Ammonia as a Fuel 101

This is the Executive Summary from the 2006 US Departement of Energy White Paper "Partial Roles of Ammonia in a Hydrogen Ecconomy" by George Thomas and George Parks.

-Editor

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
• 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.

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.

While this paper describes general advantages and disadvantages of ammonia with a focus
on on-board vehicular hydrogen storage,

...the use of ammonia as a potential hydrogen carrier for hydrogen delivery or off-board hydrogen storage is currently under evaluation by the DOE and the FreedomCAR and Fuel Partnership’s Hydrogen Delivery Technical Team.

Full Department of Energy Report

Page 5 of 23 Feb 2006