Oxidation Removal of Nitric Oxide from Flue Gas Using UV Photolysis

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Oxidation Removal of Nitric Oxide from Flue Gas Using UV Photolysis of Aqueous Hypochlorite Yangxian Liu, Yan Wang, Ziyang Liu, and Qian Wang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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Environmental Science & Technology

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Oxidation Removal of Nitric Oxide from Flue Gas Using UV

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Photolysis of Aqueous Hypochlorite

3 4

Yangxian Liu*, Yan Wang, Ziyang Liu and Qian Wang School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China

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ABSTRACT: Oxidation removal of nitric oxide (NO) from flue gas using UV photolysis of aqueous hypochlorite

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(Ca(ClO)2 and NaClO) in a photochemical spraying reactor was studied. The key parameters (e.g., light intensity,

7

hypochlorite concentration, solution temperature, solution pH, and concentration of NO, SO2, O2 and CO2),

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mechanism and kinetics of NO oxidation removal were investigated. The results demonstrate that UV and

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hypochlorite have a significant synergistic role for promoting the production of hydroxyl radicals (·OH) and

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enhancing NO removal. NO removal was enhanced with the increase of light intensity, hypochlorite concentration

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or O2 concentration, but was inhibited with the increase of NO or CO2 concentration. Solution temperature,

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solution pH and SO2 concentration have double effects on NO removal. NO is oxidized by ·OH and hypochlorite,

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and ·OH plays a key role in NO oxidation removal. The rate equation and kinetic parameters of NO oxidation

14

removal were also obtained, which can provide an important theoretical basis for studying the numerical

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simulation of NO absorption process and the amplification design of the reactor.

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Keywords: UV Photolysis; Hypochlorite; NOx; SO2; Hydroxyl radicals (·OH)

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1. Introduction

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Emissions of NOx and SO2 from coal-fired boilers and industrial furnaces have caused serious environmental

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pollution problems (e.g., acid rain, photochemical smog and regional haze).1,2 Chinese government has issued a

20

series of strict regulations and laws to control emissions of SO2 and NOx. Because of having very high solubility

21

in water, both SO2 and NO2 can be easily removed by wet flue gas scrubbing processes (e.g., the common

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Ca-based and ammonium-based wet flue gas desulfurization processes).2 NO (accounting for more than 90% of

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ACS Paragon Plus Environment


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NOx) is more hard to remove since it is only sparingly soluble in water.2 Available NO abatement technologies

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can be divided into dry method and wet method. The wet method NO abatement technologies mainly include

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oxidation absorption, complex absorption, reducing absorption, etc.2-14 The dry method NO abatement

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technologies mainly include O3 oxidation, catalytic oxidation, photocatalytic removal, selective catalytic reduction

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(SCR), selective noncatalytic reduction (SNCR), plasma removal, etc.16-21 These dry method removal technologies

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show good prospects in laboratory or pilot stage, but they also encounter several drawbacks, such as high costs,

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poor removal efficiency, high temperatures, or post-processing costs with associated secondary pollution

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problems.4,6 Therefore, it is very necessary to further develop more economically effective NO removing

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technologies.

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Because of having strong oxidizing capacity and environmentally-friendly process, ultraviolet light

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(UV)/peroxides (e.g., hydrogen peroxide, persulfate and Oxone) advanced oxidation technologies recently have

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gained very widespread attention in the area of wastewater treatment and flue gas purification.2,4,12,13,15,22--29 In the

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previous works,2,12,13,15,30 the authors and the other researchers investigated the removal of NO, SO2 and Hg0 from

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simulated flue gas using UV/H2O2, UV/persulfate and UV/Fenton (-like) advanced oxidation technologies. The

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results show that UV/H2O2, UV/persulfate and UV/Fenton (-like) advanced oxidation technologies can efficiently

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remove NO, SO2 and Hg0 from simulated flue gas, and the removal products can be recycled, which show good

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prospect.2,12,13,15,30 Nevertheless, the high prices of peroxides hinder further commercial applications of these

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technologies.

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Some studies31-34 have reported that ultraviolet light can effectively activate hypochlorite to produce

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hydroxyl radicals to oxidize organic pollutants from waste water. In China, both calcium hypochlorite and sodium

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hypochlorite are the by-products of production process in food/papermaking, chemical/electrolysis and

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disinfection/medical industries.35 These containing-hypochlorite wastewaters will cause secondary pollution if

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they are directly discharged. Adding additional post-processing processes for these containing-hypochlorite

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wastewaters will result in high post-processing costs. Based on this situation, we will use containing-hypochlorite

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wastewaters to replace peroxides to produce hydroxyl radicals to oxidize NO from flue gas. This new technology

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not only can greatly reduce the application costs of UV-based advanced oxidation technologies, but also can

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greatly reduce the post-processing costs of those enterprises that produce containing-hypochlorite wastewaters,

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which show good prospects. Moreover, the previous studies2,12,13,15,30 were mainly carried out in photochemical

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bubbling reactor. Compared with the bubbling reactor, the spraying reactor is the most widely used wet scrubbers

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in the area of flue gas purification because of its simple structure and small pressure drop, which have much better

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prospects for industrial applications. Therefore, in this study, the oxidation removal of NO will be studied in a

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photochemical spraying reactor.

