Dependence of N2O/NO Decomposition and Formation on Temperature and Residence Time in Thermal Reactor

energies

Article Dependence of N2O/NO Decomposition and Formation on Temperature and Residence Time in Thermal Reactor

Sang Ji Lee 1, Jae Geun Yun 1, Han Min Lee 1, Ji Yeop Kim 1, Jin Han Yun 2 and Jung Goo Hong 1,*

1 School of Mechanical Engineering, Kyungpook National University, Buk-gu, Daegu 41566, Korea; [email protected] (S.J.L.); [email protected] (J.G.Y.); [email protected] (H.M.L.); [email protected] (J.Y.K.) 2 Department of Environmental Machinery, Korea Institute of Machinery & Materials, Yuseong-gu, Daejeon 34103, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-53-950-6570

Abstract: Nitrogen dioxide (N2O) is a greenhouse gas that is harmful to the ozone layer and con- tributes to global warming. Many other nitrogen oxide emissions are controlled using the selective

non-catalytic reaction (SNCR) process, but N2O reduction methods are few. To avoid future air pollution problems, N2O reduction from industrial sources is essential. In this study, a N2O decomposition and NO formation under an argon atmospheric N2O gas mixture were observed in a lab-scale SNCR system. The reaction rate and mechanism of N2O were calculated using a reaction path analyzer (CHEMKIN-PRO). The residence time of the gas mixture and the temperature in the reactor were

set as experimental variables. The results conﬁrmed that most of the N2O was converted to N2 and

NO. The change in the N2O reduction rate increased with the residence time at 1013 and 1113 K, but decreased at 1213 K due to the inverse reaction. NO concentration increased with the residence time

Citation: Lee, S.J.; Yun, J.G.; Lee, at 1013 and 1113 K, but decreased at 1213 K owing to the conversion of NO back to N2O. H.M.; Kim, J.Y.; Yun, J.H.; Hong, J.G. Keywords: Dependence of N2O/NO nitrous oxide (N2O); nitric oxide (NOx); argon (Ar) ambient; thermal decomposition; Decomposition and Formation on residence time; rate of progress; GRI-Mech 3.0 Temperature and Residence Time in Thermal Reactor. Energies 2021, 14, 1153. https://doi.org/10.3390/ en14041153 1. Introduction

Nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) are considered green- Academic Editor: Mejdi Jeguirim house gases. N2O is very stable in air, and takes an average of 135 years to decompose naturally. Upon reaching the stratosphere, N2O reacts with an oxygen atom (O) to form Received: 1 February 2021 nitrogen monoxide (NO). NO reacts with ozone (O ) again in the ozone layer and destroys Accepted: 17 February 2021 3 O in a chain [1,2]. Therefore, N O has 310 times higher global warming potential (GWP) Published: 22 February 2021 3 2 than CO2. N O was designated as a greenhouse gas by the Kyoto Protocol in 1997. Afterward, Publisher’s Note: MDPI stays neutral 2 N O was regulated as a greenhouse gas under the Paris Agreement adopted at the UN with regard to jurisdictional claims in 2 published maps and institutional affil- Climate Change Conference in 2015. However, in most countries, including South Korea, iations. specific reduction policies for N2O are still inadequate [3]. N2O reduction methods can be divided into two methods. The first is the inhibition of N2O production. The second is N2O removal at the end of the exhaust [4–6]. In the first method, N2O generation can be suppressed using a fluid medium in the fluidized bed combustion process (FBC) and by using a catalyst mounted on a generation source Copyright: © 2021 by the authors. such as a vehicle [4–6]. However, this method is greatly affected by the N O temperature Licensee MDPI, Basel, Switzerland. 2 and pressure conditions. The second method involves reducing the N O generated in This article is an open access article 2 distributed under the terms and the incinerator and the industrial process at a subsequent stage [4–6]. This technology of conditions of the Creative Commons N2O reduction at the end of the process is largely divided into selective catalytic reduction Attribution (CC BY) license (https:// (SCR) and selective non-catalytic reduction (SNCR). The SCR process uses a catalyst, so the creativecommons.org/licenses/by/ facility can be operated at a lower temperature than that required for SNCR. The SNCR 4.0/).

Energies 2021, 14, 1153. https://doi.org/10.3390/en14041153 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 11

Energies 2021, 14, 1153 2 of 11

SNCR process operates at a relatively high temperature (approximately 1000–1500 K) comparedprocess operates with the at SCR a relatively process, high and temperaturethe initial investment (approximately cost is 1000–1500lower than K) other compared pro- cesseswith the[4–8]. SCR process, and the initial investment cost is lower than other processes [4–8]. TheThe SNCR SNCR process process is a is NOx a NOx reducing reducing process, process, where where the NOx the NOxgenerated generated in the com- in the bustioncombustion process process of an of incinerator an incinerator is reduced is reduced using using a reductant. a reductant. Specifically, Speciﬁcally, a areductant reductant such as ammonia (NH3) and urea aqueous solution (NH2CONH2) is injected into the such as ammonia (NH3) and urea aqueous solution (NH2CONH2) is injected into the chamber at temperatures of 1173–1373 K, thereby reducing NOx to nitrogen (N2) and wa- chamber at temperatures of 1173–1373 K, thereby reducing NOx to nitrogen (N2) and water 3 tervapor vapor [9 [9].]. In In general, general, NH NH3 isis moremore efﬁcientefficient thanthan the urea urea aqueous aqueous solution, solution, but but urea urea is is easiereasier to to handle handle and and results results in in a aless less costly costly process process [10,11]. [10,11]. FigureFigure 11 shows shows the the decomposition decomposition mechanism mechanism and and reaction reaction pathway pathway of of NH NH3 3andand urea urea aqueousaqueous solution solution in in the the SNCR SNCR process process [12]. [12]. In In the the process, process, the the Zel'dovich Zel’dovich mechanism mechanism is is usedused as as the the basic basic reaction reaction pathway pathway for for oxidation oxidation and and deoxidation deoxidation of of N N2 2[13].[13].

