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 pollu- tion 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.