- Email: [email protected]
Modelling study of NOx removal in oil-fired waste off-gases under electron beam irradiation
Radiation Physics and Chemistry 113 (2015) 20–23
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Modelling study of NOx removal in oil-fired waste off-gases under electron beam irradiation Ewa Zwolińska a, Yongxia Sun a,n, A.G. Chmielewski a, H. Nichipor b, S. Bulka a a b
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland The Joint Institute for Power and Nuclear Research, Minsk-Sosny, Belarus
H I G H L I G H T S
Modelling study of NOx removal in oil-fired off-gases under EB irradiation. Energy consumption (i.e. dose) influence on NOx removal efficiency was examined. The influence of temperature, SO2 concentration and ammonia addition was examined. NOx removal mechanism from flue gas under electron beam irradiation was elaborated.
art ic l e i nf o
a b s t r a c t
Article history: Received 1 March 2015 Received in revised form 2 April 2015 Accepted 17 April 2015 Available online 20 April 2015
Computer simulations for high concentration of NOx removal from oil-fired waste off-gases under electron beam irradiation were carried out by using the Computer code “Kinetic” and GEAR method. 293 reactions involving 64 species were used for the modelling calculations. The composition of simulated oil-fired off-gas was the same as the experimental conditions. The calculations were made for following system: (75.78% N2 þ11.5% CO2 þ 8.62% H2Oþ 4.1% O2), NOx concentration varies from 200 ppm to 1500 ppm. Calculation results qualitatively agree with the experimental results. Furthermore the influence of temperature, SO2 concentration and ammonia addition is discussed. & 2015 Elsevier Ltd. All rights reserved.
Keywords: NOx SO2 Electron beam Flue gas Kinetic Mechanism
1. Introduction Whereas the amount of pollution sources such as cars, ships, and industry is growing, the environmental protection is becoming more and more important within modern societies. As a result new stringent rules are set to limit the release of dangerous compounds such as NOx or SO2. This forces scientists and producers to invent and apply new, more efficient methods of cleaning exhaust gases from pollutants. There is a variety of technologies used to remove NOx and SO2 separately as well as simultaneously. However there is a growing interest in using non-thermal plasma technologies for abatement of dangerous pollutants instead of conventional methods. NOx is still one of the most important air pollutants, which has n
Corresponding author. E-mail address: [email protected] (Y. Sun).
http://dx.doi.org/10.1016/j.radphyschem.2015.04.008 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
to be controlled. The NOx abbreviation is a symbol for all nitric oxides, however usually refer to two of them: NO and NO2. The proportion of these two oxides in exhaust gases is 95% to 5%, respectively (Skalska et al., 2010). Until 2020 the emissions of SO2 and NOx from cargo ships on their routes around Europe are expected to exceed the emissions of these pollutants from all other sources in the European Union (EU). Although NOx removal from off-gases was widely studied, most of the studies were focused on NOx emission control from power plants, other combustion processes and chemical industry. Only a few research works were concentrated on NOx emission from diesel engines of cargo ships. Heavy fuel oil (HFO) is a main fuel used in the diesel engines of the ships with high sulphur content up to 2.5 wt%. EU set the new rules in May 2012, in Brussels that EU will confirm the International Maritime Organization (IMO) sulphur limit of 0.1 wt% for 2015 which applies to Sulphur Emissions Control Areas (SECAs) including the Baltic Sea, whereas the limit of NOx emission is under discussion.
E. Zwolińska et al. / Radiation Physics and Chemistry 113 (2015) 20–23
During the last few years an urgent need has appeared for developing technologies with good removal efficiency for high inlet concentrations of SO2 and NOx, emitted from diesel engineering, marine resources and/or industrial processes, to meet the new stricter regulations, especially in SECAs. One of the technologies, which uses radiation for the exhaust gases treatment, is electron beam. During the last few years this method has been optimized by many researchers experimentally (Basfar et al., 2008; Radoiu et al., 1998; Calinescu et al., 2013) as well as theoretically (Gogulancea and Lavric, 2014; Nichipor et al., 1995). However, only a few works were provided with high concentration of NOx (Chmielewski et al., 2012; Lakshmipathiraj et al., 2013) or high concentration of SO2 (Basfar et al., 2008; Kim et al., 2011). The computer simulation of high concentration NOx removal in oilfired off-gas under EB irradiation was carried out, in order to investigate the feasibility of NOx removal from off-gases emitted by marine ship under electron beam (EB) irradiation.
