The nitrogen oxides recovery as a new approach promoting the nitrogen circular economy
Humans are massively perturbing the global nitrogen cycle, leading to excess reactive nitrogen (Nr) in the environment. As a key Nr species, NOx mainly derived from combustion processes. The commonly employed post-combustion methods for NOx treatment are selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). Their basic principles are converting NO to harmless N2 using NH3 as reducing agent. Recently NOx recovery (ReNOx) is being developed as an alternative. With a different philosophy, NH3 was not used as a reducing agent, but generated as a byproduct. To achieve true environmental improvement, for the first time we conducted a life cycle assessment of ReNOx in comparison with traditional methods, SNCR and SCR.
The environmental impacts of ReNOx were compared with traditional methods at impact, damage, and eco-index levels as shown in Fig. 1. At the impact level, ReNOx had minimum values in acidification, urban area air pollution, and eutrophication compared with SNCR and SCR. At the damage level, ReNOx showed minimum values with respect to human health, social assets, and primary production. At the eco-index level, ReNOx ranked first, followed by SCR and SNCR. Scenario analysis for ReNOx was conducted to investigate the effect of key variables. Compared with the eco-index (the less, the better) in the condition of 200°C with a conversion rate of 95%, it would increase substantially in the condition of 160°C with a conversion rate of 80% and in the case without a sound NH3 treatment. Feedstock format change, adsorption material performance deterioration, and recovery rate decline would increase the eco-index by 8%, 12%, and 18%, respectively. The eco-index was decreased by 31% with a renewable energy source and an increased conversion rate.
Figure 1. The environmental impacts of ReNOx compared with traditional methods, SNCR and SCR.
This study provides a better understanding of the environmental characteristics of ReNOx technology, and its relative profile compared with traditional technologies. It demonstrated the usefulness of life cycle assessment for new industrial technology evaluation which features in impact, damage, and eco-index level analysis. Critical processes can be identified, which serves as a reference to improve the technology for its developers from the environmental perspective promoting its social implementation towards sustainability. It can be an important reference for other new industrial technologies preliminary assessment.
※ Xue, M., Lin, B-L., Tsunemi, K., Minami, K., Nanba, T., Kawamoto, T. Sustainability, 2021, 13 (14), 7826.