FigureFigure 1. 1. BasicBasic decomposition decomposition mechanism mechanism of of nitrous nitrous oxideoxide usingusing ammoniaammoniaand and urea urea aqueous aqueous solution solu- tionreductants reductants [13 ].[13].

InIn a aSNCR SNCR process process that that uses uses urea urea aqueous aqueous solution solution as a as reductant, a reductant, NOx NOx is reduced is reduced to Nto2 and N2 Nand2O N[14].2O[ Svoboda14]. Svoboda et al. reported et al. reported that the that reductant the reductant employed employed in the process in the processaffects theaffects production the production of N2O [15]. of N Moreover,2O[15]. Moreover, Kim reported Kim that reported a small that amount a small of amount N2O was of gen- N2O eratedwas generated when urea when aqueous urea aqueoussolution solutionwas injected was injected for NOx for removal NOx removal in the inoperation the operation of a wasteof a waste incinerator incinerator [16]. [16]. CarbonCarbon monoxide monoxide (CO) andand oxygenoxygen (O (O2)2 are) are typical typical reaction reaction gases gases that that affect affect the SNCR the SNCRprocess. process. In the In oxygen-free the oxygen-free process, process, the NOxthe NOx reducing reducing reaction reaction occurs occurs at temperatures at tempera- turesof 1400 of 1400 K or K higher. or higher. This This results results from from the NOthe NO reduction reduction proceeding proceeding at a at relatively a relatively high hightemperature temperature due todue the to low the OHlow concentration, OH concentrat therebyion, thereby leading leading to an increase to an increase in the reaction in the reactiontemperature. temperature. Im reported Im reported that thethat optimalthe optimal O2 concentrationO2 concentration in in the the SNCR SNCR process process is is5–7% 5–7% [ 17[17].]. InIn a astudy study on on the the residence residence time, time, Liang Liang et et al. al. reported reported that that the the optimum optimum temperature temperature for N O decomposition and NOx formation decreased with increasing residence time for N2O2 decomposition and NOx formation decreased with increasing residence time of theof mixture the mixture [18]. Various [18]. Various studies studies considering considering the dependence the dependence of NO reduction of NO reduction on the tem- on peraturethe temperature and residence and residencetime have used time ammonia have used as ammonia a reductant as in a reductantthe SNCR process in the SNCR [19– 22].process Duo et[19 al.–22 reported]. Duo etthat al. the reported optimum that temperature the optimum for temperatureNO reduction for decreased NO reduction with increasingdecreased residence with increasing time [23]. residence time [23]. Previous studies have conﬁrmed that the reduction efﬁciency of N2O is determined Previous studies have confirmed that the reduction efficiency of N2O is determined by complex factors such as the amount of reductant, mixture composition, and residence by complex factors such as the amount of reductant, mixture composition, and residence time. Most of those studies focused on controlling the amount of N2O generated during time. Most of those studies focused on controlling the amount of N2O generated during the NOx reduction process in the SNCR. Additionally, studies of N2O decomposition have the NOx reduction process in the SNCR. Additionally, studies of N2O decomposition have mostly used catalysts. [24–26]. In the SNCR system, N2O is less regulated than NOx; hence, mostly used catalysts. [24–26]. In the SNCR system, N2O is less regulated than NOx; hence, the reduction of N2O has rarely been investigated. Therefore, in this study, we used a the reduction of N2O has rarely been investigated. Therefore, in this study, we used a laboratory-scale thermal reactor that simulated an actual SNCR system, and observed the N2O and NO behaviors that were generated in combustion furnace.

Energies 2021, 14, x FOR PEER REVIEW 3 of 11

laboratory-scale thermal reactor that simulated an actual SNCR system, and observed the N2O and NO behaviors that were generated in combustion furnace. Energies 2021, 14, 1153 The reaction characteristics of N2O and NO were studied through thermal decompo-3 of 11 sition experiments, where the temperature and residence time in an argon (Ar) atmosphere gas mixture were varied. Furthermore, the reaction equation and reaction rate (rate of progress) of N2O and NO were examined using a chemical reaction program (CHEM- KIN-PRO)The reaction that simulated characteristics the same of N conditions2O and NO as were the studiedexperiment. through Correlations thermal decomposi- between tion experiments, where the temperature and residence time in an argon (Ar) atmosphere the reduction of N2O and NO, and the reaction rate based on the temperature and resi- gas mixture were varied. Furthermore, the reaction equation and reaction rate (rate of dence time, were investigated. progress) of N2O and NO were examined using a chemical reaction program (CHEMKIN- PRO) that simulated the same conditions as the experiment. Correlations between the 2. Experimental Methods reduction of N2O and NO, and the reaction rate based on the temperature and residence 2.1.time, Experimental were investigated. Setup Figure 2 shows a schematic of the experimental setup used to investigate the N2O thermal2. Experimental decomposition Methods characteristics. The setup consisted of a gas inlet part, a thermal reactor,2.1. Experimental a measuring Setup part, and an exhaust port. The flow of experimental gases Ar, O2, and

