Mechanisms of Formation of Soot - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

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Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture12/12_1.htm[6/15/2012 2:58:59 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 12:Mechanisms of Formation of Soot and PM Mechanisms of Formation of Soot and PM The Lecture Contains: SOOT AND PARTICULATE EMISSIONS Composition and Structure of Diesel Particulates Typical Composition of Diesel Particulate Matter Soot Structure Diesel Smoke Soot Formation Stoichiometry Conceptual Models of Soot Formation Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture12/12_2.htm[6/15/2012 2:58:59 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 12:Mechanisms of Formation of Soot and PM SOOT AND PARTICULATE EMISSIONS Soot is a carbonaceous particulate matter and is produced during combustion of the rich fuel - air mixtures. Appearance of black smoke emissions in the exhaust indicates high concentration of soot in the exhaust gases. Soot is mostly produced in the diffusion combustion systems, but overly rich premixed combustion also produces soot. As the spark ignition engines generally operate close to stoichiometric air-fuel ratio, soot emissions from these engines are not significant. With the use of unleaded gasoline, lead particulates from the SI engines have been eliminated. Here, we will discuss particulate emissions only from the diesel engines as these are of major health concern and are more difficult to control. Soot emissions have been associated with respiratory problems and are thought to be carcinogenic in nature. The particle size is important as the particles smaller than 2.5 µ can reach lungs along with the inhaled air and cause health problems. The particles smaller than 2.5 µ constitute more than 90 percent mass of the total particulate matter in the diesel exhaust. The fuel composition also is an important factor in soot production and emissions. For diffusion combustion soot- forming tendency is generally in the following order; Composition and Structure of Diesel Particulates US Environmental Protection Agency (USEPA) defines the particulate matter as any substance other than water that is collected by filtration of the diluted exhaust gases at or below 325 K (125 F). Composition of particulate matter collected on a filter is schematically shown on Fig. 2.20. Various components adsorbed on the surface of spherical soot particles are shown. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture12/12_3.htm[6/15/2012 2:58:59 PM] Figure 2.21 Typical diesels PM composition for a Euro 3engine. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture12/12_4.htm[6/15/2012 2:59:00 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 12:Mechanisms of Formation of Soot and PM Soot Structure Electron micrographs of soot have been obtained in diffusion flames and in the engine combustion chamber. Soot is seen to be aggregates of primary spherical particles. The primary soot particles are mostly in the 15 to 30 nm size range. Typical electron micrograph of soot obtained from acetylene-air diffusion flame is shown in Fig. 2.22. Primary soot particles are seen as spheroids on the micrograph. Primary particles of about 30 nm size and branched chainlike soot aggregates are clearly observed in this micrograph at 80000 magnifications. A single spherical soot particle contains 105 to 106 carbon atoms. Initially, the combustion-generated soot particles have one hydrogen atom to about 8 carbon atoms, (C8H)n with a density of approx. 1800 kg/m3.. The primary soot particles form aggregates in the combustion chamber of 100-200 nm in size containing generally 20 to over 100 primary particles. These aggregates may further agglomerate to particles as large as 1 µ. The aggregates usually resemble a cluster of spheres in branched or chain like structure. Figure 2.22 Transmission electron microscope (TEM) images of soot in acetylene-air diffusion flames. Micrograph at 80K magnification shows branched chainlike soot aggregates Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture12/12_5.htm[6/15/2012 2:59:00 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 12:Mechanisms of Formation of Soot and PM Diesel Smoke Visible black smoke emissions from diesel engine result on account of high concentration of soot in the exhaust gas. All the diesel engine design and operating variables that affect soot formation and oxidation also influence black smoke intensity. Initially, smoke emission standards for production diesel vehicles were in force to control black smoke. Smoke emissions increase with increase in engine load due to overall richer fuel-air ratios and hence, the rated engine power was specified based on the maximum permitted smoke density to curb black smoke emissions during engine operation. The rated power was also known as ‘smoke limited power’. Poor control of fuel injection rate during acceleration also increases smoke. Use of EGR reduces combustion temperatures and oxygen concentration in the burned gases. EGR also reduces oxidation of soot and hence overall effect of EGR is to increase smoke. Smoke emissions can be reduced by accelerating combustion. Higher combustion rates are obtained by increasing fuel air mixing through use of high swirl rates, by increasing injection rate and improving fuel atomization. Advancing injection timing increases combustion temperatures and allows more time for oxidation of soot thereby reducing smoke emissions. Smoke is measured by measurement of light absorbed (opacity) in a defined specific length of column of exhaust gas. The smokemeters employing this principle are known as light extinction type of smokemeter such as Hartridge or AVL smokemeters. Smoke has also been measured by filtering a fixed volume of exhaust gases through a filter paper and the smoke stain thus formed is evaluated on a grayness scale by a light reflectance meter (Bosch smokemeter). As the diesel particulate matter (PM) mainly consists of soot and the adsorbed unburned hydrocarbons (SOF) on soot core, the PM content has been related to exhaust soot content and HC concentration as below; (2.37) Soot content and HC concentration are in mg/m3. Soot content can be roughly estimated from smoke measurement from correlations developed by SAE (SAE Handbook), where soot content has been correlated with Bosch and Hartridge smoke units and opacity measured by full flow type smoke meters.