Flue Gas Denitration by Wet Oxidation Absorption Methods: Current Status and Development

Abstract

Controlling NOx is not only the key to improving air quality but it is also an important issue for air pollution control. In view of the performance of the wet oxidation absorption method in NOx removal, it has attracted extensive attention and has been widely studied. This article reviews the research progress of NOx removal from flue gas. The wet oxidation absorption method is mainly divided into three categories: direct oxidation absorption technique, catalytic oxidation absorption technique, and high energy ion oxidation absorption technique, which are discussed and analyzed comprehensively and systematically in this article. The development of this method in the future is discussed as well. It is believed that it has multiple advantages, as it possesses simple technological processes, high denitration rate, and easy to realize resource recycling and nonsecondary pollution. The wet oxidation absorption method is one of the more promising methods in terms of reducing operational costs and increasing NO oxidation degree and NOx removal rate.

Introduction

Nitrogen oxides are one of the major sources of air pollution (Guo et al., 2012); they can be represented by the generic molecular formula, that is, NOx; it includes NO, NO₂, N₂O₃, N₂O₄, N₂O₅, and so on, which are the precursors that produce the most pollution problems among all kinds of atmospheric pollutants. NOx is known as the “culprit” and “killer” among all of the atmospheric pollutants for their hazardous effects on human health and environment. Its effects are very toxic on the human body and plants. They play a key role in the formation of acid rain, fog, photochemical smog with hydrocarbons, the destruction of the ozone layer, and so on. Consequently, NOx possesses a serious threat to human beings and to the environment. Controlling the NOx is not only the key to improve the air quality but also a big problem and a hot issue to control air pollution.

The existing denitration technology mainly includes adsorption and absorption (water, acid, and alkali absorption of constant pressure or decompression), oxidation absorption and catalytic reduction, bio-oxidation, or combination of several technologies mentioned above. The advantages and disadvantages of the main denitration technologies are shown in Table 1.

Table 1.

Advantages and Disadvantages of Main Denitration Technologies

Denitration technologies	Advantages		Disadvantages	Remarks
SCR method	1. Reliable technology		1. High operating expenses.
	2. The denitration rate is so high that it can reach up to 80–90%		2. Catalyst deactivation.
	3. The equipment is reliable and easy to maintain		3. NH₃ can have leakage problems and is difficult to operate and store.
			4. In aerobic conditions, SO₃ and excessive NH₃ can react to generate corrosive and viscous NH₄HSO₄, which can cause damage to tail of the flue equipment.
SNCR method	1. Reliable technology		1. High consumption of deoxidizer	At present, most boilers do not adopt SNCR.
	2. No catalyst		2. The utilization of ammonia in SNCR process is not high, and therefore there is a possibility of excessive ammonia leakage. Ammonia leakage causes environmental pollution and forms ammonia salts, which can clog up and corrode downstream equipment
	3. Less equipment investment		3. The formation of greenhouse gas N₂O
			4. If there is a lack of operational control during the experiment, the use of urea for deoxidizer may cause more CO emissions
			5. When the low-temperature urea solution is sprayed into the furnace, which is located at the front of the boiler super heater, its temperature is more than 800°C; the continued burning of hot coal significantly increases the temperature, which eventually results in increased fly ash and unburned carbon.
Adsorption method	1. High purification rate		1. Adsorbent is consumed greatly and needs to be regenerated.	Adsorption method is only used to purify the exhaust gas with low concentration of NOx.
	2. No consumption of chemicals		2. High energy consumption
	3. The equipment is simple and easy to operate.		3. Heavy equipment makes the process very expensive.
Bio-oxidation process	1. Less equipment investment		1. The growth rate of microorganism is relatively slow. To deal with the flue gas of large flow, it needs to further screen the fungus	It is still in experimental stage.
	2. Low cost		2. The growth of microorganism needs a suitable environment. To create the appropriate training conditions in industrial application is a difficult problem to overcome
			3. The growth of microorganisms can cause blockage in the system.
			4. NO has to be converted into liquid phase as it is not effective in gas phase for the microbial adsorption of NO.
Photocatalysis method	1. No oxidant and deoxidizer required		TiO₂-based photocatalysts, therefore it can only absorb UV light with a wavelength of less than 387 nm due to their forbidden bandwidth. They are usually excited by UV light (wavelength of 300–400 nm, accounting for 4%–6% of the solar energy on the surface), and the composite probability of light-borne carrier is higher, which limits their industrial application. Therefore, it is necessary to develop a photocatalyst with higher luminous energy utilization.	1. Photocatalysis method can be divided into photocatalytic oxidation, photocatalytic reduction, and photocatalytic decomposition
	2. Mild reaction conditions			2. The photocatalytic oxidation method can be used in wet denitration
	3. Energy saving, green and environmental friendly			3. It is still in experimental stage.
	4. High oxidation-reduction ability.
Plasma activation method	EB method	1. Less land occupation	1. The energy required for ionization is wasted
		2. The process is simple and easy to operate	2. It produces a lot of x-rays
		3. No waste water and waste residue, and by-product (ammonium nitrate and ammonium sulfate, etc.) can be recycled.	3. Expensive and its target window is easy to corrode.
	Pulsed corona discharge method	1. Its by-product can be recycled.	1. High energy consumption	It is still in experimental stage.
		2. Less corrosion and scaling	2. High cost.
		3. The size of equipment is small, and it is easy to operate.
	Dielectric barrier discharge method	1. It applies to chemical reactions	The insulating medium needs to be optimized
		2. The discharge process is easy to control
		3. Highly energy efficient.
ECO method	1. The by-product can be recycled.		1. High energy consumption	Its essence is pulsed corona discharge plasma activation method.
	2. Less corrosion and scaling		2. High cost.
	3. The size of equipment is small, and it is easy to operate.
Water absorption method	Uncomplicated process and equipment		Since NO is almost insoluble in water, it is not suitable for high-level exhaust treatment.	It is often combined with (catalysis) oxidation.
Acid absorption method	1. The denitration rate is so high that it can reach up to 90%		1. It requires a small gas to liquid ratio	It mainly applies to control exhaust gas discharging from the nitric acid plants and the enterprises that produce sulfuric acid and nitric acid.
	2. The high concentrations of nitric acid or nitrous acid and sulfuric acid can be recovered; therefore this method has good economic returns.		2. A large amount of acid cycle
			3. It requires higher energy consumption.
Alkali absorption method	1. The process is relatively simple		1. The denitration rate is not high enough to meet the emission requirements	It is often combined with (catalysis) oxidation.
Alkali absorption method	2. Recyclable denitrification products (nitrite and nitrates, etc.)		2. A certain gas to liquid ratio has to be maintained	It is often combined with (catalysis) oxidation.
Complexation absorption method	The denitration rate in laboratory is so high that it can reach up to 90%.		1. It is a highly complex reaction and is slow
			2. The denitration rate of complexation absorption method in industrial experiments is very low (10–60%), and far below the laboratory scale.
			3. The recycling of chelates is difficult.
			4. Chelation is lost during the reaction. The absorption liquid is easy to deactivate and difficult to regenerate. Its utilization rate is low. Waste liquid is treated complicatedly.
			5. High operational cost.
Oxidation absorption method	1. It is a less complicated process.		Oxidants are expensive. For example, the preparation cost of O₃ on the market is high, the commonly used oxidant such as NaClO₂, NaClO, and KMnO₄ lead to equipment corrosion problems, and the raw materials for preparing them are expensive.
	2. Easy to operate.
	3. More choice of oxidant
	4. More choice of absorbent
	5. Denitrification effect is good.

