Gas-liquid mass transfer and reaction characteristics of SNCR denitration in CFB coal-fired unit

doi:10.11949/0438-1157.20230084

Abstract

Abstract:

The CFD simulation method was used to predict the reductant droplet evaporation, flue gas mixing and reaction characteristics in the SNCR denitrification process of a 300 MW circulating fluidized bed unit. The numerical results show that the flue gas is attached to the wall of the cyclone separator and forms a double-vortex structure including a quasi-free vortex and a quasi-forced vortex, which makes the droplets start to evaporate at a constant temperature after contacting the flue gas for about 0.01 s and enhances the mixing behavior between flue gas and gaseous reducing agent. NH₃ is mainly distributed above the cone of cyclone separator when aqueous ammonia is used as the reductant, whilst both HNCO and NH₃are produced with the consumption rate of HNCO higher than that of NH₃ when aqueous urea is as the reductant. Although the concentration distribution of NH₃ is similar, the denitration efficiency is about 79.5% for aqueous ammonia compared to about 76.5% for aqueous urea under both the same flue gas temperature and ammonia nitrogen molar ratio. The reaction rate of NH₃ and NO is increased and the denitration efficiency increases from 19.7% to 81.0% when the temperature is increased from 1023 K to 1173 K. However, the oxidation rate of NH₃ itself is increased significantly resulting in a reduction of denitration efficiency to 17.4% when the temperature is further increased from 1173 K to 1323 K. The denitration efficiency is increased with the increase of NH₃/NO mole ratio (NSR), but the utilization efficiency of reductant is decreased which leads to the increase of ammonia slip. Considering the SNCR denitration efficiency together with ammonia escape rate of the CFB boiler unit used in this study, the NSR of 1.25—1.50 can meet the ultra-low emission standard of NO _x emission not higher than 50 mg/m³.

Key words: circulating fluidized bed unit, SNCR denitrification, evaporation, gas-liquid mass transfer, numerical simulation

摘要：

采用CFD模拟方法预测了300 MW循环流化床机组SNCR脱硝过程中的还原剂液滴蒸发、烟气混合和反应特性。结果表明烟气在旋风分离器内贴壁旋转流动并形成外旋流为准自由涡和内旋流为刚性涡的双涡结构，使液滴与烟气接触约0.01 s后开始恒温蒸发，并强化了烟气与气态还原剂的混合效果。氨水为还原剂时，NH₃主要分布于旋风分离器锥体上方；尿素为还原剂时，蒸发后快速分解的HNCO消耗速率高于NH₃，其中NH₃浓度分布与氨水为还原剂相似，相同烟气温度和氨氮摩尔比时氨水和尿素溶液对应脱硝效率分别约为79.5%和76.5%。温度对脱硝效率的影响表现为先上升后下降趋势，当温度由1023 K提高至1173 K时NH₃与NO反应速率提高，脱硝效率由19.7%提高至81.0%；而当温度由1173 K进一步提高至1323 K时，NH₃由于自身氧化速率显著提高而导致脱硝效率降低至17.4%。脱硝效率随氨氮摩尔比（NSR）增大而升高，但还原剂利用率的降低致使氨逃逸率增大，综合考虑本台CFB锅炉SNCR脱硝效率和氨逃逸率，NSR选取1.25~1.50可以满足NO _x 排放低于50 mg/m³的超低排放标准。

关键词: 循环流化床机组, SNCR脱硝, 蒸发, 气液传质, 数值模拟

CLC Number:

X 701

Yuanyuan ZHANG, Jiangyuan QU, Xinxin SU, Jing YANG, Kai ZHANG. Gas-liquid mass transfer and reaction characteristics of SNCR denitration in CFB coal-fired unit[J]. CIESC Journal, 2023, 74(6): 2404-2415.

张媛媛, 曲江源, 苏欣欣, 杨静, 张锴. 循环流化床燃煤机组SNCR脱硝过程气液传质和反应特性[J]. 化工学报, 2023, 74(6): 2404-2415.

