Unburnt hydrocarbons

Reducing capacity is enhanced by hydrocarbons (unburnt fuel) which provide a source of hydrogen. 

A sample of the air/gas mixture is fed into the analyzer cell at a fixed rate. The cell and sensor are maintained at approximately 1500°F (812°C). the sample ignites, burners and the hot products of combustions (POC s) flow past the ceramic/platinum sensor and produce an electrical voltage proportional to the amount of excess O2 or excess hydrocarbon. The signal is amplified, conditioned and lineraized. A discrete step function occurs when excess hydrocarbon (unburnt fuel) is present in the POC s. This signal is in used an input to the PLC which has a PID control loop output to operate a gas flow control valve to maintain the specified Plasma Valve. 

Unburnt hydrocarbon (UHC) and carbon monoxide (CO) are only produced in incomplete combustion typical of idle conditions. It appears probable that idling efficiency can be improved by detailed design to provide better atomization and higher local temperatures. CO2 production is a direct function of the fuel burnt (3.14 times the fuel burnt) it is not possible to control the production of CO2 in fossil fuel combustion, the best control is the increasing of the turbine efficiency, thus requiring less fuel to be burnt for the same power produced. 

These catalysts are able to oxidize simulataneously the unburnt hydrocarbons and carbon monoxide to H20 and C02, and to reduce N02 emissions to N2. They... 

Compliance with the EuroIII standards (2000) forced the fitting of Diesel oxidation catalysts (DOC) in the exhaust line [for the after-treatment of unburnt hydrocarbons (HC) and carbon monoxide (CO)]. Additionally, the exhaust gas recirculation (EGR) was adapted to reduce the engine-out emissions of nitrogen oxides (NOx). 

However, even if this strategy seems very attractive, the EGR gases densification also shows several drawbacks (see Figure 7.3). Because of a lower temperature range, the combustion no longer allows the complete oxidation of the fuel carbon species that can be released in the exhaust line in the form of unburnt hydrocarbons. With EuroV constraints, the previous NO,/particle compromise is now substituted by a new one the NO,/IIC compromise. 

The oxidation catalyst reduces unburnt hydrocarbons and CO by combusting them over a platinum or palladium catalyst. 

Thus, rather surprisingly, the key to reducing tropospheric ozone pollution is to minimize the release of hydrocarbon vapors from such sources as unburnt fuel in automobile exhausts, vaporization of fuel at service station pumps (modern pumps recover fuel vapors from automobile fuel tanks while refilling them), kitchen exhausts from fast-food restaurants and, as in Mexico City, leakages of liquefied petroleum gas (mainly butane) used for domestic heating and cooking.26... 

R 22] This microstructured reactor will in the future be supplemented by a hydrocarbon adsorber in order to reach future zero emission regulations. A hydrocarbon adsorber equipped with a ceramic layer of zeolite will adsorb unburnt hydrocarbons during the engine start-up and desorb the gases when the pipe temperature rises above 150 °C which is obtained by the heated converter right behind the adsorber. 

Figure 11.1 (A) Dependence upon air/fuel ratio of concentration (arbitrary units) of pollutants formed in an internal combusion engine (HC = unburnt hydrocarbons). (B) Extent of removal of pollutants with the catalyst system named above the diagram.

Catalytie eonverters (eontaining preeious metal eatalysts dispersed on ceramic honeyeomb struetures that oxidize earbon monoxide and unburnt hydrocarbons to carbon dioxide and water, and reduee nitrogen oxides to nitrogen) are now fitted on more than 85% of new ears, and aehieve emission reductions of over 90%. 

The most practical and convenient method for removing NOj, gases is to reduce them to N2, using unburnt CO, H2, propene or other hydrocarbons in the exhaust gas. This can be achieved using conventional PGM catalysts, using a three-way catalyst (TWO) system, under stoichiometric conditions, where the amount of air supplied is controlled to a level just sufficient for the complete combustion of the fuel [132,489] (see Fig. 6.22). 

The pollutants covered are carbon monoxide (CO), unburnt hydrocarbons (HC) and nitrogen oxide (NOx). 

In the case of automotive gas exhaust emissions, the SCR process is Umited by technical problems, so that it has been dealt with only recently, essentially for emission control for trucks. From the general point of view, deNOx for cars is still an unresolved problem and different processes are under investigation. The development of three-way catalysts (TWC) over the last 30 years has been a remarkable technical achievement Current catalytic converters, based on various combinations of Pt, Pd and Rh as the active ingredients, provide a very high level of emission control for the removal of CO, NOx and unburnt hydrocarbons. Under normal working conditions for a stoichiometric gasoUne engine (temperatures from about 400 to 800 °C) they have proved to be efficient and reliable. 

In the early days of exhaust emissions control, it was established that, within the likely commercial range, motor gasoline and diesel fuel composition have relatively little effect on the composition of vehicle exhaust in terms of the regulated components carbon monoxide, unburnt hydrocarbons and nitrogen oxides. Legislation relating these exhaust components has therefore had little direct effect on the way in which fuels are refined and, hence, their composition. The only exception to this is where emissions controls are so severe as to demand catalytic exhaust control and hence unleaded gasoline. 

H2O, CO2 and N2 removed from air by non-cryogenic pressure swing adsorption over LiX zeolite, leaving > 95% O2 High silica zeolites adsorb unburnt hydrocarbons and desorb them as the engine and the catalytic converter warm up Shape selective Ca-A zeolite (5A) adsorbs linear but not branched hydrocarbons Shape selective MFI type zeolites (silicalite) adsorb para- but not ortho- or meta-xylenes. FAU type zeolites are also effective for this separation under simulated moving bed conditions... 

Zeolites are widely used to adsorb hydrocarbons, including unwanted volatile organic compounds (VOCs) such as solvent vapour and unburnt fuel in automobile engines. They are also used in refineries and petrochemical plants to separate and purify products on a large scale. The zeolites are chosen on the basis of cost, capacity, resistance to process conditions and, particularly in petrochemical applications, for their ability to act as true molecular sieves. 

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