Microsoft Word - CET--006.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 59, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Zhuo Yang, Junjie Ba, Jing Pan Copyright © 2017, AIDIC Servizi S.r.l. ISBN 978-88-95608- 49-5; ISSN 2283-9216 Research Progress in Flue Gas Denitrification Technology Based on Oxidation Theory Hailiang Wang*, Zhensong Tong, Yupan Yun School of Energy and Environmental Engineering,University of Science and Technology Beijing, Beijing 100083, China 407981576@qq.com Because of its mild reaction condition, relatively high denitrification efficiency and ability of combined removal of various pollutants, the flue gas denitrification technology based on oxidation theory has become a new and feasible method for flue gas denitrification. The technology has been applied extensively and studied in depth. This paper reviews the influencing factors, reaction mechanism, reaction products and subsequent absorption reaction of the oxidation of NO in flue gas with gas phase and liquid phase oxidants at high and low temperatures. On this basis, the shortcoming of the technology and the key research directions for the future are analyzed and discussed, laying the basis for industrial applications of the denitrification technology based on oxidation theory. 1. Introduction Nitrogen oxides (NOx) are major air pollutants and important causes of acid rain, photochemical smog, haze, etc (Benhorma et al., 2017; Di Iorio et al., 2016; Barajas-Solano et al., 2016; Scaglione et al., 2016; Sibilio et al., 2016). According to statistics, China’s nitrogen oxide emissions reached 23.378 million tons in 2012. Therefore, the denitrification of flue gas is an urgent requirement for environmental protection. At present, the main technologies used in flue gas denitrification include selective catalytic reduction (SCR) technology, selective non-catalytic reduction (SNCR) technology, SNCR-SCR combined denitrification technology (Gap et al., 2017). Among them, the SCR is the most popular choice for flue gas denitrification (Puccini et al., 2016; Jecha et al., 2017). Nevertheless, all these denitrification technologies only work within specific temperature ranges. What is worse, the SCR technology faces potential catalyst poisoning and ammonia escape due to the usage of a large number of catalysts. It is also troubled by high construction and operation costs. Against this backdrop, it is necessary to develop a new flue gas denitrification technology. NO accounts for over 90% of the NOX in flue gas. The rest 10% is mainly NO2. There are essential differences between the NO and NO2: NO2 is easily soluble in water; the resulting products HNO3 and HNO2 can react with the common alkaline substances (e.g. Ca(OH)2); in contrast, NO is barely soluble in water and unable to react with common alkaline absorbents. The differences call for new denitrification technology for NO, a new field in flue gas denitrification. Currently, the oxidants used in oxidation treatment of NO mainly fall into the categories of gas phase oxidants (O3, halogen gases, etc.) and liquid phase oxidants (NaClO2, KMnO4, H2O2, etc.) (Schildberg, 2016; Lima et al., 2017). The oxidation of NO can be performed with gas phase oxidants at a high temperature or liquid phase oxidants at a low temperature. However, the flue gas denitrification technology based on oxidation theory is still in the phase of lab research. The technology is not fully mature, the reaction mechanism is not thoroughly understood, and engineering applications are far from sufficient. Despite the shortcoming, the flue gas denitrification technology based on oxidation theory has attracted the attention from various researchers thanks to its mild reaction condition, relatively high denitrification efficiency and ability of combined removal of various pollutants (SO2, NOX, Hg0 ). It is poised to become a popular flue gas denitrification technology in the near future. DOI: 10.3303/CET1759030 Please cite this article as: Hailiang Wang, Zhensong Tong, Yupan Yun, 2017, Research progress in flue gas denitrification technology based on oxidation theory, Chemical Engineering Transactions, 59, 175-180 DOI:10.3303/CET1759030 175 2. Gas phase oxidants Featuring strong oxidizing properties, certain gas phase substances can be utilized to oxidize the NO in flue gas into high-valent NOX, and ultimately absorb and remove the NO. Such gas phase oxidants include O3, halogen gas, etc. Out of all these gas phase oxidants, O3 has been proved in countless studies to have the largest application potential and least likelihood of secondary pollution (Cannistraro et al., 2016; Spinelle et al., 2016). 