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Therefore, the main purpose of this article is to investigate oxidation removal of NO from flue gas using UV

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photolysis of aqueous hypochlorite (Ca(ClO)2 and NaClO) in a photochemical spraying reactor (in order to reduce

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the complexity of the study, containing-hypochlorite simulated wastewater will be prepared using Ca(ClO)2 and

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NaClO reagents and deionized water). Some experiments were carried out to study the effects of several key

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parameters (e.g., light intensity, hypochlorite concentration, solution temperature, solution pH and concentration

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of NO, SO2, O2 and CO2 ) on NO oxidation removal. The reaction products and mechanism of NO oxidation

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removal are also studied. Besides, kinetic law and rate equation are the important theoretical basis for further

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studying numerical simulation of NO removal process and the amplification design of reactor in the future.

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Therefore, the rate equation and kinetic parameters of NO oxidation removal will be also studied. The results can

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provide important support for the further development and industrial applications of this new technology.

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2. Experimental Section

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2.1 Experimental system

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(1-5) Cylinder Gases (N2/O2/SO2/NO/CO2); (6-10) Flowmeters; (11) Gas Mixer; (12-13) Gas Valves; (14)

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Silicone Plug; (15) Thermocouple; (16) Atomizing Nozzles; (17) Spray Reactor; (18) UV Lamp and Quartz Tube;

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(19) Flue Gas Distribution Plate; (20) Solution Pump; (21) Constant Temperature Water Bath; (22) Flue Gas

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Analyzer; (23) Exhaust gas processor.

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Figure 1. Schematic diagram of experimental device

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The removal device mainly contains a flue gas generation system, a photochemical spraying reactor, a

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reaction temperature regulating system, and an analysis and detection system. The flue gas generation system

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mainly contains five cylinder gases (N2/O2/SO2/NO/CO2), five flowmeters and a gas mixer. The photochemical

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spraying reactor mainly contains a spraying reactor (silicate glass, 55cm length × 10cm i.d.), three atomizing

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nozzles, a silicone plug, a solution pump, a flue gas distribution plate (sand core; pore size, 20-50 microns) and

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an UV lamp with quartz tube (the length of the used UV lamps with wavelength of 254 nm is about 40 cm, and

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their UV radiation intensities are 57, 94, 147 and 218 µW / cm , respectively). The temperature regulating

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system mainly contains a constant temperature water bath and a thermocouple. The analysis and detection

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system mainly contains a flue gas analyzer (MRU-VARIO PLUS, Germany) and an exhaust gas processor. The

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experimental device is described in detail in Figure 1.

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2.2 Experimental method

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In order to reduce the complexity of experiments, containing-hypochlorite wastewater is replaced by

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prepared hypochlorite solutions (i.e. containing-hypochlorite simulated wastewater). The hypochlorite solutions

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were prepared using deionized water and commercial Ca(ClO)2 and NaClO analytical reagents (Sinopharm

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Chemical Reagent Co.,Ltd). The solution pH values of the prepared hypochlorite solutions were adjusted using

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0.5 mol/L NaOH and 0.5 mol/L HCl solutions. The solution pH values were measured via a pH meter (PHS-25,

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Shanghai Lei magnetic Co., Ltd; Resolution is 0.1).

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N2/O2/SO2/NO/CO2-containing simulated flue gas (1200 mL/min) was prepared through the mixing of N2, O2,

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SO2, NO and CO2 based on the required proportions. Concentrations of gas components and flow rates of

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simulated flue gas were controlled by the five flowmeters. The temperatures of the prepared hypochlorite

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solutions were adjusted to the desired values using the thermocouple and constant temperature water bath. When

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the solution temperatures reached the desired values, the simulated flue gas entered the photochemical spraying

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reactor to initiate a gas-liquid reaction after UV lamp was opened. The radiation intensities of UV lamps were

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adjusted using different UV lamps with different radiation intensities. The inlet concentrations of gas components

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were analyzed using flue gas analyzer through gas bypass line. The outlet concentrations of gas components were

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measured via flue gas analyzer through the outlet of the photochemical spraying reactor. Each experiment was

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kept for 30min. Remaining pollutants in exhaust is further processed via exhaust gas processor.