1 atmospheric pressure. The G-values (molecules/100 eV) of main primary species are simplified as follows (Mätzing, 1991): 4.43N2-0.29Nn2 þ0.885N(2D) þ 0.295N(2P)þ 1.87N þ2.27N2þ þ 0.69N þ þ2.96e
(2)
5.377O2-0.077On2 þ2.25O(1D) þ 2.8Oþ 0.18O* þ2.07O2þ þ1.23O þ þ3.3e
(3)
3
7.33H2O-0.51H2 þ 0.46O( P) þ 4.25OH þ 4.15H þ1.99H2O þ þ0.01H2 þ þ 0.57OH þ þ 0.67H þ þ0.06O þ þ3.3e 3
7.54CO2 -4.72COþ 5.16O( þ0.07C þ þ0.21O þ þ 3.03e
(4)
P) þ2.24COn2 þ 0.51CO þ (5)
These primary species and thermalised secondary electrons cause NOx removal through complex chemical reactions. Key reactions involving in NOx removal and by-products formation without SO2 and NH3 presence are shown below. On2 þ H2O þM-H2O3þ þM (M is the third body in the reaction), k¼ 2.80 10 28
2. Simulation calculations High concentration of NOx removal from exhausted gas of oil fired burner under EB irradiation was experimentally studied by Licki et al. (2014) and Chmielewski et al. (2012). Simulation calculations of NOx removal under EB irradiation were carried out based on the published experimental data (Chmielewski et al., 2012; Licki et al., 2014). A computer code “Kinetic” (Bugaenko and Grichkin, 1980) and a GEAR method were used. For “Kinetic” code applied in this work, the reaction rate of Wj (molecules cm 3 s 1) of j type active species (e.g. N*2 ), which were generated from pure k type molecules (e.g. N2) that absorb 100 eV of energy, is calculated according to the folowing equation:
Wj = ΣGjk ·I·ρk
21
H2O3þ þH2O-O4 þH2O3þ , k ¼1.50 10 9 H4O2þ
þ
(7)
þH2O-H2O þOH þH3O , k ¼1.40 10
NOþHO2-OH þNO2, k ¼8.28 10 2NO2-O2 þ 2NO, k ¼2.71 10
9
(8)
12
(9)
12
(10)
OHþ NO2 þM- HNO3 þ M, k0 ¼ 2.6 10 k1 ¼ 5.2 10 11; Fc ¼exp( T/353) OHþ HNO3-H2O þNO3, k ¼1.52 10 NOþNO3-2NO2, k ¼3.00 10
30
12
11
(300/T)
2.9
; (11) (12) (13)
(1)
where Gjk is the radiation chemical yield of j type species generated from pure k type molecules that absorb 100 eV of energy (molecules/100 eV), I is the dose rate (kGy/s), ρk is the gas phase density of pure k type molecules (g/cm3), and ρ is the overall density of the gas phase applied when dose is expressed in ρ eV cm 3 (1 Gy ¼6.2415 ρ 1015 eV cm 3). The units of the simulation calculations adopted in this work are cm, g, s and eV. The model consists of 293 reactions involving 64 species. Five main groups of reactions were included, whereas the rate constants of reactions were mostly taken from the literature (Albritton, 1978; NIST, 2014; Mätzing, 1991). The units of rate constants are 1/s, m3/mol s and m6/mol2 s for first-, second- and thirdorder reactions, respectively. The input values (such as pulse duration, repetition frequency of electron pulse, pressure, temperature and initial concentration) of the simulation calculations were the same as those described in the experiments (Chmielewski et al., 2012, Licki et al., 2014). The boundary conditions of calculation in the model without addition of ammonia at time 0 are: 75.78% N2 þ 4.10% O2 þ 8.62% H2O þ11.50% CO2, SO2 (varies between 600 ppm, 700 ppm and 1250 ppm) and NOx (varies between 200 ppm, 400 ppm, 1000 ppm and 1500 ppm), temperature being 70 °C or 90 °C. Pulse duration was 400 μs, repetition frequency of electron pulse varied from 2 Hz to 25 Hz. In the model with addition of ammonia α was equal 0.9. When the energy of fast electrons from electron beam is absorbed in the carrier gas, it causes ionization and excitation processes of the nitrogen, oxygen and water molecules in the carrier gas. Primary species and secondary electrons are formed. The secondary electrons are fast thermalized within 1 ns in the air at
