Diesel exhaust stream

The previous section has evidenced that NH3-SCR technology has been used successfully for more than two decades, to reduce NOx emissions from power stations fired by coal, oil and gas, from marine vessels and stationary diesel engines. NH3-SCR technology for high-duty diesel (HDD) vehicles has also been developed to the commercialization stage and is already available as an option in the series production of several European truck-manufacturing companies starting from 2001. For mobile source applications, the preferred reductant source is aqueous urea, which rapidly hydrolyses to produce ammonia in the exhaust stream. 

Diesel particulate filters (DPF), which remove particulate from the exhaust stream. They can also be coated with catalyst, e.g. DOC, LNT or SCR, to enable removal of gaseous pollutants in the same system component. 

A great effort is underway to develop reliable aftertreatment systems for lowering NOx emissions from diesel and LB engines. A variety of approaches have been proposed for NOx aftertreatment of advanced vehicles including lean NOx catalysts (LNC), NOx storage and reduction (NSR) catalysts, selective catalytic reduction with urea (urea-SCR), and plasma-assisted catalysis (PAC). Lean NOx catalysts are mainly designed to reduce NOx with unburned hydrocarbons already included in the exhaust stream in the presence of O2 but result in... 

The selective catalytic reduction (SCR) of nitrogen monoxide by hydrocarbons in excess oxygen currently appears to be the best method for the removal of NO from exhaust streams. This is also a key issue in catalytic purification of diesel exhaust gases, but even more complex due to the presence of SOx. 

NOj sensing in a diesel engine exhaust stream. Sens. Actuators B, 107 (2), 839-48. 

Another recent advancement involving chemists and design is the development of diesel emissions fluid (DEF) systems. Diesel emissions fluid is a 35% urea and 65% water mixture injected into the exhaust stream of a diesel vehicle to improve or reduce the NOx emissions. The urea injection works by decomposition of urea into ammonia when injected into the exhaust stream ... 

These DEF systems are generally mounted on the side of the frame of the vehicle. The fill point is generally under hood. The actual diesel emissions fluid is a 35% urea-water mixture. Because this mixture freezes at -11°C, a heated EPDM (ethylene-propylene diene monomer)/nylon 6,6 line is used to deliver the urea from the tank to the exhaust stream. A sensor controls the flow of the urea to obtain the NOx reduction. This system can reduce NOx emissions by up to 40%. The urea refill rate is about once every 3 months based on typical driving schedules. 

The control of NOx emissions in lean-bum gasoline and diesel engines has become one of the most important challenges in environmental catalysis due to the difficulty of reducing nitrogen oxides in their typically humid, oxygen rich exhaust streams. Reduction with hydrocarbons is an attractive means of converting NO to N2 [1]. However, no industrially practical catalyst has been reported to date. 

The majority of the performance tests were done with an integral model gas reactor, described in a recent paper [2]. It consists of a gas mixing section, a reactor section and an analytical section. For simulation of typical diesel exhaust gas hydrocarbons several selected liquid HC-components were introduced in the exhaust gas stream by means of an HPLC-pump (Shimazu LC9A) and using an stainless steel evaporator (T>180°C). The model gas compositions used in this study are given in Table 2. 

The LA facility currently operates two units, a 25-MW combustion turbine and a set of four 3-MW (12 MW total) diesel engines. The diesel engines share a common exhaust stream. The NOx reduction can come from either unit or both. 

Periodically (e.g., every 60-120 s), the trap is regenerated either by introducing a rich pulse of reductant (e.g., diesel fuel) into the exhaust stream (157) or by switching the engine operating mode to stoichiometric or rich for 1-2 s. This is demonstrated in Figure 21. This rich pulse provides the necessary chemical reductant (H2 and CO) to convert the adsorbed nitrate to nitrogen over the Rh... 

Watson S, Huang W, Wong V (2007) Correlations among Ash-Related Oil Species in the Power Cylinder, Crankcase and the Exhaust Stream of a Heavy-Duty Diesel Engine. SAE Technical Paper 2007-01-1965... 

In this chapter, the application of the virtual testbench as part of the development process is demonstrated. In this case, the virtual testbench represents the SCR system of the aftertreatment of Diesel exhaust, i.e., the SCR catalyst itself and the necessary algorithms for control of the AdBlue dosing into the exhaust stream, cf. Fig. 22.2. These algorithms are coupled with the catalyst model in the same manner as they are implemented in the control unit. However, the injection of AdBlue, its processing, and the generation of NH3 are not directly modeled. Ideal NH3-generation is assumed instead. 

Diesel particulate filters (DPFs) (see Figure 19.1) are used to remove soot particles from the exhaust stream [1-4]. These filters usually consist of wall-flow monoliths, that is, honeycomb-like structures with 50% of the channels plugged at the gas entry side and the remaining channek plugged at the exit The gas stream enters into the filter through the open channels and is forced to pass through the porous walls where the soot particles get stuck. 

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