N2O intoFigure the2 thermalshows a reactor schematic were of controlled the experimental using a setup mass usedflow tocontroller investigate (MFC). the NThe2O reactorthermal had decomposition a cylindrical characteristics.structure (inner The diameter: setup consisted140 mm, ofand a gaslength: inlet 1000 part, mm), a thermal was anreactor, electric a measuringfurnace capable part, andof heating an exhaust up to port. 1273 TheK, and ﬂow was of experimentalheated with four gases internal Ar, O2 , heatingand N2 Ocoils. into The the temperature thermal reactor inside were the controlled reactor was using measured a mass ﬂow using controller an R-type (MFC). thermo- The couplereactor placed had a cylindricalin the center structure of each (innercoil. Af diameter:ter the thermal 140 mm, reaction, and length: the exhaust 1000 mm), gas passed was an throughelectric furnacea heat exchanger. capable of A heating non-dispersive up to 1273 infrared K, and was(NDIR) heated type with gas fourmeter internal (Sensonic heating IR- 1,coils. for the The N2 temperatureO measurement) inside and the a gas reactor analyzer was measured(MK 6000+, using com GmbH, an R-type for thermocouplethe NOx and Oplaced2 measurement) in the center were of each used coil. to measure After the the thermal gas components. reaction, the The exhaust measurement gas passed error through of thea heat NDIR exchanger. gas meter A(equipped non-dispersive with an infrared electrochemical (NDIR) sensor) type gas was meter 3%. (SensonicThe measurement IR-1, for errorthe N of2O NO measurement) and N2O of the and gas a gas analyzer analyzer (also (MK equipped 6000+, comwith GmbH, an electrochemical for the NOx sensor) and O2 wasmeasurement) 2% [27]. were used to measure the gas components. The measurement error of the NDIRFigure gas meter3 shows (equipped a photograph with anof a electrochemical laboratory-scale sensor) SNCR wasapparatus 3%. The for measurementN2O thermal decomposition.error of NO and To N 2preventO of the heat gas analyzerloss and (alsoto maintain equipped the withtemperature an electrochemical employed sensor)in the experimentalwas 2% [27]. conditions, the reactor was surrounded by an insulator.

FigureFigure 2. 2. AA scheme scheme of of the the experimental experimental setup setup [27]. [27].

Figure3 shows a photograph of a laboratory-scale SNCR apparatus for N 2O thermal decomposition. To prevent heat loss and to maintain the temperature employed in the experimental conditions, the reactor was surrounded by an insulator.

Energies 2021, 14, 1153 4 of 11 Energies 2021, 14, x FOR PEER REVIEW 4 of 11

FigureFigure 3. 3.The The laboratory laboratory scale scale selective selective non-catalytic non-catalytic reaction reaction (SNCR) (SNCR) system. system.

2.2.2.1. Experimental Experimental Conditons Conditons

TableTable1 1shows shows the the composition composition of of the the gas gas mixture mixture used used in in the the N N2O2O thermalthermal decompo-decompo- sitionsition experiment experiment [ 27[27].]. The The gas gas mixture mixture was was composed composed of of 94.96% 94.96% Ar, Ar, 5% 5% O O2,2, and and 400 400 ppm ppm ofof NN22OO toto simulate the exhaust gas gas in in a a selective selective non-catalytic non-catalytic reduction reduction process. process. The The ac- actualtual combustion combustion exhaust exhaust gas gas isis N N2 and2 and CO CO2 atmosphere.2 atmosphere. However, However, in inthis this experiment, experiment, an anAr Ar atmosphere atmosphere was was employed employed to minimize to minimize the theinvolvement involvement of nitrogen of nitrogen components components from fromsources, sources, in addition in addition to the to N the2 generated N2 generated by thermal by thermal decomposition decomposition of N2O. of Moreover, N2O. More- the over, the molecular weights of N O and CO were the same, and hence measurement molecular weights of N2O and CO2 2 were the2 same, and hence measurement errors oc- errorscurred occurred in the NDIR in the measurement. NDIR measurement. Therefore, Therefore, no hydrocarbon-based no hydrocarbon-based material material was added. was added. The chemical reaction time of NOx and N O depends on the residence time and The chemical reaction time of NOx and N2O depends2 on the residence time and tempera- temperature.ture. Accordingly, Accordingly, the residence the residence time of time the of gas the mixture gas mixture in the in thethermal thermal reactor reactor was was se- selected as an experimental variable. The dependence of the N2O thermal decomposition lected as an experimental variable. The dependence of the N2O thermal decomposition on on the residence time was investigated by fixing the composition of the gas mixture and the residence time was investigated by fixing the composition of the gas mixture and var- varying the residence time (times of 10, 20, and 40 s were employed). The gas mixture flow ying the residence time (times of 10, 20, and 40 s were employed). The gas mixture flow rate corresponding to each residence time was calculated from the equation of the reactor rate corresponding to each residence time was calculated from the equation of the reactor volume and residence time (Equation (1)). In this equation, Q is the gas mixture flow rate, volume and residence time (Equation (1)). In this equation, Q is the gas mixture flow rate, D is the inner diameter of the reactor, L is the length of the reactor, and t is the residence D is the inner diameter of the reactor, L is the length of the reactor, and t is the residence time. time. π × (D/2)2 × L Q = (1) ×/2t × Q= (1) Table2 shows the experimental conditions [27]. The experimental temperature was increasedTable in 2 20shows K intervals the experimental for temperatures conditions ranging [27]. fromThe experimental 1023 to 1223 temperature K, which is thewas generalincreased operating in 20 K temperatureintervals for oftemperatures the SNCR process. ranging In from order 1023 to maintainto 1223 K, the which residence is the timegeneral of the operating gas mixture temperature on each temperatureof the SNCR section, process. the In flow order rate to wasmaintain adjusted the basedresidence on Charles’stime of the law, gas which mixture is given on each as follows: temperature section, the flow rate was adjusted based on Charles’s law, which is given as follows:Q Q 1 = 2 (2) T T 1 =2 (2) where Q1 is the inlet flow rate, T1 is the inlet temperature, Q2 is the flow rate for each residencewhere Q1 time, is the and inletT2 flowis the rate, reactor T1 is temperature. the inlet temperature, The gas concentration Q2 is the flow was rate measured for each inresidence 1 min intervals time, and after T2 is 10 the min reactor of supplying temperature. the The gas gas mixture. concentration For gas was measurements, measured in the1 min con-centration intervals after of 10 exhaust min of gassupplying from the the rear gas endmixture. of the For reactor gas measurements, was measured. the Thecon- measuredcentration data of exhaust are reported gas from as the the average rear en valued of the of five reactor measurements was measured. performed The measured for each experimentaldata are reported condition. as the Laboratory average value ambient of five conditions measurements were maintained performed atfor atmospheric each experi- airmental (298 K,condition. 1 atm). Laboratory ambient conditions were maintained at atmospheric air (298 K, 1 atm).