2.1 Denitrification based on O3 oxidation 2.1.1 High temperature denitrification with gas phase O3 O3 may oxidize or decompose the NOX at a high temperature, reducing or even eliminating the ability of the latter to oxidize NO. Good practice indicates that only 28% of O3 is decomposed in 10s at 150C, the typical exhaust gas temperature of boilers. At this temperature, the survival time of O3 is much longer than the duration of kinetic reaction between O3 and NO. Thus, the temperature has little effect on the reaction process. Besides, the decomposition of O3 is independent of its initial concentration. In the range of 100~200C, O3 can efficiently oxidize NO and the oxidation of NO is insensitive to temperature changes. However, as the temperature continues to increase, the decomposition rate of O3 is accelerated and the NO oxidation efficiency is decreased. By the time the temperature reaches 400C, the O3 has lost all of its oxidation capacity. At a high temperature, the oxidative denitrification with gas phase O3 covers a total of 19 substances, namely O3, O2, H, O, OH, H2O, H2O2, HO2, N2, H2, N, NO, NO2, NO3, N2O, HNO2, HNO, HNO3, and N2O5, and may involve 65 kinds of elementary reactions. According to of theoretical analysis and experimental studies, the NO oxidation is a staged process: First, the NO is oxidized into NO2, plus a few amounts of NO3 and N2O5 if there is excess O3(n(O3): n(NO)≥1). After the oxidation of NO with gas phase O3 at a high temperature, the high valence NOX products are either used to prepare HNO3, or absorbed and removed by the wet flue gas washing apparatus (Wang et al., 2007). If the products are used to prepare HNO3, the generation of HNO3 in gas-liquid reaction is significantly affected by the concentrations of O3 and NO3 when the gas flue temperature falls between 100 and 160C. The molar ratio of O3/NO should be above 1.5. High molar ratio of O3/NO and low simulated gas flue temperature stimulate the production of NO3- and provide high volume fraction of O3 for the gas-liquid reaction. In this way, the formation of HNO3 is promoted and that of HNO2 is suppressed. With water as the tail absorbent, the denitrification efficiency is enhanced with the increase of (O3/NO). When (O3/NO)=0.9, the efficiency reaches 86.27%. With CaCO3 as the tail absorbent, the absorbent concentration and the gas-liquid ratio M must be maintained in a certain range. The critical point M falls between 600~700 when [CaCO 3]=0.05 mol/l, 1200~1300 when [CaCO3]=0.1 mol/l, and 1,900~2,000 when [CaCO3]=0.15 mol/l . Although the flue gas denitrification technology based on oxidation theory is still in the phase of lab research in China, it has been systematically studied and practiced in foreign countries. Developed by BOC Sciences (US), the LoTOx technology oxidizes NOX with O2/O3 gas mixture, and then carries out two-stage washing using CaCO3/NaOH. The denitrification efficiency surpasses 90% (Boc, 2000). Young et al., introduced O3 into the flue gas to oxidize NO, and used Na2S and NaOH solution for liquid phase absorption, achieving an NOX removal rate of up to 95% (Sun and Lee, 2006). 2.1.2 Low temperature oxidative denitrification with liquid phase O3 The first step to liquid phase oxidation of NO is the dissolution of the gas in solution, which is consistent with Henry’s law. The conversion of NO into an easily absorbable form is the key to denitrification because of the extremely low solubility of NO in water (the Henry’s law constant is 1.94×10-8mol·L-1·Pa-1 at 25C). The liquid denitrification procedure is roughly divided into the following stages: the absorption of NO, the liquid phase oxidation of NO into high-valent NOx, and the gas phase oxidation of NO into easily soluble high-valent NOx. In the liquid phase, O3 reacts with NO at a fast rate. The reaction generates easily water-soluble NO2, NO3, etc. and destroys the solubility equilibrium of NO, thereby promoting the dissolution, absorption and oxidation of NO. Under low temperature conditions, O3 either undergoes decomposition or oxidizes NO. At 150C, O3 can react with NO swiftly to produce NO2, and the decomposition rate of O3 at this temperature is less than 4%. Hence, the reaction is a fast, irreversible reaction. When the temperature falls below 100C, only 0.5% of O3 will be decomposed within 10s. Although the decomposition of O3 