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2.3 Analysis and detection methods

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The hydroxyl radicals were captured by electron spin resonance (ESR) spectrometer (Bruker ESP-300)

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combining with the spin trap agent 5,5-dimethy l-1- pyrrolidine N-oxide (DMPO) (>99%, Sigma) with the

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conditions as below: X-band spectrometer; operating temperature, 298K; microwave power, 10mW; modulation

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amplitude, 0.1mT; resonance frequency, 9.82GHz; modulation frequency, 100kHz; sweep time, 180s; sweep

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width, 10mT; time constant, 148ms; receiver gain, 3.0×105.

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The concentrations of O2, SO2 and NO in simulated flue gas were determined via electrochemical sensors in

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gas analyzer (MRU-VARIO PLUS, Germany). The concentration of CO2 in simulated flue gas was determined via

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non dispersive infrared sensor in flue gas analyzer (MRU-VARIO PLUS, Germany). The concentrations of anions

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(e.g., Cl-, NO2-, NO3-, SO32- and SO42-) in solution were measured using an ion chromatography (Metrohm IC-883,

112

Switzerland).

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In this study, the ranges of the studied process parameters and flue gas composition concentrations are as

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follows: Light intensity, 0 to 218 µW/cm2; Hypochlorite concentration, 0 to 0.2 mol/L; Solution temperature, 298

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to 318 K; Solution pH value, 1.53 to 12.01; O2 concentration, 0 to 10%; SO2 concentration, 0 to 3500 ppm; NO

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concentration, 200 to 1200 ppm; CO2 concentration, 0 to 16%.

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2.4 Data processing method

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NO or SO2 concentration measured by the bypass line is used as the inlet concentrations of NO or SO2 in flue

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gas. NO or SO2 concentration measured by the outlet of the photochemical spraying reactor is used as the outlet

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concentrations of NO or SO2 in flue gas. Removal efficiency of NO or SO2 can be calculated via the expression (1)

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as below:

η = removal efficiency = (Cin − Cout ) / Cin ×100%

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(1)

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where η

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equal to the NO or SO2 removal efficiency in this study); Cin represents the inlet concentration of NO or SO2 in

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flue gas; Cout represents the outlet concentration of NO or SO2 in flue gas.

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NO absorption rate can be calculated via the equation as below(2):36-38

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represents the average removal efficiency of NO or SO2 within 30 min experimental cycle, % (it is

N NO = (η NO ⋅ C NO ,in ⋅ QG ) /(60 ⋅ M NO ⋅ aNO ⋅ VL )

(2)

where N NO represents the absorption rate of NO, mol / m ⋅ s ;η NO represents the average removal efficiency 2

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of NO, %; C NO , in represents the inlet concentration of NO, mg/m3; QG represents the total flue gas flow

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rate, L / min ; M NO represents the molecular weight of NO, g/mol ; VL represents the solution volume in

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reactor, L ; a NO represents the specific interfacial area of NO, m −1 .

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3. Results and discussions

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3.1 Comparison of removal efficiency and radical yield in different removal systems

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Figure 2 (a) and (b) shows the comparison of NO and SO2 removal efficiencies in different removal systems.

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It can be seen that UV radiation can not remove NO and SO2 from flue gas. NO removal efficiency is 25.4% and

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22.6% in Ca(ClO)2 and NaClO removal systems, respectively. As shown in Figure 2(c) and (d), no free radicals

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were caught in Ca(ClO)2 and NaClO solutions using electron spin resonance (ESR) spectrometer combining with

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DMPO. The comparison results of removal efficiency and ·OH yield in different removal systems show that

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Ca(ClO)2 and NaClO solutions can oxidize NO through the following reactions (Eq.s 3-5):1,8,39,40

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NO + ClO- ↔ NO 2 + Cl -

(3)

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3NO 2 + H 2O ↔ 2HNO 3 + NO

(4)

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NO 2 + H 2O ↔ HNO 3 + HNO 2

(5)

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NO removal efficiency reaches 92.4% and 81.7% in UV/Ca(ClO)2 and UV/NaClO removal systems, respectively.

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As shown in Figure 2(e) and (f), ·OH was captured successfully in UV/Ca(ClO)2 and UV/NaClO removal systems.

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The generated ·OH has very strong oxidizing capacity, and can oxidize NO from flue gas to produce HNO3 in

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solution. The related reaction processes can be expressed by the equations (6)-(12) as follows: 2,13,31,32,34,41

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pKa - 7.6 HClO ←  → H + + ClO −

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UV photons HClO ← → ⋅ OH + Cl ⋅

k = (1.3 ± 0.3) × 10-3 s -1 31

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UV photons ClO - ← → ⋅ O − + Cl ⋅

k = (9.0 ± 0.17) × 10-3 s -1 31

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⋅ O- + H 2O ↔ ⋅ OH + OH -

k = 9.4 × 107 s -1 41

(6)

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(7) (8) (9)


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NO + ⋅OH ↔ HNO 2

(10)

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HNO 2 + ⋅OH ↔ HNO3 + ⋅H

(11)

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HNO 2 + HClO ↔ HNO3 + HCl

(12)

