E. Arato

Some more considerations on the optimization of cermet solid oxide fuel cell electrodes

Limiting behaviour and optimization considerations are discussed on the basis of an analytical model (Costamagna, Costa and Antonucci, Electrochim. Acta, 43, 375 (1997); Costamagna, Antonucci, Costa and Arato, Proc. IEA Workshop, Les Diablerets, Switzerland, January 1997, p. 196) for cermet solid oxide fuel cell electrodes. The results show that it is not possible to optimize the overall electrode conductivity, taking polarization and ohmic effects into account (Costamagna, Costa and Antonucci, Electrochim. Acta, 43, 375 (1997)), and the electrode utilization at the same time. Under conditions of optimal conductivity, the electrochemical reaction is unevenly distributed and only a fraction of the electrode thickness effectively supplies electrical current; on the contrary, thicknesses small enough for a uniform current distribution to result are sub-optimal from the point of view of the electrode conductivity.

Fluid dynamic study of fuel cell devices: simulation and experimental validation

Abstract The paper is concerned with the mass flow distribution in fuel cell stacks. In particular, the flow through the manifold system connected to the parallel arrangement of the cell channels is modelled and numerically treated. The numerical results are recognized to be more realistic than those obtained by means of an approximate analytical solution since more detailed effects could be accounted for. This evidence is confirmed by experiments carried out at a stack model device consisting of 100 cells. Pressure and velocity distributions were measured for various Reynolds numbers and geometrical shapes of the manifolds. The agreement between the experimental and numerical results is good.

Mechanism of oxidation/deoxidation of liquid silicon: theoretical analysis and interpretation of experimental surface tension data

Abstract A theoretical analysis of the behaviour of molten metals in the presence of oxygen is presented. A generalised Wagner approach has been adopted for molten metals, forming volatile oxides (Si, Sn, Al, etc.), in which the description of oxygen transfer from the gas phase to the condensed phase must account for the double contribution of molecular oxygen and oxygen linked as oxide, which leads to define the concept of ‘oxygen effective pressure’. From the analysis of the system at varying operating conditions, it was possible to relate the gas-phase composition at the surface, which is hardly measurable, to the composition in the feed or at the outlet. The application of this theory to the molten silicon-oxygen system is presented and the effectiveness of such a theory as a supporting tool for experimental work of capillary phenomena and crystal growth processes is discussed and verified.

Preliminary experimental and theoretical analysis of limit performance of molten carbonate fuel cells

The aim of this work is to investigate the limit performance of molten carbonate fuel cells due to gas diffusion phenomena in the porous electrodes when high reactant utilisation factors are used. Modelling and experimental activities are presented. An electrode kinetic micromodel has been performed accounting for the effects of the diffusion limits and has been integrated into a macromodel in order to optimise cell performance and operation. Specific tests have been carried out to identify the dependence of anodic as well as cathodic limiting current density on operating parameters using planar square cells produced by Ansaldo Ricerche (ARI). In turn, limiting current measurements have been used in order to identify overall reactant mass transfer coefficients in the complex structure of the electrodes. A good agreement between simulated and experimental results is presented and discussed.

Clean energy from sugarcane waste: feasibility study of an innovative application of bagasse and barbojo

Abstract Due to the existing difficulty of finding energy sources and reducing pollution, the use of renewable sources and highly efficient technologies for electrical energy production stands out as one of the promising solutions for the future. This paper shows the results of the combination of these two aspects, namely, a molten carbonate fuel cell system fed with biomass derived syngas. In particular, the biogas comes from bagasse and barbojo, the sugarcane residues. So far in developing countries they have been wasted or partly used with poorly efficient technology. The feasibility of such an application is studied by means of the process simulator Aspen Plus

Dynamic surface tension measurements on molten metal-oxygen systems: model validation on molten tin

Abstract A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid tin as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under low total pressure conditions (Knudsen regime), confirming that an oxide removal regime occurs under an oxygen partial pressure much higher than the equilibrium one (the “Effective Oxidation Pressure”). Experimental results are reported which give a new insight on the relative importance of the various processes due to the oxygen mass transport between the liquid metal and the gas phase. The critical aspects involved in surface tension measurements of liquid metals, related to the problem of liquid metal–oxygen interactions, are also carefully underlined.

Deoxygenation of waste cooking oil and non-edible oil for the production of liquid hydrocarbon biofuels

Deoxygenation of waste cooking vegetable oil and Jatropha curcas oil under nitrogen atmosphere was performed in batch and semi-batch experiments using CaO and treated hydrotalcite (MG70) as catalysts at 400 °C. In batch conditions a single liquid fraction (with yields greater than 80 wt.%) was produced containing a high proportion of hydrocarbons (83%). In semi-batch conditions two liquid fractions (separated by a distillation step) were obtained: a light fraction and an intermediate fraction containing amounts of hydrocarbons between 72-80% and 85-88% respectively. In order to assess the possible use of the liquid products as alternative fuels a complete chemical characterization and measurement of their properties were carried out.

Partial oxidation of CH4 in solid oxide fuel cells: simulation model of the electrochemical reactor and experimental validation

Abstract The practicability of using a substoichiometric mixture of air and methane to feed the anode compartment of a solid oxide electrochemical reactor has been tested at laboratory level through the operation of a 150W tubular stack. Fuel is partially oxidized to synthesis gas as an alternative to steam reforming of natural gas in solid oxide fuel cells. A simulation model of the electrochemical reactor, including mass, heat and charge is presented. Experimental results confirm the simulation ones and the model gives new insights into the phenomenological behaviour of the whole process.

Optimisation of the cell shape for industrial MCFC stacks

Abstract The aim of this work is to compare Molten Carbonate Fuel Cell (MCFC) stack behaviour when square and rectangular cells are used, and to discuss the optimisation of their operating conditions. In particular, experimental data have been collected for a stack with square cells of 0.1-m 2 area to validate a stack simulation model; then, the effects of area increase of up to 0.75 m 2 have been predicted, proving the advantage of working with rectangular cells in order to obtain a better temperature and pressure drop management. Moreover, the operating conditions of a rectangular stack have been analysed and optimised when reformed natural gas, as well as coal gas, are used as fuel. The results presented and discussed have been used to guide design and operation of a rectangular stack constructed by Ansaldo Ricerche (ARI).

Characteristic times of oxygen mass transfer at the liquid metal–vapour interface

The interactions of liquid metals and alloys with the environment mostly depends on the thermodynamic properties of the liquid surface. In fact, the surface tension is strongly influenced by the presence in the surrounding atmosphere of reactive gases through solution, adsorption mechanisms and/or surface reactions. In particular, oxygen, which shows a high surface activity towards a large number of metallic systems, is the most important contaminant of liquid metals and alloys.Theoretical approaches for estimating the oxygen mass transfer at the liquid–vapour interface under inert atmosphere and vacuum have been developed already in order to relate the observed physical properties to the real surface composition data.In the present work a model of the interfacial transport of a liquid metal–oxygen system under Knudsen conditions that foresees the temporal evolution of the interfacial composition is presented. The diffusion characteristic times for reaching steady-state conditions are evaluated in order to define two system “sizes” depending on the different oxygen transport mechanisms in the liquid phase.An experimental study of the interface evolution is at present under way and preliminary results show a satisfactory agreement with theoretical studies.