is accelerated with the increase of temperature, the decomposition rate is much slower than the reaction rate with NO under low temperature, liquid phase conditions. Ma Shuangchen et al., carried out a systematic study on this issue. They performed an experiment on oxidative denitrification with liquid phase O3 in a self-made bubbling reactor. The results show that the pH 176 value and temperature have a little impact on the removal rate of NO; in the range of 20~65C, the NO removal rate does not change significantly; when the molar ratio [O3]/[NO] = 0.8, the NO efficiency can reach 90%; SO2 does not have an obvious effect on the removal of NO for it is hard to be oxidized and removed by O3 in the homogeneous reaction . 3. Liquid phase oxidative denitrification 3.1 Oxidative denitrification with H2O2 Under different reaction conditions, H2O2 can be decomposed into four types of products: ·OH+·OH, HO2·+H·, H2O+O2 or H2O+HO2·. Table 1 lists the standard electrode potentials of some oxidants. As shown in the table, · OH boasts a strong oxidation capacity (its standard electrode potential is 2.8 V). To make use of the oxidation capacity of · OH, it is necessary to create the reaction conditions to decompose H2O2 into OH·. Nonetheless, the utilization of H2O2 is constrained by the lack of the ability to produce sufficient ·OH at low temperatures. Many researchers have explored how to strengthen the oxidation capacity of H2O2 and decompose it into a large number of ·OH. One of the focal points is the advanced oxidation technology. Table 1: Standard electrode potentials of some oxidants Oxidant Electrode reaction Standard electrode potential/V F2 F2+2e→2F- 2.87 ·OH ·OH+H++e→H2O 2.80 O3 O3+2H++2e→H2O+O2 2.07 H2O2 H2O2+2H++2e→2H2O 1.77 MnO4- MnO4-+8H++5e→Mn2++4H2O 1.52 ClO2 ClO2+e→Cl-+O2 1.50 Cl2 Cl2+2e→2Cl- 1.36 O2 0.5O2+2H++2e→H2O 1.23 Oxidant Electrode reaction Standard electrode potential/V Oxidant Electrode reaction Standard electrode potential/V Advanced oxidation processes (AOPs), also known as deep oxidation processes, are a set of chemical treatment procedures proposed by Glaze et al. in 1987. They stand for any chemical oxidation technologies realized through reactions with lots of hydroxyl radicals (William et al., 1987; Zhuang and Yan, 2016). The AOPs usually use the free radical (·OH) to oxidize pollutants, turning them into less toxic and easy-to-remove substances. Aiming to promote the decomposition of H2O2 into ·OH, the AOPs research at home and abroad has put emphasis on the following aspects: (1) The advanced oxidation technology of high temperature H2O2 The main reactions between H2O2 and NO in the flue gas are listed as below (Cooper et al., 2004): When the reaction temperature is above 400°C, the reaction proceeds in the direction of the formation of NO2 and HNO3. Specifically, the NO is oxidized by the H2O2 directly injected into the high-temperature flue or spray tower, and the resulting high-valent NOX are absorbed and removed (Rossi and Unfried, 1997). When the flue temperature surpasses 400°C, the H2O2 injected into the flue will release strong oxidizing groups like ·OH and HO2·, and oxidize the NO in the flue gas into high-valent NOX. In this process, the oxidation effect is improved by adjusting and optimizing the position and type of the nozzle, the molar ratio of H2O2/NO, the temperature, and the SO2 concentration. According to Collins’ research in 2001, the H2O2 solution injected into the hot flue gas can oxidize NO into easily soluble NO2, HNO2 and HNO3, and increase the NO oxidation ratio to over 90% (Collins et al., 2001; Su et al., 2016). (2) The advanced oxidation technology of UV/H2O2 The photolysis of H2O2 under UV irradiation generates ·OH through the following steps: H2O2+UV(200-280nm)→2·OH H2O2→H·+HO2· In this respect, Cooper et al. conducted an in-depth and detailed study. They placed a UV lamp in the flue to activate the release of ·OH by H2O2 and achieve the oxidation of NO with the released ·OH. The conversion rate fell between to 10% -70%. In addition, they had meaningful discussion over the mechanism of NO oxidation with H2O2 under UV irradiation (Cooper et al., 2002). 