Conceptual design of a process for the use of liquid ammonia as hydrogen vector

Conceptual design of a process for the use of liquid ammonia as hydrogen vector Ilenia Rossetti 1*, Antonio Tripodi 1, Gianguido Ramis 2 1 - Chemical Plants and Industrial Chemistry Group, Dip. Chimica, Università degli Studi di Milano, CNR-SCITEC and INSTM Unit Milano-Università, via C. Golgi 19, 20133 Milan, Italy 2 - Dip. Ing. Chimica, Civile ed Ambientale, Università degli Studi di Genova and INSTM Unit Genova, via all’Opera Pia 15A, 16145 Genoa, Italy Abstract Chemical energy storage presents a unique feature: flexibility. Chemicals can be moved, stored, and distributed easily, with many of them having a mature market already standing for over decades. Ammonia has been recently presented as a zero-carbon molecule that can provide the required energy storage medium for renewable sources. It can be stored under easy conditions (i.e., refrigerated at −33 °C at atmospheric pressure or at 0.8−1.0 MPa under atmospheric temperature), thus making it a versatile, easy to store medium. Moreover, liquid ammonia has a greater volumetric hydrogen density than liquid hydrogen itself (i.e., liquid hydrogen at 20 K has approximately 70 kg of H2/m3, while liquid ammonia at 300 K and 1.0 MPa has 106 kg of H2/m3), so that the immediate implementation of an “ammonia economy” can support the futuristic “hydrogen economy”. In this work we present the simulation of a plant for the exploitation of renewable hydrogen with production of renewable ammonia as hydrogen vector and energy storage medium The simulation and sizing of all unit operations were performed with Aspen Plus® as software. Vegetable biomass is used as raw material for hydrogen production, more specifically pine sawdust. The hydrogen production process is based on a gasification reactor at high temperature (700-800 °C), in the presence of a gasifying agent such as air or steam. At the outlet, a solid residue (ash) and a gas, which mainly contains H2, CH4, CO and some impurities (e.g. sulphur or chlorine compounds) are obtained. After the removal of the sulphur compounds through an absorption column with MEA (to avoid poisononing of the catalytic processes), 3 reactors are arranged in series: Methane Steam Reforming (MSR), High temperature Water-Gas Shift (HT-WGS), Low temperature Water-Gas Shift (LT-WGS). All the oxygenated compounds must be carefully eliminated: the remaining traces of CO are methanated while CO2 is removed by a basic scrubbing with MEA (35 wt%) inside an absorption column. The Haber-Bosch synthesis of ammonia was carried out at 200 bar and in a temperature range between 300 and 400 °C, using two catalysts: Fe (wustite) and Ru/C. In conclusion, from an hourly flow rate of 1000 kg of dry biomass and 600 kg of nitrogen, 550 kg of NH3 98.8 wt% were obtained, demonstrating the proof of concept of this newly designed process for the production of hydrogen from renewable waste biomass and its transformation into a liquid hydrogen vector to be easily transported and stored. The design of a small scale ammonia cracker is also added to demonstrate an integrated process for H2 centralised production and storage and supply to delocalized units.