Simultaneous purifying of Hg0, SO2, and NOx from flue gas by Fe3+/H2O2: the performance and purifying mechanism

The removal of NO from flue gas by NaOH-catalyzed H2O2 system: Mechanism exploration and primary experiment

Currently, most advanced oxidation denitrification technologies require long flue gas residence time to obtain ideal NO removal efficiency. The NaOH-catalyzed H₂O₂ system proposed in this paper can obtain 98% NO removal efficiency under the condition of flue gas residence time of 3 s. The mechanism of NO removal and H₂O₂ decomposition to O₂ were proposed. It was confirmed with ESR (Electron-spin-resonance), inhibitor experiments and UV-Vis spectrophotometer that the main group in the reaction process was·O₂^- radicals, which reacted with NO to form ONOO^-, and ONOO^- would be gradually transformed into NO₃^- and NO₂^- in the air. The effect of some primary factors on the NO removal efficiency and the percentage of H₂O₂ decomposition to O₂ were also investigated. The increase of initial pH has a positive effect on NO removal, while the promotion of NO removal by increasing H₂O₂ concentration and reaction temperature is limited and the increase of NO has a negative effect on NO removal. Initial pH has a dual impact on the percentage of H₂O₂ decomposition to O₂, H₂O₂ concentration and reaction temperature promote the decomposition of H₂O₂ to O₂, while NO concentration has an inhibiting effect on it.

Effect of doping Cr on NH3 adsorption and NO oxidation over the FexOy/AC surface: A DFT-D study

Activated carbon supported iron-based catalysts (Fe_xO_y/AC) show good deNO_x efficiency at low temperature. The doping of chromium (Cr) greatly improves the catalyst activity. However, the detailed effect of doping Cr over Fe_xO_y/AC surface at molecular level is still a grey area. In this study, the roles of Cr dopant on gas adsorption and NO oxidation were deeply investigated by a DFT-D3 method. Results show that the synergy of Cr-Fe bimetal improves the binding capacity of Fe₂O₃/AC and Fe₃O₄/AC surfaces after doping Cr. NH₃ can be adsorbed on Cr and Fe sites to form coordinated NH₃. Doping Cr greatly improves the NH₃ adsorption property on the Fe₃O₄/AC surface. NO molecule can combine with Cr, Fe, and O sites to form nitrosyl and nitrite. The doping of Cr increases the adsorption performance of NO on the Fe₂O₃/AC and Fe₃O₄/AC surfaces, especially for Fe₃O₄/AC surface. Furthermore, NO can be oxidized to NO₂ by adsorption oxygen or active O sites of Fe_xO_y clusters. The doping of Cr restrains the formation of insoluble chelating bidentate nitrates and greatly reduces the reaction energy barrier of NO oxidation on the Fe_xO_y/AC surface, which can promote the deNO_x reaction.

Enhanced oxidative removal of NO by UV/in situ Fenton: Factors, kinetics and simulation

A series of experiments on the oxidative removal of NO from flue gas using a novel in situ Fenton (IF) system was performed in the presence of ultraviolet light (UV). The comparison tests revealed that the in situ Fenton system facilitated by UV (UV/IF) has a better oxidation ability of NO than that of the IF system due to the photochemical effect on the generation of oxidative species like (OH). Both of the aforementioned oxidation efficiencies were higher than that of the conventional Fenton system (CF) depending on the premix of Fe²⁺ and H₂O₂ solutions, which attribute to the improvement of (OH) yield and valid utilization with continuous addition of fresh reagents and UV radiation. In follow-up experiments, the effects of UV power, gas flow rate, reagent temperature, Fe²⁺/H₂O₂ molar ratio, initial pH, initial concentration of NO and SO₂ and volume fraction O₂ and CO₂ on the oxidative removal of NO by UV/IF method were investigated respectively. Moreover, the results of kinetic analysis indicated that NO oxidation was confirmed to have a pseudo-first-order kinetics pattern. The rate constants decreased slightly with increasing liquid temperature, and then the apparent activation energy of NO oxidation reactions in the UV/IF system was calculated as -5.62 kJ/mol by the Arrhenius equation. Furthermore, the reaction mechanism and application prospects concerning NO oxidative removal by using the UV/IF system was speculated in brief. Finally, the computational fluid dynamics (CFD) simulations revealed that the improvement of axial and radial gas hold-up would enhance the gas-liquid contact and accelerate the oxidation reactions on the interface. In addition to reasonable control of process parameters, the optimization of reactor interior structure needs to be carried out via CFD simulation and experimental validation in future research, both are favourable to promote the NO oxidation efficiency and large-scale development of this technology.

Recent advances in hybrid wet scrubbing techniques for NOx and SO2 removal: State of the art and future research

Recently, the discharge of flue gas has become a global issue due to the rapid development in industrial and anthropogenic activities. Various dry and wet treatment approaches including conventional and hybrid hybrid wet scrubbing have been employing to combat against these toxic exhaust emissions. However, certain issues i.e., large energy consumption, generation of secondary pollutants, low regeneration of scrubbing liquid and high efficieny are hindering their practical applications on industrial level. Despite this, the hybrid wet scrubbing technique (advanced oxidation, ionic-liquids and solid engineered interface hybrid materials based techniques) is gaining great attention because of its low installation costs, simultaneous removal of multi-air pollutants and low energy requirements. However, the lack of understanding about the basic principles and fundamental requirements are great hurdles for its commercial scale application, which is aim of this review article. This review article highlights the recent developments, minimization of GHG, sustainable improvements for the regeneration of used catalyst via green and electron rich donors. It explains, various hybrid wet scrubbing techniques can perform well under mild condition with possible improvements such as development of stable, heterogeneous catalysts, fast and in-situ regeneration for large scale applications. Finally, it discussed recovery of resources i.e., N₂O, NH₃ and N₂, the key challenges about several competitive side products and loss of catalytic activity over time to treat toxic gases via feasible solutions by hybrid wet scrubbing techniques.

