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Computer simulation of NOx removal from diesel engine off-gases under electron beam and wet scrubber system
Journal Pre-proof Computer simulation of NOx removal from diesel engine off-gases under electron beam and wet scrubber system Y. Sun, A. Dobrowolski, A.G. Chmielewski, O. Roubinek, A. Pawelec, H. Nichipor PII:
S0969-806X(19)31306-4
DOI:
https://doi.org/10.1016/j.radphyschem.2020.108707
Reference:
RPC 108707
To appear in:
Radiation Physics and Chemistry
Received Date: 8 October 2019 Revised Date:
7 January 2020
Accepted Date: 9 January 2020
Please cite this article as: Sun, Y., Dobrowolski, A., Chmielewski, A.G., Roubinek, O., Pawelec, A., Nichipor, H., Computer simulation of NOx removal from diesel engine off-gases under electron beam and wet scrubber system, Radiation Physics and Chemistry (2020), doi: https://doi.org/10.1016/ j.radphyschem.2020.108707. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
COMPUTER SIMULATION OF NOx REMOVAL FROM DIESEL ENGINE OFF-GASES UNDER ELECTRON BEAM AND WET SCRUBBER SYSTEM
Y. Sun1,*, A.Dobrowolski1, A.G.Chmielewski1, O. Roubinek1, A. Pawelec1, H.Nichipor2 1
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland 2
IRPCP, Academy of Sciences Republic of Belarus, Minsk-Sosny, Belarus
* Corresponding author: Yongxia Sun, [email protected]; tel: 0048-22-5041368 ORCID 0000-0002-7359-3869
1
Abstract NOx emission from diesel engine off-gases in marine transportation sector is still an environmental issue to be solved in present day. Electron beam (EB) followed by a seawater wet scrubber method to removal NOx has been experimentally
studied
(Chmielewski et al., 2018). In this work numerical simulation of NOx reduction using EB followed by a water scrubber system has been carried out. The first stage was to calculate NOx removal versus dose under EB irradiation . Computer code “KINETIC” and Gear method were used, the model has been improved based our previous work ( Zwolinska E., et al., 2015) and verified by the experimental results. The calculation results of NOx removal efficiency was 4.9% at 10.9 kGy dose under EB irradiation, which was a little higher than experimental value (3.5%). Computer program MATLAB was used to study NOx absorption in water. The NOx removal efficiency was increased by 17.5% at 10.9 kGy dose for inlet concentration of NOx being 1333 ppm after EBwater absorption process . At the end, NOx reduction in the simulated marine off-gas using microwave plasma was simulated using computer program Chemical Workbench . Less than 20% NOx was removed from the off-gases in microwave plasma reactor even at 9000 kGy dose for the inlet concentration of NOx being 1500 ppm. EB is more energy efficient than microwave plasma for NOx removal.
Key words: NOx removal; off-gas; electron beam; wet scrubber; microwave plasma; simulation
1. Introduction
2
NOx emission is still an environmental problem to be solved. NOx emission from landbased sources has been reduced in recent years, while NOx emission from marine transportation sector has been increased, about 30% NOx emission came from ships (European Environment Agency, 2013). International Marine Organization (IMO) has enacted very strict MARPOL NOx Tier III regulation, it has been adopted in emission control areas (ECA) of US, and it will take effect on 1 January 2021 in ECA of Europe (IMO, 2017). EB technology has been applied to remove SO2 and NOx from flue gas emitted from coal-fired power plant Pomorzany Power Station, Poland (Chmielewski et al., 2004). The principle of this technology to remove NOx is that under EB irradiation, NO has been oxidized into NO2 and some of them into NO3, with the presence of ammonia, ammonia nitrate/nitrite has been formed which can be removed from gas phase using electron static precipitator (Zwolińska et al., 2019). Due to the limited space in the ships, this technology with ammonia addition is not practical to be applied in NOx removal from diesel engine off-gas emitted from ship. Sea water might be a good candidate to replace NH3. It is easily to be obtained and it scrubs NO2 from gas phase due to NO2 good solubility in water. This assumption has been roughly verified by our experimental results (Chmielewski et al, 2018) .
Currently, two separate technologies
have been implemented on board, wet scrubber is for removal SO2 and SCR (selective catalytic reduction) is for NOx removal from off-gases, and good removal efficiency of SO2 (99%) and NOx (90%) are achieved using these two separate technologies. However these two separate technologies possess of number of drawbacks: high installation cost, high maintenance cost, and a large space needed for two separate installations and ammonia storage (EGSCA, 2012; Jurgen S. et al, 2012). Moreover, high temperature
3
(300-400 °C) is needed for NOx reduction using the SCR method, so the flue gas after the wet scrubber for SO2 reduction needs to be reheated. Otherwise, the catalyst might be deactivated in a short time when the temperature is below 250 °C, especially when a high concentration of SO2 and PM (particle matter) are present in the off-gases (Magnusson M. et al., 2012). In this work, computer simulation of NOx reduction using electron beam (EB) followed by a water scrubber system has been studied. It consists of two stages . The first stage was to calculate NOx removal versus dose under EB irradiation. The model has been improved based our previous work ( Zwolinska E., et al., 2015) by removing ammonia and related species generated therefore, and by adding some reverse reactions which are formation of NOx.
The revised model better simulated NOx removal under EB
irradiation without ammonia addition. The second stage was to simulate NOx absorption in water using computer program MATLAB.
Microwave plasma has been reported to a
good method for waste management due to its’ many potential advantages, including energy savings and overall cost effectiveness (Schulz et al., 1993) ,
high removal
efficiency of organic pollutants , such as trichloroethylene ,1,1,1-trichloroethane ( Krause and Helt, 1993) and HFC 134a refrigerant from air (Jasinski et al., 2009) was reported . In order to find out whether there is a more energy efficient method for removal NOx, we simulated NOx removal from flue gas using microwave plasma.
2. Modelling calculation
2.1 Computer simulation of NOx reduction from flue gas under electron beam irradiation
4
Computer simulation of NOx reduction from flue gas under electron beam irradiation was carried out using Computer code “Kinetic” (Bugaenko and Grichkin, 1980) and “GEAR” method. The model was improved based our previous work ( Zwolinska E., et al., 2015) by removing ammonia and related species generated therefore, and by adding some reverse reactions which are formation of NOx thorough O atom reaction with NO2 and NO3.
