Study on the removal of nitric oxide (NO) by dual oxidant (H2O2/S2O82−) system

Removal of NO in flue gas simulated by the Fe2+/Cu2+-activated double oxidant system

⋅ O H The wet denitrification technology has a good development prospect due to its simple system and mild reaction conditions, and related research has become a hot topic in the field of flue gas purification. In this work, a novel simultaneous removal technology of NO from flue gas using Fe2+/Cu2+-catalytic H2O2/(NH4)2S2O8 system was developed for the first time. The feasibility of this new flue gas cleaning technology was explored through a series of experiments and performance analyses. The mechanism of oxidation products, free radicals and simultaneous removal of NO was revealed. The effects of the main process parameters on the removal of NO were investigated. Relevant results demonstrated that the removal efficiency of NO was elevated when the concentration of (NH4)2S2O8 or reacting temperature increased, while it was decreased after increasing the raising of Fe2+, Cu2+ and H2O2 concentrations. The main radicals were and·S O 4 - , using the electron spin resonance technique in the solution, and played a very important role in NO removal. The main products were carried out by ion chromatography and elemental N material accountancy, and the results showed that it was sulfate and nitrate in the solution, which provided theoretical guidance for the subsequent treatment and resource utilization of the absorption solution. The results of the study provided a theoretical basis for the industrial application of wet denitrification.Highlights A new green process of NO removal by a wet process with Fe2+/Cu2+ activated (NH4)2S2O8 system is proposed in this paper;Elimination mechanisms and paths of NO are elucidated;The synergistic role produced by Cu2+ and Fe2+ is beneficial to the purification of NO;The synergistic role produced by (NH4)2S2O8 and H2O2 increased the concentration of free radicals in the solution;This process jointly considers the enhanced removal of NO and recycling of transition metal ions.

Study on removing marine multiple pollutants in raw exhaust gas with a novel composited method combined with pre-agglomeration and wet scrubbing technology

Most of the existing oxidation denitrification methods need longer residence time to obtain higher NOx removal efficiency. In this study, urea peroxide (CO(NH₂)₂·H₂O₂) was first used for removing SO₂ and NOx on diesel engine bench. The addition of ferrous sulfate can enhance the oxidant capacity of the solution. The better removal efficiency and lower nitrate content in liquid can be achieved in short exhaust gas residence time. The raw gas flow and residence time contained the actual application situation in ships and have high reference value. The removal efficiency decreased with the increase of gas flow, and the reaction temperature, urea peroxide concentration, liquid-gas ratio were the main factors. The optimal Fe²⁺ concentration of 50 mmol/L and pH value of 4 were determined. The urea peroxide concentration, reaction temperature, and liquid-gas ratio were 9%, 70 ℃, and 10 L/m³ respectively. The maximum gas treatment capacity was about 100 L/min, and residence time was close to 10 s for the scrubber. The pre-agglomerating method were used to improve the particle capturing efficiency combined with spray technology. The composited method can realize the synchronous and efficient removal of multiple pollutants in a single scrubber. The possibility of application on ship was further increased.

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.

Effects of Operating and Structural Parameters on Removal of Nitric Oxide by Oxidation in a Ceramic Hollow Fiber Membrane Contactor

A numerical study on the oxidation removal of nitric oxide in a ceramic hollow fiber membrane contactor was performed. To represent the transport and absorption process, the model was created by combining multiphase, species, reaction, and porous models. The numerical results were verified by comparing them with experimental data. The tube and lumen sides both have laminar parabolic velocity distributions. The nitric oxide concentration decreases gradually from the membrane wall to axis at the cross-section except on the inner and outer sides of the membrane tube. The equivalent diffusion length was proved useful for evaluating the entrance effect. At low concentrations, the reduction efficiency was proportional to the absorbent concentration, and at large concentrations, it neared a maximum value. The reduction efficiency was positively affected by elevated operating temperature and pressure. With a gas channel width of 13 mm, the reduction flow rate achieves its maximum. The efficiency of NO reduction per area decreases as the effective membrane length increases. Increasing the operating temperature and membrane length are recommended as design priorities due to high relative enhancements. It is not recommended to improve reduction efficiency by increasing membrane tube diameter and operating pressure in design. Changing the gas flow rate, absorbent concentration and gas channel width are moderate recommended as well.

