Definition : Catalytic converter
Function: Catalytic converter
Catalyst poisoning and deactivation
Definition: Recycling


Definition: Catalytic converter

A catalytic converter (colloquially, "cat" or "catcon") is a device used to reduce the toxicity of emissions from an internal combustion engine. First widely introduced on series-production automobiles in the US market for the 1975 model year to comply with tightening EPA regulations on auto exhaust, catalytic converters are still most commonly used in motor vehicle exhaust systems. Catalytic converters are also used on generator sets, forklifts, mining equipment, trucks, buses, trains, and other engine-equipped machines. A catalytic converter provides an environment for a chemical reaction wherein toxic combustion byproducts are converted to less-toxic gases. The catalytic converter was invented at Trinity College (Connecticut).
(from: www.wikipedia.org)


Function: Catalytic converter

Three-way catalytic converters

A three-way catalytic converter has three simultaneous tasks:

1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NO_x → xO₂ + N₂
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: C_xH_y + nO₂ → xCO₂ + mH₂O

These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running at the stoichiometric point. This is 14.7 parts oxygen to 1 part fuel, by weight, for gasoline (the ratio for propane, LPG, natural gas and ethanol fuels is slightly different, requiring modified fuel system settings when using those fuels). When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and hydrocarbons) are favoured, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NO_x is favoured, at the expense of CO and HC oxidation. If an engine could be held at the strict stoichiometric point for the fuel used, it is theoretically possible to reach 100% conversion efficiencies.

Since 1981, three-way catalytic converters have been at the heart of vehicle emission control systems in North American roadgoing vehicles and are also used 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 NO_x+HC emissions from 12 gram/BHP-hour to 3 gram/BHP-hour or less, per the Environmental Protection Agency (EPA) 2004 regulations. A further drop to 2 gram/BHP-hour of NO_x+HC emissions is mandated in 2007 (note: NO_x 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). The EPA intends to introduce emissions rules for stationary Spark Ignition engines, to take effect in January 2008.

Two-way catalytic converters

A two-way catalytic converter has two simultaneous tasks:

1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
2. Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: C_xH_y + O₂ → xCO₂ + mH₂O

This type of catalytic converter is commonly used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They also were used on spark ignition (gasoline) engines in automobiles up until 1981, when they were replaced by three-way converters due to regulatory changes requiring reductions on NOx emissions.

Curiously the regulations regarding hydrocarbons vary according to the engine regulated, as well as the jurisdiction. In some cases what is regulated is "non-methane hydrocarbons" and in other cases the regulated substance is "total hydrocarbons". Technology for one application (to meet a non-methane hydrocarbon standard) may not be suitable for use in an application that has to meet a total hydrocarbon standard. Methane is more difficult to break down in a catalytic converter, so in effect a "non-methane hydrocarbon" standard can be considered to be looser. However since methane is a greenhouse gas, more interest is rising in how to eliminate emissions of it.
(from: www.wikipedia.org)


Catalyst poisoning and deactivation

Catalytic converters become ineffective in the presence of lead due to catalyst poisoning. The widespread use of catalytic converters caused the end of leaded gasoline. Catalyst poisoning occurs when a substance in the engine exhaust coats the surface of the catalyst, preventing further exhaust access to the catalytic materials. 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, zinc, manganese, silicone and phosphorus.

Zinc, phosphorus and sulfur originate from lubricant antiwear additives such as ZDDP; sulfur and manganese primarily originate from fuel impurities or from additives such as Methylcyclopentadienyl Manganese Tricarbonyl (MMT), respectively. Silicone poisoning in automotive applications is the result of engine damage, such as a faulty cylinder head gasket or cracked casting, admitting silicate-containing coolant into the combustion chamber, in stationary engines silicon poisoning is more often caused by the use of "Landfill" gas as a fuel.

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 because of its high boiling point. In particularly bad cases of catalyst poisoning by lead, the catalytic converter can actually become completely plugged with lead residue.

A variety of conditions may cause the catalyst to overheat (heat deactivation) and potentially to melt down. A non-comprehensive list of conditions that can cause this is:

• lube oil in the exhaust system (damaged rings or valves)
• engine misfires (partially burnt fuel/air mix in the exhaust)
• cracked exhaust valve (unburnt fuel/air mix in the exhaust)
• lack of spark (electrical failure - unburnt fuel/air mix in the exhaust)

Contrary to popular belief overly rich fuel mixtures are not an issue - there is too little unused oxygen for the exotherm to be large enough to cause damage, and many engine manufacturers design "rich excursions" as a catalyst protection measure in the engine control software. A slightly lean of stoichiometric mix is far more dangerous, as the oxygen level is elevated, allowing a very large exotherm.

Engine misfires can overheat and destroy the converter as the excessive amounts of unburned fuel are broken down within it, especially when the engine is under heavy loads. Vehicles equipped with OBD-II diagnostic systems are designed to alert the driver of a misfire condition, along with other malfunctions, using the Malfunction Indicator Lamp or "Check Engine" light. If the misfire and engine load can produce heating severe enough to cause catalyst damage, the MIL will flash until the misfire or engine load is reduced.
(from: www.wikipedia.org)


Definition: Recycling

Recycling is the reprocessing of materials that would otherwise become waste in order to make them into new products. This is in contrast with reuse: collecting waste such as food containers to be cleaned, refilled and resold. Recycling prevents waste being landfilled or incinerated, reduces the consumption of new raw materials, and is advocated by supporters to require less energy than virgin production. Commonly recycled materials include glass, paper, aluminium, asphalt, steel, textiles and plastic. These materials can be derived either from pre-consumer waste (materials used in manufacturing) or post-consumer waste (materials discarded by the consumer). Recycling is a key concept of modern waste management and is the third component of the waste hierarchy. To be recycled waste has to be sorted and separated. Waste sorting can be done by the waste producer, at kerbside collection sites, or at a materials recovery facility. In everyday speech the word recycling often refers to the practice of waste sorting done by consumers.
(from: www.wikipedia.org)

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