The revised model better simulated NOx removal under EB irradiation
without ammonia addition. The “Kinetic” code is designed for calculation the time evolution of the different main species involved in the kinetic reaction system at constant pressure and temperature. The kinetic reaction system is considered as self-contained and closed. It should obey the principle of mass balance and charge balance. For “Kinetic” code applied in this work, the reaction rate of Wj (molecules·cm-3·s-1) of j type active species (for e.g. N2*), which were generated from pure k type molecules (for e.g. N2) that absorb 100 eV of energy, was calculated according to an equation 1:
Wj = ∑ Gjk· I·ρk
(1)
where: Gjk - the radiation chemical yield of j type species generated from pure k type molecules that absorb 100 eV of energy (molecules/100 eV), I - dose rate (kGy/s),
ρk - gas phase density of pure k type molecules (g/cm3)
5
ρ - overall density of the gas phase applied when dose is expressed in ρ·eV·cm-3 (1 Gy = 6.2415·ρ·1015 eV·cm-3). The units of the simulation calculations of EB irradiation adopted in this work are cm, g, s and eV. The kinetic reaction system from a mathematical point of view is described by the system of ordinary differential equations (ODE). The “Kinetic” code used a Gear algorithm for numerical integration of stiff systems of ODE. In this work, the revised model consists of 287 reactions with 55species. 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/mole·s and m6/mole2·s for first-, second- and third- order reactions, respectively. The input values of the modelling simulation were the same as those described in the experiments (Chmielewski, et al., 2018). The boundary conditions of calculation in the model at time 0 are: 70.6% N2, 5.6% O2, 15.6% CO2, 8.2% H2O, 1500 ppmv NO and 700 ppmv SO2. Calculation was made at 90°C , electron beam was 400 ns pulse duration with different repetition frequency varying from 2 Hz to 15 Hz . The mechanism of NOx removal under EB irradiation has been described in details ( Chmielewski et al, 2002; Zwolinska, et al., 2015 & 2017; ). In general , under electron beam irradiation, the main components (nitrogen, oxygen, water and carbon dioxide) in flue gas are ionized and excited by the high energy electrons from electron beam , the primary species and second electrons are formed. The G-values (molecules/100 eV) of main primary species are simplified as follows (Mätzing, 1991): 4.43N2 → 0.29N2* + 0.885N(2D) + 0.295N(2P) + 1.87N + 2.27N2+ + 0.69N+ + 2.96e (2) 6
5.377O2 → 0.077O2* + 2.25O(1D) + 2.8O + 0.18O* + 2.07O2+ + 1.23O+ + 3.3e
(3)
7.33H2O → 0.51H2 + 0.46O(3P) + 4.25OH + 4.15H + 1.99H2O+ + 0.01H2+ + 0.57OH+ + 0.67H+ + 0.06O+ + 3.3e
(4)
7.54CO2 →4.72CO + 5.16O(3P) + 2.24CO2+ + 0.51CO+ + 0.07C+ + 0.21O+ + 3.03e
(5)
These primary species and thermalized secondary electrons cause NOx removal through complex chemical reactions.
The removal efficiency of the flue gas (R%) is denoted by the following equation (5), where C1 denotes the initial concentration of a certain compound of flue gas and C2 denotes the concentration of a certain compound of flue gas after the clean-up procedure. R%=[(C1-C2)/C1]×100%
(6)
2.2 Calculation of NOx removal in the water absorption process using MATLAB software
After electron beam irradiation, NOx from outlet of EB reactor was absorbed in wet scrubbing process.
A mathematical model was constructed to enable analysis of
mechanism of NOx removal in the water absorption process.
7
The main reactions of NOx in the gas and liquid phases were considered (Loutet et al., 2011): - gas phase 2
+
( )
( ) ↔
2 ( )
+
2
( )
( )
(7) (8)
( )
( ) ↔
(9)
( )
( )
+
( )
2
( )
+
( )
(10)
( ) ( )
+
( )
(11)
Liquid phase 2
3
( )
+
( )
( )
( )
+
( )
2
( )
( )
+
( )
( )
( ) ( )
+
+
(13)
+
()
(12)
( )
+2
( )
(14)
( )
(15)
The chemical reaction rates are as given by Loutet et al.(2011): k7 = 0.0102 m6kmol–2s–1, k8 = 109 m3kmol–1s–1, k9 = 109 m3kmol–1s–1, k10 = 41 000 m3kmol–1s–1, k11 = 250 m3kmol–1s–1, k12 = 46 999 m3kmol–1s–1, k13 = 17000 s–1, k14 = 93.32 s–1, k15 = 0.0468 atm2m9kmol–3s–1, k8’ = k8 · K8–1, K8 = 0.439 m3kmol–1, k9’ = k9 · K9–1, K9 = 13 699 m3kmol–1, k10’ = k10 · K10–1, K10 = 0.2763, k11’ = k11 · K11–1, K11 = 0.00002191. The absorption kinetic was described by equation (16): 8
ri = kLa · (pi – pi*)
(16)
where: i = NO, NO2, N2O3, N2O4; kLa – the mass transfer coefficient [s–1], in this work the value of kLa was set as 0.01[s–1] (Pisut et al, 2009); pi – partial pressure of i component in bulk; pi* – interface pressure of i component in liquid. Taking into account of mass balance of absorption process, the rate of accumulation of component (i) is defined by following : Rate of accumulation of component (i) = Rate of mass transport of component (i) +Rate of (inflow – outflow) of component(i)+ Rate of generation of component (i) by chemical reactions i=NOg, O2g, NO2g, N2O3g,N2O4g,HNO2g,HNO3g,H2Og, NOl, O2l, NO2l, N2O3l,N2O4l,HNO2l,HNO3l
Gas phase
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H= constant of Henry for (i) component KH,i – 1/H ci,g,in –concentration of (i) component at the inlet Vl – volume of liquid Vg – volume of gas Qg – volumetric flowrate of gas Ql – volumetric flowrate of liquid
The system of ordinary differential equations, being the mathematical model of considered process, was solved with the use of Runge-Kutta procedure in the MATLAB programming language. The simulations were made for the following conditions: - initial NO concentrations: 200, 500, 1000, and 1271 ppmv;- initial NO2 concentrations: 0, 20, and 50 ppmv. The following irradiation doses were considered: 3.5 kGy, 10.9 kGy, 44.1 kGy. The concentrations of NO and NO2 at the scrubber inlet were taken based on the our
previous calculation and experimental
results
(Zwolinska et al., 2017;
Chmielewski et al, 2002) . The volume flowrate of the flue gas (Qg ) was assumed as 5 m3/hr, volumetric flowrate of the scrubber solution (Ql) was assumed as 0.05m3/hr. The dimension of the absorption column was assumed : 0.15 m (diameter) * 0.9m (height). The temperature of the gas phase was assumed 90oC which was the temperature of the flue gas from outlet of EB reaction vessel ; the temperature of the liquid phase was assumed 15oC which was the temperature of scrubber solution used .
