Reduction of Unburned Carbon Release and NOx Emission from a Pulverized Wood Pellet Boiler Retrofitted for Fuel Switching from Coal
Abstract
:1. Introduction
2. Target Boilers and Numerical Methods
2.1. Target Boiler and Operation Conditions
2.2. CFD Modeling Methods
3. Results and Discussion
3.1. Comparison of CFD Results with Measured Data for the Reference Case
3.2. CFD Results for Flow and Reaction Characteristics in Case R
3.3. CFD Results for Influence of Air Staging (Cases 1–3)
3.4. CFD Results for Decrease in Particle Sizes (Cases 4–6)
3.5. CFD Results for Other Modifications in Boiler Operation (Cases 7 and 8)
4. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Nomenclature
A | Pre-exponential factor (s−1), surface area (m2) |
c | Specific heat (J/kg·K) |
CFD | Computational fluid dynamics |
d | Diameter (cm) |
E | Activation energy (kJ/kmol) |
ECO | Economizer |
FEGT | Furnace exit gas temperature (°C) |
FGR | Flue gas recirculation |
h | Convection coefficient (W/m2·K) |
IRZ | Internal recirculation zone |
Keq | Chemical equilibrium constant |
k | Reaction rate (g·cm−2·atm−1·s−1) |
Nu | Nusselt number |
OFA | Overfire air |
P | Pressure (atm) |
Pr | Prandtl number |
R | Universal gas constant, Reaction rate of char (g·cm−2·s−1) |
Re | Reynolds number |
SH | Superheater |
SR | Stoichiometric ratio |
T | Temperature (K) |
t | Time (s) |
UBC | Unburned carbon |
UCSM | Unreacted core shrinking model |
V | Volatile matter (kg) |
v | Velocity (m/s) |
Y | Unreacted char core to particle diameter ratio |
θR | Radiation temperature |
ε | Porosity of the ash layer, Emissivity |
σ | Stefan–Boltzmann constant (5.67 × 10−8 (W/m2·K4)) |
ax | axial direction |
char | Unreacted char core |
diff | Diffusion rate |
dash | Diffusion rate in the ash layer |
i | Index of char conversion reaction |
o | Initial |
p | Particle |
s | Surface |
t | Total pressure |
tan | Tangential direction |
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Parameter | Values |
---|---|
Wood pellet | Proximate analysis (% wet): Total moisture 8.90, volatile matter 73.77, fixed carbon 14.85, ash 2.48 Ultimate analysis (% dry, ash-free): C 49.65, H 5.62, O 44.32, N 0.41 Higher heating value (MJ/kg): 17.11 |
Fuel throughput | 76,800 kg/h |
Burner primary air | 122,920 kg/h, 159 °C at the mill inlet |
Burner secondary air | 331,888 kg/h, 315 °C |
OFA | 22,275 kg/h, 315 °C |
Excess air ratio | 21.6% |
Case | Burner Secondary Air Ratio (%) | Burner Zone Stoichiometric Ratio | OFA Ratio (%) | Note | ||
---|---|---|---|---|---|---|
F1 | F2 | R2 | ||||
R | 23.0 | 23.0 | 23.0 | 1.16 | 4.7 | Reference case |
1 | 24.6 | 24.6 | 24.6 | 1.22 | 0 | Different air staging |
2 | 21.5 | 21.5 | 21.5 | 1.10 | 9.4 | |
3 | 18.4 | 18.4 | 18.4 | 0.99 | 18.7 | |
4 | 23.0 | 23.0 | 23.0 | 1.16 | 4.7 | Smaller fuel particles (Figure 2) with different air staging |
5 | 21.5 | 21.5 | 21.5 | 1.10 | 9.4 | |
6 | 18.4 | 18.4 | 18.4 | 0.99 | 18.7 | |
7 | 23.0 | 23.0 | 23.0 | 1.16 | 4.7 | Lower swirl intensity (vtan/vax = 0.7) |
8 | 23.0 | 23.0 | 23.0 | 1.16 | 4.7 | Flue gas recirculation (20%) |
Category | Submodels |
---|---|
Discrete phase | —Lagrangian scheme with stochastic tracking for turbulence —Number of particles: 74,800 —Particle size: 10–2700 μm |
