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[ES] Proceso para la obtención de gas de síntesis por calentamiento con microondas de sustratos orgánicos. El proceso para producir gas de síntesis por calentamiento con microondas de residuos orgánicos se basa en la producción de gas de síntesis mediante calentamiento por microondas en dos etapas. Inicialmente, el sustrato orgánico, junto con un susceptor de microondas, se somete a un calentamiento por microondas, en ausencia de oxígeno, a temperaturas entre los 400ºC y 800ºC. Se obtiene un gas de síntesis, un residuo carbonoso y una fracción líquida de aceites de pirólisis. Posteriormente, los aceites procedentes de la primera etapa se mezclan y se calientan en un horno microondas, en ausencia de oxígeno, a una temperatura superior a 700ºC, obteniéndose un gas de síntesis y el susceptor enriquecido en carbono. El proceso en su conjunto da lugar únicamente a gas de síntesis y a un residuo carbonoso., [EN] The invention relates to a process for producing synthesis gas by means of the microwave heating of organic residues, based on producing synthesis gas by means of microwave heating in two steps. Initially, the organic substrate, together with a microwave susceptor, is subjected to microwave heating, without oxygen, to temperatures of between 400ºC and 800ºC, thereby producing a synthesis gas, a carbonaceous residue, and a liquid fraction of pyrolysis oils. Subsequently, the oils form the first step are mixed and heated in a microwave oven, without oxygen, to a temperature greater than 700ºC, thereby producing a synthesis gas and the carbon-enriched susceptor. The whole process produces only synthesis gas and a carbonaceous residue.

Microwave-induced pyrolysis of organic wastes has been demonstrated to be a suitable technology for the production of syngas (H2 + CO). However, there is a need for developing alternatives to manage the residual streams from this process. In this short communication, the use of the solid fraction from microwave-induced pyrolysis as a soil conditioner and its ecotoxic effect on the growth of barley and cress plants have been studied. The porous structure of these chars has been found out to be one of the possible factors related to the germination rates. This alternative has resulted to be highly promising since the toxic effect on plants germination was very limited or non-existing regardless of the municipal waste char composition.

Herein a new process is proposed to produce a syngas-rich gas fraction (>80 vol% H2 + CO) from biowaste based on microwave heating within two differentiated steps in order to avoid tars production. The first step consists of the microwave pyrolysis of biowaste induced by a char-based susceptor at 400–800 °C; tars, char and syngas-rich gas fractions being produced. The tars are then fed into the second step where a portion of the char from the first step is used as a bed material in a 0.3:1 wt% ratio. This bed is heated up by microwaves up to 800 °C, allowing thermal cracking of tars and additional syngas (>90 vol% H2 + CO) being then produced. This new concept arises as an alternative technology to the gasification of biowastes for producing syngas with no need for catalysts or gasifying reagents to minimise tars production.

Interest in microwave-induced pyrolysis has increased in recent years due to its several advantages over conventional pyrolysis. Most of these advantages are related to the presence of microplasmas in microwave heating, since the pyrolysis reactions that take place in a plasma atmosphere generally produce light molecules, such as H2 and CO. Although the exact nature of these plasmas is as yet unknown, it is likely to be dependent on the ionization of the surrounding gases. For this reason, the influence of different carrier gases (N2, He or no carrier gas) on microwave-induced pyrolysis was chosen as the subject of this study. It was found that microwave-induced pyrolysis can be used to obtain equally good gas compositions and yields after the reactor has been inertized, without the need for a carrier gas.

The specific energy consumption of six different microwave-driven processes and equipments has been studied and it was found that the scale used dramatically affects it. Increasing the amount of sample employed from 5 to 100 g leads to a reduction in the specific energy consumption of 90–95%. When the amount of sample is 200 g or higher, the specific energy consumption remains practically constant. This means that to assess the real energy efficiency of a microwave-driven process a minimum mass of about 200 g needs to be used. The energy results can then be easily extrapolated to larger scales. Otherwise, a correlation should be used to avoid overestimated energy values and inaccurate energy efficiencies.

Several studies have dealt with the microwave-induced pyrolysis of organic residues as an effective technology for maximizing the production of syngas at the expense of the production of oily tars. This liquid fraction is composed of a complex mixture of organic compounds making it difficult to upgrade for use as a biofuel and containing hazardous compounds such as polycyclic aromatic hydrocarbons (PAH). In this study the effect of the type of residue (municipal solid wastes, sewage sludges, plastic wastes and agroresidues) and the heating mechanism on the product distribution within the oil fraction has been investigated by means of GC-MS technique. Two different approaches have been performed: direct GC-MS analysis and methanolysed-oils GC-MS analysis, the later determining the non-volatile compounds. In general, quite different distributions are attained when microwave pyrolysis is conducted, providing lighter compounds. In some cases, such as the plastic-derived oil, microwave pyrolysis lead to a potential source of chemicals such as benzene, toluene and xylenes.