11
2.3 Computer simulation of NOx cleaning from ship emission off-gases by microwave plasma using Chemical workbench software Microwave plasma proved its efficiency in removing organic pollutant HFC 134a refrigerant from air (Jasinski et al., 2009). Chemical Workbench software (version 4.2) was used to simulate cleaning off-gases using microwave plasma reactor. The system of microwave plasma reactor to treat NOx was similar to the horizontal microwave waste treatment system used (Schulz, 1998) . It includes: injecting N2 gas into the microwave oven to generate N atoms ; the second stage
;
off-gas interconnected and mixed with N atoms and N2 in
off-gas treatment microwave oven.
The model consists of
thermodynamic reactor, well stirred reactor and plug flow reactor to simulate these three process . Thermodynamic reactor was used to simulate nitrogen atoms generation from N2 dissociation caused by adding enthalpy originated from the arc power , a well stirred reactor was used to simulate plasma mixing with waste gas, NOx removal in off-gas was simulated in a plug flow reactor. All process took place at 1 atmospheric pressure, No heat lost. 29 species and 155 reactions were included in this model. Only radicals’ and neutral species’ reactions were taken into consideration. The reaction rate constants were mostly taken from literatures (Baulch et al., 1992; Atkinson et al., 2004). The size of microwave plasms reactor to treat off-gas was assumed the same as the reaction vessel under EB irradiation.
The reactor was 25 cm diameter with 140 cm length. The
volumetric flow rate of flue gas was 5 Nm3/h, the temperature of the flue gas inside microwave reactor was assumed to be 90°C .
3. Results and discussion
12
3.1 NO removal in gas phase under EB irradiation Dose influencing on NOx removal was studied using computer simulation for inlet concentration of NO being 1500 ppm. The calculation results are presented in Figure 1a. It is seen that NO removal efficiency increases with the absorbed dose increasing. The similar trend was observed in experimental work (Chmielewski et al., 2018). It may be explained by that with the absorbed does increasing, more actives species ( O, OH etc.) are formed. Concentration of active species increasing enhances NOx removal efficiency. While NOx removal efficiency decreased with inlet concentration of NOx increasing under EB irradiation at the same absorbed dose (figure 1b) , It can be explained that with inlet NO concentration increasing, more oxidant species e.g. OH and O are used to oxidize NO into NO2. Concentration of OH radicals , which are generated from water radiolysis,
are almost constant
at the same absorbed dose.
The ratio between
concentration of OH radicals to total concentration of NO and NO2 decreases with increasing inlet concentration of NO, thus the NOx (NO + NO2) removal efficiency decreases significantly during the concentration range of 200 ppm to 1000 ppm. With NO concentration increases to 1.7 times, e.g. from 1000 ppm to 1700 ppm, the NOx removal efficiency decreases from 6.9% to 3.5% based on the modeling calculation results.
The calculation results of NOx removal efficiency for 1500 ppm NO were a
little higher than experimental results. At 10.9 kGy absorbed dose, 3.5% NOx was removed based on the experiment results, while
4.86% NOx was predicted to be
removed based on the computer calculation results obtained from the current model, the results obtained in the present model is much close to the experimental results. At low doses (10.9 kGy), the calculated value is similar to the experimental value, as shown, but
13
the accuracy of the calculated value decreases as the dose is increased. In our previous one, 6.77% NOx was predicted to be removed at 8.8 kGy for 1500 ppm NO while only 1.05% NOx was removed based on the experimental measurement (Figure 2 in Zwolinska et al., 2015). There is some discrepancy between calculation results and experimental data, especially at 200 ppm NO. It might be caused by following factors: first in the computer simulation, “ closed system” was assumed while “flowing system” was used in the experiment; second, energy distribution inside reaction vessel was assumed uniform in the computer simulation while average dose was used in the experiment, for e.g. 10.9kGy dose reported in the experiment, dose inside reaction vessel varied from 9.35kGy (bottom position) to 19.68 kGy ( top position, near
NOx removal efficiency(%)
accelerator window) (Figure 1 in Chmielewski et al., 2018) .
20 15 10 5 0 0
NOx(cal.)
10
20 Dose (kGy)
30
NOx (Chmielewski et.al, 2018)
Figure 1a. NOx removal efficiency versus dose under EB radiation
14
40
NOx removal efficiency(%)
40 30 20 10 0 0
500
1000
1500
2000
Inlet NOx concn. (ppm) NOx (Chmielewski et al., 2018)
NOx(Cal.)
Figure 1b. Inlet concentration of NOx influencing on its removal efficiency at 10.9 kGy absorbed dose under EB radiation
During radiolysis of flue gas , active species O, OH etc. was formed from radiolysis of air and water vapor ( see reactions 3, 4) . They play very important role for NOx removal. The key reactions of NOx removal were given below: NO + O(3P) + M → NO2 + M
(M is a third body in the reaction system)
O(3P) + O2 + M → O3 + M
(33)
NO + O3 + M → NO2 + O2 + M
(34)
NO + HO2· + M → NO2 + ·OH + M
(35)
NO2 + ·OH + M → HNO3 + M OH + NO = HNO2
(32)
(36) k= 3.12 *10-11 cm3/molecule s (37), (Fulle et al.,
1998). 15
OH + HNO2 = H2O + NO2 , k= 2.5x10-12 [cm3/molecule s] e2.16 [±2.16 kJ/mole]/RT, (290 380 K)
(38)
(Atkinson et al., 2014)
NO2 + O(3P) + M → NO3 + M
(39)
Under the EB radiation, NO is oxidized into HNO2 and NO2, NO2 reacts with OH radical to form HNO3 , HNO2 and HNO3 are removed from flue gas when NH3 is added into flue gas. HNO2 is main oxidation product of NOx formed by reaction 37. However without ammonia addition, following reverse reactions (40-44) happen which decrease NOx removal , that’s the reason why only 3.5% NOx was removed at 10.9 kGy absorbed dose under EB irradiation only. O + NO2 = O2 + NO
k= 9.7 ×10-12
(40)
O + NO3 = O2 + NO2
k= 1.7 ×10-11
(41) k=1.52×10-12
OH + HNO3 = H2O + NO3
(42)
NO + NO3 = 2NO2
k=3.00×10-11
(43)
2NO2 = O2 + 2NO
k=2.71×10-12
(44)
Besides, OH radicals take part in self-combination reaction (45) and reaction with SO2 (46) OH + OH + M = H2O2
(45)
SO2 + OH + M = HSO3 + M k0=5,0×10-31×(300/T)3,3; k∞=2,0×10-12; Fc=exp(-T/380) (46) k=1,34×10-12×exp(-330/T) (47)
HSO3 + O2 = SO3 + HO2
16
3.2 NOx removal in the EB-wet scrubber process NOx removal in hybrid system- EB irradiation followed by water scrubber process was studied. It consists of two stages, the first stage was to study NOx removal versus dose under EB irradiation, the second stage was to study NOx absorption in wet scrubber using MATLAB. Electrolyte reactions of water, HNO2 and HNO3 were omitted as done by Loutet (2011, et al.), NaCl was also omitted . The calculation was made for water instead of sea-water which contains 3.5% NaCl.
