Sustainable fuel cell integrated membrane desalination systems

According to the United Nations, between two and seven billion people will face water shortages by the year 2050. Further, it is estimated that the amount of water available per person will shrink by a third during the next two decades. Inadequate supply of good-quality water coupled with higher water demand due to rapid population growth and industrialisation in developing countries are among the major reasons for the worsening water situation. Current shortages of potable water around the world and looming water scarcity especially in the developing countries is the driving force behind the implementation of membrane technologies for seawater and brackish water desalination. Typical energy consumption in seawater reverse osmosis (RO) plants operating at 40–45% product water recovery and with energy recovery from the high pressure reject stream currently is about 3–4 kWh/m³. The near-term goal of the industry is to reduce energy consumption to less than 2 kWh/m³ by using a combination of energy efficient RO pumps, more efficient energy recovery devices, high performance low energy RO membranes, hybrid membrane systems, advanced pretreatment technologies and alternate energy integrated membrane systems. The beneficial aspects of using alternate energy systems such as on-site distributed fuel cell systems integrated with membrane desalination units in remote locations are discussed.     

Optimal design of hybrid RO/MSF desalination plants Part III: Sensitivity analysis

This is the last paper in a series of three parts entitled “Optimal design of hybrid RO/MSF desalination plants”. This research is concerned with exploring the feasibility of hybridization of multi-stage flash (MSF) and reverse osmosis (RO) technologies in order to improve the performance characteristics and process economics ofthe conventional MSF process. The research project involved an optimization study where the water cost perunit product is minimized subject to a number of constraints. In the first part, the design and cost models were presented, the optimization problem formulated and solutions for a number of cases were outlined. In the second part, results were presented and discussed. In this paper we discuss the sensitivity of water cost from the alternative plant designs to variations in some cost elements and operating conditions. In general, it is concluded that, for the same desalting capacity, hybrid RO/MSF plants can produce desalted water at a lower cost than brine recycle MSF plants, while hybrid plants are characterized, by lower specific capital costs and higher water recovery fractions. Reduction in steam cost allows MSF to compete more with hybrid RO/MSF plants. This result explains the advantage of coupling MSF plants and steam power plants where the exhaust steam from the back pressure turbine represents a relatively cheaper source of heat for the MSF process. Results showed that the RO technology exceeds all other designs over the whole range of energy, chemicals and membrane costs studied here. However, water cost of the RO process was the most sensitive to variations in membrane and electricity costs compared to other hybrid configurations.     

Optimal design of hybrid RO/MSF desalination plants Part II: Results and discussion

This paper presents the results of an optimization study, based on minimum water cost, to explore the feasibility of the hybridization of RO and MSF processes. The study explores the possible improvement of MSF process economics. Nine different scenarios for the production of the same capacity of desalted water are presented and compared from the standpoint of minimum water cost, specific capital cost and water recovery. The process and cost models, formulation of the optimization problem and solution outlines were previously presented in the first part of this study. In this work, results show that RO technology is recommended when building new desalination plants. RO technology becomes preferable at low feed concentrations and for brackish water desalination. Although they come in second position after the RO process, some hybrid plants economically exceed by far the MSF process. Computations gave a water cost of 1.1 $/m³ for the brine recycle MSF process against 0.75 $/m³ for the two-stage RO process. Water cost of the MSF process can be reduced by 17 to 24% through hybridization with RO technology.     

Conceptual design of single-feed hybrid reactive distillation columns

It is well known that reactive distillation offers benefits by integrating distillation and reaction within a single unit. While there are procedures available for the synthesis of non-reactive distillation processes and of reaction-separation systems, the design of reactive distillation columns is still a challenge. This work presents a new synthesis and design methodology for hybrid reactive distillation columns, featuring both reactive and non-reactive sections; reactive equilibrium is assumed. The approach is based on graphical techniques; therefore it is restricted to systems with two degrees of freedom according to Gibbs phase rule. The design method allows rapid and relatively simple screening of different reactive distillation column configurations. The results are useful for initialising more rigorous calculations. The methodology is illustrated for MTBE and ethyl formate production.     

Hydrogen generation in a downdraft moving bed gasifier coupled to a molten carbonate fuel cell

This paper describes the steady-state simulation of a moving bed downdraft gasifier which allows the conversion of agricultural biomass into a hydrogen-rich gas mixture so that it has an adequate composition for being used as a feedstock in a molten carbonate fuel cell (MCFC). In order to emphasize the applicability of the results, fuel specifications for a 250 kW MCFC (HM-300, MTU, Friedrichshafen, Germany) was used as a reference. The final design makes possible to produce 350 Nm³/h of a biogas in a vessel of 0.8 m diameter × 2.5 m height capable of treating 50 kg/h of dry biomass using 45 Nm³/h of air at 800 K and 1 bar.     