3. Results of calculation and discussion 3.1. Removal efficiency of NOx vs. dose Removal efficiency of NOx vs. dose under EB irradiation was calculated for the inlet concentration of NO being 200 ppm, 400 ppm and 1000 ppm at 90 °C, and the calculation results (solid line) are presented in Fig.1 and compared with the experimental results (dashed line) (Licki et al., 2014). Calculation results and experimental results showed that removal efficiency of NOx was increasing with increasing absorbed dose, while it was decreasing with increasing inlet concentration of NOx. Above 50% NOx was removed at 44.1 kGy absorbed dose for the inlet concentration of NOx being 200 ppm, while for the inlet concentration of NOx being 1000 ppm, 11% NOx and 26.5% NOx were removed at the same absorbed dose based on the experimental results (Licki et al., 2014) and calculation results, respectively. The calculation value of NOx removal efficiency is higher than the experimental value. Based on the calculation results, NOx removal mainly goes to oxidation pathway. 3.2. Removal efficiency of NOx vs. inlet waste off-gas temperature Fig. 2 presents influence of inlet waste off-gas temperature on NOx removal efficiency in the presence of 700 ppm SO2 at 8.8 kGy absorbed dose for the two initial concentrations of NOx being 200 ppm and 1500 ppm, respectively. The calculation results are compared with experimental results (Chmielewski et al., 2012). Based on the experimental results (Chmielewski et al., 2012), NOx removal efficiency was increasing with the increasing inlet off-gas temperature from 70 °C to 90 °C. Based on the modelling
E. Zwolińska et al. / Radiation Physics and Chemistry 113 (2015) 20–23
12
70
cal.
60 50 40 30 20 10 0
0
10
20 30 Dose (kGy)
200 ppm 1000 ppm 400ppm[Licki et al., 2014]
40
50
NOx removal efficiency (%)
NOx Removal Efficiency (%)
22
Fig. 1. NOx removal efficiency as a function of dose for various NOx inlet concentrations.
3.3. Removal efficiency of NOx vs. inlet concentration of SO2 Inlet concentration of SO2 influencing on NOx removal efficiency at 8.8 kGy absorbed dose has been calculated for the initial concentration of NOx being 1000 ppm and the results are presented in Fig. 3. It is seen that the calculation results agree well with the experimental results (Chmielewski et al., 2012). NOx removal efficiency increases with increasing inlet concentration of SO2. It can be explained by the following radiation-induced reaction cycle: SO2 þ OH þM¼ HSO3 þ M, k0 ¼ 5.0 10 10 12; Fc ¼exp( T/380)
31
3.3
(300/T)
; k1 ¼2.0 (14)
HSO3 þO2 ¼ SO3 þHO2, k¼ 1.34 10 12 exp(–330/T)
(15)
HO2 þ NO¼NO2 þ OH, k ¼3.7 10 12 exp(240/T)
(16)
OH radical reacts with SO2, which leads to formation of HSO3 radical (14). In the presence of O2 fast reaction with HSO3 radical
Removal efficiency of NOx (%)
30 200 ppm
1500 ppm
25
6
4 2
600
700 1250 SO2 concentration (ppm)
100 90 80 70
60 50 40 30 20 10 0 0
10
20
30
40
50
Dose (kGy) 200 ppm NO [NH3] 1000ppm NO [NH3] 400 ppm
400 ppm NO [NH3] 200 ppm 1000 ppm
Fig. 4. Influence of addition of ammonia on NOx removal efficiency.
occurs (15), in which HO2 radical is formed. Moreover, HO2 reacts with NO (16), which leads to generation of NO2. Finally the NO2 is removed through reaction (11) and increases NOx removal efficiency. 3.4. Removal efficiency of NOx with addition of ammonia
20
The influence of addition of ammonia on removal efficiency of NOx under EB irradiation was calculated and the calculation results were compared with the calculation results of removal efficiency of NOx under EB irradiation without ammonia addition. The results showed a significant increase in NOx removal efficiency under EB irradiation when ammonia was added (Fig. 4), which is due to the following reactions:
15 10 5 0
8
Fig. 3. Inlet concentration of SO2 influence on NOx removal efficiency.
