• Energy Research

The present article reports on the first application of geopolymers for the production of oxygen carriers for chemical looping combustion. Granules with different properties and in typical sizes for fluidized bed applications were produced starting from geopolymer/iron oxide slurries. These slurries were prepared according to a previously-developed formulation, modified by adding iron oxides and pore-forming agents to obtain oxygen carriers with different micro- and macrostructures. The performance of these novel oxygen carriers was tested in thermogravimetric equipment, measuring a capacity very close to the theoretical value 1.3 after repeated cycles. Tests conducted in a laboratory-scale differential reactor gave rise to a lower O-carrying capacity (<1.0%), but rather high kinetics (i.e. rate index) by comparison with other materials and published data. The analysis on the sample (ESEM, XRD and MIP) provided a reasonable interpretation of the phenomena observed, attributable mainly to the influence of internal porosity. Fluidization tests, elutriation/attrition results and the lack of any sign of particle agglomeration proved the suitability of the synthesized granules for use in fluidized beds.

The chemical looping combustion allows for inherent CO2 separation when burning fossil fuels in presence of a suitable oxygen carrier. The choice of the material to be used should take into account not only chemical/physical properties but also economical, environmental, and safety concerns, addressing for more common materials, like Fe oxides. In this research a geopolymeric oxygen carrier, based on Fe2O3, was tested for the first time in a laboratory CLC plant operated at high temperature for the combustion of a CO rich gas from char gasification in CO2. The CLC plant reliably performed in repeated cycles without decay of the CO conversion during the chemical looping combustion. The maximum CO content in the flue gas was around 1% vol. and carbon monoxide conversion achieved 97%. The calculated oxygen transport capacity was 0.66%. The plant results were confirmed by the XRD analysis that proved the presence of reduced phases in samples after chemical looping stage and by significant peaks obtained during H2 reduction in TPR equipment.

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.

Biomass residues are often considered as a resource if conveniently converted in fuel and alternative feedstock for chemical processes, and their conversion into valuable products may occur by different pathways. This work is focused on the thermochemical conversion at moderate temperature and in steam atmosphere, a mild process in comparison to hydrothermal liquefaction, followed by extraction of soluble products in a solvent. Such process has been already applied to various residues and here extended to the case of marc, the residual pomace from wine making, largely produced worldwide. A pressurized batch reactor was used for the quantitative determination of produced solid and liquid fractions, and their qualitative characterization was performed by instrumental analyses. The pressurized steam conversion of marc was effective, providing a yield in liquid fraction, upon extraction in solvent, up to 30% of the raw dried biomass. The use of polar and nonpolar solvent for the extraction of the liquid fraction was inspected. Applied operating conditions, namely residence time in the batch reactor and extraction modality, showed a significant influence on the process performance. In particular, long residence and extraction times and use of nonpolar solvent substantially improved the yield in liquid fraction.

This article reports on the use of strontium oxide (SrO)-based sorbents for chemical sorption of carbon dioxide (CO2) at high temperatures (>1000 °C). Two different sizes of the SrO granules were tested as potential sorbent materials through cycles of carbonation and calcination at high temperature in a fixed-bed reactor, under flux of argon (Ar) and Ar/CO2. Thermogravimetric analysis (TGA) assessed their carrying capacity and effectiveness at increasing numbers of cycles. Further investigation was dedicated to the SrO granules in combination with alumina or hydroxyapatite to prevent the material from sintering under high-Temperature conditions and to improve the sorbent durability. A simple kinetic analysis was also performed on the basis of TGA data. The sorbent materials, before and after the cycling steps, were characterized through mercury intrusion porosimetry, environmental scanning electron microscopy, and X-ray diffraction analysis to evaluate any change in the microstructure, thus including the pore-size distribution, material morphology, and crystallographic phases, which can influence the CO2 flowing ability and capture. The results showed that fine granules of SrO are not totally effective, owing to their tendency to break down and consolidate into a compact agglomerate for high-Temperature carbonation. Coarse granules of SrO and SrO/Al2O3, contrarily, maintained open architectures during cycling and allowed one to obtain a similar CO2 carrying capacity of around 9.4% by weight, although showing a different compaction degree. Kinetic analysis confirms the better performance of the sorbent in the form of coarse granules.

In framework of the thermochemical energy storage (TCES) in concentrating solar power (CSP) applications, great attention is focused on the SrCO/SrO system, which is characterized by remarkably high theoretical volumetric energy density (4 GJ m) and working temperatures (1200 °C). It has been shown that the incorporation of AlO in the SrO/SrCO system can successfully hinder the sintering and agglomeration phenomena, thus improving the performances of the system. Aiming at providing useful information for the design, simulation and scale up of a reactor for the energy storage, besides the multicycle carbonation conversion, the evaluation of the reaction kinetics is crucial. Thus, in this work, the kinetics of the carbonation of a SrO-AlO composite (34%wt of AlO) for TCES-CSP has been investigated for the first time using a two-stage kinetic model. In particular, tests have been performed in a thermogravimetric analyzer at operating conditions relevant for TCES, namely at 1 atm of CO partial pressure within the temperature range of 900-1050 °C. The reaction rate, the intrinsic carbonation kinetic constant, the characteristic product layer thickness and their dependence on the temperature has been evaluated in the temperature range 900-1000 °C; the activation energy has been found to be 52 kJ mol. Finally, comparison of the calculated conversion-time profiles, obtained from the applied kinetic models, with experimental data revealed a good agreement.

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.

The widespread use of natural aggregates is one of the main causes of the depletion of natural resources, as aggregates are constituents of several construction materials. Alternatively, it is, today, proven to be feasible to use mining tailings, either natural or recycled materials, to produce artificial aggregates through specific processes. A possible way to produce artificial aggregate is through the alkali activation of the powdered material in a process called geopolymerization. This study proposes to use a basalt powder and two different metakaolins as precursors for the production of an alkali-activated artificial aggregate, with a specific shape and size achieved by using 3D-printed molds. The experimental aggregates were evaluated using traditional tests for natural aggregates, such as resistance to compression, specific density and resistance to abrasion and fragmentation. Furthermore, the material was chemically analyzed in order to evaluate the geopolymerization process promoted by the two adopted metakaolins. The physical tests showed that artificial aggregates do not perform well in terms of resistance to wear and fragmentation, which can be improved. However, they revealed promising results in terms of skid, polishing and micro-texture.