The importance of nitrogen fixation is evident in every aspect of a human being’s life, from the synthesis of vital for all organisms nutrients and, in turn, the ecosystem conservation to the production of fertilizers, plastics, and many other daily usage products. However, increasing concerns about the environmental sustainability of contemporary chemical industry seem to impose, nowadays, great challenges to the industrial nitrogen fixation which is linked to immense energy consumption and burdened emissions profile. Upon these considerations, it becomes imperative to adopt a holistic approach toward the development of novel “green” process technologies for the synthesis of fixed nitrogen. A considerable effort to that direction has been made by means of plasma technology mainly at the laboratory scale. Although research studies have shown promising results, little attention has been placed on conceptualizing plasma-assisted nitrogen fixation at an industrial scale and evaluating its environmental footprint. This issue is practically addressed in the present research work which focuses on the ex-ante process design of plasma-assisted nitric acid synthesis for a modular plant and the respective life cycle assessment (LCA) incorporating renewable energy sources. In order to facilitate the analysis of the LCA study, a sensitivity analysis has been considered on the reaction yield, the plasma power consumption, the recycle of unreacted gas stream, and the energy recovery in the plasma reactor. LCA results exhibit for the plasma-assisted nitric acid, incorporating the recycle of the tail gas and solar energy, an improvement in the global warming potential of 19% as compared to the conventional production pathway.
Abstract:The expected world population growth by 2050 is likely to pose great challenges in the global food demand and, in turn, in the fertilizer consumption. The Food and Agricultural Organization of the United Nations has forecasted that 46% of this projected growth will be attributed to Africa. This, in turn, raises further concerns about the sustainability of Africa's contemporary fertilizer production, considering also its high dependence on fertilizer imports. Based on these facts, a novel "green" route for the synthesis of fertilizers has been considered in the context of the African agriculture by means of plasma technology. More precisely, a techno-economic feasibility study has been conducted for a small-scale plasma-assisted nitric acid plant located in Kenya and South Africa with respect to the electricity provision by renewable energy sources. In this study, standalone solar and wind power systems, as well as a hybrid system, have been assessed for two different electricity loads against certain economic criteria. The relevant simulations have been carried out in HOMER software and the optimized configurations of each examined renewable power system are presented in this study.
An eco-efficiency analysis has been conducted, as a sustainability performance indicator, by combining the life cycle costs (LCC) and the environmental impacts of diverse plasma-assisted ammonia and nitric acid synthesis routes, for which a detailed process design for small-scale production has been previously reported. The proposed design of the specific plasma processes involves new upstream and downstream activities, which are independent of conventional natural resources and comprise less equipment. In the context of this study, the impact of the product yield and plasma power consumption on the eco-efficiency profiles of the selected plasma processes is evaluated and benchmarked against that of the established synthesis pathways. Results show a relatively improved environmental profile of the plasma-assisted NH3 (5% NH3 yield), considering a power consumption of 17.2 g NH3 kWh−1 and energy recovery of 5%, against that of the contemporary production route. In the case of the plasma-assisted HNO3 (6% NO yield) synthesis, incorporating a power consumption of 7.77 kWh kg−1 NO and a 20% energy recovery, a better ecological footprint is displayed as compared to the conventional chemical process. Both plasma processes are characterized by higher LCC than the conventional ones, with the plasma-assisted nitric acid displaying a more competitive LCC profile. A clear contribution of the utilities (upstream and downstream equipment) to both the environmental and cost benefits is shown, and the plasma plant is the enabler of such integration. The contribution is related to both the number reduction of equipment (process simplification) and improved operation (process intensification). Given the outcomes of this study, the concept of developing modular plants incorporating the plasma technology and renewable energy sources—e.g. wind power—for synthesizing ammonia and nitric acid demonstrates promising potential and promotes a new window of opportunities for future sustainable decentralized fertilizer production; such as distributed production at the farm site, with the opportunity to react immediately to weather changes and to local conditions (soil, climate, crops, farming business model).
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