Simultaneous removal of NO and SO2 from flue gas using vaporized H2O2 catalyzed by nanoscale zero-valent iron

Synergistic removal of particles, SO2, and NO2 in desulfurized flue gas during condensation

The synergistic removal of multi-pollutants, including particles, SO₂, and NO₂, is a key concern in the process of flue gas purification, during which the supersaturated environment is an essential premise for the nucleation and deep reduction of particles. The condensation of desulfurized flue gas using heat exchangers can not only recover condensed water and latent heat but also create supersaturated environment to promote the flue gas purification. In this study, an experimental system for desulfurized flue gas condensation is established. The effect and associated mechanism of condensation process on the removal of multi-pollutions are clarified. The results show that particles with an aerodynamic diameter larger than 2.5 μm accounts for 50% in mass proportion. The flue gas temperature drop has positive influence to the increase of the ideal supersaturation degree, which is beneficial for the removal of particles (especially when the aerodynamic diameter is less than 1 μm), SO₂, and NO₂. The ideal supersaturation degree slightly reduces with the rise of inlet flue gas temperature, which can promote the removal efficiency of small particles, while weaken that of large particles, SO₂, and NO₂. Caused by the increase of flue gas flow rate, the nucleation process weakens, reducing the removal efficiency of all pollutants (particles, 45.2-28.3%; SO₂, 27.5-14.5%; NO₂, 21.5-15%). On the whole, the increase of the ideal supersaturation degree contributes to the synergistic removal of pollutants especially particles with smaller radius in the flue gas. The reduction of particles with aerodynamic diameter less than 1 μm is conductive to the synergistic removal of SO₂ and NO₂.

Simultaneous removal of SO2 and NOx with OH from the catalytic decomposition of H2O2 over Fe-Mo mixed oxides

In this paper, the simultaneous removal of SO₂ and NO_x catalyzed by Fe-Mo mixed oxides at varying Mo/Fe atomic ratios was reported for the first time with the aim of reducing H₂O₂ consumption and elucidating the roles of Fe and Mo species in the catalytic process. Fe-Mo mixed oxides with varying Mo/Fe atomic ratios were synthesized and the catalytic performances were systematically studied. The catalyst with Mo/Fe atomic ratio of 2.0 exhibited the highest activity, with which removal efficiencies of 89.4 % for NO_x and 100 % for SO₂ can be attained at extremely low H₂O₂ dosage. Products analysis revealed that SO₂ was mainly removed via wet scrubber, while the adequate oxidation resulting from OH radicals was the prerequisite for NO_x removal. The redox pair of Fe²⁺/Fe³⁺ played a significant role in decomposing H₂O₂, while Mo species had double effect on catalytic activity. Higher Mo content resulted in abundant oxygen vacancies and stronger surface acidity, which favored OH formation. However, the excessive Mo content involved severe surface Mo enrichment and remarkably reduced the active sites of Fe species. The H₂O₂/Fe-Mo catalyst system showed excellent stability and had a promising prospect for simultaneously removing SO₂ and NO_x in coal-fired flue gas.

Research on NO and SO2 removal using TiO2-supported iron catalyst with vaporized H2O2 in a catalytic oxidation combined with absorption process

Simultaneous removal of NOx and SO₂ is carried out by an oxidation-absorption process, which NO oxidized by active hydroxyl radicals (·OH) derived from catalytic decomposition of vaporized H₂O₂ over Fe₃O₄/TiO₂ and then adsorbed by NaOH solution along with SO₂. Fe₃O₄/TiO₂ synthesized by wet impregnation method with an additional reduction under H₂ atmosphere was characterized by XRD, FTIR, BET, XPS, and VSM analysis. Effects of H₂O₂ concentration, H₂O₂ injection rate, reaction temperature, gas flow rate, and flue gas component on simultaneous removal were investigated. The experimental results show that NO can be effectively oxidized by highly reactive ·OH radicals generated from H₂O₂ decomposition over Fe₃O₄/TiO₂ catalyst, and removal efficiencies of 93.31% for NO, 85.90% for NOx, and 100% for SO₂ were obtained. The surface zero-valent iron (Fe⁰) and divalent iron (Fe²⁺) are the key factors of the catalytic oxidation with hydroxyl radical. H₂O₂ adsorption and dissociation mechanism on catalyst surface was studied using DFT calculation. The calculation results demonstrate that H₂O₂ prefers to dissociate on iron containing surface, and ·OH radicals generation follow by Haber-Weiss (H-W) mechanism. The stable oxidative product of HNO₂ and HNO₃ were generated through NO/NO₂ and H₂O₂ co-adsorption on the FeO/TiO₂ (0 0 1) surface.

