Oxidation Removal of Nitric Oxide from Flue Gas Using Ultraviolet

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Oxidation Removal of Nitric Oxide from Flue Gas Using Ultraviolet Light (UV) and Heat Coactivated Oxone System Yangxian Liu, Yan Wang, Yanshan Yin, Jianfeng Pan, and Jun Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03165 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Energy & Fuels

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Oxidation Removal of Nitric Oxide from Flue Gas Using

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Ultraviolet Light (UV) and Heat Coactivated Oxone System Yangxian Liu*,a Yan Wang,a Yanshan Yin,c Jianfeng Pana and Jun Zhangb

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a.School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China

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b.Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University,

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Nanjing, 210096, China

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c.Key Laboratory of Efficient & Clean Energy Utilization of Education Department of Hunan Province, Changsha

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University of Science & Technology, Changsha 410000, China

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ABSTRACT: Oxidation removal process of nitric oxide (NO) from flue gas using UV and heat coactivated

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Oxone (Potassium peroxymonosulfate, 2KHSO5·KHSO4·K2SO4) system in an UV (254 nm)-impinging stream

11

reactor was studied. The main process parameters (e.g., light intensity, Oxone concentration, solution temperature,

12

solution pH, flue gas composition and flow rate of flue gas and solution), products, mechanism and kinetics of NO

13

removal were studied. The results show that UV and Oxone have significant synergistic effect for promoting free

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radicals production, and improving NO removal. NO removal was improved via increasing light intensity, Oxone

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concentration or solution flow rate, and was inhibited with increasing NO concentration, SO2 concentration or flue

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gas flow rate. Solution temperature and pH have double impacts on NO removal. UV-light activation for Oxone is

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the main source of SO4−· and ·OH. Heat-activation for Oxone is the complementary source of SO4−· and ·OH.

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SO4−· and ·OH are the key oxidizing agents, and play an important role in NO removal. Oxone plays a

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complementary role in NO removal. NO removal process is a fast reaction, and meets a total 1.44-order reaction

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(i.e.1.0-order for NO and 0.44-order for Oxone). The key kinetic parameters of NO removal were also determined.

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Keywords: UV and heat coactivated Oxone; NO; SO2; ·OH free radical; SO4−· free radical

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*

Corresponding author phone:Tel.: +86 0511 89720178;Fax: +86 0511 89720178;E-mail: [email protected] (Y.X. Liu)

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ACS Paragon Plus Environment

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1. Introduction

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NOx and SO2 are responsible for the generations of acid rain, photochemical smog and regional haze.

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Coal-fired boilers and industrial furnaces are considered to be the largest source of NOx and SO2.1,2 Chinese

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government has issued a series of regulations and laws to decrease the emissions of SO2 and NOx. Compared

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with SO2 or NO2, which are very easily removed via wet gas scrubbing processes, NO is very difficult to be

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removed since it is very sparingly soluble in H2O.1 Thus, exploring efficient NO control technologies is always a

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hot issue in the area of flue gas treatment. Available NO abatement methods may be divided to wet and dry two

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categories. The dry removal technologies mainly include catalytic oxidation, photocatalytic oxidation, selective

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noncatalytic reduction (SNCR) denitrification, selective catalytic reduction (SCR) denitrification, plasma

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removal, Ozonation, etc.1,3-10 The wet removal technologies mainly include oxidation, complex, reducing

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absorption, etc.2,11-18 SNCR and SCR denitrification technologies have achieved applications. Nevertheless,

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SNCR has low denitrification efficiency, which can not meet the high requirements for environmental

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protection.2,11 SCR denitrification technology often has strict process requirements and deactivated

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catalysts-inducing solid waste problems. Besides, it also lacks of the potential ability of simultaneous removal of

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multiple contaminants (e.g., SO2, NOx, Hg0, etc.), which are not suitable for some special small and

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medium-sized boilers and furnaces from metallurgy and chemical industry.17,18 The other above-mentioned

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technologies also show good prospects in the laboratory or pilot stage, but most of them also suffer from several

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disadvantages, including low conversion efficiency, harsh technical requirements, or disposal costs with several

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associated environmental problems.12,13 Therefore, it is very desirable to try to develop new NO abatement

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technologies.

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In recent years, based on strong oxidizing ability and green environmental process, H2O2-based advanced

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oxidation technologies (AOTs) have attracted the attention of many scholars in flue gas purification area. Zhao et

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al.19 and Guo et al.17 used Fenton reagent to oxidize NO from flue gas in a bubbling reactor, and the removal

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efficiency of more than 90% for NO was obtained. Liu et al.2,20 developed UV/H2O2 and UV/Fenton-like AOTs

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to oxidize NO in an UV-bubbling reactor. The removal product is recyclable nitric acid. However, for flue gas

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purification area, H2O2-based AOTs have the following disadvantages: industrial grade H2O2 solution contains

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72.5% water, which will make the reaction solutions become very dilute (with the addition of containing-72.5%

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water H2O2, a lot of waters are inevitably carried into the reaction solutions).11,21 The dilute acid solutions with

7

high water content are quite difficult to be recycled because the evaporation and crystallization of solutions

8

require a lot of energy or heat.