The calculation results is presented in
Table 1. It is seen that both NO and NO2 concentration reduce after water scrubbing. NOx removal efficiency in the EB process and EB-wet scrubber process is presented in Table 1 as well. At 10.9 kGy absorbed dose, NOx removal efficiency from 38.5% (EB irradiation process) increases to 43.7% (EB-wet scrubber process) for the inlet concentration of NOx being 500 ppm; for inlet concentration of NOx being 1271 ppm, NOx removal efficiency from 4.9% (EB irradiation process) increases to 22.4% (EBwet scrubber process),
it increases by 17.5%.
(Chmielewski et al., 2018),
Based on our previous
NOx removal efficiency
in EB-hybrid process
work with
seawater as scrubber was 50% at 10.9 kGy dose for the flue gas with the inlet concentration of NO being 1500 ppm NO, the two stages of bubbling system were used during experiments. It increased the contacting time between gas and liquid, that’s maybe the reason why the experimental result was much higher than calculation result . From Table 1 , it is seen that wet scrubber process is most efficient for high inlet concentration of NOx removal at the medium range dose.
17
Table 1. Efficiency of NOx removal in the EB process and EB-wet scrubber process
No.
Dose [kGy]
1 2 3 4 5 6
3.5 3.5 10.9 10.9 44.1 44.1
NO initial NO2 initial concentration concentration [ppmv] [ppmv] 200.0 0.0 1000.0 0.0 1271.0 52.0 500.0 20.0 200.0 0.0 1000.0 0.0
Efficiency of NOx removal in the EB process [%] 9.0 2.0 4.9 38.5 55.0 29.0
Efficiency of NOx removal in the hybrid process [%] 11.9 8.7 22.4 43.7 55.8 34. 9
Increased NOx removal efficiency [%] 2.9 6.7 17.5 5.2 0.8 5.9
Figure 2. Mechanism of NOx removal in EB- wet scrubber system Mechanism of NOx removal in EB- wet scrubber
system was shown in figure 2.
Reactions under EB process were presented inside block in figure 2. The scrubber process was drawn with green arrow. All relevant reactions have been given in previous section except N2 and N2O formation. N2* + NO = N2 + NO
(48)
NO2 + N = N2O + O
(49)
18
3.3 NOx removal from ship emission off-gases by microwave plasma Computer programme Chemical workbench was applied to simulate NOx reduction in microwave plasma reactor. The first step of the calculation was made for the following system 70.6 N2 + 15.6 CO2 + 5.6 O2 + 8.2 H2O + 1500 ppm NO without SO2 addition, it is found that only 3.03% NOx was removed in microwave plasma reactor at 32.7 kJ/kg (Figure 3), which is lower than that (experimental value being 5.5% ) in plasma reactor generated by EB. The second step of the calculation was made to the gas composition from
typical ship emission (76.84% N2, 12.4% O2, 5.2%CO2, 5.35% H2O, 1500 ppmv
NOx (1400 ppm NO + 100 ppm NO2) when desulphur fuel was used. NOx removal was calculated at different dose, 6.5 kGy, 10.9 kGy, 32.7 kGy and 9000 kGy. NOx removal efficiency was not nearly changed from 6.5 kGy till to 32.7 kGy, about 11.04% NOx was removed (Figure 4a) . When the dose was increased to 9000 kGy, NOx removal efficiency was increased to 19.75% at the outlet of the microwave reactor (Fig.4b). NO concentration decreased along the length of the microwave plasma rector, NO2 concentration increased, , HNO3 was a main product due to inlet flue gas containing 100 ppm NO2, which reacts with OH radicals to form HNO3.
19
Figure 3.
Concentration of NO, NO2 and HNO3 versus reactor length in microwave
plasma reactor at 32.7 kJ/kg. (The gas composition was similar to that made for EB irradiation)
20
Figure 4a.
Concentration of NO, NO2 and HNO3 versus reactor length in microwave
plasma reactor at 32.7 kJ/kg (Gas composition was similar to ship emission)
Figure 4b. Concentration of NO, NO2 and HNO3 versus reactor length in microwave plasma reactor at 9000 kJ/kg. (Gas composition was similar to ship emission)
Conclusions High concentration of NOx removal from flue gas was simulated under 3 different conditions: sole EB, EB + wet scrubber and micro wave plasma . When sole EB was used, NOx removal efficiency was very low due to NOx formation by the reverse reactions. About 5% NOx was removed at 10.9 kGy dose for the inlet concentration of 21
NO being 1500 ppm. When a wet scrubber (water ) was applied after EB irradiation, NOx removal efficiency was increased by absorbing NO2 from gas phase into water , HNO3 was formed.
For moderate doses (10.9kGy) and high inlet concentration of NOx
(1323 ppm), NOx removal efficiency was increased by 17.5%, from 4.9% (EB only) increased to 22.4% (EB + water scrubber) . Microwave plasma process to remove NOx from marine off-gases was simulated, without scrubber presence ( for e.g., NH3) , less than 20% NOx was removed even at very high dose (9000 kGy), EB is more energy efficient than microwave to remove high inlet concentration of NOx from off-gases.
Acknowledgements
This work is financed by the Polish NCBiR TANGO 2 project (grant No. TANGO2/341079/NCBR/2017).
References
Albritton D.L., (1978).Ion neutral reactions rates. Atomic Data and Nuclear Data 22, 1-101. Atkinson, R.; Baulch, D.L.; Cox, R.A.; Crowley, J.N.; Hampson, R.F.; Hynes, R.G.; Jenkin, M.E.; Rossi, M.J.; Troe, J., 2004. Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reactions of Ox, HOx, NOx and SOx species. Atmos. Chem. Phys. 4, 1461 – 1738. Baulch, D.L., Cox, R.A., Hampson Jr., R.F., Kerr, J.A., Troe, J. 1992. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry. Supplement IV, IUPAC
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Mätzing H., (1991). Chemical kinetics of flue gas cleaning by irradiation with electrons. Advances in Chemical Physics Volume LXXX, 315-402, John Wiley & Sons, Inc., New Jersey.
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25
Zwolinska, E.A., Sun, Y., Chmielewski, A. G., 2019. Electron beam flue gas technology for SOx and NOx simultaneous removal: its process and chemistry evolution from power plants to diesel off-gas treatment. Rev. Chem. Eng. , https://doi.org/10.1515/revce-20180055.
26
Highlights
•
Computer simulation of NOx reduction from off-gases has been studied
•
Computer code KINETIC , MATLAB and
Chemical Workbench were used for
simulation. •
NOx removal from off-gases in three different conditions was investigated.