Energies 2021, 14, 1153 5 of 11

Table 1. Test gas condition [27].

Residence Time (s) Gas Composition

10, 20, 40 Ar (94.96%), O2 (5%), N2O (400 ppm)

Table 2. Experiment conditions [27].

Condition Value Temperature (K) 1013–1213 Flow Rate (L/min) 5.44–25.81 Ambient Temperature (K) 298 Ambient Pressure (atm.) 1

2.3. CHEMKIN Calculation Conditon CHEMKIN-PRO [28] calculations were performed by simulating the thermal reactor used in the N2O thermal decomposition experiment. The 1D reaction structure analysis plug flow reactor model, a cylindrical reactor model that assumes a steady state during flow, was used for the calculations. The input gas was assumed to be perfectly mixed, and continuous chemical reactions inside the reactor were observed. The model of a reaction furnace with an 140 mm inner diameter and a 1000 mm length was built, which was the same as the actual experimental apparatus. The temperature of the thermal reactor was divided into 11 intervals, ranging from 1023 to 1223 K. The residence time of the gas mixture was calculated in the same manner as the experimental conditions at 10, 20, and 40 s. Furthermore, the N2O chemical reaction (see Table3) was calculated from the chemical reaction equation included in GRI 3.0 [29], and the chemical reaction route was confirmed via reaction path analysis.

Table 3. GRI-Mech 3.0.

k = ATb exp(−E/RT) Reaction A (cm3/gmol)/s b E (kJ/gmol) 10 N2O(+M) <=> N2+O(+M) (1) 7.9 × 10 0 56,020 12 N2O+O <=> N2+O2 (2) 1.4 × 10 0 10,810 13 N2O+O <=> 2NO (3) 2.9 × 10 0 23,150 12 NO2+O <=> NO+O2 (4) 3.9 × 10 0 −240.00 20 NO+O+M <=> NO2+M (5) 1.06 × 10 −1.41 0 13 N+NO <=> N2+O (6) 2.7 × 10 0 355 9 N+O2 <=> NO+O (7) 9.0 × 10 1 6500

3. Results and Discussion 3.1. Formation of Nitrogen and Nitrogen Monoxide by Thermal Decomposition of Nitrous Oxide Figure4 shows the gas concentration calculated using CHEMKIN-PRO. Calculation conditions of residence time: 1 s, gas composition: 99.96% Ar, 400 ppm N2O, and reactor temperature: 1020–1220 K, were employed. The results conﬁrmed that the concentration of N2O in the reactor generally decreased with increasing temperature. In the initial temperature condition (1013 K), the N2O concentration was 397 ppm. Concentrations of 375 and 253 ppm occurred at 1113 and 1213 K, respectively. The concentration of N2 was 2 ppm under the initial temperature conditions. The concentration of N2 increased from 23 ppm at 1113 K to 138 ppm at 1213 K. This resulted from N2O thermally decomposing into two N and one O (see Reaction (1) of Table3) with increasing reactor temperature. The decomposed two N collided with each other, thereby generating an activation energy, and stable N2 was formed. NO was not formed under the initial temperature condition of 1013 K, but ~8 ppm formed when the temperature increased to 1213 K. This resulted from Energies 2021, 14, x FOR PEER REVIEW 6 of 11 EnergiesEnergies 20212021,, 1414,, 1153x FOR PEER REVIEW 66 of of 11

and N being converted into NO (see Reactions (3) and (7) of Table 3). Therefore, with the and N being converted into NO (see Reactions (3) and (7) of Table 3). Therefore, with the Nreduction2O and N of being N2O, converteda small amount into NO of (seeNO Reactionswas generated. (3) and (7) of Table3). Therefore, with reduction of N2O, a small amount of NO was generated. the reduction of N2O, a small amount of NO was generated.

Figure 4. Dependence of the calculated gas concentration on the temperature. Figure 4. Dependence of the calculated gas concentration on the temperature.temperature. 3.2.3.2. Dependence ofof NitrousNitrous OxideOxide ReductionReduction RateRate onon thethe ResidenceResidence TimeTime 3.2. Dependence of Nitrous Oxide Reduction Rate on the Residence Time FigureFigure5 5shows shows the the dependence dependence of of the the N 2NO2O reduction reduction rate rate on on the the residence residence time time in the in Figure 5 shows the dependence of the N2O reduction rate on the residence time in reactor.the reactor. The rateThe israte expressed is expressed as the as ratio the of ratio the inletof the N 2inletO concentration N2O concentration (400 ppm), (400 and ppm), the the reactor. The rate is expressed as the ratio of the inlet N2O concentration (400 ppm), reducedand the reduced N2O concentration N2O concentration after the after reaction, the reaction, i.e., i.e., and the reduced N2O concentration after the reaction, i.e., Reduced N O Concentration % = 2 × 100 (3) N2O Reduction (%%) == ××100 100 (3) Inlet N2O Concentration (3) The gas mixture composition was 94.96% Ar, 5% O2, and 400 ppm N2O. Residence The gas mixture compositioncomposition was 94.96% Ar, 5%5% OO22,, andand 400400 ppmppm NN22O.O. ResidenceResidence times of 10, 20, and 40 s, and reaction temperatures of 1013, 1113, and 1213 K were em- times of of 10, 10, 20, 20, and and 40 40 s, s,and and reaction reaction temperatures temperatures of 1013, of 1013, 1113, 1113, and and 1213 1213 K were K were employed. For each residence time, the N2O reduction rate increased with increasing reaction ployed.employed. For each For eachresidence residence time, time,the N2 theO reduction N2O reduction rate increased rate increased with increasing with increasing reaction temperature because both variables (i.e., the time and the temperature) affect the forward temperaturereaction temperature because both because variables both variables (i.e., the ti (i.e.,me theand time the temperature) and the temperature) affect the affect forward the reaction of the N2O thermal decomposition process. However, in an actual reactor, in- reactionforward reactionof the N of2O the thermal N2O thermal decomposition decomposition process. process. However, However, in an inactual an actual reactor, reactor, increasing both variables infinitely is inefficient. Therefore, determining the relationship be- creasingincreasing both both variables variables infinitely infinitely is inefficient. is inefficient. Therefore, Therefore, determining determining the therelationship relationship between the residence time and the reaction temperature and finding an optimum condition tweenbetween the the residence residence time time and and the the reaction reaction te temperaturemperature and and finding finding an an optimum optimum condition between the two variables are essential. between the two variables are essential.