[EN] Carbonaceous N-containing materials derived from leather skin residues have been found to have unprecedented photocatalytic properties as compared to P25 Evonik, with an interesting degradation potential for contaminants in water (e.g. phenol). The carbonaceous materials were prepared by carbonization of the leather skin residues at different temperatures (180–600 °C). Different crystalline species of Cr2O3 and TiO2 were found in the materials obtained. In spite of a low surface area and phase crystallinity, a thermally treated material at 180 °C containing doped chromium oxides (eskolaite phase) provided remarkably improved activities with respect to classical titania derived materials.

In this article we propose the possibility of obtaining syngas from very different and complex organic wastes, such as municipal solid wastes, agricultural residues or sewage sludge, through microwave-induced and conventional pyrolysis at 400 and 800 °C. Microwave heating has proved to be an appropriate way to produce a syngas with CO + H2 concentrations as high as 90 vol% and in large yields (up to 0.83 L g−1waste). In addition, the potential of the syngas produced by this technology as fermentation substrate for the production of bioplastics is discussed. Microwave pyrolysis seems to serve as a novel route into biorefineries to produce valuable biobased products.

The variety of methodologies used to determine the electrical conductivity of carbons makes it very difficult to compare samples and establish reference values. In this study, the electrical conductivity of a wide range of carbons was determined using two different methods: four-point probe and compression. Although the methodologies and the operating conditions are very different, linear correlations between the values measured by these two methods can be established for some of the materials studied. Only materials with a very high conductivity (graphite and carbon black) could not be correlated.

An efficient methodology based on low temperature microwave-induced pyrolysis has been developed for syngas production from macroalgae. The protocol provided unprecedented hydrogen production, with switchable H2/CO ratios depending on pyrolysis conditions which were found to remarkably improve conventional pyrolysis experiments even at significantly higher temperatures (400 vs. 800 °C). Arcing effects under microwave irradiation, which result in an interesting observed pseudo-catalytic effect promoted by the metal oxides contained in macroalgae, seem to account for the improved results.

[EN] The development of integrated platforms is desirable in order to improve the efficiency of biorefineries. This study deals with an innovative all-in-one thermochemical process (drying, pyrolysis and gasification) based on the use of microwave energy, which could eliminate the need to pre-dry municipal solid waste in order to produce a high quantity of synthesis gas. Depending on the initial moisture content and pyrolysis time, it is possible to obtain different syngas compositions but only during the first 20 min. Beyond this time the syngas composition remains almost constant. In addition, the moisture content was found to improve the volumetric gas production by almost 50%. The energy consumption of this all-in-one process is discussed by comparing it with the scarce information available in the literature concerning the scaling-up of microwave-assisted processes, which may be more cost-effective at a higher scale.

[EN] Microwave pyrolysis is presented in this study as a recycling approach for municipal solid waste treatment. The process is based on the conversion of solid waste to syngas (CO + H2) by means of a microwave absorbent. Experiments to characterise the syngas produced were performed using the char obtained from the pyrolysis of a municipal solid waste as microwave absorbent in the microwave power range of 150–450 W and in an absorbent-to-waste ratio range of 0.2:1 to 1:1 (wt.%:wt.%). A rich-syngas fraction with a high H2 content (c.a. 50–55 vol.%) was obtained and analysed by means of response surface methodology through the interaction between the microwave power and absorbent-to-waste ratio. Moreover, a positive effect of the moisture content on gas production is attained since gasification of the char occurs. Thus, the simple use of a cheap waste-derived char leads to a reduction in the microwave power and economic cost of the process.

[EN] A comparative study of the pyrolysis of a macroalgae industrial solid waste (algae meal) in an electrical conventional furnace and in a microwave furnace has been carried out. It was found that the chars obtained from both pyrolyses are similar and show good properties for performing as a solid bio-fuel and as a precursor of activated carbon. Bio-oils from conventional pyrolysis have a greater number of phenolic, pyrrole and alkane compounds whereas benzene and pyridine compounds are more predominant in microwave pyrolysis with a major presence of light compounds. The bio-gas fraction from microwave pyrolysis presents a much higher syngas content (H2 + CO), and a lower CO2 and CH4 proportion than that obtained by conventional pyrolysis. Yields are similar for both treatments with a slightly higher gas yield in the case of microwave pyrolysis due to the fact that microwave heating favors heterogeneous reactions between the gases and the char.