Power generation/energy storage by a fuel cell/battery system: Regeneration of the MnO2 positive electrode with gaseous oxygen

Fuel cell/battery (FCB) systems are promising power generation/energy storage systems because of their bi-functionality as fuel cells and as secondary batteries. We investigated the required charging after the discharged manganese dioxide (MnOOH) by oxygen gas under the rest condition and during the fuel cell operation mode using manganese dioxide as a positive electrode for the FCB system. Electrochemical characterization was performed using cyclic voltammetry and galvanostatic measurements. Additionally, changes in the crystal structure and the chemical functional groups during the electrode reactions were monitored by X-ray diffractometry and Fourier transform infrared spectroscopy. The results indicated that MnOOH formed via the electrochemical discharge of manganese dioxide (MnO₂) and that the oxyhydroxide can be chemically transformed back to MnO₂ with gaseous oxygen (O₂). The recharged MnO₂ can be used as the cathode in a fuel cell with an O₂ supply and it can also be electrochemically discharged without an O₂ supply. In addition, we confirmed that MnO₂ does not convert to Mn₃O₄ during the charge/discharge cycles if the redox reaction is maintained within a restricted range where a homogeneous process exists between MnO₂ and MnOOH. The results in this study suggest that the FCB system can be constructed using MnO₂ as the positive electrode and a metal hydride (MH) as the negative electrode, which can be rapidly charged to more than 70% of the theoretical capacity within 10 min using pressurized H₂ and electrochemically discharged, in an alkaline electrolyte. This system possesses a high-power generation efficiency, a high-energy density and a high load-following capability.     

Fibrous MnO2 electrode electrodeposited on carbon fiber for a fuel cell/battery system

A novel fibrous MnO₂ electrode for a fuel cell/battery system is fabricated on carbon fiber by the electrodeposition method. The characteristics of the fibrous MnO₂ electrode are examined by electrochemical impedance spectra, galvanostatic performance and cyclic voltammetry. The experimental results indicate that the fibrous MnO₂ electrodes are superior to pasted electrodes because of the following: (i) better contact between MnO₂ and the electrical conducting material; (ii) high charge-transfer rate because of a smaller diameter than conventional electrodeposited MnO₂ particles (thus it is expected that the specific surface area would be higher); and (iii) a low overpotential. The morphology and the crystal structure of the fibrous MnO₂ electrode are investigated by scanning electron microscopy and X-ray diffraction, respectively. The entire surface of the carbon fiber is found to be coated with γ-MnO₂ after 2 h of electrodeposition at 0.01 A dm⁻² current density.     

Improved stability of mesoporous carbon fuel cell catalyst support through incorporation of TiO2

The electrochemical stability of Pt deposited on mesoporous carbon, which was either applied in its unmodified state or coated with 20 wt% TiO₂, was investigated by cyclic voltammetry in N₂ purged 0.5 M sulfuric acid. XRD analysis revealed that TiO₂ was present in the anatase phase. The mean Pt particle diameter was ∼6 and ∼4 nm for mesoporous carbon with and without TiO₂, respectively. Pt supported on TiO₂ modified substrates was more stable than Pt supported on conventional mesoporous carbon when subjected to 1000 cycles in the potential range from 0.05 to 1.25 V vs. RHE. This was evident from the observation that the support with TiO₂ retained ∼53% of the electrochemically active surface area relative to the state observed after 100 cycles, whereas ∼33% of the active area remained in the case without TiO₂. The oxygen reduction mass activity was identical for both fresh samples (i.e., ). After 1000 cycles the mass activity decreased to for the case without TiO₂, whereas with TiO₂ the deactivation was minor; i.e., the mass activity after 1000 cycles was .     