•
EB is more energy efficient for NOx removal than microwave plasma.
•
NOx removal efficiency is higher in EB-wet scrubber process than EB only.
Jay LaVerne Editor-in-Chief Radiation Physics and Chemistry
Author statement Dear Prof. Jay LaVerne: I am pleased to submit the revised version of this manuscript entitled “COMPUTER SIMULATION OF NOx REMOVAL FROM DIESEL ENGINE OFF-GASES UNDER ELECTRON BEAM AND WET SCRUBBER SYSTEM ” for consideration for publication in Radiation Physics and Chemistry. All changes have been made following reviewers’ comments. This manuscript has not been published and is not under consideration for publication elsewhere. We have no conflicts of interest to disclose.
Best regards. Sincerely yours Yongxia Sun (on behalf of all co-authors) ORCID 0000-0002-7359-3869
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
'Declarations of interest: none'
S0969-806X(19)31306-4
DOI:
https://doi.org/10.1016/j.radphyschem.2020.108707
Reference:
RPC 108707
To appear in:
Radiation Physics and Chemistry
Received Date: 8 October 2019 Revised Date:
7 January 2020
Accepted Date: 9 January 2020
Please cite this article as: Sun, Y., Dobrowolski, A., Chmielewski, A.G., Roubinek, O., Pawelec, A., Nichipor, H., Computer simulation of NOx removal from diesel engine off-gases under electron beam and wet scrubber system, Radiation Physics and Chemistry (2020), doi: https://doi.org/10.1016/ j.radphyschem.2020.108707. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
COMPUTER SIMULATION OF NOx REMOVAL FROM DIESEL ENGINE OFF-GASES UNDER ELECTRON BEAM AND WET SCRUBBER SYSTEM
Y. Sun1,*, A.Dobrowolski1, A.G.Chmielewski1, O. Roubinek1, A. Pawelec1, H.Nichipor2 1
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland 2
IRPCP, Academy of Sciences Republic of Belarus, Minsk-Sosny, Belarus
* Corresponding author: Yongxia Sun, [email protected]; tel: 0048-22-5041368 ORCID 0000-0002-7359-3869
1
Abstract NOx emission from diesel engine off-gases in marine transportation sector is still an environmental issue to be solved in present day. Electron beam (EB) followed by a seawater wet scrubber method to removal NOx has been experimentally
studied
(Chmielewski et al., 2018). In this work numerical simulation of NOx reduction using EB followed by a water scrubber system has been carried out. The first stage was to calculate NOx removal versus dose under EB irradiation . Computer code “KINETIC” and Gear method were used, the model has been improved based our previous work ( Zwolinska E., et al., 2015) and verified by the experimental results. The calculation results of NOx removal efficiency was 4.9% at 10.9 kGy dose under EB irradiation, which was a little higher than experimental value (3.5%). Computer program MATLAB was used to study NOx absorption in water. The NOx removal efficiency was increased by 17.5% at 10.9 kGy dose for inlet concentration of NOx being 1333 ppm after EBwater absorption process . At the end, NOx reduction in the simulated marine off-gas using microwave plasma was simulated using computer program Chemical Workbench . Less than 20% NOx was removed from the off-gases in microwave plasma reactor even at 9000 kGy dose for the inlet concentration of NOx being 1500 ppm. EB is more energy efficient than microwave plasma for NOx removal.
Key words: NOx removal; off-gas; electron beam; wet scrubber; microwave plasma; simulation
1. Introduction
2
NOx emission is still an environmental problem to be solved. NOx emission from landbased sources has been reduced in recent years, while NOx emission from marine transportation sector has been increased, about 30% NOx emission came from ships (European Environment Agency, 2013). International Marine Organization (IMO) has enacted very strict MARPOL NOx Tier III regulation, it has been adopted in emission control areas (ECA) of US, and it will take effect on 1 January 2021 in ECA of Europe (IMO, 2017). EB technology has been applied to remove SO2 and NOx from flue gas emitted from coal-fired power plant Pomorzany Power Station, Poland (Chmielewski et al., 2004). The principle of this technology to remove NOx is that under EB irradiation, NO has been oxidized into NO2 and some of them into NO3, with the presence of ammonia, ammonia nitrate/nitrite has been formed which can be removed from gas phase using electron static precipitator (Zwolińska et al., 2019). Due to the limited space in the ships, this technology with ammonia addition is not practical to be applied in NOx removal from diesel engine off-gas emitted from ship. Sea water might be a good candidate to replace NH3. It is easily to be obtained and it scrubs NO2 from gas phase due to NO2 good solubility in water. This assumption has been roughly verified by our experimental results (Chmielewski et al, 2018) .
Currently, two separate technologies
have been implemented on board, wet scrubber is for removal SO2 and SCR (selective catalytic reduction) is for NOx removal from off-gases, and good removal efficiency of SO2 (99%) and NOx (90%) are achieved using these two separate technologies. However these two separate technologies possess of number of drawbacks: high installation cost, high maintenance cost, and a large space needed for two separate installations and ammonia storage (EGSCA, 2012; Jurgen S. et al, 2012). Moreover, high temperature
3
(300-400 °C) is needed for NOx reduction using the SCR method, so the flue gas after the wet scrubber for SO2 reduction needs to be reheated. Otherwise, the catalyst might be deactivated in a short time when the temperature is below 250 °C, especially when a high concentration of SO2 and PM (particle matter) are present in the off-gases (Magnusson M. et al., 2012). In this work, computer simulation of NOx reduction using electron beam (EB) followed by a water scrubber system has been studied. It consists of two stages . The first stage was to calculate NOx removal versus dose under EB irradiation. The model has been improved based our previous work ( Zwolinska E., et al., 2015) by removing ammonia and related species generated therefore, and by adding some reverse reactions which are formation of NOx.
The revised model better simulated NOx removal under EB
irradiation without ammonia addition. The second stage was to simulate NOx absorption in water using computer program MATLAB.
Microwave plasma has been reported to a
good method for waste management due to its’ many potential advantages, including energy savings and overall cost effectiveness (Schulz et al., 1993) ,
high removal
efficiency of organic pollutants , such as trichloroethylene ,1,1,1-trichloroethane ( Krause and Helt, 1993) and HFC 134a refrigerant from air (Jasinski et al., 2009) was reported . In order to find out whether there is a more energy efficient method for removal NOx, we simulated NOx removal from flue gas using microwave plasma.
2. Modelling calculation
2.1 Computer simulation of NOx reduction from flue gas under electron beam irradiation
4
Computer simulation of NOx reduction from flue gas under electron beam irradiation was carried out using Computer code “Kinetic” (Bugaenko and Grichkin, 1980) and “GEAR” method. The model was improved based our previous work ( Zwolinska E., et al., 2015) by removing ammonia and related species generated therefore, and by adding some reverse reactions which are formation of NOx thorough O atom reaction with NO2 and NO3.