EB, electron beam; EBFGT, electron beam flue gas treatment; ECO, electrocatalytic oxidation; SCR, selective catalytic reduction; SNCR, selective non-catalytic reduction.

In the denitration technologies explained above, one of the most widely used technology is catalytic reduction. It is represented by selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) method, which is being widely used in flue gas cleaning field. However, due to SCR's disadvantages, including high investment, high operational cost, leakage problems, and secondary pollution, and SNCR's low denitration rate, high operational cost, higher ammonia leakage, and more consumption of ammonia liquid than SCR method, many oxidation absorption technologies have been extensively researched because it possesses simple technological processes, high denitration rate and easy to realize resource recycling, and nonsecondary pollution. For example, NaClO₂, KMnO₄/NaOH, Fe(II)EDTA, urea, O₃, and UV/H₂O₂ have been introduced into aqueous solutions as oxidative absorbents to remove NO (Sada et al., 1980; Adewuyi et al., 1999; Chu et al., 2001; Hutson et al., 2008; Liu et al., 2010; Fang et al., 2011; Zhang et al., 2016), and other technologies, including photocatalytic oxidation technologies (Yuan et al., 2012; Su et al., 2013) and nonthermal plasma technologies (Jeong and Jurng, 2007; Yu et al., 2007), have been developed.

It is well known that the main component of NOx is NO (Mok and Lee, 2006), which is hardly absorbed by water or alkali liquor. For instance, it accounts for more than 90% of NOx in coal-fired flue gas (Liu et al., 2014). Therefore, NO oxidation technology is the key to improve denitration rate in the wet oxidation absorption method, which is mainly divided into three categories: direct oxidation absorption technique, catalytic oxidation absorption technique, and high energy ion oxidation absorption technique. This article will discuss from the three aspects mentioned above and analyze the pros and cons of these relevant methods. Their comparison of the advantages and disadvantages is shown in Table 2.

Table 2.

