Combustión de CH4 en lecho fluidizado con separación inherente de CO2 por medio de transportadores sólidos de oxígeno de base cobre

Las emisiones de dióxido de carbono (CO2) de origen humano a la atmósfera han elevado su concentración desde ~270 ppm, antes de la revolución industrial (~1850), hasta ~370 ppm en la actualidad. El dióxido de carbono es uno de los principales responsables del efecto invernadero ya que se encuentra en concentraciones muy superiores a las de otros compuestos (CH4, NOx, CFCs y SF6) también perjudiciales para la estabilidad térmica de la atmósfera. El efecto invernadero y la elevación de la temperatura se encuentran asociados al cambio climático actual que padecemos y para contrarrestar este proceso se deben disminuir las emisiones de CO2 a la atmósfera. Las emisiones de CO2 antropogénicas provienen aproximadamente a partes iguales del transporte, producción de energía y fuentes diversas. Las líneas de actuación tradicionales para disminuir las emisiones de CO2 asociadas a la producción de energía son la mejora de la eficacia de los procesos y la utilización de fuentes de energía con menor contenido en carbono (gas natural en lugar de carbón, energías renovables, energía nuclear, etc.). No obstante, y teniendo en cuenta la alta dependencia de las fuentes de energía fósiles, sus reservas y la capacidad de actuación limitada de las líneas de actuación tradicionales, es preciso adoptar nuevas medidas adicionales para reducir las emisiones de CO2. En este contexto se presenta la captura y almacenamiento de CO2 como una nueva vía complementaria a las medidas ya adoptadas. La captura y almacenamiento de CO2 consiste en la separación de éste del resto de gases que lo acompañan en los procesos de producción de energía a partir de combustibles fósiles, su transporte y almacenamiento seguro definitivo. Sin embargo, para llevar a cabo la captura del CO2 es necesario separarlo de otros gases, siendo ésta la etapa de mayor consumo energético y económico que impide la puesta en marcha de esta ruta con las tecnologías actuales. En esta situación surge el proceso de combustión indirecta o “chemical looping combustión” (CLC) como una de las tecnologías más apropiadas para la producción de energía a partir de combustibles en fase gas sin costes debidos a la separación del CO2 para su posterior almacenamiento. El proceso CLC consiste en separar la combustión convencional de un gas en dos etapas con la ayuda de un transportador sólido de oxígeno, Figura 5.1. En una primera etapa, en el reactor de reducción, el combustible gaseoso se oxida a CO2 y H2O según la Reacción 5.1. En un segundo reactor, reactor de oxidación, el transportador de oxígeno se regenera con aire según la Reacción 5.2, quedando disponible para iniciarse un nuevo ciclo. De este modo, al no ponerse en contacto el combustible con el nitrógeno del aire, la mezcla de gases a la salida del reactor de reducción es solamente CO2 y H2O. Esta mezcla es fácilmente separable por condensación del agua, quedando así el CO2 listo para su acondicionamiento para el transporte y almacenamiento definitivos. Por otra parte, la energía total puesta en juego es equivalente a la de la combustión directa porque la suma de las energías en cada uno de los reactores (Reacción 5.1 y 5.2) es la correspondiente a la combustión directa (Reacción 5.3). Aire O2/N2 CnH2m CO2 H2O Me (+MeO) Reactor de oxidación Reactor de reducción MeO (+Me) MexOy-1 + ½ O2 → MexOy (2n + m) MexOy + CnH2m → →(2n + m) MexOy-1 + m H2O + n CO2 Condensador Q Figura 5.1. Esquema conceptual del sistema CLC Reactor de reducción CnH2m + (2n+m) MexOy → → (2n+m) MexOy-1 + m H2O + n CO2 AH1 Reacción 5.1 Reactor de oxidación (2n+m) MexOy-1 + (2n+m)/2 O2 → → (2n+m) MexOy AH2 Reacción 5.2 Combustión directa CnH2m + (2n+m)/2 O2 → n CO2 + m H2O AHc = AH1+AH2 Reacción 5.3Peer reviewe

Combustión de CH4 en lecho fluidizado con separación inherente de CO2 por medio de transportadores sólidos de oxígeno de base cobre

Entre las nuevas tecnologías para la separación del CO2 generado en los procesos de producción de energía se encuentra la combustión indirecta con transportadores sólido de oxígeno (chemical looping combustion, CLC), tecnología en la que se enmarca este trabajo de investigación. Los objetivos de esta tesis son: - demostrar la viabilidad del proceso CLC en una planta piloto de 10 kW - desarrollar un transportador de oxígeno de base cobre apropiado para este sistema CLC - determinar la cinética intrínseca de las reacciones redox del transportador desarrollado con metano y oxígeno - estudiar la influencia de las variables de operación (caudal de sólido circulante, temperatura del reactor de reducción, velocidad de alimentación del combustible y tamaño de partícula) sobre la conversión del combustible en una planta piloto CLC de 10 kW - estudiar la idoneidad y comportamiento del transportador desarrollado en la planta CLC en relación con su atrición, aglomeración y mantenimiento de la reactividad