Figure 5. Dependence of N O reduction on the residence time. Figure 5. Dependence of N22O reduction on the residence time. Figure 5. Dependence of N2O reduction on the residence time.

Energies 2021, 14, x FOR PEER REVIEW 7 of 11

Table 4 shows the N2O reduction rate in each temperature section of the reactor based on the change in residence time shown in Figure 5. In residence time change Section 1, the change in percentage of the reduction rate increased linearly from 9.75% to 21.25% with increasing temperature in the reactor. However, the change in percentage decreased from 27.5% to 15.5% when the temperature increased from 1113 to 1213 K in change Section 2. Energies 2021, 14, 1153 7 of 11 Table 4. Reduction rate of N2O depending on change in residence time and reactor temperature.

s T 1013 K 1113 K 1213 K Residence time Change Section 1 Table4 shows the N 2O reduction rate in each9.75 temperature % section 16 % of the reactor 21.25 % based on the change in(10 residence s → 20 s) time shown in Figure5. In residence time change Section1, the Residence time Change Section 2 change in percentage of the reduction rate increased21.25 % linearly from27.5 % 9.75% to 21.25% 15.5 % with increasing temperature(20 s → 40 in thes) reactor. However, the change in percentage decreased from 27.5% to 15.5% when the temperature increased from 1113 to 1213 K in change Section2. Figures 6 and 7 show the calculation results obtained from the reaction path analyzer. TheseTable 4. resultsReduction explain rate ofthe N 2trendsO depending observed on change in Figure in residence 5 and timeTable and 4. reactorThe reaction temperature. rate of conversion from N2O to N2 based on the residence time was calculated with CHEMKIN- T PRO. Table 5 shows the specific reaction rate (r1013ate Kof progress) 1113 values K plotted in 1213 Figure K 6. s As shown in Figure 6, the reaction rate of the N2O decomposition decreased with in- Residence time Change Section1 creasing residence time (see Reactions (1) and 9.75(2) of % Table 3). This 16 %resulted from 21.25 the %chem- (10 s → 20 s) ical reaction rate being related to the concentration of the reactants. The reaction time of Residence time Change Section2 21.25 % 27.5 % 15.5 % the N2O introduced(20 s → in40 the s) reactor increased with increasing residence time, and hence the concentration of exhausted N2O after the reaction decreased. When the N2O concentration decreased, the number of molecules per unit volume in the reactor and, consequently, the collisionsFigures between6 and7 showthese themolecules calculation decreased. results obtainedTherefore, from the thenumber reaction of pathchemical analyzer. reac- These results explain the trends observed in Figure5 and Table4. The reaction rate of tions and the reaction rate were reduced, leading to an increase in the N2O reduction rate conversion from N O to N based on the residence time was calculated with CHEMKIN- with increasing residence2 time2 and temperature, as shown in Figure 5. PRO. Table5 shows the speciﬁc reaction rate (rate of progress) values plotted in Figure6.

(a) 1013 K (b) 1113 K Energies 2021, 14, x FOR PEER REVIEW 8 of 11 2 2 33 FigureFigure 6.6.Rate Rate ofof progressprogress from from N N2OO toto NN2(M/cm(M/cm s)s) at at 1013 1013 and and 1113 1113 K. K.

Figure 7. Rate of progress from N2O to N2(M/cm3s) 3at 1213 K. Figure 7. Rate of progress from N2O to N2(M/cm s) at 1213 K.

Table 5. Rate of progress between N2O and N2.

K s 10 20 40 Forward reaction rate: 4. 6 × 10−11 Forward reaction rate: 3.64 × 10−11 Forward reaction rate: 2.37 × 10−11 1013 K Reverse reaction rate: −1.54 × 10−17 Reverse reaction rate: −2.32 × 10−17 Reverse reaction rate: −2.74 × 10−17 Forward reaction rate: 7.52 × 10−11 Forward reaction rate: 1.89 × 10−11 Forward reaction rate: 1.21 × 10−11 1113 K Reverse reaction rate: −2.14 × 10−16 Reverse reaction rate: −7.94 × 10−17 Reverse reaction rate: −5.42 × 10−17 Forward reaction rate: 6.62 × 10−13 Forward reaction rate: 1.49 × 10−15 Forward reaction rate: 5.29 × 10−17 1213 K Reverse reaction rate: −4.1 × 10−17 Reverse reaction rate: −3.63 × 10−17 Reverse reaction rate: −3.63 × 10−17

Figure 7 shows that the forward reaction rate decreased with increasing residence time (see Reaction (1) of Table 3). In addition, the inverse reaction rate, corresponding to the formation of the N2O, appeared with increasing residence time at 1213 K. Accordingly, the N2O reduction rate decreased significantly since N2 and O recombined to form N2O again. This observation was consistent with the previous experimental result.

3.3. Nitrogen Monoxide Concentration According to Residence Time Figure 8 shows the dependence of the NO concentration on the residence time. The gas composition and the experimental conditions are the same as those shown in Figure 5. The results confirmed that for a given residence time, the amount of NO formed increased with increasing temperature. This resulted from NO forming via thermal decomposition of N2O (see Reaction (3) in Table 3). At a reactor temperature of 1013 K, only a small amount of NO formed at residence times of 10 and 20 s, but the amount increased at 40 s. Increasing the residence time at a reactor temperature of 1113 K yielded only a slight increase in the NO concentration. However, at a reactor temperature of 1213 K, the NO concentration decreased with increasing residence time.

