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为缓解NOx引起的雾霾、光化学烟雾、酸雨等环境问题,中国于2015年发布《全面实施燃煤电厂超低排放和节能改造工作方案》[1-2]。作为主要人为NOx排放源,燃煤电厂普遍采用选择性催化还原(selective catalytic reduction,SCR)技术进行烟气脱硝,以满足NOx排放质量浓度低于50 mg· (m3) −1的标准[3-6]。在“双碳”能源结构转型背景下,脱硝效率和氨逃逸率作为评估燃煤电厂SCR脱硝性能的关键指标[7],容易受燃煤机组调峰运行引起的喷氨量、NOx质量浓度、烟气流量、烟气温度等波动带来的影响[8-9]。
由于实验手段难以全面评估脱硝性能变化的内在机理,数值模拟凭借方便和准确等优点被广泛应用[9]。基于数值模拟,众多学者对脱硝性能开展了大量研究。为优化烟气流场分布,凌忠钱等[7]和邹红果等[10]利用FLUENT分析了喷氨格栅和导流板布置对SCR反应器内流场分布均匀性影响,发现速度与NH3体积分数分布不均会严重影响脱硝性能,并对喷氨格栅和导流板设计进行优化,使系统流场与氨氮摩尔比(NSR)分布更加均匀。为探讨有关因素对脱硝性能的影响机理,较多学者利用FLUENT建立SCR催化剂单孔道三维模型进行了模拟研究。其中,ZHENG等[11]采用SIMPLE算法求解控制方程,系统研究了NO在单通道中轴向和径向浓度分布梯度,发现SCR反应仅发生在催化剂壁表面约0.2 mm的薄层内;LEI等[12]使用相同方法,单因素分析了入口速度、NSR和反应温度对脱硝效率的影响,发现入口速度降低、烟气温度和NSR增加均有利于提升脱硝效率;赵大周等[13]探究了不同NSR和空速下脱硝效率随温度的变化规律,以及NSR与温度对氨逃逸率的共同作用,发现NSR增加在提升脱硝效率的同时还会加剧氨逃逸,空速增大会显著降低脱硝效率;杜云贵等[14]联立烟气停留时间与NSR对脱硝效率进行了分析,并探讨了NSR与氨逃逸率间的关系;李晗天等[15]则更为细致地耦合空速与NSR模拟了出口NO与NH3的分布情况。此外,张迪等[16]和沈伯雄等[17]建立SCR催化剂单孔道一维模型,采用MATLAB进行数值仿真,重点研究了不同温度下NSR和烟气流速对脱硝效率的影响,发现NSR超1.05时氨逃逸率显著上升,后者还讨论了入口NO质量浓度及催化剂孔几何特性与脱硝效率间的紧密联系,发现蜂窝式催化剂脱硝效率最高。
以上研究表明,反应器入口参数(NSR、入口NO浓度、入口速度)和反应温度均为脱硝性能的重要影响因素。然而,目前研究考虑二者间耦合效应不够全面,多关注脱硝效率,氨逃逸也多局限于NSR影响,缺少相同耦合条件下脱硝效率与氨逃逸率的横向对比。基于此,本课题组针对各入口参数同反应温度对脱硝效率及氨逃逸率的交互作用进行系统分析,并在此基础上探究调峰运行负荷下脱硝效率及氨逃逸率随温度的变化规律,以期为燃煤电厂SCR脱硝性能的整体优化提供参考。
SCR反应器入口参数与反应温度对脱硝性能的交互作用
Interaction effect of inlet parameters and reaction temperature on denitration performance of SCR reactor
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摘要: 为系统研究选择性催化还原(SCR)反应器入口参数与反应温度对脱硝性能的交互作用,基于E-R机理,通过建立SCR脱硝反应一维模型,结合燃煤电厂调峰实际运行数据设定入口参数界限,利用MATLAB数值仿真,分别对反应温度同氨氮摩尔比(NSR)、入口NO质量浓度和入口速度对脱硝效率及氨逃逸率的耦合效应进行了重点分析。结果表明:入口参数对最佳反应温度影响较大;偏离最佳反应温度越多,NSR对氨逃逸率影响程度越低,NSR>1时,脱硝性能对NSR敏感性降低;增大入口NO质量浓度可改善整体脱硝性能,当入口质量NO浓度为1 050 mg·Nm−3时 ,脱硝效率可达82.42%,氨逃逸率低至0.33%;入口速度与反应温度的交互作用最大,降低入口速度可拓宽催化剂高活性温度窗口,显著提升脱硝性能,将入口速度由7 m·s−1降至1 m·s−1,最佳脱硝效率由69.32%升至89.17%,最低氨逃逸率由8.11%趋近于0;燃煤电厂调峰运行负荷上升会导致脱硝效率下降和氨逃逸加剧。该研究结果可为燃煤电厂SCR脱硝性能的整体优化提供参考。Abstract: In order to systematically study the interaction effects between inlet parameters and reaction temperature of selective catalytic reduction (SCR) reactor on denitration performance, a one-dimensional model of SCR denitration reaction was established based on the E-R mechanism, the inlet parameter boundaries were set by combining actual coal-fired power plant operation data of different loads in peak shaving. The coupling effects of reaction temperature with molar ratio of NH3 to NOx (NSR), inlet NO concentration, inlet velocity on denitration efficiency and ammonia escape rate were examined separately by numerical simulation in MATLAB. The results showed that all the inlet parameters had great influence on the optimum reaction temperature, and the impact of NSR on ammonia escape rate dampened with the increase of deviation from the ideal reaction temperature. When NSR>1, the denitration performance was less sensitive to NSR. By raising the inlet NO concentration, denitration performance could be made better generally. When the concentration of NO was 1 050 mg·Nm−3, the denitration efficiency reached 82.42% and the ammonia escape rate was as low as 0.33%. The interaction between the inlet velocity and reaction temperature was the largest, reducing the inlet velocity could broaden the high activity temperature window of the catalyst and significantly improve the denitration performance. When the inlet velocity was reduced from 7 m·s−1 to 1 m·s−1, the optimal denitration efficiency increased from 69.32% to 89.17%, and the lowest ammonia escape rate approached to 0 from 8.11%. The increase of peak load in coal-fired power plants would lead to the decrease of denitration efficiency and the intensification of ammonia escape. The results of this study can provide a reference for the overall optimization of SCR denitration performance in coal-fired power plants.
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表 1 催化剂几何结构参数
Table 1. Geometric structure parameters of catalyst
参数 (符号) 单位 数值 孔径(d) mm 6.5 节距(p) mm 7.4 长度(L) mm 985 内壁厚(δin) mm 0.9 外壁厚(δout) mm 1.45 表 2 入口参数范围
Table 2. Value range of inlet parameters
参数 (符号) 单位 数值 入口速度(v) m·s−1 1~7 氨氮比(NSR) — 0.6~1.2 温度(T) K 423~693 入口浓度(CNO,in) mg·Nm−3 150~1 050 表 3 化学反应动力学参数
Table 3. Kinetic parameters of chemical reaction
参数 单位 数值 A0 s−1 2.68·10−17 A1 s−1 1.00·106 A2 s−1 6.80·107 E0 J·mol−1 −2.43·105 E1 J·mol−1 6.00·104 E2 J·mol−1 8.50·104 -
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