387 research outputs found
Techno-economic comparison of 100% renewable urea production processes
No full textUrea is widely used in agriculture, industry, and food, while it is also a potential fuel. Large-scale urea production relies on fossil fuels, thus there is a strong need for green urea given the increasing penetration of renewable energy sources. A potential alternative is biomass-to-urea; however, it cannot fully convert the biomass carbon into urea. To achieve full carbon conversion, innovative integrated biomass- and power-to-urea processes are designed conceptually. The two green urea production processes are evaluated techno-economically and compared with state-of-the-art methane-to-urea. The results show that the methane-to-urea achieves a system efficiency of 58% (LHV), while biomass-to-urea only has 39% (LHV) with unconverted biomass carbon of up to 60%. The integrated power- and biomass-to-urea has outstanding heat integration performance which fixes all biomass carbon into urea, with an efficiency enhanced up to 53%. Due to the electricity demand, the levelized cost of the urea of integrated biomass- and power-to-urea is 15 - 38 and 58 - 87% points higher than those of the biomass-to-urea and methane-to-urea for the scale of 10 - 60 MWth urea production. The available annual hours and electricity price of renewable electricity have a significant impact on the levelized cost of the urea. When the available annual hours decrease from 7200 to 3600 with an electricity price of 73 /GJ with the plant capacity being 10 - 60 MWth urea. However, when electricity price is reduced from 73 / MWh with available annual hours of 3600, the levelized cost decreases on average by 15% from 59 $/GJ with the same plant capacity.SCI-STI-JVHSCI-STI-F
Solid oxide fuel cell anode degradation by the effect of hydrogen chloride in stack and single cell environments
No full textThe poisoning effect by hydrogen chloride (HCl) on state-of-the-art Ni anode-supported solid oxide fuel cells (SOFCs) at 750 degrees C is evaluated in either hydrogen or syngas fuel. Experiments are performed on single cells and short stacks and HCl concentration in the fuel gas is increased from 1 ppm(v) up to 1000 ppm(v) at different current densities. Characterization methods such as cell voltage monitoring vs. time and electrochemical impedance response analysis (distribution of relaxation times (DRT), equivalent electrical circuit) are used to identify the prevailing degradation mechanism. Single cell experiments revealed that the poisoning is more severe when feeding with hydrogen than with syngas. Performance loss is attributed to the effects of HCl adsorption onto nickel surfaces, which lowered the catalyst activity. Interestingly, in syngas HCl does not affect stack performance even at concentrations up to 500 ppm(v), even when causing severe corrosion of the anode exhaust pipe. Furthermore, post-test analysis suggests that chlorine is present on the nickel particles in the form of adsorbed chlorine, rather than forming a secondary phase of nickel chlorine. (C) 2016 Elsevier B.V. All rights reserved.GR-LUDSCI-STI-JV
Dealing with fuel contaminants in biogas-fed solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) plants: Degradation of catalytic and electro-catalytic active surfaces and related gas purification methods
No full textFuel cell and hydrogen technologies are re-gaining momentum in a number of sectors including industrial, tertiary and residential ones. Integrated biogas fuel cell plants in wastewater treatment plants and other bioenergy recovery plants are nowadays on the verge of becoming a clear opportunity for the market entry of high-temperature fuel cells in distributed generation (power production from a few kW to the MW scale). High-temperature fuel cell technologies like molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) are especially fit to operate with carbon fuels due to their (direct or indirect) internal reforming capability. Especially, systems based on SOFC technology show the highest conversion efficiency of gaseous carbon fuels (e.g., natural gas, digester gas, and biomass-derived syngas) into electricity when compared to engines or gas turbines. Also, lower CO2 emissions and ultra-low emissions of atmospheric contaminants (SOx, CO, VOC, especially NOx) are generated per unit of electricity output. Nonetheless, stringent requirements apply regarding fuel purity. The presence of contaminants within the anode fuel stream, even at trace levels (sometimes ppb levels) can reduce the lifetime of key components like the fuel cell stack and reformer. In this work, we review the complex matrix (typology and amount) of different contaminants that is found in different biogas types (anaerobic digestion gas and landfill gas). We analyze the impact of contaminants on the fuel reformer and the SOFC stack to identify the threshold limits of the fuel cell system towards specific contaminants. Finally, technological solutions and related adsorbent materials to remove contaminants in a dedicated clean-up unit upstream of the fuel cell plant are also reviewed. (C) 2017 Elsevier Ltd. All rights reserved.SCI-STI-JV
Solid oxide fuel cell anode degradation by the effect of siloxanes
