Toward minimal complexity models of membrane reactors for hydrogen production

Membrane reactors are inherently two-dimensional systems that require complex models for an accurate description of the different transport phenomena involved. However, when their performance is limited by mass transport within the reactor rather than by the selective product permeation across the membrane, the 2D model may be significantly simplified. Here we extend results previously found for methane steam reforming membrane reactors to show that such simplified two-dimensional model admits either a straightforward analytical solution for the cross-section averaged concentration profile, or can be reduced to a 1D model with an enhanced Sherwood number, depending on the stoichiometry of the reaction considered. Interestingly, the stoichiometry does not affect the expression of the enhanced Sherwood number, indicating that a versatile tool has been developed for the determination of membrane reactor performance at an extremely low computational cost and good degree of accuracy

Analysing the performance of MCECs over a wide range of operating temperatures

Hydrogen production through water electrolysis has gained significant attention in the past years as a means of tackling the problem of the imbalance between the intermittent rate of electricity production from renewable sources and the continuous electricity demand from end users. Recently, much of the effort has been shifted toward the electrolysis of steam rather than water, for example in solid oxide cells, which operate at temperatures around 800°C. In this manner, part of the energy required for the conversion to hydrogen is provided as heat rather than electricity. At the same time, the high temperature levels require the use of highly resistant materials, which increase the overall cost of the process. An interesting alternative is represented by molten carbonate electrolysis cells (MCECs), operating at temperatures well below 700°C. In the present work, a molten carbonate cell was operated in a lower temperature range (490-550°C) by changing the composition of the electrolyte mixture. The data obtained, along with experimental results at higher temperature (570-650°C) available in the literature, was analyzed using a 0D model accounting for Ohmic and activation overpotentials to determine the correlation between current and potential. It was found that, while the dependence of Ohmic losses on temperatures is discontinuous when cell operation is switched from the lower to the higher temperature range, activation losses vary with continuity. This result provides important insight on the performance of MCECs that can serve as a basis for future studies

Estimate of the height of molten metal reactors for methane cracking

Methane Cracking represents one of the most promising routes to CO2-free hydrogen production.The methane decomposition reaction is typically carried out in fixed or fluidized catalytic beds, where the metal catalyst is supported on porous ceramic particles. By proper choice of the metal catalyst, the catalytic reaction environment allows to obtain sizeable reaction rates at operating temperatures as low as 700°C. Besides, in solid catalytic beds, the catalyst is swiftly deactivated due to the massive (i.e. stoichiometric) deposition of the solid carbon product. One way to bypass carbon deposition is to use a molten metal bath (which may or may not contain catalytic metal components) as a reaction environment, where methane bubbles are introduced at the bottom of the bath and are progressively converted as they rise through the liquid metal. The key point of this process is that, owing to a large density difference between the solid carbon phase and the molten metal, the solid product of the reaction floats on top of the liquid metal and can be thus mechanically skimmed. In this article, we develop an analytical approach to the estimate of the bath height, which constitutes one of the most critical design parameters of the process. Specifically, based on the observation that in practical applications the reacting bubble is in the kinetics-controlled regime, we obtain the conversion vs time solution for a bubble of given initial size. On the assumption of ideal gaseous mixture behaviour, the knowledge of the conversion curves allows to estimate the bubble diameter as a function of time during the rise of the bubble through the molten metal. This piece of information is then post-processed to obtain the bubble motion as a function of time. The elimination of the time parameter between the two solutions allows to construct a conversion-height map for different diameters of the bubbles

A discussion of possible approaches to the integration of thermochemical storage systems in concentrating solar power plants

