Deposit Mitigation Practical Example

Ideally the injected liquid should not come in contact with any surfaces and remain airborne until a complete decomposition of urea into ammonia. However, mixing space constraints and injection design parameters make that impossible. 

Nevertheless, the contact could be limited to mixer surfaces near the center of the pipe having the highest temperatures. The onset of deposits formation will then be determined by surface temperature, exhaust gas enthalpy, and the dosed quantity [ 1 ]. 

At lower exhaust temperatures typical for city driving, deposits could be forming in excess of 2 g/h rate. As exhaust gas and surface temperatures increase, the formation rate drops below 2 g/h, and most deposits form on the pre-mixer (Fig. 15.34). This is preferred, as these surfaces have higher average temperatures, which slows down the deposits growth and makes it easier to bum them off during DPF regeneration events. 

In addition to measurements of exhaust gas temperature at various locations, knowing exhaust wall temperamres allows to gain better understanding of deposits formation pattern. For each cross-section of interest the waU temperatures were recorded at four rotational positions, 90° apart. 

The waU temperature reduction relative to the gas centerline temperatures at the SCR entry are summarized in Fig. 15.35. The data were collected at deposits rate of 1 g/h for 200, 600, and 1000 kg/h air flow. The wall temperature reduction is much higher (lower waU temperatures) at lower exhaust flows that allow for more droplet residence time resulting in more evaporation of the liquid fraction. The evaporation is also helped by smaUer amount of liquid in the mixing section to form the same amount of deposits. The delta T is increasing with gas temperature at aU exhaust Uows. The magnitude of the delta T and the rate of rise as a function of exhaust temperature diminish at higher exhaust flows, where heat content is greater. 

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