Should I Be Using Additives In My Stored Diesel Fuel?
Technical Information Report #090298-3
Gregory A. Hagopian
A common question from Uptime Users, "Should I be using additives in my stored diesel fuel"?
There are many additives available from many different sources. It is important to choose the proper additive to achieve the results that you are looking for. Recently, Chevron Products Company published, "Technical Review, diesel fuels". Section 7 of the Review is titled, "Diesel Fuel Additives, Types and Use of". We have reproduced Section 7 of the Review for this FTI Technical Information Report.
This Review is the result of a one year collaboration of employees and contractors of Chevron: John Bacha, Linda Blondis, John Freel, Greg Hemighaus, Kent Hoekman, Nancy Houge, Jerry Horn, David Lesini, Christa McDonald, Manuch Nikanjam, Eric Olsen, Bill Scott and Mark Sztenderowicz.
Please note: This information is accurate as of Spring 1998. It may be superseded by new regulations or advances in fuel or engine technology.
FTI thanks Chevron Products Company, Chevron employees and Chevron contractors for allowing us to reprint and pass this information on to you, our friends and associates.
DIESEL FUEL ADDITIVES
The first part of this chapter describes the additives that are used in diesel fuel - what they are and why and how they work. The second part describes their use in practice.
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.
2-Ethylhexyl nitrate (EHN) is the most widely used cetane number improver. It is sometimes also called octyl nitrate. EHN is thermally unstable and decomposes rapidly at the high temperatures in the combustion chamber. The products of decomposition help initiate fuel combustion and, thus, shorten the ignition delay period from that of the fuel without the additive.
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.
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.
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.
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.