Anode Outlet Research Articles

Application of solid oxide fuel cell for flare gas recovery as a new approach; a case study for Asalouyeh gas processing plant, Iran

Flare gas emission is one of the main sources of environmental pollution and global warming. Implementations of some no flaring methods have a great impact in reducing the pollutants emission. Using solid oxide fuel cell (SOFC) system is a new approach which is proposed in this study. In this work, an electrochemical model is developed for a steady-state, planar SOFC by considering the direct internal methane steam reforming. In this new configuration, there is no pre-reforming and the sweetened flare gas is fed to SOFC directly. Also a part of required steam is supplied by recycling the anode outlet gas. The present model is validated with experimental and modeling data taken from the literature. Application of SOFC technology for flare gas recovery of Asalouyeh gas processing plant not only generates about 1200 MW electrical energy, but also it decreases the equivalent mass of greenhouse gas emission from 1700 kg/s to 68 kg/s. Economical evaluations show that the total capital investment of this method is significantly lower than other no gas flaring approaches. For parametric investigation, the effects of some related parameters such as temperature, recirculation, fuel utilization and air ratios are studied. The results show that increasing the operating temperature of SOFC enhances the cell voltage and maximum power density. Decreasing the recirculation ratio from 0.6 to 0.2 increases the electrical efficiency of the cell. Although at lower current densities, changing the recirculation ratio has no significant effect, it is observed that at average current density equals to 1.5 A/cm2, increasing the recirculation ratio from 0.2 to 0.6 decreases the power density from 0.51 W/cm2 to 0.48 W/cm2. Also increasing the fuel utilization ratio improves the cell performance.

A method for the ad hoc and real-time determination of the water balance in a PEMFC

A novel method for the ad hoc and real-time determination of the water balance in a proton exchange membrane fuel cell is presented. The method requires the anode side of the fuel cell to be operated in open-ended mode and to use dry, pure hydrogen as is typical for vehicular applications. In that case there is a linear relationship between the anode outlet velocity and the effective drag coefficient of water through the membrane, rd, provided the stoichiometric flow ratio is sufficiently low (below ξ = 1.2). The anode outlet velocity can then be directly measured e.g. by using hot wire anemometry, and this method provides a voltage signal that can be fed to the board computer of a fuel cell vehicle for PEMFC diagnosis. It is also shown that the nitrogen cross-over from cathode to anode has only a small effect on the anode outlet velocity. In addition to detecting the velocity, the relative humidity may be measured which is shown to be independent of the current density, but measurement techniques suffer from lower accuracy. It is argued that this method can also be applied to quantify fuel cell degradation. Finally, it is fundamentally shown that when operating the fuel cell in steady state mode at a hydrogen stoichiometric flow ratio as low as 1.03, the molar fraction of hydrogen in the gas mixture at the anode outlet is at least 50% which means that at steady state there will be no hydrogen starvation at the anode outlet.

Development of a methodology to optimize the air bleed in PEMFC systems operating with low quality hydrogen

The use of hydrogen with lower quality than that specified in current regulation is an attractive option for stationary PEMFC power production. In this paper, the effect of CO is mitigated using air bleed levels up to 2% in an H2 PEMFC fed with CO concentrations below 20 ppm. A methodology to optimize the air bleed levels is developed using a novel arrangement of cells coupled to a gas chromatograph. The methodology relies on evaluating the distributed performance of the cell and on determining the CO and CO2 molar flow rates at the anode outlet. Furthermore, the amount of CO adsorbed onto the catalyst and the fraction of catalytic sites covered by CO are estimated. The results show that different parameters, such as the H2 volumetric flow rate, CO concentration and air bleed level, influence both the steady state and dynamics of PEMFCs operated with low quality hydrogen.

