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Ammonia, the second most produced chemical in the world (176 million tons in 2014), is manufactured at large plants (1,000 – 1,500 t/day) using Haber-Bosch process developed more than hundred years ago. A simple reaction of nitrogen and hydrogen (produced by steam methane reforming or coal gasification) consumes about 2% of world energy, in part due to the use of high pressure and temperature. With the global transition from fossil fuels to intermittent renewable energy sources there is a need for long term storage and long range transmission of energy, for which ammonia is perfect fit. To make it practical, it is necessary to match the scale of ammonia production with the scale of renewable energy sources, at the same or better capital cost per ton of NH3, and reduce the energy consumption.
The Advanced Research Projects Agency (ARPA-E) invests in transformational, high-risk, early-stage research to create America’s future technologies for energy generation, use, and storage thus ensuring the US’s technological lead. The Renewable Electricity to Fuels through Utilization of Energy-dense Liquids (REFUEL) program funds the development of transformational technologies to reduce the barriers to widespread adoption of intermittent renewable energy sources by enabling the conversion of energy from these sources, water and air to energy-dense zero-carbon liquid fuels, mainly focusing on ammonia.
The REFUEL program funds projects related to two major paths that may potentially reduce both capital and energy costs of ammonia production: dramatically improving conventional Haber-Bosch process or replacing it with direct electrochemical synthesis. The next generation of Haber-Bosch chemistry should work at lower pressure and temperature, which can be achieved by using more active catalysts or a combination of catalysts with physical activation, and by continuous ammonia removal from the reaction zone thus shifting the equilibrium. To make it competitive with conventional process, the process efficiency should be >86% and the catalyst should provide a reaction rate of at least 7×10-7 mol cm-2 s-1. Direct electrochemical ammonia synthesis from nitrogen and water has a potential for big energy savings due to fewer process steps and a reduction in pressure / temperature. In addition, electrochemical technologies are easily scaled down to allow for low capital, modular plants that match the scale of renewable energy production. The REFUEL program’s very challenging target for the electrochemical process is 60% energy efficiency at a current density of >300 mA cm-2 and 90% coulombic efficiency. In this presentation a comparison of these two future ammonia production technologies will be made and REFUEL projects targeting various approaches to ammonia synthesis as well as related projects supported by ARPA-E through OPEN program will be highlighted.