Robert W. Bradshaw

Thermodynamic analysis of hydrogen production by steam reforming

Abstract This paper presents thermodynamic analysis of hydrogen production by steam reforming. The analysis treats the chemistry at two levels: a global species balance assuming complete reaction and solution of the equilibrium composition at the specified reformer temperature. The global reaction allows for an energy balance that leads to analytical expressions for the thermal efficiency. We use this to determine the maximum efficiency, and to distinguish between various definitions of efficiency. To obtain a more realistic estimate of the efficiency, the chemical equilibrium solution is combined with a system energy balance, which compares the energy required to vaporize and heat the fuel–steam mixture to the reformer temperature with the heat available from combusting the residual fuels in the reformate stream. The equilibrium solutions are compared to experimental measurements of the species and thermal efficiency of reforming diesel fuel, obtained with prototype compact steam reformers. The observed efficiency is significantly lower than the equilibrium prediction, indicating that both incomplete reaction and heat transfer losses reduce the performance.

MOLTEN NITRATE SALT DEVELOPMENT FOR THERMAL ENERGY STORAGE IN PARABOLIC TROUGH SOLAR POWER SYSTEMS

Thermal energy storage can enhance the utility of parabolic trough solar power plants by providing the ability to match electrical output to peak demand periods. An important component of thermal energy storage system optimization is selecting the working fluid used as the storage media and/or heat transfer fluid. Large quantities of the working fluid are required for power plants at the scale of 100-MW, so maximizing heat transfer fluid performance while minimizing material cost is important. This paper reports recent developments of multi-component molten salt formulations consisting of common alkali nitrate and alkaline earth nitrate salts that have advantageous properties for applications as heat transfer fluids in parabolic trough systems. A primary disadvantage of molten salt heat transfer fluids is relatively high freeze-onset temperature compared to organic heat transfer oil. Experimental results are reported for formulations of inorganic molten salt mixtures that display freeze-onset temperatures below 100°C. In addition to phase-change behavior, several properties of these molten salts that significantly affect their suitability as thermal energy storage fluids were evaluated, including chemical stability and viscosity. These alternative molten salts have demonstrated chemical stability in the presence of air up to approximately 500°C in laboratory testing and display chemical equilibrium behavior similar to Solar Salt. The capability to operate at temperatures up to 500°C may allow an increase in maximum temperature operating capability vs. organic fluids in existing trough systems and will enable increased power cycle efficiency. Experimental measurements of viscosity were performed from near the freeze-onset temperature to about 200°C. Viscosities can exceed 100 cP at the lowest temperature but are less than 10 cP in the primary temperature range at which the mixtures would be used in a thermal energy storage system. Quantitative cost figures of constituent salts and blends are not currently available, although, these molten salt mixtures are expected to be inexpensive compared to synthetic organic heat transfer fluids. Experiments are in progress to confirm that the corrosion behavior of readily available alloys is satisfactory for long-term use.

Multicomponent Molten Salt Mixtures Based on Nitrate/Nitrite Anions

Molten salts are a promising medium for thermal energy transfer and storage. They have a very low vapor pressure and most are unreactive in air. Over the past 3 decades, Sandia National Laboratories has investigated a variety of molten salt mixtures of alkali nitrates and, most recently, quaternary mixtures of sodium, calcium, lithium, and potassium nitrate salts. This effort led to the discovery of mixtures with liquidus temperatures below 100°C. We have now extended this work to the mixed nitrate/nitrite anion system and found compositions with liquidus temperatures below 80°C. In this paper, we present experimental results exploring the lithium, sodium, and potassium compositional space with a 1:1 molar mixture of nitrate/nitrite. From our work, we have identified a five-component system with a liquidus temperature near 70°C. Physical properties of these salts, such as viscosity and density, are reported as well as thermal stability in air. Such a molten salt mixture, with a low liquidus temperature, has the potential to make parabolic trough collectors economically competitive with traditional power generation schemes.

