Purification of Exhaust Gases

1. Purification of Exhaust Gases Removal of pollutants from exhaust gas after they leave the engine cylinder can be done either by using a thermal reactor or by using a catalytic converter In order to oxidize HC in the gas phase without catalyst a residence time of the order of 50ms and temp of about 6000 C is required. Whereas to oxidize CO temp in excess of 7000C is required.

2. Thermal Reactors • HC and CO can be reduced by using thermal reactors. • Reactors are designed to reduce heat loss of the gas in the exhaust manifold and increase residence time of the gas in the exhaust manifold. • Typically a thin steel liner acts as reactor core. Heat losses are minimized by insolating the reactor core. • The reactor core is kept in a CI casing.

3. Disadvantages of Thermal Reactors • To increase the residence time of the burned gases reactor vol has to be large. • To achieve the required temp of oxidation some time is required. • Air injection requirement makes the system complicated. • NOx reduction is not possible. • Reactor material has to withstand high very temp

4. Catalytic Converter System Four basic designs have been developed • Oxidation Catalytic Converter • Dual bed Catalytic Converter • Three way Catalytic Converter and • Denox (lean burn) Catalytic Converter

5. Oxidation Catalytic Converters • Oxidation catalytic converters are the simplest form of CCs. • It can oxidize CO and HC to form CO2 and H2O. • The air required for oxidation process can be supplied either by using lean mixture or supplying secondary air injection. • Normally used in diesel engines. • They cannot remove NOx. Also they are not capable of removing soluble PMs.

6. Dual Bed Catalytic Converters • A reduction catalyst is fitted to minimize NOx and then an oxidation catalyst is used to oxidize CO and HC. • Engine must be operated at rich mixture. • Therefore the system has certain drawback.

7. Denox Catalytic Converters • Denox catalytic converters allow not only CO and HC reduction but also NOx reduction at lean mixture. They are currently at development stage.

8. Three way catalytic converters • Most of the SI engines use 3 way CC since they allow HC, CO & NOx to be converted simultaneously. • The main components of a 3 way CC are • Substrate (monolith) • Washcoat • Catalyst (noble metal) • Support and housing

9. Three way catalytic converters

10. Three way catalytic converters

11. Three way catalytic converters --Contd. (Substrate) • Substrate is a ceramic honeycomb structure held in a metal can or housing. • Noble metal (catalyst) is impregnated into a highly porous washcoat about 20 µm thickness that is applied to the passage way walls. • A typical monolith has a square section passage way of inside dimensions of about 1 mm separated by thin porous wall of thickness (0.15 to 0.3mm). Number of cells per sq. cm. varies between 30 to 60. • Ceramic mat must be highly thermal resistant. • It should have mechanical strength.

12. Metal Substrate • Now a days metallic substrates are also available. Their advantages are: • Increased conversion efficiency • Longer life • Lower wall thickness (0.04 to 0.06 mm) • More No. of cells can be use per unit area. WASHCOAT Washcoat is applied to substrate material. It has a surface area of 100-200 m2/g.

13. Catalysts • Catalysts used in a 3 way CC are Pt, Pd & Rh. • A very small amount of Pt, Pd & Rh is impregnated into highly porous alumina washcoat. • For oxidation of CO and HC a mixture of Pt and Pd is most commonly used. • For oxidation of CO, olefins and CH4 specific activity of Pd is higher than that of Pt. • For oxidation of aromatic compounds Pt and Pd have similar activity. • For oxidation of paraffinic HCs (<C3) Pt is more active than Pd.

14. Catalysts Contd. • Noble metals sinter rapidly at 5000 C to 9000 C. • Noble metals are dispersed as finely as possible in alumina which prevents particle to particle metal contact and supress sintering. • Particle size of noble metal is 50 nm. This can increased to about 100 nm when exposed to high temperature.

15. Catalysts--contd. • Pt/Pd = 2 is typical in a three way catalytic converter. • Concentration of noble metal is about 1-2 g/dm3

16. Conversion efficiency of a CC Conversion efficiency = (min – mout)/min = 1 – mout/min Conversion efficiency of a new catalytic converter is 98 – 99 % for CO and 95% for HC

17. Effect of A/F on ηconv of CC

18. Light off temperature Light off temperature of a catalytic converter is the temperature at which the catalytic converter become 50% effective. Typical light off temperature is 250 -3000 C