Lean-bum combustion

It is known that lean-bum combustion is attractive from the standpoint of fuel economy and CO emission reduction, but it hard to implement because state-of-the-art catalysts do not provide sufficient NO removal because of oxygen excess. Extensive efforts aimed at development of catalysts for NO decomposition and for lean NO reduction by hydrocarbons were not successful to date. 

Natural gas provides an attractive source of energy for various purposes. For instance, it is used to fire gas turbine combustion chambers [1] and more recently has been reported as an alternative fuel for automotive applications [2]. The main advantages are lower levels of particulate matter and nitrogen oxides in lean bum combustion [3]. The high H/C ratio reduces the net carbon dioxide emissions, when compared to other fossil fuels. 

Fluoride solid electrolytes with fluorite- 7.2.5. Sensors for lean-bum combustion 170... 

Smith LL, Karim H, Castaldi MJ, Etemad S, Pfefferle WC (2006) Rich-catalytic lean-bum combustion for fuel-flexible operation with ultra-low emissions. Catal Today 117 438-446... 

Hydrocarbons and carbon monoxide emissions can be minimised by lean air/fuel mixtures (Fig. 2), but lean air/fuel mixtures maximize NO emissions. Very lean mixtures (>20 air/fuel) result in reduced CO and NO, but in increased HC emissions owing to unstable combustion. The turning point is known as the lean limit. Improvements in lean-bum engines extend the lean limit. Rich mixtures, which contain excess fuel and insufficient air, produce high HC and CO concentrations in the exhaust. Very rich mixtures are typically used for small air-cooled engines, needed because of the cooling effect of the gasoline as it vaporizes in the cylinder, where CO exhaust concentrations are 4 to 5% or more. 

Aleifres, P.G., Y. Hardalupas, A.M.K.P. Taylor, K. Ishii, and Y. Urata, Flame chemiluminescence studies of cyclic combustion variations and air-to-fuel ratio of the reacting mixture in a lean-bum stratified-charge spark-ignition engine. Combustion and Flame, 136 72-90, 2004. 

Flammability limits Vapor-air mixtures will ignite and bum only over a well-specified range 01 compositions. The mixture will not burn when the composition is lower than the lower flammable limit (LFL) the mixture is too lean for combustion. The mixture is also not combustible when the composition is too rich, i.e., that is, when it is above the upper flammable limit (UFL). A mixture is flammable only when the composition is between the LFL and the UFL. Commonly used units are volume percent of fuel (percentage of fuel plus air). 

P.G. Kristensen, B. Karll, A.B. Bendtsen, P. Glarborg, and K. Dam-Johansen. Exhaust Oxidation of Unbumed Hydrocarbons from Lean-Bum Natural Gas Engines. Combust. Sci. Techn., 157 263-292,2000. 

There is concern about the levels of carbon dioxide emissions from cars, as well as the three main pollutants from combustion HC, CO, and NO. Lower CO2 emissions result from improved fuel economy that can be obtained in several ways. An important, more fuel efficient approach is through the use of so-called lean-bum engines that operate with excess air rather than with stoichiometric air-fuel mixtures. 

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