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As shown in Figure 2 (b), SO2 is completely removed using Ca(ClO)2, NaClO, UV/Ca(ClO)2 and UV/NaClO four

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removal systems, respectively. The related reaction processes can be explained by the following equations

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(13)-(18): 2,12, 8,39,40

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SO 2 + H 2 O ↔ HSO 3− + H +

(13)

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HSO -3 ↔ SO 32 − + H +

(14)

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HSO3- + ⋅OH ↔ ⋅SO3− + H 2O

(15)

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SO 32 - + ⋅OH ↔ ⋅SO 3− + OH -

(16)

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HClO + HSO3- ↔ 2H + + SO 24- + Cl-

(17)

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HClO + SO 32- ↔ H + + SO 24 - + Cl -

(18)

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Numerous results2-4,6-9,11-13 also reported that compared with NO, SO2 was much easier to remove by wet

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scrubbing of oxidizing and alkaline solutions owing to its very high solubility in water and good reactivity. Hence,

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the main purpose of this paper is to study the removal process of NO from flue gas.

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Figure 2. Comparison of NO removal efficiencies (a), SO2 removal efficiencies (b) in different removal systems,

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and yields of hydroxyl radicals in NaClO (c), Ca(ClO)2 (d), UV/NaClO (e) and UV/Ca(ClO)2 (f) four removal

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systems. Basic experimental conditions: Hypochlorite solution, 500 mL; Light wavelength, 254 nm; Light

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intensity, 147 µW/cm2; Hypochlorite concentration, 0.16 mol/L; Solution temperature, 318 K; Solution pH value,

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7.18; O2 concentration, 6%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 8%.

175 176

3.2 Effects of key parameters on NO removal efficiency and absorption rate

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3.2.1 Effects of light intensity

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The effects of light intensity on NO removal efficiency and absorption rate were studied, and the results are

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shown Figure 3 (a). It can be observed that NO removal efficiency and absorption rate greatly increase with the

9



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increase of light intensity, showing that UV-light plays a key role in the photochemical removal system. Based on

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the above equations (7) and (8), we can see that an increase in light intensity can effectively increase the quantum

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yield of UV-light, thereby increasing the yield of ·OH in solution.27,30 Thus increasing the light intensity can

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promote NO removal.

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3.2.2 Effects of hypochlorite concentration

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The effects of hypochlorite concentration on NO removal efficiency and absorption rate were studied, and

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the results are shown Figure 3 (b). It can be seen that with the increase of hypochlorite concentration, NO removal

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efficiency and absorption rate greatly increase. Based on the above equations (7) and (8), an increase in

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hypochlorite concentration can effectively increase the yield of ·OH in solution,31,34 thereby promoting NO

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removal. Besides, according to the above equation (3), an increase in hypochlorite concentration will enhance the

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direct oxidation removal of NO by hypochlorite.8,39

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Figure 3. Effects of light intensity (a), hypochlorite concentration (b), solution temperature (c), solution pH value

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(d) on NO removal efficiency and absorption rate. Basic experimental conditions: Hypochlorite solution, 500

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mL; Light wavelength, 254 nm; Light intensity, 147 µW/cm2; Hypochlorite concentration, 0.16 mol/L; Solution

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temperature, 318 K; Solution pH value, 7.18; O2 concentration, 6%; SO2 concentration, 1500 ppm; NO

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concentration, 400 ppm; CO2 concentration, 8%.

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3.2.3 Effects of solution temperature

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Temperature is an important parameter, which simultaneously affects chemical reaction and gas

200

solubility.42,43 The experiments about the effects of solution temperature on NO removal efficiency and absorption

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rate were carried out, and the results are shown Figure 3 (c). It is observed that with the increase of solution

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temperature, NO removal efficiency and absorption rate increase at first, and then decrease in the range of 298 K -

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338 K. Increasing temperature will promote chemical reaction42, but will decrease solubility of NO in water43. For

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UV/Ca(ClO)2 removal system, 318 K is the optimized temperature for NO removal. For UV/NaClO removal

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system, 328 K is the optimized temperature for NO removal. This shows that the optimized temperature range for

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the two removal systems are close to the common operating temperatures for most wet flue gas desulfurization

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processes.

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3.2.4 Effects of solution pH value

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The effects of solution pH on NO removal efficiency and absorption rate were studied in the solution pH

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value range 3.19–12.01, and the results are shown in Figure 3 (d). It can be seen that when solution pH value

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increases from 3.19 to 12.01, NO removal efficiency and absorption rate increase at first, and then decrease. The

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solution pH value (5.01 to 9.08) is more suitable for removal of NO in UV/Ca(ClO)2 and UV/NaClO removal

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systems. The results31,33,34 show that under acidic conditions, HClO is the main ingredient in solution. And under

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neutral and alkaline conditions, ClO- is the main ingredient in solution. According to the decomposition rate

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constants in the equations (7) and (8), ClO- will be more easily decomposed by ultraviolet light than HClO (the

216

rate constant is about 6.8 times). Hence, an appropriate increase in solution pH value will increase ·OH yield,

217

which can effectively promote NO removal. However, under strong alkaline conditions or high solution pH value,

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OH- will be generated excessively in solutions. The produced OH- can consume ·OH through the following side

219

reactions (Eq.s 19-20) with very high chemical reaction rates,31,34 which is detrimental to NO removal. Hence,

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too low and too high solution pH value are detrimental on NO removal.