[EN] Energy storage is a topic of great importance for the development of renewable energy, since it appears to be the only solution to the problem of intermittency of production, inherent to such technologies. In this paper, a new technology for energy storage, based on microwave-induced CO2 gasification of carbon materials is proposed. The tests carried out in this study on different carbon materials showed that charcoal consumes the least amount of energy. Two microwave heating mechanisms, a single-mode oven and a multimode device, were evaluated with the latter proving itself to be the more efficient in terms of energy consumption and recovery. The initial results obtained showed that this technology is able to achieve energy efficiencies of 45% at laboratory scale with every indication that these results can be improved upon to make this approach highly competitive against other energy storage technologies.

The selective production of 2-methyltetrahydrofuran from levulinic acid has been effectively conducted using designed Cu based catalysts and compared with a commercial Pd/C system under microwave irradiation. Optimised conditions for the most active catalysts Cu-MINT (>90% conversion, 75% selectivity to MTHF) and Pd/C (78% conversion, 92% selectivity to MTHF) were further translated into a continuous flow process using the proposed catalysts to find out the deactivation of Cu-MINT under flow conditions (79 vs. 13% conversion with a switch in selectivity to products after 30 min in flow), the high stability of Pd/C (73 vs. 70% conversion at stable selectivity under analogous conditions to those of Cu-MINT) but, most importantly, different relevant pathways to valuable products from levulinic acid depending on the type of catalyst employed.

[EN]The microwave induced pyrolysis of the microalga Scenedesmus almeriensis and its extraction residue was carried out at 400 and 800 ºC. The results show that it is possible to obtain a gas fraction with a high content (c.a. 50 vol.%) in H2 from both materials, regardless of the pyrolysis temperature. Furthermore, an outstanding syngas production and high gas yields were achieved. The maximum syngas concentration obtained was c.a. 94 vol.%, in the case of the pyrolysis of the residue at 800 ºC, indicating that the production of CO2 and light hydrocarbons was minimized. The same experiments were carried out in a conventional electric furnace in order to compare the products and yields obtained. It was found that microwave induced pyrolysis gives rise not only to higher gas yields but also to greater syngas and H2 production.

[EN] A novel method of producing methanol from coke oven gas (COG), involving the CO2 reforming of COG to obtain an appropriate syngas for the synthesis of methanol is proposed. This method is compared with a conventional process of methanol synthesis from natural gas, in terms of energy consumption, CO2 emissions, raw material exploitation and methanol purity. Whereas this new process requires the consumption of less energy, the conventional process allows a higher energy recovery. CO2 emissions are considerably lower with the new process, but the geographic situation of the plant plays a determinant role. From the point of view of raw material exploitation and methanol purity, the process proposed yields better results. These results suggest that methanol production from coke oven gas would be a more attractive alternative to conventional processes.

[EN] The selective production of 2-methyltetrahydrofuran from levulinic acid has been effectively conducted using designed Cu based catalysts and compared with a commercial Pd/C system under microwave irradiation. Optimised conditions for the most active catalysts Cu-MINT (>90% conversion, 75% selectivity to MTHF) and Pd/C (78% conversion, 92% selectivity to MTHF) were further translated into a continuous flow process using the proposed catalysts to find out the deactivation of Cu-MINT under flow conditions (79 vs. 13% conversion with a switch in selectivity to products after 30 min in flow), the high stability of Pd/C (73 vs. 70% conversion at stable selectivity under analogous conditions to those of Cu-MINT) but, most importantly, different relevant pathways to valuable products from levulinic acid depending on the type of catalyst employed.