No full textLifetime and durability issues connected with Solid Oxide Fuel Cell (SOFC) technology are strongly related to the amount of contaminants that reach the stack. In this study the focus is on organic silicon compounds (siloxanes) and their highly detrimental effects on the performance of SOFC Ni-YSZ anodes. The involved mechanism of degradation is clarified and quantified through several test runs and subsequent post-mortem analysis on tested samples. In particular, experiments on both Ni anode-supported single cells and 11-cell-stacks are performed, co-feeding D4-siloxane (octamethylcyclotetrasiloxane, C8H24O4Si4) as model compound for the organic silicon species which are generally found in sewage biogas. High degradation rates are observed already at ppb(v) level of contaminant in the fuel stream. Post-test analysis revealed that Si (as silica) is mostly deposited at the inlet of the fuel channel on both the interconnect and the anode side of the cell suggesting a relatively fast condensation-type process. Deposition of the Si was found on the interconnect and on the anode contact layer, throughout the anode support and the three phase boundary in the anode, correlating with the observed increase of polarization losses from the EIS analysis of tested cells. (C) 2015 Elsevier B.V. All rights reserved.SCI-STI-JVHCIM
Techno-economic optimization of biomass-to-methanol with solid-oxide electrolyzer
No full textThe purpose of this paper is to assess techno-economically the integration of solid-oxide electrolysis in biomass-to-methanol processes: (1) The hydrogen produced by electrolysis can be used to adjust the composition of syngas from gasification to increase the conversion of carbon in biomass, (2) the oxygen as a byproduct of electrolysis can be used in the gasifier to avoid expensive air separation units, and (3) the overall process can be thermally integrated. Two integration concepts are proposed with different sizing methods of the electrolyzer: (1) the case of full conversion of carbon in biomass, in which a large electrolyzer is driven by the electricity purchased from the grid, and (2) the case of zero power exchange, in which only part of the carbon in biomass is converted reaching self-sufficiency of electricity. The three cases including the state-of-the-art biomass-to-methanol process are investigated to identify (1) possible trade-offs between efficiency and costs, and (2) under which conditions, these concepts become economically viable. With a reference methanol production of 100 kton/year, the results show that there is an optimal design for the state-of-the-art case, which offers an efficiency of 53.3% due to steam cycles and a payback time of 4.8 years. For the integrated concepts, there are sharp trade-offs between the system efficiency and methanol production cost rate. The case of full carbon conversion can reach an energy efficiency of 64.5–66.0% but results in a longer payback time of over 11 years. The case of zero-power exchange can achieve a similar efficiency as the state-of-the-art case with a slightly increased payback time of over 5.5 years. The payback time of the full carbon conversion case can be shorter than 5 years with a reduction in stack cost and electricity price, and an increase in stack lifetime
Assessment of ammonia as energy carrier in the use with reversible solid oxide cells
No full textAmmonia represents one of the most promising potential solutions as energy vector and hydrogen carrier, having a higher potential to transport energy than hydrogen itself in a pressurized form. Furthermore, solid oxide fuel cells (SOFCs) can directly be fed with ammonia, thus allowing for immediate electrical power and heat generation. This paper deals with the analysis of the dynamic behavior of commercial SOFCs when fueled with ammonia. Several measurements at different temperatures have been performed and performances are compared with hydrogen and a stoichiometrically equivalent mixture of H2 and N2 (3:1 M ratio). Higher temperature led to smaller drops in voltage for both fuels, thus providing higher efficiencies. Ammonia resulted slightly more performant (48% at 760 °C) than hydrogen (45% at 760 °C), in short stack tests. Moreover, different ammonia-to-air ratios have been investigated and the stack area-specific resistance has been studied in detail by comparing numerical modeling predictions and experimental values.SCI-STI-JVHThis is an Open Access article under the terms of the Creative Commons Attribution Licens
Techno-economic optimization of CO2-to-methanol with solid-oxide electrolyzer
No full textCarbon capture and utilization are promising to tackle fossil-fuel depletion and climate change. CO2 hydrogenation can synthesize various chemicals and fuels, such as methanol, formic acid, urea, and methane. CO2-to-methanol integrated with solid-oxide electrolysis (SOE) process can store renewable power in methanol while recycling recovered CO2, thus achieving the dual purposes of storing excess renewable power and reducing lifetime CO2 emissions. This paper focuses on the techno-economic optimization of CO2 hydrogenation to synthesize green methanol integrated with solid-oxide electrolysis process. Process integration, techno-economic evaluation, and multi-objective optimization are carried out for a case study. Results show that there is a trade-off between energy efficiency and methanol production cost. The annual yield of methanol of the studied case is 100 kton with a purity of 99.7%wt with annual CO2 utilization of 150 kton, representing the annual storage capacity of 800 GWh renewable energy. Although the system efficiency is rather high at around at 70% and varies within a narrow range, methanol production cost reaches 560 /MWh, being economically infeasible with a payback time over 13 years. When the electricity price is reduced to 47 /MWh, the methanol production cost becomes 365 and 172 $/ton with an attractive payback time of 4.6 and 2.8 years, respectively. The electricity price has significant impact on project implementation. The electricity price is different in each country, leading to a difference of the payback time in different locations
Techno-economic comparison of green ammonia production processes