Energies 2021, 14, 1153 8 of 11

Table 5. Rate of progress between N2O and N2.

s 10 20 40 K Forward reaction rate: 4. 6 × 10−11 Forward reaction rate: 3.64 × 10−11 Forward reaction rate: 2.37 × 10−11 1013 K Reverse reaction rate: −1.54 × 10−17 Reverse reaction rate: −2.32 × 10−17 Reverse reaction rate: −2.74 × 10−17 Forward reaction rate: 7.52 × 10−11 Forward reaction rate: 1.89 × 10−11 Forward reaction rate: 1.21 × 10−11 1113 K Reverse reaction rate: −2.14 × 10−16 Reverse reaction rate: −7.94 × 10−17 Reverse reaction rate: −5.42 × 10−17 Forward reaction rate: 6.62 × 10−13 Forward reaction rate: 1.49 × 10−15 Forward reaction rate: 5.29 × 10−17 1213 K Reverse reaction rate: −4.1 × 10−17 Reverse reaction rate: −3.63 × 10−17 Reverse reaction rate: −3.63 × 10−17

As shown in Figure6, the reaction rate of the N 2O decomposition decreased with in-creasing residence time (see Reactions (1) and (2) of Table3). This resulted from the chemical reaction rate being related to the concentration of the reactants. The reaction time of the N2O introduced in the reactor increased with increasing residence time, and hence the concentration of exhausted N2O after the reaction decreased. When the N2O concentration decreased, the number of molecules per unit volume in the reactor and, consequently, the collisions between these molecules decreased. Therefore, the number of chemical reactions and the reaction rate were reduced, leading to an increase in the N2O reduction rate with increasing residence time and temperature, as shown in Figure5. Figure7 shows that the forward reaction rate decreased with increasing residence time (see Reaction (1) of Table3). In addition, the inverse reaction rate, corresponding to the formation of the N2O, appeared with increasing residence time at 1213 K. Accordingly, the N2O reduction rate decreased signiﬁcantly since N2 and O recombined to form N2O again. This observation was consistent with the previous experimental result.

3.3. Nitrogen Monoxide Concentration According to Residence Time Figure8 shows the dependence of the NO concentration on the residence time. The gas composition and the experimental conditions are the same as those shown in Figure5. The results conﬁrmed that for a given residence time, the amount of NO formed increased with increasing temperature. This resulted from NO forming via thermal decomposition of N2O (see Reaction (3) in Table3). At a reactor temperature of 1013 K, only a small amount of NO formed at residence times of 10 and 20 s, but the amount increased at 40 s. Increasing the residence time at a reactor temperature of 1113 K yielded only a slight increase in the Energies 2021, 14, x FOR PEER REVIEW 9 of 11 NO concentration. However, at a reactor temperature of 1213 K, the NO concentration decreased with increasing residence time.

Figure 8. Dependence of the NO concentration on the residence time.time.

Figures 9 and 10 show the calculated reaction rates of conversion from N2O to NO as determined by the CHEMKIN-PRO reaction path analyzer. The calculation conditions were the same as those employed in the experiment. The specific reaction rate values are shown in Table 6.

Table 6. Rate of progress between N2O and NO.

K s 10 20 40 Forward reaction rate: 2.38 × 10−12 Forward reaction rate: 1.63 × 10−12 Forward reaction rate: 8.01 × 10−13 1013 K Reverse reaction rate: −1.23 × 10−21 Reverse reaction rate: −3.47 × 10−21 Reverse reaction rate: −7.56 × 10−21 Forward reaction rate: 2.35 × 10−12 Forward reaction rate: 1.92 × 10−13 Forward reaction rate: 2.33 × 10−15 1113 K Reverse reaction rate: −3.0 × 10−19 Reverse reaction rate: −3.39 × 10−19 Reverse reaction rate: −3.43 × 10−19 Forward reaction rate: 7.45 × 10−16 Forward reaction rate: 1.48 × 10−18 Forward reaction rate: 5.27 × 10−20 1213 K Reverse reaction rate: −4.98 × 10−18 Reverse reaction rate: −4.98 × 10−18 Reverse reaction rate: −4.98 × 10−18

At reaction temperatures of 1013 and 1113 K, the reaction rate decreased with increasing residence time. Times of 10 and 20 s yielded only slight differences in the reaction rate, but a time of 40 s led to a significant decrease in the reaction rate. Therefore, the concentration of NO at a residence time of 40 s was higher than that at 10 and 20 s, indicating that more N2O was converted into NO. This result corresponds to the change in NO concentration shown in Figure 8, where the experimental results are presented.

(a) 1013 K (b) 1113 K

Figure 9. Rate of progress from N2O to N2(M/cm3s) at 1013 and 1113 K.

Energies 2021, 14, x FOR PEER REVIEW 9 of 11

Figure 8. Dependence of the NO concentration on the residence time.

Table 6. Rate of progress between N2O and NO.

Energies 2021, 14, 1153 9 of 11 At reaction temperatures of 1013 and 1113 K, the reaction rate decreased with increasing residence time. Times of 10 and 20 s yielded only slight differences in the reaction rate, but a time of 40 s led to a significant decrease in the reaction rate. Therefore, the con- centrationFigures of NO9 and at a 10 residence show the time calculated of 40 s was reaction higher rates than of that conversion at 10 and from 20 s, Nindicating2O to NO thatas more determined N2O was by converted the CHEMKIN-PRO into NO. This reaction result path corresponds analyzer. to The the calculation change in conditionsNO con- centrationwere the shown same as in those Figure employed 8, where inth thee experimental experiment. results The speciﬁc are presented. reaction rate values are shown in Table6.

(a) 1013 K (b) 1113 K Energies 2021, 14, x FOR PEER REVIEW 10 of 11 3 3 FigureFigure 9. 9. RateRate of of progress progress from from N N2O2O to to N N2(M/cm2(M/cms) ats) 1013 at 1013 and and 1113 1113 K. K.

Figure 10. Rate of progress from N2O to NO(M/cm3s)3 at 1213 K. Figure 10. Rate of progress from N2O to NO(M/cm s) at 1213 K.

