Hydrogen production from glycerol steam reforming for low- and high-temperature PEMFCs

This paper presents a thermodynamic study of a glycerol steam reforming process, with the aim of determining the optimal hydrogen production conditions for low- and high-temperature proton exchange membrane fuel cells (LT-PEMFCs and HT-PEMFCs). The results show that for LT-PEMFCs, the optimal temperature and steam to glycerol molar ratio of the glycerol reforming process (consisting of a steam reformer and a water gas shift reactor) are 1000 K and 6, respectively; under these conditions, the maximum hydrogen yield was obtained. Increasing the steam to glycerol ratio over its optimal value insignificantly enhanced the performance of the fuel processor. For HT-PEMFCs, to keep the CO content of the reformate gas within a desired range, the steam reformer can be operated at lower temperatures; however, a high steam to glycerol ratio is required. This requirement results in an increase in the energy consumption for steam generation. To determine the optimal conditions of glycerol steam reforming for HT-PEMFC, both the hydrogen yield and energy requirements were taken into consideration. The operational boundary of the glycerol steam reformer was also explored as a basic tool to design the reforming process for HT-PEMFC.     

ENERGY EFFICIENCY EVALUATION FOR A “GREEN” POWER GENERATION PROCESS WITH MINIMUM EFFORT ON CARBON DIOXIDE CAPTURE AND STORAGE

This study evaluates the use of cracking for the removal of carbon from fuels to be used in a power generation process. Unlike conventional power generation systems, the proposed system includes a cracking unit, the function of which is to convert primary fuels into H₂ rich syngas and solid carbon, thus avoiding the emission of CO₂ and the need for carbon capture and storage (CCS) in the power generation system. Based on the thermodynamic analysis of equilibrium reactions in the cracker, it is demonstrated that the operating temperature has a significant influence on the carbon capture rate achieved and the composition of the syngas. Carbon in the fuel can be captured in solid form from hydrocarbon fuels when operating the cracker at sufficiently high temperatures; however, only a portion of carbon can be captured in a solid form from oxygenated hydrocarbon fuels, with the maximum carbon capture rate being achieved at an optimum temperature. An energy analysis, which takes into account the energy penalty of CCS for the conventional power generation system, reveals that the net available energy from the proposed system is still not as high as that of the conventional system with CCS; however, the solid carbon can be of high commercial value when appropriate technology is employed to convert the carbon byproduct into a high-added-value carbon product such as carbon black or carbon nanotubes (CNTs).     

Thermodynamic analysis of calcium oxide assisted hydrogen production from biogas

This paper investigates calcium oxide assisted hydrogen production from biogas. Preliminary experiments were performed to compare the catalytic performance of combined carbon dioxide reforming and partial oxidation of biogas among four different adsorbent (CaO)/catalyst (Ni/SiO₂·MgO) arrangements; i.e. (i) Ni/SiO₂·MgO before CaO, (ii) CaO before Ni/SiO₂·MgO, (iii) Ni/SiO₂·MgO mixed with CaO, and (iv) Ni/SiO₂·MgO without CaO. The mixture of CaO and Ni/SiO₂·MgO was found to be the best arrangement, offering the highest hydrogen yield. Thermodynamic investigation of the integrated sorption-reaction systems for hydrogen production from biogas was performed. The system can be operated under thermal neutral condition when appropriate operating parameters are adjusted. Finally based on the thermal neutral operation, the effects of H₂O/CH₄ and CaO/CH₄ ratios on the required O₂/CH₄ ratio, hydrogen yield, hydrogen concentration and CO/H₂ ratio in product were determined. Obviously the use of CaO adsorbent can improve hydrogen production and there is an optimum H₂O/CH₄ ratio which offers the highest hydrogen production at each CaO/CH₄ ratio. Increasing H₂O/CH₄ ratio generally increases H₂/CO ratio but decreases hydrogen concentration in the product.     

