Abstract
Among renewable energies, bioenergy has one of the biggest potentials. Biomass can be used as feedstock not only for combined heat and power generation, but also for the production of biofuels, with properties similar to the ones of conventional fossil fuels. In particular, this pathway can be an attractive option for biomass gasification, in the view of generating gaseous products (syngas) and solid byproducts (char) valuable and useful for innovative applications.
It is a matter of fact that, until now, the increasing interest in small-scale biomass gasification in Europe has been mainly driven by the highly subsidization regime on renewable energy. Nonetheless, in the near future, once the subsidies will be decreased or not provided anymore, it will be necessary to find alternatives to sustain the profitability of the technology. A possible solution could be the production of second-generation biofuels through the Fischer - Tropsch (FT) synthesis, during which a gas mainly composed by H2 and CO is catalytically converted into liquid hydrocarbons and waxes. Specifically, part of the thesis is dedicated to the description of the FT process, its origins, chemistry, reaction mechanisms and kinetics.
Another critical aspect for the management of the existing gasification plants is the utilization of char, which presently has to be disposed of as a waste – representing thus an actual loss for the plant owners.
In this framework, this research project aims at exploring possible valorization routes for char in a biorefinery perspective. In particular, the utilization of char as CO2 adsorbent and catalyst support not only for the aforementioned FT synthesis, but also for the dry reforming of methane (DRM) reaction was investigated.
For this purpose, char residues were collected from different commercial biomass gasifiers operating in South Tyrol, Italy. Their characteristics were analyzed in depth using several analytical techniques. Results showed significant differences among the samples depending on initial feedstock, gasification technology and operating conditions. Moreover, many properties of char such as carbon content, porosity development, surface tailorability and surface chemistry resembled the ones of more traditional activated carbon (AC). Characterization results were used for selecting the chars to be further examined in adsoprtion and catalytic applications.
As far as CO2 adsorption is concerned, adsorption/desorption capacity of selected chars and their CO2/N2 selectivity were examined, and their performances were compared with the ones of commercial ACs. The effects of adsorption temperature, CO2 concentration, chemical activation, and adsorption cycles were investigated. The highest CO2 uptake (3.7 %) was measured for char activated with KOH, at Tads = 50 °C and CO2:N2 = 1:1.
In order to assess char suitability as catalyst support for FT synthesis, cobalt and iron catalysts were prepared, characterized and tested in collaboration with the Karlsruhe Institute of Technology, Germany. Performances of char and AC based catalysts were compared as well. The highest CO conversion (26 %) was measured for the iron based catalyst and only an 18 % decrease in the conversion was observed after 72 hours. Additionally, with the experience gained at KIT, an experimental set-up was designed and developed to be operated in the Bioenergy and Biofuels Laboratories of the Free University of Bolzano.
Char was also tested in DRM reactions. Effects of cobalt loading, HNO3 treatment of char and MgO addition were investigated. Although catalysts prepared using pure char showed low average CO2 and CH4 conversions and H2 and CO yields, MgO addition boosted the catalyst performances and stability leading to conversions and yields similar to what can be obtained using conventional supports such as Al2O3.