Mapping of the range of operational conditions for Cu-, Fe-, and Ni-based oxygen carriers in chemical-looping combustion

Chemical-looping combustion (CLC) is a two-step combustion process that produces a pure CO2 stream, ready for compression and sequestration. A CLC system is composed by two reactors, an air and a fuel reactor, and an oxygen carrier (OC) circulating between the reactors, which transfers the oxygen necessary for the fuel combustion from the air to the fuel. This system can be designed similar to a circulating fluidised bed, but with the addition of a bubbling fluidised bed on the return side. A mapping of the range of operational conditions, design values, and OC characteristics is presented for the most usual metal oxides (CuO, Fe2O3, and NiO) and different fuel gases (CH4, H2, and CO). The pressure operation of a CLC system is also considered. Moreover, a comparison of the possible use of three high reactive OCs (Cu10Al-I, Fe45Al-FG, Ni40Al-FG) previously characterised is carried out. It was found that the circulation rates and the solids inventories are linked, and the possible operating conditions are closely dependent on the reactivity of the OCs. The operational limits of the solids circulation rates, given by the mass and heat balances in the system, were defined for the different type of OCs. Moreover, a plot to calculate the solids inventories in a CLC system, valid for any type of OC and fuel gas, is proposed. The minimum solids inventories depended on the fuel gas used, and followed the order CH4 > CO > H2. Values of minimum solids inventories in a range from 40 to 133 kg / MWf were found for the OCs used in this work, excepting for the reaction of Fe45Al-FG with CH4, which needs a higher amount of solids because of its low reactivity. From the economic analysis carried out it was found the cost of the OC particles does not represent any limitation to the development of the CLC technology. © 2006 Elsevier Ltd. All rights reserved.This work was carried out with financial support from the European Coal and Steel Community (Project 7220-PR/125), and the Spanish Ministry of Education and Science (Project CTQ2004-04034).Peer Reviewe

Operation of a 10 kWth chemical-looping combustor during 200 h with a CuO-Al2O3 oxygen carrier

Chemical-looping combustion (CLC) is an attractive technology to decrease greenhouse gas emissions affecting global warming, because it is a combustion process with inherent CO2 separation and therefore without needing extra equipment for CO2 separation and low penalty in energy demand. The CLC concept is based on the split of a conventional combustion of gas fuel into separate reduction and oxidation reactions. The oxygen transfer from air to fuel is accomplished by means of an oxygen carrier in the form of a metal oxide circulating between two interconnected reactors. A Cu-based material (Cu14Al) prepared by impregnation of γ-Al2O3 as support with two different particle sizes (0.1-0.3 mm, 0.2-0.5 mm) was used as an oxygen carrier for a chemical-looping combustion of methane. A 10 kWth CLC prototype composed of two interconnected bubbling fluidized bed reactors has been designed, built in and operated at 800 °C during 100 h for each particle size. In the reduction stage full conversion of CH4 to CO2 and H2O was achieved using oxygen carrier-to-fuel ratios above 1.5. Some CuO losses as the active phase of the CLC process were detected during the first 50 h of operation, mainly due to the erosion of the CuO present in external surface of the alumina particles. The high reactivity of the oxygen carrier maintained during the whole test, the low attrition rate detected after 100 h of operation, and the absence of any agglomeration problem revealed a good performance of these CuO-based materials as oxygen carriers in a CLC process. © 2006 Elsevier Ltd. All rights reserved.This research was carried out with financial support from the Spanish Ministry of Education and Science (Projects PPQ-2001-2111 and CTQ-2004-04034) and from the Diputación General de Aragón (Project PIP023/2005).Peer Reviewe

Impregnated CuO/Al2O3 oxygen carriers for chemical-looping combustion: Avoiding fluidized bed agglomeration

