Simulation of Oxygen Transport Membranes for CPO Reactors in Small-scale Hydrogen or Syngas Production Applications

The proposed work aims at presenting a 1D finite volume steady state simulation model of an Oxygen Transport Membrane for Catalytic Partial Oxidation (OTM-CPO) reactor developed at the Group of Energy COnversion Systems (GECOS) at Politecnico di Milano. The model is able to simulate supported and unsupported perovskite-based reactive membranes by means of a lumped mass and energy transport method; the active ceramic layer is modelled throughout a generalised O2permeation equation, which depends on the micro-structure characteristics and mixed-ion conduction properties of the material. The supporting porous structure is represented by a mass diffusion model dominated by gas-gas, porous and surface exchange transport processes. The model also includes a global chemical reaction kinetic mechanism of CPO on Rh-based catalysts. The model is applied to simulate the behaviour of a membrane reactor operated upstream the Hydrogen Transport Membrane for Methane Steam Reforming (HTM-MSR) installed at the Laboratory of Micro-Cogeneration (LMC) at Politecnico di Milano. The test bench is focused on testing fuel pre-processing systems for low temperature fuel cells (PEM) applications. The simulation object of this work would allow obtaining a feasibility assessment of the system and a preliminary design of the OTM-CPO reactor

Membrane reactors for green hydrogen production from biogas and biomethane:A techno-economic assessment

This work investigates the performance of a fluidized-bed membrane reactor for pure hydrogen production. A techno-economic assessment of a plant with the production capacity of 100 kgH2/day was carried out, evaluating the optimum design of the system in terms of reactor size (diameter and number of membranes) and operating pressures. Starting from a biomass source, hydrogen production through autothermal reforming of two different feedstock, biogas and biomethane, is compared. Results in terms of efficiency indicates that biomethane outperforms biogas as feedstock for the system, both from the reactor (97.4% vs 97.0%) and the overall system efficiency (63.7% vs 62.7%) point of views. Nevertheless, looking at the final LCOH, the additional cost of biomethane leads to a higher cost of the hydrogen produced (4.62 €/kgH2@20 bar vs 4.39 €/kgH2@20 bar), indicating that at the current price biogas is the more convenient choice.</p

Potentiality of a biogas membrane reformer for decentralized hydrogen production

This paper investigates the potentiality of membrane reactor for green hydrogen production from raw biogas. The assessment is carried out both from thermodynamic and economic point of view to outline the advantages of the innovative technology with respect to the conventional one based on reforming, water gas shift and pressure swing adsorption unit. Both biogas produced by landfill and anaerobic digestion are considered to evaluate the impact of biogas composition on system design. BIONICO system model is implemented in Aspen Plus and Aspen Custom Modeler to perform respectively the balance of plant with thermal integration and a detailed fluidized bed membrane reactor design. Two permeate side configurations, sweep gas and vacuum pump, were modelled and compared. The adoption of membrane reactor increases the system efficiency by more than 20% points with respect to reference cases. Focusing on the economic results, hydrogen production cost show lower value respect to the reference cases (4 €/kgH2vs 4.2 €/kgH2) at the same hydrogen delivery pressure of 20 bar. Between the landfill and anaerobic digestion cases, the latter has the lower costs as consequence of the higher methane content

Experimental and Numerical Study of a Micro-cogeneration Stirling Engine for Residential Applications☆

