Diesel Exhaust Gas Aftertreatment - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

Download Diesel Exhaust Gas Aftertreatment - Engine Combustion - Lecture Notes and more Study notes Sustainability Management in PDF only on Docsity! Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_1.htm[6/15/2012 3:07:21 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment The Lecture Contains: DIESEL EXHAUST GAS AFTERTREATMENT DIESEL OXIDATION CATALYSTS Design Features of DOC NOx Storage-Reduction (NSR) Catalysts Selective Catalytic Reduction (SCR) NH3/NOx Ratio and Ammonia Slip SCR Catalyst System Comparison of NSR and SCR Catalyst Systems ; Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_2.htm[6/15/2012 3:07:21 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment DIESEL EXHAUST GAS AFTERTREATMENT The combustion process and hence the exhaust gas composition and its thermodynamic state in diesel engines differ from SI engines. The main differences are; The overall air-fuel ratio in the diesel engines varies from about 19:1 to 75:1 resulting in large variations in the exhaust gas composition with excess oxygen always present in the exhaust gases. Due to heterogeneous combustion in diesel engines, a large concentration of particulate matter is present in the exhaust gases. The exhaust gas temperature varies usually from 150 to 350º C. The gas temperatures at the exit turbocharger are further lower compared to temperatures at the exhaust port due to expansion in the turbine In the European heavy duty engine cycle the exhaust temperatures vary from 200 - 400º C although in the US transient cycle the gas temperatures may reach up to 600 C. On the other hand in the driving cycle for light duty vehicles the gas temperatures vary in the range of 150 350 C only. Until the year 2000, the diesel vehicle emission standards in the US and Europe were largely met by use of improved injection system, engine combustion improvements, EGR and turbocharging. The three-way catalytic converters are unable to function in diesel engines as a high amount of excess oxygen is always present in the exhaust gases. Hence, the nature of exhaust treatment in diesel engines is considerably different than for the stoichiometric SI engines. In the light duty diesel vehicle segment, diesel oxidation catalysts have found application for the Euro 2 and 3 vehicles. For the later standards such as Euro 4 and 5, advanced forms of exhaust aftertreatment like diesel particulate filters and lean de-NOx catalysts are being employed. Exhaust aftertreatment in diesel engines may be grouped in two broad categories; Diesel catalytic exhaust aftertreatment and Diesel particulate filters (DPF) Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_5.htm[6/15/2012 3:07:21 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment contd... The particulate emission reduction by DOC is influenced by the exhaust gas temperature as shown on Fig. 6. 11. The optimum temperature range for the DOC operation is observed to be from about 200 to 350º C. At lower temperatures poor oxidation of SOF and PAH is obtained and at temperatures higher than 350º C a high conversion of SO2 to sulphates results in an increase of mass of PM emissions. Figure 6.11 Effect of exhaust gas temperature on conversion ofparticulate mass by DOC. The diesel fuels during early 1990s contained 0.2 to 0.3 % sulphur by mass Due to high fuel sulphur content the DOC design has to address to the following requirements; Minimize conversion of SO2 to SO3 at high exhaust gas temperatures Minimize formation and storage of the sulphate on the catalyst. Good conversion of SOF so that DOC reduces the mass of PM emissions in addition to conversion of HC and CO. As sulphur in the diesel fuels has been reduced to around 0.03% the sulphate formation on DOC is not of serious concern. . Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_6.htm[6/15/2012 3:07:22 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment contd... Design Features of DOC The catalyst volume is typically equal to the engine swept volume. A DOC although, may appear similar to the oxidation catalysts used for gasoline vehicles but the following differences in the design features are incorporated; Ceramic monoliths of a lower cell density having 200 to 400 cpsi are normally used to keep it free of clogging by soot. As alumina is more readily gets converted to Al2 (SO4)3, different washcoat materials like titanium oxide, silicon dioxide, or mixtures of 50 % CeO2 and 50% γ-Al2O3 are used. Mainly platinum is used in DOCs with metal loading varying from 0.5 to 2.0 g/l. The diesel oxidation catalyst is placed downstream of the turbocharger and experiences much lower temperatures (100-550º C) compared to the gasoline engine catalyst (300-1100º C). The thermal deactivation of DOC is not a major problem. Diesel engines burn more lubricating oil in the cylinder and the engine oils have a higher metal additive content than the gasoline engines. The pore structure of washcoat must be formed to tolerate larger amounts of these inorganic oxides Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_7.htm[6/15/2012 3:07:22 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment