Influencia del CO2 alimentado en la corriente de combustible en pilas de combustible tipo PEM de baja y alta temperatura

De entre todas las impurezas que pueden existir en el combustible alimentado a una PEMFC, el estudio de la influencia del C02 es de gran interés debido a las altas concentraciones que del mismo pueden estar presentes en una corriente de reformado, así como por ser uno de los contaminantes menos estudiados en la actualidad. La presencia de este contaminante en la corriente de H2 que es alimentada al ánodo produce pérdidas de rendimiento que, aunque no son tan elevadas como las obtenidas por la presencia de CO, no deben ser despreciadas El objetivo central de la presente tesis es profundizar y avanzar en el estado del conocimiento del efecto que produce la presencia de C02 en el combustible alimentado a pilas de combustible tipo PEM de baja y alta temperatura. Mediante el mismo, se discutirá la posibilidad de la existencia de un proceso de contaminación del catalizador además del efecto dilución causado inevitablemente por la presencia de C02. Para alcanzar dicho objetivo, la tesis se divide en los siguientes objetivos secundarios: 1. Estudiar el efecto producido por la presencia de diferentes concentraciones de C02 en el combustible alimentado a pilas tipo PEM de baja temperatura (LT PEMFCs) y alta temperatura (HT PEMFCs). 2. Determinar el papel que juega la densidad de corriente en el proceso de contaminación por C02. 3. Evaluar la influencia de la temperatura de operación en el impacto negativo producido por el C02 en LT PEMFCs y HT PEMFCs. 4. Analizar cómo afecta la humedad relativa al proceso de electro oxidación del CO en LT PEMFCs. 5. Investigar la influencia del catalizador en el impacto originado por la presencia de C02 en LT PEMFCs. 6. Estudiar diferentes métodos de regeneración del catalizador y aportar evidencias de la posibilidad de la existencia de un proceso de contaminación derivado de la presencia de C02 en el combustible. 7. Desarrollar una metodología de ensayos en pilas de combustible tipo PEM, que permita normalizar el desarrollo de las curvas de polarización en aquellos estudios en los que CO y/o C02 están presentes en el combustible alimentado al sistema. Los resultados sugieren que las pérdidas de rendimiento son mayores que si sólo existiera un efecto dilución debido al C02 debido a la producción de CO a partir de la reacción “Reverse Water Gas Shift” (RWGS). Los resultados se muestran a partir de los sobrepotenciales calculados. A su vez, se muestra el análisis de los diferentes parámetros de operación anteriormente mencionados. Finalmente, se desarrolla una metodología de ensayos que permite estudiar el efecto ejercido por contaminantes que presentan una naturaleza acumulativa con el tiempo, como pueden ser CO y C02.Among all the impurities, the study of the influence of C02 on the performance of a Polymer Electrolyte Membrane Fuel Cells (PEMFC) is of considerable interest due to the high concentration of this contaminant in a typical reformate mix. The presence of C02 in the fuel can produce performance losses that cannot be underestimated. An experimental investigation of the performance and contamination of Low and High Temperature PEMFCs operating with different concentrations of C02 at the anode inlet is presented. An analysis of different operating parameters (current density, temperature, relative humidity, catalyst composition and operating time) influence in the contamination process for both low and high temperature operation range are investigated in this work. To reach the main objective, the thesis is divided in the next secondary objectives: 1. Study the effect of different concentrations of H2/C02 mixtures at Low and High Temperature PEMFCs. 2. Researching the current density influence into the contamination process. 3. Evaluate the temperature effect of temperature which is closely related with the ratio “CO production/CO electro- oxidation”. 4. Study the effect of relative humidity in the CO electro-oxidation for LT PEMFCs. 5. Change the composition of catalyst in order to evaluate the impact in the contamination process for LT PEMFCs. 6. Study two different regeneration methods to recover the contaminated electrode, in order to recover initial performance. 7. Develop a new and versatile method to study the effect of contaminants which show an accumulative behavior with time. The results show that the performance loss is larger than expected if only a dilution effect were considered, so that a real contamination process occurs in the cell when C02 is fed to the anode, due to the RWGS reaction. This contamination effect is analysed and quantified by comparing the polarization curves of the contaminated cell with the ones corresponding to the cell operating with pure hydrogen. The oveipotentials for different operating parameters are presented and discussed for both types of fuel cells. Two different regeneration processes (anode feeding with pure H2 and with air) are also presented and discussed. The analysis of the effectiveness of each regeneration strategy also supports that CO produced via the RWGS reaction is adsorbed onto the catalyst