Biomass combustion | —Devolatilization: bio-FLASHCHAIN [24] Dry biomass→77.80 wt.% daf volatiles + 19.62 wt.% daf C(s) (Char) Composition of volatiles: Tar 31.2, CO 34.1, CO2 5.7, H2O 11.6, H2 0.91, CH4 1.19, CxHy 8.31 wt.% daf Devolatilization rate: ; E = 18.5 kcal/mol, A= 1.03 × 107 s−1 —Char conversion: unreacted core shrinking model [25] , (R1) C(s) + 0.5 O2 → CO (R2) C(s) + H2O → CO+H2 (R3) C(s) + CO2 → 2 CO |
Species, gas reaction | —Species: Tar, CO, CO2, H2, CH4, CxHy, H2, SO2, O2, N2 —Reaction mechanism [26,27] (R4) CxHyOz (tar) + a x2+y4-z2)O2 → x CO + 0.5y H2 (R5) CnHm + 0.5n x2+y4-z2)O2 → n CO + 0.5m H2 (R6) CnHm + 0.5n Hx2+y4-z2)2O → n CO + 0.5(m+n) H2 (R7) CH4 + 0.5 x2+y4-z2)O2 → CO + 2 H2 (R8) CH4 + 0.5 Hx2+y4-z2)2O → CO + 2.5 H2 (R9) CO + H2O → CO2 + H2 (R10) H2 + 0.5 O2 → H2O —Reaction rate: kinetic rate/eddy dissipation rate model [28] |
NOx | —Thermal NOx: extended Zeldovich mechanism —Fuel NOx: De Soete model [34] —Fuel-N evenly distributed between volatile-N as NH3 and char-N as NO —NO reduction on the particle surface with a N2-BET surface area of 200 m2/g |
Parameter | Measured Data | CFD | |
---|---|---|---|
Exit O2 (% dry) | 3.72 | 4.1 | |
Exit gas temperature (°C) | 354.5 | 367.6 | |
Exit NO (ppm, 6% O2) | 81.2 | 107.6 | |
Heat absorption (MWth) | Evaporator | 148.4 | 148.8 |
Platen+ Primary SH | 65.1 | 65.4 | |
Final SH | 31.6 | 31.9 | |
RH | 46.0 | 46.5 | |
Economizer | 15.7 | 15.0 | |
UBC (wt%) | Bottom ash | 61.7 | 69.6 |
Fly ash | 1.9 | 1.3 |
Case | Burner Zone Stoichiometric Ratio | Unburned Carbon Content (%) | Bottom Ash Release (kg/h) | Carbon Conversion (%) | Boiler Efficiency (%) | Furnace Exit Gas Temp. (°C) | Exit NO (ppm, 6% O2) | |
---|---|---|---|---|---|---|---|---|
Fly Ash | Bottom Ash | |||||||
R | 1.16 | 1.3 | 69.6 | 990.0 | 97.84 | 84.3 | 1117.8 | 107.6 |
1 | 1.22 | 2.8 | 68.7 | 984.9 | 97.81 | 84.2 | 1140.9 | 120.1 |
2 | 1.10 | 1.6 | 70.7 | 1066.6 | 97.63 | 84.0 | 1115.4 | 98.5 |
3 | 0.99 | 2.4 | 76.6 | 1663.9 | 96.02 | 82.8 | 1099.4 | 84.3 |
4 | 1.16 | 0.7 | 3.5 | 143.4 | 99.95 | 86.0 | 1138.9 | 111.1 |
5 | 1.10 | 1.6 | 5.9 | 139.3 | 99.89 | 85.9 | 1135.6 | 101.3 |
6 | 0.99 | 3.5 | 31.6 | 287.4 | 99.54 | 85.6 | 1125.9 | 85.4 |
7 | 1.16 | 0.7 | 71.4 | 1079.9 | 97.63 | 84.0 | 1112.0 | 108.4 |
8 | 1.16 | 6.2 | 69.3 | 906.8 | 97.76 | 82.8 | 1086.9 | 179.8 |
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Lee, J.; Yu, S.; Park, J.; Jo, H.; Park, J.; Ryu, C.; Jeong, Y.-g. Reduction of Unburned Carbon Release and NOx Emission from a Pulverized Wood Pellet Boiler Retrofitted for Fuel Switching from Coal. Energies 2020, 13, 5077. https://doi.org/10.3390/en13195077
Lee J, Yu S, Park J, Jo H, Park J, Ryu C, Jeong Y-g. Reduction of Unburned Carbon Release and NOx Emission from a Pulverized Wood Pellet Boiler Retrofitted for Fuel Switching from Coal. Energies. 2020; 13(19):5077. https://doi.org/10.3390/en13195077
Chicago/Turabian StyleLee, Jiseok, Seunghan Yu, Jinje Park, Hyunbin Jo, Jongkeun Park, Changkook Ryu, and Yeong-gap Jeong. 2020. "Reduction of Unburned Carbon Release and NOx Emission from a Pulverized Wood Pellet Boiler Retrofitted for Fuel Switching from Coal" Energies 13, no. 19: 5077. https://doi.org/10.3390/en13195077
APA StyleLee, J., Yu, S., Park, J., Jo, H., Park, J., Ryu, C., & Jeong, Y. -g. (2020). Reduction of Unburned Carbon Release and NOx Emission from a Pulverized Wood Pellet Boiler Retrofitted for Fuel Switching from Coal. Energies, 13(19), 5077. https://doi.org/10.3390/en13195077