Potential Roles of Ammonia in a Hydrogen Economy
https://www.energy.gov/sites/prod/files/2015/01/f19/fcto_nh3_h2_storage_white_paper_2006.pdf...EXECUTIVE SUMMARY...
...Ammonia has a number of favorable attributes, the primary one being its high capacity for
hydrogen storage, 17.6 wt.%, based on its molecular structure. However, in order to release
hydrogen from ammonia, significant energy input as well as reactor mass and volume are
required. Other considerations include safety and toxicity issues, both actual and perceived, as
well as the incompatibility of polymer electrolyte membrane (PEM) fuel cells in the presence of
even trace levels of ammonia (> 0.1ppm).
Given the state of the art in ‘cracking’ ammonia to produce hydrogen, there are many issues in
the on-board use of ammonia similar to those identified for on-board fuel processors.
Specifically, these include: high operating temperature (>500° C); longevity and reliability of
catalysts and other components (at high temperatures and in the presence of impurities); start-up
time (to get the system up to operating temperature); purification requirements (to prevent
ammonia poisoning of fuel cells); complexity of the overall system; energy efficiency (on-board
ammonia would have to be burned in the cracking process); cost (currently ~$100K for 1-3 g
H2/s stationary units); and reactor weight and volume (commercial units with sufficient
throughput currently weigh about 2000-5000 kg and are about 3000-6000 liters in size). 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....