Gassificazione di biomasse, pulizia e condizionamento del gas prodotto per la produzione di biocombustibili ed energia

The main energy source used worldwide is fossil fuels. Their exhaustion, with consequent energetic, economic, and environmental problems drives research into the study of alternative renewable sources. Biomass is an advantageous renewable energy source, since unlike other it is not unpredictable and unstable, it can be transported, stored and is CO2 neutral. Among biomass conversion methods, gasification appears to be very promising, since it allows the generation of syngas, a versatile fuel gas that can be used for energy production (internal combustion engine, gas turbines, fuel cells) or for synthesis of high-quality biofuels (hydrogen, biomethane, biodiesel, etc.) or chemicals (methanol). However, biomass gasification presents some problems to its full development, the most relevant of which is tar, a mixture of heavy condensable hydrocarbons produced together with syngas. These must be removed from the gas stream because their presence causes serious damage to the gasification downstream components, drastically limiting the possible uses of syngas. Catalytic conditioning systems appear to be a very promising solution. Their action involves the conversion of tar into H2 and CO by means of steam reforming reactions. The work carried out during the PhD focused on the study of different types of catalysts through laboratory-scale tar steam reforming tests. These analyses were carried out within the European projects BLAZE and GICO which involving different gasification technologies and different uses of syngas, required the study of several materials at different operating conditions. A good compromise between process economy and efficiency can be reached with Nickel-based catalysts. As part of the European projects, it was decided to use catalytic filter candles as hot syngas cleaning and conditioning system, with the advantage of performing the operation in situ, avoiding additional vessels and therefore a more complex and expensive plant design. These can be filled or impregnated with catalyst. Laboratory-scale tests were also conducted using portions of these catalytic ceramic candles. In addition to the laboratory activity, gasification tests were conducted on a pilot scale (100 kWt). Since the tar quantity in output from gasification was always found to be high, limiting the use of syngas for downstream processes, the conditioning system composed by candles was studied and optimized through CFD simulations with Fluent software. These simulations evaluated the possibility of injecting a stream of steam and oxygen (50/50 weight %) in the freeboard of the gasifier, where the filter candles are placed, to burn a small portion of syngas and increase the operating temperature and tar conversion. Excellent results were obtained, achieving acceptable concentrations of tar for syngas downstream exploitation. Furthermore, within the AIRE project, laboratory-scale methanation tests were conducted, as a study of a possible utilization of the clean syngas obtained from gasification. Keywords: Biomass gasification, syngas cleaning and conditioning, tar removal, steam reforming, methanation