Simultaneous removal of multi-pollutants from flue gas by a vaporized composite absorbent

The effect of UV365/Fenton process on the removal of gaseous ethylbenzene in a bubble column reactor

As volatile organic compounds (VOCs), gaseous ethylbenzene has adverse effects on human health and ecology. Therefore, an effective degradation process is highly desirable. The Fenton process under UV 365 nm was selected as the first option to remove gaseous ethylbenzene in a bubble column reactor. The main parameters for the batch experiments were systematically studied, including H2O2 concentration, [H2O2]/[Fe2+], pH, UV wavelength, UV intensity, gaseous ethylbenzene concentration, gas flow rate, and process stability towards removal efficiency. The optimum conditions were found to be H2O2 concentration of 100 mmol·L-1, [H2O2]/[Fe2+] of 4, pH of 3.0, UV wavelength of 365 nm, UV power of 5 W, gas flow rate of 900 mL·min-1, and gaseous ethylbenzene concentration of 30 ppm, resulting in a removal efficiency of 76.3%. The study found that the Fenton process, when coupled with UV 365 nm, was highly effective in removing gaseous ethylbenzene. The degradation mechanism of gaseous ethylbenzene was proposed in the UV365/Fenton process based on EPR, radical quenching experiments, iron analysis, carbon balance, and GC-MS analysis. The results indicated that •OH played a crucial role in the process.

Oxidation-absorption process for simultaneous removal of NOx and SO2 over Fe/Al2O3@SiO2 using vaporized H2O2

3% Fe/Al₂O₃ and 3% Fe/Al₂O₃@SiO₂ were prepared to investigate the performance in simultaneous removal of NO_x and SO₂ using vaporized H₂O₂. Certain paraments were changed to explore the activity of catalysts, including temperature, H₂O₂ concentration, GHSV and coexistence gases component. A 24-h durability test was conducted on 3% Fe/Al₂O₃@SiO₂. Moreover, a series of characterizations were employed to analyze the physical and chemical properties of catalysts, including XRD, BET, SEM, TEM, FTIR and XPS. Compared with 3% Fe/Al₂O₃, 3% Fe/Al₂O₃@SiO₂ exhibited more excellent catalytic activity, which could achieve the peak removal efficiency of 100% for SO₂ and 93.76% for NO_x. Moreover, 3% Fe/Al₂O₃@SiO₂ kept stable simultaneous removal efficiency in a 24-h test. The characterization results indicated that the BET area was greatly improved and the core-shell structure was synthesized with the formation of more micropores and mesopores by the coating of SiO₂, which could improve the activity of catalyst at high temperature and high SO₂ concentration. Besides, the mechanism of SO₂ molecules on simultaneous removal was investigated. On one hand, a part of H₂O₂ was consumed by SO₂ molecules without catalyst, which resulted in the drop of NO_x removal by the decrease of oxidants. The main products were sulfites and bisulfites, which were broken down into SO₂ over the catalyst. On the other hand, the presence of SO₂ was beneficial for NO_x removal by increasing oxygen vacancies on the catalyst surface and facilitating the absorption of NO₂ by NaOH solution.

The boosting of microwave roasting technology on the desulfurization of phosphate rock

A green and-easy to operate method, the microwave technology, was developed to promote the desulfurization process of phosphate rock, systematically investigates the strengthening effect of microwave, and uses XRD, BET, SEM, XRF, ICP, and EDS to characterize the reactants. The results show that the main reason for the desulfurization efficiency is improved by microwave heating under microwave conditions, different thermal stress phosphate rock materials lead to the destruction of each microstructure, and a specific surface area increased 40.25% phosphate rock. In addition, after microwave irradiation, the pore size of the phosphate rock at 2-5 nm is significantly increased, and the number of mesopores is significantly increased, thereby increasing the desulfurization efficiency of the phosphate rock. By investigating the effects of temperature, oxygen content, flow rate, and solid-liquid ratio on desulfurization efficiency, the paper concludes that the optimal conditions for desulfurization of phosphate rock after microwave irradiation are C(SO₂) is 2500 mg·m^-3, temperature is 40 °C, φ(O₂) is 5%, solid-liquid ratio is 3.5 g:200 ml, and flue gas flow is 500 ml·min^-1.

Biological treatments of mercury and nitrogen oxides in flue gas: biochemical foundations, technological potentials, and recent advances

Nitrogen oxides (NO_x) and mercury (Hg) are commonly found coexistent pollutants in combustion flue gas. Ever-increasing emission of atmospheric Hg and NO_x has caused considerable environmental risks. Traditional flue gas demercuration and denitration techniques have many socioeconomic, technological and environmental drawbacks. Biotechnologies can be a promising and prospective alternative strategy. This article discusses theoretical foundation (biochemistry and genomic basis) and technical potentials (Hg⁰ bio-oxidation coupled to denitrification) of bioremoval of Hg and NO_x in flue gas and summarized recent experimental and technological advances. Finally, several specific technical perspectives have been put forward to better guide future researches.

