Sustainable three-stage chemical looping ammonia production (3CLAP) process

In the present paper, a new CO2-neutral thermodynamic process is developed based on a chemical looping principle that circulates nitrogen carrier between reactors for producing ammonia and/or hydrogen via a nitrogen fixation reaction. This study advances the knowledge on selecting plausible nitrogen carrier candidates from a wide range of metals including rare and transient elements. The proposed three-stage chemical looping ammonia production (3CLAP) system consists of two exothermic reactors: nitridation and ammoniation reactors to drive the nitrogen fixation reactions, and an endothermic thermal plasma reactor to handle dissociation of metal oxides back into a pure metal that is re-circulated in the process. The heat released in the nitridation and ammoniation reactors is recovered to generate electricity, which partially meets the energy demand of the thermal plasma unit. Several key criteria were used in the thermochemical analysis for selecting suitable metals for the 3CLAP process; the spontaneity of the reactions, melting temperature of the metals, availability of metals, energetic performance of heat recovery unit, and the nitrogen and steam economy. It was identified that chrome (Cr), magnesium (Mg), aluminium (Al), calcium (Ca), and manganese (Mn) were the best nitrogen carrier metals amongst all candidates. Chrome yielded the highest nitrogen economy of molar ratio (NH3/N2) of ~ 1.97 and the highest steam economy (NH3/H2O) of ~ 0.98. However, the lowest performance of nitrogen and steam economy was for Mn of ~0.96, and ~0.3, respectively. With chrome, the power block efficiency was ~ 32.3%, which is a competitive value to the current state-of-the-art efficiencies reported in the literature. The highest self-sustaining factor (SSF) was ~ 0.33, meaning that one-third of the total energy demand of thermal plasma reactor could be delivered via a heat recovery system.