Figures/Tables 17

Fig.1 Schematic diagram of physical model for cyclone separator

Table 1 Simplified mechanism and kinetic parameters of chemical reactions for SNCR process［7，9-10］

No.	Reaction	A	β	E_a/（J/mol）	Order
A1	4NH₃+4NO+O₂ $\overset{}{\to}$ 4N₂+6H₂O	3.1×10⁸	5.30	3.3×10⁵	3
A2	4NH₃+5O₂ $\overset{}{\to}$ 4NO+6H₂O	2.4×10⁵	7.41	4.9×10⁵	2
U1	NH₃+NO $\overset{}{\to}$ N₂+ H₂O+H	4.2×10⁸	5.30	3.5×10⁵	2
U2	NH₃+ O₂ $\overset{}{\to}$ NO+ H₂O+H	3.5×10⁵	7.70	5.2×10⁵	2
U3	HNCO+M $\overset{}{\to}$ H+NCO+M	2.4×10¹⁴	0.85	2.8×10⁵	2
U4	NCO+NO $\overset{}{\to}$ N₂O+CO	1.0×10¹³	0	-1.6×10³	2
U5	NCO+OH $\overset{}{\to}$ NO+CO+H	1.0×10¹³	0	0	2
U6	N₂O+OH $\overset{}{\to}$ N₂+ O₂+H	2.0×10¹²	0	4.2×10⁴	2
U7	N₂O+M $\overset{}{\to}$ N₂+ O+M	6.9×10²³	-2.50	2.7×10⁵	2
U8	CO(NH₂)₂ $\overset{}{\to}$ NH₃+HNCO	1.3×10⁴	0	6.5×10⁴	1
U9	CO(NH₂)₂+H₂O $\overset{}{\to}$ 2NH₃+CO₂	6.1×10¹⁰	0	8.8×10⁴	2

Table 1 Simplified mechanism and kinetic parameters of chemical reactions for SNCR process［7，9-10］

No.	Reaction	A	β	E_a/（J/mol）	Order
A1	4NH₃+4NO+O₂ $\overset{}{\to}$ 4N₂+6H₂O	3.1×10⁸	5.30	3.3×10⁵	3
A2	4NH₃+5O₂ $\overset{}{\to}$ 4NO+6H₂O	2.4×10⁵	7.41	4.9×10⁵	2
U1	NH₃+NO $\overset{}{\to}$ N₂+ H₂O+H	4.2×10⁸	5.30	3.5×10⁵	2
U2	NH₃+ O₂ $\overset{}{\to}$ NO+ H₂O+H	3.5×10⁵	7.70	5.2×10⁵	2
U3	HNCO+M $\overset{}{\to}$ H+NCO+M	2.4×10¹⁴	0.85	2.8×10⁵	2
U4	NCO+NO $\overset{}{\to}$ N₂O+CO	1.0×10¹³	0	-1.6×10³	2
U5	NCO+OH $\overset{}{\to}$ NO+CO+H	1.0×10¹³	0	0	2
U6	N₂O+OH $\overset{}{\to}$ N₂+ O₂+H	2.0×10¹²	0	4.2×10⁴	2
U7	N₂O+M $\overset{}{\to}$ N₂+ O+M	6.9×10²³	-2.50	2.7×10⁵	2
U8	CO(NH₂)₂ $\overset{}{\to}$ NH₃+HNCO	1.3×10⁴	0	6.5×10⁴	1
U9	CO(NH₂)₂+H₂O $\overset{}{\to}$ 2NH₃+CO₂	6.1×10¹⁰	0	8.8×10⁴	2

Fig.2 Schematic of geometry discretization for cyclone separator

Fig.3 Emission concentration of NO and NH3 depending on grid number

Fig.4 Streamline for flue gas in cyclone separator, vector field in xy cross section and velocity magnitude in zx cross section

Fig.5 Distribution of droplet size depending on flue gas temperature

Fig.6 Distribution of droplet temperature depending on flue gas temperature

Fig.7 Distribution of NO and NH3 molar fractions in cyclone separator using ammonia as reducing agent

Fig.8 Distribution of CO(NH2)2 and HNCO molar fractions in cyclone separator

Fig.9 Distribution of NO and NH3 molar fractions in cyclone separator using urea as reducing agent

Fig.10 Effect of reducing agent on performance of SNCR system

Fig.11 Distribution of NO molar fraction depending on flue gas temperature

Fig.12 Distribution of NH3 molar fraction depending on flue gas temperature

Fig.13 Denitration performance of SNCR system depending on flue gas temperature

Fig.14 Distribution of NO molar fraction in zx cross section

Fig.15 Distribution of NH3 molar fraction in zx cross section

Fig.16 Denitration performance of SNCR system depending on molar ratio of NH3 to NO

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