Technical and economic study of integrated system of solid oxide fuel cell,palladium membrane reactor,and CO2 sorption enhancement unit

This paper deals with the integrated system of solid oxide fuel cell (SOFC), palladium membrane reactor (PMR), and CO₂ sorption enhancement (SE) unit. Three configurations of the SOFC systems fed by biogas are considered, i.e., PMR–SOFC, SE–PMR–SOFC, and SE–PMR–SOFC with a retentate gas recycling (SER–PMR–SOFC). The SOFC system equipped with a conventional reformer (CON–SOFC) is considered as a base case. The simulation results show that the capture of CO₂ in biogas before being fed to PMR (SE–PMR–SOFC) can improve H₂ recovery. The performance of SE–PMR–SOFC can be further enhanced by recycling retentate gas from PMR to CO₂ sorption enhancement unit (SER–PMR–SOFC). Compared to CON–SOFC, both SE–PMR–SOFC and SER–PMR–SOFC give higher power density and thus require smaller stack size (the stack size reduction of 1.55% and 8.27% are observed for SE–PMR–SOFC and SER–PMR–SOFC, respectively). The economic analysis is performed to identify the potential benefits of each SOFC configuration. The results indicate that SE–PMR–SOFC and SER–PMR–SOFC are not cost-effective systems compared with CON–SOFC; however, the capture of CO₂ in these SOFC systems offers an environmental benefit. High %total CO₂ capture and low cost of CO₂ capture are achieved under these SOFC systems.     

Light-assisted synthesis of Pt-Zn porphyrin nanocomposites and their use for electrochemical detection of organohalides

Pt-Zn porphyrin nanocomposites have been synthesized using zinc porphyrin and dihydrogen hexachloroplatinate in the presence of light and ascorbic acid. TEM and AFM imaging revealed that Pt nanoparticles with an average diameter of approximately 3.5 nm were embedded within the Zn porphyrin matrix. The glassy carbon electrode was modified with Nafion-stabilized Pt-Zn porphyrin nanocomposites and used for dehalogenation of carbon tetrachloride, chloroform, pentachlorophenol, chlorobenzene, and hexachlorobenzene as five test models. The Pt-Zn porphyrin nanocomposite-modified electrode exhibited catalytic activity for the reduction of organohalides at -1.0 V versus Ag/AgCl. Raman signatures confirmed the dehalogenation of chlorobenzene by the nanocomposite-modified electrode. The above two aliphatic and three aromatic organohalides had detection limits of 0.5 microM with linearity up to 8 microM. The modified electrode was good for at least 80 repeated measurements of 4 microM chlorobenzene with a storage stability of 1 month at room temperature. The deactivation of the electrode activity was associated with the loss of platinum nanoparticles from the nanocomposite structure.     

Voltammetric sensor for general purpose organohalide detection at picogram per liter concentrations based on a simple collector-generator method

With the aim of producing a general purpose sensor for environmental analysis, we describe a simple and sensitive method for organohalide detection, based on an electrochemical collector-generator process. The sensor consists of four coplanar electrodes contacting a solution volume of 300 microL, containing organohalide. At the first working electrode (a Zn/PTFE composite), the analyte is electrolyzed to liberate halide ions. At the second working electrode (Ag), the halide ions are detected by cathodic stripping voltammetry. Using a preconcentration time of 600 s, with differential pulse voltammetry for stripping, the responses to 1-chloropropane, chloroform, carbon tetrachloride, iodoethane, and bromoethane can be plotted on a common calibration curve, with a detection limit of 0.1 nM (1.3 pg L(-1) or less depending on the organohalide). To the best of our knowledge, this is the lowest reported organohalide detection limit by an electrochemical method and is so far the only general purpose electrochemical method sensitive enough for regulatory requirements. The sensor response was invariant for approximately 40 measurements. Analysis of tap water, spiked with chloroform or carbon tetrachloride, gave recoveries within 1.0-2.6% of the recoveries by the standard GC method.     

Thermodynamic analysis of hydrogen production from glycerol at energy self‐sufficient conditions

A thermodynamic analysis based on the principle of minimising the Gibbs free energy is performed for hydrogen production from glycerol. When the operating parameters such as water/glycerol ratio (WGR), oxygen/glycerol ratio (OGR) and operating temperature (T) are carefully chosen, the energy self‐sufficient conditions can be achieved. Two levels of energy self‐sufficient, (i) within the reformer and (ii) within the overall system, are considered. Unlike the consideration in the reformer level reported in most works in literature, the consideration in the overall system level represents more realistic results based on the fact that some energy is required for heating up the feeds to a desired operating temperature. The obtained results demonstrate that the maximum hydrogen production significantly decreases from 5.65 mol H₂/mol glycerol for the reformer level to 3.31 mol H₂/mol glycerol for the system level, emphasising the significant demand of energy for feed preheating. © 2011 Canadian Society for Chemical Engineering     