Radical-induced oxidation removal of multi-air-pollutant: A critical review

Sulfur dioxide (SO₂), nitric oxide (NO) and elemental mercury (Hg⁰) are three common air pollutants in flue gas. SO₂ and NO are the main precursors for chemical smog and Hg⁰ is a bio-toxicant for human. Cooperative removal of multi-air-pollutant in flue gas using radical-induced oxidation reaction is considered as one of the most promising methods due to the high removal efficiency, low cost and less secondary environmental impact. The common radicals used in air pollution control can be classified into four types: (1) hydroxyl radical (OH), (2) sulfate radical (SO₄^-), (3) chlorine-containing radicals (Cl, ClO₂, ClO, HOCl^-, etc.) and (4) ozone. This review summarizes the generation methods and mechanism of the four kinds of radicals, as well as their applications in the removal of multi-air-pollutant in flue gas. The reactivity, selectivity and reaction mechanism of the four kinds of radicals in multi-air-pollutant removal were comprehensively described. Finally, some future research suggestions on the development of new technique for cooperative removal of multi-air-pollutant in flue gas were provided.

Simultaneous removal of nitrogen oxides and sulfur dioxide using ultrasonically atomized hydrogen peroxide

A new method was developed for denitrification and desulfurization using hydrogen peroxide with the aid of an ultrasonic nebulizer to obtain high removal efficiency of NOx and SO₂. Comparing with the atomizing nozzles having the aperture size of 0.01~0.02 mm, the droplets generated using the ultrasonic nebulizer show the smallest d₅₀ value of 7.2 μm, with 72% possessing the size less than 10 μm. Based on the numerical simulation of the vaporization rate of droplets, it is indicated that the droplets with the size of 7.2 μm can be vaporized totally at very short residence time (0.11 s) under 130 °C. Effects of influence factors including the reaction temperature, the initial H₂O₂ concentration, pH value, and the flue gas flow rate were studied on the removal efficiencies of NO and SO₂. Using the in-series double-oxidation subsystems with H₂O₂ concentration of 6 wt%, pH 5.0, and the reaction temperature of 130 °C, the removal efficiencies of SO₂ and NO are respectively 100% and 89.3% at the short residence time of 1.8 s, and the removal efficiency of NO can be increased to 100% as the residence time is longer than 3.7 s. It is confirmed that the ultrasonically atomized H₂O₂ can indeed enhance the removal efficiencies of NO and SO₂ at the optimal temperature, owing to the fast vaporization rate of fine droplets as well as the formation of more active radicals to be captured by NO and SO₂ simultaneously. The results here provide a promising route to remove effectively the emissions of NO and SO₂ simultaneously. Graphical abstract.

Photonic/magnetic hyperthermia-synergistic nanocatalytic cancer therapy enabled by zero-valence iron nanocatalysts

Traditional cancer-therapeutic modalities such as chemotherapy suffer from the low therapeutic efficiency and severe side effects. The emerging nanocatalytic therapy could in-situ catalyze the endogenous substances into highly toxic species and then efficiently kill the cancer cells, but the lack of high-performance nanocatalysts hinders their broad clinical translation. In this work, we have successfully developed, for the first time, nanosized zero-valence crystalized iron nanoparticles for in-situ triggering nanocatalytic Fenton reaction within tumor microenvironment to produce large amounts of hydroxyl radicals and subsequently kill the cancer cells, which could be further synergistically enhanced by either photonic hyperthermia or magnetic hyperthermia as assisted by these iron nanoparticles acting as photothermal-conversion or magnetothermal-conversion nanoagents, respectively. Especially, the excellent magnetic performance of these zero-valence crystallized iron nanoparticles has achieved both in vitro and in vivo contrast-enhance magnetic resonance imaging for potentially guiding the photonic/magnetic hyperthermia-synergistic nanocatalytic cancer therapy. This work not only provides the new type of iron-based nanoparticles for biomedical application, but also demonstrates the high efficiency of nanocatalytic cancer therapy as assisted by both photonic and magnetic hyperthermia.