Advantages and Disadvantages Comparison of Oxidation Absorption Flue Gas Denitration Technologies

Oxidation absorption denitration technology		Advantages	Disadvantages	Remarks
Direct oxidation absorption technique	O₃ oxidation absorption method	1. This technological process is simple	1. O₃ is highly unstable, so it has to be produced on site.
		2. Low upfront investment costs	2. High cost of production
		3. High denitration rate
		4. It can be integrated with other hybrid removal techniques and has high flexibility
		5. Does not produce secondary pollutants.
	ClO₂ oxidation absorption method	1. ClO₂ is low cost and environment friendly	1. ClO₂ needs to be prepared on site	ClO₂ is a promising oxidant for oxidation absorption flue gas denitration technology.
		2. The technological process is simple	2. Equipment corrosion problem.
		3. The system is easy to operate
		4. There are no problems such as plugging and scaling
		5. Higher denitration rate.
	NaClO₂ and its composite oxidation absorption method	1. Simple and practicable	1. NaClO₂ is very expensive with high operating cost
		2. High denitration rate.	2. The effect of flue gas composition on the desulfurization and denitration of NaClO₂ is significant.
			3. Its handling is complicated and has to be used very cautiously. If mishandled can cause secondary pollution
			4. Can cause corrosion of equipment.
	KMnO₄ oxidation absorption method	1. The effect of pH and temperature on the removal efficiency of NO is not obvious.	1. Complexity preparation process and high price of KMnO₄
	KMnO₄ oxidation absorption method		2. The resulting MnO₂ precipitation is easy to cause equipment blockage.
	H₂O₂ oxidation absorption method	1. H₂O₂ is relatively cheap and has good oxidizing property.	1. The nature of H₂O₂ is unstable, so it can be easily decomposed by heat
		2. Higher denitration rate	2. Handling is complex
		3. Does not produce secondary pollutants.	3. Large consumption of oxidants.
	Ferrate(VI) oxidation absorption method	1. Environment friendly.	1. Denitration efficiency is relatively low
	Ferrate(VI) oxidation absorption method	1. Environment friendly.	2. High price of ferrate(VI).
Catalytic oxidation absorption technique	Catalytic oxidation absorption technique for conventional catalysts	1. High oxidant utilization	1. The problem of catalyst poisoning	If the low-cost and adaptable catalyst for oxygen-free radical induction can be developed, the application of the technology in the industrial production will be promoted.
		2. Fast oxidation speed	2. Adaptability of catalyst reaction temperature.
		3. Using low-cost oxidants
		4. Higher denitration rate.
	Photocatalytic oxidation absorption technique	1. Mild reaction conditions	1. The absorption range of ultraviolet light is narrower	At present, it is still in the stage of laboratory research, and its key is the development of photocatalyst to broaden the response range of TiO₂ and improve its luminous energy utilization
		2. Low energy consumption	2. Light energy utilization is low, and is also restricted by the properties of the catalyst, ultraviolet wavelengths, and reactor.
		3. Low investment cost
		4. Does not produce secondary pollutants.
		5. Higher denitration rate.
Plasma-activated oxidation absorption technique	EB	1. Small footprint	1. High energy utilization for ionization
		2. The process is simple and easy to operate	2. A lot of X-rays are generated
		3. Waste water and residue will not be produced, and by-products are ammonium nitrate and ammonium sulfate, which are recyclable	3. It is expensive
	PPCP	1. Recyclable by-products	1. High energy consumption	It is still in the development stage.
		2. Less corrosion and scaling	2. High cost.
		3. Equipment, small size, is easy to operate.
	DBD	1. It is suitable for chemical reaction	The insulating medium needs to be optimized
		2. The discharge process is easy to control
		3. High energy efficiency.
ECO absorption technique		1. Recyclable by-products	1. High energy consumption	Its essence is pulse corona discharge plasma activation method.
		2. Less corrosion and scaling	2. High cost.
		3. Equipment, small size, is easy to operate.

DBD, dielectric barrier discharge; PPCP, pulse corona induced plasma chemical process.

Direct Oxidation Absorption Technique

This technique involves oxidation-absorption process and absorption-oxidation process (Sun et al., 2011). In oxidation-absorption process, NO is first oxidized to high-valence NOx in the gas phase and NOx is subsequently absorbed by the absorption apparatus. While in absorption-oxidation process, NO is directly introduced into the absorption device, containing the oxidative absorbent, in which NO is oxidized and absorbed. The main oxidants are O₃, ClO₂, NaClO_2, NaClO, KMnO₄, H₂O₂, and ferrate(VI).

O₃ oxidation absorption method

O₃ has proved to be an efficient gas phase oxidant with advantages of selectivity, high oxidation efficiency, fast oxidation speed, and nontoxic by-products (Sun et al., 2011), and it has lots of achievements, which have been successfully applied in the industry. O₃ can efficiently oxidize NO to high-valence NOx in a short time. Skalska et al. (2017) found that a residence time around 7 s was sufficient for almost 100% NO₂ conversion provided around one of the O₃/NOx molar ratio in the pilot scale studies on nitrogen oxides preozonation in flue gases from phosphate rock digestion.