Abstract Micro-cogeneration Stirling engines are considered promising for residential applications. The present work covers the experimental and numerical analysis of a commercial Stirling unit capable of 8 kW of hot water and 1 kW of electricity. A previously concluded experimental campaign that focused on external measurements is extended here to include internal measurements. The scope is collecting useful data to validate a detailed numerical model. Three test cases are considered by fixing the temperature of the cogeneration water at the unit inlet at alternatively: 30, 50 and 70 °C. Mass flow rate of the water is kept at the nominal value of 0.194 kg/s. This numerical model is an extension of the well-known work by Urieli and Berchowitz. The model is calibrated on the 50 °C case and compared in the other two cases. Maximum deviations with respect to experiments are about 4% on net power output, whereas they remain below 1% on heat input and rejection. The Stirling unit has shown an electrical efficiency exceeding slightly 9% and a thermal efficiency of 90% (both based on the Higher Heating Value) if the cogeneration water inlet temperature is 30 °C, which decreases down to about 84% with water inlet at 70 °C. The Primary Energy Index is remarkably positive for all cases, ranging from 17% to 22% as the temperature of the water inlet goes from 70 °C to 30 °C

Achievements of European projects on membrane reactor for hydrogen production

Membrane reactors for hydrogen production can increase both the hydrogen production efficiency at small scale and the electric efficiency in micro-cogeneration systems when coupled with Polymeric Electrolyte Membrane fuel cells. This paper discusses the achievements of three European projects (FERRET, FluidCELL, BIONICO) which investigate the application of the membrane reactor concept to hydrogen production and micro-cogeneration systems using both natural gas and biofuels (biogas and bio-ethanol) as feedstock. The membranes, used to selectively separate hydrogen from the other reaction products (CH4, CO2, H2O, etc.), are of asymmetric type with a thin layer of Pd alloy (<5 μm), and supported on a ceramic porous material to increase their mechanical stability. In FERRET, the flexibility of the membrane reactor under diverse natural gas quality is validated. The reactor is integrated in a micro-CHP system and achieves a net electric efficiency of about 42% (8% points higher than the reference case). In FluidCELL, the use of bio-ethanol as feedstock for micro-cogeneration Polymeric Electrolyte Membrane based system is investigated in off-grid applications and a net electric efficiency around 40% is obtained (6% higher than the reference case). Finally, BIONICO investigates the hydrogen production from biogas. While BIONICO has just started, FERRET and FluidCELL are in their third year and the two prototypes are close to be tested confirming the potentiality of membrane reactor technology at small scale.The research leading to these results has received funding from the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreements No 621181 (FERRET), No 621196 (FluidCELL). BIONICO has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 671459. This Joint Undertaking receives support from the European Union's Horizon 2020 research and innovation programme, Hydrogen Europe and N.ERGHY

Adoption of CO2 blended with C6F6 as working fluid in CSP plants

The adoption of CO2-based mixtures as power block working fluid for CSP plant can turn supercritical CO2 cycles into efficient transcritical cycles even at high ambient temperature, with significant performance improvement and potential power block cost reduction. In this work, the use of CO2+C6F6 mixture as working fluid for a power cycle coupled with a solar tower is analyzed. Two different cycle maximum temperatures (550°C and 650°C) are considered and for both configurations the overall plant design is performed. The yearly energy yield is computed with hourly data and the LCOE is minimized varying storage and cycle recuperator sizes. Results show comparable results for the innovative working fluid and for the sCO2 cyclesEuropean Union’s Horizon 2020 No 81498

Thermal efficiency gains enabled by using supercritical CO2 mixtures in Concentrated Solar Power applications

Supercritical Carbon Dioxide (sCO2) power cycles have been proposed for Concentrated Solar Power (CSP) applications as a means to increase the performance and reduce the cost of state-of-the-art CSP systems. Nevertheless, the sensitivity of sCO2 systems to the usually hot ambient temperatures found in solar sites compromises the actual thermodynamic and economic gains that were originally anticipated by researchers of this innovative power cycle. In order to exploit the actual potential of sCO2 cycles, the utilization of dopants to shift the (pseudo)critical temperature of the working fluid to higher values is proposed here as a solution towards enabling exactly the same features of supercritical CO2 cycles even when ambient temperatures compromise the feasibility of the latter technology. To this end, this work explores the impact of adopting a CO2-based working mixture on the performance of a CSP power block, considering hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4) as possible dopants. Different cycle options and operating conditions are studied (250-300 bar and 550-700ºC) as well as molar fractions ranging between 10 and 25%. The results in this work confirm that CO2 blends with 15-25%(v) of the cited dopants enable efficiencies that are well in excess of 50% for minimum cycle temperatures as high as 50 or even 55ºC. It is also confirmed that, for these cycles, turbine inlet temperature and pressure hardly have any effect on the characteristics of the cycle that yields the best performance possible. In this regard, the last part of this work also shows that cycle layout should be an additional degree of freedom in the optimisation process inasmuch as the best performing layout changes depending on boundary conditions.Unión Europea SI-1900/10/201