Effect of mode of operation on hydrogen production from glycerol at thermal neutral conditions: Thermodynamic analysis

Thermodynamic analysis of hydrogen production from glycerol under thermal neutral conditions is studied in this work. Heat requirement from the process can be achieved from the exothermic reaction of glycerol with oxygen in air fed to the system. Two modes of operation for air feeding are considered including (i) Single-feed mode in which air is fed in combination with water and glycerol to the reformer, and (ii) Split-feed mode in which air and part of glycerol is fed to a combustor in order to generate heat. The thermal neutral conditions are considered for two levels including Reformer and System levels. It was found that the H₂ yield from both modes is not significantly different at the Reformer level. In contrast, the difference becomes more pronounced at the System level. Single-feed and Split-feed modes offer high H₂ yield in low (600–900 K) and high (900–1200 K) temperature ranges, respectively. The maximum H₂ yields are 5.67 (water to glycerol ratio, WGR = 12, oxygen to glycerol ratio, OGR = 0.37, T = 900 K, Split-feed mode), and 3.28 (WGR = 3, OGR = 1.40, T = 900 K, Single-feed mode), for the Reformer and System levels, respectively. The difference between H₂ yields in both levels mainly arises from the huge heat demand for preheating feeds in the System level, and therefore, a higher amount of air is needed to achieve the thermal neutral condition. Split-feed mode is a favorable choice in term of H₂ purity because the gas product is not diluted with N₂ from the air. The use of pure O₂ and afterburner products (ABP) stream were also considered at the System level. The maximum H₂ yield becomes 3.75 (WGR = 5.21, OGR = 1.28, T = 900 K, Split-feed mode) at thermal neutral condition when utilizing heat from the ABP stream. Finally comparisons between the different modes and levels are addressed in terms of yield of by-products, and carbon formation.     

Performance improvement of bioethanol-fuelled solid oxide fuel cell system by using pervaporation

This work proposes an improvement in performance with respect to the electrical efficiency of a bioethanol-fuelled Solid Oxide Fuel Cell (SOFC) system by replacing a conventional distillation column by a pervaporation unit in the bioethanol purification process. The simulation study indicates that the membrane separation factor has a significant influence on the electrical power and heat energy required to generate a feed of 25 mol% ethanol in water to the reformer. The values of overall electrical efficiency of the SOFC systems with a distillation column and with a pervaporation unit are compared under the thermally self-sufficient condition (Q_net = 0) which offers their maximum electrical efficiency. At the base case, the SOFC system with a pervaporation unit provides an electrical efficiency of 42% compared with 34% achieved from the system with a distillation unit, indicating a significant improvement by using a pervaporation unit. An increase in ethanol recovery can further improve the overall electrical efficiency. The study also reveals that further improvement of the membrane selectivity can slightly enhance the overall efficiency of the SOFC system. Finally, an economic analysis of a bioethanol-fuelled SOFC system with pervaporation is suggested as the basis for further development.     

Using a membrane reactor for the oxidative coupling of methane: simulation and optimization

An oxidative coupling of methane (OCM) is a promising process to convert methane into ethylene and ethane; however, it suffers from the relatively low selectivity and yield of ethylene at high methane conversion. In this study, a membrane reactor is applied to the OCM process in order to prevent the deep oxidation of a desirable ethylene product. The mathematical model of OCM process based on mass and energy balances coupled with detailed OCM kinetic model is employed to examine the performance of OCM membrane reactor in terms of CH₄ conversion, C₂ selectivity, and C₂ yield. The influences of key operating parameters (i.e., temperature, methane-to-oxygen feed ratio, and methane flow rate) on the OCM reactor performance are further analyzed. The simulation results indicate that the OCM membrane reactor operated at higher operating temperature and lower methane-to-oxygen feed ratio can improve C₂ production. An optimization of the OCM membrane reactor using a surface response methodology is proposed in this work to determine its optimal operating conditions. The central composite design is used to study the interaction of process variables (i.e., temperature, methane-to-oxygen feed ratio, and methane flow rate) and to find the optimum process operation to maximize the C₂ products yield.