Characterizations of polybenzimidazole based electrochemical hydrogen pumps with various Pt loadings for H2/CO2 gas separation

Carbon capture and storage (CCS) technologies have been intensively researched and developed to cope with climate change, by reducing atmospheric CO2 concentration. The electrochemical hydrogen pumps with phosphoric acid doped polybenzimidazole (PBI) membrane are evaluated as a process to concentrate CO2 and produce pure H2 from anode outlet gases (H2/CO2 mixture) of molten carbonate fuel cells (MCFC). The PBI-based hydrogen pump without humidification (160 °C) can provide higher hydrogen separation performances than the cells with perfluorosulfonic-acid membranes at a relative humidity of 43% (80 °C), suggesting that the pre-treatment steps can be decreased for PBI-based systems. With the H2/CO2 mixture feed, the current efficiency for the hydrogen separation is very high, but the cell voltage increase, compared to the pure hydrogen operation, mainly due to the larger polarization resistance at electrodes, as confirmed by electrochemical impedance spectroscopy (EIS). The performance evaluation with various Pt loadings indicates that the hydrogen oxidation reaction at anodes is rate determining, and therefore the Pt loading at cathodes can be decreased from 1.1 mg/cm2 to 0.2 mg/cm2 without significant performance decay. The EIS analysis also confirms that the polarization resistances are largely dependent on the Pt loading in anodes.

Sectional electrochemical impedance analysis of a high temperature polymer electrolyte membrane fuel cell with three types of flow-fields

This work reports sectional electrochemical impedance (EIS) measurements of a high temperature polymer electrolyte membrane (HTPEM) fuel cell. These measurements were taken using a high temperature stable polybenzimidazole membrane electrode assembly (MEA) doped with phosphoric acid (PBI/H3PO4). Three different types of flow-fields (namely, a six channel parallel serpentine flow-field, a parallel straight flow-field, and a mixed-type flow-field) were analyzed in gas counter-flow and co-flow configuration under selected operating conditions. The current density distribution was also measured, and the results were presented to highlight the dominant factors that lead to inhomogeneous distributions. The results showed that the distribution mainly depended on the availability of oxygen, overlapped by the fluid-flow distribution. The situation changed once a carbon monoxide (CO) enriched gas was used at the anode side. In this case, the current density distribution decreased close to the cathode inlet and increased near the anode inlet. At high CO concentrations, the highest current densities were found close to the anode inlet. Sectional EIS measurements supported these observed trends and confirmed that the segments close to the anode inlet had a lower charge transfer resistance with a higher charge transfer resistance at segments closer to the anode outlet.

Passive hydrogen recovery schemes using a vacuum ejector in a proton exchange membrane fuel cell system

The present work describes the development of a passive hydrogen-recirculating scheme for the anode of a proton exchange membrane (PEM) fuel cell system. A vacuum ejector is fluidly connected to the stack anode outlet to entrain the unused hydrogen into the main hydrogen supply. A combination of a continuous-flow mode and a pulse-flow mode is employed to cover a wide range of power consumption. The former deals with the normal and high stack power conditions, while the latter is active only at low stack power. Transient results showed that the hydrogen anode stoichiometry has been successfully stabilized in the range of 1.4–1.6 with an entrainment ratio of 40–50% under the constant system load of 1.45 kW. In addition, the reliable operation of the PEM fuel cell system without any failure during the approximate 1-h test indicates the stability and reliability of the present hydrogen recovery scheme.

Analysis of the behavior and degradation in proton exchange membrane fuel cells with a dead-ended anode

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode (DEA) can obtain high hydrogen utilization by a comparatively simple system. Nevertheless, the accumulation of the nitrogen and the water in the anode channels can lead to a local fuel starvation, which degrades the performance and durability of PEMFCs. In this paper, the behaviors of PEMFCs with a DEA are explored experimentally by detecting the current distribution and the local potentials. The results indicate that the current distribution is uneven during the DEA operation. The local current firstly decreases at the region near the anode outlet, and then extends to the inlet region along the channels with time. The complete fuel starvation near the anode outlet leads to a high local potential and carbon corrosion on the cathode side. The SEM images of the cathode electrode reveal that the significant thickness reduction and the collapse of the electrode's porous structure happen in the cathode catalyst layer, leading to the irreversible decline of the performance. The comparison of the experiments with different oxidants and fuels reveals that the nitrogen crossover from cathode to anode is the dominant factor on the performance decline under the DEA operations.

