Direct catalytic oxidation and removal of NO in flue gas by the micro bubbles gas–liquid dispersion system

International Journal of Industrial Chemistry https://doi.org/10.1007/s40090-019-00198-6 RESEARCH Direct catalytic oxidation and removal of NO in flue gas by the micro bubbles gas–liquid dispersion system Hongrui Sun1 · Guanghui Yang1 · Tallal Bin Aftab1 · Fei Xue1 · Zhengguo Xiao1 · Qihao Guo1 · Dengxin Li1 Received: 10 December 2018 / Accepted: 3 December 2019 © The Author(s) 2019 Abstract The method of micro bubbles is widely applied in the fields of water and soil treatment. A novel treatment method of NO in flue gas through a gas–liquid two-phase system formed by micro bubbles is proposed in this study. The system depends on the generation of hydroxyl radicals. The NO removal performance of the micro gas–liquid dispersion system induced by catalysts and O 3 was explored and the reaction pathways were elucidated. Micro bubbles, F e2+, and M n2+ in solution improved NO removal performance significantly. Salinity and surfactants affected the removal performance of NO by altering micro bubbles. In the presence of F e2+, the NO removal rate reached 65.2% at pH 5, 75.8% under 0.5 g/L NaCl and 82.1% under 6 mg/L sodium dodecyl sulfate. In the presence of M n2+, the NO removal rate reached 69.2% at pH 5, 83.2% under 0.5 g/L NaCl and 92.3% under 6 mg/L sodium dodecyl sulfate. However, in the presence of both Mn2+ and Fe2+, NO conversion rate was 93.2%. The NO removal rate in the presence of O3 was further improved under the same conditions. The study provides the basis for the application and development of micro bubbles in flue gas treatments for NO removal. The results can help to solve the problems of high operating cost, large oxidant consumption, secondary pollution, and high energy consumption in traditional NO removal methods. Graphic abstract Keywords NO · Micro bubble · Oxidation · Removal · Denitration Introduction * Dengxin Li Extended author information available on the last page of the article Micro bubbles (MBs) refer to a kind of bubble mixture whose bubble diameter is between several hundred nanometers and tens of micrometers [1–3]. MBs’ residence time 13 Vol.:(0123456789) International Journal of Industrial Chemistry in water is tens of seconds to several days [4, 5]. Negatively charged ions (such as OH−) in water are prone to be adsorbed on the surface of MBs, so MBs carry some negative charges [6, 7]. As the bubble diameter reaches micrometers or even nanometers, the surface tension of the gas–liquid interface compresses the bubbles, thus increasing the specific surface area of bubbles and the oxygen transfer efficiency between gas and liquid [8]. The diameter of MBs is small and the bubble pressure is high. When MBs rupture, they release high energy and hydroxyl radicals [9]. Surfactants, salinity, and pH affect the properties of MBs [10]. Surfactants make them more stable [11]. Under the conditions of low salinity and low pH, the stability of the MBs is enhanced. Till now, MBs have been extensively explored in environmental applications, especially in surface water restoration [12, 13], agricultural production [14], and ozone oxidation [15]. The NO removal by the micro bubbles gas–liquid dispersion system (MBGLS) has not been reported yet. NOx is one of the major air pollutants and the NO concentration in flue gas is sometimes as high as 90% [16]. Due to insoluble NO [17], it is necessary to oxidize NO into soluble NO2 in wet denitration processes. NO oxidation methods mainly include photocatalysis [18], plasma oxidation [19], strong-oxidant oxidization [20], and selective catalytic oxidation (SCO) [21, 22]. SCO utilizing catalysts and O 2 in flue gas can be combined with traditional wet absorption processes to achieve efficient and integrated desulfurization and denitration and has become the most promising NO oxidation method in industrial applications [23]. However, in order to realize high denitration rate, the NO oxidation methods should be improved in the following three aspects: First, it is necessary to improve the resistances of catalysts to steam, sulfur, and other pollutants so as to ensure the stability of NO oxidation rate. Second, the catalyst cost should be lowered. Third, it is necessary to increase the recovery rate of catalysts. Advanced oxidization processes can be divided into two categories: gas phase oxidation processes and the liquid phase oxidation processes. NO oxidation reactions happen in the gas phase with common oxidants, such as O2, O3, Cl2, and CIO2−, or in the liquid phase with oxidants [24] such as Na2S2O8 [25], KMnO4 [26], NaClO2 [27], and H2O2 [28]. The absorption probability of N Ox can be increased by the addition of various agents [29, 30]. NOx removal by the oxidation–absorption method has been extensively explored. However, several problems in the NO oxidation–absorption methods remain to be resolved, such as high operation cost, non-recyclable absorption liquid, large oxidant consumption, and the comprehensive use of absorption liquid. Hence, it is imperative to develop a denitration method with high denitration efficiency, low energy consumption, non-secondary pollution, less reagent consumption, high utilization rate of oxidant and low investment. 13 In order to achieve the high denitration rate, NO oxidation–absorption removal processes should be improved in the following aspects. First, contact time between NO and oxidant should be long. Second, the oxidation ability of oxidants should be strong. Third, the oxidation reactions or catalysts should not be affected by various pollutants. Fourth, the denitration process should prevent regenerated nitric oxide from escaping for the application in different flue gas environments. The properties of MBs meet the above four conditions. In the study, a new MBGLS generation process utilizing MB generator is proposed to treat NO through inhaling water and the mixed gases of NO and O3/air. In the process, MBGLS is sprayed into the oxidation–absorption tower, in which NO is oxidized and absorbed. The effects of reaction parameters such as the amount of intake (O3, NO), transition metal ion catalyst ( Mn2+ and F e2+), concentration of salt medium (NaCl), and surfactant (sodium dodecyl sulfate, SDS) on the rate of NO oxidation–absorption were explored. This study provides a laboratory theoretical basis for the industrialization of flue gas treatment by micro-nano bubble technology and is expected to achieve flue gas reduction and resource utilization. Materials and methods Experimental materials The main experimental devices include micro bubble generator (XZCP-K-0.75), ozone generator (CFT-5G), glass rotor flow meter, UV–Vis spectrophotometer (N4, Shanghai INESA Scientific Instrument Co.), steady-state/transient fluorescence spectrometer (QM/TM*, the United States), and portable pH meter (MODEL 6010). Sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium chloride (NaCl), SDS, ferrous sulfate (FeSO4·7H2O), manganese sulfate (MnSO4·4H2O), NO mixtur (...truncated)