Chemical-looping combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2 that involves the use of an oxygen carrier, which transfers oxygen from air to the fuel avoiding the direct contact between them. An oxygen carrier in a CLC power plant must show high reaction rates and sufficient durability in successive cycle reactions. Furthermore, the oxygen carrier particles must not agglomerate. To produce materials with these characteristics, different Cu-based oxygen carriers with a range of CuO content between 10 and 26 wt % were prepared by impregnation using alumina as support. The particles were calcined at different temperatures in the range of 550-950 °C. The oxygen carriers were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray, and thermogravimetry. The samples were evaluated in a fluidized bed facility in order to determine their behavior in multicycle reduction-oxidation tests. A number of 100 cycles was done when the carrier did not agglomerate. The effects of metal content, calcination temperature, and method of preparation on the oxygen carriers reactivity, gas product distribution, attrition rate, and agglomeration were analyzed for the different oxygen carriers. It was observed that the CuO content in the oxygen carrier, the calcination temperature used in the preparation, and the conversion reached by the oxygen carrier during the reduction period affected the agglomeration process. In this work, the preparation conditions to produce Cu-based oxygen carriers with high reactivities, and small attrition rates were found. Moreover, the agglomeration of these oxygen carriers in the fluidized reactor, which is the main reason adduced in the literature to reject the Cu-based oxygen carriers for the CLC process, was avoided. © 2005 American Chemical Society.This research was carried out with financial support from the Spanish Ministry of Education and Science (Project PPQ-2001-2111) and the European Coal and Steel Community (Project 7220-PR125).Peer Reviewe

Chemical Looping Combustion in a 10 kWth prototype using a CuO/Al2O3 oxygen carrier: Effect of operating condition on methane combustion

5 pages, 10 figuresChemical Looping Combustion (CLC) is nowadays an attractive option to decrease the greenhouse gas emissions affecting global warming, because it is a combustion process with inherent CO2 separation and, therefore, without energy losses. The CLC concept is based on the transfer of oxygen from the combustion air to fuel by means of an oxygen carrier in the form of a metal oxide. The system consists of two separate but interconnected reactors, normally fluidized bed type. In the fuel reactor the oxygen carrier particles react with fuel and generate a gas stream mainly composed by CO2 and H2O. The reduced metal oxide is later transported to the air reactor where oxygen from the air is transferred to the particles, obtaining in this way the original metal oxide ready to be returned to the fuel reactor for a new cycle. In this work, a 10 kW pilot plant composed of two interconnected bubbling fluidized bed reactors has been design and built to demonstrate the CLC technology. The prototype was run during 200 h, of which 120 h burning methane, and the effect of the operating conditions (oxygen carrier to fuel ratio, fuel gas velocity, oxygen carrier particle size and fuel reactor temperature) on fuel conversion was analyzed working with a CuO-Al2O3 oxygen carrier prepared by dry-impregnation. Also, the behavior with respect to attrition, agglomeration, and reactivity of the oxygen carrier was analyzed. It was found that the most important parameter affecting the CH4 conversion was the oxygen carrier to fuel ratio. Complete methane conversion, without CO or H2 emissions, was reached with this oxygen carrier working at 800ºC and oxygen carrier to fuel ratios higher than 1.4.This research was carried out with financial support from the Spanish Ministry of Education and Science (Projects PPQ-2001-2111 and CTQ-2004-04034) and from the Diputación General de Aragón (Project PIP023/2005).Peer reviewe

Nickel-copper oxygen carriers to reach zero CO and H2 emissions in chemical-looping combustion

8 pages, 13 figures, 2 tablesNi-based oxygen carriers allow to work at high temperatures (900-1100ºC) in a Chemical-Looping Combustion (CLC) process with full CH4 conversion, although thermodynamic restrictions result in the presence of CO and H2 in the gas outlet of the fuel reactor. On the other hand, Cu-based oxygen carriers allow complete fuel combustion to CO2 and H2O, but the operating temperature is limited due to the low melting point of the metallic Cu. The objective of this research was to analyze the behavior of several Ni-Cu oxygen carriers to reduce or avoid CO and H2 emissions during a CLC process working at high temperatures. Commercial Al2O3 and Al2O3 were used as support to prepare by dry impregnation different oxygen carriers based on nickel and copper. The reactivity of these oxygen carriers was determined in a thermogravimetric analyzer (TGA). The effect of the mixture of NiO and CuO on the CO and H2 generation was analyzed in a fixed bed reactor, and the gas product distribution during reduction/oxidation reactions was studied in a batch fluidized bed reactor working with CH4 as fuel and diluted air for oxidation. The fluidization behavior of the oxygen carriers with respect to the attrition and agglomeration processes was also analyzed during the multi-cycle batch fluidized bed tests. The presence of CuO in the Ni-Cu oxygen carriers allows the full conversion of CH4 to CO2 and H2O in the batch fluidized bed reactor during the initial part of the reduction time, and this time depended on the CuO content of the oxygen carrier. TGA and X-ray diffraction studies indicated that CuO is used for the reduction reaction before NiO. In addition, it was observed that the presence of NiO stabilized the CuO and allowed to work at 950 ºC with Ni-Cu oxygen carriers with a high metal oxide utilization.This research was carried out with financial support from the CCP (CO2 Capture Project), Project subcontract No. C006 under DOE contract No. DE-FC26-01NT411145 and the Spanish Ministry of Education and Science (Project CTQ2004-04034)Peer Reviewe