The revised model better simulated NOx removal under EB irradiation
without ammonia addition. The “Kinetic” code is designed for calculation the time evolution of the different main species involved in the kinetic reaction system at constant pressure and temperature. The kinetic reaction system is considered as self-contained and closed. It should obey the principle of mass balance and charge balance. For “Kinetic” code applied in this work, the reaction rate of Wj (molecules·cm-3·s-1) of j type active species (for e.g. N2*), which were generated from pure k type molecules (for e.g. N2) that absorb 100 eV of energy, was calculated according to an equation 1:
Wj = ∑ Gjk· I·ρk
(1)
where: Gjk - the radiation chemical yield of j type species generated from pure k type molecules that absorb 100 eV of energy (molecules/100 eV), I - dose rate (kGy/s),
ρk - gas phase density of pure k type molecules (g/cm3)
5
ρ - overall density of the gas phase applied when dose is expressed in ρ·eV·cm-3 (1 Gy = 6.2415·ρ·1015 eV·cm-3). The units of the simulation calculations of EB irradiation adopted in this work are cm, g, s and eV. The kinetic reaction system from a mathematical point of view is described by the system of ordinary differential equations (ODE). The “Kinetic” code used a Gear algorithm for numerical integration of stiff systems of ODE. In this work, the revised model consists of 287 reactions with 55species. 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/mole·s and m6/mole2·s for first-, second- and third- order reactions, respectively. The input values of the modelling simulation were the same as those described in the experiments (Chmielewski, et al., 2018). The boundary conditions of calculation in the model at time 0 are: 70.6% N2, 5.6% O2, 15.6% CO2, 8.2% H2O, 1500 ppmv NO and 700 ppmv SO2. Calculation was made at 90°C , electron beam was 400 ns pulse duration with different repetition frequency varying from 2 Hz to 15 Hz . The mechanism of NOx removal under EB irradiation has been described in details ( Chmielewski et al, 2002; Zwolinska, et al., 2015 & 2017; ). In general , under electron beam irradiation, the main components (nitrogen, oxygen, water and carbon dioxide) in flue gas are ionized and excited by the high energy electrons from electron beam , the primary species and second electrons are formed. The G-values (molecules/100 eV) of main primary species are simplified as follows (Mätzing, 1991): 4.43N2 → 0.29N2* + 0.885N(2D) + 0.295N(2P) + 1.87N + 2.27N2+ + 0.69N+ + 2.96e (2) 6
5.377O2 → 0.077O2* + 2.25O(1D) + 2.8O + 0.18O* + 2.07O2+ + 1.23O+ + 3.3e
(3)
7.33H2O → 0.51H2 + 0.46O(3P) + 4.25OH + 4.15H + 1.99H2O+ + 0.01H2+ + 0.57OH+ + 0.67H+ + 0.06O+ + 3.3e
(4)
7.54CO2 →4.72CO + 5.16O(3P) + 2.24CO2+ + 0.51CO+ + 0.07C+ + 0.21O+ + 3.03e
(5)
These primary species and thermalized secondary electrons cause NOx removal through complex chemical reactions.
The removal efficiency of the flue gas (R%) is denoted by the following equation (5), where C1 denotes the initial concentration of a certain compound of flue gas and C2 denotes the concentration of a certain compound of flue gas after the clean-up procedure. R%=[(C1-C2)/C1]×100%
(6)
2.2 Calculation of NOx removal in the water absorption process using MATLAB software
After electron beam irradiation, NOx from outlet of EB reactor was absorbed in wet scrubbing process.
A mathematical model was constructed to enable analysis of
mechanism of NOx removal in the water absorption process.
7
The main reactions of NOx in the gas and liquid phases were considered (Loutet et al., 2011): - gas phase 2
+
( )
( ) ↔
2 ( )
+
2
( )
( )
(7) (8)
( )
( ) ↔
(9)
( )
( )
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( )
2
( )
+
( )
(10)
( ) ( )
+
( )
(11)
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3
( )
+
( )
( )
( )
+
( )
2
( )
( )
+
( )
( )
( ) ( )
+
+
(13)
+
()
(12)
( )
+2
( )
(14)
( )
(15)
The chemical reaction rates are as given by Loutet et al.(2011): k7 = 0.0102 m6kmol–2s–1, k8 = 109 m3kmol–1s–1, k9 = 109 m3kmol–1s–1, k10 = 41 000 m3kmol–1s–1, k11 = 250 m3kmol–1s–1, k12 = 46 999 m3kmol–1s–1, k13 = 17000 s–1, k14 = 93.32 s–1, k15 = 0.0468 atm2m9kmol–3s–1, k8’ = k8 · K8–1, K8 = 0.439 m3kmol–1, k9’ = k9 · K9–1, K9 = 13 699 m3kmol–1, k10’ = k10 · K10–1, K10 = 0.2763, k11’ = k11 · K11–1, K11 = 0.00002191. The absorption kinetic was described by equation (16): 8
ri = kLa · (pi – pi*)
(16)
where: i = NO, NO2, N2O3, N2O4; kLa – the mass transfer coefficient [s–1], in this work the value of kLa was set as 0.01[s–1] (Pisut et al, 2009); pi – partial pressure of i component in bulk; pi* – interface pressure of i component in liquid. Taking into account of mass balance of absorption process, the rate of accumulation of component (i) is defined by following : Rate of accumulation of component (i) = Rate of mass transport of component (i) +Rate of (inflow – outflow) of component(i)+ Rate of generation of component (i) by chemical reactions i=NOg, O2g, NO2g, N2O3g,N2O4g,HNO2g,HNO3g,H2Og, NOl, O2l, NO2l, N2O3l,N2O4l,HNO2l,HNO3l
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The system of ordinary differential equations, being the mathematical model of considered process, was solved with the use of Runge-Kutta procedure in the MATLAB programming language. The simulations were made for the following conditions: - initial NO concentrations: 200, 500, 1000, and 1271 ppmv;- initial NO2 concentrations: 0, 20, and 50 ppmv. The following irradiation doses were considered: 3.5 kGy, 10.9 kGy, 44.1 kGy. The concentrations of NO and NO2 at the scrubber inlet were taken based on the our
previous calculation and experimental
results
(Zwolinska et al., 2017;
Chmielewski et al, 2002) . The volume flowrate of the flue gas (Qg ) was assumed as 5 m3/hr, volumetric flowrate of the scrubber solution (Ql) was assumed as 0.05m3/hr. The dimension of the absorption column was assumed : 0.15 m (diameter) * 0.9m (height). The temperature of the gas phase was assumed 90oC which was the temperature of the flue gas from outlet of EB reaction vessel ; the temperature of the liquid phase was assumed 15oC which was the temperature of scrubber solution used .
