Engine Exhaust Aftertreatment - 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/lecture21/21_1.htm[6/15/2012 3:04:25 PM] Module 5:Emission Control for SI Engines Lecture21:Engine Exhaust Aftertreatment The Lecture Contains: INTRODUCTION TO EXHAUST AFTERTREATMENT THERMAL REACTORS Catalytic Reactors Catalyst Catalyst Substrate Palletized Catalysts Metal Monoliths Washcoat Converter Housing Space Velocity and Converter Size Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture21/21_2.htm[6/15/2012 3:04:25 PM] Module 5:Emission Control for SI Engines Lecture21:Engine Exhaust Aftertreatment INTRODUCTION TO EXHAUST AFTERTREATMENT Improvements in engine design and adjustment of engine parameters carried to control engine emissions were inadequate to meet the first set of stringent emission regulations introduced in the USA from 1975. Devices were developed to treat exhaust gas for conversion of engine emissions to harmless gases. Conversion of pollutants present in the exhaust gas is termed as ‘exhaust aftertreatment’. Two basic types of exhaust treatment systems were considered; Thermal reactors Catalytic reactors or converters Initially, thermal reactors for oxidation of HC and CO to CO2 and H2O were developed. However, as high conversion rates of pollutants could not be obtained in the thermal reactors these did not find widespread application and very soon the catalytic converters became a standard exhaust aftertreatment device for the spark ignited engine vehicles. THERMAL REACTORS If high exhaust gas temperatures are maintained and sufficient free oxygen is present in the exhaust gases, CO and HC can be oxidized in the engine exhaust system. Oxidation rate of HC can be estimated by an expression given in Module 2. Thermal conversion efficiency for HC and CO as a function of temperature is presented in Fig 5.9. Figure 5.9 Conversion efficiency versus temperature for thermal oxidationof HC and CO For 50% oxidation of CO and HC temperatures in excess of 500 and 600 C, respectively are required. For conversion of 80 percent, temperatures required are about 600 and 750º C for HC and CO, respectively. Residence time in reactor is another is another important variable. At 750º C, conversion of HC up to 90 percent may be obtained in 100 ms while at 850º C only 50 ms are required. Similarly, for 90 percent oxidation of CO 250 ms and 70 ms would be necessary at 750º C and 850º C, respectively. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture21/21_4.htm[6/15/2012 3:04:26 PM] Module 5:Emission Control for SI Engines Lecture21:Engine Exhaust Aftertreatment contd.... The oxides of base metals such as copper, chromium, nickel, cobalt etc. have been studied. The base metal oxides are effective only at higher temperatures. In addition, they sinter and deactivate when subjected to high exhaust gas temperatures experienced at high engine loads. Their conversion efficiency is severely reduced by sulphur dioxide produced by sulphur in fuel. The noble metals platinum (Pt), palladium (Pd) and rhodium (Rh) were found to meet the above mentioned performance requirements. In practice, only the noble metals are used although these are expensive. Mixtures of noble metals are used to provide higher reactivity and selectivity of conversion. Following are typical formulations; Pt : Pd in 2:1 ratio for oxidation catalysts (Pt + Pd): Rh in ratio of 5 :1 to 10: 1 for simultaneous oxidation and reduction such as in 3-way catalysts Palladium has higher specific activity than Pt for oxidation of CO, olefins and methane. For the oxidation of paraffin hydrocarbons Pt is more active than Pd. Platinum has a higher thermal resistance to deactivation. Rhodium is used as a NOx reduction catalyst when simultaneous conversion of CO, HC and NOx is desired as in the 3-way catalytic converters. The amount of noble metal used typically varies from about 0.8 to 1.8 g/l (25 to 50 g/ft3) of catalytic converter volume. For a passenger car the total amount of noble metal in the converter is typically in the range 0.8 g to 2 g. The active metal is in a highly dispersed state when impregnated on the surface of the catalyst support. The size of the noble metal particles on the fresh converter is about 50 nm. However, when used the noble metal particles sinter and may grow to a size of around 100 nm. Catalyst Substrate The active catalyst material is impregnated on the surface of catalyst substrate or support. The function of catalyst substrate is to provide maximum possible contact of catalyst with reactants. Following are the main requirements of catalyst substrate: High surface area per unit volume to keep a small size of the converter Support should be compatible with coating of a suitable material (washcoat) to provide high surface area and right size of pores on its surface for good dispersion and high activity of the catalyst. Low thermal capacity and efficient heat transfer properties for quick heat-up to working temperatures. Ability to withstand high operating temperatures up to around to 1000º C. High resistance to thermal shocks that could be caused by sudden heat release when HC from engine misfire get oxidized in the converter. Low pressure drop Ability to withstand mechanical shocks and vibrations at the operating temperatures under road conditions for long life and durability of 160,000 km and longer The following types of catalysts supports are used; Pellets Monolithic supports Objecti ves_template Ceramic monoliths e Metal monoliths [Previous Next ||> file://C\/...