Effects of nitrogen and carbon monoxide concentrations on performance of proton exchange membrane fuel cells with Pt–Ru anodic catalyst

The reformate gas is one of the commonest solutions to the problem of fuel supply for proton exchange membrane fuel cell systems. Thus, it is important to understand the behaviors of a fuel cell using reformate gases. In this work, effects of nitrogen and carbon monoxide concentrations on unsteady characteristics of a proton exchange membrane fuel cell with the Pt–Ru anodic catalyst are investigated experimentally. Simulated reformate gases with hydrogen, nitrogen and carbon monoxide are employed as the fuel. The experimental data show that a larger dilution effect of nitrogen is noted for cases with lower hydrogen stoichiometric ratios. Furthermore, increasing the carbon monoxide concentration reduces the cell performance because the elevated carbon monoxide adsorption rate results in a severer poison effect. The voltage fluctuating phenomenon is observed at high CO ppm and is due to a periodical change of coverage on the catalyst surface. Meanwhile, it is verified that the voltage fluctuation is accompanied with a periodical change of the anode outlet carbon monoxide concentration. Furthermore, the fluctuating phenomena of voltage and the anode outlet carbon monoxide concentration are more easily triggered in diluted hydrogen than in non-diluted hydrogen.

Comparison study on different SOFC hybrid systems with zero-CO2 emission

Based on a traditional SOFC (solid oxide fuel cell) hybrid power system, three different SOFC hybrid power systems with zero-CO2 emission are proposed in this paper and their performances are analyzed and compared. The anode outlet gas of SOFC burns with the pure oxygen and the concentration of CO2 is greatly increased. In order to maintain the appropriate turbine inlet temperature, three different measures (steam injection, CO2 gas injection and heat exchange layout) are taken. Because the outlet flue gas of the afterburner mainly consists of CO2 and steam, the CO2 in the flue gas can be captured easily by the condensation method after the recovery of work and heat. With the exergy analysis method, this paper studies the exergy loss distributions of every unit of SOFC hybrid systems with CO2 capture and reveals the variation rules of exergy loss distributions. The effects of the main operating parameters on the overall performances of SOFC hybrid systems with CO2 capture are also investigated. The results show that the zero CO2 emission SOFC hybrid systems still have higher efficiencies, which only decrease about 3–4% compared with that of the basic SOFC hybrid system without CO2 capture.

Employing Dew Point Diagrams to Optimize PEMFC Operating Conditions

Dew point diagrams show the dew point temperature of the anode and cathode outlet gas streams as function of the cell pressure, stoichiometric flow ratios and the effective drag coefficient of water across the membrane in a PEM fuel cell. They are a useful tool for specifying the operating temperature for a given pressure and stoichiometric flow ratio to avoid cell flooding or membrane dehydration. In this work a sequence of logical steps is presented that help determining the optimum cell operating conditions in a natural way rather than employing a trial-and-error method. Furthermore, modeling results are presented for the specified case. The results indicate that fuel cell operation on completely dry inlet gases at anode and cathode stoichiometric flow ratios of ξa = 1.03 and ξc = 1.1 is possible. However, the predicted membrane hydration level of λ ≈ 9 for this case is below expectation.

