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Emissions Control: After Treatment Systems

 

This is the easier approach to reduce the emissions. Various options are available:

  • Thermal reactors
  • Catalytic converters
  • Particulate traps (CI Engines)

 

Thermal Reactors

Thermal reactors were used extensively in the early days but have limited use nowadays with the much tighter emission legislations. They were abandonment in favour of catalytic converters.

Thermal reactors are high-temperature combustion chambers through which the exhaust gas flows. They are used to oxidise CO and HC (under oxygen rich conditions) and operate by exposing the exhaust products to temperatures above 600°C-700°C for an extended period of time. The residence time of the gases is increased by enlarging the volume of the exhaust manifold. For maximum efficiency, thermal reactors need to be designed to minimise the heat losses and increase the residence time. Therefore, a concentric assembly containing a liner of fine refractory steel plate inside a thermally-insulated cast iron cylinder is used. An example of a thermal reactor design is shown in the following figure.

Disadvantages:

  • Complexity
  • Low efficiency. Total oxidation never achieved due to incomplete mixing and insufficient residence time
  • High fuel consumption since the engine needs to operate a rich in order to increase the temperature in the reactor needed for the oxidisation.
  • No action on NOX

 

Catalytic Converters

Catalytic converters consist of a carrier (generally ceramic monolith) with impregnated active catalytic materials (usually Platinum (Pl), Palladium (Pd) and Rhodium (Rh)) in a metal casing through which the exhaust gases flow. Platinum and Palladium control CO and HC and Rhodium controls NOX. The relative amount of the active materials used is dependent on the market costs, the performance required and concern about catalyst ageing (decrease of performance over time).

The efficiencies of catalytic converters can approach 100% under the right conditions of temperature and equivalence ratio. Catalysts need to work between 300°C (minimum for good conversion efficiency) and 1000°C (max for durability). To reduce emissions during cold start, catalytic converters are being installed as upstream of the exhaust pipeline as possible.

The minimum temperature at which a conversion efficiency of 50% is achieved is known as the light-off temperature.

Catalyst performance is degraded by:

  • High temperature exposure. For example, engine misfires can cause excessive catalyst heating and premature ageing. Also, the catalyst will run hottest at full load and it is common practice to operate slightly rich under such conditions to prevent catalyst damage.
  • Certain compounds, such as lead and burnt lubricating oil can “poison” the catalyst. This is referred to as catalyst “deactivation” and is caused by sintering of the catalytic and carrier components, masking of the active sites and erosion of the catalyst materials.

As catalysts age, the light-off temperature increases and the conversion efficiency reduces.

 

Oxidising Catalysts

Early catalysts (previous 1979) were of the oxidising type. These could reduce CO and HC emissions. They are not effective in reducing the NOX emissions and a combination of lean mixtures and EGR has to be used to meet the NOX requirements.

Nowadays they are still used to oxidise CO and HC in lean burn engines including CI and two-stroke SI engines. They have high conversion efficiency for CO and HC providing the mixture is weak.

 

Dual Bed Catalysts

Dual bed catalysts were used next. A reducing catalyst was placed upstream of an oxidising catalyst with air injection between the two catalysts. The engine operated rich giving the required reducing atmosphere for the first catalyst to control the NOX emissions whilst the air injection leaned the gases producing necessary environment for the second catalyst to control CO and HC.

Dual catalysts have some disadvantages:

  • The engine needs to runs rich, which implies higher fuel consumption.
  • During the first stage NOX are oxidised to N2 and NH3.  However, this NH3 formed in the first stage may undergo more intensive oxidation on the second catalyst, returning it to the initial state of NOX.

Because of these drawbacks, dual catalysis was discarded on small engines in favour of “three-way” catalysts. Dual Bed Catalyst is used on large marine spark-ignition engines.

 

Three-Way Catalyst

The most widely used system for SI engines nowadays. Three-Way Catalysis reduces NOx and oxidise CO and HC simultaneously.

 

To achieve high conversion efficiency (approximately 80 to 90%), these need to operate with a mixture which is extremely close to stoichiometric. As seen in the following figure, AFR affects conversion efficiency considerably.

The AFR is controlled using a closed loop AFR control system based on a lamda or HEGO (Heated Exhaust Gas Oxygen) sensor. The lambda sensor is installed in the exhaust pipe and gives a sharp change in electrical output at stoichiometric and this is used by the control system to maintain stoichiometric AFR.

When the engine is running rich, an air pump is activated that pumps air into the exhaust, thereby leaning the exhaust out.

 

Specifics for CI Engines

An oxidising catalyst is used to reduce CO (40-90%) and HC (30-80%) emissions. They have little effects on solid carbon soot. Diesel fuel, also, contains sulphur impurities, and this poisons the catalyst materials

Since CI engines run lean NOX cannot be controlled via any catalyst. Therefore NOX emissions are to be controlled by design of the combustion process and control of the engine operation. Injection timing retarded from 22 to 15 degrees BTDC reduces NOx emission by a factor of 2, whereas the fuel consumption increases only by about 3%. For this reason, diesel engines are usually operated at injection timings slightly retarded from that which produces best fuel consumption. NOx is also reduced in diesel engines by the use of EGR, which keeps the maximum temperature down. Lower combustion temperature and EGR, however, contribute to an increase in solid soot.

 

Future

Catalysts are being developed which will reduce NOx emissions under conditions which are lean overall. This technology has great potential for application on CI engines and lean burn SI engines and offers a route for reduced CO2 emissions and reduced fuel consumption whilst producing low toxic emissions. These are based on a copper exchanged zeolite (Cu ZSM-5) catalyst and will convert NO to N, providing some HC is available in the exhaust.

 

 

Particulate Traps (CI Engines)

Particulate Traps or DPF (Diesel Particulate Filter) prevent particles in the exhaust of CI engines above a certain size being released into the atmosphere. They are filter-like systems often made of ceramic in the form of a monolith. Traps typically remove 60 – 90% of the particulates in the exhaust flow.

The pressure drop across the filter increases as the particulates build up. A mean of regenerating system (i.e. burning off the particles) the trap is required to restore the filter to the initial state. A temperature of at least 450°C is required for ignition and the exhaust temperature is usually below this. To regenerate, the engine injects a tiny amount of fuel during the exhaust stroke to increase the temperature of the exhaust gases. This is done automatically and the driver does not notice anything.

Efficiency of the filters decrease with time, as some ash and sulphur is always left in the filter.


 

Also interesting:

Emissions Control: In-Cylinder

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