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Catalytic converter

Catalytic converter.jpg


A catalytic converter is a device used to reduce the emissions from an internal combustion engine. Most commonly used in an automobile's exhaust system, catalytic converters are now commonly used on generator sets, forklifts, mining equipment, trucks, buses, trains, and other machines that have engines to provide an environment for a chemical reaction where unburned hydrocarbons are more completely combusted. Automobile converters use platinum or palladium and rhodium as catalysts. Hence the combustion (redox) process continues, but outside the engine's combustion chamber, so no useful energy is extracted. The catalytic converter was invented at Trinity College (Connecticut)

Purpose and function of catalytic convertors

A three-way catalytic converter has three simultaneous tasks:

  1. oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
  2. reduction of nitrogen oxides to nitrogen: NOx → O2 + N2
  3. oxidation of hydrocarbons (unburnt fuel) to carbon dioxide and water: CxHy + nO2 → xCO2 + mH2O

These three reactions are most balanced at the stoichiometric point, which is the mid-point between rich and lean operation. The amount of oxygen to fuel in the engine is in a ratio for the most complete combustion. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. The above two oxidizing reactions (oxidation of CO and hydrocarbons) are favoured. When there is more fuel than oxygen (stoichiometrically), then the engine is running rich. The reduction of NOx is favoured.

Catalytic converters are now standard fit in North America on "Large Spark Ignition" engines. LSI engines are used in forklifts, aerial boom lifts, ice resurfacing machines, and construction equipment. The converters used in these are three-way types designed to reduce combined NOx+HC emissions from 12 gram/BHPhour to 3 gram/BHPhour or less, as per the Environmental Protection Agency (EPA) 2004 regulations. A further drop to 2 gram/BHPhour of NOx+HC emissions is mandated in 2007 (note: NOx is the industry standard short form for nitric oxide (NO) and nitrogen dioxide (NO2) both of which are smog precursors. HC is the industry short form for hydrocarbons).

Catalyst Poisoning

Catalytic converters become ineffective in the presence of lead due to catalyst poisoning, and the introduction of catalytic converters triggered the end of leaded gasoline. Catalyst poisoning occurs when a substance in the fuel or lube oil of the engine coats the surface of the catalyst, masking the precious metal deposits. Poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time to raise exhaust gas temperature, which may cause liquefaction or sublimation of the catalyst poison. Common catalyst poisons are lead, sulfur, and phosphorous. Removal of sulfur from a catalyst surface by running heated exhaust gasses over the catalyst surface is often successful, however removal of lead deposits is often not possible (the lead becomes vapourized in the combustion chamber of a gasoline 4 stroke engine under the ambient temperature and pressure after charge air ignition, and condenses on the cooler catalytic converter core surface. In particularly bad cases of catalyst poisoning by lead the converter can actually become completely plugged with lead residue). Theoretically catalyst poisoning could also occur if the charge air was contaminated by a catalyst poison, however catalyst poisons are all substances that are solid at the internal temperature of the catalytic converter, and thus precipitate out of the air.

Technical details

The catalytic converter consists of several components:

  1. The core, catalyst support, or substrate. In modern catalytic converters this is most often a ceramic honeycomb, however stainless steel foil honeycombs are also used. The purpose of the core is to "support the catalyst", asnd therefore it is often called a "Catalyst Support".
  2. The washcoat. In an effort to make converters more efficient a washcoat is utilized, most often a mixture of silicon and aluminum. The washcoat when added to the core forms a rough, irregular surface which has a far greater surface area than the flat core surfaces. The irregular surface is desirable to give the converter core a larger surface area, and therefore more places for active precious metal sites. The catalyst is added to the washcoat (in suspension) before application to the core.
  3. The catalyst itself, most often a precious metal. Platinum is widely used, however platinum is not suitable for all applications because of unwanted additional reactions and/or cost. Palladium and rhodium are two other precious metals that are used. Cerium, iron, and nickel are also used, though each has its limitations, and nickel is not legal for use in the European Union (nickel hydrate formation). While copper can be used, its use is illegal in North America due to the formation of dioxin.