11
2.3 Computer simulation of NOx cleaning from ship emission off-gases by microwave plasma using Chemical workbench software Microwave plasma proved its efficiency in removing organic pollutant HFC 134a refrigerant from air (Jasinski et al., 2009). Chemical Workbench software (version 4.2) was used to simulate cleaning off-gases using microwave plasma reactor. The system of microwave plasma reactor to treat NOx was similar to the horizontal microwave waste treatment system used (Schulz, 1998) . It includes: injecting N2 gas into the microwave oven to generate N atoms ; the second stage
;
off-gas interconnected and mixed with N atoms and N2 in
off-gas treatment microwave oven.
The model consists of
thermodynamic reactor, well stirred reactor and plug flow reactor to simulate these three process . Thermodynamic reactor was used to simulate nitrogen atoms generation from N2 dissociation caused by adding enthalpy originated from the arc power , a well stirred reactor was used to simulate plasma mixing with waste gas, NOx removal in off-gas was simulated in a plug flow reactor. All process took place at 1 atmospheric pressure, No heat lost. 29 species and 155 reactions were included in this model. Only radicals’ and neutral species’ reactions were taken into consideration. The reaction rate constants were mostly taken from literatures (Baulch et al., 1992; Atkinson et al., 2004). The size of microwave plasms reactor to treat off-gas was assumed the same as the reaction vessel under EB irradiation.
The reactor was 25 cm diameter with 140 cm length. The
volumetric flow rate of flue gas was 5 Nm3/h, the temperature of the flue gas inside microwave reactor was assumed to be 90°C .
3. Results and discussion
12
3.1 NO removal in gas phase under EB irradiation Dose influencing on NOx removal was studied using computer simulation for inlet concentration of NO being 1500 ppm. The calculation results are presented in Figure 1a. It is seen that NO removal efficiency increases with the absorbed dose increasing. The similar trend was observed in experimental work (Chmielewski et al., 2018). It may be explained by that with the absorbed does increasing, more actives species ( O, OH etc.) are formed. Concentration of active species increasing enhances NOx removal efficiency. While NOx removal efficiency decreased with inlet concentration of NOx increasing under EB irradiation at the same absorbed dose (figure 1b) , It can be explained that with inlet NO concentration increasing, more oxidant species e.g. OH and O are used to oxidize NO into NO2. Concentration of OH radicals , which are generated from water radiolysis,
are almost constant
at the same absorbed dose.
The ratio between
concentration of OH radicals to total concentration of NO and NO2 decreases with increasing inlet concentration of NO, thus the NOx (NO + NO2) removal efficiency decreases significantly during the concentration range of 200 ppm to 1000 ppm. With NO concentration increases to 1.7 times, e.g. from 1000 ppm to 1700 ppm, the NOx removal efficiency decreases from 6.9% to 3.5% based on the modeling calculation results.
The calculation results of NOx removal efficiency for 1500 ppm NO were a
little higher than experimental results. At 10.9 kGy absorbed dose, 3.5% NOx was removed based on the experiment results, while
4.86% NOx was predicted to be
removed based on the computer calculation results obtained from the current model, the results obtained in the present model is much close to the experimental results. At low doses (10.9 kGy), the calculated value is similar to the experimental value, as shown, but
13
the accuracy of the calculated value decreases as the dose is increased. In our previous one, 6.77% NOx was predicted to be removed at 8.8 kGy for 1500 ppm NO while only 1.05% NOx was removed based on the experimental measurement (Figure 2 in Zwolinska et al., 2015). There is some discrepancy between calculation results and experimental data, especially at 200 ppm NO. It might be caused by following factors: first in the computer simulation, “ closed system” was assumed while “flowing system” was used in the experiment; second, energy distribution inside reaction vessel was assumed uniform in the computer simulation while average dose was used in the experiment, for e.g. 10.9kGy dose reported in the experiment, dose inside reaction vessel varied from 9.35kGy (bottom position) to 19.68 kGy ( top position, near
NOx removal efficiency(%)
accelerator window) (Figure 1 in Chmielewski et al., 2018) .
20 15 10 5 0 0
NOx(cal.)
10
20 Dose (kGy)
30
NOx (Chmielewski et.al, 2018)
Figure 1a. NOx removal efficiency versus dose under EB radiation
14
40
NOx removal efficiency(%)
40 30 20 10 0 0
500
1000
1500
2000
Inlet NOx concn. (ppm) NOx (Chmielewski et al., 2018)
NOx(Cal.)
Figure 1b. Inlet concentration of NOx influencing on its removal efficiency at 10.9 kGy absorbed dose under EB radiation
During radiolysis of flue gas , active species O, OH etc. was formed from radiolysis of air and water vapor ( see reactions 3, 4) . They play very important role for NOx removal. The key reactions of NOx removal were given below: NO + O(3P) + M → NO2 + M
(M is a third body in the reaction system)
O(3P) + O2 + M → O3 + M
(33)
NO + O3 + M → NO2 + O2 + M
(34)
NO + HO2· + M → NO2 + ·OH + M
(35)
NO2 + ·OH + M → HNO3 + M OH + NO = HNO2
(32)
(36) k= 3.12 *10-11 cm3/molecule s (37), (Fulle et al.,
1998). 15
OH + HNO2 = H2O + NO2 , k= 2.5x10-12 [cm3/molecule s] e2.16 [±2.16 kJ/mole]/RT, (290 380 K)
(38)
(Atkinson et al., 2014)
NO2 + O(3P) + M → NO3 + M
(39)
Under the EB radiation, NO is oxidized into HNO2 and NO2, NO2 reacts with OH radical to form HNO3 , HNO2 and HNO3 are removed from flue gas when NH3 is added into flue gas. HNO2 is main oxidation product of NOx formed by reaction 37. However without ammonia addition, following reverse reactions (40-44) happen which decrease NOx removal , that’s the reason why only 3.5% NOx was removed at 10.9 kGy absorbed dose under EB irradiation only. O + NO2 = O2 + NO
k= 9.7 ×10-12
(40)
O + NO3 = O2 + NO2
k= 1.7 ×10-11
(41) k=1.52×10-12
OH + HNO3 = H2O + NO3
(42)
NO + NO3 = 2NO2
k=3.00×10-11
(43)
2NO2 = O2 + 2NO
k=2.71×10-12
(44)
Besides, OH radicals take part in self-combination reaction (45) and reaction with SO2 (46) OH + OH + M = H2O2
(45)
SO2 + OH + M = HSO3 + M k0=5,0×10-31×(300/T)3,3; k∞=2,0×10-12; Fc=exp(-T/380) (46) k=1,34×10-12×exp(-330/T) (47)
HSO3 + O2 = SO3 + HO2
16
3.2 NOx removal in the EB-wet scrubber process NOx removal in hybrid system- EB irradiation followed by water scrubber process was studied. It consists of two stages, the first stage was to study NOx removal versus dose under EB irradiation, the second stage was to study NOx absorption in wet scrubber using MATLAB. Electrolyte reactions of water, HNO2 and HNO3 were omitted as done by Loutet (2011, et al.), NaCl was also omitted . The calculation was made for water instead of sea-water which contains 3.5% NaCl.