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⋅ OH + OH - ← → ⋅ O - + H 2O

222

⋅ OH + ClO - ← → OH - + ClO ⋅

223

k = 1.3 × 1010 s -1 31,34

(19)

k = 8.8 × 109 s -1 31,34

(20)

3.3 Effects of main flue gas components on NO removal efficiency and absorption rate

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NO, SO2, CO2 and O2 are the main components in actual coal-fired flue gas, and their concentrations will

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change based on the different combustion conditions, fuel types and combustion devices.6 Figure 4 (a)-(d) shows

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the effects of NO, SO2, O2 or CO2 concentration on NO removal efficiency and absorption rate. As shown in

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Figure 4 (a)-(d), an increase in NO concentration reduces NO removal efficiency. This is because increasing NO

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concentration makes the molar ratio of oxidants to NO decrease. However, an increase in NO concentration will

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increase NO partial pressure (equivalent to strengthening the mass transfer drive of NO in gas-liquid two phase),13

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and thereby increases NO absorption rate.

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Figure 4. Effects of NO concentration (a), SO2 concentration (b), CO2 concentration (c), O2 concentration (d) on

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NO removal efficiency and absorption rate. Basic experimental conditions: Hypochlorite solution, 500 mL;

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Light wavelength, 254 nm; Light intensity, 147 µW/cm2; Hypochlorite concentration, 0.16 mol/L; Solution

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temperature, 318 K; Solution pH value, 7.18; O2 concentration, 6%; SO2 concentration, 1500 ppm; NO

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concentration, 400 ppm; CO2 concentration, 8%.

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SO2 shows a double impact on NO removal efficiency and absorption rate. The previous researchers

239

indicated that bisulfite species from SO2 hydrolysis could react with nitrogen oxides to form N-S intermediates in

240

liquid phase, which were discussed in more detail in other papers.4-6 Thus appropriate addition of SO2 is able to

241

promote NO removal. However, SO2 could also consume hypochlorite or ·OH, which could be expressed by the

242

above equations (15)-(18). Hence, adding high concentration of SO2 inhibits NO removal.

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With the increase of CO2 concentration, NO removal efficiency and absorption rate reduce. CO2 can react

244

with hypochlorite (ClO-) to produce hypochlorous acid (HClO), which can be expressed by the following equation

245

(21).8,40 Based on the above discussions in Section 3.2.4 about “Effects of solution pH”, HClO has much

246

lower ·OH yield than ClO-. Thus increase of CO2 concentration will inhibit removal of NO.

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2 NaClO + CO 2 + H 2O ↔ Na 2CO 3 + 2HClO

(21)

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With the increase of O2 concentration in flue gas, NO removal efficiency and absorption rate slightly

249

increase in the low range of O2 concentration (0-3%). Some results4,13,14 showed that O2 was considered as an 13



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important ·OH radical capture intermediate, which could effectively hinder the recombination of ·OH (increasing

251

the effective use of ·OH radicals), thereby enhancing NO removal. However, when O2 concentration exceeds 3%,

252

NO removal efficiency and absorption rate almost keep constant, showing that O2 has been enough.

253

3.4 Products analysis

254 255

Table 1. Measurement of reaction products and intermediates UV/Ca(ClO)2 removal system Measured anion concentration (mg/L) UV/NaClO removal system Measured anion concentration (mg/L)

SO42-

SO32-

NO3-

NO2-

0

73.7

0

NO3-

NO2-

65.2

0

442.1 SO42-

SO320

455.7

Cl1381.5 Cl927.1

NO2 0 NO2 0

256 257

The removal products were analyzed by ion chromatography, which are displayed in Table 1. It is found that

258

NO3- and SO42- are the main detected anions in the liquid phase, suggesting that NO and SO2 in gas phase are

259

oxidized to NO3- and SO42-, respectively. Some studies44,45 reported that NO3- in solution could effectively

260

promote the production of hydroxyl free radicals in UV-based advanced oxidation system. This is a positive

261

information for the application of this process because it means that the accumulation of the reaction products will

262

not inhibit the removal process. The by-products, NO2- and SO32-, were not found in the liquid phase. To avoid

263

secondary gas phase pollution, the potential NO2 from the exhaust was monitored online, but could not be found,

264

which shows that this removal process will not lead to secondary gas phase pollution. The HNO3 and H2SO4 in

265

solutions can be converted into the recyclable nitrate and sulfate through adding alkaline substances. The

266

produced aqueous nitrate and sulfate can be further converted into solid products by evaporation and

267

crystallization using the waste heat from boilers (the solid products can be used as the industrial raw materials).

