NOx exhaust emissions

TABLE 1.2. Effect of antioxidants on nitrogen oxides (NOx) exhaust emissions."... 

Examples of multi-disciplinary innovation can also be found in the field of environmental catalysis such as a newly developed catalyst system for exhaust emission control in lean burn automobiles. Japanese workers [17] have successfully merged the disciplines of catalysis, adsorption and process control to develop a so-called NOx-Storage-Reduction (NSR) lean burn emission control system. This NSR catalyst employs barium oxide as an adsorbent which stores NOx as a nitrate under lean burn conditions. The adsorbent is regenerated in a very short fuel rich cycle during which the released NOx is reduced to nitrogen over a conventional three-way catalyst. A process control system ensures for the correct cycle times and minimizes the effect on motor performance. 

Figure 1.1. General trend of the NOx and particulate emissions in Europe, Japan and the U.S. for light- and medium-duty engines (ESC test cycle) and effect of engine tuning on NOx/particulate emissions and fuel consumption. EGR exhaust gas recirculation. ESC test cycle European stationary cycle (http //www.dieselnet.com/standards/cycles/esc.html).

Ciambelli, P Corbo, P Migliardini, F. Potentialities and limitations of lean de-NOx catalysts in reducing automotive exhaust emissions, Catal. Today, 2000, Volume 59, Issues 3-4. 279-286... 

Recent automobile exhaust emissions standards are summarized in Table III, and a review of the catalytic systems designed to meet these standards has recently appeared (26). Catalytic converters have been used as a part of emission control systems since 1975. One approach has been to use a dual bed catalytic converter where the reduction of NO to N2 occurs over the first bed, and excess O2 is provided to the second bed to oxidize the CO and hydrocarbons more completely. Typically, the exhaust contains compounds listed in Table IV plus some poisons containing Pb, P, S etc, (27). The catalytic system must reduce concentrations of CO, hydrocarbon and NOx to legally acceptable levels. 

Diesel-water emulsions are being studied extensively worldwide because of the impact these fuels have on reducing engine exhaust emissions, especially NOx and particulates. Although formulations vary, a typical diesel-water emulsion will contain approximately 80% to 90% diesel fuel, 10% to 15% water, and 1% to 5% of an emulsification additive mixture. The resulting fuel blend is transparent in appearance and has the typical appearance of diesel fuel. 

Determination of the specific reactivity of the exhaust emissions requires accurate knowledge of both the types and amounts of compounds emitted as well as how each contributes to 03 formation. The latter factor, the ozone-forming potential, is treated in terms of its incremental reactivity (IR), which is defined as the number of molecules of ozone formed per VOC carbon atom added to an initial surrogate atmospheric reaction mixture of VOC and NOx ... 

Because CNG is primarily methane, it is expected to have relatively low reactivity, with the small amounts of reactive impurities such as small olefins and alkanes being responsible for most of its reactivity (see Table 16.14). Emissions of CO are smaller than from gasoline-powered vehicles, while the effect on NOx emissions is not clear (National Research Council, 1991). As seen in Tables 16.10 and 16.11, CNG shows the highest promise for low-reactivity exhaust emissions, and this appears to be the case for its use in real vehicles (Gabele, 1995). Figure 16.40, for example, shows the estimated ozone production per mile traveled for a vehicle fueled on CNG compared to vehicles fueled on reformulated gasoline (RFG) or the alcohol fuels M85 or E85 (vide infra). These measurements and estimates based on them include the contributions from both exhaust (including CO) and evaporative emissions (Black et al., 1998). Clearly, the reactivity of the CNG-powered vehicle emissions was substantially smaller than for the other vehicle-fuel combinations. 

Ce02-supported noble-metal catalysts such as Pt, Pd and Rh are of interest because of their importance in the so-called three-way converter catalysts (TWC), designed to reduce emissions of CO, NOx and uncombusted hydrocarbons in the environment and to purify vehicle-exhaust emissions. Such catalysts are also of current interest in steam reforming of methane and other hydrocarbons. Conventional practical catalysts for steam reforming consist of nickel supported on a ceramic carrier with a low surface area and are used at high temperatures of 900 C. This catalyst suffers from coke formation which suppresses the intrinsic catalyst activity. Promoters such as Mo are added to suppress coke formation. Recently, Inui etal(l991) have developed a novel Ni-based composite... 

Variation of catalyst length Variation of NOx raw emissions Variation of exhaust temperature... 

Fio. 33. Influence of catalyst length, NOx raw emissions and exhaust temperature on the integral NO, conversion over NEDC aged NSRC (Giithenke et al., 2007b). Reprinted with permission from SAE Paper 2007-01-1117 2007 SAE International. 

Exhaust emission legislation has become more and more stringent over the last years, demanding for lower engine raw emissions and more efficient exhaust converters. Simultaneous low emission limits for different species, e.g. PM and NOx, lead to the development of combined aftertreatment systems, consisting of different catalyst technologies and particulate filter. Simulation can make a considerable contribution to shorten the time and lower the cost of the system development. In this publication, the current status of exhaust aftertreatment simulation tools used in automotive industry is reviewed. The developed models for DOC with HC adsorption, NSRC and catalyst for SCR of NOx by NH3 (urea) were included into the common simulation environment ExACT, which enables simulation of complete combined exhaust aftertreatment systems. 

NOx Oxides of nitrogen—exhaust emission created from reactions of nitrogen in the air with oxygen at peak combustion temperatures. 

Exhaust emissions of HC, CO, and NOx—the products of incomplete combustion—are controlled primarily by a catalytic converter, in conjunction with exhaust gas recirculation and increasingly sophisticated technology for improving combustion efficiency, including electronic emission controls. See emissions (stationary source),... 

Generally, four kinds of regulated exhaust emissions are analyzed when operating an engine. These species are particulate matter (PM), NOx, hydrocarbons (HC), and carbon monoxide (CO). Besides these regulated species, a host of other exhaust emissions are generated but they currently remain unregulated. 

Additional research for both ethanol and methanol showed that the amount of ignition improver could be reduced by systems increasing engine compression (63). Going from 17 1 to 21 1 reduced the amount of TEGDN required for methanol from 5% by volume to 3%. Igiution-improved methanol exhibited very low exhaust emissions compared to diesels particulate emissions were eliminated except for small amounts associated with engine oil, NOx was even lower with increased compression, and CO and hydrocarbons were also below diesel levels. 

If some of these results are encouraging for gasoline and diesel road vehicles, related emissions still represented a relatively large contribution amongst the overall exhaust emissions from fuel consumption by 1995 it accounts for 74.0% for CO 48.0% for NOx and 77.5% for VOC respectively. 

The most challenging field in achieving the exhaust emission legislative standards is the NOx and particulates control. Fig. 1 shows the comparison of the European light duty (LD) and heavy duty (HD) diesel engine NOx/particulate emission targets. From 1995/1996 on it is clear that the HSDI diesel has to simultaneously reduce NOx and particulates emission, whereas the HD diesel has to concentrate on NOx emissions control. 

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