NOx Removal Efficiency (%)
calculation results, NOx removal efficiency is almost not changed when temperature varies from 70 °C to 90 °C. Dors and Mizeraczyk (1998) studied the influence of the temperature on NOx removal by corona discharge, where they found that NO removal efficiency decreased a little when inlet gas temperature increased from 80 °C to 100 °C.
10
0
400 ppm 200ppm [Licki et al., 2014] 1000ppm [Licki et al., 2014]
exp. [Chmielewski et al., 2012]
70[calc.]
90[calc.]
70[Chmielewski et 90 [Chmielewski al., 2012] et al., 2012]
Inlet gas temperature (° C) Fig. 2. Inlet gas temperature influence on NOx removal efficiency.
NH3 þ HNO3-NH4NO3, k ¼1.05 10 8
(17)
OHþ NH3-H2Oþ NH2, k¼ 8.32 10–17 T1,6 exp( 480/T)
(18)
NOþNH2-H2O þN2, k ¼1.00 10–6 T–1.96
(19)
E. Zwolińska et al. / Radiation Physics and Chemistry 113 (2015) 20–23
The same tendency was observed also in experimental research (Chmielewski et al. 2002).
4. Conclusions The main results of this study are summarized as follows:
Calculation results of NOx removal from oil-fired off-gases un
der EB irradiation qualitatively agree well with the experimental results (Chmielewski et al., 2012, Licki et al., 2014). NOx removal efficiency increases with increasing dose and inlet concentration of SO2. The experimental results and calculation results proved it. Temperature varying from 70 °C to 90 °C has little influence on NOx removal efficiency based on the calculation results. NOx removal under EB irradiation mainly goes to oxidation pathway. Radicals such as HO2 and OH play very important role in NOx removal. Addition of NH3 significantly improves removal efficiency of NOx. Model can be improved to fit better with experimental results by introducing larger amount of reactions, which are dependent on temperature, as well as reactions in which NO is reproduced.
Acknowledgements This work is financed by Institute of Nuclear Chemistry and Technology in the frame of statute grant work task 4.4 .
23
References Albritton, D.L., 1978. Ion neutral reactions rates. At. Data Nucl. Data 22, 1–101. Basfar, A.A., Fageeha, O.I., Kunnummal, N., Al – Ghamdi, S., Chmielewski, A.G., Licki, J., Pawelec, A., Tymiński, B., Zimek, Z., 2008. Electron beam flue gas treatment technology for simultaneous removal of SO2 and NOx from combustion of liquid fuels. Fuel 87, 1446–1452. Bugaenko, W.L., Grichkin, W.L., 1980. Program for Modeling of Chemical Kinetics. Report ITEP No. 50. Institute of Theoretical and Experimental Physics, Moscow. Calinescu, I., Martin, D., Chmielewski, A.G., Ighigeanu, D., 2013. E-Beam SO2 and NOx removal from flue gases in the presence of fine water droplets. Radiat. Phys. Chem. 85, 130–138. Chmielewski, A.G., Sun, Y., Licki, J., Pawelec, A., Witman, S., Zimek, Z., 2012. Electron-beam treatment of high NOx concentration off-gases. Radiat. Phys. Chem. 81 (8), 1036–1039. Chmielewski, A.G., Sun, Y., Zimek, Z., Bułka, S., Licki, J., 2002. Mechanism of NOx removal by electron beam process in the presence of scavengers. Radiat. Phys. Chem. 65 (4-5), 397–403. Dors, M., Mizeraczyk, J., 1998. Influence of temperature and humidity on NOx removal by corona discharge. Czechoslov. J. Phys. 48 (10), 1193–1197. Gogulancea, V., Lavric, V., 2014. Flue gas cleaning by high energy electron beam – modeling and sensitivity analysis. Appl. Therm. Eng. 70 (2), 1253–1261. Lakshmipathiraj, P., Chen, J., Doi, M., Takasu, N., Kato, S., Yamasaki, A., Kojima, T., 2013. Electron beam treatment of gas stream containing high concentration of NOx: an in situ FTIR study. Chem. Eng. J. 