O₃ oxidation combined with lye absorption is the primary method used for denitration technology. Wang et al. (2007) proposed a process capable of removing NOx, SO₂, and mercury simultaneously, which utilized the injection of O₃ and assisted with a glass-made alkaline washing tower. The results showed that about 97% of NO and nearly 100% of SO₂ could be removed simultaneously. Many other researches on the technology of O₃ oxidation combined with lye absorption such as the alkaline magnesium oxide slurry (Sun et al., 2015; Wu et al., 2018b), NaOH (Skalska et al., 2012; Li et al., 2015), and Na₂S (Mok and Lee, 2006) have achieved good effect.

This method has been successfully applied commercially. In 1970's, Japan developed the O₃ oxidation combined with lye absorption denitration technology, which had been successfully implemented in power plants (Rice, 2002). Germany made significant process improvements on this technology. They replaced the lye with ammonia and recycled the by-products for fertilizer so as to further reduce the operating costs. This improved technology was successfully tested for actual flue gas treatment at a power plant (Rice, 2002). BOC company in the United States developed a low-temperature oxidation technology named LoTOx, the principle of which was to use the mixture of O₂ and O₃ into the flue to oxidize NO into high-value NOx, which was washed off with lye. Belco company combined the LoTOx technology with its EDV Wet Scrubbing system to develop LoTOx-EDV system (BOC world, 2002). It could obtain the lower NOx emission, less than 10 μg/g. This technology has been applied in marketing.

High energy consumption of O₃ generator with pulsed corona or electrolytic process preparing O₃ increases the cost of operation. The Lawrence Berkeley National Laboratory in the United States had utilized relatively inexpensive yellow phosphorus as O₃ agent to develop PhoSNOX technology that oxidized NO to NO₂ and converted NOx and SO₂ into salts and gypsum removed by lye absorption (Chang and Lee, 1992). Thermal Energy International Inc. had developed Thermalonox technology by using this technology (TEII, 2001). The Thermalonox™ emission system was applied at a 375 MW coal-fired power plant in American Electric Power Co., Inc. (AEP) in 2001 and it removed at least 75% of NOx emissions.

This method has high denitration rate and can be flexibly integrated with other hybrid removal techniques. At present, relatively high preparation cost of O₃ is still the bottleneck of this method.

ClO₂ oxidation absorption method

ClO₂ is less expensive, and it is a novel chlorine-containing oxide, possessing a strong oxidizability to convert NO into NO₂. It has been proved to be the very promising oxidant for NOx removal (Deshwal et al., 2008a; Lee et al., 2008). ClO₂ can oxidize NO with high efficiency under a wide range of process conditions. The more ClO₂ gas is added, the higher the degree of NO oxidation achieved, and the total NOx removal efficiency is increased. In an experimental investigation dealing with the oxidation of NO by gaseous ClO₂ and subsequently wet scrubbing carried out by Hultén et al. (2017), the total NOx reduction at 0.6 ClO₂:NO mole ratio was between 79% and 94%, depending upon the process conditions used.

However, since ClO₂ is easily soluble in water, more of ClO₂ aqueous solution for NO removal has been studied. Whether its acidic or alkaline, both high oxidation efficiency and high removal efficiency can be maintained as long as ClO₂ is adequate. In the study of simultaneous desulfurization and denitration with ClO₂ aqueous solution, the researchers (Jin et al., 2004, 2006; Deshwal et al., 2008a; Deshwal and Lee, 2009) found that the removal efficiency of NOx and SO₂ could reach 66–72% and 100%, respectively, under optimal conditions, and the range of pH was wide.

On account of explosion hazard (ClO₂ is explosive when its volume concentration in the air exceeds 10% and content in the water exceeds 30%; it is capable of explosive reaction with many chemicals; it is so sensitive to heat, vibration, impact, and friction that it can easily break down and explode) and equipment corrosion problem, which brings challenges for transportation, storage, and maintenance of equipment, this method is still at the stage of laboratory research, although ClO₂ is low cost and its system is easy to operate.

NaClO₂ and its composite oxidation absorption method

NaClO₂ is a high-efficiency oxidant. It is an earlier oxidant studied much more in denitration field. Hutson et al. (2008) studied simultaneous removal of SO₂, NOx, and Hg from coal flue gas using an NaClO₂-enhanced wet scrubber. The results showed that NO was almost completely oxidized. Teramoto et al. (1976) began to research absorption of NO using aqueous solutions of NaClO₂ and NaOH back in 1976. Thereafter, more studies (Sada et al., 1978, 1979; Brogren et al., 1998; Chu et al., 2003; Wei et al., 2008, 2009; Guo et al., 2013; Han et al., 2016; Fang et al., 2017a) have shown that the alkaline solution of NaClO₂ has a good removal effect on NO. Although the lower pH is not conducive to absorption of NOx with aqueous solution of NaClO₂, it is beneficial to oxidization of NO. Some scholars have studied the oxidation absorption effect of NO with NaClO₂ solution under acidic conditions. Deshwal et al. (2008b) carried out the research on the removal of NOx using acidic solutions of NaClO₂. Under optimal conditions, NOx removal efficiency could reach about 81% and 95.2%, respectively. Therefore, NaClO₂ solution, whether under alkaline conditions or under acidic conditions, can show good removal effect on NOx.

