Hydrocarbon unbumed

The modified Brayton cycle is used for both gas turbines and jet engines. The turbine is designed to produce a usable torque at the output shaft, while the jet engine allows most of the hot gases to expand into the atmosphere, producing usable thrust. Emissions from both turbines and jets are similar, as are their control methods. The emissions are primarily unbumed hydrocarbons, unbumed carbon which results in the visible exhaust, and oxides of nitrogen. Control of the unbumed hydrocarbons and the unburned... 

Natural gas is attractive as a fuel ia many appHcatioas because of its relatively clean burning characteristics and low air pollution (qv) potential compared to other fossil fuels. Combustion of natural gas iavolves mixing with air or oxygen and igniting the mixture. The overall combustion process does not iavolve particulate combustion or the vaporization of Hquid droplets. With proper burner design and operation, the combustion of natural gas is essentially complete. No unbumed hydrocarbon or carbon monoxide is present ia the products of combustioa. 

Fuel economy is measured usiag a carbon balance method calculation. The carbon content of the exhaust is calculated by adding up the carbon monoxide (qv), carbon dioxide (qv), and unbumed hydrocarbons (qv) concentrations. Then usiag the percent carbon ia the fuel, a volumetric fuel economy is calculated. If the heating value of the fuel is known, an energy specific fuel economy ia units such as km/MJ can be calculated as well. 

PGM catalyst technology can also be appHed to the control of emissions from stationary internal combustion engines and gas turbines. Catalysts have been designed to treat carbon monoxide, unbumed hydrocarbons, and nitrogen oxides in the exhaust, which arise as a result of incomplete combustion. To reduce or prevent the formation of NO in the first place, catalytic combustion technology based on platinum or palladium has been developed, which is particularly suitable for appHcation in gas turbines. Environmental legislation enacted in many parts of the world has promoted, and is expected to continue to promote, the use of PGMs in these appHcations. 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

Exhaust emissions of CO, unbumed hydrocarbons, and nitrogen oxides reflect combustion conditions rather than fuel properties. The only fuel component that degrades exhaust is sulfur the SO2 concentrations ia emissions are directly proportional to the content of bound sulfur ia the fuel. Sulfur concentrations ia fuel are determined by cmde type and desulfurization processes. Specifications for aircraft fuels impose limits of 3000 —4000 ppm total sulfur but the average is half of these values. Sulfur content ia heavier fuels is determined by legal limits on stack emissions. 

The main combustion pollutants are nitrogen oxides, sulfur oxides, carbon monoxide, unbumed hydrocarbons, and soot. Combustion pollutants can be reduced by three main methods depending on the location of thek appHcation before, after, or during the combustion. Techniques employed before and after combustion deal with the fuel or the burned gases. A thkd alternative is to modify the combustion process in order to minimise the emissions. 

Nitrogen Oxides. From the combustion of fuels containing only C, H, and O, the usual ak pollutants or emissions of interest are carbon monoxide, unbumed hydrocarbons, and oxides of nitrogen (NO ). The interaction of the last two in the atmosphere produces photochemical smog. NO, the sum of NO and NO2, is formed almost entkely as NO in the products of flames typically 5 or 10% of it is subsequently converted to NO2 at low temperatures. Occasionally, conditions in a combustion system may lead to a much larger fraction of NO2 and the undeskable visibiUty thereof, ie, a very large exhaust plume. 

Soot. Emitted smoke from clean (ash-free) fuels consists of unoxidized and aggregated particles of soot, sometimes referred to as carbon though it is actually a hydrocarbon. Typically, the particles are of submicrometer size and are initially formed by pyrolysis or partial oxidation of hydrocarbons in very rich but hot regions of hydrocarbon flames conditions that cause smoke will usually also tend to produce unbumed hydrocarbons with thek potential contribution to smog formation. Both maybe objectionable, though for different reasons, at concentrations equivalent to only 0.01—0.1% of the initial fuel. Although thek effect on combustion efficiency would be negligible at these levels, it is nevertheless important to reduce such emissions. 

Uses. MTBE and related ethers are used to add octane to gasoline. It also adds oxygen to the gasoline, which allows for more efficient combustion, and therefore less carbon monoxide and unbumed hydrocarbon ia the exhaust emissions (11,24). 

