Influence of non-uniform distribution of catalysts on the characteristics of ship SCR system
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摘要:
选择性催化还原（SCR）系统常用于降低柴油发动机尾气排放出的氮氧化物（NOx）。为进一步提高其脱硝性能，首先建立三维数值模拟模型，通过改变SCR转换器中孔隙率来实现催化剂的非均匀分布，从而研究在不同发动机负荷下的催化剂非均匀分布对SCR系统中的流动、传质和传热等性能的影响；其次，与催化剂的均匀分布进行对比，重点考察催化剂非均匀分布时对SCR系统性能的影响；最后，基于场协同原理分析SCR系统中的速度场和温度场对传热过程的影响。结果表明：催化剂的非均匀分布可以提高SCR系统的性能。在发动机25%的负荷下，与均匀分布相比，案例P-R5的压力损失降低165 Pa，脱硝率提高了0.8%，氨的逸出量减少了7 mg/L；案例P-R5的温度梯度高于均匀孔隙率催化剂温度梯度的最大峰值，这表明向外部传递的能量较少，催化剂非均匀分布的结构可以保持催化区域的温度，提高催化剂的活性，有利于脱硝反应。
Abstract:Selective catalytic reduction (SCR) systems are typically used to reduce nitrogen oxides (NOx) emitted from diesel engine exhaust emissions. To further improve its de-NOx performance, firstly, a three-dimensional numerical simulation model was established. The non-uniform distribution of the catalyst was achieved by changing the porosity in the SCR converter, and the impact of the non-uniform distribution of the catalyst under different engine loads on the flow, mass transfer, and heat transfer performance of the SCR system was investigated. Secondly, the impact of the non-uniform distribution of the catalyst on the performance of the SCR system was examined and compared with that of the uniform distribution. Finally, based on the principle of field synergy, the influence of velocity and temperature fields on the heat transfer process in the SCR system was analyzed. The results indicate that the non-uniform distribution of catalysts can improve the performance of SCR systems. At 25% engine load, compared to a uniform distribution, the pressure drop of Case P-R5 is reduced by 165 Pa, the conversion efficiency of NOx is increased by 0.8%, and the escape of ammonia is reduced by 7 mg/L. In addition, the temperature gradient of Case P-R5 is higher than the maximum peak of the temperature gradient of the catalyst with uniform porosity, indicating that there is less energy transferred to the outside. The non-uniform distribution of the catalyst structure can maintain the temperature of the catalytic region, improve the catalyst activity, and facilitate de-NOx reactions.
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图 2 不同发动机负荷下的仿真模拟结果与试验值的比较
Figure 2. Comparison of simulation results and experimental values under different engine loads
图 4 不同案例在50%发动机负荷下的压力分布云图
Figure 4. Pressure distribution cloud map of different cases at 50% engine load
图 5 50%发动机负荷条件下SCR系统中心线温度分布
Figure 5. SCR system centerline temperature distribution at 50% engine load
图 6 发动机负荷为50%时案例的温度分布
Figure 6. Temperature distribution of the cases at 50% engine load
图 7 不同发动机负荷下案例的脱硝率
Figure 7. NOx conversion efficiency of the cases under different engine loads
图 8 50%和100%发动机负荷下案例的中心平面NO含量的分布
Figure 8. Distribution of NO mass fraction in the central plane of the cases at 50% and 100% engine loads
图 9 不同发动机负荷下SCR系统出口氨的逸出量
Figure 9. Ammonia escape at the SCR system outlet under different engine loads
图 10 不同案例在50%和100%发动机负荷下氨的逸出量分布
Figure 10. Ammonia escape in different cases under 50% and 100% engine loads
图 11 不同发动机负荷下案例的轴向温度梯度峰值的变化
Figure 11. Variation in peak axial temperature gradient under different engine loads
图 12 不同发动机负荷下催化转化器前部的平均速度和均匀性系数
Figure 12. Average speed and uniformity coefficient in front of catalytic converter under different engine loads
表 1 SCR系统物理模型参数
Table 1. SCR system physical model parameters
入口和出口
截面直径
（d2）/mm入口或出口
段长度
（L1)/mm膨胀角度
（α）/（°）催化反应
段直径
（d1）/mm催化反应
段长度
（L2)/mm120 60 90 240 320 
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表 2 不同负荷下船用柴油机的催化转化器的边界条件
Table 2. Boundary conditions for catalytic converters of marine diesel engines under different loads
柴油机负荷/% 质量流量/（kg/s） 尾气压力/MPa 尾气温度/℃ NOx含量/（mg/L） H2O含量/% O2含量/% CO2含量/% 理论氨氮比 25 0.56 0.16 255 1 218 3.13 16.16 3.60 0.821 50 1.08 0.228 320 1 131 3.12 15.06 4.38 0.797 75 1.52 0.318 350 1 054 3.11 14.56 4.75 0.731 100 1.84 0.392 400 1 124 3.11 13.69 5.37 0.693
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表 3 案例设置
Table 3. Cases setting
案例 催化剂的分布 区域半径/mm 孔隙率 区域1（R1） 区域2（R2） 区域1（ε1） 区域2（ε2） P-R1 催化剂非均匀
分布40 80 0.41 0.75 P-R2 20 80 0.41 0.75 P-R3 40 100 0.41 0.75 P-R4 40 80 0.75 0.75 P-R5 40 80 0.41 0.41 P0 催化剂均匀分布 整个区域的孔隙率为0.65，半径为120 mm
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表 4 网格独立性
Table 4. Research on grid independence
网格类型 网格数量/个 出口处NO浓度/（mg/L） 粗网格 59 904 227 中网格 449 280 203 细网格 3 669 850 200
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