Thermophysical Property Measurement of Nitrate Salt Heat Transfer Fluids

Nitrate salts have been used for decades in the concentrating solar power industry as heat transfer fluids and thermal storage media. For most of this time these inorganic fluids have been restricted to use in central receiver platforms due to the useful working temperature range of the most widely researched formulation, a near eutectic mixture of sodium and potassium nitrate, which melts at 220°C and is stable in air to nearly 580°C. Recent research efforts have led to the development of nitrate salt mixtures that melt at lower temperatures and are suitable for use in parabolic trough systems. These mixtures include three or more components and generally have melting points in the range of 100°C, with stability in air up to 500°C. The design of parabolic trough systems that utilize molten salt heat transfer fluids is complicated by the fact that the properties of these fluids are considerably different from the organic heat transfer fluids that they may replace. In this paper we present measured thermophysical property data for several commercial and non-commercial molten salt mixtures that can be used in the system level design of parabolic trough and central receiver power plants. The data presented include heat capacity, density, thermal conductivity, viscosity, all as a function of temperature, along with melting point and thermal stability limits. Some properties, such as density, can be predicted by simple mixing rules. The dependence of viscosity was strongly influenced by the composition of the molten salts and, particularly, the proportion of calcium nitrate.Copyright

Desalination Utilizing Clathrate Hydrates (LDRD Final Report)

Advances are reported in several aspects of clathrate hydrate desalination fundamentals necessary to develop an economical means to produce municipal quantities of potable water from seawater or brackish feedstock. These aspects include the following, (1) advances in defining the most promising systems design based on new types of hydrate guest molecules, (2) selection of optimal multi-phase reactors and separation arrangements, and, (3) applicability of an inert heat exchange fluid to moderate hydrate growth, control the morphology of the solid hydrate material formed, and facilitate separation of hydrate solids from concentrated brine. The rate of R141b hydrate formation was determined and found to depend only on the degree of supercooling. The rate of R141b hydrate formation in the presence of a heat exchange fluid depended on the degree of supercooling according to the same rate equation as pure R141b with secondary dependence on salinity. Experiments demonstrated that a perfluorocarbon heat exchange fluid assisted separation of R141b hydrates from brine. Preliminary experiments using the guest species, difluoromethane, showed that hydrate formation rates were substantial at temperatures up to at least 12 C and demonstrated partial separation of water from brine. We present a detailed molecular picture of the structure and dynamics of R141b guest molecules within water cages, obtained from ab initio calculations, molecular dynamics simulations, and Raman spectroscopy. Density functional theory calculations were used to provide an energetic and molecular orbital description of R141b stability in both large and small cages in a structure II hydrate. Additionally, the hydrate of an isomer, 1,2-dichloro-1-fluoroethane, does not form at ambient conditions because of extensive overlap of electron density between guest and host. Classical molecular dynamics simulations and laboratory trials support the results for the isomer hydrate. Molecular dynamics simulations show that R141b hydrate is stable at temperatures up to 265K, while the isomer hydrate is only stable up to 150K. Despite hydrogen bonding between guest and host, R141b molecules rotated freely within the water cage. The Raman spectrum of R141b in both the pure and hydrate phases was also compared with vibrational analysis from both computational methods. In particular, the frequency of the C-Cl stretch mode (585 cm{sup -1}) undergoes a shift to higher frequency in the hydrate phase. Raman spectra also indicate that this peak undergoes splitting and intensity variation as the temperature is decreased from 4 C to -4 C.

Clathrate Hydrates for Production of Potable Water

Clathrate hydrates are crystalline inclusion compounds of water and a guest molecule that can form at temperatures above the freezing point of water. Such inclusion compounds exclude dissolved solutes, e.g., sodium chloride present in the aqueous phase, and thereby provide a basis for desalination. Clathrate hydrate formation experiments were performed using several guest molecules, including R141b (CFClH), a commercial refrigerant, and ethylene. Ethylene, a gaseous hydrate guest, readily formed hydrates with saline water at up to 5°C and 20 atm. of pressure. Hydrates of R141b, in the liquid state, were formed at temperatures from 2°C to 6°C and atmospheric pressure from deionized water and 2% - 7% NaCl solutions. Significant reductions in saline content were obtained with both forming agents in a batch reactor without additional separation equipment. Samples of the R141b hydrates were characterized by cold-stage x-ray diffraction and Raman spectroscopy and determined to be structure II. Proof-of-concept experiments were performed to demonstrate a novel technique of desalination using R141b as the hydrate forming agent and an inert secondary fluid.