Relative to the process streams, emissions from auxiUary equipment and flares are small. Some ethylene oxide units use gas-fired turbines to feed air or ethylene (109). These result in unbumed hydrocarbon and possible NO emissions. Also, most ethylene oxide units have flares to vent the process gas during upsets. Data is scarce, but estimates indicate that flaring of process gas occurs once to twice a year (109). 

Oxidation Catalyst. An oxidation catalyst requires air to oxidize unbumed hydrocarbons and carbon monoxide. Air is provided with an engine driven air pump or with a pulse air device. Oxidation catalysts were used in 1975 through 1981 models but thereafter declined in popularity. Oxidation catalysts may be used in the future for lean bum engines and two-stroke engines. 

Nonselective catalytic reduction systems are often referred to as three-way conversions. These systems reduce NO, unbumed hydrocarbon, and CO simultaneously. In the presence of the catalyst, the NO are reduced by the CO resulting in N2 and CO2 (37). A mixture of platinum and rhodium has been generally used to promote this reaction (37). It has also been reported that a catalyst using palladium has been used in this appHcation (1). The catalyst operation temperature limits are 350 to 800°C, and 425 to 650°C are the most desirable. Temperatures above 800°C result in catalyst sintering (37). Automotive exhaust control systems are generally NSCR systems, often shortened to NCR. 

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. 

Control of exhaust emissions for unbumed hydrocarbons and carbon monoxide has followed three routes. 

The atmosphere of the world cannot continue to accept greater and greater amounts of emissions from mobile sources as our transportation systems expand. The present emissions from all transportation sources in the United States exceed 50 biUion kg of carbon monoxide per year, 20 billion kg per year of unbumed hydrocarbons, and 20 billion kg of oxides of nitrogen. If presently used power sources cannot be modified to bring their emissions to acceptable levels, we must develop alternative power sources or alternative transportation systems. All alternatives should be considered simultaneously to achieve the desired result, an acceptable transportation system with a minimum of air pollution. 

List the following in increasing amounts from the exhaust of an idling automobile O-NO,. S(l,. Nj, unbumed hydrocarbons, CO2, and CO. 

In addition to carbon monoxide (CO) and unbumed hydrocarbons (UHC), the most significant products of combustion are the oxides of nitrogen (NOx). At high temperatures, free oxygen not consumed during combustion reacts with nitrogen to form NO and NO2 (about 90% and lO /i of total NOx, respectively). 

Gas-liquid chromatography (GLC) finds many applications outside the chemistry laboratory. If you ve ever had an emissions test on the exhaust system of your car, GLC was almost certainly the analytical method used. Pollutants such as carbon monoxide and unbumed hydrocarbons appear as peaks on a graph such as that shown in Figure 1.7. A computer determines the areas under these peaks, which are proportional to the concentrations of pollutants, and prints out a series of numbers that tells the inspector whether your car passed or failed the test. Many of the techniques used to test people lor drugs (marijuana, cocaine, and others) or alcohol also make use of gas-liquid chromatography. 

Automobile catalytic converter. Catalytic converters contain a "three-way" catalyst designed to convart CO to CO2, unbumed hydrocarbons to CO2 and H2O. and NO to N2. The activa components of the catalysts are the precious metals platinum and rhodium palladium is sometimes used as well. 

C09-0114. In the lower atmosphere, NO2 participates in a series of reactions in air that is also contaminated with unbumed hydrocarbons. One product of these reactions is peroxyacetyl nitrate (PAN). The skeletal arrangement of the atoms in PAN appears at the right, (a) Complete the Lewis structure of this compound, (b) Determine the shape around each atom marked with an asterisk, (c) Give the approximate values of the bond angles indicated with arrows. 

Neoprene (C4H5CI) is burned in air at constant pressure with the reactants and products at 25 °C. The mass yields of some species are found by measurement in terms of g/gp. These product yields are CO at 0.1, soot (taken as pure carbon, C) at 0.1 and gaseous unbumed hydrocarbons (represented as benzene, CgHg) at 0.03. The remaining products are water as a gas, carbon dioxide and gaseous hydrogen chloride (HC1). 

---