Topics on Selective Catalyst Reduction

Abstract

SCR concept is used effectively in large electric power stations to abate nitric oxides for the last fifty years. Here, simpler controls to inject ammonia in the engine exhaust over a catalyst were enough since the change in emission was only with respect to load and slow. However, application of this technique to truck and car engines was challenging and invited control and thermal problems. This chapter, after introducing the chemical technology goes in depth, explaining the engine optimization for SCR technology, the trade-off of NO_x and soot, and rail pressure and timing. The principle of NO_x reduction is explained using NO_x model with maps of exhaust flow, storage of ammonia in the catalyst and dosing ratio. A temperature model is essential for precise control of nitric oxides. Also, a model for hydrolysis of urea to ammonia and accelerated reaction rates in the presence of nitrogen dioxide is given to consider upstream diesel oxidation catalyst that not only increases the concentration of nitrogen dioxide but also oxidizes carbon monoxide as well as hydrocarbons. Understanding the specification and salient properties of urea solution is important for the success of the SCR technology. In the highly transient thermal environment, there are various types of potential failures due to creep, thermal stress and flow stress. Further, the hydrolysis of urea just after injection, nozzle clogging, crystallization in the catalyst, ammonium di sulphide plugging, active catalyst surface plugging, poisoning due to high sulphur in the fuel as well as potassium and other alkali metals from lubricating oil and other flow phenomenon are studied for the longevity of the SCR system in the engine with the optimally located urea injector. Airless injection system does away with the need for compressed air which is necessary for air assisted injection system; however, distribution of urea and its hydrolysis in the flow is more complex and hence more detailed design analysis must be carried out. Continuous development and tightening of emission limits call for Cu or Ze based catalyst that lights off at a lower temperature than the cost-effective and sulphur tolerant vanadium-based catalyst. Choice of metallic or ceramic catalyst substrate creates an intense dilemma between cost, manufacture, thermal response, and life. Coated or extruded catalyst is again a design trade-off. The high-speed electronic control or the dosing control unit communicates with engine management system. The entire application exercise involves in modelling the storage, reaction and slip of ammonia over the catalyst for transient cycles. Therefore, some companies have preferred exhaust gas recirculation which is less fuel efficient than SCR. SCR in emerging markets has its own challenges and advantages. There are challenges in the field regarding dosing system failure, HC poisoning, catalyst wash out, reliability of NO_x sensors. Application of SCR calls for long duration field trials at varying loads, at high altitudes and temperatures which are either not envisaged in the engine laboratory or not possible to simulate easily.