(181a) Evaluation of Alternate Thermochemical Cycles

Argonne National Laboratory (Argonne), as part of the U.S. Department of Energy's Nuclear Hydrogen Initiative, is evaluating the potential of alternate thermochemical cycles for producing hydrogen. Some cycles were identified as promising based on preliminary research conducted primarily in the 1970's. These cycles include the following: Cu-SO4, Cu-Cl, Mg-I, V-Cl, Fe-Cl, and the hybrid chlorine. Other cycles have garnered interest more recently, e.g., the Ce-Cl and some proprietary cycles. Argonne briefly reviewed these cycles using the scoping flowsheet methodology, described below. The results of these analyses indicate that these cycles deserve a second look. Consequently, the Nuclear Hydrogen Initiative is supporting evaluation work at eight universities.

These universities will reexamine the cycles using a three-step methodology. This approach allows the cycles to be reevaluated from a new perspective using the latest technologies. These universities have extensive process design experience and familiarity with thermodynamic databases. Some also have experience in high temperature reactions, experimental expertise in conducting high temperature and high pressure reactions and electrochemical reactions, and unique tools like sensitivity and uncertainty analysis. Each is being paired with a process that matches their capabilities. The results of the work will be flowsheets for eight cycles, an assessment of whether the challenges identified earlier might be met after 30+ years of technical advances, and the identification of critical R&D needs, which will be needed to set research priorities for future work for the most promising cycle(s). In addition to the chemistry, a promising thermochemical cycle must produce hydrogen efficiently with respect to energy consumption and competitively with respect to the cost of gasoline. A key metric will be a new calculated value for energy efficiency, which correlates with hydrogen production cost. The evaluation will be carried out in three stages.

In the first stage, it is assumed that the reactions are stoichiometric and that no competing products are formed. The results will consist of a heat balance at atmospheric pressure (heats of reaction, sensible and latent heat) to determine external heat addition requirements, evaluation of each reaction step for feasibility and suitability for a cyclical process, and commentary on the chemical viability of each reaction. Work terms for electrochemical reactions, for separations, and for chemical potential will be included. The work terms are converted to their heat equivalent. A process such as pinch analysis, or the equivalent, is used to ensure that any internal heat exchange within the process uses a feasible temperature driving force. This will determine the net external thermal heat demand for each process. This idealized analysis will indicate if the thermodynamic efficiency is high enough to support further work.

In the second stage, equilibrium data are considered. Competing product formation is allowed and excess reagents may be used to drive the reaction to the right. The heat balance is determined as before and recycle costs may be included. Again, the value of the thermodynamic efficiency will be considered as the justification for continued work.

For the third stage, a detailed process flowsheet is developed using using computer simulation software. Shaft work requirements are added to the thermal efficiency calculation. Pressures may be greater than atmospheric in order to maximize yields of desired products. Experimental conditions are chosen so that the reactions go to completion or as far to the right as possible. Further optimization of recycle streams is necessary. In addition, actual separation processes may be identified so that the energy costs may be more realistically estimated. A heat exchanger network is used to optimize heat management. The results of this work reflect a more accurate thermal efficiency value and identify additional R&D needs, such as non-idealities, critically needed thermodynamic data, and better definition of unit operations for process optimization. A preliminary cost analysis for the most promising alternate cycles is the final step.