As shown in Figure 10, the overall reaction rate was further reduced when the tem- Table 6. Rate of progress between N2O and NO. perature increased. This confirmed that the forward reaction rate decreased with increas- s ing residence time, whereas the inverse reaction rate remained the same (see Reaction (3) 10 20 40 K in Table 3). Therefore, at a temperature of 1213 K, the inverse reaction rate appeared with −12 −12 −13 Forward reactionincreasing rate: 2.38 ×residence10 timeForward and reaction NO was rate: converted 1.63 × 10 back toForward N2O; hence, reaction the rate: NO 8.01 concentra-× 10 1013 K Reverse reactiontion rate: decreased−1.23 × 10 −at21 a timeReverse of 40 reaction s. rate: −3.47 × 10−21 Reverse reaction rate: −7.56 × 10−21 Forward reaction rate: 2.35 × 10−12 Forward reaction rate: 1.92 × 10−13 Forward reaction rate: 2.33 × 10−15 1113 K −19 −19 −19 Reverse reaction4. rate: Conclusions−3.0 × 10 Reverse reaction rate: −3.39 × 10 Reverse reaction rate: −3.43 × 10 Forward reaction rate: 7.45 × 10−16 Forward reaction rate: 1.48 × 10−18 Forward reaction rate: 5.27 × 10−20 1213 K Reverse reaction rate: In−4.98 this× study,10−18 N2ReverseO reduction reaction and rate: NO−4.98 formation× 10−18 wereReverse observed reaction through rate: − 4.98thermal× 10 −de-18 composition experiments performed in a high-temperature reactor. The dependence of the chemical reaction on the residence time and temperature was confirmed by comparing At reaction temperatures of 1013 and 1113 K, the reaction rate decreased with in- the calculated reaction equation and the reaction rate with experimental results. The major creasing residence time. Times of 10 and 20 s yielded only slight differences in the reaction findings of this work are summarized as follows: rate, but a time of 40 s led to a signiﬁcant decrease in the reaction rate. Therefore, the 1.con-centration The results ofconfirmed NO at athat residence most of time the N of2O 40 was s was converted higher to than N2 and that a atsmall 10and amount 20 s, of NO via thermal decomposition in Ar atmosphere due to the high operating tem- indicating that more N2O was converted into NO. This result corresponds to the change in NOperature concentration of SNCR. shown Therefore, in Figure the8, where therma thel decomposition experimental results temperature are presented. must be con- trolledAs shown appropriately in Figure 10 to, theprevent overall the reaction generation rate of was NO. further reduced when the temper- 2.ature The increased. change in This the conﬁrmed N2O reduction that therate forward at temperatures reaction rateof 1013 decreased and 1113 with K increasingincreased with the residence time, but decreased at 1213 K and a time of 40 s. This resulted from the inverse reaction rate and the regeneration of N2O at 1213 K. Therefore, exaggerated residence times and thermal decomposition temperatures are inefficient in terms of the energy consumption compared to the reduction of N2O. 3. The NO concentration increased with residence time at temperatures of 1013 K and 1113 K, but decreased with the time at 1213 K. This resulted from the inverse reaction rate associated with the conversion of NO back to N2O. Therefore, the optimal operating conditions for the highest reduction efficiency must be studied.

Energies 2021, 14, 1153 10 of 11

residence time, whereas the inverse reaction rate remained the same (see Reaction (3) in Table3). Therefore, at a temperature of 1213 K, the inverse reaction rate appeared with increasing residence time and NO was converted back to N2O; hence, the NO concentration decreased at a time of 40 s.

4. Conclusions

In this study, N2O reduction and NO formation were observed through thermal decomposition experiments performed in a high-temperature reactor. The dependence of the chemical reaction on the residence time and temperature was conﬁrmed by comparing the calculated reaction equation and the reaction rate with experimental results. The major ﬁndings of this work are summarized as follows:

1. The results confirmed that most of the N2O was converted to N2 and a small amount of NO via thermal decomposition in Ar atmosphere due to the high operating temperature of SNCR. Therefore, the thermal decomposition temperature must be controlled appropriately to prevent the generation of NO. 2. The change in the N2O reduction rate at temperatures of 1013 and 1113 K increased with the residence time, but decreased at 1213 K and a time of 40 s. This resulted from the inverse reaction rate and the regeneration of N2O at 1213 K. Therefore, exaggerated residence times and thermal decomposition temperatures are inefficient in terms of the energy consumption compared to the reduction of N2O. 3. The NO concentration increased with residence time at temperatures of 1013 K and 1113 K, but decreased with the time at 1213 K. This resulted from the inverse reaction rate associated with the conversion of NO back to N2O. Therefore, the optimal operating conditions for the highest reduction efficiency must be studied.

Author Contributions: Conceptualization, methodology, software, validation, data curation, J.G.Y., H.M.L., J.Y.K. and J.H.Y.; writing—original draft preparation, S.J.L.; writing—review and editing, J.H.Y. and J.G.H.; supervision, J.G.H.; All authors have read and agreed to the published version of the manuscript. Funding: National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (No. 2019M1A2A2103992). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This research was supported by Technology Development Program to Solve Climate Changes through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (No. 2019M1A2A2103992). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Jang, K.S. Status and Trends of Emission Reduction Technologies and CDM Projects of Greenhouse Gas Nitrous Oxide. J. Korean Ind. Eng. Chem. 2008, 19, 17–26. 2. Choi, S.H.; Ko, J.C. Investigation of N2O Emission Characteristics and Emission Factors of Solid Fuel Incineration Facilities. J. Korean Soc. Environ. Eng. 2019, 41, 82–88. [CrossRef] 3. Yoon, I.J. Key Issues and Prospects of the Paris Agreement. Law Res. Inst. Summer Jt. Conf. 2017, 28, 113–144. 4. Jeon, S.G. N2O Reduction Technology and Catalyst Research and Development Trend. Ind. Chem. Outlook 2016, 19, 33–44. 5. Cooper, C.D.; Alley, F.C. Air Pollution Control: A Design Approach; Waveland Press: Long Grove, IL, USA, 1994; pp. 488–497. 6. Gullet, B.K.; Paul, W.G.; Lin, M.L.; James, M.C. NOx Removal with Combined Selective Catalytic Reduction and Selective Non Catalytic Reduction: Pilot-Scale Test Results. Air Waste Manag. Assoc. 1994, 44, 1188–1194. [CrossRef] 7. Fumihiko, K.; Peter, G.; Jan, E.J.; Kim, D.J. The Thermal DeNOx Process: Inﬂuence of Partial Pressures and Temperature. Chem. Eng. Sci. 1995, 50, 1455–1466. 8. Tayyeb Javed, M.; Ahmed, Z.; Asim Ibrahim, M.; Irfan, N. A comparative Kinetic Study of SNCR Process Using Ammonia. Braz. J. Chem. Eng. 2008, 25, 109–117. [CrossRef] Energies 2021, 14, 1153 11 of 11

