• Energy Research

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

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.