TYPES OF ADDITIVES

Diesel fuel additives are used for a wide variety of purposes, however they can be grouped into four major categories:

Engine performance Fuel handling

Fuel stability Contaminant control

Engine Performance Additives 

This class of additives can improve engine performance. The effects of different members of the class are seen in different time frames. Any benefit provided by a cetane number improver is immediate, whereas that provided by detergent additives or lubricity additives is typically seen over the long term, often measured in tens of thousands of miles.

Cetane Number Improvers   (Diesel Ignition Improvers) Cetane number improvers can reduce combustion noise and smoke. The magnitude of the benefit varies among engine designs and operating modes, ranging from no effect to readily perceptible improvement.

The increase in cetane number from a given concentration of EHN varies from one fuel to another. It is greater for a fuel whose natural cetane number is already relatively high. The incremental increase gets smaller as more EHN is added, so there is little benefit to exceeding a certain concentration. EHN typically is used in the concentration range of 0.05% mass to 0.4% mass and may yield a 3 to 8 cetane number benefit.

Other alkyl nitrates, as well as ether nitrates and some nitroso compounds, also have been found to be effective cetane number improvers, but they are not currently used commercially. Di-tertiary butyl peroxide was recently introduced as a commercial cetane number improver.

A disadvantage of EHN is that it decreases the thermal stability of some fuels. The effect of the other cetane number improvers on thermal stability is unknown, but it seems likely that they will be similarly disadvantaged. Several laboratories are investigating this issue.

Injector Cleanliness Additives    Fuel and/or crankcase lubricant can form deposits in the nozzle area of injectors – the area exposed to high cylinder temperatures. The extent of deposit formation varies with engine design, fuel composition, lubricant composition, and operating conditions. Excessive deposits may upset the injector spray pattern (see Figure 7-1) which, in turn, may hinder the fuel-air mixing process. In some engines, this may result in decreased fuel economy and increased emissions.

Ashless polymeric detergent additives can clean up fuel injector deposits and/or keep injectors clean (see Figure 7-2). These additives are composed of a polar group that bonds to deposits and deposit precursors, and a non-polar group that dissolves in the fuel. Thus, the additive can redissolve deposits that already have formed and reduce the opportunity for deposit precursors to form deposits. Detergent additives typically are used in the concentration range of 50 ppm to 300 ppm.

Lubricity Additives   Lubricity additives are used to compensate for the poor lubricity of severely hydrotreated diesel fuels. They contain a polar group that is attracted to metal surfaces, causing the additive to form a thin surface film. The film acts as a boundary lubricant when two metal surfaces come in contact. Two additive chemistries, fatty acids and esters, are commonly used. The fatty acid type is typically used in the concentration range of 10 ppm to 50 ppm. Since esters are less polar, they require a higher concentration range of 50 ppm to 250 ppm.

Smoke Suppressants   Some organometallic compounds act as combustion catalysts. Adding these compounds to fuel can reduce the black smoke emissions that result from incomplete combustion. During the 1960s, before the Clean Air Act and the formation of the EPA, certain barium organometallics were used occasionally as smoke suppressants. The EPA subsequently banned them because of the potential health hazard of barium in the exhaust.

Smoke suppressants based on other metals, e.g., iron, cerium, or platinum, are used in other parts of the world; but have not been approved by the EPA for use in the U.S. These additives are often used in vehicles equipped with particulate traps to lower particulate emissions even further.

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Fuel Handling Additives

Antifoam Additives   Some diesel fuels tend to foam as they are pumped into vehicle tanks. The foaming can interfere with filling the tank completely, or result in a spill. Most antifoam additives are organosilicone compounds and are typically used at concentrations of 10 ppm or lower.

De-Icing Additives   Free water in diesel fuel freezes at low temperatures. The resulting ice crystals can plug fuel lines or filters, blocking fuel flow. Low molecular weight alcohols or glycols can be added to diesel fuel to prevent ice formation. The alcohols/glycols preferentially dissolve in the free water, giving the resulting mixture a lower freezing point than that of pure water.

Low Temperature Operability Additives   There are additives that can lower a diesel fuel's pour point (gel point) or cloud point, or improve its cold flow properties. Most of these additives are polymers that interact with the wax crystals that form in diesel fuel when it is cooled below the cloud point. The polymers mitigate the effect of the wax crystals on fuel flow by modifying their size, shape, and/or degree of agglomeration. The polymer-wax interactions are fairly specific, so a particular additive generally will not perform equally well in all fuels. To be effective, the additives must be blended into the fuel before any wax has formed, i.e., when the fuel is above its cloud point. The best additive and treat rate for a particular fuel can not be predicted; it must be determined experimentally.

The benefits that can be expected from different types of low temperature operability additives are listed in Figure 7-3.