A Critical Review on Removal of Gaseous Pollutants Using Sulfate Radical-based Advanced Oxidation Technologies

Excessive emissions of gaseous pollutants such as SO₂, NO_x, heavy metals (Hg, As, etc.), H₂S, VOCs, etc. have triggered a series of environmental pollution incidents. Sulfate radical (SO₄•^-)-based advanced oxidation technologies (AOTs) are one of the most promising gaseous pollutants removal technologies because they can not only produce active free radicals with strong oxidation ability to simultaneously degrade most of gaseous pollutants, but also their reaction processes are environmentally friendly. However, so far, the special review focusing on gaseous pollutants removal using SO₄•^--based AOTs is not reported. This review reports the latest advances in removal of gaseous pollutants (e.g., SO₂, NO_x, Hg, As, H₂S, and VOCs) using SO₄•^--based AOTs. The performance, mechanism, active species identification and advantages/disadvantages of these removal technologies using SO₄•^--based AOTs are reviewed. The existing challenges and further research suggestions are also commented. Results show that SO₄•^--based AOTs possess good development potential in gaseous pollutant control field due to simple reagent transportation and storage, low product post-treatment requirements and strong degradation ability of refractory pollutants. Each SO₄•^--based AOT possesses its own advantages and disadvantages in terms of removal performance, cost, reliability, and product post-treatment. Low free radical yield, poor removal capacity, unclear removal mechanism/contribution of active species, unreliable technology and high cost are still the main problems in this field. The combined use of multiactivation technologies is one of the promising strategies to overcome these defects since it may make up for the shortcomings of independent technology. In order to improve free radical yield and pollutant removal capacity, enhancement of mass transfer and optimization design of reactor are critical issues. Comprehensive consideration of catalytic materials, removal chemistry, mass transfer and reactor is the promising route to solve these problems. In order to clarify removal mechanism, it is essential to select suitable free radical sacrificial agents, probes and spin trapping agents, which possess high selectivity for target specie, high solubility in water, and little effect on activity of catalyst itself and mass transfer/diffusion parameters. In order to further reduce investment and operating costs, it is necessary to carry out the related studies on simultaneous removal of more gaseous pollutants.

Simultaneous removal of NO and SO2 with a micro-bubble gas-liquid dispersion system based on air/H2O2/Na2S2O8

A novel environment-friendly process was proposed to conduct the simultaneous removal of NO and SO₂. In this process, a micro-bubble generator was utilized to generate the micro-bubble gas-liquid dispersion system (MBGLS) by inhaling mixed gas (NO, SO₂ and/or air) and absorption liquid. The MBGLS was then sprayed into an oxidation absorption column reactor, in which NO and SO₂ were oxidized and absorbed. As the additives, air, H₂O₂ and/or Na₂S₂O₈ were brought into the MBGLS to investigate their effects on the simultaneous removal of NO and SO₂. In addition, the effects of initial pH and temperature of the absorption liquid on the simultaneous removal of NO and SO₂ were also explored. The performance of the MBGLS in removing NO and SO₂ was excellent. Even if the MBGLS was composed of tap water, NO and SO₂, the removal efficiencies of NO and SO₂ respectively reached 78% and 94.4%. The additives significantly improved the removal performance of the MBGLS. Under the conditions of pH = 8 and room temperature and the addition of air, SO₂ was removed completely and the NO removal efficiency reached 99.5% when Na₂S₂O₈ to H₂O₂ molar ratio was 0.005/0.05. The effect of the absorbent temperature on the removal of NO and SO₂ was insignificant. With the increase in pH, the removal of NO in both H₂O₂ aqueous solution and Na₂S₂O₈ aqueous solution firstly increased and then decreased, but pH had no effect on the removal of SO₂.