Catalytic converters are used on spark ignition (gasoline; liquified petroleum gas (LPG); flexible fuel vehicles burning varying blends of E85 and gasoline; compressed natural gas (CNG)) rich burn engines; and compression ignition (diesel, lean CNG) lean burn engines. The reasons for use on each type of engine are different.

For spark ignition engines the most commonly used catalytic converter is the three-way converter, which should only be used on engines that feature electronic fuel injection (if gasoline fueled), or some form of feedback fuel system (LPG fueled engines). This is because the three-way converter works best when the air-fuel ratio of the engine is kept within a certain very narrow band (when the engine is running stoichiometric). Within that band conversions are very high, sometimes approaching the theoretical point of perfection (i.e. 100%), however outside of that band conversions tend to fall off very rapidly. A three-way catalyst reduces emissions of CO (carbon monoxide), HC (hydrocarbons), and NOx (nitric oxide and nitrogen dioxide) simultaneously. Note that unwanted reactions can occur in the three-way catalyst such as the formation of H2S (hydrogen sulfide) and NH3 (ammonia). Formation of each can be limited by modifications to the washcoat/precious metals used, however it is difficult to eliminate completely.

For compression ignition engines the most commonly used catalytic converter is the diesel oxidation catalyst. The catalyst uses excess O2 (oxygen) in the exhaust gas stream to oxidize CO (carbon monoxide) to CO2 (carbon dioxide) and HC (hydrocarbons) to H2O (water) and CO2. These converters often reach 90% effectiveness, virtually eliminating diesel odor and helping to reduce visible particulate (soot), however they are incapable of reducing NOx as chemical reactions always occur in the simplest possible way, and the existing O2 in the exhaust gas stream. To reduce NOx (nitric oxide and nitrogen dioxide) on a compression ignition engine it is necessary to change the exhaust gas - two main technologies are used for this - SCR (selective catalytic reduction) and [NOx] (NOx) traps. Another issue for diesel engines is particulate (soot). This can be controlled by a soot trap or diesel particulate filter (DPF).

Regulations

Emissions regulations vary considerably from jurisdiction to jurisdiction, as do what engines are regulated. In North America any spark ignition engine of over 19 KW (25 hp) power output built later than January 1, 2004 probably has a three-way catalytic converter installed. In Japan a similar set of regulations will come into effect January 1, 2007, while the European Union has not acted as yet. Automobiles in North America have been fitted with catalytic converters since the early 1970s, and the technology being used in non-automotive applications is generally based on it.

Diesel engine regulations are simularly varied, with some jurisdictions focusing on NOx (nitric oxide and nitrogen dioxide) emissions and others focusing on particulate (soot) emissions. This can cause problems for the engine manufacturers as it may not be economical to design an engine to meet two sets of regulations.

Another issue is that fuel quality varies widely from place to place, as do the regulations covering fuel quality. In North America both gasoline and diesel fuel are highly regulated, and there are campaigns under way to regulate CNG and LPG as well. In most of Asia this is not true - in some places sulfur content of the fuel can reach 20,000 parts per million (2 percent). Any sulfur in the fuel may be oxidized to SO2 (sulfur dioxide), SO3 (sulfur trioxide) or even SO4 in the combustion chamber. If sulfur passes over a catalyst it may be further oxidized in the catalyst (SO2 may be further oxized to SO4). Sulfur oxides are precursors to sulfuric acid, a major component of acid rain. While it is possible to add substances like vanadium to the catalyst wash coat to combat sulfur oxide formation, this will typically reduce the effectiveness of the catalyst—the best solution is further refinement of the fuel at the refinery to remove the sulfur, however the expense is such that this is not practical in many developing countries, and cities in these countries with high levels of vehicular traffic suffer high levels of damage to buildings due to acid rain eating away the stonework.

See also

Patents

  • Keith, C. D., et al., -- Template:US patent -- "Apparatus for purifying exhaust gases of an internal combustion engine" -- April 29, 1969