Titolo della tesi: Pure hydrogen production by chemical looping technology: use of iron as redox element and bioethanol as renewable reductant
The use of hydrogen as an alternative energy carrier is a promising solution to overcome the global warming issues. Hydrogen is light, storable, energy-dense, and when burned it produces no emissions of pollutants or greenhouse gases. However, since it is not naturally available, the environmental impact of hydrogen is closely linked to the type of source used for its production. The 96% of the commercial H2, mainly used in chemical and petrochemical sectors, is produced from fossil fuels, resulting globally in 900 Mt of CO2 per years. The study and optimization of alternative hydrogen production technologies based on renewable sources is therefore essential to make hydrogen a green fuel and to achieve the Zero-Net Emissions target of 2050.
One of the most interesting applications of H2 as alternative and clean fuel is in the in the automotive sector, which up to now contribute for a large part to global CO2 emissions. The use of H2 in the sector of automotive is now possible thanks to the development of Fuel Cells zero-emission vehicles. Among them, Proton Exchange Membrane Fuel Cells (PEMFC) are the most promising one, able to converts the chemical energy of H2 directly into electricity already at low temperatures, with an efficiency value three times higher than internal combustion engine powered by gasoline. However, for their correct functioning, high purity hydrogen stream is required, with a strict limit on CO concentration (CO<10 ppm), a poison of the Pt-based fuel cell catalyst.
Chemical looping hydrogen (CLH) technology allows the direct production of pure hydrogen in a totally green way. The process is based on the ability of iron oxides to transfer oxygen atoms between a fuel and an oxidant, maintaining constant its activity for high number of redox cycles. The process is composed by two steps: the iron oxides is first reduced to the metal form by feeding a fuel and then Fe is oxidized by steam to produce pure hydrogen and to restore the iron oxides, which participate in a subsequent redox cycle. The absence of purification units makes the CLH process suitable for the decentralized small-scale hydrogen production, solving the issues of hydrogen transportation and storage.
The main purpose of this work is to demonstrate the feasibility of producing pure and green H2 by a CLH process, suitable to be directly fed to PEMFC, using bioethanol as renewable fuel. The experimental work focused on the synthesis of Fe-based materials, having high activity and high resistance to deactivation, evaluating the process efficiency in a fixed bed bench-scale plant. The influence of the operative conditions on the process efficiency was investigated, focusing the attention on the effect of different redox temperature (675°-750°C) at constant pressure (1 bar) and different flow rate of reductant fuel, with the aim of identifying the optimal conditions.
The thesis is structured into 10 chapters. The first part introduces the issues related to global warming and increased energy demand, mainly based on fossil fuel. Then, chemical looping process is presented as a promising solution to overcome the criticalities of the use of H2 as fuel. The second chapter reports the recent advances in scientific literature in the field of CLH technology. Then, in Chapter 3 the experimental set-up and the synthesis methods of the iron-based oxygen carrier are described in detail; the characterization of the synthesized particles is reported in Chapter 4. In Chapter 5 the decomposition of bioethanol is studied to evaluate the feasibility of using it as renewable source of reducing agents; furthermore, tests of CLH are performed on commercial Fe2O3 powders aimed at the production of pure hydrogen by monitoring the amount of ethanol fed in reduction. Then, the influence of MnO2 addition on enhancing the iron oxides reducibility and therefore on the maximization of pure hydrogen yields is studied. In Chapter 6 the experimental results on the use of structural promoters (Al2O3, MgO and CeO2) to improve Fe oxides thermal resistance is deeply investigated focusing the attention on the influence of promoter addition on iron oxide redox activity and on the sample morphology. In Chapter 7 a dedicated study of the couple Fe/Al is performed, evaluating the influence of temperature on the process efficiency values and on Fe/Al interaction. The effect of the addition of Mn oxide to enhance the Fe oxides reduction degree and to avoid the production of hydrogen contaminated by CO, when Al2O3 is present as a structural promoter, is investigated in Chapter 8. In Chapter 9 Temperature programmed reduction (TPR) profiles of the most active samples are reported aiming to deeply study the influence of promoters in kinetics of iron oxides reduction and on the iron oxides reduction mechanism. At the end, in Chapter 10, based on the promising results obtained with OCs powders, the use of Fe-based foam, a highly porous materials suitable to be used in fixed bed reactor by keeping low pressure drop is studied with the aim of process scalability.