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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
NO (^) x Storage-Reduction (NSR) Catalysts
Selective Catalytic Reduction (SCR)
NH 3 /NO x Ratio and Ammonia Slip
SCR Catalyst System
Comparison of NSR and SCR Catalyst System s
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)
Module 6:Emission Control for CI Engines
Lecture 29:Diesel Exhaust Gas Aftertreatment
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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 SO 2 to sulphates results in an increase of mass of PM emissions.
Figure 6.
Effect of exhaust gas temperature on conversion of
particulate 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 SO 2 to SO 3 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..
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 NO (^) x to N (^2) requires a reducing atmosphere. In the diesel engines due to oxidizing atmosphere in the exhaust, a NO (^) x reduction catalyst different than the conventional 3-Way catalyst is required. For reducing NO^ x 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 NO (^) x reduction in oxygen rich atmosphere are:
NO (^) x Storage – Reduction (NSR) Catalysts Selective Catalytic Reduction (SCR)
Low temperature plasma/catalyst systems are also being developed for application to diesel engines.
NO x Storage-Reduction (NSR) Catalysts
The NO (^) x storage-reduction catalyst system or ‘NO (^) x 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 NO (^) x (NO converted to NO^2 on the catalyst itself) on rare earth metal oxides and the second
step is release of NO (^) x in presence of hydrocarbons for reduction to N 2.
For significant reduction in NO (^) x , typically 2 to 5:1 HC/NO (^) x 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.
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Lecture 29:Diesel Exhaust Gas Aftertreatment
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About 2% of the main injection quantity is injected 90 to 200º CA after the main injection in the cylinder. The common rail injection system is well suited for providing post injection. The best NO (^) x storage^ and^ conversion^ efficiency^ of^ NSR^ catalysts^ are^ obtained^ in^ a^ narrow temperature range of 200-350º C. Peak conversion efficiency may reach around 55 to 60% but overall conversion efficiency under driving cycle conditions is only around 35%. A number of catalyst modules to reduce space velocity and improve over all conversion have been employed in prototypes.
Sulphur Poisoning of NSR Catalysts
Sulphur on combustion forms sulphur dioxide, which gets oxidized to SO 3 over the catalyst and reacts
with the rare earth oxides to form their sulphates such as barium oxide present in washcoat gets converted to barium sulphate. The mechanism of sulphur poisoning is similar to the mechanism of NO (^) x
trapping by the catalyst. Hence, presence of sulphur in fuel reduces NO (^) x trapping efficiency. Even with
5 ppm sulphur in fuel the conversion efficiency has been seen to drop by half after about 25000 kms of operation. To improve the catalyst resistance to sulphur poisoning new formulation of the adsorber material are being developed. The NSR catalysts so far are not being applied in diesel engines.
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Lecture 29:Diesel Exhaust Gas Aftertreatment
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NH 3 /NO x Ratio and Ammonia Slip
Based on the stoichiometric considerations, 90% conversion of NO (^) x requires the NH 3 /NO (^) x molar ratio of about 0.9, assuming NO 2 constitutes 10% of NO (^) x. Concentration of NO (^) x 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 NH 3 /NO molar ratio and ammonia slip are shown on Fig 6.12. With increase in NH 3 /NO molar ratio NO (^) x 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
NO x conversion and ammonia slip for a SCR catalyst as
a function of NH 3 /NO^ x ratio.
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Lecture 29:Diesel Exhaust Gas Aftertreatment
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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
NO (^) x conversion efficiency can however, be improved at low catalyst temperatures (< 300º C) when all the NO (^) x is converted to NO 2 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. NO (^) x 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
Schematic layout of SCR catalyst system using pre-
oxidation catalyst.