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Numerous results22-32 indicate that the emerging solid oxidant, Oxone, can also generate ·OH and

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SO4−· radicals in solution by activation of ultraviolet light, transition metal ions, metal oxides, heat, etc. Solid

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oxidants have more advantages in transport, store, safety and concentrating the reaction products than H2O2

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solutions.21,24,29 Activated Oxone AOTs have been widely used for degradating organic pollutants from

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wastewater because of its advantages in strong oxidizing ability and environmentally friendly features.27-31 In the

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area of flue gas denitrification, Adewuyi et al.33 studied for the first time the removal process of nitric oxide by

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aqueous solutions of Oxone without activation, and studied the chemistry and key factors of NO removal.

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However, unactivated Oxone has a quite low oxidation capacity for NO (it is also proved by the comparison in

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Figure 2 (a)). Our previous works21 reported the removal of NO using aqueous Oxone with coactivation of

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Cu2+/Fe3+ and high temperature. The results showed that coactivation of Cu2+/Fe3+ and high temperature could

19

effectively activate Oxone to produce ·OH and SO4−· to oxidize NO from flue gas. Nevertheless, the comparison

20

results in this study (in Figure 2 (a)) show that under the same experimental conditions, compared with Cu2+/Fe3+

21

activation, UV-light activation for Oxone has much stronger removal capacity for NO. Moreover, flue gas

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temperature of boilers and furnaces is often more than 120 ℃,30 thus it is also quite interesting to use waste heat

3


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from flue gas to activate Oxone to generate ·OH and SO4−· to remove NO and SO2 from flue gas.

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In the previous works,34 we used vacuum ultraviolet light of 185 nm as an excitation light source to

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decompose Oxone to generate ·OH and SO4−· to oxidize NO from flue gas in an 185 nm UV-bubbling column

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reactor. However, vacuum ultraviolet light of 185 nm often has a very low penetration/spread distance in

5

solutions.11 Compared with vacuum ultraviolet light of 185 nm, shortwave UV light of 254 nm has a far farther

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penetration/spread distance in solutions (it is also the most widely used light source in the field of wastewater

7

treatment), which has a big advantage for reducing reactor volume and complexity.11 Hence, in this work, the

8

shortwave UV light of 254 nm will be used as the excitation light source of the removal system.

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Moreover, in the previous works,34 the used bubbling column reactor has a low mass transfer efficiency,

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which is usually more suitable for absorption/washing of gas with high solubility in solutions.11,35,36 Many

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results11,33,35,36 showed that NO had a very low solubility in water, thus its liquid phase mass transfer resistance

12

was very large, which reduced the absorption rate of NO. Hence, selecting/designing a gas-liquid reactor with

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high mass transfer efficiency is the key measures for promoting absorption of NO in liquid phase. Besides. in

14

order to keep the UV lamp clean, ultrasonic cavitation was added to the solution to continuously clean the

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surface of the UV lamp. However, ultrasonic cleaning often has a high demand for energy consumption and

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equipment investment costs. The concept of impinging stream was proposed by Elperin in 1961.36,37 The idea is

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to make two streams with the same flow rate collide with each other in two symmetrical accelerating tubes,

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thereby forming an opposed coaxial high speed impact in reaction zone. The high speed impact will form a

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highly turbulent zone, and thus can provide an very excellent condition for enhancing heat and mass transfer.36,37

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Besides, the high speed impact of solution also can keep the surface of the UV lamps clean through hindering the

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deposition of impurities and particulates from coal-fired flue gas. Based on this situation, we will try to use an

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254 nm UV-impinging stream reactor as a gas-liquid reactor for studying the oxidation removal of NO by UV

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Energy & Fuels

and heat coactivated Oxone system.

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In addition, for a gas-liquid reaction, studying the mass transfer reaction-kinetic law is the extremely

3

important works for strengthening the removal process of NO. The kinetic parameters and absorption rate

4

equation are also indispensable basis for guiding the design and amplification of industrial reactor, and studying

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the numerical simulation of NO removal process. Therefore, the main purpose of this article is to investigate the

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feasibility, chemistry and kinetics of NO removal from flue gas using UV and heat coactivated Oxone system in

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a 254 nm UV- impinging stream reactor. The main research contents are as follows: (a) some experiments were

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carried out to evaluate the effects of light intensity, Oxone concentration, solution temperature, solution pH, flue

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gas composition, as well as flow rate of flue gas and solution on NO removal in a 254 nm UV-impinging stream

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reactor; (b) the reaction products and free radicals in solutions were measured using ion chromatography and

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electron spin resonance (ESR) spectroscopy; (c) the mechanism and mass transfer-reaction kinetics of NO

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removal were revealed. The results may provide some meaningful theoretical basis for the follow up research

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and applications of this new technology.

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2. Experimental Section

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2.1 Experimental system

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The removal system mainly contains three key parts: simulated gas generation device, 254 nm UV

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-impinging stream reactor and analysis/post-processing device. The simulated gas generation device mainly

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includes 5 cylinder gases N2/NO/O2/SO2/CO2 (purity, 99.99%), 5 flowemeters, 2 valves and a gas mixer. The 254

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nm UV-impinging stream reactor mainly contains a thermometer, a reactor lid, a plexiglass container (high of 55

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cm and inside diameter of 12 cm), a thermostatic water bath, 2 quartz tubes, 2 UV lamps of 254 nm (the radiation

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intensities of each UV lamp for 18 W, 24 W, 36 W and 60 W are 57 µW / cm 2 , 94 µW / cm 2 , 147 µW / cm 2 and

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218 µW / cm , respectively), 2 nozzles, 2 accelerating tubes (length of 60 cm and inside diameter of 1.5 cm), and 2

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2 pumps. The analysis/post-processing device mainly contains a gas analyzer (VARIO-PLUS, Germany MRU), an

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absorption bottle and a computer. The arrangement of the UV (254 nm) lamps and the nozzles, and the key

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parameters can be seen from the auxiliary picture, which has been called as “UV and nozzle arrangement”. The

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removal system is displayed in Figure 1.