Fuel processor integrated H2S catalytic partial oxidation technology for sulfur removal in fuel cell power plants

H₂S catalytic partial oxidation technology with an activated carbon catalyst was found to be a promising method for the removal of hydrogen sulfide from fuel cell hydrocarbon feedstocks. Three different fuel cell feedstocks were considered for analysis: sour natural gas, sour effluent from a liquid middle distillate fuel processor and a Texaco O₂-blown coal-derived synthesis gas. The H₂S catalytic partial oxidation reaction, its integratability into fuel cell power plants with different hydrocarbon feedstocks and its salient features are discussed. Experimental results indicate that H₂S concentration can be removed down to the part-per-million level in these plants. Additionally, a power law rate expression was developed and reaction kinetics compared to prior literature. The activation energy for this reaction was determined to be 34.4 kJ/g mol with the reaction being first order in H₂S and 0.3 order in O₂.     

Conceptual design of a novel hydrodynamic cavitation reactor

Hydrodynamic cavitation has been increasingly used as a substitute to conventional acoustic (or ultrasonic) cavitation for process intensification owing to its easy and efficient operation. In this paper, we have put forth conceptual design of a new kind of hydrodynamic cavitation reactor that uses a converging-diverging nozzle for generating pressure variation required for driving radial motion of cavitation bubbles. Moreover, the reactor uses externally introduced bubbles of a suitable gas (argon or air) for cavitation nucleation. This design differs from earlier designs used by researchers where an orifice plate is used for creating cavitating flow. The new design offers a good control over two crucial parameters that affect the cavitation intensity produced, viz. rate of nucleation and nature of pressure variation driving bubble motion. Using numerical simulations of bubble dynamics and associated heat and mass transfer, trends in cavitation intensity produced in the reactor are assessed with varying design parameters. The results of simulation show that the externally introduced bubbles undergo transient motion in the flow through the nozzle generating moderate cavitation intensity. On the basis of results of simulation, some recommendations have been made for the effective design and scale up of the new kind of hydrodynamic cavitation reactors using concept introduced in this paper.     

Study of basic biopolymer as proton membrane for fuel cell systems

Up to now, many research groups work to improve the electrical and mechanical properties of membranes with a low cost of production. The biopolymers could be an answer to produce proton membranes at low cost. This work demonstrates that the intrinsic membrane polymer and clays properties can help to develop a novel proton exchange membranes. Biopolymer composites (chitosan-oxide compounds) present conductivity between 10⁻³ and 10⁻² S cm⁻¹. The measurements were calculated by EIS (1 MHz-0.05 Hz) using the two-electrode configuration. Different oxides were used: MgO, CaO, SiO₂, Al₂O₃. The ionic conductivities were compared with Nafion^®'s in the same conditions of P and T. The catalyst layer/membrane ensemble was made during the design with the subsequent demonstration as membrane electrode assemblies and finally the fuel cell was built. Our focus was to increase the compatibility between the proton basic polymer exchange membrane and basic clays as CaO and test a new kind of fuel cell.     

Electrocatalytic activity mapping of model fuel cell catalyst films using scanning electrochemical microscopy

Scanning electrochemical microscopy has been employed to spatially map the electrocatalytic activity of model proton exchange membrane fuel cell (PEMFC) catalyst films towards the hydrogen oxidation reaction (the PEMFC anode reaction). The catalyst films were composed of platinum-loaded carbon nanoparticles, similar to those typically used in PEMFCs. The electrochemical characterisation was correlated with a detailed physical characterisation using dynamic light scattering, transmission electron microscopy and field-emission scanning electron microscopy. The nanoparticles were found to be reasonably mono-dispersed, with a tendency to agglomerate into porous bead-type structures when spun-cast. The number of carbon nanoparticles with little or no platinum was surprisingly higher than would be expected based on the platinum-carbon mass ratio. Furthermore, the platinum-rich carbon particles tended to agglomerate and the clusters formed were non-uniformly distributed. This morphology was reflected in a high degree of heterogeneity in the film activity towards the hydrogen oxidation reaction.     

Investigation of the response of separate electrodes in a polymer electrolyte membrane fuel cell without reference electrode

The paper presents electrochemical measurements carried out in a PEMFC with a view to determining the separate kinetics of the electrode reactions. For this purpose, the separate response of one electrode (anode or cathode) was magnified by dilution of the reacting gas, respectively hydrogen and oxygen, and comparison of the experimental data in the form of steady voltage-current variations and impedance spectra. Experiments were carried out at 60 °C and ambient pressure. Water management was thoroughly controlled so that the gases leaving the cell had the same relative humidity in all experiments of one series. Hydrogen oxidation, although rapid, corresponds to overpotentials up to 50 mV at high dilution rates and current densities. Assuming a Tafel–Volmer mechanism, the exchange current density of the anode reaction at the Pt surface is of the order of 1 mA cm⁻². The two techniques employed led to Tafel slopes of oxygen reduction ranging from 120 to 150 mV/decade, with an exchange current density near 1 μA cm⁻², in good agreement with published data.     