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Moreover, chloride may be derived from hypochlorite and added hydrochloric acid. Thus, in the future practical

269

application, it is necessary to use chloride-corrosion resistant materials (e.g., duplex stainless steel, enamel, glass,

270

etc.) as the lining inside the photochemical removal device.

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3.5 Discussions on mechanism of NO removal

272

Based on the above experimental results and discussions, mechanism and pathways of NO oxidation removal

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using UV/Ca(ClO)2 and UV/NaClO removal systems can be summarized as the following steps: (1) ·OH was

274

generated in solution by UV photolysis of hypochlorite (Ca(ClO)2 and UV/NaClO) (Eq.s 6-9 ); (2) According to

275

the double film theory, NO in gas phase will enter liquid phase to form dissolved NO in liquid phase; (3) ·OH can

276

oxidize NO dissolved in liquid phase to produce nitric acid in solution (Eq.s 10-12); (4) Hypochlorite can also

277

directly oxidize NO dissolved in liquid phase to produce nitric acid in solution (Eq.s 3-5); (5) According to the

278

comparison results in Figure 2(a), ·OH plays a leading role for NO oxidation removal, and hypochlorite only

279

plays a complementary role for NO oxidation removal.

280

3.6 Mass transfer-reaction kinetics of NO oxidation removal

281

As a gas-liquid heterogeneous reaction, studying the mass transfer reaction-kinetic process is the key works

282

for further strengthening NO removal process. Besides, kinetic parameters and absorption rate equation are also

283

indispensable basis for guiding the design and amplification of industrial reactor, and studying the numerical

284

simulation of NO removal process. Based on the results of NO removal mechanism and products, the total

285

reaction of NO removal can be described by the following equation (22):

286

UV a NO + b ClO- + cH 2O → dNO3- + e Other by - products

287

where a,b,c,d and e are the stoichiometric coefficients for NO, ClO-, H2O, NO3- and by-products, respectively.

288 289

(22)

The chemical reaction rate equation of NO removal using UV/Ca(ClO)2 and UV/NaClO removal systems can be expressed as the following equation (23):42

15



mx nx rNO , x = k j x , m x , n x ⋅ CHj x2 O , x ⋅ C NO , i , x ⋅ CClO − , x

290

Page 16 of 25

(23)

291

where rNO , x is the chemical reaction rate of NO, mol /( L ⋅ s ) ; k j x , m x , n x is the pseudo-(jx+mx+nx)-order reaction

292

rate constant for the total reaction (22), L

293

mol / L ; C NO ,i , x is NO interface concentration, mol / L ; CClO − , x is ClO- concentration, mol / L ; j x ,

294

mx and nx are the partial reaction order for H2O, NO and ClO-, respectively; x is Ca or Na (Ca represents

295

UV/Ca(ClO)2 and Na represents UV/NaClO).

296 297

( j x + m x + n x −1)

⋅ mol (1− j x − m x − n x ) ⋅ s −1 ; CH 2 O , x is H2O concentration,

As a solvent, H2O is excessive, and its concentration is often recognized as a constant.42 Thus the above equation (23) can be further reduced to the following equation (24): mx nx rNO , x = km x , n x ⋅ C NO , i , x ⋅ CClO − , x

298

(24)

299

where k m x , n x = k j x , m x , n x ⋅ C Hx2 O , x represents pseudo-(mx+nx)-order reaction rate constant for NO and ClO- in

300

UV/Ca(ClO)2 and UV/NaClO removal systems, L

j

( m x + n x −1)

⋅ mol (1− m x − n x ) ⋅ s −1 .

301

Based on the chemical reaction rate equation (24), the double-film theory and the results of the other

302

researchers,12,13,44,45 for a fast reaction, the absorption rate of NO in solution can be described by the following

303

equation (25):

N NO , x

304

m x +1 nx  2 DNO , L , x ⋅ k m x , n x ⋅ C NO , i , x ⋅ CClO − , x =  mx + 1 

1/ 2

   

(25)

305

where N NO , x represents the NO absorption rate, mol / m 2 ⋅ s ; DNO , L , x represents the liquid phase diffusion

306

coefficient for NO, m 2 / s .

307 308 309

In the equation (25), N NO , x can be calculated by the aforementioned equation (2). NO interface concentration, C NO ,i , x ,can be calculated by the equation (26) as below:46,47

C NO , i , x = H NO , L , x ( p NO ,G , x − N NO , x k NO ,G , x )

(26)

310

where H NO , L , x represents the solubility coefficient of NO in the solution, mol /( L ⋅ Pa ) ; pNO , G , x represents

311

NO partial pressure in the gas phase body, Pa ; k NO ,G , x represents NO gas phase mass transfer coefficient, 16


Page 17 of 25

312


mol / s ⋅ m 2 ⋅ Pa .

