Efficient Ammonia Cracker

About

Hydrogen fuel is a clean energy carrier which has the potential to play a crucial role in global decarbonisation. However, the low volumetric energy density of hydrogen makes it expensive to store and transport, inhibiting its use as an economically viable energy vector. The use of ammonia (NH3) as a hydrogen carrier provides a promising solution to achieve effective energy storage and transportation. Ammonia has a high volumetric hydrogen density and has been produced in very large quantities for use as fertiliser for over 75 years, meaning that significant storage and transportation infrastructure already exists. Ammonia can be decomposed into a mixture of nitrogen (N2) and hydrogen (H2) which can then be used as fuel using a method known as cracking. For the distribution of hydrogen on a national scale, a decentralised model whereby ammonia is transported to the point of use and cracked onsite is expected to be more economically favourable than a centralised model where imported ammonia is cracked centrally then transported as hydrogen to the point of use. This technology provides a highly efficient and scalable solution to crack ammonia at the point of use, enabling a decentralised model where hydrogen can be used as a completely carbon-free and practical energy source. This technology is a TRL 3 instrument design for an ammonia cracking system. The instrument uses a known method for producing hydrogen from ammonia in combination with an innovative closed loop nitrogen heating system and novel catalyst technology. The main components of the cracker are an evaporator to vaporise the stored liquid ammonia, a preheater to heat the resulting ammonia gas to 400°C and a reactor filled with lithium imide where the cracking reaction is catalysed. A separator is required to obtain pure hydrogen from the cracked gas which contains a mixture of hydrogen and nitrogen. Several technologies exist for performing this separation including pressure swing adsorption and polymeric and palladium membranes. The selection of technology for this separation step depends on the purity and quantity of hydrogen required. The decoupled nature of the cracker and separator provides the opportunity to optimise the hydrogen purity - cost relationship. The design can achieve improved flexibility, efficiency and efficacy in producing hydrogen, in a more compact system compared to state of the art crackers. It also allows variable production rates without significantly affecting efficiency, making it highly scalable. This would enable the storage of large amounts of energy generated by renewable sources at times when supply exceeds demand, as well as the cost effective distribution of renewable energy from distant generation sites to end users. Electric ammonia crackers used to produce hydrogen for steel manufacturing processes are commercially available, however, these tend to use high temperature nickel catalysts and energy efficiency has not been a key requirement of primary concern. Ammonia cracking to release hydrogen as efficiently as possible on a scale needed to fuel a typical forecourt petrol station has not yet been demonstrated. The ammonia cracker technology has been computationally modelled and a detailed design of the process has been completed using simulation software. The novel Lithium Imide Catalyst has shown significantly higher catalytic activity than the current state of the art, reducing the temperature of 90% conversion by around 50°C. Additionally the feedstock elements are far more abundant than current catalytic materials. Together with the simpler catalyst formulation, the use of lithium imide is likely to result in cheaper catalyst formulations. A feasibility study has been carried out by the following organisations: Siemens, Engie, Ecuity Consulting and STFC. The study concluded that the development of this technology will provide the missing link in an otherwise mature value chain and place UK companies and expertise at the forefront of an emerging global market. It also found that a decentralised hydrogen model is more economically viable than a central model and that this effect is exaggerated when hydrogen must be transported over larger distances.