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Metal based fuel additives were first studied as smoke suppressants and cetane improvers . Several metals, including Ba, Ca, Fe, Ce, and Mn, have been found effective in lowering the amount of soot formed during combustion in both diesel and SI engines. This effect was explained by a combination of the following three mechanisms:
The most effective smoke suppressants, Ba and Ca, are believed to work primarily through the first two mechanisms. Metallic additives could also reduce PM mass and other emissions, and improve fuel economy.
Currently, the main interest in metal based fuel additives is related to diesel particulate filters, as opposed to reducing engine-out emissions. There are two major reasons for this shift of focus: (1) diminishing emission effect from additives in modern diesel engines, and (2) health concerns related to metallic particulate emissions. In old technology diesel engines, using metallic fuel additives (e.g., 10 to 100 ppm of Fe, Ce) could reduce engine-out PM emissions by as much as 30-40% and improve fuel economy by as much as 10%. However, in advanced diesel engines (say US 2004 or Euro III)—just as it was the case with the effect of fuel properties—the emission reduction and fuel economy improvement are typically small, hardly justifying the use of additives. If additives are used, the metal is emitted from the engine in the form of a metal oxide (ash) nanoparticles. Even if the mass of such emission is low, the particle numbers can be very high, making them a possible public health concern. Therefore, environmental authorities usually remain cautious about the use of metallic fuel additives, unless a highly efficient particulate filter is used, which can capture the metal ash particles.
Numerous metal additives have been investigated as soot oxidation catalysts that would facilitate regeneration in passive diesel filter systems. Fuel additives used for that purpose are also called fuel soluble catalysts. As the additive is combusted in the engine cylinder, its metal component leaves the combustion chamber in the form of the corresponding metal oxide or other inorganic compound (e.g. sulfate). These compounds can form particles of their own or can be incorporated into diesel particulates. After being collected in the particulate filter, the catalytic metal is distributed throughout the diesel particulate phase and can effectively catalyze the oxidation of carbon particles.
The mechanism of filter regeneration is similar to that of a catalyzed filter, except the use of additives provides better contact between the catalyst and the carbon particles. This is probably why additive regenerated filters regenerate at somewhat lower temperatures than catalyzed filters. However, even though fuel additive filter systems may have the lowest regeneration temperatures, they are not likely to provide the ultimate passive solution of the particulate problem, especially in light-duty applications. As illustrated in the following examples of filter systems, commercial applications require “quasi-active” or “passive-active” approaches, where additive-induced regeneration is additionally supported by certain engine management measures in order to periodically increase exhaust gas temperature.
Although many papers have been published on diesel filter additives, there is little or no information on such issues as the actual interactions between the additive and diesel particulates, catalytic mechanisms, additive particle size and its distribution within the diesel particle phase.
Fuel additives are fuel-soluble organometallic compounds. Since the additive compound is destroyed by oxidation during the combustion process, its exact chemical formulation is, from the catalytic activity point of view, of limited importance. The additive formulation is, however, very important for its stability and miscibility properties. Organometallic compounds used for diesel filter additives that were reported in the literature include fatty acid soaps, acetyl acetonates, alcoholates, beta-diketonates, sulfonates, carbonates, carboxylates, dicyclopentadienes, and naphthenates .
Tens of compounds of various transition, alkaline, and noble metals have been tested as fuel additives to facilitate diesel filter regeneration. The list of metals includes sodium, lithium, calcium, barium, manganese, iron, nickel, cerium, copper, lead, silver, vanadium, platinum, and more. For the purpose of diesel filter regeneration, additives are usually added to the fuel at concentrations from a few to about 100 ppm wt. of the respective metal. The choice of additive level depends on the activity of the particular additive, exhaust gas temperature, and the quantity and composition of particulate emissions. Generally, higher additive concentrations allow the filter to regenerate at lower temperatures. The disadvantages of high additive levels are (1) increased filter pressured drop due to the accumulation of ash and (2) higher operating cost.
Additive concentrations much higher than 100 ppm have been reported in several laboratory studies. Such high concentrations may have only academic importance and are not practical. Even the concentration of 100 ppm can result in significant pressure drop build-up in the filter due to accumulation of the additive ash and appears to be too high for practical purposes.
Fuel additive for diesel filter regeneration which have successfully passed laboratory evaluations and are either commercial products or undergo field evaluations are listed below.
|Active Metal/Compound||Brand Name||Supplier|
|Iron (Fe/ferrocene)||Satacen||Octel (Pluto)|
|Iron-strontium (Fe-Sr)||Octimax 4800||Octel|
|Cerium (Ce), cerium-iron (Ce-Fe)||Eolys||Rhodia|
|Platinum (Pt), platinum-cerium (Pt-Ce)||Platinum Plus||Clean Diesel|
|† Lubrizol copper additive is no longer commercially available. Lubrizol has exclusively licensed the Clean Diesel “Platinum Plus” additive for sale in Europe.|