313

H NO , L , x were calculated by the methods which are described in references;37,43,46,47 DNO , L , x were calculated

314

using the Wilke-Chang empirical equation.37,43,46,47 k NO , L , x , k NO ,G , x and a NO , x were determined using the

315

chemical methods and Danckwerts-plot-theory.12,37,38 The described calculation and measurement methods can be

316

found in Support Information. These key parameters were summarized in Table S1.

317 318

319

In order to determine mx value in the equation (25), the following equation (27) can be obtained through making the logarithm for the both sides of the equation (25):

log N NO , x

nx mx + 1 1  2 DNO , L , x ⋅ k m x , n x ⋅ CClO − , x = log C NO ,i , x + log 2 2  mx + 1

   

(27)

320

where N NO , x can be calculated by the aforementioned equation (2), and C NO ,i , x can be calculated by the

321

aforementioned equations (2) and (26).

322

Based on the data of NO concentration vs NO removal efficiency in Figure 4(a) and the aforementioned

323

equations (2)/(26), the plots of Log (CNO, i,x) vs Log (NNO,x) are conducted, which are indicated in Figure 5 (a) and

324

(b). As illustrated in Figure 5 (a) and (b), Log (NNO,Ca) keeps a good linear relationship with Log (CNO, i,Ca), and

325

Log (NNO,Na) also keeps a good linear relationship with Log (CNO, i,Na). The slopes of the two ﬁtted line (mCa+1)/2=

326

0.978 and (mNa+1)/2= 1.024. Thus mCa= 0.956 ≈ 1.0, and also mNa= 1.048 ≈ 1.0, suggesting that NO removal

327

processes using UV/Ca(ClO)2 and UV/NaClO removal systems in the photochemical spraying reactor can be

328

regarded as a pseudo-first-order reaction with respect to NO, respectively.

329

17



Page 18 of 25

330

331 332

Figure 5. Log(NNO,Ca) vs Log(NNO,i,Ca) (a), Log(NNO,Na) vs Log(NNO,i,Na) (b), Log(NNO,Ca) vs Log(NClO-,i,Ca) (c),

333

Log(NNO,Na) vs Log(N ClO-,i,Na) (d).

334 335 336

To calculate nx, mx=1 was brought back to the aforementioned equation (25), and the equation (25) is finally changed to the equation (28) as below with appropriate conversion:

log( N NO , x / C NO ,i , x ) =

(

)

(

nx 1 log CClO − , x + log k m x , n x ⋅ DNO , L , x 2 2

)

(28)

337

The values of CClO − are obtained using the data in Figure 3(b). Based on the data in Table S1, the values of ,x

338

N NO , x and C NO ,i , x are calculated using the aforementioned equations (2) and (26). Plots between N NO, x and

339

CClO − , x are conducted, which are displayed in Figure 5(c) and (d). As displayed in Figure 5 (c) and (d), Log

340

(NNO,Ca) has a linear relationship with Log ( CClO − ,Ca ), and Log (NNO,Na) also has a linear relationship with Log

341

( CClO − , Na ). The slopes of the two ﬁtted line nCa/2 = 0.397 and nNa/2 = 0.497. Thus nCa= 0.794 ≈ 0.8, and nNa=

342

0.994 ≈ 1.0, suggesting that NO removal processes can be regarded as a pseudo-0.8-order reaction with respect to

18


Page 19 of 25


343

ClO- in UV/Ca(ClO)2 removal system, and a pseudo-1.0-order reaction with respect to ClO- in UV/NaClO

344

removal systesm, respectively.

345

Thus, NO absorption rate equation (25) can be further represented as the following expressions (29) and (30):

346

0.4 N NO , Ca = DNO , L ,Ca ⋅ kmCa , nCa ⋅ C NO , i , Ca ⋅ CClO − , Ca

347

0.5 N NO , Na = DNO , L , Na ⋅ km Na , n Na ⋅ C NO , i , Na ⋅ CClO − , Na

(29) (30)

348

The Hatta coefficient Ha can be expressed as the equation (31) as below in accordance with the double-film

349

theory. 1/ 2

350

 2  m x −1 nx km x , n x ⋅ DNO , L , x ⋅ C NO  , i , x ⋅ CClO - , x  m +1  Hax =  x k NO , L , x

(31)

351

where Ha is the ratio between chemical reaction rate and physical absorption rate. When Ha < 0.03 , the

352

absorption process of NO in the liquid phase belongs to a slow reaction; when 3.0 > Ha > 0.03 , the absorption

353

process of NO in the liquid phase belongs to a moderate speed reaction; when Ha > 3.0 , the absorption process