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Figure 1. The schematic diagram of removal system 2.2 Experimental method

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At the beginning of the experiment, Oxone solution was prepared via deionized water and commercial Oxone

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(Sigma-Aldrich). The solution pH was adjusted by sodium hydroxide and hydrochloric acid solutions and was

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measured using an acidometer meter (PHS-25, Lei magnetic Instrument Co., Ltd, Shanghai, China). The

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simulated flue gas was produced through the mixing of independent gas components from the cylinders based on

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required proportions. The flow rate of simulated flue gas and the concentrations of gas components were regulated

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using the flowmeters. The solution temperatures were adjusted to the desired temperatures via thermometer and 6


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thermostatic water bath. When solution temperature reached desired values, simulated flue gas entered the 254 nm

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UV-impinging stream reactor to initiate a gas-liquid reaction after turning on the UV lamps. The radiation

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intensity of 254 nm UV lamps can be adjusted by using different 254 nm UV lamps. Remaining pollutants in

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exhaust will be further processed via the back absorption bottle. The inlet concentrations of gas components were

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analyzed using gas analyzer through gas bypass line. The outlet concentrations of NO were measured by gas

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analyzer through the outlet of 254 nm UV-impinging stream reactor. In these experiments, NO absorption process

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was operated under semibatch mode with circulating the solution. The basic experimental conditions are follows:

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Oxone concentration, 0.25 mol/L; Light intensity, 147 µW/cm2; Solution temperature, 328 K; Solution pH, 4.4; O2

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concentration, 5.0%; SO2 concentration, 1000 ppm; NO concentration, 300 ppm; CO2 concentration, 11%;

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Solution flow rate, 4.0 L/min ; Oxone solution volume, 0.5 L; Flue gas flow rate, 1200 mL/min. When the effect

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of a process parameter or a flue gas component is studied, all of the other parameters and flue gas components

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remain unchanged (that is the so-called single factor method).

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2.3 Detection and analysis methods

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Concentrations of NO2-, NO3-, SO32- and SO42- were determined using ion chromatography (Metrohm IC-883,

15

Switzerland) with chromatographic conditions as below: anion dual 2 anion column, aluent (1.8 mmol/L Na2CO3

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+ 1.7 mmol/L NaHCO3 ), flow rate (1.0 mL/min), injection volume (50 µl ), column temperature (303K), and

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automatic regeneration suppression system (H2O and 60 mmol H2SO4). ·OH and SO4−· were determined by ESR

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spectrometer (Bruker ESP-300), joining with the capture agent 5,5-dimethy l-1- pyrrolidine N-oxide (DMPO)

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(>99%, Sigma) under the following setting conditions: X-band spectrometer, temperature of 298 K, microwave

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power of 10 mW, receiver gain of 3.0×105, resonance frequency of 9.82 GHz, modulation amplitude of 0.1 mT,

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sweep width of 10 mT, modulation frequency of 100 kHz, sweep time of 180 s and time constant of 148 ms.

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2.4 Data processing method

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NO concentration measured by the bypass line is used as the inlet concentrations of NO. NO concentration

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measured by the outlet of 254 nm UV-impinging stream reactor is used as the outlet concentrations of NO.

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Removal efficiency of NO can be calculated via the expression (1) as below:

η NO = NO removal efficiency = (C NO ,in − C NO ,out ) / C NO ,in ×100%

4

(1)

η NO is removal efficiency for NO, %; C NO , in is inlet concentration of NO in flue gas, ppm; C NO , out is

5

where

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outlet concentration of NO in flue gas, ppm.

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3. Results and discussions

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3.1 Comparison of NO removal efficiency and radical yield in different removal systems

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Figure 2 (a) shows the comparison of NO removal efficiency in different removal systems. It was found from

10

Figure 2 (a), NO removal efficiency is 0.4% in UV/H2O (328 K) system, which shows that UV/H2O (328 K)

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system almost has no removal ability for NO. NO removal efficiency is only 6.4% in Oxone solution (298 K)

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system. As shown in Figure 2(d), no free radicals were caught in Oxone solution (298 K) system by electron spin

13

resonance (ESR) spectrometer combining with DMPO. The results show that although the removal share is low,

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Oxone can directly oxidize NO through the following reactions (Eq.s 2 and 3):21,33

15

NO + HSO 5- → NO 2 + H + + SO 24 -

(2)

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2NO + HSO5- + H 2O → 2NO-2 + 3H + + SO 42 -

(3)

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When solution temperature increases from 298 K to 328 K in Oxone solution, NO removal efficiency raises from

18

6.4% to 13.2%. As shown in Figure 2(d), the typical spectral peaks that represent SO4-· and ·OH were detected in

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Oxone solution (328 K). The previous results21 have shown that high temperature or heat can activate Oxone to

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produce SO4-· and ·OH, which are able to enhance NO removal. The high temperature-activation process can be

21

expressed as the following equation (4).