Drag Reducing Additives   Pipeline companies sometimes use drag reducing additives to increase the volume of product they can deliver. These high molecular weight polymers reduce turbulence in fluids flowing in a pipeline, which can increase the maximum flow rate by 20% to 40%. Drag reducing additives are typically used in concentrations below 15 ppm. When the additized product passes through a pump, the additive is broken down (sheared) into smaller molecules that have no effect on product performance in engines.

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Fuel Stability Additives

Fuel instability results in the formation of gums that can lead to injector deposits or particulates that can plug fuel filters or the fuel injection system. The need for a stability additive varies widely from one fuel to another. It depends on how the fuel was made – the crude oil source and the refinery processing and blending. Stability additives typically work by blocking one step in a multi-step reaction pathway. Because of the complex chemistry involved, an additive that is effective in one fuel may not work as well in another. If a fuel needs to be stabilized, it should be tested to select an effective additive and treat rate. Best results are obtained when the additive is added immediately after the fuel is manufactured.

Antioxidants   One mode of fuel instability is oxidation, in which oxygen in the small amount of dissolved air attacks reactive compounds in the fuel. This initial attack sets off complex chain reactions. Antioxidants work by interrupting the chains. Hindered phenols and certain amines, such as phenylenediamine, are the most commonly used antioxidants. They typically are used in the concentration range of 10 ppm to 80 ppm.

Stabilizers   Acid-base reactions are another mode of fuel instability. The stabilizers used to prevent these reactions typically are strongly basic amines and are used in the concentration range of 50 ppm to 150 ppm. They react with weakly acidic compounds to form products that remain dissolved in the fuel, but do not react further.

Metal Deactivators   When trace amounts of certain metals, especially copper and iron, are dissolved in diesel fuel, they catalyze (accelerate) the reactions involved in fuel instability. Metal deactivators tie up (chelate) these metals, neutralizing their catalytic effect. They typically are used in the concentration range of 1 ppm to 15 ppm.

Dispersants   Multi-component fuel stabilizer packages may contain a dispersant. The dispersant doesn't prevent the fuel instability reactions, but it does disperse the particulates that form, preventing them from clustering into aggregates large enough to plug fuel filters or injectors. Dispersants typically are used in the concentration range of 15 ppm to 100 ppm.

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Contaminant Control

This class of additives mainly is used to deal with housekeeping problems.

Biocides    The high temperatures involved in refinery processing effectively sterilize diesel fuel. But the fuel quickly becomes contaminated with microorganisms present in air or water. These microorganisms include bacteria and fungi (yeasts and molds).

Since most microorganisms need free water to grow, biogrowth is usually concentrated at the fuel-water interface, when one exists. In addition to the fuel and water, they also need certain elemental nutrients in order to grow. Of these nutrients, phosphorous is the only one whose concentration might be low enough in a fuel system to limit biogrowth. Higher ambient temperatures also favor growth. Some organisms need air to grow (aerobic), while others only grow in the absence of air (anaerobic).

The time available for growth also is important. A few, or even a few thousand, organisms don't pose a problem. Only when the colony has had time to grow much larger will it have produced enough acidic by-product to accelerate tank corrosion or enough biomass (microbial slime) to plug filters. Although growth can occur in working fuel tanks, static tanks – where fuel is being stored for an extended period of time – are a much better growth environment when water is present.

Biocides can be used when microorganisms reach problem levels. The best choice is an additive that dissolves in both the fuel and the water so it can attack the microbes in both phases. Biocides typically are used in the concentration range of 200 ppm to 600 ppm. A biocide may not work if a heavy biofilm has accumulated on the surface of the tank or other equipment, because then it doesn't reach the organisms living deep within the film. In such cases, the tank must be drained and mechanically cleaned.

Even if the biocide effectively stops biogrowth, it still may be necessary to remove the accumulated biomass to avoid filter plugging. Since biocides are toxic, any water bottoms that contain biocides must be disposed of appropriately. The best approach to microbial contamination is prevention. And the most important preventative step is keeping the amount of water in a fuel storage tank as low as possible, preferably zero.

Demulsifiers   Normally, hydrocarbons and water separate rapidly and cleanly. But if the fuel contains polar compounds that behave like surfactants and if free water is present, the fuel and water can form an emulsion. Any operation which subjects the mixture to high shear forces, like pumping the fuel, can stabilize the emulsion. Demulsifiers are surfactants that break up emulsions and allow the fuel and water phases to separate. Demulsifiers typically are used in the concentration range of 5 ppm to 30 ppm.

Corrosion Inhibitors   Since most petroleum pipes and tanks are made of steel, the most common corrosion is the formation of rust in the presence of water. Over time, severe rusting can eat holes in steel walls, creating leaks. More immediately, the fuel is contaminated by rust particles, which can plug fuel filters or increase fuel pump and injector wear. Corrosion inhibitors are compounds that attach to metal surfaces and form a barrier that prevents attack by corrosive agents. They typically are used in the concentration range of 5 ppm to 15 ppm.

