Abstract
Recent advances in technologies for the decentralized, islanded ammonia economy are reviewed, with an emphasis on feasibility for long-term practical implementation. The emphasis in this review is on storage systems in the size range of 1–10 MW. Alternatives for hydrogen production, nitrogen production, ammonia synthesis, ammonia separation, ammonia storage, and ammonia combustion are compared and evaluated. A conceptual process design, based on the optimization of temperature and pressure levels of existing and recently proposed technologies, is presented for an islanded ammonia energy system. This process design consists of wind turbines and solar panels for electricity generation, a battery for short-term energy storage, an electrolyzer for hydrogen production, a pressure swing adsorption unit for nitrogen production, a novel ruthenium-based catalyst for ammonia synthesis, a supported metal halide for ammonia separation and storage, and an ammonia fueled, proton-conducting solid oxide fuel cell for electricity generation. In a generic location in northern Europe, it is possible to operate the islanded energy system at a round-trip efficiency of 61% and at a cost of about 0.30–0.35 € kWh−1.
Highlights
Poisoning Iron-based catalysts have been the preferred alternative for conventional steam methane reforming (SMR)-based ammonia synthesis plants, in which methane is present in the ammonia synthesis loop, while oxygen-containing compounds are completely removed [27]
Technology pushes within the process design are the development of new materials (the battolyser for a combined battery function and hydrogen generation, the Ru/Ba-Ca(NH2)2 catalyst for ammonia synthesis at relatively low temperature, and the CaCl2/SiO2 absorbent for ammonia separation and storage), enabling the decentralized production of ammonia from renewables at a relatively low temperature and pressure (275 °C, 8 bar) [58,76,105,125]
Current technological developments discussed in this paper may make ammonia synthesis at temperatures of 275 °C and pressures of 8 bar feasible
Summary
A fossil carbon-free, circular economy is required to decrease the greenhouse gases emissions [1] This can be accomplished by renewable sources, such as wind power and solar power. For long-term chemical energy storage (i.e., above 1 day), ammonia is more economically stored than hydrogen [10]. Various chemical storage alternatives are discussed in the supplementary information Ammonia can both be used to store energy in time (for islanded systems) and in space (transportation from places with abundant wind hours or solar hours to other places) [13]. Various alternatives for renewable electricity generation are available, but in the current case only wind power and solar power are considered due to the location considered. The energy requirement for producing 0.66 m3 (STP) nitrogen and 1.97 m3 (STP) hydrogen from 1 kg ammonia is 2.7 MJ (equivalent to 0.75 kWh kg−1 ammonia, excluding preheating and evaporation) [27]. [21]
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