229, 344–350. Kim, J.K., Kim, Y.R., Han, B., Doutzkinov, N., Jeong, K.Y., 2011. Electron-beam flue-gas treatment plant for thermal power station “Sviloza” AD in Bulgaria. J. Korea Phys. Soc. 59 (6), 3494–3498. Licki, J., Chmielewski, A.G., Pawelec, A., Zimek, Z., Witman, S., 2014. Electron beam treatment of exhaust gas with high NOx concentration. Phys. Scr. T161, 014067–014071. Mätzing, H., 1991. Chemical kinetics of flue gas cleaning by irradiation with electrons. Adv. Chem. Phys. LXXX, 315–402. Nichipor, H.V., Dashouk, E.M., Yatsko, S.N., 1995. Radiation induced scavenging of NOx, SO2, H2S from exhaust gases. Radiat. Phys. Chem. 45 (6), 1067–1070. NIST. 〈http://kinetics.nist.gov/kinetics/index.jsp〉. Radoiu, M., Martin, D., Georgescu, I.I., Calinescu, I., Bestea, V., Indreias, I., Matei, C., 1998. A laboratory test unit for exhausted gas cleaning by electron beam and combined electron beam-microwave irradiation. Nucl. Instrum. Methods Phys. Res. Sect. B 139 (1–4), 506–510. Skalska, K., Miller, J.S., Ledakowicz, S., 2010. Trends in NOx abatement: review. Sci. Total Environ. 408, 3976–3989.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Modelling study of NOx removal in oil-fired waste off-gases under electron beam irradiation Ewa Zwolińska a, Yongxia Sun a,n, A.G. Chmielewski a, H. Nichipor b, S. Bulka a a b
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland The Joint Institute for Power and Nuclear Research, Minsk-Sosny, Belarus
H I G H L I G H T S
Modelling study of NOx removal in oil-fired off-gases under EB irradiation. Energy consumption (i.e. dose) influence on NOx removal efficiency was examined. The influence of temperature, SO2 concentration and ammonia addition was examined. NOx removal mechanism from flue gas under electron beam irradiation was elaborated.
art ic l e i nf o
a b s t r a c t
Article history: Received 1 March 2015 Received in revised form 2 April 2015 Accepted 17 April 2015 Available online 20 April 2015
Computer simulations for high concentration of NOx removal from oil-fired waste off-gases under electron beam irradiation were carried out by using the Computer code “Kinetic” and GEAR method. 293 reactions involving 64 species were used for the modelling calculations. The composition of simulated oil-fired off-gas was the same as the experimental conditions. The calculations were made for following system: (75.78% N2 þ11.5% CO2 þ 8.62% H2Oþ 4.1% O2), NOx concentration varies from 200 ppm to 1500 ppm. Calculation results qualitatively agree with the experimental results. Furthermore the influence of temperature, SO2 concentration and ammonia addition is discussed. & 2015 Elsevier Ltd. All rights reserved.
Keywords: NOx SO2 Electron beam Flue gas Kinetic Mechanism
1. Introduction Whereas the amount of pollution sources such as cars, ships, and industry is growing, the environmental protection is becoming more and more important within modern societies. As a result new stringent rules are set to limit the release of dangerous compounds such as NOx or SO2. This forces scientists and producers to invent and apply new, more efficient methods of cleaning exhaust gases from pollutants. There is a variety of technologies used to remove NOx and SO2 separately as well as simultaneously. However there is a growing interest in using non-thermal plasma technologies for abatement of dangerous pollutants instead of conventional methods. NOx is still one of the most important air pollutants, which has n
Corresponding author. E-mail address: [email protected] (Y. Sun).