177 https://en.wikipedia.org/wiki/Hydroxyl_radicals However, the research by Cooper et al. was conducted in high-temperature flue gas. At such high temperatures, the flue gas is not yet desulfurized. In the oxidation process, the presence of SO2 not only consumes a lot of H2O2, but also affects the oxidation of NO. In this background, the oxidation of desulfurized flue gas with H2O2 under UV irradiation has become a research hotspot. Ma Shuangchen et al., spent much effort on this topic. They studied the effects of H2O2 concentration, NOx initial concentration, pH value, O2 concentration, temperature, metal ion catalyst and other technological parameters on NOx removal rate in a self-made bubbling reactor under low temperature conditions (below 45°C), and optimizes these parameters. It is discovered that the NOx removal rate exceeds 95% when the pH value is around 3.3, the O2 concentration is greater than 6%, the temperature is controlled below 45°C and the metal catalytic ions are added. (3) The advanced oxidation technology of Feton method The Fenton reaction is essentially the ·OH-producing chain reaction between Fe2+ and H2O2. During the oxidative denitrification by the Fenton method, the produced ·OH fully oxidizes the NO in flue gas. The oxidation effect is influenced by the H2O2 concentration, the Fe2+ dosage, the initial pH value, UV irradiation and temperature. Under proper reaction conditions, the denitrification efficiency may go beyond 80%. In spite of some related studies, researchers are still exploring the reaction mechanism of oxidative denitrification by Fenton method. The main reactions in the Fe3+/H2O2 system are as follows (Laat et al., 2005): There are a series of mutual reactions between Fe2+ and Fe3+. The equilibrium of the two affects the generation of free radicals. The following reactions may take place between Fe2+ and Fe3+ (Laat et al., 1999; Pignatello et al., 1999): The presence of UV light will accelerate the speed of free radicals, which in turn promote the oxidation and removal of NO. At this point, the following reactions may occur in addition to the above reactions (Tokumura et al., 2008): 3.2 Oxidative denitrification with NaClO2 NaClO2 is a disinfectant and decolorizer widely used in water supply, as well as disinfection and oxidation treatment of wastewater. Prepared by in-situ electrolysis of brine, NaClO2 has a clear edge over other oxidants in that it is easily produced, strongly oxidative and safe and reliable to operate (e.g. no leak of chlorine or explosion). Studies have shown that the removal of NOx by NaClO2 oxidation is a gas film-controlled absorption-oxidation reaction (Brogren et al., 1998; Hsu et al., 1998; Chien and Chu, 2000). NOx is mainly absorbed through the hydrolysis of N2O3 and N2O4; NO can be oxidized into NO3- by NaClO2, and ClO2- is reduced to Cl- and ClO-. The acid HNO3 solution produced in the reaction dissolves NaClO2 into ClO2, which further oxidizes NO. 3.3 Oxidative denitrification with KMnO4 Under strong alkaline conditions, NO is oxidized to NO2-- by MnO4-; under weak alkaline or neutral conditions, it is oxidized to NO3- . Chu et al., (1998 and 2001) suggested that the side effects of SO2 in the flue gas on the oxidative absorption reaction should not be ignored albeit the MnO4-’s ability to oxidize and remove NO; moreover, the further promotion of the method was limited by the complex preparation process and high price of the oxidant KMnO4. 4. Conclusions and prospects The past decade has witnessed the rapid development of the flue gas denitrification technology. With more and more advantages, the technology now boasts broad application prospects. For the purpose of removing the NOx from the flue gas, the technology rests on the conversion of the NO in flue as into high-valent NOx that are easily absorbed and handled. The existing studies concentrate on the oxidation effect of different oxidants, the influencing factors of oxidation, and the possible oxidation mechanism. However, further research indicates that the following issues are yet to be solved for the flue gas denitrification technology based on oxidation theory: (1) Despite the relatively high denitrification efficiency, the flue gas denitrification technology based on oxidation theory generally usually removes about 80% of NOx. The efficiency must be further improved in future studies. (2) The oxidants like O3 and H2O2 are favored by researchers thanks to the low likelihood of secondary pollution. However, such oxidants are unstable, and the reactions are difficult to control. (3) The popular oxidants are usually very expensive, raising the need for the R&D of efficient and cheap oxidants for denitrification. 178 (4) The research into the oxidative denitrification reaction is still in the initial stage. 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