Catalyst Poisoning - 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/lecture24/24_1.htm[6/15/2012 3:05:33 PM] Module 5:Emission Control for SI Engines Lecture 24:Lean de-NOx Catalysts and Catalyst Poisoning The Lecture Contains: Lean de-NOx Catalysts NOx storage-reduction (NSR) catalyst SCR Catalysts CATALYST DEACTIVATION Catalyst Poisoning Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture24/24_2.htm[6/15/2012 3:05:33 PM] Module 5:Emission Control for SI Engines Lecture 24:Lean de-NOx Catalysts and Catalyst Poisoning Lean de-NOx Catalysts The gasoline direct injection (GDI) engine operating in stratified charge mode is a a lean-burn spark- ignited engine that gives 20 – 30% higher fuel efficiency compared to the conventional stoichiometric engine The diesel engines also operate with 40% or more excess air. The 3-way catalytic converter cannot provide NOx reduction in the lean burn SI and the diesel engines. Lean de-NOx catalyst technology has been developed to meet the needs of these engines. Two main types of lean de-NOx catalyst technology are; NOx storage-reduction (NSR) catalyst or NOx trap Selective catalytic reduction (SCR) Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture24/24_4.htm[6/15/2012 3:05:33 PM] Module 5:Emission Control for SI Engines Lecture 24:Lean de-NOx Catalysts and Catalyst Poisoning SCR Catalysts Selective reduction catalysts applied in SI engines work on the principle of direct reduction of NOx by hydrocarbons injected into exhaust stream. In the diesel engines the SCR system uses ammonia produced from urea is used as the reducing agent or ‘reductant'. The urea SCR systems would be discussed in detail in Module 6. In the lean-mixture environment, NOx may be reduced by HC as per the following reaction: (5.10) The exhaust gas stream should have right type of HC in right concentrations to complete the above reaction and reduce nitrogen oxides. Propane is effective at around 500º C and ethylene at 160-200º C. Zeolites like Cu/ZSM-5 have been studied as SCR catalysts. However, these catalysts are sensitive to water vapour and sulphur dioxide, and hence so far, have had only a limited success. CATALYST DEACTIVATION The automotive catalysts in the USA are required to meet the emission standards for 192,000/240,000 km. of life, The fresh catalysts while meet the standards but during vehicle operation their conversion efficiency deteriorates due to ageing and poisoning effects by the contaminants that may come from fuel or engine lubricating oil that burns in the cylinder. The catalyst is subjected to high temperatures exceeding 900 ºC, thermal shocks and mechanical vibrations. Contaminants originating from fuel that cause serious catalyst poisoning, are sulphur and lead (now the gasoline is free of lead), and from lubricating oil are zinc and phosphorous compounds. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture24/24_5.htm[6/15/2012 3:05:33 PM] Module 5:Emission Control for SI Engines Lecture 24:Lean de-NOx Catalysts and Catalyst Poisoning contd.. The catalyst deactivation causes ; Increase in light off temperature, and Decrease in maximum conversion efficiency Two types of catalyst of deactivation are encountered in practice: Catalyst poisoning Thermal deactivation Catalyst Poisoning The contaminants can poison the catalyst in the following manner: Deposition on the active catalyst sites chemically reacting with catalyst Accumulation of the contaminants on the outer surface of the catalyst physically restricting contact of the exhaust gases with the catalyst. This is termed as ‘blanketing effect'. Figure 5.25 Effect of lead poisoning on a 3-waycatalytic converter. Lead as tetra ethyl lead was used for many years as antiknock additive in gasoline. Lead oxides and other lead compounds formed during combustion cause very rapid degradation of the catalyst performance. About 10 to 30 percent of the lead in the fuel gets deposited on the catalyst sites and catalyst washcoat. A typical effect of lead on the conversion efficiency of a Platinum/Rhodium 3- way catalyst is shown on Fig. 5.25. A lead deposition of about 0.5 % of catalyst weight causes 50% drop in the conversion efficiency. Now, the gasoline almost all over the world is lead free. Sulphur naturally occurs and is present in small amounts in petroleum fuels It causes catalyst poisoning, Pd being more sensitive than Pt and Rh.. In a test, after 160,000 km vehicle operation fuel with sulphur of 575 ppm increased the catalyst light off temperature to 299º C from 277 º C with 40 ppm sulphur fuel. Zinc and phosphorous additives used in lubricating oil get converted to oxides during combustion and form zinc pyrophosphate glaze over large areas of the catalyst surface, Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture24/24_5.htm[6/15/2012 3:05:33 PM] which seals the passage of exhaust gas to the catalyst sites. Silicon coming from contamination of fuel clogs the protective sheath of the sensor restricting the diffusion of gases to the surface of the sensor element. It affects the response of oxygen sensor which adversely affects conversion efficiency of the closed loop controlled three-way catalysts.