Effects of marine atmosphere on the cell performance in molten carbonate fuel cells

The effects of NaCl, a main component in seawater, on molten carbonate fuel cell (MCFC) performance is investigated using a single cell test with 1, 5, and 10 wt.% NaCl-impregnated cathodes for marine applications. The cell performance increases with increasing amounts of impregnated NaCl in the cathode. This cell performance enhancement is due to the reduction in the charge transfer resistance of the electrode. From the analysis of the electrolyte composition using the inductively coupled plasma (ICP) and ion chromatography (IC) methods after cell operation, it is confirmed that the Na+ ions are accumulated in the carbonate melts, and approximately 80 at.% of the Cl- ions are emitted into the anode outlet as HCl. Expecting that the emitted HCl causes severe corrosion of the utilities, the concentrations of accumulated Na+ ions of emitted HCl in the anode outlet are calculated when air with a sea-salt particle concentration of 5–500 ugm−3 is supplied to the MCFC stack. Although HCl is a very corrosive gas, it is expected that the emitted HCl does not cause severe corrosion because, even at a high sea-salt concentration of 500 ugm−3, the emitted HCl concentration is low enough to operate the stack safely.

Optimization of an integrated electrodisinfection/electrocoagulation process with Al bipolar electrodes for urban wastewater reclamation

In this work, a novel integrated electrochemical process for urban wastewater regeneration is described. The electrochemical cell consists in a Boron Doped Diamond (BDD) or a Dimensionally Stable Anode (DSA) as anode, a Stainless Steel (SS) as cathode and a perforated aluminum plate, which behaves as bipolar electrode, between anode and cathode. Thus, in this cell, it is possible to carry out, at the same time, two different electrochemical processes: electrodisinfection (ED) and electrocoagulation (EC). The treatment of urban wastewater with different anodes and different operating conditions is studied. First of all, in order to check the process performance, experiments with synthetic wastewaters were carried out, showing that it is possible to achieve a 100% of turbidity removal by the electrodissolution of the bipolar electrode. Next, the effect of the current density and the anode material are studied during the ED–EC process of actual effluents. Results show that it is possible to remove Escherichia coli and turbidity simultaneously of an actual effluent from a WasteWater Treatment Facility (WWTF). The use of BDD anodes allows to remove the E. coli completely at an applied electric charge of 0.0077 A h dm−3 when working with a current density of 6.65 A m−2. On the other hand, with DSA anodes, the current density necessary to achieve the total removal of E. coli is higher (11.12 A m−2) than that required with BDD anodes. Finally, the influence of cell flow path and flow rate have been studied. Results show that the performance of the process strongly depends on the characteristics of the initial effluent (E. coli concentration and Cl−/NH4+ initial ratio) and that a cell configuration cathode (inlet)–anode (outlet) and a higher flow rate enhance the removal of the turbidity from the treated effluent.

Characterisation of the electrochemical water gas shift reactor (EWGSR) operated with hydrogen and carbon monoxide rich feed gas

Water gas shift units are used to raise the H2 yield of reforming processes by converting CO to CO2 and additional H2. Additional subsequent processes are required to separate H2 from the product gas stream to obtain pure H2. This study investigates a novel electrochemical membrane reactor, where the water gas shift reaction occurs electrochemically. The reactor is operated at 393K and 403K with electrical energy to enable hydrogen purification in terms of electrochemical pumping, as well as a simultaneous electrochemical CO oxidation to increase the yield of purified H2. The experimental results show the influence of several operation parameters upon its operation characteristics (e.g. cell voltage, electrochemical CO oxidation, the energy demand, etc.). The process yielded high overall exergy efficiencies of, e.g. 78.3%, whereas the anodic outlet stream contributed with 35%-units, and the purified hydrogen with 43.3%-units.

