Purification of exhaust gases
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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

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Purification of Exhaust Gases

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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.


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.


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


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


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.


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.


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.


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


Three way catalytic converters


Three way catalytic converters


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.


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.


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.


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.


Catalysts--contd.

  • Pt/Pd = 2 is typical in a three way catalytic converter.

  • Concentration of noble metal is about 1-2 g/dm3


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


Effect of A/F on ηconv of CC


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


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