Properties for Advanced Hydrogen Technologies

D.G. Friend, M.L. Huber, E.W. Lemmon, G.R. Hardin, and J.C. Rainwater

Objective: To provide industry with high quality thermophysical property surfaces for mixtures of hydrogen and methane over broad ranges of temperature, pressure, and composition.

Problem: There are currently no high accuracy models available that can handle mixtures of hydrogen and methane at high hydrogen concentrations, although fuel cells and hydrogen technologies may play a more important part in satisfying our energy needs. The fuel processing stage in fuel cells, known as reforming, involves processing the fuel to separate hydrogen from the other constituents, and mixtures of hydrogen and methane may be found in this sub-system of a fuel cell. Mixtures of hydrogen and methane have also been proposed as a fuel that may be used directly in internal combustion engines to reduce CO₂ and NOX emissions. The proposed research will develop a model for predicting the thermophysical properties of hydrogen/methane mixtures over the entire composition range from pure hydrogen to pure methane. The topic of this report relates to a project funded by the Electronics and Photonics Technology Office of the ATP program and is part of a more extensive program on fuels and, in particular, on fluids related to natural gas systems. When the cryogenic fluids, hydrogen and helium, are included in such fluids, the standard property formulations must be reconsidered in part because the typical phase envelope topology is 1^C1^Z rather than 1^C as for most other component pairs in natural gas systems.

Approach: Two models which are currently being used to establish standard reference thermodynamic surfaces are the extended corresponding states (ECS) model and a two-fluid Helmholtz mixing model. Both of these can use existing high accuracy pure fluid equations of state for methane and hydrogen, so that the mixture model will reduce to the pure fluid standards in the proper limits. The first step in the project is to perform a literature search and collect and evaluate experimental thermodynamic data (PVT relationships, heat capacities, vapor-liquid equilibria, sound speeds) for the methane/hydrogen binary system. Versions of both the ECS and Helmholtz mixing models were developed to describe the data, and the behavior of the binary interaction parameters was investigated. As the models are developed, comparisons will be made with the experimental database. Upon achieving a satisfactory optimized model, it will be incorporated into a NIST Standard Reference Data mixture database.

Results and Future Plans: There are about 3000 experimental points from 25 sources which give relevant thermodynamic data for the hydrogen-methane system. Although the data situation for the mixture is generally satisfactory, data are sparse for concentrations near the equimolar composition and at the lower temperatures; in addition, there are no caloric data which are generally required to establish the most accurate property standards. Initial results for the ECS model exhibited some numerical convergence problems; thus, much of the development and optimization work has been completed on the two-fluid Helmholtz energy model. Sample deviations between the data and model are shown in the figure. These results have been implemented in a version of NIST Standard Reference Database 14, although further testing, optimization, and quality control protocols will be required before releasing the revised database.