%20and%20Settings/iitkranal /My%20Documents/Google%20Talk% 20Received%20Files/engine_combustion/lecture2 1/21_4.htm[6/15/2012 3:04:26 PM] Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture21/21_5.htm[6/15/2012 3:04:26 PM] Module 5:Emission Control for SI Engines Lecture21:Engine Exhaust Aftertreatment Palletized Catalysts The first catalytic converters introduced in 1970s and used until early 1980s employed spherical ceramic pellets, which were packed in a catalyst bed. The pellets of 3 to 6 mm dia were made of γ- alumina (Al2O3) The porous surface of alumina pellets provides a large surface area on which the noble metal salts are impregnated to a depth of about 250 µm. The pellet catalysts are loaded with approximately 0.05% by weight of noble metals. A typical pellet type catalytic converter is shown in Fig 5.10. The gas flow through the packed bed pellet reactors is a mix of axial and radial flow so as to provide large flow area and reduce flow resistance. The gas flow through pellet bed is turbulent resulting in high mass-transfer rates. The packed bed catalysts suffer from the following disadvantages: High pressure drop Are heavy, have high thermal inertia and hence slow to warm-up. Loss of catalyst from abrasion due to rubbing of pellets against each other. Therefore during 1980s the ceramic honeycomb or metallic matrix monolith converters replaced the pellet type converters. Figure 5.10 Pellet type catalyticconverter. Ceramic Monoliths A ceramic monolith is shown schematically in Fig 5.11. It has parallel flow channels or cells of square or triangular cross section. The flow through these channels is laminar. The ceramic material commonly Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture21/21_6.htm[6/15/2012 3:04:26 PM] Frontal View of Metallic Monolith Ceramic Monolith Cells Metallic Monolith Cells Figure 5.11 Frontal view of a metal monolith and comparison of cell formation in ceramic and metallic monoliths The foil thickness of a metallic monolith of 400-cpsi is typically 0. 0.04 to 0.05 mm compared to 0.15 to 0.20 mm for an early 400-cpsi ceramic monolith. A comparison of flow and physical characteristics of ceramic and metal monoliths is given in Table 5.2. The metallic monoliths in general, have 20 to 30% higher GSA and 10 to 20 % higher OFA. The mass of metallic monolith is however, about 2 times higher than the ceramic monolith of the same volume and cell density. The thermal capacity of metallic monoliths is 15 to 80% higher than the comparable ceramic monoliths. The metallic monoliths however, have 10 to 15% lower backpressure. A disadvantage with metal monoliths is that these cool down faster at lower loads and have to be fitted as close to the engine as possible taking into account the peak temperatures that might occur. The metallic support is non-porous and special techniques to obtain adherence of wash coat and hence catalyst on the surface have to be employed. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture21/21_7.htm[6/15/2012 3:04:26 PM] Module 5:Emission Control for SI Engines Lecture21:Engine Exhaust Aftertreatment Table 5.2 Comparison of Characteristics of Metallic and Ceramic Monolith Catalyst Substrates Characteristics MetallicMonolith Ceramic Monolith Geometric Data Wall thickness, mm 0.02 - 0.04 0.06 -0.20 Range of cell density , cells / cm2 16 - 186 16 - 186 OFA (uncoated) for 62 cells / cm2, % cross section 89- 91 67 -75.0 area 3.2 2.4-2.8 GSA (uncoated) for 62 cells / cm2, m2/l Physical Data Thermal conductivity, W/m.ºK 14-22 0.1-0.8 Specific Heat capacity, kJ/kg.ºK 0.4- 0.5 0.75-1.05 Density, g/cm3 7.4 2.2-2.7 Coefficient of thermal expansion, 1/ºK 15 1 Maximum short duration operating temperature, ºC 1275-1375 1400 Maximum continuous operating temperature, ºC 900-1150 1200 Mass (105.7 mm dia. x 98 mm length) 62 cells / cm2, g 680 340 Metallic monoliths have the following advantages compared to the ceramic monoliths: Higher mechanical strength High thermal conductivity and faster warm-up Higher flow area and lower pressure drop Higher tolerance to high temperature spikes Higher conversion efficiency Smaller size No special housing is required. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture21/21_8.htm[6/15/2012 3:04:26 PM] Module 5:Emission Control for SI Engines Lecture21:Engine Exhaust Aftertreatment Washcoat Monolithic substrates have a geometrical surface area of 2.0 - 4.0 m2/l of its volume. This is too low to provide high contact area between the catalyst and reacting gases for high conversion rates. A thin layer of γ-Al2O3 seeded with other oxides as ‘washcoat’ is applied to the monolith cells to increase effective surface area for dispersion of catalyst material. About 20 percent of other oxides consisting of cerium oxide (CeO2) and stabilizers such as zirconium oxide and barium oxide are added to alumina. The washcoat has pores of varying sizes ranging from 0.2 to 1 µm. The ceramic monolith walls are porous and surface is rough so that good adhesion of washcoat is obtained. High surface area created by Al2O3 washcoat and dispersion of active catalyst deposited by solution impregnation is shown schematically in Fig. 5.12. Adhesion of washcoat to metallic monoliths requires at first pre-treatment of surface to make it rough and improve its bonding characteristics with ceramic washcoat materials. For the prefabricated metallic monoliths then, a procedure similar that for the ceramic monoliths may be followed. Figure 5.12 Conceptual model of catalytic sites dispersed on a high surface-area Al2O3 washcoat. The washcoat constitutes 5 to 15 percent of the mass of ceramic monoliths. Its thickness typically varies in the range 10-30 µm on the walls. The washcoat increases actual surface area of the catalyst substrate to 10000-40000 m2/l of monolith volume. are added. Converter Housing The ceramic monolith is mounted in a metal casing made of high quality corrosion resistant steel. A mat made of ceramic material around the ceramic monolith holds it tightly inside the casing. The mat protects the ceramic monolith against mechanical impact and vibrations, and also acts as heat insulation. The ceramic mat is made of aluminium silicate that expands as it is heated. As the temperature of mat rises, gas bubbles are formed inside the mat and it ensures proper tightening of the ceramic monolith in the housing and sealing to prevent any bypass of the exhaust gases. The metallic monolith converters are easier to mount as the metal mantle that holds the metallic honeycomb structure is welded or brazed in the exhaust system of the engine.