http://dx.doi.org/10.1016/j.radphyschem.2015.04.008 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
to be controlled. The NOx abbreviation is a symbol for all nitric oxides, however usually refer to two of them: NO and NO2. The proportion of these two oxides in exhaust gases is 95% to 5%, respectively (Skalska et al., 2010). Until 2020 the emissions of SO2 and NOx from cargo ships on their routes around Europe are expected to exceed the emissions of these pollutants from all other sources in the European Union (EU). Although NOx removal from off-gases was widely studied, most of the studies were focused on NOx emission control from power plants, other combustion processes and chemical industry. Only a few research works were concentrated on NOx emission from diesel engines of cargo ships. Heavy fuel oil (HFO) is a main fuel used in the diesel engines of the ships with high sulphur content up to 2.5 wt%. EU set the new rules in May 2012, in Brussels that EU will confirm the International Maritime Organization (IMO) sulphur limit of 0.1 wt% for 2015 which applies to Sulphur Emissions Control Areas (SECAs) including the Baltic Sea, whereas the limit of NOx emission is under discussion.
E. Zwolińska et al. / Radiation Physics and Chemistry 113 (2015) 20–23
During the last few years an urgent need has appeared for developing technologies with good removal efficiency for high inlet concentrations of SO2 and NOx, emitted from diesel engineering, marine resources and/or industrial processes, to meet the new stricter regulations, especially in SECAs. One of the technologies, which uses radiation for the exhaust gases treatment, is electron beam. During the last few years this method has been optimized by many researchers experimentally (Basfar et al., 2008; Radoiu et al., 1998; Calinescu et al., 2013) as well as theoretically (Gogulancea and Lavric, 2014; Nichipor et al., 1995). However, only a few works were provided with high concentration of NOx (Chmielewski et al., 2012; Lakshmipathiraj et al., 2013) or high concentration of SO2 (Basfar et al., 2008; Kim et al., 2011). The computer simulation of high concentration NOx removal in oilfired off-gas under EB irradiation was carried out, in order to investigate the feasibility of NOx removal from off-gases emitted by marine ship under electron beam (EB) irradiation.
1 atmospheric pressure. The G-values (molecules/100 eV) of main primary species are simplified as follows (Mätzing, 1991): 4.43N2-0.29Nn2 þ0.885N(2D) þ 0.295N(2P)þ 1.87N þ2.27N2þ þ 0.69N þ þ2.96e
(2)
5.377O2-0.077On2 þ2.25O(1D) þ 2.8Oþ 0.18O* þ2.07O2þ þ1.23O þ þ3.3e
(3)
3
7.33H2O-0.51H2 þ 0.46O( P) þ 4.25OH þ 4.15H þ1.99H2O þ þ0.01H2 þ þ 0.57OH þ þ 0.67H þ þ0.06O þ þ3.3e 3
7.54CO2 -4.72COþ 5.16O( þ0.07C þ þ0.21O þ þ 3.03e
(4)
P) þ2.24COn2 þ 0.51CO þ (5)
These primary species and thermalised secondary electrons cause NOx removal through complex chemical reactions. Key reactions involving in NOx removal and by-products formation without SO2 and NH3 presence are shown below. On2 þ H2O þM-H2O3þ þM (M is the third body in the reaction), k¼ 2.80 10 28
2. Simulation calculations High concentration of NOx removal from exhausted gas of oil fired burner under EB irradiation was experimentally studied by Licki et al. (2014) and Chmielewski et al. (2012). Simulation calculations of NOx removal under EB irradiation were carried out based on the published experimental data (Chmielewski et al., 2012; Licki et al., 2014). A computer code “Kinetic” (Bugaenko and Grichkin, 1980) and a GEAR method were used. For “Kinetic” code applied in this work, the reaction rate of Wj (molecules cm 3 s 1) of j type active species (e.g. N*2 ), which were generated from pure k type molecules (e.g. N2) that absorb 100 eV of energy, is calculated according to the folowing equation:
Wj = ΣGjk ·I·ρk
21
H2O3þ þH2O-O4 þH2O3þ , k ¼1.50 10 9 H4O2þ
þ
(7)
þH2O-H2O þOH þH3O , k ¼1.40 10
NOþHO2-OH þNO2, k ¼8.28 10 2NO2-O2 þ 2NO, k ¼2.71 10
