HC-SCR交替循环脱硝过程的解耦机制

Selective catalytic reduction of NOx by hydrocarbon (HC-SCR) could avoid ammonia slip in the widely used NH3-SCR technology. What’s more, the reducing agent, hydrocarbon, is cheaper, suggesting a very good future of this technology. However, the NOx reduction efficiency in the HC-SCR process is negatively affected by oxygen in the flue gas. In order to solve this problem, a novel rotating reactor is proposed here, which separates adsorption and reduction into two different zones. Catalyst, rotating around, plays a role of NOx carrier between the two zones. This novel reactor could take advantage of the promotion effect of oxygen in the adsorption and avoid the negative effect in the reduction, and thus is expected to have an excellent deNOx performance. Based on this adsorption-reduction swing process, mechanism of NOx adsorption and reduction is to be studied and a decoupled model will be built based on this mechanism. Governing equations include a gas phase (representing gas in the reactor) and a solid phase (representing substances on the catalyst surface). Film model will be used to simulate the mass transfer between the two phases. It could also well describe the adsorption process. The reaction terms, calculated from elemental reaction steps, will be embedded into the solid phase and make the reduction process decoupled. By using this decoupled model of the adsorption-reduction swing system, the effect of swing time and frequency could be evaluated. The optimized reaction condition of the adsorbed NOx could also be obtained. The decoupled mechanism could enrich the understanding of HC-SCR process and be used to design more novel reactors.

碳氢化合物选择性催化还原脱硝技术（HC-SCR）可避免NH3-SCR的氨逃逸，成本低，有很好的发展前景，但HC-SCR的脱硝效率受烟气中氧气浓度的负面影响较大。本项目提出一种新型回转式脱硝反应器以实现交替循环脱硝过程：将吸附区与还原区分离，催化剂作为NOx载体在两个区域内循环，充分利用氧气对吸附的促进，并巧妙避免其对NOx还原的抑制，提高整体脱硝效率。基于吸附还原交替循环过程，研究催化剂表面的NOx吸附还原解耦机制。建立气（气体）固（催化剂表面）两相摩尔守恒方程，采用界面膜模型计算气固两相的传质，基元反应项嵌入固相方程，实现反应过程的解耦。模拟催化剂表面吸附-还原分离的交替循环过程，分析循环时间及循环频率等对脱硝效率的影响，研究已吸附于催化剂表面的NOx在还原反应时的最佳反应氛围。交替循环脱硝解耦机制的建立是对已有HC-SCR机理的扩充和深入，可作为理论基础用于新型脱硝反应器的开发设计。

碳氢化合物HC和CO等燃料型气体作为还原剂进行催化还原脱硝技术可避免NH3-SCR的氨逃逸，成本低，有很好的发展前景，但HC和CO等燃料型气体的脱硝效率受烟气中氧气浓度的负面影响较大。本项目提出一种新型回转式脱硝反应器以实现交替循环脱硝过程：将吸附区与还原区分离，催化剂作为NOx载体在两个区域内循环，充分利用氧气对吸附的促进，并巧妙避免其对NOx还原的抑制，提高整体脱硝效率。基于吸附还原交替循环过程，我们研究了催化剂表面的NOx吸附还原解耦的脱硝特性和反应机制。我们筛选了一系列碳氢化合物和CO作为还原剂，其中CO表现出较好的脱硝活性。在兼顾催化剂的NOx吸附性能和还原性能的条件下，我们制备优化了Fe/ZSM-5和FeCo/ASC催化剂，并将其应用于NOx吸附还原解耦过程，Fe/ZSM-5催化剂在250-400℃、FeCo/ASC催化剂在200-250℃的温度区间内表现出了较好的脱硝性能，脱硝效率可达95%以上，且不同与传统的固定床反应器，基于回转式反应器的吸附还原解耦过程的脱硝效率在不同温度下较稳定，表现出了优异的变工况适应性。在典型的NOx吸附还原动态过程中，烟气区NOx吸附在催化剂表面，出口处NOx浓度较低，还原区内催化剂表面的NOx被还原的同时亦有一部分从催化剂表面脱附，故出口处NOx浓度较高。由吸附还原动态曲线可以计算基于进出口烟气中NOx浓度的NOx脱除率（表征烟气中NOx脱除效率）、基于整个反应器中NOx浓度的NOx还原率（表征反应器中NOx还原为N2的效率）、基于烟气中逃逸的CO浓度的CO逃逸率（表征CO经催化剂携带进入烟气区的比例）等参数。研究结果表明，即使回转式反应器的NOx脱除率较高，由于大量NOx脱附进入还原气，系统的整体还原率较低，需在还原侧加装深度还原装置；还原侧CO供给浓度越高，NOx的脱除率和还原率则越高，主要原因是过量的CO不仅可以还原NOx，还可以有效的促进催化剂表面NOx的脱附，进而强化还原侧催化剂的再生，原位DRIFT所研究的反应机理也证明了此过程；烟气侧出口检测到的CO逃逸率较低，因为即使有CO由于还原气的停滞而残留在烟气区，也可以被氧气催化氧化，故烟气的二次污染问题较小。以HC和CO为还原剂的脱硝方式不仅适用于燃煤电站，也适用于钢铁、冶金、焦化、玻璃等行业，预期此技术有很好的市场推广前景。