Potential Roles of Ammonia in a Hydrogen Economy

A Study of Issues Related to the Use Ammonia for On-Board Vehicular Hydrogen Storage

U.S. Department of Energy, February 2006
Primary Authors: George Thomas¹ and George Parks²
1. US Department of Energy (retired, Sandia National Laboratory, on assignment to DOE Hydrogen Program), and member of FreedomCAR & Fuel Partnership Hydrogen Storage Technical Team; 2. ConocoPhillips; member of FreedomCAR & Fuel Partnership Hydrogen Storage Technical Team, and co-chair of FreedomCAR & Fuel Partnership Hydrogen Delivery Technical Team

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Executive Summary

The objectives of this paper are to identify, evaluate and summarize the key issues and advantages and disadvantages associated with ammonia as an energy carrier for on-board vehicular hydrogen storage. These issues have been investigated by the U.S. Department of Energy (DOE) with input from various sources including members of the Hydrogen Storage Technical Team of the FreedomCAR & Fuel Partnership (a partnership among DOE, BP America, Chevron Corporation, ConocoPhillips, Exxon Mobil Corporation, Shell Hydrogen (U.S.), and the United States Council for Automotive Research (USCAR – a legal partnership among DaimlerChrysler Corporation, Ford Motor Company, and General Motors Corporation). The outcome of this investigation is a discussion of the potential roles that ammonia might play in a hydrogen economy, particularly with regard to the viability of ammonia as an on-board hydrogen carrier for fuel cell vehicles.

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. 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.