9
(8)
12
(9)
12
(10)
OHþ NO2 þM- HNO3 þ M, k0 ¼ 2.6 10 k1 ¼ 5.2 10 11; Fc ¼exp( T/353) OHþ HNO3-H2O þNO3, k ¼1.52 10 NOþNO3-2NO2, k ¼3.00 10
30
12
11
(300/T)
2.9
; (11) (12) (13)
(1)
where Gjk is the radiation chemical yield of j type species generated from pure k type molecules that absorb 100 eV of energy (molecules/100 eV), I is the dose rate (kGy/s), ρk is the gas phase density of pure k type molecules (g/cm3), and ρ is the overall density of the gas phase applied when dose is expressed in ρ eV cm 3 (1 Gy ¼6.2415 ρ 1015 eV cm 3). The units of the simulation calculations adopted in this work are cm, g, s and eV. The model consists of 293 reactions involving 64 species. Five main groups of reactions were included, whereas the rate constants of reactions were mostly taken from the literature (Albritton, 1978; NIST, 2014; Mätzing, 1991). The units of rate constants are 1/s, m3/mol s and m6/mol2 s for first-, second- and thirdorder reactions, respectively. The input values (such as pulse duration, repetition frequency of electron pulse, pressure, temperature and initial concentration) of the simulation calculations were the same as those described in the experiments (Chmielewski et al., 2012, Licki et al., 2014). The boundary conditions of calculation in the model without addition of ammonia at time 0 are: 75.78% N2 þ 4.10% O2 þ 8.62% H2O þ11.50% CO2, SO2 (varies between 600 ppm, 700 ppm and 1250 ppm) and NOx (varies between 200 ppm, 400 ppm, 1000 ppm and 1500 ppm), temperature being 70 °C or 90 °C. Pulse duration was 400 μs, repetition frequency of electron pulse varied from 2 Hz to 25 Hz. In the model with addition of ammonia α was equal 0.9. When the energy of fast electrons from electron beam is absorbed in the carrier gas, it causes ionization and excitation processes of the nitrogen, oxygen and water molecules in the carrier gas. Primary species and secondary electrons are formed. The secondary electrons are fast thermalized within 1 ns in the air at
3. Results of calculation and discussion 3.1. Removal efficiency of NOx vs. dose Removal efficiency of NOx vs. dose under EB irradiation was calculated for the inlet concentration of NO being 200 ppm, 400 ppm and 1000 ppm at 90 °C, and the calculation results (solid line) are presented in Fig.1 and compared with the experimental results (dashed line) (Licki et al., 2014). Calculation results and experimental results showed that removal efficiency of NOx was increasing with increasing absorbed dose, while it was decreasing with increasing inlet concentration of NOx. Above 50% NOx was removed at 44.1 kGy absorbed dose for the inlet concentration of NOx being 200 ppm, while for the inlet concentration of NOx being 1000 ppm, 11% NOx and 26.5% NOx were removed at the same absorbed dose based on the experimental results (Licki et al., 2014) and calculation results, respectively. The calculation value of NOx removal efficiency is higher than the experimental value. Based on the calculation results, NOx removal mainly goes to oxidation pathway. 3.2. Removal efficiency of NOx vs. inlet waste off-gas temperature Fig. 2 presents influence of inlet waste off-gas temperature on NOx removal efficiency in the presence of 700 ppm SO2 at 8.8 kGy absorbed dose for the two initial concentrations of NOx being 200 ppm and 1500 ppm, respectively. The calculation results are compared with experimental results (Chmielewski et al., 2012). Based on the experimental results (Chmielewski et al., 2012), NOx removal efficiency was increasing with the increasing inlet off-gas temperature from 70 °C to 90 °C. Based on the modelling
E. Zwolińska et al. / Radiation Physics and Chemistry 113 (2015) 20–23
12
70
cal.
60 50 40 30 20 10 0
0
10
20 30 Dose (kGy)
200 ppm 1000 ppm 400ppm[Licki et al., 2014]
40
50
NOx removal efficiency (%)
NOx Removal Efficiency (%)
22
Fig. 1. NOx removal efficiency as a function of dose for various NOx inlet concentrations.