[EN] The steelmaking industry is the largest energy-consuming manufacturing sector. As a consequence of this, the CO2 emissions from this sector account for about 5-7 % of the total anthropogenic CO2 emissions. For this reason, increasing efforts are being made to find solutions that might help diminish these emissions and increase energy efficiency. A better management of the coke oven gas (COG) surplus is one of the proposed solutions. This study deals with the CO2 reforming of COG surplus. By means of this technology it is possible to obtain a synthesis gas with a composition suitable for use in the production of methanol. Thus, a highly valuable product, with many applications in different industries is obtained from two residual streams: the surplus of coke oven gas and CO2. Examined from a more global perspective this process constitutes a partial recycling of CO2, since part of the CO2 emitted when methanol is used is consumed in the production process. It has been established that, from the thermodynamic point of view, the most favourable operating conditions for carrying out the CO2 reforming of COG are temperatures higher than 800 ºC and the lowest possible pressures. In addition, the CH4/CO2 ratio must be as near to the stoichiometric ratio as possible. Otherwise, the process yield will be very low and/or the syngas thus obtained will not be suitable for methanol production. Since the CO2 reforming of methane is a heterogeneous catalytic reaction, it is necessary to use an appropriate catalyst. Several catalysts were tested, and the physical mixtures of activated carbon and a conventional Ni/Al2O3 catalyst were found to be the most promising. Such mixtures have a synergetic effect that leads to higher conversions of methane and carbon dioxide than those predicted by the law of mixtures. Moreover, the production of by-products, such us water, are lower than what is predicted by this law. It was found that the CO2 reforming of COG can take place via two dif, [ES] La industria siderúrgica es el sector productivo con mayor consumo energético en el mundo. Debido a esto, sus emisiones de CO2 suponen entre el 5 y el 7% de las emisiones totales de origen humano. Por este motivo se está trabajando en la búsqueda de soluciones que permitan disminuir esas emisiones, así como mejorar la eficiencia energética. Una de esas soluciones es la mejora en la gestión y el aprovechamiento de los excedentes de gas de coquería. El reformado con CO2 de los excedentes de gas de coquería, que se estudia en este trabajo, puede suponer una salida rentable para esos gases, ya que permitiría la obtención de un gas de síntesis con una composición adecuada para la producción de metanol. Esto implicaría obtener un producto muy valioso a partir de dos corrientes gaseosas residuales: el propio excedente de gas de coquería y el CO2. De hecho, realizando un balance global al proceso, éste puede dar lugar a un reciclado parcial del CO2, ya que parte del CO2 que se produce tras la utilización del metanol se consume en la producción. Un estudio termodinámico del proceso de reformado con CO2 de gas de coquería permitió determinar que es necesario trabajar a temperaturas superiores a 800 ºC y a presiones tan bajas como la economía del proceso permita. Además, la relación de CH4/CO2 alimentada debe ser lo más próxima posible a la estequiométrica. En otras condiciones el rendimiento del proceso sería muy bajo y/o el gas de síntesis obtenido no tendría la composición adecuada para la producción de metanol. El reformado de metano con CO2 es una reacción catalítica heterogénea, por lo que es necesario encontrar catalizadores adecuados para llevarla a cabo. Se han evaluado diferentes catalizadores, siendo la mezcla física de carbón activo y un catalizador convencional Ni/Al2O3 la que ha dado lugar a resultados más prometedores. Esto se debe a que esta mezcla presenta un efecto sinérgico que permite alcanzar conversiones mayores a las que predice la ley de las mezcla

[EN] The steelmaking industry is the largest energy consuming manufacturing sector in the world and is responsible for 5–7% of anthropogenic CO2 emissions. It is therefore necessary to increase energy efficiency and reduce greenhouse gases emissions in these industries. COG, a by-product of coking plants, is one of the key ways to achieve these goals. COG, which is used as fuel in different processes of the steelmaking plants, is a H2-rich gas with a high energetic potential. However, there is a significant surplus that usually is burnt away in torches, and even directly emitted into the air. With the aim of tackling this wasting of resources and energy inefficiency, several alternatives have been proposed during recent years. In the present work, these alternatives are reviewed and their main advantages and drawbacks are discussed.

[EN] A thermodynamic study of the equilibrium of the CO2 reforming of coke oven gas (COG) was carried out with the aid of Aspen Plus® software. The influence of various operation conditions (temperature, pressure, COG composition and CO2/CH4 ratio) upon different parameters (conversions, yields, outlet composition, carbon production, by-products) was studied in order to evaluate the suitability of the process for producing a synthesis gas appropriate for methanol production. It was established that it is necessary to work at temperatures higher than 800 °C, at the lowest possible pressures and in stoichiometric conditions of CH4 and CO2. It was also found that the presence of light hydrocarbons in the COG gives rise to a syngas that is more suitable for methanol production than when they are absent. However, they were also observed to promote deactivation of the catalyst.