DIESEL DE-NOX CATALYSTS The diesel engine exhaust always has high amount of excess oxygen. Conversion of NOx to N2 requires a reducing atmosphere. In the diesel engines due to oxidizing atmosphere in the exhaust, a NOxreduction catalyst different than the conventional 3-Way catalyst is required. For reducing NOx in the oxygen rich atmosphere, the reducing agents also termed as ‘reductants’ are necessary. The reductants can be supplied either from the engine itself or added by external sources in the exhaust. Hydrocarbons or ammonia are the two most frequently used reductants. As discussed earlier, the main strategies employed for NOx reduction in oxygen rich atmosphere are: NOx Storage – Reduction (NSR) Catalysts Selective Catalytic Reduction (SCR) Low temperature plasma/catalyst systems are also being developed for application to diesel engines. NOx Storage-Reduction (NSR) Catalysts The NOx storage-reduction catalyst system or ‘NOx Trap’ was first developed for application to gasoline direct injection, lean-burn DISC spark ignited engines. It has been discussed in Module 5. In the diesel engines, diesel derived hydrocarbons are used as reductants. The principle of operation and basic features of Diesel NSR catalysts are the same as for the lean burn SI engines. The first step is to absorb NOx (NO converted to NO2 on the catalyst itself) on rare earth metal oxides and the second step is release of NOx in presence of hydrocarbons for reduction to N2. For significant reduction in NOx, typically 2 to 5:1 HC/NOx molar ratios are required. Normally, engine out hydrocarbon emissions are quite low in the diesel engines. In the diesel NSR system, hydrocarbons are added to the exhaust gas by; post injection of fuel in the cylinder after the main fuel injection event adding secondary fuel into the exhaust system. Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_10.htm[6/15/2012 3:07:22 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment contd... NH3/NOx Ratio and Ammonia Slip Based on the stoichiometric considerations, 90% conversion of NOx requires the NH3/NOx molar ratio of about 0.9, assuming NO2 constitutes 10% of NOx. Concentration of NOx in the exhaust gases varies depending upon engine operating conditions. Hence, for a vehicle continuously variable injection rate of urea is required. If more urea than stoichiometric requirements is injected, unreacted ammonia is emitted in the exhaust which is called ‘ammonia slip’. To minimize ammonia slip, a dynamic urea dosage system governed by engine operating conditions is to be employed. Even with the dynamic dosage system, ammonia slip occurs during transient operation. Typical conversion efficiency at different NH3/NO molar ratio and ammonia slip are shown on Fig 6.12. With increase in NH3/NO molar ratio NOx conversion efficiency increases and but the ammonia slip also increases. An oxidation catalyst is therefore, added to SCR system to prevent emissions of ammonia. Figure 6.12 NOx conversion and ammonia slip for a SCR catalyst as a function of NH3/NOx ratio. Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_11.htm[6/15/2012 3:07:22 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment contd... SCR Catalyst System The basic SCR system using urea consists of three catalysts viz., Hydrolysis catalyst SCR catalyst, and An oxidation catalyst to oxidize ammonia slip NOx conversion efficiency can however, be improved at low catalyst temperatures (< 300º C) when all the NOx is converted to NO2 before entering the SCR catalyst. An additional oxidation catalyst therefore, ahead of SCR catalyst is used in the modern SCR systems. A typical SCR system for heavy-duty vehicles is shown schematically in Fig. 6.13. NOx conversions of more than 70 % have been obtained with SCR over the HD driving cycle. On road, over all reductions of close to 68 % have been obtained for heavy duty trucks. Urea consumption is about 5.5% of the fuel consumption. Urea requirements for several thousand kms of operation can be stored on board. Figure 6.13 Schematic layout of SCR catalyst system using pre- oxidation catalyst. Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture29/29_12.htm[6/15/2012 3:07:22 PM] Module 6:Emission Control for CI Engines Lecture 29:Diesel Exhaust Gas Aftertreatment contd... Comparison of NSR and SCR Catalyst Systems Table 6.3 Comparison of SCR and NSR de-NOx Technologies Selective Catalytic Reduction (SCR) NOx Storage Reduction (NSR) Catalyst Advantages: High conversion rate up to 90% Technology already developed and used in stationary applications Advantages: Using HC and CO exhaust emissions as reducing agents Oxidation of SOF, HC, CO emissions possible due to the use of specially coated zeolite catalysts Disadvantages: Costly and large space requirements Injections of another substance i.e., urea/ammonia as reductant Dynamic dosage control of reducing agent needed Extra oxidation catalyst for excess ammonia and SOF necessary Additional urea distribution network required Disadvantages: Lower conversion rates (up to about 35% only) Engine/Fuel system development for providing higher HC emissions needed in order to avoid additional HC injection before catalyst