The specific hydrogen market determines the value of hydrogen from different sources.Each hydrogen production technology has its own distinct characteristics. For example, steam reforming of natural gas produces only hydrogen. In contrast, nuclear and solar hydrogen production facilities produce hydrogen together with oxygen as a by-product or co-product. For a user who needs both oxygen and hydrogen, the value of hydrogen from nuclear and solar plants is higher than that from a fossil plant because "free" oxygen is produced as a by-product. Six factors that impact the relative economics of fossil, nuclear, and solar hydrogen production to the customer are identified: oxygen by-product, avoidance of carbon dioxide emissions, hydrogen transport costs, storage costs, availability of low-cost heat, and institutional factors. These factors imply that different hydrogen production technologies will be competitive in different markets and that the first markets for nuclear and solar hydrogen will be those markets in which they have a unique competitive advantage. These secondary economic factors are described and quantified in terms of dollars per kilogram of hydrogen.

The hybrid sulfur thermochemical cycle has been proposed as a means to produce efficiently massive quantities of clean hydrogen using a high-temperature heat source like nuclear or solar.The cycle consists of two steps, one of which is electrolytic. The reversible cell potential for this step and, hence, the resulting operating potential will depend on the concentrations of dissolved SO 2 and sulfuric acid at the electrode. To understand better how these are related as functions of temperature and pressure, an Aspen Plus phase equilibrium model using the OLI Mixed Solvent Electrolyte physical properties method was employed to determine the activities of the species present in the system. These activities were used in conjunction with the Nernst equation to determine the reversible cell potential as a function of sulfuric acid concentration, temperature and pressure. A significant difference between the reversible and actual cell potentials was found, suggesting that there may be considerable room for reducing the operating potential.

The hybrid sulfur cycle has been investigated as a means to produce clean hydrogen efficiently on a large scale by first decomposing H 2 SO 4 to SO 2 , O 2 , and H 2 O and then electrochemically oxidizing SO 2 back to H 2 SO 4 with the cogeneration of H 2 . Thus far, it has been determined that the total cell potential for the hybrid sulfur electrolyzer is controlled mainly by water transport in the cell. Water is required at the anode to participate in the oxidation of SO 2 to H 2 SO 4 and to hydrate the membrane. In addition, water transport to the anode influences the concentration of the sulfuric acid produced. The resulting sulfuric acid concentration at the anode influences the equilibrium potential of and the reaction kinetics for SO 2 oxidation and the average conductivity of the membrane. A final contribution to the potential loss is the diffusion of SO 2 through the sulfuric acid to the catalyst site. Here, we extend our understanding of water transport to predict the individual contributions to the total cell potential. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3397901͔ All rights reserved. The hybrid sulfur ͑HyS͒ cycle has gained attention due to the possibility of using this process to produce clean hydrogen on a large scale at efficiencies higher than those using water electrolysis.1-19 The high temperature decomposition of H 2 SO 4 to SO 2 , O 2 , and H 2 O is suited for use with advanced gas-cooled nuclear reactor heat sources or solar receiver arrays.1-7 The electrolysis step described here is coupled with the high temperature step to complete the cycle. We developed a gas-fed anode electrolyzer in which SO 2 is oxidized to H 2 SO 4 via the following reaction 11,[14][15][16][17][18] We have successfully carried out Reactions 1 and 2 over a range of operating conditions ͑i.e., temperature, flow rate, and membrane pressure differential͒ and design variations ͑i.e., catalyst loading and membrane type and thickness͒. [14][15][16][17] We have also accurately predicted water transport and correlated the operating potential to the sulfuric acid concentration produced at the anode. [15][16][17] We have shown that the concentration of sulfuric acid produced at the anode increases with current density and that the sulfuric acid concentration at the anode influences the cell potential via the reversible cell potential U eq . 18Although we have previously correlated cell potential to acid concentration and hence water transport, a quantitative measure of the various potential losses has never been made. In this discussion, we present a comprehensive investigation of the components that make up the total cell potential ͑i.e., reversible cell potential, membrane resistance, and catalyst activity͒ to better understand and improve electrolyzer performance and operation. ExperimentalThe experimental setup was the same as that described previously.14-17 The cell was a standard 10 cm 2 cell from Fuel Cell Technologies, Inc. Reactants and products were fed to and from the cell through Kynar manifolds inst...