3.3. Removal efficiency of NOx vs. inlet concentration of SO2 Inlet concentration of SO2 influencing on NOx removal efficiency at 8.8 kGy absorbed dose has been calculated for the initial concentration of NOx being 1000 ppm and the results are presented in Fig. 3. It is seen that the calculation results agree well with the experimental results (Chmielewski et al., 2012). NOx removal efficiency increases with increasing inlet concentration of SO2. It can be explained by the following radiation-induced reaction cycle: SO2 þ OH þM¼ HSO3 þ M, k0 ¼ 5.0 10 10 12; Fc ¼exp( T/380)
31
3.3
(300/T)
; k1 ¼2.0 (14)
HSO3 þO2 ¼ SO3 þHO2, k¼ 1.34 10 12 exp(–330/T)
(15)
HO2 þ NO¼NO2 þ OH, k ¼3.7 10 12 exp(240/T)
(16)
OH radical reacts with SO2, which leads to formation of HSO3 radical (14). In the presence of O2 fast reaction with HSO3 radical
Removal efficiency of NOx (%)
30 200 ppm
1500 ppm
25
6
4 2
600
700 1250 SO2 concentration (ppm)
100 90 80 70
60 50 40 30 20 10 0 0
10
20
30
40
50
Dose (kGy) 200 ppm NO [NH3] 1000ppm NO [NH3] 400 ppm
400 ppm NO [NH3] 200 ppm 1000 ppm
Fig. 4. Influence of addition of ammonia on NOx removal efficiency.
occurs (15), in which HO2 radical is formed. Moreover, HO2 reacts with NO (16), which leads to generation of NO2. Finally the NO2 is removed through reaction (11) and increases NOx removal efficiency. 3.4. Removal efficiency of NOx with addition of ammonia
20
The influence of addition of ammonia on removal efficiency of NOx under EB irradiation was calculated and the calculation results were compared with the calculation results of removal efficiency of NOx under EB irradiation without ammonia addition. The results showed a significant increase in NOx removal efficiency under EB irradiation when ammonia was added (Fig. 4), which is due to the following reactions:
15 10 5 0
8
Fig. 3. Inlet concentration of SO2 influence on NOx removal efficiency.
NOx Removal Efficiency (%)
calculation results, NOx removal efficiency is almost not changed when temperature varies from 70 °C to 90 °C. Dors and Mizeraczyk (1998) studied the influence of the temperature on NOx removal by corona discharge, where they found that NO removal efficiency decreased a little when inlet gas temperature increased from 80 °C to 100 °C.
10
0
400 ppm 200ppm [Licki et al., 2014] 1000ppm [Licki et al., 2014]
exp. [Chmielewski et al., 2012]
70[calc.]
90[calc.]
70[Chmielewski et 90 [Chmielewski al., 2012] et al., 2012]
Inlet gas temperature (° C) Fig. 2. Inlet gas temperature influence on NOx removal efficiency.
NH3 þ HNO3-NH4NO3, k ¼1.05 10 8
(17)
OHþ NH3-H2Oþ NH2, k¼ 8.32 10–17 T1,6 exp( 480/T)
(18)
NOþNH2-H2O þN2, k ¼1.00 10–6 T–1.96
(19)
E. Zwolińska et al. / Radiation Physics and Chemistry 113 (2015) 20–23
The same tendency was observed also in experimental research (Chmielewski et al. 2002).
4. Conclusions The main results of this study are summarized as follows:
Calculation results of NOx removal from oil-fired off-gases un
der EB irradiation qualitatively agree well with the experimental results (Chmielewski et al., 2012, Licki et al., 2014). NOx removal efficiency increases with increasing dose and inlet concentration of SO2. The experimental results and calculation results proved it. Temperature varying from 70 °C to 90 °C has little influence on NOx removal efficiency based on the calculation results. NOx removal under EB irradiation mainly goes to oxidation pathway. Radicals such as HO2 and OH play very important role in NOx removal. Addition of NH3 significantly improves removal efficiency of NOx. Model can be improved to fit better with experimental results by introducing larger amount of reactions, which are dependent on temperature, as well as reactions in which NO is reproduced.
Acknowledgements This work is financed by Institute of Nuclear Chemistry and Technology in the frame of statute grant work task 4.4 .
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