While expensive NaClO₂ results in the increased operating cost of this method, to cut the cost, some researchers have studied the removal of NOx using composite absorbent (CA) configured with NaClO₂ and other cheaper oxidants such as NaClO and Na₂S₂O₈. Not only are they cheap but they also show excellent NO removal effect (Khan and Adewuyi, 2010; Mondal and Chelluboyana, 2013; Raghunath and Mondal, 2017; Kang et al., 2018). Zhao et al. (2010) conducted a study on the simultaneous desulfurization and denitrification process using CA configured with cheaper NaClO combining with NaClO₂. The results showed that the removal of NO could be achieved by 85%. Yang et al. (2012) and Wang and Zhong (2016) improved the experimental conditions and further studied the simultaneous desulfurization and denitration process of NaClO₂ and NaClO composite absorber. They increased the overall denitration efficiency up to 90%. Besides, there are more CAs, such as NaClO₂/NaBr (Zhao et al., 2015b), HA-Na/NaClO₂ (Hao et al., 2017b), and NaClO₂/Na₂S₂O₈ (Hao et al., 2017a), proved to have good NO removal effects.

This method is simple and practicable, and its removal rate is high, but its application in industry is restricted because the needed oxidants are relatively expensive, and its equipment requires a high degree of corrosion resistance.

KMnO₄ oxidation absorption method

As early as 1977 (Kobayashi et al., 1977; Sada et al., 1977), it was reported that KMnO₄ could remove NO efficiently. As it turned out that the addition of NaOH in the solution of KMnO₄ could promote the removal of NO (Uchida et al., 1983), other researchers (Brogren et al., 1997; Chu et al., 1998, 2001; Fang et al., 2013; Pan et al., 2015) further studied oxidation and absorption of NO in an alkaline solution of KMnO₄. The results showed that it was feasible to use alkaline KMnO₄ solution to oxidize and absorb NO.

However, the complexity preparation process and high price of KMnO₄ hinder its industrial application.

H₂O₂ oxidation absorption method

University of Florida (Haywood and Cooper, 1998) first conducted the study on H₂O₂ denitration. In recent years, apart from the study on wet denitration of a dual oxidant prepared by H₂O₂ and other oxidants such as Na₂S₂O₈ (Wang et al., 2017b) and the research on NO removal by catalytic reactor, in which the catalytic decomposition of gas-phase H₂O₂ over solid-phase Fe₂(SO₄)₃ occurred, synergized with bubbling reactor, in which NO was absorbed (Wu et al., 2018a), the research emphasis of H₂O₂ oxidation absorption method is mainly to use photocatalytic activation method to make H₂O₂ more effective. The relevant research results will be discussed in the Photocatalytic Oxidation Absorption Method section. In addition, some scholars have opened a new path (Fang et al., 2017b). They used polypropylene hollow fiber membrane contactor instead of ordinary absorber to improve the oxidation absorption efficiency of H₂O₂ on NO. This approach, denitration rate of which reaching 91.2%, has yielded satisfactory results.

H₂O₂ is a kind of environmental-friendly oxidant and is relatively cheap. This method does not produce secondary pollution and can reach high denitration rate. However, because of the unstable nature and thermal decomposition of H₂O₂, unstable operation of device and large oxidant consumption, large-scale industrial application of this method is restricted.

Ferrate(VI) oxidation absorption method

Ferrate(VI), possessing the strong oxidizing property, is a hexavalent iron salt, the effective part of which is the high ferric acid root. Research on the NO removal using Ferrate(VI) has only arisen in recent years. At present, it has been reported that Zhao et al. (2011, 2014) utilized Ferrate (VI) as oxidant for the removal of NO. Their experimental results showed that the removal rate of NO was more than 64.8%. Liu et al. (2016) further studied enhancement mechanism of NO absorption using ferrate (VI) (K₂FeO₄) solution. It provides a new foundation for the in-depth study of this method from the perspective of mass transfer-reaction kinetic theory.

The biggest bottleneck of this method is the high price of Ferrate(VI). It is still in its infancy.