Design, Fabrication and Testing of an Apparatus for Material Compatibility Testing in Nitrate Salts at Temperatures Up to 700°C

Thermal energy storage is one of the key differentiators between Concentrating Solar Power (CSP) and other renewable energy technologies. Molten salt is an effective and affordable method of storing thermal energy. Current salt storage systems charge at temperatures between 390°C and 585°C (oil filled parabolic trough systems to molten salt towers). It is highly desirable to increase the operating temperature of salt storage systems in order to increase the efficiency of the power cycle and to permit the use of alternative, high-temperature cycles. However, higher salt temperatures cause increased reactivity and thus increased corrosion rates in many materials. In order to utilize molten salt at higher temperature, it is necessary to test and understand these corrosion interactions at elevated temperature. A corrosion test system has been designed and built for evaluating molten salt/material interactions to 700°C. The primary components of this system are several salt containment vessels that are constructed of 6″ dia. × 24″ long stainless steel, aluminum diffusion treated pipes with flat plate welded to one end and a flanged lid on the other. The vessels are designed to operate with a charge of 10 kg of molten salt and accommodate a “sample tree” on which corrosion test coupons may be suspended. The salt vessels are heated and insulated on the bottom half, roughly to the salt fill level, and cooled on the top half to protect the flange gasket and feedthrough ports. The samples trees have a stainless plate that reduces radiative heat transfer from the molten salt to the lid. Finite element analysis was performed to determine the pipe length and heating and cooling requirements to maintain molten salt at 700°C while limiting the lid gasket to 300°C or less. The vessels are designed to have an oxygen atmosphere in the ullage region to mitigate nitrate decomposition. Oxygen systems for operation at 700°C require careful design including the sizing, routing, cleanliness, and material selection of components in order to reduce risk of fire. Additionally, the system is designed to run at 1–2 psig which requires specialized low pressure / high temperature components. In this paper we present the design of the molten salt corrosion test system including details related to the containment vessels, oxygen handling system, and control software along with a discussion of the safety considerations necessary for these high temperature, high oxygen partial pressure tests.Copyright

Assessment of disinfectants in explosive destruction system for biological agent destruction : LDRD final report FY04.

Treatment systems that can neutralize biological agents are needed to mitigate risks from novel and legacy biohazards. Tests with Bacillus thuringiensis and Bacillus steurothemophilus spores were performed in a 190-liter, 1-112 lb TNT equivalent rated Explosive Destruction System (EDS) system to evaluate its capability to treat and destroy biological agents. Five tests were conducted using three different agents to kill the spores. The EDS was operated in steam autoclave, gas fumigation and liquid decontamination modes. The first three tests used EDS as an autoclave, which uses pressurized steam to kill the spores. Autoclaving was performed at 130-140 deg C for up to 2-hours. Tests with chlorine dioxide at 750 ppm concentration for 1 hour and 10% (vol) aqueous chlorine bleach solution for 1 hour were also performed. All tests resulted in complete neutralization of the bacterial spores based on no bacterial growth in post-treatment incubations. Explosively opening a glass container to expose the bacterial spores for treatment with steam was demonstrated and could easily be done for chlorine dioxide gas or liquid bleach.

Vapor-liquid phase behavior of the iodine-sulfur water-splitting process : LDRD final report for FY03.

This report summarizes the results of a one-year LDRD project that was undertaken to better understand the equilibrium behavior of the iodine-water-hydriodic acid system at elevated temperature and pressure. We attempted to extend the phase equilibrium database for this system in order to facilitate development of the iodine-sulfur water-splitting process to produce hydrogen to a commercial scale. The iodine-sulfur cycle for thermochemical splitting of water is recognized as the most efficient such process and is particularly well suited to coupling to a high-temperature source of process heat. This study intended to combine experimental measurements of vapor-liquid-liquid equilibrium and equation-of-state modeling of equilibrium solutions using Sandias Chernkin software. Vapor-liquid equilibrium experiments were conducted to a limited extent. The Liquid Chernkin software that was developed as part of an earlier LDRD project was enhanced and applied to model the non-ideal behavior of the liquid phases.

Experimental and theoretical characterization of surfaces using infrared reflectance spectroscopy

External reflectance infrared spectroscopy has been used to study the formation of thin oxide films on metals. Experimental data have been acquired with a Fourier transform infrared (FT-IR) spectrometer using a polarized beam at high angles of incidence, and have been compared with theoretical calculations.