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.

Cost analysis of environmental protection price of coal-fired plants in China

In the paper, the achievements obtained from carrying out the policy of environmental protection price for promoting air pollution control in coal-fired power plants in China during more than a decade were summarized. Based on the situation of current electricity market reform, the role and effectiveness of environmental protection price for controlling the normal air pollutants, such as sulfur dioxide, nitrogen oxides, and dust in China's coal-fired power generation plants, were investigated, including the price structure of electricity environmental protection for coal-fired power generation enterprises in different regions, generating units, and power demands. The policy suggestions were proposed, namely, the reform of electricity environmental protection price would be carried out gradually, the relationship between electricity environmental protection price policy and other environmental protection policies would be matched under the relative overcapacity condition, and the environmental protection price regulation would be integrated into other environmental policies.

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.

A novel catalytic oxidation process for removing elemental mercury by using diperiodatoargentate(III) in the catalysis of trace ruthenium(III)

A series of experiments were conducted at a bench scale reactor to investigate the effects of key influencing factors on the Hg⁰ removal from flue gas using the prepared diperiodatoargentate (III) (DPA) as an oxidant, trace ruthenium(III) as a catalyst, respectively. The experimental results showed that the average Hg⁰ removal efficiency reached to 87.5% under the optimal conditions in which the DPA concentration was 1.03 mmol/L, catalyst concentration was 2.0 μmol/L, reaction temperature was 40 °C and solution pH was 8.5. Meanwhile, it was found from the experiments that the high concentrations of SO₂ and NO could inhibit the Hg⁰ removal due to the competition between Hg⁰ and SO₂/NO, while the lower NO concentration exhibited a slight promotion for Hg⁰ removal. The evolutions of DPA(III) and Ru(III) before and after the reaction were characterized by an ultraviolet visible spectrophotometer (UV-vis), from which, the promotional mechanism of Ru(III) on Hg⁰ removal was analyzed. The spent solution was analyzed by a cold vapor atomic fluorescence spectrometer (CVAFS), which verified that Hg⁰ was oxidized into Hg²⁺ by the catalytic system of DPA(III)-Ru(III), and DPA was converted into Ag⁺.

Efficient and stable catalyst of α-FeOOH for NO oxidation from coke oven flue gas by the catalytic decomposition of gaseous H2O2

A catalyst α-FeOOH possesses excellent catalytic activity for NO oxidation, and N₂O₅ is first found in NO oxidation products.

High adsorption activated calcium silicate enabling high-capacity adsorption for sulfur dioxide

Fly ash, with its abundant silicon sources and high porosity, is an excellent precursor of porous silica-based sorbents, which are the key to obtaining high SO₂ adsorption performance.

NO oxidation over Fe-based catalysts supported on montmorillonite K10, γ-alumina and ZSM-5 with gas-phase H2O2

The catalytic gas-phase H₂O₂ oxidation of NO was achieved over Fe-based catalysts supported on montmorillonite K10, γ-alumina and ZSM-5. ESR tests illustrate that the three catalysts can catalyze decomposition of H₂O₂ yielding highly reactive hydroxyl radicals, of which Fe/K10 has the fastest rate, followed by Fe/γ-alumina. Fe³⁺ in Fe/K10 and Fe/γ-alumina show lower density of electron cloud due to a strong interaction between Fe³⁺ and the support, which benefits the electron transfer from the H₂O₂ to Fe³⁺, thus favoring the production of hydroxyl radicals. Fe species exist on the surface of Fe/K10 mainly in the form of Fe₂O₃, whereas Fe species of Fe/γ-alumina and Fe/ZSM-5 exist mainly in the form of Fe₃O₄, and it is found that Fe₂O₃ is more active than Fe₃O₄ in catalytic gas-phase H₂O₂ oxidation of NO. Interestingly, Fe/ZSM-5 has the lowest efficiency in generating hydroxyl radicals, its NO removal efficiency is 90%, which is much higher than 47.5% for Fe/γ-alumina and 62.3% for Fe/K10. In-situ IR results suggested that Fe/ZSM-5 are dual functional in oxidation of NO, that is, whether both Fe ion sites and Brønsted acid sites collectively provide the catalytic functionality. In the meantime, a possible reaction mechanism on catalytic gas-phase H₂O₂ oxidation of NO over Brønsted acid sites is proposed.