Oxidation absorption with the vaporized composite oxidant

Some scholars carried out denitration research by preparing vaporized composite oxidant (VCO), which is economical and has a good denitration effect. The principle of this method is to first oxidize NO with VCO and then absorb NOx with lye. Zhao et al. (2015a; 2016; 2017) prepared the different VCOs, which are a vaporized enhanced-Fenton reagent made up of H₂O₂, FeSO₄, and PAA (Polyacrylic acid, excellent decomposition of CaCO₃ and Ca(OH)₂ in water), a VCO consisting of cost-effective H₂O₂ and NaClO₂, and a vaporized Fenton-based complex oxidant consisting of Fenton and NaClO, respectively, to oxidize NO into high-valence NOx absorbed with Ca(OH)₂ solution subsequently. The removal rate of NO is all above 81%. Hao et al. (2018) proposed a novel method for simultaneous deep removal of SO₂ and NO, namely H₂O₂/Na₂S₂O₈ vapor oxidation with dual absorption by Na₂CO₃-Na₂SO₃. The results indicated that the best NO efficiency of 95.3% was obtained under these optimal conditions.

Catalytic Oxidation Absorption Technique

Catalytic oxidation absorption technique for conventional catalysts

The addition of catalyst can improve the oxidation efficiency of NO. Lin et al. (2016) prepared a catalyst of MnOx loaded onto spherical alumina support material and evaluated its catalytic activity for the deep oxidation of NO by O₃. The study found that the catalyst increased the conversion efficiency of NO to N₂O₅ in comparison to noncatalytic deep oxidation. It displayed good stability and resistance to SO₂ and proved to be very effective in O₃ decomposition. The use of this catalyst reduced the required residence time, the O₃ requirement concentration, and O₃ leakage, to some extent. The NO deep oxidation efficiency exceeded 95%.

In this technology, O₂, O₃, H₂O₂, potassium monoper sulfate (PMS), and other oxidants are used to oxidize NO to high-valence NOx with catalysts, which are removed in liquid phase by absorption. The mechanism is as follows: in the gas phase, NO adsorption, oxidation reactions can occur on the surface of the catalyst, the intermediate product of which is a multimer, NO₂ is also subsequently adsorbed on the catalyst surface; to reduce occupation of the active sites of the catalyst from the complex components of the flue gas, thereby affecting the property of the catalyst, the catalysts composed of Fe, Mn, Ti, Ce, Co, Ni, or Cr are adopted, in which the valence metal oxidizes NO to the intermediate NO*, and the low-valent metal is used to absorb oxygen and activate the O²⁻, then the two react to generate NO₂; in the liquid phase, the catalyst activates O₂, O₃, H₂O₂, PMS, and high energy ions or free radical, which oxidize NO and absorb NOx.

Different catalysts show different catalytic performance under different conditions. At high temperature, iron-based catalysts could effectively activate radicals, leading to high removal efficiency of NO, which increases with the calcination temperature rising (Ding et al., 2014; Gao et al., 2015). For catalytic oxidation performance of ceria substrate composite catalysts, some studies showed that the catalyst with Co or Mn doping displayed the highest catalytic activity among the doped metals such as Co, Mn, Fe, Cr, and Ni, and the highest NO conversion efficiency could reach 93% (Wang et al., 2016; Shen et al., 2017). Wu et al. (2010) studied the catalytic oxidative activity of a series of MnOx/TiO₂ composite nanoxides by deposition-precipitation (DP) method. Results showed that the MnOx species in MnOx(0.3)/TiO₂(DP) was highly dispersed and enriched on the surface, producing more active sites. Moreover, more Mn³⁺ species were created. The maximum NO conversion over MnOx(0.3)/TiO₂(DP) could reach up to 89%. Based on the study of Wu et al. (2010), Tang et al. (2010) utilized CaSO₃ (the by-product of flue gas desulfurization process) slurry, added MgSO₄, Na₂SO₄, and MgCl₂ as an absorbent for NO₂ absorption, and studied its absorption effect. Results showed that the denitration efficiency was 86% when the concentration of MgSO₄ increased from 0 to 0.6 mol/L. Lately, Liu et al. (2018a) found that with high temperature, ultrasound (US) could enhance mass transfer and chemical reaction so as to synergize with iron-based catalysts to activate persulfate and improve the NO removal rate.

Adding catalyst to the oxidation absorption system can improve the utilization efficiency of the oxidant. The key of this process is to develop low-cost catalysts having a feasible applicability and high catalytic activity. It should be noted that the research of Wu et al. (2017) provided a new way of thinking. They used hexamminecobalt(II) solution to absorb NO and regenerated the spent absorbent by thermal decomposition and acid treatment. Results indicated that the regenerated hexamminecobalt(II) solution showed a similar NO removal performance as the fresh solution prepared from reagent cobalt. This method not only achieves good NO removal effect but also realizes the cycling use of cobalt-base absorber, which helps to reduce costs.

Photocatalytic oxidation absorption method

Photocatalytic technology is an efficient and environmental friendly purification technology. It is based on the principle that light, with certain intensity, irradiates the semiconductor catalyst to excite the valence band electron on the semiconductor material to undergo transition into the conduction band. At the same time, the valence band generates holes. Because conduction band electrons and valence band holes have strong tendency to reduce and oxidize, O₂, H₂O, NOx, etc. adsorbed on the catalyst surface will produce active radicals in the presence of a catalyst. When they are in contact with the flue gas, the NOx is converted into NO₃⁻ and removed by the catalytic oxidation reactions. Because of its excellent catalytic oxidation performance, it can be effective to develop denitration technologies having low cost, simple process, and high removal efficiency combined with the oxidation absorption method.