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heat HSO5-  → SO -4 ⋅ + ⋅ OH

8


(4)

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The produced SO4-· and ·OH have very strong oxidizing properties, and can remove NO from flue gas through the

2

following series of reactions (Eq.s 5-12):2,11-13,21

3 4

⋅ OH + NO → H + + NO −2 SO -4 ⋅ + NO → NO 2 + SO3- ⋅

(5) (6)

5

⋅ OH + NO -2 → NO 3- + ⋅H

(7)

6

3NO 2 + H 2O → 2H + + 2 NO3- + NO

(8)

7

NO 2 + ⋅OH → H + + NO3-

(9)

8

2NO 2 + HSO5- + H 2O → 2NO3- + 3H + + SO 42 -

(10)

9

NO-2 + HSO -5 → NO3- + H + + SO 42 -

(11)

10

2SO -4 ⋅ + NO -2 + H 2O → NO3- + 2H + + 2SO 42 -

(12)

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As shown in Figure 2(a), compared with Oxone (328 K) system, NO removal efficiency in UV/heat/Oxone

12

(328 K) system has a great increase (from 13.2% to 87.9%). As shown in Figure 2(d) and (e), with the addition of

13

UV radiation in Oxone solution (328 K), SO4-· and ·OH yields also have a huge increase. The above results fully

14

show that UV and Oxone have a huge synergy role in enhancing NO removal and increasing SO4-· and ·OH yields.

15

The phenomenon can be explained by the following reasons. The results of the previous researchers show that UV

16

radiation can catalyze Oxone to produce SO4-· and ·OH, which further strengthens NO removal. The activation

17

process can be described via the following equations (13) and (14):24,28-31

18

HSO 5- + hv → SO -4 ⋅ + ⋅ OH

(13)

19

SO -4 ⋅ + H 2O → ⋅OH + H + + SO 42 -

(14)

9


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2

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Figure 2. Comparison of NO removal efficiency and radical yield in different removal systems. (the solid circles

5

represent the SO4 · and the solid rectangles represent the ·OH)

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3.2 Effect of light intensity

7

-

Light intensity will affect the yield of free radicals and energy consumption of system. The effects of light

10


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intensity on NO removal were investigated, which are shown in Figure 3(a). As shown in Figure 3(a), when light

2

intensity increases from 0 to 218 µW/cm2, the removal efficiency for NO raises from 13.2% to 91.2%. It is seen

3

from the equations (13) and (14) that with increasing light intensity, the yields of SO4 · and ·OH will increase,

4

thereby being able to enhance NO removal. Figure 4(a) shows the results of SO4 · and ·OH yields measured by

5

ESR spectrometer combining with DMPO under different light intensities. This shows that when light intensity

6

increases, the yields of SO4 · and ·OH greatly increase, maintaining a good consistency with the above

7

discussions. Moreover, we also observe that when light intensity exceeds 147 µW/cm2, NO removal efficiency

8

only slightly increases. For a gas-liquid reaction, both chemical reaction and mass transfer simultaneously

9

dominate the removal process of NO.21 With the further enhancement in chemical reactions, the mass transfer

10

process will become the main rate controlling step, which has been confirmed by the back results about kinetics in

11

Section 3.8. Thus, the increasing rate of NO removal efficiency becomes not obvious with the further

12

strengthening of chemical reactions.

13

3.3 Effect of Oxone concentration

-

-

-

-

14

As the precursor of SO4 · and ·OH, the concentration of Oxone has been proven to have an important effect

15

on radical yield.21 Figure 3(b) shows the effects of Oxone concentration on removal efficiency of NO. It is

16

observed that when Oxone concentration raises from 0 to 0.3 mol/L, NO removal efficiency greatly raises from 0

17

to 88.9%. Based on the equations (2) and (3), an increase in Oxone concentration can promote NO removal by

18

oxidation of Oxone. Moreover, in view of the reactions (13) and (14), the increase of Oxone concentration also

19

will effectively increase the yields of SO4 · and ·OH, thereby improving NO removal. Figure 4(b) shows the yield

20

of SO4 · and ·OH determined by ESR spectrometer under different Oxone concentrations. This measuring result

21

of ESR proves that a higher Oxone concentration results in the higher SO4 · and ·OH yield. Nevertheless, when

22

the concentration of Oxone exceeds 0.25 mol/L, NO removal efficiency only slightly improves. This is most likely

-

-

-

11


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because at this time the mass transfer has become the main rate controlling step for NO removal.

2

Moreover, some results2,11,21,29,31 show that the following side reactions (15)-(19) with high reaction rates

3

also will occur in the Oxone solution with high concentrations, which is not conducive to the further enhancement

4

of NO removal. As shown in Figure 4(b), in the high concentrations range of Oxone solution (e.g., more than 0.18

5

mol/L), the growth rate of SO4 · and ·OH yields becomes smaller with the further increase of Oxone solution,

6

which also keeps a good consistency with the above discussions.

-

7

HSO 5- + SO -4 ⋅ → SO -5 ⋅ +SO 42 - + H +

8

HSO5- + ⋅OH → SO5- ⋅ + H 2O

9

SO -4 ⋅ + SO -4 ⋅ → S2O82 -

k = 4.4 ± 0.4 × 108 M −1s −1

10

⋅ OH + SO -4 ⋅ → HSO 5-

k = 0.95 ± 0.08 × 1010 M −1s −1

11

⋅ OH + ⋅OH → H 2O 2

12

k ≤ 1.0 × 105 M −1s −1

k = 1.7 × 107 M −1s −1

k = 5.3 × 109 M −1s −1

(15) (16) (17) (18) (19)

3.4 Effect of solution pH

13

Figure 3(c) exhibits the effects of solution pH on NO removal efficiency. It is seen that as solution pH

14

changes from 1.2 to 4.4, NO removal efficiency only has a very small increase (from 85.3% to 87.9%). But as

15

solution pH further changes from 4.4 to 12.3, NO removal efficiency greatly decreases from 87.9% to 59.5%.