Peroxymonosulfate catalyzed by rGO assisted CoFe2O4 catalyst for removing Hg0 from flue gas in heterogeneous system

The cobalt ferrite-reduced oxidized graphene (CoFe₂O₄/rGO) catalyst was synthesized by hydrothermal method and characterized by Powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscope (SEM), Brunauere Emmette Teller (BET) and Hysteresis loop. For developing a new method of removing elemental mercury (Hg⁰) from flue gas, the effects of catalyst dosage, PMS concentration, solution pH and reaction temperature on the removal efficiency were investigated experimentally by using peroxymonosulfate (PMS) catalyzed by CoFe₂O₄/rGO at a self-made bubbling reactor. The average removal efficiency of Hg⁰ in a 30-min period reached 95.56%, when CoFe₂O₄/rGO dosage was 0.288 g/L, PMS concentration was 3.5 mmol/L, solution pH was 5.5 and reaction temperature was 55 °C. Meanwhile, based on the free radical quenching experiments, in which, ethyl alcohol and tert butyl alcohol were used as quenchers to prove indirectly the presence of ^•OH and SO₄^•-, the characterizations of catalysts and reaction products, and the existing results from other scholars. The reaction mechanism was proposed.

Simultaneous mercury oxidation and NO reduction in a membrane biofilm reactor

This work demonstrates bacterial oxidation of mercury (Hg⁰) coupled to nitric oxide (NO) reduction in a denitrifying membrane biofilm reactor (MBfR). In 93 days' operation, Hg⁰ and NO removal efficiency attained 90.7% and 74.1%, respectively. Thauera, Pseudomonas, Paracoccus and Pannonibacter played dual roles as Hg⁰ oxidizers and denitrifiers simultaneously. Denitrifying bacteria and the potential mercury resistant bacteria dominated the bacterial community. Denitrification-related genes (norB, norC, norD, norE, norQ and norV) and enzymatic Hg⁰ oxidation-related genes (katG, katE) were responsible for bacterial oxidation of Hg⁰ and NO reduction, as shown by metagenomic sequencing. XPS, HPLC-ICP-MS and SEM-EDS indicated the formation of a stable mercuric species (Hg²⁺) reasulting from Hg⁰ oxidation in the biofilm. Bacterial oxidation of Hg⁰ was coupled to NO reduction in which Hg⁰ served as the initial electron donor while NO served as the terminal electron acceptor and thereby redox between Hg⁰ and NO was formed. MBfR was capable of both Hg⁰ bio-oxidation and denitrifying NO reduction. This research opens up new possibilities for application of MBfR to simultaneous flue gas demercuration and denitration.

Removal of multi-pollutant from flue gas utilizing ammonium persulfate solution catalyzed by Fe/ZSM-5

A nano-sized iron loaded ZSM-5 zeolite (Fe/ZSM-5) catalyst was firstly used to activate (NH₄)₂S₂O₈ solution for the simultaneous removal of multi-pollutant from flue gas. The simultaneous removal efficiencies 100% of SO₂, 72.6% of NO and 93.4% of Hg° were achieved under the condition that the catalyst dose was 0.8 g/L, concentration, pH and temperature of (NH₄)₂S₂O₈ solution were 0.03 mol/L, 5 and 65 °C, respectively. The stability of catalyst was checked by a continuous test, proving that the catalytic activity was maintained for 4 h and the leached iron reached low levels. Based on the catalyst characterizations, product analysis and literatures, the removal mechanism was speculated preliminarily, during which, OH and SO₄^- played key roles for oxidizing NO and Hg° into NO₃^- and Hg²⁺.

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.

Elemental mercury removal from flue gas by CoFe2O4 catalyzed peroxymonosulfate

A magnetic cobalt ferrite (CoFe₂O₄) catalyst was prepared by sol-gel method, and characterized by a X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Brunauer-Emmett-Teller (BET) and hysteresis loop method. The chemical states on surface of the fresh and spent catalysts were analyzed by a X-ray photoelectron spectroscopy (XPS). The experiments of elemental mercury (Hg⁰) removal from flue gas were conducted in a laboratory scale using activated peroxymonosulfate (PMS) catalyzed by CoFe₂O₄, and the effects of the dosage of catalyst, the concentration of PMS, initial solution pH and reaction temperature on mercury removal efficiency were investigated. The average removal efficiency of Hg⁰ could maintain steady at 85% in 45min when the concentrations of CoFe₂O₄ and PMS were 0.288g/L and 3.5mmol/l respectively, solution pH was 7 and reaction temperature was 55°C. In order to speculate the reaction mechanism, ethyl alcohol and isopropyl alcohol were used as the quenching agents to indirectly prove the existence of SO₄^- and OH.