In recent years, some scholars have also carried out studies on the application of photocatalytic oxidation technology for denitration studies. Liu et al. (2011) used TiO₂ hydrosol as a catalyst to establish a simultaneous desulfurization and denitration system combining photocatalytic oxidation with wet scrubbing. Since Liu et al. (2010) began to study wet removal of SO₂ and NO with UV/H₂O₂ and found that there was a significant cooperative effect between UV and H₂O₂, more researchers improved the reaction conditions to further study the UV/H₂O₂ process. Liu et al. (2014) found that increasing H₂O₂ concentration, NaOH concentration, energy density per unit solution, t-butanol concentration and low concentration of Fe²⁺ had a significant role in promoting the removal of NO in the simultaneous removal of NO and SO₂ from flue gas by using ultraviolet/H₂O₂/NaOH process. The maximum removal efficiency of NO was 90.3%. Other studies, such as the researches of Hao et al. (2016) and Wang et al. (2017a), had shown that UV/H₂O₂ has a favorable effect of NO removal. Liu et al. (2018b, 2018c, 2018d) not only conducted the research on removal of NO using ammonium persulfate synergistically activated by UV-light and heat but also studied the removal process of NO from flue gas using vacuum ultraviolet light (VUV)+system, including VUV + heat + PMS system, VUV + NaClO system, VUV + NaClO + US system, and VUV + NaClO + US + O₂ system. In these systems, high removal efficiency of NO (83.77–91.3%) was achieved. In recent years, many scholars (Martinez et al., 2011; Guo and Poon 2013; Dong et al., 2015; Dong et al., 2016; Luo et al., 2017; Sun et al., 2017; Chen et al., 2018a, 2018b; Jin et al., 2018; Núñez and Cruz, 2018; Wan et al., 2018; Xiang et al., 2018; Ye et al., 2018; Zhang et al., 2018) have developed various photocatalysts that have played a good role in promoting the further application of photocatalytic oxidation technology.

Photocatalytic oxidation technology, possessing mild reaction conditions, low energy consumption, and nonsecondary pollution, and being well-integrated into wet oxidation absorption denitration technology, is an effective measure to improve and optimize the wet oxidation absorption denitration technology. The key of this method is the development of the photocatalyst.

High Energy Ion Oxidation Absorption Technique

Plasma-activated oxidation absorption method

The plasma technology is one of the most promising technologies for SO₂ and NOx removal (Calinescu et al., 2013; Dors et al., 1998; Pawelec et al., 2016). Plasma-activated oxidation absorption method is to use plasma to oxidize NOx to higher oxides, which is easy to be removed by wet-method denitration technology. Its principle is to use high energy electrons to irradiate various gas molecules in flue gas to ionize or dissociate to generate free electrons and active functional groups to form plasma. These functional groups (such as OH, O, and HO₂) in the plasma can catalyze the oxidation of NOx, which is thereafter removed by a wet absorption device (Yu et al., 2014). At present, the plasma activation technologies more commonly used for denitration include the following: electron beam (EB) method, pulse corona induced plasma chemical process (PPCP), and dielectric barrier discharge (DBD) method.

Although electron beam flue gas treatment (EBFGT) is still a relatively new approach (Chmielewski, 2007), it is one of the more successful technologies among the many industrial technologies for SO₂ and NOx removal. Kawamura et al. (1979) carried out pilot plant experiment of NOx and SO₂ removal from exhaust gases by EB irradiation. Results showed that a maximum efficiency of 90% was accomplished both in NOx and SO₂ removal. In the 80s and 90s in the 20th century, pilot plants using EBFGT, obtaining a good desulfurization and denitrification effect, were built in United States, Japan, Germany, Poland, and China (Frank, 1995; Wang et al., 1995). Later it was successfully implemented and used industrially in Chengdu (China) (Doi et al., 2000) and in the north of Poland in the Electropower Station “Pomorzany” (Chmielewski et al., 2004).

PPCP was proposed by Masuda (1988), a Japanese scholar in the 1980s, during the study of EB method. From December 1987 to November 1989, the European Community funded the Italian National Electricity Commission to conduct industrial pilot test of flue gas volume of 1000 m³/h in the Marghera power plant using pulsed corona discharge, the removal efficiency was found to be 80% for SO₂ and 50–60% for NOx (Dinelli et al., 1990). Some research showed that pulsed corona discharge would be an efficient method on NO oxidization (Yamamoto et al., 2000; Moon et al., 2000; Shen et al., 2012; Sretenović et al., 2013).

