Purification of Exhaust Purification of Exhaust GasesGases

Removal of pollutants from exhaust Removal of pollutants from exhaust gas after they leave the engine cylinder can gas after they leave the engine cylinder can be done either by using a thermal reactor or be done either by using a thermal reactor or by using a catalytic converterby using a catalytic converter In order to oxidize HC in the gas phase In order to oxidize HC in the gas phase without catalyst a residence time of the order without catalyst a residence time of the order of 50ms and temp of about 600of 50ms and temp of about 6000 0 C is C is required.required. Whereas to oxidize CO temp in excess of Whereas to oxidize CO temp in excess of 70070000C is required. C is required.

Thermal ReactorsThermal Reactors

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

Disadvantages of Thermal Disadvantages of Thermal ReactorsReactors

To increase the residence time of the To increase the residence time of the burned gases reactor vol has to be large.burned gases reactor vol has to be large.To achieve the required temp of oxidation To achieve the required temp of oxidation some time is required.some time is required.Air injection requirement makes the Air injection requirement makes the system complicated.system complicated.

NONOxx reduction is not possible. reduction is not possible.

Reactor material has to withstand high Reactor material has to withstand high very tempvery temp

Catalytic Converter SystemCatalytic Converter System

Four basic designs have been Four basic designs have been developed developed

Oxidation Catalytic ConverterOxidation Catalytic Converter Dual bed Catalytic ConverterDual bed Catalytic Converter Three way Catalytic Converter andThree way Catalytic Converter and Denox (lean burn) Catalytic ConverterDenox (lean burn) Catalytic Converter

Oxidation Catalytic Oxidation Catalytic ConvertersConverters

Oxidation catalytic converters are the simplest Oxidation catalytic converters are the simplest form of CCs.form of CCs.

It can oxidize CO and HC to form COIt can oxidize CO and HC to form CO22 and H and H22O.O.

The air required for oxidation process can be The air required for oxidation process can be supplied either by using lean mixture or supplied either by using lean mixture or supplying secondary air injection.supplying secondary air injection.

Normally used in diesel engines.Normally used in diesel engines.

They cannot remove NOThey cannot remove NOxx. Also they are not . Also they are not

capable of removing soluble PMs.capable of removing soluble PMs.

Dual Bed Catalytic Dual Bed Catalytic ConvertersConverters

A reduction catalyst is fitted to A reduction catalyst is fitted to minimize NOminimize NOxx and then an oxidation and then an oxidation

catalyst is used to oxidize CO and catalyst is used to oxidize CO and HC.HC.

Engine must be operated at rich mixture.Engine must be operated at rich mixture.

Therefore the system has certain Therefore the system has certain drawback.drawback.

Denox Catalytic ConvertersDenox Catalytic Converters

Denox catalytic converters Denox catalytic converters allow not only CO and HC allow not only CO and HC reduction but also NOreduction but also NOxx

reduction at lean mixture. reduction at lean mixture. They are currently at They are currently at development stage.development stage.

Three way catalytic Three way catalytic convertersconverters

Most of the SI engines use 3 way CC Most of the SI engines use 3 way CC since they allow HC, CO & NOsince they allow HC, CO & NOxx to be to be

converted simultaneously.converted simultaneously.

The main components of a 3 way CC areThe main components of a 3 way CC are

1.1. Substrate (monolith)Substrate (monolith)

2.2. WashcoatWashcoat

3.3. Catalyst (noble metal)Catalyst (noble metal)

4.4. Support and housingSupport and housing

Three way catalytic convertersThree way catalytic converters

Three way catalytic convertersThree way catalytic converters

Three way catalytic converters Three way catalytic converters --Contd. (Substrate)--Contd. (Substrate)

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

Metal SubstrateMetal Substrate

Now a days metallic substrates are also Now a days metallic substrates are also available. Their advantages are:available. Their advantages are:

Increased conversion efficiencyIncreased conversion efficiency

Longer lifeLonger life

Lower wall thickness (0.04 to 0.06 mm)Lower wall thickness (0.04 to 0.06 mm)

More No. of cells can be use per unit area.More No. of cells can be use per unit area.

WASHCOATWASHCOAT

Washcoat is applied to substrate material. It Washcoat is applied to substrate material. It has a surface area of 100-200 mhas a surface area of 100-200 m22/g./g.

CatalystsCatalysts

Catalysts used in a 3 way CC are Pt, Pd & Rh.Catalysts used in a 3 way CC are Pt, Pd & Rh.A very small amount of Pt, Pd & Rh is impregnated into A very small amount of Pt, Pd & Rh is impregnated into highly porous alumina washcoat.highly porous alumina washcoat.For oxidation of CO and HC a mixture of Pt and Pd is For oxidation of CO and HC a mixture of Pt and Pd is most commonly used.most commonly used.

For oxidation of CO, olefins and CHFor oxidation of CO, olefins and CH44 specific activity of specific activity of Pd is higher than that of Pt.Pd is higher than that of Pt.For oxidation of aromatic compounds Pt and Pd have For oxidation of aromatic compounds Pt and Pd have similar activity.similar activity.

For oxidation of paraffinic HCs (<CFor oxidation of paraffinic HCs (<C33) Pt is more active ) Pt is more active than Pd.than Pd.

Catalysts Contd.Catalysts Contd.

Noble metals sinter rapidly at 500Noble metals sinter rapidly at 50000 C to C to 90090000 C. C.Noble metals are dispersed as finely as Noble metals are dispersed as finely as possible in alumina which prevents particle possible in alumina which prevents particle to particle metal contact and supress to particle metal contact and supress sintering.sintering.Particle size of noble metal is 50 nm. This Particle size of noble metal is 50 nm. This can increased to about 100 nm when can increased to about 100 nm when exposed to high temperature.exposed to high temperature.

Catalysts--contd.Catalysts--contd.

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

Concentration of noble metal Concentration of noble metal is about 1-2 g/dm3is about 1-2 g/dm3

Conversion efficiency Conversion efficiency of a CCof a CC

Conversion efficiency = Conversion efficiency =

(m(minin – m – moutout)/m)/minin = 1 – m = 1 – moutout/m/minin

Conversion efficiency of a new Conversion efficiency of a new catalytic converter is 98 – 99 catalytic converter is 98 – 99

% for CO and 95% for HC% for CO and 95% for HC

Effect of A/F on Effect of A/F on ηηconvconv of CC of CC

Light off temperatureLight off temperature

Light off temperature of a catalytic Light off temperature of a catalytic converter is the temperature at converter is the temperature at which the catalytic converter which the catalytic converter become 50% effective.become 50% effective.

Typical light off temperature is Typical light off temperature is 250 -300250 -30000 C C