Evidence for high performances of low Pt loading electrodes based on capped platinum electrocatalyst and carbon nanotubes in fuel cell devices

Recently we reported the preparation and electrochemical behaviour of porous electrodes based on the controlled combination of carbon nanotubes and capped platinum nanoparticles towards oxygen reduction. Due to the organic crown of the nanoparticles, the electrodes exhibited low hydrogen underpotential deposition (H upd) electroactive surface areas but significant activity towards oxygen reduction was recorded down to very low platinum loadings of few μg/cm². While the presence of organic stabilizing material, at the surface of the electrocatalyst synthesized by wet chemistry, may be considered as a potential drawback in fuel cell community, we present in this paper results showing that our capped electrocatalyst associated with carbon nanotubes can be used without any pre-treatment and exhibit high performances in fuel cell devices, in spite of low platinum loadings. Beyond the practical interest of such capped nanoparticles in fuel cell technology demonstrated here, fundamental question related to the high performances of the capped electrocatalyst are still opened and are currently under investigation.     

Development of novel chromium oxide/metallocene hybrid catalysts for bimodal polyethylene

The investigation of a multicomponent catalyst in polyolefin field came up as an alternative for synthesizing bimodal polymers in only one step process under constant reaction conditions.In the present work, new bifunctional catalysts were prepared by combining chromium and metallocene species on the same solid and tested in ethylene polymerization in order to evaluate the possibility of producing bimodal polyethylene. The catalytic system methylaluminoxane (MAO)/(nBuCp)₂ZrCl₂ was immobilized on activated chromium catalysts supported onto several inorganic carriers (silica, silica-alumina, aluminophosphate and mesostructured SBA-15-type materials). The reaction results showed a clear influence of the physicochemical properties of the support on the relative contribution of metallocene and chromium centers as well as on polymers molecular weight distribution. A bimodal polyethylene was obtained by supporting the MAO/metallocene system on a mesostructured chromium catalyst prepared by direct synthesis.     

Heat flux from the catalyst layer of a fuel cell

We report exact solutions to the problem of heat transport in the catalyst layer (CL) of a fuel cell. The solutions are obtained for the low- and high-current regimes of CL operation. The approximate equation for the heat flux from the CL valid for the whole range of current densities is suggested. This equation is suitable for CFD calculations of heat transport in cells and stacks. Heat fluxes from the catalyst layers of PEMFC, HT-PEMFC and DMFC are discussed.     

Facile in situ template synthesis of sulfonated polyimide/mesoporous silica hybrid proton exchange membrane for direct methanol fuel cells

Highly conductive and hydration retentive organic-inorganic hybrid proton exchange membranes for direct methanol fuel cells were synthesized by in situ sol-gel generation of mesoporous silica (mSiO₂) in sulfonated polyimide (SPI) via self-assembly route of organic surfactant templates for the tuning of the architecture of silica. The microstructure and properties of the resulting hybrid membranes were extensively characterized. The mesopores of about 3 nm in silica dispersion phase were formed in the SPI matrix. The existence of the mesoporous structure of silica improved the thermal stability, water-uptake and proton conductivity as well as methanol resistance of the hybrid membranes. The hybrid membrane with 30 wt.% mSiO₂ exhibited the water-uptake of 44.8% at 25 °C, and proton conductivity of 0.214 S cm⁻¹ at 80 °C at RH = 100%, while pure SPI exhibited the values of 40.6% and 0.179 S cm⁻¹ in the same test conditions, respectively. The results suggested that the highly hydrophilic character of Si-OH groups and the large surface area of mSiO₂ should contribute to the improvement of the water-uptake, meanwhile the mesoporous channels may supply the continuous proton conductive pathway in the hybrid membranes.     

An approach to improve the economy of desalination plants with a nuclear heating reactor by coupling with hybrid technologies

In order to investigate the possible approach to increase thermal efficiency of desalination plants, decrease water production costs and further optimize the coupling design of a nuclear heating reactor (NHR) with the desalination process, the coupling schemes of NHR reactors with hybrid desalination technologies were investigated. The cogeneration operation mode was adopted in this investigation. Two coupling schemes were selected for the cogeneration mode: NHR + low-temperature MED+RO and NHR + low-temperature MED+MED/VC. Technical specifications and economic aspects of the investigation are briefly presented in this paper.