Catalytic production of jet fuels from biomass

Concerns about depleting fossil fuels and global warming e ects are pushing our society to search for new renewable sources of energy with the potential to substitute coal, natural gas, and petroleum. In this sense, biomass, the only renewable source of carbon available on Earth, is the perfect replacement for petroleum in producing renewable fuels. The aviation sector is responsible for a significant fraction of greenhouse gas emissions, and two billion barrels of petroleum are being consumed annually to produce the jet fuels required to transport people and goods around the world. Governments are pushing directives to replace fossil fuel-derived jet fuels with those derived from biomass. The present mini review is aimed to summarize the main technologies available today for converting biomass into liquid hydrocarbon fuels with a molecular weight and structure suitable for being used as aviation fuels. Particular emphasis will be placed on those routes involving heterogeneous catalysts

Adaptive Vectorial Filter for Grid Synchronization of Power Converters Under Unbalanced and/or Distorted Grid Conditions

This paper presents a new synchronization scheme for detecting multiple positive-/negative-sequence frequency harmonics in three-phase systems for grid-connected power converters. The proposed technique is called MAVF-FLL because it is based on the use of multiple adaptive vectorial filters (AVFs) working together inside a harmonic decoupling network, resting on a frequency-locked loop (FLL) which makes the system frequency adaptive. The method uses the vectorial properties of the three-phase input signal in the αβ reference frame in order to obtain the different harmonic components. The MAVF-FLL is fully designed and analyzed, addressing the tuning procedure in order to obtain the desired and predefined performance. The proposed algorithm is evaluated by both simulation and experimental results, demonstrating its ability to perform as required for detecting different harmonic components under a highly unbalanced and distorted input grid voltage

Electrochemical reduction of CO2: overcoming chemical inertness at ambient conditions

Electroreduction allows for the transformation of a chemically inert molecule such as CO2 into a wide variety of useful carbon products. Unlike other approaches operating at higher temperatures, electrochemical reduction holds great promise since it achieves reduction under ambient conditions, thereby providing more control over the reaction selectivity. By controlling basic parameters such as the potential and the composition of the electrode, CO2 can be transformed into a variety of products including carbon monoxide, syngas (CO/H2), methane, and methanol. This reduction process takes place without external hydrogen, since water can be used as a source of both electrons and protons. Furthermore, this technology, when combined with renewable wind- or solar-derived electricity, has the potential to serve as a storage system for excess electricity. Despite these advantages, a number of challenges need to be overcome before reaching commercialization. New (and cheaper) electrocatalyst formulations with high faradaic selectivities are required. Impressive progress has been made on carbon-doped materials, which, in certain cases, have outperformed expensive noble metal-based materials. Research is also needed on new electrochemical reactor configurations able to overcome kinetic/mass transport limitations, which are crucial to reduce overpotentials. Fine control over the nature of the active sites and the reaction conditions is important to avoid parasitic reactions such as the hydrogen evolution reaction (HER), and therefore increases the faradaic e ciency towards the desired products

Recent advances in the electroreduction of CO2 over heteroatom-doped carbon materials

Ever-growing anthropogenic activity has increased global energy demands, resulting in growing concentrations of greenhouse gases such as CO2 in the atmosphere. The electroreduction of CO2 has been proposed as a potential solution for reducing anthropogenic CO2 emissions. Despite the promising results obtained so far, some limitations hinder large-scale applications, especially those associated with the activity and selectivity of electrocatalysts. A good number of metal catalysts have been studied to overcome this limitation, but the high cost and low earth abundance of some of these materials are important barriers. In this sense, carbon materials doped with heteroatoms such as N, B, S, and F have been proposed as cheaper and widely available alternatives to metal catalysts. This review summarizes the latest advances in the utilization of carbon-doped materials for the electroreduction of CO2, with a particular emphasis on the synthesis procedures and the electrochemical performance of the resulting materials

International Environmental Conventions and the civil society: a social network analysis in Spain

