Diesel oxidation flow

Fig. 8.3 Operational concept of a diesel oxidation flow-through catalyst.

Scheme 35.2 Diesel oxidation flow through monolith catalyst...

Diesel oxidation catalysts (Fig. 8.3) target the oxidation of soot, hydrocarbons and CO, although flow-through systems employed for this purpose are fairly inefficient at treating the solid portion of the emission (usually less than 5% of the soot becomes oxidised as a consequence of the low contact time since catalysts are designed to avoid pressure drop by clogging).1,76,78 Their oxidising power must however be tuned in order to minimise SO2... 

Fig. 20. Proposed simplified diesel exhaust after-treatment system (2010). A diesel oxidation catalyst, wall-flow filter, selective catal5dic reduction with urea injection, and an ammonia decomposition catalyst. All catalysts are deposited on monoliths.

A similar setup for 2D LIF spectroscopy was recently completed in our group at KIT (ZeUner et al., 2014), which was used to study the catalytic reduction of NO by hydrogen toward ammonia over a diesel oxidation catalyst consisting of a Pt/Al203-coated monolith. The combination with ex situ analytics and the consideration of quenching effects on the measured signal lead to quantitative NO concentration profiles in the catalytic channel only 2 mm in width. The interaction of difiusion and flow with the surface reaction could be elucidated for this system under different operating conditions (Zellner et al., 2014). 

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...

To determine the fate of formaldehyde and formic acid in a coal mine, an unused shaft about 120 m long and 6 m2 in cross sectional area was selected for study. With a ventilation air flow of 190 m3/min and an engine exhaust flow of 1.5 m3/min, complete exhaust dispersion and dilution was observed in about 10 m. Samples collected in the mine air downstream of the diesel engine indicate no significant change in formic acid concentration at increasing distances from the engine (Table VIII). This is certainly not consistent with the loss of formaldehyde in the same interval. The mechanism for loss of formaldehyde is apparently not a gas phase oxidation to formic acid. Interaction with surfaces may be a more suitable explanation of the observed reduction in formaldehyde concentrations. 

The focus for the reduction of harmful diesel emissions is mainly on particulate matter (PM) and NO,. Both components are harmful to health and environment and are present in relatively large quantities. The other regulated harmful emissions, hydrocarbons and carbon monoxide, can be removed with relatively simple measures, such as flow-through monoliths with an oxidation catalyst. Some of the techniques used for removal of particulate matter and/or NO,... 

Since the diesel engine is the workhorse of urban buses and heavy-duty trucks, worldwide, and since diesel exhaust poses health concerns, including cancer, emphysema, and other respiratory diseases, the aftertreatment of diesel exhaust is receiving worldwide attention. Both wall-flow filters and catalytic converters are being tested, the former for trapping solid particulates and the latter for converting hydrocarbons by oxidizing the soluble... 

The second technique is based on a filter to capture the soot particulates. Common filters are wall flow monoliths or ceramic foams. Cordierite wall flow monoliths are probably currently the most used particulate traps. They can capture diesel particulates with an efficiency of 99%. At normal diesel engine exhaust gas temperatures, the captured soot is not reactive enough to prevent build up on the filter, with an intolerable high pressure drop over the exhaust system as a result. The oxidation rate of the soot should, therefore, be increased which can be achieved by increasing the temperature of the filter, resulting in higher fuel consumption and thus making this solution unfavourable. The other possibility is catalytic oxidation of the collected soot. Several catalytic systems will be discussed. 

The presence of sulphur in diesel exhaust gases or particles has to be considered as a poisoning agent for the catalysts used in soot combustion reactions. Copper oxide has been reported to be sensitive towards sulphur dioxide (7) which implies a deactivation of the solid and then eventual modifications of its sinface properties. In this way, lCulCel073 sample was treated in a microflow reactor under SO2 flow (2L.h ) at room temperature for 30 minutes. 

An exploratory study was carried out with respect to the performance of a copper fuel additive in combination with monolithic wall flow filters for the removal of soot firom diesel exhaust gas. Cordierite filters, copper coated cordierite filters, and silicon carbide filters were studied. Model experiments have been performed to investigate the influence of contact between soot and catalyst on the oxidation rate. 

Most of the work cited above has dealt with treating the soot in some way before doing the combustion experiments. We wish to report experiments conducted on soot from a diesel vehicle which has been deposited onto catalytic monolithic substrates. This sooted substrate is then placed in a laboratory apparatus where a synthetic gas mixture flows over the sample, and the soot combustion is monitored as a function of temperature. The laboratory set up simulates regeneration conditions on a vehicle. Using this technique we have been able to obtain kinetic information about the oxidation of soot and gaseous products. Comparisons of base metal and noble metal catalysts were also conducted and are reported. It is intended that this work will help elucidate the mechanism involved in the catalytic combustion of soot which should help in developing improved catalytic materials. 

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