Electrocatalytic oxidation of ethanol on Pt, Pt-Ru and Pt-Sn nanoparticles in polymer electrolyte membrane fuel cell—Role of oxygen permeation

We have recently shown that in low temperature, polymer-electrolyte membrane direct ethanol fuel cells oxygen permeating from cathode is able to chemically oxidize ethanol at the anode surface [A. Jablonski, P.J. Kulesza, A. Lewera, Journal of Power Sources 196 (2011) 4714–4718]. Such process is responsible for significant amounts of acetaldehyde and acetic acid present at anode outlet, which are not related to the flow of electric current in external circuit. Here, for the first time, we report unbiased distribution of products of ethanol electrooxidation on Pt, Pt-Ru and Pt-Sn as a function of fuel cell voltage (under load) and temperature. We show here that for Pt anode and for cell voltage above 300mV oxidation of ethanol to acetic acid does not occur as a result of electric current flow, but it is caused exclusively by chemical reaction between ethanol and oxygen permeating from cathode. This explains the obvious contradiction, which can be found in scientific literature regarding ethanol electrooxidation on Pt. Namely acetic acid is not observed in spectroelectrochemical, ex situ experiments in conditions comparable to working direct ethanol fuel cell, where it is detected in case of in situ, fuel cell experiments.Relation between unbiased distribution of products and anode catalyst's type, cell voltage, cell discharge current and temperature is also discussed. Oxygen permeation via Nafion 117 at 80°C expressed in unit of electric current has been estimated to be at least 2.5–3.5mA/cm2.

Application of MCFC in Coal Gasification Plants for High Efficiency CO2 Capture

Integrated gasification combined cycles (IGCCs) are considered the reference technology for high efficiency and low emission power generation from coal. In recent years, several theoretical and experimental studies in this field have been oriented toward capturing CO2 from IGCCs through the integration of solid oxide fuel cells (SOFCs) for coal-syngas oxidation, investigating the so-called integrated gasification fuel cell cycles (IGFC). However, molten carbonate fuel cells (MCFCs) can also be a promising technology in IGFCs. After rather comprehensive research carried out by the authors on modeling and simulation of SOFC-based IGFC plants, an interesting IGFC cycle based on MCFC is assessed in this work, where plant layout is designed to exploit the capability of MCFCs of transferring CO2 and O2 from the oxidant side to the fuel side. Syngas produced in a high efficiency Shell gasifier is cleaned and mainly burned in a combustion turbine as in conventional IGCCs. Turbine flue gas, rich with oxygen and carbon dioxide, are then used as oxidant stream for the fuel cell at the cathode side, while the remaining clean syngas is oxidized at the anode side. In this way, the MCFC, while efficiently producing electricity, separates CO2 from the gas turbine flue gas as in a post-combustion configuration; oxygen is also transported toward the anode side, oxidizing the remaining syngas as in an oxy-combustion mode. A CO2-rich stream is hence obtained at anode outlet, which can be cooled and compressed for long term storage. This configuration allows production of power from coal with high efficiency and low emission. In addition, as already highlighted in a previous study where a similar concept has been applied to natural gas-fired combined cycles, a limited fraction of the power output is generated by the fuel cell (the most expensive component), highlighting its potential also from an economic point of view. Detailed results are presented in terms of energy and material balances of the proposed cycle.

Rapid degradation phenomenon of NiCu–Ce0.8Gd0.2O1.9 anode at high p(H2O) in different concentrations of dry methane

Degradation phenomena of Ni0.5Cu0.5–CGO (Ce0.8Gd0.2O1.9) bimetallic anode in different concentrations of dry methane were studied in LSM ((La0.75Sr0.25)0.95MnO3−δ)–CGO cathode supported SOFCs. According to XRD, SEM and EIS analyses, the degradation of the Ni0.5Cu0.5–CGO bimetallic anode could be mainly attributed to re-oxidation of Ni by H2O and/or by rapid sintering of Ni due to a high p(H2O) appeared at a high current density. Furthermore, it is found that the degradation of anode resulted by the re-oxidation of Ni and/or by rapid sintering of Ni in the as-prepared cells generally occurred when the partial pressure ratio of the produced H2O and CH4 at the outlet of anode increased above a threshold value in different concentrations of dry methane.