Las Convenciones Internacionales de Cambio Climático, Diversidad Biológica y Desertificación constituyen uno de los pilares para la gestión ambiental global. Se han analizado las redes sociales entre 47 ONGs españolas implicadas en la sensibilización sobre estas tres grandes convenciones. Se han identificado asimismo las ONGs con mayor actividad de comunicación y de intermediación en la red. Los resultados muestran que la Convención sobre Cambio Climático es la que cuenta con un mayor nivel de participación entre el conjunto de ONGs de la muestra, sin embargo, las organizaciones conservacionistas participan con mayor frecuencia en la Convención de Biodiversidad. La red social analizada sigue un patrón de "centro-periferia" con una baja densidad de vínculos sociales. Este estudio muestra la importancia de promover la cooperación entre las ONGs, especialmente entre las de conservación y las de desarrollo, para la creación de nuevos vínculos y así potenciar la capacidad de incidir en las convenciones y de influir en la sociedad civil española.The International Conventions on Climate Change, Biological Diversity and Desertification are one of the pillars for global environmental management. We analyzed the social networks of 47 Spanish NGOs involved in the promotion and awareness of these conventions. Furthermore we identified the NGOs with the highest levels of communication and interaction activity within the network. The results reveal that the highest levels of participation are in relation to the Climate Change Convention. However, conservation organizations have higher degree of participation in the Biodiversity Convention. The social networks analyzed follow a pattern of "core-periphery" with a low density of social ties. This study highlights the importance of promoting cooperation between the NGOs, especially among those involved in conservation and development, in order to establish new ties and thereby enhance the ability to affect the conventions and influence Spanish civil society

Interplay of Support Chemistry and Reaction Conditions on Copper Catalyzed Methanol Steam Reforming

A series of Cu catalysts supported on SiO2, Al2O3-SiO2, TiO2 rutile, and Cu/TiO2 anatase metal oxides has been studied for methanol reforming in the vapor phase. The highest activity was obtained on Cu/SiO2 catalysts (5493 μmol H2 min-1·gcat -1) followed by Cu/TiO2 rutile, Cu/Al2O3-SiO2, and anatase. XRD and HRTEM characterization after reaction revealed that on Cu/SiO2 significant sintering occurred during reaction. In contrast, the particle size growth on Cu/TiO2 rutile and anatase was less pronounced, which could be associated with the interaction between Cu clusters and TiO2. Characterization by TGA showed that on Cu/Al2O3-SiO2 the main cause of deactivation was coke deposition.</p