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A properties model was developed for use in commercial process simulators to model pyrolysis of lignocellulosic biomass. The component list was chosen to enable process simulations based on a recently published lumped pyrolysis kinetics model. Since many of the compounds involved in pyrolysis are not found in simulator databanks, estimation based on available literature data was used to establish missing parameters. Standard solid enthalpy of formation, solid heat capacity, and solid density estimates calculated from the limited experimental data available were prepared for six biomass constituents and nine intermediate and end-products of their pyrolysis. Ideal gas enthalpy of formation and heat capacity, critical property, and vapor pressure estimates were prepared for another four pyrolysis end-products and one biomass component. The estimates were all validated against the closest available experimental data in the literature. The addition of these new components and properties allows thermodynamically rigorous simulation of lumped biomass pyrolysis reactions with accurate energy balances. Enthalpies of reaction calculated from the properties model were compared with reported reaction enthalpies for the same lumped biomass pyrolysis reactions and found to be in general agreement.

Two hybrid sulfur (HyS) cycle process flowsheets intended for use with high-temperature gas-cooled reactors (HTGRs) are presented. The flowsheets were developed for the Next Generation Nuclear Plant (NGNP) program, and couple a proton exchange membrane (PEM) electrolyzer for the SO2-depolarized electrolysis step with a silicon carbide bayonet reactor for the high-temperature decomposition step. One presumes an HTGR reactor outlet temperature (ROT) of 950°C, the other 750°C. Performance was improved (over earlier flowsheets) by assuming that use of a more acidtolerant PEM, like acid-doped poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI), instead of Nafion®, would allow higher anolyte acid concentrations. Lower ROT was accommodated by adding a direct contact exchange/quench column upstream from the bayonet reactor and dropping the decomposition pressure. Aspen Plus was used to develop material and energy balances. A net thermal efficiency of 44.0% to 47.6%, higher heating value basis is projected for the 950°C case, dropping to 39.9% for the 750°C case.Highlights for "Hybrid sulfur cycle flowsheets for hydrogen production using hightemperature gas-cooled reactors" Two hybrid sulfur cycle flowsheets are presented that combine a bayonet decomposition reactor with a proton exchange membrane electrolyzer. Aspen Plus material and energy balances are provided. One flowsheet can be used with a 950°C reactor to make hydrogen at 44.0% to 47.6% net thermal efficiency, HHV basis. The other flowsheet can be used with a 750°C reactor to make hydrogen at 38.0% net thermal efficiency, HHV basis. *HighlightsHybrid sulfur cycle flowsheets for hydrogen production using high-temperature gas-cooled reactors Lower ROT was accommodated by adding a direct contact exchange/quench column upstream from the bayonet reactor and dropping the decomposition pressure. Aspen Plus was used to develop material and energy balances. A net thermal efficiency of 44.0% to 47.6%, higher heating value basis is projected for the 950°C case, dropping to 39.9% for the 750°C case.Keywords: Hydrogen production; nuclear heat application; hybrid sulfur cycle; process flowsheet; Aspen Plus; proton exchange membrane electrolyzer; bayonet decomposition reactor * Tel.: (803)

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A biomass fast pyrolysis model was developed for implementation in equationoriented modeling software. Based on a previous framework of coupled 1-dimensional mass, momentum, and heat balance equations, this model includes updated reaction kinetics to provide a more detailed representation of biomass components, intermediates, and products. A recently published derivation of thermodynamic properties for the species in this model has allowed the energy balance around the pyrolysis reactor to be rigorously redefined. With these improvements, the optimum pyrolysis temperature for bio-oil production predicted by the model is increased by up to 50 C, bringing it in line with experimental data and increasing the overall agreement. More importantly, the reactor energy balance is strictly enforced. The resulting model can be used for the design and optimization of biomass fast pyrolysis processes, and comparisons with other options for biomass utilization. K E Y W O R D S biomass pyrolysis, entrained-flow, predictive model, pyrolysis process flowsheet simulation, reactor model