Lean-burn 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. 

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. 

One of the most straightforward methods to reduce carbon dioxide emissions is to enhance the fuel efficiency of engines. The three-way catalyst, although very successful at cleaning up automotive exhaust, dictates that engines operate at air-to-fuel ratios of around 14.7 1. Unfortunately, this is not the optimum ratio with respect to fuel efficiency, which is substantially higher under lean-burn conditions at A/F ratios of about 20 1, where the exhaust becomes rich in oxygen and NOx reduction is extremely difficult (Fig. 10.1). 

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... 

Lean burn/dry low-NOx combustors can generate NOx emissions levels as low as 9 ppm (at 15% 02), while those with liquid fuel combustors have NOx emissions limited to approximately 25 ppm (at 15% 02). There is no substantial difference in general performance with either fuel. However, the different heats of combustion result in slightly higher mass flows through the expansion turbine when liquid fuels are used, and thus a very small increase in power and efficiency performance is obtained. Also, the fuel pump work with liquid fuel is less than with the fuel gas booster compressor, thereby further increasing net performance with liquid fuels. 

Nitrogen oxide emissions from hydrogen ICEs are around 90% lower than for a gasoline ICE, because the engine can operate in lean-burn mode with an excess of air, which leads to lower engine temperatures and less NO production. 

Pressure oscillations with RMS value up to 10 kPa in two models of lean-burn gas turbine combustors, with heat release around 100 kW, have been actively controlled by the oscillation of fuel flow. The flames were stabilized behind an annular ring and a step in one arrangement, and downstream of an expansion and aided by swirl in the other. Control was sensitive to the location of addition of oscillated fuel. Oscillations in the annular flow were attenuated by 12 dB for an overall equivalence ratio of 0.7 by the oscillation of fuel in the core flow and comprising 10% of the total fuel flow, but negligibly for equivalence ratios greater than 0.75. Oscillation of less than 4% of the total fuel in the annulus flow led to attenuation by 6 dB for all values of equivalence ratio considered. In the swirling flow, control was more effective with oscillations imposed on the flow of fuel in a central axial jet than in the main flow, and oscillations were ameliorated by 10 dB for equivalence ratio up to 0.75, above which the flame moved downstream so that the effectiveness of the actuator declined. The amelioration of pressure oscillations resulted in an increase in NOj, emissions by between 5% and 15%. 

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... 

A gold catalyst with low temperature activity towards carbon monoxide and hydrocarbon oxidation could be suitable to combat cold start-up emission problems and removal of nitrogen oxides from lean-burn gasoline... 

The steady-state characteristics of practical significance for a lean-burning jet combustor intended to control pollutant emissions are its lean operating limits for eflBcient, stable combustion i.e., the minimum operating equivalence ratios that yield acceptable levels of CO and un-bumed HC emissions. Those operating points that also yield acceptable... 

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