DBD, one of the discharge methods to be applied first, is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Chang et al. (1993) carried out the study on desulfurization and denitration using DBD technology combined with UV radiation technology. The results showed that the removal efficiency of SO₂ and NOx was 29% and 79%, respectively. Many other researchers (Nagao et al., 2002; Takaki et al., 2004; Liu et al., 2005; Du et al., 2009; Zhang et al., 2013; Wang and Sun, 2016; Cui et al., 2018) have analyzed the denitration process and reaction mechanism of DBD technology from different perspectives, which further promoted the development of this technology in denitration field.

At present, some progress has been made in the field of denitration using plasma-activated oxidation system combined with absorption liquid such as aqueous NaClO₂ solution, Ca(OH)₂, Ammonia (NH₃), and propylene (C₃H₆) (Lee et al., 2003; Huang and Dang, 2011; Park et al., 2015; Chmielewski et al., 2018), and obtained high denitration efficiency.

Research priorities of high energy ion oxidation absorption technique are to seek effective ways to reduce energy consumption.

Electrocatalytic oxidation absorption method

Electrocatalytic oxidation (ECO) technology is an important pollutant removal technology developed in the last decade. Compared with traditional pollutant removal methods, it has the advantages of simplified operations, easy to manage, highly controllable oxidation conditions, easy to realize automatic control, high integration of facilities, and small footprint. The principle of the denitration of ECO is that, high energy pulsed corona discharge is used to generate plasma; the active radicals are used to oxidize NO. Then, NOx is absorbed by the absorption liquid to realize its removal. The essence of ECO absorption method is still the technology of plasma activation coupled wet absorption. At present, the technology of ECO absorption denitration has already been applied to industries. The ECO Commercial Run System, installed at the R.E. Burger coal-fired power plant in Ohio, United States, consisted of three parts: oxidizing gas-phase pollutants by electrical barrier discharge; absorbing oxidative product by ammonia scrubber combined with wet electrical dust precipitator; and recycling by-products, which were mainly nitrate and sulfate. The removal efficiency of NOx was 90% (Boyle, 2002). To reduce energy consumption, Chang et al. (2001) proposed an improved ECO method based on dc corona discharge free radical shower. The method was characterized by a discharge electrode with a nozzle, which prevented the flue gas from entering the corona area so as to reduce energy loss. It also reduced the escape of ammonia.

Due to high energy consumption, becoming the main obstacle to its promotion, there are not much application and research on this method.

Conclusions and Prospects

In conclusion, the wet oxidation absorption flue gas denitration technology, possessing many advantages, such as simple process, high denitration rate, by-product recyclability, and nonsecondary pollution, can be applied to flue gas denitration for industrial kiln and boiler from thermal power, petrochemical, metallurgy, cement, glass, heating, and so on, so as to achieve environmental governance; so it is an important development direction of flue gas denitration technology. However, some problems that need to be resolved are as follows:

1. Low-cost preparation technology for O₃ oxidants

2. The expensive raw materials and equipment corrosion in direct oxidation absorption technique such as NaClO₂, NaClO, KMnO₄, and ClO₂ oxidation absorption method are needed to be further studied and resolved

3. The photocatalytic method also needs to solve the problem of catalyst preparation and production with low-cost, high efficiency, and long-term stable operations

4. Large-scale electron accelerator and long-cycle, low-cost operation should be the research focus to further study the high energy ion activation method.

In the future, the wet oxidation absorption flue gas denitration technology should be improved. The following aspects should be kept in focus, like to minimize the cost of oxidants and highly active catalysts, and more in-depth research should be carried out combined with synergistic removal of multipollutants to promote the development of low-cost, efficient, green, and environment-friendly flue gas removal technology.

Footnotes

Acknowledgments

This project is supported by National Natural Science Foundation—China BAOWU Steel Group Co., Ltd. iron and steel joint research fund (NO. U1660107). Meanwhile, the authors wish to thank for help from Fei Xue, too.

Author Disclosure Statement

No competing financial interests exist.

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Flue Gas Denitration by Wet Oxidation Absorption Methods: Current Status and Development

Abstract

Abstract

Introduction

Direct Oxidation Absorption Technique

O3 oxidation absorption method

ClO2 oxidation absorption method

NaClO2 and its composite oxidation absorption method

KMnO4 oxidation absorption method

H2O2 oxidation absorption method

Ferrate(VI) oxidation absorption method

Oxidation absorption with the vaporized composite oxidant

Catalytic Oxidation Absorption Technique

Catalytic oxidation absorption technique for conventional catalysts

Photocatalytic oxidation absorption method

High Energy Ion Oxidation Absorption Technique

Plasma-activated oxidation absorption method

Electrocatalytic oxidation absorption method

Conclusions and Prospects

Footnotes

Acknowledgments

Author Disclosure Statement

References

O₃ oxidation absorption method

ClO₂ oxidation absorption method

NaClO₂ and its composite oxidation absorption method

KMnO₄ oxidation absorption method

H₂O₂ oxidation absorption method