One of the most interesting perspectives for the development of concentrated solar power (CSP) is the storage of solar energy on a seasonal basis, intending to exploit the summer solar radiation in excess and use it in the winter months, thus stabilizing the yearly production and increasing the capacity factor of the plant. By using materials subject to reversible chemical reactions, and thus storing the thermal energy in the form of chemical energy, thermochemical storage systems can potentially serve to this purpose. The present work focuses on the identification of possible integration solutions between CSP plants and thermochemical systems for long-term energy storage, particularly for high-temperature systems such as central receiver plants. The analysis is restricted to storage systems potentially compatible with temperatures ranging from 700 to 1000 ◦C and using gases as heat transfer fluids. On the basis of the solar plant specifications, suitable reactive systems are identified and the process interfaces for the integration of solar plant/storage system/power block are discussed. The main operating conditions of the storage unit are defined for each considered case through process simulation

Calcium Looping for Thermochemical Storage: Assessment of Intrinsic Reaction Rate and Estimate of Kinetic/Transport Parameters for Synthetic CaO/Mayenite Particles from TGA Data

Mayenite-supported CaO represents an affordable and safetycompliant candidate material for thermochemical storage processes. We here analyze the thermogravimetric analysis (TGA) performance of synthetic CaO/mayenite micrometric powder under carbonatation/calcination looping and develop a model to interpret and analyze the experimental results. In the experimental campaign, calcination is run at 900 degrees C, while the carbonatation temperature is varied between 600 and 800 degrees C. For the carbonatation reaction, a generalized shrinking core model assuming a thermodynamically consistent first-order kinetic and a conversion-dependent diffusivity of CO2 inside the porous CaCO3 layer is validated through TGA carbonatation tests conducted with CO2/N-2 mixtures at different compositions. Interestingly, the kinetic constant of this reaction is found to be relatively insensitive to the temperature in the interval considered. In contrast, diffusion-limited regimes are never found for the calcination reaction so that this phase of the cycle can be predicted based on a single kinetic constant of the heterogeneous reaction. This constant is found to follow the typical Arrhenius-type dependence on temperature. Sizably different kinetic and transport parameters are obtained in the first carbonation performed on virgin CaO/mayenite particles with respect to those associated with subsequent cycles. When different parameters are afforded for the first and following cycles, the shrinking core model proposed closely predicts the TGA data over five CaO/CaCO3 cycles. The results found constitute an essential preliminary piece of information for designing equipment geometry and operating conditions of industrial-scale reactors. In this respect, knowledge of the parameters defining the intrinsic reaction rates and diffusive transport is essential in defining the optimal conversion of the material associated with minimal looping time

Mayenite-supported CaO for thermochemical storage applications: Analysis of dynamic behavior under charging/discharging cycles

The possibility of storing solar thermal energy to decouple electricity production from the availability of the solar resource is a key factor in development of concentrating solar power (CSP) technologies. In this context, a challenging perspective is the storage of solar energy on a seasonal basis through thermochemical storage (TCS) systems, as well as the use of excess summer solar energy for stabilizing the annual electricity production, thus increasing the capacity factor of the CSP plant. In this paper, we report the experimental characterization of a material initially developed within the context of CO2 capture technologies, namely calcium oxide supported on mayenite, which in previous investigations shown good sorption capacity and substantial cycling stability. The objective of this new experimental campaign is to test the performance of this material when adopted for thermochemical storage purposes. The tests confirmed that the material, synthesized through a SolGel method, remains stable over long term cycling, with a carbonation conversion higher than 80%. Furthermore, no physical/chemical interaction of the mayenite support with CO2 was observed, confirming its inertia and suitability for TCS purposes

High temperature stability of Terphenyl based thermal oils

Thermal oils are nowadays widely used as heat transfer fluids or cooling media in industrial and energy production plants. Currently, very few data are available about their thermal stability in function of the operating temperatures, which is a crucial parameter to estimate oil structural changes and their possible effects the maximum fluids lifetime. The present work is concerned with ageing tests on a commercially used thermal oil at temperatures higher than the nominal working ones, including a full post-test characterization. At this aim, a dedicated experimental set-up was designed and constructed to study the degradation kinetics, and to qualitatively and quantitatively analyze the released gases. As a result, the kinetic parameters were estimated, along with the related changes in the oil thermos-physical properties