354

of NO in the liquid phase belongs to a fast reaction.46,47

355 356

HaCa and HaNa can be obtained when mCa, nCa, mNa, and nNa are brought back to the aforementioned equation (31), and the equation (31) is finally changed to the equations (32) and (33) as below:

357

HaCa =

358

HaNa =

k mCa , nCa ⋅ DNO , L ,Ca k NO , L ,Ca km Na , n Na ⋅ DNO , L , Na k NO , L , Na

0.4 ⋅ CClO − , Ca

(32)

0.5 ⋅ CClO − , Na

(33)

359

Based on the above discussions, the key kinetic parameters such as km x , n x and Ha are calculated, and the

360

results are summarized Table 2. We can see that under all experimental conditions, HaCa > 3.0 and also

361

HaNa > 3.0 , indicating that NO removal processes using UV/Ca(ClO)2 and UV/NaClO removal systems in the

362

photochemical spraying reactor belong to a fast reaction. The chemical reaction rate is far larger than the mass

19



Page 20 of 25

363

transfer rate, and thus the mass transfer process is the rate control step of NO removal process.46,47 Thus, NO

364

removal process can be further promoted via strengthening the mass transfer step. The mass transfer and kinetic

365

data in Table 2 and Table S1 can provide an important support for the future industrial amplification and design of

366

reactor and numerical simulation of NO removal process.

367

Table 2. Mass transfer-reaction kinetic parameters of NO oxidation removal

Removal systems → Studied parameters

Chypochlorite (mol/L)

Solution temperature (K)

Solution pH

CNO (ppm)

kmCa , nCa × 105

HaCa

UV/NaClO removal system

km Na , n Na × 105

L0.8 /(mol 0.8 ⋅ s)

↓ Light intensity (µW/cm2)

UV/Ca(ClO)2 removal system

HaNa

L /(mol ⋅ s )

0

0.60

21.4

0.75

14.7

57 94 147 218 0.01

4.98 8.50 9.45 9.93

61.6 80.5 84.9 87.0

4.25 8.19 11.00 13.31

35.1 48.7 56.5 62.1

0.04 0.08 0.12 0.16 0.20 298

8.41 10.23 11.30 10.72 9.40 7.90 3.98

26.6 50.9 70.5 80.8 84.9 85.0 57.9

10.00 10.25 13.25 12.83 12.75 12.00 4.25

13.5 27.3 43.8 52.8 60.8 65.9 36.7

308 318 328 338 3.09 5.01 7.18 9.08 11.05 12.01 200 400 600 800 1000 1200

6.53 9.45 10.53 10.43 6.50 7.63 9.45 8.73 7.95 6.58 10.15 9.45 8.55 7.90 7.58 7.08

73.7 84.9 89.3 86.1 70.4 76.3 84.9 81.6 77.9 70.8 88.0 84.9 80.8 77.6 76.0 73.5

7.19 12.94 13.69 11.81 6.75 11.06 13.81 12.75 12.06 6.81 11.88 11.00 12.06 11.44 10.56 10.25

47.1 61.2 61.0 56.7 43.2 56.6 63.3 60.8 59.1 44.4 58.7 56.5 59.1 57.6 55.3 54.5

20


Page 21 of 25


CSO2 (ppm)

CO2 (%)

CCO2 (%)

0 500 1000 1500 2000 2500 3500 0 1 3 6 10 0 2 5 8 12 16

8.73 8.70 8.95 9.45 9.20 8.83 8.45 7.83 8.88 9.40 9.45 9.45 10.25 9.78 9.65 9.55 9.50 9.28

81.6 81.5 82.6 84.9 83.8 82.0 80.3 77.3 82.3 84.7 84.9 84.9 88.4 86.3 85.8 85.3 85.1 84.1

10.44 10.63 11.00 10.56 10.06 9.56 9.06 9.38 10.63 11.19 11.31 11.31 12.94 11.25 11.06 11.00 10.69 9.94

55.0 55.5 56.5 55.3 54.0 52.7 51.3 52.1 55.5 57.0 57.3 57.3 61.2 57.1 56.6 56.5 55.7 53.7

368 369

ASSOCIATED CONTENT

370

Supporting Information

371

More detailed calculation and measurement methods for physical and mass transfer parameters, and the key

372

parameters that are summarized in Table S1 can be found in the Supporting Information.

373

AUTHOR INFORMATION

374

Corresponding Author

375

*Telephone/Fax: +86-0511-89720178. E- mail : [email protected] (Y.X. Liu)

376

Notes The authors declare no competing financial interest.

377

Acknowledgements

378

This study was supported by National Natural Science Foundation of China (No.51576094), Jiangsu “Six

379

Personnel Peak” Talent-Funded Projects (GDZB-014), China Postdoctoral Science Foundation (2017M610306),

380

and Training Project of Jiangsu University Youth Backbone Teacher.

21



381

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382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423

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Oxidation Removal of Nitric Oxide from Flue Gas Using UV Photolysis

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