A simple, analytic model of polymer electrolyte membrane fuel cell anode recirculation at operating power including nitrogen crossover

A simple, analytic model is presented that describes the steady state profile of anode nitrogen concentration in a polymer electrolyte membrane fuel cell operated with anode recirculation. The model is appropriate for fuel cells with straight gas channels and includes the effect of nitrogen crossover from cathode to anode through the membrane. The key analytic simplification in the model is that this crossover rate, when scaled to the gas flows in the channels, is small. This is a good approximation when the device is used at operating power levels. The model shows that the characteristic times for the anode nitrogen profiles to reach steady state are of the order of minutes and that the dilution effect of anode nitrogen is severe for pure recirculation. The model shows additionally that a small anode outlet bleed can significantly reduce the nitrogen dilution effect. Within the framework of the model, the energy efficiency of pure recirculation can be compared to hydrogen venting or partial anode bleeding. An optimal bleed rate is identified. The model and optimization analysis can be adapted to other fuel cell designs and operating conditions. Along with operating conditions, only two key parameters are needed: a nitrogen crossover coefficient and the marginal efficiency loss to compressors for increased anode stoichiometric gas flow.

The real time dynamic water transport investigation due to net water transport coefficient (NWTC) based on measurement of water content in the outlet gas in a PEFC

The present method is expanded from the previous research [1], based on the experimental water pressure in the anode and cathode outlet. The real time NWTC in cathode and anode are obtained. The general NWTC, the peak NWTC, and the average NWTC in start-up and shut-down are discussed. The results show that, firstly, the membrane absorbs water from cathode and anode in the start-up after the current turned on and discharges water to cathode and anode after the current turned off, which causes the dynamic NWTC far from the steady state value. Secondly, there is peak value for NWTC both for start-up and shut-down, the anode peak value is nearly independent on the current in start-up, and decreases (more water transport) with increase of the current in shut-down. The asynchronous appearance moment of the peak values in the cathode side and the anode side are obtained. The average NWTC in the time interval of start-up keeps the same trend as the steady NWTC with increase of the current in start-up, and more water is discharged from the membrane for high current in shut-down.

Effect of Addition of SnOx to the Pt2Ru3/C Catalyst on CO Tolerance for the Polymer Electrolyte Fuel Cell

SnOx-modified Pt2Ru3/C catalysts were prepared by depositing SnOx nanoparticles onto a commercial Pt2Ru3/C catalyst at different pH values together with a post-treatment in reduction atmosphere to improve the catalytic activity for H2/CO electro-oxidation. The structure and morphology of Pt2Ru3/C and SnOx-modified Pt2Ru3/C catalysts were characterized by x-ray diffraction and scanning transmission electron microscopy. Electrochemical activities were evaluated by CO stripping voltammetry and a single cell test in combination with on line microgas chromatograph analysis and in situ infrared reflection–absorption spectroscopy (IRRAS). The Pt2Ru3/C and SnOx-modified Pt2Ru3/C catalysts had similar morphology and particle size. Electrochemical, on line microgas chromatograph analysis of anode outlet gas and in situ IRRAS measurements indicated that the SnOx-modified Pt2Ru3/C catalyst prepared at the pH value of 5 during the SnOx deposition process exhibited the greatest CO tolerance by improving the CO electro-oxidation efficiency.

Spontaneous oscillations of cell voltage, power density, and anode exit CO concentration in a PEM fuel cell

The spontaneous oscillations of the cell voltage and output power density of a PEMFC (with PtRu/C anode) using CO-containing H(2) streams as anodic fuels have been observed during galvanostatic operating. It is ascribed to the dynamic coupling of the CO adsorption (poisoning) and the electrochemical CO oxidation (reactivating) processes in the anode chamber of the single PEMFC. Accompanying the cell voltage and power density oscillations, the discrete CO concentration oscillations at the anode outlet of the PEMFC were also detected, which directly confirms the electrochemical CO oxidation taking place in the anode chamber during galvanostatic operating.