Pollutants unburned hydrocarbons

In a general manner, diesel engines, jet engines, and domestic or industrial burners operate with lean mixtures and their performance is relatively insensitive to the equivalence ratio. On the other hand, gasoline engines require a fuel-air ratio close to the stoichiometric. Indeed, a too-rich mixture leads to an excessive exhaust pollution from CO emissions and unburned hydrocarbons whereas a too-lean mixture produces unstable combustion (reduced driveability and misfiring). 

Since the 1960 s, two ideas have gained our attention the struggle against pollution before the first oil crisis of 1973 and the diminution of consumption since. One can consider, in fact, that the two objectives are linked. Indeed, any maladjustment of a fuel admission system will modify the equivalence ratio of the mix. The consequences are modifications, on one hand, of the consumption and on the other, of the nature and the quantity of pollutants emitted CO, NO, and unburned hydrocarbons. 

Pollutant Formation and Control in Flames Key combustion-generated air pollutants include nitrogen oxides (NOJ, sulfur oxides (principally SO9), particulate matter, carbon monoxide, and unburned hydrocarbons. 

Unbumed Hydrocarbons Various unburned hydrocarbon species may be emitted from hydrocarbon flames. In general, there are two classes of unburned hydrocarbons (1) small molecules that are the intermediate products of combustion (for example, formaldehyde) and (2) larger molecules that are formed by pyro-synthesis in hot, fuel-rich zones within flames, e.g., benzene, toluene, xylene, and various polycyclic aromatic hydrocarbons (PAHs). Many of these species are listed as Hazardous Air Pollutants (HAPs) in Title III of the Clean Air Act Amendment of 1990 and are therefore of particular concern. In a well-adjusted combustion system, emission or HAPs is extremely low (typically, parts per trillion to parts per billion). However, emission of certain HAPs may be of concern in poorly designed or maladjusted systems. 

Combustion processes are the most important source of air pollutants. Normal products of complete combustion of fossil fuel, e.g. coal, oil or natural gas, are carbon dioxide, water vapour and nitrogen. However, traces of sulphur and incomplete combustion result in emissions of carbon monoxide, sulphur oxides, oxides of nitrogen, unburned hydrocarbons and particulates. These are primary pollutants . Some may take part in reactions in the atmosphere producing secondary pollutants , e.g. photochemical smogs and acid mists. Escaping gas, or vapour, may... 

Unlike carbon dioxide and water that are the inevitable by products of complete combustion of hydrocarbons, species such as carbon monoxide, ethene, toluene, and formaldehyde can be emitted because combustion has been interrupted before completion. Many factors lead to emissions from incomplete combustion. Emitted unburned hydrocarbons and carbon monoxide are regulated pollutants that must be eliminated. In automobiles with spark ignited engines, these emissions are almost entirely removed by the catalytic converter. 

For the last decade, semiconductor diode-laser sensors have been developed at Stanford University for measurements of important parameters in laboratory-and industrial-scale gaseous flowfields. For example, a mass flux sensor was developed based on rapid measurements of O2 absorption near 760 nm in supersonic flowfields [1] and a multiplexed sensor was developed for the simultaneous measurement of various pollutants representing unburned hydrocarbons (CH4, CH3CI) near 1.65 pm [2]. 

Flames interact with the walls of a combustor through various mechanisms, which affects flame stability and pollutant emissions. For example, thermal quenching by cold walls in internal combustion engines can cause an increase of unburned hydrocarbon emissions [1-3], as has been shown by impinging a... 

The following sections describe the influence of the porous layer on exhaust concentrations of nitrogen oxides, carbon monoxide, and unburned hydrocarbons. Additional experiments using two porous layers were conducted and the second layer was found to affect pollutant emissions. Results for different operating conditions are also included. 

The chemical reactions by which these pollutants are converted to harmless emissions take place as exhaust gases pass through two chambers. Each chamber contains a catalyst that has been deposited on large numbers of very small ceramic beads or on the surfaces of a honeycomb-shaped hlter. In the hrst chamber, unburned hydrocarbons and water from exhaust gases react to form elemental hydrogen (H2). The most common catalyst in this chamber of the converter is hnely divided rhodium metal. 

Another important application of heterogeneous catalysts is in automobile catalytic converters. Despite much work on engine design and fuel composition, automotive exhaust emissions contain air pollutants such as unburned hydrocarbons (CxHy), carbon monoxide, and nitric oxide. Carbon monoxide results from incomplete combustion of hydrocarbon fuels, and nitric oxide is produced when atmospheric nitrogen and oxygen combine at the high temperatures present in an... 

FIGURE 12.19 The gases exhausted from an automobile engine pass through a catalytic converter where air pollutants such as unburned hydrocarbons (CjHy), CO, and NO are converted to C02, H20, N2, and 02. The photo shows a cutaway view of a catalytic converter. The beads are impregnated with the heterogeneous catalyst. 

Despite the differences in the chemical composition, biodiesel has physical-chemical properties that are similar to diesel, showing a decrease in the emission of C02, SOx and unburned hydrocarbons during the combustion process when compared to fossil fuels (Crabbe et al, 2001). For these reasons it is possible to use biodiesel blended with diesel without significant changes to engines, with important decrease in the emission of pollutant gases. 

Catalytic converters were first installed in U.S. cars in 1976.20 21 They were passive devices in that they were simply placed in the exhaust with no communication with the engine or its control strategy. It catalyzed the oxidation of the unburned hydrocarbons (CyHn) and carbon monoxide (CO) emitted during the incomplete combustion of the fuel. In some vehicles excess air was pumped into the exhaust to ensure sufficient oxygen to complete the catalytic oxidation. This resulted in about a 90% reduction of these two pollutants relative to the uncontrolled uncatalyzed vehicle. 

Ozone is also very reactive. It can react with the unburned hydrocarbons in the polluted air to produce chemicals that cause the eyes to water and burn and are harmful to the respiratory system. 

Accurate specifications of kinetic mechanisms for combustion often are less critical to calculation of overall rates of heat release than to estimation of amounts of pollutants produced. Pollutants of primary interest are oxides of nitrogen, oxides of sulfur, unburned hydrocarbons, and particulates such as smoke or soot [12]. A primary mechanism by which nitric oxide is formed in flames is that attributed to Zel dovich, namely,... 

Severe emission limits for motor vehicles were introduced, first in the United States and later in many other countries, starting in the mid-1960s. Meeting the increasingly stringent emission requirements in subsequent years forced the installation in motor vehicles of progressively more advanced emission control devices. The focal point of emission control is the catalytic converter, in which the desired chemical reactions occur. The pollutants carbon monoxide and unburned hydrocarbons (he) are converted by oxidation into the desired CO2 and water ... 

Another example is the solid catalyst used to reduce the emission of pollutants such as unburned hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust streams of automobile engines (Fig. 18.16). A catalytic converter is designed to simultaneously oxidize hydrocarbons and CO through the reactions... 

TVirbojet aircraft operating under most conditions produce lower concentrations of pollutants than do motor vehicles. At idle, CO and unburned hydrocarbons are higher than in motor vehicles at idle, but in the operating mode the emission index (grams of pollutant per kilogram of fuel) is ... 

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