[EN] The purpose of this work was to explore the application of microwaves for the low temperature regeneration of activated carbons saturated with a pharmaceutical compound (promethazine). Contrary to expectations, microwave-assisted regeneration did not lead to better results than those obtained under conventional electric heating. At low temperatures the regeneration was incomplete either under microwave and conventional heating, being this attributed to the insufficient input energy. At mild temperatures, a fall in the adsorption capacity upon cycling was obtained in both devices, although this was much more pronounced for the microwave. These results contrast with previous studies on the benefits of microwaves for the regeneration of carbon materials. The fall in the adsorption capacity after regeneration was due to the thermal cracking of the adsorbed molecules inside the carbon porous network, although this effect applies to both devices. When microwaves are used, along with the thermal heating of the carbon bed, a fraction of the microwave energy seemed to be directly used in the decomposition of promethazine through the excitation of the molecular bonds by microwaves (microwave-lysis). These results point out that the nature of the adsorbate and its ability to interact with microwave are key factors that control the application of microwaves for regeneration of exhausted activated carbons.

[EN] Microwave heating can be used to promote heterogeneous reactions with carbon materials, mainly because of the heating mechanism involved in microwave radiation and the phenomenon known as microplasmas. This work presents photographic evidence of plasma formation when different carbon materials are subjected to microwave heating. Two different kinds of plasmas were observed: ball lightning and arc discharge plasmas. The intensity of the plasmas in the less ordered carbon was significantly higher at the beginning of the process.

[EN] A series of carbon-based nickel (Ni) catalysts was prepared in order to investigate the effect of the preparation method on the dispersion of Ni and its final catalytic activity in the dry reforming of methane, i.e. CH4 + CO2 = 2H2 + 2CO. Three parameters were studied: (i) the influence of the surface chemistry of the carbon used as support; (ii) the method of drying (conventional vs. microwave drying); and, (iii) the temperature of the reduction stage. In order to study the role of the surface chemistry of the commercial activated carbon used as support, the active carbon was tested as received and oxidized. Although a better Ni dispersion was achieved over the oxidized support, the conversions were much lower. It was also found that microwave drying offers various advantages over conventional drying, the main one being that less time is required to prepare the catalyst. Two reduction temperatures were used (300 and 500 °C), being found that it is necessary to adjust this parameter to prevent the Ni particles from sintering.

[EN] The dry reforming of coke oven gases (COG) over an activated carbon used as catalyst has been studied in order to produce a syngas suitable for methanol synthesis. The primary aim of this work was to study the influence of the high amount of hydrogen present in the COG on the process of dry reforming, as well as the influence of other operation conditions, such us temperature and volumetric hourly space velocity (VHSV). It was found that the reverse water gas shift (RWGS) reaction takes place due to the hydrogen present in the COG, and that its influence on the process increases as the temperature decreases. This situation may give rise to the consumption of the hydrogen present in the COG, and the consequent formation of a syngas which is inappropriate for the synthesis of methanol. This reaction can be avoided by working at high temperatures (about 1000 °C) in order to produce a syngas that is suitable for methanol synthesis. It was also found that the RWGS reaction is favoured by an increase in the VHSV. In addition, the active carbon FY5 was proven to be an adequate catalyst for the production of syngas from COG.

[EN] Carbon materials are, in general, very good absorbents of microwaves, i.e., they are easily heated by microwave radiation. This characteristic allows them to be transformed by microwave heating, giving rise to new carbons with tailored properties, to be used as microwave receptors, in order to heat other materials indirectly, or to act as a catalyst and microwave receptor in different heterogeneous reactions. In recent years, the number of processes that combine the use of carbons and microwave heating instead of other methods based on conventional heating has increased. In this paper some of the microwave-assisted processes in which carbon materials are produced, transformed or used in thermal treatments (generally, as microwave absorbers and catalysts) are reviewed and the main achievements of this technique are compared with those obtained by means of conventional (non microwave-assisted) methods in similar conditions.

[EN] The thermal valorization of glycerol to produce synthesis gas has been investigated under conventional and microwave heating systems. Different processes (pyrolysis, steam reforming and dry reforming) are compared, using a commercial activated carbon as catalyst. The reforming processes that employ oxidizing agents (CO2 or H2O) were found to promote higher glycerol conversions than mere thermal decomposition. Steam reforming generates the lowest gas fraction and the highest amounts of hydrogen and syngas, while the opposite occurs in the dry reforming experiments. Microwave processing produced higher gas yields with large syngas content than conventional heating processes in all cases. The use of carbon-based catalysts appears to be highly suitable for producing synthesis gas with a H2/CO ratio close to 1, minimum CO2 emissions being an additional advantage.