The US Department of Energy (DOE) has developed a paper—Potential Roles of Ammonia in a Hydrogen Economy: A Study of Issues Related to the Use of Ammonia for On-Board Vehicular Hydrogen Storage—to identify, evaluate, and summarize the key issues and advantages and disadvantages associated with ammonia for on-board vehicular hydrogen storage. DOE is now seeking public comment on the document.
Ammonia has a high capacity for hydrogen storage—17.6 wt.%—based on its molecular structure. The release of hydrogen from ammonia on-board a light duty vehicle, however, is problematic given a number of factors, including energy input requirements, reactor size, slow start-up time, safety and toxicity issues, and the incompatibility of polymer electrolyte membrane (PEM) fuel cells in the presence of even trace levels of ammonia (> 0.1ppm).
Simply stated, most of the performance parameters of ammonia reactors would need at least two orders-of-magnitude improvements in order to be used on-board commercially viable hydrogen-powered fuel cell vehicles.
Due to the above reasons, DOE does not plan to fund R&D to improve ammonia fuel processing technologies for use on board light weight vehicles at the present time. However, a distinction may be made between conventional fuel processing of ammonia (e.g. high temperature, low efficiency, slow start-up/time response crackers) versus novel approaches to store ammonia and release its hydrogen content under conditions available on-board PEM fuel cell vehicles.
As DOE’s current portfolio in hydrogen storage evolves, breakthrough approaches that allow the safe, efficient and cost effective use of ammonia-based storage may be considered at a future date.
Ammonia (NH3) is normally produced by the catalytic reaction of nitrogen and hydrogen, according to the following basic reaction:
N2(g) + 3H2(g) → 2NH3(g)
Ammonia synthesis is usually coupled with hydrogen production from natural gas to increase efficiency. Other hydrocarbon feedstocks can be gasified to form synthesis gas (CO and H2), which can then be reacted with water and nitrogen to produce ammonia.
According to the DOE, current ammonia production based on natural gas as the feedstock produces ammonia with an energy input as low as 6.8 Gcal (LHV)/Mt.
Given the energy content of ammonia, conversion of natural gas to ammonia then is about 60-65% energy efficient; that is, 60-65% of the energy input to the process (mostly methane) is contained in the ammonia product. As stated earlier, ammonia can be produced from other feedstocks, although production is not expected to be as efficient or as cheap as natural gas-based production.
Ammonia decomposition (cracking) is simply the reverse of the synthesis reaction.
NH3(g) → 1/2 N2(g) + 3/2 H2(g)
The reaction is endothermic, with the temperature required for efficient cracking depending upon the catalyst, but typically above 650ºC—well above PEM fuel cell operating temperatures.
Were the DOE to fund research on ammonia at this time, it would focus on the following priorities:
High Efficiency Cracking Catalysts &Reactors;
Purification Systems Coupled to Reactor Designs;
Failsafe Ammonia Tank Designs; and