Potential and challenges of the utilization of CO2-mixtures in supercritical power cycles of Concentrated Solar Power plants

The potential of supercritical Carbon Dioxide power cycles to supersede subcritical steam turbine technology in Concentrated Solar Power applications is widely acknowledged. Some differential features of the former are higher efficiency at similar temperatures (in the range from 600 to 750ºC), smaller footprint, higher flexibility and lower cost. Several theoretical and experimental R&D projects are currently working on aspects such as component development (turbomachinery and heat exchangers), system integration into the solar subsystem (receiver and thermal energy storage system), operability, materials… Nevertheless, whilst progress is being made at a very high pace, there is still a great deal of uncertainty regarding how much sCO2 technology will be able to reduce the cost of solar thermal electricity with respect to contemporary CSP technology. This is mostly caused by the sensitivity of cycle performance to ambient temperature, bringing about a large efficiency drop when this temperature exceeds 35ºC. The root cause for this performance drop is the unfeasibility of compression near the critical point, where the very high density of the fluid reduces density and, therefore, compression work. The SCARABEUS project is based on the addition of certain dopants to carbon dioxide in order to yield a working mixture with higher critical pressure and temperature. As a consequence of these modified critical properties of the fluid, compression near the critical point is enabled even at ambient temperatures as high as 40-45ºC. Moreover, at these high temperatures, condensation and compression in liquid state are still possible. The characteristics of the new working fluids have been proved to enable thermal efficiencies higher than 50% for minimum cycle temperatures as high as 60ºC, hence boosting the performance of CSP plants well beyond of the capabilities of systems based on steam turbines. This implies a substantial reduction of the cost of the plant. Nevertheless, whilst the thermal and economic performances are more favourable for CO2-mixtures, new technical challenges must be faced if the technology is to be mature: thermal stability and potential hazards of the dopants, new turbomachinery and heat exchanger designs adapted to the composition of the mixture, phase separation, materials (selection, compatibility and degradation) and others. This paper introduces the main advantages and technical potential of the SCARABEUS technology along with a discussion of the main challenges faced by the consortium in order to demonstrate the technology and beyond.Unión Europea H2020-81498

Thermal Stability and Thermodynamic Performances of Pure Siloxanes and Their Mixtures in Organic Rankine Cycles

Organic Rankine cycles are often the best solution for the conversion of thermal energy. The many working fluids include silicon oils. One crucial issue that determines the choice of a working fluid is its thermochemical stability, as this sets a limit to the maximum temperature at which the fluid can be used in a power plant. A second subject, much debated today, is the use of mixtures in ORCs. In the first part of this study, an investigation into the thermal stability of siloxanes using two different approaches was carried out. The results confirmed a limit working temperature for the considered siloxanes of about 300 &deg;C, with a degradation that advanced significantly over time at 350 &deg;C. In the second part of the study, an analysis of the thermodynamic performances of some siloxane mixtures was carried out. It was found that the efficiencies of the corresponding thermodynamic cycles were substantially the same as for the pure fluids used today. By changing the composition of the mixture, it was also possible to vary, within reasonable limits, the values of the condensation pressure, adapting the thermodynamic cycle to the different situations that can be encountered in current practice