16

Numerous results11-33,21,26-29 demonstrated that SO4 · could react with OH- to generate ·OH in neutral and alkaline

17

mediums, which can be expressed as the equation (20) as below.

-

SO -4 ⋅ +OH - → SO 24 - + ⋅OH

18

(20)

-

19

The produced ·OH has far stronger oxidation capacity for NO than SO4 ·,11,21 thus NO removal was promoted

20

with appropriately increasing solution pH.

21

Nevertheless, ·OH was very unstable under strong alkaline conditions, and can be consumed by the

22

following elementary reaction (Eq. 21) with a very high reaction rate (its rate constant has been up

12


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Energy & Fuels

to 1.3 × 1010 M −1s −1 ) . 11,12,21 Thus too high solution pH will inhibit removal of NO.

⋅ OH + OH - → H 2 O + O - ⋅

2

(21)

3

Besides, some results21,25,27,32 have shown that compared with alkaline conditions, Oxone often has a stronger

4

oxidative capacity under acidic and neutral conditions. Therefore, strong alkaline conditions (pH=12.3) is also

5

quite detrimental to oxidation removal of NO by Oxone alone (Eq.s 2 and 3).

6

As shown in Figure 4(c), we determined the yields of ·OH and SO4-· under different solution pHs. The

7

results showed that higher solution pH resulted in the decrease of ·OH and SO4-· yields. During the experiments,

8

we also observed that the decomposition of Oxone significantly occurred under strong alkaline conditions

9

(pH=12.3) (some small bubbles were produced in solutions, and more O2 in exhaust was also measured).

10

Moreover, compared with the other relatively stable curves in Figure 3(c), the significant decay or reduction of

11

NO removal efficiency with operation time under strong alkaline conditions (pH=12.3) was observed. This

12

phenomenon may be caused by the self-dissociation of Oxone in strong alkaline conditions. Thus, high solution

13

pHs or strong alkaline conditions are extremely detrimental to NO removal in this removal system.

14

3.5 Effect of solution temperature

15

Experiments were conducted to evaluate the effects of solution temperature on NO removal. The related

16

results are shown in Figure 3(d). It is found that solution temperature exhibits double effect on NO removal in the

17

solution temperature range of 298 K to 348 K. As solution temperature changes from 298 K to 328 K, NO

18

removal efficiency raises from 76.1% to 87.9%. On the basis of Arrhenius equation,30 an increase for temperature

19

will improve reaction rate between NO and oxidizing agents, thereby promoting NO oxidation. Furthermore, on

20

the basis of the reaction (4), the yields of SO4 · and ·OH will increase due to activation of the higher

21

temperatures.30 The yields of SO4 · and ·OH measured by ESR spectrometer in Figure 4(d) proves the above

22

discussions.

-

-

13


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Nevertheless, it's worth noting that when the temperature exceeds 328 K, the removal efficiency for NO

2

reduces. Related studies38-40 displayed that an increase in temperature would reduce the solubility of NO in

3

reaction solutions, which was detrimental to removal of NO in liquid phase (the decrease of the gas solubility will

4

increase the mass transfer resistance of gas in liquid phase). Hence, when the temperature further raises from 328

5

K to 348 K, the removal efficiency of NO decreases from 87.9% to 84.2%.

6

7

8 9 10

Figure 3. Effects of light intensity (a), Oxone concentration (b), solution pH (c) and solution temperature (d) on NO removal efficiency.

11 12

3.6 Effect of flow rate of solution and flue gas

14


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

The effects of flow rate of solution and flue gas on NO removal efficiency were studied, which are shown in

2

Figure 5 (a) and (b). It is observed that with increasing solution flow rate, NO removal efficiency has a significant

3

increase. When flue gas flow rate keeps constant, an increase in solution flow rate is equivalent to increasing the

4

liquid-gas ratio, which is helpful for NO removal.11,21 With increasing flue gas flow rate, NO removal efficiency

5

has a significant decrease. An increase in the flue gas flow rate will decrease the liquid-gas ratio, which is

6

detrimental to removal of NO.11,21 Moreover, increasing the flue gas flow rate also will make the residence time

7

(or reaction time) of NO in the reactor greatly decrease, which is also detrimental to removal of NO.

8

9

10 11

Figure 4. Effects of light intensity (a), Oxone concentration (b), solution pH (c) and solution temperature (d) on

12

free radical yield.

15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Page 16 of 26

3.7 Effect of flue gas compositions

2

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

3

change depending on the different fuel types and combustion conditions.11 The experiments on the effects of SO2,

4

CO2, O2 and NO concentrations on NO oxidation removal were carried out. As displayed in Figure 6 (a)-(d), it is

5

observed that the change of O2 or CO2 concentrations has no significant effect on NO oxidation removal. The

6

presence of SO2 inhibits NO oxidation removal. Numerous results35-38 proved that SO2 could consume HSO5-,

7

SO4-· and ·OH through the competition with NO (Eq.s 22-29), which was detrimental to removal of NO.

