Ammonia Slip Catalysts

Dual-Layer Ammonia Slip Catalysts for Automotive SCR Exhaust Gas Aftertreatment An Experimental... 

To achieve this high level of efficiency, all aspects of the system need to be optimized. An SCR system will generally comprise an ammonia delivery system, and the catalyst system itself comprises the DOC (and typically DPF), SCR catalyst, and the ammonia slip catalyst. 

Fig. 1.10 Nitrous oxide formation in ammonia slip catalysts is promoted by high N0 NH3 ratios coming out of the SCR catalyst [45]...

Fig. 1.11 Generational improvements in ammonia slip catalysts. Early improvements dropped cost with minor compromise in selectivity. The latest generation recovers the performance with greatly reduced precious metal loading [34]...

Ammonia slip catalysts are improving in terms of cost and selectivity to nitrogen. Figure 1.11 from Walker [34] shows modem catalysts have similar performance to the first generation catalysts, but with only 20 % of the platinum loading. The latest catalyst has much better selectivity to nitrogen with less undesirable by-products, yet with half the precious metal of its predecessor. Formation of NO and NO2 is still an issue but improving. 

Figure 2.21 shows the cross-section of the installed DPF -I- SCR reactor. After entering the inlet chamber, the exhaust flow is divided into a right and a left path. First, the exhaust flows through a diesel oxidation catalyst (DOC) to convert some NO to NO2, which is a prerequisite for a continuous soot oxidation within the diesel particulate filter (DPF) and which additionally improves the SCR conversion rate, (see Fig. 2.22) [22]. The DPF placed behind the DOC reduces particulate matter (PM) with high eflftciency. Downstream of the DPF, a special designed mixing chamber gives the injected urea solution sufficient time for evaporation, mixing and thermolysis before entering the SCR catalyst. Finally, an ammonia slip catalyst is used to prevent any NH3 slip to the environment.

Fig. 2.21 Combined aftertreatment system for a shunting locomotive a DPF -I- SCR installed in one reactor housing, b gas flow (Picture MTU) with A exhaust gas inlet, 1 diesel oxidation catalyst (DOC), 2 diesel particulate filter (DPF), 3 urea solution Injection, 4 SCR catalyst, 5 ammonia slip catalyst (ASC), 6 urea mixing area, B exhaust gas outlet...

As can be seen in Fig. 3.7, at low ANR, the NH3 slip is close to zero and NOx conversion is close to proportional to ANR. At higher ANR (closer to 1), NH3 slip is increasing rapidly as the NOx conversion approaches 100 %. Since NH3 slip in the exhaust outlet needs to be minimized, this puts a limit to what NOx conversion can be achieved. By adding an ammonia slip catalyst (ASC) after the SCR catalyst, the NH3 slip can be lowered allowing for extra margin in the control [18]. 

The challenge for the NH3-SCR systems is to maximize the conversion efficiency by keeping at the same time the NH3 release to a minimum. However, under certain conditions an increase of the de-NO performance can only be achieved by increasing the reductant quantity and consequently allowing higher NH3 slip. In order to minimize the released NH3, the addition of an ammonia slip catalyst ASC (also referenced as ammonia oxidation catalyst AOC) downstream the SCR is considered as an efficient and reliable solution. 

Like for the SCR component, the reacting systems prevailing over the PGM component of the Ammonia Slip catalyst under typical NH3-SCR operating conditions have been addressed according to an order of increasing complexity,... 

Colombo M, Nova I, Tronconi E (2012) A simplified approach to modeling of dual-layer ammonia slip catalysts. Chem Eng Sci 75 (0) 75-83... 

Tang et al. [50] report on experiments of a heavy-duty aftertreatment system comprising a DOC followed by a DPF with integrated SCR functionality and an ammonia slip catalyst. The size of the DPP was 12" diameter and 12" in length... 

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