(61e) Integrated Micro System for Controlled Delivery of Hydrogen from Metal Ammines

Efficient and safe storage of hydrogen for energy production has received significant attention over the last decades. However, it still remains a substantial challenge for the prospects of a hydrogen economy to provide a safe, reversible and dense storage method for hydrogen. Most research is focused on the so-called complex hydrides based on alanates or borates. Some hydrides have promising properties, but most suffer from a too low hydrogen density, too slow kinetics or lack of reversibility.

Ammonia is an efficient hydrogen carrier. The ?reforming? of ammonia into hydrogen and nitrogen, the reverse ammonia synthesis, is well understood and can be done at a temperature level close to 650K [Sørensen et. al., 2005]. Pressurized ammonia condenses above 8 bars to a liquid with as much as 110 kgH2/m3, and as such it is a very interesting hydrogen carrier. However, the common method of storing ammonia in a pressure vessel is impractical to incorporate into a micro system, and for many applications the toxicity of ammonia can be a problem. Consequently, ammonia has great potential as hydrogen rich fuel, when combined with a safe and practical, high density storage method.

In earlier work [Christensen et. al. 2005] we have demonstrated the use of solid metal ammine complexes as hydrogen storage materials. Ammonia can be bound into a metal salt and with focus on MgCl2 as one of the most promising materials we will outline the potential of such a storage method. Mg(NH3)6Cl2 contains 9.1 wt % hydrogen at 109 kgH2/m3 in the form of ammonia. The ammonia storage is completely reversible and by combining it with an ammonia decomposition catalyst hydrogen can be delivered at temperatures below 650 K. The reversibility has been demonstrated by successively absorbing and desorbing ammonia from MgCl2. Temperature Programmed Desorption of successive runs are almost identical and the saturation reaches 99-100% within the experimental uncertainty. Thermal decomposition of the complex starts around 350 K. At 500 K, two thirds of the ammonia has been released. The remaining third is released below 620 K.

The present work presents a micro structured system, which is able to desorb ammonia from the compact metal ammine, decompose it catalytically into H2 and N2 and at the same time supply heat for both reactions by catalytic combustion of part of the decomposed ammonia or a simulated purge gas from a fuel cell running on decomposed ammonia.

Hydrogen production by decomposition of ammonia in micro reactors is a known process [Sørensen et. al., 2005], as is ammonia release from metal ammines [Christensen et. al. 2005] and catalytic combustion of hydrogen in air [Ryi et. al. 2005]. This work, however, is the first to combine these processes in an integrated catalytic unit. It is shown that heat for driving the reactions can be supplied either by part of the fuel before it enters the fuel cell, purge gas from said fuel cell or a mixture thereof.

A basic flow sheet of the process is given in fig 1, while fig 2 is a schematic representation of the setup. Initially the decomposition reactor is heated to its operating temperature. The heat dissipating from the reactor will trigger desorption of ammonia from the storage material, and the ammonia will be delivered directly to the decomposition reactor. The hydrogen rich stream leaving the reactor is then split into a part bypassing the fuel cell to supply heat for more decomposition and ammonia release, and the desired part, which fuels the fuel cell. As the hydrogen is diluted by nitrogen, some must be purged from the fuel cell at all times. This purge gas will still contain some hydrogen, and is passed through the catalytic combustion reactor to provide heat for the other reactions and to avoid release of hydrogen gas to the environment.

Figure 1: Flow sheet of a system for delivery of hydrogen to a fuel cell, employing Mg(NH3)6Cl2 for hydrogen storage.

Figure 2: Schematic representation of a catalytic micro system for delivery of hydrogen to a fuel cell, employing Mg(NH3)6Cl2 for hydrogen storage. Two catalytic micro reactors are combined with a small chamber holding the solid hydrogen storage material.

References

Claus Hviid Christensen, Rasmus Zink Sørensen, Tue Johannessen, Ulrich J. Quaade, Karoliina Honkala, Tobias D. Elmøe, Rikke Køhler, Jens K. Nørskov, Metal ammine complexes for hydrogen storage, Journal of Materials Chemistry, 15, 4106-4108, 2005.

Rasmus Zink Sørensen, Lærke J.E. Nielsen, Søren Jensen, Ole Hansen, Tue Johannessen, Ulrich Quaade and Claus Hviid Christensen: Catalytic ammonia decomposition: miniaturized production of COX-free hydrogen for fuel cells, Catalysis Communications, 6 (3), 229-232, 2005.

Shin-Kun Ryi, Jong-Soo Park, Seung-Hoon Choi, Sung-Ho Cho, Sung-Hyun Kim: Novel micro fuel processor for PEMFCs with heat generation by catalytic combustion, Chemical Engineering Journal, 113, 47?53, 2005.