No full textTo reduce the fossil-fuel consumption and the impacts of conventional ammonia production on climate change, green ammonia production processes using green hydrogen need to be investigated. For commercial production scale, potential alternatives can be based on biomass gasification and water electrolysis via renewable energy, namely biomass- and power-to-ammonia. The former generally uses entrained flow gasifier due to low CO2 and almost no tar, and air separation units are shared by the gasifier and ammonia synthesis. The latter may use solid-oxide electrolyzer due to high electrical efficiency and the possibility of heat integration with the ammonia synthesis process. In this paper, techno-economic feasibility of these two green ammonia production processes are investigated and compared with the state-of-the-art methane-to-ammonia process, considering system-level heat integration and optimal placement of steam cycles for heat recovery. With a reference ammonia production of 50 kton/year, the results show that there are trade-offs between the overall energy efficiency (LHV) and ammonia production cost for all three cases. The biomass-to-ammonia is the most exothermic but is largely limited by the large heat requirement of acid gas removal. The steam cycles with three pressure levels are able to maximize the heat utilization at the system level. The power-to-ammonia achieves the highest system efficiency of over 74%, much higher than that of biomass-to-ammonia (44%) and methane-to-ammonia (61%). The biomass-to-ammonia reaches above 450 /ton, 5 years). The power-to-ammonia is currently not economically feasible due to high stack costs and electricity prices; however, it can be competitive with a payback time of below 5 years with mass production of solid-oxide industry and increased renewable power penetration
Approaches on the Thermal Management and Parameter Estimation of Solid Oxide Fuel Cells and their Systems
No full textSolid Oxide Fuel Cells (SOFCs) are electrochemical devices that convert chemical energy from fuel directly into electrical and thermal energy. They present high electrical efficiency and because of this feature they are considered to be an important power source alternative. However, due to their high temperature of operation, SOFCs are subject to degradation, that is, their ability to produce electricity deteriorates in time. The reasons and the conditions that contribute to degradation are multiple. Among the studied factors, operating temperature is an important one. It has been found that a stable and controllable thermal environment can mitigate this undesirable phenomenon. Therefore, the pursuit of appropriate thermal conditions during their operation is of paramount importance for their lifetime and consequently for their commercialisation. Studying and obtaining such conditions is also known as thermal management. Another important point regarding the study of SOFCs is the development and use of adequate mathematical models that the engineer and generally the researcher will use as tools to analyse their behaviour and will determine conditions for safe and efficient operation. For practical applications, these models must be validated against experimental data. The result of such validation is the determination, or calibration, of the model parameters so that the output of the model fits measurements taken in the laboratory to the best possible extent. This process is called parameter estimation or system identification. This thesis tackles both aspects. In its first part it works on the thermal management of a specific product, an SOFC autonomous unit fed with methane, producing electrical power and liberating hot off-gases that can be used as a source of thermal power. A dynamic model of the physical system is developed and validated against experimental data. During this process, it was found that measurement of gas temperatures using thermocouples may be severely biased due to radiation effects of surrounding solids to the thermocouples. In order to overcome this hurdle, the phenomena that take place around the thermocouple were incorporated into the system's mathematical model. Such a systematic error may have important consequences on the thermal management and generally on the control of the SOFC system. Indeed, an investigation effectuated in this thesis revealed how incorrect gas temperature measurements affect the system's control to such an extent that, depending on the conditions, may lead to system failure. The validation of the system model proved to be particularly challenging. This fact incited the author to investigate the factors that contribute to reliable parameter estimations. This is the subject of the second part of this thesis. A first study was performed on model-based Design of Experiments (DoE) for SOFC button cells, i.e. on how measurements on small cells may be optimised. To our knowledge the study treats for the first time the issue of repetitive measurements and their impact on the quality of parameter estimation systematically. It distinguishes the notion of measurements, which can be repetitive, from that of measurement points (or design points), which are defined once for an experiment. Introducing a new type of graph that provide information on quality criteria as functions of the number of measurement points for constant number of measurements, it stresses the importance of repetitions of measurements during experiments, which up to now has been neglected. A mathematical formula is also given that helps the experimenter define the necessary number of repetitions for a specific degree of precision of the information matrix. The theoretical findings on Design of Experiments are validated experimentally with measurements on button cells. The thesis introduces a novel method with which repetitive