8

SO 2 + H 2 O ↔ HSO 3− + H +

(22)

9

HSO -3 ↔ SO 32 − + H +

(23)

10

HSO 3- + HSO 5- → 2SO 24 − + 2H +

(24)

11

HSO3- + ⋅OH → SO3− ⋅ + H 2O

(25)

12

SO 32 - + ⋅OH → SO3− ⋅ + OH -

(26)

13

2SO -4 ⋅ +SO32 - + H 2O → 2H + + 2SO 24 −

(27)

14

HSO -3 + 2SO -4 ⋅ + H 2O → 3H + + SO 24 −

(28)

15

SO32 - + HSO 5- → 2SO 24 − + H +

(29)

16

An increase in the concentration of NO has a clear negative impact on NO oxidation removal. With the

17

increase of NO concentration, the number of NO molecular through the reactor per unit time will greatly increase,

18

which will reduce the molar ratio of oxidizing agents to NO,11,20,21 and thus is detrimental to removal of NO.

16


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Energy & Fuels

1 2

Figure 5. Effects of flow rate of solution (a) and flue gas (b) on NO removal efficiency.

3

4 5

Figure 6. Effects of O2 concentration (a), CO2 concentration (b), SO2 concentration (c) and NO concentration (d)

6

on NO removal efficiency.

7 17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

3.8 Analysis of reaction products

2

The products and potential byproducts of NO oxidation removal using UV and heat coactivated Oxone

3

system were analyzed, which are exhibited in Table 1 (the running conditions are follows: Oxone concentration of

4

0.25 mol/L, light intensity of 147 µW/cm2, solution temperature of 328 K, solution pH of 4.4, O2 concentration of

5

5.0%, SO2 concentration of 1000 ppm, NO concentration of 300 ppm, CO2 concentration of 11%, solution flow

6

rate of 4.0 L/min, Oxone solution volume of 0.5 L, flue gas flow rate of 1200 mL/min, running time of 35min).

7

Nitrate ions were detected in the reaction solutions, which may originate from the oxidation products of NO from

8

flue gas. In order to verify the transfer pathways of N (from NO) balance between the flue gas and the liquid

9

phase, the calculation of a total N mass balance had been carried out, which is shown in Table 1. It is found that

10

the determined value of the concentration of N in NO3- keeps a relatively good consistency with the predictive

11

value (the relative errors is 4.37%), suggesting that NO was mainly removed by oxidation reactions, and NO3- was

12

the final oxidation removal product of NO from flue gas. The error may come from the systematic errors and

13

measurement errors (e.g, the measurement process of NO in flue gas, and NO3- in the reaction solutions).

14

Sulfate ions were also detected in the reaction solutions, which may originate from the oxidation products of

15

SO2 from flue gas and the containing-components in Oxone itself (2KHSO5·KHSO4·K2SO4). Both nitrite and

16

sulfite in solutions were also analyzed, but could be not detected owing to their instability in oxidizing medium.

17

Compared with NO, NO2 has much stronger toxicity and greater harm for human health and environment. The

18

possible formation of NO2 in the tail gas will lead to more serious secondary pollution. Therefore, it is interesting

19

to detect the NO2 in the tail gas. During the experiments, we used a NO2 gas analyzer to detect the potential

20

byproduct NO2. However, NO2 could be not detected, which showed that this removal process would not produce

21

new secondary air pollution.

22

The above analyzing results indicate that the reaction products in mixed solutions mainly include H2SO4,

18


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

HNO3, sulfates and nitrates. The H2SO4 and HNO3 in the reaction solutions can be converted to the recyclable

2

sulfates and nitrates after adding alkaline substances such as potassium carbonate. The liquid products (sulfates

3

and nitrates) can be further converted to some solid products after being evaporated and crystallized (or being

4

separated) using the waste heat from the flue gas of boilers and furnaces. The flue gas temperature from boilers

5

and furnaces is usually between 120℃ and 200℃, thus the evaporation and enrichment of the reaction solutions

6

would not need extra heat and energy (water vapor would be also condensed and recycled), which may

7

significantly reduce the post-processing costs of the products in the future application.

8 9

Table 1. Measurement of products/byproducts and mass balance calculation of NO in gas-liquid two phases SO42-

Oxone= 0.25 mol/L Measured anion concentration (mg/L)

4.32 × 104

SO32-

NO3-

NO2-

0

64.1

0

0

NO2 (ppm)

Calculated anion concentration (mg/L)

—

—

61.3

—

—

Relative error (%)

—

—

4.37%

—

—

10 11

3.9 Discussions on mechanism and route of NO oxidation removal

12

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

13

using UV and heat coactivated Oxone system can be summarized as the several steps as below: (1) The key free

14

radicals, SO4 · and ·OH, are produced in liquid phase by UV and heat coactivation of Oxone (Eq.s 4,13 and 14 );

15

(2) Based on the double film theory and the present results about kinetics in this article, the gaseous NO in the gas

16

phase body enters the liquid film to form the liquid state NO; (3) The oxidizing agents such as SO4 ·, ·OH and

17

Oxone can finally oxidize NO to nitric acid through a series of reactions (Eq.s 2,3 and 5-12) in the liquid film; (4)

18

In the presence of SO2, part of SO4 ·, ·OH and Oxone will be consumed by SO2 (Eq.s 22-29); (5) In the liquid film

-

-

-

19


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Page 20 of 26

1

and/or the liquid phase body, some side reactions (Eq.s 15-19) also will occur.

