Numerical Investigation of SCR Mixer Design Optimization for Improved Performance
Abstract
:1. Introduction
CO(NH2)2→ NH3 + HNCO | (Urea pyrolysis reaction) |
HNCO + H2O →NH3+ CO2 | (HNCO hydrolysis) |
4NH3 + 4NO + O2→ 4N2 + 6 H2O | (Standard SCR reaction) |
4NH3 + 2NO + 2NO2→4N2 + 6 H2O | (Fast-speed SCR reaction) |
8NH3 + 6NO2→7N2 + 12H2O | (Slow SCR Reaction) |
2. Computational Model Formulation
2.1. Geometric Model
2.2. Numerical Procedure
2.3. Boundary Conditions
3. Results and Discussion
3.1. Uniformity
3.1.1. Velocity Uniformity
3.1.2. Ammonia Uniformity
3.2. Droplet Residence Time
3.3. Urea Conversion
3.4. Relation of Temperature with SCR Performance
Temperature Distribution along the Mixer Downstream
3.5. Working Performance of LSM-Based SCR System
4. Conclusions
- For the in-line type mixer (LM), the uniformity index of velocity was good (0.93) but the uniformity of ammonia was poor (0.87). In contrary to LM, the swirl type mixer (SM) has good ammonia uniformity (0.94) but poor uniformity index of velocity (0.86). However, better values were observed by using combination of two mixers (LSM). The uniformity index of velocity and ammonia uniformity achieved the values of 0.95 and 0.96, respectively, for LSM-based SCR system.
- The residence time UWS was studied. The results show that the residence time of urea droplets for LSM-based SCR system was 0.064 s, which represents47% and 29% decreases compared to LM and SM, respectively. Furthermore, the conversion of urea into ammonia is highly related with the residence time of urea droplets in the pipe. Hence, urea conversion achieves the value of 95.4% by using LSM, which is 19.3% and 12.2% higher than the value of LM and SM, respectively.
- It was also observed that the combination of two mixers (LSM) have good temperature distribution than the LM and SM for radial and axial directions, the temperature at catalyst inlet in axial direction was 300 °C for LSM-based SCR system which is suitable for the catalyst reaction performance and prevents the deposit formation.
- Finally, the simulated results of the model parameters were compared and verified by using ISO 8178 standard marine Diesel engine test cycle E3. The average weighted value of NOx emission was calculated as 2.44 g/kWh for four different loads. Hence, it is concluded that the system based on using LSM strongly meets the standard of IMO Tier III NOx emission regulations effectively.
Author Contributions
Funding
Conflicts of Interest
Abbreviations
CFD | Computational Fluid Dynamics |
ECA | Emission Control Areas |
IMO | International Maritime Organization |
LM | Line Mixer |
LSM | Line Swirl Mixer |
SM | Swirl Mixer |
SCR | Selective Catalyst Reduction |
UWS | Urea Water Solution |
Nomenclature | |
Symbol | Name (Unit) |
R | Gas constant (J kg−1 K−1) |
P | Pressure (Pa) |
T | Reaction Temperature (K) |
U | Fluid velocity (m/s) |
Ρ | Droplet density (m2/s3) |
K | Turbulent kinetic energy (m2/s2) |
Vt | Velocity with time (m/s) |
Cµ | Closure coefficient |
Cs | Volume concentration (m3) |
Diffusion flux | |
DS | Component diffusion coefficient |
Sm | Chemical reaction component mass |
Gas velocity vector | |
Ri | Net production rate of species |
Si | Rate of creation from dispersed phase to user defined phase |
Yi | Species destruction molar rate |
DT | Turbulent diffusivity (m2/s) |
DT,i | Thermal diffusion coefficient (m2/s) |
DT,m | Mass diffusion coefficient |
Cwater | Molar concentration of water |
Kc | Mass transfer coefficient (m/s) |
Tp | Droplet temperature |
Ri,r | Species destruction molar rate |
mp | Droplet mass (kg) |
up | Velocity of liquid droplet |
ρp | Density of liquid droplet |
F | Force except drag force |
CP | Specific heat of liquid droplet (J/K) |
AP | Droplet surface area (m2) |
T∞ | Droplet environment temperature |
h | Convective heat transfer coefficient (W/(m2K)) |
hfg | latent heat of vaporization (KJ/kg) |
Vi | Carrier Nominal velocity (m/s) |
Vmean | Average velocity (m/s) |
Ai | Cell area (m2) |
A | Sectional area of the plane (m2) |
EFx | Weighted emission level (g/kWh) |
mi | Mass emission rate (g/h) |
WFi | Weighting factor |
Pi | Engine load |
Ar | Pre-exponential factor |
Ea | Activation energy (J/kmol) |
MW | Species molecular weight |
Greek symbols | |
Ε | turbulent dissipation (m2/s3) |
Α | viscosity coefficient |
εp | radiant heat transfer rate of liquid droplet |
µt | turbulent viscosity (m2/s) |
Psat, water | Vapour pressure of water |
Dimensionless numbers | |
Pr | Prandtl number |
Sct | Turbulent Schmidt number |
Sc | Schmidt number |
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Parameters | 25% | 50% | 75% | 100% |
---|---|---|---|---|
Flow (kg/s) | 0.516 | 1.2 | 1.63 | 1.94 |
Pressure (bar) | 1.4 | 2.17 | 3.1 | 3.8 |
Temperature (°C) | 254 | 290 | 327 | 395 |
Urea Injection (kg/s) | 0.0091 | 0.013 | 0.0161 | 0.0192 |
Parameters | Value | |
---|---|---|
Injection Conditions | Injection type model | Pressure swirl atomizer |
Injection inner hole diameter | 0.0007 m | |
Urea temperature | 313 k | |
Number of streams | 10 | |
Catalyst Conditions | Inverse absolute Permeability (m−2) | 1.85 |
Inertia resistance (m−1) | 85 | |
Porosity | 0.89 | |
Surface to volume ratio (1/m) | 1275 |
Uniformity Index | LM | SM | LSM |
---|---|---|---|
Velocity Uniformity | 0.93 | 0.86 | 0.95 |
Ammonia Uniformity | 0.87 | 0.94 | 0.96 |
Type ISO 8178 E3 Mode | 1 | 2 | 3 | 4 |
---|---|---|---|---|
Load (%) | 25 | 50 | 75 | 100 |
Speed (%) | 63 | 80 | 91 | 100 |
Weightage factor | 0.15 | 0.15 | 0.5 | 0.2 |
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Mehdi, G.; Zhou, S.; Zhu, Y.; Shah, A.H.; Chand, K. Numerical Investigation of SCR Mixer Design Optimization for Improved Performance. Processes 2019, 7, 168. https://doi.org/10.3390/pr7030168
Mehdi G, Zhou S, Zhu Y, Shah AH, Chand K. Numerical Investigation of SCR Mixer Design Optimization for Improved Performance. Processes. 2019; 7(3):168. https://doi.org/10.3390/pr7030168
Chicago/Turabian StyleMehdi, Ghazanfar, Song Zhou, Yuanqing Zhu, Ahmer Hussain Shah, and Kishore Chand. 2019. "Numerical Investigation of SCR Mixer Design Optimization for Improved Performance" Processes 7, no. 3: 168. https://doi.org/10.3390/pr7030168
APA StyleMehdi, G., Zhou, S., Zhu, Y., Shah, A. H., & Chand, K. (2019). Numerical Investigation of SCR Mixer Design Optimization for Improved Performance. Processes, 7(3), 168. https://doi.org/10.3390/pr7030168