de oxígeno obtenible a partir de un procedimiento que comprende: a) mezclar de un 30% a un 60% en peso de CuO y de un 40% a un 70% en peso de MgAl2O4; b) someter la mezcla anterior a una etapa de secado por atomización o spray drying; c) tamizar el material obtenido en la etapa anterior hasta obtener un tamaño de partícula comprendido entre 100 y 500 micras; d) calcinar las partículas tamizadas, dando lugar al material transportador de oxígeno. Es asimismo objeto de la invención la aplicación de dicho material en un proceso de combustión de sólidos con captura inherente de CO2.

Chemical-Looping with Oxygen Uncoupling (CLOU) process is a Chemical-Looping Combustion (CLC) technology that allows the combustion of solid fuels with inherent CO2 separation. As in the CLC technology, in the CLOU process the oxygen necessary for the fuel combustion is supplied by a solid oxygen-carrier, which contains a metal oxide. The CLOU technology uses the property of the copper oxide which can generate gaseous oxygen at high temperatures. The oxygen generated by the oxygen-carrier reacts directly with the solid fuel, which is mixed with the oxygen-carrier in the fuelreactor. The reduced oxygen-carrier is transported to the air-reactor where it is oxidized by air. The flue gases from the fuel-reactor are only CO2 and H2O, since fuel is not mixed with air. This work demonstrates the proof of the concept of the CLOU technology burning coal in a 1.5 kWth continuously operated unit consisting of two interconnected fluidized-bed reactors. A bituminous coal was used as fuel. An oxygencarrier prepared by spray drying containing 60 wt% CuO and MgAl2O4 as supporting material was used as oxygen-carrier. The effects of fuel-reactor temperature, coal feeding rate, and solids circulation flow rate on the combustion and on the CO2 capture efficiencies were investigated. Fast reaction rates of oxygen generation were observed with the oxygen-carrier and full combustion of coal was attained in the plant using a solids inventory ≈ 235 kg/MWth in the fuel-reactor. In addition, values close to 100 % in carbon capture efficiency were obtained at 960 ºC. Results obtained are analyzed and discussed in order to be useful for the scale-up of a CLOU process fuelled with coal.