Oxidation of nine petroleum hydrocarbon compounds by combined hydrogen peroxide/sodium persulfate catalyzed by siderite

A system consisting of hydrogen peroxide/persulfate (H₂O₂/S₂O₈^2-) catalyzed by siderite was attempted to oxidize nine representative petroleum hydrocarbon compounds [benzene, toluene, ethylbenzene, m-xylene, p-xylene, o-xylene, 1,2,4-trimethylbenzene, methyl-tert-butyl ether, and naphthalene] that tend to persist in the environment. Oxidation under different siderite dosages, H₂O₂:S₂O₈^2- ratios, and pH conditions were investigated. Results indicated that oxidation rates increased from 1.21-4.62 to 1.77-8.94 d^-1 as siderite increased from 0.16 to 0.48 g/40 mL (H₂O₂:Na₂S₂O₈ = 5:1, initial pH = 3.0), except for naphthalene (decreased from 0.58 to 0.45 d^-1 with increased siderite dosage). When the H₂O₂:S₂O₈^2- ratio was increased from 1:1 to 5:1 (siderite = 0.16 g, initial pH = 3.0), the oxidation rates increased from 0.02-0.73 to 0.33-2.19 d^-1. However, as pH increased to > 5.5 (siderite = 0.16 g, H₂O₂:Na₂S₂O₈ = 2.5:1), the oxidation rates of petroleum hydrocarbons decreased to 0.003-0.09 d^-1, which was approximately 90% less than that at pH = 3.0. The partial correlations and principal component analysis of the experimental data were conducted. Overall, both siderite dosage and H₂O₂:S₂O₈^2- ratio correlated positively with oxidation efficiency. The oxidation potential by H₂O₂/S₂O₈^2- mixtures towards the target petroleum hydrocarbon compounds seemed to be more sensitive to pH conditions than to siderite dosages or H₂O₂:S₂O₈^2- ratios.

Technologies for the nitrogen oxides reduction from flue gas: A review

The required energy of the global industry is mostly generated from fossil fuel sources, such as natural gas, gasoline, diesel, oil, and coal. Nitrogen oxides are one of the main air pollutants that are produced from the combustion of fossil fuels in stationary and mobile sources. Development of new technologies to decrease the NO_x emission from exhaust gases is essential due to the harmful effect of NO_x on the environment and human health. Compared with pre-combustion and combustion methods (with <50% NO_x removal efficiency), the post-combustion methods with higher efficiency (above 80%) have attracted more attention in NO_x elimination. This review describes the currently used technologies of NO_x abatement. Different available post-combustion methods of NO_x removal, including selective catalytic reduction (using different types of reducing reagents, including ammonia, hydrogen, hydrocarbons, and carbon monoxide), selective noncatalytic reduction, wet scrubbing, adsorption, electron beam, nonthermal plasma, and electrochemical reduction of NO_x, are discussed.

Simultaneous Removal of SO2 and NO by O3 Oxidation Combined with Wet Absorption

The effects of ozone concentration, NaOH concentration, type and concentration of additives, initial pH, temperature, and NO and SO₂ concentration on simultaneous removal of NO and SO₂ were studied through ozone oxidation combined with wet absorption. Results indicated that ozone concentration and the type and concentration of additives had the most significant effect on NO removal. The optimal ozone concentration was 250 ppm (NO/NO₂ = 1), and the best additive was KMnO₄. The removal efficiency of NO _x was as high as 97.86% when NO/NO₂ = 1, and the concentration of KMnO₄ was 0.025 mol/L. Considering economic and other factors, the KMnO₄ concentration was selected to be 0.006 mol/L. At this time, the removal efficiencies of NO _x and SO₂ were 81.35 and 100%, respectively. This method has potential application prospects for simultaneous removal of SO₂ and NO in the industrial flue gas.

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.

