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.     

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).     

Hydrothermally synthesized titanate nanostructures: impact of heat treatment on particle characteristics and photocatalytic properties

The role titanate particle structure plays in governing its characteristics upon calcining and their ensuing influence on photocatalytic performance was investigated. Titanate nanotubes and nanoribbons were prepared by hydrothermal treatment of Aeroxide P25 and then calcined at temperatures in the range 200 - 800 °C. Heat treatment directly transformed the nanotubes to anatase while nanoribbon transformation to anatase occurred via a TiO(2)(B) intermediate phase. The nanoribbon structure also provided an increased resistance to sintering, allowing for retention of the original {010} facet of the titanate nanosheets up to 800 °C. The changing material properties with calcining were found to influence the capacity of the particles to photodegrade oxalic acid and methanol. The nanotubes provided an optimum photoactivity following calcination at 500 °C with this point representing a transition between the relative dominance of crystal phase and surface area on performance. The comparatively smaller initial surface area of the nanoribbons consigned this characteristic to a secondary role in influencing photoactivity with the changes to crystal phase dominating the continually improving performance with calcination up to 800 °C. The structural stability imparted by the nanoribbon architecture during calcination, in particular its retention of the {010} facet at temperatures >700 °C, advanced its photocatalytic performance compared with the nanotubes. This was especially the case for methanol photooxidation whose primary degradation mechanism relies on hydroxyl radical attack and was facilitated by the {010} facet. The effect was not as pronounced for oxalic acid due to its higher adsorption on TiO(2) and therefore greater susceptibility to oxidation by photogenerated holes. This study demonstrates that, apart from modulating sintering effects and changes to crystal phase, the titanate nanostructure influences particle crystallography which can be beneficial for photocatalytic performance.     

Comparative study of fuel gas production for SOFC from steam and supercritical-water reforming of bioethanol

Theoretical study of fuel gas (H₂ + CO) production for SOFC from bioethanol was carried out to compare performances between two reforming technologies, including steam reforming (SR) and supercritical-water reforming (SCWR). It demonstrates that the fuel gas productions are comparable among the two reforming systems; however, SCWR requires the operation at much higher temperature and pressure than SR. The maximum hydrogen yield can be obtained at 850 K, atmospheric pressure, ethanol to water molar feed ratio of 1:20 for SR system and at 1300 K, 22.1 MPa, and ethanol to water feed ratio of 1:20 for SCWR. The use of a distillation column to purify the bioethanol feed was proven to improve the fuel conversion efficiency of both systems. The analysis reveals that SCWR is a promising system for fuel production for SOFC when a gas turbine is incorporated to the system for energy recovery. Further, it is not necessary to distil bioethanol to obtain too high ethanol recovery (i.e. >90%) as higher energy consumption at the distillation column could lead to lower overall thermal efficiency.     

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     

Integrated methane decomposition and solid oxide fuel cell for efficient electrical power generation and carbon capture

This work proposes the application of methane decomposition (MD) as a fuel processor to replace methane steam reforming (MSR) for hydrogen production for a methane-fuelled solid oxide fuel cell (SOFC) system. In this work, comparison between the MD–SOFC and the MSR–SOFC was performed in terms of SOFC performances and economic analysis to demonstrate a benefit of using MD as a fuel processor. Energy analysis of SOFC system was evaluated based on thermally self-sufficient condition where no external energy is required for the system. Although the MD–SOFC system offers lower electrical efficiency than that of the MSR–SOFC as solid carbon is generated without being further combusted to generate energy; however, the MD–SOFC stack can be operated at higher power density due to high purity of hydrogen supplied to the fuel cell, resulting in smaller size of the system when compared to the MSR–SOFC. Moreover, the MD–SOFC system is less complicated than that of the MSR–SOFC as the CCS facility is not necessary to be included to reduce CO₂ emission. Economic analysis demonstrated that the SOFC system with MD is more competitive than the conventional system with MSR when considering the valuable by-products of solid carbon even with the low-valued carbon black. It is suggested that the success of this proposed SOFC system with MD relies on the technology development on cogeneration of hydrogen and valuable carbon products.     

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.     

Partial oxidation of palm fatty acids over Ce‐ZrO2: Roles of catalyst surface area,lattice oxygen capacity and mobility

Nanoscale Ce‐ZrO₂, synthesized by cationic surfactant‐assisted method, has useful partial oxidation activity to convert palm fatty acid distillate (PFAD; containing C₁₆–C₁₈ compounds) to hydrogen‐rich gas with low carbon formation problem under moderate temperatures. At 1123 K with the inlet O/C ratio of 1.0, the main products from the reaction are H₂, CO, CO₂, and CH₄ with slight formations of gaseous high hydrocarbons (i.e., C₂H₄, C₂H₆, and C₃H₆), which could all be eliminated by applying higher O/C ratio (above 1.25) or higher temperature (1173 K). Compared with the microscale Ce‐ZrO₂ synthesized by conventional coprecipitation method, less H₂ production with relatively higher C₂H₄, C₂H₆, and C₃H₆ formations are generated from the reaction over microscale Ce‐ZrO₂. The better reaction performances of nanoscale Ce‐ZrO₂ are linearly correlated with its higher specific surface area as well as higher oxygen storage capacity and lattice oxygen mobility, according to the reduction/oxidation measurement and ¹⁸O/¹⁶O isotope exchange study. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Glycerol ethers synthesis from glycerol etherification with tert-butyl alcohol in reactive distillation

This paper presents a study of glycerol etherification with tert-butyl alcohol catalyzed by Amberlyst 15 in reactive distillation (RD). A thermodynamic analysis is firstly investigated by applying three group contribution methods, to determine the equilibrium composition by minimization of the Gibbs free energy and to compare the predicted values against measured data. Next, the kinetic model parameters are regressed by matching measured data from an autoclave reactor. The activity based Langmuir-Hinshelwood model is found to give the best representation of the reaction rate data. The regressed kinetic rate expressions are also compared against independently measured data in fixed bed reactors reported in the literature and found to give a good match. Finally, using the developed models, it is shown by simulation as well as verification by experiments, that the suitable RD configuration for the production of glycerol ethers in RD is the one consisting of 6 rectifying stages and 6 reaction stages without stripping stage.