The calculation results is presented in
Table 1. It is seen that both NO and NO2 concentration reduce after water scrubbing. NOx removal efficiency in the EB process and EB-wet scrubber process is presented in Table 1 as well. At 10.9 kGy absorbed dose, NOx removal efficiency from 38.5% (EB irradiation process) increases to 43.7% (EB-wet scrubber process) for the inlet concentration of NOx being 500 ppm; for inlet concentration of NOx being 1271 ppm, NOx removal efficiency from 4.9% (EB irradiation process) increases to 22.4% (EBwet scrubber process),
it increases by 17.5%.
(Chmielewski et al., 2018),
Based on our previous
NOx removal efficiency
in EB-hybrid process
work with
seawater as scrubber was 50% at 10.9 kGy dose for the flue gas with the inlet concentration of NO being 1500 ppm NO, the two stages of bubbling system were used during experiments. It increased the contacting time between gas and liquid, that’s maybe the reason why the experimental result was much higher than calculation result . From Table 1 , it is seen that wet scrubber process is most efficient for high inlet concentration of NOx removal at the medium range dose.
17
Table 1. Efficiency of NOx removal in the EB process and EB-wet scrubber process
No.
Dose [kGy]
1 2 3 4 5 6
3.5 3.5 10.9 10.9 44.1 44.1
NO initial NO2 initial concentration concentration [ppmv] [ppmv] 200.0 0.0 1000.0 0.0 1271.0 52.0 500.0 20.0 200.0 0.0 1000.0 0.0
Efficiency of NOx removal in the EB process [%] 9.0 2.0 4.9 38.5 55.0 29.0
Efficiency of NOx removal in the hybrid process [%] 11.9 8.7 22.4 43.7 55.8 34. 9
Increased NOx removal efficiency [%] 2.9 6.7 17.5 5.2 0.8 5.9
Figure 2. Mechanism of NOx removal in EB- wet scrubber system Mechanism of NOx removal in EB- wet scrubber
system was shown in figure 2.
Reactions under EB process were presented inside block in figure 2. The scrubber process was drawn with green arrow. All relevant reactions have been given in previous section except N2 and N2O formation. N2* + NO = N2 + NO
(48)
NO2 + N = N2O + O
(49)
18
3.3 NOx removal from ship emission off-gases by microwave plasma Computer programme Chemical workbench was applied to simulate NOx reduction in microwave plasma reactor. The first step of the calculation was made for the following system 70.6 N2 + 15.6 CO2 + 5.6 O2 + 8.2 H2O + 1500 ppm NO without SO2 addition, it is found that only 3.03% NOx was removed in microwave plasma reactor at 32.7 kJ/kg (Figure 3), which is lower than that (experimental value being 5.5% ) in plasma reactor generated by EB. The second step of the calculation was made to the gas composition from
typical ship emission (76.84% N2, 12.4% O2, 5.2%CO2, 5.35% H2O, 1500 ppmv
NOx (1400 ppm NO + 100 ppm NO2) when desulphur fuel was used. NOx removal was calculated at different dose, 6.5 kGy, 10.9 kGy, 32.7 kGy and 9000 kGy. NOx removal efficiency was not nearly changed from 6.5 kGy till to 32.7 kGy, about 11.04% NOx was removed (Figure 4a) . When the dose was increased to 9000 kGy, NOx removal efficiency was increased to 19.75% at the outlet of the microwave reactor (Fig.4b). NO concentration decreased along the length of the microwave plasma rector, NO2 concentration increased, , HNO3 was a main product due to inlet flue gas containing 100 ppm NO2, which reacts with OH radicals to form HNO3.
19
Figure 3.
Concentration of NO, NO2 and HNO3 versus reactor length in microwave
plasma reactor at 32.7 kJ/kg. (The gas composition was similar to that made for EB irradiation)
20
Figure 4a.
Concentration of NO, NO2 and HNO3 versus reactor length in microwave
plasma reactor at 32.7 kJ/kg (Gas composition was similar to ship emission)
Figure 4b. Concentration of NO, NO2 and HNO3 versus reactor length in microwave plasma reactor at 9000 kJ/kg. (Gas composition was similar to ship emission)
Conclusions High concentration of NOx removal from flue gas was simulated under 3 different conditions: sole EB, EB + wet scrubber and micro wave plasma . When sole EB was used, NOx removal efficiency was very low due to NOx formation by the reverse reactions. About 5% NOx was removed at 10.9 kGy dose for the inlet concentration of 21
NO being 1500 ppm. When a wet scrubber (water ) was applied after EB irradiation, NOx removal efficiency was increased by absorbing NO2 from gas phase into water , HNO3 was formed.
For moderate doses (10.9kGy) and high inlet concentration of NOx
(1323 ppm), NOx removal efficiency was increased by 17.5%, from 4.9% (EB only) increased to 22.4% (EB + water scrubber) . Microwave plasma process to remove NOx from marine off-gases was simulated, without scrubber presence ( for e.g., NH3) , less than 20% NOx was removed even at very high dose (9000 kGy), EB is more energy efficient than microwave to remove high inlet concentration of NOx from off-gases.
Acknowledgements
This work is financed by the Polish NCBiR TANGO 2 project (grant No. TANGO2/341079/NCBR/2017).
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26
Highlights
•
Computer simulation of NOx reduction from off-gases has been studied
•
Computer code KINETIC , MATLAB and
Chemical Workbench were used for
simulation. •
NOx removal from off-gases in three different conditions was investigated.
•
EB is more energy efficient for NOx removal than microwave plasma.
•
NOx removal efficiency is higher in EB-wet scrubber process than EB only.
Jay LaVerne Editor-in-Chief Radiation Physics and Chemistry
Author statement Dear Prof. Jay LaVerne: I am pleased to submit the revised version of this manuscript entitled “COMPUTER SIMULATION OF NOx REMOVAL FROM DIESEL ENGINE OFF-GASES UNDER ELECTRON BEAM AND WET SCRUBBER SYSTEM ” for consideration for publication in Radiation Physics and Chemistry. All changes have been made following reviewers’ comments. This manuscript has not been published and is not under consideration for publication elsewhere. We have no conflicts of interest to disclose.
Best regards. Sincerely yours Yongxia Sun (on behalf of all co-authors) ORCID 0000-0002-7359-3869
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
'Declarations of interest: none'