Simultaneous removal of NOx and SO2 from simulated marine ship flue gas in a novel wet scrubbing system based on divided diaphragm seawater electrolysis technology: efficiency optimization and economic assessment

This work constructed a divided diaphragm seawater electrolysis system with two tandem packed towers for the synergistic removal of NO_x and SO₂. The first tower was mainly used to oxidize NO and SO₂ by AC (active chlorine), and the second tower was used to further absorb NO_x. The factors affecting on NO removal, including ACC (active chlorine concentration), pH value, initial NO concentration and temperature in the oxidation tower were investigated. Moreover, the effect of different inlet gas concentrations and current values were explored. The results showed that with the increase of ACC, the NO and NO_x removal efficiency increased rapidly, but when the ACC was higher than 500 mg/L [Cl₂], the removal efficiency did not increase further in the oxidation tower. Low pH values in the oxidation tower were favorable for NO removal. NO removal efficiency reached a maximum at 40 °C. Higher NO and SO₂ concentrations were favorable for NO removal. The decline of pH in the anode cell was not conducive to the storage of AC in the continuous electrolysis removal process. NO_x and SO₂ were almost completely removed after being scrubbed in the oxidation and absorption towers. The relationship between current and removal efficiency of NO and SO₂ in the oxidation tower was also analyzed. Finally, the removal mechanism and the application prospects were discussed.

Effects of system parameters and residual ions on the oxidation removal of NO by Fenton method

In the present work, the effects of relevant system parameters on the oxidation removal of NO using the Fenton method were discussed in detail. Moreover, the impacts of ions remaining in the coal-fired process on the NO oxidation efficiency were investigated specifically. The experimental results showed that the oxidation efficiency of NO decreased with the increase of gas flow rate, reagent temperature, and CO₂ volume fraction in the evaluated range, while it increased first and then decreased with the increase of gas temperature, NO initial concentration, O₂ volume fraction, initial pH of reagent, and Fe²⁺/H₂O₂ molar ratio. In addition, the corresponding impact mechanism of the system parameters was discussed respectively. Although the SO₂ showed a competitive effect on the utilization of oxidative radicals, the Fenton system also showed an ability for simultaneous removal of NO and SO₂. Furthermore, the results indicated that the NO oxidation efficiency would be influenced by the residual ions, such as Ca²⁺, Mg²⁺, Na⁺, SO₄^2-, and Cl^-. The presence of the mentioned ions showed an inhibiting effect on the oxidation removal of NO in the first few minutes, while the NO oxidation efficiency would be enhanced in the bulk stage of the tests. The positive effect trended to be more obvious with the decline of the ion dosage. Subsequently, the influence mechanism of the aforesaid residual cations and anions was supposed and proposed preliminarily.

Removal of SO2 and NOx from flue gas using mud-phosphorus slurry

In this study, the mud-phosphorus slurry was used to simultaneously remove SO₂ and NO_x. The technology proposed new avenues for the purification and utilization of remove SO₂ and NO_x in flue gas. The effects of reaction temperature, solid-liquid ratio, and oxygen content on the efficiency of desulfurization and denitrification were studied experimentally. Results show that the parameters were solid-liquid ratio of 5.0 g/40 mL, T = 60 °C, φ (O₂) = 20%, Q = 300 mL/min under the best experimental conditions. The maximum amount of ozone generated was 563.8 mg/m³. The reaction time with desulfurization rate ≥ 99% was 340 min; the reaction time with denitrification rate ≥ 99% was 160 min. Response surface analysis method was used to perform a three-factor three-level response surface experiment. Results show that the oxygen content had a highly significant effect on the desulfurization and denitrification efficiency, and the relationship between the desulfurization and denitrification efficiency was oxygen content > mud-phosphorus slurry liquid-solid ratio > reaction temperature. The process is simple, the solid waste is used to treat the flue gas, and the removal effect is good, which is convenient for popularization.

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.

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

To remove NO and SO₂ from flue gas simultaneously, a heterogeneous catalytic oxidation system was developed with the nanoscale zero-valent iron (nZVI), vaporized H₂O₂, and sodium humate (HA-Na) acting as the catalyst, oxidant, and absorbent, respectively. The experimental results indicated that the desulfurization was mainly influenced by the absorption, and the denitrification was significantly affected by the catalytic oxidation parameters. Under the optimal conditions, the simultaneous removal efficiencies of SO₂ and NO were 100 and 88.4%, respectively. The presence of ^·OH during the removal process was proved by the scavenger tests, and the production of ^·OH with and without nZVI was indirectly evaluated by the electron paramagnetic resonance (EPR) and methylene blue experiments. Moreover, the fresh and aged nZVI were characterized by a series of techniques and the results suggested that the redox pair Fe²⁺/Fe³⁺ released by nZVI could react with H₂O₂ to provide the sustainable ^·OH, which was important for the oxidation from NO and SO₂ to NO₃^- and SO₄^2-. The removal mechanism was proposed preliminarily based on the correlative experiments, characterizations, and references.