• Energy Research

The aim of the Special Issue "Geopolymers: a new and smart way for a sustainable development" is to present an updated "state-ofthe-art" collection of original papers ranging from the base science to application-driven researches. Since geopolymers mimic clay minerals in terms of final structure and properties, the special issue contains not only papers on clay-based geopolymers, but also on materials produced from recycled aluminosilicates, assessing that: "Geopolymers are the logical consequence of the green chemistry, which operates for a sustainable development". The first part of the Issue is dedicated to basic studies of metakaolin-based model systems with the aim to define their micro- and nano-structures. Passing through the study of the processing of raw materials or the geopolymer consolidation stage, readers are accompanied to the evaluation of the properties; this latter part of the Issue appears particularly appealing because it is application-driven. Finally, several formulations of geopolymers deriving from industrial wastes are presented

The synthesis of silica-fume based foams, with a multi-range macroporosity, was obtained by alkaline activation. Foams were obtained through an in situ foaming process exploiting the gaseous production of hydrogen caused by the oxidation, in alkaline medium, of metal silicon impurities contained in silica fume. Potassium or sodium alkaline solutions were selected and a temperature of 70 degrees C was sufficient to promote the development of hydrogen bubbles, the increase of the viscosity and the consolidation of the foams. The balance of these reactions allowed to entrap hydrogen bubbles inside the structures creating highly porous foams. The foams were characterized in term of macro- and microstructure, porosity distribution, infrared spectroscopy, thermal and acoustic properties achieved. The foams showed ultramacroporous structures, with a total porosity of approximate to 80%. The average values of bulk density (0.5 g cm(-3)), thermal conductivity (0.16 Wm(-1) K-1), and the acoustic behaviors, highlighted a use of the foams as promising insulating materials. (C) 2016 Elsevier B.V. All rights reserved.

The objective of this research is to investigate the occurrence of agglomeration phenomena during fluidized bed processing of biomass fuels. These are to be ascribed to the alkali content in biomass materials that gives rise to low melting compounds in combination with SiO2, normally present in bed materials. The results obtained in a lab scale facility under operating conditions of gasification highlight that other materials may be adopted (e.g. olivine and chromite), despite they have higher density with respect to silica sand, affecting the bed fluid-dynamics. As alternative the addition of metakaolin is effective in order to delay the occurrence of bed agglomeration, as demonstrated by experiments carried out in alumina crucible at 900 °C and complemented by electronic scanning microscopy (SEM) and x-ray diffraction (XRD) analyses. The results are reported and discussed in the article, with the sake of providing further insights for contrasting the undesired agglomeration behavior in practical applications.

A novel application of geopolymers for the catalytic cleaning of biomass-derived syngas is reported. Powders of metal oxides, i.e. Fe2O3 and Mn2O3, were dispersed in a geopolymer matrix, to produce composites in granular form for fixed bed application. Additionally, a mixed Fe/Mn composite was produced to explore the combined effects of the two oxides. The activity of the new catalysts was investigated in real gasification conditions by means of a double fixed bed reactor, at 700, 800 and 900 °C. All the systems promoted an appreciable tar removal, while FE-SEM and MIP analyses demonstrated their stability at the process conditions. The best performances were obtained using the composite including both Mn and Fe oxides, which registered a tar decomposition up to 86% compared to inert sand, and 50% compared to olivine. A reasonable explanation was provided by TPR and XRD analyses, which pointed out an easier reducibility of this system.

Synthetic oxygen carriers based on geopolymer composites for chemical looping combustion have been recently proposed as alternative to traditional ones. In this work, this novel class of oxygen carriers was further developed by introducing manganese oxide (Mn2O3) as active phase. A geopolymer composite with Mn2O3 powder was produced and compared to a previously developed Fe-based system. Additionally, a mixed iron/manganese composite was fabricated to investigate possible combined effects of the two oxides. Laboratory experiments were carried out in a plant for the combustion of a CO rich gas from char gasification in CO2, consisting of two sequential fixed beds for char gasification and chemical looping combustion. The tests conducted at 900 °C pointed out the excellent performance of the Mn- and MnFe-based oxygen carriers, achieving an efficiency of CO conversion up to 99% during the first minute of operation with gas residence time less than 1 s. Moreover, the Mn-based materials exhibited the ability to release O2 during the initial stage of combustion. In the mixed iron/manganese system, the formation of the mixed phase MnFe2O4 was detected by XRD analysis, resulting in a beneficial synergistic effect. Both developed oxygen carriers (Mn- and MnFe based) exhibited similar CO conversion profiles after repeated cycles of operation.

This paper deals with a theoretical and experimental investigation on chemical looping gasification of biomass. The process was conceived by combining in a fluidized bed CO2gasification and oxygen delivery from a suitable oxygen carrier in order to increase the carbon conversion to CO. In such a way no dilution in nitrogen occurs and exceeding carbon dioxide can be further separated, e.g. by membrane. The results of the theoretical insights based on Cu oxide as O2 carrier show that producer gas with heating value higher than 8 MJ/m3 can be obtained at 900°C and equivalence ratio of 0.5. First experimental results in batch fluidized bed gasifier proved the effectiveness of a purposely developed oxygen carrier and its contribution (up to two times) in increasing the CO yield during the first stage of gasification.

Chemical looping combustion allows a simple separation of CO2 during the combustion of fossil fuels, thanks to the use of regenerable oxygen carriers. In this work, novel materials containing manganese and iron/manganese oxides have been developed via geopolymerization, and characterized in thermogravimetric apparatus and fixed bed reactor. The materials demonstrated suitable characteristics for chemical looping combustion (CLC). The tests conducted in the temperature range 800-900 °C revealed the good performance of the developed oxygen carriers, which also exhibited the ability to release O2 in inert conditions. Efficiencies in CO conversion up to 99% were achieved, as well as some synergies between Fe and Mn oxides gave a beneficial effect toward the oxygen yield. X-ray diffraction analyses of the samples confirmed the effective reduction/oxidation behavior of the materials, as well as the morphological characterization did not reveal dramatic changes of the internal microstructure up to 900 °C

Abstract Chemical looping gasification (CLG) of biomass is an emerging technology for producing synthetic gas with high content in H2, CO, and other valuable compounds in alternative to O2-enriched gasification, an oxygen carrier delivering O2 to the fuel. In the present paper, the results of CLG experiments at the bench scale are presented with a particular focus on the conversion of biomass char that is the least reactive but most energetic constituent of biomass. Synthetic Cu oxygen carrier and CO2-enriched atmosphere were used at temperatures of 900 and 945 °C in a fluidized bed. In inert conditions, the char conversion was not complete for the fixed equivalence ratio that was adopted. Conversely, char was fully converted in the presence of CO2, thanks to the inverse Boudouard reaction. The results show that higher temperature is preferable for thermodynamic reasons, although the related energy balance reduces the range of auto-thermal operability. The CO produced upon combined gasification by O2 and CO2 achieved a yield very close to the theoretical value of 78 mmol per gram of char at 100vol% CO2 and 945 °C.