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USES OF ADDITIVES

Refinery Additization   This review discusses the many factors that determine the quality of diesel fuel. Given their number, it must be obvious that the quality of all diesel fuels is not the same. But, since fuel is the single largest operating expense for a diesel truck fleet, many users make their purchase decisions based on price alone.

Refiners have a legal requirement to provide a product that meets specifications. Beyond that, reputable refiners ensure that non-specification properties, such as stability, lubricity, and low temperature operability are suitable for the intended use.

The refiner has several options on how to achieve the desired properties: choice of crude oil, refinery processing, refinery blending, or the use of additives. The balance between refining actions and additive use is driven by economics. Since there are no legal requirements that diesel fuel contain additives, except red dye in high sulfur and tax-exempt fuel, some refiners may use no additives at all and still provide a high quality fuel.

There is no published information on use of additives. The following comments represent the authors' impression of common industry practice in the U.S.:

 

	Pour point reducers are probably the diesel fuel additive most widely used by refiners. However, their use is limited to fuel made in the wintertime and destined for regions with colder ambient temperatures.
	Some refiners add one or more additives to improve fuel stability, either as a regular practice or on an "as needed" basis.
	Some refiners use a cetane number improver when the additive cost is less than the cost of processing to increase cetane number.
	Red dye is added to high sulfur diesel fuel and may be added to tax-exempt diesel fuel at the refinery.

Cloud point is the property used in the U.S. to measure the low temperature operability of diesel fuel. Most refiners control cloud point by processing changes because cloud point reducing additives have historically been relatively ineffective.

While pour point reducers may improve CFPP, U.S. refiners probably don't use additives specifically designed to lower CFPP because it is not a specification property. Since Europe uses CFPP instead of cloud point as a measure of low temperature operability, additives which reduce CFPP are used more widely there.

Antifoam additives are widely used in Europe and Asia to ensure that consumers can fill their cars and trucks without spilling fuel on their hands, clothes, and vehicles. There is less of a problem with fuel foaming in North America because of different fuel properties (lower distillation end point), vehicle tank designs, and fuel dispensing pumps.

California: A Special Case   Because of its unique diesel fuel regulations, California is a special case. California regulations restrict the aromatics content of diesel fuel in order to reduce emissions. The regulations can be met either with a low aromatics diesel (LAD) having less than 10% aromatics, or with an alternative low aromatics diesel (ALAD) formulation that gives an equivalent reduction in emissions. Many of these ALAD formulations use cetane number improvers to help achieve the necessary emissions reduction. As a result, a significant percentage of the low aromatic diesel fuel now sold in California contains some cetane number improver.¹

Reducing diesel aromatic content to 10% requires more severe hydrotreating than reducing sulfur content. As a result, the lubricity of some LAD may be low, so some refiners may treat the fuel with a lubricity additive. (In the rest of the U.S., hydrotreating to remove sulfur may reduce lubricity, but not enough to require a lubricity additive.)

Two diesel fuel lubricity guidelines have recently been proposed in the U.S.: the EMA guideline recommends a 3100 g minimum (SLBOCLE method) and the state of California recommends a 3000 g minimum (SLBOCLE method). There are ongoing discussions and investigations in the industry, which may lead to a specification. In the absence of a specification, each refiner sets its own standard.

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Distribution System Additization   When diesel fuel is distributed by pipeline, the operator may inject corrosion inhibiting and/or drag reducing additives. No additional additives are added to diesel fuel distributed by truck or marine ship or barge.

Some refiners and petroleum marketers offer a premium diesel, which can be created at the refinery by the proper choice of operating conditions, or at the terminal by treating regular diesel with additives. Usually a blend of several additives, called an additive package, is used, rather than a single additive. The package may contain: a detergent/dispersant, one or more stabilizing additives, a cetane number improver, a low temperature operability additive (flow improver or pour point reducer), and a biocide. Each refiner or marketer is likely to use a different package of additives and a different treat rate. There are good reasons for this; many additives must be tailored to the fuel in which they will be used and the requirements of the market vary from place to place.

Aftermarket Additives   It would be convenient for the user if a finished diesel fuel could satisfy all his or her requirements without the use of supplemental additives. Although this is often the case, some users must use additives because the low temperature conditions in their region are more severe than those for which the fuel was designed, or because of other special circumstances. Other users feel that they need a higher quality diesel than regular diesel. And, finally, there are users who regard the cost of an additive as cheap insurance for their big investment in equipment.

A large number of aftermarket additive products are available to meet these real or perceived needs. Some are aggressively marketed with testimonials and bold performance claims that seem "too good to be true." So, as with any purchase, it is wise to remember the advice, caveat emptor – let the buyer beware.

It may be helpful to regard additives as medicine for fuel. Like medicine, they should be prescribed by an expert who has made an effort to diagnose the problem. And they should be used in accordance with the recommendations of the engine manufacturer and the instructions of the additive supplier. Sometimes indiscriminant use of additives can do more harm than good because of unexpected interactions.

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