Municipal water demand forecasting: Tools for intervention time series

This article introduces some approaches to common issues arising in real cases of water demand prediction. Occurrences of negative data gathered by the network metering system and demand changes due to closure of valves or changes in consumer behavior are considered. Artificial neural networks (ANNs) have a principal role modeling both circumstances. First, we propose the use of ANNs as a tool to reconstruct any anomalous time series information. Next, we use what we call interrupted neural networks (I-NN) as an alternative to more classical intervention ARIMA models. Besides, the use of hybrid models that combine not only the modeling ability of ARIMA to cope with the time series linear part, but also to explain nonlinearities found in their residuals, is proposed. These models have shown promising results when tested on a real database and represent a boost to the use and the applicability of ANNs.This work has been supported by project IDAWAS, DPI2009-11591, of the Direccion General de Investigacion of the Ministerio de Ciencia e Innovacion of Spain, and ACOMP/2010/146 of the Conselleria de Educacion of the Generalitat Valenciana. As well, the authors are grateful to "Aguas de Murcia" for the collaboration in this work and for the availability of the data.This work has been supported by project IDAWAS, DPI2009-11591, of the Direccion General de Investigacion of the Ministerio de Ciencia e Innovacion of Spain, and ACOMP/2010/146 of the Conseller a de Educacion of the Generalitat Valenciana. As well, the authors are grateful to "Aguas de Murcia" for the collaboration in this work and for the availability of the data.Herrera Fernández, AM.; García-Díaz, JC.; Izquierdo Sebastián, J.; Pérez García, R. (2011). Municipal water demand forecasting: Tools for intervention time series. Stochastic Analysis and Applications. 29(6):998-1007. https://doi.org/10.1080/07362994.2011.610161S9981007296Zhou, S. ., McMahon, T. ., Walton, A., & Lewis, J. (2002). Forecasting operational demand for an urban water supply zone. Journal of Hydrology, 259(1-4), 189-202. doi:10.1016/s0022-1694(01)00582-0Bougadis, J., Adamowski, K., & Diduch, R. (2005). Short-term municipal water demand forecasting. Hydrological Processes, 19(1), 137-148. doi:10.1002/hyp.5763Jain, A., & Ormsbee, L. E. (2002). Short-term water demand forecast modeling techniques-CONVENTIONAL METHODS VERSUS AI. Journal - American Water Works Association, 94(7), 64-72. doi:10.1002/j.1551-8833.2002.tb09507.xPeña, D., Tiao, G. C., & Tsay, R. S. (Eds.). (2000). A Course in Time Series Analysis. Wiley Series in Probability and Statistics. doi:10.1002/9781118032978et al. 2000 . Mining Time Series of Meteorological Variables Using Rough Sets—A Case Study, Binding Environmental Sciences and Artificial Intelligent. BESAI 2000, Germany, 7:1–8.Herrera, M., Torgo, L., Izquierdo, J., & Pérez-García, R. (2010). Predictive models for forecasting hourly urban water demand. Journal of Hydrology, 387(1-2), 141-150. doi:10.1016/j.jhydrol.2010.04.005McLeod, A. I., & Vingilis, E. R. (2005). Power Computations for Intervention Analysis. Technometrics, 47(2), 174-181. doi:10.1198/004017005000000094Box, G. E. P., & Tiao, G. C. (1975). Intervention Analysis with Applications to Economic and Environmental Problems. Journal of the American Statistical Association, 70(349), 70-79. doi:10.1080/01621459.1975.10480264Zhang, G. P., & Qi, M. (2005). Neural network forecasting for seasonal and trend time series. European Journal of Operational Research, 160(2), 501-514. doi:10.1016/j.ejor.2003.08.037Zealand, C. M., Burn, D. H., & Simonovic, S. P. (1999). Short term streamflow forecasting using artificial neural networks. Journal of Hydrology, 214(1-4), 32-48. doi:10.1016/s0022-1694(98)00242-xWang, W., Gelder, P. H. A. J. M. V., Vrijling, J. K., & Ma, J. (2006). Forecasting daily streamflow using hybrid ANN models. Journal of Hydrology, 324(1-4), 383-399. doi:10.1016/j.jhydrol.2005.09.032Kneale , P. , See , L. , and Smith , A. 2001 .Towards Defining Evaluation Measures for Neural Network Forecasting Models; Proceedings of the Sixth International Conference on GeoComputation, University of Queensland, Australia.Peña, D., & Rodríguez, J. (2002). A Powerful Portmanteau Test of Lack of Fit for Time Series. Journal of the American Statistical Association, 97(458), 601-610. doi:10.1198/016214502760047122Peña, D., & Rodríguez, J. (2006). The log of the determinant of the autocorrelation matrix for testing goodness of fit in time series. Journal of Statistical Planning and Inference, 136(8), 2706-2718. doi:10.1016/j.jspi.2004.10.026LJUNG, G. M., & BOX, G. E. P. (1978). On a measure of lack of fit in time series models. Biometrika, 65(2), 297-303. doi:10.1093/biomet/65.2.297MONTI, A. C. (1994). A proposal for a residual autocorrelation test in linear models. Biometrika, 81(4), 776-780. doi:10.1093/biomet/81.4.77

Improved thermochemical energy storage behavior of manganese oxide by molybdenum doping

To improve the thermochemical energy storage (TCS) behavior of Mn2O3, several Mn–Mo oxides with varying amounts of MoO3 (0–30 wt%) were prepared by a precipitation method. The physico-chemical properties of the solids were studied byN2 adsorption–desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), and H2-temperature-programmed reduction (TPR), while their TCS behavior was determined by thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC). Apart from Mn2O3 and MoO3 phases, XRD revealed a mixed MnMoO4 phase for MoO3 loadings equal or higher than 1.5 wt%. All samples showed a wellformed coral-like surface morphology, particularly those solids with low MoO3 contents. This coral morphology was progressively decorated with compact and Mo-enriched MnMoO4 particles as the MoO3 content increased. TPR revealed that the redox behavior of Mn2O3 was significantly altered upon addition of Mo. The TCS behavior of Mn2O3 (mostly oxidation kinetics and redox cyclability) was enhanced by addition of low amounts of Mo (0.6 and 1.5% MoO3) without significantly increasing the reduction temperature of the solids. The coral morphology (which facilitated oxygen diffusion) and a smoother transition from the reduced to oxidized phase were suggested to be responsible for this improved TCS behavior. The samples containing 0.6 and 1.5 wt% of MoO3 showed outstanding cyclability after 45 consecutive reduction–oxidation cycles at high temperatures (600–1000 C). These materials could potentially reach absorption efficiencies higher than 90% at concentration capacity values typical of concentrated solar power plants

Use of plasmid R68.45 for constructing a circular linkage map of the Rhizobium trifolii chromosome

Plasmid R68.45 was used to promote conjugal transfer of chromosomal markers in Rhizobium trifolii RS55. Analysis of two-factor and three-factor crosses among R. trifolii strains enabled construction of a circular linkage map of the R. trifolii chromosome, containing 17 nutritional and resistance markers