2

3.10 Kinetics of NO removal

3

For a gas-liquid reaction, studying the mass transfer reaction-kinetic process is the key works for

4

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

5

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

6

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

7

chemical reaction of NO oxidation removal using UV and heat coactivated Oxone system can be described by the

8

following equation (30): activation a NO + b Oxone UV/heat  → cNO-3 + d Other by - products

9 10 11 12

(30)

where a,b,c and d are the stoichiometric coefficients for the reactants, the products and the by-products. The intrinsic rate equation of NO oxidation removal using UV and heat coactivated Oxone system can be expressed to the equation (31) as below: m n rNO = km , n ⋅ C NO , i ⋅ COxone

13

(31)

14

where rNO is the chemical reaction rate of NO removal, mol /( L ⋅ s ) ; km , n is the pseudo-(m+n)-order reaction

15

rate constant for the total reaction (30), L( m + n −1) ⋅ mol (1− m − n ) ⋅ s −1 ; C NO , i represents the interface concentration

16

for NO, mol / L ; COxone represents the Oxone concentration, mol / L ; m and n represent the partial reaction

17

orders for NO and Oxone , respectively;

18

On the basis of the rate equation (31), the double-film theory and the results of the other researchers,

19

21,33,39,42-44

20

below:

for a fast reaction, the absorption rate of NO in solutions can be described by the equation (32) as

1/ 2

21

22

N NO

m +1 n  2 DNO , L ⋅ km, n ⋅ CNO  , i ⋅ COxone   =  + m 1  

(32)

where N NO represents the absorption rate of NO, mol / m 2 ⋅ s ; DNO , L represents the liquid phase diffusion 20


Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Energy & Fuels

coefficient of NO, m 2 / s . In the equation (32), N NO can be calculated by the equation (33) as below:11,21,39,45-47

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

3

(33)

4

where QG represents the total gas flow rate, L / min ; M NO represents the molecular weight of NO, g/mol ;

5

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

6

The interface concentration of NO, C NO ,i ,can be obtained by the equation (34) as below:11,21,39,46

7

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

8

where H NO , L represents the solubility coefficient of NO, mol /( L ⋅ Pa ) ; pNO , G represents the partial pressure

9

of NO in the gas-phase, Pa ; k NO ,G represents the gas-phase mass transfer coefficient of NO, mol / s ⋅ m 2 ⋅ Pa .

10

H NO , L can be obtained by the literatures;42-44,46 DNO , L can be calculated using the Wilke-Chang empirical

11

equation.39,42-44,46 k NO , L , k NO ,G and a NO can be determined using the classical chemical methods, and the

12

described calculation and measurement methods can be found in the references39,45,46 or the “Support information”.

13

These key parameters at optimized solution temperature of 328 K were shown in Table 2.

14

(34)

Table 2. The physical and mass transfer parameters of NO.

DNO , L × 109

H NO , L × 108

k NO , L × 104

k NO ,G × 106

a NO

m2 / s

mol / L ⋅ Pa

m/s

mol / s ⋅ m 2 ⋅ Pa

m −1

3.61

1.01

3.43

3.39

85.7

15

21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

1 2 3 4

5

6

Figure 7. (a) Plot of Log (CNO, i) vs Log (NNO) and (b) plot of Log (COxone) vs Log (NNO). To determine the m value in the equation (32), the following equation (35) is obtained by taking a logarithm for both sides of the equation (32):

log N NO

n m +1 1  2 DNO , L ⋅ km , n ⋅ COxone    = log C NO ,i + log  2 2  m +1 

(35)

where N NO is calculated by the equation (33), and CNO,i is calculated by the equations (33) and (34).

7

Based on the data of NO concentration vs NO removal efficiency in Figure 6(d) and the above equations

8

(33)/(34), the plot of Log (CNO, i) vs Log (NNO) has been conducted, which is displayed in Figure 7(a). As

9

displayed in Figure 7(a), Log (NNO) has a linear relationship with Log (CNO, i). The slope of the ﬁtted line, (m+1)/2,

10

is 0.961, and thus m is 0.922 ≈ 1.0. This result suggest that NO removal using UV and heat coactivated Oxone

11

system in the reactor can be regarded as a pseudo-1.0-order reaction for NO.

12 13 14 15

To calculate n, m=1 was brought back to the above equation (32), and the equation (32) is finally changed to the equation (36) as below with appropriate conversion:

log( N NO / C NO , i ) =

(

)

(

1 n log COxone + log km , n ⋅ DNO , L 2 2

)

(36)

where N NO is calculated by the above equation (33), and C NO ,i is calculated by the equations (33) and (34).

16

Based on the data of Oxone concentration vs NO removal efficiency in Figure 3(b) and the above equations

17

(33)/(34), the plot of Log (COxone) vs Log (NNO) has been conducted, which is displayed in Figure 7(b). As 22


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Energy & Fuels

1

displayed in Figure 7(b), Log (NNO) has a linear relationship with Log (COxone). The slope of the ﬁtted line (n/2) is

2

0.221. Thus n= 0.442 ≈ 0.44, suggesting that NO removal process can be regarded as a pseudo-0.44-order reaction

3

with respect to Oxone in the UV and heat coactivated Oxone system.

