Emission control, vehicle catalytic converters

Although the naturally occurring concentration of ozone at the earth s surface is low, the distribution has been altered by the emission of pollutants, primarily by automobiles but also from industrial sources which lead to the formation of ozone. The strategy for controlling ambient ozone concentrations arising from automobile exhaust emissions is based on the control of hydrocarbons, CO, and NO via catalytic converters. As a result, peak ozone levels in Los Angeles, for instance, have decreased from 0.58 ppm in 1970 to 0.33 ppm in 1990, despite a 66% increase in the number of vehicles. 

On-Board Diagnostics. State of California regulations require that vehicle engines and exhaust emission control systems be monitored by an on-board system to assure continued functional performance. The program is called OBD-II, and requires that engine misfire, the catalytic converter, and the evaporative emission control system be monitored (101). The U.S. EPA is expected to adopt a similar regulation. 

Emission Control Technologies. The California low emission vehicle (LEV) standards has spawned iavestigations iato new technologies and methods for further reducing automobile exhaust emissions. The target is to reduce emissions, especially HC emissions, which occur during the two minutes after a vehicle has been started (53). It is estimated that 70 to 80% of nonmethane HCs that escape conversion by the catalytic converter do so during this time before the catalyst is fully functional. 

Catalytic Converter an air pollution abatement device that removes pollutants from motor vehicle exhaust either by oxidizing them into carbon dioxide and water or reducing them to nitrogen. A typical catalytic oxidizer for auto emission control is illustrated in the sidebar figure. 

Probably the most significant control technology breakthrough came m 1977, when Volvo released a computer-controlled, fuel-mjected vehicle equipped with a three-way catalyst. The new catalytic converters employed platinum, palladium, and rhodium to simultaneously reduce NO and oxidize CO and HC emissions under carefully controlled oxygen conditions. The new Bosch fuel injection system on the vehicle provided the precise air/fuel control necessary for the new catalyst to perform effectively. The combined fuel control and three-way catalyst system served as the foundation for emissions control on the next generation of vehicles. 

To counter the elevated emissions associated with enrichment, the EPA has adopted supplemental federal test procedures. The new laboratory test procedures contain higher speeds, higher acceleration and deceleration rates, rapid speed changes, and a test that requires the air conditioning to be in operation. These tests increase the probability that vehicles will go into enrichment under laboratory test conditions. Hence, manufacturers have an incentive to reduce the frequency of enrichment occurrence in the real world. Future catalytic converters and emissions control systems will be resistant to the high-temperature conditions associated with engine load, and will be less likely to require enrichment for protection. Thus, enrichment contributions to emissions will continue to decline. 

Air pollution from exhaust emissions NO t and particulate matter from exhaust emissions causing air pollution are -1% at present. By adoption of better combustion control technology, better quality fuels (or even fuel cell-based motor vehicles), and more efficient catalytic converters, researchers aim to mitigate this problem. 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

ARCO has marketed a reformulated gasoline, EC-1 Regular (emission control-1), for older vehicles without catalytic converters, in southern California. These older vehicles were only a small segment of the total car and truck population in the region but produced about a third of the air pollution. ARCO has also marketed a premium reformulated gasoline, EC-Premium. The EPA estimated that the ARCO reformulated gasolines reduced air pollution by almost 150 tons a day in southern California. 

The control of carbon monoxide emissions is based on the principle that less of the gas is produced when the efficiency of combustion is improved. One device to achieve this objective is the catalytic converter, now required on all motor vehicles sold in the United States. A catalytic converter provides a second stage of combustion in motor vehicles, allowing carbon monoxide and other unburned components of a fuel to be oxidized before release into the atmosphere. (The operation of a catalytic converter is described later in this chapter.)... 

The primary system for controlling VOC emissions from automotive vehicles is the catalytic converter, described earlier in this chapter (see "Oxides of Nitrogen on pages 24-33). A number of different technologies have been developed for removing VOCs from flue gases of stationary sources. They include thermal and catalytic incineration, adsorption, absorption, and biofiltration. 

The need for controlling the exhaust emissions from automotive vehicles has been recognized since 1975. The most effective and tested method proved to be the installation of diree-way catalysts at the exhaust emission system of cars. The development and the improvement M of such catalysts was and will be a complicated effort, since a cat-alyst placed in a vehicle should simultaneously accelerate oxida-tion and reduction reactions, under continuously changing conditions of temperature and space velocity, in contrast to industrial applications where catalysts operate under fixed and MjM controlled conditions. Generally, the catalytic converter of a vehicle has to satisfy the following requirements ... 

Severe emission limits for motor vehicles were introduced, first in the United States and later in many other countries, starting in the mid-1960s. Meeting the increasingly stringent emission requirements in subsequent years forced the installation in motor vehicles of progressively more advanced emission control devices. The focal point of emission control is the catalytic converter, in which the desired chemical reactions occur. The pollutants carbon monoxide and unburned hydrocarbons (he) are converted by oxidation into the desired CO2 and water ... 

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