Promoting Effect of H+ and Other Factors on NO Removal by Using Acidic NaClO2 Solution

In this study, NaClO2 was selected as a denitration oxidant. In order to clarify the mechanism of NaClO2 as an oxidation agent for NO removal efficiency, the effects of H+ and other factors (NaClO2 concentration, temperature, and the other gas) on the NO removal efficiency were investigated. NaClO2 showed a promotional ability on NO removal, whose efficiency increased with the increase of NaClO2 concentration. One hundred percent removal efficiency of NO could be achieved when the NaClO2 concentration was 0.014 mol/L. Furthermore, raising the reaction temperature benefited the removal of NO. The lower the pH, the better the NO removal efficiency. The promoting effect of H+ on the NO removal was studied by the Nernst equation, ionic polarization, and the generation of ClO2. Under the optimal conditions, the best removal efficiency of NO was 100%. Based on the experimental results, the reaction mechanism was finally speculated.

Simultaneous Removal of NOx and SO2 through a Simple Process Using a Composite Absorbent

In this work, the feasibility of the simultaneous removal of NOx and SO2 through a simple process using a composite absorbent (NaClO2/Na2S2O8) was evaluated. Factors affecting the removal of NOx and SO2, such as NaClO2 and Na2S2O8 concentrations, solution temperature, the initial pH of solution, gas flow rate, and SO2, NO, and O2 concentrations were studied, with a special attention to NOx removal. Results indicate that a synergistic effect on NOx removal has been obtained through combination of NaClO2 and Na2S2O8. NaClO2 in the solution played a more important role than did Na2S2O8 for the removal of NOx. The above factors had an important impact on the removal of NOx, especially the solution temperature, the initial pH of the solution, and the oxidant concentrations. The optimum experimental conditions were established, and a highest efficiency of NOx removal of more than 80% was obtained. Meanwhile, tandem double column absorption experiments were conducted, and a NOx removal efficiency of more than 90% was reached, using NaOH solution as an absorbant in the second reactor. A preliminary reaction mechanism for NOx and SO2 removal was deduced, based on experimental results. The composite absorbent has the potential to be used in the wet desulfurization and denitration process, to realize the synergistic removal of multi-pollutants.

Experimental investigation of nitric oxide removal from flue gas using hexamminecobalt(II) solution scrubbing in a pilot-scale facility

An experimental investigation of operational parameters, including liquid/gas ratio (L/G), inlet nitric oxide (NO) concentration, reaction temperature, and pH value of absorbing agent, on NO removal efficiency with hexamminecobalt(II) solution scrubbing was conducted on a pilot-scale facility to search optimal operation conditions. The experimental results show that NO removal efficiency increased with the pH value of hexamminecobalt solution, while the improving rate dropped gradually. When the reaction temperature increased, the NO removal efficiency increased first and then decreased. At the same time, NO removal efficiency increased with the increasing of L/G and hexamminecobalt concentration, while the removal efficiency did not change much at low NO concentration. The pH of 10.4 and L/G of 16 L/m³ were close to the optimal operation conditions, and the scrubbing temperature fell within a reasonable operation temperature. The experimental results can be used as a reference for the design and operation of scaled-up industrial devices.

Simultaneous removal of NO and SO2 from flue gas by combined heat and Fe2+ activated aqueous persulfate solutions

The use of advanced oxidation processes (AOPs) to integrate flue gas treatments for SO₂, NO_x and Hg⁰ into a single process unit is rapidly gaining research attention. AOPs are processes that rely on the generation of mainly the hydroxyl radical. This work evaluates the effectiveness of the simultaneous removal of NO and SO₂ from flue gas utilizing AOP induced by the combined heat and Fe²⁺ activation of aqueous persulfate, and elucidates the reaction pathways. The results indicated that both SO₂ in the flue gas and Fe²⁺ in solution improved NO removal, while the SO₂ is almost completely removed. Increased temperature led to increase in NO removal in the absence and presence of both Fe²⁺ and SO₂, and in the absence of either SO₂ or Fe²⁺, but the enhanced NO removal due to the presence of SO₂ alone dominated at all temperatures. The removal of NO increased from 77.5% at 30 °C to 80.5% and 82.3% at 50 °C and 70 °C in the presence of SO₂ alone, and from 35.3% to 62.7% and 81.2%, respectively, in the presence of Fe²⁺ alone. However, in the presence of both SO₂ and Fe²⁺, NO conversion is 46.2% at 30 °C, increased only slightly to 48.2% at 50 °C; but sharply increased to 78.7% at 70 °C compared to 63.9% for persulfate-only activation. Results suggest NO removal in the presence of SO₂ is equally effective by heat-only or heat-Fe²⁺ activation as the temperature increases. The results should be useful for future developments of advanced oxidation processes for flue gas treatments.

