Lean-burning engines

Anderson, R. and J.R. Asik, Ignitability Experiments in a Fast Burn, Lean Burn Engine. SAE, 830477, 1983. 

The PAG system utilizes plasma to oxidize NO to NO2, which then reacts with a suitable reductant over a catalyst. LNC, NSR, and PAG systems have still several challenging tasks to be solved. Gonsequently, all these technologies are not yet appropriate for commercial applications to diesel and lean-burn engine exhausts [47]. 

The fact that Fischer-Tropsch fuels contain neither sulfur nor aromatics may become a strong selling point for the process. Less sulfur in the fuel has, of course, a direct effect on the sulfur oxides in the emissions, and the newly developed exhaust purification systems for lean burning engines that can be introduced means that all emissions, including GO2 and NOx, will diminish. Aromatics promote particulate formation in the combustion of diesel fuels and are therefore undesirable. We discuss this further in Ghapter 10. 

NOx Storage-Reduction Catalyst for Lean-burning Engines... 

For similar motivations, there are limited incentives to develop an alternative SCR process for stationary sources based on methane (CH4-SCR) or other HCs, or based on NTP technologies, if not for specific, better applications. The situation is instead quite different for mobile sources, and in particular for diesel engine emissions. The catalytic removal of NO under lean conditions, e.g. when 02 during the combustion is in excess with respect to the stoichiometric one (diesel and lean-burn engines, natural gas or LPG-powered engines), is still a relevant target in catalysis research and an open problem to meet future exhaust emission regulations. 

It is believed that SCR by hydrocarbons is an important way for elimination of nitrogen oxide emissions from diesel and lean-burn engines. Gerlach etal. [115] studied by infrared in batch condition the mechanism of the reaction between nitrogen dioxide and propene over acidic mordenites. The aim of their work was to elucidate the relevance of adsorbed N-containing species for the F>cNOx reaction to propose a mechanism. Infrared experiments showed that nitrosonium ions (NO+) are formed upon reaction between NO, NOz and the Brpnsted acid sites of H—MOR and that this species is highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, the propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can further be hydrolysed to yield... 

Takahashi, N., Shinjoh, H., Iijima, T. et al. (1996) The new concept 3-way catalyst for automotive lean-burn engine NO, storage and reduction catalyst, Catal. Today, 27, 63. 

Miyoshi, N., Matsumoto, S., Katoh, K. et al. (1995) Development of New Concept Three-Way Catalyst for Automotive Lean-Burn Engines, SAE Technical Paper 950809. 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

Fig. 11.29 Schematic diagram of the oxygen sensor for a lean burn engine (Soejima and Mase, 1985).

Schalwig et al. have tested the feasibility of using a SiC MOS capacitor sensor containing a contact metal of 40-nm TaSi plus 45-nm Pt to detect NO and HC after the catalytic converter. This was carried out by simulating lean burn engine exhausts [116]. It was observed that the sensor signal increased for NO detection and decreased for HC detection. This could permit this sensor to be used in a sensor array to differentiate these two gases. 

Along with the concern in the pollutants of NOx, CO and HC, the care about the emission of C02 is also increasing. Today, fuel-efficient lean-burn engines... 

The NOx reduction for the exhaust from lean-burn engine is one of the greatest challenges in environmental protection, and a lot of researchers have strived to develop more effective catalysts by many ways. Their efforts could be possibly categorized into four approaches (1) NOx direct decomposition, (2) selective catalytic reduction on NOx with hydrocarbons (HC SCR),... 

Under excess of the second reactant (in automobile exhaust gas typically H20, C02 and for lean-burn engines exhaust specifically also 02), the effectiveness factor calculation can be simplified by approximating the reaction rate Rj by a pseudo-first-order rate law with respect to the component using new rate constant kiefj (evaluated from the original rate law)... 

Another method that has been introduced to reduce the amount of pollutants is that of the lean burn engine. Although this type of engine reduces the amounts of carbon monoxide and oxides of nitrogen produced, it actually increases the amount of hydrocarbons in the exhaust gases. 

D. Bradley, J. Hynes, M. Lawes and C.G.W. Sheppard, Limitations to Turbulence-Enhanced Burning Rates in Lean Burn Engines, International Conference on Combustion in Engines, Inst. Mech. Eng. (1988) 17. 

Gases such as HCN and CO are of course totally undesirable in principle, but will, if formed under actual three-way operation conditions, be converted to harmless products during secondary reactions on down-stream locations of the catalyst. Reduction of NO by purposely added hydrocarbons is currently investigated as a possible route to reduce NOx emissions from the exhaust of so-called lean-burning engines (operating at air-to-fuel ratios above 14.7). 

Takahashi, N., Shinjoh, H., Iijima, T., Suzuki, T., Yamazaki, K., Yokota, K., Suzuki, H., Miyoshi, N., Matsumoto, S., Tanizawa, T., Tanaka, T., Tateishi, S. and Kasahara, K. (1996) The new concept3-way catalyst for automotive lean-burn engine NOx storage and reduction catalyst. Catal. Today, 27, 63-69. 

Matsumoto, S. (1996) DeNOx catalyst for automotive lean-burn engine. Catal. Today, 29, 43-45. 

However, diesel engines and some gasoline engines are operated under lean-burn conditions, where the oxygen is fed in excess, i.e., 10-20% more than is required to meet the stoichiometry for combustion of the fuel [132,489]. Gold catalysts have therefore been examined for their potential in low-temperature activity to combat cold-start emission problems and removal of NOj, from lean-burn engines [202]. 

The concept of trapping a contaminant in low concentration by adsorption with periodic regeneration of the adsorbent-catalyst has been applied commercially by Toyota for NOx-trap catalysts used in converting NOx in diesel or lean burn engine emissions, for example, for reduction of NOx in the presence of O2 [63]. The catalyst acts as absorbent of NOx (in the form of surface nitrate-like species) in the presence of O2 (lean conditions), but a periodic switch of the air to fuel ratio to rich conditions (deficit of O2 with respect to stoichiometry for the complete oxidation of CO and hydrocarbons present in the car emissions to CO2) leads to regeneration by reducing trapped NOx to N2. 

Menil, F., CoiUard, V. and Lucat, C. (2000) Critical review of nitrogen monoxide sensors for exhaust gases of lean burn engines. Sens. Actuators B, 67, 1—23. 

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