9. Jang, J.H. Removal of NOx in SNCR Process Using Urea and Additives. Master’s Thesis, School of Environmental Engineering, The University of Seoul, Seoul, Korea, December 2003. 10. Ostberg, M.; Kim, D.J.; Johnsson, J.E. Influence of Mixing on the SNCR Process. Chem. Eng. Sci. 1997, 52, 2511–2525. [CrossRef] 11. Oxley, J.C.; Smith, J.L.; Rogers, E.; Yu, M. Ammonium nitrate: Thermal Stability and Explosivity Modifiers. Thermochim. Acta 2002, 384, 23–45. [CrossRef] 12. Smith, R.A.; Muzio, L.J.; Hunt, T. Integrated Dry NOx/SO2 Emissions Control System: Low-NOx Combustion System SNCR Test Report; Report to DOE: DOE/PC/90550-T13; National Energy Technology Laboratory: Morgantown, WV, USA, 1994. 13. Zeldovich, I.A.; Barenblatt, G.I.; Librovich, V.B.; Makhviladze, G.M. The Mathematical Theory of Combustion and Explosion; Plenum Press: New York, NY, USA, 1985; p. 597. 14. Jung, M.S. A Study on the Reduction of N2O Generation at SNCR Process Using Urea and Additives. Master’s Thesis, School of Environmental Engineering, The University of Seoul, Seoul, Korea, June 2011. 15. Svoboda, K.; Baxter, D.; Martinec, J. Nitrous Oxide Emissions from Waste Incineration. Chem. Pap. 2006, 60, 78–90. [CrossRef] 16. Kim, J.M. A Study on the Reduction Mechanism of NOx and N2O in Urea-SNCR Process. Master’s Thesis, School of Environmental Engineering, The University of Seoul, Seoul, Korea, February 2012. 17. Im, Y.I. NOx Removal by Using Urea Solutions in a Pilot-Scale Reactor. Master’s Thesis, School of Chemical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Korea, August 1996. 18. Liang, L.; Hui, S.; Pan, S.; Shang, T.; Liu, C.; Wang, D. Influence of Mixing, Oxygen and Residence Time on the SNCR Process. Fuel 2004, 120, 38–45. [CrossRef] 19. Muzio, L.J.; Teixeira, D.P. Gas Phase Decomposition of Nitric Oxide in Combustion Products. Symp. Int. Combust. 1977, 16, 199–208. [CrossRef] 20. Rota, R.; Antos, D.; Zanoelo, E.F.; Carra, S. Experimental Study and Kinetic Modelling of Nitric Oxide Reduction with Ammonia. Combust. Sci. Technol. 2001, 163, 25–47. [CrossRef] 21. Dagaut, P.; Nicolle, A. Experimental and Kinetic Modeling Study of the Effect of SO2 on the Reduction of NO by Ammonia. Proc. Combust. Inst. 2005, 30, 1211–1218. [CrossRef] 22. Wenli, D.; Dam-Johansen, K.; Qstergaard, K. Widening the Temperature Range of the Thermal DeNOx Process an Experimental Investigation. Symp. Int. Combust. 1991, 23, 297–303. [CrossRef] 23. Duo, W.; Dam-Johansen, K.; Qstergaard, K. Kinetics of the Gas Phase Reaction between Nitric Oxide Ammonia and Oxygen. Can. J. Chem. Eng. 1992, 70, 1014–1020. [CrossRef] 24. Wang, Y.; Hu, X.; Zheng, K.; Wei, X.; Zhao, Y. Effect of SnO2 on the Structure and Catalytic Performance of Co3O4 for N2O Decomposition. Catal. Commun. 2018, 111, 70–74. [CrossRef] 25. Katerina, P.; Anna, K.; Tomas, K.; Dagmar, F.; Alexandr, M. Effect of Support on the Catalytic Activity of Co3O4-Cs Deposited on Open-Cell Ceramic Foams for N2O Decomposition. Mater. Res. Bull. 2020, 129, 110892. 26. Yu, H.; Wang, X.; Li, Y. Strong Impact of Cobalt Distribution on the Activity for Co3O4/CaCO3 Catalyzing N2O Decomposition. Catal. Today 2020, 339, 274–280. [CrossRef] 27. Lee, H.M.; Yun, J.G.; Hong, J.G. A Study of Nitrous Oxide Thermal Decomposition and Reaction in High Temperature Inert Gas. J. ILASS Korea 2020, 25, 132–138. 28. CHMEKIN-PRO; Reaction Design Inc.: San Diego, CA, USA; Available online: https://www.Ansys.com/20 (accessed on 25 September 2008). 29. Smith, G.P.; Golden, D.M.; Frenklach, M.; Moriarty, N.W.; Eiteneer, B.; Goldenberg, M.; Bowman, C.T.; Hanson, R.K.; Song, S.; Gardiner, W.C., Jr.; et al. GRI Mech. 3.0. Available online: http://www.me.Berkeley.Edu/GRI_Mech/19 (accessed on 30 July 1999).