4

Thus, NO absorption rate equation (32) can be further represented as the following expressions (37):

5

0.44 N NO = DNO , L ⋅ km , n ⋅ C NO ,i ⋅ COxone

6

According to the double-film theory, the Hatta coefficient Ha can be described by the equation (38) as

7

(37)

below.38-40 1/ 2

 2  m −1 n k m, n ⋅ DNO , L ⋅ C NO  , i ⋅ COxone  m +1  Ha =  k NO , L

8

(38)

9

where Ha represents the ratio between the chemical reaction rate and the physical absorption rate.

10

When Ha < 0.03 , the absorption of NO in liquid phase is a slow reaction (i.e., chemical reaction rate is far less

11

than mass transfer rate); when 3.0 > Ha > 0.03 , the absorption of NO in liquid phase is a moderate speed

12

reaction (i.e., chemical reaction rate is approximately equal to mass transfer rate); when Ha > 3.0 , the

13

absorption of NO in liquid phase is a fast reaction (i.e., chemical reaction rate is far larger than mass transfer

14

rate).42-44

15 16

17

Ha can be obtained when both m and n values are brought back to the above equation (38), and the equation (38) is finally changed to the equation (39) as below:

Ha =

km , n ⋅ DNO , L k NO , L

0.44 ⋅ COxone

(39)

18

On the basis of the above discussions, the key kinetic parameters such as N NO , km , n and Ha are calculated

19

at the optimized temperature of 328 K, which are displayed in Table 3. As displayed in Table 3, we can see that

20

under all tested conditions, Ha > 3.0 , indicating that NO removal using UV and heat coactivated Oxone system

21

in the reactor belongs to a fast reaction, and the mass transfer is the rate control step for NO removal.42-44 The 23


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

1

mass transfer and kinetic data in Table 3 can provide important reference for industrial amplification and design of

2

the reactor and numerical simulation of NO removal process in the future application.

3

Table 3. N NO , km,n and Ha of NO removal using UV and heat coactivated Oxone system Oxone concentration, mol/L

0.04

0.12

0.2

0.25

0.30

—

N NO × 10-6 ( mol / m 2 ⋅ s )

3.79

4.80

5.44

5.73

5.85

—

km,n × 106 ( L0.44 /(mol 0.44 ⋅ s) )

1.89

1.91

1.98

2.00

1.91

—

Ha

37.3

47.8

54.5

57.6

58.8

—

57

94

147

218

—

Light intensity, µW/cm2

0

N NO × 10-6 ( mol / m 2 ⋅ s )

0.86

3.62

4.82

5.73

5.95

—

km,n × 106 ( L0.44 /(mol 0.44 ⋅ s) )

0.04

0.76

1.39

2.00

2.11

—

Ha

8.2

35.5

48.0

57.6

59.1

—

1.2

4.4

6.9

9.3

11.7

—

N NO × 10-6 ( mol / m 2 ⋅ s )

5.56

5.73

5.69

5.19

3.91

—

km,n × 106 ( L0.44 /(mol 0.44 ⋅ s) )

1.87

1.96

1.63

0.93

—

Solution pH

2.00

55.7

57.6

57.0

52.0

NO concentration, ppm

150

300

500

800

1200

1600

N NO × 10-6 ( mol / m 2 ⋅ s )

2.56

5.73

7.77

11.76

16.31

21.12

km,n × 106 ( L0.44 /(mol 0.44 ⋅ s) )

1.60

2.00

1.32

1.18

1.00

0.94

Ha

51.5

57.6

46.8

44.2

40.7

39.4

0

300

600

1000

1500

2000

N NO × 10-6 ( mol / m 2 ⋅ s )

5.97

5.93

5.76

5.73

5.48

5.31

km,n × 106 ( L0.44 /(mol 0.44 ⋅ s) )

2.19

2.02

2.00

1.82

1.71

SO2 concentration, ppm

2.16 24


39.2

—

Ha

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Ha

60.2

59.8

57.9

57.6

54.9

53.2

1 2

4. Conclusions

3

The main conclusions of this article are as below: (1) UV and Oxone have very significant synergistic effect

4

for production of free radicals and improving NO removal efficiency. NO oxidation removal was improved by

5

increasing light intensity, Oxone concentration or solution flow rate, and was inhibited with increasing NO

6

concentration, SO2 concentration or flue gas flow rate. Solution temperature and pH have double impacts on NO

7

oxidation removal; (2) UV-light activation for Oxone is the main source of SO4−· and ·OH. Heat-activation for

8

Oxone is the complementary source of SO4−· and ·OH. SO4−· and ·OH are the key oxidizing agents, and play an

9

important role in NO oxidation removal. Oxone only plays a complementary role in NO oxidation removal; (3)

10

The results of kinetics show that NO removal using UV and heat coactivated Oxone system is a fast reaction, and

11

meets a total 1.44-order reaction (i.e. 1.0-order for NO and 0.44-order for Oxone). The mass transfer is the rate

12

control step of NO removal. NO removal may be further enhanced by strengthening mass transfer rate of NO.

13

Acknowledgements

14

This study was supported by National Natural Science Foundation of China (Nos.U1710108; 51576094),

15

Jiangsu “Six Personnel Peak” Talent-Funded Projects (GDZB-014), China Postdoctoral Science Foundation

16

(2017M610306), Key Laboratory of Efficient & Clean Energy Utilization, College of Hunan Province (Changsha

17

University of Science & Technology) (No.2017NGQ002).

18

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