parameter estimations are obtained using data from polarisation curves. This method allows the calculation of histograms for the model parameters approximating this way their stochastic behaviour. Standard deviations, covariance and correlation matrices are calculated directly from the available data, avoiding the approximate calculation based on sensitivity (Jacobian) matrices. The experimental results showed again the importance of repetitions on the quality of parameter estimation. A rule-of-thumb is introduced suggesting that the number of repetitions of measurements needs to be at least equal to the number of calculated parameters plus three in order to avoid high correlations. However, repetitions do not influence the values of parameters or the fit to the experimental data, but their precision. These values are influenced by the number of measurement points. Another point of importance shown in this thesis is that there is a counterbalance between the parameters' variances and covariances and their correlations, that is, if the one improves, the other deteriorates. This is demonstrated to be in accordance with the Cramèr-Rao theorem in statistics. A consequence of this is the conclusion that parameter estimation cannot be obtained to an arbitrarily high precision. Another consequence is that assessment of system identification based on only one of the widely known criteria may not be adequate, a result that concurs with the findings of other authors in the field of DoE. Finally the available results from the afore-mentioned method revealed a potential relation between correlations of parameters and the non-uniformity of their histograms. Among the investigated examples, a case of fast degradation exhibits how the introduced parameter estimation method may be used as a diagnostics tool. However such a use requires the employment of models that describe the phenomena adequately, especially after degradation. The thesis ends with the calculation of the histograms of the parameters of the SOFC unit's heat exchanger providing a stochastic perspective of parameter estimation at an SOFC system level.LEN
Electrochemical devices and computer science: water/thermal management of proton exchange membrane fuel cells and electrolyzers in different scales
No full textThe current restrictions on the registration of combustion engines in different countries and the harmful impacts of fossil fuels on the environment and human health have motivated decision-makers to use batteries and/or fuel cells as alternatives for combustion engines in different applications. Although there has been considerable progress to commercialize batteries in the automotive industry, which demands an average range of 300 km, the low range and high weight of batteries has prevented them to be an option for maritime and aviation applications. On the other hand, proton exchange membrane fuel cells (PEMFC), known as the most commercialized type of fuel cell for mobile applications, benefit from high ranges. The main aim of this thesis was to investigate PEMFC on different scales (micro-, meso-, and macro- scale). In particular, the role of computer science using artificial neural networks has been considered. Thermal management has been treated as a shared topic in the various analyzed scales.
In the micro-level studies, the presence of capillary pressure results in the creation of water channels in the GDL/MPL, called capillary fingering. This phenomenon fills the pores of those layers and reduces cell performance by flooding. That is why an analysis of the water distribution is required to understand and limit the operating window of PEMFCs. In this regard, a simulation model has been developed to analyze the impact of changes in the GDL contact angle, porosity, and permeability on the GDL liquid water removal, which has a direct relation with the water/thermal management of PEMFCs. Focused Ion Beam- Scanning Electron Microscopy and Computational Tomography (CT) scans were used in this scale for deepened analysis.
The main objective of the meso-level study was to explore the potential for performance improvement of PEMFCs by the implementation of porous media in the gas flow channel. The effect of this layer on thermal/water management was evaluated using simulations with ANSYS software considering measurable PEMFC metrics such as voltage, power density, Nusselt (Nu) number, and pressure drop. The synergistic de-sign of the flow channel and the structure of the porous layer in the channel are key to improving thermal/water management, which was evaluated through simulations. A new parameter called Evaluation Criterion of Proton Exchange Membrane (ECPEM) was introduced using ANN modeling to optimize the performance of the system considering the voltage, power density, Nusselt (Nu) number, and pressure drop.
In the macro-level studies, the goal of this thesis was to analyze the possibility of different electrochemical technologies to provide the required power for different purposes. The system-level analyses are mainly developed with PEMFC for mobile applications, while Solid Oxide Fuel Cells (SOFC) have been suggested for stationary purposes. In mobile applications, the use of PEMFC has been analyzed for shipping and flight. Lithium-Ion (Li-Ion) batteries were combined with PEMFC and dynamic performances have been developed. It was shown that the integration of Li-Ion batteries and PEMFC can be considered as a solution for shipping applications with around 1 MW of power to transport 780 passengers for short distances (ferry). The use of integrated fuel cell-battery systems in flight applications was illustrated with a 14 kg drone in three different power demand profiles.SCI-STI-JV
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