Removal of NO with the hexamminecobalt solution catalyzed by the carbon treated with oxalic acid

NO can be removed at the same time with SO₂ by aqueous Co(NH₃)₆²⁺ solution. The reduction of Co(NH₃)₆³⁺ to Co(NH₃)₆²⁺ is catalyzed by activated carbon to regain the NO absorption ability of the scrubbing solution. Oxalic acid solution is explored to change the carbon surface to ameliorate its catalytic capability. The experimental results suggest that the best catalyst is prepared by impregnating the carbon sample in 0.7 mol l^-1 oxalic acid solution for 24 h followed by being activated at 600 °C for 5 h under nitrogen atmosphere. After being treated with oxalic acid solution, the surface area and the acidity on the carbon surface increase. The experiments show that the carbon modified with oxalic acid can get a much higher NO removal efficiency than the original carbon.

Modified Method for Trapping and Analyzing 15N in NO Released from Soils

¹⁵N isotope tracing is an effective and direct approach to investigate sources of nitric oxide (NO) formed in soils. However, NO is highly reactive and rapidly converted to nitrogen dioxide (NO₂) in the presence of ozone, making it impossible to directly measure ¹⁵N in NO. Various wet-chemical methods for conversion of NO to nitrite (NO₂^-) and nitrate (NO₃^-) have been proposed for ¹⁵N analysis in high-concentration NO sources, such as combustion processes. In contrast, NO concentrations in the soil surface-near air are usually small (ppbv-range), posing major challenges to conversion efficiency and blank correction. Here, we present a modified method in which NO is oxidized quantitatively to NO₂ by chromium trioxide (CrO₃), before conversion to NO₂^- and NO₃^- in an alkaline hydrogen peroxide (H₂O₂) solution. A denitrifier method was used to reduce NO₂^- and NO₃^- in the trapping solution quantitatively to nitrous oxide (N₂O) for subsequent ¹⁵N analysis. NO trapping efficiencies of >85% were obtained with 50 ppb NO in a 0.5 L min^-1 air stream bubbling through a solution of 1.2 M H₂O₂ and 0.5 M NaOH. In a laboratory test with distinct ¹⁵NO abundances, the overall precision was 0.29‰ (δ-values) for natural abundance NO and 0.13 atom % for labeled NO, suggesting that our method can be used for both natural abundance studies and ¹⁵N labeling experiments. In a soil incubation experiment with ¹⁵NH₄NO₃, NH₄¹⁵NO₃, or Na¹⁵NO₂ amendments, we found distinct ¹⁵N abundances in NO, indicating that our method is well suited to investigate NO sources in soils.

NOxRemoval from Simulated Marine Exhaust Gas by Wet Scrubbing Using NaClO Solution

The experiments were performed in a lab-scale countercurrent spraying reactor to study the NOxremoval from simulated gas stream by cyclic scrubbing using NaClO solution. The effects of NaClO concentration, initial solution pH, coexisting gases (5% CO2and 13% O2), NOxconcentration, SO2concentration, and absorbent temperature on NOxremoval efficiency were investigated in regard to marine exhaust gas. When NaClO concentration was higher than 0.05 M and initial solution pH was below 8, NOxremoval efficiency was relatively stable and it was higher than 60%. The coexisting CO2(5%) had little effect on NOxremoval efficiency, but the outlet CO2concentration decreased slowly with the initial pH increasing from 6 to 8. A complete removal of SO2and NO could be achieved simultaneously at 293 K, initial pH of 6, and NaClO concentration of 0.05 M, while the outlet NO2concentration increased slightly with the increase of inlet SO2concentration. NOxremoval efficiency increased slightly with the increase of absorbent temperature. The relevant reaction mechanisms for the oxidation and absorption of NO with NaClO were also discussed. The results indicated that it was of great potential for NOxremoval from marine exhaust gas by wet scrubbing using NaClO solution.