火电厂选择性催化还原（SCR）烟气脱硝数值模拟

发布时间：2018-05-01 14:39

  本文选题：氮氧化物 + 选择性催化还原　； 参考：《上海电力学院》2017年硕士论文

【摘要】：氮氧化物(NO_x)是最主要的大气污染物之一,随着世界环境形势日益严峻,我国的环保标准也日益严格。近年来,我国环保部门对火电机组NO_x排放要求不断提高,火电厂的环保压力不断增加。因此开发出高效、安全、经济的脱硝技术已成为火电行业迫在眉睫的问题。选择性催化还原(SCR)烟气脱硝技术因其具有较高的脱硝效率(最高可达90%)、对机组安全运行影响较小、系统组成简单的特点成为火电机组降低NO_x排放的首要选择。主要从微观反应机理入手建立微观反应模型,分析影响脱硝效率的主要因素,其中反应温度、流场均匀性、氨氮摩尔比等对脱硝效率有重要影响;然后以山西某厂600MW超临界燃煤机组SCR脱硝系统为研究对象建立宏观模型,通过FLUENT6.3软件对某600MW超临界燃煤机组的SCR烟气脱硝反应器内速度分布场、压力分布场、氨气浓度分布场、NO_x脱除效果、NH_3逃逸率及不同粒径的飞灰颗粒运动特点进行数值模拟。首先,主要针对优化前反应器内出现的低速三角区、高速冲刷区和低速回流区采用不同形状的导流板进行优化。模拟结果显示,“圆弧”型、“平直—圆弧—平直”型、“圆弧—平直”和“平直”型导流板可以有效抑制烟道弯头和变截面处烟气速度分离的现象。最终将AIG下游标准速度偏差从优化前的56.90%降到9.43%,小于设计标准要求的15%;首层催化剂入口标准速度偏差从优化前的67.34%降低到8.60%,小于设计标准要求的15%。其次,通过喷氨模拟对反应器内NH_3浓度分布进行模拟,模拟结果表明,由于NH_3浓度较低,受流场均匀性影响较大,微弱的流场偏差对NH_3的浓度分布都有较大的影响。采用等速喷氨时,由于近壁面烟气流速较低、扰动较小,所以,近壁面NH_3浓度偏高,首层催化剂入口 NH_3浓度分布并不理想,标准浓度偏差Cn达23.45%,远高于设计标准要求的5%,脱硝效率仅为80.90%,NH_3逃逸率为23.44uL/L。为降低NH_3浓度分布偏差提高脱硝效率,对喷氨格栅进行分区,将喷氨格栅分为三个区域,通过4种喷氨方案对氨气浓度偏差进行调整,最终将首层催化剂入口 NH_3浓度分布偏差由等速喷氨时的23.45%降低到4.73%,反应器出口 NH_3逃逸率由优化前的23.44uL/L降低到4.57uL/L,脱硝效率由等速喷氨时的80.90%提高到86.77%,优化效果显著。最后,对不同粒径下飞灰颗粒的运动特征及浓度分布进行了模拟,对合理布置灰斗,加装吹灰器的具体位置具有一定的指导意义。
[Abstract]:Nitrogen oxide (NO_x) is one of the most important atmospheric pollutants. With the increasingly severe environmental situation in the world, the environmental standards of our country are increasingly strict. In recent years, the environmental protection departments of our country have continuously improved the NO_x emission requirements of thermal power units, and the environmental pressure of thermal power plants has been increasing. The selective catalytic reduction (SCR) flue gas denitrification technology has a high efficiency of denitrification (up to 90%), which has little influence on the safe operation of the unit, and the simple composition of the system becomes the primary choice for the thermal power unit to reduce the NO_x emission. The main factors affecting the denitrification efficiency are analyzed, in which the reaction temperature, the uniformity of the flow field, the mole ratio of ammonia and nitrogen have an important influence on the denitrification efficiency. Then the macro model is established for the SCR denitrification system of 600MW supercritical coal-fired unit in a factory of Shanxi and the SCR flue gas denitrification reaction of a 600MW supercritical coal-fired unit through FLUENT6.3 software. The velocity distribution field, the pressure distribution field, the distribution field of ammonia gas concentration, the effect of NO_x removal, the escape rate of NH_3 and the movement characteristics of fly ash particles with different particle sizes are numerically simulated. Firstly, the low velocity triangle area in the pre optimized reactor, the high speed scour area and the low velocity reflux area are optimized with different shapes of diversion plates. The results show that the "circular arc" type, "flat straight arc straight" type, "circular arc straight" and "flat" type diversion plate can effectively restrain the phenomenon of flue gas velocity separation at the pipe elbow and the variable section. Finally, the standard velocity deviation of the downstream AIG is reduced from 56.90% before the optimization to 9.43%, less than the design standard, 15%; the first layer catalyst. The standard velocity deviation of the inlet is reduced from 67.34% before the optimization to 8.60%, which is less than the 15%. required by the design standard. The simulation of the concentration distribution of NH_3 in the reactor is carried out by the simulation of the ammonia injection simulation. The simulation results show that the uniformity of the flow field is greatly influenced by the low concentration of NH_3, and the weak flow field deviation has a great influence on the concentration distribution of the NH_3. When the constant velocity of ammonia is used, the near wall flue gas velocity is low and the disturbance is smaller, so the NH_3 concentration in the near wall is high, the concentration distribution of the initial catalyst inlet NH_3 is not ideal, the standard deviation is Cn up to 23.45%, which is far higher than the design standard 5%, the denitrification efficiency is only 80.90%, the NH_3 escape rate is 23.44uL/L. to reduce the NH_3 concentration distribution deviation. In order to improve the denitrification efficiency, the ammonia grille is divided into three regions, and the ammonia concentration deviation is adjusted by 4 kinds of ammonia spraying schemes. The NH_3 concentration distribution deviation of the first layer catalyst inlet is reduced from 23.45% to 4.73% at the constant velocity of ammonia injection, and the escape rate of the reactor outlet is reduced from the 23.44uL/L to 4.57u before the optimization. L/L, the efficiency of denitrification increased from 80.90% to 86.77% at the constant speed of ammonia injection. The optimization effect was significant. Finally, the movement characteristics and concentration distribution of fly ash particles under different particle sizes were simulated. It was of certain guiding significance for the rational layout of the ash hopper and the specific position of the soot blower.

【学位授予单位】：上海电力学院
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：X773

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